Falling flow pattern

In another land of ideal flow reactor, all portions of the feed stream have the same residence time that is, there is no mixing in the axial direction but complete mixing radially. It is called a.plugflow reactor (PFR), or a tubular flow reactor (TFR), because this flow pattern is characteristic of tubes and pipes. As the reaction proceeds, the concentration falls off with distance. 

In aerosol theory, is the velocity of free fall of a particle, and by extension in the current work is an empirical velocity related to the buoyancy of the contaminant in air. We further assume that the overall fluid flow pattern is unaffected by the minor quantity of the buoyant contaminant. 

The mechanism of suspension is related to the type of flow pattern obtained. Suspended types of flow are usually attributable to dispersion of the particles by the action of the turbulent eddies in the fluid. In turbulent flow, the vertical component of the eddy velocity will lie between one-seventh and one-fifth of the forward velocity of the fluid and, if this is more than the terminal falling velocity of the particles, they will tend to be supported in the fluid. In practice it is found that this mechanism is not as effective as might be thought because there is a tendency for the particles to damp out the eddy currents. 

Example 11.7 Carbon dioxide is sometimes removed from natural gas by reactive absorption in a tray column. The absorbent, typically an amine, is fed to the top of the column and gas is fed at the bottom. Liquid and gas flow patterns are similar to those in a distillation column with gas rising, liquid falling, and gas-liquid contacting occurring on the trays. Develop a model for a multitray CO2 scrubber assuming that individual trays behave as two-phase, stirred tank reactors. 

Figure 2.2 shows the cash flow pattern for a typical project. The cash flow is a cumulative cash flow. Consider Curve 1 in Figure 2.2. From the start of the project at Point A, cash is spent without any immediate return. The early stages of the project consist of development, design and other preliminary work, which causes the cumulative curve to dip to Point B. This is followed by the main phase of capital investment in buildings, plant and equipment, and the curve drops more steeply to Point C. Working capital is spent to commission the plant between Points C and D. Production starts at D, where revenue from sales begins. Initially, the rate of production is likely to be below design conditions until full production is achieved at E. At F, the cumulative cash flow is again zero. This is the project breakeven point. Toward the end of the projects life at G, the net rate of cash flow may decrease owing to, for example, increasing maintenance costs, a fall in the market price for the product, and so on.

In circulating fluidized beds two main attrition sources, namely the riser and the return leg, may be distinguished. Although a lot of information is available about solids flow patterns and flow structures inside the circulating fluidized bed risers, no systematic investigations have been found in the open literature on the influence of riser geometry and flow conditions inside the riser on attrition. With respect to attrition occurring in the return leg, the work of Zenz and Kelleher (1980) on attrition due to free fall may be mentioned (cf. Sec. 4.3). 

From a practical point of view, for improved solids distribution, the indications are for innovative design of obstructing structures next to the wall to break the falling sheet of solids in order to equalize their flow pattern across the column through repeated redistribution. 

Basic Breakup Modes. Starting from Lenard s investigation of large free-falling drops in still air,12671 drop/droplet breakup has been a subject of extensive theoretical and experimental studies[268] 12851 for a century. Various experimental methods have been developed and used to study droplet breakup, including free fall in towers and stairwells, suspension in vertical wind tunnels keeping droplets stationary, and in shock tubes with supersonic velocities, etc. These theoretical and experimental studies revealed that droplet breakup under the action of aerodynamic forces may occur in various modes, depending on the flow pattern around the droplet, and the physical properties of the gas and liquid involved, i.e., density, viscosity, and interfacial tension. 

Gravity is fiequendy important in controlling flow patterns, as lighter phases tend to rise and denser phases tend to fall in the reactor. 

If an isolated drop or bubble rises or falls in the reactor, then the flow pattern in this phase is clearly unmixed, and this phase should be described as a PFTR. However, drops and bubbles may not have simple trajectories because of stirring in the reactor, and also drops and bubbles can coalesce and breakup as they move through the reactor. 

These possible flow patterns of a drop or bubble phase are shown in Figure 12-12. At the left is shown an isolated bubble which rises (turn the drawing over for a falling drop), a clearly unrtuxed situation, ff the bubble is in a continuous phase which is being stirred, then the bubble wiU swirl around the reactor, and its residence time will not be fixed but will be a distributed function. In the limit of very rapid stirring, the residence time of drops or bubbles wiU have a residence time distribution... 

Figure 12-12 Sketches of possible flow patterns of bubbles rising through a liquid phase in a bubble column. Stirring of the continuous phase will cause the residence time distribution to be broadened, and coalescence and breakup of drops will cause mixing between bubbles. Both of these effects cause the residence time distribution in the bubble phase to approach that of a CSTR. For falling drops in a spray tower, the situation is similar but now the drops fall instead of rising in the reactor.

Internal circulation patterns have been observed experimentally for drops by observing striae caused by the shearing of viscous solutions (S7) or by photographing non-surface-active aluminum particles or dyes dispersed in the drop fluid [e.g. (G2, G3, J2, L5, Ml, SI)]. A photograph of a fully circulating falling drop is shown in Fig. 3.5a. Since the internal flow pattern for the Hadamard-Rybczynski analysis satisfies the complete Navier-Stokes equation... 

A question to be resolved in predicting efficiency concerns the liquid-flow pattern. It is usual practice to assume that the vapour is fully mixed, but there is a diversity of treatments of the liquid phase. The two limiting cases are completely-mixed-liquid and plug-flow-liquid. Achieved efficiencies on well designed trays usually fall between these cases. The assumption of a well-mixed tray liquid is only valid for the smallest trays (pilot scale). 

M 54] [P 48] CFD simulations for the flow in the separation-layer micro mixer predict a stable, almost irrotational flow pattern in the inlet region, which is in line with the experimental findings of a transparent region mentioned above [39], This pattern is maintained until the droplet end cap. Changes only occur when the droplet breaks up and falls, inducing rotational flow. 

The rovings in a plenum chamber are fed into a cutter and after being cut to the desired lengths, fall either into a plenum chamber or perforated screen where the air is exhausted from under the screen. A plastic binder of usually up to 5wt% is applied and is later cured. As the glass falls into the plenum chamber, the air flow pattern and baffles inside the screen control its distribution. Preform screen rotates and sometimes is tilted to ensure maximizing uniform deposits of the roving. 

Enclosures arc frequently encountered in practice, and heal transfer through them is of practical interest. Heat transfer in enclosed spaces is complicated by the fact that the fluid in the enclosure, in general, does not remain stationary. In a vertical enclosure, the fluid adjacent to the hotter surface rises and the fluid adjacent to the cooler one falls, setting off a rolationary motion within the enclosure that enhances heat transfer through the enclo.surc. Typical flow patterns in vertical and horizontal rectangular enclosures are shown in Figs. 9-21 and 9-22. 

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