【Weekly Topic】{November 20, 2011} What are the main factors affecting the PSA process?

The factors affecting the PSA process are as follows: (1) Water content in the feed: Liquid carried in the feed enters the bed layer, which severely affects the adsorption capacity of the adsorbent for gaseous impurities; moreover, regeneration becomes difficult. Therefore, it is necessary to carry out strict dehydration and heat preservation treatment on the feed gas. (2) Feed composition: When the hydrogen content is below the designed value (an increase in impurities), the adsorption time should be shortened accordingly, resulting in a decrease in hydrogen production and yield. If the feed composition is not within the designed range, it also damages the adsorbent and affects its service life. - (3) Feed flow rate: At low flow rates, the adsorption time should be increased to achieve a higher hydrogen yield; at high flow rates, the adsorption time should be shortened accordingly to maintain product purity. PSA has a large operating flexibility, allowing it to operate at any flow rate within 110–30% of its design capacity while still ensuring the purity of the produced hydrogen. (4) Adsorption pressure: The operating pressure for PSA is not the higher the better. Within a certain pressure range, as the pressure increases, the amount of impurities adsorbed increases and the hydrogen recovery rate improves; however, at higher pressures, the amount of hydrogen adsorbed also increases, which in turn reduces the recovery rate. All pressures are around 1.28 MPa. (5) Feed temperature: Either too low or too high a feed temperature reduces the hydrogen yield. Excessively high temperatures are unfavorable for adsorption and affect the lifespan of the adsorbent, while too low temperatures make regeneration difficult and may also lead to the presence of water, which is likewise detrimental to the adsorbent.

1. Adsorbent activity: The adsorbent has high activity, enabling strong adsorption of impurities and resulting in a high purity of the product. Since it regenerates the adsorbent through voltage transformation, the lower the reverse pressure, the better the regeneration effect ； Those with vacuum pumps: the higher the vacuum level, the better the regeneration effect. Attention should also be paid to the effects of poisoning, which are divided into sulfur poisoning and liquid poisoning. It is necessary to control the sulfur content in the raw materials ； Before shutting down the vacuum pump, the inlet valve must be closed first to prevent backflow of the working fluid; once the molecular sieve absorbs water, it becomes difficult to regenerate. 2. Adsorbent strength: High strength, resistant to crushing, and excellent adsorption performance. 3. Adsorption pressure: I disagree with the view expressed above; I know that in many cases the adsorption pressure is above 2.0 MPa. 4. Programmable valve: Programmable valves have good sealing properties, preventing gas leakage; as a result, the product purity is high and the production volume is large.

As a chemical processing unit, pressure swing adsorption gas separation technology is rapidly evolving into an independent discipline known as adsorption separation engineering. It is finding increasingly wide applications in the fields of petroleum, chemicals, metallurgy, electronics, national defense, light industry, agriculture, medicine, food, and environmental protection. Practice has proven that pressure swing adsorption technology is an effective method for gas separation and purification. Pressure Swing Adsorption (PSA) is a high-tech method within adsorption separation techniques used for separating gas mixtures. In the 1960s, amid a global energy crisis, the United Carbon Corporation (UCC) was the first to use pressure swing adsorption technology to recover high-purity hydrogen from hydrogen-containing industrial waste gases; the first industrial facility for hydrogen recovery using PSA was put into operation in 1966. Up to 1999. Around the world, at least a thousand PSA hydrogen production units are in operation, with hydrogen production capacities ranging from 20 to 100,000 Nm3/h. The Southwest Chemical Research and Design Institute of China began researching pressure swing adsorption gas separation technology in 1972, and in 1982 it built the first industrial pressure swing adsorption unit in Shanghai for recovering pure hydrogen from ammonia plant off-gases. Over the years, as research in areas such as adsorbents, process technologies, instrument control, and engineering implementation has continued to advance, the application of pressure swing adsorption technology in the field of gas separation and purification has grown increasingly widespread. The pressure swing adsorption gas separation technology, which has now been successfully developed, has been applied in nine different areas: the recovery of hydrogen from ammonia synthesis off-gases, the purification of carbon monoxide from mixtures containing it, carbon removal from ammonia synthesis conversion gases, natural gas purification, the production of oxygen-rich gas through air separation, the production of pure nitrogen through air separation, the concentration of methane from coal mine gas, the concentration of ethylene from mixtures rich in ethylene, and the purification of carbon dioxide from carbon dioxide-containing mixtures. The Southwest Chemical Engineering Research and Design Institute in China has become one of the three major research institutions in the world dedicated to the research and development of pressure swing adsorption system technologies, alongside UOP and Linde. 2.2 Applications of PSA technology: The operating modes of pressure swing adsorption gas separation units can be roughly classified as follows – depressurization, flushing for desorption; isobaric type; equilibrium separation type; depressurization and evacuation for desorption; pressure swing adsorption; non-isobaric type; rapid pressure swing adsorption; speed separation type. The equilibrium separation type separates gas mixtures based on the principle of selective adsorption due to the equilibrium adsorption properties of gases on the adsorbent, while the speed separation type achieves separation of gas mixtures by taking advantage of the differences in the adsorption rates of various components by the adsorbent. In the equilibrium separation type, the adsorption operation of the isobaric type is carried out at a constant pressure; the adsorption bed experiences minimal pressure loss under the operating pressure. Most pressure swing adsorption units in use in industry belong to the isobaric type within the equilibrium separation category. Non-isobaric rapid pressure swing adsorption is achieved through the appropriate distribution of system fluid resistance; the adsorption process requires a certain pressure gradient within the adsorption bed. The particle size of the adsorbent is small, ranging from 40 to 60 mesh (while the adsorbents used in isobaric pressure swing adsorption devices have a diameter of 1 to 4 mm), which results in high pressure losses. The fast pressure swing adsorption process is characterized by a particularly simple setup, usually comprising only one adsorption bed; moreover, thanks to the fast cycle timing, it offers a high production capacity per unit of adsorbent. Its drawback is that it cannot produce products of high purity; the recovery rate is low at the same product purity level, and it is only suitable for small-scale production facilities. Currently, there are few industrial plants of this type. The velocity-separation type of adsorption operation is achieved by the differences in the diffusion rates of different gas molecules within the micropores of the adsorbent; therefore, the adsorption process must be completed in a very short time. The adsorption and separation of air by carbon molecular sieves falls into this category. 3. Key technologies in the pressure swing adsorption process 3.1 Selection of adsorbents The adsorption capacity of adsorbents for various gas components is evaluated through experimental determination of isotherm lines under static conditions and flow curves under dynamic conditions; good adsorption performance by adsorbents is a fundamental requirement for the adsorption separation process. In the pressure swing adsorption process, the selection of the adsorbent requires consideration of the conflict between adsorption and desorption. Generally, components that are easier to adsorb are more difficult to desorb, whereas components that are more difficult to adsorb are easier to desorb. For example, for strongly adsorbable substances such as benzene and toluene, an adsorbent with weaker adsorption capacity, such as silica gel, should be used. This ensures an appropriate adsorption capacity while facilitating desorption ； For weakly adsorbed substances such as CO, CH4, N2, etc., adsorbents with stronger adsorption capacity such as molecular sieves need to be used. Another key factor in selecting an adsorbent is that the separation coefficient between its components should be as large as possible. The so-called separation coefficient refers to the fact that the total amount of a certain gas component in the adsorption bed consists of two parts. Part of it is in the dead space, and another part is adsorbed by the adsorbent; the total of these amounts is referred to as the retention amount of a particular gas component within the adsorption bed ； The ratio of the proportion of weakly adsorbed components to that of strongly adsorbed components in the dead space, relative to the total amount remaining in the bed, is called the separation factor. The separation factor between the two components separated during the pressure swing adsorption process should be no less than 2. Table 3-2 lists the separation coefficients of gas components on common adsorbents at atmospheric pressure and 20°C. Furthermore, during the operation of the adsorption bed, due to the periodic changes in pressure within the bed, gas enters or exits the adsorption bed on a short time scale, causing the adsorbent to be subjected to frequent flow-induced erosion. This requires that the adsorbent used have sufficient strength in order to minimize fragmentation and wear. Table 3-2 Separation factors of gas components on commonly used adsorbents (20°C, 1 atm). Gas components: Adsorbents: CH4/CO2, CO/CH4, N2/CH4, N2/CO, H2/CH4, H2/CO, H2/N2. Silica gel: 6.4, 1.34, 1.86, 1.42, 2.90, 2.05, 3.8. Activated carbon: 2.0, 2.07, 2.84, 1.37, 6.97, 5.10, 14.4. 5A molecular sieve: 1.79, 3.15, 1.4, 2.50, 9.65, 17.2, 6.9. Mordenite: 1.18, 2.23, 1.39, 1.65, 15.0, 18.5, 11.2. 13X molecular sieve: 1.58, 4.7, 1.52, 2.40, 8.0, 12.6, 5.25. Note: 1 atm = 101.325 kPa. Table 3-2 lists the adsorbents commonly used for separating different gas mixtures via pressure swing adsorption. For separating gas mixtures with complex compositions and a large number of components, it is often necessary to use several adsorbents. These adsorbents can be arranged in layers within the same adsorption bed in order of their adsorption separation performance to form a composite bed, or they can be placed in separate adsorption beds depending on the specific circumstances. 3.2 Programmable valves are key to ensuring the proper operation of the PSA process. High-quality programmable valves provide a reliable guarantee for the long-term stable operation of the system. Valve control technology has indeed always been a crucial element in PSA systems, as the operation of PSA relies on the frequent opening and closing of dozens of such valves in order to switch between the adsorption and regeneration states of the adsorption beds. These programmable valves need to be opened and closed tens of thousands to hundreds of thousands of times per year. According to statistics, 90% of PSA unit failures occur with programmable valves. Therefore, the operational specifications and requirements for PSA programmable valves are higher than those of ordinary valves; in addition to having good sealing performance, fast opening and closing speeds, and good regulation capabilities. It must also be able to operate reliably over a long period of time under frequent operations. Its main features are: (1) a long service life – it is required to maintain its sealing performance after 500,000 cycles of opening and closing; for some programmable valves, no leakage is required even after 1,000,000 such cycles. ⑵It features a fast opening and closing speed; depending on the valve diameter, the opening and closing time should be less than 1–3 seconds. ⑶Some valves require bidirectional flow capability. ⑷In addition to the aforementioned opening and closing properties, some valves also have a regulating function. ⑸The internal and external sealing of valves must meet the sealing requirements specified in AN-SIBl6.04 after 500,000 operating cycles. In particular, for programmed control valves used in CO plants, the external sealing requirements are even stricter. ⑹It features on-site indication of valve position status and remote signal transmission; its service life is comparable to that of programmable valves, and it meets the explosion protection requirements for Zone II. ⑺The electro-pneumatic valve positioner equipped with a programmable valve featuring regulation functions has a service life that is synchronized with that of the programmable valve, while also meeting the explosion-proof requirements for Zone II. Early PSA gas separation units primarily used pneumatic ball valves; however, as this technology continued to develop, relying solely on ball valves proved insufficient to meet the demands of the evolving pressure swing adsorption technology, which tended toward multi-level and larger-scale systems. At present, various domestic research institutions specializing in pressure swing adsorption have their own departments for valve research and production. They have successfully developed dozens of different types of valves suitable for pressure swing adsorption processes, including seven main categories such as self-compensating anti-scouring pneumatic flat valves, logic-guided valves, bellows valves, vacuum butterfly valves, and combined valves. The maximum diameter of these valves has reached 500 mm. Typically, small-scale installations use ball valves, logic-controlled valves, combination valves, and flat valves with small diameters, while large-scale installations employ vacuum butterfly valves and flat valves with large diameters. For PSA systems used in the purification of CO, where extremely low leakage rates are required, only bellows valves with very good sealing properties can be used. Currently, the programmable valves specifically designed for pressure swing adsorption and developed in China generally feature high sealing performance, reduced maintenance requirements, fast switching speed, and a long service life; the leak-free switching life of such valves can reach up to 1 million cycles. 3.3 Fatigue design of adsorbers to ensure safety: Non-standard equipment – The adsorbers in pressure swing adsorption units are classified as category II pressure vessels based on their function and pressure level. However, since they are subjected to 400,000 cycles of full-amplitude alternating pressure changes during their service life, they are considered fatigue-sensitive pressure vessels. Therefore, they cannot be designed solely in accordance with the standards for category II pressure vessels; instead, specialized testing is required to develop a design that meets the requirements of alternating pressure. Since the design codes for fatigue-pressure vessels in our country are not yet complete, in the design of adsorbers, in addition to referring to relevant domestic and international regulations and codes, the design is primarily based on data obtained from long-term fatigue tests of pressure vessels. In addition to fatigue issues, the design of adsorbers also has specific requirements regarding the structure of their air flow distributors. If the structural design of the air flow distributor is inadequate, it will result in uneven air flow distribution, prone to air flow backmixing, and insufficient utilization of the adsorbent, which directly affects the adsorption efficiency and may even lead to the degradation of the adsorbent and its loss of effectiveness. Especially for adsorbers with large diameters (greater than 3 meters), the correct design of the gas flow distributor is even more crucial for the proper operation of the device. 3.4 Computers drive pressure swing adsorption to new levels – Control systems: Pressure swing adsorption units are characterized by continuous operation, frequent switching of programmable valves, a large number of control valves, and significant requirements for sequential control; as a result, a high degree of automation is necessary. During the production of the device, the instrumentation and control systems should be monitored effectively to ensure stable and reliable operation. Domestic research institutions specializing in PSA technology can configure various types of instruments based on the characteristics of different devices and the users’ requirements regarding automation levels. These include domestic and imported instruments such as pneumatic type III, electric type III, SPEC–200, YS-800, 1151, ST3000, etc., as well as programmable controllers or distributed control systems with various functions and grades, such as the FX2, C200HS series, SYSTEM-3 series, T1535, R150, UXL series, S9000E, ROSEMOUNT, TDC-3000, Centum, and others.

(1) Water in the feed: Water contained in the feed significantly impairs the adsorption capacity of the adsorbent for gaseous impurities, and regeneration becomes difficult; therefore, the feed gas must undergo strict dehydration and heating treatments. (2) Feed composition: When the hydrogen content is below the designed value (an increase in impurities), the adsorption time should be shortened accordingly, which results in a decrease in hydrogen production and a reduction in the hydrogen yield. If the feed composition is not within the designed range, it can also damage the adsorbent and affect its service life. (3) Feed flow rate: At low flow rates, the adsorption time should be increased to achieve a higher hydrogen yield; at high flow rates, the adsorption time should be shortened accordingly to maintain product purity. PSA has a large operating flexibility, allowing it to operate at any flow rate within 110–30% of its design capacity while still ensuring the purity of the produced hydrogen. (4) Adsorption pressure: The operating pressure for PSA is not the higher the better. Within a certain pressure range, as the pressure increases, the amount of impurities adsorbed increases and the hydrogen recovery rate improves; however, at higher pressures, the amount of hydrogen adsorbed also increases, which in turn reduces the recovery rate. All pressures are around 1.28 MPa. (5) Feed temperature: Either too low or too high a feed temperature reduces the hydrogen yield. Excessively high temperatures are unfavorable for adsorption and affect the lifespan of the adsorbent, while too low temperatures make regeneration difficult and may also lead to the presence of water, which is likewise detrimental to the adsorbent.

Main factors affecting the PSA process: 1. Design-related factors: properties of the adsorbent, composition of the feed gas, capacity of the tower, number of pressure equalization cycles, and product purity. 2. Operational settings and conditions: status of programmable valves, setting of adsorption pressure, level of back-pressure, degree of vacuum, and settings for various timing parameters. Therefore, in pressure swing adsorption operations, measures such as increasing the adsorption pressure, reducing the desorption pressure, extending the adsorption time, and improving the desorption efficiency should be adopted to maximize gas recovery rates.

The factors affecting the PSA process include: feed temperature, adsorption pressure, adsorbent, composition of the feed gas, number of pressure equalization cycles, and desorption method

1. Water in the feed: Water present in the feed, once it enters the bed layer, severely affects the adsorption capacity of the adsorbent for gaseous impurities, and regeneration becomes difficult. Therefore, it is necessary to carry out strict dehydration and separation processes on the feed gas, as well as to provide heat preservation. 2. Feed composition: When the hydrogen content is below the designed value (an increase in impurities), the adsorption time should be shortened accordingly, which results in a decrease in hydrogen production and a lower hydrogen yield. If the feed composition is not within the designed range, it can also damage the adsorbent and affect its service life. 3. Feed flow rate: At low flow rates, the adsorption time should be increased to achieve a higher hydrogen yield; at high flow rates, the adsorption time should be shortened accordingly to maintain product purity. PSA has a large operating flexibility, allowing it to operate at any flow rate within 110–30% of its design capacity while still ensuring the purity of the hydrogen product. 4. Adsorption pressure: The operating pressure in PSA processes is not the higher the better. Within a certain pressure range, an increase in pressure leads to more impurities being adsorbed and thus a higher hydrogen recovery rate. However, at higher pressures, the amount of hydrogen that gets adsorbed also increases, which in turn reduces the recovery rate. All pressures should be around 1.28 MPa. 5. Feed temperature: Either too low or too high a feed temperature results in a lower hydrogen recovery rate. Excessively high temperatures are unfavorable for adsorption and affect the lifespan of the adsorbent, while too low temperatures make regeneration difficult and may also lead to the presence of water, which is likewise detrimental to the adsorbent

(1) Water in the feed: Water contained in the feed significantly impairs the adsorption capacity of the adsorbent for gaseous impurities, and regeneration becomes difficult; therefore, the feed gas must undergo strict dehydration and heating treatments. $ Y7 h: y$ g0 Y7 N: E, H(2). Feed composition: When the hydrogen content is below the designed value (an increase in impurities), the adsorption time should be shortened accordingly, which results in a decrease in hydrogen production and a lower hydrogen yield. If the feed composition is not within the designed range, it also causes damage to the adsorbent, affecting its service life. (3) Feed flow rate: At low flow rates, the adsorption time should be increased to achieve a higher hydrogen yield; at high flow rates, the adsorption time should be shortened accordingly to maintain product purity. PSA has a large operating flexibility, allowing it to operate at any flow rate within 110–30% of its design capacity while still ensuring the purity of the produced hydrogen. 7 U’ c- v1 I- J# s1 P8 k6 F (4) Adsorption pressure: The PSA operating pressure is not the higher the better; within a certain pressure range, an increase in the amount of impurities adsorbed leads to an improvement in hydrogen recovery rate. However, at higher pressures, the amount of hydrogen that gets adsorbed also increases, which in turn reduces the recovery rate. All pressures are around 1.28 MPa. ( B! c) ?9 T1 V; g: S (5) Feed temperature: Either too low or too high a feed temperature reduces the hydrogen yield. Excessively high temperatures are unfavorable for adsorption and affect the lifespan of the adsorbent, while too low temperatures make regeneration difficult and increase the risk of water contamination, which is also detrimental to the adsorbent.

This question is a bit too broad. If we use product purity, yield, and the economic efficiency of the equipment as criteria, I think there are two main factors at play: one is the adsorbent, and the other is the process conditions. For the adsorbent, its selectivity is the key factor; process conditions include adsorption pressure, the number of pressure equalization steps, whether steps such as replacement flushing and reverse operation are employed, evacuation pressure, evacuation time, and so on