This bioreactor is similar to those of the stirred tank group but differs in the manner in which mixing is achieved: the CRDB consists of a cylinder that rotates horizontally around its axis. Bioreactors of this kind fall between perfectly mixed bioreactors and plug-flow bioreactors and hence might be referred to as mixed-flow bioreactors. Indeed, as in solid-drying equipment of this shape (see Moyers et al. 1999), they can have internal devices that promote forward and backward mixing. These devices could be static mixers, like the baffles in Fig. 11.2, or dynamic mixers, which stir and transport the solid internally within the vessel.
The design and operating variables of CRDBs are similar to those of CSTBs. Nevertheless the fact that the drum rotates without the motion of an internal agitator produces particular features in the stirring mechanisms. The number, shape, and position of the baffles are important factors that affect the flow through the drum and consequently the performance of this bioreactor.
In addition to heat removal by convection to the air flowing through the head-space, different strategies can be tried for removal of waste metabolic heat from the bed of fermenting solids. For example, the lower part of the external wall of the vessel could be immersed in a water bath.
The speed of rotation of the drum and the angle of inclination of the body of the bioreactor to the horizontal are very important factors affecting solids mixing and transportation. Rotational speeds as low as 2 to 3 rpm are commonly used in batch systems (Hesseltine 1977; Pandey 1991), although higher speeds have also been reported. The substrate normally occupies 10% to 40% of the volume of the bioreactor (Stuart 1996).
Van de Lagemaat and Pyle (2001) used a 1-m-long CRDB with a diameter of 8 cm. By adjusting the baffle arrangement and the inclination of the central axis of the bioreactor to the horizontal, they achieved near-perfect mixing of un-inoculated solid substrate particles. The main goal of this design was to achieve sufficient back mixing so that the sterile feed could be inoculated by the fermented particles within the bioreactor, in such a manner as to remove the need for an external inoculation system. However, the efficiency of such "back-inoculation" has not yet been directly investigated. Further, as will be explained later, there may be problems with product uniformity in back-mixed CSSFBs.
A special kind of CRDB was tested for fermentation of a mixture of feedlot waste and coarsely cracked corn (Hrubant et al. 1989). The 91.5-cm-long bioreactor had a diameter of 22.8 cm and consisted of three chambers aligned axially and separated by bulkheads. Each bulkhead had a centrally located hole to permit unidirectional passage of fermenting substrate sequentially through the chambers. Each chamber had several baffles to ensure perfect mixing of the fermenting solids within the chamber and therefore this bioreactor acted like three perfectly-mixed continuous bioreactors in series. Fermentation runs as long as two months were conducted with this bioreactor. A pilot-scale bioreactor of this kind having three 468-liter chambers was also used at Illinois University.
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