Analysis of the importance of spray density in the production process

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In the field of wet flue gas desulfurization, it is a relatively systematic and complex engineering project with many factors affecting production. To successfully carry out this project, it is crucial to pay attention to every aspect, from gas washing and purification to the control of process parameters. Low purity and short operation cycles are common problems in wet flue gas desulfurization; in particular, it is crucial whether the equipment configuration is appropriate and can meet the production requirements. Based on years of experience in after-sales service for wet flue gas desulfurization, the author finds that a low spray density has a significant impact on production; it not only prevents the purification level from meeting the standards but also fails to ensure a proper operating cycle. What is spray density? Spray density refers to the flow rate of fluid per unit time through a unit area of the tower (m3/m2.h). So what is an appropriate spray density? Based on my own experience combining practical production with theoretical knowledge, I recommend that in production, the spraying density should be maintained at above 50 m3/m2·h. A low spray density can cause significant harm to production. Here, the spray density is low, which results in a higher level of hydrogen sulfide at the outlet, as well as a shorter operating cycle. I would like to discuss these two aspects with everyone. I. A low spraying density affects the level of purification. There are many factors that influence the degree of purification, but no matter how well other process parameters are met, if the spraying density is insufficient, the required level of purification will never be achieved. In modern wet desulfurization processes, whether in fertilizer plants or coking plants, and whether using bulk packing materials or lightweight ceramic packing, polypropylene, structured packing, etc., it is all aimed at enabling the solution to absorb hydrogen sulfide more effectively. Only when gases dissolve better in the solution can a neutralization reaction occur, ensuring that the hydrogen sulfide level at the outlet remains within specified limits and maintaining the purity of the solution. If the spray density is low, the gas cannot come into contact with the solution as it rises through the packing; as a result, the elemental sulfur, salts, and other insoluble substances produced by the reaction do not dissolve properly in the solution. These substances then adhere to the packing, and over time, as more and more of these deposits accumulate, a dry zone is formed. At that time, gas maldistribution and excessive hydrogen sulfide levels at the outlet will occur. Here is an example: On April 11, 2016, a ceramic factory in Chengdu, Sichuan, had continuously unsatisfactory levels of hydrogen sulfide at its outlet after the plant was put into operation. Technicians from Changchun Dongshi Company went to the site for an investigation and found that the main issue was too low a spraying density, resulting in poor gas-liquid contact and thus excessive levels of hydrogen sulfide at the outlet. The relevant data for this ceramic factory are as follows: 1. Gas flow rate: 25,000 Nm3/h; 2. Inlet H2S level: <3 g/Nm3; 3. Outlet H2S level: <50 mg/Nm3; 4. The diameter of the desulfurization tower is 3.4 meters, and its height is 30 meters, with three layers of bulk packing inside; 5. The regeneration section consists of a low-tower regeneration tank with dimensions of 4000*4600*5400, and the regenerator is equipped with ten nozzles that are 20 mm in diameter. 6. There are three desulfurization pumps and three regeneration pumps, each with a capacity of 200 m3/h; the design specifies two in operation and one as a backup. 7. Soda ash solution is used for absorption, and all components of the solution are within the specified limits. At the beginning of operation of this plant, the hydrogen sulfide level at the outlet was acceptable, but after three days it exceeded the allowable limit, reaching over 100 mg/Nm3. At that time, only one desulfurization pump with a capacity of 200 m3/h and one regeneration pump with a capacity of 200 m3/h were in operation on site. Through on-site inspections and calculations, it was determined that the spraying density for such a desulfurization tower was only 22 m3/m2·h; clearly, this spraying density was not sufficient to meet the production requirements. So, why is only one desulfurization pump with a capacity of 200 m³/h operating? There should be two pumps operating according to the design; if both were in operation, the spray density would reach 44 m³/m²·h. Here, another problem arises: regeneration backpressure. This is mainly evident in the ejectors equipped with ten 20-mm nozzles each (with each ejector handling approximately 20 m³/h of solution). The nozzles are too small to accommodate the flow rate generated by two pumps; in other words, the spray density fails to reach 44 m³/m²·h. To ensure stable operation, only one desulfurization pump and one regeneration pump can be operated simultaneously. If both desulfurization pumps were run, an imbalance would occur between the lean and rich liquid tanks. Therefore, for a desulfurization tower with a diameter of 3.4 meters, such a low spray density prevents proper contact between the gas and liquid phases. Naturally, this results in excessive hydrogen sulfide levels at the outlet. So how does this factory meet the standards for hydrogen sulfide? The measure taken was to replace the injectors in the regeneration tank, enlarging the nozzles to over 28 mm, to ensure that each injector can handle a flow rate of more than 40 m3/h of solution, thereby maintaining the spray density. It turned out that after replacing the injector, the circulation rate increased, ensuring the required spraying density, and the hydrogen sulfide level at the outlet dropped immediately to the specified value of 50 mg/Nm3. II. A low spray density leads to an increase in the tower resistance, which in turn affects the operating cycle. There are many factors that can cause an increase in tower resistance, such as broken light porcelain packing, high levels of impurities in the solution, collapsed corrugated packing, high levels of by-products, high levels of suspended sulfur, and excessive tar. All these factors can contribute to an increase in tower resistance, but a low spray density is also a major cause of this increase. Today, gas generators such as coke oven gas, furnace gas, and semi-water gas all require H2S treatment. Wet desulfurization and dry desulfurization are commonly used for H2S removal. Desulfurization towers typically have three layers of packing, either random packing or structured packing, in order to ensure thorough gas-liquid contact. As the gas passes through the desulfurization tower, it travels from bottom to top through the first layer of packing, where it comes into countercurrent contact with the desulfurization liquid flowing from top to bottom. Through a catalytic reaction facilitated by the 888 desulfurization catalyst, H2S is converted into elemental sulfur. The gas then sequentially passes through the second and third layers of packing, where any remaining H2S that has not been converted into elemental sulfur is further absorbed and transformed. Finally, it is carried out of the desulfurization tower along with the solution. Therefore, the solution containing H2S and elemental sulfur is passed through three layers of packing and flows out of the desulfurization tower sequentially to reach the regeneration system. If the spray density is low, as the gas rises, insoluble substances such as sulfur particles, dust, and tar adhere to the packing. Due to the low spray density, these adhered substances are not disturbed or washed away; they cannot dissolve in the solution, nor can they be carried out of the desulfurization tower along with the solution. Over time, dry areas form in these regions, and as time passes, the area of these dry areas increases while the gas flow channels become narrower, resulting in an increase in the resistance within the tower. For example, a lime desulfurization plant in Shandong Province encountered problems such as low spray density and increasing resistance in the tower. Here is some relevant production data: 1. Gas flow rate: 35,000 Nm³/h; 2. H₂S concentration at the inlet: <5 g/Nm³; 3. H₂S concentration at the outlet: <50 mg/Nm³; 4. Height of the desulfurization tower: 4.5 m; 5. Capacity of the desulfurization pump: 500 m³/h. This company purchased PDS catalyst from a supplier in Changchun for use during the startup phase. Nearly two months after startup, not only did the H₂S concentration at the outlet exceed the standard limits, but the resistance in the desulfurization tower also increased significantly—reaching over 8 kPa in severe cases. This severely impacted the gas output, forcing the plant to halt operations and have the tower emptied and its packing replaced. This incident received great attention from the leadership of Lv Electric Company. After conducting market research, it was decided to invite experts from Changchun Dongshi Company to provide on-site guidance in analyzing the causes of the incident. The conclusions drawn were as follows: 1. The designed flow rate of the desulfurization pump was too low; the spraying density was merely 31.5 m³/m²·h. This low spraying density was the main cause of tower blockage. 2. The quality of the catalyst was poor; it had weak oxidation capabilities and insufficient oxygen-carrying capacity. As a result, very fine elemental sulfur particles were formed, which hindered the regeneration process via froth flotation (no foam was generated during the two months following startup). 3. The technical support provided through after-sales services was inadequate. Following maintenance and modifications, production resumed normally, with the online SO₂ monitoring levels consistently remaining below 20 mg/Nm³. The measures taken included increasing the pump’s circulation rate; now, the desulfurization pump operates at 800 m³/h, resulting in a spraying density exceeding 50 m³/m²·h. The other uses a desulfurization catalyst from Changchun Dongshi Company. Under the guidance of the technical staff from Changchun Dongshi Company, once the catalysts provided by this company were put into use, there was an immediate improvement in production performance. The SO2 levels met the required standards within two days of monitoring, and there was no increase in tower resistance – the situation remained stable, which earned high praise from the manufacturer. Through the analysis of these two cases, what I want to illustrate is the importance of spray density in production. Only by ensuring the spray density can the purity be guaranteed. Only by ensuring the spray density can the operating cycle be guaranteed. When purchasing desulfurization catalysts, it is recommended to opt for high-efficiency catalysts that offer good cost-performance ratio, high purification efficiency, strong oxidation capabilities, and do not cause tower clogging. Additionally, there must be robust professional after-sales service expertise to provide support, thereby alleviating any concerns for enterprises; otherwise, it is impossible to ensure the long-term stable operation of production.