Polyethylene raw materials. Low pressure polyethylene production. What is XLPE

Industrial production of polyethylene was carried out in 1938 under high pressure (about 150 MPa) and 180 - 200 0 C in the presence of traces of oxygen by a radical mechanism.

An important stage in the development of the production of polyolefins was the discovery by Ziegler of catalysts - complexes of alkyl aluminum and titanium chlorides, which caused the polymerization of ethylene, propylene and other olefins during atmospheric pressure... Currently, the number of such catalysts has increased significantly. They are complexes consisting of organometallic compounds Al, Be, Mg, Zn, Cd, Ba, Na and chlorides of metals of IV, V, VI and VIII groups, i.e., elements with an unfilled intermediate electron shell. Most often, titanium chlorides TiCl 4 and TiCl 3 are used, and TiCl 4, when interacting with metal alkyls, in particular Al (C 2 H 5) 3, is reduced to compounds of lower valence. Depending on the nature of the catalyst components, as well as the number of substituents in the olefin, stereoregular polyolefins of various spatial configurations can be obtained: isotactic, syndiotactic, etc.

Different degrees of crystallinity and the nature of the structure determine the complex of valuable physical and mechanical properties of polyolefins obtained on Ziegler-Natta catalysts.

It was important to develop the polymerization of olefins (Phillips method) at a pressure of 3.5 - 7 MPa and 130 - 170 0 С in an inert hydrocarbon medium in the presence of a catalyst consisting of metal oxides variable valence, for example, chromium oxides deposited on an aluminosilicate. There are several modifications of this method, collectively known as medium pressure polymerization. Various ways industrial production makes it possible to obtain polyethylene with various properties.

The heat effect of ethylene polymerization is about 4200 kJ / kg. This number includes the heat of polymerization of ethylene, calculated from the binding energies and equal to 3653 kJ / kg, the heat released during the transition of a gaseous product (ethylene) into a solid product (polyethylene), as well as the heat of dissolution of ethylene in a liquid hydrocarbon (in the case of polymerization at low pressure).

The polyethylene formula [–CH 2 –CH 2 -] n is formal. Polyethylene is a branched polymer, its structure contains the following abnormal links:

~ CH 2 –CH ~; ~ CH 2 –CH = CH 2; ~ CH 2 –C – CH 2 ~; ~ CH 2 –CH = CH – CH 2 ~

…………… .CH 3 …………………………. CH 2

Polyethylene is a solid product. Depending on the production method, it has different properties and can be of two types: polyethylene obtained at high pressure (low density) and polyethylene obtained at low and medium pressures (high density). However, this sign is conditional. The density can be changed within the method.

The physicochemical and mechanical properties of polyethylene depend mainly on the structure of the polymer and its molecular weight. High-pressure polyethylene is characterized by unevenness, greater elasticity, less brittleness, and a lower softening point (108 - 120 0 С) in comparison with polyethylene obtained at low pressure. Polyethylene with a molecular weight of about 3 * 10 6 has extremely high strength, which is very valuable in the production of fibers and composites.

Polyethylene at room temperature is insoluble in any of the known solvents and only at 80 ° C and above it begins to noticeably dissolve in carbon tetrachloride, trichlorethylene, benzene, toluene, xylene. When the solution is cooled, the polymer precipitates.

Polyethylene is highly water and chemical resistant. At temperatures up to 60 - 80 0 С, it is resistant to the action of alkalis and acids, including hydrofluoric acid, with the exception of concentrated nitric acid.

When polyethylene is heated in air, already at 120 ° C, its oxidation begins, accompanied by cross-linking of linear macromolecules and the formation of insoluble polymers. At temperatures above 290 ° C, polyethylene is degraded with the formation of liquid oily and gaseous products, including a small amount (about 3%) of monomer. When exposed to ultraviolet rays, air oxygen and heat during processing and operation, polyethylene ages, which is manifested in the deterioration of its physical, mechanical and dielectric properties.

Polyethylene is used in various fields National economy... The most widely used high-pressure polyethylene is for the manufacture of films, sheets, pipes, hoses, barrels, buckets. It is used in the cable industry, radio engineering, chemical industry, agriculture, for channel cladding, in construction. Low-pressure polyethylene, as well as its copolymers with propylene, are used in construction for the manufacture of pipes and sanitary ware. Low pressure polyethylene is an inelastic polymer that melts and acquires the properties of an elastomer at a temperature of 130 0 C. As the propylene content in the copolymer increases, flexibility increases and crystallinity decreases. Copolymer with 20 mol. % propylene content has valuable properties and is obtained both by the Ziegler-Natta method at low pressure and at 3.5-4 MPa using metal oxides as a catalyst. With a ratio of two ethylene units to one propylene unit, a low pressure elastomer with an average molecular weight of 80,000 to 500,000 and a crystallinity of 58 to 75% can be obtained. Compared to low-pressure polyethylene, the copolymer has an increased resistance to cracking under prolonged stress.

High pressure polyethylene (low density)

In industry, high-pressure polyethylene (LDPE) is obtained by polymerizing ethylene in a condensed gas phase in the presence of radical initiators at a pressure of 150 - 300 MPa and a temperature of 200 - 280 0 С. 500,000 and a degree of crystallinity of 50 - 65%.

Regulation of polyethylene density and chain length is carried out by varying the polymerization conditions (pressure and temperature), as well as the introduction of various additives (hydrogen, propane, isobutane, alcohols, aldehydes, ketones). Since high molecular weight polyethylene is formed only at a high concentration of ethylene, polymerization is carried out at high pressures, at which the density and concentration of ethylene is 450-500 times higher than at atmospheric pressure. High pressure promotes the convergence of the reacting molecules and the homogeneity of the reaction medium. The process is carried out in the condensed phase of the monomer in the presence of oxygen or radical polymerization initiators.

When ethylene interacts with oxygen, ethylene peroxide or hydroperoxide compounds are formed:

CH 2 = CH 2 + O 2 ⟶ CH 2 –CH 2 or CH = CH 2

………………… ... O ¾ O …… ... OOH

The unstable peroxide bond –О – О– under the influence of heat undergoes homolytic rupture with the formation of bi- and monoradicals: * OCH 2 - CH 2 O * and CH 2 = CHO *. Free radicals initiate the polymerization of ethylene. Free radicals are part of the polymer and, therefore, are consumed during the polymerization process.

In the process of synthesis, a linear polymer is formed, containing side branches (short and long) with a length of 2 to 5 carbon atoms, located randomly at a distance of about 50 carbon atoms from each other. Less often, the formation of macromolecules with a side chain length comparable to the length of the main chain is possible. The ends of the chain contain CH 3 groups. The high-pressure polyethylene macromolecule can contain vinyl and diene groups in the amount of 4 - 6 per 10,000 carbon atoms.

The branching of LDPE macromolecules limits the degree of crystallinity to 55 - 60%.

High pressure polyethylene is a non-polar, amorphous - crystalline polymer with a melting point of 103 - 110 0 С. The molecular weight of industrial brands ranges from 30,000 to 500,000.

The efficiency of ethylene polymerization is due to the high reaction rate, the properties of the resulting polyethylene, and the degree of monomer conversion in one pass. Polymerization efficiency depends on temperature, pressure, initiator concentration and monomer residence time in the reactor.

As the temperature rises, the rate of polymerization and the degree of conversion of the monomer increase, but the molecular weight of the polymer decreases. As the temperature rises, the number of double bonds in polyethylene and the degree of its branching increase.

With an increase in pressure, the rate of polymerization and the degree of conversion of the monomer, as well as the molecular weight and density of polyethylene, increase, and the physical and mechanical properties of the product improve.

To increase the degree of conversion of ethylene, a new portion of the initiator is sometimes introduced into the reaction zone, which makes it possible to increase the yield of the product per unit volume of the reaction space.

In industry, for the production of high-pressure polyethylene, two types of installations are used, which differ in the design of the reactor for the polymerization of ethylene. Reactors are either tubular apparatus operating on the principle of ideal displacement, or vertical cylindrical apparatus with a stirring device - autoclaves with a stirrer, operating on the principle of ideal mixing.

To obtain polyethylene with a sufficiently high molecular weight and density, polymerization is carried out at high pressures. For this, thick-walled metal pipes will be used. In addition, polyethylene has the highest heat of polymerization among the monomers of the olefin series, which requires efficient heat removal.

To ensure high speeds of the process (and thus high productivity of the reactor with a limited volume of the reaction space), polymerization is carried out at the maximum permissible temperatures (200 - 300 0 С). The upper temperature limit depends on the operating pressure in the reactor and is limited by the explosion safety conditions (due to the possibility of ethylene decomposition at critical temperatures), the given molecular weight and molecular weight distribution.

A tubular reactor has a number of advantages over an autoclave reactor.

First, in a tubular reactor, more heat is removed through the wall than in an autoclave. The conversion of ethylene to polyethylene in an autoclave is below. In a tubular reactor, a product with a broader molecular weight distribution is obtained, which is important in the production of films, cable coatings, etc.

Secondly, during polymerization in a tubular reactor, cheap oxygen can be used as an initiator, i.e., the supply of paraffin oil with a peroxide initiator can be excluded.

The supply of different initiators to different zones of the reactor makes it possible to vary the properties of the resulting polyethylene.

Automation of the high-pressure polyethylene production process is necessary to control complex chemical-technological processes, prevent breakdowns technological equipment, abbreviations production costs and increasing the level of labor safety at the enterprise. Implementation of ACS in manufacturing process brings many benefits to enterprises in the chemical industry, solving a complex of problems associated with the control and management of production.

The automated system makes it possible to increase the productivity of the reactor, ensure a high speed of the process, reduce the harmfulness of the production area, reduce the cost of polyethylene, increase the yield of the finished product and improve its quality. Automation of highly complex chemical-technological processes, sensitive to any deviations, makes polyethylene production manageable, predictable and controlled.

Functions of the APCS for high-pressure polyethylene:

diagnostics of the state of technological equipment;
program-logical control of technol. aggregates;
predicting the quality indicators of polyethylene,
analysis of technological data, processing and storage;
visual display of information on the operator's screen;
process control, pre-alarm, blocking.

Automation is seen as a highly efficient tool that allows you to have all the necessary technological information. APCS helps to protect the production process from emergencies, remotely control processes, and control each technological stage. The composition of the hardware and software used to build an automated process control system is determined by the functions of the system and the requirements for it. Both individual processes and the production of polyethylene as a whole can be automated.

The APCS for the production of high-pressure polyethylene can be built both on a two-level and on a three-level principle. The lower level is programmable controllers, software which collects data from sensors, then generates and issues impacts on mechanisms, as well as exchanges information with the upper level. The upper level is the workplaces of technologists and operators who receive technological information in digital and graphic form. Based on the data, processes are monitored: they set the operating mode of the system, make changes to the operation of installations, etc.

Advantages of high pressure polyethylene

High pressure polyethylene (GOST 16337-77) has another name - it is "low density polyethylene". Also, the plastic material has such abbreviations as: LDPE, LDPE, LDPE, LDPE. There are more than 30 basic grades of polyethylene, in a flexible automated enterprise it is possible to quickly make changes to the product formulation and switch from the release of one grade of material to another. Polymer producers are strategically important for the development of the economy and industry.

LDPE production is carried out by: NefteKhimSevilen OJSC, Polimir OJSC, Volzhsky Polymer Plant LLC, Khimpek CJSC, Tomskneftekhim LLC, Kazanorgsintez OJSC, Shurtan Gas Chemical Complex, Angarsk Polymer Plant OJSC, Salavatnefteorgsintez OJSC, Ufaorgsintez OJSC. Products made of high-density polyethylene are used in construction, radio engineering, electrical engineering, agriculture, food and chemical production, mining, automotive, housing and communal services and other industries. LDPE is distinguished by expressive physical and mechanical properties.

Table 1: Basic physical and mechanical properties of LDPE

High-pressure polyethylene is used to produce flexible packaging, film products, plastic films, thermal films, bags, food packaging, greenhouse films, multilayer packaging. Thermoplastic ethylene polymer is used for the production of cables, sheaths of cord products, containers (cans, cans, pots, buckets, garden tools), dielectric antennas, polymer pipes, sheets, parts of technical equipment, hoses, chemical production equipment. The material is used for lining channels and lining of valves, fittings, equipment, pipelines.

Advantages of high pressure polyethylene:

Chemical resistance;
Mechanical strength;
Moisture - and heat resistance;
Impact resistance;
Electrical insulating material;
Corrosion resistance;
Radiation resistance;
Softness and elasticity;
Low gas permeability;
Non-toxic, harmless;
Resistance to temperature extremes;
Low absorbency.

The processing of high-pressure polyethylene is carried out by such methods as: injection molding, extrusion in an extruder, pressing using a hot press. For the production of polyethylene, they use: a reactor block, pumps, a refrigeration unit, a separator, compressors, a separator, a heat exchanger and other equipment. The raw material for the production of high-pressure polyethylene is ethylene (without impurities) of high purity.

There are 2 types of ethylene polymerization units - agitated autoclaves and tubular reactors. The target product is obtained by polymerizing ethylene under high pressure, temperature and mixing with oxygen. At the final stage of the technological process, the polymerized liquid mass is processed in a separator, and then in a pellet press to obtain polyethylene granules (granulation).

Functions of the APCS for high-pressure polyethylene

Strict requirements are imposed on the polymerization process. It is characterized by: high speed, temperature and pressure. The degree of polymerization, quality of polyethylene and production safety depend on these indicators. Automation of the high-pressure polyethylene production process ensures precise adherence to the parameters of the technological process, helps to reduce the influence of subjective factors (employee errors) and reduce the consumption of resources (water, steam, heat, gas). The system allows you to exclude the controlled parameters going beyond the permissible limits. The functions of the system include: Automated process control system for dosing reagents, temperature control; accounting of technical and economic indicators; changing the recipe; control of actuators and electric motors of pumps and others.

APCS allows you to control such indicators as:

- pressure in the system;
- heating temperature;
- the degree of conversion of the monomer;
- consumption of ethylene and initiator;
- working pressure in the reactor;
- polymerization rate;
- the amount of the initiating substance;
- duration of exposure.

Low density polyethylene (high pressure) is produced in granules (unpainted and colored) and in powder form. Compared to powder polyethylene, granular polyethylene has high technical and economic advantages due to the fact that it is easier to transport, store, dose, pack and move. It is easier to remove polyethylene in granules from the loading device or container bag: it does not accumulate in the units of the units, does not electrify, does not stick to the walls of the equipment. To automate the granulation process, automated lines are being introduced.

So everyone technological stages- from the reception and preparation of raw materials to granulation and packaging of the finished product, are carried out without direct human participation. The operator receives all the required information about the course of technological processes. Operational personnel in remote mode can conduct a qualitative analysis of chemical production, unload technical and economic calculations, control the operation of cutting devices, pumps, vibrating screens, conveyors, extruders-granulators and other technological units. The result of the automation of the high-pressure polyethylene plant is an increase in labor productivity and an increase in the efficiency of the entire production.

Order the introduction of an automated process control system for the production of high-pressure polyethylene

LLC "Olaysis" - a Russian engineering company offers highly efficient IT solutions for the automation of enterprises for the production of high-density polyethylene. We carry out the development and implementation of an automated process control system of a high scientific and technical level, adapted to the requirements of LDPE production. Automation significantly simplifies obviously complex technological operations, significantly improves the organization of personnel labor, ensures an increase in production volumes, helps to objectively assess the state of the enterprise and make timely decisions when technological parameters are deviated. The specialists of our company are experts in the assembly of automation cabinets, selection and assembly of explosion-proof equipment for industrial operation.

The use of worn-out and outdated management and control means increases the risk of violations of the technological regime, reduces the level of industrial safety, leads to a deterioration in quality indicators and increases the labor intensity of products. In our catalog you can order equipment, instrumentation, controllers, industrial computers and automation components for the modernization of petrochemical production. If you want to improve the processes of pyrolysis, conversion and increase the capacity of your enterprise, then contact our company.

We are ready to conduct a survey of your enterprise, identify reserves for increasing productivity, help with the choice of equipment and automation elements. For more than 10 years in the market of automated process control systems, our company has been developing and implementing economically effective systems... Our solutions take into account the needs of both high-density polyethylene plants under construction and facilities under reconstruction. We create systems from scratch, as well as work with automated control systems developed by third-party companies. We also invite partners for joint development, the development of complex solutions, the introduction of intellectual products.

Most often, the manufacturing business is associated with a large investment of initial capital. In addition, for a person unfamiliar with technological process, mastering a new business can be quite difficult. The production of polyethylene can be safely attributed to the pleasant exceptions to the general rules. For a successful start, there is no need to spend a lot of money at once, because the business quickly pays off and begins to bring stable profits. But before setting up the production of polyethylene, we will study its features, varieties, application possibilities and try to draw up a small business plan.

What is polyethylene?

This is the name of a synthetic polymer material based on ethylene - an organic colorless gas with a weak odor. It is the most productive material in the world. Such well-known products as ethyl alcohol, styrene, ethylbenzene, acetic acid, vinyl chloride and many others are synthesized from it.

Polyethylene is produced in the form of transparent or colored granules of various shapes. Their size is usually from three to five millimeters. The production of polyethylene granules consists in the process of polymerization of ethylene gas under high and low pressure conditions, as well as with the use of additional conditions. The main enterprises engaged in the manufacture polymer materials, are located in Russia, Uzbekistan, Belarus, South Korea.

Due to its special properties, the following grades of polyethylene are distinguished:

HDPE - high density;
LDPE - Low Density;
LLDPE - Linear;
mLLDPE, MPE - metallocene linear;
MDPE - Medium Density;
HMWPE, VHMWPE - high molecular weight;
UHMWPE - ultra high molecular weight;
EPE - expandable;
PEC is chlorinated.

A lot of materials are also known that belong to the category of copolymers. Let's analyze several types that are most often found in industrial processing.

Low density polyethylene

The material has a plastic and soft structure. The production of high-density polyethylene (LDPE) involves the polymerization of ethylene in a tubular reactor or autoclave. The process takes place at a temperature of about 750 o C under a pressure of 1.5-3 kgf / cm 2. The result is a low density granulate. The resulting raw materials are sent to the production of polyethylene packaging in contact with dry and bulk substances. Bags made from such material are capable of withstanding up to four kilograms of weight.

High density polymer

The production of low-density polyethylene (HDPE) is a polymerization process using catalyst systems. As a result, hard granules with a high density level are obtained - 0.960 g / cm 3. They are suitable for the production of cling film. Commercial granulate is produced colored and colorless. Sometimes the finished product is in the form of a powder.

What does expandable polyethylene look like?

This is the name of a synthetic material that has a closed-porous structure. The production of foamed polyethylene is based on strong heating of raw materials and subsequent whipping with the help of gas (butane, freon and others). In practice, polyethylene foam is widely used as an excellent universal heat insulator.

What is XLPE?

The production of extra strong granules is based on the use of ultra-high pressure. As a result of the process, a strong adhesion of the molecules of the original substance occurs. The modified polymer is distinguished by high technical characteristics:

Resistant to high temperatures. The material softens only at temperatures above 150 ° C, melts at 200 ° C, and ignites only when it reaches 400 ° C.
Increased stiffness and tensile strength.
Preservation of the main features during a sharp change in environmental conditions, as well as under the influence of chemical or biological destructors.
High steam and waterproofing properties.

Cross-linked polyethylene is actively used in the production of pressure pipes for cold and hot water supply. In addition, it is used in the manufacture of elements of heating systems and special building materials.

How does a business start?

A polyethylene plant can include several technological lines for the production of various products: plastic films, bags, lids, containers, pipes, bottle caps and much more. It is not worth organizing several directions at once. It is more expedient to enter the polymer market as a manufacturer of plastic films and bags. Having established stable work, you can gradually expand the range of products.

Practical experience shows that the production of polyethylene in Russia is guaranteed to provide a level of profitability of at least 15%. Before starting an enterprise, you need to take care of the design permits... You will have to visit the city administration, energy supervision, sanitary and epidemiological station, fire brigade, environmental service. If you tackle these issues closely, then you can fully meet the deadline for a month and a half. Overhead costs will amount to only 15–20 thousand rubles.

Residue recycling issue

Before you start organizing the production of polyethylene products, think carefully about the issue of waste disposal. Under no circumstances should plastic scraps be buried in the ground or burned. Firstly, it causes enormous harm to the environment. And secondly, for such actions, serious punishment threatens.

The easiest and cheapest way is to take polymer residues to a plastics processing plant. But it should be borne in mind that such a plant may not be in your locality... If the production of recycled polyethylene is planned, then it is best to organize the production of garbage bags. To do this, you will have to incur additional costs for the purchase of a technological line. But in the end, the costs will be paid off by the quick sale of popular goods that are in steady demand among the population.

Purchase of main equipment

The choice of production lines is quite large today. As an example, consider the list of machines and units that will be required for the production of film with the further formation of packages for household use from it.

Necessary equipment for polyethylene production:

Extruder (extrusion machine)- machine for converting raw granules into film by blowing from the bottom up. The width of the sleeve should correspond to the dimensions of the bags being produced (300–550 mm). The unit also includes a seam folding device.
Bag making machine- a machine for cutting film or sleeves into workpieces of a certain length. The device also seals the workpiece on one side, forming a finished product.
Cutting press with a set of molds for the production of T-shirt bags or bags with a slotted handle.
Machine for making plastic clamps for packaging.
Flexograph - a machine for applying printed images to the bag sleeve.

If start-up capital not very much, then at first you can completely do without a printing device. It would be wiser to apply for a service for drawing drawings in specialized printing centers.

To process industrial waste, you will need to purchase special apparatus for crushing. The approximate cost of a production line with delivery and setting up of machines is 1.5–2 million rubles.

Additional equipment items

The production of polyethylene also requires the purchase of warehouse equipment (racks, tables, stands, boxes, etc.) for storing raw materials and finished products... Do not forget about furnishing offices. Optional equipment may increase the total amount of expenses by 50-60 thousand rubles.

Production workshops need to equip a high-quality powerful ventilation unit and a fire-fighting system. Special requirements are imposed on warehouses: the primary raw material for the production of polyethylene (granulate) tends to absorb vapors and gases. Failure to comply with the rules for storing raw material can lead to a deterioration in the quality of the manufactured products.

Required raw materials

The main synthetic material for the production of polyethylene products is polymer granules. They are 3-5 mm in size and are available in the form of a ball, cube, cylinder or small chips. The second source of raw materials is the recycling of waste or process residues.

Getting the film

Polyethylene production technology includes several stages that must be passed in order to obtain bright and convenient bags from the raw material.

Polymer pellets are loaded into the hopper compartment of the extruder. From here they are taken by means of a feed auger. The container maintains a constant temperature in the range from 180 to 240 degrees. In the process of movement, the granules, heating up strongly, are melted into a homogeneous mass. The resulting mixture is forced through a forming hole, resulting in a plastic film in the form of a sleeve (or pipe). Automatic adjustment of the extruder allows you to produce the finished web of a given thickness and width.
The resulting sleeve is gradually cooled and rolled.
An automatic knife cuts the web into two strips of equal width.
The finished sleeve enters the rewinder, which rolls the film into rolls. Scraps are packed separately, and then they are allowed to be recycled.

Drawing a picture

If necessary, a color image is printed using flexography.

The special paint is diluted with alcohol and mixed constantly. This is necessary so that the solution does not lose the desired viscosity.
The dispenser directs certain portions of the dye onto the rollers, which make an impression on the film. After drawing the pattern, the polyethylene is rewound into a roll.

Packing

The next stage allows you to create a base for the bags.

The roll with the printed image is placed in a bag making machine. With the help of special devices, a "pattern" of the future bag is cut out of the film and a bottom fold is formed.
Passing polyethylene blanks through the stamping press, holes are made for the handles. The guillotine cuts off the top of the bag to further secure the plastic handles, or cuts out the shirt.
A welding knife joins the edges of the bag at a temperature of 180 degrees, resulting in a whole product.

The final process is to check the quality of the seams and fasteners.

Conclusion

As we were able to see, the production of polyethylene is a rather complex chemical process that only large-scale industrial enterprises specialized direction. And the technology for processing finished granules seems to be a fairly simple matter that does not require in-depth knowledge. Having started your business with the installation of a production line, you can fully return the money spent in 2-3 years.

Polyethylene is the cheapest non-polar synthetic polymer that belongs to the class of polyolefins. Polyethylene is a white solid with a grayish tint.

The first to study ethylene polymerization was the Russian chemist Butlerov in 1873. But an attempt to implement it was tried in 1884 by the organic chemist Gustavson.

Polyethylene production technology + video how to do

All are engaged in the production of polyethylene large companies petrochemical industry. The main raw material from which polyethylene is obtained is ethylene. Production is carried out at low, medium and high pressures. Typically, it comes in granules that have a diameter of 2 to 5 millimeters, sometimes in powder form. Today there are four main methods of polyethylene production. As a result, get: high-pressure polyethylene, low-pressure polyethylene, medium-pressure polyethylene, as well as linear high-pressure polyethylene. Let's take a look at how the production of MPE is carried out.

High pressure polyethylene is formed at high pressure as a result of the polymerization of ethylene in an autoclave or in a tubular reactor. Polymerization in the reactor is carried out by a radical mechanism under the influence of oxygen, organic peroxides, they are lauryl, benzoyl or their mixtures. Ethylene is mixed with an initiator, then heated to 700 degrees and compressed by a compressor to 25 megapascals. After that, it enters the first part of the reactor, in which it is heated to 1800 degrees, and then into the second part of the reactor to carry out polymerization, which occurs at temperatures ranging from 190 to 300 degrees and pressures from 130 to 250 megapascals. In total, ethylene is in the reactor for no more than 100 seconds. Its conversion rate is 25 percent. It depends on the type and quantity of the initiator. The ethylene that has not reacted is removed from the polyethylene obtained, after which the product is cooled and packaged.

LDPE is produced in the form of both unpainted and colored granules. The production of low-density polyethylene is carried out using three main technologies. The first is polymerization, which takes place in suspension. The second is solution polymerization. This solution is hexane. The third is gas phase polymerization. The most common method is solution polymerization. Polymerization in solution is carried out in a temperature range from 160 to 2500 degrees and a pressure from 3.4 to 5.3 megapascals. Contact with the catalyst takes about 10-15 minutes. Polyethylene is released from the solution by removing the solvent. First of all, in the evaporator, and then in the separator and in vacuum chamber granulator. Granular polyethylene is steamed with water vapor.

HDPE is produced in the form of both unpainted and colored granules, and sometimes in powder. The production of medium pressure polyethylene is carried out as a result of the polymerization of ethylene in solution. Medium-pressure polyethylene is obtained at a temperature of about 150 degrees, a pressure of no more than 4 megapascals, and also in the presence of a catalyst. PSD from the solution falls out in the form of flakes. The product obtained in the above manner has a weight average molecular weight of no more than 400 thousand, a degree of crystallinity of no more than 90 percent. The production of linear high-pressure polyethylene is carried out using the chemical modification of LDPE. The process takes place at a temperature of 150 degrees and about 30-40 atmospheres. Linear low density polyethylene is similar in structure to high density polyethylene, but it has longer and more numerous side branches. The production of linear polyethylene is carried out in two ways: the first is gas phase polymerization, the second is liquid phase polymerization. She is currently the most popular. As for the production of linear polyethylene by the second method, it is carried out in a liquefied bed reactor. Ethylene is fed into the reactor, while the polymer, in turn, is withdrawn continuously. However, the level of the liquefied bed is constantly maintained in the reactor. The process takes place at a temperature of about one hundred degrees, a pressure of 689 to 2068 kN / m2. The efficiency of this method of polymerization in the liquid phase is lower than that of the gas phase.

Video how to do:

It should be noted that this method also has its own advantages, namely: the size of the installation is much smaller than that of equipment for gas-phase polymerization, and much lower capital investment. The method in a reactor with a stirrer using Ziegler catalysts is practically the same. In this case, the maximum yield is formed. Not so long ago, for the production of linear polyethylene, a technology began to be used, as a result of which metallocene catalysts were used. This technology makes it possible to obtain a higher molecular weight of the polymer, thereby increasing the strength of the product. LDPE, HDPE, PSD and LPVD differ from each other, both in their structure and properties, respectively, and they are used to solve various problems. In addition to the above methods of ethylene polymerization, there are others, only in industry they have not received distribution.

The first experience of ethylene polymerization at the end of the 19th century was obtained by a native of Russia - scientist Gustavson by carrying out this process with the catalyst AlBr3. For many years, polyethylene was produced in small volumes, but in 1938 the British mastered the industrial production process. At that time, the polymerization method was not yet perfect.

1952 saw a breakthrough in industrial production. The German chemist Ziegler invented an efficient variant of ethylene polymerization using metal-organic catalysts. However, the real technology of polyethylene production is based on this method.

Raw materials

The starting material for obtaining is ethene - the simplest representative of a number of alkenes. The simplicity of this production method is highly dependent on the availability of ethyl alcohol, which is used as raw material. Modern industrial lines for polymer production are developed taking into account their work on oil and associated gases- readily available oil fractions.

Such gases are released during pyrolysis or cracking of petroleum products at very high temperatures and contain impurities of H2, CH4, C2H6 and other gases. The associated gas, in turn, contains such components as gases-paraffins, therefore, when subjected to heat treatment, ethylene is obtained in high yield.

High pressure polyethylene production technology

The process of obtaining PE goes by a radical mechanism. When carrying out, various kinds of initiators are used to reduce the activation threshold of the molecule. Examples of these are hydrogen peroxide, organic peroxides, O2, nitriles. The radical mechanism, in general, does not differ from conventional polymerization:

Stage 1 - initiation;
Stage 2 - chain enlargement;
Stage 3 - open circuit.

The chain is initiated by the release of free radicals during heat treatment of their source. Ethene reacts with the released radical, is endowed with a certain Eact, thereby increasing the number of monomer molecules around it. In the future, an increase in the chain is observed.

Process technology

There are two variants of the polymerization process - either polyethylene is formed in bulk or in suspension. The first has received and is a collection of processes.

Ethylene gas, which is a mixture and not pure substance, first, the filtration path passes through a fabric filter that retains mechanical impurities. Next, an initiator is supplied to the purified ethene in a cylinder, the volume of which is calculated based on the process conditions. The correction is made for the highest polymer yield.

After that, the mixture is transported, filtered and compressed in two stages. At the outlet of the reactor, practically pure polyethylene is obtained with an admixture of ethylene, which is disposed of by throttling the mixture in a receiver under low pressure.

Low-density polyethylene production technology

The sources of raw materials for obtaining this type of polyethylene are pure ethylene without impurities and the catalyst - aluminum triethylate and Ti tetrachloride. Both diethylaluminum chloride and aluminum ethylate dichloride can serve as a substitute for Al (C2H5) 3. The catalyst is obtained in 2 stages.

Process technology

This process of obtaining low-pressure PE is characterized by both periodicity and continuity. The choice of technology also depends on the process scheme, each of which is different in the design of the equipment, the volume of the reactors, the method of purifying polyethylene from impurities, etc.

The most common polymer production scheme includes three continuous stages: polymerization of raw materials, purification of the product from catalyst residues and drying. Catalyst feed apparatuses separate a 5% mixed catalyst solution into the measuring tanks, after which it enters the tank, in which it is mixed with an organic solvent to the required concentration of 0.2%. From the tank, the finished catalyst mixture is discharged into the reactor, where it is maintained at the required pressure.

Ethylene is fed into the reactor from the bottom, where it is subsequently mixed with the catalyst to form a working mixture. The production of polyethylene under reduced pressure is characterized by contamination of the product with residues of the catalyst mixture, which change its color to brown. The main product is purified by heating the mixture, resulting in the destruction of the catalyst, further separation of impurities and their direct filtration from polyethylene.

The moistened product goes to drying in drying chambers bunker, where it is completely cleaned on a fluidized bed of nitrogen (T = 373 K). Dry powder is poured from the hopper onto a pneumatic line, where it is sent to granulation. Dust with polyethylene particles remaining after nitrogen purification is sent to the same line.