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Ind. Eng. Chem. Res. 1998, 37, 998-1008
Homogeneous Azeotropic Distillation. 2. Design Procedure for Sequences of Columns Francisco J. L. Castillo, Dennis Y.-C. Thong, and Gavin P. Towler* Department of Process Integration, UMIST, Manchester M6O 1QD, U.K.
A graphical procedure based on residue-curve maps and distillation-line maps is developed for the synthesis of sequences of distillation columns for homogeneous azeotropic distillation. The feasibility of separation is determined by the overlap of product operation leaves defined as the feasible region containing all column profiles that lead to a desired product specification, which need not be a precise composition. Column sections may be constrained to operate within a subregion of the composition space, in which case the limit of operation is defined as a section boundary. Section boundaries for node-type and saddle-type products are established, and it is shown that they do not always coincide with simple-distillation or distillation-line boundaries. When the distillation line (or residue curve) and pinch-point curve from a desired product terminate at different nodes in the composition space, then distillation sequences using complex column arrangements can be proposed. 1. Introduction The existing synthesis procedures for homogeneous azeotropic distillation column sequences are based on residue curve maps (Laroche et al., 1992a; Doherty and Caldarola, 1985); therefore, they are restricted to separations operated at total reflux. Although useful, the same authors recognize that conclusions drawn at total reflux are incomplete. Although Stichlmair et al. (1989) and Stichlmair and Herguijuela (1992) present flowsheets based on distillation-line maps for staged columns, they do not develop a procedure for column sequencing at finite reflux. Wahnschafft et al. (1992, 1994) extended these approaches to finite reflux, making use of residue curves and pinch-point curves to define areas of possible liquid composition profiles in packed columns and therefore to establish feasible separations at finite reflux. However, they do not develop a systematic procedure for the synthesis of column sequences. Castillo et al. (1998) extended the concept of operation leaves to staged columns and showed how it could be used to determine the feasibility of separation for a new design or for retrofit or reuse of an existing piece of equipment. Special cases of operation leaves that occur near total reflux boundaries were examined, and it was shown that these could be used to produce designs that cross such boundaries using complex column arrangements. In this work we will describe a synthesis procedure for distillation sequences that incorporates these insights. 2. Section Boundary 2.1. Product Definition and Composition Profiles. In designing a separation process, we do not usually seek to produce an exact product composition but instead require the process to meet specifications on one or more components, for example, a minimum * Author to whom correspondence is addressed. Telephone: 0161 200 4386. Fax: 0161 236 7439. E-mail: towler@ umist.ac.uk.
Figure 1. Product specification: 90% purity of component A.
purity of the main product or a maximum level of a given contaminant. We can define a product region as the set of points in the composition space that satisfy the specification for the desired product; hence, a product region is a graphical representation of the product specifications. For example, in a ternary mixture, a desired product purity of 90% or more of component A becomes the area PA, shown in Figure 1. Although any composition in PA satisfies the product specification, normally the separation process will stop once it reaches any point on the segment Q1-Q2, since further purification is unnecessary and will increase the process cost; therefore, only the points in the segment Q1-Q2 are of practical interest to the designer. The pure components and azeotropes in a mixture can be classified as nodes (stable or unstable) or saddle points, according to the local behavior of the residue curves or distillation lines (Doherty and Caldarola, 1985). Stichlmair et al. (1989) further observed that only those compositions that are located at the end of
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Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 999
Figure 2. Product type definition: (a) node-type product; (b) saddle-type product.
Figure 3. Product type depends on the specification: (a) saddle-type product for 95% purity; (b) node-type product for 90% purity.
distillation lines, i.e., node points, can be obtained in pure form. This classification system can be extended to product regions. Node-type products are those products where residue curves and distillation lines either enter or leave the product region (Figure 2a). If residue curves and distillation lines pass through the product region and re-exit, then the product is a saddle-type product (Figure 2b). Figure 3 shows that the presence of an azeotrope of composition similar to that of the pure component makes the product type depend on the strictness of the purity specification as well as the nature of the nodes. In the example shown, for a desired product of 95% purity the product is a saddle-type (Figure 3a), but for a specification of 90%, it is a node-type (Figure 3b). A column section operation leaf is defined and can be plotted in three-component systems for a single product composition, i.e., for a single point in the composition space (Castillo et al., 1998). In a similar way, a product operation leaf can be defined as the area covered by all the operation leaves of all the product compositions that belong to a product region, i.e., all the points that satisfy the product specifications. The product operation leaf represents the location of all the possible liquid-phase composition profiles for a column section that could produce the specified product.
Theoretically, the product operation leaf must be plotted for all the product compositions in the product region. In practice this is unnecessary, as the points that belong to the segment that defines the product region (e.g., segment Q1-Q2 in Figure 1) are sufficient to indicate to the designer the area covered and, most importantly, its limits. 2.2. Section Constraints for Node-Type Products. Figure 4 shows the residue-curve map for the system chloroform, acetone, and benzene at 101.3 kPa. The system has a total reflux boundary between the chloroform-acetone maximum-boiling binary azeotrope and pure benzene. Figure 5 shows three operation leaves for a product specification of 90% purity of acetone. This product is a node-type. It can be seen that the operation leaves for different product compositions in the product region will cover all the area on the right-hand side of the total reflux boundary and that no operation leaf can go beyond this boundary. The total reflux boundary therefore acts as the column-section boundary for the acetone product. If staged columns are used, the distillationline boundary is the column-section boundary. If packed columns are used, the simple-distillation boundary is the column-section boundary.
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Figure 4. Residue-curve map for the system chloroform, acetone, and benzene at 101.3 kPa.
Figure 5. Node-type products. Specification: 90% purity acetone. System chloroform, acetone, and benzene at 101.3 kPa.
The system ethanol, water, and ethylene glycol at 101.3 kPa does not have a total reflux boundary (Figure 6). Figure 7 shows some operation leaves for a product specification of 90% purity of ethylene glycol (a nodetype product). The operation leaves for this product are not constrained and can reach any point in the composition space. In this case, the product operation leaf is the entire composition space. Although the system ethanol, water, and methanol at 101.3 kPa has a total reflux boundary between the ethanol-water binary azeotrope and pure methanol (Figure 8), the operation leaves for a product specification of 90% purity of methanol are not constrained by the boundary (Figure 9). Methanol can be produced from any feed in the composition space, despite the existence of a boundary. Figure 11 shows some operation leaves for a product specification of 90% purity of methanol in the system chloroform, acetone, and methanol at 101.3 kPa, which has three binary azeotropes and a ternary azeotrope (Figure 10). Despite having four total reflux boundaries, the product operation leaf is only constrained by two total reflux boundaries: that between the chloroformmethanol binary azeotrope and the ternary azeotrope and that between the acetone-ethanol binary azeotrope
Figure 6. Residue-curve map for the system ethanol, water, and ethylene glycol at 101.3 kPa.
Figure 7. Node-type products. Specification: 90% purity ethylene glycol. System ethanol, water, and ethylene glycol at 101.3 kPa.
and the ternary azeotrope. The total reflux boundary between pure methanol and the ternary azeotrope does not affect the methanol product operation leaves. From the examples considered, we see that for nodetype products the column sections are constrained only by total reflux boundaries that do not pass through the product region. If a total reflux boundary passes through the product region, then it does not prevent that product specification from being achieved by a column section beginning at any point in the composition space (if there is only one boundary); thus, the total reflux boundary is not active for that product. 2.3. Section Constraints for Saddle-Type Products. Now consider saddle-type products. We will begin by considering residue curves; i.e., we consider first packed columns and then extend the analysis to staged columns later. Figure 12 shows the residuecurve map for the ideal system n-pentane, n-hexane, and n-heptane at 101.3 kPa. This mixture forms no azeotropes. Figure 13 shows the product region for a product specification of 90% purity of n-hexane. Because of the saddle nature of the product, every residue curve that enters the product region must exit from it. The limit
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Figure 8. Residue-curve map for the system ethanol, water, and methanol at 101.3 kPa.
Figure 11. Node-type products. Specification: 90% purity methanol. System chloroform, acetone, and methanol at 101.3 kPa.
Figure 9. Node-type products. Specification: 90% purity methanol. System ethanol, water, and methanol at 101.3 kPa.
Figure 12. Residue-curve map for the ideal system n-pentane, n-hexane, and n-heptane at 101.3 kPa.
Figure 10. Residue-curve map for the system chloroform, acetone, and methanol at 101.3 kPa.
at total reflux is therefore defined by the residue curve that is tangent to the product region, i.e., the residue curve that passes through point T (Figure 13). We call
this residue curve the tangent residue curve. No residue curve on the concave side of the tangent residue curve enters the region. Every residue curve on the convex side of the tangent residue curve crosses the product region. At total reflux, the column-section boundary is therefore the tangent residue curve. Hexane is the intermediate-boiling component. If n-hexane is produced as the top product (e.g., using a direct sequence of columns), then the composition profile of the rectifying section will follow the direction of increasing temperature (Figure 14a). On the other hand, if n-hexane is produced as the bottom product (e.g., when the indirect sequence is adopted), the composition profile of the stripping section follows the direction of decreasing temperature (Figure 14b). Now consider the minimum reflux operation of a stripping section. Figure 15 shows pinch-point curves plotted for different points on the segment Q1-Q2, which defines the product specification. The pinch-point curves for points between Q2 and T lead out of the product region toward pure n-pentane. In contrast, the pinch-point curves for points between Q1 and T pass through the product region before going toward pure n-pentane. All of these pinch-point curves must by
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Figure 13. Saddle-type products. Specification: 90% purity n-hexane. System n- pentane, n-hexane, and n-heptane at 101.3 kPa.
definition go through point T, since T is a pinch point for all the points on Q1-Q2. The limit is given by the product pinch-point curve for point Q1. Similarly, the limit on a rectifying section at minimum reflux is given by the pinch-point curve for point Q2. The total reflux (Figure 14) and minimum reflux (Figure 15) composition profiles are combined in Figure 16 to form the product operation leaf for a saddle-type product. It is interesting to note that the product operation leaf at finite reflux is larger than that at total reflux. The limit for the product operation leaf for a rectifying section is given by the pinch-point curve for point Q2 (Figure 16a). Similarly, the limit for the product operation leaf for a stripping section is given by the pinch-point curve for point Q1 (Figure 16b). Since pinch-point curves are the same for staged and packed columns, the results can be generalized to staged columns. The section boundary for saddle-type products in both packed and staged columns is therefore defined by the pinch-point curves for the extreme points of the specification. Figure 17 shows an example of column-section boundaries for saddle-type products in a nonideal system. The product operation leaves for 95% purity acetone and 95% purity chloroform products are plotted for the system chloroform, acetone, and methanol at 101.3 kPa. The product operation leaves are constrained by the pinch-point curves, despite the fact that this system has four total reflux boundaries. Figure 18 shows the product operation leaves for 95% purity ethanol and 95% purity water for the system ethanol, water, and ethylene glycol at 101.3 kPa. Although this system does not have a total reflux boundary, the product operation leaves for the two specified products are constrained to a small fraction of the composition space, as both are saddle-type products. In this section we have shown that it is not impossible to produce saddle-type products. The condition for a feasible separation is that the feed to the column section that makes the saddle-type product must lie inside the product operation leaf defined by the pinch-point curves through the end points of the segment defining the product region.
2.4. Section Boundary-Comparison between Boundaries. We can distinguish between total reflux boundaries and section boundaries. Total reflux (distillation-line and simple-distillation) boundaries are limits for separations performed at total reflux, and they depend solely on the vapor-liquid equilibrium of the system. Column-section boundaries are limits on the composition profiles or operating lines of column sections that produce a specified product, and they depend on both the vapor-liquid equilibrium and the desired product specification. A distillation column operated at total reflux is a single section; therefore, constraints on column sections constrain the whole column. When the column is operated at finite reflux, the column rectifying and stripping sections are no longer merged. They therefore have different constraints, and it is possible to find feasible designs that are unfeasible at total reflux (Wahnschafft et al., 1992; Laroche et al., 1992b). The total reflux boundary only acts as a constraint for those products whose product region it does not enter. We will see how this can be exploited in design in the next section. 3. Sequence Synthesis Procedure 3.1. Design Philosophy. The objective of the synthesis procedure is to generate as many feasible flowsheets as possible that permit the separation of the feed into the desired products. Strictly, the search for the best flowsheet is made in a second step, once every alternative has been cost optimized. In general, however, some parameters of the design, e.g., the recycleto-feed ratio, can be used to compare the performance of different column sequences before optimization. In this way the number of alternatives that require detailed analysis is reduced. The synthesis procedure proposed is based on the synthesis of column sections rather than whole distillation columns. Using operation leaves, it is possible to establish the feasibility of a proposed separation at any reflux (when residue-curve or distillation-line maps are used, this can only be done for total reflux operation). Conventionally, we must know the feed to a distillation column before we proceed with the design. The use of operation leaves requires instead the products of the column sections. This is not the problem that it appears to be at first sight, since the existence of recycles and mixers in azeotropic distillation sequences makes all of the column feeds uncertain, while the product specifications are part of the problem data and hence are available at the start. The product specifications determine two of the column sections in the flowsheet through the product operation leaves. In a two-column sequence, for example, at least two of the four column sections in the sequence are therefore known from the start. In addition to column feasibility, the designer must also ensure that material balances are closed for every column and mixer; i.e., the column feed and products lie on a straight line in the ternary diagram. When sequences of three or more columns are used in ternary systems, subsystems can be identified where there is one stream entering and two leaving (or vice versa). The overall sequence will not be feasible if the mass balance is not closed around these subsystems as well. 3.2. Entrainer Selection. In general, the azeotropic separation is a subproblem of a bigger separation task,
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 1003
Figure 14. Saddle-type products. Specification: 90% purity n-hexane. System n- pentane, n-hexane, and n-heptane at 101.3 kPa: (a) rectifying section and (b) stripping section.
Figure 15. Stripping section composition profiles at minimum reflux for saddle-type products. Specification: 90% purity nhexane. System n-pentane, n-hexane, and n-heptane at 101.3 kPa.
involving the separation of substances that do not form azeotropes. Traditional design practice selects column sequences where difficult separations are performed in the absence of nonkey components (Douglas, 1988; Smith, 1995). Although any substance can act as an entrainer in principle, not every substance is a good entrainer, i.e., affects the vapor-liquid equilibrium enough or in a convenient way. Those species already present in the system are attractive entrainer candidates since their use does not involve the purchase and storage of additional components. Furthermore, using a substance already present in the mixture may save a distillation column in the overall flowsheet. These components will often be rejected by the conventional azeotropic distillation design methods, as these methods are based on total reflux operation and lead to conservative designs (Laroche et al., 1992; Wahnschafft et al., 1992). Once the substances present in the original mixture have been studied, entrainer screening criteria
that look for components that produce convenient residue-curve or distillation-line maps can be applied (Doherty and Caldarola, 1985; Stichlmair et al., 1989). 3.3. Synthesis Procedure. The use of operation leaves enables us to propose the following synthesis procedure: Step 1: Select a candidate entrainer. Step 2: Draw the product specifications and overall feed in the triangular diagram. Step 3: Establish the product operation leaves and the product section boundaries. The product operation leaves tell us what type of column section produces each product (i.e., strippers, rectifiers, or both, in the case of saddle-type products) and, more importantly, what constrains the column sections, i.e., the product section boundaries. If the product section leaves are of opposite type (i.e., one is a stripper and the other one is a rectifier) and they overlap, then the separation can be performed in a single distillation column. In general though, the product operation leaves do not overlap and more than one column is required to separate the feed into the desired products. Step 4: Pick potential recycle compositions. The stripping and rectifying section operation leaves can be plotted for any potential recycle composition. Let us focus first on two-column flowsheets. If the operation leaf defined by the recycle composition does not overlap with any of the product operation leaves, then the point should be rejected because the proposed recycle composition does not lead to a feasible column design with either of the products. A new point should be tried instead. If the recycle operation leaf overlaps one of the product operation leaves, then there is a feasible column (Figure 19) that produces the recycle and one of the specified products. Notice that this column can obtain only those points in the product specification region whose operation leaves overlap the recycle operation leaf. This is important for the determination of the column feed composition. If the recycle operation leaf overlaps both product operation leaves, then it is possible to propose a complex column arrangement for the separation. This is explored in detail in the next section.
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Figure 16. Saddle-type products. Specification: 90% purity n-hexane. System n- pentane, n-hexane, and n-heptane at 101.3 kPa.
Figure 17. Saddle-type products. Specification: 95% purity acetone and chloroform. System chloroform, acetone, and methanol at 101.3 kPa.
For two-column flowsheets, only one recycle composition R is needed to define the flowsheet. Every additional column requires an extra recycle. At least one of the recycles proposed must form an operating leaf that intersects a product operation leaf. Step 5: Establish flowsheet feasibility for the selected recycle composition. Once a recycle has been found that allows a feasible column to be formed with one of the products, then the feed to that column can be identified. This feed composition must be in mass balance with the recycle and the corresponding product composition. Generally, the product specification is a region (Figure 1); therefore, the feed must be located in the triangle formed by the segment bounding the product region and the recycle composition. Strictly, not all the points in this triangle are feasible feeds. There could be some compositions that satisfy the product specification, i.e., belong to the product region, whose operation leaves do not overlap the recycle operation leaf. Even for those points in the product region whose operation leaves overlap the recycle operation leaf, the intersection of the composition profiles only occurs within a certain interval of reflux
Figure 18. Saddle-type products. Specification: 95% purity ethanol and water. System ethanol, water, and ethylene glycol at 101.3 kPa.
Figure 19. Feasible column producing one of the specified products and the recycle. (expanded view).
and boil-up ratio values that constrains the feasible feed composition even further (Castillo et al., 1998). Since
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 1005
Figure 20. Evolution of the flowsheet from two columns (open leaves) to column and side rectifier arrangement.
at this point we are interested in establishing the flowsheet structure rather than the exact recycle composition, we will consider, as a first approximation, that all points of the triangle previously described are feasible feed compositions. Once a suitable column sequence has been established, the feasibility of the column feed composition is easily determined. A two-column flowsheet is feasible if and only if there is a feasible feed whose operation leaf intersects the product operation leaf of the remaining product. Normally, there will either be a region of points for which this is the case or else no such point at all. Step 6: Try another recycle composition and/or entrainer and keep promising structures. End of procedure. Although the flowsheet synthesis involves some trial and error, it is not necessary to explore a large number of recycle compositions to assess the performance of a potential entrainer. Laroche et al. (1992) suggest that it is sufficient to explore the pure components and all azeotropes as candidates for recycle. These points are adequate for analysis of total reflux designs; however, in our experience, better designs are found when intermediate points are examined, particularly when options such as complex columns are considered. The use of pure components and azeotropes as recycle compositions unnecessarily increases the cost of the separation (at finite reflux, the number of stages increases considerably with the purity of the products) and the computational effort of forming operation leaves. The effort of calculating product operation leaves can be greatly reduced by developing appropriate software tools. In essence, all that is needed is to calculate the pinch-point curve and residue curve/distillation line through the chosen points. This can easily be coded in a spreadsheet or as a FORTRAN program (e.g., Castillo and Sutton, 1996). The choice of a “good” recycle composition depends on the design objectives and will be discussed below once we have introduced a method for designing complex column arrangements for azeotropic separations. When more than two columns (i.e., more than one recycle) are considered, the same principles apply.
Notice that, in a three-column flowsheet, only one of the two recycle operation leaves must overlap one of the product operation leaves. The design of three-column sequences is therefore more complicated than that of two-column sequences, and there are more options for recycles. In general, three-column sequences are avoided unless two-column sequences are not feasible or are very uneconomical. In the next section we will show that it is usually possible to find an attractive two-column sequence. 4. Complex Columns Consider the case when the operation leaf of a potential recycle composition overlaps both product operation leaves. The double overlap means that it is possible to find feasible column designs that produce the recycle composition at one end of the column and either product at the other end, depending on what reflux or boil-up ratio is used. Necessarily, the feed to each of these two column designs must be different; otherwise, the mass balances would not be satisfied. Figure 20a shows an example where the recycle is produced in both stripping sections and the rectifying sections are making products A and B. The boil-up ratios used, s1 and s2, must be different. The design shown in Figure 20a is not affected if an additional stripping section is used to further purify the recycle R, as shown in Figure 20b. This design (Figure 20b) can be evolved to give the column and side rectifier arrangement shown in Figure 20c. Parts b and c of Figure 20 are entirely equivalent flowsheets. The design shown in Figure 20c is feasible if the appropriate boil-up ratios, s1 and s2, are used. The capital investment can be further reduced if a side reboiler is placed on the section operating at lower reflux once the critical part of the column composition profile has been overcome (Castillo et al., 1998). The final flowsheet of the column and side rectifier arrangement will have only one feed, which can be fed either to the main column or to the side rectifier. Compositions that form open leaves must by definition overlap two product operation leaves. Hence, when open leaves are found, the designer has two options.
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Figure 21. Acetone and chloroform product operation leaves for 95% purity. System: chloroform, acetone, and benzene at 101.3 kPa.
Figure 22. Operation leaves for two potential recycle compositions (R1, 0.05 acetone and 0.40 chloroform; R2, 0.10 acetone and 0.25 chloroform). System chloroform, acetone, and benzene at 101.3 kPa.
Either a two-column sequence can be specified as in Figure 20a or a side stripper or rectifier arrangement can be used to cross the total reflux boundary, as in Figure 20c. These complex designs are helped by appropriate placement of side reboilers or condensers in the main column or side column. Furthermore, in systems where the difference between the simpledistillation boundary and the distillation-line boundary is significant, there will also be an area of intersection between staged sections and packed sections. In such cases, a combination of a staged main column and a packed-bed side column (or vice versa) will enhance the separation. Recycle compositions with open leaves are not rare. In fact, it is difficult to find closed leaves in the vicinity of an azeotrope or a total reflux boundary. This suggests that the scope for using complex column arrangements in the separation of azeotropic mixtures is greater than previously thought. 5. Example Consider the separation of an equimolar feed of acetone and chloroform into products of 90% purity using benzene as the entrainer. The system has a maximum-boiling binary azeotrope between acetone and chloroform (Figure 4). Both specified products are nodetype, and their product operation leaves are shown in Figure 21. It can be seen that the total reflux boundary is the section boundary for both products. Since both products are the lowest-boiling components in their operation leaves, they will both be obtained from rectifying sections, i.e., as column top products. The separation cannot be performed in a single column because both products must be made in rectifying sections. Following the synthesis procedure, we choose two arbitrary potential recycle compositions R1 and R2, shown in Figure 22. Since the products are obtained from rectifying sections, the recycle must be the product of a stripping section. We therefore need to consider the recycle operation leaves in the direction of decreasing temperature, as both R1 and R2 are bottom products. The figure shows that the operation leaf for composition R1 overlaps the chloroform product operation leaf,
Figure 23. Feed region for R1 (0.05 acetone, 0.40 chloroform) and chloroform product (90% purity). System: chloroform, acetone, and benzene at 101.3 kPa. There is no point in the feed region whose operation leaf overlaps with the acetone product operation leaf, i.e., there is no feasible design for a recycle composition R1.
indicating that the separation R1-chloroform product is feasible. The separations R1-acetone product and R2-chloroform product are not feasible; however, the separation R2-acetone product is possible. Let us consider R1 in more detail. The feed to the column that separates R1 and the chloroform product must be in the triangle formed by R1 and the segment Q1-Q2 that defines the product, shaded in Figure 23; otherwise, the mass balance for the column will not be satisfied. To begin with, we assume that every point in this triangle is a feasible feed. In the case of R1, there is no point in the triangle that forms a stripping section operation leaf that overlaps with the acetone product operation leaf; i.e., R1 produces no feasible flowsheet. R2 does not produce a feasible flowsheet either. Now consider a third potential recycle composition R3, shown in Figure 24. This composition has an open leaf; i.e., the column section leaf overlaps both product oper-
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 1007 Table 1. HYSIM Simulation Data main column no. of trays reflux ratio feed tray side draw tray feed composition top product bottom product
acetone chloroform benzene acetone chloroform benzene acetone chloroform benzene
50 20 20 48 0.200 0.300 0.500 0.999 0.000 0.001 0.046 0.336 0.618
side rectifier 30 65 30
0.000 1.000 0.000
Figure 24. Operation leaf for R3 (0.002 acetone, 0.15 chloroform) is an open leaf; hence, it overlaps the chloroform product operation leaf. System: chloroform, acetone, and benzene at 101.3 kPa. Point F1 (0.02 acetone, 0.30 chloroform) is a feasible column feed (expanded view).
Figure 26. Novel flowsheet that performs the desired separation. System chloroform, acetone, and benzene at 101.3 kPa.
6. Conclusions
Figure 25. Two-column flowsheet that performs the desired separation. System chloroform, acetone, and benzene at 101.3 kPa.
ation leaves. R3 and the chloroform product are feasible products of a column whose stripping section operates at a high boil-up ratio. Possible feeds for this column lie within the triangular region shaded in Figure 24. Many points of this triangle form operation leaves that overlap with the acetone product operation leaf (near to R3). The point F1 in Figure 24 is one example. Once F1 is identified, it is possible to verify that it is a feasible feed. Recycle composition R3 leads to a feasible two-column flowsheet, as shown in Figure 25. The open leaf found for R3 suggests that a column with a side rectifier can be used to cross the simple distillation boundary and perform the separation. Table 1 shows the main results of a simulation of such an arrangement carried out using HYSIM (Hyprotech Ltd., Calgary, Canada). Figure 26 shows the composition profiles of the main column and side rectifier plotted in a triangular diagram. The simulation is not optimized, as the objective is only to prove that a complex column arrangement can be used to cross the simple distillation boundary.
A graphical synthesis procedure for homogeneous azeotropic distillation sequences has been proposed. The product operation leaves and the section boundary help to verify the constraints on the desired products. Instead of using residue curves or distillation lines, the feasibility of a proposed separation is established by the overlap of operation leaves of the products and recycles. In this way, the design is not constrained to total reflux operation. The procedure is particularly useful for the design of two-column sequences and complex column arrangements, such as columns with side strippers or rectifiers. Acknowledgment The authors acknowledge the support of the UMIST Process Integration Research Consortium and of Hyprotech Ltd., who provided the software used in this research. Literature Cited Castillo, F. J. L.; Sutton, C. COLOM, Azeotropic Distillation Simulation Package; Department of Process Integration, UMIST: Manchester, U. K., 1996. Castillo, F. J. L.; Thong, D.Y-C.; Towler, G. P. Homogeneous Azeotropic Distillation: Design Procedure for Single-Feed Columns at Non-Total Reflux. Ind. Eng. Chem. Res. 1998, 37, 987-997.
1008 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 Doherty, M. F.; Caldarola, G. A. Design and synthesis of homogeneous azeotropic distillations. 3. The sequencing of columns for azeotropic and extractive distillation. Ind. Eng. Chem. Fundam. 1985, 24, 474. Douglas, J. M. Conceptual design of chemical processes; McGrawHill, Inc.: New York, 1988. Laroche, L.; Bekiaris, N.; Andersen, H. W.; Morari, M. Homogeneous azeotropic distillation: Separability and flowsheet synthesis. Ind. Eng. Chem. Res. 1992a, 31, 2190. Laroche, L.; Bekiaris, N.; Andersen, H. W.; Morari, M. The curious behavior of homogeneous azeotrope distillation: Implications for entrainer selection. AIChE J. 1992b, 38, 1309. Smith, R. Chemical process design; McGraw-Hill: Inc.: New York, 1995. Stichlmair, J. G.; Herguijuela, J.-R. Separation Regions and Processes of Zeotropic and Azeotropic Ternary Distillation. AIChE J. 1992, 38, 1523.
Stichlmair, J.; Fair, J. R.; Bravo, J. L. Separation of azeotropic mixtures via enhanced distillation. Chem. Eng. Prog. 1989, 1, 63. Wahnschafft, O. M.; Koehler, J. W.; Blass, E.; Westerberg, A. W. The product composition regions of single-feed azeotropic distillation columns. Ind. Eng. Chem. Res. 1992, 31, 2345. Wahnschafft, O. M.; Kohler, J. W.; Westerberg, A. W. Homogeneous Azeotropic Distillation: Analysis of Separation Feasibility and consequences for entrainer selection and column design. Comput. Chem. Eng. 1994, 18, Suppl., S31.
Received for review July 10, 1997 Revised manuscript received November 24, 1997 Accepted November 25, 1997 IE9704814
