| In order to reduce the membrane fouling, the membrane operating pressure have to be reduced, which results in the membrane separation performance is down, in the process of practical application. Therefore, controlling membrane fouling and improving membrane flux has become a very urgent and realistic problem. Based on fluid mechanics principle, this paper designed a novel pulsed flow process via pressure relief to control membrane fouling in ultrafiltration process. In this work, soy milk was used as feed solution, PVDF hollow fiber membrane was used as filter medium. It was analyzed that the effect of the pulsation process on reducing membrane fouling and improving membrane flux. Then, draw lessons from previous research results, adding a moderate amount of diatomite filter-aid to improve the effect of reducing membrane fouling was investigated. Finally, the concept of critical operating flux was defined to investigate the membrane fouling of different process in high operating pressure.The purpose of flushing the cake layer on the membrane surface was achieved by the pressure relief pulsation process. The result showed that the flux under pulsation condition increased 14.9%, though the averaged trans-membrane pressure was 22.5% lower than that in non-pulsation process, compared with non-pulsation filtration process. It illustrated that the process had played a positive role on reducing membrane fouling, make up the flux decline caused by the pressure loss. The process was a feasible method to control membrane fouling. In addition, the process was simple and practical, was helpful for industrial application.In order to strengthen the effect of controlling membrane fouling, based on micro-particle auxiliary filtration principle, we proposed firstly that diatomite filter-aid assisted the pulsation process to strengthen the control of membrane fouling. Using the bridging role of diatomite on membrane surface, a loose cake layer was formed, which avoid the flux reduced rapidly caused by the pollutant was deposited on membrane surface to form compact cake layer. Then, the strong shear force produced by pulsed flow flushed the cake layer effectively, so as to the cake layer resistance was reduced and the flux was improved. The results showed that the cake layer resistance was reduced by 39.2% via pulsation process coupled with diatomite, compared with non-pulsation process, the percentage of cake layer to total filtration resistance was reduced from 80.1% down to 71.9%, the membrane flux improved 33.3%. The flux increased significantly by the pulsation process coupled with diatomite, compared with 8% increased by adding diatomite only and 14.7% increased by pulsation process.In order to reflect the situation of controlling membrane fouling by the proposed process, the concept of critical operating flux was proposed:in the process of filtration, the flux increases with the pressure, however, the flux was not increased any longer but decreased or maintained stable when the pressure was high enough, the biggest point was the critical operating flux, its corresponding operation pressure was defined as the critical operating pressure. The results showed that the cake layer resistance was composed of cake density and cake thickness. The cake density was decided by the material properties of the feed solution and operating pressure, the cake thickness was decided by the operating condition. The coupling process of pulsation with diatomite could reduce the cake density and thin the cake thickness. So we could improve operating pressure, and receive a higher flux:the critical operating flux was achieved 65.7 kg-m-2·h-1. Compared with non-pulsation process, the critical operation flux under the coupling process increased by 38.9%, compared with the condition of dosing diatomite, the critical operation flux increased by 16.9%, compared with pulse process, the flux increased by 17.3%. Compared with the critical flux, the critical operating flux could more truly reflect the influence of membrane fouling (especially the filter cake layer resistance) on membrane flux in the filtration process. Compared with the critical flux, the critical operating flux was more practical. |
