Chemistry of membrane separation processes in sugar industry applications

Interactions of sugarcane and sugarbeet components during membrane filtration result in complex membrane surface modifications. This is the case with synthetic organic membrane systems as well as with ceramic cross flow colloidal and ultrafiltration systems. Major factors involved 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 juice and molasses fractions entering the membrane streams, as well as for each of the fractions coming from the membrane retentates and permeates. 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. Components were further analyzed by gel permeation chromatography, compositional analysis, and physical methods such as nuclear magnetic resonance spectroscopy. Colorants, polysaccharides and other non-sugars have been classified in these separated fractions to determine their disposition in membrane filtration processes for both cane and beet sugars. Colloidal chemistry of molasses retentates indicates that affinity of the natural components produces very high molecular weight aggregates which are implicated in membrane fouling and other inefficiencies of the process. Intrinsic rejection of solutes by the membranes, and the dynamics of concentration polarization of the membrane surface, were found to be very important physical properties in estimating feasibility of using membrane processes to reduce energy input and maintenance costs, and to effect improved yields of sugar. Reduction of tightly held, colloidally suspended particulate components in sugar juices or syrups and liquors by physical means before membrane filtration is extremely important to ensure reduction of color bodies or crystallization inhibitors, and increased efficiency and product yield. In Part II treatment of sugarcane juice to remove membrane fouling materials is described. Use of a disc-stack centrifuge on clarified sugarcane juice removed over 65% of finely suspended solid material and allowed a significant increase in flux rate or cycle time.