Beds of solids

Gas—solids fluidization is the levitation of a bed of solid particles by a gas. Intense soflds mixing and good gas—soflds contact create an isothermal system having good mass transfer (qv). The gas-fluidized bed is ideal for many chemical reactions, drying (qv), mixing, and heat-transfer appHcations. Soflds can also be fluidized by a Hquid or by gas and Hquid combined. Liquid and gas—Hquid fluidization appHcations are growing in number, but gas—soHds fluidization appHcations dominate the fluidization field. This article discusses gas—soHds fluidization. 

FIG. 6-46 Friction factor for beds of solids. (From Leva, Fluidization, McGraw-Hill, New Yotk, 1959, p. 49. )... 

A pseudo-convective heat-transfer operation is one in which the heating gas (generally air) is passed over a bed of solids. Its nse is almost exchisively limited to drying operations (see Sec. 12, tray and shelf dryers). The operation, sometimes termed direct, is more aldu to the coudnctive mechanism. For this operation, Tsao and Wheelock [Chem. Eng., 74(13), 201 (1967)] predict the heat-transfer coefficient when radiative and conductive effects are absent by... 

Static This is a dense bed of solids in which each particle rests upon another at essentially the settled bulk density of the solids phase. Specifically, there is no relative motion among solids particles (Fig. 12-26). 

Gas-Solids Contacting Terms used in this section to describe the method by which gas may contact a bed of solids are the following ... 

FIG. 12-866 Vap or disengaging tray at the top of a gravity-hed catalytic reactor. This design may also he employed for the addition of gas to a bed of solids. 

Fluidization, or fluidizing, converts a bed of solid particles into an expanded, suspended mass that has many properties of a liquid. This mass has zero angle of repose, seeks its own level, and assumes the shape of the containing vessel. 

Fraction voids in bed of solids Dimensionless Dimensionless Dimensionless... 

Filtration is the separation of a fluid-solids mixture involving passage of most of the fluidthrough a porous barrier which retains most of the solid particulates contained in the mixture. This subsec tion deals only with the filtration of solids from liquids gas filtration is treated in Sec. 17. Filtration is the term for the unit operation. A filter is a piece of unit-operations equipment by which filtration is performed. The filter medium or septum is the barrier that lets the liquid pass while retaining most of the solids it may be a screen, cloth, paper, or bed of solids. The hquid that passes through the filter medium is called the filtrate. 

Granular Beds of Particulate Solids Beds of solids like sand or coal are used as filter media to clarify water or chemical solutions containing small quantities of suspended particles. Filter-grade grains of desired particTe size can be purchasea. Frequently beds will be constructed of layers of different materials and different particle sizes. 

In the steady-state operation of the OXITOX reactor, pelletized solid of catalytieally aetivated sodium earbonate slides down a Silo type reaetor. Counter-current to the solid flow, the polluted air rises through the sliding bed of solids. At reaction temperature the following reaetion oecurs ... 

When a fluid flows upward through a bed of solid particles, pressure drop across the bed increases as the flow rate increases. Eventually the pressure drop equals the weight of the bed (per unit horizontal area) at which point the particles are... 

Consider a gaseous reactant flowing through a bed of solid catalyst pellets. The physical steps involved are the transfer of the component... 

There is no striet proeedure for seale-up of a ehemieal reaetor by matliematieal modeling. Himmelblau [4] has reviewed the mathe-matieal modeling approaeh, and otliers speeifie examples are expatiated by Rase [5] and Carberry [6]. The following deseribe the steps applied to a seale-up problem as they relate to a eatalytie reaetor having a fixed bed of solid eatalysts. 

Fluidized bed A bed of solid particles floating on air or any other gas, on which combustible matter is burnt. 

Bed Density—p. The average density of a fluidized bed of solid particles and gas. Bed density is mainly a function of gas velocity and, to a lesser extent, the temperature. 

Minimum Fluidization Velocity (Umf). The lowest velocity at which the full weight of catalyst is supported by the fluidization gas. It is the minimum gas velocity at which a packed bed of solid particles will begin to expand and behave as a fluid. For an FCC catalyst, the minimum fluidization velocity is about 0.02 ft/sec. 

Ion exchange (IX) is a very useful technique for the concentration, the purification and the separation of chemically similar metallic elements present in an aqueous solution. In its most popular form of application, the metal-bearing aqueous solution is passed through a bed of solid organic resin in a particulate form wherein the sorption of the metal ions on the resin takes place by ion-exchange reactions. The pregnant resin is washed free of the entrapped feed solution and then brought into contact with an eluant of suitable composition and volume so that the resin releases the metal ions back to the eluant. The ratio of the volume of the feed and that of the eluant determines the extent of concentration that can be achieved. Purification and separation are achievable if the resin is selective or specific with respect to the metal ions of interest in comparison to impurity ions. 

When the feed is solids, it is important to present the material to the dryer in a form that will produce a bed of solids with an open, porous, structure. 

In this type of dryer, the drying gas is passed through the bed of solids at a velocity sufficient to keep the bed in a fluidised state which promotes high heat transfer and drying rates. 

In an open sorption storage system air is transporting water vapor and heat in and out of the packed bed of solid adsorbents (see Figure 235) or a reactor where the air is in contact with a liquid desiccant. In desorption mode a hot air stream enters the packed bed or the reactor, desorbs the water from the adsorbent or the salt solution and exits the bed cooler and saturated. In adsorption mode the previously humidified, cool air enters the desorbed packed bed or the... 

Figure 235. Open sorption storage system using a packed bed of solid adsorbent...

You want to fluidize a bed of solid particles using water. The particles are cubical, with a length on each side of 1/8 in. and on a SG of 1.2. 

Martin, H., Heat Transfer Between Gas Fluidized Beds of Solid Particles and the Surface of Immersed Heat Exchanger Element, Parts I II, Chem. Eng. Process, 18 157-169,199-223 (1984)... 

The low value of X indicates that the bed of solids is probably suspended above the coalesced jets. Therefore, the solids rarely come into contact with the grid. This type of design reduces the chances of grid pluggage due to sticky polyethylene solids. 

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