Particle voidage

Sauter mean size 120 pm Particle voidage 0.5 Sphericity 0.8... 

The effective diffusivity De is a characteristic of the particle that must be measured for greatest accuracy. However, in the absence of experimental data, De may be estimated in terms of molecular diffusivity, Dab (for diffusion of A in the binary system A + B), Knudsen diffusivity, DK, particle voidage, p, and a measure of the pore structure called the particle tortuosity, Tp. 

The effective diffusivity is obtained from D, but must also take into account the two features that (1) only a portion of the catalyst particle is permeable, and (2) the diffusion path through the particle is random and tortuous. These are allowed for by the particle voidage or porosity, p, and the tortuosity, rp, respectively. The former must also be measured, and is usually provided by the manufacturer for a commercial catalyst. For a straight cylinder, rp = 1, but for most catalysts, the value lies between 3 and 7 typical values are given by Satterfield. 

Ug that is, gas associated with the wake travels at the hubhle velocity, (iii) that is, the particle voidage in... 

Fig. 3. Zenz plot. Correlation of bed voidage, S, the volume fraction of the fluidized bed that is occupied by gas, for values of S from 1.0 to 0.5, and dimensionless velocity and particle properties, where p — p )/(3 PgU ). The horizontal lines represent the different values of e = 0.5...

The heat-transfer coefficient depends on particle size distribution, bed voidage, tube size, etc. Thus a universal correlation to predict heat-transfer coefficients is not available. However, the correlation of Andeen and Ghcksman (22) is adequate for approximate predictions ... 

The total voidage i in a packed bed (outside and inside particles) is... 

The value of e varies between the limits of 0 and unity however, many particles have a loosely poured voidage of approximately 0.4 to 0.45. 

Size, shape distribution Voidage, bulk density, contacts, etc. Fluid flow, particle flow, caking, etc. 

Note. Permeability is strongly dependent on both particle size and voidage of bed. Example... 

The size distribution of the particles (with particles of a uniform size, there is increased voidage between the particles and a lower tortuosity, that is deviation from the linear path for the fluid flowing between the particles, making for improved filterability), and... 

Di Felice, R., 1994. The voidage function for fluid-particle interaction systems. International Journal of Multiphase Flow, 20, 153-159. 

Fluidised beds have been used previously for the industrial-scale recovery of the antibiotics streptomycin and novobiocin.30 However, more recently, considerable interest has been shown in the use of fluidised beds for the direct extraction of proteins from whole fermentation broths.31 In a packed bed, the adsorbent particles are packed within the contactor. The voidage, that is, the inter-particle space, is minimal and thus feedstock clarification is mandatory to avoid clogging of the bed. In a fluidised/expanded bed, the adsorbent bed is allowed to expand by irrigation with feedstock. Bed voidage is increased, allowing the passage of particulates in the feed. The diameters of the adsorbent beads are exaggerated for illustrative clarity. 

The voidage (e) of abed of particles is the fraction of the bed volume occupied by the interstitial space between the particles. Its value depends upon the geometrical configuration of the beads, the pattern in which they are arranged within bed, the size distribution of the particles and the ratio of mean particle and contactor diameter. The bed voidage can be calculated using the following equation ... 

The accumulated liquid fills part of the originally free gas volume lowering the porosity of the filter to a certain extent, but typically not totally due to the transport of the liquid, if the liquid is not too viscous. On the contrary, there is no such limitation for accumulated solid particles which can fill the voidage until the filter is clogged. 

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