integrated process design instruction

7/28/2019 Integrated process design instruction

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Computers and Chemical Engineering 26 (2002) 295306

Integrated process design instructionD.R. Lewin a,*, W.D. Seider b, J.D. Seader c

a Chemical Engineering Department, Technion, Israel Institute of Technology, Haifa 32000, Israelb Chemical Engineering Department, Uni6ersity of Pennsyl6ania, Philadelphia, PA 19104, USA

c Chemical and Fuels Engineering Department, Uni6ersity of Utah, Salt Lake City, UT 84112, USA

Received 21 August 2000; received in revised form 2 January 2001; accepted 2 January 2001

Abstract

As chemical engineering education moves into the new millennium, it is incumbent on educators to provide a moderncurriculum for process design, yet mindful of the limited time for instruction that is available. This paper addresses three key

components of a chemical engineering curriculum that prepare undergraduates to be effective process designers in industry: (a) a

structured approach relying on fundamentals, integrated with instruction in the competent use of process simulators; (b) a balance

between heuristic and algorithmic approaches; and (c) instruction in the integration of design and control. It is argued that these

components should be included in an integrated fashion, with much of the material appearing gradually during the delivery of

core courses, taking full advantage of computing capability and multimedia support for self-paced instruction. In this paper, each

of the features is discussed in detail and demonstrated for the design of a typical process. 2002 Elsevier Science Ltd. All rights

reserved.

Keywords: Process design instruction; Heuristic and algorithmic approaches; Chemical process simulators; Interaction of design and control;

Multimedia and web-based instruction

www.elsevier.com/locate/compchemeng

1. Introduction

Instruction of chemical engineers should reflect the

challenges they face in industry. Young chemical engi-

neers are required to assimilate rapidly new and emerg-

ing technologies to react in a flexible manner to shorter

production cycles and strict quality regulations. They

are expected to improve product quality while at the

same time reduce operating costs and environmental

impact, improve operability, minimize waste produc-

tion, and eliminate possible hazards. It is incumbent on

chemical engineering educators to provide a modern

curriculum for process design instruction that addressesthese needs while being mindful of the limited time

available.

The first issue involves the concept of a structured

core curriculum that focuses on fundamentals as a basis

for design. Typically, design is taught in the senior year

and involves the integration and assimilation of core

course material as dictated by the needs of a designproject. Section 2 describes how the core course se-

quence has impact on the needs of instruction in design.

Furthermore, we discuss the need for students to sup-

port their developing knowledge of engineering funda-

mentals in general, and more specifically their design

activity, by mastering the use of a commercial simulator

to a high level of competence. We suggest that adopting

self-paced methods relying on multimedia tutorials,

which assist the students in preparing simulations of

process flowsheets, can support this effort. In the sec-

ond issue, which is discussed in Section 3, it is postu-

lated that the teaching of design itself should strike abalance between heuristic and algorithmic approaches.

While heuristics lay the foundations for acquiring the

experience necessary to carry out practical process cre-

ation and equipment design, the importance of the

latter is to ensure the generation of optimal designs.

The last issue is the importance of dealing with interac-

tions between the design and control of chemical pro-

cesses when learning to prepare process designs. In

Section 4, the current state of the art in the integration

of process design and process control is reviewed with

* Corresponding author. Tel.: +972-4-829-2006; fax: +972-4-823-

0476; http://tx.technion.ac.il/dlewin/pse.htm.

E-mail address: [email protected] (D.R. Lewin).

0098-1354/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 9 8 - 1 3 5 4 ( 0 1 ) 0 0 7 4 7 - 5
http://tx.technion.ac.il/~dlewin/pse.htmhttp://tx.technion.ac.il/~dlewin/pse.htmhttp://tx.technion.ac.il/~dlewin/pse.htmmailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]://tx.technion.ac.il/~dlewin/pse.htm


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D.R. Lewin et al. /Computers and Chemical Engineering 26 (2002) 295306296

particular emphasis on its impact on the education of

undergraduates.

Several textbooks are available to support a senior

course in process design. The traditional textbooks

focus on either hierarchical design relying on back-of-

the-envelope calculations (Douglas, 1988), or on de-

tailed equipment design, costing, and economics

(Ulrich, 1984; Peters & Timmerhaus, 1991). Of the

more recent texts (Smith, 1995; Woods, 1995; Turton,Bailie, Whiting, & Shaeiwitz, 1997; Biegler, Grossmann,

& Westerberg, 1997), only Seider, Seader, and Lewin

(1999) additionally provide detailed support on the use

of simulators, with an explicit treatment of the interac-

tion of design and control.

In this paper, it is an objective to discuss our view of

several key aspects of how computer-aided process

design can be taught to chemical engineering under-

graduates. This topic has been treated previously by a

number of chemical engineering educators, starting

with Westerberg (1971), and with more recent treat-

ments by Turton and Bailie (1992), Cameron, Douglas,and Lee (1994), Shaeiwitz, Whiting, and Velegol (1996),

Bell (1996), Rockstraw, Eakman, Nabours, and Bellner

(1997), Counce, Holmes, Edwards, Perilloux, and

Reimer (1997). It is not intended in this article to

provide a comprehensive coverage of instruction in

process design with emphasis on the advantages and

disadvantages of alternative approaches. Rather, it is

our purpose to extend some old ideas and introduce

some new ones that we have tested with our students.

2. A structured approach relying on fundamentals

Before discussing the building blocks that are an

integral part of the toolbox of a process designer, a

brief mention of the educational approach that we

advocate is in order. We therefore first discuss the

particular skills that need to be fostered, and the frame

of reference used to define goals for the student,

couched in terms of educational objectives.

2.1. Educational approach ad6ocated

An important goal of the undergraduate curriculum

in chemical engineering is to develop the integration,

design, and evaluation capabilities of the student. As

shown in Fig. 1, Bloom (1956), characterized the six

cognitive levels in the hierarchy: Knowledge

Comprehension Application Analysis Synthesis

Evaluation. The cognitive skills at the highest level

are synthesis and evaluation, which rely on comprehen-

sion, application, and analysis capabilities in the knowl-

edge domain, and are consequently the most difficult

and challenging to teach. However, to prepare under-

graduates to be effective designers in industry, it is

important to ensure an adequate coverage of these

higher-level skills, rather than limit their education to

one based on just knowledge, comprehension, applica-

tion, and analysis. To achieve the desired coverage in a

cost-effective manner, it is important to define instruc-

tional objectives in each undergraduate course in a

manner such that the six skills are covered by the senior

year. Note that Blooms taxonomy has been applied in

chemical engineering by Fogler and LeBlanc (1995),

Fogler (1999), Felder and Rousseau (2000).

The focus of the learning activity is placed on the

accomplishments expected from the student through the

formulation of course goals in terms of instructional

objectives. The key is to provide material that increasesthe abilities of the students, with the emphasis being on

what the student is able to achieve rather than merely

what he or she is aware of or understands. As an

example of a possible approach, the instructional objec-

tives for a typical course on process design might be:

On completion of this course, the student should be able

to:

Carry out a detailed steady-state simulation of a

chemical process using a process simulator (e.g.

HYSYS) and interpret the results.

Synthesize a network of heat exchangers for a chemi-cal process such that the maximum energy is recov-

ered or the minimum number of exchangers is used.

Synthesize a train of separation units.

Suggest reasonable process control configurations us-

ing qualitative methods.

Formulate and sol6e a small-scale process optimiza-

tion problem using a process simulator (e.g.

HYSYS).

E6aluate process alternatives at various levels: single

units, complete plants, and the conglomerate level.Fig. 1. Blooms taxonomy of educational objectives (Bloom, 1956).


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D.R. Lewin et al. /Computers and Chemical Engineering 26 (2002) 295306 297

Exercise judgment in the selection of physical prop-

erty correlations for design.

It is noted that these objectives focus on the profi-

ciency in required skills expected from the student.

Clearly, a precondition for exhibiting these skills is that

the student understands the underlying material. Fur-

thermore, it is our experience that students feel morecomfortable with clearly defined objectives that quan-

tify what is expected of them.

2.2. The design project and process simulator as means

to integrate process knowledge

A designer must have a working knowledge of math-

ematics, chemical and physical technology, biotechnol-

ogy, materials science, and economics, which are the

building blocks used by the design engineer. This

knowledge is developed in a structured fashion in the

core chemical engineering courses. It is advantageous todevelop the capabilities of the students with a process

simulator, in conjunction with the core course materi-

als, as will be discussed shortly. The integration skills of

the students are developed through their solution of

industrially-relevant design case studies. During the

design project, teams of students are expected to call

upon diverse aspects of their working knowledge to

carry out an integrated process design, determining its

feasibility with respect to environmental impact, safety,

controllability, and economics. In so doing, the student

designer integrates previously acquired knowledge in

the engineering disciplines, as well as management

skills. Due to the problem scale, this inevitably involves

the use of a process simulator to formulate and solve

the material and energy balances, with phase and chem-

ical equilibrium, chemical kinetics, etc. and to size

process equipment for cost estimation. Familiarity and

competence in the use of a simulator permit the student

to quickly develop a base-case design, which is verified

against process and thermodynamic data. The availabil-

ity of a reliable process model allows the design team to

assess rapidly the economic potential for alternative

designs, as well as to derive optimal operating condi-

tions using optimization methods that incorporate eco-

nomics. Moreover, competence in the use of thesimulator allows process evaluation to go beyond eco-

nomics alone; controllability and operability can be

assessed using dynamic simulation, while some simula-

tors automatically provide information to help deter-

mine the environmental impact of each of the product

streams.

Process simulators are an indivisible part of modern

practice in chemical process design. This has been true

for some time in the petrochemicals, bulk and fine

chemicals industry, and is rapidly becoming true in

biotechnology and microelectronics manufacturing. The

routine use of the process simulator in industry implies

that chemical engineering graduates should be com-

petent to utilize these tools in the analysis, synthesis,

and evaluation of process designs. Once students have

learned to use simulators intelligently and critically,

they appreciate how easy it is to incorporate data and

perform routine calculations, and master effective ap-

proaches to building up knowledge about a process. Asdiscussed next, the level of simulation skills required of

the students completing industrial-scale design prob-

lems imply sufficient exposure to the use of simulators

during the core courses.

2.3. Use of the simulator in core courses: opportunities

and challenges

The high level of competence in the use of simulation

expected of the students in the design project relies on

their having obtained exposure to simulation in parallel

with the core courses. One way to accomplish this is torequire students to solve at least one exercise involving

the use of simulators as part of each core course.

Indeed, recent articles by Russell and Orbey (1993),

Bailie, Shaeiwitz, and Whiting (1994) discuss the addi-

tion of design projects in the sophomore and junior

years. Table 1 provides a typical simulator-based exer-

cise for core courses in the chemical engineering cur-

riculum. Adoption of such a sequence goes far in

preparing students to use a simulator in solving large-

scale problems in the senior design course. With the

wide availability of commercial process simulators to

educators, the working knowledge of mathematics,

chemical and physical technology, and economics can

be put to effective use in solving meaningful problems,

starting in the sophomore course on material and en-

ergy balances, by solving various parts of a complete

process with a process simulator. The third author of

this paper recalls vividly his experience as a junior when

taking the first course in chemical engineering, based on

material in Chemical Process PrinciplesPart 1Ma-

terial and Energy Balances (Hougen & Watson, 1943).

The instructor first covered the fundamentals in Chap-

ters 19, with application to and homework exercises

for small closed-end problems. The last 2 weeks of the

course were spent on Chapter 10, which involved mate-rial and energy balance calculations by hand for a

complete process. Although the calculations were te-

dious and very time consuming, students developed an

appreciation of what chemical engineering was all

about and a desire to proceed to the next level of

instruction.

Today, the tediousness and time-consuming aspect of

process calculations can be eliminated and some time

can be spent on teaching synthesis and evaluation skills,

even in the sophomore year. The material and energy


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Table 1

Core course sequence and typical exercises using simulators

Course ObjectivesExercise

Analysis of methanol synthesis loopMass and Convergence of material and energy balances for processes with recycle

energy and purge streams

balances Analysis of sensitivity to degrees-of-freedom

Selection of economically optimal operating conditions

Heat-integrated toluene dehydroalkylationHeat transfer Designing a heat exchanger for vaporizing fluid (computing temperatureapproaches)(see Fig. 1(d))

Optimal selection of heat-transfer area, weighing reduced energy demands

in furnace against increased cost of exchanger

Avoidance of temperature crossovers

Thermodynamics Constructing Txy diagrams for Impact of estimation method on the accuracy of thermodynamic

properties, including K-values and enthalpies.alcoholwater systems

Simulation of a depropanizer column Impact of design variables (e.g. number of ideal trays, feed tray location)Separation

processes on performance of the column

Impact of selection of degrees of freedom on attaining column

specifications

Difficulties in converging multicomponent, multistage separation models

Dynamics and control of a binary distillationDynamics and Learning to set up a dynamic simulation

control column Definition of controlled and manipulated variables and the installation

and tuning of control loopsTesting the dynamic resiliency of the column

Process design Optimization of a multi-draw column Learning to use the simulator to set up and solve an optimization

problem

Observing the importance of selecting the appropriate manipulated

variables for optimization

Observing the impact of process constraints

balance course is taught in the sophomore year, using

textbooks such as Himmelblau (1996), Felder and

Rousseau (2000). Both of these books cover essentiallythe same fundamentals as presented in the Hougen and

Watson textbook. In addition, Himmelblau (in Chapter

6) and Felder and Rousseau (in Chapter 10) cover the

solution of material and energy balances for continu-

ous, steady-state processes with a process simulator.

Both texts leave to the instructor the choice of a process

simulator and instruction on how to use it, so unless he

or she is knowledgeable in the use of computer-aided

process simulation programs, it is probable that this

material will not be covered. In Chapters 12 and 13 of

Felder and Rousseau, two fairly complex processes are

described and problems given for making material andenergy balances, as well as other chemical engineering

calculations. Calculations for the methanol synthesis

process in Chapter 13 are particularly suitable for the

use of a process simulator and serve as an excellent

introduction in the sophomore year to process design.

The use of a process simulator in the sophomore year

introduces the student to the importance of being famil-

iar with a large number of chemical species; the use of

physical properties such as density, vapor pressure,

specific heat, enthalpy, and K-values; the ease of chang-

ing units; the ease of drawing process flow diagrams

with systematic ways of numbering streams and equip-

ment units; and methods of handling recycle.If students are introduced to the use of a process

simulator in the sophomore year, their skill in using

simulators can be further enhanced in the junior year in

courses in fluid mechanics, heat transfer, separations,

thermodynamics, and reaction engineering. The course

in fluid mechanics can include simulator calculations of

pipeline pressure drop, sieve-tray pressure drop, and

power requirements of pumps, compressors, and tur-

bines. The study of heat exchangers in the heat transfer

course can include the detailed design of a heat ex-

changer, including considerations of the complex varia-

tion of the temperature driving force, temperaturecrossover violations, and prediction of bubble and dew

points for multicomponent mixtures.

Process simulators are quite useful in the solution

thermodynamics course because the tedious calcula-

tions of activity coefficients, K-values, bubble and dew

points, vaporliquid equilibria, liquidliquid equi-

libria, and data correlation are readily carried out, and

property graphs and tables are easily prepared. When a

process simulator is used in a thermodynamics course,

less time need be spent on the myriad of equations that


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appear in the textbooks and more time can be spent in

solving practical problems that demonstrate the impor-

tance of thermodynamics to students. Regrettably, the

use of a process simulator in a solution thermodynam-

ics course does not appear to be considered in the

leading textbooks on the subject. Instead these text-

books either provide their own computer programs for

computing physical properties or suggest the use of

popular numerical-method programs. Thus, the oppor-

tunity to integrate the important lessons learned in the

solution thermodynamics course for the later benefit of

the capstone design course is often missed. The se-

parations course can profit greatly from the use of

process simulators to solve both binary and multicom-

ponent, multistage separation operations such as distil-

lation, absorption, stripping, and liquidliquid

extraction. It is suggested that less time be spent on

graphical methods that are limited to binary and

ternary mixtures, with more time spent on multicompo-

nent separations that are readily handled by process

simulators.

The reactor-engineering course also affords an excel-

lent opportunity to tackle practical problems in reactor

design after completing instruction on the ideal plug-

flow and CSTR reactors. Using an enthalpy datum of

the elements (rather than the compounds), simulators

readily handle reactor energy balances without the need

to supply heat of reaction information. Simulators also

readily compute chemical or simultaneous chemical and

physical equilibrium using either the equilibrium-con-

stant method for specified stoichiometry or the mini-

mization of free energy method for specified product

chemicals. Activity coefficients can be taken into ac-

count and complex kinetic expressions can be specified.

Here too, the use of process simulators to design chem-

ical reactors appears to be ignored in the leading text-

books on chemical reaction engineering.

As discussed by de Nevers and Seader (1992), the use

of process simulators prior to the senior design course

provides students with an opportunity to develop a

critical attitude towards chemical process calculations.

They cite a problem involving the condensation and

subsequent single-stage flash separation at 100 psia of a

vapor mixture of ammonia and water, initially at 290 Fand 250 psia. The student first solves this problem

graphically using an enthalpyconcentration diagram.

The result, which is considered to be reasonably accu-

rate, is a vapor of\99 wt.% ammonia and a liquid of

about 68 wt.% ammonia at a temperature of about 80

F. The student then solves the problem numerically

with a process simulation program. He or she is re-

quired to select at least four different pairs of K-value

and enthalpy correlations for comparison with the

graphical solution. Many students are shocked by the

widely varying results. For example, with one set of

four pairs of correlations, the flash temperature ranges

from 91.2 to 83.4 F with an average of 0.5 F. From

then on, students pay careful attention to the selection

of correlations for physical properties. The educational

importance of discussing errors is also presented by

Whiting (1987, 1991).

Students who have used process simulators through-

out the chemical engineering curriculum are in a posi-

tion in the senior design course to concentrate their

efforts on synthesis and evaluation aspects of process

design. Instructors can devote more time to instruction

in the synthesis of heat-exchanger systems using pinch

analysis, the synthesis of nearly- and non-ideal separa-

tion trains, second-law analysis, economic evaluation,

optimization, waste minimization, safety, environmen-

tal impact, and controllability. During the senior design

project, teams of students are better prepared to call

upon diverse aspects of their working knowledge to

carry out an integrated process design and determine its

feasibility from all aspects, not just economics.

2.4. Effecti6e instruction in process simulation: the role

of self-paced approaches

The quality of training may be enhanced, and in-

struction resources used more efficiently, through the

use of multimedia and web-based approaches. Such

self-paced methods of training undergraduates allow

them to obtain the details they need to use the simula-

tors effectively, saving instructors class time, as well as

time answering detailed questions as the students use

simulators to make calculations. In a typical situation,

when creating a base-case design, students can use the

examples in the multimedia tutorials to learn how to

obtain physical property estimates, heats of reaction,

flame temperatures, and phase distributions. Then, stu-

dents can learn to create a reactor section, using the

simulators to perform routine material and energy bal-

ances, and in some cases kinetic calculations, to size the

reactor. Next, they can create a separation section,

which often involves multicomponent, multistage distil-

lation-type calculations (Seader & Henley, 1998), which

almost always leads to the addition of recycle streams.

Using the coverage of process simulators in the multi-

media tutorials accompanying the textbook by Seider et

al. (1999), the instructor needs only to review the

highlights of simulator usage in class. This invariably

leaves time for the discussion of more advanced issues.

Furthermore, through installation of the multimedia

materials on the web, students gain access to the mate-

rial from remote locations. Our experience is that the

response of students to self-paced multimedia instruc-

tion has been very positive.


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3. A balance between heuristic and algorithmic

approaches

The teaching of design should strike a balance be-

tween heuristic and algorithmic approaches. Since de-

sign invariably involves significant designer

intervention, it is important to teach both heuristics as

well as computer-aided algorithmic methods. The for-

mer lay the foundations for acquiring the experiencenecessary to carry out practical process design, while

the latter is critical to ensure the generation of optimal

designs.

Process synthesis is generally introduced first by ex-

ample and by instructing students to rely on heuristics

(Douglas, 1988). These heuristic rules are important in

that they provide a framework for workable designs,

based on easy to understand rules of thumb (Walas,

1988). For example, consider the synthesis of a process

to hydrodealkylate toluene using a number of heuristic

rules, which lead to the sequence of flow diagrams

shown in Fig. 2 (Seider et al., 1999). It is noted in Fig.2(a) that an undesirable side-reaction to biphenyl ac-

companies the principal reaction, and the conversion of

toluene is incomplete. The selection of the reactions

conditions is motivated by a desire to minimize the

production of the unwanted side-product, while maxi-

mizing the yield. The reaction conditions lead to the

distribution of chemicals shown in Fig. 2(b), in which

unreacted toluene and hydrogen are recovered by in-

stallation of two material recycle streams. The two

reaction products (benzene and biphenyl) are removed

from the unreacted toluene and hydrogen by installa-

tion of a separation section. One possible arrangement

consists of the flash vessel and three distillation

columns shown in Fig. 2(c). It is noted that heuristics

dictate that column operating pressures should be se-

lected to allow the usage of cooling water wheneverpossible. Finally, Fig. 2(d) shows a possible instantia-

tion of task integration, in which a preheater is installed

to supply much of the heat duty required to bring the

reactor feed to the high temperature that favors the

primary reaction, by exchange with the hot reactor

products, which need to be cooled. This arrangement

significantly reduces the heat duty required in the

furnace.

As the heuristic ideas are mastered, the students

should be directed to computer-aided algorithmic ap-

proaches that assist them in the generation of better

designs. Several algorithmic approaches, which havegreat practical value, should be presented. These in-

clude heuristic and evolutionary synthesis of nearly

ideal vaporliquid separation sequences (Seader &

Westerberg, 1977), synthesis of separation systems for

non-ideal liquid mixtures (Malone & Doherty, 1995),

the application of second-law analysis (Seider et al.,

1999) to identify opportunities for improved energy

Fig. 2. The evolution of the flowsheet for a process to hydrodealkylate toluene.


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Fig. 2. (Continued)


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utilization, and the application of methods to compute

heat recovery targets (Linnhoff & Hindmarsh, 1983),

and to assist in the design of optimal or near-optimal

heat-exchanger networks (Smith, 1995). For example,

the following algorithmic approaches can refine the

design in Fig. 2(c):

1. Compare the separation sequence in the base-case

design to alternative sequences by branch-and-

bound search.2. Check the utility requirements against the thermo-

dynamic MER (maximum energy recovery) target

using the temperature-interval or graphical meth-

ods. Then, a mixed-integer non-linear program

(MINLP) can be implemented to derive an optimal

design for implementation. There may be additional

opportunities for energy savings. For example, a

number of alternative heat-integration configura-

tions can be considered for the column sequence

proposed in Fig. 2(c). In these configurations, the

heat of condensation in a column operating at high

pressure is used to supply the heat of vaporizationin a column operating at a lower pressure, requiring

careful selection of column operating pressures to

ensure sufficient temperature driving forces. In se-

lecting between these alternatives, the economic

benefits need to be weighed against their impact on

the operability of the process, as discussed next.

4. Integration of design and control

Traditionally, plant controllability and operability

has been considered late in the design process, often

leading to poorly performing chemical plants. The in-

disputable fact that design decisions invariably impact

the process controllability and resiliency to disturbances

and uncertainties is driving modern design methods to

handle flowsheet controllability in an integrated fash-

ion. Several recent articles, including Rhinehart, Na-

tarajan, and Anderson (1995), Edgar (1997), stress the

need to integrate process control with process design.

The model of an industrial chemical process for study-

ing process control technology presented by Downs and

Vogel (1993) has proved to be very valuable in helping

to bridge the gap. Morari and Perkins (1995) stress the

importance of steady-state and dynamic analysis in thedetermination of controllability. Perkins (2000) cites the

need for educators to develop a systematic process

systems approach that considers design, operation and

control. Lewin (1999) describes the state-of-the-art and

suggests that two alternative approaches, controllability

and resiliency (C&R) screening methods and integrated

design and control, can ensure that chemical plants meet

design specifications. While C&R analysis is used for

screening early in the design process, the integrated

design and control approaches can be applied to fully

optimize and integrate the design of the process and its

operation. Lewin focuses on three critical aspects that

are predicted to characterize future activity in inte-

grated design and control:

1. The quantitative assessment of chemical process

controllability and resiliency has generated consider-

able interest, both academically and in industry. The

vendors of commercial simulation software equate

controllability assessment with dynamic simulation,and ultimately, plant-wide operability and control-

lability needs to be verified using this tool. However,

it is more important to initiate C&R diagnosis with-

out this expensive and engineering-intensive activity.

It has been shown that controllability analysis re-

duces the alternatives early in the design process

(Perkins & Walsh, 1994; Weitz & Lewin, 1996;

Solovyev & Lewin, 2000). The challenge to the

vendors is to build these tools directly into their

simulation software.

2. Approaches for integrated design and control are

important for improving a final design (Bansal,Mohideen, Perkins, & Pistikopoulos, 1998). To ef-

fectively use a MINLP, it is necessary to develop

methods to prune the network of configurations

evaluated by the MINLP solver. The commonly used

heuristic approach for MINLP network pruning can

be replaced by adopting C&R analysis.

3. The training of chemical engineers, who should be

taught to view design and control as an integrated

activity, is a precondition to the future advancement

of this field (Seider et al., 1999; Luyben, Tyreus, &

Luyben, 1999). To this end, both the fundamentals

of process dynamics and control, and the impact of

design on control, should be covered adequately in

the undergraduate curriculum. The concern here is

the need to bridge the gap between traditional pro-

cess control courses, which emphasize theory, and

applications to actual processes.

As an illustration, consider potential control prob-

lems in the flowsheet in Fig. 2(d), and their resolution

by adopting C&R diagnosis during the design process:

1. Impact of recycle: The positive feedback loops asso-

ciated with the material recycles in the flowsheet can

amplify feed disturbances. Careful controllability

assessment indicates that the control configuration

needs to account for the dynamic interaction be-tween the process units. More specifically, to elimi-

nate the disturbance amplification caused by the

material recycles, it is recommended that the flow

rate of the recycle streams be controlled, either

directly or indirectly by manipulating the purge

stream.

2. Impact of heat-integration: The loss of degrees-of-

freedom associated with heat integration may cause

the quality of control to deteriorate, depending on

the configuration selected.


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As an example of the impact of C&R diagnosis on

the synthesis of heat-integrated designs, consider the

selection of an appropriate configuration for heat-inte-

grated columns for the separation of an equimolar

mixture of methanol and water, a typical product of a

methanol synthesis loop. To provide commercial

methanol, nearly free of water, dehydration is achieved

commonly by distillation, a process in which energy is

invested in return for separation. To reduce the sizableenergy costs, heat-integrated configurations are consid-

ered commonly as alternatives to a single distillation

column, three of which are shown in Fig. 3:

FS (Feed Split): The feed is split nearly equally

(FH:FL) between two columns to achieve optimal

operation. The overhead vapor product of the high-

pressure column supplies the heat required in the

low-pressure column.

LSF (Light-split/Forward heat-integration): The en-

tire feed is fed to the high-pressure column. About

half of the methanol product is removed in the

distillate from the high-pressure column, and thebottoms product is fed into the low-pressure column.

Heat integration is in the same direction as the mass

flow.

LSR (Light-split/Reverse heat-integration): The en-

tire feed is fed to the low-pressure column, with the

bottoms product from the low-pressure column fed

into the high-pressure column. Heat integration is in

the opposite direction to that of the mass flow.

These configurations reduce the energy costs by using

the heat of condensation of the overhead stream from

the high-pressure column (H) to supply the heat of

vaporization of the boilup in the low-pressure column

(L). Although more economical, assuming steady-state

operation, they are potentially more difficult to control

because the configurations: (a) are more interactive;

and (b) have one less manipulated variable for process

control, since the reboiler duty in the low-pressure

column can no longer be manipulated independently.

To show the energy savings, the flowsheets for a

single column and for the three heat-integrated alterna-

tives in Fig. 3 were simulated on the basis of an

equimolar feed of 45 kmol/min, producing 96 mol%

methanol in the distillate and 4 mol% methanol in the

bottoms product, assuming 75% tray efficiency and no

heat loss to the surroundings, and using UNIFAC to

estimate the liquid-phase activity coefficients. The total

energy requirements for the four alternatives were com-

puted as follows:

0.205106 kcal/LSR0.353106 kcal/SC

min min

FS 0.205106 kcal/0.222106 kcal/LSF

min min

Clearly, the LSR and FS configurations save the

most energy, and on the basis of steady-state economics

alone, one of these two configurations would be se-

lected. It makes sense to consider controllability and

resiliency diagnosis to select the most appropriateconfiguration, as did Chiang and Luyben (1988), using

the relative gain array (RGA) and minimum singular

values based upon linear approximations to detailed

non-linear process models. Although their findings were

inconclusive, they showed the FS configuration to be

far less desirable using closed-loop simulations with

their non-linear model. It is preferable, however, to

perform C&R diagnosis using the results of the steady-

state material and energy balances in a procedure sug-

gested by Weitz and Lewin (1996), involving the

following steps:

1. After the alternative flowsheets are synthesized, con-

trol structures are considered, first by selecting the

process outputs to be controlled, y6

{t}, the manipu-

lated variables, u6{t}, and the disturbance variables,

d6{t}. These are related by the model:

y{s}=P{s}u{s}+Pd{s}d{s}.

2. Steady-state simulations of the flowsheets are car-

ried out using a process simulator.

Fig. 3. Three heat-integrated alternatives to a single distillation column.


10/12


Fig. 4. DC maps for the SC, FS and LSR configurations to dehydrate methanol. The bounds on the disturbances are 920% from their nominal

values. The DC maps for each manipulated variable are computed separately.

3. The flowsheets are decomposed into component

parts. These are MIMO subsections of the flow-

sheets that are approximated by matrices of low-or-

der transfer functions (usually first order with

deadtime). This decomposition permits process units

to be modeled in sufficient detail, allowing inverse

response and overshoot phenomena to be

represented.

4. Steady-state gains for the component parts are com-

puted by perturbation of each input, one at a time.

Time constants and delay times are estimated as-

suming perfect mixing or plug flow, as appropriate,with the flow rates at steady state. At this point,

transfer-function matrices are defined for each com-

ponent part.

5. The transfer-function matrices, P{s} and Pd{s}are

generated for each complete flowsheet. This involves

computing the frequency response of each compo-

nent part, and recombining the component parts, as

dictated by the plant topology.

6. The frequency-dependent C&R measures are com-

puted using the approximate linear model, P{ j}

and Pd{ j}.

Following this approach, C&R diagnosis is carried

out on the single column, as well as the best heat-inte-

grated configurations, in terms of steady-state econom-

ics, namely the FS and LSR configurations. Fig. 4

shows the Disturbance Cost contour maps (Lewin,

1996) computed for each of the manipulated variables

associated with the configurations: SC, FS and LSR,

where the abscissa is the frequency, and the ordinate is

the direction of the disturbance [F, xF]T. For clarifica-

tion, a disturbance direction of 0 denotes a positive

disturbance in the feed flow rate alone, of 90 denotes apositive disturbance in the feed mole fraction alone,

and of 45 denotes that both disturbances in the feed

flow rate and feed mole fraction are at their maximum

positive values. Since DC=0.5 corresponds to a satu-

rated manipulated variable, the disturbances are re-

jected adequately by all of the designs at the steady

state (when 0). However, for a wide range of

disturbance directions, the FS configuration has distur-

bance costs that exceed the 0.5 constraint beyond 0.1

rad/min (LH, QRH and FH/FL), and thus, disturbance


11/12


rejection is expected to be very sluggish for this configu-

ration. The other two configurations have low distur-

bance costs, and can be expected to reject these

disturbances about as well as a single column. These

results are corroborated by dynamic simulations using

HYSYS. Plant (Seider et al., 1999), and are in agree-

ment with those of Chiang and Luyben (1988). In this

case, C&R analysis is effective for screening, enabling

the FS and SC configurations to be rejected without theneed for dynamic simulations.

This approach has been used successfully for screen-

ing flowsheets featuring exothermic reactors (Naot &

Lewin, 1995), and polymerization reactors (Lewin &

Bogle, 1996), azeotropic distillation columns (Solovyev

& Lewin, 2000) and material recycles (Lewin, Gong, &

Gani, 1996). In all cases, the conclusions obtained by

others using rigorous dynamic models have been confi-

rmed. It is promising as a short-cut diagnostic tool, and

is well suited for integration into flowsheet simulation

software. When such analysis tools become available

within the framework of commercial simulators, flow-sheet operability can be checked routinely.

5. Conclusions

We recommend that a curriculum that prepares

chemical engineering graduates for the challenges they

will face in industry should include the following

features:

1. A structured approach relying on fundamentals. In

this approach, students use process simulators start-

ing in the sophomore material and energy balance

course, applying their knowledge to practical prob-

lems. Students will then be better prepared for the

challenges of the capstone design project and can

spend more time on synthesis, controllability, safety,

environmental concerns, waste minimization, opti-

mization, and economic evaluation. Tutorials pre-

pared in multimedia format can support this goal

efficiently and spare instruction time in the

classroom.

2. A balance between heuristic and computer -aided al-

gorithmic approaches. Since design invariably in-

volves significant designer intervention, it is

important to teach both heuristics as well as al-gorithmic methods. The former lay the foundations

for acquiring the experience necessary to prepare

practical process designs, while the latter is critical

to ensure the generation of optimal designs.

3. Integrated design and control. Instruction should

reflect the current state-of-the-art in the integration

of process design and process control. The concern

here is the need to bridge the gap between tradi-

tional process control courses, which emphasize the-

ory, and applications to actual processes.

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