Hydrocarbons autoignition

M.P. Halstead, L.J. Kirsch, A. Prothero and C.P. Quinn, A Mathematical Model for Hydrocarbon Autoignition at High Pressures, Proc. Roy. Soc. Lond. A346 (1975) 515. 

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. 

Enhanced oil-recovery processes include chemical and gas floods, steam, combustion, and electric heating. Gas floods, including immiscible and miscible processes, are usually defined by injected fluids (carbon dioxide, flue gas, nitrogen, or hydrocarbon). Steam projects involve cyclic steam (huff and puff) or steam drive. Combustion technologies can be subdivided into those that autoignite and those that require a heat source at injectors [521]. 

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... 

Synthetic FT diesel fuels can have excellent autoignition characteristics. The FT diesel is composed of only straight-chain hydrocarbons and has no aromatics or sulfur. Reaction parameters are temperature, pressure and H/CO ratio. FT product composition is strongly influenced by catalyst composition the yield of paraffins is higher with cobalt catalytic ran and the yield of olefins and oxygenates is higher with ironcatalytic ran. 

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... 

The first attempts to understand quantitatively the high-temperature chemical processes were related to combustion. They took place in the first part of the last century, partly with the development of thermal theories and theories for chain reactions and partly with work on high-temperature oxidation of hydrocarbons (to understand flame propagation) and low-temperature oxidation (to understand autoignition and knock in internal combustion engines). 

For aliphatic hydrocarbons a close relationship exists between knock and low-temperature, two-stage ignition (110). In both cases two induction periods are observed. One, Ti, extends up to cool flame formation. The other, r2, follows ti and lasts up to autoignition. 

Differences in stability among hydroperoxides are probably not controlling. Moreover, the reactions immediately preceding autoignition probably involve simple entities formed in the combustion of all hydrocarbons and are only indirectly affected by structure. 

Radiation and Ignition Studies. The existence of cool flame radiation prior to the occurrence of autoignition in a motored engine was discovered early by Peletier, van Hoogstraten, Smittenberg, and Koojman (109). Since then there has been a constant effort to define the engine conditions limiting the occurrence and extent of cool flame radiation with various hydrocarbons (25, 26, 37, 71, 72, 77, 107). 

Conversion of the original hydrocarbon to other compounds appears to be extensive in the reactions preceding autoignition, as found in one investigation in which 70% or more of the inducted n-heptane was degraded to other products prior to autoignition (105). Hydrocarbon structure has an important bearing on the amount of conversion,... 

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