Adsorption gas purification

The method is based on the ability of some solids to selectively absorb gaseous components from gas mixtures. Present in gas mixture Contaminated gas or vapor molecules collect on the surface or in the pores of a solid material. The substance absorbed from the gas phase is called adsorbent, and a solid substance on the surface or pores of which the adsorption of an absorbed substance occurs - adsorbent. The gas phase in which the extracted component is located - the carrier gas, and after the extracted component has passed into the adsorbed state, it is called adsorbate.

Apply in this case:

1) when other methods are ineffective;

2) the concentration of pollutants is very low and guaranteed recovery of the extracted impurity is required due to its significant cost or danger. Using the adsorption method, SO 2, hydrocarbons, chlorine, hydrogen sulfide, carbon disulfide, and others are removed from exhaust gases.

The phenomenon of adsorption is due to the presence of attractive forces between the molecules of the adsorbent and the adsorbent at the interface between the contacting phases. The transfer of pollutant molecules from the carrier gas to the surface layer of the adsorbent occurs if the attractive forces of the adsorbent are greater than the attractive forces acting on the adsorbent from the carrier gas molecules. The molecules of the adsorbed substance, moving to the surface of the adsorbent, reduce its energy, resulting in the release of heat, approximately 60 kJ/mol (small). The forces of attraction are different - physical or chemical and, therefore, are distinguished:

Physical adsorption - in which the interaction of molecules of pollutants with the surface of the adsorbent is determined by weak dispersed, inductive forces(Van der Wals forces). In this case, the adsorbed molecules do not enter into chemical interaction with the molecules of the adsorbent and retain their individuality.

Physical adsorption is characterized by a high process speed, low bond strength and low heat. With increasing temperature, the amount of physically adsorbed substance decreases, and an increase in pressure leads to an increase in the amount of adsorption. The advantage is easy reversibility of the process by:

a) reducing pressure

b) increase in temperature. Adsorbed molecules are easily desorbed without change chemical composition, and the regenerated adsorbent can be used repeatedly. The process can be carried out cyclically, alternating the stage of absorption and release of the extracted component.

Chemical adsorption - is based on the chemical interaction between the adsorbent and the adsorbed substance. The forces acting in this case are much greater, and the released heat coincides with the heat of the chemical reaction and amounts to 20 - 400 kJ/mol.

Main differences:

)adsorbent molecules, having easily entered into chemical interaction, are firmly held on the surface and in the pores of the adsorbent;

) reaction speed, at low temperatures small, but increases with increasing temperature.

Both types of adsorption accompany each other, however, highest value for gas purification has physical adsorption.

Industrial adsorbents

Any solid has a surface and, therefore, is potentially an adsorbent.

In technology, adsorbents with a highly developed internal surface obtained as a result of (sintering), synthesis and special processing are used.

Adsorbents must have:

− large dynamic capacity (time protective action);

− large specific surface area;

− selectivity;

− thermal and mechanical stability;

− ability to regenerate;

− ease of manufacture;

− low cost;

These are activated carbons, silica gels, zeolites, clay minerals, porous glasses and others.

Adsorption capacity of adsorbents (activity)

It is used to determine the size of the devices and the efficiency of gas purification.

There are static and dynamic capacities of the adsorbent. Dimension [grams of absorbed substance/per 100g. adsorbent or mol/g]

Static capacity shows how much of a substance an adsorbent can adsorb under equilibrium conditions.

The dynamic capacity corresponds to the absorbed substance by the adsorbent layer from the beginning of the process until the beginning of the “breakthrough” of the adsorbent, i.e. when traces of adsorbent appear in the carrier gas leaving the adsorbent layer.

The adsorption capacity depends on the nature of the substance, it increases with an increase in surface area, porosity, and a decrease in pore size. It increases: with increasing concentration of pollutants in the carrier gas; pressure in the system. As temperature and humidity increase, the adsorption capacity decreases, so they are dried before use. A good adsorbent does not lose activity when performing hundreds and thousands of cycles.

Adsorption gas purification is most effective when processing large volumes of gases with low impurity content, for example, for fine purification of process gases from sulfur compounds and carbon dioxide, as well as when removing vapors toxic substances and carcinogens. The most appropriate use is when it is necessary to reduce the content of impurities to several parts per million or even lower, for example, pollutants with a strong odor can be detected when their content in the air is about 100 ppb, so it is necessary to reduce the concentration even lower.

The effectiveness of adsorption systems is determined mainly by the properties of the adsorbent, which must:

− have high adsorption capacity;

− have high selectivity;

− have high mechanical strength;

− regenerates well;

− have a low cost.

Adsorbents are divided into three groups:

) non-polar solids on the surface of which physical adsorption occurs.

) polar - chemical adsorption occurs without changing the structure of gas molecules and the surface of the adsorbent.

) substances on the surface of which purely chemical adsorption occurs and which desorb gas molecules after a chemical reaction, requiring their replacement.

The most common non-polar adsorbent is activated carbon, consisting of neutral atoms of the same type and having a surface with uniform distribution charges at the molecular level.

Issued:

) for domestic ventilation systems AG, KAU, SKT. Granule size 1 - 6 mm, ρ n =380 - 600 kg/m3.

recovery coals AR, ART, SKT - 3.

) molecular sieve coals MSC.

The amount of gas adsorbed by 1 g of adsorbent in an equilibrium state depends on the nature of the adsorbent and adsorbate, as well as on temperature and pressure. Dependence of the mass (m) of adsorbed pollutants on the adsorbate (activated carbon) at t=const.

The adsorption isotherm shows that since adsorption is an exothermic process, the amount of substance adsorbed at equilibrium decreases with increasing temperature.

Adsorbent regeneration includes:

desorption, drying, cooling

a) thermal (160 ÷ 170°)

b) at high temperatures(300 - 400°)

c) displacement (cold)

Calculation of adsorbers

The main quantities to be determined are: the diameter of the apparatus and the height of the word of the sorbent layer at a given process time.

)permissible fictitious gas velocity (velocity in the free section)

ω 0 = (0.016 r ρ us d e g / ρ g) 0.5

d e - equivalent diameter of granules, m,

ρ g - gas density, kg/m 3,

ω 0 ≤ 0.3 m/s.

Porous structure of adsorbents

The porous structure has a significant impact on the adsorption properties of the sorbent.

The sorbent surface includes:

outer surface, depending on the number of macropores and is 0.5 ÷ 2.0 m 2 / h, i.e. 2.0 ÷ 0.5% of the total surface;

the inner surface formed by the walls of micropores. It can be equal to 500 ÷ 1000 m 2 / h.

Surface of the porous body:

N A - Avagadro's number,

a m is the adsorption value corresponding to covering the surface with a continuous monolayer of adsorbed molecules,

S m - area occupied by one adsorbed molecule,

S m = 1.53 · V 2/3, V is the molar volume of the adsorbed substance.

S m N2 = 1.62 m 2.

Total porosity solid can be determined by its density.

There are true (ρ ist), apparent (ρ kazh) and bulk (ρ us) densities of porous bodies.

True - the mass of a unit volume of a densely packed body (not containing pores).

Apparent - mass per unit volume of a porous body, including pore volumes, but excluding the volume of voids between grains.

Bulk - the mass of a unit volume of a porous body, including the volume of a dense substance, the volume of pores and the volume of voids between grains.

Total pore volume:

V ∑ = 1∕ρ each = - 1∕ρ source, g∕cm 3 .

Activated carbon ρ source, = 1750 ÷ 2100 ρ each = 500 ÷ 1000 ρ us = 200 ÷ 600

Soft granular silica gel ρ ist = 2100 ÷ 2300 ρ each = 1300 ÷ 1400 ρ us = 800 ÷ 850

Coarse-grained silica gel ρ ist = 2100 ÷ 2300 ρ each = 750 ÷ 850 ρ us = 500 ÷ 600

Zeolites ρ source = 2100÷ 2300 ρ each = 1200 ÷ 1400 ρ us = 600 ÷ 800

Characteristics of the adsorbent

Active carbons- sorbents of organic origin (from coal, peat, wood materials, paper production waste, animal bones, nut shells, fruit seeds, etc.).

First, the source material is subjected to heat treatment at t = 600 ÷ 900°C, moisture and resins evaporate from the coals, and then to impart porosity it is activated - treated with steam, gases or chemical reagents (CO, CO 2, NH 3, water vapor) at t = 800 ÷ 900°С. By measuring the temperature, activator feed rate and activation time, we obtain different adsorption - structural properties brands of active carbons: BAU, DAK, AR - A, AR - B, KAD, SKT - 1,2.3,4. The main characteristic is ρ us, and the fractional composition. Produced in the form of granules, with a diameter of 2 ÷ 5 mm, H › diameter. Sometimes they are crushed into smaller fractions of 0.15 ÷ 2.5 mm and used for gas purification with stationary movement and an adsorbent layer.

Powdered coals d fr< 0,15 мм - для очистки веществ в жидкой фазе. БАУ - Березовский активный уголь, АГ - гранулированный активный уголь, АР - активный уголь рекуперационный. КАУ - косточковый, СКТ - уголь сернистокалиевой активации.

For the cleaning gas emissions(ventilation) brands AG, KAU are used. SKT, as well as coals from polymer materials and molecular sieve carbons (MSC) - have high adsorption activity in the region of low concentrations of pollutants, characterized by increased strength, so self-propelled guns - (made from uranium polymer).

Negative properties - flammability, oxidize at t = 250°C, to reduce the fire hazard, silica gels are added to coal.

Silica gels - hydrated amorphous, obtained by interaction liquid glass and sulfuric acid. This is a mineral adsorbent, a reaction product (SiO 2 nH 2 O) d fr = 0.2 ÷ 7 mm in the form of grains,

ρ us = 0.2 ÷ 7 g∕cm 3.

Cheap sorbent, has high mechanical strength to abrasion, low regeneration temperature (110-120°C), used for drying gases and trapping organic pollutants.

Silica gel, obtained in an acidic environment and washed with acidified water, has small pores. IN alkaline environment- large-porous.

Depending on the grain shape:

− lump silica gel (irregularly shaped grains);

− granular (grains of spherical or oval shape).

- for processes with a fluidized bed - 0.1 ÷ 0.25 mm

― with a moving layer - 0.5 ÷ 2.0 mm

― with a stationary layer - 2.0 ÷ 7.0 mm

Disadvantage - destruction of grains under the influence of dripping moisture

Aluminum gels - Al 2 O · n · H 2 O - active aluminum oxides, like silica gels, are hydrophilic adsorbents, they have a developed structure, large surface area and are suitable for drying gases, capturing hydrocarbons and fluorine. They are more resistant to water. They are capable of absorbing from 4 to 10% of water vapor from their own mass.

Zeolites(from Greek: boiling stones). All of the adsorbents discussed above have an irregular structure, so molecules of a wide variety of sizes can penetrate and remain in their pores, i.e. they do not have selective adsorption - this is their disadvantage.

Adsorbents with a strictly regular porous structure can selectively adsorb molecules of the same size - these are natural minerals siderite, faujasite, erionite, glabazite, mordenite, etc. By heat treatment they are converted into an adsorbent with high porosity, large surface area and uniform pore sizes. There are few natural zeolites in nature, they are contaminated with impurities, therefore approximately 100 types of zeolites have been synthesized for industrial use.

The most applicable zeolites are KA, NaA, CaA, NaX, CaX. The first letter corresponds to the cation that compensates the lattice charge (K +, Na +, Ca +), the second - the type of crystal lattice.

Zeolites are unique adsorbents that remove ammonia, SO 2, acetylene, H 2 S, CO 2, etc.

Regeneration of adsorbents

Regeneration consists of removing adsorbed substances from its pores. The effectiveness of the purification process depends on the quality and speed of release of the adsorbed substance from the adsorbent.

Adsorption methods:

― thermal (increasing the temperature of the adsorbent layer to 110 - 130 °C - at normal and 300 - 400 - elevated temperatures);

- displacement desorption (at 30 - 80°C);

- desorption with the field of acute water vapor is currently more common.

Designs of adsorption plants

) Periodic adsorbents with a fixed (stationary) layer of absorber.

Gas supply from top to bottom (or vice versa). If necessary, the adsorbent is placed on the shelves in layers, with a ring layer of adsorbent.

To carry out a continuous process, at least two devices are installed.

) The first stage is adsorption

) Desorption - the gas supply is stopped and steam is supplied. As a result of heating the adsorbent, desorption of the absorbed components occurs, which, together with steam, are removed from the separation apparatus

) Drying the adsorbent - stop the steam supply and serve hot air

) Cooling - cold air is supplied.

adsorbent gas purification

Calculation of adsorption units

It consists in determining the design dimensions (diameter, height), the volume of the adsorbent, the time of the protective action of the hydraulic resistance and some other quantities.

Where V Г is the volumetric flow rate of the threshold mixture m 3 ∕s,

Speed ​​related to the free section of the apparatus, m ∕s.

For devices with a fixed layer = 0.25 ÷ 0.3 m ∕s.

) Volume of adsorbent for one-time loading into the device

n y - number of transfer units;

ß y - volumetric mass transfer coefficient, kg ∕m 3 s.

or

u n, u k - initial and final concentration of the adsorbent in the vapor-gas mixture,

x, y - current concentrations of adsorbate in the solid and adsorptive in the vapor-gas phase, kg ∕m 3,

x x, y x - equilibrium concentrations of adsorbate, kg∕m 3.

The equation can be solved by graphical integration. Given a series of “y” values, we build a graph in coordinates 1∕(y - y *) - y, and then, by measuring the area of ​​the curvilinear trapezoid, we find the value of the desired integral, taking into account the scales: M 1 = l 1 ∕h 1 and M 2 = l 2 ∕h 2 ,

l 1 - ordinate value 1∕(y - y *),

h 1 - the value of the same ordinate in mm,

l 2 - the value of the abscissa on the graph y,

h 2 - the value of the same abscissa in mm.

To construct the graph used to obtain the number of carry units, it is necessary to determine the value of y x (x x). To do this, it is necessary to construct adsorption isotherms (line 2) and the operating line of the process (line 1). Adsorption isotherm (equilibrium curve0 at t = const serves as the main characteristic of the process and 0 = f(p),

and 0 is static activity,

p - partial pressure.

There is a Clapeyron equation between the concentration of the adsorbed substance in the gas phase and it:

Kg∕m 3 .

The adsorption isotherm is constructed based on experimental (or reference) data. To construct a working line, it is necessary to know the coordinates of at least two points that meet the operating conditions of the process.

For example, if y n, y k and x n are specified (initial concentrations of the extracted component in the solid phase), then the final concentration of the adsorbent in the solid phase x k is determined from the equation:

The volume of the adsorbent saturated with the adsorbent per unit time (the size of the working layer).

,m 3∕ s,

The value x* (equilibrium concentration of adsorbate in the solid phase), corresponding set value“y” is determined from the adsorption isotherm. Knowing the coordinates (·)A (x n;y k) and (∙)B (x k;y n), we plot them on the graph and connect them with a straight line.

To determine x *, y * we set the values ​​of “y” in the interval y n - y k. If the perpendicular from the initial (∙) y n is continued to the intersection with the equilibrium line 2 to (∙) Г and projected onto the x axis, we get equilibrium compound of the adsorbate in the solid phase x * at a given value y n. If the adsorption isotherm is unknown, then it can be constructed from the adsorption isotherm of the standard substance. The value of adsorption values ​​is recalculated using the formula:

,

Isotherm ordinate of a standard substance (usually benzene), kg/kg,

Ordinate of the determined isotherm, kg/kg,

V 1, V 2 - molar volumes of the standard and test substance in the liquid state,

M is the molar mass of the substance, kg/mol,

Affectivity coefficient,

Density of the substance in the liquid state, kg/m3.

As an adsorbent, we choose active carbon of the AR-A grade, d e = 1.3 ∙ 10 -3 m.

We accept = 0.28m/s, then ,

.

To construct an adsorption isotherm, we use a monogram to determine pressure saturated steam some substances, by which we determine the partial pressure of substances using the formula:

(1)

where P 1, P 2 - partial pressure of the standard and test substance, mm Hg (Pa),

Р S,1 - saturated vapor pressure of the standard substance at absolute temperature(mmHg),

Р S,2 - saturated vapor pressure of the test substance.

When calculating the isotherm points of the substance under study, the coordinates and are taken from the curve of the standard substance, the values ​​of Р S,1, Р S,2 are taken from the tables of saturated vapor pressure. P 2 - calculated according to formula (1).

Expressing the partial pressure in terms of the corresponding concentrations, we obtain:

(2)

Affinity coefficient for diethyl ether (Table 36, Kuznetsov).

) according to table 25 (equilibrium data on the adsorption of benzene vapors and their mixture with air on active carbons , ,

) according to the diagram (page 115), we determine the coordinates of the points of the adsorption isotherm of diethyl ether, Р S,1 - for benzene - 75 mm Hg (9997.5 Pa), Р S, 2 - for diethyl ether - 442 mm Hg ( 58918.6 Pa).

)Volume mass transfer coefficient:

Volumetric mass transfer coefficient in the gas and solid phases, respectively, s -1,

m - distribution coefficient (average slope of the equilibrium line).

Since - is usually very small, we neglect the value.

Based on this, it depends on the hydrodynamic situation in the apparatus and the physical properties of the flow.

For oriented calculations of K y, criterion equations are used:

At Re > 30

At Re = 2 - 30

At Re< 2

Where - Nusselt diffusion criterion.

D e - equivalent diameter of adsorbent grains, m

,

Gas flow speed, m/s

Porosity of the fixed adsorbent layer,

Density, kg/m 3

Dynamic viscosity, Pa s

- Prandtl diffusion criterion.

)Height of the fixed adsorbent layer in the apparatus

,

h - height of the transfer unit,

where G g is the gas mass flow rate, kg∕s

S sl - layer cross-section, m 2

Balance equations for absorbed matter;

Adsorption kinetics equations;

Adsorption isotherm equation.

: (for benzene),

:

The adsorption isotherm for solving the equations is divided into three regions:

region - linear dependence between the gas concentration and the amount of absorbed substance, and it is conventionally assumed that the adsorption isotherm obeys Henry’s law.

Then the duration of adsorption:

;

where y n is the initial concentration of the adsorbed substance, kg∕m 3

x* - equilibrium amount of adsorbed substance, kg∕kg (taken from the adsorption isotherm and multiplied by bulk density adsorbent).

area - curvilinear

where is the content of the substance in the gas flow, equilibrium with the quantity, equal to half substance maximally absorbed by the adsorbent at a given temperature, kg/m3.

region - the amount of substance absorbed by the adsorbent reaches a limit and remains constant

) Height of the mass transfer zone (height of the working layer)

,

time to equilibrium saturation, sec

time of protective action at minimum breakthrough concentration,

unused adsorption capacity,

) Pressure drop in the layer (the formula is applicable if the layer porosity E = 0.4)

∆Р - pressure drop in the layer, kg/m 3

g - 9.81 m/s 2

d e - equivalent grain diameter, m

G - gas mass velocity, kg/(m 2 ∙s)

1)- at<0,25м/с, ламинарный режим

)- transition region

)- in a layer of zeolites

- for balls, - for cylinders

Substituting the obtained values ​​into the equation

Let us express the partial pressures in terms of volumetric concentrations according to the equation

0.005125t.k. Re>30, then 80.23

Adsorption is the process of selective absorption of a gas, vapor or solution component by the porous surface of a solid (adsorbent). Adsorption is used to purify gases with a low content of gaseous or vaporous contaminants to obtain very low volumetric concentrations. Adsorption is used to capture sulfur compounds, hydrocarbons, chlorine, nitrogen oxides, organic solvent vapors, etc. from gases and ventilation emissions.

Adsorption processes are selective and reversible. Each absorber has the ability to absorb only certain substances and not others. The absorbed substance can always be separated from the absorber by desorption.

Unlike absorption methods adsorption allows for gas purification at elevated temperatures.

The target absorbed component located in the gas to be purified is called an adsorbent, the same component in the adsorbed state, i.e., the absorbed substance in the adsorbent, is called an adsorbate.

Based on the nature of the interaction of the adsorbate with the surface, physical and chemical adsorption are distinguished.

Physical adsorption is determined by the forces of intermolecular interaction (dispersion, orientation and induction effects). Intermolecular forces are weak, so during physical adsorption only slight deformation of the adsorbed particles occurs. This type of adsorption is purely physical process with an activation energy of about 4.12 kJ/mol. With physical adsorption, the absorbed molecules of gases and vapors are held by van der Waals forces, with chemisorption - chemical forces. During physical adsorption, the interaction of molecules with the surface of the adsorbent is determined by relatively weak forces (dispersive, inductive, orientational). Physical adsorption is characterized by high speed, low bond strength between the surface of the adsorbent and the adsorbent, and low heat of adsorption (up to 60 kJ/mol).

Chemical adsorption (chemisorption) is carried out due to the unsaturated valence forces of the surface layer. In this case, surface chemical compounds can be formed, the properties and structure of which are still poorly understood. It is only known that they are different from the properties of bulk compounds. When forming surface compounds, it is necessary to overcome the energy barrier, which is usually 40.100 kJ/mol. Because chemisorption requires significant activation energy, it is sometimes called activated adsorption. During physical adsorption, the interaction of molecules with the surface of the adsorbent is determined comparatively weak forces(disperse, induction, orientation). Physical adsorption is characterized by high speed, low bond strength between the surface of the adsorbent and the adsorbent, and low heat of adsorption (up to 60 kJ/mol). Chemical adsorption is based on chemical reaction between the adsorbent and the adsorbed substance. The forces acting in this case are much greater than during physical adsorption, and the released heat coincides with the heat of the chemical reaction (it ranges between 20,400 kJ/mol).

The values ​​of physical and chemical adsorption decrease with increasing temperature, but when certain temperature physical adsorption can abruptly become activated.

During adsorption, very high absorption rates and complete extraction of components are possible, the isolation of which by absorption would be impossible due to their low concentration in the mixture.

Adsorption continues to be the main method for purifying process gas emissions. In principle, adsorption can be used to remove any contaminant from a gas stream. In practice, its scope of application is limited to a number of operational, technical and economic conditions. Thus, according to fire and explosion safety requirements, gases containing explosive components exceeding 2/3 of the lower flammable concentration limit cannot be subjected to adsorption treatment.

The optimal concentrations of pollutants in gases supplied for treatment are in the range of 0.02...0.5% vol. (in terms of connections with molecular weight~100). Modern technical capabilities do not allow reducing the concentrations of pollutants through adsorption to sanitary standards. Approximately the minimum final concentrations of pollutants corresponding to the acceptable characteristics of adsorption devices, in practice, are 0.002...0.004% vol. Therefore, adsorption purification of gases with initial content a pollutant of less than 0.02% is appropriate if it is an expensive product or a substance of a high hazard class.

Treatment of waste gases with a high (more than 0.2...0.4% vol. in terms of compounds with a molecular weight of the order of 100...50) initial concentration of the pollutant requires a significant amount of adsorbent and, accordingly, large dimensions of the adsorber. The bulkiness of the apparatus is also caused by low (up to 0.5 m/s) values ​​of the flow velocity through the adsorbent layer, since at higher speeds the abrasion and entrainment of the adsorbent sharply increases. Thus, losses of adsorbent due to entrainment can reach up to 5% per day at flow rates of 1...1.5 m/s.

However, the possibilities of the adsorption process are far from being exhausted. In some cases, it can be used to create a new generation of treatment systems that meet not only sanitary standards, but also economic requirements. For example, adsorption can be used in a two-stage purification scheme to pre-concentrate highly diluted organic pollutants, which are then sent for thermal neutralization. Thus, the concentration of pollutants in ventilation emissions can be increased tens of times.

Adsorption can occur in a fixed layer, a moving (moving) layer, or a boiling (fluidized) adsorbent layer.

INTRODUCTION

This development is a presentation of the material special course lectures that the author has been delivering for a number of years to students, graduate students and graduate students specializing in the field of adsorption. The author hopes that this teaching aid will fill the gap that exists in scientific and educational literature on the topic under consideration, and will help novice researchers become acquainted with the main problems and achievements of the science of adsorption - this most complex branch of thermodynamics.

The proposed development option does not consider such important sections as adsorption on mesoporous adsorbents, accompanied by capillary condensation processes, the use of the apparatus of molecular statistical thermodynamics, complex lattice models, quantum chemical and numerical methods for the analysis of adsorption phenomena. The author hopes to implement the necessary additions in 2008-2009. and will be grateful for all comments and suggestions for improving this

Lectures 1. Gibbs excess method. Lecture 2. Method full content.

Lectures 3. Thermodynamics of adsorption. Stoichiometric theory of adsorption.

Lecture 4. Thermodynamics of adsorption of binary mixtures of fluids and thermodynamics of adsorbed solutions.

Lecture 5: Description of adsorption equilibria of gases, vapors and solutions on macro and microporous adsorbents.

Lecture 1. Gibbs excess method.

Introduction.

Adsorption is the thickening of a substance at the phase boundary, caused by the unsaturation of the bonds of surface atoms or molecules and, as a consequence of this, the existence of an adsorption field that extends, strictly speaking, to points infinitely distant from the surface of the adsorbent in the bulk phase. This circumstance leads to the need to take into account following features such systems: 1. The division of the system into adsorption and bulk phases cannot be carried out strictly1,2.

2. The adsorption phase, identified on the basis of any additional (always approximate) considerations, will be energetically inhomogeneous (it will be in an inhomogeneous adsorption field) and, since this inhomogeneity cannot be taken into account within the framework of phenomenological thermodynamics, a description of the properties of the adsorption phase must be carried out using phase-averaged parameter values ​​(concentrations, chemical potentials, etc.)3.

Parameters of the adsorption phase: concentrations - c,x, activity coefficients -γ, chemical potentials -μ are marked either with a line above the corresponding symbol or with a subscript R.

3. The presence of an adsorption field must be taken into account in the expression for the chemical potential, i.e., use complete chemical

potentials for the components of the adsorption phase3 (for more details, see methodological development for a course of lectures on physical chemistry, ch. 2: http://www.chem.msu.su/rus/teaching/tolmachev/tolmachev.pdf):

For bulk gas or vapor phase:

μ (P , T)= μ0

1)+RTln

P iγ i

P i, st.

μ i (Ci , T)= μ0 i,id. (T, Ci,st. = 1)+ RTln

C iγ i

C i, art.

1) =μ 0

1)− RTlnRT

For bulk liquid phase:

μ i (Xi , T, P)= μ o i,id. (T, P, Xst. = 1)+ RTln Xi γ i,x

μ i (Ci , T, P)= μ o i,id. (T, P, Cst. )+ RTln

C iγ i

C i,st.= 1 or C i,st =C i 0

C i, art.

For the adsorption phase:

Let's introduce new feature states:

G *= G − σ W , dG* = dG − Wd σ − σ dW = -SdT + VdP+ ∑ μ i dn i - Wdσ

where: W is the surface (pore volume) of the adsorbent, σ is the surface tension (internal pressure).

Using Maxwell's equations, we get:

	∂μi		∂W
					= −s i ,	μi (σ) = μi (σ= σ0 ) − s i (σ− σ0 )
			∂n i
	∂ σ P, T, n			P, T, σ, nj

And, accordingly (s i is the partial molar area (volume) of the adsorbate):

i = μ 0 i,id.(T, P,

i, art. )+ RT ln

− s i (σ − σ0 )

c i, art.

i = μ 0 i,x,id.(T, P,

i, art. = 1)+ RT ln

i,x − s i(σ − σ 0)

Standard states for adsorbates and reference states for γ i:

i, art. =

i,c,count. = 1 at

i, art. = 1

i,x,count.. = 1 at

i, art. = 1

In addition to the above options for choosing standard states, saturated vapor pressure and saturated solutions are sometimes considered as alternatives. When analyzing interphase equilibria, it is convenient to use standard states components in two equilibrium phases

each other, for example, saturated vapor pressure and pure concentration

liquid or adsorbate at complete saturation of the adsorbent (C i,st. = C 0 i).

Wherein:

μ i = RT ln

P iγ i

RT ln

− s i (σ − σ0

P i,s

With i,s

It is useful to note two forms of the Gibbs-Duhem equation that are widely used for adsorption solutions within the total content method. In older models, the adsorbent was often not considered as a component of the adsorption solution, but only as a source of the adsorption field (surface energy). In this case, for example, during the adsorption of one-component steam, the Gibbs-Duhem equation has the form (P,T=const.):

i + Wdσ= 0

(W is the surface area of ​​the adsorbent, μ i is the total chemical potential of the adsorbate).

IN modern models adsorbent (R) is a component of the adsorption solution. It is introduced either in the form of adsorption centers (as in the Langmuir and Tolmachev models) or in the form of vacancies (free voids of certain sizes in the adsorption solution).

In this case, the Gibbs-Duhem equation can be presented in two equivalent forms (one-component pair, P,T=const.):

(molar areas of the components - s=const., s i +s R =W ) (1.8) reduces to the form:

сi dμ i + cR dμ R − (si + sR )dσ+ Wdσ= сi dμ iR + cR dμ R = 0(1.9)

Equations (1.7), (1.8) make it possible to use the equality of total potentials in equilibrium phases, and (1.9) is more convenient for analyzing the properties of an adsorption solution.

U The above features of adsorption systems led to the development of two versions of their thermodynamic description:

1.Gibbs excess method 1.2 - thermodynamically strict description of changes during adsorption of properties the entire system as a whole based on experimentally determined excess adsorption values(see below) without dividing it into two phases. This method obviously does not allow one to obtain any information about the properties of the adsorption phase and, therefore, is not informative enough, especially when solving practical problems, since it does not provide information about the capacity of the adsorbent in relation to the components of the bulk phase, about its structure, properties, etc.

2. The total content method, 3-6 based on dividing the system into two phases (see below) and describing its properties as a heterogeneous system using the absolute concentrations of the components in each of the equilibrium phases. Thermodynamically, this method is less strict, because it is based on model approximation, determining the interface between the bulk and adsorption phases, but it is obviously much more informative, because allows one to obtain the characteristics of the adsorption phase, which is extremely important from a practical point of view, and, in addition, allows one to compare them with those calculated on the basis of various molecular models, which are necessarily associated with specifying a specific arrangement of molecules at the surface of the adsorbent.

In this regard, a significant part modern information about adsorption is presented within the framework of the total content method, and the excess method is used to obtain primary information and as creatorial (see below) when choosing a model to move to the full content method. Let's take a brief look at both of these methods:

1.2. Gibbs excess method.

Let’s begin a brief summary of the basics of the “Gibbs Excess Method” with two quotes that quite fully outline the main idea of ​​the method and reflect two approaches to assessing the value of this method in modern theory adsorption phenomena:

1. “The peculiarity of Gibbs’s approach is that he immediately abandoned the attempt to characterize adsorption in any absolute quantities, that is, to consider the interphase layer as a certain physical object that has natural boundaries and, therefore, containing a certain amount of substance in a certain volume, which could be equated to the measured adsorption value. Such consideration would be contrary to the principles of adsorption measurement. According to Gibbs, the value of adsorption (G), as well as the thermodynamic functions associated with it, are excess quantities, to calculate which, instead of one system, we need to consider two: the real system we are interested in and the one introduced in a certain way comparison system - the zero level from which adsorption properties are measured”2 and further: “The advantage of excess quantities is that they are directly measured in experiment and therefore are not associated with any models. With their help, it is possible to construct a thermodynamic theory that will include only experimental quantities”2;

2. “Some features of the proposed thermodynamic formalism for describing adsorption phenomena are, as it seems to us, in sharp discrepancy with current state doctrine of adsorption. The adsorption value is determined by Gibbs as a certain excess value, which is the difference in the amount of adsorbent in a real adsorption system and in a fictitious system characterized by the same macroscopic state parameters (volume, pressure, temperature) as real system, but in which the coexisting phases are homogeneous up to a certain mathematical interface. The excess adsorption value is determined directly from the adsorption experiment, and in any equation

Gibbs's adsorption theory is allowed to use only this value. From our point of view, the use in all cases of only excess adsorption put the Gibbs method in irreconcilable contradiction with the adsorption science of the late 20th century. In fact, in any equation of the adsorption isotherm (for example, the Langmuir equation) or the equation of state of the adsorption phase, based on molecular kinetic concepts, it is not the number of excess molecules that enters, but full number real molecules in the region of heterogeneity. The experimentally determined heats of adsorption are associated with the change in enthalpy when all, and not just excess, molecules enter the adsorbent field. In two-dimensional phase transitions not only excess, but all adsorbed molecules are involved. Finally, when using the method of statistical thermodynamics to describe adsorption phenomena, it should be remembered that in statistical physics there are no “excess” molecules at all. Thus, for almost any modern research adsorption must be introduced into

consideration of all molecules of the adsorbate, while in the thermodynamic equations according to Gibbs, in the name of ephemeral “rigor,” only excess adsorption must be taken into account"5

The essence of this method Let us first consider the example of adsorption of a single-component gas.

Let us introduce into three (I, II, III) identical vessels (Fig. 1) with volumes V 0 equal amounts of moles of gas n 0. Let the walls of vessel I absolutely not adsorb a given gas– then its pressure in vessel I will be P 0, molar density ρ 0, and the number of moles n 0 = ρ 0 V 0. Let the bottom wall of vessel II be absorbent surface. Then the gas density at the surface will increase, and far from the surface in the volume of the vessel it will decrease to ρ ρ extends down to the bottom adsorbent

surface (the adsorption phase is identified with the geometric surface located on the lower wall of vessel II).

Change in the amount of gas in the volume of vessel II compared to vessel I:

ne = V ρ		− V ρ

representing an excess of gas near the surface compared to its amount in the same volume far from the surface is called

excess adsorption value or briefly excess adsorption of a given gas. Obviously, only this quantity and can be measured in a real adsorption experiment. It is usually referred to as a unit of weight (or surface area) of the adsorbent. For example:

G =			V0 ρ0	− V 0

Let us now carry out in the vessel III division systems into the bulk and adsorption phases by drawing (the method will be discussed below) the phase boundary at a certain distance from the adsorbing surface. In this case we can calculate absolute value gas adsorption in the volume of the adsorption phase ( average over the entire volume

adsorption phase)V , and the volume of the gas phase will be equal to:
V = V0 − V

Indeed, the absolute adsorption n will be equal.

Adsorption at the solid-gas interface

A characteristic feature of solid surfaces is their porosity. The nature of the surface of the adsorbent, the size and shape of its pores affect adsorption, change its quantitative and qualitative characteristics, i.e. adsorption mechanism.

Solid surfaces as adsorbents are used for the adsorption of gases or liquids, and adsorption processes occur at the solid-gas (S-G) and solid-liquid (S-L) interfaces.

Unlike the surface of a liquid, the surface of solids is geometrically and energetically inhomogeneous - solid adsorbents may have pores. One of the main characteristics of such adsorbents is porosity P, it is equal to the ratio of the total pore volume V p to the total volume of the adsorbent V vol, i.e. . Depending on the porosity, solid adsorbents are divided into two groups: non-porous And porous.

The phase interface of non-porous adsorbents corresponds to the contour of a solid body. In a porous adsorbent, this surface is much larger due to the presence of pores. Porous adsorbents are often used in the form of powders.

Processes of absorption of gases or dissolved substances hard materials or liquids can flow through different mechanisms and are collectively called sorption.

There are four main sorption processes: absorption, adsorption, capillary condensation, chemisorption.

Absorption refers to the absorption of gas or vapor by the entire volume of a solid or liquid.

This process consists of the penetration of gas molecules into the mass of the sorbent and ends with the formation of a solid or liquid solution. The distribution of gas molecules in the solid or liquid phase occurs mainly by diffusion. Since in solids Since the diffusion rate is very low, absorption in them proceeds very slowly and considerable time is required to establish equilibrium.

Adsorption called spontaneous concentration on a solid or liquid interface of a substance with lower surface tension.

Adsorption is a purely surface process, which consists of the interaction of molecules or ions of an adsorbate (gas or solute) with the surface of the adsorbent due to van der Waals forces, hydrogen bonds, electrostatic forces. The speed of this process is high, and adsorption occurs instantly.

Capillary condensation is a process of liquefying steam in the pores of a solid sorbent. Steam can condense only at temperatures below critical. If the resulting liquid well wets the walls of the capillaries, i.e. surface of the sorbent, then concave menisci are formed in the capillaries as a result of the merging of liquid adsorption layers that appear on the walls of the capillaries. Capillary condensation occurs under the influence not of adsorption forces, but of the forces of attraction of vapor molecules to the surface of a concave meniscus of liquid in the pores. It proceeds quite quickly and ends within a few minutes.

Chemisorption- This is an adsorption process that occurs under the influence of the forces of basic valences, therefore it is classified as chemical adsorption.

Question 2. Physical and chemical adsorption and their features

Adsorption on solid surfaces can be explained by the presence of attractive force fields arising due to unbalanced bonds in the crystal lattice.

Adsorption forces are composed of valence interaction forces (chemical) and weaker van der Waals forces (physical). The role of both in different cases of adsorption is different. Thus, at the very beginning of the adsorption of most gases, when their pressure is low, chemical adsorption is observed; with increasing pressure it gives way to physical, which mainly determines the adsorption of gases. Adsorption is influenced not only by the nature of the adsorbent, but also by the adsorbent. On solid adsorbents, those gases that liquefy more easily are more strongly adsorbed, i.e. critical temperature which are higher.

Physical adsorption is a reversible exothermic process; As the temperature increases, adsorption decreases and desorption increases. The heats of physical adsorption are low and usually amount to 8-20 kJ/mol. Physical adsorption does not have a specific selective nature. Chemisorption, on the contrary, is specific. It depends both on the nature of the adsorbent and on the nature of the adsorbate. The adsorbent–adsorbate binding energy is quite high and approximately equal to the heat of formation chemical compounds(80-800 kJ/mol). With increasing temperature, chemisorption increases, obeying the laws chemical kinetics and balance heterogeneous reactions. Chemisorption is often irreversible and results in the formation of strong surface compounds between the adsorbent and the adsorbate.

It should be noted that the phenomena of physical and chemical adsorption are clearly distinguished only in extreme cases. Usually intermediate options are carried out, when the bulk of the adsorbed substance binds relatively weakly (physical adsorption) and only slightly most of is tightly bound and can be removed by prolonged heating and vacuuming (chemical adsorption). For example, oxygen on metals or hydrogen on nickel are adsorbed at low temperatures according to the laws of physical adsorption, but as the temperature increases, adsorption begins to occur with a noticeable activation energy. In a certain temperature range, the increase in chemical adsorption overlaps the decrease in physical adsorption.

Question 3. Basic principles of the theory of adsorption

There are theories: monomolecular adsorption (Langmuir's theory of monomolecular adsorption), the theory of polymolecular adsorption (Polyany's theory of polymolecular adsorption) and the generalized theory of Brunauer, Emmett and Teller (BET).

Question 4. The concept of molecular and polymolecular adsorption. Langmuir adsorption isotherm equation and its physical meaning.

In 1915 I. Langmuir proposed the theory of monomolecular adsorption. The Langmuir adsorption isotherm equation is valid for a wide range of concentrations and for interfaces, both mobile (l-g, l-g) and solid (s-g, s-g).

Derivation of the Langmuir isotherm equation for solid adsorbents is based on a number of initial premises:

1) adsorption forces are similar to the forces of basic valences and act over short distances;

2) not the entire surface has adsorption activity, but only certain active centers located mainly on convex areas of the surface: protrusions, edges, corners;

3) molecules of the adsorbed gas are fixed on adsorption centers, do not move along the surface of the adsorbent and do not interact with each other.

4) Each active center has a short range of action and is capable of being saturated. Therefore, the active center can interact with only one adsorbate molecule. As a result, only one (monomolecular) layer of adsorbate can form on the surface of the adsorbent (monomolecular adsorption).

5) Adsorbed molecules are retained by a given active site only for a certain period of time. After some time, the molecules break away from the active center and enter the gas phase.

The Langmuir adsorption isotherm equation for adsorption from solutions has the form:

(1)

for gas adsorption:

(2)

where K is the adsorption equilibrium constant. The greater the affinity of a given adsorbed substance for a given adsorbent, the greater it is. In addition to the nature of the adsorbent and adsorbate, the value of K is affected by temperature. With increasing temperature, the desorption process intensifies, as it increases kinetic energy adsorbate molecules and the constant K decreases.

And PR is the limiting adsorption. The value of A PR depends on the number of adsorption centers per unit surface or the mass of the adsorbent and the size of the adsorbate molecules. The larger the adsorbate molecules, the larger area, per molecule in the adsorption layer, and the smaller the value of A PR.

A graphical representation of the Langmuir adsorption isotherm is shown in Fig. 1. The curve is characterized by the presence of three sections: an initial linear section, a middle section in the form of a parabola segment, and a final linear section running parallel to the abscissa axis.

Fig 1 Langmuir adsorption isotherm

Analysis of the Langmuir equation shows that depending on the concentration of the adsorbate, it can take different forms.

At low concentrations, when K<<1, этой величиной в знаменателе можно пренебречь и уравнение принимает вид:

According to this expression, adsorption increases linearly with increasing concentration. On the adsorption isotherm graph, this condition corresponds to the initial section of the curve (I).

In the region of high concentrations K>>1 and unity can be neglected in the denominator of equation (2), then

The resulting equality indicates the saturation of the surface with the adsorbate. On the adsorption isotherm graph, this condition corresponds to horizontal linear section III, in which the adsorption value no longer depends on concentration. The Freundlich equation is used to describe the middle section of the curve.

The values ​​of K and A PR in the Langmuir equation are determined graphically. To do this, take the linear form of the equation. To do this, divide the unit into both sides of the equation (). We get an equation like y=a+bx:

(5)

The graph is a straight line (Fig. 2):

Fig. 2 Graphic determination of the Langmuir adsorption equation constants: OA=α=1/A pr; ОD "= 1/С 1/2=К

The segment of the ordinate axis OA=a, cut off when extrapolating a straight line, equal to the value reverse A PR.

Adsorption is a change in the concentration of a substance at the interface. Adsorption equilibrium, i.e. equilibrium distribution of matter between boundary layer and adjacent phases, is dynamic and quickly established.

Particles that are on the surface of a solid have excess energy.

Due to this, the molecules environment are attracted to the metal and concentrated on its surface. This process always occurs spontaneously and with a positive thermal effect.

There are two types of adsorption: physical and chemical.

Physical adsorption is due to van der Waals forces. The binding energy between adsorbate molecules and the metal surface is low (about 40-50 kJ/mol). Equilibrium is established quickly. Adsorbed substances can be easily removed from the surface. Physical adsorption is most clearly manifested at low temperatures close to the condensation temperature of the adsorbate.

Chemisorption is the process of adsorption accompanied by chemical reaction between the molecules of the adsorbed substance and the metal. The bond energy between atoms is estimated at 150-160 kJ/mol. The bond that occurs between the metal and the oxidizing agent is ionic in nature. The metal donates electrons to the atom of the adsorbed substance. The chemisorption process occurs very quickly (fractions of a second). The outer surface of the adsorbed film is charged negatively, and the inner surface is charged positively.

The amount of adsorbed substance per unit surface depends on the temperature of the medium and the concentration of the adsorbate in the gas or liquid phase.

called adsorption isotherm.

First theoretical basis adsorption isotherms were given by Langmuir. He made several assumptions to simplify the process model.

It was assumed that:

the surface of the adsorbent is energetically homogeneous;

adsorbate particles on the surface do not interact with each other in any way;

there is one particle of adsorbed substance per active surface center;

Only a monomolecular adsorption layer can form.

will be directly proportional to the gas pressure P and the size of the free surface of the metal

A constant characterizing the rate of the adsorption process.

Speed reverse process- desorption - will be directly proportional to the surface area occupied by reacting molecules:

the fraction of the metal surface occupied by adsorbate particles.

When equilibrium is established, the adsorption rate is equal to the desorption rate:

(3.2), we get:

Solving equation (3.4) for b, we obtain:

where b is the adsorption coefficient.

Equation (3.6) is called an isotherm. Its graphic expression is given in Fig. 3.3.

a horizontal section is obtained corresponding to

formation of a filled monolayer.

or at low gas pressure

and then from (3.6) we get:

Under these conditions, the degree of surface filling is small and proportional to the pressure.

Expression (3.7) reflects the distribution law and is called the Henry isotherm.

And

we obtain from equation (3.6):

This means that all active centers on the metal surface are completely filled with the adsorbed substance and a further increase in its partial pressure in the gas phase does not affect the amount of the substance adsorbed on the surface of the solid. The right section on the isotherm curve corresponds to this state, i.e. straight line (Fig. 3.3).

If a mixture of gases is adsorbed on the metal surface, then the degree of filling surface um gas is calculated using the equation

where the sum is taken over all n components of the gas mixture.

In most cases the surface hard metal energetically inhomogeneous. It is a series of elementary platforms with different heat adsorption.

The experimental data is more accurately described by the equation:

A coefficient reflecting the distribution function, and ao is the adsorption coefficient at the highest heat of adsorption.

Expression (3.9) is called the logarithmic adsorption isotherm. It was first described experimentally in the works of A.N. Frumkin and A.I. Shlygina. The theoretical derivation of the equation of this isotherm was made by M.I. Temkin.

Adsorption can be monomolecular or polymolecular. IN the latter case Several layers are formed on the surface of the adsorbent. The first monomolecular layer is caused by the interaction forces between the surface of the solid and the adsorbate. The second and subsequent layers are held by van der Waals forces. The adsorbate layers are distributed unevenly over the surface. In some areas there may be two or three layers. At the same time, areas may remain covered by a monolayer or completely free of adsorbate (Fig. 3.5).

In Fig. Figure 3.4 shows the oxygen adsorption isotherm. Sections ab and bc correspond to monomolecular adsorption, while the section corresponds to polymolecular adsorption. In Fig. Figure 3.5 shows a diagram of filling a metal surface with an adsorbed substance according to the Brunauer theory.