Sulfuric acid with structured packing

Thread Content

This post was last edited by springflower on 2022-8-29 at 16:18. Section on packings in the practical application of dry absorption: Background knowledge: Whether it’s absorption or distillation, both are processes involving heat and mass transfer. Therefore, it is necessary for the gas and liquid phases to undergo three processes: effective dispersion, effective mixing, and effective separation. Successfully managing these three processes means achieving good heat and mass transfer effects. Throughout the entire sulfuric acid absorption process, to achieve good absorption results, all three towers operate with an excess of liquid phase. A good advantage of this is that it **reduces the required height of the packing**, and it is possible to use the maximum height of each packing section as a design parameter. At a certain height, due to factors such as flow deviation, it is necessary to collect the liquid phase and redistribute it in order to avoid affecting the distribution efficiency. After all, as a highly corrosive operating system, one packing section implies a set of distributors and collectors, which **reduces reliability**. The disadvantage is also obvious, namely the increased operating costs. When faced with two evils, one must choose the lesser one. All discussions are based on the actual operating conditions in which sulfuric acid is absorbed as an excess liquid for spraying. Effective dispersion: When considering the process of effective dispersion, we need to take into account not only the effective dispersion in the liquid phase but also that in the gas phase. Generally, at the feed point at the bottom of the tower, a certain height is reserved for gas self-balancing. In the design, the gas velocity in the perforated towers is low enough that no special equipment is required to achieve an initial distribution of the process gas; of course, abnormal operating conditions are not taken into consideration here. Liquid-phase dispersion raises another issue: what kind of dispersion is required during the sulfuric acid absorption process. As mentioned above, the sulfuric acid absorption process is a type of excess liquid absorption process. This means that as long as more than 50% of the surface is not affected by problems (no acid production), the excess acid will essentially reach an equilibrium within the filler, allowing it to hold up until you conduct maintenance and discover the issue. It’s like during a downpour – are we still discussing where less water is being applied and where more? No, continuous heavy rainfall will cause water to accumulate directly on the ground. With such heavy rainfall as well, it’s quite amusing when we discuss the number of distribution points per unit area; regular packing materials may require 60 to 90 P/M2 of such points, yet we have been using 43 P/M2 as the design value. Is this appropriate? Is this enough? It’s enough, but merely enough – it’s likely still a bit short. But can it be increased to 60~90 P/M2? Yes, it’s possible, but the distributor needs to be improved, and few manufacturers are willing to do that. Again, in heavy rainfall conditions, when it comes to uniform distribution and the number of distribution points, there is plenty of technical discussion on this topic; however, the differences in actual application may not be as significant as suggested. Furthermore, due to the excessive amount of liquid, the area available for the gas phase channel is small, which can lead to excessively high local gas velocities; it’s as if gas is being emitted from individual holes. If the gravity or flow rate of the falling liquid is not sufficient, it can very easily be carried upward by those fast gas velocities. Effective mixing: This is the area where heat and mass transfer actually take place. The quality of this area, as well as whether it is suitable for such processes, directly determines the efficiency of absorption. The equipment provided for this facility includes trays, random packing, and structured packing. First, let’s talk about trays. As the earliest type of distillation and absorption equipment, they have a very long history. They are particularly well-suited for environments with low gas velocities and high liquid flow rates. From sieve trays to various types of bubble-cap trays, each plays an irreplaceable role in different applications. Theoretically speaking, tray towers are quite suitable for sulfuric acid absorption processes characterized by high flow rates and low gas velocities. But is this tray suitable for use in sulfuric acid equipment? The answer is that it is not suitable; as a corrosion-resistant system, the corrosion resistance of the tray is a major issue. To meet the requirement of corrosion resistance, one must either choose extremely expensive metal alloys or metal-based coated materials, or even ceramic and plastic materials. In non-metallic substrate materials, the toughness and ductility of ceramics against violent boiling gas-liquid phases may also be a problem, while plastic materials fail when exposed to high temperatures. In conclusion, it can be used; those with corrosion resistance have high costs, while those without it have a short lifespan. Ceramics are too brittle, inexpensive, and corrosion-resistant, but their brittleness makes them prone to breaking. Plastic is also cheap and corrosion-resistant, but it cannot withstand high temperatures. The next type is the bulk packing; its principle of dispersion involves wetting the surface of the packing in order to achieve heat and mass transfer. It does not require intense bubbling like in tray towers in order to facilitate effective heat and mass transfer. Among these three materials as well, ceramic bulk packing stands out. In particular, it is suitable for environments with high liquid volumes and low flow rates; moreover, the pressure drop across the tower is lower than that at the bottom of the tray, thereby effectively reducing fan energy consumption. But everything has its advantages and disadvantages, and his disadvantages are quite obvious; as a ceramic cast, it is unable to balance strength and specific surface area, and in applications where a high specific surface area is required, it may not be possible to achieve that through manufacturing processes. Fortunately, the absorption of sulfuric acid is an instantaneous process, so not many theoretical plates are required for calculations; using a sufficient number will suffice. Finally, there’s the regularly structured packing, which has become very popular recently. Invented in the 1970s as a device for separating heavy water in nuclear fusion, regularly structured packing has always been kept under strict secrecy and protection. The first type of packing invented was the mesh-type structured packing, which consists of metal woven into a dense mesh structure, followed by folding it into the pattern characteristic of structured packing. This reflects his area of application. High gas velocity, low liquid volume. The metal mesh surface exhibits self-wetting and self-diffusion; regardless of the surface tension, a liquid film forms on the mesh, enabling heat and mass transfer at the surface for the gas passing through. Because a thin liquid film can be formed, the gas-phase channels **increase**, resulting in excellent mass and heat transfer performance for regularly packed materials, very low pressure drops, and the ability to effectively separate two fluid phases with extremely small temperature differences. It’s truly a revolutionary product. With the development of structured packing, metal plate structured packing, plastic structured packing, and ceramic structured packing have also been developed. His disadvantages are quite obvious: his resistance to clogging is very poor. This is especially true for structured packing with a high specific surface area, as the channels within it are dense; over time, acid sludge can form at the bottom of the packing, resulting in a decrease in processing capacity and a reduced operating volume for the tower. Another point is that, as a medium for high-precision separation, structured packing yields a separation efficiency that is not significantly different from that of random-packed packing in applications requiring lower precision in separation and absorption. As a result, it is generally believed that structured packing performs similarly in the sulfuric acid industry, so there is little incentive to replace it. Due to their inherent defects, conventional bulk packing materials tend to cause accumulation in the packing rings when attempting to increase the specific surface area, which reduces the space available for the liquid phase to pass through and leads to an increase in pressure drop. Therefore, the typical specific surface area is only 80–200 m2/m3. Metallic structured packing can achieve 100~1000 m2/m3. Of course, ceramic structured packing also cannot exceed 300 m2/m3 due to manufacturing limitations (I wonder which company has managed to do so; I’d like to visit them. With such a manufacturing process, it would be more profitable to produce teacups and teapots rather than packing materials). ). Here is the key point: as can be seen from the above description, structured packing is highly suitable for environments with low or extremely low liquid volumes and high gas velocities. Because the surface of the filler can self-wet to form a film. What about high liquid volume and low gas velocity? That’s also possible, but it’s important to note that your gas-liquid phase channels must be relatively large; this means that you can only use packing with a density of 200 m2/m3 or less. If it’s too high, it’s easy to experience flooding. Due to the consistency of the liquid flow direction, structured packing is more likely to cause liquid deflection. The traffic is extremely high in some areas, while it is extremely low in others. It has a significant impact on the absorption effect. Moreover, the solutions can only be found by improving the verticality of the tower, the ellipticity of the tower walls and the packing, as well as the orientation in which the packing is installed. The most important factor is the distribution effect of the acid distributor on the liquid. As we mentioned earlier regarding effective dispersion, in the era of bulk packing, 40 points/m2 for the distributor is sufficient. In the era of structured packing, 40 points may not be enough. Although there was a theory about torrential rain earlier. The requirements for precise drip irrigation have increased; as a structured packing material with a specific surface area of 200 m2/M3, your drip point density must be greater than 80 points/M2. Therefore, for structured packing, 43 points are merely sufficient, and they can be used only under conditions of a certain liquid flow rate. As a plant process engineer, your focus must be on saving energy in daily production; therefore, when using packed towers, have you considered reducing the amount of acid used? This consideration is based solely on absorption efficiency and energy consumption; acid mist is not taken into account for now. I found that it couldn’t be adjusted at all; once it was adjusted, the absorption quality would decline. It might be the regularly arranged packing at a lower height and the acid distributor with poor distribution efficiency that are holding things back; if 60–80 points/m2 can be used, your operational range will definitely **increase**. Conclusion: Regularly structured packing has the advantage of a lower pressure drop. However, due to the large volume of liquid in the sulfuric acid system, packing with a high specific surface area is not suitable. A certain height is required to achieve effective absorption, and this height is even greater than that of the previously used loose packing. Yet even with an increased height, the overall tower pressure drop remains low. To further reduce energy consumption, using acid distributors with a high drip point density to decrease the amount of acid applied is the direction for the future. Effective separation: In the world, long periods of division inevitably lead to unity, and long periods of unity inevitably lead to division. Starting with a stream of gas and a stream of liquid, after they are dispersed and mixed together, it is necessary for them to be able to effectively separate their respective gas and liquid phases. When a gas phase is mixed into a liquid phase, it may result in a foamy system; in such cases, the only solutions are to let it settle or use various devices for defoaming – details on this will not be covered here. When a liquid phase is mixed in the gas phase, it is only possible to allow the droplets to settle due to gravity by using a large tower volume. Alternatively, a wire mesh mist eliminator can be installed, allowing the passing droplets to collide with the wires; they then coalesce into larger droplets that cannot be carried by the gas phase, thus settling down. Then, we will describe how the liquid phase carried by the gas phase is generated, how it can be reduced, and how it can be completely collected. Only the sulfuric acid absorption process is considered; other processes are not addressed for now. There are several aspects to the generation of sulfuric acid mist; the first is mechanically generated spray mist, with particle sizes ranging from 3 to 10 micrometers. The second is sulfuric acid-absorbed mist; the absorption of sulfuric acid releases a large amount of heat, which makes it extremely easy to form acid mist with very small particles in large quantities, with particle sizes ranging from 0 to 3 micrometers. The two types of acid mist are more abundant near the dripping holes in the downcomer or tubular acid separator. Especially with mechanically sprayed acid mist, the liquid splashes onto the packing, tower walls, and gas-phase channels, where it is generated and carried upward. Absorptive acid mist is generated at any part of the packed tower wherever an absorption exothermic process takes place; the temperature difference between the acid and the process gas can only be reduced, but not eliminated. As a solution, the best approach is to increase the amount of circulating acid and install equipment at the points where the acid is released that can block mechanical acid mist. A bulk packing of appropriate height is therefore very suitable for this requirement, as it only blocks mechanical mist. This is also why some of the random-packed material needs to be used at the top of the structured packing to cover the acid splitter and the entire tube of the tubular acid splitter. The part blocked by the bulk packing, as well as the mist that is absorbed, can only be removed using a foam trap or a candle-type demister. Conclusion: Sulfuric acid mist is composed of splashed mist and absorbed mist. When loose packing is placed at the top of a structured packing tower, I believe its only function is to prevent the sulfuric acid splashed mist from spreading, with no separation or absorption process taking place. And the absorbed mist needs to be further removed using a candle-type demister. Finally: structured packing is indeed a good option, offering the advantages of low pressure drop and excellent separation performance. However, its disadvantages are also quite obvious: it is not resistant to clogging, and it is difficult to clean when acidic sludge accumulates; it can only be replaced. Rigid packing alone cannot prevent the formation of sulfuric acid mist; therefore, some loose packing needs to be stacked on top to capture the foam. Regular packing requires a acid distributor with a droplet density of 60–90 points/m²; the existing acid distributor with 43 points/m² is barely sufficient. Other requirements regarding the installation of tower walls, the way in which the packing is stacked in layers, and the calculations for packing arrangement can very easily lead to the situation where the same type of structured packing works well at Plant A but not at Plant B. That is the technical expertise of each manufacturer. I won’t say more here, so as not to ruin someone’s livelihood. Purely AC; feel free to ask any questions. I log in randomly and reply accordingly.