Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/46—Removing components of defined structure
  • B01D53/48—Sulfur compounds
  • B01D53/52—Hydrogen sulfide
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
  • B01D53/84—Biological processes
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
  • Y02A50/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
  • Y02A50/20—Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters

Definitions

• This invention relates to a method for desulfurizing off-gases which contain a high water vapor content. More specifically, the invention comprises a method for reducing the total sulfur content of off- ⁇ gases from sulfur recovery plants.
• the preparation of elemental sulfur from hydrogen sulfide (H 2 S) by partial oxidation thereof by means of oxygen or an oxygen-containing gas such as air, followed by reaction of the sulfur dioxide (S0 2 ) formed from the hydrogen sulfide, with the residual part of the hydrogen sulfide, in the presence of a catalyst, is known as the Claus process.
• This process is frequently employed both in refineries and for the processing of hydrogen sulfide recovered from natural gas.
• a conventional Claus plant consists of a burner with a combustion chamber, the so-called thermal stage, followed by a number of - generally two or three - reactors which are filled with a catalyst. These last stages constitute the so- called catalytic stages.
• the incoming, H 2 S-rich gas stream is combusted with an amount of air at a temperature of about 1200°C.
• the amount of air is set such that one-third of the H 2 S is combusted to S0 2 according to the reaction:
• H 2 S is converted to elemental sulfur.
• the gases coming from the combustion chamber are cooled to about 160°C in a sulfur condenser, in which the sulfur formed condenses, which subsequently flows via a siphon into a sulfur sink.
• the uncondensed gases, in which the molar ratio of H 2 S to S0 2 is still 2 : 1, are subsequently heated to about 250 °C and passed through a first catalytic reactor, in which again the
• the gases coming from this catalytic reactor are subsequently cooled again in a sulfur condenser, whereafter the liquid sulfur formed is recovered and the residual gases, after re-heating, are passed to a second catalytic reactor.
• the Claus reaction is thermodynamically limited by this increase of the water vapor content and simultaneously by the decrease of the H 2 S and S0 2 concentration, with the result that the equilibrium of the Claus reaction (2) shifts to the left. Condensation of the water vapor in the process gas would be desirable to remove this limitation as much as possible. However, since the water dew point lies far below the solidification point of sulfur, condensation of water vapor in the Claus process meets with insurmountable problems, such as clogging due to the solidification of sulfur and corrosion due to the formation of sulfurous acid.
• Claus tailgas processes utilize a hydrogenation reactor, also referred to as reduction reactor, in which S0 2 , carbonyl sulfide (COS) , carbon disulfide (CS 2 ) , sulfur vapor and any entrained sulfur droplets (sulfur mist) are converted with hydrogen (H 2 ) or a reducing gas, which contains, for instance, hydrogen and carbon monoxide, to hydrogen sulfide.
• S0 2 carbonyl sulfide
• CS 2 carbon disulfide
• sulfur vapor and any entrained sulfur droplets sulfur mist
• the hydrogen sulfide is then removed by absorption in a solution or by conversion in the gas phase to elemental sulfur, using a catalyst.
• tailgas processes which, after hydrogenation, convert the resultant H 2 S in the gas phase using a catalyst
• only a few processes have been built and become known, such as MODOP, CLINSULF, BSR-Selectox, Sulfreen, SUPERCLAUS- 99.5.
• MODOP MODOP
• CLINSULF BSR-Selectox
• Sulfreen SUPERCLAUS- 99.5.
• the water formed in the Claus reaction (2) and in the selective oxidation reaction (3) is condensed, because the presence of water has an adverse effect on the subsequent H 2 S removal in an absorption liquid or in the catalytic conversion of H 2 S to elemental sulfur.
• the absorption liquids used in the above-mentioned processes are secondary or tertiary alkanolamine solutions such as Diisopropanolamine (DIPA) or Methyldiethanolamine (MDEA) or complex Redox solutions.
• thermodynamic conversion of H 2 S according to the Claus reaction (2) is strongly reduced and a situation is obtained comparable to that in the last reactor stage in the Claus process, so that a total sulfur recovery efficiency of more than 99.5% is impossible to achieve.
• H 2 S and S0 2 gas can be converted to elemental sulfur in a Claus plant. This process route, too, is costly. Another development in this field is the biological desulfurization of flue gases.
• BIO-FGD process for removing S0 2 from chimney gas from power stations and consists of an absorber where S0 2 is dissolved in a diluted sodium hydroxide solution according to the reaction
• This solution is subsequently treated in two biological reactor stages.
• the sodium bisulfite (NaHS0 3 ) formed is converted with an electron donor to sodium sulfide (NaHS) .
• Suitable electron donors are, e.g., hydrogen, ethanol, hydrogen and glucose.
• the sodium sulfide is oxidized to elemental sulfur, which is separated.
• Chimney gases contain, after combustion of coal or fuel oil, a slight amount of water vapor.
• the water content is typically between 2-15 vol.%, which corresponds to a water dew point of 20-55°C.
• BIO-FGD process were used for desulfurization of Claus off-gas which has been afterburnt and whereby all sulfur components have been converted to S0 2 , the gas must be cooled because of the high water vapor content of the Claus off-gas. This is done to prevent the water vapor from condensing in the sodium hydroxide solution, as a result of which a part of the sodium hydroxide solution would constantly have to be discharged.
• a first object of the invention is to provide a method for desulfurizing off-gases with a high water vapor content of 20 to 40 vol . % and in which condensation of this water is not necessary, thereby preventing the formation of acidic hydrogen sulfide-containing condensate which must then be discharged.
• a second object of the invention is to provide a method in which the H 2 S formed upon hydrogenation can be absorbed in an absorption liquid at a temperature above the dew point of water in the gas, so that also during the absorption of H 2 S no condensation of water occurs.
• a next object of the invention is to provide a method whereby a total sulfur recovery efficiency of more than 99.90% is achieved without the above-mentioned disadvantages occurring.
• the invention is based on the surprising insight that it is possible to absorb H 2 S from such a gas with a water content of 20 to 40 vol . % at a temperature above the water dew point, in an alkaline solution, whereafter the sulfide- containing solution formed is subjected to an aerobic biological oxidation.
• the invention accordingly relates to a method for removing H 2 S from off-gases which contain at least 20 vol . % of water vapor, comprising treating the off-gases at a temperature above the water dew point of the off-gases with an aqueous, alkaline solution, under absorption of the H 2 S, followed by subjecting the sulfide-containing solution formed to a biological oxidation of the sulfide.
• the H 2 S dissolved in the alkaline solution preferably a sodium hydroxide solution
• the alkaline solution preferably a sodium hydroxide solution
• Such gases with a water content of 20-40 vol . % have a water dew point of 60-80°C, which means that in practice the biological oxidation will occur at a temperature of at least 65°C, more specifically at a temperature of 70 to 90°C. It is particularly surprising that it is possible to carry out an efficient and proper biological oxidation at such high temperatures.
• the total sulfur content of off-gases is reduced by first raising these off-gases in temperature to a temperature above 200°C and subsequently passing them together with a hydrogen and/or carbon monoxide-containing gas over a sulfided group VI/group VIII metal catalyst on an inorganic oxidic support, whereby sulfur components such as S0 2 , sulfur vapor and sulfur mist are converted with hydrogen or another reducing gas which contains, for instance, hydrogen and carbon monoxide, to hydrogen sulfide, according to the reactions:
• a catalyst from the above group which further has the property of hydrolyzing COS and CS 2 according to the reactions
• the off-gases from the hydrogenation reactor are cooled to just above the dew point of the water vapor present in the gas, such that no condensation occurs.
• cooling proceeds to 3 to 5°C above the dew point .
• Off-gases specifically off-gases from a Claus recovery plant, with a water vapor content of 20 to 40 vol.%, have a dew point between 60-80°C.
• these off-gases are subsequently contacted directly with a diluted alkaline solution, preferably sodium hydroxide solution, with a pH between 8 and 9, whereby the H 2 S present in the gas is dissolved according to the reaction:
• a diluted alkaline solution preferably sodium hydroxide solution
• the non-absorbed part of the off-gases mentioned is, optionally after combustion, discharged to the air.
• the H 2 S present in the off-gases is completely absorbed and in this manner a total sulfur recovery efficiency of more than 99.90% can be achieved.
• the solution is passed to the biological aerobic reactor at the same temperature, preferably at the same temperature as that at which absorption has taken place, so that no heat needs to be removed or supplied.
• an amount of air is supplied, such that the dissolved H 2 S is partially oxidized with oxygen from the air, to form elemental sulfur according to the reaction:
• the sulfur is separated from the sodium hydroxide solution, whereafter the solution is recirculated to the absorber. It is possible to cool the sodium hydroxide solution having the H 2 S absorbed therein before it is fed to the biological aerobic reactor. After the sulfur separation, however, the solution is then heated again before it is supplied to the absorber.
• Fig. 1 a general process diagram is represented.
• the off-gas of a sulfur recovery plant is passed via line 1, with addition of hydrogen or another reducing gas via line 2, and adjusted to the desired hydrogenation temperature with heater 3, before being passed via line 4 into the hydrogenation reactor 5.
• the sulfur dioxide, sulfur vapor and organic sulfur compounds present in the gas are converted with H 2 to H 2 S . If oxygen is present in the gas, it is converted to H 2 0. COS and CS 2 , if present, are converted with the water vapor present , to H 2 S and C0 2 .
• the gas from the hydrogenation reactor 5 is adjusted via line 6 to the desired absorption temperature with cooler 7, before being passed via line 8 into the absorber 9 of a bioplant.
• H 2 S is washed from the gas with a diluted sodium hydroxide solution, which is subsequently passed via line 10 to an aerobic biological reactor 11, in which H 2 S, with addition of oxygen from the air supplied via line 12, is converted to elemental sulfur.
• the sodium hydroxide solution is passed into a sulfur separator 14, from which the sulfur formed is discharged via line 15.
• the solution is recirculated via line 16 to the absorber.
• the gas from the absorber which now contains only a very low content of H 2 S, is passed via line 17 to the afterburner 18 before the gas is discharged via the chimney 19.
• Fig. 2 a diagram is given for a plant according to the invention, in which off-gas from a Claus plant with a high H 2 S/S0 2 ratio is absorbed directly, without intermediate hydrogenation.
• Off-gas coming from a three-stage Claus plant 100 is added via line 101 to absorber 102.
• the Claus plant 100 is operated such that the molar H 2 S/S0 2 ratio is at least 100.
• H 2 S is washed from the gas with a diluted sodium hydroxide solution, which is subsequently passed via line 103 to an aerobic biological reactor 104, in which H 2 S, with addition of oxygen from the air supplied via line 105, is converted to elemental sulfur.
• a portion of the sodium hydroxide solution is passed into a sulfur separator 109, from which the sulfur formed is discharged via line 110.
• the solution is recirculated via lines 111 and 112 to the absorber, with a small discharge via line 113.
• the gas from the absorber which now contains only a very low content of H 2 S, is passed via line 114 to an afterburner, not drawn, before the gas is discharged via a chimney, also not drawn.
• This sour gas was fed to a Claus plant with two Claus reactors.
• the sulfur formed in the sulfur recovery plant was, after the thermal stage and the catalytic reactor stages, condensed and discharged.
• the amount of sulfur was 7768 kg/h.
• the sulfur recovery efficiency of the Claus plant, based on the sour gas, was 93.3%.
• the amount of off-gas of 29749 Nm 3 /h coming from the Claus plant had the following composition at 164 °C and a pressure of 1.14 bar abs.
• This off-gas was supplied with 103 Nm 3 /h of hydrogen as reducing gas and then heated to 280°C to hydrogenate all sulfur dioxide (S0 2 ) and sulfur vapor (S 6 , S 8 ) present to H 2 S, and further to hydrolyze carbonyl sulfide (COS) and carbon sulfide (CS 2 ) to H 2 S in the hydrogenation reactor which contains a sulfided group 6 and/or group 8 metal catalyst, in this case a Co-Mo catalyst.
• COS carbonyl sulfide
• CS 2 carbon sulfide
• the amount of off-gas from the hydrogenation reactor was 31574 Nm 3 /h and had the following composition at 317°C and 1.10 bar abs. 1.24 Vol.% H 2 S
• the off-gas was then cooled to 72°C, a temperature which is 3°C above the dew point of the water vapor present in the off-gas.
• H 2 S is washed from the off-gas with diluted sodium hydroxide solution, whereafter the solution with the absorbed H 2 S was passed to an aerobic biological reactor in which the H 2 S was converted to elemental sulfur.
• the bioplant no heat is supplied or removed, so that the absorption of H 2 S and the conversion to elemental sulfur occurred at the same temperature of 72 °C.
• This sour gas was supplied to a SUPERCLAUS® plant with two Claus reactors and a selective oxidation reactor.
• the sulfur formed in the sulfur recovery plant was, after the thermal stage and the catalytic reactor stages, condensed and discharged.
• the amount of sulfur was 8227 kg/h.
• the sulfur recovery efficiency of the Claus plant, based on the sour gas, was 98.5%.
• the amount of off-gas of 21279 Nm 3 /h coming from the Claus plant had the following composition at 129°C and a pressure of 1.14 bar abs
• This off-gas was supplied with 133 Nm 3 /h of hydrogen as reducing gas and then heated to 280°C to hydrogenate all sulfur dioxide (S0 2 ) , sulfur vapor (S 6 , S 8 ) present to H 2 S and H 2 0, and further to hydrolyze the carbonyl sulfide (COS) and carbon sulfide (CS 2 ) to H 2 S in the hydrogenation reactor which contains a sulfided group 6 and/or group 8 metal catalyst, in this case a Co-Mo catalyst.
• the amount of off-gas from the hydrogenation reactor was 22863 Nm 3 /h and had the following composition at 367°C and 1.10 bar abs.
• the off-gas was then cooled to 76°C, a temperature which is 3°C above the dew point of the water vapor present in the off-gas . Then the cooled off-gas was treated in a bioplant at 76°C, with no water condensation from the off-gas taking place.
• H 2 S is washed from the off-gas with a diluted sodium hydroxide solution, whereafter the solution with the absorbed H 2 S was passed to an aerobic biological reactor in which the H 2 S was converted to elemental sulfur.
• no heat is supplied or removed, so that the absorption of H 2 S and the conversion to elemental sulfur occurred at the same temperature of 76°C.
• the aerobic reactor was supplied with an amount of 205 Nm 3 /h of air for the partial oxidation of H 2 S to sulfur.
• the gas from the absorber was 22780 Nm 3 /h and had the following composition at 76°C and 1.05 bar abs.
• this gas was passed to the chimney.
• the amount of sulfur formed in the bioplant was 119 kg/h.
• the total amount of sulfur produced in the sulfur recovery plant and the bioplant was 8346 kg/h, which raised the total desulfurization efficiency, based on the original sour gas, to 99.97%.
• An amount of sour gas of 3500 Nm 3 /h coming from a gas purification plant had the following composition at 40 °C and 1.7 bar abs .
• This sour gas was supplied to a Claus plant with three Claus reactors.
• the air supply to this Claus plant was set such that the reaction (2) in the thermal stage and in the Claus reactors was operated with excess H 2 S, so that the H 2 S : S0 2 content after the third reactor stage is greater than 100 to 1 , so that the S0 2 content became less than 0.009 vol . % .
• the sulfur formed in the sulfur recovery plant was, after the thermal stage and the catalytic reactor stages, condensed and discharged.
• the amount of sulfur was 4239 kg/h.
• the sulfur recovery efficiency of the Claus plant, based on the sour gas, was 96.4%.
• the amount of off-gas of 10001 Nm 3 /h coming from the Claus plant had the following composition at 130°C and a pressure of 1.15 bar abs.
• the off-gas was then cooled to 78°C, a temperature which is 3°C above the dew point of the water vapor present in the off-gas. Then the cooled off-gas was treated in a bioplant at 73 °C, with no water condensation from the off-gas taking place.
• H 2 S is washed from the off-gas with diluted sodium hydroxide solution, whereafter the solution with the absorbed H 2 S was passed to an aerobic biological reactor in which the H 2 S was converted to elemental sulfur. In the bioplant, no heat is supplied or removed, so that the absorption of H 2 S and conversion to elemental sulfur occurred at the same temperature of 73 °C.
• this gas was passed to the chimney.
• the amount of sulfur formed in the bioplant was 156 kg/h.
• the total amount of sulfur produced in the sulfur recovery plant and the bioplant was 4395 kg/h, which raised the total desulfurization efficiency, based on the original sour gas, to 99.93%.
• the small amount of S0 2 was converted to sulfate in the lye solution.
• a small amount of 85 kg/h of the lye solution was discharged and replaced with a corresponding amount.

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