Mechanical aeration bioreactor

Fig. 1. Different types of bioreactors for plant cell, tissue and organs. (A) mechanically-agitated bioreactors, a aeration-agitation, b rotating drum, c spin filter. (B) air-driven bioreactors, a bubble column, b draft tube, c external loop, (C) non-agitated bioreactors, a gaseous phase (mist), b oxygen permeable membrane aerator, c surface aeration, (D) light emitting draft tube...

In the following ptu-agraphs this approach will be applied to production of an alkaloid from plant cells cultured in a bioreactor. Regime analysis can be performed by comparison of characteristic parameters of the mechanisms involved in the process. Here the characteristic time concept will be used. The characteristic time is a measure for the rate of a mechanism. A fast mechanism has a short characteristic time. Other terms used are relaxation time, process time, or time constant. A time constant is formally only defined for first-order linear processes. Not all mechanisms involved in a plant cell production process are first order, therefore the term characteristic time is used. The characteristic time is defined as the ratio of a capacity and a flow for example, the characteristic time for oxygen transfer to the liquid phase in a aerated bioreactor q.l becomes... 

For scale-up of inoculum conditions of hairy root cultivation, a 1-L bioreactor (working volume of 800 mL) was used. This bioreactor had a height/diameter aspect ratio of 7.14. The bubble bioreactors had no internal mechanical agitation parts. The supplied aeration rate was 0.1 wm at the bottom by sparger. Each bioreactor was inoculated with 0.2-2.0 % (w/v) g fresh weight of hairy roots and cultured for 32 d. 

Before any additive is to be used with a given cell type, it should first be tested to ensure that it has no detrimental effects on cell growth, metabolism, differentiation, protein expression, etc., and that it does indeed offer mechanical protection in agitated and/or aerated systems (bioreactors). 

Gaseous Phase Bioreactor. As shown in Fig. 24, this type of bioreactor is equipped with filters on which the culture is supported and with a shower nozzle for spraying on the medium (Ushiyama et al., 1984 ti l Ushiyama, 1988).1 1 Seed cultures are inoculated on the filters and the medium is supplied to the culture by spraying from a shower nozzle. The drained medium is collected on the bottom of the bioreactor. This type of bioreactor is excellent for plant cell, tissue, and organ cultures because there is no mechanical agitation (e.g., driven impeller, aerator) and, therefore, the growth rate and the secondary metabolite production are enhanced. 

Fig.1 Different fermentation modes for bioreactors. In order to achieve best aeration and mixing and to avoid high shear forces, different fermentation modes are applied, (a) Mechanical, (b) pneumatical,...

Due to the forced aeration, oxygen supply in packed bed bioreactors is usually not a problem, even if quite low aeration rates are used [138]. In contrast, temperature control can be difficult, especially in packed beds above 15 cm diameter and lacking internal heat transfer plates. In such bioreactors, the main heat transfer mechanisms are axial convection and evaporation, and radial temperature gradients are negUgible except close to the bioreactor wall. The dynamics of convective cooHng mean that axial temperature gradients are established and temperatures over 20 °C higher than the inlet air temperature... 

The various types of plug flow bioreactors were recently surveyed by Moser (1985a). They utilize surface aeration by means of a variety of rotating brushes, rotors, cone aerators, or gas or fluid jets such as are found in biological waste water treatment plants. Beyond all mechanically driven systems, reactors can also be both aerated and mixed pneumatically, or one pump can serve for both mixing and hydrodynamic stirring. 

Mechanically stirred vessels Mixing is promoted by mechanical stirring (occasionally magnetic in bench-scale vessels) using different impeller types. In addition, aeration is also employed to augment mixing. In this most commonly used type of bioreactor, the tank content is perfectly mixed and hence uniform in composition. 

For aerated systems, the mean circulation time and the standard deviation increased. However, correlations under these conditions were not reported. A lognormal CTD was also reported by Oosterhuis (1984) and van Bameveldt et al. (1987), who measured circulation times using a similar technique in a 25 m bioreactor using water. In mechanically agitated vessels, a very high degree of... 

High agitation and aeration rates can result in foam formation inside the bioreactor, which can block the exhaust gas line and cause loss of cells and media and possible contamination when the foam is washed out from the bioreactor. Mechanical foam breakers and antifoam chemicals are used to control foam formation inside the bioreactor. The working volume of the bioreactor is usually 70-80% of the vessels physical volume. 

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