Merox is an acronym for mercaptan oxidation. It is a proprietary catalytic chemical process developed by UOP used in oil refineries and natural gas processing plants to remove mercaptans from LPG, propane, butanes, light naphthas, kerosene and jet fuel by converting them to liquid hydrocarbon disulfides.
The Merox process requires an alkaline environment which, in some process versions, is provided by an aqueous solution of sodium hydroxide (NaOH), a strong base, commonly referred to as caustic. In other versions of the process, the alkalinity is provided by ammonia, which is a weak base.
The catalyst in some versions of the process is a water-soluble liquid. In other versions, the catalyst is impregnated onto charcoal granules.
Processes within oil refineries or natural gas processing plants that remove mercaptans and/or hydrogen sulfide (H2S) are commonly referred to as sweetening processes because they results in products which no longer have the sour, foul odors of mercaptans and hydrogen sulfide. The liquid hydrocarbon disulfides may remain in the sweetened products. These may be used as part of the refinery or natural gas processing plant fuel, or they may be processed further.
When dealing with kerosene, the Merox process is usually more economical than using a catalytic hydrodesulfurization process for much the same purpose. It is rarely (if ever) required to reduce the sulphur content of a straight-run kerosene to meet the 3000 ppm sulphur specification of jet fuel, because very few crude oils have a kerosene cut with a higher content of sulphur than this limit.
Types of Merox process units
UOP has developed many versions of the Merox process for various applications:
Conventional Merox for extraction of mercaptans from LPG, propane, butanes or light naphthas. Conventional Merox for sweetening jet fuels and kerosenes. Merox for extraction of mercaptans from refinery and natural gases. Minalk Merox for sweetening of naphthas. This process continuously injects just a few ppm of caustic into the feed naphtha. Caustic-free Merox for sweetening jet fuels and kerosenes. This process injects small amounts of ammonia and water (rather than caustic) into the feed naphtha to provide the required alkalinity. Caustic-free Merox for sweetening of naphthas. This process also injects small amounts of ammonia and water (rather than caustic) into the feed naphtha to provide the required alkalinity.
In all of the above Merox versions, the overall oxidation reaction that takes place in converting mercaptans to disulfides is:
4 RSH + O2 → 2RSSR + 2H2O
The most common mercaptans removed are: Methanethiol - CH3SH [m-mercaptan] Ethanethiol - C2H5SH [e- mercaptan] 1-Propanethiol - C3H7SH [n-P mercaptan] 2-Propanethiol - CH3CH(SH)CH3 [2C3 mercaptan] Butanethiol - C4H9SH [n-butyl mercaptan] tert-Butyl mercaptan - C(CH3)3SH [t-butyl mercaptan] Pentanethiol - C5H11SH [pentyl mercaptan]
In some of the above Merox process versions, the catalyst is a liquid. In others, the catalyst is in the form of impregnated charcoal granules.
Process flow diagrams and descriptions of the two conventional versions of the Merox process are presented in the following sections.
Conventional Merox for extracting mercaptans from LPG
The conventional Merox process for extraction and removal of mercaptans from liquefied petroleum gases (LPG), such as propane, butanes and mixtures of propane and butanes, can also be used to extract and remove mercaptans from light naphthas. It is a two-step process. In the first step, the feedstock LPG or light naphtha is contacted in the trayed extractor vessel with an aqueous caustic solution containing UOP's proprietary liquid catalyst. The caustic solution reacts with mercaptans and extracts them. The reaction that takes place in the extractor is:
2RSH + 2 NaOH → 2NaSR + 2 H2O
In the above reaction, RSH is a mercaptan and R signifies an organic group such as a methyl, ethyl, propyl or other group. For example, the ethyl mercaptan (ethanethiol) has the formula C2H5SH.
The second step is referred to as regeneration and it involves heating and oxidizing of the caustic solution leaving the extractor. The oxidations results in converting the extracted mercaptans to organic disulfides (RSSR) which are liquids that are water-insoluble and are then separated and decanted from the aqueous caustic solution. The reaction that takes place in the regeneration step is:
4NaSR + O2 + 2H2O → 2RSSR + 4NaOH
After decantation of the disulfides, the regenerated "lean" caustic solution is recirculated back to the top of the extractor to continue extracting mercaptans.