Lifetime, filter media

The filter medium is that critical component which determines whether or not a filter will perform adequately. Within the context of solid/Uquid separation the term filter medium can be defined as any material that, under the operating conditions of the filter, is permeable to one or more components of a mixture, solution or suspension, and is impermeable to the remaining components (Purchas and Sutherland, 2002). The principal role of a filter medium is to cause a clear separation of particulates (which may be solid particles, liquid droplets, colloidal material, or molecular or ionic species) from the liquid with the minimum consumption of energy. In order to achieve this, careful selection of the medium must take into account many factors criteria by which a medium is assessed include the permeability of the clean medium, its particle retention capability and the permeability of the used medium. Serious loss of permeability may follow plugging or blinding of pores in the filter medium, and can determine the lifetime of the medium if an uneconomic filtration rate results. Permeability and particle retention are dependent on the structure of the medium, but interaction of media structure with the shape and size distribution of the particles challenging the medium is also of crucial importance. 

Filter media are characterised by many different chemical and mechanical properties, and the right combination can usually be found for most applications. Purchas (1980) identified some 20 significant properties divided into three major categories (1) machine-orientated properties (Table 2.2), (2) application-orientated properties (Table 2.3) and (3) filtration-specific properties (Table 2.4). These and the characteristics of the feed suspension interact to affect the lifetime of the filter medium, which has implications for the process productivity and economics - as illustrated in Figure 2.1. Although... 

Figure 2.1 The feed to the filter and the filter medium Interact to an extent that depends In part on the operation of the filter, affecting process productivity and medium lifetime, and hence the process costs.

Energy transfer by the trivial mechanism is characterized by (a) change in the donor emission spectrum (inner filter effect), (b) invariance of the donor emission lifetime, and (c) lack of dependence upon viscosity of the medium. 

Filter cloth changing and replacement of product—contacted O-rings—are required when cleaning between products. Wear parts are the O-rings and filter cloths. These should be changed on a preventive maintenance basis approximately every two to three (2-3) months. The membrane has a lifetime of approximately one (1) year and, of course, must be chemically compatible with the solvents as is the filter cloth medium. Preventative maintenance is required for the vacuum and hydraulic (water) system. 

Surface coatings applied to filter fabrics can enhance one or more of the filtration properties of the fabric. The coating may be sprayed on as a liquid or laid down as a sheet which is then bonded to the fabric. Microporous polymer coatings may be used to provide a smoother and fine aperture size to the fabric surface, which may enable easier detachment of the cake and prolong the lifetime of the medium. 

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