Investigations Of Gas-Liquid Mass Transfer Process For High Viscosity Pseudoplastic Fluid In Industrial Scale Stirred Reactor

Stirred equipment is a most frequently used equipment in industrial production.It is widely used in the industry of chemical production,food fermentation and paper making.Especially in food fermentation,stirred reactor is one of the most widely used reactors.Xanthan gum solution is a kind of pseudo-plastic fluid and its fermentation process is carried out in a mechanically stirred reactor.However,during the fermentation process,a great change in the viscosity of fermentation broth affected the dissolution of oxygen,mass transfer and mixing in the reactor,seriously limiting the industrialization production of xanthan gum.In this paper,the reactors are designed into two types,with or without helical coil.With the help of commercial software Fluent,the method of Computational Fluid Dynamics(CFD)was used to study gas-liquid mass transfer and mixing behaviors of 1.0 wt% xanthan gum solution in a stirred reactor agitated by Maxblend(MB)agitator and a three-layer of six-straight blade disk turbines(TT)impeller.It is hoped that this study can improve the problems encountered in the fermentation process of xanthan gum and provide theoretical guidance and data support for the gas-mixing of high viscosity fluid.The research content and main conclusions are as follows:Firstly,the simulation were conducted under the condition of non-ventilation to obtain the basic data of the two kinds of agitators.A RNG k-? model is selected to solve turbulent flow in the mainstream field,the multiple reference frame method was used to deal with the flow field of blade area.The results showed that: the flow pattern generated by MB impeller showed a "large cycle" flow pattern,while TT impeller formed two circulations in the part of up and down of each paddle.With the increase of agitation speed,the unit volume power consumption,the shear rate and the amount of liquid discharge of the two kinds of agitators increase.The high shear rate area and high drainage area are mainly concentrated in the vicinity of the blades.After adding the coil,the flow pattern near the coil and the wall of the reactor becomes more complicated.Meanwhile,the unit volume power consumption and the shear rate of the agitator increase,but the drainage volume decreases.Secondly,flow behavior and gas-liquid parameters of xanthan solution in a gassing stirred vessel were further investigated.The Euler-Euler technique was introduced to solve these problems of gas-liquid two-phase flow,the(Population Balance Model)PBM were employed to describe the evolution of coalescence and break-up of the bubbles.The results showed that:The apparent density of the medium decreases and result in the unit volume power consumption decreases after gas was entered.Under the same unit volume power consumption,the gas holdup and the diameter of the bubbles increased with the increase of gas velocity-inlet.The gas-liquid mass transfer coefficient agitated by TT impeller is larger than that by MB impeller.After the coil is set,the existence of the coil increases the resistance to the fluid,and also promotes the breakage of the bubbles,which is beneficial to the interphase mass transfer.Therefore,the power consumption and the gas-liquid mass transfer coefficient increase,but the bubble diameter in the reactor decreases.The fields will more turbulence and the bubble is easily broken under the strong turbulent kinetic energy with larger power consumption.So,the average bubble size decreases,but the gas holdup and the gas-liquid mass transfer coefficient increases.The gas-liquid mass transfer coefficient agitated by the TT impeller is still larger than that of the MB impeller,the same thing as without ventilation.