Method for producing sulphur trioxide

The method for the production of sulphur trioxide, operating in a non-stationary regime, intend for sulphuric acid production. The aim of the present invention is to provide a method for the production of SO3 by catalytic oxidation of SO2, contained in an inlet gas flow, providing a stable regime of operation of wide limits of variation of the flow rate and the concentration of SO2 in the input gas. The essence of the invention is in this, that between the two layers of the first step of oxidation in a two-step three-layer contact apparatus, the gas flow is partially cooled in an inner heat-exchanger-mixer and part of the heat of the reaction of oxidation of SO2 in the first stage of oxidation is passed to the second stage of oxidation by means of the gas flow coming out of the intermediate absorption, containing unoxidized SO2 from the first stage.