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We have 2-stage Claus with SCOT TGTU. Whenever we have a long shutdown, we keep the Claus section & TGTU section in hot stand-by mode. The Claus Reaction furnace is fuel gas firing and diverted to an incinerator (bypassing the TGTU). The TGTU section has nitrogen circulation in hot mode. The SCOT reactor temperature is maintained at operating temperature (290C).
The problem: we observe that every time the quench pH drops after a few days. What would be the reason?