Sulphidation corrosion in a claus waste heat boiler
Simulation enabled a refiner to avoid potentially dangerous operating conditions in a sulphur recovery unit.
SIMON WEILAND and NATHAN HATCHER
Optimized Gas Treating, Inc.
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Sulphidation corrosion and thermal cycling in Claus waste heat boilers (WHB) are the two leading causes of unexpected failures that result in unscheduled shutdowns of sulphur recovery units (SRU). Sulphidation corrosion is caused by two main factors: high temperatures, typically exceeding 650°F (343°C), and hydrogen sulphide (H2S). Hot hydrogen sulphide is prevalent in an SRU, especially at the front end where the thermal reactor (originally called a reaction furnace) and WHB operate. Refractory lining is used as protection wherever metal surfaces may see high temperatures and H2S. Other special protection measures are put in place in the front end of the WHB in the form of ceramic ferrules to protect the tube-to-tubesheet transition joint as well as the first 6—12 inches of heat transfer tubing. It is in this region of the WHB where most failures occur.
Although metal temperature and H2S concentration are the two basic factors that go into determining the rate of sulphidation corrosion, tube metal temperature itself is influenced by several parameters. In a WHB, one of these factors is the heat flux. Heat flux is the rate at which heat is exchanged from the hot process gas through the tube wall to the cool utility water/steam. The faster that heat is transferred through the tube walls, the hotter the tube walls become, making it a vital consideration when designing a WHB or when managing changes in an SRU’s operating conditions. Oxygen enrichment also can have a profound effect on heat flux and capsize a border-line issue into a catastrophic failure.
Sulphidic corrosion is generally calculated based on the widely accepted Couper-Gorman curves1 which, at least for high heat fluxes, have been very closely confirmed by measurements made by Alberta Sulphur Research Limited (ASRL) for carbon steel. Couper-Gorman curves are also available for a variety of chromium-containing metallurgies and are generally used to assess corrosion in systems containing H2 and H2S. Claus WHBs are typical examples of such systems. The curves were created based on an industry survey conducted by the National Association of Corrosion Engineers (NACE) Committee T-8. An excellent NACE article2 summarises much of what is known about the corrosion of carbon steels and higher metallurgies in this context. The Couper-Gorman curves were modified downwards by a factor of between two and three by Martens3 based solely on anecdotal evidence but the original curves have been validated by the already-cited ASRL work.
A North American refinery was considering adding oxygen enrichment to the Claus unit in an effort to increase capacity. Using oxygen enrichment in an SRU can be a simple and inexpensive way to squeeze more hydraulic capacity through a plant without requiring a huge capital expenditure. This is achieved by increasing the oxygen content of the air, which replaces the diluent nitrogen that otherwise simply takes up volume. Oxygen enrichment has been successfully practised for many years, but there is an increased tendency for failures caused by the resulting higher operating temperatures.
Figure 1 shows the facility as a typical three conversion stage SRU with a front-side split design on the thermal reactor, and a two-pass WHB. In addition to amine acid gas (AAG), this plant processes sour water acid gas (SWAG) with high ammonia content. As designed, the SRU operates on air only. Consideration is being given to upgrading to low level oxygen enrichment (up to 28% oxygen). The WHB steam generation pressure in this plant operates with a somewhat elevated pressure of 657 psig (45.3 barg). Normally, WHBs are designed to operate somewhere between 300 and 600 psig (20-40 barg). This will be important later in the discussion.
To gain a better understanding of how oxygen enrichment will affect the SRU and to reveal any possible pitfalls, the unit’s present operating conditions were used in a simulation model of the facility using the SulphurPro simulator. The corrosion calculations are built into SulphurPro and are based on carbon steel tube metallurgy using recent research data from ASRL and the Couper-Gorman plots reduced to correlations. The model provided a good picture of the heat transfer characteristics along with an estimate of the present corrosion rate in the WHB. The blue line in Figure 2 shows the tube wall temperature through the first pass of the WHB under current air-only base conditions.
The front of the WHB (Pass 1) is where the tube wall temperature (and the heat flux) are highest. The tube wall temperature is slightly elevated at just over the recommended maximum tube wall temperature of 650°F (345°C). The blue line in Figure 3 shows the sulphidation corrosion rate in the first pass of this WHB for this air-only base case. The peak corrosion rate is just over 21 mils per year (m/y) based on the Couper-Gorman corrosion curves. It is a bit alarming that, on air-only operations, the peak corrosion rate is already twice the recommended limit of 10 m/y where most of the last 20 feet of tubing hovers. Returning to the steam generation pressure of just over 657 psig, the higher the steam generation pressure, the higher the heat flux needs to be to meet that demand. A higher heat flux will necessitate a higher tube wall temperature and cause the corrosion rate to increase.
With the base case understood, oxygen enrichment was added to the simulation and the acid gas rates adjusted to keep the same overall hydraulic throughput in the unit. The red lines in Figures 2 and 3 correspond to enrichment to 28% oxygen. The tube wall temperature has increased to over 700°F (370°C) at the inlet and the sulphidic corrosion rate is just over 29 m/y. Although that may not sound like a big increase, it is 50% higher than the base-case air-only operation.
This particular plant was already operating with a higher than normal WHB steam generation pressure, causing the tube wall temperatures to be in the ‘caution’ region at just over 650°F. This resulted in a much higher than advisable corrosion rate of over 21 m/y when operating on air only. Even low level oxygen enrichment compounds the problems already present, increasing the corrosion rate to over 30 m/y (by more than 50%), thereby reducing the life expectancy of the WHB tubes even further. Increased sulphidic corrosion stems in part from the unusually high utility-side pressure causing higher tube temperatures, and in part from higher temperatures consequent to using oxygen enriched air. The effect of utility-side pressure is instructive.
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