The word "filtration" is used in describing the process of passing a flow of liquid containing suspended solids through a porous medium, the fabric. It has been widely accepted as an effective, reliable and economic method for solid-liquid separation. Some critical applications include sterilization of pharmaceutical fluids; control of sub micron contaminants is de-ionized water for integrated circuit manufacture and purification of a variety of chemicals and solvents. Greater performance demands are being placed on filtration systems with particular reference to increased security, improved economy and enhanced removal efficiency. A good knowledge of the technique is, therefore, essential for the process industry personnel.
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 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.
An 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.
The fabric used as filter is critical in solid-liquid separations. The most common filter fabric materials found in liquid filtration applications are polypropylene, polyester, nylon, polyethylene, and cotton. Others include polytetrafluoroethylene (PTFE), acrylics, fiberglass, and some newer, specialized fibres. The latter are specialized and are not routinely used in most applications.
When selecting the proper filter fabric, four capabilities are important
· Retarding the passage of large particles,
· Facilitating the deposition even the smallest group within the distribution,
· Resisting plugging or blinding,
· Easing the removal of cake or cleaning.
The characteristics of a filter fabric depend partly on the yarns, the fabrication method and the finishing techniques.