PROCESS FOR PRODUCING STABILIZING HYDROPROCESSED LUBRICATING OIL STOCKS BY THE ADDITION OF HYDROGEN SULFIDE

Abstract: Lubricating oil stocks produced by catalytic dewaxing and subsequent hydrotreating are stabilized against oxidation by the addition of hydrogen sulfide to the feed to the catalytic dewaxing unit. Preferably the hydrotreater effluent is the source of H₂S for the catalytic dewaxing unit. The H₂S saturates olefins that contribute to oxidation instability and restores antioxidant sulfur compounds such as thiols to the lube stock.

Description: This invention relates to a method of stabilizing hydroprocessed lube stocks by the addition of hydrogen sulfide to the hydrogen feed, without significantly increasing catalyst aging rate. It is well known that certain types of organic compounds are normally susceptible to deterioration by oxidation or by corrosion through coming into contact with various metal surfaces. For example, it is known that liquid hydrocarbons in the form of fuels or lubricating oils tend to accumulate considerable quantities of water when maintained for long periods of time in storage vessels; and when subsequently brought into contact with metal surfaces in their functional environments, deterioration as a result of corrosion occurs. As a further example, in modern internal combustion engines and in turbojet engines, lubricants can be attacked by oxygen or air at high temperatures to form heavy viscous sludges, varnish and resins which become deposited on the engine surfaces. As a result, the lubricant cannot perform its required task effectively, and the engine does not operate efficiently. Furthermore, the sludges produced by lubricant deterioration generated by insufficient oxidative stability tend to foul and plug low tolerance hydraulic system components and interconnecting piping and valves. In addition, where such lubricating oils or other corrosion-inducing materials are incorporated into solid lubricants as in the form of greases, similar results are encountered, thus clearly indicating the necessity for improved methods of treatment which increase the oxidative stability of lubricating oils. Accompanying the deterioration of lubricants by oxidation is the resultant corrosion of the metal surfaces for which such lubricants are designed and supplied. Once a lubricant has been oxidized to produce viscous sludges and resins, acids develop which are corrosive enough to destroy most metals. Moreover, the friction between metal parts increases following lubricant breakdown due to oxidation and leads to excessive metal wear. Increasing demands on lubricants, brought about by the widespread introduction of engines operating at steadily increasing temperatures, pressures, and speeds, necessitate a constant search for new methods of hydrocarbon treatment which can provide lubricants with increased oxidation resistance. Due to the lubricant oxidative stability requirements for newer engines and other rotating or moving equipment lubrication, feedstocks which were previously suitable for lubricant production are presently unsuitable or at best marginal for such uses. Thus at a time when overall lubricant demands are increasing, the amount of suitable lubricant feedstock material is being diminished due to the oxidative stability requirements of newer machinery. In order to produce lube stocks of suitable pour point from paraffin-containing feedstocks, it is generally necessary to remove significant amounts of hydrocarbon waxes from such feedstocks. Distillate fractions of suitable boiling ranges for lube base stock can be dewaxed by extraction with solvent mixtures such as methyl ethyl ketone and toluene, or methyl ethyl ketone and methyl isobutyl ketone. Because solvent extraction processes generally require large amounts of expensive solvents, alternative or supplemental methods of dewaxing have been devised. In recent years techniques have become available for catalytic dewaxing of petroleum stocks. A process of that nature developed by British Petroleum is described in The Oil and Gas Journal dated January 6, 1975, at pages 69-73. See also U.S. Patent No. 3,668,113. In U.S. Patent No. Re. 28,398, is described a process for catalytic dewaxing with a catalyst comprising zeolite ZSM-5. Such process combined with catalytic hydrofinishing is described in U.S. Patent No. 3,894,938. In U.S. Patent No. 4,137,148 is described a process for preparing speciality oils of very low pour point and excellent stability from a waxy crude oil distillate fraction by solvent refining, catalytic dewaxing over a zeolite catalyst such as ZSM-5, and hydrotreating, under specified conditions. Hydrocarbon lubricating oils have been obtained by a variety of processes in which high boiling fractions are contacted with hydrogen in the presence of hydrogenation-dehydrogenation catalysts at elevated temperatures and pressures. One such process is disclosed in U.S. Patent No. 3,755,145, relating to catalytic lube dewaxing using a shape-selective zeolite catalyst, a large pore cracking catalyst such as clay or silica, and a hydrogenation/dehydrogenation catalyst. In U.S. Patent No. 4,181,598, a lube base stock oil of high stability is produced from a wax crude oil fraction by solvent refining, catalytic dewaxing over a shape-selective zeolite and hydrotreating under specified conditions. In both processes, there is a consumption of hydrogen and lubricating oil fractions are separated from the resulting products. The separated lubricating oil fractions differ from those obtained by fractional distillation of crude oils and the like, in that they have relatively higher viscosity index values. These lubricating oil fractions suffer from the shortcoming that they are unstable when exposed to highly oxidative environments. When so exposed, sediment and lacquer formation occurs, thus lessening the commercial value of such lubricants. The instability of catalytically dewaxed lube base stocks arises from the presence of easily oxidizable olefins in the catalytically dewaxed product. Efforts to saturate these olefins by reacting them with hydrogen, i.e., hydroprocessing, have been successful in significantly reducing the olefin content. However, prior art methods of hydroprocessing result in the removal of antioxidant sulfur compounds such as thiols or sulfides from the hydroprocessed product, thus lowering its oxidation stability. The anti-oxidation capability of sulfur compounds is known. See G.H. Denison, Jr. and P.C. Condit "Oxidation of Lubricating Oils," Industrial and Engineering Chemistry, Vol. 37, No. 11, pp 1102-1108, 1945; D. Barnard et al, "The Oxidation of Organic Sulphides, Part X: The Co-Oxidation of Sulfides and Olefins", J. Chem. Soc., pp. 5339 to 5344, 1961. Several methods have been discovered whereby sulfur in various forms is added to improve petroleum products. U.S. Patent No. 2,914,470 is directed to hydrorefining a petroleum oil fraction by contacting it with a catalyst in the presence of hydrogen sulfide for the purpose of increasing the life of the catalyst. Increases in hydrogen sulfide concentration employed result in decreases in the sulfur content of the product. U.S. Patent No. 3,972,853 relates to a process for preparing a stabilized lubricating oil resistant to oxidation which includes contacting the lubricating oil stock with a small amount of elemental sulfur (0.1 to 0.5 percent by weight) at a mild contact temperature of about 25°C to about 130°C. The elemental sulfur may be added as such or else generated in situ from sulfur precursors such as H₂S or an added organosulfur compound. U.S. Patent No. 3,904,513 is directed to a method of improving the oxidative stability of solvent refined lube stocks. These lubricating base charge stocks are contacted with a catalyst containing nickel-molybdenum on a large pore alumina catalyst in the presence of a gas mixture of about 90% H₂ and 10% H₂S under hydrofinishing conditions. Hydrofinishing under these conditions results in a product which contains significant amounts of sulfur materials which contribute to the oxidative stability of the lube stock. However, it is also known that the presence of H₂S in the hydrogen feed deleteriously affects the catalyst used in the hydrodewaxing of hydrocarbons. Excessive H₂S in the hydrogen feed results in the undersirable acceleration of of catalyst aging rate on significant increases in product pour point, see, e.g., U.S. 4,283,272. A way has now been found to obtain improved results in the catalytic hydrodewaxing of lube stocks without deleteriously affecting the aging rate of the catalyst. Accordingly, the present invention provides a method for processing a lube stock comprising catalytic dewaxing of the lube stock in a conventional catalytic dewaxing zone with hydrogen over a dewaxing catalyst comprising a shape-selective zeolite having a silica to alumina ratio of at least 12, and a constraint index of 1 to 12 and to produce a dewaxed lube stock and then charging the dewaxed lube stock to a conventional hydrotreating zone with hydrogen characterized in that the hydrogen added to the dewaxing zone and the hydrotreating zone contains 0.5 to 5 mole percent hydrogen sulfide. The present invention is particularly advantageous in that it generally requires no removal of hydrogen sulfide from the reactor effluents. This results in a simplified, more economical catalytic dewaxing process which produces a product of low olefin content having significant amounts of antioxidant sulfur-containing compounds. The addition of hydrogen sulfide to olefin materials can be represented by the following equation: This reaction serves not only to saturate the olefins present in the feed but increases mercaptan or thiol content in the feed as well. The instant process enhances the oxidation resistance of catalytically hydrodewaxed lube stocks because it saturates olefins thus reducing the concentration of these easily oxidizable compounds. Furthermore, anti-oxidant sulfur compounds lost in previous catalytic dewaxing of hydrocracking processes are replaced by the mercaptan reaction products. Consequently, the olefin content of the lube stocks is reduced without significantly diminishing the anti-oxidant sulfur compound content of the resulting product. In addition to reducing olefins and increasing anti-oxidant sulfur content, the process of the present invention advantageously permits to some extent, substitution of inexpensive sour gas (H₂S) for hydrogen. The process of the present invention is suited for lube base stock refining methods comprising hydroprocessing, that is, processes which consume hydrogen. These include the lube dewaxing processes disclosed in U.S. Patent Numbers Re 28,398, 3,894,938 and 4,181,598. Also included among the processes of the present invention are those wherein a lube stock is catalytically dewaxed with a shape-selective zeolite catalyst, such as Ni/ZSM-5 and hydrofinished with a conventional hydrofinishing catalyst such as cobalt-molybdenum. Hydrogen sulfide may be advantageously present in the hydrogen-containing feed of only the hydrofinishing step or it may be present in the hydrogen-containg feed or either or both the catalytic dewaxing and hydrofinishing steps. The lubricating oil stock which may be treated generally boil above 316°C (600°F). Such lubricating oil stock materials include those obtained by fractionation, as by, for example, vacuum distillation, of crude oils identified by their source, i.e., Pennsylvania, Midcontinent, Gulf Coast, West Texas, Amal, Kuwait, Barco, Aramco and Arabian. The oil stock may have a substantial part of these crude oils mixed with other oil stocks. Both high sulfur oil stock, i.e., stock having a sulfur content above about 0.4 weight percent, and low sulfur oil stock may be treated. In catalytic dewaxing, the lubricating oil stock, which may be subjected to a conventional solvent dewaxing step to produce a lube stock of intermediate pour point, is contacted with a catalyst in the presence of hydrogen gas. The gas has 0.5 to 5 percent, preferably 1 to 3 percent, typically 2 percent hydrogen sulfide by volume, at temperatures of 204 to 427°C (400 to 800°F), preferably 260 to 371°C (500 to 700°F) and superatmospheric pressures, from just above atmospheric to 14,000 kPa (2000 p.s.i.g.). The liquid hourly space velocity (LHSV) may range from 0.1 to 10, preferably from 0.2 to 2 volumes of oil per hour per volume of catalyst. Advantageously, hydrogen/hydrocarbon rates range from 90 to 900 volumes of gas at standard conditions per volume of oil at standard condition, V/V, [500 to 5000 s.c.f.b. (standard cubic feet per barrel)] preferably 180 to 535 V/V (1000 to 3000 s.c.f.b.). The hydrogen-containing feed may be obtained from any suitable source, preferably from the gaseous effluent of the hydrotreating zone. The dewaxing catalyst can be a composite of a hydrogenation metal, preferably a metal of Group VIII of the Periodic Table, associated with a highly siliceous zeolite having a silica/alumina ratio of at least 12 and a constrained access to the intracrystalline free space, such that the zeolite has a constraint index of 1 to 12. Dewaxing catalysts suitable for the present invention such as Ni/ZSM-5 are described in U.S. Patent No. 4,181,598. After catalytic dewaxing, the resulting lube stock can be passed to a separator where lighter components such as ammonia, hydrogen sulfide and light gas, e.g., methane, ethane and ethylene, are removed. However, a separator is not generally necessary for the removal of H₂S and the dewaxing reactor effluent can be hydrofinished in a hydrotreating step employing a conventional hydrotreating catalyst. Conventional hydrotreating catalysts consist of a hydrogenation component on a non-acidic support. Such catalysts include, for example, a cobalt-molybdate or nickel-molybdate on alumina. Suitable temperatures for hydrotreating range from 204 to 427°C (400-800°F), preferably 260 to 371°C (500 to 700°F). The feed to the hydrotreating reactor may be mixed with hydrogen sulfide so the resulting mixture contains from 0.5 to 5 percent by volume hydrogen sulfide, preferably 1 to 3 percent by volume, ideally 2 percent by volume hydrogen sulfide. Expressed as a mole ratio of H₂/H₂S in the normally gaseous phase charged to the hydrotreater, the H₂/H₂S mole ratio may range from 200:1 to 19:1, preferably 100:1 to 33:1, and ideally 50:1. The hydrogen sulfide can be obtained from various sources, e.g. the H₂S in the effluent from the dewaxing zone, derived from a gas phase from a separation step which follows dewaxing or from the hydrogen-containing effluent of the hydrotreater or any other similar source. The hydrotreating step can be run at pressures of 800 to 21,000 kPa, (100 to 3000 p.s.i.g.), preferably 2,900 to 14,000 kPa (400 to 2000 p.s.i.g.), liquid hourly space velocities ranging from about 0.1 to 10 hr⁻¹, preferably about 0.2 to 2.0 hr⁻¹, and hydrogen and hydrogen sulfide feed rates of 90 to 900 volumes of gas at standard condition per volume of liquid at standard conditions, V/V, (500 to 5000 s.c.f.b.), preferably 180 to 535 V/V (1000 to 3000 s.c.f.b.). The severity of the hydrotreating step can vary depending on the olefin content of the hydrocarbon feed, extent of saturation required, etc. The effluent of the hydrotreater can be stripped or topped by distillation, removing the most volatile components to meet flash and other product specifications. The present invention has been described in terms of current technology. As lube oil refining technology evolves, new solvents for solvent-refining, modifications of dewaxing and fractionation procedures and other innovations will occur. This invention is adaptable to such innovations. EXAMPLES 1-11 An Arabian light bright stock having the properties shown in Table I was catalytically dewaxed and subsequently hydrofinished in 11 consecutive runs employing the same catalysts. The catalytic dewaxing zone employed a ZSM-5 catalyst while the hydrofinishing zone employed a cobalt-molybdenum catalyst. The first five runs used a H₂/H₂S gas with 98 mole % H₂, 2 mole % H₂S. This gas pass charged to both the catalytic dewaxing reactor and the hydrofinishing reactor. The last six runs used pure hydrogen. Operating conditions for all runs included a pressure of 2,900 kPa (400 p.s.i.g.), H₂ or H₂ + H₂S circulation rates of 352 to 476 kPa (1970 to 2670 s.c.f.b.) and LHSVs from 0.8 to 1.1. The temperature in the catalytic dewaxing reactor increased from an initial 295°C (563°F) for the first run to 335°C (635°F) for the 11th run. The second, or hydrotreating reactor was maintained at temperatures ranging from about 288 to 294°C (550 to 562°F). The entire reactor effluent from the catalytic dewaxing reactor was charged to the hydrotreating reactor. Each run lasted about 18 hours. A noticeable overall increase in sulfur content was observed in the products of the runs wherein hydrogen sulfide was combined with the hydrogen feed gas. Table II shows the operating conditions and properties of the lube fractions resulting from the above-described runs. Figure 1 shows the effects of H₂S addition on catalyst aging. The reactor temperatures on Figure 1 have been corrected to the temperatures required to achieve a -6.7°C (20°F) pour point in the 343°C+ (650°F+) lube products (1° reactor temperature rise per 1° pour point). The fresh catalyst of the first cycle had an aging rate of about 3.3°C/day (6°F/day). Initially, the hydrogen feed of the first cycle contained 2% hydrogen sulfide. Upon changing to 100% hydrogen in the last half of the cycle, the aging rate was slightly lowered to about 2.8°C/day (5°F/day). Such a difference is not considered a significant adverse effect, particularly when the enhanced resistence to oxidation of the H₂S treated product is considered. TABLE I TABLE II TABLE II (continued) EXAMPLE 12 To compare oxidation stability of the lube fractions, four samples of the 343°C+ (650°F+) bottoms obtained from Examples 4, 5, 9, and 10 were submitted for the B-10 oxidation test. For the B-10 test, 50 c.c. of oil is placed in a glass all together with iron, copper, and aluminium catalysts and a weighed lead corrosion specimen. The cell and its contents are placed in a bath maintained at 163°C and 10 liters/hr of dried air is bubbled through the sample for 40 hours. The cell is removed from the bath and the catalyst assembly is removed from the cell. The oil is examined for the presence of sludge and the Neutralization Number (ASTM D644) and Kinematic Viscosity at 100°C (ASTM D445) are determined. The lead specimen is cleaned and weighed to determine the loss in weight. The results show that the oxidation stability of the lubes produced with 2 percent H₂S/98 percent H₂ mixture is significantly improved. The B-10 Oxidation Tests are summarized in Table III. As shown in Table II, the experiments of Examples 4 and 5 were conducted with a 2 percent H₂S/98 percent H₂ gas mixture; and the experiments of Examples 9 and 10 were conducted with 100% H₂. The lubes obtained from Examples 4 and 5 result in lower lead loss, lower neutralization number, and less viscosity change as compared to the lubes obtained from Examples 9 and 10. These results all indicate that the presence of H₂S in the gas phase improves the oxidation stability of the lube products. TABLE III

Claims: 1. A method for processing a lube stock comprising catalytic dewaxing of the lube stock in a conventional catalytic dewaxing zone with hydrogen over a dewaxing catalyst comprising a shape-selective zeolite having a silica to alumina ratio of at least 12, and a constraint index of 1 to 12 and to produce a dewaxed lube stock and then charging the dewaxed lube stock to a conventional hydrotreating zone with hydrogen characterized in that the hydrogen added to the dewaxing zone and the hydrotreating zone contains 0.5 to 5 mole percent hydrogen sulfide. 2. The method of Claim 1 wherein the hydrogen in the catalytic dewaxing zone contains 1 to 3 mole percent hydrogen sulfide. 3. The method of Claim 2 wherein the hydrogen contains 2 mole percent hydrogen sulfide. 4. The method of any of Claims 1 to 3 wherein the dewaxing zone effluent is cascaded to the hydrotreating zone. 5. The method of any of Claims 1 to 4 wherein at least a portion of the H₂S added to the catalytic dewaxing zone is obtained from the hydrotreating zone effluent. 6. A method for processing a lube stock substantially as herein before described.

Inventors: SHIH, STUART SHAN-SAN
Applicants: MOBIL OIL CORPORATION
Application year: 1985
Main classification: C10G65
IPC codes: C10G45, C10G49, C10G65, B01J27, C10G73