Paper 06yy
Biodegradable sugar mill lubricant.
What makes it suitable for a sweet but aggressive environment
Hocine Faci
Bob Cisler
BP/Castrol
and
Charles Barrett
BP Americas, Inc.
150 W. Warrenville Rd
Naperville, IL60563
Presented at the 73rd NLGI annual meeting
Lake Buena Vista, Florida
October 29 - 31, 2006
Biodegradable sugar mill lubricant.
What makes it suitable for a sweet but aggressive environment
Abstract
Sugar mill journal bearings, commonly referred to as Top Roll Bearings, Receiving Roll Bearings and Discharge Roll Bearings, are components operated in hostile environments, under extreme loads, in the presence of high volumes of aggressive water, steam, vegetable fibers, solid contaminants, etc. Most conventional mill lubricants showed limitations in maintaining a continuous film of lubricant between the wear components of these bearings. A vegetable oil based product has been developed to address this situation. A tenacious continuous lubricating/sealing film has been obtained leading to lower operating temperatures, reduced lubricant consumption and cleaner waste water.
Introduction
The process of making raw sugar consists of separating the natural sugar present in the cane stalk or beet root from the rest of the plant through a milling process. For cane sugar, the process is carried out in the following steps: the harvested cane enters the mill and is chopped into the optimum size for crushing; the chopped cane passes through a series of rollsin what is typically referred to as a tandem mill where it is crushed and pressed to extract the sugar rich juice; (see figure 1)the juice is heated under pressure and lime is added; the heated juice passes through a clarifier where the impurities precipitate and settle to the bottom and the clarified juice is drawn from the top; the clarified juice is concentrated into syrup by boiling off excess water in a series of connected evaporator vessels; the syrup from the evaporator continues to be heated in vacuum pans where small seed crystals are introduced to the concentrating syrup, the seeded crystals grow in sizeto the required size; the mixture of crystals and syrup is separated in centrifugal machines; the moist raw sugar from the centrifugal machines is tumble dried in a stream of air and transferred to bulk storage bins.[1, 2, 3, 4, 5]
The raw sugar is typically further processed in a refinery that can be an integral section of the mill or an independent facility. At the refinery the sugar can undergo several washing, filtering and re-crystalizing processes before it is packaged and distributed to the consumer market. Beet sugar processing is similar except for the absence of the machinery required to chop and crush cane. Sugar beets enter the mill where they are washed, sliced and soaked in hot water to separate the sugar containing juice from the beet fiber. After the sugar rich juice is separated from the beet fiber the concentrating, crystallizing, separation, drying and refining steps are similar to cane sugar processing.
Cane crushing mills are equipment designed to shred, crush and press the sugar cane between rollers in a series of mill stands [6]. The extraction (Figure 1) is actually conducted as a counter-current process using fresh hot water at one end being pumped in the opposite direction to the cane. The more water that is used, the more sugar is extracted but the more dilute the mixed juice is and hence the more energy that is required to evaporate the juice. The more accurately that the mills are adjusted, the drier is the residual fiber, and hence the less sugar remaining in the fiber.
The journal bearings supporting these rollers, commonly referred to as Top Roll Bearings, Receiving Roll Bearings and Discharge Roll Bearings are components that operate in an extremely hostile environment that includesheavy loads and shock loading in addition to mechanical misalignment, contamination with hot water, acidic sugar cane juice, and other debris such as cane fibers, dirt, etc. These adverse conditions constitute a major challenge to the effective lubrication of these bearings and typically require the use of application specifichigh performance lubricants. This paper will discuss the performance displayed by a non-conventional product in comparison with polymer based products that traditionally have provided adequatelubrication. The physical properties as well as the performance characteristics of this product will be covered along with the product consumption and its impact on the sugar mill rolling process.
Background
Traditionally, the criteria of lubricant selection for this type of application relied mainly on the tackiness/adhesion propertiesas well as extreme pressure characteristics. High viscosity polymeric type products fortified with solid or sulfurized compounds may provide both adequate film thickness and proper load carrying capacity. The field experience, however, showed that lubricant performance wouldn’t categorically improve with the viscosity increase or with the introduction of solids or sulfur content in the finished product. Many high molecular weight (high viscosity) polymers have a tendency to shear down under service stress leading to reduced viscosity base oils under continued operation, and thus lower film thickness and reduced adhesion. Several technologies of sulfurized compounds are available in the market, however some display extreme aggressiveness toward cupric metals. In conclusion, the ideal product for this application wouldbe a product that displays the following characteristics:
- Ability to form a strong film that is resistant toshear and squeeze out under pressure
- Sufficient tackiness/adhesion to make the product strongly adhere to the journal and bearing surfaces
- Sufficient flow to continuously carry away debris from the loaded journal and bearing surfaces
- Sufficient resistance to washout to maintain a consistent lubricant film between the journal and bearing surfacesin the presence of high volumes of hot water
- High base oil Viscosity Index (VI) to assure continuous protection at high temperatures (see Figure 2)
- Protection of ferrous and cupric metal surfaces from corrosion in presence of strong acidic sugar cane juice
- Extreme pressure and anti wear protection against wear and damage from continuous high and severe and shock loads
- A light color would be desirable to minimize discoloring of the processed product
- Preferably bio-based or biodegradable, heavy-metal free and thereforeless environmentally harmful than traditional sugar mill lubricants
Laboratory evaluation
Two samples of conventional commercially available sugar mill lubricantswere evaluated in order to determine their performance properties under laboratory test conditions. Standard test methods were used for evaluation of wear and extreme pressure characteristics, corrosion inhibition, water resistance and mechanical stability. Friction propertieswere also taken into considerationfor performance during low startup operating temperatures, and reduced bearing noise under extreme operating load conditions. The following summarizes the laboratory test methodsdetermined to be appropriate evaluating and comparing sugar mill lubricants [7]:
Water resistance
In grease lubrication applicationswater contamination may cause the structure of the grease to change. It may become softer or harder. It may adsorb water or reject it, and in some instances, it may lose its adhesiveness or sealing capabilities. The use of fresh samples of process water from the actual application(s) for which a lubricant is intended to be used is known to be critical in determining the operating properties of lubricants. The differences in chemical composition, contaminants, pH, and adsorbed gases in process waters versus the standard distilled water or synthetic seawater specified in test methods can have a significant influence on the water resistance and corrosion protection test results. The effect of process water on lubricant resistance in this situation was determined to best evaluated through the use of the following test methods:
Water Spray-off (ASTM D4049)
This method consists of subjecting an evenly distributed layer of lubricantspread at a given thickness on a stainless steel panel to water spray-off. The lubricant is sprayed with water at a given temperature for 5 minutes at a prescribed pressure. The panel is then air dried and weighed. Spray-off resistance is reported as the mass percent of lubricant removed by the water spray. (see figure 3)
Wet Roll Stability (ASTM D1831)
This method consists of running the standard ASTM D1831 roll stability test with the addition of the presence of water. Process water such as that containing sugar cane juice may be used instead of specified standard water. Visual inspection of the lubricant and its penetration change after being subjected to roll shear, along with the presence of free water and its quantity, are determining factors for the water resistance characteristics of the lubricant.
Shear stability
Shear stability is the ability of a lubricant to resist changes in consistency when subjected to mechanical work. The most common laboratory test used to evaluate shear stability is the roll stability test.
Shear stability (ASTM D1831)
In the Roll Stability Test, a small sample (50 grams) of lubricant is rolled at 165 rpm for 1 hour under a given temperature. The difference in worked penetrations (measured with ¼ scale penetrometer) before and after roll is reported.
Corrosion Resistance
In wet applications, such as those in sugar mill environments, lubricants are expected to assure protection against steel rusting and the corrosion of cupric metals.
Steel Rust (ASTM D665 modified)
This method consists of the complete immersion of a cylindrical steel test rod in alubricantsample, removing it and placing it directly into a beaker containing process water from the application for which the lubricant is to be used. The test rod remains in the beaker of process waterat ambient temperature for one week. The test rod is routinely visually examined for signs of rusting during and at the conclusion of the test period. At the conclusion of the test the rod may also be inspected and measured for the degree of rusting.
Copper corrosion (ASTM D 4048)
This method consists of the immersion of a clean polished strip of copper in a lubricant sample at 100°C for 24 hours. If desired, the test temperature and time may be varied from the standard conditions. At the end of the test, the strip is removed and cooled. The strip is then cleaned and inspected for staining. Corrosion may be qualitatively described or rated numerically.
Extreme Pressure /Anti wear
Equipment such as the Four Ball EP and Four Ball Wear tester, to name only two, have played and continue to play a critical role in the development, testing and selection of lubricants.
Four Ball EP (ASTM D2596)
This method consists of 4 balls arranged in the form of an equilateral tetrahedron. The basic elements of the tetrahedron are 3 balls held stationary in a pot to form a cradle in which the fourth (upper) ball is rotated on a vertical axis under pre-determined conditions of loads. The rotating speed is 1770 +/- 60 rpm. A series of 10-second runs are made at successively higher loads until the 4th ball seizes/welds to the 3 stationary balls.
Four Ball Wear (ASTM D2266)
This method uses the same principle as above. The test is run at a specified rotational speed under a prescribed load at a controlled temperature. The test standard duration is 1 hour. The diameter of the wear scars on the stationary balls is measured after completion of the test.
Friction
Multiple test apparatus and numerous methods are employed to measure friction at the contacting surfaces of various shapes and sizes of test specimens. Apparatus includes Four Ball, Pin and Vee, Ring on Block, Block on Ring, Disk on Disk, and Ball on Disk Testers. The SRV test machine was selected for measuring the coefficient of friction in this project because it can be adjusted and programmed to closely simulate the line contact configuration of journal bearings
Friction (ASTM D 5707 modified).
Test specimens consisting of a steel cylinder representing a mill bearing journal and a bronze disk representing a mill bearing shell were used. A detailed description of the SRV test machine can be found in references [8] and [9]. (see Figures 4 and 5)
Biodegradability
Measuring the biodegradable property of lubricants remains a work in process at both national and international standards level. ASTM D-5864, EPA Shake Flask, CEC-L-33-A-93, ISO 9439, OECD 301 A to F, and OECD 302 are a few of tests currently used to measure the biodegradability of lubricants.
Biodegradability (OECD 301B)
The OECD 301B test method has been found to be suitable for determining the ready biodegradability of soluble and insoluble organic chemicals under aerobic conditions. It is the basis for ISO 9439. This method is only applicable to those test materials which, at the concentration used in the test, have negligible vapor pressure, are not inhibitory to bacteria and do not significantly adsorb to glass surfaces. In general, bio-based greases fit this category of substances.
Percentage biodegradation is calculated by dividing the Biological Oxygen Demand (BOD) by the Theoretical Oxygen Demand (ThOD). The calculation of ThOD is based on the percentage composition of the elements C, H, Na, Cl, P, and S. A sample with high water content and/or low carbon content will have a correspondingly high oxygen value (this being calculated by difference). This will result in a low estimation of theThOD, and as a consequence, an overestimation of the percentage biodegradability. The elemental analysis for aqueous samples may also be harder to achieve. Due to these difficulties, Chemical Oxygen Demand (COD) values can be used as an alternative to ThOD
Laboratory Test Results:
Laboratory test results are summarized in Table 1 and Figures 2, 3, 4 and 5
Discussion
Test results displayed in Table 1 indicate the following:
Although the polymeric based products (PBP1 and PBP2) have a kinematic viscosity at 40°C approximately twice as high as that of Biodegradable Sugar Mill Lubricant (BSML), the gap between these viscosities becomes less and less significant at higher temperatures. At 100°C, the viscosity of PBP1 is only 10% higher than the viscosity of BSML, and at 102°C and above, the viscosity of BSML becomes higher than the viscosity of PBP1 (Figure 2). This is due to the higher VI of the BSML base oils. This suggests that the viscosity of the base fluids, and thus the film thickness, is less sensitive to variations in operating temperature.
Also, when qualitatively evaluating the tackiness/adhesion in the presence of water or in the presence of hydrocarbon solvents, BSML showed outstanding resistance to wiping even while in an extremely thin film. Even though it’s recognized that polymeric compounds, because of their non-polar characteristic, play a key role in repelling water away from lubricants, it appears that the biodegradable systemsproduced with vegetable oil provide much better water resistance than the products built with polymeric compounds without thickening systems (Figure 3). The mechanical stability test results obtained in the presence or absence of water are also more favorable in case of the bio-based product. Less shearing and less viscosity change was observed with BSML.
The results of the rust resistance testing (modified ASTM D665) applied to the three lubricants were not surprising. All products tested in this program passed the test with excellent results. However, when tested against cupric metals in the copper corrosion test (ASTM D 4048) only the BSML passed the test with outstanding results.
On the wear and extreme pressure side, when BSML wastested with steel against steel configurations, it showed equivalent to slightly better wear characteristics in Four Ball Wear,but much better results in Four Ball EP compared to the polymer based products. In SRV testsusing a steel-bronze configuration (Figures 4 and 5), the friction was significantly lower with BSML than PBP1 and PBP2. This trend was maintained when the load was increased. PBP2 failed the SRV test after 13 minutes of run time. These SRV results confirmed the results obtained on the Four Ball EP test discussed earlier.
Finally, BSML is a compound with a light amber color and displays no adverse effects on the housekeeping or surrounding environment. It is also a biodegradable compound that presentsreduced environmental risk.
Field Testing
Description
Field testing was conducted in a cane sugar mill in South Eastern Louisiana. The equipment involved in the evaluation was a Tandem mill stand with three operator’s side bearings isolated from the drive side bearings. [10]. Nomenclature for bearings was Top Roll, Receiving Roll and Discharge Roll. These were half-shell, removable bronze segments with water-cooled chocks. Ambient temperatures during the evaluation period ranged between 50°F and 90°F, and component temperaturesranged between 60°F and 120°F. Pitch line speeds of the bearings were <250 fpm and the loads ranged from very high to extremely high depending on throughput of the sugar cane. Environmental conditions included a wet atmosphere with water wash potential, and the presence of steam. There was the continuous potential for severe contamination by dirt, as well as cane fibers and dust (bagasse). The process water was primarily supplied from wells. As a result its physical and chemical properties variedfrom moderately to very aggressive withgroundwater conditions.
Testing of BSML
Several weeks prior to commencing the lubricant evaluation the ambient temperatures in close proximity to and the operating temperatures of the tandem mill stand bearings, mill loads, mill throughputs, and process water flow rates and temperatures were monitored and recorded. A special log sheet was developed and used to record these and all future mill readings and comments. This base line data also was used to produce reliable, repeatable plots of the relationship of mill loading, and mill throughput to the delta between the near proximity ambient and operating temperatures of the tandem mill bearings. (see Figure 6)