Particle Size

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:

Sphere Property
Math Expression
Diameter in Volume
Diameter in Specific Surface

Surface Diameter


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 dp is the average Sauter’s diameter [L], xi is the retained mass fraction [-] and dp 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).

Diâmetro da Superfície Projetada
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.

Diâmetros estatísticos
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


Storage and coating

English Standard


Very Fine
Very Fine
Granular MaterialBreakable Solids
Granular Powder
Granular Solid

Table 2: Classifications for solids according to different industry authors and sectors