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.