January 11, 2012 Leave a comment
Disposable Filter Media: When final product clarification is a key process objective, a general standard of particle removal is retention in the 0.2 – 200 µ range. Many disposable filter media meet this criterion. Disposable filter media, typically bags or cartridges come in a wide range of µ and fabrics. Woven and nonwoven polypropylene, cellulose, polyester, nylon, and other materials are all available.
Two types of efficiency ratings are typically used for disposable filtration media: nominal and absolute. Nominal ratings can vary from 50 – 90% removal efficiency, depending on the product and the manufacturer. Absolute ratings imply 100% removal of particles at a set rating; this actually means 98.7 – 99.99%, depending on the product and the vendor.
Bag fabrication has advanced over the past few years, improving the filtration capacity of bags and making them more efficient. Multilayer bags, some as much as an inch thick, are now on the market. The multiple layers increase the solids holding capacity of the bags and provide a larger surface area for filtration. These higher efficiency bags normally last longer, remove a higher percentage of contaminants (up to 99.9%) and can be rated as low as 1.5 µ.
Even with new designs in bag construction, cartridge filters trap particulate that a simple bag cannot, such as soft particles, which can be extruded through bags. The ‘depth’ design of cartridges (layer of rigid construction) means they have more surface area upon which to trap the dirt and enable significantly more dirt holding capacity when compares to a similar size bag. This makes cartridge the filter choice for absolute filtration.
While disposable media such as bags and cartridges usually have a relatively low initial cost, operating costs can be high if charge out is frequent. Media replacement and waste disposal costs can quickly outweigh any ‘savings’ from the lower acquisition cost. Conversely, for applications with low processing volumes or where media replacement is infrequent, bag or cartridge filtration may be the best choice.
Cleanable Filter Media: Several different types of cleanable filter media are available as alternatives to disposable media or as a prefilter in staged filtration systems. They include wire mesh, wedge wire, defined pore, perforated, and sintered metal filters. Cleanable media can often be used in the same applications as disposable bags or cartridges, sometimes with significant labor and cost savings. There are many applications where pressure or flow requirements make cleanable filter media a better choice. When comparing purchase price to operational expenditures, a typical payback can range from six months to one year.
Cleaning of this media type may be done manually, hydraulically or mechanically. Manual cleaning often requires the use of expensive cleaning compounds, and the filter media can be damaged during cleaning. Additionally, work force is obviously required for manual cleaning, and worker safety/exposure issues are raised. Hydraulic cleaning involves using either the process stream or another compatible source of liquid to backwash the filter media. This may be cost prohibitive if the liquid being filtered is very expensive, hazardous, and/or no compatible with an outside source of liquid to the backwashing. Disposal of large volumes of contaminated backwash liquid could also be prohibitive.
There are three main classes of cleanable filters: vibrating screens, backwashing filters, and mechanically cleaned filters. Of these three, only vibrating screens require manual cleaning, but they have limited use in the petroleum process, especially for water treatment. Mechanically cleaned filters are ideal for highly viscous liquids. Again, this does not apply to most water treatment applications.
Backwashing filters work well in high volume applications, typically ranging from 100 gpm and upwards. A minimum pressure of 45 psi and a small volume of liquid are required for backwashing. For these reasons, backwashing filters are often found throughout the petroleum industry.
Ultimately, whatever the process water application, careful consideration and selection of filtration equipment can significantly improve overall system performance. Although most attention for filtration in the petroleum industry traditionally focuses on refining crude, water is a key process component and can help drive optimization. It will reduce maintenance costs, repair costs, and labor requirements. It will also extend the life of expensive and valuable equipment, improve a plant’s competitive position, and help to drive profits.