Bioreactors production problems

No doubt additional effort is needed in the kinetic analysis of a process to optimize a laboratory-scale bioreactor using the criteria listed in Table 4.3. The results, however, should justify the effort. Figure 4.11 illustrates this problem showing that biokinetics seems to be dependent from the type of bioreactor. Productivity is plotted for a stirred tank with increasing rotational speed n and is compared with that in the cycle tube cyclone reactor (Ringpfeil, 1980). 

Bioreactor operation and scale-up are not completely independent processes and should be assessed as different aspects of the same problem. Bioreactor operation must then consider process scale-up not only as the next step but also, as in the classic scale-down method, as a starting point. This means that bioreaction design should be done in scalable systems. A scalable system has to be inherently simple and reliable in operation and control, for easy validation and control in a production facility [56]. 

While the batch process is the dominant one in current use, researchers and companies have attempted to create continuous bioreactor systems. Lopez et al. immobilized Candida rugosa in polymethacrylamide hydrazide beads and polyurethane foam 3 with the intent to achieve the continuous production of lipase enzymes. Despite flow problems with the polyurethane foam, it showed high lipolytic activity. Biomass buildup was problematic. Feijoo et al. immobilized Phanerochaete chry-sosporium on polyurethane foam in packed bed bioreactors under near-plug flow conditions. Continuous lignin peroxidase production was accomplished, the rate of which was studied as a function of recycle ratio. 

The present study on butanol fermentation has been focused primarily on the effects of pH and dilution rate (D) in continuous cultures of the mutant strain from C. acetobutylicum ATCC 55025. To overcome the problems of low productivity and yield of butanol, cell immobilization in a convoluted fibrous bed bioreactor (FBB) and feeding with dextrose and butyric acid as cosubstrates to produce butanol and reduce production of ancillary byproducts were used in the fermentation. By changing the dilution rate from 0.1 to 1.2 h 1 at pH 4.3 and varying the pH from 3.5 to 5.5 at the dilution rate of 0.6 hr1, the optimal conditions for high productivity and butanol yield were investigated. 

Fermentation is typically conducted in dilute suspension culture. The low concentration in such systems limits reaction efficiency, and the presence of particulate and colloidal solids poses problems for product recovery and purification. By circulating the fermentation broth through an ultrafiltration system, it is possible to recover product continuously as they are generated while minimizing loss of enzyme or cells and keeping product concentration in the bioreactor below the self-inhibition level for the biocatalyst. This process is referred to as perfusion. As the ultrafiltration unit is part of the production process, the entire system is often considered a membrane reactor. 

The use of silicon-based antifoams is common in industry (van Bonarius et al., 1993). Flowever, they should be used with care, since these substances can be toxic to the cell above certain concentrations. Furthermore, chemical antifoams can pose problems for the chromatographic purification of the product. Foam traps, which are devices mounted in the upper part of bioreactors to break the foam, have been used successfully at small and intermediate scales, but are not widely used on a large scale. On the other hand, low aeration rates using pure oxygen effectively lead to a significant decrease or even complete elimination of foam, but may result in CO2 accumulation in the medium, which is harmful to the cells (Gray et al., 1996). 

Hollow-fiber bioreactors constitute an optimized production system where it is possible to achieve higher cell concentrations (107 to 108 cells/ ml), and the product concentration can reach a level of 0.7-2.3 g/L, which is similar to what can be obtained with ascitic fluid (Hendriksen and Leeuw, 1998). This system can operate for over 3 months without affecting cell viability, but presents problems with mass transport, and the formation of nutrient gradients, which require specific solutions (Kretzmer, 2002). 

Until now, bioreactors of various types have been developed. These include loop-fluidized bed [14], spin filter, continuously stirred turbine, hollow fiber, stirred tank, airlift, rotating drum, and photo bioreactors [1]. Bioreactor modifications include the substitution of a marine impeller in place of a flat-bladed turbine, and the use of a single, large, flat paddle or blade, and a newly designed membrane stirrer for bubble-free aeration [13, 15-18]. Kim et al. [19] developed a hybrid reactor with a cell-lift impeller and a sintered stainless steel sparger for Thalictrum rugosum cell cultures, and cell densities of up to 31 g L1 were obtained by perfusion without any problems with mixing or loss of cell viability the specific berberine productivity was comparable to that in shake flasks. Su and Humphrey [20] conducted a perfusion cultivation in a stirred tank bio-... 

Integrated bioprocesses can be used to enhance the production of valuable metabolites from plant cell cultures. The in situ removal of product during cell cultivation facilitates the rapid recovery of volatile and unstable phytochemicals, avoids problems of cell toxicity and end-product inhibition, and enhances product secretion. In situ extraction, in situ adsorption, the utilization of cyclodextrin, and the application of aqueous two-phase systems have been proposed for the integration of cell growth and product recovery in a bioreactor. The simultaneous combination of elicitation, immobilization, permeabilization, and in situ recovery can promote this method of plant cell culture as a feasible method to produce various natural products including proteins. 

In some technological and medical applications protein adsorption and/or cell adhesion is advantageous, but in others it is detrimental. In bioreactors it is stimulated to obtain favourable production conditions. In contrast, biofilm formation may cause contamination problems in water purification systems, in food processing equipment and on kitchen tools. Similarly, bacterial adhesion on synthetic materials used for e.g. artificial organs and prostheses, catheters, blood bags, etc., may cause severe infections. Furthermore, biofilms on heat exchangers, filters, separation membranes, and also on ship hulls oppose heat and mass transfer and increase frictional resistance. These consequences clearly result in decreased production rates and increased costs. 

One reason for interest in plant cell culture is that over 20,000 different chemicals are produced from plants, with about 1600 new plant chemicals added each year. Also, 25% of all prescribed drugs come from plants. These chemicals can be produced in a bioreactor through suspension culture. Advantages of plant cell suspension culture, as compared to agriculture, are that plant cell suspension culture can be carried out independently of weather conditions and political problems, it does not compete with other agricultural products for land use, and it is done in a controlled environment which minimizes contamination and provides easier product validation and assurance. 

A key problem in bioreactor control is the difficulty in obtaining reliable sensors and consequently of reliable on-line process information. Demands for product consistency and process productivity produce requirements for more process information.13 Especially in the case of fermentors, rapid, accurate on-line measurement of process variables is often a complex task. As a result, much research effort has focused on methods for quantitatively estimating compositions within reactors and on using model-based control techniques. 

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