Computers and Chemical Engineering 25 (2001) 807–820
Design and synthesis of multicomponent thermally coupleddistillation flowsheets
Ben-Guang Rong *, Andrzej Kraslawski, Lars NystromDepartment of Chemical Technology, Lappeenranta Uni6ersity of Technology, P.O. Box 20, FIN-53851 Lappeenranta, Finland
B.-G. Rong et al. / Computers and Chemical Engineering 25 (2001) 807–820808
Qr
heat duty of reboiler (106 kcal/h)
R operating reflux ratioRm minimum reflux ratio
minimum reboil ratioSm
total annual cost of a flowsheet 104$ (per annum)TACTb column bottoms temperature (°C)
column top temperature (°C)Td
vapor flow rate of rectifying section (kmol/h)Vvapor flow rate of coupling stream (kmol/h)Vc
vapor flow rate of stripping section (kmol/h)V %xdi mole fraction of component i in distillate stream
mole fraction of component i in bottoms streamxbi
mole fraction of component i in feed streamxfi
Greek symbolsrelative volatility of component iaI
f underwood root
1. Introduction
Distillation is a widely used separation process and itis the largest energy consumer among process unitoperations. The task of optimal design and synthesis ofmulticomponent distillation processes is an importantand challenging issue. In many cases, so popular, HeatIntegration approach is impractical due to some limita-tions of heat flows and process operations. Then, the
novel design of new distillation configurations is verydesired.
Among all possible new schemes for multicomponentdistillation processes, the thermally coupled distillationschemes are very promising for both energy and capitalcost savings (Petlyuk, Platonov, & Slavinskii, 1965;Smith, 1995). However, due to complexity, research oncomplex distillation configurations is restricted to three-component mixtures, and only a few promising flow-
Fig. 1. Feasible configurations of complex distillation flowsheets for five-component mixtures.
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Fig. 2. Network representations of CDFs in Fig. 1.
Fig. 3. The thermodynamic equivalent schemes of CDFs in Fig. 1.
sheets can be constructed for ternary mixtures (Tedder& Rudd, 1978; King, 1980; Glinos & Malone, 1988).Many works have been realised on some specificconfigurations for ternary mixtures, aiming at the per-formance analysis and industrial applications (Tri-antafyllou & Smith 1992; Wolff & Skogestad, 1995;Agrawal & Fidkowski, 1998; Mutalid & Smith, 1998).
There are very few works on configurations of fouror more component mixtures, especially on the para-metric studies of the performance of such multicompo-nent complex distillation flowsheets by computationalwork (Agrawal, 1996; Christiansen, Skogestad, & Lien1997). Such a situation is due to the combinatorialproblem of the possible configurations for multicompo-nent separations. Moreover, there is a lack of shortcutdesign procedure as well as modelling and synthesismethods for these types of distillation schemes. In
consequence, the optimal design and synthesis of multi-component distillation processes are usually performedin a search space which excludes the considerations of
Fig. 4. Three basic units of thermodynamic equivalent schemes ofCDFs.
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Fig. 5. The shortcut design procedure of CDFs.
types of the identified configurations. The examples ofsynthesis of distillation flowsheets for both simple se-quences and complex flowsheets are given as well. Thepresented shortcut design method makes it practical foroptimal design of multicomponent distillation systemsto include the complex distillation flowsheets for indus-trial problems, and some preliminary insights are ob-tained for multicomponent thermally coupleddistillation flowsheets.
2. Feasible configurations and representation ofcomplex distillation flowsheets
2.1. The alternati6e configurations
For ternary mixtures, there are only three feasiblethermally coupled configurations called the side strip-ping column, the side rectifying column and the fullythermally coupled column or Petlyuk column, respec-tively. There are a large number of feasible configura-tions with thermally coupled to separatefive-component mixtures. When considering the feasibleconfigurations, the number of condensers and reboilersas well as the number of column sections in a flowsheetare very important factors for both operability andcapital costs (Agrawal, 1996). Moreover, the flexibilityand controllability of a system largely depend on itscomplexity concerned with the number of units andtheir interconnectivities (Westerberg & Chen, 1986;Christiansen et al., 1997). In consequence, the simpleconfigurations are more promising for practical applica-tions. Taking into account this factor, we focus on theconfigurations with side strippers and side rectifiers,while the complex configurations have the same num-ber of column sections as simple column sequences.Thus, in this work, we build a complex distillationflowsheets with the following units:1. Main Column (MC): a main column in a complex
scheme is a column with an overall condenser and areboiler while connecting with side columns. For acomplex scheme, to separate five-component mix-tures, there may be two such main columns of whichthe one with feedstock is called main column.
2. Side Stripping Column (SSC): a column with onlyone reboiler.
3. Side Rectifying Column (SRC): a column with onlyone overall condenser.
4. Simple Column (SC): a column with one feed andtwo product streams and with an overall condenserand a reboiler. There is a sharp separation realizedin the simple column.
With the above units, we construct a feasible complexconfiguration with the following constraints:1. The number of column sections in a complex
scheme is the same as of a simple column sequence,
Table 1Feed components and mole fractions of design example 1
Component Mole fraction
A 0.05Propane0.15i-butaneB0.25n-butaneC
D 0.20i-pentanen-pentaneE 0.35
complex distillation flowsheets due to the lack of expe-rience and available knowledge.
In this paper, we focus on the study of thermallycoupled distillation flowsheets for the separations ofmulticomponent mixtures. First, the feasible configura-tions of such flowsheets are analyzed. Then, a universalshortcut design method is proposed for design of any
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Table 2The design results of flowsheets of Fig. 1b and c for design example 1
7862.06� V 10 172.1627.70� Qc 32.9629.43 37.77� Qr
i.e. 2(n-1). n is the number of products in a simplecolumn sequence with all sharp separations.
2. Side stripping columns are connected with the recti-fying section of a column, while side rectifyingcolumns are connected with the stripping section ofa column.
3. A side column can be connected with the maincolumn, as well as with the other side columns in acomplex configuration.
4. The simple columns can be used anywhere in acomplex configuration.
5. There are not the same products from the differentcolumn sections in a flowsheet.
6. Only one product exists in a side column, i.e. thedistillate in a side rectifying column and bottomsproduct in a side stripping column.
Thus, four types of complex distillation flowsheets(CDF) could be constructed with the above units andconstraints as shown in Fig. 1 (where [ represents acondenser, and < represents a reboiler).1. Complex flowsheets with side columns connected in
parallel (Fig. 1a, b).2. Complex flowsheets with side columns connected in
both series and parallel (Fig. 1c).3. Complex flowsheets with simple columns included
(Fig. 1d).4. Complex flowsheets with side columns connected in
series (Fig. 1e).Although fully thermally coupled configurations for
multicomponent distillation separations (so-called Pet-lyuk type configurations) are also the feasible schemes,these configurations with only one condenser and onereboiler have too many column sections and featuredcomplex connectivities for multicomponent separations.For example, there are 20 column sections of the whole
flowsheet with eight column sections existed in the maincolumn in a five-component Petlyuk type superstructurewith one reboiler and one condenser (Agrawal, 1996).These types of configurations are not considered herefor the parametric studies at first stage.
2.2. Representation of complex distillationconfigurations
For the sharp separations of multicomponent mix-tures with the above four types complex distillationflowsheets, a network representation of such flowsheetsis presented. Throughout this paper, components in amixture are ranked according to their relative volatili-ties, i.e. for feed mixture ABCDE, A is the most volatilecomponent, the volatility decreases in successive order,with E being the least volatile. For example, the flow-sheets in Fig. 1a–e could be represented in networksshown in Fig. 2, respectively.
In any given network, a line represents a connectionbetween two units. A node is one of the four types ofunits in Section 2.1, units of the same type in a flow-sheet are distinct by their orders. Subgroups in bracketare the products of that unit corresponding to its node,to main column and simple column the former in the
Table 3Feed components and mole fractions of design example 2
Component Mole fraction
0.30A Ethanoln-propanol 0.20B
C 0.10Isobutanoln-butanol 0.10Dn-pentanol 0.30E
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Table 4The design results of flowsheets of Fig. 1b and c for design example 2
bracket is the distillate while the latter is the bottomsproduct. The connected locations for the side columnsin a flowsheet are identified according to their products.For example, there are three side rectifying columnsSRC1, SRC2, and SRC3 in flowsheet Fig. 1a with B, C,and D being their products, respectively. According totheir relative volatilities of B, C and D, SRC1 is con-nected with the main column at the top, SRC2 in themiddle and SRC3 in the bottoms. The connecting posi-tion of SSC1 is up of SSC2 in Fig. 1c based on theirproducts B and D. Thus, any alternative configurationsconstructed with the four types of units could be repre-sented in such networks.
3. Thermodynamic equivalent schemes of CDFs and thethree basic units
A feasible complex distillation configuration for afive-component mixture discussed above has eightcolumn sections which is the same number of columnsections as the simple column sequence and it is theminimum number of column sections. As definedabove, a simple column has two column sections, one isthe rectifying section with a condenser and another isthe stripping section with a reboiler. Based on thefunction of each of the column sections in a complexdistillation flowsheet, i.e. either rectifying sections or
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Fig. 6. The 14 simple column sequences for a five-component mix-ture.
stripping sections, a complex scheme can be convertedinto a flowsheet in which each unit has only onerectifying column section and one stripping columnsection. Then the connections of the units are deter-mined according to the interconnections of theirstreams. Thus, for the flowsheets in Fig. 1, five con-verted schemes could be obtained which are shown inFig. 3. These converted configurations are called thethermodynamic equivalent simple column flowsheets ofthe corresponding complex distillation flowsheets (Carl-berg & Westerberg, 1989).In Fig. 1a–e, column sectionsdesignated with 1, 3, 5 and 7 represent rectifying sec-tions, while column sections designated with 2, 4, 6 and8 represent stripping sections. The column sections arenumbered based on the separation sequence of thecomponents in the mixture. For instance, the separationsequence of Fig. 1b is: AB/CDE (column 1) � C/DE(column 2) � D/E (column 3) � A/B (column 4). Thesplit of AB/CDE corresponds to column sections 1, 2;C/DE to column sections 3, 4; D/E to column sections5, 6; and A/B to column sections 7, 8. The numbers ofcolumn sections in Fig. 3 are the same as those in Fig.1.
From the analysis of the thermodynamic equivalentschemes of any complex distillation flowsheets shown inFig. 3, three different basic units can be abstractedwhich are the basic units to construct any of thethermodynamic equivalent schemes. These three basicunits are shown in Fig. 4.
The feed in unit (a) of Fig. 4 is F, while the feeds inunit (b) and (c) are thermal coupling streams, and unit(b) with Lc inlet and Vc outlet, unit (c) with Vc inlet andLc outlet. The top and bottoms products in each unitcould be the thermal coupling streams.
Table 7The synthesis results of the simple column sequences of synthesis example 1
COP×104$ (perCOC×104$ (per� Qc×106 kcal/h Total TAC×104$ (per� Qr×106 kcal/h TotalSequenceannum) Annual capital annum) Total annualduty of reboilers duty of condensers annum) Annualnumber
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Table 8The synthesis results of the thermally coupled flowsheets of synthesis example 1
� Qr×106 kcal/h Total � Qc×106 kcal/h TotalFlowsheet COC×104$ (per COP×104$ (per TAC×104$ (perannum) Annual capital annum) Total annualNumber annum) Annualduty of condensersduty of reboilers
c 33.037.8 89.8 496.8 586.735.1 96.7 499.640.0 596.2f
4. The design procedure of CDFs
The distinct feature of a CDF with a simple columnsequence is that the units in a CDF are interconnectedby the thermal coupling streams. These units could notbe designed separately as in a simple column sequencein which each simple column could be designed sepa-rately without considering the coupling of the parame-ters. The design of a CDF must simultaneouslyconsider the constraints of the design variables resultedfrom the interconnection streams within its units.
In this work, we consider design of complex distilla-tion flowsheets with sharp separations of nearly idealmixtures — a common example in hydrocarbon pro-cessing and petrochemical industry. The design is aim-ing at giving the equipment and operating parameterssimultaneously. Thus, the design results can be used asthe initial information for rigorous simulation of aCDF. Moreover, combined with cost models, the de-sign results can be used to compare among many of thefeasible alternatives and the optimal configuration canbe obtained.
With the converted thermodynamic equivalentschemes and the three basic units, the design procedureis developed in the following steps.
4.1. The CDF is con6erted into its thermodynamicequi6alent scheme
This conversion is based on the analysis of separationsequence and the functions of the column sections in aCDF. First, the rectifying and the stripping columnsections in a CDF are identified, then, the separationsequence is determined based on the interconnections ofthe units. Next, a thermodynamic equivalent configura-tion is obtained in which each unit has only onerectifying and one stripping column section, thus four
separation units are produced in it and each unit be-longs to one of the three basic units of Fig. 4. The fourunits in a thermodynamic equivalent configuration aredistinguished from their interconnection streams andeach of them is indicated with the type of the threebasic units. For example, the CDF with its columnsections ordered in numbers of Fig. 1b, the identifiedseparation sequence is AB/CDE (column 1) � C/DE(column 2) � D/E (column 3) � A/B (column 4).Based on this, its thermodynamic equivalent configura-tion is obtained as shown in Fig. 3b. Then, based onthe interconnections of the thermal coupling streams,the four simple columns belong to the basic units of(a), (b), (b) and (c) of Fig. 4, respectively.
The structural information for the thermodynamicequivalent configuration and its corresponding originalcomplex flowsheet is stored in a database. The storedinlet and outlet streams for a complex flowsheet includethe thermal coupling streams, thus the designed resultsare the complete information which include all theoperating and equipment parameters. It can give verygood initial information for rigorous simulation andthese detailed information is necessary since theconfiguration is usually very complex and several inter-connected coupling streams are usually existing.
Table 9Feed components and mole fractions of synthesis example 2
Mole fractionComponent
Ethanol 0.25AB 0.15Isopropanol
0.35n-propanolC0.10IsobutanolD
E n-butanol 0.15
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Table 10The synthesis results of the simple column sequences of synthesis example 2
� Qr×106 kcal/h Total � Qc×106 kcal/h TotalSequence COC×104$ (per TAC×104$ (perCOP×104$ (perannum) Annual capitalduty of reboilers duty of condensers annum) Annual annum) Total annualnumber
Table 11The synthesis results of the thermally coupled flowsheets of synthesis example 2
Flowsheet � Qr×106 kcal/h Total TAC×104$ (per� Qc×106 kcal/h Total COC×104$ (per COP×104$ (perannum) Annual capitalduty of condensersduty of reboilers annum) Annual annum) Total annualnumber
costsoperating costscosts
b 29.028.2 70.3 352.6 423.0g 30.029.2 78.7 355.1 433.8
The three basic units in CDFs are designed based onUnderwood (1946) equations for traditional simplecolumns. However, since the feed, the top and bottomsproducts of a basic unit in CDFs might be the thermalcoupling streams, the design of each of the three basicunits should be based on the situations of the feed, thetop and bottoms products which are introduced asfollows, respectively.
4.2.1. The thermal qualities of feed streams of thethree basic units
The thermal quality of the feed stream is a veryimportant factor for the calculation of the minimumreflux ratio for distillation columns. For basic unit (a)in Fig. 4, there is only one feed stream and the thermal
quality of feed stream is defined by its liquid fraction ofthe feed. While for units (b) and (c), based on Carlbergand Westerberg (1989) for side-stream enricher andside-stream stripper, the feed thermal quality for unit(b) is subcooled, and for unit (c) is superheated.
4.2.2. Calculations of minimum reflux ratio of the basicunits
The Underwood equations are directly used to calcu-late the minimum reflux ratios of the basic units for thethermodynamic equivalent configurations.
%n
i=1
aih xfi
aih−f=1−q. (1)
%n
i=1
aih xdi
aih−f=Rm+1. (2)
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%n
i=1
aih xbi
aih−f= −Sm. (3)
For each of the three basic units, the value of feedthermal quality q is calculated, respectively as indicatedin Section 4.2.1.
The operating reflux ratio is chosen based on theheuristic of Douglas (1988), that for the first estimatesthe rule-of-thumb value is used so that R/Rm=1.2.This given value of R will not affect the selection of theoptimal flowsheet among a large number of alterna-tives. Then, the vapor and liquid flow rates in rectifyingand stripping sections for each unit are calculated onthe basis of the operating reflux ratio.
4.2.3. Determination of the operating pressures of theunits
The operating pressures of the units in a thermody-namic equivalent configuration are determined by thegiven cold utility. First, if there is a condenser in acolumn, then the operating pressure of this column isdetermined in such a way that its top vapor could becondensed by the given cold utility. This is done by thecalculation of the bubble point of the top vapor stream.The temperature of the top stream is determined basedon the temperature of the cold utility and the givenminimum approach temperature in the condenser. Theminimum approach temperature is given based on theheuristics of King (1980). Then, the bottom pressure ofthe column is determined based on the calculated num-ber of the theoretical trays and the given pressure dropfor a single tray. The pressure drop for a single tray isgiven based on the heuristics of Kister (1992).
The above pressure determination approach is appli-cable only for the separated units as it is rational onlyfor simple column sequences. For complex flowsheets,the units are interconnected by the thermally coupledstreams. The pressures must be determined based onthe interconnections of the units and pressure con-straints of the flowsheets.
4.3. Determination of the operating pressure for a CDF
For a thermally coupled flowsheet, the relationshipof the pressures for the units are restricted by theinterconnections of the units through the thermal cou-pling streams in the flowsheet. The vapor flow ofthermal coupling streams is usually realized by thepressure difference of the thermally coupled units. Thismeans that the pressure in the location of a unit with awithdrawn thermal coupling vapor stream must begreater than the pressure in the location of another unitto which the withdrawn thermal coupling vapor streamflows in. For example in Fig. 1b, the pressures at thethermal coupling locations have to satisfy the followingconstraints.
P %1\P1 (4)
P2\P %2 (5)
P3\P %3 (6)
Meanwhile, the pressure distribution in the thermo-dynamic equivalent scheme must satisfy the pressureconstraints of its original complex flowsheet. For exam-ple, the pressures in the thermodynamic equivalentscheme of Fig. 3b must satisfy the following inequalityconstraint of its original thermally coupled flowsheet ofFig. 1b.
Pb3\Pb2\Pb1\Pd1\Pd4 (7)
These pressure constraints are the distinct features ofthermally coupled flowsheets with simple column se-quences in which the pressures of the single simplecolumns are determined separately. Thus, thermallycoupled configuration for multicomponent mixtures isan integrated flowsheet and all the parameters of itsunits must be designed simultaneously. Therefore, thecalculated operating pressures for single units in thethermodynamic equivalent scheme must be adjustedbased on the pressure constraints in its original complexflowsheet. Then, the pressures of the units bottoms andthe thermal coupling locations are calculated based onthe pressure drops calculated from a single tray pres-sure drop and the number of trays of the correspondingcolumn sections. Finally, the single units must be re-designed based on the revised pressures and usuallyseveral iterations are needed to obtain the final designresults which satisfy the pressure constraints. This con-vergence is determined by the following convergenceFig. 7. The eight additional configurations of CDFs.
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criterion that the top and bottoms temperatures of eachunit in a thermodynamic equivalent scheme are nolonger changing along the iterations.
%N
i=1
[Tdi(m+1)−Tdi
(m)]2+ %N
i=1
[Tbi(m+1)−Tbi
(m)]250.05, (8)
where, N is the number of units in a thermodynamicequivalent scheme, and m is number of iterations.
4.4. Transfer of the designed parameters from thethermodynamic equi6alent scheme to its originalcomplex flowsheet
The operating and equipment parameters of the com-plex flowsheet are obtained through transferring of theparameters from its thermodynamic equivalent scheme.It is realized based on the stored structural informationof the thermodynamic equivalent scheme and its origi-nal complex flowsheet. This is readily done since bothof them have the same column sections and theparameters are transferred automatically by the designalgorithm.
With the above designed parameters, the synthesisof the above thermally coupled distillation flow-sheets can be implemented based on the economicevaluation. The economic evaluation is based on thetotal annual cost of a flowsheet where the operatingcost is calculated based on the cold and hot utilityconsumptions, while the capital cost is a sum ofthe costs of columns, condensers and reboilers. Thecapital cost of columns, condensers and reboilersis estimated based on the correlations and dataprovided by Douglas (1988). While the operating costis calculated based on the heating and coolingloads and the unit costs of heating and cooling utili-ties.
Thus, a shortcut design procedure for thermally cou-pled distillation flowsheets for multicomponent mix-tures is formulated as shown in Fig. 5. It is a flexibledesign procedure suitable for any types of CDFsconfigured in this work.
It must be emphasised that the above developeddesign procedure is towards the separations of realfive-component mixtures. The operating parameterssuch as temperatures and pressures as well as heatduties of condensers and reboilers for a CDF are allcalculated rigorously based on EOS. Meanwhile, thepressure drops in each unit and optimal minimumtemperature approach in reboilers and condensers arealso considered in the design process. Thus, the devel-oped design procedure can be used in industrial prob-lems to explore the possibilities of using thermallycoupled distillation flowsheets to save operating andcapital costs.
5. Design example problems
5.1. Design example 1
The separation of a five-component hydrocarbonmixture is presented in this example. It has been exten-sively studied in the synthesis of simple column se-quences (Heaven, 1969; Rathore, Wormer, & Powers,1974). Here, with the developed design procedure forthermally coupled flowsheets, we can extend the searchspace of the feasible alternatives and explore the possi-bilities of the thermally coupled flowsheets for separa-tion of this mixture. The components and the molefractions are shown in Table 1.
Feed flow rate is F=907.2 kmol/h, Five nearly pureproducts are required and the recovery of each keycomponent is 98%. The cold utility is cooling water.
The design results for the selected flowsheets of Fig.1b and c are shown in Table 2. The K-values, en-thalpies and associated thermodynamic properties arecalculated with Peng and Robinson (1976) EOS.
5.2. Design example 2
The separation of a five-component mixture of alco-hols is presented in this example. The components andmole fractions of the feed are shown in Table 3.
Feed flow rate is 200.0 kmol/h. Recovery of keycomponents in each column is 0.98. The cold utility iscooling water. The design results for flowsheets of Fig.1b and c are shown in Table 4.
5.3. Comparison of design results with rigoroussimulation
A rigorous simulation was performed to enable acomparison of the designed results for such type ofmulticomponent thermally coupled distillation flow-sheets (Rong, Li, Han, & Guo, 1996). For example, thecomparison of the shortcut design results of flowsheetof Fig. 1a for design example 2 with that of simulationresults is given in Table 5. It is shown that the shortcutdesign results of thermally coupled distillation flowsheethave a good consistence with that of rigorous simula-tion results.
6. The synthesis of CDFs and examples
The synthesis of complex distillation flowsheets isbased on the economic evaluation of both simplecolumn sequences and the thermally coupled flow-sheets. The developed shortcut design procedure can beeasily used to implement the synthesis task. First, allthe simple column sequences are represented by theproposed approach through the registrations of the
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structural information. Next, the configured thermallycoupled flowsheets are represented using the structuralinformation of the thermodynamic equivalent schemesand the original complex flowsheets. The storedconfigurations can be regarded as a constrained super-structure and the design algorithm can automaticallycalculate and economically evaluate all the alternativesembedded in the superstructure. Since it is difficult toconsider all the feasible configurations simultaneously,it is practical to generate a number of candidate alter-natives for optimal design. The generation of the candi-date alternatives is by considering the possibleconfigurations among the concerned alternative space(Agrawal, 1999; Dunnebier & Pantelides, 1999). More-over, in certain cases, if these new configurations havea potential to give results similar to the flowsheetsknown previously, then these new configurations mustbe considered as the relevant in the search for anoptimum distillation system (Agrawal, 1996). In such asituation, we can use the developed design algorithm tostudy the synthesis problems for separations of five-component mixtures to include the complex distillationflowsheets, while in some cases the previous studiesonly searched the optimal schemes among simplecolumn sequences.
The synthesis example 1, presented in Section 5.1, isa widely studied distillation synthesis problem sinceHeaven (1969). There are no complex distillation flow-sheets like thermally coupled configurations consideredfor this synthesis problem in literature. Here we studythe synthesis of this problem by taking into account theconfigurations of both simple column sequences andthermally coupled flowsheets. The 14 simple columnsequences for a five-component mixture are illustratedin Fig. 6. For preliminary synthesis studies of CDFs,apart from the five CDFs in Fig. 1, there are other eightfeasible configurations presented in Fig. 7. The avail-able utilities are given in Table 6.
The results of the shortcut design procedure for thesimple column sequences are shown in Table 7. Whilethe synthesis results for thermally coupled flowsheetsare shown in Table 8. The cost correlations are basedon Douglas (1988). A capital charge factor of 0.1 isused to annualise the installed equipment cost, and theoperating time is assumed to be 8000 h per year.
A striking result of this example is that most of thethermally coupled flowsheets are not better than thesimple column sequences. Only the configurations (m),(l) and (d) have the competitive total annual costs withthose of simple column sequences. It is due to the factthat for this light hydrocarbon mixture, the operatingpressures of thermally coupled flowsheets are muchhigher than those of simple column sequences, in conse-quence, it has caused the decrease of relative volatilitiesand the increase of minimum reflux ratio. Meanwhile,the steams with high temperature levels are required in
reboilers, and with expensive hot utilities, the operatingcosts of thermally coupled configurations are higherthan simple column sequences. For equipment cost, dueto the decrease of relative volatilities, the requiredtheoretical number of trays is increased. Meanwhile, asthermally coupled flowsheets are operated closer to theutility bounding temperatures, the required heat trans-fer areas are also increased. Thus, for this separationwith relatively higher pressure, the capital costs ofthermally coupled flowsheets are not reduced.
For comparison, synthesis example 2, as shown inTable 9, is presented (Andrecovich & Westerberg,1985). In this case, the thermally coupled flowsheets areoperated at atmospheric conditions by using coolingwater as cold utility.
Feed flow rate is 500.4 kmol/h. Recovery of keycomponents in each column is 0.98.
The synthesis results for simple column sequencesand thermally coupled flowsheets are presented inTable 10 and Table 11, respectively.
It is found that most of the thermally coupled flow-sheets are economically advantageous in comparison tosimple column sequences. The heat loads in reboilersand condensers of thermally coupled flowsheets aremuch lower than simple column sequences. For capitalcosts, the calculated equipment parameters showed thata big main column is usually required in a thermallycoupled flowsheet. Thus, the capital cost of the ther-mally coupled flowsheet is still a little higher thansimple column sequences in this case. For instance, forconfiguration (e) in Fig. 1 of synthesis example 2, thereare two big columns required. The calculated numbersof theoretical trays for the first and the second columnare 113 and 82, respectively; while the designed diame-ters for the first and the second column are 4.57 and4.27 m, respectively, thus the capital cost of this systemis much higher. This observation is somewhat oppositewith those assumptions that less condensers and/orreboilers will reduce the capital cost (Agrawal 1996) formulticomponent distillation separations.
Although the literature has paid much attention tothe total number of column sections for a distillationsystem (Sargent & Gaminibandara, 1976; Kaibel, 1987;Agrawal, 1996), and indicated that the number ofcolumn sections is a very important factor for thecomplex distillation flowsheets. Here, based on thecalculation results, we observed that the number ofcolumn sections in the main column is also a veryimportant factor for multicomponent thermally coupledflowsheets. Too many column sections will make thecolumn too high to be constructed and the constructionand instalment costs will be increased greatly. Thevapour and liquid flows in the main column will bechanged at different column sections due to the intro-duced inlet and outlet streams. Thus, the tray hy-draulics and the calculations of column diameter will be
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affected in each column section. Meanwhile, the trayefficiency will be lowered due to the uneven distributionof vapour and liquid flows, and the operation reliabilitywill be affected. Based on our calculations of thecolumn heights for these several examples, we observedthat it is more reasonable to keep the number ofcolumn sections for the main column to be less than5–6 in a complex flowsheet.
The separation sequence for multicomponent mix-tures in thermally coupled flowsheets is also an impor-tant factor for the determination of the optimalconfiguration. For instance, for configuration ( f ) inFig. 7 of synthesis example 1, the determined separa-tion sequence is ABCD/E, ABC/D, AB/C, A/B. Thedifficult split between D and E is separated firstly. Ithas required a high minimum reflux ratio in the maincolumn (Rm=192.44), thus the heat loads in condenserand reboilers are the largest among all the thermallycoupled configurations and the capital and operatingcosts are much higher.
Thus, the thermally coupled distillation flowsheetsare not always costly efficient for the separations ofmulticomponent mixtures. The possibilities for the useof thermally coupled flowsheets depends on the charac-teristics of the mixtures (boiling points and distributionof relative volatilities), it depends on available utilities(different levels and prices), as well as on the distribu-tion of the feed compositions.
7. Conclusions
The thermally coupled distillation flowsheets for theseparations of multicomponent mixtures are studied.Considering the flexible structures and the operabilityof the possible alternatives, the configurations with sidestripper(s) and side rectifier(s) are constructed with theconstraints of the same number of column sections ofthe corresponding simple column sequences. Four typesof feasible schemes are obtained. Three basic units areabstracted for the design of such multicomponent ther-mally coupled distillation flowsheets based on the anal-ysis of the thermodynamic equivalent schemes of thethermally coupled flowsheets.
A universal shortcut design procedure is developedfor any types of the configured thermally coupled flow-sheets. The examples have shown that the shortcutdesign procedure can give all the needed equipment andoperating parameters. The comparison of shortcut de-sign results with those of rigorous simulation showed,that this shortcut design procedure can give rationalequipment and operating parameters; meanwhile, itpresents good initial information for rigorous simula-tions of the complex distillation flowsheets.
Combined with cost models, the shortcut procedurepresents a practical method for the synthesis of multi-
component thermally coupled distillation flowsheets.The synthesis is based on the calculations of the capitaland operating costs of both all of the simple columnsequences and the constructed thermally coupled flow-sheets simultaneously. Thus, for the real multicompo-nent distillation separation problems, the spectrum forthe synthesis of optimal multicomponent distillationprocesses is extended to include the complex distillationflowsheets.
Two five-component mixtures are used for the syn-thesis of thermally coupled distillation flowsheets.Based on the analysis of the calculated results, somepreliminary insights are obtained for multicomponentthermally coupled distillation flowsheets. The multi-component thermally coupled flowsheets are usuallyfavourite for atmospheric operating pressures. The eco-nomic advantage of CDFs is very case-based, the possi-bilities for the use of multicomponent thermallycoupled distillation flowsheets depend on the character-istics of the mixtures, it depends on the available utili-ties, as well as on the distribution of the feedcompositions.
There is needed further research on the detailedparametric studies of thermally coupled distillationflowsheets for separations of multicomponent mixturesto understand the economic performance and to verifysome of the literature assumptions. Moreover, an im-portant research issue is to study different types ofmulticomponent thermally coupled distillation flow-sheets aiming at identifying the reasonable connectionsof the units and promising complex distillationflowsheets.
Acknowledgements
The financial support from the Academy of Finlandis gratefully acknowledged.
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