Bed voidage at minimum fluidization conditions

However, the minimum fluidization velocity represents a convenient way to characterize the interactions between a given solid powder and a given gas. As such, it is extensively used in semi-empirical models based on the two-phase theory and in empirical correlations which predict bed expansion, heat transfer coefficients, mass transfer coefficients and chemical conversions in fluidized bed reactors (most of these models and correlations also use the bed voidage at minimum fluidization conditions). 

The minimum fluidization velocity of a given powder/gas system is substantially affected by temperature and pressure. This effect is not only due to the changes in gas viscosity and density but it also results from changes in the bed voidage at minimum fluidization velocity. Chi tester et al. [23] conducted some measurements at high pressures and proposed a new correlation which took into account the observed increase with pressure in bed voidage at minimum fluidization conditions. 

The bed voidage at minimum fluidization conditions is the average value of the fraction of the bed volume occupied by gas at minimum fluidization conditions. Three different techniques are available to measure the bed voidage at minimum fluidization conditions. The first technique uses the experimental value of the minimuon fluidization velocity. It also requires knowledge of the fluidizing fluid properties, the particle apparent density, the Sauter mean particle diameter and the particle shape factor. The... 

The bed voidage at minimum fluidization conditions is affected by the same operating variables which affect the minimum fluidization velocity temperature and pressure which change the gas properties, adhesiveness or stickiness of the particles, moisture level of the gas and of the particles and electrostatic effects. 

The fluidized-bed voidage at minimum bubbling conditions (emb) also has been shown to increase with increasing temperature (Fig. 7) and increasing pressure (Fig. 8). 

Many correlations and models to calculate bed expansion, heat transfer, mass transfer and chemical conversion require knowledge of the voidage at minimum fluidization conditions. 

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... 

---