WO2013098329A1

WO2013098329A1 - Method for producing sulphuric acid

Info

Publication number: WO2013098329A1
Application number: PCT/EP2012/076958
Authority: WO
Inventors: Gerald Sprachmann; Cornelis Jacobus Smit; Yasaman MIRFENDERESKI
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 a method for producing sulphuric acid, with 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 S02, and wherein the S02 so produced is converted into sulphuric acid.

Description

METHOD FOR PRODUCING SULPHURIC ACID

This invention concerns an improved method for producing sulphuric acid.

Sulphuric acid is a highly corrosive strong mineral acid with the molecular formula H₂S0₄. It has many applications and is a commonly used in the chemical industry. Principal uses include lead-acid batteries for cars and other vehicles, ore processing, fertilizer manufacturing, oil refining, wastewater processing, and chemical synthesis. For production of sulphuric acid, sulphur dioxide (S0₂) is required. S02 is then oxidised to sulphur trioxide (S0₃) . The S0₃ may then be hydrated into sulfuric acid H₂S0₄.

Conventionally, the final step of the production of sulphuric acid is carried out by the so-called double absorption process which is described in Ullmann's

Encyclopedia of Industrial Chemistry, 5th edition, Vol. A25, pages 635 to 700. For catalyzing the oxidation of sulphur dioxide to sulphur trioxide, catalysts containing vanadium pentoxide as active component are typically used with an operating range of 380 to 640°C. Below this range, the catalyst is inactive, whereas the catalyst is irreversible damaged at temperatures above 640 °C.

S02 can be produced from a gas containing hydrogen sulphide (H₂S) such as acid gas. Thus, acid gas may be combusted into sulphur dioxide (S0₂) . It is common that hydrogen sulphide is removed from a sour gas. This may be carried out 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 EP1474218. 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 passed to a combustion unit, generating a stream containing, amongst others, sulphur dioxide (S0₂) . In a subsequent step S0₂ is separated and this is then

oxidized to sulphur trioxide using oxygen with for example the vanadium pentoxide as catalyst.

From WO201113484, for instance, a process is known for producing power from a sour gas comprising H₂S, the process comprising the steps of: (a) providing a sour gas stream comprising natural gas and H₂S to an acid gas removal unit, resulting in a cleaned natural gas and a acid gas comprising H₂S; (b) combusting the cleaned natural gas stream with an oxygen containing gas in a gas turbine to produce power and a hot flue gas; (c) sending the hot flue gas to a first heat recovery steam generator to generate steam and a clean flue gas; (d) combusting at least part of the H₂S in the acid gas comprising H₂S in the presence of an oxygen containing gas to obtain a hot gas effluent comprising S0₂; (e) sending the hot gas effluent comprising S0₂ to a second heat recovery steam generator to generate steam and a cooled gas effluent comprising S0₂; (f) leading the cooled gas effluent comprising S0₂ to a sulphuric acid unit to produce sulphuric acid, steam and a cleaned flue gas stream.

The process of WO201113484 is much more efficient than the conventional method for producing sulphuric acid from a gas stream containing H₂S wherein elemental sulphur is produced first.

On the other hand, combusting hydrogen sulphide as described in step (d) of the process discussed above is less efficient, in particular when treating acid gas streams relatively lean in H₂S content. For relatively lean acid gas streams, oxygen enrichment or gas

enrichment may be required to increase H₂S content in the acid gas stream and hence ensure stable flame temperature in the main burner of the combustion unit. However, the size of the combustion unit(s) needs to be large, given the stoichiometry of the reaction and the oxygen content of air. To maintain the temperature at combustion

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. Furthermore, sulphur dioxide still needs to be removed from the gas effluent, before it can be oxidized to sulphur trioxide. Since the sulphur dioxide content in this process can be relatively low, large investments are required with respect to the sulphur dioxide removal. In WO201113484, for instance, various processes are mentioned that can be used to concentrate the sulphur dioxide. Processes known in the art include for example liquid absorption, e.g. the

Cansolv™ process, absorption, membrane separation or by condensation of the sulphur dioxide. That the post combustion separation of C0₂, S0₂ and nitrogen may be complicated by the presence of large amounts of nitrogen is apparently accepted.

It is therefore an object of the current invention to further improve the process of WO201113484.

Chemical Looping Combustion (CLC) of a fossil fuel, with separation of C0₂ for capture and storage is known. From a paper entitled "Effect 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 C0₂ 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 different 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 CH₄ 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 (500Wth) . 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 majority of the sulphur fed into the system was released in the gas outlet of the FR as S0₂, affecting 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 fluidized 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 C0₂ produced is affected.

Although this paper discusses the possibility of using a fuel containing up to 0.13 vol% H₂S (1300 ppmv), the reader with an interest in the production of

sulphuric acid would not have appreciated that CLC may be used to further improve the already efficient process of WO201113484.

The current inventors realised that Chemical Looping Combustion of acid gas with subsequent capture of S0₂ and conversion of the captured S0₂ may result in a much more efficient process for producing sulphuric acid, as compared to the process in WO201113484. Thus, 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 C0₂ 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 object of the current invention to provide an improved process for producing sulphuric acid, with capture of produced C0₂, from gas streams that contain hydrogen sulphide (H₂S) . Accordingly, the current

invention concerns a process as claimed in claim 1.

An embodiment of the inventive 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 0₂;

(D) represents steam (and power) ;

(E) represents flue gas;

(F) represents essentially pure C0₂;

(G) represents essentially pure S0₂, and

(H) represents sulphuric acid.

Moreover

(1) represents a fuel reactor (FR)

(2) represents an air reactor (AR) wherein the

(depleted) oxygen carriers are oxidized and thereby loaded/regenerated

(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,

(4) represents an S0₂ separation unit, and

(5) represents a sulphuric acid unit.

Chemical-looping combustion technology with inherent separation of C0₂ 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 from 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 C0₂ separation,

• No need for an air separation unit,

• Lower NO_x levels in gas coming from the air reactor,

• No residual oxygen in the separated C0₂ stream,

• Potential for water recovery.

While having all the above mentioned advantages, further advantages, in case of the use of an acid gas containing H₂S and volatile sulphur compounds, include utilising the high caloric content of burning H₂S and significant size reduction of the subsequent downstream treating units.

CLC conditions are known. Preferably an atmospheric chemical-looping combustor is used, composed of two interconnected fluidized 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

C0₂ and H₂0. The solids in this respect are particles of the oxygen carrier. Depleted oxygen carrier particles overflow 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 a, preferably from 1.5 to 2.5 bar a. 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 a, preferably from 1.5 to 2.5 bar a. 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 a) and/or to facilitate for a subsequent 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 H₂S it may contain C0₂ 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. The H₂S 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 H₂S 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 flue gas stream from the FR is essentially composed of C0₂, H₂0 and S0₂, 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 used or compressed and stored.

The S0₂ so produced may be sent directly to the sulphuric acid unit or first separated from the other components. It is preferably separated using a

regenerable S0₂ capture process and more specifically 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 S0₂ and produces a concentrated water- saturated stream of S0₂ (for instance, 90% S0₂ / 10% water) . The gas stream containing S0₂ is then sent to the sulphuric acid plant. Suitably, the S0₂ may be diluted prior to conversion into sulphuric acid at the Acid

Plant .

The conversion of S0₂ into sulphuric acid is known.

In WO2011134847, for example, both the wet and the dry sulphuric acid processes are mentioned. The wet process (WSA process) is known from e.g., GB904982 and GB1074434.

In this process air may be used or any other oxygen containing stream.

Alternatively a process may be used wherein S0₂ is removed from the CLC flue gas by subjecting the gas cyclically to scrubbing in an acid stream followed by electrolysis. The process of US 4830718 comprises the steps of scrubbing the gas in a confined scrubbing zone with an aqueous sulphuric acid stream to remove sulphur dioxide from the gas and convert the thus removed sulphur dioxide to sulphurous acid; subjecting the sulphuric acid stream containing the produced sulphurous acid to

electrolysis in an electrolytic cell to oxidize the sulphurous acid to sulphuric acid, recycling the

sulphuric acid stream resulting from the electrolysis step to the scrubbing zone, and maintaining the recycled sulphuric acid within a predetermined range of

concentration by means of make-up water or acid.

Claims

C L A I M S

1. A method for producing sulphuric acid, with 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 S0₂, and wherein the S0₂ so produced is converted into sulphuric acid.

2. The method of claim 1, wherein:

(i) in a chemical-looping combustion process an oxygen carrier (OC) is oxidized and thereby

loaded/regenerated with air,

(ii) the gas stream containing H₂S 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 S0₂, and

(iii) the produced S0₂ is converted to sulphuric acid, 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-Al₂03) .

4. The method of claim 2 or 3, wherein the separated C0₂ 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 NO_x.

7. The method of claim 6, wherein the pressure in the AR is in the range from 1.1 to 5 bar a, preferably from 1.5 to 2.5 bar a .

8. The method of claim 6 or 7, wherein the temperature in the AR is in the range from 800 to 1400°C, preferably from 850 to 1050°C.

9. 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 .

10. The method of claim 9, wherein the pressure in the FR is in the range from 1.1 to 5 bar a, preferably from

1.5 to 2.5 bar a .

11. The method of claim 9 or 10, wherein the temperature in the FR is in the range from 800 to 1400°C, preferably from 850 to 950°C.

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

13. The method of claim 12, wherein the gas has an H₂S content that is ≤ 80 vol%, more preferably ≤ 60 vol%, still more preferably ≤ 40 vol%.

14. The method of any one of the preceding claims, wherein the S0₂ so produced is separated, preferably in a regenerable S0₂ capture process, more preferably a solvent based scrubbing unit, still more preferably a

Cansolv S0₂ scrubbing unit.

15. The method of any one of the preceding claims, wherein the S0₂ so produced is converted into sulphuric acid in a sulphuric acid unit, preferably using a wet sulphuric acid process.