“Analysis of the performance of the 888” desulfurizer: Key factors for achieving a positive cycle.

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Preface: The atmospheric desulfurization unit in Workshop 1 for raw material gas at Anyang Chemical Group Company in Henan Province is responsible for the desulfurization of the ammonia synthesis, methanol, and methylamine production systems in the existing plant, with a gas flow rate of 90,000 m3/h. The desulfurization fluids from the three systems are recycled together. I. Existing Problems 1. Overloading of equipment: The desulfurization unit in our company was originally designed as a \"6-to-8\" system; it was a set of desulfurization equipment installed in 1992, and its capacity has since been increased to 120,000 tons per year of synthetic ammonia production ； In 1995, another project with an annual production capacity of 30,000 tons of methanol was added, and the production capacity has since been increased to over 60,000 tons. As the gas load continues to rise, the H2S level before it enters the tower becomes high, posing significant challenges to the completion of desulfurization tasks. The regeneration tank is designed to be too small, resulting in a short residence time for the solution. This poses significant difficulties in the precipitation of elemental sulfur, the oxidation of the oxygen-carrying tannin, and the conversion of vanadium. The lean solution becomes cloudy, regeneration is incomplete, and the concentration of suspended sulfur is high, averaging 0.84 g/L. As a result, the sulfur precipitation effect is poor, and tower blockages occur very frequently. 2. High consumption of chemical raw materials for desulfurization: Although the consumption of chemical raw materials for desulfurization is lower compared to most enterprises of the same type, it still differs significantly from the theoretical values. Currently, the average cost of chemical raw materials per ton of ammonia produced is 5.5 yuan in RMB. The regeneration tank is small; whenever the hydrogen sulfide level at the inlet increases or fluctuations occur in the system, a large amount of clear liquid flows from the overflow weir into the intermediate foam tank, resulting in high solution loss and placing a significant burden on the company’s efforts to reduce costs and improve efficiency. 3. High suspended sulfur levels: Our desulfurization system uses the tannin-based desulfurization process, which was adopted in 1992; we implemented this technology earlier than our competing companies. Overall, the operation, management, and resource consumption related to this process are quite satisfactory, and there have never been any issues in our processes that affected the system’s production. However, high suspended sulfur levels remain a persistent problem, with average levels exceeding 0.8 g/L. Sulfur blockages in the towers occur frequently, requiring tower cleaning during regular maintenance work each year. Despite numerous attempts at adjusting the process parameters, it has not been possible to keep these levels within acceptable limits. Later, we improved the management of overflow during process operations, but the problem persisted. II. Specifications of main desulfurization equipment: Two desulfurization towers – for ammonia synthesis: diameter Ф4400mm, height 38850mm; for methanol production: diameter Ф2800mm, height 38850mm. The towers are equipped with Pall ring fillers. There are three regeneration tanks – two in the ammonia synthesis system: diameters Φ4800/Φ5700mm, height 6600mm; diameters Φ3500/Φ4400mm, height 6600mm; one in the methanol production system: diameter Ф4500/Φ5400mm, height 8000mm. III. Problem analysis: It can be seen from the specifications of the above equipment that the small size of these devices is the main reason for the problems in this process. Additionally, the shortage in the coal market and changes in coal quality exacerbate the situation. With the equipment already operating under overload conditions, this further contributes to the issues. The amount of solution circulated during desulfurization and the efficiency of regeneration become the main challenges in this process. Increasing the circulation rate results in a shorter residence time of the solution in the regeneration tank, while reducing the circulation rate makes it difficult to maintain an adequate desulfurization efficiency. IV. Regarding the amounts to be used and the methods involved, based on the current situation and taking into account the characteristics of tannin gum and the \"888\" desulfurization catalyst, we conducted a comprehensive comparison. Analysis showed that the \"888\" catalyst can improve desulfurization efficiency and reduce suspended sulfur levels. The required amounts were determined through theoretical calculations, and then this catalyst was introduced gradually through a step-by-step replacement process. V. Effects after implementation 1. Changes in sulfur recovery and suspended sulfur: One week after the initial use of this product, changes occurred in the sulfur foam layer in the regeneration tank; its density increased and its quality improved. The thickness of the foam layer reached 200 mm, which was very encouraging. Sulfur production increased significantly, with the daily output rising from 350 kg to over 550 kg. The calculated sulfur recovery rate was over 100%. It took nearly a month of intensified efforts to restore the normal sulfur recovery rate. In summary, sulfur production and sulfur recovery rates have increased, and a reduction in suspended sulfur is inevitable. Before the use of the \"888\" desulfurization catalyst, there were always fine sulfur particles in the lean solution that could not be removed, making filtration particularly difficult during analysis; it took 40 minutes, or even longer, to pump air using a vacuum pump. However, as the \"888\" gradually replaced other substances in the solution and began to exert its effect, the time required for filtration was significantly reduced to just five minutes. The analysis results also decreased from 0.8 g/L to below 0.3 g/L, and these values have remained stable for several years now. As a result, the resistance of the desulfurization tower remained below 450 mmH2O columns and stayed relatively stable; the packing has not been cleaned or replaced for five consecutive years. It has saved a significant amount of human, material, and financial resources for the group company, ensuring the stable operation of the system over long periods under high loads. 2. Changes in desulfurization efficiency: With the desulfurization unit in this process already operating under overload conditions, the impact of changes in coal quality on desulfurization has become a problem that cannot be ignored. An increase in sulfur content at the inlet, high tar levels along with strong odors, as well as contamination of the solution, all directly affect the proper functioning of the regeneration system, thereby compromising the desulfurization efficiency. To address this issue, we took various measures internally to increase the circulation volume of the solution; for example, we added connecting pipes in the lean liquid area of the regeneration tank and removed the filters at the pump inlets. Although the circulation volume was increased, the regeneration system still faced significant burdens. Since the introduction of the \"888\" desulfurization catalyst, this contradiction has been effectively resolved. The circulation rate dropped from 720 m3/h to 600 m3/h, while the desulfurization efficiency remained above 99.7%. This not only increased the residence time of the solution in the regeneration tank but also resolved the conflict between desulfurization and regeneration, enabling a sustainable cycle. 3. Changes in the consumption of chemical raw materials: When it comes to consumption, it is directly related to the level of hydrogen sulfide. The solution we prepare is a mixed liquid composed of four ingredients: soda ash, tannin, vanadium pentoxide, and “888”. Based on our long-term experience, the “888” desulfurization catalyst indeed has the ability to remove sulfur compounds and reduce the consumption of chemical raw materials. When we first started using this product, we didn’t notice this issue; we simply used it for sulfur removal. However, after several months of operation, we found that as the amount of various chemical raw materials used decreased, the process parameters did not decline at all, which puzzled us. In particular, when the average hydrogen sulfide concentration at the inlet was 1200 mg/m3, adding 200–300 kg of alkali per day was still sufficient to keep the parameters stable and maintain a consistent desulfurization efficiency. The current efficiency is 0.52 kg/tNH3. The amount of tannin used also decreased significantly: originally 75 kg per day was required, but now only 30 kg is needed to maintain the desired parameters ； Previously, 300 kg of vanadium pentoxide had to be added per month, but now only 75 kg is needed per month, which is more than sufficient; moreover, the side reactions are well under control. Since 2004, our consumption of chemical raw materials per ton of ammonia has averaged around 2.8, saving the company over 700,000 yuan each year, which has earned praise from leaders at all levels. VI. Management and Experience Summary 1. Management of Desulfurization Personnel No matter how high the quality of the chemical raw materials used for desulfurization, proper management of this process is essential. Most companies do not pay enough attention to the desulfurization process; in an effort to improve economic efficiency, they often reduce the number of staff employed in this area. Some companies even hire temporary workers who do not understand the relevant processes. Analysis of the components in the solution is hardly ever carried out. On the surface, this may seem to improve economic efficiency, but the dynamics of the desulfurization process are unpredictable, and it is one of the most difficult processes in ammonia synthesis. Once problems arise, it is hard to identify their causes. In severe cases, the system remains in a degraded state for a long time, preventing it from returning to normal operation promptly and resulting in increased costs. In the worst cases, this can lead to the shutdown of the entire system. 2. Management of the regeneration tank overflow operation: In the desulfurization process, the overflow from the regeneration tank is a critical aspect; it has a direct impact on the quality of the lean solution, absorption efficiency, tower resistance, sulfur recovery, and so on – ultimately affecting the overall operation of the system. We attach great importance to this aspect and assign a dedicated operator to oversee the overflow process, which ensures timely sulfur recovery and is also a key factor in maintaining the stability of suspended sulfur. Furthermore, the changes in sulfur foam in the regeneration tank are highly complex and variable; factors such as the quality of gas, the level of hydrogen sulfide, the recovery of residual liquids, the amount of air drawn in for regeneration, and the control of catalyst parameters can all affect the foam. Therefore, we must strictly control the thickness of the sulfur foam as well as the issue of insufficient sulfur foam. If the reaction layer formed by the sulfur foam thickens or there is no sulfur foam at all, the amount of suspended sulfur will increase. It is necessary to adjust the amount of air introduced, or the amount of catalyst added, in a timely manner based on the condition of the sulfur foam. Many sister companies these days use continuous sulfur melting; in order to reduce the amount of foam liquid that enters the continuous reactor, they ignore the management of overflow from the regeneration tank. I think that tower blockage is an inevitable consequence of this. 3. Formation and control of by-products in desulfurization processes: The formation of by-products is a normal phenomenon in such processes. If not properly controlled, it will lead to an increase in alkali consumption and accelerate equipment corrosion. Based on our experience, the main method to control a high rate of by-product salt formation is to regulate the solution temperature carefully; we should strengthen coordination and management in this regard, with a target temperature of 37–40°C being optimal. Furthermore, the recovery of residual liquid is also a major cause of increased by-products; especially when the continuous sulfur melting rate is high, it is best to return it to the system after pretreatment. Sodium thiocyanate, another by-product, is one that is difficult to control in this process; however, high solution temperatures and incomplete regeneration oxidation also create conditions conducive to its formation. As for how to control by-products, we attach great importance to this. First of all, it is necessary to strengthen the analysis of these by-products and closely monitor their changes; generally, their levels increase in a proportional manner. Once such an increase occurs, we add vanadium pentoxide, and the control effect is quite significant. We keep the level of sodium thiosulfate in our desulfurization solution below 3 g/L. The final product, sodium sulfate, is kept at around 20 g/L. These two values have remained stable over the past several years, ensuring proper consumption of chemical raw materials, reducing corrosion of the equipment, and extending its service life. In short, since the desulfurization equipment is relatively small, by choosing high-quality chemical raw materials and improving process management, it is entirely possible to achieve a positive cycle in the desulfurization process.