Bubble bioreactor

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. 

The specific surface, a, is also relatively insensitive to the duid dynamics, especially in low viscosity broths. On the other hand, it is quite sensitive to the composition of the duid, especially to the presence of substances which inhibit coalescence. In the presence of coalescence inhibitors, the Sauter mean bubble size, is significantly smaller (24), and, especially in stirred bioreactors, bubbles very easily circulate with the broth. This leads to a large hold-up, ie, increased volume fraction of gas phase, 8. Sp, and a are all related... 

Eig. 5. Examples of air driven bioreactors (a) bubble column, (b) draught tube, and (c) external loop. 

Eigure 6 enables a comparison to be made of kj a values in stirred bioreactors and bubble columns (51). It can be seen that bubble columns are at least as energy-efficient as stirred bioreactors in coalescing systems and considerably more so when coalescence is repressed at low specific power inputs (gas velocities). 

Fig. 6. A comparison of k a values (51). Represented are 1, stirred bioreactor using water, = 0.02 m/s, kj a (eq. 16) 2, stirred bioreactor using water, t 3 = 0.04 m/s, kj a (eq. 16) 3, bubble column using water, kj a (eq. 18) 4, stirred bioreactor using salt water, = 0.02 m/s, kj a (eq. 17) 5, stirred bioreactor using salt water, = 0.04 m/s, kj a (eq. 17) and 6, bubble column using salt water (noncoalescing).

Bubble behaviour was studied in the pilot-scale bioreactor so that a complete model of flow, OTR, mixing, cooling, energy requirement and disengagement could be developed for this system and larger production-scale vessels of similar type. 

The mass transfer, KL-a for a continuous stirred tank bioreactor can be correlated by power input per unit volume, bubble size, which reflects the interfacial area and superficial gas velocity.3 6 The general form of the correlations for evaluating KL-a is defined as a polynomial equation given by (3.6.1). 

The main part of the report describes the results of systematic investigations into the hydrodynamic stress on particles in stirred tanks, reactors with dominating boundary-layer flow, shake flasks, viscosimeters, bubble columns and gas-operated loop reactors. These results for model and biological particle systems permit fundamental conclusions on particle stress and the dimensions and selection of suitable bioreactors according to the criterion of particle stress. 

The aim of this report is to examine the principles of shear stress on particles that would allow the design of bioreactors for technical use, mainly stirred tanks, bubble columns and loop reactors. 

Special reactors are required to conduct biochemical reactions for the transformation and production of chemical and biological substances involving the use of biocatalysts (enzymes, immobilised enzymes, microorganisms, plant and animal cells). These bioreactors have to be designed so that the enzymes or living organisms can be used under defined, optimal conditions. The bioreactors which are mainly used on laboratory scale and industrially are roller bottles, shake flasks, stirred tanks and bubble columns (see Table 1). 

As is the case with pure bubble columns and gas-operated loop reactors, most bioreactors in technical use are aerated with oxygen or air. Reactors with pure surface aeration, such as roller bottles, shake flasks and small stirred reactors or special reactors with membrane aeration, are exceptions. The latter are used for the cultivation of cells and organisms which are particularly sensitive to shearing (see e. g. [28 - 29]). The influence of gas bubbles in increasing stress has been described in many publications (see e.g. [4, 27, 29, 30]). In principle it can be caused by the following processes ... 

Figure 2.27. Mixing, mass transfer and oxygen consumption in a bubble column bioreactor (Oosterhuis, 1984). Tj - reaction time constant, Xmt - ass transfer time constant, tmix -mixing time constant. ro2 - oxygen consumption rate, Vs - superficial gas velocity.

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