Multiphase Extraction: Design of Single- and Multistage Separation Using Liquid Pseudomembranes

Abstract

In this work, we consider a relatively new liquid extraction method, namely, multiphase extraction, in which the mass exchange involves more than two liquid phases. Two types of multiphase extraction can be distinguished, one of which is simultaneous extraction of components from the initial solution by several flows of extractants and the other of which is multiphase extraction using liquid membranes and liquid pseudomembranes. Here, we analyze single- and multistage linear extraction using liquid pseudomembranes and derive analytical expressions for process parameters. Mass exchange between two or more liquid (aqueous or organic) phases without direct contact between them, specifically, by transferring components from one phase into another via an exchange medium, is used to separate and purify a wide range of pharmaceutical and chemical products, including radionuclides. The components are separated by distributing them between two or more phases. Such processes can be regarded as multiphase extraction since the mass exchange involves three or more liquid phases. Two possible types of multiphase extraction can be noted: (1) simultaneous extraction of a number of components from the initial solution by several flows of extractants and (2) processes in which the exchange (i.e., transport) medium is not removed from the system of an apparatus or a set of apparatuses and the flows of the phases involved in the mass exchange pass through the system without contacting one another. In this work, we study the second type of multiphase extraction. Among processes of this type are coupled extraction processes (extraction‐stripping, extraction‐stripping‐washing, etc.), which are widely used in hydrometallurgy; processes using liquid membranes, which have not yet found noticeable industrial use [1]; and also the relatively new technique of using so-called liquid pseudomembranes [2‐13]. Multistage extraction using liquid pseudomembranes is a discrete method of extraction using supported liquid membranes. In devices with supported liquid membranes, the initial solution and the extract phase flow on different sides of a porous partition impregnated with a liquid membrane. Figure 1 presents a schematic of a single-stage three-phase extractor with natural circulation of a continuous membrane phase. The extractor consists of two vertical chambers designed as columns whose upper and lower parts are connected by overflow devices. The lower overflow device is a horizontal cylindrical settler, and the upper overflow device is a separator of similar design. The settler and the separator are divided into two compartments by a vertical partition, whose height is smaller than the diameters of the separator and the settler. In the first chamber, extraction is performed; in the second, stripping is carried out. The extractor is filled with a transport medium (liquid membrane), which serves as the continuous phase. The initial solution (raffinate phase) and the receiving phase (extract phase) are continuously fed to and removed from the extractor. With a nozzle or another atomizer, the liquids