Solids mixing patterned

There is approximately a 22% deviation between the experimental and the distribution mean residenee time. However, the main purpose was to use the information from the RTD eurve to improve the reaetor operation. The results of the RTD provided vital information eoneern-ing the effeets of operating eonditions and struetural designs on solid-mixing patterns in fluidized systems. The perfeet mixing funetion was generated by e , where 6 = t/f. Figure 8-19 shows plots of these funetions against dimensionless residenee time 6. 

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. 

Grit Chambers Industries with sand or hard, inert particles in their wastewaters have found aerated grit chambers useful for the rapid separation of these inert particles. Aerated grit chambers are relatively small, with total volume based on 3-min retention at maximum flow. Diffused air is normally used to create the mixing pattern shown in Fig. 25-44, with the heavy, inert particles removed by centrifugal action and friction against the tank walls. The air flow rate is adjusted for the specific particles to be removed. Floatable solids are removed in the aerated grit chamber. It is important to provide for... 

The solids circulation pattern and solids circulation rate are important hydrodynamic characteristics of an operating jetting fluidized bed. They dictate directly the solids mixing and the heat and mass transfer between different regions of the bed. 

Yang, W. C., Ettehadieh, B., and Haldipur, G. B., Solids Circulation Pattern and Particles Mixing in a Large Jetting Fluidized Bed, AIChE J., 32 1994 (1986)... 

Solids mixed flow Fluid questionable flow pattern... 

Fig. 7. Basalt normalized REE distributions of the salt samples R 4250 to R 4254 from 0.1 to 4.7 m from the basalt contact. The solid lines show the measured REE distributions. Note the depletion of Ce, Pr. Sm, and Eu appearing with distance. The dashed lines are calculated mixing patterns discussed in the text. See Fig. 2 for sample locations.

The spray pattern is also important. A solid cone pattern mixes less well than a hollow cone, sheet, or multiple jet patterns. Drops that enter along the edge of the spray pattern nearest the exit have less time to heat up and evaporate. Drops that enter along the edges of the spray pattern nearest the walls may not fully evaporate before hitting the walls, resulting in erosion and corrosion of the brick. 

P 28] A 3-D solid model of the cross-shaped micro mixer is meshed to a sufficiently fine scale with brick elements of 2 pm for the simulations [71]. Simulation results were intended at very short time scales, e.g. in intervals of 50 ps, to verify the mixing patterns at the initial state after application of pressure. The numerical values of the mass fraction are taken to give quantitative measures of the mixing efficiency. The pre-processor fluidics solver and post-processor of ConventorWare were used for the simulations. The software FLUENT 5 was used for verification of these results, since the former software is so far not a widely established tool for fluid dynamic simulation. 

The two extreme hypotheses on mixing produce lumped models for the fluid dynamic behavior, whereas real reactors show complex mixing patterns and thus gradients of composition and temperature. It is worthwhile to stress that the fluid dynamic behavior of real reactors strongly depends on their physical dimensions. Moreover, in ideal reactors the chemical reactions are supposed to occur in a single phase (gaseous or liquid), whereas real reactors are often multiphase systems. Two simple examples are the gas-liquid reactors, used to oxidize a reactant dissolved in a liquid solvent and the fermenters, where reactions take place within a solid biomass dispersed in a liquid phase. Real batch reactors are briefly discussed in Chap. 7, in the context of suggestions for future research work. 

A time sequence of bubbling from such a calculation of the IGT six-inch EGO-33 run is shown in Figure 3. The sequence is that of a stationary or quasi-steady pattern of bubble evolution subsequent to the start-up transient. In each individual "frame" of the time sequence a reactor section, bounded by the centerline axis on the left and the reactor radius on the right, is shown. The representative particles are indicated by the black dots while the bubbles and the voids are white. The time corresponding to discrete frames is indicated on the base of the figure and the rise of the bubbles, together with the solids mixing, can be discerned in that sequence of frames. This... 

Ibrahim, S.B. Nienow, A.W. The effect of viscosity on mixing pattern and solid suspension in stirred vessels. I. Chem. E. Symp. Ser. 1994, 136, 25-32. 

The celebrated Kunii-Levenspiel [78, 79, 81, 82] (p 289) reactor model presented in this section was designed for the particular case of fast bubbles in vigorously bubbling beds which is relevant for industrial applications with Geldart A and AB solids. In such beds there are definite gross mixing patterns for the solid, downward near the wall and upward in the central core. This has a marked effect on the gas flow in the emulsion phase, which is also forced downward near the wall. However, based on experimental data analysis, Kunii... 

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