Bioreactors airlift reactors

Recent reports on other forms of reactors are also available. Immobilized cell bioreactors, upflow sludge blanket reactors, draft-tube airlift reactor and other have been suggested. Each of these reactors has its own pros and cons and the advantages need to be evaluated with the whole process in mind. Further work is necessary in this area. Several process schemes have been considered and evaluated with the batch reactor design as the core BDS reactor ... 

There are a wide variety of three-phase fluidized bioreactor designs possible. The conventional reactor, shown in Fig. 9, is fluidized by both gas and liquid entering at the bottom of the reactor and leaving at the top and is the most common type of three-phase fluidized bed bioreactor. This reactor may be configured to operate with little axial liquid mixing or in a well-mixed mode by adding a recycle stream. The airlift reactor or draft tube fluidized bed reactor, Fig. 10, is also frequently used. In this reactor, gas is injected at the bottom of a draft tube placed in the center of the... 

Merchuk, J. C., Ladwa, N., and Bulmer, M., Improving the Airlift Reactor The Helical Flow Promoter, 3rd Int. Conf. on Bioreactor and Bioprocess Fluid Dynamics, (A. W. Nienow, ed.), p. 61, Information Press, Ltd., Oxford (1993)... 

Biomass containment in continuously operated bioreactors is an essential prerequisite for the feasibility of practical industrial-scale dye biodegradation. Biofilm airlift reactors have demonstrated excellent performance for their ability to control mixing, interphase mass transfer and biofilm detachment rate. Further studies are required to further exploit the potential of this type of reactors with either aggregated cells or biofilm supported on granular carriers. 

As microcarriers provide a good surface area for attachment per unit volume, various types have been routinely used to grow anchorage-dependent cell lines in bioreactors primarily used for suspension cultures. Airlift reactors can also be operated using microcarriers. Wang et al. reported the use of a fluidized-bed... 

Airlift reactors are an alternative bioreactor in which the agitation is accomplished by a clear cyclic pattern of air flow through both a riser and a downflow reactor compartment. Stirred tank and airlift reactors are most often used for aerobic cell cultures. Airlift reactors are often utilized to culture plant and animal cells because they typically have lower shear levels. ... 

Mechanically stirred hybrid airlift reactors (see Fig. 6) are well suited for use with shear sensitive fermentations that require better oxygen transfer and mixing than is provided by a conventional airlift reactor. Use of a low-power axial flow impeller in the downcomer of an airlift bioreactor can substantially enhance liquid circulation rates, mixing, and gas-liquid mass transfer relative to operation without the agitator. This enhancement increases power consumption disproportionately and also adds other disadvantages of a mechanical agitation system. 

Chisti, M.Y. Airlift reactor current technology. In Airlift Bioreactors, Elsevier Amsterdam, 1989 74-79. 

In a third paper Scragg et al. (625) compared three different modes of bioreactor operation batch, fed-batch, and draw-fill. The experiments were conducted using airlift fermentors of 7 and 30 liters working volume. Batch and fed-batch cultures were also performed in shake flasks. It appeared that serpentine production in batch cultures in shake flasks was highest. Batch cultures performed in airlift bioreactors showed a lower productivity. Fed-batch cultures showed a lower productivity compared with batch cultures, both in shake flasks and in airlift reactors. When the draw-fill method was used, serpentine production was negligible. 

The possibilities of surface immobilization of C. roseus cells was investigated by Archambault et al. (630), Facchini and Dicosmo (631), and Rho et al. (632). Archambault et al. (630) used a fibrous polyester sheet and studied the effect of surface immobilization on alkaloid production. The cells were grown on the polyester matrix in shake flasks, a stirred vessel, or an airlift reactor. From the results presented it is clear that surface immobilization had a negative effect on alkaloid production in these experiments (Table XXXIX). Suspension cultures were found to produce significantly higher amounts of alkaloids than surface immobilized cells. Cultivation in bioreactors further diminished alkaloid production. 

A major study should be attempted to compare the different bioreactor performance characteristics. Comparisons between BCs and airlift reactors are available, but a wider array is lacking, but needed. Research towards this end, such as Bouaifi et al. (2001) who compared stirred-tank reactors and BCs, is sparse. Different bioreactor designs have quite unique scaling abilities and associated costs, and the economic benefits and decisions would be better understood if such studies would be more common. 

Wastewater treatment plants oftentimes use vessels that combine (he (slurry) bubble column with a mechanical extractor and/or a mixer. The mechanical extractor is used to scrape heavy residue at strategically located divider walls. Such a system implies that the liquid flows across the gas flow field. Siemens proprietary Attached Growth Airlift Reactor (AGAR)-Moving Bed Bioreactor is an example of such a device. 

Figures of merit are quantities used to compare reactor performance across all reactor types or just the different designs of stirred-tank bioreactors. This section summarizes some reactor-specific figures of merit and problems that have prevented meaningful and significant figures of merit being developed as well as some possibilities that could be considered for further research. The figures of merit are also oriented toward stirred-tank reactors, bubble columns, and airlift reactors since these are the most common gas-liquid and gas-liquid-solid bioreactors. Fixed bed reactors use a much smaller gas and liquid flow rate so that they are not able to compete with the other reactors unless the microorganisms have to be suspended or otherwise protected.

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. 

A similar concept has also been used for liquid-solids and liquid-gas-solids contacting devices (Oguchi and Kubo, 1973 Fan et al., 1984) and bioreactors (Chisti, 1989). Bubble columns fitted with draft tubes have also been employed in the chemical process industries as airlift reactors for gas-liquid con-... 

Chisti MY. Airlift Bioreactors. New York Elsevier, 1989. Chisti MY, Moo-Young M. Improve the performance of airlift reactors. Chem Eng Progr 38-45, June 1993. 

The spherical type bubble column bioreactor (Figure 2.8) is another modified airlift reactor with spherical shape instead of cylindrical. Ihrough the addition of small amounts of agar to the culture medium, low shear stress is achieved as well as high oxygen transfer rates. As a result, 5.6 g.L of BC were produced after 72 hours of cultivation [54,55]. 

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. 

Airlift bioreactors are highly energy efficient relative to stirred fermenters, yet the productivities of both types are comparable. Heat and mass transfer capabilities of airlift reactors are at least as good as those of other systems, and airlift reactors are more effective in suspending solids than are bubble columns. For optimal gas-liquid mass transfer... 

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