FCC for producing low emission fuels from high hydrogen and low nitrogen and aromatic feeds

FIELD OF THE INVENTION
The present invention relates to a fluid catalytic cracking process for producing low emissions fuels. The feedstock is exceptionally low in nitrogen and aromatics and relatively high in hydrogen. The catalyst is an amorphous silica-alumina catalyst having a discrete surface silica phase. The feedstock can be characterized as having less than about 50 wppm nitrogen; greater than about 13 wt. % hydrogen; less than about 7.5 wt. % 2+ ring aromatic cores; and not more than about 15 wt. % aromatic cores overall.
BACKGROUND OF THE INVENTION
Catalytic cracking is an established and widely used process in the petroleum refining industry for converting petroleum oils of relatively high boiling point to more valuable lower boiling products including gasoline and middle distillates such as kerosene, jet fuel and heating oil. The pre-eminent catalytic cracking process now in use is the fluid catalytic cracking process (FCC) in which a pre-heated feed is brought into contact with a hot cracking catalyst which is in the form of a fine powder, typically having a particle size of about 10-300 microns, usually about 100 microns, for the desired cracking reactions to take place. During the cracking, coke and hydrocarbonaceous material are deposited on the catalyst particles. This results in a loss of catalyst activity and selectivity. The coked catalyst particles, and associated hydrocarbon material, are subjected to a stripping process, usually with steam, to remove as much of the hydrocarbon material as technically and economically feasible. The stripped particles, containing non-strippable coke, are removed from the stripper and sent to a regenerator where the coked catalyst particles are regenerated by being contacted with air, or a mixture of air and oxygen, at elevated temperature. This results in the combustion of the coke which is a strongly exothermic reaction which, besides removing the coke, serves to heat the catalyst to the temperatures appropriate for the endothermic cracking reaction. The process is carried out in an integrated unit comprising the cracking reactor, the stripper, the regenerator, and the appropriate ancillary equipment. The catalyst is continuously circulated from the reactor or reaction zone, to the stripper and then to the regenerator and back to the reactor. The circulation rate is typically adjusted relative to the feed rate of the oil to maintain a heat balanced operation in which the heat produced in the regenerator is sufficient for maintaining the cracking reaction with the circulating, regenerated catalyst being used as the heat transfer medium. Typical fluid catalytic cracking processes are described in the monograph Fluid Catalytic Cracking with Zeolite Catalysts, Venuto, P. B. and Habib, E. T., Marcel Dekker Inc. N.Y. 1979, which is incorporated herein by reference. As described in this monograph, catalysts which are conventionally used are based on zeolites, especially the large pore synthetic faujasites, zeolites X and Y.
Typical feeds to a catalytic cracker can generally be characterized as being a relatively high boiling oil or residuum, either on its own, or mixed with other fractions, also usually of a relatively high boiling point. The most common feeds are gas oils, that is, high boiling, non-residual oils, with an initial boiling point usually above about 230.degree. C., more commonly above about 350.degree. C., with end points of up to about 620.degree. C. Typical gas oils include straight run (atmospheric) gas oil, vacuum gas oil, and coker gas oil.
While such conventional fluid catalytic cracking processes are suitable for producing conventional transportation fuels, such fuels are generally unable to meet the more demanding requirements of low emissions fuels. To meet low emissions standards, the fuel products must be relatively low in sulfur, nitrogen, and aromatics, especially multiring aromatics. Conventional fluid catalytic cracking is unable to meet such standards. These standards will require either further changes in the FCC process, catalysts, or post-treating of all FCC products. Since post-treating to remove aromatics from gasoline or distillate fuels is particularly expensive, there are large incentives to limit the production of aromatics in the FCC process. Consequently, there exists a need in the art for methods of producing large quantities of low emissions transportation fuels, such as gasoline and distillates.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a fluid catalytic cracking process for producing low emissions fuel products, which process comprises:
(a) introducing a hydrocarbonaceous feedstock into a reaction zone of a catalytic cracking unit comprised of a reaction zone and a regeneration zone, which feedstock is characterized as having: a boiling point from about 230.degree. C. to about 350.degree. C., with end points up to about 620.degree. C.; a nitrogen content less than about 50 wppm; a hydrogen content in excess of about 13 wt. %; a 2+ ring aromatic core content of less than about 7.5 wt. %; and an overall aromatic core content of less than about 15 wt. %;
(b) catalytically cracking said feedstock in said reaction zone at a temperature from about 450.degree. C. to about 600.degree. C., by causing the feedstock to be in contact with a cracking catalyst for a contact time of about 0.5 to 5 seconds, which cracking catalyst is an amorphous silica-alumina material modified with a discrete surface silica phase, thereby producing lower boiling products and catalyst particles having deposited thereon coke and hydrocarbonaceous material;
(c) stripping said partially coked catalyst particles with a stripping medium in a stripping zone to remove therefrom at least a portion of said hydrocarbonaceous material;
(d) recovering said hydrocarbonaceous material from the stripping zone;
(e) regenerating said coked catalyst in a regeneration zone by burning-off a substantial amount of the coke on said catalyst, optionally with an added fuel component to maintain the regenerated catalyst at a temperature which will maintain the catalytic cracking reactor at a temperature from about 450.degree. C. to about 600.degree. C.; and
(f) recycling said regenerated catalyst to the reaction zone.
In preferred embodiments of the present invention, an added fuel component is used in the regeneration zone and is selected from: C.sub.2.sup.- light gases from the catalytic cracking unit, natural gas and any other non-residual petroleum refinery stream in the appropriate boiling range.
In preferred embodiments of the present invention the catalyst is an amorphous silica-alumina having about 10 to 40 wt. % alumina.
In other preferred embodiments of the present invention the contact time in the cracking zone is about 0.5 to 3 seconds.
DETAILED DESCRIPTION OF THE INVENTION
The practice of the present invention results in the production of less aromatic naphtha products as well as the production of more C.sub.3 and C.sub.4 olefins which can be converted to high octane, non-aromatic alkylates, such as methyl tertiary butyl ether.
Feedstocks which are suitable for being converted in accordance with the present invention are any of those hydrocarbonaceous feedstocks which are conventional feedstocks for fluid catalytic cracking and which have an initial boiling point of about 230.degree. C. to about 350.degree. C., with an end point up to about 620.degree. C. The feedstocks of the present invention must also contain no more than about 50 wppm nitrogen, no more than about 7.5 wt. % 2+ ring aromatic cores, no more than about 15 wt. % aromatic cores overall, and at least about 13 wt. % hydrogen. Non-limiting examples of such feeds include the non-residual petroleum based oils such as straight run (atmospheric) gas oil, vacuum gas oil, and coker gas oil. Oils from synthetic sources such as coal liquefaction, shale oil, or other synthetic processes may also yield high boiling fractions which may be catalytically cracked, either on their own or in admixture with oils of petroleum origin. Feedstocks which are suitable for use in the practice of the present invention may not be readily available in a refinery. This is because typical refinery streams in the boiling point range of interest, which are conventionally used for fluid catalytic cracking, generally contain too high a content of undesirable components such as nitrogen, sulfur, and aromatics. Consequently, such streams will need to be upgraded, or treated, to lower the level of such undesirable components. Non-limiting methods for upgrading such streams include hydrotreating in the presence of hydrogen and a supported Mo containing catalyst with Ni and/or Co; extraction methods, including solvent extraction as well as the use of solid absorbents, such as various molecular sieves. It is preferred to hydrotreat the streams.
Any suitable conventional hydrotreating process can be used as long as it results in a stream having the characteristics of nitrogen, sulfur, and aromatics level previously mentioned. That is nitrogen levels of less than about 50 wppm, preferably less than about 30 wppm, more preferably less than about 15 wppm, and most preferably less than about 5 wppm; a hydrogen content of greater than about 13 wt. %, preferably greater than about 13.5 wt. %; a 2+ ring aromatic core content of less than about 7.5 wt. %, preferably less than about 4 wt. %; and an overall aromatic core content of less than about 15 wt. %, preferably less than about 8 wt. %.
Suitable hydrotreating catalysts are those which are typically comprised of a Group VIB (according to the Sargent-Welch Scientific Co. Periodic Table of the Elements) metal with one or more Group VIII metals as promoters, on a refractory support. It is preferred that the Group VI metal be molybdenum or tungsten, more preferably molybdenum. Nickel and cobalt are the preferred Group VIII metals with alumina being the preferred support. The Group VIII metal is present in an amount ranging from about 2 to 20 wt. %, expressed as the metal oxides, preferably from about 4 to 12 wt. %. The Group VI metal is present in an amount ranging from about 5 to 50 wt. %, preferably from about 10 to 40 wt. %, and more preferably from about 20 to 30 wt. %. All metals weight percents are based on the total weight of the catalyst. Any suitable refractory support can be used. Such supports are typically inorganic oxides, such as alumina, silica, silica-alumina, titania, and the like. Preferred is alumina.
Suitable hydrotreating conditions include temperatures ranging from about 250.degree. to 450.degree. C., preferably from about 350.degree. C. to 400.degree. C.; pressures from about 250 to 3000 psig; preferably from about 1500 to 2500 psig; hourly space velocities from about 0.05 to 6 V/V/Hr; and a hydrogen gas rate of about 500 to 10000 SCF/B; where SCF/B means standard cubic feet per barrel, and V/V/Hr means volume of feed per volume of the catalyst per hour.
A hydrocarbonaceous feedstock which meets the aforementioned requirements for producing a low emissions fuel is fed to a conventional fluid catalytic cracking unit. The catalytic cracking process may be carried out in a fixed bed, moving bed, ebullated bed, slurry, transfer line (dispersed phase) riser, or dense bed fluidized bed operation. It is preferred that the catalytic cracking unit be a fluid catalytic cracking (FCC) unit. Such a unit will typically contain a reactor where the hydrocarbonaceous feedstock is brought into contact with hot powdered catalyst particles which were heated in a regenerator. Transfer lines connect the two vessels for moving catalyst particles back and forth. The cracking reaction will preferably be carried out at a temperature from about 450.degree. to about 680.degree. C., more preferably from about 480.degree. to about 560.degree. C.; pressures from about 5 to 60 psig, more preferably from about 5 to 40 psig; contact times (catalyst in contact with feed) of about 0.5 to 10 seconds, more preferably about 1 to 6 seconds; and a catalyst to oil ratio of about 0.5 to 15, more preferably from about 2 to 8. During the cracking reaction, lower boiling products are formed and some hydrocarbonaceous material, and non-volatile coke are deposited on the catalyst particles. The hydrocarbonaceous material is removed by stripping, preferably with steam. The non-volatile coke is typically comprised of highly condensed aromatic hydrocarbons which generally contain about 4 to 10 wt. % hydrogen. As hydrocarbonaceous material and coke build up on the catalyst, the activity of the catalyst for cracking, and the selectivity of the catalyst for producing gasoline blending stock are diminished. The catalyst particles can recover a major proportion of their original capabilities by removal of most of the hydrocarbonaceous material by stripping and the coke by a suitable oxidative regeneration process. Consequently, the catalyst particles are sent to a stripper and then to a regenerator.
Catalyst regeneration is accomplished by burning the coke deposits from the catalyst surface with an oxygen-containing gas, such as air. Catalyst temperatures during regeneration may range from about 560.degree. C. to about 800.degree. C. The regenerated, hot catalyst particles are then transferred back to the reactor via a transfer line and, because of their heat, are able to maintain the reactor at the temperature necessary for the cracking reactions. Coke burn-off is an exothermic reaction, therefore in a conventional fluid catalytic cracking unit with conventional feeds, no additional fuel needs to be added. The feedstocks used in the practice of the present invention, primarily because of their low levels of aromatics, and also due to the relatively short contact times in the reactor or transfer line, may not deposit enough coke on the catalyst particles to achieve the necessary temperatures in the regenerator. Therefore, it may be necessary to use an additional fuel to provide increased temperatures in the regenerator so the catalyst particles returning to the reactor are hot enough to maintain the cracking reactions. Non-limiting examples of suitable additional fuel include C.sub.2.sup.- gases from the catalytic cracking process itself, natural gas, and any other non-residual petroleum refinery stream in the appropriate boiling range. Such additional fuels are sometimes referred to as torch oils. Preferred are the C.sub.2.sup.- gases.
Catalysts suitable for use in the present invention are amorphous acidic catalytic materials which are modified with a discrete surface silica phase. It is preferred that the amorphous acidic material have a surface area after commerical deactivation, or after steaming at 760.degree. C. for 16 hrs, from about 75 to 200 m.sup.2 /g, more preferably from about 100 to 150 m.sup.2 /g. Amorphous acidic catalytic materials suitable for use herein include: alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania, and the like. Most preferred is a silica-alumina material having from about 10 to 40 wt. % alumina, preferably from about 15 to 30 wt. % alumina. Such materials will typically have a pore volume of at least about 0.3cc per gram. In general, higher pore volumes are preferred as long as they are not so high as to adversely affect the attrition resistance of the catalyst. Thus, the pore volume of the amorphous catalytic material will be at least about 0.3 cc per gram, preferably from about 0.4 to 1.5 cc per gram, and more preferably from about 0.8 to 1.3 cc per gram, and most preferably from about 1 to 1.2 cc per gram. Generally, the particle size of the catalyst will be in the range typically used for fluid bed catalysts. Generally this size will range from about 10 to 300 microns in diameter, with an average particle diameter of about 60 microns.
The discrete surface silica phase will constitute from about 2 to 25 wt. %, preferably from about 4 to 15 wt. %, and more preferably from about 6 to about 12 wt. %, which weight percents are based on the total weight of the catalyst. Such an amount of surface silica will correspond to a silica surface coverage of about 0.2 to 2 monolayers.
The discrete surface silica phase may be deposited onto the amorphous material by use of any appropriate reagent. Preferred reagents include the silicon halides, Na.sub.2 SiO.sub.3, the silicon alkoxides, and the like. Any appropriate deposition can be used for depositing the surface silica phase. For example, an incipient wetness technique can be used wherein the silicon-containing reagent is dissolved in a suitable wetness, which solution is used to impregnate the silica-alumina material. The so-treated material is then dried and air calcined at a temperature of about 450.degree. C. to about 550.degree. C. for an effective amount of time.
The following examples are presented to illustrate the present invention and are not to be taken as limiting the invention in any way.

EXAMPLE 1 (COMPARATIVE)
Cracking tests were conducted in a microactivity test (MAT) unit. Such a test unit is described in the Oil and Gas Journal, 1966 Vol. 64, pages 7, 84, 85 and Nov. 22, 1971, pages 60-68, which is incorporated herein by reference. Run conditions in the MAT unit were as follows:
______________________________________Temperature, .degree.C. 525Run Time, Sec. 30Catalyst Charge, gr. 4.1Amount Feed, cc. 1.1Cat/Oil ratio 4.2 to 4.5______________________________________
Tests were made with two fresh, steamed, catalysts. The catalysts were steamed for 16 hours at 760.degree. C. to simulate commercially deactivated catalysts. The first catalyst (ZA) is commercially available from Davison under the tradename Octacat-D. Catalyst ZA contains a USY zeolite (LZY-82 from Union Carbide) but no rare earths. It is formulated in a silica-sol matrix and after steaming, or commercial deactivation, it is a relatively low unit cell size catalyst. The second catalyst was an amorphous Si-Al gel catalyst, 3A, commercially available from Davison, The composition and properties of catalyst ZA and 3A are as shown below.
______________________________________ ZA 3A______________________________________CATALYSTAl.sub.2 O.sub.3 26.0 wt. % 25 wt. %SiO.sub.2 73.0 75Re.sub.2 O.sub.3 0.02 0Na.sub.2 O 0.25 --After calcination for 4 hrs at 538.degree. C.Surface Area, M.sup.2 /g 297.5 --Pore Volume, cc/g 0.24 --Unit Cell Size, .ANG. 24.44 --After steaming for 16 hrs at 405.degree. C.Surface Area, M.sup.2 /g 199.5 128Pore Volume, cc/g 0.20 0.49Unit Cell Size, .ANG. 24.25 --______________________________________
A raw and two hydrotreated Arab Light VGO (virgin gas oil) streams, were used as feeds for catalytic cracking experiments. A commercially available NiMo on alumina catalyst, available from Ketjen as catalyst KF-843, was used to hydrotreat the feeds. The hydrotreated feed is designated as HA and the 345.degree. C.+ fraction is designated HA+. The severity of hydrotreating is designated by a number indicating the hydrotreating severity which increases from HA2+ to HA1+. The raw Arab light vacuum gas oil (VGO) is designated as RA. Arab Light VGO is a typical, conventional feedstock for fluid catalytic cracking. The properties of the raw and hydrotreated feeds are set forth below.
______________________________________Properties of Raw and Hydrotreated Arab Light VGO HA2+ HA1+ RA+______________________________________Wppm N 0.7 <.5 596Wt. % S <0.01 <0.01 1.99Wt. % C 86.11 85.70 85.86Wt. % H 13.89 14.30 12.09Wt. % 93.7 95.7 47.8SaturatesWt. % 1 Ring 4.2 2.3 17.1AromaticsWt. % Total 2.0 1.3 21.5Arom. CoresWt. % 2+ 1.4 1.0 16.8Ring Cores______________________________________
The total liquid product from the MAT tests amounted to about 0.3 to 0.7 grams and was analyzed using two different gas chromatograph instruments. A standard analysis was the boiling point distribution determined by gas chromatographic distillation (GCD) to evaluate: (1) the amount of material boiling less than 15.degree. C.; (2) naphtha boiling between 15.degree. C. and 220.degree. C.; (3) light cat cycle oil (LCCO) boiling between 220.degree. C. and 345.degree. C.; and (4) bottoms boiling above 345.degree. C. For selected tests, another portion of the sample was analyzed on a PIONA instrument, which is a multidimensional gas chromatograph (using several columns) to determine the molecular types according to carbon number from C.sub.3 to C.sub.11. The types include normal paraffins, isoparaffins, naghthenes, normal olefins, iso-olefins, cyclo-olefins, and aromatics.
Detailed cracking data are given in Table I below for the raw and hydrotreated Arab Light VGO feeds.
TABLE I______________________________________Cracking of Raw Arab Light VGO with Catalyst ZA vsClean Feed with 3A @ 525.degree. C. and 4.5 Cat/Oil______________________________________Feed RA+ HA1+ HA2+Catalyst ZA 3A 3AConversion (220.degree. C.) 67.1 69.1 65.0Yields, Wt %Coke 2.35 0.37 0.69C.sub.2.sup.- Dry Gas 2.17 1.05 1.55C.sub.3 H.sub.6 4.7 8.5 6.4C.sub.3 H.sub.8 0.95 0.71 0.43C.sub.4 H.sub.8 5.9 13.7 10.5Iso-C.sub.4 H.sub.10 4.2 3.5 2.5N--C.sub.4 H.sub.10 0.88 0.49 0.2915.degree.-220.degree. C. Naphtha 45.9 41.1 42.5LCCO 15.6 2.9 6.3Bottoms 17.2 27.9 28.715.degree.-220.degree. C. NaphthaAromatics 32.4 7.5 13.3Olefins 27.6 65.6 62.7______________________________________
The above table shows that conversion obtained with the conventional fluid catalytic cracking feed RA+ and zeolitic catalyst ZA is bracketed by the conversions obtained with the two clean feeds of this invention and the amorphous silica-alumina catalyst 3A, a catalyst of this invention. Furthermore, the naphtha produced from the clean fed with a preferred low hydrogen transfer catalyst (3A) is substantially less aromatic than naphtha produced by conventional fluid catalytic cracking. Also, propylene and butylene yields are higher.
EXAMPLE 2
A series of three catalysts was investigated in duplicate in a microactivity test (MAT) unit. The tests were run at 482.degree. C. for 80 seconds, and with a catalyst to oil ratio of 2.9 using a hydrotreated light Arab virgin gas oil (VGO) feedstock containing about 40 wppm of nitrogen and 13.4 wt. % hydrogen. The feedstock contained about 82 wt. % of 345.degree. C.+ hydrocarbons. One catalyst was an amorphous silica-alumina gel catalyst, which is commercially available as 3A from Davison, and which contains about 75 wt. % silica. Two experimental catalysts were also used and were produced from this 3A silica-alumina by impregnating it with surface silica. Both experimental catalysts were fabricated by impregnating ethyl orthosilicate in isopropanol to the point of incipient wetness, followed by air drying for 16 hours at room temperature, then vacuum drying for 16 hours at 100.degree. C., then air calcination at 550.degree. C. for 3 hours. Catalyst B contained an additional 10 wt. % surface silica on the 3A starting material. This corresponded to a total (bulk plus surface) silica content of about 77 wt. %. Catalyst C contained 16 wt. % added surface silica for a total silica content of about 78 wt. %. All three catalysts were steamed at 760.degree. C. overnight prior to the microactivity tests.
Table II below summarizes catalyst activity and selectivity data from these microactivity tests. Specific coke represents the coke yield divided by the fractional conversion divided by one minus the fractional conversion and corresponds to the selectivity for producing coke. The ratio of C.sub.4.sup.= C.sub.4 represents the product ratio for C.sub.4 -olefins (C.sub.4.sup.=) to paraffins (C.sub.4) and is used to distinguish different selectivity patterns for producing olefinic light products. The olefins and paraffins were determined by on-line gas chromatography.
TABLE II______________________________________ Conver- Yields SpecificCatalyst sion % wt. % Coke H.sub.2 Coke C.sub.4.sup.= /C.sub.4______________________________________3A 66.1 1.46 0.025 0.75 1.493A 68.5 1.05 0.022 0.48 1.68B 57.6 0.62 0.023 0.46 2.50B 57.9 0.38 0.022 0.27 2.35C 55.1 0.41 0.018 0.33 2.27C 59.2 0.35 0.017 0.24 2.41______________________________________
The above table shows that the surface silica modified catalysts (B and C) exhibited a lower activity as compared to Catalyst A. However, the silica modified catalysts exhibited improved selectivities for coke production combined with higher olefin to paraffin ratios. These selectivity credits far outweigh the small activity debit. Cracking clean feeds with silica-surface modified catalysts of this invention provides more light olefins for alkylation or producing MTBE for low emissions fuels. Reduced coke yields indicate surface modified catalysts also produce less aromatic products.
EXAMPLE 3
A similar series of surface modified catalysts (D-G) was prepared in accordance with the procedure of Example 1 above, but modified with different levels of surface silica. Catalyst D contained 3 wt. % surface silica, catalyst E contained 6 wt. % surface silica, catalyst F contained 10 wt. % surface silica, and catalyst G contained 16 wt. % surface silica. This series of catalysts was also evaluated by the microactivity test at equivalent conditions used in Example 1 but using a slightly different hydrotreated Arab light VGO feedstock that contained 11 wppm nitrogen and 13.30 wt. % hydrogen. As before, all of the catalysts were steamed overnight at 760.degree. C. prior to the microactivity test. Table III below summarizes the results of these tests.
TABLE III______________________________________ Conver- Yields SpecificCatalyst sion % wt. % Coke H.sub.2 Coke C.sub.4.sup.= /C.sub.4______________________________________3A 68.8 0.973 0.043 0.44 1.07D 68.1 1.104 0.033 0.52 --D 69.8 1.270 0.031 0.55 1.06E 58.4 0.459 0.022 0.33 1.89E 55.1 0.261 0.020 0.21 1.92G 54.9 0.173 0.021 0.14 --G 54.4 0.058 0.013 0.05 1.68______________________________________
The above table shows similar trends with this series of catalysts when compared with the series of Example 1 above. That is, the catalysts which were modified with surface silica showed lower activity, reduced coke selectivity, and increased olefin to paraffin ratios. The one exception was catalyst D which contained a relatively low loading of surface silica. Based on these data, it appears that surface silica loadings greater than about 5 wt. % are needed in order to obtain favorable changes in selectivity with the feeds employed in the practice of the present invention.

FCC for producing low emission fuels from high hydrogen and low nitrogen and aromatic feeds

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