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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
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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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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.
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D.R. Lewin et al. /Computers and Chemical Engineering 26 (2002) 295306304
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
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D.R. Lewin et al. /Computers and Chemical Engineering 26 (2002) 295306 305
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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