Fluidization fluid flow through solid beds

Solids of group C are very fine-grained, cohesive powders (e.g. flour, fines from cyclones, and electrostatic filters) that virtually cannot be fluidized without fluidization aids. The adhesion forces between particles are stronger than the forces that the fluid can exert on the particles. Gas flow through the bed forms channels extending from the grid to the top of the bed, and the pressure drop across the bed is lower than the value from cq 1. Fluidization properties can be improved by the use of mechanical equipment (agitators, vibrators) or flowability additives such as Aerosil. 

Now, the actual linear velocity with respect to the retaining vessel of a fluid flowing through a fluidized bed of voidage e is m0/e, where u0 is the superficial fluid velocity, that is, the volumetric fluid flow rate divided by the horizontal cross-sectional area of the otherwise empty retaining vessel, being positive for the upward direction. Similarly, the actual linear velocity of the particles with respect to the retaining wall is ud/(l - e), where ud is the solid mass flow rate... 

The term fluidization is applied to processes in which a loose, porous bed of solids is converted to a fluid system, having the properties of surface leveling, flow, and pressure-depth relationships, by passing the fluid up through the bed. 

Spray (indirect convection) residence time 3 to 30 s gas velocity 0.2 m/s thermal efficiency 50% adiabatic efficiency 100% solid temperatnre = adiabatic satnration temperatnre volnmetric heat transfer coefficient 0.13 to 0.18 kW/m K 1.8 to 2.7 kg steam/kg water evaporated. Ap = 1.5 to 5 kPa. See size redaction sprays, Section 16.11.8.2 spray reactor, Section 16.11.6.12 heat exchange. Section 16.11.3.12 and size enlargement. Section 16.11.9.4. Flash/transported (indirect convection) 175 to 630°C, gas velocity 3 to 30 m/s or 2.5 to 3 times the terminal velocity of the particles gas reqnirement 1 to 5 Nm /kg solid or 1 to 10 kg air/kg solid exit air temperatnre 20°C greater than exit dry solid temperatnre 4000 to 10,000 kJ/kg water evaporated. See transported slnrry, transfer Une reactors. Section 16.11.6.9. Heat transfer coefficient for gas drying h = 0.2 kW/m -K. Flnidized bed (indirect convection) residence time 30 to 60 s for surface fluid vaporization 15 to 30 min for internal diffnsion 3500 to 4500 kJ/kg water evaporated. See fluidized bed reactors. Section 16.11.6.27 heat transfer. Sections 16.11.3.4 and 16.11.3.8 size enlargement. Section 16.11.9.5 and mixing. Section 16.11.7.1. Tray/gas flow through the bed 0.24 to 3.3 g water evaporated/s m tray area. Residence time 2 to 8.5 h superficial air velocity 0.2 to 1 m/s steam 2 to 6.8 kg steam/kg water evaporated. Fan power 1.6 to... 

Conventionally, fluid-solid reactions are carried out in various types of reactors, such as packed beds, fluidized/slurry, and monolith reactors as summarized in Table 6.1 [1]. Packed bed reactors are relatively simple, easy to operate and suitable for reactions that require relatively large amounts of catalyst, as they provide a high volumetric catalyst fraction of about 60%. The characteristic feature of packed bed reactors is the pressure drop of the fluid flowing through the catalytic bed. To avoid excessive pressure drop the use of catalyst pellets of 2-6 mm is necessary. But, large porous particles lower the transformation rate through diffusion limitations in the porous network and may decrease product selectivity and yield as discussed in Chapter 2. 

There are two modes of fluidization. When the fluid and solid catalyst densities are not too different, or the particles are very small, the bed is fluidized evenly. This is called smooth fluidization, and is typical of liquid-solid systems. If the fluid and solid densities are significantly different, or the catalyst particles are large, the velocity of the flow must be relatively high. In this case, fluidization is uneven, and the fluid passes through the bed mainly in large bubbles. These bubbles burst at the surface, spraying the solid catalyst above the bed. Here, the bed has many of the characteristics... 

System E consists of an isothermal PFR with a bypass stream In a fluidized bed, solid catalyst is circulated within the bed however, the fluid moves through the bed essentially in plug flow. Maldistribution of fluid, due to the formation of bubbles and voids, and channeling, may cause some fluid to bypass the catalyst. This model is used in catalytic fluidized bed applications or in any other application where the solid does not react. A portion of the reactants forms an emulsion phase with... 

The velocity at which gas flows through the dense phase corresponds approximately to the velocity that produces incipient fluidization. The bubbles rise, however, at a rate that is nearly an order of magnitude greater than the minimum fluidization velocity. In effect, then, as a consequence of the movement of solids within the bed and the interchange of fluid between the bubbles and the dense regions of the bed, there are wide disparities in the residence times of various fluid elements within the reactor and in... 

Phase Diagram (Zenz and Othmer) As shown in Fig. 17-2, Zenz and Othmer, (Fluidization and Fluid Particle Systems, Reinhold, New York, 1960) developed a gas-solid phase diagram for systems in which gas flows upward, as a function of pressure drop per unit length versus gas velocity with solids mass flux as a parameter. Line OAB in Fig. 17-2 is the pressure drop versus gas velocity curve for a packed bed, and line BD is the curve for a fluidized bed with no net solids flow through it. Zenz indicated that there was an instability between points D and H because with no solids flow, all the particles will be... 

Many reactors fall in the classification of fluid-solid catalytic units where the catalyst may be retained in a fixed-bed position in the reactor with the reactant flowing through the catalyst bed, or the unit may be operated as a fluidized-bed reactor with the catalyst particles being suspended in the flowing fluid due to motion of the fluid. A third type of reactor is one in which the catalyst particles fall slowly through the fluid by gravity in the form of a so-called moving bed. 

As the bubbles rise, mass transfer of the reactant gases takes place as they flow (diffuse) in and out of the bubble to contact the solid particles where the reaetion produet is formed. The product gases then flow back into a bubble and finally exit the bed when the bubble reaches the top of the bed. The rate at which the reaetants and products transfer in and out of the bubble affects the conversion, as does the time it takes for the bubble to pass through the bed. Consequently, we need to deseribe the veloeity at which the bubbles move through the eolumn and the rate of transport of gases in and out of the bubbles. The bed is to be operated at a superfieial veloeity Mq- To calculate these parameters we need to determine a number of fluid-mechanics parameters associated with idle fluidization proeess. Speeifieally, to detennine the velocity of the bubble through the bed we need to first ealeulate ... 

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