The performance of hollow fiber direct contact membrane distillation modules

Abstract

Computational fluid dynamics simulations were conducted in hollow fiber direct contact membrane distillation modules. The objective is to understand the impact of packing densities and incorporating flow mixers on the feed side of the system. The feed flow rate varies while the feed inlet and permeate inlet temperatures are fixed. Furthermore, finned hollow fibers are used as flow disruptors in the feed channel to mitigate temperature and concentration polarization and to enhance vapor flux. The laminar flow model is used to characterize velocity, temperature, and concentration fields. It is demonstrated that the packing ratio significantly influences when the flow rate is low. The tightly packed hollow fibers in the shell resulted in decreased permeating flux due to a lack of mixing in flow in the narrowed regions around hollow fibers. The data shows that the permeate flux was 14.5 % higher for a 25 % packing ratio compared to a 75 % packing ratio at a lower flow rate. In contrast, the flux decreases only 3 % at a higher flow rate with an increasing packing ratio. It is demonstrated that the presence of fins breaks the parallel flow and enhances the mixing performance by approximately 10 % While fin modifications can enhance flux performance and mitigate polarization issues, the decrease in the merit criterion suggests that the benefits must be weighed against the potential drawbacks, such as elevated energy consumptions due to the increase in pressure drops. This underscores the importance of a balanced approach in optimizing membrane module performance.