Bioreactors heat transfer

Activities associated with bioreactors include gas/hquid contacting, on-hne sensing of concentrations, mixing, heat transfer, foam control, and feed of nutrients or reagents such as those for pH control. The workhorse of the fermentation industry is the conventional batch fermenter shown in Fig. 24-3. Not shown are ladder rungs inside the vessel, antifoam probe, antifoam system, and sensors (pH, dissolved oxygen, temperature, and the like). Note that coils may lie between baffles and the tank wall or connect to the top to minimize openings... 

True. Pressure-cycle bioreactors have controllable and predictable flow patterns, which makes scale-up more predictable. Factors such as OTR and heat transfer are easier to arrange at large scales. 

Heat transfer is needed to operate the bioreactor at constant temperature, as the desired optimal microbial growth temperature. 

The calculation of heat transfer film coefficients in an air-lift bioreactor is more complex, as small reactors may operate under laminar flow conditions whereas large-scale vessels operate under turbulent flow conditions. It has been found that under laminar flow conditions, the fermentation broths show non-Newtonian behaviour, so the heat transfer coefficient can be evaluated with a modified form of the equation known as the Graetz-Leveque equation 9... 

Small temperature differences, AT, in a bioreactor are usually easily stabilised, unless refrigerated cooling water is used, which means that the product of overall heat transfer coefficient and the heat transfer area, UA , has to be large. Therefore the heat transfer area can be maximised by having cooling water in the baffles as well as in the jacket of the bioreactor. 

Tlie biochemical reaction rate followed the Monod rate model with a Monod rate constant of ks = 6.2 X 10 6g-cm 3 and a specific growth rate of vmlx 6.67 X 10 7g-cm 3-s. Design the bioreactor with a suitable heat transfer area. 

Overall volumetric productivity Qp (mol.m s ) (it is also common to use a yearly basis) is the average production capacity per unit volume and time of the bioreactor. The overall volumetric productivity is confined, on the one hand, by physical constraints, such as mass and heat transfer, and, on the other hand, by biocatalyst concentration... 

This physical limitation is the result of mass and heat transfer limitations, which are stoichiometrically related to product formation. The vertieal dotted line in Figure 11.1 symbolizes the limitation which is a conseqnence of the faet that the eoneentration of the biocatalyst is bound to certain defined limits, for instanee solnbihty in case of isolated enzymes and space in case of suspended eells. Fignre 11.1 also shows that the biocatalyst should have a minimum speeifie aetivity to be able to operate the bioreactor close to its physical ceiling. 

In the design and operation of various bioreactors, a practical knowledge of physical transfer processes - that is, mass and heat transfer, as described in the relevant previous chapters - are often also required in addition to knowledge of the kinetics of biochemical reactions and of cell kinetics. Some basic concepts on the effects of diffusion inside the particles of catalysts, or of immobilized enzymes or cells, is provided in the following section. 

Figure 9.19 shows typical cell concentrations reached in the main industrial bioreactors and a comparison of these values with those found in microbial fermentations. As can be observed, batch and fed-batch cultivations attain dry biomass values comparable to those of continuous cultures of microorganisms, so that mass and heat transfer capacities are not limited for these operation modes. However, high cell density cultivation in heterogeneous bioreactors, such as hollow-fiber devices, reaches dry biomass values similar to the maxima observed in microbial cultures. 

Semibatch Reactors Some of the reactants are loaded into the reactor, and the rest of the reactants are fed gradually. Alternatively, one reactant is loaded into the reactor, and the other reactant is fed continuously. Once the reactor is full, it may be operated in a batch mode to complete the reaction. Semibatch reactors are especially favored when there are large heat effects and heat-transfer capability is limited. Exothermic reactions may be slowed down and endothermic reactions controlled by limiting reactant concentration. In bioreactors, the reactant concentration may be limited to minimize toxicity. Other situations that may call for semibatch reactors include control of undesirable by-products or when one of the reactants is a gas of limited solubility that is fed continuously at the dissolution rate. 

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). 

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