Pore Model for Pervaporation

P. Sukitpaneenit, T.S. Chung, and L.Y. Jiang. (2010). Modified pore-flow model for pervaporation mass transport in PVDF hollow fiber membranes for ethanol-water separation, J. Memb. Sci. 362 393-406. 

Reverse osmosis, pervaporation and polymeric gas separation membranes have a dense polymer layer with no visible pores, in which the separation occurs. These membranes show different transport rates for molecules as small as 2-5 A in diameter. The fluxes of permeants through these membranes are also much lower than through the microporous membranes. Transport is best described by the solution-diffusion model. The spaces between the polymer chains in these membranes are less than 5 A in diameter and so are within the normal range of thermal motion of the polymer chains that make up the membrane matrix. Molecules permeate the membrane through free volume elements between the polymer chains that are transient on the timescale of the diffusion processes occurring. 

Finally, it is important to notice the effect of the support in the pervaporation flux, analyzed by Bruijn et al. [117] who proposed a model and evaluated the contribution of the support layer to the overall resistance for mass transfer in the selected literature data. They found that in many cases, the support is limiting the flux the permeation mechanism through the support corresponds to a Knudsen diffusion mechanism, which makes improvements in the porosity, tortuosity, pore diameter, and thickness necessary for an increase in the pervaporation flux. 

Finally, it is important to notice the effect of the support in the pervaporation flux, analyzed by de Bruijn et al. [164] who proposed a model and evaluated the contribution of the support layer to the overall resistance for mass transfer in the selected literature data. They found that in many cases, the support is limiting the flux the permeation mechanism through the support corresponds to a Knudsen diffusion mechanism, which makes improvements in the porosity, tortuosity, pore diameter, and thickness necessary for an increase in the pervaporation flux. In fact, the researchers of Bussan Nanotech Research Institute Inc. (BNR), Sato et al. [165], designed and patented an appropriate asymmetric ceramic porous support to suppress pressure drop, and in this case, the water flux increased dramatically compared to previous reported results. Wang et al. [166] have clearly shown that the flux of the membranes increased with the porosity of the hollow fiber supports. In spite of the thin 1 pm zeolite layer, prepared by Zhou et al. [167], the flux enhancement compared to layers 10 times thicker [168] was not significant. 

Figure 6.31 shows some experimental data for the pervaporation of water/ethanol mixtures by a silicone rubber membrane preferentially permeable to ethanol. The experiment was conducted at 23 C for downstream pressures of 667, 1200, and 2100 Pa (5,9, and 16 mmHg). As reported by Hoover and Hwang [250] and Tanigaki et al. [245], the silicone membrane showed preferential permeation to ethanol. Evidently, the downstream pressure has little effect on both permeate composition and permeation rate, supporting the calculated results shown in Figure 6.30. When the experimental data arc closely examined, however, the relative permeation rate decreases slightly with an increase in the downstream pressure. The calculated values in Figure 6.30 show exactly the same tendency, justifying the transport model on which the calculation is based. It has to be noted, however, that the saturation vapor pressure of water and ethanol at 60 C are 1.99 x l(f and 4.69 x lO Pa (149.4 and 351.9 mmHg), respectively. When the downstream pressure approaches the saturation vapor pressure, the assumption on which the theoretical calculation is based (i.e., (he vapor permeation prevails across the membrane cross-.section) becomes invalid, since liquid penetrates more deeply into the pore.

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