METHOD FOR PRODUCTION OF SULFUR AND SULFURIC ACID

Abstract

A process plant and a process for production of sulfur from a feedstock gas including from 15% to 100 vol % H.sub.2S 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 to a Claus reaction furnace operating at elevated temperature, c) cooling to provide a cooled Claus converter feed gas, d) directing to contact a material catalytically active in the Claus reaction, e) withdrawing a Claus tail gas and elementary sulfur, f) directing a stream comprising said Claus tail gas to a Claus tail gas treatment, wherein sulfuric acid directed to said Claus reaction furnace is in the form of droplets with 90% of the mass of the droplets having a diameter below 500 μm, with the associated benefit of such a process efficiently converting all liquid H.sub.2SO.sub.4 to gaseous H.sub.2SO.sub.4 and further to SO.sub.2.

Claims

1. A process for production of sulfur from a feedstock gas comprising from 15% to 100 vol % H.sub.2S and a stream of sulfuric acid, the process comprising: a. providing a Claus reaction furnace feed stream comprising 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, b. directing said Claus reaction furnace feed stream to a Claus 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 after optional reheating 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 to a Claus tail gas treatment, wherein said sulfuric acid directed to said Claus reaction furnace being in the form of droplets with a droplet size distribution characterized by 90% of the mass of the droplets having a diameter below 500 μm.

2. A process according to claim 1, wherein at least an amount of the sulfuric acid is directed to said Claus reaction furnace via at least one pneumatic nozzle, receiving sulfuric acid and an atomization medium.

3. A process according to claim 2, in which the atomization medium is compressed air and the flow is from 25 Nm.sup.3 air/ton acid to 500 Nm.sup.3 air/ton acid.

4. A process according to claim 1, wherein at least an amount of the sulfuric acid is directed to said Claus reaction furnace via at least one hydraulic nozzle.

5. A process according to claim 1, wherein the average process gas residence time in the Claus reaction furnace is less than 5 seconds.

6. A process according to claim 1, wherein the Claus reaction furnace comprises a turbulence enhancer.

7. A process according to claim 1, wherein the Claus reaction furnace comprises a means of impaction.

8. A process according to claim 1, wherein said Claus tail gas treatment comprises g. directing a stream comprising said Claus tail gas, oxygen and a fuel as a feedstock gas to a Claus tail gas combustor operating at a temperature above 900° C. or a catalytic means for oxidation providing an SO.sub.2 converter feed gas, h. 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, i. 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, j. recycling at least a part of the produced sulfuric acid to the Claus reaction furnace.

9. Process according to claim 1, in which an amount of sulfuric acid is from a source other than a Claus tail gas treatment.

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

11. A process plant comprising a Claus reaction furnace, a Claus waste heat boiler, a Claus conversion section, a Claus tail gas combustor and a sulfuric acid section, wherein the Claus reaction furnace has a furnace inlet, an acid nozzle 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 Claus tail gas combustor 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 inlet of the Claus reaction furnace is configured for receiving a feedstock gas and an oxidant, and the outlet of the Claus reaction furnace is configured for being in fluid communication with the inlet of the Claus waste heat boiler, wherein the outlet the Claus waste heat boiler is configured for being in fluid communication with the inlet of the Claus conversion section and wherein the inlet of the Claus tail gas combustor is configured for being in fluid connection with the outlet of said Claus conversion section gas outlet, the Claus tail gas combustor outlet is configured for being in fluid connection with the inlet of the sulfuric acid section, wherein the sulfuric acid outlet of the sulfuric acid section is in fluid communication with the acid nozzle inlet of said Claus reaction furnace.

12. A process plant according to claim 11, further comprising a sulfur storage tank having a volume corresponding to the amount of sulfuric acid withdrawn from the sulfuric acid outlet of the sulfuric acid section in from 1 day to 4 days.

Description

FIGURES

[0154] FIG. 1 shows a sequential Claus+ sulfuric acid process according to the prior art

[0155] FIG. 2 shows an integrated Claus+ sulfuric acid process with injection of sulfuric acid in the Claus reaction furnace according to the present disclosure

[0156] In FIG. 1 a process for production of sulfur and sulfuric acid according to the prior art is shown. Here a feedstock gas 2 rich in H.sub.2S is directed to a Claus process, from which the tail gas 26 is directed to a sulfuric acid process. The feedstock gas 2 rich in H.sub.2S is directed to a Claus reaction furnace 66 converting an amount of the of H.sub.2S to SO.sub.2, to form a Claus converter feed gas 4 having a ratio between H.sub.2S and SO.sub.2 close to 2:1. The Claus converter feed gas 4 is directed to a converter 8 containing a material catalytically active in the Claus reaction 12, providing a Claus process product 14. The Claus process product 14 is directed to a sulfur condensation unit 16, providing condensed sulfur 18 and a Claus tail gas 20. The wet Claus tail gas 20 is typically further reacted in the presence of additional material catalytically active in the Claus reaction followed by further condensation of sulfur, in one to four further Claus stages (not shown here), to provide a final wet Claus tail gas. An aqueous phase 24 may optionally be separated from the wet Claus tail gas 20 in a separator 22, providing a dried Claus tail gas 26, which is directed to a Claus tail gas combustor 32, providing a SO.sub.2 converter feed gas 34. The SO.sub.2 converter feed gas 34 is cooled and directed to an SO.sub.2 converter 40, containing one or more beds (layers) of catalytically active material 42, 44, 46 optionally with interbed cooling, from which an SO.sub.3 rich gas 48 is withdrawn. As the SO.sub.3 rich gas contains water, the SO.sub.3 may hydrate to form H.sub.2SO.sub.4. H.sub.2SO.sub.4 is condensed as concentrated sulfuric acid 52 in a sulfuric acid condenser 50. From the sulfuric acid condenser 50 a substantially pure gas 62 may be withdrawn and directed to stack 64.

[0157] In order to maintain a stable flame and sufficient high temperature for complete oxidation of H.sub.2S, CO, CS.sub.2, COS and H.sub.2, fuel gas may be directed to the Claus tail gas combustor 32. Oxygen is also supplied, typically via air and preferably hot air from the sulfuric acid condenser (50), in order to supply oxygen for both the combustion reactions in Claus tail gas combustor 32 but also the oxygen required for the oxidation of SO.sub.2 in the SO.sub.2 converter. To reduce fuel consumption, the oxygen for SO.sub.2 oxidation can be added between the Claus tail gas combustor 32 outlet and the SO.sub.2 converter 40 inlet.

[0158] In FIG. 2 an integrated Claus/sulfuric acid process with injection of sulfuric acid in the Claus reaction furnace 66 according to the present disclosure is shown. A feedstock gas 2 rich in H.sub.2S, sulfuric acid 56 and a gas rich in oxygen 72, as well as optionally a gas comprising a fuel and optionally, a second feedstock gas e.g. comprising a lower concentration of H.sub.2S and possibly NH.sub.3 are directed to a Claus reaction furnace 66 and the combustion product is directed as an O.sub.2 free Claus converter feed gas 4 to a converter 8. Between the outlet of the Claus reaction furnace 66 and Claus converter inlet 8, a waste heat boiler and optionally a sulfur condensation unit (not shown) are typically installed to reduce the temperature to the optimal working temperature for the Claus catalyst, optionally also withdrawing elemental sulfur formed in the Claus reaction furnace 66. The O.sub.2 free Claus converter feed gas 4 is directed to contact an optional material catalytically active in conversion of SO.sub.3 to SO.sub.2 10 (comprising e.g. one or more compounds of V, Mn, Fe, Co, Cu, Zn, Ni, Mo, W, Sb, Ti and Bi supported on one or more compounds of Al, Ti, Si, diatomaceous earth, Zr, Mg, and cordierite) and a material catalytically active in the Claus reaction 12, comprising e.g. aluminum oxide or titanium oxide, providing a Claus process product 14. The Claus process product 14 is directed to a sulfur condensation unit 16, providing condensed sulfur 18 and a Claus tail gas 20. The Claus tail gas 20 may optionally be further reacted in the presence of additional material catalytically active in the Claus process followed by further condensation of sulfur, in one to four further Claus stages (not shown here), to provide a final Claus tail gas. The final Claus tail gas comprising H.sub.2S 20 is directed to a Claus tail gas combustor 32, providing an SO.sub.2 converter feed gas 34. To ensure oxidation of the compounds in the Claus tail gas, an O.sub.2 rich gas 72 is directed to the Claus tail gas combustor 32.

[0159] The SO.sub.2 converter feed gas 34 is typically cooled in a waste heat boiler (not shown) to provide optimal temperature for the first catalyst layer 42 in the SO.sub.2 converter 40. The SO.sub.2 converter feed gas 34 is directed to an SO.sub.2 converter 40, containing one or more beds/layers of catalytically active material 42, 44, 46 optionally with interbed cooling, from which an SO.sub.3 rich gas 48 is withdrawn. As the SO.sub.3 rich gas contains water, the SO.sub.3 may hydrate to form H.sub.2SO.sub.4. H.sub.2SO.sub.4 is condensed as concentrated sulfuric acid 52 in a sulfuric acid condenser 50. If the amount of water is insufficient for full hydration of SO.sub.3, addition of steam in a position upstream the sulfuric acid condenser 50 may be preferred. From the sulfuric acid condenser 50 a substantially pure gas 62 may be withdrawn and directed to stack 64. Typically, all sulfuric acid 56 is recycled to the Claus reaction furnace 66, but optionally an amount of sulfuric acid may be withdrawn for other process purposes. For that purpose, an intermediate sulfuric acid tank (54) can be located between the sulfuric acid outlet of the sulfuric acid condenser 50 and the Claus reaction furnace 66, which may act as a buffer, decoupling operation of the Claus process from operation of the sulfuric acid process, which provides stability of the overall system.

[0160] An optional catalytic reactor 35 for oxidation of remaining impurities such as hydrocarbons, CO, COS, CS.sub.2, S, H.sub.2 and H.sub.2S is also shown in FIG. 2.

[0161] In a further embodiment, the entire amount of second feedstock containing NH.sub.3 and H.sub.2S 70 is directed to the Claus tail gas combustor 32, eliminating the risk of NH.sub.3-salt formation in the Claus condensers 16. In this embodiment a system for reduction of NOx 33, located between the Claus tail gas combustor 32 outlet and the inlet of the SO.sub.2 converter 40 will be installed. Typically, a so-called SCR (Selective Catalytic Reaction) catalytic reactor will be used, requiring addition of NH.sub.3 for the SCR reaction to proceed. The NH.sub.3 addition can be from an external source or could be a small stream of the second feedstock containing NH.sub.3 and H.sub.2S 70, which is then bypassed the Claus tail gas combustor.

[0162] In a further embodiment the conversion and condensation of sulfuric acid may be made in two stages, where remaining SO.sub.2 from the first stage is further oxidized, hydrated and condensed, with the associated benefit of providing increased sulfur removal.

[0163] In a further embodiment, additional SO.sub.2 conversion can be achieved by installing a tail gas cleaning plant downstream the sulfuric acid process. Numerous of these tail gas solutions exist, where alkaline scrubbers optionally combined with mist filters, are the most common type. Scrubbers using H.sub.2O.sub.2 or NH.sub.3 are preferred as the effluent from these scrubbers is H.sub.2SO.sub.4 and (NH.sub.4).sub.2SO.sub.4 respectively, both of which can be recycled to the Claus reaction furnace for thermal destruction, i.e. eliminating a waste stream.

[0164] In a further embodiment the SO.sub.2 converter feed gas 34 may be dried, such that the SO.sub.3 rich gas 48 will contain little or no water. In that case the condenser 50 may be replaced with an absorber, in which SO.sub.3 may be absorbed in sulfuric acid, to provide concentrated sulfuric acid, by a dry sulfuric acid process.

[0165] In a further embodiment an amount of elemental sulfur may also be transferred to the Claus tail gas combustor 32, which will have the effect of providing SO.sub.2 to the sulfuric acid process without introduction of water, which may be beneficial if it is desired to increase the SO.sub.3 concentration, which may be beneficial in a dry sulfuric acid process.

[0166] In a further embodiment an amount of fuel gas 68 is directed to the Claus tail gas combustor 32 to ensure sufficiently high temperature for complete oxidation of all reduced compounds in the Claus tail gas 20.

[0167] In a further embodiment, an amount of the feedstock gas 2 rich in H.sub.2S may also be split in an amount directed for the combustor of the Claus process 66 and an amount directed to the Claus tail gas combustor 32. This will reduce the need for fuel gas addition to the Claus tail gas combustor 32.

[0168] In a further embodiment a part of the Claus tail gas 20 is bypassed the Claus tail gas combustor 32 and combined with the hot off gas 34 from the Claus tail gas combustor in a gas mixing point just downstream the Claus tail gas combustor. This reduces the amount of fuel gas 68 needed for the Claus tail gas combustor to maintain a sufficiently high temperature. The combined Claus tail gas combustor off gas and bypassed Claus tail gas must have a mixed gas temperature in excess of 400° C. to ensure homogeneous (i.e. gas phase) oxidation of H.sub.2S. To ensure complete oxidation of “difficult” species such as COS and CO, an optional oxidation catalyst 35 can be installed between the gas mixing point and inlet to the SO.sub.2 converter 40. To ensure optimal control of the temperature to the oxidation catalyst, a waste heat boiler or any other heat exchanger can be installed between the gas mixing point and inlet to the oxidation catalyst. The oxidation catalyst typically comprises a noble metal such as Pt or Pd.

[0169] In a further embodiment the gas comprising oxygen 72 may be pure oxygen or atmospheric air enriched in oxygen, such that it comprises less than 50%, 20%, 10% or even 1% N.sub.2+Ar.

EXAMPLE 1

[0170] The evaporation of sulfuric acid droplets has been numerically analyzed using a detailed mathematical model including [0171] 1. Heat transfer to droplet by convective heat transfer [0172] 2. Heat transfer to droplet by radiative heat transfer from gas molecules and hot refractory walls in the reaction furnace [0173] 3. Mass transfer between droplet and process gas [0174] 4. Detailed thermodynamics for sulfuric acid/water mixtures

[0175] The atomization nozzle is assumed to be of the air assisted type, the initial droplet velocity is around 50 m/sec and the process gas flow velocity is around 10 m/sec.

[0176] The results of the simulations for 3 different reaction furnace temperatures are shown in FIG. 3. Sulfuric acid (93% w/w H.sub.2SO.sub.4) droplets enters the hot reaction furnace at low temperature and initially grows by absorbing water from the surrounding process gas. As the droplet heats up, primarily water is evaporated until the sulfuric acid concentration reaches around 98.5% w/w, which is the azeotrope concentration. From that point the droplet concentration and temperature does not change until the droplet is completely evaporated.

[0177] From the results it is seen that the initial droplet size is of relevance for the safe and long term operation of the Claus plant, especially if the residence time in the reaction furnace must be fixed at a value of 1-2 seconds. For 1 second residence time, droplets with initial diameters above 500 μm will not evaporate completely at 1,000° C. As seen, there is a temperature effect too, however the influence of the initial droplet diameter is much stronger.

EXAMPLE 2

[0178] The atomization of the sulfuric acid requires careful choice of nozzle type and operation of the nozzle. Numerous nozzles with their own characteristics regarding liquid capacity, pressure, type etc. exist in the market, ranging from very fine sprays with low capacity to coarse sprays with very large capacity, depending on the application.

[0179] For evaporation purposes, a small droplet size distribution is desired and for that purpose pneumatic nozzles (air-assisted, two phase) and hydraulic (pressure) nozzles are preferred, the former producing smaller droplets but with the “cost” of consumption of an atomization fluid, which is usually compressed air.

[0180] Based on data given in table 18-18 and 18-19 in Perry's Chemical Engineers Handbook, 4th edition (McGraw-Hill 1963), the D.sub.0.9 value has been calculated and is shown in table 1. The D.sub.0.9 defines the diameter, where 90% of the total mass (or volume) of the droplets have smaller diameters. The data are based on water as the liquid and air, in the case of pneumatic nozzles, at room temperature.

[0181] As seen in the table, the pneumatic nozzle produces the smallest droplets. The hydraulic nozzle also produces fine droplets, but with sizes up to 500 μm. Nozzle #2 and #3 only differ by the pressure of the liquid and it is seen that higher liquid pressure leads to smaller droplets. Although producing small droplets, the hydraulic nozzles may be an inferior choice if only 1 second residence time in the reaction furnace is allowed, but for 2 second residence time, the nozzles will work fine. See FIG. 3 for evaporation time of sulfuric acid droplets. For hydraulic nozzles, it should be considered to install an impaction wall or similar in the reaction furnace, such that most of the largest droplets will collide with the wall and evaporate within the reaction furnace.

TABLE-US-00001 TABLE 1 D.sub.0.9 diameters (mass/volume based) for pneumatic and hydraulic nozzles. Data taken from table 18-18 and 18-19 in Perry's chemical engineers handbook, 4.sup.th edition. Nozzle #1 Nozzle #2 Nozzle #3 Nozzle type Pneumatic Hydraulic Hydraulic Pressure of liquid/air 0.3 barg 6.9 barg 13.8 barg D.sub.0.9 (mass/volume based) 55 μm 550 μm 420 μm

[0182] To document the effect of integrating a Claus process and a sulfuric acid process, four 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.

[0183] These examples are based on the following feedstock gases:

[0184] Feed stock gas rich in H.sub.2S (stream 2 in FIGS. 1 and 2):

[0185] Total gas flow: 8190 Nm.sup.3/h

[0186] H.sub.2S concentration: 94 vol %

[0187] H.sub.2O concentration: 6 vol %

[0188] The rich H.sub.2S gas is typical for refineries, and will also contain varying amounts of light hydrocarbons.

[0189] Feed stock gas rich in H.sub.2S and NH.sub.3 (stream 70 in FIGS. 1 and 2):

[0190] Total gas flow: 3669 Nm.sup.3/h

[0191] H.sub.2S concentration: 28 vol %

[0192] NH.sub.3 concentration: 45 vol %

[0193] H.sub.2O concentration: 27 vol %

[0194] These streams comprising H.sub.2S and NH.sub.3 are typically waste gases from so-called sour water strippers and recognized as SWS-gases. They may also contain varying amounts of light hydrocarbons.

[0195] The fuel gas is a light hydrocarbon mixture (primarily CH.sub.4), with a lower heating value of 12,200 kcal/Nm3.

[0196] 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.

[0197] 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 3

[0198] Sequential Claus+ Sulfuric Acid Process According to Prior Art.

[0199] In example 3 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, Sx and CS.sub.2, are fully oxidized to CO.sub.2, H.sub.2O and SO.sub.2.

[0200] The production of concentration sulfuric acid is 2.4 t/h, calculated as 100% w/w H.sub.2SO.sub.4.

[0201] The total sulfur and sulfuric acid recovery is >99.9% of the S in the feed, in compliance with even strict environmental legislation.

EXAMPLE 4

[0202] Recycle of H.sub.2SO.sub.4 to Claus Reaction Furnace.

[0203] 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.

[0204] 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.

[0205] 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.

[0206] 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.

[0207] 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 5

[0208] Recycle of H.sub.2SO.sub.4 to Claus Reaction Furnace and SWS Gas Bypass to Claus Tail Gas Combustor.

[0209] 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 wet sulfuric acid 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 NH3 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.

[0210] 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, Sx and CS.sub.2.

[0211] 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.

[0212] With proper feed stream preheating it is still possible to achieve sufficiently high temperature in the Claus reaction furnace without needing support fuel.

[0213] 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.

[0214] 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.

[0215] 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 6

[0216] Recycle of H.sub.2SO.sub.4 and Complete Bypass of SWS Gas to Claus Tail Gas Combustor

[0217] 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.

[0218] The process gas flow out of the Claus reaction furnace is 69% of the base case, but a little higher compared to example 5 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.

[0219] The H.sub.2SO.sub.4 production in the wet sulfuric acid 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.

[0220] Even if fuel gas is needed in the Claus reaction furnace, the total flow of fuel gas is only 41% of the base case.

[0221] From a plant size and operational cost point of view, this example seems less optimal than example 5, 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.

[0222] 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.

[0223] 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.

[0224] 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 7

[0225] Recycle of H.sub.2SO.sub.4, Bypass of SWS Gas to Claus Tail Gas Combustor and Use of O.sub.2 Enriched Air.

[0226] 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.

[0227] 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.

[0228] 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.

[0229] 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 wet sulfuric acid plant.

[0230] 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.

TABLE-US-00002 TABLE 2 Example Example Example Example Example 23 4 5 6 7 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%