Simultaneous removal of CO2 and H2S from pressurized CO2-H2S-CH4 gas mixture using hollow fiber membrane contactors

In the present paper, the simultaneous removal of CO2 and H2S from their pressurized mixture with CH4 (5% CO2-2% H2S-93% CH4) using custom made membrane contactors equipped with micro-porous polymeric hollow fibers is described. Two types of commercial hollow fibers were employed and compared, i.e., expanded poly(tetrafluoroethylene) (ePTFE) and poly(tetrafluoroethylene-co-perfluorinated alkyl vinyl ether) (PFA). The feed gas mixture composition was 2% H2S, 5% CO2 in balance of CH4 to mimic the typical natural gas composition. Distilled water, aqueous sodium hydroxide and amine solutions of different concentrations were tested as absorption liquids to achieve the selective removal of the acid gases from the pressurized gas streams. The effect of pressure on the simultaneous CO2 and H2S absorption rates was investigated by the simultaneous pressurization of the absorption liquid and the feed gas (up to 50 bar) and monitoring the residual acid gases in the exit stream. The obtained experimental results indicated that the simultaneous CO2 and H2S fluxes were enhanced by increasing the inlet gas pressure for both physical and chemical absorption solvents. Up to the authors' best knowledge, this is the first report on (i) the experimental simultaneous removal of CO2 and H2S from pressurized gas streams using HFM contactors and absorption solvents up to 50 bar and (ii) the utilization of PFA fibers in custom made high pressure-membrane modules. PFA fibers exhibited impressive higher fluxes (9-10 times) for CO2 and H2S than those obtained with the common ePTFE fibers which could be attributed in part to the associated higher mass transfer coefficient. The anticipated CO2/H2S flux ratio of 2.5 (which matches the ratio of their inlet concentrations in the feed gas) was obtained whenever the co-absorption was unhindered into absorption solvent of sufficient capacity, e.g., 2.0 M NaOH solution. Analysis of the mass transfer coefficients showed that while the overall mass transfer coefficients are determined by the liquid phase mass transfer coefficients at low pressures, gas phase mass transfer resistance contributes considerably to the overall resistance at high pressures. (C) 2011 Elsevier B.V. All rights reserved.