Process simulation for the production of hydrogen and ethylene: exploitation of diluted 2nd generation bioethanol solutions as poorly expensive raw material

Process simulation has been used for the design of an integrated process for the production of hydrogen and ethylene from diluted bioethanol solutions. Heavily dehydrated ethanol is not needed for both ethanol steam reforming and dehydration, allowing the exploitation of less expensive and energy intensive feedstock. Process conditions and feed concentration have been optimised for both processes with the aim of process intensification. 1. Scope Bioethanol has gained attention as renewable fuel, but this application requires energy demanding dehydration, which impacts for 50-80% 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 supply heat and prevent coking (dehydration). 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. The investigation is based on 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. All these experimental data are the basis for process simulation to assess the feasibility of the use of diluted bioethanol for all these applications and the relative process optimisation. 2. Results and discussion Differently diluted 2nd generation bioethanol solutions (50 and 90 wt%) have been tested both for ethylene and hydrogen production. No significant effect of impurities was observed, leading to similar activity and selectivity for the less purified feedstock than for absolute ethanol. The effect of different water/ethanol ratio in the feed has been varied selecting the optimal operating temperature (625°C for steam reforming and 400°C for ethylene production) allowing to exploit a 50 wt% bioethanol solution, easily and economically obtained by flash distillation of the fermentation broth. Based on these experimental results, a cogeneration unit with residential size has been simulated by means of the Aspen Plus process simulator (Figure 1). The system was experimentally tested in the recent past and is described elsewhere 1,2. It 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 as feed of a polymer electrolyte fuel cell. During process simulation we optimized catalyst amount (contact time), temperature and water/ethanol ratio. As a result 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. Increasing the water/ethanol ratio in the feed monotonously improved hydrogen productivity due to favourable thermodynamic equilibria, but additional water in the feed must be vaporized. This implies lower electrical output from the fuel cell due to the use of a fraction of the produced reformate as fuel to heat up the reformer. At the end, higher electrical power output can be achieved with intermediate water/ethanol ratio. The total power output, including thermal energy recovery, monotonously increased with feed dilution, because excess water provided in the feed can be recovered from the plant in the form of hot water (65-80 °C), which can be valorised in the cogeneration plant. Figure 1: Flowsheet of a heat and power cogeneration unit fed with diluted bioethanol. 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 (Figure 2). A really cheap diluted bioethanol solution as feed can justify a moderate additional duty to the heater. Figure 2: Heat recovery scheme for an ethylene dehydration reactor. 3. Conclusions The proof of concept for the use of cheap diluted bioethanol solutions is provided for the production of hydrogen/syngas and ethylene. Taking as basis experimental data, we optimized process conditions (temperature, water/ethanol ratio and contact time). Heat recovery is a must to improve process efficiency. References 1. I. Rossetti, J. Lasso, M. Compagnoni, G. De Guido, L. Pellegrini, Chem. Eng. Trans. 2015, 43, 229. 2. I. Rossetti, M. Compagnoni, M. Torli, Chem Eng. J. 2015, 281, 1024 (part I) and 1036 (part II).