Maximizing Diesel Production in a Refinery
Coking is a thermal cracking process that converts vacuum residue, a heavy oil fraction, into valuable lighter products such as fuel gas, LPG, naphtha, light coker gas oil (LCGO), heavy coker gas oil (HCGO). Petroleum coke is also produced in the process. LCGO boils in the boiling range of diesel and, therefore, undergoes the process of hydrotreating to produce diesel. HCGO boils in the boiling range of vacuum gas oil (VGO). Hence, it can be fed to the FCC unit or hydrocracker unit. Hydrocracker produces diesel as a major product. The FCC unit also produces diesel through hydrotreating of LCO. But, diesel from FCC is a secondary product, primary being gasoline.
There are two basic types of coking, delayed coking and fluid coking. However, delayed coking is most commonly used in the refining industry. Combined yield of LCGO and HCGO in a delayed coking unit is typically close to 50 per cent (weight basis) depending upon the nature of the crude processed in the refinery.
In a visbreaking unit, vacuum residue or a heavy petroleum fraction is converted to lighter products such as light gases, naphtha, visbreaker gas oil (VBGO) and fuel oil in a thermal cracking process. VBGO can be fed to the hydrocracker to produce diesel as a major product. As fuel oil is a major product of a visbreaking unit as well as a low value product now-a-days, visbreaking unit is not preferred in modern refineries.
Maximization of Diesel Yield
Overall diesel yield from a refinery can be maximized by installing a hydrocracker unit and a coker unit in addition to CDU and VDU, plus a hydrotreating unit. The LVGO and HVGO from VDU should be processed in the hydrocracker unit. The alternative route of processing LVGO and HVGO in FCC unit results into lower diesel yield and hence cannot maximize the overall refinery diesel yield. The vacuum residue should be processed in the coker unit.
The alternative route of processing vacuum residue in visbreaker results into much lesser diesel yield and hence does not achieve the objective of maximization of overall diesel yield from the refinery. The straight-run diesel from CDU and LCGO from cokerunit can be treated in a hydrotreater unit to reduce the sulfur level to meet regulatory requirements, whereas HCGO from coker unit is processed in a hydrocracker unit.