利用電場抑制掃流微過濾之膜結垢

Application of electric field to reduce the fouling in crossflow microfiltration

鄭志豪(Chih-Hao Cheng)

指導教授 ： 莊清榮

中原大學/工學院/化學工程研究所/碩士(2006年)

https://doi.org/10.6840/cycu200600528

摘要

摘要

於掃流過濾程序中外加一電場，來提升分離效能，係藉粒子電泳作用及伴隨之濾餅及濾材的電滲透效應，以達提昇濾速及濾液品質之效果。若選用具高電性之膜材進行電場掃流過濾，其絕大部分之濾速提升，係由電滲透所貢獻，本研究為了深入了解電滲透作用，首先進行電滲透穩態流之微觀分析，模擬管壁電性，複合管之管徑比及電場施加方向等對電滲透流態之影響；另一方面亦進行附加電場掃流過濾，利用0.1及0.2μm孔徑之Nylon薄膜，以自行配置之SiO2懸浮液，進行電場薄膜微過濾實驗，探討電場施加方式(連續式及反向脈衝式)、過濾壓差及掃流速度對過濾效能的影響。
於模擬電滲透中，吾人所採用之方法，係以流體運動方程中另加一電場作用力進行模擬小孔徑(0.2μm)單一圓管中之流態，其結果顯示，模擬之全展流截面的平均速度與理論電滲透平均流速值，其相對誤差約為6~10%：再以較大孔徑(0.8μm)模擬與以滑動邊界條件解析電滲透流之方法進行比較，兩者之模擬結果甚相近。而於兩層複合管之模擬中，發現管徑比及壁上界達電位比對大管與小管連接處之流態影響甚鉅，當機械壓力梯度與電場力作用同向時，其穩定之出口流量皆大於未施加電場者，而電場作用與機械壓差相反時，管壁轉角區出現循環流之情形較為明顯，若將電場作用及機械壓差同時存在之出口流量扣除僅機械壓差之滲透流，其結果顯示，電場與壓力梯度同向之滲透流量皆大於電場與壓力梯度異向者。
於電場掃流過濾實驗中，掃流速度對濾速影響甚微，本實驗所配置之SiO2懸浮液，於壓差0.1bar，掃流速度0.403 m/s操作下，在電場強度14000V/m時，該電滲透佔擬穩態濾速的82%。反向脈衝電場係利用類似電滲透逆洗之概念，於過濾過程中抑制膜面上之濾餅量，其結果顯示，當反向電場開啟時，其濾速為零，但掃流剪應作用力仍無法完全刮除以附著的粒子且部分粒子已滲入膜孔中，故無法使濾速回復到100%，以0.2μm孔徑之Nylon薄膜在過濾壓差0.2bar、掃流速度0.806m/s下，可獲得之濾液收集量約為未施加電場者的1.66倍，且其平均濾速回復率隨掃流速度增加而提高。在0.2μm孔徑之Nylon薄膜在過濾壓差0.1bar、掃流速度0.403m/s下，以三種不同電場(連續、正向脈衝及反向脈衝)施加模式下，於30分鐘後所收集到之濾液量為2263.2、450.26及184.89 ，其平均濾速之關係為JE(continuous) > JE(Pulse) > JE(Reverse pulse)。

關鍵字

電場掃流過濾

並列摘要

ABSTRACT

Among the various techniques developed to reduce the external and/or internal fouling of membranes, the so-called crossflow electrofiltration using a combination of shearing action and electric field to reduce both concentration polarization and membrane deposition has been attracting an increasing amount of attention. In such operations, electroosmosis induced through the membrane may play a vital role in enhancing the filtration rate, but so far, there is inadequate knowledge in the electroosmotic phenomena within the fine pore membranes. The first objective of this study was to simulate the microflow in the fine channels imposed with electric fields. The effects of wall zeta potential, pore dimension ratios for two layers composite membrane and the direction of electric field etc. on the electroosmotic flow were investigated numerically. The secondary part of this study was to investigate experimentally the characteristics of electro-microfiltration of CMP graded SiO2 suspensions. The effects of electric field mode(continuous, pulse and reverse pulse field ) and other operation parameters such as crossflow velocity and transmembrane pressure etc. on the filtration performance were analyzed. Emphasis was placed on the potential of applying reverse pulse electric field to clean the membrane during crossflow filtration of CMP suspensions.
The simulation was implemented using Fluent software in which an electric force distribution function as a source term was added into the Navier-Stokes Equation. As compare the simulated results with that evaluated from electrokinetic model for single capillary and fully developed flow, both has a 6~10 % relative difference in average velocity. In this study, the simulation with slip boundary condition was also carried out and showed a result very close to that obtained with no-slip boundary condition only for large pores where the electric double layer effect is negligible. With respect to the simulated results for two layers composite channels, it was found that circulation flow is obvious in the enlargement region between the two layers if both wall zeta potentials have opposite polarities. The electroosmotic flow rate induced in the composite channels imposed with forward electric field (fluid forced to flow from small channel to the large channel) is higher than that with reverse electric field.
Experiments of electro-filtration showed that the crossflow velocity has a little influence on the filtration rate. It was found that about 82% of the pseudo-steady state filtration is contributed by the electroosmotic flux at E=14000 V/m under the given conditions in the study. Although the instantaneous filtrate flux can approach zero or even negative value when a reverse pulse electric field was applied, the shearing action used still can not sweep off all the deposit due to some particles have been penetrated into the membrane pore. The capability of using reverse pulse electric field to clean membrane depends on the crossflow velocity. Based on the permeate amount received in a 30 min filter run, the relative ratios in average filtration flux between that from continuous electric field, pulse field and reverse pulse field are 12.2 : 2.4 : 1.0.

並列關鍵字

electrofiltration