Principle of deodorization process

The basic principle of alkali-free desulfurization of thiols is that thiols, oxygen in the feed oil, and the catalyst form complexes, after which an oxidation reaction takes place within these complexes. This eliminates the need for phase transfer due to the presence of two phases, and it avoids low removal rates resulting from the too slow transfer rate of high-molecular-weight thiols； On the other hand, since no alkaline solution is used in this process, contamination of the catalyst by phenates and naphthenates is eliminated, and there is no discharge of alkaline waste.

From 1991 to 1993, China University of Petroleum (Beijing) developed the alkali-free desulfurization alcohol (II) process. This process utilizes a new type of catalyst called AFS-12; this catalyst is insoluble in alkaline solutions but soluble in a certain solvent, making it particularly suitable as a fixed-bed catalyst. Its performance is comparable to that of UOP’s Merox 8, 10, and 21 catalysts. Pilot tests as well as evaluations of the process flow and operating conditions were carried out jointly at Wuhan Petrochemical Complex during 1993–1994. The pilot test passed the expert evaluation organized by Sinopec in July 1996, and industrial trials were carried out at Anqing Petrochemical Complex in 1997 (600,000 tons per year). Two calibrations were carried out on September 5, 1997, and October 14, 1998, respectively; the process was found to be reasonable, and the catalyst-adjuvant system exhibited high activity and a long service life. In recent years, alkali-free desulfurization alcohol (II) has seen rapid development; to date, more than 30 sets of alkali-free desulfurization alcohol (II) processing units have been successfully put into operation in China. In addition, the fiber-film desulfurization patent technology developed by the American company Merichem is specifically used for the desulfurization of light oils and the alkaline washing purification process for desulfurization of alcohols. It involves a non-dispersive reaction between the alkaline solution and the oil to be treated within a longitudinal liquid film, which greatly improves the efficiency of mass transfer and reaction. The fiber-film contactor of this technology was put into industrial use in 1974; to date, more than 500 such industrial units have been installed worldwide across over 30 countries in the petrochemical industry, making it one of the dominant processes for removing sulfur and other impurities from hydrocarbons. It is a cylindrical device containing countless extremely fine proprietary metal fibers. It features a large contact area between the two phases, eliminates oil emulsification, reduces the amount of alkali used, and minimizes the transfer of alkali to the oil or oil to the alkali. However, compared with the conventional domestic dispersion-based alkaline washing and refining process for oil products, the investment required for this process (patent fees, cost of purchasing fiber-film contactors) is higher. This process, used for the catalytic desulfurization of gasoline, is marketed under the name MERICATS. Its key equipment is also a fiber membrane contactor. The mechanism of desulfurization is similar to that of the conventional Merox process: pre-alkaline washing of gasoline and extraction-oxidative desulfurization are carried out in one fiber membrane contactor. Air and catalyzed gasoline are introduced at the top of the contactor, while the catalyst alkaline solution enters from the side. Hydrogen sulfide and thiol compounds undergo catalytic oxidation reactions on the liquid film formed on the metal fibers of the contactor. Subsequently, the mixture separates at the bottom of the separation tank; the catalyst alkaline solution is recycled using a pump, while the refined gasoline is discharged from the top of the tank. Fresh alkaline solution is added intermittently, and the alkaline residue is removed. The catalytic reaction equation for hydrogen sulfide removal is as follows: H2S + 2NaOH → Na2S + 2H2O; 2Na2S + H2O + 2O2 → Na2S2O3 + 2NaOH. The overall equation for the catalytic hydrogen sulfide removal reaction is: 2H2S + 2NaOH + 2O2 → Na2S2O3 + 3H2O. The catalytic reaction for mercaptan removal is as follows: RSH + NaOH → RSNa + H2O; RSNa + 1/2O2 → RSSR + H2O. The overall equation for the catalytic mercaptan removal reaction is: RSH + 1/2O2 → RSSR + H2O. Compared with the conventional process of pre-alkaline washing of gasoline followed by Merox extraction-catalytic oxidative mercaptan removal, this process has the following significant advantages: ① It combines hydrogen sulfide removal through alkaline washing with mercaptan removal through extraction-oxidative processes in a single fiber membrane contactor, resulting in a simpler process flow, easier operation, and reduced space requirements. ② The Na2S produced by the pre-alkaline washing of catalytic gasoline to remove hydrogen sulfide can be deeply oxidized to Na2S2O3, which can reduce the amount of alkaline slag generated by 60–70%. ③ It overcomes the problems associated with the conventional Merox method, such as oil emulsification and oil-alkali entrainment (oil carrying alkali or alkali carrying oil) during operation, and offers significant economic benefits in terms of preventing losses of oil and alkali solutions due to such entrainment

The principle of gasoline deodorization is, simply put, to convert thiol RSH into thioether RSSR. For details, see floor 3.