WO1995011952A1

WO1995011952A1 - Catalyst and process for producing low-aromatics distillates

Info

Publication number: WO1995011952A1
Application number: PCT/US1994/009485
Authority: WO
Inventors: Minas Robert Apelian; Thomas Francis Degnan, Jr.; Chang-Kuei Lee; Stuart Shan-San Shih
Original assignee: Mobil Oil Corporation
Priority date: 1993-10-26
Filing date: 1994-08-24
Publication date: 1995-05-04
Also published as: AU7953494A

Abstract

The invention provides a process for decreasing the aromatics content of a hydrodesulfurized feedstock containing up to 80 weight percent aromatics in the presence of a catalyst comprising a Group VIIIA metal and ultrastable zeolite Y having a unit cell size of form 2.410 to 2.429 nm to evolve product containing less than 20 weight percent aromatics at a 199 ºC+ conversion of less than 30 weight percent.

Description

CATALYST AND PROCESS FOR PRODUCING LOW-AROMATICS

DISTILLATES

The invention relates to a catalyst and process for decreasing the aromatics content of a desulfurized petroleum distillate without substantially altering the boiling range. The catalyst for this process comprises a VIIIA metal, ultrastable Y, and, preferably, a binder. Under present conditions, petroleum refineries are finding it necessary to convert increasingly greater proportions of crude to premium fuels including gasoline and middle distillates such as diesel and jet fuel. Catalytic cracking processes, exemplified by the fluid catalytic cracking (FCC) process and Ther ofor catalytic cracking (TCC) process together, account for a substantial fraction of heavy liquids conversion in modern refineries. Both are thermally severe processes which result in a rejection of carbon to coke and to residual fractions; during catalytic cracking high molecular weight liquids disproportionate into relatively hydrogen-rich light liquids and aromatic, hydrogen-deficient heavier distillates and residues.

Catalytic cracking therefore produces significant quantities of highly aromatic, light and middle distillates which not only have high sulfur and nitrogen levels, but which may contain as much as 80 wt.% or more of aromatics. Generally, the level of heteroatom contaminants increases with the boiling point of the fraction. For example, the light cycle oil produced as a typical FCC main column bottoms stream contains about 80% aromatics, 4.6% sulfur compounds, 1500 ppm nitrogen compounds, and 9.1% hydrogen (in proportions and percentages by weight, as in the remainder of this specification unless otherwise defined) . Present market requirements make highly aromatic product streams such as these particularly difficult to dispose of as commercially valuable products. Formerly, the light and heavy cycle oils could be upgraded and sold as light or heavy fuel oil, such as No. 2 fuel oil or No. 6 fuel oil. Upgrading the light cycle oil was conventionally carried out by a relatively low severity, low pressure catalytic hydrodesulfurization (CHD) unit in which the cycle stock would be admixed with virgin middle distillates from the same crude blend fed to the catalytic cracker. At many petroleum refineries, the light cycle oil (LCO) from the FCC unit is a significant component of the feed to the catalytic hydrodesulfurization (CHD) unit which produces No. 2 fuel oil or diesel fuel. The remaining component is generally virgin kerosene taken directly from the crude distillation unit. The highly aromatic nature of LCO, particularly when the FCC unit is operated in the maximum gasoline mode, increases operational difficulties for the CHD and can result in a product having marginal properties for No. 2 fuel oil or diesel oil, as measured by cetane numbers and sulfur content. Further, increasingly stringent environmental regulations limiting the aromatics content of diesel fuel have prompted refiners to focus research efforts on economical methods for producing the required low-aromatics fuels.

An alternative market for middle distillate streams is automotive diesel fuel. However, diesel fuel has to meet a minimum cetane number specification of about 45 in order to operate properly in typical automotive diesel engines. Because cetane number correlates closely and inversely with aromatic content, the highly aromatic cycle oils from the cracker typically with aromatic contents of 80% or even higher have cetane numbers as low as 4 or 5. In order to raise the cetane number of these cycle stocks to a satisfactory level by the conventional CHD technology described above, substantial and uneconomic quantities of hydrogen and high pressure processing would be required.

Thus from an economic and operational standpoint, it would be desirable to rely upon the CHD unit for desulfurization, and to provide a more effective and less costly method for reducing aromatics content while providing a product which closely matches the boiling ranges of the feedstock.

Accordingly, this invention provides a process for decreasing the aromatics content of a hydrodesulfurized feedstock containing up to about 80 weight percent aromatics in the presence of a catalyst comprising a Group VIIIA metal and ultrastable zeolite Y having a unit cell size of from 24.10 to 24.29 Angstroms to evolve product containing less than 20 weight percent aromatics at a 199^βC+ (390°F+) conversion of less than 30 weight percent. The catalyst of the invention comprises a Group VIIIA metal on ultrastable Y having a unit cell size equal to or less than 2.429 nm (24.29 Angstroms (A)), and which ultrastable Y contains less than about 1000 ppm Na. The catalyst composition preferably includes a binder or a matrix material. The term "Group VIIIA metal" as used herein refers to Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt as shown in the Periodic Table of the Elements published as Catalog No. S-18806 by Sargent-Welch Scientific Company, 7300 North Linder Avenue, Skokie, Illinois, 60077. The Group VIIIA metal content is preferably at least about 0.1 wt% and preferably at least 0.3 wt%. The ultrastable Y unit cell size is preferably from 2.410 to 2.429 nm (24.10 to 24.29 A), more preferably from 2.415 to 2.429 nm (24.15 to 24.29 A), and most preferably from 2.420 to 2.427 nm (24.20 to 24.27 A). The process of this invention is operated under low to moderate pressure, typically 2860 - 8380 kPa (400 -1200 psig) hydrogen pressure. At the relatively low severity conditions employed temperatures will generally be in the range 204°C - 455°C (400° -

850°F), more typically 204^βC - 400°C (400° - 750"F), with space velocity adjusted to obtain the desired conversion. The operating pressure is preferably within the range of from 7000 to 8400 kPa (1000 to 1200 psig) , and is preferably maintained with makeup and/or recycled hydrogen. Pressures of at least 7000 kPa (1000 psig) are particularly preferred for decreasing the aromatics content of the feed.

The hydrotreating catalyst of this invention has an unusually low activity for boiling range conversion, thus producing a product having essentially the same boiling range as the feed. In one embodiment, the process of this invention is characterized by a 199^βC+ conversion (390°F+ conversion) of less than about 30 wt%, preferably less than about 20 wt%, and more preferably less than about 10 wt%. As used herein, the term "199°C+ conversion" ("390°F+ conversion") is defined as follows:

₁1_ft9_Λ9_O C„+ conversion = i f-(199°C+ in f ~ —eed) - - ( -199° C+ in p -roduct) -]'

(199° C+ in feed)

3_n9_r0-o F_r,+ conversion = i f-(390° F+ in f - —eed) - (390° F+ in product) -]V)

\ 390° F+ in feed )

In another embodiment, the process of this invention characterized by a 149°C+ conversion (300°F+ conversion) of less than 30 wt%, preferably less than 20 wt%, and more preferably less than 10 wt%. As used herein, the term "149^βC+ conversion" ("300^βF+ conversion") is defined as follows:

_Λ 14_AI9_O C_r,+ conversion = i f-(149° C+ in f —eed) - — - (149° C+ in product) -]*-

(149° C+ in feed)

f(300° + in feed) - (300° + in product)])

300° F+ convers iioonn == {- — - \

\ 300° F+ in feed ) The feed is preferably hydrotreated (hydrodesulfurized) upstream from the process of the present invention to reduce sulfur content in the feed to less than 5000 ppmw, preferably less than 1000 ppmw, more preferably less than 300 ppmw. In a particularly preferred embodiment, the feed is hydrodesulfurized to less than 50 ppmw sulfur.

The catalyst of the invention comprises a Group VIIIA metal and ultrastable Y having a unit cell size of less than or equal to 2.429 nm (24.29 A), and which ultrastable Y contains less than about 1000 ppm Na. The catalyst composition preferably includes a binder or a matrix material. More specifically, the amount of the Group VIIIA metal is controlled so that it is present in the catalyst composition in an amount which is at least directly proportional to the zeolitic framework aluminum contained in the ultrastable Y.

The catalyst contains at least one noble metal component. Suitable hydrogenation components include at least one metal of Group VIIIA such as nickel, cobalt, rhodium, palladium, and platinum in an amount between 0.1 and 25 wt %, typically from 0.3 to 20 wt %, and preferably from 0.3 to 5 wt %. The most preferred Group VIIIA metals include platinum and palladium. These components can be exchanged or impregnated into the composition or added via other methods well known to those skilled in the art, using suitable compounds of the metals. The compounds used for incorporating the metal component into the catalyst can usually be divided into compounds in which the metal is present in the cation of the compound or compounds in which it is present in the anion of the compound. Compounds which contain the metal as a neutral complex may also be employed. The compounds which contain the metal in the ionic state are generally used. The original cations associated with the crystalline ultrastable Y herein may be replaced by the cations, according to conventional techniques. Typical replacing cations including hydrogen, ammonium and metal cations, including mixtures of these cations. Typical ion-exchange techniques are to contact the particular zeolite with a water-soluble salt of the desired replacing cation. Although a wide variety of salts can be employed, particular preference is given to chlorides, nitrates and sulfates. Representative ion-exchange techniques are disclosed in a wide variety of patents, including U.S. Patents Nos. 3,140,249; 3,140,251; and 3,140,253.

Following contact with a solution of the desired replacing cation, the zeolite containing catalyst is then preferably washed with water and dried at a temperature ranging from 65° to 315°C (150° to 600°F) , and thereafter calcined in air, or other inert gas, at temperatures ranging from about 260° to 815°C (500° to 1500°F) for periods of time ranging from 1 to 48 hours or more.

The calcined zeolite Y may then be contacted with an aqueous solution containing rare earth chlorides. The aqueous solution may contain any mixture of rare earth elements in the Lanthanide series. These compounds may be added by ion exchange, impregnation, or by other methods well known to those skilled in the art.

In accordance with the invention, the Group VIIIA metal is present in the composition in an amount directly proportional to the framework aluminum content of the ultrastable Y. The ultrastable Y has a silica:alumina framework molar ratio exceeding 15. The catalyst should have some acidic functionality, i.e., an alpha value greater than 1 for the cracking function. The alpha value, a measure of zeolite acidic functionality, is described together with details of its measurement in U.S. Patent No. 4,016,218 and in J. Catalysis. 61, p. 395 (1980) and reference is made to these for such details. The catalyst of this invention effectively reduces sulfur and aromatics content for low sulfur feeds containing up to 5000 ppmw sulfur, but hydrodesulfurized feeds containing less than 1000 ppmw sulfur are preferred. As used herein, the terms "ppm" and "ppmw" are defined as parts per million based upon weight, unless otherwise stated.

The silica:alumina ratio of the final ultrastable zeolite Y catalyst may be varied by initial zeolite synthesis conditions, or by subsequent dealuminization as by steaming or by substitution of framework aluminum with other trivalent species such as boron, iron or gallium. The alkali metal content should be held at a low value, preferably below 1% and lower, e.g. below 0.5% Na. This can be achieved by successive sequential ammonium exchange followed by calcination. It has been found that preferred catalysts for this process require controlled metal/acid ratios. Furthermore, this can be described by the ratio of Group VIIIA metal to the zeolite framework Al content. The molar ratio of Group VIIIA metal:framework aluminum (provided by the ultrastable Y) , in the catalyst of the invention, is greater than 0.01. Preferably, the Group VIIIA metal:framework aluminum ratio (provided by the ultrastable Y) ,in the catalyst of the invention, ranges from 0.01 to 10.

Preferably, the catalyst composition includes a matrix comprising another material, other than the ultrastable Y, resistant to the temperature and other conditions employed in the process. The matrix material is useful as a binder and imparts greater resistance to the catalyst for the severe temperature, pressure and reactant feed stream velocity conditions encountered in the process. Useful matrix materials include both synthetic and naturally occurring substances, such as clay, silica, alumina, silica-alumina, zirconia and/or metal oxides. The latter may be either naturally occurring or in the form of synthetic gelatinous precipitates or gels including mixtures of silica and metal oxides such as alumina and silica-alumina. The matrix may be in the form of a cogel. Naturally occurring clays which can be composited with the zeolite include those of the montmorillonite and kaolin families. Such clays can be used in the raw state as originally mined or initially subjected to calcination, acid treatment or chemical modification. The relative proportions of zeolite component and the matrix, on an anhydrous basis, may vary widely with the zeolite content ranging from between about 1 to about 99 wt %, and more usually in the range of about 5 to about 80 wt % of the dry composite. The binder is preferably composited with the zeolite prior to treatments such as steaming, impregnation, exchange, etc., in order to preserve mechanical integrity. The process of the invention may suitably be conducted using a single catalyst in one processing stage. However, a two stage dual-catalyst system, employing both a conventional hydrocracking catalyst and the catalyst of the invention, can be employed, if it is desirable to shift the boiling range of the feed to produce a lower average boiling product. In such a two stage process, the two catalysts are in sequential beds and the process is operated in the cascade mode without interstage separation to remove ammonia and hydrogen sulfide.

Feedstock The feedstocks used in the present process are hydrocarbon fractions which are highly aromatic, which may also be hydrogen deficient, but which have preferably been hydrotreated (hydrodesulfurized) to contain no more than 5000 ppmw sulfur. The feedstocks preferably contain less than 1000 ppmw sulfur, more preferably less than 300 ppmw sulfur, and most preferably less than ppmw sulfur. These feedstocks generally comprise fractions which have been substantially dealkylated, as by a catalytic cracking operation, for example, in an FCC or TCC unit. Catalytic cracking characteristically removes alkyl groups (generally bulky, relatively large alkyl groups, typically but not exclusively C₅~C_g alkyls) , which are attached to aromatic moieties in the feed. These detached alkyl groups form the bulk of the gasoline product from the cracker. The aromatic moieties such as benzene, naphthalene, benzothiophenes, dibenzothiophenes and polynuclear aromatics (PNAs) such as anthracene and phenanthrene form the high boiling products from the cracker. The mechanisms of acid-catalyzed cracking and similar reactions remove side chains of greater than 5 carbons while leaving behind short chain alkyl groups, primarily methyl, but also ethyl groups on the aromatic moieties. Thus, the "substantially dealkylated" cracking products include those aromatics with small alkyl groups, such as methyl, and ethyl, and the like still remaining as side chains, but with relatively few large alkyl groups, i.e., the C₅-C₉ groups, remaining. More than one of these short chain alkyl groups may be present, for example, one, two or more methyl groups.

Feedstocks of this type have an aromatic content in excess of 50 wt. percent; for example, 70 wt. percent or 80 wt. percent or more, aromatics. Highly aromatic feeds of this type typically have hydrogen contents below 14 wt. percent, usually below 12.5 wt. percent or even lower, e.g. below 10 wt. percent or 9 wt. percent. The API gravity is also a measure of the aromaticity of the feed, usually being below 30 and in most cases below 25 or even lower, e.g. below 20. In most cases the API gravity will be in the range 5 to 25 with corresponding hydrogen contents from 8.5-12.5 wt. percent. The nitrogen content of the feed typically falls within the range of from 50 to 1000 ppmw, more usually from 50 to 650 ppmw. More severely hydrotreated feeds may have lower aromatic contents within the range of from 25 to 50 weight percent.

Suitable feeds for the present process are substantially dealkylated cracked product fractions which have been hydrotreated to reduce sulfur to below 1000 ppmw, preferably below 300 ppmw. Feeds of this type include mildly hydrotreated cycle oils from catalytic cracking units. Full range cycle oils may be used, for example, full range light cycle oils with a boiling range of 196° - 400°C (385° - 750°F), e.g., 205" - 370°C (400° - 700^βF) or, alternatively, cycle oil fractions may be employed such as heavy cycle oil or light cycle oil fractions. When operating with an extended boiling range feed such as a full range light cycle oil (FRLCO) , conversion should be limited so as to avoid excessive catalyst aging; a maximum conversion to a product containing 5 wt% aromatics is preferred. However, if a light cut cycle oil is used, higher conversions may be tolerated. For this reason, lower boiling range fractions of that type are preferred. Thus, cycle oils with end points below 345°C (650^βF), preferably below 315°C (600°F) are preferred. Initial boiling point will usually be 150°C (300°F) or higher, e.g. 165° (330°F) or 195^βC (385^βF).

Feeds having higher endpoints tend to age the catalyst of the invention more rapidly. For this reason, less severe operating conditions (principally LHSV and temperature) are preferred for higher endpoint feeds. In a preferred embodiment, the maximum feedstock endpoint is about 399°C (750^βF), more preferably 370°C (700^βF) . Light cycle oils generally contain from 60 to 80% aromatics and, as a result of the catalytic cracking process, are substantially dealkylated, as described above. Other examples of suitable feedstocks include the dealkylated liquid products from delayed or fluid bed coking processes. If a cycle oil fraction is to be used, it may be obtained by fractionation of a FRLCO or by adjustment of the cut points on the cracker fractionation column.

While the aromatics content of the product stream varies with process severity, the product typically contains less than 20 weight percent aromatics, preferably less than 15 weight percent aromatics, and more preferably below 10 weight percent aromatics.

This process decreases aromatics content without substantially changing the boiling range of the feedstock. In contrast, U.S. Patent 5,219,814 to Kirker et al. teaches a process using an ultrastable zeolite Y catalyst which cracks the feedstock to lighter products. The '814 Kirker et al. patent is incorporated by reference as if set forth at length herein. The invention will now be illustrated by the following Examples.

EXAMPLES Example 1

A USY zeolite with an elemental Si0₂/Al₂0₃ ratio of 1600 and framework Si0₂/Al₂0₃ ratio of 3300 (as measured by Al-NMR) with an unit cell size of 2.420 nm (24.20 Angstroms) and no detectable silanol content (as measured by Si-NMR) was exchanged with a rare earth chloride solution at pH = 5. 65 parts by weight of this RE- exchanged USY was mixed with 35 parts by weight A1₂0₃ on a dry basis. Enough water was added to form an extrudable paste. This mixture was formed into 1.59 mm (1/16") extrudates. The extrudates were dried at 121°C (250°F) and air calcined at 538"C (1000°F) for 3 hours. The calcined extrudates were humidified and then exchanged with a solution containing Pd(NH₃)₄ ⁺⁺. The Pd-containing extrudates were then calcined at 288°C (550^βF) . This material is referred to as Catalyst A and has properties summarized in Table 1.

Example 2

A USY zeolite with an elemental Si0₂/Al₂0₃ ratio of 14 and a framework Si0₂/Al₂0₃ ratio of 200 (as measured by Al- NMR) with a unit cell size of 2.430 nm (24.30 Angstroms) and no detectable silanol content (as measured by Si-NMR) was exchanged with a rare earch chloride solution at pH = 5. 65 parts by weight of this RE-exchanged USY was mixed with 35 parts by weight A1₂0₃ on a dry basis. Enough water was added to form an extrudable paste. This mixture was formed into 1.59 mm (1/16") extrudates. The extrudates were dried at 121°C (250^βF) and air calcined at 538°C (1000°F) for 3 hours. The calcined extrudates were humidified and then exchanged with a solution containing Pd(NH₃)₄ ⁺⁺. The Pd-containing extrudates were then calcined at 288°C (550°F) . This material is referred to as Catalyst B and has properties summarized in Table 1.

Example 3

The catalysts described above in Examples 1 and 2 were used to treat a low-sulfur (<10 ppmw) distillate fuel containing 45 wt% aromatics (Feed-I, Table 2). Example 3 demonstrates catalyst performance for hydrogenation of low-sulfur fuels derived from a conventional high-pressure (>7000 kPa (>1000 psig)) hydrotreating process. Each catalyst was crushed and sized to 14/24 mesh, and evaluated at 1 LHSV and 7000 kPa (1000 psig) H₂ pressure. Each catalyst was reduced in 2520 kPa (350 psig) H₂ at 177°C (350^βF) for three hours prior to introducing the feed. The results for the two catalysts are summarized in Table 3. While both catalysts reduce aromatics content to below 5 wt%. Catalyst A gives a higher yield of low-aromatics distillate while consuming less hydrogen.

Example 4

Catalyst A and Catalyst B were evaluated using a feedstock produced from a high-pressure (12515 kPa (1800 psig)) vacuum gas oil (VGO) hydrotreater. The feedstock (Feed-II, Table 2) produced from the VGO hydrotreater contained 12 ppmw sulfur, 31.8 wt% aromatics, and 62 ppmw nitrogen. Each catalyst was crushed and sized to 14/24 mesh and evaluated at 1.0 LHSV and 7000 kPa (1000 psig) H₂ pressure. Results are shown in Table 4. Catalyst A is effective in reducing aromatics to 10 wt%. Catalyst B failed to reduce aromatics content to below 10 wt%. Both catalysts have low boiling range conversion activities and produce high yields of 199°C+ (390°F+) boiling range distillates.

Example 5

Catalyst B was evaluated with a distillate feed which was obtained by desulfurizing a typical feed for a commercial catalytic hydrodesulfurization (CHD) unit under conventional conditions (4410 kPa (625 psig) ) . The desulfurized distillate contained 500 ppmw sulfur, 41.3 wt% aromatics, and 200 ppmw nitrogen.

Example 5 compared the sulfur-resistance of the catalysts, and showed that Catalyst B is effective in reducing aromatics content with a small yield loss (Table

5) . Further, Catalyst B was shown to provide a high degree of sulfur removal.

Table 1 Catalyst Properties

Table 2 Feedstocks Properties

Feed-I Feed-II Feed-Ill

Gravity, API 31.8 30.5 32.1

Sulfur, ppmw <10 12 500

Nitrogen, ppmw 2 62 200

Aromatics 45 31.8 41.3 (ASTM-M1539)

Distillation (ASTM-D2887)

Weight Percent Temperature, °C (°F)

5 172 (341) 204 (399) 184 (363)

30 253 (488) 263 (506) 237 (458)

50 283 (541) 273 (523) 267 (513)

70 311 (592) 290 (555) 298 (569)

95 369 (697) 320 (609) 356 (672)

Table 3 Summary of Example 3

Yield and Conversion based on 199"C (390°F) cut point Catalyst A: Pd/(3300/1) USY/A1₂0₃ Catalyst B: Pd/(200/1) USY/A1₂0₃

Table 4 Summary of Example 4

Yield and Conversion based on 199°C (390°F) cut point Catalyst A: Pd/(3300/1) USY/A1₂0₃ Catalyst B: Pd/(200/1) USY/A1₂0₃

Table 5 Summary of Example 5

Yield and Conversion based on 199°C (390°F) cut point Catalyst B: Pd/(200/1) USY/A1₂0₃

Feed Product from reaction with

Catalvst B

LHSV - 1.0

Temperature, °C - 343 (650) (^•F)

Pressure, kPa - 4600 (650) (psig)

Conversion, wt.% - 15.5

Yield, wt.% 91.0 76.9

H₂ Consumption, 33354 (1853) v/v feed (SCF/B (Standard Cubic Feet per Barrel of Feed) )

Sulfur, ppmw 500 <10

Desulfurization, % - 96

Aromatics, wt.% 41.3 8.3

Claims

CLAIMS ;

1. A process for decreasing the aromatics content of a hydrodesulfurized feedstock containing up to 80 weight percent aromatics comprising contacting the hydrodesulfurized feedstock in the presence of hydrogen with a catalyst comprising ultrastable zeolite Y, which catalyst has been calcined at a temperature of from 260"C to 288^βC after exchange with a solution containg a Group VIIIA metal cation, the ultrastable zeolite Y having a unit cell size of from about 2.410 to 2.429 nm to evolve product containing less than 20 weight percent aromatics at a 199°C+ conversion of less than 30 weight percent.

2. The process of claim 1 wherein the ultrastable zeolite Y unit cell size is from about 2.415 to 2.429 nm.

3. The process of claim 1 wherein the 199°C+ conversion is less than 20 weight percent.

4. The process of claim 1 further characterized by a 149°C+ conversion of less than 30 weight percent.

5. The process of claim 1 wherein the endpoint of the hydrodesulfurized feedstock is less than 399°C.

6. The process of claim 1 wherein the catalyst comprises at least 0.1 wt% of the Group VIIIA metal.

7. The proces of claim 1 wherein the Group VIIIA metal is selected from platinum and palladium.

8. The process of claim 1 wherein the hydrodesulfurized feedstock contains up to 70 weight percent aromatics.

9. The process of claim 1 wherein the product contains less than 15 weight percent aromatics.

10. A hydrodesulfurization catalyst comprising a Group VIIIA metal and ultrastable zeolite Y having a unit cell size of from 2.410 to 2.429 nm to evolve product containing less than 20 weight percent aromatics at a 199°C+ conversion of less than 30 weight percent.