Types, structures, and properties of tower packing

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The function of tower packing is to provide a sufficient contact surface between the gas and liquid phases, and to create conditions that enhance turbulence (mainly in the gas phase), thereby facilitating mass transfer (including heat transfer). They should enable a large gas-liquid contact area and high mass transfer coefficients, while also providing high flow rates with low resistance; therefore, it is required that the packing layer have a high porosity, a large specific surface area, good surface wettability, and a structure that facilitates close contact between the two phases to promote choking flow. The manufacturing materials must be corrosion-resistant to the materials being processed, and possess sufficient mechanical strength to prevent the bottom of the packing layer from breaking or deforming under pressure. Common tower packing can be divided into two main categories: random packing and structured packing. a. Bulk packing: Bulk packing includes hollow ring-shaped packing, saddle-shaped packing with an open surface, etc. Common construction materials include ceramics, metals, glass, graphite, etc. The characteristics of several major bulk packing materials are as follows. (1) Lassi ring: A Lassi ring is a ring whose height is equal to its diameter; the commonly used diameters range from 25 to 75 mm (there are also those as small as 6 mm and as large as 150 mm, but these are less common). The wall thickness of ceramic rings is 2.5 to 9.5 mm, while that of metal rings is 0.8 to 1.6 mm. The packing is piled up in a disordered manner inside the tower; those with larger diameters can also be arranged neatly to reduce resistance and minimize the tendency of the liquid to flow toward the tower walls. The Lasi ring has a simple structure, but compared to other packing materials, it has a low gas flow capacity and high resistance; it is difficult for liquid to reach the interior of the ring, resulting in insufficient wetting and poor mass transfer efficiency, which is why it is used less frequently in recent years. By adding a partition at the diameter position within the inner space of the Lasi ring, a Lysin ring is formed ； Adding a spiral partition inside the ring results in a spiral ring. The partition serves to improve the packing capacity and increase the surface area. (2) Arc saddle: Also known as the Berl saddle, it is an early-type saddle-shaped packing that resembles a horse saddle; sizes ranging from 25mm to 50mm are commonly used. The surface of the arc saddle has no inner or outer side; it is entirely open, with the fluid distributed evenly on both surfaces. Another feature of it is that when stacked inside the tower, it exerts less lateral pressure on the tower walls compared to annular packing. However, because the configurations of the two surfaces are identical, the filler tends to overlap when stacked, thereby reducing the exposed surface area; it has recently been gradually replaced by rectangular saddle fillers with improved configurations. Arc saddle packing is often made of ceramic. (3) Intalox saddle: The surfaces on either side of the Intalox saddle do not overlap, it is relatively resistant to pressure, has a simple design, and is easier to manufacture than an arc saddle; it is often made of ceramic. Among fillers made of ceramics, this type of filler exhibits superior hydraulic and mass transfer properties. The walls of these various bulk fillers have no holes or grooves, and they are usually made of ceramic. In addition, there are those with holes or grooves in the wall, usually made of metal or plastic. The performance of the latter is much improved compared to the former, which is why it is called a \"high-efficiency\" filler. The common bulk open-pore fillers are as follows. (4) Pall ring: The structure of the Pall ring consists of one or two rows of square or rectangular holes made in the wall of a metal Lassie ring. When these holes are created, three of the four edges are severed while the other edge remains intact, causing the original metal sheet to bend inward in the form of tabs; these tabs almost meet up inside the ring. The porosity and specific surface area of the filler did not increase as a result. However, when stacked in layers, the flow of gas and liquid is smooth, which facilitates their flow and helps them enter the ring. Therefore, both the gas throughput and the volumetric mass transfer coefficient of the Pohl ring are significantly improved compared to the Lassie ring, while the resistance is also reduced. Bauer rings can also be made of plastic. (5) Cascade miniring: The cascade miniring is an open-ring packing with a flared end; the ratio of its height to diameter is slightly less than 1. It contains ribs that serve to reinforce it and increase the contact area. The flared end prevents the packing from freezing together, thereby increasing the porosity and making the surface more exposed. The manufacturing materials are mostly metal or plastic. (6) Metal Intalox saddle: A rectangular saddle made of metal, with two narrow strips punched out on the back of the saddle and bent into ring-shaped ribs; four small claws are then punched out on these ribs and bent inward to form a ring. In terms of its structure, it is a combination of a saddle and a ring; it combines the advantages of the saddle-shaped packing, which ensures uniform liquid distribution, with those of the open-ring packing, which offers high gas flow rates and low resistance. Hence, it is referred to as a ring-moment saddle. b. Structured packing: Structured packing differs from bulk packing in that it has a structured, block-like form that allows it to be stacked layer by layer inside the tower. The earliest regular packing materials were grills made of arranged wooden slats; later, those made of metal strips or plastic slats were also used. Grid packing has low air flow resistance, but its mass transfer efficiency is relatively poor; it is now rarely used in gas-liquid mass transfer equipment, though it is still employed in cooling towers. The screen corrugated packing and plate corrugated packing, developed after the 1960s, are the most widely used structured packings at present. Their configurations and characteristics are described as follows: (1) Wire mesh corrugated filler – Metal wire mesh is cut into rectangular strips 50–100 mm wide, and corrugations are formed on them; the angle between these corrugations and the long edges is 30°, 45°, or 60°. Small holes are made in the strips to allow gas to pass through. Then several mesh strips are arranged side by side to form a disk slightly smaller than the cross-section of the tower, with the height of the disk equal to the width of the strips; many such disks are stacked inside the tower to achieve the desired height. If the tower diameter is large, one disc is divided into several parts, which are then combined during installation. Within one tray, the mesh strips of the adjacent trays on the left and right also intersect at 90°. The advantages of this structure are: 1) The pieces are arranged neatly with large gaps between the peaks and valleys, resulting in low air resistance ； 2) The direction of the channels between the corrugations changes frequently, exacerbating airflow slippage ； 3) The interlacing of mesh strips between sheets and between discs promotes continuous redistribution of the liquid ； 4) The screen has a fine mesh, allowing the liquid to form a stable film on its surface; even at a low spraying density, complete wetting is easily achieved. These characteristics result in a high throughput for this type of packing layer, and its use in large-diameter columns avoids the disadvantages of uneven liquid distribution and poor wetting of the packing surface. Disadvantages of mesh corrugated fillers: 1) High cost ； 2) High requirements are placed on the installation process; strict accuracy is required regarding the verticality of the tower body, and the gaps between the discs and the tower walls must be sealed properly ； 3) The internal channels of the filler are narrow, prone to blockage, and difficult to clean. However, due to its high mass transfer efficiency and low resistance, it is very suitable for use in precision distillation and vacuum distillation. Initially, it was mainly used in towers with relatively small diameters; nowadays it can be applied to towers with diameters of several meters, and its usage is no longer limited to distillation. (2) Plate wave filler: To overcome the disadvantages of high cost and stringent installation requirements associated with mesh corrugated fillers, the mesh strips are replaced with plate strips; the configuration of the filler remains the same, and in addition to metal, plastics can also be used as the construction material. Although the mass transfer performance of plate wave fillers is lower than that of wire mesh corrugated fillers, they still belong to the category of high-performance fillers. The trade names for such fillers include Mellapak, Fle*pac, etc.