Abstract
The outstanding capabilities of membrane distillation (MD) process to handle highly saline solutions are still driving research efforts to take this process to the industrial level. MD scale-up remains a challenge as the objective of increasing performance faces several design issues imposed by intrinsic physical constraints. Among the major hurdles encountered with an increase in membrane active area stands temperature polarization, a phenomenon that drastically penalizes MD efficiency. It is shown that in a typical permeate flux distribution over the membrane of an MD module, the highest local permeate flux values contribute to less than 1% of the total module production, while average values add up to half the production, suggesting a dramatic underuse of the membrane active area. Therefore, this investigation explores design opportunities to enhance module performance by reducing polarization for large-scale modules. Using state of the art CFD calculations, it is shown that combined lateral flow and progressive feeding from perforated top wall of the feed channel, and eventually the permeate channel in case of direct contact membrane distillation (DCMD) variant, can considerably increase the productivity of the module. Results further confirm the critical importance of flow distribution at the vicinity of the membrane and its effect on permeate flux. Careful analysis reveals that there is an optimum flow rate ratio between the two feed paths and that the design of the holes of the perforated plate plays a critical role to ensure flow smoothness and reduce temperature polarization. Simulations show that the productivity of a 1 m2 flat sheet module can be enhanced by more than 50% under the same operating conditions, which opens up new opportunities for MD scale up.