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In an ammonia synthesis system with an annual output of 300kt, in order to extend the service life of the catalyst, increase output and reduce energy consumption, ethanolamine cycle desulfurization is used for the natural gas entering the plant, cobalt and molybdenum decompose organic sulfur, and zinc oxide A and B furnaces are refined to remove the total sulfur in the natural gas to less than 5×10-7. According to the design of Kellogg Company, the total sulfur after ethanolamine must be less than 25×10-6. Since January 1999, the total sulfur of the natural gas entering the factory of Lutianhua Group Company is less than 25×10-6, the organic sulfur is about 1×10-6, and sometimes the total sulfur is less than 10×10-6. The total sulfur of the natural gas entering the plant has reached the ethanolamine index, so the ethanolamine desulfurization system can be cancelled. From the perspective of saving materials and costs, the wet desulfurization process can also be cancelled. In September 1999 and August 2000, the ethanolamine unit was shut down twice due to equipment reasons. The total sulfur at the zinc oxide furnace outlet and other data were continuously monitored. It was found that the total sulfur at the zinc oxide furnace outlet continued to rise, reaching a maximum of 5×10-6. This shows that the cancellation of the ethanolamine process will have a serious impact on subsequent refined desulfurization. 1 The impact of carbon dioxide on the rough removal device. According to the composition analysis of the natural gas entering the plant, the CO2 content is much greater than H2S. In this way, in the ethanolamine system, alkaline ethanolamine first reacts with CO2 acidic gas, so the presence of CO2 has a great impact on the absorption of H2S. The lower the CO2 content in the gas, the better the absorption of H2S. During the long-term production process, it was found that the total sulfur phase difference of natural gas before and after the ethanolamine system is very small, and it has basically no effect on H2S, but the solution temperature increases significantly, which shows that CO2 has a serious impact on the absorption of H2S by ethanolamine. Therefore, some plants use molecular sieves or activated carbon for desulfurization. From the two ethanolamine unit shutdowns, the following conditions were found. 1.1 After the first shutdown of the ethanolamine cycle on September 22, 1999, the changes in total sulfur at the zinc oxide furnace outlet were monitored. At that time, the zinc oxide furnace B was saturated with sulfur and was about to be overhauled and replaced. Furnace A played a major role. The analysis data are shown in Table 1. The difference before and after the ethanolamine shutdown was CO2. CO2 was not detected before and after the ethanolamine shutdown, and the CO2 in the main pipe was 0.65%. 9 hours after the ethanolamine shutdown, the total sulfur of the zinc oxide furnace B began to rise. At this time, the inorganic sulfur and organic sulfur in the main pipe remained basically unchanged, and the cobalt and molybdenum exports reached a maximum of 4.32×10-6. The data in Table 1 shows that after CO2 enters the cobalt and molybdenum, the total sulfur at the cobalt and molybdenum outlet increases, and the sulfur in zinc oxide moves back. http://www.nmtech.com.cn/jishuwang/upload/0608231507462861.jpg1.2 The second shutdown was on July 25, 2000. Due to HIC-400 failure, ethanolamine could not circulate and was forced to shut down. This time the zinc oxide furnace A was in front. After 8 hours, the total sulfur of cobalt and molybdenum reached 15.85×10-6. At this time, the CO2 of the natural gas entering the plant was between 0.85% and 1.20%, and the CO2 of the cobalt and molybdenum export was around 0.5%. The analysis data is shown in Table 2. http://www.nmtech.com.cn/jishuwang/upload/0608231508279243.jpg It can be seen from the data in Table 2 that during the shutdown period from the 25th to the 26th at 16:00, the total sulfur of cobalt and molybdenum fluctuated greatly. The highest value was almost equal to the total sulfur of natural gas. It reached the highest value of 24.10 the next day. The organic sulfur of the cobalt and molybdenum outlet was higher than that of the main pipe. The cobalt and molybdenum had a re-sulfidation process. According to analysis, 80% of the sulfur content of the zinc oxide outlet was organic sulfur. The excessive sulfur came from both the cobalt and molybdenum organic sulfur and possibly itself. Only when CO2 drops to a certain level, cobalt and molybdenum begin to sulfide, and after the sulfurization is complete, organic sulfur begins to decompose. The total sulfur in zinc oxide furnaces A and B increased successively, with a process of moving backward, and returned to normal after a few days. 2 Theoretical analysis of the effect of CO2 on cobalt-molybdenum and zinc oxide 2.1 The effect of CO2 on cobalt-molybdenum catalyst The cobalt-molybdenum catalyst itself becomes active after being sulfurized. The active center of the catalyst is CoS and MoS2. When CO2 is present on the active center, the following reaction occurs: http://www.nmtech.com.cn/jishuwang/upload/0608231509071684.jpg During the study of desulfurizers, the Southwest Research Institute of Chemical Industry found that the incoming gas did not contain COS, while the cobalt and molybdenum outlet gas contained (1 to 5) × 10-7 COS. Zinc oxide has a very poor COS removal effect. When cobalt and molybdenum encounter CO2 during production, sulfur will be released. After the CO2 is eliminated, the cobalt and molybdenum will be sulfurized again. Therefore, it is possible for CO2 to form COS and replace H2S on the catalyst surface. The analytical data after the wet desulfurization was stopped are shown in Table 3. http://www.nmtech.com.cn/jishuwang/upload/0608231509366735.jpg The CO2 loss of the incoming natural gas CO2 minus cobalt and molybdenum is 0.27%, and 3% H2 is not considered. Part of the lost CO2 is converted into COS, and the other part is replaced by H2S, and the carbonate produced is unstable. At 610K, the relevant physical and chemical data found to form COS are as follows:: http://www.nmtech.com.cn/jishuwang/upload/0608231510165737.jpg This reaction is not affected by pressure and inert gas, and KP increases significantly as the temperature increases. Assume that the heat of reaction and entropy values do not change into the equilibrium constant of 670K: http://www.nmtech.com.cn/jishuwang/upload/0608231510466861.jpg Assuming that cobalt and molybdenum exports reach equilibrium, the analysis and calculation data are shown in Table 4. http://www.nmtech.com.cn/jishuwang/upload/0608231511225341.jpg It can be seen from the calculated data that when the water vapor partial pressure is constant, the COS concentration increases with the increase in temperature, H2S, and CO2 concentrations. COS is not easily absorbed by zinc oxide, so the total sulfur at the zinc oxide outlet increases. 2.2 Impact on zinc oxide The wet desulfurization process is used to remove CO2 and H2S roughly. If this process is removed, CO2 will penetrate cobalt and molybdenum and enter zinc oxide. See the analysis data in Table 3. The operating temperature of zinc oxide is 400°C, which is 673K. This temperature is the equilibrium temperature for carbonate decomposition.: http://www.nmtech.com.cn/jishuwang/upload/0608231511534334.jpg For the reaction of formula (4), △G0298=56.68kJ/mol. This shows that CO2 and water vapor together can displace a small amount of H2S, especially for zinc oxide that is about to expire. When zinc sulfide generates zinc carbonate, it can easily decompose into ZnO and absorb H2S. Therefore, the H2S in the previous furnace of zinc oxide is replaced by CO2 and moved to the later furnace. Perform theoretical calculations based on equilibrium constants: http://www.nmtech.com.cn/jishuwang/upload/0608231512584921.jpg It was measured that the zinc oxide outlet CO2 was 0.56% and H2S was 0.10×10-6 at equilibrium. http://www.nmtech.com.cn/jishuwang/upload/0608231513276830.jpg For the reaction (4): http://www.nmtech.com.cn/jishuwang/upload/0608231513588261.jpg When the zinc oxide outlet CO2 is 1.00%, under the same conditions: http://www.nmtech.com.cn/jishuwang/upload/0608231514265934.jpg This shows that when CO2 doubles, the total sulfur at the outlet of zinc oxide saturated with sulfur absorption will double. 3 Conclusions and Suggestions Through the theoretical analysis of the phenomenon and theoretical analysis of zinc oxide exceeding the standard after stopping ethanolamine twice, carbon dioxide is not removed. It can displace H2S under high temperature and high pressure, and can also synthesize carbonyl sulfide with H2S, causing the total sulfur at the zinc oxide outlet to exceed the standard, thus poisoning the catalyst. The ethanolamine system cannot be eliminated without changing the process. According to the current stage, the total sulfur of natural gas is less than 25×10-6. If the ethanolamine unit is to be shut down, a nickel-molybdenum catalyst can be used. CO2 can be converted into methane and CO2 is recovered. But we should pay attention to two issues: ①Methanation of carbon dioxide is an exothermic reaction, so avoid excessive temperature rise of the catalyst ； ②The natural gas preheating coil heat is moved backward and over-temperature. As long as the above two problems are solved, it is completely feasible to cancel the ethanolamine system and save materials and energy.