MXPA99011426A - Hydrocarbon cracking with positive reactor temperature gradient - Google Patents

Abstract

The invention provides an improvement in processes for cracking hydrocarbons to prime olefins, ethylene and propylene, which comprises applying a positive temperature gradient across a catalyst contact zone. The temperature gradient may be uniform or stepwise, may be one reactor or a series of reactors so long as at least one down stream cracking zone is operated at a higher temperature than an upstream zone.

Description

CRAQUEO OF NAFTAS WITH POSITIVE GRADIENT OF REACTOR TEMPERATURES Field of the Invention The invention provides a process for increasing the yields of ethylene and / or propylene in a catalytic cracking process by the use of a positive gradient of reactor temperatures. Background of the Invention The thermal and catalytic conversion of hydrocarbons into olefins is an important industrial process that produces millions of kilograms of olefins each year. Due to the large volume of production, small improvements in operating efficiency translate into considerable profits. Catalysts play an important role in the more selective conversion of hydrocarbons into olefins. Particularly important catalysts are found among natural and synthetic zeolites. Zeolites are complex crystalline aluminosilicates that form a network of tetrahedra of A104 and Si04 linked by shared oxygen atoms. The negative charge of the tetrahedra is balanced by the inclusion of protons or cations such as alkali metal or alkaline earth ions. The spaces or interstitial channels formed by the crystalline network allow the zeolites to be used as molecular sieves in separation processes. The ability of the waxes to adsorb materials also allows them to be used in catalysis. There is a large number of natural and synthetic ceolithic structures. The great amplitude of such numbers can be understood considering the work "Atlas of Zeolite Structure Types", by W.M. Meier, D.H. Olson and C.H. Baerlocher (4th edition, Butter orths / Intl. Zeolite Assoc. [1996]). Catalysts containing zeolites have been found to be active in the cracking of light naphtha in ethylene and propylene, the primordial olefins. Of particular interest are the acidulated zeolites which are effective for the conversion of light hydrocarbons such as low boiling point naphtha in the primordial olefins. Typical catalysts include zeolite ZSM-5, described and claimed in U.S. Patent No. 3,702,886, and ZSM-11, described in U.S. Patent No. 3,709,979, and the numerous variations of these catalysts disclosed and claimed in US Pat. later patents. Previous uses of multiple temperature zones in the production of the primordial olefins have used hydrocracking or hydrogenolysis to produce ethane and propane with little production of primordial olefins in the early stages. Franck et al., U.S. Patent No. 4,137,147, used multiple stages of hydrogenolysis, each stage being operated at 5-25 ° C more than the preceding stage. The light hydrocarbons, up to C3, in the effluent of the hydrogenolysis steps were then cracked into water vapor in primordial olefins while the production of C4 + was separated and at least part sent to further hydrogenolysis for production of additional ethane and propane. The steam cracking unit was supplied with a fraction consisting essentially of ethane and propane for conversion to ethylene. Lionetti et al., U.S. Patent No. 4,388,175, disclose a two-stage system for production of aromatics from heavy oil. The second stage is operated at a higher temperature than the first to produce light naphthas, gasoline and needle coke. There was no indication of any application to the production of primordial olefins. Tabak, U.S. Patent No. 4,487,985, and its divisional patent No. 4,560,536, teaches oligomerization of lower olefins in a series of multi-stage reactions where the partially inactivated catalyst in the first stage is employed at higher temperature in a secondary stage before the regeneration of the catalyst. In the European patent application 0 023 802, a step of hydrocracking produces C2 to C5 alkanes from which primordial olefins are produced by thermal cracking downstream at a higher temperature. The GB 2 patent, 105,362 teaches a two-stage thermal cracking process in a catalyst-free system, where the first reaction zone heats the feed / steam material from 800 to 1,000 ° C and then passes the feed material to a second zone. catalyst free where it is heated from 850 to 1,150 ° C. Mauleon et al., U.S. Patent Nos. 5,506,365 and 5,264,115, teach a multi-zone process where hot catalyst is used in a thermal cracking process with warm water vapor and further downstream is reacted with additional catalyst at a lower temperature in a process destined to the production of gasoline. European Patent Application 0 262 049 teaches hydrocarbon cracking in water vapor (propane is exemplified), followed by contact with a catalyst containing multiple component zeolite, with added metal oxides having a hydrogenation / dehydrogenation function. The thermal cracking unit is operated at a higher temperature than the catalytic cracking reactor. Adams, U.S. Patent No. 3,360,587, also teaches a steam cracking step followed by a catalytic cracking reactor, again the catalytic cracking reactor being at a lower temperature than the upstream catalytic cracking reagecpr. In the European patent application 0 023 802, a catalytic reaction step (hydrocracking) produces mainly C2 to C5 paraffins which are subsequently fed to a higher temperature optional thermal cracking unit for conversion into primary olefins. The published patent application WO 95/13255 discloses an integrated system where a light fraction is separated from the effluent of a deep catalytic cracking unit operating on a relatively heavy petroleum fraction and recycled to a thermal cracking unit to produce olefins primordial. The published patent application WO 86/02376 discloses heavy oil cracking including a pre-pyrolysis cracking step followed by separation of a thermal cracking head stream for production of primordial olefins. Burich, United States Patent No. 3,702,292, discloses an integrated refinery apparatus where various streams are separated and fed to both a hydrocracking unit and a thermal cracking unit. The document Der ent WPI access No. 88-053890 / 08 for Japanese patent 60235890 discloses thermal cracking of hydrocarbons in a two-stage system. Derwent WPI Access No. 86-011144 / 02 for Japanese Patent 63010693 discloses feeding secondary light oil containing olefins from a catalytic cracking unit to a thermal cracking furnace with primordial olefins recovered in high purity. • * Up to now, the state of the art has not recognized that a gradient of temperatures or temperature stages through multiple reactors with suitable catalysts can result in considerable increases in the production of primordial olefins without the previous separation of components or removal of C4 + materials from the feed stream and without the need to use cracking units in downstream water vapor or in recycle turns. SUMMARY OF THE INVENTION The present invention provides a process for improving the conversion of a hydrocarbon feedstock into light olefins, comprising the step of contacting a hydrocarbon feedstock with a cracking catalyst in a reactor operated at a Positive temperature gradient. In an alternate embodiment, the invention provides a process for producing ethylene and propylene in a catalytic cracking process where a hydrocarbon feedstock is contacted with a cracking catalyst, the improvement comprising providing a first contact zone of catalyst at a first temperature and providing a second catalyst contact zone at a second temperature, downstream of the first catalyst contact zone, and maintaining the second catalyst contact zone at a higher temperature than the first catalyst contact zone and passing the cracking hydrocarbon feedstock from the first catalyst contact zone to the second catalyst contact zone. In any of the embodiments, the downstream catalyst may be the same or a different catalyst to produce the desired product mixture. The second embodiment may comprise a plurality of reactor vessels while at least one downstream vessel is operated at a higher temperature than an upstream reactor, and both provide areas of catalytic cracking. Detailed Description of the Invention Definitions "" Light naphtha "means a distilled fraction of hydrocarbons that is predominantly C5 to C7 hydrocarbons" Virgin naphtha "means a distilled fraction of hydrocarbons obtained from crude oil or natural gas without additional conversion processing. "catalytic" means a distilled fraction of hydrocarbons obtained by the catalytic cracking of a heavier hydrocarbon fraction "BTX" means a mixture containing benzene, toluene and xylenes "Light olefins" or "primordial olefins" means ethylene, propylene or its mixtures "Improving the conversion" means producing an increase in production that is a higher yield of light olefins within the accuracy of the measurement system on the cracking of the same feed material with the same or the catalysts at a constant temperature. " Hydrocarbon feed material "means a stream of hydrocarbons comprising one or more hydrocarbons by breaking into fragments by thermal, chemical or catalytic action, the fragments forming light olefins.

"Positive temperature gradient" means a change in temperature from a lower temperature in a first place to a higher temperature in a second place, the second place being downstream of the first place. Reaction Conditions and Catalysts Substantial amounts of ethylene and propylene are produced by cracking hydrocarbon feedstocks such as light catalytic naphtha (LCN) or light virgin naphtha (LVN) on catalysts, particularly zeolite containing catalysts, containing ZSM-5 . The present invention provides a method for increasing the ethylene and propylene yields comprising contacting a feed stream with a catalyst in a reactor bed operated with a positive temperature gradient. Preferably, the feed stream is LCN or LVN, but any hydrocarbon stream that can be cracked by means of a catalyst can be used. Any cracking catalyst capable of being operated to selectively produce primordial olefins can be improved by using the reactor temperature gradient method. Suitable zeolites for use as a cracking catalyst are typically the acid form of any of the naturally occurring or synthetic crystalline zeolites, especially those having a silica-alumina ratio in the range of about 2.0: 1 to 2,000: 1. In general, any catalyst capable of cracking hydrocarbons in light olefins, having an improved conversion through the use of a temperature gradient, is suitable for use in the process. Using the simple bank test described below, a person skilled in the art can quickly determine if a catalyst exhibits improved conversion by temperature stages. Examples of zeolites useful in the claimed process include gallium silicate zeolites such as those described in U.S. Patent No. 5,096,686, beta zeolite, rho zeolite, ZK5, titanosilicate, ferrosilicate; borosilicate zeolite; zeolites designated by the letters X, Y, A, L by the Linde division of Union Carbide (these zeolites are described in U.S. Patent Nos. 2,882,244, 3,130,007, 3,882,243, and 3,216,789, respectively); crystalline zeolites occurring naturally such as phillipsite, ferrierite, mazzita, heulandite, faujasite, chabazite, erionite, mordenite, offretite, gmelinite, analcita, etc., and ZSM-5, as described in United States Patent No. 3,702,886 . Particularly suitable catalysts are found between the medium and small pore zeolites. It is considered that such medium pore zeolites have a constriction index of about 1 to about 12. The method by which the constriction index is determined is fully described in U.S. Patent No. 4,016,218. Zeolites that conform to the specified values of the constriction index for medium pore zeolites include ZSM-5, ZSM-11, intermediate ZSM-5 / ZSM-11, ZSM-12, ZSM-21, ZSM-22, ZSM- 23, ZSM-35, ZSM-38, ZSM-48, ZSM-50, MCM-22 and zeolite β, which are described, for example, in U.S. Patent Nos. 3,702,886 and Re. No. 29,949, 3,709,979 , 3,832,449, 4,046,859, 4,556,447, 4,076,842, 4,016,245, 4,229,424, 4,397,827, 4,954,325, 3,308,069, Re. 28,341 and EP 127,399, to which reference is made for details of these catalysts. These zeolites can be produced with different molar ratios of silica to alumina varying from 12: 1 upwards. In fact, they have been produced from reaction mixtures of which alumina is intentionally excluded, so as to produce materials having extremely high silica to alumina ratios which, theoretically, can at least extend to infinity. Preferred medium pore zeolites include ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-35 and MCM-22. Particularly preferred is ZSM-5. Small pore zeolites include crystalline aluminosilicate zeolites such as erionite, chabazite, phillipsite and their synthetic counterparts such as zeolites A and ZK5, as described in U.S. Patent Nos. 2,882,243 and 3,247,195, respectively. Preferably, the zeolite catalyst is selected from the group consisting of faujasite, chabazite, erionite, mordenite, offretite, gmelinite, analcita, phillipsite, ZSM-5, ZSM-11, intermediate ZSM-5 / ZSM-11, ZSM-12 , ZSM-21, ZSM-22, ZSM-23, ZSM-35, ZSM-38, ZSM-48, ZSM-50, MCM-22, gallium silicate zeolite, ß-zeolite, rho zeolite, ZK5, and titanosilicate, and zeolites having a molar ratio of silica to alumina within the range of about 2.0: 1 to 2,000: 1; ferrosilicate; borosilicate zeolite, as described in the Belgian patent No. 859656 (4584091), and zeolites designated by the letters X, Y, A, L by the Linde division of Union Carbide. A specially favored zeolite is ZSM-5. The preparation of catalysts containing zeolite can be carried out as described in the preceding references or purchased from commercial supplies well known to those skilled in the art. The cracking process can be carried out with any conventional equipment, which can be a fixed bed, fluidized bed, such as a fluid bed system of elevator or dense or stationary fluid bed system, and with typical hydrocarbon feed streams . Preferably, the catalyst is contacted at a temperature in the range of 400 to 750 ° C; more preferably, in the range of 550 to 725 ° C; most preferably, in the range of 600 to 700 ° C. The rector's temperature gradient will preferably be in the range of 10 to 150 ° C, more preferably in the range of 50 to 100 ° C; most preferably, in the range of about 60 to 80 ° C. The gradient may be uniform across a portion of the catalyst bed, where each downstream portion is at a slightly higher temperature than the adjacent upstream portion. Alternatively, the gradient may not be uniform, where the temperature of a downstream zone is greater than the temperature of an upstream zone. The preference process is carried out at a space hourly speed in weight (WHSV) in the range of about 0.1 to about 100 hr "1, more preferably in the range of about 1.0 to about 50 hr" 1 , with the greatest preference in the range of around 1.0 to around 30 hr_1. Examples of hydrocarbon streams that can be used to obtain high yields of light olefins include ethane, propane, butane, naphthas, gas oils, Fischer-Tropsch liquids, and streams containing olefins or diolefins such as butenes, butadiene, naphtha cracked in water vapor , cracked catalytic naphtha and coke and kerosene reactor naphtha. A preferred feedstock is light catalytic naphtha (LCN) or light virgin naphtha (LVN). An alternative embodiment to a positive temperature gradient within a single reaction vessel is the use of a plurality of reactors in series with at least one downstream reactor maintained at a higher temperature than the upstream reactor. In this alternative embodiment, several reactors can be used to approximate a uniform gradient, or large temperature steps can be made between reactors producing olefins. The catalyst in each reactor can be the same or different, depending on the preferred mixture of products to be obtained. By selecting the catalyst and the magnitude of the temperature gradient, the selection between ethylene and propylene can be improved. Example 1 A series of runs in a small bank reactor was carried out in a light catalytic naphtha. A control run was conducted at 650 ° C, WHSV of 1.9 hr "" 1, on a fixed bed of 2.4 g of ZCAT40, a zeolite ZSM-5 commercially available from Intercat, Inc. of Sea Girt, New Jersey, United States. . Prior to cracking tests, ZCAT40 was subjected to water vapor with 100% water vapor, at 704 ° C, and an atmosphere for 16 hours, to age the catalyst. In the test runs, a water vapor to hydrocarbon weight ratio of 0.85 was used while controlling the reactor pressure at a gauge pressure of 6 psi. The effluent stream was analyzed by on-line gas chromatography. A 60 meter column, packed with fused silica in a Hewlett-Packard model 5880A GAS, double-FID, was used for the analysis. The test runs were conducted with the same feed material at a WHSV of 1.9 and 2.5 hr-1 on two fixed beds, the first stage at 610 ° C and the second at 680 ° C. The data for yields of the key product are illustrated in Table 1. The positive temperature gradient of 65 ° C results in an increase to a yield of 32.0% by weight for ethylene, compared to an average of about 26.9% in weight at 650 ° C uniform. The yield of propylene in this run is slightly lower, about 21.7% by weight, compared to 23.2% by weight at the uniform temperature. The run at a WHSV of 2.5 hr "1 and slightly less catalyst shows that the selectivity for propylene is a function of the temperature and residence time (other factors being constant) In the run at a WHSV of 2.5 hr-1 , the yield of ethylene fell to 28.3% while the propylene was raised to 30.9%, with a small punishment in the conversion, reflecting the shorter residence time.This data shows the advantage of the invention over the reactions at uniform temperature of the state of the technique.

Table 1 Effect of the Temperature Gradient on Key Product Returns Example 2 An additional series of runs was conducted in a small bank reactor, as described for Example 1, in the hexane model compound. The conditions were as described in Example 1, except that the WHSV was 12 hr-1. The manometric pressure of the reactor was 6 psi while the ratio of water vapor to hydrocarbons was maintained at 0.33. As can be seen from Table 2, there are considerable benefits in the yield of light olefins from staging the reaction temperature of the catalyst bed. When the first half of the bed was operated at 610 ° C and the second half at 680 ° C, the ethylene yield was 28.9% by weight, while the yield of propylene was 31.9% by weight. These yields compare favorably with the yields obtained when the bed was operated in an isothermal manner, which ranged from 15.6 to 26.4% by weight for ethylene and 21.1 to 23.5% by weight for propylene. Table 2 The preceding examples are presented to illustrate the invention and not as limitations. There are many variations of the invention that will be apparent to those skilled in the art. The invention is defined and limited by the claims set forth below.

Claims (18)

1. CLAIMS 1. A process for improving the conversion of a hydrocarbon to light olefins, comprising: contacting a hydrocarbon feedstock with a cracking catalyst in a reactor operated at a positive temperature gradient.
2. 2. In a process for producing ethylene and propylene in a catalytic cracking process, where a hydrocarbon feedstock is contacted with a cracking catalyst, the improvement comprising providing a first catalyst contact zone at a first temperature and providing a second catalyst contact zone at a second temperature downstream of the first catalyst contact zone and maintaining the second catalyst contact zone at a higher temperature than the first catalyst contact zone and passing a feed material of cracked hydrocarbons from the first catalyst contact zone to the second catalyst contact zone.
3. 3. In a process for producing ethylene and propylene in a catalytic cracking process, where a hydrocarbon feedstock is contacted with a cracking catalyst, the improvement comprising providing a first catalyst contact zone at a first temperature and providing a second catalyst contact zone at a second temperature downstream of the first catalyst contact zone and maintaining the second catalyst contact zone at a higher temperature than the first catalyst contact zone and passing the catalyst feed material. cracked hydrocarbons from the first catalyst contact zone to the second catalyst contact zone, wherein the first catalyst contact zone is at a temperature in the range of from about 500 to about 750 ° C, the second contact zone of catalyst is at a temperature in the range of 10 to 150 ° C more than the temperature of the first contact zone of catalyst, and the feed material flows to a space velocity hour by weight in the range of about 0.1 to about 100 WHSV.
4. 4. The process of claim 2 or 3, wherein the first catalyst contact zone is at a temperature in the range of about 500 to about 750 ° C, the second catalyst contact zone is at a temperature in the range from 10 to 150 ° C more than the temperature of the first catalyst contact zone, and the feed material flows to a space velocity hour by weight in the range of about 0.1 to about 100 hr "1. The process of claim 2 or 3, wherein the catalyst is contacted at a first catalyst contact zone temperature within the range of 600 to 700 ° C and the feed material flows to a space velocity hour by weight in the range from 1.0 to 30 hr "1. 6. The process of claim 2 or 3, wherein the temperature increase between the first catalyst contact zone and the second catalyst contact zone forms a uniform gradient. The process of claim 2 or 3, wherein the temperature increase between the first catalyst contact zone and the second catalyst contact zone forms a non-uniform gradient. The process of any of the preceding claims, wherein the cracking catalyst comprises a zeolite having a silica to alumina ratio in the range of about 2.0: 1 to 2,000: 1. The process of any of the preceding claims, wherein the zeolite is selected from the group consisting of phillipsite, heulandite, mazzite, ferrierite, faujasite, chabazite, erionite, mordenite, offretite, gmelinite, analcita, ZSM-5, ZSM-11 , intermediate ZSM-5 / ZSM-11, ZSM-12, ZSM-18, ZSM-21, ZSM-22, ZSM-23, ZSM-35, ZSM-38, ZSM-48, ZSM-50, MCM-22, gallium silicate zeolite, zeolite ß, rho zeolite, ZK5, titanosilicate, ferrosilicate; borosilicate; zeolites designated by the letters X, Y, A, L by the Linde division of Union Carbide. The process of claims 1 to 7, wherein the catalyst comprises ZSM-5. The process of claims 1 to 7, wherein the catalyst comprises ZCAT40. 12. The process of any of the preceding claims, wherein the feedstock is selected from the group consisting of ethane, propane, butane, naphthas, gasoils, Fischer-Tropsch liquids, butenes, butadiene, naphtha cracked in steam, cracked catalytic naphtha , coke reactor naphtha, and kerosene. The process of any of the preceding claims, wherein the catalyst is contacted in a reactor having a catalyst contact inlet zone and a catalyst contact outlet zone, and the temperature of the contact inlet zone of catalyst is within the range of 500 to 750 ° C and the catalyst contact exit zone is maintained at a temperature in the range of 10 to 150 ° C higher than the temperature of the catalyst contact inlet zone and the Feeding material flows to a space velocity hour by weight in the range of 0.1 to 100 hr. "1 14. The process of any of the preceding claims, wherein the catalyst is brought into contact at an inlet temperature within the range of 550. at 725 ° C. 15. The process of any of the preceding claims, wherein the flow of the feedstock is within the range of a space velocity hour by weight of 1.0 to 50 hr "1. 16. The process of any of the preceding claims, wherein the catalyst is contacted at an inlet temperature in the range of 600 to 700 ° C. 17. The process of any of the preceding claims, wherein the flow of feedstock is in the range of a space velocity hour by weight of 1.0 to 30 hr. "18. The process of any of the preceding claims, wherein the Increase in temperature forms a uniform gradient.