Gasoline liquid film desulfurization process

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Principle of the gasoline liquid film desulfurization process: The sulfur in gasoline is mainly in the form of large molecular thiols, with small amounts of sulfides, carbonyl sulfides, thiophenes, etc. also present. Thiols in gasoline react with oxygen in the air in the presence of a catalyst and an alkaline solution as follows: 2RSH + 1/2O2 → RSSR + H2O. This reaction actually occurs through the following two steps: RSH + NaOH → NaSR + H2O; 2NaSR + 1/2 O2 + H2O → 2NaOH + RSSR. Low-molecular-weight thiols in gasoline react more easily with sodium hydroxide, while high-molecular-weight thiols are difficult to remove using conventional alkaline washing methods. By utilizing the efficiency of liquid film mass transfer technology and catalytic oxidation deodorization processes, the removal of large-molecule thiols from gasoline can be easily achieved. However, the resulting disulfides dissolve in the product gasoline, so the degassing process cannot reduce the total sulfur content in the product gasoline. Liquid film mass transfer is a novel technology widely used in the petrochemical industry both domestically and internationally. This technology utilizes the principles of surface tension and gravity to cause the alkaline solution to spread across special hydrophilic fibers, forming a liquid film. Gasoline passes through this film of alkaline solution, thereby increasing the contact area between the alkaline solution and gasoline significantly. This leads to a marked increase in both the reaction rate and depth of the reaction between thiols in gasoline and sodium hydroxide in the alkaline solution, thus achieving the removal of thiols from gasoline. Gasoline generally also contains acidic impurities such as hydrogen sulfide and phenols; these impurities have a slightly higher acidity than thiol compounds, and they react more easily with sodium hydroxide in alkaline solutions. The sodium sulfide and sodium phenolate formed as a result cannot be converted under the deodorization conditions of this process. Therefore, the hydrogen sulfide and phenols in gasoline cause permanent consumption of sodium hydroxide in the alkaline solution, which will significantly increase the consumption of alkaline solution and the emission of alkaline sludge. The oxidation reactions for the alkaline elution of hydrogen sulfide and sodium sulfide are as follows: H2S + 2NaOH → Na2S + 2H2O; 2Na2S + 2O2 + H2O → Na2S2O3 + 2NaOH. Combining these two reactions, it can be seen that one molecule of hydroxide is required to remove one molecule of hydrogen sulfide. The reaction for alkali-elution of phenol is as follows: C6H5OH + NaOH → C6H5ONa + H2O. The phenoxide salt formed cannot be regenerated under the conditions of this procedure. Results from the operation of similar industrial units show that when hydrogen sulfide is removed through primary alkaline washing, as well as phenol and mercaptans being removed, and 15% wt of fresh alkali solution is used until the sodium hydroxide concentration reaches 11–13% wt, the product gasoline fails to pass the Drabek test ； The process involves primary pre-alkaline washing for hydrogen sulfide removal and phenol removal, as well as primary liquid film alkaline washing for desulfurization. The alkali solutions used in the pre-alkaline washing and liquid film alkaline washing can reach concentrations of 7% wt and 9% wt, respectively. Therefore, this scheme adopts a two-stage process of pre-alkaline washing to remove hydrogen sulfide and phenol, followed by desulfurization and denitrogenation in an alkaline liquid membrane reactor, which helps to reduce the amount of alkali used for washing as well as the consumption of catalysts. Other forms of sulfur in gasoline are difficult to remove under the conditions of this scheme. 1. Low operating costs: The operation cost of the device is low, with the cost of gasoline refining at approximately 2.48 yuan per ton. 2. Advanced technology, reliable operation, concise processes, and reasonable investment. 3. The quality of refined gasoline meets standards and remains stable. 4. Low energy consumption and low operating costs. 5. High efficiency: By utilizing liquid film mass transfer technology and catalytic deodorization processes, it is possible to keep the thiol sulfur content in gasoline products below 5 ppm, or to ensure successful completion of the Drabek test. 6. Low consumption: a) Low alkali consumption: Hydrogen sulfide is removed through pre-alkaline washing, and phenols are removed as well; odor removal is achieved via liquid film alkaline washing. The sodium hydroxide concentration in the alkali solution can be set at 7% and 9% respectively, both values being lower than the 11–13% used in the primary treatment process. This results in high efficiency in the use of the alkali solution and reduced consumption of it. b) Low catalyst consumption: No catalyst needs to be added to the pre-alkaline wash solution, significantly reducing catalyst usage.