Hydrocarbons autoignition temperature

Properties of the principal hydrocarbons found in commercial hexane are shown in Table 9. The flash point of / -hexane is —21.7 °C and the autoignition temperature is 225°C. The explosive limits of hexane vapor in air are 1.1—7.5%. Above 2°C the equiUbrium mixture of hexane and air above the Hquid is too rich to fall within these limits (42). 

Propylene is a colorless gas under normal conditions, has anesthetic properties at high concentrations, and can cause asphyxiation. It does not irritate the eyes and its odor is characteristic of olefins. Propjiene is a flammable gas under normal atmospheric conditions. Vapor-cloud formation from Hquid or vapor leaks is the main ha2ard that can lead to explosion. The autoignition temperature is 731 K in air and 696 K in oxygen (80). Evaporation of Hquid propylene can cause skin bums. Propylene also reacts vigorously with oxidising materials. Under unusual conditions, eg, 96.8 MPa (995 atm) and 600 K, it explodes. It reacts violentiy with NO2, N2O4, and N2O (81). Explosions have been reported when Hquid propylene contacts water at 315—348 K (82). Table 8 shows the ratio TJTp where is the initial water temperature, and T is the superheat limit temperature of the hydrocarbon. 

In applying the requirement for snuffing steam connections, 315°C should be used as an autoignition temperature criterion for typical hydrocarbon streams. 

Autoignition temperature of hydrocarbons decreases with the length of the hydrocarbon chain. 

For straight paraffinic hydrocarbons (i.e., methane, ethane, propane, etc.) the commonly accepted autoignition temperatures decrease as the paraffinic carbon atoms increase (e g., methane 540 °C (1004 °F) and octane 220 °C (428 °F)). 

Semi-empirical formulae, based only on molecular structure, have been derived which allow flammability limits to be calculated for hydrocarbons and alcohols. Flash points, autoignition temperatures and boiling points may also be calculated from molecular structure for these classes. Quoted examples indicate the methods... 

Diesel fuel is produced by distilling raw oil, which is extracted from bedrock. Diesel is a fossil fuel, consisting of hydrocarbons with between 9 and 27 carbon atoms in a chain, as well as a smaller amount of sulfur, nitrogen, oxygen and metal compounds. It is a general properly of hydrocarbons that the autoignition temperature is higher for more volatile hydrocaibons. The hydrocarbons present in the diesel fuels include alkanes, naphthenes, olefins and aromatics. 

Hydrocarbons heated above their autoignition temperatures... 

MTBE and related ethers are used to add octane to gasoline. MTBE also adds oxygen to the gasoline, which allows for more efficient combustion, and therefore less carbon monoxide and unbumed hydrocarbon in the exhaust emissions A relined grade of MTBE is used in the solvents and pharmaceutical industries. Its higher autoignition temperature and narrower flammability range also make it relatively safer to use compared to other ethers. 

Fig. 3.4. Autoignition temperatures of paraffin hydrocarbons at 1 atm. (Data from Bodurtha 1980P)...

When higher hydrocarbons are desired for use as reaction solvents, mineral oil (bp = 260 to 330°C) is recommended because the autoignition temperature of mineral oil (260 to 370°C) is much higher than those of the n-paraffins. Since it is produced in large quantity, it is quite cheap, and is a mixture of various branched aliphatic hydrocarbons. 

Values of these properties, autoignition temperature, and adiabatic flame temperature are given for hydrogen and some hydrocarbons in Table 9.3, which is based on data from Dugdale (1985). 

Egolf, L.M. and Jurs, P.C. (1992). Estimation of Autoignition Temperatures of Hydrocarbons, Alcohols and Esters from Molecular Structure. Ind.Eng.Chem.Res., 31,1798-1807. 

TABLE 2-317 Lower and Upper Flammability Limits, Flash Point, and Autoignition Temperature for Selected Hydrocarbons... 

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