CHEMISTRY OF MEMBRANE SEPARATION PROCESSES IN SUGAR-INDUSTRY APPLICATIONS

Interactions of sugarcane and sugarbeet components during membrane fil tration result in complex membrane surface modifications. This is the case with synthetic organic membrane systems as well as with ceramic c ross flow colloidal and ultrafiltration systems. Major factors involve d in solute separation by membranes are 1) intrinsic rejection by the membrane of a specific solute, and 2) the nature of the concentration polarization layer at the surface of the porous filtration medium. In this study, factors involved in determining the behavior of membranes in sugar processing have been defined for raw materials, including jui ce and molasses fractions entering the membrane streams, as well as fo r each of the fractions coming from the membrane retentates and permea tes. Subfractionation of the separated materials by membrane ''nominal molecular weight cutoff'' types afforded both enriched high molecular weight polymer retentates, and permeates of lower molecular weight. C omponents were further analyzed by gel permeation chromatography, comp ositional analysis, and physical methods such as nuclear magnetic reso nance spectroscopy. Colorants, polysaccharides and other non-sugars ha ve been classified in these separated fractions to determine their dis position in membrane filtration processes for both cane and beet sugar s. Colloidal chemistry of molasses retentates indicates that affinity of the natural components produces very high molecular weight aggregat es which are implicated in membrane fouling and other inefficiencies o f the process. Intrinsic rejection of solutes by the membranes, and th e dynamics of concentration polarization of the membrane surface, were found to be very important physical properties in estimating feasibil ity of using membrane processes to reduce energy input and maintenance costs, and to effect improved yields of sugar. Reduction of tightly h eld, colloidally suspended particulate components in sugar juices or s yrups and liquors by physical means before membrane filtration is extr emely important to ensure reduction of color bodies or crystallization inhibitors, and increased efficiency and product yield. In Part II tr eatment of sugarcane juice to remove membrane fouling materials is des cribed. Use of a disc-stack centrifuge on clarified sugarcane juice re moved over 65% of finely suspended solid material and allowed a signif icant increase in flux rate or cycle time.