Further uses of the heat of oxidation in chemical hazard assessment

Abstract

Flammability: The “net heat of oxidation” technique described in an earlier publication is extended to predicting the lower flammable limits, lower limit flame temperatures, and limiting oxygen concentrations of chlorinated organic fuels having H:Cl ratios greater than unity. A new Rule is derived for predicting the effect of initial temperature on the lower flammable limits and limiting oxygen concentrations of organic fuels. It is suggested that this Rule be used in preference to the modified “Burgess‐Wheeler” Rule. The effect of initial pressure is discussed. Instability: Net heats of oxidation (kcal/mol oxygen) for a series of disparate fuel groups are compared with “ΔHD,” the maximum heat of decomposition (cal/g) calculated using CHETAH methodology. Given the reasonable assumption that CHETAH's “maximum heat of decomposition” cannot exceed the net heat of combustion “ΔHC,” examination is made as to whether the ratio of these parameters (each expressed in units of kcal/mol), coined the “Reaction Heat Ratio” (RH), provides a useful new indicator for instability assessment. Of these parameters, the net heat of oxidation (ΔHC/S) is the best indicator to help assign NFPA Instability Ratings. However, ΔHC/S cannot generally be used to assign ratings for organo‐peroxides. Also, its performance as an indicator for hazardous polymerization depends on the ΔHC/S difference between the reacting monomer and the polymer product, so it should become increasingly unreliable as the monomer ΔHC/S approaches ‐100 kcal/mol oxygen. The ranking method tacitly assumes organic polymers to have a constant heat of oxidation of about ‐100 kcal/mol oxygen. Errors in this assumption must invalidate the ranking approach where ΔHC/S differences are small. Finally, separate “cut‐offs” must be used at each NFPA Instability Rating for organo‐nitrates versus other organics containing combinations of CHON atoms. Additional materials need to be examined to extend this preliminary analysis. The net heat of oxidation would be a useful additional output parameter of the CHETAH program, if only for its application in flammability assessment. No conclusions are drawn regarding the usefulness of net heat of oxidation or RH in conducting CHETAH hazard assessments, since this procedure requires consideration of several variables. However, the analysis may be helpful to the ASTM E 27.07 subcommittee responsible for developing the program. For example, the ‐ΔHD ⩾ 700 cal/g cut‐off used to assign a “high” CHETAH hazard rating typically corresponds to organic materials rated NFPA 1, the second to lowest hazard rating.

On the temperature dependence of flammability limits of gases

“…In connection with the temperature dependence of flammability limits, White reported that the limit flame temperature is kept constant independently of the experimental temperature [10]. If it is the case, the lower flammability limits change linearly to temperature [11][12][13]. Among others, Zabetakis investigated systematically to demonstrate the linear temperature dependence of lower flammability limits of saturated hydrocarbons [12].…”

Estimation of temperature‐dependent lower flammability limit of pure organic compounds in air at atmospheric pressure

“…This equation is shown to be valid only for linear hydrocarbons [13]. To estimate the effect of temperature on the LFL, Britton and Frurip [14] recommend the following equation: where L T is the LFL (in mol %) at the temperature of interest T (in K), L 0 is the known experimental or estimated value of the LFL at a temperature T T (in K), and LLFT is the lower limit flame temperature (in K). Most of the previous studies did not discuss the reliability of the experimental LFL data reported in the literature (except briefly in Britton's studies).…”

Experimental flammability limits and associated theoretical flame temperatures as a tool for predicting the temperature dependence of these limits

“…Britton and Frurip (2003) have summarized and amplified the above findings for the temperature dependence of the LFL. They explicitly included a variable limiting flame temperature (T f ) to give (5) where L o and T o refer to the ambient or test temperature (all temperatures are consistently either in °C or K, and C L = 1/(T f − T o ).…”

“…Britton and Frurip (2003) used T f = 1440 K for "typical" hydrocarbons, T f = 1505 K for other "typical" organic (CHON) fuels, and T f = 1580 K for chlorinated organic compounds. Hence the single limit flame temperature used previously has been replaced by Britton and Frurip with three such limits for the various types of organic fuels (hydrocarbons, oxygenated organics, and halogenated organic fuels).…”

“…The combustion of hydrogen (H 2 ), carbon monoxide (CO), acetylene (C 2 H 2 ), formaldehyde (H 2 CO), ethylene oxide (C 2 H 4 O), and hydrazine (N 2 H 4 ) are examples of common exceptions to the above energy ranges (Britton, 2002). More recently, reliable and somewhat conservative estimates of both the lowest flammable concentration and limiting oxygen concentration for many fuels have been made using the heat of oxidation, or a combination of the heat of oxidation and the limit heat of combustion of fuel-air mixtures (Britton, 2002;Britton & Frurip, 2003). Despite such success, exceptions to the predicted limits may be found, e.g., hydrogen in air.…”