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Problems in LPG Merox extraction unit

Problems in LPG Merox extraction unit

Problems in LPG Merox extraction unit

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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