In taking a process that operates well in the laboratory and establishing a commercial process, the first step will be to identify what type of bioreactor will be suitable at large scale. Many of the laboratory experiments may have been undertaken in erlenmeyer flasks or thin packed-beds, but larger versions of these will probably not be appropriate for large-scale production.
Many considerations must be kept in mind when selecting a bioreactor, but a key question is: "What criterion do I use to compare different bioreactors and different operating conditions in order to be able to end up with the best system possible for my particular process?" Obviously the best criterion to use is the economic performance of the process. However, SSF processes have not been analyzed sufficiently to enable accurate estimates of capital and operating costs for new processes. In fact, at present the only way to compare the economic performance of bioreactors would be to build and operate a full-scale version of each bioreactor and record their capital and operating costs. It is likely to be some time before it will be possible to use economic performance as a criterion to guide bio-reactor selection.
In the absence of sufficient information about the economics of SSF processes, the aim should then be to maximize the productivity of the bioreactor, in terms of product formation, which might be biomass or a metabolite. In other words, the criterion is the rate of production in kg of product per m3 of bioreactor volume. Of course, if the substrate bed is not homogeneous, it will be necessary to calculate the productivity based on the evolution of the "volume-weighted biomass (or product)" curve. What is being sought is the combination of bioreactor operating strategy and harvesting time that will give the greatest value for:
pr _ * ' harvest initial t process Vbioreactor where Pr is the productivity (kg h-1 m-3), Xharvest is the amount of biomass (or product) at the time of harvesting (kg), Xinitial is the amount of biomass (or product) at zero time (kg), tprocess is the overall process time (the time between successive harvests, h), and Vbioreactar is the bioreactor volume (m3).
Therefore bioreactor selection will be guided by the answers to several key questions about factors that affect the productivity. These are discussed in the following subsections.
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