Light mercaptans

This section covers the removal of COS, CS2, and light mercaptans from gas streams by amine solutions. These are the principal organic sulfur compounds normally encountered in fuel and synthesis gases. The removal of organic sulfur compounds from liquid hydrocarbons is discussed in the next section. The presence of the above components (and many other reactive species) in a gas to be treated raises two questions (1) How much, if any, of the material will be removed during the treating operation and (2) will the impurities cause deterioration of the amine solution The question of solution deterioration by reaction with various gas impurities is discussed in detail in Chapter 3 this discussion is concerned primarily with removal of carbonyl sulfide, carbon disulfide, and mercaptans from the gas by amine solutions. 

The efficiency of mercaptan removal is highest for light mercaptans (i.e R is a light hydrocarbon radical) and concentrated caustic solutions. The circulating caustic solution contains a dispersed Merox catalyst udiich promotes the conversion of mercaptans to disulfides at a relatively low tenoperatuie without ptmooting otho reactions and does not affect the formation of mercaptides. 

This second example of application is taken from a complete thermodynamic study devoted to the adsorption of light mercaptans, paraffins and aromatics on FAU type zeolite [28-30]. The aim of this work was to find a good adsorbent able to perform a deep desulphurization of natural gas by selective adsorption. Results presented below concern the adsorption and coadsorption of ethylmercaptan (ESH), n-heptane (HEP) and toluene (TOL) on the NaX zeolite a 298 K. NaX zeolite has exactly the same stmcture as NaY excepted that it contains more compensation cations located in sites II and also in sites HI around the dodecagonal window (Fig. 7.27). 

Of the general formula, R - S — H, where R represents an aliphatic or cyclic radical, the thiols —also known as mercaptans— are acidic in behavior owing to their S—H functional group they are corrosive and malodorous. Their concentration in crude oils is very low if not zero, but they are created from other sulfur compounds during refining operations and show up in the light cuts, as illustrated in Table 8.6. 

Distribution of mercaptan sulfur among the different cuts of Arabian Light crude oil. 

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. 

Hydrogen sulfide reacts with olefins under various conditions forming mercaptans and sulfides (108,109). With ethylene it can react to ultimately give diethyl sulfide (110). With unsymmetrical olefins, the direction of addition can be controlled by the choice of either a free-radical initiator, including ultraviolet light, or an acidic catalyst (110) ... 

Sulfur reacts with mercaptans ia the presences of basic catalysts at temperatures of 75—105°C, forming sulfides. These sulfides are usually light ia color and are formed without cross-linking. The sulfurization of mercaptans leads to di-, tri-, or higher polysulfides, depending on the mole ratio used (eqs. 5 and 6). An extensive Hst of references to the sulfurization of mercaptans is available (8). 

Potential Processes. Sulfur vapor reacts with other hydrocarbon gases, such as acetjiene [74-86-2] (94) or ethylene [74-85-1] (95), to form carbon disulfide. Higher hydrocarbons can produce mercaptan, sulfide, and thiophene intermediates along with carbon disulfide, and the quantity of intermediates increases if insufficient sulfur is added (96). Light gas oil was reported to be successflil on a semiworks scale (97). In the reaction with hydrocarbons or carbon, pyrites can be the sulfur source. With methane and iron pyrite the reaction products are carbon disulfide, hydrogen sulfide, and iron or iron sulfide. Pyrite can be reduced with carbon monoxide to produce carbon disulfide. 

Water and sulfur compounds are the principal non-hydrocarbon impurities present in light ends which frequently require removal. The sulfur compounds of concern are concerned with here are hydrogen sulfide and mercaptans, both of which have to be removed almost quantitatively from any light ends cut which is going to be marketed. There are two reasons for this First, they have an objectionable odor, even in minute concentrations. Second, they may cause corrosion either by themselves or through their combustion products. 

Mercaptans are slightly acidic organic sulfur compounds. They can be removed from light ends mixtures by caustic washing in a Merox extraction... 

When both HjS and mercaptans are to be removed in the light ends treating plant, the amine scrubbing tower precedes the caustic washing step. If the order were reversed, the large quantity of HjS present would "fix" the caustic. Also, the caustic will act to remove any final traces of HjS when it follows the scrubbing tower. 

Washing light hydrocarbons with water is a common refinery practice. It finds application on the feed to catalytic polymerization plants. It is used to remove any entrained caustic from the mercaptan removal facilities as well as any other impurities such as amines which tend to poison the polymerization catalyst. Another use for water wash is in alkylation plants to remove salts from streams, where heating would tend to deposit them out and plug up heat exchanger surfaces. Water washing can be carried out in a mixer- settler, or in a tower if more intimate contacting is necessary. 

Originally cresylic acid was obtained from caustic waste streams that resulted from treating light distillates with caustic solutions to reduce H2S and mercaptans. Currently, most of these streams are hydrodesulfurized, and the product streams practically do not contain phenolic compounds. 

Caustic extraction can remove mercaptan sulfur in light fractions, but not higher carbon number mercaptans or other types of sulfur molecules that are in the FCC gasoline. 

A. M. Fakhriev, M. M. Latypova, V. 1. Nasteka, A. I. Berdnikov, and V. Ya. Klimov. Odorising agent for compressed hydrocarbon gas— contains ethyl-mercaptan or mixed mercaptans, and additionally waste from process of oxidising de-mercaptanisation of light hydrocarbons. Patent RU 2009178-C, 1994. 

Isoamyl ether and -amyl ether formed In preparation of amyl alcohols from amyl chloride very slightly soluble In water used mainly as a solvent. am-al e-thar) amyl mercaptan org chem C5H11SH A colorless to light yellow liquid with a boiling range of 104-130°C soluble In alcohol used In odorant for detecting gas line leaks. am-ol mor kapitan ... 

Reforming is a relatively clean process. The volume of wastewater flow is small, and none of the wastewater streams has high concentrations of significant pollutants. The wastewater is alkaline, and the major pollutant is sulfide from the overhead accumulator on the stripping tower used to remove light hydrocarbon fractions from the reactor effluent. The overhead accumulator catches any water that may be contained in the hydrocarbon vapors. In addition to sulfides, the wastewater contains small amounts of ammonia, mercaptans, and oil. 

Example 2-8 Suppose the room you are in now actually contains 9.5% CH4. [You could not detect the presence of CH4 in air unless the natural gas contained a mercaptan odorizer.] If someone turned on the light switch and created a spark, what would be the temperature and pressure in the room before the windows and walls burst ... 

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... 

Light is an excellent catalyst for the promotion of free-radical reactions. For example, the light-initiated reaction of an olefin with a mercaptan to form an organosulfide can occur in fuel as follows ... 

---