Polypropylene:  Technology Review 

The shale-driven shift to light feedstocks, while dampened by lower oil prices, continues to drive numerous investments in the polyolefins value chain ranging from greenfield polyethylene plants to on-purpose propylene and methanol-to-olefins units driving the investment in new, greenfield, polypropylene plants.  First in a multi-part series, the purpose of this article is to provide a high level overview of nine major commercial polypropylene technologies.  
Commercial production of polypropylene (PP) began in 1957. Today, PP is the second largest global volume polymer business and accounts for roughly 25% of total polymer demand.  Over 61 million tonnes are consumed by markets ranging from automotive to medical, with China consuming almost one-third of total PP demand. 
To manufacture homopolymer polypropylene, the following are required:
  • Polymer grade propylene
  • Catalyst
  • Polymerization reactor
  • Extruder/pelletizer
To manufacture polypropylene copolymer, a second monomer is required – typically ethylene. The manufacture of homopolymer or random copolymer can be accomplished within a single reactor, whereas the manufacture of impact copolymer or TPOs necessitates at least two reactors in series.
Propylene monomer, also known as 1-propene, is one of the smallest stable unsaturated hydrocarbon molecules used in the gas industry.  Roughly two-thirds of global propylene production goes to produce polypropylene. The majority of all propylene monomer is manufactured in commercial quantities via (1) as a byproduct of catalytic cracking in a petroleum refinery or (2) as a by-product of the dominant olefin ethylene in the steam pyrolysis process of a chemical cracker.  The balance is produced via on-purpose routes including propane dehydrogenation (PDH), coal-to-olefins (CTO), methanol-to-olefins (MTO), methanol -to-propylene (MTP) and metathesis. 
Three grades of propylene are commonly traded throughout the world, based primarily on the propylene content, but also upon the level of certain critical impurities.  These grades are their usual purity levels are:
  • Polymer Grade (PGP):  99.5% propylene (minimum), used to produce polypropylene
  • Chemical Grade (CGP):  ~92-96% propylene, used to produce solvents and intermediate chemicals
  • Refinery Grade(RGP):  ~60-70% propylene, upgraded (split) into purer grades or utilized to produce cumene or alkylate
There are five basic polypropylene production processes:
  1. Solution polymerization
  2. Slurry (or diluent) polymerization
  3. Gas phase polymerization
  4. Bulk (or liquid polypropylene) polymerization
  5. Hybrid (bulk plus gas phase) polymerization
The catalyst is the key component that influences the productivity and economics of the process and determines certain properties of the polymer.  Historically, catalyst improvements have been commercialized progressively, as follows:

  • High activity Ziegler-Natta catalysts have increased the productivity of the process and eliminated the need for catalyst removal from the polypropylene, thereby reducing production costs;
  • High stereospecificity Ziegler-Natta catalysts have increased the yield of isotactic polypropylene in the process, thereby eliminating the need to remove atactic polypropylene and further reducing production costs; and
  • Metallocene catalysts have improved certain polymer properties for isotactic polypropylene, and have also provided the opportunity to produce on-purpose syndiotactic and atactic polypropylene.        bnnnb
Ziegler-Natta catalyst suppliers are continuing to make improvements in catalyst productivity and selectivity. Metallocene catalysts were first used to produce quantities of polypropylene in 1995, brand name Achieve by Exxon.  Since then, metallocene polypropylene grades have been produced commercially and are delivering superior properties which are valuable in certain end-use market segments especially high MFI fiber grade and injection molded parts.
There are nine major suppliers of polypropylene technology, offering unique variations of the five basic production processes.  Currently, Spheripol has the largest share of global installed capacity, followed by Unipol PP and Novolen. 


LyondellBasell (formerly Basell Polyolefins, Himont, Montell)
In Basell's Spheripol process, homopolymerization is carried out in the liquid phase using a high activity/high stereospecificity catalyst system, which includes a MgCl2 supported titanium catalyst in spherical form. Liquid propylene and the catalyst system are fed continuously into a loop reactor together with hydrogen (for controlling molecular weight); ethylene is added to produce random copolymers. Polymerization occurs at temperatures of 60 to 80oC and at pressures of 3,500 to 4,000 kPa. The reaction vessel is cooled by using a water jacket. Spheres of polymer form as a slurry; the granules can then be fed to a gas phase copolymerization reactor, and unreacted monomer is condensed and recycled.  Gaseous ethylene and propylene are copolymerized on a fluid bed of polymer particles; the composition of the spherical impact copolymer can be controlled by changing the amount of gases that are recycled. Solid products are finally treated on a fluid bed to deactivate the residual catalyst and to remove volatiles.  The Spheripol process was developed by Himont/Montedison using technology devised under a cooperative research and development agreement with Mitsui Petrochemical of Japan.  In 2002, Basell commercialized the Spherizone process. This new process uses a multizone-circulating reactor with bulk and gas phase zones separated by a barrier fluid.
Borealis offers for license Borstar PP, a multiple reactor polymerization technology based upon an extension of their Borstar polyethylene process. Borstar PP consists of a slurry loop reactor, followed by a number of gas phase reactors in series. Two, three or four reactor configurations are available depending on the type of polypropylene required. Borealis has demonstrated the feasibility of this new technology by constructing their first 200 thousand tonnes line at Schwechat, Austria.
Ineos (formerly BP Chemicals)
In 1999, BP Chemicals acquired Amoco Chemicals and has since restructured its polypropylene and polyethylene businesses. As part of this restructuring, Amoco's and BP Chemicals'€™ licensing businesses were combined. The polypropylene technology originally developed by Amoco is now offered under the brand Innovene PP. This gas phase polymerization technology uses a horizontal stirred bed process operating at 2,200 kPa. Only one reactor is used to produce homopolymers and random copolymers; two gas phase reactors in series are used to produce impact copolymers. The unique reactor reportedly allows near plug flow (a linear flow of the propylene/polypropylene through the reactor, versus a random removal with recycling). The near plug flow, in turn, provides a much narrower molecular weight distribution than a back mixed reactor, as the residence time of each molecule is very nearly the same.
Chisso co-developed the horizontal gas phase polymerization process with Amoco, and they co-licensed this technology for a number of years. This cooperative arrangement has since ended, and Chisso under JPP now offers its own version for license.
Dow entered the polypropylene licensing business in 2001 through its acquisition of Union Carbide. The Unipol PP process is based upon an extension of Union Carbide's gas phase polymerization technology which was originally developed to produce polyethylene. The SHAC Catalyst technology, which Union Carbide acquired from Shell, involves a high activity titanium chloride/triethyl aluminum-based catalyst. The dual reactor configuration can produce a complete range of commercial polypropylene grades. Dow then developed Advanced Donor Technology (ADT) and produced non-phthalate PP catalysts. Grace purchased Dow's PP catalyst/licensing business on October 11, 2013.  All Dow's PP technologies now belong to Grace.
Mitsui Petrochemical
The Hypol polymerization process utilizes a high activity, high stereospecificity catalyst which was developed jointly by Himont/Montedison and Mitsui. First-generation Hypol technology is based upon a hybrid process, which uses a bulk stirred tank reactor for the first stage and a gas phase reactor for the second stage. In second-generation Hypol II units, a bulk loop reactor is used for the first stage. In either case, homopolymers and random copolymers are made in an auto-refrigerated adiabatic vessel using the bulk polymerization process. Impact copolymers are then made in the second stage gas phase reactor. Line configurations with more than two reactors have been used in order to manufacture polymers with specific characteristics or properties.
Novolen Technologies
As a consequence of the merger between Montell Polyolefins and Targor to form Basell, licensing rights to the Novolen process were divested to a separate company called Novolen Technologies Holdings, an 80/20 percent joint venture between ABB Lummus and Equistar. Originally developed by BASF, the Novolen process is based upon gas phase polymerization technology using a vertical stirred powder bed at 50 to 105­oC. and 2,500 to 4,000 kPa. A modified Ziegler-Natta catalyst is used for high polymerization yield. Only one reactor is necessary to produce homopolymers and random copolymers, while a cascade of two gas phase reactors is used to produce impact copolymers.
A two-reactor system can manufacture strictly homopolymer at up to 130 percent of the designed plant capacity as both reactors are utilized to manufacture homopolymer. Cooling is provided by vaporizing liquid propylene, which is injected into the reactor, condensed, re-injected and recycled continuously.  Polypropylene powder is discharged periodically from the reactor into a vessel for downstream cleansing, finishing, and pelletizing.
In the Rexene or El Paso process, polypropylene homopolymers and copolymers can be produced in a single-train plant (up to 150,000 tonnes/year). Homopolymers and random copolymers are produced via a liquid pool process in a single stirred tank reactor. Impact copolymers are made by on-line transfer of the polymers to a gas phase copolymer reactor system. The process uses high-yield catalysts, and the removal of catalyst residues and atactic polymer is not required.
Sumitomo Chemical
Sumitomo offers gas phase polymerization technology using a fluidized bed reactor. Only one reactor is necessary to produce homopolymers and random copolymers; a cascade of two gas phase reactors is used to produce impact copolymers. Proprietary fourth generation catalyst technology has also been developed. The combination of the catalyst and process technology has enabled Sumitomo to manufacture very high MFR grades, highly crystalline PP, and high ethylene-propylene rubber content copolymers.
The long-term outlook for polypropylene is positive  as the promise of abundant, low-priced propylene supports continued PP growth and attractive margins.  
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