Maleic anhydride synthesis in millistructured fixed-bed reactors
With an annual production of approx. 2.8 million tons, the synthesis of maleic anhydride (MA) represents an important industrial process. MA is produced through selective catalytic oxidation of n-butane with air in the presence of vanadium-phosphorus-oxide (VPO) catalysts at temperatures in the range of 390 to 420°C. Due to the high heat of reaction released, the industrially widespread salt-bath cooled multi-tubular fixed-bed reactors develop pronounced hot spots, which cause significant risks regarding stability and safety of the process. This effect is intensified by the phosphorus dynamics of the catalyst, which leads to a gradual increase in the activity of the catalyst while at the same time reducing selectivity and yield. Precise control of the reaction process is therefore essential for economical and safe reactor operation.
The aim of ongoing research is the development of a mathematical description of the process, including phosphorus dynamics, in order to enable a model-based optimization of the process. A prerequisite for this is a precise description of the reaction kinetics, which, in addition to the calculation of the conversion, also enables a correct description of the selectivities to the by-products CO, CO2 and other organic by-products that are usually neglected in the literature. Due to its almost isothermal behavior, the salt-bath temperature-controlled millistructured fixed-bed reactor developed at the ICVT offers ideal conditions for studying the kinetics of strongly exothermic reactions. For this purpose, systematic experiments are carried out by varying the reaction conditions. Furthermore, methods for the addition of the reaction products, in particular the target product MA, which is present as a solid, are to be developed in order to enable an investigation of the over-oxidation.