sd article 34

Exergy load distribution approach for multi-step processdesign

M. Sorin*, A. Hammache, O. Diallo

Energy Diversification Research Laboratory (CANMET), 1615 Lionel-Boulet Blvd., P.O. Box 4800, Varennes, Que.,

Canada J3X 1S6

Abstract

The purpose of this study is to develop a new procedure for process synthesis based on a reduciblesuperstructure, and exergy load distribution analysis. The latter makes it possible to evaluate the impactof each competitive process included in the superstructure according to the specific performancecriterion of the overall flowsheet. This criterion, utilizable exergy coecient, is a function of threeimportant aspects of the process design: ecient use of raw materials, energy eciency and wastereduction. The procedure starts by building a specific reducible structure of the process flowsheet calledthe competitive process superstructure. The exergy load distribution analysis is carried out on thecompetitive process superstructure to reduce it to a final optimal flowsheet topology. The newprocedure is tested for the design of a benzene synthesis chemical plant and is compared with previouslypublished solutions found by the hierarchic and mathematical methods. The new flowsheet consumes theleast amount of raw materials and produces the least amount of discharged gas as waste. 7 2000Elsevier Science Ltd. All rights reserved.

Keywords: Process design; Superstructure; Exergy; Load distribution

1. Introduction

The purpose of process design is to transform raw materials into desired products. Processdesign involves many steps including choosing process units, determining operating parameters

Applied Thermal Engineering 20 (2000) 13651380

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* Corresponding author. Tel.: +1-450-652-3513; fax: +1-450-652-0999.E-mail address: [email protected] (M. Sorin).

and conceiving the process flowsheet. The designer usually has several flow topology solutionsfor any given product. The problem is to find the optimal flow topology with adequateoperating conditions. Two main approaches have been suggested for process design: thehierarchic approach [1] and the mathematical approach [2]. The hierarchic approach is basedon an irreducible structure flowsheet. Process units and their operating parameters are fixed byheuristic methods and the design process follows a continuous pattern where local decisions aremade at each conceptual design stage. The mathematical approach is based on a reduciblestructure flowsheet. A mathematical formulation of the problem is defined by using designequations and design variables. The idea is to search for an economical optimal process flowtopology through simultaneous simulation of dierent process flowsheets.In this study we propose a solution based on the reducible superstructure approach. The

reducing process is achieved using load exergy distribution analysis [3] within a specialflowsheet called the competitive process superstructure.

2. Exergy load distribution method

Exergy optimization of multi-operation processes can be dicult because the local ecienciesof individual operations are interdependent in a way which was heretofore dicult toanticipate. Moreover, a local improvement could result in an overall degradation of the processthermodynamic performance. Sorin and Brodyyansky [3] have shown that there is a simplerelationship between local and overall exergy eciencies. It is based on the concepts of primaryand transformed exergy. A unit operation can consume either type of exergy or both. Theprimary exergy load of an operation, lp, i, is the fraction of the total primary exergy that itconsumes. Its transformed exergy load, lt, i, is the ratio of the transformed exergy that itconsumes to the total primary exergy. The relationship between the individual eciencies, Zi,and the overall eciency, Ze, is thus:

Ze Xi

Zilp, i

1 Zi

lt, i

Xi

Ai 1

The expression AiZilp; i 1 Zi lt; i characterizes the net contribution of each process unitto overall system eciency.It has been demonstrated by Sorin and co-workers [4] that Eq. (1) is consistent with the

definition of utilizable exergy coecient, which is a measure of the three performance optionsin chemical process design: energy eciency, waste reduction and ecient use of raw materials.An eective way to improve the overall exergy eciency of a process is to increase the

primary load of the units with the largest eciencies at the expense of those with the lowesteciencies or to decrease the transformed exergy loads of the units with the lowest eciencies[3]. Eq. (1) lends itself to a simple graphical interpretation as shown in Fig. 1, which is veryuseful in its practical application to process analysis [5]; each unit is represented by tworectangles of width lp, i and lt, i and height Zi and 1 Zi), respectively. When the primary loadrectangles are drawn on the positive x axis and the transformed load rectangles on the negativeaxis, the dierence between the total areas of the right and left rectangles is the overall processeciency, Ze, given by Eq. (1).

M. Sorin et al. / Applied Thermal Engineering 20 (2000) 136513801366

The concept of load distribution is illustrated in Fig. 1. Exergy eciency of unit 1 is greaterthan exergy eciency of unit 2 Fig. 1. In order to increase the impact of A1 on the overallexergy eciency, the primary load of unit 1 may be increased by redistributing it from unit 2to unit 1 and reducing the transformed load of unit 2. Subsequently, the impact of unit 2 onthe overall exergy eciency decreases (Fig. 1).So far, the exergy load distribution method has been subjected to parameter optimization by

changing operating conditions within the given process structure. In this study the method willbe broadened to synthesize the alternative process structures. The criterion of optimization isthe utilizable exergy coecient of the overall process.

Fig. 1. Load distribution from a process unit 2 (lower exergy eciency) to a process unit 1 (higher exergy

eciency).

M. Sorin et al. / Applied Thermal Engineering 20 (2000) 13651380 1367

3. Benzene synthesis process

Benzene is produced from toluene and hydrogen. Liquid toluene and a gaseous mixture ofhydrogen and methane constitute the feed. Methane represents an inert impurity. Diphenyl isalso produced as a by-product. All design specifications and constraints were taken fromDouglas [6].

3.1. Basic structure for benzene synthesis

The various operations were classified into five groups by Douglas [6] and are shown inFig. 2 Inside each group, several alternative design options were retained by Kocis andGrossman [7] to realize the desired operation and are shown in Fig. 3. The basic superstructureproposed by the same authors includes all the design options shown in Fig. 3.

4. New approach for process synthesis

First, the competitive process superstructure must be built. The subsequent exergy loaddistribution analysis makes it possible to cut o process operations whose impact on theutilizable exergy coecient of the overall process is smaller than other competitive options. The

Fig. 2. Classified groups for the benzene synthesis process.


procedure is done for each design option presented in Fig. 3 through the numerical assessmentof parameter Ai in Eq. (1).

4.1. Building the competitive process superstructure

Four design options are possible for each Group 4 and 5, as demonstrated in Fig. 3.Therefore, a total of 16 flowsheet structures for benzene synthesis are obtained. Meanwhile,there is a restriction for the application of the stabilization flash unit (Group 5). The unit maybe used only if the absorber from Group 4 is used to recover benzene from the gas euent. Onthe other hand, the absorber can be used without the stabilization flash unit. This means that,among the identified 16 structures, four structures are infeasible. Twelve structures remain andare presented in Fig. 4. Analysis of the twelve alternative flowsheets in Fig. 4 shows that thereactor yield depends mainly on the benzene concentration in the gas recycling to the reactor.Therefore, two major cases for the reactor system should be distinguished, namely with andwithout benzene recycling. In order to build the competitive process superstructure, basicrules are applied to the topology tree (Fig. 4) to bring together the process units for allstructures.

Basic rule 1 (BR-1): If alternative design options inside each of the groups presented inFig. 3 have the same components but dierent flowrates in the outlet flows, it is possible tobring together all the outlet flows and adjust the flowrates through feed consumption.Obviously, it is feasible only if the feed is the corresponding chemical component. The

Fig. 3. Design options for the benzene synthesis process within each group.


objective is to bring together the reactors within each of the four structures depending onthe two reactor cases. The rule is applied for the design options inside Group 5 by adjustingthe toluene flowrates and the design options inside Group 4 by adjusting the hydrogenflowrates.Basic Rule 2 (BR-2): The second rule is based on the expression (1) of the overall utilizableexergy coecient, Zu as a function of factor Ai, which characterizes the impact of each unitoperation. If the value Ai for the same process units in the dierent structures is constant,then the process units may be brought together in the superstructure. The rule is applied tothe flash after the reactor within the 12 structures of Fig. 4.Basic Rule 3 (BR-3): Factors Ai of some process units, such as a flash, are independent offlow composition. Meanwhile, the values Ai of some downstream process units may dependon flow composition. This means that after mixing, the molar composition of some flowsgoing to the downstream units from the dierent structures must be restituted. If comparedwith the cases of separate flashes, the streams mixing ahead of the flash will modify themolar composition of both vapor and liquid flows. It will influence the downstream

Fig. 4. Twelve flowsheets embedded within the superstructure.


processes of gas and liquid separation. Thus, both the vapour and liquid exits of the flashshould be corrected by restituting their initial concentrations. Fictitious ideal separators thatrestitute the initial concentration for each chemical component can be used. Since theutilizable exergy coecients of these ideal units equal 1, the fictitious separators do not haveany impact on overall process eciency.

The application of BR-1, BR-2 and BR-3 provided the final topology for the competitiveprocess superstructure shown in Fig. 5. The new topology integrates stream groupings aheadof the flash and streams separation (SEP) after it. The superstructure has embedded all theprocess operations and their interconnections presented initially in Fig. 4.

4.2. Reduction of the superstructure through exergy load distribution analysis

Exergy load distribution analysis is carried out on the competitive process superstructure(Fig. 5) in order to reduce it to the optimal final flowsheet. Process simulation has been doneusing ASPEN PLUS code. The criterion used for comparison of competitive processes is thelocal eciency contribution given by the expression Ai Ze; ilp; i 1 Ze; i lt; i: The value Aicould be negative or positive depending on the utilizable exergy coecient and the type of

Fig. 5. Final topology of the competitive process superstructure.


exergy consumed, i.e., primary or transformed. Processes showing the non-maximum localeciency contribution to the overall eciency, Zu, are discarded from the superstructure. Theremaining competitive process for the same operation is kept for the final flowsheet.To carry out the reduction, the competitive processes or assemblies of processes

characterized by identical chemical and thermomechanical parameters at both ends (inlet andoutlet) should be identified. These processes or assemblies are called blocks.Blocks may include the dierent combinations of unit operations within two groups of

separation processes (4 and 5 from Fig. 2):

. Competitive unit process operations within each group;

. Competitive assembly of process operations within each group;

. Competitive assembly of processes within dierent groups.

The three reducing steps needed in order to reduce the superstructure in Fig. 5 to the finalflowsheet are given below.

4.2.1. First reduction stepThe five identified blocks for the first reduction step are shown in Fig. 5. Two blocks (1 and

2) may be identified within gas separation Group 4. The design options with benzene recyclingare included in block 1 and the design options without benzene recycling are included in block2. Block 1 is made up of two competitive process units, 1a and 1b. Unit 1a is the gas purgeand unit 1b is the membrane. Block 2 is made up of two process assemblies, 2a and 2b.Assembly 2a is the combination of the gas purge and the absorber, and assembly 2b is thecombination of the membrane separation and the absorber.Three blocks (3, 4 and 5) may be identified within liquid separation Group 5. Operation a

is the distillation in the column for toluene separation. Operation b is the flash process fortoluene separation. The dierence between blocks 3, 4 and 5 consists in their inlet streams,which are quite dierent depending on whether an absorber, a stabilization flash or astabilization column is used.The values of the utilizable exergy coecients, Zu, i, for each competitive process assembly as

Table 1

Utilizable exergy coecients Zu,i and local eciency contributions Ai for the first reduction step

Process unit or assembly Zu, i (%) Ai

1a 1.4 0.0821b 2.3 0.0532a 3.6 0.0792b 5.7 0.0513a 61.7 2.5 1053b 57.6 5.9 1054a 64.6 1.3 1054b 57.4 3.6 1055a 64 1.4 1055b 57 3.6 105


well as their impact on the overall process eciency, expressed by the local eciencycontribution Ai, are shown in Table 1The process unit or assembly within the same block demonstrating the lowest value of factor

Ai is discarded from the corresponding block and the competing process unit/assembly isretained. For example, because A1a A1b, the purge is discarded as a design option from thesuperstructure. As a result, only the membrane is retained as a design option for furtheranalysis.By discarding the purge (1a) and the toluene flash (3b), three topological structures (1a

3a,1a3b and 1b3b) for the case using a reactor with benzene recycling are eliminated. Bydiscarding the purge-absorber assembly (2a) and two process units, the toluene flash (4b and5b), six topological structures (2a4a, 2a4b, 2a5a, 2a5b, 2b4b and 2b5b) for the caseusing a reactor without benzene recycling are eliminated. A total of nine topological structuresare thus eliminated from the original superstructure, which initially embedded twelvestructures. Thus, as presented in Fig. 6 the new reduced superstructure contains only threetopological structures. One of them includes the reactor with benzene recycling and two othersthe reactor without benzene recycling.

Fig. 6. Defined block and the competitive assembly of units (a) and (b) after the first reduction step.


4.2.2. Second reduction stepThe second reduction is applied to the remaining three competitive topological structures

within the superstructure in Fig. 6. Just one block, 6, is identified. It includes the two processassemblies in the case of the reactor without benzene recycling. The first assembly (6a) includesan absorber and stabilization, benzene and toluene columns. The second assembly (6b) includesthe same unit operations except that it has a stabilization flash instead of a stabilizationcolumn. The values of Zu, i and Ai for the process options are presented in Table 2Although the utilizable exergy coecient of the column is higher than that of the flash,

factor A6a has a more negative impact on overall eciency A6a A6b). This is primarily due tothe large transformed chemical exergy consumed in process assembly 6a compared with processassembly 6b. In the case of process assembly 6b, the transformed chemical exergy consumed isreduced because the gases from the stabilization flash are trapped in the absorber. In the caseof process assembly 6a, the gases from the stabilization column are discharged into theatmosphere. Therefore, design option 6a is discarded from the superstructure, and this reducesthe number of topological structures to two. The new superstructure is shown in Fig. 7.

4.2.3. Third reduction stepThe third reduction step is applied to the two remaining competitive topological structures.

One topological structure uses a reactor with benzene recycling; the other uses a reactorwithout benzene recycling. To be analyzed, they are embedded into the last block, 7, asindicated in Fig. 7. It includes the two competitive process assemblies 7a and 7b. Assembly 7arepresents the reactor with benzene recycling, hydrogen recuperation through the membrane,and stabilization, benzene and toluene columns. Assembly 7b represents the reactor withoutbenzene recycling, hydrogen recuperation through a membrane, benzene recuperation throughan absorber, a stabilization flash, and benzene and toluene columns. The corresponding valuesof Zu, i and Ai have been calculated and are presented in Table 3Since the structure represented by assembly 7a has a lesser impact on the overall exergy

eciency A7a A7b), it is discarded from the superstructure. The final topology, shown inFig. 8, includes hydrogen recuperation through the membrane, benzene recuperation using anabsorber, a flash stabilization, and benzene and toluene columns. The utilizable exergycoecient of the overall process is 76.6%. This value translates into 285 kmol/h of freshtoluene and 280.5 kmol/h of fresh hydrogen consumed by the process.

4.3. New design option

Throughout the reduction analysis of the superstructure, process units of the gas separation

Table 2Utilizable exergy coeients Zu, i and local eciency contributions Ai for the second reduction step

Process assembly Zu, i (%) Ai

6a 40.1 93.3 1056b 36.8 1.22 105


group (Group 4) exhibited the most negative impact on overall process eciency. This isclearly illustrated by the negative values of expression Ai in Table 1. Therefore, the designersattention should be focused on the impact of the remaining units of Group 4, such as themembrane and the absorber, on the overall eciency of the flowsheet in Fig. 8. The utilizableexergy coecients and exergy loads of the membrane and the absorber are detailed in Table 4The negative impact of the absorbermembrane subsystem on the overall eciency, as a sum

of the local eciency contributions Ai, is 0:198: In order to reduce the negative impact of thesubsystem, the main guidelines of the exergy load distribution method may be applied. Thismeans that the primary exergy load consumption to the most ecient unit, the absorber,should be increased. The transformed exergy load on the least ecient unit, the membrane,should be reduced.

Fig. 7. Defined block and the competitive assembly of units (a) and (b) after the second reduction step.

Table 3Utilizable exergy coecients Zu, i and local eciency contributions Ai for the third reduction step

Process assembly Zu, i (%) Ai

7a 2.0 0.360

7b 4.5 0.422


The utilizable exergy coecient of the absorber is high and the unit does not consume thetransformed exergy. On the other hand, the membrane exergy eciency is very low and itconsumes both the primary and transformed exergy due to high pressure loss and gasdischarge. One way to achieve load distribution from the membrane to the absorber is torelocate the absorber ahead of the membrane, just after the vapor flash unit, as illustrated inFig. 9. In this case, all benzene in vapor phase will be recovered by absorption. It results in thereduction of benzene losses from the membrane and an increase of gas flowrate to theabsorber. In terms of the exergy analysis it means that the transformed exergy load on themembrane, that is the function of benzene losses, is reduced. Therefore, the membranesnegative impact on the overall process is reduced, too. Moreover, the absorber relocation willresult in toluene flowrate increase to the absorber. This means an increase in the primaryexergy load on the absorber and, therefore, an increase in overall process eciency.A new process simulation should be performed to confirm the benefits of the new design

option suggested by the exergy analysis. Remember that the option was not included in thepreviously developed superstructure. Therefore, a simulation of the flowsheet, shown in Fig. 10,with the new option for the gas separation subsystem is required.This simulation was also done using ASPEN PLUS code. The net result of the utilizable

exergy coecient Zu for the overall process is 77.6%. This value is slightly higher than thatfound previously for the flowsheet in Fig. 8. Nevertheless, it translates into 276.3 kmol/h offresh toluene and 269.6 kmol/h of fresh hydrogen. It represents an improvement of about 3%reduction in fresh toluene consumption and 4 % reduction in fresh hydrogen consumption. As

Fig. 8. Topology structure for the benzene synthesis process after the third reduction step.

Table 4Eciency factors of the membrane and the absorber placed within the flowsheet in Fig. 9

Equipment Zu, i (%) lp, i lt, i Ai

Absorber 99.4 81.9 106 0 81.4 106Membrane 2.4 1.7 103 0.203 0.198


mentioned, this improvement has been provoked by the exergy loads distribution between themembrane and the absorber due to the relocation of the absorber. The results are presented inTable 5The negative impact of the absorber-membrane subsystem on overall exergy eciency,

calculated as the sum of the two factors Ai, is 0.188. Compared with the flowsheet in Fig. 8,

Fig. 9. Relocation of absorber prompted by the exergy load distribution method.

Table 5Exergy loads to the membrane and the absorber after the relocation of the absorber

Equipment Zu, i (%) lp, i lt, i Ai

Absorber 99.4 151.1 106 0 150.2 106Membrane 2.4 7.4 103 0.193 0.188


the negative impact of the absorber-membrane subsystem on the overall exergy eciency hasbeen reduced.The comparison of the flowsheet in Fig. 10 with those obtained by using the hierarchic and

mathematical approaches is given in the next section.

5. Comparison with solutions found by hierarchic and mathematical approaches

Hierarchic approach: The detailed description of this approach applied to benzene synthesiscan be found in Refs. [1,6]. The final structure uses hydrogen recycling and gas purge, andstabilization, benzene separation and toluene separation columns. Thus,the major dierencefrom the flowsheet suggested by the exergy analysis consists in gas recycling and liquidstabilization. The flowsheet in Fig. 10 uses the combination membrane-absorber for gasseparation and a stabilization flash instead of a column. The exergy eciency of a flash islower than that of a column; however, the membrane-flash-absorber assembly is more

Fig. 10. Optimal flowsheet for the benzene synthesis process.


ecient than the purge-column assembly proposed by Douglas [6].Mathematical approach: The application of the approach to benzene synthesis was performedby Kocis and Grossman [7]. The final structure uses a membrane for hydrogen recuperation,and stabilization, benzene separation and toluene separation columns. There is no absorberin the flowsheet. In the flowsheet in Fig. 10 the absorber is used and stabilization is done bya flash. The mathematical approach made it possible to choose the final structure among thedesign options initially embedded in the superstructure. The approach used in this paper ledto the new option for the process design initially not included in the superstructure, namelythe relocation of the absorber within the flowsheet. Meanwhile, it should be mentioned thatthe mathematical approach allows one to find the optimal operating parameters in asystematic way. In the new approach, the operating parameters are fixed at the start of thedesign procedure by using heuristic methods. The latter are based only on the knowledgeand experience of a designer.

In order to compare the performance of the various flowsheets, all of them have beensimulated using the same constraints, i.e., benzene production is 265 kmol/h and the ratio ofhydrogen over aromatics at reactor inlet is equal to 5. The results raw materialconsumption, gas emission flowrates and the utilizable exergy eciency coecients aresummarized in Table 6Due to more ecient hydrogen and benzene recuperation, the new process consumes less

feed and produces fewer emissions.The new process for benzene synthesis demonstrates the higher value of the utilizable exergy

eciency coecient. This result is not surprising, since the new approach is based on exergyoptimization. However, the approach proposed here does not take into account the capital costof the new process, a subject which is beyond the scope of the study.

6. Conclusion

A new approach for chemical process synthesis has been developed. It is based on exergyoptimization of a reducible superstructure. The approach is distinct from previous methods intwo respects: (1) it is based on a special superstructure without application of the designinteger variables and (2) the reduction of the superstructure to the final optimal flowsheettopology is done by using the exergy load distribution technique. The exergy load distribution

Table 6Comparative analysis of raw material consumption, gas emissions and exergy eciency for the solutions obtained

by dierent methods

Solution Fresh hydrogen

(kmol/h)

Fresh toluene

(kmol/h)

Discharged gas

(kmol/h)

Utilizable exergy eciency coecients

(%)

Hierarchic method 442.1 279.5 482.3 69.8

Mathematical method 277.7 280.1 296.5 76.7New method 269.6 276.3 293.9 77.6


method is a tool to establish the impact of each option on overall process performance. Themethod also indicates ways to relocate some equipment and provides new design options thatwere not included in the initial superstructure.Compared with solutions found in the literature, a more exergy ecient benzene synthesis

process has been conceived using this approach. The new flowsheet for benzene synthesis isdierent from the ones proposed by the hierarchic and mathematical methods. It consumesfewer raw materials and discharges fewer emissions.

References

[1] J.M. Douglas, A hierarchical design procedure for process synthesis, Aiche J. 31 (1985) 353362.[2] I.E, Grossman, Mixed-integer programming approach for the synthesis of integrated process flowsheets,

Comput. Chem. Engng. 9 (1985) 463482.[3] M.V. Sorin, V.M. Brodyansky, A method for thermodynamic optimization. Part I: Theory and application to

an ammonia-synthesis plant, Energy 17 (1992) 10191031.

[4] M.V. Sorin, J. Lambert, J. Paris, Exergy flows analysis in chemical reactors (Part A), Trans. IChemE. 76 (1998)389395.

[5] M.V. Sorin, J. Paris, Exergy load redistribution method for multi-step process analysis, Thermodynamics and

the Design, Analysis, and Improvement of Energy Systems AES-35 (1995) 143160.[6] J.M. Douglas, Conceptual Design of Chemical Processes, McGraw-Hill, New York, 1988.[7] G.R. Kocis, I.E. Grossman, Global optimization of nonconvex mixed-integer nonlinear programming (MINLP)

problems in process synthesis, Ind. Eng. Chem. Res 27 (1988) 14071421.


sd article 34

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