Introduction

Based on a slurry process for High density Polyethylene (HdPE) DSM developed a second generation process, taking it all the way from bench scale through pilot plant to commercial scale. This became the so-called COMPACT Polyethylene Solution Process. This catalyst is a proprietary one, although all components are commercially available.

DSM operates its own plant for over 30 years. Based on that proven design Stamicarbon has granted several licenses of the COMPACT LLdPE/HdPE Solution Process.

Benefits

• The COMPACT process makes commercial production of polymers possible with a wide range of melt indices (1.0-100 dg/min) and densities (900 to 965 kg/m 3) for virtually any application.
• The COMPACT process uses a wide variety of comonomers, so that products can be optimized (based on price-performance ratios) to suit their final applications.
• For film applications - for example - mechanical as well as optical properties of STAMYLEX grades (DSM's trade name for LLdPE/HdPE) are judged as the best in the market, so that substantial down gauging is possible, with films retaining extreme toughness and very good optical properties.
• Consistency as well as reproducibility of products from the COMPACT process is pre dominant. This results in very stable and easy controllable processing conditions.

Process features

• Stamicarbon COMPACT polyethylene solution process has numerous features, which substantially reduce production costs and result in highly competitive product qualities.
• Construction material for most equipment is made of carbon steel.
• No purification step is required for the raw materials; normal polymerization grades can be used.
• Major part of the (co)monomer and solvent recycle does not require additional purification.
• No catalyst removal step is necessary.
• The polymerization reactor is extremely small resulting in short residence time.
• Catalyst handling is very simple.

Low operating costs

• Ethylene conversion per pass is well over 90% (92-96 %), so recycle of raw materials is small.
• Utilities consumption’s are low, due to a good energy balance and use of reaction heat for flashing off solvent and LP steam generation.
• Production of byproducts, e.g. wax-like polymers, is negligible.
• Grade change-over time can be very short, due to the low residence time of the process. Most of the grade changes are performed without any off-spec or 'wide'-spec material.

Process description

The process is to be operated using a proprietary Ziegler Natta catalyst system. The catalyst reactants are commercially available. From these reactants, the high active catalyst is prepared in a dedicated reactor.

Feedstock ethylene and (recycled) comonomer is absorbed in the solvent and fed to the agitated, liquid filled reactor.

In over 30 years of operational experience DSM has developed a special-design reactor, which can meet requirements and is capable of producing grades of extremely constant quality under very stable production conditions.

Downstream small amounts of unconverted ethylene, major part of comonomer and solvent are separated in a flash vessel. The overhead vapors are condensed and recycled to the reactor feed without any purification. Low catalyst consumption results in high purity polymer without a dedicated de-ashing step. A catalyst deactivator stops residual activity of the catalyst.

Desolventizing is executed in a vented extruder. Additives are supplied to the molten polymer in this extruder before pelletizing. On leaving the pelletizer, pellets are cooled with circulated water. After drying, pellets are sent to the blending and storage area by pneumatic transport. The polyethylene is shipped to the client in bulk or bagged as required.

Remaining solvent, flashed off in the second flash step and the final desolventizing, shall be purified before being recycled to polymerization.

Unconverted octene comonomer is separated from solvent via distillation. Recycle solvent is extracted to remove excess catalyst deactivator and dried.