Mass transfer and kinetics of H2O2 direct synthesis in a batch slurry reactor

Abstract

A model of a gas bubbling, batch slurry reactor for H2O2 direct synthesis is presented. Experimental measurements were carried out in the absence of halides and acids at temperatures between 258 and 297K (pressures 14–20bar, depending on temperature) with H2 and O2 diluted in CO2 outside flammability limits (gas phase composition of CO2, O2 and H2 was 77%, 21% and 2%, respectively). Kinetic experiments performed on a commercial 5% Pd/C catalyst (0.15g in 400ml methanolic solution) have been used to identify the intrinsic kinetics and assess the influence of mass transfer. The simplest rate equations compatible with the acknowledged reaction network has been included in a reactor model, which accounts for mass transfer resistances between gas and liquid and bulk of the liquid-catalyst surface.The corresponding Arrhenius parameters were estimated from direct synthesis experiments for all the reactions. Comparable temperature dependence was observed for H2O production, hydrogenation and decomposition (activation energies close to 45kJmol−1), while H2O2 synthesis has a much lower activation energy (close to 24kJmol−1), suggesting that a higher selectivity is achievable at low temperature. Decomposition had a very limited influence on the overall peroxide production rate, being quite slow (its rate is approx. 40% the direct synthesis rate at H2 full conversion). Hydrogenation was the most rapid side reaction, depressing H2O2 production as H2 conversion increased. Independent investigation on the H2O2 hydrogenation in the absence of O2 highlighted significant difference in the kinetics, apparently due to a different oxidation state of the catalyst.A sensitivity analysis on the mass transfer coefficients to allow for uncertainties in the correlations proved that no resistances in the liquid occur, while gas–liquid H2 transfer rate may be limiting, although unlikely, requiring that literature coefficients overestimates the real transfer rate by an order of magnitude.