CSTRs scaleup

Also assume that the pilot- and full-scale vessels will operate at the same temperature. This means that A(o-out,bout, . )and/i/2 will be the same for the two vessels and that Equation (1.49) will have the same solution for provided that 7 is held constant during scaleup. Scaling with a constant value for the mean residence time is standard practice for reactors. If the scaleup succeeds in maintaining the CSTR-like environment, the large and small reactors will behave identically with respect to the reaction. Constant residence time means that the system inventory, pV, should also scale as S. The inventory scaleup factor is defined as... 

A volumetric scaleup by a factor of 512 is quite large, and the question arises as to whether the large vessel wiU behave as a CSTR. The concern is due to the factor of 4 increase in mixing time. Does it remain true that tmix h/i and tmix t If so, the assumption that the large vessel wiU behave as a CSTR is probably justified. The ratio of internal circulation to net throughput—which is the internal recycle ratio—scales as the inverse of the mixing time and will thus decrease by a factor of 4. The decrease may appear worrisome, but if the increase in mixing time can be tolerated, then it is likely that the decrease in internal recycle ratio is also acceptable. 

There is one significant difference between batch and continuous-flow stirred tanks. The heat balance for a CSTR depends on the inlet temperature, and Tin can be adjusted to achieve a desired steady state. As discussed in Section 5.3.1, this can eliminate scaleup problems. 

Example 11.18 Consider a gas-sparged CSTR with reaction occurring only in the liquid phase. Suppose a pilot-scale reactor gives a satisfactory product. Propose a scaleup to a larger vessel. 

Copolymerizations. The uniform chemical environment of a CSTR makes it ideally suited for the production of copolymers. If the assumption of perfect mixing is justified, there will be no macroscopic composition distribution due to monomer drift, but the mixing time must remain short upon scaleup. See Sections 1.5 and 4.4. A real stirred tank or loop reactor will more closely... 

The pilot reactor is a CSTR. The large reactor will be geometrically similar to the small one, and the scaleup wiU be done at constant power per unit volume. This form of scaleup exploits the fact that small vessels typically... 

One of the most challenging aspects of chemical engineering is the problem of scaling up a process unit from a small laboratory or pilot plant to a large commercial size. Reactors are perhaps one of the more difficult to deal with. In this section we show quantitatively what the heat transfer scaleup problem is for a CSTR. 

Example 1.7 Suppose a pilot-scale reactor behaves as a perfectly mixed CSTR so that Equation (1.49) governs the conversion. Will the assumption of perfect mixing remain valid upon scaleup ... 

However, each set of factors entering in to the rate expression is also a potential source of scaleup error. For this, and other reasons, a fundamental requirement when scaling a process is that the model and prototype be similar to each other with respect to reactor type and design. For example, a cleaning process model of a continuous-stirred tank reactor (CSTR) cannot be scaled to a prototype with a tubular reactor design. Process conditions such as fluid flow and heat and mass transfer are totally different for the two types of reactors. However, results from rate-of-reaction experiments using a batch reactor can be used to design either a CSTR or a tubular reactor based solely on a function of conversion, -r ... 

A factor of 10 scaleup in the linear dimensions of a CSTR, 5 = 10, requires a factor of 100,000 increase in total power and a factor of 100 increase in power per unit volume if fniix is held constant. Such a scaleup would be absurd. A more reasonable scaleup rule is to maintain constant power per unit volume so that a 1000-fold increase in reactor volume requires a 1000-fold increase in power. What happens to fniix in this case ... 

A volumetric scaleup by a factor of 512 is quite large, and the question arises as to whether the large vessel will remain well mixed on the large scale. The concern is the fact that the mixing time increases by a factor of 4. Does it remain true that [Pg.144]

The removal of condensation byproducts becomes increasingly difficult upon scaleup. Some commercial PET processes use CSTRs for the early stages of the reaction where most of the byproduct ethylene glycol is removed. They use only the top, visible surface of the liquid for mass transfer and rely on jacket heating to supply the latent heat of vaporization. The surface area scales as and limits the production rate in some processes because the previous limit, a downstream finishing reactor, has been... 

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