Summary And Conclusions

ALRs are popular in modern bioprocess research and development. These reactors are particularly suitable for biological processes in which a high mass transfer rate is re quired, but excessive power input may lead to damage of the cells due to shear effects. ALRs also have very appealing characteristics for bioprocesses for low-value products in which efficiency of energy utilization may become the key point for design. Such is the case for ALRs for wastewater treatment. The ALR is particularly effective for solid fluidization, which is important in many biological processes in which the biocatalyst is available in the form of pellets or is immobilized on a solid support.

The distinctive characteristics of ALRs are conferred by the fluid dynamics of the liquid-gas or liquid-gas-solid mixtures in it. These characteristics are expressed as gas holdup, liquid velocity, and mixing in each of the zones of the ALR. It is important to recognize the differences in the fluid dynamic characteristics of these zones: the riser, the gas separator, and the downcomer. Only a correct understanding of behavior and interconnection of these regions can make possible the correct design of a new reactor or the scale-up of a laboratory device up to pilot or industrial size.

The purpose of scale-up is to conserve and repeat on a larger scale the fluid dynamics of the reactor. Therefore, one of the most important factors in the design and scale-up of reactors is the influence of the geometric characteristics of the system on the flow of the different phases present.

The variables affecting the performance of the reactor are geometric design, operation variables, and fluid properties. Several correlations are available in the technical literature for the prediction of the fluid dynamic characteristics of the reactor (gas holdup, liquid velocity, mixing time of axial dispersion coefficient) and of the mass and heat transfer coefficients. Although attempts have been made to study all the aspects described, no single research group has managed to cover all the variables over a wide range. It is therefore extremely important that the engineer confronting scale-up or de novo design of an ALR analyzes in depth the range of validity of the correlations used for the calculations.

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