Mixing Scale-up

Dimensionless Numbers. With impeller diameter D as length scale and mixer speed N as time scale, common dimensionless numbers encountered in mixing depend on several controlling phenomena (Table 2). These quantities are useful in characterizing hydrodynamics in mixing tanks and when scaling up mixing systems. 

Sizing, 451, 453, 455, 459, 462 Sonic flow, 461 Types, illustrations, 411-421 Rupture disk, liquids, 462, 466 Rupture disk/pressure-relief valves combination, 463 Safely relief valve, 400 See Relief valve Safety valve, 400, 434 Safety, vacuum, 343 Scale-up, mixing, 312, 314—316 Design procedure, 316-318 Schedules/summaries Equipment, 30, 31 Lines, 23, 24 Screen particle size, 225 Scrubber, spray, 269, 270 Impingement, 269, 272 Separator applications, liquid particles, 235 Liquid particles, 235 Separator selection, 224, 225 Comparison chart, 230 Efficiency, 231... 

Establishing the process sensitivity with respect to the above-mentioned factors is crucial for further scale-up considerations. If the sensitivity is low, a direct volume scale-up is allowed and the use of standard batch reactor configurations is permitted. However, many reactions are characterized by a large thermal effect and many molecules are very sensitive to process conditions on molecular scale (pH, temperature, concentrations, etc.). Such processes are much more difficult to scale up. Mixing can then become a very important factor influencing reactor performance for reactions where mixing times and reaction times are comparable, micromixing also becomes important. 

It is seldom realized that many rules of thumb utilized for scale-up of different types of equipment are represented by quantities, which fulfill only a partial similarity. As examples, only the volume-related mixing power P/ V widely used for scaling-up mixing vessels and the superficial velocity V which is normally used for scale-up of bubble columns, should be mentioned here. 

Meier SJ. (2005) Cell culture scale-up mixing, mass transfer, and use of appropriate scale-down models. Biochem. Eng. XIV. Harrison Hot Springs, Canada. 

This is a step in a process that was recently developed at AstraZeneca. The original process description specified 10 mole equivalents of chlorine to be used. A reaction mechanism was postulated suggesting that only 3 mole equivalents were required, therefore the rest was effectively wasted. Attention was focused around improving the mass transfer of the chlorine. Laboratory experiments showed that the reaction kinetics is extremely fast and highly exothermic. Scale-up mixing utilities, provided with DynoChem, were used to provide required agitation rates in the laboratory to ensure the gas was fully dispersed. 

The third step consists of calculating scaled-up mixing parameters (shear, pumping capacity, etc.) according to the results of the previous study. 

Liquid-liquid complex reactions have been classified as one of the most difficult—if not the most difficult—reaction scale-up mixing problem (Leng, 1997). The complexities of drop formation and coalescence both change with scale. They both depend on the location in a vessel and on very subtle changes in the composition of the fluids. These variations can cause problems like Bill s when a complex reaction occurs between reagents in separate liquid phases. 

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