Gas Flow Pattern

The gas flow pattern in cyclones is fairly well known from experimental evidence collected over decades. For particle trajectories, on the other hand, very little experimental data are available, so for this we shall resort to computational fluid dynamics (CFD) simulations. 

The velocity field in cyclones has been measured using hot-wire anemometers, pitot tubes and, recently, laser-Doppler anemometry (LDA). See Chap. 10 for more information about these techniques. 

To the right in Fig. 3.1.1 the radial profiles of the axial and tangential gas velocity components are sketched. The former shows the outer region of downwardly directed axial flow and the inner one of upwardly directed flow. As mentioned, the downward velocity at the wall is the primary mechanism for particle transport out the dust outlet. The axial velocity often shows a dip aroimd the center hne. Sometimes this is so severe that the flow there is downwardly directed. The tangential velocity profile resembles a Rankine vortex a near loss-free swirl surrounding a core of near solid-body rotation. 

As the discussion in Sect. 2.1.1, and Eq. (2.A.12) show, in order for a rotating fluid element to maintain its equilibrium (static position in the r-direction), the pressure on its surface at higher r must exceed that on its surface a lower r. Thus the static pressure must increase monotonically with increasing radius. This, in fact, is borne out by experiment—a classic example of which is the data of Ter Linden (1953), a sample of which is presented in Fig. 3.1.2. Here the lower curves contained within each set of curves represents the variation in static pressure, p, with radial position the upper curves, the total pressure, p+(l/2)y0 (static plus dynamic). Comparing with Eq. (2.1.3) and realizing, as before, that the second term in Bernoulli s trinomial is small, we see from the profiles of total pressure in Fig. 3.1.2 that Bernoulli s trinomial is almost constant in the outer, nearly loss-free part of the vortex, while it decreases significantly in the center. This is as we would have expected. 

These data also show that the static pressme within the vortex finder also increases with radial position. This is also what we would expect, since there is still swirl present there. In this core region the velocity is approximately that of solid-body rotation, i.e. Eq. (2.1.1). Additionally, the static pressure is 

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Fig. 5. Schematic representation of gas flow pattern in the IMHEX design.

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

Processing variables that affect the properties of the thermal CVD material include the precursor vapors being used, substrate temperature, precursor vapor temperature gradient above substrate, gas flow pattern and velocity, gas composition and pressure, vapor saturation above substrate, diffusion rate through the boundary layer, substrate material, and impurities in the gases. Eor PECVD, plasma uniformity, plasma properties such as ion and electron temperature and densities, and concurrent energetic particle bombardment during deposition are also important. 

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. 

The introduction of changes in the gas flow pattern of the unit could provide a means for improvement in both, single and two-reactor systems. It is important to keep in mind that in counter-current designs measures have to be taken to prevent hot spots within the reactor bed and to improve vapor-liquid contact. 

One discouraging problem is the decrease in reactor or combustor performance when a pilot plant is scaled up to a larger commercial plant. These problems can be related to poor gas flow patterns, undesirable solid mixing patterns and physical operating problems (Matsen, 1985). In the synthol CFB reactors constructed in South Africa, first scale-up from the pilot plant increased the gas throughput by a factor of 500. Shingles and McDonald (1988) describe the severe problems initially encountered and their resolution. 

The existence and the movement of gas bubbles in a fluidised bed has an appreciable influence on the flow pattern within the bed. Several studies have been made of gas-flow patterns, although, in most cases, these have suffered from the disadvantage of having been carried out in small equipment where the results tend to be specific to the equipment used. 

The selection of a specific fuel cell pressure will affect numerous design parameters and considerations such as the current collector width, gas flow pattern, pressure vessel size, pipe and insulation size, blower size and design, compressor auxiliary load, and the selection of a bottoming cycle and its operating conditions. 

L.J. Giling. Gas Flow Patterns in Horizontal Epitaxial Reactor Cells Observed by Interference Holography. J. Electrochem. Soc., 129(3) 634-644,1982. 

Next, we focus on the effects of gas flow pattern. When the air inlet temperature in the co-flow pattern is the same as that in the standard case (counter-flow pattern),... 

The design of the jet spouted bed requires the rigorous definition of the gas flow pattern in order for the residence time distribution to be considered. In previous papers, the regime of jet spouted bed and its hydrodynamics correlations have been defined [2-8]. The minimum jet spouting velocity is calculated by the following correlation [7]. 

Figure 9.7. Bubble configurations and gas flow patterns around a bubble in gas-solid fluidized beds (a) Fast bubble (clouded bubble) Ub > /mf/ mf (b) Slow bubble (cloudless bubble)...

The reactant gas disperser is assembled from anodized aluminum plates of alternating geometry. A cross sectional view normal to the flow direction illustrating the gas flow pattern is shown in Figure 4. 

Gamblin, B., Newton, D., and Grant, C. X-Ray Characterization of the Gas Flow Patterns from FCC Regenerator Air Rising Nozzles, in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 595-600. Somerset, Pennsylvania (1993). 

Numerous studies on the kinetics and mechanisms of CVD reactions have been made. These studies provide useful information such as activation energy and limiting steps of deposition reactions which are important for the understanding of deposition processes. The main problem in the CVD kinetics studies is the complexity of the deposition process. The difficulty arises not only from the various steps of the CVD process but also from the temperature and concentration gradient, geometric effects, and gas flow patterns in the reaction zones. Exact kinetic analysis is therefore usually not possible as the kinetic data are reactor dependent. There are several possible rate-limiting factors but mass transport and surface kinetics control are the most... 

Gas flow pattern—cross-flow, cocurrent, or countercurrent... 

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