Fluid Bed Pressure Drop

The pressure drop of a fluid bed can be simply calculated by dividing the gravitational force resulting from the particle bulk by the cross-sectional area Ag, of the fluid bed [134]. The gravitational force must be corrected by buoyancy and requires the mass of the solid bulk nts. 

Also the void fraction of a fluid bed eg, and its height Hg, are related to the pressure drop. 

All other variables used are the same as explained in Section 3.12.4.3. 

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FIG. 12-69 Fluid-bed pressure drop versus fluidizing velocity. (Niro A/S.)... 

Fluid-bed pressure drop and temperature Fluidizing steam Heating steam Coal grain size (feed) Coal grain size (output) Dry coal moisture... 

Darey s law (Darey, 1856) relates fluid flowrate to bed pressure drop, depth and permeability... 

Again, an alternative approaeh to the predietion of bed pressure drop and fluid flow in porous media is to use frietion faetors (the analogue of the drag eoeffieient developed for partiele flow above). 

So far, some researchers have analyzed particle fluidization behaviors in a RFB, however, they have not well studied yet, since particle fluidization behaviors are very complicated. In this study, fundamental particle fluidization behaviors of Geldart s group B particle in a RFB were numerically analyzed by using a Discrete Element Method (DEM)- Computational Fluid Dynamics (CFD) coupling model [3]. First of all, visualization of particle fluidization behaviors in a RFB was conducted. Relationship between bed pressure drop and gas velocity was also investigated by the numerical simulation. In addition, fluctuations of bed pressure drop and particle mixing behaviors of radial direction were numerically analyzed. 

Figure 3.53 corresponds to an upflow operation, where the fluidized-bed pressure drop remains constant after the minimum fluidization velocity. On the contrary, if a fixed bed is operated in downflow mode, the pressure drop continues to increase by increasing the fluid velocity (dense line). This is the reason that fluidized beds may exhibit a lower pressure drop and thus the power cost is lower, for high fluid velocities. 

The first four types, pellets, extrudates. spheres, and granules, are primarily used in packed bed operations. Generally, the larger the particle diameter, the cheaper the catalyst. But this is usually not a significant factor for the process designer. More important are uniform fluid flow, pressure drop, and diffusional effects. 

External Stresses Caused by Fluid FloWf Pressure Drop Catalyst Bed Weight, and Thermal Cycling... 

Figure 6W1.1 Plot of packed bed pressure drop versus superficial fluid velocity...

Equating this expression for pressure drop across the packed bed to the fluidized bed pressure drop, we determine superficial fluid velocity at incipient fluidization, Umf. 

Gas mixing in laboratory internal recycle reactors used for gas-solid catalytic studies may be assessed from pressure drop (Berty, 1974 Berty, 1979), temperature drop measurements across the bed (Mahoney, 1984), or from mass transfer coefficient estimations (Caldwell, 1983). For a given impeller speed, the first method involves comparing the bed pressure drop of the recycle reactor v/iih pressure drop of a calibrated fixed catalyst bed conducted in a separate unit. Then knowing the fluid velocity versus pressure drop for the calibrated bed, the impeller speed versus fluid velocity can be drawn. The recycle rate can also be determined from thermodynamics based on the ratio of the adiabatic temperature change and the measured temperature difference. This method requires the measurements of temperatures across the bed and the mass flow rate. 

For dense suspensions of s<0.8 the following Ergun [6] correlation for the bed pressure drop in a steady state (i.e. dy/dt=0) can be used (note if. fluid viscosity) ... 

Pressure Drop. The pressure drop across a two-phase suspension is composed of various terms, such as static head, acceleration, and friction losses for both gas and soflds. For most dense fluid-bed appHcations, outside of entrance or exit regimes where the acceleration pressure drop is appreciable, the pressure drop simply results from the static head of soflds. Therefore, the weight of soflds ia the bed divided by the height of soflds gives the apparent density of the fluidized bed, ie... 

Solid Desiccants. The sohd desiccants used in dynamic appHcations fad into a class caded adsorbents (see Adsorption). Because they are used in large packed beds through which the gas or Hquid to be treated is passed, the adsorbents are formed into soHd shapes that adow them to withstand the static (fluid plus sohd head) and dynamic (pressure drop) forces imposed on them. The most common shapes are granules, extmded pedets, and beads. 

Porous Media Packed beds of granular solids are one type of the general class referred to as porous media, which include geological formations such as petroleum reservoirs and aquifers, manufactured materials such as sintered metals and porous catalysts, burning coal or char particles, and textile fabrics, to name a few. Pressure drop for incompressible flow across a porous medium has the same quahtative behavior as that given by Leva s correlation in the preceding. At low Reynolds numbers, viscous forces dominate and pressure drop is proportional to fluid viscosity and superficial velocity, and at high Reynolds numbers, pressure drop is proportional to fluid density and to the square of superficial velocity. 

Fluidization occurs when the fluid flow rate is great enough so that the pressure drop across the bed equals the weight of the bed. As stated previously, the solids film thickness adjacent to the wall is difficult to measure and/or predict. Wen and Fan [Chem. Eng., 64(7), 254 (1957)] give external walls ... 

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