Minimum fluidizing velocity Reynolds number

The exponent m is a function of particle Reynolds number based on the minimum fluidization velocity. It can be estimated by the following correlation ... 

The minimum Reynolds number for fluidization Remf as well as the minimum fluidization velocity Umf may be calculated according to the correlation of Riba et al. [20], Mv defining the ratio of particle to liquid density... 

R pmf Particle Reynolds number at minimum fluidization velocity... 

Specific drying performance (dry product) related to the cross section of the dryer, kg/m h Bulk velocities in spaces 1, 2, and 3, respectively (see Figure 15.5), m /s Bulk velocities in points a and b, respectively, mVs Conveying rate of the screw, kg/s Reynolds number at minimum fluidization velocity... 

Archimedes number is the ratio of gravitational forces to viscous forces. The number is about 3,500 for water, 9,000 for air and 700,000 for supercritical carbon dioxide. This leads to Reynolds numbers two orders of magnitude higher at the minimum fluidization velocity and fluidized bed heights that are an order of magnitude smaller than liquids and gas counter parts. 

In this equation, Re is tlie particle Reynolds number based on the minimum superficial velocity for fluidization. Moreover, for fixed-beds, we can set = 1 and sf = s. The correlation is applicable for void fractions between 0.4 and 0.8 with particle density up to 480 lb/ft3. Note that by changing the Rep number, the fluidized bed voidage ef is changed. 

The performance of a fluidized bed combustor is strongly influenced by the fluid mechanics and heat transfer in the bed, consideration of which must be part of any attempt to realistically model bed performance. The fluid mechanics and heat transfer in an AFBC must, however, be distinguished from those in fluidized catalytic reactors such as fluidized catalytic crackers (FCCs) because the particle size in an AFBC, typically about 1 mm in diameter, is more than an order of magnitude larger than that utilized in FCC s, typically about 50 ym. The consequences of this difference in particle size is illustrated in Table 1. Particle Reynolds number in an FCC is much smaller than unity so that viscous forces dominate whereas for an AFBC the particle Reynolds number is of order unity and the effect of inertial forces become noticeable. Minimum velocity of fluidization (u ) in an FCC is so low that the bubble-rise velocity exceeds the gas velocity in the dense phase (umf/cmf) over a bed s depth the FCC s operate in the so-called fast bubble regime to be elaborated on later. By contrast- the bubble-rise velocity in an AFBC may be slower or faster than the gas-phase velocity in the emulsion... 

From the critical Reynolds number Re one can calculate minimum (critical) velocity of fluidization as... 

When velocity is increased beyond that for minimum fluidization, the bed s expansion is given by (e — e j)/(l — e), where e is the bed voidage at the appropriate expansion. The fluid velocity at any expansion can be estimated from u/u f = (e/e y) , where n is a function of the Reynolds number and the ratio dp/D. At low Reynolds numbers, n is independent of this ratio and is approximately 4.6. The effect of bed expansion on the mass transfer coefficient can thus be determined. The Reynolds number, here is too high for a constant value of n as a result, calculated values of the electrolyte velocity are too high compared with velocities determined by experiments (Table 2.3). Values of for various expansions have been calculated using in the rearranged Eq. (2.37). 

Rowe (1961) performed experiments on water flow through a regular array of spheres and found that the force on the single sphere in the array was 68.5 times the force on an isolated sphere at the same superficial velocity. If we assume that the same factor of 68.5 is also applicable at the minimum fluidization condition when the drag force of the fluid on the particles is just balanced by the net downward force on the particles, we have, for the low-Reynolds number case,... 

Remf Reynolds number at minimum fluidization conditions TDH transport disengaging height, m U dimensionless gas velocity,... 

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