Conceptual design of chemical process (1988) - Tài liệu text (2024)

No part o tlh is publication may be reproduced or distributed
in any form or by any means, or !>Imed In II data base or
retrieval system, without the prior written permission of
the publisher.
This book was sel in Times Roman.
The editors were B.J .Oat1l: and James W.Bradtey.
The production supervisors were Diane Renda and LDuise Karam.

Ubrary of Congress Cataloging-in-Publieation Dala
Douglas,James M.(James Merrill)
CoooeptuaJ design 01 chemical processes.

(McGraw-Hili chemical engineering series)

Bibliography:p.
Includes index.
1. O"Iemical prooesse5. I. Tille. II. Series.

TPI55.7.D67 1988
ISBN 0-07.{}ln62·7

660.2'81

87·21359

When ordering this title use ISBN O-07· 100195--S

Printed In Singapore

James M . Douglas. Ph.D.. is currently a professor of chemical engineering at the

University of Massachusetts. Previously he taught at the University of Rochester
and al the University of [klaware. Before entering leaching. he spen t five years at
ARea, working on reactor design and control problems. l-Ie has published
extensively in areas of reacting engineering, process control (including two books).
and conceptual process design. He won the Post-Doctoral FellowshIp Award al
AReO, the Faculty Fellowship Award at the University of MassachusellS, and the
Computing and ChemIcal Engmeering Award of A1ChE.

DEDI CATED TO:
The loves of my life,
M y lovely wife. Mary E. (Betsy) Douglas.
M y mo ther, Ca rolyn K .• and the memory of my
father. Merrill H. Douglas,
My two wonderful kids, Lynn and Bob,
and to my colleagues, wbo bave taugbt me so much about
design and con trol,
Mike Doherty, Mike Malone, Ka Ng. and Erik Ydstie,
and to my st udents. who have suffered so much.

CONTENTS

Preface

Part )

"

A Strategy for Process Synthesis and

Analysis
The Nature of Process Synthesis and Analysis
I-I

1-'
1-'

2
2-1
'-2

,-,
'-4
2-'

,-.
'-7

3
3-1
3-'

Creall\c A5pc!;IS of I'roces~ Design
A Ihcrarchl!;al Approach to Com::c:plual Design
Summary, Eac:rc~, lind Nomenclature

Engineering Economics
CoSI Information Requned
Estimating Capital and Opcrallog Costs
TOlal Capilal Investment and TOlal Produce Costs

Time Value of M onty
Measures of Process Profitability
SlmphfYlng the EconomIC AnalysIs for Conceptual Designs
Summary, E.trcises. and Nomenclature

•

I'

"

2J
32
37

...
."

Economic Decision Making : Design of a
Solvent Recovery System

72

Problem Definition and General Considerations

72

Design or II Gas Absorber. Flnwshccl, Malena1 and Energy

Balances, and Stream

3-3
3-4
3-'

,

COSIS

equipment Design Considerat ions
Rules o f Thumb

Summary, ExercISeS, and Nomenclatllre

""

"

90

x;

xii

00""""

~,

Part II

4
4-1
4-2
4-3

5
'-I
'-2
'-3

,-4

6
6-1
6-2
6-3
6-4
6-'
6-6
6-7
6-.

7
7-1
7-2
7-J
7A
7-'
7-6

8
'-1
'-2
'-J

,-4

.-,
'-6
'_7

.-..-,

8-10
.-11

Developing a Conceptual Design and
Finding the Best Flowsheet
Input Infonnation and Batch versus Continuous
Input InConnation
level-I Decision - Balch venus Continuous
Summary, ElerC1scs, and Nomenclature

9

Cost Diagrams and the Quick Screening of
Process Alternatives
Cost Diagrams
Cost Diagrams for Complex Processes
QUICk Screening of Process Ahemati,-es

HDA Process
Summary, Exerclsc, and Nomenclature

289
289
297
303
30S
31'

Part III Other Design Tools and Applications

117

97

"

"

107
III

Inpul·Qutput Structure of the Flowsheet

11 6

Decisions for the Input-Output Structure
Design Variable!, ~ral1 Material Balances. and Stream CoslS
Pl"ocess Altematives

SUmmal'}', Elerciscs, and Nomenclature

116
l2l
132
132

Recycle Structure of the Flowsheet
Oec1Slons thaI Determine the Recycle Slructure
Recycle Material Balances
Rea ctor Heal Errects
Equilibnum Limitations
Compressor DesIgn and Cmu
Reactor DesIgn
Recycle EconomIC EvaluatIOn
Summary. Elelc1SCS, and Nomenclatule

117
137
142
146
149
III
136
138
1>9

Separation System

163

General Structure of the Separallon System
Vapor Reoovery System
Uquid Separation System
AzcotroptC Sy5lems
Rtgorous Material Balances
Summary, Elerciscs, and Nomenclature

163
168
172
189

""

Heat·Exchanger Networks

216

Minimum Heating and Cooling Requin:ments
Mmimum Number of ElchangeD
Area Esumates
Design of Minimum-Energy tleat-ElChaDger Networks
Loops and Paths
Reducing the Number of Elchan~n
A More Complete Design Algorithm Stream Splitting
Heat and Power Integration
Il eal and DISlillalion
HOA Process
Summary, E.ercises, and Nomenclatule

216
230
2JJ
236

'-I
'-2
'-3
,A

,-,
10
10-1
10-2
10-3
10-4
10-'

11
11 · 1
11 -2
11 -3

12
12·'
12-2
12·)
12A

13

211

24'
251

'"
261
264
21J
284

xiii

13-1
13-2
13·)

Preliminary Process Optimization
DesIgn Variables and Economic Trade-offs
COSt Models for Process Units
A Cmt Model for a Simple Process
Approlimate Optlmiution Analysis
Summar), Elercisc5. and NomencialUre

A- I
A-2
A-J
AA

'"

Process Retrofit s

ll3

A Syslematic Procedure for Process Fetrofits
HDA Process
Summary and ElerClSCS

354
338
368

Computer· Aided Design Programs

(FLOWTRAN)

J69

General Structure of Computer-Aided Design Programs
Malenal Balance Calculallons
Com plete Plant Simulation
Summary and E.erciscs

370

Summary or the Conceptual Des ign Procedure
and Extensions of the Method

A Revie ...· of the Hierarchical Decision Procedure ror
Petrochemical Processes
Deugn of Sohds Processes and Batch Processes
Other Significant Aspects of the Design Problem

Part IV Appendixes
A

31'
320
321
lJ2
340

Shortcut Procedures for Equipment Design
Number o(Trays fOI. Gas Absorber
Distillation Columns : Number of Trays
Design o f Gas Absorbers and Distillation Columns
Distillation Column 5cquencing

J7l
J97
4"

40'
406

40'
41 2

423

'"
'"
436

'"
461

xit'

CONTEP'lT$

A·'
H
A·7

...

A·'
A·lO
A-II

Complex Distillation Columns
Energy Integralion of Distillation Columns
Heal_Exchan ger Design
Gas Compressors
Design of Refrigeration SyStems
Reactors

Summary of Shortcut Equipment Design GUidelines and
Nomenclature for Appendix A

466

.18

.86
.90
.90
S07
S07

B

HDA Case Study

'"

C

Design Data

543

Hydrocarbon Vapor-Liquid Equilibria
Temperature Ranges for some Materials

543
547

FLOWTRAN Input forms

,

C·I
C·,

D
D·I
D·'
D·3
D4
D·'
D·6
D·7
D·8
D·'
D -IO
D-ll

E
E·I
E·'

F

Component List
IFLSH
A.FLSH

SEPR
ADD
SPLIT
PUMP
GCOMP
SCVW
DSTWU

REACT

Cost Data

PREFACE

.

548

""
'"
'"
'56
'"
'"
""
SSl

'54

'6'

S68

Operating Costs
Summary of Cost CorrelaUons

'"
56'

Conversion Factors

S78

Indexes
Author Index
Subject Index

"I
,,3

'"

llLis book describes a systematic procedure for the conceptual design of a limited
class of chemical processes. The goal of a conceptual design is 10 find the best
process fiowsheet (i.e., to select the process units and the interconnections among
Ihese: unils) and estimate the optimum design conditions. The problem is difficult because very many process alternatives could be considered. In additIOn,
experience indicates that less than I % of ideas for new designs ever become
commercialized. Thus, there are many possibilities 10 consider with only a small
chance of sUCGCss.
In man} cases the processing costs associated with the various process

alternatives differ by an order of magnitude or more, so that we can use shoncut
calculations to screen the alternatives. However, we must be certain that we are in
the neighborhood ortne o ptimum design conditions for each alternative, to prevent
discarding ao alternative because of a poor choice of design variables. Hence, we
use cost studies as an initial screening to eliminate ideas for designs that are
unprofitable. If a process appears to be profitable, then we must consider other
factors, including safety. environmental constraints, controllability, etc.
We approach tne synthesis and analysis problem by cstablishjng a hierarchy
of design decisions. With this approach, we decompose: a very large and complex
problem into a number of smaller problems that are much simpler to handle. By
focusing on the decisions that must be made at each level in the hierarchy (e.g., Do
we want to add a solvent recovery system?), we can identify the existing
technologies that could be used to solve the problem (e.g., absorption, adsorption.
condensation) without precluding the possibility that some new teth nology (e.g., a
membrane process) might provide a better solution. Moreover, by 'listing the
alternative solutions we can propose: for each decision, we can systematically
generate a list of process alternatives.
In some cases it is possible to use: design guidelines (rules of thumb or
heuristics) to make some deciSIOns about the structu re of the flowshect and/or to
set the values of some of the design variables. We usc order-of-magnitude

"

arguments to denve many of these heuristics, and .....e use a simple analysis of this
type to identify the III111ta1l0ns of the heunstics Tn many cases, no heuristics are
available, and therefore we de\elop shortcut design methods that can be uscd as a
basis for makmg decisions.
By followmg this hierarchical decision procedure. a beginning designer can
substItute the evaluation of a number of extra calculations for experience during

the devdopmenl of a conceptual desIgn Since <;horteu t calculatIons are used.
however, the penalty paid in the time required to screen more alternatives is not
\ery high Of course. as a designer gains experience. she or he will be able 10
recognize what alternatives do not need to be considered for a parlicular type of
process and thereby obtain an increase m effiCiency. Note also that experience
normally '5 required for assessing the operability of a design, and therefore a
beginner should always get an experienced designer to review the results of the
des.gn study.
O rgan iza tion of the Text
The text is meant to be used m a o ne-semester. scDlor-level course in process design
for chem.cal engineering students. We present the material as a lecture course. A
single case study is carried throughout the text to illustrate the ideas. and the
homework assignments include the evaluation of alternatives for the central case
study. as well as several other case studies_ The purpose of these other case studies
is to help the student understand the similarities and d ifferences betwttn various
types of processes (e_g_. Single reactIOns ~ersus product d istnbution problcms. cases
where gas·reqcle costs dominate, cascs where liquid separation costs dommate,
the choice bet~een recycling or removing by-products formed by reversible
reactions, the economic trade-ofTs encountered when a gas recycle and a purge
stream is used, etc.). The focus is on screening calculations., although a computer~
aided design program is eventually used to verify the approximations
Part I discusses a strategy of synthesis and analysis. In Chap. I it is noted that
only about I % of ideas for new designs ever become commercialized, so that .....e
need an efficient procedure for eliminating poor projects. Similarly, since design
problems arc always underdefined and we can often generate Icr to 10' alternati\

processes even for a single-product plant, we noed an efficient way of screening
process alternatives. These discussions provide the motivation for the use of
shortcut calculations. Also. a procedure for decomposing process flowshoets into a
hierarchical sct of simpler problems is presented.
Chapter 2 presents an introduction to engineering economics, including a

discussion of various measures of profitability. Tn addition, a simple economic
model that is useful for conceptual designs .s developed
Chapter 3 presents a very simple design problem (actually a subsystem of
what could be a larger de~igl1 problem) This example illu~trates how simple it i~ to
gencrate process altcrnatives. the need for des.gn heunstics. the origlll of deSIgn
heuristics. the limitatIOns of dcslgn heuristiCS. the intcractions among processing
units, the need for a systems Viewpoint in placc of a unit operations viewpoint, and
how shortcut de~ign methods can he developed

Part II presents the details of the hierarchicat decision procedure for the
sYnlhesis and analYSis of conceptual designs. Chapter 4 describes the infonnation
needed to get started. and the decision of designing a batch versus a contmuous
process is discussed Chapter 5 presents the important decisions for the input and
output structure, the identification of the important design variables at this level of
complexity, and shortcut procedUres to calculate the stream costs and the costs of a
feed compressor (if one is reqUIred). Chapter 6 introduces the additional deciSions
required to fix the overall recycle st ructure of the fl owsheet . i.e., the interaction of
the r~aClor system(s) with th~ remainder of the process The reactor cost and any
gas-recycle compressor costs are evaluated in tenos of the design variables This
discussion is limited to single-product plants.
At present. the systematic preliminary design procedure is also limited to
vapor.liquid processes. For this class of processes, the structure of the separation
system (i.e., the general structure, vapor recovery system alternatives. and the
decisions for the liquid separation system) is described in Chap. 7. Chapter 8 then
presents a synthesis procedure for the heat-<:xchanger network. At this point. a
base-case design and an estimate of the optimum design conditions are available
Our basic design strategy is to develop a base-case design as rapidly as
possible. simply listing the process alternatives as .....e go along. to detennine
whether there is something about the process that will make all the alternatives
unprofitable. Provided that our base·case design appears to be promising. we use

the methods in Chap 9 to scrttn the process alternatives. Thus., at this point we
attempt to identify the best process flowshttt .
Part III presents some other design tools and applications. In the procedure
presented in Chaps 4 through 9, we used case-study cakulations to estimate thc
optimum design conditions because we were continually changing the structure of
the flowshee\.. Once we have identified the best flowsheet. we can use more
sophisticated optimization procedures. However. to assess the degree of soph.stication that is desirable, .....e present an approximate optimization analysis in Chap. 10.
This approximate optimization procedure helps to identify the dominant economic
trade-offs for each design variable. the dominant design variables, and an indication of how far a design variable IS away from the optimum without knowing the
exact value of the optimum This approximate optimization analysis is also very
useful for retrofit studies and for optimum steady-state control calculations.
In Chap II we use the same techniques for process retrofits that we used to
develop a design for a new plant. A systematic procedure is presented for
retrofitting processes, Including completely replacing the existing plant with either
the same or a better process alternative.. The approximate optimization procedure
is used to help identify the dominant operating variables and the equipment
constraints that prevent the operating costs from being minimized. Then, based on
these results, additional equipment capacity is added until the incremental,
annualized equipment cost balances the incremental decrease in operating
costs
In Chap 12 we dlscu~<; the use of a compu ter-aIded design program to
improve the accuracy of the shortcut calcul ations Chapter 13 prc<;ents a ~um mary
of the design procedure, brief outlines of hierarchical decision procedures for solids

and batch processes. and a brief dIscussIOn of what remams to be done aftcr a
conceptual design has been completed
The appendixes prescnt some auxIliary information. The shortcut mOt.lels for
equipment design arc dISCUSsed in AppendIX A, and the complete details of a case
study aregl\en in AppendIX B Some samples of design data and cost data afe given

in Appendixes C and E.
Acknowledgments
I am very appreciative of the effo rts of A. Enc Anderson (formerly with ARCO),
Duncan Woodco*ck of Imperial C hemical Industries, Edward C. I-laun of UOP
Inc.,JeffKantor, University of Notre Dame; Carl F. King from duPont, E L Sherk
from Exxon. R. Hoch (formerly with !-Ialcon International). John Scinfcld,
California Institute of Tcchno logy and J. 1. Sirola from Tennessee Eastman Co. for
their careful review of the text. Similarly, I am grateful 10 the chemical engineenng
students at the University of Massachusetts and to tbe students from Imperial
Chemical Industries (United Kingdom), Rohm and Haas, Monsanto, Union
Carbide and Celanese, [or many valuable comments concerning the course
material. In addition, I must acknowledge the numerous contributions that my
colleague Mike Malonc made to the text, and I want to thank my o ther colleagues
Mike Doherty, Erik Ydstie, and Ka Ng for their feedback when they taught the
material. The contributions of my graduate students, partIcularly Wayne Fisher
and Bob KirkWood. also need to be ackno .... lcdged.
Of course. I am especially grateful to m} lovely .... ife. Betsy, to my children.
Lynn and Bob. and to my mother, Carolyn K. Douglas, for theu support during
the preparation of the text. SImilarly, Pat UII.'IS, my admmistrative assistant. and
Pat 8arschcnski, who did the typmg, prOVIded much needed support.
James M . Douglas

CONCEPTUAL DESIGN OF
CHEMICAL PROCESSES

PART

I
THE

STRATEGY
OF PROCESS
SYNTHESIS
AND ANALYSIS

CHAPTER

1
THE
NATURE
OF PROCESS
SYNTHESIS
AND ANALYSIS

J.I CREATIVE ASllECTS OF PROCESS
DESIGN
The purpose of engmecnng is 10 crealc new malenal .....ealth We attempt to
accomplish this goal in chemical engmeering via the chemical (or biological)
transfonnation and/ or separation of materials.. Process and plant design is the
creative aCll vily whereby we generate ideas and then translate them inlO equipment
and processes for producing new malerials or for significantly upgradmg the value
of uisling materials..
In any panicu lar company, we mIght try 10 generate new Ideas :

To produce a purchased raw material
To convert a waste by-product 10 a valuable product
To create a complelely new malenal (synthetic fibers, food. bioproces~mg)
T o find a new way of producing an existing produCI (3 new cata lyst. a
bioprocessi ng alternative)

To exploit a new technology (genetic engineenng. expert systems)
To exploit a new malerial or construclion (hlgh.lcmperalUre- or hlghpressure-opera tion, specia lty polymers)

3

S£CTIOf'l • I

As an indIcatIon of thc tremendous sucG:ess of the engineering effort. we note thai
over 50 % or the products sold by most chemical companies were devdoped during
the last decade or two.
Success Rates
Despite thiS ClIccllent record of success, we should realize that very few new idea~.
either fGr Improving existing processes or for developing new processes, lead to new
wealth In fa ct. the chances of commerciahzat ion at the research stage for a new
process are only about I to 3 %. at the development stage they arc about 10 to 25 %.
and at the pilot plant stage they arc about 40 to 6O Y..• Of course. we expect that
the success rate for process modifications Will be higher than that for completely
new processes, but the economic rewards associated with these safer projects will
have a significantly lower potential
It is not surprising that so few ideas m engineering ever prove to be fruitful ;
the same pallern holds for any type of creative activity. Since experience indicates
that only a small number of ideas ever will ha ve a payout. we see that et'o/UOlion is
one of the most slgmficant components of any design methodology. I n fact, process
syn thesis, i.e.. the selection of equipment and the interconnections between that
equipment which will achieve a certain goal. is really a combination of a synthesis
and analysis activity.
Synthesic; and Analysis
Perhaps the majo r feature that d,stmgulshes deSIgn problems from o ther t)'pts of
engmcermg problems IS that they are underdefined: i.e., only a very smilll rracllon

or the IIlfonnatlon nceded to define a design problem is available from the problem
statement. For example, a chemist mIght dIscover a new reaction to make an
existing product or a new catalyst for an existing. commercial reaction. and we
want to translate these discoveries to a new process. Thus, we start with only a
knowledge of the reaction conditions that we obtain from the chemiSI. as well as
some information about available raw materials and products that we obtain from
our marketing organization, and then we need to supply all the other information
that we n~ to define a design problem.
To supply this missing infonnation, we must make assumptions about what
types of process units should be used, how those process units will be inter~n­
neeted. and what temperatures, pressures. and process How rates will be required.
This is the synthesis activity. Synthesis is difficult because there are a very large
number (10'" to 10"') of ways that we might consider to accomplish the same goal
I-Ience, design problems are very open-(:nded

• Theso: values l ep!~nl lhe ave,p&eJ of CSll ..... les supplied by sla r• .....w
evalua.,on l' o UI'S of maJOr ChemlCat and r<1. o!eum OOmp"nltS

WO ,~

nl In ecOnom IC

CIlUlIV!!

ASl'fCTll o~ I'IlOCUS DESIGN

5

Nonnally, .....e want to find the process alternative (out of the 10'" to IO~
possibilities) that has the lowest cost. but we must also ensure that the process IS

safe. will satisfy environmental constraints, is easy to start up and operate, etc. In
some cases, we can usc: rules of thumb (heuristics) 10 eliminate certain process
alternatives from further consideration. bUI in many C8§/!S it is necessary to design
various alternatives and then to compare then costs Experienced designers can
minimize the effort required for tIllS type of evaluation becau§/! they can often guess
the costs of a partIcular Ulllt, or group of uniu, by analogy to another proce~s
However, beginning deSigners normally must design and evaluate more alternati,"es in order to find the best alternative.
When experienced deSIgners consider new types of problems, where they lack
uperience and where they cannot identify analogies. they try to use shortcu t (back of-the-envelope) design procedures as the basis for com paring alternatives. These
back-of-the-cnvelopt ca lculations an: used only to screen alternatives. Then if the
process appears to be profitable, more rigorous design calculations are used to
develop a final design for the best alternative. or the best few alternatives.
Because of the underdefined and open-ended nature of design pmb1c:ms. and
because of the low success rates. it is useful to develop a strategy for solving design
proble ms. We expcet that the strategy that a beginning designer would uS!': fOI
synthesis and analysis would be different from that of an experienced designer.
because a beginner must evaluate many more process alternatives. However, b}
using shortcu t design procedures we can minimize the effort required to undertake
these addllional calculatio ns

Engi neering Method
If .....e refleet on the nature of process synthesis and analysis, as discussed above, we
recognize that process design actually is an art, i.e.• a creative process. There!ore.
we might try to approach design problems in much the same way as a pamter
develops a painting. In other words, our o riginal design procedures shou ld
correspond to the development of a pencil sketch, where we wan\ to suppress all
but the most significant details of the design; i.e., we want to discover the most
upensive parts or a process and the significant economic trade-offs. An artist next
e"aluates the prcliminar) painting and makes modifications. using only gross
outlines of the subjects. SImilarly. we want to evaluate our first guess at a design

and generate a number of process alternatives that might lead to impro"ements In
this way......e hope to gel:..::rate a "reasonabk-looking." rough process d~ign before
we Slart adding much detail.
Then the artist adds color, shading, and the details of various objects in the
painting and reevaluates the results. Major modificalions may be introduced if they
socm to be wflrranted . In an analogous manner, the engineer uses more rigorous
design and costing procedures for the most expensive equipment items. improves
the accuracy or the approximate-material and energy-balance calculations, and
adds detail in terms of the small. inexpensive equipment items that arc necessary for

6

SECTJO'II I I

CIl:A TlVE ASPECTS

or

SECTION I I

CaEA1WE ASPfCT'S Of PloctSS DESION

7

PlOCESS OESIGN

TABU: 1. 1-1

the process operallons but do not ha\e a majo r impact on the tOlal plant cost, eg,

pumps, flash drums, etc.
Thus, we s...e that both a painting and a process design proceed through a
series of successively more detailed synthesis and evaluation stages. Thatcher refer~
to a solution strategy of this type as successive refinements, and he calls it the
tngineerin9 method· Note that as we make successive refinements, we should
always maimam a focu s on the overall problem.
If we accept t!lIS analogy between engineering design and art, then we can
recognize some other interesting fealurtS of the design process. An artist never
really compleles a painting; no rmally the work is lemunaled whenever the
additional effort reaches a point of diminishing returns; i.e., if little added value
comes from much additional effort, the effort is not worthwhile. Another feature of
art is that there is never a single solution 10 a problem; i.e.., there are a variety of
ways of painting a "great" Madonna and Child or a landscape; and in process
engir.ccring nonnally different processing routes can be used to produce the same
chemical for essentially the same cost. Still another analogy betwec:n engineering
design and art is that it requires judgment to decide how much detail should be
included in Ihe various stages of painting, just as it does in a process design.
Of course, numerous scientific principles are used in the development of a
design, but the overall activity is an art. In facL, it is this combination of scie nce and
art in a creative aCllvity that helps to make process deSign such a fascinating
challenge 10 an engineer.

uvels of Enginei.'ring Designs
Now we see that there are a number of levels of englllcering designs and cost
eSlima tc:s that we expeCt to undertake. These vary from very simple and rapid, but
not very accurate, estimates to very dctailed calculations that are as accurate as we
can make. Pikulik and Diazt classify thcsc design estimates by the categories given
in Table 1.1-1.
They also give the relative costs required 10 obtain these estimates, as shown
in Table 1.1-2_ From this table we sec how rapidly engllleering costs increase as we

include more detail III the calculations. Obviously, we want to avoid large deSign
COSIS unless they can be economically Justified.

Types of design estim.tes
I. Onkr-of·mlpllude eshmlte (rlIOOc:lill".. le) boose<! on mnollr prevlOul 00s1 da .. , probable locuraey
uc:ccds i40~
1.. Study eilimale (faelored ... Ilmlle) luwf on knowledge of mlJOI lIems of equipment . probable
aocuTaCy up 101 25 %
l. Prdmunary al,mlle (bt.ld j,et lutho nlilion esumale. K:Opc ct.hmale) based on sull'1cocnl dltl 10
pcnru t 1M dOlimale 10 be budj,cled , probable aocuracy wllhm i t1 %
.. [klimll~ ellirnale (pc~1 co~lroi esllmatc) bued OQ almo$t compicle dala, bul before complellon
of draWings and spe.;,j'Quom. probable I<:cuney Wllhln i 6~
1. [klailcd UltmllC (CO.l'lln,,:::Ior', allmale) based on compicte enJlDOCnnj, drllwmgs.. 1pc<:lfiQIIOM.
lAd Nle .urveys: ptobably &CQIraq wlth,n ± 3%

For the case or a new process, where previous cost data are not available, it
seems as if it would not be possible to develop an order-of-magnitude estimate
However, an experienced designer can overcome this difficully by drawing analogies between the new process and other existing processes for which some data arc
available in the company files. Procedures for developing order-of-magnitude
estimates have been described in the literature,· bUI normally it requires some
experience to evaluate the resulls obtained from this type of calculatIOn.
For a beginning deSigner, wilh little or no experience, it would be useful to
have a systematic approach for dc ..dopmg order-of· magnitude estimates We ean
usc order-of-magnitude arguments to Simplify many oflhe design calculallons, and
.....e can limn our allen lion to the majo r pieces of process equipment as we ca rry out
a prehmmary process design The goal of this text IS to develop a systematic

• J. H Tlyk>r, - Procc:u slep-Soonn, Melhod for Makin' Quick Capri.! EmmalQ, ~ C.u/ £iIg., p. 207.
bty·AuJUSI t980 0 H. Allen. Ind II; C. PIce. - ReVISed Tedln>quc for PrWcs.gn Cost fl;hmahng,CMm. Eng ., 12(S) t-4 2 (Mlteh 1, 1975).

TABLE 1.1-2

Enginttring costs to prepare eslimlte5 (1977)

........

SI II1iJIiM

• C. M Thllcher. fu

F~,,',,1s

P/CIte ...~aI Engi>wr,"'fI, Merrill Cotumbus, OhIO, 1962, chiP 1

1 A P,kuhlr Ind H E DIu., -C~I &onmaunl M'Jor Process Equlpment.- C.vm. Eng~ 114(2t): 106
(t911). NOI~ Thac accuracy bounds ..,tl \'lITy from one compa ny 10 anotiKr. and the Iccuracy of t M
de"lkd e~llmllQ Will nO{ be lh ls j,ood dunn, pcnod$ ofhlgb mill 110.1'1 (the elton mlghl be as much IS 8
10 10".. eyen for a detlllcd esl mUlle) Abo. norm~U) Ihe ch4ncc of oblal mna pOSIlJVe errou u; j,rca lcr
Ihln Ihal for I1Cllamt tr,ou., Wlhll lbe ordcr-of·magnJluUc esllmalc, 1t ., liem I , would be repolled at
+40 10 ~ 2~ % (dQlifI engJl>C!I'rs 5oC1dom oycratunlle CO$tJ;) SImilarly, hl&hcr co~tingcTlCy fees mly be
Uldudcd In lbe eall~r kvtlS{Ihai I$, 20 10 2j }~ In lIem ) dropprollo 10" ID ,Iem -4) 10 accounl for COt ..
.1'101 Inctuded m Ihe lnalysu ( .. hlCh IS somc .. bal dllfcrefll rrom Ihe accuracy oit be esurwlle)

T,.,.. of fttimalc
Study (S IhollUnd$)
Pl'I:lupmlrY ($ Ihotlsdlxh)
[kli .... u~e (S Ihow;onds)

S5- l.5O

PI...

""-

......
,"
IS JS

",.
,.60

2>60

601"

Fro. A. ,.,tu!i\..." H E. Du.z,

&,, 101(11) 106 (1971)

SI - S! ",,1li1Nl

..uwo.

,",0

lO90
IOO 1l0

-ea., Eo......''''' MaJOf Pt...,... Eq",~- aw...

8

SECfION I I

SI!.CTlON U

... HIU .... CHlCAl "'''PROACH TO CONCEI'TU4L OESIGN

procedure of this type and then to show ho w the results can be extended to a study
estimate.
Detailed estimates are considered to be beyond the scope of this text
However, as noted before, the chance that a new idea ever becotnes commercialized
is only about I ~... so that we expect to undertake roughly 100 preliminar} designs
for every detailed design. Hence. the methodolo gy of conceptual procas design
should be mastered in considerable detail.
O ther A pplica tions or the !\1('1 hodology
Dcsplle the fact that our primary focus is dllccted to the design and evaluation of
new processes. much of the methodology we develop is useful for other engineering
tasks. including basic research and technical service. In basic research, we want to
spend most of our effort studying those variables that will have the greatest
economic impact on the process, and rough process designs will help to identify the
high-aJSt parts of the process and the dominant design variables. Similarly. in
technical service activities, .....e look for ways of improving an existing process. To
accomplish this goal, we need to understand the significant economic trade-ofTs in
the process, and it is useful to have procedures available for obtaining quick
estimates of the potential payout of new ideas. Thus, the methodology we develop
will have numerous applications in the process industries.

The engineenng method (or the artlst"s approach) indicates that we should solve

design problems by first de\'eloping very simple solutio ns and thcn adding
successive layers of detail. To see how we can use this approach for process design
problems. we consider a typical flo wshert (o r a petrochemical process, and then .....e
look for ways of stripping away layers of detail until we obtain the simplest
problem of interest. By applying this procedure to a number of different types of
processes, we might be able to recognize a generaJ pattern that we can use as the
basis ror synthesizing new processes.

Exa mple : HydrodealkylatioQ or Tolu~n e (HD A
Process)
The example we consider is the hydrodealkylation or toluene to produce benzene.'"
The reactions of interest are

+ CH ..
2Benzene~Diphenyl + Hz

ROd

+

H, ..... Benzene

'~r

(12-1)
(I 2-2)

1967 AlIKnean I m h!u!~ 01 C herwcal Engm«.s
J J M c Ken .. EIIt)"c.d,a ttl Clw ......al PrOC~s.Jtng ."d /H ug".
.. 01 • • Dl:kker, New YOl k. 1911. pi n. re r the OflJlIUII problem.OO . Mlluhon

• Tlns ell"" .luod y

(AtChE) SlUden! Con!esi

Probkm . 11ft

I

Purge
Heat

Coolut

Reactor

IH"'J

IH,,' I

,L

-.l

Toluene

].

"•
"&

I

u

~

Diphcnyl

'.I

Compressor : .

T

-

Flash

H2. e H"
Ben=<

•

~

."

:E

•

~

T

nclJR.[

1.2- 1
HDA rrocess lA.ft .... 1 M Dot.llitu. A.IChE 1. JJ JH (l94.S).]

The: hom*ogeneous reactions take place

1.2 A HI E RARC HI CAL APPROACH TO
CONCE PTUAL D ESIGN

Toluene

Hl'tH..
IH,,' I

... HIU....CHIC4L ... "PRO ... CH TO CONCErTUU I)ESKlN

G

the range (rom II50 F (below this
temperature the reactIOn rate 15 too slow) to 1300~ F (above this temperature a
significant amount of h)drocracking lakes place) and al a pressu~e of about
500 psia. An excess of hydrogen (a 5/ 1 ratio) is needed 10 prevenl co*kmg. and !he
reactor emuent gas must be rapidly quenched to I 150°F in order to prevent co*kmg
m the heat exchanger following the reactor.

One poSSible flowsheet for the process is sbown in Fig. 1.2-1. The toluene and
bydrogen raw-material streams are heated and combined with recycled toluene
and bydrogen streams before they are fed to the reactor. The product stream
leaving the reactor contains hydroge n, methane, benzene, tol uene, and the unwa nted diphenyl. We attempt to separate most of the hydrogen and metbane from the
aromatics by wing a partial condenser to condense the aromatics., and then we
flasb away the light gases We use tbe liquid leaving this Hash drum to supply
quench cooling of the bot reactor gases (not sbown on the Howshect).
We would like: to recycle the hydrogen leaving in the fl ash vapor, but the
methane. which enters as an impurity in the hyd rogen feed stream and is also
produced by reaction J.2- 1, will accumulate in the gas-recycle loop. Hence, a pu rge
stream is required to remove both the feed and the product metbane .from the
process. Note that no rules of thumb (design guidelines) can be used to estlma.te the
optimum concentration of methane that should be allowed to accum~late In the
gas-recycle loop. We discuss this design variable in much greater detalll~te~.
N ot all the hydrogen and methane can beseparated from the aromatics m the
flash drum, and therdore we remove most of the remaining amount in a distillation
column (the stabilizer) 10 pre\ent them from contaminating our benzene product
In

10

SECTION U

~

A HIE .... JI;C HICAl A""lO ACti lO CONCEPTUAL OESIGN

.l!

The benzene is then recovered in a second dlsullalion column, and finally, Ihe
recycle toluene is separaled from the unwanted diphenyl. Other, alternative
flowsheets ca n also be drawn, and we discuss some of these as we go through the
analysis.

:t"

-'

Energy Integration
The process f10wsheet shown In Fig 12·1 IS not very reahstlc bttause 11 implies
thaI the heating and coohng requirements for every process st ream will take place
In separate heat exchangers uSing external utilities (coolmg water, steam, fuel, etc.).
In the last decade, a new design proexdure has bun developed that makes II
possible to find the minimum heating and cooling loads for a pr()(;CSS and the heatCIlchanger network that gIVes the "best " energy iDlegration. This procedure is
described in detail in Chap. 8.
To apply this new design procedure, we must know the flow rate and
composition of each process slream and the inlet and outlet temperatures of each
process stream. One alternative flowsheet that results from this energy integration
analysis is shown in Fig. 1.2-2. · Now we see tbat first the reactor product stream IS
used to partially preheat the feed entering Ihe reactor. Then the hot reactor gases
are used to drive the toluene recycle column reboiler, to preheat some more feed, to
drive the stabilizer column reboiler, to supply part of the benzeoe prod uct column
re boi ler load, and to preheat some mo re feed before the gases enter the partial
condenser. Also the toluene column is pressurized, so that the condensing
temperature for toluene is higher than the boiling point oflhe bott om Slream in Ihe
benzene column Wilh this arrangement, condensing to luene can be used to supply
some of Ihe benzene reboder load, instead of using steam and cooling water from
external sources of utilities.
If we compare the energy-integrated flowsbeet (Fig.. 1.2-2) with the flowsheet

indicating only the need for heating and cooling ( Fig. 1.2·1), then we see that the
energy integration analysis makes the Howsheet more com plicated (i.e .. there are
many more interconnections). Moreover, to apply the energy integration aoalysis,
we must kno w the How rate and composition of every process stream, i.e., all the
process heat loads including those of the separation system as well as all the stream
temperatures. Since we need to fix almost aU the Bowsheet before we can design the
energy integration system and since it adds the greatest complication to the process
fl owsheet, wc consider the energy integration analysis as the last step in our proexss
design procedure.

.J"
· ~

~~
sr.

"

e~

] H

" !I.

"

O I"

S,

•u
•0E

'-T

"
t.

"

~

~

~

•

u:•

~~

u

,

.lll

r

,

I

/:0

y:::

~
~

~

uwnl0;)

1

~:nuag

)

)

I
:t•

.-; )

;..

,I

uWfll0:;) J:»'l!lIqIfIS

r

U

'"

,

1,
,

uwnlOO au:»nl°.L

>.
e

"0-

~

r
I

~

is

L

~
0

Distillation Train

u

Let us now consider the train o f distillation columns shown in Fig. 1.2- 1. Since the
unwanted dlphenyl is fo rmed by a reversible reaction (Eq. 1.2-2), we cou ld recycle

'"

~

"-

"-

c

,

• ThUi wJulion wu developed by 0 W To .. n!iC.tld al Impenal
Km&dom

<:"MmlOlltndLl$lf1CS, Ru ncom.,

United

t

"

"••
E

•"

~

"

II

12

SECTION 1.1

SEC TION II

A HIEIIAIIC HICAl A'PIIOM' H 10 COI'I('(PTUAl DESIGN

Food
Toluene

(To recycle)

A mE ..... IICHlCAl AI'1'1I0ACH 10 CONO:I'TUAL DUIGN

13

mIght be cheaper than uSing the configuratIon s hown in the original flowsheet
(Fig. 1.2- 1).
The heuristIcs (design guidelines) for separation ~ystems require a koowledge
of the feed compoSItion of the stream entering the distillation train. Thus. bdore we
consider the deci~ions associated with the design of the distillatioo train, we must
specify the remainder of the flowsheet and estimate the process flows. For this
reason \\ e con ~ ide r the design of the distillation train bero re we consider the design
of the heat-exchanger network

Vapor Recm'ery System
DiphenyJ
FIGURE 1.2.J

Altemate dislillallon Iralns.

the diphenyl with the toluene and let it build up to an eqUilibrium lc\·el. ThiS
alternative would make it possible to eliminate one of the distillation columns,
although the now ra te th rough the reactor would increase
)fwe decide to recover the diphenyl as Fig. 1.2-1 indicates, we expect that the
toluene-diphenyl split WIll be \ery easy. Therefore, we might be able 10 u~ a
sldestream column to accomplish a benzene-toluene-diphenyl split. That is, we
could recover the benzt'ne oH:rhead. remove the toluene as a sidestream below the
feed, and recO\'er the d'phenyl as a bollom stream (sec Fig. 1.2-3) We can still
obtain very pure benzt'ne o\erhead if we take the toluene sldestream off below the

feed . The purity of the toluene recycle will decrease, howe\'er, if II is reco\'ered as a
sidestream, as compared to an overhead producL Since there is no specification for
the recycle toluene, the purity might not be important and the savings might be
worthwhile. Similarly, we expect that the methane-benzene split in the stabilizer is
easy. Then, recovering benzeoe as a sidestream in a H} and CH.
benzene-toluene and diphenyl spliner (a pasteurizatioo column) (see Fig. \.2-4)

Toluene

Referring again to Fig 1.2-1. we consider the vapor flow leaving the flash drum. We
know that we never obtain sha rp splits in a flash drum and therefore that some of
the aromatics will leave wilh the flash vapor. Moreover, some of these aromatics
",iiI be losl in the purge stream. Of course, we could recover these aromatics by
IOstaIling a vapor recovery system either on the fl ash vapor stream or on the purge
stream
As a vapor recovery system we could use one of these .
Condensation (high pressllre, or low temperature. or both)
Absorption
Adsorption
A membrane process
To estimate'" hether a vapor recovery system can be economically justified.
"'e must estimate the flow rates of the aromatics 1051 in the purge as well as the
h~drogen and methane flow in the purge. Hence, before we consider the necessity
and/ or the design of a vapor recovery system. we must specify the remainder or the
Howsheet and "'e must estima te the process flows. We consider the design of the
'apor rcwvery system before tha t for the liquid separa tion system because the exit
stream s from the options for a vapor reco\'ery system listed above (e.g.• a gas
absorber) nonnally include a liquid st ream that is sent to the liquid separation
system.

[fo recycle)

Simplified

FIGURE 1.1....

Diphenyl

A1ternale dl~unation ' "inS

Flowsh~t

for the s.-paration Systems

Our goal is to find a way of simplifying flowsheets. II is obvious that Fig. 1.2-1 is
much simpler than Fig. 1.2-2, and therefore we decided to do the energy iotegration
last. Similarly. sin~ we have to know the process Dow rates to design the vapor and
liquid rocovery systems, we decided to consider these design problems just before
the energy integration. Thus, we can simplify the flowsheet shown in Fig. 1.2- 1 by
drawing it as shown," I· ig. 1.2-5. The connc<::tions between the vapor and liquid
recovery systems ~hown In Fig. '-2-5 are discussed in more detail later.
We now ask oursehes whether all processes can be represented by the
Simplified flowsheet ~hown In Fig. 12-5. SIO~ this flowsheet con tams both gas- and

14

SECTtOW tl

A ItIEUl(:HtfAl APPlOAOI TO C'OI'OCEPlUAlDf..sIGN

S[CJlt)t; Il

p","

Vapor recovery
system

1

11 2• CH,

---..[--'-----'-1-

Benzene

-

Dtphen)I

Toluene

H, • CH"

Reactor
s)':>tem

'---------'

Phase

split

To lucne

fiGURE 1.1.,
HDA

FIGURE 1.l-5
HDA M:~flIIiOn syslffil. [Afm J !of DougllU, AICIt[ J. 31 JjJ (11185).]

Ilquld+rccycle loops. but some processes do not contain any gaseous components,
we do not expect Ihe results to be general (See Sec. 7.1 for other alternatIves.)
H owever, we can Simplify the ft owsheet stIli more by lumping the vapor and hquid
sepa ration systems in a Single box (see Fig. 1.2· 6) Thus, ~e consider the
specificatton of the general structure of the separation system before we consider
the specification of either the vapor or the liquid recovery systems.

Recycle Struclure of Ihe Flowshet'l
Now we have obtained a very simple flowsheet for the process (Fig. 1.2·6). We can
use this simple representation to cslimate the recycle fl ows and their el'fecl on the
reacto r cost and the cost of a gas-recycle compressor, if any. Moreover, we can Ify
Gas recycle

-

1

Reactor
syslem

I

I

r-

r---

understand what design questions are important to obtam thIs simplified
withoul wonying about tile additional complexities caused by the
separauon system or the energy integration network . For example, we can study
the factors that determine the number o f recycle streams, heat effects In the reactor
equi~ibrium limitauons in the reactor, elc. Thus, conl1nuing to Strip away levels of
detail, we see that we want to st ud y the recycle structure of the flowsheet before
considering the details of the separation system.

Inpul-OUlpUI S t,.uclure of Ihe F lo"'-sheet
Figu~e 1.2-6 provides a very simple flow~heel , but ~e consider the possibility of
obtalnmg an even SI mpler representallon. Obviously, If ~e dra w a box around the
compJ.ete process, we WIll be lefl with the feed and product Slrea ms. At first glance
(see Fig. 1. 2-7), thts representat io n might seem to be iooslmp1c, but 11 <'\'111 a id us III
understand 109 the deSign vanables thai atrecllhe overall malenal balances without
introducing any other complica tions Since raw-mal erial costs normally filII in the
range fro m JJ 10 8.s ~~ of the Imal product costs, · the meral! material balances are
a dominanl fa cto r In a design Also, we do not want to spend any time investigating
the design variables in the ranges where the products and by-products are worlh
less than the raw materials. TIl us, we consider the IOpUt-oulput structure of the
flo wsheet and the decisions that affect this struclUre before <'\" e consider any recycle
systems.

P ossible L imitations

H2, C

Separation
system

~IfUCI"re [Aff~' J It D<... ~I,,~. AIC~t: J . 31 JH (l98j)]

represe~tation ,

0; phenyl

I

mp"H)"tp"t

10

"'

Liquid separalion
s)'Stem

-

1.5

,
11 2. CH"

Toluene

A HIUAlUI,o.t APPlOACH TO C'OI\IClYIlJAL DU1<.oN

Benzene
Diphcnyl

By su~ssively simplifying a fl o wsheet, we can develop a general procedure for
auackmg dcslgn problems. However, o ur onglOal fl owshcct described a conlinuous, vapor-lIquid process Ihat produced a sll1glc product and involved only
simple chemicals (no polymers o r hydrocarbon cuts) There are a large number of
processes tilal satls r) these Itnlllatlo ns, and ~ we try 10 develop this systematic

Toluene recycle
FIGURE 1.2-6

HDA recycle

~lfUCl"re

• E L GrulTICr. -Se llmll Pnc:c ~~ H..... M .'m~l COIiI.- eN," &t.j. 79(9) 190 (Ap,,1 14, 1961) Also
II E. Kyle, C~"," . £"9 P'og . II1( S) J7 (t9116). ror some dall c;on' p&nns commodllY chemICal
prodlKhon co spcclahl y cheflllQl~
iec:

[#u,

J

M

Dougllu,

A/C~£

J.

JI

JjJ

(/985).]

16

SE(."l10N Il

A HIFUIICIUCAl A,."1I0ACH TO CONCEI'TUAI DESIGN

procedure rn greater detail. However. batch procc:sscs may ha~e a somewhat
difTerent underlYlllg structu re (we oflen ca rry out multIple operations In a smgle
\essel). and certamly they a~ described differently in terms of mathematICal
models (normally ordllltlry differential or partial d ifferen tIal equations rnstead of
algebraic equations or ordinary different ia l equa tions) li enee. our first decIsion
probably sho uld be to dlstlllgulsh between batch and continuous processes.
Hierarch y of De<:isions
If we collect the results diSCUssed aboH:. we can develop a systematic approach to
process design by reducing the deSign problem 10 a hIerarchy of deciSions; sec
Table 1.2·1. One great advantage of this approach to design is that it allows us to

calculate equipment sizes and 10 estimate costs as we proceed through the levels 10
the hierarchy. Then if the potential profit becomes negative at some level, we can
look for a process alternative or terminate the design project without baving 10
obtain a complete solut ion to the problem.
Another advantage of the procedure arises from the fa ct that as we make
deciSions abou l Ihe structu re of the fl owsheet at various levels, we know that if .... e
change these decisions, we wilt generate process alternatives. Thus. with a
systematic design procedure for idenllfying alternatives we are much less likely 10
overlook some importa nt choices. The goal of a conceptual design is to find the
"best" al teroat i\"e

Shorccut Solulions
Experience indicates that it is usually possible to generate a very large number (i.e.,
often 10" to 10°) of alternative fl owsheets for any process if all the possibilities are
considered. Hence, it is useful to be: able to quickly reduce the number of
alternatives that we need to consider. We nonnally screen these alternatives, using
order-of-magnitude arguments to simplify the process material balances, the
equipment design equations. and the cost calculations_These shortcut calculations
often are sufficiently accurate to eliminate the 90%, or so, of tbe alternatives that
do not correspo nd to profitable operation. Then if our synthesis and analysis lead

~

A HIUAkCHICAl A,.,.kOAC.. TO COr
1Io.~~t

Recycle structu re of lire lI o,",slittt

to a profitable solut ion, we repeat all the calculations more rigo rously, because

then we can justify the add itional engrneering effort.
The use of shortcut solu tions and the hierarchical decision procedure also
makes it possible to provide mo re rapid feedback 10 the chemist who is attempting
10 develop a process. That is. a lternate chemical roules could be used to make the
same product, ..... ilh a largc number of nowsheet alternatives for each route. l lence.
quick estimates of the range of conversions, molar ralios of reaClants, etc., that arc
close 10 Ihe economic o ptimum for the various rou tes help the chemist 10 take dala
in the range where Ihe mosl pro fitable operation might be: obtained and to
lenmnate experiment s that a re ou tside the range of profitable operation.
Decomposition Procedures for Existing Processes
Of course, we can also usc the approach presented above as a decomposition
procedure fo r existing processes, to simplify the understanding of the process, 10
understand Ihe decisio ns made to develop the process, or to systematically develop
a list of process altema ti\·es. The decomposition procedure we suggest is as follows :
I. Remove all the heal exchangers, drums, and storage vessels.
2. Group all the dlstiltation columns (liquid separa tion syslem block).

J. Simplify the general structure of the separation system (similar to Fig. 1.2-5).
4. Lump (group a ll units m a smgle box) the complete separation system (simi lar
to Fig. 1.2-6)
5. Lump the complete process
This decomposition procedure is different from those that break down Ihe
flowsheet inlo dIscrete subsystems which always retain their identity, i.e., into
indi vidual unit operations. To develop process alternatives. we want to modify the
subsystems. With o ur approach we accomplish this task within a framework where
we always consider the lOll'll plant. altbough the amount of detail included at
various levels changes.
Hierarchical Planning

The essence of Ihis approach IS 10 utilize a means for discriminating bet ..... een

imporlanl ,"formallon and details In problem spaa: 8y planning in .. hierarchy or
abstraction spaces in which suocc:s.sive levels or detail are inlrodueed. sigmlitan t
increases in problem-solving power have been achieved

.. Geneu151rucIUle or lhe $ep~r.tion 5Y51,.n,
II. Vapor recovery 'yslCIl1
b uquld A:OO'tIY Sf$tem
50 lleal-n chanse' network

11

Our Slrategy of successive refinements and our hierarchical design procedure lire
si milar to the hierarchical planning strategy discussed in the artificial intelligence
(AI ) literature. Sacerdoli - states,

TARtE 1,2.1
Hierarchy or dedsions
1. Ibleh ~,.n;us continuous
Z. Inpul-ouCPU I ~tr"':lun: 0( Ihe

SEcnON I J

• E. D. Saao.dol~ -Plannrnl!

In

a Hierarchy or AbstractIon SPllnes.- AmI /..",_ 5 liS (19H~

18

ncnoN U

SU!oI!oIAIY ANO EXUClSf.lI

The ooncep! Clin be readily el!ended 10 a hierarchy of lpaca, each deallDg wllh
fewer delails than !he ground space. below II and wuh more details than the
abstraction space lbo~e II. By consldenng details only when a sua:;cssful plan in a
higher level space lives strong evidence or thelf Importance, a heunnic search procc.s.s
will invl:S ugate a greatly reduced portion of the search space.

1.3

~

In our hierarchy, the ground state represen ts the energy-integrated flowsheet,
and each level above II contains fewer detaib. M oreover, if the process appears to
be unprofitable as we proceed th rough the levels in Table 12-1, we look for a
profitable alternative or we terminate the project before we proceed to the next
level. As noted by Sacerdoti, the hierarchy provides an efficient approach for
developing a design.

"

'"

§

•u

'"

::;j

~l

SUMMARV AND EXERCISFS

8

•

IIUlflIO;)

:KI~~

E
"")-

t X'

Summary

~ uwnlOO pnprud)-

'¢

H,O

•

"

"
'""'

j
<

"

~

~
J~IU:>S

Acetone

l l--------.J

..

I------~

y

~"'

•

X

U"

X

c;-"'

t

u

Cool water

;lJ"''u."

Process design problems are underdefined, and only about I ~ of the Ideas for new
designs ever become commercialized. Hence, an efficient strategy for developing a
design is initially to consider only rough, screening-type calculations; I.e., we
eliminate poor projects and poor process alternatives with a minimum o f effort

H,

•

~

~

~

uwoloo 8 30 ) -

'f

"''"

Lo

.•
u.

I

.,~[
u

u

<
~
<

Ol

Walcr _ _J
FIG URE l.3-t
IPA planL (A/ler /fUl AICII£ SludDtl C""USI Proble",)

19

20

nnION!'

n'M"'''.'

",..IJ HUOSF5

Sff'TJOto! IJ

Thcn If the rc~ult~ of this preliminary analysis seem pro mismg. we add detail to thc
calculallons and we use more ngorous computallonal proced ures.
Wc can simplify the des1gn problem by brealmg it down into a hierarc hy of
decisions, as 111 Table 1.2·1. In this text .... e discuss this hierarchy o f de cisio n s in
detail

SIJIolWUY ..",0 EJlUClSU

1.3-5. An en~rgy - Incegraced nowsheec for the prodUc:lion of echylbenzene: IS
I 1·2 The prnnary ruclions .r~
Elhylenc:

+ Bc:nzenc:

---0

&I~en

21

In Fig

Ethylbenune

Ethylene ... Ethylbc:n7.c:ne:;:,! Dic:thylbc:nzc:nc:
Ethylcne:

+ Dtc:thylben1.c:nc::;:'! Tnc:thylben«ne:

2Ftbylbc:nzene:;:'! Bc:nl.c:ne .. Dlethylbenl.c:flC

Exerci..eo;
Recommended e'\clcise:s ar(' pre<:td<'d by an asterisk'
1.3- 1. If enginccnng time costs SIOO/hr, CS \llnate th~ wOlkel-ho ur; reqUired to complete
each type of deslSn study In Table I I - I for a small plant
1.3-2. According to the engineering m('thod, what .... ou ld be the best .... ay to read a
te~lbook that co\ers a field you ha"e not studied befo rt, (i.e., blOtechnology,
elccnocheminry, etc.)"
-1 .3-3. If the diphcnyl In Ihe h),drodc:alkyla.,on of toluene (1I 0A) process: is Iccyded 10
nhnchon. ins tead or being n:coveled. show onl': altl:mallve for the hierarch y of
nowsheets, 10::., mpu t-output, recycll:, separauon system, d istillation train (do nol
conSider energy Integrahon).
1.3-4. A Oowsheet for a pr0CZ5S 10 produtt atttone from lsoplopanol is &J~en in Fig. I ]·1
The: reaction is Isopropanol_ aCC::lone: + II ,. and an azeOUOPIC mi~lure of
I PA- H,O 15 used as tht feed stream. The reaC110n takes place at I atm and S72"F
Show the: hterarchy of Oowsheen. .

The: reacuo n IS lun ....·lth an u.ccss of benzene: and almOlIl complc:te con>eBlOn
of Ihe ethylene, to Iry to minImize: the formation of dl- and tr1c:thylben7.c:nc. and it
takes place.t ]00 psigand 82

stream and the o ther being regenerated because of co*ke formation). There IS 0.94 Yof ethane: In Ihe c:thylcne fea:l and O.l8Y- water In the bc:1lttnc: feed. De~elop the
hierarchy of Oowsheets for this process.
1.J...4.. A no wshcct fO I cthanol synthesis is shown in Fig. 1.3·3. The primary reactions an:
Ethylcne: + II ,O:;:'! Ethanol
2 Ethanol:;:,!Dlethyl Elher

+ HI O

The: reactio n takes platt at 560 K and 69 bars, and about 7/,; conversIOn of the
ethylene IS obtamed The equlhbrium constan t fOI dielhyl ether production atth~
conditions is about K _ 0.2. The feed s t r~ ms are pure waler and an ethylene stream
contamlng 90~" I':thylcne, S/,; ethane, and 2 /,; methane:. Show the hierarchy of
ftowshec:ts.

Reaction section
Heater

Reactor

Separator ~_ _~

Scrubb<'r

Vent

Water

Benzoic acid
Condensate

SUrtI>-----__"
Feed ethylene

~--~

DEE

EtOH- H20
ucotrope

E
,

8
FI GU RE 1.3-3

Ethanol synthc:m

CataJYSI

Reactor

Toluene
stri pping

Benzoic Acid

rectification

Fl GU RE t.J...4
lkTll.OOC IIad ",oductoon LAfi~ JlJ..J'QCarb P'or~ 48(11) 1M (No~~ 19M) J

22

SU~"'AIlY A,,"O U(ltClsa

UCT'OH U

1.3-7. A Howshcci for benlolC ii10d p,oducllon I~ shown In h&- I.}·4 (from SNIA
VISCOSA I'roce~s, IIJ'drociJrb P'IX ., " 8(lJ): 156 (Nov., 1964)) The prim.. ')

reaction

CHAPTER

I)

2

Toluene ... 150: . UcnlOlcAc,d ... 1i 1 0
lI a""e~er.

re'trslble by.prvducts (ben~ald~hydc and hclllyll..: ~lcahal) ,is 'WC:II ,J)
ones (assume phenyl ben;mate and benz)'1 benw3 le) are ~Isa formed at the
n:aetlon condUJons of J60' e and 10 atm Pure toluene and au are used as the "' ....
matenals. and the toluene con'erSIOIl IS lept at 30 to ]5 ~'~ As shown on the

Hawsheet. the wluene IS recovered "nd recycled in one column, and the re"chlb1e byprodU<.:ts are recycled from the overhead af a second The product is rcco\'ered as I
vapor sidestream ( wlIh greater Ihln 99 y. pumy), and the heavy componen ls :..re SCn!
10 fuel. Shaw the hierarchy of Hawshects.
1.3-8. Select a Ho wshcet from Hydrocarbon Procuring (sec the November i.ssue of any
year). Develop the hierarchy of Howshccu far the process
hc:a~ler

ENGINEERING
ECONOMICS

In C hap I we deSCribed a syste matic approach thai can be used 10 de\'ciop a
concepl ual design In add ilion. we hsted Ihe Iypes of design e5limales Ihal nonnal1y
are underlaL.en over the hfe of a prOJecl. The goa l of these eSllmales is 10 generale
COSI data , all hough the accuracy of the calculalion procedures and the amount o f
de lail conside red are different for each Iype of estimale.
Since. cost estimates are 1I11! driVing force for any deSIgn study, we need 10
undeTSland the various faclors to Include. We describe a procedure for generating a
cost estimate for a conceptual deSign 10 Ihis chapter We ~gin by prescnung the
results from a published casc Stud }. 10 order 10 gam an o\'erall perspecU\'e on the
t)pts of cost dala requirc:d, and then we discuss the de tails of Ihe COSI analysis.
Remember Ihat the cost modds that we develop should be used ollly for
screemng process alternatives. The COS I estima tes that are reporled to management
should be prepared by t he app ropriate economic s peciahsts in the company,
because they will mdude (:onllngency factors based on expenence: and Will include
the costs of more ilems t han " e consider. Thus, our cost estimates normally will be
100 o ptimist ic, and they should be kept ronfidentia l unli lthey have been venfied.

2.1 COST INFORMATION REQUIRED
By conside ri ng the results of a published case study, we can get an overview of the
"ind of informalion thai we need to develop a cost eSllmate for a conceplUal design.

Moreover, the framework rdatmg the material and e nergy balances, eq UIpment
size5 and utility fl ows, capItal a nd operating costS, and process profitability should
become more apparen!. Tbe parllcular case siudy we consider ,"volves the
productio n o { cyd o hexane by the hydroge nation of benzene ·
IknU:lle

+ 3H 1 ;;:: Cycloheune

• J R.. Fall, C,.doito.

D Smith, Wash,,,.'''n

U"I\''''5,ly. St I ou.s. Mo,

UD1Yc~ly

AUg.

I.

(2.1·1)

DcsIp C ..", Silldy No . 4, ed.led by B

1%7

23

OUT purpose here is nOI to discuss the details of the design. but merely to see
what type of results are generated

Flowsheet and S tream Table
One of the most important items that we develop during a design is a process
flowsheci (see Fig 21-1). The nO~sheel shows the major PlettS of equipment. and
usually each piece of equipment is given a special number or name. as In Fig. 2.1-1
Nonnally each strea m on Ihc: nowsheet is also lettered or numbered, and a stream
table that contains these letters or numbers often appears al the boUom of the
Oowsheet. The stream table conlains the flows of each component in every stream
as well as the stream temperatures and pressures. In some cases, en thalpies.
densities, and other information for each stream are included in the stream table.

©

Operaring Cosls
~C~

ON~

or..:...:

....

........ "'~

Once we know the stream flow rates and the stream temperatures, we can calculate
th~ utility flo ws for the various units shown on the flowsheet ; see Table 2.1-1. Then
if we know the unit costs orthe utilities. we can calculate Ibe total utility costs. We
combine these utilities costs wi th the raw-materials costs and other operat1l1g

expenses 10 obtain a summary of the operating cosIs; see Table 2.1-2.
TABLE 1.1- 1

Utilities summary: Base cue

u..••

-

10-

Urili,y

Boder fcnl".lt.
Steam, 50 lb.

u

""'"

R_'
R-'

Eq,uPfMa' .... OM

R. I.

RQC10f (coolant)

".

Waste-hal boikr

Elec:1'10 I)'

t

_ _ _ _ _ _ _ _ _~1l

'"

Col
Co,
pol
po'
P-J
p~

I....ightmg

F~ comp'eMc)r
Reo.;yde compressor
Be.....,ot feed pump
Boikr Ieed pump
RQCIOf KnU~ pump
Filler pump
12 hrldar

Tot.r
E-'

E_'
E_J
TOlal

10

Iblb·
,,,.
••
JI6
J_'

""

0.4

,

,....
'""
"
3"

Cookr~n'ltl'

Compr(:$$(l' inle'OQ(Ilcr
Comr

.......
Mpl

',lIII0
Mib
45.SOO
kwh •

l62O,OOO
26.000
<8,lIII0
3,lIII0

2>000
2,719,000
Mpl
I~,ooo

9,SOO
9,SOO
141,000

r-....... J R. rolf. Wulunl'OfI Va,on<'y DosIp C... 5(vdy No ' . cd"e-d It, B 0 S"",b,
",_. h,nCl"'" U .....".."y. S, I.ou ... Mo. 1961

Il:

TAB LE 2.1-2

Opef1lling COS I summary: Cyc:loheunt - basc cast
ESTI M AT ED P RODUCTION COST AT ARNOLD, CONSOLIDATED C HEMI CAL CO
C. H

'1

OUTPUT _ 10,000.000 GAL (65.000,000 LR)

P RODUCT DELIVE RED AS LIQUI D, 99,9+

Pf R yr.AR (8322 HOURS)

%
_ ssrO.flOO

TOTAL MFG CAPITAL
TOTAL FIXED &. WORKING CA PITAL _

693,000

UNIT

QU .... NTITy
PER YEA R

UNIT ['R ICE

COST
PER YEAR

('OST
PER 100 Lil

RAW MATE RI ALS
~,2'n.om

SO.D

Sl.R91.{XX)

'1011....

0.23

207,000

Ih

In,Hm

'00

21.600

Ih

10,Roo

OSO

- 5,400

R~NZENE

.. I

HYDROGEN

M eF

CATALYST

R M H .... NDLING
TOTAL R M
CREDITS SPENT C .... T .... LYST

NET R.... W M .... TI: RIALS

2.116.200

Sl26

DIRECT EXPENSE

Lobo.

'<300

SUpet~ISIOI'I

9.600

,.

.."

Payroll Charlie.
Steam (SO PSIG-CREDIT)

hl lh

4.5.SOO

OSO

- 22.800

Eleeln~ny

kwh

2.719.000

0.01

21,200

Compo AIr

'0,<00

Rcpalr1@4 % MFG CAP
WI!er -Coolm ll

Mgll

1"7,000

Mllal

'.000

0.015

~200

OJO

1.5011

WI!ct-Pro~cu

Waler - BOILER FEEDWATE R
Fuel -Ga.-O,I
Fuel -CO li

10.200

Faclory SUPPhU}
2% MFG C .... P
Laboralory
TOT .... L

,
DF~

80.000

012

!::l
(Co"",,~ed)

•

lADLE l.I -3

J

Equipmenl schedule

1;;8

g

0u~

w

.,

linn

~

~

.

N..
Sin (nodi)

C I

~

'i

<
w
>

~
~

~

8

~

.,
.,,..
C ·2

~

"

8w

~

PI

~

P-2

P-'
w

U
~
~

I:
~<

~

S2~

Inlcrcookr
Aftrrcook.

I ~~ fl'

Ikrurnc Iced pump
Boric. kuI pump
Reftu1 pump

Fille' pump
Ik nttnc sur~
RenU1 dlllm

'"
T-.

~

Rec) ck romp' CHo'
Coolel

P4

T4

z

4 So," dram " 28
<101 bhp. two- SIIIC'

T-I

T-'
T-'

t:

2
2
2

r.

Re.cro,'
reed compressor

bhp
fl'

ISSfl'
11 APf'l. 860 n
II IP"'. 116 ft
1) gpm, 93 ft
2S Wm, 62 n
S7,OOO ga l
930 pi

,

!.mc "'p"Talo'
Slu m drum

12.m diam. " ) n

2

I'roducl otO. ISC'

13.11.000 ,_I

FIlter

char~

,.nl;

F>

Catalyst filler

" IE- }}

RdC10f C....hnf coil

L~&&I

lOO .. ,

)s (,'

r,ORl J R F." . w _I''''' U"'>a1IIl ~ ea.c S...dJ No 4. ~" ... b,
8 0 SmIth. I'oa,h,,'ttOfl UII" n .. '~. St. l ou,,- Mo.. 1961

~

<
w
>
~

~~

o·

C. pilal COSIS
After .....e have determined the st ream no ws and stream temperatures. we can
calculate the equipment siltS; soc Table 2.1-3. Then we can use cost correlations
(which are discussed in Sec. 2.2) to estimate the delivered equipment costs. Next we
use installation factors 10 estimate the installed equipment costs (see Table 2.1-4)
We must also estima te the wo rking capital required for thc plant (sec Table 2. 1-5)
Combining all these costs, we obtain an estimate of the total capital requirements
(see Table 2. 1-6).

t:

z

~

~

u<

~

<
u

~

~

~

zw

~

x

w

t;
w
~

"
~

18

~

~

I

<

g

"

-"

~

•

~

• £
<

I

~

<

;
0

~

~

~

u

•
'"
£

•• .5"

~

We combine the opcraung and capital costs, along with some other costs, and v.e
usc these results to estImate the profitability of the process (see Table 2.1 -7). The
rdurn on investment is used as criterion of profitability in the case study, but a
number of other criteria can be used. These arc discussed in Sec. 2.4

~

~

os"

Profitability Estimate

~

•

~

•
! g
0

~

J

•

~

~

~

~

~

w

U

"'9
J

~

g

8
0
0

~
~

J

~

~

•

Engine-erillg Economics

Now thaI we ca n see whatlYpes of costs arc mcluded in an economic analYSIS. how
ca ll we generllle thcse cost data ? First we consider some of the methods for

30 S~CHON

con !NfOu.tA 'nON IEQHIXEIl

l!

HCTlON II

cosr

ISroU'4TION UQUIUO

TAIU t: 2.1 -4

J\1anurll cu.ring capillll : 8 a",e

TABLE

C illoe

1.1-"'

Estimate of capital requirt mtllts: Base cast:
o..Un rPII

..

h e,",

1>0-

CI

fael.,.

76,000
J.OOO
S,IOO

' ·1

E·2
E·J
P-Ia
P-Ib
P-2.
P.lb
P_J.

P-Jb

I.""
I.""
1.200
1.200

p~

1.200

T· I
T·2
T· ]

,>00
>00
600

T -Sa

10,800
10,800

T-5 b
T-6

m

1·1

"00
S143.3 70

J

"""
•.",

"
.,
"
•••
••
46

...

""
"
"

• f.", .....w"',o .... em,e,.,,}

S'ud) No 4, .... "ed b) B
... " S, Lou .. Mo. 1961

TOlal COSI
S 9,700

RC~<:I""

S 44.600
212,~UO

".000

Com presson

10,(0)

EKhlon", ••

8."'"

9.000

Pllmps
Tanh

20,400
10,000
10,000

32.670

FII1~r

2.900

Total proc:cu CQlllpmcnl
TOlll ma nulaclu nn, CIIplu l buc:d on b.nd (aClon
Total mallufaclunnllXKl e$Um.lle

8.800
8.800
'.>00

'.""
'.""
"'.000
J.700

P,oporlOOl1lllc ,ha,e ull"ng CllplW osumaled al 15 % m.anufaClunng capllal

J. "'"

.1. TOlal Fued C.pol.l
Sum of I ao.j 2

>JOO
2.800

"'.000
"'.000

S. TOIal F ,. cd and

SSIO.lOO
Ui

Wor~ln~

~

IroSi ulcs

Cap""1

r,,,,,,, J It F .... W•• lun.' .... e"".nool

o.:...p c..",

OnIpI C...

Smllb,. ...... hld""" Ub".,

TABU 1.1.7

"""w e....,
10' ,I.IIY.

Working capilal

J. P rodUCI !IIvcnlo,), (SO Y. (IIU)
Cyclohuanc; 14 3.000 pi @SO.2Jesumllc.d
.... Olher,al Sy' WCUS ules

'.600

Ml.n waClllnn! (:I.l" lal
TOla. F&W CIIp,I.I·
Gnw gJu per yea r
MlnwlClllnn, COlt
Grou profit
SARE' @ 1 0~~

........
]].000
120.000

SIS9,OOO
From J It, F... _ w ......... _ U"' ...... '1 Dc.op C- SHld} No 4.
cdn ... b1 8 0 Sm."b. WuhID,'011 U ... ..., ... , . 51 Lou-. Mo~ 1967

S SIO.OO)
693,000
2.400.000

2.257.400
14l.600
14.300
121,300

64.200

,~

64,100

Nel pro fit

Rel ... rn o n 10111 f&. W

9J y'

r,om J R F.". Wqlulo"oll UN"~1y 0<>11" C. ",
So.,.j) 1'10 ", cd".d by II 0 SID.I'b. WI. hlOl'OD UD"".ny. S,_ Lowa, Mo. 1967

. f"

W .. ID lCIonY'"

eo. 6. ... lad .. .,. l on, ""JII'••

• URE IS ... lC7O<Iym Jot .. Ie>.

•"" ...,._nn,

.~u. , ......

".000

12.000
107.000
S69J,OOO

S,.,.j) No • cd" ... b, B D Smnh_ Wulnn"oa

L",,'~""" 51. Louu.. Mo. 1961

TABLE 1.1 -5

76,(0)

'.600
400

TOla' ,",orkin, caplt.1

].600

11 ,500

2. Goods In pr~
U I I7SO p i @ SOB

510.000

"'.000

" Wor kllli Qt.p... 1
Rlw-maICful lll >cnlo')'
Goods in Pfoceu
Fims hc.d poodllCI ,"w.nl0Iy
SlOr. , " pphQ Ind all olber Ite .... II ]

12,400

1_ Ra w mllena l (SO ~~ (1111)

C.II . 24,jOO pl@51U3

143,37Q

2. NonmanwaClllnn, c.p,a'

Profitability of cydohtxane ntllflufaClure

10.000,000(024 )(005)

on construction in 1967

I. Manllradlmna Capllal
Equ'pmenl

"

800
800

»00

TfIt ..1

40

2.>00
2.500

T~

r,om

.."

=,

, '."'"

C·2

bll ~d

lIu d

'

31