Effect of Chemical Structure on Spontaneous Ignition of Hydrocarbons.

Abstract

S P O N T A N E O U S ignition properties of combustible materials, particularly fuels and hydrocarbons, have been an important area of combustion research for over 50 years. This interest in spontaneous ignition is based on the important role which this phenomenon plays in the fire hazard in the handling and storage of combustibles, the performance of various types of combustion engines, and the elucidation of oxidation and combustion mechanisms of fuels, hydrocarbons, and related substances.The spontaneous ignition temperature (SIT) of a substance is defined as that lowest temperature a t which the substance will ignite in air without the aid of a spark or flame. Based on the thermal theory of ignition and on classical reaction-rate theory, spontaneous ignition temperature can be regarded as that temperature to which a combustible mixture must be raised so that the rate of heat evolved by the exothermic oxidation reactions of the system will just overbalance the rate a t which heat is lost to the surroundings. However, the criterion that is usually taken to indicate ignition-i.e., visible and/or audible combustion observed under ordinary laboratory conditions-is quite subjective. Also, the spontaneous ignition temperature of a substance should be a quantity related to some characteristic chemical property of the material, yet its experimentally determined value is markedly dependent on the method and apparatus employed for its determination. I n a recent monograph, Mullins (1 7) reviews this subject, including the importance, definition, and meaning of spontaneous ignition and spontaneous ignition temperature and describes numerous methods for its determination and the factors which influence results.Earlier work on the effects of oxygen concentration ( 2 , 12-14) indicated that it would be of interest to study spontaneous ignition in finer detail using better instrumentation with particular emphasis on preignition behavior. A static system was chosen for its simplicity and to make temperature-time measurements. An inherent fault of such a system is the problem of uniform gas mixing after introducing the hydrocarbon sample. This problem should be minimized by discharging the sample as a fine spray and by a long time delay before ignition occurs.The general purpose of this investigation was to study the influence of chemical structure on the spontaneous ignition processes. Specifically, the influence of chain length, chain branching, unsaturation, and of cyclic and aromatic structures on the preignition processes was examined by measuring internal gas temperature and oxygen consumption.I n addition to minimum ignition temperature in air for cool-or hot-flame ignition, minimum reaction temperature, preignition temperature range, temperature rise, and oxygen consumption a t ignition were useful. A correlation between these values and ease of oxidation and ignition was noted. The findings were as follows: I n general, decreasing chain length, addition of methyl groups, unsaturation, and particularly, chain branchin...

Short communication: Autoignition temperature of trichlorosilanes

“…The AIT decreased as the chain length increased, and started to level off at the chain length of eight. This result is consistent with the result of Affens and his coworkers 5 for the effect of chain length on the AIT of hydrocarbons: the longer the chain length, the lower the AIT. The AIT of 3-chloropropyltrichlorosilane was about 368C higher than that of n-propyltrichlorosilane.…”

Low temperature autoignition of Jet A and surrogate jet fuel

“…The nature of the data also hampers the development of models for the prediction of the AIT based on molecular properties alone. Affens et al (1961) used the same apparatus as Setchkin (1954) in an effort to correlate ignition behavior to chemical structure for various classes of alkane and aromatic substances. They noted some correlations between a decrease in chain length, methyl groups, unsaturation, and chain branching with an increase in the minimum AIT for aliphatic hydrocarbons as well as a correlation between existing side chains and side chain length and increased AIT for alicyclic compounds and aromatics (Affens et al, 1961).…”

“…Affens et al (1961) used the same apparatus as Setchkin (1954) in an effort to correlate ignition behavior to chemical structure for various classes of alkane and aromatic substances. They noted some correlations between a decrease in chain length, methyl groups, unsaturation, and chain branching with an increase in the minimum AIT for aliphatic hydrocarbons as well as a correlation between existing side chains and side chain length and increased AIT for alicyclic compounds and aromatics (Affens et al, 1961). More recently, a study by Tsai et al (2012) attempted to fit a model to the AIT of 820 compounds reported in the DIPPR database based on a set of molecular descriptors.…”

Prediction of autoignition temperatures of organic compounds by the structural group contribution approach

“…Thus, the AIT of a compound is highly affected by its molecular structure because the stability of the free radicals that are formed is related directly to the molecular structure of the compound. For hydrocarbons, decreasing the chain length, addition of methyl groups, unsaturation, branching, and cyclic and aromatic structures will elevate the AIT [18]. The relationship between the AITs of lower alkanes and their corresponding alcohols and aldehydes is in the following order: alkane > alcohol > aldehyde [17].…”