Numerical analysis of spacer impacts on forward osmosis membrane process using concentration polarization index

Understanding the mass transfer in membrane separation processes plays a crucial role in improving the process performance. In this study, we numerically scrutinized the impacts of spacers—a structure enhancing the mass transfer in the vicinity of the membrane—on the concentration polarization (CP) in a forward osmosis (FO) process. To quantitatively measure the extent of CP, we introduce a concentration polarization index (CPI), which is proportional to the degree of CP and inversely proportional to the permeate flux. Based on the CPI, we simulated the impact of spacers on FO performance at various concentration conditions and found that the FO spacers generally reduce the extent of the CP; subsequently, this shows that FO spacers are important factors for the advancement of FO technology. In FO processes it was found that the boundary layer compression is more efficient for diminishing the CP than the boundary layer disruption, whereas the disruption mechanism was more successful in alleviating the CP in reverse osmosis (RO) processes. This contradiction was subsequently attributed to the intrinsic characteristics of the boundary layer disruption, which inevitably caused a dead zone near the spacer attachment and thereby boosted the CP in the FO process. Moreover, it was observed that spacers in feed channels increase the CP even when the concentration of the feed solution is zero, unless the membrane completely rejects salt, because any solute penetrating through the active layer of the membrane by a reverse diffusion mechanism accumulates near the spacer attachment. Hence, it is crucial to minimize the adverse impacts of FO spacers to maximize the beneficial effects of spacers to decrease the CP and improve the performance of FO processes.