Method for Non-Oxidative Hydrogen Reactivation of Zeolite Dewaxing Catalysts

FIG. 1 depicts a schematic of the method of the present invention.

FIG. 2 depicts the effect of metal function on carbonaceous deposition of a ZSM-5 catalyst used in lube dewaxing.

FIG. 3 depicts the effect of deoiling and hydrogen reactivation on NiZSM-5 lube dewaxing catalyst.

FIG. 4 depicts the effect of deoiling and hydrogen reactivation on PdZSM-5 lube dewaxing catalyst.

The present invention relates to a process employing a catalyst comprising noble metal, such as platinum or palladium, preferably palladium, and a zeolite having a constraint index ranging from about 1 to 12, and silica-to-alumina mole ratio of at least about 12. The process includes the steps of catalytically dewaxing a hydrocarbon feedstock and non-oxidatively regenerating the catalyst by hydrogen reactivation, to provide a catalyst of lower carbonaceous residue content and enhanced activity.

The reactivation procedure of the present invention utilizes hydrogen, with any remainder being an inert gas, such as nitrogen, carbon monoxide or carbon dioxide, from fuel processing streams located in a typical refinery. Regeneration of the coked catalyst material is achieved by contacting it with hydrogen under suitable reactivation conditions. These include temperatures ranging from about 315.degree. to 538.degree. C. (600.degree. to 1000.degree. F.), preferably about 371.degree. to 454.degree. C. (700.degree. to 850.degree. F.), and contact times ranging from about 1 to 48 hours, preferably about 5 to 24 hours, such that the catalyst coke content is reduced to less than 10, 5, 2 or even 1 weight percent of the catalyst. The hydrogen treatment step is carried out at pressures ranging from about 791 to 6996 kPa (100 to 1000 psig), preferably about 2170 to 3548 kPa (300 to 500 psig). The hydrogen feed rate may range from about 0.2 to 400 standard liters per hour per gram of catalyst, preferably about 2 to 50 standard liters per hour per gram of catalyst. Hydrogen may be taken from any hydrogen stream in a refinery, such as recycle streams or streams from a steam reformer. Typically, the refinery streams will have a hydrogen concentration of greater than 50 volume percent, with the remainder being inert gas. Preferably, the reactivating agent forms a stream containing greater than 70 volume percent hydrogen (H.sub.2), with the remainder being inert gas.

The noble metal-containing catalyst, which can be regenerated by the regeneration process described above, is preferably incorporated into a cyclic dewaxing, regeneration process, as shown by FIG. 1. The dewaxing conditions may include a temperature from about 232.degree. to 454.degree. C. (450.degree. to 850.degree. F.), a pressure of about 791 to 6996 kPa (100 to 1000 psig), and a space velocity from about 0.1 to 20 LHSV. The hydrogen feed rate generally ranges from about 3.times.10.sup.5 to 3.times.10.sup.6 liters per liter (1,000 to 10,000 scfb). For lube dewaxing, conditions may include a temperature ranging from about 249.degree. to 399.degree. C. (480.degree. to 700.degree. F.), a pressure ranging from about 791 to 6996 kPa (100 to 1000 psig), a space velocity ranging from about 0.2 to 5 LHSV, and a hydrogen feed rate ranging from about 3.times.10.sup.5 to 3.times.10.sup.6 liters per liter (1,000 to 10,000 scfb). Lube dewaxing conditions are described in greater detail in U.S. Pat. No. 4,181,598. For distillate dewaxing, conditions may include temperatures ranging from about 288.degree. to 468.degree. C. (550.degree. to 875.degree. F.), pressures of about 791 to 6996 kPa (100 to 1000 psig), a space velocity of about 0.1 to 20 LHSV and a hydrogen feed rate ranging from about 3.times.10.sup.5 to 3.times.10.sup.6 liters per liter (1000 to 10,000 scfb). Preferably, said temperatures range from about 315.degree. to 427.degree. C. (600.degree. to 800.degree. F.). Said pressures range from about 1477 to 4229 kPa (200 to 600 psig), said space velocity ranges from about 0.25 to 4.0 LHSV and said hydrogen feed rate ranges from about 3.times.10.sup.5 to 3.times.10.sup.6 liters per liter (1,000 to 10,000 scfb) U.S. Pat. No. 4,419,220, incorporated herein by reference gives further details of distillate dewaxing.

The process of the present invention is particularly suited to dewaxing hydrocarbon feedstocks containing straight chain paraffins to form an effluent stream containing greater than about 80 volume percent of product having a boiling point at atmospheric pressure greater than about 200.degree. C., preferably above about 230.degree. C. or even 315.degree. C. Such product is particularly useful as lube base stock.

Prior to dewaxing, a preliminary hydrotreating step to remove nitrogen and sulfur and to saturate aromatics to naphthenes without substantial boiling range conversion will usually improve catalyst performance and permit lower temperatures, higher space velocities, lower pressures, or combinations of these conditions, to be employed. However, care should be taken to remove the lighter products including hydrogen sulfide and ammonia prior to dewaxing in order to avoid poisoning of the dewaxing catalyst.

With reference to FIG. 1, the dewaxing and regeneration process would include catalytically dewaxing a first stream of hydrocarbon feedstock 1 comprising waxy components in a first catalyst dewaxing step 2, by contact with a stream of hydrogen 2a in the presence of the catalyst under the dewaxing conditions described above to produce a first dewaxed effluent stream 3 having a desired pour point. Preferably, the pour point is less than about 0.degree. C. for lubes and about -17.degree. C. for distillates. The dewaxing may continue until the catalyst is substantially deactivated. Deactivation is indicated when the catalyst cannot produce an effluent having the desired pour point at the above-described conditions.

The deactivated catalyst is then non-oxidatively regenerated by catalyst reduction step 4, in which a hydrogen-containing stream 5 contacts the catalyst and thereafter emerges as effluent stream 6, at suitable conditions as described above.

The regenerated catalyst may then be employed for catalytically dewaxing a second stream of the waxy hydrocarbon feedstock 7 in a second catalytic dewaxing step 8, by contact with the regenerated catalyst under the dewaxing conditions of step 2. Hydrogen may be provided by a hydrogen stream 9 which thereafter emerges as effluent 10, while the dewaxed product exits the step as product effluent 11.

The dewaxing catalyst used in the method of the present invention may be used to dewax a variety of feedstocks ranging from relatively light distillate fractions up to high boiling stocks, such as whole crude petroleum, reduced crudes, vacuum tower residua, cycle oils, FCC tower bottoms, gas oils, vacuum gas oils, deasphalted residua and other heavy oils. The feedstock will normally be a C.sub.10.sup.+ feedstock, since lighter oils will usually be free of significant quantities of waxy components. The process is particularly useful with waxy distillate stocks, such as gas oils, kerosenes, jet fuels, lubricating oil stocks, heating oils and other distillate fractions whose pour point, freeze point, and cloud point need to be reduced to certain specification levels. Lubricating oil stocks will generally boil above 230.degree. C. (450.degree. F.), more usually above 315.degree. C. (600.degree. F.). Hydrotreated stocks are a convenient source of these materials as well as other distillate fractions since they normally contain significant amounts of waxy n-paraffins which have been produced by the removal of polycyclic aromatics.

The catalyst used in the process comprises a crystalline zeolite material which may be shape-selective. Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials include ordered, porous crystalline metallosilicates or metallogermanates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of small cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.

Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline materials such as aluminosilicates. These materials can be described as a rigid three-dimensional framework of XO.sub.4 and YO.sub.4 wherein X is silicon and/or germanium, and Y is one or more of aluminum, gallium, iron, chromium, vanadium, molybdenum, arsenic, manganese, or boron. This framework is comprised of tetrahedra which are cross-linked by the sharing of oxygen atoms whereby the ratio of the total Y and X atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing Y, e.g. aluminum, is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of Y to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given zeolite by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.

The crystalline zeolites utilized by the method of the present invention are members of a special class of zeolitic materials which exhibit unusual properties. Although these zeolites have unusually low Y atom contents, i.e., high X to Y mole ratios, e.g., high silica-to-alumina molar ratios, they are very active even when the X to Y mole ratio exceeds 30. The activity is surprising, since catalytic activity is generally attributed to framework Y atoms and/or cations associated with these atoms. These zeolites retain their crystallinity for long periods in spite of the presence of steam at high temperatures which induces irreversible collapse of the framework of other zeolites, e.g., of the X and A type.

An important characteristic of the crystal structure of these zeolites is that they provide constrained access to, and egress from, the intracrystalline free space by virtue of having a pore dimension greater than about 5 Angstroms and pore windows of about a size such as would be provided by 10 membered rings of oxygen atoms. It is to be understood, of course, that these rings are those formed by the regular disposition of the tetrahedra making up the anionic framework of the crystalline silicate, the oxygen atoms themselves being bonded to the silicon or aluminum atoms at the centers of the tetrahedra. Briefly, the preferred zeolites useful in this invention possess, in combination: an XO.sub.4 to YO.sub.4 mole ratio, e.g., a silica to alumina mole ratio, of at least about 12; and a structure providing constrained access to the crystalline free space.

The zeolites useful in this invention freely sorb normal hexane and preferably have a pore dimension greater than about 5 Angstroms. In addition, their structure must provide constrained access to some larger molecules. It is sometimes possible to judge from a known crystal structure whether such constrained access exists. For example, if the only pore windows in a crystal are formed by 8-membered rings, then access by molecules of larger cross-section than normal hexane is substantially excluded and the zeolite is not of the desired type. Zeolites with windows of 10-membered rings are preferred, although excessive puckering or pore blockage may render these zeolites substantially ineffective.

Rather than attempt to judge from crystal structure whether or not a zeolite possesses the necessary constrained access, a simple determination of the "constraint index" may be made by continuously passing a mixture of equal weight of normal hexane and 3-methylpentane over a small sample, approximately 1 gram or less, of zeolite at atmospheric pressure according to the following procedure. A sample of the zeolite, in the form of pellets or extrudate, is crushed to a particle size about that of coarse sand and mounted in a glass tube. Prior to testing, the zeolite is treated with a stream of air at 540.degree. C. (1000.degree. F.) for at least 15 minutes. The zeolite is then flushed with helium and the temperature adjusted between 290.degree. C. and 510.degree. C. (550.degree. F. and 950.degree. F.) to give an overall conversion between 10 and 60 percent. The mixture of hydrocarbons is passed at 1 liquid hourly space velocity (i.e., 1 volume of liquid hydrocarbon per volume of catalyst per hour) over the zeolite with a helium dilution to give a helium to total hydrocarbon mole ratio of 4:1. After 20 minutes on stream, a sample of the effluent is taken and analyzed, most conveniently by gas chromatography, to determine the fraction remaining unchanged for each of the two hydrocarbons.

The "constraint index" is calculated as follows: ##EQU1##

The constraint index approximates the ratio of the cracking rate constants for the two hydrocarbons. Catalysts suitable for the present invention are those which employ a zeolite having a constraint index from about 1.0 to 12.0.

Crystalline zeolite material having the structure of zeolite beta, ZSM-5, ZSM-5/ZSM-11 intermediate, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38, and ZSM-48 are examples of crystalline silicates for use herein, and are more particularly described in U.S. Pat. Nos. 3,308,069 (zeolite beta), 3,702,886 (ZSM-5), 4,229,424 (ZSM-5/ZSM-11), 3,709,979 (ZSM-11), 3,832,449 (ZSM-12), 4,076,842 (ZSM-23), 4,016,245 (ZSM-11), 4,046,859 (ZSM-38), and 4,375,573 (ZSM-48) respectively, the entire contents of each being incorporated herein by reference.

The zeolitic dewaxing catalyst comprises a hydrogenation-dehydrogenation component. The hydrogenation/dehydrogenation component is a noble metal such as platinum or palladium, preferably palladium.

The metal may be incorporated into the catalyst by any suitable method, such as impregnation or exchange onto the zeolite resulting in intimate association of noble metal with the zeolite. Alternatively, the catalyst may be prepared by physically mixing the metal salt with the zeolite and an inorganic matrix component. The noble metal need not be directly associated with the zeolite component of the catalyst mixture. The metal may be incorporated into the catalyst in the form of a cationic, anionic or neutral complex, such as Pd(NH.sub.3).sub.4.sup.2+.

The amount of the noble metal component is less than about 2 weight percent, preferably about 0.1 to 0.9 weight percent, e.g., about 0.3 to 0.6 weight percent of the total catalyst.

It may be desirable to incorporate within the catalyst a non-zeolitic inorganic oxide binder material resistant to the temperature and other conditions employed in the process. Such matrix materials include synthetic or natural substances as well as inorganic materials, such as clay, silica and/or metal oxides. The latter may be either naturally occurring or in the form of gelatinous precipitates. Naturally occurring clays which can be composited with the catalyst include those of the montmorillonite and kaolin families. These clays can be used in the raw state as originally mined or initially subjected to calcination, acid treatment or chemical modification.

The catalyst may be composited with a porous matrix material, such as alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania, as well as ternary compositions, such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia, and silica-magnesia-zirconia. The matrix may be in the form of a cogel with the zeolite. The relative proportions of zeolite component and inorganic oxide gel matrix may vary widely, with the zeolite content ranging from between 1 to 99 weight percent, more usually 5 to 80 weight percent of the composite, say about 70 weight percent zeolite and 30 weight percent alumina. The matrix may itself possess catalytic properties, generally of an acidic nature.

This invention will be better understood by reference to the following examples.

HZSM-5, NiZSM-5 and PdZSM-5 dewaxing catalysts having the characteristics set out in Table 1 were employed in dewaxing. The HZSM-5 and NiZSM-5 catalysts were steamed to reduce their alpha activity to about 70 as measured by the alpha test set out in U.S. Pat. No. 4,326,994, incorporated herein by reference. All three catalysts were employed in dewaxing heavy neutral or bright stock feeds for at least 30 days.

The deactivated catalysts of Examples 1 to 3 were each hydrogen reactivated at 482.degree. C. (900.degree. F.) for 24 hours in 4.7 liters of hydrogen per gram of catalyst per hour (75 scf H.sub.2 per pound of catalyst per hour), once through hydrogen at 2860 kPa (400 psig). Table 2 compares the residual organic contents and elemental compositions of organic material remaining on the various lube dewaxing catalysts. The amount of organic material remaining on the PdZSM-5 catalyst of Example 3 was considerably less than either the conventional NiZSM-5 catalyst of Example 2 or the HZSM-5 catalyst of Example 1.

The Pd catalyst was prepared by physically mixing 0.37 wt % Pd as an aqueous Pd(NH.sub.3).sub.4 Cl.sub.2 solution with 65% ZSM-5 and 35% Al.sub.2 O.sub.3, extruding to 0.16 cm diameter by 0.63 cm long extrudates, drying at 110.degree. C., and then calcining in nitrogen to decompose the organic directing agent, exchanging with an ammonium salt (NH.sub.4 NO.sub.3), and then calcining in air to 538.degree. C. The Pt containing catalyst was prepared identically except with Pt(NH.sub.3).sub.4 Cl.sub.2 as the metal salt. The Ni catalyst was prepared by extruding the 65% ZSM-5/35% Al.sub.2 O.sub.3 mixture to 0.16 cm by 0.63 cm extrudates, drying at 100.degree. C., calcining in nitrogen to 538.degree. C., exchanging with the ammonium salt, rinsing with deionized water, exchanging with 0.5 M Ni(NO.sub.3).sub.2 for 4 hours at 82.degree. C. and calcining in air to 538.degree. C. for three hours. The Ni catalyst used in this example was unsteamed (alpha=170).

The coked catalysts of Examples 7 to 9 were deoiled at 482.degree. C. (900.degree. F.) in helium and then analyzed for coke (ignited basis) by thermogravimetric analysis (TGA). As shown in Table 3 the amount of neutral stock desorbed from the catalyst during heat-up in helium is 34 to 37 weight percent. Approximately the same amount of coke, i.e., material remaining immediately after the catalyst has been heated to 482.degree. C. (900.degree. F.), is deposited on the platinum and palladium catalysts, 7.2 and 7.5 weight percent, respectively, compared to the 10.3 weight percent deposited on the Ni catalyst. Thus, there is approximately 30% less coke make on the noble metal containing catalysts.

FIG. 2 shows that there is a significant difference in the temperature for maximum rate of coke removal in air. A temperature of 540.degree. C. (1000.degree. F.) is required for maximum rate of coke removal of the helium treated NiZSM-5 catalysts compared to 500.degree. C. (932.degree. F.) for PtZSM-5 and 460.degree. C. (860.degree. F.) for PdZSM-5. This would indicate that the carbonaceous material formed on noble metal catalysts, especially palladium catalysts, is easier to remove by oxygen regeneration as well as hydrogen reactivation. While not wishing to be bound by theory, it is believed that the noble metal not only catalyzes the removal of carbonaceous residues formed but inhibits the formation of highly refractory carbonaceous residues as well.

Fresh samples of the used catalysts were treated in a thermogravimetric analyser (TGA) with hydrogen for 2 hours at 480.degree. C. (900.degree. F.) and 101.3kPa (1 atmosphere). Comparison of the He and H.sub.2 reactivation data in Table 3 indicates that an additional 50 weight percent of the coke from all three samples was removed during hydrogen reactivation. Platinum and palladium catalysts retained 3.9 and 3.5 weight percent coke respectively after hydrogen reactivation while the Ni catalyst retained 5.4 weight percent coke. As before, the temperature of maximum rate of coke removal for PdZSM-5 is about 100.degree. C (210.degree. F) lower than that of NiZSM-5. However, as shown in FIG. 3, the temperature maximum for coke removal from NiZSM-5 is shifted about 40.degree. C. (72.degree. F.) higher than that observed during the helium reactivation. This suggests that only the hardest coke remained after H.sub.2 reactivation. As shown in FIG. 4, the temperature for maximum rate of coke removal remained about the same after the H.sub.2 and helium reactivation indicating that hydrogen reactivation of the Pd catalyst produced a "softer" coke than processed by hydrogen reactivation of the Ni catalyst. Analysis of the remaining residue as shown in Table 2 substantiates this observation, since the H/C ratio of the coke after lube dewaxing over PdZSM-5 is higher than for NiZSM-5.

The examples show that noble metal catalysts exhibit advantages over conventional nickel or non-metal dewaxing catalysts for hydrogen reactivation in that: (1) less coke remains on the Pd and Pt catalysts after reactivation at equivalent severities, (2) lower temperatures are required to remove the coke from these catalysts and (3) the coke that does remain on the Pt and Pd catalysts is a "softer" coke that should be easier to remove by going to higher temperatures.