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Sulphur Recovery Process

The project involved the optimization of 5 sulphur recovery plants (1, 3, 4, 5 and 6) by means of precise control of the H2S:SO2 ratio in the tail gas from the Claus process. Two acid gas streams: amine acid gas and ammonia acid gas, flow to two separate headers which subsequently feed the amine acid gas to all 5 sulphur recovery plants and the ammonia acid gas to 4 of the sulphur recovery plants (3, 4, 5, and 6). The acid gas streams flow from Hydro-desulphurisation plants 1 and 2, Catalytic plants 1 and 2 and one Hydro Diesel plant. In addition, streams from the sour water exhaust columns 5, 6, 7,and 8 commingle in the ammonia acid gas headers. From the headers, the acid gas streams are fed to the 5 sulphur recovery units which burn part of the H2S in the presence of air, supplied via a combination of blowers. The tail gas then goes through three sulphur condensation phases at the end of which a tail gas analyser measures the ratio of H2S:SO2 which must be kept under precise control for optimization to be accomplished.

Optimization fundamentals

  • Keep the flow rate of both acid gases in recovery units equal to the flow rate from contributing plants while maintaining an appropriate pressure in the headers.

  • The total amount of acid gas treated by the recovery units distributed in proportion to their treatment capacities.

  • Air flow controlled by fine control valves. Coarse control valves to keep fine valves operating within desired range.

  • Set point of the air flow rate determined using adaptive predictive expert control so that the ratio H2S:SO2 is kept stable and within narrow limits around 2.

Optimisation Strategies

  • Pressure of amine and ammonia acid gas. ADEX gas pressure controller determines the acid gas flow which needs to be absorbed by each of the sulphur recovery plants.
  • Combustion air flow. Two ADEX controllers determine the position of the fine and coarse control valves in such a way that the fine valve operates within a suitable operating range
  • Ratio H2S:SO2 : Calculates the set point of combustion air flow for each plant using flow rates of amine acid and ammonia acid gases.

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