Process Complexities in Scale-up

Successful scale-up means that larger scale operations are fiiUy anticipated and understood. Usually, the performance will be poorer than witnessed on a smaller scale. Scale-up must address several interdependent, flow-sensitive physical processes occurring simultaneously. These are dispersion, dispersion kinetics, coalescence, and drop suspension, as mentioned previously. 

1 Drop Suspension. A completely suspended condition is necessary to control and ensure a steady and predictable DSD. Segregation and layering in all cases lead to inferior results. The ease with which a suspension forms depends on phase density differences, agitation rate, impeller type/size, and its 

1 Flow Regintes Laminar, Transition, and Turbulent. Flow regimes are separated by the value of the Reynolds number. Re, the ratio of inertial to viscous forces. The impeller Reynolds number is 

Dilute Systems (]) 0.01. Ideally, a dilute system is one in which dispersion is affected only by hydrodynamics, and each drop is a single entity experiencing continuous phase fluid forces. Coalescence is neglected because few collisions occur. These simplifications enable a fairly fnndamental treatment of dispersion to be made. Coalescence can become significant for clean systems at c ) 0.05. 

Moderately Concentrated Systems ( ) 0.2. The behavior and technical treatment of systems in this concentration range depend on coalescence behavior. Ideal dilute dispersion theories may still apply, particularly if the system is noncoalescing. A simple test to detect coalescence is to agitate or shake a sample for 5 min and then watch it settle and coalesce. If only a dace of coalesced layer appears on the surface after 5 min, the system can be considered to be stable. The system is considered to be strongly coalescing if complete separation occnrs in less than 30 s. Obviously, many results fall between these limits. More details coalescence tests are given in Section 12-3.1.5. 

---