This research study aimed to synthesize polybenzimidazole (PBI) FO flat sheet membranes for seawater desalination in high temperature and salinity regions like the UAE. The fabrication conditions that were considered to improve membrane performance included casting thickness, oven temperature and duration. The impact of draw and feed solution flow rates, draw solution type and concentration was also investigated. The ideal fabrication conditions for 15 wt% PBI FO membranes were found to be 150 μm casting thickness, heat treatment for 1.5 min at 165 °C and an immersion duration of 10 min in coagulation bath at 23 °C. Performance results showed that the optimal membrane achieved a water flux and salt rejection of 4.2 LMH and 97.4%, respectively with 2 M MgCl₂ draw solution, which increased 21.3 LMH and 98.8%, respectively when the draw solution was allowed to flow in the top compartment of the membrane cell (orientation B). Additionally, computational fluid dynamics (CFD) modeling was used to study the impact of concentration gradient across membrane film on water flux and salt rejection at different FO cell orientation when operated at low cross-flow velocities. CFD modeling results demonstrated that solution density, gravity and diffusion rate affected FO performance at low cross-flow velocity.