Oxidative desulfurization process

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Optimization of the ZL wet oxidation desulfurization process at Tangshan Jianlong Li Xinmei, Wang Feili, Jia Yingui, Ren Hongwu, Wang Sijie (Tangshan Jianlong Coking and Refractory Plant, Zunhua 064200); Li Zhenbo (Tangshan Jianlong Technology Department, Zunhua 064200). The coke oven gas produced by the Coking and Refractory Plant of Tangshan Jianlong Industrial Co., Ltd. is primarily used for power generation and in the heat treatment processes of cold-rolling mills, and high standards are required for its purification level; therefore, desulfurization and decyanidation are very important steps in the gas purification process. Our plant’s desulfurization section is located after the blower and before gas ammonia removal. The wet oxidation method using a ZL catalyst with ammonia as the base source was employed, in which the concentrated ammonia obtained by evaporating the remaining ammonia water was added to the desulfurization solution. This section is equipped with two desulfurization towers and two regeneration towers; the desulfurization system is designed such that the two desulfurization towers operate in series for desulfurization, while the two regeneration towers operate in parallel for regeneration. After more than a year of operation with a single tower, the hydrogen sulfide content in the gas downstream of the desulfurization tower has been kept at a low level. The extracted sulfur is pressed into cakes using a filter press and then sold externally, while the desulfurization wastewater is reused in coal blending, thereby essentially addressing issues such as high treatment costs, poor efficiency, and secondary pollution. The gas desulfurization process in our plant is shown in Figure 1. http://www.meijiaohua.com/WebUpLoadFile/ArticlePhoto/200892330079125.gif Figure 1 Schematic diagram of the gas desulfurization process 1 Equipment operation status, process improvements, and operational control 1.1 Control and adjustment of desulfurization solution parameters As can be seen from the chemical equation for ammonia-based desulfurization, the level of free ammonia in the desulfurization solution directly affects the efficiency of desulfurization; the higher the free ammonia content, the better the desulfurization effect, and vice versa. In addition to the ammonia contained in the coke oven gas itself, concentrated ammonia water distilled from an ammonia distillation tower is also used as a supplementary liquid as the ammonia source. The amount of supplementary liquid directly affects the level of free ammonia in the desulfurization solution; too little supplementation results in poor desulfurization efficiency ； If too much is added, the desulfurization efficiency improves, but the circulation rate of the desulfurization liquid increases rapidly. The inclusion of desulfurization waste liquid in the coal mixture poses difficulties in coal preparation processes and in the operation of charging coal cakes into coke ovens; it also accelerates the corrosion of related equipment and results in a loss of desulfurizing agents. To reduce the amount of water containing ammonia that enters the desulfurization liquid without losing the ammonia source, this can be achieved by lowering the temperature of the cooling water in the top reductor of the ammonia evaporation tower and increasing the reflux ratio. However, in practice, due to the high operating temperature of the ammonia condenser, scale tends to form on the inner wall of the condenser, preventing it from maintaining a normal low temperature. To this end, we installed a vapor-liquid separator on the high-concentration ammonia vapor pipeline between the reductor of the ammonia vaporization tower and the ammonia vapor condensation cooler. This separator diverts a portion of the low-concentration ammonia solution (about 40 g/L) to the inlet pipeline of the remaining ammonia water pump, while only the ammonia vapor with a higher concentration and lower flow rate (about 200 g/L) is condensed before being fed into the desulfurization mother liquor circulation tank. The amount of concentrated ammonia water entering is regulated using valves to control the ammonia content in the desulfurization solution. Through adjusted control, the amount of desulfurization liquid delivered to the coal yard was reduced from 60 tons per day to 45 tons. Meanwhile, keeping the ammonia content in the desulfurization solution at 10–12 g/L yields the best desulfurization effect. Since no cooling tower is installed before the gas enters the desulfurization tower, the temperature remains basically stable at 40±2°C. The desulfurization liquid inlet temperature is adjusted by heating it with steam through a heat exchanger. Since gas desulfurization is an exothermic reaction, lowering the temperature can improve desulfurization efficiency and reduce side reactions, but too low a temperature is not conducive to the regeneration process. After prolonged adjustment, the inlet temperature of the desulfurization liquid to the tower was controlled at 33±1°C. Sufficient oxygen is a necessary condition for the catalytic regeneration of the desulfurization solution; by appropriately reducing the blower volume while ensuring that the regeneration reaction proceeds, the occurrence of side reactions can be minimized and energy consumption can be reduced. The original design specified an air volume of 2,000–3,500 m3/h for the single tower; through practical experimentation, this value was adjusted to around 3,250 m3/h, with a gas-to-liquid volume ratio of 2.4 in the regeneration tower. The desulfurization efficiency is directly related to the circulation rate of the desulfurization fluid. After some testing, our plant maintained a circulation rate of 1330 m3/h for this fluid, with a gas-to-liquid volume ratio of 30 in the desulfurization tower. Through the control of these indicators, the temperature inside the tower and the amount of regenerative air were stabilized, ensuring effective desulfurization while also appropriately reducing energy consumption. 1.2 Addition of catalyst and control and adjustment of concentration: The ZL catalyst used in our factory is a blue-black powder with a particle size of less than 15 mesh, and a water-insoluble content of less than 3%. The ZL catalyst possesses special oxygen-carrying capabilities, and its catalytic activity is 0.06/min. During the desulfurization process, the ZL catalyst adsorbs dissolved oxygen to form highly active large ions. When encountering hydrogen sulfide in gas, it can be adsorbed on the surface of highly active macroparticles; the sulfur ions in hydrogen sulfide are converted into elemental sulfur or polysulfides, and then desorbed from the surface of the ZL catalyst. The deactivated ZL catalyst regains its oxygen-carrying capacity after being regenerated and oxidized with air. To control the concentration of the ZL catalyst in the mother liquor, the catalyst was activated using compressed air for 24 hours before being added gradually to the desulfurization system. The amount of loss of the ZL catalyst is closely related to the volume of desulfurization wastewater discharged and the output of sulfur paste; therefore, the amount of catalyst added should be adjusted in a timely manner based on the volume of wastewater discharged and the production volume of sulfur paste. Our plant keeps the consumption at 9 kg per day; the composition of the desulfurization liquid remains relatively stable, with the average values shown in Table 1. Table 1 Composition of desulfurization liquid: ZL content in ppm, free ammonia in g/L, hydrogen sulfide in g/L, thiosulfate in g/L, ammonium thiocyanate in g/L, total salt content in g/L, pH value, suspended sulfur in g/L – 92–119, 12–140, 0.15–0.25, 51–70, 95–113, 146–188, 8.7–8.9, 0.98–1.45, 1.3. Energy-saving measures for sulfur foam treatment: The sulfur foam generated from coal gas desulfurization in our plant is processed using a filter press to produce sulfur paste, which is then sold to nearby sulfuric acid factories. This method for producing sulfur avoids the harmful waste gases and liquids generated by the sulfur melting process, reducing environmental pollution; it also decreases steam consumption, which is beneficial for energy savings and reduced waste. 2 Conclusion The design specification for hydrogen sulfide content in the cleaned gas after desulfurization is less than 200 mg/m3, and our plant actually maintains this level at below 50 mg/m3. Production experience shows that by applying the ammonia-based wet oxidation regeneration-ZL catalytic desulfurization process, and by properly regulating the relevant process parameters, a high desulfurization efficiency can be achieved, keeping the hydrogen sulfide level in the gas after the desulfurization tower at a low level. By controlling the catalyst and free ammonia levels, it is possible to effectively regulate the rate of formation of by-product salts and reduce the amount of waste liquid discharged, thereby lowering costs and improving the environment. This post was last edited by ryn on 2009-4-21 08:17]