qualification of new technology for capture of co2 in coal-fired power plants

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MSC THESIS BJØRN UTGÅRD:

QUALIFICATION OF NEW TECHNOLOGY

FOR CAPTURE OF CO2 IN COAL-FIRED

POWER PLANTS

FOREWORD

This article is a result of a cooperative project between DNV Research and Innovation and

The Norwegian University of Science and Technology (NTNU) undertaken from September 2006

to September 2007. In agreement with the supervisors of the project, the scope of work has been

changed since the outset. The scope of the article therefore does not overlap fully with the scope

defined in the thesis assignment. For one, the report is written in the article format, with the

consequences that implies for formulation and structure. The tables of content and lists of

figures and tables are therefore given before the start of the actual article. Furthermore, the

intended application of the membrane was changed from gas-fired to coal-fired power plants. It

was also felt that the knowledge basis for technology qualification of the membrane needed to

be explored and synthesized, meaning that actually carrying out the qualification work would be

outside scope. Rather, the objective was changed to provide recommendations for the upcoming

qualification process. Negotiating access to business-sensitive information sufficient to actually

carry out qualification work was further a restraint. Finally, the desire to make sure that the

findings could be published openly in academic fora was an important consideration.

The work has been made possible due to three important instances. First of all, Dr. Håvard

Thevik and Dr. Tore Myrvold at DNV Research and Innovation had the original idea for the work.

They are gratefully thanked for their encouragement and direction. Secondly, Professor May-

Britt Hägg and Dr. David Grainger at the MEMFO research group at NTNU deserve gratitude for

their open and helpful attitude in the venture into new and unknown territory. And, most

importantly, this work would not have been possible without the insight, patience, support and

encouragement of Professor Marvin Rausand at the department of Production and Quality

Engineering of NTNU, who has been the mentor and academical anchor of the work. The author

is grateful for his generous attitude, flexibility and openness, the valuable personal and

professional advice, as well as the friendship.

MSc Thesis: Qualification of new technology for capture of CO2 in coal-fired power plants Bjørn Utgård, Norwegian University of Science and Technology, September 2007

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CONTENTS

1 Introduction ...................................................................................................................................................... 1

2 Qualification of new technology ............................................................................................................... 3

2.1 Classification of technology .............................................................................................................. 5

2.2 Reliability engineering ........................................................................................................................ 6

2.2.1 Systems and their interfaces ....................................................................................................... 6

2.2.2 Approach .............................................................................................................................................. 7

2.2.3 Reliability specification .............................................................................................................. 10

2.2.4 Reliability characteristics .......................................................................................................... 11

2.2.5 Operational regime and conditions ....................................................................................... 13

2.3 Reliability assurance ......................................................................................................................... 13

2.3.1 Functional analysis ....................................................................................................................... 15

2.3.2 Failure analysis .............................................................................................................................. 18

2.3.3 Reliability prediction ................................................................................................................... 25

2.3.4 Risk classification and ranking ................................................................................................ 32

2.4 Technology Qualification in practice ......................................................................................... 34

3 IGCC coal fired power plants and carbon capture .......................................................................... 35

3.1.1 IGCC power plants......................................................................................................................... 35

3.2 Carbon capture .................................................................................................................................... 36

3.2.1 Carbon capture concepts............................................................................................................ 37

3.2.2 Gas separation methods ............................................................................................................. 38

3.2.3 Bench-marking of carbon capture systems ........................................................................ 39

3.3 Carbon capture in IGCC power plants ....................................................................................... 40

3.3.1 System performance bench-mark .......................................................................................... 41

3.3.2 Process adaption for CO2 capture ........................................................................................... 41

4 Gas separation membranes ..................................................................................................................... 42

4.1 Membrane performance and economics .................................................................................. 42

4.1.1 Performance targets..................................................................................................................... 42

4.1.2 Performance determinants ....................................................................................................... 43

4.2 Membrane types ................................................................................................................................. 44

4.2.1 Separation driving forces ........................................................................................................... 44

4.2.2 Membrane materials .................................................................................................................... 45

4.2.3 Physical structure ......................................................................................................................... 46

4.3 Composite polymeric facilitated transport membranes .................................................... 47

4.3.1 Facilitated transport .................................................................................................................... 47

4.3.2 Membrane design and configurations .................................................................................. 49


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4.4 Module and process design ............................................................................................................ 51

4.4.1 Membrane modules ...................................................................................................................... 51

4.4.2 Flow patterns .................................................................................................................................. 53

4.4.3 Process design ................................................................................................................................ 54

4.5 Physics-of-failure of polymeric membranes ........................................................................... 55

4.5.1 Corrosion (chemical degradation) ......................................................................................... 56

4.5.2 Physical aging (structural degradation) .............................................................................. 57

4.5.3 Plasticization (swelling) ............................................................................................................. 58

4.5.4 Compaction ...................................................................................................................................... 59

4.5.5 Fouling ............................................................................................................................................... 59

4.5.6 Dehydration ..................................................................................................................................... 60

4.5.7 Concentration polarization ....................................................................................................... 60

4.5.8 Assembly issues ............................................................................................................................. 61

4.6 Membrane performance evaluation and testing ................................................................... 62

4.6.1 Performance simulation ............................................................................................................. 62

4.6.2 Membrane testing ......................................................................................................................... 62

5 Recommendations for qualification of a fixed site carrier PVAm membrane .................... 63

5.1 Background ........................................................................................................................................... 64

5.2 Qualification approach and methodology ................................................................................ 64

5.3 Qualification Basis ............................................................................................................................. 65

5.3.1 System description and boundaries ...................................................................................... 65

5.3.2 Operational regime and interface conditions .................................................................... 68

5.3.3 Performance and reliability requirements ......................................................................... 69

5.4 Technology Assessment .................................................................................................................. 70

5.5 Failure Mode Identification and Risk Ranking ....................................................................... 71

5.6 Selection of qualification methods .............................................................................................. 72

6 Concluding remarks .................................................................................................................................... 73

7 Bibliography................................................................................................................................................... 75


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LIST OF FIGURES

Figure 1 : The commercial rationale for developing and qualifying new technology ................... 4 Figure 2 : A technical system and its interfaces ............................................................................................ 7 Figure 3 : Design flexibility decreases throughout the technology development process. ......... 8 Figure 4: Uncertainties in qualification of new technology. .................................................................. 14 Figure 5 : Examples of Function tree, FAST and SADT diagrams. ....................................................... 16 Figure 6 : Failure mechanisms, modes and effects. ................................................................................... 18 Figure 7 : Failure process flow .......................................................................................................................... 19 Figure 8 : HAZOP process. ................................................................................................................................... 24 Figure 9 : Time and cost-efficient process for acquisition of adequate qualification and

reliability data. ................................................................................................................................................................. 25 Figure 10 : Structural similarity analysis process ..................................................................................... 27 Figure 11 : The physics-of-Failure process .................................................................................................. 28 Figure 12 : Risk ranking process....................................................................................................................... 32 Figure 13 : Technology qualification stages ................................................................................................. 35 Figure 14 : CO2 capture concepts for coal-fired power plants. ............................................................. 37 Figure 15 : General schemes of the main separation processes relevant for CO2 capture ...... 38 Figure 16 : IGCC power plant process blocks .............................................................................................. 41 Figure 17 : Membrane performance parameters. ...................................................................................... 43 Figure 18 : Total flux versus driving force in facilitated transport membranes ........................... 49 Figure 19 : A proposed mechanism for facilitated transport of CO2 in a fixed-site carrier

membrane. ......................................................................................................................................................................... 50 Figure 20 : Hollow fiber asymmetric polymer membrane .................................................................... 52 Figure 21 : Schematic of a typical hollow fiber membrane module and the case (pressure

vessel) that holds it ........................................................................................................................................................ 53 Figure 22 : Idealized flow patterns in membrane separation. ............................................................. 54 Figure 23 : Membrane modules set up in parallel in a series of stages. ........................................... 55 Figure 24 : Molar fraction profiles of the more (left) and less (right) permeable gases on the

feed side under steady separation condition ...................................................................................................... 61 Figure 25 : IGCC power plant with CO2 capture integrated in the process. .................................... 66 Figure 26 : Process diagram for an IGCC process with sour water-gas shift and membrane

CO2 capture ........................................................................................................................................................................ 66 Figure 27 : Physical system breakdown. ....................................................................................................... 68

LIST OF TABLES

Table 1: Technology classification (DNV 2001). ........................................................................................... 6 Table 2 : Reliability clauses in a specification (BS 5670). ...................................................................... 11 Table 3 : Operational regime and conditions (BS 5670). ........................................................................ 13 Table 4 : Function types. ...................................................................................................................................... 17 Table 5 : Failure causes (Rausand and Høyland 2004). .......................................................................... 19 Table 6 : FMECA table. ........................................................................................................................................... 21 Table 7 : Failure probability classification. .................................................................................................. 33 Table 8 : Consequence severity classification. ............................................................................................ 33 Table 9 : Risk ranking matrix. ............................................................................................................................ 34 Table 10 : Selected technology demonstration projects of IGCC with CCS ..................................... 36 Table 11 : Interface conditions and requirements. ................................................................................... 69 Table 12 : Newness classification of main elements of a membrane module. ............................... 70 Table 13 : Evaluation of the relevance of known polymer membrane failure mechanisms. ... 73

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QUALIFICATION OF NEW TECHNOLOGY FOR

CAPTURE OF CO2 IN COAL-FIRED POWER PLANTS

Bjørn Olsrud Utgård

Department of Production and Quality Engineering

Norwegian University of Science and Technology, 7491 Trondheim, Norway

Phone: +47 92088057, e-mail: [email protected]

ABSTRACT

New technology is needed to overcome the challenge of mitigating climate change in a world

heavily dependent on the utilization of fossil fuels for energy supply. To meet this need a CO2 –

selective polymeric gas separation membrane that enables capture of CO2 from fossil-fuelled power

plants is under development. Key to successful development and commercialization of this

technology is that it can be demonstrated that it will work in the intended application for the

intended time. In other words, it must be qualified. To do this, both the process of technology

qualification and the science of the membrane and the process of which it will be part must be

understood. Based on thorough surveys of these area s of knowledge, the current article provides

recommendations for the forthcoming qualification process.

1 INTRODUCTION

The Fourth Assessment Report of the International Panel on Climate Change (IPCC) leaves

little doubt; global warming is indeed taking place and its consequences - both those observed

and those anticipated – are very likely (probability of occurrence higher than 90%) to pose

serious threats to both human society and the ecosystems it relies on. And, “most of the

observed increase in globally averaged temperatures since the mid-20th century is very likely

due to the observed increase in anthropogenic greenhouse gas concentrations.” (IPCC 2007) It is

therefore necessary to stabilize and reduce anthropogenic emission of greenhouse gases.


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An effective strategy for reducing anthropogenic emission of greenhouse gases must address

both socioeconomic and technological aspects. While large reductions in greenhouse gas

emissions certainly is within reach through increased energy efficiency throughout the value

chain, economic development has historically been closely coupled with increased energy

consumption. Access to sufficient supply of energy without sacrificing national independence

and security is therefore a key national policy objective. An abundant and relatively cheap

source of energy, coal currently fuels some 40% of global electricity generation (IEA 2005). Until

an alternative that is both sufficiently cheap and abundant is found, large reductions in

consumption of coal and other fossil fuels seems unlikely. A way to allow continued exploitation

of fossil energy resources for power generation while strongly reducing its emission of

greenhouse gases is therefore needed.

A promising solution to this challenge is to capture CO2 from power plants and store it safely

in reservoirs in the earth’s crust (IEA 2006). The concept studied in this paper captures CO2

prior to the combustion in an Integrated Gasification Combined Cycle (IGCC) power plant by

means of a Fixed Site Carrier (FSC) cross-linked polyvinyl-amine membrane (PVAm) supported

on polysulfone (these terms are further described in Section 4.) The membrane acts as a semi-

permeable barrier between two phases, which in this case are both gases. Driven by a partial

pressure difference (solution-diffusion mechanism) and facilitated by a reversible chemical

reaction (absorption) with the membrane itself, the permeate is transported through the

membrane and separated from the remaining feed mixture, the retentate (Noble and Koval

2006).

Large-scale CO2 capture by means of polymer membranes in power plants is not yet a proven

concept. Even though computer simulations and laboratory experiments show that the concept

does have a large potential (Grainger and Hägg 2007; Favre 2007), what quality, reliability and

availability that can be expected in the field still remains uncertain. Qualification of the concept


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is therefore necessary to reduce uncertainty, mitigate risks, and ensure feasibility. The objective

of the current work is to establish a solid basis for cost- and time-efficient qualification of the

technology, and provide recommendations for the choice of technology qualification approach

and methods. The provision of precise cost and performance predictions for the proposed

membrane is a substantive exercise on its own, and is beyond scope of the current work.

Broadly speaking, qualification of new technology rests on two fundamental pillars of

knowledge: For one, qualification makes use of knowledge and analytical methods developed in

various academic fields, such as reliability engineering, risk management and project

management. An outline of the most relevant concepts and methods from these fields is given in

Section 2. Secondly, a thorough understanding of the technology at hand is necessary.

Developing membrane technology requires an understanding of polymer chemistry to develop

new membrane structures, physical chemistry and mathematics to model transport properties

and predict membrane separation performance, and chemical process engineering to design

separation processes for large scale industrial utilization (Strathmann 1990). Qualifying a new

polymeric membrane for capture of CO2 in power plants requires that these fields are

understood, and an outline is given in Sections 3 and 4. Recommendations for qualifying the

membrane are given in Section 5.

2 QUALIFICATION OF NEW TECHNOLOGY

Successful new products must be better (superior performance), faster (get to market

faster), and cheaper (lower factory cost, prices, and operating costs). “The problem is that these

three requirements seem to be mutually exclusive, particularly ‘better’ and ‘faster’ – it is very

difficult to achieve high reliability at low cost with shortened design and life cycles that leave no

time for testing and refinements” (Moss 1996). The desired effect of new technology (grey

arrows in Figure 1) is to increase the performance (quality, reliability, service life) of products

and reduce capital (CAPEX) and operating (OPEX) expenditure, and in so doing increase


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revenue. The undesirable side effect of introducing new technology is the increased risk it

induces. Technology qualification (black arrows) is therefore necessary to understand and

reduce risks to acceptable levels.

Figure 1 : The commercial rationale for developing and qualifying new technology (DNV 2006).

The starting point of any technology development project is a specification of the

requirements or intentions of the intended user of the technology in terms of function and

reliability. The specification must be in compliance with relevant standards and governmental

regulations. The design task of the developer is then to make optimum compromises among

elements such as performance, appearance, ease of construction or manufacture, and reliability

and maintainability (BS 5670). To enable transfer of the concept to the manufacture, marketing

and installation phases of the product cycle, the developer must be able to demonstrate and

document that the new technology in the end is able to live up to the specification. This is done

through technology qualification, which is defined as “confirmation by examination and

provision of evidence that the new technology meets the specified requirements for the intended

use” (DNV 2001). Qualification should be viewed as an iterative process in which design, trial,

analysis and improvement cycles until the desired performance (in terms of function and

Time

$

Technology Qualification

First

Production

CAPEX

Revenue

OPEX

Risk Exp.

Risk Exp.

New Technology


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reliability) is demonstrated and documented. The sooner in the development cycle attention is

given to technology qualification, the better.

Hence, the overall objective of technology qualification is twofold:

a. Identify all ways in which the new technology can conceivably fail; evaluate risks,

that is, the combination of likelihood and consequence of failure; suggest concept

improvements to tackle risks; and document compliance with specifications and

standards.

b. Minimize the time and capital penalty on the development project due to

qualification.

Note that economical investment analysis usually lies beyond the scope of technology

qualification.

2.1 CLASSIFICATION OF TECHNOLOGY

The choice of qualification methods depends on the level of newness of the technology. If

field experience shows that a technology is able to meet the specified requirements in a defined

environment, qualification rather simple; it merely requires documentation of this experience.

In this case the technology is proven (class 1 in Table 1). At the other end of this scale is new

technology (class 4 in Table 1), that is, “technology that is not proven” (DNV 2001). This is in line

with the technology development guideline of a major supplier of technology to the energy

industry, in which new technologies comprise “technologies and processes outside the current

capability range (often shown by wide risks or high uncertainty of success), technology used

outside its verified range or domain of performance, safety or reliability; and capabilities that

need to be acquired to feed into the development of one or more products.”

New products are rarely completely new; some elements (parts, processes) are usually

known. To keep the time and cost penalty of qualification to a minimum, it is important that

attention is focused on the truly new elements of the technology and not on elements that


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already have been qualified and proved to work in the field. To do this, the technology should be

divided into manageable elements that can be subject to further analysis, see Sections 2.3.1 and

2.3.2.

Table 1: Technology classification (DNV 2001).

Tech. status Application

Proven Limited field

history

New or unproven

The classes indicate:

1. No new technical uncertainties

2. New technical uncertainties

3. New technical challenges

4. Demanding new challenges

Known 1 2 3

New 2 3 4

2.2 RELIABILITY ENGINEERING

To be acceptable, a product must be able to operate satisfactorily for a specified period of

time in the actual application for which it is intended. Reliability assurance is therefore an

important part of technology qualification. Reliability is defined as “the ability of an item to

perform a required function, under given environmental and operational conditions and for a

stated period of time” (ISO 8402). From this, it is clear that function and failure are core concepts

of reliability engineering; the goal is to understand the mechanisms and impact of failure and

make informed choices throughout the life cycle to maximize performance.

2.2.1 SYSTEMS AND THEIR INTERFACES

Reliability engineering is based on the study of technical systems and their interfaces. For

example, a gas separation membrane constitutes a technical system in itself. One level up, it can

be regarded as a subsystem of a larger CO2 capture system, which again is a subsystem of the

power plant it is applied in. For the sake of analysis, a frame of reference is established to define

the boundary of the system of interest, that is, what lies within the system subject to study and

what lies beyond. The system will usually comprise several subsystems and components

interconnected in such a way that the system is able to perform the required functions in line


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with the specification. Elements of the system are termed functional blocks (Rausand and

Høyland 2004).

Sub-

system 1

Sub-

system 3

Sub-

system 2Wanted

outputs

Unwanted

outputs

Wanted

inputs

Unwanted

inputs

Boundary

conditions

External

threats

Support

System

Figure 2 : A technical system and its interfaces (Rausand and Høyland 2004).

The reliability of a technical system depends on its interfaces with the rest of the world. The

system is intended to perform a specific function to create a desired output from a specific input

(artifacts, material, energy, information). Beyond this wanted output, the system unfortunately

will most often produce outputs that are not wanted and can impact the surroundings adversely.

Unwanted inputs, for example particulate matter in a gas feed, further complicate the operation

of the system. Also influencing the system at its interface are certain boundary conditions, such

as environmental regulations, standards and risk acceptance criteria. The system may also need

some support, such as maintenance, cleaning or repair. External threats include the impact of

natural environmental catastrophes (earth quakes, floods and the similar), infrastructure

deficiencies and breakdowns, social events (sabotage etc), and other surrounding systems.

While the distinction between external threats and unwanted inputs is not always clear, the

essence is that the interaction across the system interface induces risks that must be managed.

Figure 2 illustrates a technical system and its interfaces (Rausand and Høyland 2004).

2.2.2 APPROACH

Reliability engineering and technology qualification can be done reactively, proactively,

qualitatively, or quantitatively. In practice a combination is usually necessary.


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Reactive vs. proactive

With a reactive approach, one examines the final product as a ‘black box’ by evaluating its

properties compared to the specification as a ‘final’ or ‘incoming inspection’ after product

development. This approach is reactive in the sense that it adds an additional step at the end of

the development phase, “and hardly differentiates in giving focus on those issues which really

need to be qualified.” The core method here is to test for the existence of known failure modes

which have been observed in products of matured technologies, usually by means of stress tests

of the product at elevated conditions. The necessity of field data makes “the meaningfulness of

the results questionable in the case of new or changed materials or technologies” (Gerling,

Preussger and Wulfert 2002). Another limitation to this approach is that identification of the

mechanisms by which failure occurs is difficult. This, combined with low design flexibility at the

time of qualification (see Figure 3), makes identifying and implementing potential design

improvements difficult.

Figure 3 : Design flexibility decreases throughout the technology development process. In the reactive

approach, qualification is done at the end of the design phase.

A supposedly more efficient approach is to make better use of existing knowledge and

integrate qualification into the innovation process instead of carrying out qualification as a

separate activity (Pecht 1993). In this proactive approach, qualification measures are carried out

Quality determined & costs committed

Life Cycle Phase

100%

50%

Pro

ble

m

Defin

itio

n

Concept

Desig

n

Deta

il D

esig

n

Ma

nufa

ctu

re

Use

Design Flexibility

Perc

enta

ge o

f to

tal


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throughout the development cycle, making proactive use of Physics-of-Failure (PoF) based

testing with respect to the product construction and the operational conditions (Gerling,

Preussger and Wulfert 2002). PoF is discussed further in Section 2.3.2.

Qualitative vs. Quantitative

A qualitative approach, in which reliability is thought of as a subjective characteristic, is the

traditional approach to reliability specification. Here, reliability has been achieved by use of

suitable codes of practice or methods of working established over decades of observing

experience. A qualitative specification of reliability requirements should clearly describe the

methods used to qualify the technology and the criteria against which reliability is to be judged.

Essential here is that relevant operating and maintenance conditions in particular are met (BS

5670).

The qualitative approach does however fall short when the importance of in-service

availability and cost of maintenance is significant. In this case, a quantitative approach is

necessary. Here, reliability specification includes defining the task (time or other measure of

usage) against which a reliability measure can be expressed, the conditions under which the

item is to function, and the performance at which the item’s function ceases to be satisfactory

(failure criterion). The requisite for this approach is that the ultimate conditions of use and

maintenance and the failure criteria that will be applied can be controlled, or that they can be

predicted adequately. Without this step, a quantitative assessment is not likely to be

trustworthy. Another prerequisite complicating the quantitative approach is that adequate

failure data must be available. These data can only be obtained from reliability testing, from field

data (statistics), or from physics-of-failure knowledge (physical processes leading to failure); the

greater the volume of data the greater the confidence (BS 5670). For new technology, field data

is by definition non-existent. Physics-of-failure knowledge may to some extent be available if the

materials used already have been thoroughly characterized and stresses and loads are known.

Designing realistic reliability tests is anything but straight forward, since it usually requires


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some sort of acceleration of usage (time, number of cycles, load). From this it is clear that while

quantitative analysis is preferable, qualification of new technology will have to rely on

considerable amounts of qualitative engineering judgment.

2.2.3 RELIABILITY SPECIFICATION

A technology is qualified when it is documented that the specified performance criteria are

met. A prerequisite for technology qualification is therefore that the requirements are clearly

communicated in writing, which is done in a specification. A specification may define general

characteristics or it may be specific to the reliability and maintainability features of a product,

such as service life at various performance levels, conditions of use, installation,

acceptance/rejection criteria and definitions of failure. The function of a specification is to

provide a basis of understanding between two parties so that both agree on the criteria to be

met (BS 5670).

A specification should be formulated in a way that makes it possible to demonstrate

compliance with the defined characteristics, which means that it should have the following

properties:

concise and precise;

adequate or complete;

unambiguous, particularly with regard to acceptance/rejection criteria

corresponding to the desired functions and reliability of the product in question;

BS 5670 gives direction for the formulation of reliability specifications. The clauses that

should be covered in a specification are shown in Table 2.


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Table 2 : Reliability clauses in a specification (BS 5670).

Reliability clauses Description

Definition of terms It is suggested that BS4778 is quoted

Function or functions of the item

Since any reliability specification is based on the failure of an item to perform its function, all functions must be stated. (See Section 2.3)

Criteria for failure of the item Failure criteria might follow implicitly from the definition of a function; in any case it is essential that all failure criteria are defined explicitly. (See Section 2.3)

Reliability characteristic or characteristics that are appropriate to the circumstances

Reliability characteristics are quantities used to express reliability in numerical terms. (See Section 2.2.4)

Required value of the reliability characteristics and, if relevant, the form of distribution of failures in time that is to be assumed

The required values are the numerical values of the chosen reliability characteristics.

The time during which, and the conditions in which, the item is required to perform its function or functions

The conditions in which the item will operate must be clearly stated. Conditions include stress, environmental and loading conditions. (See Section 2.2.5)

Maintenance policy In some cases, maintenance and operational procedures are expected to be important for reliability. Maintenance requirements and procedures, including periodic testing, maintenance periods and down time for maintenance, should therefore be specified to the user.

The means by which reliability assurance is to be attained

The methods used to assure compliance with the specified requirements, that is to qualify the technology, should also be specified. (See Sections 2.3)

Procedures for concessions and variations

Procedures for failure reporting and classification

2.2.4 RELIABILITY CHARACTERISTICS

Reliability characteristics are quantities used to express reliability in numerical terms. These

can be expressed in terms of success or failure of an item to perform a function for a given

period of time. The choice of characteristics and their value will depend on the application of the

product. Air-craft components will typically require a probability of failure (F(t)) lower than a

certain level for a 10-hour mission time, while safety systems will typically require a sufficiently

low probability of failure on demand (PFD), or a specific Safety Integrity Level (SIL). An example


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of the latter case is subsea technologies in the oil and gas industry. Measures such as mean time

to failure (MTTF), mean time between failures (MTBF), or availability are also common, and

time can be replaced by distance, cycles, throughput or other usage-related parameters

(Rausand and Høyland 2004).

Reliability clauses may specify that (BS 5670):

i. An item or equipment shall operate successfully (for X hours/cycles/other) on Y % of

the occasions on which it is required, with Z % confidence; or

ii. An equipment shall not fail more frequently than X times in Y equipment running

hours, with Z % confidence, given preventive maintenance to schedule...; or

iii. Assuming X distributed failures, the mean life of a population of similar items shall

be equal to or greater than, Y hours, with a standard deviation of not more than S

hours or with Z % confidence; or

iv. The probability that the item will be available (not undergoing corrective

maintenance) at any instant during the operational period shall be not less than X %.

These statements need to be coupled with the probability density functions of the

corresponding failure patterns. If these are known, every test can be related to available

information. This simplifies qualification procedures and also makes it possible to estimate the

confidence of the results. When the failure patterns are unknown, as usually is the case for new

technology, they must either be established from field data of structurally similar items or

assumed for the purpose of assurance (BS 5670).

The choice of characteristics and their required value influences the choice of reliability

assurance (qualification) methods. With characteristics i, ii or iii above, testing will be required

to provide proof that the specification has been met. The time and cost of such testing should be

considered when judging what should be required. In the case of characteristic iv, the required


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value will be assessed from some source(s) agreed upon between the supplier and purchaser

(BS 5670). With very stringent reliability requirements, as the case is in for instance the subsea,

aircraft and space industries, “box level testing is infeasible” (Pecht 1993), making alternative

approaches and methods necessary. And when only a small number of units will be made,

“testing to the extent required for statistical proof of the required reliability frequently becomes

inappropriate and assessments have to be made.” (BS 5670) Testing is further discussed in

Section 2.3.3.

2.2.5 OPERATIONAL REGIME AND CONDITIONS

The intended use and operation of the technology must be clearly defined for qualification to

be possible, since the appropriateness of the technology depends on its application. Important

conditions are shown in Table 3.

Table 3 : Operational regime and conditions (BS 5670).

Condition Description

Environmental conditions

Includes the conditions the product may be subjected to during transport, storage or operation. Examples include

i. Extremes of heat, cold, humidity and pressure ii. Exterior use, in which case rain proofing and resistance to

sunlight are important iii. Mechanical shock and vibration iv. Electromagnetic environment v. Use in contaminating or corrosive atmospheres

Stress conditions The nature and direction of the loading that is expected.

Lifespan/durability The targeted or required serviceable life of the product given adequate maintenance

Envisaged use time Share of time the item is expected to be in operation

Protection from misuse The means of protecting the product against the effect of misuse at any time of its total life

Maintenance The nature and frequency of the maintenance the product needs

Storage life The maximum shelf-life

2.3 RELIABILITY ASSURANCE

With reliability requirements specified, the task of the developer is to confirm “by

examination and provision of evidence that the new technology meets the specified


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requirements for the intended use” (DNV 2001). In the case of new technology, the volume of

data available is small, which means that the confidence of the evidence provided is reduced.

Confidence is further reduced due to possible discrepancies between specification, design,

manufacture, installation, commissioning and use. This means that the predicted performance

demonstrated through the qualification process may be different from the actual performance

realized in the field (Pecht 1993). This is due to unanticipated failure modes, unanticipated

operating conditions, unforeseen failure mechanisms and causes, epistemic uncertainties or

aleatory uncertainties, see Figure 4.

Figure 4: Uncertainties in qualification of new technology.

Unanticipated operating conditions stem either from incorrect specification or from

unexpected changes in the actual operating conditions. A thorough failure analysis is necessary

to avoid uncertainties related to failure. Epistemic uncertainty arises from the inability to obtain

complete knowledge about a matter, while aleatory uncertainty arises from the inability (of the

analyst) to provide perfect deterministic forecast of events. Epistemic and aleatory uncertainties

are by nature of their definition impossible to totally remove, so the only way to reduce their

impact is to enable the technology to cope with them. This is done by building robustness into the

design, so as minimize the impact of any remaining uncertainty (Clausing and Fey 2004).

≠

Unanticipated failure modes Unanticipated operating conditions Unforeseen failure mechanisms/causes Epistemic uncertainty Aleatory uncertainty (random noise)

Requirements

Field performance

(quality & reliability)

Inherent (design) performance

(quality & reliability)

Qualification


15 | P a g e

‘Examination and provision of evidence’ is done through functional and failure analysis, and

reliability testing, and can be supplemented with experience from proven, structurally similar

technologies.

2.3.1 FUNCTIONAL ANALYSIS

Failure can be defined as termination of the ability of an item to perform the required

(specified) function (BS4778). Hence, an analysis of the ways in which a technology can fail

requires that all of its functions and the performance criteria related to them are thoroughly

defined and understood. This is done through a functional analysis, which aims to (Rausand and

Høyland 2004):

1. Identify all the functions of the system

2. Identify the functions required in the various operational modes of the

system

3. Provide a hierarchical decomposition of the system functions

4. Describe how each function is realized

5. Identify the interrelationships between the functions

6. Identify interfaces with other systems and with the environment

The functions of a system are performed jointly by the physical components (subsystems) of

the system. An analysis of the system’s reliability may therefore take a structural or a functional

focus. With a structural focus, the system is broken down according to its physical structure

(subsystems and components). With a functional focus, the various functions of the system and

how these are fulfilled is of interest. Which focus is preferable depends on the use of the

analysis; while a structural focus often is more practical for existing systems, the functional

approach is useful in the early design process of a new system (Rausand and Høyland 2004).


16 | P a g e

System

function

System

function 3

System

function 2

System

function 1

Function 1.1

Function 1.1

Function 1.1

Function 1.1

Function 1.1

Function 1.1

Function 1.1

Limit of fuctional analysis

Why?How?

OR

AND

Function

(verb + noun)How?

When?

When?

Why?

More level 3 functions



System functions

Function 1 Function 2

Function

1.1

Function

1.2

Function

2.1

Function

2.2

Function

1.1.1

Function

1.1.2

Function

2.1.1

Function

2.1.2

Function

2.3

a) Function tree

b) FAST diagram

Control valves

Mechanism

Unstimulated

oil flowFunction Control

Mechanism

Input

Sensor

information

Function

Function

Control

Output

Mechanism

Input

Function

b) SADT diagram

Figure 5 : Examples of Function tree, FAST and SADT diagrams.

Various diagrams can be used for functional analysis. In a function tree, the overall system

function is analyzed by asking how the system function is carried out. The answer to this

question is a number of subfunctions that together make out the system function. This is

repeated for each of these subfunctions until the lowest level of functions is reached. A tree


17 | P a g e

diagram can also be used for a physical system breakdown in the case of a structurally focused

analysis. Other diagrams include functional block diagrams (SADT diagrams) and FAST

diagrams. Figure 5 shows examples of such diagrams.

It is important that all functions are identified, but all functions are not equally important.

For the purpose of functional analysis, the following classification of types is useful: Essential

functions, auxiliary functions, protective functions, information functions, interface functions,

and superfluous functions (Rausand and Høyland 2004).

Table 4 : Function types.

Function type Description

Essential Functions required to fulfill the intended function of the functional block

Auxiliary Functions required to support the essential functions

Protective Functions required to safeguard the system and its surroundings

Information Functions required for monitoring and managing the system

Interface Functions required across system interfaces by other systems

Superfluous Functions not really required or not used

Essential functions are those required to fulfill the intended purpose of the functional block;

without them, the item does not fulfill its raison d’être. For example, the essential function of a

CO2 separation membrane is, as indicated by its name, to separate CO2 from a gas mixture.

Auxiliary functions are those required to support the essential functions, and are usually less

obvious than essential functions. Yet, in many cases auxiliary functions are just as important as

the essential functions, and failure of auxiliary functions may be more safety critical than failure

of essential functions. For a membrane module, an auxiliary function may for example be to

‘transport gas’. To safeguard life, property and the environment from damage and injury,

protective functions such as safety functions, environment functions and hygiene functions.

Information functions include condition monitoring, alarms, gauges and control signaling, and

are needed to operate and manage the system. Interface functions are needed across the systems

interface with other systems, and can either be passive or active. In addition to these functions,

which all to some extent are really necessary for the system to work, superfluous functions may


18 | P a g e

be included in the functional block. These functions are ‘nice-to-have’ functions that are not

really necessary or never used (Rausand and Høyland 2004).

2.3.2 FAILURE ANALYSIS

A core activity of technology qualification is identifying all the ways in which the system can

fail to perform as required, that is, experience unreliability. This is the case when one or more

required functions are terminated (i.e. exceeding the acceptable limits). The event when this

happens is called a failure; the resulting state is termed a fault. A fault can be observed as a

failure mode. Each function may have several failure modes, and each failure mode may have

several different causes, mechanisms and effects, see Figure 6. (Rausand and Høyland 2004)

Function i

Failure

mode 1

Failure

mode 2

Failure

mode n

Failure

mode j

Failure

mechanism 1

Failure

mechanism 2

Failure

mechanism m

Failure

effect 1

Failure

effect 2

Failure

effect k

Figure 6 : Failure mechanisms, modes and effects.

The failure mode of a component will act as a failure cause of the subsystem, whose resulting

failure mode then causes failure of the next level and so on, see Figure 7.


19 | P a g e

Part failure

mechanism

Component

failure

mechanism

Assembly

failure

mechanism

Subsystem

failure

mechanism

System

failure

mechanism

part

component

assembly

subsystem

system

mode

cause

mode

cause

mode

cause

mode

cause

Figure 7 : Failure process flow (IEEE 2003).

In order to avoid failure, it is necessary to understand what circumstances during design,

manufacture or use which can lead to failure. Such failure causes are shown in Table 5.

Table 5 : Failure causes (Rausand and Høyland 2004).

Failure cause Description

Design failure A failure due to inadequate design of a functional block Weakness failure A failure due to weakness in the functional block itself when

subjected to stresses within the stated capabilities of the functional block

Manufacturing failure A failure due to nonconformity during manufacture to the design of a functional block or to specified manufacturing processes

Ageing failure A failure whose probability of occurrence increases with the passage of time, as a result of processes inherent in the functional block

Misuse failure A failure due to use for a purpose or at intensity or stress levels not envisaged in the design specification (outside stated capacity).

Mishandling failure A failure caused by incorrect handling or lack of care of the functional block.

Generally speaking, failure results either from the application of a single overstress (such as

shock, temperature extremes or electrical overstress) or by the accumulation of damage over

time from lover level stresses (such as temperature cycling, abrasion or material aging). Root


20 | P a g e

causes are the most fundamental factor or factors that, if corrected or removed, will prevent the

recurrence of the failure. (IEEE 2003) Failure mechanisms are “physical, chemical or other

processes that have lead to failure” (IEC n.d.). One of the reasons for identifying these root

causes and failure mechanisms in the design phase is to fix the problem at its most fundamental

source rather than merely fixing failure symptoms is that recurrence can be avoided (IEEE

2003). Another purpose of root cause and failure mode identification is to predict to predict the

probability of occurrence of the failure in real life (IEEE 2003).

Failure Modes, Effects, and Criticality Analysis

The systematic technique most commonly used to make sure the failure analysis captures all

conceivable failure modes and their effects is the Failure Modes, Effects, and Criticality Analysis

(FMECA). Usually carried out in the design phase of a system, the objective of an FMECA is to

reveal weaknesses and potential failures at an early stage, and based on their criticality rank

suggest corrective and mitigative modifications to the design. The level of detail and focus of

analysis depends on when the FMECA is carried out. In the early design phase, when no or very

few hardware solutions are known, a functional FMECA is done by identifying potential failures

for each function according to the hierarchy established in the function tree. In the embodiment

design phase, an interface FMECA is useful to verify compliance with requirements across the

interfaces between components and subsystems. When hardware and software solutions are

decided for the various functions in the detailed design phase, a System Breakdown Structure

showing the hierarchy of components and subsystems, much in the same way as for Function

Trees as shown in Figure 5. With the breakdown structure as input, a detailed FMECA identifies

system failures by starting with the failure modes at the lowest level of indenture and then

proceeding upwards in the hierarchy until the system level is reached.

The FMECA is done by answering a set of questions:

1. How can each part conceivably fail?


21 | P a g e

2. What mechanisms might produce these modes of failure?

3. What could the effects be if the failures did occur?

4. Is the failure in the safe or unsafe direction?

5. How is the failure detected?

6. What inherent provisions are provided in the design to compensate for the failure?

An FMECA may be done by individuals or by teams of experts, much in the same way as

HAZOPs, see below. The information is gathered in worksheets and updated throughout the

design process as new information becomes available (Rausand and Høyland 2004). It is

important that the FMECA is continually updated to keep pace with the evolving design so that it

can be used effectively throughout the product cycle. Methods for prediction of failure rates,

evaluation of consequence of failure and risk-reducing measures are outlined in Sections 2.3.3

and 2.3.4.

While an FMECA usually is very effective when applied to a system where system failures

most likely are results of single component failures, Fault Tree analysis may be a better

alternative for systems with a fair degree of redundancy.

Table 6 : FMECA table.

Description of unit Description of failure Effect of failure

Ref

. no

Fu

nct

ion

Op

erat

ion

al

mo

de

Fai

lure

m

od

e

Fai

lure

ca

use

or

mec

han

ism

Det

ecti

on

of

fail

ure

On

th

e su

b-

syst

em

On

th

e sy

stem

fu

nct

ion

Fai

lure

rat

e

Sev

erit

y

ran

kin

g

Ris

k

red

uci

ng

mea

sure

s

Co

mm

ents


22 | P a g e

Fault tree analysis

A Fault Tree is constructed by means of logic operators to display the interrelationships

between a potential critical event in a system and the causes of the event. Depending on the use

of the fault tree, it may be quantitative or qualitative in approach. The output may for example

be the probability of a critical event occurring during a specified time interval or a list of possible

combinations of environmental factors, human errors, normal events and component failures

that may result in a critical event in the system.

Fault tree analysis is a deductive technique in the sense that a system failure is the starting

point, and the causes for it are identified backwards in the causal chain until the root causes of

the failure are identified. The steps of constructing a fault tree are:

1. Define boundary conditions for the analysis

a. Physical boundaries of the system

b. Initial conditions (mode of operation)

c. Boundary conditions with respect to external stresses

d. Level of resolution (how far down in detail should the analysis go?)

2. Identify all critical/fault events and for each describe

a. What type of critical event it is

b. Where the event occurs (in the system)

c. When the event occurs (mode of operation)

3. Evaluate fault events (primary failures, secondary failures or command faults?)

4. Complete the gates before moving down the tree to the next gate

The gathered information is logically arranged in a tree shape, with logic operators

connecting the lower level causes to the TOP event. The finished fault tree is then qualitatively

and/or quantitatively analyzed to identify the various combinations of events that may lead to


23 | P a g e

the TOP failure, and weigh the importance/criticality of the possible events (Rausand and

Høyland 2004).

HAZard and OPerability study (HAZOP)

The limited amount of experience data that is available for qualifying new technology means

that it is particularly important to gather as much qualitative expert judgment on the potential

ways in which a system may fail as possible. A technique for systematically doing this is the

HAZOP technique. Commonly applied in the concept design phase of process plants to analyze

the risks involved in the system, its primary objective is to identify potential problems that can

arise during operation and maintenance of the system.

The main activity of a HAZOP is a managed creative process carried out through a series of

sessions by a team usually built up by 4-6 experts of relevant disciplines, a secretary and a

HAZOP facilitator charged with managing the process. By conducting the sessions as brainstorm

sessions, the creative potential of the team members can be utilized to identify all possible ways

the system can conceivably fail (Rausand 1991).

The HAZOP process is shown in Figure 8. After the process system has been divided into

individual processes (study nodes), each process can be studied in turn. Guide words are

selected and combined with process parameters to guide or stimulate the brainstorming

process. For example, the guide word less combined with the process parameter pressure helps

the team identify the consequences of this particular deviation. If the consequences are thought

to cause safety or operability problems beyond the limits of the design robustness, this is noted

as a potential hazard. Recommendations for mitigative measures may also be given.

A HAZOP analysis requires that the information (drawings, figures, other) used as basis for

the analysis is complete and accurate, and that the technical skill and insight of the team

members is adequate. The role of the facilitator is then to manage the team working process in


24 | P a g e

accordance with the principles of effective brainstorming, and help keeping the focus on the

serious hazards that are identified (Rausand 1991).

Select a process

Explain design intention of the process

Select a process variable or task

Repeat for all process sections

Repeat for all process variables or tasks

Repeat for all guide words

Develop action items

Apply guide word to process variable or task to develop

meaningful deviation

List possible causes of deviation

Assess acceptability of risk based on consequences,

causes and protection

Examine consequences associated with deviation

Identify existing safeguards to prevent deviation

Divide the system into processes (study nodes)

Figure 8 : HAZOP process.

HAZOP is a well recognized and commonly applied technique, but some limitations should

be considered. It is for example not well suited for identifying component failures and

environmental stresses as causes for deviation. Unforeseen hazards are not included in the

study, and the results rely heavily on the team’s composition (Rausand 1991). Combining

HAZOP with other techniques, such as FMECA and Fault Tree analysis, is therefore

recommended.


25 | P a g e

2.3.3 RELIABILITY PREDICTION

With all failure modes, mechanisms and causes identified, information must be gathered to

predict the probability and consequence of failure. This can be already existing knowledge about

failure mechanisms of structurally similar technologies or new information in the form of results

from tests and analysis of the technology at hand.

Structural similarity analysis

Gathering new information through analysis and in particular testing can be quite costly and

time consuming, and should therefore be preceded by checking the validity of existing

information. Qualification and reliability data acquisition should therefore follow the flow chart

shown in Figure 9.

Existing qualification

and reliability data for

the technology at hand

Sufficient for

qualification?

Qualification and

reliability data from

structurally similar

technologies

No

Sufficient for

qualification?

Concept

improvement

and reiteration

Yes

Yes

New/additional

information from

analysis and/or

testing

No

Compliance with

specification?

Technology

qualifiedYesNo

Predict field reliability

by combining gathered

information and

mathematical models

Figure 9 : Time and cost-efficient process for acquisition of adequate qualification and reliability data.

New technology may have properties (materials, architecture, design) similar to proven

technology in several ways. Given that the structural similarity can be validated, using

knowledge about such structurally similar technologies will make significant contributions to


26 | P a g e

simplifying qualification of the new technology. Structural similarity analysis is widely used in

the highly competitive semiconductor industry to define effective qualification and reliability

monitoring programs (van Driel, Zhang og Ernst 2005). The technique is also widely used in the

energy industry (Sunde and Bjeglerud 2006).

The structural similarity analysis process consists of six consecutive steps as shown Figure

10 (IEEE 2003):

1. Select an in-service item that has similarities with the item of interest. Examine

physical and functional characteristics of the items to confirm sufficient similarity.

The appropriate system hierarchy (based on for example an established function

tree) for comparison is selected in this step. A close design and operational similarity

will naturally improve reliability prediction accuracy.

2. Analyze and compare failure modes, mechanisms and root causes of the new and in-

service product. This information will typically come from failure analysis (Section

2.3.2). Failure mechanisms and root causes of high criticality should be examined in

detail, while those of lower criticality can be aggregated or approximated. Non-

similar failure modes, mechanisms and root causes are studied further in Step 3,

while those that are similar are followed by step 4.

3. Select appropriate reliability prediction method. For those failure modes,

mechanisms and causes that are not sufficiently similar, structural similarity analysis

does not apply, and other qualification methods should be applied.

4. Determine field reliability prediction of new and in-service items. For similar failure

modes, mechanisms and root causes, a field reliability prediction is performed for

the in-service item. If the failure modes, mechanisms and root causes are identical,

the field reliability prediction for the in-service item may used for the new item. If


27 | P a g e

they are similar but not identical, field reliability prediction may be adjusted as

described in Step 5.

5. Adjust field reliability prediction based on similarity between new and in-service

items. This adjustment is done based on qualitative assessment of the direction in

which the differences between the new and the in-service item is likely to influence

system reliability.

6. Combine reliability predictions from similarity analysis with reliability predictions

from other methods to create a reliability prediction for the new item

1. Select in-service item

structurally similar to the

item of interest

2. Analyze failure modes/

mechanism/causes of new

and in-service items

For similar failure modes/

mechanisms/causes

4. Determine field reliability

prediction of new and in-

service items

5. Adjust field reliability

prediction based on

similarity between new and

in-service items

3. Select appropriate

reliability prediction method

For non-similar failure

modes/mechanisms/causes

6. Combine reliability

predictions to create new

item reliability prediction

Figure 10 : Structural similarity analysis process (van Driel, Zhang og Ernst 2005).

In step 4 of the structural similarity analysis, mathematical models are applied to calculate

reliability predictions for each specific failure mechanism. If the data used is statistical data from

the in-service item or from tests of the new item the prediction will be empirical. If the

calculation is based on knowledge about physical material-load interactions and their influence

on product reliability with respect to the use conditions, the prediction is deterministic. The

validity of the models used can be tested by means of accelerated aging tests. If no models are

available, or if existing models are found to be inaccurate, then new models are developed using


28 | P a g e

a series of experiments, statistically designed to identify the most important design and

environmental factors governing failure and the mathematical relationship linking those factors

to the time to failure. Reliability prediction by means of testing is described below.

Physics-of-Failure

Physics-of-Failure (PoF) is a proactive, deterministic approach to reliability prediction. This

approach has along with structural similarity analysis been successfully applied in the

semiconductor industry, see Pecht and Dasgupta (1995) and Gerling, Preussger and Wulfert

(2002). The use of the PoF concept for qualification is based on the understanding that reliability

depends on the stability of product elements properties, their materials and the interaction

between them, and that these change in response to (external or internal) stresses applied in

operation by physical/chemical processes (Gerling, Preussger and Wulfert 2002). Figure 11

shows the PoF workflow.

Operational inputs

Pressure, temperature,

mole fraction, flow rate

Environment conditions

Temperature, relative

humidity, shocks and their

cyclic ranges, rate of

change and time and

spatial gradients. The life

cycle includes

transportation, storage,

handling and application

environments

Product materials,

geometry and architecture

Inputs

Manufacture,

Test and Screen

Conditions

Environment and

Application

conditions

Life Cycle

Stress Profiles

Stress Analysis

Thermal

Thermo-

mechanical

Hygro-

mechanical

Electro-magnetic

Vibration Shock

Diffusion

Reliability Assessment

Determines the

appropriate failure

mechanism model(s),

and calculates time-to-

failure for each failure

mechanism

Stress Sensitivity

Analysis

Evaluate sensitivity

of the product life to

application stresses

Derive the safe-

operating region for

the desired life cycle

profile

Derive screening

and accelerated test

conditions

Ranked list of

expected time-to-

failure with associated

failure mechanisms

and sites

Stress-margin conditions

Accelerated test

conditions

Outputs

Screening conditions

Figure 11 : The physics-of-Failure process (Pecht and Dasgupta 1995)

The PoF approach consists of the following seven steps (DoD 2005):

1. Select subsystem or component to analyze


29 | P a g e

a. Determine which subsystem or components are most functionally critical to

the operation of the system

b. Identify which subsystem or component has the highest likelihood of failure.

2. Examine operational and environmental loads and the preliminary design to identify

potential failure sites and failure modes. This is similar to the failure analysis

methods discussed in Section 2.3.2.

3. Analyze the stresses (including thermal stresses, thermal cycling, vibration,

mechanical shock, humidity, humidity cycling, voltage and current) that affect the

potential failure mechanisms.

4. Identify appropriate failure models (stress-life relationships) and their input

parameters (material characteristics, damage properties, relevant geometry at

failure sites, manufacturing flaws and defects, environmental and operating loads).

When possible, the variability for each design parameter is identified here.

5. After the model is developed, predict reliability characteristics (time-to-failure or

lifetime) of the potential failure mechanisms in the test, operational or usage

environment. Probabilistic time-to-failure estimates should preferably be calculated

for the variability of input parameters and process characteristics.

6. Perform physical testing (life testing, accelerated-life testing, instrumented terrain

tests, instrumented drop tests or other tests) to validate the modeling process. The

objective here is to validate the stress analysis, verify that the identified failure

mechanisms will occur, and to determine if there are unexpected failure

mechanisms.

7. Redesign to eliminate failure mechanisms, or if this is not feasible, plan a preventive

maintenance program to replace the affected items before they cause a system

failure.


30 | P a g e

Performance and reliability testing

Usually used in combination with other methods, or as a validation of these (see for example

stage 6 of the PoF process), the benefit of tests is that they include actual equipment operational

experience. This makes possible the discovery of unexpected failure modes and experimentation

for robustness development (Clausing and Fey 2004). The main disadvantage of testing is its

time and cost penalty on the development cycle.

One of the most critical aspects of reliability or qualification testing is planning. Some

general test considerations in this respect are given below (DoD 2005):

1. A structured system for collecting and storing gathered data is highly desirable. The

database should include test start and stop times and dates, environmental

conditions, transients, transient durations, unit responses, etc. If the event of failure

the results of the subsequent root cause analysis along with identified corrective

actions and design modifications should also be stored in the database. This

information is useful to understand the circumstances leading to failure.

2. Tests can be performed at material, component, subsystem or fully integrated system

level. Testing of the integrated system at full scale in actual environmental conditions

is usually prohibitively expensive. Tests are therefore often conducted at lower

levels of the system. If failure due to interaction of various components is negligible,

reliability data at lower levels can be combined to infer reliability at the next higher

level of the system hierarchy.

3. Some failures can be excluded from the results if it after rigorous failure analysis is

proved that they can be ascribed to conditions related to circumstances of the test

(test fixture, test software or environmental conditions that will not be present in the

actual use environment).


31 | P a g e

4. Multiple failures due to the same cause or exhibiting the same failure mode should

not be aggregated and should instead be counted as individual events.

Testing can be done at the specified loading levels and environmental conditions, which is

the case for reliability demonstration tests and various kinds of manufacturing tests. Common

for this kind of tests is that they are performed at the later stages in development cycle, typical

to the reactive approach discussed in Section 2.2.2.

To verify within a short time the life-cycle reliability of items that are expected to last for a

long period of time (order of several years), some sort of acceleration (elevation) of test

conditions (number of cycles, loads and stresses, temperature, pressure) is necessary. The goal

of accelerated testing is to accelerate the damage accumulation rate for the relevant wear-out

failure mechanisms. The level of acceleration is indicated by an acceleration factor which is

defined as the ratio of the life under real life-cycle conditions to the life under accelerated test

conditions. The data obtained must be quantitatively extrapolated from the test results to

provide a prediction of reliability in use conditions. In this case, uncertainty is added to the

qualification since it is not necessarily given that the up-scaled data accurately indicates the

performance that will be delivered by the integrated product in the actual conditions. Hence,

confidence in results from tests performed with accelerated conditions is expectedly lower than

if the tests were performed in realistic conditions, but may still be acceptable (DoD 2005).

A comprehensive account and evaluation of the appropriateness of the various types of tests

available for testing performance and reliability is beyond the scope of the current work. It is

however appropriate to emphasize the fact that testing is both time-consuming and costly. It is

therefore commendable to put effort into optimizing the utility of their results and evaluate

precisely the extent to which testing is strictly necessary, that is, identify the parts of the

technology that actually are new (see Section 2.1).


32 | P a g e

2.3.4 RISK CLASSIFICATION AND RANKING

With all possible failure modes identified and their probability of occurrence and

consequences to the overall system function and reliability predicted, an evaluation of all risks

induced by the technology can be done. The risk represented by a failure is the combination of

the failure probability and its consequence.

While the body of research on risk assessment is voluminous, the essential activities for the

current purpose can be summarized as shown in Figure 12. The input to failure probability and

consequence classification is information from the preceding failure analysis and probability

prediction activities. Probability of occurrence and consequence for all failure modes are

combined to form a risk ranking matrix, which is useful for focusing on the most critical issues.

Risks that fall outside the defined risk acceptance criteria should be reduced through corrective

measures or design modifications.

Classify failure

probability

Classify failure

consequences

Failure analysis:

identification of

potential failure

Risk ranking

matrix

Risk acceptance

criteria

Risk

acceptable?Risk reduction

No

Yes

Figure 12 : Risk ranking process.

Failure probability

Failure probability can in most cases be classified in rather broad classes, an example of

which is shown in Table 7.


33 | P a g e

Table 7 : Failure probability classification.

Description Failure rate

5 Frequent Once per month or more often

4 Probable Once per year

3 Occasional Once per 10 years

2 Remote Once per 100 years

1 Very unlikely Once per 1000 years or more seldom

Consequence classification

The consequence of failure can be measured by its impact on safety and health, environment

and production. An evaluation of the severity (intensity of consequence) of a failure must hence

take into account the specific operating environment. An example of severity classification

categories is shown in Table 8. The classes should be given definitions relevant to the specific

case studied.

Table 8 : Consequence severity classification.

1 2 3 4 5

Description Insignificant Low Significant Serious Catastrophic

Safety and health

No injury Potential for minor injury, no fire potential

Potential for serious injury, potential for fire in non-classified area

Potential for fatalities, potential for small fire in classified area

Potential for several fatalities, potential for large fire in classified area

Environment No pollution < 1 ton > 1 ton > 10 ton > 100 ton

Production No effect Partial capacity reduction

Outage up to 1 day

Outage up to 1 week

More than 1 week outage

Risk ranking matrix

Table 7 and Table 8 can now be combined to form a risk ranking matrix, an example (DNV

2001) of which is shown in Table 9. The risk level increases upwards from left to right. The risk

ranking matrix allows each failure mode to be classified in terms of its risk, and then compared

to the defined risk acceptance criteria. Failure modes with medium and high risk, termed failure


34 | P a g e

modes of concern, are then subject to further investigation, while those of low risk can be

assessed qualitatively by qualified personnel (DNV 2001).

Table 9 : Risk ranking matrix.

Consequence

Probability

1 2 3 4 5

Insignificant Low Significant Serious Catastrophic

5 Frequent Medium risk Medium risk High risk High risk High risk

4 Probable Low risk Medium risk Medium risk High risk High risk

3 Occasional Low risk Low risk Medium risk Medium risk High risk

2 Remote Low risk Low risk Low risk Medium risk Medium risk

1 Very unlikely Low risk Low risk Low risk Low risk Medium risk

2.4 TECHNOLOGY QUALIFICATION IN PRACTICE

The knowledge base on which technology qualification rests is so comprehensive, the

perhaps most difficult issue for the engineering team seeking to get a new technology qualified is

not if there are methods available but rather which ones to use and how to use them. To provide

instruction and guidance to the team, companies therefore develop generic guidelines for

technology development and qualification. In addition to guiding the technology development

process, technology qualification is also intended to facilitate communication between

subsuppliers, suppliers, buyers and users. This creates a need for consistency across companies’

technology qualification efforts, which drives the development of industry standard practices.

One such recommended practice, the DNV-RP-A203, was developed by Det Norske Veritas

and energy industry partners in 2001. It arranges the technology qualification stages in a

workflow shown in Figure 13 (DNV 2001). For each stage in this managed workflow, decisions

must be made regarding which methods (outlined above) to use. It should be noted that the

linear appearance of the figure discriminates the fact that technology qualification in real life is

an iterative process, in which the stages are updated throughout the process as more


35 | P a g e

information is made available through analysis and testing. Suggestions for improvement will

typically also be identified and implemented throughout the process.

Figure 13 : Technology qualification stages.

3 IGCC COAL FIRED POWER PLANTS AND CARBON CAPTURE

Coal fired power plants account for about 60 percent of worldwide large stationary CO2

sources emitting more than 0.1 Mt CO2 per year (IPCC 2005). At the same time, about 40 percent

of world electricity generation is currently fuelled by coal. The IEA World Energy Outlook 2005

predicted global coal demand to experience a 1.4 % annual growth rate until 2030, which adds

up to a 36% growth from the present demand (IEA 2005). Part of the explanation for the

growing demand for coal lies in increased oil and gas prices, but economics is not the only

driver. Energy security concerns are also encouraging countries such as China and India to

increase the use of coal as an alternative to import of other sources of energy (Ricketts 2006).

Hence, drastically cleaner coal technologies must be implemented if the targeted reductions in

CO2 emissions are to be feasible.

3.1.1 IGCC POWER PLANTS

One of the most promising “clean coal” technologies is the Integrated Gasification Combined

Cycle (IGCC) power plant. The concept is rather new, and only four plants (two in the US, one in

Spain, one in the Netherlands) are currently in operation. It is viewed as one of the “cleanest and

Functionality Assessment

Data Collection (Analysis and Testing)

Selection of Qualification Methods

Failure Mode Identification and Risk Ranking

Technology Assessment

Qualification Basis


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most efficient clean-coal power-production technologies,” and has the advantage of being

flexible both in terms of input (all carbonaceous feedstocks, including coal, petroleum coke,

residual oil, biomass and municipal solid waste can be processed) and output (any mix of

electricity and hydrogen is possible) (IEA 2006).

Table 10 : Selected technology demonstration projects incorporating carbon dioxide capture and storage

(CCS) (Ricketts 2006). Refer to Figure 14 and Figure 16 for explanations of process terms.

Company/project name

Fuel Plant output (and cost)

Technology Start date

RWE, Germany coal 450 MW (€1bn)

IGCC + shift + pre-combustion, storage in saline reservoir

2014

Siemens, Germany coal 1000 MW €1.7bn

IGCC + shift + pre-combustion 2011

Stanwell, Queensland, Australia

coal 100 MW IGCC + shift + pre-Combustion, storage in saline reservoir

2012

BP DF2, Carson, USA petcoke 500 MW ($1bn)

IGCC + shift + pre-combustion, storage in oilfield – EOR

2011

China Huaneng Group (CHNG), GreenGen, China

coal 100 MW IGCC + shift + pre-combustion 2015

E.ON, Killingholme, Lincolnshire coast, UK

coal 450 MW (£1bn)

IGCC + shift + pre-combustion? (may be capture ready)

2011

FutureGen, USA coal 275 MW ($1bn)

IGCC + shift + pre-combustion 2012

Nuon, Eemshaven, Netherlands

coal/ biomass/ natural gas

1200 MW IGCC with option to capture 2011

GE / Polish utility coal 1000 MW IGCC + shift + pre-combustion

Powerfuel, Hatfield Colliery, UK

coal ~900 MW IGCC + shift + pre-combustion 2010

Centrica / Progressive Energy, Teesside, UK

coal (petcoke)

800 MW ($1.5bn)

IGCC + shift + pre-combustion (+ H2 to grid)

2009

A technology foresight scenario prepared by IEA (2006) shows that by 2050, more than

5000 TWh electricity globally can be provided by IGCC plants, which with an assumed average

plant size of 700 MW and 7000 operating hours per year amounts to 1000 new plants. This

optimism seems to be reflected in the industry, as a host of IGCC projects are currently being


37 | P a g e

planned (Holt 2003). Many of the projects are planned to include carbon capture, a selection of

which is shown in Table 10 (Ricketts 2006).

3.2 CARBON CAPTURE

The purpose of CO2 capture is to produce a concentrated stream that can readily be

transported by pipelines or ships to sequestration sites where the CO2 can be permanently

deposited in geological formations in the earth’s crust. The economics of CO2 transport requires

that the CO2 is compressed (typically up to 110 bar) before transport (IPCC 2005).

3.2.1 CARBON CAPTURE CONCEPTS

Power & Heat CO2 Separation

GasificationReformer +

CO2 separation

Power

& Heat

H2

Coal

Coal

Air

Air/O2

Steam

CO2

CO2

Air/N2

N2, O2

Coal

Air Separation

Power & Heat

O2

N2, O2

AirN2

CO2

Transport

& Storage

Postcombustion (Pulverized Coal)

Pre-combustion (IGCC)

Oxy-fuel combustion

CO2

Compression

& Dehydration

Figure 14 : CO2 capture concepts for coal-fired power plants. Figure based on IPCC (2005).

Several processes for capturing CO2 from power plants are under investigation. These are

typically grouped with respect to the time or place of separation in the process sequence, that is,

whether the capture happens before or after combustion of the fuel (pre- or post combustion) or

if combustion happens with pure oxygen separated from air (oxy-fuel). While pre- and post

combustion both involves separating CO2 from flue gas or synthesis gas, oxy-fuel combustion


38 | P a g e

requires separation of O2 from air. A conceptual overview of these concepts applied to coal-fired

power plants is given in Figure 14 (IPCC 2005).

3.2.2 GAS SEPARATION METHODS

Several separation mechanisms and technologies are applicable in each of these processes.

The three main concepts are (i) absorption/adsorption with sorbents or solvents, (ii) cryogenic

distillation, and (iii) separation with membranes, see Figure 15.

a) absorption/adsorption with sorbents/solvents

c) Separation by cryogenic distillationb) Membrane gas separation

Figure 15 : General schemes of the main separation processes relevant for CO2 capture. The gas removed

in the separation may be CO2, H2 or O2. In b) and c) one of the separated gas streams (A or B) is a

concentrated stream of CO2, H2 or O2 and the other is a gas stream with all the remaining gases in the

original gas (A+B) (IPCC 2005).

In the first concept, CO2 is removed by passing the CO2 -containing gas in direct contact with

a liquid absorbent or solid sorbent that is capable of capturing CO2, usually an amine. The

sorbent is then regenerated to be used again, releasing the captured CO2 that is then sent for

storage. This concept is applicable for pre- and post combustion capture. Cryogenic distillation

uses the difference in thermodynamic properties to separate O2 or CO2 from a gas mixture. The

gas mixture is first liquefied through a series of compression, cooling and expansion steps,


39 | P a g e

making separation of the gases possible through a distillation column. The process is currently

used commercially at large scale for separation of O2 from air, and is applicable both for oxy-fuel

combustion and pre-combustion capture. The third concept is gas separation with membranes.

A membrane can be viewed as a semi-permeable barrier that only lets part of the gas feed

through. The gas that permeates (migrates) through the membrane is called the permeate, while

the (non-permeating) gas that is retained on the feed side is called the retentate. Being the

concept of interest in this study, membrane gas separation is discussed further in Section 4.

3.2.3 BENCH-MARKING OF CARBON CAPTURE SYSTEMS

An economic comparison of the contender technologies and concepts is complicated. Since

actual field experience is almost completely lacking, such a comparison would have to be based

on published estimates for CO2 capture costs. These vary greatly, mainly as a result of different

assumptions regarding technical factors related to plant design and operation as well as

economic and financial factors such as fuel cost, interest rate and plant lifetime (Rubin, et al.

2006). Even so, the IPCC Special Report on Carbon Dioxide Capture and Storage (2005)

concluded that absorption processes based on chemical solvents are the preferred solution for

post-combustion capture. The rather limited breadth of scientific evidence referred to support

this conclusion (it was based on three comparative studies, the latest dated in 2000) gives

reason to question its validity, and certainly should not discourage further development of other

concepts, such as membranes. An extensive discussion of this argument is provided by Favre

(2007).

Although none of the CO2 capture technologies as such have yet been put into large-scale

application in power plants, several of the concepts have reached the commercial stage of

operation for CO2 capture, “albeit not on the scale required for power plants” (IPCC 2005, p114).

A techno-economic analysis of contending solutions for CO2 capture at a planned gas-fired

power plant in Norway found an upscale in the order of 10 from the largest plant currently in

operation to be necessary for that particular application (Svendsen, Thorsen and Selfors 2005).


40 | P a g e

Confidence in estimates of cost of capture is low due to the fact that no full-scale coal- or gas-

fired power plants have yet been commissioned. IPCC (2005) expects it to be in the range of 15 -

75 US$/t CO2 net captured. This level uncertainty is certainly not acceptable to potential investors, and

underlines the importance of further investigation and technology qualification.

3.3 CARBON CAPTURE IN IGCC POWER PLANTS

Figure 16a) shows the main blocks of an ordinary IGCC process. Pure O2 is separated from

air by the Air Separation Unit (ASU) and supplied to the gasifier, where coal is gasified at

pressures normally in the range of 30-80 bar and 1000-1500oC. The high pressure, high

temperature raw synthesis gas, consisting mainly of H2 and CO as energy carriers, is cooled to

allow for removal of dust particles, sulphur, chlorides, mercury and other contaminants. The

cleaned synthesis gas is then mixed with nitrogen from the ASU (in order to suppress NOx

emissions) and burned in a gas turbine. A Heat Recovery Steam Generator (HRSG)

boiler/superheater uses the exhaust heat to generate steam, which in turn drives a steam

turbine. Electricity is generated by both generators. The overall plant efficiency is improved by

this combination of gas and steam turbines, hence the term ‘Combined Cycle’ (Starr, Tzimas and

Peteves 2006).

The high pressure of the syngas makes IGCC power plants well suited for pre-combustion

capture of CO2. An example of a modified IGCC process with capture of CO2 is shown in Figure

16b. The syngas is run through a Water Gas Shift (WGS) reaction in which CO and H2O is

converted to CO2 and H2, before sulfur and other contaminants are removed as in the ordinary

case. Alternatively to this ‘sour gas shift’, sulfur (bound as H2S) can be removed from the syngas

before WGS; this is known as ‘sweetened gas shift’. After the WGS, the gas typically has a

pressure of around 20 bar and contains 40 mol% CO2. High pressure (separation driving force)

and concentration of CO2 is advantageous for CO2 capture (see Section 4).


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GasifierCombined Cycle

Power BlockPowerO2 Syngas

Air

Separation

Unit (ASU)

GasifierO2

Air

Air

Coal

Slag

Coal

Slag

Gas Clean

Up

Combined

Cycle Power

Block

PowerWater Gas

Shift

CO2 H2

N2

N2

Sulfur, other H2

Gas Clean

Up

a) ordinary IGCC process

b) IGCC process with CO2 capture

Syn-

gas

Sulfur, other

Air Separation

Unit (ASU)

CO2 capture

Figure 16 : IGCC power plant process blocks. Concept a) is the ordinary concept, b) includes CO2 capture.

3.3.1 SYSTEM PERFORMANCE BENCH-MARK

IGCC plants (without CO2) capture currently in operation have efficiencies of about 43%, a

figure that is expected to increase up to 55% due to further technology development in the

coming decades (IEA 2006). Most of the first generation IGCC plants have experienced

availabilities of less than 80% after a number of years in operation, about 5-7% less than

contending coal technologies (e.g. Pulverized Coal plants) (Dalton 2005). This relatively low

figure is attributable to the newness of the technology and can be expected to increase as

experience from the first generation plants is implemented in future installations (Elcogas n.d.).

3.3.2 PROCESS ADAPTION FOR CO2 CAPTURE

While IGCC plants are well suited for carbon capture, several process adaptations must be

made to accommodate carbon capture in IGCC plants. The most notable adaptations are (Dalton

2005):

Gasifiers and ASU need to be oversized to match later CO2 removal (more syngas is

needed)


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More moisture is needed for WGS CO in syngas to CO2, favoring different gasifier

designs

Turbines must be able to handle the modified synthesis gas feed

New burners may be needed as lean pre-mixed low-NOx burners are not suitable for

H2 meaning N2 (from the ASU) would need to be injected

4 GAS SEPARATION MEMBRANES

The fact that gas can permeate through a dense, non-porous material was first scientifically

observed in the mid 19th century (Pandey and Chauhan 2001). Since then, the growing

understanding of the mechanisms by which gas permeates through membranes has made

possible the development of membranes for a host of industrial applications. However,

widespread use was limited by the generally unreliable, non-selective and low-flux performance

of the membranes available. It was not until the late 1970s that separation of gas mixtures of

industrial interest with membranes became economically competitive for certain applications.

Core to this was the invention of new production processes that enabled asymmetric

membranes, which combined the properties of different materials to drastically improve the flux

of the membranes. The further development of gas separation membranes was accelerated in

the 1980s with the application of membranes for large-scale separation of hydrogen from

hydrocarbons and CO2 from air (Fritzsche and Kurz 1990).

4.1 MEMBRANE PERFORMANCE AND ECONOMICS

4.1.1 PERFORMANCE TARGETS

For CO2-capture, target separation performance is typically expressed through the CO2

recovery ratio 𝑅 =𝑄𝑝 ∙𝑦

𝑄𝑖𝑛 ∙𝑥𝑖𝑛 (the fraction of CO2 in the feed that is recovered) and the CO2 mole

fraction y in the permeate (the purity of the CO2 recovered), see Figure 17. The specification of


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purity requirements depends on what the recovered CO2 is used for (EOR, sequestration,

industrial uses), and higher purities and recovery ratios is traded off against increased capital

(membrane area) and operation (energy) costs. Typical requirements for CO2 capture

technologies are recovery ratios above 80%

(R > 0.8) and permeate CO2 mole fractions above 80% (y > 0.8) (Favre 2007).

The energy requirement of the separation process is another important parameter. Pressure

ratio (driving force) and pressure drop over the membrane leads to compressor duty. In the case

of H2/ CO2 separation, permeation of H2 (fuel) adds to the loss of overall plant energy efficiency.

Retentate:

Mole fraction xout

Flow rate Qout

Pressure P’

Permeate:

Mole fraction y

Flow rate Qp

Pressure P”

Feed:

Pressure Pin

Mole fraction xin

Flow rate Qin

CO2H2

P”

P’

Membrane

selectivity α

Figure 17 : Membrane performance parameters.

Another important performance issue is service lifetime (Rautenbach 1990). Service lifetime

depends particularly on the durability and mechanical integrity of the membrane at its operating

conditions (Koros and Mahajan 2000). These issues are discussed in Section 4.5.

4.1.2 PERFORMANCE DETERMINANTS

The key membrane properties influencing separation performance are the selectivity

(separation efficiency) and the permeation rate (Pandey and Chauhan 2001). Selectivity is

commonly measured as 𝛼𝑖𝑗 =𝑃𝑖

𝑃𝑗 , that is, the ratio between the pure gas permeabilities of the

targeted gas i (here CO2) and the non-targeted gas j (here H2) (Kim, Koros and Paul 2006). In

practical application of the membrane, the feed is a mixed gas stream which means that the

permeability of components i and j in practice will differ from pure gas permeabilities. The


44 | P a g e

definition 𝛼𝑖𝑗 =𝑦𝑖 𝑦𝑗

𝑥𝑖 𝑥𝑗 , where y and x are the compositions of i and j on the permeate and feed

sides respectively, is therefore more correct.

Selectivity depends on material properties and operating factors, such as temperature,

concentration of CO2 in the feed (xin) and pressure ratio (permeate side pressure over retentate

pressure) across the membrane (𝜓 = 𝑃"/𝑃′). The higher the CO2 concentration, the lower the

necessary selectivity to achieve targeted separation performance (Favre 2007). High selectivity

is desirable because it improves process efficiency, lowers the driving force required to achieve

a given separation, and hence reduces operating costs (Koros and Mahajan 2000).

The rate of permeation (migration) of the permeate through the membrane is decided by the

permeability, which depends on the material properties and the thickness of the membrane. The

lower the thickness, the higher the permeation rate, the smaller the required membrane area,

and the lower the capital cost of the membrane system (Koros and Mahajan 2000). High

selectivity and high permeability are thus key objectives when developing membranes for gas

separation. Target values for selectivity and permeance are typically set by use of so-called

tradeoff curves, which enable balancing ideal selectivity versus fast component permeance

(Favre 2007).

4.2 MEMBRANE TYPES

Several distinctions can be made when describing gas separation membranes, such as

transport mechanisms, driving forces, membrane materials, and physical structure. Transport

mechanisms are discussed in line with the outline of facilitated transport in Section 4.3.1; the

latter three are outlined below:

4.2.1 SEPARATION DRIVING FORCES

Three different mechanisms lead to transport of a species through a membrane (Strathmann

1990):


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a) A hydrostatic pressure difference between two phases separated by a membrane leads to

a separation of chemical species when the hydrodynamic permeability of the membrane is

different for different components.

b) A concentration difference between two phases separated by a membrane leads to a

separation of various chemical species when the diffusivity and the concentration of the

various chemical species in the membrane are different for different components.

c) A difference in the electrical potential between two phases separated by a membrane

leads to a separation of various chemical species when the differently charged particles

show different mobilities and concentrations in the membrane.

4.2.2 MEMBRANE MATERIALS

Gas separation membranes can be made from organic materials (typically polymers), non-

organic materials (ceramic, alloy, glass, carbon, zeolite), or hybrids (combinations) of the two.

Polymeric membranes tend to be more economical than other materials and thus dominate

traditional gas separation processes. The economical advantage stems from the ease at which

polymeric materials can be spun into hollow asymmetric fibers or spiral wound modules (Koros

and Mahajan 2000). A disadvantage with polymer materials is that their efficiency tends to

decrease with time due to fouling, compaction, chemical degradation, and thermal instability

(see Section 4.5). This limits their applicability in separation processes where hot reactive gases

are encountered, and has for these processes led to a shift of interest toward ceramic and alloy

materials (Pandey and Chauhan 2001). While these membranes have shown promising

performance, manufacture is more complex. Even with improvements in manufacturing

efficiency, “it is reasonable to expect ceramic, glass, carbon, zeolitic and other inorganic

membranes to cost between one- and three-orders of magnitude more per unit of membrane

area compared to polymeric membranes” (Koros and Mahajan 2000).

The membrane of interest in this study is made from the polymeric materials

polyvinylamine and polysulfone. These polymers are thermoplasts, that is, they have long-chain


46 | P a g e

linear molecules that can be easily formed by heat and pressures at temperatures above a

critical temperature referred to as the glass temperature, Tg (Schweitzer 2007). The glass

temperature is marked by a step change in thermodynamic and mechanic properties of the

polymer when it is cooled at a finite rate. Above Tg, the polymer is said to be in the rubbery state;

below Tg, the polymer is in the glassy state (Pfromm 2006).

Polymers are built up by many units (monomers) chemically bound into long molecule

chains (thereof the name polymer – ‘many parts’ [gr.]). Polymerization, the process of chemically

binding monomers together to polymers, can take place through various methods. Important in

deciding the physical and chemical properties of polymers is the chosen formation method

(Schweitzer 2007). This underlines the importance of careful control of the membrane

manufacture process to ensure the desired gas separation performance.

4.2.3 PHYSICAL STRUCTURE

The physical structure of membranes is either porous or non-porous, or a combination

(asymmetric). Porous membranes are rigid, highly voided structures with randomly distributed

interconnected pores. In structure and function, porous membranes are similar to conventional

filters; they rely on and utilize differences in molecular size to separate gases (Fritzsche and

Kurz 1990). Permeation happens through molecular migration through microvoids in the

material. While their high permeability (which gives a high permeate flux through the

membrane) gives porous membranes an advantage, their selectivity or separation efficiency is

low (Pandey and Chauhan 2001).

The characteristics of non-porous (dense) membranes are generally opposite to porous

membranes; selectivity is usually high while the flux is low. Dense membranes uses differences

in solubility in the membrane to separate gases and are therefore capable of separating gases of

similar molecular sizes (Fritzsche and Kurz 1990). Since the permeability of dense membranes


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is inversely related to the thickness of the membrane, it is a design objective to make the dense

membrane as thin as possible while maintaining its mechanical integrity.

An asymmetric membrane is designed to combine the strengths of porous and non-porous

membranes; one way to do this is to combine two different polymers. In such a composite

membrane, a thin, dense and selective skin (barrier) layer, termed the permselective layer,

provides high separation efficiency. This skin layer can be made very thin and hence relatively

permeable since it is physically supported by a much thicker, porous and highly permeable

substrate layer (Strathmann 1990). The permselective layer may be supported on one or both

sides by a substrate layer; support on both sides is advantageous if the membrane needs to be

reversed for cleaning purposes (Smith 2005). This enables design of membranes that are at the

same time highly permeable and highly selective.

4.3 COMPOSITE POLYMERIC FACILITATED TRANSPORT MEMBRANES

The membrane which is the subject of the current study is a Fixed Site Carrier (FSC) cross-

linked polyvinyl-amine membrane (PVAm) supported on polysulfone. As the term indicates, the

membrane is an asymmetric composite membrane, with a permselective layer made from a

dense polyvinyl-amine providing selectivity, and a substrate layer made from polysulfone

providing structural support. Polyvinylamine and polysulfone are polymeric materials. The

concepts facilitated transport, Fixed Site Carrier and cross-linking are explained below.

4.3.1 FACILITATED TRANSPORT

Transportation of a permeant through a dense, polymeric membrane is commonly described

as happening by sequential gas dissolution and diffusion through the polymeric material

(Matteucci, et al. 2006). According to this ‘solution-diffusion’ model (the term ‘sorption-

diffusion’ is also common), transport through the membrane happens in three steps. The


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permeate is first dissolved into the membrane. It then diffuses1 across the membrane due to a

concentration gradient, and desorbs (disassociates) from the membrane at the permeate side

(Koros and Mahajan 2000). While a range of other models have been proposed as well, this

model is popular because the same model and the same set of assumptions can be used to

describe permeation in a variety of seemingly different processes. The mathematics of the model

is straight forward to apply, and the resulting transport equations are easy to use because the

contributions of diffusion and sorption are conveniently combined into one permeability

coefficient (Wijmans and Baker 2006).

In addition to the solution-diffusion mechanism, which suffices to explain gas permeation in

dense polymeric membranes of the simplest kind, transport in facilitated transport membranes

is enhanced by a reversible complexing reaction between the permeant and the membrane. This

additional reaction is analogous to a chemical absorption process (Section 0) on the feed side

and a stripping process on the permeate side of the membrane. After the solute is dissolved

(sorbed) in the membrane, it will either diffuse down its own concentration gradient, or react

chemically with the complexing (carrier) agent and diffuse down the concentration gradient of

the carrier-solute complex concentration gradient. Provided that the complexing agent is not

reactive with any other gas component in the feed, this additional mechanism makes facilitated

transport membrane highly selective (Koros and Mahajan 2000).

The total flux of a permeant A (here CO2) through a facilitated transport membrane is

calculated as the sum of the flux of the carrier-solute complex and the uncomplexed solute

according to the following equation: 𝐽𝐴 =𝐷𝐴

𝑙 𝑐𝐴,0 − 𝑐𝐴,𝑙 +

𝐷𝐴𝐶

𝑙(𝑐𝐴𝐶 ,0 − 𝑐𝐴𝐶 ,𝑙)

where the first term on the right hand side represents the contribution of the solution-diffusion

1 Diffusion: “gradual dispersal of one substance through another substance” (Atkins and Jones 2002)


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mechanism (DA) and the second term represents the carrier mediated diffusion (DAC). The

thickness of the membrane is l, and c is the concentration of the component A and its complex AC

at the interfaces of the membrane, 0 and l, on the feed and permeate side respectively (Kim, Li

and Hägg 2004).

Figure 18 : Total flux versus driving force in facilitated transport membranes (Noble and Koval 2006).

Figure 18 shows a plot of the contribution of the two mechanisms to the total flux of the

permeate in facilitated transport membranes. Line ‘a’ refers to the contribution by solution-

diffusion, while line ‘b’ shows the added transport due to facilitation. At a certain level ΔPL, all

carrier species are bound to solute molecules (carrier saturation), and an increase in the driving

force (pressure difference, ΔP, across the membrane) will not increase the flux by the reactive

pathway. As the plot shows, the contribution of the two mechanisms depends on the driving

force. At very low driving force conditions, the majority of the transport is due to diffusion of the

carrier-solute complex, and the flux is not linearly proportional to the driving force. At very high

driving forces, transport of unbound solute by diffusion is the main contributor, and the flux is

linearly proportional to the driving force (Noble and Koval 2006).

4.3.2 MEMBRANE DESIGN AND CONFIGURATIONS

Facilitated transport membranes can have one of three general configurations: an

immobilized liquid film, a solvent-swollen polymer, and a solid polymer film containing reactive

functional groups. The membrane in this study is of the latter kind, which is described below.


50 | P a g e

An issue that potentially will limit the application of facilitated transport membranes for

large-scale gas separation is their stability. Instability can arise due to the complexation

chemistry, the support configuration or both. A way to mitigate this problem is to attach the

complexation agent (the carrier) to the polymer chains that make up the membrane material.

This concept is known as the fixed-site or chained-carrier membrane, and has the potential to be

both highly selective and stable (Noble and Koval 2006).

Figure 19 : A proposed mechanism for facilitated transport of CO2 in a fixed-site carrier membrane. The

figure shows ordinary diffusion to the right and carrier mediated diffusion to the left (Kim, Li and Hägg 2004)

The fixed-site carriers (complexing agents) transport gas molecules across the membrane by

what has been termed the ‘Tarzan swing’ mechanism. The name stems from visualizing the

permeating gas molecules as ‘swinging’ from one carrier ‘vine’ to the next, see Figure 19. For

this to be possible, two carriers must be close enough to each other to pass the permeate

molecules to each other across the membrane (Cussler, Aris and Bhown 1989). This and other

models for fixed-site carrier facilitated transport are outlined by (Noble and Koval 2006).

Most of the reported research on fixed-site carrier facilitated membranes for separation of

CO2 involves the use of amines or anionic bases as complexing agents. Due to their high


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reactivity with CO2, amines are commonly used as solvents in chemical absorption processes

(IEA 2004).

Another challenge with asymmetric polymeric membranes is maintaining their properties in

the presence of aggressive feeds (see Section 4.5). Change in membrane properties will usually

cause degraded membrane performance, and must hence be avoided. A method to overcome this

potential problem is to cross-link the polymer structures, which means linking polymer chains

to each other across their longitudinal direction. Cross-linking can be implemented by

incorporating cross-linkable functional groups in the polymer chains after the membrane is

spun. The effect is reported to be higher resistance to plasticization (since cross-linking prevents

swelling of the material in the presence of plasticization agents) and increased thermal and

chemical stability (Koros and Mahajan 2000).

4.4 MODULE AND PROCESS DESIGN

4.4.1 MEMBRANE MODULES

Module design is another important part of designing membrane systems of optimal

performance. The ideal design results from a trade-off of the following factors:

flow conditions along the membranes,

ratio of membrane area to pressurized vessel volume,

module price, and

the possibility of cleaning the membrane.

Which factor is most important depends on the process application, and for this reason, a

range of different modules have been designed (Rautenbach 1990). The two most common

arrangements are spiral wound and hollow fiber. In the spiral wound arrangement, flat

membrane sheets are separated by spacers to allow space for the feed and permeate to flow,

then wound into a spiral. In the hollow fiber arrangement, the membrane is made into thin,

cylindrical, hollow fibers (see Figure 20) which are stacked close together. The permselective


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layer is usually on the outside of the fibers and the feed typically enters the membrane on the

outside of the fibers to permeate toward the center of the fibers (Smith 2005).

Highly porous

support layer

Thin selective

skin layer

Figure 20 : Hollow fiber asymmetric polymer membrane (dimensions not necessarily in actual scale)

(Koros and Mahajan 2000).

The design of choice in the case studied here is the hollow fiber arrangement. Typically,

modules of hollow-fiber membranes consist of bundles of randomly or regularly packed thin,

hollow fibers. The surface area of hollow fibers per volume is typically in the range 10 000 –

30 000 m2/m3. The ends of the fiber bundle are potted to form tube sheets which are then sliced

to provide access to the inside of the fibers. The bundle is inserted into a case (pressure vessel)

and a seal is formed between the tube sheet and the case. Gas can be fed to and removed from

the inside of the hollow fibers through ports at the lumen access ports on either side of the

module; introduction and removal to the shell space (the region outside the fibers) is done

through shell access ports on the periphery of the module. In operation, mass transfer happens

between the fluid flowing through the hollow fibers (lumen) and the shell (Bao and Lipscomb

2003). Figure 21 shows a typical hollow fiber membrane module design.


53 | P a g e

TubesheetFiber Bundle

Case

Lumen

header

Bolts to attach

header to case

Lumen

access port

Distribution collar

for shell fluid

Shell access port

Fiber Bundle

Tubesheet

Fiber ends

open along

tubesheet

face

Figure 21 : Schematic of a typical hollow fiber membrane module and the case (pressure vessel) that

holds it (Bao and Lipscomb 2003).

Predicting and measuring the separation efficiency and throughput of the membrane is

complicated. Membrane material properties and transport mechanisms are still not completely

understood. “Therefore, almost all transport models proposed in the literature are

phenomenological in nature and contain one or more adjustable parameters which must be

determined experimentally” (Pandey and Chauhan 2001). Concentration boundary layers in the

lumen and shell spaces further complicates the analysis, as does the complex geometry of the

shell and the flows within it (Bao and Lipscomb 2003). This implies that the sensitivity to

random variations in the shell and lumen flow must be considered especially when selecting

qualification methods (tests, analysis) for the module.

4.4.2 FLOW PATTERNS

The performance of a membrane is also influenced by the way in which the permeate and

retentate flow in the membrane. Three common flow patterns for gas separation are shown in

Figure 22. Parametric studies of these flow patterns have shown that given equal operating

conditions, the countercurrent flow pattern is most efficient (meaning less membrane area is

needed), followed by the cross flow pattern. Which of these idealized flow patterns is assumed in

practice is not always clear, since the flow pattern depends on both the geometry of the

membrane module design and the permeation rate (Smith 2005).


54 | P a g e

Retentate

Permeate

Feed

b) Countercurrent flow

Retentate

Permeate

Feed

c) Cross flow

Retentate

Permeate

Feed

a) Cocurrent flow

Figure 22 : Idealized flow patterns in membrane separation.

4.4.3 PROCESS DESIGN

Membranes are modular in construction, and this gives high flexibility for process design. In

cases where it is impossible to achieve the desired permeate purity or recovered fraction of a

given mixture in one stage, the separation process will be done in several stages. To handle the

desired feed volume, each stage can be built up by several membrane modules in series or

parallel configuration.

Figure 23 shows a setup of several stages with several membrane modules in parallel in each

stage. In this setup, each module is assumed to achieve the desired permeate purity. The number

of modules is adjusted to accommodate the feed rate. Since part of the feed permeates through

the membranes in each stage, the volume flow decreases from stage to stage. Assuming equally

sized modules, this means that the necessary number of modules decreases from stage to stage

(m ≥ n ≥ k in the figure). The targeted recovery (fraction of the gas that actually is captured from

the feed) is achieved by adjusting the number of stages; the retentate from one step becomes the

feed of the next. Compression of the retentate might be necessary between stages to maintain

the necessary driving force (Smith 2005). If the desired permeate purity is not achievable in one

stage, a cascade set-up, in which the permeate from one stage is (recompressed and) fed to the


55 | P a g e

next, is applicable (Rautenbach 1990). Such additional compressor duty would increase the

energy required to drive the separation, hence reducing overall plant efficiency.

Feed

a1

a2

am

b1

bn ck

b2 c1

Retentate

Permeate

Figure 23 : Membrane modules set up in parallel in a series of stages.

The modular nature of membranes also has implications when scaling up from small-scale

test systems to full-scale application. On one hand, the linear relationship between membrane

area and separation capacity means that the economy of scale can be expected to be

considerably smaller than in large single unit cases (such as absorption scrubbers). On the other

hand, it is likely that predicting the performance of the full-scale membrane system is more

straight-forward in the case of modular membrane systems. Furthermore, a set up of several

modules in parallel, and stages in series, has active redundancy built in, which means that

unavailability or failure of one module is less dramatic for the overall system reliability. This

implies that the actual process design (number of modules in parallel, number of stages, etc) is

an important input for technology qualification to be possible.

4.5 PHYSICS-OF-FAILURE OF POLYMERIC MEMBRANES

Factors such as the separation performance, reliability and service lifetime of membranes

are important when evaluating the appropriateness of a membrane system. In application, loss

of performance and failure can happen due to physical (structural) and chemical changes in

membrane properties. These processes are intrinsic (wear-out) failure mechanisms (see


56 | P a g e

sections 2.3.2 and 2.3.3) which ultimately may cause failure of the product if limits required for

the product function are exceeded.

Published studies of polymer membranes tend to focus on performance assurance, giving

little or no attention to assurance of long-term reliability. Furthermore, an all-inclusive account

of failure mechanisms relevant in polymer gas separation membranes has not been found in the

literature, and terms tend to be used without proper definitions. While this may be because they

are widely understood within certain academic disciplines, it does not promote cross-

disciplinary understanding and analysis. The following listing therefore aims to briefly describe

failure mechanisms reported at various places in the literature. Note that the effect of the

mechanisms depends largely on membrane properties; some mechanisms may actually have a

positive effect on membrane performance. The relevance of each mechanism to the particular

membrane of interest is addressed in Section 5.5.

4.5.1 CORROSION (CHEMICAL DEGRADATION)

Unlike the case with metallic materials, polymers do not experience specific corrosion rates

in a particular corrosive environment. This makes prediction of performance degradation with

time due to corrosion less straight-forward for polymers than for metals. Within specific

temperature ranges, polymers are usually completely resistant to a specific corrodent or they

deteriorate rapidly. Corrosion failure mechanisms can be classified as follows (Schweitzer

2007):

1. Disintegration or degradation of a physical nature because of absorption, solvent

action, or other factors

2. Oxidation, where chemical bonds are attacked

3. Hydrolysis, where ester linkages are attacked

4. Radiation (may also lead to cross-linking and hence have a positive effect on

performance)


57 | P a g e

5. Thermal degradation involving depolymerization and possibly repolymerization

6. Dehydration (rather uncommon)

7. Any combination of the above

The results of these corrosive attacks on the polymer material can be observed as softening,

charring, crazing, delamination, embrittlement, dissolving or swelling (Schweitzer 2007).

For well-known polymeric materials, information about the resistance to a whole range of

corrodents is readily available; see for example Schweitzer (2007). In the case of novel

polymeric materials or combinations of well-known materials, testing is necessary to acquire

this information.

4.5.2 PHYSICAL AGING (STRUCTURAL DEGRADATION)

Physical aging (or structural relaxation) has been defined as the change of a chosen property

with time in the absence of chemical changes of the polymer, and in the absence of highly

sorbing components such as CO2 and hydrocarbons (which can cause plasticization). Physical

aging happens in glassy polymers due to their non-equilibrium state and is observed as a

gradual approach to thermodynamic equilibrium. Physical aging at constant temperature leads

to increased stiffness, brittleness and volume contraction with time, and hence influences the

separation performance of the membrane. Describing the impact of aging of polymeric materials

is complicated by the observation that the thickness of the tested sample is of high importance

for aging; aging in the glassy state of polymers proceeds very rapidly in thin films compared to

thick films (Pfromm 2006). Aging rate has also been found to correlate with the level of free

volume of the polymer; the higher the free volume, the faster the aging. The aging rate is also

dependent on aging temperature (Kim, Koros and Paul 2006).

The implication of thickness-dependent aging for practical membrane gas separations is that

performance of practical gas separation membranes with selective layers made from glassy

polymers is time-dependent. It should hence not be surprising if a gas separation module looses


58 | P a g e

a significant amount of productivity while gaining some selectivity during the first months of

operation. This is the case even in the complete absence of fouling or presence of contaminants

in the feed, simply by physical aging. Another implication is that the (thermal) history of samples

used for testing should be carefully considered, as re-testing after some time is likely to show

different results for thin films, while thick films may remain unchanged. Data for design of

membrane gas separation systems should therefore be obtained with membranes that have

aged sufficiently (in the order of weeks) if glassy polymers are used (Pfromm 2006).

4.5.3 PLASTICIZATION (SWELLING)

Loss of selectivity due to plasticization is mentioned as one of the challenges for polymeric

gas separation (Wallace, et al. 2006). Plasticization happens when the polymer matrix is swollen

with highly sorbing penetrants (such as CO2 and hydrocarbons), forcing the polymer chains

apart to accommodate the penetrant molecules. This can be thought of as partially pushing the

polymer toward its liquid state. Penetrant molecules can also ‘lubricate’ the polymer matrix,

hence facilitating chain motion. The effect of this tends to be increased permeability but reduced

selectivity, making long-term performance somewhat unpredictable since the effects may be

time-dependent (Kim, Koros and Paul 2006). For polyimide membranes, for example,

plasticization has been observed to greatly reduce (to the order of 6-8) selectivity in industrial

application with gas mixtures at high, near saturation, pressures compared to that measured in

pure gas tests in idealistic conditions (Merkel, et al. 2006).

Plasticization influences membrane performance; in some cases, a certain level of swelling is

necessary to achieve the desired separation performance. Treatment of the membrane to control

plasticization is therefore necessary. Several strategies, such as heat treatment, blending,

reactively formed interpenetrating networks and crosslinking, have been tried to developed

plasticization resistant polymeric membranes. Of these, crosslinking has been found to be an

attractive option, since it combines relatively easy implementation within acceptable processing


59 | P a g e

conditions and produces significantly improved plasticization resistance (measured by

improved selectivity under demanding feed conditions (Wallace, et al. 2006).

4.5.4 COMPACTION

In the presence of high trans-membrane pressures, membrane performance can be reduced

due to compaction of the porous asymmetric hollow fiber membrane substructure. Compaction

is defined as a compression of the membrane structure under a trans-membrane pressure

difference causing a decrease in membrane permeability. The reduced permeability is believed

to be mostly attributable to densification of the substrate support layer (observed as lower

porosity), but thickening of the permselective layer also contributes (Reinsch, et al. 2000).

It has also been hypothesized that compaction can happen when the membrane is exposed

to large concentrations of highly sorbing feed stream contaminants that sorb into the glassy

polymer matrix. Compaction here is believed to be caused by reduced fiber substructure

modulus, which is a measure of the physical stiffness of the material, equaling the ratio applied

load (stress) to resultant deformation of the material (high modulus indicates a stiff material). In

other words, the ability of the membrane to resist stress (transmembrane pressure difference)

is reduced (Madden 2005).

4.5.5 FOULING

Fouling happens when particulate matter in the feed deposits on or within the membrane

pore structure in a wetted environment. The particulate matter is contaminants stemming from

some stage earlier in the process upstream from the membrane, and process considerations are

hence necessary to avoid fouling. The effect of fouling is increased hydraulic resistance to flow.

Fouling is reported to be a major problem with microfiltration polymer membranes (Zydney, Ho

and Yuan 2003). In gas separation applications, fouling can be avoided by removing fouling

matter (particles) in the feed gas prior to entering the membrane. Such pretreatment may either

be done by a designated functional block (such as a feed gas filtration unit), or as a side effect of

other processes that the gas passes through upstream from the membrane.


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4.5.6 DEHYDRATION

For some membranes and gas separations, water is required as a mobile medium for the

permeate. Such hydrophilic membranes must be swollen to work optimally, which means that

the gas feed at all times should be saturated with water vapor to keep the membrane from

drying out (Kim, Li and Hägg 2004). As Figure 19 shows, the facilitated transport concept

designed for the membrane of interest in the current work will not work if the membrane is not

sufficiently swollen with water. Dehydration should therefore be regarded as a potential failure

mechanism for failure analysis. As with fouling, careful control of the feed (i.e. ensuring a certain

level of humidity of the feed) is a possible preventive measure.

4.5.7 CONCENTRATION POLARIZATION

As the most permeable species (CO2 in a CO2-selective membrane) in the feed gas migrates

through the membrane, there is an accumulation of the less permeable species and a depletion

of the more permeable species in the boundary layer adjacent to the feed side of the membrane,

see Figure 24. The effect of this concentration polarization is a decrease in the available cross-

membrane driving force for the more permeable gas and an increase in the available driving

force for the less permeable species. This means that the permeation rate of the most permeable

gas (here: CO2) decreases while the permeation rate of the less permeable gas increases, and

hence that the overall separation efficiency is reduced (Bhattacharya and Hwang 1997).

Concentration polarization will in theory take place in all membrane separation processes.

Due to the high diffusivity of gases, however, concentration polarization has until recently

(justifiably or not) been thought not to be a concern for gas separation membranes. In a

parametric study of the factors influencing concentration polarization in gas separation

membranes, He, Yongli, Yue and Chen (1999) showed that this assumption no longer safely can

be made. Having established a mathematical model to analyze the effects of concentration

polarization on membrane performance, they found permeation rate to be a dominating factor


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positively affecting concentration polarization. The effect of operating pressure was found to be

relatively small, and increased gas feed velocity was found to decrease, but not completely

eliminate, concentration polarization (He, et al. 1999).

Membrane

x

u

k (x-xs)

N = N1+N2

xs

Permeate

side

Membrane

Bulk feed

1-x

u

k (x-xs)

N = N1+N2

1-xs

Boundary

layer

Permeate

sideBulk feedBoundary

layer

Figure 24 : Molar fraction profiles of the more (left) and less (right) permeable gases on the feed side

under steady separation condition (He, et al. 1999). In a CO2-selective membrane, the left profile would show

the concentration profile of CO2. N1 and N2 represent the permeation fluxes of the more and less permeable

gases

4.5.8 ASSEMBLY ISSUES

When two or more materials are combined as in the case of asymmetric polymer

membranes (the substrate and permselective layers), differing behavior (expansion, stretching,

etc) of the materials in the presence of physical and thermal stresses may cause physical damage

to the membrane.

Another potential structural problem specific to the case of facilitated transport membranes

is loss of carriers (complexing agent) from the selective layer. As the term fixed site carrier

indicates, the carriers are fixed to the polymer chains to prevent loss of carriers. Crosslinking

has been reported to mitigate this potential failure mechanism (Koros and Mahajan 2000).

The pressure difference across the membrane represents another potential problem if it

exceeds the structural strength of the capillary (hollow fiber) structure. If the high pressure side

is on the outside of the capillary (lumen), which is the case if the feed gas enters the to the shell

space of the module, the result will be that the capillaries collapse. If the feed enters the module


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on the lumen side, failure can occur by puncture of the capillary. These potential failures can

possibly be mitigated by means of safety over-pressure valves.

4.6 MEMBRANE PERFORMANCE EVALUATION AND TESTING

4.6.1 PERFORMANCE SIMULATION

In order to evaluate what system performance can be expected from the membrane and the

various possible process designs it can be integrated into, a computer simulation program has

been developed and integrated into the well-known process simulation program Aspen HySys®

(Hägg and Lindbråthen 2005). The model offers flexibility in terms of flow directions, modular

architecture and feed compositions useful for process design and techno-economic performance

evaluations, but does not include information about degradation rates or failure mechanisms. Its

utility on its own for reliability assurance is therefore limited.

The transport mechanisms in polymer membranes are however still not completely

understood. “Therefore, almost all transport models proposed in the literature are

phenomenological in nature and contain one or more adjustable parameters which must be

determined experimentally” (Pandey and Chauhan 2001). Deterministic modeling of membrane

performance is therefore difficult, and should commendably be supplemented with

experimental models such as performance and reliability tests.

4.6.2 MEMBRANE TESTING

Published permeability values for a specific polymer have been found to be varying “very

significantly”. This scatter of permeability values is attributable to two major causes: the

variation in physical properties of a polymer (due to different manufacture processes and its

history-dependent properties), and the differences in permeability measurement techniques

(Kruczek and Matsuura 2000). For qualification of polymer membranes, this implies that it is

important that the manufacture method of tested components (prototypes) should be equivalent


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to the method used for serial manufacture. It further implies that the testing method used for

qualifying the membrane’s performance should be chosen with caution.

The most elementary method for evaluating gas separation performance of a membrane

material is to measure the permeability of various gases through it. In its simplest form, this

method tests the permeability of pure gases. As the discussion of concentration polarization

(Section 4.5.7) showed, the permeation rate of one gas is however influenced by other gases in

the mixture. The real-life permeance and selectivity of a gas will usually be lower than shown in

pure gas tests. Membrane performance is also strongly influenced by testing conditions

(pressure and temperature), so testing should be performed at conditions as close as possible to

actual operating conditions (Fritzsche and Kurz 1990).

In general, three types of measurement techniques are available: constant pressure, constant

pressure with sweep gas, and constant volume techniques. Variations in testing schemes also

exist in terms of the order and duration of tests. Considerations regarding the limitations of the

various test protocols are discussed by Kruczek and Matsuura (2000). A method for

simultaneous, real-time measurement of membrane compaction and performance during

exposure to high-pressure gas was suggested by Reinsch et al. (2000).

5 RECOMMENDATIONS FOR QUALIFICATION OF A FIXED SITE CARRIER

PVAM MEMBRANE

A foundation for qualification of the proposed fixed site carrier polyvinylamine CO2-selective

membrane applied for pre-combustion capture of CO2 in IGCC power plants has been established

in Sections 2, 3 and 4. Based on this foundation, the objective of the current section is to provide

recommendations for the forthcoming qualification process. Main sources of information are

journal articles and documentation (published and non-published) provided by the research and

development (R&D) team. Supplemental information has further been acquired through

interviews and conversations with the research team. Beyond scope has been to carry out


64 | P a g e

interviews and workshops involving external resources (such as polymer membrane

manufacturers and potential customers) to gather the information necessary for carrying out the

actual qualification work.

5.1 BACKGROUND

The proposed membrane is a CO2-selective membrane intended for capturing CO2 generated

from combustion of fossil fuels at power plants. The membrane is under development by the

Membrane Research Group (MEMFO) at the department of Chemical Engineering of the

Norwegian University of Science and Technology. R&D has been going on for several years, and

an international patent for the membrane and its production process was granted in 2005

(Hägg, Kim and Li 2005). The membrane itself is described by Kim, Li and Hägg (2004), and

industrial applications of the membrane are proposed and analyzed by Hägg and Lindbråthen

(2005), Grainger, Lindbråthen and Hägg (2006), and Grainger and Hägg (2007). Industrial

demand for cost-efficient technologies for CO2-capture technology is a strong driver in the

development, and a lab-scale pilot plant is being planned in close cooperation with leading

industrial actors, both within membrane manufacture and process integration and application.

The membrane can be adapted for a range of carbon capture applications, including pre- or

post combustion at coal- or natural gas-fired power plants, as well as various industrial

processes. The application studied here is pre-combustion capture integrated in an IGCC power

plant as described by Grainger and Hägg (2007). Even so, the following discussion is intended to

be relevant also when qualifying the membrane for other process concepts, such as integrated

CO2 capture in a Natural Gas Combined Cycle (NGCC) plant.

5.2 QUALIFICATION APPROACH AND METHODOLOGY

Following the discussion in Section 2.2.2, a proactive approach in which attention is given to

technology qualification early on is advisable. Quantitative analysis is furthermore

recommended, supplemented with qualitative engineering judgment from relevant scientific and


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engineering disciplines. Existing knowledge from technologies similar in structure and materials

(Physics-of-Failure) should be identified and utilized in order to minimize the cost of

development and testing (see Figure 9). This will also help directing attention to the aspects of

the membrane which are truly unknown.

Technology suppliers are increasingly being required by major energy companies prove that

technology qualification has been carried out in compliance with the DNV RP-A203, see Section

2.4 and DNV (2001). The following discussion of the forthcoming qualification process will

therefore follow the main steps of the guideline, as shown in Figure 13.

5.3 QUALIFICATION BASIS

Setting the Qualification Basis (QB) is the first step of the qualification process, and includes

Reliability specification (Sections 2.2.3). Here, the goal is to understand and document the needs

for the given system capability so that the technology development process can fulfill those

needs. For this, the QB should describe the technology and its interfaces with other system

components (Section 2.2.1), describe the operational regime and conditions (Section 2.2.5), and

set required values for key operational parameters (Section 2.2.4).

5.3.1 SYSTEM DESCRIPTION AND BOUNDARIES

A conceptual sketch of an Integrated Gasification Combined Cycle power plant with CO2

capture integrated in the process is shown in Figure 25 (refer back to Figure 16 for the ordinary

IGCC process), and Figure 26 shows the main components of each functional block. Coal and

pure oxygen (from an Air Separation Unit) enters the gasifier to produce syngas. The syngas is

then run through a Water Gas Shift (WGS) reaction in which CO and H2O is converted to CO2 and

H2. After the WGS, the gas typically has a pressure of around 20 bar and contains 40 mol% CO2.

The shifted syngas is then cooled by means of heat exchangers to about 35oC to remove sulfur

and other contaminants in Venturi scrubber, Selexol and Claus units. The membrane section

then splits the syngas stream in two by separating CO2 from H2. Some electric power is needed

here for recompression of the feed gas between stages of the membrane cascade. The CO2 is then


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compressed and transported off to permanent storage, while the retentate (mainly H2) is diluted

with N2 from the ASU and fed to the Combined Cycle Power block to generate electric power.

IGCC in general are discussed in Section 3, and the specific process studied here is further

described in Grainger and Hägg (2007).

Gasifier

Coal

Slag

Gas Clean

Up

Combined

Cycle Power

Block

PowerWater Gas

Shift

CO2 H2

Syn-

gas

Sulfur, other

CO2 capture

O2

Figure 25 : IGCC power plant with CO2 capture integrated in the process.

Coal dust

O2

Steam

Syngas

Gasifier

25 Co

1200-1600

bar

Quench gas

IP Steam

boiler

235 C

24.9 bar

Candle

filters

336 C

35 C21.0 bar

Waste N2 from ASUSteam

Compressed air to

ASU

Air

Exhaust gas to HRSG

CO2 compression train

CO2

110 bar30 C

Gas Turbine

Membrane

section

Selexol and

Claus units

Water

25 C 24.3

bar

302 C

Venturi

Scrubber

LP steam

260 C

CO

Shift

Water

260 C

WaterHalogenated

compounds

CO

Shift

IP steam

516 C

29 bar

466 C

235C

24.2 bar

35 C

H2 rich

CO2 rich

70 C

24.7 bar

22 C

24.2 bar

18.2 bar

Figure 26 : Process diagram for an IGCC process with sour water-gas shift and membrane CO2 capture

(Grainger and Hägg 2007).

As discussed in Section 2.2.1, system boundaries must be set at the outset of the analysis to

define what to consider the system of interest and what lies beyond. The technology of interest


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here is the CO2 capture unit in general, and the membrane modules in particular. Therefore,

system boundaries should either be set around the whole membrane section, or around a single

membrane module in the membrane section, see Figure 26. For the following discussion, system

boundaries will be assumed to be set around a single membrane module as illustrated in Figure

21. Once this has been done, the physical structure and functions of the system should be

described.

Functional description

A thorough description of all the various functions expected from the system is necessary in

order to qualify the technology. As discussed in Section 2.3.1, various methods are available for

such a functional analysis. The list of function types given in Table 4 can be used to make sure all

expected functions are identified, and some sort of function diagram would be used to visualize

the hierarchy of functions and their interconnection, see Figure 5. As an example, the essential

function of a membrane module can be described as to “separate 85% of the CO2 from the feed

gas”. Several functions are necessary for the membrane to perform this level 1 function, such as

“lead feed gas into the module,” “maintain gas pressure,” “keep the permeated gas separate from

the feed stream,” and “lead separated CO2 away from the module.”

Physical system breakdown

The detailed design of the membrane CO2 capture section has not yet been carried out, and

decisions regarding module dimensions, structure and materials, and process design (such as

number of units and stages, and operating conditions and requirements) are still to be made.

The physical system breakdown shown in Figure 27 is therefore assumed on the basis of the

outlines given in Sections 3.3, and 4.4, and must be updated as decisions on module and process

design are made.


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Figure 27 : Physical system breakdown.

As indicated in the figure, the four main components of the CO2 capture unit are the

membrane modules, compressors, pipes connecting the modules to each other and to the rest of

the process, and sensor and controls. The latter three components would lie outside the system

boundaries if the system boundaries are set around a single membrane module. A membrane

module would in turn typically be built up by 5 main parts, including bundles of membrane

fibers, tube sheets holding the membrane fibers in place, lumen headers, a pressure vessel, and

access ports. The membrane fibers are in turn made by combining a substrate layer of

polysulfone (PSO) and a permselective layer made from Polyvinylamine. Each of the components

and parts within the system boundaries should to the extent possible be described in terms of

materials and structure. The discussion of membrane materials, structure and materials of the

membrane modules, and process design provided in Section 4 can be referenced for this

purpose.

5.3.2 OPERATIONAL REGIME AND INTERFACE CONDITIONS

Operational regime and conditions

As discussed in Section 2.2.5, the operational regime and conditions that the membrane

module will be subject to during transport, storage and operation must also be defined as part of

the Qualification Basis. For this purpose, it is recommendable to use Table 3 as a checklist to

make sure all important parameters are described quantitatively or qualitatively.

Parts

Parts

Components

Subsystems

Power plant IGCC

ASU Gasification WGSGas Clean

UpCO2

capture

Compressors Membrane modules

Pressure vessel

Membrane fibers

Substrate layer Permselective layer

Tube sheets

Lumen Headers

Access ports

Pipes Sensors & control

CO2compression

CC power block


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Interface conditions and requirements

Conditions at the interfaces of the system should be described, for example following the

grouping shown in Figure 2. Requirements across the system boundaries should then be

specified by considering the input from the Gas Clean Up block upstream, and the output to the

Combined Cycle Power and CO2 compression blocks downstream.

A summary of key interface conditions that can be expected for the system at hand are

shown in Table 11. The indicated values are assumed on the basis of Grainger and Hägg (2007),

and are here intended as examples. In the end, the values should be traded off against

production and operation costs, and updated to account for changes as membrane development

and process integration advances.

Table 11 : Interface conditions and requirements.

Interface condition Input from Gas Clean Up block

Output to CC Power block

Output to CO2 Compression block

Wanted

Components and concentrations

H2: 50%mol

CO2: 39%mol CO: 2%mol N2+Ar: 8 – 9%mol H2Og (vapor)

H2: 65 – 70%mol CO2: 16 – 20%mol CO: 3 – 4%mol N2+Ar: 11 – 12%mol

CO2: >95%vol As little H2 as possible (typically 1 – 3%vol)

Mass flow

Temperature 25 oC – 35oC 25 oC – 35oC 25 oC – 35oC

Pressure ~21 bar as high as possible as high as possible

Unwanted

Components Aggressive gases Particulate matter

Anything else than CO2

5.3.3 PERFORMANCE AND RELIABILITY REQUIREMENTS

The required performance and reliability of the membrane must also be specified in the

Qualification Basis. This should be done in close interaction between the membrane

development and process design teams to make sure overall plant performance is optimized.

The values assigned to the various performance and reliability parameters will also have to be

traded off against manufacture and operation expenditure. It can therefore be expected that this

part of the Qualification Basis needs to be updated as more information from the development


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and qualification process becomes available. Relevant in this respect are the outlines of key

performance parameters of gas separation membranes (Section 4.1.1) and reliability

specification (Sections 2.2.3 and 2.2.4). Authority requirements and industry standards should

also be taken into account at this point.

5.4 TECHNOLOGY ASSESSMENT

Table 12 : Newness classification of main elements of a membrane module.

Part Technology status

Application Newness classification

Comments

Pressure vessel 1 1 1 Assuming that commercially available pressure vessels can and will be used.

Membrane fibers

Substrate layer 1 2 2 PSO hollow fiber membranes are commercially available, but have never been used in this application.

Permselective layer

3 3 4 PVAm is a new kind of polymer and has never been used for this application.

Tube sheets 1 2 2 Tube sheets are commercially available, but have not been used for this kind of membrane fibers.

Lumen headers 1 1 1 Assuming that commercially available lumen headers can and will be used.

Access ports 1 1 1 Assuming that commercially available access ports can and will be used.

As discussed in Section 2.1, it is important that qualification is focused on the truly new

elements of the technology and not on elements that already have been qualified and proved to

work in the field. The physical breakdown structure (Figure 27) is an example of how the

membrane section might be divided into manageable elements that may then be classified in

terms of newness (Table 1). Based on the knowledge on gas separation membranes gathered in


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Section 4, the main elements of a fixed site carrier polyvinylamine CO2-selective membrane

module might be classified as shown in Table 12.

The assumptions made to classify the pressure vessel, tube sheets, lumen headers, and

access ports need to be validated, that is, it has to be shown that the operational regime these

parts will be exposed to lies within their proven capability range. Even so, it is clear that the

membrane fibers in general and the permselective polyvinylamine layer in particular pose

demanding new challenges, and hence should be considered a main concern for the qualification

process.

5.5 FAILURE MODE IDENTIFICATION AND RISK RANKING

As pointed out in the discussion on Reliability assurance (Section 2.3), an inquiry into how

an item might conceivably fail requires that all of its functions and the performance criteria

related to them are thoroughly defined and understood. Such a functional analysis should by

now have been carried out as part of the Qualification Basis. The aim of the failure mode

identification and risk ranking stage of the qualification process is then to identify both the

failure mechanisms (physical, chemical or other process) leading to failure and the resulting

failure modes (the manner in which failure is observed). Combining this information with an

assessment of the probability of the individual failure modes occurring and their consequence,

the technical risks of the technology can be evaluated and ranked.

A sensible starting point for doing this is to prepare and carry out a HAZOP and/or FMECA

session. Apart from a trained facilitator, such a session should involve experts of polymer

membrane manufacture, polymer chemistry, chemical process engineering, and mechanical

engineering. In addition to the membrane R&D team, such expertise could be found within

potential manufacturing companies and companies interested in using the membrane. The

objective of the sessions would be to make sure that all functions the membrane is expected to

perform have been identified, and identify all conceivable failure modes and mechanisms and

their consequence for membrane performance and reliability. Elements of failure tree analysis


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are expected to be useful at this point in order to understand the logic of each failure, see Section

2.3.2. The likelihood of each failure occurring and their consequence should at this point be

assessed qualitatively based on the experience of the gathered expertise. The findings of the

analysis should then be fed into a risk ranking matrix, see Section 2.3.4. Later in the qualification

process, the findings should be sought verified through further analysis and testing, see Section

2.3.3.

For the system components that were found to be proven technology (classed 1 in Table 12),

qualification documentation and statistical information on failures and performance should be

gathered. For those items that pose new technical uncertainties, new technical challenges, or

demanding new challenges (classed 2-4 in Table 12), the Structural similarity and Physics-of-

Failure techniques should be used see Section 2.3.3. To aid this analysis, the knowledge on

physics of failure of polymeric hollow fiber membranes gathered in Section 4.5 is discussed in

relation to qualifying the fixed site carrier PVAm membrane in Section 5.6 below.

5.6 SELECTION OF QUALIFICATION METHODS

Using the input of the previous steps in the qualification process, the aim of this step is to

select qualification methods that will address each of the identified failure mechanisms and

modes. As discussed in Section 4.6, a combination of deterministic and experimental methods is

necessary. Table 13 indicates the extent to which each of the physics of failure issues identified

in Section 4.5 is expected to be an issue for the given membrane, and suggests strategies for

addressing each of them.


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Table 13 : Evaluation of the relevance of known polymer membrane failure mechanisms.

Failure mechanism

Comments

Corrosion (chemical degradation)

For the PSO substrate layer, information about the conditions in which corrosion is an issue is readily available. The PVAm permselective layer is not proven, making testing with respect to corrosion necessary.

Physical aging (structural degradation)

Information about physical aging of PSO is available. For the PVAm layer, testing is necessary, taking into account the thickness-, time, and temperature dependent property of the aging process.

Plasticization (swelling)

Cross-linking is used to achieve the controlled swelling necessary for the membrane to work. As long as the cross-linking method can be qualified, plasticization is expected not to be a problem.

Compaction Testing is necessary to analyze the ability of the membrane to resist compaction at the pressure it is expected to operate in.

Fouling Particulate matter in the feed will be removed in process stages upstream from the membrane section, but could be added to the feed from piping due to corrosion. This should be tested and analyzed.

Dehydration A direct consequence of dehydration would be strongly reduced separation performance as swelling is necessary for the membrane to work. This immediate consequence can be avoided by controlling the humidity of the feed before entering the membrane. Testing is expected to be necessary to discover any long-term effect of dehydration on performance.

Concentration polarization

The conditions in which concentration polarization will be an issue can be established by a combination of theoretical analysis (using a model developed by He et al. (1999)) and tests.

Assembly issues Physical and thermal stress tests are expected to be necessary in order to explore the extent to which differing behavior of the substrate and permselective materials in the presence of physical and thermal stresses will be a problem. As long as the cross-linking method can be qualified, loss of carrier is not expected to be a problem. The operating pressure range in which the membrane fibers are able to avoid collapse or punctuation should be identified by means of testing.

6 CONCLUDING REMARKS

As has been shown, most indicators suggest that coal and other fossil energy sources are likely to

remain the key fuel for electricity generation. At the same time, the need to reduce anthropogenic

emissions of CO2 to avoid the substantial negative impacts of climate change is pressing. This

combination of circumstances strongly promotes the business case for large scale carbon capture

technology. In this light, the potential for the novel fixed site carrier polyvinylamine membrane

discussed in the current work is encouraging. Large-scale capture of CO2 by means of fixed site


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carrier polyvinylamine membranes in power plants is however not yet a proven concept. While

the concept is likely to have a large potential, it remains to be proven that it in application will

work within acceptable ranges of quality, reliability and cost. In other words, the technology

must be qualified.

The survey of relevant scientific knowledge in Sections 2 through 4 provides a sound basis

for qualifying the technology. Guided by the recommendations provided in Section 5, the next

step is then to plan a qualification process that takes into account the issues raised. A proactive

approach that addresses qualification from the very start of the upcoming pilot plant project is

strongly recommended. This will ensure that the qualification process is as efficient as possible,

and that the cost and time penalty on the development and commercialization process is

minimized.


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