Study On Seawater Desalination Device Of Solar Membrane Distillation By Using PTFE Hollow Fiber Membrane

Membrane distillation(MD) is a membrane separation technology which mass transfer driving force is the vapor pressure difference.MD has higher rejection for inorganic salts、macromolecules and other non-volatile components, lower investment cost, simpler operation compared with other separation techniques.Coupling solar heat with membrane distillation can reduce energy consumption which has great prospects in the desaliIlation for seawater and brackish water.The key of MD process is the high temperature resistant porous hydrophobic membrane materials.Polytetrafluoroethylene(PTFE) with good hydrophobicity, chemical and temperature stability is an ideal material for membrane distillation.In this paper, PTFE hollow fiber membranes were produced by“Mix-Push-Extrusion-Sintering”method,and its microstructure, water contact angle and pore size distribution were characterized;The membrane modules with internal heat exchange were designed and prepared for air gap membrane distillation(AGMD),which took PTFE hollow fiber membrane as the hot membrane and PTFE hollow solid drainpipe as the cold membrane.The effects of structural and operating facts on the water flux and salt rejection of AGMD process were stuied,including membrane thickness, pore size,porosity,the hot feed inlettemperature, the cool feed inlet emperature and the feed flow rate and so on;Desalination was studied by coupling solar water heater with air gap membrane distillation(MD) and using NaCl aqueous solution as feed. The effects of hot feed temperature, rate of flow, cold feed temperature,different weather conditions and the connecting styles of the modules on permeate flux and salt rejection of MD were investigated. The main contents and conclusions are as follows:(1) The PTFE hollow fiber membranes,made by“Mix-Push-Extrusion-Sintering”method,has an asymetric structure of dense outsideand porous inside;The average membrane pore size is 0.22-0.32?m and the pore size distribution is uniform; The membrane porosity is 35.3-45.6%;The membrane surface water contact angle are 120-127.8°, so its hydrophobic property was excellent;The tensile strength is more then 160 N.(2) The membrane modules with internal heat exchange were designed and prepared for air gap membrane distillation(AGMD),which took PTFE hollow fiber membrane as the hot membrane and PTFE hollow solid drainpipe as the cold membrane.The effects of PTFE membrane thickness,membrane pore size,porosity and other structural facts on water flux and salt rejection of AGMD process were studied. The experient result showed that,within a certain range, a lower membrane thickness was advantageous for membrane distillation desalination. The larger the porosity of the membrane was, the greater membrane flux was. Rejection stays at99.99%,.not affected by the membrane thickness or porosity.(3) The effects of hot feed temperature T1, the cool feed emperature T4, feed flow rate Q and so on on water flux and salt rejection of AGMD were studied. The experient result showed that: With the increase of cool feed temperature and the decrease of feed flow rate, the permeate flux decreased but GOR increased. With the increase of feed flow rate, the permeate flux increased but GOR decreased. And the rejection was stabled at 99.9% in the MD process.(4) Desalination was studied by coupling solar water heater with air gap membrane distillation(MD) and using 3% NaCl aqueous solution as feed. The effects of hot feed temperature and rate of flow, cold feed temperature and different weather conditions on permeate flux and conductivity of MD were investigated. The results showed that the permeate flux significantly increased with the increase of hot feed temperature and rate of flow. However, the permeate flux decreased with the increase of cold feed temperature. The permeate flux of MD had significant difference among different weather conditions. The maximum permeate flux was obtained in sunny day and the minimum permeate flux in overcast. The largest permeate flux of system and daily permeate quantity were 2.7L·m-2·h-1 and 11.8kg, respectively, when the collector area of solar water heater, membrane area and rate of flow were 1.85m2, 1m2 and150L·h-1, respectively. The permeate conductivity was range from 5 to 15 ?s·cm-1 and the salt rejection was over 99.99%. The result of this paper has a certain extend reference meaning for solving drinking water problem in China’s remote islands.