Hydraulics and Mass Transfer Efficiency of a Commercial-Scale Membrane Extractor

Abstract

In recent years there has been significant interest in utilizing microporous hollow fiber membranes for liquid-liquid extraction. The membrane extractor resembles the shell and tube heat exchanger with the tube section composed of 1000–2500 fibers/in². The diameter of each fiber is approximately 300 microns. In this process, the feed may be passed through the shell side, while the solvent is passed through the fiber side, or vice versa. Mass transfer occurs across the liquid-liquid interface formed in the pores of the fiber wall. The advantages of this technology are high throughput capacities, independence of density difference between the feed and solvent, and potentially high mass transfer areas. The mass transfer performance of an available commercial scale nonbaffled membrane extraction module was determined to be lower than expected from results obtained in smaller scale modules. Mass transfer studies of a commercial-scale membrane extraction module at the Separations Research Program have shown that a significant portion of the fibers are bypassed by the shell side fluid and consequently only a fraction of the total fiber surface area is utilized. A hydraulic study using a dye tracer technique verified this finding with an aqueous flow on the shell side. A model which incorporates mass transfer correlations reported by others has been developed and shown to have excellent agreement with the experimental data obtained. In this paper, the efficiency of the membrane extractor is compared with conventional spray, sieve tray, and packed columns; the effect of shell side bypassing is also presented.