干旱区生态环境知识资源中心

Scarcity of potable water nowadays, presents a serious problem all over the world. Environmental changes are taking place at a rapid pace, resulting in greenhouse effects, desertification, and lack of fresh water. Accordingly, scientists are working hard toward inventing new techniques for desalinating sea water, which presents the only alternative solution to this problem. In this regard, desalination techniques are mainly divided into thermal and membrane techniques. However, the latter are superior in that they are modular in shape, and consume less energy. In the present work, desalination by a novel recent membrane separation technique, the so-called sweeping air pervaporation (PV), which has been very sparsely applied to desalination of sea water, has been conducted in our laboratory. The technique is simple, straightforward, cost-effective and does not suffer from limitations as regards low water recovery, such as reverse osmosis. An innovated deacetylated cellulose acetate (CA) membrane was prepared by the phase inversion technique, from a specially formulated casting solution mixture composed of CA, acetone (A), dioxane (D), dimethyl formamide, dimethyl phthalate, and maleic anhydride in definite proportions, and used in all the experiments. Two PV cells of different configuration and aspect ratio (AR) were designed and constructed, in order to investigate the effect of hydrodynamics on membrane performance. Numerous variables were studied for their effect on the pervaporate flux (J) and % salt rejection (%SR), and these were: initial salt solution concentration (C-i), PV temperature (Tpv), and PV cell configuration. It was found that the flux obtained was directly proportional to Tpv, and that the J was almost independent of C-i at low Tpvs, except in the case of Tpv=80 degrees C for both small and large configurations, where J was inversely proportional to C-i (in the range 15-120g/l), and that higher fluxes were obtained under the same conditions in case of the PV cell of higher AR. The activation energy (E-a) for permeation through the membrane in both the cells was computed from the Arrhenius equation, and was found to be 20.89-23.06kJ/molK at C-i=120g/l, for the cell with higher and lower AR, respectively, denoting facile permeation of water through the membrane, for the two cell designs, in particular with higher AR. The results indicate that at all concentrations tested for large cell below 70 degrees C, the product, after a once-through operation, was exceptional potable water with very low salinity (%SR 98.9) even when C-i was 120.8g/l, and maximum flux obtained was 5l/m(2)h at Tpv=70 degrees C. The separation factor (), PV separation index (PSI), overall mass transfer coefficient (K-ov), diffusion coefficient (D-i), salt diffusivity (D-s), and salt permeability (P), through the membrane were all calculated and their values are stated inside the paper text. Nevertheless, varied between 5.6 and 853.4, and PSI reached 3494.5l/m(2)h.