(39g) Multiscale Modeling for High Temperature Heterogeneous Gas Phase Synthesis Reactions: HCN Synthesis

The climate change challenge does not only affect energy production and mobility but also chemical production as a whole. While process design is crucial to meet long-term future challenges, process optimization of existing plants can boost resource efficiency and ecological footprint of the chemical industry today. Energy-intensive processes such as high-temperature synthesis reactions stand out in that context because small process improvements have a potentially large impact on the ecological footprint as a whole.

Steam reforming and the synthesis of hydrogen cyanide (HCN) via the BMA route (Blausäure aus Methan und Ammoniak) are classical examples for high temperature heterogeneously catalyzed gas phase reactions. HCN is an important chemical intermediate that is used in a variety of industrial processes such as the synthesis of herbicides, polymer precursors and most importantly the synthesis of food additives such as methionine. While other processes for the synthesis of HCN exist such as the Andrussow or Shawinigan process, none of those meet the high yield and product purity that is achieved in the BMA process. In the industrial process, educts methane and ammonia react in aluminum oxide tubes that are impregnated with a platinum catalyst and are suspended in stacks inside an oven that provides the energy required for the endothermic reaction. The temperature of the reaction is around 1500K to achieve optimum yield at the thermodynamic equilibrium of the involved species. HCN is the major product and stoichiometric amounts of hydrogen are produced as byproducts. The process is operated with slight ammonia excess to prevent coking of the narrow tubes [1]. Optimum heat transfer from the oven chamber to the active catalyst sites on the inside of the tube walls but also mass and heat transport inside the reactor tubes are crucial to achieve the desired yield at the minimum cost of heating fuel. For these reasons a better understanding of the tube-oven interactions is essential that can only be achieved via a coupled model of the reactor tubes and the oven chamber which constitutes an extension of the multiscale design approach [2].

In this work, a two-dimensional model of the synthesis tubes is developed based on the reaction kinetics of Hasenberg et al. [3]. The high temperature and large temperature gradients between inlet and outlet discourage the use of standard Navier-Stokes equations to describe the fluid flow inside the tubes and require the use of compressible flow and the addition of buoyancy and radiation terms in the gas phase. Application of the finite volume method for discretization ensures overall conservation of energy and mass. The tube model is used to predict the required minimum wall temperatures or respective heat flows to obtain a sufficient yield and that information serves as boundary condition of a second model in three dimensions of the oven chamber in order to account for the heating of the tubes. Additional boundary conditions are the design parameters of the oven chamber and the arrangement of the stack of tubes inside the oven. In that manner the two essential goals of the process are accounted for: a maximum synthesis yield at a minimum energy cost. At the high temperatures inside the oven, adequate description of convective and radiative heat transfer are essential and accounted for using different models for radiative heat transfer in participating media. Different reactor designs are simulated and evaluated against the industrially applied benchmark design of the reactor. References:

[1]	Diefenbach M., Brönstrup, M., Aschi, M., Schröder, D., Schwarz, H. (1999), HCN Synthesis from Methane and Ammonia: MEchanisms of Pt+-Mediated C-N Coupling, Journal of the American Chemical Society
[2]	Karst, F., Freund, H., Maestri, M., Sundmacher, K. (2014), Multiscale Chemical Process Design Exemplified for a PEM Fuel Cell Process, Chemie Ingenieur Technik
[3]	Hasenberg, D., Schmidt, L. (1986), HCN Synthesis from CH4 and NH3 on Platinum, Journal of Catalysis