Ind. Eng. Chem. Res. 1993,32, 1121-1141
1121
A Problem Decomposition Approach for the Synthesis of Complex Separation Processes with Recycles Oliver M. Wahnschafft,’*+ Jean-Pierre Le Ruddier, and Arthur W. Westerberg Engineering Design Research Center and Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
We consider the synthesis of separation processes to create relatively pure products while using separation devices that are incapable of sharply splitting their products. The use of distillation for separating azeotropic mixtures is, perhaps, the most prominent example of this class of problems. Economic designs contain recycles, a feature not required, and generally not present, in processes which separate ideally behaving mixtures. Focusing on separating azeotropic mixtures, we propose and illustrate an approach for the systematic conceptual design of these complex separation schemes. Strategic use of recycles permits the design of processes requiring fewer units and often also permitting separation without additional mass separating agents. A key step in our proposed synthesis methodology is the assessment of the products attainable in each separation step. Thus, the synthesis of separation schemes for azeotropic mixtures is an important application of the methods presented in two previous papers to determine the product composition regions of homogeneous azeotropic distillation columns. 1. Introduction
A major part of the effort in process synthesis since the late 1960’s has addressed separation systems because of the large number of alternative designs and the significant investment and operating expenses associated with separation tasks in the chemical, petrochemical, and related industries. Most of this research has dealt with heuristic or optimization-based search methods to tackle the combinatorial nature of the problem-certainly the main difficulty in devising distillation sequences for more or less ideal mixtures (e.g., Nishida et al., 1981). However, many industrially important separation methods, including extraction, decantation, crystallization, membrane permeation, and the distillation of azeotrope-forming mixtures are not generally able to produce sharp separations and are often used in combination with mass separating agents, calling for complex separation schemes with recycles to achieve the desired functionality. All previous work on the conceptual design of such complex separation systems of which we are aware has dealt with distinct types of processes only, e.g., azeotropic distillation schemes, and was restricted to the problem of separating binary mixtures by means of additional species acting as mass separating agents (e.g., Doherty and Caldarola, 1985; Stichlmair et al., 1989; Rajagopal et al., 1991; Laroche et al., 1992b). The approaches proposed thus far essentially assume a flowsheet structure and test its feasibility using a graphical representation if separability limits in ternary diagrams. This kind of technique is difficult to automate and does not readily generalize to the synthesis of hybrid separation schemes, i.e., processes based on various different separation methods, nor to systems with more species because of the difficulty of visualizing the product compositions achievable in the individual separation steps. The primary objective of this work is to lay a foundation for the computer-aided synthesis of general separation processes, including in particular the kind of complex process schemes not considered in most separation sequencing studies. While the methodology presented here t Present address: Aspen Technology,Inc., Ten Canal Park, Cambridge, MA 02141.
lends itself to automation, it is also suitable as an outline for systematic separation process synthesis even if not all steps are automated. The strategy we propose builds upon the traditional approach to consider streams sequentially and to choose the separation methods and, if necessary, mass separating agents before determining which splits are actually feasible and fixing the operating conditions. Despite this problem decomposition,it is possible to arrive at quite intriguing flowsheets, especially when species already present in a problem can function as mass separating agenta. We shall show that recycling may be strategically employedto improve separationsand possibly decrease the number of units required. To illustrate the methodology and to emphasize the practical significance of the methods to determine the feasible products of azeotropic distillation columns presented in two previous papers (Wahnschafft et al., 199213; Wahnschafft and Westerberg, 1993),we concentrate on example problems involving the use of distillation-based methods for the separation of azeotropic mixtures. It should be pointed out that we are more concerned here with the systematic generation of alternate feasible process flowsheets aa a presynthesis requirement than with the flowsheet evaluation and the efficiency of the search. The proposed methodology is meant to provide a framework within which one may use whatever technique appears most suited to narrow down the number of separation methods and agents to be considered and to determine the range of feasible product compositions for each separation step. For suggestions concerning the implementation of the proposed strategy, the reader is referred to the descriptions of a prototype system named SPLIT (Wahnschafft et al., 1991, 1992a). To introduce the fundamental idea on which we will base a methodology for the systematic synthesis of general separation systems, let us fiist consider a simple example problem. Suppose one has to separate a binary mixture of n-butanol and water consisting, for example, of 35 mol % alcohol and 65% water. The physicalproperty behavior of the butanol-water system has been studied extensively and is reported to exhibit a minimum-boiling, heterogeneous azeotrope (e.g., Perry, 1950). For total liquid compositions between about 55 and 97 mol % water, a partial liquid immiscibility leads to the formation of an
0888-588519312632-1121$04.00/0 0 1993 American Chemical Society
1122 Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993
-
---
Distillation
Distillation
f
(a)
Butanol
\
Decantation
-Water
Feed Fa.
Az. Butanol
(c)
A~
Fam
Az. s5 s4 Water
Butanol
Water
Butanol
Water
Butanol
&Water
Figure 1. Synthesis of a simple heterogeneous azeotopic distillation configuration based on the separability limit for the individual operations.
alcohol-rich and a water-rich phase. For now we will consider only distillation and decantation as possible separation steps. The question is which process options exist to separate the mixture into essentiallypure products. Figure l a shows the separability limits for distillation and decantation dependent on the mixture composition. Since the feed composition is in the immiscibilityregion, one option would be to decant it (Figure lb). Neither of the resulting streams meets a product specification, but each can provide a product through distillation. For both these streams, SIand SZ,the maximum possibleseparation by distillation yields the azeotrope as the top product, whose composition is in the range of the split performed by the previous decanter. Thus it is possible to recycle the two distillate streams to the condenser/decanter,which results in the first feasible flowsheet of a simple heterogeneous azeotropic distillation process (Figure IC,left). An apparent alternative is to first distill the feed to obtain some n-butanol and the azeotrope (Figure lb). For the distillate, which is at almost azeotropic composition,only decantation is appropriate as the next separation step. The composition of the organicphase leaving the decanter is in the range that is covered in the first distillation, allowing for recycling. The aqueous phase, however, contains too much alcohol. A second column can separate water from the water/alcohol azeotrope, removing the excess alcohol, therefore, as part of the near-azeotrope top product. Observingthe separation ranges depicted in Figure la, this stream can be recycled to the decanter. The overlappingseparationranges meet the product purity specification,which can be specified as a lower bound on the higher fraction of butanol and an upper bound on its lower fraction. The resulting process configuration is shown as the second scheme in Figure IC(right). Admittedly, a separation problem with only two components does not appear very challenging, but some important conclusions can be drawn from this example. To ensure that all promising flowsheet configurationsare taken into account, we should be able to generate the process alternatives systematically, for which currently no approach appears possible other than a decomposition into subtasks which can be dealt with in a sequential manner to make the problem tractable. Also, to configure processes sequentially that employ combinations of incomplete separations to accomplish sharper splits, a representation is needed of the separation functionality of every step. To decide if the compositionof a nonproduct stream is one which could be processed in a step already
existingin the proposed partial solution,we shall generalize the notion of binary splits. The use of a part of the existing process configurationto completethe separation required for astream generatedby that structure makes the problem decomposition approach somewhat recursive in nature. In this sense, we believe that the strategy we propose is fundamentally different from earlier approaches. The paper begins with an outline of the main steps of our algorithm for the systematic synthesis of complex separation systems. We then discuss each step in more detail, drawing upon a number of examples. Lastly, we address the complicationsinvolved in keeping a record of the alternative separationsystem configurationsdeveloped in a step-by-step fashion and present results from the application of our approach to the separation of an extremely nonideal quaternary mixture. 2. The Algorithm
Figure 2 shows the main steps of the proposed problem decomposition approach which allows for the sequential synthesis of complex separation systems. At the points marked with a "v-, multiple choicesmay be made, resulting in different flowsheets. Processes are created by choosing separation steps until all outlet streams meet product specificationsor can be recycled. The search for suitable separation steps to process a nonproduct stream is guided by an analysis that determines which splits still have to be performed, i.e., which splits have not been carried out anywhere in the partial flowsheet under consideration. Separation methods and operating conditions are essentially chosen such that either (i) the separations between components with different final destinations are maximized or (ii) certain separations are performed to the minimum only so that the outlet streams can eventually result in products after separating sharply between other key components. If sharp splits are feasible, only the first criterion needs to be considered which then leads to the generation of simple separation sequences. In cases where sharp separations are infeasible or very difficult, both criteria should be considered independently to synthesize alternate process schemes. Processes which employ nonsharp splits to accomplish sharp separations usually require internal recycles. These recycles can serve two distinctly different purposes. Streams may be recycled because they facilitateseparation, i.e., because they contain species functioning as mass separating agents, or because a flowsheet already has
Ind. Eng. Chem. Res., Vol. 32, No. 6,1993 1123
Figure 2. The equential procadwe for the synthesis of altsrnate complex separation schemes.
separationsteps whichlead to alldesiredproducta. Thus, in our sequential design approach, recycle decisions are made based on two different considerations: (i) mixing requirements which describe the conditions a stream must meet in order to be used to supply a mass separating agent to a separation step (the representation of mixing goals will have to take into account that separation agents do not always have to be provided pure hut may, for example, berecycledasazeotropes) and (ii) “relative”concentrations which allow one to determine which combination of units that perform nonsharp splits can lead to a complete separation. We will term a recycle stream satisfying the first criterion aprimary recycle because it is essential for the function of a separation step. Streams that are recycled, not because they contain species acting as separationagents but toavoid redundantsplits inaproceas, aretermedsecondaryrecycles.A specialcase of secondary recycling is what we will call a “range-extending recycle” in which one makes use of a device existing in a partial flowsheet to carry out separations for which it was not intended initially. In the following, we will discuss the individual steps of the synthesis procedure from the problem specification to the selection of alternative splits and recycle decisions in more detail and illustrate their application to several example problems. Fixing the Separation Requirements. There are various ways in which a separation problem can be stated. In practice, simple purity requirements are often inadequate to express the nature of a problem. The reason is that recycle, waste, or even product streams often do not have to be totally pure but may tolerate certain species in smaller amounts than others. Thus, one frequently has
to combine purity and recovery requirements, or, as advocated here, one can also impose a combination of purity and impurity constraints. Such specifications are as easily transformed into split requirementsfor individual separators as recovery specifications but are more useful forthe sequential synthesis of complex separation systems because the internal flow rates and recoveries may change significantly once recycle loops are closed while the target compositions can remain more or less constant. Two basic concepts in the representation of separation functionality and requirements used in this work are relatiue concentrations and binaryseparation ranges. To explain these, let us consider the separation of a hypothetical ternary mixture (Figure 3). This feed mixture consists of 36% A, 24% B, and 40% C. The split into the products PI and PZwould perform separations between A and B, and A and C as indicated by the projections of the mass balance line onto the left and the bottom side of the composition triangle. As seen on the left axis, Pz contains A and C in a ratio of about 8 parts to 92 parts, while PI is essentially pure A. The relative fraction between A and B in PZis about 12%, as seen from the projectionthroughPptothebottomsideofthecomposition diagram. Thus one could say that the split into PI and Pp carries out the A/B split with relative fractionsbetween 0.12 and 1,and the A/C split with fractions between 0.08 and 1. The relative concentration between B and C in both products is unchanged, as no separation is performed between these species. In general, the functionality of a separation device performing a nonsharp separation of a feed with several components can be characterized in terms of the separation ranges, Le., the upper and lower relatiue concentrations for all pairs of components present in the product streams. Relative concentrations (mole fractions) x;j in a multicomponent mixture can be calculated using the component flow rates f i i or absolute fractions x;j:
On the other hand, separation requirements may be stated as bounds on the relative fractions between the component pairs in the desired products. Unlike process Specificationsbased on recoveries,binary separationlimits do not refer to the feed composition(s) but simply require any viable flowsheet to have operations which together cover all ranges of relative concentrations prescribed in binary separation tasks! An example of a ternary mixture separation problem that was introduced in a previous paper (Wahnschafftet al., 1992b) shall illustrate the use of purity/impurity specifications: the separation of a mixture consisting of 36% acetone, 24% chloroform, and40% benzene. In this system, chloroform and acetone form a maximum-boiling azeotrope,and there is a residue curve boundary traversing between this binary azeotrope and the heaviest species, benzene. The product requirements shall now be the following (Figure 4): In the chloroform product, practically no benzene can be tolerated (ppm range only) and the minimum chloroform concentration is to be 99%. This specification means that the chloroform product can contain some acetone. The two other outlets of the separation system shall consist of acetone and benzene, respectively, with minimum purities of 99.9 % We can now express the problem in terms ofthe relative or binary concentrations that must be achieved between each pair of species to be separated to produce these outlet streams, calculated from eq 1.
.
1124 Ind. Eng. Chem. Res., Vol. 32, No.6, 1993
Q
Feed
Figure 3. Relative concentrationa and separation ranges illustrated in a ternary disgram.
2 4 % ChbmfOrm 4 0 % 8ewene
1:
Figure 4. Specification for the separation of a mixture of acetone, chloroform, and benzene.
A viable process for our example problem must have operations which achieverelative concentrationsof acetone to chloroform of more than 99.9% for the acetone product and less than 1%for the chloroform product. Similarly, the separation between benzene and chloroform must be performed to relative fractions above 99.9% and below lo-'%,while the acetone/benzene split has to be carried out to relative acetone concentrationsof above 99.9% and below 0.1% (Figure 5). Note that these relative concentration limits definethe minimum separation requirements for each binarysplit. However, tomeet the overall product specifications, not all binary separations may be carried out to theminimumsplitrequirementsonly. Nevertheless, knowing the minimum separation requirements for each combinationof components isvery useful for the synthesis of complex separation processes where often not all splits are equally difficult. As the example will demonstrate, performing the most difficult splits up to the minimum separation requirements only can lead to interesting process schemes. Determinationof Binary Concentrations. For every stream that is not a desired product of the separation system, we first calculate its binary or relative concentrations between all the species present in a nonnegligible
ACSMn$
FSBd
Chbmlorm
ACeMns
BsnzeM,
F e e d !
FBsd
01%
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Benzene
Chlomlorm
F i y m 6. Binary ssparation taaka for the acetonddIlomfoW benzene example and relative concentrations between the Pairs of species in the feed.
amount, using eq 1. Figure 5 also shows these relative concentrations on the vertical axes for the combinations of components present in the feed mixture of acetone, chloroform,and benzene. The analysis of streams in terms of binary concentrations is the key to determine which splitsremain to be carried out. Only for those splits which are not performed by any of the units already existing in a partial solution, additional separators are required. Relative concentrations also enable one to make recycling
Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993 1125 decisionsbased on the separation functionalityof the units in a partial flowsheet. To represent the functionality of a separator that can perform only partial splits, we suggest the use of the so-called separation ranges, Le., the upper and lower relative concentrations between the pairs of components present in its outlet streams. Irrespective of whether or not mass separating agents are used, a nonproduct stream can be recycled or, more generally, can be processed in a unit already existing in a partial flowsheetif all its relative concentrations that do not meet product specifications fall in separation ranges covered by the combination of separation steps in the partial flowsheet considered. If such a stream is not recycled, a process is generated with more units than required. Since at this point in our example problem recycling is not yet an issue, we shall continue this discussion later. Selectionof Separation Methodsand Agents. Once the separations required for a given mixture, i.e., feed or intermediate outlet stream, have been determined,suitable separation methods and, if necessary, mass separating agents must be identified. Selecting the most promising methods and separating agents, i.e., entrainers, solvents, membranes, adsorbents, etc., is obviously of crucial importancewithin the general separation system synthesis problem,but the overall synthesis methodologypresented here is applicable regardless of the way these choices are made. King (1980) and Kelly (1987) addressed the general separation system synthesis problem, assembling lists of general processing considerations that suggest, for example, the early removal of hazardous,corrosive,or thermally unstable substances, and other considerations useful to prioritize the required separations. Moreover, as a guideline for the method selection problem, King classified separation methods according to the underlying physical principles and significant property differences. More recently, several prototypes of knowledge-based systems for separation process synthesis have already been reported, for which criteria and strategies for the selection of separation methods have been captured in relatively simple rules (e.g., Engelmann et al., 1989; Barnicki and Fair, 1990; Garg et al., 1991). Thus, this topic is not addressed in this paper. As for the selection of separating agents for specific methods, the literature also is quite extensive but not of central interest to this work. To illustrate the concepts developed here for the systematic synthesis of complex, hybrid separation processes, we will focuson homogeneous and heterogeneousazeotropicdistillationand liquid-liquid extraction, probably the most widely used vapor-liquid and liquid-liquid equilibrium-based separation methods for azeotropic mixtures. Homogeneous Azeotropic Distillation. The selection of entrainers for homogeneous azeotropic distillation has been discussed in two preceding papers (Wahnschafft et al., 1992b;Wahnschafftand Westerberg, 1993). According to this analysis,both the trajectories of residue curves and the effect of candidate entrainers on the relative volatility between the species to be separated may have to be considered to identify promising separating agents. Vapor-liquid K-values at infinite dilution are particularly convenientto evaluate the impact of a potential entrainer on the volatilities between the azeotrope-formingspecies. Heterogeneous Azeotropic Distillation. The principle of heterogeneous azeotropic distillation processes for the separation of azeotropic or closely-boiling mixtures is to exploit the immiscibility between one (or some) of the speciesto be separated and a component actingasentrainer
to avoid distillation in the zone where the relativevolatility between the originalspecies approaches unity. To separate a binary azeotropic mixture, an azeotroping agent must induce a heterogeneous minimum-boilingazeotropewhich must have the lowest boiling point in the multicomponent system so that it is possible to remove it overhead in a distillation column. In addition, it has to be ensured that the azeotropic composition is in an immiscibility region that covers parts of the two distillation regions in which the azeotropic constituents can be obtained as bottom products of distillation columns. Matauyama (1975) examined the question of which criteria entrainers must satisfy for heterogeneous azeotropic distillation by generating all conceivable combinations of residue curve topologieswith possible locations of miscibility gaps. He showed that there is a considerable number of such combinations which allows for the separation of the azeotropic constituents. However, all these cases appear to be based on the principle described above. Liquid-Liquid Extraction. Extraction is applied mainly to carry out dilute separations for which large amounts of light species would have to be removed overhead in distillation. As a bulk separation method it is best suited to separate species with similar molecular structure from the rest of the species in the feed mixture, as is the case in the separation of alkanes and aromatics. Extraction is usually performed at temperatures well below the boiling points of the species involved which makes activity coefficients (7)a better means of evaluation than vaporliquid K-values. Suitable solvents must be largely immiscible with the primary material in a stream and have to effect a reasonably high distribution of the species to be separated (say A and B). For a quick estimate of the separation factors a solvent can produce, Hampe (1986) suggested one consider the limiting selectivitythat would be obtained if the solute species were infinitely dilute in both the extract (’) and the raffinate (”1 phase: SP;, = ( Y A m / 7 ” B w ) /
(Y’A~/Y‘B-)
(2)
It should be pointed out that the purpose of this brief discussion is only to provide some background on the separation methods consideredin the examplespresented here. A comprehensivereview of the work on the selection of mass separating agents, even if it was only for heterogeneous azeotropic distillation and liquid-liquid extraction, could be the subject of a publication in itself and is beyond the scope of this paper. Searching of Reachable Products. As long as only methods that can provide sharp splits in every separator are considered, once a separation unit operation is chosen one can simply decide on the split location and quickly obtain a base-casedesign. However,when it is not obvious which separations are actually feasible for a selected method, the scope of process synthesis broadens from the selection between competing alternatives to the prior generation of feasible separation processes. As the products attainable in any single separation step depend on the composition of its feed@),one of the basic problems in the synthesisof these separation schemesis to determine the separations achievable for a fixed feed composition. For the distillation of ternary azeotropic mixtures, a method to assess the feasible product compositions has been presented by Wahnschafft et al. (1992b). Using this method,we can quickly determinethe product composition regions for our example feed mixture, as shown in Figure 6. However, because of the limitation the graphical representation in a ternarydiagram imposes on the number
1126 Ind. Eng. Chem. Res., Vol. 32, No.6, 1993 Benzene 80.1 % L
O
o Feed
,
0
Chloroform
Acetone 56.5OC
1
0.6
0.4
0.6
0.2
61.2k
+ XAc.IOn.
Finure 6. Feasible omducts for distillation of the acetanelchlorofonn/bene feed mixture determined using the method presented by wahnschafft et al. cisszs).
of species that can be handled and because separation metbodsotherthandistillation may have to beconsidered, we need to conceive other, more general techniques to determine which separations are feasible. One way of searching the space of feasible product compositions for a single separation step is the use of a simulation model with varied design and operating parameters. Since one is generally interested in the best separations achievable, some parameters may be set to practical limits. Thus, for most unit operations only one or two parameters have to be varied to search for the interesting products. According to our experience with the Aspen Plus simulation program (Aspen Tech, 1992) installed on a DEC 5000 RISC Station, such a search can be performed in reasonable time, Le., a few seconds or at the most some minutes, even when one deals with extremely complex mixtures. The most complicated example studied in the context of the work reported here is the separation of a hypothetical six-componentmixture consisting of acetone, water, chloroform, pentane, hexane, and methanol (Thomas, 1991). Eleven of the 15 binary combinations of its constituents are known to form azeotropes, both maximum- and minimum-boiling, bomogeneous and heterogeneous. Moreover, there are numerous ternary azeotropes, of which only some are reported by Horsley (1973), and even apparently some quaternary azeotropes. The use of simulations to determine extreme designs has the advantage that no preliminary knowledge of separation boundaries is required and that there is no limit to the technique with respect to the number of componentsin a problem. However, this is not to say that it would not be better to have more efficient, dedicated methods to find the best separations possible. The separations feasible in azeotropic distillation provide a
good example for the trade-off between the computational effort invested in the search for feasible splits by simulations and the possibilitythat interesting separations are overlooked. One might expect that by setting the reflux ratio of a continuous distillation column to a very high value it can be ensured that the best possible separations are realized, but we know now that this is not necessarily true for azeotropicsystems(e.g. Van Dongen, 1983;Laroche et al., 1992a; Wahnschafft et al., 1992b). Distillation boundaries for totalreflux columns can be crossed at finite reflux ratios when the relative volatilities between the azeotrope-forming speciesdepend on the presence of other Components in the mixture. Such a situation is shown in Figure 6. As could be predicted on the basis of K-values at infinite dilution (Wahnschafftand Westerberg, 1993), benzene increases the relative volatility between acetone and chloroform, which makes it possible to surpass the total reflux boundary. For efficient synthesis and design of general separation systems, however, such a qualitative assessment is not informative enough, and methods like the ones presented by Wahnschafft et al. (199213) and WahnschafftandWesterberg (1993)todetermine the exact product composition regions for ternary nonideal distillation would be extremely useful. However, for many unit operations, includingthe distillationof mixtures with more than three species, we presently know of no technique other than repeated rigorous performancesimulations-or the obtaining of experimental results. Selecting Interesting Splits. Given the ability to determinewhich splitsa separation methodcanaccomplish for any particular stream under consideration, one has to decide which of these splits will lead to different flowsheet structures and are thus the most interesting. Instead of analyzing the absolute compositions of the possible products of the separation step, we suggest one consider
Ind. Eng. Chem. Res., Vol. 32, No. 6,1993 1127 0.2
1
0.9
0.1 6
0.8 cn
1
p
ac .-
e
e
s
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E
0.7
0.12
i
8g
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0
0.08
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0.5 0.04 0
0.4
0
-
Acetone Benzene Chloroform-Benzene
0
0.3 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Figure 7. Absolute separation ranges between the pairs of components in the products of a distillation column operated at high reflux and varied D:F ratios.
the relative concentrations between the pairs of components present in the products. The operating conditions of interest are those where either (i) the Separations between components with different final destinations are maximized, or (ii)the outlet streams can result in products after separating sharply between other key components; i.e., certain separations are carried out only to the minimum necessary. The idea underlying these criteria is best illustrated using our example problem. Figure 7 shows a plot of the absolute separation ranges = Ixij+&
1
- x .vproduot . 2I
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
D:F
D:F
sij
0
(3)
Le., the differencesin the relative concentrations between acetone and chloroform,acetone and benzene, and benzene and chloroform, obtained by simulating a distillation column with feed F (Figure 6) with a fairly large number of stages and high reflux at varied distillate-to-feed ratios (D:F). If the distillation were not confined by the azeotropicbehavior of the mixture, we would observe that two of the separation ranges reach a value very close to 1 at discrete D:F ratios, corresponding to sharp splits between adjacent key components. Distillation boundaries limit separation ranges to smaller values but there are still discrete conditions under which binary separations are maximized, as seen in Figure 7. At a D:F ratio of about 0.31, a maximum in the separation between acetone and chloroform is reached. At higher D:F ratios, the column moves from performing the direct split toward the indirect split. Increasing the distillate-to-feed ratio to 0.6 forces all acetone and chloroform into the distillate, resulting in a maximum separation between benzene and the other two components. As D:F is increased further, benzene must appear in the distillate, too, and all separations are reduced. Note that only the separation ranges between adjacent key components exhibit sharp maxima, in this case the separations between acetone and chloroform and between chloroform and benzene. By analyzing the absolute separation ranges or, as Thomas (1991)suggested,the differencesin the recoveries between the (adjacent key) pairs of components in the outlet streams of a separation device like a distillation column, one can quickly identify the extreme separations
Figure 8. Relative concentrationsbetween the pairs of componenta in the bottoms product of the column distilling the acetone/ chloroform/benzenemixture.
achievable. Performing some separations beyond the minimum requirements enables other splits, in particular the most difficult ones, to be carried out just to the minimum. To recognize splits that just lead to acceptable purities, one can compare the relative concentrations between the speciesin an outlet stream to the binary split requirements postulated at the beginning. In our example problem, the most difficult separation is the one between acetone and chloroform. According to the binary separation tasks (Figure5), the maximum relative concentration of acetone to chloroform tolerable on the chloroform-richside is 1%. Figure 8 is a plot of the relative concentrations between pairs of components in the bottom product of the distillation column with the feed F. We observe that at aD:F ratio of about 0.57 the relativeconcentrationbetween acetone and chloroformreaches the 1% limit. Thus, there is a third operating condition of interest at which the distillation column produces a bottoms compositionwhich could be further processed into desired outlet streams simply by separating between chloroform and benzene. Figure 9 depicts the mass balance lines representing the three distinct splits found by simulating a total reflux column, searching for maxima in the absolute separation ranges, and identifying operating conditions where minimum separation requirements are met. If the indirect split is carried out first, one is left with the acetone/chloroform mixture for which a suitable separation method (and most likely a mass separating agent) would have to be found. The only distillationbasedmethod for azeotropicmixturesthat does not require an agent is pressure-swingdistillation. However,not many mixtures exhibit sufficient difference in the slopes of the pure component vapor pressure curves to allow for a significant shift of the azeotropic composition by pressure change (Barnicki and Fair, 1990). In any case, because we are more interested in investigating if the presence of benzene could not be exploited in other schemesto separate the mixture without external mass separating agents, we will not pursue the option of the indirect split as the first separation any further. For the direct split shown in Figure 9, the distillate is clearly a desirable product but the bottoms still contains
1128 Ind. Eng. Chem. Res., Vol. 32, No. 6,1993 Benzene 80.1 Oc 4
0
>
0
Chloroform
Acetone 56.5 O C
1
0.8
0.6
* Acetono
0.4
02
61.2%
Figure 9. The three most interesting splits for the feed mixture found by simulation.
all three species and thus does not satisfy any split requirement. In the case of the intermediate split, on the other hand, the bottom product contains only a small amount of acetone-this outlet stream satisfies the requirements for the recovery of acetone from the benzene and chloroform products. Since these two products are in principle compatible because one is a distillate and one is a bottom product, there isa fourth interesting separation option, namely to change the overall inlet composition to be able to produce the two desired outlet compositions in one step. Such a composition change is typically accomplished by recycling material, as we shall now discuss. Recycles. Primary Recycles. As indicated in Figure 10, to accomplish a shift of the total inlet composition onto the mass balance line between the two desired products, in principle any stream with a composition in the shaded area could be added. Assuming that B1 is obtained in the first separation step, a second distillation column readily produces sufficiently pure benzene and chloroform. Figure 10 also shows the resulting process flowsheet, in which recycled benzene is used as the added material to enable the separation in the first column-this flowsheet is the one analyzed in more detail by Wahnschafft et al. (199213). They showed that the recycle rate of benzene needed to recover enough acetone in the first column is actually not as high as a first analysis might suggest becausethe residue curve boundary can be crossed, an effect that can be enhanced through intermediate heat exchange. While in this particular example we have used a pure component as a recycled mass separating agent, it is important to note that, in the systematic synthesis of complex hybrid processes, one may have more flexibility with regard to possible compositionsof streams functioning as mass separating agents. Consider, for example, the
separation of a mixture of toluene, ethanol, and water. Figure 11shows the distillation boundaries and the region of liquid immiscibility at ambient temperature for this ternary system. If all three components are desired in essentially pure form, three complete binary separations have to be performed. As in the butanol/water example the feed can be decanted into a water-rich phase (1)and another, in this case,toluene-richphase (2). Distillationof the former could provide pure water (4) and the mixture (31, which lies on a distillation boundary. To achieve further separationof this mixture by distillation,a separatingagent is needed. Because toluene satisfies the criteria for an azeotropingagent for the separation of ethanol and water by heterogeneous azeotropic distillation, we will first investigate which separations become possible when toluene is added to 3. It turns out that addition of toluene would allow for the production of pure ethanol (6) as the bottoms of a second distillation column. The distillate composition is limited by the distillation boundary and could at best correspond to the composition of the ternary azeotrope (5). If the stepwise synthesis procedure is continued assuming that the streams 5 and 6 are available, we must keep in mind that a stream will have to be mixed with 3 to achieve an overall concentration on the mass balance line between the compositions of 5 and 6. Just as in the above example, this mixing requirement actually defines a quite broad range of concentrations for suitable recycle streams, shaded gray in Figure 11. Stream 2, for instance, is in this composition range. However, this stream has to be subjected to further separation, since otherwise there is no operation providing a product with the necessary high relative fraction of toluene. The recovery of toluene can be realized by another distillation column, whose top product could be at the most on the
Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993 1,129
o Feed e
Intermediate and
Figure IO. Smtheaisof a two-eolumnsequence for the separation of the acetonelchloroformlbmne mixture wing some benzene 88 primary recle.
other distillation boundary a t the composition labeled 7. All separation tasks are now performed, and it is possible to mix stream 7 with 4 to provide the separating agent that allows for recovery of ethanol. Finally, we must deal with the stream with the composition close to that of the ternary azeotrope (5). For reasons to be explained in the section on secondary recycles, this stream ought to be recycled to the decanter. Figure 12 shows the developed flowsheet structure. Note that we would have arrived at the same process flowsheet if we had first considered distillationof 2 and posteda mixingrequirement torecover ethanol from the distillate. In either case, the separating agent is not provided as a pure component! Theuseof recyclingor, moregenerally, mixingto reduce the number of separation steps required is a concept of wide applicability in the synthesis of complex separation systems. Quite frequently it is possible to use species present in a multicomponent mixture as mass separating agenta for splits between other components, but these species are rarely present in the exact ratio needed. To obtain two outlet streams of desired composition as products of a single separation step, the totalcomposition of the inlets has to be changed to satisfy the mass balance for the intended separation by providing one or more auxiliary streams, usually as recycles. Note that moving the mass balance line does not imply that an auxiliary stream actually has to be mixed with the main feed-it might be supplied as a separate feed, depending on the separation method in question. To ensure that it is in principle possible to obtain two productsof desired composition within one separation step, one should check that each of the species present in the inlets is expected in at least one of these product streams.
In our previousproblem, the acetonelchlorofodbenzene separation (Figure lo), this would, for example, not be the case if the distillate of the direct split (pure acetone) was to be paired with the bottoms product of the indirect split (pure benzene), because chloroform does not appear in either of these two products. While this consideration is trivial for mixtures of few components, the situation is not necessarily the same for the separation of multicomponent mixtures. For the theoreticallyfeasible combinationsof interesting outlet compositions of a separation step, one can assume the availability of auxiliary streams that will enable these separations. A flowsheet built upon such an assumption can function correctly only if it produces this auxiliary stream somewhere else, astream whose composition often does not have to be fixed a priori. The test whether or not a stream can function as a primary recycle or, in other words, whether or not it can be used to supply the separating agent to the separation step entails the consideration of the following: (i) The first consideration is the mass balance. Mixing with the main feed F to a separation step must bring the overall feed composition onto the mass balance line between the desired products. This requirement defines a constraint on the range of feasible compositions for a primary recycle stream. (ii)The second consideration is the physical constraints arising from the nature of the separation method. Aa an example, if the agent fed close to the top of an extractive distillation column is contaminated with the species it is supposed to make less volatile so as to remove them with the bottoms of the extractive column, the separation will not work as desired because some of these contaminating
1130 Ind. Eng.Cham. Res., Vol. 32, No. 6, 1993
Ethanol 78.5 O C
/
p - 1 bar
84.0 C
Toluene 110.6 O C
Water 100.0 OC
F i y r e 11. Synthesis of an seeotropic distillation proceae for the separation of a mixture of a mixture of ethanol, toluene, and w a t e d e species used 88 entrainer me not required in pure form.
F
Figure 12. The resulting proceas flowsheet for example problem 11.
species will, hy being at the top of the column, escape with the distillate. A similar argument holds for liquid-liquid extraction or, in general, all "countercurrent" processes in whichthe agent and the primary feed have to he introduced at different stages. To determine if a stream could in principle he used to shift the overall composition of the inlets to a separation step onto a desired mass balance line in the n-composition space, i.e., to satisfy a mixing goal, we first have to test if the line connecting the original feed composition with the auxiliary stream composition in question (the mixing balance) intersects with the mass balance line of the intended separation (Figure 13). Using the composition vectors of the two products PI and Pz,XI and XZ, and of the original feed (XF) and the recycle ( x d , an intersection implies that the set of equations x,
+ S(XR - X,)
= x,
+ t(XZ - x,)
(4)
has a solution where the parameters s and t are between 0 and 1. The closers is to 1,the higher is the recycle rate that would be required to achieve the desired change of the inlet composition.
C2
P1 c1
Figure 13. Test for suitability of a candidate recycle eompcmition R to achieve an overall feed compaeition on the mans balance line between two desired products. PIand Pn.
In a ternary system, the two balance lines intersect if eq 4 is satisfied for two species, iand j . In general, n 1 component mass balances have to he tested to ensure intersection in the n - 1dimensional composition space of n-component mixtures. However, in separation prohlems involving more than three species it can he too restrictive to demand an exact intersection of the two balance lines because process schemes based on nonsharp splits may still work if the internal concentrations deviate slightly from the ones assumed initially in the sequential synthesis approach in which we "break" the recycle loops.
Ind. Eng. Chem. Rea., Vol. 32, No. 6,1993 1131 Benzene 80.1
J
Acetone 56.5
OC
/ \\\ 1 \I\
\
D
Intermediate and Drodwt comDosllions open loop
-
Intermediate and product compoaltlons converged with recycle
-
07
61.2 C '
'C
Acetons
Figure 14. A threecolumn sequence requiring a secondary recycle for the acetonelchloroformlbeneene separation.
Thus, one can generalize the test for the suitability of a stream to achieve a desired change of composition to calculatingthe minimum distance between the separation mass balance (the vector connecting the two desired products) and the mixing mass balance (the vector connecting the original feed and the potential recycle composition). This distance, illustrated in Figure 13 for a four-component mixture, is the length of the vector Ip - q1 determined from (p - q ) ( x R
- XF) = 0
(5a)
and
(P- P)(X, - XI) = 0 (5b) where p and q are the pointa of closest approach on the two balance lines at which the vector joining p and q is perpendicular to the directionsof both mass balance lines (Figure 13). The compositions p and q can he replaced by the parametric forms of the equations of the mass balance lines
xF + s(xR- xF) and x1+ t(x2- x1) so that the parameters s and t have the same significance
as in eq 4. A stream can be considered a candidate for a primary recycle if s and t are between 0 and 1 for the pointa of closest approach and the minimum distance between the mixing and the separation mass balance lines is sufficientlysmall (e.g., less than 0.01) in the normalized composition space. Again, for certain separation steps additional restrictions on feasible recycle compositions may have to he observed. Thomas (1991, p 30) suggested an alternative approach todetermine what should or could be mixed wth aprocess stream so as to make a desired separation feasible from a mass balance point of view. A linear programming problem formulation may be used that takes into account
the possibility of mixing (fractions of) several candidate recycle streams toensure thattheinitiallypostulatedmass balance is satisfied exactly. However, such an approach may produce processes with more recycle streams than necessary, causing control problems. Secondary Recycles. To illustrate the significance of what we have defined as secondary recycling, let us return again to the acetonelchlorofordbenzene problem. A second process alternative for this problem is based on using the first column to perform the direct split, i.e., to produce acetone as overhead (Figure 14). The corresponding bottom product still contains a nonnegligible amount of acetone. This stream can be further separated in a distillation column whose most interesting products are found to be a bottoms of pure benzene and a distillate mixture of acetone and chloroform. This latter distillate stream cannot be recycled because the separation range required between acetone and chloroform is not yet recovered; Le., no separation produces sufficiently pure chloroform. However, a third column can yield pure chloroform as distillate product. Ita bottoms composition can lie at best close to the maximum-boiling azeotrope between acetone and chloroform. Now all separation ranges are covered. Thus, no additional separation step is needed. The azeotropic stream is fed back into the first column (as a secondary recycle) since ita relative composition between acetone and chloroformfalls in the range of separation performed in the first unit (Figure 14). Figure 14 also shows the resulting flowsheet and the separation ranges between acetone and chloroformrealized in column 1and column 3 in the open sequence and for the converged process, as determined by rigorous simulation. Lastly, there is a third option that resulta from performing the intermediate split first, i.e., the separation into two binary mixtures. The benzenechloroform mix-
1132 Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993
Acetone 56.5 OC
Figure 15. The second three-column sequence.
ture is then readily separated into its constituents by distillation,while acetone could be distilled overhead from the acetone-chloroform mixture. Because the separation between acetone and chloroform is accomplished again by the combination of the first and the third columns, the maximum-boilingazeotrope obtained in the bottom of the third column has to be recycled to the first unit. The synthesis steps and this last flowsheet alternative are illustrated in Figure 15. Since complex separation processes can exhibit quite complicated trade-offs (e.g., Ryan and Doherty, 1989),it is not obvious which of the process schemes will be economically more attractive. The trade-offs in this example are likely to depend on the separation requirements. The fist three-column sequencecan easily produce highly pure chloroform whereas the recycle rates required in the two-column sequence increase with the purity constraints, countering the investment cost advantage of the two-column process. In any case, a decision should probably not be made by neglecting flowsheet options, which could happen without a systematic synthesis approach. Range-Extending Recycles. The three process schemes synthesized for the acetone/chloroform/benzene example illustrated the concepts of primary and secondary recycling. It was said before that recycling is possible once all binary separation ranges between the componentspresent in a stream either meet the split requirements or fall in ranges covered by the units in the partial flowsheet considered. However, there are cases where one might attempt to recycle a stream although not all splits required for it are performed in the sequence. The reason is that a secondary recycle can change the functionality of a separation unit to perform additional splits because of the change of the total inlet composition caused by the recycle. It is a case of a column already in the sequence being able to do more if more were asked of it.
To illustrate the idea, we will apply the synthesis methodology outlined here to another example problem, namely the separation of a mixture of ethanol and water. For the drying of alcohol, both extractive distillation and heterogeneous azeotropic distillation processes are used widely in industry. The principle of heterogeneous azeotropicdistillation was first suggested around the turn of the century. Since that time, a number of variations of the separation sequence proposed initially have been reported in the literature based on two, three, and four distillation columns. Using benzene as the azeotroping agent, Ryan and Doherty (1989)analyzed these sequences to obtain insights into their economics. Interestingly enough, although the ethanol/water separation has been studied for many decadesand it is actually a rather simple process synthesis problem, the process scheme suggested most recently is one of the potentially most economic, depending on the composition of the ethanol/water feed. Followingthe steps of the synthesis approach described so far, one readily arrivesat all flowsheetoptions discussed by Ryan and Doherty (1989), with two exceptions. One is the originallyproposed four-columnsequence. However, in this process there are two columns that perform identical tasks, which can be avoided by recycling. Indeed, the economicanalysiscarried out by Ryan and Dohertyshowed that the four-column sequence is never competitive. The second process configuration which we would not synthesize using the conceptsdiscussed so far is the one which requires two columns only. As we will see, it is the use of a range extending recycle that makes this flowsheet feasible. Figure 16 shows the residue curve map of the ternary system, where we used toluene instead of benzene. However, this makes little difference. The ways in which benzene and toluene enable separation by azeotropic distillation are essentially identical. Toluene forms minimum-boilingazeotropesboth with ethanol and with water.
Ind. Eng.Chem. Res., Vol. 32,No. 6,1993 1133 Ethanol 78.5 OC
p = 1 bar
internal and
-
Water 100.0 O c 84.0 C Figure 16. Synthesisof an azmtropic distillationsequence consistingof two columna only to separate an ethanol-watermixture wing toluene as the agent. This sequence works due to the fact that the first column, initially designed to recover water from the binary feed mixture. can also accomplish some aeparation between toluene and water. Toluene 110.6 OC
Moreover,thereisatemaryazeotropewhich has thelowest boiling point in this system. As opposed to the problem discussed above (Figure ll), the feed now is only a binary mixture. AsshowninFigure16,itisrichinwatercompared to the ethanollwater azeotrope and can thus he concentrated in a first distillation column that produces pure water aa the bottoms while its distillateis of near-azeotropic composition. At this point, an entrainer is needed to perform further separation. Iftolueneisaddedtotheazeotropicmixture,itispossihle to recover ethanol in the bottoms of a second column. The maximum separation is achieved if just enough toluene is added so that the ternary azeotrope can be obtained as the distillate. From a mass balance point of view this maximum separation is realizable whenever the overall composition of all streams entering the column lies on the line between pure ethanol and the ternary azeotrope (Figure 16), which again defines a mixing requirement. Streams with compositions in the shaded region could be used to provide the entrainer to the second column. Because this time an external species is introduced as the separating agent, we also have to create additional separation tasks, namely for the splits of toluene and ethanol and toluene and water. These tasks essentially prescribe that toluene must not appear in either product. Note that on the other side, i.e., as far as the removal of ethanol and water from toluene is concerned, there is no such strict requirement because toluene is not allowed as a product anyway and does not have to be pure to function as the entrainer. Figure 17 shows the separation ranges between water and ethanol and between toluene and water covered by the individual separation steps in the open loop. The combination of column 1 and column 2 accomplishes a complete separation between water andethanol. However, the ternary azeotropic stream leaving step 2 has to he
I
Ethanol
Toluene
F-re 11. Separationranges of the units in the tw*column sequence before introduction of the range-extending recycle.
subjected to further separation because the entrainer requirement for the second column is not satisfied yet and a portion of the separation range between toluene and water is uncovered. The ternary azeotrope, i.e., the distillate of the second column, can be decanted into a toluene-rich and a waterrich phase. This decanter initially carries out separations between toluene and ethanol (not shown in the binary diagrams) and splits between toluene and water with relative toluene concentrations of ahout 90% and 13% (Figure 17). Lastly, the decanter separates between ethanolandwatertosomeextent,butthisseparationrange is covered already hy the two columns. For the organic phase formed in the decanter, no additional separations are needed, and its composition satisfies the entrainer composition constraint. Thus, the toluene-rich layer should be recycled to column 2. In practice this stream would likely be used as the liquid reflux. In principle, it could also be introduced lower in the column hut only at the expense of an increase in the reflux ratio required. To
1134 Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993
determine the most appropriate design for given specifications, one could use design methods such as the one proposed by Pham et al. (1989). The aqueous stream leaving the decanter D contains a nonnegligible amount of entrainer, i.e., toluene. All required separation ranges are now covered, except for the toluene-water separation. However, the gap is not very large (Figure 17). Thus, we test if the aqueous stream can be recycled anyhow. The only unit that may be considered a potential recycle destination of the first column because (i) the relative composition between ethanol and water in the aqueous stream is in the range of separation performed by this column (Le., its main separation function), and (ii) because the relative concentrations between toluene and water in the products of the first column (both 0 at this time) are on the other side of the separation range still uncovered. When the aqueous phase from the decanter is recycled to the first column, it changes the inlet composition so that this column also carries out some separation between toluene and water. Figure 16 shows the completed two-column flowsheet. In the more conventional process flowsheets, a third distillation column is used instead of the range-extendingrecycle for entrainer recovery (see Ryan and Doherty, 1989). Again, range-extending recycles can work if only small gaps in separation ranges are left to be covered because the extent of additional separations a unit can perform when its inlet composition is slightly changed is usually small. However, if such a recycle is possible, it allows for separation in a process with a minimum number of units.
3. Keeping Track of the Process Alternatives To illustrate how structural flowsheet alternatives emerge when the proposed synthesis procedure is applied, let us consider the separation of a mixture F of pyridine, water, and toluene into pure components (Figure 18a-d). Rather than describingthe separation functionalityof each unit in terms of quantified binary concentrations, as has been done sofar and is required to automate the procedure, we will depict the separation ranges on the sides of ternary composition diagrams. The pyridine/water/toluenesystem exhibitsthree binary minimum-boiling azeotropes and an immiscibilityregion between water and toluene. These basic characteristics could, for example, quickly be derived from the K-values at infinite dilution discussed by Wahnschafft and Westerberg (1993). However, cooling the feed (Figure 18a) to ambient conditions does not result in two liquid phases. An analysis of the separations achievable by distillation identifies two extreme designs, one of which performs a complete separation between pyridine and toluene. Its distillate (1)(Figure 18a) cannot be further separated by distillation as it corresponds to the azeotrope between water and toluene,but it could be decanted,which provides practically pure toluene (3) and water (4). The bottom product (2) can be further separated by distillation into pyridine (5) and a distillate consisting of pyridine and water (6). As all required separation ranges are naw covered, stream 6 is recycled as a secondaryrecycle to the first distillation column. It should be noted that the compositions shown in Figure 18a-d correspond to those obtained in the open loop. The compositions of streams in and downstream of recycle loops usually change when the flowsheetis simulated with closed loops, so that design specifications must be set for the individual units to maintain their intended function. Instead of separating the feed into two two-component fractions, a distillation column could be used to provide
pure pyridine (1)as bottoms (Figure 18b-d). However, it is found impossible to remove all pyridine from the correspondingdistillate,whose composition(2) must hence lie on or close to a distillation boundary. The repeated test for liquid-phaseimmiscibilityshowsthat the distillate (2) corresponds to the two phases (3 and 4) when cooled, which suggests decantation as a possible next separation step. Since none of the two phases meets a product specification, two additional distillation columns are necessary to produce toluene (5) and water (7). Although we already know that there is an alternative flowsheet which only requires two distillation columns altogether, we will complete the second solution, shown in Figure 18b. This process produces both toluene and water at the bottom of distillation columns, whose top products can be recycled to the first column. Since a distillation column with the feed F can produce the desired pure pyridine (1)as a bottom product on the one hand, and on the other hand a mixture (3) free of pyridine as top product, one may also investigate the question whether something could be mixed with F such that the overall concentration of the feed to the column lies on the mass balance line between 1and 3, Le., postulate a mixing goal. The azeotropicmixture (3)obtained as the distillate of the first column (Figure 18c) could again be separated by decantation, which yields sufficiently pure water (4) and toluene (5). Obviously, toluene also satisfies the compositionconstraint so that a fraction of the toluene is recycled to provide the additional entrainer needed in the distillation column (Figure 18c, first option). The analogous considerations apply to the distillate stream (2) of a first column that recovers some of the pyridine in the mixture. Such a column would act as a preconcentrator, as the resulting flowsheet shows (left side in Figure 1812). It should be noted that, while the process configuration in Figure 1&(right) is attractive because it only consists of one column and one decanter, the distillation column that is to produce pyridine and the toluene-water azeotrope is difficult to control. The reason is that there is a possibility multiple steady states for the composition profile may swing toward the toluene-rich or the waterrich side. In general,such instabilityis likelyto be observed when the two products of a distillation column are stable nodes of the residue curve map. It is not impossible to overcomesuch problems, but a more sophisticated control scheme is needed. The last alternative to be taken into account in this process synthesis study results from mixing water instead of toluene with the distillate stream (2). Figure 18dshows the mass balance line representing the distillative separation between the two minimum-boiling azeotropes (3 and 4). Since the water/toluene azeotrope is the lowest boiling pseudocomponent of the water-rich composition region, it appears as the top product of the second distillation column. Again,azeotrope 3 can be cooled down to provide the toluene and water products. However, this time some water has to be recycled to the second column, whose bottom product is in the range of the f i t distillation. Hence the last flowsheet is completed by recycling azeotropic stream 4 to the first column. Due to the complex nature of the general separation system synthesis problem, it would be hard to prove whether or not a synthesis methodology is in principle capable of generating all process alternatives comprising a specified set of separation methods. There is in fact practically no reference literature which could be used to thoroughly validate the approach presented here-the
Ind. Eng. Chem. Res., Vol. 32, No.6, 1993 1135 Pyridine 115.4 C
immiscibility at ambient rondilions
Pyridine 115.4 C
0
-Toiuene 110.7 C
'
Water
minimum
IOLOC
P
P
a. Pyridine 115.4 C
" Toluene 110.7 C
F
3
Pyridine 115.4 C
Water 100.0 C
pFm
C.
Toluene 110.7 C
84yl C Water 100.0 C
F
d.
Figure 18. ( a d ) Synthesis of alternate proceea flowsheet for the aeparation of a mixture of pyridine. water. and toluene.
state of the art in complex separation process synthesis and design is at the level of ad-hoc, trial-and-error approaches and adaptation of solutions previously developed for similar problem specifications. In any case, the relatively simple ternary mixture separation problems discussed so far already indicate that the number of flowsheetoptions that may be generated by o w synthesis approach can be expected to be quite large, ohviously
growing with the number of separation methods taken into account and the number of components to be separated. Probably the main questions arising from this realization are the following: 1. How can one efficiently keep track of the complex process alternatives developed? 2. How could the approach be implemented on a
1136 Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993 s.20 S-14
S-4
EX
Path U-6 5-7 U-3 S-4 U-2 S-1
EX
.1
w
Figure 19. Compatibility between units in process sequences represented as an and/or tree.
computer so that the routine consideration of a larger number of alternatives becomes practical? 3. How could the search be organizedin sucha computer program to not just randomly produce all conceivable options but to focus on the most promising alternatives? Since one should distinguish between the synthesis methodology, which sets a framework for the systematic development of complex,hybrid separation processes,and issues strictly related to implementation, the latter two problems are not addressed here. The first question, however, is of interest irrespective of the way the synthesis is actually carried out. When process alternatives are generated by applying the sequential synthesis approach, the nontrivial problem arises to ensure consistency among the alternative partial solutions when recycles are introduced. As long as there are no recycles,the alternate flowsheets can be embedded in a simple and/or tree. In such a tree, the and-nodes correspond to the devices used in the flowsheet, in particular separators and stream splitters, which have outlet streams that must ultimately all be assignedto other units unless they are acceptable products. Streams, on the other hand, correspond to or-nodes since they can be subjected to different processing steps, which leads to flowsheet alternatives. The decision whether a particular stream can be processed in an already existing separation step, or which separations are required for the stream before it can be recycled, depends on the partial flowsheet considered. An efficient mechanism is thus needed to identify the partial solutions that are compatible with a stream in question. Figure 19 illustrates that, in an and/or tree, or stream is compatible with all units and streams that branch off an and-node of the flowsheet path that led to the stream in question. Stream 5-18could, for example, be assigned to
unit 2 or unit 10, but not unit 8. By keeping track of the paths through the process sequences it is thus easily psosibleto identify compatible partial solutions,regardless of the unit and stream naming conventions. Once a stream is actually assigned to a separation step that is not just downstream of the unit it originates from, the situation becomes more complicated. In principle, we then have to lumpthe partial solutionslinked by the stream assignment to a new and-node and place this lumped partial flowsheet at the position in the tree where the process paths diverged. As an example, assume it was recognized that stream S-18 could be fed as a secondary stream into unit U-10. The process paths leading to S-18 and U-10 diverge at unit 3, so that the whole combination of units 3, 6, 9, and 10 with a stream from U-9 to U-10 becomes a new and-node, placed next to U-3 with S-4 as the input stream. Without such lumping, it would not be possible to recognize incompatibilities any more. For example, stream S-8 can coexist in a flowsheet with unit 10, but not when stream 5-18 is also fed to U-10 because U-4 and U-9 cannot occur together in a single flowsheet. Conceptually, the problem of maintaining multiple flowsheet versions with recycles that may be introduced subsequently is related to the theory of nonmonotonic reasoning, which deals with the difficulties arising when the assertions on which a line of reasoning is based can change with time (e.g., Winston, 1984). At first sight, the problem may appear somewhat esoteric. However, it is crucial to the formalization of design methodologies that are to cope with the simultaneousdevelopment of multiple solutions for complex tasks which cannot be decomposed into totally independent subproblems. The synthesis of process flowsheets with recycles certainly falls in this category, and we have shown how the concept of and/or
Ind. Eng. Cbem. Res., Vol. 32, No.6,1993 1137
11.34% H U h a r d 1 A? .* W U W
F i r e 20. Specification of the separation of a mixture of acetone, pentane,water, and methanol. This mixture exhibitshighlyeompler nonideal behavior. Table I. K-VaIuedActivity Coefficients of Crat Infinite Dilution in C, CjICi
acetone methanol pentane water
pentane
water
7.9
0.98
.
...
A..7
29.6 14.4 1.0 1.0 8106
3213
74
0.4 1.6 71.4 1537 1.0 1.0
trees can be generalizedto keep track of multiple complex process flowsheets designed in a sequential fashion. 4. A Hypothetical Solvent Recovery Problem
The last example problem to be discussed here is the separation of a mixture of n-pentane, acetone, methanol, and water. This separation problem has not been studied elsewhere before; it was conceived to test the approach proposed here. The feed composition shown in Figure 20 is actually a product of previous separation steps which split an extremely nonideal, hypothetical six-component mixture into four-component fractions (Thomas, 1991). As mentioned before, the initial mixture also contained chloroform and hexane. Eleven of the 15 binary combinations of species are azeotropic, and numerous ternary and apparently also quaternary azeotropes are formed. Compared to this initial system, the four-component mixture analyzed here could be considered well behaved. However, the complexity of this separation problem probably still matches that of many industrial solvent recovery problems. Table I presents the K-value of componentCi at infinite dilution in component Cj, determined at the boiliig temperature of the pure component Cj for each possible pair i, j . The table also shows activity coefficients at infinite dilution for every pair of components, determined at ambient temperature. The upper values in each cell are theK-values;thelower oneaare theactivitycoefficients. For atmosphericpressure,the K-valuesindicateazeotropic behavior of the binary mixtures of acetonemethanol, pentanewater, pentane-methanol, and pentaneacetone. Acetone and water are also strongly nonideal; there is a tangent pinch close to pure acetone, as indicated by the K-value of water in acetone of practically unity. Based on the parameters from the DIPPR database of Aspen Plus (Aspen Tech, 1992). the UNIFAC method (e.g., Fredenslund et al., 1977)also predicts a ternary azeotrope in the system methanol, acetone, and pentane. The only unproblematic, i.e. nonazeotropic, binary combination is water-methanol. In the following,we will sumarize the most important considerationsthat lead to a process scheme which does not require an external mass separating agent. This process was generated by following the synthesis meth-
odologydescribedhere, implementedinaprototypesyatem called SPLIT (Wahnschafft et al., 1991,1992a). Since pentane and water exhibit an immiscibility,one could think of a decantation as the first step, but a threephase equilibrium calculation predicts that in the presence of acetone and methanol the small water fraction in the feed does not lead to the formation of a second liquid phase. The calculation also reveals that the feed mixture is at almost azeotropic composition. The only method found suitable (among distillation-based methods and extraction) with a separating agent that is present in the mixture to accomplish separations between pentane and other speciesis liquid-liquid extractionwith water. Water is largely immisciblewith the dominant species, pentane, and it yieldsgood selectivitiesfor therecoveryoftheother species from pentane. The selectivitiescan be estimated on the basis of the activity coefficientsat infinite dilution. According to eq 2, with water as the extract phase (') and pentane as the raffinate phase ("), we obtain for the selectivity between methanol and pentane in water:
The selectivitybetween acetone and pentane is not quite as high (~1850)but still allows for separation between theaespecies. While theK-valuesofacetoneand methanol at infinite dilution in water also indicate that water could be used asanextractivedistilionagentfortheseparation of these species (Wahnschafft and Westerberg, 1993),the splits involving pentane have a higher priority because they remove the dominant component. Using a rigorous simulation to asses what separations are feasible using water as a solvent, it turns out that, for an extractor with 10theoreticalstages,rather little water is needed to recover all methanol from the pentane leaving the extractor with the raffinate. A t higher solvent flow rates the water-rich extract would contain more and more acetone but could not produce a completeseparation of acetoneand pentane. Thus, the solvent flow is selected at which the methanolpentane separation is sufficiently sharp. As shown in Figure 21, the raffinate stream leaving such an extractor (stream F11) essentially contains pentane and acetone. This mixture can be separated in a distillation column which produces pentane as bottoms while the distillate composition is limited by the azeotrope. Figure 22 depicts the flow rates and relative concentrations between the major components of the streams leaving the extractor and the distillation column labeled DI-2. Since the relative concentration between acetone and pentane in the distillate stream F21 (about 22%) falls in the separation range covered by the extraction step (from about 5% to 85%), the stream F21 ought to be recycled to the extractor. The extract stream F12 still contains all componentsin nonnegligible fractions. However, this mixture can now be distilled. A simulation run with varied D F ratios suggests that one option would be to completely remove water from the mixture. However, because water is a candidate separating agent for the acetone/methanol split (examinethe infinitedilutionK-valuesfor acetonein water and methanolin water.namely38.5vs 7.8), another option, which thesesimulationsalsodiscover,ispreferable,namely to recover all the pentane. Due to the azeotropes with acetone and methanol, the pentane cannot be removed without also taking fractions of these components overhead. The corresponding distillate stream F31 (Figures 21 and 22) can also be recycled to the extractor as it too has a pentane to acetone relative concentration of about 22% which is in the range of 5 4 5 % .
1138 Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993
Fiure 21. Flow rates in the process eequenee generated by the sequential Synthesis method.
The bottom product of column DI-3 consists of acetone, methanol, and wateyand no pentane. Water boils higher than the two remaining species and does not form an azeotrope with either of them. As just noted above, the K-values of acetone and methanol at infinite dilution in water suggestusing water as an extractivedistillationagent for the separation of these species. In such a column, pure acetone is recovered in the distillate. It should be noted that, because of the tangent pinch between acetone and water, this column might be operated below ambient pressure to exploit the favorablevapodiquid equilibrium. Finally,the bottom product ofthe extractive distillation column can be separated in a simple column since it contains the nonazeotropic components methanol and water only. Now the process sequence covers all separation ranges required (Figure 22). However, we need to add twoprimaryrecycles. Thus,wesplit stream F52 to provide water as the separating agent to the extraction and to the extractive distillation. Figure 23 shows the simulation results for the complete process scheme that has been generated by searching for feasible and desirable separationsin the manner described until all outlet streams are products or can he recycled. This process features two primary and two secondary recycles. It appears noteworthy that, despite the complexity of the nonideal mixture behavior, it is possible to obtain all components in highly pure products without adding a separating agent. The extraction step accomplishes separations across three relative azeotropic compositions, namely between acetone and pentane, between methanol and pentane, and also between water and pentane. It should also be noted that, since the extractor does not have to perform sharp separations,it is a question of an overall optimization as to how many stages should be used. In principle, a single stage or, in other words, a simple decanter should sufficeto make the process feasible. The comparison between the simulation results for the open-Impflowsheet(Figure 21) and theclosed-loop process
(Figure 23) shows that it is indeed possible to maintain the separation functionality of the sequence, and even to slightly improve the achievablepurities, when the recycles are introduced-one of the essential premises of our sequential synthesis approach. 5. Summary
To date, most of the effort on separation process synthesis has dealt with sharp split separation sequences. In this paper, a methodology has been presented for the general separation system synthesis problem which accounts for the fact that several industrially important separation methods areunable to realizesharp separations in single-unit processes. To accomplish the overall separation process functionality by combining nonsharp splits, complex separation systems with internal recycles have to be devised, where as much use as possible is made of species present in a problem as mass separating agents. The problem of the synthesis of complex separation systemsis significantly more complicated than that of the synthesis of simpleseparationsequencesbecause theeffed of recycleswith initially oftenunknown compositionsmust be taken into account. The tree search algorithms developed for conventional separation sequence synthesis are not readily applicable to the synthesis of separation schemes in which recycles are needed to facilitate separations and to combine nonsharp splits. We demonstrated how the problem of the synthesis of such complex separation systems may be broken down into subtasks to facilitate the systematic development of the potentially interestingprocess schemes. Our synthesis methodology builds upon the traditional approach to consider streams sequentially and to choose the separation methods and, if necessary, mass separating agents before determining which splits are actually feasible and fixing the operating conditions. By analyzing the feasible separations at each separation step, the possibility to
Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993 1139
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exploit species present in a mixture as mass separating agents for other difficult splits may be discovered. Thus, quite intriguing process flowsheets can be devised. To illustrate the synthesis approach, a number of examples have been discussed, for which the synthesis steps were visualized in ternary diagrams. However, the proposed strategy does not depend on ternary diagrams andis thusin principleapplicable regardlessofthe number of species present in a problem. The overall synthesis methodologyprovideaa framework within which one may use whatever technique appears most suitable to narrow down the number of separation methods and agents to be considered and to determine the range of feasible product compositions for each separation step. These subproblems actually still constitute research areas in their own right, in particular the development of special methods for the quick search for feasible products for different separation methods. So far, this task can only be accomplished for the most widely used unit operations usingconventional simulation models. Theproposedapproachfocusesonfeasibilityratherthan economiccomparisons, and thusstill requires a subsequent stage in which, for example, thermodynamic analysis or rigorous optimization could be used to select from among the alternative configurations synthesized. However, the latter techniques are more of an analytical nature and could not be employed systematically without prior synthesis of a set of feasible process schemes or, in other words, a reasonable superstructure of alternatives.
to thank Aspen Technology Inc. for providing us with the Aspen Plus simulation program. Nomenclature fij
= flow rate of component i or j in mixture
K = vector of K-values W i * / x i ) S = selectivity = dimensionless parameter = absolute separation range between components i and j t = dimensionless parameter x = liquid composition vector (mole fraction) x i j = fraction of component i or j in mixture x i j = relative fraction of component i to j y = vapor composition vector (mole fraction) 8
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= activity coefficient
Literature Cited AapenTwh. AspenPlw UserGuidetoRek~e8.5;AapenTeehoology
Inc.: Ten Canal Park. Cambridge, MA 02141.1992. Bamicki, S. D.; Fair, J. R. Separation Syntem Synthesis: A Knowledge-BaaedApproach. 1. Liquid Mixture Separations. Ind. Eng. Chem. Res. 199Q,ZS, 421-432.
Doherty, M. F.;Caldarola,G. A. Design and Synthssis of Homogb neow Azeotropic Distillations. 3. The Sequencing of Columns for Azeotropic and Extractive DintiUations. I d . Eng. Chem. Fundom. 1986,24,474-485.
Engelmann, H.-D.; Erdmann, H.-H.; Funder. R.;Simmrock, K.-H. The Solving of Complex h e s a Synthesis Problem Using DistributedExpertSystems. Comput. Chem.Eng. 1989.13 (4/5), 451t465.
Acknowledgment
Fredennlund, A,; Gmehling,J.; Rasmussen, P. Vapor-Liquid Equi-
Theauthorswouldliketothankthe Engineering Design Research Center, an NSF-sponsored center at Carnegie Mellon University, and the Eastman Chemicals Division of the Eastman Kodak Company for their support of the work reported here. Funds provided to O.M.W. by the Ernest-Solvay Foundation and the 'Studienstiftung des deutschen Volkes"arealsoacknowledged. Finally,we wish
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libria Uaing UNIFAC; Elsevier: Amnterdam, 1917. Douglas, P. L.;Lindera, J. G. An Expert System for IdentifyingSeparation Processes. Can. J . Chem. Eng. 1991,69 (2), 67-75.
Hampe, M. J. Selection of solvents in LiquiQLiquid Ertraction Accordingto PhysicochemicalAapeeta. Ger. Chem. Eng. 1986.9, 251-263.
Horsley, L. H.Azeotropic Data I I t Advance8in Chemistry Series 116; American Chemical Society Washingon, DC, 1973.
Ind. Eng. Chem. Res., Vol. 32, No. 6,1993 1141 Kelly, R. M. General Processing Considerations. In Handbook of SeparationProcess Technology;Wiley: New York, 1987;pp 197225. King, C. J. Separation Processes, 2nd ed.; McGraw-Hilk New York, 1980. Laroche, L.; Bekiaris, N.; Andersen, H. W.; Morari, M. The Curious B e h a v i o r of H o m o g e n e o u s A z e o t r o p i c D i s t i l l a tion-ImDlications for Entrainer Selection. AZChE J. 1992a.38 (9),130411328. Laroche, L.; Bekiaris, N.; Andersen, H. W.; Morari, M. Homogeneous AzeotroDic Distillation: Separability and Flowsheet Synthesis. Znd. Erag. Chem. Res. 1992b,31 (9),-21*2209. Matauyama, H. Synthesis of Azeotropic Distillation Systems. Presented at the Japan-US Joint Seminar, Kyoto, Japan, 1975. Nishida, N.; Stephanopoulos, G.; Westerberg, A. W. A Review of Process Synthesis. AZChE J. 1981,27,321. Perry, J. H., Ed. Chemical Engineers’ Handbook, 3rd ad.; McGraw Hill. New York, 1950,p 632. Pham,H.N.;Ryan,P. J.;Doherty,M.F.DesignandMinimumReflux for Heterogeneous Azeotropic Distillation Columns. AIChE J. 1989,s (lo), 1585-1591. Rajagopal, S.;Ng, K. M.; Douglas, J. M. Design and Economic TradeOffs of Extractive Crystallization Proceeses. AZChE J. 1991,37 (31,437-447. Ryan, P. J.; Doherty, M. F. Design/Optimization of Ternary Heterogeneoush t r o p i c Distillation Sequences. AZChE J.1989, 35 (lo), 1592-1601. Stichlmair, J.; Fair, J. R.;Bravo, J. L. Separation of Azeotropic Mixtures via Enhanced Distillation. Chem. Eng. h o g . 1989,s (1),63-69.
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Van Dongen, D. B. Distillation of Azeotropic Mixtures. The Application of Simple-Distillation Theory to the Design of Continuous Processes. Ph.D Dissertation. University of Massachusetts, Amherst, MA, 1983. Wahnschafft, 0. M.; Westerberg, A. W. The Product Composition Regions of Azeotropic Distillation Columns. 2. Separability in Two-Feed Columns and Entrainer Selection. Znd. Eng. Chem. Res. 1993,preceding paper in this issue. Wahnschafft, 0. M.; Jurain, T. P.; Westerberg, A. W. SPLIT a Separation Process Designer. Comput. Chem. Eng. 1991,I5 (e), 565-581. Wahnschafft, 0. M.; Le Ruddier, J. P.; Blania, P.; Westerberg, A. W. SPLIT 11. Automated Synthesis of Hybrid Separation Processes. Comput. Chem. Eng. 1992a,16 (S),305-312. Wahnschafft, 0. M.; KBhler, J. W . ; B h , E . ; Westerberg,A. W. The Product Composition Regions of Single-Feed Azeotropic Distillation Columns. Znd. Eng. Chem. Res. 1992b,31 (lo),2345-2362. Winston, P. H. Artificial Intelligence; Addison-Wesley: New York, 1984.
Received for review October 21, 1992 Revised manuscript received March 5, 1993
