Heat and mass transfer characteristics of a direct contact membrane distillation process for desalination

The direct contact membrane distillation (DCMD) process was applied for water desalination. The main objective was to study the heat and mass transfer of the process. The experiments were performed on a DCMD unit using a flatsheet PVDF membrane with a pore size of 0.22 µm. Feed solutions were aqueous NaCl solutions, 5000–35,000 mg/L in concentration, brackish water, and seawater. Other experimental parameters studied were: feed temperature, 40– 70°C; crossflow velocity, 1.85–3.7 m/s (turbulent flow region); and the temperature and crossflow velocity of the receiving distillate were fixed at 20°C and 2.92 m/s, respectively. The permeation fluxes increased with feed temperature and velocity, but decreased with feed concentration. The membrane distillation coefficient was evaluated from the pure water flux data and was subsequently used to estimate water fluxes. It was found that the estimated fluxes agreed well with the experimental data, except at a high feed concentration. The analysis of heat transfer of the process permitted the calculation of heat transfer coefficients, membrane surface temperatures, heat flux components of the process, and also the temperature polarisation coefficients. The temperature polarisation coefficients increased with feed velocity, concentration, and decreased with increasing temperature. The values obtained were 0.88–0.93, higher than most reported values, indicating the effective heat transfer of the system.

The value of C (the membrane distillation coefficient) obtained from pure water feed (0.0024 kg/m2.h.Pa) (see Section 5.2) was used together with the predicted values of T1 and T2 for aqueous salt solutions (P1 and P2 were calculated from T1 and T2, accordingly) to predict fluxes. The prediction was precise with only a small discrepancy at a high concentration. The concentration polarization and the reduction of water vapor pressure are believed to be responsible for the results. For comparison, C was also calculated for the aqueous salt feed of 35,000 mg/L data (see Fig. 9) and was found to be 0.0021 kg/m2.h.Pa. The difference was approximately 12%. Therefore, C obtained from pure water data may be used for flux prediction except at high feed concentrations.