WO2007003618A1

WO2007003618A1 - Process for producing a gas stream depleted of mercaptans

Info

Publication number: WO2007003618A1
Application number: PCT/EP2006/063786
Authority: WO
Inventors: Anders Carlsson; Thijme Last; Jayantilal Bhagvanji Rajani
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Canada Limited
Priority date: 2005-07-04
Filing date: 2006-07-03
Publication date: 2007-01-11
Also published as: CA2614169A1; CN101213009B; CN101213009A; EA200800234A1; EA014385B1; CA2614169C

Abstract

The invention provides a process for producing a gas stream depleted of RSH from a feed gas stream comprising natural gas, RSH and aromatic compounds selected from the group of benzene, toluene, o-xylene, m-xylene and p-xylene, the process comprising the steps of: (a) contacting the feed gas stream with absorbing liquid comprising a physical solvent in an aromatic compound removal zone to obtain loaded absorbing liquid comprising aromatic compounds selected from the group of benzene, toluene, o-xylene, m-xylene and p-xylene and a gas stream depleted of these aromatic compounds; (b) contacting the gas stream obtained in step (a) with solid adsorbent in a mercaptan removal zone to obtain solid adsorbent loaded with RSH and the gas stream depleted of RSH.

Description

PROCESS FOR PRODUCING A GAS STREAM DEPLETED OF

MERCAPTANS

The invention relates to a process for producing a gas stream depleted of mercaptans (RSH) from a natural gas stream comprising RSH and aromatic compounds selected from the group of benzene, toluene, o-xylene, m-xylene and p-xylene.

Producing a gas stream depleted of RSH from a natural gas stream comprising RSH and aromatic compounds selected from the group of benzene, toluene, o-xylene, m-xylene and p-xylene involves removal of RSH. Removal of RSH from gas streams comprising these compounds has always been of considerable importance in the past and is even more so today in view of continuously tightening environmental regulations .

Numerous natural gas wells produce what is called "sour gas", i.e. natural gas comprising RSH, often in combination with aromatic compounds selected from the group of benzene, toluene, o-xylene, m-xylene and p- xylene and additional sulphur-containing compounds such as H2S, sulphides, disulphides and thiophenes. The total amount of compounds is generally too high, making the natural gas unsuitable for direct use. In addition, natural gas may further comprise varying amounts of carbon dioxide. Depending on the intended use of the natural gas, carbon dioxide often also has to be removed. Generally, a total concentration of sulphur- containing compounds of less than 30 ppmv is desired. Sales gas specifications often mention total sulphur concentrations lower than 10 ppmv, or even as low as less than 4 ppmv.

One situation where removal of RSH from a natural gas stream is of importance is in cases whers the gas stream is intended for domestic use. Mercaptans, due to their odorous nature, can be detected at parts per million concentration levels. Thus, it is desirasle to have concentrations of RSH lowered to e.g. less than 10, or even less than 5 ppmv. Another situation where mercaptan removal is important is in the event that the natural gas stream is intended for use in a process involving a catalytic step. For example, natural gas can be used for the generation of synthesis gas, typically in a gasifier unit. The thus-formed synthesis gas is generally converted to hydrocarbons in a catalytic process, known in the art as a Fischer-Tropsch process. If RSH are present in the natural gas stream, they will react to form H2S in the gasifier, resulting in a synthesis gas stream comprising H2S. The thus-formed H2S may bind irreversibly on catalysts and cause sulphur poisoning. This results in a deactivated catalyst, which severely hampers the catalytic process. Hence, in cases where the process involves a catalytic step, removal of RSH to very low levels, as low as less than 2 ppmv or even in the ppbv range, is required.

Processes for the removal of RSH from a gas stream comprising natural gas and RSH are known in the art and are generally based on solid bed adsorption or physical and/or chemical absorption.

Physical absorption processes suffer from the fact that they frequently encounter difficulties in removing RSH to low concentration. Generally, large contactors are needed to achieve the desired low concentrations.

Chemical absorption processes in general are able to remove carbon dioxide and/or hydrogen sulphide without much difficulty. However, they suffer from the fact that they do not effectively remove RSH and often produce large amounts of waste. For example, in EP 229,587 a process is described wherein the gas stream is treated with an alkaline aqueous liquid. In the process described in EP 229,587, a high consumption of alkaline chemicals is needed to remove RSH.

Solid bed adsorption processes generally are suitable for adsorption of RSH. However, they can suffer from the fact that compounds other than RSH can also be adsorbed, resulting in the adsorption of only limited amounts of RSH or in the use of very large adsorbent beds. Producing a gas stream depleted of RSH from a gas stream comprising other compounds, especially when low levels of RSH are desired, is difficult. Regeneration of the adsorbent is relatively cumbersome. Especially large solid adsorbent beds take relatively more time for regeneration and disproportionately high quantities of regeneration gas are needed.

Therefore, there remains a need in the art for a simple and efficient process to remove PSH from a gas stream comprising RSH and aromatic compounds selected from the group of benzene, toluene, o-x^lene, m-xylene and p-xylene, thereby obtaining a purified gas stream having a low concentration of RSH. To this end, the invention provides a process for producing a gas stream depleted of RSH from a feed gas stream comprising natural gas, RSH and aromatic compounds selected from the group of benzene, toluene, o-xylene, - A -

m-xylene and p-xylene selected from the group of benzene, toluene, o-xylene, m-xylene and p-xylene, the process comprising the steps of:

(a) contacting the feed gas stream with .absorbing liquid comprising a physical solvent in an aromatic compound removal zone to obtain loaded absorbing Liquid comprising aromatic compounds selected from the grojp of benzene, toluene, o-xylene, m-xylene and p-xylene and a gas stream depleted of these aromatic compounds; (b) contacting the gas stream obtained in step (a) with solid adsorbent in a mercaptan removal zone to obtain solid adsorbent loaded with RSH and the gas stream depleted of RSH.

It has been found that aromatic compounds selected from the group of benzene, toluene, o-xylene, m-xylene and p-xylene, known collectively as BTX compounds, interfere with adsorption of RSH, especially when using zeolites as adsorbent. It is believed that preferential adsorption of these aromatic compounds takes place, especially when using zeolites as adsorbent. Therefore, a feed gas stream comprising RSH and BTX compounds is considered a difficult feed gas stream from which to produce a gas stream depleted of RSH using an adsorption process, especially when using zeolites as adsorbent. The process according to the invention offers a simple and effective way of producing a gas stream depleted of RSH even from such a feed gas stream, because BTX compounds are removed to a large extent prior to subjecting the gas stream to the adsorption process. This results in a considerably better adsorption of RSH onto the zeolite adsorbent, producing a gas stream depleted of RSH with a low concentration of RSH. A further advantage of the process according to the invention is that it enables the sizing Df the aromatic removal zone and the mercaptan removal ZDΠΘ according to need. This enables a versatile way of operating the process. In addition, by designing both zones according to requirement, process optimization can also be achieved.

In step (a) of the process accordinc_f to the invention, aromatic compounds selected from the group of benzene, toluene, o-xylene, m-xylene and p-xylene are transferred from the feed gas stream comprising natural gas, aromatic compounds selected from the group of benzene, toluene, o-xylene, m-xylene and p-xylene and RSH to the absorbing liquid. A loaded absorbing liquid comprising BTX compounds and a gas streaτi depleted of BTX compounds are obtained.

Natural gas is a general term that i 3 applied to mixtures of light hydrocarbons and optionally other gases (nitrogen, carbon dioxide, helium) derived from natural gas wells. The main component of natural gas is methane. Further, often ethane, propane and butane are present. In some cases (small) amounts of higher hydrocarbons may be present, often indicated as natural gas liquids or condensates. When produced together with oil, the natural gas is usually called associated gas. Other compounds that may be present in natural gas in varying amounts include H2S, aliphatic RSH, sulphides, disulphides, thiophenes and aromatic RSH.

Reference herein to RSH is to aliphatic mercaptans, especially C^-Cg mercaptans, more especially C1-C4 mercaptans; aromatic mercaptans, especially phenyl mercaptan; or mixtures of aliphatic and aromatic RSH. The invention especially relates to the removal of methyl mercaptan, ethyl mercaptan, normal- and Lso-propyl mercaptan and butyl mercaptan isomers from the feed gas stream. Mercaptans having 3 or more carbon atoms are henceforth referred to as C3+ RSH. The concentration of RSH and the type of RSH present in the feed gas stream can vary and depe ids on the source from which the first gas stream originates. Suitably the total feed gas stream comprises in the range of from 1 ppmv to 1 vol% RSH and from 1 ppmv to 1 vol% BTX compounds, preferably from 20 ppmv to 1 vol% RSH and from 5 ppmv to 1 vol% BTX compounds, based on the total feed gas stream. A feed gas stream comprising RSH and BTX compounds in these concentration ranges is considered a very difficult feed gas stream from which to produce a gas stream depleted of RSH using an adsorbent process.

Optionally, the feed gas stream may further comprise H2S, preferably in the range of from 1 ppmv to 10 vol%, more preferably from 0.1 to 10 vol% based on the total feed gas stream. It has been found that the presence of H2S hinders the removal of RSH in a conventional adsorption process, especially when using zeolites as adsorbent, since H2S is preferentially adsorbed on zeolites. Therefore, a gas stream comprising RSH, BTX compounds and H2S, wherein the concentration of H2S is in the range of from 0.1 to 10 vol% H2S is regarded as a very difficult feed gas stream for producing a gas stream depleted of RSH. The process according to the invention enables the production of a gas stream depleted of RSH even from such a feed gas stream, because in step (a) H2S will be removed from the feed gas stream to a large extent, resulting in a gas stream depleted of BTX compounds and of H2S. The feed gas stream may also comprise carbon dioxide, preferably in the range of from 0 to 40 vol%, more preferably from 0 to 30 vol%, based on the total feed gas stream. It is often desired to reduce the concentration of carbon dioxide, especially in cases where the feed gas stream comprises natural gas and the gas stream depleted of RSH is intended to be processed to liquefied natural gas (LNG) . The process according to the invention enables the production of a gas stream depleted of carbon dioxide having a low concentration of carbon dioxide, because in step (a) carbon dioxide will be removed from the feed gas stream to a large extent.

The absorbing liquid comprises a physical solvent capable of removing BTX compounds and optionally C3+ RSH from the feed gas stream. Preferably, ths amount of physical solvent is in the range of from 10 to 80, more preferably from 15 to 50, still more preferably from 20 to 35 parts by weight, based on the total absorbing liquid. The remainder of the absorbing liquid is suitably another solvent, preferably water and/or an amine solvent .

Preferred physical solvents are sele^cted from the group of sulfolane (cyclo-tetramethylenesulfone and its derivatives) , aliphatic acid amides, N-methylpyrrolidone, N-alkylated pyrrolidones and the corresponding piperidones, methanol, ethanol and mixtures of dialkylethers of polyethylene glycols. The most preferred physical solvent is sulfolane.

An advantage of using an absorbing liquid comprising a physical solvent is that removal of BTX compounds is achieved to low levels, even in the ppmv range. BTX compounds are carcinogenic and their emission must therefore be below certain levels. It is therefore necessary to reduce the concentration of BTX compounds, in the gas stream. More importantly, it ias been found that the presence of BTX compounds hinders the adsorption of RSH onto adsorbents, especially onto some types of zeolites. Hence, producing a gas stream 'depleted of RSH from a feed gas stream comprising RSH and BTX compounds through a conventional adsorption process, especially when using a zeolite adsorbent, is combe csome. Low levels, typically in the ppmv range, of ^SH in the RSH depleted gas stream cannot be achieved t irough a process using a zeolite adsorbent.

Another advantage of using an absorbing liquid comprising a physical solvent is that RS i comprising three carbon atoms or more (C3+ RSH) , considered as difficult to remove via a conventional adsorption process, will also be taken up in the ph/sical solvent and thereby removed from the feed gas stream.

Preferably, the absorbing liquid is a mixed solvent comprising a physical solvent and a chemical solvent. Absorption liquids comprising both chemical and physical solvents are preferred because they show good absorption capacity and good selectivity for H2S against moderate investment costs and operational costs. In addition, in the event that the feed gas stream comprises carbon dioxide, carbon dioxide can also be removed in the mixed absorption liquid depending on the solvent composition, resulting in a gas stream depleted of H;?S and of carbon dioxide. Another advantage of mixed systems is that they perform well at high pressures, especially between 20 and 90 bara. Hence, in the case that the feed gas stream is pressurised, for example if the feed gas stream is a natural gas stream obtained at high pressure, no depressurising step is needed. Yet another advantage is that the use of a combined physical/chemical absorbing liquid, rather than a physical absorbing liquid only, also results in the possibility of flashing any carbon dioxide at relatively high pressures (i.e. between 5 and 15 bara) . This reduces re-compression requirements, e.g. for re-injection.

Suitable chemical solvents are primary, secondary and/or tertiary amines, especially amines that are derived of ethanolamine, especially monoethanol amine (MEA) , diethanolamine (DEA) , triethanolamine (TEA) , diisopropanolamine (DIPA) and methyldiet ianolamine (MDEA) or mixtures thereof. The preferred chemical solvent is a secondary or tertiary amine, preferably an amine compound derived from ethanol amine, more especially DIPA, DEA, MMEA (monomethyl-ethanolamine) , MDEA, or DEMEA (diethyl- monoethanolamine) , preferably DIPA or MDEA.

The absorbing liquid may also furthe c comprise a so- called activator compound, optionally in combination with a chemical solvent. The addition of an activator compound to the absorbing liquid system is believed to result in an improved removal of acidic compounds . This is especially useful in the event that the feed gas stream further comprises H2S and/or carbon dioxide. Suitable activator compounds are piperazine, methyl ethanol amine, or (2-aminoethyl) ethanol amine, especially piperazine.

A preferred absorbing liquid system comprises sulfolane and a secondary or tertiary amine, preferably an amine compound derived from ethanol amine, more especially DIPA, DEA, MMEA (monomethyl-ethanolamine) , MDEA, or DEMEA (diethyl-monoethanolamine) , preferably DIPA or MDEA. In the aqueous absorbent in the present process the amount of water is preferably between 20 and 45 parts by weight, the amount of sulfolane is preferably between 20 and 35 parts by weight and the amount of amine is preferably between 40 and 55 parts by weight, the amounts of water, sulfolane and amine together being 100 parts by weight. The preferred ranges result in optimum carbon dioxide removal in most cases.

Another preferred absorbing liquid comprises in the range of from 15 to 45 parts by weight, preferably from 15 to 40 parts by weight of water, from 15 to 40 parts by weight of sulfolane, from 30 to 60 parts by weight of a secondary or tertiary amine derived from ethanol amine, and from 0 to 15 wt%, preferably from 0..5 to 10 wt% of an activator compound, preferably piperazine, all parts by weight based on total solution and the added amounts of water, sulfolane, amine and activator together being 100 parts by weight. This preferred absorbing liquid enables removal of carbon dioxide, hydrogen sulphide and/or COS from a gas stream comprising these compounds. This offers an advantage over other processes, which do not enable removal of carbon dioxide., When compared with the same absorbing liquid without tie addition of a primary or secondary amine compound, especially a secondary amine compound, one or more of the following advantages are obtained: the carbon dioxide absorption rate is faster, the loading amount is higher, the solvent/gas ratio is lower, the design of the plant is smaller and the regeneration heat requirement is lower (resulting in less cooling capacity) . When compared with an absorbing liquid comprising aqueous amines, especially DMEA and piperazine, the addition of sulfolane enables the production of a gas stream comprising carbon dioxide having intermediate pressures, e.g. pressures between 3 and 15 bara, preferably between 5 and 10 bara. - li lt is an advantage of the invention that by choosing a specific absorbing liquid in step (a) , the process can be adjusted to enable producing a gas stream depleted of RSH from feed gas streams further comprising other compounds, in particular H2S and/or carbon dioxide. The process also enables producing a gas stream depleted of RSH from feed gas streams having different concentrations of BTX compounds and/or other compounds, such as hydrogen sulphide or carbon dioxide. A choice can be made whether or not to remove certain compounds, for example carbon dioxide, and to what extent to remove these compounds. Hence, different compositions of the gas stream obtained in step (a) can be achieved.

Suitably, step (a) is carried out at a temperature in the range of from 15 to 90 ⁰C, preferab../ at a temperature of at least 20 ⁰C, more prefsrably from 25 0 80 ⁰C, still more preferably from 40 to 55 ⁰C, and even still more preferably at about 55 °C. Stsp (a) is suitably carried out at a pressure between 10 and 150 bar, especially between 25 and 90 bara.

Step (a) is suitably carried out in a zone having from 5-80 contacting layers, such as valve trays, bubble cap trays, baffles and the like. Structured packing may also be applied. The extent of Cθ2-removal can be optimised by regulating the solvent/feed gas ratio. A suitable solvent/feed gas ratio is from 1.0 to 10 (w/w) , preferably between 2 and 6.

As a result of the transfer of BTX compounds from the feed gas stream to the absorbing liquid, the gas stream obtained in step (a) is depleted of BTX compounds, and optionally of C3+ RSH, meaning that the concentration of

BTX compounds and optionally of C3+ RSH in the gas stream obtained in step (a) is lower than the concentration of these compounds in the feed gas stream, ^"t will be understood that the concentration of BTX compounds and optionally of C3+ RSH in the gas stream obtained in step

(a) depends on the concentration of these compounds in the feed gas stream. Suitably, the total concentration of BTX compounds in the gas stream obtained in step (a) is less than 500 ppmv, preferably in the range of from 1 ppmv to 100 ppmv, more preferably from 1 ppmv to 50 ppmv. Suitably, the concentration of C3+ RSH in the gas stream obtained in step (a) is less than 10 ppmv, preferably less than 5 ppmv.

It will be understood that the mercaptan concentration in the gas stream obtained after step (a) will depend on the mercaptan concentration in the feed gas stream. Suitably, mercaptan concentrations in the gas stream obtained after step (a) will be in the range of from 100 ppbv to 5 vol% .

In step (a) , loaded absorbing liquid is obtained comprising BTX compounds and optionally other compounds such as C3+ RSH, hydrogen sulphide and ODtionally carbon dioxide and other sulphur compounds such as carbonyl sulphide. Step (a) will usually be carπ.d out as a continuous process, which process also comprises the regeneration of the loaded absorbing liqαid. Therefore, in a preferred embodiment the aromatic compound removal zone comprises at least one regenerator therein loaded absorbing liquid is regenerated by transferring at least part of the compounds to a regeneration gas stream, typically at relatively low pressure and high temperature. Suitably, the regeneration temperature is in the range of from 70 to 150 ⁰C. The desired temperature is preferably achieved by heating with steam or hot oil. Preferably, a stepwise temperature increase is done. Suitably, regeneration is carried out at a pressure in the range of from 1 to 2 bara. The loaded absorbing liquid comprises BTX compounds and may further comprise C3+ RSH and optionally CO2 , H2S and/or COS. Appreciable amounts of other compounds from the feed gas gas stream, e.g. hydrocarbon condensates, may also be present in the loaded absorbing liquid. It may be advantageous to remove these compounds at least partially from ^H:he loaded solvent by flashing to a pressure higher that the sum of the partial pressures belonging to the compounds. Usually the flash is carried out at a pressure between 1 and 15 bara, preferably between 1 and 10 bar,a, more preferably ambient pressure. Flashing at atmospheric pressure is preferred. The temperature in the flashing operation is suitably in the range of from 50 to 120 ⁰C, preferably between 60 and 90 °C.

The regeneration process results in regenerated absorbing liquid and a loaded regeneration gas stream comprising BTX compounds, and optionally C3+ RSH, hydrogen sulphide, carbon dioxide and/or carbonyl sulphide. Suitably, sulphur compounds are removed from the loaded regeneration gas stream in a sulphur recovery unit, for example via the Claus process, Suitably, at least part of the regenerated liquid absorbent is used for removal of BTX compounds as described hereinbefore.

Preferably, at least part of the lean absorbent solvent is used again in the absorption stage of step (a) . Suitably, the lean solvent is heat exchanged with loaded solvent to use the heat elsewhere. In step (b) , the gas stream depleted of BTX compounds, obtained in step (a) is contacted with a solid adsorbent. Thereby, remaining RSH is transferred from the gas stream to the solid adsorbent. Suitable solid adsorbent materials include materials based on silica, silica gel, alumina or silica-alumina.

Preferably, zeolites are used in the mercaptan- adsorbent beds. Zeolites are solid adsorbents having openings capable of letting a species enter or pass. In some types of zeolites, the opening is suitably defined as a pore diameter whereas in other types the opening is suitably defined as openings in a cage structure. Zeolites having an average opening (pore diameter) of 5 A or more are preferred, more preferably zeolites having an average opening (pore diameter) in the range of from 6 to 8 A. Especially preferred are 13X zeolites having an average opening (cage structure) of aboir: 7.4 A. It is believed that zeolites having an average opening of 5 A or more allow adsorption of methyl mercaptan, ethyl mercaptan and n-propyl mercaptan and hence enable their removal from the gas stream. It is believed that zeolites having an average opening in the range of from 6 to 8 A, especially 13X zeolites, also allow adsorption of branched RSH, for example i-propyl mercaptan. Thus, zeolites having an average opening in the range of from 6 to 8 A, especially 13X zeolites offer an advantage over zeolites having a smaller average diameter because they allow removal of all RSH, including branched RSH. However, it has been found that BTX compounds also adsorb onto zeolites having an average opening in the range of from 6 to 8 A, especially 13X zeolites and hinder adsorption of RSH due to their preferential adsorption. It would therefore not be possible to produce a gas stream depleted of RSH, including C3+ RSH, from a feed gas stream comprising BTX compounds, using a solid adsorbent comprising zeolites having an average opening in the range of from 6 to 8 A, especially 13X zeolites. However, the process according to the invention enables the use of zeolites having an average opening in the range of from 6 to 8 A, especially 13X zeolites, as solid adsorbent because BTX compounds are removed prior to contacting the gas stream with solid adsorbent. Although at least part of the C3+ RSH will be removed in step (a) , typically there will be still some C3+ RSH present in the gas stream obtained in step (a) . The use of zeolites having an average opening in the range o.i from 6 to 8 A, especially 13X zeolites, as solid adsorbent in step (b) enables the removal of remaining C3+ RSH to achieve very low levels of total RSH concentration in the gas stream depleted of RSH. Levels as low as less than 1 ppmv can be thus achieved. In the case that water is present in the hydrocarbon stream, a more efficient process is obtained when the water is removed in a water removal unit preceding the removal of RSH, preferably by adsorbing ;he water on a zeolite molecular sieve having a opening of 5 A or less, preferably a opening of 3 or 4 A. In suci zeolites hardly any RSH are adsorbed, mostly water is adsorbed. In general, the capacity of such zeolites is higher than larger pore zeolites. The amount of water to be removed may be small or large, but preferably at least 60 wt% of the water is removed, preferably 90 wt% . Very suitably water is removed to a level of less than 1 %v in the gas stream leaving the water removal unit, preferably to a level less than 100 ppmv, more preferably to a level less than 5 ppmv. The operating temperature of the solid adsorbent beds in the mercaptan removal zone may vary between wide ranges, and is suitably between 0 and 80 °c, preferably between 10 and 60 °c, the pressure is suitably between 10 and 150 bara. The superficial gas velocity is suitably between 0.03 and 0.6 m/s, preferably between 0.05 and 0.40 m/s.

An especially preferred adsorbent is an adsorbent comprising 13 X zeolites. The advantage of using

13 X zeolites is that all RSH, including C3+ RSH, can be adsorbed. Although at least part of C3+ RSH will be removed in step (a) , depending on the conditions in step (a) and on the concentration of C3+ RSH in the feed gas stream, the gas stream obtained in s :ep (a) will typically still comprise some C3+ RSH. This offers the possibility of removing RSH to low level.3, suitably resulting in a gas stream depleted of RSJi having a concentration of less than 1 ppmv RSH. Typically, the process in the mercap^~:an adsorption unit results in a purified gas stream substantially free of RSH and mercaptan-adsorbent beds now Loaded with RSH. It will be understood that the process according to the present invention is preferably carried uut in a continuous mode, which will involve regeieration of the loaded adsorbent beds.

The adsorption of RSH on the mercaptan adsorbents can be reverted by contacting the mercaptan- loaded beds with a gas stream at elevated temperature or reduced pressure. Thereby, RSH are transferred from the mercaptan-adsorbent beds to the regeneration gas stream, resulting in a gas stream loaded with RSH, which gas stream is a first gas stream according to the invention. Suitable gas streams to this purpose are for example inert gas streams or hydrocarbonaceous gas streams. For the purposes of the invention, it is preferred to use as a regeneration gas stream a hydrocarbonaceous stream, especially part of the purified hydrocarbonaceous stream leaving the mercaptan- adsorption unit.

Preferably, two or more adsorbent beds comprising solid adsorbent, preferably zeolites, are used. Typically, at least one adsorbent bed is in an adsorbing mode and at least one adsorbent bed is a desorbing mode. Depending on the actual situation there may be combinations of two, three, four or even more adsorbent beds, one in absorbing mode, the others in different stages of desorbing mode.

Reference herein to a mercaptan-depleted gas stream is to a gas stream wherein the concentration of both H2S and the RSH has been reduced to a level which is acceptable for the intended purpose of the gas stream. The mercaptan-depleted gas stream can be processed further in known manners, for example by catalytic or non-catalytic combustion, to generate electricity, heat or power, or as a feed gas for a chemical reaction, or for the production of liguefied natural gas (LNG) , or for residential use. The invention also provides LNG obtained from liquefying the gas stream depleted Df RSH. The LNG thus-obtained typically has very low concentrations of compounds other than natural gas. It is an advantage of the process according to the invention that the gas stream depleted of RSH comprises very low levels of contaminants, especially of sulphur contaminants, enabling the production of LNG without the need of additional steps for removal of sulphur contaminants. In the process according to the invention, a gas stream having a very low concentration of RSH, suitably in the ppmv range, is produced in an efficient way. As only part of the RSH, namely C3+ RSH, is removed in the step (a) , the process avoids the use of very large and inefficient reactors. In step (b) a relatively small solid adsorption bed can be used to remove the remaining part of the RSH. This is due to the fact that almost all hydrogen sulphide has already been removed in the step (a) together with part of the RSH. Regeneration of such a bed is not very laborious or cumbersome. Thus, the combination of steps of the process acco :ding to the invention results in an overall efficient process for producing a gas stream depleted of RSH from a feed gas stream comprising RSH and BTX compounds, even in the presence of hydrogen sulphide, and carbon dioxide, while avoiding the disadvantages of only one technology or other technologies. In addition, treating the regeneration gas of the solid bed adsorber in a dedicated absorber optimises the process. The loaded solvent of the dedicated absorber is then regenerated in the same regenerator as is used for the main process.

The invention will now be illustrated by the following, non-limiting examples. Example 1 (comparative)

A feed gas stream having a composition as shown in table 1, column A, was contacted with an absorbing liquid comprising MDEA and piperazine in an absorber unit at a temperature of 45 ⁰C and a pressure of 63 bar g. The composition of the gas stream leaving the absorber unit is shown in table 1, column B. The gas stream leaving the absorber unit was contacted with a large adsorbent bed comprising 13 X zeolites. The gas stream leaving the adsorbent bed, measured using gas chromatography, still comprised RSH in concentrations above 10 ppmv. Example 2 (according to the invention)

A feed gas stream having a composition as shown in table 1, column A, was contacted with an absorbing liquid comprising a physical solvent at a temperature of 45 ⁰C and a pressure of 60 bar g. The composition of the gas stream leaving the absorber unit is shown in table 1, column C. The gas stream leaving the absorber unit was contacted with a small adsorbent bed comprising 13 X zeolites. The concentration RSH in the gas stream leaving the adsorbent bed, measured using gas chromatography, was below 2 ppmv.

Table 1: concentrations of components in mol%. Total BTX compounds and total RSH in ppmv.

From the examples it is evident that the process according to the invention enables producing a gas stream depleted of RSH having a concentration of RSH below 2 ppmv. The comparative processes result in gas streams having an RSH concentration above 10 ppmv.

Claims

C L A I M S

1. A process for producing a gas streair depleted of RSH from a feed gas stream comprising natural gas, RSH and aromatic compounds selected from the group of benzene, toluene, o-xylene, m-xylene and p-xylene selected from the group of benzene, toluene, o-xylene, m-xylene and p-xylene, the process comprising the steps of:

(a) contacting the feed gas stream with absorbing liquid comprising a physical solvent in an aromatic compound removal zone to obtain loaded absorbing ..iquid comprising aromatic compounds selected from the group of benzene, toluene, o-xylene, m-xylene and p-xylene and a gas stream depleted of these aromatic compounds;

(b) contacting the gas stream obtained in step (a) with solid adsorbent in a mercaptan removal zone to obtain solid adsorbent loaded with RSH and the gas stream depleted of RSH.

2. A process according to claim 1, wherein the solid adsorbent comprises a zeolite.

3. A process according to claim 2, wherein the zeolite has an average opening in the range of from 6 to 8 A.

4. A process according to any one of claims 1 to 3, wherein the absorbing liquid further comrises one or more solvents selected from the group of monoethanol amine (MEA), diethanolamine (DEA), triet ianolamine (TEA), diisopropanolamine (DIPA) and methyldiet ianolamine (MDEA) .

5. A process according to any one of claims 1 to 4, wherein the physical solvent comprises one or more compounds selected from the group of sulfolane, aliphatic acid amides, N-methylpyrrolidone, N-alkylated pyrrolidones and the corresponding piper:.dones, methanol, ethanol and dialkylethers of polyethylene glycols.

6. A process according to any one of claims 1 to 5, wherein the absorbing liquid further comprises water.

7. A process according to any one of claims 1 to 6, wherein the absorbing liquid comprises sulfolane, a secondary or tertiary amine, preferably an amine compound derived from ethanol-amine, more especially DIPA, DEA, MMEA (monomethyl-ethanolamine) , MDEA, or DEMEA (diethyl- monoethanolamine) , preferably DIPA or MDEA and optionally water .

8. A process according to any one of claims 1 to 7, wherein the concentration of RSH in the ::eed gas stream is in the range of from 1 ppmv to 1 vol%, based on the total feed gas stream.

9. A process according to any one of claims 1 to 8, wherein the concentration of aromatic compounds selected from the group of benzene, toluene, o-xyLene, m-xylene and p-xylene is in the range of from 1 ppmv to 1 vol%.

10. Liquefied natural gas, produced by cooling the gas stream depleted of RSH obtainable by a process according to any one of claims 1 to 9.