Improved process for dehydrogenation of hydrocarbons

Abstract

The present invention relates to a process for dehydrogenating hydrocarbon chains in the presence of a catalyst in at least one reactor; the method comprises the step of adding a metal M, preferably zinc, upstream or at the inlet of the one or more reactors, said metal M being selected from the group consisting of zinc (Zn), tin (Sn), indium (In), zirconium (Zr), cerium (Ce), germanium (Ge), gallium (Ga), lead (Pb) and thallium (Tl).

Description

Technical Field

[0001] The present invention relates to an improved process for dehydrogenating hydrocarbon chains. The invention also relates to a composition making it possible in particular to utilize such a process. Background Art

[0002] The dehydrogenation of hydrocarbon chains, and in particular alkanes, is of great industrial interest because of the growing demand for dehydrogenated hydrocarbon chains (hereinafter referred to as alkenes or olefins), which are essential synthetic intermediates for the chemical industry.

[0003] The conversion of propane into propylene and isobutane into isobutylene is therefore of very particular interest to manufacturers. Propylene is primarily polymerized to give polypropylene (PP), a highly consumed polymer. It is used, for example, for the manufacture of molded parts in the automotive sector, food packaging, furniture fabrics, disposable work clothes or as an additive for concrete. Isobutylene itself can be used to produce methyl tert-butyl ether or polymerized to give polyisobutylene (PIB). This polymer is used to make inner tubes or tubeless tire linings. Secondary applications are varied: the manufacture of chewing gum, sealants, vibration damping putties, adhesives, coatings, cosmetics, insulating oils, etc.

[0004] The catalytic dehydrogenation of hydrocarbon chains has long been known, however, to date these catalytic processes have yields of between 50% and 85%. In view of the growing demand for olefins, it is desirable to increase these yields.

[0005] Therefore, there is a need to improve existing dehydrogenation processes, and in particular to improve their yields. There is a need for more efficient, more economical and easier to implement dehydrogenation processes. Summary of the invention

[0006] It is an object of the present invention to provide an improved process for dehydrogenating hydrocarbon chains, preferably with improved yields.

[0007] Another object of the present invention is to provide an improved dehydrogenation process which is easy to implement and in particular does not involve any modifications to previously existing industrial facilities.

[0008] It is an object of the present invention to provide a composition which is easy to use for the process operator and which allows the dehydrogenation reaction to be improved while being compatible with industrial facilities.

[0009] The present invention responds to the above objectives in whole or in part.

[0010] The inventors have found that the introduction of at least one metal M as defined below upstream or at the inlet of the dehydrogenation reactor improves the yield of the reaction and / or increases the life of the catalyst. In particular, the introduction of the metal M according to the invention makes it possible to obtain a yield increase of between 1% and 10%, preferably between 2% and 5%. The addition of the metal M enables a controlled improvement in the yield to be achieved, in particular by means of a specific and suitable formulation of said metal M.

[0011] Without wishing to be bound by theory, the introduction of metal M makes it possible to produce more active catalytic sites on the dehydrogenation catalyst. In addition, when the catalyst comprises several metals, the organization of the metal catalytic sites can prove difficult to control. Therefore, the addition of metal M according to the present invention makes it possible to improve and / or control the activity of the catalyst. The present invention can also make it possible to use and handle catalysts that are simpler and less toxic, which may be easier to regenerate.

[0012] The addition of the metal M separate from the catalyst, and optionally continuously, also makes it possible to combat the gradual deactivation of the catalyst, a problem well known to the manufacturers in this type of process.

[0013] The inventors have also found that the metal M in the form of an organometallic compound defined below improves the yield of the reaction while being perfectly adapted to the process and the installation in which it is employed. This is because the organometallic compound according to the invention can be dissolved in all or part of the reactants (that is to say in the hydrocarbon chain to be dehydrogenated) and / or in other additives introduced into the process.

[0014] In a very particularly preferred and surprising manner, these organometallic compounds can be dissolved in compositions that limit or prevent coke formation (especially on the catalyst) while maintaining a yield gain. Such compositions generally contain sulfur compounds that can passivate the metallurgy of the reactor and / or selectively poison the catalyst to limit or prevent coke formation.

[0015] Thus, an organometallic compound (such as according to the present invention) may be dissolved in a composition comprising a sulphur compound as defined below. Alternatively, a sulphur compound to which an organometallic compound is added may be mentioned. For example, dimethyl disulfide (hereinafter referred to as DMDS) to which an organometallic compound is added is mentioned.

[0016] These compositions present the advantage of being able to be introduced into the process at the point of introduction of the additives already present in the dehydrogenation unit and making it possible to control the metering of the introduced metal M. The dehydrogenation process can thus be improved without changing the installation, which is a great advantage for the producer. DETAILED DESCRIPTION

[0017] It should be noted that “from… to… (… to…)” and “(between)… and…” used in this specification should be understood to include each of the limit values ​​mentioned.

[0018] It is also stipulated that, unless expressly stated otherwise, ppm values ​​represent weight fractions.

[0019] The present invention relates to a process for preparing olefins, which can also be called a dehydrogenation process. The process comprises a stage of dehydrogenating a hydrocarbon chain in the presence of a catalyst in at least one reactor (hereinafter referred to as stage b));

[0020] The process comprises a stage of introducing the metal M upstream or at the inlet of the reactor (hereinafter referred to as stage a)), and

[0021] The metal M is selected from zinc (Zn), tin (Sn), indium (In), zirconium (Zr), cerium (Ce), germanium (Ge), gallium (Ga), lead (Pb) and thallium (Tl), preferably zinc.

[0022] The present invention also relates to a composition comprising:

[0023] ■ Sulfur compounds having the following general formula (I):

[0024] [Chemical formula 1]

[0025]

[0026] in:

[0027] - R is selected from linear or branched alkyl groups containing 1 to 4 carbon atoms and linear or branched alkenyl groups containing 2 to 4 carbon atoms,

[0028] -n is an integer equal to 0, 1 or 2,

[0029] - x is an integer selected from 0, 1, 2, 3 or 4, and

[0030] - R' is selected from a linear or branched alkyl radical containing 1 to 4 carbon atoms, a linear or branched alkenyl radical containing 2 to 4 carbon atoms and, only when n=x=0, is a hydrogen atom, and

[0031] ■The metal M is selected from zinc (Zn), tin (Sn), indium (In), zirconium (Zr), cerium (Ce), germanium (Ge), gallium (Ga), lead (Pb) and thallium (Tl), preferably zinc.

[0032] The invention also relates to the use of said metal M in a process for dehydrogenating hydrocarbon chains, preferably in order to increase the yield thereof.

[0033] The invention also relates to the use of said composition in a process for dehydrogenating hydrocarbon chains, preferably in order to increase the yield thereof.

[0034] Preferably, the metal M is in the form of an organometallic compound of the general formula (II) defined below.

[0035] The dehydrogenation of hydrocarbon chains is a well-known reaction (see, for example, WO 2020 / 081421 and EP 3 240770). This reaction allows the formation of C=C double bonds and in particular the production of monoolefins from alkanes and / or dienes from monoolefins. Preferably, alkanes are used to obtain monoolefins.

[0036] The term "hydrocarbon chain" is understood to mean in particular an alkane or an alkene. It may have between 2 and 30 carbon atoms, preferably between 2 and 10 carbon atoms.

[0037] The term "alkane" is understood to mean, in particular, a saturated, linear, branched or cyclic hydrocarbon chain, preferably a linear one.

[0038] The term "olefin" is understood to mean in particular an unsaturated hydrocarbon chain which may be linear, branched or cyclic, preferably a linear, monoolefin or diolefin.

[0039] The term "monoolefin" is understood to mean an unsaturated hydrocarbon chain comprising a single C=C double bond. The term "diolefin" is understood to mean an unsaturated hydrocarbon chain comprising two C=C double bonds.

[0040] The alkane to be dehydrogenated may contain between 2 and 30 carbon atoms, for example between 2 and 10 carbon atoms, preferably between 3 and 6 carbon atoms. Propane, butane, isobutane and mixtures thereof are preferred, and propane, isobutane and mixtures thereof are particularly preferred.

[0041] The monoolefin to be dehydrogenated may contain between 3 and 30 carbon atoms, for example between 3 and 10 carbon atoms, preferably between 3 and 6 carbon atoms. Mention may be made of but-1-ene.

[0042] The hydrocarbon chains may be in liquid or gaseous form or as a gas-liquid mixture. They may be introduced into the dehydrogenation reactor as a mixture with a diluent. The mixing with the diluent may be carried out upstream of the reactor, at the inlet or in the reactor. As diluents, hydrogen, steam, methane, ethane, carbon dioxide, nitrogen, argon or any other inert gas and mixtures thereof for the dehydrogenation reaction may be mentioned in particular. Preferably, hydrogen and steam are used. The molar ratio (diluent / hydrocarbon chain) generally used is between 0.1:1 and 40:1, preferably between 0.4:1 and 10:1.

[0043] The dehydrogenation reaction may be carried out at a temperature between 400°C and 900°C, preferably between 520°C and 650°C.

[0044] The dehydrogenation reaction may be carried out at a pressure between 0.01 and 10 bars, preferably between 1 and 6 bars, more preferably between 1 and 3 bars.

[0045] The catalyst used is a dehydrogenation catalyst, preferably a heterogeneous dehydrogenation catalyst, which in particular comprises a supported metal.

[0046] More particularly, it comprises at least one metal selected from Groups 6, 8, 9 and 10 (formerly Groups VIA and VIII) of the Periodic Table of the Elements. Preferably, the catalyst comprises a metal selected from chromium, platinum or palladium, more preferably platinum. The chromium, platinum or palladium present may be the main metal in the catalyst (by weight relative to all the metals contained in the catalyst). The catalyst may also comprise other metals such as rhenium, germanium or tin, for example one, two or three metals. The catalyst may also comprise a promoter, preferably a metal selected from alkali metals or alkaline earth metals, and preferably potassium.

[0047] According to one embodiment, the catalyst comprises less than 5% by weight, preferably less than 1% by weight, of metal M relative to the total weight of the catalyst before stage a) of introducing the metal M. In particular, the catalyst does not comprise zinc before stage a) of introducing the metal M.

[0048] The metals of the above catalysts may be present as elements or as compounds, for example in oxidized or reduced form.Such catalysts are known to those skilled in the art and are described, for example, in document US Pat. No. 3,723,557.

[0049] The support of the catalyst may be of any type known to those skilled in the art and is preferably selected from porous supports. For example, it may be selected from silica, alumina, silica-alumina, molecular sieves, titania and zirconia, preferably alumina.

[0050] A particularly preferred catalyst is one comprising platinum supported on alumina, preferably a catalyst consisting essentially of platinum supported on alumina.

[0051] Such catalysts may undergo activation, such as reduction and / or sulfidation, carried out in situ or ex situ, before using the catalyst in a dehydrogenation reaction. Such operations are known to those skilled in the art and may be carried out according to conventional techniques.

[0052] The reaction can be carried out in one or more reactors. The reactors can be placed in parallel or in series. The reactors can be fixed, mobile or fluidized catalytic bed reactors. Mobile catalytic bed reactors in series are preferably used.

[0053] In the presence of several reactors, a heating device can be installed between each reactor to ensure that the temperature of the reactants entering the reactor is the desired reaction temperature. In this case, the metal M can be introduced before or after the heating device, preferably before the heating device.

[0054] After the dehydrogenation reaction, the process according to the invention may comprise further stages until the olefins and hydrogen (H2) produced are recovered. It makes it possible to obtain in particular olefin-rich streams (hereinafter streams A and D) which can then be purified if necessary.

[0055] The stream S leaving the reactor may contain hydrogen, olefins and possibly unreacted hydrocarbon chains. When the reactor is a moving catalytic bed reactor, the heterogeneous catalyst can also be recovered separately from the exiting stream S at the reactor outlet.

[0056] Once the exit stream S has been collected, its separation can be performed by any technique known to those skilled in the art. For example, it can be cooled and then condensed to obtain:

[0057] - a gas stream H comprising hydrogen; and

[0058] - Stream A comprising olefins and possibly unreacted hydrocarbon chains, which is preferably liquid.

[0059] Stream A can then be collected and separated to obtain:

[0060] - a liquid stream D comprising olefins; and

[0061] - a liquid stream E comprising unreacted hydrocarbon chains.

[0062] Streams H and / or E can be recycled in whole or in part to the reaction stage. In the presence of several reactors, they can be reintroduced into one, several or all reactors. If necessary, liquid stream D is collected and then optionally purified.

[0063] The dehydrogenation process may also include a stage of regenerating the catalyst (called stage e)). This is because the catalyst may gradually become deactivated, in particular due to the formation of coke on the catalyst. Therefore, a regeneration stage may prove necessary in order to fully or partially restore its activity. This regeneration can be carried out according to conventional techniques.

[0064] For example, the catalyst may be regenerated by incinerating the coke (at at least 600°C) and then oxychlorinating to redisperse the catalytic metals. Oxychlorination is generally carried out by contacting the catalyst with a gas containing a halogen, preferably chlorine, at elevated temperatures (e.g. between 500°C and 550°C).

[0065] After the regeneration stage, the catalyst can be reduced by contact with hydrogen to reduce the metals oxidized during the regeneration. The regenerated and optionally reduced catalyst can be recycled to the reaction stage alone or as a mixture with new catalyst.

[0066] According to one embodiment, the process according to the invention comprises at least the following stages:

[0067] a) introducing the metal M upstream or at the inlet of the reactor;

[0068] b) dehydrogenating the hydrocarbon chains in a moving catalytic bed reactor to obtain an exit stream S comprising olefins and hydrogen;

[0069] c) taking and separating the exiting stream S from stage b) to obtain:

[0070] - a stream A comprising olefins, which is preferably liquid, and

[0071] - a gaseous stream H comprising hydrogen;

[0072] d) optionally recovering the catalyst;

[0073] e) optionally regenerating the catalyst;

[0074] f) optionally recycling the regenerated catalyst to stage b).

[0075] More particularly, the process according to the invention comprises at least the following stages:

[0076] a) introducing the metal M upstream or at the inlet of the reactor;

[0077] b) dehydrogenating the alkane in a moving catalytic bed reactor to obtain an exit stream S comprising olefins, unreacted alkane and hydrogen;

[0078] c) taking and separating the exiting stream S from stage b) to obtain:

[0079] a stream A comprising olefins and unreacted alkanes, which is preferably a liquid, and

[0080] - a gaseous stream H comprising hydrogen;

[0081] d) optionally recovering the catalyst;

[0082] e) optionally regenerating the catalyst;

[0083] f) optionally recycling the regenerated catalyst to stage b);

[0084] g) collecting and separating stream A to obtain:

[0085] - a stream D comprising olefins; and

[0086] - a stream E comprising unreacted alkanes;

[0087] h) optionally recycling stream E to stage b).

[0088] As indicated above, streams H and / or E can be recycled in whole or in part to the reaction stage. In the presence of several reactors, they can be reintroduced into one, several or all reactors.

[0089] The process according to the invention can be carried out batchwise or continuously, preferably continuously. Several dehydrogenation processes are therefore known and mention may be made of those mentioned in the publication "Propylene Production by Propane Dehydrogenation (PDH) (Process Review)" by Amir Razmi, 2019 (https: / / www.academia.edu / 39604411 / Propylene_Production_by_Propane_Dehydrogenation_PDH_Process_Review), such as the following processes: "Star Dehydrogenation" by ThyssenKrupp Uhde " and Lummus UOP-Honeywell"Oleflex TM ".

[0090] The metal M according to the present invention is selected from zinc (Zn), tin (Sn), indium (In), zirconium (Zr), cerium (Ce), germanium (Ge), gallium (Ga), lead (Pb) and thallium (Tl). In particular, the metal M is selected from zinc (Zn), tin (Sn), lead (Pb) and indium (In). Very preferably, the metal M is zinc (Zn).

[0091] The metal M is more particularly in the form of an organometallic compound, preferably an organometallic compound of the following general formula (II):

[0092]

[0093] in:

[0094] - M is a metal as defined above;

[0095] - p is an integer between 1 and 6, preferably between 2 and 4; and

[0096] The -R1 groups are identical or different and independently represent a hydrocarbon group containing 1 to 12 carbon atoms.

[0097] Preferably, the R1 groups are identical or different and are independently selected from:

[0098] Linear, branched or cyclic alkyl groups containing between 1 and 10 carbon atoms;

[0099] linear, branched or cyclic alkenyl containing between 1 and 10 carbon atoms;

[0100] an aryl group containing between 6 and 10 carbon atoms, said aryl group being optionally substituted with one or more linear or branched alkyl groups containing from 1 to 10 carbon atoms; and

[0101] Alkylaryl groups containing between 6 and 12 carbon atoms.

[0102] In particular, the R1 groups are identical or different and are independently selected from:

[0103] Linear, branched or cyclic alkyl groups containing between 1 and 10 carbon atoms; and

[0104] An aryl group containing between 6 and 10 carbon atoms, said aryl group being optionally substituted with one or more linear or branched alkyl groups containing from 1 to 10 carbon atoms.

[0105] In particular, the R1 radicals are identical or different, independently selected from linear, branched or cyclic alkyl radicals containing between 1 and 10 carbon atoms.

[0106] Preferably, the R1 groups are identical. Preferably, the alkyl group contains between 1 and 6 carbon atoms, more preferably between 1 and 3 carbon atoms. For example, the alkyl group is selected from: methyl, ethyl, n-propyl and isopropyl. For example, the aryl group is phenyl and / or the alkylaryl group is benzyl and / or the alkenyl group is cyclopentadienyl.

[0107] In particular, the organometallic compound is selected from those of the formula:

[0108] Zn(R1)2(IIa)

[0109] Sn(R1)4(IIb)

[0110] In(R1)3(IIc)

[0111] Zr(R1)4(IId)

[0112] Ce(R1)4(IIe)

[0113] Ge(R1)4(IIf)

[0114] Ga(R1)3(IIg)

[0115] Pb(R1)4(IIh)

[0116] Tl-R1(IIi)

[0117] wherein the R1 groups are identical or different and are as defined for the general formula (II).

[0118] In particular, the metal M is zinc and is in the form of an organometallic compound having the following general formula (IIa):

[0119] Zn(R1)2(IIa)

[0120] wherein the R1 groups are identical or different and are as defined for the general formula (II).

[0121] As the organometallic compound having the general formula (II), mention may be made of:

[0122] Tetramethyltin, trimethylindium, tetrabenzylzirconium, tetraethylgermanium, trimethylgallium, tetramethyllead, cyclopentadienylthallium, dimethylzinc (Zn(CH3)2), diethylzinc (Zn(C2H5)2), diisopropylzinc (Zn(i-C3H7)2), dipropylzinc (Zn(C3H7)2) and diphenylzinc (Zn(C6H5)2).

[0123] The organometallic compound is particularly selected from: Zn(CH3)2, Zn(C2H5)2, Zn(i-C3H7)2, Zn(C3H7)2 and Zn(C6H5)2. The CAS numbers of these compounds are as follows: Zn(CH3)2: 544-97-8; Zn(C2H5)2: 577-20-0; Zn(C3H7)2: 628-91-1; Zn(i-C3H7)2: 625-81-0 and Zn(C6H5)2: 1078-58-6.

[0124] More preferably, the metal compound is selected from Zn(CH3)2, Zn(C2H5)2 and Zn(C3H7)2; more particularly Zn(CH3)2 and Zn(C2H5)2.

[0125] The organometallic compound can be formulated in a solvent, in particular an organic solvent, in particular selected from toluene and alkanes containing between 5 and 10 carbon atoms. Preferably, the solvent is selected from toluene, heptane, hexane or a mixture thereof. The above-mentioned organometallic compounds are commercially available in the form of 1M or 2M solutions.

[0126] The introduction of the metal M, optionally in organometallic form as defined above (stage a)) can be carried out by any known means, for example by injection.

[0127] It is to be understood that the metal M does not form part of the dehydrogenation catalyst during its introduction in stage a)).

[0128] In particular, the metal M is introduced separately from the dehydrogenation catalyst.

[0129] The introduction may be one-time, semi-continuous or continuous, preferably continuous. It may be carried out at the start-up of a unit producing olefins or during the entire production or partial production of olefins. The amount of the metal M introduced may or may not change during the addition duration, for example, having a gradient that decreases or increases during the addition duration.

[0130] In the case of a series of reactors, it will be appreciated that the introduction of the metal M may occur at the inlet or upstream of the first reactor in the series, and / or at the inlet or upstream of any one or more of the subsequent reactors. The introduction of the metal M may therefore occur at different places in the process facility.

[0131] Said metal M, optionally in said organometallic form, is preferably contained in a composition further comprising a sulphur compound having the following general formula (I):

[0132] [Chemical formula 1]

[0133]

[0134] in:

[0135] - R is selected from linear or branched alkyl groups containing 1 to 4 carbon atoms and linear or branched alkenyl groups containing 2 to 4 carbon atoms,

[0136] -n is an integer equal to 0, 1 or 2,

[0137] - x is an integer selected from 0, 1, 2, 3 or 4, and

[0138] -R' is selected from a linear or branched alkyl group containing 1 to 4 carbon atoms, a linear or branched alkenyl group containing 2 to 4 carbon atoms and, only when n=x=0, is a hydrogen atom.

[0139] Preferably, R and R' are identical or different, selected from linear or branched alkyl groups containing 1 to 4 carbon atoms. Preferably, R and R' are identical.

[0140] In particular, n is equal to 0. In particular, x is selected from 1, 2, 3 or 4, preferably 1 or 2 and more preferably 1.

[0141] More particularly, the sulfur compound has the following general formula (Ia):

[0142] RSS-R'(Ia),

[0143] wherein R and R' groups are as defined above.

[0144] According to the present invention, it is understood that mixtures of two or more sulphur compounds of formula (I) may be used. In particular, mixtures of disulfides or polysulfides may be used, for example mixtures of disulfides, such as disulfide oil (DSO).

[0145] Preferably, the sulfur compound of general formula (I) is selected from dimethyl disulfide (DMDS), dimethyl sulfide (DMS), dimethyl sulfoxide (DMSO) and di(tert-butyl) polysulfide. In particular, the sulfur compound of general formula (I) is selected from dimethyl disulfide (DMDS), dimethyl sulfide (DMS) and di(tert-butyl) polysulfide, preferably dimethyl disulfide.

[0146] Completely preferably, the sulfur compound is dimethyl disulfide. In particular, the sulfur compound is commercially available. Mention may be made of DMDS sold by Arkema. E2.

[0147] Very particular preference is given to compositions comprising:

[0148] -DMDS; and

[0149] - an organometallic compound having the following general formula (II):

[0150] M[R1] p (II)

[0151] wherein M, R1 and p are as defined above.

[0152] In particular, the composition comprises at least 80%, preferably at least 95%, more preferentially at least 99% by total weight of sulphur compounds relative to the total weight of the composition. It may contain one or more odour masking agents (see, for example, International Application WO 2011 / 012815A1).

[0153] The composition may also contain a solvent, especially an organic solvent, in particular selected from toluene and alkanes containing between 5 and 10 carbon atoms. Preferably, the solvent is selected from toluene, heptane, hexane or a mixture thereof. Preferably, the composition contains between 0.001% and 10% by weight, such as between 0.005% and 1.5% by weight, of the solvent, relative to the total weight of the composition.

[0154] The amount of metal M is preferably between 1 and 10 000 ppm, preferably between 1 and 1000 ppm, for example between 1 and 500 ppm, more preferably between 5 and 200 ppm, relative to the total weight of metal M and sulfur compound. It may be between 5 and 100 ppm, more particularly between 10 and 100 ppm, relative to the total weight of metal M and sulfur compound.

[0155] The amount of composition introduced (during stage a)) may be between 5 and 200 ppm, preferably between 10 and 90 ppm, for example 25, 50 or 75 ppm, relative to the weight of the hydrocarbon chains to be dehydrogenated.

[0156] Said metal M, optionally in organometallic form, or said composition, may be mixed beforehand with all or part of the hydrocarbon chain to be dehydrogenated, before introducing it upstream or at the inlet of the reactor.

[0157] The compositions defined above comprising a sulphur compound and a metal M are novel and themselves form part of the present invention.

[0158] Very particular preference is given to compositions comprising:

[0159] -DMDS; and

[0160] - an organometallic compound having the following general formula (II):

[0161] M[R1] p (II)

[0162] wherein M, R1 and p are as defined above.

[0163] Such compositions are themselves particularly suitable for introduction into the process according to the invention. They are in particular liquid (at ambient temperature, for example between 5° C. and 40° C.). The organometallic compounds are clearly soluble in DMDS, which makes handling and control of the amount of metal M introduced extremely convenient.

[0164] The composition according to the invention can be prepared by simple mixing of the sulphur compound and the metal M. The metal M may be in organometallic form, optionally dissolved in a solvent as defined above.

[0165] They may also be in the form of a kit comprising a sulphur compound and a metal M, so as to be formed upstream, at the inlet or in situ of the dehydrogenation reactor.

[0166] The invention also relates to the use of said metal M, optionally in organometallic form, in a process for dehydrogenating hydrocarbon chains to provide olefins, preferably for increasing their yield. It is understood that said metal M may be included in the composition defined above. BRIEF DESCRIPTION OF THE DRAWINGS

[0167] Figure 1 The yield of the dehydrogenation reaction obtained as a function of time is shown.

[0168] DMDS alone is represented by a curve containing squares, DMDS with added Zn(CH3)2 is represented by a continuous curve and DMDS with added Zn(C2H5)2 is represented by a curve containing diamonds.

[0169] Example

[0170] Example 1 : The process according to the present invention

[0171] The dehydrogenation of propane was carried out in a reactor at a temperature of 625° C., a pressure of 3 bars and in the presence of a catalyst of the Pt / Al 2 O 3 type (Pt / Al 2 O 3 catalyst from Strem Chemicals Inc.) on a fixed bed for a period of 20 h.

[0172] The propane flow rate was 2.2 Sl / h.

[0173] Continuous injections of 75 ppm of different DMDS compositions were carried out upstream of the reactor.

[0174] The composition according to the invention was prepared by adding 50 ppm of the following commercial solution (supplier Aldrich) to DMDS:

[0175] - 1 M solution of dimethylzinc,

[0176] -1 M solution of diethylzinc.

[0177] A clear solution of the added DMDS was obtained.

[0178] A comparative composition of DMDS without added metal M was also tested.

[0179] The results obtained by Figure 1 supply.

[0180] The cumulative amount of propylene obtained in 20 h is as follows:

[0181] -6.08Sl, DMDS alone;

[0182] -6.23Sl, DMDS with Zn(CH3)2 added, i.e. the final yield increased by 2.6%; and

[0183] -6.37Sl, DMDS with the addition of Zn(C2H5)2, that is, the final yield increased by 4.8%.

[0184] Thus, the propylene yield is improved by the present invention.

Claims

1. A process for preparing olefins, comprising a stage of dehydrogenating hydrocarbon chains in the presence of a catalyst in at least one reactor; The process comprises a stage of introducing the metal M upstream or at the inlet of the reactor, and The metal M is selected from zinc (Zn), tin (Sn), indium (In), zirconium (Zr), cerium (Ce), germanium (Ge), gallium (Ga), lead (Pb) and thallium (Tl), preferably zinc.

2. The process of claim 1, wherein the metal M is in the form of an organometallic compound having the following general formula (II): M[R 1 ] p (II) in: - M is a metal as defined in claim 1; - p is an integer between 1 and 6, preferably between 2 and 4; and -R 1 The radicals are identical or different and independently represent a hydrocarbon radical containing 1 to 12 carbon atoms.

3. The process of claim 2, wherein the organometallic compound is selected from the group consisting of tetramethyltin, trimethylindium, tetrabenzylzirconium, tetraethylgermanium, trimethylgallium, tetramethyllead, cyclopentadienylthallium, dimethylzinc, diethylzinc, diisopropylzinc, dipropylzinc and diphenylzinc.

4. A process as claimed in any one of the preceding claims, wherein the metal M is contained in a composition which also comprises a sulphur compound having the following general formula (I): [Chemical formula 1] in: - R is selected from linear or branched alkyl groups containing 1 to 4 carbon atoms and linear or branched alkenyl groups containing 2 to 4 carbon atoms, -n is an integer equal to 0, 1 or 2, - x is an integer selected from 0, 1, 2, 3 or 4, and -R' is selected from linear or branched alkyl groups containing 1 to 4 carbon atoms, linear or branched alkenyl groups containing 2 to 4 carbon atoms and, only when n=x=0, is a hydrogen atom.

5. The process of claim 4, wherein the sulfur compound of formula (I) is selected from dimethyl disulfide (DMDS), dimethyl sulfide (DMS) and di(tert-butyl) polysulfide, preferably dimethyl disulfide.

6. The process according to any one of the preceding claims, comprising at least the following stages: a) introducing a metal M as defined in any one of claims 1 to 5 upstream or at the inlet of the reactor; b) dehydrogenating the hydrocarbon chains in a moving catalytic bed reactor to obtain an exit stream S comprising olefins and hydrogen; c) taking and separating the exiting stream S from stage b) to obtain: - a stream A comprising olefins; and - a stream H comprising hydrogen; d) optionally recovering the catalyst; e) optionally regenerating the catalyst; and f) optionally recycling the regenerated catalyst to stage b).

7. A composition comprising: n is a sulfur compound having the following general formula (I): [Chemical formula 1] in: - R is selected from linear or branched alkyl groups containing 1 to 4 carbon atoms and linear or branched alkenyl groups containing 2 to 4 carbon atoms, -n is an integer equal to 0, 1 or 2, - x is an integer selected from 0, 1, 2, 3 or 4, and - R' is selected from a linear or branched alkyl radical containing 1 to 4 carbon atoms, a linear or branched alkenyl radical containing 2 to 4 carbon atoms and, only when n=x=0, is a hydrogen atom, and The metal M is selected from zinc (Zn), tin (Sn), indium (In), zirconium (Zr), cerium (Ce), germanium (Ge), gallium (Ga), lead (Pb) and thallium (Tl), preferably zinc.

8. The composition of claim 7, wherein the sulfur compound having the general formula (I) is selected from dimethyl disulfide (DMDS), dimethyl sulfide (DMS) and di(tert-butyl) polysulfide, preferably dimethyl disulfide.

9. A composition as claimed in claim 7 or 8, wherein the metal M is in the form of an organometallic compound having the following general formula (II): M[R 1 ] p (II) in: - M is a metal as defined in claim 1; - p is an integer between 1 and 6, preferably 2, 3 or 4; and -R 1 The radicals are identical or different and independently represent a hydrocarbon radical containing 1 to 12 carbon atoms.

10. The composition of any one of claims 7 to 9, further comprising an organic solvent.

11. Use of a composition as defined in any one of claims 7 to 10 in a process for dehydrogenating hydrocarbon chains to provide olefins.

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