Concentration of xylose reaction liquor by nanofiltration for the production of xylitol sugar alcohol

Xylitol, a five-carbon sugar alcohol, is used commercially as a natural sweetener in several food products including soft drinks and confectionery. Xylitol is produced by catalytic dehydrogenation of xylose, an intermediate sugar compound, which is extracted from sugar cane bagasse, corncobs or rice husk. The reaction liquor of xylose contains 1.5-2% of d-xylose besides water as the major constituent. The current process of evaporation for concentration of xylose reaction liquor can be effectively replaced by the more economical nanofiltration technique. Pilot scale nanofiltration (NF) experiments were carried out using a polyamide (PA) spiral membrane module having 300 molecular weight cut-off (MWCO) and 1 m2 effective area. At a feed pressure of 20 bar, xylose was concentrated from 2 to 10% at a reasonably high average flux of 24 l/(m2 h) and rejection of >99% which indicated negligible losses of the sugar in the permeate. The efficiency of NF was compared with another membrane methods, reverse osmosis (RO) for concentration of xylose reaction liquor. Effect of fouling on NF membrane performance and life span as well as methods for cleaning the membrane for restoring flux and rejection were evaluated. An economic comparison between nanofiltration and evaporation for a feed capacity of 1 m3/h of xylose reaction liquor showed that NF capital investment was 75% of the evaporator installation cost, whereas the operating cost for NF was only 0.04 times the evaporation cost.