Avoiding scaleup problems

Scaleup problems are sometimes avoidable. A few simple possibilities are  

Use enough diluents so that the adiabatic temperature change is acceptable. 

Depart from geometric similarity so that V and A xt both increase in direct proportion to the throughput scaling factor S. Scaling a tubular reactor by adding length is a possibility for an incompressible fluid. 

Use temperature-control techniques that inherently scale as S e.g., cold feed to a CSTR, or autorefrigeration. 

Intentionally degrade the performance of the small unit so that the same performance and product quahty can be achieved upon scaleup. 

Scale in parallel, for example, shell-and tube designs. 

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Section 5.3 discusses a variety of techniques for avoiding scaleup problems. The above paragraphs describe the simplest of these techniques. Mixing, mass transfer, and heat transfer aU become more difficult as size increases. To avoid limitations, avoid these steps. Use premixed feed with enough inerts so that the reaction stays single phase and the reactor can be operated adiabatically. This simplistic approach is occasionally possible and even economical. 

The main problem with a living polymer is maintaining the strict cleanliness that is demanded by the chemistry. This is a particularly severe problem for large-scale batch polymerizations, but it is a problem more of economics than technology. Living polymerizations are usually run to near completion, so that end-point control is not a problem. Most living polymerizations operate at low temperatures, —40 to - -40°C, to avoid chain transfer reactions. Thus temperature control is a significant scaleup problem. The usual approach is to use 85-95 w % solvent and to rely on sensible heat transfer to the vessel walls. 

Gas absorption can be carried out in a column equipped with sieve trays or other types of plates normally used for distillation. A column with trays is sometimes chosen instead of a packed column to avoid the problem of liquid distribution in a large diameter tower and to decrease the uncertainty in scaleup. The number of theoretical stages is determined by stepping off plates on a y-x diagram, and the number of actual stages is then calculated using an average plate efficiency. The plate and local efficiencies are defined in the same way as for distillation [Eqs. 

Section 1.5 described one basic problem of scaling batch reactors namely, it is impossible to maintain a constant mixing time if the scaleup ratio is large. However, this is a problem for fed-batch reactors and does not pose a limitation if the reactants are premixed. A single-phase, isothermal (or adiabatic) reaction in batch can be scaled indefinitely if the reactants are premixed and preheated before being charged. The restriction to single-phase systems avoids mass... 

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