Carbon dioxide capture using sodium hydroxide solution : comparison between an absorption column and a membrane contactor

Since the industrial revolution, human activities have contributed to increase drastically the atmospheric concentration of greenhouse gases. Carbon dioxide, even though it is not the more armful of them, is by far the most problematic of these gases. Indeed, its concentration is so high that its contribution is accountable to two third of the total contribution of warming gases. In order to mitigate the CO2 atmospheric concentration, the focus is made on the flue gas at the exit of the power plants. Indeed, the large increase of the last decades in atmospheric CO2 is attributable by 75% to burning fossil fuels to produce power. Being part of an overall work on the ability to capture and convert carbon dioxide into a valuable product – sodium carbonate – by using membrane contactors for every step involved in the process, this work aims to compare the absorption column and the membrane contactor technologies for the chemical absorption of CO2 in liquid NaOH. The influence of operating parameters was studied on the efficiency of the absorption for an absorption column designed by Armfield Ltd. and a membrane contactor designed by Liqui-Cel®. The conducted experiments lead to the conclusion that the most impacting operating parameter is the concentration of the liquid absorbent. Indeed, the continuous providing of fresh hydroxide to react with the injected CO2 is the key parameter ensuring an efficient absorption. Therefore, the increase in liquid flow rate also have a higher impact than the increase of gas flow rate. An increased liquid flow rate allows to retard the fast saturation of the absorbent. The increase in gas concentration in CO2 lowers the efficiency of the absorption because of the accelerated saturation in liquid film it involves. After having analysed the influence of the operating parameters for both the devices, a comparison of performance was performed. The effective interfacial area of the membrane is twice as small as the one of the column and the liquid and gas flow rates achievable with the setup are lower than the one used with the column. Nonetheless, the overall mass transfer coefficient obtained while using the membrane contactor lies in the same range than the column coefficient. This highlights the power of the intensification which is a strong argument in favour of the membranes to be used as CO2 capture devices.