Acid gas control using hollow fiber membrane contactor: experimental and modeling study

Membrane gas separation is a reliable alternative to traditional acid gas absorption techniques using direct contact of gas stream and scrubbing solvent without dispersion of one phase in another. The success of this technology is highly dependent on the types of membranes used in the gas-liquid contactor process. Microporous hollow fiber membranes act as the barrier between the gas and liquid phases and prevent the limitations caused by channeling, foaming, and flooding. Additionally, a high surface area to volume ratio of the membrane module can provide a substantial saving on space and weight requirements. This dissertation investigated the potential of using commercially available hydrophobic PTFE hollow fiber membranes for acid gas control applications, in particular separation of CO2 from a CO2/CH4 gas mixture as well as removal of CO2 and NO2 from simulated flue gas streams. A gas-liquid membrane contacting apparatus was constructed and comprehensive experimental studies were performed to determine the gas absorption performance of the membrane system. Deionized-water and aqueous solutions of sodium hydroxide (NaOH), monoethanolamine (MEA), diethanolamine (DEA) and triethylamine (TEA) were employed as the absorbents. Operating parameters such as: gas and liquid cross flow velocities, feed gas concentration, temperature, nature of absorbent, scrubbing solvent concentration, packing density, module configuration, flow pattern, and long-term performance of modules were systematically evaluated in order to better understand the overall performance of the membrane system. Results indicated that acidic gases (CO2 and NO2) absorption performance of the membrane system enhanced with increasing the liquid phase velocity, decreasing the gas phase velocity, and using chemical scrubbing absorbents. For the physical absorbent, the CO2 separation efficiency decreased with increasing the temperature, whereas increasing the temperature was in favor of system performance when using chemical absorbents. Moreover, it was shown the sodium hydroxide to be a superior absorbent as compared to alkanolamine solutions for the co-capture of CO2 and NO2 species. It was observed that low concentrations of NO2 in the feed gas had a minimal impact on the decarbonization of gas-liquid membrane contacting system. On the other hand, the module with smaller size hollow fibers showed the best performance for separation of acid gas compounds (CO2 and NO2) from the gas mixture. The long-term performance of the membrane system was also evaluated by running the simultaneous gas removal experiments over a 24-h period. The consistency of the absorption efficiency results confirmed the potential of using PTFE membrane system for the simultaneous absorption of CO2 and NO2 gases. A comprehensive two-dimensional mass-transfer simulation model was developed to evaluate the separation process of CO2 from CO2/CH4 gas mixture using computational fluid dynamics (CFD). The governing mass transfer equations for all the three compartments of the membrane system (i.e. lumen-side, membrane, and shell-side) were solved using COMSOL Multiphysics. Convection and diffusion flux vectors and concentration gradient of CO2 species in the radial and axial directions of the membrane system were investigated. The simulation results reveal that increasing the absorbent velocity improved the CO2 separation performance of the membrane module, while increasing the gas mixture velocity deteriorated the CO2 removal of the system. The impact of hollow fibers geometry on the removal of CO2 from the gas mixture was investigated and the results indicate that hollow fibers with smaller inner diameters showed a superior CO2 removal in the membrane system. Furthermore, the predictions for the CO2 separation performance of the modules exhibited a good agreement with the experimental data under various ratios of gas to liquid volumetric flow rate