Stagnant flow zones

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. 

Vapor maldistribution. Most popular theoretical models (such as the AIChE and the Chan and Fair models, Sec. 7.2.1) postulate perfectly mixed vapor flow. In larga-diameter columns, vapor is more likely to rise in plug flow. Modeling work showed (143,179,180) that in the absence of stagnant zones on the tray, vapor flow pattern has generally little effect on tray efficiency. When column efficiency exceeds 30 percent (143), or when stagnant liquid zones exist (171,173,180), vapor plug flow reduces tray efficiency. 

Flow of water from fractures is often encountered in mines at different depths. Significant variations in the water properties suggest locations in the downflow zone. In contrast, in mines located in the lateral flow zone or in stagnant systems, uniform water properties are expected. 

In undisturbed systems pollutants reach the base flow zone and do not penetrate into deeper (below sea level) stagnant parts of the system. Thus the deeper stagnant parts below through-flow systems may serve as a strategic reserve of clean water. Lowering of the water table may ruin this reserve because it lets pollutants reach the deeper parts of the system. 

The construction of a combined model starts with one image (created, supposed or seeded) where it is accepted that the flow into the device is composed of distinct zones which are coupled in series or parallel and where we have various patterns of flow flow zones with perfect mixing, flow zones with plug flow, zones with stagnant fluid (dead flow). We can complete this flow image by showing that we can have some by-pass connections, some recycled flow and some slip flow situations in the device. 

It was assumed that after the step signal, essentially all the particles in the flowing zone are tracer particles (because the front moves very fast when compared with the smdied exchange phenomena). In contrast, the flow of particles from the stagnant zone contains both tracer and inert particles, that is,

stagnant zone, while refers to the whole stagnant zone. If

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In Chapter 2, the design of the so-called ideal reactors was discussed. The reactor ideahty was based on defined hydrodynamic behavior. We had assumedtwo flow patterns plug flow (piston type) where axial dispersion is excluded and completely mixed flow achieved in ideal stirred tank reactors. These flow patterns are often used for reactor design because the mass and heat balances are relatively simple to treat. But real equipment often deviates from that of the ideal flow pattern. In tubular reactors radial velocity and concentration profiles may develop in laminar flow. In turbulent flow, velocity fluctuations can lead to an axial dispersion. In catalytic packed bed reactors, irregular flow with the formation of channels may occur while stagnant fluid zones (dead zones) may develop in other parts of the reactor. Incompletely mixed zones and thus inhomogeneity can also be observed in CSTR, especially in the cases of viscous media. 

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, ... 

The zoning method offers better contaminant removal and thermal effectiveness than with mixing, limited control of the flow patterns in the ventilated zone, and the ability to avoid stagnant areas with high local concentrations in the ventilated zone. However, partial mixing of contaminants in the ventilated zone decreases its effectiveness. 

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. 

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