Principles of filtration
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Depth filters capture contaminants within their structures, while membrane filters trap contaminants on their surface above the pore size. Depth filters have advantages of lower cost, higher throughput, and dirt capacity, but can experience media migration and inconsistent pore sizes. Membrane filters can achieve absolute sub-micron pore sizes and are generally integrity testable, but have lower flow rates and higher cost. Combination filters combine depth and membrane technologies to provide economical serial filtration.
Principles of filtration
- 1. Principles of filtration Important Filter Media Properties • Pore Size Rating • Porosity (Flow rate) • Chemical Compatibility • Hydrophilic or Hydrophobic • Extractables • Binding • Surface Area Depth vs. Membrane Filtration Depth Filter Depth media is a filter consisting of either multiple layers or a single layer of medium that has depth which captures contaminates within its structures as opposed to on the surface. Advantages • Lower cost • Higher throuputs • High dirt holding capacity • Protection of final filters Potential disadvantages • Media migration • Nominal pore size • Particulate unloading with increase in differential pressure
- 2. • Leeching • No integrity testing Membrane filter Typically traps contaminates larger then the pore size on the surface of the membrane, contaminates smaller then the rated pore may pass through or be captured within the membrane via other mechanisms Membrane filters are typically used for critical applications i.e. sterilising and final filtration Membrane filters are characterized as: Thin, Highly porous, Polymeric sponge like material. A membrane filter trapping contaminates larger then its pore size Advantages • Absolute sub micron pore sizes is possible • Generally integrity testable • Lower extractables Potential Disadvantages • Lower flow rates • Higher cost • Relatively low dirt holding capacity Combination filters Combination filters combine different membrane pore sizes or it combines depth media and a membrane filter to create a self contained serial filter units. They can offer an economical alternative to using individual pre-filters and final filters, Sometimes these filters are used in series.
- 3. Pore Size It is important that pore size does not change during service life of filter and that all the pores are uniform size. Pore size ratings Absolute rated – Specifies the pore size at which a challenge organism of a particular size will be retained with 100 % Efficiency under specific test conditions. Nominal size ratings Depth Media Membran e
- 4. Specifies the pore size at which a challenge particle of a particular size will be retained with a efficiency of 60 – 98 %. Nominally rated filters vary significantly in the filtration industry from supplier to supplier. Porosity • The measure of all the open spaces (pores) in the membrane • Membranes are generally 50% to 90% open space • Directly proportional to the flow rate of the membrane (more pores, higher flow) • A membrane with a high porosity will have a high flow rate – BUT… the structure will be weaker, as it will have more open spaces. Therefore its important to make a membrane with a high porosity but retaining material strength. Chemical Compatibility The filter media’s resistance to certain chemicals. Pore structure should not be adversely affected by chemical exposure. A media’s resistance or lack of resistance can be used for different applications
- 5. Hydrophilic or Hydrophobic Hydrophobic filters will not wet in water but will wet in low surface tension liquids (i.e. organic solvents will pass through). Once a hydrophobic filter has been wetted, aqueous solutions may also pass through. Hydrophobic filters are best suited for gas filtration, low surface tension solvents and venting. In certain applications hydrophobic filters are used to filter aqueous solutions because of chemical compatibility requirements
- 6. Hydrophilic or Hydrophobic Hydrophobic filters will not wet in water but will wet in low surface tension liquids (i.e. organic solvents will pass through). Once a hydrophobic filter has been wetted, aqueous solutions may also pass through. Hydrophobic filters are best suited for gas filtration, low surface tension solvents and venting. In certain applications hydrophobic filters are used to filter aqueous solutions because of chemical compatibility requirements