Fluidization minimum bubbling rate

For group B and D particles, nearly all the excess gas velocity (U — U,nj) flows as bubbles tnrough the bed. The flow of bubbles controls particle mixing, attrition, and elutriation. Therefore, ehitriation and attrition rates are proportional to excess gas velocity. Readers should refer to Sec. 17 for important information and correlations on Gel-dart s powder classification, minimum fluidization velocity, bubble growth and bed expansion, and elutriation. 

This equation has been experimentally verified in liquids, and Figure 2 shows that it applies equally well for fluidized solids, provided that G is taken as the flow rate in excess of minimum fluidization requirements. In most practical fluidized beds, bubbles coalesce or break up after formation, but this equation nevertheless gives a useful starting point estimate of bubble size. 

The velocity at which gas flows through the dense phase corresponds approximately to the velocity that produces incipient fluidization. The bubbles rise, however, at a rate that is nearly an order of magnitude greater than the minimum fluidization velocity. In effect, then, as a consequence of the movement of solids within the bed and the interchange of fluid between the bubbles and the dense regions of the bed, there are wide disparities in the residence times of various fluid elements within the reactor and in... 

Some smoothed data of expansion ratio appear in Figure 6.10(c) as a function of particle size and ratio of flow rates at minimum bubbling and fluidization. The rather arbitrarily drawn dashed line appears to be a conservative estimate for particles in the range of 100 pm. 

Ordinarily under practical conditions the flow rate is at most a few multiples of the minimum fluidizing velocity so the local maximum bed level at the minimum bubbling velocity is the one that determines the required vessel size. The simplest adequate equation that has been proposed for the ratio of voidages at minimum bubbling and fluidization is... 

Due to their complexity, the model equations will not be derived or presented here. Details can be found elsewhere [Adris, 1994 Abdalla and Elnashaie, 1995]. Basically mass and heat balances arc performed for the dense and bubble phases. It is noted that associated reaction terms need to be included in those equations for the dense phase but not for the bubble phase. Hydrogen permeation, the rate of which follows Equation (10-51b) with n=0.5, is accounted for in the mass balance for the dense phase. Hydrodynamic parameters important to the fluidized bed reactor operation include minimum fluidization velocity, bed porosity at minimum fluidization, average bubble diameter, bubble rising velocity and volume fraction of bubbles in the fluidized bed. The equations used for estimating these and other hydrodynamic parameters are taken from various established sources in the fluidized bed literature and have been given by Abdalla and Elnashaie [1995]. 

Calculate the minimum fluidization velocity minimum bubbling velocity and minimum slugging velocity (u ). As the calculation of requires a value of the reactor diameter, which is not known, assume a value and go through the calculations. This velocity represents the maximum permissible velocity and therefore the minimum diameter. From the given volumetric flow rate, calculate the maximum diameter by assuming a velocity of, say, Choose a suitable diameter between these limits. Iteration may be needed. 

The above discussion identifies the growth-rate gradient of short waves, dajde, evaluated at Smb, as a further measure of fluidization quality. It provides the necessary additional dimension to the quantification in terms of the minimum bubbling void fraction, distinguishing between systems having the same Smb but different perturbation-amplitude growth rate characteristics. This gradient may be readily evaluated from eqn (10.4). 

As is the case for emb> the parameter Aa may be readily evaluated for any defined system. Because it reflects the amplitude growth-rate gradient of perturbations at the critical void fraction that separates stable from unstable fluidization, it provides a direct measure both of the extent of instability for unstable systems and the robustness of the stability manifested by those stable systems for which a minimum bubbling point exists. Systems that fluidize homogeneously for all void fractions cannot be... 

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. 

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 ... 

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