Reasons for sulfur foam in gas after desulfurization

Thread Content

This post was last edited by Cao Jianqing on 2009-7-29 21:41. 1. Sulfur foam in the gas after desulfurization: During the operation of the desulfurization system, the desulfurization tower, acting as a gas-liquid contact device, often experiences foam entrainment, to varying degrees of severity. Systems with light mist entrainment have virtually no impact on subsequent processes. If foam entrainment is severe, sulfur foam will be carried into the subsequent system, causing the gas valves in the compressor section to become clogged with sulfur foam, as well as the heat exchange tubes in the water cooler to become blocked by sulfur. Mist entrainment in desulfurization towers is different from that in other gas-liquid contact devices. The sulfur foam carried in the mist has strong adhesiveness and poor fluidity. During the mist separation process, sulfur mist tends to adhere to the surfaces of separation devices and similar components. No demister is installed at the top of the desulfurization tower; if mist is carried along, it can have a significant impact on subsequent processes. If swirl plates or mesh-type demisters are used, the resistance increases due to the adhesion of sulfur foam. Installing a section of packing above the liquid distributor at the top of the tower can be effective. It should be noted that sulfur blockage can also occur if the surface of the filler is not flushed by liquid. The situation will be relatively better if a baffle demister is used, but the spacing and angle of the baffles need to be carefully calculated based on the empty-tower gas velocity of the desulfurization tower. A large spacing with a small angle results in poor separation effects, while a small spacing with a large angle leads to high resistance and a risk of the material being blown over. Water is added after the desulfurization tower for tower washing, and wastewater treatment is rather difficult; the wastewater containing desulfurization solution and sulfur foam cannot be treated. Adding a primary electrostatic precipitator after desulfurization can indeed effectively remove the entrained sulfur foam, but its operating cycle is relatively short. Once sulfur foam adheres to the corona electrode, its ability to remove sulfur foam is immediately lost, and the cleaning effort required for the electrode wire also becomes considerable. It appears that, to deal passively with the issue of foam entrainment, using a baffle demister is the best option. To completely solve the problem of sulfur foam, solutions must be found at the source. The reasons for sulfur foam in the gas are as follows: First, the gas velocity in the desulfurization tower is high; otherwise, the amount of entrained sulfur would be low ； Secondly, the high content of by-products in the desulfurization solution causes it to foam easily ； Third, the desulfurization lean liquid has a high level of suspended sulfur, resulting in a large amount of sulfur foam in the foam. And these three situations exist simultaneously most of the time. The occurrence of this series of problems is due to inherent flaws in the entire system, and it has little to do with production management or the workers’ operations. 2. Accumulation of by-products The by-products in the desulfurization liquid are mainly NaSCN, Na2S2O3, and Na2SO4. Among these, NaSCN is formed due to the presence of HCN in the feed gas, which is generated during the coal-to-gas production process; this HCN leads to the formation of NaSCN in the desulfurization liquid. The formation of NaSCN is uncontrollable. NaSCN can be further oxidized to Na2SO4. During the regeneration process of the desulfurization solution, NaS2O3 is formed; this side reaction is a controllable one in the desulfurization process. Further oxidation of Na2S2O3 also produces Na2SO4; the level of Na2SO4 in the desulfurization solution is related to the amounts of Na2S2O3 and NaSCN formed in that solution. Furthermore, the tar present in the gas cannot be completely removed; it reacts with the desulfurization liquid through saponification, causing foaming in the liquid being separated. This has already caught the attention of most manufacturers, so there is no need to elaborate further. The side reaction that produces Na2S2O3 during the regeneration of the desulfurization solution is related to the regeneration temperature, the pH value of the desulfurization solution, and the residence time in the rich liquid tank (sometimes called the reaction tank). The reason for the high regeneration temperature is the high temperature of the gas entering the desulfurization tower; therefore, the cooling effect of the cooling tower needs to be taken into account. The desulfurization solution is actually a buffer solution of Na2CO3-NaHCO3; a higher pH value leads to greater absorption of CO2 from the gas, so the pH value of the desulfurization solution does not get too high. On the contrary, an increase in the content of by-products leads to a lower pH value of the desulfurization solution. A short residence time in the rich liquid tank indicates that the tank is too small; it is necessary to modify the tank in order to increase its volume. Another aspect is the sulfur melting process; as for the reactions that take place during this process, there are currently no authoritative publications providing credible analyses. However, what everyone can agree on is that adding the sulfur-melted waste liquid back into the system has a significant impact on it, and there is little difference whether it is added to the lean liquid tank or the rich liquid tank. As a result, many manufacturers are seeking ways to deal with sulfur-containing waste liquids, such as precipitation, filtration, cooling, etc., but the results have been mixed. The above methods can only control the amount of side reactions that occur, but they cannot eliminate their occurrence altogether. Even if the equipment for alkaline desulfurization is properly matched and production operations are well controlled, it is only possible to reduce the amount of side reactions; it is not possible to prevent their occurrence altogether. And in accordance with the principle of the conservation of matter, the by-products in the desulfurization solution will only continue to accumulate and will not disappear. In some units, when the accumulation of by-products reaches a certain level, appropriate measures are taken, and after a period of adjustment the system can return to normal. So where do these by-products go? If there is no device for extracting by-products. The conclusion should be understood without words, right! As for how to control the accumulation of by-products, it simply involves removing them from the system. There are a few units whose desulfurization process operates stably, with no accumulation of by-products. There is one common factor that merits consideration: they do not produce sulfur themselves, but rather filter the sulfur paste and sell it directly. In fact, this is also a form of indirect replacement. Even without considering the sale of sulfur paste, it should be considered to filter the sulfur foam before melting it. In this way, the amount of waste liquid is **reduced, making it easier to handle. For boilers equipped with flue gas desulfurization systems, the desulfurized substance can be sprayed onto the fuel coal for combustion; those with coal ball systems can also add it to the raw coal and send it back into the furnace. If the wastewater treatment system has sufficient capacity, the waste water can also be sent there for treatment. The accumulation of by-products increases the specific gravity of the desulfurization fluid. The specific gravity of fresh desulfurization fluid is generally below 1.1 g/l, and when it exceeds 1.2 g/l, the performance of the desulfurization process deteriorates. It can be measured using an ordinary hydrometer, which is also quite convenient. 3. Sulfur capacity and the volume of the rich liquid tank/regeneration tank: The level of sulfur capacity can indicate the quality of the desulfurization solution; however, there may be a misconception at present, which is the one-sided pursuit of high sulfur capacity. The purpose of desulfurization is to reduce the hydrogen sulfide content in gas to specified levels; for example, in the nitrogen fertilizer industry, the required level of H2S at the outlet of semi-water gas after desulfurization is 0.068 g/Nm3, and it is essential to meet this target. It is not possible to simply increase the alkalinity or the catalyst concentration in order to boost the sulfur capacity, as this requires ensuring that there is a corresponding residence time in the rich liquid tank to match it. A high sulfur capacity implies a high content of NaHS in the rich liquid; if there is no corresponding residence time in the rich liquid tank to allow for adequate oxidation of NaHS, then when this liquid comes into contact with large amounts of air during spray regeneration, a side reaction occurs resulting in the formation of Na2S2O3. Furthermore, when the residence time in the rich liquid tank is favorable, it is possible to control the high sulfur capacity of the desulfurization liquid, which in turn allows for a reduction in the volume of liquid that needs to be circulated. However, this raises the issue of whether the liquid-to-gas ratio and spraying density meet the required standards; if the spraying density is too low, it will not be possible to wash away the sulfur foam formed during the desulfurization process, as well as the suspended sulfur carried in by the lean liquid. Over time, this can lead to tower blockages. Insufficient attention is paid to the downtime of the rich liquid tank; if its volume is too small, numerous side reactions occur. This leads to a decrease in desulfurization efficiency. To maintain the required desulfurization standards, it is necessary to increase the circulation rate, which in turn shortens the residence time in the rich liquid tank, creating a vicious cycle. The investment required to modify the rich liquid tank is not large, but it can solve major problems. The regeneration tank is a device for restoring the oxygen-carrying capacity of the desulfurizer and oxidizing HS to S, while also enabling the flotation of sulfur foam. If the regeneration tank is too small, not only will the desulfurization solution not be regenerated properly, but it will also result in high suspended sulfur levels in the lean solution. Furthermore, the volume of the regeneration tank refers to the volume dedicated solely to gas-liquid contact. For the spray regeneration tank, the volume of the lean liquid in the annular tank is not included. The recommended residence time for the rich liquid tank/regeneration tank is 15±5 minutes, which may not be in line with common opinions. 4. Tower blockage: After desulfurization using alkaline solutions, a common problem at present is tower blockage. However, if we look back, this contradiction was not prominent in the early 1990s; on one hand, some manufacturers were still using ammonia for desulfurization, while few used alkaline solutions. On the other hand, the problem of manufacturers using alkaline solutions for desulfurization clogging the towers at that time was indeed not as severe as it is today. The problem of blockages in desulfurization towers became widespread around the year 2000; due to the rapid development of the fertilizer industry, the supply of anthracite began to tighten and its prices started to rise. To reduce production costs, it has become common for fertilizer manufacturers to use high-sulfur coal for gas production. Additionally, some companies do not pay enough attention to desulfurization; as their production scale continues to expand, they fail to make the necessary modifications to their desulfurization systems. This ultimately led to the desulfurization system operating at high load or even beyond its capacity. As a result, the phenomenon of desulfurization causing tower blockages has gradually become the \"norm,\" with towers that do not get blocked becoming the exception. In recent years, numerous attempts have been made in various areas to address the issue of tower blockages. Treatment of the sulfur melting residue, modification of the liquid distributor, replacement of the column packing from structured packing to random packing, catalyst replacement, and so on – yet in the end, the blockage still occurs. Currently, some plants use desulfurization towers with air tower spraying, which eliminate the problem of tower blockage; however, their desulfurization efficiency is low, so they can only be used for pre-desulfurization. A unit uses 250-regular packing in a tower of Ф4200; the tower can handle a gas flow rate of 20,000 m3/h, with an inlet H2S concentration of 1.0~1.5 g/Nm3. After two years of use, there has been no significant change in resistance. Later, the capacity was increased to 30,000 m3/h, with an inlet H2S concentration of 2.0~2.5 g/Nm3. Even when using 125 regular packing, it basically can’t last more than a year. Later, a Ф76 plastic step ring was used, but there was no significant improvement either. The long operating cycle per hour for the desulfurization tower can be attributed to the following reasons: 1. The tower itself has a certain margin of capacity; even if there is a tendency for blockage, the flow rate within the tower can be maintained for a longer period given certain input conditions. 2. When there is a surplus in the tower, there is also a corresponding surplus in the circulation volume of the desulfurization liquid, allowing for regular flushing of the packing at a high circulation rate. 3. If the imported H2S level is high, more sulfur will precipitate inside the desulfurization tower, thereby increasing the likelihood of sulfur blockages. 4. Given a constant level of suspended sulfur in the lean liquid, the circulation rate of the desulfurization system is high, resulting in a relatively larger amount of sulfur being introduced into the tower. Furthermore, when the desulfurization system operates under high load, poor regeneration, high levels of by-products, and high levels of suspended sulfur are more likely to occur. When such situations occur, it also has an adverse effect on the desulfurization tower. For example, high levels of suspended sulfur can result from inadequate treatment of sulfur melting residues; this can lead to blockages in the liquid distributors, resulting in less liquid flow in certain areas of the packing. As a consequence, the necessary flushing effect is not achieved, and blockages occur quickly. Furthermore, if the desulfurization tower operates under overload conditions and the solution circulation volume exceeds the designed flow rate for the liquid distributor, this will lead to uneven distribution of the liquid. Not only will this prevent effective scouring, but blockages can also occur quickly due to low liquid levels in certain areas. Under normal conditions, the resistance of the desulfurization tower is related to the tower diameter and the tower load. The increase in resistance of a new tower generally follows a pattern that is slow at first, then faster, and finally slow again. After a significant change in the column resistance, the gas-liquid distribution within the column had changed, and some of the packing elements became clogged. The desulfurization tower begins to operate under higher loads, and as mentioned in the previous section, the resistance increases significantly. In general, when the resistance in a desulfurization tower reaches 80 mmHg, it becomes very difficult for the system to operate. Not only does power consumption increase, but the pressure at the outlet of the Roots blower also rises along with the temperature. If the cooling capacity of the tower at the blower’s outlet is insufficient, the overall performance of the process will deteriorate as well. When the resistance exceeds 100–120 mmHg, it is necessary to remove the tower.