One-stage ethanol to butadiene process: Analysis and design of a multi-tubular fixed bed reactor

The paper develops a design feasibility study of a multi-tubular polytropic fixed bed reactor for the one-stage ethanol to 1,3-butadiene (ETB) process employing a modified MgO-SiO2 catalyst. The analysis quantifies the effect of the internal diffusion on scaling up the catalyst activity and selectivity by considering a complex chemistry of seven parallel/series reactions described by detailed kinetic modelling. The efficient numerical solution takes advantage from the MATLABTM function bvp5c. The simulation model allows scanning the catalytic process by profiles of species' concentrations, reaction rates and effectiveness factors, inside the catalyst particle and over the reactor. The butadiene yield is controlled by the accumulation in particle of ethanol and key intermediates, as acetaldehyde and crotonaldehyde, related to internal chemical reaction with diffusion and external mass and heat transport rates. The catalyst performance depends significantly on the pellet size that becomes optimization variable for catalyst selection and reactor design. Catalyst particles in the range 2 to 3 mm with tubes of to 25 to 40 mm diameters are the most efficient for achieving 80% butadiene carbon selectivity suitable for an industrial process. The modelling approach is generic for calculating the catalyst effectiveness and designing catalytic chemical reactors involving complex reactions.