Flash atmospheric distillation unit

Jet fuel is kerosene-based aviation fuel. It is medium distillate used for aviation turbine power units and usually has the same distillation characteristics and flash point as kerosene. Jet fuels are manufactured predominately from straight-run kerosene or kerosene-naphtha blends in the case of wide cut fuels that are produced from the atmospheric distillation of crude oil. Jet fuels are similar in gross composition, with many of the differences in them attributable to additives designed to control some fuel parameters such as freeze and pour point characteristics. For example, the chromatogram (Figure 27.4) of a commercial jet fuel (Jet A) is dominated by GC-resolved n-alkanes in a narrow range of n-C-j to n-Cig with maximum being around n-Ci. The UCM is well dehned. 

Specialised units are used to simulate complex fractionation processes in petroleum refining. Typical configuration consists of a main column with pump-around and side strippers (Fig. 3.14). Among applications, we may cite pre-flash tower, crude atmospheric distillation, or Fluid Catalytic Cracking (FCC) main fractionator. 

In two stage units, it is often economical to distill more gas oil in the vacuum stage and less in the atmospheric stage than the maximum attainable. Gas formed in the atmospheric tower bottoms piping at high temperatures tends to overload the vacuum system and thereby to reduce the capacity of the vacuum tower. The volume of crude vaporized at the flash zone is approximately equal to the total volume of distillate products. Of course, the vapor at this point contains some undesirable heavy material and the liquid still contains some valuable distillate products. The concentration of heavy ends in the vapor is reduced by contact with liquid on the trays as the vapor passes up the tower. This liquid reflux is induced by removing heat farther up in the tower. 

Since cracking stocks generally do not have to meet the color specifications that lube distillates do, higher flash zone temperatures (up to 8(X)°F) can be tolerated. Fuel units are normally designed to distill material boiling up to 1100°F (at atmospheric pressure) from the feed, and some units have distilled beyond 12(X)°F at low feed rates. 

Figure 1-11 General flow diagram for a distillation section (atmospheric and vacuum), including a primary flash unit, in a refinery. 

The liquid stream passes a separator (2), then a let-down valve (3) for pressure release, and enters a flash evaporator (4) where the major part of inerts and unconverted reactants is taken overhead. The flashed-off gases are compressed and returned to the reactor, whereas the liquid is heated and fed to a first distillation column (5), from which vaporized aldehydes are taken as head stream. As the bottoms still contain aldehydes, a second distillation column (6) with sub-atmospheric pressure is required to concentrate the catalyst solution. The gaseous aldehydes from both units are condensed and sent to the upgrading section the separated gases (7) are recycled (after compression) or vented. In order to limit... 

The VPS is generally the first process unit. The VPS s goal is to fractionate the atmospheric resid or reduced crude so that the base stock will have the desired viscosity. The fractionation also controls the volatility and the flash point. The boiling point separation is accomplished by using high efficiency distillation/fractionation hardware. Secondary effects include asphalt segregation in the Vacuum Resid from the VPS (potential by-product), reduction in Conradson carbon and color improvement. 

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