chlorine or bromine are produced from the respective hydrogen halide in a reaction system , wherein a stoichiometric excess of active nitrogen is maintained within the reaction system over the amount of hydrogen halide in the system . preferably the reaction system is a single tower wherein sulfuric acid is introduced to the top of the tower where it absorbs the active nitrogen while conditions are so set that at the bottom of the tower the gases , introduced there , strip the active nitrogen away from the acid . it is a further characteristic of the process , that the nitrogen compounds stripped at the bottom , are mainly stripped as a mixture of nitrogen monoxide and nitrogen dioxide . a major portion of the nitrogen monoxide stripped , is oxidized with the oxygen , present in the gases fed in the bottom , to form nitrogen dioxide . if hydrogen halide is fed in the bottom , it will also strip nitrogen compounds , but mostly in the form of nitrosyl - halide . oxidation of nitrosyl - halide with oxygen , either direct or indirect , does not play a significant role in the present process . in the reaction section of the process , where practically all the halogen is formed , the characteristic phenomena is , that the chlorine or bromine made there in gas phase , is all formed by reaction of nitrogen dioxide and hydrogen halide or nitrosyl - halide , while the sulfuric acid absorbs the active nitrogen compounds mainly in the form of nitrosylsulfuric acid , thus helping the reaction to proceed . a small amount of hydrogen halide is preferably left unconverted .