External-loop airlift

Eig. 23. Airlift reactors (a) spHt cylinder internal loop, (b) draft tube internal loop, and (c) external loop (94). 

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. 

The cooling duty can be provided by either making the draught tube an internal heat exchanger or with a heat exchanger in an external circulation loop. The mass transfer coefficient for external loop airlift Fermenter is estimated as 8... 

The external-loop slurry airlift reactor was used in a pilot plant (3000 t/a) for one-step synthesis of dimethyl ether (DME) from syngas. Specially designed internals were used to intensify mass transfer and heat removal. This new technology is highly efficient and easy to scale-up to industrial. 

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,... 

In some respects, airlift reactors (airlifts) can be regarded as modifications ofthe bubble column. Airlift reactors have separate channels for upward and downward fluid flows, whereas the bubble column has no such separate channels. Thus, fluid mixing in bubble columns is more random than in airlift reactors. There are two major types of airlift reactors, namely, the internal loop (IL) and the external loop (EL). 

As this trend levels off with larger columns, it is recommended that values estimated for a 60 cm column are used. If heat transfer is a problem, then heat transfer coils within the column, or even an external heat exchanger, may become necessary when operating a large, industrial bubble column-type fermentor. Scale-up of an internal loop airlift-type fermentor can be achieved in the same way as for bubble column-type fermentors for external loop airhfts see Section 7.7. 

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. 

C. Bentifraouine, C. Xuereb, J.R Riba, An experimental study of the hydrodynamic characteristics of external-loop airlift contactors, J. Chem. Technol. Biotechnol. 69 (1997) 345-349. 

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. 

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). 

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. 

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. ... 

Fig. 3 External loop-jet nozzle airlift fermenter. (View this...

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. 

Al-Masry [43] investigated the effects of siHcone antifoams in tap water in airlift reactors with an external loop with regard to the gas holdup and volumetric mass transfer coefficient. They recommend the following relationship for this special case ... 

Multimodule external-loop airlift PBR for hatcheries (GEPEA, University of Nantes)... 

The culture system described earlier is based on cylindrical tubes, which makes it difficult to calculate radiative transfer in the culture volume, which has to be solved numerically (Lee et al, 2014). As already described, the one-dimensional hypothesis where light attenuation occurs along only one main direction serves to obtain analytical relations to represent the light attenuation field (as with the two-flux model, Eq. 12). This enables accurate and easy determination of light attenuation conditions for any operating conditions and thus greater system control. Based on this statement, researchers designed a specific PBR. Like the multimodule external-loop airlift PBR, this system is of industrial size (130 L), but the unit is a flat panel with front illumination so as to respond to the one-dimensional hypothesis. It is also illuminated on both sides to increase specific illuminated area... 

Aden Fernandez FG, SevillaJM, Sanchez Pdez JA, Mohna Grima E, Chisti Y Airlift-driven external-loop tubular photobioreactors for outdoor production of microalgae assessment of design and performance, Chem Eng Sci 56(8) 2721—2732, 2001. 

Loubiere K, Ohvo E, Bougaran G, Pruvost J, Robert R, LegrandJ A new photobioreactor for continuous microalgal production in hatcheries based on external-loop airlift and swirhng flow, Biotechnol Bioeng 102(1) 132—147, 2009. 

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,... 

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. 

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