CATALYTIC CRACKING

Crude oil in itself is a very poor fuel for internal combustion engines, and considerable chemical transformation is necessary to convert it to gasoline. Modern refineries contain, in addition to distillation columns, process units such as thermal and catalytic crackers (to make smaller molecules out of larger ones), hydrotreaters (to convert undesired sulfur compounds into sulfur-free compounds), reformers (to make aromatics for high-octane fuel), and alkylation and isomerization units (to make higher-octane molecules). Central to the modern refinery is the catalytic cracker. Improved zeolite cracking catalysts, developed since the early 1960s, have allowed the United States to squeeze more gasoline from each barrel of crude oil, saving to date the equivalent of about 60 percent of the total production from Alaska's North Slope. Over 6 mil

Framework of a zeolite catalyst used in the catalytic cracking process for gasoline production. Because only molecules of a particular size can fit into their pores and channels, zeolites have highly selective chemical activity.

lion barrels of feed-stock are processed in fluid catalytic cracking units in the United States daily, with more than 70 percent of the cracked products ending up as transportation fuel. Cracking catalysts greatly benefit our balance of payments: the zeolite-based cracking catalysts have allowed the United States to reduce crude-oil imports by more than 400 mil lion barrels per year (relative to pre-zeolite technologies).

The first cracking catalysts were introduced in 1936, when acid-washed natural clays were employed. Over the years, amorphous silica-alumina catalysts were developed, and the introduction of fluid cracking technology enabled scaling of the process to large units that processed over 100,000 barrels per day. The new zeolite-based cracking catalysts have regular microscopic channels as part of their molecular structure.

Because the dimensions of these channels are very similar to the size of the hydrocarbon molecules that constitute gasoline, the zeolite catalysts are very efficient in converting the much larger molecules of crude oil to the desired range of small hydrocarbons for gasoline.

Zeolitic cracking catalysts have had a major impact on the design and operation of modern fluid catalytic cracking units; these units were extensively redesigned to take full advantage of the higher activity and selectivity of the zeolites as well as their ability to process heavier feeds. A growing research challenge for chemists and chemical engineers is the design of catalysts that can reduce the emissions of oxides of nitrogen and sulfur during refining, and can at the same time produce cleaner-burning transportation fuels. Another major challenge is to modify cracking catalysts to accommodate the changing process needs of reformulated fuels dictated by environmental considerations. As an example, catalysts that yield increased quantities of isobutylene and isoamylene may be desirable for producing oxygenated blending stocks that can replace more environmentally harmful gasoline additives.



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