SHEAR CONTROLLED MODEL OF THE ULTRAFILTRATION OF SOY SUSPENSIONS

The ultrafiltration of the aqueous extract of full-fat soy flour was i nvestigated as a complex suspension of proteins, lipids and carbohydra tes where the foulant layer is the major resistance. Ultrafiltration w as performed on both homogenised and unhomogenised soy extracts using a flat plate module and two polysulphone membranes. The permeate flux profiles for constant recirculation flow exhibited a combination of fa lling-flux and constant-flux regions. The mass transfer model was not applicable to these profiles. Both regions were modeled by the shear c ontrolled model J = J(i) tau(r)(n)ae(-bC). The model was also effectiv e in correlating the flux where the retentate flow was reduced for inc reasing concentration at constant pressure drop over the module. Wall shear stress had a larger effect on the permeate flux for the homogeni sed extract (n = 0.7) than for the unhomogenised extract (n = 0.5). Th e wall shear stress was shown to have minimal effect on displacing the foulant layer. The major effect of the wall shear stress was to sweep away the gel layer preventing it from binding to the foulant layer.