Distillation is used predominately in the process industries for separating the components of a liquid mixture. Liquid-liquid extraction (LLE) has some similarities to distillation; but whereas distillation employs differences in the boiling point to make a separation, extraction relies on the differences in chemical structure. LLE applications have a broad range and are growing.

LLE is seldom a stand-alone operation. It nearly always requires at least one distillation step as an adjunct because the extract stream contains not only the component but also some solvent. Most LLE processes distill this stream to purify the component and recover the solvent.

If a liquid-liquid separation problem can be conveniently handled by distillation alone, that option is simpler and better than LLE. Extraction becomes necessary in the separation of components that are either difficult or im-

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