Studies About The Effect Of Membrane Modules On The Filtration Process Of Chinese Herbal Decoctions And Sub-critical Flux Operation Optimization

Membrane separation technology has been promoted and applied in the separation and purification of Chinese herbal decoction in some extent. As Chinese herbal decoction has complex environment structure, serious membrane fouling will be produced in the process of membrane filtration, causing the flux decline and reducing the efficiency of membrane system.Membrane modules and operating conditions have important impacts on the membrane filtration process. The filtration processes of some kinds of Chinese herbal decoctions with different physicochemical properties in four common modules (hollow fiber membrane, spiral wound membrane module, tubular membrane module and flat membrane with a stirrer) are researched and compared from many aspects. At the same time, blocking model is used to analyze the experimental data in order to know of the fouling mechanism of Chinese herbal decoctions under different membrane modules and operating conditions. The content and results of this paper are as follows:Part one:The filtration experiment of five kinds of single herbs and compound chinese herbal decoctions with four different membrane modules are carried out. The membrane filtration processes of various Chinese herbal decoctions in four membrane modules are well analyzed and compared from several aspects including the permeate flux, flux decay rate and fouling resistance. Experimental results show that under the same pressure, temperature and concentration, the tubular membrane and flat membrane with a stirrer are superior to the hollow fiber membrane and spiral wound membrane in permeate flux, flux decay rate and fouling resistance. While the extent of differential performance between four membrane modules is different in different Chinese herbal decoctions, which should be related to the different complex solution environments between Chinese herbal decoctions. Overall, the filtration performances of the tubular membrane and flat membrane model are better than the hollow fiber membrane and spiral wound membrane model. The permeates obtained by the four membrane modules are not significant differences in the physicochemical properties and macromolecules retention rate. Part two:The Chinese herbal decoction of Huanglianjiedu decoction is chosen as an experimental subject to further examine the impact of different membrane modules and operating conditions on the filtration process. Experimental results show that the flow of fluid in the membrane module has an important effect on filtration behaviors. Because of higher turbulent state in the tubular membrane and flat membrane with a stirrer,the permeate flux is relatively higher and fouling resistance is relatively smaller than the hollow fiber and spiral wound membrane where the fluid only reaches lower laminar state.Operating conditions have an important impact on the membrane filtration process. When the operating pressure of the system is greater than the critical pressure, serious fouling hinders the flux from increasing continually, reducing the system membrane permeability, and keeps it relatively stable finally. Increasing fluid Reynolds number (Re) and reducing the concentration can improve membrane flux in a certain extent.Results also show that operating conditions have influences on the retention rate of macromolecule. The retention rate is higher under higher pressure. It may be that the serious fouling phenomenon on the membrane under the high pressure, in some extent, hinders macromolecule through membrane and increases the retention rate.Part three:Analyzing and comparing the liquid characteristic chromatography between various Chinese herbal decoctions before and after filtrating by four different modules, the similarities between them are more than95%. It illustrates from an aspect that membrane filtration processes under different operating conditions have only weak effects on the overall chemical nature of Chinese herbal decoctions. The membrane separation technology can maintain the overall characteristics of Chinese herbal decoctions well.Part four:Membrane flux has an important impact on membrane fouling. Under super-critical flux operation, the high flux accelerates the mass transfer rate of the particles and macromolecules to the membrane, enhances the concentration polarization phenomenon, accelerates cake layer formation on the membrane surface and increases the cake layer specific resistance, which all result serious membrane fouling. Under sub-critical flux operation, the low flux weakens those factors causing serious fouling under high flux and extends the operating time and improves the efficiency of the system.We compare the two different modes of operation, constant transmembrane pressure (TMP) operation and constant flux operation. The results found that, under the same membrane flux, the system pressure under constant transmembrane pressure operation is higher than that under constant flux operation. It shows that when achieving the same membrane flux, fouling resistance under the constant transmembrane pressure operation is higher than that under constant flux operation. This is due that in the initial stage of the constant transmembrane pressure operation, the flux exceeds the critical flux, causing a relatively severe membrane fouling in a short period, which results in higher fouling resistance under the constant transmembrane pressure operation.Part five:Blocking model is used to analyze the experimental data. It finds that the membrane filtration processes of Chinese herbal decoctions under different membrane modules and different operating conditions fit blocking-cake model.Therefore, we can do the following explanation for the phenomenon of membrane fouling in the membrane filtration process of Chinese herbal decoctions from the angle of the pollution mechanism:In the initial stage of the membrane filtration, part of materials, which diameter smaller than that the membrane pore, enter the pores and deposit along the surface of the pores which reduces the effective diameter of the pores.With the process proceeded, materials superimposition in the membrane pores or on surface and completely block at the entrance of the film hole, which gradually leads to a decrease in membrane permeability;finally, macromolecules with relatively larger diameter gradually deposited on the surface to form cake resistance layer, and the fouling of the system gradually reaches a steady state and permeate flux keeps stable relatively.The innovation of this paper:(1) From the view of practical applications, the membrane filtration processes of Chinese herbal decoctions are comprehensive studied under different modules and operating conditions.(2) The two membrane filtration modes of operation, constant transmembrane pressure operation and constant flux operation, are compared to study the infection of the operation mode on the membrane filtration process in order to provide a reference for the optimal choice of the operation mode.(3) The sub-critical flux is applied to study its applicability in reducing the membrane fouling in the filtration process of Chinese herbal decoctions.(4) Fouling mechanism of Chinese herbal decoctions in membrane filtration process is analyzed to provide an explanation for membrane fouling,which has a certain guiding significance for the further cognition to membrane fouling phenomenon and reduce membrane pollution.