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John Amos Plant in St. Albans, WV
The mean time between scheduled outages is becoming longer and the duration of the outages is growing shorter. Welcome to the new world of equipment maintenance. Reliability managers and engineers are looking for items to remove from the shutdown task list. This naturally creates pressure to eliminate unnecessary oil changes, leading many in the power generation industry to look at in-situ lubricant reconditioning options, especially in the larger systems such as turbine generators.
If the lubricant can be effectively reconditioned while the machine is in service, an oil change during the outage can be avoided. More importantly, the ability to recondition while online gives options to plants that find themselves with degraded oil, and no opportunity for a shutdown in sight.
The demand for more service life from lubricants has increased interest in filtration and separation technologies. Ion exchange is among the more promising technologies for chemical reconditioning. The technique has some advantages over conventional adsorptive acid removal technologies such as fuller’s earth and activated alumina. Ion exchange does not leave metal salts of calcium, aluminum and/or magnesium in the oil, which can cause deposits.
Rather, it actually removes the salts. In contrast to either fuller’s earth or activated alumina, ion exchange resins do not contain extractable metals that can migrate into the oil. Ion exchange allegedly does not remove additives. In fact, it has been shown that some aggressive resins do remove certain phosphate-based additives from certain fluids, such as polyolesters.
Ion exchange has been used successfully to recondition phosphate ester-based fire resistant hydraulic fluids. Because there is some disagreement about the technology’s ability to de-acidify other synthetic fluids and mineral oils, we have asked two noted experts, Dr. John Duchowski and Peter Dufresne, Sr. to offer their opinions about what ion-exchange technology can and cannot do.
Dr. Duchowski graduated from Simon Fraser University in Vancouver, BC and obtained his doctorate at Carnegie Mellon in Pittsburgh, Penn. In 1992, he joined Pall Corp.’s scientific and laboratory services department where he held the position of senior staff scientist for seven years. He then moved to Pall Machinery & Equipment where he holds the position of technical director. His work focuses on projects aimed at better understanding the effects of physical and chemical contamination on the performance of hydraulic and lubricating systems and on fluids employed in those systems. Duchowski is a member of the American Chemical Society, the American Physical Society and the Society of Tribologists and Lubrication Engineers.
Peter Dufresne, Sr. graduated from Northrop University, Inglewood, Calif. in 1968 with a degree in aircraft maintenance engineering. He has earned numerous diplomas and technical certificates related to gas turbines. A licensed FAA airframe and power plant technician, he holds two U.S. patents related to the use of ion-exchange resins for the reprocessing of synthetic and hydrocarbon fluids. Dufresne has worked at Environmental and Power Technologies Ltd. (EPT) since 1994. He currently serves as president and chief technical officer.
ML presented the following questions independently to John and Peter. Here are their responses:
Thus far we have conducted only limited experiments with mineral oils. In general, I would say ion exchange does not remove acid from mineral oil because the ion-exchange process requires the surrounding medium to exhibit at least a limited measure of polarity. We have data that shows the treatment is more effective with more polar fluids (phosphate esters) than with less polar fluids (polyolesters) and nearly completely ineffective with nonpolar fluids (transformer oils).
We plan to publish these findings shortly. On the other hand, we have carried out limited experiments with wet paper machine oils where we have found that ion exchange can remove acidic species as manifested by reduced acid number in the water phase. I must emphasize, however, that the treatment’s effectiveness under these circumstances is most readily explained by the presence of water.
Yes, acids can be removed from mineral oil; however, some qualification is required. We have established that hydrocracked mineral oils respond quite well when treated with selected ion-exchange resins. On a 72-hour test run of a hydrocracked mineral oil, the acid number (AN) was reduced from 0.46 to 0.06, and rotating pressure vessel oxidation test (RPVOT) increased 30 percent (Figures 1 and 2).
Improvement in RPVOT is a good indication that the resins are removing some of the oil degradation products. Solvent-refined oils do not seem to respond as well to ion-exchange resins, although more testing will be required to prove this as the rule.
Again, this depends on the polarity of surrounding media. The less polar the environment, the less effective the treatment. As the name of the medium itself (ION) implies, the process does depend on the treated material’s ability to detect and respond to the presence of electric charge. Ionic processes do not occur in nonpolar environments.
Although it is true that oxidation products are usually more polar than the base stock from which they were derived, they still need to be able to detect the ionic substrates in the ion-exchange resin. Again, simple physics as well as experience dictates that this is not possible in nonpolar environments that do not support charge separation. For example, it is well-known that oil degradation proceeds through free radical rather than ionic mechanisms.
Because phosphate ester degradation products are water-soluble, ion-exchange resins are able to remove these types of products. In the mineral oil environment, existing varnish would not be removed. We know that varnish deposits require significant oxidation and acids to electrolytically plate-out on hot surfaces. Removal of acids and scavenging of acids as they are produced clearly reduces the fluid’s ability to coke and produce further deposits.
Ion-exchange resins contain 65 percent to 70 percent water by weight. This is how they are manufactured and stored. This water, which can enter the system being treated, needs to be removed to prevent any unwanted side reactions. Consequently, Pall Corp. recommends that ion-exchange treatment be carried out in conjunction with mass transfer/vacuum dehydration to ensure that this excess water is removed.
Ion exchange (I-X) does add water to the oil, due to the viscous nature of the fluid. The amount of water will depend on the number of I-X cartridges used and their operational interval. Data clearly shows that if the cartridges are operated for a sufficient time, that the water will be rescavenged by the resin (Figure 3).
Some resins are severe desiccants. The data shows that the water content during the test reached 1200 parts-per-million, and without any form of water removal, fell below the starting water level. To be absolutely safe, we recommend that vacuum dehydration be incorporated into the acid scavenging system.
EPT 1208EHC Ion-Exchange Cartridge for Alstrom/ABB Steam Turbine
Given that mineral or synthetic hydrocarbon oils (such as PAOs) do not lend themselves to this treatment for reasons explained above, I would say that I see only a limited potential for application of this treatment in typical lubrication applications. However, if a lubricating system is equipped with a phosphate or a polyolesters, such a system could be effectively treated with ion exchange.
Due to the low cost of mineral oils, one would not normally consider the use of expensive ion-exchange resins to reprocess them. The case is made in applications that will experience very costly loss-of-production hits associated with fluid replacement and flushing requirements (two to five days).
Steam Turbine Installation at Rutenberg Plant in Israel
“Ion exchange essentially trades a water molecule for an acid molecule in the host oil. This results in a decrease in the oil’s acid number (AN), and the addition of water to the oil.” This is NOT a true statement. In nonpolar environments, there is no exchange; the process works on adsorption. Water is released from resin which is stored wet and does NOT form as a result of the exchange process. The true exchange process is operative only in aqueous, protic (hydrogen-ion containing) environments, not even in ester-based oils, to say nothing of petroleum-based oils.
The principles of ion exchange in nonaqueous solutions continue to be hotly debated. When I started researching the degradation process of phosphate ester fluids in the mid-1980s, I realized that the only media that might resolve the chronic problems associated with fuller’s earth, activated alumina and to a lesser degree, Selexsorb, was I-X resins.
When I discussed my thoughts with the specialists who manufactured the fluids and the resins, I was confronted with the same response from both: It won’t work. To date, Environmental and Power Technologies has more than 1,000 equipment years of absolute success at solving the problems associated with other media.
The most common problem associated with the use of resins in oils is the false assumption that resins are water-treatment technology and therefore must behave the same in the oil environment. This is a bad assumption! In the oil environment, water is not produced and acid neutralization does not take place. The requirement of water in the resin is critical because it activates the ion exchange reaction. Our experience has shown that resin moisture content of 50 percent by weight is ideal.
Resin water content decreases due to the viscous nature of the oil, but is recovered by the resin in fairly short order. I-X cartridges can last up to 27 months with water levels remaining at or below historical reservoir water levels. Generally, reservoir desiccants or fluid vacuum dehydration is not required, but in some cases can be application-specific.
I-X is proving to be an effective acid scavenging media for other synthetic fluids: polyolesters, diesters, PAOs and water-glycol solutions.
Rutenberg Plant, Israel
For Further Reading
Collins, K. and Duchowski, J. “Ion Exchange/Vacuum Dehydration Treatment of Phosphate Ester Fluid.“ Machinery Lubrication magazine, September-October 2001.
Dufresne, P. “Mega Savings Using Ion-Exchange Filtration Process.” Lubrication and Fluid Power, February 2002.