Experimental analysis of polymeric membranes for CO2/CH4 separation in biogas upgrade

The increase in temperature in last decades has been strictly related to carbon dioxide emission: a sharp decrease in fossil fuel use is needed to limit irreversible damages to the global ecosystem. EU is committed to achieve net zero emissions by 2050, as announced in the European Green Deal. Between fossil fuel sources, methane is the one with the least environmental impact. This led EU to choose NG as the ideal fossil fuel to start the energy transition. In 2021, around 40% of imported NG derived from Russia. In response to the invasion in Ukraine new NG contracts with other suppliers were stipulated, increasing energy security; in parallel a rise in other sustainable sources and alternative fuel to NG was programmed. Biogas production from waste water treatment and municipal solid waste can be a solution to increment energy security and deal with climate change. Biogas is mainly composed by 60% of CH4 and 40% CO2. In order to achieve a product with similar characteristic to the NG it is necessary to upgrade biogas into biomethane via CO2 separation. Biomethane from upgraded biogas has identical characteristics to NG, hence no more investment in infrastructure is needed. The upgrade of biogas is mostly related to separation of CO2 from CH4. Different techniques for CO2 capture from biogas stream are in operation stages, such as the high-pressure water scrubbing, pressure swing adsorption or cryogenic separation. Each of them has some weakness, like high amount energy demand, expansive investment and operational costs or extreme working temperature. An emerging field that can be applied in the separation of gas mixture is the membrane technology. They have different advantages, such as simple construction and operation, high reliability and good scalability. Membrane performance is mainly referred to 2 parameters: permeability and selectivity. These values are related by the Robeson upper bound, which sets the limits of membrane performance: the higher the selectivity, the lower the CO2 permeability and vice versa. In this work different membrane were tested applying many gradient pressures between the two sides of the membrane In isothermal condition. The experimental test rig used in this thesis is located in the CO2 circle lab in Environment Park (Turin, Italy). First of all, a Matrimid membrane (polymide commercial membrane) with known performance (PCO2= 8 Barrer, SCO2/CH4= 30) was used to validate the experimental setup. Then two blend-polymer membranes (PES-WC and PEEK-WC) are tested with and without a cyclodextrin (beta-CD NSI): these new membranes were synthesized by the chemical department of UniTo with interesting results. After a brief section about the material and the production technique of these membranes, a description of the test bench with two different way to acquire data permeability is explained (open and closed volume). A section about data filtering and the function to correctly evaluate the results is exposed. Finally, permeability and selectivity calculation are carried out managing the experimental results with the MATLAB software.