Circulation time, shear rates

If the blending process is between two or more fluids with relatively low viscosity such that the blending is not affected by fluid shear rates, then the difference in blend time and circulation between small and large tanks is the only factor involved. However, if the blending involves wide disparities in the density of viscosity and surface tension between the various phases, then a certain level of shear rate may be required before blending can proceed to the required degree of uniformity. 

When comparing different impeller types, an entirely different phenomenon is important. In terms of circulation time, the phenomena shown in Figs. 18-18 and 18-19 stiU apply with the different impellers shown in Fig. 18-5. When it comes to blending another factor enters the picture. When particles A and B meet each other as a result of shear rates, there has to be sufficient shear stress to cause A and B to blend, react, or otherwise participate in the process. 

Solid-Liquid Mass Transfer There is potentially a major effect of both shear rate and circulation time in these processes. The sohds can either be fragile or rugged. We are looking at the slip velocity of the particle and also whether we can break up agglomerates of particles which may enhance the mass transfer. When the particles become small enough, they tend to follow the flow pattern, so the slip velocity necessary to affect the mass transfer becomes less and less available. 

Emulsions Almost eveiy shear rate parameter affects liquid-liquid emulsion formation. Some of the efrecds are dependent upon whether the emulsion is both dispersing and coalescing in the tank, or whether there are sufficient stabilizers present to maintain the smallest droplet size produced for long periods of time. Blend time and the standard deviation of circulation times affect the length of time it takes for a particle to be exposed to the various levels of shear work and thus the time it takes to achieve the ultimate small paiTicle size desired. 

The measured values of LH appreciably increased with increasing shear rate, and those at the highest shear rate were 3-6 times as large as the diameter of a free polymer in the bulk solution. To explain this result Gramain and Myard considered that there should exist a net circulation of solvent in the adsorbed layer to elongate adsorbed polymers axially. This effect reduces the number of trains to about 3 or 4 per polymer chain. 

If it appears that upon a qualitative examination that the role of circulation time, blend time, and shear rate may not be important to the process on scaleup, then go ahead and use geometric similarity on the pilot plant study and all of these differences noted above will play no part in the results of the scaleup prediction. It may be that there are compensating effects that while circulation time becomes longer and shear rates become larger, there is a compensating effect that makes the process result satisfactory. 

Figure 24 illustrates possible flocculation and chaining of particles in flow conditions. Larger particles close to the walls of the vessels experience greater shearing forces because of the nature of the flow patterns shown. Particles that adhere to er5dhrocytes move with them until detachment, often prolonging their own circulation times. Adhesion, seen as a prerequisite to cellular uptake from blood and interstitial fluids is not a foregone conclusion. The probability of adhesion, Padheaon can be written phenomenologically as in Figure 24. The factors include particle diameter, flow rate, the density of receptors, and the force of attraction between particle and receptor.

If data are available on a fermentation in a production-size tank, scaleup may be made by increasing, in a relative proportion, the various mass transfer, blending and shear rate requirements for the full-scale system. For example, it may be determined that the new production system is to have a new mass transfer rate of x% of the existing mass transfer rates, and there may be specifications put on maximum or average shear rates, and there may be a desire to look at changes in blend time and circulation time. In addition, there may be a desire to look at the relative change in CO2 stripping efficiency in the revised system. 

The shear experiments were carried out with a carefully constructed plane-parallel flow cell. Details of the shear circuit have been reported previously (12). The recirculation and roller pump sections, accounting for much of the circulation duty cycle, had 3-5 times the test section wall shear rate. The shear system loading, exposure, and wash steps were analogous to those for the static exposure studies. Test surfaces were exposed to one of the following calculated wall shear rates 0, 100, 500, 800, and 1500 s 1, for 1 h. Wash steps were carried out at a calculated wall shear rate at the test section of 25 s-1. The exposed surfaces were critical-point dried, as described for the static exposure studies. The wall shear rate calculation assumed a steady, plane-parallel flow with no edge effects, and a parabolic velocity profile. 

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