Application Example: Feeding & Conveying in Polyolefin Production

Polyolefins are produced by the polymerization of olefins or alkenes (molecules with the general formula CnH2n) such as ethylene, propylene, butene, isoprene, pentene, etc. The name “olefin” means “oil-like” and refers to the oily character of the materials.


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Introduction

The most important commercial polyolefins are polyethylene (PE), polypropylene (PP) and ethyl vinyl acetate (EVA). Polyethylene is classified according to its density as Very Low Density Polyethylene (VLDPE), Low Density Polyethylene (LDPE), Linear Low Density Polyethylene (LLDPE), Medium Density Polyethylene (MDPE), and High Density Polyethylene (HDPE).

Polyethylene and polypropylene are two of the top polymer resins used in the world today. These resins can be tailored to achieve wide range of mechanical and chemical properties, making it possible to use them in a myriad of applications such as agricultural films, garments, tapes, stretch films, retail bags, bottles, containers, pipes, etc.

The selection of a polyolefin for a particular application depends on the resin’s type and grade, which are determined by the manufacturing technology or process, the catalyst, and the raw feedstock used in production. In turn, the polyolefin grade is determined by key properties such as molecular weight, molecular weight distribution (MWD), crystallinity, branching and density that affect how and where each resin is used.

Processes

Two distinctive process sections can be identified in polyolefin production, a Wet-end and a dryend process.

Wet-End Process
The wet-end process for the manufacturing of polyolefins can be high pressure (autoclave and tubular) or low pressure (slurry, solution or gas phase). It includes:

  • Feed preparation
  • Catalyst handling
  • Polymerization reaction
  • Diluent recovery
  • Polymer separation, where the polymer is cleaned and dried.
Wet-End Process High Pressure Technology Slurry Phase Process (Low Pressure)

High Pressure Technology

Slurry Phase Process

Gas Phase Process

Wet End Product
Regardless of the process used, the polymerization reaction produces a powder (sometimes called fluff or flake) that is transferred to a bin that serves as a buffer between the Wet End and Dry End processes.

Density and Applications of Polyolefin Products

Name Abbr. SPI Resin ID Code Density g/cm3 End Use Examples
High Density Polyethylene HDPE >0.941 Plastic lumber, fuel tanks, furniture, storage sheds, chemical & heat resistant piping & containers
Medium Density Polyethylene MDPE n/a 0.926 to 0.940 Containers with good shock and drop resistance, gas pipes, shrink film, packaging films
Linear Low Density Polyethylene LLDPE n/a 0.915 to 0.925 Industrial containers, trash cans, automotive parts, packaging materials under FDA regulations
Low Density Polyethylene LDPE 0.910 to 0.940 Trays & general purpose containers, weldable & machinable parts, computer parts, plastic bags, playground equipment
Very Low Density polyethylene VLDPE n/a 0.880 to 0.915 Blown films, molded parts, industrial & general rubber, stretch wrap
Polypropylene PP 0.855 to 0.946 Packaging, textile fibers, carpets, ropes, thermal clothing, automotive components, reusable containers
Ethyl Vinyl Acetate EVA n/a 0.93 Biomedical applications for time release medications, foam padding for sport equipment, flotation devices, sandals, hot melt adhesives

Dry-End Process
The polymerized powder (also called “fluff” or “flake”) needs to be cleansed of residual monomers, catalysts and solvents. This can be done using a decanter centrifuge, purge bin, degassing vessel, low-pressure and highpressure separators, or deodorizing silos, depending on the individual process.

The dry section of a polyolefin manufacturing process begins once the clean, dry powder has been stripped of these impurities and is pneumatically transferred with nitrogen (to prevent oxidation) to an intermediate storage or powder surge hopper situated several levels above a melt extruder.

In some technologies, the polymer is transferred in a molten state directly from the reaction area into the melt extruder.

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