Low density polyethylene - Research Paper Example

In recent years, another dimension of environmental aspect has been added to reduce the global warming and related factors. (LDPE PERP 2010, p4) This paper deals with mainly the production processes of LDPE and its effect as life cycle assessment. Production Processes Installed capacities of LDPE based on production processes employed can be presented schematically as per the following diagram. Source: http://www.chemsystems.com/about/cs/news/items/PERP%200809_1_LDPE.cfm The technologies based on tubular and autoclaves reactors are used for the polymerization of ethylene.

Polymerization process is carried out at high pressure between 1500 and 3000 bar and at temperature between 120 and 320 degree centigrade. A large variety of grades are produced using a single reactor line. Tubular Reaction Process The major outlines of the process can be described as per the following. Ethylene feedstock is compressed and mixed with the recycled ethylene and co monomer such as ethyl acrylate, vinyl acetate, or methacrylic acid. The mixture is then pressurized in the tubular reactor at certain pressure.

Polymerization of the monomer takes place in the presence of organic peroxides. Reactor has several zones to increase the conversion rates. Per reactor pass conversion rate is found to be between 15 and 35 percent. The separation is done in two stages. The mixture of ethylene and polyethylene enters the high pressure separator and splits into an ethylene rich gas and polymer rich liquid. The polymer rich liquid at the bottom of the high pressure separator is taken out to the low pressure separator.

The ethylene gas leaving the separator is sent back to the compressor and again mixed with fresh ethylene. The liquid stream is taken to the extruder for making the pellets of the polymer compound. (Pladis 2006) ExxonMobil has a long history of working with tubular reactors to manufacture LDPE. As claimed by ExxonMobil, tubular technology has a lower operating cost than autoclave technology. Currently, 14 plants are in operation based on tubular technology as provided by ExxonMobil giving production output of 2500kta at different locations.

(LDPE tubular…2010) Autoclave Process The compressed ethylene feedstock is mixed with comonomer such as ethyl acrylate, vinyl acetate, or methacrylic acid. Then the mixture is allowed to enter secondary compressor where it is compressed at very high pressure of 45,000 psi before entering autoclave reactor. Initiator accelerates the polymerization process. The molten polyethylene is taken out through separator and taken to extruder for making pellets and further processing. (Lyondellbasell 2010) A typical flow diagram of autoclave process for producing LDPE can be represented as per the following.

Source: http://www.lyondellbasell.com/techlit/techlit/Tech%20Topics/Plant%20Profiles/Autoclave%20LDPE%20Technology.pdf Tubular versus Autoclave Reactor Processes Both, tubular and autoclave, are proven processes but there is some subtle difference in the product structure of resins produced from these processes. The difference lies in the level and type of long-chain branching. The autoclave process provides more long-chain branching compared to those provided by tubular reactor. That is due to higher level of backmixing possible in case of tubular reactor.

In contrast, tubular reactor provides little backmixing as the reaction flow carries the polymer down the tube as reaction progresses; however, such subtle