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Processes and apparatus chemical technology

1. Subject and objectives of the course "Processes and apparatus of chemical technology"

1.1 Objectives of the PACT course

1.2 Classification of the main processes of chemical technology

2. Theoretical foundations of chemical technology 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 apparatus 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 ones (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, etc. natural sciences, and into production or technological, the study of which is the subject and task of technology (i.e. art, skill, skill).

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, form of raw material, material or semi-finished product carried out during the production process. Production technology includes a number of similar physical and physicochemical processes characterized by general laws. These processes in various industries are carried out in devices similar in operating principle. The processes and apparatuses common to various branches of the chemical industry are called the basic processes and apparatuses of chemical technology.

The PACT discipline consists of two parts:

· theoretical foundations of chemical technology;

· standard processes and apparatuses of chemical technology.

The first part outlines the general theoretical principles of typical processes; fundamentals of the methodology of approach to solving theoretical and applied problems; analysis of the mechanism of the main processes and identification of general patterns of their occurrence; generalized methods of physical and mathematical modeling and calculation of processes and apparatus 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 apparatus.

These sections provide theoretical justification for each typical technological process, discuss the basic designs of devices and the methodology for their calculation.

1.1 Objectives of the PACT 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 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 - settling, filtration, centrifugation, etc.).

Thermal processes, the speed 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 rate of chemical processes is determined by the laws of chemical kinetics.

Mechanical processes are described by the laws of solid 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 one apparatus, but in different time. Continuous processes occur simultaneously, but are separated in space.

Chemical technology processes can be stationary (steady) and non-stationary (unsteady).

If the parameters (temperature, pressure, etc.) of the process change with changes in the spatial coordinates in the apparatus, remaining constant in time at each point (space) of the apparatus - a steady-state 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 a batch process, one or more stages of which are carried out continuously.

Most chemical technological processes include several sequential 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 rate-limiting stage. If the stages of the process proceed 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 technology processes

2.1 Basic laws of science about processes and apparatuses

The theoretical foundation of the science of processes and apparatuses of chemical technology is the following basic laws of nature:

The laws of conservation of mass, momentum and energy (substance), according to which the arrival 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 technological processes.

The laws of transfer of mass, momentum and energy determine the flux density of any substance. The transfer laws make it possible to determine the intensity of ongoing processes and, ultimately, the performance of the devices used.

The laws of thermodynamic equilibrium determine the conditions under which the transfer of any substance comes to completion. The state of the system in which there is no irreversible process of substance transfer is called equilibrium. Knowledge of the equilibrium conditions allows us to determine the direction of the transfer process, the boundaries of the process and the magnitude of the driving force of the process.

2.2 Transfer phenomena

Any process of chemical technology is caused by the transfer of one or more types of substance: mass, momentum, energy. We will consider the mechanisms of substance transfer, the conditions under which the transfer occurs, as well as the transfer equations for each type of substance.

Transfer Mechanisms

There are three mechanisms of substance transfer: molecular, convective and turbulent. Energy transfer can also occur through radiation.

Molecular mechanism. The molecular mechanism of substance transfer is determined by the thermal movement of molecules or other microscopic particles (ions in electrolytes and crystals, electrons in metals).

Convective mechanism. The convective mechanism of substance transfer is determined by 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 some part of space, is called the field of this quantity (field of density, concentrations, pressures, velocities, temperatures, etc.).

The movement of macroscopic volumes of the medium leads to mass transfer With, impulse With and energy sE unit volume ( With - density or mass of a unit volume, cW- impulse of unit volume, WithE- energy of a unit volume).

Depending on the reasons causing convective movement, free and forced convection are distinguished. The transfer of a substance under conditions of free convection is due to the difference in densities at different points in the volume of the medium due to differences 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 in the case of mixing it with a stirrer).

Turbulent mechanism. The turbulent transport mechanism occupies an intermediate place between the molecular and convective mechanisms in terms of the spatiotemporal scale. Turbulent motion occurs only under certain conditions of convective motion: sufficient distance from the phase interface and heterogeneity 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. This movement is called laminar. If the speed inhomogeneity and the distance from the phase boundary exceed a certain value, the stability of movement is violated. Irregular chaotic motion of individual volumes of the medium (vortices) develops. This 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. With laminar movement, a thin tinted stream did not mix with the main mass of the moving liquid and had a straight trajectory. As the flow speed or pipe diameter increased, the stream acquired a wave-like motion, which indicates the occurrence of disturbances. With a further increase in the above parameters, the stream mixed with the bulk of the liquid, and the colored indicator was washed out over the entire cross-section of the pipe.

The concept of turbulence scale, which determines the size of the eddies, is used here. Unlike, for example, molecules, vortices are not stable, clearly limited in space formations. They originate, decay into smaller vortices, and die out with the transition of energy into heat (energy dissipation). Therefore, the scale of turbulence is an averaged statistical value. Various approaches to describing turbulent motion are possible.

One approach consists of temporal averaging of the values ​​of physical quantities (velocities, concentrations, temperatures) over intervals significantly exceeding the characteristic periods of pulsations of even large-scale eddies.

3. Laws of thermodynamic equilibrium

If the system is in a state of equilibrium, then macroscopic manifestations of substance transfer are not observed. Despite the thermal movement of molecules, each of which transfers mass, momentum and energy, there are no macroscopic flows of substance due to the equal probability of transfer in each direction.

Equilibrium in a single-phase system, not subject to external forces, is established when the values ​​at each point in the space of macroscopic quantities that characterize the properties of the system are equal: speed -

(x,y,z,t) = const;

temperatures - T(x,y,z,t) = const; chemical potentials of components

- m i(x,y,z,t) = const.

We can distinguish separately the conditions of hydromechanical, thermal and concentration equilibrium.

Hydromechanical balance:

Thermal (thermal) equilibrium:

T=const;

Concentration equilibrium:

mi=const,

Here is the differential operator nabla operator

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 desire of the system to a state of equilibrium, i.e. transfer processes lead to equalization of speed, temperature and chemical potentials of system components. The inhomogeneities of these quantities are necessary conditions for the occurrence of transfer processes and are called driving forces.

In order to carry out the process, it is necessary to remove the system from a state of equilibrium, i.e. exert influence from the 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

Flow of substance- 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 speed by the following relation

[kg/m 2 s] (2)

It is often more convenient to use a 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/m3].

Molecular mechanism. The basic 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, depending both on the properties of the medium and on the inhomogeneity of the velocity and the distance from the interphase 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 a system can be divided into 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). Macroscopic energy consists of kinetic energy ( E k), caused by the convective movement 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

E" = U" + E" k+ E" P[J/kg] (6)

The prime indicates energy 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 flow transferred 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 time through a unit surface area.

Molecular mechanism. The molecular mechanism carries out energy transfer at the microscopic level, i.e. in the form of heat. The heat flow due to the molecular mechanism under conditions of mechanical and concentration equilibrium can be represented

, (8)

where is the coefficient of molecular thermal conductivity [W/mK].

This equation is called Fourier's law.

Turbulent mechanism. Turbulent energy transfer can be considered by analogy with molecular transfer by introducing the coefficient of turbulent thermal conductivity

T (9)

As well as the turbulent diffusion coefficient T will be determined by the properties of the system and the mode of movement. The total energy flow in the laboratory reference frame can be written

.

4. Momentum transfer

Convective transport. Let us consider the case when the medium moves with a certain convective speed W x in the direction of the axis X. In this case, the impulse 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 time through a unit surface will be equal to

= [Pa] (10)

X, transferred per unit time through a unit surface along the axis Y, will be equal

(11)

Similarly, momentum transfer in all directions gives 9 components of the convective momentum flux tensor,

(12)

(13)

Molecular transfer. Amount of motion directed along the axis X, (W x), axially portable 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. Momentum transfer 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, the elements of which include both molecular and turbulent momentum transfer

(17).

So, the equations for the transfer of mass, energy and momentum are considered. It is easy to see the analogy of these equations. Convective flow represents the product of the transported substance in a unit volume (With,E", With) to convective speed. Flows due to molecular or turbulent mechanisms are the product of the corresponding transfer coefficient (D, m, m T) on the driving force of the process. This analogy allows you to use the results of studying some processes to describe others.

Main literature

1. Dytnersky Yu.I. Processes and apparatus of chemical technology. M.: Chemistry, 2002. T.1-400 p. T.2-368 p.

2. Kasatkin A.G. Basic processes and apparatuses of chemical technology. 9th ed. M.: Khimiya, 1973. 750 p.

3. Pavlov K.F., Romankov P.G., Noskov A.A. Examples and tasks for the course on processes and apparatus of chemical technology. L.: Chemistry, 1987. 576 pp.

4. Razinov A.I., Dyakonov G.S. Transference phenomena. Kazan, KSTU publishing house, 2002. 136 p.

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Classification of basic processes and apparatuses of chemical technology

Depending from patterns Characterizing the flow, chemical technology processes are divided into five main groups.

1. Mechanical processes , the speed of which is related to the laws of solid state physics. These include: grinding, classification, dosing and mixing of solid bulk materials.

2. Hydromechanical processes , the flow rate of which is determined by the laws of hydromechanics. These include: compression and movement of gases, movement of liquids, solid materials, sedimentation, filtration, mixing in the liquid phase, fluidization, etc.

3. Thermal processes , the flow rate of which is determined by the laws of heat transfer. These include the following processes: heating, evaporation, cooling (natural and artificial), condensation and boiling.

4. Mass transfer (diffusion) processes , the intensity of which is determined by the rate of transition of a substance from one phase to another, i.e. laws of mass transfer. Diffusion processes include: absorption, rectification, extraction, crystallization, adsorption, drying, etc.

5. Chemical processes associated with the transformation of substances and changes in their chemical properties. The rate of these processes is determined by the laws of chemical kinetics.

In accordance with the listed division of processes, chemical apparatuses are classified as follows:

– grinding and classifying machines;

– hydromechanical, thermal, mass transfer devices;

– equipment for carrying out chemical transformations – reactors.

By organizational and technical structure processes are divided into periodic and continuous.

IN periodic process individual stages (operations) are carried out in one place (device, machine), but at different times (Fig. 1.1). IN continuous process (Fig. 1.2) individual stages are carried out simultaneously, but in different places (devices or machines).

Continuous processes have significant advantages over periodic processes, including the possibility of specializing equipment for each stage, improving product quality, stabilizing the process over time, ease of regulation, automation capabilities, etc.

When carrying out processes in any of the listed devices, the parameters of the processed materials change. The parameters characterizing the process are pressure, temperature, concentration, density, flow rate, enthalpy, etc.

Depending on the nature of the movement of flows and changes in the parameters of substances entering the device, all devices can be divided into three groups: devices ideal (full )mixing , devices ideal (full )repression and devices intermediate type .

It is most convenient to demonstrate the features of flows of various structures using the example of continuous heat exchangers of various designs. Figure 1.3a shows a diagram of a heat exchanger operating on the principle of ideal displacement. It is assumed that in this apparatus there is a “piston” flow of the flow without mixing. The temperature of one of the coolants changes along the length of the apparatus from the initial temperature to the final temperature as a result of the fact that subsequent volumes of liquid flowing through the apparatus do not mix with the previous ones, completely displacing them. The temperature of the second coolant is assumed to be constant (condensing steam).

In the device perfect mixing subsequent and previous volumes of liquid are ideally mixed, the temperature of the liquid in the apparatus is constant and equal to the final temperature (Fig. 1.3, b).

In real devices, neither the conditions of ideal mixing nor ideal displacement can be ensured. In practice, only a fairly close approximation to these circuits can be achieved, so real devices are intermediate type devices (Fig. 1.3, c).

Rice. 1.1. Apparatus for carrying out a periodic process:

1 – raw materials; 2 – finished product; 3 – steam; 4 – condensate; 5 – cooling water

Rice. 1.2. Apparatus for carrying out a continuous process:

1– heat exchanger-heater; 2 – apparatus with a stirrer; 3 – heat exchanger-refrigerator; I – raw materials; II – finished product; III – steam; IV – condensate;
V – cooling water

Rice. 1.3. Temperature change when heating liquid in devices various types: a – complete displacement; b – complete mixing; c – intermediate type

Driving force of the liquid heating process under consideration for any element of the apparatus is the difference between the temperatures of the heating steam and the heated liquid.

The difference in the course of processes in each type of apparatus becomes especially clear if we consider how the driving force of the process changes in each type of apparatus. From a comparison of the graphs it follows that the maximum driving force occurs in complete displacement devices, the minimum in complete mixing devices.

It should be noted that the driving force of processes in continuously operating ideal mixing apparatus can be significantly increased by dividing the working volume of the apparatus into a number of sections.

If the volume of an ideal mixing apparatus is divided into n apparatuses and the process is carried out in them, then the driving force will increase (Fig. 1.4).

With an increase in the number of sections in ideal mixing apparatuses, the value of the driving force approaches its value in ideal displacement apparatuses, and with a large number of sections (about 8–12), the driving forces in apparatuses of both types become approximately the same.

Rice. 1.4. Changing the driving force of the process during partitioning

Preface
Introduction
1. Subject of chemical technology and course objectives
2. Classification of processes
3. Material and energy calculations
General concepts of material balance. Exit. Performance. Intensity production processes. Energy balance. Power and coefficient useful action.
4. Dimension of physical quantities
PART ONE. HYDRODYNAMIC PROCESSES
Chapter first. Basics of hydraulics
A. Hydrostatics Engineering graphics Descriptive geometry Fundamentals of life safety Industrial practice Physical education (Physical education) Psychology and pedagogy Political science Sociology Air conditioning and ventilation Structural mechanics and strength Thermodynamics Physics Philosophy General chemical technology Processes and apparatus of chemical technology Chemistry Environmental impact assessment and environmental assessment Industrial ecology Environmental audit and environmental management Environmental monitoring Ecology Economics and forecasting of industrial environmental management Energy saving and resource conservation Accounting In-house planning and controlling General planning Marketing Management Organization and production planning Economics Economics and enterprise organization Electrical engineering Legal science Business law Environmental law English language French language