Group III base stocks

Fortunately the API group system has brought some badly needed uniformity and simplicity to the nomenclature, and at least the new terms 11+ and III+ fit in with the system and what they mean is easily recognized or inferred. 

There are three routes to manufacture these base stocks, which are basically routes to meet the VI targets since the saturates and sulfur contents fall into place automatically due to the processes employed. These are 

The compositional goal is to make base stocks whose structures are dominated by isoparaffins and monocycloparaffins with long hydrocarbon chains attached. These molecules do not exist in natural distillates in sufficient concentration to obtain them by solvent extraction (and no solvent with the requisite selectivity appears to have been developed), therefore catalytic methods must be used. Group III+ base stocks will be largely isoparaffins in composition and therefore will use wax, preferably of Fischer-Tropsch origin, as feed to an isomerization process unit. In this section we will consider only the hydrocracking option the other options will be discussed elsewhere. 

FIGURE 7.18 VI versus viscosity at 100°C for model compounds of several different compositional types. 

Source A. Billon, M. Derrien, and J. C. Lavergne, Manufacture of New Base Oils by the I.F.P. Hydrofining Process, Proceedings of the Division of Refining of the American Petroleum Institute 49 522-548 (1969). With permission. 

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Group III base stocks contain >90% and <0.03% sulphur with a viscosity index... 

FIGURE 7.22 Mitsubishi group III base stock production scheme. 

Typical Properties of Mitsubishi Group III Base Stocks of Two Viscosity Grades Compared to Those of PAOs Group III Base Stocks PAOs ... 

Group III base stocks contain greater than or equal to 90 percent saturates and less than or equal to 0.03 percent sulfur and have a viscosity index greater dian or equal to 120. 

Figure 3. Gas chromatograph comparison of a 4 cSt PAO with a 4 cSt Group III base stock...

A number of papers have looked at the development of relationships between base stock composition as measured by NMR and either physi-cal/chemical properties or their performance.22 27 Most of this work has been focused on group II and III base stocks, with less or little attention paid to solvent extracted ones. These have all relied on various techniques to simplify the spectra and the assignments of peaks and make peak integration more reliable. These have many acronyms,23 for example, GASPE (gates spin echo), PCSE (proton coupled spin echo), INEPT (insensitive nuclei enhancement by polarization transfer), DEPT (distortionless enhancement by polarization), QUAT (quaternary-only carbon spectra), 2D COSY (two-dimensional homo-nuclear spectroscopy), and HETCOR (heteronuclear shift correlated spectroscopy)]. Table 4.10 provides an example of some of the chemical shift data generated26 and employed in this type of work, and Adhvaryu et al.25 were able to develop the correlations between base stock properties and carbon types in Table 4.11, whose main features correspond to intuition (e.g., the values of API and aniline points are both decreased by aromatic carbon and increased by the... 

Both of these produce group II base stocks and the LHDC route can also lead to group III as required. 

PAO is classified by itself as a Group IV base stock. In addition to the differences listed in Table 3, PAO also contains no cyclic paraffins, naphthenes or aromatics, whereas Group I, II and III base stocks contain different amoimts of aromatics ranging from <1% to >40% ". With the increasing presence of aromatics and/or naphthenes, oxidative stability and low temperature properties of these fluids are typically degraded. Also, as shown earlier in Figure 3, PAO have discrete carbon numbers with relatively long linear hydrocarbon branches, whereas mineral base stocks contain a continuum of carbon number. As a result, PAO usually have lower volatility. 

Data in Table 5 show that PAO has lower volatility than Group I to III base stocks. This lower volatility is the result of the unique chemical compositions of PAO - 100% relatively linear paraffin, little low molecular weight hydrocarbons of less than C30 (Figure 3). Low volatility is advantageous for decreased oil consumption and reduced emissions. 

It has been demonstrated that the un-formulated PAO base stock treated with 0.5 wt% antioxidant resists oxidation for more than 2500 minutes in a standard rotary bomb oxidation test (RBOT, D2272 method). In comparison, similarly treated Group II and III base stock started to oxidize much earlier, at less than 800 or 1700 minutes, respectively . 

More recently, British Petroleum (BP)37 39 produced two group III stocks, a 4 cSt BP HC-4 and a heavier HC-6 from the fractionator bottoms of their fuels hydrocracker at BP s Lavera, France, refinery. The total bottoms are solvent extracted to stabilize the final products, fractionated, and finally solvent dewaxed. The hydrocracker is operated in a severe mode (relative to lube hydrocracking) at a once-through conversion of 90% (in comparison, lube hydrocrackers may operate at conversions of only about 20%, and perhaps less with a very good quality feed). Like other hydrocracker-sourced group HI products, their compositions are virtually independent of feed source due to the extent of the molecular reorganization that occurs and the molecular structures that are required for those Vis. The properties of the HC-4 are compared in Table 7.26 with those of some competitive group I, II, and IV base stocks. It can be seen that the HC-4 closely resembles the more... 

Ushio et al. of Nippon Oil (Japan) have described some of their research work undertaken to produce group III stocks via hydrocracking distillates.12 Their process was similar to BP s, employing a fuels hydrocracker to produce the waxy lubes stream(s) by downstream fractionation of the hydrocracker bottoms. Base oil products were stabilized by furfural extraction. 

Comparison (Table 7.33) of compositional analyses on Yubase 6 with group I, II, and IV base stocks of similar viscosities shows the expected compositional differences—the Yubase 6 has a much higher isoparaffin content than either groups I or II and less 2+ ring cycloparaffins.44 The other benefits expected for group III stocks, namely higher flash point and decreased volatility as measured by the Noack test, also materialized. 

S-Oil and ExxonMobil have described their joint work on modifying a fuels hydrocracker to make group III lubes, in which the bottoms stream was fractionated, dewaxed using MSDW2 hydroisomerization technology followed by aromatic saturation with ExxonMobil s MAXSAT catalyst.44 The properties of the base stocks are shown in Table 7.35, and like SK Corporation s products, these also meet U.S. food-grade white oil specs. 

Table 5 compares the basic properties of low and high viscosity PAO versus Group I to Group III minei oil base stocks. 

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