Principle of pressure swing adsorption

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This post was last edited by liuquan1100 on 2017-12-15 at 16:51. Knowledge of Pressure Swing Adsorption: 1. What is adsorption? Answer: When gas molecules move to the solid surface, due to the residual gravitational force of the atoms on the solid surface, some molecules in the gas remain temporarily on that surface. The concentration of these molecules on the solid surface increases, and this phenomenon is known as the adsorption of gas molecules on a solid surface. The solid that adsorbs substances is called an adsorbent, while the substance that is adsorbed is called the adsorbate. Based on the nature of the attractive forces between the adsorbate and the adsorbent, adsorption can be classified into chemical adsorption and physical adsorption. 2. What is the principle of gas separation? When the gas is a mixture, the difference in the attraction exerted by the solid surface on different gas molecules results in a composition of the adsorbed phase that differs from that of the gas phase. This difference in density and composition between the gas phase and the adsorbed phase forms the basis of gas adsorption separation technology. The heat released during the adsorption process is called adsorption heat, while the heat absorbed during the desorption process is called desorption heat. The heat of adsorption of a gas mixture is the sum of the heat of condensation and the heat of wetting of the adsorbate. The adsorption heat of different adsorbents for various gas molecules varies. 3. Chemical adsorption and physical adsorption: Chemical adsorption refers to adsorption that is accompanied by a chemical reaction. In chemical adsorption, the adsorbate molecules react with the surface of the adsorbent to form surface complexes, and the adsorption heat is close to the heat of chemical reaction. Chemical adsorption requires a certain activation energy to occur. Under normal conditions, the adsorption or desorption rate of chemical adsorption is slower than that of physical adsorption. The adsorption of chlorine by limestone and the adsorption of ethylene by zeolites are both cases of chemical adsorption. Physical adsorption: also known as van der Waals adsorption, it is caused by the attractive forces between the adsorbate molecules and the surface molecules of the adsorbent; these forces are also referred to as van der Waals forces. Since the molecules on the solid surface differ from those inside the solid, a residual surface free energy field exists. When gas molecules come into contact with the solid surface, some of them get adsorbed, releasing adsorption heat in the process. Among the adsorbed molecules, only those whose kinetic energy from thermal motion is sufficient to overcome the potential energy of the adsorbent’s gravitational field can return to the gas phase; therefore, many adsorbed molecules remain on the solid surface in contact with the gas. Due to adsorption caused by intermolecular attractive forces, the heat of adsorption is relatively low, close to the heat of vaporization or condensation of the adsorbate; both the adsorption and desorption rates are also relatively fast. The adsorbed gas can also be desorbed from the solid surface relatively easily; therefore, physical adsorption is reversible. Physical adsorption is usually divided into temperature-dependent adsorption and pressure-dependent adsorption. 5. What are the common adsorbents used in pressure swing adsorption? What is their respective role? Common adsorbents used in pressure swing adsorption include silica gel, activated alumina, activated carbon, molecular sieves, etc. There are also adsorbent materials developed for the selective adsorption of specific components. The success of gas adsorption separation depends to a large extent on the properties of the adsorbent; therefore, selecting an appropriate adsorbent is the primary issue in determining the adsorption process. Silica gel is a hard, amorphous polymeric particle with chain-like and network-like structures; its molecular formula is SiO2·nH2O, and it is a hydrophilic, polar adsorbent. It is obtained by treating an aqueous solution of sodium silicate with sulfuric acid to form a gel; after washing away sodium sulfate and drying, glassy silica gel is produced. It is mainly used for drying, as well as for the separation of gas mixtures and petroleum components. Silica gel used in industry is divided into two types: coarse-pored and fine-pored. Under conditions of saturated relative humidity, the adsorption capacity of coarse-pore silica gel can reach over 80% of the weight of the adsorbent; whereas under low-humidity conditions, its adsorption capacity is **lower than that of fine-pore silica gel. Activated alumina is produced by heating and dehydrating aluminum hydrates; its properties depend on the structural state of the initial hydroxide. It is generally not pure Al2O3, but rather a partially hydrated, amorphous porous material that contains not only amorphous gels but also crystals of hydroxides. Due to the high reactivity of its pore channel surface, it is also known as active alumina. It has a strong affinity for water and is an adsorbent used for the deep drying of trace amounts of water. Under certain operating conditions, its drying depth can reach below the dew point of -70°C. Activated carbon is produced by carbonizing and activating carbon-containing materials such as charcoal, fruit shells, and coal. Activation methods can be divided into two main categories: chemical activation and gas activation. The chemical activation method involves adding chemicals such as zinc chloride and potassium sulfide to the raw material, followed by heating in a non-reactive atmosphere to carry out carbonization and activation. The gas activation method involves heating activated carbon raw materials in an inert atmosphere; typically, after removing volatile components at temperatures below 700°C, steam, carbon dioxide, flue gas, air, etc., are introduced, and a reaction is carried out within the temperature range of 700–1200°C to activate the material. Activated carbon contains many capillary pore structures, which gives it excellent adsorption capabilities. Therefore, its applications are widespread in areas such as water treatment, decolorization, and gas adsorption. Zeolite molecular sieves, also known as synthetic zeolites or molecular sieves, have the general chemical formula: O.Al2O3.nSiO2.mH2O. Here, M(Ⅰ) and M(Ⅱ) represent monovalent and divalent metal ions, usually sodium and calcium respectively. n denotes the silica-alumina ratio of the zeolite; silica comes from sodium silicate and silica gel, while aluminum comes from sodium aluminate and Al(HO)3. These substances react with an aqueous solution of sodium hydroxide to form a colloid, which, after drying, becomes zeolite. Generally, n ranges from 2 to 10, and m ranges from 0 to 9. Zeolites are characterized by their molecular sieve properties, featuring uniform pore sizes such as 3A0, 4A0, 5A0, and 10A0 pores. 4A0 zeolite with a 4A0 pore size can adsorb methane and ethane, but not n-alkanes with more than three carbon atoms. It has been widely used in gas adsorption and separation, gas and liquid drying, and the separation of n-alkanes and isoalkanes. Carbon molecular sieves are essentially a type of activated carbon. What sets them apart from ordinary carbon-based adsorbents is that the pore sizes of their micropores are uniformly distributed within a narrow range; these pore sizes are comparable to the diameter of the gas molecules to be separated. The specific surface area of these micropores accounts for more than 90% of the total surface area of the carbon molecular sieve. The pore structure of carbon molecular sieves is mainly organized as follows: large pores have diameters that connect to the outer surface of the carbon particles; transitional pores branch off from the large pores; and micropores branch off from the transitional pores. During the separation process, the macropores mainly serve as transport channels, while the micropores act as molecular sieves. Methods for producing carbon molecular sieves from coal as a raw material include carbonization, gas activation, carbon deposition, and impregnation. Among them, the carbonization method is the simplest, but to produce high-quality carbon molecular sieves, these methods must be used in combination. Carbon molecular sieves have achieved success in the field of air separation for nitrogen production, and they also hold great potential for other gas separations. 7. Temperature-dependent adsorption and pressure-dependent adsorption 1) Temperature-dependent adsorption: Adsorption takes place at lower temperatures (room temperature or lower), while raising the temperature causes the adsorbed substance to desorb. Isothermal adsorption involves adsorption and desorption by moving up and down between two isothermal adsorption lines at different temperatures. 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 enough 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. 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. 8. The feed gas entering the PSA unit must have its water removed; if the separated gas contains free water and saturated water, only a certain amount of the saturated water can be removed in the adsorption bed. Therefore, during the adsorption process, allowing a large amount of water to enter the adsorption tower can lead to a decline in the performance of the adsorbent or even its failure. At this point, the failed adsorbent needs to be reactivated or replaced. The activation of the adsorbent requires the circulation of an inert gas at a temperature of over 150°C, which is time-consuming, labor-intensive, and quite difficult. Therefore, during operation, it is essential to strictly control the water content entering the PSA unit, ensuring that it contains no mechanical water. 9. Must the feed gas entering the PSA unit be deoiled? 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 reduction. Therefore, the oil content in gases must be strictly limited; in some cases, additional oil removal facilities are required to prevent the adsorbent from becoming ineffective during use. 10. Why is it necessary to control the appropriate airflow speed? During the operation of the adsorption bed, as the pressure inside 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. When the airflow velocity is too high, causing the adsorbent to become suspended, wear increases, leading to the fragmentation of the adsorbent and affecting its service life. For different adsorbents, different airflow velocities are selected. This mainly depends on the strength of the adsorbent and the size of the adsorbent particles. 11. What is dead space? The so-called dead space refers to the space remaining within the adsorption bed after deducting the volume occupied by the adsorbent. In other words, at the adsorption equilibrium for a certain component, the total amount of that component within 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 is referred to as 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 factor. The larger the separation coefficient, the easier the separation. 12. What is the impact of adsorbent selection on pressure swing adsorption units? The adsorption performance of the adsorbent for various gas components is evaluated by experimentally determining the isothermal adsorption lines 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 requires taking into account the conflict between adsorption and desorption. It is necessary to choose an adsorbent with high adsorption capacity and easy desorption in order to reduce the power consumption for desorption under reduced pressure. Another key point in selecting an adsorbent is to have as large a separation coefficient between the components as possible, thereby reducing the loss of useful gases. 13. What are the basic working steps of pressure swing adsorption? In single fixed adsorption bed operation, whether it is temperature-swapped adsorption or pressure-swapped adsorption, the adsorption 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 reduction desorption: Depending on the properties of the adsorbed components, several of the aforementioned methods—pressure reduction, vacuum evacuation, flushing, and displacement—are selected 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.) Pressure increase: After the regeneration of the adsorbent is complete, the adsorption bed is pressurized with weakly adsorbable components until the adsorption pressure is reached. Then adsorption is carried out under pressure. 14. How many methods are there for regenerating adsorbents? To make adsorption separation economically viable, in addition to the adsorbent having good adsorption properties, the method for regenerating the adsorbent is of critical importance. The degree of adsorbent regeneration determines the purity of the product and also affects the adsorbent’s adsorption capacity ； The regeneration time of the adsorbent determines the length of the adsorption cycle, which in turn 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 increases ； 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, the adsorption and separation cycle process is divided into two categories according to the regeneration method of the adsorbent: 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. a. Pressure reduction: The adsorption bed adsorbs 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 insufficient, and the degree of regeneration of the adsorbent is low. 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. In 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. Selection of pressure swing adsorbents: The adsorption capacity of adsorbents for various gas components is evaluated by experimentally determining the isotherm lines 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 requires taking into account 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 adsorbable substances such as methane, nitrogen, carbon monoxide, etc., 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 given by: α = (Ka + 1)/(Kb + 1). In other words, at equilibrium under adsorption conditions, 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 factor. 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 factors 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 CH4 N2 CH4 N2CO H2 CH4 H2CO 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 CH4 N2CO H2 CH4 H2CO 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 inside the bed changes periodically and gases enter and exit within a short time frame, 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 reduction. 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