Membrane Contactors for Post Combustion Carbon Dioxide Capture: Wetting Resistance On Long Time Scales

Carbon dioxide (CO₂) is one of the largest contributors to the greenhouse effect. Because of the raise of the energy needs, the CO₂ concentration in the atmosphere could increase by a factor of 2 from today to 2100 if no action is engaged to reduce emissions. Postcombustion CO₂ capture by gas-liquid absorption in a chemical solvent is usually considered as the easiest way to remove CO₂ from power plants. In place of packed columns, which show a large footprint and may be subject to several disadvantages such as flooding, foaming, channelling and entrainment, membrane contactors, which are based on hydrophobic microporous hollow fibers, have recently attracted increased attention.

Membrane contactors are indeed considered as one of the most effective strategy for intensified gas-liquid absorption processes. A major complication has however to be taken into account when a membrane contactor is used for gas-liquid absorption purposes: the gradual wetting of the membrane by the solvent can significantly increase the mass transfer resistance and annihilate the interest in terms of process intensification. The influence of wetting on membrane contactors performances is well documented, essentially through simulation studies but, surprisingly, no comparison of the wetting resistance of different types of hollow fibers over long time scales (i.e. months) has been reported.

 In this study, post-combustion CO₂ capture by absorption in a chemical solvent (MonoEthanolAmine, MEA) has been investigated based on laboratory scale experiments under steady state conditions. Two standard membrane contactor materials (microporous polypropylene PP and polytetrafluoroethylene PTFE) have been compared to two novel composite hollow fiber materials, which show a dense skin layer on the liquid side in order to prevent wetting (a polymethylpentene, PMP and a Teflon-AF dense skin coated on a microporous PP fiber). The four different membrane contactors have been tested over 1000 hours for CO₂ absorption in a 30% MEA solution. The results in terms of stability of the CO₂ capture efficiency over time and the overall mass transfer performances are presented and discussed. A spectacular wetting protection effect of the dense skin membranes is obtained (Figure 1). Microporous membrane materials show a significant decrease of the capture efficiency over time; a very large decrease of the capture efficiency is observed with PP after 100 hours contact time with the solvent, while a slight decrease only is obtained after 300 hours for the PTFE membrane. The interest of the different types of fibers for process intensification purposes with a special emphasis on long time use is discussed. More specifically, it will be shown that dense skin composite fibers only can ensure a significant intensification factor (higher than 4) to be attained on long time scales.