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

Understanding the effects of in-service microstructural changes in metal alloys on the mechanical performance and remaining life of equipment is a critical part of integrity management in industrial plants. However, the effects of processes such as carburization or nitridation are not accounted for in industry-standard life assessment methodologies such as those provided in the API 579-1/ASME FFS-1: Fitness-For-Service standard. This is problematic for the austenitic stainless steel Alloy 800H, which typically operates in the creep regime and has been reported to suffer nitriding during high-temperature service in air. Despite this being one of the most common service environments for 800H, the impact of nitriding on in-service performance and implications for remaining life assessment have not been well-studied for this alloy. In this work, we characterize the microstructures of as-received, aged, and nitrided Alloy 800H tube material, and correlate observations with room-temperature tensile properties and high-temperature creep behavior. We show that while the creep properties of aged 800H material can be captured by the widely-used MPC Project Omega creep model and API 579-1 Omega properties for 800H, nitrided material properties fall outside of the expected bounds, and therefore remaining life cannot be reliably predicted using this method without experimental data.

Internal nitridation kinetics were determined for a UNS N08810/800H alloy using a general model of the form x n = kt. Nitridation behavior was studied at service-relevant temperatures 800°C-1000°C in a 95% N 2 /5% H 2 atmosphere for times 50 h-750 h. Optical and scanning electron microscopy were used for microstructural characterization and measurement of nitride penetration. AlN, Cr 2 N, and CrN were formed, and the experimentally observed precipitation sequence was consistent with equilibrium calculations for this alloy using Thermo-Calc. A combination of diffusivity data determined using DICTRA and experimentally verified equilibrium calculations showed that Wagner's analysis for internal oxidation kinetics was valid for AlN penetration. Parabolic kinetics closely approximated measured AlN penetration. This suggests that extension of AlN penetration models to other temperatures and Fe-Ni-Cr-Al alloy systems is reasonable. Cr 2 N penetration did not conform to Wagner's analysis. Deviation from parabolic behavior was evident, and general model penetration predictions for Cr 2 N were experimentally validated. Using the experimentally determined models, time-temperature-precipitation diagrams for AlN and Cr 2 N penetration were constructed.

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