Kinetic modelling and process simulation for H2 production by steam reforming of diluted bioethanol solutions

INTRODUCTION An experimental kinetic investigation has been carried out for ethanol steam reforming (ESR). The selected catalyst was a 9 wt% K2O-10 wt% Ni/ZrO2 sample prepared by flame pyrolysis, which revealed particularly active and stable for this application based on previous investigations of the group. Ethanol conversion, selectivity to the main possible byproducts (methane, ethylene and acetaldehyde), hydrogen productivity and the CO/CO2 ratio, as a measure of the contribution of the water gas shift reaction, were correlated to the experimental variables chosen: temperature, water/ethanol ratio and space velocity varied according to a central composite experimental design. The parametric dependence of the reaction outcomes helped the qualitative assessment of the best operating conditions and suggested hypotheses on the reaction mechanism. A more quantitative parametric analysis was carried out by multivariate analysis. Particularly dramatic experimental conditions have been adopted in order to highlight the formation and further evolution of possibly critical intermediates, such as ethylene and acetaldehyde. Indeed, to keep ethanol and intermediates conversion below 100% at least at 550°C to guarantee coke-free operation, the space velocity and feed dilution were increased. An extended microkinetic model has been proposed for the ethanol steam reforming reaction that includes kinetic steps for the formation of important byproducts such as ethylene, acetaldehyde and coke. The model, initially drawn on our own experimental data, was also validated against literature data collected over a different catalytic system and without the presence of the selected byproducts. EXPERIMENTAL The catalyst was tested under the following experimental conditions1, in order to highlight the formation of important byproducts and intermediates, such as ethylene, acetaldehyde and coke: Min Max T (°C) 550 650 Catalyst (mg) 26 132 EtOH : H2O (mol/mol) 1 : 5 1 : 3 GHSV (h-1) 25000 125000 RESULTS AND DISCUSSION An increase of temperature did not adequately improved H2 selectivity, whereas the water/ethanol ratio was an effective parameter to push forward H2 productivity. The reforming reactions of ethanol and of acetaldehyde and ethylene were dependent on the three parameters (kinetically controlled), whereas the CO/CO2 ratio was substantially independent on the space velocity. The data were elaborated by setting up a Matlab script for data regression. A Plug-flow reactor model was implemented, neglecting radial dispersion and back mixing. Isothermal operation was assumed and checked experimentally, due to the very low amount of catalyst, diluted in SiC. The same model was also implemented in the Aspen Plus process simulator, obtaining comparable results. The proposed microkinetic model returned very straightforward predictions of catalyst performance for the literature data: in this case the lump sum of all the square residues (calculated vs. tabulated mol fractions) was 2.88 x 10-2 over a dataset of 408 points2,3. For our own kinetic data, the model optimally predicted the conversion of ethanol and the evolution of the main byproducts, which include ethylene and acetaldehyde. The evolution of coke was interpreted less satisfactorily because of a missing step for coke gasification. Rate-limiting steps have been identified together with very rapid reaction steps, which can be considered to be substantially equilibrated. CONCLUSION A parametric study and kinetic testing for ethanol steam reforming has been carried out on a K-promoted Ni/ZrO2 catalyst prepared by flame pyrolysis. An original microkinetic reaction mechanism was drawn to interpret these data. REFERENCES 1. M. Compagnoni, et al., Appl. Catal. B: Environ. 203, 899 (2017) 2. A. Tripodi, et al., ChemCatChem 8, 3804 (2016) 3. I. Llera, et al., Chem. Eng. Sci. 71, 356 (2012)