Sweetening processes

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. 

Carbonyl sulfide occurs as a by-product ia the manufacture of carbon disulfide and is an impurity ia some natural gases, ia many manufactured fuel gases and refinery gases, and ia combustion products of sulfur-containing fuels (25). It tends to be concentrated ia the propane fraction ia gas fractionation an amine sweetening process is needed to remove it. 

The formation of sulfuric acid cannot be economically retarded in the combustion process. The best method of eliminating sulfuric acid as a combustion product is to remove sulfur from the incoming fuel gas. Two separate sweetening processes are used to remove all sulfur from the fuel gas that will be burned. 

J Consider the coke-oven gas COG sweetening process shown in Fig. 3.22. The basic objective of COG sweetening is the removal of acidic impurities, primarily hydrogen sulfide, from COG (a mixture of H2, CH4, CO, N2, NH3, CO2, and H2S). Hydrogen sulfide m undesirable impurity, because it is corrosive and contributes to SO2 emission when the G... 

Refiners use sweetening processes to remove mcr-captans that give a vei y unpleasant odor to gasolines and middle distillates (the skunk uses mercaptans to protect itself). This is done by washing the hydrocarbon stream with a caustic solution followed by a wash with water to remove die caustic. 

The sweetening process utilizes a caustic solution, catalyst, and aii. Mercaptans are converted to disulfides in a mixing vessel or fiber film contactor. The reactions take place according to the following equations... 

Perco An early petroleum sweetening process, using an adsorbent bed containing copper sulfate and sodium chloride. 

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 ... 

Selective Absorption in the Sweetening Process. In recent years removal of the acid gas (H2S, CO2) components from a gas stream has increasingly been by absorption in a solvent system containing amines. While non-reactive solvent sweetening processes are in use, the ability of the basic amine to react chemically with the acid gas to yield water soluble salts has favored the chemical sweetening system. Thus... 

The production of COS in the front end reaction furnace presents special problems since sulfur in this form may be difficult to remove in the downstream catalytic beds under conditions that are optimal for the Claus redox reaction between H2S and SO COS (and CS2) were known to be generated from hydrocarbon impurities carried over in the acid gas feed thus the efficiency of the up-stream sweetening process became an important factor. The reaction of CO2, a common constituent of the acid gas feed, with H2S and/or sulfur under furnace temperature conditions has also been shown to be an important source of COS. 

Copper sweetening processes involving the oxidation of mercaptans to disulfides by oxygen in the presence of cupric chloride. 

The pathways for production of the various sweeteners share many common steps. A generalized sweetener process is shown in Figure 21,2.7 Production of each of the sweeteners discussed will utilize one or more steps in this process. 

Alkanolamines are used during the sweetening process in the oil and gas industry to remove toxic levels of gases such as hydrogen sulfide and carbon disulfide from raw gas condensates. Various alkanolamines such as A-methyldiethanolamine (23) and triethanolamine (25) may be utilized. They are therefore of concern as environmental contaminants of groundwater and wetland areas. Headley et al. (66)... 

Function Nutritive sweetener processing aid humectant (anhydrous form) texturizer. 

When the sulfur content of the crude oil is low (usually less than one percent by weight), the crude oil is known as a sweet crude, while crude oil with higher concentrations of sulfur is called sour crude. Removal of sulfur and other impurities form part of the treating processes and sulfur itself can form a valuable by-product in a refinery as an input into the chemical industry. Other impurities include nitrogen, oxygen, and salt, as well as small quantities of metals such as vanadium and nickel that are common in certain of the heavier crude oils. As well as extraction processes to purify oil and its products of impurities, specific additives are also used to react with corrosive or odiferous constituents to produce harmless and odorless substances. Such processes are generally termed sweetening processes. 

FIGURE 9.6 Flowsheet giving the schematic details of Girbotol and related sour gas sweetening processes. Twenty to twenty-four plates are used in the absorbers and strippers [25] for efficient contacting and desorption with monoethanolamine (MEA),... 

Elf Activated MDEA (1994) An important improvement in natural gas sweetening processes. 19th World Gas Conference, Milan. 

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