New insights into Claus waste 
heat boilers

The real plant performance of a waste heat boiler depends on many factors besides heat transfer.

Optimized Gas Treating

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Article Summary

The Claus waste heat boiler (WHB) is a critical piece of equipment in the sulphur recovery unit (SRU). As processors move towards higher sulphur feedstocks, more load is placed on the SRU, and WHB failures are becoming more common. Higher failure rates have come at the very time when uptime metrics and environmental constraints have also become stricter.

A set of case studies is reported using a newly developed rate based heat transfer and chemical reaction model of the WHB which provides quantitative insights into several aspects of the WHB that affect SRU performance:
• Recombination reactions that occur at the front of the WHB are:

H2 + ½ S2 ⇌ H2S
CO + ½ S2⇌ COS

These reactions not only influence sulphur recovery, air demand, and hydrogen production in the SRU, but they also affect 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 them tends to increase heat flux near the critical tube to tubesheet joint.

• Radiation affects heat transfer, 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, heat flux, and corrosion rate predictions from the model are examined down the length of the tubes for an oxygen enriched and 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 sulphidic corrosion and the resulting impact on boiler tube life and sulphur plant reliability are examined with this new information.

The WHB (see Figure 1) is arguably the most fragile part of an SRU and is subject to sudden and very costly failure. The most common failure point is the tube to tubesheet joint where temperatures can become unacceptably high, causing the welds there to fracture and the joints to fail. To provide operability, this region of the WHB is protected by ceramic ferrules (see Figure 2) inserted a short distance into the tubes and which usually also completely cover the face of the tubesheet (see Figure 3). On the utility side, high or medium pressure steam is usually generated (heat recovery) by cooling the hot gas on the process side. Sulphur is not usually condensed in the WHB except at turndown conditions. 
As heat is removed in the WHB, a number of interesting reactions take place (see Equations 1-4). The S2 vapour allotrope is exothermally converted into the S6 and S8 forms as the gas is cooled (see Equations 1 and 2). Reactions of at least equal importance involve hydrogen recombination with S2 vapour (see Equation 3) and COS formation from carbon monoxide and S2 vapour (see Equation 4). These reactions are also exothermic and take place primarily at the WHB’s front end:1-5

3S2 ⇌ S6 + heat                      (1)
4S2 ⇌ S8 + heat                      (2)
2H2 + S2 ⇌ 2H2S + heat         (3)
2CO + S2 ⇌ 2COS + heat        (4)

Because of the high inlet temperature of the process gas, radiation also plays a significant role in heat transfer in the WHB. This is quite unlike the heat exchangers further downstream.

Approaches to recombination modelling
The recombination reactions can generate significant heat near the front of the WHB (close to the fragile tube to tubesheet joint area), so getting the simulated temperature there as correct as possible is important. Until very recently, the models used by all commercially available SRU simulators handled recombination by one of several obfuscation techniques:
• Ignore local recombination and assume the reaction furnace is at equilibrium
• Lump these reactions into the reaction furnace effluent
• Freeze the reactions by assuming they reach equilibrium at a user supplied quench temperature.

The only correct approach is to model the reactions as they truly are: fully reaction kinetics rate based. With the advent of the SulphurPro simulator, this approach is now available.

The SulphurPro simulator uses a first principles, rate based model that incorporates the effects of
• Reaction kinetics
• Rigorous heat transfer (including temperature, composition, and geometry dependent radiation)
• Condensation calculations of liquid sulphur (including thermodynamic and physical property effects resulting from the varying distribution of sulphur allotropes).

The interdependency of physical properties, reaction rates (and their heats of reaction/redistribution), bulk heat transfer, and stream enthalpies (both latent and sensible) are all considered together to provide a consistent and powerfully predictive modelling tool. The set of equations governing the WHB, including recombination reactions, is numerically integrated along the boiler tube length. Adaptable segmentation is used to yield more accurate results by placing more segments in the locations where properties are changing fastest and consequently require greater numerical resolution.

Reaction kinetics modelled in SulphurPro are based on work whose original purpose was exploration of the two main recombination reactions that occur in the WHB, and in which Arrhenius kinetics parameters were tuned to match sets of experimental, pilot, and full-scale SRU data. Implementation of kinetics in SulphurPro are consistent with the ProTreat simulator’s thermodynamics, with additional refinements made to match internal sets of plant data for both normal operations and off-spec conditions in real operating sulphur plants. All other transport coefficients and physical properties are calculated from proprietary or well-established literature correlations. Case studies will illustrate the importance and relevance of these reactions.

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