Studies On The Purification And Concentration Of Tea Polyphenols Based On Multi-stage Membrane Filtration

This paper took low-grade oolong tea as experimental material, explored the process of thepurity and concentration of tea polyphenols by multi-stage membrane filtration.The performanceof the purification and concentration of tea polyphenols by ultrafiltration, nanofiltration andforward osmosis for tea polyphenols from tea extract solution were investigated, membranefouling, the performance of membrane flux and the effect of membrane cleaning in the processof membrane separation were analyzed. To explore the possibility of using membrane separationtechnology to purificate tea polyphenols, so as to provide reference for further research andapplication.In the process of separation and purification with ultrafiltration, the performance ofultrafiltration membranes with different molecular weight cutoff (MWCO) for tea polyphenolsfrom tea extract solution was investigated. The different cleaning strategies, including pure waterbackwashing, NaOH, ultrasonic and the combination of NaOH and ultrasonic on the membranefouling were also investigated resepectively. The results showed that the purity of teapolyphenols could be increased to33.37%from18.1%; membrane fouling is mainly caused bythe concentration polarization of gel layer, its resistance is about9.5×1013m-1,93.9%of the totalresistance. The effect of pure water backwashing is negligible, foulants in fouled membranecould be removed effectively by the combination of NaOH and Ultrasonic,97.4%recovery fluxcould be achieved by the combinative treatment of15min of Ultrasonic and NaOH.The ultrafiltration dialysis fluid was used for further separation and purification bynanofiltration. Molecular weight cutoff of nanofiltration membrane for the200and800Da wereused respectively, and the experimental was run under the constant pressure of0.6MPa and theconstant cross-flow velocity of0.22m/s. After the filtration by800Da nanofiltration membrane,the purity of the tea polyphenols in infiltration was almost no change, but after filtration by200da nanofiltration membrane, the purity of tea polyphenols in infiltration increased from33.37%to43.2%, but the yield is lower, just about0.3%.In the process of separation and concentration with forward osmosis, the concentrationperformance of tea polyphenols from tea solution at different draw solution concentration anddifferent membrane materials using NaCl as draw solution were investigated. The filtration performance, membrane fouling, the performance of membrane cleaning and the retention rate oftea polyphenols in concentrate of forward osmosis (FO) were also investigated. The resultsshowed that steady flux and concentrate effect were obtained using NaCl as draw solution.PA-4M, CTA-4M and CTA-2.7M were used to concentrate the tea solution until theconcentration ratio was up to10times. Then the fouled membrane were cleaned with NaOH(pH=10) for25min. The flux recovery rate were100%,95.9%,96.1%, and the retention rate oftea polyphenols in the concentrate were70%,79.3%and83.8%respectively. The results showedthat CTA membrane is more suitable for concentration using2.7M NaCl as draw solution.