When a fluid contacts a porous solid, one or more components in the fluid accumulate on the solid surface, which is called adsorption.Adsorption also refers to the phenomenon that the molecules or ions in the surrounding medium (liquid or gas) are adsorbed on the surface of a substance (mainly a solid substance).
The formation of a layer of atoms or molecules on the surface of a liquid or gas.The adsorbed atoms or molecules are often firmly adsorbed by chemical bonds, that is, chemisorption.In chemical adsorption, the adsorbed layer is usually a thin layer as thick as a molecule.Adsorption can also occur through weak physical forces, that is, physical adsorption, which usually forms several molecular layers.[1]
Adsorption is a mass transfer process. The molecules inside the substance and the surrounding molecules attract each other, but the molecules on the surface of the substance, in which the force relative to the outside of the substance is not fully exerted, can absorb other liquids or gases on the surface of liquid or solid substances, especially when the surface area is large, this adsorption force can have a great effect,Therefore, in industry, a large area of materials are often used for adsorption, such as activated carbon, water film, etc.
Adsorption is the adsorption of gas or solute on the surface of solid or liquid.The adsorption due to the action of chemical bond is chemisorption.asNickel catalystFor hydrogen adsorption, chemical bonds are generated and destroyed in the chemical adsorption process. The adsorption heat absorbed or released is relatively large, and the activation energy required is also large. It needs to be carried out under high heat and selectively.Physical adsorption is determined byIntermolecular forceAdsorption due to interaction.For example, the physical adsorption of activated carbon on gas is generally carried out at low temperature, with fast adsorption speed, low adsorption heat and no selectivity.[2]
AdsorbentsAdsorbent: The components accumulated on the solid surface are called adsorbents orAdsorbatePorous solid is called adsorption.
Broadly speaking, it refers to the absorption of gas or liquid on the solid surface.The solid is called adsorbent, and the adsorbed substance is called adsorbate.According to the intermolecular relationship between adsorbate and adsorbent surfaceBinding forceThe nature of can be divided intophysical adsorptionAnd chemisorption.Physical adsorption is caused by the intermolecular attraction between adsorbate and adsorbent, and the binding force is weak,Heat of adsorptionSmall, easyDesorption, such as the adsorption of activated carbon on gas.Chemisorption is caused by the chemical bond between the adsorbate and the adsorbent, as ifchemical reactionThe adsorption is often irreversible, and the adsorption heat is usually large.In chemical production, adsorption refers to the treatment of fluid mixture with solid adsorbent to adsorb one or more components on the solid surface so as to separate the components of the mixtureMass transfer and separation processOfUnit operationPhysical adsorption is mainly involved.Adsorption separation is widely used in chemical, petroleum, food, light industry and environmental protection sectors.
Adsorption classification
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physical adsorption
also known asVan der WaalsAdsorption, which is the adsorbate and adsorbentIntermolecular forceMainly adsorption.Physical adsorption, which is strictly defined as the enrichment and concentration of a component in the phase boundary layer region.The force of physical adsorption is the van der Waals gravity between the solid surface and the gas molecule, as well as between the adsorbed molecule and the gas molecule, including electrostatic force and dispersion force.The physical adsorption process does not produce chemical reaction, electron transfer, atomic rearrangement and the destruction and formation of chemical bonds.Due to the weak effect of intermolecular gravity, the structure of adsorbate molecules has little change.During the adsorption process, the substance does not change its original nature, so the adsorption energy is small, and the adsorbed substance is easy to be separated again, such as using activated carbonAdsorbed gasAs long as the temperature is raised, the adsorbed gas can be expelled from the surface of activated carbon.
chemical adsorption
Chemisorption refers to the adsorption of adsorbate and adsorbent mainly based on intermolecular chemical bond, which refers to the adsorption caused by chemical interaction between adsorbent and adsorbate and the formation of chemical bond. In the process of adsorption, there is not only gravity, but also the force of chemical bond, so the adsorption energy is large, and the adsorbedMaterial needsHigh temperature, and even if the adsorbed material is expelled, it has also producedChemical changeIt is no longer the original material. Generally, catalysts work in this way.
Difference between two kinds of adsorption
Physical adsorption and chemical adsorption are not isolated and often occur together.In sewage treatment technology, most of the adsorption is often the result of the combined action of several adsorption.Due to the influence of adsorbate, adsorbent and other factors, some kind of adsorption may play a leading role.
The adsorption generated under the action of chemical bond force is chemisorption.Chemical adsorption can only occur under certain conditions, such asinert gasNo chemisorption.If the valence bond of the surface atom has formed saturated bond with the adjacent atom, chemisorption cannot be generated.During chemical adsorption, the chemical bond force plays a role, and its force is much larger than van der Waals' gravity, so the adsorption potential well is deeper and the action distance is shorter.In the process of chemisorption, electron transfer occurs between gas atoms and surface atoms.In fact, the chemisorption process usually does not occur when the molecules are dissociated in advance.Physical adsorption is similar to the condensation of molecules on the surface, and it has no selectivity.Due to the van der Waals attraction between molecules in the adsorbed phase and molecules in the gas phase, multiple adsorption layers can be formed.[3]
Fundamentals
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When the liquid or gas mixture is in full contact with the adsorbent for a long time, the system reaches equilibrium. The equilibrium adsorption capacity of the adsorbate (the adsorption mass of the adsorbent per unit mass when reaching the adsorption equilibrium) first depends on the chemical composition and physical structure of the adsorbent, and at the same time, it depends on the temperature and pressure of the system and the concentration orpartial pressureofFor mixtures containing only one adsorbateFunctional relationIs calledadsorption isotherm。For the gas mixture with low pressure, the inert component has little effect on the adsorption isotherm;The solvent of liquid mixture usually affects the adsorption isotherm.The adsorption isotherms of the same system change with temperature.The higher the temperature, the smaller the equilibrium adsorption capacity.When the mixture contains several adsorbates, the equilibrium adsorption capacity of each component is different, and the concentration ratio of each component adsorbed is generally different from that of the original mixture, that is, the separation factor (see mass transfer separation process) is not equal to 1.The better the selectivity of the adsorbent, the more conducive to adsorption and separation.
The separation of mixtures containing only one adsorbate is the simplest process.When the content of adsorbate in the raw material is very low and the equilibrium adsorption capacity is quite large, the adsorbate can be completely adsorbed once the mixture contacts the adsorbent.The adsorbent is recycled after desorption and regeneration, and adsorbate products are obtained at the same time.However, some situations often encountered in industry are that the mixed materials contain several kinds of adsorbates, or the selectivity of the adsorbent is not high, and the balance adsorption capacity is not large. If the mixture only contacts the adsorbent once, it cannot meet the separation requirements, or if the amount of adsorbent is too large, multi-level or differential contact must be usedMass transfer equipment。
Adsorption separation
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The method of using some porous solids to selectively adsorb one or several components in the 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.
Examples of adsorption and separation:
(1) Dehydration and deep drying of gas or liquid, such as removing the moisture in ethylene gas to trace amount, and then polymerization.
(2) Deodorization and decolorization of gas or solution, and recovery of solvent vapor, such as in the paint industry, often have a large number ofOrganic solventThe discharged gas is treated by activated carbon, which not only reduces environmental pollution, but also recycles valuable solvents.
(3) Adsorption and separation of trace substances in gas, such as the preparation of pure nitrogen and pure oxygen.
(4) Separation of some systems difficult to be separated by distillation, such as alkanesolefin, aromatic hydrocarbonfractionSeparation of.
The indicators for evaluating adsorption and separation are:
(1) Recovery rate of adsorbate (when adsorbate is valuable material) or purification rate of adsorbate (when adsorbate is harmful impurity).
(2) The operating strength of the equipment, that is, the unit equipment volume can handlemixed gas Or the flow rate of the solution.
(3) Energy consumption, including energy consumption for transporting materials and adsorbents, and heat consumption for temperature rise during desorption.
The equilibrium adsorption capacity andAdsorption selectivityIt has a decisive impact on the above indicators of adsorption operation. The selection of adsorbent with large balance adsorption capacity and high adsorption selectivity can significantly improve the economy of the process.In addition, the amount of adsorbent and the operating temperature and pressure have an important impact on the above indicators and must be carefully determined.
Equipment type
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(1) Adsorption tank.Stirring tank for adsorption operation, such as refining with activated clay in the adsorption tankOils Or liquid sugar.
(3) Fluidized bed adsorption equipment.Adsorbent onFluidizationAdsorption is carried out under the state, such as removal from tail gas of nitric acid plant by fluidized bedOxides of nitrogen。When higher recovery rate of adsorbate is required, multi-layer can be usedFluidization equipment。Fluidized bed adsorption is easy to operate continuously, but materialBackmixingAnd adsorbent are seriously worn.
(4) Moving bed adsorption column.also calledSupersorptionColumn, used as moving bed mass transfer equipment in adsorption, was once used as an intermediate plant for separating olefins.
