Bed pressure drop

Design Considerations. For a perforated plate, the pressure drop across the distributor should be at least 30% of the bed pressure drop when operating at the lowest expected gas velocity. The number of holes in the distributor should exceed 10 per square meter. The pressure drop, AP, across the distributor is given by... 

In pipe distributors, the pressure drop requited for good gas distribution is 30% of the bed pressure drop for upward facing holes, but only 10% for downward facing ones. The pressure drop calculation and the recommended hole density are the same as for a perforated plate. To maintain good gas distribution within the header system, it is recommended the relation... 

AP = Bed pressure drop, inches of water per foot of packing... 

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). 

Leva [40] has correlated the data of Lubin into correction factors to apply to a non-irrigated bed pressure drop to end up vith pressure drop for a liquid-gas system in the loading to flooding range. In general this does not appear any more convenient to use than Figure 9-2 ID. 

Dry bed pressure drop values usually run 0.1 to 0.5 in. water/ft of packing [96]. Use Equation 9-3 IB when Lf is below 20,000. Packings operate essentially dry when Lf is below 1,500 (about 3 gpm/ft2) at Fp = 20. Pressure drop at flooding is suggested to be predicted by Kister and Gill s relationship [93] presented in this text. 

AP(j = dry bed pressure drop, in. water/ft AP = operating pressure drop, in. liquid/ft e = base of natural logarithms Xi,X2 = curve fit coefficients for C2, Table 9-32. 

AP = Air pressure loss, in. of water APflood = Pressure drop at flood point for all random packings, in. of water/ft of packing height APd = Dry bed pressure drop, in. water/ft packed height... 

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. 

Fig. 6 shows the FFT spectrum for calculated bed pressure drop fluctuations at various centrifugal accelerations. The excess gas velocity, defined by (Uo-U ,, was set at 0.5 m/s. Here, 1 G means numerical result of particle fluidization behavior in a conventional fluidized bed. In Fig. 6, the power spectrum density function has typical peak in each centrifugal acceleration. However, as centrifugal acceleration increased, typical peak shifted to high frequency region. Therefore, it is considered that periods of bubble generation and eruption are shorter, and bubble velocity is faster at hi er centrifugal acceleration. 

The effect of gas velocity on the bed pressure drop (-APbed) with a uniform distributor (Fopen = 1.68 %) in the beds with decreasing and increasing Ug is shown in Fig. 5. As can be seen, -APbed maintains almost a constant value rmtil the minimum velocity of full fluidization (Unur) and then it decreases with decreasing Ug. As shown, Umfd is the maximum velocity of full defluidization, Umpf is the minimum velocity of partial fluidization, and Umu is the minimum velocity of full fluidization [6]. 

Figure 20. The variance of bed pressure drop versus superficial gas velocity.

The plenum, or windbox, is the chamber immediately below the grid. If the bed-pressure-drop-to-grid-pressure-drop ratio is high enough, the plenum design will probably not be that important. However, for the case where this ratio is marginal, the plenum design may determine whether the bed will operate satisfactorily. 

Magnetic particles may form much more stable beds when subjected to a magnetic field. Saxena and Shrivastava(51) have examined the complex behaviour of spherical steel particles of a range of sizes when subjected to fields of different strengths, considering in particular the bed pressure drop, the quality of fluidisation and the structure of the surface of the bed. 

The relafionship befween bed pressure drop and superficial fluidizing velocify is shown in Figure 1.3. As fhe gas velocity increases, the... 

Figure 1.3 Relationship between bed pressure drop and superficial fluidizing velocity.

The pressure drop across the plate AP, should be high to promote even gas distribution and stable fluidization and is usually some fraction of bed pressure drop AP although Kunii and Levenspiel (1991) point out that an excessive AP, has the disadvantage of significantly... 

For a shallow bed, fhe pressure drop across fhe disfribufor should be of fhe same order as fhe bed pressure drop (Richardson, 1971). 

At the point of incipient fluidization the drag force exerted on a particle is equal to its net weight. For the whole particle bed the drag force can be equafed fo the product of bed pressure drop AP and bed cross-secfional area A. The nef bed weighf is fhen the product of bed volume, nef densify, fhe fracfion of fhe bed (1 - e) which is occupied by parficles and fhe accelerafion due to gravity Thus, at minimum fluidizing velocity... 

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. 

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