External-Loop Airlift Bioreactor

The ELALR can be used for these processes because gas disengagement is very efficient. The bnbbles have a relatively fast rise velocity and slow radial velocity. Hence, bubble-bnbble interactions are diminished in the external-loop variant relative to the bnbble colnmn or stirred-tank bioreactor, which, in turn, leads to higher gas holdnp sensitivity to liquid property variations in bubble columns than in ELALRs (Chisti, 1989 Joshi et al., 1990 Shariati et al., 2007). In other words, the bnbble-bnbble collision frequency is lower in ELALRs, which makes coalescence-adjnsting liqnid properties, such as viscosity, surface tension, or ionic strength, less important. So, while bubble column and internal-loop airlift bioreactor gas holdnp are nsnally similar, the downcomer gas holdup in an external-loop airlift bioreactor is only 0-50% of the riser gas holdup (Bello et al., 1984), which leads to much lower global gas holdup in ELALRs. 

The bioreactor geometry effects in an ELALR can be quite complex and dynamic. As the area ratio increases, the liquid circulation velocity decreases. Hence, the gas-phase circulation time decreases and gas holdup increases. The increase in gas holdup leads to an increase in the interfacial area. Some bubble dynamics are reflected in the growth, but, due to the lower bubble-bubble interactions in ELALRs, the increase is fairly continuous, but at a relatively slow rate. For example, Joshi et al. (1990) showed that by increasing the area ratio from 0.25 to 1.0 using a 10-m ELALR at 0.3kW/m yielded a negligible increase 

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External humidification, 12 213 External interface management, in technology transfer, 24 366 External loop airlift bioreactors, 1 741, 742 Externally manifolded fuel cells, 12 200 External magnetic field, 23 835 External mass transfer, 15 728-729 External mass transfer resistance dimensionless parameter and,... 

Vendruscolo (2005) used an external loop airlift bioreactor for chitosan production by G. butleri CCT 4274 on the watery extract of apple pomace. The experiments using higher levels of aeration (0.6 volume of air per volume of liquid per minute) provided greater concentrations of biomass, attaining 8.06 and 9.61 g/L, in the production of 873 and 1062 mg/L of chitosan, respectively. These findings demonstrated the adequacy of the airlift bioreactor for the cultivation of microorganisms with emphasis on the production of chitosan. 

Meng, A.X. Hill, G.A. Dalai, A.K. Hydrodynamic characteristics in an external loop airlift bioreactor containing a spinning sparger and a packed bed. Ind. Eng. Chem. Res. 2002, 41, 2124-2128. 

The gas separator is an important design feature that is often ignored. The simple reason is that internal-loop airlift bioreactors have only a few options, and the design is essentially the same a vented headspace, which is similar to the tank separator shown in Figure 8.7c. The external-loop airlift bioreactor, however, is presented with additional design options (shown in Figure 8.7a and b), which provides the external-loop airhft bioreactor with some advantages for certain processes. 

Common gas separator designs for external-loop airlift bioreactors (Jones,... 

Gas-liquid mass transfer coefficients follow the same gas holdup trends. As shown in Figure 8.9, the gas-Uquid mass transfer coefficient increases monotoni-cally with riser superficial gas velocity. The correlations by Chisti etal. (1988b) (as cited by Murchuk and Gluz (1999) and Popovic and Robinson (1984) were developed using external-loop airlift bioreactors, while the others used the draught tube internal-loop airlift bioreactor. The DT-ILALR has much better performance than the ELALR. It is unfortunate to note that gas-liquid mass transfer correlations are much fewer in number than their gas holdup counterpart. 

Chisti, Y, Kasper, M., and Moo-Young, M. (1990), Mass transfer in external-loop airlift bioreactors using static mixers, Canadian Journal of Chemical Engineering, 68(2) 45-50. 

Kawase, Y, Tsujimura, M., and Yamaguchi, T. (1995), Gas hold-up in external-loop airlift bioreactors. Bioprocess and Biosystems Engineering, 12(1-2) 21-27. 

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. 

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. 

Zhang, E, Jing, W., Xing, W. and Xu, R. (2009) Experiment and calculation of filtration processes in an external-loop airlift ceramic membrane bioreactor. Chemical Engineering Science, 64,2859-2865. 

In applications of airlift bioreactor there are various types of fermenter. The most common airlift bioreactors are pressure cycle, internal and external loop bioreactors. 

A modified type of airlift system widi gas and liquid flow patterns in which a pump transports the ah and liquid through die vessel. Here, an external loop is used, with a mechanical pump to remove the liquid. Gas and circulated liquid are injected into the tower through a nozzle. Figure 6.2 shows an airlift bioreactor diat operates widi an external recirculation pump. 

Flower [25], Panda et al. [26], Doran [27]. and Payne et al. [28]. Several kinds of bioreactors, such as the stirred tank bioreactor with hollow paddle and flat blade impellers, the bubble column, the airlift bioreactor with internal and external loops, the rotating drum bioreactor, the stirred-tank with a draft tube, and the mist bioreactor have been attempted for plant cell, tissue and organ cultures (Fig. 1). 

There have been many simple modifications to airlift bioreactors for specific applications. For example, a novel airlift loop fermenter (schematic unavailable in the literature) utilizes a side arm. The external loop in this integrated system overcomes the problem of ethanol inhibition by continuously stripping ethanol from the fermentation broth and recovering it by condensation. This is suitable for the simultaneous production and recovery of ethanol. ... 

Airlift bioreactors are used as an alternative to bubble columns the configuration of the former permits more defined liquid flows, since upward- and downward-moving streams are physically separated. Airlift bioreactors typically allow better mixing than bubble columns, particularly if they are of the external-loop configuration, but at low liquid velocity. 

Figure 19.1 shows an airlift bioreactor contains external loop which is made of Pyrex glass. The bioreactor was fed with sweet cheese sterilized and deprotenized whey. The cell suspension was aseptically transferred to the bioreactor. Airlift bioreactor was operated at working volume of 7 liters that included 10% pre-culture. The regulation system allows for temperature control at 30 1°C foam-level and pH controlled by addition of antifoam and ammonia, respectively. The set-point fixed at pH 5.0 0.1. The system was aerated with filtered air at a different flow rate of 0.1, 0.4, and 0.8 vvm that was controlled using an aeration pump controller Each run was achieved in duplicates the average values of lactose, ethanol, and biomass concentrations were calculated and monitored with respect to time. 

FIGURE 5 Airlift bioreactors (a) draft-tube internal-loop configuration, (b) a split-cylinder device, and (c) an external-loop system. 

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