WO2008145680A2

WO2008145680A2 - Process for producing a purified gas

Info

Publication number: WO2008145680A2
Application number: PCT/EP2008/056571
Authority: WO
Inventors: Frank Michael ÖHLSCHLÄGER
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2007-05-31
Filing date: 2008-05-29
Publication date: 2008-12-04
Also published as: WO2008145680A3

Abstract

A process for producing purified gas from a sour gas comprising H2S, the process comprising the steps of : (a) contacting a sour gas comprising H2S with an absorbing liquid for H2S to transfer H2S from the sour gas to the absorbing liquid, thereby producing an H2S- rich absorbing liquid and the purified gas; (b) transferring H2S from the H2S-rich absorbing liquid to a stripping gas, thereby obtaining a feed acid gas stream enriched in H2S; (c) providing the feed acid gas stream enriched in H2S and a gas stream comprising SO2 to a sulphur recovery system comprising two or more Claus catalytic stages in series, each Claus catalytic stage comprising a Claus catalytic reactor coupled to a sulphur condenser, wherein either the feed acid gas stream enriched in H2S or the gas stream comprising SO2 is completely routed to the first Claus catalytic stage while the other stream is split into two or more substreams and each of the two or more substreams are supplied to a different Claus catalytic stage, wherein the amount of feed acid gas stream enriched in H2S or the amount of gas stream comprising SO2 is supplied to the Claus catalytic stages in dependence of the temperature in the Claus catalytic reactors to enable that the temperature in the Claus catalytic reactors is moderated; (d) reacting at least part of the H2S in the feed acid gas stream enriched in H2S with SO2 to elemental sulphur in the two or more Claus catalytic reactors.

Description

PROCESS FOR PRODUCING A PURIFIED GAS

The invention relates to a process for producing a purified gas from a sour gas comprising H2S.

Sour gas comprising H2S can originate from various sources. For example, numerous natural gas wells produce sour natural gas, i.e. natural gas comprising H2S and optionally other contaminants. Natural gas is a general term that is applied to mixtures of light hydrocarbons and optionally other gases (e.g. nitrogen, carbon dioxide, helium) derived from natural gas wells. The main component of natural gas is methane. Further, often other hydrocarbons such as ethane, propane, butane or higher hydrocarbons are present. Carbon dioxide may be present as well. It is desirable to remove H2S to low concentrations from the natural gas in order to be able to use the natural gas, for example for domestic purposes, or to produce liquefied natural gas (LNG) .

Another example of a sour gas is sour synthesis gas. Synthesis gas mainly comprises carbon monoxide and hydrogen, while contaminants such as carbon dioxide, water vapour, ammonia or nitrogen may be present as well. Synthesis gas is generally used as feedstock for chemical processes. In particular, synthesis gas can be used for the preparation of hydrocarbons in a catalytic process, for example the well-known Fischer-Tropsch process. Removal of H2S from synthesis gas to low levels is of considerable importance, because H2S may bind irreversibly on catalysts and cause sulphur poisening. This results in a deactivated catalyst, which severely hampers the catalytic process. A process known in the art for producing a gas stream depleted of H2S from a gas stream comprising H2S uses the partial oxidation of H2S to SO2 according to:

2 H₂S + 302 → 2H₂O + 2SO₂ (1) The SO₂ formed can be ( catalytically) converted to elemental sulphur according to the Claus reaction:

2 H₂S + SO₂ → 2H₂O + 3/n S_n (2)

The combination of reactions (1) and (2) is known as the Claus process. The Claus process is frequently employed both in refineries and for the processing of H₂S recovered from natural gas .

As conventional Claus installations are costly, both in terms of capital expenditure as well as in terms of operational costs, alternative processes have been reported.

For example, in US 5,628,977 a Claus process is described using a single catalytic Claus reactor combined with a Claus tail-gas treatment step, wherein unreacted H₂S is combusted to SO₂ and the SO₂ is concentrated and recycled to the Claus catalytic reactor. A drawback of the process described in US 5,628,977 is that it is suitable only for gas streams containing up to 20 volume% of H₂S. In most processes, the amount of H₂S in the gas streams will be higher. This drawback is said to be overcome in the process described in EP 1,230,149, wherein Claus tail gas is partly sent back to the Claus catalytic reactor to act as a diluent gas, thereby moderating the temperature in the Claus reactor. However, the drawback of the process described in EP 1,230,149 is that a relatively large volume of tail gas needs to be recycled in order to achieve temperature moderation. As a result, a larger Claus reactor is needed. In addition, the process lacks flexibility because only a single temperature control means is used.

It has now been found that a purified gas can be produced using a simple line-up with two or more catalytic Claus reactors, wherein the temperature in the Claus reactors can be controlled easily by controlled addition of the reactants to the two or more catalytic Claus reactors. Accordingly, the invention provides a process for producing purified gas from a sour gas comprising H2S, the process comprising the steps of:

(a) contacting a sour gas comprising H2S with an absorbing liquid for H2S to transfer H2S from the sour gas to the absorbing liquid, thereby producing an H2S- rich absorbing liquid and the purified gas;

(b) transferring H2S from the H2S-rich absorbing liquid to a stripping gas, thereby obtaining a feed acid gas stream enriched in H2S; (c) providing the feed acid gas stream enriched in H2S and a gas stream comprising SO2 to a sulphur recovery system comprising two or more Claus catalytic stages in series, each Claus catalytic stage comprising a Claus catalytic reactor coupled to a sulphur condenser, wherein either the feed acid gas stream enriched in H2S or the gas stream comprising SO2 is completely routed to the first Claus catalytic stage while the other stream is split into two or more substreams and each of the two or more substreams are supplied to a different Claus catalytic stage, wherein the amount of feed acid gas stream enriched in H2S or the amount of gas stream comprising SO2 is supplied to the Claus catalytic stages in dependence of the temperature in the Claus catalytic reactors to enable that the temperature in the Claus catalytic reactors is moderated; (d) reacting at least part of the H2S in the feed acid gas stream enriched in H2S with SO2 to elemental sulphur in the two or more Claus catalytic reactors .

Suitably, the sour gas comprising H2S is sour natural or associated gas. Alternatively, the sour gas comprising H2S is sour synthesis gas. The process according to the invention is especially suitable for sour gas comprising H2S and optionally also significant amounts of carbon dioxide, as both compounds are efficiently removed in the liquid absorption process in step (a) . Suitably the total sour gas comprises in the range of from 0.05 to 80 vol% H2S and from 0.5 to 90 vol% carbon dioxide, based on the total sour gas. Preferably, the sour gas comprises from 10 to 40 vol% H2S and from 50 to

80 vol% carbon dioxide, based on the total sour gas. In step (a), the sour gas is contacted with absorbing liquid to transfer H2S from the sour gas to the absorbing liquid. This results in an absorbing liquid loaded with contaminants and the purified gas .

The absorbing liquid is any liquid capable of removing H2S from the sour gas. A preferred absorbing liquid comprises a chemical solvent as well as a physical solvent .

Suitable chemical solvents are primary, secondary and/or tertiary amines. A preferred chemical solvent is a secondary or tertiary amine, preferably an amine compound derived from ethanol amine, more especially DIPA, DEA, MEA, DEDA, MMEA (monomethyl-ethanolamine ) , MDEA, or DEMEA (diethyl-monoethanolamine) , preferably DIPA or MDEA. It is believed that these chemical solvents react with acidic compounds such as H2S and/or CO2, thereby removing

H2S and/or CO2 from the sour gas. 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. It is believed that H2S and/or CO2 will be taken up in the physical solvent and thereby removed from the sour gas. Additionally, if mercaptans (RSH) are present, they will be taken up in the physical solvent as well.

Preferably, the absorbing liquid comprises sulfolane, MDEA or DIPA, and water .

A 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 20 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, amine, optionally sulfolane and optionally activator together being 100 parts by weight. This preferred absorbing liquid enables removal of hydrocarbons, carbon dioxide, hydrogen sulphide and/or COS from sour gas comprising these compounds.

Suitably, step (a) is carried out at a temperature in the range of from 15 to 90 ⁰C, preferably at a temperature of at least 20 ⁰C, more preferably from 25 to 80 ⁰C, still more preferably from 40 to 65 ⁰C.

Step (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 or random packing may also be applied. The amount of CC>2-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.

The purified gas obtained in step (a) is depleted of H2S, meaning that the concentration of H2S in the purified gas stream is lower than the concentration of H2S in the sour gas. It will be understood that the concentration of H2S in the purified gas obtained in step (a) depends on the concentration of H2S in the sour gas. Typically, the concentration of H2S in the purified gas stream is in the range of from 0.0001% to 80%, preferably from 0.0001% to 20%, more preferably from 0.0001% to 10% of the H2S concentration in the sour gas.

Suitably, the concentration of H2S in the purified gas obtained in step (a) is less than 10 ppmv, preferably less than 5 ppmv.

The purified gas can be processed further in known manners. For example, the purified gas can be subjected to catalytic or non-catalytic combustion, to generate electricity, heat or power, or can be used as a feed gas for a chemical reaction or for residential use.

In the event that the sour gas comprises natural gas, the purified natural gas is preferably cooled to obtain liquefied natural gas (LNG) . Therefore, the invention also provides LNG formed by cooling the purified natural gas obtained by the process according to the invention.

In the event that the sour gas comprises synthesis gas, the purified synthesis gas stream is preferably converted to normally liquid hydrocarbons in a hydrocarbon synthesis reaction. Therefore, the invention also provides the products obtained in a hydrocarbon synthesis reaction, including distillates and hydroconverted products, e.g. fuels such as naphtha, kero and diesel, base oils and n-parafins, lower detergent feedstocks and wax.

In step (b), the H2S-rich absorbing liquid is regenerated by transferring at least part of the H2S to a stripping gas. Suitably, step (b) takes place at relatively low pressure and high temperature. Step (b) is suitably carried out by heating the H2S-rich absorbing liquid in a regenerator at a relatively high temperature, suitably in the range of from 70 to 150 ⁰C. The heating is preferably carried out with steam or hot oil. Preferably, the temperature increase is done in a stepwise mode. Suitably, step (b) is carried out at a pressure in the range of from 1 to 2.5 bara.

In step (b), regenerated absorbing liquid is obtained and a feed acid gas stream enriched in H2S.

Preferably, regenerated absorbing liquid is used again step (a) for H2S removal. Suitably the regenerated absorbing liquid is heat exchanged with H2S-rich absorbing liquid to use the heat elsewhere. In step (c), the feed acid gas stream enriched in H2S and a gas stream comprising SO2 are provided to a sulphur recovery system comprising two or more Claus catalytic stages in series. Each of the Claus catalytic stages comprises a Claus catalytic reactor coupled to a sulphur condenser. In the Claus catalytic reactor, the Claus reaction between H2S and SO2 to form elemental sulphur takes place. A product gas effluent comprising elemental sulphur as well as unreacted H2S and/or SO2 exits the

Claus catalytic reactor and is cooled below the sulphur dew point in the sulphur condenser coupled to the Claus catalytic reactor to condense and separate most of the elemental sulphur from the Claus reactor effluent. The reaction between H2S and SO2 to form elemental sulphur is exothermic, normally causing a temperature rise across the Claus catalytic reactor with an increasing concentration of H2S in the incoming feed gas stream enriched in H2S. At an H2S concentration in the feed acid gas above 30% or even above 15%, it is believed that the heat generated in the Claus catalytic reactors will be such that the temperature in the Claus reactors will exceed the desired operating range if sufficient SO2 is present to react according to the Claus reaction. Preferably, the operating temperature of the Claus catalytic reactor is maintained in the range of from about 200 to about 500 ⁰C, more preferably from about 250 to 350 ⁰C. In order to enable operating the process at higher

H2S concentrations in the feed acid gas, generally above

15%, temperature modification in the Claus reactors is needed. The amount of sour gas comprising H2S or the amount of gas comprising SO2 that is supplied to the Claus catalytic stages is such that the temperature in the Claus catalytic stage is moderated. This is suitably done by monitoring the temperature in the Claus catalytic stage and adjusting the amount of sour gas comprising H2S or the amount of gas comprising SO2 that is supplied to the Claus catalytic stages in dependence of the temperature in the Claus catalytic stages. Preferably, the temperature is moderated such, that the operating temperature of the Claus catalytic reactor is maintained in the range of from about 200 to about 500 ⁰C, more preferably from about 250 to 350 ⁰C. Thus, the process can handle feed acid gas streams enriched in H2S comprising in the range of from 15 to 80 vol% of H2S, preferably from 20 to 80 vol% of H2S, based on the total feed acid gas stream.

In step (d), at least part of the H2S is converted to elemental sulphur in the Claus catalytic reactors. Preferably, the catalyst used in the Claus catalytic reactor is non-promoted spherical activated alumina or titania.

The gas stream comprising SO2 may be supplied from an external source to the system comprising two or more Claus stages or may be generated in the system comprising two or more Claus stages. In a preferred embodiment, the gas stream comprising SO2 is generated in the system comprising the two or more Claus stages, by combusting H2S to SO2. Preferably, unconverted H2S in an off-gas stream exiting one or more of the Claus reactors is combusted to obtain a combustion gas effluent comprising SO2, followed by concentrating the SO2 to obtain the gas stream comprising SO2. Concentration of SO2 may be done by any know means in the art, for example by using liquid absorption, adsorption or membrane separation. A most preferred manner is by contacting the combustion gas effluent comprising SO2 with a absorbing liquid for SO2 in a SC>2 absorption zone to selectively transfer SO2 from the combustion gas effluent to the absorbing liquid to obtain SC>2-enriched absorbing liquid and subsequently stripping SO2 from the Sθ2~enriched absorbing liquid to produce a lean absorbing liquid and the gas stream comprising SO2. Suitable absorbing liquids for SO2 are physical SO2 absorbing liquids. Preferably, the absorbing liquid for SO2 comprises at least one substantially water immiscible organic phosphonate diester. Alternatively, the absorbing liquid for SO2 comprises tetraethyleneglycol dimethylether . Another preferred absorbing liquid for SO2 comprises diamines having a molecular weight of less than 300 in free base form and having a pKa value for the free nitrogen atom of about 3.0 to about 5.5 and containing at least one mole of water for each mole of SO2 to be absorbed.

Steps c and d of the process will now be illustrated using the following non-limiting embodiments with reference to the Figures. In figure 1 is shown a two-stage Claus process wherein an acid feed gas comprising H2S is led via line 1 to heat exchanger 2. The preheated acid feed gas comprising H2S is combined with part of a stream comprising SO2 received via line 29 and led via line 3 to a first Claus catalytic stage comprising a first Claus catalytic reactor 4 and a first sulphur condenser 5. In the first Claus catalytic reactor, reaction between H2S and SO2 to form elemental sulphur takes place. A first Claus reactor effluent stream comprising elemental sulphur and unreacted H2S and SO2 and other non-sulphur components, exits the first Claus catalytic reactor via line 6 to the first sulphur condenser 5, where it is cooled below the dewpoint of sulphur. Low pressure steam exits the sulphur condenser via line 7 and condensed elemental sulphur is lead from the sulphur condenser via line 8 to sulphur accumulation vessel 9. At least part of a gas stream comprising unreacted H2S and SO2 is combined with part of a stream comprising SO2 received via line 30 and led via line 10 to heat exchanger 11 and then via line 12 to a second catalytic Claus stage comprising a second catalytic Claus reactor 13 and a second sulphur condenser 14. Another part of a gas stream comprising unreacted H2S and SO2 and other non-sulphur components is optionally led via line 15 to combuster 16 for control purposes. In the second Claus catalytic reactor 13, remaining H2S and SO2 react to form elemental sulphur. A second Claus reactor effluent stream comprising elemental sulphur and unreacted H2S and SO2 exits the second Claus catalytic reactor via line 17 to the second sulphur condenser 14 where it is cooled below the dewpoint of sulphur. Low pressure steam exits the sulphur condenser via line 18 and condensed elemental sulphur is lead from the sulphur condenser via line 19 to sulphur accumulation vessel 9. A gas stream comprising unreacted H2S and SO2 is led via line 20 to coalescer 21, where remaining elemental sulphur is removed and led to the sulphur vessel 9 via line 22. A gas stream comprising H2S and SO2 is led via line 23 to combustor 16, where H2S is combusted to SO2 using air supplied via line 24. A flue gas stream comprising SO2 is led via line 25 to an SO2 recovery unit 26, where a concentrated stream of SO2 is generated. This concentrated stream of SO2 is led from the SO2 recovery unit via line 27 and heat exchanger 28 and split into two streams. One stream is led to the first Claus catalytic reactor via line 29 and the other stream is led to the second Claus reactor via line 30.

The embodiment shown in figure 2 shows the same process steps as described for figure 1, except that the concentrated stream of SO2 is led from the SO2 recovery unit via line 27 and heat exchanger 28 only to the first Claus catalytic reactor via line 29. In this embodiment, temperature moderation is achieved by splitting the feed acid gas stream into two streams, one stream being led via line 3 to the first Claus catalytic stage and the other stream is being led to the second Claus catalytic stage via line 30.

Claims

C L A I M S

1. A process for producing purified gas from a sour gas comprising H2S, the process comprising the steps of:

(b) transferring H2S from the H2S-rich absorbing liquid to a stripping gas, thereby obtaining a feed acid gas stream enriched in H2S; (c) providing the feed acid gas stream enriched in H2S and a gas stream comprising SO2 to a sulphur recovery system comprising two or more Claus catalytic stages in series, each Claus catalytic stage comprising a Claus catalytic reactor coupled to a sulphur condenser, wherein either the feed acid gas stream enriched in H2S or the gas stream comprising SO2 is completely routed to the first Claus catalytic stage while the other stream is split into two or more substreams and each of the two or more substreams are supplied to a different Claus catalytic stage, wherein the amount of feed acid gas stream enriched in H2S or the amount of gas stream comprising SO2 is supplied to the Claus catalytic stages in dependence of the temperature in the Claus catalytic reactors to enable that the temperature in the Claus catalytic reactors is moderated;

(d) reacting at least part of the H2S in the feed acid gas stream enriched in H2S with SO2 to elemental sulphur in the two or more Claus catalytic reactors .

2. A process according to claim 1, wherein at least part of the gas stream comprising SO2 is obtained by combusting H2S to SO2.

3. A process according to claim 2, wherein combusting H2S to SO2 involves combusting H2S in an off-gas stream exiting one or more of the Claus reactors to obtain a combustion gas effluent comprising SO2.

4. A process according to claim 3, wherein the combustion gas effluent comprising SO2 is contacted with a absorbing liquid for SO2 in a SO2 absorption zone to selectively transfer SO2 from the combustion gas effluent to the absorbing liquid to obtain SC>2-enriched absorbing liquid and stripping SO2 from the Sθ2~enriched absorbing liquid to produce a lean absorbing liquid and the gas stream comprising SO2.

5. A process according to claim 4, wherein the absorbing liquid for SO2 is one or more liquids selected from the group of water immiscible organic phosphonate diesters, tetraethyleneglycol dimethylether and diamines having a molecular weight of less than 300 in free base form and having a pKa value for the free nitrogen atom of about 3.0 to about 5.5 and containing at least one mole of water for each mole of SO2 to be absorbed.

6. A process according to any one of the preceding claims, wherein the feed acid gas stream comprises at least 15 mole % of H2S, preferably at least 30% mole % of

H2S, more preferably at least 50 mole % of H2S.

7. A process according to any one of the preceding claims, wherein the operating temperature of the Claus catalytic reactor is maintained in the range of from about 200 to about 500 ⁰C, preferably from about 250 to 350 ⁰C, and wherein the temperature is maintained in these ranges preferably by adjusting the amount of feed acid gas stream enriched in H2S or the amount of gas stream comprising SO2 supplied to the Claus catalytic stages .

8. A process according to any one of the preceding claims, wherein the sour gas is a sour natural gas and the purified gas is purified natural gas.

9. A process according to claim 8, wherein the purified natural gas is cooled to form liquefied natural gas.

10. A process according to any one of claims 1 to 7, wherein the sour gas is sour synthesis gas and the purified gas is purified synthesis gas.