薄膜結垢與流線電位關係之探討

Analysis of the Relationship between Membrane Fouling and Streaming Potential

蔡宗諺(Tsung-Yen Tsai)

指導教授 ： 莊清榮

中原大學/工學院/化學工程研究所/碩士(2007年)

https://doi.org/10.6840/cycu200700818

摘要

摘 要

膜結垢的發生會導致濾速嚴重衰減及選擇穿透性下降等，在密閉過濾系統中，以往僅能由濾速之衰減判定膜結垢阻力，但膜結垢變化是漸近的，如何於過濾程序中即時監控膜結垢生成是極重要之課題。
目前欲藉由電動量測監控膜結垢已有數位學者提出，本研究以電動現象之量測，探討薄膜表面有濾餅附著或膜孔內有結垢發生時之垂直膜面流線電位與膜結垢之關係，並理論分析濾餅過濾過程中，其電動現象與濾餅成長之關係。
結果指出，於濾餅過濾過程中，以垂直膜面流線電位偵測濾餅過濾過程中濾餅成長情形，初期膜面有濾餅附著時，其流線電位變化呈大幅度遞增，之後隨濾餅厚度成長而緩緩增加，至某一濾餅厚度時，流線電位趨於定值，理論解析亦呈現相同趨勢，但定量上則有差異。對於欲過濾粒子之界達電位與膜材帶相同電性，但粒子電位與膜材相當或較小時，其流線電位變化隨濾餅成長而呈遞減；反之，粒子電位甚大於膜材時，其流線電位變化呈遞增。如粒子電位與膜材電性相異時，其流線電位變化均呈遞減。
當粒子界達電位與膜材電性相同且電位甚大於膜材時，假設不同過濾壓差下，濾餅平均孔隙度為定值，則隨操作壓差增加，其流線電位變化幅度明顯增加。反之，若濾餅孔隙度隨過濾壓差增加而降低，其流線電位變化受濾餅孔隙度在不同範圍內，而有所增減。於本研究模擬條件下，在恆壓濾餅過濾時，於濾餅孔隙度小於0.5範圍內，當濾餅孔隙度愈小，因電雙層重疊效應與濾速衰減，其流線電位變化愈小；反之，於濾餅孔隙度大於0.5範圍內，當濾餅孔隙度愈小，其通過濾餅層之壓降比例較大，增加其總流線電位變化量。
濾餅層粒子之界達電位大於膜材甚多時，初始濾速衰減，流線電位變化呈大幅度遞增，其主要受濾餅層粒子電性影響而遞增，但濾速衰減使流線電位遞增趨勢減緩。
以PC及MCE膜材過濾粒子粒徑小於膜孔徑時，因PC膜材結構為直通型孔道，其結垢多附著於膜內上層，亦有些許附著於膜內中層；MCE膜材結構為網狀交錯型，其結垢附著於膜內分佈較均勻，兩者膜內界達電位及膜阻皆有增加，且MCE單位膜阻變化之膜內界達電位變化比例大於PC膜材。由膜內界達電位變化比例對膜阻變化比例作圖，斜率愈大者，其膜內結垢粒子附著分佈較均勻。分析過濾前後膜阻及界達電位變化，因膜材結構不同，膜結垢的阻塞並非一定均勻分佈，目前只能以定性分析界達電位與膜阻之相關性，難以定量關聯。

關鍵字

界達電位 ； 薄膜結垢 ； 濾餅過濾 ； 流線電位

並列摘要

ABSTRACT

Membrane fouling is one of the major factors limiting the application of membrane filtration. Membrane fouling typically results in an unacceptable filtration rate and sometime also leads an unacceptable rejection of targeted component in selective separation. In a closed filtration chamber, the extent of fouling is generally determined based on the decline of the permeate flux. Since the rate of fouling is time dependent, how to on-line monitoring the fouling is very important for membrane processes.
Several studies have applied the electrokinetic method to characterize membrane fouling, however, very few paid attentions to a quantitative analysis between the electrokintetic properties and the membrane fouling. This study aims to investigate the relationship between the filtration streaming potential and the membrane fouling when a cake deposited on the membrane surface or the fouling occurred in membrane pore. Theoretical models to relate the streaming potential to the cake growth and filtration rate in the cake filtration process are developed and compared with the experimental data.
Experimental results from constant pressure cake filtration show that the change of streaming potential increases rapidly at the initial stage of filtration, and then a slight increase in streaming potential is observed with the growth of cake. As the cake grows up to a certain thickness, the streaming potential approaches a constant. Such a tendency is agreement with the simulation result based on the theoretical model. However, both still have some differences in quantitative values. When the deposited particles and the membrane both have the same charged polarity, the change of the streaming potential is decreasing with the cake growth if the zeta potentials of the particles are similar to or smaller than that of the membrane. On the contrary, the streaming potential increases with cake growth as the deposited particles have higher zeta potentials. When the particles and the membrane have the opposite polarity on zeta potential, the change of the streaming potential during constant pressure filtration is always decreasing.
Theoretical prediction under the conditions given in the study shows that the change of streaming potential decreases with the decrease of cake porosity when the porosity is lower than 0.5, due to the overlap of electric double layer and the decrease of the filtration rate. As the porosity of the cake is higher than 0.5, there is a reverse effect of the cake porosity on the change of streaming potential, due to a higher pressure drop through a denser cake.
When using PC and MCE membrane to filter the particles which size are smaller than the membrane pore, the zeta potential of the fouled membranes increases associated with the increase of hydraulic resistance through the membrane. The internal fouling occurs mainly on the top or even the medium layer of the PC membrane, while there is a comparatively uniform fouling through the MCE membrane. This difference may be due to that the PC pore is like the capillary structure but the MCE membrane has a pore structure like interconnected mesh. Comparison of the relative zeta potential and the relative hydraulic resistance of the virgin and fouled membranes shows that the change of zeta potential of the MCE membrane is higher than that of PC membrane. Because the internal fouling generally are quite non-uniform and depends on the structure of the membranes, it doesn’t seem to obtain a quantitative relationship between the change of the zeta potential and the hydraulic resistance of the fouled membrane.

並列關鍵字

cake filtration ； membrane fouling ； streaming potential ； zeta potential

參考文獻

江宜蓁 (民92)。薄膜電荷量測與膜過濾電動現象分析。私立中原大學化學工程學系碩士學位論文，桃園縣。

Benavente, J., Jonsson, G. (1998). Effect of Adsorbed Protein on the Hydraulic Permeability, Membrane and Streaming Potential Values Measured across a Microporous Membrane. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 138, 255-264.

Chong, T.H., Wong, F.S., Fane, A.G. (2007). Fouling in reverse osmosis: Detection by non-invasive techniques. Desalination, 204, 148-154.

Chun, Myung-Suk, Cho, Hong Il, Song, In Kyu (2002). Electrokinetic Behavior of Membrane Zeta Potential During the Filtration of Colloidal Suspensions. Desalination, 148, 363-367.

Happel, J., Brenner, H. (1965). Low Reynolds Number Hydrodynamics with Special Applications to Particulate Media. (2nd ed). New Jersey: Prentice Hall.