Desalination by salt replacement and ultrafiltration.

Abstract

The replacement of solutes in a saline solution by a replacer chemical across an osmotic membrane, and the subsequent removal of the chemical by virtue of its special removal characteristics, comprises salt replacement desalination. Any of a number of removal processes may be coupled to the replacement step, the process being determined by characteristics of the replacer. Ultrafiltration is examined as a removal process with sucrose as the replacer chemical. A theoretical treatment of osmotic flow across semipermeable membranes is presented in terms of phenomenological and first-order transport equations. The pore flow and solution-diffusion models of the transport kinetics of such osmotic flows are derived. An experimental examination of the ultrafiltration of sucrose shows that higher flux membranes operated at lower pressures than reverse osmosis offer comparable product fluxes. Such a system would, thus, not require the high-pressure apparatus required for reverse osmosis. Experimental results also show that a constant separation relationship exists between concentrate and permiate at operating pressures above the osmotic pressure of the retinate, and that the separation characteristics of membranes of different cutoff levels with solutes of molecular weight well below the cutoff level being filtered are similar. These findings indicate that salt replacement with ultrafiltration has strong possibilities for development as a large-scale desalination method.