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The Magic Media, Part 1: Cellulose

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AGC REFINING & FILTRATION

THE MAGIC MEDIA, PART 1: CELLULOSE 2

Contents What is Varnish? 3

Why Solids Filters Don’t Work 3

So…What’s a Mother to Do? 3

Activated Carbonized Cellulosic Fibers 4

The Allen Cellulose Adsorbent Filter Cartridges 4

References 5



Cellulose is the common material of plant cell walls. It occurs in almost pure form in cotton fiber and in combination with other materials such as lignin and hemi-cellulose, in wood, and plant leaves and stalks. Cellulose is a long-chain molecule (polymer) made up of recurring units of glucose, a simple sugar. The structural unit is shown below in Figure 1.

Because of the strong hydrogen bonds that exist between cellulose chains, cellulose does not dissolve in common solvents. The positions of the hydroxyl (-OH) groups protrude laterally along the molecule chain. Those positions make them readily available for “hydrogen bonding.” As a consequence, cellulose can adsorb large quantities of polar compounds such as water and other contaminants. Bundles of cellulose molecules are aggregated together in the form of micro-fibrils in which highly ordered (crystalline) regions exist with less ordered amorphous regions. When the fibers adsorb water or other polar contaminants both intra-crystalline and inter-crystalline swelling occurs. As the fiber swells, inter-molecular bonds are broken as a result of internal stresses produced by swelling. With very strong swelling agents it is possible to reach a critical point where the entire crystalline structure is disrupted and the fiber structure is lost. Some binary mixtures of liquids can produce more swelling than either compound alone. This effect is particularly true when water is one of the liquids.

When used in bulk filter cartridges, the bulk density of the compressed fibers has to be around 1.00 grams per cubic centimeter. At this density the swelling is very rapid. The swelling power of water reaches in excess of 90%, while that of diesel or fuel oil only reaches 2%. This is what makes cellulose such a good adsorbing material when it is used to remove water from fuel.

A lesser known application of cellulose adsorbing filter elements is the removal of varnish and sludge from lubrication oil.

What is Varnish?

It is that thin film that deposits itself on servo valves and bearings. It is a high-molecular weight substance that is insoluble in oil. When suspended in oil, it is made up of 75% soft contaminants that are less than 1-micron in size. They cannot be measured by traditional particle count nor can they be filtered out. However, these insoluble compounds have polar affinities and as such they migrate from the oil to machine surfaces under the influence of environmental factors such as temperature and pressure. The presence of other contaminants such as water and wear metal particles further enhances varnish and sludge production.

Why Solids Filters Don’t Work

In an effort to remove the varnish and sludge precursor’s operators are using finer filtration. While this is totally ineffective, it has the undesirable effect of creating static charge buildup in the oil system. The discharge of the static charge creates arcs with extremely high temperatures. This auto-degradation effect further damages the oil.

So…What’s a Mother to Do?

Remember cellulose? Remember the hydroxyls (-OH) sticking out all over the millions of square feet of crystals? AHA!! Varnish and sludge particles are polar compounds that will be attracted to these hydroxyl spikes. They will bond with them in a process called adsorption, which is the chemical bonding of particles to a surface called the adsorbent. Adsorption is dependent on temperature, flow rate, concentration, etc. that solids filters are less sensitive to.

Cellulose fiber filter elements can sustain higher flow rates than normal solids filter elements and are used in a “kidney-loop” continuous filtration on an oil tank. They are inexpensive and dependably remove moisture, acids, varnish, and sludge.



Activated Carbonized Cellulosic Fibers

Cellulose fibers, such as cotton, are hollow as is shown in the photomicrograph below.

Cotton fiber also has a hollow structure that helps increase surface area and porosity. Use of ACF for nonwoven production will greatly enhance nonwoven performance and to expand end-use applications including military protective clothing, solvent recovery, wastewater treatment, water purification, air cleaning, and acoustic insulation. A photo-micrograph of a cotton fiber is shown below.

Studies have shown that activated carbon fiber with high adsorption capacity and micro-porosity can be prepared from rayon and cotton nonwoven fabrics by heating at 800 °C for four hours. A photo-micrograph of carbonized cotton fiber is shown below.

The Allen Cellulose Adsorbent Filter Cartridges

Standard filters are manufactured using a cellulose filtration medium. A protective polypropylene netting is applied to the exterior of the cartridge. Plate end caps that engage most standard industrial filter housings. Nominal particulate rating (20 μm) is for >85% of a given size as determined from single-pass

particle counting results. †

Standard Allen filter cartridge dimensions are 7” x 18” and 11” x 18”. They are sold in cases of four (4) elements per case.

Common applications are to remove moisture from diesel, kerosene, and jet fuel as well as transformer and breaker insulating oil.

†Nominal Filter Rating: Filter rating indicating the approximate size particle, the majority of which will not pass through the filter. It is generally interpreted as meaning that 85% of the particles of the size equal to the nominal micron rating will be retained by the filter.



References 1. Nan Jiang Doctor’s Thesis. Louisiana State University, December 2008.

2. Mantanis, G.I. et al. “Swelling of Compressed Cellulose Fiber Webs in Organic Liquids, Cellulose.” 1995, vol. 2, 1–22.

3. Magats, S. “Varnish in Turbine Oils—Causes, Effects and Solutions.”

4. Atherton, B.G. ”Discovering the Root Cause of Varnish Formation.” Practicing Oil Analysis, March 2007.

5. Stover, J. “ Adsorption: A Simple and Cost-Effective Solution to Remove Varnish.” Practicing Oil Analysis, March 2008.

Figure 1: The Structural Unit of Cellulose

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