Polyethylene processes, bimodal

Reactors used in ethylene polymerizations range from simple autoclaves and steel piping to continuous stirred tank reactors (CSTR) and vertical fluidized beds. Since the 1990s, a trend has emerged wherein combinations of processes are used with transition metal catalysts. These combinations allow manufacturers to produce polyethylene with bimodal or broadened molecular weight distributions (see section 7.6). 

Univation Technologies, LLC Polyethylene Ethylene UNIPOL Polyethylene process produces the widest array of linear low-density polyethylene (LLDPE), medium-density polyethylene (MDPE) and high-density polyethylene (HDPE) having unimodal or bimodal molecular weight distribution using single, low-pressure, gas-phase reactor 100 NA... 

Two-stage Large-Scale Plants for the Processing of Bimodal Polyethylene... 

Figure 15.12 First ZSK 320/380 NT process section for compounding of 55 t/h bimodal polyethylene...

Description The Hostalen process is a slurry polymerization method with two reactors parallel or in series. Switching from a single reaction to a reaction in cascade enables producing top quality unimodal and bimodal polyethylene (PE) from narrow to broad molecular weight distribution (MWD) with the same catalyst. 

Borstar A catalytic process for polymerizing ethylene or propylene, subdivided into Borstar PE and Borstar PP. Use of two reactors — a loop reactor and a gas-phase reactor — allows better control of molecular weight distribution. The loop reactor operates under supercritical conditions to avoid bubble formation. Either Ziegler-Natta or metallocene catalysts can be used. The latest version, Borstar PE 2G, uses a single, multizone gas-phase reactor to make polymers that have bimodal molecular weight distributions. Developed by Borealis A/S. The first commercial unit, for polyethylene, was installed in Porvoo, Finland, in 1995. The first polypropylene plant was operated by Borealis in Schwechat, Austria, in 2000. In 2005, Borstar s total capacity for PE and PP was 1.3 million tons. 

Application The Borstar polyethylene (PE) process is used when producing bimodal and unimodal linear low density (LLDPE), medium density (MDPE) and high density (HDPE) polyethylene using loop and gas-phase low pressure reactors in series. All products can be produced in one cycle. 

Mitsui Chemicals, Inc. Polyethylene Ethylene/comonomers CX process yields bimodal-molecular weight polymers 40 2001... 

Equistar Chemicals L. D. Polyethylene, Bimodal HDPE Ethylene Equistar-Maruzen low-pressure sluury process producees bimodal HDPE resins for film, blow molding, pipe and injection molding 3 1985... 

Bimodal molecular weight distribution may be achieved by several techniques. The simplest method is post-reactor blending of polyethylene with different melt indices. Two other methods involve in-reactor production of polyethylene. One approach involves use of mixed catalyst systems that polymerize ethylene in different ways to produce polyethylene with different molecular weights. The latter requires that the catalysts are compatible. Another technique employs use of reactors in series operated under different conditions (see section 7.6 in Chapter 7). Figure 1.9 illustrates polyethylene with a bimodal molecular weight distribution produced with a single site catalyst system in a Unipol gas-phase process. 

Figure 1.9 Bimodal MWD polyethylene produced with SSC in Unipol gas phase process P. J. Ferenz, 2" Asian Petrochemicals Technology Conference, May 7-8,2002, Seoul, Korea.

The most suitable properties for polyethylene used in each fabrication method vary. For example, for blow molding applications, it is preferred to use FFDPE having broad or bimodal molecular weight distribution. Figures 8.4-8.6 show how the major types of polyethylene are processed. 

Some polyethylenes are supplied as so-called bimodal grades a mix of high- and low-molecular weight components in similar concentrations. They often show quite different processing characteristics compared to normal, monomodal (unmodified) polyethylenes. Bimodal grades PE often have higher die swell. 

Application To produce bimodal and multimodal high-density polyethylene (HOPE) using the stirred-tank, heavy-diluent Hostalen ACP process. 

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