Each of us has a magical gift by nature and many face...
Thermal processes are processes whose rate of occurrence is determined by the rate of heat supply or removal. At least two environments with different temperatures take part in thermal processes, and heat is transferred spontaneously (without the cost of work) from a medium with a higher temperature T1 to a medium with a lower temperature T2, i.e. if the inequality T 1 >T 2 is observed.
In this case, the medium with temperature T 1 is called the coolant, and the medium with temperature T 2 is called the refrigerant. For thermal processes used in chemical production, these temperatures fluctuate over a very wide range - from close to 0K to thousands of degrees.
The main characteristic of the thermal process is the amount of heat transferred, from which the heat transfer surface of the apparatus is calculated. For a steady process, the amount of heat transferred per unit time is determined by the formula:
Q = KDT*F, (10.4)
K – heat transfer coefficient, T – average temperature difference between media,
F – heat exchange surface.
The driving force of thermal processes is the temperature gradient
DT = T 1 – T 2. (10.5)
Thermal processes include: heating, cooling, condensation, evaporation and evaporation, heat exchange.
1. Heating– the process of increasing the temperature of processed materials by supplying heat to them. Heating is used in chemical technology to accelerate mass transfer and chemical processes. According to the nature of the coolant used for heating, they are distinguished:
– heating with sharp water vapor through a bubbler or with dead water vapor through a coil or jacket;
– heating by flue gases through the wall of the apparatus or by direct contact;
– heating with preheated intermediate coolants water: mineral oils, molten salts;
– heating by electric current in electric furnaces of various types (induction, arc, resistance);
– heating with a solid granular coolant, including a catalyst in a gas flow.
Heating scheme with granular coolant coolant
Furnace
heated
| |
cold transport component
1 – firebox, 2 – apparatus for heating granular material, 3 – apparatus for heating gas, 4 – loading device, 5 – separator for granular material
2.Cooling– the process of lowering the temperature of processed materials by removing heat from them. The following refrigerants are used for cooling: water, air, refrigerants. Cooling devices are divided into:
– devices for indirect contact of the cooled material with the coolant through the wall (refrigerators) and
– devices for direct contact of the cooled material with the refrigerant (cooling towers or scrubbers).
The choice of device design is determined by the nature of the material being cooled and the coolant.
3.Condensation- the process of liquefying vapors of a substance by removing heat from them. Based on the principle of contact of the refrigerant with the condensed vapor, the following types of condensation are distinguished:
– surface condensation, in which vapor liquefaction occurs on the surface of the water-cooled wall of the apparatus, and
– condensation by mixing, in which cooling and liquefaction of vapors occurs through direct contact with cooling water. Devices of the first type are called surface capacitors, devices of the second type are called mixing capacitors and barometric capacitors. Condensation by mixing is used in cases where the evaporated liquid does not mix with water.
4. Evaporation- the process of concentrating solutions of solid non-volatile substances by removing from them a volatile solvent in the form of a feather. Evaporation is a type of thermal evaporation process. The condition for the evaporation process to occur is that the steam pressure above the solution is equal to the steam pressure in the working volume of the evaporator.
If this condition is met, the temperature of the secondary vapor formed above the boiling solvent is theoretically equal to the temperature of the saturated vapor of the solvent. Evaporation can be done under pressure or in a vacuum, which allows the process temperature to be reduced. Evaporation can be carried out in two variants: multiple evaporation and evaporation with a heat pump.
Repeated evaporation is the process of evaporation using secondary steam as heating steam. To do this, evaporation is carried out in a vacuum or using high-pressure heating steam.
The number of installation buildings is determined by economic considerations, in particular, the costs of steam production and maintenance, and depends on the initial and final concentration of the evaporated solution.
The heat pump evaporation process is based on the fact that the secondary steam is heated to the temperature of the heating steam by compressing it in a turbocharger or injector and is then used again to evaporate the solvent in the same evaporator.
Multiple evaporation scheme.
Condensate condensate
1 – first evaporator, 2 – second evaporator, p gr1 – pressure of the heating steam of the first apparatus (fresh steam), p at1 – pressure of the secondary steam from the first apparatus, equal to p gr2 – pressure of the heating steam of the second apparatus, p at2 – pressure of the secondary pair from the second device.
Evaporation scheme with heat pump.
Evaporated liquid
| |
Evaporated liquid
1 – evaporator, 2 – device for heating secondary steam.
End of work -
This topic belongs to the section:
Chemical Technology
Federal State Educational Institution.. Higher Professional Education.. Novgorod State University named after Yaroslav the Wise..
If you need additional material on this topic, or you did not find what you were looking for, we recommend using the search in our database of works:
What will we do with the received material:
If this material was useful to you, you can save it to your page on social networks:
| Tweet |
All topics in this section:
11. 2 Basic principles of homogeneous processes 12.1 Characteristics of heterogeneous processes 12 Heterogeneous processes 12.1 Characteristics of heterogeneous processes
Environment
The primary source of satisfying the material and spiritual needs of man is nature. It also represents his habitat – the environment. In the environment there are natural
Human production activity and the planet's resources
The condition for the existence and development of humanity is material production, i.e. social and practical attitude of man to nature. Diverse and gigantic scale of industrial production
Biosphere and its evolution
The environment is a complex multicomponent system, the components of which are interconnected by numerous connections. The environment consists of a number of subsystems, each of which
Chemical industry
Based on the purpose of the products produced, industry is divided into sectors, one of which is the chemical industry. Share of chemical and petrochemical industries in total production
Chemical Science and Manufacturing
3.1 Chemical technology - the scientific basis of chemical production Modern chemical production is a large-scale, automated production, the basis
Features of chemical technology as a science
Chemical technology differs from theoretical chemistry not only in the need to take into account the economic requirements for the production it studies. Between the tasks, goals and content of the theoretical
Relationship between chemical technology and other sciences
Chemical technology uses material from a number of sciences:
Chemical raw materials
Raw materials are one of the main elements of the technological process, which largely determines the efficiency of the process and the choice of technology. Raw materials are natural materials
Resources and rational use of raw materials
The share of raw materials in the cost of chemical products reaches 70%. Therefore, the problem of resources and rational use of raw materials during their processing and extraction is very relevant. In the chemical industry
Preparation of chemical raw materials for processing
Raw materials intended for processing into finished products must meet certain requirements. This is achieved through a set of operations that make up the process of preparing raw materials for processing.
Replacement of food raw materials with non-food and vegetable mineral ones
Advances in organic chemistry make it possible to produce a number of valuable organic substances from a variety of raw materials. For example, ethyl alcohol, used in large quantities in the production of synthetic
Use of water, properties of water
The chemical industry is one of the largest consumers of water. Water is used in almost all chemical industries for a variety of purposes. Water consumption at individual chemical plants
Industrial water treatment
The harmful effects of impurities contained in industrial water depend on their chemical nature, concentration, dispersed state, as well as the technology of specific production of water use. Sun
Energy use in the chemical industry
In the chemical industry, various processes occur that are associated either with the release, or with the consumption, or with the mutual transformations of energy. Energy is spent not only on chemical
The main sources of energy consumed by the chemical industry are fossil fuels and their products, water energy, biomass and nuclear fuel. Energy value department
Technical and economic indicators of chemical production
For the chemical industry, as a branch of large-scale material production, not only technology matters, but also the economic aspect closely related to it, on which the
Economic structure of the chemical industry
Indicators such as capital costs, production costs and labor productivity are also important for assessing economic efficiency. These indicators depend on the economic structure
Material and energy balances of chemical production
The initial data for all quantitative calculations made when organizing a new production or assessing the effectiveness of an existing one are based on material and energy balances. These
The concept of the chemical technological process
In the chemical production process, starting substances (raw materials) are processed into the target product. To do this, it is necessary to carry out a number of operations, including the preparation of raw materials for transferring them into the reaction
Chemical process
Chemical processes are carried out in a chemical reactor, which represents the main apparatus of the production process. The efficiency of the chemical reactor depends on the design of the chemical reactor and its operating mode.
Chemical reaction rate
The rate of the chemical reaction occurring in the reactor is described by the general equation: V = K* L *DC L-parameter characterizing the state of the reacting system; K-const
Overall rate of chemical process
Since for heterogeneous systems the processes in reactor zones 1, 3 and 2 obey different laws, they proceed at different rates. The overall rate of the chemical process in the reactor is determined by
Thermodynamic calculations of chemical technological processes
When designing technological processes, thermodynamic calculations of chemical reactions are very important. They allow us to draw a conclusion about the fundamental possibility of this chemical transformation,
Equilibrium in the system
The yield of the target product of a chemical process in a reactor is determined by the degree to which the reaction system approaches a state of stable equilibrium. Stable equilibrium meets the following conditions:
Calculation of equilibrium from thermodynamic data
Calculation of the equilibrium constant and the change in the Gibbs energy makes it possible to determine the equilibrium composition of the reaction mixture, as well as the maximum possible amount of product. The calculation is based on cons
Thermodynamic analysis
Knowledge of the laws of thermodynamics is necessary for an engineer not only to carry out thermodynamic calculations, but also to assess the energy efficiency of chemical technological processes. The Value of Analysis
Chemical production as a system
Production processes in the chemical industry can differ significantly in the types of raw materials and products, the conditions for their implementation, the power of equipment, etc. However, with all the diversity of specific
Simulation by chemical engineering system
The problem of a large-scale transition from laboratory experiment to industrial production when designing the latter is solved by the modeling method. Modeling is a research method
Process flow selection
The organization of any chemical process includes the following stages: – development of chemical, principle and technological process diagrams; – selection of optimal technological parameters and settings
Process Parameter Selection
The parameters of the chemical processing plant are selected in such a way as to ensure the highest economic efficiency not of its individual operation, but of the entire production as a whole. So, for example, for the production considered above
Chemical production management
The complexity of chemical production as a multi-factor and multi-level system leads to the need to use a variety of control systems for individual production processes,
Hydromechanical processes
Hydromechanical processes are processes that occur in heterogeneous, at least two-phase systems and obey the laws of hydrodynamics. Such systems consist of a dispersed phase,
Mass transfer processes
Mass transfer processes are processes whose speed is determined by the rate of transfer of matter from one phase to another in the direction of achieving equilibrium (mass transfer rate). In the process of massoo
Chemical Reactor Design Principles
The main stage of the chemical technological process, which determines its purpose and place in chemical production, is implemented in the main apparatus of the chemical technological scheme, in which the chemical process takes place
Chemical reactor designs
Structurally, chemical reactors can have different shapes and structures, because they carry out a variety of chemical and physical processes that occur under difficult conditions of mass and heat transfer
Construction of contact devices
Chemical reactors for carrying out heterogeneous catalytic processes are called contact devices. Depending on the state of the catalyst and the mode of its movement in the apparatus, they are divided into:
Characteristics of homogeneous processes
Homogeneous processes, i.e. processes occurring in a homogeneous medium (liquid or gaseous mixtures that do not have interfaces separating parts of the system from each other) are relatively rarely encountered
Homogeneous processes in the gas phase
Homogeneous processes in the gas phase are widely used in the technology of organic substances. To carry out these processes, organic matter is evaporated, and then its vapor is processed by one or the other.
Homogeneous processes in the liquid phase
Of the large number of processes occurring in the liquid phase, the processes of neutralization of alkali in the technology of mineral salts without the formation of solid salt can be classified as homogeneous. For example, obtaining sulfate
Basic principles of homogeneous processes
Homogeneous processes, as a rule, occur in the kinetic region, i.e. the overall rate of the process is determined by the rate of the chemical reaction, therefore the laws established for reactions are also applicable
Characteristics of heterogeneous processes
Heterogeneous chemical processes are based on reactions between reagents in different phases. Chemical reactions are one of the stages of a heterogeneous process and occur after movement
Processes in the gas-liquid system (G-L)
Processes based on the interaction of gaseous and liquid reagents are widely used in the chemical industry. Such processes include absorption and desorption of gases, evaporation of liquids
Processes in binary solid, two-phase liquid and multiphase systems
Processes involving only solid phases (S-T) usually include sintering of solid materials during firing. Sintering is the process of producing hard and porous pieces from fine powders.
High temperature processes and apparatus
An increase in temperature affects the equilibrium and speed of chemical technological processes occurring both in the kinetic and diffusion regions. Therefore, temperature control
Essence and types of catalysis
Catalysis is a change in the rate of chemical reactions or their excitation as a result of the influence of catalyst substances, which, while participating in the process, remain chemically non-chemical at the end of the process.
Properties of solid catalysts and their production
Industrial solid catalysts are a complex mixture called a contact mass. In the contact mass, some substances are the actual catalyst, while others serve as activators.
Hardware design of catalytic processes
Homogeneous catalysis apparatuses do not have any characteristic features; carrying out catalytic reactions in a homogeneous medium is technically easy to implement and does not require special apparatuses.
The most important chemical production
In present day Over 50,000 individual inorganic and about three million organic substances are known. Only a small part of the discovered substances is obtained under industrial conditions. Actually
Application
The high activity of sulfuric acid, combined with the relatively low cost of production, predetermined the large scale and extreme variety of its use. Among the mineral
Technological properties of sulfuric acid
Anhydrous sulfuric acid (monohydrate) H2SO4 is a heavy oily liquid that mixes with water in all proportions, releasing a large amount
Methods of obtaining
Back in the 13th century, sulfuric acid was produced by the thermal decomposition of iron sulfate FeSO4, which is why even now one of the varieties of sulfuric acid is called oil of vitriol, although sulfuric acid has long been
Raw materials for the production of sulfuric acid
The raw material in the production of sulfuric acid can be elemental sulfur and various sulfur-containing compounds, from which sulfur or sulfur oxide itself can be obtained. Natural deposits
Contact method for the production of sulfuric acid
The contact method produces large quantities of sulfuric acid, including oleum. The contact method includes three stages: 1) gas purification from impurities harmful to the catalyst; 2) contact
Production of sulfuric acid from sulfur
Burning sulfur is much simpler and easier than burning pyrites. The technological process for producing sulfuric acid from elemental sulfur differs from the production process
Fixed Nitrogen Technology
Nitrogen gas is one of the most persistent chemicals. The binding energy in a nitrogen molecule is 945 kJ/mol; it has one of the highest entropies per a
Raw material base of the nitrogen industry
The raw materials for obtaining products in the nitrogen industry are atmospheric air and various types of fuel. One of the components of air is nitrogen, which is used in semi-chemical processes.
Obtaining process gases
Synthesis gas from solid fuel. The first of the main sources of raw materials for the production of synthesis gas was solid fuel, which was processed in water gas generators according to the following methods:
Ammonia synthesis
Let's consider an elementary technological scheme of modern ammonia production at average pressure with a productivity of 1360 tons/day. Its operating mode is characterized by the following parameters: temperature
Typical salt technology processes
Most MUs represent various mineral salts or solids with salt-like properties. Technological schemes for the production of MU are very diverse, but, in most cases, the warehouse
Decomposition of phosphate raw materials and production of phosphate fertilizers
Natural phosphates (apatites, phosphorites) are used mainly for the production of mineral fertilizers. The quality of the obtained phosphorus compounds is assessed by their P2O5 content
Phosphoric acid production
The extraction method for the production of phosphoric acid is based on the decomposition reaction of natural phosphates with sulfuric acid. The process consists of two stages: decomposition of phosphates and filtration of the product.
Production of simple superphosphate
The essence of the production of simple superphosphate is the transformation of natural fluorapatite, insoluble in water and soil solutions, into soluble compounds, mainly monocalcium phosphate
Production of double superphosphate
Double superphosphate is a concentrated phosphorus fertilizer obtained by decomposing natural phosphates with phosphoric acid. It contains 42-50% of digestible P2O5, including in
Nitric acid decomposition of phosphates
Obtaining complex fertilizers. A progressive direction in the processing of phosphate raw materials is the use of the nitric acid decomposition method of apatites and phosphorites. This method calls
Production of nitrogen fertilizers
The most important type of mineral fertilizers are nitrogen fertilizers: ammonium nitrate, urea, ammonium sulfate, aqueous solutions of ammonia, etc. Nitrogen plays an extremely important role in life.
Production of ammonium nitrate
Ammonium nitrate, or ammonium nitrate, NH4NO3 is a white crystalline substance containing 35% nitrogen in ammonium and nitrate forms, both forms of nitrogen are easily absorbed
Urea production
Urea (urea) ranks second among nitrogen fertilizers in terms of production volume after ammonium nitrate. The growth in urea production is due to its wide range of applications in agriculture.
Ammonium sulfate production
Ammonium sulfate (NН4)2SO4 is a colorless crystalline substance, contains 21.21% nitrogen, when heated to 5130C completely decomposes into
Calcium nitrate production
Properties Calcium nitrate (lime or calcium nitrate) forms several crystalline hydrates. Anhydrous salt melts at a temperature of 5610C, but already at 5000
Production of liquid nitrogen fertilizers
Along with solid fertilizers, liquid nitrogen fertilizers are also used, which are solutions of ammonium nitrate, urea, calcium nitrate and their mixtures in liquid ammonia or in concentrate
general characteristics
More than 90% of potassium salts extracted from the bowels of the earth and produced by industrial methods are used as fertilizers. Potash mineral fertilizers are natural or synthetic
Obtaining potassium chloride
Flotation production method The flotation method for separating potassium chloride from sylvinite is based on flotation-gravity separation of water-soluble minerals of potassium ore in an environment
Typical silicate materials technology processes
In the production of silicate materials, standard technological processes are used, which is due to the similarity of the physical and chemical principles of their production. In the most general form, the production of any silicate
Air lime production
Air or construction lime is a silicate-free binder based on calcium oxide and hydroxide. There are three types of air lime: - boiling lime (quicklime
Glass production process
The raw materials for glass production are a variety of natural and synthetic materials. According to their role in the formation of glass, they are divided into five groups: 1. Glass formers that create the basis
Production of refractories
Refractory materials (refractories) are non-metallic materials characterized by increased fire resistance, i.e. ability to withstand high temperatures without melting
Electrolysis of aqueous solutions of sodium chloride
The electrolysis of aqueous solutions of sodium chloride produces chlorine, hydrogen and caustic soda (caustic soda). Chlorine at atmospheric pressure and normal temperature is a yellow-green gas with
Electrolysis of sodium chloride solution in baths with a steel cathode and graphite anode
Electrolysis of a sodium chloride solution in baths with a steel cathode and a graphite anode makes it possible to obtain caustic soda, chlorine and hydrogen in one apparatus (electrolyzer). When passing constant
Electrolysis of sodium chloride solutions in baths with a mercury cathode and a graphite anode makes it possible to obtain more concentrated products than in baths with a diaphragm. When skipping
Production of hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water. Hydrogen chloride is a colorless gas with a melting point of –114.20C and a boiling point of –85
Electrolysis of melts. Aluminum production
During the electrolysis of aqueous solutions, only substances can be obtained whose release potential at the cathode is more positive than the potential for hydrogen release. In particular, such electronegative
Alumina production
The essence of alumina production is the separation of aluminum hydroxide from other minerals. This is achieved by using a number of complex technological techniques: converting alumina into a soluble solution
Aluminum production
Aluminum is produced from alumina dissolved in Na3AlF6 cryolite. Cryolite, as an alumina solvent, is convenient because it dissolves Al quite well
Metallurgy
Metallurgy is the science of methods for obtaining metals from ores and other raw materials and the branch of industry that produces metals. Metallurgical production arose in ancient times. Once again at dawn
Ores and methods of their processing
Raw materials in metal production are metal ores. With the exception of a small number (platinum, gold, silver), metals are found in nature in the form of chemical compounds that are part of metallic materials.
Iron production
The raw materials for the production of cast iron are iron ores, divided into four groups: Ores of magnetic iron oxide or magnetic iron ores, contain 50-70% iron and consist mainly
Copper production
Copper is a metal that is widely used in technology. In its pure form, copper has a light pink color. Its melting point is 10830C, its boiling point is 23000C, it is
Chemical fuel processing
Fuel refers to naturally occurring or artificially produced combustible organic substances that are a source of thermal energy and raw materials for the chemical industry. By nature percent
Coking of hard coals
Coking is a method of processing fuels, mainly coal, which involves heating them without air access to 900-10500C. In this case, the fuel decomposes with the formation of
Production and processing of gaseous fuels
Gaseous fuel is a fuel that is in the gas state at the temperature and pressure of its operation. By origin, gaseous fuels are divided into natural and synthetic
Basic organic synthesis
Basic organic synthesis (BOS) is a set of productions of organic substances of relatively simple structure, produced in very large quantities and used as a
Raw materials and environmental protection processes
The production of environmental protection products is based on fossil organic raw materials: oil, natural gas, coal and shale. As a result of various chemical and physico-chemical pre
Syntheses based on carbon monoxide and hydrogen
Organic synthesis based on carbon monoxide and hydrogen has received widespread industrial development. Catalytic synthesis of hydrocarbons from CO and H2 was first carried out by Sabatier, synthetist
Synthesis of methyl alcohol
For a long time, methyl alcohol (methanol) was obtained from tar water released during the dry distillation of wood. The alcohol yield depends on the type of wood and ranges from 3
Ethanol production
Ethanol is a colorless mobile liquid with a characteristic odor, boiling point 78.40C, melting point –115.150C, density 0.794 t/m3. Ethanol is mixed in
Formaldehyde production
Formaldehyde (methanal, formic aldehyde) is a colorless gas with a pungent irritating odor, with a boiling point of 19.20C, a melting point of –1180C and a density (in liquid
Preparation of urea-formaldehyde resins
Typical representatives of artificial resins are urea-formaldehyde resins, which are formed as a result of the polycondensation reaction that occurs during the interaction of urea molecules and forms
Acetaldehyde production
Acetaldehyde (ethanal, vinegar
Production of acetic acid and anhydride
Acetic acid (ethanoic acid) is a colorless liquid with a pungent odor, with a boiling point of 118.10C, a melting point of 16.750C and a density
Polymerization monomers
Monomers are low-molecular compounds of predominantly organic nature, the molecules of which are capable of reacting with each other or with molecules of other compounds to form
Production of polyvinyl acetate dispersion
In the USSR, industrial production of PVAD was first carried out in 1965. The main method of obtaining PVAD in the USSR was continuous-cascade, however, there were productions in which periodic
High molecular weight compounds
Natural and synthetic high-molecular organic compounds are of great importance in the national economy: cellulose, chemical fibers, rubbers, plastics, rubber, varnishes, adhesives, etc. How p
Pulp production
Cellulose is one of the main types of polymer materials. More than 80% of wood used for chemical processing is used to produce cellulose and wood pulp. Cellulose, sometimes
Manufacturer of chemical fibers
Fibers are bodies whose length is many times greater than their very small cross-sectional dimensions, usually measured in microns. Fibrous materials, e.g. substances consisting of fibers and
Production of plastics
Plastics include a wide group of materials, the main component of which are natural or synthetic IUDs, capable of becoming plastic at elevated temperatures and pressures.
Obtaining rubber and rubber
Rubbers include elastic IUDs, which are capable of being significantly deformed under the influence of external forces and quickly returning to their original state after the load is removed. Elastic properties
1. Classification of main processes and apparatuses
Classification of processes according to the method of creating the driving force:
Mass transfer
Hydromechanical
Mechanical
Thermal
Chemical
1) Mass transfer - the transition of a substance from one phase to another is carried out due to diffusion.
Depending on the transition from phase to phase, the process is called:
solid → l (melting) g → l (condensation, absorption)
f → tv (crystallization) tv → g (sublimation)
l → l (extraction) g → tv (adsorption)
g → g (evaporation, desorption) g ↔ p (rectification)
The driving force in mass transfer processes is the difference in concentrations, and the speed of the process is determined by the laws of mass transfer.
2) Hydromechanical – associated with the processing of suspensions (a heterogeneous system consisting of
liquids or gases and liquid/solid particles suspended in it. bodies.
Movement of liquid or gas;
Mixing in a liquid medium;
Separation of liquid heterogeneous systems (sedimentation, filtration, centrifugation);
Gas purification from dust;
The driving force of such processes is the pressure difference, caused by the difference in the densities of the materials being processed, and the speed is determined according to the laws of hydromechanics of inhomogeneous systems.
3) Mechanical - associated with the processing and movement of a solid body. Includes: grinding, dosing, mixing, screening, transportation. Driving force is the difference in forces, pressure, or stress gradient (compression, shear, tension). The speed of the process is determined by the laws of solid mechanics.
4) Thermal processes are associated with the transfer of heat from one body to another. Heating, cooling, evaporation, condensation, melting, solidification, evaporation, crystallization. The speed is determined by the laws of heat transfer. The driving force is the temperature difference.
5) Chemical - associated with chemical transformations of substances involved in the process and the production of new compounds. They include catalytic cracking, hydrotreating, reforming, pyrolysis, coking, polymerization, alkylation. The driving force is the difference in concentrations of reacting substances. The rate of the process is determined by the laws of chemical kinetics. According to the way various processes are carried out over time:
Devices have the same classification as processes:
1) Mass transfer - absorbers, adsorbers, desorbers, distillation columns, extractors, dryers, crystallizers.
2) Hydromechanical - filters, cyclones, electric dehydrators, settling tanks, centrifuges, mixers
3) Mechanical – crushers, sieve, mixers, dispensers.
4) Thermal - heat exchangers, refrigerators, evaporators, condensers, melting furnaces.
5) Chemical - reactors of various types (with a fixed bed of catalyst, with a fluidized bed, with a gushing bed).
2. Main features of mass transfer processes
The main features of mass transfer processes are:
Used to separate mixtures
At least 2 phases involved
A substance passes from one phase to another due to diffusion
Driving force - concentration difference
All processes are reversible, the direction of the process is determined by the laws of phase equilibrium, the actual concentrations of the component in the phases and external conditions (P, t).
The transition of a substance from one phase to another ends when dynamic equilibrium is achieved.
3. Basic mass transfer equation
The rate of the mass transfer process is equal to, where is the mass of the substance transferred through 1 tsu of the surface in 1 tsu of time
The driving force, - mass transfer resistance, - mass transfer coefficient, characterizes the mass of a substance transferred from phase to phase through a unit of surface per unit of time with a driving force equal to unity. The larger K, the smaller the apparatus needed to transfer a given amount of substance.
the same for the liquid phase.
The basic mass transfer equation is used to find the phase contact surface, the working volume of the apparatus, and the number of theoretical plates
4.Material balance of the mass transfer process
Carrying out any process in chemical technology involves the use of various materials and types of energy transmitted in the form of heat. Material balance is based on the law of conservation of mass. The purpose of the compilation is to identify all the flows of matter and energy involved in the process, taking into account losses. Mathematical balance allows you to calculate external flows of matter and energy (flows entering and leaving a given system).
Introduction
Mechanical processes of chemical technology
Mixing processes
1 Main characteristics of the mixing process
3 Mixing methods
Mixing devices
1 Paddle mixers
2 Sheet mixers
3 Propeller mixers
4 Turbine mixers
5 Special stirrers
6 Selecting a stirrer
Conclusion
List of sources used
Applications
Introduction
Any technology, including chemical technology, is the science of methods for processing raw materials into finished products. Recycling methods must be economically and environmentally beneficial and justified.
Chemical technology arose at the end of the 18th century and almost until the 30s of the 20th century consisted of a description of individual chemical production facilities, their main equipment, material and energy balances. With the development of the chemical industry and the increase in the number of chemical production facilities, the need arose to study and establish general principles for the construction of optimal chemical technological processes, their industrial implementation and rational operation. In chemical technology, it is necessary to clearly distinguish the flows of substances with which transformation occurs, first from raw materials, then through the step-by-step formation of intermediate products until the final target product is obtained.
The main task of chemical technology is the combination in a single technological system of various chemical transformations with physical, chemical and mechanical processes: grinding and sorting of solid materials, formation and separation of heterogeneous systems, mass and heat transfer, phase transformations, etc.
Mechanical processes occupy one of the main places in production, as they are involved at every stage. In this work, a special place is given to the most common process - mechanical mixing. Depending on the conditions of the process, containers and apparatus with mixing devices (stirrers) of various designs are used in production.
The main goals of the work are a detailed study of the basic mechanical processes, mixing devices, their operation and technological purpose.
1. Mechanical processes of chemical technology
Mechanical processes include processes that are based on mechanical effects on the product, namely:
Sorting.
There are two types of product separation: sorting by quality depending on organoleptic properties (color, surface condition, consistency) and separation by size into separate fractions (sorting by grains and shape).
In the first case, the operation is carried out by organoleptic examination of the products, in the second - by sifting.
Sorting by sifting is used to remove foreign impurities. When sifting, product particles whose sizes are smaller than the sieve openings pass through the holes (passage), and particles with sizes larger than the sieve openings remain on the sieve as waste.
For sifting, the following are used: metal sieves with stamped holes; wire sieves made of round metal wire, as well as sieves made of silk, nylon threads and other materials.
Silk sieves are highly hygroscopic and wear out relatively quickly. Nylon is insensitive to changes in temperature, relative humidity and sifted products; The strength of nylon threads is higher than silk.
Grinding.
Grinding is the process of mechanically dividing the processed product into parts for the purpose of its better technological use. Depending on the type of raw material and its structural and mechanical properties, mainly two grinding methods are used: crushing and cutting. Products with low moisture content are crushed, and products with high moisture content are cut.
Crushing in order to obtain coarse, medium and fine grinding is carried out on grinding machines, fine and colloidal - on special cavitation and colloid mills.
During the cutting process, the product is divided into parts of a certain or arbitrary shape (pieces, layers, cubes, sticks, etc.), as well as the preparation of finely ground types of products.
To grind solid products with high mechanical strength, band and circular saws and cutters are used.
Pressing.
Product pressing processes are mainly used to separate them into two fractions: liquid and dense. During the pressing process, the structure of the product is destroyed. Pressing is carried out using continuous screw presses (extractors of various designs).
Mixing.
Mixing promotes the intensification of thermal biochemical and chemical processes due to an increase in surface interaction between the particles of the mixture. The duration of mixing of mixtures determines their consistency and physical properties.
Dosing and formation.
The production of enterprise products and their release are carried out in accordance with GOSTs or technical specifications or internal technological standards and collections of recipes, with standards for the laying of raw materials and the yield of finished products (weight, volume). In this regard, the processes of dividing the product into portions (dosing) and giving them a certain shape (molding) are essential. The dosing and forming processes are carried out manually or with the help of machines depending on the production.
2. Mixing processes
.1 Main characteristics
Mixing is one of the most common processes in the food and chemical industries. When mixing, particles of a liquid or bulk material move repeatedly in the volume of the apparatus or container relative to each other under the action of an impulse that is transmitted to the stirred medium from a mechanical stirrer or a jet of liquid, gas or steam
Mixing purposes:
acceleration of chemical reactions or processes;
ensuring uniform distribution of solid particles in the liquid;
ensuring uniform distribution of liquid in liquid;
intensification of heating or cooling;
ensuring a stable temperature throughout the liquid.
There are many designs of mixing devices, but the most common are mechanical mixers with rotary movement of mixing elements. Along with this, mixing with gas or steam, mixing by circulating liquid, vibration or pulsation mixing is carried out.
Each of the listed types of mixing devices has its own specific advantages and disadvantages and a specific area of application.
When selecting a mixing device or mixing method, the following basic concepts are used:
The degree of mixing or the degree to which two or more substances or liquids are mutually distributed after the entire system has been mixed. The degree of mixing, sometimes called the homogeneity index, is determined experimentally from samples taken and is used to determine the effectiveness of mixing.
The intensity of mixing, expressed using certain quantities, such as the speed of rotation of the mixer, the power consumed for mixing, reduced to a unit volume or density of the product. In practice, the intensity of mixing is determined by the time to achieve a specific technological result, i.e. uniform mixing.
Mixing efficiency, determined by the ability to achieve the required quality of mixing in the shortest time and with minimal energy consumption. Thus, of two apparatuses with mixers, the one in which the result is achieved with the least energy consumption works more efficiently.
2.2 Mixtures
Any raw materials and intermediate products are certain technical products that are processed: separation into pure substances or vice versa, adding other components to them to create new mixtures.
Mixtures of substances are divided into homogeneous (homogeneous) and heterogeneous (heterogeneous). Table-1 provides examples of various mixtures.
Aggregate state of the constituent parts (before the formation of the mixture) Homogeneous mixture (homogeneous system) Heterogeneous mixture (heterogeneous system) Solid - solid Solid solutions, alloys (for example, brass, bronze) Rocks (for example granite, mineral-containing ores, etc.) Liquid - liquid Liquid solutions ( for example, vinegar - a solution of acetic acid in water) Two- and multi-layer liquid systems, emulsions (for example, milk - drops of liquid fat in water) Aggregate state of the constituent parts (before the mixture is formed) Homogeneous mixture (homogeneous system) Heterogeneous mixture (heterogeneous system) Solid - liquid Liquid solutions (for example, aqueous solutions of salts) Solid in liquid - suspensions or suspensions (for example, clay particles in water, colloidal solutions) Liquid in solid - liquid in porous bodies (for example, soils, soils) Solid - gaseous Chemisorbed hydrogen in platinum , palladium, steels Solid in gaseous - powders, aerosols, including smoke, dust, smog Gaseous in solid - porous materials (for example, brick, pumice) Liquid - solid Solid liquids (for example, glass - solid, but still liquid) Can take different shapes and fix it (for example, dishes - different shapes and colors) Liquid - gaseous Liquid solutions (for example, a solution of carbon dioxide in water) Liquid in gaseous - aerosols of liquid in gas, including mists Gaseous in liquid - foam (for example, soap foam) Gaseous - gaseousGas solutions (mixtures of any quantities and any number of gases), for example. air. A heterogeneous system is impossible
In homogeneous mixtures, the constituent parts cannot be detected either visually or using optical instruments, since the substances are in a fragmented state at the micro level. Homogeneous mixtures are mixtures of any gases and true solutions, as well as mixtures of some liquids and solids, such as alloys.
In heterogeneous mixtures, either visually or using optical instruments, it is possible to distinguish regions (aggregates) of different substances delimited by the interface; each of these areas is homogeneous within itself. Such areas are called phase.
A homogeneous mixture consists of one phase, a heterogeneous mixture consists of two or more phases. Heterogeneous mixtures in which one phase in the form of individual particles is distributed in another are called dispersed systems. In such systems, a distinction is made between a dispersion medium (distribution medium) and a dispersed phase (substance crushed in the dispersion medium).
It is necessary to distinguish between mixtures and complex substances. Mixtures as opposed to complex substances:
are formed through a physical process - mixing pure substances;
the properties of the pure substances from which the mixture is composed remain unchanged;
pure substances (simple and complex) can be present in a mixture in any mass ratio.
Mixtures are formed by mixing various components. Mixing is one of the most common processes in chemical technology and related industries. Mixing can occur:
under the influence of external forces created by the working bodies of mixing machines;
as a result of both factors.
Mixing and stirring are synonymous words. It is customary to use the term mixing for solid bulk and pasty materials. For liquid media and gaseous substances, the term stirring is used.
When mixing, the distribution of particles of individual components in the mixed medium is random and occurs under the influence of many forces, such as gravity, inertial and various hydrodynamic and mechanical forces. In this case, their distancing and segregation, distribution in volume and sedimentation can occur simultaneously.
When mixing, they strive to achieve a perfect mutual distribution of particles. Perfect, or complete, mixing is such a mixing, as a result of which infinitesimal samples of the mixture taken anywhere in the mixed system will have the same composition or the same temperature. Since it is not possible to achieve such a state, in practice, various mixture quality criteria are used to qualitatively characterize the mixing process.
Ready-made mixtures are most often represented by solutions, emulsions, suspensions, pastes, granular compositions, and gas-liquid mixtures.
Solutions are a homogeneous (homogeneous) mixture formed by at least two components, one of which is called a solvent and the other a soluble substance; it is also a system of variable composition that is in a state of chemical equilibrium.
Emulsions are dispersed systems with a liquid dispersion medium and a liquid (less often gas) dispersed phase.
Suspensions are coarse systems with a solid dispersed phase and a liquid dispersion medium.
Granular mixtures are mixtures consisting of a large number of granular particles.
Gas-liquid mixtures are multiphase disperse systems, the physicochemical properties of which depend on the volume ratio of the liquid and gaseous phases in the mixture.
2.3 Mixing methods
Methods of mixing depending on the physical state of the components being mixed.
1.Circulating and in-line mixing. When transporting liquid through these pipes at high speed, intense mixing occurs - turbulization of the flow. Therefore, to mix the liquids contained in the device, it is enough to place a circulation pump next to the device, which will pump the liquid for some time. This mixing is called circulation. The efficiency of mixing increases significantly if the liquid in the apparatus is sprayed or introduced tangentially. The intensity of circulation mixing depends on the fluid flow rate in the circulation pump and the volume of the apparatus itself. Jet pumps are used to mix pure liquids, for example, raw alcohol and water during alcohol rectification. In this case, mixing occurs in a flow and is called in-line mixing. To mix non-viscous liquids in pipelines, mixers are installed, the working element of which is made of sequentially installed differently twisted screws or turbines. In-line mixing is carried out due to the kinetic energy of the flows. These same devices can be used to mix liquids and gases. Shelf mixers are used in fermentation industries. Molasses and water are mixed on the shelves. In this case, cold and hot water is supplied to different shelves in zones, which allows you to maintain the set temperature. 2.Gravity mixing In the preparatory workshops of chemical production, it is often necessary to create a mixture of several dry bulk components. In this case, solid bulk material rises to a certain height and falls under the influence of gravity, describing more or less complex trajectories, mixing at the same time. The most common for these purposes are screw mixers, the working body of which is one or more screws. Good mixing of bulk materials is achieved in rotating drums. The axis of rotation of the drum is inclined to the horizon, and this ensures the movement of material not only in the vertical plane, but also along the axis of the drum. The drums rotate, as a rule, at a low frequency (5...10 rpm). To increase the lifting height of the material, special blades are installed on the inner surface of the drum. The processes of mixing bulk materials can be intensified by using mechanical vibrations that accompany mixing with screws or blades rotating on a shaft. Such devices are called vibrating mixers. 3.Mechanical mixing. Mechanical stirring is the most common method of mixing in liquid media. It is produced using special devices - propeller, blade, turbine, anchor and frame mixers. As a rule, technical fluids have different characteristics, therefore the mixing mechanisms differ in their characteristics and operating parameters. Pneumatic mixing Pneumatic mixing with compressed inert gas or air is used when the stirred liquid is highly chemically active and quickly destroys mechanical stirrers. Mixing with compressed gas is a low-intensity process. Energy consumption with pneumatic mixing is greater than with mechanical mixing. Pneumatic mixing is not used for processing volatile liquids due to significant losses of the mixed product. Mixing with air may be accompanied by oxidation or tarring of substances. Mixing with compressed gas is carried out in devices equipped with special devices - a bubbler or a central circulation pipe. The bubbler consists of pipes located along the bottom of the apparatus with holes, through which gas is bubbled through the layer of the liquid being processed. During circulation (airlift) mixing, gas is fed into a circulation pipe. Gas bubbles carry the liquid in the vessel up the pipe, which then falls down in the annular space between the pipe and the walls of the apparatus, providing circulation mixing of the liquid. Electromagnetic stirring This type of mixing can be used in methods for intensifying technological processes in liquid metals. According to the proposed method, mixing of electrically conductive melts in mixers and furnaces is carried out by simultaneous exposure to a traveling electromagnetic field and one or more pulsating electromagnetic fields located in the traveling field zone, acting along the entire height of the melt column on the side of the mixer. The fields acting on the melt create its movement in one direction or alternately in one direction or the other throughout the entire mixing time in a plane parallel to the side of the mixer or oven. By varying the intensity of pulsating electromagnetic fields at the input and output of the traveling electromagnetic field, it is possible to change the trajectory of the electrically conductive melt during the mixing process. Electromagnetic stirring in open or closed glass vessels is often carried out using electromagnetic stirrers. The operating principle of these mixers is based on the fact that an electromagnet mounted on the axis of a vertically located motor rotates at a frequency of up to 24 sec. -1sets in motion a soft iron anchor. The latter is placed in a graphite, glass or polymer ampoule, which is sealed and placed at the bottom of the apparatus. Electromagnetic stirrers are used for mixing low-viscosity liquids (during hydrogenation, electrolysis, titration, etc.), when working in high vacuum, etc. If necessary, isolate the reaction mixture from the action of water and air, as well as to prevent leakage of volatile substances, the stirrers are sealed with rubber or cork stoppers, liquid seals (mercury or glycerin), cylindrical glass sections. The disadvantages of this method are: ) low efficiency of mixing the melt in the “dead zone” between the inlet and outlet of the channel and in the corners of the mixer and furnace; ) devices implementing the method, in particular a thin-walled channel or rolled metal, have low reliability when exposed to high-temperature metal melts. Mixing methods depending on the organization of the process itself. With periodic mixing, all individual stages of the process occur sequentially, at different times. The nature of the change in the concentrations of reacting substances is the same at all points of the reaction volume, but different over time for the same point in the volume. In such a process, the reaction time can be measured directly, since the reaction time and the residence time of the reactants in the reaction volume are the same. Process parameters change over time. With continuous stirring, all individual stages of the process of biochemical transformation of a substance (supply of reactants, biochemical reactions, output of the final product) are carried out in parallel, simultaneously. The nature of the change in the concentrations of reacting substances in the reaction volume is different at each moment of time at different points in the volume of the apparatus, but is constant over time for the same point in the volume. The process parameters are constant over time. With semi-continuous mixing, one of the reactants is supplied continuously, and the other - periodically. Options are possible when the reagents are supplied periodically, and the reaction products are unloaded continuously. This method is used when changing the rate of supply of reagents allows you to regulate the speed of the process. sorting mixture stirring stirrer 3. Mixing devices
Mechanical mixing devices consist of three main parts: the mixer itself, the shaft and the drive. The stirrer is a working element of the device, mounted on a vertical, horizontal or inclined shaft. The drive can be carried out either directly from an electric motor (for high-speed mixers), or through a gearbox or V-belt drive. Based on the design of the blades, there are different types of mixers: blade, sheet, propeller, turbine and special. Based on the type of liquid flow created by the stirrer in the apparatus, stirrers are distinguished that provide predominantly tangential, radial and axial flows. In tangential flow, the liquid in the apparatus moves predominantly in concentric circles parallel to the plane of rotation of the mixer. Mixing occurs due to vortices arising at the edges of the mixer. The quality of mixing will be worst when the speed of rotation of the liquid is equal to the speed of rotation of the mixer. Radial flow is characterized by the directed movement of liquid from the stirrer to the walls of the apparatus perpendicular to the axis of rotation of the stirrer. The axial flow of the liquid is directed parallel to the axis of rotation of the mixer, which determines the areas of their application. At high speeds of rotation of the mixers, the stirred liquid is drawn into a circular motion and a funnel is formed around the shaft, the depth of which increases with an increase in the number of revolutions and a decrease in the density and viscosity of the medium. To prevent the formation of a funnel, reflective partitions are placed in the apparatus, which, in addition, contribute to the emergence of vortices and increase the turbulence of the system. The formation of a funnel can also be prevented when the apparatus is completely filled with liquid, i.e., in the absence of an air gap between the liquid being mixed and the lid of the apparatus, as well as when the stirrer shaft is installed eccentrically to the axis of the apparatus or when a device of rectangular cross-section is used. In addition, reflective partitions are installed in all cases when mixing in gas-liquid systems. The use of reflective baffles, as well as an eccentric or inclined arrangement of the mixer shaft, leads to an increase in its power consumption. 3.1 Paddle mixers
Paddle mixers are devices consisting of two or more rectangular blades mounted on a rotating vertical or inclined shaft (Fig. 1). Paddle mixers also include some special-purpose mixers: anchor, frame and sheet. Due to the insignificance of the axial flow, paddle mixers mix only those layers of liquid that are in close proximity to the mixer blades. The development of turbulence in the volume of the stirred liquid occurs slowly, and the circulation of the liquid is small. Therefore, paddle mixers are used for mixing liquids whose viscosity does not exceed 103 ppm. sec/m 2. These mixers are not suitable for mixing in a flow, for example in continuous machines. A slight increase in the axial fluid flow is achieved when the blades are tilted at an angle of 30-45° to the shaft axis. Such a mixer is capable of keeping particles in suspension, the settling rate of which is low. In order to increase the turbulence of the medium when mixing with paddle mixers, in devices with a large height-to-diameter ratio, multi-row two-blade mixers are used with several rows of mixers installed on the shaft, rotated 90° relative to each other. The distance between individual rows is chosen within the range (0.3-0.8d), where d is the diameter of the mixer, depending on the viscosity of the mixed medium. For mixing liquids with a viscosity of no more than 104 ppm. sec/m2, as well as for mixing in apparatus heated by a jacket or internal coils, in cases where sedimentation or contamination of the heat transfer surface is possible, anchor (Fig. 2) or frame (Fig. 3) mixers are used. They have a shape that matches the internal shape of the apparatus and a diameter close to the internal diameter of the apparatus or coil. When rotating, these mixers clean the walls and bottom of the apparatus from adhering contaminants. Advantages of paddle mixers: ) simplicity of the device and low cost of production; ) quite satisfactory mixing of moderately viscous liquids. Flaws: ) low intensity of mixing of viscous liquids; ) unsuitability for mixing easily separated substances. Main applications of paddle mixers: ) mixing liquids of low viscosity; ) dissolving and suspending solids; ) rough mixing of liquids. Figure 1 - Paddle mixer Figure 2 - Anchor stirrer Figure 3 - Frame mixer 3.2 Sheet mixers
Sheet mixers (Fig. 4) have blades that are wider than paddle mixers and are classified as mixers that provide tangential flow of the mixed medium. In addition to the purely tangential flow, which is predominant, the upper and lower edges of the mixer create vortex flows, similar to those that arise when liquid flows around a flat plate with sharp edges. At high rotation speeds of the sheet mixer, the tangential flow is superimposed on the radial flow caused by centrifugal forces. Sheet mixers are used for mixing low-viscosity liquids (viscosity less than 50 ppm sec/m2), intensifying heat transfer processes during dissolution. For dissolution processes, sheet mixers with holes in the blades are used. When such a mixer rotates, jets are formed at the outlet of the holes, promoting the dissolution of solid materials. The main dimensions of paddle mixers vary depending on the viscosity of the medium. Typically, the following size ratios are accepted for paddle mixers: mixer diameter d = (0.66-0.9) D (D is the internal diameter of the apparatus), stirrer blade width b = (0.1 - 0.2) D, liquid level height in the vessel H = (0.8-1.3)D, the distance from the stirrer to the bottom of the vessel h d 0.3D. For sheet mixers d = (0.3-0.5) D, b = (0.5-1.0) D, h = (0.2-0.5) D. Peripheral speed of paddle and sheet mixers depending on depending on the viscosity of the stirred medium, it can vary within wide limits (from 0.5 - 5.0 sec-1), and with increasing viscosity and blade width, the rotation speed of the stirrer decreases. At high rotation speeds of paddle mixers, reflective partitions are installed in the apparatus. Sheet mixers, as a rule, are not used without reflective partitions. Figure 4 - Sheet mixer 3.3 Propeller mixers
The working part of a propeller mixer is the propeller (Fig. 5) - a device with several shaped blades curved along the profile of the propeller. Three-blade propellers are the most common. One or more propellers are installed on the mixer shaft, which can be located vertically, horizontally or obliquely, depending on the height of the liquid layer. Due to their more streamlined shape, propeller mixers consume less power at the same Reynolds number than other types of mixers. Figure 5 - Propeller mixer Device body Propeller Diffuser Figure 6 - Propeller mixer with diffuser: To improve the mixing of large volumes of liquids and organize the directional flow of liquid (with a large ratio of the height to the diameter of the apparatus), a guide apparatus, or diffuser, is installed in the vessels (Fig. 6). The diffuser is a short cylindrical or conical glass, inside of which a stirrer is placed. At high speeds of rotation of the mixer in the absence of a diffuser, reflective partitions are installed in the apparatus. Propeller mixers are used for mixing liquids with a viscosity of no more than 2.103 ppm. sec/m2, for dissolution, formation of suspensions, rapid mixing, formation of low-viscosity emulsions and homogenization of large volumes of liquid. For propeller mixers, the following ratios of the main dimensions are accepted: mixer diameter d= (0.2-0.5) D, screw pitch s=(1.0-3.0) D, distance from the mixer to the bottom of the vessel h=(0, 5-1.0) d, height of the liquid level in the vessel H = (0.8-1.2) D. The speed of the propeller mixers reaches 40 per second, the peripheral speed is 15 m/sec. Advantages of propeller mixers: ) intensive mixing; ) moderate energy consumption, even at a significant number of revolutions; ) low cost. Flaws: ) low efficiency of mixing viscous liquids; ) limited volume of intensively mixed liquid. Propeller mixers are mainly used for the following purposes: ) intensive mixing of low-viscosity liquids; ) preparation of suspensions and emulsions; ) resuspension of sediments containing up to 10% of the solid phase, consisting of particles up to 0.15 mm in size. 3.4 Turbine mixers
These mixers are shaped like water turbine wheels with flat, inclined or curved blades, usually mounted on a vertical shaft (Fig. 7). In devices with turbine mixers, predominantly radial fluid flows are created. When turbine mixers operate at high speeds, along with a radial flow, a tangential (circular) flow of the contents of the apparatus and the formation of a funnel may occur. In this case, reflective partitions are installed in the apparatus. Closed turbine mixers (Fig. 7), in contrast to open ones (Fig. 7 a, b, c), create a more clearly defined radial flow. Closed agitators have two disks with holes in the center to allow liquid to pass through; the disks at the top and bottom are welded to the flat blades. The liquid enters the mixer parallel to the shaft axis, is thrown out by the mixer in the radial direction and reaches the most remote points of the apparatus. Turbine mixers provide intensive mixing throughout the entire volume of the apparatus. For large values of the height to diameter ratio of the apparatus, multi-row turbine mixers are used. The power consumed by turbine mixers operating in devices with baffles under turbulent mixing conditions is practically independent of the viscosity of the medium. Therefore, this type of mixer can be used for mixtures whose viscosity changes during mixing. Turbine mixers are widely used for the formation of suspensions (the particle size for closed mixers can reach 25 mm), dissolution, absorption of gases and intensification of heat transfer. For mixing in large volumes (for example, when homogenizing liquids in storage facilities with a volume of up to 2500 m3 or more), turbine mixers are less suitable than propeller mixers or nozzles. Depending on the application, turbine mixers usually have a diameter d = (0.15-0.65) D with a ratio of the height of the liquid level to the diameter of the apparatus no more than two. For large values of this ratio, multi-row mixers are used. The stirrer speed ranges from 2-5 per second, and the peripheral speed is 3-8 m/sec. a - open with straight blades b - open with curved blades c - open with inclined blades g - closed with a guide vane Turbine mixer Guide vane Figure 7 - Turbine mixer Advantages of turbine mixers: ) speed of mixing and dissolution; ) effective mixing of viscous liquids; ) suitability for continuous processes. The disadvantage of turbine mixers is their relative complexity and high manufacturing cost. Application areas of turbine mixers: ) intensive stirring and mixing of liquids of various viscosities, which can vary over a wide range (open-type mixers up to 105 spz., closed-type mixers up to 5 * 105 spz); ) fine dispersion and rapid dissolution; ) agitation of sediments in liquids containing 60% or more solid phase (for open mixers - up to 60%); permissible solid particle sizes: up to 1.5 mm for open mixers, up to 25 mm for closed mixers. 3.5 Special stirrers
This group includes mixers that have a more limited use than the types of mixers discussed above. Drum mixers (Fig. 8) consist of two cylindrical rings connected to each other by vertical blades of rectangular cross-section. The height of the stirrer is 1.5-1.6 times its diameter. Mixers of this design create a significant axial flow and are used (if the ratio of the height of the liquid column in the apparatus to the diameter of the drum is at least 10) for carrying out gas-liquid reactions, producing emulsions and roiling sediments. Figure 8 - Drum mixer. Disc mixers (Fig. 9) are one or more smooth disks rotating at high speed on a vertical shaft. The fluid flow in the apparatus occurs in the tangential direction due to the friction of the fluid against the disk, and the tapering disks also create an axial flow. Sometimes the edges of the disc are made jagged. The diameter of the disk is 0.1-0.15 of the diameter of the apparatus. The peripheral speed is 35 m/s, which corresponds to very high speeds with small disk sizes. Energy consumption ranges from 0.5 kW for low-viscosity media to 20 kW for viscous mixtures. Disc mixers are used for mixing liquids in volumes up to 4 m3. Figure 9 - Disc mixer Vibrating mixers have a shaft with one or more perforated disks attached to it (Fig. 10). The discs perform a reciprocating motion, which achieves intensive mixing of the contents of the apparatus. The energy consumed by this type of mixer is low. They are used for mixing liquid mixtures and suspensions, mainly in pressure apparatus. The time required for dissolution, homogenization, dispersion when using vibrating mixers is significantly reduced. When stirred with these mixers, the surface of the liquid remains calm and no funnel is formed. Vibrating mixers are manufactured with a diameter of up to 300 mm and are used in devices with a capacity of no more than 3 m3. Figure 10 - Design of vibrating mixer disks 3.6 Selecting a stirrer
The choice of one type of mixer or another is determined by the intended purpose of the mixing devices and the specific conditions of the process. Any clear recommendations on this issue cannot yet be formulated. Therefore, when choosing a particular type of mixing device, you can use the approximate characteristics of the conditions for the appropriate use of various types of mixers given in Table 2. Table 2 - Approximate characteristics for selecting a mixer Type of mixers Volume of liquid mixed with one mixer, m3 Solid phase content during suspension, % Dynamic viscosity of the stirred liquid, kg/(m*s) Peripheral speed of the mixer, m/s Rotation speed of the mixer Blade<1,5<5< 0,011,7-5,00,3-1,35Пропеллерные<4,0<10<0,064,5-17,08,5-20,0Турбинные: - Открытые - Закрытые <10,0 <20,0 <60 60 и больше <1,00 <5,00 1,8-13,0 2,1-8,0 0,7-10,0 1,7-6,0Специальные<20,0<75< 5,006,0-30,01,7-25,0Conclusion
During the mixing process, there is close contact of particles and continuous renewal of the surface of interaction of substances. As a result, when stirring, mass transfer processes are significantly accelerated, for example, such as the dissolution of solids in liquid, the occurrence of most chemical reactions and the heat transfer process. Stirring promotes the process of accelerating absorption, evaporation and basic processes of chemical technology. Mixing is the process of repeated movement of particles of a heterogeneous fluid medium relative to each other throughout the entire volume of a container or apparatus, occurring due to pulses in an environment with a stirrer, a stream of liquid or gas. Mixing with a stirrer is a prerequisite for the successful implementation of many different technological operations. In production, mixing using a mixer is carried out for the following purposes: a) ensuring uniform distribution and crushing, grinding to a given dispersion (dispersion) of gas in liquid or liquid in liquid, as well as uniform distribution of solid particles in the volume of liquid; b) intensifying the heating or cooling of the processed masses in a container or apparatus, as well as ensuring uniform temperature distribution in the stirred container or apparatus; c) intensification of mass transfer in a stirred medium, as well as uniform distribution of the dissolved substance in the stirred mass. Thus, stirring with a mechanical stirrer has a decisive influence on the rate of various chemical transformation processes, since in industrial conditions the rate of these processes is determined not only by chemical kinetics, but to a large extent by the conditions of heat and mass transfer. Depending on the purposes and conditions of the process, containers and apparatus with mixing devices of various designs are used. The mixing process using a stirrer is widely used in many industries such as chemical, paint and varnish, energy, petroleum, asphalt, food and others for the production and preparation of suspensions, suspensions, solutions, reagents and emulsions, carrying out reactions, homogenizing, suspending, dissolving, mixing, stirring, etc. List of sources used
1.#"justify">2. #"justify">. Kafarov V.V., Dorokhov I.N., Arutyunov S.Yu., System analysis of chemical technology processes .
M.: Chemistry, 1988. - 214-298 p. . #"justify">. #"center"> Appendix A
Table 1 - Options for mixtures of substances in different states of aggregation Aggregate state of the constituent parts (before the formation of the mixture) Homogeneous mixture (homogeneous system) Heterogeneous mixture (heterogeneous system) Solid - solid Solid solutions, alloys (for example, brass, bronze) Rocks (for example granite, mineral-containing ores, etc.) Liquid - liquid Liquid solutions ( for example, vinegar - a solution of acetic acid in water) Two- and multi-layer liquid systems, emulsions (for example, milk - drops of liquid fat in water) Solid - liquid Liquid solutions (for example, aqueous solutions of salts) Solid in liquid - suspensions or suspensions (for example, clay particles in water, colloidal solutions) Liquid in solid - liquid in porous bodies (for example, soils, soils) Solid - gaseous Chemisorbed hydrogen in platinum, palladium, steels Solid in gaseous - powders, aerosols, including smoke, dust, smog Gaseous in solid - porous materials (for example, brick, pumice) Liquid - solid Solid liquids (for example, glass - solid, but still liquid) Can take different shapes and fix it (for example, dishes - different shapes and colors) Liquid - gaseous Liquid solutions (for example , a solution of carbon dioxide in water) Liquid in gaseous - aerosols of liquid in gas, including mists Gaseous in liquid - foams (for example, soap suds) Gaseous - gaseous Gas solutions (mixtures of any quantity and any number of gases), for example. air. A heterogeneous system is impossible Appendix B
An example of calculating material flows when mixing solutions Task. Mix 50 ml of 45% NaOH solution ( r = 1.480 g/ml) and 70 ml of 1.8N Na solution 2CO 3 (r = 1.180 g/ml). Calculate material flow. Solution. mole. mole mole mol/l mol/l mol/kg mol/kg mol∙eq/l mol∙eq/l
Component nameWeight, g n,mole ω
i, %
χ
i,%NaOH33,3000,83321,311,8Na2
CO3
13,3560,1268,51,8H2
O109.9446.10870.286.4Total156.6007.067100100 Material balance of mixing solutions loaded received name of component mass, g. name of component mass, g. technical in 100% calculation technical in 100% calculation A) Raw materials, including: 1) solution NaOHH
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 changes when heating a liquid in devices of various types: a – complete displacement; b – complete mixing; c – intermediate type
The 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
Introduction
Any technology, including chemical technology, is the science of methods for processing raw materials into finished products. Recycling methods must be economically and environmentally beneficial and justified.
Chemical technology arose at the end of the 18th century and almost until the 30s of the 20th century consisted of a description of individual chemical production facilities, their main equipment, material and energy balances. With the development of the chemical industry and the increase in the number of chemical production facilities, the need arose to study and establish general principles for the construction of optimal chemical technological processes, their industrial implementation and rational operation. In chemical technology, it is necessary to clearly distinguish the flows of substances with which transformation occurs, first from raw materials, then through the step-by-step formation of intermediate products until the final target product is obtained.
The main task of chemical technology is the combination in a single technological system of various chemical transformations with physical, chemical and mechanical processes: grinding and sorting of solid materials, formation and separation of heterogeneous systems, mass and heat transfer, phase transformations, etc.
Mechanical processes occupy one of the main places in production, as they are involved at every stage. In this work, a special place is given to the most common process - mechanical mixing. Depending on the conditions of the process, containers and apparatus with mixing devices (stirrers) of various designs are used in production.
The main goals of the work are a detailed study of the basic mechanical processes, mixing devices, their operation and technological purpose.
Mechanical processes of chemical technology
Mechanical processes include processes that are based on mechanical effects on the product, namely:
Sorting
There are two types of product separation: sorting by quality depending on organoleptic properties (color, surface condition, consistency) and separation by size into separate fractions (sorting by grains and shape).
In the first case, the operation is carried out by organoleptic examination of the products, in the second - by sifting.
Sorting by sifting is used to remove foreign impurities. When sifting, product particles whose sizes are smaller than the sieve openings pass through the holes (passage), and particles with sizes larger than the sieve openings remain on the sieve as waste.
For sifting, the following are used: metal sieves with stamped holes; wire sieves made of round metal wire, as well as sieves made of silk, nylon threads and other materials.
Silk sieves are highly hygroscopic and wear out relatively quickly. Nylon is insensitive to changes in temperature, relative humidity and sifted products; The strength of nylon threads is higher than silk.
Grinding
Grinding is the process of mechanically dividing the processed product into parts for the purpose of its better technological use. Depending on the type of raw material and its structural and mechanical properties, mainly two grinding methods are used: crushing and cutting. Products with low moisture content are crushed, and products with high moisture content are cut.
Crushing in order to obtain coarse, medium and fine grinding is carried out on grinding machines, fine and colloidal - on special cavitation and colloid mills.
During the cutting process, the product is divided into parts of a certain or arbitrary shape (pieces, layers, cubes, sticks, etc.), as well as the preparation of finely ground types of products.
To grind solid products with high mechanical strength, band and circular saws and cutters are used.
Pressing
Product pressing processes are mainly used to separate them into two fractions: liquid and dense. During the pressing process, the structure of the product is destroyed. Pressing is carried out using continuous screw presses (extractors of various designs).
Mixing
Mixing promotes the intensification of thermal biochemical and chemical processes due to an increase in surface interaction between the particles of the mixture. The duration of mixing of mixtures determines their consistency and physical properties.
Dosing and forming
The production of enterprise products and their release are carried out in accordance with GOSTs or technical specifications or internal technological standards and collections of recipes, with standards for the laying of raw materials and the yield of finished products (weight, volume). In this regard, the processes of dividing the product into portions (dosing) and giving them a certain shape (molding) are essential. The dosing and forming processes are carried out manually or with the help of machines depending on the production.
