Multiple phase SO3/SO2/H2O/H2SO4 electrolyzer

Abstract

An electrolyzer (16) having opposed electrodes (44, 46) to produce hydrogen gas operates at a pressure greater than 5.5 MPa, where SO2 is liquefied, and a reaction phase (48) of liquid SO2 and H2O which is passed to the electrolyzer where liquid SO3 is formed as a bottom phase (54), and where the amount of water present is less than two moles per mole of liquid SO2.

Description

FIELD OF THE INVENTION

This invention describes improvements that can be incorporated into sulfur oxides based processes that use high temperature heat for all or a portion of the energy needs for a hydrogen production facility. These improvements include the use of multiple liquid phases in a hydrogen electrolyzer operating with or without a membrane, where the major liquid phases are pure SO₂, H₂O, H₂SO₄ and SO₃ or a mixture of these components. These improvements dramatically increase the process energy efficiency of the sulfur oxides processes and reduce costs associated with the design and construction of the processes by reducing the amount of water required. The processes are usually the “Westinghouse Sulfur Process” and the “General Atomic Iodine/Sulfur Process.”

BACKGROUND OF THE INVENTION

Sulfur cycles are a group of thermochemical processes that can make hydrogen, mainly using thermal energy from a high temperature heat source. Two sulfur cycles are the so called “Westinghouse Sulfur Process” and the “General Atomic Iodine/Sulfur Process”. The Westinghouse Sulfur Process (“WSP”), is described in Proceedings of ICONE 12, “Optimization of the Westinghouse Sulfur Process for Hydrogen Generation and Interface with an HTGR” by E. J. Lahoda et al. 12^th Intl. Conference on Nuclear Engineering, Arlington, Va., dated Apr. 25-29, 2004, pp 1-3; and in AICHE Spring Meeting “Interfacing the Westinghouse Sulfur Cycle with the PBMR for the Production of Hydrogen” by R. Matzie et al., New Orleans, dated Feb. 27, 2004 pp 1-10. The Iodine/Sulfur (“S/I”) Process is described in Proceedings of ICONE-11, “Nuclear Energy in Non-Electric Power Applications” by E. J. Lahoda et al., 11^th International Conference on Nuclear Engineering, Tokyo, Japan, Apr. 20-23, 2003, pp 1-9. Both processes are compared in American Nuclear Society Global Paper #88017, “Improvements in the Westinghouse Process for Hydrogen Production” by J. E. Goossen et al., American Nuclear Society Annual Winter Meeting, New Orleans, La. November 2003, pp 1-5. At 1000° C. only the WSP system reaches 50% efficiency. Both cycles require temperatures in excess of 760° C. to have at least 40% efficiency.

The Westinghouse Sulfur Process (“WSP”) generates hydrogen using high temperature process heat and electricity. The high temperature heat sources are any that produce heat available for use above about 760° C., such as an HTGR (High Temperature Gas Cooled Reactor, such as a Pebble Bed Modular Reactor “PBMR”), a high temperature solar concentrator, a natural gas fired combustor or any combination of these heat sources. The portion of the process where sulfur trioxide is decomposed into sulfur dioxide (SO₂) and oxygen typically takes place at these high temperatures. The final step in this process is absorption of the SO₂ in water at room temperature to form sulfurous acid and a SO₂ free stream Of O₂. The one issue with these cycles is to have an efficient method for capturing the SO₂. This is due to the relatively low solubility of the SO₂ in water. The Westinghouse Sulfur Process produces hydrogen in a low-temperature electrochemical step, in which sulfuric acid and hydrogen are produced from sulfurous acid. This reaction can be run at between 0.17 and 1.6 volts with a current density of 200 to 2000 ma/sq.cm at about 60° C. to 100 C. This is a well known process which is hereby defined as ∂standard WSP”=
SO₃+H₂O⇄SO₂+H₂O+O.5O₂(>760° C. heat required);   (1)
SO₂+2H₂O+0.50₂⇄H₂SO₃+H₂O+O.5O₂(T<100° C.); and   (2)
H₂O+H₂SO₃→H₂+H₂SO₄(electrolyzer at about 100° C. or less).   (3)

The Iodine/Sulfur Process also starts with a reversible reaction where sulfuric acid is decomposed at over 760° C. to form sulfur dioxide as above, followed by reaction of the sulfur dioxide with Iodine to form HI. This is a well known process which is hereby defined as “standard S/I”=
SO₃+H₂O⇄SO₂+H₂O+O.5O₂(>760° C. heat required not shown in FIG. 2)   (1)
I₂+SO₂+2H₂O+O.5O₂+excess H₂O⇄2HI+H₂SO₄+O.5O₂+excess H₂O (about 100° C. to 200° C. heat generated) defined as the “Bunsen Reaction.”  (2)
2HI⇄H₂+I₂(greater than 400° C. heat required)   (3)

The Westinghouse Sulfur Process has also been described in U.S. Department of Commerce Document Contractor (U.S.D.C.) Report, www.ntis.gov, Westinghouse Astronuclear Lab, contact NAS 3-18934 “The Conceptual Design of an Integrated Nuclear Hydrogen Production Plant Using the Sulfur Cycle Water Decomposition System, April 1976, pp. iii-x, 130-165 and 198-199. The highest efficiency at about 100 psi, 689 KPa shown in FIG. 5.3.8 was about 45%. The process taught there used 70 wt.% sulfuric acid as the product from the electrolyzer. The electrolyzer used excess water with SO₂ dissolved in the water. The excess water diluted the sulfuric acid to 70%.

In a later U.S.D.C. report FE226215, www.ntis.gov, Westinghouse Advanced Energy Systems, contract EX-76-C-01-2262 ”Hydrogen Generation Process,” G. H. Farbman and V. Koump, June 1977, pp. i-iii and 10-31, problems of migration of sulfurous acid into the catholite, causing preferential sulfur formation at the cathode were recognized. A solution taught was to slightly over pressurize the catholite and use an appropriate diaphragm (liquid porous membrane), such as a microporous rubber material. In the processes taught there, slight over-pressure caused water flow into the anolite. Although that prevented sulfur deposition on the cathode, it also diluted the sulfuric acid that was formed in the anodic compartment. This excess water increased the amount of energy required to evaporate the sulfuric acid (sulfur trioxide and water) for the high temperature decomposition step.

In a June 1978 report for EPRI, U.S.D.C. Report EPRIEM 789, www.ntis.gov, Westinghouse Advanced Energy Systems “Economic Comparison of Hydrogen Production Using Sulfuric Acid Electrolysis and Sulfur Cycle Water Decomposition Final Report,” pp. iii-viii, and 1-1 to 4-23, A7, B1-B10, showed that low cell voltages could be obtained and that the “WSP,” using ion exchange membranes, would show an energy efficiency increase of from about 41% to 47%.

In U.S. Ser. No. 11/054235, filed on Feb. 9, 2005, (Docket No. RDM 2004-006) the idea was advanced to increase the pressure to above 1100 psi (7.58 MPa) to lower the amount of excess water required to remove all of the SO₂ from an O₂ stream at increased scrubbing temperatures. This results in a significant energy efficiency increase. However, a sulfuric acid product is still produced as a product of the electrolysis step. This sulfuric acid and any excess water must be vaporized before it can be sent to the high temperature decomposition step. The energy requirements of this step are significant and while efficiency is very good, 50% to 56% at 1450 psi (10.0 MPa), even higher efficiencies are desirable. In addition, exotic materials of construction are required for this vaporizer.

The S/I process has a significant technical issue in separating the products of the Bunsen Reaction because of the mutual solubility of the reactants and products in an aqueous phase reactor.

What is needed is an improvement to the WSP and S/I systems to improve efficiency to over 50% and up to 65% to reduce costs. It is a main object of this invention to provide such an efficient, cost effective system.

SUMMARY OF THE INVENTION

The above needs are met and issues solved by providing a method of operating an electrolyzer, having an anode electrode and an opposed cathode electrode, to produce hydrogen gas at a pressure greater than 800 psi (5.5 MPa) comprising/processes the steps, in no particular order: (1) liquefying SO₂ at a pressure greater than 5.5 MPa in the presence of H₂O to provide a reaction phase which is passed to the anode of an electrolyzer, where the liquid SO₂ is substantially immiscible in the H₂O where the amount of water present is less than or equal to two moles per mole of liquid SO₂; (2) forming liquid SO₃ from oxidizing the liquid SO₂ to liquid SO₃ on the anode, in the electrolyzer, where the liquid SO₃ is reactive to excess H₂O, and where 2H⁺ and 2e⁻ are provided at or near the cathode; (3) formation of a liquid phase in the electrolyzer from the SO₃ and excess H₂O where the liquid phase is selected from the group consisting of dense H₂SO₄ solution and liquid SO₃ both of which are more dense than liquid H₂O or liquid SO₂; (4) recovering a liquid from the electrolyzer where the liquid is selected from the group consisting of liquid SO₃ and liquid H₂SO₄ and (5) recovering hydrogen gas formed at or near the cathode of the electrolyzer by reaction of the 2H⁺ and 2e⁻ provided in step (2).

The invention also includes a method of operating the standard Westinghouse Sulfur Process (standard WSP,) containing an electrolysis oxidation step or the standard Iodine Sulfur Process (standard S/I) containing an iodine/sulfur oxidation step at a pressure greater than 800 psi (5.5 MPa) to allow the formation of a feed with a minimal amount of water to the oxidation step, where SO₃ is generated to form a solution layer denser than water, and water plus immiscible liquid SO₂, where the liquid SO₃ is recovered or is reacted with excess water to form liquid H₂SO₄, where the product H₂O:SO₃ mole ratio is lower than 1:1 (less H₂O than SO₃) and the product of the oxidation step contains pure SO₃ and/or H₂SO₄.

Preferably the pressure will be greater than 800 psi (5.5 MPa) up to 1700 psi (11.7 MPa), most preferably from 1000 psi (6.9 MPa) to 1700 psi (11.7 MPa.) Preferably the liquid SO₃ will be recoverable at from 20° C. to 75° C. This also allows both systems as noted above to reduce the number of excess moles H₂O required in the reactions by up to 150%, preferably by 15% to 150%:
WSP: (1+x)H₂O+SO₂→H₂+(1−x)SO₃+xH₂SO₄(x<1)
SI: I₂+SO₂+(1+x)H₂O⇄2HI+(1−x)SO₃₊xH₂SO₄(x<1).

The prior art processes always produced dilute H₂SO₄ in the electrolysis or I₂ oxidation steps which had to be vaporized and then sent to the H₂SO₄ decomposition reactor. If pure SO₃ is produced in the electrolysis or I₂ oxidation step, energy requirements and water requirements would be reduced and efficiency could be raised from 50% to about 70%.

BRIEF DESCRIPTION OF THE DRAWINGS

A full understanding of the invention can be appreciated from the following detailed description of the invention when read with reference to the accompanying drawings wherein:

FIG. 1 is a schematic diagram of one embodiment of the so called Westinghouse Sulfur Process Cycle (“WSP”)

FIG. 2 is a schematic diagram of the Sakuri embodiment of the so called Iodine/Sulfur Process (“S/I”);

FIG. 3, which best describes the invention, is a block diagram of an electrolyzer operating at high pressure to provide liquid SO₃;

FIG. 4 is a block diagram of an electrolyzer operating at high pressure to provide liquid H₂SO₄;

FIG. 5 is a block diagram of an electrolyzer containing an ion-exchange or microporous membrane, operating at high pressure to provide liquid SO₃; and

FIG. 6 is a block diagram of an electrolyzer containing an ion-exchange or microporous membrane, operating at high pressure to provide liquid H₂SO₄.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to prior art FIG. 1, a standard WSP process 2 is shown operating at less than 800 psi. In FIG. 2, the standard S/I process 4 is shown operating at less than 800 psi. In FIG. 1, thermal energy 10 at about 760° C. to 1000° C. is passed into oxygen generator 12 to provide the reaction shown, passing H₂O, SO₂ and O₂ to an absorber unit 14, by way of vaporizer 20, where H₂O and SO₂ are passed to an electrolyzer 16 energized with D.C. electricity 18 to provide H₂ shown as 6 and H₂SO₄, where the latter is vaporized in vaporizer 20 by thermal energy 22 and the energy in the H₂O and O₂ stream, to feed vaporized H₂SO₄ to the oxygen generator/sulfuric acid decomposition reactor 12, as shown.

As shown in prior art FIG. 2, a “Sakuri 2000” process schematic of the reactions for S/I vs. temperature, sulfuric acid 26 is vaporized in vaporizer 28 and then passed to decomposition reactor 30 at about 760° C. to 810° C. to generate O₂ and pass O₂, H₂O and SO₂ to iodine reactor 32 which generates HI which is decomposed in second decomposition reactor 34 to provide H₂ shown as 6 after passing through the excess I₂ separator 36. The decomposition reactor 34 provides the main source of I₂, shown as 38 for the iodine reactor 32, also known as a Bunsen reactor.

By operating the entire WSP or SI cycle at a high pressure of most preferably greater than 1000 psi (6.9 MPa-mega pascals), we have found that SO₂ can be removed from the system at temperatures above 20° C., preferably at 20° C. to 75° C., without the use of energetically inefficient refrigeration systems or excess water. In addition, operation of the cycle at a higher pressure would allow higher removal efficiency of the SO₂ using less water. High system pressure operation has other advantages. It has now been shown that the use of pressure greater than 1000 psi (6.9 MPa) greatly increases overall process efficiency by allowing for the direct liquid phase conversion of sulfurous acid (H₂SO₃) to sulfur trioxide and hydrogen in the electrolyzer, the hydrogen generation portion of the Westinghouse Sulfur Process (“WSP”). Consequently, for every mole of H₂ produced, only one mole of H₂O and one mole of SO₂ is required.

A diagram of a liquid SO₂ electrolysis cell operating at 800 psi to 1000 psi or higher, without a cell dividing membrane is shown in preferred option FIG. 3 and option FIG. 4. As shown, electrolyzer 16 is fed with a liquid reaction phase SO₂ stream 40 with dissolved H₂O with hydrogen 6 being generated. Electrodes are shown as cathode 44 and anode 46. The electrolyzer uses multiple liquid phases of SO₂ and SO₃ and H₂O for the electrolysis of the SO₂ of stream 40 to liquid SO₃ stream 54, to provide hydrogen 6.

This process operates under high pressure where SO₂ becomes a liquid that is immiscible in H₂O, reaction phase, shown generally as 48, which provides liquid SO₂ that is oxidized to SO₃. If excess H₂O is present, the resulting SO₃ then reacts with water, reaction 50, dissolved in the SO₂ and forms a dense H₂SO₄ solution 52 as shown in FIG. 4 that settles to the bottom of the electrolysis cell. If the SO₃ does not react with water, as in the preferred FIG. 3, then a liquid SO₃ layer 54 is formed at the bottom of the cell. This process reduces the costs associated with eletrolytically producing hydrogen in the sulfur family of hydrogen production methods by reducing the amount of water required in the process, reducing the heat required for vaporizing this stream and eliminating the need for a separating membrane, as shown in FIGS. 3 and 4. In both cases, SO₂ settles with dissolved H₂O in a top chamber 55, and hydrogen 6 is formed by the reaction 2H⁺ 2e⁻→H₂. Thus, the reactions are
SO_2(liq)+H₂O (liq dissolved)→SO_3(liq)+2H⁺+2e⁻  (1)
2H⁺+2e⁻→H₂(gas)   (2)
H₂O+SO₃→H₂SO_4(liq)(possible if excess water is available)   (3)
In scrubber 60, upper phase 62 sits on a bottom phase 64 SO_2(liq) that settles with dissolved H₂O.

The same reactions take place if a conventional ion exchange or microporous membrane is used in the electrolyzer, as shown in FIGS. 5 and 6, where the membrane separates an anode chamber 65 and cathode chamber 66. The membrane is shown as 56. Hydrogen is formed at the cathode electrode 44. This membrane should have high mechanical strength and be chemically stable and H+ ion permeable or microporous. Particularly useful membranes are 0.2 mm to 1.0 mm thick microporous rubber or polyvinyl chloride (PVC) for microporous membranes. A variety of ion exchange membranes are available. As shown, the membrane is disposed between electrode 46, the anode and liquid SO₂ anolite on one side with water plus permeating H⁺ hydrogen ion and the cathode 44 in which catholite reaction (2) occurs at or near the cathode: 2H⁺+2e⁻→H₂, providing hydrogen, as shown in FIGS. 3-6.

In this process, in an electrolyzer used to produce H₂ from H₂O and SO₂, sufficient pressure is used to liquefy the feed SO₂, reducing the water flow to a SO₂ scrubber 60 and increasing the pressure to the point where two liquid phases are formed at the bottom of the scrubber. The lighter phase is water with dissolved SO₂, upper phase 62 and the lower, heavier phase 64 is liquefied SO₂ with dissolved water. The upper phase 62 is recirculated as shown within the SO₂ scrubber column 60 while the lower phase is withdrawn and sent to the electrolysis reactor 16. The electrolysis reactor is now operated using liquid SO₂ with dissolved H₂O.

The potential exists to also eliminate the separator membrane 56, shown in FIGS. 5-6, that is used in the current water based electrolysis cells, which will lower capital costs and reduce cell resistance and voltage losses. This approach allows for the product of the electrolysis cell to be liquid SO₃, concentrated H₂SO₄ 52 or a mixture of both. In any case, the energy required to evaporate the resulting liquid is much reduced over the current aqueous based cells since the excess water does not have to be evaporated. The SO₂ scrubber absorber 60 operates at pressures >800 psi, preferably between 800 psi and 1700 psi, most preferably from 1000 psi to 1700 psi and with excess water being <1 mole per mole of SO₂. An SO₂ scrubber similar to 60 can be used in the S/I process. The resulting SO₂ phase with dissolved water, phase 64, is mixed with an I₂ phase with same dissolved water. The resulting Bunsen Reaction occurs and leaves two immiscible phases of SO₃ and HI in I₂ respectively.

Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.

Claims

1. a method of operating an electrolyzer having an anode electrode and an opposed cathode electrode to produce hydrogen gas at a pressure greater than 5.5 MPa comprising:

(1) liquefying SO2 at a pressure greater than 5.5 MPa in the presence of H2O to provide a reaction phase which is passed to the anode of an electrolyzer, where the liquid SO2 is immiscible in the H2O, where the amount of water present is less than or equal to two moles per mole of liquid SO2;

(2) forming liquid SO3 from oxidizing the liquid SO2 to liquid SO3 on the anode in the electrolyzer where the liquid SO3 is reactive to excess H2O, and where 2H+ and 2e− are provided at or near the cathode;

(3) formation of a liquid phase in the electrolyzer from the SO3 and excess H2O where the liquid phase is selected from the group consisting of dense H2SO4 solution and liquid SO3 both of which are more dense than liquid H2O or liquid SO2;

(4) recovering a liquid from the electrolyzer where the liquid is selected from the group consisting of liquid SO3 and liquid H2SO4; and

(5) recovering hydrogen gas from the cathode of the electrolyzer by reaction of the 2H+ and 2e− provided in (2).

2. The method of claim 1, wherein the pressure is from 5.5 MPa to 11.7 MPa.

3. The method of claim 1, wherein the pressure is from 6.9 MPa to 11.7 MPa.

4. The method of claim 1, wherein liquid SO3 is recovered at a temperature from 20° C. to 75° C.

5. The method of claim 1, wherein the following reactions occur: SO2(liquid)+H2O (liquid)→2H++2e−+SO3(liquid) 2H++2e−→H2O (gas); and optionally SO3(liquid)+H2O (liquid)→H2SO4(liquid)

6. The method of claim 1 where the electrolyzer is used in the operation of the Westinghouse Sulfur Process.

7. The method of claim 1, wherein H+ and H2O pass through a microporous or ion exchange membrane separating the electrolyzer into an anode chamber and a cathode chamber, to form H2 at or near the cathode electrode.

8. The method of claim 1, wherein liquefaction of SO2 in (1) occurs in a scrubber where two phases form; a lighter phase of water with dissolved SO2 and a heavier phase of liquefied SO2 with dissolved water, which heavier phase is passed to the electrolyzer.

9. The method of claim 8, wherein the scrubber operates at pressures greater than 5.5 MPa.

10. The method of claim 8, wherein the scrubber operates at pressures from 5.5 MPa to 11.7 MPa.

11. The method of claim 8, wherein the scrubber operates at pressures from 6.9 MPa to 11.7 MPa.

12. A method of operating a Westinghouse Sulfur Process containing an electrolysis oxidation step or an Iodine Sulfur Process containing an iodine/sulfur oxidation step, at a pressure greater than 5.5 MPa to allow the formation of feed with a minimal amount of water used in the oxidation step, where SO3 is generated to form a solution layer denser than water, and water plus immiscible liquid SO2, where the liquid SO3 is recovered or is reacted with water to form liquid H2SO4, where the H2O:SO3 mole ratio is lower than 1:1 (less H2O than SO2) and the product of the oxidation step contains pure SO3 and/or H2SO4.

13. The method of claim 12, wherein the pressure is from 6.9 MPa to 11.7 MPa.