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Discussion on the blockage problem in gas wet desulfurization towers: Many enterprises suffer from varying degrees of blockage issues in their gas wet desulfurization towers. Since the 1990s, cobalt phthalocyanine sulfonate desulfurization methods represented by PDS, TS-8505, and 888 have been successfully developed, which has alleviated the problem of tower clogging to a certain extent. Production experience shows that even using cobalt phthalocyanine sulfonate, which has a \"tower cleaning\" effect, as a desulfurization agent, it cannot solve all problems. 1 Shape of the blockages and deposition process. Studies on the shape of blockages in desulfurization towers using the ADA method and the earlier PDS method have shown that the blockages formed in such towers are mainly of three types: sulfur, salts, and salt-sulfur mixtures. During desulfurization using the ADA method, the main substance that clogs the desulfurization tower is sulfur. When transitioning from the ADA method to the PDS method, the blockages are mainly salts. If PDS is disabled and the ADA method is reinstated, the blockage is mainly a salt-sulfur mixture. If the blockages form rapidly over days or months, their composition is mostly salts with a small amount of sulfur. However, it is very common for the resistance of desulfurization towers to gradually increase over 1 to 2 years. These blockages are usually composed mainly of sulfur, with a small amount of salts, and form ‘annual ring’-like deposition layers of varying thicknesses. Surveys of various enterprises have shown that it is common for sulfur foam not to form during the regeneration process over periods ranging from a few hours to several days. Factors such as an inappropriate composition of the desulfurization circulation fluid, impurities introduced from previous processes, and temperature changes can all directly affect the regeneration process. When the depleted liquid, which has not undergone sufficient regeneration, enters the absorption tower, the retention and adsorption of sulfur paste on the surface of the packing are inevitable. If these sulfur pastes are not transferred to the liquid phase in a timely manner, they will gradually deposit and solidify on the surface of the filler, gradually forming ring-shaped deposition layers. 2 The impact of equipment and operating procedures on blockages: In normal production, the use of cobalt phthalocyanine sulfonates as desulfurizers helps to reduce or prevent the formation of sulfur-based blockages. However, for severe, old-fashioned sulfur blockages, using desulfurizing agents with a \"tower cleaning\" effect to remove them during the production process yields very limited results. During the period when our company transitioned from the ADA method to the PDS method from 1992 to 1995, used the TS-8505 desulfurizer from 1995 to 2000, and switched from TS-8505 to 888 from November 2000 to January 2001, the initial goal in each case was to remove old deposits inside the tower, but none of these approaches were completely effective. Therefore, it can be seen that relying solely on desulfurizing agents capable of producing Sx to remove sulfur deposits accumulated on the surface of the filler is not very effective; even if the operation is carried out correctly, the removal process is lengthy. To this end, it is also necessary to strengthen operational management in the following areas. (1) Maintain an appropriate air velocity and spray rate. Some people believe that the larger the equipment, the greater the operational flexibility, which seems to make it easier to manage. Since the air velocity has a significant impact on the gas-liquid distribution in the absorption process, an excessively low air velocity (<0.5 m/s) is one of the main factors causing blockages in the desulfurization tower. Generally, when the gas treatment volume is low, the spray volume is often reduced while still ensuring the purification efficiency. As the spray volume was reduced, the residence time in the tower increased accordingly, leading to excessive regeneration and the premature growth of sulfur particles, which then adhered to the packing and gradually formed blockages. By the time that a severe blockage is detected and the spraying volume is increased, it is often already too late. (2) Reduce the oxygen content in the gas. During the production of semi-water gas, equipment failures and operational errors caused by the quality of raw materials can lead to an excessive oxygen content in the semi-water gas, which can sometimes exceed 2%. Since a regeneration process occurs alongside the absorption process, the higher the oxygen content in the gas, the stronger the regeneration process in the desulfurization tower, which accelerates the formation of blockages. Therefore, the oxygen content in the raw gas must be strictly controlled. (3) Prevent high liquid levels in the oxidation tank. During normal production, the liquid level in the oxidation tank should be at or slightly below the level of the partition. However, improper coordination between preceding and subsequent processes often leads to an increase in pressure inside the desulfurization tower, causing the liquid level to rise above the partition; as a result, some of the rich liquid takes a shortcut, which hinders the precipitation and flotation of elemental sulfur. Air bubbles tend to aggregate as they rise, reducing air utilization and lowering the regeneration efficiency. Furthermore, when the liquid level is above the partition, it accelerates the mixing of the solution and the sulfur foam, causing the already formed foam to be broken apart by the air bubbles, which makes it difficult to form a stable foam layer and hinders the flotation and separation of the sulfur foam. (4) Resume using the reaction tank. In the trough regeneration process, a reaction tank is usually also provided between the desulfurization tower and the oxidation tank. The main purpose is to give the HS in the rich liquid sufficient time for the sulfur precipitation reaction under the action of NaVO3. At that time, due to our company’s low gas processing capacity and corresponding low solution circulation rate, the residence time of the solution in the reaction tank was as long as 36 minutes (the original design value was 17.4 minutes). This often led to overflow incidents caused by the excessive growth and floating of sulfur particles. To this end, the reaction tank was discontinued in August 1987, but this led to the following problems: first, due to insufficient reaction time for sulfur oxidation, the rich liquid entered the oxidation tank directly. On the one hand, the unconverted HS– in the rich solution reacts with oxygen in the oxidation tank to produce Na2S2O3, which is further oxidized to Na2SO4. As a result, the concentration of Na2S2O3 in the system decreased from 50 g/L to around 10 g/L, while the concentration of Na2SO4 reached around 30 g/L. On the other hand, the unconverted portion of HS directly enters the desulfurization tower to release sulfur. Secondly, when the desulfurization tower becomes clogged, it is more difficult for sulfur foam to form in the oxidation tank, which in turn exacerbates the clogging. The sulfur precipitation reaction in the desulfurization tower is weakened, reducing the amount of elemental sulfur produced, and as a result, sulfur foam cannot form continuously. To achieve the overflow of sulfur foam as quickly as possible, it is necessary to increase the amount of V2O5 and Na2CO3 added. This further accelerated the formation rate of by-products such as NaCNS and Na2SO4. Therefore, shutting down the reaction tank makes the desulfurization system extremely vulnerable. Once the external conditions change, the clogging rate of the desulfurization tower increases significantly. Thus, the reaction tank was used again. (5) Restore the two-stage sedimentation and clear liquid separation functions of the sulfur foam. After being floated and separated from the oxidation tank (or regeneration tower), sulfur foam still contains a large amount of lean liquid; before entering the sulfur melting vessel, it generally undergoes two stages of sedimentation separation, with the clear liquid being returned to the oxidation tank. However, the two-stage sedimentation separation system in many manufacturers is essentially ineffective; the heating and stirring equipment in the second foam tank no longer exists, and both the foam and the clear liquid end up entering the sulfur melting vessel without being separated. The main problem that arises is the degradation of the active components in the solution into harmful substances. When desulfurization is carried out using methods such as ADA, PDS (or TS-8505, 888), the active components in the solution may degrade into inactive and harmful components at high temperatures (around 1500°C). Practice has shown that the waste liquid after sulfur melting, just like the waste liquid and filter residues generated during the extraction of NaCNS, exhibits defoaming effects when returned to the oxidation tank. When these waste liquids and filter residues are added to the oxidation tank, it becomes difficult for sulfur bubbles to grow, aggregate, and float. The more of these substances that are added, the longer it takes to eliminate the bubbles; in extreme cases, no sulfur bubbles can form for up to 6 days, resulting in a suspended sulfur concentration as high as 17 g/L in the weak acid solution, which accelerates the clogging of the packing. For example, in 1988, when the Na2SO4 concentration in the desulfurization wastewater from which by-products were extracted reached around 30 g/L, it was found that the filter residue was fine and highly hydrated, making filtration difficult and exacerbating corrosion of the system. These tiny sulfur particles act as seeds in the oxidation tank to combine with elemental sulfur; due to their extreme small size and abundance, it is difficult for elemental sulfur to grow, preventing the formation of stable sulfur bubbles over time. Another issue is the increased steam consumption in sulfur production. 3 Improving the process and tower structure: As mentioned above, in addition to restoring the two-stage sedimentation and separation functions of the reaction tank and sulfur foam, it is also necessary to improve the desulfurization process and tower structure in order to minimize the regeneration reactions during the absorption process. Taking our company as an example, coke oven gas undergoes two-stage desulfurization in a turbulent tower and a packed tower, while semi-water gas is desulfurized using a single packed tower. The lean liquid is fed into the turbulent tower at its top and middle sections as well as into the upper part of the packed tower, while the rich liquid is collected and then sent to an oxidation tank for regeneration. After secondary sedimentation, the sulfur foam is fed into the sulfur melting tank; the waste liquid resulting from sulfur melting is sent to the oxidation tank after being settled in a sedimentation tank. 10 years of production experience have shown that turbulent flow towers have never experienced clogging, whereas packing towers suffer from frequent clogging, especially in the lower section. Studies show that wall flow effects exist in almost all packed columns, ranging from 70% in severe cases to over 10% in milder cases. In a packed tower with wooden packing, a wall flow effect is inevitable, and a centripetal effect exists in the gas phase, which directly affects its mass transfer efficiency and leads to the formation of blockages. To this end, process improvements should be made according to Figure 1. After the modification as shown in Figure 1, the semi-rich liquid drawn from the middle of the two packed towers enters the middle of the turbulent tower to serve as the absorption liquid in its lower section, thereby avoiding the severely clogged lower part of the packed towers and making full use of the advantage of the turbulent tower’s resistance to clogging. Two packed towers operate in parallel, with gas-liquid redistributors added at the middle of the upper and lower sections of each tower, thereby increasing the number of gas-liquid redistributions from 2 to 4. This helps to reduce the tendency of the liquid to flow toward the walls and the tendency of the gas to flow toward the center.