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With about 55 billion lb of ethylene produced in the U.S. each year, commodity chemical reaction engineering would seem to be set in its ways. But thanks to recent developments, that mature technology may learn a thing or two yet. Researchers have come up with a catalyst and procedure to convert ethane to ethylene in a highly efficient manner that is friendlier to the environment than the process most manufacturers currently use.
Today s chemical plants generally use steam cracking of alkanes--mainly ethane--to make ethylene. The process typically runs near 85% selectivity (to form a single product) at roughly 60% ethane conversion. And while these plants are cleverly designed to derive much of the heat they require from burning unwanted by-products, that energy-saving feature helps convert more than 10% of the ethane into carbon dioxide--a greenhouse gas. The units also emit harmful nitrogen oxides.
AUTHORS for ethylene production by coating centimeter-scale alumina monoliths (left) with a platinum-tin film. With use, the metal aggregates into micrometer-sized platinum-tin particles (right).
But now a group of chemical engineers from the U.S. and Italy have demonstrated that with a certain platinum-tin catalyst and large amounts of hydrogen they can produce ethylene by partial oxidation of ethane at greater than 85% selectivity and 70% conversion. The development may lead to much smaller and simpler chemical plants that produce less CO2 and other pollutants.
"Production of ethylene by steam reforming is believed to be the petrochemical industry s biggest contributor to greenhouse gases." comments.. "If one were able to find a way around that problem, that would certainly be a significant contribution."
COMMENT AUTHOR emphasizes, however, that before the new technology can be commercialized, several issues--especially in the area of process safety--need to be thoroughly addressed. Although the researchers report that they never observe flames during experiments, the procedure uses hydrogen under conditions that are generally regarded as explosive.
The researchers prepare catalysts by coating porous, one-piece alumina supports (monoliths) with 1 to 5% platinum and tin by weight. The group flows ethane, oxygen, and hydrogen in a 2:1:2 ratio over a catalyst heated to near 950 C, then analyzes the products with gas chromatography and mass spectrometry.
A number of features of the new catalytic process are surprising, the group notes. First, as COMMENT AUTHOR points out, the reaction ought to be dangerous because of the hydrogen-to-oxygen ratio--especially in the presence of platinum. Yet the researchers find that the two equivalents of ethane in the mixture make it nonflammable.
Also unexpected is the high olefin selectivity. High temperatures usually lead to many products, they explain, because entropy effects cause all reaction channels to open. Yet at 950 C, the reaction proceeds toward a single product.
"Another surprising aspect of this reaction," AUTHORS remarks, "is that it yields as much hydrogen in the products as is fed into the reactor." That means that an external source of hydrogen may be unnecessary if a reactor is designed with a recycle feature.
The team proposes that the reaction occurs by way of mechanisms that are very different from conventional homogeneous and heterogeneous catalytic processes. The group examined catalytic surface-only mechanisms, purely gas-phase (homogeneous) mechanisms, and catalytic hydrogen oxidation followed by homogeneous ethane decomposition. But the researchers note that none of the scenarios agrees satisfactorily with their observations.
AUTHOR and coworkers say that additional experiments and simulations are required before a comprehensive mechanistic model can be developed. In the meantime, the group asserts that extreme conditions such as these "may provide the environments to carry out similar reactions to produce chemicals with high efficiency, improved energy use, and less pollution."
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