Process for recovering elemental sulfur from gases having a high carbon dioxide content and containing sulfur compounds
FIELD OF THE INVENTION 
     This invention relates to a process for recovering elemental sulfur from gases which have a high carbon dioxide content and contain inorganic and/or organic sulfur compounds, unsaturated hydrocarbons, hydrogen cyanide and other impurities. 
     BACKGROUND OF THE INVENTION 
     When fossil fuels are gasified to produce fuel gases and synthesis gases, the resulting gases have a high CO 2  content and in addition thereto contain H 2  S and organic sulfur compounds and other impurities, such as HCN. When produced from coal, such gases usually also contain unsaturated hydrocarbon which tend to polymerize and are referred to as resinogetic substances, and NH 3 . 
     These gases are mostly produced under pressure and are almost always scrubbed to remove the above-mentioned impurities. It has become conventional in gas-purification technology to scrub the gases with physically absorbing scrubbing agents. 
     Such a process, in which organic liquids, such as methanol, are used as scrubbing agents, has been described, e.g. in German Pat. No. 935,144. 
     It is also known to remove impurities from gases by chemical scrubbing. 
     German Pat. No. 1,145,740, for example, describes a process of scrubbing gases with water under pressure to remove H 2  O and CO 2  from said gases. An ethanolamine, preferably triethanolamine, and a polybasic material acid in which the terminal hydrogen atom has no longer an acid activity, such as phosphoric acid, are added to the aqueous scrubbing liquor in a very small amount, which totals about 0.02-0.2 moles and in which is so selected that the pH value of the scrubbing liquor is slightly below or not in excess of 8. 
     In another known process, a mixture of organic liquids, such as methanol, with water and amines is used to scrub the gases (Opened German specification -- Offenlegungsschrift -- No. 1,815,1137). 
     In physical scrubbing processes it is easy to remove the above-mentioned impurities but the regeneration of the scrubbing agent laden with said impurities results in exhaust gases which can be eliminated or destroyed only at high expense and in a difficult manner particularly in view of ecological requirements. Whereas these impurities, which are generally combustible, in the regeneration exhaust gases having in most cases a high CO 2  content can be burnt, this results in combustion gases which have such a high SO 2  content that the problem is not solved by the combustion alone. 
     The removal of impurities such as HCN, polymerizable hydrocarbons, mercaptans, and NH 3  from gases by chemical scrubbing requires a high expenditure and involves considerable difficulty. Specifically, it is very difficult to regenerate the scrubbing liquor and to recover the chemicals. 
     OBJECT OF THE INVENTION 
     It is the object of this invention to avoid these diadvantages of the known processes and provide a process which is economically and ecologically satisfactory. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention the process for recovering elemental sulfur from gases which have a high carbon dioxide content and contain inorganic and/or organic sulfur compounds, unsaturated hydrocarbons, hydrogen cyanide, and other impurities comprises adding a combustion-promoting gas to the gases, the resulting mixture being burned with a slight excess of air. 
     The resulting gas is caused to flow in contact with with coke at temperatures of 300°-450° C to remove residual oxygen, sulfur trioxide, and nitrogen oxides. 
     The prepurified gas is cooled to a temperature of 20°-80° C and is then subjected in one or two stages to counterflow scrubbing with an aqueous absorbent solution of alkali salts of weak inorganic and/or organic acids or amines to remove sulfur dioxide, the laden solution is heated and stripped with gases or water vapor to strip off the sulfur diodide. 
     The stripped-off sulfur dioxide is cooled, excess hydrogen is added thereto, and the mixture is hydrogenated at temperatures of 200°-450° on a catalyst which contains oxides of cobalt, of nickel, of molybdebum or of tungsten, to form sulfur and/or hydrogen sulfide, the resulting elemental sulfur is removed by cooling. 
     Sulfur dioxide in a stoichiometric proportion is added, if required, to hydrogen sulfide which has been formed, the mixture is reacted at temperatures of 200°-300° C in contact with a catalyst consisting of alumina or activated carbon to form elemental sulfur, which is recovered by cooling, and all or part of the resulting exhaust gas is recycled to the process. 
     The process according to the invention comprises the burning of the combustible constituents of the exhaust gases which have been released by the regeneration of the laden absorbent solution and have a high CO 2  content. The combustion may be effected in contact with a catalyst or as a strictly thermal process. In both cases, a combustion-promoting gas having a high calorific value is required to increase the calorific value of the gas, this calorific value must be kept above a lower limit to ensure that the combustion is upheld. This combustion-promoting gas may be a common fuel gas (e.g. methane) or a fraction which has a high calorific value. Such fraction is released by the fractionating flashing and regeneration of the laden scrubbing liquors used for physical scrubbing treatments. The use of such fractions is particularly advantageous because they contain hydrocarbons as well as mercaptans and HCN in high concentrations. 
     In accordance with the invention, the combustion is carried out with a slight excess of air, i.e. a slight excess of O 2 . The excess of O 2  is adjusted by control means and in accordance with the invention is up to 0.2% by volume. The combustion causes all sulfur compounds to be burned to SO 2 . Traces of SO 2 , H 2  S and COS are left. NH 3  is converted to N 2  nitrogen oxides. 
     In the process according to the invention, the flue gas produced by the combustion is passed through waste heat-utilizing means or coolers, wherein the gas temperature is adjusted to 300°-450°, preferably 360°-400° C, and the gas is then passed through a bed of granular coke which has preferably a particle size of 5-20 mm. The coke consists preferably of petroleum of coke or low-temperature coke, which has been produced from peat, brown coal or hard coal by low-temperature coking. The following reactions take place on the coke: 
     
         O + C → CO.sub.2 
    
     
         2 so.sub.3 + c → 2 so.sub.2 + co.sub.2 
    
     
         h.sub.2 s + 3/2 o.sub.2 → so.sub.2 + h.sub.2 o (catalytic) 
    
     By these reactions, said constituents, which are undesirable for the further processing of the gas, are eliminated or converted into harmless substances. The gas now contains sulfur only as SO 2 . 
     In accordance with the invention this gas, which is free from O 2  SO 3 , is suitably cooled first to a temperature of 20°-80° and is then scrubbed with the aqueous solution of alkali salts of weak inorganic and/or organic acids to remove SO 2 , which is to be recovered in concentrated form. 
     The alkali cations may consist of ammonium, sodium or potassium alone or in a mixture. The acid anions may consist of those of sulfurous acid, arsenious acid, boric acid, phosphoric acid, vanadic acid, phenols and the like. To ensure a high absorption capacity for SO 2 , the aqueous solutions are used at the highest concentration which ensures that the saturation limited will not be exceeded at a temperature of 0° C. Depending on the nature of the compound used in the scrubbing solution, the SO 2  is combind therein to form alkali hydrogen sulfite, and the weak acid which is contained in the scrubbing solution and combined with the alkali metal is released, e.g., in accordance with the reaction equation: 
     
         NA.sub.2 HPO.sub.4 + SO.sub.2 +H.sub.2 O = NaH.sub.2 PO.sub.4 + NaHSO.sub.3 
    
     if Na 2  HPO 4  is used as an active scrubbing agent. 
     The above reaction takes place in the reverse direction during the regeneration of the SO 2  -laden scrubbing solution. The scrubbing solution may be regenerated by being directly heated with water vapor or by being indirectly heated with a heating gas or a hot exhaust gas. 
     Aliphatic amines, such as ethylene diamine, alkanolaines or aromatic amines, e.g. toluidine, xylidine, aniline and the like may also be used as scrubbing agents. 
     The scrubbing solution used to remove SO 2  from the gas is regenerated and recirculated. Depending on the degree to which the laden scrubbing solution is regenerated, the regeneration exhausted gas contains 100-150 ppm SO 2 . The SO 2  which has been stripped from the laden scrubbing solution is concentrated and free from other gaseous constituents. 
     To eliminate water vapor, the SO 2  is cooled to normal temperature and is then processed to form sulfur. For this purpose, hydrogen or a hydrogen-containing gas is added, and the mixture is heated to a temperature of 200°-450° C, and is then caused to flow in contact with a sulfur-resisting hydrogenating catalyst, which contains metals of the 6th and/or 8th group(s) of the periodic system, preferably CO, Ni, Mo, and W, and on which the SO 2  is reduced at least in part to form H 2  S and in part to form sulfur. 
     This gas can be fed in known manner to a Claus process plant, if such plant is available. Alternatively, the gas can be fed to a specially adapted Claus process reactor in which the gas, to which SO 2  or H 2  S in a stoichiometric proportion has been added if required, is reacted to elemental sulfur in contact with a catalyst. This reactor contains a known catalyst consisting of activated alumina or the like. Granular activated carbon may also be used for this purpose. The gas leaving the Claus process reactor contains the resulting elemental sulfur in vapor form and also contains unreacted residual H 2  S and SO 2  and residual hydrogen, and is cooled to a temperature of about 130°-140° C to effect a condensation of the sulfur. The gas left after this condensation is admitted to the exhaust gases released by the regeneration of the laden absorbent solution and is burned and processed together therewith. 
     According to a further preferred feature of the invention, the sulfur dioxide stripped from the laden absorbent solution is reacted only in part with hydrogen to form hydrogen sulfide, part of said sulfur dioxide is added to the resulting hydrogen sulfide, and the mixture is subsequently reacted to form elemental sulfur. 
     Within the scope of the invention, hydrogen sulfide may be added to the sulfur dioxide which has been stripped from the laden absorbent solution, whereafter the mixture is reacted to form elemental sulfur. 
     In all cases it must be ensured that the conditions of the reaction represented by the equation: 
     
         2H.sub.2 S + SO.sub.2 = 3 S + 2H.sub.2 O +Q 
    
     where Q is heat evolved, are maintained. 
     The process according to the invention will be described by way of example with reference to the accompanying flow scheme and enables a processing of ecologically unsatisfactory exhaust gases and a complete conversion of otherwise dangerous gaseous constituents with a simultaneous recovery of elemental sulfur. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above and other objects, features and advantages of this invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which the sole FIGURE is a flow diagram illustrating the process as it applies to the specific example given hereinafter. 
    
    
     SPECIFIC DESCRIPTION 
     The drawing shows a combustion chamber 4 which has a burner 4a supplied with a combustion-promoting gas (fuel) through line 2, a combustion-sustaining gas (e.g. air) supplied through line 3 and exhaust gas containing sulfur compounds to be processed in accordance with the invention which is supplied through line 1. A line 29 also serves to recycle a portion of clean gas to the burner as a diluent. 
     The gaseous combustion products are delivered via line 5 to a reactor 6 packed with coke from which the gases pass through a waste heat recovery system represented diagrammatically at 8 and serving to cool the gases. An aftercooler 9, supplied with precooled combustion gases from the waste heat utilizer 8 via line 8a, brings the gases to the desired temperature before they enter the first scrubbing column 10. A gas 11 emerging from this scrubbing column can be discharged into the atmosphere since it contains only a small concentration of sulfur as the oxide. A pump 12 draws the scrubbing liquid from the sump of column 19 and sprays it into the head of a regenerating column 14, the sump of which is provided with a heater 15 capable of bringing the scrubbing liquid to a temperature which enables it to release the absorbed gages. The regenerated liquid is drawn from the sump of the column 14 by a pump 17 and passes through in heat exchanger 13 in which it preheats the liquid sprayed into the head of this column 14. Pump 17 delivers the regenerated scrubbing liquid to the head of column 10. 
     The desorbed gases are cooled at 19, passed through the heat exchanger 8 and supplied to a reactor 21 containing a reducing catalyst. The gases then pass through a condenser 22 for the sulfur which is collected at tank 30 and enters a Claus reactor in which sulfur is formed from the H 2  S. Another condenser 25 again separates out sulfur and the gas, after passing through a particle separator 26 from which atomized or finely divided sulfur is recovered, is recycled via a blower 28 to the line 29 or the reactor 21. Line 24 can deliver SO 2  containing gas to the Claus reactor together with the H 2  S gas from the reactor 21 so that the Claus process reaction of H 2L  S and SO 2  can proceed in the usual manner. 
     EXAMPLE 
     Gaes produced by the pressure gasification of coal is subjected to a physical scrubbing under pressure and at low temperature. Methanol is used as a scrubbing agent and is flashed and regenerated to release a regeneration exhaust gas having the following composition: 
     
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H.sub.2 S   0.47% by vol.                                                 
CO.sub.2    98.27% by vol.                                                
CH.sub.4    0.48% by vol.                                                 
C.sub.2 H.sub.6                                                           
            0.69% by vol.                                                 
C.sub.3 H.sub.8                                                           
            0.09% by vol.                                                 
COS         180 ppm                                                       
HCN         206 ppm.                                                      
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     Combustion-promoting gas is supplied through conduit 2 and preheated air is supplied through conduit 3 to this regeneration exhaust gas as the same is fed through conduit 1 to a combustion chamber 4, in which the gas is burned to form a gas having the following composition: 
     
         ______________________________________                                    
H.sub.2 O   2.98% by vol.                                                 
SO.sub.2    0.37% by vol.                                                 
CO.sub.2    79.22% by vol.                                                
N.sub.2     17.35% by vol.                                                
O.sub.2     0.08% by vol.                                                 
SO.sub.3    180 ppm                                                       
NO.sub.x     57 ppm.                                                      
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     This gas may be cooled, if desired, and is fed through conduit 5 into a reactor 6, which is packed with granular petroleum coke and in which, at a temperature of about 380+-400° C, the residual oxygen is reduced to form CO 2 , the SO 3  is reduced to form SO 2 , and nitrogen oxides are reduced to N 2 . The resulting gas flows through conduit 7, a heat exchanger 8 and an aftercooler 9, and then enters a scrubbing column 10 and flows through the same from bottom to top. In said scrubbing column the gas is scrubbed at a temperature of 65° C with a counterflowing aqueous solution of Na 2  HPO 4  and is thus freed from SO 2 . The gas leaving the scrubbing column through conduit 11 contains less than 250 pm SO 2 . 
     By a pump 12, the SO 2  -laden scrubbing solution is withdrawn from the sump of the scrubbing column and is fed through a heat exchanger 13 to the top of a regenerating column 14. In the sump of column 14, the laden scrubbing solution is heated by a heating system 15 to the regeneration temperature and is thus caused to release the absorbed SO 2 , which leaves the column through conduit 16. By means of a pump 17, the regenerated scrubbing solution is fed through a conduit 18 and the heat exchanger 13 to the top of the scrubbing column 10. 
     SO 2  leaves the regenerating column 14 through conduit 16 and flows through a cooler 19, in which water vapor is condensed and through the outlet of said cooler and conduit 20 and the heat exchanger 8 into a reactor 21, which is packed with reduction catalyst. Hydrogen or a hydrogen-containing gas is added from conduit 32 to the gas before it enters the reactor 21, in which at least part of the gas is reduced at a temperature of 200°-400° C to form H 2  S and elemental sulfur. When the resulting sulfur has been separated in a cooler 22, the gas consisting of H 2  S, SO 2  and H 2  O enters a Claus process reactor 23, in which the gas is reacted in contact with an alumina catalyst to form elemental sulfur. The SO 2  required for this purpose is also removed through conduit 24 from the SO 2  main stream 16 and is added in stoichiometric proportions. The exhaust gas from the Claus process reactor is conducted through a cooler 25 and a separator 26, whereby the resulting sulfur is condensed, and the gas is then fed by a blower 28 through conduit 27 into the conduit 31 to cause the residual hydrogen to be processed. 
     The liquid sulfur which has been separated in the reactors 21 and 23 and in the coolers 22 and 25 flows into the collecting tank 30.