Fluidized beds minimum velocity

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,...

Fluidized Beds When gas or liquid flows upward through a vertically unconstrained bed of particles, there is a minimum fluid velocity at which the particles will begin to move. Above this minimum velocity, the bed is said to be fluidized. Fluidized beds are widely used, in part because of their excellent mixing and heat and mass transfer characteristics. See Sec. 17 of this Handbook for detailed information. 

This reaction is carried out in tall fluidized beds of high L/dt ratio. Pressures up to 200 kPa are used at temperatures around 300°C. The copper catalyst is deposited onto the surface of the silicon metal particles. The product is a vapor-phase material and the particulate silicon is gradually consumed. As the particle diameter decreases the minimum fluidization velocity decreases also. While the linear velocity decreases, the mass velocity of the fluid increases with conversion. Therefore, the leftover small particles with the copper catalyst and some debris leave the reactor at the top exit. 

For fluidized beds, part of the gas flows through the emulsion at minimum fluidization velocity Uo, leaving U - Ug to influence bubble behavior. Then equation (4) is modified to read ... 

Fluidized bed dryers work best on particles of a few tenths of a mm dia, but up to 4 mm dia have been processed. Gas velocities of twice the minimum fluidization velocity are a safe prescription. In continuous operation, drying times of 1-2 min are enough, but batch drying of some pharmaceutical products employs drying times of 2-3 hr. 

FIGURE 11.9 Fluidization regimes in a batch fluidized bed at low multiples of the minimum fluidization velocity. 

The effect of gas velocity on the bed pressure drop (-APbed) with a uniform distributor (Fopen = 1.68 %) in the beds with decreasing and increasing Ug is shown in Fig. 5. As can be seen, -APbed maintains almost a constant value rmtil the minimum velocity of full fluidization (Unur) and then it decreases with decreasing Ug. As shown, Umfd is the maximum velocity of full defluidization, Umpf is the minimum velocity of partial fluidization, and Umu is the minimum velocity of full fluidization [6]. 

One of the basic parameters to be determined when designing bubbling fluidized-bed systems is the minimum fluidization velocity, Ump The effect of temperature and pressure on IJhas been investigated by many researchers (Botterill and Desai, 1972 Botterill and Teoman, 1980 ... 

Temperature can also affect fluidized beds if it causes the particles to sinter or become sticky. Siegell (1984) has shown that above a certain critical temperature, the fluidizing gas velocity must be increased above the minimum fluidization velocity in order to maintain fluidization in the bed. Siegell reported that the increase in velocity required to maintain fluidization was linearly proportional to the temperature increase. Increasing bedL/D, and smaller particle sizes, increased the tendency to agglomerate. 

The results of an example calculation for a recirculating fluidized bed coal devolatilizer of 0.51 m in diameter handling coal of average size 1200 pm at 870°C and 1550 kPa are presented in Fig. 11. The calculation is based on operating the fluidized bed above the draft tube at 4 times the minimum fluidization velocity. It is also based on the selection of a distributor plate to maintain the downcomer at the minimum fluidization condition. If the two-phase theory applies, this means that the slip velocity between the gas and the particles in the downcomer must equal to the interstitial minimum fluidizing velocity as shown below. 

Group A powders show a limited tendency to form bubbles and generally exhibit considerable bed expansion between the minimum fluidization velocity Vmp and the minimum bubbling velocity Vmb. These powders also retain aeration and the fluidized bed collapses very slowly when the gas is turned off. 

Group B materials fluidize readily and tend to form bubbles, which grow rapidly by coalescence. However, bed expansion is small. That is, the minimum bubbling velocity, Vmb, usually is approximately equal to (or only slightly greater than) the minimum fluidizing velocity Vmj, The fluidized bed does not retain its aeration and collapses quickly when the gas supply is removed. 

A first-order reaction, A -> products, is conducted in a fluidized-bed reactor. Before fluidization, the bed is 1.8 m deep (Lpa), with a voidage (epa) of 0.42. The feed gas (P/ = 0.45 kgm-3 p,y = 4.2 x 10-5 Pa s) flows through the bed at a velocity three times the minimum fluidization velocity. The following data are also available ... 

As the fluid velocity is increased the drag on the particles increases and a point is reached where the pressure drop balances the effective weight of bed per unit cross-sectional area. At this point the fluid drag just supports the solid particles. A small increase in the flow rate causes a slight expansion of the bed from its static, packed state. Further increase in the flow rate allows the bed to expand more and the particles become free to move around and the bed is said to be fluidized. The state when the bed just becomes fluidized is known as incipient, or minimum, fluidization. The fluid velocity required to cause incipient fluidization is called the minimum fluidization velocity and is denoted by umf. 

The best method of determining the minimum fluidization velocity umf is experimentally, by measuring the pressure drop across the bed over a range of fluid velocities. The pressure drop increases linearly until fluidization occurs and then increases very slowly indeed up to about twice the minimum fluidization velocity the pressure drop may appear to be constant within experimental error. When a bed is initially fluidized, there is a tendency for the pressure drop across the bed to be rather high and to go through a peak as incipient fluidization occurs. It is possible that this is caused by a need to unstick the particles. If the fluid velocity of an already fluidized bed is reduced, the peak in the pressure drop is not observed and a much clearer transition to the linear pressure drop—flow... 

Two-Phase Theory of Fluidization The two-phase theory of fluidization assumes that all gas in excess of the minimum bubbling velocity passes through the bed as bubbles [Toomey and Johnstone, Chem. Eng. Prog. 48 220 (1952)]. In this view of the fluidized bed, the gas flowing through the emulsion phase in the bed is at the minimum bubbling velocity, while the gas flow above U j, is in the bubble phase. This view of the bed is an approximation, but it is a helpful way... 

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