Kasatkin processes and apparatuses of chemical technology. Basic processes and apparatuses of chemical technology. Mass, molar and volumetric composition

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Kasatkin A.G. Basic processes and apparatuses of chemical technology. – M.: 1973, 754 p.
Skoblo A.I., Tregubova I.A., Molokanov Yu.K. Processes and devices of the oil refining and petrochemical industries. – M.: Chemistry, 1982, 584 p.
Molokanov Yu.K. Processes and apparatuses of oil and gas processing. - M., Chemistry, 1980, 408 p.
Pavlov K.F., Romankov P.G., Noskov A.A. Examples and tasks in the course of processes and apparatuses of chemical technology. Textbook for universities. - L .: Chemistry, 1987, 576s

1 GENERAL SIGNS OF MASS TRANSFER PROCESSES
Mass transfer or diffusion processes are associated with the transition of components from one phase to another in order to separate them.

All mass transfer processes have a number of common features.

They are used to separate mixtures.
In any process, at least two phases are involved: liquid and vapor (distillation and rectification), liquid and gas (absorption), solid and gas-vapor (adsorption), solid and liquid (adsorption, extraction), two liquid (extraction).
The transition of a substance from one phase to another is carried out by diffusion.
The driving force of mass transfer processes is the concentration difference or concentration gradient. The process proceeds in the direction of the phase in which the component concentration is lower.
The transfer of matter from one phase to another occurs through the phase boundary, on which the equilibrium state of the phases is assumed.
Diffusion processes are reversible, i.e. the direction of the process is determined by the laws of phase equilibrium.

7. The transition of a substance from one phase to another ends when dynamic equilibrium is reached.

The state of equilibrium should be understood in such a way that the exchange between phases does not stop, however, the rates of transition of components from one phase to another are aligned.
Classification of mass transfer processes

Phase source
Phase receiver		G	F	T
	G	Membrane processes	distillation rectification Desorption I	Desorption II
	F	absorption	extraction	Desorption II
	T	adsorption		Phase transition of the 2nd kind

Rectification- the process of multiple countercurrent contacting of oncoming nonequilibrium flows of vapor and liquid in order to separate liquid homogeneous mixtures into fractions.

Absorption- the process of selective absorption of the components of the gas mixture by a liquid absorbent - absorbent.

Extraction- the process of selective extraction of components from a liquid mixture (or from a solid) by a liquid extractant.

Adsorption- the process of selective absorption of the components of a gas or liquid mixture by a solid absorber - adsorbent.

Drying - the process of removing liquid (moisture) from solid materials

Membrane processes - selective extraction of mixture components or their concentration using a semi-permeable partition membrane.

Basic mass transfer equation

Two types of substance transfer are known - molecular and convective diffusion. Molecular diffusion is caused by the transfer of substance molecules from a region with a higher concentration to a region with a lower concentration and proceeds in a stationary medium or laminar boundary layers.

The rate at which a substance is transferred from one phase to another dM is proportional to the driving force of the process D, which characterizes the degree of deviation of the systems from the equilibrium state, and the phase contact surface dF . Hence:

where K is the mass transfer coefficient. (similarly with heat transfer)

The mass transfer coefficient characterizes the mass of a substance transferred from one phase to another per unit of time through a unit of the phase contact surface with the driving force of the process equal to unity.

The mass transfer coefficient reflects the level of process intensification: the larger the value of K, the smaller their size is required for the apparatus to transfer a given amount of substance. At the same time, it is necessary to influence the size of the phase contact surface, striving for its maximum development and renewal per unit volume of the apparatus. The greatest influence on the intensity of mass transfer is exerted by hydrodynamic and structural factors.
3. GIBBS PHASE RULE APPLICATION TO MASS TRANSFER PROCESSES

At equilibrium, pressure and temperature must be constant in all parts of the system, otherwise mass and heat transfer processes will occur.

For equilibrium systems, the Gibbs phase rule is satisfied, which establishes the dependence of the number of degrees of freedom (N)

where N is the number of degrees of freedom of the system; To- number of components; F- number of phases.

The number of degrees of freedom of a system is the number of independent variables (temperature, pressure, concentration of components) that can be changed arbitrarily within certain limits without changing the equilibrium of the system.

In an equilibrium system (N = 0), the number of coexisting phases cannot be more than F=K+2.

For two-phase systems, the number of degrees of freedom of the system is equal to the number of components (N = K). Basically, the course will further consider two-phase systems.

Binary mixture K=2, N=2, you can change the temperature and concentration, at a constant external pressure. The apparatus must have a gradient of t and x at a constant π.

For multi-component systems (oil) K → ∞ and N → ∞, Therefore, for multi-component systems (typical for oil refining), the number of degrees of freedom can be very large.

Mass, molar and volumetric composition

The mass fraction of a component is determined by the ratio of the mass of this component to the mass of the entire mixture

(1)

Considering that the total mass of the mixture is equal to the sum of the masses of the individual components of the mixture, i.e.

you can write

those. the sum of the mass fractions of all components of the mixture is equal to one

The mole fraction of any component of a mixture is defined as the ratio of the number of moles of this component to the total number of moles of the mixture

(4)

where N i the number of moles is determined by the following relationship:

(5)

The volume fraction of a component in a mixture is equal to the ratio of the volume of this component to the volume of the entire mixture

(7)

(8)
Volume fractions are used in cases where mixing does not change the volume of the components.

For mutual conversion of mass and mole fractions, the following ratios are used:

(9)

(10)

When converting volume concentrations to mass or mole concentrations (for example, when converting distillation curves plotted in volume fractions), the corresponding calculation formulas are used:

( 11)

where ρ cm is the average density of the mixture.

L 2
4 ESSENCE OF THE RECTIFICATION PROCESS

There are various approaches and methods for substantiating the technology of distillation and rectification, as well as the principles for choosing the design of an apparatus for separating a binary mixture.

As an example, consider the separation of a binary mixture of benzene-toluene. The composition and properties of the components of the initial mixture are known. Let's conduct a series of experiments. Let us place in a distillation flask (Figure 4.1) a liquid with a known composition (40% benzene and 60% toluene), while it is necessary to obtain a condensate from this mixture with a composition of benzene 99.9% and toluene 0.01%.

After gradual evaporation and condensation (Figure 4.1), we determine the composition of the condensate, we got 85% benzene and 15% toluene. Those. a sufficiently good degree of separation of the components during gradual evaporation is not achieved. This design is unacceptable in practice.

The principle of single evaporation (condensation) is implemented in a hollow apparatus called a gas separator or vapor separator (Figure 4.1), the resulting condensate composition is unacceptable (benzene 65% and toluene 35%), however, the design of the apparatus is more successful compared to the previous apparatus.

Let us repeat the processes of single evaporation and condensation several times by placing a series of such devices (Figure 4.1). In this case, the desired compositions of the vapor and liquid phases are achieved, but the mass of the condensate is insignificant compared to the mass of the initial mixture. Also, with this technology, more cumbersome and expensive hardware design.

All previous shortcomings are realized in one apparatus, which includes the processes of multiple evaporation and condensation at each stage of contact, called plates. On any plate of the column, there is contact between the vapors rising to this plate and the liquid flowing onto this plate (Figure 4.2)

Obviously, a change in the composition of the phases will occur if there is a gradient of concentrations and temperatures. Since the pressure in the column is constant, this condition will be met if the temperature of the liquid flow is less than the vapor temperature. The lowest temperature should be at the top of the column and the highest at the bottom of the column. When these flows come into contact, the composition of the phases changes to equilibrium. Heat is needed at the bottom of the column, and cooling is needed at the top.

The contacting of the counter flows of the phases is carried out until the desired compositions of the products of the column are reached. This process is called distillation, and the column is called distillation. The upper part will be concentration or strengthening, and the lower part will be stripping or exhaustive, the place where raw materials are introduced into the column is called the feed section.

Fig.4.1. The main types of evaporation and condensation processes:

I-evaporation processes; a-gradual; b - single (OI); in-multiple;

II - condensation processes; a - gradual; d- single (OK); in - multiple; 1, 1" - evaporator; 2, 2" ~ capacitor; 3 - receiver; 4, 4" - evaporator; 5 , 5" - separating vessel (separator).
Depending on the purpose, the columns can be complete, which have a concentration and stripping sections, or incomplete: the strengthening column does not have a stripping section, and the stripping column does not have a concentration section. In addition, there are simple and complex columns. In a simple column, the raw material is divided into two products, and in a complex column, the number of selected products is more than two.

Thus, the design of the apparatus for separating a binary mixture has been substantiated, and it is necessary to prove by calculation methods that this apparatus is acceptable.
No

Fig.4.2. Scheme of a distillation column.
5 ISOBAR TEMPERATURE CURVES
Let us construct the isobars of liquid and vapor (at constant pressure). The abscissa shows the concentrations of the liquid and vapor phases, the ordinate shows the temperature (Figure 7.1, lower curves). Two curves are obtained that have two common points: a point BUT at corresponding to the boiling point of benzene and the point AT at, corresponding to the boiling point of toluene. Curve AA 1 BUT 2 AT, the determining relationship between the temperature of the system and the composition of the liquid phase is called the boiling line. Curve AB 1 AT 2 AT, which determines the relationship between the temperature of the system and the composition of the vapor phase, is called the line of condensation or saturated vapor.

Liquid vapors can be saturated and superheated. A saturated vapor is a vapor that is in equilibrium with a liquid. The higher the temperature, the higher the pressure at which this equilibrium system is located. For saturated vapors, there is an unambiguous relationship between vapor pressure and temperature. Unsaturated (superheated) vapors are vapors that, at a given temperature and pressure, form a single-phase system. The liquid phase is absent.

The equilibrium vapor and liquid phases have the same temperature and therefore, on the isobaric temperature curves, the equilibrium compositions of the phases will be determined by the intersection points of the horizontals, with the lines of boiling and condensation, these horizontal segments are called conodes (for example BUT 1 AT 1 ).

Chart area under the curve AA 1 BUT 2 AT, corresponds to a non-boiling liquid (point F).Diagram area above the condensation curve AB 1 AT 2 AT, corresponds to superheated vapors (point E).

Any point lying between the condensation and boiling curves, such as point C, characterizes a two-phase system (vapor-liquid).
5.2 Raoult-Dalton's Law

Isobaric curves can be constructed experimentally, as well as by calculation.

Dot BUT 1 on the boiling curve of a liquid can be found by Raoult's law. Partial pressure of the component p i ideal solution is equal to the product of saturated vapor pressure P i at a given temperature per molar concentration of the component in the liquid phase x':

(20)

The saturation vapor pressure of each component is calculated by empirical formulas. For example, according to Antoine's formula

(21)

where A, B, C- constants depending on the properties of the substance and determined experimentally;

t is the temperature.

It is known that a liquid begins to boil at a temperature at which its saturated vapor pressure becomes equal to the external pressure.

Boiling liquid condition:

- received the equation of the lower isobar. (23)

According to Dalton's law, the partial pressure of a component of a gas mixture is equal to the product of the pressure in the system and the mole fraction of the component in the gas mixture

(24)

At equilibrium, the pressure is the same at all points in the system. The combined Raoult-Dalton law

p i = P B x' i = Py' i . (25)

, - the equation of the upper isobar (26).

Consequently, at the given temperature and pressure of the system, the equilibrium compositions of the vapor and liquid phases are uniquely determined by the pressures of saturated vapors of the mixture components.
6 EQUATION AND EQUILIBRIUM CURVE OF PHASES OF A BINARY MIXTURE

Lineups x' and y' equilibrium liquid and vapor phases for a binary mixture can be represented graphically at a given system pressure (Figure 6.1). The Raoult-Dalton law can be represented in the following form:

For the low boiling component:

, (29)

For the high boiling component:

(30)
Divide the equation by the equation, denote P 1 /P 2 = a- relative elasticity

(31)

The phase equilibrium equation is a hyperbola passing through the origin (Fig. 4.2) of the diagram x' - y'(point 0 and point BUT with coordinates x' = y' = 1).

The relative volatility coefficient increases with decreasing pressure.

Figure 6.1 Equilibrium curve

7 Enthalpy diagram

To analyze and calculate the distillation and rectification processes, enthalpy diagrams are used, which give the relationship between the compositions of the liquid and vapor phases and their enthalpies.

The enthalpy (or heat content) of a liquid is equal to the amount of heat required to heat the liquid from 0°C to a given temperature. The enthalpy of steam is equal to the amount of heat required to heat a substance from 0°C to a given temperature, taking into account the heat of evaporation and overheating of vapors.

The enthalpy value is determined empirically from tables or from approximate formulas:

(27)
(28)

Enthalpy diagrams are used in calculations of distillation and rectification, when it is necessary to simultaneously take into account material and heat flows.

The enthalpy diagram shows the curves of the enthalpy of a boiling liquid and the enthalpy of saturated vapor as a function of concentration.

Any point BUT on the enthalpy diagram, lying below the enthalpy curve of the liquid phase characterizes a system consisting only of the liquid phase. Any point A4 lying above the vapor phase enthalpy curve consists of superheated vapors. Points located between curves, for example BUT 2 characterizes two-phase systems.

The vertical segments between the enthalpy curves of the vapor and liquid phases correspond to the latent heat of evaporation (condensation) of a mixture of a certain composition.

The physical meaning of the value of the latent heat of evaporation

Because Since the latent heat of vaporization does not coincide for different substances, then the enthalpy curves of liquid and vapor are not parallel.

On the enthalpy diagram, the conodes are represented by oblique straight lines. Since the conodes are arranged horizontally on the isobar graphs, i.e. their position is easy to determine, and on the enthalpy diagram - obliquely at different angles to the abscissa axis, then for the convenience of construction, the enthalpy diagram is usually combined with the graph of isobaric temperature curves (Figure 7.1).

Fig.7.1. Representation of the process of RI (OK) of a binary mixture on the enthalpy diagram and isobaric temperature curves

Textbook for universities - 10th ed., stereotyped, revised. Reprinted from ed.
1973 - M .: OOO TID "Alliance", 2004. - 753 p., illustrations - ISBN 5-98535-004-5, dpi300, navigator. The book discusses the theoretical foundations of the processes of chemical technology, methods for their calculation and describes the design of typical apparatuses.
The book is a textbook for chemical-technological universities. It can also be used as a manual for engineers and technicians in the chemical and related industries. General information
The subject of the course "Processes and devices"
The emergence and development of the science of processes and apparatuses
Classification of the main processes
General principles of analysis and calculation of processes and devices
Hydromechanical processes
Fundamentals of hydraulics. General issues of applied hydraulics in chemical equipment
Hydrostatics
Hydrodynamics
The main characteristics of the movement of liquids
Fundamentals of the theory of similarity and analysis of dimensions. Modeling principles
Hydrodynamic similarity
Hydrodynamics of boiling (fluidized) granular layers
Elements of hydrodynamics of two-phase flows
The structure of the flows and the distribution of the residence time of the liquid in the apparatus
Moving liquids (pumps)
Basic parameters of pumps
Centrifugal pumps
Piston pumps
Special types of piston and centrifugal pumps
Comparison and applications of pumps of various types
Movement and compression of gases (compressor machines)
Thermodynamic foundations of the gas compression process
Comparison and applications of compressor machines of various types
Separation of heterogeneous systems
Separation of liquid systems
settling
Filtration
centrifugation
Separation of gas systems (gas treatment)
Comparative characteristics and selection of gas cleaning equipment
Mixing in liquid media
Mechanical agitation
Mechanical agitators
Pneumatic agitation
Mixing in pipelines
Mixing with nozzles and pumps
Thermal processes
Fundamentals of heat transfer in chemical equipment
Thermal balances
Heating, cooling and condensation
Cooling down to ordinary temperatures
Cooling to low temperatures
Designs of heat exchangers
Comparative characteristics of heat exchangers
Mixing condensers
Calculation of heat exchangers
Calculation of vapor condensers
Evaporation
The device of evaporators
Calculation of multi-shell evaporators
Mass transfer processes
Mass transfer basics
Calculation of the main dimensions of mass transfer apparatus
Solid state mass transfer
Absorption
The device of absorption devices
Calculation of absorbers
Desorption
Schemes of absorption plants
Distillation of liquids
Characteristics of two-phase liquid-steam systems
Simple distillation
Rectification
Special types of distillation.
Extraction
Extraction processes in liquid-liquid systems
Extraction and dissolution processes in solid-liquid systems
Extraction and dissolution methods
The device of extraction devices
Calculation of extraction devices
Adsorption
Desorption
The device of adsorbers and schemes of adsorption plants
Calculation of adsorbers
Ion exchange processes
Drying
Drying options
Drying speed
Dryer device
Special types of drying and types of dryers
Crystallization
The device of crystallizers
Mold calculations
Refrigeration processes
artificial cooling
Moderate cooling
Deep cooling
Cycles with gas throttling
Cycles based on a combination of throttling and gas expansion in an expander
Stirling cycle for liquefying gases
Heat pump cycles
Comparison of basic deep chill cycles
Gas separation methods
Mechanical processes
Grinding of hard materials
coarse crushing
Medium and fine crushing
fine grinding
Ultrafine grinding
Classification and sorting of materials
Screening
Hydraulic classification and air separation
Mixing solid materials in
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