Chapter 9. General information about mixing gases.
Goals and objectives of the chapter:
Learn about fire safety rules when working with oxygen
Learn about the rules for handling and working with oxygen
Learn about the application of the "40% rule"
Learn about different systems for mixing gases.
New terms in this chapter.
Flammable (fire hazardous) triangle
Oxygen-compatible grease
Adiabatic heating (Diesel process)
Oxygen cleaning
40% rule
Mixing partial pressures
Constant flow mixing
Absorption with periodic cleaning of the absorbent
Membrane separation.
As a diver using enriched mixtures in your dives, you must be able to obtain these mixtures. You do not need to know how to prepare nitrox yourself, however, you should have an understanding of how it is prepared and the cleaning requirements of your equipment that nitrox imposes. Some of the commonly used methods for producing fortified mixtures are reviewed in this chapter, and their advantages and disadvantages are discussed. The mixture you breathe must have the appropriate oxygen content.
1. Handling and working with oxygen.
Oxygen is an amazing gas. He can be both a friend and an enemy. When mixing gases for scuba use, the operator must obtain the appropriate oxygen content in the high-pressure mixture. This can be done by mixing pure oxygen with nitrogen or air, or by removing some of the nitrogen from the air. The main problem with mixing high-pressure oxygen is the fire hazard. Anything that is not completely oxidized - and that means practically everything - will burn in high-pressure oxygen if an ignition source is present. There is some risk when handling mixtures, but handling pure compressed oxygen poses a much greater risk. A diver using enriched mixtures does not need to be proficient in handling pure oxygen, but should have some understanding of the associated risks as oxygen is used as the diver's activities become more complex and extensive.
2. Flammable (fire hazardous) triangle.
To prevent a fire, you need to know what components cause and support a fire. These components are shown in the figure
in the form of a so-called “flammable or fire-hazardous triangle”. Fire is a rapid chemical reaction between fuel and oxygen (oxidizer) that can only occur if there is an ignition source (heat). Oxidation can occur without combustion, as, for example, during the rusting process. Fire occurs when there is a source of ignition (heat). After ignition, a chemical combustion reaction releases energy (heat), which supports further combustion. If we remove one of the components (fuel, oxygen, ignition source), fire cannot occur. If, therefore, all three components are not present at the same time, fire will be prevented. If a flame already exists, removing one of the components will cause the flame to go out. These are the basics of fire fighting theory. Another important point is that fire must spread in order to maintain its existence. Sometimes the desire to spread fire is even added as another component of the “triangle” described above.
3.Oxygen.
In the situations discussed below, oxygen is present in concentrations greater than its concentration in air. This means that the oxidizer in the “flammable triangle” is always present by default and cannot be removed from this “fire formula”. Everyone knows that atmospheric oxygen can actively participate in combustion reactions under appropriate circumstances, so it should not be surprising that higher concentrations can only increase the risk. Further, it is necessary to remember that an increased oxygen content in the air means a reduced inert gas content. For this and some other reasons, the combustion intensity does not depend linearly on the percentage of oxygen. It depends on both the percentage (share) of oxygen in the mixture and its partial pressure and increases significantly as these parameters increase.
4.Fuel.
In this paragraph we will talk about the fuel available in the gas system that provides the use of gas for breathing. At high oxygen pressures, if a fire occurs, the system itself can become the fuel for a chemical reaction, but something more flammable is needed to start a fire. This could be some separate part of the system, a solvent, a lubricant, or soft components of the system (rubber, plastic).
Some fuels found in gas systems may be virtually non-flammable under normal conditions and highly flammable in an oxygen-enriched environment. These types of fuel include silicone grease, silicone rubber, neoprene, compressor lubricants, plastic and metal shavings and burrs, organic substances and materials, dust of various types, even grease on hoops. Perhaps the most dangerous fuels are various lubricants. There is a common misconception that silicone (probably due to the exotic name) is safe when used with oxygen. Actually this is not true. There are special oxygen-compatible lubricants, such as Christo-lube, Krytox, Halocarbon. It is precisely these self-lubricants that should be used in an oxygen-enriched environment.
5. Ignition.
Some ignition sources are obvious, however, most of them are outside the gas system and are not considered by us. The two main sources of ignition within a system are friction and compression of the gas as it passes through the system. The term "friction" is used here in a general sense: in the sense of the presence of any particles in the gas flow or in the sense of the movement of the gas flow itself and its collision with the corners of gas pipelines or other obstacles. Another phenomenon - the same one that causes the cylinder to heat up - can also cause a fire (if enough heat is released). This is the same effect that causes fuel to ignite in the cylinders of a diesel engine without a spark plug. This effect is called "adiabatic heating (Diesel process)".
The sudden opening and closing of a cylinder valve during gas compression can cause an increase in temperature to the ignition point, and if there are contaminants in the gas flow, the ignition itself. Therefore, compressors do not use quick changeover valves (“ball valves”).
6.Use of oxygen systems.
The important message of this chapter is that the risk of handling oxygen can be minimized by following certain rules in the design and handling of systems. In particular, it is important to avoid sharp corners and quick change valves and to use appropriate materials. The metals used to make air systems are also suitable for making oxygen systems. As for “soft components”, such as gaskets, flexible joints, diaphragms, they must be replaced with oxygen-compatible ones. In some cases the main criterion is less flammability in oxygen, but in most cases it is increased resistance to oxygen under high pressure. Special kits are available that allow you to convert air equipment into equipment for using nitrox.
These include proper cleaning and maintenance of equipment, use of appropriate lubricants, handling gases in a manner that does not cause ignition, and opening valves slowly and smoothly.
7.Cleaning equipment for use with oxygen. Some considerations regarding equipment cleaning.
The concept of “oxygen cleaning” causes some confusion among amateur divers. The reason is that it is not entirely clear whether equipment needs to be cleaned for use with mixtures containing 21% to 40% oxygen. This problem has deeper roots: there are no developed and standardized industrial procedures for handling mixtures containing some intermediate amount of oxygen in the range from 21% (air) to 100% (pure oxygen). Standards exist only for the handling of pure oxygen; Thus, any mixture containing more than 21% oxygen is equivalent to pure oxygen by current standards. Therefore, in order to perform all operations in accordance with industry standards, any enriched mixture must be treated as pure oxygen.
The Compressed Gas Association CGA, the National Fire Protection Association NFPA, NASA and several other organizations recommend treating gases with intermediate concentrations as pure oxygen. This does not mean that they have performed any studies in this concentration range. This only means that there are no industrially developed and accepted standards, and these organizations prefer to take a conservative position. On the other hand, the US Navy has developed procedures stating that mixtures with an oxygen concentration of up to 40% can be treated as air for handling purposes. No test results have been published that would suggest that this conclusion is true, however, this approach has been practiced for many years and there have been no reports of accidents related to this issue. NOAA has adopted this concentration limit when working with fortified mixtures; NAUI, in general, too, however, with some restrictions.
Clean compressed air.
Another confusion arises in relation to the concept of “air purity”. The different "grades" of breathing gas purity used by various associations and organizations (CGA, US Navy) are confusing when it comes to the purity of the enriched mixture. Standards allow for the presence of some oil (hydrocarbon) vapor in compressed air (usually 5 mg/cu.m.). This amount is safe from a breathing point of view, but can be dangerous from a fire point of view when working with compressed oxygen.
Thus, there are no generally accepted and agreed upon gradations of air purity that determine its suitability for mixing with pure oxygen. Industry standard setters have agreed that hydrocarbon levels are on the order of 0.1 mg/m3. m can be considered acceptable for air, which "must further be mixed with oxygen." In the last few years, filter systems (pictured) have become available to produce compressed air that meets these requirements. Compressors that prevent air from contacting the lubricant, of course, cope with this task better, but they are significantly more expensive. A formalized approach to oxygen cleaning.
The phrase “oxygen cleaning” also sounds scary for the reason that its industrial implementation requires compliance with fairly strict procedures. These periodic procedures are published by the CGA and other organizations. They are designed to maintain safety when working with compressed oxygen.
NAUI states that any equipment intended for use with pure oxygen or with mixtures containing more than 40% oxygen at pressures greater than 200 psi (approximately 13 atm) must be oxygen-compatible and purified for use with oxygen. The cylinder, the first stage of the regulator and all hoses must be cleaned. Some pieces of equipment can be converted to handle such mixtures by using components from special kits.
8. Informal approach to oxygen cleaning: “40% rule”
Despite the lack of formal testing, the so-called "40% rule" has been used quite successfully in the diving industry, and its application has not revealed any problems. Numerous fires in diving gas mixing systems have occurred but were caused by higher oxygen concentrations.
NAUI accepts this rule but requires that equipment be oxygen-cleaned and that oxygen-compatible lubricants be used. This approach is less strict than the formal one, however, when done correctly it is very effective. Cleaning must be performed by qualified technicians.
The equipment must be cleaned of all visible dirt and grease, then brushed or ultrasonically cleaned using a strong detergent in hot water. Liquid cleaning products like Joy are good for home use. Cleanliness should be no less than that expected of plates and silverware. After drying, the soft components must be replaced with oxygen-compatible ones, after which the equipment is lubricated with an oxygen-compatible lubricant.
After cleaning, the equipment should only be used for enriched mixtures and should not be used with compressed air, otherwise it will have to be cleaned again.
9. Preparation of enriched mixtures.
The traditional scheme for constructing a gas mixing system is based on adding oxygen to the air in one way or another. Two new methods have recently been developed and become available that enrich the air in a different way - by removing nitrogen. This section will cover 3 oxygen addition methods: weight mixing, partial pressure mixing, constant flow mixing; and 2 methods with nitrogen removal: absorption with periodic cleaning of the absorbent, membrane separation (Ballantyne and Delp, 1996).
The type of gas mixing system used is important to the end user in that it determines the cylinder filling procedures and the range of possible oxygen concentrations in the resulting mixture.
Mixing gases by weight.
The simplest and most reliable method of obtaining mixtures that are accurate in composition is to purchase ready-made mixtures. Industrial gas producers typically mix pure oxygen and pure nitrogen rather than pure oxygen and air.
Gases mix by weight. This makes it possible to ignore many anomalies in the behavior of gases caused by their differences from ideal ones and provides a very accurate gas composition of mixtures. Mixing can be done in cylinders, cylinder banks or tanks. It is necessary to have accurate scales, which are quite expensive, since they must be able to measure small changes with large weights. This method of mixing gases is the most accurate, and the resulting mixtures are carefully analyzed to ensure that the actual composition matches the declared one. When preparing such mixtures, the industrial company is forced to use pure oxygen, but the retailer of the mixtures can avoid this. This method is quite expensive, and its cost is increased by the fact that the containers for storing the mixtures belong to the supplier of the mixtures, and therefore are rented by the seller of the mixtures.
Mixing of partial pressures.
As the name of the method itself says, it is based on the ratio of partial pressures. The technician fills the tank with the specified amount of oxygen (measured by the pressure value), then tops it up with ultra-pure air to the desired final pressure. First of all, oxygen is pumped in when the cylinder is still empty, which reduces the fire hazard of the procedure, since there is no need to manipulate oxygen at the full pressure of the filled cylinder. Since pure oxygen is used, the entire system, including the cylinder being filled, must be oxygen compatible and cleaned. Since pressure depends on temperature, and the cylinder heats up when filling, it is necessary to either allow the cylinder to cool or take into account the influence of temperature when measuring pressure. Since the final adjustment of the composition is often made after the cylinder has completely cooled, the entire process of preparing the mixture takes quite a lot of time. This process can also be used to refill a container of a mixture of known composition to obtain a mixture of the same or a different specific composition.
A compressor for mixing using this method is not required if the air is supplied at a pressure sufficient to fill scuba tanks without additional compression. To achieve maximum utilization of the bank of refill cylinders, they use the so-called “cascade technology”, which consists in using the refill cylinder with the lowest pressure first, followed by the cylinder with the highest pressure, and so on. Sometimes the method itself is called the “cascade mixing method.”
Compressors are also often used with this method. They must not use oil lubricants or must provide ultra-high purity air suitable for mixing with oxygen. Another way to pump air into a cylinder is to use a pneumatic pump that compresses air in a set of cylinders of different diameters, the pistons of which are connected to the same camshaft. Ogna of the most popular models is Haskel.
Partial pressure mixing is very popular among diving centers, which prepare many different mixtures in small volumes for various purposes of recreational and technical diving, including mixtures with an oxygen content of more than 40%. In this case, a significant portion of the cost of the system is a high-precision pressure gauge. In this case, the use of a pneumatic pump is very effective. This method is used in remote diving sites. Because oxygen is added at low pressure, some technicians do not clean the oxygen cylinders. This practice should be avoided: the cylinder should always be cleaned for use with oxygen.
10.Constant flow mixing.
This method (also called the atmospheric loading method) was first developed by NOAA (1979, 1991) and is the most user-friendly method (Figure 9-7). In this method, oxygen at low pressure is added to the inlet air stream entering the compressor with a high degree of oil vapor removal. The effluent stream is continuously analyzed for composition and the result of this analysis is used to adjust the oxygen admixture into the inlet stream accordingly. The output flow can bypass the bank of filling cylinders while the mixture composition is adjusted. Once the mixture is pumped into the refill cylinders, it can then be transferred to the scuba cylinders by bypass or using an air pump. A constant flow plant may also use an absorption subsystem as the oxygen source, with periodic purification of the PSA absorbent.
There is another class of constant flow units that provide air to the commercial diver through an air supply hose. Such installations have means of monitoring the constancy of the mixture composition - various flow meters and regulators. Their output pressure is typically less than 200 psi (13 atm).
11. Absorption with periodic cleaning of the absorbent (PSA).
This method is based on the use of a material called a "molecular sieve" - a synthetic porous clay-like material whose pores provide a very large surface area. This surface adsorbs gases (“adsorb” means “absorb on a surface”). Nitrogen is adsorbed faster than oxygen, so the air passing through the adsorbent becomes richer in oxygen (more precisely, poorer in nitrogen). Two adsorbing plates are used, between which the air flow is switched. When the flow is directed to one plate, it adsorbs nitrogen, the second plate at this time is cleared of previously adsorbed nitrogen. Then the plates switch roles.
By changing the pressure and the frequency of cleaning the plates, it is possible to obtain different values of the oxygen content in the output mixture. The maximum achievable oxygen content is 95%, the rest is argon. Argon behaves in relation to this type of adsorbent almost like oxygen (i.e., it is not adsorbed), therefore it will be contained in the outlet mixture in almost the same proportion to oxygen as in the inlet air. This argon has no effect on the diver.
Installations of this type do not require oxygen under high pressure, but they are complex and quite expensive in terms of acquisition and maintenance; the output flow must be pumped into cylinders using an oxygen-compatible purified compressor or air pump (pictured).
12. Membrane separation.
This method is based on the use of a membrane, which, when clean air passes through it, allows oxygen molecules to pass through better than nitrogen molecules. The output mixture is thus enriched with oxygen, and the oxygen concentration is determined by the input flow. The maximum achievable oxygen content in commercially available systems is about 40%. The same technology, by the way, is used to separate helium in some other processes.
Similar to PSA units, there is no need to use high pressure oxygen. The effluent must be pumped into cylinders using an oxygen-compatible purified compressor or air pump. Membrane systems are quite reliable and do not require special maintenance, provided that the purity of the inlet flow is sufficient.
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Let them mix n chemically non-reacting between 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 the 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 get the formula for the temperature of the mixture of gases 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 the average heat capacities for which the mixture temperature 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.
(In fact 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, consequently, the adiabatic exponents must also 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 the mixing of 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.
Solving a large number of technical problems often involves mixing different gases (liquids) or different amounts of the same gas (liquid) in different thermodynamic states. To organize displacement processes, a sufficiently large number of a wide variety of mixing devices and apparatuses has been developed.
In the thermodynamic analysis of mixing processes, the problem is usually reduced to determining the state parameters of the mixture from the known state parameters 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 in 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 as occurring without heat exchange between the mixing system and the environment, i.e., proceeding adiabatically. Mixing in the presence of heat transfer can be divided into two stages: adiabatic mixing without heat transfer 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).
|
p 1 , T 1, u 1, U 1 , m 1 |
p 2 , T 2, u 2, U 2 , m 2 |
|
p, T,u, U, m |
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 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 1ln T 1 – R ln p 1) + m 2 (C p 2ln T 2 – R ln p 2)]–
–m(R 1 g 1ln r 1 + R 2 g 2ln 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 .
The diagram of exergy and anergy at 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 surface of the Earth, intensively giving off heat, is greatly cooled. Warm moist air comes into contact with the cooling Earth or with cold air near its surface and liquid droplets are formed 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. Further, the water vapor cools rapidly, the water in it condenses, and already at a short distance from the teapot spout we see a milky cloud - a 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 stronger aerosol is formed when oil boiled in a frying pan creates a gas (oil aerosol) in the room, which can only be removed by well-ventilated room.
In addition, condensation aerosol can be formed as a result of gas reactions leading to the formation of non-volatile products:
During the combustion of fuel, flue gases are formed, the condensation of which leads to the appearance of furnace 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
Dispersion aerosols are formed during grinding (spraying) of solid and liquid bodies in a gaseous medium and during the transition of powdered substances in suspended states under the action of air flows.
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 domestic settings, almost the only means of obtaining liquid and powdered aerosols is a device called "aerosol packaging" or "aerosol can". The substance in it is packed 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 at the time 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 existence of a substance in the state of aggregation. Thus, one molecule of water cannot form either a gas, or 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 be smaller than 1 10 -3 µm. To consider a gas as a continuous medium, it is necessary that the particle sizes be much larger than the free path of the 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:
A low concentration of particles of the dispersed phase - so, if 1 cm 3 of gold hydrosol contains 10 16 particles, then in the same volume of gold aerosol there are 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 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 first due to adhesion to the walls or sticking together, the second - as a result of settling to the bottom. Particles of intermediate sizes have maximum stability. Therefore, no matter how great the numerical concentration of particles 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. DES does not appear 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, which are formed in the gas phase as a result of gas ionization by cosmic, ultraviolet or radioactive rays.
3. The charge of particles is random, and for particles of the same nature and the same size it can be different both in magnitude and in sign.
4. The charge of a particle changes over time both in magnitude and sign.
5. In the absence of specific adsorption, the particle charges 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 strongly polar substance, since in this case a sufficiently large potential jump occurs on the interfacial surface, due to the surface orientation of the molecules. For example, on the interfacial surface of water or snow aerosols, there is a positive electric potential of about 250 mV.
It is known from practice that aerosol particles of metals and their oxides usually carry a negative charge (Zn, ZnO, MgO, Fe 2 0 3), and aerosol particles 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, aerosols do not have any interaction between the surface of particles and the gaseous medium, which means that there are no forces that prevent the adhesion of particles to each other and to macroscopic bodies upon impact. 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 highly diluted systems.
| Dependence of the coagulation rate on an increase in the number of aerosol concentrations | |
| Initial numerical concentration in 1 cm 3 | The time required to reduce the aerosol concentration by 2 times |
| Fractions of a second | |
| 15-30 s | |
| 30 min | |
| Several days | |
METHODS FOR DESTROYING AEROSOLS
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 dissipation of clouds and fog.
Aerosols are broken down by
· dispersion under the influence of air currents or due to charges of particles of the same name;
· sedimentation;
Diffusion to vessel walls
· 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 re-entering 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 operation 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.
EVACUATE 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).
As compressed gases, N 2, N 2 O, CO 2, etc. are used.
ADVANTAGES OF AEROSOL PACKS
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 nozzles 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 fix the dosage of the sprayed substance in advance.
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 has a high reactivity. 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 act on large surfaces with the drug (acute respiratory diseases, burns, etc.). A great effect is given by dosage forms containing liquid film-forming substances in their composition. When such a 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 packages.
The first group: household chemicals.
Insecticides are preparations for the destruction of insects.
Anti-moth products.
Insecticides for pets.
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:
Perfumes and cosmetics. « Hair care products (varnishes, shampoos, etc.).
Shaving foams and gels.
Creams for hands and feet.
Oil for and against tanning.
Deodorants.
Perfumes, colognes, toilet water.
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 decorating cakes and pastries and were only used by restaurants who returned them to be refilled. 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 freon C318 with these products, although it is much more expensive.
Table 18.4 Examples of formulations for various food aerosols
| Ingredients included in aerosols | Quantity, % mass | |
| 1. Cream for snack sandwiches | ||
| Cottage cheese with cream | 50-60 | |
| 25-30 | ||
| Vegetable oil and aromatic additives | 6-10 | |
| Freon S318 | 7 | |
| 2. Sugar glaze for finishing confectionery products | ||
| Sugar | 55-60 | |
| Water | 15-25 | |
| Vegetable oil | ||
| solid | 9-14 | |
| liquid | 3-5 | |
| Table salt | 0,1-0,3 | |
| Microcrystalline cellulose | 1,0 | |
| Fragrances | 1-4 | |
| Emulsifiers | 0,5-1 | |
| Freon S318 | 7 | |
| 3. Mousse | ||
| Honey or fruit syrup | 78-83 | |
| Water | 7-9 | |
| Vegetable oil (solid) | 3-5 | |
| Microcrystalline cellulose | 1-2 | |
| Monoglycerides | 0,5-1 | |
| Sorbitol polyesters | 0,05-1 | |
| Freon SZ18 | 7 | |
| Continued from Table 18.4 | ||
| Ingredients included in aerosols | Quantity, % mass | |
| 4. Decorative sauce in the form of foam | ||
| Mustard (finely ground powder) | 0,94 | |
| Lemon juice | 4,72 | |
| Vinegar | 9,44 | |
| Water | 34 | |
| Polysorbate 80 | 0,5 | |
| Emulsifying mixture | 2,25 | |
| Microcrystalline cellulose | 2,5 | |
| Additives - foam stabilizers | 4,59 | |
| Freon C318 + nitrous oxide (P=8 atm) | 7 | |
| 5. Oil-vinegar dressing in the form of foam | ||
| Water | 11,80 | |
| Salt | 1,96 | |
| Sugar | 1,47 | |
| Vinegar | 22,81 | |
| Olive oil | 61,75 | |
| Polysorbate 80 | 0,10 | |
| Garlic oil | 0,12 | |
| Black pepper oil | 0,10 | |
| Freon S318 | 10,0 | |
| 6. Dressing for roasted corn kernels | ||
| Salt (extra) | 10,00 | |
| Vegetable oil | 58,97 | |
| Other oil additives | 0,03 | |
| Dye | 1,00 | |
| Freon-S318 | 10,00 | |
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 is the future not only in flour milling, but also in other industries associated with the use of bulk materials and powders.
Aerosols are microheterogeneous systems in which solid particles or liquid droplets are suspended in a gas (S/G or L/G),
According to the state of aggregation of the dispersed phase, aerosols are divided into: fog (F/G); smoke, dust (T/G); smog [(F+T)/G)].
According to dispersion, aerosols are: fog, smoke, dust.
Like other microheterogeneous systems, aerosols can be obtained from true solutions (condensation methods) or from coarse systems (dispersion methods).
Water droplets in fogs are always spherical, and particulate smoke can have different shapes depending on their origin.
Due to the very small size of the particles of the dispersed phase, they have a developed surface on which adsorption, combustion, and other chemical reactions can actively proceed.
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 no EDL occurs 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 \u003d 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 is 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 us imagine three horizontal layers A, B and C of our gas column, with layer B located above A, and A above C. It is always possible to obtain any amount of a mixture of composition A by mixing a certain volume from layer C with a volume from layer B. Conversely, Any quantity of a mixture of composition A can be split into two mixtures of composition B and C.
This mixing and separation of the two gases can also be achieved in a reversible way by strengthening horizontal pipes in A, B and C. The end of each such pipe that comes out of the gas column is closed with a piston. We will now push the pistons inward in layers B and C, moving them, say, from left to right, and at point A, on the contrary, we will push the piston outward, i.e., from right to left. Then in B and C some masses of gas will leave the column, and in A, on the contrary, some volume of the mixture will enter. We will assume that each such pipe contains a certain mass of a mixture of the same composition as the horizontal layer of the gas column with which this pipe communicates.
The values will then be determined from the equations
It follows that
Let us now divide the mixture in some reversible way and calculate the work expended.
Let us introduce into A the unit volume of the mixture, and from B we will derive, accordingly, the volumes
The total work expended in this process is equal to
Substituting the values here we see that this work is equal to zero.
There is some subtlety here: mixtures B and into which mixture A broke up were raised to different heights and thus acquired different potential energy. But since the work is zero and the temperature of the system is constant, this is possible only if the system has given or received a certain amount of heat. Knowing the change in potential energy, we will find the amount of heat imparted to the system, and hence the change in entropy.
The increment in potential energy will be
but it is equal to the amount of heat imparted to the system, so the increase in entropy will be equal to
By this amount, the sum of the entropies of the volume of mixture B and the volume of mixture C is greater than the entropy of a unit volume of mixture A. From here we can find the volumes of mixtures B and C, the sum of the entropies of which is equal to the entropy of a unit volume of mixture A; To do this, we bring the volumes of mixtures B and C in a reversible isothermal way to volumes and equate the sum of the increments in the entropies of both mixtures during this process to expression (75), taken with the opposite sign.
The entropy increment for mixture B will be
Let us substitute into equation (76) the expression for pressures in terms of densities
