Base oils: Manufacture, properties, performance and applications
Stuart F. Brown, Contributing Editor | TLT Webinars March 2014
How to select the correct base oil before formulating a lubricant.
KEY CONCEPTS
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Four base oil properties should be considered in evaluating lubricant performance.
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Base oils are classified into five groups according to their ingredients and performance characteristics.
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Both synthetic and rerefined base oils have generated some demand in recent years.
MEET THE PRESENTER
This article is based on Webinars originally presented by STLE University. “Base oils: Manufacture, properties, performance and applications” is available at
www.stle.org: $39 to STLE members, $59 for all others.
John Rosenbaum has a bachelor’s degree in chemistry from San Diego State University and a doctorate in materials science and mineral engineering from the University of California at Berkeley. He is the senior project development engineer at Chevron Global Base Oils in Richmond, Calif., where he has spent 32 years working for Chevron. He has 17 years of experience researching catalysts and process technologies, fuels and lubricant base oils. He is also responsible for spearheading Chevron’s experimental program to make liquid fuels and lubricant base oil from natural gas and worked on the ISODEWAXING® Catalyst System, as well is the author/co-author of over 60 U.S. Patents and several publications. You can reach John at
rosj@chevron.com.
John Rosenbaum
ANALYZE THE CONTENTS OF A QUART OF MOTOR OIL and you will discover that approximately 75 to 85 percent of its volume consists of what are known as base oils. The remainder is a package of additives that confers properties essential to protecting the moving parts of gasoline and diesel engines. Viscosity modifiers, antioxidants and corrosion inhibitors are a few of the ingredients ensuring that modern motor oils meet the demanding performance specifications of vehicle manufacturers.
A supplier of motor oil additives will be happy to tell you that the additives are obviously the important part of the product and the base oils are just there to deliver these important ingredients to the bearings, piston rings and other rotating and reciprocating engine parts requiring continuous lubrication.
But wait just a minute. “The base oils themselves confer important properties to lubricants and are much more than mere carriers for other ingredients,” says STLE-member Dr. John Rosenbaum, senior project development engineer at Chevron Global Base Oils. With lots of marketing hoopla in the air about synthetic and even renewable lubricants, this is a good time for an overview of the predominant categories of mineral oils—the emerging “boutique” base oils, and of the manufacturing processes used to make them.
In a November 2011 STLE University Webinar, Dr. Rosenbaum offered an illuminating review of the evolving market for base oils, their properties and the feedstocks from which they are created. Following is a condensation of Dr. Rosenbaum’s Webinar.
KEY BASE OIL PROPERTIES
When producers formulate lubricants, they need to consider several variables that will affect the performance of the finished product. The four major criteria are:
1.
Pour Point: This is the lowest temperature at which a given oil can be poured.
2.
Viscosity: The resistance to flow exhibited by an oil is its viscosity.
3.
Viscosity Index (VI): The change in an oil’s viscosity as temperature changes is known as the viscosity index. The viscosity of a high-VI oil changes less with temperature than that of a low-VI oil. High-VI base oils are needed to formulate multigrade engine oils and other automotive lubricants. High-VI oil will also generally have lower volatility in a given viscosity grade, which is important for most modern engine oils.
4.
Purity: Levels of sulfur, nitrogen and polycyclic aromatic compounds must be held within strict limits for many lubricants.
CLASSES OF BASE OILS
Base oils are classified by the American Petroleum Institute into five groups (
see Figure 1) according to their ingredients and performance characteristics.
Figure 1. Classification of Base Oils
Group I base oil stocks contain greater than 0.03 percent sulfur, less than 90 percent saturates and have a viscosity index ranging from 80-119 VI. Demand for Group I oils is declining, although they are still the largest single category in the global market. These oils are commonly used in industrial and marine lubricants and in engine oils for older engines.
Groups II and III are both hydroprocessed oils with comparable purity and typically 99 percent saturates. The big difference between them is that Group III has a VI of 120 or greater. More than 90 percent of all the volume of lubricants in the world can be made from Group II oils, which are really the workhorses, especially in North America, where Group II dominates the base oil supply. Group III oils come into their own when a higher VI, along with low volatility, is needed. The Group IIIs are growing in use as fuel economy lubricants become more important.
Group IV consists of polyalphaole-fins (PAOs), which are the traditional synthetic stocks. The use of these oils is severely limited by the low availability of feedstocks, which face competition from other applications. As a result, the Group IVs are destined to remain a niche category.
Group V embraces everything else, ranging from some very low-quality napthenics all the way up to some very exotic synthetic oils.
MANUFACTURING PROCESSES
Group I oils are produced by solvent extraction and dewaxing processes (
see Figure 2). Solvent extraction starts with a heavy refinery feedstock from a crude distillation unit that’s called vacuum gas oil. A very selective solvent removes much of the sulfur, aromatic and nitrogen compounds. The resulting oil stream comes out of the extractor still containing paraffins that need to be removed to make it a base oil. A light solvent is added and the mixture is refrigerated to a low temperature at which the paraffins precipitate and are filtered out. While the entire process is simple in concept, the maintenance and operating costs are relatively high.
Figure 2. Solvent Extraction Process
Hydroprocessing is a method that typically uses three catalytic reactors to produce Group II and III base oils (
see Figure 3). The process begins with the same vacuum gas oil feedstock as Group I but runs it through a hydrocracker at very high pressures, frequently above 2,000 pounds per square inch. The hydrocracker reshapes the molecules, saturating a lot of the aromatic compounds. It converts sulfur into hydrogen sulfide gas and nitrogen into ammonia, creating a product called a waxy base oil that’s a lot cleaner than the output of a solvent extraction and contains more than 90 percent saturated compounds.
Figure 3. Hydroprocessing Conversion
The second reactor is a catalytic dewaxer (also known as a hydroisomerization unit), where the wax molecules are converted to isoparaffins and even more saturation occurs. Finally there is a lower temperature but very highpressure finishing step in which the final few percent of residual aromatic compounds are saturated to form Group II base oils. A more severe hydrocracking process is used to produce Group III oils.
SYNTHETIC LUBRICANTS
Although they account for just 1 to 2 percent of the volume of base oils produced, much of the buzz in recent years has focused on synthetic base oils and on oils produced from renewable and rerefined feedstocks.
The traditional synthetics are polyalphaolefins (PAOs) for which there is a limited production capacity in the world. As a result, they are expensive and tend to find specialized applications where their unique properties such as good performance in very high- or low- temperature conditions make them the ideal choice. In general, PAOs can be quite stable and deliver very good performance under extreme conditions; hence they find uses in exotic applications such as spacecraft or at the top of tall windmills where very long-lasting lubrication is needed.
“Synthetics is really more of a marketing term than anything that conforms to a scientific definition,” Rosenbaum says. “The only place where a synthetic lubricant is really defined as a PAO is Germany. So there are a lot of other definitions of synthetics out there, and the buyer probably needs to beware of something that just calls itself synthetic or semisynthetic. What you really want is performance, and the sophisticated customer will look beyond the claim using the word synthetic to the actual performance service category and OEM approvals.”
A synthetic can be thought of as something that is created, where the end material does not resemble the starting material. This applies to the PAOs and also to esters that are made from vegetable products, including canola oil. In the past 20 years, a new class of lubricants derived from vegetable oils has come on the scene. Traditional unmodified vegetable oils have long been used as lubricants, but their poor oxidation stability and the fact that they are often solid or nearly so at room temperature has made them poor performers.
Modified vegetable oils are usually esters, a new type of synthetics that are frequently created by reacting a fatty acid derived from vegetable oil with an alcohol. Esters can have excellent low-temperature properties and stability. Vegetable oils can also be fed directly to a base oil refinery where oxygen is stripped off to create a paraffin that can usually be transformed to an isoparaffin. But the resulting molecules are fairly small, making the product a low-viscosity base oil.
Several methods of these biostocks are amenable to having having their properties modified. Vegetable oils can be made into really premium lubricants that may derive marketing advantages from being claimed as a synthetic. The ultimate goal would be to make some of these natural feedstocks out of biowaste like corn stalks or wood waste. Being able to utilize those kinds of products instead of food products for making base oils, however, is still a ways away.
There are several other types of synthetic base stocks such as phosphate esters, polyalkylene glycols, alkylated aromatics and silicone oils, to name a few. Although these oils combined do not add up to 1 percent of the total global base oil market, there are important niche applications that require them. (You can find more information on these materials in the book,
Synthetic Lubricants and High- Performance Functional Fluids (1999), L.R. Rudnick and R.L. Shubkin eds., Marcel Dekker Inc., New York.)
REREFINED BASE OILS
Rerefined base stocks are another category generating a lot of buzz lately. “I remember when I was a teenager buying rerefined oil and putting it in my car,” Rosenbaum recalls. “I’m not sure there were any additives in it, and that it was not simply used oil which had been filtered. But in recent years, rerefined stocks have gotten quite good, if for no other reason but that the starting material, high-quality engine oil, is often so much better. Some of the new rerefineries can turn out decent Group II base oil, although saturate content is frequently less than virgin Group II, and some customers have expressed concern over the variability of rerefined oil properties.”
When used motor oil is brought to a rerefinery, the first step is usually separating out the water that’s almost always in it. Sludge, consisting of soot and some of the heavier additives, is also separated, usually in some sort of short-path distillation process. Then the remaining oil is fed to a low-pressure hydroprocessing unit for a process called hydrotreating. This process operates at 700-800 pounds of hydrogen pressure and saturates many of the aromatic compounds in the used oil. Sulfur compounds are transformed into hydrogen sulfide gas from which sulfur can be reclaimed.
One of the major challenges in rerefining is collecting the used oil. A rerefinery can have more than a hundred trucks circulating in its area, collecting oil and delivering it to a central collection facility for refining. In several states, used motor oil is classified as a hazardous waste, which adds testing and documentation requirements.
There has also been a movement to closed-loop rerefineries. These are smaller facilities processing a couple of hundred barrels daily coming from dedicated customers such as large vehicle- fleet operators that bring them used oil for rerefining. The plants buy hydrogen by the truckload for the rerefining process, then blend the rerefined product with fresh oil and additives and return it for use in the vehicle fleet. There is the potential to realize significant operating cost savings through operating or purchasing from rerefineries. However, most fleet customers will need proof that engine oil quality can be maintained at a level required to satisfy an OEM’s standards through multiple rerefining cycles.
CONCLUSION
Having provided a tour of the boutique base stocks, Rosenbaum reminds us that about 98 percent of base oils are still derived from petroleum. The lion’s share of these are paraffinic or neutral oils derived from paraffinic crude stocks. About 15 percent of the world’s base oils are napthenes or pale oils, composed largely of cyclic compounds made from napthenic crude stocks.
As automotive lubricant specifications continue to evolve and the availability of feedstocks changes around the world, lubricant producers will need to stay on their toes to satisfy the demanding auto industry. Hydroprocessing will be the technology of choice in all significant new base oil plants due to the appeal of converting undesirable molecules into desirable ones, rather than just extracting the unwanteds. Hydroprocessing gives an operator much greater flexibility in running a plant and in selecting raw materials.
In spite of the market changes now underway, Group I still dominates, accounting for more than half of the world’s base oil supply. Many of the plants producing these oils are small and old, however, and will be shutting down in the next 20 years. Growth will be the trend for the Group II and III oils, which are certainly preferred in most automotive formulations. As demand increases for fuel-economy lubricants, the higher VI oils will have the advantage of being suitable for making low-viscosity lubricants while maintaining the needed volatility properties.
In closing, Rosenbaum encourages engineers to recognize the role base oils play in finished lubricants. “Additive technology is very important,” he asserts. “But selecting the right base oil is the first step in formulating a premium lubricant.”
Stuart F. Brown is a free-lance writer who can be reached at www.stuartfbrown.com.