LDPE Production from Ethylene (High-Pressure Tubular Process)

The process under analysis comprises four major sections: (1) Compression; (2) Reaction; (3) Separation; and (4) Extrusion & Pelletizing.

Compression. Fresh polymer-grade ethylene is mixed with a low-pressure gas stream, containing unreacted monomer recovered downstream, and fed to the primary compressor where the ethylene is compressed to 300 bar. The ethylene stream from the Primary Compressor is mixed with chain transfer agents and recycled ethylene monomer recovered and fed to the secondary compressor, where it is compressed to 3,000 bar, the reactor operating pressure.

Reaction. The compressed ethylene is divided into several streams, fed to the tubular reactor at different injection points. The organic peroxide solutions from the Initiator feed system, also injected at multiple points along the length of the reactor, initiate the polymerization reaction. The reaction is carried out at high temperatures (220 °C) and high pressures (3,000 bar). Reaction heat is removed by (i) the increase in the temperature of ethylene, (ii) the addition of fresh monomer in side-stream entry points, and (iii) heat transfer through a jacket water cooling system.

Separation. The reactor outlet is expanded to 300 bara and fed to a high-pressure separator, where nonreacted 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. Here, the molten polymer phase passes through an additional flash step, in which remaining unreacted ethylene is removed.

Extrusion & 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. Finally, LDPE product is packed in bags.