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Design of pressure swing adsorption nitrogen generation systems: Over the past five to six years, as domestic pressure swing adsorption nitrogen generation technology has continued to mature, its application areas have also expanded. It has been used for purposes such as nitrogen sealing for fire suppression in coal mining systems, nitrogen atmosphere heat treatment in mechanical and electrical systems, and nitrogen sealing for the processing and purification of raw materials in the chemical industry. It is also utilized in the food industry for preserving fruits and vegetables, as well as in the beer industry to compensate for shortages of carbon dioxide, and it has been well received by users in all these applications. To help more users understand pressure swing adsorption nitrogen generation technology and make use of such devices, this article provides a brief overview of the design of pressure swing adsorption nitrogen generation units, the faults that may occur during commissioning, and the issues that need to be taken into consideration during operation. 1 Working principle of the pressure swing adsorption nitrogen generation unit The design of the pressure swing adsorption nitrogen generation unit is based on the pressure swing adsorption theory. The adsorption matrix consists of various adsorbents, such as aluminum gel, silica gel, activated carbon, and molecular sieves. In pressure swing adsorption nitrogen production units, adsorbents are mostly various types of molecular sieves (zeolite molecular sieves, carbon molecular sieves). Each type of molecular sieve has a specific selectivity for adsorption; for example, 3A carbon molecular sieves are commonly used for nitrogen production and oxygen removal, while 13X zeolite molecular sieves are often chosen for adsorbing H2O and CO2. Working principle of the pressure swing adsorption nitrogen production unit: Clean, pressurized air (an oxygen-nitrogen mixture) passes through a type of carbon molecular sieve; most of the oxygen and any remaining trace amounts of moisture are adsorbed by the molecular sieve, while nitrogen is not adsorbed and is thus released – this is the adsorption nitrogen production stage. When the pressure is reduced to atmospheric pressure or negative pressure, the molecular sieve’s ability to adsorb oxygen decreases, allowing the oxygen and water it has absorbed under pressure to be released; this is the desorption and regeneration (oxygen production) stage. The regenerated molecular sieve can then be used in the next cycle of nitrogen adsorption, and this process is repeated to produce nitrogen of a certain purity, as well as oxygen-enriched air of a certain purity. The block diagram of its device is shown in Figure 1. 2 Design: Based on pressure swing adsorption technology, nitrogen generation units with various technical performance parameters are designed and manufactured to meet the different needs of various users. Below, taking a nitrogen generation unit with a capacity of 200 m3/h and a purity of 99.9% used in a hydrogenated white oil project at a plant in the petrochemical industry as an example (the process flow is shown in Figure 2), a brief discussion of the relevant aspects is provided. 2.1 Main technical parameters of the device: Nitrogen production rate: 200 m3/h (at 20°C and absolute pressure of 0.1 MPa) ； Nitrogen purity: 99.9% (oxygen content less than 0.1%) ； Nitrogen supply pressure: 0.6 MPa. 2.2 Selection of molecular sieve and determination of its quantity: Based on past experience, experimental data for various molecular sieves, as well as the technical parameters required by the device, this device uses the latest 185-type carbon molecular sieve from the German company BP. The performance data for this sieve under certain operating conditions are as follows: (1) Overall density: 610–630 g/l ； (2) Average particle diameter: 2.3~2.5mm ； (3) PSA nitrogen production rate: greater than 115 m3/h·t at 99.9% N2 ； (4) PSA extraction rate: greater than 27% N2/air. The data in the above aspects impose certain requirements on the pressure, dew point, and oil content of the process air. The reason for choosing BF molecular sieve is that if domestic molecular sieves are used, a larger amount of such sieves would be required, which in turn leads to an increased volume of the adsorption cylinder; moreover, it is difficult to achieve the desired purity level through single-stage adsorption. By referring to the relevant parameters of molecular sieves and making comparisons, the amount of molecular sieve Q1 required at a pressure of 0.6 MPa and with a N2 purity of 99.9% can be determined. 2.3 Selection of air compression systems and cold dryer systems: Since molecular sieves have high requirements regarding the pressure, dew point, and oil content of the air to be processed, oil-free lubricated air compressors or air compressors with oil lubrication coupled with multiple stages of oil mist filters are commonly used to meet these requirements regarding oil content ； A cold dryer is used to meet its requirements regarding the dew point of the air. In this device, an oil-free lubricated air compressor is used, with the following technical specifications: air processing capacity of 10 m3/h (Q2) and outlet pressure of 0.8 MPa. The pressure dew point required for the cold dryer should not exceed 5°C. 2.4 Parameters and structural design of the adsorber system: The adsorber system is the core of the entire installation, and the quality of its design and manufacturing directly affects the performance and lifespan of the whole system. Moreover, the different parameters and structural design methods of adsorbers also have their own advantages and disadvantages. Different manufacturing businesses have varying views on how to achieve the best design. 2.4.1 Parameter design of the adsorber ① Determination of the minimum diameter D0 of the adsorber: The design of the adsorber begins with considering its empty tower velocity V0; different adsorbents have varying allowable values for this velocity. The minimum diameter D0 of the adsorber can be calculated based on the processed air volume Q2 and the empty tower velocity V0. ②Determination of the adsorber volume V: The adsorber volume V consists of three parts: the volume V1 required to fill the molecular sieve Q1, the volume V2 occupied by related structural components, and the volume V3 needed for other purposes such as buffering and ensuring a stable airflow ； That is, V=V1+V2+V3 ； Among them, V is determined by parameter design, while V2 and V3 are determined by structural design. 2.4.2 Structural design of the adsorber ① Determination of the height-to-diameter ratio of the adsorber The height-to-diameter ratio of the adsorber determines whether it will be short and stout or tall and slender. It features a short and stout low-bed design, offering high molecular sieve utilization, low absorption resistance, and minimal pressure loss; however, the manufacturing of the device is relatively difficult. The elongated high-bed design results in high adsorption resistance, but it allows the airflow to spread evenly along the axis of the adsorber, enabling isothermal adsorption as well as meeting the requirements for high purity in adsorption. ②Determination of the number of adsorbers: Adsorption systems can be classified into single-tower, double-tower, and multi-tower systems based on the number of adsorbers. Considering the actual conditions of the entire installation, such as the economic efficiency of compressor-driven gas supply, the continuity of exhaust gas flow, the complexity of the required equipment, and manufacturing costs, double-tower and triple-tower configurations are commonly used. The double-tower configuration is particularly common in nitrogen production plants; such systems consist of two adsorbers and one buffer tank. ③Design of the internal structure of the adsorber: The design of the internal structure of the adsorber includes the determination of the bed layers, as well as the design of various auxiliary structures such as upper and lower filters, flow guides, compression mechanisms, and gas distributors. Adsorbers can generally be divided into single-bed and double-bed types; the structure is shown in Figure 3. Both types have filters and gas distributors at their upper and lower ventilation ports. The single-layer bed structure is equipped with a mesh perforated plate and a cylinder compression device on the molecular sieve adsorbent. When the adsorber is in operation, the piston of the cylinder generates a downward force due to the pressure difference, and this force is used to compress the molecular sieve through the mesh perforated plate. This prevents the molecular sieve from experiencing boiling and fluidization caused by excessive airflow, as well as prevents damage to the filter mesh, thereby extending the lifespan of the molecular sieve and ensuring the proper operation of the adsorber. This structure is simple and reliable, and it can effectively overcome the problem of molecular sieve dust boiling within the stroke range permitted by the cylinder piston. The double-bed structure features a double layer of packing; compressed packing is added above the molecular sieve, with a wire mesh separating the two layers. During operation of the adsorber, the weight of the compressed packing presses down on the wire mesh and molecular sieve, thus serving the same function as a single-bed compression device, without being restricted by the limitations mentioned earlier. However, if this structure is not designed or assembled properly, the intermediate wire mesh may tilt during operation, leading to mixing of the molecular sieve and the compressed packing and consequently increased wear on the molecular sieve. ④Selection of the actuation valve: In the double-tower process, the periodic alternation between the two adsorbers is achieved through electromagnetic (pneumatic) valves controlled by a program controller. Due to the short switching cycle and frequent valve operation, it is required that the selected valves have a service life of over 500,000 cycles, and some valves must possess bidirectional ventilation capability. After determining the type of valve, its diameter and specifications are determined based on the volume of gas passed through each valve and the flow rates commonly used. If this device uses the DN50, DN80, and DN100 electro-pneumatic butterfly valves manufactured by our company, they feature a long service life and excellent sealing performance. 2.5 Design of the product gas buffer tank and selection of the production rate measurement system: Due to the use of a dual-tower process, there are significant pressure fluctuations in the discharge of product gas; therefore, using a buffer tank with an appropriate capacity is highly beneficial for improving the performance of the adsorbers as well as ensuring a stable supply of product gas. The size of the buffer tank volume is related to the gas production rate of the system. The principle for selecting the buffer tank volume is that the amount of pulsating gas generated in each cycle should not cause excessive fluctuations in the buffer tank pressure; generally, the pressure fluctuation Δp should not exceed 0.1 MPa. For small and medium-sized devices, the measurement of flow rate is typically accomplished using flanged orifice plates. The pressure difference generated as air flows through the orifice plate is converted into an electrical current signal by a differential pressure transmitter, and this current signal is then displayed in terms of flow rate by secondary instruments. Through this structure, it is possible to automatically control the product flow to the desired value. 2.6 Filtration System Depending on the operating environment of the user’s product gas (nitrogen), a filtration system is sometimes essential. This device is equipped with both coarse and fine filters. The coarse filter features a switchable structure with backwashing, while the fine filter is a disposable precision filter with a replaceable element, achieving a filtration accuracy of 0.01μm. 2.7 Control System Carbon molecular sieves possess notable characteristics: First, when exposed to moisture, their adsorption capacity for other gases decreases and their surface hardness drops. This results in a reduction in purity of the output gas, as well as the inclusion of powder in the product gas, which can cause failures in the control valves. Second, affected by oil components, the molecular sieve eventually fails because the oil components cannot be analyzed and regenerated. To enable the molecular sieve to perform its normal functions and ensure the proper operation of the adsorber, in addition to the regular control and protection systems of each unit, the installation includes various measures such as interlocks for setting the air temperature entering the adsorber, alarms for faults related to the discharge of condensate water from the cold dryer, and alarms when the product purity exceeds the specified limits. There is also programmatic automatic control of the control valves, along with signal display controls in the control room. 3 Common Faults During Debugging and Their Main Causes During on-site debugging, in addition to errors caused by a lack of familiarity with the procedures, other common fault phenomena include: (1) Faults in various associated moving components: ① Insufficient air supply volume or too low air supply pressure from the air compressor ； ②The cold dryer is not removing water properly or is not compatible with the entire system. (2) Fault symptoms of the adsorber unit: ① Abnormal pressure readings related to the adsorber unit ； ②The actuation valve switches slowly or does not operate ； ③Powder is entrained in the product gas and the purge gas. (3) Indicators of system failure in product gas analysis: ① The analysis system indicates a low purity level ； ②The flow rate does not meet the design specifications. The causes of the above faults, as analyzed, are: unreasonable design and selection, as well as defects in the units or instruments themselves ； The actuation valve does not seal properly when closed ； The solenoid valve is faulty, or powder has caused problems with the actuation valve ； Fluidization occurs in the adsorber bed, resulting in severe frictional impact on the molecular sieve and the generation of powder ； Severe water and oil contamination significantly reduces the hardness of the molecular sieve, accelerating its pulverization or leading to its failure ； Improper setting of time for various stages of the operation cycle, and so on. 4 Issues to Note During Use: After the device is delivered for debugging, it is essential for users not only to know how to operate it but also to maintain it and understand its operational principles. After understanding the performance of various supporting units, the following points should be taken into consideration in order to maintain and make good use of the adsorber, which is the core component. As mentioned earlier, molecular sieves are sensitive to water and oil and prone to wear; therefore, special attention must be paid to the quality of the air (level of moisture and oil content) during operation. In daily operations, it is necessary to regularly check whether the air compressor is being lubricated properly, whether the oil filtration system is functioning correctly, and whether the refrigeration and water removal systems of the cold dryer are operating as intended. It is also necessary to regularly check the product gas discharge filter and the purge gas vent for any accumulation or excessive entrainment of molecular sieve powder ； Regularly check the wear level of the molecular sieve in the adsorber, and determine whether it is necessary to add more of it or take other actions; if any of the above issues occur, they should be resolved promptly. If it is operated and maintained properly, its service life will be **extended**. Author profile: Ji Zongsheng, male, engineer, born in December 1966. He graduated with a bachelor’s degree in chemical engineering machinery from Zhejiang University in 1988. Currently, he is engaged in product design work at the military products factory of Hangzhou Oxygen Generator (Group) Company.