A Theoretical Fluid Dynamic Model for Estimation of the Hold-up and Liquid Velocity in an External Loop Airlift Bioreactor (original) (raw)
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Gas hold-up in external-loop airlift bioreactors
Bioprocess Biosyst Eng, 1995
A new model of gas holdup is proposed for externalloop airlift bioreactors. It is based on the similarity between the liquid circulation due to the local variation of gas holdup in airlift bioreactors and the natural convection due to temperature difference. The model is developed to include the case of non-Newtonian fermentation media which are involved in many industrially bioprocesses. The capability of the model is examined using a wide range of experimental results including the present data. Reasonable agreement is obtained between the proposed model and the experimental data both for Newtonian and non-Newtonian media.
Liquid circulation in external-loop airlift bioreactors
Biotechnology and Bioengineering, 1990
A simple model for prediction of liquid velocity in externalloop airlift bioreactors has been developed. Theoretical correlations for friction factor of gas-non-Newtonian two-phase flows and for liquid velocity in the riser were derived using the concept of an eddy diffusivity. The predictions of the proposed model were compared with the available experimental data for the friction factor and the liquid velocity in the riser of external-loop airlift contactors. Satisfactory agreement was obtained.
Applied Research Journal, 2018
In this study, a modeling and comprehensive 2D simulation of airlift bioreactor with the external flow loop is presented with the purpose of achieving the required physical properties such as the velocity of gas and liquid phase and also viscosity. In order to simulate, an airlift bioreactor with a length of 1.75 m, a width of 0.5 m and a thickness of 0.08 m is applied. Fine mesh is used in this modeling and simulation to increase the accuracy and reduce the relative error of calculation. Simulation illustrates that the flow rate of gas at the inlet of the sparger is maximum and its amount decreases during the flow in bioreactor. Also, simulation suggests a direct relationship between the amount of gas hold up and its velocity of gas. According to the simulation, the highest dynamic viscosity value was 0.8 Pa/s and the lowest value was 0.1 Pa/s.
Biochemical Engineering Journal, 2005
In this work, the effect of alcohol addition on gas hold-up, liquid circulation velocity and gas-liquid volumetric mass transfer coefficient was studied in a split-rectangular airlift bioreactor using air-water as a system to which propanol was added in concentrations ranging from 0.01 to 0.1% (v/v). In order to compare the effect of the propanol with other alcohols, methanol and butanol were also used. The experimental results showed that, at high superficial gas velocities, the addition of small amounts of alcohols decreases the difference between the gas hold-up in the riser and the downcomer, which is due to smaller bubble diameter than with the tap water system. This behaviour is enhanced by alcohols with long carbon chain lengths, which has been explained on the basis of surface tension effects. As a result, the addition of alcohol solutions results in a significant decrease in the liquid circulation velocity, which is mainly due to the decrease in the circulation driving force, but also in a strong decrease in the volumetric gas-liquid mass transfer coefficient for superficial gas velocities higher than 0.033 m/s. This surprising result has been explained by a strong decrease in K L values due to the presence of alcohol and by oxygen depletion due to the increasing amount of small recycled bubbles at high superficial gas velocity.
Study on Mixing Time and Gas Hold-up in an Airlift Bioreactor
— The performance of rectangular draft tube airlift bioreactor was studied by measurement of gas holdup and mixing time using conductivity method. The conductivity data were recorded with time using data acquisition software. The parameters studied were the air flowrate, the viscosity of liquid and the ratio of down-comer to rise area (Ad/Ar). It was observed that increasing gas flow rate reduces mixing time and vice versa while increasing Ad/Ar ratio and viscosity of the water will cause an increase in mixing time. Gas holdup was found to be increased on increasing the gas flowrate and Ad/Ar ratio while when the viscosity of liquid increased, the gas holdup decreased.
Hydrodynamics of a three-phase external-loop airlift bioreactor
Chemical Engineering Science, 2000
The e!ect of the distributing plate ori"ce diameter, air#ow rate, solids loading and solids density on the hydrodynamic characteristics * gas holdup, circulation time and liquid velocity * of a three-phase external-loop airlift reactor was characterized. It was observed that the gas distributor has a small e!ect on riser gas holdup, circulation time and downcomer liquid velocity. On the contrary, the air#ow rate, solids loading and solids density signi"cantly a!ect the hydrodynamic characteristics of the external-loop airlift reactor. A previously described model was used to estimate simultaneously both the riser gas holdup and the downcomer linear liquid velocity. The model simulated with high-accuracy experimental data obtained with three di!erent distributing plate ori"ce diameters, two solids density and solids loading up to 30% (v/v).
Korean Journal of Chemical Engineering, 2010
The effect of mode of sparging gas on the mixing parameters of an internal loop airlift bioreactor was investigated. Two bioreactors of identical volume of 14×10 3 cm 3 and the optimum riser to downcomer cross sectional area ratio of 0.6 were studied. In one bioreactor a gas sparger was located in the draft tube and in the annulus in another. Liquid mixing characteristics, i.e., mixing time and circulation time, were employed to describe the performance of the bioreactors. The tracer injection method was used to determine the mixing parameters. A mathematical modeling based on the tanks-in-series model was employed to characterize the hydrodynamics behavior of the bioreactors. Matlab 7.1 software was used to solve the model equations in the Laplace domain and determine the model parameter, the number of stages. A comparison between the simulation results and experimental data showed that the applied model can accurately describe the behavior of the bioreactors. The results showed that when the gas sparger was located in the draft tube, the liquid mixing time, circulation time, and the number of stage were less than while the gas sparger was located in annulus. This is due to more wall effects, more energy losses and pressure drop in the case of gas injection in the annulus.
Airlift bioreactors: Analysis of local two-phase hydrodynamics
AIChE Journal, 1991
Local two-phase flow measurements were obtained in a pilot-scale, external-loop airlift bioreactor using hot-film anemometry and resistivity probe techniques. The radial dependence of both gas and liquid velocities and of the void fraction was substantial. Developing flow effects were pronounced, as evidenced by distinct changes in the radial profiles of fluid flow properties with axial position. For high gas flow rates, liquid acceleration effects near the sparger resulted in greatly reduced slip velocities in a substantial portion of the riser. A significant reduction in mass transfer may occur under such conditions. The point equations of continuity and motion were used to develop a differential, two-fluid model for two-phase flow in airlift risers. The only empirical parameters in the model represent frictional effects. The developing two-phase flow characteristic of airlift risers was observed to create significantly higher frictional effects at the wall than is routinely observed for fully-developed flow. Model predictions were compared to our own experimental results as well as those of Merchuk and Stein (1981). Agreement between the predicted and measured values was typically within 10% for both cases.
Chemical Engineering Science, 1999
The in#uence of small bubbles on the behaviour of airlift reactors during citric acid fermentation was studied. The small bubbles formed up to 70% of the total gas holdup , so their distribution strongly in#uenced the predicted liquid circulation velocities. A periodically changing air #owrate could be used to signi"cantly improve the oxygen transfer capacity of airlift reactors. This could be achieved by the e!ective utilisation of the oxygen content of small bubbles, which originated from the foam. A procedure based on the evaluation of transient pO and gas holdup pro"les was suggested for the determination of the period of sparging. 1999 Elsevier Science Ltd. All rights reserved.
Chemical Engineering Science, 2011
Several studies have shown a strong relationship between morphology and agitation . The shear stress distribution and mass transfer are the important parameters which can improve the performance of bioreactor. In this work, a mathematical model using computational fluid dynamics (CFD) techniques is used to study the gas-liquid dispersion in an airlift reactor. Multiple rotating frame (MRF) technique is used to approximate the movement of the impeller in the stationary reactor. Population balance modeling (PBM) is used to describe the dynamics of the time and space variation of bubble sizes in the reactor. The PBM equation is solved using an approximate method known as the class method (CM) and the bubble sizes are approximated through a discrete number of size 'bins', including transport, and different bubble phenomena. These equations of the CM are then written as scalar transport equations and added to the multiphase fluid mechanical equations describing the dynamics of the flow. All these equations are solved using control volume formulation through the use of an open-source CFD package OpenFOAM. The model is used to analyze an existing geometry of an airlift bioreactor and validate the modification on the initial design. The new design of airlift gives a clear performance by the increase of the global and local mass transfer and the decrease of the shear stress.