Exploiting diluted 2nd generation bioethanol solutions for the production of hydrogen and ethylene

Bioethanol has gained attention as renewable fuel, but this application requires energy demanding dehydration, which impacts on its production cost. Two catalytic processes, steam reforming and dehydration, can lead to industrially important products from this renewable source, i.e. syngas/hydrogen and ethylene. In both cases, steam is co-fed as reactant (steam reforming) or, to a lower extent, to supply heat and prevent coking (dehydration to ethylene). Thus, the aim of this work is to provide a proof of concept for the use of diluted (2nd generation) bioethanol solutions as a much less expensive raw material, as a way to improve the economic sustainability of these very promising processes. Taking advantage of previous experimental data collected on Ni-based catalysts for the ethanol steam reforming, zeolites with tuned acidity for ethylene production and on a demonstrative testing of a 5 kW cogeneration unit, based on fuel cells, we assessed the feasibility of the use of diluted bioethanol for all these applications and the relative process optimization through process simulation with Aspen Plus. In particular, a cogeneration unit with residential size has been simulated. The system is constituted by an ethanol steam reformer, followed by high and low temperature water gas shift reactors and a methanator. The achieved hydrogen purity is < 20 ppmv CO, suitable to feed a polymer electrolyte fuel cell. During process simulation we highlighted the dependence of the electrical and thermal power output from the water/ethanol ratio in the feed, reactor temperature and contact time/space velocity in the reformer. The detailed modelling of the reformer and the optimization of the heat exchange network were the key to attain the overall sustainability and stability of the system. The same plant concept has been also designed for the centralized hydrogen production from 40,000 ton/y bioethanol, with full sizing of the plant and the relative economic evaluation under three different scenarios. Process simulation has been applied also to design an ethanol dehydration reactor. Water addition is thermodynamically detrimental for ethanol dehydration to ethylene, imposing to increase temperature (from 200 to 250°C to achieve full ethanol conversion when cofeeding water/ethanol = 3:1 mol/mol instead of pure ethanol. Furthermore, the addition of water in the feed requires additional heat input for its vaporisation. However, process design evidences that most of the energy required to vaporize the feed can be recycled from the product stream, by condensing back excess steam (the amount originally fed plus the amount produced through dehydration) in a heat recovery exchanger. A really cheap diluted bioethanol solution as feed can justify a moderate additional duty to the heater. The feasibility of the use of diluted bioethanol streams (e.g. 50 wt%) has been proven at least at the conceptual design level. The experimental application of 2nd generation bioethanol on bench scale plants for both reactions confirmed that the lower purity of the feedstock is sufficient to avoid catalyst deactivation, opening the way to the use of a much less expensive feedstock.