This study is focused on enhancing secondary vapor entrainment and direct-contact condensation in a water jet eductor for the purpose of developing a compact, medium-scale desalination system. It encompasses an extended investigation of an eductor as a condenser, or heat exchanger, for the entrained phase. Exergy study, experimental measurement, and computational analysis are the primary methodologies employed in this work. The target parameters of the optimization work were set through exergetic analysis to identify the region of maximum exergy destruction. In the case of water and water vapor as primary and secondary fluids, mixing and condensation initiates in the mixing chamber of the eductor and is where the maximum exergy destruction was calculated. Therefore, multi-jet primary nozzle eductors were studied to determine the effect of increased interphase interaction area on the exergy destruction and the maximum suction and cooling capacities. Increases in the entrainment ratio, condensation rate and heat transfer coefficient were noted for increasing numbers of nozzles when comparing one-, two- and three-jet eductors.

Thermal desalination technologies involve two primary processes: vapor generation from saline water, and effective recovery of the resulting condensate. Membrane distillation (MD) systems are among the emerging thermal desalination technologies which use a hydrophobic membrane to recover condensate through either direct or indirect contact (with the cooling fluid) condensation. The specific process technology (for thermal energy transfer and condensate recovery) depends on the type of MD. Direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD) are two significant MD processes, with DCMD having the advantage of direct condensation and simple design, while VMD systems have high yield through sub-atmospheric vapor generation. This work focuses on developing an eductor-based MD process incorporating the strengths of both DCMD and VMD. It is an experimental study with a water jet eductor replacing the vacuum pump and condenser in a typical VMD system for active permeate vapor transfer and condensation. Unlike the exiting VMD systems, the proposed design recovers condensate by direct contact condensation. The sub-cooled water acts as a motive flow which entrains the secondary vapor into the stream, causing mass transfer via condensation at the interface. The modified VMD was found to have achieved better flux compared to the conventional VMD system. The performance of the eductor, sensitivity to parameters, and the practicality of the technology have been analyzed.

In the direct contact membrane distillation (DCMD) system, the temperature polarization due to boundary layer formation limits the system performance. This study presents the experimental results and heat transfer analysis of a DCMD module coupled with a salinity gradient solar pond (SGSP) under three different flow channel configurations. In the first case, the feed and permeate channels were both empty, while in the next two cases, the feed and permeate channels were filled with a porous spacer material. Two different spacer geometries are examined: 1.5 mm thick with a filament angle of 65°, and 2 mm thick with a filament angle of 90°. The study considers only the heat transfer due to conduction by replacing the hydrophobic membrane normally used in a DCMD module with a thin polypropylene sheet so that no mass transfer can occur between the feed and permeate channels. The Reynolds number for all three configurations was found to be between 1000 and 2000, indicating the flow regime was laminar. The flow rate through both the feed and permeate sides was the same, and experiments were conducted for flow rates of 5 L/min and 3 L/min. It has been found that the highest overall heat transfer coefficient was obtained with the spacer of 2 mm thickness and filament angle of 90°.

The mass transfer mechanism within commercial PTFE membrane with various nominal pore sizes in spacer-filled direct contact membrane distillation (DCMD) was determined. Mass fluxes and membrane permeability (MP) values for each commercial PTFE membrane were experimentally measured. The MP values increased insignificantly with 2.3% and 4% when the inlet temperature at feed side rose from 400C to 500C in different membrane pore sizes. All investigated mass transfer models except from Dusty Gas model were good enough to simulate the mass transfer inside smaller membrane pore sizes (0.22 µm, and 0.45 µm). Compared to combined diffusion model, the predicted mass fluxes using the overall mass transfer model including the contribution of Poiseuille flow obtained better agreement with experimental results for larger membrane pore size (1 µm). The mean absolute percentage error (MAPE) values for combined diffusion model were up to 16.5% compared with the Ding et al. model or Schofield et al. model (under 3%). Regarding the root mean square error (RMSE), the combined diffusion model obtains larger values than the mass transfer model considering the Poiseuille flow contribution. Consequently, the contribution of Poiseuille flow in mass transfer mechanism within smaller PTFE membrane pores (0.22 µm, and 0.45 µm) could be ignored, however, the contribution of Poiseuille flow to the overall mass transfer should be included in case of larger membrane pores (1 µm), or the case of applied transmembrane hydrostatic pressure.

This study aims to clarify the salinization degree of irrigation water in Quang Dien and Phu Vang districts in Winter-Spring crop season and to propose a potential technology to treat saline water on lab-scale. The majority of irrigation water was brackish water (70%) at Quang Phuoc, Quang Loi, and Phu Dien villages with water concentration of up to nearly 7.1‰. For Quang Thai and Phu An villages, the salinization degree is much lower when the percentage of brackish water was from 30% to 40%. Direct contact membrane distillation (DCMD) was implemented to treat 20‰ - 40‰ concentrations of saline water. The experimental results revealed that the freshwater production by DCMD met the requirements of irrigation water when the salinity was under 0.1‰. Additionally, feed inlet temperature was the most effective factor to produce the highest amount of freshwater compared to volume flowrate and feed concentration factors.