Base Stocks from Crudes

FIGURE 1.1 Schematic of a refinery crude fractionation train and deasphalting unit. 

The waxy distillates and DAO require three further processing steps to obtain acceptable base stock  

Representative Boiling and Carbon Number Ranges for Lube Feedstocks 

Fraction Approximate Boiling Point Range (°F) Carbon Number Range1 

There are two strategic processing routes by which these objectives can be accomplished  

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Sulfur Sulfur is present in all lube plant feedstocks fractionated from crude oil and its content may be up to several percentage points. Solvent refining removes some but not all, therefore such stocks with no further treatment can contain up to several mass percent of sulfur. Hydrofinishing of solvent refined stocks can reduce this level substantially. Base stocks from conversion processes will have sulfur levels in the low parts per million (ppm) range since sulfur is relatively easily removed in severe hydroprocessing. 

Murray et al.2728 (Imperial Oil) used a number of these tests to develop methods for predicting performance from composition. They found that ASTM D943 lives for solvent refined base stocks from a single crude correlated well with VI and therefore with severity of solvent extraction. However, when applied to base stocks from different crudes and different processes, the correlation with VI no longer held (Figure 5.13). 

Inspections on Hydrofinished and Clay-Treated Base Stocks from Western Canadian Crude... 

Paraffin sulfonates, 24 146 Paraffin wax(es), 28 671 26 214—215 separation from crude oil, 26 216 as barrier coating, 28 125 in lubricating oil base stocks, 15 216—217 Paraffin wax vapor barriers, in finish removers, 28 80... 

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. 

As described in Chapter 1, mineral oil base stocks are derived from crude oil and are complex mixtures of naturally occurring hydrocarbons. Synthetic ester lubricants, on the other hand, are prepared from man-made base stocks having uniform molecular structures giving well-defined properties which can be tailored to specific applications. 

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. 

Processing steps which act by chemical separation The undesirable chemical compounds (e.g., polyaromatics) are removed using solvent-based separation methods (solvent refining). The by-products (extracts) represent a yield loss in producing the base stock. The base stock properties are determined by molecules originally in the crude, since molecules in the final base stock are unchanged from those in the feed or... 

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... 

This concept of optimum aromaticity and the role of sulfur compounds as inhibitors were further established by a study by Bum and Greig (British Petroleum) of the oxidation of solvent extracted base stocks.18 They chose samples from a North African (Sahara) and three Middle East (Iran, Abu Dhabi, and Kuwait) crudes. The aromatic + heterocyclic (A + II) and paraffin + naphthene (P + N) components were separated by alumina chromatography from each base stock (Table 5.9 includes their composition and sulfur contents) and recombined in several ratios and the resistance of the blends to oxidation measured by the oxygen uptake method. 

Comparison of Base Stock Yields and Their Distributions from 20,000 bpd Reduced Crude Feed to a Solvent Refined... 

Advances in processing methods are not only improving the quality and yield of lubricant base stocks, they are also reducing the dependence on more expensive crude oil starting materials. Process Chemistry of Lubricant Base Stocks provides a comprehensive understanding of the chemistry behind the processes involved in lubricant base stock production from crude oil fractions. 

The VPS is generally the first process unit. The VPS s goal is to fractionate the atmospheric resid or reduced crude so that the base stock will have the desired viscosity. The fractionation also controls the volatility and the flash point. The boiling point separation is accomplished by using high efficiency distillation/fractionation hardware. Secondary effects include asphalt segregation in the Vacuum Resid from the VPS (potential by-product), reduction in Conradson carbon and color improvement. 

With an acceptable assessment of the new crude, the refiner may elect to validate the crude for Base stock manufacture. This may entail running a plant test to make Base stock products from the new crude. The products made from the plant test are typically blended into formulated oils and subjected to testing to demonstrate acceptable product performance. 

Results from the manufacturing test will determine if the crude will be accepted. The certification test must have been acceptable and the crude processed as expected. There must not be any evidence that Base stock quality is unacceptable. If the above is completed successfully, the crude may be approved and added to the manufacturer s list of approved crudes. 

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