Paul M. Thoen

Single component and mixed gas transport in a silica hollow fiber membrane

Abstract The permeances of gases with kinetic diameters ranging from 2.6 to 3.9 A were measured through silica hollow fiber membranes over a temperature range of 298 to 473 K at a feed gas pressure of 20 atm. Permeances at 298 K ranged from 10 to 2.3· 10 5 Barrer/cm for CH 4 and He, respectively, and were inversely proportional to the kinetic diameter of the penetrant. From measurements of CO 2 adsorption at low relative pressures, the silica hollow fibers are microporous with a mean pore size estimated to be between 5.9 and 8.5 A. X-ray scattering measurements show that the orientation of the pores is completely random. Mass transfer through the silica hollow fiber membranes is an activated process. Activation energies for diffusion through the membranes were calculated from the slopes of Arrhenius plots of the permeation data. The energies of activation ranged from 4.61 to 14.0 kcal/mol and correlate well with the kinetic diameter of the penetrants. The experimental activation energies fall between literature values for zeolites 3A and 4A. Large separation factors were obtained for O 2 N 2 and CO 2 CH 4 mixtures. The O 2 N 2 mixed gas separation factors decreased from 11.3 at 298 K to 4.8 at 423 K and were up to 20% larger than the values calculated from pure gases at temperatures below 373 K. Similar differences in the separation factors were observed for CO 2 CH 4 mixtures after the membrane had been heated to at least 398 K and then cooled in an inert gas flow. The differences between the mixture and ideal separation factors is attributed to a competitive adsorption effect in which the more strongly interacting gases saturate the surface and block the transport of the weakly interacting gases. Based on Fourier transform infrared (FTIR) spectroscopy results, this unusual behavior is attributed to the removal of physically adsorbed water from the membrane surface.

Reactive polymer membranes for ethylene/ethane separation

Abstract Olefin/paraffin separations by distillation are highly energy intensive. Facilitated transport, or reactive membranes have long been investigated as an alternative and/or complementary separation technology to conventional distillation. However, stability problems associated with facilitated transport membranes have been the primary obstacle in the development of commercial FT processes. In this paper, we report the development of a polymer membrane containing silver(I) ion that facilitates the transport of ethylene in the absence of solvent. Blends of ionically conductive and electrically conductive polymers were found to have the appropriate electronic environment to allow reaction of silver(I) ion and ethylene. The results for the ethylene/ethane separation were obtained with composite membranes of silver(I)-form Nafion ® and 2 wt% poly(pyrrole). Permeation measurements were performed with ethylene/ethane mixtures at total feed pressures ranging from 760 to 1900 mmHg and at temperatures of 40°C to 70°C. Pure gas permeability measurements were obtained at a total feed pressure of 1900 mmHg and temperatures of 30°C and 40°C. Ethylene/ethane separation factors with the silver(I)-form Nafion-poly(pyrrole) composite membranes increased from 8 to 15 as temperature decreased. Ethylene permeabilities increased from 0.2 to 1 Barrer over the temperature range of 30°C to 70°C. An ethylene/ethane mixed gas permeability ratio of about 2 was observed with non-reactive proton-form Nafion-poly(pyrrole) composite membranes. Ethylene permeation measurements as a function of membrane thickness suggested that the facilitated transport of ethylene approached the reaction-limited regime at membrane thickness of 5 μm. The complexation between ethylene molecules and silver(I) ions in Nafion-poly(pyrrole) composite membrane was observed with FTIR spectroscopy.

Palladium-Copper and Palladium-Gold Alloy Composite Membranes for Hydrogen Separations

Electroless plating was used to fabricate PdCu and PdAu alloy composite membranes using tubular Al2O3 and stainless steel microfilters to produce high temperature H2 separation membranes. The composite membranes were annealed and tested at temperatures ranging from 350 to 400°C, at high feed pressures (≤250 psig) using pure gases and gas mixtures containing H2, carbon monoxide (CO), carbon dioxide (CO2), H2O and H2S, to determine the effects these parameters had on the H2 permeation rate, selectivity and recovery.