Study On A Model Of Membrane Fouling And Cleaning Methods In Processing MPC

Ultrafiltration is a membrane technology, UF has been utilized widely in the dairy industry,especially for preparing MPC. In UF process, there is a critical problem—membrane fouling. Thedecrease of flux was caused by fouling, it limited the use of UF in mass production. Therefore,membrane fouling has been widely concerned. It is significance to study about basic theory andcleaning of fouling. Now in China, there is no scale processing of MPC, and also lack of theorystudies on basic theory and cleaning methods. In this study, a model of the decrease of flux due toprotein fouling was described in preparing MPC. The model was obtained by improving andcombining, and the results were compared with experimental data; Compared with differentcleaning agents, found the best concentration, cleaning time, temperature by calculating WFR. Theresearch content and results as follows:1. Raw milk was centrifuged to get skim milk, study on ultrafiltration effect with different fatcontent, results shows: Fat content had significant influence（P<0.05） on flux, membrane pollutionand protein content; after defatting, the fat content of skim milk is0.0365%, while flux was9.39L/m²·h²3.96L/m²·h, the pure water flux was67.28L/m²·h when it works2hours, this was42.48%of pure water flux which was measured before ultrafiltration.2. A model was combined on the basis of classical models, result shows:（1）Pore blockage: Pore blockage was the critical fouling in the first stage. The permeate fluxis expressed as follow: Due to the presence of cake layer,P_mfis not equal to Δ P. The pressure distribution in thecake layer should be determined.（2）Compressive cake layer: After pore blockage, cake layer formed, and became the criticalfouling, this is the second stage. Volume fraction distribution, permeate flux and compressive cakeheight are functions of time t. The relation between t and V, is calculated by the summation ofΔ tiandΔ Vi.The water flux of each membrane（J₀=ΔP/ηRm）is introduced to remove the effect ofifference in the resistance of each membrane:3. Compared with experimental data at difference conditions, result shows: the value oftJ₀/Vincreases gradually at the beginning of the experiment as blockage dominant period. Afterhat, it increases sharply and it might be the characteristic of cake dominant period. Finally, theradient of the tJ₀/Vcurve becomes gradual again. The reproducibility was broadly confirmed,specially in the later period. The difference between two experiments is within20%in the initialeriod and less than10%in the later. It suggests that with the changing within20%of α and within0%of Rp0and υ, the calculation results of flux also vary within20%in the initial period andithin10%in the later. The model is stable enough within20%change.4. In order to study the main effect operating condition to the process, the average value of theitted parameters in all experiments with the same level of that factor was calculated, results show:he α seems to decrease slightly when ΔP increases, υ has a slight tendency to decrease whilep0tends to increase with increase of ΔP. Therefore, ΔP is not the bigger the better, noffect of flux with more ΔP after cake forming. The α and υ decrease slightly when feedlow rate increase, but Rp0decreases. Therefore, rate is the bigger the better.5. According to the properties of PES, five cleaning agents was chosen: NaOH, HCl, NaClO,DS and alkaline protease. results show:（1）The best cleaning agent is NaOH, WFR is79.2%. The WFR of alkaline protease, NaClO,DS are69.1%,62.6%,46.1%. HCl is the worst, WFR is only39.1%.（2）The best concentration ofaOH, HCl, NaClO, SDS and alkaline protease are0.5-0.7%、0.3%、250ppm、0.1%、0.2%.（3）Theest time: HCl is30min, NaOH and NaClO are40min, and SDS and alkaline protease are0-50min.（4）The best temperatures of all agents are45℃.（5）Compound agent is better thaningle agent. The group of0.5%NaOH and250ppm NaClO is the best, WFR is96.8%.