MXPA00006839A - Process for the preparation of c5-/c6-olefins - Google Patents

Abstract

The production of 5C/6C olefins from a 4C olefin-containing product stream involves disproportionation of 1-, 2- and iso-butene on a Group VIb, VIIb or Sub-Group VIII metal-based catalyst followed by distillation to give a 4-6C fraction which is then distilled to give hexene and methylpentene as a high-boiling fraction. A process (A) for the production of 5C/6C olefins from 4C olefins involves:(a) disproportionation of 1-butene, 2-butene and isobutene in a 4C olefin product stream in presence of a disproportionation catalyst containing Group VIb, VIIb or Sub-Group VIII metal compound(s), to give a mixture of 2-6C olefins and butanes;(b) distillation to give a low-boiling fraction (A) comprising 2-3C olefins or 2-4C olefins and butanes, which is removed, and a high-boiling fraction consisting of 4-6C olefins and butanes;(c) distillation of the high-boiling fraction to give a low-boiling fraction (B;butenes and butanes), a middle fraction (C;pentene and methylbutene) and a high-boi ling fraction (D;hexene and methylpentene), and (d) separation of (D) as product, with complete or partial return of fractions (B) and/or (C) to stage (a). An Independent claim is also included for a process (B) for the production of 5C/6C olefins and propene from steam cracker streams or refinery 4C streams, involving:(1) removal of butadiene and acetylenes by optional extraction with a butadiene-selective solvent and subsequent or alternative selective hydrogenation of butadienes and acetylenic impurities in the crude 4C fraction, to give a product containing n-butene and iso-butene;(2) removal of isobutene by reaction of the above product with an alcohol in presence of acid catalyst to give an ether, with separation of the ether and alcohol to give a product containing n-butene and possibly oxygenated impurities, and removal of the ether or re-cleavage for the production of pure isobutene by subsequent distillation with optional separation of entrained 3C, iso-4C and 5C hydrocar bons;(3) removal of oxygenated impurities on suitable adsorbers;and (4) disproportionation of the refined product (II) stream as in stages (a)-(d) above.

Description

PREPARATION OF OLEFINS OF C5-C6 The present invention relates to a process for converting C 4 olefinic hydrocarbons, for example, from steam thermofractioners or FCC plants, into pentenes and hexenes by means of a metathesis reaction. Propene is obtained as the desired coproduct of the process. The olefinic metathesis (molecular disproportion) describes, in its simplest form, the reversible catalyzed metal catalyzed transalkylidenation of olefins by breaking and reforming the C = C double bonds according to the following equation: In the special case of the acyclic olefin metathesis a distinction is made between self-synthesis in which an olefin is transformed into a mixture of two olefins of different molar mass (for example: propene - ethene + 2-butene) and crossed metathesis or co-metathesis that describes the reaction of two different olefins (propene + l-butene -> ethene + 2-pentene). If one of the reactants is ethene, it is generally known as an ethenoisis.

The catalysts for suitable metathesis are, in principle, homogeneous and heterogeneous transition metal compounds, in particular of transition groups VI to VIII of the Periodic Table of the Elements, and also homogeneous and heterogeneous catalyst systems in which they are present these compounds. Different metathesis processes have been described starting from currents of C4. US 5,057,638 refers to a process for preparing 1-hexene, the process comprising the steps of: a) metathesis of l-butene to obtain a mixture of 1-hexene and ethene, b) separation of 3-hexene from the product mixture obtained in step a), c) reaction of 3-hexene with an el-ectrophile containing reactive hydrogen, preferably obtained from water or carboxylic acid, under acidic conditions that allow the addition of the electrophilic components in the C = C bond ( for example, hydration), and d) thermofracting the product of step c), for example, by dehydration, to prepare a mixture of N-hexenes in which l-hexene is present in economically acceptable amounts. US 69-821945 (May 5, 1969), Gulf Res. & Dev. Co. discloses the conversion of short chain C4-C12 olefins, preferably α-olefins, into higher olefins by metathesis. The process consists of putting the initial olefin in contact with a catalyst containing aluminum, molybdenum or rhenium and silver or copper from 100 to 240 ° C, with byproducts of relatively low boiling point, for example, ethene, being possible to be eliminated of the equilibrium in situ. The present invention furthermore relates to a combined process for preparing C5-C6 olefins together with propene as a by-product from the C4 fractions of the steam thermofractioners or FCC plants. Steam thermofractioners represent the main source of basic petrochemicals, for example ethene, propene, C4 olefins and higher hydrocarbons. In the thermo-fractionation process, it is necessary to introduce large amounts of energy at high temperatures within a sufficient time for the pyrolysis or thermo-fractionation to occur but not allow another reaction of the products of the thermo-fractionation. In the thermocracking of hydrocarbons, the yield of ethene and propene, therefore, is mainly determined by: • the type of hydrocarbons used (naphtha, ethane, LPG, gas oil and similar), • the temperature of the thermofraction, • the time of stay • and the partial pressures of the respective hydrocarbons. The highest yield of ethene and propene is obtained at thermo-fractionation temperatures from 800 to 850 ° C and residence times from 0.2 to 0.5 seconds. In this range the main product is always ethene, and the product ratio C3 / C2 can increase from about 0.5 to 0.7 by a slight variation in the conditions of the thermofraction. The global demand for propene increases faster than for ethene. This has the consequence, among others, that processes for downstream utilization of the higher hydrocarbons formed in the fractionation process, for example, C4 hydrocarbons, are becoming increasingly important with respect to the optimization of propene performance. An objective of the present invention, in the context of the work on the improvement of the added value derived from the by-products of the steam thermofraction, is to develop a catalytic process that can be controlled in a flexible manner to obtain pure C5-Ce olefins streams at Starting from mixtures of C4 hydrocarbons containing olefins, inexpensive, without the introduction of ethene. We have found that this objective is achieved through a process to prepare C5-C6 olefins from an initial olefinic stream containing C4 hydrocarbons, which consists of: a) performing a metathesis reaction in the presence of a metathesis catalyst containing less a compound of a transition group metal VIb, Vllb or VIII of the Periodic Table of the Elements to convert l-butene, 2-butene and isobutene present in the initial stream to a mixture of C2-C6 olefins and butanes , b) first fractionating the resulting product stream by distillation to obtain a fraction A of low boiling substances containing C2-C4 olefins and butanes, which is discharged, and a fraction of higher boiling point substances containing C4-C6 olefins and butanes, c) subsequently fractionating the high-boiling substances fraction from b) by distillation to obtain a fr action B of low-boiling substances containing butenes and butanes, and the C fraction of substances with an intermediate boiling point containing pentene and methylbutene, and a higher boiling fraction D containing hexene and methylpentene. d) where all or part of fractions B and / or C are recirculated to process step a), and fraction D is discharged as product. The term "containing" means the presence of relatively small amounts of other hydrocarbons. In this process, carried out in a single step, a fraction containing C4 olefins, preferably n-butenes, isobutene and butanes is converted onto a homogeneous or preferably heterogeneous metathesis catalyst in a mixture of butane (inert) products 1- unreacted butepo, 2-butene and possibly isobutene and also the products of the metathesis ethene, propene, 2-pentene, possibly 2-methyl-2-butene, 3-hexene and possibly 2-methyl-2-pentene in a reaction of metathesis according to the following equation: ^ X JM - x ^. Eíhep Propen 2-Pente .. 3-Hexßft l-Buten 2-Buten lsobuten * y * y ^. 2-Mett? Yl-2 -.uteri 2-Mßthy1-2-penten The amount of branched hydrocarbons in the metathesis product depends on the isobutene content and the C4 feed and is preferably kept as small as possible (< 3%). To explain the process of the present invention in its different variations in greater detail, the above equilibrium reaction (without taking isobutene into account) will be divided into three important individual reactions: 1. Cross-metathesis of l-butene with 2-butene [Kat] 1 -Buten 2-Butep Propßn 2-Penten Auto-synthesis of l-butene [Ka j ^ 1 -Buten Ethen 3-HexBn 3. Etheolysis of 2-butene ** and i = ík 2 ¿P ^ - 2-Buten Ethen Propen Depending on the prevailing demand for the target products propene, 2-pentene and 3-hexene (the term 2-pentene includes any of the isomers formed, for example cis / trans or 2-methyl-2-butene, and the same applies to 3-hexene), the balance of the external mass of the process can be influd in a directed way by shifting the equilibrium by recirculating the particular subcurrents.

Thus, for example, the yield of 3-hexene is increased when the cross-metathesis of l-butene with 2-butene is suppressed by the recirculation of 2-pentene in the metathesis step, so that cross-metathesis is consumed very little, if any, of 1-butene. The autoethesis of l-butene to 3-hexene which then proceeds preferentially forms additional ethene which reacts in a subsequent reaction with 2-butene to form the desired product propene. Mixtures of olefins containing l-butene, 2-butene and isobutene are obtained, inter alia, in different thermo-fractionation processes such as steam thermofraction or FCC as a C4 fraction. Otherwise, it is possible to use butene mixtures as obtained in the dehydrogenation of butenes or by dimerization of ethene. The butanes present in the C4 fraction behave as inert. The dienes, alkynes and eninos are separated before the metathesis step of the present invention by the customary methods such as extraction or selective hydrogenation. The butene content of the C4 fraction used in the process is from 1 to 100% by weight, preferably from 60 to 90% by weight. The butene content refers to l-butene, 2-butene and isobutene. Prefer is given to the use of a C4 fraction as obtained in steam thermofraction or FCC or in the dehydrogenation of butane. The refining I or Il ^ can * be used as the C4 fraction, and the C4 stream is freed of interfering impurities before the metathesis reaction by an adequate treatment on protective adsorbent beds, preferably on aluminum oxides of area of Elevated surface or molecular sieves. The low boiling point fraction A obtained, in particular the C2 / 3 fraction, can be directly processed as such, fed to the treatment sequ of a steam thermoformer or FCC plant to obtain pure ethene and propene, or it can be recirculated completely or in part to the metathesis step to increase the yield of pentene / hexene, or it can be used separately for the isolation of ethene and propene as a pure component (in particular, as a fraction of C2 / 3). The preferred metathesis reaction is carried out in the pres of heterogeneous, non-active or only slightly active metathesis catalysts for isomerization selected from the class consisting of transition metal compounds of the metals of groups VIb, Vllb and VIII of Periodic Table of the Elements applied to inorganic supports. The catalyst for the preferred metathesis is rhenium oxide on a support, preferably on an α -aluminium oxide or on combined supports AI2O3 / B2O3 / SiO2. Particular prefer is given to the use of Re2? 7 /? - Al2? 3 having a content of rhenium oxide from 1 to 20%, preferably from 3 to 15%, particularly preferably from 6 to 12% (% by weight) as catalyst. In the case of a liquid phase process, the preferred metathesis is carried out from 0 to 105 ° C, particularly preferably from 20 to 80 ° C, and a pressure from 2 to 200 bar, particularly preferably from 5 to 30 bar . When the metathesis is carried out in the gas phase, the temperature is preferably from 20 to 300 ° C, particularly preferably from 50 to 200 ° C. The pressure in this case is preferably from 1 to 20 bar, particularly preferably from 1 to 5 bar. Another objective of the invention, in the context of the work in the improvement of the added value to the byproducts of the steam thermoformer, is to develop a process sequence that can be controlled in a flexible way for the use of a C4 fraction. The objective is to add value by converting C4 olefins into olefinic fractions of higher price. The crude C4 fraction of steam thermofractioners or FC plants are available as raw materials for food.

We have found that this goal is achieved through a process to prepare C5-C6 olefins and propene from the steam thermofraction or refinery C4 currents, which consists of the sub-steps of: 1) eliminating butene and acetylenic compounds, if it is desired, by extracting butadiene with a butadiene selective solvent and then, or otherwise, selectively hydrogenating the butadienes and acetylenic impurities present in the crude C4 fraction to obtain a reaction product containing N-butenes and isobutene and essentially without butadienes and acetylenic compounds. 2) removing isobutene by reacting the reaction product of the preceding step with an alcohol in the presence of an acid catalyst to obtain an ether, and separating ether and alcohol simultaneously with or after etherification to obtain a product of reaction containing N-butenes and possibly oxygen-containing impurities, where the ether formed can be discharged or redissolved to obtain pure isobutene, and the etherification step can be followed by a distillation step to remove isobutene, where, if desired, the C3, i-C4 and C5 hydrocarbons introduced can be separated by distillation in the ether treatment, or the oligomerization or polymerization of isobutene from the reaction product of the previous step in the presence of an acid catalyst whose acidity is suitable for selectively separating Isobutene such as oligoisobutene or polyisobutene to obtain a stream containing from 0 to 15% isobutene residual ethene, 3) elimination of oxygen-containing impurities from the product of the previous steps on suitably selected adsorber materials, 4) metathesis of the resultant refining stream II as described. The sub-step of selective butadiene hydrogenation and acetylenic impurities present in the crude C4 fraction is preferably carried out in two steps by placing the crude C4 fraction in contact with a catalyst containing at least one metal selected from the group consisting of nickel, palladium and platinum on a support, preferably palladium on aluminum oxide, in the liquid phase from 20 to 200 ° C, a pressure from 1 to 50 bar, a mass flow from 0.5 to 30 m 3 of the fresh feed per m3 of catalyst per hour and a ratio of the recycle to the feed stream from 0 to 30 at a molar ratio of hydrogen to diolefins from 0.5 to 50 to obtain a reaction product in which, in addition to isobutene, n-butenes are present, 1-butene and 2-butene, in a molar ratio from 2: 1 to 1:10, preferably from 2: 1 to 1: 2, and in which essentially no diolefins or acetylenic compounds are present. The sub-step of the butadiene extraction of the crude C4 fraction is preferably carried out using a butadiene selective solvent selected from the class consisting of polar aprotic solvents such as acetone, furfural, acetonitrile, dimethylacetamide, dimethylformamide and N-methylpyrrolidone for obtaining a reaction product in which the n-butenes, l-butene and 2-butene are present in a molar ratio from 2.1 to 1:10, preferably from 2: 1 to 1: 2. The isobutene etherification sub-step is preferably carried out in a cascade of reactors in three stages using methanol or isobutanol, preferably isobutanol in the presence of an acid ion exchanger, in which the reaction mixture flows from the top downwards through the flooded fixed bed catalyst, where the reactor inlet temperature is from 0 to 60 ° C, preferably from 10 to 50 ° C, the outlet temperature is from 25 to 85 ° C, preferably from 35 to 85 ° C. 75 ° C, the pressure is from 2 to 50 bar, preferably from 3 to 20 bar and the ratio of isobutanol to isobutene is from 0.8 to 2.0, preferably from 1.0 to 1.5, and the total conversion corresponds to the conversion in the Balance. The sub-step of removing isobutene is preferably carried out by oligomerization or polymerization of isobutene starting from the reaction product obtained from the step described above of extraction and / or selective hydrogenation of butadiene and in the presence of a catalyst selected from the class consisting of in homogeneous and heterogeneous Brónsted acids, preferably heterogeneous catalysts containing a metal oxide of the transition group VIb of the Periodic Table of the Elements and an acid inorganic support, preferably O3TIO2, to produce a stream having a residual isobutene content of less than 15%.

Selective hydrogenation of the crude C4 fraction Alkynes, alkylennes and alkadienes are, because of their tendency to polymerize or their pronounced tendency to form complexes with transition metals, undesirable substances in many industrial syntheses. Sometimes these have a very strong adverse effect on the catalysts that are used in these reactions. The C4 stream of a steam thermoformer contains a high proportion of multiple unsaturated compounds such as 1,3-butadiene, 1-butyne (ethylacetylene) and butenin (vinylacetylene). Depending on the downstream processing, the multiple unsaturated compounds are extracted (butadiene extraction) or selectively hydrogenated. In the first case, the residual content of the multiple unsaturated compounds is usually from 0.05 to 0.3% by weight, while in the latter case it is usually from 0.1 to 4.0% by weight. Since the residual amounts of the multiple unsaturated compounds in the same way interfere in other processing, further purification by selective hydrogenation at values of < 10 ppm. To obtain the highest possible proportion of valuable butenes, over-hydrogenation to butanes should be kept as low as possible. Suitable catalysts for hydrogenation are described in: • J. P. Boitiaux, J. Cosyns, M. Derrien and G. Léger, Hydrocarbon Processing March 1985, p. 51-59 Description of bimetallic catalysts for selective hydrogenations of hydrocarbon streams of C2, C3, C4, C5 and Cs +. Particularly bimetallic catalysts containing metals of group VIII and group Ib show improvements in selectivity compared to pure Pd catalysts, with support DE-A-2 059 978 The selective hydrogenation of unsaturated hydrocarbons in the liquid phase on a Pd / alumina catalyst. To produce the catalyst, the alumina support having a BET surface area of 120 m / g is first subjected to a steam treatment at 300 ° C and subsequently calcined at 500-1200 ° C. Finally, the Pd compound is applied and the catalyst is calcined at 300-600 ° C.

EP-A-0 564 328 and EP-A-0 564 329 Catalysts containing, among others, Pd and In or Ga on supports. It is possible to use catalyst combination without addition of CO at high activity and selectivity.

EP-A-0 089 252 Pd, Au catalysts with support. The production of the catalysts comprises the following steps: impregnation of a mineral support with a compound of Pd calcination under gas containing O2 treatment with a reducing agent impregnation with a halogenated Au compound treatment with a reducing agent washing the halogen by means of a basic compound calcination under gas containing 0¿-, US 5,475,173 Catalyst containing Pd and Ag and alkali metal fluoride on an inorganic support. Advantages of the catalyst: the addition of KF provides a higher conversion of butadiene and better selectivity to butenes (ie, reduced hydrogenation to n-butane).

EP-A-0 653 243 In this catalyst, the active component is located mainly in the esoporos and macropores. The catalyst also has a large pore volume and a low packing density. Thus, the catalyst of Example 1 has a packing density of 383 g / 1 and a pore volume of 1.17 ml / g.

EP-A-0 211 381 Catalyst containing a group VIII metal (preferably Pt) and at least one metal selected from Pb, Sn or Zn on an inorganic support. The preferred catalyst contains Pt / Zn l? 4. The specified promoters Pb, Sn and Zn improve the selectivity of the Pt catalyst.

EP-A-0 722 776 Catalysts containing Pd and at least one alkali metal fluoride and, if desired, Ag on inorganic supports (AI2O3, TiO2 and / or Zr2). The combination of the catalyst makes possible a selective hydrogenation in the presence of sulfur compounds.

EP-A-0 576 828 Catalyst based on noble metal and / or noble metal oxide on AI2O3 supports having a definite X-ray diffraction pattern. The support consists of n-Al203 and / or? - l2? 3. Due to the specific support, the catalyst has a high initial selectivity and therefore can be used immediately for the selective hydrogenation of unsaturated compounds.

JP 01110594 Pd catalyst with support Another electron donor is also used. This is a metal deposited on the catalyst, for example, Na, K, Ag, Cu, Ga, In, Cr, Mo or La, or in addition the hydrocarbon feed, for example, alcohol, ether or N-containing compounds. described make it possible to achieve a reduction in isomerization of 1-butene.

DE-A-31 19 850 Catalyst comprising the support of SIO2 or AI2O3 having a surface area from 10 to 200 m / g or < 100 m / g and Pd and Ag as active components. The catalyst is used primarily for the hydrogenation of hydrocarbon streams having a low butadiene content.

EP-A-0 780 155 Catalyst containing Pd and a metal of group IB on an AI2O3 support, where at least 80% of the Pd and 80% of the metal of group IB are applied in an outer shell between ri (= radius of the granulate ) and 0.8-r ?.

Alternative: extraction of butadiene from the raw C4 fraction The preferred method for isolating butadiene is based on the physical principle of extractive distillation. The addition of selective organic solvents reduces the volatility of the specific components of a mixture, in this case butadiene. These, therefore, remain together with the solvent in two residues of the distillation column, while the accompanying substances that could not have been previously separated by distillation can be eliminated in the upper part. The main solvents used for the extractive distillation are acetone, furfural, acetonitrile, dimethylacetamide, dimethylformamide (DMF) and N-methylpyrrolidone (NMP). Extractive distillations are particularly useful in the case of C-fractions rich in butadiene having a relatively high proportion of alkynes, for example methylacetylene, ethylacetylene and vinylacetylene, and also methylalene. The simplified principle of a solvent extraction of the crude C4 fraction can be described as follows. The fully vaporized C4 fraction is fed to the lower end of an extraction column. The solvent (DMF, NMP) flows down from the top in countercurrent to the gas mixture and on its way drags down the more easily soluble butadiene and small amounts of butenes. At the lower end of the extraction column, part of the isolated pure butadiene is introduced to distill most of the butenes. The butenes leave the separation column at the top. In another column, known as degassing column, butadiene is separated from the solvent by boiling and then distilled to recover it in pure form.

The reaction product of an extractive butadiene distillation is usually fed to the second step of a selective hydrogenation to reduce the residual butadiene content to values of < 10 ppm. The current of C4 that remains after the separation of butadiene is known as C4 refined or refined I and consists mainly of the components isobutene, l-butene, 2-butene and also n-butane and Isobutane.

Separation of isobutene from refining I In the other fractionation of the C4 stream, isobutene is then preferably isolated since it differs in terms of its branching and its greater reactivity from the remaining C4 components. One possibility is to separate it by means of selective molecular sieves of the form, which makes it possible to isolate isobutene having a purity of 99% and in which the n-butenes and butane adsorbed in the pores of the molecular sieve can be desorbed again by means of a relatively high boiling hydrocarbon, but the separation is usually carried out by distillation using a deisobutenizer by means of which isobutene is separated together with l-butene and isobutene in the upper part and 2-butenes and n-butane including residual amounts of isobutene and l-butene remains in the waste, or extractively by reaction of Isobutene with alcohols on acidic ion exchangers. In this last method preference is given to the use of methanol (- >; MTBE) or isobutanol (IBTBE). The preparation of MTBE from methanol and isobutene is carried out from 30 to 100 ° C and slightly superatmospheric pressure in liquid phase on acidic ion exchangers. This is done in two reactors or in a two-stage shot or well reactor to obtain almost quantitative isobutene conversion (> 99%). Due to the pressure-dependent formation of a methanol / MTBE azeotrope, the isolation of pure MTBE requires the use of multi-stage pressure distillation or is obtained using more recent technology including adsorption of methanol on adsorbent resins. All other components of the C4 fraction remain unchanged. Since small amounts of diolefins and acetylenes can shorten the life of the ion exchanger as a result of polymer formation, preference is given to the use of bifunctional Pd-containing ion exchangers, on which only diolefins and acetylenes are hydrogenated in the presence of small amounts of hydrogen. This has no influence on the etherification of isobutene. MTBE is used first and foremost to increase the gasoline octane number. As an alternative, MTBE and IBTBE can be redisolated in the gas phase from 150 to 300 ° C over acid oxides to obtain pure isobutene. Another possible way of separating isobutene from raffinate I is the direct synthesis of oligoisobutene / polyisobutene. Isobutene conversions of up to 95% can be obtained in this form on homogeneous and heterogeneous acid catalysts, for example, tungsten trioxide on titanium dioxide, to obtain a product stream having a residual isobutene content of not more than 5%.

Purification of the feed of the refining stream II on adsorbent materials In order to improve the operating life of the catalysts used in the subsequent metathesis step, it is necessary, as described above, to use a current purification step (protective bed) ) to separate catalyst poisons such as water, oxygen-containing compounds, sulfur or sulfur compounds or organic halides. The processes for adsorption and purification by adsorption are described, for example, in W. Kast, Adsorption aus der Gasphase, VCH, Weinheim (1988). The use of zeolite adsorbents is described in D. W. Breck, Zeolite Molecular Sieves, Wiley, New York (1974). The separation of acetaldehyde specifically from C3-C15 hydrocarbons in the liquid phase can be carried out as described in EP-A-0 582 901.

Selective hydrogenation of the raw C4 fraction In a two-stage process, the butadiene (1,2- and 1,3-butadiene) present in the crude C4 fraction of a steam thermoformer or a refinery is selectively hydrogenated first, then of which the alkynes and alkenines present in the C4 fraction are selectively hydrogenated. The C4 stream of a refinery can also, in one embodiment, be fed directly to the second step of the selective hydrogenation. The first hydrogenation step is preferably carried out on a catalyst containing from 0.1 to 0.5% by weight of palladium on aluminum oxide as support. The reaction is carried out in the gas / liquid phase in a fixed bed (descending flow mode) using a liquid circuit. The hydrogenation is carried out from 40 to 80 ° C and a pressure from 10 to 30 bar, a molar ratio of hydrogen to butadiene from 10 to 50 and an LHSV of up to 15 m 3 of fresh feed per m3 of catalyst per hour and a ratio of recycling to the feed stream from 5 to 20. The second hydrogenation step is preferably carried out on a catalyst containing from 0.1 to 0.5% by weight of palladium on aluminum oxide as support. The reaction is carried out in the gaseous / liquid phase in a fixed bed (descending flow mode) using a liquid circuit. The hydrogenation is carried out from 50 to 90 ° C and a pressure from 10 to 30 bar. A molar ratio of hydrogen to butadiene from 1.0 to 10, and a LHSV from 5 to 20 m 3 of fresh feed per m3 of catalyst per hour and a ratio of recycle to feed stream from 0 to 15. Hydrogenation is carried out under conditions X? ISOM low "under which occurs very little, if any, isomerization C = C of l-butene to 2-butene The residual butadiene content can be from 0 to 50 ppm, depending on the severity of the hydrogenation conditions The reaction product obtained in this way it is known as refined I and contains, in addition to isobutene, 1-butene and 2-butene in a molar ratio from 2: 1 to 1:10, preferably from 2: 1 to 1: 2.

Alternative: separation of butadiene from the crude C fraction by extraction The extraction of butadiene from the crude C4 fraction is carried out according to BASF technology using N-methylpyrrolidone.

The reaction product of the extraction, in one embodiment of the invention, is fed to the second step of the selective hydrogenation described above to remove residual amounts of butadiene, with precautions being taken to ensure that little, if any, isomerization of l-butene to 2-butene.

Separation of isobutene by etherification with alcohols In the etherification step, isobutene is reacted with alcohols, preferably with isobutanol, on an acid catalyst, preferably on an acid ion exchanger, to form ethers, preferably isobutyl terbutyl ether. In one embodiment of the invention, the reaction is carried out in a three stage reactor cascade in which the reaction mixture flows from the top downwards through flooded fixed bed catalysts. In the first reactor, the intake temperature is from 0 to 60 ° C, preferably from 10 to 50 ° C; the discharge temperature is from 25 to 85 ° C, preferably from 35 to 75 ° C, and the pressure is from 2 to 50 bar, preferably from 3 to 20 bar. At a ratio of isobutanol to isobutene from 0.8 to 2.0, preferably from 1.0 to 1.5, the conversion is from 70 to 90%. In the second reactor, the intake temperature is from Q to 60 ° C, preferably from 10 to 50 ° C; the outlet temperature is from 25 to 85 ° C, preferably from 35 to 75 ° C and the pressure is from 2 to 50 bar, preferably from 3 to 20 bar. The total conversion over the two stages increases from 85 to 99%, preferably from 90 to 97%. In the third and largest reactor, equilibrium conversion is achieved at equal admission and discharge temperatures from 0 to 60 ° C, preferably from 10 to 50 ° C. The etherification and separation of the ether formed is followed by the dissociation of the ether: the endothermic reaction is carried out on acid catalysts, preferably on heterogeneous acid catalysts, for example, phosphoric acid on a SiO2 support, at an inlet temperature of from 150 to 300 ° C, preferably from 200 to 250 ° C and an outlet temperature from 100 to 250 ° C, preferably from 130 to 220 ° C. When the C4 fraction of FCC is used, it should be expected that about 1% by weight of propane, about 30-40% by weight of isobutene and about 3-10% of C5 hydrocarbons can be introduced, and these can affect adversely affect the sequence of the subsequent process. Therefore, the possibility of separating these components by distillation is obtained in the ether treatment.

The reaction product obtained in this manner, known as refining II, has a residual isobutene content of 0.1 to 3% by weight. If relatively large amounts of isobutene are present in the product, for example when fractions of FCC C4 are used or when isobutene is separated by acid-catalyzed polymerization to polyisobutene (partial conversion), the stream of the remaining raffinate can, according to a embodiment of the invention, be subjected to a distillation before further processing.

Purification of the stream of the refining II on adsorbent materials The stream of the raffinate II 'obtained from the etherification / polymerization (or distillation) is purified on at least one protective bed comprising aluminum oxides-of high surface area, silica gels, aluminosilicates or molecular sieves. The protective bed serves to sedate the C4 stream and separate substances that can act as catalyst poisons in the subsequent metathesis step. The preferred absorbent materials are Selexsorb CD and CDO and 3Á molecular sieves and NaX (13 X). The purification is carried out in drying towers at temperatures and pressures that are chosen so that all the components are present in the liquid phase. If desired, the purification step is used to preheat the feed for the subsequent metathesis step. . The stream of the remaining refining II is almost free of water, of compounds containing oxygen, organic chlorides and sulfur compounds. When the etherification step is performed using methanol to prepare MTBE, the formation of dimethyl ether as a secondary component may make it necessary to combine a plurality of purification steps or use them sequentially. To maximize the yield of 2-pentene and 3-hexene, the following variants of the process of the present invention are preferred, which are shown in simplified schematic form in Figure 1, Figure 2, and Figure 3. For clarity, the reactions in each case are described without significant amounts of isobutene in the C4 feed. In the figures: C2 = ethene C3 = = propene C4 = = 1- and 2-butene C4 ~ = n- and i-butene C5 ° = 2-pentene c6 = = 3-hexene C4-Re = recycled from C4 n-Bu = n- butenes / 5 ~ R = recycled C4 / 5 C5 ~ Re = recycled C5 • Metathesis is two-stage distillation and partial recycling of C4. Insulation of 2-pentene and 3-hexene (Figure 1) The product stream from the metathesis reactor R, containing C2-C6 olefins and butanes, is fractionated in DI distillation to obtain a fraction containing ethene, propene and from 0 to 50% unreacted butenes and butanes, which may, if desired, be fed for the treatment sequence of a thermo-fractionator, and a high boiling fraction containing residual C4 and the 2-pentene and 3-hexene formed. This last fraction is distilled in a column D2 to obtain 2-pentene in the lateral intake and 3-hexene. Both streams are obtained in a purity of > 99% The fraction of C4 is taken in the upper part and recycled to the metathesis reactor R. The column D2 can also be designed as a column of separation plates. The reactor R and the column DI can be coupled to form a reactive distillation unit. To increase the yield of the C5-C6 olefins, the product in the upper part of the DI distillation column can, if required, be recycled to the metathesis reactor R.

• Metathesis step with two-stage distillation and recycling of partial C and C5 Maximum hexene yield (Figure 2) The product stream from the metathesis reactor R, containing C2-C6 olefins and butanes, is fractionated in the DI distillation for obtain a fraction containing ethene, propene and from 0 to 50% unreacted butenes and butanes, which can, if desired, be fed to the treatment sequence of a thermo-fractionator, and a high boiling fraction containing residual C4 and the -pentene and 3-hexene formed. This last fraction is distilled in a column D2 to obtain 3-hexene which is isolated in a purity of >; 99% The C4 fraction together with pentene is taken in the upper part and recycled to the metathesis reactor R. The reactor R and the columns DI and D2 can be coupled to form a reactive distillation unit. • Metathesis step with three-stage distillation / partial recirculation of C4 and C5 to maximize the hexene yield (Figure 3) The product stream from the metathesis reactor R, containing C2-C6 olefins and butanes, is fractionated into DI distillation to obtain a low boiling fraction containing ethene and propene, which can be fed to the treatment sequence of a thermofractionator from 0 to 50% unreacted butenes and butanes, which can, if desired, be fed to the sequence of the treatment of a thermo-fractionator, or, preferably, in the pure components ethene and propene in another distillation column D3, and a high boiling fraction containing olefins of C4 and butanes and the 2-pentene and 3-exine formed . This latter fraction is fractionated by distillation on a D2 column to obtain 3-hexene which is isolated in a purity of> 0.05. 99%, which can, if desired, be designed as a column of lateral collection or column with dividing wall, to obtain a low boiling fraction containing olefins of C4 and butanes, all or part of which can be recycled to the step of metathesis, an intermediate boiling fraction preferably containing 2-pentene, all or part of which can be recycled to the metathesis step, and a high boiling fraction containing the desired product 3-hexene (purity of> 99%) , which is downloaded. As catalysts, preference is given to the heterogeneous rhenium catalysts known in the literature, for example Re207 on? -l2? 3 or in mixed supports as SIO2 / AI2O3, B2O3 / SIO2 / AI2O3 or Fe2? 3 / l2? 3 having different metal contents. Despite the chosen support, the content of rhenium oxide is from 1 to 20%, preferably from 3 to 10%. The catalysts are used in freshly calcined form and do not require further activation (for example, by means of alkylating agents). The deactivated catalyst can be regenerated on different occasions by burning the coke residues above 400 ° C in a stream of air and cooling under an inert gas atmosphere. Less suitable, but nevertheless, can be used according to the present invention, are the homogeneous catalysts that are sometimes more active but have a significantly shorter operant life: K. J. Ivin, J. Organoment. Catal. A: Chemical 1998, 133, 1-16; K. J. Ivin. I.C. Mol, Olefin Metathesis and Metathesis Polymerization, 2nd edition, Academic Press, New York, 1996; G. W. Parshall, S. D. Ittel, Homogeneous, Catalysis 2nd edition, 1992, John Wiley & Sons, New York, Chichester, Brisbane, Toronto, Singapore, p. 217 ff; RH. Grubbs in Prog. Inorg. Chem., S. Lippard (editor), John Wiley & Sons, New York, 1978, vol 24, 1-50; R. H. Grubbs in Comprehensive Organomet. Chem., G. Wilkinson (editor), Pergamon Press, Ltd., New York, 1982, vol. 8, 499-551; D.S. Breslow, Prog. Polym. Sci. 1993, vol. 18, 1141-1195, and also catalysts for homogeneous metathesis which are stable to the protic medium and atmospheric oxygen, for example, the ruthenium-alkylidene compounds defined in the formula RuX (= CHR) (PRr3) 2 (R = R '= alkyl , aryl) described by RH Grubs et al., in WO 93/20111, WO 96/04289, WO 96/06185, WO 97/03096 and WO 98/21214 and also mixtures generated in situ from [Ru (n -aryl ) X2-22 / PR3 phosphines and diazo compounds RCHN2, whose suitability as catalysts for metathesis has been reported by AF Noels in J. Chem. Commun. 1995, 1127-1128. In comparison, heterogeneous catalysts, in particular molybdenum oxides, tungsten oxides and rhenium oxides on inorganic oxidic supports, which may have been previously treated with alkylating agents, are more often more sensitive to impurities in the feed. Its advantage over homogeneous catalysts having superior activity is the simplest catalyst regeneration, which is usually done by burning coke residues in a stream of air. The comparison of the heterogeneous catalysts to each other shows that Re2? 7 / Al2? 3 is active under very moderate reaction conditions (T = 20-80 ° C) while MO3 / SiO2 (M = Mo, W) become active only at temperatures above 100-150 ° C and therefore it is possible that isomerizations of C = C double bonds occur as side reactions. Other catalysts that may be mentioned are: • 'WO3 / SiO2, prepared from (C5H5) W (CO) 3C1 and Si0 J. Mol. Catal. 1995, 95, 75-83; • Three component system containing [MO (NO) 2 (OR) 2] n, SnEt4 and AICI3 in J. Mol. Catal. 1991, 64, 171-178 and J. Mol. Catal 1989, 57, 207-220; • Complexes of nitridomolybdenum (VI) [sic] of highly active pre-catalysts in J. Organomet. Chem. 1982, 229, C19-C23; • M0O3 and WO3 catalysts with heterogeneous SiO2 support in J. Chem. Soc. Faradey Trans. / 1982, 78, 2583-2592; • Mo catalysts with support in J. Chem. Soc. Faradey Trans, / 1981, 77, 1763-1777; • Precursor of the active tungsten catalyst in J. Am. Chem. Soc. 1980, 102 (21), 6572-6574; • Acetonitrile (pentacarbonyl) tungsten in J. Catal. 1975, 38, 482-484; • Trichloro (nitrosyl) olibdene (II) as a catalyst precursor in Z. Chem. 1974, 14, 284-285; • W (CO) 5PPH3 / EtAlCl2 in J. Catal. 1974, 34, 196-202; • WCl6 / n-BuLi in J. Catal 1973, 28, 300-303; • WCl6 / n-BuLi in J. Catal 1972, 26, 455-458; FR 2 726 563 03ReO [Al (OR) (L) xO] nRe03 where R hydrocarbon of C? -C4Q, n = 1-10, x = 0 or l and L = solvent, EP-A-191 0 675, EP- A-129 0 474, BE 899897 catalyst systems containing tungsten, two substituted phenoxide groups and four other ligands, including a halogen, an alkyl or carbene group, FR 2 499 083 catalyst system containing an oxo-tungsten transition metal complex, molybdenum or rhenium with a Lewis acid; US 4,060,468 catalyst system containing a tungsten salt and an oxygen containing aromatic compound, for example, 2,6-dichlorophenol and possibly molecular oxygen; BE 776,564 catalyst system consisting of a transition metal salt, an organometallic compound and an amine. To improve the cycle time of the catalysts used, especially supported catalysts, it is advisable to purify the feed using adsorber beds (protective beds). The protective bed serves to dry the current of C4 and separates substances that can act as catalyst poisons in the subsequent metathesis step. The preferred adsorbent materials are Selexsorb CD and CDO and also molecular sieves 3Á and NaX (13 X). The purification is carried out in drying towers at temperatures and pressures that are preferably chosen so that all the components are in the liquid phase. The purification step can also be used to preheat the feed for the subsequent metathesis step. It may be convenient to combine a plurality of purification steps with one another or use them sequentially. The pressure and temperature in the metathesis step are chosen so that all the reactants are present in liquid phase (usually from 0 to 150 ° C, preferably from 20 to 80 ° C, p = 2-200 bar). However, as an alternative, it may be convenient, particularly in the case of the feed streams having a relatively high isobutene content, to carry out the reaction in the gas phase and / or to use a catalyst having a relatively low acidity. In general, the reaction is completed after 1 second to one hour, preferably after 30 seconds to 30 minutes. This can be done continuously or in batches in reactors such as pressure gas vessels, abductor tubes or feeders or reactive distillation apparatus, giving preference to the abductor tubes.

Examples Example 1 Continuous experiment of the two-stage selective hydrogenation of the crude C fraction The crude C4 fraction having a composition of 43.7% of butadiene (including butenin and butyne), 14.3% of 1-butene, 7.8% of 2-butenes and 7.2% of n-butane is reacted with 175 1 standard / h over a heterogeneous catalyst of 0.3% Pd / Al2? 3 in a continuous abductor tube reactor with a fresh feed flow of 1 kg / h of the fraction of raw C4 and a circulation of 8.2 kg / h at an LHSV of 9.0 h at a reactor inlet temperature of 20 ° C. At a butadiene conversion of 95.2%, the first stage of selective hydrogenation under these conditions obtains a total butene selectivity of 99.6% and a l-butene selectivity of 56.5%. A common reaction product of the first stage of selective hydrogenation containing 0.61% butadiene (including butenino and butino) 26.9% of l-butene, 14.9% of 2-butenes and 11.6% of n-butane, is reacted with 16 1 standard / h of hydrogen over a heterogeneous catalyst of 0.3% Pd / Al2? 3 (H0-13L) in a continuous abductor tubular reactor to a fresh feed flow of 2.2 kg / h of a reaction product of the first stage and a circulation of 4.0 kg / h to a LHSV of 20 h ~ at a temperature of reactor inlet of 60 ° C and a reactor outlet temperature of 70 ° C. At a butadiene conversion of 99.2% and a yield of l-butene of 58.2%, under these conditions a refining stream I having a residual butadiene content of 48 ppm was obtained.

Example 2 Continuous experiment on the separation of isobutene by etherification with isobutanol In a three-stage reactor cascade, the refining of isobutanol is passed from top to bottom through a fixed bed flooded with acidic ion exchanger, with the ratio of isobutanol to isobutene in the feed being established at 1.2. The temperature of the reactor inlet is 40 ° C, the outlet temperature of the reactor is 65 ° C and the reaction pressure is 8 bar. The conversion of isobutene after the first stage is measured as 85%. In the second reactor with similar dimensions, the conversion increases to 95% at a reactor inlet temperature of 40 ° C, a reactor outlet temperature of 50 ° C and a reaction pressure of 8 bar. In the third, significantly longer reactor, the equilibrium conversion is achieved at a reactor inlet temperature and a reactor outlet temperature of 40 ° C in each case and a reaction pressure of 8 bar. The stream of the refining remaining under these conditions after separating isobutyl terbutyl ether by distillation has a residual isobutene content of 0.7%.

Example 3 Continuous experiment on the single-step metathesis of raffinate II After purification of the feed on a 13X molecular sieve adsorbent bed, a C4 fraction containing 43.5% l-butene, 36.2% 2-butene, 2.0 % of isobutene and 18.3% of butanes are passed continuously at a mass rate of 1300 g / h and a residence time of three minutes through an abductor tube containing the heterogeneous catalyst Re2? 7 / l2? 3 at 40 ° C and 10 bar (liquid phase). The reaction product is fractionated in a two-stage distillation sequence, with a low boiling phase of C2 / C3 / C4 containing 1.2% ethene, 38.7% propene, 31.3% butenes, 2.9% isobutene and 25.9% of butanes being taken at the top of the first column at 10 bar. The residues containing 28.0% of butenes, 1.3% of isobutene, 20.4% of butanes, 27.8% of 2-pentene and 21.9% of 3-hexene are subsequently passed to a second column operated at 2 bar, in which the fraction of Low boiling of C4 / C5 is taken at the top and everything is recycled to the metathesis reaction. The high boiling fraction obtained in two residues contains 99.5% of 3-hexene. The percentages are in each case in mass. The determined butene conversions are 91% with respect to l-butene and 50% with respect to 2-butene. The space-time yields determined were, on average, 700 g / 1 per hour of propene and 760 g / 1 per hour of hexene. .

Claims (11)

1. A process for preparing Cs-C6 olefins from an initial olefinic stream containing C4 hydrocarbons, which comprises: a) performing a metathesis reaction in the presence of a metathesis catalyst containing at least one metal group compound of transition VIb, Vllb or VIII of the Periodic Table of the Elements to convert the l-butene, 2-butene and isobutene present in the initial stream into a mixture of C2-C6 olefins and butanes, b) first fractionate the product stream resulting by distillation to obtain a fraction A of low boiling substances containing C-C4 olefins and C2-C3 butanes or olefins, which is discharged, and a higher boiling point fraction containing olefins of C-C6 and butanes, c) Subsequently fractionating the fraction of high-boiling substances from b) by distillation to obtain a fraction B of substances of point of low boiling containing butenes and butanes, and the C fraction of substances with intermediate boiling point containing pentene and methylbutene, and a higher boiling fraction D containing hexene and methylpentene. d) where all or part of fractions B and / or C are recirculated to process step a), and fraction D is discharged as product.

2. The process as claimed in claim 1, wherein the C4 fraction is obtained from a steam thermoformer or FCC plant or in the dehydrogenation of butane.

3. The process as claimed in claim 1 or 2, wherein the C4 fraction used is the raffinate I or II and is freed of interfering impurities by suitable treatment on adsorbent protective beds before the metathesis reaction.

4. The process "as claimed in any of claims 1 to 3, wherein the metathesis reaction is carried out in the presence of catalysts for heterogeneous metathesis selected from the class consisting of transition metal compounds of the metals of the groups VIb, Vllb and VIII of the Periodic Table of the Elements applied to inorganic supports

5. The process as claimed in claim 4, wherein the metathesis catalyst used is rhenium oxide on α-aluminum oxide or on mixed supports A1203 / B203 / SI02

6. A process for preparing C5 / C6 olefins and propene from the steam thermoformer or refinery C streams, comprises the substeps: 1) removing the butene and acetylenic compounds, if desired, by extracting butadiene with a butadiene selective solvent and then, or otherwise, selectively hydrogenate the butadienes and acetylenic impurities present in the crude C4 fraction to obtain the ener a reaction product containing N-butenes and isobutene and essentially without butadienes and acetylenic compounds. 2) removing isobutene by reacting the reaction product of the preceding step with an alcohol in the presence of an acid catalyst to obtain an ether, and separating ether and alcohol simultaneously or after etherification to obtain a product of reaction containing N-butenes and possibly oxygen-containing impurities, where the ether formed can be discharged or redissolved to obtain pure isobutene, and the etherification step can be followed by a distillation step to remove isobutene, where, if desired, the hydrocarbons of C3, C4 and C5 introduced can be separated by distillation in the treatment of ether, 3) elimination of oxygen-containing impurities from the product of the previous steps on appropriately selected adsorbent materials, 4) metathesis of the stream of the resulting refining II as it is claimed in any one of claims 1 to 6.

7. The process as claimed in claim 6, wherein the sub-step of selective hydrogenation of butadienes and acetylenic impurities present in the crude C fraction is carried out in two steps putting the fraction of crude C in contact with a catalyst containing at least one metal selected from the group consisting of nickel, palladium or platinum on a support, from 20 to 200 ° C, a pressure from 1 to 50 bar, a mass flow from 0.5 to 30 m3 of the fresh feed per m3 of catalyst per hour and a ratio of the recycle to the flow of feed from 0 to 30 at a molar ratio of hydrogen to diolefins from 0.5 to 50 to obtain a reaction product in which, in addition to isobutene, n-butenes, l-butene and 2-butene are present in a molar ratio from 2: 1 to 1:10, preferably from 2: 1 to 1: 2, and in which practically no diolefins or acetylenic compounds are present. The process as claimed in claim 6 or 7, wherein the sub-step of extracting butadiene from the crude C4 fraction is carried out using selective butadiene solvent selected from acetone, furfural, acetonitrile, dimethylacetamide, dimethylformamide and N-methylpyrrolidone to obtain a reaction product in which the n-butanes, l-butene and 2-butene are present in a molar ratio from 2: 1 to 1:10, preferably from 2: 1 to 1: 2. 9. The process as claimed in any of claims 1 to 8, wherein the isobutene etherification sub-step is carried out in a cascade of reactors in three stages using methanol or isobutanol in the presence of an acidic ion exchanger, in which the Extraction mixture flows from the top down through the flooded fixed bed catalyst, where the inlet temperature of the reactor is from 0 to 60 ° C, the outlet temperature is from 25 to 85 ° C, the pressure is from 2 to 50 bar and the ratio of isobutanol to isobutene is from 0.8 to 2.0, and the total conversion corresponds to the conversion at equilibrium. The process as claimed in any of claims 1 to 8, wherein the isobutene separation sub-step proceeds by oligomerization or polymerization of isobutene starting from the reaction product obtained from the above-described step of extraction and / or selective hydro-reduction. of butadiene in the presence of a catalyst selected from the group consisting of heterogeneous catalysts containing an oxide of a metal of the transition group VIb of the Periodic Table of the Elements on an acidic inorganic support to produce a stream having a residual isobutene content less than fifteen%. The process as claimed in claim 3 or 6, wherein the sub-step of purification of the feed is carried out on at least one protective bed consisting of aluminum oxides, silica gels, aluminosilicates or molecular sieves of high surface area . SUMMARY OF THE INVENTION A process for preparing C5-C6 olefins from an initial olefinic stream containing C hydrocarbons, which comprises: a) performing a metathesis reaction in the presence of a metathesis catalyst containing at least one metal group compound of transition Vlb, Vllb or VIII of the Periodic Table of the Elements to convert the l-butene, 2-butene and isobutene present in the initial stream in a mixture of C2-C6 olefins and butanes, b) first fractionate the product stream resulting by distillation to obtain a fraction A of low boiling substances containing C2-C4 olefins and C2-C3 butanes or olefins, which is discharged, and a fraction of higher boiling substances containing olefins of C4-C6 and butanes, c) subsequently fractionate the fraction of high-boiling substances from b) by distillation to obtain a -B fraction of punctate substances or low boiling containing butenes and butanes, and the C fraction of substances with an intermediate boiling point containing pentene and methylbutene, and a higher boiling fraction D containing hexene and ethylpentene. where all or part of fractions B and / or C are recirculated through process a), and fraction D is discharged as product.