Process for converting light alkanes to aromatic compounds with improved selectivity

Abstract

In a method for catalytically converting lower hydrocarbons into aromatic compounds comprising benzene, toluene, and xylene, a process stream containing the lower hydrocarbons is contacted with a zeolite catalyst having an MFI framework and containing 0.1-10% by weight of a zinc compound. The process stream also contains one or more sulfur compounds, particularly hydrogen sulfide, to improve selectivity.

Description

[0001] The present invention relates to a process for converting light alkanes, particularly low-value, ethane-rich streams, into aromatics, particularly high-value BTX, a mixture of benzene, toluene, and xylenes (all of which are aromatic hydrocarbons). More particularly, the present invention relates to a process wherein an ethane-rich stream can be efficiently converted into a product rich in benzene, toluene, and xylenes in the presence of small amounts (on the order of about 50 ppm) of sulfur (e.g., H2S) with minimal formation of higher (C 9+ ) aromatic compounds, and the selectivity for methane is greatly reduced. The preferred catalyst comprises a zeolite, preferably of the MFI type, containing 0.1-10 wt% Zn and optionally 1-5 wt% of a phosphorus compound. The zeolite is preferably embedded in a binder matrix such as alumina.

[0002] Natural gas primarily contains methane, with smaller amounts of ethane, propane, butane, and heavier hydrocarbons, as well as varying amounts of water vapor, carbon dioxide, sulfur compounds, and other non-hydrocarbons. Ethane, propane, butane, and propane are referred to as associated gases. These gases may need to be removed from raw natural gas to meet consumer specifications or to isolate the hydrocarbons from the natural gas. Various technologies, such as compression, refrigeration, absorption, adsorption, or a combination of these, can be used to recover associated gases from natural gas.

[0003] Recovery of natural gas liquids (NGLs) from natural gas is very common in natural gas processing. The purpose of recovery is usually to:

[0004] - produce transportable gas (containing sufficiently low amounts of heavy hydrocarbons to avoid condensation in pipelines),

[0005] - meet sales gas specifications, and / or

[0006] - Maximizing liquid recovery (when liquid products are more valuable than methane).

[0007] Natural gas liquids (NGLs) are hydrocarbons that belong to the same molecular family as natural gas and crude oil. They consist solely of carbon and hydrogen, with the exception of trace amounts such as carbon dioxide and sulfur compounds. Lower alkanes above methane, namely ethane, propane, n-butane, isobutane, and pentane, are NGLs. NGLs have many uses, covering almost every technical field. NGLs can be used as feedstock for petrochemical plants, burned for heating and cooking, and blended into motor vehicle fuels. High NGL values ​​provide an incentive to drill for liquid-rich resources with high NGL contents.

[0008] The chemical composition of these individual hydrocarbons is similar, but their applications vary greatly. Ethane accounts for the largest share of NGL production from oil fields. Most of it is used to produce ethylene, which can then be turned into plastics. In contrast, most propane is burned for heating purposes, although a considerable amount is used as a petrochemical feedstock. A mixture of propane and butane, called LPG, is a very popular fuel in some parts of Europe and Asia. Natural gasoline (C 5+ ) can be blended into various internal combustion engine fuels and can also be used to recover energy from wells and oil sands.

[0009] NGLs can be removed from raw natural gas by various gas processing technologies. These NGLs are ethane (C2), propane (C3), butane (C4) and pentane (C5) found in natural gas. 5+ , natural gasoline). Mixtures of NGLs can be separated into individual compounds by fractional distillation.

[0010] Typically, sulfur is also removed because it is considered an impurity, i.e. a contaminant for downstream catalytic processes.

[0011] The present invention discloses a catalyst and a method by which an ethane-rich stream can be efficiently converted to a BTX product in the presence of a small amount (about 50 ppm) of sulfur (e.g., H2S) with minimal formation of higher (C 9+ ) aromatic compounds, and the selectivity to methane is greatly reduced.

[0012] The present invention converts low-value ethane into a liquid product rich in aromatics and a gaseous product rich in ethylene, thereby providing an outlet for ethane-rich streams, which are in substantial surplus due to shale gas production. Ethylene itself is a valuable product that can be separated from the effluent or recycled to the reactor where it is ultimately converted to aromatics.

[0013] The standard solution for dealing with excess ethane is to feed it into a steam cracker to produce ethylene. However, the capital investment in a steam cracker is substantial, and the market is already oversaturated with excess ethane. Some of this ethane is liquefied and exported, or simply used as fuel. It can be converted to BTX over, for example, a Zn / ZSM-5 catalyst, although this process has not yet reached commercial scale. This is likely due to the large amounts of heavy aromatic compounds, such as C, formed as byproducts. 9+ Aromatic compounds.

[0014] According to the present invention, preferred catalysts comprise a zeolite, preferably of the MFI type, containing 0.1-10 wt% Zn and optionally 1-5 wt% P. The zeolite is preferably embedded in a binder matrix, such as alumina.

[0015] The technical features of the process of the present invention can be summarized as follows: temperature T: 550-600° C.; pressure P: 3-20 bar absolute; S in the feed stream: 10-100 ppm; optionally, unconverted olefins are recycled.

[0016] Regarding the prior art, US 2012 / 0036889 describes a method for converting a methane feed into aromatic hydrocarbons, which is integrated with LNG and / or pipeline natural gas production. The hydrocarbon feed is supplied to a conversion zone containing a dehydrogenation aromatization catalyst, where it is converted into a gaseous effluent comprising at least one aromatic compound, unreacted methane, and hydrogen. The effluent is separated into a first stream comprising the at least one aromatic compound and a second stream comprising methane and hydrogen. The methane is sent to liquefied LNG and / or pipeline natural gas production. The gaseous hydrocarbon feed has at least one of the following properties: (i) a sulfur content of at least 25 ppmv, (ii) a CO2 content of at least 25 ppmv, and (iii) a dew point of at least -70.15°C. Preferred catalysts used in the conversion zone include Mo, W, Zn, Re, and compounds and combinations thereof supported on ZSM-5, silica, or alumina.

[0017] US Pat. No. 7,057,084 B2, filed by the applicant, describes a method for removing higher hydrocarbons from natural gas that also contains sulfur compounds. This method is based on the use of specific crystalline aluminosilicates as catalysts for the simultaneous conversion of hydrocarbons into aromatic compounds and methane, thereby removing higher hydrocarbons from natural gas by conversion (aromatization) and subsequent separation of the aromatized molecules from the methane.

[0018] US 2005 / 0143610 A1 describes a process for converting C1-C4 alkanes into aromatic compounds, such as BTX, using a crystalline zeolite catalyst having platinum deposited thereon. Specifically, the catalyst may be a Pt-containing ZSM-5 catalyst, which suppresses methane formation and increases selectivity for BTX. The high ethane content relative to methane in the light gas fraction allows the process effluent to serve as a feed stream for a steam cracker.

[0019] EP 3110 919 B1 describes a process for producing BTX from mixed hydrocarbon streams, which process comprises pyrolysis, aromatic ring opening and recovery of the BTX produced.

[0020] The applicant's own US 2003 / 0118496 A1 discloses a process for removing hydrocarbons from natural gas also containing sulphur compounds by simultaneously converting the hydrocarbons into aromatic compounds and methane in the presence of a catalyst comprising a crystalline aluminosilicate, such as H-ZSM-5 zeolite.

[0021] US 2010 / 0048969 A1 discloses a method for producing aromatic compounds from lower alkanes by using a zeolite catalyst such as ZSM-5, which reduces the production of methane. The catalyst comprises Pt and an attenuating metal consisting of tin, lead, and germanium.

[0022] US 2011 / 0301394 A1 discloses a fixed bed process for aromatizing lower alkanes using a catalyst diluted with a second inert solid material.

[0023] WO 2019 / 164610 A1 discloses a comprehensive process for converting a crude natural gas feed containing hydrogen sulfide and ethane into aromatic compounds, particularly by upgrading a light hydrocarbon stream without removing methane and / or ethane from the light hydrocarbon stream prior to catalytic processing. The conversion results in a product having a higher concentration of methane and a lower concentration of ethane than the feed. Thus, the reference does not, at least not mention, how to suppress methane formation during aromatic compound formation.

[0024] WO 2017 / 052858 A1 discloses a general process for converting alkanes (e.g., feeds containing ethane) while improving selectivity to desired aromatic compounds, whereby aromatic compounds (particularly benzene) produced in the first stage (aromatic compound formation process) are further alkylated to form xylenes. A number of feeds containing alkanes are cited, one of which is a feed containing less than 10 wt% hydrogen sulfide as an impurity. This citation does not at least mention how to improve selectivity to BTX and how to suppress the production of methane and C in the aromatic compound formation process. 9+ Formation of aromatic compounds.

[0025] The effects and advantages of the present invention can be summarized as improved selectivity to the final product of BTX, while methane and C 9+ The formation of aromatic compounds (indan / indene and naphthalene) was minimal.

[0026] Aromatization of hydrocarbons is an endothermic reaction and it has been proposed to carry out the exothermic hydrocracking and the endothermic aromatics synthesis simultaneously in a catalytic reaction zone according to the following reaction (using propane as an example of a higher hydrocarbon to be removed from natural gas):

[0027] 9C3H8<->2C6H6+15CH4

[0028] The reaction is essentially thermoneutral with an enthalpy of -5 kcal / mole.

[0029] The above-mentioned simultaneous endothermic and exothermic reaction has been applied and mentioned in US 4,260,839 for the production of LPG, gasoline and aromatic compounds by ethane conversion via contact with a ZSM-5 type catalyst.

[0030] The process according to the present invention involves subjecting raw natural gas to a gas treatment process in a manner known per se, thereby obtaining pure methane and Y-grade NGLs. However, the NGLs are not fractionated, but instead used as a feedstock for an aromatics synthesis process, thereby producing a mixture of valuable aromatic compounds, more specifically, primarily benzene, toluene, and xylenes. Further quantities of methane can be added to the pure natural gas from the gas treatment process, if necessary after purification, or used separately for other purposes, such as energy production. Instead of fractionating the NGLs in the conventional manner, it can sometimes be more economically attractive to aromatize at least part of the feed gas in the presence of an aromatization catalyst, thereby converting the NGLs in the feed gas into a mixture of aromatic compounds.

[0031] Although NGL stands for natural gas liquids, it doesn't mean that all natural gas liquids are derived from raw natural gas. In fact, the term "natural gas liquids" specifically refers to the lighter (but heavier than methane), condensable hydrocarbon fractions within the hydrocarbon stream in question. These "lighter" hydrocarbons are those with only a few carbon atoms. Methane is the lightest hydrocarbon fraction, with only one carbon atom in its molecule. However, methane is not an NGL because it cannot be condensed under normal processes due to its very low boiling point of -164°C.

[0032] Hydrocarbons are often classified as either "X-grade" or "Y-grade" hydrocarbons. Y-grade is a general term used in the industry for hydrocarbons that "easily" condense. Both X-grade and Y-grade hydrocarbons can be stored as liquids under pressure, but Y-grade hydrocarbons are stored at much lower pressures than X-grade hydrocarbons, making them easier to move around in liquid form.

[0033] In particular, the present invention relates to a process for the catalytic conversion of a gaseous mixture of lower hydrocarbons containing at least 50% by volume of ethane into aromatic compounds consisting mainly of, i.e. comprising, benzene, toluene and xylenes, the process comprising the step of contacting a process stream containing lower hydrocarbons with a zeolite catalyst, wherein the catalyst has an MFI framework and contains 0.1 to 10% by weight of zinc, and wherein the process stream also contains one or more sulfur compounds.

[0034] It is understood that the term "lower hydrocarbons" refers to lower alkanes above methane, i.e. ethane, propane, n-butane, isobutane and pentane, as well as optionally C 5+ (natural gasoline); these are NGLs.

[0035] It should also be understood that the volume percentage of ethane in the gas mixture is relative to the other lower hydrocarbons therein, i.e., excluding diluents such as nitrogen (N2). It should also be understood that the term "process stream" refers to the feed stream in the process, which contains lower hydrocarbons and may include diluents such as N2. For example, a process stream containing lower hydrocarbons in the form of 10% by volume ethane (C2H6) in N2 means 100% by volume ethane in the gas mixture of lower hydrocarbons, and 10% by volume ethane in the process stream.

[0036] Thus, the present invention can also be described as a method for catalytically converting a process stream containing lower hydrocarbons into aromatic compounds comprising benzene, toluene and xylenes, such as a mixture of benzene, toluene and xylenes (BTX), by contacting the process stream with a zeolite catalyst having an MFI framework and containing 0.1 to 10 weight percent of a zinc compound, wherein the process stream comprises a gaseous mixture of lower hydrocarbons containing at least 50 volume percent ethane, and wherein the process stream further contains one or more sulfur compounds.

[0037] Preferably, the one or more sulphur compounds are present in an amount of 10-100 ppm.

[0038] For the purposes of this application, the ppm unit is based on volume, ie, ppmv.

[0039] Preferably, the process further comprises forming a gaseous outflow of unconverted olefins and recycling at least a portion thereof back into the process. This can increase the BTX production in the process.

[0040] Therefore, this method not only produces the aromatic compounds that comprises benzene, toluene and dimethylbenzene (BTX), but also produces the gas stream effluent of unconverted olefins.The gas stream of unconverted olefins is for example a gas stream rich in ethene, for example comprises the gas stream of 90 volume % or more ethene.Ethylene itself is a kind of valuable product, can separate it from the gas stream effluent, also can it be recycled among the above-mentioned method, wherein it is finally converted into aromatic compounds.

[0041] Optionally, the catalyst may comprise Cu instead of Zn, or it may comprise a mixture of copper and zinc.

[0042] Preferably, the zeolite catalyst is ZSM-5 and the zinc compound is metallic zinc and / or zinc oxide. In a preferred embodiment, the zeolite catalyst is embedded in a binder matrix. This binder matrix may advantageously include alumina.

[0043] According to another preferred embodiment, the zeolite further contains 1 to 5 wt% of a phosphorus compound, such as 1-5 wt% P.

[0044] According to one embodiment, the zeolite catalyst is ZSM-5 containing 0.1-10 wt% Zn and optionally 1-5 wt% P. In another embodiment, the zeolite catalyst comprises 5 wt% Zn supported on H-ZSM-5 (silica to alumina ratio of 40).

[0045] Preferably, the feed stream in the process comprises 90 to 100% by volume ethane. Thus, in one embodiment of the invention, the process stream comprises 90 to 100% by volume ethane. Furthermore, preferably, the one or more sulfur compounds in the process stream are H2S, for example 10-100 ppm H2S.

[0046] According to another preferred embodiment, the process is carried out at a temperature of 550-600° C. and a pressure of 3-20 bar absolute.

[0047] In another preferred embodiment, the weight hourly space velocity (WHSV) is in the range of 2-6, for example 3.

[0048] In yet another preferred embodiment, the process stream containing lower hydrocarbons is derived from subjecting raw natural gas to a gas treatment step selected from compression, refrigeration, absorption, adsorption or a combination thereof, thereby obtaining pure methane and said process stream. The process stream is typically free of sulphur compounds and therefore requires the addition of one or more sulphur compounds.

[0049] Therefore, in another preferred embodiment, one or more sulfur compounds are added to the process stream.

[0050] The mixture of organic compounds consisting primarily of benzene, toluene, and xylenes, i.e., the aromatic compounds produced by the process of the present invention, can be fractionated to obtain pure grades of benzene, toluene, and xylene products. These products can then be upgraded, for example, to obtain o-xylene, m-xylene, and p-xylene, or they can be used as a feedstock or a portion of a feedstock in a process for producing p-xylene.

[0051] In addition to being present as part of NGLs obtained from gas processing of raw natural gas, C 5+ The fractions may also come from processing equipment in various industries. For example, heavy C 5+ The stream is obtained as a by-product of the production of ethylene by pyrolysis (steam cracking). 5+ The stream is known in the industry as pyrolysis gasoline or pyrolysis gas (pygas). Due to its high reactivity and low stability, the C 5+ The stream has little commercial value. However, the stream contains many high-value components such as isoprene, benzene, toluene, and xylenes.

[0052] Therefore, another option is to add the mixture of organic compounds produced by the method of the present invention (a mixture of benzene, toluene and xylene, i.e. BTX) to a similar mixture of organic compounds obtained by extracting cracking gas, rather than directly fractionating the mixture. The cracking gas may come from existing processing equipment, such as a steam cracking plant. The resulting mixture of organic compounds from the method of the present invention and existing equipment can then be fractionated as described above to obtain pure grades of benzene, toluene and xylene products.

[0053] Thus, in another embodiment, the process further comprises combining the aromatic compounds comprising benzene, toluene and xylenes with a cracked gas obtained as a by-product in a separate ethylene production by steam cracking, thereby forming a combined stream comprising benzene, toluene and xylenes, and optionally subsequently subjecting the combined stream to one or more fractionation steps to produce pure grades of benzene, toluene and xylene products.

[0054] The present invention is further illustrated in the following examples. The C2-C5 fraction from mixed gas sources (e.g., shale gas) is a valuable hydrocarbon feedstock that can be used to synthesize a mixture of aromatic compounds by aromatization of Y-grade natural gas liquids. Among the C2-C5 hydrocarbons, ethane (C2H6) is the most difficult to convert.

[0055] As mentioned above, BTX is a valuable aromatic compound product that can be used as a feedstock for the production of paraxylene. It can also be used as a high-octane reforming product for gasoline blending. Compared with BTX, aromatic compounds with higher carbon numbers, i.e., C 9+ , which is considered to be of lower value. Therefore, in the process for preparing aromatic compounds, high selectivity for BTX is required.

[0056] As shown in the examples, the conversion of ethane results in the formation of appreciable amounts of methane and higher aromatic compounds, i.e., aromatic compounds having a carbon number of 9 and higher. These higher aromatic compounds are primarily naphthalenes (e.g., 1- and 2-methylnaphthalene), indanes and indene, and methyl-substituted indanes / indenes, which are mixtures of minor interest.

[0057] As shown in Example 1, by actively adding a small amount of sulfur (in the form of H2S) to the ethane feed stream (process stream), the carbonylation of methane and C 9+ Selectivity of hydrocarbons. Example

[0058] Catalyst: 5 wt% Zn supported on H-ZSM-5 (silica to alumina ratio of 40).

[0059] Conditions: 550°C, 3 bar absolute pressure, WHSV=3, 10 vol% C2H6 in N2.

[0060] The figures show the evolution of ethane conversion and product selectivity as a function of time on stream (TOS); closed symbols are the results without H2S, ie, no H2S addition; open symbols are the results with 60 ppm H2S in the feed.

[0061] The accompanying figure shows that the conversion is essentially the same over time with and without H2S in the feed ( Figure 1 Of particular note is that the conversion in the presence of H2S is initially lower but quickly (after approximately 20 hours) exceeds the conversion of the same catalyst operating in the absence of sulfur.

[0062] The BTX yield with H2S in the feed is slightly lower ( Figure 2 ), but having H2S in the feed results in significantly higher selectivity to ethylene (and other light olefins) ( Figure 3 ). It can also be seen that methane ( Figure 4 ) and C 9+ ( Figure 5 ) selectivity decreased significantly in the presence of H2S.

Claims

1. A process for the catalytic conversion of a gaseous mixture of lower hydrocarbons containing at least 50% by volume of ethane into aromatic compounds comprising benzene, toluene and xylenes, the process comprising the step of contacting a process stream containing lower hydrocarbons with a zeolite catalyst, wherein the catalyst has an MFI framework and contains 0.1 to 10% by weight of a zinc compound, wherein the process stream further contains one or more sulfur compounds, wherein the content of the one or more sulfur compounds is 10 to 100 ppm, and wherein the temperature is in the range of 550 to 600° C. and the pressure is in the range of 3 to 20 bar absolute.

2. The process of claim 1 further comprising forming a gas stream effluent of unconverted olefins and recycling at least a portion thereof back to the process.

3. The method according to claim 1 or 2, wherein the zeolite catalyst is ZSM-5.

4. The method according to claim 1 or 2, wherein the zinc compound is metallic zinc and / or zinc oxide.

5. The method according to claim 1 or 2, wherein the zeolite further contains 1 to 5 wt% of a phosphorus compound.

6. The method of claim 1 or 2, wherein the zeolite catalyst is embedded in a binder matrix.

7. The method of claim 6, wherein the binder matrix comprises aluminum oxide.

8. The process of claim 1 or 2, wherein the process stream comprises 90-100% by volume ethane.

9. The method according to claim 1 or 2, wherein the sulfur compound is H2S.

10. The process according to claim 1 or 2, wherein the process stream containing lower hydrocarbons is derived from subjecting raw natural gas to a gas processing step selected from compression, refrigeration, absorption, adsorption or a combination thereof, thereby obtaining pure methane and the process stream.

11. The process of claim 1 or 2, wherein the one or more sulfur compounds are added to the process stream.

12. The process of claim 1 or 2, further comprising combining the aromatic compounds comprising benzene, toluene and xylenes with pyrolysis gasoline obtained as a by-product in a separate ethylene production by steam cracking, thereby forming a combined stream comprising benzene, toluene and xylenes, and optionally subsequently subjecting the combined stream to one or more fractionation steps to produce pure grades of benzene, toluene and xylene products.

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