掃流過濾分離微藻懸浮液之研究

A Study on Cross-Flow Microfiltration of Algae Suspension

巫佩樺(Pei-Hua Wu)

指導教授 ： 莊清榮

中原大學/工學院/化學工程研究所/碩士(2012年)

https://doi.org/10.6840/cycu201200831

摘要

由於全球經濟的蓬勃發展，使人類面臨了地球暖化與石油危機兩大問題，而利用藻類來吸收二氧化碳並轉換成生質柴油是目前最受矚目的方法。但藻類細胞相當微小，且比重與水接近，因此如何有效的回收藻類也成為一門值得研究的課題。
薄膜過濾為單純的物理性分離，較不會破壞被分離物質，因此已廣泛的應用於化工、食品、生化、水處理等程序中的濃縮分離。直接以薄膜過濾法處理微藻溶液阻擋率可達98%以上，但培養液中的微小顆粒、細胞代謝產生的大分子及細胞碎片容易造成膜阻塞，膜結垢是導致濾速大幅下降的主因，而形成結垢的因素眾多，包含了薄膜的材質與操作參數等。
本研究分別以0.22μm MCE、0.2μm PC及0.2μm PTFE三種不同材質與特性的薄膜來對擬球藻懸浮液進行掃流過濾實驗，不僅針對顆粒與薄膜之間的作用力，也藉由改變掃流速度、壓力等參數及外加電場產生的作用，分別對其通量及膜結垢的影響做一探討，並進行膜阻力、濾餅阻力等分析，以了解膜阻塞情形。實驗結果顯示，三種膜中MCE微濾膜有較大的通量，且當掃流速度從0.004m/s提高至0.052m/s時可使濾速明顯提高，且高掃流速度之擬穩態濾速也較高。過濾前後膜阻之變化率以流速0.017m/s疏水性PTFE膜865.1%為最高。而透膜壓力的改變並不會對濾速造成明顯之影響，但壓力的提高會降低膜阻的變化率、提升濾餅阻力。於所有過濾過濾實驗中，濾餅是主要的過濾阻力。
離心後、自由沉降及原始之微藻懸浮液等三種不同進料條件，其濾速及擬穩態濾速以離心後之溶液1.88 10-5 m/s為最高，而原始培養液及自由沉降之微藻懸浮液，其雜質、EOM含量較高，濾餅堆積較緻密，αav為1.93 1014 m/kg，約為離心後的6倍。
在過濾系統施加一外加電場，與無電場相比，電場的施加有效的提升了起始通量，但由於溶液的電導度很高及電極電解產生的氣泡影響了有效電場，因此操作50分鐘後，電場的效應則變的較不明顯。離心後之微藻懸浮液，其電導度降低，顆粒界達電位提高至 -39.5 mV，當外加電場強度為4900V/m時，其初始通量約為無電場的兩倍，因此利用外加電場提升過濾效能的效果非常顯著。

關鍵字

微藻 ； 掃流過濾

並列摘要

Global warming and energy crisis are currently major problems of the world, and in recent years the use of algae to absorb carbon dioxide and convert it into biodiesel has attracted many attentions. However, due to the algal cells have a quite small size and its bulk density is close to water, how to effective recovery of algae cell from fermentation suspension is very important on the practical applications. Although direct membrane filtration method for treatment of microalgae solution can remove more than 98% algae, but the small size of cells, the cellular metabolism of macromolecules and cell debris are likely to cause serious membrane fouling. To give a better understanding of the effect of operation conditions on the filtration rate and fouling resistance of this bio-suspension is important for algae harvesting by membrane separation.
In this study, 0.22 μm MCE, 0.20 μm PC and 0.20 μm PTFE three different membranes were used for cross-flow filtration experiments of Nannochloropsis oculata suspension. The effect of applied pressure, cross flow velocity and interactions between algae cell and membrane on the filtration rate and fouling resistance are analyzed. In addition, experiments with an electric field imposed on the filter chamber will also carried out to investigate the electrokinetic effect on the filtration performance.
Experimental results of 4000 ppm algae suspension with cross flow velocity ranging from 0.004 to 0.052 m/s and pressure drop of 0.4 bar showed that among the three membranes MCE membrane has a larger filtration rate and also has an obvious rise in filtration rate as the increase of cross flow velocity. After 1 hr of filtration, the PTFE membrane will exhibit significant increase in membrane resistance, even up to 8.6 times the virgin membrane. Transmembrane pressure applied in the range of 0.2~0.6 bar showed that the pressure variation will not cause obvious impact on the filtration rate, but the pressure increasing will make a raise in cake resistance and a decrease of fouled membrane resistance. All the experimental results of the study showed that the overall filtration resistance is mainly contributed by cake resistance.
Comparison of the filtration results between the feed with the original microalgae suspension and the feeds with pretreatments of the original suspension by centrifugation or gravity sedimentation showed that the centrifuged microalgae suspension has the highest filtration rate . Due to the original culture medium contains more impurities and EOM, the average specific cake filtration resistance exhibits a very high value as 1.93 x1014 m/kg which is about six times that with centrifugation pretreatment.
An external electric field imposed on the filtration system can effectively improve the initial flux. But, due to the high conductance of the solution will cause a large amount of bubbles generated by water electrolysis and therefore reduce the effective electric field strength, it appeared that after 50 min operation the influence of applying electric field on flux enhancement becomes less obvious. If the microalgae suspension was pretreated with centrifugation, the solution conductivity decreases and the zeta potential of algae cells increases to -39.5 mV. Therefore, there is a significant effect by applying electric field to enhance filtration rate for the pretreated suspension.

並列關鍵字

microalgae ； cross-flow filtration

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