Study On Concentrating The Extract Of Traditional Chinese Medicine By Direct Contact Membrane Distillation

Membrane distillation is one kind of new separation techniques in recent years. Because of its high efficiency, low energy consumption and working at room temperature, the research and application of membrane distillation have got more and more attention. This study investigated the process of concentrating the extract of traditional Chinese medicine by direct contact membrane distillation and methods of improving flux through changing experiment conditions. Different pretreatments and membrane fouling restraining methods were used in controlling membrane fouling. The experiments were conducted to find out the reasons of the trans-membrane flux decline during the process of concentration, and influence of manipulate conditions on flux. Different methods of mitigating membrane fouling were also investigated.The content of effective ingredients of distilled Chinese traditional medicine was well maintained which confirmed the feasibility of this technology. The result of granularity analysis experiment indicated that membrane fouling was caused by the deposition of pollutants on membrane surface. Measurement of vapor pressure confirmed that concentration multiple had negligible effect on vapor pressure of the extract. The result of measurement of viscosity confirmed that viscosity increased remarkably with CR when CR was above 5.0.By decoupling the factors affecting the trans-membrane flux decline, it was found that the observed flux decline throughout the process can be attributed to the membrane fouling, the reduction of water vapor pressure and the increase of transport resistance at feed side. Membrane fouling works on flux throughout concentration process, and the other two affect the flux only when CR increases to be higher than 6.0 (in this study).It was found that the trans-membrane water flux could be greatly improved if the concentration was carried out under higher extract temperature and flow velocity within membrane module.Both filtration and centrifugation pretreatments had positive effect on mitigating membrane fouling. Sedimentation had lower effect than the above. But filtration and centrifugation had remarkable limitations of being time-consuming and uneconomical. It was found that both gas-liquid two-phase flow at the feed side and gas back-washing within membrane module were effective ways to control membrane fouling. Gas back-washing seemed to give better fouling control than gas bubbling at feed side.