Fundamental studies
The VK catalysts are so-called supported liquid phase (SLP) catalysts, where the oxidation of SO2 takes place as a homogeneous reaction in a liquid film consisting of V2O5 dissolved in alkali-metal pyrosulphates on an inactive porous silica support made from diatomaceous earth. Through comprehensive characterisation and advanced fundamental studies Topsoe continuously strives to uncover the rate-determining mechanisms of the working catalyst, eg catalytic steps, melt distribution, vanadium oxidation states and diffusional transport restrictions.
Rate models
Since every VK catalyst has unique properties, thorough experimental mapping of catalyst activity covering the full range of relevant industrial operating conditions is carried out in bench scale reactors and used in subsequent development of reaction rate models. Combined with pilot-scale testing and comprehensive feedback from industrial installations these rate models constitute a first-class basis for prediction of the required VK catalyst volumes.
Development of novel catalysts
The desire to reduce pressure drop and energy consumption in industry has inspired Topsoe to develop a number of new catalyst sizes and shapes which reduce pressure drop while maintaining activity and mechanical strength. The development from cylinders to rings and subsequently Daisy-shapes increases the effectiveness factor of the pellets for SO2 oxidation and increases the void fraction of the bed thus reducing pressure drop.
Topsoe was first to utilise the enhanced stabilisation of the catalytic melt by the caesium promoter when the low-temperature catalysts VK59 and VK69 were developed for increased total conversion and lower ignition temperatures.
Testing and development of sulphuric acid catalysts for operation in wet gases in eg WSA plants is a special focus area of Topsoe. For this purpose a new VK-W series of catalysts has been developed and optimised.
Dust protection
Since the 70’s Topsoe has installed top-layers of large 20 mm ring-shaped VK pellets for better distribution of dust in the first bed. Based on mechanistic modelling of dust deposition, comprehensive industrial feedback from dust protection layers and excellent extrusion competencies, an improved 25 mm Daisy-shaped dust protection catalyst has been developed recently. This catalyst provides higher dust capacity due to higher void fraction and lower specific external surface area.