Residence time distribution, small-scale processes

In the development of these processes and their transference into an industrial-scale, dimensional analysis and scale-up based on it play only a subordinate role. This is reasonable, because one is often forced to perform experiments in a demonstration plant which copes in its scope with a small produdion plant ( mock-up plant, ca. 1/10-th of the industrial scale). Experiments in such plants are costly and often time-consuming, but they are often indispensable for the lay-out of a technical plant. This is because the experiments performed in them deliver a valuable information about the scale-dependent hydrodynamic behavior (arculation of liquids and of dispersed solids, residence time distributions). As model substances hydrocarbons as the liquid phase and nitrogen or air as the gas phase are used. The operation conditions are ambient temperature and atmospheric pressure ( cold-flow model ). As a rule, the experiments are evaluated according to dimensional analysis. 

A continuous tubular-loop process has been patented (10) and used for relatively small-scale production. The loop process consists of a tube-pump system in which the rate of latex circulation around the tube loop is considerably greater than the throughput rate. Thus, the distribution of residence times should be nearly the same as that of a single CSTR. 

Since the separate time scale condition is clearly valid for most of the polymerization process, one may say that each polymer chain is formed inside a particle of unchanging siz wherein all rate coefficients are constant and the distribution of free radicals has its steady-state value, for each volume V. Any residual effect of the PSD on the MWD would reside presumably in the effects of the PSD on the kinetic parameters (e.g., p. c, and to a lesser extent fc). Conversely, the MWD would possibly influence the PSD through its effects on the swelling of the particles by the monomer the effect, if it exists, is likely to ha small. 

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