Maximising crude value in base oil production
Retrofitting a combination of all-hydroprocessing routes delivers lower capital and operating costs in the production of premium base oil stocks.
SUBHASIS BHATTACHARYA, GUAN-DAO LEI and WOODY SHIFLETT
Advanced Refining Technologies and Chevron Lummus Global
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The production of base oils for lubricants has a long history with humble and rather simple beginnings. Base oils comprise about 85% of a typical lubricant formulation with various additives making up the remainder. Hence, base oil quality is central to defining final lubricant quality. In the far past, base oil production was simple but inefficient: choose the proper crude oil, do some fractionation and conduct some simple treating steps, and a useful product emerged. As demands on lubricant properties and more and more applications arose, needs for continuous process evolution and optimisation arose, concurrent with pressures to utilise continuously more diverse crude sources and employ economic processes to enable this. Processing advances were made possible by innovative inventions of new and novel catalytic materials and catalyst systems that became able to literally reformulate and match the molecular structures and compositions to stringent physical and chemical requirements for the most premium of base oil stocks. The catalytic chemistry and shape-selective zeolite structures required, on a nanostructure scale, are now synthesised not by Edisonian experimentation but by the careful architectural design of catalytic materials scientists. This is a short traverse through that rather noteworthy, if not almost amazing, technology story.
Chemical and physical requirements of base oils
Base oils, and particularly premium base oils, must meet a variety of chemical and physical requirements and specifications that, when combined with the appropriate additive package components, allow final lubricants to perform robustly in applications varying from lubrication of vehicle motors and drives and stationary machinery, to food, pharmaceutical and cosmetic products use. On a very fundamental level, these requirements focus on specific flow (viscosity) properties, cold flow behaviour, low contaminant levels, limited volatilisation propensity and appearance (colour). Specifically, a given base oil grade must be able to exhibit a limited range of viscosity variance over a significant range of operating temperatures. This is quantified by the viscosity index (VI) property (defined by ASTM D2270). The higher the VI, the better the base oil’s viscosity behaviour with temperature change. The API has established a general classification for base oils (see Table 1). Note that there are less formal classifications such as Group II+ and Group III+ that demark intermediate base oil types. It should also be noted that there are viscosity grade classifications for base oils that vary from producer to producer. Simply summarised, they are denoted as light neutral, medium neutral, heavy neutral, and bright stock and the progression exhibits higher and higher mid-boiling point and viscosity. The first three may be additionally described with numbers such as ‘100 Neutral, 300 Neutral’ and so on. The numbers used typically related to the viscosity measured at 38°C (100°F) in SUS viscosity units. Bright stock will typically have a boiling point above 540°C and a viscosity of 20-30 cSt at 100°C.
Table 1 indirectly indicates that aromatics are to be kept below 10%. Figure 1 gives a general view of the type of molecules that are needed to meet VI targets. Also critical to performance is the ability of the base oil to flow at low temperatures that may be encountered in final lubricant usage. This need is captured by specifications or requirements for pour point and/or cloud point. In general, the lower the better, and this favours limiting paraffinic hydrocarbons. Hence, desired molecular components tend to be isoparaffins, and aromatic compounds are the least favoured. Petroleum feedstocks appropriate for base oil production are typically vacuum gasoil (VGO) distillation fractions. Fischer-Tropsch products from gas-to-liquids (GTL) and coal-to-liquids (CTL) processes are also seeing usage for high VI base oils.
Base oils specifications vary by their end use and by a number of specialised applications, all of which add to create a complex set of formal and informal specifications across the world. Suffice it to note that, apart from viscosity and VI, most specifications centre on colour (for instance, ASTM D1500), aromatics levels, NOACK volatility (for instance, ASTM D5800), and oxidation stability (several ASTM methods).
Recent history of base oil production1
Certainly, by the second decade of the 20th century, it became apparent that rudimentary fractionation and simple clay treating or acid treating of ‘lube-friendly’ crudes such as those from western Pennsylvania or Arabian Light crude would not address quality lubricant demand economically. By the early 1930s, solvent extraction was used and refined to remove undesirable components, particularly aromatic compounds. Common choices for solvents include furfural, n-methyl pyrrolidone (NMP) and the double-solvent Duo-Sol process. Solvent extraction only removes 50-80% of impurities such as polar compounds, aromatics and sulphur or nitrogen containing compounds. Pour point requirements dictated that waxy (paraffinic) compounds needed subsequently to be removed, and the raffinate from the initial extraction process was diluted with a second solvent, chilled below -10°C and filtered to remove precipitated wax crystals. Such solvent combinations as methyl ethyl ketone (MEK)/toluene and MEK/methyl isobutyl ketone are employed for dewaxing. Such processes produced Group I base oil stocks. Little in the fundamentals of the solvent processes changed over several decades, and improved performance of base oils relied upon additives. By the 1960s, mild hydrotreating using base metal catalysts (typically Ni-Mo/alumina or Ni-W/alumina) was employed to follow the solvent processes to remove more of the remaining sulphur and nitrogen contaminants and modestly reduce aromatics levels. Base oil yields were not high and there remained much room to optimise. However, hydroprocessing was to play a greater and greater role.
Hydrocracking is utilised to produce lower boiling and more valuable products from heavier feeds as well as to significantly reduce levels of sulphur, nitrogen and aromatics species, and finds it roots in Germany in the early part of the 20th century. After the Second World War, the heritage rudimentary hydrocracking process technologies from Germany were evaluated elsewhere, and Chevron commercialised the first commercially practical application of the technology, Isocracking, as a conversion process and for clean fuels production, in 1959. It became clear that hydrocracking had the ability to appreciably increase the VI of base oil process VGO feedstocks across a broad range. Gulf technologies, later to be incorporated into Chevron after the 1984 merger with Gulf Oil, were deployed to commercialise hydrocracking for base oil production in 1969 at Idemitsu’s Chiba, Japan refinery and at Sun Oil’s Yabacoa, Puerto Rico refinery in 1971. Hydrocracking greatly increased the feedstock flexibility of base oil plants and addressed needs for contaminant reduction to produce higher value base oils.
Catalytic dewaxing and hydroisomerisation of wax were the next key processing steps to be introduced into the base oil production tool kit. Pure catalytic dewaxing was a clear move in the desired direction from solvent dewaxing because it hydrocracked undesirable normal paraffins into smaller molecules that could cascade to valuable lighter base oil products or fuels components, and waxy side chains could be removed from base oil components. All of these lowered base oil pour point as did solvent dewaxing, but also moved processes toward better optimisation of feedstock usage. Hydroisomerisation was a highly beneficial inclusion into the process because it tended to restructure normal paraffins into lower pour point base oil molecules rather than hydrocracking them to lighter products outside of base oil boiling ranges. Hydroisomerisation also had the added benefits of significant saturation of remaining aromatic compounds plus removal of residual sulphur and nitrogen. Catalytic dewaxing and hydroisomerisation technologies were commercialised in the 1970s. Mobil used catalytic dewaxing to replace solvent dewaxing but continued to use solvent extraction for the production of many conventional neutral base oils. Shell employed wax hydroisomerisation in conjunction with solvent dewaxing to produce extra high VI base oils, predominantly in Europe.
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