Flow regime complete dispersion

Mixing processes involved in the manufacture of disperse systems, whether suspensions or emulsions, are far more problematic than those employed in the blending of low-viscosity miscible liquids due to the multi-phasic character of the systems and deviations from Newtonian flow behavior. It is not uncommon for both laminar and turbulent flow to occur simultaneously in different regions of the system. In some regions, the flow regime may be in transition, i.e., neither laminar nor turbulent but somewhere in between. The implications of these flow regime variations for scale-up are considerable. Nonetheless, it should be noted that the mixing process is only completed when Brownian motion occurs sufficiently to achieve uniformity on a molecular scale. 

In actual practice, a truly plug-flow or completely mixed-flow regime in a reactor is not attained because of longitudinal dispersion and nonideal mixing conditions. The equations reported here approximate the actual conditions in the held. 

For a given gas flow rate, the dispersion pattern of gas bubbles in the working media depends on the interplay between the isothermal expansion energy of the gas and the hydraulic power of the impeller. One of three bubble dispersion scenarios is typically expected, i.e., flooding, dispersion in the upper region only, or complete dispersion. Fig. 7 illustrates each of these flow regimes. 

Dispersed droplet flow. In this regime, which can only normally exist in heated systems, the liquid phase is completely dispersed as droplets in the vapor. 

Figure 6 Loading regimes and transitions for radial flow impellers, where AlpL indicates a transition from flooded to loaded regimes and A/ d defines the transition to completely dispersed flow regime (Jade et al., 2006).

CD Impeller speed at which a transition in flow regime occurs from loaded to completely dispersed (rev/ min)... 

The residence time distribution (RTD) is a probability distribution function used to characterize the time of contact and contacting pattern (such as for plug-flow or complete backmixing) within the reactors. Excessive retention of some elements and shortdrcuiting of others due to backmixing and other dispersive phenomena lead to a broad distribution in the residence times of individual molecules in the reactor. This tends to decrease conversion and exerts a negative influence on product selectivity/yield. The RTD depends on the flow regime and is characterized by Reynolds (Re) and Schmidt (Sc) numbers. 

The value of flow regime on the tray. The tray is said to operate in the spray regime when the liquid is dispersed almost completely into small droplets by the action of the gas jets. This occurs at high gas velocities and low liquid loads. In the spray regime... 

CD Complete dispersion F flooding HB Herschel-Bulkley t turbulent flow regime... 

---