An Experimental Study of a Hollow-Fiber Membrane-Contacting System for Carbon Dioxide/Methane Separation

Abstract

Summary Separation of carbon dioxide (CO2) from methane (CH4) using a gas/liquid membrane-contacting system is a promising alternative to conventional absorption techniques, such as wet scrubbers. The main objective of this research was to design, develop, and implement a hollow-fiber membrane-contacting system to absorb and separate CO2 from CH4 in a simulated flare-gas stream. A gas/liquid contacting system was constructed using microporous polytetrafluoroethylene (PTFE) hollow fibers as a highly hydrophobic membrane. The module used for the experimental studies had 51-mm diameter and 200-mm effective length. The membrane module had a packing density of 60%, and the PTFE hollow fiber used in this module had a mean pore size of 0.48 µm. Experiments were conducted in a laboratory-scale plant fed with a simulated flare-gas mixture containing 2.5% CO2 balanced with CH4 that could produce varying concentrations of inlet gas using a mass-flow controller. CO2-separation-experimentation studies were performed, and the effect of operational variables on the separation efficiency of the system has been studied. To optimize the gas-separation performance of the membrane module, the effects of gas/liquid-flow rates, the concentration of absorbent, and the nature of the scrubbing liquid were examined. The absorption efficiency of deionized (DI) water and aqueous solutions of sodium hydroxide (NaOH) and diethanolamine (DEA) as the physical and chemical absorbents were compared. Results indicated that increasing the flow rate and concentration of the scrubbing liquid can enhance the separation efficiency; however, increasing the flow rates of the gas phase had a negative effect on the CO2-absorption performance of the system. The conventional chemical-absorption processes for the separation of CO2 have many drawbacks, such as flooding, channeling, and foaming that can impair the mass transfer between gas and liquid, and possible equipment failure caused by corrosion. Membrane processes can offer attractive opportunities for gas-treatment applications, including removal of acidic-gas compounds from flare-gas streams that can help to mitigate the adverse health effects associated with burning the waste gases.