Solid bioreactor design

There is good heat transfer in agitated gas-liquid-solid slurry reactors see, e.g., van t Riet and Tramper for correlations (Basic Bioreactor Design, Marcel Dekker, 1991). 

Mass transfer limitations can be relevant in heterogeneous biocatalysis. If the enzyme is immobilized in the surface or inside a solid matrix, external (EDR) or internal (IDR) diffusional restrictions may be significant and have to be considered for proper bioreactor design. As shown in Fig. 3.1, this effect can be conveniently incorporated into the model that describes enzyme reactor operation in terms of the effectiveness factor, defined as the ratio between the effective (or observed) and inherent (in the absence of diffusional restrictions) reaction rates. Expressions for the effectiveness factor (rj), in the case of EDR, and the global effectiveness factor (t ) for different particle geometries, in the case of IDR, were developed in sections 4.4.1 and 4.4.2 (see Eqs. 4.39-4.42,4.53,4.54,4.71 and 4.72). Such functions can be generically written as ... 

The bioreactor is the central point of a fermentation process. It is here that the biotransformation takes place, that a raw material is turned into a desired and valued product Optimization of the rate of formation and yield of product within the bioreactor is a key part of optimizing the production process. Although the field of bioreactor design for submerged liquid fermentation systems is well developed, many of the principles cannot be directly translated to SSF systems. Solid beds and liquid broths are different solid beds are not as easy to mix as liquid broths, and due to poor heat transfer properties of solid substrate beds, heat removal is much more difficult in SSF than it is in SLF. 

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 this chapter, open-pond operations or other open systems are not considered as a topic of discussion. There are roughly 20 different closed bioreactor design variants Table 1.2 summarizes the basic design principles. Two major aspects are important one is whether the cells are growing as individual single cells ( planktonic ) suspended in a culture medium, or whether they need to adhere to solid surfaces or to each other (state of biomass in Table 1.2) the second is how the... 

Some technical limitations to solid-phase PAH bioremediation exist. As compared with bioreactor operations, extended periods of treatment time will predictably be required with solid-phase PAH bioremediation operations. In many cases this is quite acceptable, especially when solid-phase systems are designed to... 

Various adsorbents have been examined for their potential to increase in situ product separation in plant cell culture. Suspended solid adsorbents were popular, and the use of immobilized adsorbent has been investigated recently [17-20]. The advantages of immobilized adsorbent are that it is easy to use in a bioreactor operation and that it allows adsorbents to be easily separated from culture broth for the repeated use of cells and adsorbents [21, 22]. The design and optimization of in situ separation process for phytochemicals using immobilized adsorbent required a detailed mathematical model. It was difficult to achieve an optimal design based on purely empirical correlations, because the effects of various design parameters and process variables were coupled. 

General design considerations for mechanically agitated gas-liquid and gas-liquid-solid mechanically agitated reactors described earlier in Sections II and III are applicable here. In this section, however, we evaluate additional design considerations that are specific to bioreactors. Novel reactors to overcome specific needs of biological processes are also evaluated in this section. The characteristics of the bioreactors and other chemical/ petrochemical gas-liquid and slurry reactors are compared in Table XX. 

Dilution rate is a major factor in design, operation and profitablity of a continuous bioreactor. When the main nutrient is a solid, dilution rate and detention time of the solids are two separate but interrelated issues. Hydrolysis of the cellulose continues as long as it is in the bioreactor, but the rates should change as the amorphous regions react more quickly leaving a higher proportion of the... 

Four basic types of SSF bioreactors have been developed (Figs. 3 and 4). These are (1) drum bioreactor [16], (2) packed bed bioreactor [17,18], (3) tray bioreactor [4, 16], (Ahmed et al. 1987 Hesseltme 1987 Pandey 1991 cited in [19]), and (4) column bioreactor (Raimbault and Germon 1976 cited in [20]). The structure and nature of the solid matrix used, type of microorganism involved, environmental conditions needed for the process, the type of use (research or industrial applications), and type of product should all be considered for the selection of appropriate design of bioreactor [19]. 

Fig. 1. Comparison of typical solid-state fermentation (SSF) and submerged liquid fermentation (SLF) systems. A stirred-bed SSF bioreactor of the design of Durand and Chereau [2] is compared with a typical stirred SLF bioreactor. For each bioreactor an expanded view of the microscale is also shown, in order to highlight differences between the micro-structure of the two systems. The relative scales make it clear that mixing is possible on much smaller scales in SLF than in SSF, since in SSF mixing cannot take place at scales smaller than the particle size. Note that particle sizes in SSF are commonly larger than 1 mm...

Sterilization of solid substrates could be done in specialized vessels, in which case transfer into the bioreactor must be done aseptically. This is more problematic to achieve than with liquid media, and therefore it is preferable to sterilize in situ in the bioreactor, meaning that the bioreactor must be designed to enable this. In most processes batch sterilization will be used [80]. For processes operated in continuous mode a continuous steriUzer will be required at the inlet end of the bioreactor, with provision for sterile transfer into the inoculation chamber [81,82]. 

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