Summary of tannin desulfurization

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Summary of the Operation of the Tannin-Based Gas Desulfurization Unit Huang Changsheng, Qi Dong, Wang Xiuping, Li Chengfeng (Yanzhou 272100, Shandong) Shandong Yankuang International Coking Co., Ltd. is a coal chemical enterprise jointly established by Yankuang, Japanese Itochu, and Brazilian CVRD, with an annual production capacity of 2 million tons of coke and 200,000 tons of methanol. The company replaced its existing 7.63-meter coke ovens along with their associated chemical recovery and gas purification systems, and built a new facility for producing methanol from 200,000 tons of coke oven gas per year. The coke oven began operations in June 2006, while the methanol plant started production on December 26 of the same year. After more than a year of operation, our understanding of the coke oven’s functioning has gradually improved, and the associated supporting facilities have also been progressively enhanced. Since the gas originally designed in Germany was intended for combustion, while the current production process uses coke oven gas to produce methanol, higher standards are required regarding the quality of this gas. In practice, insufficient purity of the gas poses significant challenges to methanol production; therefore, further purification of the gas is both necessary and urgent. Operational data from 2007 showed that the quality of the gas emitted from the chemical production system was highly variable; hydrogen sulfide levels often exceeded permissible limits, while ammonia content was consistently below acceptable standards. The oil present in the gas caused blockages and sticking in the valves of the coke oven gas compressors, resulting in significant maintenance requirements and short operational cycles. The accompanying NHD desulfurization system suffered from reduced absorption capacity due to the high amount of organic substances in the solution, as well as severe corrosion, which led to frequent shutdowns and posed a serious threat to production. After carefully analyzing the underlying causes and examining the purification processes used in other facilities for converting coke oven gas into methanol, it became clear that it was necessary to install an additional gum-based gas desulfurization unit in order to ensure the stable operation of the methanol production facility. 1 Process flow of coal gas desulfurization using tannin method. The coal gas desulfurization unit using the tannin method is located at the end of the coal gas purification system, that is, it is placed after the benzene washing tower. Considering the relationship between system resistance and gas processing volume, not all of the coal gas was processed through the tannin desulfurization unit. The rich liquid discharged from the bottom of the tannin desulfurization tower enters the reaction tank; after staying there for a sufficient reaction time, it is pumped to the injection regenerator using a rich liquid pump. After being regenerated, it goes into the lean liquid tank in the injection regeneration tank, where the solution undergoes oxidative regeneration. The regenerated lean liquid is pressurized by a lean liquid pump and sent to the top of the tannin desulfurization tower. To ensure that the temperature of the lean liquid entering the tannin desulfurization tower is appropriate, a bypass is installed before the lean liquid enters the tower; the lean liquid flowing through this bypass goes to a lean liquid heat exchanger where heat exchange occurs, thereby regulating the temperature of the lean liquid. In winter, to prevent the solution temperature from dropping too low, the heat exchanger is heated using low-pressure steam ； In summer, to prevent the solution temperature from rising too high, circulating cooling water is used to cool the desulfurization solution. The sulfur foam suspended in the spray regeneration tank flows by gravity to the sulfur foam tank, and then is pumped to the sulfur foam high-level tank under nitrogen pressure. After being heated in the sulfur foam sump, the sulfur foam is pressurized by a sulfur foam pump and sent to the molten sulfur tank ； The return fluid flows into the underground tank, where a pump in that tank sends it back to the system. Sulfur paste is continuously heated in a sulfur melting furnace to produce molten sulfur, which is then fed into a sulfur casting tank to be cast into solid sulfur. The rated pressure of the gas blower is 18 kPa, and the maximum outlet pressure during operation is 13 kPa. The design value for the resistance in each of the three towers for hydrogen sulfide, ammonia, and benzene washing is 1.5 kPa. It is required that the outlet gas pressure not fall below 6 kPa; therefore, (13 – 1.5) × 3 – 6 = 2.5 kPa. As can be seen from the above data, this is the maximum resistance permitted for the tannin desulfurization tower. In actual design, the resistance of the tannin desulfurization tower is less than 1 kPa. 1.1 Selection of desulfurization method: Based on the experience of our sister manufacturers, we have decided to use a desulfurization method that involves tannin + vanadium pentoxide + soda ash + PDS. Based on the requirement of 65,000 m3/h of gas to be processed for methanol production, and a solution circulation rate of 500–700 m3/h, a large circulation volume and a large tower diameter are employed to reduce system resistance; this also enables optimal gas purification results at an effective spraying area. The composition of the desulfurization solution is shown in Table 1, while the consumption amounts of raw materials, catalysts, and chemicals are shown in Table 2. Table 1 Components of the gum desulfurization solution: Sodium bicarbonate, Sodium carbonate, Total alkalinity, Gum, Sodium thiosulfate, Sodium sulfate, Suspended sulfur, Total vanadium, pH value. Values in g/L: 20, 4.5, 21.2, 1.0–1.5, <150, <150