Vacuum Membrane Distillation For Desalination Research Articles

Numerical study on performance and efficiency of batch submerged vacuum membrane distillation for desalination

Harvesting clean water from seawater is widely studied in thermal-driven vacuum membrane distillation (VMD) application. Although the conventional VMD promotes excellent flux due to the deaeration by means of vacuum, the external heat supply of the system suffers the heat loss during pumping high feed volume. The batch submerged VMD (S-VMD) which has internal heat supply was introduced in this work to provide uniform feed temperature in the feed tank. This simple and space-saving design makes it particularly suitable to supply water for small community with space limitation. To evaluate the energy efficiency of the S-VMD in desalinating seawater, the performance of the system had been simulated and verified with the experimental results. The effects of main operating parameters including feed temperature and its circulation rate, heat transfer coefficient, vacuum degree on permeate side, packing density of membrane and temperature polarization on the membrane permeation flux using saline water feed had been experimentally investigated. Energy efficiency in terms of gained output ratio (GOR) and thermal loss of the heat supply had also been examined. The simulated results showed that the flux enhanced with the increase of feed temperature, circulation rate, vacuum degree and packing density of membrane. The combination of feed temperature with circulation flow and packing density of membrane played dominant role to improve permeate flux and GOR of the system. This finding is important for the future implementation of S-VMD system in space-constraint remote area. The simulation results were in good agreement with experimental results, recording less than 5% error.

Vacuum membrane distillation for desalination: Scaling phenomena of brackish water at elevated temperature

Membrane fouling or scaling is one of the main operational problems plaguing membrane distillation (MD) process during desalination. Specifically, scaling by the inorganic compounds is a serious and also common problem in MD, particularly at high temperature. In lieu of this, the effects of temperature, inorganic compounds and the membrane matrix on the membrane fouling phenomena require further investigation. In this work, a commercial hydrophobic polypropylene (PP) hollow fibre membrane with a smaller pore size (0.1 µm) was performed for its scaling and wettability propensity under the elevated feed temperatures condition. Brackish water that contains inorganic compounds was used as feed. Based on the membrane fouling in the brackish water jar test study, it was found that the deposits on the membrane surface were predominantly magnesium hydroxide [Mg(OH)2], magnesium silicates (MgSiO3), magnesium sulphate (MgSO4), silica (SiO2), and organic matter traces. The critical temperature for the significant deposition of Mg-based foulants on the membrane surface is 333 K. In the VMD experiments with feed temperature of 333 K, the commercial PP hollow fibre with smaller pore size (0.1 µm) and lower contact angle (102°) suffered more severe magnesium-based surface scaling compared to the membrane with larger pore size (0.2 µm) and higher contact angle (128°). These findings are important for the subsequent operational design of the membrane in vacuum membrane distillation (VMD) for the desalination process.

Recovery and development of correlations for heat and mass transfer in vacuum membrane distillation for desalination

AbstractThis study evaluates the recovery of pure water by vacuum membrane distillation (VMD). The VMD process has been experimentally studied using a polytetrafluroethylene (PTFE) membrane for aqueous sodium chloride solution containing salt concentration of 20,000 ppm and permeate pressure of 7 kPa. The percentage recovery was observed to increase with time of operation and in 183 h, it became 86%. A mathematical model for recovery has also been developed and validated with experimental data. Heat transfer and mass transfer correlations have been developed for the range of feed flow rate of 0.5–2 lpm and feed bulk temperature of 40–60°C. The effect of feed salt concentration and membrane fouling was also studied on permeate flux. Permeate flux was found to decrease by 15 and 10% at 55 and 60°C, respectively, on increasing feed salt concentration from 5 to 40 g/l. The permeate flux was observed to decrease by 8% from 26.1 to 24 kg/m2 h after continuous operation of 180 h at 60°C and 20,000 ppm feed salt ...

Mathematical and CFD modeling of vacuum membrane distillation for desalination

In this work, a mathematical model has been developed for vacuum membrane distillation incorporating molecular diffusion–Knudsen diffusion–Poiseuille flow and validated with the experimental data. The feed-side membrane surface temperature used in mathematical model was estimated by computational fluid dynamics modeling. The effects of different operating parameters such as feed flow rate, feed bulk temperature, inlet concentration, and permeate-side pressure on water flux and salt rejection have been studied using PTFE membrane. It has been shown that with variation in feed flow rate from 0.5 to 2 lpm, the permeate flux increased from 20.1 to 25.6 kg/m2 h. Similarly, with increase in feed bulk temperature from 40 to 60°C, the permeate flux increased from 2.10 to 26.6 kg/m2 h. The permeate flux showed the variation from 17.8 to 19.5 kg/m2 h on decreasing permeate pressure from 7 to 5.5 kPa. No significant effect of feed salt concentration was observed on permeate flux. More than 99% rejection was obtained at the feed salt concentration of 40,000 ppm of NaCl. Few runs were also carried out by taking multi-ion feed (Ca++, Mg++, Na+, K+, Cl−, , Fe++, , F−) equivalent to that in ground water; in which case also more than 99% rejection was obtained.

Performance evaluation of vacuum membrane distillation for desalination by using a flat sheet membrane

Vacuum membrane distillation (VMD) process received a great deal of attention by many investigators because of its promising applications in several separation areas. It is a rising technology for seawater or brine desalination process. The process simply consists of a flat sheet hydrophobic microporous PTFE membrane and diaphragm vacuum pump without a condenser for the water recovery or trap. In this work, VMD performance was investigated for aqueous NaCl solution. In order to enhance the performance of the VMD process in desalination, that is, to get more flux, it is necessary to study the effect of operating parameters on the yield of distillate water. The influence of operational parameters such as feed flow rate, feed temperature, feed salt concentration and permeate pressure on the membrane distillation (MD) permeation flux have been investigated. The VMD performance showed that this device could reach a desalting degree of 99.99% which was not affected by feed concentration. The membrane distillati...