Membrane Distillation (MD) is a separation process driven by the vapour pressure difference established across hydrophobic membrane. In order to combine the advantages of conventional MD configurations, Permeate Gap Membrane Distillation (PGMD) modules were developed. The objectives of this study were to systematically evaluate the performance of several new hollow fibre PGMD modules.
This study consisted of four components. First of all, the membrane was systematically characterized. The membrane dimension and morphology were investigated using Scanning Electron Microscope (SEM). The membrane porosity was measured using the wetting method. The membrane hydrophobicity was determined by measuring the contact angles of the inner and outer hollow fiber surfaces. Finally, Liquid Entry Pressure (LEP) was investigated. Based on the membrane characterization, it was confirmed that the employed hollow fibre membrane was suitable for MD application.
Next, a single PGMD module was built with 8 gap channels and 1 hollow fibre within each gap channel. This module was operated in different modes (PGMD, DCMD and SGMD) to compare their performance. The results showed that the maximum flux of hollow fiber PGMD was 27% and 1.6% lower than the maximum flux of DCMD and SGMD respectively. This phenomenon was due to the higher coolant velocity for DCMD and applied air flow in the gap channel for SGMD. The mass transfer coefficient was also used as an indicator to compare performance. For PGMD, the mass transfer coefficient increased initially at the lower feed inlet temperature and then decreased when the feed inlet temperature was higher than 60˚C, which could be attributed to the combined effects of transverse vapor flux and temperature non-uniformity of the bulk flow. On the contrary, the global mass transfer coefficients of DCMD and SGMD decreased slightly as a function of feed inlet temperature. Compared to other studies, our results demonstrated that PGMD has the potential to effectively combine the advantages of different conventional MD processes.
Afterwards, we have investigated the impacts of different PGMD module designs on water productivity and energy efficiency. The results showed that module with lower hollow fibre packing density or gap channel density had a higher flux and better energy efficiency, while modules with higher hollow fibre packing density or gap channel density exhibited more energy efficient use of the membrane surface area and higher productivity. Additionally, the module with a more conductive cooling plate had a higher flux and lower Specific Thermal Energy Consumption (STEC), which was mainly attributed to the lower thermal resistance of the cooling plate. Due to the nearly stagnant velocities within the gap and coolant channels, the impact of cooling plate material on PGMD performance was greater than that of hollow fibre packing density and gap channel density. The Gain Output Ratio (GOR) obtained for the hollow fibre PGMD module was relatively low compared to other MD studies, however, the PGMD module performance cannot be assessed purely based on GOR. A trade-off exists between GOR and flux for MD modules, and the flux obtained from our hollow fibre PGMD module was relatively high.
Finally, a set of mathematical models were developed to simulate the mass and heat transfers phenomenon in hollow fibre PGMD process. The validated model was employed to evaluate the impacts of important MD design parameters on module performance. The modelling results showed that coolant velocity and coolant temperature had less impact on flux compared to those of DCMD, because the coolant of DCMD contacts with membrane directly. The model also suggested that increasing the cooling plate thermal conductivity resulted in a higher flux. However, when the cooling plate thermal conductivity was higher than 5 W/m.K, further increases in the thermal conductivity of the cooling plate had a negligible impact on flux. A sensitivity study was undertaken to analyze the combined effects of gap channel inner/outer diameters and gap channel thermal conductivity on flux. It is concluded that the gap thermal conductivity played a more important role in PGMD performance compared to the hydrodynamic flow in permeate and coolant channels.
To further improve hollow fibre PGMD performance, six recommendations are provided for the future work.