(635a) First European Demonstration Plant for the Synthesis of Renewable Ome Fuels

Poly(oxymethylene) dimethyl ethers (OME) are oligomers of the structure CH₃-O-[CH₂O]_n-CH₃. OME of a chain length of 3 to 5 (OME_3-5) are particularly interesting, because they have diesel-like properties. It has been demonstrated that OME_3-5 significantly reduce soot formation in engine test stands [1] and under real driving conditions [2]. OME_3-5 are produced in all industrially feasible processes exclusively from the raw material methanol, which presents a flexible raw material base, including biomass, carbon dioxide, other organic waste, and natural gas. Within the framework of the project consortium “NAMOSYN” funded by the German Federal Ministry of Education and Research (BMBF), the Technical University of Munich has erected Europe’s first continuous production demonstration plant for OME synthesis with a capacity of 1 kg/h (= 8 tonnes/a) OME_3-5 at the Campus Straubing [3]. Its process flow diagram is shown in Figure 1 [4]. The feed, a methanolic formaldehyde solution with about 10 mass-% water, is mixed with two liquid recycle streams and fed to the reactor. In the heterogeneously acid-catalyzed reactor, a stream comprising formaldehyde (FA), water (WA), methanol (ME), and OME of various chain lengths is obtained. The isolation of the desired product fraction OME_3-5 is carried out in two distillation columns C1 and C2. The first distillation column C1 separates OME of chain lengths n ≥ 3 as bottom product from a stream containing mainly FA, ME, WA and OME_1-2. The second distillation column C2 separates the product fraction OME_3-5 as overhead product from OME of longer chain lengths n ≥ 6 which are recycled back to the reactor. Before the distillate of the column C1 is recycled to the reactor, water is removed in a membrane unit.

The most challenging unit of the process is the reactive distillation column C1 because of the high number of components present and because formaldehyde reacts uncatalyzed with methanol and water. Further, formaldehyde may easily precipitate since its solubility in these mixtures is limited. An equilibrium stage model for the reactive column C1 was developed, which is illustrated in Figure 2. This model includes a novel solid-liquid equilibrium model for formaldehyde conceived by the University of Kaiserslautern [5]. For the first time, the column is verified experimentally at process-relevant conditions including high concentrations of formaldehyde in the feed (> 0.10 g/g).

A further challenge is the separation of water from mixtures containing formaldehyde, methanol and OME. Because of the presence of multiple azeotropes, distillation is not the preferred option for this task [4]. Therefore, our plant uses a membrane unit, developed by the German company DBI Gas- und Umwelttechnik GmbH, that contains an innovative inorganic membrane material to selectively remove water from formaldehyde-containing mixtures as it was also but on a smaller scale investigated by Schmitz et al. [6].

In this contribution, we introduce the demonstration plant for OME synthesis at the Campus Straubing of the Technical University of Munich. The main challenges regarding the operation of the different units are discussed. We present the results of long-term steady-state experiments of the plant running in closed loop. In detail, we are looking at the performances of the reactor, the distillation column C1, and the membrane unit. Experimental profiles are presented and compared to process simulations. The present work shows that the individual as well as the combined operation of all process units is feasible despite the existing challenges and presents a relevant step on the way to a non-fossil transportation era.

References

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[2] M. Münz, A. Mokros, D. Töpfer, C. Beidl. MTZ Worldw. 79 (3) (2018) 16-21. DOI: 10.1007/s38313-017-0185-8

[3] A. Ferre, J. Voggenreiter, Y. Tönges, J. Burger. MTZ Worldw. 82 (5) (2020) 26-42. DOI: 10.1007/s38313-021-0639-x

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[5] C. F. Breitkreuz, J. Burger, H. Hasse, Ind. Eng. Chem. Res. 61 (2022) 1871-1884. DOI: 10.1021/acs.iecr.1c04275

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