Bed Pressure

At the completion of adsorption, the less selectively adsorbed components have been recovered as product. However, a significant quantity of the weaMy adsorbed species are held up in the bed, especially in the void spaces. A cocurrent depressurization step reduces the bed pressure by allowing dow out of the bed cocurrendy to feed dow and thus reduces the amount of product retained in the voids (holdup), improving product recovery, and increases the concentration of the more strongly adsorbed components in the bed. The purity of the more selectively adsorbed species has been shown to depend strongly on the cocurrent depressurization step for some appHcations (66). A cocurrent depressurization step is optional because a countercurrent one always exists. Criteria have been developed to indicate when the use of both is justified (67). 

The Flat-bed pressure filter (Hydromation Engineering Co. Ltd.) (19) is based on the above principle. The pressure compartment consists of two halves, top and bottom. The bottom half is stationary while the top half can be raised to allow the belt and the cake to pass out of the compartment, and can be lowered onto the belt during the filtration and dewatering stage. The filter can be considered as a horizontal filter press with an indexing cloth in comparison with a conventional filter press, however, this filter allows only the lower face of the chamber to be used for filtration. 

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... 

If the solution is allowed to flow through a granular bed such as sand, the larger particulate matter remains on the surface, while the smaller material is collected in the thickness of the granular bed. Pressurization of the filter accelerates the process. Besides sand, other materials used as filtering media are anthracites, manganese dioxide, and activated carbon. 

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). 

Figure 2.12 Packed bed pressure loss Euler number versus particle Reynolds number (Ergun, 1952)...

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... 

Returning now to the subject of the chapter, in addition to appropriate retentive characteristics, a potential stationary phase must have other key physical characteristics before it can be considered suitable for use in LC. It is extremely important that the stationary phase is completely insoluble (or virtually so) in all solvents that are likely to be used as a mobile phase. Furthermore, it must be insensitive to changes in pH and be capable of assuming the range of interactive characteristics that are necessary for the retention of all types of solutes. In addition, the material must be available as solid particles a few microns in diameter, so that it can be packed into a column and at the same time be mechanically strong enough to sustain bed pressures of 6,000 p.s.i. or more. It is clear that the need for versatile interactive characteristics, virtually universal solvent insolubility together with other critical physical characteristics severely restricts the choice of materials suitable for LC stationary phases. 

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]. 

Fitzgerald et al. (1984) measured pressure fluctuations in an atmospheric fluidized bed combustor and a quarter-scale cold model. The full set of scaling parameters was matched between the beds. The autocorrelation function of the pressure fluctuations was similar for the two beds but not within the 95% confidence levels they had anticipated. The amplitude of the autocorrelation function for the hot combustor was significantly lower than that for the cold model. Also, the experimentally determined time-scaling factor differed from the theoretical value by 24%. They suggested that the differences could be due to electrostatic effects. Particle sphericity and size distribution were not discussed failure to match these could also have influenced the hydrodynamic similarity of the two beds. Bed pressure fluctuations were measured using a single pressure point which, as discussed previously, may not accurately represent the local hydrodynamics within the bed. Similar results were... 

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. 

Jiang, L., V.G. Fox, and L.T. Biegler, Simulation and optimal design of multi-Bed pressure swing adsorption systems, AIChE ]., 50, 2904-2917, 2004. 

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