Filtration

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Which grade of cellulose is right for my application?

Too Course Too Fine Right

Cellulose too course:
some dirt trapped but filtrate still dirty; filtration not achieved.

Cellulose too fine:
dirt all trapped on surface; inefficient removal known as "surface blinding."

Cellulose grade right:
dirt penetrates filter cake but does not pass through; fitrate clear.

As illustrated above, the most efficient grade of cellulose for your application is the grade that allows particulate penetration, ultimately trapping all dirt and providing the required cleanliness level. Through laboratory bench testing of representative samples, your Purifiber technical support team can recommend the grade of cellulose that is just right for you.

Key Areas to consider:

Sufficient filter aid must be introduced in the system to adequately coat all of the filter septum area. The cellulose filter aid is necessary to protect the septum (screens, cloth, etc.) from becoming plugged by the application contaminate.

Proper grade of cellulose: Once the type and quantity of particulate contaminate of the influent stream are identified, then the proper grade of cellulose can be selected. It is important to utilize the grade of cellulose that will achieve the greatest efficien- cies to minimize your disposable media costs.

Dosage Rates: In general, when starting new filtration equip- ment, use 1/3 pounds of cellulose per 10 square feet of filter surface. Using the correct quantity of cellulose in your filtration equipment is key to establishing long filtration cycles and trouble free equipment operation.

*Depending on the type of filtration equipment, it may be possible to gradually reduce the amount of cellulose filter aid to optimize the system. The reduction should be a gradual process and not implemented until the filter system is at full flow or maximum operating capacity and equilibrium has been established.

Degree of Filtration Vs Grade

The relationship between the "grade" of cellulose filter aid and the resultant filtrate clarity are directly related to the density and average particle size of the fibers. Use of the coarser grades, i.e. longer fibers, lower density powders, in filtration results in a lower degree of filtration achieved or less particles retained. Use of the finer grades, shorter fibers, more dense powders, results in a higher degree of filtration clarity, or more particles captured from the filtrate stream.

Permeability Vs Cellulose Grade

Permeability is the ability to achieve flow at a given differential across a wet bed of cellulose filter aid. As the grade of cellulose becomes finer, the density increases, resulting in reduced perme- ability for flow. While the clarity achieved is better, the ability to flow through (the torturous path) is reduced. Conversely, use of the more coarse grades of cellulose filter aid results in higher permeable filter cakes and greater flow at a given differential. The gain in permeability is the loss in degree of filtration.

Flow Rate Vs Cycle Times

For all grades of cellulose filter aid, the cycle time of the filter (time between cleaning) decreases exponentially as the rate of filtration, or flux rate is increased. Simply stated, lowering the flux rate (or rate of filtration) for any given application makes the filter more efficient. By slowing down the rate of filtration, the cellulose filter aid cake is able to capture and hold more contaminates. As this is an exponential relationship, the size of the filter provided for any application is extremely critical. Your Purifiber® technical support team is ready to assist you to make your filtration systems as efficient and cost effective as possible with the right cellulose grade, and the proper rate of filtration.

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