Diesel atmospheric distillation unit

Although gas oils obtained from the atmospheric distillate still remain the main source of diesel fuels, in order to cope with the increased consumption of naphtha and middle distillates almost all refineries in Romania use conversion processes such as fluid catalytic cracking on vacuum distillates and coking or visbreaking on residue. These processes generate middle distillates with higher olefins, diolefins, sulphur, nitrogen and aromatics content compared to gas oil obtained from an atmospheric distillation unit... 

The distillated products of atmospheric distillation unit (ADU) are limited to the boiling fractions under 350 C such as gasoline and diesel because pietroleum fractions tend to thermally degrade in high tempieratures. To recover additional distillates and gas oils, the refinery uses vaaium distillation unit (VDU) following the ADU. The reduced operating pressure ofVDU allows recovery of heavy boiling fraction above 560 C from the atmospheric residue. 

The catalytic cracking unit is often referred to as the gasoline workhorse of a refining unit. As shown in Fig. 18.9, feeds to the catalytic cracking unit are gas oils from the atmospheric and vacuum distillation columns and delayed coker. These heavier fractions also carry metals such as nickel, vanadium, and iron. More important, sulfur compounds concentrate in the heavier product fractions. Table 18.8 lists a typical mass balance for sulfur.25 FCC blend-stocks comprise 36 percent of the volume of the gasoline pool. However, this stream also contributes 98 percent of the sulfur concentration to blended procucts.25 As specifications on sulfur concentrations in diesel and gasoline tighten, more efforts are focused on how feeds and product streams from the FCC are pre- and posttreated for sulfur concentrations. 

Conversion units may employ a full-fledged fractionation train, with a preflash tower to remove light ends an atmospheric fractionator to separate light naphtha, heavy naphtha, middle distillates, and unconverted oil and a vacuum tower to maximize the recovery of diesel. Some hydrocrackers use the atmospheric tower to produce full-range naphtha, which is then separated into light and heavy fractions in a naphtha splitter. 

---