Mercaptans, removal process

Figure 5"49. Flow sheet for mercaptan removal process with thermal regeneration. (Wanams, 1964t...

Figure 5-52. Block diagram of Merichem mercaptan removal process. Wizig, 1983i...

Sulfuric acid was used extensively several decades ago to reduce fuel mercaptans, olefins, and to some extent, aromatics. Today, sulfuric acid processing has been almost entirely replaced by more effective and safer processes. Mercaptan removal was accomplished by converting mercaptans to disulfides through the following reaction ... 

The ability of water to function as a solvent for many compounds also makes it useful in the fuel refining and processing industry. It is utilized to wash and remove salts from crude oil prior to distillation, remove H2S and mercaptans from processed fuel (as caustic water), and function in the removal of trace amounts of acid from acid-catalyzed fuel processing reactions. 

Butyl methacrylate, lauryl methacrylate, and cetyl methacrylate were combined with maleic anhydride, lauryl mercaptan, and process oil and then charged into a 2-liter reaction vessel equipped with two mixing impellers rotated at 300 rpm during the reaction. The mixture was preheated to 85°C and then treated with 2,2 -azoisobutyronitrile and heated for 4 hours at 85°C followed by 1 hour at 100°C. In some cases additional oil was added to make the product more easily pourable. Unreacted maleic anhydride and other low-molecular-weight products were removed by heating the reaction mass to 120°C while applying a vacuum. Reaction scoping results are provided in Table 1. 

Ferrocyanide process a regenerative chemical treatment for mercaptan removal using caustic-sodium ferrocyanide reagent. 

The Selexol process removes acid gases from hydrocarbon gas streams by selective absorption in polyethylene glycol dimethyl ether (DMPEG). It absorbs H2S, C02, COS, and mercaptans. The process has been used to remove carbon dioxide from syngas, natural gas, and coal gas. The process was developed by Allied Chemical in the 1960s and is now... 

Adsorption of methyl mercaptan in moist conditions was performed on numerous samples of activated cartons of various origins. Methyl mercaptan adsorption was tested by a dynamic method. The amount of products of surface reaction was evaluated using thermal analysis. The results revealed that the main product of oxidation, dimethyl disulfide, is adsorbed in pores smaller than SO A. There is apparent competition for adsorption sites between water (moist conditions) and dimethyl disulfi. The comp ition is won by the latter molecule due to its strong adsorption in the carbon pore system. Althou dimethyl disulfide has to compete with water for the adsorption sites it can not be formed in a significant quantity without water. Water facilitates dissociation of methyl mercaptan and thus ensures the efficient removal process. 

An objective of this paper it to describe the results of our further investigation of the competition for adsorption sites between water and dimethyl disulfide molecules during methyl mercaptan adsorption on activated carbons. Moreover, we attempt to indicate the apparent borderlines between the conditions of adsorption processes leading to different adsorption/oxidation paths. Those working conditions have a significant effect on the feasibility of methyl mercaptan removal. 

In the practical application of the process for the purification of large volume gas streams, it is very important that H2S and CO2 concentrations be reduced to very low levels before the gas enters the mercaptan absorber. Both of these impurities are stronger acids than mercap-tans and consume caustic by forming nonregenerable salts. When the gas stream is processed in a conventional amine plant ahead of the mercaptan removal unit, the H2S concentration is normally reduced to less than 0.2S grain/100 scf (about 4 ppmv) to meet product specifications but the carbon dioxide content is often over 100 ppm. As a result modifications to the amine plant may be necessary to provide more complete carbon dioxide removal and make the removal of mercaptans with caustic economical. Alematively, the mercaptan removal unit should have an upstream caustic wash to remove trace amounts of CO2 and H2S. 

Two undesirable aspects of FCC naphtha quaUty are that it may contain unacceptably high amounts of foul smelling mercaptans, and that its thermal stabiUty may be too low. Mercaptans are usually found in the light FCC naphtha and may be removed or converted to sulfides and disulfides by a sweetening process such as Merox, developed by UOP. Thermal stabiUty is improved in sweetening processes through removal of cresyUc and naphthenic acids. It may be further improved by clay treating and by addition of oxidation inhibitors such as phenylene diamine. 

Alkali Treatment. Caustic washing is the treatment of materials, usually products from petroleum refining, with solutions of caustic soda. The process consists of mixing a water solution of lye (sodium hydroxide or caustic soda) with a petroleum fraction. The treatment is carried out as soon as possible after the petroleum fraction is distilled, since contact with air forms free sulfur, which is corrosive and difficult to remove. The lye reacts either with any hydrogen sulfide present to form sodium sulfide, which is soluble in water, or with mercaptans, foUowed by oxidation, to form the less nocuous disulfides. 

The Lo-Cat process can be used to sweeten or convert H2S to sulfur. It removes H2S only and will not remove CO2, COS, CS2, or mercaptans. Iron is held in dilute solution (high circulation rates) by the common chelating agent EDTA (ethylene diamine tetra acetic acid). The iron oxidizes the H2S to sulfur. The solution is circulated batchwise to an oxidizer for regeneration. 

The iron sponge process is very old (introduced in England in the mid-19th century) and very simple. It removes only H S and mercaptans. It is good only for streams containing low H2S concentrations at pressures of 25 to l,200psig. Hydrated iron oxide containing water and of proper pH is supported on wood chips or other material. Water is injected with the gas. 

Mol sieve processes can be developed to do almost anything desired remove H2S, sweeten and dehydrate at the same time, remove CO2. remove mercaptans, etc. Regeneration is done by switching beds and sending hot... 

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