72- For the demisting and defoaming devices in sulfuric acid (nitric acid) plants, screen, fiber, electrostatic capture, and vane-type separation internals are used in the design and selection

Based on the types of demisting and defoaming separators currently used in enterprises that produce acid from ore or sulfur, with an emphasis on statistics, traditional types such as metal wire meshes and non-metallic fibers predominate. Some enterprises also use swirl plate and vane-type demisting and defoaming separators, primarily in high-end industries such as petrochemicals, coal chemicals, and fine chemicals. It is necessary for industries to engage in horizontal exchanges, learn from each other’s experiences and lessons, and strive for improvement.

Judging simply from the development timeline of sulfuric acid production facilities in China, this industry can be considered a traditional sector with mature technologies; there are dozens of companies in the country capable of carrying out system design, and dozens or even hundreds of such facilities of various sizes are in operation. However, traditional industrial equipment technology can regain new vitality if it is improved by drawing on techniques from other industries. For example, Topsoe has improved and innovated upon the traditional sulfuric acid production process by using a mixture of acidic gas and sulfur paste to produce acid, and it promotes its proprietary process package to domestic companies that intend to set up new sulfuric acid production facilities.

Based on existing sulfuric acid plants, demister and mist separator devices are primarily used in processes such as drying, absorption, and exhaust gas treatment. In the drying stage, traditional defoaming internals often choose a mesh type. This is mainly because the intake air may, due to various factors, carry dust, sludge, or even liquid-solid sulfur, which can cause blockages in the demisting components; therefore, it is necessary to clear these blockages promptly and even replace the wire mesh demisting components on a regular or irregular basis. Metal wire mesh demisters with lower costs and higher porosity are often used. It is particularly often chosen in plants with a smaller scale of sulfuric acid production and in plants that produce acid from ore. It is said that in some companies, due to operational failures of the equipment, the pressure drop across the demister caused by blockages can reach 10 kPa, which certainly has a severe impact on production. I’ve also heard that in earlier pyrite-based acid production plants, due to frequent clogging, demisters were not even used in the drying unit.

If no demister is used in the drying unit, the corrosion of the inlet and outlet fans, as well as the costs associated with maintenance and repair, will be high, and the yield will not be high.

This post was last edited by luoli519 on 2017-1-8 at 18:28. In the absorption stage and exhaust gas treatment stage, traditional demisting internals often use non-metallic fiber types. Mainly considering that the intake air may be free of particulates and viscous substances, thereby eliminating the risk of clogging the demisting internals, fiber demisting internals with higher density and lower porosity, obtained through weaving, are often used. It is particularly often chosen in plants with a larger scale of sulfuric acid production as well as in sulfur-based sulfuric acid production facilities. However, its cost is higher, and the operating pressure drop is also relatively higher.

This post was last edited by luoli519 on 2017-1-8 at 18:52. Due to the high cost and high operating pressure drop of non-metallic fiber demisters (whether in the form of fiber mats or fiber filters), it might lead one to assume that their mechanism for removing foam and mist is fundamentally different from that of metal mesh demisters. In fact, the fiber-based demister operates on the same principle as other barrier-type demisters such as mesh demisters and media-based demisters; that is, it creates tiny pores through the interconnection of components within the demister, thereby trapping, coalescing, and separating mist droplets. The difference is that the internal structures within different demisters form bridges with each other, resulting in pores of varying sizes; fiber demisters create smaller and more dense internal pores, which gives them better efficiency in blocking, aggregating, and separating mist and foam. Accordingly, fiber foam eliminators have a higher operating pressure drop compared to mesh foam eliminators; they are also more prone to clogging, and their operation and maintenance costs are higher.

This post was last edited by luoli519 on 2017-1-17 at 11:13. For the purpose of extending the service life of fiber-based and mesh-type demisters as well as reducing procurement costs, many companies wash and regenerate the aforementioned demisters that use pore-blocking mechanisms before reusing them. In terms of actual performance, it is not as satisfactory as expected. For soluble crystalline saline soils, the regeneration effect after repeated rinsing with hot water or even boiling is only barely satisfactory ； However, for materials clogged with insoluble particulate matter, the regeneration effect and number of times it can be achieved through repeated rinsing with hot water or even boiling are very limited, or even impossible. Structurally, the cellular barrier type demister contains internal components, which are divided into a surface layer, a middle layer, and a deep layer. Blockages such as particulate matter located on the surface can have most of them removed through water and air flushing methods. However, in the middle and especially deep endoplasm, the particulate matter is trapped by the fibers in an opposing manner, making it difficult to remove the blocking substances using water and air flushing methods. Some on-site operators mistakenly believed that regeneration was successful, thinking that most of the blockages had been removed from the surface ； As a result, after being installed in the tower and operated for a short time, it causes a high pressure drop, leading to deformation and collapse of the fiber filter element. In fact, fiber demister manufacturers with extensive experience in design and operation also do not want to see the embarrassment that arises when some site operators mistakenly install demisters that they think have been successfully regenerated into the towers for use.

This post was last edited by luoli519 on 2017-1-17 at 11:15. Can an airflow carrying mist and droplets meet the requirements for mist and foam removal after passing through fiber mist eliminators and wire mesh mist eliminators? The answer is not necessarily. After analyzing the operating mechanism of the Kungge barrier-type demister, one can arrive at the above conclusions. Take the fiber mist eliminator as an example; its mist and fog removal involves two main processes: the coalescence and growth of mist droplets, and their sedimentation and separation. The sedimentation and separation process is the controlling process. The coalescence and growth process of fog droplets and mist particles: An airflow containing a large number of such droplets and particles (with sizes following a Gaussian distribution and varying in size) is intercepted by the \"micro-pores\" formed by fibers interlocking with each other, which have varying equivalent diameters. As a result, these droplets and particles collide with one another as well as with the fibers, and they coalesce due to the surface tension of the liquid film, forming larger droplets and particles whose size distribution remains Gaussian and varies across different sizes. Control process for the sedimentation and separation of liquid droplets and bubbles: As the air flow passes through the internal components of the fiber demister, the liquid droplets and bubbles of varying sizes coalesce and grow larger, after which they settle out of the air flow. The settling process of droplets and bubbles is governed by the famous Stokes, Allen, and Newton laws. Only those droplets and bubbles whose equivalent size is greater than a certain critical size specified in these laws can successfully settle out of the gas stream and reach the liquid collection area before the gas leaves the outlet of the tower. Liquid droplets that are smaller than the critical size fail to reach the liquid phase collection area as the gas stream exits the tower outlet, and thus remain suspended in the gas stream, preventing gas-liquid separation. The higher the gas flow velocity, the larger the critical size of the droplets and mist that can successfully settle into the liquid phase collection area for gas-liquid separation, and the lower the efficiency of this gas-liquid separation. It should be noted that the volumetric flow rate of the gas stream before entering the fiber demister is not very different from its volumetric flow rate after exiting the fiber demister. Moreover, if the cross-sectional shapes of the flow channels entering and leaving the fiber demister are similar, then the linear velocity of the gas stream will be in the same order of magnitude, and the critical size of the liquid droplets and bubbles suspended in the gas stream will also be of the same order of magnitude. The difference is that before entering the fiber demister, the number of critical-size droplets and mist entrained in the gas flow is very large; after passing through the fiber demister, this number is significantly reduced, thereby achieving gas-liquid demisting and separation.

The last edit to this post was made by luoli519 on 2017-1-8 at 21:00. It can be seen that in order for the fiber demister to meet the technical requirements regarding gas-liquid mist removal and separation, the following conditions must be taken into account and satisfied: 1. The distribution of the small sizes of the “pores” in the structure of the fiber demister, as well as their equivalent sizes, must be less than the critical size values, in order to ensure effective blocking, interception, and coalescing of mist droplets. Otherwise, larger mist droplets will directly penetrate the material inside the demister, causing the fiber demister to lose its ability to block, intercept, and coalesce mist droplets of the target size. 2. The total number of fiber demisters and their total surface area must be greater than the critical values required by dynamic separation techniques, so as to ensure that the velocity of the airflow passing through the fiber demisters does not exceed a specific critical value; this in turn ensures that liquid droplets and bubbles of a certain critical size can be successfully separated through sedimentation. 3. The critical size of liquid droplets that can be successfully separated through sedimentation, as ensured by the fiber demister, may not meet the separation and demisting requirements of the plant’s process technology. Especially in situations where the designers of demisters lack a precise dynamic calculation and design system platform, or where there are fluctuations in actual operating conditions, the critical size of liquid droplets that can be separated by fiber demisters is much larger than the size of droplets that need to be separated according to the requirements of the demisting system. As a result, secondary demisting and defogging measures must be implemented after the fiber demisters.

This post was last edited by luoli519 on 2017-1-17 at 11:16. Secondary demisting devices such as electrostatic demisters and vane demisters are added after fiber-type and mesh-type demisters in petrochemical, coal chemical, coking, and fine chemical plants, in order to further reduce the critical size of residual liquid droplets and foam in the airflow separated by the fiber-type and mesh-type demisters, thereby achieving more thorough treatment. The optimal operating condition for an electrostatic demister is one in which there are very few residual liquid droplets and particles in the gas stream; the size of these droplets and particles is small, even on the order of aerosol particles; and these droplets, particles, and aerosols can easily become charged. Conversely, electrostatic defoamers are less suitable for use. It is undeniable that, as a secondary demister installed behind the fiber demister, the electrostatic demister can achieve excellent deep treatment results. People will say that electric demisters require significant investment; when used as a secondary demister in the absorption towers of sulfuric acid production plants, their components’ resistance to high temperatures and corrosion, along with the costs associated with operation and maintenance, are factors that need to be carefully considered.