Applications of pressure swing adsorption units

Collected from Haiyou; hope it’s useful. Design and Operation of a 2500 m3/h Nitrogen Purification Unit 1 　Introduction With the development of high-tech technologies and new products such as integrated circuits, lasers, new materials, polymer synthesis, and controlled-atmosphere heat treatment, there is an increasing demand in the market for high-purity nitrogen, as well as high-purity helium and argon, and the requirements regarding gas purity are also becoming more stringent. The air separation equipment currently in use at Zhejiang Juhua Co., Ltd. is of the type produced by Hangyang in the 1980s. After more than 20 years of operation, its production capacity has declined, and the purity of nitrogen can only be maintained between 99.197% and 99.1995%. Meanwhile, customers have increasingly higher requirements regarding the purity of liquid nitrogen; some even require that the oxygen content in liquid nitrogen be below 1 × 10^-6. This will inevitably cause the company’s existing market share to shrink gradually, let alone lead to an expansion of the market. In light of the growing market demand for liquid nitrogen and the increasing requirements regarding its purity, Zhejiang Juhua Co., Ltd. built a set of high-purity nitrogen liquefaction equipment with a capacity of 2,500 m3/h in September 2005. The nitrogen purification unit was supplied by Hangzhou Hangyang Co., Ltd. (hereinafter referred to as Hangyang); its design pressure is 613 MPa, which is the highest among similar products in China. Such nitrogen purification units can purify bottled nitrogen, pipeline nitrogen, and liquid nitrogen into high-purity nitrogen. They can also be used in conjunction with pressure swing adsorption (PSA) nitrogen generation systems to directly extract high-purity nitrogen from air, thereby meeting the various requirements of users. Such purification devices are also suitable for the purification of carbon dioxide, ethylene, propylene, as well as inert gases such as argon and helium. 2 　Key performance indicators: The nitrogen purification unit can produce 2500 m3/h of pure nitrogen. The purity of the input nitrogen is 99.999%, with an average oxygen content of ≤100 ×10^-6; the purity of the output nitrogen is 99.99998%, with an oxygen content of ≤2 ×10^-6. The pressure of the pure nitrogen is ≥413 MPa (with a design pressure of 613 MPa). The dew point at atmospheric pressure is ≤-60 °C. The system’s switching time is ≥120 hours, the hydrogenation time is about 15 hours, and the service life of the deoxidizer is approximately 3 years under normal usage conditions. The 2500 m3/h nitrogen purification unit can operate under variable conditions to meet production requirements. 3 Process organization: Considering cost factors, the 2500 m3/h nitrogen purification unit uses chemical adsorption to remove oxygen. The selection of a deoxidizer is key to removing oxygen from the nitrogen in the raw material. After on-site testing of different types of deoxidizers produced by various domestic manufacturers, the 506HN21 type deoxidizer manufactured by a domestic company, which possesses both oxidizing and reducing properties, was ultimately selected. This deoxidizer belongs to the metal oxide category, and its deoxidation mechanism is as follows: the reducing low-valent metal oxides (manganese-based) react with the trace amount of oxidizing oxygen present in nitrogen to form high-valent metal oxides (manganese-based compounds are polyvalent both before and after the reaction), thereby achieving deoxidation. Once it loses its effectiveness, the deoxidizer can be regenerated by passing hydrogen gas over it while heating it; the high-valent metal oxides are reduced back to low-valent metal oxides, restoring its activity once again. To ensure continuous gas production, two vertical adsorbers are specially installed and used in rotation, with one adsorbing while the other is regenerated. The chemical equation for the redox reactions involving manganese is as follows: Oxidation of manganese species: Mn X+ + O2 → Mn Y+ + O2-. Reduction of manganese species: Mn Y+ + O2- + H2 → Heating → Mn X+ + H2O. The main features of the nitrogen purification device are: oxygen removal at room temperature, high oxygen-removal efficiency, and a high degree of oxygen elimination; the regeneration temperature is relatively high, around 300 ℃. If the deoxidation temperature (operating temperature) is around 100 ℃, the deoxidation performance will increase by nearly a factor of two. An internal heating method using an electric heater is employed; if other heat sources (such as steam) are available, external heating can also be used to heat the regenerated nitrogen to around 300 °C. Considering the high operating pressure and high regeneration temperature of the 2500 m3/h nitrogen purification unit, as well as the characteristics of its associated equipment, some targeted improvements were made on the basis of the conventional process; the process is shown in Figure 1. The specific improvement measures are as follows: (1) The feed gas for the 2500 m3/h nitrogen purification unit is the nitrogen gas under pressure at the booster end of the expander; usually, a post-cooler is installed to cool this pressurized nitrogen gas to room temperature. To investigate the relationship between the operating temperature and deoxidation capacity of the 506HN21 type deoxidizer, the post-cooler was removed from the entire nitrogen liquefaction system in order to increase the temperature of the raw nitrogen entering the adsorber. This not only reduces equipment investment but also extends the adsorption cycle by nearly 1/3 while keeping the amount of deoxidizer used unchanged. It reduces the heating frequency of electric heaters, the consumption of regeneration gas, and the usage rate of deoxidizer; as a result, the average energy consumption of the device is reduced by 1/3, while the lifespan of the deoxidizer is extended. (2) To ensure thorough regeneration of the deoxidizer, the control of the regenerating gas flow rate and temperature is particularly important. On the one hand, a pressure reducing valve is installed in the regeneration gas flow path; this valve reduces the pressure of the product gas, which is then mixed with hydrogen before entering the adsorber. This arrangement helps to control the flow rate of the regeneration gas as well as its uniform distribution, and it allows full utilization of the heat generated by the reaction between hydrogen and oxygen to produce water during regeneration. As a result, the deoxidizer undergoes regeneration in layers, with the focus of regeneration moving downward layer by layer. On the other hand, an internal heating method is used to control the temperature variations of the electric heating elements, thereby keeping the regeneration temperature of the deoxidizer within a reasonable range. (3) Considering the high regeneration temperature, a water cooler is installed at the outlet of the adsorber. This not only improves operational safety but also ensures the appropriate temperature of high-purity nitrogen entering the liquefaction equipment, thereby preventing excessive temperature differences at the hot side of the plate-fin heat exchanger. (4) Pressurize and depressurize in stages to reduce the pulverization of the deoxidizer. The raw nitrogen gas is first pressurized to a certain level using pressure regulator V8012, and then further pressurized to the operating pressure via pressure regulators V8007 and V8008, thereby preventing high-pressure nitrogen from directly impacting the deoxidizer. By adjusting the opening degree of vent valves V8005 or V8006, it is possible to achieve staged pressure relief in adsorbers A8001 or A8002. 4 　Structural features of the equipment: (1) The upper and lower parts of the absorber are equipped with umbrella-shaped airflow distribution devices. On the one hand, it prevents high-pressure gas from directly impacting the deoxidizer, thus protecting the deoxidizer, **reducing its rate of pulverization and improving the quality of the nitrogen in the product**. On the other hand, it reduces the dead zones in the adsorption bed, as well as direct impacts of the airflow on the deoxidizer and the ineffective volume of the adsorber, resulting in a more optimal distribution of the airflow. This improves the efficiency of use of the deoxidizer and extends its service life. (2) The main requirements for deoxidizer regeneration are minimal heat loss, low power consumption, thorough deoxidization, and reduced amount of regeneration gas used. This poses certain challenges in terms of the design of the adsorber structure, the arrangement of electric heating elements, and their wiring; it is necessary to take into account both uniform heating and the service life of the electric heating elements as well as the ease of replacing them. To this end, the design features electric heating elements arranged in a circular pattern, as well as electric heating rods, in order to reduce the heat generation per unit length and lower the failure rate of these heating elements. The electric heating elements are of a plug-in type and are distributed throughout the top area, which facilitates their replacement. 5 　Instrument and electrical control system: (1) The instrument and electrical control is implemented using PLC programmable controllers, enabling automatic operation. It can also be switched to manual control in the event of a failure in the automatic control system, to ensure the normal operation of the system. (2) There is 1 control knob on the instrument control cabinet, with 3 settings: A8001 for adsorption, A8002 for regeneration/stop, and A8002 for adsorption and A8001 for regeneration. Indicator lights are used to show the operating status of each adsorber. When the “A8001 Absorption” or “A8002 Absorption” button is in the engaged state, a start signal is sent to the program controller; the program controller then begins to operate and keep timing, while also controlling the opening degree of valve V8012 and the hydrogen supply valve V9003. Once the specified time has been reached, an alarm signal is generated. When the knob is switched, the timing starts from zero. (3) One platinum resistor is installed at the upper, middle, and lower parts of the adsorber to monitor the temperature of the adsorption bed; a telescopic thermocouple is placed at the center of the cylinder to control the regeneration temperature. The two adsorbers, with their 8 temperature measurement points, share one temperature monitoring instrument. As soon as the measurement value at any of these 8 points exceeds 350 °C, an alarm is triggered automatically, and a signal is sent to the program controller, which in turn shuts down valve V9003, thereby enabling synchronized switching between temperature control and the control buttons. (4) An 380VAC electric heating element is used to extend the service life of the heating element and ensure the stable operation of the device. 6 　Operation status To date, the 2500 m3/h nitrogen purification unit has been operating continuously and stably for 27 months. Based on more than two years of follow-up testing of the device, the nitrogen purification unit has maintained good operational performance, and its capacity to handle varying loads meets the requirements of production. The key performance parameters under various operating conditions are shown in Table 1. Table 1 Main operating parameters of the 2500 m3/h nitrogen purification unit under different operating conditions
Condition Raw nitrogen flow rate / (m3/h) Oxygen content in raw nitrogen Regeneration temperature / °C Operating cycle / h
Ⅰ 2700 50 ×10^-6～60 ×10^-6 ～270 ～240
Ⅱ 2850 100 ×10^-6 ～300 125
Ⅲ 2650 Approximately 200 ×10^-6 ～310 62
Ⅳ 2700 Approximately 300 ×10^-6 ～325 40～48

Note: The operating cycle refers to the cycle time of the 2500 m3/h nitrogen purification unit when the oxygen content in the high-purity nitrogen is ≤2 ×10^-6. 　　Due to the influence of ambient temperature on air separation equipment, the purity of nitrogen produced in high-temperature environments during summer is only 99.197% (with an oxygen content of 300 ×10^-6, Condition IV); this oxygen content is three times the value specified in the design parameters. Moreover, the switching time for the nitrogen purification unit is reduced to 40–48 hours, which is far from the designed value of 120 hours. According to statistics, the annual average purity of nitrogen processed by the nitrogen purification unit is approximately 150 ×10 - 6 O2. Converted to the design operating conditions, the deoxidizer in the unit is equivalent to having been in use for over 3 years; however, the unit’s processing capacity remains at the design level, and no signs of degradation of the deoxidizer have been observed. Up to October 2008, the nitrogen purification unit had been operating normally, with the exception of a few cases where electric heating elements burned out during the commissioning and trial operation phases. 7 　Areas requiring improvement: (1) The pressure and purity of the hydrogen gas used for regeneration vary significantly; human intervention is needed during debugging and actual operation, which causes inconvenience in operations. To achieve automated control, an automatic adjustment device that controls the amount of hydrogen based on its purity should be installed. (2) The regeneration temperature is controlled by a thermocouple inserted at the center of the cylinder; it is affected by the depth of insertion, resulting in significant differences in the regeneration temperature at the upper, middle, and lower parts of the cylinder. This requires continuous improvement during debugging and actual operation. If a deoxidizer of type 506HN with a regeneration temperature of 160 ℃ is used, its relatively uniform regeneration temperature and low value allow for reduced energy consumption during operation of the equipment; however, its initial investment cost is high, exceeding what users can afford. 8 　Conclusion The nitrogen purification unit with a capacity of 2500 m3/h features energy efficiency, stability, and safety. Its successful operation provides new ideas for the renovation of older air separation units and other gas separation equipment in the future, and it will hold great significance for the production of high-purity gases in China. In today’s era focused on energy conservation and environmental protection, how to improve the utilization rate and recycling rate of deoxidizers will remain a technical challenge that needs to be overcome in the future

The concept of pressure swing adsorption and adsorbents; the concept of adsorption. Pressure swing adsorption (PSA) technology is a new type of gas separation and purification technique that has been developed over the past 30 years. In 1942, Began published the first patent document on heat-free adsorption for air purification. In the early 1960s, United Carbon Corporation was the first to industrialize the pressure swing adsorption four-bed process technology. Due to its low investment costs, low operating expenses, high product purity, simple and flexible operation, minimal environmental impact, and wide adaptability to various feed gas sources, pressure swing adsorption technology has been widely used in fields such as petrochemicals, metallurgy, light industry, and environmental protection since the 1970s. Adsorption refers to the process in which, when two substances in different phases come into contact, the molecules of the substance with lower density accumulate on the surface of the substance with higher density. Substances with adsorption properties (usually porous solids with relatively high density) are called adsorbents, while the substances that are adsorbed (usually gases or liquids with relatively low density) are called adsorbates. Based on their different properties, adsorption can be divided into four main categories: chemical adsorption, active adsorption, capillary condensation, and physical adsorption. The adsorption in pressure swing adsorption (SPA) gas separation units is primarily physical adsorption. Physical adsorption refers to the adsorption that occurs through molecular forces between the adsorbent and the adsorbate molecules, including van der Waals forces and electromagnetic forces. Its characteristics are: there are no chemical reactions during the adsorption process, the adsorption occurs very rapidly, the dynamic equilibrium between the various phases involved in the adsorption is established in an instant, and this type of adsorption is completely reversible. The pressure swing adsorption gas separation process is made possible by two fundamental properties of the adsorbent in this type of physical adsorption: first, it has different adsorption capacities for various components; second, the adsorption capacity of the adsorbate on the adsorbent increases as the partial pressure of the adsorbate rises, and decreases as the adsorption temperature increases. By utilizing the first property of the adsorbent, it is possible to achieve preferential adsorption of certain components in a mixed gas, thereby purifying the other components ； By taking advantage of the second property of adsorbents, it is possible for them to absorb gases at low temperatures and high pressures, and to release those gases for regeneration at high temperatures and low pressures. This creates a cycle of absorption and regeneration of the adsorbent, thereby enabling continuous gas separation. There are many posts on the forum related to pressure swing adsorption; the poster can search using several keywords such as pressure swing adsorption, PSA, and carbon removal. Here are a few links that may be helpful to the poster: http://bbs.hcbbs.com/viewthread.php?tid=521564&highlight=%B1%E4%D1%B9%CE%FC%B8%BD http://bbs.hcbbs.com/viewthread.php?tid=158541&highlight=PAS http://bbs.hcbbs.com/viewthread.php?tid=409804&highlight=PAS http://bbs.hcbbs.com/viewthread.php?tid=544802&highlight=PAS

Brothers on the 2nd and 3rd floors, it seems like what the original poster is looking for isn’t this. What do you think?