Liquid Mixing

For the design, modeling, and operation of ALRs, a thorough knowledge of mixing behavior is necessary. This is of particular importance during the process of scale-up from laboratory-scale to industrial-scale reactors. The optimum growth rate of a microorganism or the optimum production rate of a specific secondary metabolite usually relates to well-defined environmental conditions, such as pH range, temperature, substrate level, limiting factors, dissolved oxygen, and inhibitor concentration in a specific well-mixed laboratory-scale vessel. Because of the compromises made during scale-up, it is difficult to keep, at different scales of operation, the same hydrodynamic conditions established in the laboratory; mixing on an industrial scale may not be as good as mixing on a laboratory scale (5). In smaller-scale reactors it is easier to maintain the optimal conditions of pH, temperature, and substrate concentration required for maximum productivity of metabolites in a fermenter.

Furthermore, in fermentation systems efficient mixing is required to keep the pH within the limited range, giving maximum growth rates or maximum production of the microorganism during addition of acid or alkali for pH control. Mixing time—or the degree of homogeneity—is also very important in fed-batch fermentation, where a required component, supplied either continuously or intermittently, inhibits the microorganisms or must be kept within a particular concentration range (114,115). A large number of commercially important biological systems are operated in batch or fed-batch mode. In this operation mode, fast distribution of the incoming fluid is required, and the necessity for understanding the dynamics of mixing behavior in these vessels is obvious. Even for batch systems, good control of the operating conditions, such as pH, temperature, and dissolved oxygen, require prior es

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