Research On Gas Dispersion And Mass Transfer Characteristics Of Wide-viscosity Impellers In Shear-thinning Fluid Systems

Xanthan gum is a complex polysaccharide made from the fermentation of Xanthomonas.It is a pseudoplastic fluid and widely used in the fields of food,petroleum exploitation and chemical industry.In the middle and later stages of fermentation of the xanthan gum,due to the continuous accumulation of products,the viscosity of the fermentation broth increases,and the gas-liquid mixing and mass transfer are more and more difficult.The xanthan gum solution with typical shear-thinning property is used as research system.The gas-liquid dispersion and oxygen mass transfer characteristics of impellers such as novel large-double-blade,Maxblend,Fullzone are researched in shear-thinning fluid systems.The effects of gas flow rate,impeller speed,mass fractions of xanthan gum and gas distributor on aerated power input,global gas holdup and oxygen mass transfer coefficient are investigated,which have important theoretical significance and industrial application value for improving the fermentation efficiency and product properties of xanthan gumThe experimental results show that,the aerated power input,global gas holdup and oxygen mass transfer coefficient increase with increasing impeller speed;The global holdup and oxygen mass transfer coefficient increase with increasing gas flow rate,while the aerated power input slightly decreases;The global gas holdup increases with the increase of the xanthan gum concentration,while the oxygen mass transfer coefficient is opposite.Under the same conditions,FZ impeller has better performance on the gas dispersion and global gas holdup than LDB impeller and MB impeller.The mass transfer performance of FZ impeller is best as the xanthan gum's concentrations are 0.25%and 0.5%.However,the oxygen mass transfer coefficient of MB impeller is higher than that of others when the xanthan gum's concentration is 1.0%.When the novel spherical micro-orifice gas sparger is used,the aerated power inputs of the wide-viscosity impellers are small,and the global gas holdup and oxygen mass transfer coefficient are high.The gas-liquid dispersion and oxygen mass transfer characteristics of the novel spherical micro-orifice gas sparger are better than the ring gas sparger.For the single-phase simulations,the macroscopic flow field,the distribution of shear amount and pumping amount are obtained.The maximum error between the simulated power input values and the experimental values is less than 20%.The computational results of the aerated power input well conform to those from the experiments for numerical simulation of the aerated systems.Furthermore,the distributions of local gas holdup in the stirred vessel are obtained,and the different factors that influence the results of the oxygen mass transfer coefficient are analyzed.This paper describes the gas dispersion and mass transfer performance of these three wide-viscosity impellers under the shear-thinning fluid systems in detail and intuitively.