Revamping of a claus plant with a sulfuric acid plan

Abstract

A revamp process for modifying a sulfur abatement plant including a Claus process plant, the Claus process plant including a Claus reaction furnace and one or more Claus conversion stages, each Claus conversion stage including a conversion reactor and a means for elemental sulfur condensation, and a means of Claus tail gas oxidation configured for receiving a Claus tail gas from said Claus process plant and configured for providing an oxidized Claus tail gas, the process revamp including: a) providing a sulfuric acid producing tail gas treatment plant producing sulfuric acid, and b) providing a means for transferring an amount or all of the sulfuric acid produced in said sulfuric acid producing tail gas treatment plant to said Claus reaction furnace, wherein the moles of sulfur in the transferred sulfuric acid relative to the moles of elemental sulfur withdrawn from the Claus process plant is from 3% to 25%.

Claims

1. A revamp process for modifying a sulfur abatement plant comprising a Claus process plant producing elemental sulfur, said Claus process plant comprising a Claus reaction furnace and 1 or more Claus conversion stages, each Claus conversion stage comprising a conversion reactor and a means for elemental sulfur condensation, and a means of Claus tail gas oxidation configured for receiving a Claus tail gas from said Claus process plant and configured for providing an oxidized Claus tail gas, said process revamp comprising the steps of a. providing a sulfuric acid producing tail gas treatment plant producing sulfuric acid, and b. providing a means for transferring an amount or all of the sulfuric acid produced in said sulfuric acid producing tail gas treatment plant to said Claus reaction furnace, wherein the moles of sulfur in the transferred sulfuric acid relative to the moles of elemental sulfur withdrawn from said Claus process plant is from 3% to 25%.

2. The revamp process of claim 1, further comprising the step of limiting the number of Claus conversion stages to two if the Claus process plant comprises more than two Claus conversion stages.

3. A revamp process according to claim 1, wherein step (a) involves providing a sulfuric acid producing tail gas treatment plant comprising a catalytic SO.sub.2 converter, comprising a material catalytically active in oxidation of SO.sub.2 to SO.sub.3, configured for receiving said oxidized Claus tail gas and configured for providing a SO.sub.2 converter off gas and a sulfuric acid unit, being configured for receiving said SO.sub.2 converter off gas, producing concentrated sulfuric acid either by condensation of hydrated SO.sub.3 or by absorption of SO.sub.3 in sulfuric acid.

4. A revamp process according to claim 1, further comprising the step of providing a means for temporary storing sulfuric acid produced in said sulfuric acid producing tail gas treatment plant, having a volume corresponding to the amount of sulfuric acid produced in said sulfuric acid producing tail gas treatment plant in from 4 hours, 8 hours or 24 hours to 50 hours or 120 hours, wherein said means for storing sulfuric acid is configured for receiving sulfuric acid from said sulfuric acid producing tail gas treatment plant and for directing sulfuric acid to said Claus reaction furnace.

5. A revamp process according to claim 1, comprising the step of providing a means for flow control, user-configurable for allowing the outlet from said means of Claus tail gas oxidation to by-pass said sulfuric acid producing tail gas treatment plant.

6. A revamp process according to claim 1, comprising the step of providing a means for catalytic oxidation of at least an amount of said Claus tail gas.

7. A revamp process according to claim 6, wherein said means for catalytic oxidation is configured for receiving at least an amount of the Claus tail gas and in combination with one or more of an oxidant, a heated gas, a recycled process gas and a heat exchanger, such that the temperature at the inlet of the means of catalytic oxidation is above 200° C. and the temperature at the outlet of the means of catalytic oxidation is below 500° C.

8. A revamp process according to claim 1, wherein said Claus reaction furnace is provided with one or more inlet nozzles configured for atomizing said amount of sulfuric acid to a droplet size distribution, in which 90% of the mass of droplets have a diameter smaller than 500 μm.

9. A revamp process according to claim 8, wherein said inlet nozzles are pressure nozzles or two phase nozzles.

10. A revamp process according to claim 8, wherein said inlet nozzles are positioned in a distance downstream the gas inlet of the Claus reaction furnace, corresponding to at least 1 second residence time of the process gas.

11. A revamp process according to claim 1, wherein said Claus reaction furnace is extended by a volume providing an increase of residence time of the process gas of at least 1 second.

12. A revamp process according to claim 1, wherein the Claus reaction furnace is provided with a means for impaction.

13. A revamp process according to claim 1, wherein the Claus reaction furnace is provided with a means for turbulence enhancement.

14. A revamp process according to claim 1, wherein the control scheme for the amount of oxygen directed to the Claus reaction furnace is modified to be dependent on a combination of feed forward control based on the amount and composition of feedstock and sulfuric acid and feedback control based on the ratio of H.sub.2S to SO.sub.2 in the inlet stream to the final Claus conversion stage or the outlet stream from the final Claus conversion stage.

15. A revamp process according to claim 1, wherein the outlet from the Claus reaction furnace is directed to an SO.sub.3 guard, absorbing or converting SO.sub.3, providing a substantially SO.sub.3 free Claus reaction furnace outlet gas.

16. A revamp process according to claim 1, wherein the concentrated sulfuric acid contains from 90% w/w to 98.5% w/w H.sub.2SO.sub.4.

Description

(1) In the event of an acid injection trips, the atomization media will continue flowing, such that the nozzle is cooled and process gas ingress to the nozzle is avoided. Figures:

(2) FIG. 1 depicts a well-known Claus layout with tail gas treatment plant (TGTP)

(3) FIG. 2 depicts a Claus layout with a sulfuric acid TGTP and recirculation of acid to the Claus reaction furnace.

(4) FIG. 3 depict a Claus layout with a partly catalytic sulfuric acid TGTP and recirculation of acid to the Claus reaction furnace.

(5) FIG. 4 depicts a Claus layout with a purely catalytic sulfuric acid TGTP and recirculation of acid to the Claus reaction furnace.

(6) FIG. 5 depicts the control system for combustion air addition to the Claus burner.

(7) In FIG. 1, showing a well-known Claus layout with tail gas treatment plant (TGTP), 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). Depending in the requirement for sulfur recovery, the so-called pit vent gas (72), comprising H.sub.2S, can also be directed to the Claus reaction furnace (6). In cases with low sulfur recovery requirements, the pit vent gas can be sent to the tail gas incinerator (42). 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

(8) After the second catalytic stage (30) and condenser (34), the sulfur recovery is about 94-97% and depending on the local requirements for sulfur recovery (emissions), additional means for sulfur recovery is installed. Such means are typically called Claus tail gas treatment plants (TGTP) (38) and numerous types of TGTPs exist. They can be a further Claus reactor followed by a sulfur condenser, which may be a so-called sub dew point reactor, a catalyst system for selective oxidation of H.sub.2S to elemental sulfur or amine based H.sub.2S scrubbing units, which capture the H.sub.2S in the tail gas (36) and returns it to the Claus reaction furnace (6). Such plants allow a high sulfur recovery, but with costs up to the cost of the Claus plant itself, depending of the complexity and efficiency of the TGTP.

(9) Downstream the TGTP, the off gas 40 is directed to an incinerator (42), which thermally oxidizes all sulfur compounds to SO.sub.2 and after cooling in a waste heat boiler (typically a part of the incinerator) the incinerator off gas (48) is sent to the stack (50). The tail gas flow from the Claus plant has low caloric value and no oxygen, so fuel gas (44) and combustion air (46) is required to operate the incinerator.

(10) In case the TGTP unit fails, the tail gas can be bypassed the TGTP via line 52 and be directed to the incinerator (42), such that the Claus plant can be kept in operation.

(11) All liquid sulfur 60, 62, 64 from the condensers (10,22,34) is collected in the sulfur pit (66). The pit is vented/flushed with atmospheric air (70) to drive off H.sub.2S dissolved in the sulfur. The sulfur pit vent gas (72) contains a little H.sub.2S and can either be directed to the Claus reaction furnace (6) or the incinerator (42). A combined flow of liquid sulfur is withdrawn in line 68.

(12) In FIG. 2, a wet type sulfuric acid plant is the TGTP, 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). 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, fuel (44) and the pit vent gas (72). The pit vent gas (72) can also be directed to the Claus reaction furnace (6). The temperature and residence time in the incinerator 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 74,76 and 78 to a heat exchanger (80) 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 (80) and the further heated air is directed to the incinerator via line 100 and some of the further heated air (98) is added to the incinerator off gas (76) to ensure that there is sufficient oxygen available 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 (82) downstream the air heater (80), with the associated benefit of reducing the size of the heat exchanger.

(13) Should the SO.sub.2 converter (84) or the sulfuric acid condenser (88) somehow fail, the incinerator off gas (74) can be directed to the stack via line 102, allowing the Claus plant to be kept in operation, which will ensure 94-97% sulfur abatement during the failure period.

(14) 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-fluid) nozzles.

(15) FIG. 3 show a sulfuric acid plant as the TGTP, in which a part of the sulfur compounds in the Claus tail gas (36) are catalytically oxidized.

(16) 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.

(17) 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 Claus tail gas 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), 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) to form a mixed process gas (110), which will comprise an amount of combustible Claus tail gas compounds such as H.sub.2S, COS, CS.sub.2, H.sub.2, S.sub.8 and CO. The mixed process gas 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, which typically is 300° C. to 400° C. to the catalytic oxidation reactor (120) is achieved. The catalyst in (120) is of a type which 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 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).

(18) 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 released 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 130 to a position between the catalytic oxidation reactor (120) and the SO.sub.2 converter inlet (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).

(19) 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-fluid) nozzles.

(20) In case of failure 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) directly to the stack (50), allowing the Claus plant to be kept in operation.

(21) FIG. 4 show a sulfuric acid TGTP, in which the Claus tail gas is 100% treated by catalytic means.

(22) 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.

(23) 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 a partial exothermal oxidation especially of H.sub.2S and CS.sub.2 is carried out, but some of the compounds in the mixed process gas are not oxidized. The partly converted process gas (138) is then optionally cooled in a heat exchanger (not shown), mixed with 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 (notably H.sub.2, CO and 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. Converted process gas from any stage in the SO.sub.2 converter (84) could also be used as a recycle gas, in order to optimize the temperature of the catalytic converters.

(24) 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 positions 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 streams (164, 168, 164, 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, possibly by heat exchange with superheated steam or hot process gas 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-fluid) nozzles.

(25) In case of failure 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 cooled in an integrated waste heat boiler and directed to the stack (50) via line 48.

(26) As the process gas temperature is insufficient for starting 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.

(27) FIG. 5 shows the Claus reaction furnace configuration and a related control scheme. The Claus reaction furnace (6) receives the following feed streams: one or more feed gas(es) comprising H.sub.2S (2), a fuel gas (3), sulfuric acid (108) and an optional atomizing media (109), which is typically compressed air but can also be e.g. N.sub.2 and steam. Each line is equipped with a flow measurement device, FI, which gives information about the flow to a calculation unit (1). The output from the calculation unit is the air demand, and this value is sent to the flow controller, FIC, on the combustion air line (4). The main part of the combustion air flow is controlled with the air demand calculated in the calculation unit (1) and a minor part is adjusted by a measurement of the H.sub.2S/SO.sub.2 ratio in the Claus tail gas (e.g. stream 36 in FIG. 1).

Example 1: Calculation of Combustion Air

Example 1.1: Calculation of Combustion Air Demand for a Fuel Gas

(28) The flow of combustion air to the Claus reaction furnace is typically controlled by measuring the other feed gas flows and by use of their composition, the air demand per unit feed gas flow can be calculated and the total air demand is calculated by adding air demands for each of the feed streams:
G.sub.air=ΣK.sub.i.Math.G.sub.i

(29) K.sub.i is the air demand factor with the unit flow of air per flow of feed stream i and G.sub.i is the flow of feed stream i. The units can be chosen freely, depending on the nature of the feed stream, however care should be given to ensure that the units for each air demand number (K.sub.i.Math.G.sub.i) are equal. For gas streams, typical flow units are Nm.sup.3/h, kg/h and kmol/h and for liquid streams, m.sup.3/h and kg/h are most common.

(30) The air demand factor, K.sub.i, is calculated from the composition of the feed stream. For a feed stream containing 100 vol % CH.sub.4, the air demand can be calculated from the following chemical reaction:
CH.sub.4+2O.sub.2.fwdarw.CO.sub.2+2H.sub.2O

(31) Each CH.sub.4 molecule requires 2 oxygen molecules for complete conversion to CO.sub.2 and thus the O.sub.2 demand factor, K, will be 2 Nm.sup.3O.sub.2 per Nm.sup.3 CH.sub.4. 1 Nm.sup.3 is defined as 1 m.sup.3 gas at 0° C. and 1 atm and corresponds to 44.6 moles.

(32) K can also be calculated as Nm.sup.3 air per Nm.sup.3 CH.sub.4 by dividing with the mole fraction of O.sub.2 in air. For a dry air, the mole fraction of O.sub.2 in air is 0.21 and thus K becomes 2/0.21=9.52 Nm.sup.3 air per Nm.sup.3 CH.sub.4

(33) If the concentration of CH.sub.4 in the fuel gas was only 80%, the O.sub.2 demand would also only be 80% and thus the K value would become 9.52.Math.0.80=7.62 Nm.sup.3 air per Nm.sup.3 fuel gas.

(34) If the fuel gas contains more than one oxygen demanding compounds (e.g. also C.sub.2Hs is present), the air demand for each compound are just added to a total oxygen demand.

Example 1.2: Calculation of Combustion Air Demand for a H.SUB.2.S Gas

(35) The O.sub.2 demand for converting H.sub.2S in a feed gas is calculated by taking into account that not full H.sub.2S oxidation is desired, but rather a ratio of H.sub.2S to SO.sub.2 in the range 2-4 is desired.

(36) The H.sub.2S to SO.sub.2 ratio, R, is defined as:

(37) $\begin{array}{cc}R=\frac{n_{H2Sin\mathrm{the}\mathrm{Claus}\mathrm{tail}\mathrm{gas}}}{n_{\mathrm{SO}2in\mathrm{the}\mathrm{Claus}\mathrm{tail}\mathrm{gas}}}=\frac{n_{H2S}.Math.\left(1-\alpha \right)}{n_{H2S}.Math.\alpha }& \end{array}$

(38) In the equation, n is the number of moles and a is the fraction of conversion of H.sub.2S into SO.sub.2. On the right hand side of the equation, nH.sub.2S is the number of moles of H.sub.2S in the feed gas.

(39) The chemical reaction of H.sub.2S oxidation is
H.sub.2S+3/2O.sub.2.fwdarw.SO.sub.2+H.sub.2O

(40) Using the stoichiometry and rearranging the equation for R, the O.sub.2 demand becomes

(41) $n_{O2}=\frac{3.Math.n_{H2S}}{2.Math.\left(R+1\right)}$

(42) Where n.sub.H2S is the number of moles H.sub.2S in the feed gas and nO.sub.2 is the required number of moles O.sub.2 needed to obtain the given H.sub.2S to SO.sub.2 ratio, R, in the converted gas. Taking into account the H.sub.2S concentration in the feed gas, yH.sub.2S (mole fraction) and O.sub.2 in the combustion air, YO.sub.2 (mole fraction) the air demand, G.sub.air, becomes:

(43) $G_{air}=K_{ag}.Math.G_{ag}=\frac{3.Math.y_{H2S}^{ag}}{2.Math.\left(R+1\right).Math.y_{O2}^{\mathrm{air}}}.Math.G_{ag}$

(44) K.sub.ag is the acid gas air demand factor (Nm.sup.3 air/Nm.sup.3 acid gas) and G.sub.ag is the acid gas flow (Nm.sup.3/h).

Example 1.3 Calculation of Combustion Air Demand for Sulfuric Acid

(45) The sulfuric acid decomposes into SO.sub.2, H.sub.2O and O.sub.2 according to the overall reaction:
H.sub.2SO.sub.4.fwdarw.SO.sub.2+H.sub.2O+½O.sub.2

(46) The sulfuric acid both directly supplies O.sub.2 to the reaction system, but also indirectly by supplying SO.sub.2, which does not need to be formed by oxidation of H.sub.2S (see example 2). Using the same principles as described in example 2, the O.sub.2 demand becomes:

(47) $n_{O2}=-\left(\frac{1}{2}+\frac{3.Math.R}{2.Math.\left(R+1\right)}\right).Math.n_{H2SO4}$

(48) R is the H.sub.2S to SO.sub.2 ratio as defined in example 2 and n.sub.H2SO4 is the number of moles of H.sub.2SO.sub.4 in the sulfuric acid feed. The negative sign indicates that less O.sub.2 is needed when H.sub.2SO.sub.4 is added.

(49) Usually the flow of sulfuric acid is measured in mass flow and based on sulfuric acid concentration, C.sub.sa (kg H.sub.2SO.sub.4/kg acid), the air demand is calculated as

(50) $G_{air}=K_{sa}.Math.M_{sa}=-C_{sa}.Math.\frac{1}{98.Math.0.0446.Math.y_{O2}^{\mathrm{air}}}.Math.\left(\frac{1}{2}+\frac{3.Math.R}{2.Math.\left(R+1\right)}\right).Math.M_{sa}$

(51) M.sub.sa is the mass flow (kg/h) of sulfuric acid, C.sub.sa is the weight fraction of H.sub.2SO.sub.4 in the sulfuric acid, Y.sub.O2 is the O.sub.2 molar vapor fraction in the combustion air and R is H.sub.2S to SO.sub.2 ratio in the process gas.

(52) Most often the sulfuric acid is atomized into the Claus reaction furnace through two-fluid nozzles using compressed air as the atomization media. In such a case, the atomization air flow will also result in a decrease in combustion air demand:

(53) $G_{air}=K_{aa}.Math.G_{aa}=-\frac{Y_{O2}^{aa}}{Y_{O2}^{\mathrm{air}}}.Math.G_{aa}$

(54) If N.sub.2 or steam is used as atomization media, the air demand factors will be 0.

Examples 2-6: Combinations of Claus Process and Sulfuric Acid Process

(55) Five further examples have been analyzed for the process shown in FIG. 2, in comparison with the process of prior art as shown in FIG. 1.

(56) These examples are based on the following feedstock gases:

(57) Feed stock gas rich in H.sub.2S (stream 2 in FIGS. 1 and 2):

(58) Total gas flow: 8190 Nm.sup.3/h

(59) H.sub.2S concentration: 94 vol %

(60) H.sub.2O concentration: 6 vol %

(61) The rich H.sub.2S gas is typical for refineries, and will also contain varying amounts of light hydrocarbons.

(62) Feed stock gas rich in H.sub.2S and NH.sub.3 (stream 70 in FIGS. 1 and 2):

(63) Total gas flow: 3669 Nm.sup.3/h

(64) H.sub.2S concentration: 28 vol %

(65) NH.sub.3 concentration: 45 vol %

(66) H.sub.2O concentration: 27 vol %

(67) These streams comprising H.sub.2S and NH3 are typically waste gases from so-called sour water strippers and recognized as SWS-gases. They may also contain varying amounts of light hydrocarbons.

(68) The fuel gas is a light hydrocarbon mixture (primarily CH.sub.4), with a lower heating value of 12,200 kcal/Nm.sup.3.

(69) Feed streams, combustion air and Claus tail gas are preheated to the extent possible by utilizing heat evolved in the combined Claus+sulfuric acid process.

(70) In these examples the Claus process operates with 94-95% recovery of sulfur from the feed, i.e. can be a well operated Claus plant with only 2 catalytic stages.

Example 2: Sequential Claus+Sulfuric Acid Process According to Prior Art

(71) In example 2 all feed streams are treated in the Claus process, providing a stream of 11.7 t/h elemental sulfur and a Claus tail gas comprising ˜5% of the S in the feed gases. In the Claus tail gas combustor, the sulfur species present in the Claus tail gas are oxidized and fuel gas is provided to maintain a combustor temperature of 1,000° C., such that all reduced species, such as CO, COS, H.sub.2, H.sub.2S, S.sub.x and CS.sub.2, are fully oxidized to CO.sub.2, H.sub.2O and SO.sub.2.

(72) The production of concentrated sulfuric acid is 2.4 t/h, calculated as 100% w/w H.sub.2SO.sub.4.

(73) The total sulfur and sulfuric acid recovery is >99.9% of the S in the feed, in compliance with even strict environmental legislation.

Example 3: Recycle of H.SUB.2.SO.SUB.4 .to Claus Reaction Furnace

(74) In this example H.sub.2SO.sub.4 is not desired as a product and the entire acid production from the sulfuric acid process is recycled to the Claus reaction furnace. The amount of H.sub.2SO.sub.4 recycle corresponds to ˜6% of the total S in the feed streams.

(75) The total elemental sulfur product flow is now equal to the S in the feed streams, corresponding to 107% of the base case as described in example 3.

(76) The temperature in the Claus reaction furnace decreases by ˜200° C. due to the evaporation and decomposition of the H.sub.2SO.sub.4, but the temperature is still well above the minimum for complete burnout of hydrocarbons and NH.sub.3. No fuel gas is needed in the Claus reaction furnace.

(77) As H.sub.2SO.sub.4 is an excellent O.sub.2 carrier, the combustion air requirements decrease and thus the process gas volume decreases as the flow of inert N.sub.2 decreases. Overall the process gas flow out of the Claus reaction furnace decreases to 94% of the base flow and the process gas flow out of the Claus tail gas combustor decreases to 93% due to this reduction in N.sub.2 flow. As less process gas needs to be heated to 1,000° C. in the Claus tail gas combustor, the fuel gas consumption is only 92% of the base case.

(78) The benefit of recycling H.sub.2SO.sub.4 has been found surprisingly high, as not only has the sulfur forming capacity of the Claus plant increased by 7% but at the same time the process gas volume has been decreased by 6-7%. This corresponds to a Claus plant capacity increase of ˜15%, provided that the process gas flow is at 100% of the base case.

Example 4: Recycle of H.SUB.2.SO.SUB.4 .to Claus Reaction Furnace and SWS Gas Bypass to Claus Tail Gas Combustor

(79) In this example, fuel gas consumption in the Claus tail gas combustor has been minimized by bypassing a fraction of the SWS gas to the Claus tail gas combustor. The SWS gas has a high heating value and can easily act as a fuel gas. The concentrated H.sub.2S feed gas could also have been used, but since the SWS gas can be problematic in the Claus process and is unproblematic in the WSA process, the bypassing of SWS gas has greater benefits than bypassing the H.sub.2S gas. Process gas wise there will also be a reduction in gas volume as the NH.sub.3 in the SWS gas will increase the process gas volume in the Claus process due to the oxygen (air) requirements for combustion of NH.sub.3 to N.sub.2 and H.sub.2O.

(80) The amount of SWS gas recycled is adjusted such that 1,000° C. is achieved in the Claus tail gas combustor, ensuring complete burnout of reduced species from the Claus tail gas, such as H.sub.2S, COS, CO, H.sub.2, S.sub.x and CS.sub.2.

(81) Combustion of NH.sub.3 will primarily form N.sub.2, but minor amounts of NO.sub.x may also form and depending on the concentration, a SCR catalyst for reduction of NO.sub.x may be required.

(82) The SCR catalyst requires NH.sub.3 for the NO.sub.x reduction and that can be supplied from a storage of NH.sub.3 or taken from the SWS gas.

(83) Since the fuel gas in the Claus tail gas combustor now contains H.sub.2S, the H.sub.2SO.sub.4 production will increase, now accounting for ˜13% of the S in the feed streams. This large amount of sulfuric acid recycle result in a significant reduction in Claus reaction furnace temperature.

(84) With proper feed stream preheating it is still possible to achieve sufficiently high temperature in the Claus reaction furnace without needing support fuel.

(85) The effect on the size of the Claus process is substantial: the process gas volume is reduced to 65% of the base case, still with 107% elemental sulfur production. This process gas volume reduction can be either used for capacity boosting of an existing plant or significant cost reduction of a new plant.

(86) Also the sulfuric acid plant will become smaller as the process gas flow is only 90% of the base case flow. This is surprising as the H.sub.2SO.sub.4 production has been more than doubled compared to the base case, but it is mainly due to the large reduction in Claus tail gas flow.

(87) What is most remarkable is the reduction in fuel gas consumption that is now only 16% of the base case flow, contributing to a significantly lower operational cost of the integrated Claus+sulfuric acid process.

Example 5: Recycle of H.SUB.2.SO.SUB.4 .and Complete Bypass of SWS Gas to Claus Tail Gas Combustor

(88) This example focus on the complete elimination of the SWS gas to the Claus plant, ensuring that ammonia salt formation in the sulfur condensers is impossible and thus decreases the risk of failure of the Claus plant.

(89) The process gas flow out of the Claus reaction furnace is 69% of the base case, but a little higher compared to example 3 where only a fraction of the SWS gas is bypassed. The increase in process gas flow is due to requirement of fuel gas addition to the Claus reaction furnace to maintain the high operating temperature.

(90) The H.sub.2SO.sub.4 production in the WSA plant has now increased to 17% of the S in the feed gases, recycling of the entire production now quenches the Claus reaction furnace temperature to an extent where fuel gas is required. The process gas from the Claus tail gas combustor has increased to 107% of the base case, due to the increased sulfur feed to the sulfuric acid plant.

(91) Even if fuel gas is needed in the Claus reaction furnace, the total flow of fuel gas is only 41% of the base case.

(92) From a plant size and operational cost point of view, this example seems less optimal than example 3, i.e. there is an optimum of H.sub.2SO.sub.4 recycle ratio which depends on the actual feed gas flows and compositions. Bypassing even more feed stock gas will result in an increased sulfuric acid production, which will quench the Claus reaction furnace even more which again will require more fuel gas and therefore the Claus tail gas flow will increase.

(93) For the feed gas compositions and flows described above, the optimum with regard to plant sizes and fuel consumption is with a H.sub.2SO.sub.4 recycle flow between 13% and 17% of the S feed in the feed streams.

(94) In general, the optimal feed stock gas bypass is close to the point where the Claus reaction furnace operates at the minimum allowable temperature, i.e. the feed stock can be bypassed to produce more sulfuric acid until the Claus reaction furnace temperature reaches the limit for thermal destruction of hydrocarbons and sulfuric acid. Increasing the feed stock bypass ratio will reduce the fuel gas need in the Claus tail gas combustor, but will increase the fuel gas consumption in the Claus reaction furnace by a much larger ratio as the fuel gas in the Claus reaction furnace need to evaporate and decompose the sulfuric acid and heat up the process gas, whereas in the Claus tail gas combustor only heating up of process gas is required.

(95) For a feed stock gas with e.g. 50 vol % H.sub.2S, the optimal H.sub.2SO.sub.4 recycle flow is ˜7% of the S feed in the feed stream. The acid gas bypass to the Claus tail gas combustor is only 2% as the relatively low H.sub.2S concentration result in a low temperature in the Claus reaction furnace and thus the sulfuric acid will quickly reduce the temperature and require fuel gas addition in the Claus reaction furnace. Using O.sub.2 enriched air in the Claus reaction furnace will allow for a higher H.sub.2SO.sub.4 recycle flow.

Example 6: Recycle of H.SUB.2.SO.SUB.4., Bypass of SWS Gas to Claus Tail Gas Combustor and Use of O.SUB.2 .Enriched Air

(96) To boost Claus plant capacity, a well-known revamp option is to install special burners which can handle enriched air with >21 vol % O.sub.2, a common O.sub.2 quality is 93-99 vol % O.sub.2.

(97) In this example an enriched air with 80 vol % O.sub.2 is used as in the Claus process, whereas atmospheric air is used in the sulfuric acid process.

(98) The effect of the enriched air is a significantly reduced process gas flow out of the Claus reaction furnace, mainly due to the reduced amount of N.sub.2 associated with the O.sub.2 flow. Also the lower process gas flow enables operation of the Claus reaction furnace without fuel addition, as less inert gas has to be heated.

(99) Since the process gas flow out of the Claus reaction furnace is now reduced to only 38% of the base case, the Claus tail gas feed to the Claus tail gas combustor is also significantly decreased. The process gas out of the Claus tail gas combustor is only 56% of the base case, it is relatively higher than the Claus plant flow due to the large amount of SWS gas bypass to the WSA plant.

(100) With this layout it is possible to operate without fuel gas in both Claus and sulfuric acid processes, even with this high recycle flow of H.sub.2SO.sub.4 from the sulfuric acid process.

(101) Examples 2-6 are summarized in Table 2, which show the potential for increased sulfur production, reduced fuel gas consumption and possibly reduced plant size by the well considered revamping of a Claus process plant with integration of a sulfuric acid process. It is furthermore seen that the saving in fuel gas consumption becomes reduced when more sulfuric acid is recycled.

(102) TABLE-US-00001 TABLE 2 Example# 2 3 4 5 6 Sulfur production 100% 107% 107% 107% 107% H.sub.2SO.sub.4 production  6% No No No No H.sub.2SO.sub.4 recycle  0%  6%  13%  17%  13% Acid gas feed to Claus 100% 100% 100% 100% 100% SWS gas feed to Claus 100% 100%  33%  0%  19% Process gas out Claus 100%  94%  65%  69%  38% reaction furnace Process gas out Claus 100%  93%  90% 107%  56% tail gas combustor Fuel gas consumption 100%  92%  16%  41%  0%