Grid region

Particle attrition not due to thermal or chemical reaction effects (i.e., mechanical attrition) occurs much more rapidly in the grid region of fluidized beds than in the bulk of the bed. This is due to high-velocity gas... 

Figure 11 Time-lapse sequence (side-view cross section from the center of a slit) of a sample plug as it is transported through the bead trap of the grid-SMEC and into the exiting flow channel. The red color indicates the higher flow rates of the grid region. Same four flow image of sample passage as in Figure 10.

Phases. Both two-phase and three-phase representations are widely used as shown schematically in Figure 1. In two-phase representations the dilate phase may represent bubbles alone, jets (in the grid region), or bubbles plus clouds. Three-phase representations generally use the scheme followed by Kunii and Leven-spiel (19) whereby bubbles, clouds, and "emulsion" (i.e. that part of the non-bubble bed not included in the clouds) are each treated as separate regions. As shown in Table II, all of these possibilities are represented in the models adopted by the authors in this symposium. There appears, however, to be an increasing tendency to adopt three phase models, probably as a result of experimental results (20) which showed that the Kunii and Levenspiel model gave a better representation of measured concen-... 

Elutriation or dilute phase phenomena Reactions in freeboard or grid region Models... 

Various grid region seiection methods have recently been proposed with the aim of significantly reducing the number of field variables and their mutual correlation. 

Grid Cell Occupancy Descriptors grid-based QSAR techniques (0 4D-QSAR analysis) grid region selection methods variable selection... 

As a useful polynomial expansion method we show the Chebyshev expansion scheme below. We consider the general case such as a dissociation process, in which a wavepacket may proceed out of the grid region set in advance. The time propagation operator of the wavepacket x) is expanded in terms of the modified Chebyshev polynomial [255],... 

It is worth pointing out that a general observation made by Chavarie and Grace (1975) was that none of the models (tested correctly) accounted for the considerable end effects observed at both the inlet and the outlet, i.e., near the distributor (grid region) and in the space above the bed surface (freeboard region). 

The grid region near the distributor and the freeboard space above the bed surface have bed hydrodynamics significantly different from the main body of the bubbling bed reactor. A number of authors have observed the abnormally high rate of reaction near the distributor, apparently due to the very high rate of interphase... 

Behie LA, Kehoe P. The grid region in a fluidized bed reactor. AIChE J 19 1070-1072, 1973. 

Their results indicated that the convective mechanism has a greater effect on the grid region mass transfer. Since bubbles are also smaller near the grid, they concluded that favorable gas-solid contacting occurs in this region primarily because of convective outflow followed by recapture. 

Pyle [1972] has compared several of these models, as is shown in Fig. 13.5.3-3. Johnsson et al. [1987] compared various bubbling bed models for the simulation of an industrial phthalic anhydride synthesis reactor. Simulation models have been extended to account for the flow of gas in the form of jets in the grid region [Behie and Kehoe, 1973 Grace and de Lasa, 1978]. The flow of solids in the freeboard region above the bed and prior to the cyclones was also studied [George and Grace, 1978]. 

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