A systematic analysis of operating parameters for CO2 capture from seawater by Bipolar Membrane Electrodialysis (BPMED)

With the escalating environmental impact of carbon dioxide (CO2) emissions, effective CO2 capture is of paramount importance. Fully electrochemical processes can play a key role in this endeavor. In particular, bipolar membranes can induce the necessary ΔpH to convert the bicarbonate present in water into dissolved CO2, which can later be extracted. The primary goal of this study is to better understand the working mechanisms of the BPMED pH swing carbon capture process. Factors such as flow rate, voltage, current density, feed water salinity and alternative electrolytes are investigated to optimize carbon capture from water. Here, a pilot scale BPMED membrane stack and a membrane contactor were used with model water similar to seawater. The extent of bicarbonate removal, mass flow of CO2 gas and energy intensity were measured. By using a new electrolyte solution (0.1 M K3/K4[Fe(CN)6]), a 20 % reduction in energy consumption was observed (by avoiding the water dissociation reaction in cathode and anode). While at low salinities (about 2 mS/cm) CO2 production was limited and resulted in high energy consumption, at salinities > 9 mS/cm an increasing energy demand was observed due to increased ohmic losses and limited bicarbonate concentration. Increasing the flow rate in the membrane stack allowed more bicarbonate and consequently more CO2 gas to be extracted. Changing the velocity from 1 to 3 cm/s resulted in a reduction in energy consumption from 3.7 to 2.5 kWh/kgCO2. The study concludes that further research is needed to increase the efficiency of the BPMED process, particularly in long-term operation and to mitigate scaling/fouling effects on the bipolar membrane. Despite its current limitations, BPMED provides a fully electrified alternative for CO2 capture from water.