If waste streams can be recycled directly, this way is clearly the simplest to reduce waste and should be considered first. Most often, the waste streams that can be recycled directly are aqueous streams which, although contaminated, can be substituted for part of the freshwater feed to the process.
Figure 3.8.2 is a flowsheet for the production of iso-propyl alcohol by the direct hydration of propylene. Propylene containing propane as an impurity is reacted with water to give a mixture which contains propylene, propane, water, and isopropyl alcohol. A small amount of by-products, principally di-isopropyl ether, is formed. Unreacted propylene is recycled to the reactor, and a purge is taken so that the propane does not build-up. The first distillation column (C1) removes the light ends (including di-isopropyl ether). The second distillation column (C2) removes as much as water as possible to approach the azeotropic composition of the isopropyl alcohol-water mixture. The final column (C3) performs an azeotropic distillation using di-isopropyl ether as an entrainer.
Water A A
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