Study of mass and heat transfer in the osmotic evaporation process using hollow fibre membrane contactors

摘要

In this work, the mass and heat transfer in the osmotic evaporation process was studied using a hollow fibre membrane contactor. A calcium chloride dihydrate solution (2.8-6 M) was circulated in the fibres and a sucrose solution (20% and 45%, w/w) in the shell side of the module. The respective mass transfer correlations were determined using a single-step methodology involving the mass and heat transfer equations, simultaneously: Sh_t = 2.66 Re_t~(0.25)Sc_t~(0.33)(d/l)~(0.33) and Sh_s = 15.4Re_s~(0.92) Sc_s~(0.33) (d/l). The correlations are valid for Reynolds numbers between 0.5 and 45 and are in agreement with that obtained for liquid-liquid extraction and gas absorption. It was observed that the mass transfer resistance in the fibres could be easily minimized, even when high driving forces are applied. Regarding the shell side boundary layer mass transfer resistance, it can be significant when processing concentrated sucrose solutions (50% of the overall resistance for a 61% (w/w) sucrose concentration). The temperature profile was calculated using a model, which combines the heat and mass transfer equations. Although the temperature gradient through the boundary layers was not significant, the temperature difference between the membrane interfaces can be important for high driving forces, it reached almost 3 ℃ when using a 6 M CaCl_2 solution. The mass and heat transfer model was also used to calculate the water flux and the overall mass transfer coefficient during the concentration process of a sucrose solution. The values predicted are in agreement with the experimental results for the range of Reynolds and Schmidt numbers studied.