Selective separation of CH4 and CO2 using membrane contactors

In this paper, the behavior of micro-porous hollow fiber membrane contactors is analyzed for biogas upgrading under different operational conditions. Physical solvents, as deionized water and sodium chloride solution, were initially used. In these cases, carbon dioxide absorption was dependent upon liquid phase flow. After these experiments, sodium hydroxide was used as chemical absorbent in order to compare results. In this case, an increasing in gas velocity resulted in a higher CO2 mass transfer. This effect was associated to the increase of reactive hydroxide ions available in the solvent. Under optimum conditions, with two membrane contactors operating in series, it was possible to obtain a gas stream with more than 99% of pure methane using deionized water as solvent. The same yields were obtain with NaOH as solvent, but in this case working with just one membrane module, that is, reducing by half the available surface area for molecules diffusion. At constant liquid to gas flow ratios, better separation behavior was targeted at pressures higher than 2.0 barg. When dealing with solvent reuse without regeneration, NaOH is the only absorbent allowing a high number of solvent recirculation cycles. This aspect could be very interesting for the economy of the process.