Process and equipment design for membrane-based extraction: Basic considerations

The basic issues involved in designing membrane-based extraction processes are systematically treated. The main objectives of liquid–liquid extraction operations (i.e. maximization of solute recovery and of solvent saturation by solute) serve as the basis of this presentation. Analyzing the operating characteristics of a single membrane module (comprised of a bundle of parallel fibers), it is shown that two parameters determine process performance; i.e. the extraction factor E (reflecting thermodynamic limitations) and a lumped parameter A which takes into account geometrical features of the module as well as kinetic and thermodynamic limitations. A narrow range of parameter values for E and A, leading to near-optimum module performance, as well as a procedure for preliminary design are recommended. The relative significance and inter-relation of geometric and operating variables is also appraised. Using dimensionless quantities, closed form expressions of general validity are developed, for a variety of continuous process options (single module, multi-module, counter-current and “cross-flow” arrays). These expressions allow direct comparative assessment of design alternatives, thus facilitating process design and performance optimization. Batch operations are also treated for the two practically significant cases of solvent re-circulation and “once-through” mode of operation. Limitations of the former and merits of the latter mode are identified.