Open-cell solid foams as catalyst carrier for structured reactors
Due to the energy-intensive processes of material conversion, the chemical and petrochemical industries are among the largest industrial consumers of energy. Albeit coupling plants by energy and resources in chemical parks, more than on third primary energy are related to those industrial sectors. The largest amounts of energy are accounted to the thermal downstream processing of product mixtures, whose energetic recovery is limited by technical means . In the course of the Helmholtz Energy Alliance ‘Energy Efficient Chemical Multiphase Processes’, different approaches were adopted for the optimization of the example process of the hydrogenation of nitrobenzene to aniline. By switching from the current gas phase process to a multiphase process, large savings are expected at descending gas-liquid flows. To cope the vast amount of reaction heat produced by the process, the application of open-cell solid foams as catalyst carrier denotes a promising option. The continuous network of thermal conductive ceramics and hydraulic accessible void volumes is beneficial for multiphase processes demanding large specific surface areas, high thermal conductivity and low pressure drop.
Within the Energy Alliance joint project, open-cell solid foams were characterized at descending gas-liquid multiphase flows. Known for their high mechanical and thermal strength as well as their good thermal conductivity, SiSiC solid foams (silicon infiltrated silicon-carbide, IKTS Fraunhofer, Dresden) with pore densities of 20, 30 and 45 ppi (pores per linear inch) were applied for experimental investigations.
At first, the hydraulic characterization was carried out at via conventional flow regime measurements, which covered a wide range of liquid and gas flow rates. The generated flow maps reveal the effect of capillary forces, available flow paths and phase homogeneity on the transition from trickle to pulse flow. The shift of the regime transition to larger gas and liquid flow rates opens up the operation mode of fast trickle flow, which combines long liquid contact times with low pressure drops.
For the detailed analysis of the flow mechanism, the ultrafast X-ray computed tomography was applied. Coarse foams were found to suffer from significant maldistribution of fluids at trickle flow, which decreased with higher pore densities and stronger capillary forces. Regardless of the flow regimes, very large liquid contents were observed in solid foams. In further studies, the flow morphology and characteristic parameters were examined for the highly dynamic pulse flow. The frequent, permanent change of liquid distribution after the pulse passage is promising for reactive applications. Volumetric CT measurements – similar to medical spiral CT – were performed to analyze the liquid dispersion at the packing inlet.
To complement the performance assessment of the new packing structure, the limiting current technique was adapted for the continuous direct measurement of the liquid-solid mass transfer in solid foams. With the electrochemical method, the intensive interaction of liquid and packing was proven for trickle and pulse flow. Additionally, improved understanding on the pulse formation and propagation was generated.
Funded by the Helmholtz Association in the course of Helmholtz Energy Alliance – Energy Efficient Chemical Multiphase Processes (HEA-E0004).
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Industrial & Engineering Chemistry Research, DOI: 10.1021/acs.iecr.7b01578