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Chemical and Process Engineering Resources

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Category: Separation Technology
Question: What factors should be considered when choosing a solvent for liquid-liquid extraction?
Keywords: v1i12,extraction,liquid,solvent,selection,criteria
Answer: The choice of solvents for liquid-liquid extraction is an important decision. The following factors should be considered:A. Distribution Coefficient - A high distribution coefficient means that the solvent will have a high affinity for the solute. This will allow lower solvent/solute feed ratios.B. Separation Factor - A high value of the separation factor reduces the number of equilibrium stages required. Note: Vendor software and Perry's Chemical Engineers' Handbook are good sources of Distribution Coefficients and Separation Factors for many systems encountered in industry.C. Density and Viscosity - Large differences in densities between the extract and the raffinate phases allows for maximum capacities in devices using gravity for phase separation. High viscosites can cause difficulties in pumping and dispersion. D. Ability to Recover Solvent - A solvent with a boiling point that is "as different as possible" from the system components leads to an enhanced ability to recover the solvent.E. Solubility of Solvent - A high solubility of the solvent in the system components can leads to difficulties in the solvent recovery. Another distillation step may be necessary.F. Availability and Cost - Naturally, the solvent that is selected should be readily available at a reasonable cost. The intial expense of charging the system should be considered as well the operating expense involved with makeup solvent.G. Toxicity and Flammability - Additional costs may be incurred in a system is to employ a solvent that represents a hazard.H. Reactivity - It's important that the selected solvent is stable in the system and that it does not polymerize.I. Corrosivity - Be sure that the solvent in compatible with the same materials of construction as the system components.J. Compatibility - Be sure that the final product would not be contaminated by the solvent. This is especially important in the food and pharmaceutical industries.Reference: Separation Process Technology, ISBN: 0070311730
Links: Liquid-Liquid Extraction Article