Scale-up example

The following roller compaction scale-up examples illustrate technology strategies that identify equipment design features, process parameters, and evaluations defining roller compaction scale-up processes. 

Scheme 5.7 Scalable synthesis of 1,2,4-triazoles developed at Hoffman-La Roche (a) general reaction conditions and (b) scale-up example.

A scale-up example from a hypothetical process in a 24 in. pan is shown in Table 8.4. 

In practice is a small number and the sing-around frequencies are scaled up for display. In one example, for a pipe 1 m in diameter and water flowing at 2 m/s, the frequency difference is 1.4 Hz (10). Frequency difference transit time meters provide greater resolution than normal transit time ultrasonic meters. The greatest appHcation is in sizes from 100 mm to 1 m diameter. 

When choosing the scale-up method, changes in other flow/power parameters and their impact on the process result must be considered. Figure 11 shows changes in important parameters for different scale-up bases. For example, scale-up based on same tip speed maintains the T / Ubut decreases P/ Uby 80%. T / Uis almost always increased on scale-up. Scale-up based on the same P/ Umeans a reduction in mixer speed by 66%, which also... 

Process plant design has come a long way from the early 1930s when process designers used the rule-of-thumb that a process faciUty could not be scaled-up more than 10-fold (2). American Oil s Ultracracking unit (Texas City, Texas) for example, was designed from data from a small pilot plant with a scale-up factor of 80,000 (3). 

Over 25 years ago the coking factor of the radiant coil was empirically correlated to operating conditions (48). It has been assumed that the mass transfer of coke precursors from the bulk of the gas to the walls was controlling the rate of deposition (39). Kinetic models (24,49,50) were developed based on the chemical reaction at the wall as a controlling step. Bench-scale data (51—53) appear to indicate that a chemical reaction controls. However, flow regimes of bench-scale reactors are so different from the commercial furnaces that scale-up of bench-scale results caimot be confidently appHed to commercial furnaces. For example. Figure 3 shows the coke deposited on a controlled cylindrical specimen in a continuous stirred tank reactor (CSTR) and the rate of coke deposition. The deposition rate decreases with time and attains a pseudo steady value. Though this is achieved in a matter of rninutes in bench-scale reactors, it takes a few days in a commercial furnace. 

Herbst et al. [International J. Mineral Proce.ssing, 22, 273-296 (1988)] describe the software modules in an optimum controller for a grinding circuit. The process model can be an empirical model as some authors have used. A phenomenological model can give more accurate predictions, and can be extrapolated, for example from pilot-to full-scale apphcation, if scale-up rules are known. Normally the model is a variant of the popiilation balance equations given in the previous section. 

Few mechanisms of liquid/liquid reactions have been established, although some related work such as on droplet sizes and power input has been done. Small contents of surface-ac tive and other impurities in reactants of commercial quality can distort a reac tor s predicted performance. Diffusivities in liquids are comparatively low, a factor of 10 less than in gases, so it is probable in most industrial examples that they are diffusion controllech One consequence is that L/L reactions may not be as temperature sensitive as ordinary chemical reactions, although the effec t of temperature rise on viscosity and droplet size can result in substantial rate increases. L/L reac tions will exhibit behavior of homogeneous reactions only when they are very slow, nonionic reactions being the most likely ones. On the whole, in the present state of the art, the design of L/L reactors must depend on scale-up from laboratoiy or pilot plant work. 

A few excellent books are also available on reaction engineering in the widest sense and from a fundamental point of view. These books treat the subject with mathematical rigor, yet are too inclusive to have any space left for details on experimental procedures. Here, the reader can find more insight and practical examples on the development and scale-up of... 

Chemical reaction hazards must be considered in assessing whether a process can be operated safely on the manufacturing scale. Furthermore, the effect of scale-up is particularly important. A reaction, which is innocuous on the laboratory or pilot plant scale, can be disastrous in a full-scale manufacturing plant. For example, the heat release from a highly exothermic process, such as the reduction of an aromatic nitro compound, can be easily controlled in laboratory glassware. Flowever,... 

The failure of conventional criteria may be due to the fact that it is not only one mixing process which can be limiting, rather for example an interplay of micromixing and mesomixing can influence the kinetic rates. Thus, by scaling up with constant micromixing times on different scales, the mesomixing times cannot be kept constant but will differ, and consequently the precipitation rates (e.g. nucleation rates) will tend to deviate with scale-up. 

The conventional scale-up criteria scale-up with constant stirrer speed , scale-up with constant tip speed and scale-up with constant specific energy input are all based on the assumption that only one mixing process is limiting. If, for example, the specific energy input is kept constant with scale-up, the same micromixing behaviour could be expected on different scales. The mesomixing time, however, will change with scale-up as a result, the kinetic rates and particle properties will be different and scale-up will fail. 

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