Technical viability and exergy analysis of membrane crystallization: Closing the loop of CO2 sequestration

The potential of membrane crystallization has been studied for the recovery of Na2CO3 from aqueous streams, as a tool that can be used in CO2 sequestration. The influence of various crystallization conditions (i.e., concentration and flowrate of the Na2CO3 solution, concentration and flowrate of the osmotic solution and kind of osmotic solution: NaCl or MgCl2) on the process performance has been determined. The concentration of the osmotic solution was found to be the key parameter that may condition the applicability of the system. The flowrate of the Na2CO3 solution has also an important influence on the mass transfer through the membrane. Thus, concentration polarization may occur since the transmembrane flux increases with the flowrate. In addition, the characterization of crystals demonstrated that Na2CO3•10H2O crystals were obtained. An exergy analysis was made for the membrane contactor. The higher the concentration of NaCl, the higher the exergy of the outlet streams. It yields a positive variation of exergy, reaching a maximum value at a specific Na2CO3 concentration. In addition, a range of concentrations can be established in order to operate under conditions of positive exergy variation. Increasing the flowrates of the feed or osmotic solutions has also an effect on the exergy variation, reaching an asymptotic value.