REMOVAL OF HYDROGEN SULFIDE AND SULFUR RECOVERY FROM A GAS STREAM BY CATALYTIC DIRECT OXIDATION AND CLAUS REACTION

Abstract

A process for the removal of hydrogen sulfide and sulfur recovery from a H.sub.2S-containing gas stream by catalytic direct oxidation and Claus reaction through two or more serially connected catalytic reactors, wherein a specific control of the oxygen supplement is operated. The control and improvement of the process is obtained by complementing, in each major step of the process, the H.sub.2S-containing gas stream by a suitable flow of oxygen, namely before the H.sub.2S-containing gas stream enters the Claus furnace, in the first reactor of the process and in the last reactor of the process. Especially in application in a SubDewPoint sulfur recovery process the H.sub.2S/SO.sub.2 ratio is kept constant also during switch-over of the reactors R1 and R by adding the last auxiliary oxygen containing gas directly upstream the last reactor R so that the H.sub.2S/SO.sub.2 ratio can follow the signal of the ADA within a few seconds.

Claims

1. A process for the removal of hydrogen sulfide (H.sub.2S) from a H.sub.2S-containing gas stream through two or more serially connected catalytic reactors, which process comprises: a) mixing the H.sub.2S-containing gas stream with a main oxygen-containing gas stream to obtain a gas stream containing both H.sub.2S and oxygen, b) introducing the obtained gas stream containing both H.sub.2S and oxygen into a furnace whereby a gas stream depleted in H.sub.2S is obtained, c) transferring the gas stream depleted in H.sub.2S to a sulfur condenser to obtain a gas stream depleted in sulfur, d) introducing the gas stream depleted in sulfur, optionally together with a first auxiliary oxygen-containing gas stream, into a first catalytic reactor R1 containing a catalyst system which catalyzes the Claus reaction of H.sub.2S with sulfur dioxide (SO.sub.2), the hydrolysis of COS and CS.sub.2 and optionally direct oxidation of H.sub.2S with oxygen to sulfur, said reactor being operated at a maximum temperature T.sup.R1.sub.max between 290 and 350° C., whereby a gas stream depleted in H.sub.2S is obtained, e) transferring the gas stream depleted in H.sub.2S to a sulfur condenser to obtain a gas stream depleted in sulfur, f) optionally introducing the gas stream depleted in sulfur obtained from reactor R1 through a series of reactors and condensers, preferably 1 or 2, each reactor containing a catalyst system which catalyzes the Claus reaction of H.sub.2S with sulfur dioxide (SO.sub.2) before reaching the last reactor R of the process, g) introducing the gas stream depleted in sulfur together with a last auxiliary oxygen-containing gas stream into the last catalytic reactor R containing a catalyst system which catalyzes the Claus reaction of H.sub.2S with sulfur dioxide (SO.sub.2) and the direct oxidation of H.sub.2S with oxygen to sulfur, said reactor being operated at a maximum temperature T.sup.R.sub.max below the maximum temperature T.sup.R1.sub.max of reactor R1, whereby a gas stream depleted in H.sub.2S is obtained, h) optionally transferring the gas stream depleted in H.sub.2S to a sulfur condenser to obtain a gas stream depleted in sulfur, i) measuring the volumetric ratio of H.sub.2S/SO.sub.2 at the exit of the last catalytic reactor R, wherein the flow rate of oxygen in the main oxygen-containing gas stream and in the optional auxiliary oxygen-containing gas streams represents 96 to 99.9 vol. % of the total flow rate of the oxygen supplemented in the process, preferably 98 to 99.8 vol. %, and more preferably 98.5 to 99.5 vol % the flow rate of oxygen in the last auxiliary oxygen-containing gas stream represents 0.1 to 4 vol. % of the total flow rate of the oxygen supplemented in the process, preferably 0.1 to 2 vol. %, and more preferably 0.5 to 1.5 vol. % and wherein the flow rate of oxygen in the last auxiliary oxygen-containing gas stream is adjusted depending on the value of the volumetric ratio of H.sub.2S/SO.sub.2 measured at the exit of the last catalytic reactor R in step i) so that the volumetric ratio of H.sub.2S/SO.sub.2 measured in step i) remains between 1.9 and 2.2.

2. Process according to claim 1, wherein the flow rate of oxygen in the last auxiliary oxygen-containing gas stream is increased when the value of the volumetric ratio of H.sub.2S/SO.sub.2 measured in step i) is above 2, and is decreased when the volumetric ratio of H.sub.2S/SO.sub.2 measured in step i) is below 2.0.

3. Process according to claim 1, wherein, in step a), the flow rate of oxygen in the main oxygen-containing gas stream and in the optional auxiliary oxygen-containing gas streams is calculated so that the volumetric ratio of H.sub.2S in the H.sub.2S-containing gas stream/O.sub.2 in the oxygen-containing gas stream be above the stoichiometric value of the reactions operated in the furnace of 2, in particular between 2.002 to 2.5, preferably 2.002 to 2.2, more preferably 2.002 to 2.08.

4. Process according to claim 3, wherein the volumetric ratio of H.sub.2S in the H.sub.2S-containing gas stream/O.sub.2 in the main oxygen-containing gas stream is maintained above the stoichiometric value of the reactions operated in the furnace of 2 during the whole process.

5. Process according to claim 1, wherein the furnace is operated at a temperature of 900° C. to 1400° C., more preferably 1100° C. to 1300° C.

6. Process according to claim 1, wherein the gas stream depleted in sulfur obtained in step c) further passes through a heater located between the condenser of step c) and the reactor R1 of step d).

7. Process according to claim 1, wherein the flow rate of oxygen in the first auxiliary oxygen-containing gas stream is adjusted to ensure that the maximum temperature T.sup.R1.sub.max in reactor R1 remains between 290 to 350° C., preferably 310 to 340° C., and more preferably 315 to 330° C.

8. Process according to claim 7, wherein the temperature T.sup.R1.sub.max in reactor R1 is maintained between 290 to 350° C., preferably 310 to 340° C., and more preferably 315 to 330° C. during the whole process.

9. Process according to claim 1, wherein the catalyst system of reactors R1 and R comprises at least one catalyst selected from titanium dioxide (TiO.sub.2), cobalt molybdenum, nickel molybdenum, iron and/or Al.sub.2O.sub.3, preferably titanium dioxide (TiO.sub.2).

10. Process according to claim 1, wherein the reactor R1 is composed of two catalytic sections: a first section containing a first catalyst suitable for direct oxidation of H.sub.2S and/or hydrolysis of COS and/or CS.sub.2, preferably titanium dioxide (TiO.sub.2), operated as an adiabatic bed without cooling at a maximum temperature T.sup.R1.sub.max, and a second section containing a second catalyst suitable for Claus reaction of H.sub.2S, preferably Al.sub.2O.sub.3, operating as a pseudo-isotherm bed with an internal heat exchanger where the outlet temperature T.sup.R1.sub.o is not higher and preferably lower than T.sup.R1.sub.max but is higher than the dew point of the sulfur.

11. Process according to claim 1, wherein the gas stream depleted in sulfur obtained in step e) further passes through a heater located between the condenser of step e) or f) and the reactor R of step g).

12. Process according to claim 1, wherein the volumetric ratio of H.sub.2S/SO.sub.2 at the exit of the last reactor R is maintained from 1.9 to 2.2 during the whole process.

13. Process according to claim 1, wherein the reactor R is composed of two catalytic sections: a first section containing a first catalyst suitable for direct oxidation of H.sub.2S, preferably titanium dioxide (TiO.sub.2), operated as an adiabatic bed without cooling at a maximum temperature T.sup.R.sub.max ranging from 180 to 240° C., preferably 190 to 210° C., and a second section containing a second catalyst suitable for Claus reaction of H.sub.2S, preferably Al.sub.2O.sub.3, operating as a pseudo-isotherm bed with an internal heat exchanger where the outlet temperature T.sup.R.sub.o is higher than water dew point and lower than sulfur dew point, preferably ranging from 105 to 140° C., and more preferably 110 to 125° C.

14. Process according to claim 1, wherein the operating conditions between the serially connected reactors are switched, and the gas flow is also switched so that the previous last reactor R is operated in the conditions of previous reactor R1, thus becoming new reactor R1, and the previous first reactor R1 is operated in the conditions of previous reactor R, thus becoming new reactor R.

15. Process according to claim 1, wherein the volumetric ratio of H.sub.2S/SO.sub.2 at the exit of the new last catalytic reactor R reaches the desired value between 1.9 and 2.2 within 1 seconds to 2 minutes during the whole process and in particular after the switch of the reactors by adjustment of the flow rate of the last auxiliary oxygen-containing gas stream.

16. Process according to claim 1, wherein the sulfur recovery efficiency is above 99 vol. %, more preferably above 99.5 vol %, and even more preferably up to 99.8 vol. % of H.sub.2S or above, based on the initial amount of H.sub.2S present in the H.sub.2S-containing gas stream treated.

17. Process according to claim 1, wherein if the oxygen demand in the last auxiliary oxygen-containing gas stream is higher than 2.5 vol. % of the total flow rate of the oxygen supplemented in the process, in particular from 2.8 to 4 vol. %, preferably from 3 to 3.6 vol. %, a signal is sent to the main oxygen-containing gas stream to increase the flow rate of oxygen in the main oxygen-containing gas stream in proportion.

18. Process according to claim 1, wherein if the oxygen demand in the last auxiliary oxygen-containing gas stream is lower than 1.5 vol. %, of the total flow rate of the oxygen supplemented in the process, in particular from 0.1 to 1.5 vol. %, preferably from 0.4 to 1.2 vol. %, a signal is sent to the main oxygen-containing gas stream to decrease the flow rate of oxygen in the main oxygen-containing gas stream in proportion.

Description

DETAILED DESCRIPTION OF THE DRAWINGS

[0134] Referring now to FIG. 1, a process involving the oxygen control according to the invention in a SMARTSULF™ preferred embodiment is illustrated.

[0135] The H.sub.2S-containing gas stream (line 1) is mixed with a main oxygen-containing gas stream (line 2) and introduced in a furnace (3) without catalyst.

[0136] The H.sub.2S flow rate in the feed gas is measured and the flow rate of the main oxygen-containing gas stream sent to the furnace is controlled in proportion to this value. The content of H.sub.2S in the feed gas is measured by an Analysis Indicator Control QIC (32 in FIG. 1) as well as the flow rate of the H.sub.2S-containing gas stream (not shown), which gives the flow rate of H.sub.2S introduced in the furnace. The flow rate of the main oxygen-containing gas stream sent to the furnace is controlled by the main air valve in proportion to the H.sub.2S flow rate.

SO2 is produced by the reaction 2H2S+3O2.fwdarw.2SO2+2H2O.

[0137] The stream of gas leaving the furnace thus contains SO.sub.2, remaining H.sub.2S, and impurities generated in the furnace such as COS, CS.sub.2 . . . .

[0138] The stream is cooled by passing through a condenser (4) where liquid sulfur is condensed and withdrawn (line 5), and the stream of gas is recovered at the top of the condenser (line 6) at a temperature of about 130° C. The sulfur removed corresponds to 50-70% of the sulfur present initially in the acid gases.

[0139] The recovered stream of gas is reheated in one or more heater (7) and optionally mixed with a first auxiliary oxygen-containing gas stream (through valve 36 and line 30) before entering the first reactor (8). This first reactor (8) is filled with titanium oxide or another suitable catalyst bed (9) which catalyzes both the Claus reaction of H.sub.2S with sulfur dioxide (SO.sub.2), the hydrolysis of COS and CS.sub.2 and optionally the direct oxidation of H.sub.2S with oxygen to sulfur. Usually the temperature of the first reactor (8) reaches 315 to 330° C. which is of particular interest to better achieve the hydrolysis of COS and CS.sub.2 which is improved at such high temperature.

[0140] The first auxiliary oxygen-containing gas stream sent to the first catalytic reactor (through valve 36 and line 30) is controlled with the maximal temperature reached in the reactor (350° C.) through the Temperature Indicator Control (TIC) device which controls the opening of valve (36). Indeed, residual H.sub.2S oxidizes with oxygen coming from line 30 when contacted with the TiO.sub.2 based catalyst in reactor (8). This reaction is exothermic and results in an increase of the reactor's (8) temperature. Sufficiently high temperatures can be obtained thus permitting COS and CS2 hydrolysis, and this is of particular interest if the heater (7) is unable to provide high enough temperature in a simple and economic manner.

[0141] The separation of the catalytic system of reactor (8) into two sections (SMARTSULF™ reactor) is of particular interest in this configuration. In this embodiment, the first adiabatic area (8A) of the reactor can be operated at high temperature (290-340° C.) to enhance previously said hydrolysis, and the second pseudo-isotherm area (8B) can be operated at much lower temperature (200-280° C.) to improve sulfur recovery rate through Claus reaction. An external or internal heat exchanger (thermoplates for example) ensures the cooling of the second area which behaves as pseudo-isotherm.

[0142] Depending on the maximal acceptable sulfur residual concentration, extra catalytic reactors can be added in order to decrease the H.sub.2S concentration in the treated vapor effluent (not shown on the figure).

[0143] The stream of gas leaving the first reactor (8) containing SO.sub.2 and remaining H.sub.2S is cooled by passing through a condenser (11) and a sulfur trap (12) where liquid sulfur is condensed and withdrawn (line 13), and the stream of gas is recovered at the top of the condenser (line 14) at a temperature of about 130° C. The sulfur removed corresponds to 80 to 95 vol. % of H.sub.2S, based on the initial amount of H.sub.2S present in the H.sub.2S-containing gas stream treated.

[0144] The recovered stream of gas is reheated in one or more heater (15) and mixed with a last auxiliary oxygen-containing gas stream (through valve 37 and line 31) before entering the last reactor. This last reactor (16) is filled with titanium oxide or another suitable catalyst bed (17) which catalyzes both the direct oxidation of H.sub.2S with oxygen to sulfur and the Claus reaction of H.sub.2S with sulfur dioxide (SO.sub.2).

[0145] The separation of the catalytic system of reactor (16) into two sections (SMARTSULF™) is of particular interest in this configuration. In this embodiment, the first adiabatic area (16A) of the reactor can be operated at a temperature ranging from 180 to 240° C., and the second pseudo-isotherm area (16B) can be operated at much lower temperature (105 to 140° C.) to improve sulfur recovery rate through Claus reaction. An external or internal heat exchanger (thermoplates for example) ensures the cooling of the second area which behaves as pseudo-isotherm.

[0146] The volumetric ratio of H.sub.2S/SO.sub.2 at the exit of the last reactor R is measured by well-known Air Demand Analyzers, also called ADA (33 in FIG. 1). Deviation from the stoichiometric value, i.e from the instruction range of H.sub.2S/SO.sub.2 volumetric ratio of 1.9 to 2.2, is rectified by a signal to the air valve (37) which will adjust the flow rate of the last auxiliary oxygen-containing gas.

[0147] Since the last auxiliary oxygen-containing gas stream is much smaller than the main oxygen-containing gas stream it can react a lot faster and thus allows a much more precise control of the H.sub.2S/SO.sub.2 ratio.

[0148] As previously indicated, the specific distribution and control of the oxygen supplemented in the claimed process improved the sulfur recovery rate of a conventional Claus unit substantially. Additionally, the long-term average values were also improved.

[0149] Downstream effluent (line 18) can be cooled by passing through a condenser where liquid sulfur (not shown) is condensed and then the effluent is withdrawn (line 22).

[0150] It is conventional to separate the sulfur which leaves the reactor in gaseous form in a downstream condenser. According to another configuration of the invention illustrated in FIG. 1, a common sulfur condenser can be used for each two reactors by using a multiway valve (21) being installed between a first reactor and the downstream sulfur condenser. This means that the installed sulfur condenser is always flowed through in the same direction regardless of the position of the reactors.

[0151] It is possible to easily regenerate the catalyst in the process of the invention. To do so, two 4 way valves (20-21) are connected to the entrance and the exit of both SMARTSULF™ reactors, and allow to switch the position of the reactors. There is in this configuration, most preferably a unique condenser (12) to collect liquid sulfur. The first reactor is working above the sulfur dew point and needs the condenser (12) to collect sulfur as liquid element. Then the last reactor is working at sub dew point, to be able to form sulfur from lower H.sub.2S and SO.sub.2 partial pressures. This last reactor accumulates liquid sulfur which condenses on the catalyst, thus after some time plugging the process. Liquid sulfur condensed on the catalyst needs to be evaporated (warm-up of the reactor) to allow the catalyst being fully regenerated. This step is done by switching the position of the two reactors together with the internal cooling. Directly after the switching the temperature of the previous last (cold) reactor is increased, allowing the liquid sulfur to evaporate and to be further recovered after being cooled down in the condenser (12).

[0152] Referring now to FIG. 2, a process involving the oxygen control according to the invention in a classic Claus Unit (without SMARTSULF™ reactor) is illustrated.

[0153] In this embodiment, the difference with FIG. 1 is that both catalytic reactors (8) and (16) are adiabatic and contain no internal heat exchanger to control the temperature. The temperature in the first reactor (8) is therefore between 290-350° C. at the outlet, and the temperature in the last reactor (16) is between 180-240° C. at the outlet.

[0154] In addition, in this embodiment, it is not possible to regenerate the catalyst in the process of the invention since no 4-way valves (20-21) are connected to the entrance and the exit of the reactors. Therefore, the reactors should not operate below the dew point of elementary sulfur to avoid sulfur condensation on the catalyst and thus, plugging of the whole process.

[0155] The control of the oxygen distribution in this classic Claus unit provides better desulfurization than what would be obtained in the same unit without oxygen supplement.

[0156] However, the process of the invention is operated in the best conditions in the preferred embodiment illustrated in FIG. 1, thus maximizing the H.sub.2S removal.