Fluidization velocity range

There are two types of FBC unit distinguished by their operating flow characteristics bubbling and circulating. These two types operate at atmospheric pressure, AFBC, or at elevated pressure, PFBC. Pressures for PFBC are in the range 0.6 to 1.6 MPa (90 to 240 psia). Typical superficial fluidizing velocities are tabulated as follows. 

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... 

Single particles will tend to be carried out of the bed if the fluid velocity exceeds the terminal falling speed u, of the particles given by equation 9.5. Thus the normal range of fluidization velocity is from umf to a,. However, it may be found that the fluid velocity required to bring about fast fluidization is significantly higher than u, because particles tend to form clusters. 

This equation allows the prediction of minimum fluidizing velocity from a knowledge of the mean particle diameter, the particle density, the density of fluidizing medium and fhe viscosity of fluidizing medium (SI units). Couderc (1985) quotes data which show that the inaccuracy of Leva s equation increases significantly outside the range 2 < Re < 30. 

The Richardson-Zaki equation has been found to agree with experimental data over a wide range of condifions. Equally, if is possible fo use a pressure drop-velocity relationship such as Ergun to determine minimum fluidization velocity, just as for gas-solid fluidizafion. An alternative expression, which has the merit of simplicify, is fhaf of Riba ef al. (1978)... 

In a similar application Thomas et al. (1993) developed a process for the separation of stones and sand particles from coriander seeds using a bed fluidized wifh air. Coriander seeds between 2 and 5 mm in diameter, and with a minimum fluidizing velocity of 0.8ms , were separated from denser stones of a similar size range and from sand parficles ranging in size from 0.3 to 1 mm. Fluidization for 5 minutes was sufficient to effect complete separation. 

Air rates in Table 9.15 range from 13 to 793 SCFM/sqft, which is hardly a guide to the selection of an air rate for a particular case. A gas velocity twice the minimum fluidization velocity may be taken as a safe prescription. None of the published correlations of minimum fluidizing velocity is of high accuracy. The equation of Leva (Fluidization, McGraw-Hill, New York, 1959) appears to be as good as any of the later ones. It is... 

A wide range of operating conditions is used commercially. Performance data are in Table 12.19. Gas velocities cover a range of 3-20 times the minimum fluidizing velocity or 0.1-2.5 m/sec. Bed expansion ratios are up to 3 or so. As in fluidized bed drying, bed depths are low, usually between 12 and 24 in. Evaporation rates are in the range 0.005-1.0 kg/(sec)(mz). 

Fluidized bed granulation is conducted in shallow beds 12-24 in. deep at air velocities of 0.1-2.5 m/s or 3-10 times the minimum fluidizing velocity, with evaporation rates of 0.005-1.0kg/m2sec. One product has a size range 0.7-2.4 mm dia. 

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