Why Merox and no other technologies?
The mercaptans extraction it’s very common reaction: 2RSH + 2NaOH pass to 2NaSR + 2H2O but the problem is that caustic regeneration step is very very slow: 2NaSR + 1/2 O2 + H2O pass to RSSR + 2NaOH which made the process unfeasible. In ’60 UOP released on the market the so called MEROX - Mercaptans Oxidation improving the regeneration time by using a special formulated catalyst WS generation.
The process can accommodate different feed such as gas, natural and fuel gas, LPG, straight light / heavy naphtha streams, FCC naphtha, thermally cracked naphtha, kerosene and even distillates with FBP of 360°C.
Merox technology can be used to remove mercaptans by EXTRACTION (classical Merox) or by SWEETENING where mercaptans are transformed in disulfides which remains in final product (the total sulfur remain unchanged).
Due to very simple process and lower CAPEX and OPEX is still the best available technology to remove or to sweet the mercaptans.
The process was continuously improved, and a new generation of catalyst was developed, the WS-2, more stable than the classical WS.
Merox equipment:
1. Pretreatment Section: for moderate level of COS, CO2, H2S a vertical vessel is used fit with a special distributor to enhance distribution and contact between acid feed and caustic solution. If the level of mentioned contaminants is higher that the pretreatment section is an Amine Unit. In the latest design of Merox the Pretreatment section is made by a static mixer and a vertical vessel part of Extractor assembly which contact the acid feed with MEA or DEA and caustic solution. The MEA/DEA concentration is adjusted considering the level of H2S, CO2 and COS in acid feed.
2. Extractor: column with proprietary trays to increase the mass transfer and the reactions velocity to extract mercaptans using counter current contact with caustic solution. The last generation of Extractor is foreseen with a coalescer near the top and in comparison with the old Merox, the Extractor contains Pretreatment Section, Extractor, COS Settler or Caustic KO Drum and Sand Filter all in just 1 (one) vessel
3. Caustic KO Drum: horizontal vessel which minimize the Extractor carryover in downstream units, wash water, static mixer and an water wash pump is used to wash the Extractor free mercaptans stream. In new Units is replaced by the coalescer section part of Extractor.
4. Sand Filter part of old Merox, filled with silica quartz sand, is used to remove any caustic to downstream units. The feed is entering from the top using a special distributor and exist from bottom lateral free of caustic.
5. Water Wash Vessel: vertical with SS mesh on top with the main goal to remove any traced of amine or caustic present in LPG.
6. Oxidizer: vertical vessel where the caustic with mercaptans is contacted with air under special catalyst injection to increase the reaction time. The vessel is full fill with raschig rings under water to not destroy them.
7. Disulfide Separator: a horizontal vessel with a coalescer pack and a stack vent near the end. The contaminated air is flowing from the stack and the liquid is passing through the anthracite coal which act as a coalescer and separated due to density difference in regenerated caustic and disulfide oil streams.
8. Vent tank: small vertical vessel used to remove any liquid carryover from disulfide separator, remember that the contaminated air is routed to fire box heater and not to flare due to higher oxygen content.
9. Spent Caustic Degassing Drum: vertical vessel in which spent caustic is entering near the top and is falling down to vessel to disengage any vapor or hydrocarbons before sending the caustic to disposal unit.
10. Wash Oil System: is used to remove any disulfide oil from regenerated caustic, remember that regenerated caustic is returning to Extractor and the disulfide oil contains sulfur compounds which need to be minimized.
Enjoy!
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