Applied Research On Ultrasonic Enhanced Direct Contact Membrane Distillation Technology

In this paper, ultrasonic technology was introduced and coupled with membranedistillation technology to construct ultrasonic enhanced direct contact membranedistillation system. In view of key problems restraining practical application ofmembrane distillation such as low permeate flux, membrane fouling and relativelybad stability of long-term operation, the effects of ultrasound irradiation on masstransfer and the improvement of anti-fouling property in membrane distillationprocess were investigated. The following results were obtained:(1) The application of common membrane materials, which were PP, PVDF andPTFE, to ultrasonic enhanced direct contact membrane distillation system was studied.The results showed that in the aspect of permeate flux enhancement affected byultrasonic irradiation, PVDF membrane> PP membrane> PTFE membrane. But thereductions in solute rejection and mechanical intensity of PVDF membrane wereobvious, and more energy was wasted due to lower heat resistance of PP membranethan others. PTFE membrane was a suitable selection for ultrasonic enhanced directcontact membrane distillation system, which not only met the demand of mechanicalproperty and energy consumption, but improved membrane flux about30%on thepremise of ensuring a good rejection.(2) The influence of salt concentration and fluid parameter on permeate flux indirect contact membrane distillation process was studied. With the increase of saltconcentration in the feed, the permeate flux would decline. It was beneficial to thepermeate flux enhancement by improving the feed inlet temperature and flow velocity.Lowering the permeate inlet temperature and improving velocity of the cold side hadlittle effect on the flux enhancement.(3) The influences of parameters, such as ultrasound frequency and power,operation condition, the feed concentration and flow path, on membrane performancein ultrasonic enhanced direct contact membrane distillation system were investigated.It was found that permeate flux increased by32%under current experimentalcondition with ultrasound wave. When the feed flowed through the shell side of the membrane module, the effect of ultrasound wave on permeate flux enhancement wasdirectly proportional to the feed concentration and ultrasonic power, and in inverseproportion to the feed temperature and flow rate and ultrasonic frequency. Duringcontinuous240h ultrasonic enhanced direct contact membrane distillation process, theflux and rejection rate had little change. The test results of the membrane propertiesindicated that ultrasonic irradiation had slight influence on the mechanical propertycrystal structure and hydrophobicity of membrane materials(4) In view of different effect of various substance, like inorganic salt, colloid,organic solute and composite system including calcium chloride (CaCl2), onmembrane distillation process, sodium chloride (NaCl), calcium sulfate (CaSO4),calcium carbonate (CaCO3), silica solution （SiO2）, humic acid (HA), bovine serumabumin (BSA), CaCl2-SiO2, CaCl2-HA and CaCl2-BSA were selected as solute tostudy reasons for permeate flux decline under conditions of4cycles of concentration.The results indicated that it was mainly due to the increase in resistance to masstransfer by the deposition of NaCl during NaCl solution concentration process. Theflux declines in the concentration of saturated CaSO4solution and saturated CaCO3were both because of crystallization on membrane surface. The reason for the fluxdecline in concentration process of silica solution was the sorbtion of SiO2onmembrane surface caused by low stability of colloidal dispersion. While Ca2+wasadded, more serious membrane fouling was observed. Humic acid was hydrophilicand electronegative when dissolving in water. Compact deposits were hard to form onthe highly hydrophobic PTFE membrane surface. Hence the flux had little change. IfCa2+existed in the HA solution, coordination compound consisting Ca2+and HAwould cover most of surface pores to make the flux decrease a lot. BSA had slightsignificant impact on the flux because BSA was electronegative in neutral solution.But addition of Ca2+was able to aggravate scaling of BSA to reduce the flux.Inaddition, BSA molecule can easily entered membrane pore because of strong sorbtion.As a result, the decline in rejection would be more obvious gradually.(5) According to the above studies, Scale inhibition of ultrasound wave wasstudied systematically. The investigation results showed that acoustic streaming madesolute away from membrane surface, which was the reason for preventing the flux decline. So ultrasound wave hardly alleviated the flux decline in concentrationprocess of NaCl solution, CaCO3solution, HA solution, and BSA solution,respectively.