Improving Liquid Entry Pressure of Polyvinylidene Fluoride (PVDF) Membranes by Exploiting the Role of Fabrication Parameters in Vapor-Induced Phase Separation VIPS and Non-Solvent-Induced Phase Separation (NIPS) Processes

Faisal Abdulla Almarzooqi,Hassan Ali Arafat,Muhammad Roil Bilad

doi:10.3390/app7020181

Abstract

Polyvinylidene fluoride (PVDF) is a popular polymer material for making membranes for several applications, including membrane distillation (MD), via the phase inversion process. Non-solvent-induced phase separation (NIPS) and vapor-induced phase separation (VIPS) are applied to achieve a porous PVDF membrane with low mass-transfer resistance and high contact angle (hydrophobicity). In this work, firstly, the impacts of several preparation parameters on membrane properties using VIPS and NIPS were studied. Then, the performance of the selected membrane was assessed in a lab-scale direct-contact MD (DCMD) unit. The parametric study shows that decreasing PVDF concentration while increasing both relative humidity (RH) and exposure time increased the contact angle and bubble-point pore size (BP). Those trends were investigated further by varying the casting thickness. At higher casting thicknesses and longer exposure time (up to 7.5 min), contact angle (CA) increased but BP significantly decreased. The latter showed a dominant trend leading to liquid entry pressure (LEP) increase with thickness.

Highlights

The membrane distillation (MD) process is driven by the vapor pressure difference between a hot and a cold stream in the feed and permeate side of a membrane module, respectively
When calculating the liquid entry pressure (LEP) of membranes prepared using both processes, they all range from 103 to 107 kPa, which makes the membranes vulnerable to wetting. This low LEP range, mostly caused by the high bubble-point pore size (BP) values for all membranes, disallows the application of high operational pressure that is required to overcome pressure-drop in both the feed and the permeate streams in a module system
These findings suggest that extending the exposure time can significantly change the surface hydrophobicity without substantially changing the BP

Summary

Introduction

The membrane distillation (MD) process is driven by the vapor pressure difference between a hot and a cold stream in the feed and permeate side of a membrane module, respectively. This driving force is created by the temperature difference between the hot and cold streams. Both streams are separated by a membrane that ideally features the following properties: high hydrophobicity, high porosity, low tortuosity, and low thickness. LEP of a membrane is a function of maximum pore size ( called bubble point, BP) and surface hydrophobicity, expressed in terms of the surface contact angle (CA) with water. LEP can be estimated using the Cantor–Laplace equation [1]

Methods

Results

Conclusion