Material and flow engineering approach in improving the membrane distillation process

• Main Author: Tan, Yong Zen
• Other Authors: Chew Jia Wei
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Chemical engineering::Processes and operations; DRNTU::Engineering::Civil engineering::Water resources
• Online Access: https://hdl.handle.net/10356/106502; http://hdl.handle.net/10220/48100
• Institution: Nanyang Technological University

Membrane distillation (MD) is a thermally-driven process involves distillation through a microporous hydrophobic membrane, which acts as a physical interface between the hot feed and cool permeate. Despite being a promising easy to integrate, low-cost, energy-saving alternative to conventional separation processes like distillation and reverse osmosis (RO), MD have not gained much commercial attention due to low fluxes or output per unit energy and pore-wetting problems, limiting its uses to desalination and tertiary wastewater treatment. In view of the problems which plagues the MD process, a series of research were carried out to understand the underlying mechanisms contributing to the problems and investigate the various proposed solutions through a two-pronged approach to the problem. Firstly, in chapter 2, fouling and its effect was studied in depth by correlating a previously developed theoretical model with experimental results using a laboratory direct-contact membrane distillation (DCMD) setup. This in turn helped proved that significant decrease in MD flux in fouling occurred due to the vapor-pressure pressure reduction largely attributed to the small pore size of the fouling layer rather than the heat- and mass-transfer resistance of fouling layer. Hence, the effect of fouling in MD can be significantly reduced by increasing the pore size of the fouling layer. Furthermore, membrane modification of membranes used in MD via coating which is essentially a fouling layer could be designed with bigger pores to minimize the negative impact on MD flux. After which, a proposed approach of altering the module orientation and design to solve the problem of fouling and wetting was investigated in chapter 3. The results show that fouling and wetting could be mitigated by changing the module orientation and manipulating the flow trajectory within an MD module. In line with module orientation and design, in chapter 4 a sweep-gas MD (SGMD) hybridized with a heat pump was studied to evaluate the feasibility of such system in being able to simultaneously cool and carry out water treatment without the need of an additional condenser and at no additional power. Results obtained confirms the feasibility of this hybridized system through the improvement in heat pump electrical efficiency from the evaporative cooling occurring at the MD modules. The material engineering solution of the two-pronged approach starts in chapter 5, in which a facile synthesis method was proposed to synthesize ligand-carrying nanoparticles (LC-NPs) with polystyrene cores and branched polyethyleneimine (b-PEI) which are cross-linked to form stable LC-NPs that can be reused after desorption of absorbed heavy metal ions. These LC-NPs can be used to remove heavy metal ions from wastewater. However, due to incompatibility of LC-NPs with solvents used in the membrane coating matrix, the coating of MXene with photothermal properties to provide localized heating in MD was studied in chapter 6. Results demonstrated the practical use of MXene for coating membranes to improve the performance of a lab-scale DCMD setup by providing localized photothermal heating and anti-fouling effects. Lastly, to compare the effect of membrane coating and spacer coating of photothermal material, in chapter 7, the growth of photothermal material on metallic spacer was carried out to provide localized heating. Since there was lack of studies done on the use of metallic spacers along the feed-membrane interface, simulations and experimental study was carried out to compare the performance of metallic spacers in MD before comparing them to the surface-modified metallic spacer with photothermal material. The results show that metallic spacers improve the energy efficiency of the MD process by improving temperature uniformity on the surface of the feed-membrane interface, and by modifying the metallic spacer with photothermal material, solar-assisted MD could be carried out to reduce the heating load of external heaters.