Continuous processes bioreactor operations

Single-Cell Protein. Systems involving single-cell proteins are often very large throughput, continuous processing operations such as the Pmteen process developed by ICI. These are ideal for air-lift bioreactors of which the pressure cycle fermenter is a special case (50). 

The bioreactor operation mode is normally defined at the outset of process configuration. Insect cells have been cultured in almost all known cultivation modes batch [10], repeated-batch [70], perfusion [71-74], fed-batch [75, 76], semi-continuous [77,78] and continuous [79]. In spite of this multitude of different strategies, the batch or, eventually, fed-batch mode is normally preferred due to the lytic infection cycle of the baculovirus. 

Batch cultivation is perhaps the simplest way to operate a fermentor or bioreactor. It is easy to scale up, easy to operate, quick to turn around, and reliable for scale-up. Batch sizes of 15,000 L have been reported for animal cell cultivation [2], and vessels of over 100,000 L for fermentation are also available. Continuous processes can be classified into cell retention and non-cell retention. The devices typically used for cell retention are spin filters, hollow fibers, and decanters. Large-scale operation of continuous processes can reach up to 2,000 L of bioreactor volume. Typically, the process is operated at 1-2 bioreactor volumes... 

Batch, fed-batch, repeated fed-batch, and continuous modes of bioreactor operation have been used in SSF processes, although batch processes are by far the most common. Fed-batch or continuous operations that involve the addition of fresh, uninoculated substrate particles require interparticle colonization to occur, which is a relatively slow process, and will lead to bioreactor dynamics very different from those obtained during similar processes in SLF. Despite this, laboratory studies have demonstrated potential advantages of such operation. Abdullah et al. [68] compared batch, fed-batch, and repeated fed-batch culture of Chaetomium cellulolyticum on wheat straw. Under optimal conditions in batch culture, protein production ceased after three days, with a maximum protein level of 12 g per gram of solids. In fed-batch culture fresh straw was added at three-day intervals. Protein productivity was maintained for 12 days at a slowly declining rate. In the repeated fed-batch culture half of the fermenting straw was removed and replaced with fresh straw at three-day intervals. Protein productivity was maintained for 12 days at a steady rate and the protein level reached a maximum level of 14 g per gram of solids. 

Fed-batch and continuous modes of operation require agitation, either to mix in fresh substrates or nutrients, or to move the substrate bed along the bioreactor from inlet to outlet. Therefore these modes of bioreactor operation can only be used if the process microorganism tolerates mixing. 

Many different experimental techniques are available to characterize and quantify bioreactor hydrodynamics and gas-Uquid mass transfer rates. Some of these techniques, including advantages and disadvantages, were outlined in this chapter. Current experimental methods are continually being modified and refined, and new techniqnes will always be developed. Hence, the development of experimental methods relevant to bioreactor operation is an evolutionary process and an area rich in application. 

The numbering-up method also introduces another potential advantage. The scaled model operates as a continuous bioreactor rather than batch while providing the operator with the same control advantages of the batch operation at the same time. Therefore, the process time is expected to be shorter with miniaturized bioreactors since most standardized bioreactors use a process time that is longer than the kinetic minimum. Safety is also increased tremendously since the process can be stopped at any point in the process flow (Ehrfeld et al., 2000). These controls can be instituted automatically without the need for human supervision. 

The success of an ISPR process does not depend only on the chosen separation technique but also on the configuration of the bioreactor/separation units and mode of operation. Previous reviews have shown the various possible modes of operation (continuous, batch) and the use of a separation unit outside of the reactor or separation techniques that act right inside the fermenter [19,22,31]. Freeman and coworkers introduced a classification scheme for ISPR process based on batch/continuous operation and internal (within the reactor)/external (outside the reactor) removal of the product [3]. 

Adapting a reactor batch-operation stage to a continuous processing line (can also be applied to, for example, bioreactors, autoclaves, and dryers). 

In the production of bioethanol, sugar is fermented, yielding low concentrated alcohol solutions that are recovered from the broth by ultrafiltration or pervaporation membranes. Ultrafiltration and pervaporation of bioethanol from fermentation broth are IG biofuel processes. As in all bioreactor-coupled membrane processes, membrane fouling and drop in permeate fluxes during continuous operation are the main concerns. 

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