The gas fluidization process is important as it is widely used in the pharmaceutical, food, and fertilizer industries. In those processes, the initial characterization of solid particles is vital to obtain the best efficiency rate. The physical characterization of particles is based on size, shape (sphericity), specific mass, and porosity.
1. Particle Size
The most important feature is the particle size. For a solid particle with irregular geometry, different size definitions are available: Equivalent Diameters, Diameters of an Equivalent Circle, and Statistic Diameters.
 Equivalent Diameters
In practice, they are used as the solid diameters that correspond to the same sphere, which presents the same properties of the particle in volume, the same projected surface, and the same terminal velocity. Table 1 shows the main diameters equivalent to one sphere:
Symbol

Designation

Sphere Property

Math Expression

dv

Diameter in Volume

Volume


ds

Diameter in Specific Surface

Surface


dsv

Surface Diameter 
Surface/Volume

In particulate system (flows through porous media), kinetics and catalysis, the most used definition is Sauter’s, which relates the diameter of the particle whose surface/volume relation is the same for all the particles:
Where d_{p} is the average Sauter’s diameter [L], x_{i} is the retained mass fraction [] and d_{p} is the average arithmetic diameter between two particle sizes [L].
Diameters of an Equivalent Circle
The diameters of an equivalent circle are the ones corresponding to a circumference presenting the same properties of the particle when it is designed. The diameter of the designed area is defined as the circle diameter, which has the same area as the designed particle’s (Figure 1).
Figure 1: Diameters of the surface projected
Statistic Diameters
These diameters correspond to the measure of a linear dimension made in a parallel with a fixed direction, being called statistic diameters and used in microscopy. A few definitions for the size of the solid are displayed by Figure 2: Martin's Diameter, Feret’s Diameter, Crofton’s Diameter.
Figure 2: Statistic Diameters
As the size of solids is defined, a few industry authors or sectors propose different classifications for such solids. According to the sieve opening, Table 2 presents some of these classifications.
Solid Conversion 
Drying 
Storage and coating 
English Standard 

Classification 
Very Fine FineGranulated Agglomerate 
Very Fine Fine Average Large Agglomerate 
Powder Granular MaterialBreakable Solids 
Granular Powder Granular Solid 
Table 2: Classifications for solids according to different industry authors and sectors