Gas dense-phase

With type B solids, Reay and Baker suggest that bed depth does have a significant effect increasing bed depth increases the humidity of fhe ouflet gas. Therefore, in fhese cases, even fhe dense-phase gas is nof af equilibrium with the moisture in the bed particles. In contrast to type A solids, gas velocity has no effect on the drying rate but there is a significant effect of parficle size drying rate is proportional to the diameter squared. These observations are explained by the high internal resistance to the movement of water and the slow diffusion of water to the particle surface. 

This term is restricted here to equipment in which finely divided solids in suspension interact with gases. Solids fluidized by liquids are called slurries. Three phase fluidized mixtures occur in some coal liquefaction and petroleum treating processes. In dense phase gas-solid fluidization, a fairly definite bed level is maintained in dilute phase systems the solid is entrained continuously through the reaction zone and is separated out in a subsequent zone. 

The interchange between bubble-phase gas and dense-phase gas is by bulk flow and diffusion. 

R.J. Hitt, P. Marjanovic, A computer technique to develop a model of vertical dense phase gas-solids flow from experimental data. Proceedings of the 1st International Symposium on Two-Phase Flow Modelling and Experimentation, vol. 1, Rome Italy, October 1995, pp. 67-74. 

Cai, P. The transition of flow regime in dense phase gas-solid fluidized bed, Ph. D. Thesis, Tsinghua University (China) and Ohio State University (U.S.A.) (1989). 

Evidence for the two-phase model came from measurements of the gas concentration profile in a commercial catalytic cracking regenerator 40 ft in diameter with a 15-ft bed [8]. The exit gas had 1% O2, but samples drawn from different bed depths had only 0.1-0.4% O2. The bed samples also showed 12-14% CO2, compared to 10% CO2 in the exit gas. Although most of the gas flow was in the bubbles, the probe saw mainly dense-phase gas, where the conversion was higher than in the bubbles. Samples taken very rapidly showed wide fluctuations in oxygen content, since the probe was sometimes in a bubble and sometimes in the dense bed. 

Corresponding to the bimodal distribution, at superficial gas velocity higher than the transport velocity (Grace et al, 2006), a monodisperse, air-fluidized gas—sohd two-phase flow mixture can be classified into dense clusters (denoted by subscript c) and dilute broth (denoted by subscript f) (Li and Kwauk, 1994). The dense clusters are dispersed in the continuous dilute phase. We may refine this broth-cluster structure by defining four continua or structural subelements, as shown in Fig. 6, namely, the dense-phase gas (denoted by subscript gc), the dense-phase solid (denoted by subscript pc), the dilute-phase gas (denoted by subscript gf), and the dilute-phase solid (denoted by subscript pf). The volume fraction of the dense phase is defined by f. The void fractions in the dilute and dense phases are denoted by ggf and e c, respectively. Then the sofids volume fraction is pf=l-egf in the dilute phase and pc = l- gc in the dense phase. The velocities with respect to the gas/sofids in the dilute and dense phases are tigf, tipf. tigc, and Upc, respectively. 

As presented in Section 5.1, the gas—sohd flow in a local ceU can be divided into four parts dilute-phase gas, dilute-phase soHd, dense-phase gas, and... 

Model of Davidson and Co-workers Hovmand and Davidson ( ) proposed a two-phase model and compared the predictions of that model with the experimental data of Hovmand and Davidson (T) and Hovmand, Freedman and Davidson (9 ). The model assumes co-current piston-flow of bubble gas (U-U ) and dense-phase gas with... 

In dilute-phase regimes the principle of using a venturi and orifice meter in series (the Graczyk or BCR meter) can be employed. In this arrangement the venturi meter will measure the acceleration of the solids and the orifice meter will measure only the gas flow, since with closely placed pressure tops the particles will undergo little acceleration. This can be a relatively inexpensive solution to a gas-solid flow problem, especially if a venturi is already on hand (see Fig. 7-2). For dense-phase gas-solid flow or systems having sizable upsets, the orifice meter-venturi setup is subject to frequent plugging with solids. The unit is also applicable only for small-diameter lines. 

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