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Low density polyethylene (LDPE) is polymerized ethylene as the polymerization monomer, peroxide as the initiator, the thermoplastic resin obtained by free radical polymerization reaction, the molecular weight is generally in 100000~500000, the density is 0.91~0.93g/cm3, is the lightest variety of polyethylene resin.

It has good softness, extensibility, electrical insulation, transparency, easy processing and certain air permeability.  Good chemical stability, alkali resistance, resistance to general organic solvents, has a wide range of uses, including extrusion coating, blow film, wire and cable coating, injection molding and blow molding hollow molding, etc.

Due to the short life of the free radicals produced by the initiator, ethylene is highly compressed by increasing the reaction pressure (110~350MPa), so that its density is increased to 0.5g/cm3, which is similar to the liquid that can not be compressed again. In order to shorten the ethylene molecular spacing and increase the collision probability between free radicals or active growing chains and ethylene molecules, free radical polymerization reaction is carried out.  Low density polyethylene is produced, so low density polyethylene is also called high pressure low density polyethylene.

Low density polyethylene production process

The production process of low density polyethylene mainly includes ethylene two-stage compression, initiator and regulator injection, polymerization reaction system, high and low pressure separation and recovery system, extrusion granulation and post-treatment system, etc.

According to the different type of reactor, it can be divided into two kinds: high-pressure tube method and autoclave method.

The tubular process and the kettle process have their own characteristics: the tubular reactor has simple structure, convenient manufacture and maintenance, and can withstand higher pressure;  The structure of the reactor is complicated, and the maintenance and installation are relatively difficult. At the same time, the volume of the reactor is usually small due to the limited heat removal capacity.

Generally speaking, the tube method is used for large-scale installations, while the kettle method is used for installations producing high value-added products such as EVA of special grade and high content of vinyl acetate.

Due to the characteristics of different processes, the kettle method has more branched chains and better impact strength, which is suitable for extruding coating resin.  The tube method has wide molecular weight distribution, less branched chain and good optical property, which is suitable for processing into thin films.

High pressure tube method low density polyethylene production technology

The inner diameter of the tubular reactor is generally 25~82mm, the length is 0.5~1.5km, the aspect ratio is greater than 10000:1, the outer diameter to inner diameter ratio is generally not less than 2mm, and the water jacket is used to remove part of the reaction heat.

So far, the pipe is approximately the same method to process the basic flow, the adoption of different reactor feed point, different molecular weight regulator, initiator and its injection location, and different ways of fertilizer injection, product processing, the amount of return of ethylene and sends out the position, has formed the different features of process.

At present, the mature tubular process technology mainly includes LyondellBasell’s Lupotech T process, Exxon Mobil’s tubular process and DSM’s CTR process.

Lupotech T process

The LyondellBasell Lupotech T process is used for approximately 60% of the domestic production capacity of low density polyethylene plants.  The reaction pressure 260~310MPa, reaction temperature 160~330℃, one-way conversion rate of 35%, product density 0.915~0.935g/cm3, melting index 0.15~50g/10min, single line production capacity 45× 104T/A, the process has five technical characteristics:

(1) Pulse reactor technology is adopted to realize the valve opening, valve opening duration and switching frequency at the end of the reactor.  Pulse operation can improve the mixing effect in the reactor, good reaction stability, high conversion rate, reduce the reactor wall adhesion, improve the heat transfer coefficient, and better heat removal effect of jacket water;

(2) Peroxides were injected into different areas of the reactor at four points to form four sections of the reaction zone;

(3) With propylene, propanaldehyde as molecular weight regulator, introduced by a compressor inlet, with ethylene into the reactor, wide product range;

(4) The high-pressure circulating gas system can realize self-cleaning, dissolving and dewaxing operation through sequential control, reducing the impact on normal production operations;

(5) Set up the hot water station system to reduce the consumption of cooling water, and recover the heat of the polymerization reaction and high-pressure circulating gas system for other devices.

Exxon Mobil tubular process

The reaction pressure of Exxon Mobil tube process is 250~310MPa, the reaction temperature is 215~310℃, the conversion rate is up to 40%, the product density is 0.918~0.934g/cm3, the melting index is 0.2~50g/ (10min), and the single line production capacity is 50× 104T/A.  The process has six technical characteristics:

(1) The horizontal push flow tube reactor is adopted, and the diameter of the reactor is expanded step by step along the axial direction to optimize the gas flow rate and the reactor pressure drop.  Increase the stability of the reaction, reduce the decomposition reaction, reduce the scale inside the reactor, improve the heat transfer efficiency of the reactor;

(2) The initiator is injected along the axial direction of the reactor, which can form 4~6 reaction zones, improve the conversion rate and operational flexibility, and wide product range;

(3) Generally use propylene as a regulator to control the melting index, production of medium-density products using propanaldehyde as a regulator, regulator through high pressure diaphragm pump injected to the compressor inlet twice, and then with ethylene into the reactor;

(4) using the hot tubular reactor of ethylene vinyl forward feed and cold multipoint feeding combination of lateral, uniform heat release and may also have the effect of removing the heat of reaction, the reactor optimized jacketed cooling load, reduce the length of the reactor, and make the reactor temperature distribution in a smooth, improve ethylene conversion rate.  At the same time, due to the multi-point lateral feed, the forward hot ethylene feed quantity of the reactor is reduced, the heat load of the reactor inlet preheater is reduced, and the consumption of high pressure and medium pressure steam is reduced.

(5) The closed temperature regulating water system is used to supply water to the reactor jacket to remove the reaction heat. By optimizing the water supply temperature of the jacket water, the heat transfer efficiency is improved, the reactor length is shortened, and the conversion rate is increased;

(6) Recovery and utilization of high pressure and high heat fluid energy discharged from the top of the high pressure separator.

CTR process

DSM CTR process reaction pressure is 200~250MPa, reaction temperature is 160~290℃, conversion rate is 28%~33.1%, the maximum can reach 38%, product density is 0.919~0.928g/cm3, melting index is 0.3~65g/ (10min),  The maximum single wire capacity can reach 40× 104T/A.  The process has five technical characteristics:

(1) Using non-pulse operation, the reactor operating pressure is low and keeps constant, the flow rate in the reactor is high, it has good scouring effect, does not produce wall sticking phenomenon, the reactor does not need cleaning and descaling, and reduces the operation cost;

(2) The reactor pipe diameter is kept constant, the direct “one-pass” principle is adopted, there is no complex side line feeding system, the reactor and support design is simpler, and the investment is lower;

(3) the reactor jacket is cooled by cold water, which can produce steam by product;

(4) The use of peroxide initiator, product gel composition is small, no catalyst residue, environmental protection effect is good;  Less oligomers were generated, and the recycling process of circulating gas was simplified.

(5) Good operating conditions and no pressure fluctuation during polymerization make the products of high quality, especially the film products, with excellent optical properties and processing properties, can produce the minimum film thickness of 10μm film products, but the product range is narrow, can not produce copolymer (EVA) products with low melt index.

Production technology of low density polyethylene by autoclave method

The autoclave process uses a tank reactor with a stirring system, the aspect ratio can be from 2:1 to 20:1, the tank reactor volume is 0.75~3m3.  The reaction temperature is generally 150~300℃, the reaction pressure is generally 130~200MPa, the conversion rate is 15%~21%.

Since the kettle reactor is a thick-walled vessel, the heat transfer through the reactor wall is more restricted than the tubular reactor, so the reaction is basically an adiabatic process, and no obvious heat is removed from the reactor. The reaction temperature is mainly controlled by multi-point injection of cold ethylene feed to balance the reaction heat.  The reactor is equipped with a motor driven stirrer to make the mixture in the reactor uniform and avoid local hot spots.  The initiator is organic peroxide, which can be injected at different locations along the axial direction of the reactor to form multiple reaction sections with different operating temperatures. There is no backmixing between reaction sections, flexible operation and wide product range, which can produce copolymerized EVA with up to 40% vinyl acetate content.

Lupotech A process

Lupotech A process uses A stirred tank reactor, the reactor volume is 1.2m3, raw materials and initiator are injected into the reactor by multiple points, the reaction pressure is 210~246MPa, the highest reaction temperature is 285℃, the regulator is propylene or propane, added by the secondary compressor inlet, can produce A variety of LDPE/EVA products,  The product density is 0.912~0.951g/cm3, the melting index is 0.2~800g/ (10min), the content of vinyl acetate can be up to 40%, the one-way conversion rate of the reactor is 10%~21%, the maximum single line design scale can reach 12.5×104t/a.

LupotechA process can not only produce extruded coated resin with more branched chain and better impact, but also produce thin film products with wide molecular weight distribution.  The melting index and density of LDPE/EVA products can be controlled well by APC control system, and uniform products can be obtained.  The main domestic introduction of this process are Sirbon petrochemical, Yangzi petrochemical, Shanghai petrochemical, etc., the device capacity is 10× 104T /a.

Exxon Mobil kettle process

The Exxon Mobil tank process adopts a self-designed 1.5m3 multi-zone tank reactor. The reactor has a larger aspect ratio, longer retention time, higher initiator efficiency and narrower product molecular weight distribution, which is conducive to the production of thin film products with similar quality to the tube process.

The regulator is different from the Exxon Mobil tube method. Isobutene or n-butane is used, which is boosted to 25~30MPa through the high-pressure diaphragm pump, injected twice at the compressor inlet, and enters the reactor with ethylene.

The reactor pressure range is wide, and the maximum reaction pressure is 200MPa, which can produce LDPE homopolymer with low melt index and EVA copolymer with high vinyl acetate content.

The Exxon Mobil tank process can produce LDPE homopolymer products with melting index of 0.2~150g/ (10min) and density of 0.910~0.935g/cm3.  Melt index 0.2~450g/ (10min) vinyl acetate content up to 35% ethylene – vinyl acetate copolymer (EVA) products.  The main domestic introduction of this process is Lianhong Group (formerly Shandong Hauda), the device capacity is 10× 104T /a, TRINA, the device capacity is 12× 104T /a, etc.