Sulphuric acid is usually obtained in a continuous double contact process. Firstly, liquefied sulphur is combusted as a raw material with dry air to make sulphur dioxide. The second stage involves further catalytically oxidising the sulphur dioxide in the contact furnace, the heart of the plant, using vanadium oxide to make sulphur trioxide. The exothermal balance reaction must be maintained in a temperature range between 400 and 600°C. To this end, the catalyst is layered in the furnace and the gas mixture flowing through the layers is repeatedly cooled to the right temperature by heat exchange and external gas cooling. As it is not usually possible to convert all of the sulphur dioxide in the first contact furnace, the non-absorbed sulphur dioxide is recycled through an intermediate absorber and passes through another contact layer after the first process in the double contact process. This ensures that much of the sulphur dioxide is converted. The sulphur trioxide produced in this way dissolves as sulphuric acid before being poured into tanks.

As the temperature in the contact furnace has to be kept within narrow limits during the reaction, very precise heat management and exact recording of the amounts of substances circulating are crucial for the efficiency of the process. This requires reliable and sturdy measuring equipment, which is what KROHNE has been providing for many years, including for example resistance thermometers with protective fittings and electromagnetic flowmeters for complex process applications with aggressive liquids such as sulphuric acid. These guarantee reliable, high-precision flow measurements, even in extremely harsh process conditions and at high temperatures.