Air lift systems

Cells growing in suspension or on microcarriers can be maintained in suspension by introducing a gas at the bottom of the culture. As 

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Fluid cracking retained the moving bed concept of the catalyst transported regularly between the reactor and regenerator. And as with the air-lift systems, the fluid plant rejected mechanical carrying devices (elevators) in favor of standpipes through which the catalyst fluid traveled. 

Figure 4.5 Fermentor types used in the production of SCP from sugar sources, (a) Stirred baffled system, (b) and (c) air lift systems...

The use of air lift fermentors has been advocated as a means of overcoming the sensitivity to shear while still maintaining adequate oxygen and mixing characteristics (24). Scale-up of such fermentors has shown mixed results in that the productivity has either decreased (22-25) or increased (24) in going to larger sizes. The hydrodynamics of such systems are extremely complex to model. Some work on modeling of air lift systems for microbial fermentations has been carried out (26-29) however, none of the models so far proposed has been tested with plant cell systems. 

Gas-liquid contacting under conditions of slug flow occurs in many pratical situations, e.g. in air-lift systems, and it is likely to be more easily amenable to physical modelling. This is because gross circulation of the liquid is not present, radial mixing is not normally a problem, bubble rise velocities are more easily predicted, and distributions of bubble sizes are easier to characterize. 

Other than the biofilter and culture chambers, recirculating systems typically also employ one or more settling chambers or mechanical filters to remove soHds such as unconsumed feed, feces, and mats of bacteria that slough from the bio filter into the water. Each recirculating system requires a mechanical means of moving water from component to component. That usually means mechanical pumping, though air-lifts can also be used. 

Plant Cell Culture. Air-lift bioreactors have been favored for plant cell systems since these cultures were first studied (4). However, they can give rise to problems resulting from flotation of the cells to form a meringue on the top. It is interesting to note that some reports indicate that stirred bioreactors do not damage such cells (4). 

Single-Cell Protein. Systems involving single-cell proteins are often very large throughput, continuous processing operations such as the Pmteen process developed by ICI. These are ideal for air-lift bioreactors of which the pressure cycle fermenter is a special case (50). 

An improved design undertaken by Sacony used high-velocity gases to replace the mechanical elevator systems as catalyst carriers. These so-called air-lift units improved upon the Thermofor process both in terms of economies and octane numbers. It was, however, only with the fluid cracking process that catalytic technology realized fully continuous production. 

The high sulfur-containing feedstock and the biocatalyst, usually suspended in the aqueous phase have to be contacted with each other in a bioreactor. A homogeneous, continuous phase would be preferred, which would imply formation of an emulsion, preferably a microemulsion. Several bioreactor designs have been suggested for biodesulfurization of petroleum feedstocks including impeller-mixed systems [65,202], electro-spray bioreactor [220,261,262], and draft tube air-lift bioreactor [263],... 

The system used by Stevens et al. [79] consisted of an air-lift reactor with an external lamella settler. They did not need to pump the cell suspension through the settler, since free flow convection was achieved by cooling the cell suspension (20 °C) before entering the sedimentation device. As also found by other authors, they could achieve selective retention of viable cells by varying the perfusion rate. 

Optimal fermentation parameters have been well established and air-lift, stirred tank, and hollow fibre systems have all been used. At commercial scale, fermentation volumes in excess of 1000 litres can be used, which can yield 100 g or more of final product. While hybridoma growth is straightforward, production levels of antibody can be quite low compared with ascites-based production systems. Typically, fermentation yields antibody concentrations of 0.1-0.5 mg/ml. Removal of cells from the antibody-containing media is achieved by centrifugation or filtration. An ultrafiltration step is then normally undertaken in order to concentrate the filtrate by up to 20-fold. 

Air-lift 1. No moving parts 2. Simple 3. High gas absorption efficiency 4. Good heat transfer 1. Poor mixing 2. Excessive foaming 3. Limited to low viscosity system... 

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