Modeling and Scaleup of Reverse Osmosis Separation

Abstract

Over the past years reverse osmosis has become a commercially attractive microsolute separation process. This success is largely due to the development of new and more reliable polymeric membrane materials and of novel module configurations. Flat-sheet membrane units are preferred for laboratory experiments to determine the intrinsic membrane properties with respect to permeate flux and separation efficiency. These units are not expensive and allow different membrane sheets to be tested with the same unit because of the easy membrane replacement. The industrial use of these units is excluded due to the low membrane surface area to volume ratio. The separation characteristics of reverse osmosis also depend to a large extent on the geometry of the membrane elements and the associated hydrodynamic phenomena. Hence, residence time distribution experiments have been performed on industrial-scale spiral-wound modules to study the flow regime. The detailed results of these experiments are reported elsewhere, and allow determination of suitable expressions to describe the pressure drop and concentration polarization phenomena in the concentrate channel of the module. The modeling procedure that will be outlined in the paper incorporates the underlying hydrodynamic and transport phenomena, and is based on an integrated discretisation approach. The procedure is applicable for the simulation of a membrane plant with "christmas tree" configuration. The basic concept of the procedure is to stagewise determine the performance of one membrane element based on its inlet flow conditions. Eventually the outlet permeate flux and concentration are calculated by properly averaging the permeate fluxes of the individual membrane elements. Industrial experiments were carried out to evaluate the simulative accuracy of the modeling procedure for spiral-wound modules. This plant was designed to treat wastewater issued from producing microelectronic chemicals. Finally, a sensitivity analysis is performed to determine the critical parameters for the design of reverse osmosis separation by spiral-wound modules.