Concentration of aqueous glycerol solution by using continuous-effect membrane distillation

In the present work, continuous-effect membrane distillation (CEMD) process with a function of in-situ internal latent-heat-recovery was applied to concentrate aqueous glycerol solution by using a hollow fiber air-gap membrane distillation module. Compared with other conventional membrane distillation processes, the CEMD process enabled higher energy efficiency and lower operational cost, and thus greater potential commercializing application. The deep-concentrating experiments revealed that a feed of 10 g/L could be successfully concentrated up to about 400 g/L with a rejection efficiency still greater than 99.9%. The maximum value of gain output ratio (GOR) experimentally obtained could reach 16.2, and even though the glycerol concentration was up to about 350 g/L, the GOR value could still be 5.3, which corresponded to the energy efficiency of a conventional seven-effect evaporator. When the feed was concentrated up to around 300 g/L, the GOR and the trans-membrane flux (Jw) decreased sharply. The increment of glycerol viscosity and the reduction of water vapor pressure resulted from the increase of feed concentration were the critical reasons that led to deterioration in GOR and Jw. Four-factor two-level full factorial analysis showed that for a certain feed-in concentration, the heated feed-in temperature was the most significant factor to obtain higher GOR and Jw. The value of GOR and Jw basically maintained invariable in the long-term stability test lasting for 60 days, which demonstrated that CEMD process was suitable for concentrating aqueous glycerol solution in a high-efficiency and energy-saving way.