Study On Osmotic Distillation For Concentrating Tea Polyphenols By Using PTFE Hydrophobic Microporous Membrane

Tea polyphenols (TP) were extracted from tea, which were one of the main ingredients thathad health function in tea. TP can be used for medical care, beauty and so on. At present, TPextraction enrichment process included the solvent extraction, ultrafiltration clarification, reverseosmosis concentration and finally spray drying into powder. In the process of reverse osmosisconcentration, with the concentration continuous improvement, the osmotic pressure of TPsolution increased significantly, so it easily lead to excessive energy consumption. Osmoticdistillation (OD) was developed in the1980s, which can implement high concentration ofmaterial under the atmospheric pressure and would not be restricted by the high osmotic pressureas a new type of membrane separation process. Therefore OD particularly suited forconcentrating heat-sensitive materials. In the OD process, the key was hydrophobic membranematerial. Polytetrafluoroethylene (PTFE) had a natural hydrophobic property with water contactangle of128°and was resistance to acid and alkali. So PTFE was an ideal material to OD. Thepaper mainly studied the preparation of PTFE hydrophobic microporous membrane andstructural characterization using in OD. The membrane modules prepared by the PTFEmicroporous membranes were examined for concentrating the solution of TP using osmoticdistillation. TP solution and CaCl2solution served as feed and osmotic solution, respectively. Thewall thickness and mean pore diameters of membrane, feed temperature, concentration of feedand osmotic solution, flow of feed and osmotic solution were explored to influence on thepermeate flux and rejection. The following three aspects were studied in this paper:(1) PTFE hydrophobic hollow fiber membranes were prepared through a cold pressingmethod including pushing, stretching and sintering. The wall thickness and the mean porediameters of the PTFE hollow fiber membranes prepared through changing the parameters ofextrusion head and stretching ratio varied from400m to750m and from0.22m to0.45m,respectively. The prepared PTFE hollow fiber membranes had a sea-island structure which wasdense outside and porous inside. The parameters of extrusion head had greatly influenced thewall thickness, but less impacted the mean pore diameters and porosities. With the increase ofstretching ratio, the diameters of fibrils elongated, the pore size and porosity increased, the rangeof pore size distribution widened. The membrane modules prepared by the PTFE hollow fibermembranes were examined for concentrating the solution of TP using OD. The results indicatedthat the decrease of wall thickness of PTFE hollow fiber membranes and the increase of pore size of PTFE hollow fiber membranes, concentration of osmotic solution, and flow of osmoticsolution and feed can improve the permeate flux. The increase of feed concentration can reducethe permeation flux. In this study, the tea rejection achieved99.9%and the operation conditionshave little effects on the tea rejection.(2) PTFE hydrophobic flat-sheet membranes were prepared through a cold pressing methodincluding paste extrusion, rolling and stretching. The mean pore diameters and porosities of theprepared PTFE flat-sheet membranes varied from0.25m to0.80m and from46.9%to78.3%,respectively. The PTFE flat-sheet membranes showed the microstructures of nodesinterconnected by fibrils. The nodes became small and the diameters of fibrils decreased with theincrease of stretching ratio. Increasing stretching ratio increased the pore size and porosity of thePTFE flat-sheet membranes. The membrane modules prepared by the PTFE flat-sheetmembranes were examined for concentrating the solution of TP using osmotic distillation. Theresults indicated that the increase of pore size of PTFE flat-sheet membranes, concentration ofosmotic solution, and flow of osmotic solution and feed can improve the permeate flux. In thisstudy, the tea rejection achieved99.9%and the operation conditions have little effects on the tearejection.(3) The stability of PTFE hollow fiber membrane modules and PTFE flat-sheet membranemodules were tested by a continuous OD experiment of200hours. The changes of permeate fluxand tea rejection with time were examined in the OD process. The study found that the permeateflux decreased with the extension of working time and gradually stabilized after100hours. Thetea rejection achieved99.9%in this study. For the hydrophobic microporous membrane, it hadgood cleaning effect with distilled water,0.5%HCl solution or0.5%NaOH solution circulationflushing. It was beneficial to wash with alkali first and then with acid agents. The permeate fluxcan be restored to over95%of the initial flux.