(45f) Development and Optimization of a Microstructured Reactor for the Partial Oxidization of Propane to Propene

Motivation

So far, mainly fluid phase reactions have been investigated in terms of their transferability into microstructured reactors. However, this type of reactor is also excellently suited to be used for heterogeneously catalyzed gas phase reactions. Taking the oxidative dehydrogenation of propane to propene over a VO_x/Al₂O₃-catalyst as a model reaction, a concept for a microstructured reactor was developed and optimized which outperforms a conventional tubular fixed bed reactor under certain reaction conditions.

Experimental

In order to deposit catalytic materials as active coatings (wall catalysts) onto the microstructure, several alternatives for the catalyst preparation were developed. Simultaneously, different binder systems for stabilizing the coatings were investigated. Furthermore, a manufacturing method for the microstructured reactors had to be developed to ensure a high-temperature and pressure stability of the modules.

In order to investigate the influence of the reactor design on the chemical process, tests on residence time, thermal and catalytic behavior were performed to compare a tubular fixed bed reactor (inner diameter 15 mm) with the microstructured reactor. Catalytic tests were carried out under the following standard reaction conditions: The composition of the reactant flow was 55% N₂, 30% C₃H₈ and 15% O₂, nitrogen acting as inert diluent. The catalyst loading of the reactors varied from 20?200 mg. Volume flows were kept between 30?240 ml_n/min in a temperature range from 400?600 °C.

Results

The catalytic coatings were optimized with regard to a good adhesion of the ceramic support material on the metallic reactor surface guaranteeing a mechanically and thermally stable wall catalyst. By implementing a new manufacturing process, in which the structured and catalyst coated steel platelets are soldered at 400 °C, a substantial connection of the single platelets is obtained providing a very effective heat transfer which results in a uniform heat distribution.

The characterization of the reactors in terms of residence time distributions is not significantly different for the tubular fixed bed reactor and the microstructured reactor. However, looking at the thermal behavior pronounced differences can be observed. For similar reaction conditions the tubular fixed bed reactor shows a temperature profile of more than 100 K while the microstructured reactor works under isothermal conditions.

Using standard reaction conditions, the tubular fixed bed reactor and the microstructured reactor show a similar catalytic behavior. However, the binder system that was used to stabilize the active coating in the microstructured reactor greatly influences the performance of the catalyst. Alumina- and silica-containing binders decrease activity and selectivity. In contrast, organic binders do not show any influence on the catalytic performance. Distinct differences between the investigated types of reactors can only be observed when standard reaction conditions are changed (higher temperature, less inert diluent and distributed oxygen feed), making the microstructured reactor the better choice compared to the conventional tubular fixed bed reactor. Particularly, the distributed oxygen feed has a positive effect on propene selectivity decreasing the oxygen partial pressure over the catalyst's active sites.

Conclusions

Taking the oxidative dehydrogenation of propane to propene as a model reaction, a concept for a microstructured reactor was developed and optimized which outperforms a conventional tubular fixed bed reactor under certain reaction conditions. It was shown that further research on the transferability of heterogeneously catalyzed gas phase reactions into microstructured reactors is reasonable and suggested.