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1 Introduction With the development of society, people pay more and more attention to environmental protection, forcing the petroleum processing industry to pay more and more attention to the development and research of desulfurization technology and the improvement of traditional desulfurization processes to reduce the sulfur content of fuels such as liquefied petroleum gas, gasoline, and diesel and produce clean fuels. This is a very urgent issue facing every domestic petroleum processing enterprise. As the proportion of liquefied gas as urban vehicle fuel increases, low-sulfur or even sulfur-free liquefied gas has become a demand in modern cities. In order to adapt to the production of clean fuels for oil refining enterprises, Beijing Sanju Environmental Protection New Materials Co., Ltd. (hereinafter referred to as Sanju Company) has developed a series of patented technologies - liquefied gas fixed bed alkali-free deodorization combined process and related catalysts, which provides a new way for oil refining enterprises to improve the traditional process of liquefied gas desulfurization and deodorization and produce low-sulfur or sulfur-free liquefied gas. 2 Current status of liquefied gas desulfurization and production processes in refineries Currently in petrochemical enterprises, liquefied gas from catalytic cracking, delayed coking and other devices contains a large amount of sulfide. In addition to H2S, it also contains various forms of organic sulfur, such as CH3SH, C2H5SH, CH3SCH3, C3H7SH, COS, etc. The main ones are CH3SH, C2H5SH and COS. The current principle flow of liquefied gas desulfurization and demercaptan technology is shown in Figure 1. That is, the liquefied gas first passes through the MDEA (N-methyldiethanolamine) extraction tower to remove H2S, and then uses pre-alkaline elution to remove the residual H2S and part of the mercaptans. Then, the liquefied gas is extracted with an alkali solution in which the sulfonated cobalt phthalocyanine catalyst is dissolved to remove the mercaptans in the liquefied gas, and then goes to the gas separation device to be used as raw material or to the liquefied gas tank area. The alkali liquid at the bottom of the demercaptan extraction tower enters the regeneration tower and is oxidized through ventilation. Under the catalysis of sulfonated cobalt phthalocyanine, sodium mercaptide is oxidized into disulfide, so that the alkali liquid is regenerated and recycled, and the disulfide is separated. Its reaction formula is as follows: RSH + NaOH RSNa + H2O 4RSNa + O2 +2H2O 2RSSR +4NaOH After the liquefied gas is separated, C2, propane and propylene are separated through the gas fractionation device, and the propylene is sent to the polypropylene device. The separated C4 is used as raw material for MTBE and alkylation. After isobutylene and isobutane are consumed, the remaining C3 and C4 are mixed and shipped as liquefied gas. 3 Problems existing in the current liquefied gas demercaptanization process 3.1 Incompleteness of demercaptanization. Since the disulfide formed after ventilation and oxidation of the alkali liquid coming out of the extraction tower cannot be well separated from the alkali liquid, a large amount of disulfide is dissolved in the alkali liquid and returns to the liquefied gas with the circulating alkali liquid. The total sulfur content of the liquefied gas exceeds the standard. In addition, the alkali residue pre-alkali washed contains a large amount of RSNa and enters the alkali residue tank along with the alkali liquid, which is the main source of pollution causing the stench of the refinery. 3.2 Production Unsafety Since disulfide cannot be effectively separated from the alkali liquid, the alkali liquid is forced to be replaced frequently. When the alkali is removed during the production process, liquefied gas leakage accidents occur. In some cases, liquefied gas enters the alkali slag tank. 3.3 Secondary pollution occurs. This process produces a large amount of malodorous alkali residue, causing secondary pollution. Alkali residue treatment is a very difficult issue in refineries. At present, most alkali residue treatment uses sulfuric acid neutralization method, which not only wastes acid resources and increases production costs, but also releases a large amount of H2S and RSH gases after acid neutralization. The operating environment is harsh, causing air pollution and becoming the main source of odor in the refinery. The alkali residue treatment device and tank area of each refinery have become the most difficult pollution source for enterprises to control. 4 Fixed-bed alkali-free deodorization combined process for liquefied gas. In view of the current problems in the production of liquefied gas demercaptans, Sanju Company has developed a new catalyst (JX-2A) for mercaptan conversion of liquefied gas, and developed a corresponding fixed-bed alkali-free deodorization combined process for liquefied gas. This process uses 3 fixed beds (or 2) to replace the pre-alkaline washing and catalyst alkali liquid liquefied gas demercaptanization process, and realizes the fixed bed removal of residual H2S, COS hydrolysis conversion and mercaptan conversion in the liquefied gas, simplifying the process flow and eliminating alkali residue. 4.1 Desulfanization mechanism of mercaptan conversion catalyst JX-2A mercaptan conversion catalyst The oxidation structure of the active component A1-xA＇xB1-xB＇xO3 belongs to the ABO3 type compound in crystallization chemistry, and its B or B' position is in a high valence state. When they act on liquefied gas mercaptan conversion, they play a bridge role in electron transfer, and the liquid In the presence of oxygen, the high valence state of B or B' positions of mercaptans in chemical gas causes the thiol group of the mercaptan to be oxidized, and the valence bond between sulfur and hydrogen in the thiol group is broken. Hydrogen combines with O2 to form water with the participation of high valence ions at B or B' position to form water. The remaining parts of the two mercaptan molecules are combined into a disulfide molecule to achieve the purpose of removing mercaptans. The reaction is as follows: 2RSH + O2 = RSSR + 2H2O This catalyst needs to be used when the molar concentration of oxygen is greater than the molar concentration of mercaptans. When carrying out the catalytic reaction, there is no need to add activators, organic bases or inorganic bases, thus achieving truly alkali-free demercaptanization and alkali residue that does not produce secondary pollution. Since in fact, during the production process of liquefied gas, mercaptans in the liquefied gas are approximately 100 to 200 ppm, while trace amounts of oxygen in the liquefied gas are around 1000 ppm. Calculated into molar concentration, the molar concentration of oxygen in the liquefied gas is 2 to 5 times higher than the concentration of mercaptans. Therefore, the reaction is completely feasible, and the laboratory results prove the correctness of its conclusion. 4.2 The fixed-bed alkali-free deodorization process removes the H2S liquefied gas through the MDEA (N-methyldiethanolamine) extraction tower. It first enters the liquid-liquid separation unit to remove the amine liquid carried by the liquefied gas to avoid the high concentration of H2S in the amine liquid from impacting the rear part, causing the copper sheet of the liquefied gas to corrode and fail. The deamined liquefied gas enters the first fixed-bed reactor for COS hydrolysis (removing COS, CS2 unit), and then enters the second fixed-bed reactor to remove H2S (removing the residual H2S after amine washing and the H2S unit generated after the hydrolysis of COS, etc.) to reduce the impact on the downstream mercaptan conversion catalyst. The liquefied gas after fine desulfurization enters the third fixed-bed reactor (mercaptan conversion or deodorization unit), and a catalytic oxidation reaction occurs with the help of the oxygen content of the liquefied gas itself and the action of the catalyst, converting the mercaptans into disulfides. The mercaptan conversion unit means that the liquefied gas containing disulfide in the deodorization unit enters the gas separation device. The schematic flow chart of the fixed bed alkali-free deodorization process is shown in Figure 2. 5 Characteristics of the liquefied gas fixed bed alkali-free deodorization combined process 5.1 The process is simple and the one-time investment is small. 5.2 Complete reaction of mercaptans and no alkali residue pollution. 5.3 COS is removed and the propylene refining process is simplified. 5.4 The production operation is simple and the operating cost is low. 5.5 Low-sulfur and sulfur-free liquefied gas can be produced. The implementation of the liquefied gas fixed-bed alkali-free deodorization combined process has made it a reality for refineries to obtain low-sulfur or even sulfur-free liquefied gas. The boiling points of some sulfides are shown in Table 1. Table 1 Boiling points of some sulfides Name Boiling point, ℃ COS 50.3 Methyl mercaptan 5.9 Ethyl mercaptan 35.0 CH3SCH3 37.3 Isopropyl mercaptan 52.6 n-propyl mercaptan 67.9 CH3-S-S-CH3 117.9 C2H5-S-S-C2H5 164.13 The organic sulfides in the liquefied gas raw materials mainly include COS, CH3SH, CH3SCH3, C2H5SH, C3H7SH, etc., of which CH3SH accounts for about 90%, and the COS content is about 2 to 90 ppm. After the combined process, the COS hydrolysis rate in the liquefied gas is above 99. After refined desulfurization, the COS