WO2013098328A1

WO2013098328A1 - Improved method for recovery of elemental sulphur

Info

Publication number: WO2013098328A1
Application number: PCT/EP2012/076956
Authority: WO
Inventors: Gerald Sprachmann; Yasaman MIRFENDERESK; Cornelis Jacobus Smit
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Oil Company
Priority date: 2011-12-27
Filing date: 2012-12-27
Publication date: 2013-07-04

Abstract

The invention concerns an improved method for recovery of elemental sulphur, with improved capture of co-produced carbon dioxide (CO₂), if any, from a gas stream containing hydrogen sulphide (H₂S), wherein the H₂S is converted with air by chemical-looping combustion into thermal energy, steam and SO₂, and wherein the SO₂ so produced is converted into elemental sulphur.

Description

IMPROVED METHOD FOR RECOVERY OF ELEMENTAL SULPHUR

This invention concerns an improved method for recovery of elemental sulphur .

The Claus process is widely used to recover the elementary sulphur from gas streams that contain hydrogen sulphide (H₂S) . In this process hydrogen sulphide is removed from a sour gas , typical ly by means of gas scrubbing , in an absorber, using an absorption liquid . This step is typically referred to as gas treating . A process for treating fuel gas may for instance be found in EP-B-1474218 . The loaded absorption liquid from the absorber is regenerated in a regenerator and, hydrogen sulphide is produced in concentrated form, referred to as acid gas . The acid gas is then pas sed to the Claus plant . The core piece of the Claus plant is thermal stage having a combustion chamber, where hydrogen sulphide is reacted with controlled amounts of oxygen in the air, at high temperatures of more than 800 °C, thus forming sulphur dioxide (S0₂ ) and steam. In the subsequent CLAUS

reaction , H₂S and SO₂ react to form elemental sulphur and steam . The process gas that leaves the thermal stage of the Claus plant is cooled to the temperature required for condensation of the sulphur . This process gas contains residual amounts of H₂S and SO₂. Subsequently the process gas is fed to one or more catalytic stages , wherein the Claus react ion continues in a catalytic step with for instance act ivated aluminum { II I ) or titanium ( IV) oxide , in order to increase the sulphur recovery . In between the stages the stream is cooled to collect additional sulphur . I f high sulphur recoveries are required a Claus tail gas treating process such as the SCOT process is added . In the SCOT process all non-converted sulphur species are catalytically converted to H₂S and recycled to the Claus unit .

The Claus process , however, is less ef f icient when treating acid streams relatively lean in H₂S content .

Gases with an H₂S content of over 40% are suitable for the recovery of sulphur in straight- through Claus plants . For leaner acid gases , oxygen enrichment or acid gas enrichment may be required to increase H2S content in the acid gas and hence ensure stable f lame temperature in the main burner, however these systems could increase the overall costs of the treating signif icantly . Moreover, the sizes of the thermal and catalytic stages of the Claus plant need to be large , given the stoichiometry of the reaction and the oxygen content of air . To maintain the temperature at Claus reaction conditions , generally a hydrocarbon fuel stream must be used, which gives rise to C0₂ production . The leaner the acid gas the more fuel gas is needed to increase the temperature to the required level . Moreover, the acid gas quality must be maintained to ensure stable flame temperature . The C0₂ from a Claus plant may be captured, compressed and stored, but not easily . Furthermore , in accordance with the reaction equilibrium about 70% of the hydrogen sulphide is

converted into elemental sulphur in the thermal stage . Subsequent catalytic stages are required, to convert residual contents of hydrogen sulphide and sulphur dioxide to sulphur applying catalysts .

Chemical Looping Combustion (CLC) of a fossil fuel , with separation of C0₂ for capture and storage is known . From a paper entitled "Ef fect of gas composition in

Chemical -Looping Combustion with copper-based oxygen carriers : Fate of sulphur" , by Forero et al in

International Journal of Greenhouse Gas Control 4 ( 2010 ) 762-770 it is known that CLC is an emerging technology for CO₂ capture because separation of this gas from the other flue gas components is inherent to the process and thus no energy is wasted for the separation . It is indicated that natural or refinery gas can be used as gaseous fuels and they may contain dif ferent amounts of sulphur compounds , such as H₂S and COS . This paper by Forero et al presents the combustion results obtained with a Cu-based oxygen carrier using mixtures of C¾ and H₂S as fuel . The influence of H₂S concentration on the gas product distribution and combustion efficiency, sulphur splitting between the fuel reactor ( FR) and the air reactor (AR) , oxygen carrier deactivation and material agglomeration was investigated in a continuous CLC plant ( SOOWth) . The oxygen carrier to fuel ratio was the main operating parameter affecting the CLC system. Complete fuel combustion were reached at 1073 K working at fuel ratio values ≥ 1 . 5 . The presence of H2S did not produce a decrease in the combustion efficiency even when working with a fuel containing 1300 ppmv H₂S . At these conditions , the great maj ority of the sulphur fed into the system was released in the gas outlet of the FR as S0₂ , af fecting the quality of the C0₂ produced . Formation of copper sulphide, Cu₂S, and the subsequent reactivity loss was only detected when working at low values of fuel ratio ≤ 1 . 5 , although this fact did not produce any agglomeration problem in the fluidi zed beds . In addition , the oxygen carrier was fully regenerated in a H₂S-free environment . It can be concluded that Cu-based oxygen carriers are adequate materials to be used in a CLC process using fuels containing H₂S although the quality of the CO₂ produced is af fected .

Although this paper discusses the poss ibility of using a fuel containing up to 0 . 13 vol% H₂S ( 1300 ppmv) , the reader would still assume that a sweetening of the sour gas is needed, and that the acid gas so produced then has to be treated in a Claus /SCOT plant .

The current inventors realised that Chemical Looping Combustion of acid gas with subsequent capture of S0₂ and conversion of the captured SO₂ may result in a much more efficient recovery of elemental sulphur , as compared to the Claus process in a conventional Claus/SCOT plant . Not only the recovery of elemental sulphur is improved, but also the plant economics are significantly more

attractive due to the use of significantly smaller and hence cheaper equipment and the lesser energy

requirements . Moreover, any CO₂ produced in the CLC combustion of the acid gas can be recovered efficiently, and optionally compressed and stored to avoid releasing greenhouse gasses in the atmosphere . In addition lean acid gases may be handled .

It is an obj ect of the current invention to provide an improved method for recovery of elemental sulphur, with improved capture of produced C0₂, from gas streams that contain hydrogen sulphide ( H₂S ) . Accordingly, the current invention concerns a method as claimed in claim 1 .

An embodiment of the invent ive method is

schematically represented in Figure 1 . In this figure :

(A) represents acid gas used as source of hydrogen sulphide ;

(B) represents air;

(C ) represents air depleted in O₂;

( D) represents steam ( and power) ;

(E) represents f lue gas ;

(F) represents essentially pure CO₂, and

(G) represents essentially pure sulphur .

Moreover ( 1 ) represents a fuel reactor ( FR)

(2 ) represents an air reactor (AR) wherein the oxygen carriers are loaded.

( 3 ) represents a loop wherein oxygen loaded carriers and depleted oxygen carriers are re-circulated from (AR) to

( FR) and from ( FR) to (AR) respectively, and

( 4 ) represents an S0₂ to sulphur conversion line-up .

Chemical-looping combustion technology with inherent separation of CO₂ is known . It has been described in the aforementioned paper and the references cited therein . In principle , a metal oxide is used as an oxygen carrier to transfer oxygen f rom an air reactor to a fuel reactor . Direct contact between fuel and combustion air is avoided . The products of the combustion reaction , carbon dioxide and water, are kept separate from nitrogen and any remaining oxygen . Thus , compared with other carbon capture technologies for steam generation, the potential advantages of CLC include :

- No additional energy for C(¾ separation,

- No need for air separation unit ,

- Lower NO_x levels in gas coming f rom the air reactor,

- No residual oxygen in the separated CO₂ stream,

- Potential for water recovery.

While having all the above mentioned advantages , further advantages , in case of the use of an acid gas containinHg₂S and volatile sulphur compounds , include utilising the high caloric content of burning H₂S and significant si ze reduction of the subsequent condensers and catalytic stages , since the gas volume to be cleaned is reduced signi ficantly ( no nitrogen originating from air present) with respect to the process gas that leaves the thermal stage of a conventional Claus plant . CLC condit ions are known . Preferably an atmospheric chemical-looping combustor is used, composed of two interconnected f luidi zed bed reactors, a fuel reactor ( FR) and an air reactor (AR) , separated by a loop seal . The chemical-looping combustor may further comprise a riser for solids transport to the fuel reactor, a cyclone and a solid valve to control the solids fed to the fuel reactor ( FR) , or similar equipment . The FR preferably consists of a bubbling fluidized bed . In this reactor the fuel combustion is performed by an oxygen carrier, giving

CO2 and H₂O . The solids in this respect are particles of the oxygen carrier . Depleted oxygen carrier particles overf low into the AR through another loop seal ,

preferably through a U-shaped fluidized loop seal , to avoid gas mixing between fuel and air .

The loading of the oxygen carrier takes place at the AR, which preferably consists of a bubbling fluidized bed . The regeneration of the oxygen carrier happens in the AR, preferably in a dense bed of the AR allowing residence times high enough for the complete oxidation of the reduced carrier . Secondary air may be introduced at the top of the bubbling bed, for instance , to help particle entrainment . N₂ and unreacted 0₂ leave the AR, for instance, passing through a high-efficiency cyclone and a filter or similar equipment . The recovered solid particles may be sent to a reservoir of solids setting the oxygen carrier ready to start a new cycle and avoiding the mixing of fuel and air out of the riser . The regenerated oxygen carrier particles may be returned to the FR by gravity from the reservoir of solids located above a solids valve . Fine particles produced by

fragmentation/attrition in the plant are preferably recovered, for instance in filters that are located downstream of the FR and AR . It is a preferred feature of the combustor to have the possibility to control and/or measure the solids circulation rate at any moment through the solids valves located above the FR.

Various oxygen carriers in CLC processes are known and suitable. Suitable metals include Fe, Ni, Mn and Cu.

The oxygen carrier (OC) in the current process is preferably a Cu-based oxygen carrier, with the Cu deposited on a solid support. Various supports may be used. Preferably silica or alumina supports are used, more preferably γ-Α1₂0₃.

The reaction conditions in the AR are such as to convert the OC without adversely affecting the OC itself and without the generation of NO_x. Preferably, the pressure is close to atmospheric pressure, albeit that a slightly higher or lower pressure may be use, e.g., from

0.1 to 5 bar g, preferably from 0.5 to 1.5 bar g. The temperature may vary from 700 to 1200°C, preferably from 850 to 950°C.

The reaction conditions in the FR are such as to convert at least 90 vol%, preferably at least 95 vol% of the acid gas with the oxidized OC without adversely affecting the OC itself and without the generation of partially combusted products. Preferably, the pressure is close to atmospheric pressure, albeit that a slightly higher or lower pressure may be used, e.g., from 0.1 to 5 bar g, preferably from 0.5 to 2.5 bar g. Suitably, the pressure in the FR and the AR are substantially the same. Indeed, it may be beneficial to operate the FR (and optionally the AR) at higher pressures, to accommodate for the already elevated pressure of the acid gas, for instance as supplied from a preceding gas treating step (using an amine regenerator at a pressure of 1.7 - 2.2 bar g) and/or to facilitate for a subsequently C0₂ recompression, if any . The temperature may vary from 700 to 1200 °C, preferably from 850 to 950 °C .

The acid gas contains H₂S and/or volatile sulphur compounds . In addition to H₂ S it may contain CO₂ and some water . Moreover , it may contain volatile hydrocarbons with up to 8 carbon atoms and organic derivatives thereof , as may be found in natural gas or fuel gases . For instance, it contains volatile hydrocarbons and oxygenated derivatives thereof as may be found in natural gas , refinery gas or synthesis gas from which the acid gas originates . TheH₂S content is preferably ≥ 1 vol% , more preferably ≥ 5 vol% , still more preferably ≥ 10 vol% . Even pure H₂S may be used ( 100 vol% ) , but suitably the f¾S content is ≤ 80 vol% , more suitably ≤ 60 vol% still more suitably ≤ 40 vol% . For instance, a suitable acid gas stream comprises 15 ± 5 vol%H₂S ; 10 ± 10 vol% C0₂ and the remainder being CH₄ and other hydrocarbons .

Oxygen carrier to fuel ratios suitable for full combustions are known in the art and may be easily determined when carrying out a series of experiments .

Suitably a ratio ≥ 1 . 5 is used .

The waste stream from the AR is composed of N₂ with a reduced content of 0₂ . It may be released to the

atmosphere or converted into pure N₂ and used in

applications where inert gas is needed .

In addition to power/steam, the CLC f lue gas stream from the FR is essentially composed of CO₂, H₂0 and SO2 , optionally with no more than 10 vol% of other components . Such other components may comprise inert components of the acid gas , and/or oxygenates derived from contaminants of the acid gas . Since C0₂ is an important greenhouse gas , it is preferably captured and either used or compressed and stored . The H_zO may be condensed and separated . The SO2 so produced is subsequent ly converted into elemental sulphur, for example by biological processes and/or catalytic processes . For instance in a catalytic CLAUS react ion the S0₂ so produced may be converted into elemental sulphur using adde d H₂S in about stoichiometric amounts . This recovery of sulphur may be carried out using a line-up comprising conventional catalytic stages . The adde d H₂S may for instance originate from a non- converted acid gas stream, wherein about 30 vol% is converted to the CLC, or from a tail gas downstream of the sulphur conversion line-up .

The catalytic CLAUS reaction is known, as are suitable conditions and catalysts . Claus catalysts including alumina catalysts , activated alumina catalysts (such as S-100 SR catalysts ) , s ilica-alumina catalysts , alumina/titania catalysts , and/or titania catalysts , or any other catalyst systems which are employed in the Claus process . In the catalytic stage, H₂S is reacted with SO2 at temperatures , ranging from 200-350 °C and subsequent ly sulphur i s produced .

As indicated above, the gas stream leaving the catalytic stages may for instance , be subj ected to a complementary finishing treatment wherein all of the sulphur components of this gas stream are converted into hydrogen sulphide in a Claus of f-gas treating unit .

Various technologies have been proposed and used to carry out this type of finishing treatment . One of the most commonly used methods employs reducing gases (hydrogen, or a mix of hydrogen and CO, known as syngas ) in the presence of a suitable catalyst . The hydrogen sulphide thus obtained after this catalytic reduction stage is separated f rom the gas stream and recycled to the catalytic stages . The gas stream from which the hydrogen sulphide is removed, is essentially pure C0₂ , that may be captured, compressed and stored .

The gas stream of the catalytic stages of the Claus reaction may alternatively be subj ected to a combustion step or second CLC step, resulting in a flue gas stream comprising CO2 and SO2 . The SO2 may be separated from this flue gas stream by convent ional techniques and recycled . For instance, SO2 can be captured either in a non- regenerable system which results in a byproduct or in a regenerable system where SO₂ can be recovered as a product .

Preferably the SO₂ is separated using a regenerable SO2 capture process and more specif ically a solvent based scrubbing unit such as a Cansolv© S0₂ scrubbing unit . A Cansolv scrubbing unit typically uses a regenerable amine-based solvent , which is highly selective for SO2 and produces a concentrated water-saturated stream of SO₂ ( for instance, 90% S0₂ / 10% water .

Alternatively , catalytic stages may be used that convert S0₂ directly into elemental sulphur . For this a reducing gas such as hydrogen or syngas is needed . Direct reduction catalysts and suitable conditions are known . The direct reduction catalyst can include such catalyst compositions as bauxite-bentonite catalyst , NiO/Al₂0₃ catalyst , C0₃O₄/AI₂O₃ catalyst , mixed oxide catalysts , including alumina supported metal oxide catalysts in which the metal oxide is selected from oxides of the metals of chromium, molybdenum, copper, cobalt , and nickel , and the sulphidized metal oxide catalysts all as disclosed in US 6 , 297 , 189 . The direct reduction reaction conditions under which the S02~containing gas stream is contacted with the direct reduction catalyst may be any suitable process conditions that provide for the

conversion of at least a portion of the SO2 to elemental sulphur . For instance, the direct reduction reaction temperature can be in the range from 200 to 1000 ° C, and the direct reduction reaction pressure can be in the range of from atmospheric upwardly to 70 bar g .

Claims

C L A I M S

1. A method for recovery of elemental sulphur, with improved capture of co-produced carbon dioxide (C0₂) , if any, from a gas stream containing hydrogen sulphide (H₂S) , wherein the H₂S is converted with air by chemical- looping combustion into thermal energy, steam and SO2, and wherein the S0₂ so produced is converted into

elemental sulphur.

2. The method of claim 1, wherein:

(i) in a chemical-looping combustion process an oxygen carrier (OC) is oxidized with air,

(ii) the gas stream containHi₂nSg is oxidized with the oxidized OC, resulting in a (partially) spent oxygen carrier that is recycled to step (i) and a flue gas stream comprising the produced SO₂, and

(iii) the produced S0₂ is separated and converted into elemental sulphur, and

(iv) optionally, any co-produced C0₂ is compressed and stored.

3. The method of claim 2, wherein the oxygen carrier is a copper-based oxygen carrier, more preferably CuO on a support (most preferably gamma~Alz03) .

4. The method of claim 2 or 3, wherein the separated CO2 is essentially pure.

5. The method of any one of claims 2 to 4, wherein the separated C0₂ is compressed and stored.

6. The method of any one of the preceding claims, wherein a chemical-looping combustor is used comprising an air reactor (AR) and wherein the reaction conditions in the AR are such as to convert the OC without adversely affecting the particles itself and without the generation of N0_X.

7. The method of claim 6, wherein the pressure in the AR is in the range from 0.1 to 10 bar g, preferably from 0.5 to 1.5 bar q and wherein the temperature in the AR is in the range from 800 to 1400°C, preferably from 850 to 1050°C.

8. The method of any one of the preceding claims, wherein a chemical-looping combustor is used comprising a fuel reactor (FR) and wherein the reaction conditions in the FR are such as to fully convert the sour gas with the oxidized OC without adversely affecting the particles itself and without the generation of partially combusted products .

9. The method of claim 8, wherein the pressure in the FR is in the range from 0.1 to 5 bar g, preferably from 0.5 to 2.5 bar g and wherein the temperature in the FR is in the range from 800 to 1400°C, preferably from 850 to 950°C.

10. The method of any one of the preceding claims, wherein the gas stream containing H₂S, has anH₂S content that is ≥ 1 vol %, more preferably ≥ 5 vol%, still more preferably ≥ 10 vol%.

11. The method of claim 10, wherein the gas has an H2S content that is ≤ 80 vol%, more preferably ≤ 60 vol%, still more preferably ≤ 40 vol% .

12. The method of any one of the preceding claims, wherein the SO₂ so produced is converted into elemental sulphur using a conversion line-up comprising

conventional catalytic stages .

13. The method of claim 12, wherein the SOj produced by said method is converted to elemental sulphur by reaction with H₂S in a catalysed Claus reaction.

14. The method of claim 12 or 13, wherein the gas stream leaving the catalytic stages is subjected to a catalytic reduction stage wherein all of the sulphur components of this gas stream are converted into hydrogen sulphide .

15 . The method of claim 12 , wherein the S0₂ produced by said method i s converted to elemental sulphur by reaction with a reducing gas such as hydrogen or syngas .