Superficial velocity fluidized beds

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,...

Terminal Velocity. The single-particle terminal velocity, U, is the gas velocity required to maintain a single particle suspended in an upwardly flowing gas stream. A knowledge of terminal velocity is important in fluidized beds because it relates to how long particles are retained in the system. If the operating superficial gas velocity in the fluidized bed far exceeds the terminal velocity of the bed particles, the particles are quickly removed. 

Fundamental models correctly predict that for Group A particles, the conductive heat transfer is much greater than the convective heat transfer. For Group B and D particles, the gas convective heat transfer predominates as the particle surface area decreases. Figure 11 demonstrates how heat transfer varies with pressure and velocity for the different types of particles (23). As superficial velocity increases, there is a sudden jump in the heat-transfer coefficient as gas velocity exceeds and the bed becomes fluidized. 

Flue particles ia a fluidized bed are analogous to volatile molecules ia a Foiling solution. Therefore, the concentration of particles ia the gas above a fluidized bed is a function of the saturation capacity of the gas. To calculate the entrainment rate, it is first necessary to determine what particle sizes ia the bed can be entrained. These particles are the ones which have a terminal velocity less than the superficial gas velocity, assuming that iaterparticle forces ia a dilute zone of the freeboard are negligible. An average particle size of the entrainable particles is then calculated. If all particles ia the bed are entrainable, the entrained material has the same size distribution as the bed material. 

Circulating fluidized-beds do not contain any in-bed tube bundle heating surface. The furnace enclosure and internal division wall-type surfaces provide the required heat removal. This is possible because of the large quantity of soflds that are recycled internally and externally around the furnace. The bed temperature remains uniform, because the mass flow rate of the recycled soflds is many times the mass flow rate of the combustion gas. Operating temperatures for circulating beds are in the range of 816 to 871°C. Superficial gas velocities in some commercially available beds are about 6 m/s at full loads. The size of the soflds in the bed is usually smaller than 590 p.m, with the mean particle size in the 150—200 p.m range (81). 

Particulate Fluidization Fluid beds of Geldart class A powders that are operated at gas velocities above the minimum fluidizing velocity (L/, y) but belowthe minimum bubbhngvelocity (L/, i) are said to be particulately fluidized. As the gas velocity is increased above L/, y, the bed further expands. Decreasing (p, — Py), d and/or increasing increases the spread between L/, yand U, b until at some point, usually at high pressure, the bed is fully particulately fluidized. Richardson and Zald [Trans. Inst. Chem. Eng., 32, 35 (1954)] showed that U/U = E , where /i is a function of system properties, = void fraction, U = superficial fluid velocity, and Uj = theoretical superficial velocity from the Richardson and Zald plot when = 1. 

A new process for the partial oxidation of n-butane to maleic anhydride was developed by DuPont. The important feature of this process is the use of a circulating fluidized bed-reactor. Solids flux in the rizer-reactor is high and the superficial gas velocities are also high, which encounters short residence times usually in seconds. The developed catalyst for this process is based on vanadium phosphorous oxides... 

Fixed-bed systems are the most common, but some countercurrent fluidized beds are in use. Flow diagrams are given in reference 47. The superficial velocities of gases in fixed beds should be about 1 ft/sec (0.3 m/sec) and those for liquids about 1 ft/min (0.3 m/min).48 See references 48 and 49 for more design information. 

A fluidized bed reactor contains catalyst particles with a mean diameter of 500 pm and a density of 2.5 g/cm3. The reactor feed has properties equivalent to 35° API distillate at 400°F. Determine the range of superficial velocities over which the bed will be in a fluidized state. 

Table 1 gives the values of design and operating parameters of a scale model fluidized with air at ambient conditions which simulates the dynamics of an atmospheric fluidized bed combustor operating at 850°C. Fortunately, the linear dimensions of the model are much smaller, roughly one quarter those of the combustor. The particle density in the model must be much higher than the particle density in the combustor to maintain a constant value of the gas-to-solid density ratio. Note that the superficial velocity of the model differs from that of the combustor along with the spatial and temporal variables. 

The interaction of parametric effects of solid mass flux and axial location is illustrated by the data of Dou et al. (1991), shown in Fig. 19. These authors measured the heat transfer coefficient on the surface of a vertical tube suspended within the fast fluidized bed at different elevations. The data of Fig. 19 show that for a given size particle, at a given superficial gas velocity, the heat transfer coefficient consistently decreases with elevation along the bed for any given solid mass flux Gs. At a given elevation position, the heat transfer coefficient consistently increases with increasing solid mass flux at the highest elevation of 6.5 m, where hydrodynamic conditions are most likely to be fully developed, it is seen that the heat transfer coefficient increases by approximately 50% as Gv increased from 30 to 50 kg/rrfs. 

A model was developed to describe this phenomenon by assuming that the gas leaks out through the bubble boundary at a superficial velocity equivalent to the superficial minimum fluidization velocity. For a hemispherical bubble in a semicircular bed, the rate of change of bubble volume can be expressed as ... 

In most commercial fluidized bed processes, the bed is much higher than the jet penetration length. There are several parameters that affect attrition in the jetting region, namely the design parameters of the distributor (i.e., orifice diameter, dor, open surface area, Aa, number of orifices, Nor) and the operating parameters (i.e., gas density, pg, volumetric flow rate, vg, superficial gas velocity, t/g, orifice velocity, uor). It holds... 

The anticipated pressure drop is calculable from the relationships in Fig. 21. Assuming the cyclone is located within, or attached externally to, the shell of an 8 diameter fluidized bed reactor operating at a superficial gas velocity of 2 fi/sec, then ... 

Figure 5.9 Effect of crystallinity on the solid-state polycondensation of PET, shown as the number-average molecular weight as a function of time. Conditions fluidized bed polymerization at 230°C particle size, 35-48 mesh superficial velocity of nitrogen, 43cm/s [6]. From Chang, T. M., Polym. Eng. Sci., 10, 364 (1970), and reproduced with permission of the Society of Plastics Engineers...

The secfion above fhe fluidized bed surface is offen referred fo as fhe freeboard. This secfion may have a gradually increasing column diamefer, rafher like an inverfed cone, which is designed fo reduce fhe gas velocify and fhus disengage gas and particles. Particles then fall back to a level where the superficial gas velocity is sufficient to support their weight. 

The superficial velocity of gas in a fluidized bed, relative to the minimum fluidizing velocity, is the quantity which has the greatest influence on the behaviour of a given particle bed. Consequently, a knowledge of minimum fluidizing velocity is vital to the operation of fluidized beds and much research effort has been expended in attempting to predict it. 

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