Diesel fuel distillation characteristics

The necessity of carrying out injection at high pressure and the atomization into fine droplets using an injector imposes very precise volatility characteristics for the diesel fuel. French and European specifications have established two criteria for minimum and maximum volatility therefore, the distilled fraction in volume % should be ... 

The properties of straight run diesel fuels depend on both nature of the crude oil and selected distillation range. Thus the paraffinic crudes give cuts of satisfactory cetane number but poorer cold characteristics the opposite will be observed with naphthenic or aromatic crudes. The increasing demand for diesel fuel could lead the refiner to increase the distillation end point, but that will result in a deterioration of the cloud point. It is generally accepted that a weight gain in yield of 0.5% could increase the cloud point by 1°C. The compromise between quantity and quality is particularly difficult to reconcile. 

Today s diesel fuel grades are blends which may contain straight-run distillate, cycle oil, various gas oils, and heavy cracked distillates. Kerosene or jet fuel may be blended into the diesel to improve the low-temperature viscosity and handling characteristics of the fuel. 

Diesel fuels are complex mixtures of hydrocarbons defined by physical and chemical properties. Petroleum diesel fuels are based on molecules with 9 to 20 carbon atoms and a boiling range between 170°C and 350°C (10). These fuels are produced by sequential chemical treatments and refining of heavy petroleum oils followed by distillation. In general, specifications for fuels are inclusive so as not to exclude compositions with similar operational characteristics. However, environmental concerns regarding toxic emissions from diesel engines have led to legislation that has forced manufacturers to modify diesel fuel chemistry (11). 

Distillation (or volatility) characteristics of a diesel fuel exert a great influence on its performance, particularly in medium- and high-speed engines. Distillation characteristics are measured with a procedure (ASTM D-86, IP 123) in which a sample of the fuel is distilled and the vapor temperatures are recorded for the percentages of evaporation or distillation throughout the range. Other procedures are also available that are applicable to kerosene (Chapter 7). 

Fractional distillation The process of boiling off, at its own characteristic boiling point, the different parts or fractions in a mixture of liquids. Each fraction can then be condensed back to a pure separate part of the liquid. It is a process used in separating the different constituents from crude oil to make fuels and separate the petrol from diesel oils and heavy lubrication oils. 

Mid-range distillates are typically products in a relatively broader carbon range (Ce to C26) and include kerosene (a flammable pale yellow or colorless oily liquid with a characteristic odor intermediate in volatility between gasoline and diesel oil that distills between 125 and 260°C), aviation jet (turbine) fuels, and lighter diesel products. 

The typical quality of the diesel from these plants is presented in Table 4 based on a sample of fuel obtained from the Kuhlmann plant located in Hames, France. ITiis was analyzed after the war (9). As indicated, this fuel was obtained by distillation of the FT products, whose composition includes mostly linear paraffins. This translates directly in a high Cetane number, as well as in poor cold temperature characteristics. 

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