Azeotropes, minimum-boiling BACK

Go back to the temperature-mole fraction diagram for the isopropyl alcohol-isobutyl alcohol system (Fig. 175). The composition of the vapor is always different from that of the liquid, and we can separate the two compounds. If the composition of the vapor is the same as that of the liquid, that separation is hopeless. Since we ve used the notions of an ideal gas in deriving our equations for the liquid and vapor compositions (Clausius-Clapeyron, Dalton, and Raoult), this azeotropic behavior is said to result from deviation from ideality, specifically deviations from Raoult s law. Although you might invoke certain interactive forces in explaining nonideal behavior, you cannot predict azeotrope formation a priori. Very similar materials form azeotropes (ethanol-water). Very different materials form azeotropes (toluene-water). And they can be either minimum-boiling azeotropes or maximum-boiling azeotropes. 

An example of azeotropic distillation is the use of benzene to permit the complete separation of ethanol and water, which forms a minimum-boiling azeotrope with 95.6 weight percent alcohol. The alcohol-water mixture with about 95 percent alcohol is fed to the azeotropic distillation column with a benzene-rich stream added at the top. The bottom product is nearly pure alcohol, and the overhead vapor is a ternary azeotrope. The overhead vapor is condensed and separated into two phases. The organic layer is refluxed, and the water layer is sent to a benzene recovery column. All the benzene and some alcohol is taken overhead and sent back to the first column, and the bottoms stream is distilled in a third column to give pure water and some of the binary azeotrope. 

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. 

Isopropyl alcohol and water form a minimum boiling point azeotrope at 88 wt% isopropyl alcohol and 12 wt% water. Vapor-liquid equilibrium (VLE) data are available from several sources and can be used to back-calculate binary interaction parameters or liquid-phase activity coefficients. The process presented in Figure B.3 and Table B.6 was simulated using the UNIQUAC VLE thermodynamics package and the latent heat enthalpy option in the CHEMCAD simulator. This package correctly predicts the formation of the azeotrope at 88 wt% alcohol. 

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