EFFECT OF HYDROPROCESSING SEVERITY ON SOLVENCY AND STABILITYAND STABILITY

The preferred molecules for lube oil manufacture are iso-paraffins having a high viscosity index (VI) and low pour point. The viscosity of base stocks is defined by the boiling range of hydrocarbons while their VI is defined by their chemistry.

The typical VI of solvent refined paraffinic base stocks is in a range of 85–95 and many crude oils are not suitable for the production of lube oil base stocks. Both paraffinic and iso-paraffinic hydrocarbons are high VI components of petroleum while hydrocarbons containing ring structures, such as aromatic and some poly-naphthenic hydrocarbons, are low VI components. The processes of hydrotreat-ing, hydrocrackhydrotreat-ing, and hydroisomerization were introduced to meet the ever increasing demand for higher quality mineral base stocks having a higher VI.

Hydrogenation processes for the conversion of petroleum fractions can be classi-fied as nondestructive and destructive. Nondestructive hydrogenation is generally used for the purpose of improving product quality without changing the boiling range. Destructive hydrogenation, such as hydrocracking, is characterized by the cleavage of carbon–carbon bonds (Speight, 2006).

Some petroleum products require the hydrogen treatment to saturate olefins.

Hydrogenfinishing processes, known as hydrofining, are mild hydrogenation pro-cesses. The commercial hydrofining processes are based on heating the feedstock in a furnace and passing it with hydrogen through a reactorfilled with catalyst. After passing through the reactor, the treated oil is cooled, separated from the excess hydrogen and pumped to a stripper tower, where hydrogen sulfide is removed by steam, vacuum, orflue gas. The finished product leaves the bottom of the stripper tower and has improved color, odor, and lower sulfur content (Speight, 2006). Under mild hydrofining conditions, nitrogen, sulfur, and oxygen compounds undergo hydrogenolysis to split out ammonia, hydrogen sulfide, and water, respectively.

Olefins are saturated and the aromatic contents of finished products are usually not affected (Speight, 2006). The hydrofining process is used to finish naphthas, gas oils, and lube oil base stocks.

The typical hydrotreating operation requires the reactor temperature of 2708C–3408C and the pressure of 100–3000 psig (Gary and Handwerk, 2001).

Hydrocracking is used to reduce the boiling point of the feed. The typical

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hydrocracking operation requires the reactor temperature of 2908C–4008C and the reactor pressure of 1200–2000 psig. During hydrocracking, heat cracking leads to olefins which are saturated in the presence of hydrogen to form paraffins.

Without the use of hydrogen, the catalytically and thermally cracked petroleum products have a tendency to form a sediment (Gary and Handwerk, 2001). The typical hydrocracking operation requires the circulation of a large amount of hydrogen to prevent the excessive catalyst fouling. Polynuclear aromatics, such as coronenes, were reported present in some hydrocracked base stocks (Gary and Handwerk, 2001). Commercially available coronene is described as an unsub-stituted 6-ring polynuclear aromatic structure (Aldrich, 2007). The effect of hydroprocessing severity on VI and solvency properties of base stocks is shown in Table 5.1.

During the hydrocracking process, the aromatic content can be reduced to low levels through many different reactions such as heteroatom removal, aromatic ring saturation, dealkylation of the aromatic rings, ring opening, straight chain and side chain cracking, and wax isomerization (Cody et al., 2002). The typically low quality feedstocks used in hydrocracking, and the consequent severe condi-tions required to achieve the VI and volatility might result in the formation of toxic polynuclear aromatic molecules (Cody et al., 2002). Heavy polynuclear aromatics were reported to form in small amounts from hydrocracking reactions and, when the fractionator bottoms are recycled, can build up to concentra-tions that cause fouling of heat exchanger surfaces and equipment (Gary and Handwerk, 2001).

The main objective of the hydroisomerization reactions is to convert the naphthenic and aromatic ring structures into straight chain and branched chain compounds. It is necessary to crack the side chains on these ring structures to reduce the chain length and saturate the molecules thus formed. Not all ring TABLE 5.1

Effect of Hydroprocessing Severity on VI and Solvency Properties of Base Stocks Feedstock Solvent refined Solvent refined Distillates Slack wax Purpose Saturate olefins Saturate olefins Saturate olefins Saturate olefins

Remove S and N Remove S and N Remove S and N Remove S and N Saturate aromatic Ring opening Ring opening

VI 90–105 >105 95–130 >140

Solvency Excellent Very good Moderate Poor

Source: From Pirro, D.M. and Wessol, A.A., Lubrication Fundamentals, 2nd ed., Marcel Dekker, Inc., New York, 2001. Reproduced by permission of Routledge=Taylor & Francis Group, LLC.

structures will open (Phillips, 1999). In the case of Shell extra high viscosity index (XHVI) and Exxon Exxsyn base stocks hydroisomerization processes, the feedstock is slack wax. The unconverted wax is removed by solvent dewaxing and it is recycled with the hydrotreated slack wax feed to the reactor (Phillips, 1999). With an increase in VI of petroleum derived base stocks, a decrease in aromatic and naphthenic hydrocarbons with an increase in paraffinic and iso-paraffinic contents will decrease their solvency properties. The use of high VI base stocks in lubricating oils and a decrease in their solvency properties will lead to a decrease in the solubility of additives affecting the lubricant perform-ance. The advantages and disadvantages of the hydrocracking process are shown in Table 5.2.

Olefins are formed during the catalytic and thermal cracking of crude oils and were reported to affect the products stability. Olefins are easily oxidized and polymerize. Diolefins are more reactive and quickly form high molecular weight polymers leading to sludge formation (Sequeira, 1994). Hydrocracked base stocks were reported to darken and form sediment on exposure to light. The severely hydrotreated base stocks, produced from vacuum distillate or deas-phalted vacuum residue, were reported to form a flocculant precipitate upon prolonged exposure to ultraviolet (UV) light known as daylight stability (Bijwaard and Morcus, 1985). The oxidation stability and daylight stability of severely hydrotreated base stocks are shown in Table 5.3.

Oxidation of base stocks, leads to an increase in acidity, viscosity, and sludge.

Oxidized base stocks, having a tendency to form sludge, were reported to have daylight stability of 2–9 days. Other oxidized base stocks, having no tendency to form sludge, were reported to have a daylight stability of over 15 days. The use of mild catalytic hydrotreatment and solvent extraction was reported effective in improving their oxidation stability and daylight stability (Bijwaard and Morcus, 1985). The literature reported that in lubricating oils, under low temperature oxidation conditions, peroxides, alcohols, aldehydes, ketones, and water are formed. Under high temperature oxidation conditions, acids are formed (Bardasz and Lamb, 2003). Hydroconversion of under-extracted raffinate was reported to produce base stocks, having a high VI and low volatility for a given viscosity, TABLE 5.2

Advantages and Disadvantages of Lube Oil Hydrocracking Process

HC Process Advantage HC Process Disadvantage

Use of poor quality crudes Catalytically dewaxed bright stocks are hazy

Higher yields Tendency to darken on exposure to light

Conversion of residual oils to distillate oils Tendency to form sludge

Higher VI Exhibit additive solubility problems

Source: From Sequeira, A. Jr., Lubricant Base Oil and Wax Processing, Marcel Dekker, Inc., New York, 1994. Reproduced by permission of Routledge=Taylor & Francis Group, LLC.

with improved oxidation stability and solvency properties (Cody et al., 2002).

Different processing techniques are used to meet the volatility targets of base stocks at the max yields. The effect of different processing on volatility and yields of 100N waxy raffinate is shown in Table 5.4.

The volatility can be improved by removing the low boiling front end, known as topping, which increases the viscosity of the oils. Another route to improving the volatility is to remove the high boiling and low boiling ends, known as heart-cut, which maintains a constant viscosity (Cody et al., 2002). Topping of waxy raffinate can lead to a decrease in volatility from 27.8% to 26.2% off, with a decrease in yield from 100% to 95.2%. Heart-cut distillation can lead to waxy raffinates having a lower volatility of 21.7%–22.7% off with a lower yield of only 38%–58% (Cody et al., 2002). The distillate feeds to the extraction zone can be of poor quality and contain

TABLE 5.3

Source: From Bijwaard, H.M.J. and Morcus, A., Lubricating Base Oil Compositions, CA Patent 1,185,962, 1985.

TABLE 5.4

Effect of Processing on Volatility and Yields of 100N Waxy Raffinate

Kinematic Viscosity,

3.9 cSt at 1008C Waxy Raffinate,

Volatility, % off Waxy Raffinate,

Source: From Cody, I.A. et al., Raffinate Hydroconversion Process, CA Patent 2,429,500, 2002.

over 1 wt% of sulfur and nitrogen. The use of raffinate hydroconversion (RHC) technology is based on hydrotreating, redistilling, and solvent refining. The raffinate from the solvent extraction unit is stripped of solvent and sent to the hydroconversion unit. For the same viscosity base stocks, the use of different processing affects the volatility and yields of base stocks. The effect of a two-stage hydrocracking process and the RHC process on the yield of lube oil fraction is shown in Table 5.5.

The use of the RHC was found to significantly increase the yield of the lube oil fraction, having the same viscosity of 6.5 cSt at 1008C and a similar volatility of 3.3%–3.6% off from 30% to 70%. The use of hydrocracking technology suffers from yield debits and was reported to require high capital investments (Cody et al., 2002).

The patent literature reported that hydroconversion of under-extracted raffinate was found to remove multiring aromatics which are known to have an adverse effect on the viscosity, VI, toxicity, and color of base stocks. The dewaxed oil was reported to be suitable for use as a lubricant base stock (Cody et al., 2002). The properties, VI, and the aromatic contents of RHC lube oil base stocks are shown in Table 5.6.

TABLE 5.5

Effect of Hydrocracking and Raffinate Hydroconversion on the Yield of Lube Oil

Properties Hydrocracking,

Two-Stage Process Raffinate Hydroconversion

Viscosity at 1008C, cSt 6.5 6.5

Volatility, % off 3.3 3.6

Yield, % 30.5 69.7

Source: From Cody, I.A. et al., Raffinate Hydroconversion Process, CA Patent 2,429,500, 2002.

TABLE 5.6

VI, Aromatic Content, and Volatility of RHC Base Stocks

Properties RHC Oil RHC Oil

Viscosity at 1008C, cSt 4.5 5.9

VI 116 114

Pour point,8C
18
18

Saturates, wt% 98 97

Aromatics, wt% 2 3

Volatility 14 8

Source: From Cody, I.A. et al., Raffinate Hydroconversion Process, CA Patent 2,429,500, 2002.

8C, was reported to have a VI of 116, 2 wt% of aromatics to assure good solvency properties, and a volatility of 14% off. Another RHC oil, having a higher viscosity of 5.9 cSt at 1008C, was reported to have a VI of 114, 3 wt% of aromatics, and a lower volatility of 8% off.

The literature reported that the quality of RHC oils, in terms of VI, was better than that of mineral base stocks, however, not as good as the quality of XHVI base stocks produced by hydroisomerization of slack wax which have a higher VI, above 140, and have a lower Noack volatility when compared to other hydro-crackates of the same viscosity (Phillips, 1999). The use of hydroprocessing can involve the hydrotreatment upgrade of solvent raffinate followed by dewaxing and hydrofinishing. The toxicity of the base stock, for a given VI, is controlled during the cold hydrofinishing step by adjusting the temperature and the pressure.

The hydroconverted raffinate is subjected to a cold finishing step and sent to a vacuum stripper to separate the low boiling components (Cody et al., 2002).

During the RHC step, the processes of hydrocracking and hydroisomerization are minimized.

5.2 EFFECT OF HYDROPROCESSING SEVERITY ON UV