Shallow fluid beds

Activated Solids Shallow Fluid Bed Heat Exchanger... 

Figure 31 (Liu, Liu, Li and Kwauk, 1986) shows a cylindrically shaped shallow fluid bed tubular heat exchanger. Solid particles are fluidized with a small stream of activating gas Ga, so as to insure maximal heat transfer between the particles and the exchanger tube wall. The waste gas Gw, from which heat is to be extracted, passes through the solid...

Figure 31. Activated Solids Shallow fluid-bed heat exchanger. (Liu, Liu, Li, and Kwauk, 1986.)...

For slow reactions, the shallow fluid beds have been organized into a cocurrent multistage fluid bed (MSFB) reactor as shown in Fig. 33 (Yan, Yao, Wang, Liu and Kwauk, 1983). In this reactor, solids are carried up by the flowing gas stream, and once they reach the top, they are collected through a funnel and recirculated to the bottom by means of a pneumatically controlled downcomer. 

Liu, D., Liu, J., Li, T., and Kwauk, M., Shallow-Fluid-Bed Tubular Heat Exchanger, Fifth Int. Fluidization Conf, p. 401M08, Elsinore, Denmark... 

The deep bed, as its name implies, is similar to the shallow bed but in this case may be up to 3 meters deep in its fluidized state, making it suitable only for large boilers. Similarly, the recirculating fluid bed is only applicable to large watertube boilers. 

The catalytic oxidation of benzene to maleic anhydride in a fluid bed reactor has been investigated by Kizer, Chavarie, Laguerie and Cassimatis (9). Their reactor had an inner diameter of 18 cm. Shallow beds of catalyst with bed heights less then 10 cm were used with reaction temperatures between 420 and 460 °C. The catalyst was silica supported VjOj. 

The extraction of toluene and 1,2 dichlorobenzene from shallow packed beds of porous particles was studied both experimentally and theoretically at various operating conditions. Mathematical extraction models, based on the shrinking core concept, were developed for three different particle geometries. These models contain three adjustable parameters an effective diffusivity, a volumetric fluid-to-particle mass transfer coefficient, and an equilibrium solubility or partition coefficient. K as well as Kq were first determined from initial extraction rates. Then, by fitting experimental extraction data, values of the effective diffusivity were obtained. Model predictions compare well with experimental data and the respective value of the tortuosity factor around 2.5 is in excellent agreement with related literature data. 

There is another factor that needs to be taken into consideration the need to reduce oil vapor degradation (Wish 1), requiring us to reduce the oil vapor residence time in the reactor. In bubbling fluidized beds (R-2) and turbulent fluid beds (R-3), the bubble rise velocities are of the order of 1.5 m/s, and in a shallow bed of, say, 2 m the oil vapor residence time may be restricted to below 2 s. 

Shallow beds (less than IS cm deep) are constructed in the form of a long channel of length, L, width, W. Reay has shown that fluid bed dryers up to 10 cm deep can be considered as consisting of a number, n, of perfectly mixed beds in series, where ... 

The use of large deep trays or crucibles, probably covered, for calcination accentuates these problems, and different sections of the sample will react at different temperatures and times. Shallow trays in a flowing atmosphere are an improvement, but tumbling and rotating tube furnaces or fluid bed calciners are obviously superior. For some reversible reactions it may be beneficial to perform a vacuum calcination for the ultimate removal of the product gases. 

Comparison with Mechanical Blenders or Homogenizers. Mechanical blenders normally have recirculation systems either within the blender or placed outside it. There are several types, but they have in common steep hopper half-angles with respect to vertical as compared to fluid bed blenders. This is required to allow all the mixed material to flow out of the blender. The silo geometry has to ensure mass flow conditions, which accounts for even withdraw of products, but at the same time, the angles have to be somewhat shallower to allow shear between the flowing layers of products to cause blending. If the outer hoppers are shallower, then normally, blenders are equipped with inserts, such as hopper-in-hopper types. These inserts can work in both axisymmetric and plane flow blenders, although the former types are more common. 

In low pressure gas phase applications such as vent stream cleaning and solvent recovery, high pressure drops are liable to occur due to the low density and high velocity of the fluid. To overcome this problem very shallow adsorption beds are sometimes used. Adsorption beds with low aspect ratios, however, may result in flow distribution problems, and therefore some of the pressure drop that is saved must be sacrificed by adding flow distribution systems of manifolds, baffles and screens. Other ways of keeping the pressure drop to a minimum in low pressure gas phase applications include the use of specially shaped adsorbents such as the trilobe and monolithic materials shown in Figure 2.7 in Chapter 2. 

The flow pattern of fluids in gas-liquid-solid (catalyst) reactors is often far from ideal. Special care must be taken to avoid by-passing of the catalyst particles near the reactor walls, where the packing density of the catalyst pellets is lower than in the centre of the bed. By-passing becomes negligible if the ratio of reactor to particles diameter is larger than 10 a ratio of 20 is recommended. Flow maldistributions might be serious in the case of shallow beds. Special devices must be used to equalize the velocity over the cross-section of the reactor before reactants are introduced onto the catalyst bed. 

In many respects, the solutions to equations 12.7.38 and 12.7.47 do not provide sufficient additional information to warrant their use in design calculations. It has been clearly demonstrated that for the fluid velocities used in industrial practice, the influence of axial dispersion of both heat and mass on the conversion achieved is negligible provided that the packing depth is in excess of 100 pellet diameters (109). Such shallow beds are only employed as the first stage of multibed adiabatic reactors. There is some question as to whether or not such short beds can be adequately described by an effective transport model. Thus for most preliminary design calculations, the simplified one-dimensional model discussed earlier is preferred. The discrepancies between model simulations and actual reactor behavior are not resolved by the inclusion of longitudinal dispersion terms. Their effects are small compared to the influence of radial gradients in temperature and composition. Consequently, for more accurate simulations, we employ a two-dimensional model (Section 12.7.2.2). 

Analytical solutions for x and y as functions of the bed-length, z, and time, t, are available [45,52], The expressions are a useful extension of two-phase model applied to plug-flow. These two models are appropriate in describing the extraction of crushed or broken seeds to recover the seed oil, either in shallow beds or in plug flow. As shown by Sovova [52], applying the plug-flow model requires corrections for non-ideal residence-time distribution (non-plug flow) of the fluid in contact with the solid. 

The Tadmor model assumes Newtonian fluids and shallow channels. The channel cross section and that of the solid bed are assumed to be rectangular. The width of the solid bed profile is denoted by X(z), which is the the main objective that we are seeking with the model. The solid bed that develops at steady state conditions is the focal interest here. Furthermore, Tadmor assumed that melting only occurs at the barrel surface and the solid bed is homogeneous, continuous and deformable. 

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