Stirred Beds with Mechanical Agitators

Some mechanically agitated bioreactors involve a substrate bed that sits on a perforated plate, such that air is blown through the whole cross-section of the bed. A mechanical agitator embedded in the bed mixes the bed. In the case of the 50-L bioreactor of Chamielec et al. (1994) and Bandelier et al. (1997) the bed is mixed with a planetary mixer, that is, the mixer blade rotates around its central axis while this central axis simultaneously rotates around the central axis of the bioreactor (Fig. 9.2(a)). This bioreactor has only been used for intermittently-stirred operation but can be used with continuous stirring. The modified solids-mixer of

Mechanically Agitated Bioreactor

Fig. 9.2. Mechanically-agitated bioreactors that can readily be used in either the continuously-mixed or intermittently-mixed mode because they give good aeration of the bed when it is static. (a) A bioreactor with a planetary mixer (Chamielec et al. 1994; Bandelier et al. 1997); (b) A conical solids mixer aerated from the top (Schutyser et al. 2003b)

Fig. 9.2. Mechanically-agitated bioreactors that can readily be used in either the continuously-mixed or intermittently-mixed mode because they give good aeration of the bed when it is static. (a) A bioreactor with a planetary mixer (Chamielec et al. 1994; Bandelier et al. 1997); (b) A conical solids mixer aerated from the top (Schutyser et al. 2003b)

Schutyser et al. (2003b) has a helical blade that scrapes the inside wall of the bioreactor with a lifting action (Fig. 9.2(b)). It has a capacity for 20 kg of cooked wheat grain. No data is available for fermentations in this bioreactor. Schutyser et al. (2003b) used it to study mixing in the absence of the microorganism.

Other mechanically agitated bioreactors have been built in such a way that air only enters at specific points, and not over a wide cross-section of the bed. In this case, the efficiency of the aeration of the bed depends on the degree of mixing achieved by the agitation system, because it is the mixing action that brings the substrate particles into the aeration zone. Such bioreactors would not be particularly appropriate for operation in the intermittently mixed mode.

Nagel et al. (2001a) used a bioreactor that consisted of a 35-L horizontal drum, with paddles mounted on a central axis (Fig. 9.3(a)). The bed was aerated by forcing high-pressure air through holes in the ends of the paddles. They were able to control the temperature at 35°C in this continuously mixed bioreactor during the growth of Aspergillus oryzae on 8 kg of cooked wheat grains. In one experiment, they showed that temperature control could be achieved by heat removal through the wall to a cooling coil wrapped around the outside of the drum, with cooling water temperatures needing to be as low as 18°C during the time of peak heat production (Fig. 9.4(a)). In another experiment they promoted evaporative cooling by using high flow rates of dry air. For adequate temperature control at the time of peak heat production, the air flow rate needed to be about 75 L min-1 (Fig. 9.4(b)), or two volumes per volume per minute (vvm, m3-air m-3-total-bioreactor-volume min-1). Despite this success at small scale, it is not clear how such a bioreactor would perform at large scale, the most important question being the efficiency of aeration of the bed.

Ellis et al. (1994) adapted a Z-blade mixer with an internal volume of 28 L as an SSF bioreactor (Fig. 9.3(b)). However, they only studied the mixing behavior, in the absence of microbial growth. The degree of mixing achieved did not depend on the rotational speed, but rather on the number of revolutions. It is not clear how effective the distribution of air will be in such a bioreactor, with air being introduced through four relatively small holes in the bottom of the bed.

Berovic and Ostroversnik (1997) designed a stirred bed bioreactor in which a horizontal cylindrical drum was filled to two-thirds depth with substrate and air was introduced through a perforated central shaft embedded in the substrate bed and upon which mixer blades were mounted (Fig. 9.3(c)). If desired, the reactor could be rotated 90° to a vertical position to aid in loading or unloading operations and could even be operated in this orientation. This bioreactor was used for optimization of inoculation, sterilization, mixing, aeration, and temperature and humidity control during the production of pectinolytic enzymes by Aspergillus niger Berovic and Ostroversnik (1997) and later for the production of fungal polysaccharides by Ganoderma lucidum (Habijanic and Berovic 2000).

The bioreactors of Nagel et al. (2001a), Berovic and Ostroversnik (1997), and Ellis et al. (1994) had water jackets. However, if such designs were to be used at larger scale with geometrically similar proportions, the effectiveness of the water jacket would decrease, due to the decrease in the ratio of the surface area for heat transfer to the volume of the substrate bed.

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