Abstract:
A process for stabilizing a lubricating oil base stock derived from a nitro-aromatic-containing hydrocracked bright stock, comprising a three-step stabilizing process utilizing hydrodenitrification followed by mild hydrofinishing followed by nonhydrogenative acid stabilization.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a process for improving the bulk oxidation stability and storage stability of lube oil base stocks derived from hydrocracked bright stock. 
     The term &#34;oxidation stability&#34; refers to the resistance of the oil to oxygen addition, in other words, how rapidly is oxygen picked up by and added to molecular species within the oil. Oxidation stability is indicated by the oxidator BN measured in hours. Oxidator BN is thoroughly described in U.S. Pat. No. 3,852,207 granted Dec. 3, 1974 to B. E. Stangeland et al at column 6, lines 15-30. Basically, the test measures the time required for 100 grams of oil to absorb one liter of oxygen. The term &#34;storage stability&#34; refers to the resistance of the oil to floc formation in the presence of oxygen. 
     The process comprises three steps. In the first step a hydrocracked bright stock is hydrodenitrified to reduce its heteroatom, particularly nitrogen, content using, for example, a sulfided nickel-tin catalyst having a siliceous matrix or a nickel-molybdenum hydrotreating catalyst having an alumina matrix. In the second step, the hydrocracked bright stock, having a reduced nitrogen content, is hydrofinished using, for example, an unsulfided nickel-tin or palladium hydrotreating catalyst having a siliceous matrix. In the third step the hydrotreated-hydrofinished bright stock is subjected to nonhydrogenative acid stabilization. 
     The first two steps are carried out at an unusually low liquid hourly space velocity (LHSV), about 0.25 Hr -1 . In the first step, a low LHSV permits the desired hydrodenitrification reaction to proceed at relatively low temperatures, about 700° F. Under these conditions hydrocracking is minimized. In the second step a low LHSV permits thorough saturation of aromatics which are floc-forming species. Thus, in general, the first step removes nitrogen and sulfur, known catalyst poisons, and improves oxidation stability; the second step saturates aromatic floc precursors, and improves storage stability; and the third step alkylates any remaining floc precursors increasing their solubility and resistance to floc formation, further improving storage stability. Accordingly, it has been found that the stability of the resultant lube oil base stock is significantly improved. Moreover, since the second and third steps are both directed to improving storage stability, a beneficial effect of the third step is that of reducing the severity of the hydrofinishing conditions required by the second step. This permits either a longer catalyst life or smaller reactors. 
     Lubricant refining is based upon the fact that crude oils, as shown by experience or by assay, contain a quantity of lubricant base stocks having a predetermined set of properties such as, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The process of refining to isolate a lubricant base stock consists of a set of unit operations to remove or convert the unwanted components. The most common of these unit operations include, for instance, distillation, hydrocracking, dewaxing, and hydrogenation. 
     The lubricant base stock, isolated by these refining operations, may be used as such as a lubricant, or it may be blended with another lubricant base stock having somewhat different properties. Or, the base stock, prior to use as a lubricant, may be compounded with one or more additives which function, for example, as antioxidants, extreme pressure additives, and viscosity index improvers. As used herein, the term &#34;stock&#34;, regardless whether or not the term is further qualified, refers to a hydrocarbon oil without additives. The term &#34;dewaxed stock&#34; will refer to an oil which has been treated by any method to remove or otherwise convert the wax contained therein and thereby reduce its pour point. The term &#34;base stock&#34; will refer to an oil refined to a point suitable for some particular end use, such as for preparing automotive oils. 
     In general, refineries do not manufacture a single lube base stock but rather process at least one distillate fraction and one residuum fraction to produce several lube base stocks. Typically, three distillate fractions differing in boiling range and the residuum of a vacuum distillation operation are refined. These four fractions have acquired various names in the refining art, the most volatile distillate fraction often being referred to as the &#34;light neutral&#34; oil. The other distillates are called &#34;medium neutral&#34; and &#34;heavy neutral&#34; oils. The residuum fraction, is commonly referred to as &#34;bright stock&#34;. Thus, the manufacture of lubricant base stocks involves a process for producing a slate of base stocks, which slate may include a bright stock. 
     Processes have been proposed to produce lubricating oil base stocks by refining bright stocks. Most such refining processes require hydrocracking the bright stock to produce a hydrocrackate which is in turn dewaxed to produce a dewaxed bright stock. The problem is that lubricating oil base stocks derived from hydrocracked stocks are unstable in the presence of oxygen and light. 
     Various stabilizing steps have been proposed. U.S. Pat. Nos. 3,189,540, 3,256,175 granted June 15, 1965 and June 14, 1966, respectively, to Kozlowski et al, describe a typical stabilization. The proposed stabilization uses a series of process steps employing a severe catalytic hydrogenation step to convert the remaining aromatic constituents into desirable lubricating oil con- stituents. 
     The goal of hydrogenation is to hydrogenate the unstable species, which are thought to be partially saturated polycyclic compounds. Unfortunately, severe hydrogenation of hydrocracked bright stocks not only hydrogenates the undesirable polycyclic constituents, but also further hydrocracks desirable constituents resulting in the loss of valuable lubricant base stock. Thus, recent processing schemes have suggested several alternatives to severe hydrogenation. 
     Refiners often now use mild hydrogenation (sometimes referred to as hydrofinishing) to produce more stable lubricating oils. Obviously, mild hydrogenation requires a compromise between the desired stabilization and the undesired hydrocracking. Consequently, thorough stabilization is often not accomplished. 
     As an alternative to hydrofinishing, stabilizing agents, such as olefins, alcohols, esters, or alkylhalides can be added to the hydrocracked base stock in the presence of acidic catalysts having controlled alkylation activity. The resulting alkylation stabilizes the aromatic floc formers. Some of the earlier efforts concentrated on the addition of stabilizing agents to a dewaxed hydrocrackate while in the presence of a heterogeneous acidic catalyst. Several issued patents relate to stabilizing hydrocracked lubricant base stocks by adding stabilizing agents such as olefins, alcohols, esters or alkylhalides to the lube stock while in the presence of a heterogeneous acidic catalyst such as acid resins, clays, and aluminosilicates having controlled alkylation activity. For instance, U.S. Pat. No. 3,928,171, Yan et et al, Dec. 23, 1975 and U.S. Pat. No. 4,181,597, Yan et et al, Jan. 1, 1980 disclose processes for stabilizing hydrocracked lube oils which have been dewaxed, preferably solvent dewaxed, by contacting them with stabilizing agents such as C 6  to C 10  olefins. The stabilizing catalyst is a heterogeneous acidic catalyst; the olefins which are not consumed in the reaction are distilled off from the product and combined with other olefins to be added into the stabilizing reaction zone feed. As discussed in U.S. Pat. No. 4,181,597 the mechanism responsible for the benefits obtained when using a stabilizing agent was not entirely understood. However, since the stabilizing agent is consumed during the stabilization reaction, it is likely that a reaction occurs between one or more components of the dewaxed lube oil stock and the stabilizing agent. In particular, conditions during the stabilization process are conducive to alkylation. While these and other processing schemes have achieved some success, in the case of highly aromatic stocks, such as bright stock, none of the previously known schemes is entirely satisfactory. 
     Thus, in general, at the time of the present invention, the literature relating to lube oil stabilization taught the use of severe hydrogenation or, alternatively, mild hydrofinishing and/or alkylation to stabilize a hydrocracked bright stock. However, in spite of the large amount of research into developing lubricant base stocks and stabilizing them, there continues to be intensive research into developing a more efficient and more convenient method for achieving those goals, especially for lubricant base stocks derived from hydrocracked bright stocks. The object of the present invention is to provide such a process. 
     It has now been discovered that a three-step hydrogenation/alkylation process comprising a first step to reduce the nitrogen and sulfur content, a second step to hydrogenate unstable polycyclics, and a third step to alkylate floc precursors increasing their solubility and resistance to floc formation will produce a more stable lubricating oil base stock from hydrocracked bright stock. Thus, rather than employing a single severe hydrogenation step, the present invention employs a relatively milder three-step hydrofinishing and nonhydrogenative stabilization for hydrocracked bright stocks. 
     SUMMARY OF THE INVENTION 
     The discovery of the present invention is embodied in an improved process for stabilizing a lube base stock derived from hydrocracked bright stock, comprising: 
     (a) contacting said hydrocracked bright stock with hydrogen in the presence of a catalyst having hydrodenitrification activity under conditions, including a low LHSV, effective to reduce the nitrogen content of said bright stock to less than about 50 ppm by weight, preferably less than 10 ppm by weight, and most preferably less than 3 ppm; 
     (b) contacting the denitrified product of step (a) with hydrogen in the presence of a catalyst having hydrogenation activity under conditions, including a low LHSV, effective to reduce the level of unsaturated polycyclic compounds to produce a lubricant base stock; and 
     (c) contacting the product of step (b) with a heterogeneous acidic catalyst substantially free of hydrogenation activity in the presence of a minor amount of an olefinic stabilizing agent. 
     DETAILED DESCRIPTION 
     The hydrocarbonaceous feeds from which the hydrocracked bright stocks used in the process of this invention are obtained usually contain aromatic compounds as well as normal and branched paraffins of very long chain lengths. These feeds usually boil in the gas oil range. Preferred feedstocks are vacuum gas oils with normal boiling ranges above about 350° C. and below about 600° C., and deasphalted residual oils having normal boiling ranges above about 480° C. and below about 650° C. Reduced topped crude oils, shale oils, liquefied coal, coke distillates, flask or thermally cracked oils, atmospheric residua, and other heavy oils can also be used as the feed source. 
     Typically, the hydrocarbonaceous feed is distilled at atmospheric pressure to produce a reduced crude (residuum) which is then vacuum distilled to produce a distillate fraction and a vacuum residuum fraction. According to the present process the residuum fraction is then hydrocracked using standard reaction conditions and catalysts in one or more reaction zones. The resulting hydrocracked bright stock can be further refined, for instance dewaxed, or used as such as the feed stock to the two-step process of this invention. 
     In the first step of the present process, the hydrocracked bright stock is hydrodenitrified to reduce its nitrogen level. Conventional hydrodenitrification catalysts and conditions can be used when carrying out this step. However, in order for the second step, detailed below, to achieve complete, or nearly complete aromatic saturation, of the hydrocracked bright stock which is essential to the present process; in the first step a combination of catalysts and hydrogenation conditions which will reduce the nitrogen level of the hydrocracked bright stock to below about 50 ppm by weight without substantially increasing the quantity of aromatic unsaturates by hydrocracking side reactions are essential. In addition, it will be desirable to select catalysts and conditions which inherently result in cleavage of carbon-sulfur bonds with formation of hydrogen sulfide to achieve some level of hydrodesulfurization. Organic sulfur, like nitrogen, is deleterious to the activity of the hydrogenation catalysts used in the second step. It is desirable to reduce the sulfur level to less than about 50 ppm, preferably less than about 10 ppm, and most preferably less than about 3 ppm. Typical first step hydrodenitrification catalysts comprise a Group VIIIA metal, such as nickel or cobalt, and a Group VIA metal, such as molybdenum or tungsten (unless otherwise noted references to the Periodic Table of Elements are based upon the IUPAC notation) with an alumina or siliceous matrix. These and other hydrodenitrification catalysts, such as nickel-tin catalysts, are well known in the art. U.S. Pat. No. 3,227,661 granted Jan. 4, 1966 to Jacobson et al, describes a method which may be used to prepare a suitable hydrodenitrification catalyst. 
     Typical hydrodenitrification conditions which are useful in the first step of the present process vary over a fairly wide range, but in general temperatures range from about 600° F. to about 850° F., preferably from about 650° F. to 800° F., pressures range from about 500 psig to about 4000 psig, preferably from about 1500 psig to about 3000 psig, contact times expressed as LHSV range from about 0.1 per hour to about 3 per hour, preferably from about 0.1 per hour to about 0.8 per hour, and hydrogen rates range from about 5000 cu. ft. per barrel to about 15,000 cu. ft. per barrel. U.S. Pat. No. 3,227,661 describes those conditions required for various processing schemes using the denitrification catalysts taught in that patent. A general discussion of hydrodenitrification is available in U.S. Pat. No. 3,073,221 granted on Feb. 19, 1963 to Beuther et al. As previously discussed, the overlying consideration, when selecting suitable denitrification conditions from the general conditions taught in these patents and the art generally, is the use of a relatively low LHSV and temperature in order to achieve nearly complete denitrification with minimal hydrocracking. 
     In the second step of the present process the denitrified, &#34;clean&#34; stock is hydrofinished using a mild hydrogenation catalyst and conditions. Suitable catalysts can be selected from conventional hydrofinishing catalysts having hydrogenation activity. Since this step can also be carried out under relatively mild conditions when a low LHSV is employed, it is preferable to use a hydrogenation catalyst such as, for example, a noble metal from Group VIIIA, such as palladium, on a refractory oxide support, or unsulfided Group VIIIA and Group VI, such as nickel-molybdenum, or nickel-tin catalysts. U.S. Pat. No. 3,852,207 granted on Dec. 3, 1974 to Stangeland et al, describes suitable noble metal catalysts and mild conditions. 
     As mentioned already, suitable hydrofinishing conditions should be selected to achieve as complete hydrogenation of unsaturated aromatic as possible. Since the first step has removed the common hydrogenated catalyst poisons and the third step will alkylate remaining floc precursors, the second step run length can be relatively long affording the opportunity to use a relatively low LHSV and mild conditions. Suitable conditions include a temperature ranging from about 300° F. to about 600° F., preferably from about 350° F. to about 550° F., a pressure ranging from about 500 psig to about 4000 psig, preferably from about 1500 psig to about 3000 psig, and an LHSV ranging from about 0.1 to about 2.0 per hour, preferably from about 0.1 per hour to about 0.5 per hour. Thus, in general terms the clear hydrodenitrified effluent of the first step is contacted with hydrogen in the presence of a hydrogenation catalyst under mild hydrogenation conditions. Other suitable catalysts are detailed, for instance in U.S. Pat. No. 4,157,294 granted June 5, 1979 to Iwao et al and U.S. Pat. No. 3,904,513, granted Sept. 9, 1975 to Fischer et al, both incorporated herein by reference. 
     In the third and final step of the present process, the hydrotreated-hydrofinished bright stock is further stabilized by contacting the stock with a minor amount of an olefinic stabilizing agent in the presence of a heterogeneous acidic catalyst substantially free of hydrogenation activity. If the bright stock has been catalytically dewaxed at some point, some olefin may be present in the contact zone as a product of the dewaxing step, however, it will generally be necessary to add additional olefinic stabilizing agent to the contact zone. 
     The heterogeneous acid catalyst used in the final stabilizing step is an acidic, or acid-acting catalyst. The classes of suitable catalysts include acid resins, clays, aluminosilicates, such as zeolites, and other acidic oxides. These solid catalysts which are useful in fluid and fixed bed catalysis, being heterogeneous to the reactants, are readily separable therefrom. 
     Of particular interest are the resins, clays and zeolites. The resins, preferably cation exchange resins, consist of a polymeric resin matrix having acidic functional group, such as sulfonic, phosphonic, phenyl sulfonic, phenylphosphonic, phenolsulfonic, and the like. The matrix may be any resin; the resin preferably is porous, or becomes porous during the process. Resins include phenolic resins, polystyrene, copolymers of styrene with polyfunctional polymerizable monomers or partial polymers, such as styrene-divinyl aryls, e.g., divinyl benzene, or acrylates, polyacrylic or polymethacrylic acid resins, and the halogenated variations of these above said resins. Acidic clays, such as attapulgite and montmorillonite are among the preferred catalysts. Crystalline aluminosilicates, at least partially in the hydrogen form, are also very good catalysts. They generally have the formula: 
     
         M.sub.2 /.sub.n O:Al.sub.2 O.sub.3 :ySiO.sub.2 :zH.sub.2 O 
    
     where M is a cation, n is its valence, y is the moles of silica, and z is the moles of water. Crystalline zeolites are well-known. Modified zeolites, such as rare-earth-exchanged or other metal exchanged zeolites may be used. Rare-earth-exchanged Y-types (y is over 3) and X-types (y is under 3) are examples of these. Both natural aluminosilicates, such as levynite, faujasite, analcite, noselite, phillipsite, datolite, chabazite, leucite, mordenite, and the like, and synthetic aluminosilicates are examples of satisfactory catalysts. Other catalysts which may be used in this invention include inorganic oxides such as alumina, silica-alumina, thoria, zirconia, silica-zirconia, silica-magnesia, titania, germania, ceria, halogenated alumina, and mixtures thereof and these with any of the other foregoing classes of catalysts. 
     Those skilled in the art of heterogeneous catalysis recognize that there are many insoluble solids with a porous structure that are effective in inducing acid-catalyzed reactions, and that, because of this property, are referred to as &#34;acidic&#34;. It is also recognized that these heterogeneous, acidic catalysts vary in efficacy, depending on catalyst preparations in some cases, and on other factors such as the particular reactions involved. These heterogeneous acidic catalysts may be organic, such as sulfonated polystyrene, or inorganic, such as silica-alumina cracking catalysts. The inorganic acidic catalysts exemplified in the foregoing paragraphs and elsewhere are simple or mixed inorganic oxides, which may be polymeric or hydrated to varying extents, as will be recognized by one skilled in the art. With few exceptions, the heterogeneous inorganic acidic oxide catalysts useful in this invention comprise an oxide selected from the group consisting of silicon and aluminum. Included in this group are the acidic clays, the acidic forms of crystalline aluminosilicate zeolites, and the known acidic cracking catalysts based on mixed oxides such as silica-alumina, silica-magnesia and silica-zirconia. 
     The hydrotreated-hydrofinished effluent, or at least part of it, is mixed with the catalyst and a minor amount of an olefinic stabilizing agent, typically in a continuous operation, for example, in a fixed bed reactor which contains the catalyst. Catalyst contact may be carried out at between about 20° C. to 400° C., preferably from 25° C. and 350° C., and most preferably from 90° C. to 280° C. The stabilizing step may be carried out batchwise or continuously at a temperature of about 20° C. to about 400° C., an LHSV of about 0.1 to about 10 (when carried out continuously) and any convenient pressure. The preferred ranges for the above conditions are temperature of 90° C. to about 280° C., an oil LHSV of 0.2 to 2, and pressure of from about 1 to about 35 bar (gauge). 
     The olefinic stabilizing agents employed in the final stabilization step are from either or both of two sources. First, the effluent of the catalytic dewaxing step may contain suitable olefins. These can be separated from the dewaxer effluent ahead of the hydrofinisher by conventional means, e.g., distillation. Second, suitable olefins may be added to the final stabilization reaction zone. Suitable olefins are characterized by the moiety: ##STR1## in which the dangling valences are attached to hydrogen, or alkyl, aryl, aralkyl, alkaryl or other alkenyl, said groups containing at least 3 carbon atoms, and preferably from 3 to about 20 carbon atoms. Olefins such as propylene, butene, hexene, decene, octadecene, butadiene, styrene, phenyl propylene, propylene and other polymers, such as propylene tetramer, and the like are suitable. The olefin can also be a mixture of olefins. Isobutylene is particularly effective either alone or in admixture with other olefins. 
     As used herein, the term &#34;a minor amount&#34; refers to the amount of olefinic stabilizing agent relative to the amount of hydrofinished effluent. Suitable minor amounts will range from as little as 0.2 weight percent of effluent to about 5.0 weight percent of effluent. The preferred range for the preferred stabilizing conditions detailed above is from about 0.4 weight percent of effluent to about 2.0 weight percent of effluent. 
     The product of the process of the present invention is suitable for use as a lubricant base stock. Typically, it is dewaxed, if that has not already been done, prior to final blending. 
    
    
     The present invention is exemplified below. The example is intended to illustrate a representative embodiment of the invention and results which have been obtained in laboratory analysis. Those familiar with the art will appreciate that other embodiments of the invention will provide equivalent results without departing from the essential features of the invention. 
     EXAMPLE 
     EXAMPLE 1 
     A hydrocracked and solvent dewaxed bright stock (Table I) was hydrotreated over a sulfided nickel-tin on silica-alumina hydrogenation catalyst at 675° F., 0.25 LHSV, 2000 psig, and 12M SCF/bbl H 2 . The product was then hydrofinished in a second stage over an unsulfided nickel-tin on silica-alumina catalyst at 430° F., 0.5 LHSV, 2200 psig, and 8 MSCF/bbl H 2 . The oxidator BN was 16 hours, compared to 3.8 hours for the feed. 
     The product from the second-stage hydrofinisher was also tested for storage stability by placing 40 cc of oil in an unstoppered cylindrical glass bottle of 13/8 inches diameter and putting the bottle in a forced convection oven controlled at 250° F. The sample was examined once per day for floc. The test was ended when a moderate to heavy floc could be observed. The product formed heavy floc in 7 days, versus 2 days for the feed. 
     In order to exemplify the advantages of the present invention, a sample of the second stage hydrofinisher product was subjected to nonhydrogenative acid stabilization by mixing it with 1 weight % isobutene and treating it over an acidic montmorillonite clay (Filtrol Corporation F-24) at 0 psig, 400° F., and 1 LHSV. 
     The product was tested for storage stability at 250° F. using the procedure described above. The oil went for 30 days without forming floc, showing a marked improvement using the present invention. The oxidator BN remained at 16 hours 
     As a comparison to the three-step process exemplified above, a two-step process was carried out as follows. 
     A hydrocracked and solvent dewaxed bright stock similar to that used in Example 1 was hydrofinished in a single stage over a sulfided nickel-tungsten on silica-alumina catalyst at 1 LHSV and 2000 psig to produce a 5.3-hour oxidator BN product with a 2-day 250° F. storage stability. A sample of the product oil was treated by nonhydrogenative stabilization as in Example 1 at 0 psig, 400° F., and 1 LHSV after mixing with 50 LV % propylene. The product from this step was topped at 950° F. and tested for 250° F. storage stability, which was found to be less than three days. 
     
                       TABLE I______________________________________Solvent Dewaxed Bright Stock______________________________________Gravity, °API               23.8Sulfur, ppm         102Nitrogen, ppm       335Pour Point, °F.               +20Oxidator BN, hr.    3.8Viscosity, cS, 40° C.               589.9Viscosity, cS, 100° C.               33.30VI                  87Distillation, D1160, LV %, °F.ST/5                950/97710/30               991/101950                  1049______________________________________