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Basic requirements for adsorption separation courseware: 1. Contents to be mastered: Concepts of physical adsorption and chemical adsorption, characteristics of adsorbents, concepts of adsorption equilibrium, gas phase monolayer physical adsorption isotherms. 2. Familiar content: Classification of adsorbents, adsorption process and adsorption process rate control. 3. What you understand: Two-component adsorption isotherms in the gas phase, adsorption equilibrium in the liquid phase, various adsorption operations and ways to enhance the adsorption process. 9.2.1 Overview of adsorption: When a fluid comes into contact with a porous solid, one or more components in the fluid accumulate on the surface of the solid. This phenomenon is called adsorption. adsorbate, adsorbent: The components accumulated on the solid surface are called adsorbates or adsorbates, and porous solids are called adsorbents. adsorption operation: The method of using certain porous solids to selectively adsorb one or several components in a fluid to separate the mixture is called adsorption operation. It is one of the important unit operations for separating and purifying gas and liquid mixtures. In fact, people have discovered and utilized the adsorption phenomenon very early, such as using charcoal for dehumidification and deodorization in daily life. With the development of new adsorbents and research on adsorption separation process conditions, the adsorption separation process has shown outstanding features such as energy saving, high product purity, removal of trace substances, and low operating temperature, making this process widely used in chemical, pharmaceutical, food, light industry, environmental protection and other industries, such as: (1) Dehydration and deep drying of gas or liquid, such as dehydrating the water in ethylene gas to trace amounts and then polymerizing it. (2) Deodorization and decolorization of gas or solution and recovery of solvent vapor. For example, in the spray painting industry, a large amount of organic solvents often escape. Using activated carbon to treat the discharged gas not only reduces environmental pollution, but also recovers valuable solvents. (3) Adsorption and separation of trace substances in gas, such as the production of pure nitrogen and pure oxygen. (4) Separate certain systems that are difficult to separate by distillation, such as the separation of alkanes, olefins, and aromatic hydrocarbon fractions. (5) Treatment of waste gas and waste water, such as recovering carbon monoxide and carbon dioxide from blast furnace waste gas, and removing phenol and other harmful substances from refinery waste water. Classification of adsorption Physical adsorption: Also known as van der Waals adsorption, it is an adsorption in which the adsorbate and adsorbent are dominated by intermolecular forces. chemical adsorption: It is an adsorption in which the adsorbent and adsorbent are mainly based on chemical bonds between molecules. 9.2.2 Adsorbents and their characteristics 1. The effect of adsorbent adsorption and separation depends largely on the performance of the adsorbent. Industrial adsorption requires the adsorbent to meet the following requirements: (1) It has a large inner surface and a large adsorption capacity ； (2) Highly selective adsorbents have different adsorption capacities for different adsorbates. The more significant the difference, the better the separation effect. ； (3) Have certain mechanical strength and wear resistance ； (4) It has good physical and chemical stability, thermal shock resistance and corrosion resistance. ； (5) Easy to regenerate ； (6) Easy to obtain and cheap. Adsorbents can be divided into two categories. One is natural adsorbents, such as diatomaceous earth, clay, natural zeolite, etc. The other type is artificial adsorbents, which mainly include activated carbon, activated alumina, silica gel, synthetic zeolite molecular sieves, organic resin adsorbents, etc. The following is an introduction to several widely used artificial adsorbents. (1) Activated carbon Activated carbon is the most commonly used adsorbent. It has a non-polar surface, a large specific surface area, good chemical stability, acid and alkali resistance, high thermal stability, and easy regeneration. After carbonization, synthetic fibers can be made into activated carbon fiber adsorbents, which can increase the adsorption capacity dozens of times. Because activated carbon fibers can be woven into various fabrics, the fluid flow resistance is reduced. Activated carbon can also be processed into carbon molecular sieves, which have the function of molecular sieves and are often used in air separation and nitrogen production, improving the smell of beverages, and cigarette filters. (2) Silica gel The molecular formula of silica gel is usually expressed as SiO2·nH2O. Its specific surface area reaches 800 m2/g. There are four types of industrial silica gel: spherical, amorphous, processed and powdered. Silica gel is a hydrophilic polar adsorbent with obvious selectivity for unsaturated hydrocarbons, methanol, moisture, etc. Mainly used for drying gases and liquids and dehydrating solutions. (3) Activated alumina Activated alumina is a polar adsorbent with strong adsorption capacity for moisture. Its specific surface area is about 200~500 m2/g. Using different raw materials and under different process conditions, activated alumina with different structures and properties can be produced. Activated alumina is mainly used for drying gases and dehydrating liquids, such as gasoline, kerosene, aromatics and other chemical products. ； Drying of gases such as air, helium, hydrogen, chlorine, hydrogen chloride and sulfur dioxide. (4) Synthetic zeolite molecular sieve Zeolite molecular sieve refers to the crystal of aluminosilicate metal salt. It is a highly polar adsorbent and has great affinity for polar molecules, especially water. Its specific surface area can reach 750 m2/g, and it has strong selectivity. It is often used in the separation of petroleum fractions, drying of various gases and liquids, etc., such as separating paraxylene from mixed xylene and separating oxygen from the air. (5) Organic resin adsorbent Organic resin adsorbent is a polymer material. It can be made into highly polar, weakly polar, non-polar, neutral, and is widely used in wastewater treatment, vitamin separation, and hydrogen peroxide refining. 2. Adsorbent properties: The adsorbent has good adsorption characteristics, mainly because it has a porous structure and a large specific surface area. The basic properties related to the pore structure and specific surface area are introduced below. (1) Density 1) Packing density B (also called volume density) refers to the mass of adsorbent per unit filling volume. Usually, the dried adsorbent is put into a measuring cylinder and shaken until the volume remains unchanged. At this time, the ratio of the mass of the adsorbent to the volume occupied by the adsorbent is called the packing density. 2) Apparent density P (also called particle density) is defined as the mass of the adsorbent particle itself per unit volume. 3) The true density t refers to the mass of the adsorbent per unit volume after deducting the pore volume within the particle. (2) Specific surface area of the adsorbent The specific surface area of the adsorbent refers to the adsorption surface area per unit mass of the adsorbent, m²/g. The pore size of the adsorbent pores directly affects the specific surface area of the adsorbent. The pore size can be divided into three categories:: Macropores, transitional pores, and micropores. The specific surface area of the adsorbent is mainly the surface area provided by the micropores, and is often measured by the gas phase adsorption method. (3) Adsorption capacity Adsorption capacity refers to the adsorption capacity when the adsorbent is full of adsorbate (the mass of adsorbate adsorbed per unit mass of the adsorbent). It reflects the adsorption capacity of the adsorbent. The amount of adsorption can be measured by observing the change in volume or mass of the adsorbate before and after adsorption. It can also be measured by observing changes in the solid surface of the adsorbent using an electron microscope. 9.2.3 Adsorption equilibrium equilibrium adsorption capacity: When the temperature and pressure are constant, the adsorbent contacts the fluid for a long time, the adsorption capacity no longer increases, and the adsorption phase (adsorbent and adsorbed adsorbate) reaches equilibrium with the fluid. The adsorption capacity at this time is the equilibrium adsorption capacity. adsorption isotherm: The adsorption equilibrium relationship is commonly expressed as the relationship between the equilibrium adsorption amount and the adsorbate partial pressure or concentration at different temperatures. The relationship curve is called an adsorption isotherm. 1． Gas phase adsorption isotherm (1) Gas phase single-component adsorption equilibrium 1) Monolayer physical adsorption Assuming that the adsorbent surface is uniform, there is no interaction between the adsorbed molecules, and the adsorbent only forms a uniform monomolecular layer on the surface of the adsorbent, then the adsorption amount increases smoothly with the increase of the adsorbent partial pressure. * * Weigh the amount of adsorption. For example, at -193°C, the adsorption isotherm of nitrogen on activated carbon is shown as I in Figure 9-6. 2) Multi-molecular layer adsorption assumes that the adsorbed molecules are arranged hierarchically on the adsorbent, the force between the adsorbed molecules is negligible, the adsorbed molecules can be stacked, and the adsorption of each layer obeys the Langmuir adsorption mechanism. This adsorption is multi-molecular layer adsorption. For example, the adsorption isotherm of water vapor on activated carbon at 30°C is shown in II in Figure 9-6. 3) Adsorption isotherms under other circumstances Some people believe that adsorption is caused by capillary condensation. The adsorption isotherms are shown in Figure 9-6 as III, IV, and V in Figure 9-6. (2) Gas phase two-component adsorption When the adsorption performance of the adsorbent on the two components in the mixed gas is similar, it can be considered as two-component adsorption. In this case, the adsorption capacity of a certain component by the adsorbent is not only related to temperature and pressure, but also changes with changes in the composition of the mixture. Generally, an increase in temperature and a decrease in pressure will cause the adsorption capacity to decrease. Figure 9-7 reflects the use of graphite carbon to adsorb CFCl3-C6H6 mixed gas and the effect of gas phase composition on the adsorption capacity. It can be seen that the relationship between the mole fraction of a certain component in the adsorption phase and the gas phase is very similar to the relationship between the mole fraction of a certain component in the gas and liquid phases in distillation. Therefore, some people use the adsorption separation coefficient to describe the adsorption equilibrium, is defined as where - are the mole fractions of component B in the gas phase and adsorption phase respectively. It can be seen that the greater the deviation of the adsorption separation coefficient from 1, the more conducive it is to adsorption separation. 2. Adsorption equilibrium in the liquid phase (1) Single-component adsorption equilibrium in the liquid phase When the adsorption of the solvent in the solution by the adsorbent is ignored, it constitutes the adsorption of a single component in the liquid phase, such as using activated carbon to adsorb organic matter in an aqueous solution. Giles et al. classified the liquid-phase single-component adsorption isotherms into four types: S, L, H, and C based on the slope of the initial part of the isothermal adsorption curve, and each type is divided into 5 families, as shown in Figure 8-8. The abscissa in the figure is the concentration of the component in the liquid phase, and the ordinate is the adsorption amount of the component. The S-type indicates that the adsorbed molecules are adsorbed in a vertical orientation on the adsorbent surface. L-type adsorption is Langmuir adsorption, which means that the adsorbed molecules are in a parallel state on the surface of the adsorbent. H-type adsorption is a high-affinity adsorption between the adsorbent and the adsorbate. Type C is an adsorption in which the adsorbate has a certain distribution ratio between the solution and the adsorbent. (2) Adsorption equilibrium of two components in the liquid phase. After the solution containing adsorbates A and B is in contact with fresh adsorbent for a long time, the adsorption amount no longer increases and the adsorption reaches equilibrium. The adsorption isotherm curve in this case is generally U-shaped or S-shaped. The U-shaped curve is a curve in which the adsorbent always preferentially adsorbs one component during the adsorption process. For example, if -Al2O3 is used to adsorb CH3Cl-benzene solution, CH3Cl is preferentially adsorbed. Type S is a situation where the solute and solvent adsorption amounts are equivalent. For example, carbon black is used to adsorb an ethanol-benzene solution. When the ethanol mole fraction is in the range of 0 to 0.4, ethanol is preferentially adsorbed, and in the range of 0.4 to 1, benzene is preferentially adsorbed. 9.2.4 Control of adsorption process and adsorption rate The adsorption rate is an important basis for designing the adsorption device. adsorption rate: It refers to the adsorption capacity per unit time when the fluid contacts the adsorbent, kg/s. ] The adsorption rate is related to the material system, operating conditions and concentration. When the material system and operating conditions are certain, the adsorption process includes the following three steps:: (1) The adsorbate is transferred from the fluid body to the outer surface of the solid adsorbent in the form of convective diffusion. This process is called external diffusion. (2) The adsorbate enters the micropores of the adsorbent from the outer surface of the adsorbent, and then diffuses to the inner surface of the solid. This process is internal diffusion. (3) The adsorbate is adsorbed by the adsorbent on the inner surface of the solid, which is called surface adsorption process. Usually adsorption is physical adsorption, and the surface adsorption rate is very fast, so the total adsorption rate mainly depends on the internal and external diffusion rate. Outdiffusion controlled adsorption: When the external diffusion rate is smaller than the internal diffusion rate, the total adsorption rate is determined by the external diffusion rate, and this adsorption is external diffusion-controlled adsorption. Internal diffusion controlled adsorption: When the internal diffusion rate is smaller than the external diffusion rate, the adsorption is internal diffusion-controlled adsorption, and the total adsorption rate is determined by the internal diffusion rate. 9.2.5 Adsorption operation The adsorption separation process includes adsorption process and desorption process. There are many forms of adsorption operations due to the different concentrations, properties and degree of adsorption required of the fluids to be treated. 1． Contact filtration operation This operation is to add the liquid to be processed and the adsorbent into the adsorption tank with a stirrer, so that the adsorbent and the solution are fully contacted, and the adsorbent in the solution is adsorbed by the adsorbent. After a period of time, the adsorbent reaches saturation, and the slurry is sent to the filter. The adsorbent is filtered out from the liquid phase. If the adsorbent is available, it is properly desorbed and recycled. Because during the contact adsorption operation, stirring is used to make the solution turbulent and the membrane resistance on the outer surface of the particles is reduced, the operation is suitable for the mass transfer process controlled by external diffusion. The equipment used in contact filtration and adsorption operations mainly includes kettle type or tank type. The equipment has a simple structure and is easy to operate. It is widely used in activated carbon to remove color from sugar liquid. 2. Fixed bed adsorption operation Fixed bed adsorption operation is to stack the adsorbent evenly on the porous support plate in the adsorption tower. The fluid containing adsorbates can flow through the adsorbent from top to bottom or from bottom to top. During the adsorption process, the adsorbent does not move. Usually the adsorption process and the regeneration process of the fixed bed are carried out alternately in two tower equipment, as shown in Figure 9-9. • means the valve is closed, and o means the valve is open. The adsorption is carried out in adsorption tower 1. When the concentration of adsorbates in the fluid exiting the tower is higher than the specified value, the material is switched to adsorption tower 2. At the same time, adsorption tower 1 uses temperature change or pressure reduction methods to regenerate the adsorbent, and then adsorbs in tower 1 and regenerates in tower 2, and so on. The fixed bed adsorption tower has a simple structure, easy processing, convenient and flexible operation, the adsorbent is not easy to wear, there is less back-mixing of materials, high separation efficiency, and good recovery effect. Therefore, the fixed bed adsorption operation is widely used in the recovery of solvents in gas, gas drying, and solvent dehydration. However, the heat transfer performance of the fixed bed adsorption operation is poor, and when the adsorbent particles are small, the pressure drop of the fluid passing through the bed is large. Since adsorption, regeneration, cooling and other operations require a certain amount of time, the production efficiency is low. 3． Moving bed adsorption operation Moving bed adsorption operation means that the fluid to be treated flows from top to bottom in the tower. When in contact with the adsorbent, the adsorbent is adsorbed. The saturated adsorbent is continuously or intermittently discharged from the bottom of the tower, and at the same time, fresh or regenerated adsorbent is replenished in the upper part of the tower. Compared with fixed bed, moving bed adsorption operation has low equipment investment costs because the adsorption and regeneration processes are carried out in the same tower. 4. Fluidized bed adsorption operation and fluidized bed-moving bed combined adsorption operation. Fluidized bed adsorption operation is an adsorption operation that makes the fluid flow from bottom to top. The flow rate of the fluid is controlled within a certain range to ensure that the adsorbent particles are lifted up but not taken out and are in a fluidized state. The production capacity of this operation is large, but the wear of the adsorbent particles is severe, and the operating range is narrowed due to the limitation of fluidization. The fluidized bed-moving bed combined adsorption operation integrates adsorption and regeneration into one tower, as shown in Figure 9-10. The upper part of the tower is a multi-layer fluidized bed, where the raw materials are in full contact with the fluidized adsorbent. The adsorbed adsorbent enters the moving bed with a heating device in the middle of the tower, and after heating up, enters the regeneration section at the bottom of the tower. In the regeneration section, the adsorbent is regenerated by countercurrent contact with the inert gas. Finally, it is transported to the top of the tower by pneumatic force and re-enters the adsorption section. The regenerated fluid can recover the adsorbate through the cooler. Fluidized bed-moving bed combined adsorption bed is often used in the recovery of solvents in mixed gas, removal of CO2 and water vapor, etc. This operation has the characteristics of continuous and good adsorption effect. Since adsorption is carried out in a fluidized bed and heating is required before regeneration, this operation has the problem of severe adsorbent wear and easy aging and degeneration of the adsorbent. 5. The adsorption operation of the simulated moving bed takes into account the advantages of good filling performance of the fixed bed and continuous operation of the moving bed, and keeps the adsorption tower operating under isothermal conditions to facilitate automatic control. A tower composed of many small tower sections is designed. Each tower section has an inlet and outlet for materials. A special multi-channel (such as 24-channel) rotary valve is used. Microcomputer control is used to regularly open and close the valves of the feed liquid and desorbent of the adsorption tower, so that the feed liquid inlet and outlet of each layer are continuously changed and connected to four main pipes. These four main pipes are the feed (A+B) pipe, the extraction liquid (A+D) pipe, the raffinate (B+D) pipe and the desorbent (D) pipe, see Figure 9-11. Generally, the entire adsorption tower is divided into four sections: The adsorption section, the first distillation section (referred to as the first distillation section), the desorption section and the second refinement section are shown in the schematic diagram 9-12 of the simulated moving bed adsorption separation operation. What is carried out in the adsorption section is the adsorption of component A. The mixed liquid flows from bottom to top and comes into counter-current contact with the adsorbent that has adsorbed desorbent D. Components A and D undergo adsorption exchange. As the fluid flows upward, adsorbate A and a small amount of B are continuously adsorbed, and D is continuously desorbed. The solution at the outlet of the adsorption section is mainly components B and D, which are discharged from the outlet of the adsorption section as raffinate. The refining of component A and the desorption of component B are completed in a refining section. The descending adsorbent at the top of this section is in contact with the fresh solution, and components A and B are adsorbed. The adsorbent that has adsorbed a large amount of A and a small amount of B at the bottom of this section is in counter-current contact with the fluid (A + D) flowing in from the upper part of the desorption section. Since the adsorbent has a stronger adsorption capacity for A than component B, a small amount of B on the adsorbent is replaced by A, and component B is gradually replaced, and A is refined. The desorption of component A is completed in the desorption section. The adsorbent that adsorbs a large amount of A is in counter-current contact with the fresh desorbent D introduced from the bottom of the tower. A is desorbed as the extractant, and then enters the distillation tower for rectification to obtain product A and desorbent D. The purpose of the second fine section is to partially recover D and reduce the amount of desorbent. The adsorbent containing only desorbent D from the desorption section is sent to the second fine section to contact the solution mainly containing B from the adsorption section in counter-current contact. B and D are replaced on the adsorbent. Component B is adsorbed, D is desorbed, and enters the adsorption section together with fresh desorbent to form a continuous cycle operation. Judging from the above operation, the relative movement of fluid from bottom to top and solid from top to bottom in the opposite direction is formed in the adsorption tower. Each short section of the bed is a small stationary fixed bed. The entire adsorbent solid and fluid in the adsorption tower move continuously. This is a simulated moving bed adsorption and separation process. The application of simulated moving bed adsorption to separate mixtures first started with the separation of para-xylene from mixed xylene, and was later applied to the separation of n-alkanes from kerosene fractions, and the separation of hexylbenzene from C8 aromatics, etc. It solved the difficulty of separation of some systems using methods such as distillation and extraction. 9.2.6 Strengthening of the adsorption process The strengthening of the adsorption process can be started from two aspects. One is to develop and improve the adsorbent, and the other is to develop a new adsorption process. 1. Modification of adsorbents and the development of new adsorbents. The adsorption effect and the scale of the adsorption process are closely related to the performance of the adsorbent. Although there are many types of adsorbents, practical adsorbents are limited. Various adsorbents with different properties can be obtained through modification or grafting methods. The industry hopes to develop adsorbents with large adsorption capacity, strong selectivity, and easy regeneration. At present, the small adsorption capacity of most adsorbents limits the processing capacity of the adsorption tower, causing the adsorption process to frequently undergo adsorption, desorption, and regeneration. Recently developed newer adsorbents such as carbon molecular sieves, activated carbon fibers, metal adsorbents and various special adsorbents have solved the shortcomings of small adsorption capacity and weak selectivity to varying degrees, making it possible to separate certain organic isomers, heat-sensitive substances, and mixtures with similar properties. 2. Develop new adsorption separation processes. With the development of food, medicine, fine chemicals and biochemical industries, it is necessary to develop new adsorption separation processes. The need to improve and enlarge the adsorption process has become an important issue. The adsorption separation process is related to the regeneration and desorption method, and the regeneration method depends on factors such as the adsorption performance of the components on the adsorbent and the size of the feed. With the continuous development of various adsorbents with good performance, the adsorption separation process has also developed rapidly, such as large-scale industrial chromatography adsorption separation to produce carotene, lutein and chlorophyll. The rapid pressure swing adsorption process is used to manufacture oxygen for aviation high-altitude aircraft. Parametric pump adsorption separation is used to separate hemoglobin-albumin systems, enzymes and treat phenol-containing wastewater.