Solvent Extraction with Microporous Hydrophilic and Composite Membranes z Dispersion-free solvent extraction using microporous hydrophobic membranes has been extended to hydrophilic and composite hydro- phobic-hydrophilic membranes. Excess phase pressure conditions, if needed for dispersion-free operation, have been identified. Boundary layer and membrane resistances to solute transport have been isolated and simple relations developed for the overall mass transfer coefficient in such systems. A variety of flat microporous membranes have been utilized. Previous investigations by others had interpreted the mem- brane mass transfer resistance using the notion of unhindered diffusion through tortuous pores of the membrane. We have studied here the applicability and limitations of such a model for a number of membrane- solute-solvent systems. zyxwvutsrq Ravi Prasad and K. K. Sirkar Department of Chemistry and Chemical Engineering Stevens Institute of Technology Castle Point, Hoboken, NJ 07030 Introduction zyxwvutsrqp Highly efficient dispersion-free techniques using microporous hydrophobic membranes are increasingly being considered for a variety of conventional equilibrium continuous contact opera- tions. A few of the candidate operations studied are gas absorp- tion (Qi and Cussler, 1985a) and separation of volatile materials from liquids (Qi and Cussler, 1985b; Imai et al., 1982). Solvent extraction without dispersion has also been studied recently using a microporous hydrophobic membrane (Kiani et al., 1984; Prasad et al., 1986). In this technique, the interface of the immiscible aqueous and organic phases is immobilized at the pore mouths of a microporous hydrophobic membrane wetted by the organic solvent by maintaining the aqueous phase at a pres- sure higher than that of the organic phase. Several advantages of this technique with membranes in a hollow-fiber configura- tion have been pointed out (Kiani et al., 1984; Qi and Cussler, 1985a; Prasad and Sirkar, 1987). These are: very high interfa- cial area per unit extractor volume, ability to handle solids, no need to have a difference in phase densities, well-defined inter- facial area, independent variation in phase flow rates, and there- fore lack of flooding and ease in scale-up due to modular nature. The application of this technique with Celgard microporous hydrophobic hollow fibers has been illustrated in a fermentor- extractor where alcohol is locally extracted from a fermentation broth using dibutyl phthalate (Frank and Sirkar, 1985). Prelimi- nary performance details of a hydrophobic hollow-fiber extrac- Correspondence concerning this paper should zyxwvutsrqp bc addressed to K. K. Sirkar tor without the complications of fermentation are available else- where (Prasad and Sirkar, 1987). To explore the versatility of the phenomenon of dispersion- free solvent extraction with microporous membranes, the above technique has been extended to microporous hydrophilic and microporous hydrophobic-hydrophilic composite membranes. In the case of a hydrophilic membrane, the aqueous-organic inter- face is immobilized in the pore mouths by having the organic phase at a pressure higher than that of the aqueous phase, with the pores of the membrane filled with the aqueous phase prefer- entially wetting the hydrophilic membrane. Dispersion-free solvent extraction has also been studied with composite hydrophilic-hydrophobic membranes where neither the organic nor the aqueous phase need be at a higher pressure. The hydrophobic section of the composite membrane is wetted by the organic phase, while the hydrophilic section is preferen- tially wetted by the aqueous phase; the hydrophilic and hydro- phobic natures of the membrane (which therefore has asymmet- ric wetting properties) are sufficient to prevent breakthroughs of the organic phase through the hydrophilic membrane and of the aqueous phase through the hydrophobic membrane unless the differences between phase pressures exceed some critical values characteristic of the solute-solvent-membrane system. Such a technique without dispersion or coalescence has been studied here with the following solvent-water-solute systems: xylene-water-acetic acid, methyl isobutyl ketone (MIBK)- water-acetic acid, n-butanol-water-succinic acid. A variety of flat hydrophilic and stacked composite hydrophilic-hydrophobic membranes with a wide variation in porosity, pore size, thick- AIChE Journal July 1987 Vol. 33, No. 7 1057