Formation of Side Products in the Synthesis of the Synthetic Fuel Poly(oxymethylene) Dimethyl Ethers and the Implications for Process Design

A substantial reduction of greenhouse gas emissions in each economic sector is essential in order to limit global warming. Especially the transportation sector lacks of quickly deployable alternative technologies due to the wide-spread use of internal combustion engines. Poly(oxymethylene) dimethyl ethers (OME) can be used as fuel in conventional diesel engines, offer excellent fuel properties like soot-free combustion and also reduce local NOx emissions [1]. Additionally, they can contribute to a sustainable transportation sector since they are based on renewable carbon and energy sources [2].

Several synthesis routes for the production of OME usually starting from methanol or dimethyl ether as feedstock are known. Methanol is converted to formaldehyde, which is then reacted with further methanol or dimethyl ether to OME in an acid-catalyzed equilibrium reaction. A recently developed water-tolerant OME process concept uses a methanolic formaldehyde solution as educt and, thus, avoids expensive intermediate process steps [3]. The main reactions leading to the formation of OME are well‑studied in literature [4].

This presentation discusses the routes toward OME comprehensively. It is highlighted which lines of research are currently done in Europe. Further, novel experimental results for the formation of the most relevant side products (trioxane, methyl formate and formic acid) during the synthesis of OME from methanolic formaldehyde solutions under industrial conditions are presented. The acidic ion exchange resin Amberlyst 46 was thereby used as heterogeneous catalyst. Long-term experiments under elevated temperature and pressure were carried out in a batch reactor and the influence of the reaction parameters temperature, educt ratio, water concentration and catalyst amount was studied. A developed pseudo-homogeneous kinetic model shows good agreement with the experimental profiles.

Based on the model, the implications for process design are evaluated within a process simulation. All three side products, trioxane, methyl formate, and formic acid, accumulate within the process. Unlike the concentration of trioxane that is limited to chemical equilibrium, the concentrations of methyl formate and formic acid increase steadily and, therefore, a concept for their removal is needed. Considering a simple purge without further purification, a trade-off between reactor temperature, or catalyst amount, and the resulting loss of product in the purge is detected. An optimal operating point for the process is identified.

References

[1] M. Härtl, P. Seidenspinner, E. Jacob, G. Wachtmeister, Fuel 2015, 153, 328-335. DOI: 10.1016/j.fuel.2015.03.012

[2] S. Deutz, D. Bongartz, B. Heuser, A. Kätelhön, L. Schulze Langenhorst, A. Omari, M. Walters, J. Klankermayer, W. Leitner, A. Mitsos, S. Pischinger, A. Bardow, Energy Environ. Sci. 2018, 11 (2), 331-343. DOI: 10.1039/C7EE01657C

[3] N. Schmitz, E. Ströfer, J. Burger, H. Hasse, Ind. Eng. Chem. Res. 2017, 56, 11519−11530. DOI: 10.1021/acs.iecr.7b02314

[4] a) N. Schmitz, J. Burger, H. Hasse, Ind. Eng. Chem. Res. 2015 54 (50), 12553-1256. DOI: 10.1021/acs.iecr.5b04046. b) D. Oestreich, L. Lautenschütz, U. Arnold, J. Sauer, Chem. Eng. Sci. 2017, 163, 92-104. DOI: 10.1016/j.ces.2016.12.037