Date of this Version
A finite element model was developed to investigate the performance of a vacuum membrane distillation module under various operating conditions and membrane parameters. Porosity, tortuosity, pore diameter, membrane thickness, and fiber length were varied along with feed temperature, velocity, and flow configuration. In all cases, boundary layer polarization phenomena were seen to inhibit the performance of the module. At certain conditions, for a 7.5 cm fiber, the reduction in permeate flux from 65 LMH (Liter/m2/h) at the inlet to below 45 LMH at the outlet of the fibers was observed. In most cases, salt concentration polarization was the rate determining phenomenon. The increase in salt concentration from a mass fraction of 0.035 to the saturation value within the boundary layer, led to 12.5% reduction in the driving force of separation. After salt concentration reached saturation within the boundary layer, heat loss continued to reduce the driving force for separation. Changing the feed from the shell to the lumen side of the membrane was seen to result in a significant decrease in permeate flux. Adding a baffling scheme to the surface of a shell side feed was seen to suppress concentration polarization and enhance membrane performance as did an increase in the feed velocities. Exergy efficiency tended to increase with feed temperature but decreased with an increase in average permeate flux. All changes in membrane parameters and design considerations had a minimal effect on overall exergy efficiency. Individual unit operations balances revealed the solar collector to provide more than 80% of all the exergy losses of the process units. It was found that the exergy loss of the solar collector was significantly dependent on process design. These findings revealed the need for continued optimization of various process designs to improve the exergy efficiency of the processes and improve the usefulness of vacuum membrane distillation for use in a solar desalination scheme.
Advisor: Siamak Nejati