黄原胶发酵过程中流变学行为和气液传质的研究

在10L搅拌式发酵罐中,研究了黄原胶发酵的流变学行为和气液传质特征。发酵液呈现典型的假塑性行为,后期发酵液稠度系数K大于20Pa.snn,流变指数n低于0.2,随着胶浓度的提高,搅拌所带来的剪切力不够,使有效搅拌区域减小,发酵液几乎不随搅拌浆而转动。考察了表观粘度ηm、搅拌转速N和通气速率Ug对体积氧传质系数kLa值的影响,将kLa值与转速N和表观粘度ηm进行了无因次关联。实验条件较高搅拌下的高剪切,不仅能提高整体混合,而且能够促进细胞周边分泌的黄原胶粘液层向主体扩散,有利于强化传递。在所研究的气速范围内,通气速率的提高似乎对气液传质的贡献不大。具有较高剪切和较大有效搅拌区域的反应器被认为更适合于黄原胶发酵过程。

Studies on Fluid Rheological Properties and Mass Transfer During the Fermentation of Xanthomonas Campestris in a Stired Reactor

Xanthan gum biosynthesis was studied in a 10L stirred reactor.Rheological properties and oxygen mass transfer coefficient(k_La) were investigated during the X.campestris fermentation.The fermentation broths behaved as pseudoplastic non-Newtonian fluids.In the final period of fermentation,the value of consistency coefficient K exceeded 20Pa·sn,while the corresponding rheological index n was below 0.2.With the increase of xanthan gum concentration,the shear stress became not enough to satisfy the mixing.It resulted in small 'well-mixed zones' and quiescent fermentation broth.The effects of apparent viscosity η_m,agitation speed N and gas flow rate Ug on k_La were investigated,and the exponential equation describing the relationship among k_La,N and η_m was obtained.Under the experimental conditions,high impeller speeds increased the 'well-mixed zones',enhanced the moving of xanthan gum slime layer around the cells and improved the mass transfer.Gas flow rate seemed to have little contribution to gas-liquid mass transfer in the studied range.It is suggested that the reactors with high shearing strength and large effective agitation zone are useful for this fermentation process.