The extent to which metallocene catalysts replace traditional catalysts in polymer production will depend greatly on the relative costs of making the products. Because metallocene catalysts often can be substituted in existing processes, the most significant cost factor, say polymer and catalyst producers, has been the catalyst and, even more specifically, the cocatalyst used.
Metallocene catalysts cost from a few thousand to several thousand dollars per pound, many times the cost of traditional Ziegler-Natta catalysts. But metallocene systems are significantly more productive - in some situations producing as much as one to two orders of magnitude more polymer per pound of catalyst. Thus, with high productivity and more efficient comonomer incorporation, the catalyst cost per pound of polymer produced starts to approach that of traditional catalysts.
Norman F. Brockmeier of Argonne National Laboratory has developed scenarios comparing the capital and operating costs of using metallocene catalysts. His analysis, presented at the MetCon '95 conference in Houston in May, looks at the production of methyl aluminoxane (MAO) cocatalyst and three specific zirconium-based catalysts used to produce new polymeric materials - high-crystallinity polypropylene, high-impact ethylene-propylene rubber, and syndiotactic polypropylene.
Brockmeier concludes that "a reduction in cost or amount of MAO has the potential for greatly reducing the cost to employ metallocene catalysts." His model already assumes that a "commercially attractive catalyst activity," or mass ratio of about 20:1 MAO:Zr, has been achieved, as compared with more common ratios of between 50:1 and 300:1. Still, MAO is probably used most often as a cocatalyst, and the amounts needed are being improved upon, although other noncoordinating anions are also finding use.
Aluminoxanes are produced by the hydrolysis of trialkylaluminum compounds, such as trimethylaluminum (TMA) to produce MAO. In his hypothetical $6 million, 300,000-lb-per-year U.S. MAO plant, Brockmeier has found that TMA synthesis accounts for two-thirds of MAO production costs and targets this for cost cuts.
"As we take advantage of economies of scale, the catalyst prices will come down so that MAO systems are competitive with cationic activators," comments Roy D. Simmons, group leader for metallocene technology at Witco. "Witco is hoping that its strength will be in providing an entire metallocene/aluminoxane system such that it wouldn't be advantageous to buy the components separately."
Actual catalyst production costs depend upon the specific structure and the difficulty of synthesis. Brockmeier foresees supplies coming from a flexible, batch-process plant that, at a capital cost of about $9 million, could produce as much as 15,000 lb per year of metallocene catalyst - enough to supply eight to 12 world-scale polymer plants. In the best cases, Brockmeier finds that metallocene catalysts contribute about 2 to 5 cents per lb to the cost of polymer production, not including licensing and royalty fees, or selling, administrative, and research costs.
Hoechst, a major developer of polypropylene metallocene technology, says the catalysts are "already in a cost-performance domain where the economic scale-up of [their synthesis] becomes an important factor" for long-term competition. But, it adds, MAO still is a significant cost factor.
Along with estimated catalyst costs, polymer production economics also depend on the scale of a plant, with more favorable economics for 220 million-lb-per-year or larger plants. Again, under the best circumstances, metallocenes add about 1 to 1.5 cents per lb of polymer relative to comparable polymers produced with other catalysts. But, Brockmeier concludes, "demand for the differentiated, value-added product may be sufficient to support the cost increment."
In polyethylenes, companies such as Dow Plastics, Exxon Chemical, and BP Chemicals call the cost issue, "old paradigm thinking," saying instead that the economics have reached the point where catalyst costs per pound of product are nearing those of traditional catalysts. Many producers expect that in time metallocene catalyst systems will be cheaper to use than conventional catalysts.
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