Packed-bed

A packed bed is understood to be the ordered or irregular arrangement of individual bodies of different shapes. As an example of this Fig. 3.36 shows a packed bed of particles of different sizes. A pipe register is also a packed bed in the sense of this definition. 

In fluidised beds the particles are mixed up by a flowing fluid and kept in suspension. They then have properties similar to that of a fluid. Chemical reactions, drying or other mass transfer processes take place rapidly in fluidised beds as a result of the brisk movement of the particles. 

Packed beds serve as regenerators in heat transfer. As so-called packed columns they are frequently implemented as mass transfer apparatus. This normally involves the introduction of a liquid mixture at the top of a column with a gas of different composition flowing in the opposite direction, as illustrated in Fig. 3.37. Through mass transfer one or more components of the gas are transferred into the 

3 Convective heat and mass transfer. Single phase flow 

In order to characterise the flow space within filling we will consider packing of equally sized spheres of diameter dP. A suitable parameter for the description of packing is the void fraction 

The forms of filter media discussed in the previous parts in this section have all been constrained as pieces of material or structures. The remaining group of media materials, of particular interest to clarifying processes, are unconstrained (except by the walls of their containing vessel), being masses of coarse particulate substances, used as packed beds within which contaminants are removed by what can truly be called depth filtration. 

Deep-bed filtration involves filtration vertically through a packed bed of granular or fibrous material, whose height is considerably greater than even the thickest of continuous filter media It is typified by the conventional sand filter, which clarifies water by depth filtration mechanisms as it flows through a bed of graded sand that may be up to one metre in depth. 

Deep-bed filters are of very simple construction a vessel (usually cylindrical), a supporting grid at the base of the vessel, and the bed of granules - plus the necessary inlet and outlet piping. Effectively, the medium is the filter. 

A size classification of one material does give the finest particles at the top, and therefore is more easily clogged than if there was a decrease in the size of flow channels downwards through the bed. The proper reclassification is best achieved by using a multi-layer filter. Two or more materials of different densities and sizes make up the bed, so that the hydraulic classification of cleaning places the finer, denser particles below the coarser, less dense particles (with filtration flow downwards). 

The most modem version of the rapid sand filter is that which uses a moving bed of sand, whereby both filtration and cleaning proceed continuously and simultaneously. Recent evidence suggests that such filters can be as effective as membrane filtration plants in the removal of such pathogens as Cryptosporidia and Giardia from water intended for drinking. 

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Gas-liquid mixtures are sometimes reacted in packed beds. The gas and the liquid usually flow cocurrently. Such trickle-bed reactors have the advantage that residence times of the liquid are shorter than in countercurrent operation. This can be useful in avoiding unwanted side reactions. 

Fixed-bed reactors in the form of gas absorption equipment are used commonly for noncatalytic gas-liquid reactions. Here the packed bed serves only to give good contact between the gas and liquid. Both cocurrent and countercurrent operations are used. Countercurrent operation gives the highest reaction rates. Cocurrent operation is preferred if a short liquid residence time is required. 

In packed beds of particles possessing small pores, dilute aqueous solutions of hydroly2ed polyacrylamide will sometimes exhibit dilatant behavior iastead of the usual shear thinning behavior seen ia simple shear or Couette flow. In elongational flow, such as flow through porous sandstone, flow resistance can iacrease with flow rate due to iacreases ia elongational viscosity and normal stress differences. The iacrease ia normal stress differences with shear rate is typical of isotropic polymer solutions. Normal stress differences of anisotropic polymers, such as xanthan ia water, are shear rate iadependent (25,26). 

External Fluid Film Resistance. A particle immersed ia a fluid is always surrounded by a laminar fluid film or boundary layer through which an adsorbiag or desorbiag molecule must diffuse. The thickness of this layer, and therefore the mass transfer resistance, depends on the hydrodynamic conditions. Mass transfer ia packed beds and other common contacting devices has been widely studied. The rate data are normally expressed ia terms of a simple linear rate expression of the form... 

N. Wakao, Heat and Mass Transfer in Packed Beds, Gordon Breach, New York, 1982. 

Fig. 22. Performance cut diameter predictions for typical dry packed bed particle collectors as a function of bed height or depth, packing diameter and packing porosity (void area) S. Bed irrigation increases collection efficiency or decreases cut diameter (271). SoHd lines, = 25 mm dashed lines,...

Taub, "Filtration Phenomena ia a Packed Bed Filter," Ph.D. Thesis, Camegie-MeUon University, Pittsburgh, Pa., 1970. 

Ca.rma.n-KozenyEfjua.tion, Flow through packed beds under laminar conditions can be described by the Carman-Kozeny equation in the... 

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. 

Fig. 1. Fluidized-bed behavior where U is the superficial gas velocity and is the minimum fluidization velocity (a) packed bed, no flow (b) fluid bed,...

Once fluidized, the bed behaves as if it were a fluid. A level is maintained and a static pressure head is generated. No flow of soHds through a side outlet occurs in a packed bed however, flow through the opening does occur after a fluidized state has been achieved (Fig. Ic). 

Fig. 22. Standpipe and standpipe pressure profiles showiag (—) fluidized flow and (--------) packed bed or defluidized flow.

Semicontinuous and continuous systems are, with few exceptions, practiced in columns. Most columnar systems are semicontinuous since flow of the stream being processed must be intermpted for regeneration. Columnar installations almost always involve the process stream flowing down through a resin bed. Those that are upflow use a flow rate that either partially fluidizes the bed, or forms a packed bed against an upper porous barrier or distributor for process streams. 

Olefin Amination (Method 6). The most recent technology for the production of lower alkylamines is olefin amination (14). This is 2eohte-cataly2ed reaction of ammonia with an olefin, eg, isobutylene, and is practiced in a packed-bed reactor system in the vapor phase. 

T. R. Melli, W. B. Kolb, J. M. deSantos, and L. E. Scriven, "Cocurrent Downflow in Packed Beds Microscale Roots of Macroscale Plow Regimes,"... 

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