assignment 2 final biolink

8/12/2019 Assignment 2 Final Biolink

1/41

Biomass Gasification to Ethanol Control System Design 2014

Damien Naidu 42661782

Kritik Prasad 42355894

Divya Sholaga 42332738

Caroline Yassa 42004747

BiomassGasification to

Ethanol Control

System Design


2/41


7 May 2014

Dr. Liu Ye

Managing Director

YTY Control

School of Chemical EngineeringUniversity of Queensland

Brisbane, QLD

Australia 4072

SUBJECT: Biomass Gasification to Ethanol Control Strategy

Dear Dr. Liu,

Please find attached the following report which outlines the control process required to synthesize fuel-

grade ethanol from biomass. The team at BIOLINK has conducted a preliminary analysis which fit the

specifications required of 100t/day of biomass feed to produce ethanol purity of 99.8%.

Please find included in the report as per your request:

Number and type of control loops; Instrumentation (sensing elements, controllers and final control element); Size final control element and cost of the control loops for a selected unit in the process flow

sheet.

We look forward to hearing from you regarding further project developments. If you have any questions

of queries, please do not hesitate to contact us.

Kind Regards,

Biolink Executive Team


3/41


Date: May 7, 2014

Invoice # 090

BioLink Engineering

The University of Queensland

Brisbane, St Lucia, QLD, 4067 To: Dave Perkins

Manager

Multinat Engineering

Brisbane, St Lucia, QLD, 4067

+61 7 334 68730

Salesperson Job Payment Terms Due Date

BIOLINK Chief Financial Officer Due on Receipt 8 May 2014

Quantity Description Unit Price Line Total

18.5 Managing Director hourly rate 120 2,220

18.5 Chief Financial Officer hourly rate 120 2,220

18.5 Chief Technical Officer hourly rate 120 2,220

18.5 Chief Operations Engineer hourly rate 120 2,220

Subtotal $8800

Sales Tax $888

Total $9768

Make all checks payable to BioLink Engineering


4/41


Executive Summary

The purpose of this report is to follow the previously completed analysis on the synthesis of ethanol

from biomass. The primary purpose is to identify appropriate control structure which can be

implemented by the process plant to ensure that the process occurs under the specified conditions.

Initially, an overview of the potentially required control system design was undertaken. This included an

overview of the control system architecture, design method and instrumentation used. This allows for

an understanding of the terminology to be established prior to further analysis.

The development of mass and energy was then carried out on each of the unit operations so that the

required control inventory was identified. By analyzing the controlled and manipulated variables in each

scenario, the number of required control loops was found. Subsequently a process control diagram was

produced and can be found in the appendix of the report.

Sizing of the valves is imperative for the overall analysis of the costing and operation of the process. Due

to the significant amount present in the process, an example of each of the different ones wereanalysed and are represented below.

Type Flow Size Cost

Fluid Flow Valve 46.8 kg/h Cvmax: 1 $ 380

Gas Flow Valve 792 kg/h Cvmax: 5 $ 590

Pump Sizing 57,600 hg/h $ 8800

The instrumentation costs were individually evaluated by consultation with industry professionals and

literature values. The cost per control loop is summarized below. Included in the price are the sensor,

transmitter and control element costs for each.

Temperature Control $1980

Pressure Control $ 30,000

Level Control $ 6,450

This analysis was undertaken with regard to some assumptions which consequently lead to limitations in

the design. The following recommendations were suggested to further increase the control capacity.

Implement multiple sensors to increase the accuracy of the measurement taken. Implementation of pumps and isolation valves to allow for increased flow diversion. Incorporation of alarms incase unavoidable faults occur to increase safety. More rigorous implementation of cascade control loops. Inclusion of more control loops to determine product quality throughout the process. Generate more extensive models for the controller software to apply. More extensive control throughout the process.

Justification of the defined parameters can be found throughout the report with extensive calculations

to be found in the appendix.


5/41


Contents

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

1.1 Purpose of Report ......................................................................................................................... 1

1.2 Process Description ....................................................................................................................... 1

1.3 Scope ............................................................................................................................................. 1

2. Control System Design .......................................................................................................................... 2

2.1 Control Overview .......................................................................................................................... 2

2.1.1 Process Terms ....................................................................................................................... 2

2.1.2 Elements and Instrumentation ............................................................................................. 2

2.2 Control System Architecture ......................................................................................................... 4

2.2.1 Control Loops ........................................................................................................................ 4

2.2.2 Hardware............................................................................................................................... 6

2.3 Control System Design Method .................................................................................................... 7

2.3.1 Selection of Controlled variables .......................................................................................... 7

2.3.2 Selection of Manipulated Variables ...................................................................................... 8

2.3.3 Selection of Measured Variable ............................................................................................ 9

3. Inventory Control Loops ....................................................................................................................... 9

3.1 Gasifier Reactor (R-101) .............................................................................................................. 10

3.1.1 Mass Inventory Control ....................................................................................................... 10

3.1.2 Energy Inventory Control .................................................................................................... 11

3.2 Compressor (P-101) .................................................................................................................... 11



3.3 Ethanol Gas-Shift Reactor (R-103) .............................................................................................. 13



3.4 Distillation Columns (D-101 & D-102) ......................................................................................... 14


3.5 Molecular Sieve (S-101) .............................................................................................................. 16



6/41


3.6 Condenser Balance (C-301 & C-302) ........................................................................................... 17


3.7 Re-boiler Balance (B-101) ........................................................................................................... 18


4. Process Control Diagram ..................................................................................................................... 18

5. Instrumentation and Costing .............................................................................................................. 19

5.1 Instrumentation Selection .......................................................................................................... 19

5.1.1 Temperature Controls ......................................................................................................... 19

5.1.2 Pressure Controls ................................................................................................................ 19

5.1.3 Level Controls ...................................................................................................................... 20

5.1.4 Ratio Controls ...................................................................................................................... 20

5.2 Control Element Sizing ................................................................................................................ 21

5.2.1 Gasifier Steam Inlet Valve ................................................................................................... 21

5.2.2 Distillation Fluid Discharge Valve ........................................................................................ 22

5.2.3 Cooling Water Pump for Ethanol Reactor ........................................................................... 22

6. Discussion ............................................................................................................................................ 23

6.1 Assumptions and Limitations ...................................................................................................... 23

6.2 Control Difficulties ...................................................................................................................... 24

6.2.1 Gasfier Inlet ......................................................................................................................... 24

6.2.2 Ethanol Synthesis Temperature .......................................................................................... 25

6.2.3 Distillation Column Parameters .......................................................................................... 25

6.3 Recommendations for Control Modifications ............................................................................ 25

7. Conclusion ........................................................................................................................................... 26

8. References .......................................................................................................................................... 27

9. Appendix ............................................................................................................................................. 29

9.1 Appendix A .................................................................................................................................. 29

9.2 Appendix B .................................................................................................................................. 30

9.3 Appendix C .................................................................................................................................. 31

9.4 Appendix D .................................................................................................................................. 32


7/41


Tables and Figures

Table 1: Control Instruments and Measurements (Control Systems Inc. 2013), (Honeywell, 2012) ........... 3

Table 2: Final Control Elements (Control Systems Inc. 2013) ....................................................................... 3

Table 3: Control Type Advantages and Disadvantages (Control Systems Inc. 2013) .................................... 6

Table 4:0 Gasifier Reactor Mass Inventory Variable Selection & Justification ........................................... 10Table 5: Gasifier Reactor Energy Inventory Variable Selection & Justification .......................................... 11

Table 6: Compressor Reactor Mass Inventory Variable Selection & Justification ...................................... 12

Table 7: Compressor Reactor Mass Inventory Variable Selection & Justification ...................................... 13

Table 8: Compressor Energy Inventory Variable Selection & Justification ................................................. 14

Table 9 Distillation Columns Mass Inventory Variable (Liquid & Vapour) Selection & Justification .......... 15

Table 10: Distillation Column Mass Inventory Master Loop Selection & Justification ............................... 15

Table 11: Molecular Sieve Mass Inventory Variable Selection & Justification ........................................... 16

Table 12: Condenser Mass Inventory Variable Selection & Justification.................................................... 17

Table 13: Re-Boiler Mass Inventory Variable Selection & Justification ...................................................... 18

Table 14: Temperature Control Selection ................................................................................................... 19

Table 15: Pressure Control Selection .......................................................................................................... 19

Table 16: Level Control Selection ................................................................................................................ 20

Table 17: Ratio Control Selection................................................................................................................ 20

Table 18: Steam Valve Sizing Parameters ................................................................................................... 21

Table 19: Fluid Valve Sizing Parameters ..................................................................................................... 22

Figure 1: Process Overview ........................................................................................................................... 1

Figure 2: Feedback Control Diagram ............................................................................................................. 4Figure 3: Feedforward Control Diagram ....................................................................................................... 5

Figure 4: Cascade Control Diagram ............................................................................................................... 5

Figure 5: Ratio Control Diagram.................................................................................................................... 5

Figure 6: General mass and energy balance for process unit ....................................................................... 9
http://c/Users/s4266178/Downloads/Assignment2FinalBiolink.docx%23_Toc387225423http://c/Users/s4266178/Downloads/Assignment2FinalBiolink.docx%23_Toc387225424http://c/Users/s4266178/Downloads/Assignment2FinalBiolink.docx%23_Toc387225424http://c/Users/s4266178/Downloads/Assignment2FinalBiolink.docx%23_Toc387225423


8/41


1

1. Introduction1.1 Purpose of ReportThe purpose of the following report is to design an appropriate process control system for the proposed biomass

to ethanol production process. Following the successful completion of the initial analysis, the development of an

effective control strategy was undertaken to further justify and expand on the viability of the proposed project.

As with any process design development, the controlling of the process needs to reflect the caliber of the design

and must be simultaneously considered. Multinat have requested that the control loops which are required for

the successful implementation of the process are identified and evaluated. The instrumentation required for

each of these loops are also investigated as well as the required sizing and approximate costing associated with

the control loops. Following this, the process plant is able to operate within the specified limits which will ensure

that losses are minimized, profits are maximized and employee safety is capitalized.

1.2 Process DescriptionThe previously developed process flow diagram identifies how biomass was converted to high-grade ethanol by

means of gasification. The gasification section was fed with 100 t/d of straw (biomass) as well as steam and

oxygen to facilitate the formation of syngas. The syngas produced contained high levels of nitrogen and sulphur

impurities which are removed from the process. By use of a catalytic tar reformer, undesired tar is broken down

and the remainder of the gas is passed onto the ethanol production stage. It is here where ethanol is produced

by manipulating the reactor operating conditions to produce ethanol. To further separate the desired product

from the other heavy alcohols and water, the use of distillation columns is employed. A molecular sieve is the

final unit in the production process and is used to further refine the ethanol to the required 99.8% purity. In

order to ensure that the process functions correctly and efficiently, control loops need to be systematically used

in the process to ensure that the final product obtained is of optimal purity in order to maximize production

values. Figure 1 provides a graphical overview of the process with the process flow diagram in Appendix A.

Figure 1: Process Overview

1.3 ScopeAs this is a preliminary analysis of the control of the ethanol production process, there are several parameters

which have not been included in the investigation.

The brief of the process is to provide an overview of the control loops which are required over the process. This

includes a preliminary investigation into;

Number and type of control loops required; Instrumentation (sensing elements, controllers and final control element); Sizing final control element and cost of the control loops

GasificationSyngas

Conditioning

Ethanol

Production

Ethanol

Separation and

RefineryDry Straw100t/d

Unconditioned

Syngas

Ethanol

99.8%

N0x , S0x , CO2

Heavy Alcohols + Water

Optimized

SyngasAlcohols

Char

Steam + O2


9/41


2

In order to simplify the initial analysis, the sizing and costing will only take place over the following control loops;

Gasifier steam supply Ethanol synthesis reaction temperature Final distillation column liquid level

The development of the initial process flow sheet as well as corresponding mass and energy balances were

prepared in the preceding report and subsequently will not be reiterated. As this is intended to be a preliminary

investigation into control strategy, the PCD will be kept simply and a piping and instrumentation diagram will

not be developed.

2. Control System DesignProcess control in an industrial setting refers to methods used to control process variables when manufacturing

a product. For instance, in this case, it is crucial that variables such as temperature, pressure and flow should be

maintained during the production of ethanol from biomass. This stringent control of the process is required to

reduce variability which ensures that ensures a high quality end product which is essential in this case. Processcontrols also help increase efficiency especially if the reaction is temperature dependant. More importantly,

implementation of process controls ensures safety throughout the process. Essentially, control system design is

dictated hierarchically by the following objectives:

Maintenance of safe operations Maintenance of steady operation and mitigation of process disturbances Maintenance of optimal operation

2.1 Control Overview2.1.1 Process TermsImplementation of process control is a combinatorial use of various control system elements, control system

architecture and utilization of strict guidelines for the design of control loops based on mass and energy

inventories. The three main components of any process control are that it aims to measure, decide and adjust

within the process. The process variable is the condition of the process fluid (gas or liquid) that impacts the

manufacturing process in some way. Example of process variables relevant to this case are temperature,

pressure, level and flow. Other process variables include density, pH, conductivity or even mass. A controlled

variable is ideally maintained throughout the system operation at a particular set point to prevent unwanted

outcomes. A measured variable is dependent on the controlled variable and is detected using a sensor, which is

the initial measurement step of a control. Utilizing the data from the measurement phase, the adjustments are

carried out by altering the manipulated variable of the system. A major manipulated variable in this process is

the flow which is altered using final control elements such as valves and pumps.

2.1.2 Elements and InstrumentationIn alignment with the three main tasks of measuring, deciding and adjusting several elements such as sensors,

transducers and transmitters are employed that satisfy each stage.

SensorsAlso referred to as primary elements since they are the first element in the control loop tomeasure process variable. Sensors essentially respond to changes in the process fluid or disturbances in


10/41


3

a predictable way which then produces a signal that can be interpreted by downstream instruments

such as transmitters.

TransducersTransducers are devices that translate a mechanical signal from the sensor into anelectrical one. It essentially amplifies this signal and sends it to a controller which is prior to the

adjustment stage. In this proposal, transducers are assumed to be part of the transmitters.

TransmittersThis device converts the reading from a sensor or a transducer into a standard signalwhich is then transmitted to a controller. There are several types of transmitters for pressure, flow and

temperature to name a few.

ControllerThe control element in the loop exerts a direct influence on the final control element, whichthen carries out the actions on the system. The final control element, accepts input from the controller

and translates it to some operation that is then performed on the

The table displayed below highlights the different types of sensors, transducers and transmitters that can be

utilized.

Table 1: Control Instruments and Measurements (Control Systems Inc. 2013), (Honeywell, 2012)

Control Type Instrument

Sensor Transducer

Flow Volumetric

Magnetic flow meters, Positive displacement

metering, Orifice plates

Mass

Thermo Mass

N/A

Level Ultrasonics , Radiation or microwaves,

Pressure (static head), Radio Frequency

Capacitance, Guided Wave Radar

N/A

Pressure N/A Pressure Transducer Spring loaded/ elastic member

Temperature Resistance Temperature Detector (RTD),

Thermocouples , Infrared (IR), Thermistors

N/A

Final control elements dictate the final adjustment stage of the control process, by physically altering the

process variables. The use of a valve could be to regulate, divert or interrupt fluid flow of a system. The use of

valves is usually coupled with actuators that direct the motion of the valve. Other final control elements include

pumps which reply on pressure or suction to move a body of fluid. These pumps could be positive displacement

pumps or centrifugal. Choice of final control element used for a system depends to a large extent on the stream

characteristics. This is further expanded inTable 2Error! Reference source not found.below.

Table 2: Final Control Elements (Control Systems Inc. 2013)

Instrument Motion Valve Characteristics

Globe Valve Linear High pressure drop across valve Throttling applications

Gate Valve Linear Flow shut off applicationsDiaphragm Valve Linear Flexible surface

Shut-off applications


11/41


4

Throttling applicationsPinch Valve Linear Low pressure drop

Shut-off applicationsBall Valve Rotary Shut-off applications

Throttling applicationsButterfly Valve Rotary Shut-off applications

Not suitable for throttlingPlug Valve Rotary Shut-off applicationsCentrifugal Valve Back flow present

Rate of fluid flow is sensitive to pressuredrop downstream of pump e.g. valve

Positive Displacement Pump

(Plunger, Piston, Diaphragm,

Rotary pumps)

Flow independent of system head No backflow

2.2 Control System Architecture2.2.1 Control LoopsFeedback Control

Feedback control is a closed loop system, which measures the output signal downstream of the control process.

The figure below highlights the main elements of a feedback control loop (Figure 2). The plant is the system

being controlled, sensors measure the output signal of the controlled variable, and the controller evaluates the

signal received while the actuator is the final control element that adjusts variable to a desired set point. The

error junction displayed is where the desired system output and measured system outputs are compared to

generate error. The system operates in either maintaining the process at its current state (regulatory control) or

changing the process from one state to another (servo control). A major disadvantage of feedback control is that

disturbances in the system go undetected until process variables are measured downstream and adjusted to the

desired set point.

Figure 2: Feedback Control DiagramFeedforward Control

Feedforward control is an open loop system that relies on measuring and compensating the disturbance variable

before any deviation in the controlled variable occurs. The disturbance in the system could be attributed to

abnormal conditions, operator actions, maintenance, sequence and changes in feed (load).Figure 3,displayed

below, is a block diagram of feedforward control.


12/41


5

Figure 3: Feedforward Control DiagramIntegration of feedback and feedforward control systems can minimize disturbances by a factor of ten compared

to a single feedback control (Smith, 2006).

Cascade Control

Cascade control is utilized in the presence of multiple disturbances to divide the system into primary and

secondary control sections which help simplify the feedback loops (Smith, 2006). Essentially, a multi-loop system

is created where the secondary loop addresses major disturbances and the output of this loop acts as a master

loop which is the set point for the primary loop or slave loop which deals with the remaining minor

disturbances. This allows for disturbance reduction and improved control of the primary variable. Refer toFigure

4 below for a diagrammatical depiction of cascade control

Figure 4: Cascade Control DiagramRatio Control

A ratio control system is a feedforward type control that commonly applies to flows. The purpose of the control

system is to ensure two or more process variable such as material flows are maintained at a specific ratio

although their individual values might be changing. This control plays an important role when two or more feed

streams are mixed to obtain a specific final composition. In the proposed control for this scenario, ratio control

is utilized for the inputs entering the gasifier.

Figure 5: Ratio Control Diagram


13/41


6

The two methods for ratio control are displayed above; with the only difference is the divider element that is

present in method 1.

All these control loops have various advantages and disadvantages, these have been summarized the in the

table below

Table 3: Control Type Advantages and Disadvantages (Control Systems Inc. 2013)

Control Type Advantages Disadvantages

Feedback (Closed Loop) Data noise reduction Increased accuracy due to complex

construction

Undisturbed in the presence of non-linearitys

Higher costs Reduces overall system gain Does not account for disturbances to

the process

Feedforward (Open

Loop) Higher stability and increased simplicity More economical construction Accounts for system disturbances

Reduced accuracy in terms or resultoutput

Unable to remove disturbance fromexternal sources due to absence offeedback mechanism

Ratio Allows for defining and maintaining adesired ratio between streams

Economical non-complex set-up Simplifies a control system

One controlled stream Is not measureddirectly, increasing control error

Not applicable for variables other thanflow rates

Cascade Simplifies a feedback process withcomplex disturbances

Accounts for disturbances in primaryvariable faster

Reduce lag time effects on system Improved overall dynamic performance

High costs due to increased instrumentand equipment

Increased difficulty during controltuning

2.2.2 HardwareTotally distributed hardware

A system architecture where sensors, controllers and controlled equipment are in close proximity with many

controllers is linked to a single sensor and final control element. The hardware is capable of accepting inputs

from supervisory controller in order to initiate or terminate automatic sequence. It is also capable of adjusting

set points as determined by the local controller. A major advantage is that if a single controller goes off-line,

other controllers are not affected. However, identifying the malfunctioning unit would be difficult if an extensive

amount of controllers are used since it is a complex process.

Centralized Hardware

Refers to hardware where a number of control processes are linked to one high capacity controller or group

controller with a single CPU that is able to account for all loops. This helps increase the speed of responses to

contingencies and operator knowledge of system conditions. Another advantage is that all process control

information is present at a single location (Ye, 2013). However a disadvantage is that a malfunction of a single

control unit affects the whole process. Also problems with the CPU may cause data loss. Routing and installing


14/41


7

control systems at a central location also incurs a higher cost. This hardware choice is most suited to a smaller

facility that does not require the use of numerous control systems.

Distributed Hardware

In contrast to centralized hardware, distributed hardware architecture consists of a number of different control

modules for systems or groups of equipment but simultaneously networked to one or more operator station in a

central location through a digital communication circuit. Although the crucial control actions are carried out at

local controllers however, the status of all systems is visible to the operator station. Furthermore, the structure

of a distributed system allows it to intervene in the control logic of the local controllers if necessary. Overall, a

distributed hardware would be the best choice for the ethanol plant being investigated. This is because it allows

for efficient monitoring for the local controllers but also allows overriding process operations if necessary.

2.3 Control System Design MethodBefore constructing control loops the following steps must be carried out chronologically to ensure efficient

design and control:

1. Mass inventories performed prior to energy inventories. Therefore, qualitative mass balancesconstructed for a particular unit operation.

2. Using the guidelines (expanded on below), the controlled, manipulated and measured variables shouldbe selected.

3. Perform energy balances, similar to mass inventories2.3.1 Selection of Controlled variablesThe three key engineering practices that play a crucial role in the selection of control variables are:

The use of mechanical energy (e.g. pumps) rather than gravity to move materials Minimization of vessel design Small design margins on pressure vessels

These practices coupled with four guidelines dictate the choice of the control variable. It should be noted that

the guidelines discussed below are not hard and fast rules, rather just provide a logical method in selecting the

variables relevant to a system.

Guideline 1 (G1):Always select state variables representing inventories which are not self-regulatory

A self-regulatory variable refers to a variable which modulates it output to a new steady state with respect to

any changes caused in the input. In contrast, a non-self-regulatory variable require the use of pumps or valves inorder to reach new steady states. Instead, these variables do not automatically modulate their steady states

with changes in the input.

Guideline 2 (G2): Select state variables which, although self-regulatory may exceed equipment orprocess

constraints


15/41


8

This guideline essentially hopes to maintain the operational constraints of equipment are adhered to, this

ensures safety maintained throughout the process. Operational constraints referred to are maximum and

minimum flow rates for instance in pumps and compressors.

Guideline 3 (G3):Select state variables which self-regulatory may interact with other inventories

This guideline suggests that the variable chosen should be controlled in such a way that it is not passed on to

other inventories

Guideline 4 (G4):Select state variables that are direct measures of product quality or that strongly affect it.

This ensures that the controlled variable selected is directly related to final product quality ensuring product

specifications are met.

2.3.2 Selection of Manipulated VariablesAfter utilizing Guidelines 1-4, for the selection of controlled variable, the manipulated variable should now be

selected using the following guidelines.

Guideline 5 (G5):The manipulated variable should affect the state variable directly

An important objective associated with the selection of manipulated variable since it ensures shorter response

times. Shorter response times are achieved since it does not rely on indirect or inferential measurements.

Guideline 6 (G6):The value of the gain between the manipulated and controlled variable should be as large as

possible, known as sensitivity.

This guideline outlines the how the control element should respond in order to bring about the desired change.

It implies that if the sensitivity of the manipulated variable to the controlled variable should be large, which

means it would not be necessary to produce a pronounced change. Similarly, if the sensitivity between the

controlled and manipulated variable was small, the control element would be required to produce major

changes.

Guideline 7 (G7):Speed of response. Any delays or lags associated with a possible manipulated variable should

be small compared with the state variable time constant.

Although in reality lag is inevitable with response times however it should be small compared to the state

variable time constant. Minimizing this ensures that the sensor and the controller are not working out of sync.

That is, any change in the manipulated variable is evident without an unreasonable delay.

Guideline 8 (G8):The extent of interactions with other balances should be minimized

This ensures that the manipulated variable is not responsible for heavily interacting with other variables in the

system, since the relationship between the manipulated and controlled variable is of interest.

Guideline 9 (G9):Pass the disturbances downstream. Take care to make sure disturbances are not recycled.

Choosing an outlet stream that either removes or absorbs disturbances is preferable. For instance inlet streams

essentially recycle disturbances, which is not ideal when selecting a manipulated variable.


16/41


9

2.3.3 Selection of Measured VariableThe Measured variable is the third variable that must be chosen. With respect to feedback control, the

measured variable is the controlled variable since it is reacting to a measured change in the process. However,

feedforward control utilizes a disturbance as a measured variable for the implementation of a control. A number

of different state variables are utilized as a measured variable depending on the product phase. Liquid level,

weight and pressure are the usual measured variables for liquids, solids and gases respectively. However, it

should be stated that component mass inventories are fairly difficult to measure, hence although inferential

measurement techniques need to be utilized. The two important guidelines for measured variable selection are:

Guideline 10 (G10): The selected measured variable must be sensitive to underlying changes in the state

variable.

This guideline highlights that it is necessary to consider the operational range of the measuring instrument and

also its physical location.

Guideline 11 (G11):Select measurement points that minimise time delays and time constants

Similar to G7 for the selection of manipulated variable, minimising the time delays, allows for a quick inference

of the effect of the measured variable on the controlled variable. It also means the controller element and the

transmitter are in sync throughout the process.

3. Inventory Control LoopsThe guidelines listed in section 2.3 were used in conjunction with qualitative mass and energy balances (and the

assumptions that underpin them) to control selected major units (see Figure 6 for general equation used for

balances).

It is important to consider that the guidelines are implemented such that the control system is able is run the

unit safely, steadily and optimally (in this order of priority). However, it should be considered that is often notpossible to select a controlled variable that satisfies all guidelines so experience and the specific workings of

individual unit operations were considered. Unless the unit was considered important in the control of the major

selected units (e.g. reboilers and condensers for distillation columns) its control system was not designed.

Figure 6: General mass and energy balance for process unit


17/41


10

3.1 Gasifier Reactor (R-101)Key Process Conditions

Gasifier Pressure = 2.5MPa Gasifier Temperature = 1100K H2 : CO = 1.6

Assum pt io ns

System can be described by the ideal gas equation of state (no liquid phase) Well insulated reactor so no exchanged with surroundings Changes in kinetic and potential energy are small compared to enthalpy changes No leaks or losses of material Equilibrium conversion due to favourable process conditions (high temperature and pressure) Ash is inert (no side reactions)

3.1.1 Mass Inventory ControlThere are gas and solid phases present in the reactor; heuristics indicate that treating each phase separately

would be the ideal approach, however the complex mass and energy transfer associated with the Gasifier make

this difficult so overall mass and energy balances are done.

(1)

( )

(2)

Table 4:0 Gasifier Reactor Mass Inventory Variable Selection & Justification

Variable Type Variable Chosen Justification

Controlled

Pressure The safety of the process depends on operating at a pressure that does not

exceed material of construction constraints (satisfies G1 & G2). Pressure is a

good measure of product quality as it relates to amount straw gasified (G4).

Manipulated

Syngas Flowrate This is an appropriate choice as it affects the controlled variable directly

(G5); similarly the response is fast as it removes excess gas (pressure)

directly (G6). The amount of straw fed could also be used but would not

respond as fast as the syngas flowrate as a variable. Oxygen and steam

flowrate represent further choices however they relate to product quality(ratio they are fed) so would not be as good a choice. There will be a limit to

how much (gain) pressure can be dissipated by the syngas flowrate so a

pressure relief valve is also placed on the reactor. Though not designed

here the changed syngas flowrate due to the control valve will need to be

checked to ensure there are no adverse interactions (recycling or safety)

with other units.

Measured

Pressure Since pressure can be measured directly this naturally represents a good

choice of the measured variable (satisfies G10 & G11)


18/41


11

3.1.2 Energy Inventory ControlThere are gas and solid phases present in the reactor; heuristics indicate that treating each phase separately

would be the ideal approach, however the complex mass and energy transfer associated with the Gasifier make

this difficult so overall mass and energy balances are done.

(3)

(4)

Table 5: Gasifier Reactor Energy Inventory Variable Selection & Justification

Quality & Stability Control

There are two ratio control loops placed on the process to control quality and steady production of product gas

at a specified composition. Both ratio controllers measure the flow of straw entering the Gasifier, one ratio loop

then adjusts the steam flow and the other the oxygen flow to achieve an optimum ratio and consistent product

composition.

3.2 Compressor (P-101)Assum pt io ns

System can be described by the ideal gas equation of state (no liquid phase) Well insulated compressor so no heat lost to surroundings Changes in kinetic and potential energy are small compared to enthalpy changes so neglected No leaks or losses of material No chemical reaction


Controlled

Temperature The safety of the process depends on operating at a temperature that does

not exceed material of construction constraints (satisfies G1 & G2).

Temperature is also a good measure of product quality as it indicatesequilibrium conversion in the unit (satisfies G4).

Measured

Temperature Since temperature can be measured directly this naturally represents a good

choice of the measured variable (satisfies G10 & G11).

Manipulated

Temperature

(enthalpy) of

oxygen by

manipulating

the flow of

heating fluid in

H-101

The mass inventory control of pressure could also act to control the

temperature of the reactor however this would increase the interaction with

other balances (G8), though if outlet flow were selected the response would

be more rapid compared to the delayed response of manipulating heating fluid

in H-101. Therefore, it could it said that this control loop is more a redundancy

for the mass inventory control. It is also important to consider that the ratio

control loop is also placed on the oxygen flow, if there was a reduced flow of

straw this valve would reduce the flow of oxygen which poses a problem

because there would be less oxygen available for cooling the reactor if it was

overheated, however if the flow of straw was reduced this means it is less

likely that the reactor is overheated.


19/41


12

3.2.1 Mass Inventory ControlThe compressor will be controlled to deliver constant discharge pressure. This also implies that the use of

compressed gas will be intermittent (Perry & Green, 2008).

(Condenser Mass Balances)

(5)

(Compressor Mass Balance)

(6)

Table 6: Compressor Reactor Mass Inventory Variable Selection & Justification

3.2.2 Energy Inventory Control

(6)

( )(Compressor Energy Balance)

( )

( )

(Condenser Energy Balance)

(7)

(8)

(9)


Controlled

Outlet Pressure The safety of the process depends on operating at a pressure that does not

exceed material of construction constraints (satisfies G1 & G2). If the flowrate

of this stream is too low it can also lead to surging which can destroy the

compressor, therefore controlling pressure is important. Maintaining constant

discharge pressure also increases process stability.

Manipulated Work input to

the compressor.

Manipulated by a control valve on the steam flowrate into the turbine that

causes the blades in the compressor to rotate. The pressure generated by the

compressor is generated through the addition of work from the turbine so the

value of gain is large (G6). The compressor work directly dictates the outlet

pressure so meets G5. It would be expected that the response would be fastand pressure deviations (disturbances) are not passed downstream (G9).

Measured

Inlet

Pressure/Flow

The inlet pressure or flow is directly related to the discharge pressure on a

variable speed pump so meets G10. A further advantage of this feed-forward

loop is that inlet pressure disturbances can be rejected before the outlet

pressure varies in response to the disturbance.


20/41


13

The outlet temperature of the compressor is an important variable to control to ensure that the

ethanol reactor runs at an optimal temperature for conversion. The outlet pressure is controlled

through the mass inventory which will also act regulate temperature. To reduce the interaction but

also achieving control of the outlet temperature the flowrate of cooling water in the condenser after

the compressor can be manipulated by measuring the outlet temperature from the compressor.

3.3 Ethanol Gas-Shift Reactor (R-103)Assum pt io ns

System can be described by the ideal gas equation of state (no liquid phase) Well insulated reactor so no exchanged with surroundings Changes in kinetic and potential energy are small compared to enthalpy changes No leaks or losses of material Equilibrium conversion due to favourable process conditions (high temperature and pressure) Ash is inert (no side reactions)

3.3.1 Mass Inventory ControlThe compressor will be controlled to deliver constant discharge pressure. This also implies that the use of

compressed gas will be intermittent (Perry & Green, 2008).

(5)

(6)

Table 7: Compressor Reactor Mass Inventory Variable Selection & Justification

Variable

Type

Variable Chosen Justification

Controlled

Outlet Pressure The safety of the process depends on operating at a pressure that does

not exceed material of construction constraints (satisfies G1 & G2). If

the flowrate of this stream is too low it can also lead to surging which

can destroy the compressor, therefore controlling pressure is

important. Maintaining constant discharge pressure also increases

process stability.

Manipulated Product flowrate from

reactor

This is an appropriate choice as it affects the controlled variable directly

(G5); similarly the response is fast as it removes excess gas (pressure)

directly (G6). However, it is important not to recycle the disturbance, as

this excess pressure is passed onto another unit. Therefore a purge

stream of steam may be needed on the flash column, though the valve

before the flash column that is used to generate two phases (not on

PCD as it is not a control valve) will have an associated pressure drop

which can remove excess head pressure.

Measured Inlet Pressure/Flow

Since pressure can be measured directly this naturally represents a

good choice of the measured variable (satisfies G10 & G11)


21/41


14

3.3.2 Energy Inventory Control

(6)

( )

(8)

(7)

Table 8: Compressor Energy Inventory Variable Selection & Justification

3.4 Distillation Columns (D-101 & D-102)Assum pt io ns

Constant molar overflow Negligible heat effects Perfect separation of heavy and light alcohols (D-101) Perfect methanol separation (D-102)

3.4.1 Mass Inventory ControlLiquid phase mass balance:

[ ]


Controlled

Reactor

Temperature Outlet

The safety of the process depends on operating at a temperature that

does not exceed material of construction constraints. Also, avoiding

unwanted side reactions and the stability of the process are also sound

reasons to pick this variable (satisfies G1 & G2).

Measured

Reactor

Temperature and

Jacket Temperature

Since temperature can be measured directly this naturally represents a

good choice of the measured variable (satisfies G10 & G11). However,

the reactor set-up presents an opportunity for cascade control as the

temperature of the jacket can also be measured (feed-forward) and the

values of the reactor and jacket temperature compared, which

generally provides better control. However, cascade control is more

hardware intensive so more costly.

Manipulated

Flow of cooling

water through

pump

This deals with the disturbance directly so meets G9. The speed of the

response depends on the overall heat transfer coefficient so could be a

fast response. The value of gain is also large as the flow of the cooling

water can be varied with a pump.


22/41


15

Vapour phase balance:

[ ]

Table 9 Distillation Columns Mass Inventory Variable (Liquid & Vapour) Selection & Justification

Although the current mass inventory control is outlined above, it should be noted that this is in fact a slave loop,

which is linked to the master loop at the reflux stream of the distillation columns. Integration of these loops is

essentially an example of a cascade control being utilized. The slave loop that modulates the level in the

distillation column is an operational control to prevent excessive liquid hold-up in the column. The master loop

at the reflux stream is responsible for quality control. Altering reflux automatically influences the final distillate

product leaving the column. This is achieved by employing ratio control over the reflux stream and the

condensate entering the second distillation column. However, increasing reflux, would then increase the liquid

level present in the column. In order to maintain a specific level that prevents flooding the level controller loop

is linked to the master loop at the reflux. The PCD does not display the DCS that utilizes the information from

the RC and LC to accordingly modulate the controllers and thus the final control valves depending on whatever

steps are necessary. DCS has not been included since it out of the scope of this report, in reality however,

without a DCS such a complex cascade control would fail.

Therefore, the mass inventory control of the master loop is:

Table 10: Distillation Column Mass Inventory Master Loop Selection & Justification

Variable Chosen Justification

Liquid level in the

reboilers (B-101 and B-

102)

R1, LR2

The efficiency of the process is dependent on the level the liquid reaches in

the bottoms section of the distillation columns. The chosen variable in this

case also plays a major role in the efficiency of the columns (G4). Choosing the

liquid level as a controlled variable also satisfies G1 and G2.

Flow of heavy alcohol and

water leaving the

Reboiler B-101

The flow of the heavy liquid and water exiting the reboiler directly impacts the

level of liquid present in the distillation column. By adjusting this variable

appropriately the amount of boil-up liquid entering the bottoms can be

maintained as required. Since this can be easily performed, there is minimal

lag in responses satisfying G7. Considering that has no other crucialinteractions within the system it also meets G8 and G9.

Level in the distillation

columns D-101 and D-102

respectively

D1, LD2

Since level can be measured directly it is a reasonable choice for a measured

variable (satisfies G10 & G11). Furthermore, this measured variable is directly

dependant on the manipulated variable. The measurement can be performed

with minimal time delays as the manipulated variable is modified (G11).


Controlled Ratio of Reflux

and Condensate

flow

In contrast to the previous control variables, in this case the ratio

between the reflux flow and the condensate flow is the controlled

variable. Since flows are self-regulatory, this variable does not meet G1,


23/41


16

3.5 Molecular Sieve (S-101)Assum pt io ns

Isothermal Steady State No chemical reaction No loss of material through leaks Gas phase pressure drop is constant Liquid obeys Raoults law

3.5.1 Mass Inventory ControlLiquid phase balance

[ ]

Table 11: Molecular Sieve Mass Inventory Variable Selection & Justification

however, it satisfies G2 and G3.

Manipulated

Reflux The controlled variable is maintained by manipulating the reflux being

fed into the reactor. The reflux strongly influences the final, therefore itsatisfies G6, G7 and G8.

Measured

Reflux andCondensate

flow

Since the controlled variables in this case is the ratio of reflux flow andcondensate flow, in order to adequately control this measurements

would have to be taken at each of these streams. However, since its a

ratio loop, the final control element only needs to be on one of the

streams. Both these satisfy G10 and G11.


Controlled Liquid level in the

molecular sieve

LM

The controlled variable for the mass inventory of the Molecular sieve,

is the level of liquid in the sieve. In order to maintain efficient

operation of the equipment, it is crucial overflow of liquid or flooding

does not occur. Since it is the liquid level, it does not satisfy G1 but

meet G2, G3 and G4.Manipulated

Stream 316 on the

PCD, the stream

which stores excess

liquid in a storage

tank

The manipulated variable of choice is the flow going into the storage

tank. If excessive liquid is entering the sieve, it is ideal that the input

stream (stream 315) is drained instead of the output. This is because

the molecular sieve works by exposing the incoming liquid to the

packing within the column, draining the output product is redundant.

Therefore, the decision was made that a bypass stream would be best

positioned at the inlet of the sieve. satisfies G5, G6, G7, G8and G9.


24/41


17

It should be noted that although not displayed in the PCD there should be a back up molecular sieve that is

available on site. This is because over time, molecular sieves will fail due to fouling of the packing. In order to

avoid any complications down the track and ensure maintenance of the process, having a spare molecular sieve

available would be beneficial in the long run.

3.6 Condenser Balance (C-301 & C-302)Assum pt io ns

Constant pressure and temperature Steady State operation KE = PE = 0 Saturated liquid at condenser outlet and incompressible

3.6.1 Mass Inventory ControlLiquid-phase balance

[ ]

Table 12: Condenser Mass Inventory Variable Selection & Justification

Measured

Stream 315 on the

PCD, which is the

reboiler product.

Utilizing stream 315 as a measurement variable allows for a quick

inference if the liquid entering sieve would lead to flooding in the tank.

Since it has a direct link to the controlled variable, choosing

would make for a measured variable. Therefore, itsatisfies G10 and G11.


Controlled

Level The level of liquid in the condenser sump is imperative in order to avoid

overflow of liquid condensate, which is a safety and operational issue.

However although using level as a mass inventory variable does not satisfy

G1 since it is self-regulatory to an extent, it satisfies G2, G3 and G4.

Manipulated

Flow of

condensate

The manipulated variable of choice for the mass inventory, however

is it not directly related to a state variable therefore it does not

meet G5. Nevertheless, modifying the flow of the condensate

directly impacts the level in the condenser sump, thus meeting G6

and G7.

Measured

Level In order to appreciate the effect on modifying the manipulated

variable on the controlled variable, in this case it is best to select the

level in the condenser as the measured variable. This mass inventory

variable satisfies both G10 and G11.


25/41


18

3.7 Re-boiler Balance (B-101)Assum pt io ns

Constant molar overflow No liquid hold-up Steady state Constant relative volatilities

3.7.1 Mass Inventory ControlLiquid-phase mass balance

[ ]

Table 13: Re-Boiler Mass Inventory Variable Selection & Justification

4. Process Control DiagramFrom the previous inventory control analysis, a process control diagram was produced and can be found in

Appendix A. Assumptions which were used to develop the diagram are listed below and their limitations are

discussed in section 6.

Biomass feed composition is consistent and doesnt vary All valves have linear characteristics Black box process has internal control loops which function independently of external streams Utilities provided are at required operating conditions Units exhibit consistent properties and require one sensor to dictate conditions No heat losses to the external environment which need to be recovered Negligible pressure drop over heat exchanger units


Controlled Liquid Level in

the distillation

DThe controller from the slave loop for the distillation column mentioned

earlier is also linked to a flow control loop at the reboiler. Hence L D

would be the controlled variable in this case. The liquid level is

considered to be self-regulatory but is not a state variable, it does not

meet G1. However, this controlled variable satisfies G2, G3 and G4.

Manipulated

Flow out of the

reboiler

The manipulated variable of choice for the mass inventory, is directly

related to LD therefore it does meet G5. Nevertheless, modifying the

flow of the condensate directly impacts the level in the condenser

sump, thus meeting G6 and G7.Measured

Level In order to appreciate the effect on modifying the manipulated variable

on the controlled variable, in this case it is best to select the level in the

condenser as the measured variable. This mass inventory variable

satisfies both G10 and G11.


26/41


19

5. Instrumentation and CostingAn investigation to the required control instrumentation, sizing of final control elements for the steam supply,

distillation liquid levels and ethanol reactor cooling system was conduction. The costing of the control loops for

these three systems was also determined. These control loops consisted of various sensors, transmitters,

controllers and valves.

5.1 Instrumentation Selection5.1.1 Temperature ControlsTemperature controls are implemented in multiple areas of the process, such as in the gasifier, compressor,

reactor and distillation control loops. The desired temperatures of the systems can be achieved, by altering flow

rates or pressures. A main consideration when determining the ideal temperature controls was that high

operating temperatures are required.

Table 14: Temperature Control Selection

Element Type Brand Description Cost Justification

Sensor RTD Oakton

CTEMP16

Air Temperature RTD Sensor

Measures Air / Gas, High

Temperatures(-200 to 850C),

Fast Response, Good for

pressurized systems, High

accuracy (2C)

$313 RTD is used a range of

temperature uses. It is

suited to this purpose as it

has a fast response with

good accuracy, and higher

temperature stability.

Transmitter Head

Mount

Intech

XU2HN

RTD Input

4-20mA output, 12V DC, High

Accuracy 1%, Linear output

with Temperature,

Programmable

$105 Mountable transmitter with

4-20mA signal conversion. It

is easy to remove and

reprogram, so can be used

in for multiple RTD

applications.Controller Auto

Tuning PID

Control

Digi-Sense

CPTEMCTP

STD02

On/Off Control

High accuracy reading (0.2%)

$155

5

Direct Plug in installation,

with multiple types of

thermocouple compatibility.

5.1.2 Pressure ControlsPressure controls are important as poor control can lead to rupturing of vessels, poor product quality and overall

downtime of the process. The ethanol reactor requires 2.5MPa of pressure for production of ethanol. The

controls are required to withstand these pressures. As these pressures are fairly high, incorrect selection of

sensors and transmitters can result in damage to the elements, as well as unsafe and poor operating control.

Table 15: Pressure Control Selection

Element Type Brand Description Cost Justification

Sensor Digital

Pressure

Gauge

Wika

CPG500

Digital Pressure Gauge

1 to 1000 bar, 0.25%

Accuracy

$794 Suitable for high pressure

conditions, with good

accuracy. Digital for easy

reading.

Transmitter Industrial

Pressure

Transmitter

Ashcroft

A20/VAC

Pressure Transmitter

30 Hg to 0 psi, 4-20mA signal

$840 Good operating range


27/41


20

Controller Pressure

Controller

Brooks

SLA5840

Digital Pressure Controller

1% flow rate error, Quick

Response time

$28360 As high pressures can lead

to explosions of vessels, a

quick response time is

ideal.

5.1.3 Level ControlsLevel controls are necessary to ensure that tanks and other vessels do not overflow. The distillation columns

have a tank after the condensers. By installing a level control loop here, it can be ensured that there is no

overflow. Level controls generally, manipulate the flow leaving the vessel, in order to control the level in the

vessel. In the distillation column, a level control loop is also located around the reboiler. This will ensure that if

the liquid in the reboiler is too low or high, the valve position after the reboiler will change to account for this.

These sensors are to be selected to operate at the temperatures and pressure at which the vessels they are

installed in operate at.

Table 16: Level Control Selection

Element Type Brand Description Cost JustificationSensor +

Transmitter

Ultrasonic

Level

Sensor with

Transmitter

EchoSafe

Ultrasonic

XP89

Utrasonic Sensor

10m range

Transmitter included, 4-20

mA signal, 0.2% Accuracy,

Explosion Proof, Corrosion

Resistant, Low Noise

$2268 This model has a transmitter

combined with the sensor. It

has 4-20mA output. As we

have multiple flucations in

temperatures in tanks and

vessels, this model has a

temperature compensation

mechanism.

Controller Process

Level

Controller

Omega

LVCN-300

Level Controller

4-20mA input, Multicoloured

bar graph, for visual

representation of system.

$4180 Good visual representation of

levels. Can be used for

multiple signal tupes. Easy to

program.

5.1.4 Ratio ControlsRatio controls are important to ensure that particular flow rates are entering at the correct ratio. The oxygen,

steam and straw are to be fed the correct ratios. The flow rates of steam and oxygen are dependent on the flow

rate of the straw entering. Therefore flow sensors are needed on both manipulated streams, which connect to

each individual flow controllers. These controllers then based on their set point will manipulate the valves to

maintain a specific ratio in the flow rates.

Table 17: Ratio Control Selection

Element Type Brand Description Cost JustificationSensor +

Transmitter

High Flow

Rate Mass

meter

GF90 Gas

Mass Flow

Transmitter

Built in sensor

69 Bar Max Operating

Pressures, Up to 500 standard

litres per second

$700 High operating pressures

and high operating

temperatures. Also very

sensitive, and has a built in

sensor.

Controller Multi

Parameter

Controller

GF Signet

8900

Multi Parameter Controller

Measures multiple variables

(flow, temperature, pressure)

$801 This controller accepts 4-

20mA signals from other

devices.


28/41


21

5.2 Control Element SizingThe sizing of the final control valves for different control loops in the process was determined. To determine this

size, the Cvvalue was to be deduced and compared with values from literature and valve manufacturers. Using

this Cvthe possible valves for control applications were evaluated.

In order to determine the Cv, there was particular system factors considered, which were:

Gas flows may vary between 70-110%. Pressure drop for liquid process due to minor losses is 2 bar.

Other assumptions which were made were:

Valves have linear characteristics. 25% of total pressure drop of system is due to the control valve. Ideal Gas behaviour in all gasses. Liquids have the same material properties as water at 25C

There are three primary control valves which are sized in the report. These include the compressible steam

leaving the boiler, the fluid exiting the distillation column and the pump supplying the cooling jacket.

5.2.1 Gasifier Steam Inlet ValveIn order to calculate the size of the valve required to control the steam leaving the boiler, the following

relationship from Perry & Green (2008) was used (see Appendix B for calculations and variable definition);

Table 18 below shows the know parameters which were obtained from the previously calculated mass and

energy balances and the calculated parameters. Steam properties were obtained from NIST.

Table 18: Steam Valve Sizing Parameters

Steam

Known Parameters Calculated Parameters

m 792 kg/h mHIGH 871.2 kg/h

mw 18 g/mol mLOW 554.4 kg/h

T 682 K PT 8.784 bar

Cp 41.485 J/mol.K PV 6.784 bar

Cv 30.832 J/mol.K x 0.2

PD(Discharge) 25 bar 0.934

PS (Stream) 33.784 bar Y 0.857

xT 0.7 CVMAX 5

Z 1

From the calculated Cv value of 5, a Saunders Weir Diaphragm Valve (1 1/4)was chosen at a cost of AUD590


29/41


22

5.2.2 Distillation Fluid Discharge Valve5.3 In order to calculate the size required for the valve from which the ethanol

following relationship was used with reference to the parameters identified in

Table 19.(SeeAppendix B

Controller Sizing- Compressible

Perry & Green, 2008


30/41


23

Appendix C for calculations)

Table 19: Fluid Valve Sizing Parameters

Fluid

Known Parameters Calculated Parameters

m 46.8 kg/h mHIGH 0.015 kg/h

1000 kg/m3 mLOW 0.010 kg/h

f(x) 0.7 PT 2.1 bar

PD(Discharge) 3.5 bar PV 0.1 bar

PS (Stream) 5.6 bar CVMAX 1

From the calculated Cv value of 5, a Saunders Weir Diaphragm Valve (1/4) was chosen at a cost of AUD380.

5.3.1 Cooling Water Pump for Ethanol ReactorIn order to size the pump, the flow rate of water through the pump, as well as the required head pressure was

to be determined. In order to do this, the enthalpy change across the reactor was used to determine the heat

produced. In order to maintain steady state conditions, the cooling water is required to remove this same

amount of heat. Assuming a temperature increase of 50C, a C p value of 4180 J/kg.K , the mass flow rate of

water required was able to be determined as 16.74 hg/h. The work required to operate this pump at the 57.6


31/41


24

m3/h flow rate of water and to achieve 21m of differential head pressure is 3.3kW. See Appendix D for

calculations.

Using the Extended Bernoulli Equation, the head pressure was able to be determined. It was assumed that there

were no velocity changes or static head.

The volumetric flow rate was therefore 57m3/hr

Using the Davey Pump Curves (Appendix D), the intersection of the cooling water flow rate and the head

pressure was found. The closest pump operating curve above this point was determined to be the most ideal

pump for application.

After consulting Davey Water Products Pty Ltd and discussing with the Customer Service and Sales Team, the

use of this pump was validated. The price of the DT110KZ(N)-H which was reported to Biolink was $8800. It was

recommended that if flow increases, this pump may be required to overwork and that investigation into a

DT150KZ may be necessary, in the case of inconsistent flow. This DT150KZ pump was priced at $10000

6. Discussion6.1 Assumptions and LimitationsIn conjunction with the assumptions which were made in order to develop the process flow diagram which the

process flow diagram was based on, the following assumptions were made with their resulting limitations

outlined

Black box process is self regulatingo As the extraction of the impurities in the process is modeled by an overall process, there are

some simplifications made in terms of control. For the purpose of this report, control of this was

negated and deemed to be independent of external parameters. However, in reality this would

not be the case as the control of these units would need to be analysed and will be affected by

things such as utilities etc.

Feed of biomass is consistento This neglects the changes in the organic feed composition and assumes that all the straw will

consistently have the same properties. Doing so means that the inlet feed rate will not need to

be regulated. However this will not occur realistically and there will be changes in the feed

which will need to be controlled so as to ensure that the gasification process is feed with the

correct components so that the subsequent syngas outlet is consistent.

Process operates within control limitations


32/41


25

o This assumption means that all aspects of the process will operate within the designspecification of the control units. In the case that the operating parameters are exceeded, such

as runway reactions and huge pressure accumulation, the control system will not fail. The

implementation of alarms, pressure relief valves and pump isolation mechanisms ensure that

that they are considered.

Measurements taken by sensors indicative of entire unito This may not be the case in particular with temperature and level measurements as the

temperature is measured at a particular point which may not be consistent throughout the

vessel. A similar issue occurs with the level measurements as vapours and foam which may be

present may distort these readings and result in inaccuracies.

Ratio controllers are instantly functioningo The used of ratio controllers can be troublesome particularly in startup and shutdown of the

process. When these unsteady conditions are prevalent, the controller doesnt function as

required and will result in a delay of the information transmitted until steady state is achieved

and maintained.

Limited time delays in processo Feedback control does not function effectively when there are significant time delays occurring.

Instead of reaching an appropriate steady state, the system continuously operates in a transient

state resulting in faulty control (Seborg, 2005).

Measurement of disturbances as requiredo In order for feed forward loops to function adequately, the disturbance must be directly

measured in order for a control response to be output. As a result, any unmeasured

disturbances, no matter how small, may result in inaccuracies due to controller sensitivity.

Follows ideal gas modelo This was assumed as the temperatures of the gases are quite high. Although there are no

quantitative balances for the process, this assumption may not hold for the conceptual analysis

of the control structure. Realistically, the individual gas properties need to be considered

All rectors reach chemical equilibriumo In reality this may not occur as the conversion may be hindered by external and unavoidable

circumstances as well as the process itself. Control measures would need to be considered to

ensure that equilibrium is reached prior to further control strategy being implemented.

Sizing properties for liquids based on water and gases based on ideal gaseso The sizing for the control elements are based on generalized parameters for the process. To

further increase accuracy, the fluid properties for the alcohols leaving the distillation columns

need to be identified. This may result in slight discrepancies for sizing the control units. No leaks or losses

o All units and piping are self-contained and will not result in leakages into the environment. Inreality this may not be the case and as a result further control measures such as bunding needs

to be considered.

No heat loss to environment


33/41


26

o This is taken into account when developing the mass and energy balances and consequentcontroller selection. in the process there may be losses to the environment which would need to

be recovered to ensure process efficiency.

Prefect separation of alcohols in distillationo It is assumed that there will be 100% separation of the heavier alcohols from the ethanol in the

distillation columns. Ideally this will not occur and as a result further control mechanisms will be

required to ensure that product quality is not compromised.

6.2 Control DifficultiesAs previously stated, the stringent control of the process is imperative to ensure that optimal operating

conditions are followed. There is however some aspects of controlling the process in which some difficulties

arise and need to be closely monitored.

6.2.1 Gasfier InletThe gasifier is one of the most imperative units and needs to be closely controlled in terms of flow into the

reactor and reactor temperature. The flow of oxygen into the reactor needs to be closely regulated to ensurethat there is a sufficient amount available to react with the biomass, yet ensure that there is not excess

provided to the reactor. The use of ratio control ensures that the flow is regulated depending on the feed rate of

the biomass. The feed rate of biomass is also used to control the amount of steam which is fed to the reactor in

the same way. Although the calculation of the ratio is done outside the control loop and doesnt interfere with

the loop response, difficulties may arise in the initial ramp up where ratio controllers struggle to adjust to the

unsteady conditions.

Another aspect of the gasifier inlet in which difficulties can arise is the control of the temperature of the vessel.

Ideally this would be done with the use of a heating jacket or similar, but is deemed inefficient due to lack of

effective control. As a result the temperature of the oxygen fed to the reactor is adjusted by manipulating theflow of steam used to manipulate the temperate of the oxygen. Although this is not a direct measure of the

manipulated variable, it is inherent to the process control and safety as well as the product quality.

6.2.2 Ethanol Synthesis TemperatureThe temperature of the ethanol synthesis reactor is another vital unit which needs to be stringently controlled.

Deviation from operating conditions will result in decreed conversion to ethanol and subsequently decreased

final production rate. By implementing a cascade controller, the temperatures of both the heating jacket and

reactor contents can be considered in the final control decision. The added complexity of the control

architecture requires increased equipment and instrument cost. One of the difficulties that arise from the

cascade controller is the inconsistent temperature of the liquid in the heating jacket. The set point for this is

determined by a predefined model and may not necessarily be indicative of the actual process. The

measurement of the actual temperature of the reactor adds to the accuracy of the temperature control and

subsequently conversion.

6.2.3 Distillation Column ParametersAs the distillation columns employee the use of both ratio and cascade control to control the reflux flow and

liquid level respectively, similar design issues that were previously expected will occur. One of the difficulties

that are encountered with the control is the placement of the sensors for both the pressure of the tank and the


34/41


27

temperature of the liquid hold up in the column. Pressure sensors should ideally be located throughout the

vessel to ensure an accurate overall representation of the system instead of a localized measurement. The same

is required for the temperature sensors for the liquid holdup.

6.3 Recommendations for Control ModificationsAs this is a preliminary control analysis, there are further modifications which can be made in order to increasehow effective the control strategy is. The following recommendations are suggested to improve the overall

control strategy.

Implement multiple sensors to increase the accuracy of the measurement taken. This ensures that ameasurement which is indicative of the entire situation is taken instead of a localized and possible

inaccurate measurement

Implementation of pumps and isolation valves to allow for increased flow diversion competency ifrequired.

Incorporation of alarms incase unavoidable faults occur to increase safety. This allows for employees tobe made immediately aware of issues which may arise and allow for sufficient action to be taken. Thisshould also be linked to automatic systems which will cease the process continuation so as not to result

in catastrophic equipment failures.

More rigorous implementation of cascade control loops. Although there are increased costs, there aresignificant benefits which will allow for a more stringent and accurate control strategy.

Inclusion of more control loops to determine product quality throughout the process so as to limit theamount of disturbances which are to be passed downstream.

Generate more extensive models for the controller software to apply. This increases the accuracy ofhow the controllers function.

More extensive control throughout the process. There are sections which have been assumed to be self-regulating or have been negated as they were not deemed crucial to control. Further analysis of all

sections is recommended to ensure that all parts of the process are considered.

7. ConclusionIn order to successfully expand on the previously completed biomass to ethanol synthesis plant, appropriate

control structure was developed. There are several components of a single control loop which must work in

unison in order to ensure that the process runs optimally and efficiently. Although the developed control

structure is adequate for the implemented system, there are a few recommendations which are outlined above

which should be applied in a more rigorous study.


35/41


28

8. ReferencesBlackmonk Engineering 2009, How To Size A Pump, viewed 4 May 2014,

Brooks Instrument 2014, Delta Class Remote Pressure Controller / Flowemeter, viewed 5 May 2014,

Brooks Instrument 2014, Pressure Controllers, viewed 5 May 2014,

Cole Parmer 2014, Digi-Sense Standard Temperature Controller 110V, viewed 5 May 2014,

Cole Parmer 2014, Oakton Temp-16 Precision RTD Thermometer, viewed 5 May 2014,

Davey 2007, Sump Pumps, viewed 5 May 2014,

Doherty, MF, Fidkowski, ZT, Malone MF & Taylor, R 2007, Distillation in Green DW (ed.), Perrys Chemical

Engineers Handbook, McGraw Hill


36/41


29

Techmation Inc, 1999, Control Strategy: Ration Control Systems, Application Note, Arizona USA

The Valve Shop, 2014, Saunders Weir Valve Specification

WIKA Australia 2014, Digital Pressure Gauge, viewed 5 May 2014,

Ye, L 2014, Introduction to Process Control Course Reader. Australia, The University of Queensland.


37/41


30

9.Appendix9.1 Appendix A


38/41


31

9.2 Appendix BController Sizing- Compressible

Perry & Green, 2008


39/41


32

9.

assignment 2 final biolink

Documents