Performance improvement of PVDF hollow fiber-based membrane distillation process

Abstract

The performance of membrane distillation depends on both membrane and module characteristics. This paper describes strategies to improve the performance of hollow fiber membrane modules used in direct contact membrane distillation (DCMD). Three different types of hydrophobic polyvinylidene fluoride (PVDF) hollow fiber membrane (unmodified, plasma modified and chemically modified) were used in this study of direct contact membrane distillation (DCMD). Compared to the unmodified PVDF hollow fiber membrane, both modified membranes showed greater hydrophobicity and mechanical strength, smaller maximum pore sizes and narrower pore size distributions, leading to more sustainable fluxes and higher water quality (distillate conductiviy < 1 μs cm −1) over a one month test using synthetic seawater (3.5 wt% sodium chloride solutions). Comparing the plasma and chemical modification the latter has marginally better performance and provides potentially more homogeneous modification. MD modules based on shell and tube configuration were tested to identify the effects of shell and lumen side flow rates, fiber length and packing density. The MD flux increased to an asymptotic value when shell-side Re f was larger than 2500, while the permeate/lumen side reached an asymptotic value at much lower Re p (>300). By comparing the performance of small and larger modules, it was found that it is important to utilize a higher shell-side Re in the operation to maintain a better mixing near the membrane surface in a larger module. Single fiber tests in combination with heat transfer analysis, verified that a critical fiber length existed that is the required length to assure sufficient driving force along the fiber to maintain adequate MD performance. In addition, for multi-fiber modules the overall MD coefficient decreased with increasing packing density, possibly due to flow maldistribution. This study shows that more hydrophobic membranes with a small maximum pore size and higher liquid entry pressure are attainable and favorable for MD applications. In order to enhance MD performance various factors need to be considered to optimize fluid dynamics and module configurations, such as fiber length, packing density and the effect of module diameter and flow rates.