The system formaldehyde-water-methanol thermodynamics of solvated and associated solutions

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 Continu- ously 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-Velocity Fluidization. In Fluidiza- tion, 2nd ed.; Davidson, J. F., Clift, R., Harrison, D., Me.; Aca- demic 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 superimposing a 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 methanol- formaldehyde. 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 sta- bilizer.

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 water- and formaldehyde-containing multicomponent mixtures. Following another approach, Brandani et al. (1987a) describe vapol-liquid equilibrium of formaldehyde-water- methanol mixtures using the Wilson equation for the activity coefficients.

More recently, Brandani et al. (1991) present a ther-

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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 formaldehyde and 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 vapor- liquid 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 this work, 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-

@ 1992 American Chemical Society

Ind. Eng. Chem. Res., Vol. 31, No. 7,1992 1793

102.w Table I. Representation of Isothermal VLE in the Water-Formaldehyde System

no. of d a b av abs dev authors T, OC points AP, Torr Ay

Koaan et al. (1977) 40 9 0.70 0.0150 I

50 9 0.35 70 10 2.52 80 11 3.91 90 13 3.91

Credali et al. (1965) 60 5 3.10 70 6 2.94 75 5 3.59 80 7 5.97 85 6 5.34 90 7 11.45 95 3 9.77

Olsson and Svensson (1975) 80 6 100 8 120 8 130 8

0.0186 0.0214 0.0186 0.0076 0.0116 0.0179 0.0209 0.0198 0.0145 0.0083 0.0041 0.0141 0.0073 0.0327 0.0481

Table 11. Representation of Isobaric VLE in the Water-Formaldehyde System

authors Green and Vener (1955) Tsochev and Petrov (1973) Olevsky and Golubev (1954)

Korzhev and Rossinskaya (1935)

Blazhin et al. (1977)

Farberov and Speranskaya (1955)

no of d a b av abs dev P.Torr Dointa AT. OC Av 760 760 760 350 200 100 60 753

735 2280 3800 2942

10 0.23 17 0.91 10 0.77 10 1.42 10 1.71 10 1.66 10 1.82 6 0.34

5 0.47 7 2.29 6 5.03 10

0.0095 0.0245 0.085 0.0030 0.0162 0.0294 0.0284 0.0119

0.0118 0.0453 0.1359 0.1173

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 experimental data of Green and Vener (1955). The model predicts the existence of an azeotrope in agreement with the data of Green and Vener

101.00 - P=760 t o r r

- Predicted

97.00 0.00 .IO 2 0 .30 .40 .SO .60

'F ,k Figure. 1. Vapor-liquid equilibrium for the water-formaldehyde system at 760 Torr.

110 0 , I / , I

I 1 1 1 I I I 000 10 20 30 40 50 60 70 80

' F ~ Y F Figure 2. Vapor-liquid equilibrium for the methanol-formaldehyde system at 760 Torr.

Table 111. Representation of Isobaric VLE in the Methanol-Formaldehyde System

authors

~~

no of d a b av abs dev P, Torr pointa AT, OC Ay

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

authors T, OC no. of data points AP, Torr AYW AYYY &F

Kogan and Ogorodnikov (1980b) 60 27 17.9 0.0393 0.0283 0.0111 70 29 28.6 0.0372 0.0313 0.0069 _. ~. . ,- 80 26 37.3 0.0366 0.0398 0.0061

Table V. Representation of Isobaric VLE in the Water-Methanol-Formaldehyde System av abs dev

authors P, Torr no. of data points AT, O C AYW &hi AYF

Green and Vener (1955) 760 30 1.15 0.0385 0.0325 0.0072 Blazhin et al. (1976) 760 28 2.41 0.0397 0.0371 0.0052

200 10 -_.

Tunik et al. (1977) 760 7 200 3

~ ~~

2.29 0.0482 0.0529 0.0084 0.0721 0.0676 0.0061 0.0778 0.0863 0.0086

Xw is the water mole fraction on a formaldehyde free basis. The true mole fractions u in the vapor phase are related

to the apparent mole fractions y by

-- - YF

@P(l-uF) yW Kpp(1-uF) yM

1 + @ ? h ~ 1 - Y F + 1 f K y h ~ -1 1 - Y F (9)

The true mole fractions z in the liquid phase are related to the apparent mole fractions x by

XW

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

1.00

.70 Formaldehyde

100

90 A W o t e r

6 0

50

0

Formaldehyde / Methanol

0 0

.10 I/ '

000 10 20 30 40 50 60 70 80 90 I 0 0

yew

Figure 3. Comparison between experimental and calculated vapor compositions for the ternary system water-methanol-formaldehyde at 60 O C (Kogan and Ogorodnikov, 1980b).

100 I A

W a t e r

Methanol F o r ma l d e h;/!

000 - 000 IO 20 30 40 50 60 70 80 90 1 0 0

YC.P


ternary system water-methanol-formaldehyde, at three different temperatures. The average absolute deviation in pressure ranges from 18 Torr at 60 "C to 37 TOR at 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 pres- sures 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 im- provement in the description of these complex mixtures can be obtained.

i

80 "C 1 I f i - I

0 0 0 10 20 30 40 50 60 70 80 90 100

Y*XP


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 aoi = Henry's constant at zero pressure of formaldehyde in

aok = Henry's constant at zero pressure of formaldehyde

&? = equilibrium ratio of the partial preasurea for the reaction

P = pressure

Q? = equilibrium ratio of the true mole fr'actions in the liquid

@ = equilibrium ratio of the true mole fractions in the liquid

= equilibrium ratio of the true mole fractions in the liquid

solvent A

in mixed solvent

A + F = A F

= vapor pressure of solvent A

phase for the reaction A + F = AF phase for the reaction F + AF = AF2

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

:! = molar volume of pure solvent A in the liquid phase = UNIQUAC parameter

1796 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992

i7;h - - partial molar volume at infinite dilution of form-

uF,M = partial molar volume at infinite dilution of form-

XA = mole fraction of component A on a formaldehyde free

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

Greek Letters yF* = true activity coefficient of formaldehyde (physical

yi* = true activity coefficient of component i (physical con-

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] =

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

L = liquid Q) = infinite dilution S = saturation V = vapor A = active solvent t = true value of fugacity coefficient

Subscripts A, F, M, W = component index i = degree of association j = component index

aldehyde in solvent A

aldehyde in mixed solvent

basis

-_

contribution)

tribution)

exp(*ij/T)

coefficienta and solute-free basis

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) =

642)

h YA = h rA(c0mbinatorid) + In yA(residual) (A3)

The expressions for the activity coefficient of solutes j are given by

In y,*(combinatorial) = In ( - F:) + rj( - k) +

The van der Waals parameters rj and qj are reported by Brandani et al. (1991). Parameters of the UNIQUAC equation

7rnk = exP[-(u,k - %)/RT] ( A l l )

where calculated according to Abrams and Prausnitz (1975)

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 ukk is 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

Computer programs are available upon request.

Registry No. Formaldehyde, 50-00-0; methanol, 67-56-1.

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

values recommended by Prausnitz et al. (1980). The apparent surface area fractions of WF2 and MF2 are given by

Literature Cited

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 Va- por-Liquid Equilibrium Data for the Water-Formaldehyde Sys- tem. 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 Vapor- Liquid Equilibria for the Water-Formaldehyde System. A Pre- dictive Thermodynamic Model. Znd. Eng. Chem. Process Des. Dev. 1980, 19, 179.

Brandani, V.; Di Giacomo, G.; Mucciante, V. Vapor-Liquid Equilib- rium 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 Ther- modynamic Consistency of VLE Data for the Systems Water- Formaldehyde and Methanol-Formaldehyde. Znd. Eng. Chem. Res. 1987b, 26,1162.

Brandani, S.; Brandani, V.; Di Giacomo, G. A Physical Theory Su- perimposed onto the Chemical Theory for Describing Vapor- Liquid 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-CH2O e 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 Form- aldehyde-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 Form- aldehyde Solutions. Zh. Khim. Prom. 1935,12,610.

Maurer, G. Vapor-Liquid Equilibrium of Formaldehyde- and Water-Containing Multicomponent Mixtures. 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: En- glewood Cliffs, NJ, 1980.

1798

Tsochev, V.; Petrov, P. Dae Dampf-Flibigkeitagleichgewicht des 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

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the Methanol-Formaldehyde-Water System 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 el- evated temperatures where the oxygen in the air m begin 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 flam- mability assessment scale. The ability of a substance to spontaneously ignite introduces potential safety hazards for all who handle, transport, and store combustible ma- terials. Since combustibles have 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 prematurely- before 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). At present, it is known that it is the structural differences in the

0888-5885/92/2631-1198$03.00/0

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 effectively altered to yield low-knock, high-efficiency fuels.

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 spontaneous ignition 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 sum- marize, 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

0 1992 American Chemical Society

Documents

The system formaldehyde-water-methanol thermodynamics of solvated and associated solutions