DE3781528T2 - METHOD FOR INCREASING OCTANES AND REDUCING THE SULFUR CONTENT OF OLEFINIC GASOLINES. - Google Patents

Description

This invention relates to a process for increasing the octane number while reducing the sulfur content of olefinic gasolines derived from cracking processes, particularly catalytic cracking processes.

New regulations requiring the reduction of lead content in gasoline are creating a need for a higher average octane gasoline tank. In addition, there is likely to be continued interest in reducing sulfur oxide (SOX) emissions, particularly as gasolines produced by fluid catalytic cracking (FCC) processes are more widely incorporated into unleaded gasoline tanks for cars equipped with a catalytic converter.

The possibility of catalytic reforming of FCC gasoline to improve the gasoline stock was considered by L.A. Gerritsen, "Catalytic Reforming of FCC Naphtha for Production of Lead-Free Gasoline", Ketjen Symposium, Amsterdam, 1984. This prior art describes reforming the FCC gasoline fraction over a bimetallic Pt-Re catalyst, but it was found that the higher severity (hereafter referred to as severity) and increased throughput conditions of this process resulted in a deterioration of the catalyst cycle length in the reformer.

Many crystalline silicate zeolites are known in the state of the art. However, the direct reforming of olefinic gasolines from catalytic cracking of gas oils leads to rapid aging of conventional reforming catalysts, This is due to the relatively high sulphur content of these gasolines (0.05 to 0.5 wt%). The olefinic composition of these gasolines also leads to a relatively high hydrogen consumption and a corresponding exotherm during the desulfurization required prior to reforming with conventional catalysts.

Certain hydrothermally stable catalysts, such as those described in US Patent No. 3,493,519, use a crystalline aluminosilicate ammonium-Y which is calcined in the presence of rapidly flowing steam. The resulting steamed product is base exchanged with an ammonium salt and treated with a chelating agent capable of combining with aluminum at a pH between 7 and 9. These aluminum-deficient catalysts are reported to exhibit extremely high activity (α-value).

Other treatments of synthetic faujasite (NH4Y) prepared by ammonium exchange of sodium faujasite are described in U.S. Patent No. 3,591,488. These steam treated zeolites, after heat treatment, are base exchanged with cations such as an ammonium ion and/or metal ions selected from Groups IIA, IB through VIIB, VIII of the Periodic Table and rare earth ions having atomic numbers from 51 to 71 such as ions of Mg, Ca, Sr, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, W, Re, Os, Ir, Pt, Au and Hg, preferably ions of Groups IIA, VIII and rare earths. A final zeolite product having an alkali metal content of less than about 0.5 wt.%, preferably less than about 0.2 wt.%, is reported. The resulting product has, depending on the type of zeolite, silica/alumina molar ratios of typically more than 5 to 10, preferably more than 10 and more preferably more than about 50.

The problem of sulfur contamination of catalysts is well known in the art, as is mentioned, for example, in U.S. Patent No. 4,456,527. However, in the prior art, the problem of catalyst contamination is solved by using separate sulfur removal steps to reduce the sulfur content to below 500 parts per trillion (ppb), preferably to less than 250 ppb, more preferably to less than 100 ppb, and most preferably to less than 50 ppb.

Although the problems of catalyst contamination with high sulfur feedstocks have been recognized in the prior art, none of the prior art proposals allow for the direct reforming of an olefinic gasoline from FCC or TCC catalytic cracking of gas oils while minimizing or avoiding the rapid aging of the reforming catalyst due to the relatively high sulfur content of these gasolines. It is therefore an object of the present invention to provide such a reforming process.

Reference is made to EP-A-0 014 291 and EP-0 186 479. The former describes the conversion of hydrocarbon feedstocks containing sulphur and nitrogen impurities with a catalyst comprising a synthetic crystalline zeolite ZSM-20 combined with a hydrogenation function, in particular palladium. The particular applications of this process are polymerisation, aromatisation, hydrocracking and catalytic cracking. EP-A-0 186 479 describes catalysts with advanced shape selectivity based on zeolites of the ZSM-5 and ZSM-11 type; these catalysts are described as being effective in the Aromatization of paraffins is particularly advantageous.

Accordingly, this invention consists in a one-step process for reducing the sulfur content and increasing the octane number of an olefin-containing feedstock comprising contacting olefin-containing feedstock and hydrogen gas with a catalyst comprising a large pore crystalline zeolite intimately associated with 0.1 to 5 weight percent of a noble metal, said zeolite having a Constraint Index of less than 2 and a lattice silica/alumina molar ratio of not less than 50:1, said contact being conducted under a combination of effective conditions to form a product having a higher octane number than the feedstock, said conditions comprising a temperature of 700 to 1000°F (370 to 540°C), a pressure of 100 to 500 psig (790 to 3350 kPa), an LHSV of 2 to 16, and a hydrogen circulation rate of 1125 to 5260 scf/bbl (200 to 1000 Nm³/m³).

The process of the present invention is directed to the reforming and desulfurization of olefinic gasolines derived from catalytic or other cracking processes. Without being limited to any particular process, the present invention is primarily directed to catalytic cracking processes. By using a catalyst comprising a noble metal-containing, large pore, high silica/alumina molar ratio zeolite, the olefinic gasoline can be processed at high temperatures and a product with increased octane and reduced sulfur content can be obtained.

Reformation

By definition, reform generally involves a A method of increasing the octane number of naphtha or gasoline oil to a usable octane number. SR naphtha from crude oil may, for example, have an octane number of 40, which is too low for use as gasoline. This unsuitable property can be improved by reforming. The naphtha may also have an unsuitable sulfur value, e.g. 50 parts per million (ppm), which is reduced by reforming under the conditions set out in this invention.

Most current reforming catalysts, e.g. platinum on alumina, require that the sulfur in the fuel oil be reduced to about 0.2 ppm or less to prolong the catalyst life. Conventional reformers operate at temperatures between 900 and 1000ºF (482 - 538ºC) and pressures between 100 and 500 psig (790 - 3550 kPa). Hydrogen is fed at the same time as the naphtha in a ratio of about 5:1. Conventional reformers feed small amounts of chlorine and occasionally water at the same time as the hydrocarbon feed to keep the catalyst active.

The need for chlorine and the requirement to reduce sulfur are detrimental to the reforming process. Therefore, standard reforming processes cannot be used with olefinic feedstocks, which are the starting material used in the present invention.

The present invention has a number of advantages over conventional reforming. First, most reforming catalysts are limited to feedstocks with a naphtha endpoint of 350ºF (177ºC). Any feedstock with an endpoint of more than 350°F (177°C) will cause the catalyst to age too quickly. However, the catalyst of the present invention can tolerate much higher endpoint feedstocks. Second, there is no need to pass chlorine over the catalyst to keep the catalyst active since the zeolite of this invention provides the acidity required to carry out the reaction. Third, there is no need to reduce the sulfur content by a preliminary hydrotreating step since use of the catalyst of the present invention at conventional reforming conditions both removes the sulfur and increases the octane number of the product from the feedstock.

Source material

The feedstock for the present invention is generally a gasoline derived from catalytic cracking or thermoforcracking. The catalytic cracking process can be either a fluid catalytic cracking (FCC) process or a thermoforcracking (TCC) process. The feedstock can contain sulfur at levels greater than about 100 ppm, which is typically reduced by hydrotreatment to enable the feedstock to be processed over a conventional reforming catalyst. In addition, the feedstock contains olefins which require additional hydrotreatment to enable them to be passed over a conventional reforming catalyst. In addition, the feedstock for the present invention has a boiling range that exceeds the boiling range of feedstocks heretofore processed with conventional reforming catalysts.

Catalysts

The catalysts used in the process of the invention are large pore zeolites having a Constraint Index of less than 2 and a silica/alumina lattice molar ratio of at least 50:1 and preferably greater than about 500:1. The Constraint Index represents a suitable value for the extent to which the zeolite controls the entry of molecules of different sizes into its internal structure. Zeolites which have highly restricted access to and exit from their internal structure have a high Constraint Index value and zeolites of this type typically have small sized pores, e.g. less than 5 Angstroms. Zeolites which, on the other hand, allow relatively free access to their internal zeolite structure have a high Constraint Index value and typically have large pores, greater than 8 Angstroms. The method by which the constraint index is determined is described in detail in US Patent No. 4,016,218.

Zeolites having a Constraint Index of less than 2 are well known in the art and generally have a pore size of greater than 7 Angstroms; this is sufficiently large to admit the bulk of the components normally present in the feedstock. Large pore zeolites suitable for use in the process of the invention are zeolite beta, zeolite L, zeolite Y (e.g. ultrastable Y and dealuminated Y), mordenite, ZSM-3, ZSM-4, ZSM-18 and ZSM-20.

Values for the constraint index (CI) for representative large pore zeolites are as follows: CI (at test temperature) Mordenite Dealuminized Y (trade name Y) Zeolite Beta

The Constraint Index appears to vary somewhat with the severity of the process (conversion) and the presence or absence of binders. Other variables such as the crystal size of the zeolite, the presence of entrapped impurities, etc., may also affect the Constraint Index. It is therefore considered that it is possible to select the test conditions such as temperature to produce more than one Constraint Index value for a given zeolite. This explains the range of Constraint Index for Zeolite Beta.

Preferred zeolites for use in the process of the present invention are zeolite ZSM-4 (described in U.S. Patent No. 3,923,639), zeolite ZSM-20 (described in U.S. Patent No. 3,972,983), zeolite beta (described in U.S. Patent No. 3,308,069 and Re. 28,341), zeolite Y (described in U.S. Patent No. 3,130,007), and modified forms of zeolite Y such as ultrastable zeolite Y (described in U.S. Patent Nos. 3,293,192 and 3,449,070), dealuminized zeolite Y (U.S. Patent No. 3,442,795), and zeolite UHP-Y (described in U.S. Patent No. 4,401,556). The most preferred zeolite is zeolite Y, which can be treated by known methods, by steam treatment and/or acid dealumination, to increase its silica/alumina ratio to at least 50:1.

When carrying out the method according to the invention, It may be advantageous to incorporate the crystalline zeolites described above into a matrix comprising another material which is resistant to the temperature and other conditions used in this process. Such a matrix material is advantageous as a binder.

Advantageous matrix materials include both synthetic and naturally occurring 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 or gels, including mixtures of silica and metal oxides. Naturally occurring clays which may be compounded with the zeolite include those of the montmorillonite and kaolin groups, which groups include the subbentonites and kaolins commonly known as Dixie, McNamee Georgia and Florida clays, or others in which the major mineral constituent is haloysite, kaolinite, dickite, nacrite or anauxite. These clays may be used in the raw state as originally mined or may be initially subjected to calcination, acid treatment or chemical modification.

In addition to the above materials, the zeolites used here can be composed with a porous matrix material such as: alumina, silica, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia and silica-titania, as well as ternary compositions such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia and silica-magnesia-zirconia. The matrix can be in the form of a cogel. The relative proportions of the zeolite component and the inorganic oxide gel matrix may vary widely, with the zeolite content on an anhydrous basis ranging from 1 to 99% by weight, and more typically from 5 to 80% by weight of the dry composite material.

The original cations associated with any of the crystalline zeolites used herein can be replaced by a wide variety of other cations by methods well known in the art. Typical replacement cations include hydrogen, ammonium, alkylammonium and metal cations, including mixtures thereof.

The crystalline zeolite used in the process of the invention is used in close combination with a noble metal in an amount between 0.5 and 5 wt.% and preferably 0.3 to 3 wt.%, such as platinum or platinum in combination with other Group VIII metals, e.g. platinum-rhenium or platinum-iridium. This component can be exchanged for this composition, impregnated thereon or physically closely mixed therewith. This component can be impregnated into or onto the zeolite, as in the case of platinum, by treating the zeolite with an ion containing a platinum metal. Suitable platinum compounds thus include chloroplatinic acid, platinum chloride and various compounds containing the platinum amine complex.

Procedural conditions

The process according to the invention is essentially a reforming process in which the reactions taking place are reforming reactions. However, the process cannot be described as a reforming process per se, because it passes an olefin-containing feed over the catalyst at high temperature and cyclizes the olefins directly to aromatics. Automatically, this process increases the octane number and reduces the sulfur content of the olefin-containing feedstock. Unlike conventional reforming processes, the process of the present invention accepts (1) olefin-containing feedstocks, (2) sulfur-containing feedstocks, and (3) high boiling point feedstocks, namely above 350ºF (177ºC).

The feedstock is contacted with the catalyst in the presence of hydrogen under conditions of temperature, pressure, space velocity and hydrogen ratio similar to those used in conventional reforming processes. As described above, these conditions include temperatures of 700 to 1000ºF (370 - 540ºC), pressure of 100 to 500 psig (790 to 3550 kPa), space velocities of 2 to 16 LHSV and a hydrogen circulation rate of 1125 to 5620 scf/bbl (200 to 1000 Nm3/m3).

The process can be conveniently carried out in conventional equipment, in a series of reactors with interstage heating to maintain the desired reactions and thermal balance. As already stated, a particular advantage of using high silicon zeolite supports is that the need to maintain acidity by chlorination, use of co-feed water, etc., can be substantially reduced and, under favorable circumstances, eliminated. If experience shows that the use of these conventional concomitant measures is necessary or desirable, they can still be used. Consequently, water in conventional quantities can be used with the starting material. , typically 1 to 100 ppm, or halogenation may be used to maintain activity, e.g. by incorporating the halogen in the form of an acid or salt, or by adding the halogen or halide compound during the reforming process itself in a conventional manner. The preferred halogen is chlorine. Details of the process for maintaining activity using halogen are described in U.S. Patent Nos. 4,261,810, 4,049,539, 3,835,063, 6,661,768 and 3,649,524.

The invention is illustrated by the following examples, in which all parts, ratios and percentages are by weight unless otherwise indicated.

example 1

The catalyst used was prepared by steam treatment and acid dealumination of an ultrastable Y from Linde followed by impregnation with platinum as a tetraamine.

The resulting dealuminated zeolite Y was analyzed and found to have a SiO2/Al2O3 mass ratio of 45 on an approximate grid, namely tetrahedral alumina, SiO2/Al2O3 of 2600 by MAS-NMR (nuclear magnetic resonance with rapid rotation of the sample through the "magic angle"). The activity of this sample, represented by the α-value, was found to be 1.5, which is in good agreement with the aluminum content of the approximate grid. The platinum loading was found to be 0.48% and had an essentially uniform distribution of 96%, as determined by chemisorption with hydrogen.

An olefinic gasoline sample from fluid catalytic cracking of "Arab Light" vacuum gas oil (VGO) with RON=91 and 2950 ppm sulfur was distilled into three fractions and analyzed for octane number (RON) and sulfur. The analysis showed that sulfur increased significantly with increasing boiling point, whereas octane number decreased, as shown below. Table 1 Properties of FCC gasoline Total range Boiling range Yield, wt.% RON+O¹ Sulphur, ppmw Hydrogen, wt.% ¹ Research octane number (RON)

The mid-boiling fraction (180-300°F) was used as the feed to the process of this example. Additional analysis showed that this fraction consisted of 17 wt.% paraffins, 44 wt.% olefins and 27 wt.% aromatics. Process conditions of 900°F (482°C), 250 psig (1825 kPa), 4.0 LHSV and about 4000 scf/bbl (712 Nm3/m3) hydrogen flow were used to simultaneously desulfurize the olefinic FCC gasoline and increase its octane value. Prior to feeding the FCC gasoline fraction, the dealuminized zeolite Y catalyst of this invention was used to reform a number of conventional feeds for 18 days.

The initial values showed that a yield of about 83 % gasoline with RON+O of 99 were obtained when the middle fraction was treated at the above conditions. Analyses showed that the product consisted of 25% paraffins, 0% olefins, 2% naphthenes and 73% aromatics. At this reaction severity, it was estimated that there was a net hydrogen production of about 150 scf/bbl (27 Nm³/m³). The product compositions and properties are shown in Table 2. Table 2 Compositions and properties of the product Feed Product Octane number, RON+O Sulphur, ppmw Paraffins, wt.% Olefins Naphthenes Aromatics

During a six-day supplemental treatment of the sulfur-containing olefinic FCC gasoline feed, essentially no catalyst aging was observed.

Comparison of yield and octane values indicates that treatment with the new inventive dealuminated catalyst zeolite Y can achieve similar overall results as conventional hydrotreating (HDT) followed by conventional reforming. In addition, the relatively low hydrogen content of the olefinic FCC gasolines indicates that the net endotherm associated with conventional reforming can be reduced.

Example 2

The catalyst used in Example 2 was the same as in Example 1. The feedstock for Example 2 was FCC gasoline distilled and fractionated at 180ºF (82ºC) and 300ºF (149ºC). The complete analysis of the FCC gasoline over the entire range and these fractions is shown in Table 3. Table 3 Analysis of FCC gasoline Fraction Fraction (separated) Total range Yield, vol.% API gravity Hydrogen, wt.% Sulfur, ppmw Nitrogen, ppmw Paraffins, wt.% Olefins Naphthenes Aromatics ¹ Motor octane number ² Reid vapor pressure ³ Actual boiling point

It is noteworthy that fractions 2 and 3 contain significant amounts of olefins (16 and 8% respectively) and aromatics (37 and 61% respectively). As a result, their octane numbers are already high (RON+O 88 and 90). However, during hydrotreatment to remove sulfur and nitrogen, the octane number drops considerably. Consequently, the purpose of this example is to find a way to maintain or even increase the octane value while removing sulfur and nitrogen.

At the beginning of the run, the catalyst was heated to 300ºF (149ºC) under a hydrogen atmosphere, held at that temperature for 2 hours, and then slowly heated to 660ºF (349ºC) at a rate of 90ºF (50ºC) per hour. Feeding began after the catalyst had been at 660ºF (349ºC) for 2 hours. The temperature was then increased to the desired reaction temperature. The heart fraction of the FCC gasoline (Fraction 2) was added and kept running at a temperature of 900ºF (482ºC) for 3 weeks. The feed was then changed to the higher boiling FCC gasoline fraction (Fraction 3) over a 2 week period. Although these fractions were treated separately, it is believed that they can be fed simultaneously. The lower boiling olefinic fraction, which already had a high octane rating (RON 92.5) and a relatively low sulfur content (0.04 wt%), did not appear to require further upgrading, although it can be treated by conventional means to reduce the mercaptans. These runs were compared with runs using a standard chlorinated platinum on alumina reforming catalyst.

The results of treatment of the cardiac fraction (fraction 2) are shown in Table 4 below. Table 4 Upgrading of 180-300ºF Fraction from FCC Gasoline Catalyst/Process Feed Zeolite² H₂, Net Consumption, scf/b Yields and Properties, wt. % C₅⁺ Gasoline, vol. % RON+O (Research Octan.) MON+O (Motor Octan.) RVP (Reid Vapor Pressure) Sulfur, ppmw Nitrogen, ppmw Aromatics, wt. % Olefins Density at 60ºF (16ºC) ¹Hydrotreating ²Process conditions: 900ºF (482ºC), 250 psig (1825 kPa), 4.0 LHSV, 4000 scf/b (712 Nm³/m³)H₂ ³Includes HDT consumption 400 scf/b (71.2 Nm³/m³)H₂ �sup4;Isobutane �sup5;n-Butane �sup6;USDY = ultra-stable dealuminized zeolite Y

When the mid-boiling fraction was treated, yields comparable to those of conventional hydrotreating and reforming were achieved, as shown in Table 4. It is noteworthy that the yield of isobutane over the zeolite catalyst was considerably higher, mainly due to the reduction in light gas production, and represented a potential source of additional alkylate. This reduction in light gas production may result in increased hydrogen purity in the recycle gas.

As shown in Table 5 below, the treatment of the higher boiling FCC gasoline fraction seems particularly interesting. Table 5 Upgrading of FCC gasoline fraction at 300ºF⁺ (149ºC) Catalyst/Process Feed Zeolite 0.5% Pt-USDY¹ Yields and properties, wt% C₅⁺ gasoline, vol% RON+O MON+O RVP sulfur, ppmw Nitrogen, ppmw Aromatics, wt. % Olefins Density at 60ºF (16ºC) ¹Process Conditions: 900ºF (482ºC), 250 psig (1825 kPa), 2.0 LHSV, 4000 scf/b (712 Nm³/m³)H₂

The higher boiling FCC gasoline fraction is currently hydrotreated at a number of refineries to remove sulfur. Both the high sulfur level of about 8000 ppm and the high end point of about 450ºF (232ºC) preclude conventional hydrotreating/reforming. Here, the results show that net gasoline yields of this fraction, reaching 97 vol%, can be achieved with an octane gain of about 13 RON and a reduction in product sulfur to less than 10 ppm.

The combination of these treated fractions with the untreated olefinic light gasoline results in a significant octane gain of the total FCC gasoline with minimal yield loss. As shown in Table 6 below, it is estimated that about 90 vol.% yield of C5+ gasoline can be obtained with an octane gain of 7 RON and a reduction in sulfur from 0.30 wt.% to less than 0.02 wt.%. The olefin content of the gasoline is significantly reduced with increasing aromatics. Table 6 Estimated Total Yields and Gasoline Properties FCC Gasoline "Joliet" Net Product¹ C₅⁺⁺ Gasoline, Vol% C₅⁺⁺ Properties RON+O MON+O RVP+O Sulfur, Wt% Nitrogen, ppmw Aromatics Olefins

Linear mixture of untreated lower boiling FCC benzene in and¹ treated middle and higher boiling fractions

²Is lower when olefinic light petrol "Merox" is treated.

Consequently, the present invention demonstrates the ability to both increase the octane number of the entire FCC gasoline pool and reduce the sulfur value. The direct treatment of the middle and higher boiling FCC gasoline fractions Using a process of this type can represent an attractive alternative to conventional hydrotreating/reforming or to conventional hydrodesulfurization of FCC feedstocks.

In addition, the process of the invention allows the catalyst to operate in the presence of higher sulfur levels without significant catalyst degradation. Conventional reforming processes can tolerate olefins in the feed which normally tend to coke very quickly under reforming conditions over conventional catalysts. The net result of this process is a high octane, low sulfur gasoline with minimal yield loss.

Claims (7)

1. A one-step process for reducing the sulfur content and increasing the octane number of an olefinic gasoline derived from fluidized bed or thermoforcing catalytic cracking of gas oils, which comprises contacting olefin-containing feedstock and hydrogen gas with a catalyst comprising a large pore crystalline zeolite intimately associated with 0.1 to 5 weight percent of a noble metal, said zeolite having a constraint index of less than 2 and a lattice silica/alumina molar ratio of not less than 50:1, said contact being conducted under a combination of effective conditions to form a product having a higher octane number than the feed, said conditions comprising a temperature of from 370 to 540°C (700 to 1000°F), a pressure of from 790 to 3350 kPa (100 to 500 psig), an LHSV of 2 to 16 and a hydrogen circulation rate of 200 to 1000 Nm³/m³ (1125 to 5260 SCF/bbl). 2. Process according to claim 1, characterized in that the sulfur is present in the starting material at a concentration of more than 100 ppm. 3. Process according to claim 1 or 2, characterized in that the olefin-containing starting material is derived from the catalytic cracking process. 4. Process according to claim 2, characterized in that the zeolite has a SiO₂/Al₂O₃ lattice ratio of more than 500:1. 5. Process according to one of the preceding claims, characterized in that the crystalline zeolite is derived from zeolite beta, zeolite L, zeolite Y, mordenite, ZSM-3, ZSM- 4, ZSM-18, ZSM-20. 6. Process according to claim 6, characterized in that the zeolite is zeolite Y. 7. Process according to one of the preceding claims, characterized in that the noble metal is platinum or platinum in combination with another metal of group VIII.