DEWAXING PROCESS

Abstract: The process of the invention for converting a hydrocarbon oil includes the following steps: (1) contacting a hydrocarbon oil feedstock in the presence of added hydrogen gas with a catalyst selected from the group consisting of a SAPO-11, SAPO-31 or SAPO-41 intermediate pore size silicoaluminophosphate molecular sieve and a hydrogenation component, and mixtures thereof, wherein at least a portion of the feedstock is converted; and (2) passing at least a portion of the converted feedstock to a fractionator, wherein at least a portion of the converted feedstock is fractionated, thereby producing at least one overhead fraction and one bottoms fraction; and (3) mixing at least a portion of the bottoms fraction with the hydrocarbon oil feedstock in step (1).

Description: DEWAXING PROCESS |. FIELD OF THE INVENTION The present invention relates to a process for catalytically dewaxing lube oils. More specifically, the invention relates to a process for dewaxing a hydrocarbon oil feedstock wherein at least a portion of fractionator bottoms is recycled to the feedstock. ||. BACKGROUND OF THE INVENTION Certain processes for dewaxing petroleum distillates are well known. Dewaxing is required when highly paraffinic oils are to be used in products which must be mobile at low temperatures, e.g., lubricating oils, heating oils, and jet fuels. The higher molecular weight straight chain normal, substituted and slightly branched paraffins present in such oils are waxes that cause high pour points and high cloud points in the oils. If adequately low pour points are to be obtained, the waxes must be wholly or partially removed. In the past, various solvent removal techniques were employed to remove such waxes, such as propane dewaxing and MEK dewaxing; however, these have high operating costs, significant environmental impacts and produce oils which are inferior to catalytically-dewaxed oils. Catalytic dewaxing processes are more economical and remove the waxes by selectively isomerizing and cracking paraffinic components to produce lower molecular weight products, some of which may be removed by distillation. Because of their selectivity, known dewaxing catalysts generally comprise an aluminosilicate zeolite having a pore size which admits the straight chain n-paraffins either alone or with only slightly branched chain paraffins, but which excludes more highly branched materials, larger cycloaliphatics and aromatics. Zeolites such as ZSM-5, ZSM-11 , ZSM-12, ZSM-23, ZSM-35 and ZSM-38 have been proposed for this purpose in dewaxing processes. Their use is described in U.S. Pat. Nos. 3,700,585; 3,894,938; 4,176,050; 4,181 ,598; 4,222,855; 4,229,282 and 4,247,388, the disclosures of which are incorporated herein by reference. Since many dewaxing processes of this kind function by means of cracking reactions, a number of useful products become degraded to lower molecular weight materials. For example, waxy paraffins may be cracked down to butane, propane, ethane and methane and so may the lighter n-paraffins which do not contribute to the waxy nature of the oil. Because these lighter products are generally of lower value than the higher molecular weight materials, it is desirable to limit the degree of cracking which takes place during a catalytic dewaxing process. European Patent Application No. 225,053 discloses a process for producing lubricant oils by partially dewaxing a lubricant base stock by isomerization dewaxing followed by a selective dewaxing step. The isomerization dewaxing step is carried out using a large pore, high silica zeolite dewaxing catalyst such as high silica Y or zeolite beta which isomerizes the waxy components of the base stock to less waxy branched chain isoparaffins. The selective dewaxing step may be either a solvent, e.g., MEK dewaxing operation or a catalytic dewaxing, preferably using a highly shape zeolite such as ZSM-22 or ZSM-23. U.S. Pat. No. 4,437,976 discloses a two-stage hydrocarbon dewaxing hydrotreating process wherein the pour point of a hydrocarbon charge stock boiling from 400°F to 1050°F is reduced by catalytically dewaxing the charge stock in the presence of a zeolite catalyst and subsequently subjecting at least the liquid portion thereof to hydrogenation in the presence of a hydrotreating catalyst comprising a hydrogenating component and a siliceous porous crystalline material from the class of ZSM-5, ZSM-11 , ZSM-23 and ZSM-35 zeolites. U.S. Pat. No. 4,575,416 to Chester et al. discloses a hydrodewaxing process with a first zeolitic catalyst having a Constraint Index not less than 1 , a second catalytic component of specified characteristics and a hydrogenation component. U.S. Pat. No. 5,149,421 teaches a dewaxing catalyst which provides superior selectivity with respect to the nature of the products obtained in a dewaxing process. By using an intermediate pore size silicoaluminophosphate molecular sieve catalyst in the dewaxing process, hydrocarbon oil feedstocks are effectively dewaxed and the products obtained are of higher molecular weight than those obtained using the other aluminosilicate zeolites. The products obtained from the dewaxing process have better viscosities and viscosity indexes at a given pour point as compared to the above-described prior art process using aluminosilicate zeolites. Nevertheless, it would be advantageous to have a process which provided increased yield over the yield obtained in known processes, or increased pour point reduction at the same yield. The present invention provides such a process. III. SUMMARY OF THE INVENTION The present invention overcomes the problems and disadvantages of the prior art by providing a process for catalytically dewaxing a hydrocarbon oil feedstock which produces a superior lube oil yield. The process of the invention for converting a hydrocarbon oil includes the following steps: (1 ) contacting a hydrocarbon oil feedstock in the presence of added hydrogen gas with a catalyst selected from the group consisting of a SAPO-11 , SAPO-31 or SAPO-41 intermediate pore size silicoaluminophosphate molecular sieve and a hydrogenation component, and mixtures thereof, where at least a portion of the feedstock is converted; and (2) passing at least a portion of the converted feedstock to a fractionator, where at least a portion of the converted feedstock is fractionated, thus producing at least one overhead fraction and one bottoms fraction; and (3) mixing at least a portion of the bottoms fraction with the hydrocarbon oil feedstock in step (1 ). IV. BRIEF DESCRIPTION OF THE DRAWING Figure 1 depicts a simplified schematic flow chart of one embodiment of the process of the invention. V. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS A. Steps of the Process The process of the invention for converting a hydrocarbon oil includes the following steps: (1) contacting a hydrocarbon oil feedstock in the presence of added hydrogen gas with a catalyst system containing catalyst selected from the group consisting of a SAPO-11 , SAPO-31 or SAPO-41 intermediate pore size silicoaluminophosphate molecular sieve and a hydrogenation component, and mixtures thereof, where at least a portion of the feedstock is converted; and (2) passing at least a portion of the converted feedstock to a fractionator, where at least a portion of the converted feedstock is fractionated, thus producing at least one overhead fraction and one bottoms fraction; and (3) mixing at least a portion of the bottoms fraction with the hydrocarbon oil feedstock in step (1 ). The catalyst system optionally further includes a catalyst selected from the group consisting of an intermediate pore size aluminosilicate zeolite catalyst, an amorphous catalyst, and mixtures thereof. For pre-treatments, the feed may be hydrocracked or solvent extracted and hydrotreated. This type of process and typical hydrocracking conditions are described in U.S. Patent No. 4,921 ,594, issued May 1 , 1990 to Miller, which is incorporated herein by reference in its entirety. Post-treatments can include hydrofinishing, discussed below. Without being limited by theory, in one embodiment, the dewaxing mechanism is isomerization and/or cracking of waxy compounds. Typically, catalytic dewaxing, e.g., Chevron's ISODEWAXING catalytic dewaxing process, operates to improve the pour point and viscosity index of a feedstock, compared to solvent dewaxing. B. Feedstock The process of the invention may be used to dewax a variety of hydrocarbon oil feedstocks classified generally as any waxy hydrocarbon feed, lube oil feedstock, or middle distillate oil. The feedstocks include distillate fractions, e.g., hydrocrackates, up to high boiling stocks such as deasphalted and solvent extracted oils. The feedstock will normally be a C 10 + feedstock generally boiling above about 350°F, since lighter oils will usually be free of significant quantities of waxy components. However, the process is particularly useful with waxy distillate stocks such as middle distillate stocks including gas oils, kerosenes, and jet fuels, lubricating oil stocks, heating oils and other distillate fractions whose pour point and viscosity need to be maintained within certain specification limits. Lubricating oil stocks will generally boil above 230°C (450°F), more usually above 315°C (600°F). Hydroprocessed stocks are a convenient source of stocks of this kind and also of other distillate fractions since they have a higher hydrogen content over solvent-processed stocks and are usually relatively free of heteroatoms (e.g., sulfur and nitrogen compounds) which can impair the performance of the dewaxing and hydrofinishing catalysts. The feedstock of the present process will normally be a C 10 + feedstock containing paraffins, olefins, naphthenes, aromatics and heterocyclic compounds and a substantial proportion of higher molecular weight n-paraffins and slightly branched and substituted paraffins which contribute to the waxy nature of the feedstock. During processing, feed molecules undergo some cracking or hydrocracking to form liquid range materials which contribute to a low viscosity product. The degree of cracking which occurs is, however, limited to preserve the yield of the valuable liquids. Typical feedstocks include light gas oils, heavy gas oils and reduced crudes boiling above 350°F. In one embodiment, the feedstock contains a major portion of a hydrocarbon oil feedstock boiling above about 350°F and contains straight chain and slightly branched chain hydrocarbons. The term "major portion" means more than 50 weight percent. While the process of the invention can be practiced with utility when the feed contains organic nitrogen (nitrogen-containing impurities), it is preferred that the organic nitrogen content of the feed be less than 50 ppmw, more preferably less than 10 ppmw. Particularly good results, in terms of activity and length of catalyst cycle (period between successive regenerations or startup and first regeneration), are experienced when the feed contains less than 10 ppmw of organic nitrogen. C. Silicoaluminophosphate Molecular Sieve Catalyst Compositions 1. Generally: Intermediate pore size silicoaluminophosphate molecular sieves (SAPOs) are catalysts used in the process of the invention. Suitable SAPOs are any conventional intermediate pore SAPO. The SAPOs are used separately or in combination with zeolites and/or amorphous catalysts. Examples of silicoaluminophosphate molecular sieves which can used in this invention are described in U.S. Pat. Nos. 4,440,871 and 5,149,421 , the disclosures of which are incorporated herein by reference. The intermediate pore size silicoaluminophosphate molecular sieve catalyst is employed in the process of the invention to convert the waxy components to non-waxy components and reduce their pour point by about 30°F to about 60°F. The amount of catalyst employed is dependent on the reaction conditions. In a preferred embodiment, the final catalyst will be a composite and includes an intermediate pore size silicoaluminophosphate molecular sieve, a platinum or palladium hydrogenation metal component and an inorganic oxide matrix. The preferred intermediate pore size silicoaluminophosphate molecular sieves suitable for use in the process of this invention include SAPO-11 , SAPO-31 and SAPO-41. The most preferred silicoaluminophosphate is SAPO-11 , the most preferred metal component is platinum, and the most preferred binder is alumina. Descriptions of SAPO-11 , SAPO-31 and SAPO-41 and methods of making them are given in the above referenced patents and in R. Szostak, Handbook of Molecular Sieves (Van Norstrand Reinhold 1992), pages 410-413, 415-416, 419-420, the disclosures of which are incorporated herein by reference. 2. Special Preparations: The molecular sieve can be composited with other materials resistant to the temperatures and other conditions employed in the dewaxing process. Such matrix materials include active and inactive materials and synthetic or naturally occurring zeolites as well as inorganic materials such as clays, silica and metal oxides. The latter may be either naturally occurring or in the form of gelatinous precipitates, sols or gels including mixtures of silica and metal oxides. Inactive materials suitably serve as binders or as diluents to control the amount of conversion in the dewaxing process so that products can be obtained economically without employing other means for controlling the rate of reaction. The silicoaluminophosphates may be combined with naturally occurring clays, e.g., bentonite and kaolin. These materials, i.e., clays, oxides, etc., function, in part, as binders for the catalyst. It is desirable to provide a catalyst having good crush strength because in petroleum refining the catalyst is often subjected to rough handling and large forces in the reactor. This tends to break the catalyst down into fragments which can plug the reactor. Naturally occurring clays which can be composited with the silicoaluminophosphate include the montmorillonite and kaolin families, which families include the sub-bentonites, and the kaolins commonly known as Dixie, McNamee, Georgia and Florida clays or others in which the main mineral constituent is halloysite, kaolinite, dickite, nacrite, or anauxite. Fibrous clays such as halloysite, sepiolite and attapulgite can also be used as supports. Such clays can be used in the raw state as originally mined or initially subjected to calcination, acid treatment or chemical modification. In addition to the foregoing materials, the silicoaluminophosphates can be composited with porous matrix materials, e.g., inorganic oxide matrix, and mixtures of matrix materials such as silica, alumina, titania, magnesia, silica- alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica- titania, titania-zirconia as well as ternary compositions such as silica-alumina- thoria, silica-alumina-titania, silica-alumina-magnesia and silica-magnesia- zirconia. The matrix can be in the form of a cogel or an intimate physical mixture. The silicoaluminophosphate catalysts used in the process of this invention can also be composited with other zeolites such as synthetic and natural faujasites, (e.g., X and Y) erionites and mordenites. They can also be composited with purely synthetic zeolites such as those of the ZSM series. The combination of the zeolites can also be composited in a porous inorganic matrix. P. Zeolites Exemplary suitable aluminosilicate zeolite catalysts for use in the process of the invention include ZSM-22, ZSM-23 and ZSM-35. These are taught in R. Szostak, Handbook of Molecular Sieves (Van Norstrand Reinhold 1992), at pages 538-542 and 545-546, which are incorporated herein by reference, and in U.S. Patent Nos. 4,481,177; 4,076,842; and 4,016,245, the disclosures of which are incorporated herein by reference. The silicoaluminophosphate molecular sieve catalyst and the aluminosilicate zeolite catalyst are employed in the process of the invention in an effective weight ratio of the intermediate pore size silicoaluminophosphate molecular sieve to the intermediate pore size aluminosilicate zeolite molecular sieve to increase yield of converted feedstock. Preferred ratios are from about 1 :5 to about 20:1. The zeolite used in the process preferably has a Constraint Index measured at from about 400°C to about 454°C of from about 4 to about 12. In another embodiment of the process of the invention, SSZ-48, preferably predominantly in the hydrogen form, can be used in the dewaxing process of the invention. Without being limited by theory, SSZ-48 is believed to dewax by selectively removing straight chain paraffins. Typically, the viscosity index of the dewaxed product is improved (compared to the solvent dewaxed feed) when the waxy feed is contacted with SSZ-48 under isomerization dewaxing (also referred to as hydrodewaxing) conditions. In preparing SSZ-48 zeolites, a decahydroquinolinium cation is used as a crystallization template. The decahydroquinolinium cation may have the following structure: The anion (X " ) associated with the cation may be any anion which is not detrimental to the formation of the zeolite. Representative anions include halogen, e.g., fluoride, chloride, bromide and iodide, hydroxide, acetate, sulfate, tetrafluoroborate, carboxylate, and the like. Hydroxide is the most preferred anion. In general, SSZ-48 is prepared by contacting an active source of one or more oxides selected from the group consisting of monovalent element oxides, divalent element oxides, trivalent element oxides, and tetravalent element oxides with the decahydroquinolinium cation templating agent. SSZ-48 is prepared from a reaction mixture having the composition shown in Table 1 below. TABLE 1 Reaction Mixture Typical Preferred YOVW β O b 10 - 100 15 - 40 OH-/YO 2 0.10 - 0.50 0.20 - 0.30 Q YO 2 0.05 - 0.50 0.10 - 0.20 M^/YO, 0.01 - 0.10 0.03 - 0.07 H 2 OAO 2 20 - 80 30 - 45 wherein Y is silicon, germanium or a mixture thereof; W is aluminum, gallium, iron, boron, titanium, indium, vanadium or mixtures thereof; c is 1 or 2; d is 2 when c is 1 (i.e., W is tetravalent) or d is 3 or 5 when c is 2 (i.e., d is 3 when W is trivalent or 5 when W is pentavalent); M is an alkali metal cation, alkaline earth metal cation or mixtures thereof; n is the valence of M (i.e., 1 or 2); and Q is at least one decahydroquinolinium cation, and a is 1 or 2, and b is 2 when a is 1 (i.e., W is tetravalent) and b is 3 when a is 2 (i.e., W is trivalent). In practice, SSZ-48 is prepared by a process comprising: (a) preparing an aqueous solution containing sources of at least one oxide capable of forming a crystalline molecular sieve and a decahydroquinolinium cation having an anionic counterion which is not detrimental to the formation of SSZ-48; (b) maintaining the aqueous solution under conditions sufficient to form crystals of SSZ-48; and (c) recovering the crystals of SSZ-48. Accordingly, SSZ-48 may comprise the crystalline material and the templating agent in combination with metallic and non-metallic oxides bonded in tetrahedral coordination through shared oxygen atoms to form a cross-linked three dimensional crystal structure. The metallic and non-metallic oxides comprise one or a combination of oxides of a first tetravalent element(s), and one or a combination of a second tetravalent element(s) different from the first tetravalent element(s), trivalent element(s), pentavalent element(s) or mixture thereof. The first tetravalent element(s) is preferably selected from the group consisting of silicon, germanium and combinations thereof. More preferably, the first tetravalent element is silicon. The second tetravalent element (which is different from the first tetravalent element), trivalent element and pentavalent element is preferably selected from the group consisting of aluminum, gallium, iron, boron, titanium, indium, vanadium and combinations thereof. More preferably, the second trivalent or tetravalent element is aluminum or boron. Typical sources of aluminum oxide for the reaction mixture include aluminates, alumina, aluminum colloids, aluminum oxide coated on silica sol, hydrated alumina gels such as AI(OH) 3 and aluminum compounds such as AICI 3 and AI 2 (SO 4 ) 3 . Typical sources of silicon oxide include silicates, silica hydrogel, silicic acid, fumed silica, colloidal silica, tetra-alkyl orthosilicates, and silica hydroxides. Boron, as well as gallium, germanium, titanium, indium, vanadium and iron, can be added in forms corresponding to their aluminum and silicon counterparts. A source zeolite reagent may provide a source of aluminum or boron. In most cases, the source zeolite also provides a source of silica. The source zeolite in its dealuminated or deboronated form may also be used as a source of silica, with additional silicon added using, for example, the conventional sources listed above. Use of a source zeolite reagent as a source of alumina for the present process is more completely described in U.S. Patent No. 5,187,132, issued February 16, 1993 to Zones et al. entitled "Preparation of Borosilicate Zeolites", the disclosure of which is incorporated herein by reference. Typically, an alkali metal hydroxide and/or an alkaline earth metal hydroxide, such as the hydroxide of sodium, potassium, lithium, cesium, rubidium, calcium, and magnesium, is used in the reaction mixture; however, this component can be omitted so long as the equivalent basicity is maintained. The templating agent may be used to provide hydroxide ion. Thus, it may be beneficial to ion exchange, for example, the halide for hydroxide ion, thereby reducing or eliminating the alkali metal hydroxide quantity required. The alkali metal cation or alkaline earth cation may be part of the as-synthesized crystalline oxide material, in order to balance valence electron charges therein. The reaction mixture is maintained at an elevated temperature until the crystals of the SSZ-48 zeolite are formed. The hydrothermal crystallization is usually conducted under autogenous pressure, at a temperature between 100°C and 200°C, preferably between 135°C and 160°C. The crystallization period is typically greater than 1 day and preferably from about 3 days to about 20 days. Preferably, the zeolite is prepared using mild stirring or agitation. During the hydrothermal crystallization step, the SSZ-48 crystals can be allowed to nucleate spontaneously from the reaction mixture. The use of SSZ-48 crystals as seed material can be advantageous in decreasing the time necessary for complete crystallization to occur. In addition, seeding can lead to an increased purity of the product obtained by promoting the nucleation and/or formation of SSZ-48 over any undesired phases. When used as seeds, SSZ-48 crystals are added in an amount between 0.1 and 10% of the weight of silica used in the reaction mixture. Once the zeolite crystals have formed, the solid product is separated from the reaction mixture by standard mechanical separation techniques such as filtration. The crystals are water-washed and then dried, e.g., at 90°C to 150°C for from 8 to 24 hours, to obtain the as-synthesized SSZ-48 zeolite crystals. The drying step can be performed at atmospheric pressure or under vacuum. SSZ-48, as prepared, has a mole ratio of an oxide selected from silicon oxide, germanium oxide and mixtures thereof to an oxide selected from aluminum oxide, gallium oxide, iron oxide, boron oxide, titanium oxide, indium oxide, vanadium oxide and mixtures thereof greater than about 40; and has the X-ray diffraction lines of Table 2 below. TABLE 2 As-Synthesized SSZ-48 2 Theta (a) d Relative Intensity 6.55 13.5 S 8.0 11.0 VS 9.4 9.40 M 11.3 7.82 M-W 20.05 4.42 VS 22.7 3.91 VS 24.1 3.69 VS 26.5 3.36 S 27.9 3.20 S 35.85 2.50 M (a) ± 0.3 (b) The X-ray patterns provided are based on a relative intensity scale in which the strongest line in the X-ray pattern is assigned a value of 100: W(weak) is less than 20; M(medium) is between 20 and 40; S(strong) is between 40 and 60; VS(very strong) is greater than 60. SSZ-48 further has a composition, as synthesized and in the anhydrous state, in terms of mole ratios, shown in Table 3 below. TABLE 3 As-Synthesized SSZ-48 Y0 N c 0 ά 40 - 100 M^AO, 0.01 - 0.03 Q/YO 2 0.02 - 0.05 wherein Y is silicon, germanium or a mixture thereof; W is aluminum, gallium, iron, boron, titanium, indium, vanadium or mixtures thereof; c is 1 or 2; d is 2 when c is 1 (i.e., W is tetravalent) or d is 3 or 5 when c is 2 (i.e., d is 3 when W is trivalent or 5 when W is pentavalent); M is an alkali metal cation, alkaline earth metal cation or mixtures thereof; n is the valence of M (i.e., 1 or 2); and Q is at least one decahydroquinolinium cation. A method of increasing the mole ratio of silica to boron is by using standard acid leaching or chelating treatments. Lower silica to alumina ratios may also be obtained by using methods which insert aluminum into the crystalline framework. For example, aluminum insertion may occur by thermal treatment of the zeolite in combination with an alumina binder or dissolved source of alumina. Such procedures are described in U.S. Patent No. 4,559,315, issued on December 17, 1985 to Chang et al, the disclosure of which is incorporated herein by reference. SSZ-48 zeolites, as-synthesized, have a crystalline structure whose X-ray powder diffraction pattern exhibit the characteristic lines shown in Table 2 above and is thereby distinguished from other known zeolites. After calcination, the SSZ-48 zeolites have a crystalline structure whose X-ray powder diffraction pattern include the characteristic lines shown in Table 4: TABLE 4 Calcined SSZ-48 2 Theta (a) d Relative Intensity <b) 6.55 13.5 VS 8.0 11.0 VS 9.4 9.40 S 11.3 7.82 M 20.05 4.42 M 22.7 3.91 M 24.1 3.69 M 26.5 3.36 M 27.9 3.20 W 35.85 2.50 W (a) ± 0.3 The X-ray powder diffraction patterns were determined by standard techniques. The radiation was the K-alpha/doublet of copper. The peak heights and the positions, as a function of 2Θ where θ is the Bragg angle, were read from the relative intensities of the peaks, and d, the interplanar spacing in Angstroms corresponding to the recorded lines, can be calculated. The variation in the scattering angle (two theta) measurements, due to instrument error and to differences between individual samples, is estimated at ±0.30 degrees. The X-ray diffraction pattern of Table 2 above is representative of "as-synthesized" or "as-made" SSZ-48 zeolites. Minor variations in the diffraction pattern can result from variations in the silica-to-alumina or silica-to-boron mole ratio of the particular sample due to changes in lattice constants. In addition, sufficiently small crystals will affect the shape and intensity of peaks, leading to significant peak broadening. Representative peaks from the X-ray diffraction pattern of calcined SSZ-48 are shown in Table 4. Calcination can also result in changes in the intensities of the peaks as compared to patterns of the "as-made" material, as well as minor shifts in the diffraction pattern. The zeolite produced by exchanging the metal or other cations present in the zeolite with various other cations (such as H + or NH 4 + ) yields essentially the same diffraction pattern, although again, there may be minor shifts in the interplanar spacing and variations in the relative intensities of the peaks. Notwithstanding these minor perturbations, the basic crystal lattice remains unchanged by these treatments. Crystalline SSZ-48 can be used as-synthesized, but preferably will be thermally treated (calcined). Usually, it is desirable to remove the alkali metal cation by ion exchange and replace it with hydrogen, ammonium, or any desired metal ion. The zeolite can be leached with chelating agents, e.g., EDTA or dilute acid solutions, to increase the silica to alumina mole ratio. The zeolite can also be steamed; steaming helps stabilize the crystalline lattice to attack from acids. SSZ-48, and any other zeolite used in this process, can be used in intimate combination with hydrogenating components, such as tungsten, vanadium molybdenum, rhenium, nickel, cobalt, chromium, manganese, or a noble metal, such as palladium or platinum, for those applications in which a hydrogenation-dehydrogenation function is desired. Platinum and palladium are preferred. Metals may also be introduced into the zeolites by replacing some of the cations in the zeolite with metal cations via standard ion exchange techniques (see, for example, U.S. Patent Nos. 3,140,249 issued July 7, 1964 to Plank et al.; 3,140,251 issued July 7, 1964 to Plank et al.; and 3,140,253 issued July 7, 1964 to Plank et al., the disclosures of which are incorporated herein by reference). Typical replacing cations can include metal cations, e.g., rare earth, Group IA, Group IIA and Group VIII metals, as well as their mixtures. Of the replacing metallic cations, cations of metals such as rare earth, Mn, Ca, Mg, Zn, Cd, Pt, Pd, Ni, Co, Ti, Al, Sn, and Fe are particularly preferred. The techniques of introducing catalytically active metals to a molecular sieve are disclosed in the literature, and pre-existing metal incorporation techniques and treatment of the molecular sieve to form an active catalyst such as ion exchange, impregnation or occlusion during sieve preparation are suitable for use in the present process. Such techniques are disclosed in U.S. Pat. Nos. 3,236,761 ; 3,226,339; 3,236,762; 3,620,960; 3,373,109; 4,202,996; 4,440,781 and 4,710,485, the disclosures of which are incorporated herein by reference. The amount of metal ranges from about 0.01 % to about 10% by weight of the zeolite, preferably from about 0.2% to about 5%. The hydrogen, ammonium, and metal components can be ion-exchanged into the zeolites. They can also be impregnated with the metals, or, the metals can be physically and intimately admixed with the zeolite using standard methods known to the art. Typical ion-exchange techniques involve contacting the synthetic zeolite with a solution containing a salt of the desired replacing cation or cations. Although a wide variety of salts can be employed, chlorides and other halides, acetates, nitrates, and sulfates are particularly preferred. The zeolite is usually calcined prior to the ion-exchange procedure to remove the organic matter present in the channels and on the surface, since this results in a more effective ion exchange. Representative ion exchange techniques are disclosed in a wide variety of patents including U.S. Patent Nos. 3,140,249 issued on July 7, 1964 to Plank et al.; 3,140,251 issued on July 7, 1964 to Plank et al.; and 3,140,253 issued on July 7, 1964 to Plank et al, the disclosures of which are incorporated herein by reference. Following contact with the salt solution of the desired replacing cation, the zeolite is typically washed with water and dried at temperatures ranging from 65°C to about 200°C. After washing, the zeolite can be calcined in air or inert gas at temperatures ranging from about 200°C to about 800°C for periods of time ranging from 1 to 48 hours, or more, to produce a catalytically active product especially useful in hydrocarbon conversion processes. Regardless of the cations present in the synthesized form of SSZ-48, the spatial arrangement of the atoms which form the basic crystal lattice of the zeolite remains essentially unchanged. The hydrogenation component is present in an appropriate amount to provide an effective hydrodewaxing and hydroisomerization catalyst preferably in the range of from about 0.05 to 5% by weight. The catalyst may be run in such a mode to increase isodewaxing at the expense of cracking reactions. Any two or more zeolites utilized in this process may be utilized as a dewaxing catalyst in the form of a layered catalyst. That is, the catalyst comprises a first layer comprising, e.g., zeolite SSZ-48 and at least one Group VIII metal, and a second layer comprising another aluminosilicate zeolite, e.g., one which, optionally, is more shape selective than zeolite SSZ-48. The use of layered catalysts is disclosed in U.S. Patent No. 5,149,421 , issued September 22, 1992 to Miller, which is incorporated by reference herein in its entirety. The layering may also include a zeolite bed, e.g., SSZ-48, layered with a non-zeolitic component designed for either hydrocracking or hydrofinishing. Instead of layering, intimately mixed catalyst systems represent another useful variant on this concept. E. Amorphous Catalysts The amorphous catalysts useful in the invention are any amorphous catalysts having hydrogenation and/or isomerization effects on the feedstock. Such amorphous catalysts are taught, e.g., in U.S. Patent No. 4,383,913, the disclosure of which is incorporated herein by reference. These include, e.g., amorphous catalytic inorganic oxides, e.g., catalytically active silica-aluminas, clays, synthetic or acid activated clays, silicas, aluminas, silica-aluminas, silica-zirconias, silica-magnesias, alumina-borias, alumina-titanias, pillared or cross-linked clays, and the like and mixtures thereof. F. Process Conditions The process is conducted at catalytic dewaxing conditions. Such conditions are known and are taught for example in U.S. Patent Nos. 5,591 ,322; 5,149,421 ; and 4,181 ,598, the disclosures of which are incorporated herein by reference. The catalytic dewaxing conditions are dependent in large measure on the feed used and upon the desired pour point. Hydrogen is preferably present in the reaction zone during the catalytic dewaxing process. The hydrogen to feed ratio, i.e., hydrogen circulation rate, is typically between about 500 and about 30,000 SCF/bbl (standard cubic feet per barrel), preferably about 1000 to about 20,000 SCF/bbl. Generally, hydrogen will be separated from the product and recycled to the reaction zone. The percent of fractionator bottoms recycled to the feed is an effective amount to enhance overall yield. Preferably, the percent recycle is from about 1 to about 100, or more preferably from about 10 to about 50. The ratio of fractionator bottoms to the raw feed is an effective ratio to either reduce pour point with no loss in yield or to enhance overall yield while maintaining pour point. Preferably, the ratio is from about 1 :100 to about 60:100, or more preferably from about 1 :100 to about 40:100. An intermediate pore size aluminosilicate zeolite catalyst and/or amorphous catalyst are optionally used in the same reactor as the silicoaluminophosphate molecular sieve catalyst, or may be used in a separate reactor. When two or more catalysts are used in the same reactor, they may be sequentially layered or mixed. When sequentially layered, the SAPO is optionally the first the first or second layer. When two or more catalysts are used in the same reactor, they may also me intimately mixed. Any conventional catalyst bed configuration can be used in the process of the invention. The catalytic isomerization step of the invention may be conducted by contacting the feed to be dewaxed with a fixed stationary bed of catalyst, with a fixed fluidized bed, or with a transport bed, as desired. A simple and therefore preferred configuration is a trickle-bed operation in which the feed is allowed to trickle through a stationary fixed bed, preferably in the presence of hydrogen. The catalytic dewaxing conditions employed depend on the feed used and the desired pour point. Some generalizations of process conditions for various catalytic processes are shown in Table 5 below: 1 Several hundred atmospheres 2 Gas phase reaction 3 Hydrocarbon partial pressure Liquid phase reaction 5 WHSV In the process of this invention, generally, the temperature is from about 200°C and about 475°C, preferably between about 250°C and about 450°C. The pressure is typically from about 15 psig and about 3000 psig, preferably between about 200 psig and 3000 psig. The liquid hourly space velocity (LHSV) preferably will be from 0.1 to 20, preferably between about 0.2 and 10. Hydrogen is preferably present in the reaction zone during the catalytic isomerization process. The hydrogen to feed ratio is typically between about 500 and about 30,000 SCF/bbl (standard cubic feet per barrel), preferably from about 1000 to about 20,000 SCF/bbl. Generally, hydrogen will be separated from the product and recycled to the reaction zone. G. Post-Treatments It is often desirable to use mild hydrogenation (sometimes referred to as hydrofinishing). The hydrofinishing step is beneficial in preparing an acceptably stable product (e.g., a lubricating oil) since unsaturated products tend to be unstable to air and light and tend to degrade. The hydrofinishing step can be performed after the isomerization step. Hydrofinishing is typically conducted at temperatures ranging from about 190°C to about 340°C, at pressures of from about 400 psig to about 3000 psig, at space velocities (LHSV) of from about 0.1 to about 20, and hydrogen recycle rates of from about 400 to about 1500 SCF/bbl. The hydrogenation catalyst employed must be active enough not only to hydrogenate the olefins and diolefins within the lube oil fractions, but also to reduce the content of any aromatics present. Suitable hydrogenation catalysts include conventional, metallic hydrogenation catalysts, particularly the Group VIII metals such as cobalt, nickel, palladium and platinum. The metals are typically associated with carriers such as bauxite, alumina, silica gel, silica-alumina composites, and crystalline aluminosilicate zeolites and other molecular sieves. Palladium is a particularly preferred hydrogenation metal. If desired, non-noble Group VIII metals can be used with molybdates. Metal oxides or sulfides can be used. Suitable catalysts are disclosed in U.S. Pat. Nos. 3,852,207; 4,157,294; 4,921 ,594; 3,904,513 and 4,673,487, the disclosures of which are incorporated herein by reference. VI. DETAILED DESCRIPTION OF THE DRAWING Figure 1 depicts a simplified schematic flow chart of one embodiment of the process of the invention. Lube oil feedstock stream 5 and hydrogen stream 10 are passed to catalytic dewaxing unit 15, e.g., an ISODEWAXING catalytic dewaxing unit. The effluent 20 from catalytic dewaxing unit 15 is passed to hydrofinishing unit 25. The effluent 30 from hydrofinishing unit 25 is passed to atmospheric distillation column 35 for initial fractionation. Various product streams, e.g., light gases stream 36, naphtha stream 37, jet fuel stream 38, and bottoms stream 40, are removed from atmospheric distillation column 35. Bottoms stream 40 from atmospheric distillation column 35 is passed to vacuum distillation column 45 for further fractionation. Various product streams, e.g., diesel fuel stream 50, 60 Neutral Oil stream 55, 100 Neutral Oil stream 60, and bottoms stream (or 300 Neutral Oil) 65, are removed from vacuum distillation column 45. A portion of bottoms stream 65 is removed as 300 Neutral Oil stream 70 and a portion is recycled through stream 75 to mix with fresh lube oil feedstock stream 5. VII. ILLUSTRATIVE EMBODIMENTS The invention will be further clarified by the following examples, which are intended to be purely exemplary of the invention. • The benefits of fractionator bottoms recycle operation have been demonstrated in large-scale pilot-plant testing. For the demonstration run, the Isodewaxing reactor contained about 5,000 cc of a precious-metal- impregnated SAPO-11 catalyst and the hydrofinishing reactor contained about 5,000 cc of a Chevron proprietary hydrofinishing catalyst. On-line distillation produced 3 lube cuts plus a mid-distillate cut. The pilot plant was configured to simulate the flow scheme illustrated in Figure 1. • Two broad-boiling hydrocracked feedstocks were tested. Inspections for these feedstocks are shown in Table 6. Following Isodewaxing and hydrofinishing, the whole liquid product was fractionated into 3 finished base oil cuts - a 60 Neutral Oil, a 100 Neutral Oil and a 300 Neutral Oil. Table 7 summarizes the performance improvements obtained by recycling a portion of the fractionator bottoms. • For Tests 1 through 4, the fresh feed rate was maintained approximately constant, while the percent of fractionator bottoms recycle and the Isodewaxer weighted average bed temperature (WAT) were varied. The hydrofinisher was operated at approximately constant temperature during these runs. A. Comparing Test 1 and Test 2 shows that when recycling a large proportion of the fractionator bottoms, it is possible to reduce the Isodewaxer WAT and, although, the dewaxing severity is not quite the same in both cases, dramatically increase the total lube yield - from 70% to 80%. Recycling also moved the pour points of the 100N and the 300N much closer together - the difference between the pour points is 18°C in Test 2, but only 9°C in Test 1. This means that with recycling, the 100N does not have to be overdewaxed as much to make an acceptable pour point on the 300N. B. Test 3 was run to the same 300N pour point as Test 2. With recycling (Test 3), we were able to increase the overall lube yield by 3%, and increase the yield of the low-pour point, high-value, 100N by 4%. Here again, with recycling, the degree of overdewaxing of the 100N is reduced. C. Test 4 was run at the same Isodewaxer Weighted Average Bed Temperature as Test 2, but in Test 4, the Isodewaxer feed contained 13 fractionator bottoms (recycle). Although the total lube yield is the same, in Test 4, there is 2% less 300N and 2% more 60N. More importantly, in Test 4, the pour points of the finished lube fractions are substantially lower. Thus, recycling can improve the product properties of the finished lubes without changing the overall yield. • For Tests 5 through 7, the fresh feed rate, as well as the percent of fractionator were varied. Here again, the hydrofinisher was operated at approximately constant temperature. A. For Test 6, the fresh feed rate was increased by 23% without any recycling. To maintain the same approximate pour point on the 300N, the Isodewaxer WAT had to be increased by 5°F, but in this case (without recycling), the total lube yield remained the same, while the pour points of the lube fractions increased slightly (got worse). B. For Test 7, the fresh feed rate was essentially maintained and the Isodewaxer feed contained 14% fractionator bottoms. In this case, we were able to lower the Isodewaxer catalyst temperature slightly, while maintaining close to the same product pour points, and we see the total lube yield increased by 2%. Perhaps more importantly, the 100N yield increased by 4% while the 100N pour point dropped slightly. The above comparative examples show the unexpected improved performance when recycling a portion of the fractionator bottoms. Because increasing the amount of bottoms recycle will eventually limit the amount of fresh feed that can be processed, economic limits will usually dictate the maximum amount of recycle. Table 6 FEEDSTOCKS FOR PILOT TESTING Table 7 Pilot Plant Summary Corrected Lube Yields, % Test No. Feed Total % Recycle IDW 100 N 300N 60N 100N 300N Total Feed in Reactor W.A.T., Pour Pour LHSV Feed °F Point, °C Point, °C 1 Feed A 1.39 37 683 -18 -9 27 39 14 80 2 Feed A 1.03 0 700 -31 -13 24 31 15 70 3 Feed A 1.18 15 696 -28 -13 24 35 14 73 4 Feed A 1.17 13 700 -33 -18 26 31 13 70 5 Feed B 1.00 0 694 -27 -11 17 39 25 81 6 Feed B 1.23 0 699 -25 -10 18 40 23 81 7 Feed B 1.11 14 689 -30 -9 19 44 20 83 With both feeds, there is a Yield and/or Pour Point Advantage to Recycling.

Claims: VIII. CLAIMS WHAT IS CLAIMED IS: 1. A process for converting a hydrocarbon oil comprising: (a) contacting a hydrocarbon oil feedstock in the presence of added hydrogen gas with a catalyst system comprising an intermediate pore size silicoaluminophosphate molecular sieve and a hydrogenation component, wherein at least a portion of said feedstock is converted; (b) passing at least a portion of said converted feedstock to a fractionator, wherein at least a portion of said converted feedstock is fractionated, thereby producing at least one overhead fraction and one bottoms fraction; and (c) mixing at least a portion of said bottoms fraction with said hydrocarbon oil feedstock in step (a). 2. The process of claim 1 , wherein said catalyst is selected from the group consisting of SAPO-11 , SAPO-31 or SAPO-41. 3. The process of claim 1 , wherein said catalyst further comprises a catalyst selected from the group consisting of an intermediate pore size aluminosilicate zeolite catalyst, an amorphous catalyst, and mixtures thereof. 4. The process of claim 2 wherein said silicoaluminophosphate sieve comprises SAPO-11 and said hydrogenation component comprises platinum. 5. The process of claim 4 wherein said catalyst system consists essentially of a SAPO-11. 6. The process of claim 1 wherein said hydrogenation component is present in an amount of from about 0.01 % to about 10% based on the weight of said molecular sieve. 7. The process of claim 2 wherein the catalyst system further comprises an intermediate pore size aluminosilicate zeolite catalyst and is predominantly in the hydrogen form. 8. The process of claim 7 wherein said catalyst further comprises a hydrogenation component. 9. The process of claim 8 wherein said hydrogenation component comprises a Group VIII metal. 10. The process of claim 9 wherein said hydrogenation component is selected from platinum, palladium, and mixtures thereof. 11. The process of claim 7 wherein said intermediate pore size aluminosilicate zeolite has a Constraint Index measured at from about 400°C to about 454°C of from about 4 to about 12. 12. The process of claim 7 wherein said intermediate pore size aluminosilicate zeolite is selected from the group consisting of ZSM-5, ZSM-11 , ZSM-12, ZSM-22, ZSM-23, ZSM-35, SSZ-48, and mixtures thereof. 13. The process of claim 7 wherein said intermediate pore size aluminosilicate zeolite catalyst further comprises a metal selected from Group VIII metals consisting of platinum, palladium, and nickel, and mixtures thereof, or Group VIB metals consisting of molybdenum, chromium, tungston, and mixtures thereof. 14. The process of claim 7 wherein the weight ratio of said intermediate pore size silicoaluminophosphate molecular sieve to said intermediate pore size silicoaluminophosphate zeolite molecular sieve is from about 1 :5 to about 20:1. 15. The process of claim 1 wherein said process is a dewaxing process and wherein said contacting is under dewaxing conditions. 16. The process of claim 15 wherein said contacting is carried out at a temperature of from about 200°C to 475°C, a pressure of from about 15 psig to about 3000 psig, a liquid hourly space velocity of from about 0.1 hr ~1 to about 20 hr ' \ and a hydrogen circulation rate of from 500 to about 30,000 SCF/bbl. 17. The process of claim 15 wherein the amount of said fractionator bottoms mixed with said hydrocarbon oil feedstock is an effective amount to increase yield of said converted feedstock or reduce the pour point of said converted feedstock. 18. The process of claim 17 wherein from about 1 weight percent to about 80 weight percent of said fractionator bottoms is mixed with said hydrocarbon oil feedstock. 19. The process of claim 15 wherein the weight ratio of said fractionator bottoms mixed with said hydrocarbon oil feedstock to said hydrocarbon oil feedstock is an effective ratio to increase yield of said converted feedstock or reduce the pour point of said converted feedstock. 20. The process of claim 19 wherein the weight ratio of said fractionator bottoms mixed with said hydrocarbon oil feedstock to said hydrocarbon oil feedstock is from about 1 :100 to about 60:100. 21. The process of claim 1 wherein said hydrocarbon oil feedstock is a middle distillate oil. 22. The process of claim 21 wherein said feedstock is a lube oil feedstock. 23. The process of claim 22 wherein said hydrocarbon oil feedstock contains less than 50 ppmw organic nitrogen. 24. The process of claim 23 wherein said hydrocarbon oil feedstock contains less than 10 ppmw organic nitrogen. 25. The process of claim 1 wherein said hydrocarbon oil feedstock is waxy bright stock. 26. The process of claim 1 wherein said hydrocarbon oil feedstock comprises a lube oil range raffinate and wherein the process is a process for hydrodewaxing said raffinate comprising contacting said raffinate in the presence of added hydrogen under hydrodewaxing conditions with the catalyst system. 27. The process of claim 1 wherein said hydrocarbon oil feedstock comprises a waxy hydrocarbon feed and wherein the process is a process for improving the viscosity index, relative to conventional solvent dewaxing, of a dewaxed product of said waxy hydrocarbon feed comprising contacting the catalyst with said waxy hydrocarbon feed under isomerization dewaxing conditions. 28. The process of claim 1 wherein at least a major portion of said hydrocarbon oil feedstock boils above about 350°F and contains straight chain and slightly branched chain hydrocarbon and wherein the process is a process for catalytically dewaxing said hydrocarbon oil feedstock boiling above about 350°F and containing straight chain and slightly branched chain hydrocarbons comprising contacting said hydrocarbon oil feedstock in the presence of added hydrogen gas at a hydrogen pressure of about 15-3000 psi under dewaxing conditions with the catalyst system. 29. The process of claim 1 wherein the process is a process for preparing a lubricating oil: (a) wherein said hydrocarbon oil feedstock is the effluent of hydrocracking in a hydrocracking zone a hydrocarbonaceous feedstock to obtain an effluent comprising a hydrocracked oil; and (b) wherein said contacting step comprises catalytically dewaxing said effluent at a temperature of at least about 200°C and at a pressure of from about 15 psig to about 3000 psig in the presence of added hydrogen gas with the catalyst. 30. A process for dewaxing a hydrocarbon oil comprising: (a) contacting, under dewaxing conditions, a lube oil feedstock in the presence of added hydrogen gas with a catalyst system comprising an intermediate pore size silicoaluminophosphate molecular sieve and a hydrogenation component, wherein said hydrogenation component is present in an amount of from about 0.01 % to about 10% based on the weight of said silicoaluminophosphate molecular sieve, and comprising a catalyst selected from the group consisting of a zeolite, an amorphous catalyst, and mixtures thereof, wherein at least a portion of said feedstock is dewaxed; (b) passing at least a portion of said dewaxed feedstock to a fractionator, wherein at least a portion of said dewaxed feedstock is fractionated, thereby producing at least one overhead fraction and one bottoms fraction; and (c) mixing an effective amount to increase yield or reduce the pour point of said dewaxed feedstock of said bottoms fraction with said hydrocarbon oil feedstock in step (a). 31. The process of claim 30 wherein said silicoaluminophosphate molecular sieve comprises SAPO-11 and said hydrogenation component comprises platinum. 32. The process of claim 31 wherein said silicoaluminophosphate molecular sieve consists essentially of a SAPO-11. 33. The process of claim 30 wherein said aluminosilicate zeolite catalyst further comprises a Group VIII metal hydrogenation component. 34. The process of claim 33 wherein said intermediate pore size aluminosilicate zeolite is selected from the group consisting of of ZSM-5, ZSM-11 , ZSM-12, ZSM-22, ZSM-23, ZSM-35, SSZ-48, and mixtures thereof. 35. The process of claim 30 wherein the weight ratio of said intermediate pore size silicoaluminophosphate molecular sieve to said intermediate pore size silicoaluminophosphate zeolite molecular sieve is from about 1 :5 to about 20:1. 36. The process of claim 30 wherein said process is a dewaxing process and wherein said contacting is under dewaxing conditions. 37. The process of claim 36 wherein said contacting is carried out at a temperature of from about 200°C to 475°C, a pressure of from about 15 psig to about 3000 psig, a liquid hourly space velocity of from about 0.1 hr 1 to about 20 hr " \ and a hydrogen circulation rate of from 500 to about 30,000 SCF/bbl. 38. The process of claim 30 wherein from about 1 weight percent to about 80 weight percent of said fractionator bottoms is mixed with said hydrocarbon oil feedstock. 39. The process of claim 38 wherein the weight ratio of said fractionator bottoms mixed with said hydrocarbon oil feedstock to said hydrocarbon oil feedstock is from about 1 :100 to about 60:100. 40. The process of claim 30 wherein said hydrocarbon oil feedstock is a middle distillate oil. 41. The process of claim 30 wherein said hydrocarbon oil feedstock contains less than 50 ppmw organic nitrogen. 42. A process for dewaxing a hydrocarbon oil comprising: (a) contacting, at a temperature of from about 200°C to 475°C, a pressure of from about 15 psig to about 3000 psig, a liquid hourly space velocity of from about 0.1 hr 1 to about 20 hr 1 , and a hydrogen circulation rate of from 500 to about 30,000 SCF/bbl., a lube oil feedstock containing less than 50 ppmw organic nitrogen, in the presence of added hydrogen gas, with catalysts consisting essentially of a SAPO-11 intermediate pore size silicoaluminophosphate molecular sieve and a hydrogenation component, wherein said hydrogenation component is present in an amount of from about 0.01 % to about 10% based on the weight of said silicoaluminophosphate molecular sieve, and a SSZ-32 zeolitic catalyst containing a Group VIII metal hydrogenation component, wherein the weight ratio of said SAPO-11 to said SSZ- 32 is from about 1 :5 to about 20:1 , wherein at least a portion of said feedstock is dewaxed; (b) passing at least a portion of said dewaxed feedstock to a fractionator, wherein at least a portion of said dewaxed feedstock is fractionated, thereby producing at least one overhead fraction and one bottoms fraction; and (c) mixing from about 1 weight percent to about 80 weight percent of said dewaxed feedstock of said bottoms fraction with said hydrocarbon oil feedstock in step (a).

Inventors: HOWELL ROBERT L, ROSENBAUM JOHN M, HOWELL, ROBERT, L., ROSENBAUM, JOHN, M.
Applicants: CHEVRON USA INC, CHEVRON U.S.A. INC.
Application year: 1998
Main classification: C10G45
IPC codes: C10G45, C10G47