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Modification of the additive-group method to describe the fire hazard indicators of paraffin hydrocarbons: flash point and spontaneous ignition temperature, vaporation enthalpy

Journal: Pozharovzryvobezopastnost/Fire and Explosion Safety (Vol.26, No. 5)

Publication Date:

Authors : ;

Page : 21-28

Keywords : ;

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Abstract

The aim of this study was to determine the relationship between flash point, spontaneous ignition temperature and vaporation enthalpy of saturated hydrocarbons with the composition and structure of their molecules in the framework of additivity of chemical bonds. Analysis of data on flash point has shown that for saturated hydrocarbons the contribution of a methylene group CH-bond is observed as a constant for compounds, starting from dodecane. The spontaneous ignition temperature of paraffin hydrocarbons of normal structure, starting from n-nonane does not change and equal to (204±2) °C. In this case the contribution of the methylene group CH-bond is formally equal to zero. Generally for normal paraffins asymptotic dependence of the contribution of the methylene group CH-bond in the flash point and spontaneous ignition temperature from the size of the molecules has been observed. It is found that some other properties of liquids (density, thermal coefficient of volume expansion) also reveal the asymptotic dependence from the molecular size. In the present study, we exploit the exponential model with three fitting coefficients to describe the asymptotic dependence of the flash point and spontaneous ignition temperature of saturated hydrocarbons. The corresponding correlation equations have been obtained. It has been suggested that the same type of dependencies caused by various reasons. The asymptotic dependence of the flash point temperature, associated mainly with the process of partial evaporation of the liquid, due to competition of intermolecular and intramolecular interactions in the liquid phase. The asymptotic dependence of the spontaneous ignition temperature of hydrocarbons related to the substance oxidation process, caused by the change of the mechanism of chemical interaction. The analysis of literature data has shown that there is a good linear dependence of the vaporization enthalpies of n-alkanes at 25 °C from the size of their molecules. Therefore, the contribution of methylene groups and the structural fragment CH s in the vaporization enthalpy is constant, regardless of the size of the molecule of the n-alkanes. The regression equation for describing and predicting the flash point of paraffin hydrocarbons and their isomers has been obtained. For the spontaneous ignition temperature in the framework of the used approach it has not been made.

Last modified: 2018-10-17 21:57:33