Ind. Eng. Chem. Res. 2000, 39, 1061-1063
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Circumventing an Azeotrope in Reactive Distillation Jae W. Lee, Steinar Hauan, and Arthur W. Westerberg* Department of Chemical Engineering and Institute for Complex Engineered Systems, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
Using a McCabe-Thiele diagram, we explain nonintuitive behavior in a binary reactive distillation column where we react and separate two isomers displaying a maximum boiling azeotrope. Placing sufficient reaction on only two trays in the top section of the column, we shift the top operating line to lie below the 45° line. On lower nonreacting trays, we then easily step past the azeotrope. Once past the azeotrope, we even find that the column temperature decreases as we step down the column. Introduction Azeotropes give rise to boundaries that limit the separation possible for mixtures when using distillation.1-3 At times we can circumvent these limits by combining reaction with distillation, a technique industry uses to produce methyl tert-butyl ether (MTBE)4 and methyl acetate.5 In this paper we use a visual approach6,7 we recently developed that allows us to see exactly how to use an isomerization reaction to circumvent the azeotrope between p-ethylphenol and 2-phenylethanol. The two isomers, p-ethylphenol (boiling point 218.8 °C) and 2-phenylethanol (boiling point 219.4 °C)8-10 display a maximum boiling azeotrope at a composition of 0.55 mol fraction of p-ethylphenol, a pressure of 1 atm, and a temperature of 228.5 °C. The UNIFAC11 (universal quasi-chemical functional-group activity coefficients) property set in AspenPlus predicts this azeotropic composition and is consistent with experimental values.8-10 Chemical equilibrium favors the almost complete isomerization of 2-phenylethanol (2-PHE) to p-ethylphenol (P-ETP) where the reaction equilibrium constant is around 103 on the basis of using ideal gas Gibbs energies of formation12 for each species:
C6H5-CH2CH2OH (2-PHE) w CH3CH2-C6H4-OH (P-ETP) (1) The Reactive Distillation Process We introduce the staged reactive distillation column shown in Figure 1 to allow reaction and distillation to occur simultaneously for these two species. The numbers on this plot, which we shall discuss shortly, are from a column simulation using the AspenPlus simulator with the reflux ratio of 4 and under total condenser. The goal of this reactive separation is to produce pure p-ethylphenol (P-ETP) and 2-phenylethanol (2-PHE) at the top and bottom by breaking the azeotrope. Figure 2 is a plot of vapor composition vs liquid composition (a McCabe-Thiele diagram13) for the product, p-ethylphenol (P-ETP). It is on this plot that we show how reaction, strategically placed, allows us to bypass the azeotropic composition when distilling. In Figure 2, the S-shaped curve gives the vapor composition of P-ETP in equilibrium with its corre* To whom correspondence should be addressed. Phone: 412-268-2344.Fax: 412-268-7139.E-mail:
[email protected].
Figure 1. A seven-stage reactive distillation column for converting 2-phenylethanol (2-PHE) into p-ethylphenol (P-ETP) and simultaneously separating the product from unreacted reactant. The numbers are from a computer simulation using AspenPlus. All compositions are for p-ethylphenol (P-ETP).
sponding liquid composition. For our column, we assume the vapor stream leaving each stage is in equilibrium with the liquid leaving. For example, vapor composition y2 () 0.93) is in equilibrium with liquid composition x2 () 0.86) in Figure 1, and this point will lie on the vapor/ liquid equilibrium curve at the point marked 2 in Figure 2. Figure 2 also features a set of “operating lines” that represent column material balances. Along these lines, possible compositions of the vapor and liquid streams pass each other between stages (e.g., between stages 2 and 3 y3 () 0.76) is on the first shifted operating line vs x2 () 0.86) between the points marked 2 and 3). Finally, for convenience, one also plots a diagonal “45° line” where vapor and liquid compositions are equal. In our earlier work, we showed how to adjust the slope and the intersection of the operating lines with the 45° line whenever we allow reaction to occur within the column.6,7,14,15
10.1021/ie990447k CCC: $19.00 © 2000 American Chemical Society Published on Web 03/09/2000
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Ind. Eng. Chem. Res., Vol. 39, No. 4, 2000
Figure 2. A McCabe-Thiele Plot for the column in Figure 1. This plot illustrates how the combination of reaction and separation allows one to step past a binary azeotrope while distilling.
For this reaction and separation, the operating lines are nearly parallel in Figure 2 as the heat of reaction is negligible. Equation 2 is a material balance and is the equation for an operating line above the feed stage. Equation 3 indicates how reaction shifts the operating line.
yn+1 )
DxD ξn L xn + V V V
δn ) xD -
ξn D
(2) (3)
Reaction extent ξn is the total of the product A (in moles/second) formed on all stages above and including stage n, which we can also call the sum of reaction molar turnover flow rate until the stage n from the top. Compositions yn+1 and xn are the vapor and liquid compositions passing each other between stages n+1 and n. L and V are the liquid and vapor molar flow rates. D and xD are the top product molar flow rate and its composition. Each difference point (δn in eq 3 and points δ2 and δ3 in Figure 2) is a composition where a shifted operating line intersects the 45° line. We supply pure 2-phenylethanol as a mixed phase in the feed stream. Our goal is to obtain almost pure p-ethylphenol at the top and unconverted 2-phenylethanol at the bottom. Without reaction, we cannot operate a binary column in such a way that top and bottom products are on opposite sides of an azeotropic composition. The top operating line starts where the top product composition, xD, intersects the 45° line in Figure 2. By material balance, compositions y1 and xD are equal; both are on the top operating line. Composition x1 is in equilibrium with y1; it is on the equilibrium line. We do not allow reaction on the first stage so we find y2 on the top operating line vs x1. On stage 2 (see Figure 1), we allow a reaction turnover of ξ2 ) 240 mol/s. The intersection of the first shifted operating line with the 45° line moves left by ξ2/D ) 240/810 ) 0.30, i.e., from 0.95 to 0.65. We allow a reaction turnover of a further 572 on stage 3. The intersection point for the second
shifted operating line, δ3, moves left by a farther 572/ 810 ) 0.71 or to a point to the left of the origin. We show no further reaction below stage 3. We continue our tray-by-tray calculations by stepping between the second shifted operating line and the equilibrium line until we reach stage 6. It is for these stages that we encounter the nonintuitive behavior we mentioned at the start of this paper. Stages 4 and below have no reaction occurring on them. Reaction above these stages shifts their operating line downward below the equilibrium line which itself is below the 45° line, allowing us to step past the azeotrope as if it were not there. To the left of the azeotrope in Figure 2, the product is heavier than the reactant (e.g., x4 > y4 in Figure 1), and yet is being depleted (e.g., y5 < y4 in Figure 1) as we move down the column from stage 4 to stage 6. When it enriches, the temperature decreases. We finish the diagram by including the bottom operating line, which is for the stages below the feed stage. We switch to it to step off stage 7. We find that this stage is counterproductive as the column reverts to its intuitive behaviorswhich occurs when the operating line moves between the equilibrium curve and the 45° line. The composition for the bottom product from the column increases from below 10% to about 18%. In an actual column, we should not include this bottom stage. We have included it here for illustrative purposes only. Use of the commercial simulator AspenPlus verified our results. We used the UNIFAC property set for physical property estimation (which, as we noted earlier, predicts correctly the location of the azeotrope reported in the literature). The reaction turnover rates we used are insufficient to bring us to chemical equilibrium on stages 2 and 3; thus, with the right catalyst they are entirely plausible. We report all the flows, compositions, and temperatures that the simulator computed in Figure 1. Note especially that the temperature goes down as we move down the column from stage 4 to stage 6, and the vapor stream has more heavy species than the liquid stream on those stages, consistent with the construction in Figure 2. In short, we had available and used sufficient reaction on stages 2 and 3 to shift the column material balance to cause this behavior. Graphically we used sufficient reaction to shift the operating line below the equilibrium line which itself was below the 45° line on stages 4 through 6. By far the most important use of the McCabe-Thiele method for binary distillation is to provide scientists and engineers with a tool to sketch column behavior. From these sketches, they can then see how best to design and operate such columns. We have shown here that our extensions that permit this plot to handle reactions allows us to see when and how we can break a binary azeotrope through the use of reaction. We needed only sketches, not accurate drawings, to gain these insights. Literature Cited (1) Foucher, E. R.; Doherty, M. F.; Malone, M. F. Automatic Screening of Entrainers for Homogeneous Azeotropic Distillation. Ind. Eng. Chem. Res. 1991, 30, 760. (2) Wahnschafft, O. M.; Koehler, J. W.; Blass, E.; Westerberg, A. W. The Product Composition Regions of Single-feed Azeortopic Distillation Columns. Ind. Eng. Chem. Res. 1992, 31, 2345.
Ind. Eng. Chem. Res., Vol. 39, No. 4, 2000 1063 (3) Westerberg, A. W.; Wahnschafft, O. M. Synthesis of Distillation-based Separation Systems. Adv. Chem. Eng. 1996, 23, 63. (4) Smith, L. A. Catalytic distillation process. U.S. Patent 4,307,254, 1981. (5) Agreda, V. H.; Partin, L. R. Reactive distillation process for the production of methyl acetate. U.S. Patent 4,435,595, 1984. (6) Lee, J. W.; Hauan, S.; Lien, K. M.; Westerberg, A. W. A Graphical Method for Designing Reactive Distillation Columns Is The Ponchon-Savarit Method. Proc. R. Soc. London 2000, in press. (7) Lee, J. W.; Hauan, S.; Lien, K. M.; Westerberg, A. W. A Graphical Method for Designing Reactive Distillation Columns IIs The McCabe-Thiele Method. Proc. R. Soc. London 2000, in press. (8) Lecat, M. Tables Azeotropiques: Azeotropes Binares Orthobares; l’Auteur: Bruxelles, 1949. (9) Horsley, L. H. Azeotropic Data-III; American Chemical Society: Washington, DC, 1973. (10) Gemehling, J.; Menke, J.; Fisher, K.; Krafczyk, J. Azeotropic data, 1st ed.; John Wiley & Sons: Weinheim, 1994.
(11) Fredenslund, A.; Gemehling, J.; Rasmussen, P. VaporLiquid Equilibria Using UNIFAC; Elsevier: Amsterdam, 1977. (12) Daubert, T. E.; Danner, R. P.; Sibul, H. M.; Stebbins, C. C. Physical and Thermodynamic Properties of Pure Chemicals: Data Compilation; Hemisphere Pub. Corp.: New York, 1989. (13) McCabe, W. L.; Thiele, E. W. Graphical Design of Fractionating Columns. Ind. Eng. Chem. 1925, 17, 605. (14) Hauan, S.; Westerberg, A. W.; Lien, K. M. Phenomena Based Analysis of Fixed Points in Reactive Separation Systems. Chem. Eng. Sci. 1999, 55, 1053. (15) Lee, J. W.; Hauan, S.; Lien, K. M.; Westerberg, A. W. Difference Points in Extractive and Reactive Cascade II- Generating Design Alternatives by Lever Rule for Reactive Systems. Chem. Eng. Sci. 1999, in press.
Received for review June 18, 1999 Revised manuscript received December 20, 1999 Accepted December 21, 1999 IE990447K
