The present invention relates to a process for the conver- sion of light alkanes, especially low-value, ethane-rich streams, to aromatic compounds, especially high-value BTX, i.e. mixtures of benzene, toluene and xylenes, all of which are aromatic hydrocarbons. More specifically, the invention relates to a process,in which a stream, rich in ethane, can be efficiently converted into a product rich in benzene, toluene and xylenes, with only a minimum formation of higher (Cg ₊₎ aromatics and with strongly reduced selectiv ity to methane, in the presence of small amounts (in the order of 50 ppm) of sulfur, e.g. ¾S. The preferred cata- lyst comprises a zeolite, preferably of MFI-type, contain ing from 0.1-10 wt% Zn and, optionally, from 1-5 wt% of a phosphorus compound. The zeolite is preferably embedded in a binder matrix, such as alumina. Natural gas mainly contains methane and smaller amounts of ethane, propane, butane and heavier hydrocarbons along with varying amounts of water vapor, carbon dioxide, sulfur com pounds and other non-hydrocarbons. Ethane, propane, butane and propane are known as associated gases. The removal of these gases from raw natural gas may be desirable to meet consumer specifications of natural gas or to obtain indi vidual hydrocarbons from natural gas. Various techniques, such as compression, refrigeration, absorption, adsorption or combinations of those, can be used to recover associated gases from natural gas. Recovery of natural-gas liquids (NGL) from natural gas is quite common in natural gas processing. Recovery is usually done with the purpose of: - producing transportable gas (containing a sufficiently low amount of heavier hydrocarbons to avoid condensation in the pipeline),

- meeting a sales gas specification and/or

- maximizing liquid recovery (when liquid products are more valuable than methane).

Natural gas liquids (NGLs) are hydrocarbons belonging to the same family of molecules as natural gas and crude oil. They are composed exclusively of carbon and hydrogen except for minute amounts of e.g. carbon dioxide and sulfur com pounds. Lower alkanes above methane, i.e. ethane, propane, n-butane, isobutane and pentane, are all NGLs. There are many uses for NGLs, spanning nearly all technical areas. NGLs can be used as inputs for petrochemical plants, they can be burned for heating and cooking purposes, and they can be blended into vehicle fuels. High NGL values have provided incentives to drill in liquids-rich resources with significant NGL contents. The chemical composition of these individual hydrocarbons is similar, yet their fields of application vary widely. Ethane occupies the largest share of NGL field production. The majority is used to produce ethylene, which can then be turned into plastics. Much of the propane, by contrast, is burned for heating purposes, although a substantial amount is used as petrochemical feedstock. A blend of propane and butane, called LPG, is a quite popular fuel in some parts of Europe and Asia. Natural gasoline (C5+) can be blended into various kinds of fuel for combustion engines, and it is also useful in energy recovery from wells and oil sands. NGLs can be removed from raw natural gas by various gas processing techniques. These NGLs are ethane (C2), propane (C ₃₎ , butane (C4) and pentane plus (C5+, natural gasoline) found in natural gas. The mixture of NGLs can be separated into the individual compounds by fractionation.

Normally also, sulfur is removed since it is regarded as an impurity i.e. contaminant for downstream catalytic pro cesses. The present invention discloses a catalyst and a process, by which a stream rich in ethane can be efficiently con verted into a BTX product with only minimum formation of higher (C9+) aromatics and with a strongly reduced selec tivity to methane in the presence of small amounts (about 50 ppm) of sulfur, e.g. as ¾S.

With the present invention, it becomes possible to convert ethane of low value to a liquid product, which is rich in aromatic compounds, and a gaseous product, which is rich in ethylene, thereby providing an outlet for ethane-rich streams, which are in considerable surplus due to shale gas production of natural gas. Ethylene itself is a valuable product, which may either be separated from the effluent, or it may be recycled to the reactor where it will, eventu ally, be converted into aromatics. The standard solution regarding what to do with the surplus ethane consists in feeding the ethane to steam crackers for making ethylene. However, steam crackers are capital exten sive and, moreover, the market is already over-saturated with surplus ethane. Some of this ethane is liquefied and exported, or simply used as fuel. It may be converted to BTX over e.g. Zn/ZSM-5 catalysts, although this approach has not yet reached commercial scale. This is possibly due to the fact that appreciable amounts of heavy aromatics, e.g. Cg ₊ aromatics, are formed as by-products.

According to the invention, the preferred catalyst com prises a zeolite, preferably of MFI type, containing 0.1-10 wt% Zn and, optionally, 1-5 wt% P. The zeolite is prefera- bly embedded in a binder matrix, such as alumina.

The technical features of the process of the invention can be summarized as follows: Temperature T: 550-600°C; pres sure P: 3-20 bar abs.; 10-100 ppm S in feed stream; option- ally, recycle of unconverted olefins.

Regarding prior art, US 2012/0036889 describes a process for converting a methane feed to an aromatic hydrocarbon, said process being integrated with LNG and/or pipeline gas production. The hydrocarbon feed is supplied to a conver sion zone comprising a dehydroaromatization catalyst, where it is converted to 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 passed to LNG and/or pipeline gas production. The gaseous hydrocarbon feed has at least one of the following properties: (i) a sulfur level of at least 25 ppmv, (ii) a CO2 level 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.

A process for the removal of higher hydrocarbons from natural gas further containing sulfur compounds is described in US 7,057,084 B2 belonging to the applicant. The process is based on simultaneous conversion of the hydrocarbons to aromatic compounds and methane using specific crystalline alumino silicates as catalysts for the removal of higher hydrocarbons from natural gas by conversion (aromatization), allowing a subsequent separation of the aromatized molecules from me thane.

A process for converting C1-C4 alkanes to aromatic com pounds, such as BTX, using a catalyst of a crystalline zeo- lite, on which platinum has been deposited, is described in US 2005/0143610 A1. The catalyst can specifically be a Pt- containing ZSM-5 catalyst, which suppresses the formation of methane and increases the selectivity to BTX. The high content of ethane relative to methane in the light gas fraction allows the process effluent to be the feed stream for a steam cracker.

EP 3110 919 B1 describes a process for producing BTX from a mixed hydrocarbon stream, which comprises pyrolysis, aro- matic ring opening and recovery of the BTX produced. Applicant's own US 2003/0118496 A1 discloses a process for the removal hydrocarbons from natural gas, which further contains sulfur compounds, by simultaneous conversion of the hydrocarbons to aromatic compounds and methane in the presence of a catalyst comprising a crystalline alumino silicate, such as a H-ZSM-5 zeolite.

US 2010/0048969 A1 discloses a process for producing aro matics from lower alkanes with reduced production of me- thane by using a zeolite catalyst such as ZSM-5 comprising Pt and an attenuating metal consisting of tin, lead and germanium.

US 2011/0301394 A1 discloses a fixed bed process for the aromatization of lower alkanes using a catalyst diluted with a second inert solid material.

WO 2019/164610 A1 discloses an overall process for conver sion of a raw natural gas feed comprising hydrogen sulfide and ethane to aromatics, in particular by upgrading light hydrocarbon streams that do not remove methane and/or ethane from the light hydrocarbon stream before catalytic processing. The conversion results in products having greater concentrations of methane than the feed, but lesser concentration of ethane. This citation is therefore silent at least about how to suppress the formation of methane in the aromatic formation process.

WO 2017/052858 A1 discloses an overall process for convert- ing alkanes such as a feed containing ethane while generat ing improved selectivity to desired aromatics, whereby aro- matics generated in a first stage (aromatic formation pro cess), in particular benzene, are further alkylated to form xylenes. A number of feeds containing alkanes are cited, one being a feed comprising less than 10 wt% hydrogen sul- fide as impurity. This citation is at least silent about how to improve the selectivity to BTX and how to suppress the formation of methane and C9+ aromatics in the aromatic formation process. The effects and advantages of the present invention can be summarized as an improved selectivity to BTX end product with minimum formation of methane and C9+ aromatics (in- danes/indenes and naphthalenes). Aromatization of hydrocarbons is an endothermic reaction, and it has been proposed to carry out exothermic hydro cracking and endothermic aromatic synthesis simultaneously in a catalytic reaction zone according to the following re action, taking propane as an example of the higher hydro- carbons to be removed from natural gas:

9 C ₃ H ₈ <-> 2 C ₆ H ₆ + 15 CH ₄

The reaction is substantially thermo-neutral with an en- thalpy of -5 kcal/mole.

The above simultaneously endo- and exothermic reaction has been applied and mentioned in US 4,260,839 for ethane con version in production of LPG, gasoline and aromatics by contact with a catalyst of ZSM-5 type. The method according to the present invention consists in subjecting raw natural gas to a gas processing procedure in a manner known per se, thereby obtaining pure methane and Y-grade NGLs. However, instead of said NGLs being subjected to fractionation, they are used as feed for an aromatic synthesis process leading to a mixture of valuable aromatic compounds, more specifically - and mainly - benzene, tolu ene and xylenes, as well as a further amount of methane which can either be added to the pure natural gas from the gas processing procedure, if necessary after a purifying treatment, or be used separately for other purposes, such as energy production. It may sometimes be more economically attractive to subject at least part of the feed gas to aro- matization in the presence of an aromatization catalyst, thereby converting the NGLs in the feed gas to a mixture of aromatic compounds, instead of fractionating the NGLs the traditional way.

Even though NGL stands for a natural gas liquid, this does not mean that all natural gas liquids originate from raw natural gas. In fact, the term "natural gas liquid" specif ically refers to the lighter (yet heavier than methane), condensable hydrocarbon fractions of the hydrocarbon stream in question. These "lighter" hydrocarbons are those with only a few carbon atoms. Methane is the lightest hydrocar bon fraction, having only one carbon atom in the molecule. Nevertheless, methane is not an NGL because it cannot be condensed during normal processes, which is due to its very low boiling point of -164°C. Hydrocarbons are often classified as "X-grade" or "Y-grade" hydrocarbons. Y-grade is a common term in the industry for the "easily" condensable hydrocarbons. Both X-grade and Y- grade hydrocarbons can be stored in a liquid state under pressure, but Y-grade hydrocarbons can be stored at a pres sure much less than that required for X-grade ones, which makes the Y-grade hydrocarbons easier to move in a liquid state.

Specifically, the present invention relates to a process for the catalytic conversion of gas mixtures of lower hy drocarbons, which contain at least 50 % by volume of ethane, to aromatic compounds consisting essentially of, i.e. comprising, benzene, toluene and xylenes, said process comprising contacting a process stream containing lower hy drocarbons with a zeolitic catalyst having an MFI framework and containing 0.1 to 10 percent by weight of zinc, wherein the process stream further contains one or more sulfur com pounds.

It would be understood that the term "lower hydrocarbons" refer to lower alkanes above methane, i.e. ethane, propane, n-butane, isobutane and pentane, and optionally also C ₅₊ (natural gasoline) found in natural gas; all being NGLs.

It would also be understood that the % by volume of ethane of the gas mixture is with reference to the other lower hy drocarbons therein, i.e. without including a diluent such as nitrogen (N ₂₎ . It would also be understood that the term "process stream" means the feed stream in the process which contains the lower hydrocarbons and may include a diluent such as N ₂ . For instance, a process stream containing lower hydrocarbons as 10% by volume of ethane (0 ₂ ¾) in N ₂ means 100 percent by volume ethane in the gas mixture of lower hydrocarbons, and 10 percent volume ethane in the process stream.

Accordingly, the invention can also be recited as a process for the catalytic conversion of a process stream containing lower hydrocarbons, to aromatic compounds comprising ben zene, toluene and xylene e.g. a mixture of benzene, toluene and xylene (BTX), by contacting said process stream with a zeolitic catalyst having an MFI framework and containing 0.1 to 10 percent by weight of a zinc compound, said pro- cess stream comprising a gas mixture of lower hydrocarbons which contains at least 50 % by volume of ethane, and wherein the process stream further contains one or more sulfur compounds.

Preferably, the content of the one or more sulfur compounds is 10-100 ppm.

For the purposes of the present application, ppm units are in a volume basis, i.e. ppmv.

Preferably, the process further comprises forming a gas stream effluent of unconverted olefins and recycling at least a portion thereof back to the process. This enables increasing the BTX-yield in the process.

Hence, the process generates not only the aromatic com pounds comprising benzene, toluene and xylenes (BTX), but also a gas stream effluent of unconverted olefins. The gas stream of unconverted olefins is for instance an ethylene- rich stream, e.g. a stream comprising 90% vol. or more eth ylene. Ethylene itself is a valuable product, which may ei ther be separated from the gas stream effluent, or it may be recycled to the process as described above where it will, eventually, be converted into aromatics.

Optionally, the catalyst may contain Cu instead of Zn or it may contain mixtures of copper and zinc.

Preferably, the zeolite catalyst is ZSM-5, and the zinc compound is metallic and/or oxidic zinc. In a preferred em bodiment, the zeolite catalyst is embedded in a binder ma trix. This binder matrix may advantageously comprise alu- mina.

According to another preferred embodiment, the zeolite fur ther contains 1 to 5 percent by weight of a phosphorus com pound, e.g. 1-5 wt% P.

According to a specific embodiment, the zeolite catalyst is ZSM-5, contains 0.1-10 wt% Zn and, optionally, 1-5 wt% P.

In another specific embodiment, the zeolite catalyst com prises 5 wt% Zn supported on H-ZSM-5 having a silica to alumina ratio of 40. Preferably, the feed stream in the process contains 90 to

100 percent by volume ethane. Accordingly, in an embodiment of the invention the process stream contains 90 to 100 per cent by volume ethane. Further, it is preferred that the one or more sulfur compounds in the process stream is ¾S, for instance 10-100 ppm ¾S According to another preferred embodiment, the process is conducted at a temperature in the range 550-600°C, and pressure in the range 3-20 bar abs. In another preferred embodiment, the weight hour space ve locity (WHSV) is in the range 2-6, for instance 3.

In yet another preferred embodiment, the process stream containing lower hydrocarbons is derived from subjecting raw natural gas to a gas processing step selected from com pression, refrigeration, absorption, adsorption or combina tions thereof, thereby obtaining pure methane and said pro cess stream. The process stream will normally be free of sulfur compounds and thus the one or more sulfur compounds need to be added.

Accordingly, in another preferred embodiment, the one or more sulfur compounds are added to the process stream. The mixture of organic compounds, primarily consisting of benzene, toluene and xylenes, i.e. the aromatic compounds produced by the process of the present invention can be fractionated to obtain pure grade benzene, toluene and xy lene products. These products can subsequently be upgraded, e.g. to obtain o-, m- and p-xylenes, or they may be applied as feed or part of the feed in processes for producing p- xylene.

Besides being present as part of the NGLs obtained from gas processing of raw natural gas, C ₅₊ fractions may also orig inate from processing plants within various industries. For instance, a heavy C ₅₊ stream is obtained as a by-product in ethylene production through pyrolysis (steam cracking).

This C5+ stream is referred to in the industry as pyrolysis gasoline or pygas. Left in its raw form, said C5+ stream has little commercial value owing to its high reactivity and low stability. However, the stream contains many high-value components, such as isoprene, benzene, toluene and xylenes.

Another option is therefore to add the mixture of organic compounds (mixture of benzene, toluene and xylene, i.e. BTX), which is produced by the process of the present in vention, to a similar mixture of organic compounds obtained by extraction of pygas instead of fractionating the mixture directly. The pygas may come from an existing processing plant, such as a steam cracking plant. The resulting mix- ture of organic compounds from the process of the invention and from an existing plant can subsequently be fractionated as described above to obtain pure grade benzene, toluene and xylene products. Accordingly, in another embodiment, the process further comprises combining said aromatic compounds comprising ben zene, toluene and xylene, with a pyrolysis gasoline (pygas) obtained as a by-product in a separate ethylene production through steam cracking, thereby forming a combined stream comprising benzene, toluene and xylene, and optionally sub sequently subjecting the combined stream to one or more fractionation steps for producing pure grade benzene, tolu ene and xylene products. The invention is illustrated further in the example which follows. The C2-C5 fractions from mixed gas sources, such as shale gas, are valuable hydrocarbon feeds for synthetiz- ing a mixture of aromatic compounds by aromatization of Y- grade natural gas liquids. Among the C2-C5 hydrocarbons, the most difficult one to convert is ethane (02¾ ).

As mentioned above, BTX is a valuable aromatic product that may be used as a feed for the production of p-xylene. It may also be applied as a high-octane reformate for gasoline blending. In contrast to BTX, aromatics with higher carbon numbers, i.e. C9+, are considered less valuable. Conse quently, in a process for making aromatics, a high selec tivity to BTX is desirable.

As illustrated in the example, conversion of ethane leads to the formation of appreciable amounts of methane and higher aromatics, i.e., aromatics with carbon numbers 9 and higher. These higher aromatics are predominantly napthtalenes, such as 1- and 2-methylnaphthalene and in- danes and indenes as well as methyl-substituted indanes/in- denes which are mixtures of little value.

By actively adding a small amount of sulfur (as H ₂ S) to the ethane feed stream (process stream), the selectivity to wards methane and C9+ hydrocarbons is greatly suppressed, as illustrated in Example 1.

Example

Catalyst: 5 wt% Zn supported on H-ZSM-5 having a silica to alumina ratio of 40.

Conditions: 550°C, 3 bar abs., WHSV = 3, 10 vol% C ₂ H ₆ in N ₂ . In the figures are shown ethane conversion and product se lectivity over time on stream (TOS); closed symbols are re sults without ¾S, i.e. without ¾S being added; open sym- bols are results with 60 ppm ¾S in the feed.

The figures reveal that the conversion over time is essen tially the same with and without ¾S in the feed (Fig.l).

In particular, it is noted that the conversion in the pres- ence of ¾S is lower in the beginning, but soon (after about 20 hours) surpasses the conversion of the same cata lyst operating under sulfur-free conditions.

The BTX yield with ¾S in the feed is somewhat lower (Fig. 2), but the selectivity to ethylene (and other light ole fins) is significantly higher with ¾S in the feed (Fig.3). Further it is seen that the methane (Fig.4) and C9+ (Fig.5) selectivities are dramatically lower in the presence of H ₂ S.