The most common form of azeotropic distillation is adding a third component to the azeotropic mixture in a distillation column. This third component essentially changes the vapor/liquid relationship between the two components and allows separation. Using ethanol/water as the example, the column is usually operated with a continuous feed of the azeotrope into the column, which contains the third component. This causes a ternary azeotrope to form in the vapor at the top of the column. When this vapor is condensed, the condensate splits into two liquid phases. The organic layer is pumped back to the top of the column as reflux. The aqueous layer is pumped to a smaller third column where the entrainer is recovered and pumped back to the dehydration column. Thus, the entrainer is continuously recycled and losses are low. In the base of the dehydration column, the entrainer is removed from the ethanol to give high purity ethanol as the base product. The process for the removal of water from the isopropyl alcohol/water azeotrope is essentially the same.
APV has supplied over 10 systems to remove water from both ethyl alcohol and isopropyl alcohol. Cyclohexane, isopropyl ether and benzene have been used as the entraining component. A large system for anhydrous ethyl alcohol production is shown in Figure 22.
Many ethyl alcohol systems involve processing a feed of about 10% w/w ethyl alcohol. This is concentrated to 93% w/w in a binary column, followed by a concentration step to over 99% w/w in the azeotropic column. A flow sheet for a typical large system is shown in Figure 23. This system consists of a binary
column, dehydration column and entrainer recovery column. On these systems, the two larger columns are often operated at different pressures so that the vapor from one column can be used as the heating medium for the reboiler of the second column, which operates at a lower pressure. A column of the size illustrated above, in Figure 22, can process about 230,000 tons per year of ethyl alcohol.
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