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Estimation of the Flash Points of Saturated and Unsaturated Hydrocarbons

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Indian Journal of Engineering & Materials Sciences Vol. 19, August 2012, pp. 269-278 Estimation of the flash points of saturated and unsaturated hydrocarbons M H Keshavarz* Department of Chemistry, Malek-ashtar University of Technology, Shahin-shahr P.O. Box 83145/115, Islamic Republic of Iran Received 26 May 2011; accepted 11 June 2012 This work introduces a novel, reliable and simple correlation for predicting the flash point of various types of saturated and unsaturated hydrocarbons containing cyclic and acyclic paraffins, olefins, alkynes and aromatic hydrocarbons. It is shown that the new method is based on the number of carbon and hydrogen atoms and some specific molecular moieties, which can easily be used for any types of hydrocarbons. The largest available experimental data consisting of 441 diverse hydrocarbons are used here in order to derive and test the general correlation. For a data set containing 423 of these 441 hydrocarbons, the root mean square (rms) and average absolute deviations are 7.7 and 5.7 K, respectively. The estimated flash points for 18 further hydrocarbons containing complex molecular structures have been compared with one of the best available new group additivity methods, which give much lower values of the rms and average absolute deviations. Keywords: Flash point, Hydrocarbons, Molecular moieties, Additivity methods The flash point can be defined as the lowest approaches for prediction of the flash points. The first temperature at which the mixture of vapor and air approaches are not widely implemented because they above the surface of the liquid can be ignited. It depend on specialized software, such as Dragon13, and characterizes the flammability of combustible liquids, quantitative structure-property relationships (QSPR). which may be used to asses the fire hazard associated They often need unusual molecular descriptors, which with designing process, transportation and storage are available only through specialized software. The systems of these compounds. Since there are many other disadvantage of these methods is that the important chemicals or new synthesized compounds compounds with similar molecular structure in for which no flash point data are given, predictive training set of the QSPR procedure should be used as methods for evaluating reported values of flash point test set. Prediction of flash point based on structural and estimating the flash points of new substances contributions are advantageous because this approach continue to be of interest. Moreover, the is applicable even when thermodynamic data for measurements of flash points are more difficult for desired organic compounds are not available and there some classes of organic compounds that have toxicity, is no need to use specialized software. capable of detonation upon heating for explosives and 1-6 Due to the importance of flash points of saturated explosion of their vapors with air . and unsaturated hydrocarbons, different approaches Many methods have been developed to predict the have been reported to predict flash points of these flash point of different classes of pure organic 14-23 7 compounds in recent years . Some of these studies compounds . The most predictive methods require were restricted to acyclic, cyclic or simultaneously accurate values of thermodynamic properties relating cyclic and acyclic hydrocarbons14-18. Among these to volatility of the compounds, typically vapor 7-9 approaches, group contribution methods were also pressure, boiling point and enthalpy of vaporization . used to predict flash point of saturated and Reliable methods for estimation of thermodynamic unsaturated hydrocarbons19,20. Group contribution properties related to volatility of solid or liquid methods require large and reliable experimental data organic compounds, e.g., heats of sublimation of 10-12 in order to consider various structural environment energetic compounds, may be useful . Neural effects. Their restrictions are: (i) the effect of networks and methods of estimating properties based specified groups cannot be applied for some organic on structural contributions are two important compounds in which the specific group values have ____________________ not been defined and (ii) the predicted flash points *E-mail: [email protected]; [email protected] may be large for some of hydrocarbons. 270 INDIAN J. ENG. MATER. SCI., AUGUST 2012 It is important to have a reliable simple method for predicted flash point on the basis of the number of predicting flash point of various types of cyclic and carbon and hydrogen atoms. For some hydrocarbons, acyclic hydrocarbons containing alkanes (paraffins), the flash point depends on the number, type, length and alkenes (olefins), alkynes and aromatic hydrocarbons location of branching along the hydrocarbon chain as (arenes). The purpose of this work is to use the largest well as the number of carbon atoms attached to double available experimental data of flash points of various or triple bonds. Equation (1) can be revised by two hydrocarbons in order to introduce a reliable simple correcting functions as increasing (P(+)) and decreasing general correlation. It will be shown that the novel (P(-)) parameters for predicting the flash point: method provides reliable predictions for any hydrocarbons containing different molecular structures FP=FP(0) + 51.12 P(+) – 49.63 P(-) … (2) and geometries. The estimated results of this method for several hydrocarbons with complex molecular The values of two correcting functions P(+) and P(-) structures will be compared with the calculated values have been chosen on the basis of deviations of FP(0) 20 of group additivity method of Rowley and coworkers from the measured values. In Eq. (2), the structural as one of the best available general methods. parameters P(+) and P(-) are equal to zero if the conditions for giving them various values are not met. Results and Discussion The values of P(+) and P(-) can be easily specified on Flash points of different categories of hydrocarbons the basis of some molecular fragments. including cyclic and acyclic paraffins, alkenes and Structural parameter P(+) alkynes as well as aromatics depend on various (i) Cycloalkanes: This parameter can be used only for structural parameters. A carful examination of the large cycloalkanes that have more than six-membered flash point of many hydrocarbons reveals its complex (+) rings for which the value P is equal to 0.4. dependency on the total number of carbon and (ii) Normal alkanes: For normal alkanes with a≥ 14, hydrogen atoms. Experimental data of different (+) the value of P is 0.25. hydrocarbons were taken from the chemical database (iii) Methyl substituted aromatics: This parameter can of the department of chemistry at the University of be applied only to polymethyl aromatics for which Akron (USA)24. This reference collects experimental P(+) is 0.1 n where n is the number of methyl data for many hazardous chemicals from a large CH3 CH3 number of scientific sources. However, the flash point groups attached to aromatic ring. of different classes of hydrocarbons with general Structural moieties affecting P(-) formula CaHb on the basis of the experimental data given in Table 1 can be expressed as follows: (i) Small cycloalkanes: For cycloalkanes with three- or four-membered rings without any substituent or (0) with the attachment of only methyl groups, the value FP =158.7+19.86a – 2.40b … (1) (-) of P is 0.2. where FP(0) is the main function that has the major (ii) Small acyclic hydrocarbons: If a≤ 5, then P(-)=1.40 contribution to the predicted flash point. A multiple – (a–1.3)×0.35. linear regression method was used to derive Eq. (1) in (iii) The attachment of isopropyl and t-butyl to which its coefficient of determination or the R2- benzene ring: The values of P(-) are 0.85 and 1.5 for value25 is equal to 0.94. As seen in Eq. (1), increasing the attachment of more than two isopropyl or t-butyl the ratio of a/b can increase the flash point of a directly to aromatic ring, respectively. desired hydrocarbon. For example, the measured flash (iv) The attachment of normal alkyl group to benzene point of methylcyclohexane and toluene are 269 and ring: The values of P(-) are 0.5 and 1.0 for the 280 K, respectively. For some compounds belong to attachment of large normal alkyl group with C10 to various categories of hydrocarbons, deviations of C12 and greater than C12, respectively. predicted results from experimental values are large (v) Alkynes with the general formula R-C≡C-H: The and Eq. (1) requires correcting functions. In addition value of P(-) equals − 18.016.1 a where a≤ 6. (-) to elemental composition, the flash point of (vi) Alkenes with formula R1-C=C-R2: P is equal to hydrocarbons is a function of the presence of some 1.55 – 0.25a where a≤ 4. specific molecular structure fragments. Some (vii) The existence of two double bonds: For the molecular moieties can increase or decrease the compounds with formula R1-C=C-C=C-R2 and R1- KESHAVARZ: SATURATED AND UNSATURATED HYDROCARBONS 271 (-) C=C=C-R2, the values of P are 1.75-0.3a and 0.95- the new method provides the simplest and 0.1a, respectively, for which only one of alkyl groups quick pathway for reliable perdition of (R1 or R2) should be methyl group and the other different classes of hydrocarbons. hydrogen atom. The calculated flash points of different categories Different behaviour of mono-alkyl substituted cyclopentane hydrocarbons compounds are given in Table 1 and and cyclohexane (+) (-) compared with corresponding experimental data. Both P or P can be participated for prediction of Since the measured data from different authors as flash point of mono-alkyl substituted cyclopentane well as organizations can differ by as much as 30 K, and cyclohexane, which depend on the number of the reliability of the present method is very good.15 As carbon atoms in alkyl group.
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