Combining CFD Derived Information and Thermodynamic Analyses to Investigate Waste Heat Boiler Characteristics

A series of computational fluid dynamics (CFD) and numerical analyses were performed to investigate operational characteristics in a sulfur recovery unit waste heat boiler (WHB). Similar analyses of WHBs have been reported by the authors (Porter et. al. [1, 2]). The initial focus for the current investigation was to determine the reason for metal loss on the inside of the tube. This required extending the focus of the previous analyses that concerned a) the departure from nucleate boiling (DNB) leading to critical tube temperatures, and b) the downstream fluxes and temperatures from the inlet ferrule, to also investigate high inside surface temperatures of the tubes caused by shell-side tube outer diameter (OD) fouling. The results of the investigations were combined to provide future operational guidance for the boiler. As in the previously reported analyses, CFD submodels of the WHB process-side inlet were constructed and analyzed to determine the fluxes and temperatures that occur during several operational conditions. Queried results of these analyses were combined with the WHB’s historical operational data to predict the nominal operational temperatures, and associated corrosion rates on the inner diameter (ID) of the tube. A second set of submodels was used to determine inside tube operating temperatures resulting from external fouling. The queried results of these analyses were combined, using an expansion of standard thermodynamic analysis techniques, to study possible fouling regimes based on the standard fouling growth equation. Additionally, a 3-dimensional CFD analysis was conducted on the shell-side of the boiler. This analysis allowed the determination of the margin of safety (MOS) from a fall-off-the-cliff (FOC) event [1]. The results of the submodels, numerical analyses and the 3D shell-side analysis of the boiler were combined to determine operational limit curves for the boiler that were based on measurable process parameters including mass flow rate and thermal reactor temperature. It should be noted that the procedures and analyses detailed in this paper do not comprise the complete analyses performed to qualify the past performance of the boiler and to determine future boiler operational limits. Additionally, due to the proprietary nature of the investigations, the specific numerical values related to the boiler’s operation are not presented. Only the derivation of the equations and logic associated with the investigation and the derivation of operational guidance are given. Complete engineering to determine these limits requires additional analyses not detailed in this paper.Copyright © 2011 by ASME