Bioreactor continuous plug-flow

A continuous bioreactor type plug flow reactor (PFR) with immobilized cell of Saccharomyces cerevisiae in spherical particle was used in the work to take into account the variations of concentrations at the reactor length and inside spherical particle. 

FIGURE 25 Bioreactor operational modes (a) a continuous plug flow bioreactor, (b) a continuous plug flow bioreactor with biomass recycle, (c) multipoint feeding of a plug flow bioreactor. 

The bioreactor has been introduced in general terms in the previous section. In this section the basic bioreactor concepts, i.e., the batch, the fed-batch, the continuous-flow stirred-tank reactor (CSTR), the cascade of CSTRs and the plug-flow reactor, will be described. 

In the ideal plug-flow reactor (Figure 11.16) the continuous phase flows as a plug through the reactor i.e., there is no mixing or, in other words, no axial dispersion. Consequently, if a compound is consumed or produced, a concentration gradient will exist in the direction of flow. The mass balance is therefore first set up over an infinite small slice perpendicular to the direction of the flow with volume dV of the bioreactor. Assuming steady state and F =Fq=F, Equation (11.5) then is reduced to ... 

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. 

Figure 8.5 Schematic of bioreactor and two extremes used for modeling the contacting pattern, (a) Complete backmix. (b) Plug flow. [Reproduced from AnaKsis of a Continuous, Aerobic Fixed-Film Bioreactor. I. Steady-State Behavior. b Y. Park. M. E. Davis, and D. A. Wallis, Biotech. Bioeng., 26 (1984) 457, copyright 1984 Wiley-Liss, Inc., a subsidiary of John Wiley and Sons, Inc ]...

Continuous SSF processes are usually operated in plug flow mode. Such processes will require pasteurization or sterilization of the substrate as it enters the bioreactor, mixing with an inoculum, and at the outlet end of the bioreactor, continuous removal of spent substrate. Such a process was operated on a pilot scale for the production of ethanol from fodder beets by Saccharomyces cerevisiae [81,82]. The bioreactor had a screw within a 4.7 m long and 15.25 cm diameter tube. The screw was rotated intermittently to mix the substrate and move it along the tube. At the front end was a hammer-mill and a pasteurization chamber for substrate preparation and a port for inoculation. New substrate was added, inoculated, and the screw rotated at 12-h intervals, resulting in a residence time of 72 h. 

Steady-state flow reactors, with a constant supply of reactants and continuous removal of products, can be operated as both a continuous stirred-tank bioreactor (CSTB) and as a plug flow bioreactor (PFB). It is possible to have different configurations of the membrane bioreactor where the biocatalyst is immobilized in the fractionated membrane support (Katoh and Yoshida, 2010). In Fig. 1.6 the scheme of a CSMB in which the biocatalyst is immobilized on the surface of the membrane beads is presented. The biocatalyst immobilized in the porous structure of a fractioned membrane can also be operated in CSMB. For example, two configurations are shown in Fig. 1.7 (a) for flat-sheet and (b) for spherical porous structures, respectively. Such structures could also be adopted for PFB, where a bed of membrane support with the immobilized biocatalyst could be utilized, in either a fixed or fluid configuration. 

Several bioreactor designs are used to produce bioproducts, and include, but are not limited to batch reactors, fed-batch reactors, continuous cultivation reactors, plug flow reactors, recycle bioreactor systems, immobilized cell reactors, biofilm reactors, packed bed reactors, fluidized-bed reactors, and dialysis cultivation reactors (Williams 2002). These reactor types can contain either mixed or pure cultures, and can stimulate heterotrophic and/or phototrophic cellular functions depending on the specific reactor design. Additionally, these reactor schemes can be used to produce products directly, or to harvest biomass or other products for downstream processes. Due to the complex nature of bioreactors, particularly anaerobic digesters, the use of metagenomics is helpful to understand the physiology of such systems. 

Continuous flow processes sometimes use a plug flow bioreactor in which there is little mixing of fluid elements in the direction of flow (Fig. 25a). This type of flow is typically achieved in long tubes and channels. The composition of the broth does not change with time at a fixed... 

Beeause F is the volume processed in time f in a continuous flow bioreactor and Vl is the corresponding volume in a batch reactor, a comparison of Eqs. (11) and (13) shows that batch and plug flow (i.e., packed bed) bioreactors containing the same amount of enzyme will achieve equal conversions in a given time. This is a general conclusion, irrespective of the reaction kinetics. A continues flow packed bed enzyme bioreactor may be advantageous relative to batch reactor, as the unproductive time for batch preparation could be eliminated in the continuous flow unit. However, the batch reactor may have other important advantages such as the ease of pH control in a well-mixed device. 

When the substrate concentration S is much greater than Km, Eqs. (12) and (13) reduce to the same form. In this case, the continuous flow stirred reactor and the plug flow device achieve similar conversion values in a given time. In contrast, when S Km, the reaction rate becomes first order in the substrate concentration (see Eq. (9)), and the plug flow reactor provides higher conversion values in comparison with the well-mixed continuous flow device. In the latter bioreactor, all the enzyme would be exposed to the same low concentration of the substrate which is not useful except when the reaction is inhibited by the substrate. 

Cells in suspension Plug flow reactor with hydrodynamic stirring with or without static mixing elements in tube or flat-bed bioreactor Continuous... 

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