Pressure swing distillation

In some situations, azeotropic points are sensitive to moderate changes in pressure. When this is the case, pressure-swing distillation can be used in place of azeotropic distillation to permit the recovery of two nearly pure species that are separated by a distillation boundary. This section introduces pressure-swing distillation. 

5 Sequencing of Operations for the Separation of Nonideal Fluid Mixtures 279 

When designing pressure-swing distillation sequences, the recycle ratio must be adjusted carefully. Note that it is closely related to the differences in the compositions of the azeotrope at Pj and P2. Horwitz (1997) illustrates this for the dehydration of ethylenediamine. 

Since the bottoms products are pure, B, = 50 kmol/hr HjO and = 50 kmol/hr THF. To determine the distillate flow rates, the following species balances apply. 

Solving these two equations simultaneously, Di = 117.9 kmol/hr and D2 = 67.9 kmol/hr. Exercise 7.19 examines the effect of pressure on the internal flow rates.  

Methyl ethyl ketoneAvater (1 bar/7bar) Water/2-butanone 

Methanol/methyl ethyl ketone (1 bar/7.6 bar) Ethanol/2-pentanone 

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Of these five methods all but pressure-swing distillation can also be used to separate low volatiUty mixtures and all but reactive distillation are discussed herein. It is also possible to combine distillation and other separation techniques such as Hquid—Hquid extraction (see Extraction, liquid-liquid), adsorption (qv), melt crystallization (qv), or pervaporation to complete the separation of azeotropic mixtures. 

Fig. 12. Pressure-swing distillation of a minimum boiling binary azeotrope, (a) Temperature—composition phase diagram showing the effect of pressure on...

Only a fraction of the known azeotropes are sufficientiy pressure-sensitive for the conventional pressure-swing distillation process to work. However, the concept can be extended to pressure-insensitive azeotropes by adding a separating agent which forms a pressure-sensitive azeotrope and distillation boundary. Then the pressure is varied to shift the location of the distillation boundary (85). 

FIG. 13-66 Conceptual sequence for separating maxinuTm-hoiling binary azeotrope with pressure swing distillation. 

Ethanol-water Minimum-hoiling azeotrope Ethylene glycol, acetate salts for salt process Alternative to azeotropic distillation, pressure swing distillation... 

Methyl acetate-methanol Minimum-hoiling azeotrope Ethylene glycol monomethyl ether Element of recovery system for alternative to production of methyl acetate hy reactive distillation alternative to azeotropic, pressure, swing distillation... 

Pressure-swing distillation, 3 817-819 general separation heuristics for,... 

When the composition of an azeotrope is sensitive to moderate changes in pressure, then pressure-swing distillation may be used to separate almost pure components. The first column separates pure A in top or in bottoms, if this is a minimum or maximum boiler, respectively. Then the second column makes possible the separation of pure B while recycling the A/B azeotrope. 

After recovering the acetonitrile the problem is breaking its azeotrope with water. VLE investigation shows that this is sensitive to the pressure change. For example, at 0.4 bar the azeotropic point is x(ACN) = 0.80 and T = 326 K, while at 6 bar this shifts to x(ACN) = 0.57 and T = 412 K. Consequently, pressure-swing distillation may be applied as indicated in a recent patent [15]. 

Because a large amount of water is entrained in the side stream, this is removed in the column C-3. Raw acetonitrile, namely a binary azeotrope with 20% water, separates in top. The bottom stream contains water with heavy impurities. Vacuum distillation at 0.5 bar is adequate to limit the bottom temperature. In the next step pure acetonitrile can be obtained by using pressure-swing distillation. 

The composition of many azeotropes varies with the system pressure (Horsley, Azeotropic Data-Ill, American Chemical Society, Washington, 1983 Gmehling et ah. Azeotropic Data, VCH Publishers, Deerfield Beach, Fla., 1994). This effect can be exploited to separate azeotropic mixtures by so-called pressure-swing distillation if at some pressure the azeotrope simply disappears, such as does the ethanol-water azeotrope at pressures below 11.5 kPa. However, pressure sensitivity can still be exploited if the azeotropic composition and related distillation boundary change sufficiently over a moderate... 

General information on pressure-swing distillation can be found in Van Winkle (Distillation, McGraw-Hill, New York, 1967), Wankat (Ejmilibrium-Stased Separations, Elsevier, New York, 1988), and Knapp and Doherty [Ina. Eng. Chem. Res., 31, 346 (1992)]. Only a relatively small fraction of azeotropes are sufficiently pressure-sensitive for a pressure-swing process to be economical. Some applications include the minimum-boiling azeotrope tetrahydrofuran-water [Tanabe et al. ... 

Azeotropic distillation and pressure-swing distillation. Methods that cause or exploit azeotrope formation or behavior to alter the boiling characteristics and separability of the mixture. 

J.-U. Repke, F. Fomer, A Klein, 2006, Separation of Homogeneous Azeotropic Mixtures by Pressure Swing Distillation - Analysis of the Operation Performanee, Chem. Eng. Techn., vol. 28, 1151-1557. 

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