Modelling temperature effects in a membrane cascade system for oligosaccharides

Open nanofiltration of mixtures of fructo-oligosaccharides was assessed by experiment and by modelling the overall permeation behaviour of 3 different membranes. The temperature effect was modelled using the steric pore model, incorporating the molecular volumetric expansion of fructo-oligosaccharides as solutes, the decrease in the solution viscosity and the volumetric expansion of the membrane with increasing temperature. The thermal expansion of the solute was described as a linear increase in the bare molecular volume plus a non-linear decrease in its hydration number. The viscosity reduction was modelled by incorporating the temperature as a variable into an existing exponential relation derived by Chirife and Buera. The thermal expansion of membranes was described with a linear increase in the pore size and a linear decrease in its hydrodynamic resistance. Although the purity of the oligosaccharide product was hardly affected by the temperature, the yield was much lower at higher temperatures. The yield can therefore be improved by decreasing the temperature while maintaining the product purity. This behaviour was also observed in a 3-stage filtration cascade. The temperature effect is closely related to the increase in fluxes with temperature, leading to a different split of the feed into permeate and retentate. In a membrane cascade, the lower yield with higher temperatures was seen most strongly at the top stage, and much less at the middle and lower stages, which can be explained by the configuration of the cascade.