Quality vacuum distillation unit

As a general rule for a given feedstock, an increase in the conversion level of the vacuum residuum (i.e., the material boiling 565°C+/1050°F+) will lead to an increase in the quantity of the distillate but there will be a decline in the quality of the distillate. If the unit is followed by vacuum distillation, the gas oil fraction... 

The heavy bottoms from vacuum distillation may be sent to a FLEXICOKING unit along with air and steam to produce additional distilled liquid products and a low quality fuel gas for process furnaces. Light hydrocarbon gases coming from the distillation unit are steam reformed to produce hydrogen. The total liquid yield is thus a blend of streams from liquefaction and flexi-coking. 

Atmospheric distillation is least effective in converting heavier products into lighter components. A second distillation column under vacuum is needed to further separate the heavier parts of crude oil into lighter fractions. Some fractions from the vacuum units have better quality than atmospheric distillation cuts because the metal-bearing compounds and carbon-forming materials are concentrated in the vacuum residue. 

In the two-step rectification unit, the feed 1 in the first tower is distilled to obtain the following fractions light oil 2, middle oil 3, and partly distilled vacuum residue 6. The vacuum residue from the first tower passes through oven 01 to rectification tower T2, where it is fractionated to narrower fractions. In comparison with the one-step scheme, the two-step scheme requires more energy for production, but the quality of the oil fractions is much higher. 

In their hybrid scheme (Figure 7.7), the light neutral fraction from the vacuum tower and solvent extraction steps goes directly to the solvent dewaxing unit, by-passing the hydrotreater, since their experience was that on such good quality streams, solvent extraction sufficed to make acceptable base stocks. The other two heavier waxy distillate streams are subjected to quality upgrading... 

Slurry-phase hydrocracking systems convert heavy vacuum residues however, these processes are not yet fully commercialized. The feed to this type of reactor is the petroleum residue plus a solid carrier (commonly known as additive). The purpose of the additive is to provide a surface for the deposition of converted asphaltenes and metals, as the residue is hydrocracked. Slurry reactors operate at high temperature and pressure, and residue conversions higher than 90% (Kressmann et al., 1998). Unfortunately, these units produce poor-quality, hydrogen-deficient distillate and vacuum products that cannot be used as fuel, unless blended with something else, for example, coal or heavy fuel oil, due to their high content of sulfur and metals (Ancheyta and Speight, 2007). 

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