Computational Fluid Dynamics Study For The Influence Of Flow Field Environment On Microbial Fermentation In Stirred Bioreactors

Stirred tank bioreactors are widely applied and dominate in microbial fermentations.The design and scale-up of stirred bioreactors are always the key research points for fermentation engineering.In this thesis,the computational fluid dynamics(CFD)simulation combined with experiments are employed to investigate mixing,mass transfer and shear formed in stirred bioreactors.These characteristics of flow fields are associated with microbial fermentations to predict fermentation process.This method will help design and scale-up process of bioreactors.Firstly,a mixing time simulation method based on mean age theory is established.The effects of different parameters,such as unstructured mesh element number,inlet flux and Matlab post-processing procedure,on simulation results are investigated.Conclusions are as follows:1)To get better predict mixing time,the unstructured mesh element number should guarantee that the power calculated by energy dissipation rate should exceed 80%of the power calculated by the torque.2)The inlet flux should be two orders lower in magnitude than the impeller pumping flow rates.3)For Matlab post-processing,the number of mean age groups should be more than 12500.Reply on the above conditions,mixing time simulation results are close to experimental value.When the reactor has a height-to-diameter ratio of more than 1.5;1,the Reynolds number less than 2.1×105 will cause difference between simulation result and experimental value to become larger,and error will reach 12%.Comparing with PBTD and PBTU,the simulation results of PBTD are all within the experimental data range,and the simulation results of PBTU are higher than the experimental values.The maximum error is up to 14%.In addition,many mixing time empirical formulas do not consider the volume factor.Through the investigation of this thesis,it is found that N?number obtained by simulation is not affected by scales of reactors.Gas-liquid oxygen transfer capacity has a relevant effect on fermentation processes.The influence of impeller structure on the mass transfer capacity is studied using the steady-state method for formulating mass transfer coefficient empirical equations.For single-impellers,radial impellers(Rushton turbine,half-circular-blade disk turbine,parabolic-blade disk turbine),axial impellers(three-narrow-blade hydrofoil impeller,45°pitched blade turbine,three-wide-blade hydrofoil impeller pumping down,hydrofoil-blade disk turbine)and wide-blade hydrofoil impeller with two pumping down blades and two pumping up blades are compared.The gas-liquid mass transfer coefficients of radial flow impellers are higher than those of axial flow impellers under same rotation speed and gas flow rate.The mass transfer efficient(defined as the volumetric mass transfer coefficient per unit power input)of radial flow impellers is also better than that of axial flow impellers.An empirical correlation for single radial impeller kLa is derived as kLa=0.02x(Pg/V)0.47(ug)0.48,and that for single axial impeller is derived as kLa=0.018×(Pg/V)0.44(ug)0.45.As for double impellers,RT+RT provided highest mass transfer capacity under certain rotation speed and gas flow rate,while WHD+WHD gives the highest values of gas-liquid mass transfer coefficient with the same power consumption.The mass transfer empirical formula of WHD+WHD is kLa=0.01x(PG/V)0 59(Ug)0.47,and that of RT+RT is KLa=0.026x(Pg/V)0.5(ug)0.58.Based on the study of gas-liquid mass transfer,effects of model selection of interphase momentum transfer model,population balance model and gas-liquid mass transfer model on the simulation results of gas-liquid two-phase flow field are studied in detail,in order to form a method for accurate simulation.Firstly,the Brucato model describing the inter-phase drag force was modified to obtain better results based on experimental data(error on gas holdup within 1%).Based on this,effects of virtual mass force,lift and wall lubrication force are investigated.It is found that all three forces have little influence on the average gas holdup.CFD simulations show that virtual mass force has little influence on the vertical slip velocity.Lift has only an effect in the regions near the impellers and sparger and wall lubrication force just play a role near the wall.For simulation of local gas-liquid property,incorporating the population model give better results near tank wall and impeller tip.Compared to experiment results showed that the upwind scheme gives better results(with 10%relative error)than the high-resolution scheme(with relative error of 15%).For simulations of gas holdup,the simulation results obtained by upwind scheme are consistent with the experimental results(less than 10%)in the region with low gas holdup,while in high gas holdup region(greater than 5%),the results obtained by upwind scheme are higher than experimental data(from 10%to 60%).Almost consistant results are gotten for using the upwind scheme for triple simulation cases.However,different similation cases with high-resolution scheme do not show an obvious pattern in the gas holdup distribution.For local bubble Sauter diameter simulation,better agreement with experimental data are reported by using upwind scheme(with standard error less than 10%),and the difference between three different cases is small(in range of 1%-2%).When using high-resolution scheme,the CFD simulation results are lower than the experimental data in most cases(relative error to the extent of 20%),and there are certain differences between different calculation cases(in range of 1%-20%).Non-drag forces have effect on the simulation results,but no obvious rule is found.Among the different mass transfer model,eddy cell model gives the best results,with a relative error only 3%.Based on the flow field investigation,a novel CFD/CM model is then proposed for simulating the effects of mixing capaicity to the fermentation process.The new proposed model,integrating flow field and biokinetics mdoel,implemented the simulation of fed-batch fermentation process.In the simulation process,both volume and velocity changing are considered at the same time,which solves the problem that the CFD cannot simulate the volume change process simulation for a long time.Substrate consumption and product generation are compared among different reactor scales(3 L and 400 L)with different impeller configurations(2RT and RT+45°PBT).Relative to industrial-scale reactor(400 L),lab-scale reactor(3 L)gets higher based on same substrate consumption(Yp/s of 3 L is 159.7 mol PenG/mol glu,400 L 152 mol PenG/mol glu).RT+45°PBT(482 W/m3)combination achieves the same yield as 2RT(609 W/m3)with less energy consumption.A substrate mixing index is proposed to illustrate substrate inhomogeneity in reactors.In the early stage of fed-batch,an obvious peak of substrate mixing index appears at 28 h.This peak means the overall inhomogeneity of the substrate in stirred tanks is maximum at this time point.This is a system characteristic of the bioreactor system depending on both mixing efficiency and substrate consumption rate.Finally,CFD simulations were applied for process analysis for two different bioprocesses,i.e.,glucoamylase production and nemadectin production.For glucoamylase production,two double-impeller configurations are employed,3RT and 3WHu.Compared to 3RT,3WHu achieves higher yields by consuming less energy.3WHu with lower shear rate can better retain the spherical structure of Aspergillus niger strain,which is requirement for higher glucoamylase production.In terms of mixing,the apparent viscosity of 3WHu is lower,and the dead zones within the reactor are less,which is advantageous for the uniform distribution of nutrients.The dissolved oxygen should be maintained above 25%to reach no O2-limitation condition in the fermentation of nemadectin production.Two double-impeller configurations are compared in 5 L bioreactors,2RT and RT+WHd respectively.The energy dissipation rate of 2RT is 43%higher under the same dissolved oxygen condition,which leads to a looser bacterial morphology.This is unfavorable to the product synthesis.The energy utilization of RT+WHd is also higher,with production of 593.6 ?g/W,and 264.71?g/W at 2RT.For nemadectin production,when the dissolved oxygen reaches more than critical dissolved oxygen concentration(25%),average shearing in tank should not exceed 110.3 s-1,otherwise excessively high shear will destroy bacterial shape and reduce final yield.