Concentration of Clarified Thin Sugar Juice through Multistage-NF/RO Membrane Technology

Sugar Manufacturing is one of the most energy intensive processes. Increasing fuel prices and stringent environmental regulations are convincing sugar manufacturers to search for alternative energy efficient and environmentally friendly processes. The conventional concentration of thin juice by evaporation needs about 50% of the total energy, making it the most energy consuming step in sugar manufacturing. RO/NF has been studied for concentration of thin sugar juice. However, it cannot reach concentrations higher then 20-25°brix in single-stage RO/NF system due to high osmotic pressure limitation.

In the present work a new multi stage NF/RO process has been developed for concentrating the liquid solutions to higher concentrations with moderate operating pressures. In experiments, sugar solution was concentrated from initial feed concentration of 15wt% to 50wt% with moderate transmembrane pressure of 32bar. The experiments were performed at 80°C because the thin sugar juice after clarification is available at 80°C so decreasing the temperature from 80°C to 25-40 °C for membrane filtration and then increasing again for evaporation will not only require heat exchangers and energy but this low temperature is also very favourable for microbial growth which degrade the solution. In classical single stage membrane concentration processes, an operating pressure of more than 102bar would be required as the osmotic pressure of sucrose solution at 50w% at 80°C is about 102bar. This very high transmembrane pressure is not economically feasible, additionally, at high temperatures of 75-80°C it is not possible to find operating conditions for the presently available reverse osmosis and nanofiltration membranes. The application of this technology presented in this work will result in significant reduction of the energy consumption compared to the conventional evaporation.