WO2016005402A1

WO2016005402A1 - Process for producing a purified gas stream by two stage absorption

Info

Publication number: WO2016005402A1
Application number: PCT/EP2015/065513
Authority: WO
Inventors: Raimo Edwin Gregor Poorte; Renze Wijntje
Original assignee: Shell Internationale Research Maatschappij BV; Shell Oil Co
Current assignee: Shell Internationale Research Maatschappij BV; Shell USA Inc
Priority date: 2014-07-07
Filing date: 2015-07-07
Publication date: 2016-01-14
Anticipated expiration: 2017-01-07

Abstract

The invention relates to a process for producing a purified gas from a sour feed gas comprising the following steps: (a) co-currently contacting sour feed gas with a loaded liquid absorbent for absorbing CO2 and/or H2S and/or SO2 to obtain a mixture of pre-treated sour feed gas and further loaded liquid absorbent; (b) separating the further loaded liquid absorbent; (c) contacting the pre-treated sour feed gas obtained in step (b) counter-currently with a liquid absorbent for absorbing CO2 and/or H2S and/or SO2 to obtain the purified gas and loaded liquid absorbent; and (d) supplying loaded liquid absorbent obtained in step (c) to step (a). The loaded liquid absorbent obtained in step (c) that is supplied to step (a) is cooled.

Description

PROCESS FOR PRODUCING A PURIFIED GAS STREAM BY TWO STAGE ABSORPTION

Field of the invention

The invention relates to a process for producing a purified gas from a sour feed gas comprising carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide.

Background to the invention

Gas streams such as from natural gas wells typically comprise contaminants such as carbon dioxide, hydrogen sulphide, mercaptans, carbonyl sulphide and carbon

disulphide that need to be removed before that gas stream can be further used. Processes for removing sulphur- comprising contaminants and carbon dioxide from so-called "sour gas", i.e. a gas stream contaminated with acid gass such as hydrogen sulphide (H₂S) , carbon dioxide (CO₂) and sulphur dioxide (SO₂) , are well known in the art. Such processes typically comprise an absorption step for removing hydrogen sulphide and other sulphur compounds and/or carbon dioxide from the gaseous feed stream by contacting such gaseous feed stream with a solvent, for example an amine solvent, in an absorption column. Thus a purified gaseous stream, often referred to as 'sweet gas' is obtained and a solvent loaded with contaminants. The loaded solvent is typically regenerated in a stripper to obtain lean solvent that is recycled to the absorption column .

Under certain circumstances there is a need for a high absorption capacity or to increase the capacity of an existing absorption column. Example of circumstances requiring a high absorption capacity are high volumes of sour gas and/or high concentrations of acid gases such as carbon dioxide, hydrogen sulphide or sulphur dioxide in the sour gas feed stream. Examples of circumstances requiring a capacity increase comprise an increase of the acid gas concentration in the sour feed gas, an increased supply of sour gas or a decrease in feed gas pressure (resulting in an increased feed gas volume) . The capacity of a single absorption column is limited, so that an additional absorption column may be needed if capacity needs to be increased .

It has therefore been proposed in the art to remove the bulk of acid gases from a sour feed gas, prior to supplying such gas to an absorption column. Such bulk removal of a portion of the acid gas allows sufficient removal of acid gases in an existing absorption column in situations where a capacity increase is needed, without the need for an additional absorption column.

In L. Lyddon and H. Nguyen, Analysis of Various Flow Schemes for Sweetening with Amines, Proceedings of the 78^th GPA Annual Convention, Nashville, TN: Gas Processors

Association, 1999, p. 177-184 is for example described a pre-contactor in the form of a static mixer or jet educator mixer in the sour gas feed stream to an absorber for bulk removal of acid gas in order to increase plant capacity with low capital expenditure. In the pre-contactor, the sour feed gas is contacted with lean amine. A separator is placed downstream of the pre-contactor to separate rich amine from the pre-treated sour gas. The pre-treated sour gas is supplied to a conventional counter-current

absorption column. It is mentioned that the bottom of the absorption column may be used as a separator provided the gas/liquid feed into the bottom of the column does not cause a problem with the inlet vapour distributor. The rich amine is regenerated in a regenerator, together with rich amine from the absorption column.

In C.R. Isom and J. A. Rogers, Utilizing High

Efficiency Co-Current Contactors to Optimize Existing Amine Treating Units, Laurance Reid Gas Conditioning Conference proceedings, Norman, Oklahoma, 1994 is disclosed a line-up wherein a sour feed gas is contacted in a co-current

contactor with rich amine from an absorption tower, to obtain a mixture of pretreated sour gas and very rich amine. This mixture is supplied to the bottom of the counter- current absorption column. Lean amine is supplied to the absorption column. There is no mention of separation of loaded liquid effluent from the co-current contactor. A disadvantage of the process disclosed in Isom and Rogers is that acid gases absorbed in the very rich amine easily flash out of the amine when introduced in the absorption column. This especially occurs when the temperature of the (very) rich absorbent is increased.

There is a need for an improved method for bulk

removal of acid gases from a sour feed gas upstream of an absorption column.

Summary of the invention

It has now been found that bulk removal of acid gases from a sour feed gas stream upstream of a counter-current absorption column (or "counter-current absorber") by means of a co-current pre-contactor can be improved by using cooled loaded absorbent (rich absorbent) from the

absorption tower in the pre-contactor and by separating the bulk of the absorbent from the pretreated gas prior to subjecting the pre-treated gas to further acid gas removal in the counter-current absorber.

Accordingly, the invention relates to a process for producing a purified gas from a sour feed gas comprising carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide, the process comprising the following steps:

(a) co-currently contacting at least part of the sour feed gas in a co-current contactor with a loaded liquid absorbent for absorbing part of the carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide to obtain a mixture of pre-treated sour feed gas and further loaded liquid absorbent;

(b) separating the further loaded liquid absorbent from the mixture obtained in step (a) to obtain pre- treated sour feed gas and a liquid stream of further loaded liquid absorbent;

(c) contacting the pre-treated sour feed gas obtained in step (b) in an absorption column counter- currently with a liquid absorbent for absorbing carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide to obtain the purified gas and loaded liquid absorbent; and

(d) supplying at least part of the loaded liquid

absorbent obtained in step (c) to step (a) , wherein the loaded liquid absorbent that is supplied to step (a) is cooled prior to being supplied to step (a) and/or during contacting with the sour feed gas in step (a) . An important advantage of the process according to the invention is that use is made of the very efficient heat transfer conditions that exist in a co-current liquid-gas contactor. By cooling the loaded absorbent that is supplied to the pre-contactor either prior to or during the pre- contacting, use is made of these conditions and a large part of the exothermic heat produced in the absorption of acid gases can be efficiently cooled away. Thus, at any given temperature, a larger mole fraction of acid gas per volume absorbent can be loaded. Accordingly, the counter- current absorber can be operated at a relatively low

temperature, therewith improving the absorber efficiency and reducing the risk of corrosion in the absorption column

An advantage of the process according to the invention is that sour feed gas with high concentrations of acid gases, in particular carbon dioxide, can be purified using a single absorption column. In plant with an existing absorption column, the process can be used for retrofitting such plant if an increased plant capacity is needed.

Moreover, the process according to the invention can deal with large variations in feed gas volume or composition, in particular acid gas content, since the process can be operated by (partially) bypassing step (a) and dynamic variations in feed gas flow and/or acid gas concentration can be overcome without the need for adapting operation of the counter-current absorber.

Drawings

Fig 1: Process line-up.

Fig 2: Series of contactor/separation units.

Detailed description of the invention

In the process according to the invention a purified gas is produced from a sour feed gas that comprises carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide. The feed gas may be any sour feed gas from which carbon dioxide and/or hydrogen sulphide need to be removed, optionally together with other compounds such as sulphur dioxide (SO₂) , carbonyl sulphide (COS) , carbon disulphide

(CS₂) or mercaptans, and/or water. Examples of suitable sour feed gases include natural gas, refinery gases, associated gas, biogas, synthesis gas (syngas) or other industrial process gases. The process is particularly suitable for sour feed gases that are available at elevated pressure.

Preferably the feed gas comprises in the range of from 40 to 99 v/v %, more preferably 50 to 98 v/v %, even more preferably 60 to 95 v/v % of methane.

In the process according to the invention, at least part of the sour feed gas is contacted in a co-current contactor with a loaded liquid absorbent to obtain a gas/liquid mixture of pre-treated sour feed gas and further loaded liquid absorbent (pre-contactor step (a) ) . The further loaded liquid absorbent is then separated from the gas/liquid mixture in separation step (b) and a gaseous stream of pre-treated sour feed gas and a liquid stream of further loaded liquid absorbent are obtained. In absorption step (c) , the pre-treated sour feed gas obtained in

separation step (b) is counter-currently contacted with a lean liquid absorbent to obtain a purified gas and loaded liquid absorbent. In step (d) , at least part of the liquid absorbent thus obtained is supplied to pre-contactor step

(a) to serve as the loaded liquid absorbent. The loaded liquid absorbent that is supplied to step (a) is at least partially cooled prior to being supplied to step (a) or during contacting with the sour feed gas in step (a) . Preferably, the process further comprises a regeneration step (e) wherein the liquid stream of further loaded liquid absorbent obtained in separation step (b) is regenerated in a regeneration unit to obtain a gas stream comprising acid gases that have been desorbed from the liquid absorbent, and a regenerated (lean) liquid absorbent, wherein the regenerated liquid absorbent is the liquid absorbent in step (c) .

In pre-contactor step (a) , at least part of the sour feed gas is contacted in a co-current contactor with a loaded liquid absorbent for absorbing part of the carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide.

Depending on inter alia the concentration or mass flow rate of acid gases in the sour feed gas, the capacity of the counter-current absorber in step (c) , all or part of the sour feed gas that is to be treated in absorption step (c) is contacted with the loaded liquid absorbent in pre- contactor step (a) . If only part of the sour feed gas is supplied to pre-contactor step (a) , the remainder of the sour feed gas is directly supplied to the counter-current absorber in step (c) , suitably together with the pre- treated sour gas.

The co-currently contacting of sour feed gas and

liquid absorbent in step (a) is carried out such that acid gases can be absorbed in the liquid absorbent. Suitable ways for absorption under co-current contacting conditions and suitable co-current contactors are well-known in the art .

The co-current contactor may for example comprise one or more tubes through which the sour feed gas and the

loaded liquid absorbent are co-currently flowing.

Preferably, the sour feed gas and the loaded liquid absorbent are co-currently flowing under annular-dispersed flow conditions. The co-current contactor may comprise one or more tubes provided with static mixers or other

internals to create turbulence and/or mixing. This allows improved gas/liquid contact area and mixing and more

efficient mass transfer. If static mixers or other

internals are employed, pressure drop is suitably

maintained within reasonable limits.

The one or more tubes may have any suitable

configuration, preferably they are configured such that the pressure drop of the gas is at most 0.5 bar, more

preferably at most 0.2 bar. In order to avoid excessive pressure drops, the one or more tubes preferably are

straight tubes.

The one or more tubes may have any suitable

orientation. Preferably, the orientation is such that

liquid hold-up is promoted in order to increase liquid residence time, whilst significant pressure drop is

prevented. A suitable system is described in WO 2013/041515, said system comprising two or more separation units in series, said separation units comprising a rising conduit for transporting a contaminated gas stream into a separator and a descending conduit for removing separated liquid absorbent, and wherein the the descending conduit of a second separation unit is fluidly connected to the rising conduit of a first separation unit and wherein the height of the descending conduit of the second separation unit is selected such that during use the hydrostatic force in the descending conduit of the second separation unit can induce the liquid absorbent in the rising conduit of the first separation unit to flow to the inlet of the first separator. Preferably, the sour feed gas is co-currently

contacted with the loaded liquid absorbent at a gas flow velocity of at least 2 meters per second (m/s) and at most 30 m/s, preferably in the range of from 2 to 10 m/s, most preferably in the range of from 4-10 m/s. Preferably, the gas flow velocity is maintained such that a dispersed annular flow regime is obtained comprising a multitude of small liquid droplets in a gas-continuous phase. Typically, the annular dispersed (turbulent) flow conditions are characterized by a Reynolds number (based on gas density, gas velocity and gas viscosity) that exceeds 3000, more preferably exceeds 100000.

Preferably, the sour feed gas is co-currently

contacted with the loaded liquid absorbent at a pressure of at least 1 bar absolute (bara) and at most 150 bara, more preferably in the range of from 30 to 130 bara, even more preferably in the range of from 40 to 120 bara, most preferably in the range of from 60 to 80 bara. It is preferred that the operational pressure of the co-current contactor section is either equal to the upstream

production system pressure, or equal to the downstream pressure. Preferably, the highest of the upstream and downstream pressure is chosen, since this allows to have maximum sour gas treatment capacity in equipment of a given volume .

Preferably the co-current contactor comprises one or more conduits and the sour feed gas is co-currently

contacted with the loaded liquid absorbent in the one or more conduits at annular-dispersed flow conditions.

Within the context of the present invention, a co- current or compact contactor is defined as a conduit for gas and liquid, where gas and liquid flow in a co-current direction, and wherein the liquid contains a liquid

absorbent that can absorb the gaseous contaminant by

chemical reaction or physical absorption or a combination thereof. The shape and dimensions of the conduit in a

compact contactor can be selected from a range of options, including straight circular conduits in either horizontal or vertical orientation, straight circular conduits in combined horizontal and vertical orientations using

multiple bends, circular conduits in helical orientation, circular conduits in zig/zag orientation, square conduits in any of the orientations described above and/or flexible conduits in any of the above orientations. To promote heat and/or mass transfer between gas and liquid phases, promote chemical reaction in the liquid solvent, and/or promote physical absorption in the liquid solvent it is

advantageous to enhance the turbulence in the compact

contactor .

In a preferred embodiment, the co-current contactor is a tube-and-shell heat exchanger comprising a multitude of parallel conduits and having a conduit side and a shell side. In such co-current contactor, the sour feed gas is co-currently contacted with the loaded liquid absorbent at the tube side and a coolant is flowing at the shell side. Thus, the loaded liquid absorbent that is supplied to pre- contactor step (a) is cooled during contacting with the sour feed gas in the co-current contactor.

In such tube-and-shell heat exchanger, the conduits (tubes) suitably contain the high-pressure sour gas flow. Thus, the shell only needs to contain the coolant, which is generally of much lower pressure than the sour process flow; this allows for designing the shell as a light weight

structure. Since the sour process gas flow tends to be corrosive, the tubes are preferably pepared of a material that is corrosion resistant. The coolant is preferably non^¬ toxic, non-flammable and/or non-corrosive, thus allowing for a low cost material for the shell side. The shell-in- tube cooling topology is advantageous as it gives a high area for heat transfer between coolant and sour gas, while allowing for high process gas velocity, thus maximizing sour gas mass flow rate capacity. The coolant can be suitably selected to be a liquid, a refrigerant, or a boiling flow, a phase change material. These can be

selected as water, seawater, air, refrigerant R-XXX (where XXX designates the international code for commercial refrigerants), oil, synthetic oil, ionic mixtures "salts", or any combination thereof.

As explained herein, reaction of liquid absorbent with C0₂ and/or H₂S and/or S0₂ is an exothermic process which tends to increase the temperature of both gas and liquid. Increasing temperature reduces gas density, thus increasing volumetric gas rates, which reduces the mass flow rate capacity, increases pressure drop and/or excessive

avaporation of absorbent. It is beneficial that the liquid absorbent has a large thermal capacity and large latent heat. Conventional design approach selects the solvent always warmer than the feed gas in order to prevent

condensation of hydrocarbon components that may cause foaming .

It is an advantage of the process according to the invention that at least part of the heat produced in pre- contacting step (a) is taken away by using cooled loaded absorbent in step (a) or by cooling the loaded absorbent during the pre-contacting in step (a) . By using cooled absorbent or by cooling the absorbent during step (a) , use is made of the efficient heat transfer conditions that typically exist in a co-current liquid-gas contactor, in particular under conditions of high gas flow velocity and high gas pressure at which sour feed gases such as natural gas are usually available.

Typically in acid gas removal absorbers, the absorber is operated at a temperature of at least 25 °C in order to avoid undesired formation of so-called gas hydrates. It has, however, been found that in the co-current contactor of the process according to the invention, the conditions are such that gas hydrates formed are decomposed as a result of exothermic heat produced in the absorption reaction.

Therefore, the co-current contactor may be operated at a temperature at which gas hydrates may be formed, in

particular if operated at high gas velocities in tubes having a sufficient large diameter (to avoid blocking of the tubes due to gas hydrate formation) .

Preferably, the temperature of the pre-treated sour feed gas obtained in step (b) which is fed to the counter- current absorption column in step (c) is in the range of 20-50 °C, more preferably 25-40 °C, most preferably about 30 °C. It is further preferred that liquid absorbent

entering the co-current contactor in step (a) has a

temperature in the range of from 0-30 °C, more preferably in the range of from 10-25 °C. It may be advantageous to supply cooled loaded absorbent to the pre-contractor at multiple points of supply (injection points) along the length of the pre-contactor . In this way, exothermic heat can be removed promptly upon creating such heat. Suitably, cooled loaded absorbent is injected in the warm gas/solvent mixture in such a proportion that the heat capacity of the cooled loaded absorbent is just sufficient to compensate for the exothermic heat of reaction of solvent and gaseous contaminant. This effectively allows to operate the co- current absorption process (near) isothermally . It is therefore advantageous to inject solvent in multiple axial locations of the co-current contactor, where the injection rates at each location and/or the temperature of injected absorbent is adjusted in such a way that the temperature is maintained at the optimum level.

Cooling of loaded liquid absorbent may be done by any means known in the art, for example by heat exchanging loaded liquid absorbent against a coolant such as air, water, seawater, or a refrigerant. The process according to the invention is particularly suitable to be applied for purifying sour natural gas that is to be liquefied to produce liquefied natural gas (LNG) . Cooling means or refrigerants that are available in a process for liquefying the natural gas can then advantageously be used to cool the loaded liquid absorbent prior to or during pre-contacting step (a) . The process according to the invention can suitably be applied on a platform or floating vessel, in particular a platform or floating vessel for production of LNG, since these commonly have space and/or weight

restrictions that mandate compact solutions.

In step (a) sour feed gas is co-currently contacted with loaded liquid absorbent and a gas/liquid mixture of pre-treated sour feed gas and further loaded liquid

absorbent is obtained. In subsequent separation step (b) , further loaded liquid absorbent is separated from the gas/liquid mixture, such that a gaseous stream of pre- treated sour feed gas and a liquid stream of further loaded absorbent are obtained. The gaseous stream may still comprise minor amounts of further loaded liquid absorbent ("liquid carry-over") , preferably in an amount of at most 5 %, more preferably in the range of from 0-2 %, based on the mass flow of lean solvent entering the main (counter- current) absorber. Such remainders of liquid absorbent will be removed from the gaseous stream in subsequent absorption step (c) .

Step (b) may be carried out in any suitable gas/liquid separator. Suitable gas/liquid separators are well-known in the art. The separator may for example by a cyclonic

separator or a centrifugal separator. The separator may be an axial cyclone. The separator may be an inline separator.

The loaded liquid absorbent obtained in step (c) , or a part thereof, is sent to step (a) . This loaded liquid

absorbent stream contains less loaded absorbent than the further loaded liquid absorbent obtained in separation step (b) . The further loaded liquid absorbent obtained in separation step (b) can be regenerated in regeneration step (e) .

In a process according to the present invention, a part of the removal of carbon dioxide and/or hydrogen

sulphide and/or sulphur dioxide thus is performed in

steps (a) and (b) . Hence, a part of the sour feed gas used in step (a) is not processed in the absorption column of step (c) . This saves capacity in the absorption column of step (c) . With a process according to the present invention optimal removal can be achieved with at a relatively low solvent circulation rate.

The loaded liquid absorbent that is supplied to

step (a) is cooled prior to being supplied to step (a) and/or during contacting with the sour feed gas in step (a) .

This enhances the absorption capacity of the loaded liquid absorbent that is supplied to step (a) . This thus further reduces the amount of sour feed gas that is processed in the absorption column of step (c) . Additionally, when cooler pre-treated sour feed gas is obtained in step (b) , the absorption process in the absorption column of step (c) is more efficient.

Preferably, the further loaded liquid absorbent is separated from the mixture obtained in step (a) directly downstream of the co-current contactor and upstream of the counter-current absorber. By separating the mixture

directly downstream of the co-current contactor, use can be made of the typically high gas flow velocity of the

gas/liquid mixture exiting the co-current contactor, resulting in effective cyclonic separation. Steps (a) and (b) may be carried out in a combined contactor/separation unit or in a series of multiple combined

contactor/separation units, for example in a series of combined contactor/separation units as disclosed in WO 2013/041545. Preferably in such series-combined

contactor/separation units, the sour feed gas is co- currently contacted with liquid absorbent that is separated from the subsequent combined contactor/separation unit. In this way counter-current gas-liquid extraction is effected, whilst gas and liquid flow co-currently through the

contactor in step (a) . Pre-treated sour feed gas obtained after gas/liquid separation in the final combined

contactor/separation unit is then supplied to the counter- current absorber in step (c) .

In an embodiment of the invention, steps (a) and (b) are carried out in a system as disclosed in WO2013/041545 comprising at least two contactor/separation units in series. An advantage of using the system of WO2013/041545 is that it can be used subsea, or more general underwater. If used subsea, the loaded liquid absorbent is suitably cooled by seawater and no additional means for cooling might be needed.

Alternatively, separation step (b) may be carried out in a bottom section of the counter-current absorber. The gas/liquid mixture of pre-treated sour feed gas and further loaded liquid absorbent is then introduced in the bottom section of the counter-current absorber. It is important that flashing of carbon dioxide or other acid gases from the further loaded absorbent upon introduction of the mixture into the counter-current absorber, is prevented. Therefore, the gas/liquid mixture is preferably introduced into the bottom section of the counter-current absorber.

In a preferred embodiment, flashing out of gas from the further loaded liquid absorbent is prevented by

minimizing thermal and spatial contact between said further loaded liquid absorbent and the liquid absorbent supplied to the absorption column in step (c) . This is suitably achieved by providing a liquid tray in a bottom section of the absorption column, wherein said liquid tray is

positioned above the inlet for the stream comprising pre- treated sour feed gas and further loaded liquid absorbent, and wherein the further loaded liquid absorbent remains in the bottom section under the influence of gravity while gaseous compounds are allowed to rise in the absorption column .

In absorption step (c) , the pre-treated sour feed gas obtained in separation step (b) is counter-currently contacted with a liquid absorbent for absorbing carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide in an counter-current absorber to obtain the purified gas stream and loaded liquid absorbent. Such absorption steps are well-known in the art.

The liquid absorbent may be any liquid capable of removing carbon dioxide and/hydrogen sulphide and/or

sulphur dioxide from the sour feed gas stream. The choice of absorbing liquid depends inter alia on the type of

contaminants to be removed. A preferred liquid absorbent comprises a chemical solvent as well as a physical solvent.

Suitable chemical and physical solvents are known in the art. Any suitable solvents known in the art may be used.

Examples of suitable chemical solvents are primary,

secondary and/or tertiary amines. A preferred chemical solvent is a secondary or tertiary amine, more preferably an amine compound derived from ethanol amine, more

especially DIPA, DEA, MMEA (monomethyl-ethanolamine ) , MDEA, or DEMEA (diethyl-monoethanolamine) , preferably DIPA or

MDEA. Suitable physical solvents are sulfolane (cyclo- tetramethylenesulfone) and its derivatives, aliphatic acid amides, N-methylpyrrolidone, N-alkylated pyrrolidones and the corresponding piperidones, methanol, ethanol and

dialkylethers of polyethylene glycols or mixtures thereof. The preferred physical solvent is sulfolane. The liquid absorbent may further comprise a so-called activator

compound. Suitable activator compounds are piperazine, methyl-ethanolamine, or (2-aminoethyl) ethanolamine,

especially piperazine. A particularly preferred liquid absorbent comprises sulfolane, MDEA and piperazine.

The liquid absorbent typically comprises water,

preferably in the range of from 15 to 45 parts by weight, more preferably of from 15 to 40 parts by weight of water.

Suitably, absorption step (c) is carried out at a temperature in the range of from 0 to 100 °C, more preferably from 25 to 40 °C, still more preferably from 30 to 45 °C. The liquid absorption is suitably carried out at a pressure between 10 and 150 bar (absolute) , preferably between 25 and 90 bar (absolute) . Suitably, absorption is carried out in the dense phase. Absorbing liquids

comprising a chemical and a physical solvent perform well at high pressures, especially between 20 and 90 bar

(absolute) .

In step (d) , at least a part of the loaded liquid absorbent obtained in step (c) is supplied to step (a) . Preferably at least 30%, more preferably in the range of 30 to 100%, even more preferably 50 to 100%, even more

preferably 70 to 100%, still more preferably 85 to 100%, still more preferably 95 to 100%, of the loaded liquid absorbent obtained in step (c) to step (a) .

The purified gas stream obtained in step (a) is depleted in carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide, meaning that the concentration of carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide in the purified gas stream is lower than the concentration of hydrogen sulphide in the feed gas stream. It will be understood that the concentration of carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide in the purified gas stream obtained in step (c) will depend on the carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide concentration in the sour feed gas and the conditions applied in steps (a) to (c) . Typically, the sour feed gas comprises carbon dioxide in a concentration in the range of from 5 to 90 mol%, preferably of from 10 to 90 mol%, more preferably of from 20 to 60 mol%. In the process according to the invention, it is for example possible to reduce the content of carbon dioxide to a concentration in the range of from 50 ppmv to 6 mol% in the purified gas by using a single counter-current absorber.

Preferably the co-current contactor of step (a) is placed in series with the absorption column of step (c) .

In the absorption step (c) , loaded liquid absorbent comprising contaminants such as carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide and optionally other contaminating compounds such as carbonyl sulphide, carbon disulphide or mercaptans is obtained. At least part of the loaded liquid absorbent is supplied to step (a) to serve as the loaded liquid absorbent in step (a) . If not all loaded liquid absorbent is supplied to step (a) , the remainder of the loaded liquid absorbent is preferably regenerated in optional regeneration step (e) , together with the further loaded liquid absorbent obtained in separation step (b) .

In optional regeneration step (e) , the liquid stream of further loaded liquid absorbent obtained in separation step (b) , optionally together with part of the loaded liquid absorbent obtained in step (c) , is regenerated by transferring at least part of the contaminants to a

regeneration gas stream. Regeneration of loaded liquid absorbent is well-known in the art and any suitable

conditions and configurations may be applied. Suitably, regeneration takes place at relatively low pressure and high temperature. The regeneration is suitably carried out by heating in a regenerator at a relatively high

temperature, suitably in the range of from 70 to 150 °C. The heating is preferably carried out with steam or hot oil in a reboiler. Preferably, the temperature increase is done in a stepwise mode. Suitably, regeneration is carried out at a pressure in the range of from 1 to 10 bara, more suitably 1-3 bara.

In a preferred embodiment, prior to regeneration part of the further loaded liquid absorbent is supplied to a flash vessel to produce a semi-loaded liquid absorbent; this semi-loaded liquid absorbent is supplied to the co- current contactor in step (a) . Thus, the co-current

contactor receives both loaded liquid absorbent from counter-current absorption step (c) and semi-loaded liquid absorbent obtained by flashing part of the further-loaded liquid absorbent obtained in step (b) prior to regeneration of the remaining further-loaded liquid absorbent.

After regeneration, regenerated liquid absorbent (lean absorbent) is obtained and a sour gas stream comprising carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide. The regenerated absorbing liquid thus obtained is preferably used as the liquid absorbent in step absorption step (c) . In order to reduce the heat and steam duty of regeneration step (e) , it may be advantageous to heat exchange the stream of (further) loaded liquid absorbent to be regenerated against regenerated liquid absorbent from the regenerator and/or against condensate from the reboiler of the regenerator. Alternatively or additionally the heat and steam duty may be reduced by flashing regenerated liquid absorbent to obtain a stream of liquid lean

absorbent that may be used in absorption step (c) and a gaseous stream of lean absorbent that may be recycled, after recompression to the pressure of the regenerator, to the regenerator to provide additional steam. A further possibility to reduce heat and steam duty is to flash steam condensate from the reboiler of the regenerator and to introduce flashed steam condensate as additional stripping steam into the regenerator.

In another embodiment of the invention, a sour feed gas comprising carbon dioxide and hydrogen sulphide is supplied to step (a) to obtain a mixture of pre-treated sour feed gas comprising carbon dioxide and further loaded liquid absorbent comprising absorbed hydrogen sulphide, and wherein the pre-treated sour feed gas comprising carbon dioxide obtained in separation step (b) is contacted in step (c) in an absorption tower counter-currently with a liquid absorbent for absorbing carbon dioxide, and wherein the further loaded liquid absorbent comprising absorbed hydrogen sulphide obtained in separation step (b) is

supplied to a Claus separation process for converting

hydrogen sulphide to elemental sulphur and regenerated liquid absorbent, and wherein preferably at least part of said regenerated liquid absorbent is the liquid absorbent in step (c) .

Detailed description of the drawings

The invention is illustrated by the following non- limiting examples.

Figure 1 illustrates a process line-up according to the invention. Sour gas (1) is contacted co-currently with a loaded liquid absorbent (2) in a co-current contactor (3) .

C02 and/or H2S and/or S02 is/are absorbed by the

loaded liquid absorbent (2) . A mixture of a pre-treated sour feed gas and a further loaded liquid absorbent is obtained .

The obtained mixture is separated in a separator (4) into a pre-treated sour feed gas (5) and a liquid stream of further loaded liquid absorbent (11) . In an absorption column (6), pre-treated sour feed gas (5) is contacted counter-currently with a liquid absorbent (7) . Purified gas (8) and loaded liquid

absorbent (2) are obtained. Loaded liquid absorbent (2) is cooled (10) prior to being supplied to the co-current contactor (3) .

The further loaded liquid absorbent (11) is

regenerated in a regeneration unit (12) . A sour gas stream (13) and a regenerated liquid absorbent (7) are obtained. Regenerated liquid absorbent (7) is used in absorption column (6) .

Figure 2 illustrates a series of contactor/separation units according to the invention. Sour gas (1) is contacted co-currently with a loaded liquid absorbent (40) in a co- current contactor (3) .

C02 and/or H2S and/or S02 is/are absorbed by the loaded liquid absorbent (40) . A mixture of a pre-treated sour feed gas and a further loaded liquid absorbent is obtained. The obtained mixture is separated in a

separator (4) into a pre-treated sour feed gas (15) and a liquid stream of further loaded liquid absorbent (50) .

The further loaded liquid absorbent (50) is

regenerated in a regeneration unit (not shown) .

Pre-treated sour feed gas (15) is sent to a further contactor/separation unit (13, 14) . A liquid stream of loaded liquid absorbent (40) is recycled to co-current contactor (3) .

Further pre-treated sour feed gas (25) is contacted co-currently with a loaded liquid absorbent (60) from an absorber (not shown) in co-current contactor (23) . The obtained mixture is separated in a separator (24) . Even further pre-treated sour feed gas (35) is sent to an absorber (not shown) .

Claims

C L A I M S

1. A process for producing a purified gas from a sour feed gas comprising in the range of from 40 to 99 v/v % methane, and carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide, the process comprising the following steps:

(a) co-currently contacting, at a pressure in the range of from 30 to 130 bar (absolute) , more preferably in the range of from 40 to 120 bar, most preferably in the range of from 60 to 80 bar, at least part of the sour feed gas in a co-current contactor with a loaded liquid absorbent for absorbing part of the carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide to obtain a mixture of pre-treated sour feed gas and further loaded liquid absorbent;

(c) contacting the pre-treated sour feed gas obtained in step (b) in an absorption column counter- currently with a liquid absorbent for absorbing carbon dioxide and/or hydrogen sulphide and/or sulphur dioxide to obtain the purified gas and loaded liquid absorbent; and (d) supplying at least part, preferably at least 30%, more preferably in the range of 30 to 100%, even more preferably 70 to 100%, still more preferably 95 to 100%, of the loaded liquid absorbent obtained in step (c) to step (a) , wherein the loaded liquid absorbent that is supplied to step (a) is cooled prior to being supplied to step (a) and/or during contacting with the sour feed gas in step (a) ; and wherein the co-current contactor of step (a) is placed in series with the absorption column of step (c) .

2. A process according to claim 1, wherein the loaded

liquid absorbent that is supplied to step (a) is cooled during contacting with the sour feed gas in step (a) , and wherein the co-current contactor is a tube-and-shell heat exchanger comprising a multitude of parallel tubes and having a tube side and a shell side, and wherein the sour feed gas is co-currently contacted with the loaded liquid absorbent at the tube side and a coolant is flowing at the shell side.

3. A process according to claim 1 or 2, wherein in step (a) the at least part of the sour feed gas is co-currently contacted with the loaded liquid absorbent at a gas flow velocity of at least 2 m/ s and at most 30 m/s,

preferably in the range of from 2 to 10 m/s, most preferably in the range of from 4-10 m/s.

4. A process according to any one of the preceding claims, wherein the liquid absorbent supplied to the co-current contactor in step (a) has a temperature in the range of from 0-30 °C, more preferably in the range of from 10- 25 °C and/or wherein step (a) is carried out such that the temperature of the pre-treated sour feed gas obtained in step (b) and supplied to the absorption column in step (c) is in the range of 20-50 °C, more preferably 25-40 °C, most preferably about 30 °C.

5. A process according to any one of the preceding claims, wherein the process further comprises the following step

(e) regenerating the further loaded liquid absorbent in a regeneration unit to obtain a sour gas stream and a regenerated liquid absorbent, wherein the

regenerated liquid absorbent is the liquid absorbent in step (c) ; and wherein preferably prior to regeneration part of the further loaded liquid absorbent is supplied to a flash vessel to produce a semi-loaded liquid absorbent and wherein the semi-loaded liquid absorbent is supplied to the co-current contactor in step (a) .

6. A process according to any one of the preceding claims, wherein the further loaded liquid absorbent is separated from the mixture obtained in step (a) directly

downstream of the co-current contactor and upstream of the counter-current absorber using a gas/liquid

separation process, preferably using an axial cyclone.

7. A process according to any one of the preceding claims, wherein separation step (b) is carried out in a bottom section of the absorption column, wherein flashing out of gas from the further loaded liquid absorbent is prevented by minimizing thermal and spatial contact between said further loaded liquid absorbent and the liquid absorbent supplied to the absorption column in step (c) ; and wherein preferably thermal and spatial contact between said further loaded liquid absorbent and the liquid absorbent supplied to the absorption column in step (c) is minimized by providing a liquid tray in a bottom section of the absorption column, wherein said liquid tray is positioned above the inlet for the stream comprising pre-treated sour feed gas and further loaded liquid absorbent, and wherein the further loaded liquid absorbent remains in the bottom section under the influence of gravity while gaseous compounds are allowed to rise in the absorption column.

A process according to any one of the preceding claims, wherein the sour feed gas is sour natural gas and wherein the purified gas is liquefied to obtain

liquefied natural gas; and wherein preferably cooling medium or refrigerant used in the liquefying process is used for cooling the loaded liquid absorbent.

A process according to any one of the preceding claims, wherein a sour feed gas comprising carbon dioxide and hydrogen sulphide is supplied to step (a) to obtain a mixture of pre-treated sour feed gas comprising carbon dioxide and further loaded liquid absorbent comprising absorbed hydrogen sulphide, and wherein the pre-treated sour feed gas comprising carbon dioxide obtained in separation step (b) is contacted in step (c) in an absorption tower counter-currently with a liquid absorbent for absorbing carbon dioxide, and wherein the further loaded liquid absorbent comprising absorbed hydrogen sulphide obtained in separation step (b) is supplied to a Claus separation process for converting hydrogen sulphide to elemental sulphur and regenerated liquid absorbent, and wherein preferably at least part of said regenerated liquid absorbent is the liquid absorbent in step (c) .

A process according to any one of the preceding claims, wherein the process is carried out offshore on a platform or on a floating vessel.