E. L. Cussler

Microporous hollow fibers for gas absorption. I. Mass transfer in the liquid

Abstract Microporous hollow fiber modules offer a larger area per volume between gas and liquid than that commonly encountered in packed towers. This larger area can be sustained at very low flows, where packed towers will not be loaded, and at very high flows, where packed towers will flood. As a result, the modules offer the potential of faster mass transfer. This potential can be compromised by the resistance to mass transfer of the membrane itself, a resistance which is increased if the liquid wets the membrane. The results presented in this two-part series show when the advantage of the increased area is greater than the disadvantage of the membrane resistance. In this part, a theory for the operation of hollow fiber membrane modules is developed, and mass transfer coefficients in the liquid phase are investigated.

Pervaporation membranes in direct methanol fuel cells

Abstract The membranes in direct methanol fuel cells must both conduct protons and serve as a barrier for methanol. Nafion, the most common fuel cell membrane, is an excellent conductor but a poor barrier. Polyvinyl alcohol pervaporation membranes are good methanol barriers but poor conductors. These and most other pervaporation membranes offer no significant advantages over Nafion in methanol fuel cell applications. However, polybenzimidazole membranes have demonstrated characteristics that suggest up to a 15-fold improvement in direct methanol fuel cells. This improvement may be due to an alternate form of proton conduction in which protons travel via a Grotthus or “hopping” mechanism.

Temperature sensitive gels as extraction solvents

Crosslinked poly(N-isopropylacrylamide) gel and crosslinked copolymers of 97% N,N-diethylacrylamide and 3% sodium methacrylate which are swollen with water collapse abruptly at 33°C and near 55°C, respectively. These collapses apparently occur because each gel is near its lower critical solution temperature. The gels can be used to extract water and low molecular weight solutes from macromolecular solutions. After they have extracted the water, the gels can be regenerated by warming to release the absorbed water. The resulting separation process is effective for proteins and other polymers, and may be operated with waste heat which is usually discarded.

Mass transfer in various hollow fiber geometries

Abstract Mass transfer coefficients in commercial modules, including blood oxygenators, agree with literature correlations at high flows but are smaller at low flows. The smaller values at low flows probably result from channelling in the hollow fiber bundle. For the special case of flow within the fibers, the slight polydispersity of the hollow fibers causing this channelling can be used to predict deviation from the Leveque limit. These deviations can not be predicted from extensions to the Leveque analysis, or the analysis by Graetz. For the special case of flow outside the fibers, the mass transfer coefficients in commercial modules of various geometries are surprisingly similar, and fall below those of carefully handmade modules. These results can be used to develop still better membrane module designs.

Microporous hollow fibers for gas absorption : II. Mass transfer across the membrane

Abstract Microporous hollow fiber modules offer a larger area per volume between gas and liquid than that commonly encountered in packed towers. This larger area can be sustained at very low flows, where packed towers will not be loaded, and at very high flows, where packed towers will flood. As a result, the modules offer the potential of faster mass transfer. This potential can be compromised by the resistance to mass transfer of the membrane itself, a resistance which is increased if the liquid wets the membrane. The results presented in this two-part series show when the advantage of the increased area is greater than the disadvantage of the membrane resistance. In this part, overall mass transfer coefficients are studied, including resistances in both liquid and membrane, and the performance of hollow fibers is compared with that of packed towers.

Self-Assembled Block Copolymer Thin Films as Water Filtration Membranes

Nanoporous membranes containing monodisperse pores of 24 nm diameter are fabricated using poly(styrene-b-lactide) block copolymers to template the pore structure. A 4 mum thin film of the block copolymer is cast onto a microporous membrane that provides mechanical reinforcement; by casting the copolymer film from the appropriate solvents and controlling the solvent evaporation rate, greater than 100 cm(2) of a thin film with polylactide cylinders oriented perpendicular to the thin dimension is produced. Exposing the composite membrane to a dilute aqueous base selectively etches the polylactide block, producing the porous structure. The ability of these pores to reject dissolved poly(ethylene oxide) molecules of varying molecular weight matches existing theories for transport through small pores.

On the limits of facilitated diffusion

Abstract Facilitated diffusion normally involves mobile carriers, reactive groups which react with specific solutes and diffuse with them across a liquid membrane. Facilitated diffusion can also occur via carriers which are covalently bound to a solid membrane if the carriers still have some limited mobility. The solute flux across such chained carrier membranes varies with solute concentration just like the flux across mobile carrier membranes. The solute flux across chained carrier membranes shows different behavior vs. carrier concentration, including a percolation threshold.

Liquid-liquid extractions with microporous hollow fibers

Abstract Liquid—liquid extractions in microporous hollow fiber modules are unusually fast because of the large surface area per volume in these modules. Extractions of p -nitrophenol into amyl acetate show no membrane resistance, and hence are especially rapid. Extractions of acetic acid into methyl amyl ketone are controlled by the membrane resistance. The results show that extractions in hollow fibers will be fastest when the fibers are wet by the fluid in which the solute is more soluble. They also show when fiber modules are a sound inexpensive alternative to centrifugal extractors.

Estimating diffusion through flake-filled membranes

Monte Carlo calculations of diffusion across membranes containing impermeable flakes show effects of tortuous paths around the flakes, of diffusion through slits between flakes, and of constricted transport from entering these slits. The calculations show that a simple analytical equation developed by Aris reliably predicts these three effects. Both the calculations and this equation show the separate conditions when each of these effects is most important. The results have value in barrier packaging, in clothing useful in chemical warfare, and in transport across human skin.

Diffusion in surfactant solutions

Abstract The diffusion coefficients in water of Triton X-100 and sodium dodecyl sulfate were measured as a function of concentration using the Taylor dispersion technique. For Triton X-100, a nonionic surfactant, the diffusion coefficient drops from 7.4 × 10 -7 cm 2 /sec at 0.45 g/liter to 6.45 x 10 -7 cm 2 /sec at 5 g/liter. The diffusion coefficient of methyl yellow solubilized in Triton X-100 is close to that of the surfactant. This behavior is quantitatively consistent with a chemical equilibrium between monomer and micelle. For sodium dodecyl sulfate, an anionic surfactant, the diffusion coefficient increases from 1.76 x 10 -6 cm 2 /sec at 0.01 M to 4.53 x 10 -6 cm 2 /sec at 0.125 M . The increase is less when 0.1 M NaCl is added. The diffusion coefficient of the methyl yellow solubilized by the SDS is significantly less than that of the surfactant, particularly at low ionic strength. This behavior can be quantitatively explained by including electrostatic coupling between monomer, micelle, and counterion.