Core-shell catalyst design for improved heat management with highly exothermic reactions

Heat management for fixed-bed reactors with exothermic reactions is a critical challenge. The catalyst can be irreversibly destroyed due to sintering or other degradation effects if a certain temperature is exceeded (in instance within localized hotspots).
Common countermeasures (e.g., feed or catalyst dilution) to avoid hotspots in fixed-bed reactors often lead to a significant decrease in space-time-yield and, thus, higher process costs.
Especially for discontinuous production scenarios, a robust catalyst reactor system with improved heat management is essential to allow for load flexible reactor operation that always ensures highest space time yields and lowest downtimes. Such scenarios are intensively discussed with regard to Power to X processes based on renewable energy sources like wind and solar energy.

Based on dynamic simulations for the highly exothermic methanation of CO₂ with hydrogen Prof. Kai Sundmacher and co-workers at the Max-Planck-Institute for Dynamics of Complex Technical Systems developed a novel core-shell catalyst design with a highly active catalyst core surrounded by an inert, low-permeability shell. At low temperatures, the active core is mainly determining the reaction rate, whereas the inert shell has no significant influence. At higher temperatures, the inert shell acts as a mass-transport barrier and prevents the catalyst particle from exceeding a certain effective reaction rate and, thus, inhibits thermal runaway.

Manufacturing Core-Shell Catalysts

The core-shell catalyst design can be produced at low costs and at any scale using standard coating techniques. Shell properties such as porosity, pore size, and shell thickness can be tailored for the given catalyst-reactor system (Figure 2).

Case Study CO₂ Methanation

Several computational and experimental studies have been conducted to evaluate performance and technical feasibility of the new core-shell catalyst particle concept [1,2]. CO₂ methanation represents a highly exothermic example case for which heat management plays a crucial role for the overall process performance. At catalyst scale, Figure 3 (left) illustrates the effect of an inert outer shell on the reaction rate applied to an industrial catalyst.

These trends, seen in simulations as well as lab-scale experiments, confirm the pronounced influence of the shell's mass transfer limitation selectively at high temperatures. At reactor scale, this catalytic behavior leads to a well-balanced fixed-bed temperature field and inhibits hot-spot formation and reactor runaway, while utilizing larger amounts of the loaded catalytic material (Figure 3, right).
In comparison to other state-of-the-art methanation process concepts (using e.g., catalyst dilution, recycle compressors, intercooling, distributed feed injection) the novel core-shell catalyst design offers several advantages [2]:

• Higher space-time-yield: up to 3x higher compared to catalyst dilution
• Lower pressure loss: up to 3x lower compared to catalyst dilution
• CO₂ conversion > 95% already after the first reactor stage
• Simpler process configuration: less or no need for recycle compressors, reactor cascades, intercoolers, distributed feed injection
• Prevents temperatures excursions causing catalyst deactivation and reactor runaway
• Reduced thermal sensitivity with regard to changing operating conditions
• Increased flexibility: faster reactor start-up and shut-down and suitable for rapid load changes.

With these features, product qualities as achieved with the widely used TREMP™ technology by Haldor Topsøe (using a recycle compressor and three to four reactor stages with intercooling) also become accessible by the use of a one-stage multi-tubular fixed-bed reactor filled with the novel core-shell catalyst particles. Ultimately, the simpler process configuration will lead to lower capital and operating costs, less downtimes, as well as higher load flexibilities.

The novel core-shell catalyst design as a general concept should be also useful for other exothermic processes, e.g., Fischer-Tropsch GTL, sulfuric acid synthesis, methanol synthesis, or ammonia synthesis.

• European priority patent application filed in May 2019
• PCT patent application filed in May 2020

1. R. Zimmermann, J. Bremer, K. Sundmacher: "Optimal catalyst particle design for flexible fixed-bed CO₂ methanation reactors", Chemical Engineering Journal 387 (2020) 123704
2. R. Zimmermann, J. Bremer, K. Sundmacher: "Load-flexible fixed-bed reactors by multi-period design optimization", Chemical Engineering Journal 428 (2022) 130771