Liquid extraction is considered when separation by distillation is in effective or very difficult. The substance that have close boiling point and the substance that cannot bear high temperature in distillation even under a vacuum then these substances are separated from impurities by extraction.
Liquid extraction utilizes chemical difference instead of vapor pressure differences. For example penicillin is separated from the fermentation broth by extra with a solvent such as butyl acetate.
It is done after lowering the pH to get a favorable partition coefficient. The solvent is then treated with buffered phosphate solution to absorb penicillin from the solvent and thus gives away a purified aqueous solution.
Penicillin is recovered after drying. Acetic acid is recovered from dilute aqueous solution by extraction. Although distillation process can also be utilized but extraction process reduces the amount of water to be distilled.
Petroleum products are largely separated by extraction. The product containing almost same boiling point but different chemical structures are separated by liquid extraction. For example lube oil fraction having boiling point greater than 300 °C are treated with low boiling polar solvents such as phenol furfural or methyl pyrrolidone. In this way aromatics are extracted with oil containing paraffin and naphthenes. Aromatics cannot be separated from distillation because of over lapping of boiling point. Even they have low temperature- viscosity characteristic. In the same way aromatics are also recovered from catalytic reformate using a high boiling point polar solvent. After this the extract is distilled to get benzene, xylene and toluene which are used as chemical intermediate. The solvent that is best for extracting aromatics is Sulfolane. It has very high selectivity for aromatics and very low volatility.
When both distillation and extraction can be carried out then distillation is preferred. In spite of the fact that heating and cooling are needed. In extraction process the solvent is recovered by distillation. The combine process is generally more complicated/complex and even expensive than a single distillation process. Although, extraction provides greater flexibility in choice of operating conditions, since the type and amount of solvent can be varied as the operating temperature. In this sense, extraction is considered for gas absorption over distillation.
Extraction can be used for separating more than two components and mixture of solvents.
Liquid Extraction Equipment
In this process two phases must be brought in good contact as in absorption and distillation to ensure material transfer and then separation. In absorption the mixing and separation of two phases is relatively easy and rapid. However, in case of extraction the two phases have comparable densities, so that the energy required for mixing and separation (when gravity flow is used) is small and even much smaller than when one phase is a liquid and other phase is a gas. The two phases are often hard to mix and harder to separate. The viscosities of both phases are relatively high and linear velocities through most equipment are low. Therefore energy needed for mixing and separation is supplied mechanically.
Extraction equipment is of two types i.e. batch and continuous. A quantity of liquid is mixed with a solvent in an agitated vessel. When the layers are settled it is separated. The extract contains the solvent with the extracted solute while the raffinate contains the layer from which the solute is removed. The extract could be lighter or greater than the raffinate. The operation can be repeated depending on the amount of solute extract. In case the quantity is large and several contact is needed then the continuous operation is carried out. Large numbers of extraction equipment are continuous with either successive stage contacts or differential contacts
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