Effect of microstructure, thermodynamic and operating conditions on performance of membrane distillation

Membrane distillation (MD) is a thermally driven process which only allows vapour molecules to be transported via diffusion through hydrophobic membranes. Although MD has been studied for a few decades since its’ first introduction in 1967, no commercial scale process has been established. Several major factors contribute to the delay in commercialization of MD; namely lack of thorough understanding on the factors affecting MD performance, absence of commercial membranes giving excellent MD performance, lack of good design of membrane modules with good fluid dynamics, lack of membranes with high thermal efficiency and limited information and data on energy requirements and potential application areas. This work was focused on two main technical issues; (i) factors affecting MD performance including terms of membrane microstructure, thermodynamic and operating conditions, and (ii) MD energy requirement. In addressing these factors, PTFE membranes with different pore size, thickness and porosity were subjected to direct contact and vacuum membrane distillation (DCMD and VMD) experiments at different temperatures, feed flow rates, stream flow directions, feed concentrations and vacuum levels. Numerical modeling using Polymath software was carried out to predict how these parameters affect MD performance. The second part of this work presents an engineering process modeling for the estimation of thermal and electrical energy requirements for DCMD and VMD. Finally, the technical feasibility of MD in treating solutions containing volatile organic compounds (VOCs) for application in the pulp and paper industry was studied. Synthetic and actual foul pulping condensates were treated with sweep gas and vacuum membrane distillation (SGMD and VMD) and the flux, VOCs separation factors and removal efficiency were compared. The energy requirements in these two processes were also estimated. Results obtained by this study confirm that membrane porosity is the most important factor affecting MD performance. The structure and porosity of membrane support materials also play important roles in determining the performance of DCMD. Higher fluxes and lower temperature polarizations were observed over membranes with large pore size, low thickness, high porosity and low tortuosity. Membrane porosity, pore size and the presence of support materials were found to have significant effects on flux and temperature polarization. The thermal energy requirement for DCMD is higher compared to VMD, but its electrical energy requirement is significantly lower. For DCMD to be competitive with commercial desalination processes, the system needs to operate in a circulation mode with an available heat source and 85% heat recovery. For VOCs removal from pulp mill’s condensates, SGMD provides higher separation factor and lower risk of membrane wetting. At operating temperatures below 45C, electrical energies between 0.005 and 1.67 kWh/m3 are required. In comparison to SGMD, VMD requires a significant less electrical energy but the separation efficiency is lower.