Development of a self-sustained model to predict the water desalination performance of the membrane distillation process

• Author / Creator

  Noamani, Sadaf

• The water crisis, as a result of the rapid increase in world population, industrialization, and extreme drought, has become one of the most global risks to the environmental, social, and economic development of many countries. The release of contaminants from the industries into water resources also causes more unreliable and aggravating situations for freshwater availability, especially in water-stressed regions. Therefore, it is essential to develop novel techniques for producing freshwater from seawater or contaminated wastewater and improve the current desalination and water treatment processes. Membrane distillation (MD) has gained a lot of attention in the last decade due to its high potential to produce clean water. MD is a thermally driven separation process in which vapor pressure difference across a hydrophobic membrane acts as a driving force for water desalination. It is an alternative solution for conventional seawater desalination methods, such as distillation and multi-stage flash due to its simple design, high production, and efficiency.
  In this study, comprehensive experimental and theoretical studies of mass and heat transfer in different configurations of MD systems were investigated to provide a strong framework for a deeper understanding and optimization of the process. Inspired by the ∈-NTU method, a new theoretical model was first developed based on heat and mass transfer analyses of the direct contact membrane ditillation (DCMD) process. Although extensive research has been carried out on the modeling of the DCMD process, they mostly relied on some experimentally-determined parameters. The results from our model, which is independent to the experimentally measurement values (self-sustained model), were in good agreement with experimental results, with only a maximum 10% deviation. The results showed that feed temperature and membrane porosity, pore size, and thickness were the most effective parameters on the permeate flux and energy efficiency on the DCMD system. A 60% increase in the temperature of the feed solution increased the permeate flux and energy efficiency by 181% and 20%, respectively. Also, by almost a 20% increase in membrane porosity, the permeate flux and energy efficiency increased about 30% and 21%, respectively. The developed model was also used to minimize the undesirable effects of temperature and concentration polarizations and allowed for the proposal of optimum conditions for achieving higher performance in terms of energy efficiency and permeate flux. Following the same method, in the second part of this thesis, another important configuration of the MD process, i.e., air gap membrane distillation (AGMD), was modeled. The results of our model matched well with AGMD experimental results, with less than 4% deviation. Using the developed model, the AGMD performance was also systematically investigated in terms of permeate flux, energy efficiency, and temperature and concentration polarization effects, and the results are compared with the DCMD configuration. The results showed that feed temperature, thickness of the air-gap and flow rate had the most significant impact on the permeate flux and energy efficiency. In contrast, the membrane thermal conductivity and porosity did not play a determining role. A 60% increase in the feed temperature increased the permeate flux by 200 %. By increasing the flow rate from 0.2 to 8 LPM, the permeate flux was enhanced 67.19%. The air-gap thickness increment from 0.6 to 5.6 mm caused a 36.8% reduction in permeate flux. In our comparative study, the permeate flux and gained output ratio (GOR) for DCMD were 56.6% and 27.3% higher as compared to AGMD at the same conditions. However, the fraction of the energy transferring by the vapor molecules through the membrane and air gap (thermal efficiency) of the AGMD process was 24.7% larger than that of the DCMD process. The developed model provides solutions to minimize the undesirable effects of temperature and concentration polarization and proposes an optimum design map to achieve higher energy efficiency and permeate flux.

• Subjects / Keywords

  • Membrane distillation
  • Water treatment
  • Modeling
  • Desalination

• Graduation date

  Spring 2021

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-38sm-vx59

• 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.