Abstract
Membrane distillation (MD) is a hopeful desalination technique for brine (salty) water. In this research, Direct Contact Membrane Distillation (DCMD) and Air Gap Membrane Distillation (AGMD) will be used. The sample used is from Shat Al –Arab water (TDS=2430 mg/l). A polyvinylidene fluoride (PVDF) flat sheet membrane was used as a flat sheet form with a plate and frame cell. Several parameters were studied, such as; operation time, feed temperature, permeate temperature, feed flow rate. The results showed that with time, the flux decreases because of the accumulated fouling and scaling on the membrane surface. Feed temperature and feed flow rate had a positive effect on the permeate flux, while permeate temperature had a reverse effect on permeate flux. It is noticeable that the flux in DCMD is greater than AGMD, at the same conditions. The flux in DCMD is 10.95LMH, and that in AGMD is 7.14 LMH. In AGMD, the air gap layer made a high resistance. Here the temperature transport reduces in the permeate side of AGMD due to the air gap resistance. The heat needed for AGMD is lower than DCMD, this leads to low permeate flux because the temperature difference between the two sides is very small, so the driving force (vapor pressure) is low. 
Highlights
Water covers 71% of the Earth's surface and plays a vital role in sustaining life on earth
3.1 Effect of Feed Temperature The driving force in all membrane distillation configurations is the difference of vapor pressure, which depends on the temperature difference from the bulk liquids to the membrane surface
The temperature transport reduced in the permeate side of Air Gap membrane distillation (AGMD) due to the air gap resistance, so the needed heat for AGMD is lower than Direct Contact membrane distillation (DCMD), that leads to low permeate flux because the temperature difference between the two sides is very small, Adham, Samer, & Altaf Hussain, 2013
Summary
Water covers 71% of the Earth's surface and plays a vital role in sustaining life on earth. There are several technologies used for desalination, but usually, the region that suffers from lack of water in developing countries cannot provide highly cost desalination technologies, so the necessity for a cost-effective desalination process in which producing pure and drinkable water leads to the study of Membrane distillation (MD). A bench scale unit of flat sheet DCMD process for the PVDF membrane is shown in the Fig.. DCMD unit consists of (1) membrane cell, (2) reservoir tanks, (3) pumps for both streams, (4) hot (feed), and (5) cold (permeate) water streams, and (6) temperature and pressure sensors. Results were measured every 30min, with 180 min as operating time, through all the time of experiment random TDS test was done by portable and bench type TDS meter for permeate to be sure there was no penetration for feed solution through the membrane
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