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Filtration Basics

Filtration processes typically fall into three categories: refining a fluid or slurry, recovering one or more of the components, and clarifying a liquid or filtrate. Most filtration is inexpensive in comparison to many industrial applications, but there are situations that require more than standard treatment. For example, the liquid downstream of some processes has to be clean enough for expulsion into an adjoining waterway. However, for successful application of this separation technique, knowledge of particle characteristics, fluid properties, equipment and pretreatment needs is essential.

Filter Media

Filter media are porous materials that capture and retain particles using these mechanisms: adsorption - mechanical means, such as sieving or straining, and adsorption - surface forces, such as electrostatic or molecular adhesion.

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Ideal Filter

Ideal Filter provides maximum restriction to the passage of entrained contaminants, offers minimum resistance to the flow of system fluid, and does not lose structural integrity through out its service life. If structural deficiencies exist in the filter, all other good features are of little value. Bypass and surge flow behavior of a filter emphasizes the importance of filter location.

In essence, filter should not be placed in lines where intermittent flow can occur; otherwise surge proof filters with low contaminant release characteristics should be chosen. Hence, the value of last chance and off-line filtration is becoming more and more recognized. Filter elements are designed to hold a given amount of contaminant and exhibit pressure-flow characteristics for specified fluid viscosity. The only problem in practice is that the magnitude of the pressure differential across the filter varied with the degree of contaminant loading and temperature of the system.

The filter is the contaminant controller of the system - the only component capable of capturing, retaining, and removing contaminants. The service life of the fluid components depends on the maintenance of a contamination level that satisfies their contaminant tolerance.

Filtration Principle

Although the filtration equipment takes several forms, its operation is basically the same. The solid-liquid feed stream passes through a porous screen that retains the solids but passes the liquid. As the flow continues, a cake builds up on the screen. The cake has a complex pore structure determined by the nature of the solid particles. The structure removes additional particles by a simple straining mechanism. A French geologist named d'Arcy formulated the equation that describes the flow of Newtonian fluids through a porous bed in 1856. Neglecting the gravitational effects on the fluid, a form of d'Arcy equation adopted for filtration can be written as:

Q = K A ?P / µL ---------------------- (1)

Where, Q is the liquid flow rate, K is the permeability of the filter cake, ?P is pressure drop across the cake, A is the filtration area, µ is the liquid viscosity and L is filter cake thickness. The equation (1) is valid for constant filter cake thickness. During filtration, however, the cake thickness increases as more and more liquid is filtered. To account for the changing thickness, the d'Arcy equation can be modified to give the differential equation:

dV / dT = KA ?P / µL --------------------- (2)

The value of L changes with the volume of liquid filtered as follows:

L = C V / D A ------------------------------- (3)

Where C is the solid concentration, ie mass of solid per unit volume of liquid, D is the bulk density of the solid deposited; V is the volume of the liquid filtered. Substituting the expression for L into the modified d'Arcy equation gives the following differential equation describing cake filtration:

V dV = (K A2 ?P D / µC) / dT --------------------(4)

This equation can be integrated for various flow conditions encountered by the process engineers. The equation, however, is applicable to only dilute slurries. When the solids volume is more than 5% of the total volume of liquids, filtered, other flow equations must be developed. However, the great majority of filter cake filtration in the process industries do treat slurries having solid content within 5%.

Considering operating parameters, two special cases are industrially important. One is constant rate filtration wherein the rate of feed delivery to the filter stays the same during the filtration cycle. The other is constant pressure filtration, where the feed is delivered under a pressure that does not vary over the cycle.