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Many factors must be evaluated when developing a liquid-liquid extraction process. Here are some of the important parameters to consider as you go from laboratory testing to commercial-scale operation.
Liquid-liquid extraction is an important separation technology for a wide range of applications in the chemical process industries (CPI). Unlike distillation, which is based on boiling point differences, extraction separates components based on their relative solubilities in two immiscible liquids. Extraction is typically chosen over distillation for separation applications that would not be cost-effective, or even possible, with distillation.
This article discusses the basics of liquid-liquid extraction and provides guidance on how to select the appropriate solvent and extraction equipment. It introduces key concepts associated with designing a liquid-liquid extraction process. The article also touches on unit design and the importance of placing temperature, pressure, and level control devices in the correct location.
The basics
In a liquid-liquid extraction unit, a liquid stream (carrier) containing the components) to be recovered (solute) is fed into an extractor, where it contacts a solvent. The two liquids must be immiscible or only slightly miscible; this allows them to form a dispersion, with one liquid dispersed as droplets in the other.
Mass transfer occurs between the droplets (dispersed phase) and the surrounding liquid (continuous phase). In order for the two liquids to be subsequently separated, they must have different densities. The droplets then accumulate above or below the continuous phase, depending on the liquids' relative densities. The boundary between the continuous phase and the droplet dispersion is referred to as the interface, and can be at the top or bottom of the extraction column.
Figure 1 illustrates the general concept of liquid-liquid extraction. More fundamental information about liquidliquid extraction can be found in engineering textbooks as well as in Perry's Handbook (1).
Extraction is typically carried out in continuous, staged units, which can be operated in either of two modes: with co-current mixing or with counter-current mixing. The co-current mixing mode is generally limited to one theoretical stage per extraction unit, whereas counter-current mixing is amenable to multiple stages per unit. For this reason, counter-current mixing is usually preferred over co-current mixing.
Counter-current extractors can be arranged in one of two ways (Figure 2), the choice of which depends...