External loop airlift reactors

Effect of internal on the hydrodynamics in external-loop airlift reactors... 

Small bubbles and flow uniformity are important for gas-liquid and gas-liquid-solid multiphase reactors. A reactor internal was designed and installed in an external-loop airlift reactor (EL-ALR) to enhance bubble breakup and flow redistribution and improve reactor performance. Hydrodynamic parameters, including local gas holdup, bubble rise velocity, bubble Sauter diameter and liquid velocity were measured. A radial maldistribution index was introduced to describe radial non-uniformity in the hydrodynamic parameters. The influence of the internal on this index was studied. Experimental results show that The effect of the internal is to make the radial profiles of the gas holdup, bubble rise velocity and liquid velocity radially uniform. The bubble Sauter diameter decreases and the bubble size distribution is narrower. With increasing distance away from the internal, the radial profiles change back to be similar to those before contact with it. The internal improves the flow behavior up to a distance of 1.4 m. 

K.H. Choi, W.K. Lee, Circulation liquid velocity, gas hold-up, volumetric oxygen transfer coefficient in external-loop airlift reactors, J. Chem. Technol. Biotechnol. 56 (1993) 51-58. 

M. Gavrilescu, R.Z. Tudose, Study of the liquid velocity in external-loop airlift reactor, Bioprocess Eng. 14 (1995) 183-193. 

J. Lin, M.H. Han, T.F. Wang, T.W. Zhang, J.F. Wang, Y. Jin, Experimental study on the local hydrodynamic behavior of a three-phase external-loop airlift reactor, Ind. Eng. Chem. Res., in press. 

Researchers have also combined reactor types. Guo et al. (1997), for example, designed an external loop airlift reactor that incorporates a fluidized bed within the downcomer section, shown in Figure 10.1. The fluidized bed section is used to immobilize microorganisms on carrier particles in order to protect them from damage. The design is meant for the production of enzymes, biofluidization, and wastewater treatment. Although shear rates were minimized, bubbles were not entrained within the downcomer. Furthermore, the gas-liquid mass transfer coefficient was observed to increase with gas holdup. The result was that the gas-liquid mass transfer was limited due to the fact that global gas holdup for the reactor was strictly defined by the riser gas holdup without any addition by the downcomer. 

Figure 10.1 Novel external-loop airlift reactor designed by Guo et al. (1997).

External loop airlift reactor (ELALR) ELALR has distinct conduits for fluid flow which are connected at the base and gas separator sections. + +-M- +++ ++ ++ Even more control and design flexibility better hydrodynamics than ILALR... 

Bendjaballah, N., Dhaouadi, H., Poncin, S., Midoux, N., Homut, J.-M., and Wild, G. (1999), Hydrodynamics and flow regimes in external loop airlift reactors, Chemical Engineering Science, 54(21) 5211-5221. 

Choi, K.H. (2001), Hydrodynamic and mass transfer characteristics of external-loop airlift reactors without an extension tube above the downcomer, Korean Journal of Chemical Engineering, 18(2) 240-246. 

Guo, Y.X., Rathor, M.N., and Ti, H.C. (1997), Hydrodynamics and mass transfer studies in a novel external-loop airlift reactor, Chemical Engineering Journal, 61(31) 205-214. 

Posarac, D., and Petrovic, D. (1988), An experimental study of the niinimum fluidization velocity in a three-phase external loop airlift-reactor, Chemical Engineering Science, 43(5) 1161-1165. 

Roy, S., Dhotre, M.T., and Joshi, J.B. (2006), CFD simulation of flow and axial dispersion in external loop airlift reactor, Transactions ofIChemE, Part A, Chemical Engineering Research and Design, 84(A8) 677-690. 

Snape, J.B., Zahradnik, J., Fialova, M., and Thomas, N.M. (1995), Liquid-phase properties and sparger design effects in an external-loop airlift reactor, Chemical Engineering Science, 50(20) 3175-3186. 

In the case of airlift reactors, the flow pattern may be similar to that in bubble columns or closer to that two-phase flow in pipes (when the internal circulation is good), in which case the use of suitable correlations developed for pipes may be justified [55]. Blakebrough et al. studied the heat transfer characteristics of systems with microorganisms in an external loop airlift reactor and reported an increase in the rate of heat transfer [56], In an analytical study, Kawase and Kumagai [57] invoked the similarity between gas sparged pneumatic bioreactors and turbulent natural convection to develop a semi-theoretical framework for the prediction of Nusselt number in bubble columns and airlift reactors the predictions were in fair agreement with the limited experimental results [7,58] for polymer solutions and particulate slurries. 

Essadki AH, Gourich G, Vial C, Delmas H, Bennajah M (2009) Defluoridation of drinking water by electrocoagulation/electroflotation in a stirred tank reactor with a comparative performance to an external-loop airlift reactor. J Hazard Mater 168 1325... 

In airlift bioreactors the fluid volume of the vessel is divided into two interconnected zones by means of a baffle or draft-tube (Fig. 5). Only one of these zones is sparged with air or other gas. The sparged zone is known as the riser the zone that receives no gas is the downcomer (Fig. 5a-c). The bulk density of the gas-liquid dispersion in the gas-sparged riser tends to be less than the bulk density in the downcomer consequently, the dispersion flows up in the riser zone and downflow occurs in the downcomer. Sometimes the riser and the downcomer are two separate vertical pipes that are interconnected at the top and the bottom to form an external circulation loop (Fig. 5c). External-loop airlift reactors are less common in commercial processes compared to the internal-loop designs (Fig. 5a, b). The internal-loop configuration may be either a concentric draft-tube device or an split-cylinder (Fig. 5a, b). Airlift reactors have been successfully employed in nearly every kind of bioprocess—bacterial and yeast culture, fermentations of mycelial fungi, animal and plant cell culture, immobilized enzyme and cell biocatalysis, culture of microalgae, and wastewater treatment. 

An application is then presented to show the efficiency of electrocoagulation/ electroflotation in removing colour from synthetic and real textile wastewater by using aluminum and iron electrodes in an external-loop airlift reactor. The defluoridation is also showed. The time, pH, conductivity and current density are the most parameters for the removal efficiency and energy consumption. 

Fig. 11. Airlift reactors, (a) internal loop airlift reactor, (b) external loop airlift reactor.

Fig. 12. External-loop airlift reactor (1 downcomer section 2 riser section 3 conductivity probes 4 junction column 5 separator 6 conductimeter 7 analog output/input terminal panel (acquisition system) 8 50-way ribbon cable kit 9 data acquisition system 10 electrodes).

Fig. 19. Photo showing decolourization by EC in external loop airlift reactor of real textile...

External loop airlift reactor as a continuous reactor... 

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