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Submitted Chris Haslego, Nov 21 2011 11:21 AM | Last updated Nov 21 2011 01:29 PM




Category: Refining
Question: What are the most common methods of cracking crude streams via a catalyst?
Keywords: cracking,crude,oil,catalytic,catalyst,hydrocracking,methods,streams
Answer: There are two main cracking processes in petroleum refining that use catalyst - catalytic cracking and hydrocracking. Catalytic cracking, uses catalyst and heat to break large hydrocarbon molecules into smaller molecules. The reaction proceeds through carbenium ion formation followed by beta scission of the ion. This forms an olefin and a second carbenium ion. The primary use of catalytic cracking is to produce gasoline (aka petrol) blend stocks from vacuum gas oils and atmospheric resids. Paraffins, Olefins and Naphthene molecules can be cracked, but aromatic rings cannot. Thus, aromatics tend to concentrate in the heavier products. For this reason, the diesel range material produced in the cat cracker is of poor quality. A major by product of catalytic cracking reactions is a solid carbonacious material - coke. This is deposited on the catalyst and burned off in the regenerator. The heat from coke combustion is used to raise the feed oil to reaction temperature and to provide the endothermic heat of cracking. Catalytic cracking is a carbon rejection process. That is, it improves the quality of the feed oils by removing carbon (coke). Most cat crackers use a powdered catalyst that is kept in a fluidized state. These units are refered to as fluid catalytic cracking units or FCCU's. There are still a few moving bed units in operation. These are refered to as TCC's or Houdry units. Hydrocracking uses catalyst and heat to crack hydrocarbon molecules in the presence of hydrogen. The cracking reaction mechanism is similar to that found in fluid catalytic cracking, but both the feed and the products are also subject to hydrogenation reactions. These reactions saturate both the olefins produced by cracking and many of the aromatic molecules found in the feed. Hydrocrackers can produce a wide range of products from LPG to diesel. Because the products are hydrogenated, they contain no olefins and few aromatics. Naphtha from hydrocrackers must be treated further to raise its octane value before it can be used as a gasoline blend stock. The kerosene and diesel range materials, however, are of superior quality. Hydrocrackers are widely used to produce jet fuel and low sulfur diesel. Most hydrocrackers are fixed bed units and the feed is limited to materials boiling below 510 0C. Ebulating bed hydrocrackers (H-Oil, LC-Fining) can process heavier feeds but generally produce somewhat lower quality products.


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