Internal-Loop Airlift Bioreactor

The draught tube ILALR (DT-ILALR) is more efficient than the baffled ILALR at gas-liquid mass transfer, and much more research has been directed toward this 

---

Internal loop airlift bioreactors, 1 742 Internal manifolding method, 12 200 Internal microwave field, 16 513 Internal olefins, sulfonation of, 23 527 Internal-pair formation (IPF),... 

Figure 8.1 Internal-loop airlift bioreactor with (a) a baffle separating the riser and downcomer, (b) a continuous draught tube separating the riser and downcomer, and (c) a sectioned draught tube separating the riser and downcomer.

Figure 8.4 Circulation regime progression in a draught tube internal-loop airlift bioreactor (van Benthum et al., 1999b), where is the downcomer liquid velocity and is the gas slip velocity.

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. 

The vented tube connector minimizes the problem of gas accumulation, and gas is allowed to separate fairly efficiently. Unfortunately for some processes, the additional separator volume increases the gas separation efficiency relative to an internal-loop airlift bioreactor for the same reasons as for the tank separator ... 

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. 

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. 

Determination of gas hold-up from Equation 26 requires a knowledge of the superficial liquid circulation rate, given by Equation 9 and the single bubble terminal rise velocity Most researchers have used = 0.25 msThe gas holdup and liquid circulation data in 250 L pilot-scale internal-loop airlift bioreactor for Saccharopolyspora erythmea (n = 0.55) were satisfactorily correlated by this model. 

LinTJ, Lee YC. High-content fructoohgosaccharides production using two immobihzed microorganisms in an internal-loop airlift bioreactor. J Chin Inst Chem Eng 2008 39(3) 211—7. 

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

The gas is circulated by means of pressurised air. In airlift bioreactors, circulation is caused by die motion of injected gas through a central tube, with fluid recirculation through the annulus between die tube and the tower or vice versa. Figure 6.1 shows an airlift bioreactor widi an internal loop cycle of fluid flow. 

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). 

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. 

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

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. 

---