FIELD OF THE INVENTION

This invention relates to a process for producing sulfur from acid gases which contain hydrogen sulfide and other sulfur compounds, in which the hydrogen sulfide is partly burnt to produce sulfur and sulfur dioxide, the hydrogen sulfide contained in the resulting process gas is catalytically reacted with sulfur dioxide at low temperature to produce sulfur, the sulfur is condensed (Claus process), the exhaust gas is purified with recovery of sulfur in that the exhaust gas is caused to flow over active adsorbents at a still lower temperature, and the laden adsorbent is desorbed with hot gases, wherein process gas is used as a gas for desorbing the laden adsorbent and is conducted in a closed cycle, and the residual sulfur and the residual hydrogen sulfide are afterburned with oxygen and reacted to produce sulfur dioxide.

BACKGROUND OF THE INVENTION

It is known that hydrogen sulfide contained in gas mixtures can be catalytically reacted with sulfur dioxide in the Claus process to produce sulfur (Opened German Specification-Offenlegungsschrift-DT-OS 2,253,806corresponding to U.S. Pat. No. 3,896,215).

It is also known to cause the resulting exhaust gases to flow over an activated-carbon adsorbent at elevated temperatures to form elementary sulfur. The laden activated carbon can be regenerated by a treatment at temperatures of 350° to 550° C with an inert gas which is virtually free from oxygen, carbon dioxide, and water (German Pat. No. 1,667,636).

It has also been suggested to use alumina, which may be impregnated, if desired, or mixtures of alumina and silica, rather than activated carbon as an adsorbent. In that case the time of contact of the exhaust gas to be desulfurized is maintained between 1 and 25 seconds and the adsorbent is subsequently regenerated by a treatment with oxygen-free gas at temperature between 200° and 350° C (Opened German Specification (Offenlegungsschrift) DT-OS 2,319,532).

A process of lowering the content of combined sulfur in Claus process exhaust gases has been proposed in which the Claus process exhaust gases are cooled and the sulfur is condensed, the resulting gas, which is substantially free from elementary sulfur, is catalytically reacted to produce sulfur, the flow of gas into the catalyst bed is stopped when the degree of conversion has decreased, and a hydrogen-sulfide-containing gas at temperatures above the dewpoint of sulfur is passed through the catalyst bed to evaporate elementary sulfur from said bed until the activity of the catalyst has been restored (Opened German Specification Offenlegungsschrift - DT-OS 2,021,111).

Claus process exhaust gas or another H₂ S-containing gas is used as a desorption gas in that process, in which the H₂ S contents are relatively high and may be as high as 50% by volume. That process requires a special control of the gaseous constituents to be reacted and an exact supervision.

Specifically, the separate heating of the regenerating gas creates high energy requirements. Another disadvantage is that the recycling of the H₂ S-reducing gas to the Claus process plant requires a high pressure and a separate gas blower.

Those known processes must be carried out in a plurality of stages, as a rule. For instance, the process according to German Pat. No. 1,667,636 comprises a first stage in which sulfur is formed in the presence of activated carbon, and a succeeding stage, in which the H₂ S-containing gas, to which oxygen-containing gas has been added, is reacted to produce sulfur also in the presence of activated carbon.

In the process according to Opened German Specification No. 2,319,532 the regenerated adsorbent is purged at a temperature below 180° C with oxygen-free gas and the purge gas contains added water vapor at least during part of the purging operation.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a simple method in which sulfur can be recovered by the Claus process and, in the same operation, the remaining exhaust gases can be purified to such a high degree that they can be discharged into the atmosphere without hesitation.

It is another object to provide a process of this type which enables a recovery of elemental sulfur such that virtually all sulfur compounds contained in the starting material have been reacted to form elemental sulfur.

Still another object is to provide such a process which is economical and, contrary to the known processes, requires only a low energy consumption.

SUMMARY OF THE INVENTION

These objects are accomplished according to the invention by a combination of steps which we have found to be vital for the improved energy balance and efficiency, namely:

a. the heat required for the desorption of the laden adsorbent is recovered by subjecting the desorption gas to heat exchange with the exhaust gas which is heated by the heat of reaction generated by reacting the residual sulfur and residual hydrogen sulfide with oxygen in an afterburner,

b. the sulfur is separated from the desorption gas in that the latter is cooled by the cooling system used for the preceding Claus process (i.e. in the sulfur condenser thereof) and with generation of wwater vapor,

c. the water vapor is condensed, and

d. the condensate is fed by gravity to the sulfur-condensing stage and is used therein to condense the sulfur.

Advantageously, the amount of hydrogen-sulfide-containing acid gases added to the process gas used for the desorption of the laden adsorbent may be so large that the H₂ S content of the process gas amounts preferably to 5-10% by volume.

The advantages afforded by the invention reside particularly in that the process is highly economical and has only a low energy consumption and that a perfectly pure exhaust gas is obtained which can be discharged into the atmosphere without hesitation. The sulfur yield is almost 100%.

The process permits of a matching of the dimensions of the Claus process equipment and of the exhaust gas purifier so that one step can be matched to the other and a fully continuous operation is achieved.

According to a preferred feature of the invention the amount of H₂ S added to the process gas used for the desorption of the laden adsorbent is so large that the H₂ S content is between approximately 5% and 10% by volume. This feature affords the advantage that the sulfate content of the catalyst can be limited so that the catalyst preserves its activity for a long time.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE of the drawing is a flow diagram of a plant according to the invention.

SPECIFIC DESCRIPTION AND EXAMPLE

Acid constituents coming from a scrubber are reacted in a Claus process combustion chamber 1 with atmospheric oxygen, which is fed by a blower 2. The resulting heat of reaction is used to generate high-pressure steam in a waste-heat boiler 3, which succeeds the combustion chamber. Depending on its COS and CS₂ contents, the process gas is cooled to 260°-300° C and is then fed to a first Claus process reactor 4.

The temperature is automatically controlled by means of hot gas coming from the combustion chamber under an automatic control effected by suitable bypass valves. On a suitable catalyst, the H₂ S is then further reacted with SO₂ to produce elementary sulfur. COS and CS₂ are hydrolyzed and in dependence on the H₂ S content of the process gas are partly hydrogenated.

The process gas leaving the Claus process reactor 4 is passed through the tubes of a shell-and-tube heat exchanger 5 and then through a sulfur condenser 6 and is thus cooled to temperatures below 165° C to remove the sulfur which is contained in the gas. The waste heat is utilized to generate medium-pressure steam.

The process gas which has left the sulfur condenser 6 is passed through a sulfur separator 7 and is then passed through the shell passage of the heat exchanger 5 and is thus heated to the operating temperature (about 200°-220° C) of a second Claus process reactor 8. The temperature is automatically controlled by a bypass arrangement. In the second Claus process reactor 8, the H₂ S is further reacted with SO₂ to produce sulfur at a temperature which is within a range in which the conversion of COS to CS₂ is negligible.

Because the reaction of H₂ S with SO₂ to produce S is an exothermic process, a decrease in temperature will result in an equilibrium ratio which is more favorable for the formation of S. For this reason the second reactor is operated only a few degrees above the dewpoint of sulfur so that a satisfactory continuous operation is ensured. In this way a sulfur recovery up to 95%, depending on the H₂ S content of the acid gas, can be accomplished in continuous operation.

The process gas leaving the second reactor 8 is fed to another sulfur condenser 9 and is cooled therein below 135° C but not below 120° C. The sulfur contained in the process gas is thus precipitated down to a content below 1 gram per standard cubic meter.

This sulfur condenser 9 is also succeeded by a sulfur separator 10, which is designed like the unit 7. The process gas which leaves the separator 10 is fed at a temperature of about 135° C to the one of the reactors 11 and 12 which is then in operation. In that reactor, a further Claus process reaction is effected in the sulfur-condensing temperature range.

That reactor contains impregnated alumina as an active adsorbent.

At the prevailing temperatures (about 135° C), H₂ S and SO₂ react to form sulfur and a sulfur recovery up to 99% can be accomplished in continuous operation.

Because the sulfur formed in the reactor 11 or 12 is adsorbed by the catalyst, the latter must be thermally regenerated when it has been laden to a certain degree.

The process gas leaving the reactor 11 or 12 is fed at a temperature of about 135° C to an afterburner chamber 13, in which the residual contents of H₂ S, COS, CS₂, S, H₂, and CO are completely oxidized with a suitable excess of air. The regulations for the prevention of air pollution require the operation of the afterburner chamber at temperatures between 600° and 800° C and an H₂ S content not in excess of 10 milligrams per standard cubic meter in the exhaust gas. These temperatures are ensured by a burning of fuel gas.

The gas which leaves the afterburner chamber 13 is quenched with cold air (or passed through another waste-heat boiler, which is not shown) and is then fed through a preheater 14 for the regenerating gas to a chimney 15 for discharge into the atmosphere.

The laden adsorbent is regenerated in two steps consisting of desorption and subsequent cooling.

Desorption is accomplished with an oxygen-free desorption gas, which is circulated in a closed cycle. This is connected by a breather line A to the process gas conduit leading to the reactor 11 or 12.

A gas blower 16 is operated to feed the desorption gas over the gas heater 14, where the desorption gas is heated to an automatically controlled temperature of about 300° C. The gas is then fed through a three-way valve to the laden reactor 11 or 12. During its passage through the catalyst bed, the desorption gas becomes saturated with sulfur in a degree which depends on the sensible heat that is available. The gas then leaves the reactor through a three-way valve IV.

The desorption gas is cooled in the sulfur condenser 9 below 135° C but not below 120° C and is thus freed from sulfur. The gas is then passed through a sulfur separator 17, which is designed like the separator 7 and 10, and is then fed back to the gas blower 16.

Desorption is considered as terminated when sulfur no longer flows from the sulfur condenser 9 and the sulfur separator 17. The need for visual supervision can be eliminated if a schedule is prepared on the basis of empirical data.

To lower the sulfate content of the catalyst, H₂ S from the acid gas is added to the desorption gas in such an amount that an H₂ S content of, e.g. 5-10% by volume is ensured during the desorption. Cooling is effected when the desorption has been completed. The cooling gas consists of a partial stream of the purified gas which leaves the reactor 11 or 12 that is then in operation. That gas is at a temperature of about 135° C.

Cooling gas at a suitable rate controlled by valve V or VI is fed through the reactor 11 or 12 which is to be cooled . That gas flows through the three-way valve IV, the sulfur condenser 9 and subsequently through the separator 17 and is fed by the gas blower 16 over the gas heater 16 and through the three-way valve I to the gas to be fed to the afterburner chamber 13. Cooling is continued until the reactor 11 or 12 which is then in operation is disconnected from the process gas stream.

Before the cooling gas is fed through the desorbed catalyst of the reactor 11 or 12 at the beginning of the cooling period, the desorption gas having a high H₂ S content is blown out through the conduit B by means of process gas which is withdrawn through breather line A from the process gas to be fed to reactor 11 or 12 and is returned to the process gas stream to be fed to reactor 11 or 12. The heats of reaction and desorption which become available in the sulfur condenser 9 are converted to saturated steam at 2 bars, which is condensed in an air-cooled steam condenser 18. The water (condensate) formed therein used to in part cool condenser 9, flowing thereto by gravity.

The process is automatically controlled by a gas chromatograph or analyzer, which analyzes the H₂ S:SO₂ ratio of the gas to be fed to reactor 11 or 12 and delivers the required signals to an automatic control system. The H₂ S:SO₂ ratio is almost 2.

The sulfur which becomes available in various units (3, 5, 6, 7, 8, 10, 17) is removed from the plant through jacket-heated pipes, which are subjected to a special dipping operation and lead to a sulfur collector 19, which is connected to a sulfur pit 20. The sulfur is fed to a loading station by a pump 21 immersed in the sulfur.

EXAMPLE

50 metric tons per day Sulfur-Plant

Acid-Gas to combustion chamber 1

______________________________________rate:            1975 Nm 3/hpressure:        1.4   bar abs.composition:  H2 S      1490 Nm 3/h  H2 O       163 Nm 3/h  CO2        275 Nm 3/h  CH4        47 Nm 3/h______________________________________

Air to combustion chamber 1

______________________________________rate:            3596 Nm 3/hpressure:        1.4   bar abs.______________________________________

Temperature of Claus combustion chamber 1 1120° C.

______________________________________  Process gas to            Process gas to                        Process gas to  reactor 4 reactor 8   reactor 11/12______________________________________rate:    5205 Nm 3/h 4974.5 Nm 3/h                            4925.7 Nm 3/hpressure:    1.25 bar abs.                1.16 bar abs.                            1.09 bar abs.Temperature entrance    280° C                210° C                            135° C exit    325° C                233° C                            140° CCompositionvol. % H2 S    5.1354      2.4313      0.5592 SO2    2.9818      1.2420      0.3062 H2 O    26.1322     30.2848     32.4813 S2 0.0083      0.0001      0.0000 S3 0.0011      0.0000      0.0000 S4 0.0009      0.0000      0.0000 S5 0.0335      0.0006      0.0000 S6 0.5970      0.0135      0.0014 S7 0.4398      0.0065      0.0006 S8 1.6035      0.0331      0.0090 COS     0.6278      0.0341      0.0344 CS2    0.1015      0.0106      0.0107 CO      0.9202      0.9629      0.9724 CO2    4.5369      5.4654      5.5196 H2 2.2966      2.4030      2.4268 N2 54.5835     57.1121     57.6784______________________________________

Conversion of H₂ S to S in % overall

______________________________________Process gas toreactor 4 reactor 8 reactor 11/12 incinerator______________________________________68.74     87.55     96.95         99.42______________________________________

rate of Sulfur recovery in % overall

______________________________________Process gas behindreactor 4      reactor 8   reactor 11/12______________________________________86.23          96.67       99.11______________________________________

Process gas to incinerator 13 Waste gas to chimney 15

______________________________________rate        4913.4 Nm 3/h 1749.2 Nm 3/hpressure    1.0 bar abs.  0.998 bar abs.temperature 135° C 380° Ccomposition vol. %        vol. %H2 S   0.0608        0.0SO2    0.0588        0.0755H2 O   33.0593       11.6313S8     0.0116        0.0COS         0.0345        0.0CS2    0.0108        0.0CO          0.9749        0.0CO2    5.5334        2.9641H2     2.4329        0.0N2     57.8230       72.7791O2     0.0000        12.55Fuel gas to incinerator 13               air to incinerator 13calorific value 13832 Kcal/Nm3rate:       106 Nm 3/h    rate: 2378 Nm 3/hquench air to wastegasrate:       10134 Nm 3/hdesorption gas for reactor 11/12rate:       2500 Nm 3/hcooling gas for reactor 11/12rate:       2500 Nm 3/hWaste heat for stem productionWaste heat boiler 3 2.367.645 Kcal/h for               high pressure steamsulfur-condenser 6   493.398 Kcal/h for               middle pressure               steam.______________________________________