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Heterogeneous Reactions in Bioreactors - Lesson Summary

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Heat transfer is the process of transfer of heat from a high-temperature system to a low-temperature system. In the thermodynamic system, heat transfer is the movement of heat across the boundary of the system due to the temperature difference between the system and the surrounding. There are three modes of heat transfer which include: convection, conduction, and radiation.

The two types of common heat transfer application in bioreactor operation are:

• In situ batch sterilization of liquid medium: in this process, a vessel containing medium is heated using steam and held at sterilization temperature for a period of time.

• Temperature control during reactor operation: the metabolic activity of cells generates heat and some microorganisms need extreme temperature conditions.

Heat exchangers are devices designed to transfer heat between two or more fluids such as liquids, vapors, or gases of different temperatures. Heat exchanger configurations include the following descriptions:

• External jacket and coil give a low heat transfer area. They are rarely used on an industrial scale.

• Internal coils are frequently used in production vessel

• External heat exchanger unit is independent of the reactor, easy to scale up and provides the best heat transfer capability.

The first-order reaction is where the reaction rate depends on a single reactant and the value of the exponent is one.
The following points should be considered in practice;

• The particle size should be as small as possible

• The maximum reaction rate is determined by enzyme activity and concentration in the support.

• High enzyme content will result in high enzyme activity per unit of reaction volume but low effectiveness factor.

Thiele`s modulus measures the reaction rate to the diffusion rate. For the low value of Thiele’s modulus, the rate of the enzyme reaction is slow compared to the diffusion rate. On the other hand, for higher values of modulus, the reaction rate is faster than the diffusion rate.

In batch sterilization, it is difficult to get the ideal temperature-time profile and there are three periods of sterilization:

• Period of heating the liquid medium

• Period of holding at a constant temperature

• Cooling period

In most biological processes, the larger the surface area the more diffusion can occur. When there is more volume and less surface area, diffusion takes longer and is less effective.
Diffusion effects in surface-bound enzymes on non-porous support materials have the following assumptions:

• Assume a situation where enzymes are bound and evenly distributed on the surface of non-porous support material.

• All enzymes molecules are equally active

• Substrate diffuses through a thin liquid film surrounding the support surface to reach the reactive surfaces.

• At steady-state, the reaction rate is equal to the mass transfer rate.

Assumptions of diffusion effects in immobilized cell systems include:

• The presence and significance of diffusional limitations depend on the relative rates of bioconversion and diffusion as given by the Damkohler number.

• A differential material balance for the rate-limiting substrate within the biofilm yields at steady-state.