October 8, 2012
Combine processes to lower propylene and butadiene costs. Shale gas is a double-edged sword for the US petrochemical industry. On the positive side, the abundance of inexpensive ethane is revitalizing the competitiveness of American ethylene and ethylene-derivative producers and is stimulating investment in new ethylene crackers for the first time in more than 10 years.
On the downside, however, the shift to ethane as the principal feedstock for olefin production in the United States is increasing propylene and butadiene prices. As more and more ethane is cracked, less and less propylene and butadiene are coproduced. This reduction is tightening the supply–demand balance for the two olefins and pushing their prices to high levels.
K. Dukandar, D. Spence, and S. Panditrao disclose a sequence of known process chemistries that allows butane to be converted to mixtures of propylene and butadiene in good yields. Typical propylene yields from steam cracking butanes are only 15–20%, and butadiene yields are even lower. With the inventors’ route, butane can be converted to these olefins in yields as high as 50–70%. The route combines butane dehydrogenation and olefin metathesis technologies.
In the patent’s examples, a butane feed stream is fed to Lummus Technology’s proprietary Catadiene butane dehydrogenation unit to produce a mixture of butadiene and butenes. The mixed-olefin stream is then sent to an extraction unit to separate the butadiene from the other C4 hydrocarbons. The raffinate that contains the butenes and small amounts of butadiene is directed to a hydrogenation unit to convert the butadiene to butenes. This is necessary because butadiene tends to deactivate the metathesis catalyst in the next stage of the process.
The butene stream is sent to another separation unit to remove isobutene and isobutane. Finally, the n-butenes are metathesized with ethylene to produce polymer-grade propylene.
For a 100-t butane feed, 24 t ethylene is needed to produce 72 t propylene and 16 t butadiene with a combined yield of 71%. (Lummus Technology [Bloomfield, NJ]. US Patent 8,258,357, Sept. 4. 2012; Jeffrey S. Plotkin)