LIQUID- SOLID AND LIQUID-VAPOR PHASE EQUILIBRIUM IN SECONDARY ALCOHOL - N-ALKANE SYSTEM
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Abstract (English):
Primary and secondary alcohols, obtained as a product of processing of plant raw materials, can be used as additives in fuel. Mixtures of n-alkanes, cyclic alkanes and aromatic hydrocarbons can act as a model for gasoline and diesel fuel. Therefore, it is necessary to study the characteristics of mixtures of alcohols with normal, cyclic and aromatic hydrocarbons. To simulate liquid-solid and liquid-vapor phase equilibria, a method is used to minimize excess Gibbs energy by the solvation parameter. The authors developed the PCEAS (Phase Charts Eutectic and Azeotropic Systems) software. The input data in the case of constant pressure are the temperature T 0 and the enthalpy of the phase change H 0 of the pure components. Prediction of the thermodynamic parameters of secondary alcohols is used to calculate the eutectic and azeotropic parameters of the secondary alcohol - n-alkane mixture: composition, temperature, melting enthalpy and evaporation. The model makes it possible to determine the average value of the association parameter in the liquid phase k . Experimental data for azeotropic mixtures made it possible to establish the association parameter in the vaporphase  of the systems under study. The results of calculations can be used to select the optimal composition and obtain the requiredcharacteristics of biofuel.

Keywords:
Biofuel, secondary alcohols, minimization of excess Gibbs energy, eutectic, azeotrope
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INTRODUCTION The study and promotion of new types of biofuels, which use renewable energy sources obtained during the processing of plant raw materials, will contribute to improving the environmental situation and make it possible to reduce the cost of fuel production. Primary and secondary alcohols can be used as additives in fuel, constituting up to 10% of gasoline and diesel fuel. The properties of these new types of biofuels vary compared to traditional fossil fuels. Mixtures of n-alkanes, cyclic alkanes and aromatic hydrocarbons can act as a model for gasoline and diesel fuel. Therefore, it is necessary to study the characteristics of mixtures of alcohols with normal, cyclic and aromatic hydrocarbons. In [1-6], 1-pentanol and 2-pentanol are considered as oxygen-containing additives to fuel due to such properties as high octane number and high heat of combustion. As the hydrocarbon, n-hexane, cyclohexane or toluene was chosen. In this paper we study the liquid-solid and liquid- vapor phase equilibrium in the secondary alcohol - n- alkane system at constant pressure. To simulate liquid- solid and liquid-vapor phase equilibria, a method is used to minimize excess Gibbs energy by the solvation parameter. Eutectic and azeotropic parameters are found in the secondary alcohol - n-alkane binary system: composition, temperature and enthalpy of the phase change, the association parameter in the liquid phase. To simulate the thermodynamic properties of solutions, it is necessary to calculate the activity coefficients of the mixture components in the liquid and vapor phases. The nonideality of the solution is largely due to the interaction of the molecules. The influence of the nonideality of the solution on the activity coefficients is taken into account by constructing the corresponding thermodynamic models. For alcohol- based polar systems, the following models are used: Van Laar equation, Wilson equation, NRTL, UNIQUAC equation, UNIFAC equation, etc. [7-10]. Most models are not predictive; they require information on the interaction of molecules in binary mixtures. MATERIALS AND METHODS The justification of the Gibbs free energy minimization method according to the solvation parameter is provided in [11]. The solvation parameter λ characterizes the ratio of the number of molecules A to Please cite this article in press as: Esina Z.N., Miroshnikov A.M., and Korchuganova M.R. Liquid- solid and liquid-vapor phase equilibrium in secondary alcohol - n-alkane system. Science Evolution, 2017, vol. 2, no. 2, pp. 33-39. DOI: 10.21603/2500-1418-2017-2-2-33-39. Copyright © 2017, Esina et al. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/), allowing third parties to copy and redistribute the material in any medium or format and to remix, transform, and build upon the material for any purpose, even commercially, provided the original work is properly cited and states its license. This article is published with open access at http:// science- evolution.ru/. i the number of molecules B in the compound. Based on this method, the PCEAS [12-14] model allowing to calculate activity coefficients and a liquid-solid and liquid-vapor equilibrium with a constant pressure or at a constant temperature, and also the parameters of eutectic and azeotropic points is proposed. The input data in the case of constant pressure are the temperature T 0 and the Here F(z1) = z1 F1(z1)+(1 - z1) F2(z1) - is a function that is chosen from the condition of thermodynamic consistency of the model by the method of Herington and Redlich-Kister [15]. According to this method, the areas limited by the curve lg γ1/γ2 and the axes of coordinates must be equal. The minimization of excess energy (2) according to the internal parameter λ provides the Bernoulli equation: enthalpy of the phase change i H 0 of the pure 2 components, i = 1, 2. The model makes it possible to determine the average value of the association parameter in the liquid phase k. In this paper, we present the results where dT / dz1 + f1(z1) T=f2(z1) T , (3) 0 / R - α z ), f (z ) = of the calculation of liquid-solid and liquid-vapor phase f1(z1) = α / (∆H1 1 2 1 diagrams in secondary alcohol - n-alkane systems and water-secondary alcohol using the PCEAS model. 0 = - (β + ln (1 - z1)/z1)/(∆H1 /R - α z1), 0 - ∆H 0 )/R, β = ∆H 0 /RT 0 - ∆H 0 /RT 0. PCEAS model refers to models of the equation of α = (∆H1 2 2 2 1 1 state, is predictive, because it is based only on data on the parameters of pure components and does not use The solution of the equation (3) is written as: 0 z + ∆H 0 (1 - z )]/[∆H 0 z /T 0 + information about the parameters of binary interactions. T(z1) = [∆H1 1 2 1 1 1 1 To construct a mathematical model of the liquid-solid and liquid-vapor phase equilibrium, the method of minimizing the excess free Gibbs energy GE according to the solvation parameter λ [11] is applied. The difference of the equations of state of the binary system for the real and ideal equilibrium phases can be presented in the form [15]: (- H E/RT 2) dT+(V E/RT) dP = Σ xi d ln γi , (1) where H E - enthalpy of mixing; V E - excess volume; P - solution pressure; γi - activity coefficient of the i-th component; xi - molar fraction of the i-th component. In the majority of real solutions, the interaction of components providing the formation of type AB molecular compounds emerges. The solvation parameter λ = λ1/λ2 characterizes the ratio of the number of molecules A to the number of molecules B in the compound. The average ratio of the number of molecules of components in solvates characterizes the stable 0 (1 - z )/T 0 - R (z ln z + (1 - z ) ln (1 - z ))], + ∆H2 1 2 1 1 1 1 where T(z1) is the solution phase change temperature. The molecules of the components A and B can also form the clusters consisting of molecules of the same type. We will characterize the relation of the number of molecules of the component A to the number of molecules of the component B aggregated into associates in the liquid phase using the association parameter k  k1 k2 . The coefficient k1 shows how many molecules of component A in the liquid phase were combined into a cluster of type AA, the coefficient k2 shows how many molecules of component B in the liquid phase were combined into a cluster of type BB. Similarly, for the association characteristic in the pair, the parameter   1  2 is introduced. To calculate the liquid-solid and liquid-vapor phase equilibria, the thermodynamic coordination of the activity coefficients in the liquid and vapor phases is of particular structure of the solution. If solvation of molecules of pure importance. The solvation parameter   1 2 , as well as components takes place, the effective molar mass of the the association parameters in the liquid phase k  k1 k2 , component in the solution can be calculated by the can be determined by checking the thermodynamic formula: Mi  i Mi , where Mi is the molar mass of the consistency of the activity coefficients of the components component before mixing. For the real systems in which the formation of the molecular compounds called solvates is possible it is necessary to make a transition to effective mole fractions in the equation (1). The effective mole fractions of the components of the binary mixture are calculated by the formulas: z1 = x1/(x1 + λ x2), z2 = x2 /(x1/ λ + x2) [14]. The partial activity coefficients at constant pressure can be represented as: by the method of Herington and Redlich-Kister [15]. If n eutectics or azeotropes with close phase change temperatures are formed, their parameters must be averaged. The statistical method for determining the average parameters was proposed by Gibbs in 1901. The average temperature, composition and enthalpy of the phase change are determined by the formula: 0 0 0 H j n  RTj H j n  RTj ln γi = ∆Hi (1 - Ti /T) / (RTi ) - ln zi + ln Fi(zi), 0 X   X j e j1 / e j 1 , where ∆Hi is the enthalpy of the phase transition of the T pure i-th component; Fi(zi) is an arbitrary function of the composition; 0 - phase change temperature of the pure i where X j is the solvation parameter, the association i-th component, i = 1, 2. For a small range of boiling points, the excess free Gibbs energy: coefficient, the eutectic temperature or the azeotrope ( Tj ), the mole fraction of the binary mixture component in the eutectic or azeotropic point ( xj ) and the enthalpy of the GE = z1 ∆H1 (T / T1 - 1) + z2 ∆H2 (T / T2 - 1) - phase change H , and n is the number of eutectics or 0 0 0 0 j - RT (z1 ln z1 + z2 ln z2) + F(z1). (2) azeotropes in the system. To calculate the average characteristics of a binary mixture, the initial value of the solution temperature in a temperature is determined in the range from T 0  T to T 0  T . In this range, possible eutectics or azeotropes eutectic or azeotrope T 0 is introduced, which is close to the with temperature Tj and the molar enthalpy of the phase experimental one, and the error T with which the change H j (j = 1, 2, ..., n) are determined in this system. Table. Eutectic and azeotropic parameters are found in the secondary alcohol - n-alkane binary system System x1 eut, mas. calc. teut, °С calc. Heut , J/mol x1az, mas. calc. taz, °С calc. Haz , J/mol x1 az, mas. exp. [16] taz, °С exp. [16] k1 / k2 2-Propanol * Pentane 0.127 -136.53 7491 0.014 35.79 52068 0.060 35.50 5/12 2-Propanol * Hexane 0.520 -112.12 8404 0.182 64.56 67448 0.230 62.70 30/29 2-Propanol * Heptane 0.569 -109.05 8395 0.584 77.80 75069 0.505 76.40 6/5 2-Propanol * Octane 0.965 -93.56 7050 0.867 81.38 79191 0.840 81.60 9/2 2-Butanol* Pentane 0.217 -137.21 8599 not az. not az. not az. not az. not az. 4/7 2-Butanol* Hexane 0.875 -120.56 10152 0.073 67.42 64982 0.080 67.20 5/2 2-Butanol* Heptane 0.918 -119.12 9884 0.367 88.26 71396 0.380 89.00 10/3 2-Butanol* Octane 0.999 -114.98 9191 not az. not az. not az. - - 27/1 2-Pentanol* Pentane 0.142 -135.78 7963 not az. not az. not az. - - 3/8 2-Pentanol* Hexane 0.624 -112.89 9152 0.024 68.38 63921 not az. not az. 8/7 2-Pentanol* Heptane 0.723 -110.23 9141 0.170 94.69 66978 0.15 96.00 13/9 2-Pentanol* Octane 0.989 -98.62 7640 0.500 110.29 76367 0.56 114.8 8/1 2-Hexanol* Pentane 0.002 -130.38 8606 not az. not az. not az. - - 1/18 2-Hexanol* Hexane 0.155 -101.35 13595 not az. not az. not az. - - 3/7 2-Hexanol* Heptane 0.219 -97.99 14388 0.055 97.45 64588 - - 4/7 2-Hexanol* Octane 0.894 -81.87 16375 0.239 119.96 73013 - - 8/3 2-Heptanol* Pentane 0.244 -136.67 8195 not az. not az. not az. - - 7/15 2-Heptanol* Hexane 0.850 -117.04 9281 not az. not az. not az. - - 9/4 2-Heptanol* Heptane 0.893 -115.06 9226 0.021 98.12. 63850 - - 9/4 2-Heptanol** Octane 0.973 -108.17 8077 0.110 123.46 70723 - - 17/1 2-Octanol * Pentane not eut. not eut. not eut. not az. not az. not az. - - - 2-Octanol * Hexane 0.002 -95.85 13262 0.033 68.74 63488 - - 1/21 2-Octanol * Heptane 0.003 -91.26 14319 0.091 95.78 32626. - - 1/17 2-Octanol * Octane 0.151 -62.41 21166 0.046 124.85 69686 - - 3/7 The mean relative error of azeotropic parameters: 8T = 0.86%; 8x= 6.98%. Note. * refers to the calculations of eutectic and azeotropic parameters based on the results of the prediction of the melting enthalpy and the enthalpy of evaporation of secondary alcohols, formulas (4, 5). The equation of the melting enthalpy model for secondary alcohols: RESULTS AND DISCUSSION The result of this work is the calculation of the H melt  ( N  1)MTmelt 1.85 N  520 N (1)N  3800 / N , liquid-solid and liquid-vapor phase equilibrium in the secondary alcohol - n-alkane system. The liquid-solid and liquid-vapor phase equilibrium in binary systems H H  173,  melt melt 1.36%. (4) containing secondary alcohol was calculated using the PCEAS software [13; 14]. The mean square deviation σX and the mean relative error δX of the value X are calculated using the formulas: m ( X exp  X mod )2 The table presents the results of calculating the composition and temperature in eutectic and azeotropic points of binary secondary alcohol - n-alkane systems, the enthalpy of the phase change, the association parameter in the liquid phase, and experimental data on the j j  X  j 1 , m 1 composition and temperature at azeotropic points [16]. Experimental data for azeotropic mixtures [16] X X m  exp  mod   j j  allowed to establish the association parameter in the  X   mX exp  100%, vapor phase for all systems presented in the table: j 1  j    1  2  (k1 / 2)k2 / 2 . The phase equilibria, taking Where Xj is the enthalpy or temperature of phase transition; j =1, 2, ..., m; m is the number of experimental data. The predictive model for the vaporization enthalpy based on the data on the critical temperature: into account the association and solvation of the molecules, were studied at normal atmospheric pressure. Fig. 1 shows the dependence of the composition of the first component in the azeotropic point of the 2-propanol - alkane system on the number of carbon H  4MTc  (1)N  210  5010, atoms in the molecule. vap N  0.47 Fig. 2 shows the dependence of the temperature in the azeotropic point of the 2-propanol - n-alkane system H H   211,  vap vap  0.31%. (5) on the number of carbon atoms in the molecule. 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 90 80 x1az, mas 70 Taz, C 60 50 40 30 5 6 7 8 5 6 7 8 N N Fig. 1. Dependence of the composition of the first component in the azeotropic point of the 2-propanol - n-alkane system on the number of carbon atoms in the n-alkane molecule: series 1 () - experiment [16]; series 2 (Δ) - calculation. Fig. 2. Dependence of the temperature in the azeotropic point of the 2-propanol-n-alkane system on the number of carbon atoms in the n-alkane molecule: series 1 () - experiment [16]; series 2 (Δ) - calculation. 180 175 170 165 T, K 160 155 150 145 140 135 350 345 340 T, K 335 330 325 320 315 310 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 x1, y1, mas x1, y1, mas Fig. 3a. Liquid-solid phase diagram of 2-propanol* - pentane system. Fig. 3b. Liquid-vapor phase diagram of 2-propanol* - pentane system. Fig. 3-4 shows liquid-solid and liquid-vapor phase diagrams of the system 2-propanol* - pentane and 2-propanol* - hexane at a pressure of 0.101 MPa. Fig. 5 shows liquid-solid and liquid-vapor phase diagrams of the 2-butanol* - hexane system at a pressure of 0.101 MPa. Fig. 6 shows liquid-solid and liquid-vapor phase diagrams of the 2-pentanol* - hexane system at a pressure of 0.101 MPa. Fig. 7 shows liquid-solid and liquid-vapor phase diagrams of the 2-octanol* - octane system at a pressure of 0.101 MPa. 180 178 176 174 T, K 172 170 168 166 164 162 160 354 352 350 T, K 348 346 344 342 340 338 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 x1, y1, mas x1, y1, mas Fig. 4a. Liquid-solid phase diagram of 2-propanol* - hexane system. Fig. 4b. Liquid-vapor phase diagram of 2-propanol* - hexane system 176 174 172 170 168 T, K 166 164 162 160 158 156 154 370 365 T, K 360 355 350 345 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 x1, y1, mas x1, y1, mas Fig. 5a. Liquid-solid phase diagram of 2-butanol*- hexane system. Fig. 5b. Liquid-vapor phase diagram of 2-butanol* - hexane system. 176 174 172 T, K 170 168 166 164 162 390 385 380 375 T, K 370 365 360 355 350 345 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 x1, y1, mas x1, mas Fig. 6a. Liquid-solid phase diagram of 2-pentanol* - hexane system. Fig. 6b. Liquid-vapor phase diagram of 2-pentanol* - hexane system 232 230 228 226 T, K 224 222 220 218 216 214 212 232 230 228 226 T, K 224 222 220 218 216 214 212 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 x1, y1, mas x1, y1, mas Fig. 7a. Liquid-solid phase diagram of 2-octanol* - octane system. Fig. 7b. Liquid-vapor phase diagram of 2-octanol* - octane system CONCLUSIONS AND RECOMMENDATIONS Models for the enthalpy of melting and enthalpy of evaporation, as well as the melting point and boiling point, make it possible to calculate the phase diagrams of systems based on secondary alcohols in the absence of information or the presence of incomplete data on the properties of pure components. The results of the calculation given in the table are in agreement with the experimental data [16]. The minimum eutectic temperature (formation of the solid phase) of the mixtures of secondary alcohols and alkanes studied is -62.4оC (2-octanol - octane), which is below the cold filter plugging point for cold (-20оC) and arctic (-38оC) climate. Thus, there is no obstacle to the creation of fuel compositions in order to control the flammability of diesel fuels and other purposes.
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