Scaleup Approach

While the simplest method for scaleup of a chemical reactor is to start with a small working unit that provides the desired results and then progressively build larger units, this method is usually too expensive and time-consuming. Consequently, more semiempirical approaches, such as dimensional similitude, are taken. Other approaches include mathematical modeling or a combined dimensionless similitude/mathematical modeling method but are not involved in the scaleup example discussed here. 

Dimensional similitude is based on principles of similarity and uses dimensionless ratios of the physical and chemical parameters that govern the working model to design the scaled-up prototype. This method is followed when the prototypical unit is expected to be dimensionally similar to the small-scale unit. The degree of success in using dimensional similitude as an approach to scaleup depends mainly on the extent to which the physical parameters necessary to achieve a desired result influence the process. 

Frequently, complete dimensional similitude cannot be reached between two reactors with widely different scales as a result, this method is usually limited in practice to relatively simple chemical reaction systems. For a cleaning vessel in which the reaction rate is very fast and the process is governed by the physical rates of the process, e.g., mass transfer, heat transfer, etc., the dimensionless groups describing the process consist of fluid mechanic and thermodynamic quantities. The cleaning process can usually be a relatively simple mechanism to describe, making dimensional similitude more easily achievable. 

The following discussion introduces the considerations applicable to initial design and subsequent scaleup of any supercritical or near-critical fluids parts cleaning vessel, then centers on dimensionless variables and principles for applying these variables to actual scaleup of a parts cleaning vessel. The last part of the discussion focuses on the application of these concepts to an agitated, supercritical carbon dioxide, parts cleaning vessel. 

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The scaleup process is not just a matter of plugging values into prescribed equations, nor can exact scaleup criteria be obtained from generalized correlations for certain types of equipment. Instead, in a good scaleup approach, all variables that describe the process are determined desired process conditions and the magnitude of the scaleup taken into account and a design selected. Scaleup designs are based almost exclusively on the principle of similarity. 

The procedure for using this scaleup approach is summarized as follows ... 

Manufacturing approaches for selected bioproducts of the new biotechnology impact product recovery and purification. The most prevalent bioseparations method is chromatography (qv). Thus the practical tools used to initiate scaleup of process Hquid chromatographic separations starting from a minimum amount of laboratory data are given. 

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 primary goal of scaleup is to maintain acceptable product quahty. Ideally, this will mean making exactly the same product in the large unit as was made in the pilot unit. To this end, it may be necessary to alter the operating conditions in the pilot plant so that product made there can be duplicated upon scaleup. If the pilot plant closely approaches isothermal piston flow, the challenge of maintaining these ideal conditions upon scaleup may be too difficult. The alternative is to make the pilot plant less ideal but more scaleable. 

Solution The approach is similar to that in Example 3.7. The unknowns are Sl and (Em)2. Set (Poudi = (Pout) - Equation (3.40) is used to calculate iPm)2 nd Equation (3.41) is used to calculate Sl- Results are given in Table 3.2. The results are qualitatively similar to those for the turbulent flow of a gas, but the scaled reactors are longer and the pressure drops are lower. In both cases, the reader should recall that the ideal gas law was assumed. This may become unrealistic for higher pressures. In Table 3.2 we make the additional assumption of laminar flow in both the large and small reactors. This assumption will be violated if the scaleup factor is large. 

This section has based scaleups on pressure drops and temperature driving forces. Any consideration of mixing, and particularly the closeness of approach to piston flow, has been ignored. Scaleup factors for the extent of mixing in a tubular reactor are discussed in Chapters 8 and 9. If the flow is turbulent and if the Reynolds number increases upon scaleup (as is normal), and if the length-to-diameter ratio does not decrease upon scaleup, then the reactor will approach piston flow more closely upon scaleup. Substantiation for this statement can be found by applying the axial dispersion model discussed in Section 9.3. All the scaleups discussed in Examples 5.10-5.13 should be reasonable from a mixing viewpoint since the scaled-up reactors will approach piston flow more closely. 

Previous chapters have discussed how isothermal or adiabatic reactors can be scaled up. Nonisothermal reactors are more difficult. They can be scaled by maintaining the same tube diameter or by the modeling approach. The challenge is to increase tube diameter upon scaleup. This is rarely possible and when it is possible, scaleup must be based on the modeling approach. If the predictions are satisfactory, and if you have confidence in the model, proceed with scaleup. 

When two or more phases are present, it is rarely possible to design a reactor on a strictly first-principles basis. Rather than starting with the mass, energy, and momentum transport equations, as was done for the laminar flow systems in Chapter 8, we tend to use simplified flow models with empirical correlations for mass transfer coefficients and interfacial areas. The approach is conceptually similar to that used for friction factors and heat transfer coefficients in turbulent flow systems. It usually provides an adequate basis for design and scaleup, although extra care must be taken that the correlations are appropriate. 

The pilot reactor is a tube in isothermal, laminar flow, and molecular diffusion is negligible. The larger reactor wiU have the same value for t and will remain in laminar flow. The residence time distribution will be unchanged by the scaleup. If diffusion in the small reactor did have an influence, it wiU lessen upon scaleup, and the residence time distribution will approach that for the diffusion-free case. This wiU hurt yield and selectivity. 

A checklist approach is used to gather process safety information (PSI) prior to scaleup to pilot plant. 

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