Synthesis of 1,3-butadiene from ethanol in an ohmic reactor

Within the framework of the Fraunhofer lead project "ShaPID", the three institutes Fraunhofer ISC, ITWM and UMSICHT are pooling their expertise to demonstrate new energy- and resource-saving synthesis routes for basic chemical building blocks using the synthesis of 1,3-butadiene from ethanol as an example. Core elements of the research: the development and construction of a novel fixed-bed reactor with direct electrical resistance heating (UMSICHT) and the development of functionalized electrically conductive catalyst moldings (ISC and UMSICHT). The development process is supported by an accompanying mathematical reactor modeling and simulation (ITWM). The realizable resource and energy savings will be quantified after successful demonstration of the synthesis process.

With a 33 percent share of primary energy consumption in the manufacturing sector, the chemical industry is one of the most energy-intensive sectors and faces major challenges in making its contribution to achieving climate protection targets. Challenging targets such as defossilization of production processes and greenhouse gas neutrality can only be achieved through a comprehensive transformation process. This specifically also requires the development of alternative synthesis routes for basic chemicals.

Direct resistance-heated – so-called – ohmic reactors can be operated with high energy efficiency at 100 percent with electricity from renewable sources. In addition to lower heat losses, they offer further advantages such as greater flexibility and better controllability of the reaction conditions. If – as in the present example – the use of a biogenic raw material base in the form of bioethanol is possible, the use of fossil resources can be dispensed completely. Ohmic reactors thus represent a promising reactor technology to support the necessary transformation process in the chemical industry. The aim of this work is to demonstrate and balance the synthesis of basic chemicals using the example of 1,3-butadiene synthesis from ethanol in an ohmic reactor.

Electrically conductive catalysts are used to realize direct electrical resistance heating. These can be extruded, e.g. with the addition of electrically conductive particles to the extrusion compound, as ceramic solid catalysts in various forms (e.g. pellets, honeycombs). Alternatively, the catalytically active component can also be coated to electrically conductive molded bodies (e.g., highly porous SiC foams). Both methods were successfully implemented for the model reaction of the synthesis of 1,3-butadiene from ethanol.

Synthesis is executed in special reactors, which can be individually adapted to the different applications. Besides the variation of the usual process parameters such as pressure and temperature, the voltage supply of the conductive materials poses a challenge here. In the current experiments, different setups and designs are being investigated with regard to their suitability for subsequent industrial implementation. For these investigations, the catalyst supports are used directly in various forms in order to be able to determine the properties of the reactor design, catalyst support and also their interactions at an early stage of development.

The microstructure of the reactor (fill of pellets, foams...) is virtually reproduced and thus usable for simulation and optimization calculations.

Based on this, various aspects of the processes in the ohmic reactor can be simulated, investigated and optimized. Thus, the electrical resistance heating can first be investigated on the basis of the applied voltage. This already provides an initial estimate of the energy required for suitable heating. By means of a kinetic description of the reactions taking place, the synthesis of 1,3-butadiene from ethanol is calculated in advance. For this purpose, a flow simulation is first performed to determine the convective and diffusive transport through the reactor. The overall reaction is endothermic, which is why heating is necessary. The final temperature distribution thus depends on the heating, the heat of reaction as well as the convective mass transport. From the simulation results, conversion, selectivity and yield can be calculated. The simulation studies thus support reactor development in advance with regard to the selection of suitable geometry adaptations or process conditions to quickly achieve high throughput rates.

• Fraunhofer Institute for Silicate Research ISC
• Fraunhofer Institute for Industrial Mathematics ITWM