Let in separate thermostated vessels under the same pressure p there are gases A And IN, taken in quantities and moles. When these vessels are connected, spontaneous mixing of gases will occur until a homogeneous composition of the gas mixture is established throughout the entire volume of the system. We will assume that the source gases and their mixtures obey the equations of state of ideal gases. Then, while maintaining a constant total gas pressure p the partial pressures of gases in the resulting mixture will be equal

When ideal gases are mixed, there are no thermal effects, so there is no heat exchange between the gases and the thermostat, and the change in the entropy of the system will be completely determined by the irreversibility of the processes within the system.

To find the desired change in entropy, it is necessary to contrast the described spontaneous process with a mental equilibrium transition between the same initial and final states of the system.

For equilibrium mixing of gases, we will use a special hypothetical device, by analogy with a thermostat, called a chemostat . This device consists of a thermostatically controlled cylinder equipped with a friction-free moving piston; at the base of the cylinder there is a membrane that is selectively permeable only to a given individual chemical; the latter separates the individual substance loaded into the chemostat from the mixture of substances being studied located in another vessel. Unlike a thermostat, designed to maintain a given temperature of a body immersed in it, or to heat or cool the latter in an equilibrium mode, with the help of a chemostat they ensure the maintenance of a certain value of the chemical potential of a given individual substance in the mixture of substances under study, as well as the equilibrium supply and removal of the substance from mixtures. Chemical Potential i - the chemical component in the chemostat is uniquely determined by temperature T and the pressure created on the piston. By changing the pressure on the piston, it is possible to change the direction of transition of a given component through the selective membrane: if is the chemical potential of the component in the mixture under study, then when the substance will be added to the mixture, when – it will be removed from the mixture, and when chemical equilibrium is maintained between the chemostat and the mixture. A quasi-equilibrium change in the composition of the mixture corresponds to the diffusion transfer of a substance through the membrane under the influence of a very small difference in the chemical potential values ​​on both sides of the membrane.

The chemical potential of an ideal gas, regardless of whether this gas is in an individual state or in a mixture with other ideal gases, is expressed by the simple relation where p i is either the pressure of the pure gas or its partial pressure in the mixture. Therefore, when an ideal gas is transferred through a semipermeable membrane, the equilibrium between the mixture and the chemostat is characterized by the equality of the pressure in the chemostat and the partial pressure of the gas in the mixture.

Rice. 2.3. Equilibrium mixing of two gases using chemostats: a– initial state of the system; b– state of the system after isothermal expansion of gases; V– final state after mixing gases through membranes; 1 – individual gas chemostats A and B ; 2 – semi-permeable membranes; 3 – a vessel for equilibrium mixing of gases.

Equilibrium mixing of ideal gases A And B will be carried out in a thermostated system consisting of two chemostats of individual components A And B, connected to a third vessel - a collection of the resulting mixture, equipped, like chemostats, with a movable piston (Fig. 2.3).

Let at the initial moment the chemostats contain, respectively, moles of the component A and moles of component B under the same pressure p ; the piston in the mixture collector is in the zero position (the volume of gas under the piston is zero). The mixing process is carried out in two stages. At the first stage, we perform a reversible isothermal expansion of gases A And B; while the pressure A reduce from p to the set pressure and pressure B accordingly from p to . The volumes occupied by gases in the first and second chemostats will change, respectively, from to and from to . The work done by the expanding gas in the first chemostat is equal to ; in the second . Thus, at the first stage, total work is performed in our hypothetical device. Since during isothermal expansion of an ideal gas its internal energy does not change, this work is carried out due to the equivalent supply of heat from the thermostat. Hence the reversible change in entropy in the system will be equal to

At the second stage of the process (mixing itself), we transfer gases from the chemostats through selective membranes into the mixture reservoir by synchronized movement of three pistons. At the same time, a constant pressure is maintained on each of the pistons, respectively, both in the chemostats and in the collector, which ensures an equilibrium transition of gases through the membranes (more precisely, a pressure is created in the collector that is slightly less p , maintaining a non-zero driving force for diffusion through membranes). The reversibility of the mixing process in this case is ensured by the possibility of synchronously changing the direction of movement of all three pistons, which would lead to the reverse division of the mixture into individual components. After the operation is completed, the mixture will obviously occupy a volume of .

Since in the case of ideal gases mixing is not accompanied by any thermal effect, there is no heat exchange between our device and the thermostat at the second stage of the operation. Consequently, there is no change in the entropy of the system at this stage.

It is useful to verify by direct calculation that the work done by the gases in the second stage is zero. Indeed, work is consumed to move the pistons in chemostats, while at the same time the same amount of work is performed in the gas collector. From here.

So, the total increase in entropy during mixing of gases is determined by expression (2.9), . If, in the equilibrium version of mixing, this increase is associated with the return supply of heat and the production of an equivalent amount of work , then with direct (irreversible) mixing of gases, the same increase in entropy occurs due to its generation inside the system; the system does not perform any work.

After substitution (2.8), expression (2.9) can be rewritten as

. (2.10)

This relationship is given a mandatory place in thermodynamics courses due to its apparent paradox. It is noteworthy that for changes in entropy (when mixing ideal gases!) it does not matter what is mixed with what, as well as at what pressure and temperature. Essentially, here is an informal derivation (2.10).

Let us supplement the conclusion (2.10) with its useful consequences. Introducing mole fractions of components and , we obtain an expression for the change in entropy per 1 mole of the resulting mixture:

. (2.11)

The maximum of this function occurs at an equimolar mixture of gases, 0.5.

From the point of view of the theory of separation of mixtures of substances, it is of interest to trace the change in entropy production when adding a sufficiently large number of moles of a component B to one mole of component A. Setting and in (2.10), we obtain

When deriving (2.12), the mathematical representation of the logarithmic function was used

.

Formula (2.12) shows that successive dilution of the mixture is accompanied by an infinite increase in entropy per mole of impurity component.

Formula (2.10) gives the integral value of the entropy increment when mixing finite amounts of gas. In order to arrive at a compact differential expression similar to formula (2.7) for heat transfer, we modify the component mixing model (see Fig. 2.4). We will assume that mixing occurs through a membrane permeable to both components, or through a sufficiently narrow valve separating the vessels filled with mixtures A And B of different composition. The system is thermostated, and constant pressure is maintained in both vessels using pistons p . With a limited mixing rate, the composition of the mixture in each of the vessels can be considered homogeneous over the volume of the vessel. Thus, this system is similar to a heat exchange system with a weakly conductive partition.

Mixing of gases. Molecular and molar (turbulent) diffusion

Molecular diffusion- the process of mutual penetration of molecules of one gas into another, leading to the formation of a perfect mixture, is observed in stationary gases and in laminar flows.

In molecular diffusion, the mixing of gases is determined by the thermal movement of molecules. Although the speed of movement of molecules W on average is very large, the free path length / is small. Therefore, molecular diffusion proceeds quite slowly. The amount of gas diffusing from one layer to another, according to Fick's law, is equal to

where is the molecular diffusion coefficient, m 2 /s; dC/dn -

concentration gradient of the diffusing gas, kg/m4.

As the temperature rises D and diffusion intensity increase. Size D can be determined using the Sutherland formula modified by N.D. Kosova:

where D)12 is the diffusion coefficient of one gas (1) into another (2) gas at pressure p Q and temperature 7o; Q and C2 are the Sutherland coefficients for the components of the mixture, K (for methane C = 198, air - 119, nitrogen - 107.0 2 - 138, C0 2 - 255); p 0, G 0 - the value of pressure and temperature, respectively, under normal physical conditions (po= 1.01 10 5 Pa; T 0= 273 K).

Often used to determine the molecular diffusion coefficient D a simple power formula is used

Where n- empirical coefficient

The dependences for the diffusion coefficients of a multicomponent mixture are more complex (see, p. 80).

In a turbulent flow, diffusion, as well as heat transfer and internal friction, is associated with turbulent transfer and mixing of finite macroscopic masses of gas - turbulent moles. The sizes of these moles and the paths of their movement before mixing are varied; there is a spectrum of values ​​​​of these quantities. The movement of moths is pulsating in nature, the speeds of their movement are the speeds of pulsations across the flow. At low Re numbers, large-scale pulsations are observed; turbulent velocities change significantly only at large distances. Under pulsation scale(turbulence) understand the order of the length over which a significant change in speed occurs. The frequencies of large-scale pulsations are low.

As Re increases, along with large-scale ones, high-frequency small-scale pulsations also appear. The scale of large-scale pulsations is of the order of the determining dimensions of the system (. D, I channel or free jet, etc.). Large-scale pulsations determine the processes of turbulent mixing: internal friction, diffusion and heat transfer. Small-scale pulsations carry out viscous dissipation. Energy from large-scale moles is transferred to small-scale ones and dissipated by them. Mixing during turbulent diffusion is completed due to molecular diffusion.

Using dimensional considerations and analogy with molecular transfer processes, the concept turbulent transfer coefficient A T, which characterizes internal friction, diffusion and heat transfer in a turbulent flow:

Where G- scale of turbulence, length of turbulent movement

praying until mixed (analogue /); - root mean square

pulsating speed.

Coefficient A t is also the coefficient of turbulent diffusion D T turbulent thermal diffusivity a t and viscosity (v T). It does not depend on the properties of the gas and is determined by the characteristics of turbulence.

Substituting (3.57) into (3.56), we obtain Prandtl’s formula

Relation (3.58) allows us to estimate the transfer coefficients in a turbulent flow. To calculate transfer (diffusion) processes, you can use relations (equations) related to molecular processes, replacing them D, a, V on D T, and t, vx. When the influence of turbulent and molecular transport is comparable, total coefficients are introduced.

Solving a large number of technical problems often involves mixing different gases (liquids) or different quantities of the same gas (liquid) in different thermodynamic states. To organize displacement processes, a fairly large range of a wide variety of mixing devices and apparatus has been developed.

In thermodynamic analysis of mixing processes, the task usually comes down to determining the parameters of the state of the mixture from the known parameters of the state of the initial mixing components.

The solution to this problem will be different depending on the conditions under which this process is carried out. All methods for the formation of mixtures of gases or liquids that occur under real conditions can be divided into three groups: 1) the process of mixing in a constant volume; 2) the process of mixing in a stream; 3) mixing when filling the volume.

Mixing processes are usually considered to occur without heat exchange between the mixing system and the environment, i.e., occurring adiabatically. Mixing in the presence of heat exchange can be divided into two stages: adiabatic mixing without heat exchange and heat exchange in the resulting mixture with the environment.

In order to simplify the conclusions, let us consider the mixing of two real gases. The simultaneous mixing of three or more gases can be found using calculation formulas for two gases by sequentially adding a new component.

All cases of mixing are irreversible processes, if only because separating the mixture into its components necessarily requires an expenditure of work. As in any irreversible process, during mixing there is an increase in entropy S c systems and corresponding loss of performance (exergy): De = T o.s. S c , where Tо.с – ambient temperature.

When mixing gases that have different pressures and temperatures, additional losses in performance arise from irreversible heat exchange between the mixed gases and from the failure to use the difference in their pressures. Thus, an increase in entropy during mixing occurs both as a result of the actual mixing (diffusion) of gases or liquids that are different in nature, and due to the equalization of temperatures and pressures of the mixed substances.

Let's look at possible mixing methods.

2.1. Constant volume mixing processes

Let some thermally insulated vessel of volume V divided by a partition into two compartments, one of which contains gas (liquid) with parameters p 1, u 1, T 1 , U 1, in the other – another gas (liquid) with parameters p 2, u 2, T 2 , U 2, (Fig. 2.1).

Rice. 2.1. Mixing process diagram

in a constant volume

We denote the mass of gas in one compartment and the volume of this compartment respectively m 1 and V 1, and in the other compartment - m 2 and V 2. When the dividing partition is removed, each gas will spread through diffusion to the entire volume, and the resulting volume of the mixture will obviously be equal to the sum V = V 1 + V 2. As a result of mixing, the pressure, temperature and density of the gas throughout the entire volume of the vessel are equalized. Let us denote the values ​​of the gas state parameters after mixing p,u, T, U.

According to the law of conservation of energy, the resulting mixture of gases will have internal energy equal to the sum of the internal energies of each gas:

U = U 1 + U 2

m 1 u 1 + m 2 u 2 = (m 1 + m 2) u = mu. (2.1)

The specific internal energy of the gas after mixing is determined as follows:

. (2.2)

Similarly, the specific volume of the mixture is equal to:

. (2.3)

As for the remaining parameters of the gas after mixing ( p, T, S), then for gases and liquids they cannot be calculated analytically in a general form through the values ​​of the parameters of the mixture components. To determine them you need to use U, u-diagram on which isobars and isotherms are plotted or U, T- a diagram with isochores and isobars marked on it (for mixing the same gas), or tables of the thermodynamic properties of gases and liquids. Having determined using relations (2.2) and (2.3) u of the gas after mixing, one can find from diagrams or tables p, T, S.

Values p, T And S gases after mixing can be directly expressed through the known values ​​of the state parameters of the mixed portions only for ideal gases. Let us denote the average value of the heat capacity of the first gas in the temperature range from T 1 to T through , and another gas in the temperature range from T 2 to T through
.

Considering that
;
;
from expression (2.2), we obtain:

T =
or T =
, (2.4)

Where g 1 and g 2 – mass fractions of ideal gases making up the mixture.

From the equation of state of ideal gases it follows:

m 1 = ;m 2 = .

After substituting the mass values ​​into (2.4), the temperature of the gas mixture can be found from the expression

T =
. (2.5)

We define the pressure of a mixture of ideal gases as the sum of the partial pressures of the components of the gas mixture
, where the partial pressures And are determined using the Clapeyron equation.

Entropy increment S c systems from irreversible mixing are found by the difference in the sums of entropy of the gases included in the mixture after mixing and the initial components before mixing:

S = S – (m 1 S 1 + m 2 S 2).

For a mixture of ideal gases when two gases are mixed.

S c = m[(g 1 C p 1 + g 2 C p 2)ln T – (g 1 R 1 + g 2 R 2)ln p]–

– [m 1 (C p 1 ln T 1 – R ln p 1) + m 2 (C p 2 ln T 2 – R ln p 2)]–

–m(R 1 g 1 ln r 1 + R 2 g 2 ln r 2),

Where r i– volume fraction of ideal gases making up the mixture;

R– gas constant of the mixture, determined by the equation:

R = g 1 R 1 + g 2 R 2 .

A diagram of exergy and anergy for mixing in a constant volume is shown in Fig. 2.2.

Rice. 2.2. Diagram of exergy and anergy at

mixing in a constant volume:
– loss of specific exergy during mixing

2. Mixing of gases and vapors having different temperatures.

This is how atmospheric fogs are formed. Most often, fog appears in clear weather at night, when the Earth's surface, intensively giving off heat, cools down greatly. Warm, moist air comes into contact with the cooling Earth or with cold air near its surface and droplets of liquid form in it. The same thing happens when warm and cold air fronts mix.

3. Cooling of the gas mixture containing steam.

This case can be illustrated by the example of a kettle in which water has boiled. Water vapor escapes from the spout, which is invisible because it does not scatter light. Next, the water vapor quickly cools, the water in it condenses, and already at a short distance from the spout of the kettle we see a milky cloud - fog that has become visible due to the ability to scatter light. A similar phenomenon is observed when we open the window on a frosty day. A more durable aerosol is formed when oil boiling in a frying pan creates a gas (oil aerosol) in the room, which can only be removed by well ventilating the room.

In addition, condensation aerosol can be formed as a result of gas reactions leading to the formation of non-volatile products:

· during fuel combustion, flue gases are formed, the condensation of which leads to the appearance of combustion smoke;

· when phosphorus burns in air, white smoke is formed (P 2 O 5);

· the interaction of gaseous NH 3 and HC1 produces smoke MH 4 C1 (sv);

· oxidation of metals in air, which occurs in various metallurgical and chemical processes, is accompanied by the formation of fumes consisting of particles of metal oxides.

DISPERSION METHODS

Dispersive aerosols are formed during the grinding (spraying) of solid and liquid bodies in a gaseous environment and during the transition of powdery substances into suspended states under the action of air currents.

Spraying of solids occurs in two stages:

grinding and then spraying. The transfer of a substance into an aerosol state must be carried out at the time of application of the aerosol, since, unlike other dispersed systems - emulsions, suspensions, aerosols cannot be prepared in advance. In household conditions, almost the only means of obtaining liquid and powder aerosols is a device called an “aerosol package” or “aerosol can.” The substance in it is packaged under pressure and sprayed using liquefied or compressed gases.

GENERAL CHARACTERISTICS OF AEROSOLS

The properties of aerosols are determined by:

The nature of the substances of the dispersed phase and the dispersion medium;

Partial and mass concentration of aerosol;

Particle size and particle size distribution;

Shape of primary (non-aggregated) particles;

Aerosol structure;

Particle charge.

To characterize the concentration of aerosols, like other disperse systems, mass concentration and numerical (partial) concentration are used.

Mass concentration is the mass of all suspended particles per unit volume of gas.

Numerical concentration is the number of particles per unit volume of aerosol. No matter how great the numerical concentration is at the moment of aerosol formation, after a few seconds it cannot exceed 10 3 particles/cm 3 .

AEROSOL PARTICLE SIZES

The minimum particle size is determined by the possibility of the substance existing in a state of aggregation. Thus, one molecule of water cannot form a gas, a liquid, or a solid. To form a phase, aggregates of at least 20-30 molecules are required. The smallest particle of a solid or liquid cannot have a size less than 1 10 -3 microns. To consider a gas as a continuous medium, it is necessary that the particle sizes be much larger than the free path of gas molecules. The upper limit of particle size is not strictly defined, but particles larger than 100 microns are not able to remain suspended in the air for a long time.

MOLECULAR-KINETIC PROPERTIES OF AEROSOLS

Features of the molecular kinetic properties of aerosols are due to:

Low concentration of dispersed phase particles - so, if 1 cm 3 of gold hydrosol contains 10 16 particles, then the same volume of gold aerosol contains less than 10 7 particles;

Low viscosity of the dispersion medium - air, therefore, low coefficient of friction (B) arising during the movement of particles;

Low density of the dispersion medium, therefore ρ part » ρ gas.

All this leads to the fact that the movement of particles in aerosols occurs much more intensely than in lyosols.

Let's consider the simplest case, when the aerosol is in a closed container (i.e., external air flows are excluded) and the particles have a spherical shape with radius r and density p. Such a particle is simultaneously acted upon by a gravity force directed vertically downward and a friction force in the exact opposite direction. In addition, the particle is in Brownian motion, the consequence of which is diffusion.

To quantify the processes of diffusion and sedimentation in aerosols, you can use the values

specific diffusion flux i diff and

specific sedimentation flux i sed. .

To find out which flow will prevail, consider their ratio:

In this expression (p - p 0) » 0. Consequently, the size of the fraction will be determined by the size of the particles.

If r > 1 μm, then i sed » i diff, i.e., diffusion can be neglected - rapid sedimentation occurs and the particles settle to the bottom of the vessel.

If r< 0,01 мкм, то i сед « i диф. В этом случае можно пренебречь седиментацией - идет интенсивная диффузия, в результате которой частицы достигают стенок сосуда и прилипают к ним. Если же частицы сталкиваются между собой, то они слипаются, что приводит к их укрупнению и уменьшению концентрации.

Thus, both very small and very large particles quickly disappear from the aerosol: the former due to adhesion to the walls or adhesion, the latter as a result of settling to the bottom. Particles of intermediate sizes have maximum stability. Therefore, no matter how large the numerical concentration of particles is at the moment of aerosol formation, after a few seconds it does not exceed 10 3 parts/cm 3 .

ELECTRICAL PROPERTIES OF AEROSOLS

The electrical properties of aerosol particles differ significantly from the electrical properties of particles in lyosol.

1. EDL does not occur on aerosol particles, since due to the low dielectric constant of the gaseous medium, electrolytic dissociation practically does not occur in it.

2. The charge on the particles arises mainly due to the indiscriminate adsorption of ions that are formed in the gas phase as a result of ionization of the gas by cosmic, ultraviolet or radioactive rays.

3. The charge of particles is random in nature, and for particles of the same nature and the same size it can be different both in magnitude and sign.

4. The charge of a particle changes over time both in magnitude and sign.

5. In the absence of specific adsorption, the charges of the particles are very small and usually exceed the elementary electric charge by no more than 10 times.

6. Specific adsorption is characteristic of aerosols, the particles of which are formed by a highly polar substance, since in this case a fairly large potential jump occurs on the interphase surface, due to the surface orientation of the molecules. For example, at the interfacial surface of water or snow aerosols there is a positive electric potential of the order of 250 mV.

It is known from practice that particles of aerosols of metals and their oxides usually carry a negative charge (Zn, ZnO, MgO, Fe 2 0 3), and particles of aerosols of non-metals and their oxides (SiO 2, P 2 O 5) are positively charged. NaCl and starch particles are positively charged, while flour particles carry negative charges.

AGGREGATIVE STABILITY. COAGULATION

Unlike other dispersed systems, in aerosols there is no interaction between the surface of the particles and the gaseous medium, which means there are no forces that prevent the particles from adhesion to each other and to macroscopic bodies upon collision. Thus, aerosols are aggregatively unstable systems. Coagulation in them occurs according to the type of rapid coagulation, i.e., each collision of particles leads to their sticking together.

The coagulation rate increases rapidly with increasing aerosol numerical concentration.

Regardless of the initial concentration of the aerosol, after a few minutes there are 10 8 -10 6 particles in 1 cm 3 (for comparison, in lyosols there are ~ 10 15 particles). Thus, we are dealing with very highly dilute systems.

METHODS FOR AEROSOL DESTRUCTION

Despite the fact that aerosols are aggregatively unstable, the problem of their destruction is very acute. The main problems, the solution of which requires the destruction of aerosols:

Purification of atmospheric air from industrial aerosols;

Capturing valuable products from industrial smoke;

Artificial sprinkling or dispersal of clouds and fog.

Aerosols are destroyed by

· dispersion under the influence of air currents or due to charges of particles of the same name;

· sedimentation;

· diffusion to the walls of the vessel;

· coagulation;

· evaporation of dispersed phase particles (in the case of aerosols of volatile substances).

The most ancient of the treatment facilities is the chimney. They try to release harmful aerosols into the atmosphere as high as possible, since some chemical compounds, entering the ground layer of the atmosphere under the influence of sunlight and as a result of various reactions, are converted into less dangerous substances (at the Norilsk Mining and Metallurgical Combine, for example, a three-channel pipe has a height 420 m).

However, the modern concentration of industrial production requires that smoke emissions be pre-treated. Many methods have been developed for destroying aerosols, but any of them consists of two stages:

the first is the capture of dispersed particles, their separation from the gas,

the second is to prevent particles from reentering the gaseous environment; this is due to the problem of adhesion of captured particles and the formation of a durable sediment from them.

AEROSOL CYLINDERS

The principle of operation of an aerosol can is that the drug placed in the package is mixed with an evacuating liquid, the saturated vapor pressure of which in the temperature range at which the package is operated is higher than atmospheric.

The mixture is released from the cylinder under the influence of saturated vapor pressure above the liquid.

It is known that the saturated vapor pressure of any stable substance is determined only by temperature and does not depend on volume. Therefore, during the entire operating time of the cylinder, the pressure in it will remain constant, therefore, the flight range of the particles and the angle of the spray cone will remain almost constant.

Depending on the nature of the interaction of the sprayed substance with the evacuating liquid and its state of aggregation, systems in aerosol packaging will consist of a different number of phases. In the case of mutual solubility of the components, a homogeneous liquid solution is formed, in other cases - an emulsion or suspension, and, finally, a heterogeneous system, when the drug and the evacuating liquid form a macroscopically heterogeneous system. Obviously, in the first case, the aerosol package contains a two-phase system - liquid and saturated vapor. When an emulsion or suspension is released into the atmosphere, only the dispersion medium is crushed - the resulting particles, at best, will have the dimensions that they had in the liquid phase.

When the drug and the evacuation liquid do not mix or mix with each other to a limited extent, with one of the liquids dispersed in the other in the form of small droplets, emulsions are formed.

The nature of the system formed when the product leaves the packaging into the atmosphere depends on which of the liquids is the dispersed phase. If the dispersed phase is a drug, then an aerosol is formed. If the dispersed phase is an evacuating liquid, then foam is obtained. The size of particles obtained using aerosol cans depends on the physico-chemical properties of the substances included in the preparation, the ratio of components, the design features of the can and the temperature conditions of its operation.

The degree of dispersion can be adjusted: “by varying the size of the outlet;

By changing the saturated vapor pressure of the evacuating liquid;

By changing the quantitative ratio of the drug and the evacuation agent.

EVACING SUBSTANCES

The most important auxiliary component is a substance that ensures the release of the drug into the atmosphere and its subsequent dispersion. These substances are called propellants (Latin “pro-peilere” - to drive). The propellant must perform two functions:

Create the necessary pressure to release the drug;

Disperse product released into atmosphere. Freons and compressed gases are used as propellants. Freons are low molecular weight organofluorine compounds of the aliphatic series.

The following system of notation for freons has been adopted: the last digit (number of units) means the number of fluorine atoms in the molecule, the previous digit (number of tens) means the number of hydrogen atoms increased by one, and the third (number of hundreds) means the number of carbon atoms decreased by one. For example: F-22 is CHC1F 2, F-114 is C 2 C1 2 F 4.

Substances consisting of molecules of a cyclic structure also have a numerical designation, but the letter “C” is placed before the numbers, for example: C318 - C 4 F 8 (octafluorocyclobutane).

N2, N2O, CO2, etc. are used as compressed gases.

ADVANTAGES OF AEROSOL PACKAGINGS

1. The transfer of the drug into a finely dispersed state occurs due to the potential energy of the liquefied propellant and does not require the use of any extraneous devices.

2. No attachments are needed to create aerosols.

3. In a unit of time, a significant amount of substance can be dispersed to produce small particles - if other methods were used, much more energy would be required.

4. The fogging mode is stable: the size of the resulting particles, their flight range, and the angle at the apex of the cone change little during the entire period of operation.

5. You can pre-fix the dosage of the sprayed substance.

6. You can set the particle size.

7. The degree of polydispersity of the aerosol is low.

8. All particles have the same chemical composition.

9. The sterility of sprayed drugs is ensured.

10. The drug in the package does not come into contact with air oxygen, which ensures its stability.

11. Automatically closing valve eliminates the possibility of loss due to spillage or evaporation of unused portion of the product.

12. The packaging is always ready for use.

13. Packaging is compact. Allows individual or collective use.

The first aerosol packages appeared in the 80s. XX century in Europe. During World War II, the United States took the initiative in their development. In 1941, aerosol packaging was created - an insect killer packaged in a glass container. The propellant was Freon-12.

Production on an industrial scale began after World War II in the United States and then in other countries around the world.

PRACTICAL APPLICATION OF AEROSOLS

The widespread use of aerosols is due to their high efficiency. It is known that an increase in the surface of a substance is accompanied by an increase in its activity. A small amount of a substance sprayed in the form of an aerosol occupies a large volume and is highly reactive. This is the advantage of aerosols over other dispersed systems.

Aerosols are used:

In various fields of technology, including military and space;

In agriculture; "in healthcare;

In meteorology; in everyday life, etc.

Recently, the preparation of dosage forms in the form of aerosols has been widely used in pharmaceutical practice. The use of medicinal substances in the form of aerosols is convenient in cases where it is necessary to apply the drug to large surfaces (acute respiratory diseases, burns, etc.). Dosage forms containing liquid film-forming substances have a great effect. When this drug is sprayed onto the affected area, it is covered with a thin, transparent film that replaces the bandage.

Let us dwell in more detail on the use of aerosol packaging.

Currently, there are more than 300 types of products in aerosol packaging.

First group: household chemicals.

Insecticides are preparations for killing insects.

Anti-moth products.

Insecticides for treating domestic animals.

Means for protecting indoor plants and fruit and berry crops from fungal diseases and pests.

Varnishes and paints.

Air fresheners.

c Polishing and cleaning compounds.

Second group:

Perfumery and cosmetics. “Hair care products (sprays, shampoos, etc.).

Shaving foams and gels.

Creams for hands and feet.

Oil for and against tanning.

Deodorants.

Perfumes, colognes, eau de toilette.

Third group: medical aerosols.

Fourth group: technical aerosols.

Lubricating oils.

Anti-corrosion coatings.

Protective films. “Dry lubricants.

Emulsions for cooling cutters on drilling machines.

Fifth group: food aerosols.

FOOD AEROSOLS

The first food containers appeared in 1947 in the USA. They contained creams for finishing cakes and pastries and were only used by restaurants, which returned them for refilling. Mass production of this type of aerosol packaging began only in 1958.

Aerosol food packaging can be divided into three main groups:

packages requiring storage at low temperatures;

packaging with subsequent heat treatment;

packaging without subsequent heat treatment.

Three types of food products are produced in aerosol packages: creams, liquids, pastes. In aerosol packages you can buy salad dressings, processed cheese, juices, cinnamon, mayonnaise, tomato juice, 30% whipped cream, etc.

The growth in food aerosol production is due to the following:

advantages over conventional types of packaging;

development of new propellants;

improvement of filling technology.

Advantages of aerosol food packaging:

ease of use;

saving time;

food is packaged in a ready-to-eat state and is released from the package in a uniform form;

no product leakage;

moisture is not lost or penetrates into the packaging;

the aroma is not lost;

the product is kept sterile.

The following requirements apply to food aerosol formulations:

1. Propellant must be of high purity, non-toxic, tasteless and odorless. Currently, carbon dioxide, nitrous oxide, nitrogen, argon and C318 freon are used.

2. Compressed gases, which have very limited solubility in aqueous solutions, cannot participate in the formation of foam, and this is necessary for whipped cream, decorative creams, mousses, etc. It is preferable to use C318 freon with these products, although it is much more expensive.

Table 18.4 Examples of formulations for various food aerosols

3. The use of freons provides another advantage: liquefied gases are introduced into product formulations, which are released in the form of foam, in an amount of no more than 10% by weight, while they occupy a relatively small volume. This allows you to load significantly more products into the cylinder - 90% of the cylinder capacity (in packages with compressed gas only 50%) and guarantees complete release of the product from the package.

4. The choice of propellant is dictated by the type of food product and the intended delivery form (cream, liquid, paste). Mixtures of CO2 and high-purity nitrous oxide have proven themselves well. To obtain foam, mixtures of C318 freon with nitrous oxide are used. Cake finishing cream packaged with this mixture produces a stable foam that retains color well. For syrups, CO2 is considered the most suitable propellant.

The quality of dispensing the contents from the cylinder depends on the following factors:

Product preparation technologies;

Stabilizer (microcrystalline cellulose is widely used);

Correct choice of cylinder and valve.

For cinnamon and lemon juice, a controlled spray head has been developed that can dispense the products either as drops or as a stream as desired. For artificial sweeteners, dosing valves are used, one dose they dispense corresponds to one piece of sawn sugar, etc.

AEROSOL TRANSPORT

Pneumatic transport is widely used in the flour-grinding, cereal, and feed milling industries, which creates conditions for the introduction of automation, increasing labor productivity and reducing costs. However, the use of pneumatic transport is associated with a large expenditure of electricity to move a large volume of air (1 kg of air moves 5-6 kg of bulk material).

More progressive is aerosol transport, in which a large concentration of material in the air flow is achieved due to aeration of flour at the beginning of transportation and high air pressure. Aeration breaks the adhesion between flour particles, and it acquires the property of fluidity, like a liquid; as a result, 1 kg of air moves up to 200 kg of flour.

The aerosol transport installation consists of a feeder, a supercharger, a material pipeline and an unloader. The main element is the feeder, in which air is mixed with the material and the initial speed is imparted to the mixture, which ensures its supply to the material pipeline.

The introduction of aerosol transport makes it possible to increase the productivity of mills and reduce specific energy consumption.

Aerosol transport holds the future not only in flour milling, but also in other industries related to the use of bulk materials and powders.

Aerosols are microheterogeneous systems in which solid particles or liquid droplets are suspended in a gas (T/G or L/G),

According to the aggregate state of the dispersed phase, aerosols are divided into: fog (L/G); smoke, dust (T/G); smog [(F+T)/G)].

According to their dispersion, aerosols are divided into: fog, smoke, dust.

Like other microheterogeneous systems, aerosols can be obtained from true solutions (condensation methods) or from coarsely dispersed systems (dispersion methods).

Water droplets in fogs are always spherical, while solid smoke particles can have different shapes depending on their origin.

Due to the very small particle sizes of the dispersed phase, they have a developed surface on which adsorption, combustion, and other chemical reactions can actively occur.

The molecular-kinetic properties of aerosols are determined by:

low concentration of dispersed phase particles; low viscosity of the dispersion medium; low density of the dispersion medium.

Depending on the size of the particles of the dispersed phase, they can either quickly sediment (at r < 1 μm), or stick to the walls of the vessel or stick together (at r < 0.01 μm). Particles of intermediate sizes have the greatest stability.

Aerosols are characterized by the phenomena of thermophoresis, thermoprecipitation, and photophoresis.

The optical properties of aerosols are similar to the properties of lyosols, but the scattering of light by them is much more pronounced due to the large differences in the refractive indices of the dispersed phase and the dispersion medium.

The specificity of the electrical properties of aerosols is that EDL does not appear on the particles, the charge of the particles is random and small in magnitude. When particles approach each other, electrostatic repulsion does not occur and rapid coagulation occurs.

The destruction of aerosols is an important problem and is carried out by sedimentation, coagulation, dust collection and other methods.

Powders are highly concentrated disperse systems in which the dispersed phase is solid particles and the dispersion medium is air or other gas. Symbol: T/G.

In powders, particles of the dispersed phase are in contact with each other. Traditionally, most bulk materials are classified as powders, however, in a narrow sense, the term “powders” is applied to highly dispersed systems with a particle size less than a certain critical value at which the forces of interparticle interaction become commensurate with the mass of the particles. The most common are powders with particle sizes from 1 to 100 microns. The specific interfacial surface of such powders varies from several m11.09.2011 (soot) to fractions of m2/g (fine sands).

Powders differ from aerosols with a solid dispersed phase (also T/G) by a much higher concentration of solid particles. The powder is obtained from an aerosol with a solid dispersed phase during its sedimentation. The suspension (S/L) also turns into powder when it is dried. On the other hand, both an aerosol and a suspension can be obtained from a powder.

CLASSIFICATION OF POWDERS

1. According to the shape of the particles:

Equiaxial (have approximately the same dimensions along three axes);

Fibrous (the length of the particles is much greater than the width and thickness);

Flat (length and width are much greater than thickness).

2. According to interparticle interaction:

Connectively dispersed (particles are linked to each other, i.e. the system has some structure);

Freely dispersed (shear resistance is due only to friction between particles).

3. Classification by particle size of the dispersed phase:

Sand (2≤10 -5 ≤ d ≤ 2∙10 -3) m;

Dust (2∙10 -6 ≤ d ≤ 2∙10 -5) m;

Powder (d< 2∙10 -6) м.

METHODS FOR OBTAINING POWDERS

Powders, just like any other dispersed system, can be obtained by two groups of methods:

On the part of coarse systems - by dispersion methods;

On the part of true solutions - by condensation methods.

The choice of method depends on the nature of the material, the purpose of the powder and economic factors.

DISPERSION METHODS

The raw materials are crushed in roller, ball, vibration or colloid mills, followed by separation into fractions, since as a result of grinding, polydisperse powders are obtained (for example, flour of the same type may contain particles from 5 to 60 microns).

Effective dispersion can be achieved by grinding very concentrated suspensions.

To facilitate dispersion, hardness reducers are used, which are surfactants. In accordance with the rule of polarity equalization, when adsorbed on the surface of the ground solid, they reduce surface tension, reducing energy consumption during dispersion and increasing the dispersion of the ground phase.

In some cases, the material is pre-treated before dispersion. Thus, titanium or tantalum is heated in a hydrogen atmosphere, converted into hydrides, which are crushed and heated in a vacuum - pure metal powders are obtained.

When producing flake powders, which are included in paints and pyrotechnic compositions, ball mills are used for grinding. The balls flatten and roll the particles of the crushed material.

Powders with spherical particles made of refractory metals (tungsten, molybdenum, niobium) are obtained in low-temperature plasma of an arc and high-frequency discharge. Passing through the plasma zone, the particles melt and take a spherical shape, then cool and solidify.

During dispersion, the chemical composition of the material does not change.

CONDENSATION METHODS

These methods can be divided into two groups.

The first group of methods is associated with the deposition of particles due to the coagulation of lyophobic sols. As a result of evaporation of the solution or partial replacement of the solvent (decrease in solubility), a suspension is formed, and after its filtration and drying, powders are obtained.

The second group of methods is associated with chemical reactions (chemical condensation). Chemical condensation methods can be classified based on the type of reaction used:

1. Exchange reactions between electrolytes. For example, precipitated chalk (tooth powder) is obtained as a result of the reaction:

Na 2 CO 3 + CaC1 2 = CaCO 3 + 2 NaCl.

2. Oxidation of metals.

For example, highly dispersed zinc oxide, which is the main component of zinc white, is obtained by oxidizing zinc vapor with air at 300°C.

3. Oxidation of hydrocarbons.

Various types of soot, which are used in the production of rubber, plastics, and printing ink, are produced by burning gaseous or liquid hydrocarbons in the absence of oxygen.

4. Reduction of metal oxides.

Reduction with natural gas, hydrogen or solid reducing agents is used to produce highly dispersed metal powders.

And much more, without which life itself is unthinkable. The entire human body is a world of particles that are in constant motion strictly according to certain rules that obey human physiology. Colloidal systems of organisms have a number of biological properties that characterize a particular colloidal state: 2.2 Colloidal system of cells. From the point of view of colloid-chemical physiology...

Let them mix n chemically non-reacting among themselves ideal gases It is assumed that the initial thermodynamic parameters of the state of all components before mixing and the mixing conditions (conditions of interaction with the environment) are known. Need to find equilibrium parameters of the state of gases after mixing.

Let us consider two cases of mixing, for simplicity assuming that this process occurs without heat exchange with the environment .

2.1. Mixing at W=Const

In this case, the mixing conditions are such that the volume of the resulting mixture W cm is equal to the sum of the initial volumes of the mixture components W H i:

(Not to be confused W H i with partial volumes W i, discussed in paragraph 1.4.3.)

Let's denote:

P H i– initial pressure i th gas;

T H i,t H i– initial temperature i-th gas respectively at 0 TO or 0 WITH.

Because the whole system from n gases when mixed under conditions W=Const does not perform external work, then in accordance with the first law of thermodynamics for this case () we can write:

Here: U cm – internal energy of a mixture of gases weighing m cm kilograms

with temperature T 0 K;

U H i- internal energy i th gas mass m i kilograms

with initial temperature T H i .

Let us introduce the following notation:

u cm – specific internal energy of a mixture of gases at temperature T 0 K;

u H i – specific internal energy i-th gas with initial temperature T H i .

Then equation (2.1.1) takes the following form:

(2.1.2)

As is known, for an ideal gas du=C v dT, from where, when counting the internal energy from 0 0 K can be written:

Here: - average in the range 0 T 0 K mass isochoric heat capacity of a mixture of gases;

Average in range 0 T H i 0 K mass isochoric heat capacity i th gas.

After substituting (2.1.3) into (2.1.2) we get:

But in accordance with paragraph 1.4.10, the true mass heat capacity of a mixture of gases is expressed in terms of the mass fractions of the components g i and their true heat capacities as follows:

Similarly, the average in the range 0 T 0 K The mass isochoric heat capacity of a mixture of gases is determined as:

Substituting this expression into the left side of equation (2.1.4) we obtain:

from where (2.1.5)

Because from the equation of state, then after substitution m i into equation (2.1.5) we finally obtain the formula for the temperature of the mixture n gases:

As is known, therefore formula (2.1.6) can be written in the following form:

(It should be recalled that the product is the average in the range 0- T H i 0 Kmolar isochoric heat capacity i th gas.)

In reference literature, empirical dependences of heat capacity on temperature are often given for the range 0 t 0 C .

After substituting (2.1.8) and (2.1.9) into equation (2.1.2) we obtain:

Replacing m i its value, we finally obtain the formula for the temperature of the gas mixture in degrees Celsius :

Expressing R i through the molecular mass, we get another formula:

The denominators of formulas (2.1.6), (2.1.7), (2.1.10) and (2.1.11) contain average heat capacities, for which the temperature of the mixture is used as the upper limit of averaging ( t or T), to be determined. Because of this, the temperature of the mixture is determined by these formulas method of successive approximations .

2.1.1. Special cases of gas mixing during W=Const

Let us consider several special cases of formulas (2.1.6), (2.1.7), (2.1.10) and (2.1.11).

1. Let gases be mixed, for which the dependence of the adiabatic exponent K i temperature can be neglected.

(Actually TO decreases with increasing temperature, because

Where s o r , A are empirical positive coefficients.

For technical calculations in the range from 0 to 2000 0 C, you can use the following formulas:

a) for diatomic gases TO 1,40 - 0,50 10 -4 t;

b) for combustion products TO 1,35 - 0,55 10 -4 t.

From these formulas it is clear that the effect of temperature on the adiabatic index TO becomes noticeable only at temperatures on the order of hundreds of degrees Celsius.)

Thus, if we assume that

then formula (2.1.6) will take the following form:

Formula (2.1.12) can be used as a first approximation for formulas (2.1.6), (2.1.7), (2.1.10) and (2.1.11)

2. Let gases be mixed whose molar isochoric heat capacities are equal and the dependence of these heat capacities on temperature can be neglected, i.e.:

Then equation (2.1.7) takes on a very simple form:

If gases have equal molar isochoric heat capacities, then in accordance with Mayer’s equation

The molar isobaric heat capacities must be equal to each other, and, therefore, the adiabatic exponents must be equal, i.e.

Under this condition, equation (2.1.12) turns into (2.1.13).

2.1.2. Pressure after mixing gases at W=Const

The pressure established after mixing the gases can be determined either by the formulas of paragraph 1.4.2 or from the condition:

R cm W cm = m cm R cm T= m cm T.