(Share) Introduction to the performance and characteristics of regular packing for air separation

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Structured packing has advantages such as a large specific surface area, regular structure, high porosity and flow rate, low pressure drop, and great operational flexibility. As the diameter of the packed tower increases, there is greater emphasis on optimizing the structure of the packing, developing packing materials of various specifications and types that offer high efficiency, low pressure drop, and high flow rates, ensuring an optimal match between the tower packing and its internal components, as well as studying the flow and distribution of liquid within the packing layer. The screen corrugated packing and plate corrugated packing, developed after the 1960s, are the most widely used structured packings at present. For modern structured packing, Sulzer should be the top choice. In the 1960s, the company developed metal wire corrugated packing; in 1977 it introduced the plate-type corrugated Mellapak packing, and in 1994 it created the Optiflow packing, which features a novel structure with multiple channels, demonstrating that research and development on structured packing continue. The Mc-pak packing introduced by Japan’s Mitsubishi Corporation is based on Mellapak packing as its model. In recent years, the Rombopak packing from the Swiss company KUHNI and the Raschig-Superpak packing from the German company RASCHIG have seen relatively successful development and application. Since the 1960s, our country has carried out systematic research and development on structured packing, and has established a relatively complete system for scientific research and production. Tianjin University and the University of Aston in the UK have jointly developed a pulsed structured packing named UnaPak ； Tsinghua University and the Shanghai Research Institute of Chemical Industry have developed corrugated packing for calendered plate nets respectively ； Sinopec Luoyang Engineering Company has developed the LH-type structured packing. Among them, the National Engineering Research Center for Distillation Technology at Tianjin University has developed a new type of high-efficiency corrugated packing based on high-efficiency packing materials – the bidirectional corrugated packing. This type of packing has a large mass transfer specific surface area, its gas and liquid flow paths are optimized, it possesses strong lateral diffusion capabilities, and it far outperforms metal perforated corrugated packing in terms of anti-clogging ability, stiffness, pressure drop, and throughput. In recent years, with the vigorous development of industries such as fine chemicals, petrochemicals, and fertilizers, research on various new types of tower fillers has attracted considerable attention. The development and industrial application of high-efficiency packing have shown that structured packing, especially corrugated packing, possesses significant advantages, and it is highly effective in replacing conventional packing as well as in the technical upgrading of some plate columns. The development of new fillers and their large-scale industrial application represent an important achievement in the research and application of towers worldwide today. 1. The pressure drop of structured packing is significantly low. Since the gas-liquid phases in structured packing are in membrane contact, unlike the bubbling contact between the two phases in tray towers, the pressure drop of a packed tower is only 1/4 to 1/6 that of a tray tower. If the operating pressure drop in the column equipped with structured packing is 3.5–4.2 kPa, and the operating pressure at the bottom is only 35–45 kPa, a sieve plate column is generally still used in the lower section of the column, with no change in the operating pressure drop. As a result, the operating pressure in the lower section decreases by 0.05–0.06 MPa, resulting in a value of around 0.44–0.48 MPa. This allows the shaft power required by the air compressor to be reduced by 5%–7%. 2. Regularly structured packing provides high separation efficiency. The lower the operating pressure in the upper column, the more favorable it is for the separation of oxygen, nitrogen, and argon, especially for the separation of oxygen and argon. Generally, the oxygen extraction rate can be increased by 1%–3%, while the argon extraction rate can be increased by 5%–10%. Practice has shown that in air separation units, the oxygen extraction rate can reach over 99%, and the argon extraction rate can reach over 80%. The extraction efficiency of the distillation column depends to a large extent on the amount of expanded air fed into the column; this has a significant impact, in particular, on the extraction efficiency of argon. Therefore, continuously improving the isentropic efficiency of the turbine expander and the pressure ratio of the booster is key to enhancing the extraction efficiency of the distillation column. 3. Regular packing has a low liquid holdup. The liquid holdup in a tower equipped with regular packing is generally only 1% to 6% of the tower’s volume, whereas in a tray tower it is 8% to 10% of the tower’s volume. A low liquid holdup means that the liquid stays in the tower for a short time, resulting in a low operating pressure drop, which is advantageous for operation under varying conditions. The design range for structured packing towers can reach 40% to 120%. For the 12,000 m3/h air separation unit at No. 5 Steel Plant, the oxygen production rate in the tower using structured packing can be adjusted within the range of 9,000–14,000 mm3/h, with the operating load ranging from 75% to 117%. 4. Regular packing has large voids; the void ratio of regular packing is over 95%. In a sieve plate tower, the area of the perforated plates accounts for 80% of the tower’s cross-sectional area, with a porosity of 8% to 12%, which is far less than the void ratio of a packed bed. For the same load, the diameter of the tower in a packed tower is smaller than that in a tray tower ； Under normal circumstances, its cross-sectional area is only about 70% that of a sieve plate tower; for large air separation units, a smaller tower size facilitates transportation. 5. The startup time of the equipment is significantly reduced. The startup process of air separation units involves operation without any product output; therefore, reducing the startup time is one of the ways to save energy and reduce consumption in such units. The startup time refers to the time required from starting the expander to the point when oxygen can be produced. With the use of structured packing in the upper column, the amount of liquid retained during normal distillation is greatly reduced, which in turn significantly shortens the startup time of the air separation unit. Generally, the startup time is only 26–30 hours. 6. The oxygen content in the argon fraction is approximately 90%. To reduce it to 1–2×10-4% using low-temperature distillation, about 180 theoretical plates are required for a distillation column, and over 300 plates are needed for a sieve tray column; the pressure drop in such systems is around 100 kPa. Clearly, crude argon cannot be removed from the column under these conditions. As for structured packing, its height would be around 45 meters. The resistance is only 14–16 kPa, thus making it possible to implement a full distillation process for argon production.