Merox process

K. M. Brown, Commercial Kesults with UOP MEROX Process for Mercaptan Extraction in US. and Canada, UOP Booklet 267, UOP, Inc., Des Plaines,... 

Oxidation or "sweetening" is used on gasoline and distillate fractions. A common oxidation process is also a Merox process that uses a solid catalyst bed. Air and a minimum amount of alkaline caustic ("mini-alky" operation) is injected into the hydrocarbon stream. As the hydrocarbon passes through the Merox catalyst bed, sulfur mercaptans are oxidized to disulfide. In the sweetening Merox process, the caustic is not regenerated. The disulfide can remain with the gasoline product, since it does not possess the objectionable odor properties of mercaptans hence, the product has been sweetened. 

However, a strong caustic solution is used to remove mercaptans from gas and liquid streams. In the Merox Process, for example, a caustic solvent containing a catalyst such as cobalt, which is capable of converting mercaptans (RSH) to caustic insoluble disulfides (RSSR), is used for streams rich in mercaptans after removal of H2S. Air is used to oxidize the mercaptans to disulfides. The caustic solution is then recycled for regeneration. The Merox process (Fig. 1-3) is mainly used for treatment of refinery gas streams. ... 

Figure 1-3. The Merox process (1) extractor, (2) oxidation reactor.

Removal of the objectionable odors due to the presence of H2S and mercaptans is the objective of the fuel sweetening process. Several methods can be utilized to remove these undesirable compounds including caustic washing, copper chloride sweetening, sulfuric acid treating, Merox processing, and hydrotreating. These methods will be discussed below. 

This process is used frequently to sweeten gasoline and light distillate streams. In the first phase of Merox processing, mercaptans are extracted from the fuel with caustic. A sweetening process follows and involves sparging a caustic/fuel mixture with air in the presence of a catalyst. Remaining mercaptans then react to form... 

A combined Merox process operation is also utilized to sweeten fuel. This countercurrent method involves feeding fuel into the bottom of a vertical reactor and feeding caustic and catalyst from the top. Mercaptans are extracted into the caustic and react to form disulfides. The caustic and disulfides are later separated. The following reactions occur during the Merox sweetening process ... 

Oxidations of hydrocarbons (cycloalkanes, cycloalkenes, aromatics) photo-catalyzed by metallotetrapyrroles lead to the formation of epoxides, aldehydes, ketones, alcohols, and carboxylic acids both in solutions and polymer matrices. These processes frequently occur as selective (one-product formation) reactions. Irradiation with visible light has a pronounced accelerating effect on such important industrial processes as the oxidation of thiols to disulfides (Merox process [265]) in a treatment of petroleum distillates or waste water cleaning. 

Minalk A variation of the Merox process in which a small quantity of a weak aqueous alkali is continuously added to the catalyst bed in order to promote the oxidation of mercaptans to disulfides. 

Sweetfrac A process for removing sulfur compounds from naphtha. The process is essentially the UOP Merox process, positioned upstream of the naphtha splitter in conventional petroleum refining. 

Any mercaptans boiling below 80°C are also readily dissolved in alkaline solutions (Eq. 18.32). A common process for the removal of dissolved mercaptans of this kind, especially from catalytically cracked gasoline and liquefied petroleum gas, is the Universal Oil Products (UOP) Merox process that also uses caustic soda for extraction. In this process, however, the mercaptans are then oxidized to disulfides using air assisted by a metal complex catalyst dissolved in the caustic soda (Eq. 18.34). 

Production of cresols and xylenols from spent caustic washes of the petroleum refineries has been confined mostly to the Untied States since cresolates feedstocks have been inadequate in other countries. Besides as a result of use of UOP s Merox process of sweetening which does not use NaOH solution, or of hydrotreating process, less and less cresols... 

Oxidation of mercaptanes is possible as well [25], while applications in the sweetening of the petroleum fractions in the MEROX process [26] seem possible. 

Progress in this area has been considerable in recent years and hopefully some non precious metal catalyst will be used in fuel cells in the near future. On the other hand, the stability of metallophthalocyanines makes them appropriate for applications in various fields such as chemical catalysis (such as the MEROX process for the sweetening of oils), dye stuffs, coloring for plastics and metal surfaces, sensors, chromatographic detectors, photoconducting agents, and so on. These complexes are also used for photobiology and photodynamic cancer therapy, electrochemical... 

In the Merox process, cold mild oxidation of RSH by air in an alkaline environment is catalyzed by a chelating agent, cobalt phthalocyanine. Four main reactions are involved the first two are avoided or curtailed as the reagents are expensive ... 

In 1958, UOP introduced a catalyst based process to accelerate the oxidation of mercap-tans to disulfides at or near ambient temperature. The process, which is licensed by UOP as the Merox process, is used to remove mercaptans from liquid or gaseous hydrocarbon streams. It operates by either converting the mercaptans to less objectionable disulfides in the flowing stream or removing the mercaptans with a caustic wash and then converting them to disulfides (Staehle et al., 1984). Only the approach involving both caustic absorption and subsequent oxidation to disulfides is applicable to gas purification. 

When used for gas treating, the Merox process simply involves the absorption of mercaptans from the gas by countercurrent contact with caustic soda solution, oxidation of the dissolved mercaptans to disulfides by contact with air, and separation of the disulfide liquid from the caustic solution by settling and decanting off the disulfide phase. Note that both CO2 and H2S are removed in a caustic prewash prior to contacting the gas with the Merox caustic solution. 

A simplified flow diagram of the Merox process as applied to gases or light liquid hydrocarbons is shown in Figure 5-50. The key reactions in the process are... 

Figure 5-50. Flow sheet of Merox process. (Staehle etal., 198 ...

Although the Merox process is plicable to gases, most units have been designed to process liquid hydrocarbons. As of January, 1991, more than 1,450 Merox units of all types had been commissioned (UOP, 1992A). 

Merox process A catalytic process used in petroleum refineries to remove mercaptans from LPG, propane, butane, naphtha, kerosene, and jet fuel by oxidation to form hydrocarbon disulphides. An abbreviation of mercaptan oxidation, the process uses an aqueous solution of sodium hydroxide and a water-soluble catalyst to remove the foul-smelling mercaptans. 

Sweetening processes oxidize mercaptans to less odoriferous disulfides without actually removing sulfur. The most common sweetening processes are the Merox processes others include the lead sulfide, the hydrochloride, and the copper chloride processes. In the Merox process, a catalyst composed of iron group metal chelates is used in an alkaline environment to promote the oxidation of mercaptans to disulfides using air as a source of o gen. 

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