Absorption of a substance by the surface of the adsorbent is called. Absorption and adsorption apparatuses. Physical and chemical adsorption
Absorption is a separation process based on the selective absorption of gases or vapors by liquid absorbers - adsorbents.
In physical absorption, the absorbed gas (absorptive) does not chemically interact with the absorbent. If the absorbent forms a chemical compound with the absorbent, then the process is called chemisorption.
Physical absorption is reversible. This property of absorption processes is based on the release of absorbed gas from the solution - desorption.
The combination of absorption and desorption makes it possible to reuse the absorbent (absorbent) and isolate the absorbed component in its pure form.
Examples of the use of absorption processes in chemical technology and engineering can be the separation of hydrocarbon gases in oil refineries, the production of hydrochloric acid, ammonia water, the purification of exhaust gases in order to capture valuable products or the neutralization of gas emissions, etc.
adsorption
Adsorption is an increase in the concentration of a solute at the interface of two phases (solid phase-liquid, condensed phase-gas) due to the uncompensated forces of intermolecular interaction at the phase separation. Adsorption is a special case of sorption, the reverse process of adsorption - desorption.
Basic concepts
The absorbed substance, which is still in the bulk of the phase, is called the adsorbate, and the absorbed substance is called the adsorbate. In a narrower sense, adsorption is often understood as the absorption of an impurity from a gas or liquid by a solid (in the case of gas and liquid) or liquid (in the case of gas) - an adsorbent. In this case, as in the general case of adsorption, the impurity is concentrated at the adsorbent-liquid or adsorbent-gas interface. The process, the reverse of adsorption, that is, the transfer of a substance from the interface to the volume of the phase, is called desorption. If the rates of adsorption and desorption are equal, then one speaks of the establishment of adsorption equilibrium. In a state of equilibrium, the number of adsorbed molecules remains constant for an arbitrarily long time if the external conditions (pressure, temperature, and composition of the system) remain unchanged.
physical adsorption
Adsorption is caused by non-specific (that is, not dependent on the nature of the substance) van der Waals forces. Adsorption complicated by chemical interaction between adsorbent and adsorbate is a special case. Phenomena of this kind are called chemisorption and chemical adsorption. "Ordinary" adsorption in the case when it is required to emphasize the nature of the interaction forces is called physical adsorption.
Adsorption is a universal and ubiquitous phenomenon that takes place always and everywhere where there is an interface between phases. Of greatest practical importance is the adsorption of surfactants and the adsorption of impurities from a gas or liquid by special highly effective adsorbents. Various materials with a high specific surface can act as adsorbents: porous carbon (the most common form is activated carbon), silica gels, zeolites, and also some other groups of natural minerals and synthetic substances.
Adsorption (especially chemisorption) is also important in heterogeneous catalysis. An example of adsorption plants is given on page nitrogen plants.
The adsorption plant is called an adsorber.
Crystallization
Crystallization - obtaining (formation) of a substance in a crystalline form. Of the three main cases of crystal formation - during sublimation, from a molten state, from solutions - the last one is of the greatest importance. Usually use:
- 1. Slow evaporation of the solvent.
- 2.. By adding a third substance that mixes with the solvent and reduces the solubility of the crystallized substance in it; a precipitant is added to the solution (usually hot) until turbidity appears and left to stand; for example, water is added to an alcoholic solution, petroleum ether to an ether solution, alcohol to phenol, etc.
- 3. Cooling the saturated hot solution; the substance is dissolved in a suitable solvent with heating and stirring, and the solvent is taken only a little more than is necessary for dissolution, and filtered hot (preferably through a heating funnel); crystals separate out on cooling.
Rest and slow cooling promote the growth of crystals, but the size of the crystals also depends on the nature of the substance. If you want to get more crystals than is possible when cooled to room t °, use a cooling mixture, but it is necessary to use liquids that do not freeze at low t ° as a solvent, for example. carbon disulfide, alcohol, ether, petroleum ether. It is often possible to cause K. of a substance that has been released in the form of an oil, introducing into it (“infecting”) a crystal of this substance, and sometimes even a substance that is close to it in chemical terms. structure. Rubbing the vessel wall with a glass rod also accelerates or causes crystallization. Crystallization is used to purify a substance or obtain it freshly crystallized with a certain amount of a "crystallization" solvent - water, alcohol, chloroform, etc.
In some cases, to isolate a substance in a chemically individual form, they resort to K. of its well-crystallized simplest derivatives: salts, acetyl, benzene, and other derivatives. Very rare is the formation of a well-crystallized double compound of an indifferent organic substance, for example. compounds of glucose and sodium chloride: 2C6H1206 + MaCl + HgO. Some substances, such as proteins, can be obtained in crystalline form by salting out (see). When purifying a substance K. (often repeatedly), they proceed from the assumption that a crystallizing substance can be separated from an impurity due to unequal solubility in a suitable solvent. In some cases, it is possible to obtain a pure substance only by fractionated K. There are cases of mixtures inseparable by crystallization and the formation of mixed crystals. - K. usually carried out in crystallizers - thin-walled low glasses - or in cups. - The crystals obtained are freed from the mother liquor by washing on a Buchner funnel or spread on an unglazed porcelain plate or filter paper, absorbing the mother liquor, and, if necessary, , squeezed between the leaves filtering. paper.
By thickening the mother liquor or by adding a precipitant to it, or by the combined action of both, further portions of crystals can be obtained. When choosing a solvent, it must be borne in mind that it does not chemically affect the substances to be crystallized and does not contain impurities harmful to K. and that in the case of K. by cooling a hot solution, the solubility of a substance in a hot solvent differs quite sharply from the solubility in a cold one. The most common solvents are water, ethyl, methyl and amyl alcohols, ether, benzene, chloroform, acetone, acetic acid, petroleum ether, phenol, pyridine, carbon disulfide, H2S04 and others.
For a microscope, the substance is crystallized on a glass slide, because even with careful transfer to a glass slide, the crystals are damaged. A drop of a concentrated solution of the test substance is applied to a glass slide, covered with a cover slip and left in air or, if the substance is easily deliquescent, in a desiccator, and the formed crystals are examined under a microscope.
If absorption is a physical process that is not accompanied by other physical or chemical processes, it usually follows the Nernst distribution law:
"at equilibrium, the ratio of the concentrations of the third component in two liquid states is a constant.";
The volume of the constant K N depends on the temperature and is called the distribution coefficient. This equality is true provided that the concentrations are not too high and if the "x" molecules do not change their shape in any other of the two states. If such a molecule undergoes association or dissociation, then this equality still describes the equilibrium between "x" in both states, but only for the same form - the concentrations of all remaining forms must be calculated taking into account all other equilibria.
In many technologically important processes, chemical absorption is used instead of a physical process, such as the absorption of carbon dioxide by sodium hydroxide - such processes do not follow the Nernst distribution law.
For some examples of this effect, extraction can be considered, in which a component can be extracted from one liquid phase of a solution and transferred to another without a chemical reaction. Examples of such solutions are noble gases and osmium oxide.
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Synonyms:See what "Sorption" is in other dictionaries:
Sorption- - the general name of the phenomenon and processes of mass transfer, in which the absorption by a solid or liquid (sorbent) of a substance (sorbent) from the environment occurs. [Usherov Marshak A. V. Concrete science: lexicon. Moscow: RIF Building Materials. 2009 ... Encyclopedia of terms, definitions and explanations of building materials
- (from Latin sorbeo I absorb) absorption by a solid or liquid of any substance from the environment. The main types of sorption are adsorption, absorption, and chemisorption. The absorbing body is called a sorbent, absorbed by a sorbate (sorbate). ... ... Big Encyclopedic Dictionary
- (from Latin sorbeo I absorb), TV absorption. body or liquid (sorbent) liquid in va or gas (sorbate) from the environment. Absorption in VA from the gas phase by the entire volume of the liquid sorbent called. absorption, absorption in the surface layer ... ... Physical Encyclopedia
Adsorption, chemisorption, absorption, sorption, absorption, chemical absorption Dictionary of Russian synonyms. sorption noun, number of synonyms: 7 absorption (5) ... Synonym dictionary
sorption- - the ability of one substance to absorb (concentrate) another. General chemistry: textbook / A. V. Zholnin Sorption is the general name for the phenomena and processes of mass transfer in which a solid or liquid absorbs a substance from ... ... Chemical terms
The process of absorption by the whole mass (absorption) or surface (adsorption) of a solid or liquid of substances from the environment. To intercellular interactions (virus cell, macrophage lymphocyte, etc.), the term is applicable in the case of adding ... ... Dictionary of microbiology
The process by which a body absorbs gases, vapors, or solutes from its environment. Includes absorption and adsorption, which may also be accompanied by chem. the interaction of the absorbed substance with the absorber ... ... Geological Encyclopedia
SORPTION- physical. chem. processes of absorption of gases, vapors and dissolved substances by solids or liquids, called (see). Distinguish the following types of S.: (see); (see), (see); capillary (see), as well as ion-exchange S., when selective ... ... Great Polytechnic Encyclopedia
AND; and. [from lat. sorbere absorb] Phys., chem. Absorption by a solid or liquid of some kind. substances from the environment. ◁ Sorption, oh, oh. C th processes. C. pump. * * * sorption (from Latin sorbeo I absorb), absorption by a solid body or ... ... encyclopedic Dictionary
sorption- ▲ absorption in direction, condensed state, out, medium sorption selective absorption by a solid or liquid to l. substances from the environment. sorbent. absorption volumetric sorption. absorbent. absorb. adsorption ... ... Ideographic Dictionary of the Russian Language
Books
- Ion-exchange sorption of biologically active substances, Demin A., Chernova I., Shataeva L. The monograph is devoted to the latest achievements in the field of synthesis of highly permeable polyelectrolyte networks with fractal morphology and high hydrophilic pore surface. Comparison…
Adsorption is a universal method that makes it possible to almost completely extract an impurity from a gaseous or liquid medium. In the chemical industry, in particular in the fuel pump, the adsorption method is widely used for smooth cleaning and drying of process streams, improving the quality of raw materials and products, and is one of the methods for protecting the environment.
Adsorption is the concentration of substances on the surface or in the volume of a solid. At least two components are involved in the adsorption process. A solid substance, on the surface or in the volume of which the absorbed substance is concentrated, is called adsorbent. Absorbed substance in the gas or liquid phase is called adsorbent, and after it has passed into the adsorbed state, adsorbate. Any solid has a surface and therefore is potentially an adsorbent. However, solid adsorbents with a developed inner surface are used in technology. The development of the inner surface in a solid is achieved by creating special conditions during its synthesis or as a result of additional processing.
From a thermodynamic point of view, adsorption manifests itself with a decrease in the Gibbs free energy (G). Like all processes accompanied by a decrease in the Gibbs energy, adsorption is a spontaneous process. The transition of a substance from the gas or liquid phase to the adsorbed state is associated with the loss of at least one degree of freedom (three-dimensional bulk gas or liquid phase two-dimensional surface phase), which leads to a decrease in the entropy of the system (S). Since enthalpy (Н) is related to the Gibbs energy and entropy by the equation Н = G + TS, it decreases during adsorption, and therefore adsorption is an exothermic process.
Adsorption phenomena are divided into two main types: physical adsorption and chemisorption (sorption based on the forces of chemical interaction). physical adsorption caused by the forces of molecular interaction: dispersion and electrostatic. Dispersion forces make the main contribution to the interaction energy of molecules. Thus, the molecules of any adsorptive have fluctuating dipoles and quadrupoles, which cause instantaneous deviations of the electron density distribution from the average distribution. When adsorbent molecules approach adsorbent atoms or molecules, the movement of fluctuating dipoles acquires a systematic and strictly ordered character, which leads to the appearance of attraction between them. In a number of cases, dispersion forces are enhanced by electrostatic forces - orientational and induction. Orientation forces arise during the interaction of polar molecules with a surface containing electrostatic charges (ions, dipoles), and induction forces are caused by a change in the electronic structure of the adsorbent and adsorbent molecules under the action of each other.
Unlike physical adsorption, chemisorption the individuality of the adsorbent and adsorbent is not preserved. When the molecules of the adsorbent approach the surface of the adsorbent, the electrons of the interacting components are redistributed with the formation of a chemical bond. If physical adsorption can be compared with condensation, then chemisorption is considered as a chemical process occurring at the interface.
Physical adsorption and chemisorption can be distinguished based on the numerical value of the heat of adsorption. The heat of adsorption of components of industrial gases is commensurate with the heat of their condensation and does not exceed 85-125 kJ/mol. The heat of chemisorption of one mole of a substance reaches several hundred kJ. Chemisorption, as a rule, proceeds at a low rate; this circumstance is often used to recognize it. In addition, chemisorption can proceed at high temperatures, when physical adsorption is negligible. During chemosorption, a sharp, abrupt change in the absorption capacity of the extracted component is characteristic during the transition from an adsorbent of one chemical nature to an adsorbent of another nature. During chemisorption, the adsorbed molecules cannot move over the surface of the adsorbent, their position is fixed, and such adsorption is called localized. Physical adsorption can be either localized or non-localized. Usually, as the temperature rises, molecules become mobile and the nature of the process changes: localized adsorption becomes non-localized.
Lecture #20
Adsorption call the absorption of gases, vapors and liquids by solid porous bodies, called adsorbents; adsorbed substance in a gas or liquid is called adsorbent, and after its transition to the adsorbent phase, adsorbate. Adsorbents used in practice have a highly developed inner surface (up to 1000 m 2 /g), which is formed by special treatment or synthesis of solid materials.
The mechanism of the adsorption process differs from the mechanism of absorption, since the extraction of a substance is carried out by a solid rather than a liquid absorber.
Adsorption is divided into two types: physical and chemical. physical adsorption is mainly due to surface van der Waals forces, which manifest themselves at distances that are much larger than the sizes of adsorbed molecules; therefore, several layers of adsorbate molecules are usually retained on the surface of the adsorbent. At chemical adsorption the absorbed substance enters into chemical interaction with the adsorbent with the formation of conventional chemical compounds on its surface.
Attractive forces arise on the adsorbent surface due to the fact that the force field of surface atoms and molecules is not balanced by the interaction forces of neighboring particles. According to the physical nature, the forces of interaction between the molecules of the absorbed substance and the adsorbent are mainly dispersive, arising due to the movement of electrons in approaching molecules. In a number of cases of adsorption, electrostatic and inductive forces, as well as hydrogen bonds, are of great importance.
Filling the surface of the adsorbent with an adsorbate partially balances the surface forces and, as a result, reduces the surface tension (free specific surface energy). Therefore, adsorption is a spontaneous process, the course of which is accompanied by a decrease in the free energy and entropy of the system.
Adsorption processes are selective and reversible. The reverse process of adsorption is called desorption, which is used to release absorbed substances and regenerate the adsorbent.
It is most rational to use adsorption for the treatment of mixtures with a low concentration of extractable substances. In this case, the duration of the adsorption apparatus increases - adsorber– at the stage of proper adsorption before its switching to desorption.
Typical examples of adsorption are the drying of gases and liquids, the separation of hydrocarbon mixtures, the recovery of solvents, the treatment of ventilation emissions and wastewater, etc. Recently, the importance of adsorption has increased significantly, especially in connection with the solution of environmental problems and the problems of obtaining highly pure substances.
8.1. Main industrial adsorbents and their properties
The main industrial adsorbents are porous bodies with a large volume of micropores. The properties of adsorbents are determined by the nature of the material from which they are made and the porous internal structure.
In industrial adsorbents, the main amount of the absorbed substance is sorbed on the walls of micropores ( r < 10–9 м). Роль переходных пор (10–9 < r < 10–7 м) и макропор (r> 10–7 m) is mainly reduced to the transport of the adsorbed substance to the micropores.
Adsorbents are characterized by their absorption, or adsorption, ability determined by the maximum possible concentration of the adsorbent per unit mass or volume of the adsorbent. The value of the absorption capacity depends on the type of adsorbent, its porous structure, the nature of the absorbed substance, its concentration, temperature, and for gases and vapors - on their partial pressure. The maximum possible absorption capacity of the adsorbent under given conditions is conventionally called equilibrium activity.
According to the chemical composition, all adsorbents can be divided into carbon And non-carbon. The carbon adsorbents are active(activated) coals, carbon fiber materials, and some solid fuels. Non-carbon adsorbents include silica gels, active alumina, alumina gels, zeolites and clayey rocks.
Activated carbons, consisting of many randomly arranged graphite microcrystals, are usually used to absorb organic substances in the purification and separation of liquids and gases (vapors). These adsorbents are obtained by dry distillation of a number of carbon-containing substances (wood, coal, animal bones, fruit pits, etc.) in order to remove volatiles. After that, the coal is activated, for example, it is calcined at a temperature of 850–900 °C, which leads to the release of pores from resinous substances and the formation of new micropores. Activation is also carried out by extracting resins from pores with organic solvents, oxidation with atmospheric oxygen, etc. A more uniform structure of coals is obtained by activating them by chemical methods: by treating them with hot salt solutions (for example, sulfates, nitrates, etc.) or mineral acids (sulfuric, nitric and etc.).
The specific surface area of activated carbons is very high and amounts to 6×105–17×105 m2/kg, and their bulk density is 200–900 kg/m3. Activated carbons are used in the form of irregularly shaped particles 1–7 mm in size, cylinders 2–3 mm in diameter and 4–6 mm high, and powder with a particle size of less than 0.15 mm. The last type of active carbon is used to separate solutions.
The main disadvantages of activated carbons are their flammability and low mechanical strength.
Silica gel - dehydrated silicic acid gel () - is used to adsorb polar compounds. It is used in the processes of drying gases and liquids, in the separation of organic substances in the gas phase and in chromatography. Silica gel is obtained by treating a solution of sodium silicate (soluble glass) with sulfuric acid (sometimes hydrochloric acid) or with acidic salt solutions. The resulting gel is washed with water and dried to a final moisture content of 5–7%, since silica gel has the highest adsorption capacity at this moisture content. The specific surface of silica gel is 4×105–7.7×105 m2/kg, bulk density is 400–800 kg/m3. The size of particles of irregular shape varies in a fairly wide range - from 0.2 to 7 mm.
The advantages of silica gels include their incombustibility and greater mechanical strength than active carbons. The disadvantage of silica gels compared to active carbons is, in addition to their lower specific surface, a sharp decrease in the absorption capacity with respect to vapors of organic substances in the presence of moisture.
In terms of sorption properties, silica gel is closely adjacent to alumogels obtained by heat treatment of aluminum hydroxide at temperatures of 600–1000 °C. The pores of the obtained sorbent (92%) have a diameter of 1–3 nm, a specific surface area of 2×10 5 –4×10 5 m 2 /kg; the bulk density of such a sorbent is 1600 . Alumogels are used for drying gases, purification of aqueous solutions and mineral oils, used as catalysts and their carriers.
Zeolites are natural or synthetic minerals that are aqueous aluminosilicates containing oxides of alkali and alkaline earth metals. These adsorbents are distinguished by a regular pore structure, the dimensions of which are commensurate with the sizes of absorbed molecules. A feature of zeolites is that the adsorption surfaces are interconnected by windows of a certain diameter, through which only smaller molecules can penetrate. This is the basis for the separation of mixtures with molecules of different sizes, which was the reason for calling zeolites molecular sieves.
For the separation of gas mixtures, zeolites are used in the form of balls or granules ranging in size from 1 to 5 mm, and for the separation of liquid mixtures - in the form of a fine-grained powder.
Zeolites are especially widely used for deep drying of gases and liquids, in the purification and separation of mixtures of substances with similar molecular weights, and also as catalysts and their carriers.
To purify liquids from various impurities, natural clay rocks are used as adsorbents. For their activation, these clays are treated with sulfuric or hydrochloric acids and an adsorbent with a specific pore surface of the order of (1.0÷1.5)·10 5 m 2 /kg is obtained. Also, some types of peat can be used to purify liquids.
Note that adsorbents are also characterized by static and dynamic activity. Under static activity understand the amount of substance absorbed by a unit mass or volume of the adsorbent from the beginning of adsorption to the establishment of equilibrium. This type of activity is determined under static conditions, i. without the movement of a mixture of gases or solution. When the mixture moves through the adsorbent layer, after a certain period of time, the adsorbent ceases to completely absorb the extracted component, and this component "leaks" with a subsequent increase in the concentration of the component in the mixture leaving the layer until equilibrium is reached. The amount of substance absorbed by a unit mass or volume of the adsorbent before the breakthrough is called dynamic activity adsorbent. Dynamic activity is always less than static, so the amount of adsorbent is determined by its dynamic activity.
Equilibrium in adsorption
The equilibrium concentration (kg/kg of pure adsorbent) of the absorbed substance in the adsorbent can be represented as a function of concentration With and temperature T:
or as a function of partial pressure R and temperature T in the case of gas adsorption:
Where With is the adsorbate concentration in the bulk phase, kg/m3; R is the partial pressure of the adsorptive in the bulk phase, Pa.
Between concentration 
adsorbed substance in the gas mixture and its partial pressure R, according to the Clapeyron equation, there is a direct proportionality:
Where R is the gas constant, J/(kg K).
Dependence or at constant temperature is called adsorption isotherm.
Adsorption isotherms are represented by curves whose shape is determined mainly by the nature of the adsorbate and adsorbent and its porous structure. From the whole variety of forms of isotherms for the analysis of adsorption processes, one should distinguish convex and concave (Fig. 8.1). It is important to note that the initial segments of the isotherms are linear.
Equilibrium dependencies are described by a number of empirical and theoretical equations. The most fruitful for describing the equilibrium of adsorption processes was the theory of volume filling of pores, which was the development of the potential theory of adsorption.
Under the adsorption potential A understand the work done by adsorption forces during the transfer of one mole of an adsorbate from an equilibrium gas phase by pressure R on the surface of the adsorption film, the pressure above which is assumed to be equal to the pressure of the saturated vapor of the adsorbate PS under consideration T.
Rice. 8.1. Convex and concave adsorption isotherms
The adsorption potential is expressed by the relation
(8.2)
During adsorption, the volume of micropores V n is filled with an adsorbate, the volume of which can be calculated in terms of the equilibrium adsorption value:
(8.3)
Where M is the molecular weight of the adsorbate; V is the molar volume of the adsorbate.
It has been established that for different substances adsorbed on the same adsorbent, the ratio of adsorption potentials at the same values V n constantly and equally affinity coefficient b, which is the ratio of the molar volumes in the liquid state, or parachores, of a given and standard substance, the value of which is found in the handbook.
For a number of microporous adsorbents, the distribution of various filled areas of adsorption volumes has the form of a Gaussian distribution:
(8.4)
Where W 0 is the total volume of micropores; E is the parameter of the distribution function.
By jointly solving equations (8.2) and (8.3), taking into account the affinity coefficient, an equation was obtained that describes the adsorption isotherms for microporous adsorbents with a homogeneous porous structure (synthetic zeolites):
For adsorbents with complex microporous structures (microporous silica gels, active carbons)
(8.6)
where are the constants characterizing the adsorbent; T- temperature.
Along with relations based on the theory of volume filling of pores, a number of other equations are used to describe the adsorption equilibrium, among which the Langmuir equation is best known.
ADSORPTION(from Latin ad-on, at and sorbeo-absorb), a change (usually an increase) in the concentration of matter near the surface of the phase separation ("absorption on the surface"). In the general case, the cause of adsorption is the uncompensated intermol. forces near this surface, i.e. the presence of adsorption force field. The body that creates such a field is called. adsorbent, in-in, molecules to-rogo can be adsorbed, and d sorb t and in om, already adsorbed. in-in-adsorbate. The reverse process of adsorption, called. desorption.
The nature of the adsorption forces m. very different. If these are van der Waals forces, then adsorption is called. physical, if valence (i.e., adsorption is accompanied by the formation of surface chemical compounds), - chemical, or chemisorption. Distinguish. features of chemisorption - irreversibility, high thermal effects (hundreds of kJ / mol), activated character. Between physical and chem. adsorption, there are many intermediates. cases (eg, adsorption due to the formation of hydrogen bonds). Also possible diff. types of physical adsorption max. universal manifestation of dispersion intermol. forces of attraction, since they are approximately constant for adsorbents with a surface of any chemical. nature (the so-called non-specific adsorption). Phys. adsorption can be caused by electrostatic. forces (mutually between ions, dipoles or quadrupoles); while adsorption is determined by chem. the nature of the adsorptive molecules (the so-called specific adsorption). Means. role in adsorption the geometry of the surface of the section also plays: in the case of a flat surface, they speak of adsorption on an open surface, in the case of a slightly or strongly curved surface, adsorption in the pores of the adsorbent.
In the theory of adsorption, a distinction is made between statics (the adsorbent-adsorbate system is in thermodynamic equilibrium) and kinetics (there is no equilibrium).
Adsorption statics
Because the system is in equilibrium, then the potentials of the adsorbate and adsorbate are the same; the entropy of the adsorbate due to the decrease in the mobility of molecules during adsorption is less than the entropy of the adsorbate. Therefore, with an inert adsorbent, the enthalpy is always negative, i.e. adsorption is exothermic. Taking into account the change in the entropy of the adsorbent can change this conclusion. For example, during sorption by polymers in-in, in which the polymer swells, the entropy of the latter (due to an increase in the mobility of macromolecules) can increase so much that adsorption becomes endothermic. In the future, the article considers only exothermic. adsorption .
There are integral, differential, isosteric. and average heat of adsorption. The integral heat Q is equal to the loss of enthalpy (at V = const - internal energy) when adsorption changes from a 1 to a 2 (in a particular case, maybe a 1 \u003d 0): Q \u003d - (H 2 - H 1) This the value is usually referred to the mass of the adsorbent and is expressed in J/kg.
There is another mechanism leading to additional adsorption of adsorbents below their critical. t-ry on porous adsorbents at relatively high values of p/p s . This is capillary condensation. If a concave adsorbate meniscus is formed in a pore, condensation begins in it at p/p s<1.
Согласно
ур-нию Кельвина:
where is the surface tension of the adsorbate, V is its molar volume, r is the radius of curvature of the meniscus. Capillary condensation leads to a sharp rise in the adsorption isotherm. In this case, the so-called is often (but not always) observed. adsorption hysteresis, i.e. adsorption mismatch. and desorbts. branches of the isotherm. As a rule, this is due to the fact that the shapes of the menisci do not coincide during adsorption and desorption.
Using the potential theory, M.M. Dubinin proposed and developed the theory of volumetric filling of micro-pores (TOZM). It has been postulated that this theory only applies to microporous adsorbents. A feature of such adsorbents, in which the linear dimensions of the pores are r1 nm, is that the entire volume of their pores is "filled" with adsorbents. field. Therefore, during adsorption, they are filled not in layers, but volumetrically. The value in the case under consideration is not adsorption. potential, and up to the sign of the chemical. adsorbate potential, measured from the level of chemical. potential of a normal liquid at the same t-re. The entire set of adsorbent pores is divided into three classes: micropores (r0.6 nm), mesopores (0.6 nm-20 nm) and macropores (r20 nm). Adsorption in micropores occurs according to the TOZM scheme, i.e. volumetrically, in mesopores - by the mechanism of layer-by-layer filling, completed by capillary condensation. Macropores during adsorption. equilibrium play no role.
Introducing the concept of f-tsii distribution of pore volumes on the values of chemical. adsorbate potential in them, M.M. Dubinin and L. V. Radushkevich received the equation for the TOZM adsorption isotherm, which is usually written down as a trace. form: 
where p, E and a 0 are parameters (a 0 \u003d a for p \u003d p s). Temperature dependence a 0:
where = -(da 0 /dT); a 0 0 \u003d a 0 at T \u003d T 0. The parameters n and E are practically independent of t-ry. In most cases, n \u003d 2. Only for cases where the initial heats of adsorption are very large, n\u003e 2. To recalculate adsorption isotherms from one adsorbent to another, it is approximately assumed that E 1 /E 2 P 1 /P \u003d and that a 01 / a 02 V 1 /V 2, where P i is a parachor, V i is the molar volume of the adsorbent.
Using the notion that in a real adsorbent there are pores of different sizes, and introducing the distribution of E values with a dispersion equal to F. Stekli, he proposed a generalization of equation (23), called the Dubinin-Stöckli equation: 
Adsorption kinetics
Adsorption, like any real process, occurs in time. Therefore, a complete theory of adsorption should contain a section on adsorption kinetics. The elementary act of adsorption is carried out almost instantly (the exception is chemisorption). Therefore, the time dependences of adsorption are determined in the main. diffusion mechanism, i.e., the supply of an adsorbent to the place of adsorption. If adsorption on an open surface is not instantaneous, such a process occurs in an external diffusion region; in this case, the laws of diffusion are not specific to adsorption. In the case of porous adsorbents, in addition to ext. diffusion, an important role begins to play vnutr. diffusion, i.e. transfer of the adsorbent in the pores of the adsorbent in the presence of a concentration gradient in them. The mechanism of such transfer may depend on the adsorbate concentration and pore sizes.
There are molecular, Knudsen and surface (Volmer) diffusion. Molecular diffusion is carried out if the length is free. the range of molecules in the pores is less than the pore size, the Knudsen length is if this length exceeds the pore size. During surface diffusion, molecules move along the surface of the adsorbent without transition to the bulk phase. However, the values of the coefficient diffusions are not the same for different diffusion mechanisms. In many cases, it is not possible to establish experimentally exactly how diffusion occurs, and therefore the so-called. effective coefficient. diffusion describing the process as a whole.
Main experimental material on the kinetics of adsorption is the so-called. kinetic curve, i.e. f-tion \u003d a / a equals \u003d f (t) where is the relative adsorption equal to the ratio of the current adsorption value a to a equal to its value at time t. To interpret the kinetic curve in the simplest case, it is assumed that the adsorbent grain has a completely uniform porous structure in volume (this model is called quasi-homogeneous). Means. improvement of the quasi-homogeneous model - the notion that each grain contains regions with larger and finer pores. Diffusion in such a grain is described by two dec. coefficients.
In the case of an open surface, taking the Langmuir model, it is easy to obtain a kinetic. adsorption level. The rate of approach to equilibrium is the difference between the rates of adsorption and desorption. Assuming, as usual in kinetics, that the rates of processes are proportional to the concentrations of reacting substances, we have:
where k ads and k dec are the rate constants respectively. adsorption and desorption. The pressure in the gas phase is assumed to be constant. When integrating this equation from t = 0 to any value of t, we get:
Hence, for f we have:= equal. So we finally have:
where k = k ads + k dec.
The effect of t-ry on the rate of adsorption is expressed by an equation similar to the Arrhenius equation. With increasing t-ry k ads exponentially increases. Because diffusion in the pores of the adsorbent is associated with overcoming activation. barriers, the temperature dependences of k ads and k des are not the same.
Knowledge of diffusion rates is important not only for the theory of adsorption, but also for calculating prom. adsorption processes. In this case, they usually deal not with individual grains of the adsorbent, but with their layers. The kinetics of the process in the layer is expressed by very complex dependencies. At each point of the layer at a given time, the amount of adsorption is determined not only by the type of equation of the adsorption isotherm and the laws of the kinetics of the process, but also by aero- or hydrodynamic. conditions for the flow of gas or liquid around grains. The kinetics of the process in the adsorbent layer, in contrast to the kinetics in a single grain, is called. adsorption dynamics, the general scheme for solving problems is as follows: a system of differentials is compiled. ur-tion in partial derivatives, taking into account the characteristics of the layer, the adsorption isotherm, diffusion characteristics (diffusion coefficient, types of mass transfer over the layer and inside the grains), aero- and hydrodynamic. flow features. Initial and boundary conditions are set. The solution of this system of equations, in principle, leads to the values of the adsorption values at a given point in time at a given point in the layer. As a rule, analytical the solution can be obtained only for the simplest cases; therefore, such a problem is solved numerically with the help of a computer.
In an experimental study of the dynamics of adsorption, a gas or liquid stream with specified characteristics is passed through the adsorbent layer and the composition of the outgoing stream is examined as a function of time. The appearance of the absorbed in-va for a layer called. breakthrough, and the time to breakthrough - the time of the protective action. The dependence of the concentration of this component behind the layer on the time called. output curve. These curves serve as the main experimental material that makes it possible to judge the patterns of adsorption dynamics.
Hardware design of adsorption processes
There are many technologies. adsorption techniques. processes. Widespread cyclic. (periodic) installations with a fixed adsorbent bed, osn. node to-rykh - one or several. adsorbers made in the form of hollow columns filled with granular adsorbent. The gas (or liquid) stream containing the adsorbed components is passed through the adsorbent bed until breakthrough. After that, the adsorbent in the adsorber is regenerated, and the gas flow is sent to another adsorber. Adsorbent regeneration includes a number of stages, of which the main one is desorption, i.e. release of previously absorbed matter from the adsorbent. Desorption is carried out by heating, depressurizing in the gas phase, displacement (for example, with sharp water
