Failure of commercial extruded catalysts in simple compression and bulk thermal cycling

Abstract

The mechanical performance of three commercial extruded catalyst pellets was characterized by single particle compression testing in five orientations and bulk crush testing during thermal cycles from 20°C to 900°C in a reformer tube. Failure loads were analyzed with Weibull statistics, and fragment shapes were cataloged. Maximum principal stresses from finite element simulations were consistent with the shapes of fragments from single pellet tests. In smaller scale bulk tests, excluding pellets in contact with the top and bottom plates, the most common location of damaged pellets was in contact with the reformer wall. In one large-scale test (280 pellets), damage was most common in pellets at the reformer wall. The most common fragment shapes from ex-service single pellet tests and bulk tests are similar, but differ from those from single pellet tests. Neither single pellet compression testing nor conventional bulk crush testing is a sufficient analog for the loading conditions on catalyst pellets in reformer tubes. This study demonstrates that catalyst pellet damage occurs in each full thermal cycle, and that using thermal cycling to reproduce the boundary conditions in service is essential for future studies. K E Y W O R D Scatalysts/catalysis, finite element analysis, fracture, thermal treatment | INTRODUCTIONCatalyst pellets are used in gas phase reactions including methane steam reforming where the conversion takes place inside a reformer furnace operating at around 900°C by passing methane and steam through hundreds tubes filled with the pellets. The operating temperature of the reformers is limited at the high end by the decreasing creep life of the alloy reformer tubes and at the low end by the decreasing efficiency of the desired reaction. The temperature of reformers is monitored as part of process control and is observed to fluctuate during operation. All temperature cycles induce thermal expansion on heating and contraction on cooling in the steel reformers. When the plant shuts down, there is a complete temperature cycle. It is anticipated that catalyst slumping on heating and crushing on cooling will damage catalyst pellets and adversely affect the reforming process.Pellets are designed for optimum heat transfer, gas flow, catalytic efficiency, and stability.1 There is a large literature on alumina scaffolds including some studies of mechanical performance, 2-4 but less on calcium aluminate. 5-8 Catalyst pellet damage and fracture affects the pressure drop, local temperature, and overall efficiency of the reforming process. 9 Understanding the relation of temperature cycles to pellet damage is important for plant operators in order to prioritize control of processing variables. Robustness of pellets is assessed through single particle strength (SPS) or side crushing strength (SCS) where a pellet is loaded in compression between two platens, multiple particle crushing strength (MPCS) where a spherical pellet is loaded in compression between a platen and a jig with

Modeling Kinetic, Thermodynamic, and Operational Effects in a Steam Methane Reformer. Part A: Reformer Output

“…In this novel approach (detailed in the Supporting Information), the fraction of crushed particles is estimated from the number of cold events; this is geometrically related to catalyst bed properties ( D p , ε and ρ cat ). The crushing mechanism has been well covered by Baharudin and Watson and Young et al…”