Separation of xylose from glucose by nanofiltration from concentrated monosaccharide solutions

Abstract

Complex separation of monosaccharides from each other is commercially carried out by chromatographic methods. The possibility of nanofiltration in a demanding separation of a pentose sugar, xylose, from a hexose sugar, glucose, is studied here. Xylose is an intermediate product in xylitol production and glucose interferes in the process. Feed solutions were made of xylose and glucose in different mass ratios and total monosaccharide concentrations. The mass ratios of xylose to glucose in solutions were 1:9, 1:1 and 9:1 and the monosaccharide concentrations of the solutions were 2, 10 and 30 wt.%. Desal-5 DK, -DL and NF270 membranes were used. Filtrations were done in total reflux mode (i.e. both permeate and retentate were recycled back to the feed tank) at 50 °C and the applied pressures were from 2 to 40 bar. The results indicate that the separation of xylose from glucose by nanofiltration is possible to a limited extent. The mass ratio of xylose to glucose in the permeate was 1.5–3.0 times higher than their ratio in the feed. The observed monosaccharide retentions depend highly on permeate flux, and retentions increase to certain reproducible level as pressure and consequently flux is increased. The observed xylose retentions were from 0 to 80% and the glucose retentions were from 10 to 90%. The effect of total monosaccharide concentration on the observed retention is smaller than the effect of flux. The largest difference between xylose and glucose retentions was detected at permeate fluxes between 5 to 30 kg m −2 h −1. The ratio of xylose to glucose in the feed had an influence on permeate flux and on xylose retentions. Xylose retentions decreased as the proportion of glucose increased in the feed. The higher the proportion of xylose in the feed the higher was the total permeate flux.