A process for preparing sulfuric acid may involve oxidizing sulfur to sulfur dioxide by way of dried air in a first oxidation stage. The sulfur dioxide may then be oxidized to sulfur trioxide in a second oxidation stage. The sulfur trioxide may be absorbed by sulfuric acid in at least one absorption stage. Further, heated sulfuric acid may be drawn off from the absorption stage and used for generating steam. Process gas from an intermediate absorption stage may be recycled to the second oxidation stage and, in some cases, a final absorption stage after the process gas flows through the second oxidation stage.

In a converter for the catalytic oxidation of SO.sub.2 to SO.sub.3 in a sulfuric acid plant, which comprises a boiler section for the cooling of process gas between catalytic layers (beds), one or more water tube boilers (inter-bed boilers) having horizontal or slightly sloped tubes are used to cool the process gas between the catalytic layers (beds) in the converter. Each water tube boiler is provided with a process gas side bypass to control the temperature to the down-stream catalyst layer.

In a converter for the catalytic oxidation of SO.sub.2 to SO.sub.3 in a sulfuric acid plant, which comprises a boiler section for the cooling of process gas between catalytic layers (beds), one or more water tube boilers (inter-bed boilers) having horizontal or slightly sloped tubes are used to cool the process gas between the catalytic layers (beds) in the converter. Each water tube boiler is provided with a process gas side bypass to control the temperature to the down-stream catalyst layer.

In a method for cooling of process gas between catalytic layers or beds in a sulfuric acid plant, in which sulfuric acid is produced from feed gases containing sulfurous components like SO.sub.2, H.sub.2S, CS.sub.2 and COS or liquid feeds like molten sulfur or spent sulfuric acid, one or more boilers, especially water tube boilers, are used instead of conventional steam superheaters to cool the process gas between the catalytic beds in the SO.sub.2 converter of the plant. Thereby a less complicated and more cost efficient heat exchanger layout is obtained.

In a method for cooling of process gas between catalytic layers or beds in a sulfuric acid plant, in which sulfuric acid is produced from feed gases containing sulfurous components like SO.sub.2, H.sub.2S, CS.sub.2 and COS or liquid feeds like molten sulfur or spent sulfuric acid, one or more boilers, especially water tube boilers, are used instead of conventional steam superheaters to cool the process gas between the catalytic beds in the SO.sub.2 converter of the plant. Thereby a less complicated and more cost efficient heat exchanger layout is obtained.

In a production mode a process for preparing sulfuric acid may involve oxidizing sulfur to sulfur dioxide in a first oxidation stage, and catalytically oxidizing the sulfur dioxide to sulfur trioxide in a second oxidation stage. The sulfur trioxide may be absorbed in at least one absorption stage. In the production mode, process gases from a last of the at least one absorption stage with respect to a flow direction are discharged. In a standby mode of the process, at least one heating stage for heating the process gases is connected. The process gases exiting from the at least one absorption stage are conveyed to the heating stage, and the process gases are circulated via the heating stage, the second oxidation stage, and the absorption stage.

Sulfur trioxide is produced from a feed stream comprising sulfur-containing compounds and dissolved metals, such as alkali metals, in a plant comprising an incineration furnace, a waste heat boiler, a dilution air heater, a dust removal absorber and an SO.sub.2 converter. The plant may further comprise a sulfuric acid condenser for the production of sulfuric acid.

Sulfur trioxide is produced from a feed stream comprising sulfur-containing compounds and dissolved metals, such as alkali metals, in a plant comprising an incineration furnace, a waste heat boiler, a dilution air heater, a dust removal absorber and an SO.sub.2 converter. The plant may further comprise a sulfuric acid condenser for the production of sulfuric acid.

The invention is a method and a sulfuric acid plant design for reduction of start-up SO.sub.2, SO.sub.3 and H.sub.2SO.sub.4 emissions in sulfuric acid production, in which SO2 is converted to SO.sub.3 in n successive catalyst beds, where n is an integer >1. The final catalytic beds are used as absorbents for SO.sub.2 to SO3 during the start-up procedure, and one or more of the m beds downstream the first bed are purged, either separately or simultaneously, with hot gas, where m is an integer >1 and m<n, during the previous shut-down. Also, one separate purge with hot gas is used on the final bed.

Commercial production of sulfuric acid is almost entirely accomplished nowadays using the contact process. And the trend is to increase conversion efficiency and reduce emissions of unconverted sulfur dioxide. By using a special combination of contact catalyst beds, a single contact single absorption (SCSA) system can be engineered to achieve the conversion and emission capabilities of conventional double contact double absorption systems. Thus, the complexity and cost of incorporating a second absorption tower and associated heat exchanger in the system can be omitted. In the SCSA system, the initial catalyst bed or beds comprise vanadium oxide catalyst and the last catalyst bed or beds comprise platinum catalyst operating at a much lower temperature than the initial beds.