Resistance-model hollow fibers

Figure 3. Cross-section of a resistance-model hollow-fiber and electrical circuit analog...

The history of the membrane developments for reverse osmosis and gas permeation shows that because of inherent differences, it is not possible to simply apply the techniques and materials from one separation technology to the other. The success of the resistance-model hollow-fiber technology which is based on the glassy-fiber technology invented for reverse osmosis, demonstrates the necessity to search for advanced techniques to prepare more selective membranes free of imperfections, rather than to look for new, unavailable materials. 

Henis, J.M.S. Tripodi, M.K. Composite hollow fiber membranes for gas separation resistance model approach. J. Membr. Sci. 1981, 8, 233-246. 

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. 

Various parameters in Equation 31.17 have been defined earlier. Danesi et al. [92] described a simple correlation between permeability coefficient in FSSLM and HFSLM configuration. At very large values of ( ) (as compared to 1), Equation 31.17 is transformed into the one used for FSSLM by Danesi et al. [92]. Hence, the smaller the value of ( ), the higher will be the negative value of the left-hand side of Equation 31.17, which suggests the higher rate of mass transfer. Later on, D Elia et al. [93] considered the resistance in series model where they have studied the mass transport across hollow-fiber contactors in NDSX mode. They showed that the overall mass transfer resistance is equal to the sum of individual mass transfer resistances across the aqueous boundary layer and membrane phase. Mathematically, it can be written as follows ... 

Since the Henis and Tripodi s resistance model led to the development of the Prism hollow fiber, their model is examined in detail in the following example. 

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