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In the nitrogen fertilizer industry, particle-shaped packing and plate towers have long dominated the field of mass transfer and separation equipment. In recent years, structured packing has gradually been adopted in small and medium-sized nitrogen fertilizer plants. This article provides a brief analysis and commentary on its applications. 1 Main types of applications and characteristics 1.1 Metal wire mesh corrugated packing: Composed of corrugated plates formed by pressing metal wire mesh; it features a low wave height, a large specific surface area, good surface wettability, and extremely high separation efficiency – up to 10 or more theoretical plates per meter of packing layer. However, due to the high cost of its manufacturing materials such as stainless steel, the high expense of filling material per unit volume, and its relatively low strength, it is generally suitable for vacuum distillation, atmospheric distillation, and absorption of systems that are difficult to separate or are thermosensitive, in applications where the tower diameter is small (under 2 meters) and where clogging and corrosion are unlikely to occur. There are single-layer and double-layer metal mesh corrugated packing types. The single-layer metal wire mesh corrugated packing comes in types 250, 500, and 700, among which type 500 is the most widely used. 1.2 Mesh (plate mesh) corrugated packing – Mesh corrugated packing is a new type of corrugated packing that has been developed recently. It is created by appropriately processing thin strips of materials such as stainless steel and drawing them into diamond-shaped mesh plates of specific specifications. Its cost is significantly lower than that of wire mesh packing, yet it retains the excellent properties of wire mesh corrugated packing. Its industrial use is on the increase, and its application areas are similar to those of wire mesh corrugated packing as well as in special operating conditions. 1.3 Metal perforated corrugated packing – It is typically composed of stainless steel corrugated sheets, with many small holes of about 5 mm drilled in these sheets. Its performance lies between that of metal mesh corrugated packing and loose packing. Thanks to its reasonable structure, high strength, lower cost compared to metal wire mesh corrugated packing, good corrosion resistance, strong resistance to contamination, low pressure drop, and high flow rate, it is particularly suitable for the distillation of organic substances under normal pressure and moderate vacuum conditions where there is a risk of contamination. It is also suitable for countercurrent absorption processes at normal pressure and under pressure. In the design of new towers and the renovation of old ones, it is used to replace dispersed packing such as Pall rings and certain tray towers. Especially in the renovation of large-scale towers, it yields significant economic benefits in terms of increasing product output and quality as well as reducing energy consumption. 1.4 Plastic orifice plate corrugated packing: Its structure is similar to that of metal orifice plate corrugated packing, and the materials used include polypropylene, polyvinylidene fluoride, polyvinyl chloride, etc. The main advantages are corrosion resistance, light weight, low cost, low resistance, high efficiency, and a small amplification effect. Currently, there are mainly two specifications: 125 Y and 250 Y. They are suitable for various absorption and desorption processes, as well as for waste gas purification and processes involving large liquid loads and high operating pressures. They can also increase the production capacity of existing towers, and are appropriate for systems prone to foaming. It has achieved good results when applied in both the fertilizer and environmental protection sectors. 2 Engineering Application Examples and Results In recent years, small and medium-sized nitrogen fertilizer plants, especially the smaller ones, have achieved significant success in using structured packing during technical upgrades and capacity expansions. 2.1 Comprehensive Gas Cleaning Tower: When designing the comprehensive gas cleaning tower for the Fufeng Nitrogen Fertilizer Factory in Shaanxi, regular packing materials with low pressure drop and resistance to clogging, developed by the Shanghai Institute of Chemical Technology, were selected. The tower is 2 meters high; it can handle an average volume of semi-water gas ranging from 11,500 to 12,000 m³/h. The volume of circulating cooling water is 120 m³/h. The inlet gas temperature is between 100 and 120°C, while the outlet gas temperature is between 40 and 50°C. The actual total pressure drop across the tower is 490 Mpa. Before the modification, the temperature at the semi-water gas outlet was above 80°C, which severely affected the normal operation of gas production and the increase in gas output. Although the pressure drop increased, gas production operations were stabilized and the gas production rate was improved. 2.2 Renovation of the desulfurization unit: Replacing the desulfurization tower’s packing with structured packing can improve production capacity and desulfurization efficiency, stabilize production, and enhance economic benefits. An improved desulfurization efficiency can reduce catalyst poisoning in the conversion process, decrease the formation of CuS precipitates in the copper melt, enhance the quality of the copper melt, and reduce copper consumption; moreover, minor leaks and liquid carryover in the copper washing stage can be **reduced**. In 1993, Hubei Yihua Group Co., Ltd. carried out a thorough renovation of the desulfurization unit by adopting the tannin gum desulfurization process, using 250Y polypropylene perforated corrugated packing in the 4,000 cubic meter desulfurization tower and the 3,600 cubic meter cooling and cleaning tower. The desulfurization efficiency and production capacity are **improved**; the H2S level at the outlet of semi-water gas is below 0.01 g/m3, and sometimes it cannot be detected at all. The improved desulfurization efficiency is particularly beneficial for the diol production section, as it doubles the service life of the diol catalyst. It is important to note that regular packing is used in desulfurization towers, and it must be used in conjunction with other equipment such as electrostatic coking removers; each plant should select different specifications and models based on its actual conditions. 2.3 Transforming Hot Water Saturation Towers – Metal plate corrugated packing can be used in direct heat transfer equipment. The Leting Fertilizer Plant upgraded the ceramic rectangular saddle rings in its 400-ton hot water tower to 250Y-type stainless steel corrugated packing and internal components; as a result, the packing height was reduced from 6 m to 4 m. No major repairs were carried out during the first two years after operation; the inlet temperature of the shift gas increased from 160°C to 210–230°C, while the outlet temperature decreased from 135°C to 110–120°C. This improved the efficiency of the hot water tower, increased the amount of steam generated by the shift system, and enhanced its capacity by over 35%, resulting in good economic benefits. In a hot water tower, water and steam come into direct contact; the limit to the temperature increase of the hot water is determined by the wet-bulb temperature. A hot water tower alone is not sufficient – other devices such as heat exchangers and water heaters must also be in good condition. If the saturated tower is upgraded simultaneously, the energy-saving effect will be more significant. 2.4 Ammonia Recovery Tower: The Fufeng Nitrogen Fertilizer Plant upgraded its plastic orifice plate corrugated packing in this tower. For the 1,800-bubble tower, the concentration of ammonia water produced increased from well below 40 t·t to 40–60 t·t. The ammonia content in the gas stream leaving the tower before the upgrade was greater than 2.3 g/m³; after the upgrade, at constant operating pressure, the ammonia content entering the tower was 8 g/m³ with a CO₂ content of 0.4%. The volume of liquid sprayed in the circulation process was 45 m³/h, and the ammonia content in the gas stream leaving the tower dropped to 1.0–1.2 g/m³, with a CO₂ content of 0.2%. Meanwhile, the system pressure drop decreased by 0.0202 Mpa. After passing through the cleaning tower, the ammonia content in the carbonized gas is less than 0.1 g/m³, which increases the concentration of ammonia water, meets the requirements for carbonization balance, and reduces environmental pollution. 2.5 Refined copper washing tower: In 1992, Tianjin University successfully upgraded the 1,000-ton copper washing tower of Hubei Yihua Group Co., Ltd. by using 250Y stainless steel orifice plate corrugated packing and internal components. The copper washing tower was originally designed with Pall ring inserts of 50 mm × 50 mm × 1.5 mm; the gas flow rate is 33,540 m³/h; and the copper liquid flow rate is 43–48 m³/h. Design calculations show that the tower can meet the demand for 80,000 tons of ammonia per year. However, its internal structure design is not entirely optimal: the packing is not segmented, which leads to uneven gas-liquid flow; the liquid distribution device consists of nozzles, resulting in uneven initial liquid distribution; and its excess capacity is limited, making it difficult to cope with the large fluctuations in the refining load required in hydroxylamine production. The copper washing tower after technical renovation has the following advantages: it ensures that the outlet gas meets the quality standards even in the face of process fluctuations; the spraying density of the copper solution can be adjusted over a wide range, providing great operational flexibility and strong adaptability; there are no incidents of liquid carryover; and the amount of copper solution used can be adjusted according to the gas load entering the tower as well as the CO% level. The copper melt circulation rate was reduced from the originally designed value of 4.2 m³/t NN3 to 2.3 m³/t NH3. The consumption of copper, glacial acetic acid, ammonia, and steam was significantly decreased; fewer copper pumps were needed, the load on the chillers was reduced, and the amount of cooling water required was lowered. These energy-saving measures brought considerable economic benefits. 2.6 New Product Development Many small and medium-sized nitrogen fertilizer plants across the country use the methanol synthesis process to produce methanol, which is then further processed to yield formaldehyde, pentaerythritol, and other products. In these areas, structured packing has been applied. According to the literature, in methanol purification and dewaxing towers, 5.6 m 700-type metal mesh corrugated packing is used in place of traditional float valve towers with a height of 18 m and 60 trays, which improves separation efficiency and ensures better quality and yield. By using 500-type metal mesh corrugated packing to upgrade the old reactors in pentaerythritol production facilities, the tower height was reduced from 28 m to 12 m, resulting in a doubling of production volume and significant energy savings. In the past, Raschig rings were used as packing in formaldehyde absorption towers; after the 1970s, Pall rings and stepped rings gradually replaced them, with rectangular saddle rings also being used. The material of these packing elements is usually ceramic. In recent years, metal perforated corrugated fillers and metal mesh corrugated fillers have been used, with stainless steel being the most common material. Using high-efficiency packing can reduce the tower diameter and height under the same conditions. 3 Application Trends and Recommendations 3.1 Carbon Dioxide Absorption Towers: Many small nitrogen fertilizer plants are now converting to urea production. The decarboxylation towers used for propylene carbonate production employ random packing; to increase production capacity, improve decarboxylation efficiency, and reduce consumption, structured packing can be used, along with modifications to the gas-liquid distribution systems. Calculations show that by replacing the 50 mm×50 mm Pall ring packing in carbon dioxide absorption towers with metal orifice plate corrugated packing, the processing capacity can be increased by 30% under the same conditions of tower diameter, packing height inside the tower, operating pressure, pressure drop, and separation efficiency. 3.2 Desorption Tower of Urea Plant: For a urea plant with an annual production capacity of 40,000 tons using the full-circulation method for aqueous solutions, the design requirement for the waste liquid discharged from the desorption system is that its ammonia content shall not exceed 0.07% (wt), with an analytical control target of 0.06%. It is difficult to meet the targets in actual production, and the excesses are quite severe. By modifying the plate-type desorption tower with structured packing, the ammonia content in the waste liquid discharged after desorption can be reduced to 0.03%–0.05%, resulting in significant economic benefits in terms of ammonia recovery and energy savings. This is a question worth studying. 3.3 Aluminum-impregnated carbon steel perforated plate corrugated packing: Tianjin University developed and produced this type of aluminum-impregnated carbon steel perforated plate corrugated packing in March 1989, filling a gap in domestic production. It not only possesses the excellent comprehensive properties of plate corrugated packing, but also has good heat resistance and corrosion resistance. The high-temperature oxidation resistance of 20# aluminized steel is 146–340 times that of 20# steel, and 2–3 times that of 18–8 stainless steel; its resistance to H2S corrosion is 289 times that of carbon steel and 60 times that of 18–8 stainless steel. In terms of the cost per cubic meter of packing material, it is 20% to 50% cheaper than stainless steel corrugated packing; when considering the annual average cost (cost/life span), it is even cheaper than carbon steel corrugated packing. This filler has been used in industries such as fertilizers, and it is worth promoting for wider use. 3.4 Precautions for Engineering Applications When regular packing towers are used in industry, the tower diameter, the inclination of the tower body, the segmentation of the packing, and its installation must all meet specified requirements. Internal tower components such as liquid distribution, liquid collection and redistribution, packing support rings, and packing retention rings must all be properly designed and manufactured and installed with precision. For large-diameter towers or when the ratio of the dynamic pressure head at the gas inlet to the pressure difference across the tower is greater than 2.5 (such as in pressurized towers), a gas-phase distribution device should be considered. When the material to be treated contains suspended solids, screen-type corrugated packing is generally not suitable. In cases where polymers or tar are likely to form inside the tower, not only are screen packings unsuitable, but care must also be taken when using perforated plate corrugated packings as well; certain measures should be taken. Once the filler is contaminated, it should only be cleaned using chemical methods. Wavy packing has a very large specific surface area; after the tower stops operating, a significant amount of liquid remains on the surface of the packing. At this time, the temperature inside the tower has not yet dropped. For materials that are prone to oxidation, if a large amount of air enters rapidly, the temperature inside the tower can rise further, resulting in the destruction of the packing. In such a situation, without stopping the process, nitrogen should be introduced into the tower for protection, or the medium should first be cleaned with another solvent before stopping the process. In most driving conditions, the mesh packing must be thoroughly pre-wetted to form a good liquid film in order to achieve high efficiency. Generally, there are three ways of pre-foaming: ① foaming after flooding the tower; ② operating with full reflux at a flux higher than the foaming point; ③ operating with full reflux at a flux greater than the foaming point. The third method is usually adopted. Packed towers outperform tray towers and random-packed towers in terms of separation efficiency and pressure drop, but they still suffer from the same limitations as random-packed towers: difficult heat exchange, trouble in making side-stream extractions, relatively poorer corrosion resistance compared to tray towers, and difficulties in process control. In engineering, specific problems need to be analyzed on a case-by-case basis. 4 Conclusion Although the use of regular packing results in higher initial costs, these costs can be recouped quickly considering the reduction in tower volume and the energy savings achieved. The application scope of current structured packing has expanded from general chemical and petrochemical industries to fields such as fertilizer production and environmental protection, providing a new approach for the fertilizer industry to save energy, reduce consumption, and increase output. A comprehensive evaluation and the rational use of structured packing will enable technological upgrades and advancements to play a greater role and yield better results. (Originally published in Chemical Engineering Design Communications, Issue 4, 1994; excerpts included)