Lean gas phase

The lean/gas phase convection contribution has received the least attention in the literature. Many models in fact assume it to be negligible in comparison to dense phase convection and set hl to be zero. Compared to experimental data, such an approach appears to be approximately valid for fast fluidized beds where average solid concentration is above 8% by volume. Measurements obtained by Ebert, Glicksman and Lints (1993) indicate that the lean phase convection can contribute up to 20% of total... 

Fig. 7. Three systems engineering solutions for high-temperature catalytic combustion. A multiple monolith catalyst design B partial catalytic combustion C hybrid (catalytic + thermal) combustion LGC Lean gas-phase combustion.

Two independent pathway for selective and total oxidation exist. Under conditions of lean gas phases the total oxidation pathway exceeds in a rapid reaction the selective oxidation. 

Many different models and correlations have been proposed for the prediction of the heat transfer coefficient at vertical surfaces in FFBs. At time of this writing, no single correlation or model has won general acceptance. The following discussion presents a summary of some potentially useful approaches. It is helpful to consider the total heat transfer coefficient as eomposed of convective contributions from the lean-gas phase and the dense-particle phase plus thermal radiation, as defined by Eqs. (15) and (16). All eorrela-tions based on ambient temperature data, where thermal radiation is negligible, should be considered to represent only the convective heat transfer coefficient hr. 

The surface renewal model accounts for convective heat transfer by the dense-packet phase (clusters) and by the lean-gas phase. Calculate for the above case by this model and compare with the data of Dou et al. (1993) at Gs = 60kg/m s... 

Almost all of the models proposed to date are based on the two phase theory of fluidization originally proposed by Toomey and Johnstone (97) and later modified by Davidson and Harrison (98). According to the theory, the fliiidized bed is assumed to consist of two phases, viz., l) a continuous, dense particulate phase (emulsion phase) and 2) a discontinuous, lean gas phase (bubble phase) with exchange of gas between the bubble phase and emiilsion phase. The gas flow rate through the emulsion phase is assumed to be at minimum fluidization and that in excess of the minimum fluidization velocity passes throu the bubble phase. This formulation of the two phase theory is based on the ass mq)tion that the voidage of the emulsion phase remains constant. However, as pointed out by Rowe (22) and Horio and Wen (lOO) this assumption may be an over-simplification. In particular, experiments with fine powders (dp < 60 ym) conducted by Rowe show that the dense phase voidage changes with gas velocity, and as much as 30 percent of the gas flow occurs interstitially. This effect can be... 

In eatalytie eombustion of a fuel/air mixture the fuel reaets on the surfaee of the eatalyst by a heterogeneous meehanism. The eatalyst ean stabilize the eombustion of ultra-lean fuel/air mixtures with adiabatie eombustion temperatures below 1500°C. Thus, the gas temperature will remain below 1500 °C and very little thermal NO will be formed, as ean be seen in Figure 10-21. However, the observed reduetion in NOx in eatalytie eombustors is mueh greater than that expeeted from the lower eombustion temperature. The reaetion on the eatalytie surfaee apparently produees no NOx direetly, although some NOx may be produeed by homogeneous reaetions in the gas phase initiated by the eatalyst. 

Throughout this bocdt, several mass-exchange operations will be considered simultaneously. It is therefore necessary to use a unified terminology such that y is always the composition in die rich phase and x is the composition in the lean phase. The reader is cautioned here that tiiis terminology may be different ftom other literature, in which y is used for gas-phase composition and x is used for liquid-phase composition. 

Figure 26.56 is the corresponding plot for 12% inlet H2 in air. In this case, there is an extinction at about 1000 K for both reactors. The qualitative features are similar to that of the PSR discussed above for 28% H2 in air. For such fuel-lean mixtures, the flame is attached to the surface. As a result, the thermal coupling between the surface and the gas phase is strong, and reduction in surface temperature affects the entire thermal boundary layer resulting in significant reduction of NOj,. These results indicate that the bifurcation behavior, in terms of extinction, determines the role of flame-wall thermal interactions in emissions. 

The separation section will change accordingly (Figure 10.11). The lean gas with only 25% VAM from the initial gas and free of acetic acid is sent to the absorption column (C-l). The column (C-2) is no longer necessary. The bottom from (T-l) is led directly to (C-3). Less water means lower reflux from the three-phase decanter. In consequence, the duty of the column (C-3) can be reduced from 30 MW to about 8.5 MW, which represents a considerable energy saving of about 70%. In this way the reboiler duty of (C-3) can be covered entirely by the steam produced in the reactor. 

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