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22SCIENTIFIC PROGRESS VOLUME 2 I ISSUE 3 I 2021
ISSN: 2181-1601
THERMAL PYROLYSIS. TECHNOLOGY, MAIN PRODUCTS AND FAR
BEYOND
Olim Rustamovich Abdurakhmonov Abdukhakim Rustamovich Muminov
Bukhara engineering technological institute
ABSTRACT
The aim of the article is to identify the most appropriate heat degree of the process to get maximum efficiency and maintain quick respond of the system by inculcating information communication systems.
Keywords: pyrolysis, low olefins, pipe furnace, coil, pyrogas.
At present, the petrochemical potential of industrialized countries is determined by the volume of production of lower olefins. The main source of their production is the process of thermal pyrolysis of hydrocarbons with steam. This process is a modification of thermal cracking of petroleum products, the development of which with the use of tube furnaces began in 10-20 years at US refineries. The first industrial syntheses of modern petrochemistry were carried out on the basis of ethylene and propylene isolated from cracking gases. It is at pyrolysis plants that primary products are obtained today, which provide raw materials for the production of plastics, synthetic resins, rubbers and fibers.
Theoretical foundations of thermal pyrolysis. Thermal decomposition of hydrocarbons is a complex process that can be represented as a series of chemical reactions proceeding sequentially and in parallel with the formation of a large number of products. The energy characteristics of the reactions, expressed by thermodynamic relations, determine the directions and the maximum equilibrium degree of conversion of the initial substances according to them. The equilibrium degree of conversion by a chemical reaction can be calculated from the equation of the dependence of the equilibrium constant Ke on the change in the standard Gibbs energy (free energy, Go):
ln
e R x T
The degree of conversion of the starting materials according to the reaction is an unambiguous function of the equilibrium constant Kp, the analytical expression of which is determined by the stoichiometry of the reaction.
As a result of the thermal decomposition of hydrocarbons, various products are obtained, including lower olefins, methane, and other alkanes with a lower molecular weight than the original one. So, when describing the pyrolysis of ethane by molecular reactions, the main one is the dehydrogenation reaction with the formation of ethylene.
In the pyrolysis of propane, along with dehydrogenation to propylene, decomposition to ethylene and methane occurs [1].
The decomposition of n-butane can be represented analogously to the reactions of dehydrogenation and cleavage in two directions. Alkanes C2-C4 decompose according to molecular reactions [1]:
n ■ Cflw ^ C3H4 + CH4 (1.1)
2C2H6 ^ 2CH4 + C2HA (1.2)
C3hs ^ C2H + CH4 (1.3)
n ■ C4H10 ^ C2H4 + C2H6 (1.4)
CH ^ CH + Hva n ■ Cflw ^ n ■ CH + H (1.5)
C2H6 ^C2H4 + H2 (1.6)
According to the calculations [1], the equilibrium dehydrogenation of C3-C4 alkanes can go to the end at a temperature of 800-850 °C, and the dehydrogenation of ethane - only at 900-950°C. Alkane cleavage reactions can be completed at a lower temperature, of the order of 250-450°C and the more C atoms in the molecule of the initial hydrocarbon, the lower the temperature corresponds to its complete equilibrium cleavage [1].
Thus, the use of pyrolysis catalysts makes it possible to significantly increase the reaction rate of the actual decomposition (cracking) of hydrocarbons without increasing the rate of secondary reactions of the addition type, in which ethylene, other alkenes, and alkadienes are consumed. This leads to an increase in the yields of alkenes and makes it possible to apply milder conditions than in thermal pyrolysis.
To date, the only method of pyrolysis that has been mastered and widely used in industry is thermal pyrolysis in tube furnaces. Of the known limitations of the pyrolysis process in tube furnaces, difficulties with the use of raw materials prone to increased coke formation are of no small importance. The need to expand the raw material base, as well as specific energy and material costs, led to the development of new process modifications, mainly designed for pyrolysis of heavy hydrocarbon raw materials. Not only the processes of pyrolysis of weighted feedstock (fuel oil, vacuum gas oil, oil) are being developed, but also fundamentally new methods with a significant increase in ethylene yields.
The main goals of ethylene producers have always been to optimize capital investments, provide flexibility in products and raw materials, high reliability and energy efficiency. One way to reduce production costs is to increase the capacity of the plant. Large scale plants have lower capital and operating costs per tonne of ethylene.
The main objectives at the moment are: increasing the strength and durability of pipes and minimizing capital investment and operating costs. Here are some technical solutions in this direction:
a) The use of hardening coolers with straight pipes, which made it possible to reduce the number of mechanical cleanings;
b) Reduction of the number of burners due to the use of burners of higher capacity, giving a flame directed upwards;
c) Manufacturing of radiant pipes from materials with a high content of nickel (48%) and tungsten, which made it possible to raise the temperature of the pipe wall and increase the creep resistance of the pipe material;
d) Modernization of bearings for radiant coils, for example, the use of suspension rods with a constant load to increase the strength;
e) The use of improved control, which allows to optimize the rigidity of the process, the furnace load, the dosage of the steam diluent, the distribution of raw materials along the furnace flows, the control of the fuel combustion process;
f) Grinding of the inner surface of the pipes, which made it possible to reduce the processes of coke formation.
As a result of the analysis of the state of thermal decomposition, the following recommendations were proposed:
- in the convection section, in principle, it is possible to place pipes of any length necessary to provide a given furnace power, but it is limited to 12 m in order not to have intermediate welds;
- due to an increase in the mass velocity of flue gases and a decrease in the length of pipes (the convection section in these furnaces is shorter than the radiant section), a high thermal efficiency is ensured with a small heat exchange area;
- an increase in the power of a pyrolysis furnace and a decrease in capital investments can be achieved by increasing the length of a single-chamber furnace (with one radiant section) or by combining two radiant sections in one two-chamber furnace (in the second design, two radiant sections have a common convection section);
- the furnace can be made the most flexible in terms of raw materials and load by pyrolysis of individual raw materials in separate coils of one pyrolysis furnace.
Table 1- Pyrogas composition (production data)
Component % mass. % mol.
C2H6 10,23 8,76
C2H4 30,52 27,99
CH4 20,02 32,13
C3H8 3,39 1,98
C3H6 15,84 9,69
C4H10 7,61 3,37
C4H6 5,91 2,81
C5H12 7-10-5 2-10-5
C2H2 0,13 0,13
H2 0,71 9,12
Polymers 3,76 —
Coke 1,88 4,02
Total 100 100
Automatic control and regulation. The development of the industry of petrochemical and organic synthesis in our time is impossible without the use of automatic control. New devices, analyzers, automatic machines, and computer technology are put into operation every year. The plant is moving from partial production automation to complex automation systems, which ensures the efficiency of these enterprises. Further increase in the level of automation of processes and production is carried out in the following main areas:
- control of several similar installations from one operator room;
- increasing the level of automation of installations by using industrial automatic and semi-automatic analyzers of the quality of basic and intermediate products;
- replacement of outdated instruments and automation equipment with new, improved ones;
- the introduction of computer technology.
The design of the catalytic pyrolysis unit provides for a technological process using modern automatic control and regulation technology in order to facilitate the work of maintenance personnel, ensure normal operation and prevent accidents, maintain an optimal technological regime, increase labor productivity, product quality with a minimum number of maintenance personnel and raw material costs. and materials.
Selection and justification of control and regulation parameters
A prerequisite for the normal conduct of the pyrolysis process is to maintain a constant consumption of raw materials, steam, cooling water, control and regulation of temperature, and maintain a given pressure. To obtain quality products and avoid undesirable hazardous consequences, strict adherence to the established process parameters is necessary.
Pyrolysis is a process of deep decomposition of hydrocarbon feedstock under the influence of high temperatures. The main goal of the process is to produce as much ethylene and propylene as possible. The pyrolysis reaction takes place in the radiant part of the tube furnace coil. The temperature and contact time have a great influence on the composition of the products of the process. Violation of the temperature regime leads to a decrease in the yield of target products. Maintaining the temperature of the pyrolysis gas at the outlet of the furnace at 800 ° C is achieved by regulating the fuel supply to the furnace. The product yield also depends on the pressure. The process is carried out by diluting the raw material with water vapor and thereby reducing the partial pressure of hydrocarbon vapors. Steam in the amount of 50% by weight of the incoming raw
SCIENTIFIC PROGRESS VOLUME 2 I ISSUE 3 I 2021
ISSN: 2181-1601
material is mixed with the raw material at the entrance to the furnace, the control valve is installed on the steam supply line.
The main task of the quenching and evaporation apparatus is the rapid cooling of the pyrogas with water. Maintaining the temperature of the pyrolysis gas at the outlet of the ZIA is achieved by regulating the supply of water condensate, the valve is installed on the line for supplying water condensate.
In the washing column by refluxing with light tar, additional cooling of the pyrolysis gas, condensation of heavy tar, and washing of pyrogas from coke are carried out. The temperature of the top and bottom of the column is regulated by the supply of light resin, respectively, to the top of the column and to the distributor device between the upper and lower trays from the pump . It is necessary to maintain a certain liquid level in the cube of the columns. A significant change in the liquid level can lead to overfilling or emptying of the apparatus, making the process impossible.
REFERENCES
1. Mukhina T.N., Barabanov N.L., Babash S.E. Hydrocarbon pyrolysis. M .: Chemistry, 1987 .-- 240p.
2. Torosyan G.H., Isakov A.A. Technology of producing liquid fuel through processing carbon-containing waste and fuel oil
3. Krichko A.A., Lebedev V.V., Farberov I.L. Non-fuel use of coal. M.: Nedra, 1978. p. 215.
4. Baykenov M.I., Omarbekov T.B., Ma Feng Yun, Amerkhanova Sh.K., Uali A.S. // Chemistry of Solid Fuel. 2011. No. 4. p. 56.
