Abstract:
In the continuous production of oleum having a concentration of 10 to 45% by weight SO 3  and/or sulfuric acid having a concentration of 94 to 100% by weight H 2  SO 4  by burning sulfur with atomospheric oxygen on the principle of overstoichiometric or understoichiometric burning, cooling the resulting SO 2  -containing gas to 390°-480° C., catalytically reacting the cooled gas to SO 3  -containing gas on a vanadium-containing catalyst on the principle of single- or double-contact catalysis, absorbing the SO 3  -containing gas after cooling and, optionally, separating liquid from the gas after absorption, followed by recovering energy, the improvement which comprises effecting the burning of the sulfur with atomospheric oxygen in the presence of a dry SO 2  -containing gas which contains up to 5,000 ppm (NO) x  expressed as NO.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a process for the production of oleum having a concentration of 10 to 45% by weight SO 3  and/or sulfuric acid having a concentration of 96 to 100% by weight H 2  SO 4  by burning of sulfur with atmospheric oxygen, reaction of the SO 2  -containing gases to SO 3  -containing gases on vanadium catalysts and absorption of the SO 3  -containing gases for the production of oleum and/or sulfuric acid. 
     Various processes for the production of oleum and sulfuric acid are known from, and described in detail in, the literature. 
     In the thermal processes (burning of sulfur with atmospheric oxygen, roasting of pyrites, cleavage of sulfuric acid), nitrogen oxides (NO) x  are formed in addition to gases containing sulfur dioxide. These nitrogen oxides are absorbed in the same way as the gases containing sulfur trioxide which are formed from the SO 2  -containing gases on the vanadium catalyst, so that the sulfuric acid formed or the oleum is contaminated with absorbed (NO) x . For many applications, however, the sulfuric acid or the oleum should be free from absorbed (NO) x . In addition, the (NO) x  absorbed in oleum or sulfuric acid leads to increased corrosion of steel parts in the sulfuric acid plants. Accordingly, there has been no shortage of attempts to remove the impurities from the sulfuric acid or oleum. 
     Thus, sulfuric acid free from nitrous vitriol and having a concentration of &gt;80% by weight can be produced by addition of hydrazine compounds at 50 ° to 100° C. (DE-OS 1 792 348). Although the process of adding aqueous hydrazine sulfate solutions is basically suitable, it has been found that secondary reactions involving hydrazine sulfate take place at relatively high contents of absorbed (NO) x  and merely create a false impression of a reduction in the (NO) x  content. In addition, this process is very expensive. 
     In addition, it is known that monomeric or polymeric compounds containing NH 2  groups are added to remove nitrous acid from waste sulfuric acids (DE-OS 2 831 941). Hydroxylammonium sulfate, urea or amidosulfonic acid are preferably used as suitable compounds. However, this process can only be carried out for sulfuric acids having a concentration of approximately 50 to 90% by weight. In addition, the use of the particularly active amidosulfonic acid, for example, involves high costs. Moreover, considerable effort is involved in the two above-mentioned processes in analyzing the sulfuric acid to monitor its nitrogen content in order to avoid under- or overdosage of the compounds added. 
     Accordingly, the problem addressed by the present invention was to reduce the content of nitrous gases formed during the burning of sulfur for the production of oleum and/or sulfuric acid and absorbed in oleum and/or sulfuric acid. 
     This problem has now been solved by the process according to the invention. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a process for the continuous production of oleum having a concentration of 10 to 45% by weight SO 3  and/or sulfuric acid having a concentration of 94 to 100% by weight H 2  SO 4  by burning of sulfur with atmospheric oxygen on the principle of overstoichiometric or understoichiometric burning, cooling of the resulting SO 2  -containing gases to 390°-480° C., catalytic reaction of the cooled gases to SO 3  -containing gases on a vanadium-containing catalyst on the principle of single- or double-contact catalysis, absorption of the SO 3  -containing gases after cooling and, optionally, separation of liquids from the gases after absorption, followed by energy recovery, characterized in that the burning of sulfur with atmospheric oxygen is carried out with addition of dry SO 2  -containing gases which contain up to 5,000 ppm (NO) x  and preferably less than 2,000 ppm (NO) x , expressed as NO. 
     The dry SO 2  -containing gases normally contain more than 50 ppm (NO) x , expressed as NO. 
     Dry cleavage gases from the thermal cleavage of waste sulfuric acids are preferably used as the SO 2  -containing dry gases which may contain up to 5,000 ppm (NO) x , expressed as NO. These cleavage gases preferably contain 5 to 10% by volume O 2 , 5 to 8% by volume SO 2 ,&lt;200 ppm CO, &lt;1,000 ppm (NO) x , &lt;50 ppm hydrocarbon compounds and 82 to 90% by volume H 2  O, N 2  and CO 2 . 
     In another variant of the process, combustion gases from the burning of sulfur-containing materials are used as the SO 2  -containing dry gases which may contain up to 5,000 ppm (NO) x . 
     The ratio between the quantity of SO 2  from the dry SO 2  -containing gases added and the quantity of SO 2  formed from the burning of sulfur is preferably from 1:5 and 3:1. 
     To carry out the process in accordance with the invention, the dry SO 2  -containing gases which may contain up to 5,000 ppm (NO) x  may be directly introduced into the furnace chamber in which sulfur is burnt with oxygen or may be introduced into the furnace chamber after mixing with combustion air. It is important that the SO 2  -containing gases which may contain up to 5,000 ppm (NO) x  (nitrous-vitriol-containing SO 2  -containing gases) are burnt together with the sulfur. Up to 95% by weight of the total nitrogen oxides can be removed by the process according to the invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In one preferred embodiment of the process, the sulfur is burnt at temperatures in the range from 500° C. to 1,000° C. (as measured at the exit of the furnace chamber before cooling of the gases) and preferably at temperatures in the range from 700° C. to 950° C. 
     The SO 3  -containing gases are preferably absorbed in oleum having a concentration of 15 to 45% by weight SO 3  and preferably 20 to 30% by weight SO 3 . 
     The energy released during the burning of sulfur may be recovered in the usual way with formation of wet steam at different pressure stages. 
     The liquids removed from the gases after absorption may be recycled to sulfuric acid processing plants, preferably sulfuric acid cleavage plants. 
     The catalysis conditions in the practical application of the process are not critical. The known methods of regulating concentration may be used for the absorption of SO 3  to oleum. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The figure is a flow sheet of one possible embodiment of the process according to the invention although the process according to the invention is by no means limited to this embodiment. The reference numerals identify the following elements: 
     
         ______________________________________ 1       air dry (drying tower) 2       gas dryer (drying tower 3       fan (blower) 4       heat exchanger 5       sulfur burner 6       heat exchanger 7       primary catalyst (pre-contact 3 bed converter) 8       heat exchanger 9       waste heat exchanger10       oleum absorber11       intermediate absorber12       filter (mist eliminator)13       secondary catalyst (post-contact)14       final absorber15       filter (mist eliminator)16       liquid17       air18       SO.sub.2 -containing gas19       sulfur20       oleum21       sulfuric acid22       flue gas. (off-gas)______________________________________ 
    
    
    
     The following examples are intended to illustrate the process according to the invention. without limiting it in any way. 
     EXAMPLE 
     Example 1 
     In an industrial plant for the production of oleum having a concentration of 15 to 38% by weight free SO 3  and sulfuric acid having a concentration of 95.5 to 99.0% by weight H 2  SO 4 , which is based on the burning of elemental sulfur on the principle of the overstoichiometric burning of sulfur, the double contact process and double absorption, 18,600 Nm 3  /h SO 2  -containing gas having a temperature of 62° C. and a content of 6.3% by volume SO 2 , 5.3% by volume O 2  and 200 mg NO/Nm 3  (remainder CO 2  and N 2 ), which has been dried with 96.7% by weight sulfuric acid in a gas dryer, are mixed with air-dried at 42° C. in an air dryer--to form a total gas volume of 46,000 Nm 3  /h. This mixture is taken in under suction by a fan. The SO 2  -containing gas mixture is heated to 263° C. in a heat exchanger utilizing the heat from the first and second fixed bed trays is mixed and burnt with 4.42 t/h finely divided liquid sulfur in a sulfur burner. The liquid sulfur is dispersed via a nozzle stock surmounted by a nozzle. The hot gas (920° C.) is cooled to 435° C. and fed to a first catalyst consisting of three fixed-bed trays. The gas entering fixed bed tray 1 has the following composition: 8.5% by volume SO 2 , 9.5% by volume O 2 , 38 mg NO/Nm 3  (remainder CO 2  and N 2 ), corresponding to an NO reduction of 53%. 
     After leaving the 3rd fixed bed tray with a temperature of 512° C., the SO 3  - and SO 2  -containing gas is cooled to 215° C. in another heat exchanger and in a waste heat exchanger. A partial gas stream of approximately 50% is fed to the oleum absorption stage. The composition of the circulating oleum (62° C.) was 20.7% by weight free SO 3 , 12 ppm N (as determined by the Devarda method) and 28 ppm N (as determined by the Kjeldahl method). The remaining partial gas stream is fed together with the gas stream from the oleum absorption stage to the intermediate absorber in which a sulfuric acid having a concentration of 98.95% by weight H 2  SO 4  and a temperature of 82° C. is present. The remaining SO 3  is completely absorbed there. The remaining SO 2  -containing gas is brought to a temperature of 440° C. in heat exchangers and oxidized on the secondary catalyst in another fixed bed tray. After reaction of the remaining SO 2 , the gas leaves the tray at 420° C., is cooled in the heat exchanger and introduced into the final absorber (containing 99.9% by weight H 2  SO 4  at a temperature of 87° C). After absorption of the SO 3 , the gas is cleaned in a Brinkmist filter and emitted with an SO 2  content of 146 ppm. In the absorbers, the nitrite value is &lt;1 mg/kg oleum or sulfuric acid. The oleum accumulating is fed to the intermediate absorber. The total production of approx. 358 tons SO 3  per day is discharged as 98.95% by weight H 2  SO 4 . The liquid of H 2  SO 4  containing approx. 2.5% nitrosylsulfuric acid which accumulates in the waste heat exchanger in a quantity of approx. 9 1/24 h is delivered together with the filter effluents to the-acid cleavage plant. 14 t/h 30 bar steam/300° C. are produced. 
     Example 2 
     As in Example 1, 2.6 t/h liquid sulfur were processed with 25,600 Nm 3  SO 2  -containing gas consisting of 6.4% by volume SO 2 , 5.0% by volume O 2 , 200 mg NO/Nm 3  (rest N 2  and CO 2 ) and with air in a total gas quantity of 47,000 Nm 3  /h. The furnace temperature in the burner was 760° C. and the composition of the gas (temperature 446° C.) on entering tray 1 was 8.1% by volume SO 2 , 7.1% by volume O 2  and 28 mg NO/Nm 3  (rest N 2  and CO 2 ), which corresponds to an NO reduction of 74%, based on the NO content of the SO 2  -containing gas used (the NO 2  -content is approximately 1 to 2 mg/Nm 3 ). The absorbers had been filled with 26.7% by weight oleum, 99.07% by weight H 2  SO 4  and 99.04% by weight H 2  SO 4 . The emission comprised 130 ppm SO 2 . 300 tons SO 3  were produced in the form of 99.07% by weight H 2  SO 4 . The output of 30 bar steam was 8.5 t/h. 
     Example 3 
     As in Example 1, 4.2 t/h liquid sulfur were processed with 24,400 Nm 3  SO 2  -containing gas consisting of 6.9% by volume SO 2 , 5.6% by volume O 2  and 150 mg NO/Nm 3  (rest N 2  and CO 2 ) and with air in a total gas quantity of 47,000 Nm 3  /h. The furnace temperature in the burner was 930° C. and the composition of the gas (temperature 454° C.) on entering tray 1 of the primary catalytst was 9.2% by volume SO 2 , 8.0% by volume O 2  and 27 ppm NO/Nm 3 , which corresponds to an NO degradation of 65%, based on the NO content of the SO 2  -containing gas used. The absorbers had been filled with 24.70% by weight oleum containing 7 ppm N (as determined by the Devarda method) and 19 ppm (as determined by the Kjeldahl method) , 99.05% by weight H 2  SO 4  and 99.10 % by weight H 2  SO 4 . The emission comprised 230 ppm SO 2 . 400 t SO 3  /24 h were produced in the form of 99.10% by weight H 2  SO 4 . The output of 30 bar steam was 14 t/h. 
     Example 4 
     Production of SO 2  -containing gases from the cleavage of sulfuric acid 
     68% by weight waste sulfuric acid, salt content 0.5% by weight, was fed into a lined furnace heated with fuel oil S (1.8% by weight sulfur). The furnace temperature was 1,050° C. The throughputs per unit of time were as follows: air 12,000 m 3 , fuel oil 1,000 kg, cleavage acid 7,000 kg. At the same time, a waste gas from an intermediate organic production process was introduced in a quantity of 50 m 3  per unit of time: SO 2  45% by volume, hydrocarbon compounds 2.5% by volume, N 2  and O 2  52.5% by volume. The efficiency of the H 2  SO 4  →H 2  +SO 2  +1/2O 2  reaction is 98% of the theoretical. The gas leaving the cleavage furnace is cooled to 800° C. in a recuperator, the burner air used for cooling being heated to 420° C. In a wet quench, the hot cleavage gas is cooled to 80° C. by introduction of an excess of water. The water deposited is removed by a stripper. The steam-saturated gas with a temperature of approximately 80° C. enters a packed washing tower which may be operated in co-current or countercurrent. A 2% by weight sulfuric acid is used as the washing liquid. The sensible.heat is dissipated by external, indirect cooling with water. Liquids deposited are removed from the system by a stripper. The gas mixture (temperature 35° C.) leaving the washing system, which has the following composition: O 2  6.5% by volume, SO 2  6.0% by volume, NO x  125 ppm, CH compounds&lt;1 ppm, H 2  O, CO 2  and N 2   87.5% by volume, is after-cleaned (wet EGC) and dried with 97.5% by weight sulfuric acid. 
     It will be appreciated that the instant specification is set forth by way of illustration and not limitation, and that various modifications and changes may be made without departing from the spirit and scope of the present invention.