WO1993020008A1

WO1993020008A1 - Desulfurization process using bromine

Info

Publication number: WO1993020008A1
Application number: PCT/US1993/003143
Authority: WO
Inventors: Paul F. Schubert; David W. Schubert; Suresh Mahajan; Thomas Rostrup-Nielsen
Original assignee: Catalytica, Inc.
Priority date: 1992-04-03
Filing date: 1993-04-02
Publication date: 1993-10-14
Also published as: AU3973393A

Abstract

This invention is a combination process for removal of sulfur oxides (SOx) from gases containing both the SOx and oxygen (106). The fluid used to remove the SOx contains sulfuric acid and bromine. The SOx is converted to sulfuric acid and the bromine is thereafter converted to hydrobromic acid. The hydrobromic acid is concentrated and catalytically converted to bromine for ultimate recycling to the SOx removal step (132). The SOx is finally recovered as a strong sulfuric acid (104).

Description

DESULFURIZATION PROCESS USING BROMINE

DESCRIPTION Technical Field

This invention is a process for the removal of sulfur oxides (SO_χ) from flue gases and other gases containing SO_χ. The fluid used to remove the SO_χ contains bromine. The SO_χ is converted to H₂S0₄ and the bromine is thereafter converted to HBr. The HBr is concentrated and catalytically converted to bromine for ultimate recycling to the SO_χ removal step. The SO_χ is finally recovered as a strong sulfuric acid.

Background Art

This invention is a process for desulfurization of gases containing sulfur oxides or SO_χ. These sulfur oxides are typically found in flue gases emanating from processes in which a sulfur-containing fuel is combusted. SO_χ is an equilibrium mixture of S0₂ and an extremely small amount of S0₃. S0_χ is typically present in flue gases formed from heavier or solid fuels. For instance, the use of high-sulfur coal in England caused the "killer fogs" of the last century. The elimination of SO_χ, either by burning low sulfur fuels or by removing the SO_χ from the flue gases after production has been a constant environmental goal.

There are a variety of processes known for removing SO_χ, e.g., scrubbing with various basic alkali metal and alkaline earth metal suspensions. For instance, use of calcium oxides is known, but produces a rather ungainly and voluminous mass of calcium sulfites and sulfates. Disposal of these materials is not performed with ease. A process that converts the SO_χ found in flue gas to concentrated sulfuric acid is highly desirable. High concentrations of sulfuric acid is, of course, a commodity chemical of value. The production of sulfuric acid having economic value rather than disposal costs is of high interest.

There are a few processes which use a mixture of sulfuric acid and bromine to remove S0₂ from flue gases; specifically, U.S. patent 4,668,490 to van Velzen et al. shows such a process. In that process, S0₂ is absorbed in a solution containing sulfuric acid (15% by weight) , hydrobromic acid (15% by weight) , and bromine (minor amounts) . The bromine is converted to HBr. The mixture of HBr and sulfuric acid is then transported to an electrolytic cell, where it is converted into bromine and hydrogen.

Similar disclosures are found in European Patent Application 0,016,290 (published October 1, 1980). This process is supported by the European Atomic Energy Community (EURATOM) and is seen generally as a process for the production of hydrogen rather than as an antipollution process. See, for instance: van Velzen et al., "The Mark XIII Process for Hydrogen Production", Proc. Intersoc.

Energy Convers. Enσ. Conf.. 15th(2) , pages 1716-1720 (1980) . van Velzen et al., "The Oxidation of Sulfur Dioxide by Bromine and Water", Int. J. Hydrogen Energy. 5(1), pages 85-96 (1980). van Velzen et al., "Status Report on the

Operation of the Bench Scale Plant for Hydrogen Production by the Mark XIII Process", Adv. Hydrogen Energy. 2 (Hvdrogen Energy Prog.. Vol. 1) . pages 423-438 (1981) . van Velzen et al., "Development, Design, and

Operation of a Continuous Laboratory-Scale 5 Plant for Hydrogen Production by the Mark

XIII Cycle", Adv. Hydrogen Energy. 1 (Hydrogen Energy Svst.. Vol. 2). pages 649-665 (1979). De Beni et al. , "The Reaction of Sulfur Dioxide 10 with Water and Halogen. The Cases of

Bromine and Iodine", Adv. Hydrogen Energy. 1 (Hydrogen Energy Syst.. Vol. 2 ) . pages 617-648 (1979). van Velzen et al., "Status Report on the 15 Operation of the Bench-Scale Plant for

Hydrogen Production by the Mark XIII Process", Int. J. Hydrogen Energy. Vol. 7, pages 629-636 (1982) . van Velzen et al., "Development and Design of a 20 Continuous Laboratory-Scale Plant for

Hydrogen Production by the Mark XIII Cycle", Int. J. Hydrogen Energy. Vol. 5, pages 131-139 (1980) . van Velzen et al. , "Hydrogen Bromide 25 Electrolysis in the ISPRA Mark XIIIA Flue

Gas Desulfurization Process: Electrolysis in a DEM Cell", J. APPI. Electrochem.. Vol. 20, pages 60-68 (1990). van Velzen et al., "Thermochemical Hydrogen 30 Production by the Mark XIII Process: A

Status Report", American Chemical Society. 799163, pages 783-789 (1979). None of these disclosures suggest the concept of using a catalyst rather than an electrochemical cell 35 for regenerating the HBr to bromine. The independent step of converting HBr to elemental bromine by catalytic oxidation is well known. There are a number of processes described in the open literature which produce bromine according to the equation:

4 HBr + 0₂ —> 2 Br₂ + 2 H₂0 One such process (British Patent 930,341) involves the conversion of hydrobromic acid solutions using dissolved metal ion catalysts. The soluble metal may be gold, cerium, chromium, nickel, platinum, thorium, titanium, or vanadium, but preferably is iron or copper. A gas containing oxygen is passed through the acidic solution containing HBr and the dissolved metal, all at a temperature below the boiling point of the liquid. The gaseous effluent is then separated via condensation and distillation into product bromine, water, and HBr for recycle to the oxidation step.

Similarly, U.S. Patent No. 3,179,498 to Harding et al. discloses a process in which a nitrite catalyst is employed in an acidic, aqueous solution of HBr to effect the oxidation of the HBr to Br₂. The temperature of the liquid is maintained between 0° and 100°C. Although any inorganic or organic nitrite is said to be suitable, preferred catalysts are alkali metal or alkaline earth metal nitrites.

There are a number of processes which use heterogeneous catalysts to effect the conversion of HBr to Br₂.

U.S. Patent No. 2,536,457, to Mugdan, teaches such a process. The conversion is carried out at a temperature between 800° and 1200^βC (preferably between 800° and 1000°C) with an excess of oxygen. The catalyst is preferably cerium oxide and may be supported on pumice granules or other refractory materials. If excessive water is included in the reactor, combustible gas such as hydrogen is included to maintain the reaction temperature. Clearly the reaction temperature for this process is quite high.

U.S. Patent No. 3,273,964 to De Rosset shows a process in which the effluent from a dehydrobromination reaction is contacted with a catalyst-adsorbent composite. The effluent contains olefinic hydrocarbons and is produced by a series of steps in which an alkane is brominated to form a bromoalkane; the bromoalkane is then dehydrobrominated to form the effluent of olefinic hydrocarbons and HBr. The catalyst-adsorbent composite adsorbs the HBr in a first step and, during regeneration, catalyzes the oxidation of HBr to form the desired Br₂. The composite contains an adsorbent of a basic metal oxide such as magnesium, calcium, or zinc oxide, and a catalyst of a Group IV-B metal oxide such as titania, magnesia, or zirconia. The preferred composite contains magnesia and zirconia in a ratio from about 0.5:1 to about 5:1. U.S. Patent No. 3,260,568 to Block et al. teaches a process in which a stream containing substantially dry HBr with a solid adsorbent containing a metal "subchloride" and which is the reaction product of refractory metal oxide and a metal chloride. The contact takes place at conditions where the HBr replaces at least a portion of the chloride in the adsorbent. When the adsorbent has reached about six percent by weight, the adsorbent is regenerated by contacting it with a dry hydrogen chloride gas. The patent does not appear to suggest the conversion of the adsorbed HBr to Br₂. The adsorbent is suggested to be selected from metal chlorides such as aluminum, antimony, beryllium, iron, gallium, tin, titanium, and zinc chlorides.

U.S. Patent No. 3,310,380 to Lester discloses a process for the adsorption of combined bromine (e.g., HBr and alkyl bromides) on a catalytic-adsorbent composite, recovering unsaturated hydrocarbons, and when the adsorbent is filled, contacting the composite with an oxygen-containing gas at a temperature between 50° and 450°C to produce a Br₂ stream also containing water and unreacted HBr. This stream (also in admixture with an oxygen-containing gas) is then contacted with a second stage rector also containing the composite but at a temperature between 200° and 600°C. The composite in the first stage comprises, preferably, 0.1 to 10% by weight of copper or cerium oxide composited on magnesium oxide: the second stage composite comprises, preferably, 2.0 to about 50% by weight of copper or cerium oxide composited on an alumina or zirconia support. Similarly, U.S. patent No. 3,346,340 by Louvar et al. suggests a process for the oxidation of HBr to Br₂ using a catalyst-inert support composite. The composite comprises a copper or cerium oxide on an inert support having a surface area between 5 and 100 square meters per gram and containing less than about 50 micromoles of hydroxyl per gram. The supports may be alpha- or theta- alumina or zirconia. The preferred temperature is between 300° and 600°C.

U.S. Patent No. 3,353,916 to Lester discloses a two-stage process for oxidizing HBr to form Br by the steps of mixing the HBr-containing gas with an oxygen- containing gas and passing the mixture at a temperature of at least 225°C over a catalyst selected from the oxides and salts of cerium, manganese, chromium, iron, nickel, and cobalt and converting a major portion of the HBr to Br₂. The partially converted gas, still containing excess oxygen, is then. assed through a second stage catalyst comprising a copper oxide or salt at a temperature of at least about 225°C but not exceeding a "catalyst peak temperature" of 350^βC to convert the remaining HBr. The preferred support appears to be zirconia.

This two-stage arrangement is carried out to prevent loss of the copper catalyst. Because the preferred copper oxide is apparently converted to copper bromide during the course of the reaction and copper bromide volatilizes at "temperatures in excess of about 350^βC", the "copper bromide migrates through the catalyst mass in the direction of flow with eventual loss of copper bromide and premature deactivation." Use of a first catalyst stage which is tolerant of high temperatures, although apparently not as active a catalyst as is copper, allows a cooler second catalyst stage containing copper to complete "quantitative conversion of bromine from hydrogen bromide."

U.S. Patent No. 3,379,506 to Massonne et al. teaches a process for the selective oxidation of hydrogen bromide to bromine in the presence of fluorocarbons by passing the mixture of gases over a Deacon catalyst at a temperature of 250° to 500°C, preferably between 300° and 400°C. The Deacon catalyst is said to be a "mostly porous carrier such as pumice, alumina, silica gel, clay, or bentonite, impregnated with a solution of bromides or chlorides of metals such as copper, iron, titanium, vanadium, chromium, manganese, cobalt, molybdenum, tungsten, or mixtures thereof." The preferred catalyst is said to be a chloride of copper. The patent notes that:

"[a] very efficient and stable catalyst is an oxidation catalyst which is prepared by impregnating active alumina with chlorides of copper, rare earths and/or alkali metals, drying at about 120°C and subsequent activation at a temperature of 300° to 450°C." One example shows the production and use of a catalyst of alumina, potassium, copper, and an amount of "rare earths of the cerite group as chlorides."

Another patent which notes the problem with the volatilization of copper bromide in the oxidation of hydrogen bromide to bromine is U.S. Patent No. 3,437, 45 to Hay et al. The solution is to eliminate the copper in favor of a noble metal, such as platinum and palladium. The reaction is carried out at a temperature between about 175° and about 700 -C with a contact time of at least about 0.1 second, "but for best operation a contact time of about five and 25 seconds is preferred." The yield of bromine is only between 28 and 78 molar percentage. U.S. Patent No. 4,131,626 to Sharma et al. suggests a process in which bromide salts are heated in the presence of an oxygen-containing gas, silicon dioxide, and an oxidation catalyst at a temperature of about 500° to 1000°C. The bromine is produced in conjunction with sodium silicate.

None of these disclosures suggest a combined process for the removal of S0_χ from oxygen-containing gases using a bromine-containing fluid and regenerating the bromine supplied to the process through the use of catalytic oxidation.

sn-imna- of the Invention

This invention is a process for removing S0_χ from gas streams containing that S0_χ. The liquid medium used to remove the S0_χ from the SO_χ-containing gas contains bromine (Br₂) and, typically, sulfuric acid (H₂S0₄) . The S0_χ is absorbed into the liquid as sulfuric acid. Hydrobromic acid (HBr) is also formed from this reaction. A portion of the liquid circulating in the desulfurization reactor is removed and subjected to a vaporization step, where the hydrobromic acid is substantially removed from the liquid. The vaporous hydrobromic acid stream is then stripped of S0₂, mixed with oxygen, and subjected to a catalytic oxidation step. The catalytic oxidation step converts the HBr to Br₂ and water. This process also provides a concentrated, high strength sulfuric acid as an end product.

Brief Description of the Drawings Figure 1 shows, in schematic fashion, the major steps of the inventive process.

Figure 2 shows a schematic process showing the detailed operation of one variation of the invention.

Description of the Invention

In this process, a gas containing sulfur oxides, predominantly sulfur dioxide but with a minor amount of sulfur trioxide, is treated by reaction with solution containing aqueous bromine, sulfuric acid, and hydrobromic acid to oxidize the SO_χ to S0₃. The S0₃ is absorbed into the liquid as H₂SO. as seen in the following reaction:

S0₂ + Br + 2H₂0 —> H₂S0₄ + 2HBr The hydrobromic acid is in turn oxidized with oxygen to regenerate the bromine, according to the following reaction:

2HBr + 1/2 0₂ —> Br₂ + H₂0 Therefore, the overall reaction in this process is: S0₂ + 1/2 0₂ + H₂0 —> H₂S0₄. The SO_χ-containing gas enters the process at elevated temperatures and this heat is used in acid concentrators to produce a high-strength sulfuric acid. The hydrobromic acid is catalytically oxidized with oxygen to regenerate the bromine for its subsequent recycle and reuse in oxidizing the sulfur dioxide into sulfur trioxide and concomitant absorption in the liquid stream in the desulfurization reactor.

The process is described in general fashion as follows: As shown in Figure 1, a gas stream containing S0_χ (102) is introduced into the unit. The gas stream may be a flue gas such as would be produced by the burning of a sulfur-containing fuel such as coal or heavy oils. The S0_χ-containing gas may be an off gas from a chemical process such as a sulfuric acid production process. In any event, the source of the gas is not so important as its content. The gas must contain sulfur oxides. The entering SO_χ-containing gas will normally be at elevated temperatures in the range of 120^β-150°c. Although not shown in either Figures 1 or 2, the entering SO_χ-containing gas may be directed through a heat exchanger where its heat is transferred to the clean flue gas (108) before the clean flue gas is returned to the chimney.

Referring again to Figure 1, the SO_χ-containing gas is introduced into both a sulfuric acid concentrator (104) and a desulfurization reactor (106) . The relative amounts of SO_χ-containing gas sent to the acid concentrator (104) and the desulfurization reactor (106) depends upon the heat content of the SO_χ-containing gas and other variables. These other variables include the amount of sulfur oxides to be removed from the gas, the S0_χ concentration in the gas, and other known chemical engineering parameters. Typically, however, most of the SO_χ-containing gas is directed to the acid concentrator (104) .

The sulfuric acid concentration step, which is performed in the acid concentrator (104) , uses a medium strength sulfuric acid stream (110) that is taken from the hydrogen bromide vaporizer (112) . The medium strength sulfuric acid stream (110) is concentrated using -li¬

the latent heat of the SO_χ-containing gas to produce a strong acid stream (114) . This strong acid stream may be used for any appropriate purpose. Strong sulfuric acid is used in a variety of chemical reactions and is a desirable article of commerce.

The HBr vaporizer (112) receives a medium- strength sulfuric acid stream (116) from the sulfuric acid concentration step and a slipstream (118) taken from the aqueous phase formed in the desulfurization reactor (106) . The combination of medium-strength sulfuric acid stream (116) and slipstream (118) , which contains sulfuric acid, bromine and hydrobromic acid, are heated in the HBr vaporizer (112) . The HBr and any bromine and sulfur oxides are vaporized and removed as stream (120) . The remaining liquid phase is split and sent as the acid to be concentrated in medium-strength sulfuric acid stream (110) or used in the desulfurization reactor as a liquid makeup stream (122) .

The vaporous stream (120) from the HBr vaporizer (112) may contain a small amount of S0_χ formed by the reaction of HBr with sulfuric acid. In any event, the vaporous stream (120) is then treated with bromine (128) in sulfur oxide absorber (124) to remove the sulfur oxides by converting those sulfur oxides to H₂S0₄ which are absorbed by the reflux water of absorber (124) .

Sulfur oxides have been found to have deleterious effects on the catalyst used in the following HBr oxidation step. The sulfur oxide-free HBr stream (126) is then mixed with an oxygen-containing gas (130) prior to being introduced into the HBr oxidation reactor (132) .

The HBr oxidation step, which occurs in the HBr oxidation reactor (132) , is catalytic in nature and oxidizes the hydrobromic acid formed in the S0_χ reactor (106) according to the following reaction: 2 HBr + 1/2 0₂ —> Br₂ + H₂0. The HBr oxidized stream (134) may be mixed with the medium strength, sulfuric acid makeup stream (122) taken from the HBr vaporizer (112) and introduced into the desulfurization reactor (106) . Alternatively, these two streams (134 and 122) may be separately introduced into the desulfurization reactor.

The desulfurization reactor (106) is charged with a portion of the entering SO_χ-containing gas (102) and aqueous bromine which enters in the HBr oxidized stream (134) . As noted earlier, the medium-strength sulfuric acid makeup stream (122) from the HBr vaporizer (112) is also charged to the desulfurization reactor (106) . The S0_χ is removed from the flue gas by reaction with aqueous bromine to form sulfuric acid and hydrobromic acid according to the following reaction:

S0₂ + Br₂ + 2H₂0 —> H₂S0₄ + 2HBr The gas leaving the desulfurization reactor (106) is a generally clean flue gas stream (108) , although it may be scrubbed using a simple water scrubber in a step not shown in Figure l. A portion (118) of the hydrobromic acid that is formed in desulfurization reactor (106) is eventually directed to the HBr oxidation reactor (132) for regeneration to bromine, as discussed above.

Additionally, the desulfurization reactor (106) system includes a recycle stream (136) which contains sulfuric acid, bromine, and hydrobromic acid. The recycle stream (136) circulates through the desulfurization reactor (106) and maintains the amount of liquid in the reactor at a level sufficient to adequately treat the volume of gas flowing through it.

Figure 2 shows another embodiment of this invention which shows in more detail an energy-integrated process for removing S0_χ from flue gas. In this variation, the S0_χ-containing gas stream (102) is split into three portions. The major portion (202) is introduced into a first acid concentrator (204) . A second portion (210) is sent to a preheater (212) where it is heated to about 310° C. This heated SO_χ-containing stream (214) is then introduced into a second acid concentrator (208) . The third portion (216) of the SO_χ- containing feed gas (102) is introduced into a desulfurization reactor (220) .

The largest portion of the entering SO_χ- containing gas (202) is charged to the first acid concentrator (204) along with the exit gas stream (206) from the second acid concentrator (208) . The heat contained in the SO_χ-containing gas stream is used to concentrate a medium-strength sulfuric acid stream (222) which is taken from the HBr vaporizer (232) . The medium strength sulfuric acid entering the first acid concentrator (204) has a strength of 60 to 70% and is concentrated to 80 to 90% sulfuric acid in bottom stream (224) .

The bottom stream (224) from the first acid concentrator (204) is split and a portion (226) is introduced into the second acid concentrator (208) . The 80 to 90% sulfuric acid bottom stream (226) is concentrated to a product acid stream of 93-100% sulfuric acid (228), preferably 96-98.5%, by contact with the heated SO_χ-containing stream (214) in the second acid concentrator (208) . The remaining portion of the bottom stream (230) is introduced into the HBr vaporizer (232) . This acid stream (230) is fairly pure sulfuric acid containing only water as diluent. The first acid concentrator exit gas (218) is combined with a portion of the SO_χ-containing gas (216) and is fed to the desulfurization reactor (220) . The SO_χ reacts with aqueous bromine that is introduced into the desulfurization reactor (220) through regenerated bromine stream (243) to form sulfuric acid and hydrobromic acid according to the following reaction:

S0₂ + Br₂ + 2H₂0 —> H₂S0₄ + 2HBr The bottom stream (234) from the desulfurization reactor (220) is split into two portions: recycle (or recirculating) stream (236) which is mixed with a portion of the regenerated bromine stream (243) and introduced into the top of the desulfurization reactor (220) as a stream for the oxidation of S0₂ to Sθ₃ and the concomitant absorption of that S0₃ as sulfuric acid. Another portion of the desulfurization reactor bottom stream (238) is fed to the HBr vaporizer (232) . The desulfurization reactor (220) utilizes a recycle loop (236) of an aqueous solution containing preferably about 64% water, 20% sulfuric acid, 15% hydrobromic acid, and 1% bromine. For purposes of convenience only, we refer to the-combination of the recycle stream (236) , a portion of the regenerated bromine stream (243) , and a portion of the medium strength sulfuric acid stream from the HBr vaporizer

(239) as lean stream (241) . The desulfurization reactor bottoms stream (234) is referred to as the rich stream. These compositions may, obviously, vary within wide limits, but a sulfuric acid concentration between 7.5% and 30% is desirable to provide an appropriate drive towards equilibrium in the reaction described immediately above. The low acid concentration drives the reaction to the right and helps to remove the sulfur dioxide concentration from the incoming gas. The sulfuric acid concentration in the de-So_χ reactor can be controlled by controlling the amount of stream (239) added to the de- So_χ reactor.

The desulfurization reactor (220) desirably is operated at about one atmosphere since the entering flue gas or other source of SO_χ-containing gas stream is not normally under significant pressure. In addition, the desulfurization reactor operates at a temperature in the range of 50 to 70°C.

The reaction in the desulfurization reactor (220) is quite rapid and removes most of the S0₂ found in any of these streams, provided that the appropriate amounts of reacting chemicals are found in the desulfurization reactor. The desulfurization reactor exit gas (242) may have small amounts of hydrobromic acid and bromine and is therefore sent to a vent gas scrubber (244) , where it is scrubbed of Br₂ and HBr with a small stream of scrubber water (246) . A portion of the liquid bottoms stream from the vent gas scrubber (244) is recycled through stream (248) . The remainder (250) is directed to the HBr vaporizer (232) for recovery of the bromine. The cleaned desulfurized gas (252) from the vent gas scrubber is then passed through a heat-exchanger (256) where it is heated by stream (254) . This heat is added to raise the cleaned desulfurized gas temperature a few degrees above the dew point and to provide buoyancy to the flue gas so as to allow it to leave a stack or other appropriate discharge means.

In order to regenerate bromine from the hydrobromic acid formed by the reaction of S0₂ with bromine, the HBr vaporizer (232) receives a portion of the liquid stream (250) from the vent gas scrubber (244) containing minor amounts of bromine and hydrobromic acid, and the liquid stream (238) from the desulfurization reactor (220) containing approximately 15% hydrobromic acid and 1% bromine. As noted earlier, a portion of first acid concentrator bottom stream (230) is also charged to the HBr vaporizer (232) . These streams are heated in the vaporizer (232) to produce vapor stream (258) containing mainly hydrobromic acid, some water, and a minor amount of sulfur dioxide. The sulfur dioxide must be removed before the hydrobromic acid oxidation step which occurs in the HBr oxidation reactor (262) . The sulfur dioxide would react with bromine in the HBr oxidation reactor to form sulfuric acid. The sulfuric acid is a poison to the catalyst that is used in the hydrobromic acid oxidation step.

The sulfur dioxide is removed by adding makeup bromine (264) to a S0₂ absorber (260). The makeup bromine may be supplied from a portion (267) of the condensed, regenerated bromine stream from the HBr oxidation reactor or from an independent source (265) . The reaction of the bromine and S0₂ will produce sulfur trioxide and eventually sulfuric acid through the absorption/reaction of the sulfur trioxide with water. The sulfuric acid is washed back into the HBr vaporizer (232) by a small reflux generated by partial condensation of the 48% hydrobromic acid stream (266) in a cooling condenser (268) . The condensate (270) may be then returned to the HBr vaporizer (232) . The noncondensed hydrobromic acid stream is superheated to about 300°C in heat exchanger

(272) . The superheated hydrobromic acid stream (274) is then mixed with an oxygen-containing gas (276) prior to being introduced into the HBr oxidation reactor (262) .

Although the HBr vaporizer (232) may be heated in a variety of ways, we have found that one desirable way is to use a hot-oil system in which a slipstream (278) , is passed through a heat exchanger (280) . The other side of the heat exchanger (280) utilizes a hot oil stream (282) which is pumped through a fired furnace (284) and back to the heat exchanger in a circulating loop to transfer heat to slipstream (278) . The hot oil system is used to recover the heat of reaction from the HBr oxidation reactor (262) , and is used to superheat the 48% Hbr vapor stream in heat exchanger (272) . Other convenient heat sources, e.g., steam, may obviously be used in this service.

The mixture of oxygen and 48% hydrobromic acid is fed to the HBr oxidation reactor (262) , where the following reaction takes place:

2 HBr + 1/2 0₂ —> Br₂ + H₂0. The HBr oxidation reactor (262) may be any of a multitude of designs, but we have found that a ultitube reactor with a cooling system or a multibed adiabatic reactor with interstage cooling is desirable because of the ferociously exothermic nature of the reaction. The reaction is preferably carried out at atmospheric pressure. The temperature may range from 100° to 500°C, preferably between 200° and 350 °C, and most preferably between 250° and 350°C. The reaction is desirably carried out with an excess of oxygen to ensure almost complete oxidation of the hydrobromic acid to bromine. The catalyst used in this reactor may be any of those discussed in detail below. The preferred catalysts are promoted copper bromide on a zirconia support or cerium bromide on a zirconia support most desirably containing a major amount of baddeleyite phase zirconia.

The regenerated bromine stream (286) is cooled in a heat exchanger (288) and condensed in gas exchanger (256). A portion of the regenerated bromine stream (243) is then recycled to the desulfurization reactor (220) . Another portion of the regenerated bromine stream (267) may be directed to the S0₂ absorber as a source of makeup bromine (264) . This process is very energy efficient in that very little fuel is required to operate the process. The majority of the energy consumed in the process is created by the hydrobromic acid oxidation reaction or is carried in with the flue gas entering the unit. The only significant energy sources are found in the fuel used to heat the HBr vaporizer and the fuel used to preheat a portion of the SO_χ-containing gas introduced into the second acid concentrator.

Suitable Catalysts for HBr Oxidation

Catalysts suitable for the HBr oxidation step of this process include a wide variety of supported or homogeneous materials. For instance, the active catalyst may be selected from the metals, and the oxides, halides (particularly chlorides and bromides) and oxyhalides of the following metals: Group IB (particularly Cu) , Group IVB (particularly Ti and Zr) , Group VB (particularly V) , Group VIB (particularly Cr, Mo, and W) , Group VIIB (particularly Mn and Re) , Group VIII (particularly Fe, Co, Ni, Pt, and Pd) , and the rare earth lanthanides series (particularly Ce) . The active catalyst may be promoted with one or more Group IIA (particularly Ca) metals or lanthanides, if so desired. The active catalyst and the promoter, if any, may be supported on known catalyst supports such as MgO, A1₂0₃ (particularly in eta or delta form) , Zr0₂, Hf0₂ , Si0₂ (particularly in silica gel form) , clays such a bentonite or attapulgite, and natural materials such as pumice.

The active catalysts listed above should be present in a catalytic amount, that is to say, the minimum amount sufficient to catalyze the reaction of HBr and oxygen to produce Br₂. Active catalytic metals, depending upon the metal selected, may be present in the amount of 0.1% to 35% (by weight) of the overall composition; 1.0% to 20% (by weight) of catalytic metal is more desirable and 3.0% to 10.0% (by weight) of catalytic metal is most desirable.

The promoters/stabilizers may be any salt, oxide, or complex of the noted metals, whether oil or water soluble. The promoter/stabilizer can be impregnated onto the catalyst support or mixed with the support, e.g., as by ball milling with the support precursor. The bromide salts are especially suitable, but other halides (iodide or chloride) , oxyhalides, oxides, phosphates, sulfides, sulfates; complexes such as acetylacetonates and the like are also suitable. The bromides, oxybromides, oxides and mixtures are useful and conveniently available. The promoter/stabilizer metal- bearing material should be present in an amount such that the overall content (in whatever form) is desirably between 0.1% and 20% (by weight) of the overall composition; 1.0% to 6.0% (by weight) is more desirable; 1.0% to 4.0% (by weight) is most desirable.

If the catalyst support is zirconium- containing, it desirably contains more than about 50%

(wt) of zirconia. A minor amount of other metal oxides, e.g., alumina, titania, hafnia, yttria, silica, etc., may be included as a binder or extrusion aid or to increase surface area or pore volume if so desired. Whether the support is zirconium-containing or not, it is desirable to use a support which has significant porosity in the range between 30 and 100 A, e.g., >0.01 cc/gm pore volume in the range of 30 and 100 k.

Preferred Catalysts

The preferred catalysts are selected from promoted copper bromide on a zirconia support or cerium bromide on a zirconia support containing a substantial or major amount of baddeleyite phase zirconia. At the temperatures of operation contemplated in this process, the copper and the cerium bromide do not substantially migrate from the catalyst composition nor among different regions of the catalyst and are very active. This high activity permits the use of comparatively lower temperatures thereby enhancing, even more, the catalyst's stability.

Copper Bromide-Based Catalyst The preferred copper catalyst is produced by placing copper bromide directly onto the support, and is not made by converting another copper-bearing material into copper bromide on the support. The direct addition of the copper bromide to the support appears to be critical to the exceptional stability and activity of the catalyst; nevertheless, we do not wish to be bound to that theory. Additionally, the addition of certain promoters to the supported copper bromide catalyst appears to add substantial stability to the catalyst. Finally, although the support most desirably comprises a zirconium-containing material such as zirconia, other supports are suitable.

Specifically, the preferred copper-based catalyst is a composite comprising or desirably consisting essentially .of copper bromide; promoter/ stabilizer selected from materials containing one or more salts, oxides, or complexes of metals selected from Calcium (Ca) , Yttrium (Y) , Neodymium (Nd) , or Lanthanum (La) or of metals having an ionic radius between about 0.9 and -1.4 A; and an oxidic zirconium-containing catalyst support. The preferred promoters are Nd and La. Most preferred is La.

The copper bromide should be present in at least a catalytic amount, that is to say, an amount sufficient to catalyze the reaction of HBr and oxygen to produce Br₂. We have found that copper bromide in the amount of 0.1% to 20% (by weight) of the overall composition is desirable; 1.0% to 10% (by weight of copper bromide is more desirable and 3.0% to 6.0% (be weight) of copper bromide is most desirable. The introduction of the copper catalyst onto the zirconium-containing catalyst support in the form of copper bromide results in a catalyst composition which is both more stable and more active than compositions in which the catalyst is introduced in another form, such as by the oxide. We have additionally found that the X-ray diffraction spectrum (Cu_α) of the catalyst composition does not show the presence of crystalline CuBr₂. Specifically, the X-ray diffraction spectrum of crystalline CuBr₂ contains the following lines:

?θ (°) Oo

14.485 1.0

29.063 1.0 36.041 0.85

The absence of the most distinctive line (2Θ = 14.485°) demonstrates the substantial absence of copper bromide crystallinity. Catalyst compositions prepared using CuO, which converts to copper bromide in the HBr oxidation process, show the presence of that distinctive line (2Θ ^•**** 14.485°). We believe this to indicate that copper bromide introduced to the zirconium-containing support, in contrast to copper bromide produced on the support from another material, is essentially amorphous.

The source of the promoters/stabilizers is not of significance. Any salt or complex of the noted metals, whether oil- or water-soluble, which can be impregnated onto the zirconium-containing support or mixed with the zirconium support, e.g., as by ball milling with the zirconium support precursor, is suitable. The bromide salts are especially suitable, but other halides (iodide or chloride) , oxyhalides, oxides, phosphates, sulfides, sulfates; complexes such as acetylacetonates, and the like are also suitable. Lanthanu bromide, oxybromide, oxide, and mixtures are useful and conveniently available. The promoter/ stabilizer metal-bearing material should be present in an amount such that the overall content (in whatever form) is desirably between 0.1% and 10% (by weight) of the overall composition; 1.0% to 6.0% (be weight) is more desirable; 1.0% to 4.0% (by weight) is most desirable.

The zirconium-containing support typically should contain more than about 50% (wt) of zirconia. A minor amount of other metal oxides, e.g., alumina,

-titania, hafnia, yttria, silica, etc., may be included as a binder or extrusion aid or to increase surface area or pore volume if so desired. We have found that it is very desirable to use zirconia support which has significant porosity in the range between 30 and 100 A, e.g., >0.01 cc/gm pore volume in the range of 30 to 100 A.

Cerium Bromide-Based Catalysis The cerium-based catalyst is a composite comprising or desirably consisting essentially of cerium bromide and a major amount of a baddeleyite-phase zirconia-containing catalyst support, optionally intermixed with one or more catalyst binders.

The cerium bromide should be present in at least a catalytic amount, that is to say, an amount sufficient to catalyze the reaction of HBr and oxygen to produce Br₂. Cerium content in the amount of 0.01 to 1 mmoles-Ce/g of the overall composition is desirable; 0.2 to 0.6 mmoles-Ce/g of the overall composition is more desirable; and 0.3 to 0.6 mmoles-Ce/g of the overall composition is most desirable.

The introduction of the cerium catalyst onto the catalyst support in the form of cerium bromide results in a catalyst composition which is both more stable and more active than compositions in which the catalyst is introduced in another form, such as by the oxide. When produced in this way, the X-ray diffraction spectrum (Cu_α) of the catalyst composition does not show the presence of crystalline CeBr₃. Specifically, the X- ray diffraction spectrvun of crystalline CeBr₃ contains the following lines:

2θ (°) U∑o 33.153 1.0 62.539 1.0

The absence of these distinctive lines demonstrates the substantial absence of cerium bromide crystallinity.

The zirconia support typically will contain a major amount, e.g., more than about 50% (wt) of zirconia. The crystalline structure of the zirconia is preferably tetragonal, i.e., in the baddeleyite form. A minor amount of other metal oxides, e.g., cubic zirconia, onoclinic zirconia, alumina, titania, hafnia, yttria, silica, thoria, etc., may be included as a binder or extrusion aid or to increase surface area if so desired; but again, the preferred form is the baddeleyite phase if all of the various benefits of the catalyst are to be enjoyed. We have found that it is desirable to use a zirconia support having significant porosity in the range between 30 and 600 A, e.g., >0.01 cc/gm pore volume in the range of 30 to 600 A pore diameter and even more preferred to have >0.01 cc/gm pore volume in the range of 30 and 100 A pore diameter. The catalyst support material may be utilized in any physical form convenient to the process in which it is utilized. Such forms may include tablets, extrudates, raschig or Pall rings, or the like. The reaction is very exothermic, and consequently the relative external surface area may be an important consideration in some reactor/process configurations.

We have also observed that the most preferred form of the catalyst, CeBr₃ on a neat Zr0₂ support, does not exhibit a phase transition in the range of 720-770°C when a thermogravimetric analysis is performed on the catalyst composition. Catalysts prepared using prior art methods often show this phase transition.

General

The catalyst material may be utilized in any physical form convenient to the process in which it is utilized. Such forms may include tablets, extrudates. Pall rings, or the like. The reaction is very exothermic and consequently the relative external surface may be an important consideration in some reactor/process configurations.

The catalyst desirably is prepared by dissolving the appropriate catalyst metal and the promoter/stabilizer metal compounds or complexes independently in aqueous acid, preferably HBr solution and impregnating them into the catalyst supports. The catalyst supports should be dried at 110° to 135°C in air before impregnation so as to allow accurate measurement of the metal content added to the support. The method and sequence of impregnating the support has not been found to be critical. Xf the various compounds are compatible, e.g., they do not react together and do not precipitate from solution, a single solution containing the metals may be used as the impregnating solution. Depending upon the impregnating procedure chosen, the solutions may be saturated or not. If an incipient wetness method is selected, the amount of solution will match the pore volume of the support requiring that the composition of the solution be adjusted to assure that the amount of metal added to the support is appropriate. If other procedures are elected, saturated solutions may be used and a particular amount of the solutions chosen. The impregnated support is dried at a temperature desirably between 100° and 300°C and preferably between 120° and 200°C and is then ready for use.

This invention has been disclosed both by description and by illustration. The illustrations are only examples and should not be used to limit the claimed invention in any way. Additionally, it will be apparent to a reader having ordinary skill in this art that other variations and equivalents will operate in the same way in removing SO_χ from gases containing both S0_χ and oxygen and yet be within the spirit of these claims.

Claims

WE CLAIM AS OUR INVENTION:

1. A process for removing sulfur oxides from a sulfur oxide-containing gas comprising the steps of: contacting a sulfur oxide-containing gas with a lean liquid comprising sulfuric acid and bromine to produce a lean gas containing a diminished amount of sulfur oxides, and a rich liquid containing hydrobromic acid and additional sulfuric acid, heating at least a portion of the rich liquid to remove at least a portion of the hydrobromic acid as an hydrobromic acid stream, adding an oxygen-containing gas to the hydrobromic acid stream, catalytically converting the hydrobromic acid stream to produce a bromine stream.

2. The process of claim 1 where the hydrobromic acid stream contains sulfur dioxide and additionally comprising the step of removing sulfur dioxide from the hydrobromic acid stream before adding an oxygen-containing gas.

3. The process of claim 1 where the bromine stream is at least partially recycled to the contacting step.

4. The process of claim 1 additionally comprising the additional step of scrubbing remaining sulfur oxides, bromine and hydrobromic acid from the sulfur oxide-containing gas.

5. The process of claim 1 where the catalyst in the hydrobromic acid conversion step comprises copper bromide.

6. The process of claim 5 where the catalyst in the hydrobromic acid conversion step comprises copper bromide on a zirconia support.

7. The process of claim 6 in which the catalyst additionally contains a promoter comprising compounds or complexes of calcium, yttrium, neodymium, or lanthanum.

8. The process of claim 7 where the promoter is selected from lanthanum bromide, lanthanum oxybromide, or mixtures of the two.

9. The process of claim 1 where the catalyst in the hydrobromic acid conversion step comprises cerium bromide.

10. The process of claim 9 in which the catalyst in the hydrobromic acid conversion step comprises cerium bromide on a zirconia support.

11. The process of claim 10 in which the zirconia support comprises a substantial amount of baddeleyite phase zirconia.

12. The process of claim 1 where a portion of the rich liquid is recirculated and mixed with the lean liquid.

13. The process of claim 1 in which at least a portion of the sulfur oxide-containing gas is initially contacted with a medium-strength sulfuric acid stream in a first acid concentration step at a temperature sufficient to concentrate the medium-strength sulfuric acid stream, prior to the contact of the sulfur oxide- containing gas with the lean liquid.

14. The process of claim 13 in which a second portion of the sulfur oxide-containing gas is contacted with at least a portion of the acid stream produced in the first acid concentration step to produce a concentrated acid stream.

15. The process of claim 14 where the concentrated acid stream is 93 to 100% sulfuric acid.

16. The process of claim 15 where the concentrated acid stream is 96 to 98.5% sulfuric acid.

17. The process of claim 14 where the sulfur oxide-containing gas is heated prior to the second acid concentration step.

18. A process for removing sulfur oxide from a sulfur oxide-containing gas comprising the steps of: separating the sulfur oxide-containing gas into three portions, contacting one portion of the sulfur oxide- containing gas with a medium-strength sulfuric acid stream in a first acid concentration step at a temperature sufficient to concentrate the medium-strength sulfuric acid stream to produce a partially concentrated sulfuric acid stream, heating a second portion of the sulfur oxide- containing gas and contacting that heated portion in a second acid concentration step with the partially concentrated sulfuric acid produced in the first acid concentration step and thereby producing a concentrated acid stream containing 93 to 100% sulfuric acid. contacting the third portion of the sulfur oxide-containing gas with a lean liquid comprising sulfuric acid and bromine to produce a lean gas containing diminished amount of sulfur oxides and a rich liquid containing hydrobromic acid and additional sulfuric acid, heating at least a portion of the rich liquid to remove at least a portion of the hydrobromic acid as a vaporous hydrobromic acid stream, absorbing any remaining sulfur dioxide from the vaporous hydrobromic acid stream, adding an oxygen-containing gas to the hydrobromic acid stream, catalytically converting the mixture of hydrobromic acid and oxygen-containing gas to bromine using a catalyst selected from cerium bromide or copper bromide on a zirconium-containing catalyst support, recycling the bromine to the lean liquid.

19. The process of claim 18 where the gas leaving the second acid concentration stage is mixed with a portion of the sulfur oxide-containing gas introduced to the first acid concentration stage.

20. The process of claim 19 where the gas from the first acid concentration stage is mixed with a portion of the sulfur oxide-containing gas introduced to the lean liquid.

21. The process of claim 18 including the step of recirculating at least a portion of the rich liquid to the lean liquid.

22. The process of claim 18 in which the hydrobromic acid oxidation catalyst comprises cerium bromide and a baddeleyite phase zirconium-containing catalyst support.

23. The process of claim 18 in which the hydrobromic acid oxidation catalyst is copper bromide on a zirconia support and shows no lines on an X-ray diffraction spectrum at 2Θ=14.485°.

24. The process of claim 23 where the hydrobromic acid oxidation catalyst additionally comprises a promoter selected from compoτinds or complexes of calcium, yttrium, neodymium, or lanthanum.

25. The process of claim 24 where the promoters are selected from lanthanum bromide, oxybromide, or their mixtures.