Membrane Mass Transfer Resistance Research Articles

Influence of matrix on external mass transfer resistance in immobilized urease membranes

The influence of seven kinds of chemical modification of acrylonitrile copolymer membrane over both: transport characteristics of modified membranes and the diffusion-limitations of substrate in immobilized urease membranes were studied. It was found that the modification improved the hydrophilicity of the membranes and urea diffusion coefficient. The kinetic parameters V max and K m of the immobilized urease were also determined. Besides this, a good correlation between diffusion coefficient of urea through the modified membranes and K m was observed. For all the membranes studied, diffusion-limitations of substrate inside of the porous membrane were not found because there was not immobilized urease in membrane pores, but there was external membrane mass transfer resistance. To establish the external mass transfer resistance, the values of mass transfer coefficient ( K L) and Nernst diffusion layer were determined. In the present experiments, a good correlation between hydrophilicity and the thickness of the boundary layer on membrane surface and K L, respectively, was observed. This results proved the influence of the modified matrix on mass transfer coefficient. All the immobilized systems studied were found to have weaker external mass transfer resistance and the values of the effectiveness factor calculated were higher than 0.95. A common feature of all immobilized systems was their quite high effectiveness factor, which clearly showed that the modified acrylonitrile copolymer membranes studied were suitable carriers for immobilization of the enzyme urease.

Mass transfer enhancement in the membrane aromatic recovery system (MARS): experimental results and comparison with theory

This paper describes an experimental investigation into mass transfer enhancement by chemical reaction in the membrane aromatic recovery system (MARS). The MARS process has recently been commercialised for recovery of cresols from aqueous process streams. In this paper, mass transfer enhancement as a function of the pH and organic concentration of the stripping solution is examined for constant bulk solution conditions. The application of two models of mass transfer enhancement (the Olander and Hatta models) is evaluated by determining the model parameters through independent experiments, limiting cases, and from literature. Model predictions are then compared to experimental data. Three organic compounds are used in this work: phenol, 4-chlorophenol (4CP), and triethylamine (TEA), which span a range of properties of typical molecules recovered by the MARS process. It is concluded that for compounds such as phenol, for which membrane mass transfer resistance dominates the system, there is no reaction enhancement. For compounds such as 4CP and TEA, enhancement effects are important. These were found to be best described through application of the Olander model, which allows for a second-order instantaneous reversible reaction, although the Hatta model, allowing for a second-order instantaneous irreversible reaction, was found to give a good description in some cases as well.

Mass and momentum transfer in commercial blood oxygenators

In modern blood oxygenators, microporous membranes (flat sheet or hollow fibre) are used to separate the blood and gas phases. Blood flows on one side of the membrane while gas (usually oxygen or air) flows on the other side. Since the membranes used are hydrophobic (e.g., polypropylene, Teflon), the pores are gas filled resulting in a negligible membrane mass transfer resistance. The major resistance to gas transfer is due to the blood side mass transfer boundary layer. In this study mass transfer and friction factor correlations for commercially available hollow fibre and flat sheet blood oxygenators have been determined experimentally. Water was used as a substitute for blood. The diffusion of oxygen into and out of water has been studied. Two different flow configurations used commercially have been investigated: flow outside and across woven hollow fibre bundles and flow in thin channels. The results for flow across bundles of woven hollow fibres may be compared to analogous results for flow in cross flow heat exchangers. The results obtained here can be used to further optimize the design of blood oxygenators.

A study of mass transfer in the membrane air-stripping process using microporous polyproplylene hollow fibers

The mass transfer of water and chloroform in membrane air-stripping (MAS) was studied using a microporous polypropylene hollow fiber membrane module, with air flow on the lumen side and liquid cross-flow on the shell side. Water transport experiments showed that its mass transport decreased significantly when the membrane had been in contact with water for prolonged periods. It was hypothesized that the increased mass transfer resistance was due to water condensation in a fraction of the membrane pores. MAS of chloroform from aqueous solutions confirmed the additional mass transfer resistance with prior exposure to water. It was concluded that membrane pores were either completely air-filled or partially wetted with water during the MAS process. Existing models are able to predict the performance only for either completely air-filled or liquid-filled pores. A modified model was proposed to take into account the diffusion through partially wetted pores. The model described the data well. This hypothesis also provided a plausible explanation to the conflicting literature values of the membrane mass transfer resistance. It was found that the membrane mass transfer resistance of the partially wetted pores is two orders of magnitude higher than that of air-filled pores. The overall mass transfer coefficient was constant for initial feed chloroform concentrations ranging from 50 to 1000 ppm.

Solvent extraction with microporous hydrophilic and composite membranes

AbstractDispersion‐free solvent extraction using microporous hydrophobic membranes has been extended to hydrophilic and composite hydrophobic‐hydrophilic membranes. Excess phase pressure conditions, if needed for dispersion‐free operation, have been identified. Boundary layer and membrane resistances to solute transport have been isolated and simple relations developed for the overall mass transfer coefficient in such systems. A variety of flat microporous membranes have been utilized. Previous investigations by others had interpreted the membrane mass transfer resistance using the notion of unhindered diffusion through tortuous pores of the membrane. We have studied here the applicability and limitations of such a model for a number of membrane‐solute‐solvent systems.