Conrad P B, Lee S S
Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53705, USA.
Biotechnol Bioeng. 1998 Mar 20;57(6):631-41. doi: 10.1002/(sici)1097-0290(19980320)57:6<631::aid-bit1>3.0.co;2-m.
Recovery of an aqueous bioconversion product from complex, two-phase Pseudomonas putida broths containing 20% (v/v) soybean oil presents a significant challenge for downstream processing. Although not used before in multiple-phase separation for complex biotech products, crossflow filtration employing ceramic filters is one of the most attractive options which allow the design of integrated, continuous bioconversion processes. As a first attempt, we studied multichannel, monolithic ceramic membranes of different nominal pore sizes and lumen diameters under steady-state conditions. The best performance was obtained with 0.2-microm-pore/3-mm-lumen membrane, which completely rejected both cells and oil droplets from the permeate, creating a clear aqueous product stream. Although the same separation was achieved, the 50K molecular weight cut-off (MWCO) ultrafilter showed greater irreversible but similar reversible resistance, in addition to an order-of-magnitude higher membrane resistance. Larger nominal pore microfilters, such as 0.45 and 1.0 microm, experienced both cell and oil leakage even at low transmembrane pressure (10 psig). Attributed to greater shear at the same recirculation rate, smaller lumen filters did provide greater permeate flux. However, for practical purposes, the 0. 2-microm-pore/4-mm-lumen ceramic membrane was chosen for further evaluation. Transmembrane pressures up to 50 psig provided only marginal gains in filtration performance, whereas increasing shear rate resulted in linear increases in steady-state flux, presumably due to formation of shear-sensitive, complex gel/oil/cell layer near the membrane surface. A nominal shear rate of 9200 s-1 and 20 psig transmembrane pressure were chosen as optimal operating conditions. Additional studies in a clean system revealed that as low as 5% (v/v) soybean oil in deionized (DI) water resulted in an order-of-magnitude decline in steady-state permeate flux. Breakthrough of oil droplets occurred at 35 psig transmembrane pressure. The severe fouling and breakthrough phenomena disappeared in the presence of washed cells for transmembrane pressure up to 43 psig, implying an oil/cell layer coating the membrane surface, thus preventing oil penetration. Serious membrane fouling was also experienced in microfiltration of oil-free, cell-free supernatant and oil-free whole broth. Consequently, soluble proteins/surfactants were suspected to be the major membrane foulants. Interestingly, soybean oil up to 30% (v/v) enhanced the flux, presumably through complicated interactions with the major foulants. Regeneration of membrane was best achieved with protease and hot caustic/bleach treatments, supporting the hypothesized fouling mechanisms mentioned above. This work provides process and system information for batch microfiltration runs in the future, to be reported elsewhere as Part II of this work.
从含有20%（v/v）大豆油的复杂两相恶臭假单胞菌发酵液中回收水性生物转化产物，对下游加工来说是一项重大挑战。尽管陶瓷过滤器错流过滤此前未用于复杂生物科技产品的多相分离，但它仍是最具吸引力的选择之一，有助于设计集成、连续的生物转化工艺。作为首次尝试，我们在稳态条件下研究了不同标称孔径和管腔直径的多通道整体式陶瓷膜。孔径为0.2微米、管腔直径为3毫米的膜性能最佳，能使透过液中完全不含细胞和油滴，从而得到清澈的水性产物流。虽然实现了相同的分离效果，但50K截留分子量（MWCO）的超滤膜显示出更大的不可逆阻力以及相似的可逆阻力，此外其膜阻力要高一个数量级。较大标称孔径的微滤膜，如0.45微米和1.0微米的，即使在低跨膜压力（10 psig）下也会出现细胞和油泄漏。由于在相同循环速率下剪切力更大，管腔直径较小的过滤器确实能提供更高的透过通量。然而，出于实际目的，选择了孔径为0.2微米、管腔直径为4毫米的陶瓷膜进行进一步评估。跨膜压力高达50 psig时，过滤性能仅有微小提升，而增加剪切速率会使稳态通量呈线性增加，这可能是因为在膜表面附近形成了对剪切敏感的复杂凝胶/油/细胞层。选择标称剪切速率为9200 s-1和跨膜压力为20 psig作为最佳操作条件。在清洁系统中的额外研究表明，去离子（DI）水中大豆油含量低至5%（v/v）会导致稳态透过通量下降一个数量级。在跨膜压力为35 psig时会出现油滴穿透现象。在跨膜压力高达43 psig时，存在洗涤过的细胞时，严重的污染和穿透现象消失了，这意味着膜表面有一层油/细胞层，从而防止了油的渗透。在对无油、无细胞的上清液和无油全发酵液进行微滤时，也出现了严重的膜污染。因此，怀疑可溶性蛋白质/表面活性剂是主要的膜污染物。有趣的是，高达30%（v/v）的大豆油能提高通量，可能是通过与主要污染物的复杂相互作用实现的。用蛋白酶以及热碱/漂白剂处理能最好地实现膜的再生，这支持了上述假设的污染机制。这项工作为未来的分批微滤运行提供了工艺和系统信息，将作为本工作的第二部分在其他地方报道。
