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Polyethylene production

Polyethylene ranks first in global production of polymers, synthetically produced by polymerization method. One of production methods is polymerization of ethylene under high pressure. Ethylene is produced by means of cracking of saturates in cracking furnaces with recovery of cracked gas.

All large companies of petrochemical industry produce polyethylene. The major feed stock for polyethylene is ethylene. Production is carried out under low, medium and high pressure. As a rule, it is produced in granule form with granule diameter of 2-6 mm, or sometimes it is produced in powder form. Today there are four main methods of polyethylene production. And as result the following is produced:

  1. High pressure polyethylene or low-density polyethylene (LDPE)
  2. Low pressure polyethylene or high-density polyethylene (HDPE)
  3. 5Medium pressure polyethylene or medium pressure polyethylene (MDPE)
  4. 5Linear high pressure polyethylene or linear low-density (LLDPE)

High pressure polyethylene is formed at high pressure and as a result of polymerization of ethylene, compressed up to high pressure, in an autoclave or in a tube reactor. Polymerization in the reactor is carried out according to radical mechanism under influence of oxygen, organic peroxides, that is, lauryl, benzoyl, and their mixtures. Ethylene is mixed with an initiator, and then heated up to 700 °С, and compressed to 25 MPa by the compressor. After this it comes to the first section of the reactor, where it is heated up to 1 800°С, and then to the second section of the reactor for polymerization, which takes place at temperature of 190-300°С and pressure of 130-250 MPa. Ethylene is in the reactor in total for not more than 100 seconds. Degree of conversion is 25%. It depends on initiator type and amount. Ethylene, which has not reacted, is removed from the polyethylene produced, and then the product is cooled and packed. LDPE is produced in the form of undyed or dyed granules.

Production of low pressure polyethylene is carried out by means of three basic technologies:

  • Polymerization taking place in suspension
  • Polymerization taking place in solution. The solution is hexane
  • Gas-phase polymerization

The most wide-spread method is considered to be пpolymerization in solution. Polymerization in solution is carried out in temperature range of 160-2500°С and under pressure of 3.4-5.3 MPa. Contact with catalyst is carried out for about 10-15 minute. Isolation of polyethylene from the solution is carried out with solvent in a vaporaizer, and after that in a separator and a vacuum chamber of the granulator. Granulated polyethylene is steamed with moisture vapor. HDPE is produced in the form of undyed or dyed granules.

Production of medium pressure polyethylene is carried out as a result of ethylene polymerization in solution. Medium pressure polyethylene is produced at temperature of about 150°С and under pressure not more than 4 MPa with catalyst. MDPE comes out of solution in the form of flakes. The product produced by means of the method described above is distinguished by weight-average molecular weight not more than 400 thousand and crystallinity not more than 90%.

Production of linear high pressure polyethylene is carried out by means of chemical modification of LDPE: The process takes place at temperature of 150°С and under pressure of about 3.0-4.0 MPa. Linear low density polyethylene resembles in its structure high density polyethylene, but it differs in terms of long and numerous side branches. Production of linear polyethylene is carried out by means two methods:

  • Gas-phase polymerization
  • Liquid-phase polymerization. It is the most popular method today. It is carried out in the reactor with fluidized layer. Ethylene is constantly fed to the reactor, and polymer is removed with keeping constant level of the fluidized layer in the reactor. The process takes place at temperature of 100°С and under pressure of about 0.689-2.068MPa

Efficiency of this liquid-phase polymerization method is less than of gas-phase one, but it is characterized by the following advantages: the size of the plant is significantly less than the equipment of gas-phase polymerization, also capital expenses are lower.

The method in the reactor with a mixer with usage of Ziegler catalysts is almost the same. In this case there is maximum product output. A technology has been recently introduced for production of linear polyethylene, that resulted in metallocene catalyst usage. That technology enables to get higher molecular polymer mass, through which product strength is increased. LDPE, HDPE, MDPE and LLDPE differ from each other in terms of their structure and properties, correspondiLPGy, and are used for addressing different tasks. Besides the above listed ethylene polymerization methods, there are other ones, which are not yet commonly used in the industry.

Currently two major types of polymer LDPE and HDPE are produced.

There are other types of polyethylene, each of which has its properties and application fields. Different additives, enabling to get black, red or another polyethylene, are added to granulated polymer during production process.

Production of high pressure polyethylene is carried out in autoclaves and in tube reactors. There are eight polymer grades produced in autoclave according to GOST. Twenty one types of high pressure polyethylene are produced from tube reactor.

For high LDPE synthesis, the following conditions should be followed:

  1. temperature conditions - from 200 to 250°С
  2. catalyst - pure oxygen, peroxide (organic)
  3. pressure - from 150 to 300 MP

Polymerized stock in the first phase has liquid form, and then it is transferred to a separator, and afterwards in granulator, where finished stock granules formation takes place. Properties of LDPE are used for production of packing films, thermal films, multi-layer packaging. Also high pressure polyethylene is used in automobile, chemical, food industries. It used for production of qualitative strong pipes for household application.

Flow chart
Flow chart

The major tasks of polyethylene manufacturing companies are equipment upgrade, improvement of cracking and conversion technologies, increase of production capacity. LENNIIHIMMASH carries out the following types of works in the direction:

  • development of equipment for cracking furnaces fitting and upgrade
  • inspection of the current condition of facilities
  • analysis, feasibility study and selection of the optimal option for revamping
  • equipment upgrade
  • building and structures design

Major equipment of polyethylene production:

  • reactor section
  • compressors
  • recycle units of high and medium pressure (separator, heat exchanger)
  • hot water station with pumps
  • cooling unit
  • pumps
  • vessels, including. mixer

Preliminary inspection of the current condition of equipment

Recycle coolers of high pressure
Recycle coolers of high pressure

Tube reactor
Tube reactor

Low pressure separator      Confectioning unit
Low pressure separator V=12 м3 Confectioning unit



Experience of LENNIIHIMMASH

During the period of active construction of plants for ethylene and propylene recovery from cracked gas for further production of polymer materials in the USSR, LENNIIHIMMASH was the main developer and supplier of columns and heat exchangers of cold boxes for plants of different capacity from 45 to 300 thous.t ethylene per year (E-45, EP-60, E-100, E-200, EP-300). In later years, LENNIIHIMMASH carried out works on the rebuild of the operating facilities in order to increase capacity for processed cracked gas; implemented technical solutions on plants operation stabilization, reduction of target products losses (increase of recovery factor), increase of products quality. Besides, plants were equipped with additional equipment, internals were replaced, and process schemes were improved. Results of LENNIIHIMMASH's research works, the developed procedures of tray hydraulic calculations, results of boxes inspections of the equipment developed at ethylene facilities were used in cold boxes of ethylene facilities and during development of column equipment. LENNIIHIMMASH developed special equipment such as reciprocating ethylene compressors (booster compressor, compressor of high pressure ethylene on opposed base (1st cascade - up to pressure of 25 MPa, and 2nd cascade - up to pressure of 230 MPa), reactor equipment, vessels for high pressure polyethylene production for Novopolotskiy,Sumgaitskiy, Tomskiy mills and production facility in Germany. This equipment continues to operate successfully nowadays.

In 2010, a proposal on process lines rebuild for the purposes of production capacity increase, technology improvement, obsolete equipment replacement, and economic efficiency was developed for LDPE production at LukoilNeftehimBurgas AD (Bulgaria).

The operating production facility includes:

  • LDPE production unit with tube reactor with capacity of 50 thous. t/year (process of ATO company, France)
  • LDPE production unit with autoclave reactor (for process line with capacity up to 15 thous. t/year, with total capacity of 30 thous. t/year, process of ICI company, ELPGand)

LENNIIHIMMASH specialists carried out an examination and found out the following reserves for major and auxiliary equipment:

As for tube reactor, there is a reserve in terms of capacity, and this fact makes it appropriate not to replace the plant entirety. Partial upgrade with increasing the capacity of major process blocks is possible:

  • reactor section without reactor disassembly
  • compression block with partial replacement of equipment without changing construction part
  • recycle unit of low pressure to be kept without large changes
  • recycle unit of high pressure requires significant rebuild

Design of a new cooler, which will significantly increase the capacity, was proposed, a list of new and upgraded equipment of blocks with major technical characteristics was drawn up.

tube reactor rebuild
Option of tube reactor rebuild - shifting to three-zone
reactor in rebuild options 2 and 3 with injection of liquid
initiation

Cooling unit operation scheme
Cooling unit operation scheme
Compressors upgrade
Compressors upgrade - Booster multi-compressor / first cascade
of Burckhardt company

Three rebuild options are proposed. Depending on the rebuild scope, total capacity of two facilities can be increased from 80 thous.t of polyethylene per year:

  • Option 1 – 90 thous. t/year
  • Option 2 – 130 thous. t/year
  • Option 3 – 128 thous. t/year

In 2016 in connection with the rebuild of the cracking and gas purification workshop at the Ethylene plant of PJSC Kazanorsintez, the main technical solutions were developed, and in 2017 the external block of the Three-cell cracking furnace P-819/815/820 as a part of the cracking unit of ethane and propane fractions in tube furnaces is being engineered. The goal is connection of 3-chamber furnace, designed and supplied by Technip company, to the process utilities of the Ethylene plant of PJSC Kazanorgsintez and construction of auxiliary facilities for providing compliance of parameters and process lines quality, and consumption indicators, required for operation of furnace block, taking into future construction of additional fourth chamber of the furnace.

The project includes development of the heating and preparation unit of feed stock and fuel gas, steam reduction unit, dosing unit of dimethyldisulfide (DMDS) - inhibitor of coke formation, feed water preparation and pumping system, blow-off water unit.