Modeling of electrically assisted ultrafiltration of whey
Abstract
Concentration polarization and fouling are two problems that limit applications and performance of ultrafiltration processes. This work was an attempt to reduce concentration polarization and increase permeate flux by applying electric fields during ultrafiltration of whey. A Minitan-S Millipore ultrafiltration cell was modified by adding two electrodes, one on each side of the membrane (polysulfone, 30kD molecular weight cutoff). Bovine whey was used to produce solutions with varying protein concentrations. Several experimental designs (two-level factorials, central composite designs, etc.) were used to vary concentrate flow rate, transmembrane pressure, protein concentration and electric field strength. Electric fields increased permeate flux in a non-linear manner. Electric field strength of 2500 V/m strength resulted in a 44% increase in permeate flux. Permeate concentration also increased with electric field strength suggesting that electric fields have a negative effect on membrane retentivity. Concentrate pH decreased, while permeate pH increased, due to the passage of electric current through the whey solution. Response surface analysis showed that all independent variables and several two way interactions were significant. Operating conditions for maximum permeate flux and minimum overall consumption were located. Pulsating electric fields further increased permeate flux (up to 100%) and/or reduced energy consumption. Existing theoretical models failed to explain the non-linear effects of electric field strength on permeate flux. Secondary effects, such as electroosmosis and electrophoretic mobility changes, could be responsible for the non-linear behavior. However, a computer simulation showed that similar non-linear effects were present even when secondary effects were ignored and electrophoretic mobility was constant. Non-linear effects can be attributed to the parabolic velocity profile of the protein solutions flowing over the membrane. Overall, application of electric fields improved permeate flux during ultrafiltration of complex biochemical solutions. This improvement was significant at high electric field strengths and during application of pulsating electric fields.
Degree
Ph.D.
Advisors
Floros, Purdue University.
Subject Area
Food science|Chemical engineering
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