Diluted Bioethanol Solutions for the Production of Hydrogen and Ethylene

Abstract

The possibility to use diluted bioethanol solutions is investigated. Ethanol purification and anhydrification impacts for 50-80 % of its production cost, but it is compulsory only when bioethanol is used for gasoline blending. Bioethanol can be converted to hydrogen, an increasingly important energy vector, through the steam reforming process. In this case, water must be fed at least in stoichiometric ratio (i.e. 3:1 mol/mol). Nevertheless, higher water/ethanol ratio is usually suggested to achieve CO conversion through the water gas shift reaction and to prevent catalyst deactivation by coke deposition. In this light, the use of dehydrated ethanol is detrimental for process efficiency and cost sustainability, opening the way to the use of more diluted bioethanol solutions. The effect of bioethanol purity and concentration is here investigated both experimentally and through process simulation with Aspen Plus®. Two solutions of second generation bioethanol, kindly supplied by Mossi&Ghisolfi, with ethanol concentration 90 and 50 wt% and obtained after different separation processes (distillation and flash, respectively) were compared with pure ethanol 99.9 vol%. We have concluded that, if the steam reforming process is carried out at temperatures above 600 °C, all the feeds brought to the same results. If temperature is lowered to 500 °C (with the aim of process intensification), different results have been obtained depending on catalyst formulation, depending on materials resistance towards some possibly poisoning compounds contained in the 50 wt% feed. The same approach was followed by considering the possibility to produce bioethylene trough the dehydration of bioethanol. At first, we carried out a thermodynamic study to understand the effect on ethanol conversion and products distribution of increasing water amount in the feed. We observed that the equilibrium conversion is shifted by ca. 50 °C towards higher temperature when using 50 wt% ethanol rather than the pure one. Furthermore, we simulated a possible process configuration accounting for heat recovery, in order to heat up and vaporize the inlet water thanks to the heat recovery with the products stream. Finally, we compared different catalyst formulations, mainly constituted by acidic BEA zeolites, in case added with small Ni amounts to improve selectivity to ethylene. 100 % conversion of diluted ethanol and 99 % selectivity to ethylene have been achieved under the best reaction conditions. Also in this case bioethanol solutions with different purity have been compared with negligible effect on the results.