Overall circulation

In gas-liquid systems, the tendency for an increase in the gas superficial velocity upon scale-up can further increase the overall circulation time. 

The overall circulating pattern, including the circulation time and the deviation of the circulation times, can never be neglected. No matter what else a mixer does, it must be able to circulate fluid throughout an entire vessel appropriately. If it cannot, then that mixer is not suited for the task being considered. 

This implies that, superimposed on the circulatory flow at a fixed radius, there is an overall circulation perpendicular to the constant r plane. This overall circulation helps in randomizing the composition non-uniformity. 

The critical mixing factors in a stirred tank at e impeller speed and type, as well as their influence on local turbulence and overall circulation. Since all aspects of these factors cannot be maintained constant on scale-up either locally or globally, the extent to which changes in the crystallizing environment will affect nucleation is extremely difficult to predict. To the mixing issue must be added the uncertainties caused by soluble and insoluble impurities that may be present in sufficiently different concentrations from batch to batch to cause variation in induction time, nucleation rate, and particle size. 

The workhorse impeller is the pitched-blade mrbine because of its ability to create good circulation at relatively low shear. These attributes help reduce secondary nucleation and crystal breakage while achieving good suspension and circulation. The flat-blade turbine is less versatile because of high shear and less overall circulation. The Ekato Intermig has proven to have superior performance in some crystaUization operations because of its combination of excellent circulation and low shear. 

A latitude-height schematic cross section of the upper troposphere and lower stratosphere is shown in Figure 5.34. The lowermost stratosphere is sometimes referred to as the middleworld and that above the 380 K potential temperature level as the overworld. Tropospheric air enters the stratosphere principally in the tropics, as a result of deep cumulus convection, and then moves poleward in the stratosphere. By overall mass con-servation, stratospheric air must return to the troposphere. This return occurs in midlatitudes the overall circulation is called the Brewer-Dobson circulation. The midlatitude... 

As shown in Table 4 and Figure 5, the single PBTU impeller (Case 2) achieved a better performance with a higher initial rate and n/i aldehyde ratio accompanied by a lower power consumption than the single DT impeller (Case 1) under the same conditions of operation. This was possibly due to the better capability for overall circulation and lower agitation intensity of the PBTU impeller, which favored maintenance of the orderly micelle structure. When the novel SRFB was used (Cases 3 and 5), the initial rate of the biphasic hydroformylation increased remarkably from 0.630 10 kmol m (Case 1) to 1.24 10 and 2.03 10 kmol m respectively, by a factor of almost 2-3. However, the conversion and regioselectivity after 2 h of reaction were almost the same. Therefore, it is clear that an SRFB can increase the initial rate of 1-dodecene hydroformylation, but fails to further enhance... 

While the overall circulation of the mesocyclone extends outward 5-10 km from its axis of rotation, peak rotational wind speeds of 20-25 m sec typically are found at a radius of 2.5-3 km. During a mesocyclone s organizing stage, such rotational wind speeds, or alternatively vertical vorticity (>1 x 10 sec ), are found primarily at middle altitudes. Some mesocyclones do not develop beyond this stage. Those that do reach maturity ... 

The first two types of stirrers create an effective overall circulation in the vessel, the third one does not. They all create shear stresses, resulting in velocity gradients and deformation of the liquid. 

In all these cases, the mechanical agitating device serves two purposes at the same time it creates an overall circulation, which results in macro-mixings and it creates eddies or small scale circulations, causing shear or elongational flow which combined with molecular diffusion results in micro-mixing. 

In turbulent flow in stirred tanks, the overall circulation a) is usually the slowest process. The breaking down of eddies (b and c) proceeds faster the smaller the eddies become, and the diffusion within the smallest eddies d) is often the fastest process. The smallest eddies are usually smaller than 0.1 nun, so that the diffusion times are generally much smaller than 1 s (see p. 65). When the agitation rate is increased, die overall circulation and the bresddng down processes are enhanced, and at the same time the size of the smallest eddies is reduced, so that the diffusion process is also promoted. 

Laminar flow conditions are usually caused by high liquid viscosities. Despite considerable agitation, the flow rates are much reduc, so that the overall circulation a) is slower than under turbulent conditions. Much more important, however, is the fact that there is no turbulence. The size reduction of liquid volume elements is brought about mostly by elongation and shear b), combined with folding and rupturing. The thickness of the smallest volume elements may be on the order of severed mm, resulting in diffusion times d) that can be several minutes. 

A better construction for acquiring both shear and overall circulation is the use of a helix impeller. To promote effective circulation this is sometimes combined with a screw impeller, or with one or several propellers mounted on a separate shaft in the center. The screw or the propellers rotate with a higher speed than does the helix (figure 4.7). 

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