This column is based the report “LDPE via High-Pressure Tubular Process – Cost Analysis,” published by Intratec. It can be found at the following URL: www.intratec.us/analysis/ldpe-production-cost.
Polyethylene (PE) is the world’s largest-volume commodity polymer, due in large part to the widespread availability of ethane, the raw material for manufacturing ethylene, as well as ethylene’s relatively low cost and broad range of applications. Low-density polyethylene (LDPE) is one of the three main types of polyethylene. The other two are high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE). LDPE differs from HDPE and LLDPE primarily in its unique molecular structure — large amounts of long-chain branching impart the material with different rheological behavior in both shear and extension.
The following paragraphs describe a high-pressure polymerization process for the production of LDPE, using tubular reactors. Figure 1 presents a simplified flow diagram for this process.
Compression. Fresh polymer-grade ethylene is mixed with a low-pressure gas stream that contains unreacted monomer recovered from downstream in the process, and is fed to the primary compressor. There, the ethylene is compressed to 300 bars. The ethylene stream from the primary compressor is mixed with chain-transfer agents and recycled ethylene monomer and fed to the secondary compressor, where it is compressed to 3,000 bars, the reactor operating pressure.
Reaction. The compressed ethylene is divided into several streams and fed into the tubular reactor at several different injection points. The organic peroxide solutions from the initiator feed system, which are also injected at multiple points along the length of the reactor, initiate the polymerization reaction. The reaction is carried out at a high temperature (220°C) and high pressure (3,000 bars). Reaction heat is removed by the following: (a) increasing the temperature of ethylene, (b) adding fresh monomer in side-stream entry points, and (c) by heat-transfer through a jacketed water-cooling system.
Separation. The reactor outlet is expanded to 300 bars (absolute) and fed to a high-pressure separator, where unreacted ethylene gas is removed from the polymer by adiabatic flash. The molten polymer phase from the bottom of the high-pressure separator is routed to the low-pressure separation section. Here, the molten polymer phase passes through an additional flash step, in which remaining unreacted ethylene is removed.
Extrusion and pelletizing. The molten polymer and additives are fed to a short single-screw extruder with an underwater pelletizer. Here, the mixture is pelletized and then conveyed to silos for homogenization. The pellets are sent to bagging silos that serve the stationary bagging and packaging lines. At this point, LDPE product is packed into bags.
LDPE production involves the polymerization of ethylene, carried out generally through either tubular or autoclave technologies (Figure 2).
The total operating cost (raw materials, utilities, fixed costs and depreciation costs) to produce LDPE was estimated to be about $1,480 per ton of LDPE in the second quarter of 2014. The analysis was based on a plant constructed in the U.S. with capacity to produce 400,000 metric ton per year of LDPE.
Edited by Scott Jenkins
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