Diesel fuel smoke point

The jet fuel and diesel obtained from hydrocracking are lower in aromatics than the straight-run products and have better engine performance characteristics, such as jet-fuel smoke point and diesel cetane number, these numbers are even higher than for the jet fuel and diesel obtained from FCC. 

Aromatics, olefins and in general, unsaturated compounds undergo hydrogenation reactions, usually unwanted due to their detrimental effect on the operating costs, derived from an excessive consumption of hydrogen. Aromatic saturation, however, is used in jet fuel to improve the smoke point and in diesel for cetane enhancement. In the case of gasoline, extreme hydrogenation leads to a deterioration of the fuel performance parameters. 

In addition to CN and ON, the smoke point (SP), which is the maximum smoke-free laminar diffusion flame height, has been employed widely to evaluate the tendency of different fuels to form soot. This tool was first applied to kerosenes, later diesel, and then jet engine fuels.19,20 Researchers have tried to relate smoke points of pure compounds to their molecular structure. It was found that the inverse of smoke point, which measures the potential of a fuel to form soot, increases from n-paraffins to iso-paraffins to alkylbenzenes to naphthalenes.21,22 Since smoke points vary with experimental conditions, the concept of a threshold soot index (TSI), which is calculated from the smoke point, molecular weight, and experimental constants, has been used to compare the soot-formation tendencies of different fuel molecules.23... 

Diesel fuel with a high T-50, for example >575°F (>302°C), will tend to bum with more smoke, soot, and hydrocarbon odor than fuel with a lower T-50. This is basically due to the incomplete combustion and oxidation of a great number of high-boiling-point, high-carbon-content fuel components in a limited-oxygen-content environment. 

Four important tests which are used to characterize an engine fuel are the spontaneous ignition temperature (SIT), flash point, fire point, and smoke point. These tests are standardized, and specialized fuels have specific requirements as defined by these tests. The SIT is dependent on the composition of the fuel and the conditions of the walls of the cylinder. Diesel fuels require low SIT with short delay times of the order of 1-2 ms. The SIT of heptane (CN = 60) is 330°C, whereas benzene with CN = —10 has a SIT of 420°C. 

Middle distillates such as diesel fuel, kerosene, jet fuel, domestic heating oil, and other gas oils, to remove sulfur for enviromnental reasons. Hydrotreating is also used to increase the smoke point or cetane number by hydrogenating aromatic components. 

The process has been applied to kerosene to improve the smoke point and for desulfurization. It is applied to selected naphthas for the recovery of high-octane-number blending stocks and for the recovery of lacquer diluents (aromatic hydrocarbons). Diesel fuels of high Diesel Index and ignition quality can be made from selected gas oils. 

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