Potential use of membrane contactors for CO2/CH4 separation by facilitated CO2 transport in ionic liquids

The presence of CO2 (30 to 50 %) reduces the calorific value of the biogas produced by anaerobic digestion. For this reason, CO2 removal and further bio-methane purification is fundamental, and membrane technology is seen as one of the possible solutions. In this study, removal of CO2 from CH4 by a facilitated transport mechanism in a membrane contactor is evaluated. The process is based on the reversible reaction between carbonic anhydrase (reactive carrier) and CO2 in an ionic liquid ([BMIM+][OTF-]. ILs are evolving as a new type of non-aqueous solvents for biocatalysis, mainly due to their physical properties as molten salts at room temperature and their negligible vapour pressures. This characteristic prevents loss of the carrier by evaporation. Moreover, in this way the carrier can be mobile, which may stabilize it against deactivation of the enzyme, as often occurs when the enzyme is bound to the membrane. A membrane contactor is used, consisting of PVDF hollow fibers embedded in a glass tube. The carbonic anhydrase enzyme, solubilized in the ionic liquid with a defined water activity, is fed into the lumen of the hollow fibers. The CO2 is transformed into HCO3- by the reaction between CO2 and the water present in the ionic liquid . The enzyme, reacting with CO2, facilitates its transport. On the contrary, methane has no affinity for the enzyme and can be purified in the shell side of the hollow fiber membrane contactor. The influence of the enzyme and the operation conditions on the transport parameters of CO2 and CH4 in the ionic liquid and on the overall separation performance will be evaluated.