Pressure-swing azeotropic distillation

Some azeotropic systems exhibit the desirable property that pressure has a strong effect on the composition of the azeotrope. When this occurs, a two-column system can be used to achieve the desired separation. The basic idea is to operate one column at low pressure and a second column at high pressure. One of the components comes out from one end of the low-pressure column. If the azeotrope is minimum boiling, the azeotrope will come out of the top and the product stream out the bottom. The composition of this distillate stream will be close to that of the azeotrope at the low pressure. If the azeotrope is maximum boiling, the azeotrope will come out the bottom and the product stream out the top. The azeotrope is then fed to the second high-pressure column in which a similar separation occurs, except now the other component is removed from one end of the column and a stream with composition close to the high-pressure azeotrope is removed from the other end. This azeotropic stream is fed back to the low-pressure column. 

This chapter gives several examples of pressure-swing azeotropic distUlation systems. All of the examples involve minimum-boiling azeotropes, so the azeotropes come overhead in the distillate streams and the bottom streams from the two columns are the high-purity products. 

The two azeotropic separation methods considered in Sections 5.1 and 5.2 required the addition of a third component to alter the vapor-liquid equilibrium. Another factor that sometime affects the phase equilibrium is pressure. If the composition of a binary azeotrope is a strong function of pressure, a two-column process can be used to achieve separation without adding a third component, which is desirable because small levels of impurity of this third component in the product streams are unavoidable. The two columns operate at different pressures, so the azeotropic compositions are different. 

The two bottoms specifications are the required product purities. The two distillate compositions are design optimization variables to be established by economics. As the distillate specifications get closer and closer to the corresponding azeotropic compositions, the separations in each column become more difficult and more trays are required (higher capital investment). However, the flow rates of the recycle stream D and D2 decrease as the difference between the two distillate compositions increases, so energy consumption 

PRESSURE-SWING AZEOTROPIC DISTILLATION yx for acetone/MeOH 

The only complication in setting up the pressure-swing distillation simulation is the recycle stream of the high-pressure distillate back to the low-pressure column. One approach is to make a guess of the conditions of this second feed to the low-pressure column and then set up the both columns sequentially, starting with the low-pressure column with its two design specification xb = 0.005 and = 0.74) and then moving to 

However, some simple total molar and component balances for this binary system can be used to obtain precisely the flow rates of all the distillate and product streams since we have specified the compositions of all four streams. 

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Figure 5.25 Pressure-swing azeotropic distillation flowsheet minimum-boiling azeotrope.

Figure 5.27 Pressure-swing azeotropic distillation xy diagram.

Luyben W. L., Design and control of a fully heat integrated pressure-swing azeotropic distillation system, Ind. Engng. Chem. Res., 47, 2681-2695 (2008). 

The final pressure-swing azeotropic distillation system presented in this chapter is the separation of pentanes from methanol. This application arises in the production of tert-amyl methyl ether (TAME), which is used as a high-octane gasoline blending component. TAME is produced in a reactive distillation column by the reaction of methanol with the unsaturated five-carbon iso-amylenes (2-methyl-1-butene and 2-methyl-2-butene). 

The designs of two different types of methanol recovery sections are compared. In the first, pressure-swing azeotropic distillation is used. In the second, extractive distillation is used. 

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... 

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... 

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