Abstract:
Process for preparing sulphuric acid from gasses containing SO 3  and gaseous nitrosylsulphuric acid (NO + HSO 4   −  or HO 3 SONO) by contact with 94% to 98% sulphuric acid, wherein the gases flow through at least one cooler and are cooled down to 160° C. to 130° C. to condense out nitrosylsulphuric acid before being brought into contact with the sulphuric acid.

Description:
DESCRIPTION 
     This invention relates to a process for producing sulfuric acid from gases containing SO 3  and gaseous nitrosyl sulfuric acid by contacting with highly concentrated sulfuric acid. 
     BACKGROUND OF THE INVENTION 
     Processes for producing sulfuric acid are known. In Rompps Chemie-Lexikon, 8th edition, 1987, pages 3760 to 3764, there is described a process for producing sulfuric acid. As raw material sulfur dioxide is used, which is contacted with a catalyst and catalytically converted to SO 3 . The SO 3  discharged from the catalytic furnace is subsequently introduced into a 98% sulfuric acid, where first of all disulfuric acid (H 2 S 2 O 7 ) is formed, which by adding water is converted to sulfuric acid. In this way, particularly highly concentrated sulfuric acids as well as various types of oleum can be obtained. This process, however, has the disadvantage that in the catalytic conversion of SO 2  to SO 3 , in dependence on the content of nitrogen oxide after the SO 2  production, there is also formed nitrosyl sulfuric acid, which likewise gets into the highly concentrated sulfuric acid and must be removed with a relatively great technical effort. In the DE-OS 17 92 348 there is described a process for producing sulfuric acid free from ammonia and nitrose, where it is provided to add hydrazine compounds to the sulfuric acid for the purpose of denitration. However, this is a relatively time- and cost-consuming process. The DE-195 16 303 describes a process for reducing the NO x  content of sulfuric acid, where NO x  is reduced with SO 2  to form N 2 , and N 2  is discharged, so that SO 2  is used for denitration. This process has the disadvantage that the nitrosyl sulfuric acid obtained in the absorption towers cannot be removed due to the high concentration of sulfuric acid. 
     It is the object underlying the invention to create a process for producing sulfuric acid from gases containing SO 3  and gaseous nitrosyl sulfuric acid by contacting with highly concentrated sulfuric acid, where there is no enrichment of nitrosyl sulfuric acid in the absorption towers. 
     SUMMARY OF THE INVENTION 
     The object underlying the invention is solved in that before contacting with highly concentrated sulfuric acid the gases are passed through at least one cooler and are cooled to 160 to 130° C. “Nitrosyl sulfuric acid” is understood to be the gaseous nitrosyl hydrogensulfate (NO + HSO 4   −  or HO 3 SONO). The concentration of the highly concentrated sulfuric acid generally lies in the range between 94 and 98%. As coolers, conventional gas coolers or heat exchangers may be used, such as shell-and-tube heat exchangers or ribbed tube-coil heat exchangers. It was surprisingly found out that the gases containing SO 3  and gaseous nitrosyl sulfuric acid can advantageously be cooled in at least one cooler, so that the nitrosyl sulfuric acid condensates out of the gases and does therefore no longer come in contact with the highly concentrated sulfuric acid in the absorption tower. The condensate containing nitrosyl sulfuric acid can advantageously be denitrated by means of the process described in DE-195 16 303. 
     A preferred embodiment of the invention consists in that the gases are cooled to 160 to 150° C. Advantageously, the nitrosyl sulfuric acid condensates out almost exclusively in the cooler. Since the gases containing SO 3  and gaseous nitrosyl sulfuric acid generally also contain a certain amount of steam, the gases will generally also have a content of gaseous sulfuric acid, which at lower cooler temperatures would condensate out together with the nitrosyl sulfuric acid, which is, however, not desired. But when the gases are cooled to 160 to 150° C., it is ensured that the gaseous sulfuric acid will not condensate out, but can subsequently be introduced into the absorption tower. 
     In accordance with a further preferred aspect of the invention it is provided that the gases are catalytically oxidized after contacting with highly concentrated sulfuric acid for the enrichment of SO 3 , are subsequently again passed through at least one cooler and are cooled to 160 to 130° C., and thereafter are again contacted with highly concentrated sulfuric acid. The advantage is that the gases which are removed from the absorption tower and contain still larger amounts of SO 2  can again be used for the production of sulfuric acid. Furthermore, the concentration of SO 2  in the pure gas is reduced. 
     The invention will subsequently be explained in detail and by way of example with reference to the drawing (FIG.  1  and  2 ), wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the flow diagram of the process for producing sulfuric acid from gases containing SO 3  and gaseous nitrosyl sulfuric acid; 
     FIG. 2 shows the flow diagram of a variant of the process for producing sulfuric acid from gases containing SO 3  and gaseous nitrosyl sulfuric acid. 
    
    
     In FIG. 1, the flow diagram of the process for producing sulfuric acid from gases containing SO 3  and gaseous nitrosyl sulfuric acid is shown in a simplified and exemplary representation. The SO 2 -containing gases  1  are introduced into the reactor  2 , in which a conversion of SO 2  into SO 3  is performed. This conversion takes place at about 500° C., where as catalyst vanadium pentoxide can be used for instance. However, due to the content of-nitrogen oxide in the SO 3  gas in the reactor  2 , nitrosyl sulfuric acid is formed, which is then present in the form of a gas. The gases containing SO 3  and gaseous nitrosyl sulfuric acid are removed from the reactor  2  and passed through a heat exchanger  3 , in which the gases are precooled to 340 to 250° C. Then, the gases are passed through the cooler  4  and cooled to 160 to 130° C. As cooling medium  5  there can likewise be used gases, which leave the cooler  4  via line  6 . The nitrosyl sulfuric acid condensated out is discharged via line  7 . The gases liberated from the nitrosyl sulfuric acid subsequently flow into the absorption tower  9 , in which SO 3  is converted to sulfuric acid, which is then discharged from the absorption tower  9  (not represented). The gases leave the absorption tower  9  as clean gas via line  8 . 
     In FIG. 2 a variant of the process for producing sulfuric acid from gases containing SO 3  and gaseous nitrosyl sulfuric acid is represented. The SO 2 -containing gases  1  are first of all introduced into the reactor  2 , in which the formation of SO 3  is realized. At the same time, gaseous nitrosyl sulfuric acid is formed. The gases containing SO 3  and gaseous nitrosyl sulfuric acid are subjected to a precooling in a further heat exchanger  10  and then flow into the further cooler  11 . As cooling media  12  there may advantageously be used gases which leave the further cooler  11  via line  13 . The gaseous nitrosyl sulfuric acid condenses in the further cooler  11  and is discharged via line  14 . Via line  15 , the gases liberated from the nitrosyl sulfuric acid flow into the further absorption tower  16 , in which SO 3  is converted to sulfuric acid. The gases leaving the further absorption tower  16  contain even larger amounts of SO 2  and via line  17  are at first counter-currently passed through the further heat exchanger  10  and then again flow into the reactor  2 . In the reactor  2 , the SO 2  is again converted to SO 3 , where there is, however, also formed gaseous nitrosyl sulfuric acid. The gases containing SO 3  and gaseous nitrosyl sulfuric.acid are then passed through the heat exchanger  3  and are subjected to a corresponding treatment in the subsequently disposed cooler  4  and absorption tower  9 . The clean gas leaving the absorption tower  9  merely contains minor amounts of SO 2  and is discharged from the system via line  8 .