Effect of membrane surface wetting on the performance of direct contact membrane distillation for seawater desalination

Abstract

Membrane distillation (MD) is a standalone process to generate fresh water and is especially attractive when low-grade waste heat or renewable thermal energy is available. Surface wetting hinders the commercialization of Membrane distillation (MD) technology by deteriorating the permeate quality, thermal efficiency, and transmembrane flux. There is still a lack of understanding on how and to what extent the partially wetted membranes affect the performance of direct contact MD (DCMD) systems. It is of great importance to optimize the operating conditions under such conditions. The DCMD performance was addressed in the literature by considering non-wetted or fully wetted membranes. This study for the first time proposes a computational model to investigate the effect of membrane surface wetting ratio (R = L wet /L total ) on the transmembrane (J) and thermal efficiency (η) of a DCMD module. Parametric and sensitivity analyses were performed to display the effect of system parameters (feed temperature, feed velocity, permeate temperature, permeate velocity, membrane thickness, and membrane surface wetting ratio) on the Key Performance Indicators (KPIs) of the DCMD module. The obtained results indicate that the permeate side temperature has more effect (more than twice) on KPIs in wetted membranes (∼30% and ∼15% rise in J and η) compared to the non-wetted ones (∼15% and ∼5% enhancement in J and η), and the negative effect of membrane surface wetting could be minimized by adjusting the permeate side operational conditions. The effect of membrane wetting ratio on the performance of the DCMD module in thin membranes (≤0.2 mm) and thick membranes (≥0.25 mm) strongly depends on the permeate and feed temperatures. The parametric and sensitivity analysis performed in this study will be beneficial to optimizing the operational conditions of MD systems for maximizing their performance and could serve as valuable guidelines in the development of efficient water desalination systems. • A numerical model was developed to investigate the performance of the partially wetted DCMD module. • The effect of surface wetting ratio (R = L wet /L total ) was investigated using a parametric study. • A sensitivity analysis was performed to quantify the relative impact of operating conditions. • The effect of permeate properties is more dominant for surface wetted membranes. • Permeate and feed temperatures dramatically alter the R ratio effect in thin and thick membranes, respectively.