Fluidization conditions

In vertical disc-stirred mills the media should be in a fluidized condition (White, Media Milling, Premier Mill Co., 1991). Particles can pack in the bottom if there is not enough stirring action or feed flow or in the top if flow is too high. These conditions are usually detected by experiment. 

This may be solved to give the expansion compared to minimum fluidization conditions. 

Equation 6-108 is also a good approximation for a fluidized bed reactor up to the minimum fluidizing condition. However, beyond this range, fluid dynamic factors are more complex than for the packed bed reactor. Among the parameters that influence the AP in a fluidized bed reactor are the different types of two-phase flow, smooth fluidization, slugging or channeling, the particle size distribution, and the... 

The process essentially involves passing air through a bottom furnace distributor plate and a fixed bed of sand. As air flow rates increase, the fixed bed becomes more unstable and bubbles of air appear (minimum fluidized condition). Above this minimum level, higher air flow rates produce—depending on design—either bubbling fluidized beds or circulating fluidized beds, and the fuel is introduced onto these beds. 

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. 

It was also shown both theoretically and experimentally (Tardos et al., 1985a,b) that there is a strong correlation between the excess temperature above the minimum sintering point, T-Ts, and the excess gas velocity above minimum fluidization conditions (measured below the sintering point) U-lJm required to maintain fluidization. A general correlation was developed between the excess temperature and the excess gas velocity, which takes the form... 

The obvious measures of lowering superficial gas velocity in the bed and the selection of wall and/or particulate materials certainly exist. But, in a given process, there is often no practical way to implement such solutions. Fluidization conditions are dictated by the application for which the bed is intended furthermore, the choice of particulate materials can seldom be made on the basis of electrostatic considerations. 

Inlet temperature is 898 K, operating pressure is 1.2 atm, charge rate is 20 lbmol of ethylbenzene and 400 lbmols of steam. Under fluidizing conditions bulk density of the bed is 25 lb/cuft. Conditions are adiabatic. 

Wilhelm and Kwauk (1948), in one of the earliest papers on fluidization, suggested that the Froude number, written for the minimum fluidizing condition, could be used to indicate the prevalent fluidization behaviour. Thus... 

Writing Ergun s equation for minimum fluidizing conditions, and assuming the particles to be spherical ( =1), gives... 

The emulsion stays at minimum fluidization conditions, thus the relative G/S velocity stays constant in the emulsion. 

The emulsion phase stays at minimum fluidizing conditions. Thus, the relative velocity of the gas and solid remains unchanged. 

The concentration of solids in the wake is the same as the concentration of solids in the emulsion phase, and therefore, the gaseous void fraction in the wake is also the same as in the emulsion phase. Because the emulsion phase is at the minimum fluidizing condition, the void fraction in the wake is equal to film. The wake, however, is quite turbulent, and the average velocities of both the solid and gas in the wake are assumed to be the same and equal to the upward velocity of the bubbles. 

The minimum fluidization condition refers to the situation in which AP, the pressure drop across the bed, is equal to the bed weight. 

All the advantages of minimum fluidization conditions. Disadvantages of fluidized beds... 

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