Analysis of retention and flux decline during ultrafiltration of limed sugarcane (clarified) juice

The flux decline behaviour of sugarcane juice solution during ultrafiltration (UF) process was studied both experimentally and theoretically. Rate of flux decline was analyzed under varying operating conditions such as pressure (550, 690, and 830 kPa) and feed concentration (11, 14, 17, 20 brix). A solution of the clarified limed sugarcane juice containing low molecular weight (LMW) solutes, mainly sucrose, and high molecular weight solutes (HMW) like proteins, fats and wax, etc., was subjected to UF in a stirred cell employing a cellulosic membrane (Spectra Por). Under typical experimental conditions, solution flux was found to reach a steady state value in about three hours. High molecular weight solutes (e.g., proteins) retained by the membrane formed a get like porous layer on top of the membrane surface. Relatively lighter molecular weight solutes (e.g., sucrose) were mostly permeable, though also partly retained within the gel layer. A mathematical model was developed to describe the flux decline behaviour during UF of sugarcane juice. The model analysis takes into account the formation of polarized concentration boundary layers of both types of solutes, permeable as well as non-permeable. Decline in the flux of sugarcane juice was attributed to the combined effects of gel layer formation as per the cake filtration theory and the development of osmotic pressure of sucrose in boundary layer; although contribution of osmotic pressure exerted by sucrose in flux decline was found to exist only during initial stage of filtration. The model predictions were found to agree well with the experimental data.