Design of multibed reactors for ammonia synthesis plants

Design of multibed reactors for ammonia synthesis plants Ilenia Rossetti1, Antonio Tripodi1, Elnaz Bahadori2, Gianguido Ramis2 1 Chemical Plants and Industrial Chemistry Group, Dip. Chimica, Università degli Studi di Milano, INSTM Unit Milano-Università and CNR-ISTM, via C. Golgi, 19, I-20133 Milano, Italy; 2 Dip. di Ingegneria Civile, Chimica e Ambientale, Università degli Studi di Genova and INSTM Unit Genova, Via all’Opera Pia 15A, 16100, Genova, Italy *Corresponding author: ilenia.rossetti@unimi.it Highlights • Ammonia synthesis loop designed with multibed configuration and simulated • Different catalysts used: Fe from magnate, wustite and Ru/C • Validation of kinetic models for the three catalysts • Optimisation of energy supply and recovery 1. Introduction Ammonia synthesis is a complex heterogeneous catalytic process, where N2, H2 and NH3 adsorption/desorption on catalyst surface are fundamental steps. Although the underlying chemistry is relatively well known, very demanding operating conditions for pressure and temperature are needed. For this reason, the technology is very sophisticated and its study continued during the last century, both in the scientific and industrial communities. Iron based catalyst can be used at 150-250 bar and 380-520°C, they are sufficiently active and not expensive, but the maximum conversion achievable is limited by inhibition by ammonia. The research to decrease the operating pressure is still in progress, relying basically on the search of more active materials. The process includes specifically designed reactors, where a multibed configuration is the most employed. For instance, with traditional Fe-magnetite 4 beds of catalyst, with inter-cooling after each pass lead to a conversion rate up to 15% and a recycling mechanism to achieve up to 98% conversion. Ruthenium showed higher activity than Fe for ammonia production and it is not inhibited by the product. This work proposes the simulation of a relatively low pressure ammonia synthesis reactor (below 120 bar). The kinetic description of the process was based on an original kinetic model, derived on a patented promoted Ru/C catalyst, obtained as a result of 20 years of research in our laboratory: the model was developed modifying the Temkin equation with the addition of H2 and NH3 adsorption terms, in order to consider their possible concurrent inhibiting effect. Different multi-bed arrangements were compared and discussed. A sensitivity analysis was performed to identify optimal operating conditions and catalyst bed assembly. 2. Methods Fe-based benchmark catalyst from magnetite, a newer commercial material from wustite and a proprietary Ru/C catalyst were considered as materials, differently couples in a multibed configuration. The kinetics of the process was validated and the ammonia synthesis loop was simulated with Aspen Plus©, including three adiabatic beds with intercooling. A modified Redlich-Kwong-Soave EOS was used. The target yield was set on a ton/day scale as model unit (55 kg/h of ammonia for 60 kg/h of fresh stoichiometric gases, plus a small quantity of methane). Despite the configuration simplicity, the single loop contains two highly non-linear blocks (the reactor and the separator) which may compromise the calculation convergence at too low purge fractions. 3. Results and discussion Figure 1. Sketch of the ammonia synthesis loop. A detailed study of the ammonia reaction yield with newly available kinetic and thermodynamic parameters was performed. The previously developed kinetic models for Ru/C, Fe-magnetite and Fe-wustite based catalysts were adapted for use in the Aspen Plus plug flow reactor model. The preliminary validation of the calculations against the available experimental data was done to check consistency and to assess the best operating conditions for each material. The best conditions for the single-pass operation of the Ru/C material were selected in the 400 – 450 °C range. Coupling different catalysts and reactor sensitivity analysis, allowed maximum NH3 yield at 100 bar, 400°C inlet temperature of a three-bed converter with intercooling. 4. Conclusions A multibed reactor configuration, with intercooling, was selected, holding different catalyst amounts and types. Mixing initial iron-based catalysts in the first bed(s) with Ru-based catalyst in the last one(s) revealed the best option to improve the yield with optimised catalyst loading. Accordingly, less demanding operating conditions can be envisaged for mixed multibed configurations.