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Ind. Eng. Chem. Res. 1999, 38, 2482-2484
Composition Profile of an Azeotropic Continuous Distillation with Feed Composition on a Ridge or in a Valley Yonghong Li,* Hongfang Chen, and Jiaqi Liu Department of Chemical Engineering, Tianjin University, Tianjin 300072, People’s Republic of China
The steady-state composition profiles of ternary azeotropic mixtures in a continuous distillation column were investigated with feed compositions on the ridge or in the valley of the boiling temperature surfaces of the systems. The success of the crossing ridges and valleys was presented experimentally. The overheads and the bottoms are in the different regions divided by the ridge or the valley. Crossing paths are not tangent to the ridge or the valley but go through it in an inclined direction. The temperature along the column changes monotonically when the composition profile crosses the ridge or the valley. Introduction When separation of a binary azeotropic mixture into its pure components is attempted, an entrainer is added to affect the properties of the mixture in such a way as to make the resulting ternary system separable. Strong nonlinearity of vapor-liquid equilibrium in the ternary mixture often causes a valley or a ridge on the boiling temperature surface of the composition triangle. This leads to complicated structures of the composition profiles in steady-state distillation columns and limits the product compositions to a certain narrow region. The phenomenon mentioned above is called a “distillation anomaly”. It is not possible for engineers to design and control a separation system without a thorough understanding of the characteristics of composition profiles in an azeotropic distillation affected by the valley and the ridge. It was of widespread opinion that valleys and ridges could not be crossed and that they were the boundaries to distillation regions because temperature changed monotonically along a column. In recent years, there has been an idea4 proposed relating to crossing of valleys and ridges, so that overheads and bottoms are on both sides of the ridges or valleys. Although the ridge/valley crossing is well-known and this was discussed on the basis of simulation results in several references,1,5-7 it has not been verified by experiment up to now. This paper presents experimental work to verify the crossing and to show the effect of feed composition on the crossing. It deals with the composition profiles of a homogeneous azeotropic distillation with feed compositions on the ridge or in the valley of the systems chloroform-methanol-acetone and chloroform-ethanol-acetone. Experiment Chloroform-methanol-acetone (C-M-A) and chloroform-ethanol-acetone (C-E-A) systems vary nonlinearly because of strong interactions between the components. Both systems have typical structures of boiling temperature surfaces. The azeotropic data of the systems at atmospheric conditions were listed in Table * To whom correspondence should be addressed. Telephone: 022-27405825. E-mail:
[email protected].
Figure 1. Ridges and valleys of boiling temperature surfaces for the C-M-A system.
Figure 2. Ridges and valleys of boiling temperature surfaces for the C-E-A system. Table 1. Azeotropic Data3 system C-M C-E C-A M-A C-E-A C-M-A
compositions (mole fraction) M: E: A: A: E: A: M: A:
0.347 0.162 0.366 0.802 0.190 0.352 0.441 0.317
temp (°C) 53.43 59.35 64.40 55.50 63.20 57.50
1. The valleys and the ridges obtained by bubble-point and dew-point calculation are denoted by dashed lines and dot-dashed lines in Figures 1 and 2. The activity coefficients in the vapor-liquid equilibrium calculations were calculated by the Wilson model. The Wilson parameters were reported by Gmehling et al.2 The continuous distillation experiments of the above systems were carried out with laboratory-scale equipment. The column consists of a total condenser, a reboiler, and 32 Oldershow plates. The diameter of each plate is 50 mm, and the feed stream is introduced on the 15th plate counted from the bottom. The temperatures on the 3rd, 7th, 11th, 15th, 19th, 23rd, and 27th plates, on the top, and in the reboiler are measured by EA-2 thermocouples and are recorded by digital thermometers with an accuracy of 0.1 °C. Table 2 shows a
10.1021/ie970937h CCC: $18.00 © 1999 American Chemical Society Published on Web 04/17/1999
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Figure 3. Composition profile in a continuous distillation column for the C-E-A system: 2, feed; ×, bottoms; 9, distillate. Figure 5. Composition profile in a continuous distillation column for the C-M-A system: 2, feed; ×, bottoms; 9, distillate.
Figure 4. Composition profile in a continuous distillation column for the C-E-A system: 2, feed; ×, bottoms; 9, distillate. Table 2. Experimental Conditions and Temperature Profile for Figure 3 experimental condition feed
mol fraction
temp (°C) rate (cm3/h) reflux ratio pressure drop (Pa) distillate (cm3/h) bottoms (cm3/h)
C: 0.250 E: 0.185 A: 0.565 60 740 6 2552 263 477
plate number
temp (°C)
top 31 27 23 19 11 7 3 bottom
56.8 57.2 59.5 61.4 62.1 62.7 63.3 63.9 65.3
group of experimental conditions. The liquid streams are sampled on the same plates. The concentrations are determined by gas chromatography. The compositions on the top, on every plate, and in the reboiler are linked, in turn, to obtain a composition profile. The experimental results show that when a feed composition is located near the ridge or the valley, the direction of the composition profile of the distillation column for a ternary homogeneous azeotropic system is the same as that of the ridge or the valley that the feed composition is closed to. If a feed composition is assigned on the ridge or in the valley, the composition profile can cross the ridge or the valley; the overheads and the bottoms are in different regions by the ridge or the valley (shown in Figures 3-5). The composition profiles cross the ridge/valley in an inclined direction. The temperature along the column changes monotonically (see Table 2) although the composition profile crosses the ridge or the valley. There has, until now, been no experimental information published about this result.
column. If the direction of the tie lines in a ternary system were the same as that of the temperature gradient, then crossing the valley or the ridge would be impossible. Fortunately, the tie lines do not behave in such a way. A ternary boiling point temperature surface is not necessarily a scalar potential function of the vector field y(x) - x but only a Liapunov function that suits -dx/dt ) y(x) - x (where t is an arbitrarily selected parameter, e.g., time or volume, in the pot). The tie line vectors in a valley or a ridge are usually not zero. In fact, the deviation characteristics of a ternary mixture are altered with a change in composition, such that the three binary mixtures have different characters of deviation from the ideal. There are effects of both positive deviation and negative deviation on every point in a phase diagram. Some areas of the diagram have positive deviation in one direction but negative deviation in another direction, so that an inclined valley and ridge is formed, which results in the fact that the temperature of some points on a ridge will be lower than the temperature of those not on the ridge and the points on a valley will be higher than those not on the valley. This causes a monotonic change of the temperature along a certain path (but not all paths). The composition profiles cross a ridge or a valley through the path. Therefore, when the composition in a column crosses the ridge or the valley, the temperature profile in the column should not violate the general distillation principle. On the other hand, the composition profile in a distillation column is, of course, assigned by the tie lines y(x) - x. The fact that more volatile components on one side of a valley or a ridge change to less volatile ones on the other side can cause a variation in the direction of the tie lines on both sides of the valley or the ridge. This causes an unstable equilibrium on the valley and the ridge, so that a slight alteration in a liquid composition point (x) on the ridge probably will lead to a considerable variation in the equilibrium vapor concentration (y). So, the slight alteration of x on a ridge can make y run from one side of the ridge line to the other side. For the valley, x should be in the same manner as y mentioned above. Once the above cases occur and the temperature changes monotonically, the crossing of ridges or valleys will take place.
Explanation In binary systems, an azeotropic point exists at a peak or a pit in the T(x) function. An azeotropic point cannot be crossed by simple distillation or by rectification because of a monotonic change of temperature along a
Conclusions The composition profiles in a continuous distillation column for ternary homogeneous azeotropic systems were studied by experiments that emphasize the effect
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of feed compositions on a ridge or a valley. The following conclusions can be drawn: (1) When a feed composition is assigned on the ridge or in the valley of a boiling temperature surface, the composition profile in a distillation column can intersect with and cross over the ridge or the valley. In this case, the overheads and the bottoms will be in different regions divided by the ridge or the valley. So, further separation of the top and bottom products can be carried out. This information is very important in the case of the azeotropic distillation system synthesis. (2) Crossing paths are not tangent to the ridge or the valley but go through it in an inclined direction. The temperature along the column changes monotonically though the composition profile crosses the ridge or the valley. Nomenclature T ) temperature, °C x ) concentration in liquid, mole fraction y ) concentration in gas, mole fraction
Literature Cited (1) Fidkowski, Z. T.; Doherty, M. F.; Malone, M. F. Feasibility of Separations for Distillation of Nonideal Ternary Mixture. AIChE J. 1993, 39 (8), 1303-1321. (2) Gmehling, J. L.; Onken, U. Vapor-Liquid Equilibrium Data Collection; DECHEMA: Frankfurt, Germany, 1977. (3) Horsley, L. H. Azeotropic Data III; American Chemical Society: Washington, DC, 1973. (4) Rev, E. Crossing of Valleys, Ridges and Simple Boundaries by Distillation in Homogeneous Ternary Mixtures. Ind. Eng. Chem. Res. 1992, 31 (3), 893-901. (5) Stichlmair, J. G.; Herguijuela, J. R. Separation Regions and Processes of Zeotropic and Azeotropic Ternary Distillation. AIChE J. 1992, 38 (10), 1523-1535. (6) Wahnschafft, O. M.; Koehler, J. W.; Blass, E.; Westreberg, A. W. The Product Composition Regions of Single-Feed Azeotropic Distillation Columns. Ind. Eng. Chem. Res. 1992, 31, 2345-2362. (7) Widagdo, S.; Seider, W. D. Azeotropic Distillation. AIChE J. 1996, 42 (1), 96-130.
Received for review December 30, 1997 Revised manuscript received January 11, 1999 Accepted January 19, 1999 IE970937H
