Exractive Distillation:
Exractive Distillation is carried out for such azeotropic solutions in which relative volatility is very low. In this case continuous distillation of the mixture to give nearly pure products will require high reflux ratios with correspondingly high heat requirements. In addition, it will necessitate a tower of large cross-section containing many rays.
Basic principle for separation of this type of solutions is to add a substance that will alter the relative volatility of the original constituents, thus permitting separation. The added solvent should be of lower volatility as compared to the components and hence it does not appreciably vaporizes in the fractionation column. This solvent must be continuously fed near the top of the column and it runs down the column as reflux and is present in appreciable concentrations on all the plates.
Actually, this extractive agent differentially affects the activities (activity coefficients) of the components, and hence alters the relative volatility of the mixture. It is important to note that the solvent must not form an azeotrope with any of the components.
Comparison With Azeotropic Distillation
Extractive distillation is usually more desirable than azeotropic distillation since no large quantities of solvent have to be vaporized. In addition, a greater choice of added component is possible since the process is not dependent upon the accident of azeotrope formation. It cannot, however, be conveniently carried out in batch operation.
Exractive Distillation is carried out for such azeotropic solutions in which relative volatility is very low. In this case continuous distillation of the mixture to give nearly pure products will require high reflux ratios with correspondingly high heat requirements. In addition, it will necessitate a tower of large cross-section containing many rays.
Basic principle for separation of this type of solutions is to add a substance that will alter the relative volatility of the original constituents, thus permitting separation. The added solvent should be of lower volatility as compared to the components and hence it does not appreciably vaporizes in the fractionation column. This solvent must be continuously fed near the top of the column and it runs down the column as reflux and is present in appreciable concentrations on all the plates.
Actually, this extractive agent differentially affects the activities (activity coefficients) of the components, and hence alters the relative volatility of the mixture. It is important to note that the solvent must not form an azeotrope with any of the components.
Comparison With Azeotropic Distillation
Extractive distillation is usually more desirable than azeotropic distillation since no large quantities of solvent have to be vaporized. In addition, a greater choice of added component is possible since the process is not dependent upon the accident of azeotrope formation. It cannot, however, be conveniently carried out in batch operation.
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