Process modelling of methanol synthesis via membrane-based carbon capture, alkaline electrolysis and catalytic hydrogenation

Methanol plays an important role in the energy and chemistry sectors, since it can be directly used as fuel and energy carrier or involved as precursor in the production of chemicals like dimethyl ether (DME) or olefins (e.g., ethylene and propylene). Methanol demand shows an increasing trend which is fulfilled by involving large scale production facilities. Nowadays methanol is mainly produced from the catalytic conversion of a syngas (i.e., a mixture of hydrogen, carbon monoxide and carbon dioxide) obtained via methane reforming or coal gasification. As an alternative, green/renewable methanol can be produced when hydrogen is obtained via water electrochemical splitting (electrolysis) by exploiting surpluses from intermittent renewable electricity. H2 can react with carbon dioxide to produce methanol as a part of Power-to-X pathways. CO2 can be obtained through carbon capture from industrial flue gases or as a by-product of biogas-to-biomethane upgrading. In this work, a process modelling for the catalytic hydrogenation of carbon dioxide into methanol was carried out. A CO2-rich stream is obtained through biogas upgrading by means of membranes, it is mixed with hydrogen produced via low-temperature alkaline electrolysis and then sent to a catalytic reactor for methanol synthesis. For this purpose, two different models have been developed. The first one consists of a MATLAB code simulating a set of membranes for the biogas upgrading and to obtain a by-product gaseous stream with a sufficiently high amount of carbon dioxide. The CO2-rich stream is the input of a second model developed in ASPEN Plus environment, where it is mixed with hydrogen from alkaline electrolysis. The reacting mixture is sent to a Plug Flow Reactor (PFR) incorporating the kinetic model for methanol synthesis. The obtained gas is sent to an upgrading section where methanol is separated from water and unreacted gases. Overall energy efficiency of the whole process was carried out, accounting for the produced methanol energy content as well as electric and thermal energy demand. An experimental campaign for the acquisition of data to evaluate the permeability of membranes both to CO2 and CH4 was carried out within the CO2 Circle Lab at Environment Park (Turin).