Archive for July, 2013

What is needle coke?

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Needle coke is formed from condensed aromatic compounds. High boiling aromatic compounds (typically 950°F and higher) react in the coke drum by cross-linking the aromatic rings. The resulting crystalline, silver-gray colored coke is called needle coke because of its erratic, but clearly noticeable structure resembling broken needles.

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What is shot coke?

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Shot coke is composed of precipitated asphaltenes. Asphaltenes are large hydrocarbon molecules that are dispersed in the lighter aromatic and paraffinic oils in vacuum residuum. When these lighter molecules thermally crack and vaporize in the coke drum, the asphaltenes agglomerate and precipitate out of solution. The coke is called shot coke because the precipitated asphal­tenes …

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COKE VCM

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Petroleum coke sold for fuel may have a high VCM; coke sold for calcining must have a low (10%—13%) VCM. Making coke too soft (high VCM) leads to potential foamover problems and increased coke yields at the expense of more valuable gas oil. Hard coke (low VCM) takes longer to cut. The VCM of coke …

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Preflash Towers Save Energy

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A refinery designed to run a 30° American Petroleum Institute (API) crude must unexpectedly switch to a 40°API crude. Sometimes, large amounts of natural gas condensate (butane and naphtha) are commingled with the crude supply. Such changes can lead to a reduction in crude running capacity. This happens because: The fractionator tower becomes overloaded and …

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Exchanger Train Fouling

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Heat transfer in crude exchangers often declines because of tube-side fouling. The difference in crude preheat may be 50°F for a dirty vs a clean exchanger train. A sudden drop in furnace feed temperature is commonly caused by a slug of bottoms sediment and water (F3S&W) in crude charge. This happens when a tank in …

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Overhead Condenser Corrosion

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Crude oil contains various chloride salts. Some of these salts decompose in the furnace to produce hydrochloric acid. The HCI boils overhead in the fractionator and dissolves in the water as it precipitates in the condenser. The resulting low pH water is very corrosive, and frequent overhead con­denser tube failures can occur. If such failures …

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Jet Fuel Stripper

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One of the important functions of a crude unit is to meet the flash­point specification for the jet fuel product. This is best done by varying the steam rate to the jet fuel stripper. However, the operators on this crude unit were adjusting the heavy virgin naphtha draw rate to meet the jet fuel flash …

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Stripping Steam Rates

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Stripping Steam Rates

Referring again to Figure 1-6, we can see that jet fuel can be downgraded to lower value products as follows: Heavy virgin naphtha. Poor stripping of jet fuel will cause a low flash point of the jet fuel product. To raise the flash point, the end point of the heavy virgin naphtha product is increased …

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Causes of Inadequate Stripping

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Causes of Inadequate Stripping

Several reasons have been observed for inefficient stripping. Operators note correctly that preheat is improved and steam is saved by minimizing stripping steam. However, far more energy is wasted by unnecessarily running naphtha and kerosene through the FCCU. Perhaps someone has retrofitted the stripper feed to preheat crude before it flows to the stripper. This …

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Steam Stripping Cat Cracker Feed

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Steam Stripping Cat Cracker Feed

A side-stream product drawn from a crude tower is only half fractionated. Refluxing below the drawoff tray controls the product end point. Front-end fractionation takes place in the side-stream stripper. A typical stripper configuration is shown in Figure 1—4. Steam enters the bottom of the stripper and reduces the hydrocarbon partial pressure. The product partially …

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