New Insights with Rate-Based Claus Waste Heat Boiler Modelling
The Claus Waste Heat Boiler (WHB) is a critical piece of equipment in the sulphur recovery unit (SRU). As refiners and gas producers have pushed towards higher
sulfur feedstocks placing more load on the SRU, failures have become more common. These higher failure rates have come at a time when uptime metrics and environmental constraints have become concurrently stricter.In this work, a set of case studies were run using a newly developed rate-based heat transfer and chemical reaction rate model of the Waste Heat Boiler. The rate-based model provides quantitative insights into several aspects of the WHB that impact the sulphur plant performance:
H2 + ½ S2 --> H2S CO + ½ S2 --> COS
- Recombination reactions that occur at the front of the WHB.
These reactions not only influence sulphur recovery, air demand, and hydrogen production in the SRU, they also impact the heat flux and performance of the WHB. These reactions occur towards the front (inlet) side of the WHB and are exothermic. The “hidden” heat associated with these reactions tends to increase heat flux near the critical tube-to-tube sheet joint.
- Radiation impacts on heat transfer also occur primarily towards the inlet of the WHB.
Radiative heat transfer, coupled with the exothermic recombination reactions collectively increase the peak heat flux at the front of the boiler well above predictions from models that ignore or discount these factors. Tube wall temperatures, pressure drop, and heat flux predictions from the model are examined down the length of the tubes for an oxygen-enriched and an air-only sulphur plant as a function of tube size and mass velocity. Surprising findings show elevated tube wall temperatures well downstream of the area of protection provided by ceramic ferrules for the higher mass velocity cases, validating documented failures in the industry. The implications of sulfidic corrosion and the resulting impact on boiler tube life and sulphur plant reliability economics are examined with this new information.