Knowledge related to packed towers

Information on packing materials 　　1. Bulk packing materials: Bulk packing materials are granular substances with specific geometric shapes and sizes, which are generally piled up in the tower in a random manner; they are also known as randomly piled packing materials or granular packing materials. Depending on their structural characteristics, bulk packing materials can be further divided into ring packing, saddle packing, ring-saddle packing, and spherical packing, among others. Here are several typical types of bulk packing materials: Raschig rings, Pall rings, stepped rings, arc saddle packing, rectangular saddle packing, metal ring rectangular saddle packing, and spherical packing. (1) Raschig ring packing: This type of packing was invented in 1914 by F. Rashching; it consists of circular rings whose outer diameter is equal to their height. Lasi ring packing has poor gas-liquid distribution, low mass transfer efficiency, high resistance, and low flux; as a result, it is less used in industry nowadays. 　　(2) The Pall ring packing is an improvement over the Raschig ring; two rows of rectangular openings are made in the side walls of the Raschig ring. One side of the cut wall remains connected to the surrounding surface, while the other side bends inward to form inward-extending tabs, the sides of which meet at the center of the ring. Due to the openings in the wall of the Bower ring, **it improves the utilization of the space inside the ring as well as its inner surface; there is less air resistance, and the liquid is distributed evenly. Compared to Raschig rings, the gas flux of Pall rings can increase by over 50%, while the mass transfer efficiency improves by about 30%. Bauer rings are a widely used type of packing. 　　(3) The stepped ring packing is an improvement over the Bower ring; compared to the Bower ring, its height is reduced by half and a conical flange is added at one end. The reduction in the height-to-diameter ratio significantly shortens the average path of the gas around the outer wall of the packing, thereby reducing the resistance to gas flow through the packing layer. The conical flanging not only increases the mechanical strength of the packing but also changes the contact between the packing elements from primarily linear to primarily point contact. This not only increases the gaps between the packing elements but also serves as points where liquid flows along the surface of the packing, thereby facilitating the renewal of the liquid film and improving mass transfer efficiency. The comprehensive performance of stepped rings is superior to that of Pall rings, making them the best type of ring packing in use today. 　　(4) Arch saddle packing: Arch saddle packing is a type of saddle-shaped packing; it has a shape similar to that of a saddle and is generally made of ceramic material. The arc-saddle filler is characterized by a completely open surface with no distinction between inner and outer areas; the liquid flows evenly on both sides of the surface, resulting in high surface utilization. The flow channel is arc-shaped, which reduces flow resistance. Its drawback is that intussusception is likely to occur, causing part of the filler surface to overlap and reducing the mass transfer efficiency. Arc saddle packing has poor strength and is prone to fragmentation, so it is not widely used in industrial production. 　　(5) Rectangular saddle packing: By changing the arc-shaped surfaces at both ends of the arc saddle packing to rectangular surfaces, with the two surfaces having different sizes, rectangular saddle packing is obtained. When the rectangular saddle packing is piled up, it does not overlap, resulting in a more even distribution of the liquid. Rectangular saddle packing is generally made of ceramic material, and its performance is superior to Raschig rings. At present, in the vast majority of applications in China where ceramic Lassie rings are used, they have been replaced by ceramic rectangular saddle packers. 　　(6) Metal ring-shoulder packing: Ring-shoulder packing (referred to as Intalox abroad) is a new type of packing designed to combine the characteristics of both ring and shoulder structures. Since it is generally made of metal, it is also known as metal ring-shoulder packing. The toroidal saddle packing combines the advantages of both ring packing and saddle packing; its overall performance is superior to that of Pall rings and stepped rings, and it is widely used among bulk packings. 　　(7) Spherical packing: Spherical packing is generally made by plastic injection molding, and it comes in various structures. The characteristic of spherical packing is that the sphere is hollow, allowing gas and liquid to pass through its interior. Due to the symmetry of the spherical structure, the packing density of the filler is uniform, making it difficult for voids and bridges to form; as a result, the gas-liquid dispersion performance is good. Spherical packing is generally only suitable for certain specific applications and is less used in engineering. 　　In addition to the aforementioned typical bulk packing materials, new types of packing materials with unique configurations have been developed in recent years, such as conjugate ring packing, Herring ring packing, Knott ring packing, and others. The characteristic data of bulk fillers commonly used in industry can be found in relevant manuals. 　　2. Regular packing Regular packing refers to packing that is arranged in a specific geometric pattern and stacked neatly. There are many types of structured packing, which can be classified into grid packing, corrugated packing, pulse packing, etc., based on their geometric structure. 　　(1) Grid packing: Grid packing is composed of strip-shaped units arranged in a certain pattern, and it comes in various structural forms. The earliest grid packing used in industry was wooden grid packing. Commonly used types include Gritsch grid packing, mesh grid packing, and honeycomb grid packing, among which Gritsch grid packing is the most representative. 　　Grid fillers have a low specific surface area and are mainly used in applications where low pressure drop, high load, and anti-clogging properties are required. 　　(2) Among the structured packing materials used in industry today, corrugated packing accounts for the vast majority. It is a disc-shaped packing made up of numerous corrugated thin plates; the angle between the corrugations and the tower axis is either 30° or 45°. During assembly, adjacent corrugated plates are stacked in opposite directions. The packing in each tray is installed vertically inside the tower, with adjacent trays arranged at 90° to each other. 　　Based on their structure, corrugated packing can be divided into two main categories: mesh corrugated packing and plate corrugated packing. The materials used for these packings include metals, plastics, and ceramics. 　　Metal wire mesh corrugated packing is the main type of mesh corrugated packing, and it is made of metal wire mesh. The wire mesh corrugated packing has a low pressure drop and high separation efficiency; it is particularly suitable for precision distillation and vacuum distillation units, offering an effective method for the distillation of difficult-to-separate systems and thermosensitive systems. Despite its high cost, it is still widely used due to its excellent performance. 　　Metal plate corrugated packing is a major type of plate corrugated packing. The corrugated plates of this filler are stamped with numerous small holes of about 5 mm in diameter, which help to roughly distribute the liquid across the plates and enhance lateral mixing. Fine grooves are rolled onto the corrugated sheet, which helps to distribute the liquid evenly across the sheet and improve surface wetting properties. Metallic perforated corrugated packing has high strength and excellent corrosion resistance, making it particularly suitable for large-diameter towers and applications with high gas-liquid loads. 　　Metal-rolled perforated plate corrugated packing is another representative type of plate corrugated packing. Its main difference from metal perforated plate corrugated packing is that the surface of the plates does not have punched holes; instead, there are small holes with a diameter of 0.4–0.5 mm, which are created on the plates by rolling. Its separation capacity is similar to that of mesh corrugated packing, but it has better resistance to clogging than mesh corrugated packing; moreover, it is inexpensive and widely used. 　　The advantages of corrugated packing include a compact structure, low resistance, high mass transfer efficiency, large processing capacity, and a large specific surface area (common values include 125, 150, 250, 350, 500, 700, etc.). The disadvantage of corrugated packing is that it is not suitable for handling materials with high viscosity, those prone to polymerization, or those containing suspended solids; moreover, it is difficult to load and unload as well as to clean, and its cost is high. 　　(3) Pulsed packing: Pulsed packing is a structured packing made up of hollow prismatic elements with necks, which are assembled in a specific manner. After the pulse packing is assembled, porous prismatic channels with necking are formed; the flow channels in their longitudinal direction alternate between contraction and expansion, resulting in intense turbulence as the gas-liquid phases pass through. In the necking section, the gas velocity is highest and turbulence is intense, thereby enhancing mass transfer. In the expansion section, the gas velocity is reduced to a minimum, enabling the separation of the two phases. The alternating contraction and expansion of the flow channel result in a \"pulsed\" mass transfer process. 　　Pulse packing is characterized by a high processing capacity and low pressure drop, making it an ideal packing for vacuum distillation. Due to its excellent liquid distribution properties that reduce the amplification effect, it is particularly suitable for applications with large tower diameters.

I. Structure of the packed tower and characteristics of the packing 1. Structure of the packed tower (see Figure on Page 1) The structure of a typical packed tower is shown in the figure; its main components include the tower body, packing and supports, fluid distributors and redistributors, as well as a demister. During operation, the liquid enters from the upper part of the tower and is evenly sprayed over the tower cross-section by a liquid distribution gas, flowing in a film-like manner over the surface of the packing ； The gas enters from the bottom of the tower and exits from the top through the gaps in the packing layer. Mass transfer occurs at the surface of the liquid film in the gas-liquid two-phase system. 2. Evaluation of filler properties: Fillers not only provide a contact surface between the gas and liquid phases but also promote the dispersion of these phases, resulting in continuous renewal of the liquid film. The performance of fillers can be evaluated from the following three aspects. ⑴ Specific surface area a: The filler should provide as much surface area as possible; this property of the filler, expressed in terms of the surface area per unit of filling volume, is called the specific surface area a, with the unit being m2/m3. ⑵ Porosity ε: The volume of voids per unit volume of the filler is referred to as porosity. Gas flows within the gaps between the packing; to reduce the flow resistance of the gas and increase the allowable gas velocity in the packing tower, the packing layer should have as high a porosity as possible. ⑶ Geometric shape of the filler: Two types of fillers with similar specific surface areas and porosities but different shapes can exhibit significant differences in fluid dynamic and mass transfer properties; however, there is currently no quantitative expression for the geometric shape of fillers. 3. Several common packing types: Common packing includes bulk packing and structured packing, with materials ranging from solid types to mesh-type materials. II. Flow of gas-liquid two-phase flow within the packing layer 1. Liquid: The ideal flow pattern is from top to bottom, in the form of a film flowing along the surface of the packing, with this liquid film being continuously renewed as it moves from one packing element to another. It is required that the liquid forms a film on the surface of the packing, that its distribution within the tower be uniform, and that the thickness of the liquid film be appropriate. Liquids have a certain ability to self-distribute within randomly packed packing materials. Therefore, for small towers, the self-distribution capability can be utilized, allowing for lower requirements for pre-distribution ； For large towers, it is difficult to achieve a uniform distribution across the entire cross-section of the tower by utilizing the self-distribution capability of the filler; therefore, high requirements are placed on the initial distribution ； Furthermore, channeling or wall flow may occur within the packing layer, requiring redistribution of the liquid. The thickness of the liquid film in the tower is related to the liquid holdup, which is the amount of liquid per unit of filled volume. The spray volume is high, the liquid retention capacity is also high, and the thickness of the liquid film increases ； Within the normal operating range of gas velocity, an increase in gas velocity has little effect on the thickness of the liquid film. 2. Gas: Within the packed tower, gas rises from bottom to top through the gaps in the packing driven by a pressure difference, and comes into contact with the liquid film to facilitate mass transfer. The pressure drop of gas through the packing layer is related to factors such as gas velocity and liquid flow rate. When the liquid volume is zero, the pressure drop Δp of the dry filler increases as the gas velocity u increases. When liquid is sprayed, the amount of liquid remains constant; as the gas velocity u increases, the pressure drop Δp also increases. At the same gas velocity, the pressure drop Δp is higher than that of dry packing. When the gas velocity u is low, as u increases, the thickness of the liquid film changes little. When the gas velocity u increases to a certain value, the thickness of the liquid film begins to increase, as does the amount of liquid held by it; this leads to liquid retention. At this point, the slope of the curve relating the pressure drop across the packing layer to the empty tower velocity increases, and this point is known as the loading point. Beyond the critical point, as the gas velocity u continues to increase to a certain value, the amount of liquid held in suspension rises sharply, and liquid accumulation leads to flooding; this value of gas velocity is known as the flooding velocity. As the liquid volume increases, the bubbling velocity decreases; at the same gas velocity, a larger liquid volume results in a greater pressure drop. 3. Flood: Flood is an abnormal operation of a packed tower. When flooding occurs, the liquid cannot flow down properly, and gas-liquid mass transfer cannot take place normally. Before the onset of flooding, the gas is the continuous phase and the liquid is the dispersed phase ； After homogenization, the gas becomes the dispersed phase while the liquid becomes the continuous phase. The transition point, also known as the phase transition point, is a stage at which the pressure drop Δp increases sharply; liquid backmixing and gas-liquid droplet entrainment become severe, resulting in extremely poor mass transfer efficiency. During design, the operating gas velocity is set at 50% to 80% of the critical velocity. The span point gas velocity can be estimated using the span point correlation diagram. 4. Operating range of the packed tower: When the liquid volume remains constant, if the gas volume is very low, the mass transfer process relies mainly on diffusion, resulting in poor mass transfer efficiency ； The large amount of gas will lead to flooding. When the amount of gas is constant, if the amount of liquid is very small, some of the packing will not be wetted, resulting in poor mass transfer efficiency ； If the amount of liquid is too large, flooding will occur. The maximum gas volume or maximum liquid volume can be estimated based on the critical gas velocity ； The minimum amount of gas and the minimum amount of liquid must be determined empirically. 5 Auxiliary structures of the packing tower ⑴ Support plates: They mainly serve to support the packing inside the tower, while also ensuring smooth passage of both gas and liquid phases. ⑵ Liquid distributor: It distributes the liquid entering the tower, ensuring a uniform distribution of the liquid across the tower’s cross-section. ⑶ Liquid redistributor: A device installed at regular intervals within the packing layer to improve the uneven distribution of liquid caused by wall-induced flow bias. ⑷ Demister: Used to remove liquid droplets from the gas escaping from the top of the packing layer, and is installed above the liquid distributor. III. Comparison between plate towers and packed towers For many counter-current contact processes, both packed towers and plate towers can be used. Different tower types have their own advantages and disadvantages, and a choice should be made based on a comprehensive consideration of the system in question. ⑴ The operating range of a packed tower is limited, and it is particularly sensitive to changes in the liquid load. ⑵ Packed towers are not suitable for treating materials that are prone to polymerization or contain solid suspensions. ⑶ Packed towers are not suitable when cooling is required during the gas-liquid contact process to remove reaction heat or heat of solution. Furthermore, when there is a side-line discharge, packed towers are also less convenient than plate towers. ⑷ The diameter of a packed tower can be very small, but the diameter of a plate tower is generally not less than 0.6 m. ⑸ Design data for plate towers is more readily available and more reliable, allowing for smaller safety factors. ⑹ When the tower diameter is not very large, the cost of a packed tower is low. ⑺ For systems prone to foaming, a packed tower is more suitable. ⑻ For corrosive systems, a packed tower is more suitable. ⑼ For thermosensitive systems, a packed tower is preferable. ⑽ The pressure drop of a packed tower is lower than that of a plate tower, making it more suitable for vacuum operation.

Thank you to the original poster for sharing; you’ve worked hard. Study*Study*

Great material! These are the basic things; learning them through practice will leave a deeper impression

Could the original poster recommend a few more common grades of regular packing materials, such as BX, CX, JSW, JBK, MalPAK, ZUPAK, etc.? I’m quite confused about them

Thank you to the original poster for the introduction; keep learning more!

I have only used the loose-type and HY type; I haven’t used the others, shame on me. .

1. Packed tower: It contains packing of a certain height; the liquid flows downward along the packing, while the gas flows upward and comes into countercurrent contact with the liquid film to facilitate mass transfer. It is commonly used in operations such as distillation, water absorption, and extraction. Based on their structural characteristics, they are divided into random-packed fillers (such as stepped rings and Pall rings, which are granular fillers) and regular-packed fillers (such as mesh corrugated fillers and wave-plate corrugated fillers). 2. Structural characteristics of packed towers: A packed tower is a mass transfer device in which the fillers inside the tower serve as the components for contact between the gas and liquid phases. The tower body of a packed tower is an upright cylinder, with a packing support plate at its bottom; the packing is placed on this support plate either in a random pile or in an organized manner. A packing press plate is installed above the packing to prevent it from being blown by upward airflow. The liquid is sprayed from the top of the tower through a liquid distributor onto the packing, and flows down along the surface of the packing. Gas is fed in from the bottom of the tower; after being distributed by a gas distribution device (which is generally not used in towers with small diameters), it flows counter-currently to the liquid through the gaps in the packing layer. On the surface of the packing, the gas and liquid phases come into close contact for mass transfer to occur. A packed tower is a type of continuous-contact gas-liquid mass transfer device, in which the composition of the two phases changes continuously along the height of the tower. Under normal operating conditions, the gas phase acts as the continuous phase, while the liquid phase is the dispersed phase. As the liquid flows downward through the packing layer, it tends to concentrate toward the tower wall, resulting in an increasing flow rate of liquid near the wall; this phenomenon is known as wall flow. The wall flow effect causes uneven distribution of the gas-liquid phases in the packing layer, thereby reducing the mass transfer efficiency. Therefore, when the packing layer is high, it needs to be segmented, with a redistribution device installed in between. The liquid redistribution device consists of a liquid collector and a liquid redistributor. The liquid that flows down from the upper packing is collected by the liquid collector and then sent to the liquid redistributor, where it is redistributed before being sprayed onto the lower packing. Packed towers have advantages such as high production capacity, high separation efficiency, low pressure drop, low liquid holdup, and great operational flexibility. Packed towers also have some disadvantages, such as the high cost of the packing ； When the liquid load is low, it cannot effectively wet the surface of the packing, resulting in a reduced mass transfer efficiency ； It cannot be used directly with materials containing suspended solids or those prone to polymerization ； Complex distillations such as counter-current feeding and discharging are not very suitable. There are many types of tower equipment, which is one of the most important devices in chemical, petrochemical, and refining industries. It enables close contact between gas-liquid or liquid-liquid phases, thereby achieving mass and heat transfer between them. Common unit operations that can be carried out in tower equipment include distillation, absorption, desorption, and extraction.

Packed tower packing: Packing is a mass transfer element installed inside a packed tower; it serves as an important internal component used to increase the contact area between the liquid and gas phases and to enhance the separation efficiency of the tower. There are mainly two categories of commonly used packing: random packing and structured packing. 　 　 Wavy packing　　　　　Regular wavy packing　　　Random packing 1) Basic principles for designing the structure and specifications: to continuously improve separation efficiency and enhance the contact and uniform distribution of fluids (liquid phase and gas phase) within the tower ； Gradually increase its throughput capacity to boost production capacity, in order to meet the demands of industrial production. The proper selection of packing is of great significance for improving the operational efficiency and economic performance of the tower. 2) Basic requirements for packing: high mass transfer efficiency, requiring the packing to provide a large gas-liquid contact area. That is, a large specific surface area is required, as well as surfaces of the filler that are easy to be wetted by liquids. Only the wetted surface is the gas-liquid contact surface. Large production capacity with low gas pressure drop. Therefore, a high porosity of the packing layer is required. It does not cause drift and gully flow. It is durable, features good corrosion resistance, high mechanical strength, and the necessary heat resistance. It is easy to obtain and inexpensive. 3) Regular packing: It is packing that is arranged in a uniform geometric pattern and stacked neatly inside the tower. Feature: It can minimize the pressure drop at each theoretical stage, thereby reducing the temperature of the material at the bottom of the tower and saving energy; it is particularly suitable for separations that require multiple stages as well as for the separation of heat-sensitive substances. High separation efficiency, low resistance, high throughput, large operational flexibility, and no significant scaling effect. At the same time, it can overcome the random flow of liquids in bulk fillings, enabling the liquids to distribute more evenly. Practice has shown that structured packing has been successfully applied in large towers. Types: Regular packing includes metal orifice plates, metal perforated plates, metal mesh sheets (with openings), wire meshes, plastic orifice plates, and grids (Gritsch), among others. (1) Metal mesh (plate mesh) corrugated packing: Metal mesh corrugated packing is a type of packing made by stamping and stretching thin metal sheets into rolled mesh sheets of specific specifications, resulting in regular diamond-shaped mesh patterns on its surface; it is then shaped into a corrugated pattern. This type of packing combines the advantages of wire mesh packing and plate wave packing, featuring low weight, low pressure drop, and high efficiency. (2) Metal mesh corrugated packing: Metal mesh packing is a highly efficient packing type that is widely used around the world. Its main advantages are: (1) high theoretical plate number, large flow capacity, and reduced pressure drop ； ⑵It exhibits good performance at low loads; the number of theoretical plates increases as the gas load decreases, with virtually no low-load limit ； ⑶High operational flexibility ； ⑷The amplification effect is not easy to discern ； ⑸It can meet the requirements of precision, large-scale, high-vacuum distillation units. It provides favorable conditions for the distillation separation of difficult-to-separate systems, heat-sensitive systems, and high-purity products. (3) Metal perforated plate corrugated packing: Metal perforated plate corrugated packing is made by drilling holes in the surface of a metal sheet, rolling out small ridges and large corrugations, and then assembling them together. It is most widely used in industry, and can be applied to ultra-large towers with diameters of over ten meters. This type of packing is used in negative pressure, atmospheric pressure, and pressurized operations. It is a traditional industrial high-efficiency packing. (4) Metal rolled hole corrugated packing: It is a corrugated packing made from metal sheets with a thickness of 0.1mm to 0.12mm, which are perforated and rolled into a corrugated shape. Due to the many small pores on the surface, the residence time of vapor and liquid on the packing surface is increased, resulting in more thorough vapor-liquid exchange within the tower and improved separation efficiency. This packing has geometric dimensions similar to those of the perforated corrugated packing; the diameter of its small pores is 0.4–0.5 mm. It is usually made from corrosion-resistant stainless steel and is used in the oil industry, as well as in equipment for fine chemicals and pharmaceuticals. (5) Ceramic plate corrugated packing: It possesses excellent resistance to acid and alkali corrosion as well as good surface wettability, and can be used at high temperatures. 4) Bulk packing: Bulk packing consists of granular materials with specific geometric shapes and sizes, which are piled up in a bulk form inside the tower. Features: high flow rate, low resistance, and easy maintenance. Type: Bulk packing materials can be divided into metal and plastic types. The products include internally curved ribbed flat rings, rectangular saddle rings, stepped rings, double-flanged flat rings (CMR), Bower rings, Lassi rings, and others. (1) Internally curved ribbed flat ring packing: The flat ring packing is the result of an optimal design developed by referring to various types of packing available abroad; it further reduces the pressure drop associated with the packing. It boasts advantages such as high mechanical strength, large processing capacity, low mixing, and high mass transfer efficiency. There are now series products such as Type 1, Type 2, Type 3, etc. (2) Rectangular saddle ring packing: Rectangular saddle ring packing is a type of packing that combines the advantages of both ring packing and saddle packing. This product was jointly developed by Tianjin University and our company. The rectangular saddle ring packing has advantages such as high flow rate, reduced pressure drop, good mass transfer performance, high strength, and resistance to damage. Its overall performance is significantly improved compared to packing materials such as Raschig rings and Pall rings. It has superior fluid dynamic and mass transfer properties compared to stepped ring packing, making it a packing with excellent performance. (3) Step ring packing: Step ring packing is a type of randomly arranged ring packing that has been developed based on Pall ring packing. It introduces two improvements to the Bower ring: the first improvement is that the high aspect ratio of the stepped ring is 1:2 ； The second improvement is a shorter, trumpet-shaped opening at one end of the ring. Such a design improves the gas-liquid distribution within the packing layer and increases the number of gas-liquid contact points, which facilitates the aggregation and dispersion of the liquid as well as the continuous renewal of the membrane surface. This enhances mass transfer and **improves** the separation efficiency. (4) Double-flanged short-loop packing (CMR): Double-flanged short loops possess all the advantages of stepped-ring packing; since the double flanges increase the strength of the packing itself, it is possible to reduce the thickness of the material, thereby lowering costs. The filler surface is further treated with sandblasting for even better results. (5) Pall ring packing: Pall ring packing features rectangular windows cut into the walls of the Raschig ring packing, with additional components added inside the ring. As a result, the fluid distribution and the effective utilization rate of the inner surface of the ring were improved, leading to a reduced pressure drop, an increased flux, and enhanced mass transfer performance.