Leo R. Aalund
Managing Editor-Technology
Exxon Chemical Co. has dedicated a second high density polyethylene (HDPE) train at its Mont Belvieu, Tex., polyethylene plant.
The new train, with capacity of 265 million lb/year expandable to 330 million lb/year, started up in October.
The new train uses Mitsui Chemical Co.'s HDPE process. Exxon is the exclusive licensee for the Mitsui process in North America.
In addition to increased flexibility in supplying different grades of HDPE, the new plant incorporates Mitsui's corporate philosophy on quality.
MONT BELVIEU COMPLEX
The first polyethylene train at Mont Belvieu started up in 1982 and uses Union Carbide's Unipol process.
It makes linear low density polyethylene (Lldpe), with capacity recently expanded to 1 billion lb/year.
The Mont Belvieu Unipol plant can also make HDPE, but the material has a narrower molecular weight range compared to that produced with Mitsui technology, Exxon said.
The plant was designed to run 8,000 hr/year, leaving almost 32 days for maintenance. But the company said the plant typically is down only half that time during a year.
MITSUI PROCESS
Mitsui's process can yield high molecular weight (HMW) as well as broad molecular weight (BMW) HDPE resins.
Exxon said films made of the HMW material have outstanding properties for high tensile strength and elongation and excellent puncture resistance and toughness. These properties permit a thin film that is stronger than thicker gauges.
Blow molded bottles made from Mitsui BMW/HDPE have 300-1,000% more environmental stress crack resistance while maintaining 8-12% more top load strength than a bottle made with 1 00% virgin conventional HDPE, Exxon said.
These qualities are also important in meeting the challenge of recycling plastics, said George A. Rizzo, president of Exxon's polymers group.
Rizzon said BMW and HMW HDPE meet the waste reduction objectives for source reduction and recyclability, requiring less material to make blow molded bottles and sacks and being strong enough to permit blending of greater quantities when making a recycled product.
MAJOR POSITION
Ray Nesbitt, executive vice-president of Exxon Chemical, detailed his company's rise to its major position in polyethylene.
The company has production at sites in the U.S., Canada, Belgium, Saudi Arabia, and Australia.
Last July, it bought a 50% interest in the Allied-Signal HDPE plant at Baton Rouge, La. This plant is currently being expanded to about 1.5 billion lb/year.
Exxon is also constructing a joint venture Lldpe plant in France with Royal Dutch/Shell Group. It will start up in 1992.
By 1995 Exxon expects to be operating polyethylene plants with a combined capacity of more than 7 billion lb/year.
Worldwide Exxon currently has 5 billion lb/year of polyethylene capacity, including joint ventures,
Nesbitt said Exxon expects worldwide polyethylene growth to climb 4%/year.
NEW PHILOSOPHY
George Fitzpatrick, Exxon's HDPE project executive, noted the difficult transition to Mitsui's philosophy after generations of Exxon engineers had done things the Exxon way.
"This was a hard legacy for many of us to overcome. But once we saw the simplicity and effectiveness of the Mitsui way, we quickly became converts."
The plant was designed, built, and will be run under the Mitsui philosophy. Part of the project team has worked at a Mitsui plant outside Tokyo. The Mitsui philosophy is reflected in the emphasis placed on keeping equipment in good running order and the lack of storage for finished HDPE.
The vessels that appear to be traditional storage silos at the end of the HDPE train are actually blenders to give the pelletized HDPE one more mix for consistency before delivery.
An Exxon technical manager said that in the unlikely event there were no hopper cars available for product, the plant would have to shut down after a day's run.
The plant can load out a day's production in 16 hr, so around the clock loading isn't necessary. Each of the hopper cars can hold 190,000 lb of HDPE.
THE PROCESS
The ethylene feed for the Mont Belvieu plant comes from Exxon Chemical's Baytown, Tex., olefins plant, about 15 miles away.
Another possible feedstock supply source is the Mont Belvieu salt dome, which contains the world's largest hydrocarbon storage facility with links to much of the Texas Gulf Coast petrochemical industry.
In the Mitsui process, the catalyst and hexane-a diluent for the polymer-are mixed and join the ethylene in the first of two reactors operating in a series.
Agitators mix the components, which polymerizes them into fine particles in solution. The reactors can be operated in parallel if another molecular distribution is required.
The highly active and expensive catalyst, which is used at a magnitude of about 1 lb/20,000 lb of product, Exxon said, remains in the product.
The slurry is then centrifuged into a wet cake, and most of the hexane is returned to the hexane purification section for cleanup and reuse.
The wet cake is then dried in a rotary drum and pneumatically transported as a fine powder to the top of the finishing structure.
There the powder goes through secondary drying, melting, extrusion, and pelletizing. The final step takes place in the pellet blenders just before loading into hopper cars.
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