ADSORPTION (from the Latin ad - on and sorbeo to absorb), the absorption of a substance from a gas phase or liquid solution by a surface layer of a solid or liquid.
The phenomenon of adsorption is caused by the presence of adsorption force field created due to the uncompensated intermolecular forces in the surface layer. A substance that creates such a field is called an adsorbent, a substance whose molecules can be adsorbed is called an adsorbate, and an already adsorbed substance is called an adsorbate. The reverse process of adsorption is desorption.
Adsorption - special case sorption. The use of adsorption processes dates back to the end of the 18th century, when three publications appeared independently and almost simultaneously: the Italian naturalist F. Fontana and C. Scheele described the absorption of gases by charcoal in 1777; acids, absorbing organic impurities.
Physical adsorption is caused by forces of molecular interaction. The main contribution to the interaction energy is made by dispersion forces. Their value is approximately constant for adsorbents of any chemical nature; therefore, the interaction caused by these forces is nonspecific. Sometimes the dispersion interaction is enhanced by electrostatic - orientation and induction. Electrostatic interaction depends on the chemical nature of the adsorbent, therefore, is specific. Specific interaction can be enhanced by the formation of hydrogen bonds between adsorbed molecules and polar groups located on the surface of the adsorbent (for example, hydrogen bonds are formed during the adsorption of water and alcohols on silica gel, the surface of which is covered with hydroxyl groups). The heat of physical adsorption is, as a rule, 8-25 kJ/mol. Physical adsorption can be reversed by lowering the pressure of the gas or the concentration of the solute. Physical adsorption does not cause changes in the individual properties of the adsorbate molecules. Absorption of a substance may be due to the formation chemical bond between adsorbate molecules and the surface layer of the adsorbent. This absorption is called chemisorption. Chemisorption is irreversible, its heat is over 80 kJ/mol. During chemisorption, adsorbate molecules form surface chemical compounds with adsorbent.
Equilibrium adsorption. If the rates of adsorption and desorption are equal, then this indicates the establishment of adsorption equilibrium. Curves of dependence of equilibrium adsorption on the concentration or pressure of the adsorptive at a constant temperature are called adsorption isotherms. The simplest adsorption isotherm is a straight line starting from the origin of coordinates, where the adsorbate pressure p (or concentration c) is plotted on the abscissa axis, and the adsorption value a is plotted along the ordinate axis. This region of adsorption is called the Henry region: a = Gr, Г is the Henry coefficient.
I. Langmuir proposed (1914-1918) the theory of monomolecular localized adsorption (adsorbate molecules do not move over the surface) under the following assumptions: the surface is homogeneous, that is, all adsorption centers have the same affinity for adsorbate molecules; adsorbate molecules do not interact with each other. The Langmuir equation has the form: a = a = a max bp / (1 + bp) or p = a / b (a max - a), where a is the amount of adsorbed substance, and max is the limiting value of adsorption in a dense monolayer, p - adsorptive pressure, b - adsorption coefficient. Polymolecular, or multilayer, adsorption, in which vapor molecules, being adsorbed, form a film several monolayers thick, is described by the Brunauer - Emmett - Teller equation (BET equation, 1938):
where p 0 - pressure saturated steam at the adsorption temperature, C is a constant. The BET equation is used to determine the specific surface area of adsorbents.
In 1914, M. Polanyi proposed a potential theory of adsorption, according to which, near the surface of the adsorbent, there is a potential adsorption field that decreases with distance from the surface; the pressure of the adsorptive, which is equal to p far from the surface, increases near it and at a certain distance reaches the value p 0 at which the adsorptive condenses.
Adsorbents are usually divided into non-porous (the radii of curvature of the surfaces of which are very large and tend to infinity) and porous. Porous adsorbents contain micro-, supermicro-, meso- and macropores (see Porosity). In macropores, adsorption is extremely low; it is usually not taken into account when assessing the adsorption properties of adsorbents. Feature adsorption in micro- and supermicropores - an increase in the energy of adsorption compared to the absorption of a substance on a non-porous adsorbent of the same chemical nature. This effect is the result of superposition of the surface force fields of opposite pore walls. In micro- and supermicropores, adsorption occurs volumetrically, in mesopores - according to the mechanism of layer-by-layer filling, completed by capillary condensation.
For microporous adsorbents, M. M. Dubinin developed the theory of volumetric filling of micropores (TOZM). Having introduced the concept of the distribution function of pore volumes according to the values of the chemical potential of the adsorbate in them, Dubinin and L. V. Radushkevich obtained (1947) the adsorption isotherm equation, which is written as: W / W 0 = exp [- (A / βE 0) 2 ], where W and W 0 are the current and limit values of vapor adsorption per unit volume, A is the differential molar work of adsorption, A = RTln(p 0 /p), R is the universal gas constant, T is the absolute temperature, E 0 - the characteristic adsorption energy of a standard vapor (usually benzene or nitrogen), β is the similarity coefficient, approximated by the ratio of the parachors of the adsorbed and standard substances.
The Dubinin-Radushkevich equation is applicable to describe adsorption isotherms in the range of relative equilibrium pressures from 5×10 -4 to 0.4 on adsorbents with a homogeneous microporous structure, that is, adsorbents in which there are no supermicropores. Since microporous adsorbents are most widely used in adsorption technology, TOZM is used not only in physicochemical studies, but also in engineering calculations.
Kinetics and dynamics of adsorption. The elementary act of adsorption is carried out almost instantly. Therefore, the time dependences of adsorption are limited mainly by the mechanism of substance diffusion to the site of adsorption. Diffuse processes are determined by the concentration of the adsorbent, temperature, the chemical nature and porous structure of the adsorbent, and the concentration of other substances in the volume and on the surface. Adsorption in the pores proceeds much more slowly than on the open surface. Adsorption from liquid solutions occurs at a slower rate than from gas mixtures. The dependence of the adsorption value on time is called the adsorption kinetic curve.
The kinetics of adsorption in a gas stream is studied using single adsorbent granules and a layer one granule thick. In practice, adsorbent layers are usually used, the thickness of which significantly exceeds a layer of one grain, that is, adsorption is studied under dynamic conditions. When studying the dynamics of adsorption, a gas or liquid stream containing adsorbed substances is passed through the adsorbent layer, and the increase in the concentration of the adsorbed substance behind the adsorbent layer is measured as a function of time. The appearance behind the layer of the absorbed substance is called a breakthrough, the time before the breakthrough is called the time of the protective action. The dependence of the concentration of this component behind the layer on time is the output curve, from the analysis of which complete information about the efficiency of the adsorption process is obtained.
Technological design of adsorption processes. Installations with a fixed adsorbent bed are widespread, the main unit of which is adsorbers - hollow columns filled with adsorbent. A gas or liquid stream containing adsorbed components is passed through the mixture (adsorbent layer) until the adsorbent breakthrough; the flow is then directed to another adsorber. The target components absorbed by the mixture are recovered by regenerating the adsorbent (heating the adsorber, displacement with water vapor, etc.). Adsorption units with a fluidized (“boiling”) adsorbent bed are characterized by high productivity, in which the gas flow enters the adsorber from below, bringing the adsorbent into a suspended state, which reduces the time of adsorption and desorption. Installations with a moving adsorbent bed are used. In them, the adsorbent slowly descends under the action of gravity; The gas stream containing vapors of adsorbed substances enters the middle part of the adsorber and moves up to the adsorbent. In the upper part of the column adsorption continuously takes place, in the lower part - regeneration of the adsorbent. The so-called short-cycle plants are widely used: during adsorption, gas is supplied to the adsorber under significant pressure, desorption occurs due to pressure relief, then the pressure is raised again.
Substances with a developed surface are used as adsorbents: activated carbons, silica gels, aluminum oxide, zeolites; from non-porous adsorbents - carbon black (soot) and highly dispersed SiO 2 (aerosil). See also sorbents.
Adsorption in nature and technology. Adsorption plays an important role in many natural (for example, soil enrichment, the formation of secondary ore deposits) and biological (functioning cell membranes) processes. Adsorption technologies are widely used for purification, drying, separation of gas and liquid mixtures: purification of industrial emissions and wastewater, including emissions nuclear power plants, detoxification of polluted soils, conditioning drinking water, separating oils, extracting precious metals from solutions and pulps, obtaining oxygen-enriched air, and purifying medicines. Adsorbents are used as fillers in the production of polymers, carriers in catalysis, in chromatography, and also in medicine to extract harmful substances that have entered the gastrointestinal tract of the body (enterosorption) or to purify blood (hemosorption). The phenomenon of adsorption is used in dyeing fabrics, in the printing and food industries, in radio-electronic engineering, etc.
Lit.: Brunauer S. Adsorption of gases and vapors. M., 1948. T. 1; Bur Ya de. Dynamic nature of adsorption. M., 1962; Dubinin M. M. Adsorption and porosity. M., 1976; he is. Current state theory of volumetric filling of micropores of carbon adsorbents//Izvestia of the Academy of Sciences of the USSR. Ser. chemical. 1991. No. 1; Keltsev NV Fundamentals of adsorption technology. 2nd ed. M., 1984; Zhukhovitsky A. A., Shvartsman L. A. Physical chemistry. 5th ed. M., 2001.
Basic concepts
The absorbed substance, which is still in the volume of the phase, is called adsorbent, absorbed - 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) - 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 that is 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 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) are unchanged.
adsorption and chemisorption
At the interface between two phases, in addition to adsorption, which is mainly due to physical interactions (mainly van der Waals forces), there can be chemical reaction. This process is called chemisorption. A clear distinction between adsorption and chemisorption is not always possible. One of the main parameters by which these phenomena differ is the thermal effect: for example, the thermal effect of physical adsorption is usually close to the heat of adsorbate liquefaction, while the thermal effect of chemisorption is much higher. Moreover, unlike adsorption, chemisorption is usually irreversible and localized. An example of intermediate options that combine the features of both adsorption and chemisorption is the interaction of oxygen on metals and hydrogen on nickel: at low temperatures they are adsorbed according to the laws of physical adsorption, but as the temperature rises, chemisorption begins to occur.
Similar phenomena
In the previous section, we discussed the case of a heterogeneous reaction occurring on a surface—chemisorption. However, there are cases of heterogeneous reactions throughout the volume, and not just on the surface; this is the usual heterogeneous reaction. Absorption over the entire volume can also take place under the influence of physical forces - this case is called absorption.
physical adsorption
| Physical adsorption models | |
| Monolayer formation | energy diagram |
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| Rice. one: a) adsorbent, b) adsorbate, c) adsorbent (gas phase or solution) | Rice. 2: a) adsorbent, b) adsorbate, c) gas phase, d - distance, E - energy, E b - adsorption energy, (1) desorption, (2) adsorption |
| Polycondensation | Selective adsorption |
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| Rice. 3: a) adsorbent, b) adsorbate, c) condensate, d) adsorbent (gas phase or solution) | Rice. 4: a) adsorbent, b) adsorbate, c) adsorbents (gas phase or solution): predominant adsorption of blue particles is shown |
Adsorption is caused by non-specific (that is, substance-independent) van der Waals forces. Adsorption complicated by chemical interaction between adsorbent and adsorbate is special occasion. 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.
Physical adsorption is a reversible process, the equilibrium condition is determined by equal adsorption rates of adsorbate molecules P on vacant sites of the adsorbent surface S* and desorption - release of the adsorbate from the bound state S-P:
;the equilibrium equation in this case:
, ,where is the proportion of the adsorbent surface area occupied by the adsorbate, is the Langmuir adsorption coefficient, and P is the adsorbate concentration.
Since and, respectively, , the adsorption equilibrium equation can be written as follows:
The Langmuir equation is a form of the adsorption isotherm equation. The adsorption isotherm equation (the abbreviated term adsorption isotherm is more often used) is understood as the dependence of the equilibrium adsorption value on the adsorbate concentration a=f(С) at a constant temperature ( T=const). The concentration of the adsorbent for the case of adsorption from a liquid is expressed, as a rule, in mole or mass fractions. Often, especially in the case of adsorption from solutions, the relative value is used: C / C s , where C is the concentration, C s is the limiting concentration (saturation concentration) of the adsorptive at a given temperature. In the case of adsorption from the gas phase, the concentration can be expressed in units of absolute pressure, or, which is especially typical for vapor adsorption, in relative units: P/P s , where P is the vapor pressure, P s is the pressure saturated vapors this substance. The adsorption value itself can also be expressed in units of concentration (the ratio of the number of adsorbate molecules to total number molecules at the interface). For adsorption on solid adsorbents, especially when considering practical problems, the ratio of the mass or amount of absorbed substance to the mass of the adsorbent is used, for example, mg/g or mmol/g.
Adsorption value
Adsorption is a universal and ubiquitous phenomenon that takes place always and everywhere where there is an interface between phases. Greatest practical value has adsorption of surfactants and adsorption of impurities from gas or liquid by special highly effective adsorbents. Various materials with a high specific surface area can act as adsorbents: porous carbon (the most common form is activated carbon), silica gels, zeolites, and some other groups of natural minerals and synthetic substances.
An adsorption plant is called an adsorber.
see also
- Nitrogen adsorption plants
Notes
Literature
- Frolov Yu. G. Course of colloid chemistry. Surface phenomena and disperse systems. - M.: Chemistry, 1989. - 464 p.
- Keltsev NV Fundamentals of adsorption technology. - M.: Chemistry, 1984. - 592 p.
- Greg S., Sing K. Adsorption, surface area, porosity. - M.: Mir, 1984. - 310 p. *
- Adamson A. Physical chemistry of surfaces. – M.: Mir. 1979. - 568 p.
- Oura K., Lifshits V. G., Saranin A. A. et al. Introduction to surface physics / Ed. V. I. Sergienko. - M.: Nauka, 2006. - 490 p.
- Karnaukhov A.P. Adsorption. Texture of dispersed and porous materials. - Novosibirsk: Science. 1999. - 470 p.
- Chemical encyclopedia. T. 1. - M.: Soviet Encyclopedia, 1990. - 623 p.
- Poltorak O.M. Thermodynamics in physical chemistry. - M.: graduate School, 1991. - 319 p.
Links
- // Encyclopedic Dictionary of Brockhaus and Efron: In 86 volumes (82 volumes and 4 additional). - St. Petersburg. , 1890-1907.
ADSORPTION(from Latin ad-on, at and sorbeo-absorb), a change (usually an increase) in the island 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. , in-in, to-rogo can be adsorbed, and d s r b t and in o m, already adsorbed. in-in-adsorbate. The reverse process of adsorption, called. .
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. Distinguish. features - irreversibility, high thermal effects (hundreds of kJ /), activated character. Between physical and chem. adsorption, there are many intermediates. cases (eg, adsorption due to the formation). Also possible diff. types of physical adsorption max. universal manifestation of dispersion intermol. forces of attraction, since they are approximately constant for any chem. nature (the so-called non-specific adsorption). Phys. adsorption can be caused by electrostatic. forces (mutually between, dipoles or quadrupoles); while adsorption is determined by chem. the nature of the adsorptive (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 pores.
In the theory of adsorption, statics (the adsorbent-adsorbate system is in thermodynamic) and kinetics (no) are distinguished.
Adsorption statics
Because the system is in equilibrium, then the potentials of the adsorbate and adsorbate are the same; the adsorbate due to the decrease in mobility during adsorption is less than the adsorbate. Therefore, when inert, it is always negative, i.e. adsorption is exothermic. Accounting for change can change this conclusion. For example, when in-in, in which it swells, the latter (due to an increase in mobility) 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 (at V \u003d 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 value usually referred to as mass and expressed in J/kg.
There is another mechanism leading to additional adsorption of adsorbents below their critical. t-ry on porous at relatively high values of p/p s . This is - . If a concave adsorbate is formed in a pore, then it begins 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. 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 adsorption forms and do not coincide.
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. The peculiarity of such, 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. at the same t-re. The entire set of 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 - according to the mechanism of layer-by-layer filling, completed. Macropores during adsorption. 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 idea that in reality 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 the kinetics of adsorption. Elemental adsorption is carried out almost instantly (the exception is chemisorption). Therefore, the time dependences of adsorption are determined in the main. mechanism, i.e., the supply of the adsorbent to the place of adsorption. If adsorption on an open surface is not instantaneous, such a process occurs in an external diffusion region; the laws are not specific to adsorption. In the case of porous , except for ext. , an important role begins to play ext. , i.e. adsorptive transfer in pores in the presence of a gradient in them. The mechanism of such transfer may depend on the adsorbate and pore sizes.
Distinguish between molecular, Knudsen and surface (Volmer). Molecular is carried out if the length is free. run in the pores smaller size pores, Knudsen - if this length exceeds the pore size. At the surface, they move along the surface without transition to the bulk phase. However, the values of the coefficient not the same for different mechanisms. In many cases, it is not possible to establish experimentally exactly how it happens, and therefore the so-called. effective coefficient. 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 value of adsorption a to a equal to its value at time t. To interpret the kinetic curve in the simplest case, it is assumed that the 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. in such a grain is described by two decomp. 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 is the difference between the adsorption rates and . Assuming, as usual in kinetics, that the rates of the processes are proportional to the reacting substances, we have:
where k ads and k dec - resp. adsorption and . in the gas phase is considered 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 in the pores is associated with overcoming activation. barriers, the temperature dependences of k ads and k des are not the same.
Knowing the rates is important not only for the theory of adsorption, but also for calculating the prom. adsorption processes. In this case, they usually deal not with individual grains, 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 point in 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 a layer, in contrast to the kinetics in a separate grain, is called. adsorption dynamics, the general scheme for solving problems is as follows: a system of differentials is compiled. ur-tions in partial derivatives, taking into account the characteristics of the layer, the adsorption isotherm, diffusion characteristics (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 the experimental study of the dynamics of adsorption, a gas or liquid stream with specified characteristics is passed through the 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 this component behind the layer on time is 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 bed, osn. node to-rykh - one or several. , made in the form of hollow columns filled with granular. The gas (or liquid) stream containing the adsorbed components is passed through the bed until breakthrough. After that, they are regenerated, and the gas flow is sent to others. includes a number of stages, of which the main one is desorption, i.e. the allocation of previously absorbed in-va from. carried out by heating, blowing in the gas phase, displacement (eg, sharp water), or a combination of these methods. Since the times of adsorption and do not coincide, they select such a number of simultaneously working and regenerated ones that, on the whole, the process goes on continuously.
According to tech. and economic considerations are not carried through. Therefore, the working capacity
1. ADSORPTION, ADSORPTION PROCESSES
Adsorption- the process of absorption of gases (vapors) or liquids by the surface of solids (adsorbents). The phenomenon of adsorption is associated with the presence of attractive forces between the molecules of the adsorbent and the absorbed substance. Compared to other mass transfer processes, adsorption is most effective in the case of a low content of extractable components in the initial mixture.
There are two main types of adsorption: physical and chemical(or chemisorption). Physical adsorption is caused by the interaction forces of the molecules of the absorbed substance with the adsorbent (dispersion or van der Waals). However, molecules, in contact with the surface of the adsorbent, saturate its surface, which worsens the adsorption process. Chemical adsorption is characterized by chemical interaction between the medium and the adsorbent, which can form new chemical compounds on the surface of the adsorbent. Both types of adsorption are exothermic. However, if the heat of physical adsorption of industrial gases and vapors is commensurate with their heat of condensation (85-125 kJ / kmol), and in the case of solutions even less, then the heat of chemical adsorption reaches several hundred kilojoules per kilomol. Chemical adsorption usually proceeds at a low rate and is possible at high temperatures, when physical adsorption is negligible.
The transition of a substance from the gas and liquid phases to the adsorbed state is associated with the loss of one degree of freedom, i.e., it is accompanied by a decrease in the entropy and enthalpy of the system, hence, the release of heat. In this case, differential and integral heats of adsorption are distinguished; the first expresses the amount of heat released when absorbing a very small amount of substance (2 g/100 g of adsorbent), the second - when absorbing until the adsorbent is completely saturated. The temperature increase in each adsorption process depends on the heat of adsorption and the mass velocity of the gas (vapor) flow, on the thermal diffusivity of this flow and the adsorbent, the amount of the adsorbed substance and its concentration. Since the adsorption capacity of the adsorbent decreases with increasing temperature, the exothermicity of the process must be taken into account in engineering calculations. With large heat releases, they resort to cooling the adsorbent layer.
Adsorption processes are selective and reversible, allowing one or more components to be absorbed (adsorbed) from gas (vapor) mixtures and solutions, and then, under other conditions, to isolate (desorb) them from the solid phase. At the same time, selectivity depends on the nature of the adsorbent and adsorbed substances, and the limiting specific amount of the absorbed substance also depends on its concentration in the initial mixture and temperature, and in the case of gases, also on pressure.
Adsorbents are porous bodies with a highly developed pore surface. The specific surface of the pores can reach 1000 m 2 /g. Adsorbents are used in the form of tablets or balls ranging in size from 2 to 6 mm, as well as powders with a particle size of 20 to 50 microns. Activated carbon, silica gel, aluminosilicates, zeolites (molecular sieves), etc. are used as adsorbents. An important characteristic of adsorbents is their activity, which is understood as the mass of the adsorbed substance per unit mass of the adsorbent under equilibrium conditions. The activity of the adsorbent is
a=M/G,(1)
where M- mass of absorbed components; G is the mass of the adsorbent.
Adsorbents are also characterized protective action time which is understood as the time during which the concentration of absorbed substances at the exit from the adsorbent layer does not change. With a longer time of operation of the adsorbent, a breakthrough of the absorbed components occurs, associated with the exhaustion of the activity of the adsorbent. In this case, regeneration or replacement of the adsorbent is necessary.
Achievement of equilibrium between the solid and mobile gas phase -
mi corresponds to the absorption of the maximum amount of the substance. The equilibrium conditions are described as a dependence of the absorbing capacity (amount of substance M, absorbed by a unit mass or volume of the adsorbent) on temperature T and concentration With absorbed substance in the equilibrium mobile phase, i.e. M = f(T, C). Usually, the conditions of adsorption equilibrium are studied at a constant temperature. Dependence M = f(G) called adsorption isotherm. The specific form of this dependence is determined by the properties and mechanism of interaction between the adsorbent and the adsorbed substance.
Due to the variety of adsorbents and adsorbed substances unified theory adsorption has not yet been developed. The regularities of adsorption processes, in which the van der Waals forces of attraction play a decisive role, can be satisfactorily described by the so-called potential theory of adsorption. According to this theory, a polymolecular adsorption layer is formed on the surface of the adsorbent, the energy state of the molecules in which is determined by the value of the adsorption potential, which is a function of the distance from the surface, and does not depend on temperature. The adsorption potential has the greatest value on the surface of the adsorbent. The potential theory is applicable to adsorption processes on adsorbents, the pore sizes of which are commensurate with the sizes of absorbed molecules. In such cases, not layer-by-layer, but volumetric filling of pores occurs.
To describe the process of monomolecular adsorption, the most widely used is the Langmuir theory, according to which, due to uncompensated forces at the surface atom or adsorbent molecule, the adsorbed molecule is retained for some time for some time.
surfaces. Adsorption occurs at special points on the surface - adsorption centers. The material flows involved in the processes of adsorption and desorption contain transportable and "inert" components. The first refers to substances that pass from one phase to another, and the second - those that do not participate in such a transfer. In the solid phase, the "inert" component is the adsorbent.
The rate of the adsorption process depends on the conditions of transport of the adsorbed substance to the adsorbent surface (external transfer), as well as on the transfer of the adsorbed substance inside the adsorbent grains (internal transfer). The external transfer rate is determined by the hydrodynamic conditions of the process, and the internal transfer rate is determined by the structure of the adsorbent and the physicochemical properties of the system.
2. DEVICE OF ADSORBENTS AND SCHEMES OF ADSORPTION PLANTS
Adsorption processes are carried out mainly in the following ways: 1) with a fixed adsorbent bed;
2) with a moving layer of adsorbent;
3) with a fluidized adsorbent bed.
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 a gaseous or liquid phase 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 influence 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.