May 2025

Petrochemical Technologies

Overview on low-density polyethylene manufacturing technologies

This article provides an overview of low-density polyethylene manufacturing technologies.

IP: 10.3.32.10

Low-density polyethylene (LDPE) is a thermoplastic formed from an ethylene monomer and is the oldest polymer in the polyethylene (PE) family. LDPE was discovered in 1933 by a scientist at Imperial Chemicals Industries Ltd. (ICI) during a study on high-pressure ethylene polymerization. A patent was granted to ICI in 1937, and commercial production of LDPE began in 1939. During World War II, LDPE products played a vital role in insulation applications (e.g., radar cables). LDPE has a density range between 0.915 grams (g)/cubic centimeters (cm3) and 0.935 g/cm3, with a melting point between 110°C and 115°C (230°F and 239°F), and a melt index ranging from 0.2 g/10 min–70 g/10 min, depending on the application. Medium-to-broad molecular weight distribution products are produced with short- and long-chain branching, which distinguishes LDPE from other polymers. 

LDPE is produced using two processes: the autoclave and tubular methods. These processes are similar, differing only in the type of reactor used. The tubular process accounts for 70% of the global installed LDPE capacity. Initiators used in LDPE production include oxygen or organic free radicals, particularly peroxides—the latter are more commonly used today. The injection of initiators polymerizes the ethylene and initiates propagation. Molecular weight and narrow molecular weight distribution are controlled by adding olefins (propylene/butene), alkanes (butane) or propionaldehyde, referred to as modifiers or chain transfer agents. A co-monomer like vinyl acetate can be injected, enabling ethylene vinyl acetate (EVA) production in the same plant with minor modifications (FIG. 1). 

FIG. 1. Producing LDPE with different reactor types. 

Polymerization reactions take places at high pressure and temperature in the presence of initiators and modifiers. Recycled ethylene gas is compressed along with fresh ethylene in the first compressor and is further compressed in the second compressor to reach polymerization pressure. Reactor pressure is controlled by a specially designed high-pressure control valve—namely the kick or let-down valve—which is a key process controller in an LDPE plant.  

A modifier or chain transfer agent is added into the first compressor or at the suction of the second compressor. Initiators are injected along with the process gas from the second compressor into the reactor via single or multiple injection points. The polymer melt is cooled and separated in high- and low-pressure product separators. Unconverted monomers from both separators are flashed, cooled and recycled after separating waxes to the first and second compressor for recovery. Waxes are generated as a byproduct in the LDPE production process. 

Extruded polymer pellets are transferred into silos where they are degassed to remove residual hydrocarbons [volatile organic compounds (VOCs), < 1,000 parts per million (ppm)]. Pellets are degassed prior to the transfer to storage silos to avoid explosive gas mixtures. The degassing time is approximately 12 hr, using a high volume of air designed to keep the gas mixture inside silos below the low explosive limit (LEL) of ethylene. Degassing vents are sent to oxidizers to reduce VOCs.   

The heat of the polymerization reaction/exotherm is controlled by a circulating cooling medium. Low-pressure steam is generated as a byproduct in the heat recovery section from the circulating cooling medium in most of the available technologies (FIG. 2).

FIG. 2. Typical LDPE process flow. 

The key attributes of the autoclave and tubular methods are detailed below. 

TUBULAR TECHNOLOGY 

• Polymerization pressure for tubular technology ranges from 2,500 bars–3,200 bars, with a temperature of 155°C–300°C   

• The exotherm is controlled by circulating water at different temperatures and pressures 

• A high conversion process ranges between 30% and 36%; conversions up to 40% may be achieved with changes in reactor configuration  

• Long residence time and degree of back mixing (plug flow reactor) 

• Products produced in a tubular reactor are used for film, foam, wires, cables and miscellaneous purposes 

• Tubular plant capacities are larger, typically between 400,000 tpy and 500,000 tpy 

• Tubular technologies are available from LyondellBasell, ExxonMobil, SABIC, Versalis and Sumitomo, among others (FIGS. 3 and 4). 

FIG. 3. A tubular reactor.

FIG. 4. Tubular technology licensor’s market share. 

AUTOCLAVE TECHNOLOGY 

• Polymerization pressure for autoclave technology ranges from 1,300 bars–2,100 bars, with a temperature of 155°C–300°C  

• The exotherm in a multizone mixed adiabatic vessel is controlled and cooled by the reactants 

• Conversion ranges between 18% and 21%  

• Short residence time and high degree of back mixing (stirred reactor) 

• Products from autoclave reactors are best suited for extrusion coating 

• Autoclave plant capacities are typically between 125,000 tpy and 150,000 tpy  

• Autoclave technologies are available from ECI-Simon Carves, LyondellBasell, ExxonMobil, Sumitomo and Versalis, among others (FIGS. 5 and 6). 

FIG. 5. An autoclave reactor.

FIG. 6. Autoclave technology licensor’s market share. 

Process safety aspects. Decomposition is a reaction where ethylene and polyethylene decompose into carbon, hydrogen and methane, resulting in extremely high pressures and temperatures.1 Decomposition may occur several times during a plant’s lifecycle due to process fluctuations or contaminants like acetylene and metals. Decomposition can occur in the compression, reactor and recycle sections, as well as in pipelines and initiator injection during startups. To prevent damage to the surrounding areas during process safety events, the reactor, blowdown vessel and other high-pressure equipment are installed in a blockhouse as part of the plant’s design. 

Plants are monitored by an array of temperature and pressure sensors that initiate shutdowns if abnormalities are detected. In the event of process upset/decomposition, quick isolation and venting of the system through a blowdown vessel stack is an essential feature of the high-pressure technology. Emergency relief valves are specifically designed hydraulic components with precise installation guidelines. An incorrect installation may lead to internal seat and packing damage. Failing to open/close the valves on demand may lead to catastrophic failure. The correct installation and precise testing of opening/closing within the desired time to isolate and depressurize the plant are key to process safety. Typical valve designs are supplied with packing cooling. If improper cooling occurs packing failure and hydrocarbon leakage probability increases. A packing cooling system may be improved and made reliable with the installation of temperature and flow monitoring systems, improving packing life and quickly identifying packing deterioration.  

Emergency relief valves are required with an intended relief speed, which is important for reactor and plant integrity. Rapid isolation and venting are governed by an emergency shutdown (ESD) system and special logics programs to protect the plant from high pressures and temperatures, and fluctuations that lead to decomposition. Extruded pellets are transferred to the degassing silo to reduce VOCs and safeguarded with an ESD system to isolate the silo in case of a process safety event. 

ESD systems and special logics programs protect the plant from process safety and machine/equipment damage. In-depth understanding of processes are required during factory and site acceptance tests, in addition to consultations with process and technology experts. Understanding the logics programs and interlock functional testing is key to prevent process safety events.  

Adding a pyro technology system during atmospheric venting at the reactor blowdown stack can prevent aerial decomposition in the event of emergency isolation and plant venting, in addition to the steam dilution that is normally provided at the vent stack. 

Design consideration. The robust safety and design features of an LDPE plant make it unique compared to other polymer plants. The technology has become mature, and designs have been improved to achieve the best efficiency, reliability and safety, enabling various end use polymers with superior quality.  

Production depends on the feed rate and conversion of ethylene, and the capacity of the second compressor determines the feed rate. In the process of LDPE production, inherent issues with fouling decrease production and increase the plant’s downtime.  

Fouling is an unwanted formation consisting of built-up elements on the equipment’s surface. Fouling occurs in combination with the temperature [> 90°C (> 194°F)] and pressure (> 900 barg─1,000 barg), in the presence of compressor oils and other reactants. In high-capacity plants, it is important to keep the first stage discharge temperature of the second compressor [< 85°C─90°C (< 185°F–194°F)] to avoid fouling in the inter-stage coolers and other components of the second compressor. 

The provision of chilled water on the first-stage suction system of the second compressor helps to minimize the fouling by limiting the first-stage discharge gas temperature [< 85°C─90°C (< 185°F–194°F)] and chilling system on intercoolers to improve the life of the second compressor’s second-stage component. 

Installing improved dead space-free pressure transmitters on the second compressors and reactors prevent blockage and erroneous measurements, improving the plant’s reliability and uptime. 

Installing an improved designed, fast response thermocouple on the reactor loop with a measuring range [> 400°C (752°F)] ensures process safety by accessing the mechanical integrity of the reactor prior to the plant’s startup after decomposition. 

Takeaway. LDPE plants (being highly energy intensive) require reliable and safe operations, with high conversion rates. The integration and recovery of waste heat and energy efficient processes with a wide grade slate are key factors for LDPE technology selection. A piping stress analysis study of all high-pressure pipes, pipe supports, blowdown vessel structures, detail vibrations and pulsation studies of the compressor area are essential. Through good workmanship regarding the installation of high-pressure components, high-pressure blowing of the piping and hydraulic oil network cleaning reduces the commissioning duration and ensures a safe startup of LDPE plants. 

LITERATURE CITED 

¹ ResearchGate, “Decompositions in the production of low density polyethylene: Causes, consequences and prevention,” June 2007, online:

https://www.researchgate.net/publication/345060491_DECOMPOSITIONS_IN_THE_PRODUCTION_OF_ LOW_DENSITY_POLYETHYLENE_-_CAUSES_CONSEQUENCES_AND_PREVENTION_version_in_English 

 

The Authors

Related Articles

From the Archive

Comments