1226 † To whom correspondence should be addressed. E-mail: Behin@razi.ac.ir Korean J. Chem. Eng., 27(4), 1226-1232 (2010) DOI: 10.1007/s11814-010-0184-9 RAPID COMMUNICATION Mixing parameters for an airlift bioreactor considering constant cross sectional area of riser to downcomer: Effect of sparging gas location Jamshid Behin † and Azade Ahmadi Department of Chemical Engineering, Faculty of Engineering, Razi University, Kermanshah, Iran (Received 31 July 2009 • accepted 18 November 2009) Abstract−The effect of mode of sparging gas on the mixing parameters of an internal loop airlift bioreactor was in- vestigated. 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 in- jection in the annulus. Key words: Airlift Bioreactor, Circulation Time, Gas Sparger, Liquid Mixing Time, Mathematical Modeling, Tank-in-series Model INTRODUCTION Airlift bioreactors (ALRs) are a special type of multiphase, pneu- matic contactors. Industrial applications of these bioreactors are in hydrogenation, oxidation, epoxidation, fermentation, production of single cell protein, cultivation of microorganisms and wastewater treatment in chemical and biochemical processes [1-6]. Airlift biore- actors have attracted considerable attention due to their simple mech- anical design with no moving part, high capacity, good mixing, low cost, low shear stress and low power input [7-9]. The important hydrodynamic parameters of the airlift bioreac- tors are the liquid mixing time and the liquid circulation time (or liquid circulation velocity). These parameters are sensitive to super- ficial gas velocity and physical properties of the fluids, gas flow rate and bioreactor geometry and have been extensively studied be- cause of their influence on mass transfer phenomena [10]. Muthu- kumar et al. [7], Freitas et al. [11] and Fadavi et al. [12] investi- gated the hydrodynamic behavior of concentric draft-tube type airlift bioreactors. They demonstrated that an increase in the air-flow rate decreases mixing and circulation times. Bando et al. [13] have studied the effect of mode of sparging gas on the liquid mixing time and derived an equation for the liquid mixing time. The liquid mixing time was reported shorter in the mode of sparging gas into the draft tube than in the mode of sparging gas into the annulus when the equipment is the same. Pollard et al. [14] have made comparisons between two ring sparger locations, draft tube and annulus, in a con- centric pilot scale airlift bioreactor. The hydrodynamic performance of the bioreactor was improved by using a draft tube ring sparger rather than the annulus ring sparger. This was due to the influence of the ratio of the cross sectional area of the riser and downcomer (A r /A d ) in conjunction with the effect of liquid velocity. In the studies of Muthukumar et al. [7], Bando et al. [13], Koide et al. [15] and Weiland [16], independently of the column diameter and mode of sparging gas, the liquid mixing time has a minimum when the diame- ter ratio is between 0.5 and 0.6, i.e., A r /A d : 0.33-0.56. The residence time distribution (RTD) of a chemical reactor is a characteristic of the flow pattern that occurs in the reactor, being one of the most informative characterizations of the reactor. In actual operation, most airlift bioreactors lie somewhere between two ex- tremes: plug flow and perfectly mixed flow. Deviation from the two ideal patterns can be caused by channeling of fluid, by recycling of fluid or by creation of stagnant regions in the vessel. The specific sections of the airlift bioreactors (riser, downcomer, gas-separator and bottom zones) are different hydrodynamically. When analyzing non-ideal reactors, the RTD alone is not sufficient to determine its performance and more information is needed. To understand RTD quantitatively the experimental data have to be fitted to an adequate model (the axial dispersion model or the tanks-in-series model) to describe non ideal reactor flow pattern [17,18]. The mixing model used in most of the previous investigations dealing with airlift bio- reactors is an axial dispersion model. It should be noted that the axial dispersion model could describe satisfactorily only mixing, which slightly deviates from the plug flow. A tanks-in-series model used in this work is applicable to the whole mixing extent including per- fect mixing and plug flow mixing. Moreover, the tanks-in-series model provides a set of first order differential equations that can be solved by using rather simple numerical techniques [19]. Prokop et