What is the basic working principle of pressure swing adsorption carbon removal?

1. Basic principle of pressure swing adsorption for carbon removal: Utilizing the fact that adsorbents have different adsorption capacities, adsorption rates, and adsorption forces for adsorbates at various partial pressures, as well as their ability to selectively adsorb various components of a gas mixture to be separated under certain pressures. Pressure is applied to adsorb impurity components from the feed gas, and then the pressure is reduced to desorb these impurities, thereby regenerating the adsorbent. 2. Type of PSA unit: A device for producing decarburized purified gas and carbon dioxide gas with a purity of 98%. It consists of a purification system and a cleaning system, both of which employ a multi-tower PSA process. The transformed gas passes through a purification system to concentrate the carbon dioxide concentration to over 98.5%, for use in the urea plant. The intermediate gas from the purification system enters the purification unit, where carbon dioxide in this gas is further reduced to levels below 0.2%, in order to meet the requirements of ammonia synthesis. 3. Common adsorbents used in PSA: silica gel, activated alumina, activated carbon, molecular sieves, etc. Isothermal adsorption curves and breakthrough curves.

This post was last edited by ZZJJAA70 on 2009-9-8 at 22:25. This section already contains this material; the specific link is http://bbs.hcbbs.com/viewthread.php?tid=420441&highlight=%B1%E4%D1%B9%CE%FC%B8%BD. The concept of adsorption: Pressure Swing Adsorption (PSA) is a new type of gas separation and purification technology that has been developed over the past 30 years. In 1942, Bèi Guó 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 suitability for various raw 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 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: no chemical reaction occurs during the adsorption process, the adsorption takes place extremely quickly, 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 the mixed gas, thereby purifying the other components ； By utilizing the second property of the adsorbent, it is possible for the adsorbent 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. The working principle of pressure swing adsorption: I. Methods of adsorbent regeneration. In order to make adsorption separation a cost-effective process, in addition to the adsorbent having good adsorption properties, the methods used for its regeneration are also of critical importance. The degree of adsorbent regeneration determines the purity of the product and also affects the adsorption capacity of the adsorbent ； The regeneration time of the adsorbent determines the length of the adsorption cycle, and thus also determines the amount of adsorbent required. Therefore, selecting the appropriate regeneration method plays an important role in the industrialization of adsorption separation methods. It is known from the isotherm lines describing adsorption equilibrium that, at a constant temperature, the amount of adsorbate adsorbed on the adsorbent increases as the partial pressure of the adsorbate rises ； At the same adsorbate partial pressure, the amount of adsorbate adsorbed on the adsorbent decreases as the adsorption temperature rises ； In other words, increasing pressure and lowering temperature facilitate the adsorption of the adsorbate, while reducing pressure and raising temperature facilitate the desorption of the adsorbate or the regeneration of the adsorbent. Thus, based on the methods of regenerating the adsorbent, the adsorption and separation cycle process is divided into two categories: temperature-swapped adsorption and pressure-swapped adsorption. 1. Temperature-swapped adsorption: Adsorption takes place at a lower temperature (room temperature or lower), while raising the temperature causes the adsorbed components to desorb. As can be seen from Figure 2-9, isothermally adsorption occurs by moving up and down between two isothermally adsorption lines at different temperatures for adsorption and desorption. Due to the relatively low thermal conductivity of commonly used adsorbents, heating and cooling take a long time (often several hours), which results in larger adsorption beds. Additionally, corresponding heating and cooling facilities are required, leading to high energy consumption and high investment costs. Furthermore, large periodic temperature changes can also affect the lifespan of the adsorbent. However, the temperature-variable adsorption method can be applied in many situations, with minimal product loss and high recovery rates; therefore, it remains a widely used method to this day. 2. Pressure swing adsorption: Adsorption occurs under pressure, while desorption takes place under reduced pressure. Due to the short cycle time, the adsorption heat does not have time to dissipate and can therefore be used for desorption. As a result, the temperature changes in the adsorption bed caused by adsorption heat and desorption heat are generally small, with fluctuations of only a few degrees, allowing it to be approximated as an isothermal process. As can be seen from Figure 2-1, the operating condition of pressure swing adsorption varies only along an isadsorption line. The common pressure-reduction adsorption methods include the following; all of them aim to reduce the partial pressure of the adsorbed components on the adsorbent, thereby enabling its regeneration. a. Pressure reduction: The adsorption bed absorbs at a higher pressure, and then the pressure is reduced to a lower level, usually close to atmospheric pressure; at this point, a portion of the adsorbed substance desorbs. This method is simple to operate, but the desorption of a single adsorbed component is incomplete, resulting in low regeneration efficiency of the adsorbent. b. Vacuum pumping: After the adsorption bed is reduced to atmospheric pressure, vacuum pumping can be used to further lower the pressure in the adsorption bed in order to achieve a better regeneration effect; however, this method increases power consumption. c. Flushing: A weakly adsorbing component or another appropriate gas is used to pass through the adsorption bed that needs to be regenerated; as the flushing gas flows through, the partial pressure of the adsorbed component decreases. The degree of regeneration of the adsorbent depends on the amount and purity of the purge gas. d. Displacement: Using a gas with stronger adsorption capacity to displace the originally adsorbed component from the adsorbent. This method is often used in cases where the product components have strong adsorption capacity while the impurity components do not, that is, to obtain the product from the adsorbed phase. During the pressure swing adsorption process, the regeneration method to be used is determined based on the properties of the various components in the gas to be separated, the requirements for the product, the characteristics of the adsorbent, and the operating conditions; usually, a combination of several regeneration methods is employed. It should be noted that regardless of the regeneration method used, at the end of the regeneration process, the residual amount of the adsorbent in the adsorption bed will not be zero; in other words, it is impossible to completely regenerate the adsorbent in the bed. This residual amount is also not evenly distributed throughout various parts of the adsorption bed. Curve-2 in Figure 2-10 shows the distribution of this residual amount within the bed. Curve-1 is the aforementioned adsorption load curve. The difference in the areas formed by the two curves and the coordinates is known as the effective adsorption capacity of the adsorption bed. An increase in this property is beneficial for adsorption operations. Once the adsorption conditions are determined, the effective adsorption capacity depends on the degree of regeneration of the adsorption bed. From this, it can be seen the importance of regeneration in adsorption operations. II. Basic steps of pressure swing adsorption operation: In the case of operating a single fixed adsorption bed, whether it is temperature swing adsorption or pressure swing adsorption, the adsorption process is intermittent because the adsorbent needs to be regenerated. Therefore, in industry, two or more adsorption beds are used to alternate (or cycle sequentially) between adsorption and regeneration of the beds, ensuring continuity throughout the entire adsorption process. For the pressure swing adsorption cycle, there are three basic operating steps: 1. Adsorption under pressure – The gas mixture to be separated is fed into the adsorption bed at the highest pressure of the process; the components that are strongly adsorbed are selectively absorbed by the adsorbent, while the components with weaker adsorption properties flow out from the other end of the adsorption bed. 2. Pressure-reduced desorption: Depending on the properties of the adsorbed components, several of the methods mentioned earlier—such as reducing pressure, evacuating, flushing, and replacement—are used to regenerate the adsorbent. In general, for pressure-reduced desorption, the pressure is first reduced to atmospheric pressure, and then flushing, vacuuming, or displacement is used. 3. Pressurization: After the adsorbent regeneration is complete, the adsorption bed is pressurized with a weakly adsorbing component until the desired adsorption pressure is reached. Then adsorption is carried out under pressure

Working principle of pressure swing adsorption I. Methods for regenerating the adsorbent In order to make adsorption separation a cost-effective process, in addition to the adsorbent having good adsorption properties, the methods used for its regeneration are of crucial importance. The degree of adsorbent regeneration determines the purity of the product and also affects the adsorption capacity of the adsorbent ； The regeneration time of the adsorbent determines the length of the adsorption cycle, and thus also determines the amount of adsorbent required. Therefore, selecting the appropriate regeneration method plays an important role in the industrialization of adsorption separation methods. It is known from the isotherm lines describing adsorption equilibrium that, at a constant temperature, the amount of adsorbate adsorbed on the adsorbent increases as the partial pressure of the adsorbate rises ； At the same adsorbate partial pressure, the amount of adsorbate adsorbed on the adsorbent decreases as the adsorption temperature rises ； In other words, increasing pressure and lowering temperature facilitate the adsorption of the adsorbate, while reducing pressure and raising temperature facilitate the desorption of the adsorbate or the regeneration of the adsorbent. Thus, based on the regeneration methods of adsorbents, the adsorption and separation cycle process is divided into two categories: temperature-swapped adsorption and pressure-swapped adsorption. Figure 2-9 illustrates the concepts of these two methods, with the horizontal axis representing the partial pressure of the adsorbate and the vertical axis representing the amount of adsorption per unit of adsorbent. The upper line is the isothermal adsorption curve at room temperature, while the lower line is the isothermal adsorption curve at high temperature. 1. Temperature-dependent adsorption: Adsorption occurs at lower temperatures (room temperature or lower), while raising the temperature causes the adsorbed species to desorb. As can be seen from Figure 2-9, isothermally adsorption occurs by moving up and down between two isothermally adsorption lines at different temperatures for adsorption and desorption. Due to the relatively low thermal conductivity of commonly used adsorbents, heating and cooling take a long time (often several hours), which results in larger adsorption beds. Additionally, corresponding heating and cooling facilities are required, leading to high energy consumption and high investment costs. Furthermore, large periodic temperature changes can also affect the lifespan of the adsorbent. However, the temperature-variable adsorption method can be applied in many situations, with minimal product loss and high recovery rates; therefore, it remains a widely used method to this day. 2. Pressure swing adsorption: Adsorption occurs under pressure, while desorption takes place under reduced pressure. Due to the short cycle time, the adsorption heat does not have time to dissipate and can therefore be used for desorption. As a result, the temperature changes in the adsorption bed caused by adsorption heat and desorption heat are generally small, with fluctuations of only a few degrees, allowing it to be approximated as an isothermal process. As can be seen from Figure 2-1, the operating condition of pressure swing adsorption varies only along an isadsorption line. The common pressure-reduction adsorption methods include the following; all of them aim to reduce the partial pressure of the adsorbed components on the adsorbent, thereby enabling its regeneration. a. Pressure reduction: The adsorption bed absorbs at a higher pressure, and then the pressure is reduced to a lower level, usually close to atmospheric pressure; at this point, a portion of the adsorbed substance desorbs. This method is simple to operate, but the desorption of a single adsorbed component is incomplete, resulting in low regeneration efficiency of the adsorbent. b. Vacuum pumping: After the adsorption bed is reduced to atmospheric pressure, vacuum pumping can be used to further lower the pressure in the adsorption bed in order to achieve a better regeneration effect; however, this method increases power consumption. c. Purging: A weakly adsorbing component or another appropriate gas is used to pass through the adsorption bed that needs to be regenerated; as the purge gas flows through, the partial pressure of the adsorbed component decreases. The degree of regeneration of the adsorbent depends on the amount and purity of the purge gas. d. Displacement: Using a gas with stronger adsorption capacity to displace the originally adsorbed component from the adsorbent. This method is often used in cases where the product components have strong adsorption capacity while the impurity components do not, that is, to obtain the product from the adsorbed phase. During the pressure swing adsorption process, the regeneration method to be used is determined based on the properties of the various components in the gas to be separated, the requirements for the product, the characteristics of the adsorbent, and the operating conditions; usually, a combination of several regeneration methods is employed. It should be noted that regardless of the regeneration method used, at the end of the regeneration process, the residual amount of the adsorbent in the adsorption bed will not be zero; in other words, it is impossible to completely regenerate the adsorbent in the bed. This residual amount is also not evenly distributed throughout various parts of the adsorption bed. Curve-2 in Figure 2-10 shows the distribution of this residual amount within the bed. Curve-1 is the aforementioned adsorption load curve. The difference in the areas formed by the two curves and the coordinates is known as the effective adsorption capacity of the adsorption bed. An increase in this property is beneficial for adsorption operations. Once the adsorption conditions are determined, the effective adsorption capacity depends on the degree of regeneration of the adsorption bed. From this, it can be seen the importance of regeneration in adsorption operations. II. Basic steps of pressure swing adsorption operation: In the case of operating a single fixed adsorption bed, whether it is temperature swing adsorption or pressure swing adsorption, the adsorption process is intermittent because the adsorbent needs to be regenerated. Therefore, in industry, two or more adsorption beds are used to alternate (or cycle sequentially) between adsorption and regeneration of the beds, ensuring continuity throughout the entire adsorption process. For the pressure swing adsorption cycle, there are three basic operating steps: 1. Adsorption under pressure – The gas mixture to be separated is fed into the adsorption bed at the highest pressure of the process; the components that are strongly adsorbed are selectively absorbed by the adsorbent, while the components with weaker adsorption properties flow out from the other end of the adsorption bed. 2. Pressure-reduced desorption: Depending on the properties of the adsorbed components, one or more of the methods mentioned earlier—such as reducing pressure, evacuating, purging, and displacing—is used to regenerate the adsorbent. In general, for pressure-reduced desorption, the pressure is first reduced to atmospheric pressure, and then purging, vacuuming, or displacement is used. 3. Pressurization: After the adsorbent regeneration is complete, the adsorption bed is pressurized with a weakly adsorbing component until the desired adsorption pressure is reached. Then adsorption is carried out under pressure. Figure 2-11 shows the relationship between the adsorption load of the adsorbent and pressure in a pressure swing cycle. III. Selection of adsorbents The adsorption capacity of adsorbents for various gas components is evaluated by experimentally determining the isothermal adsorption curves under static conditions and the effusion curves under dynamic conditions. Good adsorption performance of the adsorbent is a fundamental requirement for the adsorption separation process. In the pressure swing adsorption process, the selection of the adsorbent also needs to take into account resolving the conflict between adsorption and desorption. For strong adsorbates such as benzene and toluene, adsorbents with weaker adsorption capacity, like silica gel, must be used in order to maintain an appropriate adsorption capacity, which also facilitates the desorption process. For weakly adsorbed substances such as methane, nitrogen, and carbon monoxide, adsorbents with stronger adsorption capacity, such as molecular sieves, need to be used in order to achieve a higher adsorption capacity. Another key point in selecting an adsorbent is to maximize the separation coefficient between components. The definition of the so-called separation coefficient is as follows: Consider a two-component system in which A is the strongly adsorbed component ； B is a weakly adsorbed component. Their contents are Xa and Xb respectively, while the adsorption coefficients Ka and Kb for components A and B are given by: Ka = qa·T·p0/(v·T0·p·Xa); Kb = qb·T·p0/(v·T0·p·Xb). Here, q represents the volume of component A or B adsorbed by the adsorbent in the bed under standard conditions ； p, T—pressure and temperature under operating conditions ； v is the dead volume of the adsorption bed (i.e., the volume that can be occupied by the fluid within the bed); its value is given by: v = 1/db – 1/dg. The separation coefficient α is then: α = (Ka + 1)/(Kb + 1). In other words, at adsorption equilibrium, the total amount of a certain component in the adsorption bed consists of two parts: one part is in the dead space, and the other part is adsorbed by the adsorbent. The sum of these two amounts represents the retention amount of that component in the adsorption bed ； The ratio of the amount of each, the weakly adsorbed component and the strongly adsorbed component, in the dead space to the amount remaining in the bed is called the separation coefficient. The larger the separation coefficient, the easier the separation. The separation factor between the two components separated by pressure swing adsorption should not be less than 2. Tables 2-2 and 2-3 list the separation coefficients of common major components at atmospheric pressure and 20°C. Table 2-2 Separation coefficients of common components for various adsorbents
Component Adsorbent CH4 CO2 CO CH4N2 CH4N2CO H2 CH4H2CO H2N2
Silica gel 6.40 1.31 1.86 1.42 2.90 2.05 3.8
Activated carbon 2.00 2.07 2.84 1.37 6.97 5.10 14.4

Table 2-3 Separation coefficients of common components for various adsorbents
Component Adsorbent CH4 CO CO2 N2 CH4N2 CH4N2CO H2 CH4H2CO H2N2
5A molecular sieve 1.79 3.15 1.40 2.50 9.65 17.2 6.9
Mordenite 1.18 2.23 1.39 1.65 15.5 18.5 11.2
13X molecular sieve 1.58 4.70 1.52 2.40 8.0 12.6 5.25

Furthermore, during the operation of the adsorption bed, as the pressure within the bed changes periodically and gases enter and exit in short periods of time, the adsorbent must have sufficient strength to minimize fragmentation and wear. If the separated gas contains substances such as organic mechanical lubricants or coal tar, then during the adsorption process, these oily substances will adhere to the outer surface of the adsorption particles, blocking the channels within the adsorbent and causing it to lose its adsorption capacity. Any adsorbent contaminated with oils cannot be regenerated, whether using a regeneration method involving temperature increase or vacuum evacuation. Therefore, the oil content in the gas must be strictly controlled; in some cases, oil removal facilities need to be added to prevent the adsorbents from losing their effectiveness over time.