Hydrocarbons vacuum distillation unit

The atmospheric reduced crude is the feedstock for the vacuum distillation unit. To prevent thermal decomposition (cracking) of the higher boiling point hydrocarbons in the crude oil, the pressure in the vacuum distillation fractionation column is reduced to about one-twentieth of an atmosphere absolute (one atmosphere pressure is 14.7 psia or 760 mm Fig). This effectively reduces the boiling points of the hydrocarbons several hundred degrees Fahrenheit. The components boiling below about 1050°F (565°C) are vaporized and removed as vacuum gas... 

Atmospheric and vacuum distillation units (Figures 4.3 and 4.4) are closed processes, and exposures are expected to be minimal. Both atmospheric distillation units and vacuum distillation units produce refinery fuel gas streams containing a mixture of light hydrocarbons, hydrogen sulfide, and ammonia. These streams are processed through gas treatment and sulfur recovery units to recover fuel gas and sulfur. Sulfur recovery creates emissions of ammonia, hydrogen sulfide, sulfur oxides, and nitrogen oxides. 

While originally designed for cracking the overhead stream from vacuum distillation units, known as vacuum gas oil (4), most FCC units currently operate with some higher boiling vacuum distillation bottoms (Resid) in the feed. Table 5.1 illustrates the difficult challenges faced by refiners, process licensors and FCC catalysts producers the resid feeds are heavier (lower API gravity), contain many more metals like Ni and V as well as more polyaromatic hydrocarbons prone to form coke on the catalysts (Conradson Carbon Residue, or CCR). 

Vacuum distillation increases the amount of the middle distillates and produces luhricating oil base stocks and asphalt. The feed to the unit is the residue from atmospheric distillation. In vacuum distillation, reduced pressures are applied to avoid cracking long-chain hydrocarbons present in the feed. 

Demex [Demetallization by extraction] A process for removing metal compounds from heavy petroleum fractions, after vacuum distillation, by solvent extraction and supercritical solvent recovery. The solvent is typically a mixture of octanes and pentanes. Developed jointly by UOP and the Institute Mexicano del Petroleo seven units were operating in 1988. Hydrocarbon Process., 1988, 67(9), 66. 

The atmospheric bottom, also known as reduced oil, is then sent to the vacuum unit where it is further separated into vacuum gas oil and vacuum residues. Vacuum distillation improves the separation of gas oil distillates from the reduced oil at temperatures less than those at which thermal cracking would normally take place. The basic idea on which vacuum distillation operates is that, at low pressure, the boiling points of any material are reduced, allowing various hydrocarbon components in the reduced crude oil to vaporize or boil at a lower temperature. Vacuum distillation of the heavier product avoids thermal cracking and hence product loss and equipment fouling. 

In atmospheric or straight-run distillation the crude oil is first pumped into the fractional distillation unit. This is the refinery s tallest unit and some of its columns are used for atmospheric distillation while others are for vacuum distillation. Heated to about 680°F in the gas furnaces, the petroleum reaches the first atmospheric column, which is divided into compartments for fractional distillation. The lighter and more volatile hydrocarbons rise to the upper part. Those that are heavier and less volatile collect in the lower part. While rising, a volatile mass tends to shed its less volatile elements. 

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. 

The aqueous solution rich in ethylene oxide is sent to purification. It passes through a stripping column, which operates under vacuum and separates the ethylene oxide at the top. The aqueous effluent leaving at the bottom is recycled to the absorption stage. It can be treated in an auxiliary unit to recover the glycol it contains. The top effluent which, in addition to carbon dioxide, contains acetaldehyde and hydrocarbon traces, is sent to two distillation columns in series, one for dehydration ( = 20 trays), and the second for purification (S 50 trays). These columns produce high-purity ethylene oxide with a very low acetaldehyde content. The product is stored in liquid form in tanks under nitrogen pressure. 

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