Stagnant zone

For turbines at Reynolds numbers less than 100, toroidal stagnant zones exist above and below the turbine periphery. Interchange of hq-uid between these regions and the rest of the vessel is principally by molecular diffusion. 

Slime is a network of secreted strands (extracellular polymers) intermixed with bacteria, water, gases, and extraneous matter. Slime layers occlude surfaces—the biological mat tends to form on and stick to surfaces. Surface shielding is further accelerated by the gathering of dirt, silt, sand, and other materials into the layer. Slime layers produce a stagnant zone next to surfaces that retards convective oxygen transport and increases diffusion distances. These properties naturally promote oxygen concentration cell formation. 

Beyond the third jet zone, there is a stagnant zone in which the velocity values are relatively uniform and have an unstable direction. There is reverse flow in zones I through III which is located between the jet boundaries and the cylinder walls. The maximum value of the velocity in the reverse flow is in the cross-section at the end of zone I at the distance Xj. The following equation was derived to calculate the length of the first zone X, ... 

This section will describe general features of airflow patterns and then present information on the dimensions and locations of recirculating (stagnant) zones around the building envelope, which determine wind pressures and contaminant dilution. This knowledge allows one to select the locations of stacks and air intakes and to calculate infiltration and natural ventilation rates. 

Design faults in two-pass condensers and heat exchangers that can cause corrosion include poor division plate seals allowing the escape of water at high velocity between the passes, and flow patterns that produce stagnant zones. 

FIGURE 15.7 Effect of a stagnant zone in a stirred tank reactor according to the side capacity model. 

Indeed, the fact that there are no dead zones indicates that there will be no accumulation of reactants, which increases the safety of the process by avoiding the development of hot spots. Moreover, a good mixing and the absence of stagnant zones will have a positive impact on yields and selectivity. 

More complex situations can be imagined, for example, the combination of stirred-tanks, bypassing and stagnant zones. Care must be taken to establish the model parameters carefully however and not to merely fit the tracer response data. Experimental E-curve data is given in Fig. 3.27 and a possible model for this is shown in Fig. 3.28. 

Hole Size. To increase the gas residence time in the bed, it is desirable to introduce the greatest number of small gas bubbles as possible into the bed. This can be achieved by maximizing N at the expense of dh in Eq. (10) (within the limits of mechanical, cost, and scale-up constraints). To minimize stagnant zones, the number of grid holes per m2 should be >10. In practice, the number of grid holes per square meter is generally about 2 to 3. 

At present, the chief value of RTD studies is for the diagnosis of the performance of existing equipment, for instance maldistribution of catalyst in a packed reactor, or the presence of bypassing or stagnant zones in stirred tanks. No correlations have been achieved for cr2(tr) or njrriang in terms of operating conditions, and only limited correlations for De. 

Stirred tank performance often is nearly ideal CSTR or the model may need to take into account bypassing, stagnant zones or other parameters associated with the geometry and operation of the vessel and the agitator. Sometimes the vessel can be visualized as a zone of complete mixing in the vicinity of the impellers followed by a plug flow zone elsewhere, thus a CSTR followed by a PFR. 

Use an agitated vessel equipped with a scraper for processing emulsions that are sensitive to heating or cooling, to minimized stagnant zones in the mixer. 

DISCUSSION AND CONCLUSIONS In this study a general applicable model has been developed which can predict mass and heat transfer fluxes through a vapour/gas-liquid interface in case a chemical reaction occurs in the liquid phase. In this model the Maxwell-Stefan theory has been used to describe the transport of mass and heat. A film model has been adopted which postulates the existence of a well-mixed bulk and stagnant zones where the principal mass and heat transfer resistances are situated. Due to the mathematical complexity the equations have been solved numerically by a finite-difference technique. In this paper (Part I) the Maxwell-Stefan theory has been compared with the classical theory due to Pick for isothermal absorption of a pure gas A in a solvent containing component B. Component A is allowed to react by a unimolecular chemical reaction or by a bimolecular chemical reaction with... 

The scale-up of trickle beds presents many difficulties mainly due to maldistribution of fluids, which leads to different routes for the liquid and gas, stagnant zones, and hot spots. In trickle-bed reactors, the particle diameter and residence time are the same for all scales. The consequence is that in different scales we have different Reynolds numbers and velocities. 

Thus soils which contain kaolinite in well-drained profiles will be montmorillonite-bearing in stagnant zones. This evolution is accounted for by a movement towards higher sodium activity in Figure 36. 

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