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Problems in LPG Merox extraction unit

Problems in LPG Merox extraction unit

(OP)
Dear All,

We are having problems in an LPG Merox extraction unit of one of our refineries. A summary of the problema:
- Mercaptan content in LPG is too high compared with design: design 400 ppmw mercaptans in LPG and actual feed is 20000 ppmw mercaptans
- We do not see SH2 after the caustic 10ºBe prewash (Dragger test)
- LPG processed in Merox unit is off-spec due to high sulphur content. Cupper strip corrosión is OK (1A). There is no SH2 in LPG out of Merox unit
- Efficiency in mercaptan extraction is quite variable. Although mercaptan content is quite far from design, there are days with mercaptan extraction > 99%, but others with 50-60%. It could be related with Na(OH)make-up: high extraction efficiency when Na(OH) is fresh
- We do not see troubles with caustic, everything seems to be OK: percent spent <10%, Beume density close to target (20º)
- We do not see trouble with air: %O2 in spent air (out ot the oxidizer) is between 9 and 12%vol
- We do not see troubles with catalyst concentration, that is on the target value: 200 - 300 ppm
- Temperature in the oxidizer is in the right value: 38-43ºC
- It seems to be very low regeneration activity: delta temperature across the oxidizer is quite low (2ºC or lower) despite the high mercaptan loading, shake test is negative (<120 seconds).
- Regenerated caustic has black color, instead of greenish-blue

It seems there is a problem with oxidation and caustic regeneration, but inlet temperatura to oxidizer, air and catalyst concentration seems to be OK, and we don't see problems with catalyst concentration (200-300 ppm according to lab results) but shake test is negative. Perhaps we need more concentration because we are operating over design limit?

I would appreciatte if anybody can give me an idea on how to solve this situation

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