Zero emissions sulphur recovery process with concurrent hydrogen production

Abstract

Disclosed is a process for the concurrent production of hydrogen and sulphur from a H.sub.2S-containing gas stream, with zero emissions. The method comprises the thermal oxidative cracking of H.sub.2S so as to form H.sub.2 and S.sub.2. Preferably, the oxidation is conducted using oxygen-enriched air, preferably pure oxygen. The ratio H.sub.2S/O.sub.2 in the feedstock is higher than 2:1, preferably in the range of 3:1-5:1.

Claims

1. A plant suitable for conducting thermal oxidative cracking of a H.sub.2S-containing gas stream, said plant comprising a reaction chamber which comprises a mixing zone, a burner and a thermal oxidative cracking zone downstream of said mixing zone and an inlet coupled to a source of a H.sub.2S-containing acid gas stream, which source is an amine regeneration unit, a sour water stripper unit or both, and an inlet coupled to a source of an oxygen-comprising stream, both of said inlets configured to introduce said H.sub.2S containing gas and said oxygen-comprising stream into said mixing zone to obtain a mixture, wherein the mixing zone comprises said burner; wherein said reaction chamber further contains downstream of said reaction zone a quench zone for effluent from the thermal oxidative cracking reaction zone which quench zone comprises a spray nozzle for direct injection of water; and a hydrogen purification section configured to receive hydrogen downstream of said quench zone and wherein the thermal oxidative cracking reaction zone does not contain a catalyst.

2. The plant of claim 1 which further comprises a waste heat boiler, downstream of the quench zone, for generating steam, and a preheater for preheating said H.sub.2S containing gas and said oxygen-comprising stream using said steam.

3. The plant of claim 1 which further contains a hydrogenation reactor for effluent from the thermal oxidative cracking reaction zone.

4. The plant of claim 1 which further contains a subsequent Claus catalytic section comprising a gas reheater, a Claus catalytic reactor and a sulfur condenser, downstream of the thermal oxidative cracking zone.

5. The plant of claim 1 wherein said thermal oxidative cracking reaction zone comprises a reaction chamber lined to withstand temperatures up to 1,550 C.

6. The plant of claim 1 wherein said plant further comprises a waste heat boiler positioned downstream of the reaction chamber and a sulfur condenser downstream of the waste heat boiler.

7. The plant of claim 1 wherein the H.sub.2S-containing acid gas stream is derived from both an amine regeneration system and a sour water stripper unit said system and unit connected to an inlet to a thermal oxidative cracking unit, and wherein said thermal oxidative cracking unit comprises an inlet coupled to said amine regeneration unit and an inlet coupled to said sour water stripper unit.

8. The plant of claim 1 wherein the hydrogen purification section comprises a pressure swing absorber.

9. The plant of claim 7 wherein the hydrogen purification section comprises a pressure swing absorber.

10. The plant of claim 1 which further comprises a tail gas pre-heater receiving a gas stream from the quench zone, a hydrogenation reactor receiving preheated tail gas from the tail gas pre-heater, a waste heat boiler configured for cooling gas leaving the hydrogenation reactor, a quench tower configured for receiving cooled gas from the waste heat boiler, an amine absorber connected to receive quenched gas from the quench tower and configured for absorption of H.sub.2S from the quenched gas, and wherein the hydrogen purification section is configured for receiving gas from the absorber.

11. The plant of claim 7 which further comprises a tail gas pre-heater receiving a gas stream from the quench zone, a hydrogenation reactor receiving preheated tail gas from the tail gas pre-heater, a waste heat boiler configured for cooling gas leaving the hydrogenation reactor, a quench tower configured for receiving cooled gas from the waste heat boiler, an amine absorber connected to receive quenched gas from the quench tower and configured for absorption of H.sub.2S from the quenched gas, and wherein the hydrogen purification section is configured for receiving gas from the absorber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 depicts a simplified flow scheme of a typical traditional Claus Plant comprising a thermal stage, two catalytic stages, a subsequent reductive Tail Gas Treatment section, and a thermal incineration section.

(2) FIG. 2 presents a simplified flow scheme of an H.sub.2S Thermal Oxidative Cracking Plant according to the invention, comprising a thermal oxidative cracking stage, optionally a Claus catalytic stage, and a subsequent reductive Tail Gas Treatment section.

DETAILED DESCRIPTION OF THE INVENTION

(3) In a broad sense, the invention is based on the simultaneous occurrence of cracking and partial oxidation of H.sub.2S so as to provide concurrent production of sulphur and of a significant amount of hydrogen. This serves to address the problem of gas emissions into the atmosphere and producing at the same time a valuable hydrogen export stream.

(4) It is emphasized that the thermal oxidative cracking in accordance with the invention is a fundamentally different process from both the thermal stage and the catalytic stage in an existing Claus-type process. With reference to the reaction equations (1) to (5) mentioned above, the Claus processes are directed to driving the above reaction (3) to near completion. The present invention is based on the judicious insight to provide a process based on the side reactions (4) and (5), and to promote these reactions for the production, from a H.sub.2S-containing gas-stream, of both hydrogen and sulphur.

(5) In the present invention, a Thermal Oxidative Cracking (TOC) stage substitutes the Claus thermal stage. The process of the invention thus favors H.sub.2S dissociation and partial oxidation instead of complete oxidation and Claus reaction.

(6) The thermal oxidative cracking is conducted in one or more reaction zones, preferably provided in a single reaction chamber. A single reaction zone, in a single reaction chamber is preferred.

(7) The invention presents the skilled person with the insight to promote the above-mentioned reactions (4) and (5). The fact that thereto the gas stream is to be subjected to thermal oxidative cracking, implies a clear message to the skilled person as to how to carry this out.

(8) The reaction is carried out at a temperature between 1100 C. and 1550 C., under the influence of oxygen. The ratio between H.sub.2S and oxygen is above 2:1, and preferably in the range of from 3:1 to 5:1, more preferably in the range 4:1-4.5:1, and wherein the oxygen is provided in a gas stream comprising at least 40% of oxygen.

(9) The Thermal Oxidative Cracking reaction zone is provided with oxygen. The oxygen is preferably provided as a gas enriched with oxygen as compared to air. Preferably, this is an oxygen-containing gas-stream comprising at least 40 vol. % oxygen, preferably at least 60 vol. % oxygen. More preferably, this oxygen is provided as substantially pure oxygen, viz. 90 vol. %-99 vol. % of oxygen, or as close to 100% as available.

(10) The use of oxygen-enriched gas, and preferably pure oxygen, is not only related to optimizing the thermal oxidative cracking process, it also presents advantages such as the avoidance of unnecessarily large equipment, which would be needed on account of the presence of large volumes of inert (nitrogen) gas. Moreover, with reference to the invention's purpose to produce hydrogen, in addition to sulphur recovery and with reduced emissions, it will be advantageous to reduce, and preferably avoid, the presence of nitrogen in the tail gas of the process.

(11) The quantity of oxygen fed to the reactor is selected so as to achieve a ratio H.sub.2S/O.sub.2 in the feedstock higher than typical figure of about 2:1. Preferably, H.sub.2S/O.sub.2 ratio in the feedstock should be in the range 3:1 to 5:1, more preferably in the range 4:1-4.5:1, for example about 4.4.

(12) In the preferred embodiment of operating the thermal oxidative cracking on the basis of a ratio H.sub.2S/O.sub.2 between 4:1 and 4.5:1, preferred reaction temperatures to obtain simultaneously cracking and partial oxidation of H.sub.2S are in the range of from 1100 to 1400 C., preferably about 1200 C.

(13) In one embodiment, the feedstock to the Thermal Oxidative Cracking reaction zone (H.sub.2S-containing acid gas and oxygen-containing gas) is preheated in order to increase the reaction temperature, to boost hydrogen production and to depress SO.sub.2 formation. In a preferred embodiment, such preheating is performed with high pressure steam at about 45 barg from a waste heat boiler in order to achieve a feedstock inlet temperature of about 240 C.

(14) It should be noted that the reaction preferably is conducted autothermally. This refers to the fact that, whilst the process is preferably adiabatic, heat exchange takes in fact place, since the oxidation reaction is exothermal, and the cracking reaction is endothermal, whereby heat made available through the exothermal reaction is utilized in the endothermal reaction.

(15) All in all, the process of the invention is believed to favor reactions (4) and (5) relative to reactions (1) and (2), leading to lower H.sub.2S conversion, but on the other hand to significantly higher H.sub.2 formation and to much lower SO.sub.2 formation. As a consequence of the lower H.sub.2S conversion, a higher acid gas recycle rate from H.sub.2S-containing gas source (e.g., an amine regenerator) to reaction chamber is obtained as compared to a traditional Claus Plant.

(16) The thermal oxidative cracking process of the invention is conducted at an optimum temperature so as to provide the minimum approach to maximum possible equilibrium figures.

(17) This results in increasing the hydrogen yield and minimizing SO.sub.2 formation, which in turn serves to minimize hydrogen consumption in the Tail Gas Treatment section to reduce SO.sub.2 to H.sub.2S.

(18) Preferably, the H.sub.2S-containing acid gas and the oxygen-containing gas are mixed in a mixing zone prior to entering the thermal oxidative cracking zone. In one embodiment the mixing zone comprises a burner mounted in front of the reaction chamber.

(19) The gas effluent from the reaction chamber is preferably quenched so as to avoid recombination of H.sub.2 and S.sub.2 to form H.sub.2S, viz. by the inverse reaction of (4), which would make the process sub-optimal in terms of overall conversion. Preferably this quenching is done substantially instantaneously. The quenching is preferably to a temperature lower than 950 C., more preferably in the range 850750 C. The residence time in the quench zone is preferably as short as possible, typically of from 10 ms to 300 ms, preferably from 10 ms to 100 ms, more preferably from 10 ms to 50 ms.

(20) The quench zone (which preferably is a zone of the reaction chamber) is preferably followed by a waste heat boiler and a sulphur condenser to cool down the process gas and to recover liquid sulphur. The latter is preferably done by raising high pressure steam in the waste heat boiler and low pressure steam in the sulphur condenser.

(21) In a preferred embodiment, the quenching of the gas effluent from the reaction chamber is achieved by mixing with water in the final part of the reaction chamber. The mixing may be done by direct injection of water into the reaction chamber through a spray nozzle.

(22) Although the process of the invention substantially reduces the formation of SO.sub.2, it will be inevitable that some SO.sub.2 is formed. In order to remove such SO.sub.2, the Thermal Oxidative Cracking stage is preferably followed by a Tail Gas Treatment section. Therein a part (e.g. about 10-15 vol. %) of the produced hydrogen is consumed in order to reduce residual SO.sub.2 to H.sub.2S in a hydrogenation reactor. Due to the much higher hydrogen content and to the much lower SO.sub.2 content in the tail gas compared to traditional Claus Plant, the reduction step of the Tail Gas Treatment section can be performed without any hydrogen import.

(23) The tail gas is preferably preheated and fed to a hydrogenation reactor. Therein the SO.sub.2, as well as other residual sulphur compounds, such as COS and CS.sub.2, are converted into H.sub.2S, which is then removed. This removal can be done in a conventional manner, e.g., by scrubbing the gas with a lean amine solution in an absorber.

(24) In one embodiment, the Thermal Oxidative Cracking stage is followed by one Claus catalytic stage, comprising a gas reheater, a Claus catalytic reactor and sulphur condenser, in order to convert most of the SO.sub.2 into sulphur, thereby minimizing H.sub.2 consumption for SO.sub.2 reduction in the Tail Gas Treatment section.

(25) In one embodiment, the hydrogen stream obtained from the TGT absorber is sent to end users, like hydrotreaters, hydrocrackers or hydrodesulphurizers. It should be noted that the composition of hydrogen rich stream from the top of the TGT absorber may be different depending on variables such as SRU feedstock quality, plant configuration and operating conditions, and may include traces or percentages of H.sub.2O, N.sub.2, CO, CO.sub.2, H.sub.2S, COS and CS.sub.2.

(26) In a preferred embodiment, a hydrogen stream obtained from the TGT absorber is further purified in a Hydrogen Purification section (for example a Pressure Swing Absorber). It should be noted that, prior to purification, the composition of a hydrogen rich stream from the top of the TGT absorber may be different depending on variables such as SRU feedstock quality, plant configuration and operating conditions, and may include traces or percentages of H.sub.2O, N.sub.2, CO, CO.sub.2, H.sub.2S, COS and CS.sub.2.

(27) The purified hydrogen is sent to end users, like hydrotreaters, hydrocrackers or hydrodesulphurizers.

(28) The invention, in one aspect, also relates to a plant suitable for conducting the thermal oxidative cracking of a H.sub.2S-containing gas stream, said plant comprising an inlet for a H.sub.2S-containing acid gas stream, an inlet for an oxygen-comprising stream, and a Thermal Oxidative Cracking reaction zone. Preferably, the plant further comprises a gas quench zone.

(29) In one embodiment, the Thermal Oxidative Cracking reaction chamber is refractory lined in order to withstand temperatures up to 1550 C.

(30) The invention will be illustrated with reference to the following, non-limiting Figures and Examples.

DETAILED DESCRIPTION OF THE FIGURES

(31) Looking at FIG. 1, in a traditional Claus Plant, acid gas from one or more Amine Regeneration Unit(s) 1 is fed together with acid gas from Sour Water Stripper Unit(s) 2 and with a combustion air stream 3 to a thermal reactor burner (or Claus main burner) 4, directly connected to a thermal reactor (or reaction furnace) 5, where one third of H.sub.2S is converted to SO.sub.2 and all other compounds such as hydrocarbons and ammonia are completely oxidized. The furnace effluent, after an adequate residence time in the thermal reactor, is cooled down in a Claus waste heat boiler 6, where heat is recovered generating high pressure steam. The process gas from the Claus waste heat boiler is fed to a first sulphur condenser 7, where gas is cooled generating low pressure steam and sulphur 8 is condensed and is sent to degassing and storage. The process gas from the first sulphur condenser is preheated in a first Claus reheater 9 before entering a first Claus catalytic reactor 10, where the reaction between H.sub.2S and SO.sub.2 to produce sulphur vapors continues until equilibrium. The process gas from reactor 10 is sent to a second sulphur condenser 11, where gas is cooled generating low pressure steam and sulphur 8 formed in the reactor is condensed and is sent to degassing and storage. The process gas from the second sulphur condenser is preheated in a second Claus reheater 12 before entering a second Claus catalytic reactor 13, where the reaction between the H.sub.2S and SO.sub.2 to sulphur vapours continues until equilibrium. The process gas from reactor 13 is fed to a third sulphur condenser 14, where gas is cooled generating low pressure steam (generally 4.5-6 barg), or low pressure steam (generally about 1.2 barg) and sulphur 8 formed in the reactor is condensed and is sent to degassing and storage. Claus tail gas 15 from third sulphur condenser is sent to Tail Gas Treatment section.

(32) Looking at FIG. 2, in a H.sub.2S Thermal Oxidative Cracking Plant according to the invention, acid gas from one or more Amine Regeneration Unit(s) 1 is fed together with acid gas from one or more Sour Water Stripper Unit(s) 2 and with a pure oxygen stream 41 (or an oxygen-enriched air stream) to a Thermal Oxidative Cracking reaction chamber 42, where H.sub.2S is partially oxidized to S.sub.2 and partially dissociated into H.sub.2 and S.sub.2, while all other compounds such as hydrocarbons and ammonia are completely oxidized and only a very small amount of SO.sub.2 is formed. The reactor effluent is cooled down in a waste heat boiler 6, where heat is recovered generating high pressure steam. The process gas from the waste heat boiler is fed to a sulphur condenser 7, where gas is cooled generating low pressure steam and sulphur 8 is condensed and is sent to degassing and storage; tail gas 15 from sulphur condenser is sent to a Tail Gas Treatment section.

(33) In one embodiment, the process gas from the waste heat boiler 6 is fed to a first sulphur condenser 7, where gas is cooled generating low pressure steam and sulphur 8 is condensed and is sent to degassing and storage. The process gas from the first sulphur condenser is preheated in the first Claus reheater 9 before entering a first Claus catalytic reactor 10, where the reaction between H.sub.2S and SO.sub.2 to produce sulphur vapors continues until equilibrium, so removing almost all SO.sub.2. The process gas from reactor 10 is sent to a second sulphur condenser 11, where gas is cooled generating low pressure steam and sulphur 8 formed in the reactor is condensed and is sent to the degassing and storage. Tail gas 15 from the second sulphur condenser (or from the first sulphur condenser in the first embodiment) is sent to a Tail Gas Treatment section.

(34) In both Plant configurations shown in FIG. 1 and FIG. 2, and also in the embodiment of the present invention comprising a further Claus catalytic step, tail gas 15 from final sulphur condenser is first preheated in the tail gas preheater 16. In the traditional Claus Plant, as shown in FIG. 1, tail gas is mixed as necessary with hydrogen obtained from an external network 17, while in the novel H.sub.2S Thermal Oxidative Cracking Plant according to the invention, as shown in FIG. 2, separate import of hydrogen is not necessary, and tail gas is directly sent to a hydrogenation reactor 18. In the hydrogenation reactor (or reduction reactor) all sulphur compounds contained in the process gas are converted to H.sub.2S under slight hydrogen excess. The tail gas leaving the reactor is cooled down first in a TGT waste heat boiler 19 generating low pressure steam and then in a quench tower 20, where the process gas cooling is achieved by circulation of the condensate 21 generated in the gas cooling. Quench water pumps 22 provide water circulation to the tower, while heat is removed from the system by a quench water cooler 23. The excess sour water 24 generated in the gas cooling is sent to battery limits for treatment in the Sour Water Stripper (SWS) Unit. The cooled tail gas from the quench tower is fed to the absorber 25. The absorption of the H.sub.2S contained in the tail gas is accomplished using a selective lean amine solution 26 coming from an amine regenerator 27. The rich amine solution 28 from the bottom of the absorber is pumped by means of the rich amine pumps 29 to a lean/rich amine heat exchanger 30, where the rich amine is preheated using as heating medium the hot lean amine from the bottom of the amine regenerator prior of being fed to amine regenerator 27 itself. The lean amine from the bottom of the amine regenerator is pumped by means of the lean amine pumps 31, is first cooled in the lean/rich amine heat exchanger 30 and then in the lean amine cooler 32 prior of being fed to the absorber 25. The acid gas 33 from the top of regenerator is recycled back to the Claus thermal reactor burner 4 in the traditional Claus Plant (FIG. 1), while it is recycled to Thermal Oxidative Cracking reaction chamber 42 in the novel H.sub.2S Thermal Oxidative Cracking Plant of the invention (FIG. 2).

(35) In the traditional Claus Plant (FIG. 1), the tail gas from the absorber 34 is sent to an incinerator burner 35, directly connected to an incinerator 36, where all residual sulphur compounds are oxidized to SO.sub.2. The combustion of the tail gas is supported with fuel gas combustion, therefore a fuel gas stream 37 and a combustion air stream 38 are also fed to the incinerator burner. The incinerator effluent (or flue gas) 40, after an adequate residence time in the thermal incinerator, is discharged into the atmosphere via a dedicated stack 39. In the novel H.sub.2S Thermal Oxidative Cracking Plant (FIG. 2), the hydrogen rich stream from absorber 34 is sent to users outside the Sulphur Recovery Unit.

(36) In a preferred embodiment, the hydrogen rich stream from the absorber, containing some amount of impurities such as N.sub.2, CO.sub.2, H.sub.2S, COS and CS.sub.2, is sent to a further Hydrogen Treatment section 43, where it is further purified. A substantially pure hydrogen stream 44 from Hydrogen Treatment section is finally sent to different end-users.

Example 1

(37) A possible plant configuration is the following. Acid gas and oxygen are fed to a reaction furnace where the oxidation reactions take place. The reaction products are partially quenched and enter a Waste Heat Boiler for the recovery of the reaction heat. After heat recovery, the process gas enters a Sulphur Condenser for the separation of the produced sulphur and for a further heat recovery.

(38) The completion of the reactions is obtained in a catalytic block, which consists of a process gas preheater, a catalytic reactor and a final sulphur condenser.

(39) From the final condenser the tail gas is sent to a traditional reductive Tail Gas Treatment.

(40) The selected conditions are the following:

(41) H.sub.2S/O.sub.2 ratio 4.4

(42) Adiabatic temperature 1200 C.

(43) The feedstock has been preheated to 240 C.

(44) Considering these operating conditions, the H.sub.2S conversion is 56%, where

(45) 15.8% is converted to H.sub.2 and S.sub.2 according to reaction 4)

(46) 39.9% is converted to H.sub.2O and S.sub.2 according to reaction 3)

(47) 0.3% is converted to SO.sub.2 and H.sub.2O according to reaction 1)

(48) while 44% of H.sub.2S remains unconverted.

(49) SO.sub.2 is a substantial hydrogen consumer in the Hydrogenation Reactor and in order to reduce it to very low concentration, a Claus catalytic reactor has been considered downstream the Waste Heat Boiler and the Sulphur Condenser.

(50) The tail gas, coming out from the final condenser, after preheating is fed to the Hydrogenation Reactor, where the sulphur vapors are transformed to H.sub.2S, COS and CS.sub.2 are hydrolyzed and CO is shifted to hydrogen.

(51) The reactions are the following:
S.sub.n+nH.sub.2.fwdarw.nH.sub.2S
COS+H.sub.2O.fwdarw.CO.sub.2+H.sub.2S
CS.sub.2+2H.sub.2O.fwdarw.CO.sub.2+2H.sub.2S
CO+H.sub.2O.fwdarw.CO.sub.2+H.sub.2

(52) The remaining small concentration of SO.sub.2 shall react as follows:
SO.sub.2+3H.sub.2.fwdarw.H.sub.2S+2H.sub.2O

(53) The tail gas coming from the Hydrogenation Reactor is cooled down in a Quench Tower where the water generated in the oxidation reactions is condensed.

(54) Finally, the cool gas is washed in an Amine Absorber. From the top of the Amine Absorber a hydrogen rich stream, containing impurities such as H2S, CO.sub.2 and N.sub.2, is released.

(55) The rich amine from the bottom of the Amine Absorber is sent to the Amine Regeneration section generating an H.sub.2S and CO.sub.2 stream, which is recycled to the reaction furnace.

(56) Therefore, the sulphur lost is only the H.sub.2S contained in the hydrogen stream leaving the Amine Absorber, so the sulphur recovery efficiency can be higher than 99.9%.

(57) The balance shows that from a feedstock containing 100 kmol of H.sub.2S, 30 kmol of hydrogen can be recovered, leading to a good saving in hydrogen consumption of the hydrotreating. It has to be noted that a traditional plant has instead a hydrogen consumption of about 1-2 kmol per 100 kmol of H.sub.2S for the Tail Gas hydrogenation step.

(58) Other differences between a traditional plant and a plant with hydrogen recovery can be noted by the comparison of the relevant heat and material balances. For this purpose the material balance of the two plant configurations has been carried out for a capacity of 100 T/D of sulphur product. The relevant process gas flow rates in crucial parts of the plant are shown in Table 1.

(59) TABLE-US-00001 TABLE 1 Process gas flow rates Traditional Plant H.sub.2 Recovery Plant Kg/h Kmol/h Kg/h Kmol/h Reaction Furnace 14425 481 9900 314 Final Condenser 10255 422 5729 256 Quench Tower Outlet 8087 304 3938 157 Absorber Outlet 7832 295 335 48

(60) Table 1 shows that the process gas flow rates of the hydrogen recovery plant are lower compared to the one of the traditional plant. Therefore, equipment sizes will be smaller and less expensive.