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A Comparison of Mass Transfer - Vacuum Dehydration and Other Fluid Conditioning Methods

Kal Farooq, Pall Corporation, Port Washington, NY

William Herguth, Herguth Laboratories, Inc., Vallejo, CA

Fluids in hydraulic and lubrication systems get contaminated with water, air/gases and in some applications with lighter hydrocarbons, refrigerants and solvents. Fluid conditioning, the terms used for the removal of these contaminants combined with filtration to remove particulate contaminant, is a relatively straightforward and cost effective way to make these fluids suitable for continued use, thus extending their service life and reducing waste. Notable among the methods commonly used for fluid conditioning are the mass transfer vacuum dehydration and flash distillation-vacuum dehydration methods. This paper discusses the relative merits of the common fluid purification methods and presents experimental data that demonstrates the effects of temperature and pressure on water and gas removal, and its impact on the chemical and physical properties and the additive packages of hydraulic and lubrication fluids. Key Words: water, mass transfer, vacuum dehydration, hydraulic fluids, lubrication fluids, and additives. Introduction: In addition to particulate contaminants, fluids in hydraulic and lubrication systems get contaminated with water, air/gases and in some applications, such as compressors and pumped lube systems, with lighter hydrocarbons, refrigerants and solvents. Fluid conditioning or purification, the term used for the removal of these contaminants along with filtration for the removal of solid contaminants, is a relatively straightforward and cost effective way to make these fluids suitable for continued use, thus extending their service life and reducing the waste stream. Methods commonly employed for the purification of the fluids include dry air purge, coalescence, centrifugation, absorbent filtration, settling (settling tanks), mass transfer vacuum dehydration and flash distillation vacuum dehydration. Methods such as centrifugation and coalescence rely on purely mechanical means, based on phase separation. Whereas flash distillation vacuum dehydration methods use a more aggressive approach, employing flash evaporation of the volatile contaminants including water at temperatures well above those normally found under standard system operating conditions, and at significantly reduced pressures, the mass transfer vacuum dehydration method, on the other hand, employs moderate level of vacuum and virtually little or no additional heating. The technique is referred to as mass transfer because it predominantly relies on the transfer of water into a steady stream of dry air under moderate vacuum and temperature conditions. The intent of this paper is to compare and contrast the various technologies employed for removal of water and other volatile contaminants from hydraulic and lubricating systems, and to examine the impact of vacuum and temperature on the chemical and physical properties of the hydraulic and lubricating fluids.

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Water contamination: Water contamination in hydraulic and lubrication systems is commonplace due to waters ubiquitous nature and its ingression into these systems through breathers, condensation, leaks, during storage, transportation and handling. Presence of water adversely affects the fluid and the system components. Water affects the viscosity and load carrying ability of the base fluid, and promotes its oxidation or hydrolytic breakdown in the case of ester base stocks, which in turn generates acids and sludge. Water causes corrosion of metallic surfaces and promotes fatigue failure of bearings by hydrogen embrittlement (Reference 6). Water may be present in the form of free, emulsified or dissolved water in the base fluid. Free and emulsified water exist when the amount of water in the fluid is in excess of fluids capacity to dissolve water. Free water is composed of discrete droplets in the 0.1 to 10 m size range, that, under favorable conditions, coalesce and settle to the bottom of the reservoir. The presence of free water, for extended periods of time, at the reservoir bottom often results in biological growth and its associated problems. Emulsified water is a form of free water, which exists as a colloidal suspension, forming a stable heterogeneous phase that does not separate from the fluid by gravity even at elevated temperatures. Soluble water is the water dissolved in the fluid phase as a homogeneous, single phase. The solubility of water in hydraulic and lubrication fluids is dependent on the type of fluid base stock and the additive package, and generally increases with temperature and the age of the fluid. Figure 1 shows the solubility of water in common industrial fluids. Figure 1 Water Solubility of Common Industrial Fluids

Proactive maintenance requires setting target water concentration limits for the fluid. This target will vary depending on the application. Lubricant applications such as steam turbines, diesel engines, dryer rollers (paper mills), screw compressors, and industrial

Temperature vs. Saturation Point (ppmw) for Various Fluids

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gearboxes, each have their own unique requirements when it comes to moisture control. As a general rule, 100 ppm is a reliable limit for many applications in terms of lubricant and bearing life. However, in view of the ingression potential of moisture in certain applications, higher limits may be more practical. Aeration of hydraulic and lubrication fluids can be caused by a myriad of reasons including poor system design, fluid degradation, suction side air leaks, etc. Fluid aeration is undesirable since it affects the response and control of hydraulic actuators, causes cavitation of valves and pumps, and results in loss of lubrication film, reduced fluid viscosity, dieseling (adiabatic compression of air bubbles resulting in thermal degradation of the fluid) and accelerated oxidation of the fluid. In the case of gas compressors and internal combustion engines, gases from the process are introduced in the lubrication fluid causing chemical and physical degradation of the lubricant. In the case of gas compressors, where hydrocarbons blow by piston rings, the blow by gases can result in dilution of the fluid, and reduced viscosity and flash point. Fluid conditioning methods: Mechanical separation methods that include settling, coalescence, absorbent filtration, and centrifugation are limited to the removal of free and emulsified water. Mass transfer and flash distillation type vacuum dehydrators, on the other hand, remove not only the free water but also the dissolved water, free / dissolved air and other gases, and lighter hydrocarbons, solvents and refrigerants. Dry air purge will not remove dissolved air but will strip dissolved non-environmental gases, lighter hydrocarbons, solvents, and refrigerants, in addition to free/dissolved water. Following is a brief discussion on each of the commonly used fluid conditioning method. Coalescers remove free water entrained in the oil phase by capturing and coalescing water droplets into larger droplets and separating them from the oil phase. Specific gravity, viscosity, and interfacial tension of the fluid are key parameters in the process. Levels as low as 10 ppm free water can be obtained with influent conditions of 10 % water by weight and an interfacial tension of 2 dyne/cm and higher. Coalescers tend to disarm (become ineffective) in the presence of surface-active agents in the fluid. Coalescers also need fine filtration for protection against fouling by solid contaminants. The process is most effective with low viscosity fluids. Centrifugal separators utilize the difference in specific gravity between the fluid and the water for the separation. Industrial centrifuges are designed to generate centrifugal forces on the order of 3,000 to 10,000 times higher than gravitational force, hence speeding up the separation of water by the same magnitude as compared with gravitational separation, for example, in a settling tank. Centrifuges can also remove some emulsified water depending upon the relative strength of the emulsion vs. the centrifugal force of the separator. Centrifugal separators do not remove dissolved water. Centrifuges are well suited for applications where continuous decontamination of fluids with excellent demulsibility (water separating characteristics) is required. Water absorbent filters remove free and emulsified water by super absorbent polymers impregnated in the media of the filter cartridge. The water is absorbed by the polymer, causing it to swell, and remains trapped in the filtration medium. Super absorbent filters can remove only a limited volume of water before causing the filter to go into pressure-

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drop induced bypass. They are not well-suited for removing large volumes of water, but are a convenient method to maintain dry conditions in systems that dont normally ingest a lot of water. These filters do not remove dissolved water. Mass transfer vacuum dehydration type purifiers work on the principle of mass transfer of the liquid and gaseous contaminants from the oil to a constant stream of dry filtered air under vacuum. The process, using techniques such as sheet-metal rings, nozzles, spinning disc, etc. generates a large surface area of the fluid. The vacuum draws ambient air into the chamber, expanding the air volume several times consequently decreasing its relative humidity by the same ratio. For example, the vacuum inside the Pall HVP series purifiers expands the air about 5 times, hence drying the air drawn into the chamber to 5 times its value at ambient condition. At a typical 50 % ambient air relative humidity (RH), the air is dried to ~ 10 % RH inside the vacuum chamber. When the thin layer of the fluid comes in contact with the 10 % saturated air, moisture and gases transfer into the air phase until equilibrium is achieved between the fluid and air phases. Vacuum also lowers the vapor pressure of the water thus helping with its transfer into the dry air. The process exposes the oil to relatively mild vacuum (22-24 Hg) and near ambient temperatures. The purifiers can be fitted with in-line heaters if needed in cases such as colder environments and for high viscosity oils. This technique effectively removes free, emulsified and dissolved water along with other volatile contaminants. Figure 2 shows a schematic diagram of Pall HNP series mass transfer type purifier.

Figure 2 Schematic Diagram of Pall HNP Series Fluid Purifier

High vacuum / heat purifiers (flash distillation vacuum dehydration) utilize higher vacuum and temperature conditions inside a chamber to rapidly boil off water and other volatile materials. The absolute temperature vs. pressure plot based on the Clausius-Clapeyron equation for water, shown in Figure 3, indicates the vacuum and temperature

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conditions required for the water to transition from the liquid phase to vapor phase and hence, boil-off. Flash distillation type equipment is often operated at vacuum and temperature conditions that are well within the vapor phase region of the plot for faster removal of water. Unlike the mass transfer type equipment, there is no purging of the vacuum chamber with dry air and the vacuum and temperature levels are more severe. Vacuum levels of >26 Hg and temperatures >160 F are commonly used in these equipment. Vapor condensers are often used to remove the vapors before they get to the vacuum pump. By virtue of higher vacuum and temperature levels, these units offer higher water removal efficiencies for each pass of the fluid compared with that of the mass transfer vacuum dehydration type purifiers, but they also expose the fluid to higher thermal stresses in the process.

Figure 3 Plot for Water Vapor Pressure vs Temperature Based on Clausius-Clapeyron Equation

Clausius-Clapeyron Equation for Water

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Table I provides a summary of the common fluid conditioning methods, their contamination removal capabilities and the various pros and cons associated with each method.

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Table I Summary of Common Fluid Purification Methods

Phases of Water Removed

Purification Method

Free Emulsified Dissolved

Gases / Solvents Removed

Pros and Cons

Centrifuge X X1 High removal rate of free water only. High initial cost and maintenance.

Coalescer X X1 Removes free water, sensitive to surfactants and dirt (requires fine filtration)

Absorbent Filter

X X1 Removes free water only. Limited water holding capacity.

Dry Air Purge

X X X X Low initial cost, removes dissolved water also. Slow process requiring dry air source.

Flash Distillation -

Vacuum Dehydration

X X X X High single pass removal of free/dissolved water, gases, solvents. Severe operating conditions stress the fluid.

Mass Transfer vacuum

dehydration

X X X X Removes free/dissolved water, gases, and solvents. Mild operating conditions; gentler on the fluid.

1 Removal of emulsions is obtained only under appropriate conditions. Experimental Work: Tests were conducted using Pall Model HNP021, mass transfer vacuum dehydration type purifier to determine the effect on the additives and the fluid base stock. Tests were conducted on a popular brand of anti-wear hydraulic fluid and a rust & oxidation, double inhibited steam turbine lubrication fluid. Both fluids were mineral based with a viscosity rating of ISO VG 32. Tests were conducted on a 50 gallon fluid volume with the starting water concentration of 1 % by volume. The HNP series purifiers employ nozzles to generate large surface area of the fluid for the efficient transfer of moisture and other volatile contaminants to the dry air. The purifier was operated under six vacuum/temperature combinations with the maximum vacuum of 26 Hg and maximum temperature of 158 F. The extreme vacuum and temperature conditions were utilized to mimic the typical operating conditions inside flash distillation- vacuum dehydration type equipment. Pall HNP purifiers normally operate in the vacuum range of 22-24 Hg and ambient temperatures. An external heater was used since the purifier does not include a built-in heater.

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Table II lists the tests conducted on the fluid samples and the objectives of the test.

Table II List of the Tests Conducted on the Fluid Samples Test ID Objective

Karl Fischer per ASTM D1744 Total water content

Dissolved Gas Analysis per ASTM D3612-01

Dissolved gas (air) content

RULER per ASTM D6810-02 Remaining phenolic antioxidant level

Rotating Pressure Vessel Oxidation Test (RPVOT) per ASTM D2272-98

Oxidation stability of the fluid

Pressure Differential Scanning Calorimetry (PDSC) per ASTM D6186-98

Time for the onset of fluid oxidation

Fourier Transform-Infrared (FT-IR) Spectroscopy

Presence of hindered phenol anti-oxidant additives

Experimental Results: The samples of the hydraulic and turbine lube fluids treated under the extreme vacuum and temperature conditions (26 Hg and 158 F) and the new fluid samples (for baseline) were analyzed for oxidation resistance. Table III lists the results for the four fluid samples. The magnitude of the phenolic peaks obtained through the FT-IR analysis shows a 5.7 % decrease for the processed turbine fluid sample. For the hydraulic fluid samples, the phenolic peaks show an increase of 7.9 % in the processed sample compared with the new sample. In a discussion with the laboratory personnel it was concluded that both changes are within the margin of error of the experimental technique, and, therefore, are not significant.

Table III - Results of the Fluid Analysis

Sample ID FT-IR

(Phenolic Peak) RPVOT Test PDSC Test RULER

Turbine Fluid- New 0.088 422 minutes 80.7 minutes N/A

Turbine Fluid-Treated (26 Hg, 158 F) 0.083 317 minutes 90.8 minutes

Yellow solution A mode: 90%

Hyd. Fluid-New 0.0267 128 minutes No clear breaks N/A

Hyd. Fluid -Treated (26 Hg, 158 F) 0.0288 206 minutes No clear breaks

Green solution V mode: 101%

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The results of the RPVOT test, which is a measure of the fluids resistance to oxidation, show a decrease of 25 % in the time required for oxidation of the treated turbine fluid compared with the new fluid sample. The results of the RPVOT oxidation stability test for the hydraulic fluid show a large increase (61 %) for the processed sample, most likely indicative of the fact that the test is not suitable for this specific fluid. The test for the onset of fluid oxidation by the PDSC method also measures the oxidation resistance of the fluid, like the RPVOT test, but does not use copper catalyst. The results of the PDSC test show a longer oxidation onset time for the treated turbine fluid sample, and no onset of oxidation up to 210 F for the treated hydraulic fluid sample. The significant decrease in time (25 %) for oxidation observed for the treated turbine fluid sample by the RPVOT test, in contradiction with longer oxidation onset time by the PDSC test, is an anomaly that needs to be studied further. The test to determine the remaining phenolic antioxidant, using RULER, shows a 10% decrease in the current flow, compared to new turbine fluid sample. There was no decrease in the current flow for the processed hydraulic fluid sample. The RULER results, shown in Table III, are for current transmittance in the processed samples, using the fresh samples as baseline (100 % current transmittance). A drop of 10 % in the remaining phenolic antioxidant would not be considered significant in this case. The above results suggest that the two fluids treated under vacuum and temperature conditions of 26 Hg and 158 F, respectively, typical for the flash distillation vacuum dehydration type process, are not significantly impacted in terms of the antioxidant properties, as measured by FT-IR, PDSC and RULER methods. The results of the dissolved gas analysis, performed on the new turbine lube fluid sample from the drum and the processed fluid samples, obtained in syringes at the outlet of the purifier, are shown in Figure 4. The results indicate that temperature had no effect on the reduction of the dissolved air content of the purified fluid for the temperature range examined. The values indicate 69 % decrease in the dissolved air content as a result of the purifier treatment. The removal of gases from the fluid is determined by the partition coefficient of the particular gas between the fluid and the dry air under the operating temperature and vacuum conditions. Had the oil been contaminated by a non-environmental gas or solvent / refrigerant, the removal of the gas or solvent would have been much higher due to the higher mass transfer gradient (the dry air would have zero concentration of the contaminants when it entered the vacuum chamber).

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Figure 4 Dissolved Air Content of the Turbine Lube Fluid Samples

Reduction in Dissolved Air Content at 26" Hg

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The water concentration levels in the hydraulic fluid samples, following 3 passes of the fluid volume (20 minutes of operation) through the purifier, are shown in Figure 5. The starting water concentration was 1 % (10,000 PPM). As the results, plotted below, indicate, the higher vacuum and temperature resulted in lower water concentration. The water concentration value obtained for the 18 Hg and 158 F trial was similar to the values obtained at higher vacuum trials. The values obtained at the typical HNP021 purifier operating conditions of 22 Hg and 113 F were similar to more severe, flash distillation vacuum dehydration type equipment condition of 26 Hg and 158 F. Figure 5 Water Concentration of the Hydraulic Fluid Samples

Water Concentraion of the Hydraulic Fluid Under Various Operating Conditions

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Conclusions: 1. The purifier test results suggest that the anti-wear hydraulic fluid and the R&O turbine lube fluid treated under vacuum and temperature conditions of 26 Hg and 158 F, respectively, typical for the flash distillation vacuum dehydration type process, are not significantly impacted in terms of their antioxidant properties, as measured by FT-IR, PDSC and RULER methods. The contradictory results for the RPVOT test for the turbine lube fluid, showing reduction in oxidation resistance, is an anomaly that needs to be studied further. 2. The results indicate that temperature had no effect on the reduction of the dissolved air content of the purified fluid for the range of temperatures examined. The results also indicate a 69 % decrease in the dissolved air content as a result of the purification treatment. 3. The reduction in water concentration obtained through the mass transfer vacuum dehydration process, operating at relatively mild conditions of 22 Hg and 113 F, were similar to the more severe, flash distillation vacuum dehydration type equipment conditions of 26 Hg and 158 F. References: 1. Bloch, Heinz P. Exxon Chemical Company, USA Baytown, Texas 77520, "Criteria for Water Removal from Mechanical Drive Steam Turbine Lube Oils." Lubrication Engineering. Volume 36, 12, 699-707 (Presented at the 35th Annual Meeting in Anaheim, California, May 5-8, 1980). 2. Cantley, Richard E. (Member, ASLE) The Timken Company Canton, Ohio 44706 "The Effect of Water in Lubricating Oil on Bearing Fatigue Life." ASLE Transactions, Volume 20, 3, 244-248. 3. D6810-02 Standard Test Method for Measurement of Hindered Phenolic Antioxidant Content in HL Turbine Oils by Linear Sweep Voltammetry 4. D2272-98 Standard Test Method for Oxidation Stability of Steam Turbine Oils by Rotating Pressure Vessel 5. D6186-98 Standard Test Method for Oxidation Induction Time of Lubricating Oils by Pressure Differential Scanning Calorimetry (PDSC) 6. Hydrogen Degradation of Ferrous Alloys. (p. 243) Edited by: Oriani, Richard A., Hirth, John P.; Smialowski, Michael 1985 William Andrew Publishing 7. Perrys Chemical Engineer Handbook, 7th Edition (p. 2-349)