Retrofitting the thermal stage of a SRU unit (TiA)
The original burner at a site was a multi-flame burner with low pressure drop on the Claus gas side.
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There is a ceramic block for each individual flame (48 flames) with undefined mixing of oxidation air and Claus gas. The ceramic blocks are not centrical hence the Claus gas is not distributed evenly. The flame is unstable during start-up and lower load cases. Corrosion is prevalent due to condensing of Claus gas.
Even though the original reactor was equipped with a checker wall in the first third of the chamber, the temperature distribution was not homogenous and refractory would burst over time due to stress in the material. The checker wall is also positioned too close to the burner and can lead to problems with long flames. The other main purpose of the checker wall was to improve the H2S to S conversion, which was also not met as it was inefficient.
The original waste heat boiler (WHB) was a smoke tube boiler with an internal bypass with a capacity of 75 t/h at 36 barg and 250°C. The steam drum was directly installed on the boiler. The biggest issue was weld cracking, which arose due to thermal stress caused by an inhomogeneous temperature profile. The challenges were:
• Process parameters up- and downstream should not be impaired (remain the same).
• Only low pressure on the Claus gas side is available. (High turbulence is difficult with low pressure availability.)
• Low pressure availability on the oxidation air side
• High H2S to sulphur conversion rate
• Overall size and dimensions should remain the same (reactor and waste heat boiler).
Our aim was a maximum pressure drop of 100 mbar(g) through the complete unit (burner, reactor and WHB). Of utmost importance was generating a high turbulence and mixture of oxidant and Claus gas, despite low pressure availability. A guaranteed conversion rate of 65% was required, as well as increasing the maximum sulphur capacity to 120%.
The following steps were taken to meet these requirements:
• CS low pressure swirl burner We implemented our low pressure double staged swirl burner, which is based on our existing and proven CS burners.
• Additional mixing section using a VectorWall By utilising a VectorWall with low pressure drop we increased the swirl and turbulence further. Reactor has the same dimensions as the previous one but with a VectorWall instead of a checker wall. The VectorWall generates separate mixing zones for an even distribution over the entire cross section.
• New waste heat boiler and CFD analysis A new waste heat boiler was designed by a partner to keep the pressure drop as low as possible. Like the original waste heat boiler, it was a smoke tube type boiler with the steam drum installed on top. The design of the heat exchangers, bypass and internal pipings focused on keeping the pressure drop as low as possible, while ensuring the necessary steam capacity. Multiple CFD analysis instances were done to verify the design and to ensure that the requirements are met.
The results were a new thermal process stage for a Claus plant for 700 t/d sulphur output with sulphur conversion ≥65%. Pressure drop is low throughout the unit which is based on a state of the art design with low maintenance.
This short case study originally appeared in PTQ's Technology In Action Feature - Q4 2021 Issue.
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