Molecular sieve adsorption

Basic principle: Adsorption refers to the phenomenon and 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 a relatively high density) are called adsorbents, while the substances that are adsorbed (usually gases or liquids with a 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. Chemical adsorption refers to the adsorption process in which a chemical reaction occurs between the adsorbent and the adsorbate, resulting in the formation of compounds on the surface of the adsorbent. Its adsorption process generally proceeds slowly, and the desorption process is very difficult. Active adsorption refers to the adsorption process in which surface complexes are formed between the adsorbent and the adsorbate. Its desorption process is generally also difficult. Capillary condensation refers to the condensation phenomenon that occurs within the pores of a solid adsorbent when it adsorbs vapor. It generally requires heating to be fully regenerated. Physical adsorption refers to the adsorption that occurs through molecular forces between the adsorbent and the adsorbate molecules, namely van der Waals forces. Its characteristics are: no chemical reaction occurs during the adsorption process, the adsorption takes place extremely rapidly, the equilibrium between the various phases involved in the adsorption is established in an instant, and this type of adsorption is completely reversible. The adsorption in PSA hydrogen production units is primarily physical adsorption. In physical adsorption, the ability of various adsorbents to adsorb gas molecules is due to the special structure of the gas molecules at the gas-solid interface. Generally speaking, gas molecules that are only in the gas phase experience the same molecular attractions from all directions, and these molecules are in a state of free motion ； When gas molecules move to the interface between the gas and solid phases (that is, when they collide with the surface of the adsorbent), they are subjected to the attractive forces exerted by molecules in both the solid and gas phases; the force from the solid-phase molecules is stronger. When the molecular energy of the gas molecules is not sufficient to overcome these attractive forces, the molecules get adsorbed on the surface of the solid adsorbent. The gas molecules adsorbed on the surface of a solid adsorbent are also known as the adsorbed phase; their molecular density is much higher than that of the gas phase, and it can generally be close to the density of a liquid. Adsorption force – The attraction exerted by the molecules on the surface of a solid adsorbent on gas molecules in the adsorbed phase can be described by the following formula: Where C1 represents the attraction constant, which is related to the size and structure of the molecules; C2 represents the electromagnetic force constant, which is primarily related to the polarity and instantaneous dipole moment of the molecules; R represents the distance between the molecules. As can be seen from this formula, for different gas components, due to differences in their size, structure, polarity, etc., the adsorbent’s ability to adsorb them as well as its adsorption capacity vary accordingly. Pressure swing adsorption technology makes use of this property of adsorbents. Since the adsorbent has a weak adsorption capacity for hydrogen in the mixed gas but a stronger capacity for other components, various impurities can be adsorbed through a mixed adsorption bed filled with different adsorbents, thereby yielding purified hydrogen. The figure below symbolically shows the order of adsorption strength of different components on the molecular sieve. Adsorption equilibrium refers to the process in which, at a certain temperature and pressure, the adsorbent comes into full contact with the adsorbate, until the distribution of the adsorbate between the two phases reaches equilibrium. During the actual adsorption process, the adsorbate molecules continuously collide with the surface of the adsorbent and are bound in the adsorbed phase by the molecular attraction at the adsorbent surface ； At the same time, the adsorbate molecules in the adsorbed phase continuously obtain energy from the adsorbent molecules or other adsorbate molecules, thereby overcoming molecular attraction and leaving the adsorbed phase ； When the number of molecules entering the adsorbed phase within a certain period equals the number of molecules leaving the adsorbed phase, the adsorption process reaches equilibrium. In the case of physical adsorption, the dynamic adsorption equilibrium is reached very quickly, and at a certain temperature and pressure, for the same adsorbent and adsorbate, the equilibrium adsorption amount is a constant value. Pressure swing adsorption and temperature swing adsorption: According to the theory of adsorption equilibrium, the higher the pressure, the greater the number of gas molecules that strike the surface of the adsorbent per unit time; therefore, a higher pressure results in a larger equilibrium adsorption capacity ； The higher the temperature, the greater the kinetic energy of the gas molecules, and fewer molecules can be held by the attractive forces of the surface molecules of the adsorbent; therefore, the higher the temperature, the lower the equilibrium adsorption capacity. This relationship is described using adsorption isotherms at different temperatures, as shown in the figure below. As can be seen from B→A and C→D in the figure, at a constant temperature, the adsorption capacity increases gradually as pressure rises ； As can be seen from B→C and A→D in the figure above, at a constant pressure, the adsorption capacity gradually decreases as the temperature rises. The working principle of pressure swing adsorption separation is to utilize the properties of the adsorbent in the A→B stage shown in the diagram to achieve the adsorption and desorption of gases. At normal temperature and high pressure (i.e., point A), the adsorbent absorbs large amounts of the impurity components in the feed gas other than hydrogen, thereby producing hydrogen of higher purity. Then, the pressure is reduced (to point B) to allow these various impurities to be desorbed, enabling the adsorbent to be reused. In practical industrial applications, adsorption separation is generally divided into two main categories: pressure swing adsorption and temperature swing adsorption. The adsorption isotherms of the adsorbent show that it has a high adsorption capacity for impurities at high pressures and a low adsorption capacity at low pressures. At the same time, from the adsorption isotherms of the adsorbent, it can also be seen that at the same pressure, the adsorbent has a higher adsorption capacity at low temperatures and a lower adsorption capacity at high temperatures. Adsorption separation utilizing the former property of the adsorbent is called pressure swing adsorption (PSA), while adsorption separation utilizing the latter property is called temperature swing adsorption (TSA). In practical industrial applications, the TSA, PSA, or TSA+PSA process is generally selected based on the composition of the gas source, its pressure, and the requirements of the product. The variable-temperature adsorption process requires heating, which results in long cycle times and high capital costs; however, it achieves thorough regeneration and is typically used for the purification of trace impurities or those that are difficult to desorb ； The pressure swing adsorption process features short cycle times, high utilization of the adsorbent, relatively low amounts of adsorbent required, and no need for external heat exchange equipment; it is widely used for the separation and purification of large volumes of multi-component gases. Pressure swing adsorption: In industrial pressure swing adsorption (PSA) processes, the adsorbent is used to adsorb the components of a mixed gas that are readily adsorbed, under normal temperature and high pressure. The components that are not easily adsorbed flow out from one end of the adsorbent bed. Afterwards, the pressure in the adsorbent bed is reduced, causing the adsorbed components to desorb and be released from the other end of the bed. This process enables the separation and purification of gases, while also allowing the adsorbent to be regenerated.

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