Terminology And Governing Equations

There are essentially four important physical parameters that characterize a filter media and are used as a basis for relating the characteristics of the material to the system flow dynamics. These are porosity, permeability, tortuosity and connectivity.

We may begin by describing any porous medium as a solid matter containing many holes or pores, which collectively constitute an array of tortuous passages. Refer to Figure 1 for an example. The number of holes or pores is sufficiently great that a volume average is needed to estimate pertinent properties. Pores that occupy a definite fraction of the bulk volume constitute a complex network of voids. The manner in which holes or pores are embedded, the extent of their interconnection, and their location, size and shape characterize the porous medium. The term porosity refers to the fraction of the medium that contains the voids. When a fluid is passed over the medium, the fraction of the medium (i.e., the pores) that contributes to the flow is referred to as the effective porosity of the media. In a general sense, porous media are classified as either unconsolidated and consolidated and/or as ordered and random. Examples of unconsolidated media are sand, glass beads, catalyst pellets, column packing materials, soil, gravel and packing such as charcoal. Examples of consolidated media are most of the naturally occurring rocks, such as sandstones and limestones.

Materials such as concrete, cement, bricks, paper and cloth are manmade consolidated media.

Ordered media are regular packings of various types of materials, such as spheres, column packings and wood. Random media have no particular correlating factor. Porous media can be further categorized in terms of geometrical or structural properties as they relate to the matrix that affects flow and in terms of the flow properties that describe the matrix from the standpoint of the contained fluid. Geometrical or structural properties are best represented by average properties, from which these average structural properties are related to flow properties.

Figure 1. SEM of pores on surface of activated carbon particle
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