Design and development of hollow fiber membrane for desalination and salt recovery via integrated membrane distillation-crystallization

Abstract

Over the past few years, the water research worldwide has carried a similar theme of water crisis with the continuous increment of world population and limited availability of fresh water resources were identified as the major causes. In this regard, desalination of saline water has great potencies to suffice the world's increasing demand of fresh water considering the high percentage of saline water on earth. Among other membrane-based desalination process, reverse osmosis (RO) technology has been proven reliable and becomes the most preferred option for desalination. Nevertheless, the large scale application of RO is often questioned for the brine disposal issue and its unfavorable impacts on the environment. These issues have urged the development and innovation of more sustainable desalination processes with the ultimate aim to maximize the water recovery and the process efficiency. For these reasons, various hybrid desalination processes have been proposed, for instance membrane distillation (MD) based hybrid desalination technology. MD process is an integration of membrane-based and thermal-based desalination processes in which a porous hydrophobic membrane is employed as a thermal insulator and physical barrier between the hot feed where water evaporation occurs and the cold distillate where the diffused water vapor is condensed. Therefore, the permeation flux in MD process is less dependent on the solution concentration as the transport of water vapor is driven by the vapor pressure gradient across the membrane. This allows MD operation to concentrate the saline solution up to saturation level enabling the subsequent recovery of the salt by means of the crystallizer. Therefore, by integrating membrane distillation with crystallizer (MDC), not only the valuable compounds can be harvested but the issue of brine disposal can be also mitigated. As compared to MD operation with a low concentration of saline solution, membranes operated with a high concentration are more prone towards salt scaling facilitated membrane wetting which results in flux decay and deterioration of separation performance. Therefore, in this study we investigated the performance of various hollow fiber membrane configurations under MDC operation with a highly concentrated sodium chloride (NaCl) aqueous solution. The membranes were developed in our laboratory which include single- and dual-layers membrane configurations along with combination of hydrophobic-hydrophilic characteristics. Heat and mass transfers along membrane module were modeled to determine the change of membrane permeability with increasing salt concentration. Upon MDC operation, the membrane performances were retested to define the recovery of membrane permeability. It was found that the morphology underneath membrane surface played important role in mitigating scaling facilitated membrane wetting and membrane with cellular structure exhibited a better wetting resistance as compared to globular structure. Moreover, the performance of membrane with cellular structure was maintained even at the widest trans-membrane temperature gap at which the scaling propensity is the highest. Lastly, NaCl was crystallized and recovered from the supersaturated MDC retentate solution. The effects of crystallizer cooling rate on the resultant salt crystal nucleation and growth were investigated.