Scaleup criterion

Power per unit volume is frequently used as a scaleup criterion for gas liquid reactions, but there is disagreement about its merit for homogeneous reactions with imperfect mixing. If micromixing effects are most important, the selectivity should not change at scaleup at constant P/V, since e would be constant and (p should be the same at comparable positions in the reactors, giving the same value of parallel reactions in 1- and 20-liter tanks, Fournier and coworkers... 

A theoretical justification for using the blending time as a scaleup criterion is that is usually proportional to t and is the critical mixing time if most of the extra byproduct formation occurs in or near the feed plume. If the ratio dg/D is kept the same, with d = aD, and the average rate of energy dissipation is e = then Eq. (6.7) becomes... 

It is often essential to maintain the same rate of heat transfer in the large-scale unit. Nevertheless this scaleup criterion is impractical because it demands excessive impeller tip speeds and high power costs. For this reason, to compensate the negative effect in the heat-transfer rate when another scaleup criterion is selected, additional cooling devices are necessary in most cases [123]. 

A price calculation is relatively easy for a product with a track record of a regular industrial-scale production. On the other hand, it is difficult, if only a laboratory procedure exists and a calculation has to be made based on the virtual scaleup to industrial-scale production. The ability to perform this desk exercise in a quick and reliable fashion is an important competence criterion of a fine-chemical manufacturer. When setting a price, a separation of tasks must be made between the controller, who calculates the manufacturing cost, and the sales manager, who determines the sales price. If mistakes have been made and prices have to be changed, you will need facts to support your request. If a pivotal product is supplied, a supply contract is concluded. 

Complete suspension of solids would be satisfactory for most purposes, and the correlations developed to predict conditions for suspension have generally used this criterion. Some of these correlations are discussed here along with guidelines for scaleup. Keep in mind that these correlations give the minimum agitation condition for suspension, and that requirements for dispersion of a gas or good heat transfer to a coil or jacket may indicate higher power inputs for some cases. 

Large-scale tests of consecutive reactions were carried out by Rice and Baud [16], who used five Pfaudler reactors from 3 to 2680 liters in size. For all reactors, the amount of secondary product decreased with increasing P/V but at equal P/V, it was four times greater for the largest reactor than for the smallest reactor. They also found less secondary product when Da/Df was increased at constant P/V. No single criterion for satisfactory scaleup was suggested. 

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