OLEFINS INTERCONVERSION AND ETHERIFICATION PROCESS

Abstract: A process for producing liquid fuel from olefins comprising converting an olefinic hydrocarbon feedstock rich in C2+ n-alkenes over a catalyst comprising an intermediate-pore molecular sieve under mild olefin interconversion conditions in an olefin interconversion zone (115) to a product stream containing C4-C6 alkenes rich in isoalkenes, C7+ olefinic gasoline boiling range hydrocarbons, and unconverted hydrocarbons; reacting said C4-C6 alkenes with a stoichiometric excess of lower aliphatic alcohol in the presence of an acid etherification catalyst in an etherification zone (112) to produce a mixture of tertiary alkyl ethers, a light hydrocarbon fraction containing unreacted alkenes, and unreacted alcohol; and converting a feed comprising said light hydrocarbon fraction, said unreacted alcohol, said unconverted hydrocarbons and said C7+ hydrocarbons over a catalyst comprising an intermediate-pore zeolite in oxygenate and oligomerization conversion zone (122) to a liquid product which includes C10+ distillate range hydrocarbons, aromatic and/or C5-C9 gasoline boiling range hydrocarbons.

Description: OLEFINS INTERCONVERSION AND ETHERIFICATION PROCESS This invention relates to a process for converting olefinic hydrocarbons to liquid fuels, in particular to an integrated process for the production of tertiary alkyl ethers and high octane gasoline by interconversion of lower olefins and the conversion of olefins to higher hydrocarbons. An important focus of current research into the production of high-octane gasoline is the blending of stocks with lower aliphatic alkyl ethers as octane boosters and supplementary fuels. C5-C7 methyl alkyl ethers, especially methyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME) , have been found particularly useful for enhancing gasoline octane. It is known that isobutylene may be reacted with methanol over an acidic catalyst to provide MTBE and isoa ylenes may be reacted with methanol over an acidic catalyst to product TAME. In these etherification processes a problem of major importance is that methanol is not totally converted. Separation of methanol from the etherification reaction product is made difficult by the tendency of methanol to form a very dilute azeotropic mixture with hydrocarbons, and by the strong solubility of methanol in both water and hydrocarbons. Due largely to these factors, the cost associated with conventional methanol separation and recycling in the etherification reaction represents approximately 30% of the cost of the total etherification process. US-A-4,684,757 discloses that the well-known ability of zeolite type catalyst to convert methanol to olefins may be utilised by directing unreacted methanol from an etherification reaction to zeolite-catalysed conversion to olefin, thereby obviating the need to separate and recycle methanol. US-A-3,960,978 and 4,021,502 disclose conversion of C2-C5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Reaction conditions of moderate severity favour the conversion of olefins to predominantly gasoline boiling range products with little paraffins conversion. Milder reaction temperatures and high operating pressures can produce distillate range fuels as well from lower olefins. US-A-4,150,062, 4,211,640, 4,227,992, 4,456,779, 4,497,968, 4,433,185, 3,931,349, 4,404,414, and CD. Chang, Catal. Rev.-Sci. Eng., 25, 1 (1983) describe processing techniques in this field which may employ methanol or olefins as feed. It is an object of the present invention to provide an improved process for the production of liquid fuel mixtures from olefin containing feedstock and lower alkyl alcohols by etherification and olefin conversion and interconversion reactions. A further object of the instant invention is an integrated liquid fuels process wherein the etherification reaction is conducted with excess alcohol but which eliminates the need to recycle excess alcohol in the etherification reactor and unreacted paraffins and olefins are converted to aromatics. We have now discovered that the process of interconverting lower olefins such as C2+ olefins to produce higher lower olefins rich in iso-olefins can be advantageously integrated with the process to produce tertiary alkyl ethers such as MTBE and TAME and processes to convert lower oxygenates and olefins to higher hydrocarbons. In the integrated process excess methanol used in the etherification reaction is not recovered and recycled to the etherification reaction but passed to an oxygenates and olefins or olefins and paraffins conversion zone, for conversion to liquid fuels or aromatics. Olefins from the interconversion step or the entire interconversion reaction effluent are used as feedstock to the etherification reaction either alone or in combination with another hydrocarbon feedstream rich in iso-alkenes. According to the present invention a process for producing liquid fuels from olefins comprises converting an olefinic hydrocarbon feedstock rich in C2+ n-alkenes over a catalyst comprising an intermediate-pore molecular sieve under mild olefin interconversion conditions to a product stream containing C4~Cg alkenes rich in isoalkenes, C7+ olefinic gasoline boiling range hydrocarbons, and unconverted hydrocarbons; reacting said C4~Cg alkenes with a stoichiometric excess of lower aliphatic alcohol in the presence of an acid etherification catalyst to produce a mixture of tertiary alkyl ethers, a light hydrocarbon fraction containing unreacted alkenes, and unreacted alcohol; and converting a feed comprising said light hydrocarbon fraction, said unreacted alcohol, said unconverted hydrocarbons and said C7+ hydrocarbons over a catalyst comprising an intermediate-pore zeolite to a liquid product which includes C10+ distillate range hydrocarbons, aromatics and/or C5-C9 gasoline boiling range hydrocarbons. The mixture of tertiary alkyl ethers is usually recovered as an ether-rich gasoline stream, and the mole ratio of lower alcohol to C4+ isoalkenes contacting the etherification catalyst is preferably between 10:1 and 1:1, more preferably between 1.4:1 and 1.1:1. Unreacted alcohol usually constitutes between 1 and 20 weight percent of the aliphatic content of the mixture produced by the etherification reaction. The C4-C5 alkenes rich in isoalkenes advantageously include isobutene, and in a favoured embodiment are co-fed to the etherification catalyst with fresh C4-C7 iso-olefins which may also include butenes. In fact isobutylene may constitute 10 to 50 weight percent of said fresh C4-C7 iso-olefins. The lower aliphatic alcohol is preferably methanol, and the ethers are of the formula R-O-R' in which R is methyl or ethyl and R* is tertiary butyl, pentyl, hexyl, heptyl, octyl and/or nonyl, preferably C5 to Cβ ethyltertiaryalkyl ethers such as MTBE and/or TAME. The unconverted hydrocarbons cofed with said light hydrocarbon fraction, unreacted alcohol and C7+ hydrocarbons can include paraffins, which are converted to aromatics. The mixture produced by the etherification reaction may be fractionated in a depentaniser or debutaniser coluπfo. to provide an overhead vapour stream rich in C5- or C4- lower olefin and containing a minor amount of excess unreacted methanol, and to provide a liquid product stream consisting essentially of C5+ or C5+ liquid hydrocarbon and C5+ methyl tertiary alkyl ether. The etherification catalyst is suitably a sulfonic acid resin, the reaction over it being conducted at 40 to 80°C. The intermediate-pore molecular sieve is preferably a zeolite, in particular one characterised by a Constraint Index of 1 to 12 and a silica/alumina mole ratio of at least 12, preferably 12 to 1600. The most preferred zeolites are of silica/alumina mole ratio 30 to 70 and are exemplified by zeolite ZSM-5 containing hydrogen cations. The olefin interconversion conditions typically include a temperature of 200 to 400°C and a pressure of 100 to 1500 kPa. The conditions under which the feed comprising light hydrocarbon, unreacted alcohol, unconverted hydrocarbons and C7+ hydrocarbons is subjected to conversion may vary according to the product sought. Reaction regimes adopted in various embodiments of the invention comprise a temperature of 300 to 750°C and a pressure from atmospheric to 3600 kPa; a temperature of 225 to 325°C and a pressure of 4000 to 8000 kPa; a temperature of 204 to 500°C and a pressure of 420 to 2100 kPa. From another aspect, a process for producing liquid fuel mixtures from olefin feedstock and lower alcohols by multistage etherification, olefin interconversion and oligo erisation reactions, comprises the steps of: (a) contacting an olefinic hydrocarbon feedstock rich in C2+ n-alkenes with acidic, medium pore metallosilicate particles in an olefin interconversion i zone under mild olefin interconversion conditions whereby a first effluent stream comprising C4-C6 alkenes rich in iso-alkenes, a second stream comprising C7+ olefinic gasoline boiling range hydrocarbons and a third stream comprising unconverted hydrocarbons is produced; (b) reacting said first stream with a stoichiometric excess of lower aliphatic alcohols in the presence of an acid etherification catalyst under reaction conditions effective to produce a mixture of tertiary alkyl ethers; (c) recovering a light hydrocarbon fraction containing unreacted alkenes from the reaction effluent of step (b) along with unreacted alcohol; (d) contacting the recovered light hydrocarbon and alcohol fraction from step (c) and step (a) second and third stream with an acid oligomerisation and oxygenate conversion catalyst to convert at least a portion of said unreacted alkenes and alcohol to heavier liquid hydrocarbon product, including C_Q+ distillate range hydrocarbons, aromatics and/or C5-C9 gasoline boiling range hydrocarbons. Optionally, alkanes may also be converted in step (d) as well as unreacted alkenes. Figure 1 is a schematic drawing of the process flow diagram of an embodiment of the invention. In the preferred embodiment of the invention methanol is reacted with a hydrocarbon feedstock containing olefins and particularly isoolefins such as isobutene to produce methyl tertiary butyl ethers and other ethers. Methanol may be readily obtained from coal by gasification to synthesis gas and conversion of the synthesis gas to methanol by well-established industrial processes. As an alternative, the methanol may be obtained from natural gas by other conventional processes, such as steam reforming or partial oxidation to make the intermediate syngas. Crude methanol from such processes usually contains a significant amount of water, usually in the range of 4 to 20 wt%. The etherification catalyst employed is preferably an ion exchange resin in the hydrogen form; however, any suitable acidic catalyst may be employed. Varying degrees of success are obtained with acidic solid catalysts such as sulfonic acid resins, phosphoric acid modified kieselguhr, silica-alumina and acid zeolites such as Zeolite B. Typical hydrocarbon feedstock materials for etherification reactions include olefinic streams, such as FCC light naphtha and butenes rich in iso-olefins. These aliphatic streams are produced in petroleum refineries by catalytic cracking of gas oil or the like. The major reaction units are operatively connected in a synergistic combination whereby etherification reaction effluent is utilised to provide reactive olefins for zeolite catalysed conversion in conjunction with olefin interconversion products. Isomerisation, oligomerisation, alkylation and aromatisations reactions may be controlled in the acidic zeolite catalysis zone to obtain a desirable distribution of normally liquid hydrocarbons useful in making gasoline and/or distillate range fuels or aromatics such as BTX for petrochemical feedstock. Advantageously, at least a portion of the gasoline range hydrocarbons are recovered with C5+ etherate octane enhancers useful in quality motor fuels. MTBE and TAME are preferred ethers. The C4-C6 alkene fraction from the olefin interconversion zone is utilised as etherification feedstock, optionally with fresh feedstock rich in C4-C6 iso-olefins. The reaction of methanol with iso-butylene and iso-a ylenes at moderate conditions with a resin catalyst is established technology, described by R.W. Reynolds, et al in The OjLJ. and gas Journal, June 16, 1975, S. Pecci and T. Floris in Hydrocarbon processing. December 1977, and J.D. Chase, et al, in The Oil aμά Gas Journal. April 9, 1979, pages 149-152. A preferred catalyst is a bifunctional ion exchange resin which etherifies and isomerises the reactant streams. A typical acid catalyst is Amberlyst 15 sulfonic acid resin. MTBE and TAME are known to be high octane ethers. The article by J.D. Chase, et al., 0 _and.,gg5 _l_-_i_l___-l t April 9, 1979, discusses the advantages one can achieve by using these materials to enhance gasoline octane. The octane blending number of MTBE when 10% is added to a base fuel (R+O = 91) is about 120. For a fuel with a low motor rating (M+O = 83) octane, the blending value of MTBE at the 10% level is about 103. On the other hand, for an (R+O) of 95 octane fuel, the blending value of 10% MTBE is about 114. Processes for producing and recovering MTBE and other methyl tertiaryalkyl ethers from C4-C7 isoolefins are disclosed in US-A-4,544,776 and 4,603,225. Suitable extraction and distillation techniques for recovering ether and hydrocarbon streams from etherification effluents are likewise well known in the art. In the conversion of oxygenate and/or olefins to heavier hydrocarbons by catalytic oligomerisation using an acid crystalline zeolite, such as ZSM-5, process conditions can be varied to favour the formation of either gasoline, distillate, lube range products or aromatics. In the MOGD process (the subject of the aforenoted US-A- 4,456,779. 4,497,968 and 4,433,185) light olefins are oligomerised to high molecular weight distillate range olefins over ZSM-5, olefin molecular weight growth through a sequence of oligomerisation and cracking reactions being thermodynamically forced at relatively high pressures of about 4000 to 8000 kPa (800 psia) and relatively low temperatures of about 225 to 325°C. At much lower pressure and higher temperature, thermodynamics restrict the olefin distribution to low molecular weight. This is the basis for the olefin interconversion process, i.e., to operate under conditions where lower olefins, such as C2- C4 olefins can be converted to an equilibrium distribution of olefins with iso-butenes and iso-pentenes maximised. The olefin interconversion process as utilised in the present invention can use fixed bed, moving bed or fluid bed reactors containing intermediate pore size (0.5. to 0.7 π ) molecular sieve type catalysts which may be zeolites, such as ZSM-5, or silicoalu inophosphates, such as a SAPO. Operating conditions encompass temperatures between 200 and 400°C and low pressures, generally between 100 and 1500 kPa. Optionally, conversion conditions in the present invention may be controlled to favour the formation of aromatics, typified by the process known as M-2 Forming in which light aliphatic, paraffinic/olefinic feed can be converted under the conditions described in US-A- 3,760,242. The operating conditions are temperatures from 300 to 750°C and pressures from atmospheric to 3600 kPa (500 psig) , preferably about 500 to 600°C and 200 to 500 kPa. The reaction products comprise an effluent stream of light gases such as hydrogen and methane, unconverted light hydrocarbons and C5+ aromatics. The conversion of methanol, methanol equivalent, olefins and paraffins to gasoline, alkylated aromatics, and aromatics occurs under a broad range of operating conditions, and is preferably catalysed by intermediate pore size zeolite catalyst having acidic functionality. The preferred zeolites are characterised by a Constraint Index of 1 to 12 and a silica:alumina ratio of at least 12:1 and preferably higher, e.g, 30:1, 70:1, 500:1, 1600:1 or even higher. (The significance and manner of determination of Constraint Index are described in US-A- 3,998,889.) Preferred zeolites which have the specified values of Constraint Index and silica:alumina ratio include zeolites ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-48, defined respectively by the x-ray diffraction data set forth in US-A-3,702,886, 3,709,979, 3,832,449, 4,076,842 and 4,016,245 and EP-A-15132. The zeolite catalyst used is at least partly in the hydrogen form, e.g HZSM-5, but other cations, e.g. rare earth cations, may also be present. The zeolite may be activated by heating in an inert atmosphere at 500°C or more for 1 hour or more. The hydrogen form can be obtained by base exchange with ammonium salts followed by calcination, e.g. at 500°C in air. Other cations, e.g. metal cations such as Ga, Zn, Cu, Pt and Fe, can be introduced by conventional base exchange techniques. When a zeolite is employed as catalyst in the olefin interconversion stage of the invention it, too, will satisfy the aforesaid prescriptions. Referring to Figure 1, hydrocarbon stream 110 rich in C2+ normal olefins is passed to an olefins interconversion zone containing acidic ZSM-5 catalyst for the conversion of lower olefins to higher olefins rich in C4-C5 iso-olefins plus C7+ olefinic gasoline and unconverted hydrocarbons. The C4-C6 stream 111 is passed to etherification zone 112 together with a fresh iso- olefin C4-C6 stream 113 and methanol stream 114. The etherification zone effluent 116 is cooled in exchanger 117 and passed to debutaniser or depetaniser 118 where a hydrocarbon stream 119 rich in C5+ ethers is separated and an overhead stream 120 containing unreacted methanol and olefins. The overhead stream, in conjunction with C7+ olefinic steam 121 from interconversion zone 115 and, optionally, unreacted olefins containing interconversion light gas by-product from zone 115 are passed to oxygenates and olefins conversion zone 122 containing metallosilicate zeolite catalyst. Conversion zone 122 may be operated under oligomerisation conditions and/or aromatisation conditions to convert methanol and olefins or olefins/paraffins to higher hydrocarbons containing distillate or aromatics-rich hydrocarbons. Preferably, C5+ gasoline boiling range hydrocarbons 123 are produced rich in aromatics. Line 124 recovers C4- hydrocarbon products.

Claims: £____-_____ 1. A process for producing liquid fuels from olefins comprising converting an olefinic hydrocarbon feedstock rich in C2+ n-alkenes over a catalyst comprising an intermediate-pore molecular sieve under mild olefin interconversion conditions to a product stream containing C4-C5 alkenes rich in isoalkenes, C7+ olefinic gasoline boiling range hydrocarbons, and unconverted hydrocarbons; reacting said C4-C5 alkenes with a stoichiometric excess of lower aliphatic alcohol in the presence of an acid etherification catalyst to produce a mixture of tertiary alkyl ethers, a light hydrocarbon fraction containing unreacted alkenes, and unreacted alcohol; and converting a feed comprising said light hydrocarbon fraction, said unreacted alcohol, said unconverted hydrocarbons and said C7+ hydrocarbons over a catalyst comprising an intermediate-pore zeolite to a liquid product which includes CI Q + distillate range hydrocarbons, aromatics and/or C5-C9 gasoline boiling range hydrocarbons. 2. A process according to claim 1 wherein said mixture of tertiary alkyl ethers is recovered as an ether- rich gasoline stream. 3. A process according to claim 1 or claim 2 wherein the mole ratio of lower alcohol to C4+ isoalkenes contacting the etherification catalyst is between 10:1 and 1:1. 4. A process according to claim 3 wherein said ratio is between 1.4:1 and 1.1:1. 5. A process according to any preceding claim wherein said unreacted alcohol constitutes between 1 and 20 weight percent of the aliphatic content of said mixture produced by the etherification reaction. 6. A process according to any preceding claim wherein said C4-C6 alkenes rich in isoalkenes include isobutene. 7. A process according to any preceding claim wherein fresh C4-C7 iso-olefins are reacted, together with said C4-C6 alkenes, with the alcohol in the presence of the etherification catalyst. 8. A process according to claim 7 wherein said fresh C4-C7 iso-olefins include butenes. 9. A process according to claim 8 wherein isobutylene constitutes 10 to 50 weight percent of said fresh C4-C7 iso-olefins. 10. A process according to any preceding claim wherein said alcohol is methanol. 11. A process according to any preceding claim wherein said ethers are of the formula R-O-R* in which R is methyl or ethyl and R' is tertiary butyl, pentyl, hexyl, heptyl, octyl and/or nonyl. 12. A process according to any preceding claim wherein said ethers are C5 to Cβ methyltertiaryalkyl ethers. 13. A process according to any preceding claim wherein said ethers are MTBE and/or TAME. 14. A process according to any preceding claim wherein said unconverted hydrocarbons cofed with said light hydrocarbon fraction, unreacted alcohol and C7+ hydrocarbons include paraffins which are converted to aromatics. 15. A process according to any preceding claim wherein the mixture produced by said etherification reaction is fractionated in a depentaniser or debutaniser column to provide an overhead vapour stream rich in C5- or C4- lower olefin and containing a minor amount of excess unreacted methanol, and to provide a liquid product stream consisting essentially of C5+ or Cg+ liquid hydrocarbon and C5+ methyl tertiary alkyl ether. 16. A process according to any preceding claim wherein the etherification catalyst is a sulfonic acid resin. 17. A process according to any preceding claim wherein the reaction over the etherification catalyst is conducted at 40 to 80°C. 17. A process according to any preceding claim wherein said molecular sieve is a zeolite. 18. A process according to claim 17 wherein the intermediate pore size zeolite is characterised by a Constraint Index of 1 to 12 and a silica/alumina mole ratio of at least 12. 19. A process according to claim 18 wherein the zeolite has a silica/alumina mole ratio of 12 to 1600. 20. A process according to claim 19 wherein the silica/alumina mole ratio is 30 to 70. 21. A process according to any preceding claim wherein the zeolite is zeolite ZSM-5 containing hydrogen cations. 22. A process according to any preceding claim wherein said olefin interconversion conditions include a temperature of 200 to 400°C and a pressure of 100 to 1500 kPa. 23. A process according to any of claims 1 to 22 wherein said feed comprising light hydrocarbon, unreacted alcohol, unconverted hydrocarbons and C7+ hydrocarbons is subjected to conversion at a temperature of 300 to 750°C and a pressure from atmospheric to 3600 kPa. 24. A process according to any of claims 1 to 22 wherein said feed comprising light hydrocarbon, unreacted alcohol, unconverted hydrocarbons and C7+ hydrocarbons is subjected to conversion at a temperature of 225 to 325°C and a pressure of 4000 to 8000 kPa. 25. A-process according to any of claims 1 to 22 wherein said feed comprising light hydrocarbon, unreacted alcohol, unconverted hydrocarbons and C7+ hydrocarbons is subjected to conversion at a temperature of 204 to 500°C and a pressure of 420 to 2100 kPa.

Inventors: HARANDI MOHSEN NADIMI, HARANDI, MOHSEN, NADIMI
Applicants: MOBIL OIL CORP, MOBIL OIL CORPORATION
Application year: 1989
Main classification: C07C1
IPC codes: C07C41, C10G3, C10G29, C10G50, C10L1