Method for production of elemental sulfur by part or fully catalytic oxidation of Claus tail gas

Abstract

A process and a process plant for production of elemental sulfur from a feedstock gas including from 15 vol % to 100 vol % H2S and a stream of sulfuric acid, the process including a) providing a Claus reaction furnace feed stream with a substoichiometric amount of oxygen, b) directing s to a reaction furnace operating at elevated temperature, c) cooling, d) directing to contact a material catalytically active in the Claus reaction, e) withdrawing a Claus tail gas and elemental sulfur, f) directing to a means for sulfur oxidation, g) directing to contact a material catalytically active in SO2 oxidation to SO3, h) converting to concentrated sulfuric acid, i) recycling to the Claus reaction furnace, wherein an amount of combustibles, in the Claus tail gas, is oxidized in the presence of a material catalytically active in sulfur oxidation, at an inlet temperature below 400° C.

Claims

1. A process for production of elemental sulfur from a feedstock gas comprising from 15 vol %, to 100 vol % H.sub.2S and a stream of sulfuric acid, the process comprising a. providing a Claus reaction furnace feed stream comprising an amount of said feedstock gas, an amount of sulfuric acid, an amount of oxygen and optionally an amount of fuel, wherein the amount of oxygen is substoichiometric with respect to the Claus reaction, b. directing said Claus reaction furnace feed stream to a reaction furnace operating at elevated temperature, providing a Claus converter feed gas, c. cooling said Claus converter feed gas to provide a cooled Claus converter feed gas and optionally withdrawing elemental sulfur from the gas, d. directing said cooled Claus converter feed gas to contact a material catalytically active in the Claus reaction, e. withdrawing a Claus tail gas and elemental sulfur, optionally by cooling the effluent from said material catalytically active in the Claus reaction, f. directing a stream comprising said Claus tail gas, oxygen and optionally a fuel as a feedstock gas to a means for sulfur oxidation, providing a SO.sub.2 converter feed gas, wherein an amount of combustibles, in the Claus tail gas, is oxidized in the presence of a material catalytically active in sulfur oxidation at an inlet temperature below 400° C., g. directing said SO.sub.2 converter feed gas to contact a material catalytically active in SO.sub.2 oxidation to SO.sub.3, providing an SO.sub.3 rich gas, h. converting said SO.sub.3 rich gas to concentrated sulfuric acid, either by absorption of SO.sub.3 in sulfuric acid or by hydration of SO.sub.3, cooling and condensation of sulfuric acid, i. recycling at least a part of the produced sulfuric acid to the Claus reaction furnace, wherein an amount of the Claus tail gas is directed to an incinerator, providing a combusted Claus tail gas, which is combined with a further amount of Claus tail gas.

2. A process according to claim 1, wherein the concentration of said concentrated sulfuric acid is at least 80 w/w % H.sub.2SO.sub.4.

3. A process according to claim 1, wherein the amount of sulfur in the stream of concentrated sulfuric acid is higher than 1% and less than 25% of the total amount of elemental sulfur withdrawn from the process.

4. A process according to claim 1, in which the combined combusted Claus tail gas and further amount of Claus tail gas in the presence of at least 2 vol % O.sub.2 is directed to a homogeneous reaction zone with a temperature exceeding 400° C. and at least 0.5 second residence time is provided.

5. A process according to claim 4 in which a turbulence enhancer is installed in said homogeneous reaction zone.

6. A process according to claim 4 in which a steam generating heat exchanger is provided at the outlet of said homogeneous reaction zone.

7. A process according to claim 1, in which the amount of the Claus tail gas which is directed to the incinerator is controlled such that the temperature increase from the oxidation of remaining combustibles in the Claus tail gas in the catalytic part of the means of sulfur oxidation is kept below 200° C.

8. A process according to claim 1, in which none of the Claus tail gas is directed to a non-catalytic means of oxidation.

9. A process according to claim 8 in which an amount of fuel and oxidant is directed to an incinerator, wherein the process may be reconfigured during operation to direct all of said Claus tail gas to the incinerator.

10. A process according to claim 9 in which one or both of the catalytic Claus tail gas oxidation reactors are internally cooled reactors.

11. A process according to claim 8 in which the temperature increase in the catalytic Claus tail gas oxidation reactors is kept below 200° C. by dilution of the catalytic Claus tail gas oxidation reactor feed gas with an amount of recycled oxidized Claus tail gas and/or an amount of oxidant.

12. A process according to claim 1, in which oxidant and an optional process gas volume are added to the Claus tail gas in proportions keeping the mixture of Claus tail gas, an optional process gas volume and oxidant below the lower flammability level (LFL) of the mixture.

13. A process according to claim 1, in which oxidant is added to the Claus tail gas in two or more stages in proportions keeping the mixture of Claus tail gas, an optional process gas volume and oxidant below the limiting oxygen concentration (LOC) of the mixture.

14. A process plant comprising a Claus reaction furnace, a Claus waste heat boiler, a Claus conversion section, a means for sulfur oxidation, and a sulfuric acid section, wherein the Claus reaction furnace has a feedstock inlet, a sulfuric acid inlet, and an outlet, the Claus waste heat boiler has a gas inlet, a gas outlet, and an elemental sulfur outlet, the Claus conversion section has a gas inlet, a gas outlet, and an elemental sulfur outlet, the means for sulfur oxidation comprises an incinerator having an inlet and an outlet, a material catalytically active in sulfur oxidation, and has an inlet, and an outlet and the sulfuric acid section has a gas inlet, a gas outlet, and a sulfuric acid outlet, and wherein the feedstock inlet of the reaction furnace is configured for receiving a feedstock gas, and an oxidant, wherein the outlet of the Claus reaction furnace is configured for being in fluid communication with the gas inlet of the Claus waste heat boiler, wherein the gas outlet of the Claus waste heat boiler is configured for being in fluid communication with the gas inlet of the Claus conversion section, wherein the inlets of the incinerator and the material catalytically active in sulfur oxidation are configured for being in fluid communication with the gas outlet of said Claus conversion section gas, and wherein the outlets of the incinerator and the material catalytically active in sulfur oxidation are configured for being in fluid communication with the gas inlet of the sulfuric acid section, wherein the sulfuric acid outlet of the sulfuric acid section is configured to be in fluid communication with the sulfuric acid inlet of said Claus reaction furnace.

Description

FIGURES

(1) FIG. 1 depicts a Claus plant layout with a sulfuric acid tail gas plant with incineration of the Claus tail gas and recirculation of acid to the Claus reaction furnace.

(2) FIG. 2 depicts a Claus plant layout with a partly catalytic sulfuric acid tail gas plant and recirculation of acid to the Claus reaction furnace.

(3) FIG. 3 depicts a Claus plant layout with a fully catalytic sulfuric acid tail gas plant and recirculation of acid to the Claus reaction furnace.

(4) In FIG. 1, a wet type sulfuric acid plant is the Claus tail gas plant, characterized by at least part of the produced sulfuric acid is recycled to the Claus reaction furnace. The Claus plant receives a feed gas comprising H.sub.2S (2), which is combusted with atmospheric or enriched air (4) in the Claus reaction furnace (6). An optional fuel gas (3) can also be directed to the Claus reaction furnace. In the Claus reaction furnace, H.sub.2S is partly oxidized to SO.sub.2 and forms sulfur. Any content of NH.sub.3 and/or hydrocarbons in the feed gas is decomposed to N.sub.2 and H.sub.2O and CO.sub.2 and H.sub.2O respectively. The temperature of the Claus reaction furnace is typically 1,000-1,400° C. with residence times in the range 1-2 seconds. The Claus reaction furnace gas is typically cooled to around 300° C. in a waste heat boiler, located just at the furnace outlet, and the off gas (8) is optionally directed to a sulfur condenser (10) in which elemental sulfur is condensed and withdrawn to the sulfur pit (66) via line 60. The condenser off gas (12) is reheated in a heat exchanger (14) or by means of an in-duct burner and the reheated process gas (16) enters the first Claus reactor (18), which is filled with catalyst comprising activated alumina or titania to react H.sub.2S with SO.sub.2 to form elemental sulfur. The reactor off gas (20) is directed to a further sulfur condenser (22), where elemental sulfur is condensed and withdrawn via line 62 to the sulfur pit (66). The process gas (24) then passes a further catalytic Claus stage via process gas reheater (26), Claus reactor (30) and sulfur condenser (34), connected by lines 28 and 32. Condensed sulfur is withdrawn to the sulfur pit (66) via line 64. The tail gas (36) is heated in a heat exchanger (37), preferably using excess heat from the sulfuric acid plant, typically in the form of high pressure steam. The heated Claus tail gas (39) is directed to an incinerator (42), where it is mixed with hot air (100) from the downstream sulfuric acid plant, support fuel (44) and the pit vent gas (72). The pit vent gas is formed by flushing/purging the sulfur pit with atmospheric air supplied via line 70. The pit vent gas is primarily humid air with small amounts of H.sub.2S and SO.sub.2. The pit vent gas (72) can also be directed to the Claus reaction furnace (6). The temperature and residence time in incinerator 42 is sufficiently high to allow for complete conversion of all sulfur containing species to SO.sub.2, a few percent of the SO.sub.2 is further oxidized to SO.sub.3. The incinerator off gas is cooled in a waste heat boiler, which is usually an integrated part of the incinerator, and is directed via line 74 to a heat exchanger (75) to be cooled further to the desired temperature at the inlet to the SO.sub.2 converter (84). In the SO.sub.2 converter, 1-3 layers of SO.sub.2 oxidation catalyst comprising vanadium oxide are installed, each layer is separated by a heat exchanger to remove heat of reaction. The fully converted SO.sub.2 converter off gas (86) is directed to a sulfuric acid condenser (88), in which sulfuric acid is condensed, concentrated and separated from the process gas, leaving at the bottom of the condenser via line 104 and is cooled and pumped to a sulfuric acid storage tank (106). The clean condenser off gas (90) is directed to the stack (50). The sulfuric acid condenser (88) uses indirect air cooling, where cold cooling air (92) enters in the top and hot air leaves in the bottom (94). At least a part of the hot air may be further heated in heat exchanger (75) and the further heated air 96 is directed to the incinerator via line 100 and some of the further heated air (98) is added to the cooled incinerator off gas (80) to ensure that there is sufficient oxygen available in the SO.sub.2 converter feed gas (82) for the SO.sub.2 oxidation in the SO.sub.2 converter (84). The further heated air (98) can also be added to the process gas (74) upstream the air heater (75).

(5) Should the SO.sub.2 converter (84) or the sulfuric acid condenser (88) somehow be forced to an unplanned shut down, the incinerator off gas (74) can be directed to the stack (not shown), allowing the Claus plant to be kept in operation, which will ensure 94-97% sulfur abatement during the failure period.

(6) The sulfuric acid from the sulfuric acid storage tank (106) passes through a pump and is directed to the Claus reaction furnace (6) via line 108. The sulfuric acid is atomized into the furnace either via hydraulic nozzles or preferably via pneumatic (two-phase) nozzles.

(7) FIG. 2 shows a sulfuric acid plant as the Claus tail gas plant, in which a part of the sulfur compounds in the Claus tail gas (36) are catalytically oxidized.

(8) The Claus plant receives a feed gas comprising H.sub.2S (2), which is combusted with atmospheric or enriched air (4) in the Claus reaction furnace (6). In the Claus reaction furnace, H.sub.2S is partly oxidized to SO.sub.2 and forms sulfur. Any content of NH.sub.3 and/or hydrocarbons in the feed gas is decomposed to N.sub.2 and H.sub.2O and CO.sub.2 and H.sub.2O respectively. The temperature of the Claus reaction furnace is typically 1,000-1,400° C. with residence times in the range 1-2 seconds. The Claus reaction furnace gas is typically cooled to around 300° C. in a waste heat boiler, located just at the furnace outlet, and the off gas (8) is optionally directed to a sulfur condenser (10) in which elemental sulfur is condensed and withdrawn to the sulfur pit (66) via line 60. The condenser off gas (12) is reheated in a heat exchanger (14) or by means of an in-duct burner and the reheated process gas (16) enters the first Claus reactor (18), which is filled with catalyst comprising activated alumina or titania to react H.sub.2S with SO.sub.2 to form elemental sulfur. The reactor off gas (20) is directed to a further sulfur condenser (22), where elemental sulfur is condensed and withdrawn via line 62 to the sulfur pit (66). The process gas (24) then passes a further catalytic Claus stage via process gas reheater (26), Claus reactor (30) and sulfur condenser (34), connected by lines 28 and 32. Condensed sulfur is withdrawn to the sulfur pit (66) via line 64.

(9) The Claus tail gas (36) is heated in heat exchanger (37) preferentially by means of excess energy from the sulfuric acid plant, preferably in the form of high pressure steam. Downstream the tail gas heater (37), the preheated Claus tail gas (39) is split into two parts: one part (41) is directed to the incinerator (42) and one part is via line 43 directed to a position just downstream the incinerator (42). The incinerator (42) receives oxygen from hot air from the sulfuric acid plant (128), support fuel (44) and optionally pit vent gas (72) from the sulfur pit (66) and all sulfur compounds are oxidized to SO.sub.2. The hot process gas from the incinerator (45) is mixed with the bypassed part of the Claus tail gas (43) and optionally an amount of preheated air via line 129 to form a mixed process gas (110), characterized by having a content of Claus tail gas compounds. The mixed process gas is allowed time to react by homogeneous gas phase reactions before the homogeneous reaction zone off gas (110) is cooled in one or two steps via a waste heat boiler (112) and/or a gas/air heat exchanger (116), such that the desired inlet temperature to the catalytic oxidation reactor (120) is achieved. The homogeneous reaction of the mixed process gas may be assigned a specific zone, such as an extension of the incinerator combustion chamber typically with a residence time of 0.5 to 2 seconds, but it may also be a metallic duct. The catalyst in (120) oxidizes all Claus tail gas compounds such as H.sub.2S, COS, CS.sub.2, H.sub.2, S.sub.8 and CO to SO.sub.2, H.sub.2O and CO.sub.2. The catalyst can be a single type or two types as described in EP 2878358. The oxidized process gas (122) is then directed to the SO.sub.2 converter (84), in which the SO.sub.2 is oxidized to SO.sub.3. The air/gas heat exchanger (116) could also be positioned downstream the catalytic reactor (120), depending on the exact composition of the Claus tail gas (36) and the need for air preheating.

(10) The SO.sub.2 converter (84) contains 1-3 catalyst layers for SO.sub.2 oxidation with coolers installed between the layers in order to remove heat of reaction. The converted and cooled process gas (86) is directed to the sulfuric acid condenser (88) in which concentrated sulfuric acid is withdrawn via line 104 and directed to the sulfuric acid storage tank (106) and the clean process gas (90) is directed to the stack (50). Cooling air for the indirectly cooled condenser (88) is supplied via line 92 and hot cooling air is withdrawn via line 94. Optionally, at least a fraction of the hot cooling air (94) is further heated in gas/air heat exchanger 116 and the further heated cooling air (126) is directed to the incinerator via line 128 and some of the air may be directed via line 129 to a position just downstream the incinerator (42) or via line 130 to a position between the catalytic oxidation reactor (120) and the SO.sub.2 converter feed gas (124), supplying sufficient oxygen for the SO.sub.2 oxidation in the SO.sub.2 converter (84). The further heated air (130) can also be added upstream the catalytic oxidation reactor (120).

(11) The sulfuric acid from the sulfuric acid storage tank (106) passes through a pump and is directed to the Claus reaction furnace (6) via line 108. The sulfuric acid is atomized into the furnace via hydraulic nozzles or preferably via pneumatic (two-phase) nozzles.

(12) In case of a forced shut down of the SO.sub.2 converter (84) and sulfuric acid condenser (88), it will for a limited time be possible to direct the oxidized Claus tail gas (122) or alternatively the partly oxidized tail gas (110, not shown), directly to the stack (50), allowing the Claus plant to be kept in operation.

(13) FIG. 3 shows a sulfuric acid Claus tail gas plant, in which the Claus tail gas is oxidized by catalytic means only.

(14) The Claus plant receives a feed gas comprising H.sub.2S (2), which is combusted with atmospheric or enriched air (4) in the Claus reaction furnace (6). In the Claus reaction furnace, H.sub.2S is partly oxidized to SO.sub.2 and forms sulfur. Any content of NH.sub.3 and/or hydrocarbons in the feed gas is decomposed to N.sub.2 and H.sub.2O and CO.sub.2 and H.sub.2O respectively. The temperature of the Claus reaction furnace is typically 1,000-1,400° C. with residence times in the range 1-2 seconds. The Claus reaction furnace gas is typically cooled to around 300° C. in a waste heat boiler, located just at the furnace outlet, and the off gas (8) is optionally directed to a sulfur condenser (10) in which elemental sulfur is condensed and withdrawn to the sulfur pit (66) via line 60. The condenser off gas (12) is reheated in a heat exchanger (14) or by means of an in-duct burner and the reheated process gas (16) enters the first Claus reactor (18), which is filled with catalyst comprising activated alumina or titania to react H.sub.2S with SO.sub.2 to form elemental sulfur. The reactor off gas (20) is directed to a further sulfur condenser (22), where elemental sulfur is condensed and withdrawn via line 62 to the sulfur pit (66). The process gas (24) then passes a further catalytic Claus stage via process gas reheater (26), Claus reactor (30) and sulfur condenser (34), connected by lines 28 and 32. Condensed sulfur is withdrawn to the sulfur pit (66) via line 64.

(15) The tail gas from the Claus plant (36) is heated in a heat exchanger (37), using excess heat from the sulfuric acid plant. The heated tail gas (39) is mixed with pit vent gas (72) and hot air (166) and optionally an amount of recycled process gas (162). This mixed process gas (134) is directed to a first catalytic reactor (136), in which some of the compounds in the mixed process gas are oxidized and some are not. The partly converted process gas (138) is then optionally cooled in a heat exchanger, such as a boiler, (not shown), mixed with an amount of recycled process gas (160) and/or optionally an amount of hot air (168) to produce a partly converted process gas (140) to the second catalytic reactor (142), in which all combustible compounds (such as H.sub.2, CO, COS) are completely oxidized to CO.sub.2, H.sub.2O and SO.sub.2. The reactor off gas (144) is then split into a recycle fraction (154) and a fraction (146), which is directed to a process gas cooler (148) and further to the SO.sub.2 converter (84). The recycled process gas (154) is optionally cooled in heat exchanger 156 and the cooled recycle gas (158) is directed to a position upstream the second catalytic reactor (142) via line 160 and optionally via line 162 to a position upstream the first catalytic reactor (136). A process gas recycle blower will overcome the pressure differences of the process gas, recycle stream and control dampers (not shown). The purpose of the recycling of oxidized process gas is temperature moderation in the catalytic converters 136 and 142.

(16) Converted process gas from any stage in the SO.sub.2 converter (84) could also be used as a recycle gas.

(17) The cooled converted process gas (150) from the process gas cooler (148) is optionally mixed with hot air (174) such that the process gas to the SO.sub.2 converter (152) has an appropriate temperature and contains sufficient oxygen for the SO.sub.2 oxidation reaction. The process gas (152) is then directed to the SO.sub.2 converter (84), in which the SO.sub.2 is oxidized to SO.sub.3. The converter contains 1-3 catalyst layers with coolers installed between the layers in order to remove heat of reaction. The converted and cooled process gas (86) is directed to the sulfuric acid condenser (88) in which concentrated sulfuric acid is withdrawn via line 104 and directed to the sulfuric acid storage tank (106) and the clean process gas (90) is directed to the stack (50). Cooling air for the indirectly cooled condenser (88) is supplied via line 92 and hot cooling air is withdrawn via line 94. Parts of the hot cooling air (94) can be supplied to one or more position in the sulfuric acid plant: upstream the first catalytic reactor 136 via line 166, between outlet of first catalytic reactor (136) and inlet of second catalytic reactor (142) via line 168 and/or upstream the SO.sub.2 converter (84) via line 174. The entire hot air stream (94) or any of the individual streams (164, 168, 166, 170) can be further heated in a heat exchanger, as shown for the hot air to the SO.sub.2 converter (170), which is further heated in heat exchanger 172, before mixed with the process gas 150, preferably by heat exchange with superheated steam or hot process gas.

(18) The sulfuric acid from the sulfuric acid storage tank (106) passes through a pump and is directed to the Claus reaction furnace (6) via line 108. The sulfuric acid is atomized into the furnace either via hydraulic nozzles or preferably via pneumatic (two-phase) nozzles.

(19) In case of a forced shut down of the sulfuric acid plant, the heated Claus tail gas (39) can be diverted to a thermal incinerator (42) via line 131. Fuel (44) and combustion air (46) is supplied to ensure heating value and oxygen for complete oxidation of combustible species in the Claus tail gas. The incinerator off gas is optionally cooled in a waste heat boiler and directed to the stack (50) via line 48.

(20) As the Claus tail gas temperature is insufficient for initiating the catalytic H.sub.2S oxidation, a start-up heater (not shown) is required to start up the sulfuric acid plant and it is preferably positioned just upstream the second catalytic reactor (142). The heater can either be electrical, fuel gas fired or receive a heating media from another process plant.

EXAMPLE 1: CLAUS TAIL GAS OXIDATION WITH COMBINATION OF THERMAL INCINERATOR AND CATALYTIC OXIDATION

(21) To investigate the effect of the combination of thermal incineration, homogeneous and catalytic oxidation of a Claus tail gas, an example is given for a normal operation of the combined Claus tail gas oxidation layout and an operation in which the H.sub.2S concentration increases due to an upset in the operation of the upstream Claus plant. The plant layout is similar to that shown in FIG. 2.

(22) The Claus tail gas has the following composition during normal operation:

(23) TABLE-US-00001 1.0 vol % CO 0.11 vol % CS.sub.2 0.01 vol % COS 1.2 vol % H.sub.2 0.64 vol % H.sub.2S 0.01 vol % S.sub.x Balance N.sub.2, CO.sub.2, SO.sub.2 and H.sub.2O

(24) The Claus tail gas is preheated to 240° C. and the combustion air to the incinerator is preheated to 400° C. The concentration of combustibles in the Claus tail gas is ˜3 vol %.

(25) Around 30% of preheated Claus tail gas is directed to the incinerator, which is controlled to ˜1,000° C. by support fuel addition, the O.sub.2 concentration in the incinerator off gas is 3 vol %.

(26) The remaining 70% of the preheated Claus tail gas is mixed with the 1,000° C. incinerator off gas and an amount of preheated air, producing a mixed gas with a temperature of 550° C. ignoring homogeneous reactions. The dilution effect reduces the concentration of combustibles by a factor 2.3, bringing the mixture well below the Lower Flammability Level. The mixed gas temperature and O.sub.2 concentration are sufficient for homogeneous oxidation of H.sub.2S and CS.sub.2 (to COS) to take place, increasing the gas temperature to 600° C. at the outlet of the homogeneous reaction zone. The hot process gas is then cooled in a waste heat boiler to a temperature around 400° C., before it enters the catalytic oxidation reactor for complete oxidation of all combustibles species (primarily CO, H.sub.2 and COS) to CO.sub.2, H.sub.2O and SO.sub.2. The catalyst preferably comprises noble metals, such as Pd and Pt. The reactor outlet temperature becomes 480° C., and thus is increased by 80° C., of which H.sub.2 and CO contributes by ˜35° C. each and S.sub.X and COS are responsible for the last 10° C. The fully converted process gas is then cooled to 420° C. in a gas/air heat exchanger before the gas is directed to the SO.sub.2 converter for catalytic oxidation of SO.sub.2 to SO.sub.3.

(27) In principle the catalytic oxidation reactor could have operated at 340° C. inlet temperature and have delivered the process gas to the SO.sub.2 converter directly at a temperature of 420° C., but in this example the process gas/air heat exchanger is used to increase the combustion air to 400° C., decreasing the need for support fuel and decreasing the size of the incinerator.

(28) In the situation in which the homogeneous oxidation of H.sub.2S and CS.sub.2 would not take place (e.g. due to low O.sub.2 concentration, reaction time and/or temperature), the 50° C. temperature increase would be released in the catalytic oxidation reactor instead. By lowering the inlet temperature from 400° C. to 350° C., the outlet temperature of 480° C. would be unchanged and the layout would work well.

(29) However, such a layout would be vulnerable to changes in the composition of the Claus feed gas. In a situation where the H.sub.2S concentration suddenly increases from the normal 0.64 vol % to 1.64 vol %, the additional temperature increase in the mixed gas will be ˜75° C. Provided the right conditions for homogeneous reactions are fulfilled, the temperature of the mixed gas increases from 600° C. to 675° C. and the waste heat boiler will be well suited to absorb most of this extra heat, possibly only increasing the waste heat boiler outlet temperature by 20° C. to 420° C. and the catalytic oxidation reactor outlet temperature would become 500° C., just at the temperature limit of the catalyst. However, if the homogeneous reactions were not taking place and the oxidation would take place in the catalytic oxidation reactor, the temperature increase would be ˜155° C. compared to the normal 80° C. With an inlet temperature of 400° C., the outlet temperature would become 555° C. thus exceeding the maximum operating temperature of the catalyst. Furthermore, the temperature at the inlet to the SO.sub.2 converter would also increase, with the consequence of lower conversion of SO.sub.2 to SO.sub.3 and increased emissions to the atmosphere.

EXAMPLE 2: PLANT SIZE AND ENERGY CONSUMPTION FOR THREE LAYOUTS OF CLAUS TAIL GAS SULFURIC ACID PLANTS

(30) To investigate the effect of the catalytic Claus tail gas oxidation, either partial or fully, process layouts have been calculated for a thermal Claus tail gas oxidation as shown in FIG. 1, a catalytic/thermal layout as shown in FIG. 2 and a fully catalytic layout as shown in FIG. 3.

(31) The Claus tail gas composition is:

(32) TABLE-US-00002 1.7 vol % CO 0.12 vol % CS.sub.2 0.02 vol % COS 1.1 vol % H.sub.2 0.57 vol % H.sub.2S 0.01 vol % S.sub.x Balance N.sub.2, CO.sub.2, SO.sub.2 and H.sub.2O

(33) The fuel gas is primarily CH.sub.4 with a heating value of 11,800 kcal/kg.

(34) The combustion air is atmospheric air, preheated to 400° C. when a thermal incinerator is used and 240° C. in the fully catalytic layout.

(35) The Claus tail gas is preheated to 240° C. in the case when a thermal incinerator is used and 195° C. in the case of the fully catalytic layout.

(36) All energy for preheating is taken from the energy released in the Claus tail gas sulfuric acid plant by oxidation of combustibles in the Claus tail gas, SO.sub.2 to SO.sub.3 oxidation and formation of sulfuric acid.

(37) Table 1 shows the comparable relative numbers for fuel gas consumption, amount of Claus tail gas being admitted in a thermal incinerator and the total amount of combustion air (for fuel, Claus tail gas combustibles and SO.sub.2 to SO.sub.3 oxidation). The sulfuric acid section size is the relative process gas flow to the catalytic SO.sub.2 converter—the three layouts are more or less similar from the inlet to the SO.sub.2 converter to the stack and the size of the SO.sub.2 converter, heat exchangers and sulfuric acid condenser are proportional to the process gas flow. Much of the surplus energy is converted into steam and exported. In this example, 40 barg superheated steam is produced.

(38) In column “thermal”, the reference data for a Claus tail gas oxidation plant is shown, in which the entire amount of Claus tail gas is thermally incinerated at 1000° C., efficiently oxidizing all combustible species to CO.sub.2, H.sub.2O and SO.sub.2. The fuel gas consumption is high and requires a lot of combustion air, producing a large volume of process gas to be further treated in the downstream sulfuric acid section. Nearly all the energy supplied via the support fuel is converted into high pressure steam, thus the export flow of high pressure steam is high.

(39) In column “thermal+catalytic” it is seen that by combining a small thermal incinerator with a catalytic tail gas oxidation solution, the fuel gas consumption has been decreased to 31% of the base case, where the entire Claus tail gas is admitted into the incinerator. This leads to a significantly lower O.sub.2 demand, resulting in a 59% combustion air flow requirement compared to the base case. The benefit of a lower air flow is both a lower air blower operating cost and a smaller process gas flow to the sulfuric acid section, enabling a smaller and thus cheaper section. A process gas flow (sulfuric acid section size) of 78% compared to the base case flow is estimated to reduce the cost of the sulfuric acid section by 15-20%.

(40) The consequence of supplying less fuel gas to the tail gas incinerator is also a decrease in steam export, since up to 90-100% of the energy in the support fuel is converted into high pressure steam in the sulfuric acid plant.

(41) In column “Catalytic” it is seen that by optimized design of the sulfuric acid tail gas plant, using Claus tail gas preheating and two different reactors for catalytic oxidation of the combustibles in the Claus tail gas, it is possible to operate the purely catalytic plant without need for external energy supply.

(42) Since there is no support fuel consumption, the need for O.sub.2 for the conversion of the combustibles into energy, CO.sub.2, H.sub.2O and SO.sub.2 is considerably decreased and is now only 40% of the O.sub.2 input compared to the thermal incineration layout. The 40% corresponds to the lowest possible O.sub.2 addition for proper oxidation of the species in the Claus tail gas to CO.sub.2, H.sub.2O and SO.sub.3, including a margin to compensate for small fluctuations in Claus tail gas compositions and to ensure that the catalysts will not experience too low O.sub.2 concentration to function efficiently.

(43) As the catalytic layout uses the minimum amount of O.sub.2, the resulting process gas flow to the SO.sub.2 converter and sulfuric acid condenser is the lowest possible. As seen in row 5 (sulfuric acid section size), the catalytic layout produces only 68% of the process gas to the sulfuric acid section, thus significantly reducing the size of the equipment in the sulfuric acid section and thus saving capital cost.

(44) In row 6 (steam export) it is seen that there is a small energy surplus in the purely catalytic sulfuric acid plant and that is exported as steam. As there is no support fuel addition, the steam export is only 20% of the steam export from the thermal incineration layout.

(45) Not included in table 1 is the effect of the backup incinerator for the purely catalytic layout, as such an incinerator may be desired in case the purely catalytic sulfuric acid plant experience a forced shut down. If the Claus tail gas cannot be oxidized before emitted to the atmosphere, it may become necessary to shut down the Claus plant and upstream plants.

(46) The size of the backup incinerator will depend on several factors, but typically they can operate at much reduced load during hot standby periods. Assuming that the incinerator must operate at 25% of the design load, the fuel gas consumption may increase from the relative value of 0 to a value of 30. That would bring the fuel gas consumption up to the same level as for the combined thermal and catalytic layout. The sulfuric acid section size will still be lower for the purely catalytic solution, thus decreasing the cost of the acid section. The cost reduction will be decreased by installing a back-up incinerator and only a more detailed cost analysis will determine whether the thermal+catalytic or the purely catalytic sulfuric acid plant layout will be the most cost effective.

(47) TABLE-US-00003 TABLE 1 Thermal + Thermal catalytic Catalytic Corresponding figure 1 2 3 Fuel gas consumption 100 31 0 Tail gas to thermal incinerator 100 31 0 Total combustion air 100 59 40 Sulfuric acid section size 100 78 68 40 barg steam export (400° C.) 100 44 20