Packed beds pressure drop

Irreversible, gas-phase, adiabatic, no pressure drop, packed bed. Part (a) Isothermal... 

Ford6 measured the pressure drop in beds packed with approximately l-mm-diameter particles. He identified the flow regime based on the flow that takes place inside the pores, namely, single-phase pore flow and two-phase pore flow. He presented correlations... 

An approximate method for determining pressure drop in beds of random or structured packings has been given (Figure 13.35). All... 

In this equation AT is a constant, characteristic of packing type and size. Pressure drops through beds of random packings have been correlated by these equations,21-23 but it is not yet possible to point toward a particular piece of work that gives reliable results for the foil range of possible packings,... 

Example 4.3 Pressure Drop in Beds Packed with First- and Third-Generation Random Packings... 

Macroscopic properties often influence tlie perfoniiance of solid catalysts, which are used in reactors tliat may simply be tubes packed witli catalyst in tlie fonii of particles—chosen because gases or liquids flow tlirough a bed of tliem (usually continuously) witli little resistance (little pressure drop). Catalysts in tlie fonii of honeycombs (monolitlis) are used in automobile exliaust systems so tliat a stream of reactant gases flows witli little resistance tlirough tlie channels and heat from tlie exotlieniiic reactions (e.g., CO oxidation to CO,) is rapidly removed. 

Venturi scmbbers can be operated at 2.5 kPa (19 mm Hg) to coUect many particles coarser than 1 p.m efficiently. Smaller particles often require a pressure drop of 7.5—10 kPa (56—75 mm Hg). When most of the particulates are smaller than 0.5 p.m and are hydrophobic, venturis have been operated at pressure drops from 25 to 32.5 kPa (187—244 mm Hg). Water injection rate is typicaUy 0.67—1.4 m of Hquid per 1000 m of gas, although rates as high as 2.7 are used. Increasing water rates improves coUection efficiency. Many venturis contain louvers to vary throat cross section and pressure drop with changes in system gas flow. Venturi scmbbers can be made in various shapes with reasonably similar characteristics. Any device that causes contact of Hquid and gas at high velocity and pressure drop across an accelerating orifice wiU act much like a venturi scmbber. A flooded-disk scmbber in which the annular orifice created by the disc is equivalent to a venturi throat has been described (296). An irrigated packed fiber bed with performance similar to a... 

The basic concepts of a gas-fluidized bed are illustrated in Figure 1. Gas velocity in fluidized beds is normally expressed as a superficial velocity, U, the gas velocity through the vessel assuming that the vessel is empty. At a low gas velocity, the soHds do not move. This constitutes a packed bed. As the gas velocity is increased, the pressure drop increases until the drag plus the buoyancy forces on the particle overcome its weight and any interparticle forces. At this point, the bed is said to be minimally fluidized, and this gas velocity is termed the minimum fluidization velocity, The bed expands slightly at this condition, and the particles are free to move about (Fig. lb). As the velocity is increased further, bubbles can form. The soHds movement is more turbulent, and the bed expands to accommodate the volume of the bubbles. 

When two phases are present the situation is quite complex, especially in beds of fine soHds where interfacial forces can be significant. In coarse beds, eg, packed towers, the effects are often correlated empirically in terms of pressure drops for the single phases taken individually. 

Packed fiber bed mist eliminators can be designed to operate at almost any desired particle collection efficiencies, depending on the allowable pressure drop and cost. A good discussion of sulfuric acid mist generation, control, and mist eliminator design is available (109,110). 

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. 

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