Reclaiming water from a direct air capture plant using vacuum membrane distillation – A bench-scale study

In the present work, vacuum membrane distillation (VMD) was evaluated for its ability to be integrated with a direct air capture (DAC) process as a sustainable combined water-CO2 recovery approach. Experiments were performed using four different VMD modules over a range of feed temperatures, feed flowrates, feed CO2 loadings, and vacuum pressures (50–60 °C, 7.6–17.6 L/h, 0–0.3 mol CO2/mol amine, and 7–12 kPa, respectively). The results showed that a commercially available LiquiCel module and a custom-made hollow fibre module (developed for this study) were the most durable (in terms of their tolerance towards the feed solvent that consisted of a basic CO2 capture solvent) among the four studied modules. These modules also produced the highest distillate fluxes (of 0.17 and 0.91 kg/m2.h for the LiquiCel and custom-made modules, respectively) under a 60 °C feed temperature, a feed flowrate of 17.6 L/h, and a 7 kPa (abs.) vacuum pressure. Moreover, under the same operating conditions, the LiquiCel and the custom-made modules exhibited salinity and dissolved oxygen (DO) removals of 96.47% and 50.98%, and 99.95% and 71.58%, respectively. The pH values of the distillates under these conditions were 10.87 and 10.37 for the custom-made and the LiquiCel modules, respectively, indicating that some basic species moved across the membranes. Furthermore, the results show that, when a CO2-loaded absorption liquid was used as the feed stream, the VMD flux was lower, but had lower conductivity and neutral pH than when a CO2-free solvent was used as the feed. Taken together, this work indicates that coupling DAC with VMD represents a viable pathway for sustainably reclaiming water from industrial processes and carbon capture systems for power plants.