This column is based on “HDPE Production via Slurry Loop Process – Cost Analysis,” published by Intratec. The report can be found at the following URL: www.intratec.us/analysis/hdpe-production-cost.
High-density polyethylene (HDPE) is one of the three main types of polyethylene, and is among the most commonly used polymers worldwide. HDPE applications include blow-molding, injection molding, blown and cast film, pipes and tubing, wire and cable coating and more.
The following paragraphs describe a slurry-loop process for HDPE homopolymer production. Figure 1 presents a simplified flow diagram.
Feed Preparation. This area comprises a catalyst activation system (fluidized-bed activator) and fixed-bed treaters for purification of ethylene monomer and isobutane, which are used as diluent in the polymerization.
Polymerization. In a loop reactor, the monomers and the catalyst suspension are mixed and circulated. As the polymer particles precipitate, they do not dissolve in the slurry. Cooling water in the reactor jacket removes reaction heat. The slurry is continuously discharged to a flash chamber, for removing residual monomers. The polymer product and residual dissolved hydrocarbons are fed to the purge column.
Separation. The flashed stream passes through a bag filter for polymer fines removal. While the fines are sent to the purge column, the vapor stream is sent to the diluent recovery unit. In diluent recovery, the vapors from the flash chamber are sent to a first partial condenser, where heavier hydrocarbons and part of the diluent are condensed and transferred to the heavies column. The uncondensed stream passes through a second condenser at lower temperatures for recovering the diluent, which is reused in the process. The remaining gaseous stream is sent to the heavies column.
In the purge column, the residual hydrocarbons are stripped from the polymer product with heated nitrogen gas. The resulting purge gas is sent to an isobutane-nitrogen recovery unit. Nitrogen is recovered and reused in the column, while gaseous isobutane is transferred to the heavies column.
In the heavies column, isobutane and unreacted monomers are separated in the overhead and sent to the lights column. Fresh isobutane make-up is also routed to the lights column for removing light impurities. The bottom product from this column, an olefin-free isobutane, is recycled to the feed-preparation stage.
The overhead material from the lights column is partially condensed by low-temperature refrigerant for recovering ethylene and light hydrocarbons, which are separated in an ethylene plant. Nitrogen gas and non-condensable material from the overhead are sent to a flare.
Finishing. The polymer powder from the purge column, additives and antiblocking agents are fed to a pelletizing system, where the mixture is melted, homogenized and pelletized. The pellets are homogenized in blending silos and finally packed in bags.
HDPE homopolymer production routes are based on ethylene manufacturing. Figure 2 presents different pathways for HDPE production.
The total operating cost (raw materials, utilities, fixed costs and depreciation costs) estimated to produce HDPE was about $1,350 per ton of HDPE in the fourth quarter of 2014. The analysis was based on a plant constructed in the U.S. with the capacity to produce 450,000 metric ton per year of HDPE.
Edited by Scott Jenkins
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