1792
Ind. Eng. Chem. Res. 1992,31, 1792-1798
in Drying; Mujumdar, A. S., Ed.; Hemisphere: Washington, D. C., 1987;Vol. 4,pp 359-396. Piccinini, N.; Cancelli, C. Mixture Composition Control in Continuously Operating Spouted Beds. In Fluidization; Kunii, D., Toei, R., Us.; Engineering Foundation: New York, 1983;pp 533-539. Romankov, P. G.; Rashkovskaya, N. B. Drying in a Suspended State; Chemistry Publishing House: Leningrad, 1968. San JosB, M. J.; O h ,M.; Aguayo, A. T.; Arandes, J. M.; Bilbao, J. Design and Hydrodynamics of Conical Jet Spouted Beds. In RBcenta Progds en GBnie des Pm&ddC, La Fluidisation;Laguerie, C., Guigon, P., Eds.; Lavoieier-Technique et Documentation: Paris, 1991; Vol. 5, pp 146-153. Tamir, A. Process and Phenomena in Impinging-Stream Reactors. Chem. Eng. h o g . 1989,86 (9),53-61.
Tsvik, M. Z.; Nabiev, M. N.; R i e v , N. U.; Merenkov, K. V.; Vyzgo, V. S. The Velocity for External Spouting in Then Combined Prowas for Production of Granulated Fertilizer. Uzb. Khim. Zh. 1967,21 (2),50. Wan-Fyong, F.; R~mankov,P. G.; Raehkovskaya, N. B. Research on the Hydrodynamics of the Spouting Bed. Zh. Prikl. Khim. 1969, 42 (3), 609-617. Yerushalmi, J.; Avidan, A. High-VelocityFluidization. In Fluidization, 2nd ed.; Davidson, J. F., Clift, R., Harrison, D., Me.; Academic Press: Duluth, MN, 1985;pp 225-291.
Receiued for reuiew October 3,1991 Revised manuscript received February 18,1992 Accepted March 4,1992
The System Formaldehyde-Water-Methanol: Thermodynamics of Solvated and Associated Solutions Stefan0 Brandani, Vincenzo Brandad,* and Gabriele Di Giacomo Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universitd de’L’Aquila, I-67040 Monteluco di Roio, L’Aquila, Italy
The vapor-liquid equilibria in the binary mixtures water-formaldehyde and methanol-formaldehyde are satisfactorily correlated by superimposinga physical model onto the chemical theory for describing the liquid phase. In a previous work, a thermodynamic model was built up which requires three adjustable parameters to describe the behavior of the liquid phase a t isothermal conditions. The description of the isothermal vapor-liquid equilibrium requires an additional adjustable parameter: Henry’s constant of formaldehyde in the active solvent. In this work, using the parameters obtained by fitting the experimental data of two selected sets of data, the prediction of the model is compared with all the existing literature data for the binary systems water-formaldehyde and methanolformaldehyde. Moreover, we present an extension of the model for predicting vapor-liquid equilibria of the ternary system water-methanol-formaldehyde. From binary data alone the model is capable of accurately predicting vapor-liquid equilibria in a ternary mixture.
Introduction The description of the vapor-liquid equilibrium (VLE) behavior of systems which contain water-formaldehyde or methanol-formaldehyde or their ternary mixtures is of great importance for the design of separation processes in the chemical industry. In fact, formaldehyde is an important raw material in the production of plastics and adhesives. Because of its extremely high reactivity, formaldehyde is usually produced, stored, and processed in the form of aqueous solutions with methanol added as a stabilizer. In a recent publication, Maurer (1986) describes a model which accounts for physical and chemical iteractions between species in the binary mixtures of water-formaldehyde and methanol-formaldehyde. Maurer (1986)usea the UNIFAC model for the activity coefficients and two equilibrium constants for describing the liquid phase: the model requires five adjustable parameters, two UNIFAC parameters for interaction between water and formaldehyde and the three equilibrium constants. Moreover, the vapor pressure of methylene glycol is required for the description of the vaporliquid equilibrium. The principal characteristic of the model proposed by Maurer (1986) is the possibility of its extension to ternary mixtures using binary information alone. This extension was the first attempt at modeling the vapol-liquid equilibrium in waterand formaldehyde-containing multicomponent mixtures. Following another approach, Brandani et al. (1987a) describe vapol-liquid equilibrium of formaldehyde-watermethanol mixtures using the Wilson equation for the activity coefficients. More recently, Brandani et al. (1991) present a ther-
modynamic model which, for each binary system containing formaldehyde and an active solvent, requires three adjustable parameters to describe the behavior of the liquid phase at isothermal conditions. The three parameters are the thermodynamic equilibrium constant for the solvation reaction between formaldehydeand the active solvent, the thermodynamic equilibrium constant for polymer formation, and a physical parameter in the equations for the activity coefficients described by the UNIQUAC model. The description of isothermal vaporliquid equilibrium requires an additional adjustable parameter: Henry’s constant of formaldehyde in the active solvent. This approach was preferred to that proposed by Maurer (1986) since the equation for the vapor pressure of .formaldehyde is reported for a temperature range up to 251 K, while the experimental data for the vapor-liquid equilibrium of binary mixtures taken from the literature are at temperatures between 310 and 400 K. In thiswork, using the parametera obtained by Brandani et aL (1991),the prediction of the model is compared with all the existing literature data for the binary systems watepfomddehyde and methanol-formaldehyde reported in the DECHEMA collection. Moreover, we present the extension of the model for predicting the vapor-liquid equilibria of the ternary system water-methanol-formaldehyde. The extension from binary to ternary mixtures is rigorous and simple.
Comparison of Calculated and Measured VLE in the Binary Systems There are many measurements in the literature for the vapor-liquid equilibrium in the binary systems water-
0888-5885/92/2631-1792$03.00/0@ 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 7,1992 1793 Table I. Representation of Isothermal VLE in the Water-Formaldehyde System no. of dab av abs dev authors T,OC points AP,Torr Ay Koaan et al. (1977) 40 9 0.70 0.0150 50 9 0.35 0.0186 70 10 2.52 0.0214 80 11 3.91 0.0186 90 13 3.91 0.0076 Credali et al. (1965) 60 5 3.10 0.0116 70 6 2.94 0.0179 75 5 3.59 0.0209 80 7 5.97 0.0198 85 6 5.34 0.0145 90 7 11.45 0.0083 95 3 9.77 0.0041 Olsson and Svensson (1975) 80 6 0.0141 100 8 0.0073 120 8 0.0327 130 8 0.0481
102.w
101.00
-
P=760 t o r r
I
Table 11. Representation of Isobaric VLE in the Water-Formaldehyde System no of dab av abs dev P.Torr Dointa AT.OC Av authors 10 0.23 0.0095 Green and Vener (1955) 760 17 0.91 0.0245 760 Tsochev and Petrov (1973) 10 0.77 0.085 Olevsky and Golubev (1954) 760 10 1.42 0.0030 350 10 1.71 0.0162 200 10 1.66 0.0294 100 60 10 1.82 0.0284 6 0.34 0.0119 Korzhev and Rossinskaya 753 (1935) 735 5 0.47 0.0118 7 2.29 0.0453 Blazhin et al. (1977) 2280 3800 6 5.03 0.1359 10 2942 0.1173 Farberov and Speranskaya (1955)
- Predicted 97.00
20
.IO
0.00
.30 'F
.SO
.40
.60
,k
Figure. 1. Vapor-liquid equilibrium for the water-formaldehyde system at 760 Torr. 110 0
000
,
I
,
I
1
1
1
I
I
I
10
20
30
40
50
60
70
80
/
I
'F~YF
formaldehyde and methanol-formaldehyde. However, in some cases there are appreciable differences between theae measurements. As discussed by Brandani and Di Giacomo (1984) and Brandani et al. (1987b), the majority of measurements are affected by thermodynamic inconsistency. We have, however, compared the results predicted using our model (Brandani et al., 1991),the parameters of which were obtained by fitting the experimental data of Brandani et al. (1980) for the water-formaldehyde system (temperature range 40-90 "C) and the thermodynamically consistent (Brandani et al., 1987b) experimental data of Kogan and Ogorodnikov (1980a) for the methanol-formaldehyde system (temperature range 60-80 "C), with those reported in the literature for these two systems. Table I gives the representation of isothermal VLE in the water-formaldehyde system. The results are quite satisfactory, with the exception of the data of Credali et al. (1965). In particular, the model predicts quite well the experimental data of Olsson and Svensson (19751, even though temperatures as high as 130 "C are clearly outside the temperature range within which parameters of the model were fitted. Table II gives the representation of isobaric VLE in the waterformaldehyde system. In this case, too,the representation is poor only for the data well outside the range of temperature within which the model was parametrized. Figure 1 shows the bubble point and the dew point curves at 760 Torr for the waterformaldehyde system predided by the model and the experimentaldata of Green and Vener (1955). The model predicts the existence of an azeotrope in agreement with the data of Green and Vener
Figure 2. Vapor-liquid equilibrium for the methanol-formaldehyde system at 760 Torr. Table 111. Representation of Isobaric VLE in the Methanol-Formaldehyde System no of dab av abs dev authors P, Torr pointa AT,OC A y Blazhm et al. (1976) 760 6 0.82 0.0087 200 6 1.20 0.0219 Olevsky and Golubev (1954) 760 6 1.90 0.0180 350 6 3.29 0.0122 200 6 4.37 0.0066 100 6 5.55 0.0056 60 6 6.38 0.0060 ~~
(1955) and in contrast with the data of Tsochev and Petrov (1973). The various experimental boiling points differ by about 3 "C. Table I11 gives the representation of isobaric VLE data in the methanol-formaldehyde system. The experimental data of Blazhin et al. (1976) are well reproduced, while those of Olevsky and Golubev (1954) are less accurate. Figure 2 shows the bubble point and the dew point curvea at 760 Torr for the methanol-formaldehyde system predicted by the model compared with the existing experimental data.
The Ternary System Water-Methanol-Formaldehyde: Extension of the Model In the ternary system watermethanol-formaldehyde,
1794 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 Table IV. Representation of Isothermal VLE in the Water-Methanol-Formaldehyde System av aba dev
T,OC
authors Kogan and Ogorodnikov (1980b)
60
no. of data points 27
70 ,-
29 _.
80
26
AP,Torr
AYW
AYYY
17.9 28.6 ~. . 37.3
0.0393 0.0372 0.0366
0.0283 0.0313 0.0398
&F
0.0111 0.0069 0.0061
Table V. Representation of Isobaric VLE in the Water-Methanol-Formaldehyde System av abs dev authors Green and Vener (1955) Blazhin et al. (1976)
P, Torr 760 760 200
-_.
Tunik et al. (1977)
760 200
no. of data points 30 28 10 7 3
AT, O C 1.15 2.41 2.29 ~
~~
AYW
AYF
&hi
0.0385 0.0397 0.0482 0.0721 0.0778
0.0325 0.0371 0.0529 0.0676 0.0863
0.0072 0.0052 0.0084 0.0061 0.0086
Xw is the water mole fraction on a formaldehydefree basis. The true mole fractions u in the vapor phase are related to the apparent mole fractions y by
--YF @P(l-uF)
yW
1 + @ ? h1 ~ -YF
Kpp(1-uF)
+
-1
yM
1 f K y h ~1 - Y F (9)
The true mole fractions z in the liquid phase are related to the apparent mole fractions x by
- =X W
1 - xl?
- =ZM 1 - xl?
In
YF,M
);(
= ln
+1
- rF +
which reduce to equations quoted by Brandani and Di Giacomo (1985) when QF = QT and s";" = Qf. The equations for the true activity coefficients are reported in the Appendix.
Comparison of Calculated and Measured VLE in the Ternary System Water-Methanol-Formaldehyde Table IV shows the representation of the isothermal VLE data of Kogan and Ogorodnikov (1980b) for the
Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 1796 100
90
A
Woter
0
Methanol
1.00
/
Formaldehyde 60 0
Formaldehyde
.70
0
i
50
.10
I/ 000 10
80 "C I
1 I
' 20
30
40
50
60
70
80
90
0 0 0 10
I00
yew
100 I
A
Water
F o r ma ld e h Methanol !/;
20
30
40
20
30
40
50
60
70
80
90
100
Y*XP
Figure 3. Comparison between experimental and calculated vapor compositionsfor the ternary system water-methanol-formaldehyde at 60 O C (Kogan and Ogorodnikov, 1980b).
000 000 IO
f i -
50
60
70
80
90 1 0 0
YC.P
Figure 4. Comparison between experimental and calculated vapor compositionsfor the ternary system water-methanol-formaldehyde at 70 O C (Kogan and Ogorodnikov, 1980b).
ternary system water-methanol-formaldehyde, at three different temperatures. The average absolute deviation in pressure ranges from 18 Torr at 60 "C to 37 TORat 80 "C. The model accurately predicts the formaldehyde vapor composition, while the predicted vapor compositions of water and methanol are less accurate. Table V gives the representation of the isobaric VLE data for the water-methanol-formaldehyde system. The average absolute deviations in the boiling temperatures range from 1.2 "C for the data of Green and Vener (1955) to 2.4 "C for the data of Blazhin et al. (1976) at 760 Torr. The experimental data of Maurer (1986), taken at pressures ranging from 239 to 780 Torr, show an average absolute deviation in temperature of 2 "C and an average absolute deviation in formaldehyde mole fraction of 0.0098. However, the experimental water mole fraction is much higher than calculated values, while the opposite occurs for methanol. In general the vapor compositions of formaldehyde are very well represented, while the model calculates vapor compositions of water and methbol which are generally lower and higher, respectively, than experimental values, as can be seen by the diagrams of Figures 3-5. This systematic error is common also to the model proposed by Maurer (1986); therefore we believe that further improvement in the description of these complex mixtures can be obtained.
Figure 5. Comparisonbetween experimental and calculated vapor compositionsfor the ternary system water-methanol-formaldehyde at 80 O C (Kogan and Ogorodnikov, 1980b).
In our opinion, since the experimental VLE data of Kogan and Ogorodnikov (1980b) for the ternary system water-methanol-formaldehyde are well represented by the proposed model, the discrepancies found for the isobaric data can be ascribed to experimental uncertainties.
Conclusions Comparison of calculated and measured vapor-liquid equilibria in the binary systems water-formaldehyde and methanol-formaldehyde confirmed that the thermodynamic model (Brandani et al., 1991) is able to accurately correlate phase behavior in solvated and associated solutions, provided that physical interactions are taken into account. Similar comparisons for the ternary system water-methanol-formaldehyde confirmed that the extension of the model to multicomponent mixtures is able to predict the vapor-liquid equilibrium accurately in such a complex system. Acknowledgment We are indebted to the Italian Minister0 dell'Universit.4 e della Ricerca Scientifica e Technologica for the financial support.
Nomenclature aij = UNIQUAC parameter, K B = second virial coefficient = Henry's constant at zero pressure of formaldehyde in solvent A aok= Henry's constant at zero pressure of formaldehyde in mixed solvent &?= equilibrium ratio of the partial preasurea for the reaction
aoi
A+F=AF
P = pressure = vapor pressure of solvent A
Q? = equilibriumratio of the true mole fr'actionsin the liquid phase for the reaction A + F = AF
@ = equilibriumratio of the true mole fractions in the liquid
phase for the reaction F + AF = AF2 = equilibrium ratio of the true mole fractions in the liquid phase for the reaction F + AF,-*= AF,;i 1 3 q = UNIQUAC surface parameter r = UNIQUAC size parameter R = gas constant T = temperature, K u, = true mole fraction of species i in the vapor phase = UNIQUAC parameter = molar volume of pure solvent A in the liquid phase
:!
1796 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992
-
- partial molar volume at infinite dilution of formaldehyde in solvent A -_ = partial molar volume at infinite dilution of formuF,M aldehyde in mixed solvent XA= mole fraction of component A on a formaldehyde free basis x i = apparent mole fraction of component i in the liquid phase yi = apparent mole fraction of component i in the vapor phase zi = true mole fraction of species i in the liquid phase i7;h
In y,*(combinatorial) = In
(F:)
+ rj(
-
k)
+
Greek Letters yF* = true activity coefficient of formaldehyde (physical
contribution) yi* = true activity coefficient of component i (physical con-
tribution) 8A* = surface area fraction of A in a solute-free solution ei = surface area fraction of component i Xi = latent heat of vaporization of component i 4 = physical parameter in the UNIQUAC equation T ~ ,= UNIQUAC parameter, T ~ ,= exp[-(uij - uj,)/RT] = exp(*ij/T) a** = volumetric fraction of A in a solute-free solution ai= volumetric fraction of component i Superscripts (0) = zero pressure * = unsymmetric convention in the normalization of activity coefficienta and solute-free basis L = liquid = infinite dilution S = saturation V = vapor A = active solvent t = true value of fugacity coefficient Q)
Subscripts A, F, M, W = component index i = degree of association j = component index
Appendix. Model for True Activity Coefficients To express the activity coefficienta which take into account physical forces between molecules of true species, we use the UNIQUAC model (Abrams and Prausnitz, 1975; Maurer and Prausnitz, 1978) with only one adjustable parameter for each binary system. The parameter #- = 0.036 was obtained by fitting three seta of experimental data for the water-methanol system, a t three different temperatures, taken from Gmehling and Onken (1977). The expression for the activity coefficient of solvent A is given by In rA(combinatorial)=
The van der Waals parameters rj and qj are reported by Brandani et al. (1991). Parameters of the UNIQUAC equation
= exP[-(u,k - %)/RT] (All) where calculated according to Abrams and Prausnitz (1975) 7rnk
642)
h YA = h rA(c0mbinatorid) + In yA(residual)
(A3)
The expressions for the activity coefficient of solutes j are given by
where X k is the latent heat of vaporization of component
k. The values of Xk are reported by Brandani et al. (1991). To simplify we assume that ukkis the same for aLl polymers AFi with i 1 2 and for each solvent. Denoting W with
Ind. Eng. Chem. Res., Vol. 31, No. 7,1992 1797
s d f i 1, M with 2, F with 3, WF with 4, MF with 5, WF2 with 6, and MF2 with 7, it results that
Computer programs are available upon request. Registry No. Formaldehyde, 50-00-0; methanol, 67-56-1.
Literature Cited
values recommended by Prausnitz et al. (1980). The apparent surface area fractions of WF2 and MF2 are given by
Abrams, D. S.; Prausnitz, J. M. Statistical Thermodyanamics of Liquid Mixtures: A New Expression for the Excess Gibbs Energy of Partly or Completely Miecible Systems. MChE J. 1975,21,62. Blazhin, Y.M.; Kogan, L. V.; Vagina, L. K.; Pastor, V. E.; Morozova, h I.; Ogorodnikov, S. K. Liquid-Vapor Equilibrium in the System Formaldehyde-Methanol-Water under Atmospheric and Reduced Pressure. J. Appl. Chem. USSR 1976,49,167. Blazhin, Y.M.; Kogan, L. V.; Vagina, L. K.; Pastor, V. E.; Morozova, h I.; Ogorodnikov, S. K. Liquid-Vapor Equilibrium in the System Formaldehydewater under Increased Pressure. Zh. Prikl. Khim. 1977,50, 39. Brandani, V.; Prausnitz, J. M. Thermodynamics of Gas Solubility in Liquid Solvent and Solvent Mixtures. Fluid Phase Equilib. 1981, 7,259. Brandani, V.; Di Giacomo, G. Thermodynamic Consistency of Vapor-Liquid Equilibrium Data for the Water-Formaldehyde System. Znd. Eng. Chem. Fundam. 1984,23,126. Brandani, V.; Di Giacomo, G. Effect of Small Amounts of Methanol on the Vapour-Liquid Equilibrium for the Water-Formaldehyde System. Fluid Phase Equilib. 1985,24,307. Brandani, V.; Di Giacomo, G.; Foscolo, P. U. Isothermal VaporLiquid Equilibria for the Water-Formaldehyde System. A Predictive Thermodynamic Model. Znd. Eng. Chem. Process Des. Dev. 1980,19, 179. Brandani, V.; Di Giacomo, G.; Mucciante, V. Vapor-Liquid Equilibrium of Formaldehyde-Water-Methanol Mixtures. Quad. Zng. Chim. Ztal. 1987a,23 (7,8), 1. Brandani, V.; Di Giacomo, G.; Mucciante, V. A Test for the Thermodynamic Consistency of VLE Data for the Systems WaterFormaldehyde and Methanol-Formaldehyde. Znd. Eng. Chem. Res. 1987b,26,1162. Brandani, S.;Brandani, V.; Di Giacomo, G. A Physical Theory Superimposed onto the Chemical Theory for Describing VaporLiquid Equilibria of Binary Systems of Formaldehyde in Active Solvents. Znd. Eng. Chem. Res. 1991,30,414. Credali, L.; Mortillaro, L.; Galiazzo, G.; Russo, M.; De Checchi, C. Compizione del sistema H2O-CH2Oe relazione con la pressione di formaldeide. Chim. Znd. 1965,47,732. Farberov, M. I.; Speranskaya, V. A. Concentrating Dilute Solutione of Formaldehyde under Pressure. Zh. Prikl. Khim. 1955,28,222. Gmehling, J.; Onken, U. Vapor-Liquid Equilibrium Data Collection, Aqueous-Organic Systems. DECHEMA Chem. Data Ser. 1 1977, Z. Green, S. J.; Vener, R. E. Vapor-Liquid Equilibria of Formaldehyde-Methanol-Water. Znd. Eng. Chem. 1955,47,103. Kogan, L. V.; Ogorodnikov, S. K. Liquid-Vapor Equilibrium in the System Formaldehyde-Methanol. J. Appl. Chem. USSR 1980a, 53,98. Kogan, L. V.; Ogorodnikov, S. K. Liquid-Vapor Equilibrium in the System Formaldehyde-Methanol-Water. J. Appl. Chem. USSR 1980b,53,102. Kogan, L. V.; Blazhin, Y. M.; Ogorodnikov, S. K.; Kafarov, V. V. Liquid-Vapor Equilibrium in the System Formaldehyde-Water (Thermodynamic Verification with Chemical Interaction in the Liquid Phase Taken into Account. Zh. Prikl. Khim. 1977,50, 2682. Korzhev, P. P.; Rossinskaya, I. M. The Concentration of Formaldehyde Solutions. Zh. Khim. Prom. 1935,12,610. Maurer, G. Vapor-Liquid Equilibrium of Formaldehyde- and Water-Containing MulticomponentMixtures. MChE J. 1986,32, 932. Maurer, G.;Prausnitz, J. M. On the Derivation and Extension of the UNIQUAC Equation. Fluid Phase Equilib. 1978,2,91. Olevsky, V. M.; Golubev, I. F. Vapor-Liquid Equilibrium in the System Methanol-Formaldehyde-Water. Tr. Ciap. Vyp. 1954,4, 36. Olason, B.; Svensson, S. G. Formalin Distillation: Vapor-Liquid Equilibria and Tray Efficiences. Trans. Znst. Chem. Eng. 1975, 53,97. Prausnitz, J. M.; Anderson, T. F.; Grens, E. A.; Eckert, C. A.; Hsieh, R.; OConnell, J. P. Computer Calculations for Multicomponent Vapor-Liquid and Liquid-Liquid Equilibria; Prentice-Hall: Englewood Cliffs, NJ, 1980.
1798
Ind. Eng. Chem. Res. 1992, 31, 1798-1807
Tsochev, V.; Petrov, P. Dae Dampf-Flibigkeitagleichgewichtdes Systems Wasser/Formaldehyd. Z . Phys. Chem. ( L e i p i g ) 1973, 252, 337.
Tunik, S. P.; Lavrova, 0. A.; Lesteva, T. M. Liquid-Vapor Phase Equilibrium in Formaldehyde-Water-Alcohols Systems. 11. Methods of Treating Data on the Liquid-Vapor Equilibrium in
the Methanol-Formaldehyde-WaterSystem Complicated by a Chemical Reaction. Zh.Fiz. Khim. 1977, 51, 2707. Received for review October 10, 1991 Revised manuscript received January 30, 1992 Accepted March 24, 1992
Estimation of Autoignition Temperatures of Hydrocarbons, Alcohols, and Esters from Molecular Structure Leanne M. Egolf and Peter C. Jurs* Department of Chemistry, The Pennsylvania State University, 152 Davey Laboratory, University Park, Pennsylvania 16802
Computer-assisted methods are used to develop equations relating molecular structural features
to the autoignition temperatures (AITs)of diverse seta of hydrocarbon, alcohol, and ester compounds. The calculated values of AIT correlate well with experimental data (R= 0.94-0.98), and the standard deviations of the regressions closely approach experimental uncertainties. Results obtained in this study provide evidence to support claims that there exist two different mechanisms for the autoignition of hydrocarbon compounds. It is shown that the low-temperature, very structure-based mechanism could be better modeled with structure-based descriptors than the high-temperature, less structure-dependent mechanism could be. Finally, the developed models are examined to gain insight into how various structural features may affect autoignition processes.
Introduction Autoignition temperature (AIT),as shown in Figure 1 (adapted from Hilado (1970)), is defined as the lowest temperature at which a substance will ignite in the absence of a spark or flame. This phenomenon is initiated at elevated temperatures where the oxygen in the air mbegin to interact with the combustible material, resulting in an exothermic oxidation reaction. When the rate of heat production exceeds the rate at which the heat can be dissipated to the surroundings, autoignition occurs. Researchers are constantly striving to better understand the autoignition process in order to control ita behavior in two areas of tremendous practical importance. The first is to establish a more complete and less ambiguous flammability assessment scale. The ability of a substance to spontaneously ignite introduces potential safety hazards for all who handle, transport, and store combustible materials. Since combustibleshave proven to be so esaential, it is increasingly imperative that the risks associated with these chemicals be accurately defined. Stringent, while not overly restrictive, regulations must be established. With both goals in mind, safety could be ensured without needlessly discouraging further development and new applications. The second is to optimize the performance of internal combustion engines through identifying or designing more efficient fuels and fuel blends. The efficiency of combustion in a gasoline engine hinges on a delicately timed sequence of events. Normally the desired combustion reaction is initiated when a spark is applied to the fuel as the piston just reaches the top of its compression stroke. With autoignition,though, the fuel ignites prematurelybefore the piston can be fully extended. Consequently, engine power and efficiency are severely reduced, and the car exhibits what is known as engine knock. Control of engine knock (autoignition) has been the subject of much research throughout the years (Kirsch and Quinn, 1985, Morley, 1987; Affens et aL, 1961). A t present, it is known that it is the structural differences in the
combustibles themselves that determine knock tendency (Morrison and Boyd, 1983). Therefore, studies have been geared toward determining how susceptible individual structural features axe to oxidative attack and autoignition. Through this knowledge, combustible compositions can be effectivelyaltered to yield low-knock,high-efficiencyfuels. Studies by earlier investigators provide much valuable insight into the structural significance and AIT trends of hydrocarbons. For instance, numerous sources agree that a high degree of branching helps to stabilize a molecule against spontaneous ignition (Affens et al., 1961; Swarts and Orchin, 1957; Frank and Blackham, 1952). Some researchers believe that the explanation for this lies in the number of methylenes in the molecule (Affens et d,1961). Since methylene groups seem to increase the potential for autoignition, this eventuality can be averted if branching is used to limit the number of such moieties. Not only is the amount of branching important but also the relative location of this branching. Frank and Blackham (1952) as well as Swarta and Orchin (1957) contend that the likelihood of spontaneousignition increases with the length of the uninterrupted methylene chain. Therefore, to raise the autoignition temperature of paraffins, it seems logical to intersperse branching between any methylenes that are present in the molecule. Other structural features that offer varying degrees of stability to a molecule include cyclic, aromatic, and multiple bond moieties. To summarize, a molecule’s tendency to react has been reported to increase in the following order: aromatics < branched < cyclics < alkenes < alkanes (Swarta and Orchin, 1957). Interestingly, this general structural sequence closely parallels another chemically important sequence: the ease of free radical formation in hydrocarbons. For this reason, it is not surprising that the AIT mechanism is said to proceed by a free radical reaction (Morley, 1987; Frank and Blackham, 1952; Swarts and Orchin, 1957). The ease of free radical formation, and consequently of oxidation, is directly governed by the stability of the radical formed. For aliphatic hydrocarbons,this stability follows the trend
0888-5885/92/2631-1198$03.00/00 1992 American Chemical Society
