رسائل ماجيستير
English
2014
Desalination Using Air Gap Membrane Distillation
Atia E. Khalifa , داهيرو عمر لاوال
كلية الهندسة-جامعة الملك فهد للبترول والمعادن · السعودية
Membrane distillation (MD) is a developing thermally driven membrane desalination technology that has been applied in four different basic configurations. In membrane distillation, a hot, saline feed stream is passed over a hydrophobic membrane. The temperature difference between the two sides of the membrane leads to a vapour pressure difference that causes water vapour to permeate through the membrane pores, and then condensed on the cold side of the membrane. The hydrophobicity of the membrane prevents the liquid from passing through the pores, while the water vapour is allowed to pass through. The technique offers the attractiveness of operation at atmospheric pressure and low temperature (40– 90oC), and has the theoretical ability to achieve 100% salt rejection. Thus, low-grade energy like solar and waste energy can be used for desalination.In this work, an experimental investigation of the performance of Air Gap Membrane Distillation (AGMD) system was performed for seawater and laboratory prepared salt feed water solutions. The influences of system operating parameters such as feed temperature, feed flow rate, coolant temperature, coolant flow rate and air gap width on permeate flux were studied. The effects of membrane pore size as well as the concentration of feed solution on permeate flux were also investigated. The performance of the AGMD unit was statistically optimized using design of experiment (DOE) and Taguchi technique. Furthermore, theoretical model describing heat and mass transfer analysis in AGMD was developed and discussed in detail.The permeate flux was found to increase with increasing feed temperature and feed flow rate. However, it decreased with increasing air gap width and coolant temperature. The system performance tends to increase marginally with increasing coolant flow rate. The system performance is mostly dominated by the effect of both feed temperature and air gap width. Feed flow rate and coolant temperature have relatively considerable effect on flux. Increasing the membrane pore size from PTFE 0.22µm to PTFE 0.45µm leads to about 10% increment in flux production. While increasing the feed concentration from 0.075g/L to 60g/L lead to about 11% drop in permeate flux. The tested double-stage AGMD design was capable of achieving a maximum permeate flux of 128.46kg/m2hr, which is almost twice that of single stage unit, under the same experimental conditions.In general, the theoretical model results were found to be in good agreement with the experimental data as the maximum deviation of model results was within 15%. The model was also used to predict thermal efficiency and temperature polarization of the AGMD system.Regarding system optimization using Taguchi methodology, the developed model proved to be in good agreement with the experimental data with a maximum deviation of about 10%. According to Taguchi orthogonal arrays, the experimental and model optimum system performance was found to be 76.0457 kg/m2h, and 74.5916 kg/m2h respectively. The conditions for the optimum performance are 80oC feed temperature, 5 L/min feed flow rate, air gap width of 3mm and coolant .temperature of 20oC