Limitations of the Models

Of course, the models provided have various limitations that mean that, although they are certainly valuable tools for increasing the understanding of the principles of bioreactor operation, they are not yet the powerful and flexible tools that we would like to have available in the bioreactor design process. For example:

• The models only simulate performance in terms of growth. They do not give any analysis of the economic consequences of certain operating modes. They do not even simulate performance in terms of product formation.

• The models do not impose practical limits on operating variables. For example, the packed-bed model does not limit the superficial air velocity to remain at values below the velocity that would fluidize the substrate particles in the bed.

• The flexibility in changing the kinetics of growth is greatly limited. It is possible to specify the optimum temperature for growth and the value of the specific growth rate constant at this temperature, but not the form of the curve describing the specific growth rate constant as a function of temperature. It is only possible to choose from two different types of dependence of growth on water activity (the "Rhizopus-type" has optimum growth at a water activity of 1.0, while the "Aspergi//us-type" has an optimum water activity of 0.95).

• Various properties of the substrate are pre-determined and cannot be changed (e.g., the absorption/desorption isotherm).

• Although it is possible to change the size and often the length to diameter ratio of the bioreactor, it is not possible to change the shape (e.g., you cannot change from a circular cross-section to a rectangular cross-section).

Obviously, much more needs to be done in order to improve models as tools in the bioreactor design process. We hope that the current book plays a role in stimulating the continued improvement of models.

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