Development of Joule Heating Coating Layers for Membrane Distillation Process

  • Author / Creator
    Enayat, Arian
  • Membrane desalination processes have attracted much attention in the last few decades with regards to water purification. While some methods such as reverse osmosis (RO) are demonstrated at the commercial scale, some such as membrane distillation (MD) have not been deployed, mainly due to issues associated with energy efficiency requirements. Some attempts have been recently made to improve energy consumption of the MD setup by developing heat-generating membranes. Although the methods explored in the literature have yielded promising results, the materials and techniques incorporated in those studies were not financially viable.
    In this M.Sc. study, the application of Joule heating coating layers deposited via flame spraying to develop heat-generating MD membranes was investigated. In the first stage of this work, stainless steel meshes were chosen to serve as the base material. Prior to nichrome deposition, alumina was coated on the steel substrate to function as a dielectric intermediary medium. To fabricate hydrophobic MD membranes, polyvinylidene fluoride (PVDF) was cast on the steel substrate's backside. Scanning electron microscopy (SEM) characterization was conducted to determine the deposited particles' morphology and the thickness of each layer.
    Even though, at the best practice, the resistive heating tests showed a 16○C temperature difference across some samples, most could not generate the desired heat due to short-circuiting. To resolve this problem, the mesh substrates were replaced with carbon steel sheets, and instead of fabricating the polymeric layer, commercial hydrophobic polytetrafluoroethylene (PTFE) membranes were used. The Joule heating performance of the developed samples was examined in air, deionized (DI) water, and saline water. For the experiments conducted in water, a commercial H-bridge was integrated into the electrical circuit to minimize electron leakage. It was shown that the higher H-bridge frequencies provided more efficient resistive heating performance. The Joule heating coating layers generated enough heat to increase the water temperature and provide the necessary driving force to distill water.
    The MD experiments showed that 80 W of power increased the Joule heating sample's temperature and its surroundings to 80○C, resulting in water desalination at 2.24 kg/hr.m2 rate. Although the recorded flux was lower than that of conventional MD tests, the amount of energy consumed in this novel method makes this new generation of MD membranes promising. Even after hours of operation, the samples' electrical conductivity did not deteriorate, suggesting that any oxides found in the system originated from the steel substrate rather than the nichrome/alumina layer. Also, the chemical composition analysis of the permeate water showed that its quality remained intact. These results indicate that Joule heating coating layers fabricated inexpensively by flame spraying can improve the MD process's energy consumption, making it a more viable option to be used commercially.
    Lastly, a mathematical model was developed to predict and evaluate the performance of the Joule heating coating layers as part of the MD setup. Even though the developed model was quite simple, in most cases, the predicted values were in close agreement with the results obtained from the experimental study. The modeling results suggested that the temperature polarization was 2- to 3 times lower than which has been recorded by other investigators for conventional MD process. Reflection on both the experimental and theoretical elements indicated that the Joule heating element could increase the temperature of its surroundings as desired. But the feed tank not being thermally insulated wasted most of the heat generated by the Joule heating element, resulting in its low efficiency. It could be said that with the right optimization made in the design of the MD setup and cell, the Joule heating element could become much more efficient.

  • Subjects / Keywords
  • Graduation date
    Spring 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.