Base Stocks General

Lubricants have been used by mankind from the very early days of civilization to assist in reducing the energy needed to slide one object against another. The first lubricants were animal fats, and much later whale oil was used. It was not until crude oil was discovered in commercial quantities in Oil Springs, Ontario, Canada, in 1858 and in Titusville, Pennsylvania, in the United States in 1859 that the concept of petroleum-based lubricants could be seriously considered on a large scale. The first petroleum refinery to produce base stocks (the petroleum distillates fractions used in lubricants) in the Western Hemisphere was built by Samuel Weir in Pittsburgh in the 1850s. One of the earliest lubricant producers (to reduce waste production) was the Standard Works in Cleveland, Ohio, owned in part by John D. Rockefeller, whose company subsequently became Standard Oil. 

The subject of this book is the chemistry of petroleum base stocks and of their manufacturing processes from crude oil fractions. Petroleum base stocks are hydrocarbon-based liquids, which are the major component (80% to 98% by volume) of finished lubricants, the remaining 2% to 20% being additives to improve performance. Therefore this book does not deal with the manufacture of nonpetroleum base stocks such as synthetics (from olefins such as 1-decene), ester-based ones, and others. 

Base stocks usually have boiling ranges between 600°F and 1100°F at atmospheric pressure (some are lighter) and lube feedstocks therefore come from the 

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FEEDSTOCKS AND BASE STOCKS GENERAL COMPOSITIONAL ASPECTS... 

This is an analysis frequently conducted on oil lubricants. Generally, the additive is known and its concentration can be followed by direct comparison of the oil with additive and the base stock. For example, concentrations of a few ppm of dithiophosphates or phenols are obtained with an interferometer. However, additive oils today contain a large number of products their identification or their analysis by IR spectrometry most often requires preliminary separation, either by dialysis or by liquid phase chromatography. 

All types of crudes have been used in the production of spray oils. On the West Coast the asphaltic base crudes occurring locally have been generally used, while oils derived from mixed base stocks are now most commonly used east of the Rockies. 

The corrosive activity on copper/lead bearings for typical carboxylic acids, such as decanoic, lauric, palmitic, stearic, and oleic acids, as 1 % w/w solutions in a lubricating oil base stock with excess of hard-core RMs, measured by infrared spectroscopy, supports the observation for the corrosive activity of used lubricating oils. An increase in total acidic number (TAN) is generally either an indication of contamination with acidic combustion products or the result of oil oxidation. Corrosion of bearing metals by used lubricating oils requires the presence of both acids and peroxides and probably takes place by a two-step mechanism. In the first step, the peroxide reacts with the metal to form a metal... 

Paraffinic and naphthenic (cycloparaffinic) stocks may be used for the formulation of lubricating oils, each with favorable characteristics for particular uses. Paraffinic stocks are generally preferred for their superior lubricating power and oxidation resistance. Naphthenic stocks, on the other hand, have naturally lower pour points, i.e, they maintain flow characteristics at lower pour-points than paraffinics (Table 18.8) and are better solvents, features which are more important for applications such as heat transfer, metal working, and fire-resistant hydraulic fluids [33]. Any residual aromatics in the lubricating base stock will have been removed before formulation by solvent extraction, using N-methylpyrrolidone, furfural, or less frequently today, phenol (Eq. 18.39). 

Synthetic lubricants have been available for many years in the early 1930s, synthetic hydrocarbon and ester technologies were simultaneously developed in Germany and the United States. Development of a catalytic polymerisation process of olefins in the United States led to the formulation of automotive crankcase lubricants with improved low-temperature performance [1,2]. These products were not commercialised due both to the inherent cost of these new synthetic base fluids and to performance improvements of mineral oil-based lubricants. In Germany, low-temperature performance drove the development of similar products [3], although the main objective was to overcome the general shortage of petroleum base stocks. 

Other than the special supply circumstances of the Second World War, synthetic lubricants were not commercially significant until after the war. In general, the improved properties of lubricants achieved with early synthetic base stocks could be obtained more cost effectively by improved formulations based on mineral oils. But the requirement for lubricants to perform over increasing temperature ranges, led by military and aero-engine performance, stimulated continuing development of synthetic lubricant technology. Synthetic lubricants are now found in all areas of... 

Base stocks are manufactured to specifications that place limitations on their physical and chemical properties, and these in turn establish parameters for refinery operations. Base stocks from different refineries will generally not be identical, although they may have some properties (e.g., viscosity at a particular temperature) in common. At this point it is worth briefly reviewing what measurements are involved in these specifications, what they mean, and where in the process they are controlled. 

Table 1.3 provides inspection results for a range of base stocks of different origins. Since the values are representative of those types, some commentary is worthwhile. First, the general format of these tables is to list the inspections (tests) performed in the left-hand column, with the column to the right of that identifying... 

Sulfur-containing compounds These may be thiols, sulfides, thiophenes, benzo- and dibenzo-thiophenes, and more complex structures. Solvent extraction reduces measured sulfur levels and therefore the content of sulfur compounds in solvent refined lubes in solvent refined lubes, oxidation studies show that there appears to be an optimum level for sulfur compounds. Lube hydrocracking generally will reduce sulfur to about 10 ppm or less in the base stocks. The 4,6-di-alkyl... 

Finally, one would expect that the pour point would be dependent on the amount of wax present and the carbon number distribution of the wax molecules—n-paraffins of higher carbon number, for example, would be expected to have a greater effect on pour point, and the more of them, the greater the effect as well. Also, n-paraffins would be expected to have a greater effect than isoparaffins. Krishna et al.18 studied the effect of n-paraffins on pour point, not on waxy lubes, but on two gas oils (250°C to 375°C and 375°C to 500°C) from Bombay High crude oil. However, we would expect their results to describe in a general way the behavior of waxy base stocks. The broad cuts they distilled were each further fractionated into five 25°C narrow cuts and pour, cloud, and cold filter plugging points measured for each. Urea adduction (Chapter 9) was employed... 

The VI of a base stock depends on its chemical composition the value for a sample reflects the components present and proportions and boiling range, but there is no published procedure for calculating VI based on composition. The converse, calculating composition from VI, will someday materialize, but will have to solve the issue that there generally are multiple compositions that will give the same VI. 

These conclusions were very important not only because of their general application, but also because the tests employed were real world ones in everyday use for evaluation of base stocks and formulated products. The results thus carried great weight. 

VI droop (Table 7.2), in which the distribution of dewaxed VI versus viscosity for the total base stock product shows a sharp drop below about 5 cSt (about 150 SUS) at 100°C. This means that the nominal light base stock has a lower VI than the medium neutral. This is in contrast to solvent refined stocks, whose Vis are generally considered to remain essentially constant. 

Aromatic levels in lubricant feedstocks and base stocks are obvious parameters of interest to both the processor and the user. Table 8.1 shows how the compositions of fractions can vary depending on their distillation positions in a crude oil, in this case heavy crude.1 The analysis here should not be taken as representative of any crude used for lubes, but the general trends are typical for most crudes, that is, what are usually labeled as the impurities —nitrogen, sulfur, aromatics, and polars—all increase as boiling point increases. These components are essentially those which must be reduced in whatever lubes manufacturing process is employed. In hydroprocessing, these steps all consume hydrogen. 

As we have seen, reduction of aromatics levels in lubricating oil base stocks relative to the feed has always been a significant part of their overall processing. The extraction step in the traditional solvent refining technology removes some aromatics, particularly the low VI polycyclic aromatics that contribute to oxidation instability and deposit formation. Hydrofinishing further reduces the levels of polynuclear aromatics, since conditions are generally too mild to reduce mono and... 

As in the traditional acid extraction process, the feedstock is generally dewaxed solvent refined base stock, since levels of the aromatics, polynuclear aromatics, and nitrogen and sulfur compounds are already reduced relative to a straight-run gas oil. This facilitates hydroprocessing by lightening the load on the catalysts and extending their lives. Equally important is that this is an already dewaxed feed, so the white oil producer does not have to bear the capital costs of crude fractionation and dewax units. 

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