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Processes and apparatuses of chemical technology
1. Subject and objectives of the course "Processes and apparatuses of chemical technology"
1.1 Objectives of the PAKT course
1.2 Classification of the main processes of chemical technology
2. Theoretical foundations of chemical engineering processes
2.1 Basic laws of science about processes and apparatuses
2.2 Transfer phenomena
3. Laws of thermodynamic equilibrium
4. Momentum transfer
Main literature
1. Subject and objectives of the course "Processes and apparatuses of chemical technology"
Processes are understood as changes in the state of natural and technological substances that occur under certain conditions. Processes can be divided into natural (these include the evaporation of water from the surfaces of reservoirs, heating and cooling of the earth's surface, etc.), the study of which is the subject and task of physics, chemistry, mechanics, and other natural sciences, and production or technological, the study of which is the subject and task of technology (i.e. art, craftsmanship, ability).
Technology is a science that determines the conditions for the practical application of the laws of natural sciences (physics, chemistry ...), i.e. a set of methods of processing, manufacturing, changing the state, properties, composition of a substance, the form of raw materials, material or semi-finished products, carried out in the process of manufacturing products. The production technology includes a number of similar physical and physico-chemical processes characterized by common patterns. These processes in various industries are carried out in devices similar in principle of operation. The processes and apparatuses common to various branches of the chemical industry are called the main processes and apparatuses of chemical technology.
The PAH discipline consists of two parts:
· theoretical bases of chemical technology;
· standard processes and devices of chemical technology.
The first part outlines the general theoretical patterns of typical processes; fundamentals of the methodology of the approach to solving theoretical and applied problems; analysis of the mechanism of the main processes and identification of general patterns of their course; generalized methods of physical and mathematical modeling and calculation of processes and devices are formulated. technological chemical apparatus thermodynamic
The second part consists of three main sections:
· hydromechanical processes and devices;
thermal processes and devices;
Mass transfer processes and devices.
In these sections, theoretical substantiations of each typical technological process are given, the main designs of apparatuses and the method of their calculation are considered.
1.1 Objectives of the PAKT course
1. Determination of the optimal technological regime for carrying out chemical technology processes on specific equipment.
2. Calculation and design of the design of devices for carrying out the technological process.
1.2 Classification of the main processes of chemical technology
Depending on the laws that determine the speed of the processes, they are divided into five groups:
Hydrodynamic processes, the speed of which is determined by the laws of hydromechanics (movement of liquids, compression and movement of gases, separation of liquid and gas heterogeneous systems - sedimentation, filtration, centrifugation, etc.).
Thermal processes, the rate of which is determined by the laws of heat transfer (heating, cooling, vapor condensation, evaporation).
Mass transfer processes, the rate of which is determined by the laws of mass transfer from one phase to another through the phase interface (absorption, rectification, extraction, etc.).
Chemical processes. The speed of chemical processes is determined by the laws of chemical kinetics.
Mechanical processes are described by the laws of solids mechanics and include grinding, transportation, sorting (classification by size) and mixing of solids.
All processes according to the method of organization are divided into periodic, continuous and combined. Periodic processes take place in the same apparatus, but at different times. Continuous processes proceed simultaneously, but are separated in space.
The processes of chemical technology are stationary (settled) and non-stationary (non-stationary).
If the parameters (temperature, pressure, etc.) of the process change with a change in the spatial coordinates in the apparatus, remaining constant in time at each point (space) of the apparatus - a steady process. If the process parameters are functions of coordinates and change at each point in time - an unsteady process.
A combined process is either a continuous process, the individual stages of which are carried out periodically, or such a batch process, one or more stages of which are carried out continuously.
Most chemical-technological processes include several successive stages. Usually one of the stages proceeds more slowly than the others, limiting the speed of the entire process. To increase the overall speed of the process, it is necessary to influence, first of all, the limiting stage. If the stages of the process run in parallel, then it is necessary to influence the most productive stage, since it is limiting. Knowledge of the limiting stage of the process allows us to simplify the description of the process and intensify the process.
2. Theoretical foundations of chemical engineering processes
2.1 Basic laws of science about processes and apparatuses
The theoretical foundation of the science of the processes and apparatuses of chemical technology are the following basic laws of nature:
The laws of conservation of mass, momentum and energy (substance), according to which the income of a substance is equal to its consumption. Conservation laws take the form of balance equations, the compilation of which is an important part of the analysis and calculation of chemical and technological processes.
The laws of mass, momentum and energy transfer determine the flux density of any substance. The laws of transfer make it possible to determine the intensity of the ongoing processes and, ultimately, the productivity of the devices used.
The laws of thermodynamic equilibrium determine the conditions under which the transfer of any substance comes to an end. The state of the system, in which there is no irreversible process of substance transfer, is called equilibrium. Knowledge of the equilibrium conditions makes it possible to determine the direction of the transfer process, the boundaries of the process flow, and the magnitude of the driving force of the process.
2.2 Transfer phenomena
Any process of chemical technology is conditioned by the transfer of one or several types of substance: mass, momentum, energy. We will consider the mechanisms of substance transfer, the conditions under which the transfer is carried out, as well as the transfer equations for each type of substance.
Transfer mechanisms
There are three mechanisms of substance transfer: molecular, convective and turbulent. The energy transfer can be carried out, in addition, due to radiation.
Molecular mechanism. The molecular mechanism of substance transfer is due to the thermal motion of molecules or other microscopic particles (ions in electrolytes and crystals, electrons in metals).
convective mechanism. The convective mechanism of substance transfer is due to the movement of macroscopic volumes of the medium as a whole. The set of values of a physical quantity, uniquely defined at each point of a certain part of space, is called the field of a given quantity (the field of density, concentrations, pressures, velocities, temperatures, etc.).
The movement of macroscopic volumes of the medium leads to mass transfer with, momentum with and energy cE unit volume ( with - density or mass of a unit volume, cW- momentum of unit volume, withE is the energy of a unit volume).
Depending on the causes of convective motion, free and forced convection are distinguished. The transfer of a substance under conditions of free convection is due to the difference in densities at various points in the volume of the medium due to the difference in temperatures at these points. Forced convection occurs when the entire volume of the medium is forced to move (for example, by a pump or if it is mixed with a stirrer).
Turbulent mechanism. The turbulent transport mechanism occupies an intermediate place between the molecular and convective mechanisms in terms of space-time scale. Turbulent motion occurs only under certain conditions of convective motion: sufficient distance from the phase boundary and inhomogeneity of the velocity field.
At low speeds of movement of the medium (gas or liquid) relative to the phase boundary, its layers move regularly, parallel to each other. Such a movement is called laminar. If the inhomogeneity of the speed and the distance from the phase boundary exceeds a certain value, the stability of the movement is violated. An irregular chaotic motion of individual volumes of the medium (vortices) develops. Such a movement is called turbulent.
The first studies of motion modes were carried out in 1883 by the English physicist O. Reynolds, who studied the movement of water in a pipe. During laminar motion, a thin tinted stream did not mix with the main mass of the moving liquid and had a rectilinear trajectory. With an increase in the flow rate or pipe diameter, the trickle acquired a wave-like motion, which indicates the occurrence of disturbances. With a further increase in the above parameters, the trickle mixed with the bulk of the liquid, and the colored indicator was blurred over the entire cross section of the pipe.
Here the concept of the scale of turbulence is used, which determines the size of the eddies. Unlike, for example, molecules, vortices are not stable formations clearly limited in space. They are born, break up into smaller vortices, and decay with the transition of energy into heat (energy dissipation). Therefore, the scale of turbulence is an averaged statistical value. Various approaches to the description of turbulent motion are possible.
One of the approaches consists in temporal averaging of the values of physical quantities (velocities, concentrations, temperatures) over intervals significantly exceeding the characteristic periods of fluctuations even of large-scale eddies.
3. Laws of thermodynamic equilibrium
If the system is in a state of equilibrium, then no macroscopic manifestations of substance transfer are observed. Despite the thermal motion of molecules, each of which transfers mass, momentum and energy, there are no macroscopic flows of substance due to the equiprobability of transfer in each direction.
Equilibrium in a single-phase system, not subject to external forces, is established with the equality of values at each point in space of macroscopic quantities characterizing the properties of the system: speed -
(x,y,z,t) = const;
temperature - T(x,y,z,t) = const; chemical potentials of components
- m i(x,y,z,t) = const.
It is possible to distinguish separately the conditions of hydromechanical, thermal and concentration equilibrium.
Hydromechanical balance:
Thermal (thermal) equilibrium:
T=const;
Concentration balance:
mi= const,
Here is the differential operator operator nabla
The condition for the manifestation of transfer processes and the emergence of macroscopic flows of mass, momentum and energy is the nonequilibrium of the system. The direction of the transfer processes is determined by the spontaneous aspiration of the system to a state of equilibrium, i.e. transfer processes lead to equalization of the speed, temperature and chemical potentials of the system components. The inhomogeneities of these quantities are necessary conditions for the flow of transfer processes and are called them driving forces.
In order to carry out the process, it is necessary to bring the system out of equilibrium, i.e. influence from outside. This is possible due to the supply of mass or energy to the system or the action of external forces. For example, settling occurs in the field of gravity, evaporation occurs when heat is supplied, and absorption occurs when an absorber is introduced into the system.
Transport equations
Substance flow- the amount of substance transferred per unit of time through a unit of surface.
Mass transfer
convective mechanism. The mass flow due to the convective mechanism is related to the convective velocity by the following relation
[kg/m 2 s] (2)
It is often more convenient to use the flow of matter rather than mass
[kmol/m 2 s] (3)
here m i- molar mass of the component i[kg/kmol], c i- molar concentration [kmol / m 3].
Molecular mechanism. The main law of the molecular mechanism of mass transfer is Fick's first law, which for a two-component system has the form:
, n=2 (4)
where D ij- coefficient of binary (mutual) diffusion ( D ij= D ji) .
Turbulent mechanism. Turbulent mass transfer can be considered by analogy with molecular transfer as a consequence of the chaotic movement of vortices. The coefficient of turbulent diffusion is introduced D t, which depends both on the properties of the medium, and on the inhomogeneity of the velocity, and the distance from the interfacial surface.
. (5)
The ratio of the coefficients of turbulent and molecular diffusion in the near-wall region reaches D t/D i ~ 10 2 - 10 5 .
Energy transfer
The energy of the system can be subdivided: microscopic and macroscopic. Microscopic, which is a measure of the internal energy of the molecules themselves, their thermal motion and interaction, is called the internal energy of the system ( U). The macroscopic energy is the sum of the kinetic energy ( E k), due to the convective motion of the medium, and the potential energy of the system in the field of external forces ( E P). Thus, the total energy of the system per unit mass can be represented as
E" = U" + E" k+ E" P[J/kg] (6)
The prime means that the energy is per unit mass.
Energy can be transferred in the form of heat or work. Heat is a form of energy transfer at the microscopic level, work is at the macroscopic level.
convective mechanism. The energy flux carried by the convective mechanism has the form
[J/m2s] = [W/m2] (7)
This is the amount of energy transferred by a moving macroscopic volume per unit of time through a unit of surface.
Molecular mechanism. The molecular mechanism carries out energy transfer at the microscopic level, i.e. in the form of heat. The heat flux due to the molecular mechanism under conditions of mechanical and concentration equilibrium can be represented as
, (8)
where is the coefficient of molecular thermal conductivity [W/mK].
This equation is called Fourier law.
Turbulent mechanism. Turbulent energy transfer can be considered by analogy with molecular energy transfer by introducing the turbulent thermal conductivity coefficient
t (9)
Like the turbulent diffusion coefficient t will be determined by the properties of the system and the mode of motion. The total energy flux in the laboratory frame of reference can be written
.
4. Momentum transfer
convective transport. Consider the case when the medium moves with some convective velocity W x in axis direction X. In this case, the momentum or momentum of a unit volume will be equal to W x. Then the amount of motion W x, transferred due to the convective mechanism in the direction of the axis X per unit of time through a unit of surface will be equal to
= [Pa] (10)
X, transferred per unit time through a unit surface along the axis Y, will be equal to
(11)
Similarly, momentum transfer in all directions gives 9 components of the convective momentum flux tensor,
(12)
(13)
Molecular transfer. Amount of movement directed along the axis X, (W x), transferred along the axis Y per unit time through a unit surface due to the molecular mechanism, can be represented as
(14)
where m[Pa s] and [m2/s] are the coefficients of dynamic and kinematic molecular viscosity, respectively. This equation is called Newton's law of viscosity. If the viscosity coefficients do not depend on the value of the derivative W x/ y, i.e. addiction xy from W x/ y linear, the medium is called Newtonian. If this condition is not met - non-Newtonian. The latter include polymers, pastes, suspensions, and a number of other materials used in industry.
turbulent transport. The transfer of momentum due to the turbulent mechanism can be considered by analogy with the molecular one.
(15)
where m t and t- dynamic and kinematic coefficients of turbulent viscosity, determined by the properties of the medium and the mode of motion t~ D t.
The total momentum flux can be written
(16),
where is the viscous stress tensor whose elements include both molecular and turbulent momentum transfer
(17).
So, the equations of transfer of mass, energy and momentum are considered. It is easy to verify the analogy of these equations. The convective flow represents the product of the transferred substance in a unit volume (with,E", with) to convective speed. Flows due to molecular or turbulent mechanisms is the product of the corresponding transport coefficient (D, m, m t) to the driving force of the process. This analogy makes it possible to use the results of studying some processes to describe others.
Main literature
1. Dytnersky Yu.I. Processes and apparatuses of chemical technology. Moscow: Chemistry, 2002. Vol. 1-400 p. T.2-368 p.
2. Kasatkin A.G. Basic processes and apparatuses of chemical technology. 9th ed. Moscow: Chemistry, 1973. 750 p.
3. Pavlov K.F., Romankov P.G., Noskov A.A. Examples and tasks in the course of processes and apparatuses of chemical technology. L.: Chemistry, 1987. 576 p.
4. Razinov A.I., Dyakonov G.S. transfer phenomena. Kazan, publishing house of KSTU, 2002. 136 p.
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Foreword
Introduction
1. The subject of chemical technology and the objectives of the course
2. Classification of processes
3. Material and energy calculations
General concepts of material balance. Output. Performance. The intensity of production processes. Energy balance. Power and efficiency.
4. Dimension of physical quantities
PART ONE. HYDRODYNAMIC PROCESSES
Chapter one. Fundamentals of hydraulics
A. Hydrostatics)
