Lecture DRYING.
Drying is the process of removing moisture from solids by evaporating it and removing the resulting vapors.
Heat drying is often preceded mechanical methods removal of moisture (squeezing, settling, filtering, centrifugation).
In all cases, drying in the form of vapors removes the volatile component (water, organic solvent, etc.)
According to the physical essence, drying is a process of joint heat, mass transfer and is reduced to the movement of moisture under the influence of heat from the depth of the dried material to its surface and its subsequent evaporation. In the process of drying, a wet body tends to a state of equilibrium with environment, so its temperature and moisture content is generally a function of time and coordinates.
In practice, the concept is used humidity v, which is defined as:
(5.2)
If then then
According to the method of heat supply, there are:
Convective drying, carried out by direct contact of the material and the drying agent;
Contact (conductive) drying, heat is transferred to the material through the wall separating them;
Radiation drying - by transferring heat by infrared radiation;
Freeze drying, in which moisture is removed from the material in a frozen state (usually in a vacuum);
Dielectric drying, in which the material is dried in the field of high frequency currents.
With any drying method, the material is in contact with moist air. In most cases, water is removed from the material, so a system of dry air - water vapor is usually considered.
Parameters humid air.
A mixture of dry air and water vapor is moist air. Humid air parameters:
Relative and absolute humidity;
Heat capacity and enthalpy.
Humid air, at low P and T, can be considered a binary mixture of ideal gases - dry air and water vapor. Then, according to Dalton's law, we can write:
(5.3)
where P– vapor-gas mixture pressure , p c g is the partial pressure of dry air, is the partial pressure of water vapor.
Free or superheated steam - given T and R it does not condense. The maximum possible vapor content in the gas, above which condensation occurs, corresponds to saturation conditions at a certain T and partial pressure .
Distinguish absolute, relative humidity and moisture content of air.
Absolute humidity is the mass of water vapor per unit volume of moist air (kg / m 3). The concept of absolute humidity coincides with the concept of vapor density at temperature T and partial pressure .
Relative Humidity is the ratio of the amount of water vapor in the air to the maximum possible, under given conditions, or the ratio of vapor density under given conditions to the density of saturated vapor under the same conditions:
According to the equation of state of an ideal gas Mendeleev - Klaiperon for steam in a free and saturated state, we have:
and 
(5.5)
Here M p is the mass of one mole of vapor in kg, R is the gas constant.
Taking into account (5.5), equation (5.4) takes the form:
Relative humidity determines the moisture content of the drying agent (air).
Here G P is the mass (mass flow rate) of steam, L is the mass (mass flow rate) of absolutely dry gas. We express the quantities G P and L through the equation of state of an ideal gas:
, 
Then relation (5.7) is transformed to the form:
(5.8)
Mass of 1 mole of dry air in kg.
Introducing 
and considering 
we get:
(5.9)
For air-water vapor system 
, 
. Then we have:
(5.10)
So, a relationship has been established between the moisture content x and the relative humidity φ of the air.
Specific heat wet gas is taken as an additive heat capacity of dry gas and steam.
Specific heat of wet gas c, referred to 1 kg of dry gas (air):
(5.11)
where is the specific heat of dry gas, the specific heat of steam.
Specific heat capacity, referred to 1 kg vapor-gas mixture:
(5.12)
Usually used in calculations With.
Specific enthalpy of moist air H refers to 1 kg of absolutely dry air and is determined at a given air temperature T as the sum of the enthalpies of absolutely dry air and water vapor:
(5.13)
The specific enthalpy of superheated steam is determined by the following expression.
Drying is the process of removing moisture from materials.
Moisture can be removed mechanically(squeezing, filtering, centrifuging) or thermal, i.e., by evaporating moisture and removing the resulting vapors.
In its physical essence, drying is a combination of heat and mass transfer processes related to each other. The removal of moisture during drying is reduced to the movement of heat and moisture inside the material and their transfer from the surface of the material to the environment.
According to the method of supplying heat to the dried material, the following types of drying are distinguished:
– convective drying- direct contact of the dried material with a drying agent, which is usually used as heated air or flue gases (usually mixed with air);
– contact drying- transfer of heat from the coolant to the material through the wall separating them;
– radiation drying- transfer of heat by infrared rays;
– dielectric drying– heating in the field of high-frequency currents;
– freeze drying– drying in a frozen state under high vacuum.
Bond form of moisture in the material
The mechanism of the drying process is largely determined by the form of moisture bonding with the product: the stronger this bond, the more difficult the drying process. The process of removing moisture from the product is accompanied by a violation of its connection with the product, which requires a certain amount of energy.
All forms of communication of moisture with the product are divided into three large groups: chemical bond, physical-chemical bond, physical-mechanical bond. In the process of drying food products, as a rule, physicochemically and physicomechanically bound moisture is removed.
Chemically bonded water is retained most firmly and is not removed when the material is heated to 120 ... 150 ° C. Chemically bound moisture is most firmly attached to the product and can only be removed by heating the material to high temperatures or by performing a chemical reaction. This moisture cannot be removed from the product during drying.
Physical-mechanical bound moisture is the liquid in the capillaries and the wetting liquid.
Moisture in capillaries is subdivided into moisture macrocapillaries and microcapillaries. Macrocapillaries are filled with moisture in direct contact with the material. Moisture enters the microcapillaries both by direct contact and as a result of its absorption from the environment.
Physico-chemical bond combines two types of moisture: adsorption and osmotically bound moisture. Adsorption moisture is firmly held on the surface and in the pores of the body. Osmotically bound moisture, also called swelling moisture, is located inside the cells of the material and is held by osmotic forces. Adsorption moisture requires much more energy for its removal than swelling moisture.
Basic parameters of humid air
During convective drying, the heat carrier (drying agent) transfers heat to the product and removes moisture evaporating from the product. Thus, the drying agent plays the role of a heat and moisture carrier. The state of moist air is characterized by the following parameters: barometric pressure and partial vapor pressure, absolute and relative humidity, moisture content, density, specific volume, temperature and enthalpy. Knowing the three parameters of humid air, you can find all the others.
The absolute importance of air called the mass of water vapor in 1 m 3 of moist air (kg / m 3).
Relative humidity
, i.e. degree of air saturation
, is the ratio of absolute humidity to the maximum possible mass of water vapor ( 
), which can be contained in 1 m 3 of moist air under the same conditions (temperature and barometric pressure),
, i.e. 
100.
(1)
The mass of water vapor, kg, contained in moist air and per 1 kg of absolutely dry air is called the moisture content of the air:
,
(2)
Enthalpy I humid air refers to 1 kg of absolutely dry air and is determined at a given air temperature t°C as the sum of the enthalpies of absolutely dry air 
and water vapor 
(J/kg
dry air):
,
(3)
where With r.v– average specific heat capacity of absolutely dry air, J/(kgK); i n is the enthalpy of water vapor, kJ/kg.
I d -diagram of humid air. The main properties of moist air can be determined using I x-diagram, first developed by L.K. Ramzin in 1918. Diagram I-X(Fig. 1) built for constant pressure R= 745mm Hg Art. (about 99 kN / m 2).
On the vertical axis of ordinates, the enthalpy is plotted on a certain scale I, and on the abscissa axis - moisture content d. The abscissa axis is located at an angle of 135 to the ordinate axis (to increase the working part of the chart field and the convenience of turning the curves = const).
The lines on the diagram are:
constant moisture content (d= const) are vertical lines parallel to the y-axis;
constant enthalpy ( I\u003d const) - straight lines parallel to the abscissa axis, i.e., going at an angle of 135 ° to the horizon;
constant temperatures, or isotherms (t= const);
constant relative humidity ( = const);
partial pressures of water vapor R P in moist air, the values of which are plotted on the scale on the right y-axis of the diagram.
Rice. one. I–d- diagram
Atmospheric air, and therefore indoor air, always contains a certain amount of water vapor.
The amount of moisture in grams contained in 1 m 3 of air is called the volumetric vapor concentration or absolute humidity f in g / m 3. Water vapor, which is part of the vapor-air mixture, occupies the same volume v as the mixture itself; temperature T of steam and mixture is the same.
The energy level of water vapor molecules contained in humid air is expressed by the partial pressure e
where M e is the mass of water vapor, kg; μ m - molecular weight, kg / mol: R - universal gas constant, kg-m / deg mol, or mm Hg. st m 3 / deg mol.
The physical dimension of partial pressure depends on the units in which pressure and volume are expressed, which are included in the universal gas constant.
If pressure is measured in kg/m2, then partial pressure has the same dimension; when measuring pressure in mm Hg. Art. partial pressure is expressed in the same units.
In building thermophysics, for the partial pressure of water vapor, the dimension expressed in mm Hg is usually taken. Art.
The value of the partial pressure and the difference between these pressures in adjacent sections of the considered material system are used to calculate the diffusion of water vapor inside the building envelope. The value of partial pressure gives an idea of the amount and kinetic energy of water vapor contained in the air; this quantity is expressed in units that measure the pressure or energy of the steam.
The sum of the partial pressures of steam and air is equal to the total pressure of the steam-air mixture
The partial pressure of water vapor, as well as the absolute humidity of the vapor-air mixture, cannot increase indefinitely in atmospheric air with a certain temperature and barometric pressure.
The limiting value of the partial pressure E in mm Hg. Art. corresponds to the complete saturation of air with water vapor F max in g / m 3 and the occurrence of its condensation, which usually occurs on material surfaces adjacent to moist air or on the surface of dust particles and aerosols contained in it in suspension.
Condensation on the surface of building envelopes usually causes undesirable wetting of these structures; condensation on the surface of aerosols suspended in moist air is associated with the slight formation of fogs in an atmosphere polluted by industrial emissions, soot and dust. Absolute values of E in mm Hg. Art. and F in g / m 3 are close to each other at normal air temperatures in heated rooms, and at t \u003d 16 ° C they are equal to each other.
As the air temperature rises, the values of E and F increase. With a gradual decrease in the temperature of moist air, the values of e and f, which took place in unsaturated air from an initial high temperature, reach limiting maximum values, since these values decrease with decreasing temperature. The temperature at which air reaches full saturation is called the dew point temperature or simply the dew point.
The values of E for humid air with different temperatures (at a barometric pressure of 755 mm Hg) are indicated in
At negative temperatures, it should be borne in mind that the pressure of saturated water vapor over ice is less than the pressure over supercooled water. This can be seen from fig. VI.3, which shows the dependence of the partial pressure of saturated water vapor E on temperature.
At point O, which is called triple, the boundaries of three phases intersect: ice, water and steam. If we continue the curved line separating the liquid phase from the gaseous (water from steam) with a dotted line, it will pass above the boundary of the solid and gaseous phases (steam and ice), which indicates more high values partial pressures of saturated water vapor over supercooled water.
The degree of saturation of humid air with water vapor is expressed as relative partial pressure or relative humidity.
Relative humidity cp is the ratio of the partial pressure of water vapor e in the air medium under consideration to the maximum value of this pressure E, possible at a given temperature. Physically, the value of φ is dimensionless and its values can vary from 0 to 1; in construction practice, the relative humidity is usually expressed as a percentage:
Relative humidity has great importance both hygienically and technically. The value of φ is related to the intensity of moisture evaporation, in particular, from the surface of human skin. Relative humidity in the range of 30 to 60% is considered normal for a permanent stay of a person. The value of φ also characterizes the process of sorption, i.e., the absorption of moisture by porous hygroscopic materials in contact with an air humid environment.
Finally, the value of φ determines the process of moisture condensation both on dust particles and other suspended particles contained in the air, and on the surface of enclosing structures. If air with a certain moisture content is subjected to heating, then the relative humidity of the heated air will decrease, since the value of the partial pressure of water vapor e remains constant, and its maximum value E increases with increasing temperature, see formula (VI.3).
Conversely, when air with a constant moisture content is cooled, its relative humidity will increase due to a decrease in E.
At a certain temperature, the maximum value of the partial pressure E will be equal to the value of e in the air, and the relative humidity φ will be equal to 100%, which corresponds to the dew point. With a further decrease in temperature, the partial pressure remains constant (maximum), and the excess amount of moisture condenses, i.e., passes into a liquid state. Thus, the processes of heating and cooling air are associated with changes in its temperature, relative humidity, and, consequently, the initial volume.
For the main values at sharp changes in the temperature of moist air (for example, when calculating ventilation processes), its moisture content and heat content (enthalpy) are often taken.
where 18 and 29 are the molecular weights of water vapor and dry air P \u003d P e + P in - the total pressure of moist air.
At a constant total pressure of moist air (for example, P = 1), its moisture content is determined only by the partial pressure of water vapor
The density of humid air decreases with increasing partial pressure in a linear fashion.
A significant difference in the molecular weights of water vapor and dry air leads to an increase in absolute humidity and partial pressure in the warmest zones (usually in the upper zone) of the premises, in accordance with the laws, .
where c p is the specific heat capacity of moist air, equal to 0.24 + 0.47d (0.24 is the heat capacity of dry air; 0.47 is the heat capacity of water vapor); t - temperature, °C; 595 - specific heat of vaporization at 0°С, kcal/kg; d is the moisture content of humid air.
The change in all parameters of moist air (for example, with fluctuations in its temperature) can be established from the I - d diagram, the main values \u200b\u200bof which are the heat content I and moisture content d of air at an average value of barometric pressure.
On the I - d diagram, the heat content I is plotted along the ordinate axis, and the moisture content projections d - along the abscissa axis; true values of moisture content are projected onto this axis from an inclined axis located at an angle of 135 ° to the y-axis. An obtuse angle is adopted in order to more clearly plot the air humidity curves on the diagram (Fig. VI.4).
Lines of the same heat content (I=const) are located on the diagram obliquely, and the same moisture content (d = const) - vertically.
The curve of full saturation of air with moisture φ=1 divides the diagram into the upper part, in which the air is not completely saturated, and the lower one, where the air is completely saturated with moisture and condensation processes can occur.
In the lower part of the diagram, there is a line p e =f(d) built in the usual grid of coordinates according to the formula (VI.4) of the growth of partial pressures of water vapor, expressed in mm Hg. Art.
Diagrams of heat and moisture content are widely used in heating and ventilation practice when calculating the processes of heating and cooling air, as well as in drying technology. Using I - d diagrams, you can set all the necessary parameters of humid air (heat content, moisture content, temperature, dew point, relative humidity, partial pressure), if only two of these parameters are known.
Notes
1. This pressure is sometimes referred to as water vapor pressure.As known, dry air(CB) consists of 78% nitrogen, 21% oxygen and about 1% carbon dioxide, inert and other gases. If there are in the air, then such air is called humid air(VV). Taking into account that the composition of the dry part of the air practically does not change during ventilation of premises, and only the amount of moisture can change, in ventilation it is customary to consider explosives as a binary mixture consisting of only two components: SW and water vapor (WP). Although all gas laws apply to this mixture, however, during ventilation, it can be assumed with sufficient accuracy that the air is almost always under atmospheric pressure, since the pressures of the fans are quite small compared to barometric pressure. The normal atmospheric pressure is 101.3 kPa, and the pressures developed by the fans are usually no more than 2 kPa. Therefore, heating and air in the ventilation occur at a constant pressure.
From the thermodynamic parameters of explosives, which are operated in the course of ventilation, one can single out the following:
- density;
- heat capacity;
- temperature;
- moisture content;
- partial pressure of water vapor;
- relative humidity;
- dew point temperature;
- enthalpy (heat content);
- wet bulb temperature.
Under the influence of various factors, it can change its parameters. If the air enclosed in a certain volume (for example, a room) is in contact with hot surfaces, it heats up i.e. its temperature rises. In this case, those layers that border on hot surfaces are directly exposed to heating. Changes due to heating, and this leads to the appearance convective currents: a process of turbulent exchange occurs. Due to the presence of turbulent mixing of air in the process of vortex formation, the air absorbed by the boundary layers is gradually transferred to more distant layers, as a result of which the entire volume of air is somehow raises your temperature.
From the example considered, it is clear that the layers close to the hot surfaces will have a higher temperature than the remote ones. In other words, the temperature by volume is not the same (and sometimes differs quite significantly). Therefore, temperature, as an air parameter, at each point will have its own individual, local value. However, it is extremely difficult to predict the nature of the distribution of local temperatures over the volume of the room, so in most situations one has to talk about a certain average value of one or another air parameter. Temperature average It is derived from the assumption that the perceived heat will be evenly distributed over the volume of air, and the air temperature at each point in space will be the same.
The issue of temperature distribution along the height of the room has been studied more or less, however, even in this issue, the distribution pattern can change greatly under the influence of individual factors: jet streams in the room, the presence of shielding surfaces of building structures and equipment, temperature and size of heat sources.
humid air is a mixture of dry air and water vapour. In fact, atmospheric air always contains a certain amount of water vapor, i.e. is wet.
The water vapor contained in the air is usually in a rarefied state and obeys the laws for an ideal gas, which allows these laws to be applied to moist air as well.
State of vapor in air (overheated or saturated) is determined by the value of its partial pressure p, which depends on the total pressure of moist air p and partial pressure of dry air p:
Saturated air– air with the highest water vapor content at a given temperature.
Absolute air humidity is the mass of water vapor contained
in 1 m humid air (vapor density) at its partial pressure and temperature of humid air:
Relative humidity- the ratio of the actual absolute humidity of the air to the absolute humidity of saturated air at the same temperature:
At a constant temperature, the air pressure changes in proportion to its density (Boyle-Mariotte law), so the relative humidity of the air can also be determined by the equation:
where p is the saturation pressure of air at a given temperature;
p is the partial vapor pressure at a given temperature:
For dry air = 0, for saturated air - = 100%.
Dew point- temperature t, at which the vapor pressure p becomes equal to saturation pressure p. When air cools below the dew point, water vapor condenses.
air (11.5)
Using the ideal gas equation of state for the components of moist air (steam and dry air), dependences (11.2), (11.3) and (11.5), as well as the molecular weights of air (= 28.97) and steam (= 18.016), we obtain the calculation formula :
air (11.6)
For the case when moist air is at atmospheric pressure,: p=B.
Heat capacity of humid air at constant pressure is defined as the sum of heat capacities 1 kg dry air and d, kg water vapor:
(11.7)
You can take into account: 
Enthalpy of moist air at a temperature t defined as the sum of enthalpies 1 kg dry air and d, kg water vapor:
Here r– latent heat of vaporization, equal to ~2500 kJ / kg. Thus, the calculated dependence for determining the value of the enthalpy of moist air takes the form:
(11.9)
Note: magnitude I refers to 1 kg dry air or to (1+ d) kg humid air.
In technical calculations, to determine the parameters of moist air, it is usually used I–d moist air diagram proposed in 1918 by Professor L.K. Ramzin.
V I–d diagram (see Fig. 11.2) graphically related the main parameters that determine the heat and moisture state of air: temperature t, relative air humidity , moisture content d, enthalpy I, partial vapor pressure P contained in the vapor-air mixture. Knowing any two parameters, you can find the rest at the intersection of the corresponding
lines I-d-diagrams.
2. Scheme of the laboratory setup ( instrument )
The relative humidity of the air in laboratory work is determined using a psychrometer of the type: "Psychrometric hygrometer VIT-1".
The psychrometer (Fig. 11.1) consists of two identical thermometers:
"dry" - 1 and "wetted" - 2. The wetting of the thermometer ball 2 is carried out with the help of a cambric wick 3, lowered into a vessel 4 with water.
2 1
 |
3 t
 |
4t and air humidity φ for this device is established experimentally. Based on the results of the experiments, a special psychrometric table (passport) was compiled, placed on the front panel of the laboratory psychrometer.
The intensity of evaporation is significantly affected by the speed of air flow around the cambric wick, which introduces an error in the readings of a conventional psychrometer. This error is taken into account in the calculations by introducing corrections in accordance with the instrument's passport.
Note: the psychrometer is free from the considered disadvantage august, in which both the dry and wet bulbs are blown at a constant speed by a stream of air generated by a spring-powered fan.
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