NL1006339C2 - Process for desulfurizing waste gases. - Google Patents

Description

Title: Method for desulfurizing waste gases

The invention relates to a method for desulfurizing waste gases containing a high water vapor content. More particularly, the invention includes a method for reducing the total sulfur content of waste gases from sulfur recovery plants.

The preparation of elemental sulfur from hydrogen sulfide (H2S) by partial oxidation thereof by oxygen or an oxygen-containing gas such as air, followed by reaction of the sulfur dioxide (SO2) formed from the hydrogen sulfide with the remainder of the hydrogen sulfide, in the presence of a catalyst, is known as the Claus process. This process is frequently used both at refineries and for the processing of hydrogen sulphide extracted 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. The latter steps form the so-called catalytic steps. In the combustion chamber, the incoming gas stream, rich in H2S, is burned with an amount of air at a temperature of approximately 1200 ° C. The amount of air is adjusted so that 1/3 of the H2S is burned to SO2 according to the reaction: 25 2H2S + 302 -> 2H20 + 2S02 (1)

After this partial combustion of H 2 S, the unreacted part of the H 2 S (ie about 2/3 of the amount offered) and the SO 2 formed reacts largely further according to the Claus reaction: 100 6339 2 4H2S + 2S02 <-> 4H20 + 3S2 (2)

Thus, in the thermal stage, about 60% of the H2S is converted into elemental sulfur. The gases coming out of the combustion chamber are cooled to about 160 ° C in a sulfur condenser, in which the sulfur formed condenses which then flows through a siphon into a sulfur pit. The uncondensed gases, in which the molar ratio of H 2 S: SO 2 is still 2: 1, are then warmed to about 250 ° C and passed through a first catalytic reactor, in which the equilibrium 4H, S + 2S0, <is again - »4H, 0 + - Sn set.

n

The gases coming from this catalytic reactor are then cooled again in a sulfur condenser, after which the liquid sulfur formed is recovered and the residual gases after reheating are passed to a second catalytic reactor.

Depending on the number of catalytic stages, the sulfur recovery percentage in a conventional Claus plant is 94-97%. So an amount of H2S and SO2 remains.

One of the main limitations of the Claus process is the increase in the water content in the process gas as the conversion of H2S to sulfur proceeds.

The Claus reaction is thermodynamically limited by this increase in the water vapor content and at the same time by a decrease in the H2S and SO2 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 overcome this limitation as much as possible. However, since the water dew point is far below the solidification point of sulfur, condensation of water vapor in the Claus process encounters insurmountable problems, such as clogging by the solidification of sulfur and corrosion due to sulfurous acid formation.

In the past, off-gas from the Claus process was burned in an afterburner; however, due to the 5 increased environmental requirements, this is no longer allowed.

This has led to improvements in the Claus process and the development of Claus waste gas removal processes. An improvement of the Claus process is known as the SUPERCLAUS * process, in which the efficiency of the Claus process is increased from 94-97% to more than 99%. The SUPERCLAUS * process is described in "SUPERCLAUS®11, the answer to Claus plant limitations" 1, publ. 38th Canadian Chem. Eng. Conference, October 25th, 1988, Edmonton, Alberta, Canada.

In the SUPERCLAUS * -99 process, reaction (2) is operated with excess H2S in the thermal stage and in the Claus reactors, so that the H2S content in the gas from the last Claus reactor is about 1 vol. % and the SO2 content is about 0.02% by volume. In a subsequent reactor stage 20, the H 2 S is selectively oxidized to elemental sulfur according to the reaction: 2 2 H 2 S + 2 2 H 2 O + - Sn 3) n 25 in the presence of a special selective oxidation catalyst. These catalysts are described, inter alia, in European patents 0242920 and 0409353.

As mentioned, increased environmental requirements have not only led to improvements in the Claus process, but also to developments in Claus tailgas processes, for the further desulphurisation of waste gas from sulfur recovery plants.

Most Claus tailgas processes use a hydrogenation reactor, also known as a reduction reactor, in which S02, hydrocarbon oxidesulfide (COS), carbon disulfide (CS2), sulfur vapor and possibly 1006339 4 entrained sulfur droplets (sulfur mist) are converted with hydrogen (H2) or a reducing gas, e.g. hydrogen and carbon monoxide, in hydrogen sulfide.

The hydrogen sulfur is then removed by absorption in a solution or by conversion to the gaseous phase using a catalyst to elemental sulfur.

Only a few tailgas processes have been developed that absorb S02 from chimney gas after the combustion of Claus tailgas. These processes are not discussed further. Of the Claus tail gas processes that absorb the H2S formed in a solution after hydrogenation, SCOT, BSR-Stretford, BSR-MDEA, Trencor-M and Sulften are the best known. These processes are described in a publication by B.G. Goar: "Tail Gas Clean-Up Processes, a 15 review11, presented at the 33rd Annual Gas Conditioning Conference, Norman, Oklahoma, March 7-9, 1983 and in Hydrocarbon Processing, February 1986.

The best known and most effective tail gas desulfurization process to date is the SCOT process 20 described in Maddox "Gas and liquid sweetening" (1977).

process achieves a sulfur recovery of 99.8 to 99.9%.

Of the tailgas processes which, after hydrogenation, convert the formed H2S into the gas phase with the aid of a catalyst, only a few processes have been built and become known, such as MODOP, CLINSULF, BSR-Selectox, Sulfrene, SUPERCLAUS-99.5. These processes are described in the aforementioned publication of B.G. Goar, in C&EN magazine of May 11, 1987, Sulfur magazine Jan / Feb 1995, and in DE-A 2648190.

In all these Claus tail gas processes, after the hydrogenation, the water which is formed in the Claus reaction (2) and in the selective oxidation reaction (3) is condensed, because the presence of water has a negative effect on the subsequent H2S removal in an absorption liquid or in the catalytic conversion of H 2 S to elemental sulfur. The absorption liquids used in the above processes are secondary or tertiary alkanolamine solutions such as Di-isopropanol amine (DIPA) or Methyl diethanol amine (MDEA) or complex Redox solutions. Without removal of water, the absorption process would be thoroughly disrupted, namely, either by the excessively high temperatures at which little or no absorption takes place, or because the water in the absorber condenses during the absorption and the circulating solution is continuously diluted, whereby 10 no more absorption can take place.

In the case of HZS conversion in the gas phase using a catalyst, the thermodynamic conversion of H 2 S according to the Claus reaction (2) is greatly reduced without water removal 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% cannot be achieved.

The use of a selective oxidation catalyst as used in the SUPERCLAUS process, although it gives a higher efficiency, but also with SUPERCLAUS-99.5 a total sulfur recovery efficiency of more than 99.5% has not been feasible in practice.

In general, it can be stated that the disadvantage of Claus tail gas processes, in which after hydrogenation the H2S in the gas phase is converted into elemental sulfur by means of a catalyst, is that the current requirements of a total sulfur recovery efficiency of more than 99.90 % cannot be reached.

Claus tail gas processes with hydrogenation followed by condensation of water after which the H2S is absorbed in an absorption liquid such as in the SCOT process can achieve more than 99.90% total sulfur recovery efficiencies, but have the major drawback that the investment costs and energy costs are enormous. be high. Newer versions of the SCOT process, such as SUPERSCOT and LS-SCOT, achieve a total 1006339 6 sulfur recovery efficiency of 99.95%, but are even more expensive.

Another drawback of these processes is that acidic hydrogen sulphide-containing condensate must be discharged and treated, for example, in a Sour Water Stripper, whereby the dissolved acid gas is separated with steam. This is also expensive.

Not only have the environmental requirements had an influence on the development of Claus and Claus tailgas processes, but also on the development of chimney gas processes, also known as flue gas processes, for power plants. Various processes for 1flue gas desulphurization '(FGD) are known, whereby S02 with lime milk is converted into gypsum (Ca2S04). Because a surplus of gypsum has arisen, processes have been sought in which SO2 can be converted into elemental sulfur. The Wellman Lord trial, described in Gas Purification, fourth edition 1985, A.L. Kohl, F.C. Riesenfeld, page 351-356, is one such example where SO2 20 is eventually released as a concentrated gas. After two thirds of the SO2 is converted to H2S in a hydrogenation step, the H2S and SO2 gas in a Claus plant can be converted to elemental sulfur. This process route is also expensive. Another development in this field is the biological desulphurisation of chimney gases.

Biological desulfurization of chimney gases, also referred to as flue gas, is described in the journal Air, issue 4, December 1994. The BIO-FGD process described therein is for removal of SO2 from chimney gas 30 of power plants and consists of an absorber where SO2 is dissolved in a dilute sodium hydroxide solution according to the reaction S02 + NaOH - »NaHS03 (4) 35 1006339 7

This solution is then treated in two biological reactor steps.

In the first biological step, in an anaerobic reactor, the sodium bisulfite (NaHSO 3) formed is converted into sodium sulfide (NaHS) with an electron donor.

NaHSOj + 3H2 -► NaHS + 3H20 (5)

Suitable electron donors are e.g. hydrogen, ethanol, hydrogen and glucose. In the second step, in an aerobic reactor, the sodium sulfide is oxidized to elemental sulfur which is separated.

NaHS + HOj NaOH + S (6)

Chimney gases contain a small amount of water vapor after burning coal or fuel oil. The water content is usually between 2-15 vol.%, Which corresponds to a water dew point of 20-55 ° C.

If the BIO-FGD process were to be used for desulphurisation of Claus off-gas which has been afterburned and where all sulfur components have been converted into SO2, the gas must be cooled because of the high water vapor content of the Claus off-gas. This is to prevent the water vapor from condensing in the caustic soda solution, which would result in a portion of the caustic soda solution having to be constantly discharged.

Claus waste gas must therefore be cooled to form acid condensate and must be removed.

This problem does not arise with desulphurisation of waste gas from a coal or oil-fired power plant, because the water dew point is below the working temperature in the absorber. Cooling of this waste gas can thus take place in a simple manner without condensation of water occurring.

A first object of the invention is to provide a method for desulphurizing waste gases with a high water vapor content of 20 to 40% by volume, whereby condensation of this water is not required, so that acid 100 6339 containing hydrogen sulphide-containing condensate is prevented and must be disposed of.

A second object of the invention is to provide a method in which the H2S formed after hydrogenation can be absorbed in an absorption liquid at a temperature above the dew point of water in the gas, so that no condensation of water occurs.

A further object of the invention is to provide a method in which a total sulfur recovery efficiency of more than 99.90% is achieved without the above drawbacks 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% by volume at a temperature above the water dew point, in an alkaline solution, after which the sulphide-containing solution is formed on a aerobic biological oxidation.

The invention therefore relates to a method for removing H2S from waste gases, which is at least 20 vol. % water vapor comprising treating the off-gases at a temperature above the water-dew point of the off-gases with an aqueous, alkaline solution while absorbing the H 2 S, followed by subjecting the formed sulfide-containing solution to a biological oxidation of the sulfide.

Surprisingly, it has now been found that it is possible to oxidize the H2S dissolved in the alkaline solution, preferably a sodium hydroxide solution, with air in a biological aerobic reactor 30 to elemental sulfur at a temperature which is preferably the same as that at which the absorption occurred.

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 take place at a temperature of at least 65 ° C, more in particular 1006339 9, at a temperature of 70 to 90 ° C . It is particularly surprising that it is possible to perform an efficient and good biological oxidation at such high temperatures.

In the process according to the invention, the total sulfur content of waste gases is reduced by first raising these waste gases in temperature to a temperature above 200 ° C and then together with a hydrogen and / or carbon monoxide-containing gas over a sulfided group 10 VI / group VIII metal catalyst on an inorganic oxidic support in which sulfur components such as SO2, sulfur vapor and sulfur mist are reacted with hydrogen or another reducing gas, e.g. contains hydrogen and carbon monoxide, to hydrogen sulphide according to the 15 reactions: S02 + 3H2 -> H2S + H20 (7) S + H2 -> H2S (8) 20

If oxygen is present in the waste gases, a catalyst from the above group is used, which also has the property of hydrogenating oxygen according to the reaction 25 02 + 2H2 -> 2H20 (9)

Preferably, a catalyst from the above group is used, which also has the property of hydrolyzing COS 30 and CS2 according to the reactions COS + H20 - »H2S + CO2 (10) CS2 + 2H20 2H2S + CO2 (11) 35 1006339 10

In the process according to the invention, the waste 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. Preferably 5 is cooled to 3 to 5 ° C above the dew point.

Waste gases, in particular waste gases from a Claus recovery installation, with a water vapor content of 20 to 40% by volume, have a dew point of between 60-80 ° C.

These off-gases are then directly contacted in an absorber 10 with a dilute alkaline solution, preferably caustic soda, with a pH between 8 and 9, the H2S present in the gas being dissolved according to the reaction: 15 H2S + NaOH -> NaHS + H20 (12)

The non-absorbed part of the aforementioned waste gases is discharged to the air, possibly after combustion.

Since the regenerated alkaline solution does not contain H 2 S 20, the H 2 S present in the off-gas is completely absorbed and a total sulfur recovery efficiency of more than 99.90% can be achieved in this way. In the method according to the invention, the solution is fed to the biological aerobic reactor at the same temperature, preferably at the same temperature at which absorption has taken place, so that no heat has to be removed or supplied. An amount of air is introduced into the aerobic reactor, such that the dissolved H2S with oxygen from the air is partially oxidized to elemental sulfur according to the reaction: H2S + H 02 -► S + H20 (13) 35 In a sulfur separator again preferably at the same temperature the sulfur is separated from the sodium hydroxide solution and the solution is recycled to the absorber. It is possible to cool the caustic soda solution with the H2S absorbed therein before it is fed to the biological aerobic reactor. However, after the sulfur separation, the solution is then reheated before being fed to the absorber.

10 FIGURE DESCRIPTION

The invention is now elucidated with reference to a figure 1, in which the method according to the invention is described in the form of a block diagram. The waste gas from a sulfur recovery installation (not shown) is brought via line 1 with the addition of hydrogen or another reducing gas via line 2 to the desired hydrogenation temperature with heater 3, before being led via line 4 to the hydrogenation reactor 5.

In the hydrogenation reactor 5, the sulfur dioxide, sulfur vapor and organic sulfur compounds present in the gas are converted into H2S with H2. If oxygen is present in the gas, it is converted into HaO. COS and CS2, if present, are converted into H2S and CO2 with the water vapor present.

The gas from the hydrogenation reactor 5 is brought via line 6 to the desired absorption temperature with cooler 7, before being led via line 8 into the absorber 9 of a bio-installation. In the absorber, H 2 S 30 is washed from the gas with a dilute caustic soda solution, which is then passed via line 10 to an aerobic biological reactor 11, in which H 2 S is converted to elemental sulfur with line oxygen added from the air via line 12. The sodium hydroxide solution is led via line 13 into a sulfur separator 14, from which the sulfur formed is discharged via line 15.

1006339 12

The solution is recycled through line 16 to the absorber. The gas from the absorber, which currently still contains a very low content of H2S, is led via line 17 to the afterburner 18 before the gas is removed via the chimney 19.

EXAMPLE 1

An amount of acid gas of 9700 Nm1 / h from a gas treatment plant had the following composition at 45 ° C and 1.6 bar abs

60.0 Vol.% H2S

3.0 Vol. % NH3 15 30.0 Vol.% C02 5.0 Vol. % H 2 O 2.0 Vol.% CH4

This acid gas was fed to a Claus plant with two Claus reactors. The sulfur formed in the sulfur recovery plant was condensed and discharged after the thermal and catalytic reactor steps. The amount of sulfur was 7,768 kg / h. The sulfur recovery efficiency of the Claus plant based on the sour gas was 93.3%.

The amount of waste gas of 29749 Nm1 / h from the Claus installation had the following composition at 164 ° C and a pressure of 1.14 bar abs.

1006339

0 0.47 Vol.% H2S

0.24 Vol.% SO2

0.03 Vol.% COS

0.04 Vol.% CS2 0.01 Vol.% S6

35 0.04 Vol.% SB

1.38 Vol.% CO

13 I, 53 Vol. % H2 II, 37 Vol.% CO2 55.96 Vol.% N2 0.66 Vol.% Ar 5 28.27 Vol.% H20

103 Nm3 / h hydrogen as reducing gas was fed to this waste gas and then heated to 280 ° C to hydrogenate all sulfur dioxide (SO2) and sulfur vapor (S6, Se) present to H2S and also the carbonyl sulfide (COS) and carbon sulfur ( CS2) to hydrolyze to H2S in the hydrogenation reactor containing a sulfided Group 6 and / or Group 8 metal catalyst, in this case a Co-Mo catalyst.

15

The amount of off-gas from the hydrogenation reactor was 31574 Nm3 / h and had the following composition at 317 ° C and 1.10 bar abs.

1.24 Vol.% H2S

28 ppm COS

2 ppm CS2 2.02 Vol.% H2 12.64 Vol.% C02 25 56.62 Vol.% N2 0.67 Vol.% Ar 26.80 Vol.% H20

The waste gas was then cooled to 72 ° C, a temperature 3 ° C above the dew point of the water vapor contained in the waste gas.

The cooled waste gas was then treated in a bio-installation at 72 ° C, with no water condensation from the waste gas. In the absorber 35 of the bio-installation, H2S is washed from the waste gas with dilute sodium hydroxide solution, after which the solution with the 1006339 14 absorbed H2S is fed to an aerobic biological reactor in which the H2S is converted into elemental sulfur.

No heat is added or removed in the bio-installation, so that the absorption of H2S and conversion to elemental sulfur took place at the same temperature of 72 ° C.

An amount of 945 NmVh of air was supplied to the aerobic reactor for the selective oxidation of H 2 S to sulfur. The amount of gas from the absorber was 31,389 Nm3 / h and had the following composition at 72 ° C and 1.05 bar abs.

2 50 ppm H2S

2 8 ppm COS

15 2 ppm CS2 2.04 Vol.% H2 12.80 Vol.% CO2 57.32 Vol.% N2 0.68 Vol.% Ar 20 27.13 Vol.% H 20

This gas was fed to the chimney through afterburning. The amount of sulfur formed in the bio plant was 551 kg / h. The total amount of sulfur produced in the sulfur recovery plant and the bio plant was 8319 kg / h, increasing the total desulfurization efficiency based on the original acid gas to 99.87%.

3 0 EXAMPLE 2

An amount of acid gas of 6481 Nm3 / h from a gas purification plant had the following composition at 45 ° C and 1.6 bar abs. 35 1006339 15

90.0 Vol. % H2S

3.0 Vol. % NH3 5.0 Vol.% H20 2.0 Vol. % CH4 5

This acid gas was fed to a SUPERCLAUS® installation with two Claus reactors and a selective oxidation reactor. The sulfur formed in the sulfur recovery plant was condensed and discharged after the thermal step and the 10 catalytic reactor steps. 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 exhaust gas of 21279 Nm3 / h from the 15 Claus installation had the following composition at 129 ° C and a pressure of 1.14 bar abs

0.03 Vol.% H2S

0.20 Vol.% SO2

20 20 ppm COS

3 0 ppm CS2 10 ppm S6 0.01 Vol.% S8

0.15% by volume of CO

25 1.72 Vol.% H2 1.14 Vol. % C02 62.45 Vol.% N2 0.74 Vol.% Ar 33.05 Vol.% H20 3 0 0.50 Vol.% 02

133 Nm3 / h hydrogen as reducing gas was fed to this waste gas and then heated to 280 ° C to hydrogenate all sulfur dioxide (SO2), sulfur vapor (S6, S8) present to H2S and H2O and also the carbonyl sulfide (COS) present and sulfurize carbon (CS2) 1006339 16 to H2S in the hydrogenation reactor containing 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 5 was 22863 Nm3 / h and had the following composition at 367 ° C and 1.10 bar abs.

0.37 Vol.% H2S

2 ppm COS

10 0.82 Vol.% H2 1.90 Vol.% CO2 62.89 Vol.% N2 0.75 Vol.% Ar 33.27 Vol.% H 2 O 15 The exhaust gas was then cooled to 76 ° C, a temperature which 3 ° C above the dew point of the water vapor present in the waste gas.

The cooled waste gas was then treated in a bio-installation at 76 ° C, with no water condensation from the waste gas taking place. In the absorber of the bio-installation, H 2 S is washed from the waste gas with a dilute sodium hydroxide solution, after which the solution with the absorbed H 2 S is passed to an aerobic biological reactor in which the H 2 S is converted into elemental sulfur.

No heat is added or removed in the bio-installation, so that the absorption of H2S and the conversion to elemental sulfur took place at the same temperature of 76 ° C. 205 30 NmVh of air was supplied to the aerobic reactor for the partial oxidation of H2S

to sulfur. The gas from the absorber was 22780 Nm3 / h and had the following composition at 76 ° C and 1.05 bar abs.

7 5 ppm H2S

35 2 ppm COS

0.82 Vol.% H2 1UU6öö9 17 1.91 Vol.% C02 63.12 Vol.% N2 0.75 Vol.% Ar 33.39 Vol.% H20 5

This gas was fed to the chimney through afterburning. The amount of sulfur formed in the bio plant was 119 kg / h. The total amount of sulfur produced in the sulfur recovery plant and the bio plant was 8346 kg / h, increasing the total desulfurization efficiency based on the original acid gas to 99.97%.

1006339

Claims (8)

A method for removing H2S from off-gases containing at least 20% by volume 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 H2S, followed by subjecting the formed sulfide-containing solution to a biological oxidation of the sulfide. The method of claim 1, wherein the absorption and oxidation occur at substantially the same temperature. A method according to claim 1 or 2, wherein the waste gases to be treated originate from a sulfur removal installation. The method of claim 3, wherein the off-gases are hydrogenated prior to absorption. A method according to claims 1-4, wherein the waste gases contain 20 to 40% by volume of water vapor. A method according to claims 1-5, wherein the sulfides in the aerobic biological oxidation are converted into elemental sulfur. A method according to claims 1-6, wherein the sulfur is separated from the liquid after the biological oxidation. 8. A method according to claim 7, wherein the liquid after recycling of the sulfur is recycled as an absorption liquid. 1006339