WO2009014584A1

WO2009014584A1 - Production of hydrogen gas from sulfur-containing compounds

Info

Publication number: WO2009014584A1
Application number: PCT/US2008/007524
Authority: WO
Inventors: Paul S. Northrop
Original assignee: Exxonmobil Upstream Research Company
Priority date: 2007-07-23
Filing date: 2008-06-17
Publication date: 2009-01-29
Also published as: EP2167423A4; AU2008279815A1; US20100230296A1; EP2167423A1; CA2689461A1

Abstract

The described invention is a process for producing hydrogen gas comprising (a) combusting sulfur (S) or hydrogen sulfide (H2S) with oxygen (O2) to obtain sulfur dioxide (SO2) and water (H2O), plus heat; (b) adding water to the product of (a) to obtain a sulfurous acid solution (H2SO3 and H2O); (c) applying electrical current to the sulfurous acid solution of (b) to obtain sulfuric acid (H2SO4) and hydrogen gas (H2); and, (d) using gas-liquid separation to separate the sulfuric acid from the hydrogen gas to obtain separated components of the sulfuric acid and the hydrogen gas; and, wherein the heat generated in (a) is used to generate at least a portion of the electricity for the electrical current of (c).

Description

PRODUCTION OF HYDROGEN GAS FROM SULFUR-CONTAINING

COMPOUNDS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 60/961 ,639, filed 23 July 2007.

TECHNICAL FIELD

[0002] This invention relates to the preparation of hydrogen gas using electrolytic processing of sulfurous acid.

BACKGROUND OF THE INVENTION [0003] Hydrogen (H₂) is a valuable feedstock in refineries for an assortment of hydrogenation processes. Hydrogen is also seeing greater value as a fuel source, both in direct combustion and in fuel cells. Public discussions addressing the reduction of carbon dioxide (CO₂) emissions often refer to an alternative of a "hydrogen economy." Thus, methods for the economic generation and recovery of hydrogen while avoiding or reducing the co-production of CO₂ are currently of interest.

[0004] In theory, the most "clean" method of producing hydrogen is the dissociation of water into the sought hydrogen with a by-product of oxygen (O₂). However, the direct thermal dissociation of water is cost prohibitive in view of the high temperatures and energy required for this highly endothermic reaction. Direct electrolysis of water is similarly energy intensive. Thus, the major current route to large-scale hydrogen generation is through steam reforming of hydrocarbonaceous fuels. Here, the hydrocarbons are partially oxidized to carbon monoxide (CO) and hydrogen gas (H₂), creating what is known as "syngas". Steam reforming of methane yields the greatest amount of H₂ per mole of methane:

CH₄ + H₂O — > CO + 3 H₂ ΔH°₂₉₈ = 210 kJ/mole (1) Note that (1) is still strongly endothermic, and thus requires heat. The CO is used to generate more hydrogen through the water-shift reaction:

H₂O + CO — > CO₂ + H₂ ΔH °₂₉₈ = -42 kJ/mole (2)

Theoretically, it is possible to generate 4 moles of H₂ per mole of CO₂ emitted. In practice, hydrocarbonaceous fuels are combusted to generate heat for reaction (1), so the actual H₂:CO₂ ratio is less than 4:1. It is desirable to increase the H₂:CO₂ ratio to reduce the relative amount of CO₂ and other greenhouse gases emitted. It would be even better to eliminate the co-production of carbon containing gases, including CO₂, all together.

[0005] To this end, a number of different routes for abstracting hydrogen directly from hydrogen sulfide (H₂S) have been proposed, since the theoretical dissociation energy for H₂S is much lower than that for water. See, for example, "Hydrogen for H₂S: Technologies and Economics," Luinstra, E. A., Alberta Energy-Alberta Hydrogen Research Program, Calgary, May 26, 1995. Many of the processes for generating hydrogen from H₂S suffer from problems associated with the co-generation of elemental sulfur. While elemental sulfur is a relatively benign solid at ambient conditions, it can be a problem in terms of disposal. Most sulfur is used in the formation of sulfuric acid, an important industrial product. However, local markets may be saturated with sulfur, and long term storage of solid, block sulfur is undesirable. An improved process would generate a co-product that is more easily disposed of than elemental sulfur. [0006] An early method of producing hydrogen by the electrolytic processing of sulfurous acid is described in United States Patent 3,888,750. Here the source of the sulfurous acid is based on a high temperature process for the recycle of sulfuric acid where the by-product of the electrolytic conversion of the sulfurous acid is the sulfuric acid that is subsequently converted into sulfur dioxide and oxygen by thermal decomposition. The sulfur dioxide is separated from the oxygen by vapor-liquid separation. The oxygen is taken off for other uses or vented to the atmosphere and the sulfur dioxide is recycled to the electrolytic cell. The thermal decomposition step requires a high energy input.

[0007] U.S. patent 5,843,395 teaches the thermal disassociation of H₂S containing waste gas at about 1,000⁰C to about 1,900⁰C to provide hydrogen and sulfur, the hydrogen taken off by membrane separation. Any waste heat is used to pre-heat the waste gas prior to the dissociation step. U.S. patent 4,836,992 teaches the use of an electrolytic cell to make H₂ from H₂SO₃, made from mixing SO₂ with NaOH, NaSO₃ or H₂SO₄, thus yielding by-products Of H₂SO₄ and NaOH upon the electrolytic conversion. The SO₂ is provided from the burning of waste gases. U.S. patent 4,519,881 teaches the use of a three-chamber electrolytic cell to regenerate sodium hydroxide from caustic solutions containing sulfides. The process produces hydrogen sulfide by the electrolysis of sodium sulfide and hydrogen ions and also produces hydrogen. However, the process is not compatible with a natural gas stream and does not generate hydrogen gas directly from the sulfurous acid solution. [0008] Accordingly, there is still a need for an effective process for the production of hydrogen from readily available sources while reducing or eliminating any co-production of carbon dioxide or other by-products that are difficult to dispose of in a environmentally effective manner. SUMMARY OF THE INVENTION

[0009] The described and claimed invention is a process for producing hydrogen gas comprising (a) combusting sulfur (S) or hydrogen sulfide (H₂S) with oxygen (O₂) to obtain sulfur dioxide (SO₂) and water (H₂O), plus heat; (b) adding water to the product of (a) to obtain a sulfurous acid solution (H₂SO₃ and H₂O); (c) applying electrical current to the sulfurous acid solution of (b) to obtain sulfuric acid (H₂SO₄) and hydrogen gas (H₂); and, (d) separating the sulfuric acid from the hydrogen gas to obtain separated components of the sulfuric acid and the hydrogen gas, wherein the heat generated in (a) is used to generate at least a portion of the electricity for the electrical current for (c). [0010] In another preferred embodiment, the oxygen in (a) is provided at least in part by the introduction of air, e.g., by providing an air stream into the combustion reaction. Oxygen for combustion can be generated from a cryogenic air separation unit (ASU), or from a membrane process, or from a pressure-swing adsorption unit, all of which reduce the nitrogen content of the gas to be processed. Alternatively, the SO₂ can be captured in a relatively pure form by using a liquid solvent, such as a diamine, applied to various flue sources containing the SO₂, for example from combusting acid gas, disulfide oil, or coal, which will all contain CO₂. In a further preferred embodiment, the sulfur or hydrogen sulfide is provided in (a) by the introduction of combustible compositions, gaseous, liquid or solid, which compositions contain sulfur and/or hydrogen sulfide. Examples include "sour natural gas", acid gas from petroleum and natural gas recovery, sulfur-laden heavy oil and bitumen, and one or more of hard or soft coal and coke products.

[0011] In an alternative embodiment, a system for producing hydrogen gas is provided. The system includes: a) an acid gas stream containing hydrogen sulfide; b)an air stream; c) a burner configured to burn the acid gas stream and air stream and produce a product stream having at least sulfur dioxide, water, and heat; d) a boiler unit operatively connected to the product stream of the burner and configured to capture heat from the burner, and generate at least a steam stream and an exhaust gas stream having at least SO₂; e) an SO₂ separation unit operatively connected to the exhaust gas stream of the boiler unit and configured to remove SO₂ from the exhaust gas stream and generate an SO₂ exit stream; and f) an electrolytic cell operatively connected to the SO₂ exit stream and configured to produce hydrogen gas (H₂).

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 presents a block diagram of one embodiment of the invention process for converting hydrogen sulfide and/or elemental sulfur into hydrogen and sulfuric acid. DETAILED DESCRIPTION

[0013] The invention described and claimed below comprises a combination of hydrogen sulfide (H₂S), or elemental sulfur (S), combustion, electricity generation from recycled heat, and electrolytic processing of sulfurous acid (H₂SO₃) to generate hydrogen and sulfuric acid with little CO₂ (or SO₂) being emitted. There is a plentiful supply of sulfur and H₂S in many parts of the world, so SO₂ does not have to be recycled from H₂SO₄ with large expenditure of energy, as it is in related technologies. The required SO₂ can be generated from a number of one-pass processes. For example, elemental sulfur can be captured from a Claus sulfur recovery process, wherein SO₂ is combined with H₂S in a molar ratio of 1 :2 to generate approximately 3 moles of elemental sulfur and 2 moles of water. Since the Claus reaction does not typically go to completion, remaining H₂S and SO₂ must be handled in a "tail gas treating unit" where the remaining gases are captured for further treatment, i.e., combusted in accordance with the invention. Alternatively, the tail gas could be combusted to convert all sulfur-containing compounds to SO₂, while capturing the SO₂ in a special solvent, as described in WO-A-2006/016979. Another example is the combustion of sulfur-containing coal, or bitumen that generates SO₂. Again, selective capture of SO₂ by a diamine or other solvent can generate a fairly pure SO₂ stream for processing. Disulfide oils, mercaptans, etc., which can be separated from liquid hydrocarbons using caustic processes, may also be considered as combustible feed for this process. [0014] The ready availability of sulfur and/or hydrogen sulfide, plus oxygen, enables an efficient process for producing both sulfuric acid and hydrogen. The hydrogen gas can be cycled for use in hydrogenation processes or transported and/or packaged for sale and used for developing fuel processes using hydrogen. The sulfuric acid by-product is a commodity product that can be recycled or sold for further use, e.g., chemical process use, or it can be readily disposed of in saline aquifers generally available where sulfur-containing natural gas is recovered. Since the sulfuric acid is a liquid, it is amenable to downhole disposal, whereas solid elemental sulfur must be kept in stockpiles on the surface, or buried at some effort and expense. [0015] In one possible embodiment, relatively pure H₂S is combusted in oxygen according to:

H₂S + 3/2 O₂ — > SO₂ + H₂O + 123.3 kcal/mol, (5) or elemental sulfur is combusted according to:

S + O₂ — -> SO₂ + 70.96 kcal/mol. (5a) [0016] According to the invention, the heat generated by these reactions can be used to produce steam, which then can be used to generate electricity for step (c) in the process. The SO₂-containing stream is then quenched with water to generate a sulfurous acid solution:

SO₂ + H₂O — > H₂SO₃ (6) The sulfurous acid solution is then routed to an electrolytic cell. Some of the electricity generated via recovery of heat from reaction (5) powers the electrolytic cell, where hydrogen is liberated, and sulfuric acid is generated according to the following:

H₂SO₃ + H₂O + electricity — > H₂SO₄ + H₂ (7)

[0017] So, the overall result is that water, oxygen, and H₂S are consumed, and hydrogen, H₂SO₄ and excess electricity are generated according to:

H₂S + H₂O + 3/2 O₂ — > H₂SO₄ + H₂ (8)

[0018] Thus when elemental sulfur (or H₂S) is used as a fuel, and pure oxygen as the oxidant, only SO₂ (and only a little SO₃) is produced, with no need for further purification. However, since H₂S is usually contaminated with CO₂ or hydrocarbon, there may in fact be some CO₂ in the resulting SO₂ product.

[0019] The advantage of electrolyzing sulfurous acid instead of pure water is that only 0.17 V is required at 25°C, compared to 1.23 V required for pure water. In principle, only about one-seventh of the power is thus required to generate a given quantity of hydrogen compared to the electrolysis of water. EXAMPLE

[0020] An example heat and material balance for the process of FIG. 1 follows. This illustrative example starts with 1000 kg-mol/hr of acid gas containing 25% H₂S, equating to 189 long tons/day (192x10³ kg/day) of elemental sulfur. The case assumes that 2% of the H₂S combusted is immediately converted to SO₃, and must be purged out ahead of the separation unit.

[0021] Heat is recovered in the waste heat boiler at 600°F (315.6°C), well above the dewpoint of the SO₃/H₂SO₄ present. Ninety-five percent heat recovery and 30% efficient conversion to electricity are assumed. Adequate heat is recovered to regenerate the scrubbing solvent, plus generate enough electricity for the electrolysis. Surplus heat can be used for additional steam or electricity.

[0022] In FIG. 1 acid gas 1 containing hydrogen sulfide at 250.0 kgmols/hr, carbon dioxide at 705.0 kgmols/hr, water in the form of moisture in an amount of 45.0 kgmol/hr, air 2 containing oxygen in amount of 426.8 kgmols/hr, nitrogen in an amount at 1740.2 kgmols/hr, and water in the form of moisture in an amount of 33.0 kgmols/hr is introduced into a burner, or combustion chamber 20. After combustion, stream 3 comprises the nitrogen and carbon dioxide in the same amounts as in the feed gas (unreacted), with sulfur dioxide at 245 kgmols/hr, with 5.0 kgmols/hr sulfur trioxide, 328.0 kgmols/hr water, and 49.3 kgmols/hr oxygen. [0023] This hot, gaseous stream 3 is passed to a boiler unit 21 (i.e., heat exchanger) where 863.3 kgmol/hr water, comprising at least in part condensed steam from the electrical generator 24, is introduced via stream 13 to provide water for the boiler. Alternatively, a separate water stream (not shown) can be used to source the boiler. In yet another embodiment, water is condensed from the combusted stream 3 to provide the water reagent for reaction (6). Steam containing 3116.2 kgmols/hr water vapor is withdrawn in stream 10, and a gaseous, SO₂-containing stream 4, containing additionally the CO₂, O₂, N₂, water vapor, and SO₃ of stream 3 exits the boiler unit 21. Stream 4 will generally be maintained at temperatures above the SO₃ dewpoint, to avoid condensation of corrosive sulfuric acid, H₂SO₄. The operating temperature may be as high as 600°F. [0024] Stream 4 is then quenched by contact with liquid water in a device such as a DynaWave scrubber from MECS, Inc., to reduce the temperature to 100-150 F. Reactive SO₃ and H₂SO₄ dissolve in the aqueous quench stream. A small amount of this stream is purged from the system as a weak solution Of H₂SO₄ in purge stream 15. The balance of the stream is cooled, and recycled to the scrubbing device. The cooled, H₂SO₄-free gas stream is then provided for separation where SO₂ is preferentially removed by absorbing it into an SO₂- selective solvent in a gas-liquid contacting device 22a. For example, Cansolv Technologies, Inc. markets a diamine solvent that is selective for SO₂ over CO₂ and other combustion gases. Gases not absorbed into the solution (including trace SO₂) are vented through stream 5. The rich solution is then regenerated in a separate vessel, generally requiring steam (12) to heat the solvent to 250-300°F. The exit stream 6, cooled to 100-150°F, comprises 240.6 kgmols/hr SO₂ and flows from the regeneration unit 22b at 17-25 psia to the electrolytic cell 23 at near-atmospheric pressure. A portion of the steam (1,992.2 kgmols/hr) from the boiler unit 21 in exit stream 10 is diverted in stream 12 to the regeneration unit 22b to provide heat. Thus stream 11 to the electricity generator comprises 862.3 kgmols/hr steam. Excess steam in an amount of 261.8 kgmols/hr water is removed from stream 12 via stream 16 prior to provision to the regeneration unit 22b. A separate stream 15 takes off 5.0 kgmols/hr H₂SO₄, 3.4 kgmols/hr SO₂ (as H₂SO₃), and 25.0 kgmols/hr water from the scrubbing unit 22a. Unreacted and by-product gases are vented from the separation unit and removed in stream 5, this stream thus comprising all of the CO₂, residual O₂, and N₂, plus 1.0 kgmol/hr SO₂, and 328.0 kgmols/hr water vapor. Separate stream 14 returns 1,992.2 kgmols/hr water from the regenerator unit 22b as condensate to the boiler unit 21.

[0025] Electricity is applied to the sulfurous acid solution (H₂SO₃) in the electrolytic cell 23 from the electrical generator 24 that is driven in major part by the steam stream 11. Additional water (264.7 kgmols/hr) is added to the electrolytic cell 23 in stream 7. Thus with the SO₂ in stream 6, the electrolysis reaction (7) above, produces 240.6 kgmols/hr H₂ which is removed in stream 9 and 240.6 kgmol/hr H₂SO₄ which is removed in stream 8. [0026] The spent steam from the electrical generator is condensed in the generator 24 or in a separate condensing unit 25, and returned via stream 13 to the boiler unit 21 as the boiler feed water.

[0027] The total flow rates, temperatures and pressures in the combined streams above are contained in the following Table 1 by stream number.

Table 1

[0028] The foregoing application is directed to particular embodiments of the present invention for the purpose of illustrating it. It will be apparent, however, to one skilled in the art, that many modifications and variations to the embodiments described herein are possible. All such modifications and variations are intended to be within the scope of the present invention, as defined in the appended claims.

Claims

CLAIMS: I claim:

1. A process for producing hydrogen gas comprising: a. combusting sulfur (S) or hydrogen sulfide (H₂S) with oxygen to obtain sulfur dioxide (SO₂) and water (H₂O), plus heat; b. adding water to the product of (a) to obtain a sulfurous acid solution (H₂SO₃ and H₂O); c. applying electrical current to the sulfurous acid solution of (b) to obtain sulfuric acid (H₂SO₄) and hydrogen gas (H₂); d. using gas-liquid separation to separate the sulfuric acid from the hydrogen gas to obtain separated components of the sulfuric acid and the hydrogen gas; and, wherein the heat generated in (a) is used to generate at least a portion of the electricity for the electrical current of (c).

2. The process of claim 1 where said oxygen is provided by introduction of air.

3. The process of claim 1 wherein said hydrogen sulfide in (a) is provided as a raw acid gas from the production of petrochemical products, said gas comprising hydrogen sulfide and carbon dioxide (CO₂), and optionally, gaseous hydrocarbons.

4. The process of claim 3 wherein before said water in (b) is added, the product sulfur dioxide is separated from the other gaseous combustion products and unreacted gases.

5. The process of claim 1 wherein said sulfur in (a) is comprised in a combustible, carbon-containing compound, or mixture of compounds.

6. The process of claim 5 wherein said combustible, carbon-containing compound or compounds is selected from one or more of hard coal, soft coal, or coke products.

7. The process of claim 5 wherein said combustible, carbon-containing compound or compounds is a sulfur-laden heavy oil or bitumen.

8. The process of claim 1, wherein the electrical current is applied using a voltage below 0.50 volts.

9. The process of claim 1, further comprising removing the sulfuric acid from the process.

10. The process of claim 1, wherein step (b) is operated at a temperature of about 125°F (about 52⁰C).

11. A system for producing hydrogen gas comprising: a. an acid gas stream containing hydrogen sulfide; b. an air stream; c. a burner configured to burn the acid gas stream and air stream and produce a product stream having at least sulfur dioxide, water, and heat; d. a boiler unit operatively connected to the product stream of the burner and configured to capture heat from the burner, and generate at least a steam stream and an exhaust gas stream having at least SO₂; e. an SO₂ separation unit operatively connected to the exhaust gas stream of the boiler unit and configured to remove SO₂ from the exhaust gas stream and generate an SO₂ exit stream; and f. an electrolytic cell operatively connected to the SO₂ exit stream and configured to produce hydrogen gas (H₂).

12. The system of claim 11, further comprising: an electrical generator driven by at least a portion of the steam stream and configured to provide electricity to the electrolytic cell.

13. The system of claim 11, wherein the SO₂ separation unit is a solvent based SO₂ separation unit.

14. The system of claim 11 , further comprising a water inlet stream configured to provide water to the electrolytic cell.

15. The system of claim 11, further comprising a regeneration unit operatively connected to the SO₂ separation unit and configured to produce at least one water stream, wherein at least a portion of the steam stream is supplied to the regeneration unit and at least a portion of the water stream is supplied to the boiler unit.

16. The system of claim 11 , further comprising: a. a sulfuric acid purge stream configured to take off at least sulfuric acid from the SO₂ separation unit; and b. a sulfuric acid exit stream configured to remove sulfuric acid from the electrolytic cell.

17. The system of claim 11, wherein the SO₂ separation unit is configured to operate at a temperature of about 100⁰F (about 38⁰C).

18. The system of claim 15, wherein the regeneration unit is configured to operate at a temperature of about 300⁰F (about 149°C).