Fuel Maximizing Diesel Production in a Refinery
Diesel is a key product of a petroleum refinery which accounts for nearly 40 per cent of the road transportation sector’s fuel demand globally. This is likely to increase to 45 per cent by 2040 according to World Oil Outlook 2015 published by Organization of the Petroleum Exporting Countries (OPEC).
It is the most demanded product among all the petroleum products globally and accounts for about 30 per cent of the global demand for petroleum products. Its demand is expected to rise in the future at a rate higher than that of other petrol products except jet fuel/kerosene. Over the period 2015–2040, its global demand is likely to grow at a compounded annual rate of 1.0 per cent, slightly lower than 1.2 per cent for jet fuel/kerosene but significantly higher than 0.5 per cent for gasoline. Therefore, maximization of diesel yield remains the major objective of petroleum refiners. Diesel can be produced in a refinery through various routes (see picture). The processes associated with the various routes for diesel production are described in the following paragraphs.
Crude Distillation Unit (CDU) is the most basic unit of a refining process. Here, crude is separated into various products such as light gases, liquefied petroleum gas (LPG), naphtha, kerosene, diesel and residue through a process known as fractionation or distillation. Diesel produced from CDU is known as straight-run diesel. Straight-run diesel may require further treatment in diesel hydrotreating unit or diesel hydrodesulfurization unit to reduce the sulfur level to the specified limit as per regulatory requirements. The typical yield of diesel from CDU is 15–20 per cent depending upon the nature of the crude.
Hydrocracking is a catalytic process in which heavier hydrocarbons are converted to lighter hydrocarbons such as light gases, LPG, naphtha, kerosene and diesel. Some portion of the feed remains unconverted. Hydrocracking can be of three types viz once through, single stage with recycle and two stage. The yield of unconverted oil is least (almost zero) in the two stage process and highest in the once through process. Or, conversion is highest (almost 100 per cent) in two-stage process and lowest in the once through process (typically approximately 60 per cent). Single stage with recycle yields a typical conversion of approximately 98 per cent. Diesel is the major product of hydrocracking and its yield can be typically as high as 75 per cent (weight basis) in a two stage hydrocracking. Diesel obtained from hydrocracking can be almost free from sulfur and can therefore meet stricter regulatory specifications such as Euro V without further treatment. Feed for hydrocracking can be light vacuum gas oil (LVGO) and heavy vacuum gas oil (HVGO) from vacuum distillation unit (VDU), heavy coker gas oil (HCGO) from coker unit, or visbreaking gas oil (VBGO) from the visbreaking unit.
Fluidized Catalytic Cracking
Fluidized catalytic cracking (FCC) is a process in which a heavier oil fraction is converted to lighter fractions such as LPG (rich in C3 and C4 olefins), FCC gasoline, light cycle oil (LCO) in a catalytic process. The unconverted fraction is removed as heavy cycle oil (HCO), which is used as a cutter stock for fuel oil blending. LCO has a boiling range of diesel but has a lower cetane index due to higher aromatics contents. It may also have a higher sulfur quantity than required by regulatory specifications. Therefore, it has to be hydrotreated to reduce the sulfur level and increase its cetane index by the saturation of aromatics. The hydrotreated LCO is added to diesel pool.
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.