Separation of Binary Azeotropes by Hybrid Processes
More Examples of Distillation-Decantation
As another example, consider the operation of the azeotropic distillation of acetic acid-water mixture using n-butyl acetate (a type of ester) as the entrainer, as shown in the Figure below. The boiling point of pure acetic acid is 118.1oC, and for water 100oC.
Addition of n-butyl acetate (boiling point 125 oC) will result in the formation of a minimum-boiling azeotrope with water (boiling point 90.2oC). The azeotropic mixture therefore will be distilled over as vapour product from the high-boiling acetic acid, which leaves as bottoms product.
When the overhead vapour is condensed and collected in a decanter, it forms two insoluble layers: top layer of nearly pure butyl acetate saturated with water, and a bottom layer of nearly pure water saturated with butyl acetate.
The liquid from the top layer is returned to the distillation column as reflux and source of entrainer. The liquid from the bottom layer is sent to another column for the recovery of the entrainer by removing the water using steam stripping.
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A more complicated system is the separation of ethanol (78.3 oC) and water (100 oC).
The ethanol-water system forms a minimum-boiling azeotropic mixture at 89.4 mole% ethanol at 78.15 oC. As noted earlier, starting with a mixture containing lower proportion of ethanol, it is not possible to obtain a product richer than 89.4 mole% ethanol using conventional distillation.
Adding benzene (80.2 oC) serves as the entrainer that forms a new minimum-boiling azeotrope with 22.8 mole% ethanol, 23.3 mole% water and 53.9 mole% benzene. This mixture boils at 64.86 oC which leaves as the overhead product from the first column. Relatively pure ethanol leaves the column as bottoms product.
The schematic flow diagram is shown in Figure below.
The condensed overhead vapour forms two liquid phases in the decanter: a top layer rich in benzene and a bottom layer rich in water (and still substantial amount of ethanol). The top layer is returned to the first column as reflux and source of entrainer.
The bottom layer is send to a second distillation column where the residual benzene is stripped from the liquid and leaves as overhead vapour. This is then returned to the first column.
The bottoms from the second column contain essentially mixture
of ethanol and water, and are fed to a third column for further distillation.
At this third column, essentially pure water is obtained from the bottoms, while the distillate is the azeotropic mixture of ethanol-water that is returned to the first column for further processing.
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