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Liquid—liquid equilibrium in methanol + gasoline blends
Fluid Phase Equilibria, 32 (1986) 27-47 Elsevier Science Publishers B.V., Amsterdam
LIQUID-LIQUID EQUILIBRIUM GASOLINE BLENDS VLASTIMIL
RU?ICKA,
Jr., RENATA
27 -
Printed
in The Netherlands
IN METHANOL
FR?DOVA
+
and JAROMIR
NOVAK
Departments of Physical Chemistry, and Petroleum Technology and Petrochemistry, Institute of Chemical Technology, 166 28 Prague 6 (Czechoslovakia) (Received
March 24, 1986; accepted
in final form June 16, 1986)
ABSTRACT RhiiEka Jr., V., Fjdovi, R. and Novak, J., 1986. Liquid-liquid gasoline blends. Fluid Phase Equilibria, 32: 27-47.
equilibrium
in methanol+
An investigation has been carried out to study the limited miscibility of methanol and gasoline blends over the temperature range - 20 to 20 o C. Two liquid phases in equilibrium were analysed by mass spectrometric methods and their composition reported, in addition to the methanol content, in terms of six principal classes of hydrocarbons. Liquid-liquid equilibrium was predicted using the UNIFAC group contribution model. In liquid-liquid equilibrium calculations, gasoline was represented by a set of model compounds. The number of different groups that comprise each model molecule was determined using the results of a distillation analysis and the paraffin-naphthene-aromatic composition. Estimation of conjugate phase composition using the UNIFAC model is reasonable at temperatures above 0°C. To describe correctly the limited miscibility of methanol+ gasoline blends over the whole temperature range studied, we found that ‘specific’ UNIFAC interaction parameters were necessary.
INTRODUCTION Pure alcohols, especially methanol and ethanol, and their blends with petroleum based fuels are receiving serious consideration as alternative fuels for both spark ignition and compression ignition engines (Jawetz, 1979; McCallum et al., 1982; Rajan, 1984). The simplest way of utilising alcohols in spark ignition engines is to blend up to 20 ~01% of alcohol with gasoline. The use of gasohol (a mixture of about 10 ~01% of methanol or ethanol and 90 ~01% of gasoline) has achieved widespread acceptance in some countries, e.g., Brazil (Carvalho et al., 1979). Methanol is probably the most attractive alternative vehicle fuel as it can be produced economically from coal or biomass using thermochemical conversion processes (Moreira and Antal, 1979). The use of pure methanol and its blends with gasoline is subject to investigation (Bardon, 1979; Amadi and Graham, 1983).
0378-3812/86/$03.50
0 1986 Elsevier
Science Publishers
B.V.
28
The main disadvantage in the utilization of alcohol + gasoline blends as vehicle fuel is the separation of the blend into two liquid phases at low temperatures. The miscibility characteristics of alcohol + gasoline blends is influenced strongly by the presence of water. The relative paraffinic, naphthenic and aromatic content of the gasoline and the structure and the concentration of the alcohol have a marked effect on the miscibility and on the water tolerance of blends. The miscibility of gasoline with alcohols increases with an increasing number of carbon atoms in the alcohol. This work has been concerned with obtaining experimental data on liquid-liquid equilibrium (LLE) in the methanol + gasoline blends and with the evaluation of different procedures for the prediction of LLE for these blends. Experimental data on the mutual solubility of methanol and saturated hydrocarbons are given by Sorensen and Arlt (1979). They include mixtures of methanol and paraffins (pentane, hexane, heptane, octane, nonane, decane, tetradecane, 2,2_dimethylbutane, 2,3-dimethylbutane, 3-methylpentane, 2,2,4_trimethylpentane, 2,2,5_trimethylhexane, 3-methylheptane) and of methanol and naphthenes (cyclopentane, methylcyclopentane, cyclohexane, methylcyclohexane). Sorensen and Arlt present only one ternary and one quaternary data set on systems containing methanol and a mixture of paraffinic, naphthenic and aromatic hydrocarbons similar to those contained in gasoline. The methanol + n-heptane + benzene system was studied at the temperatures 6.8, 13.8 and 32.8”C. LLE in the methanol + n-hexane + nheptane + benzene system was measured at 32.8”C. KubiCek (1984) measured LLE in the ternary system methanol + n-heptane + toluene at the temperatures -19.2, -0.1, 15.0 and 31.O”C. LLE in the above mentioned mixtures of well-defined compounds was most frequently correlated by using a suitable GE model (e.g., UNIQUAC, NRTL). The main difficulty in modelling LLE in systems that contain a fossil fluid, e.g., a petroleum fraction, is the proper representation of the complex multicomponent mixture. The conventional approach is based on representation of the complex fossil fluid by a few well-defined key compounds. Leeper and Wankat (1982) reported the use of heptane, toluene and a mixture of heptane and toluene to represent gasoline in an investigation of the extraction of ethanol from water using gasoline as solvent. In the study of the extraction of aromatics from a multicomponent hydrocarbon mixture, Mukhopadhyay and Dongaonkar (1983) approximated a multicomponent mixture by a paraffinic or a naphthenic and aromatic compound. Rahman et al. (1984), in an investigation of the solvent extraction of aromatics from medium petroleum fractions, considered the multicomponent hydrocarbon mixture to be composed of 72 (detailed model) and of 12 (simplified ‘model) components. The representative compounds were selected on the basis of mass spectrometric analysis which identified the principal classes of hydro-
29
carbons making up the parent mixture and their apparent carbon number. Very frequently, the choice of representative compounds is somewhat arbitrary. However, representative compounds should be selected preferably on the basis of some inspection property, e.g., distillation curve and paraffin-naphthene-aromatic (PNA) content (Ruiic’ka et al., 1983a) or mass spectrometric analysis (Rahman et al., 1984). To keep the computer time and memory requirements reasonably low, the number of representative compounds should be kept as low as possible. REiCka et al. (1983a) proposed a method for describing distillable fossil fuels in terms of model compounds determined by the application of the group-contribution concept. This method assumes that the fossil fuel fraction is composed of three representative compounds, viz. a paraffin, a naphthene and an aromatic. The number of different groups that each model molecule is made of is determined using the true boiling point distillation curve and PNA analysis. EXPERIMENTAL
Materials
Methanol (Lachema Brno, Czechoslovakia) was purified by the Grignard reaction and distilled in a packed column. It contained 6 x 10e3 wt.% of water as determined by the Fischer titration method. Gasoline was an atmospheric distillation cut which was dried by annealed calcium chloride. The amount of water in gasoline was 7 X lop3 wt%. Gasoline was thoroughly characterised using several experimental techniques. Firstly, two distillation assays, viz. the ASTM D86 and the ASTM D2892-73 true boiling point (TBP) distillation procedure were performed. During the ASTM D2892-73 procedure, we distilled the sample of gasoline at a constant rate of 5 cm3 s-l and divided it into 19 fractions of an approximately equal volume. Further, the density, viscosity and molar mass of gasoline and of each of its TBP fractions were measured. Molar mass was determined by the cryoscopic method. The ASTM D86 distillation curve and the TBP data are presented in Tables 1 and 2, respectively. Gasoline
TABLE 1 ASTM D86 distillation curve of gasoline Volume % distilled off Temperature (“C) Volume % distilled off Temperature ( ’ C)
0 30 55 99
5 46 60 100
10 56 65 110
15 59 70 115
20 70 75 119
25 73 80 125
30 81 85 125
35 85 90 137
40 86 94 148
45 93 100 151
50 97
30 TABLE 2 TBP distillation curve and properties of gasoline and of its fractions Fraction number
Volume % distilled off
IBP 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Gasoline
5 10 15 20 25 30 35 40 45 50 55 60 67.8 75 77.5 89.3 97 97.8 99.5
wt. % distilled off
TBP temperature (O C) 24.6 35.8 50.8 68.2 74.4 81.0 87.0 93.6 96.2 102.8 108.2 111.8 116.6 124.2 132.5 134.6 145.1 158.0 161.2
4.5 8.9 13.4 18.0 22.6 27.4 32.2 37.1 42.0 47.0 52.0 57.1 62.5 70.0 72.6 86.3 96.0 96.9 98.8
Density at 25°C (g cmm3)
Viscosity at 25°C (mPa s)
Molar mass (g mol- ‘)
0.6474 0.6437 0.6514 0.6730 0.6845 0.6872 0.6966 0.7028 0.7094 0.7153 0.7202 0.7246 0.7303 0.7356 0.7396 0.7416 0.7535 0.7600 0.7785 0.7018
0.250 0.253 0.266 0.311 0.337 0.347 0.374 0.395 0.417 0.435 0.456 0.474 0.498 0.528 0.549 0.565 0.660 0.692 1.040 0.397
15.3 76.8 19.3 85.2 92.7 89.2 93.9 92.7 97.1 91.2 98.9 105.0 104.9 110.3 111.9 115.4 119.6 129.0 130.4 96.4
was also analysed by mass spectrometry to determine the concentration of principal types of hydrocarbons. Using the ASTM D2789-71 (Anon, 1975) procedure, we calculated the concentration of the following compound types and their apparent carbon number: paraffins, monocycloparaffins, dicycloparaffins, alkylbenzenes, indans and tetralins, naphthalenes. Results of the mass spectrometric analysis are given in Table 3
TABLE 3 Concentration of principal types of hydrocarbons in gasoline
Cont. (mol%) Apparent carbon number
Paraffins
Monocycleparaffins
Dicycloparaffins
Alkylbenzenes
Indans tetralins
Naphthalenes
64.6
26.2
1.4
6.8
0.5
0.5
7.8
7.8
7.8
8.0
8.0
8.0
57.5
46.5 56.4 81.1
64.2 70.4 75.3
52.5 65.6 77.5
10
0 0 0
- 10 - 10 -10
-20 -20 -20
57.5 60.4 58.9
56.8 57.8 56.9
51.6 52.0 54.4
49.1
57.7 58.5 59.4
57.1 57.7 58.2
54.8 55.3 57.6
53.9
25.6 24.7 25.0
26.0 26.9 26.2
25.3 27.6 26.7
25.0
21.6 22.2 22.3 23.3
24.2 24.1 23.9
23.5 23.4 23.3
23.1 23.0 22.4
22.4
21.8 21.8 21.8 21.7
Pred.
7.8 7.3 8.6
6.4 4.7 6.7
8.7 7.2 5.5
7.6
6.9 4.0 5.4 4.8
5.4 4.6 3.8
4.6 4.1 3.7
5.5 5.0 3.1
4.9
5.7 5.5 4.9 4.1
Pred.
9.1 7.6 7.4
10.8 10.5 10.2
14.5 13.2 13.2
18.5
20.9 20.5 22.4 20.5
Exp.
12.8 12.8 12.9
14.7 14.8 14.8
16.6 16.7 16.9
18.8
21.0 21.0 21.1 21.2
Pred.
1.1 0.7 0.7
2.8 2.3 1.9
2.8 2.5 2.4
3.6
4.8 5.5 5.5 5.4
Exp.
2.9 3.0 3.0
3.2 3.2 3.2
3.4 3.4 3.5
3.7
4.0 4.0 4.0 4.0
Pred.
Exp.
51.5 51.7 52.2 53.0
Pred. ’
Exp.
50.5 53.3 49.9 51.4
Exp.
0.4 0.3 0.2
0.7 0.6 0.3
0.8 0.7 0.7
1.1
1.6 1.9 1.8 1.9
Exp.
1.8 1.8 1.7
1.9 1.9 1.8
2.0 2.0 1.9
2.1
2.3 2.3 2.3 2.2
0.4 0.3 0.1
0.6 0.3 0.2
0.8 0.4 0.4
0.6
1.0 1.1 1.0 0.9
Exp.
2.0 1.7 1.2
1.7 1.6 1.4
2.1 1.9 1.2
1.9
2.4 2.2 2.0 1.7
Pred.
Aromat.
98.2 98.7 99.0
95.9 96.9 97.6
95.5 96.3 96.4
94.6
92.7 91.5 91.7 91.8
Exp.
93.3 93.6 94.1
93.2 93.4 93.6
92.4 92.6 93.5
92.2
91.3 91.5 91.7 92.1
Pred.
Methanol
determined by the minimization
Pred.
Naphth.
Paraff.
Aromat.
Naphth.
Paraff. Methanol
Methanol-rich phase
Methanol-lean phase
a Owing to the round-off error, the percentage may not add to exactly 100.0. b Composition of bulk solution. ’ Composition predicted by the UNIFAC model, gasoline represented by three model compounds procedure A.
35.2 43.0 54.9 68.0
20 20 20 20
Bulk soln. mol% MeOH b
Comparison of experimental and predicted conjugate phase compositions in methanol + gasoline blends (composition in mol%) ’
TABLE 4
32
Procedure
Liquid-liquid equilibrium in methanol + gasoline blends was determined by the analytical method. The apparatus used was a modification of that described by Fabries et al. (1977). Briefly, all experiments were carried out in a glass cell provided with a magnetic stirrer and surrounded by a thermostatted jacket. To prevent condensation of air moisture on the outer surface of the vessel at subzero temperatures, we surrounded the thermostatted jacket by another evacuated jacket. LLE experiments were conducted in the temperature range - 20 to 20°C. Temperature was controlled to within 0.03” C at 20 oC and 0.05 oC at - 20 oC. Equilibrium phases were analysed by mass spectrometric methods as it enabled both the concentration of methanol and the composition of the hydrocarbon mixture, expressed in terms of six principal classes of hydrocarbons, to be determined. The gas chromatograph mass spectrometer LKB 9000, equipped with a heated inlet system kept at a temperature of 200°C was used. The amount of sample injected was 1 ~1. Ions were produced by electron impact with energies of 70 eV in the ion source which maintained a temperature of 250 oC and a pressure of lop4 Pa. The concentration of methanol in the equilibrium phases was determined by an internal standard method. A weighed amount of dioxane was added to a precisely known amount of the sample and the relative amount of methanol and dioxane in the mixture with gasoline was found from the calibration curve. The calibration curve gives the ratio of the intensity of ions m/t 32 (methanol) and m/z 88 (dioxane) versus the weight fraction of methanol in the binary mixture of methanol and dioxane. Methanol concentration measurement was reproducible to within +2-3 mol%. Reproducibility of measurements of the hydrocarbon type concentration is somewhat lower as noted in Anon (1975). Experimental conjugate phase compositions in methanol + gasoline blends are presented as a part of Table 4 at temperatures of 20, 10, 0, - 10 and - 20 oC. Composition of the bulk solution is also given here,
RESULTS
Representation of gasoline by model compounds
To incorporate gasoline in any standard computational procedure for calculation of LLE, we found it necessary to represent this multicomponent petroleum fraction, which contained around 100 different chemical sub-
33
stances, by a few properly averaged model molecules. Group contribution methods present a versatile and effective tool for predicting phase equilibrium in systems containing components for which no experimental data are available. The application of group contribution methods can even be extended to artificial compounds that do not exist in nature but have a defined molecular structure. If the structures of the artifical model compounds are determined on the basis of some inspection data (e.g., TBP distillation curve, molar mass distribution, paraffin-naphthene-aromatic content) these compounds can represent a complex fossil fluid. To estimate the structure of model compounds, we resolve the fossil fluid into a number of fractions. Each fraction may be represented by one or more model compounds depending on the number of different families of chemical species present in the mixture. The structure of model compounds is defined in terms of groups. The number of groups in each model molecule is then determined by the adjustment procedure that entails the minimisation of the selected objective function. The equality of the mid-volume boiling point of the fraction and of the bubble point of the mixture of model compounds was the minimisation criterion selected by RBZi&a et al. (1983a). Therefore, the group contribution models for predicting pure component vapour pressures and liquid phase activity coefficients were required. UNIFAC models (Fredenslund et al., 1977; Jensen et al., 1981) were used. As the characterisation method was developed for application to petroleum fluids that contain three main families of hydrocarbons, each fraction was represented by three model compounds, viz. a paraffinic, a naphthenic and an aromatic one. The composition of the model mixture was determined on the basis of the PNA analysis. The adjustment procedure, which results in the determination of the number of groups in each model molecule, was repeated for all fractions in which the original complex fluid was resolved. Thus, the complex fluid was represented by a mixture of model compounds; each model compound was characterised by a definite number of groups. In contrast to real molecules, the model molecules were often described by noninteger values of group numbers. The crucial point of the adjustment procedure was the definition of the structure of model molecules. Some group assignments gave unrealistic results, especially in the ratio of pure component vapour pressures of model compounds. To improve the representation of the parent multicomponent fluid by model compounds, in particular to ensure a better distribution of model compounds along the TBP distillation curve, we slightly modified the characterisation procedure previously developed (RtiZiEka et al., 1983). The number of different groups that comprise model molecules was determined
34
so as to fulfil the following three criteria P-
6
XiP,O(T,
g;(n,
a,
p))
=o
0) (4
x,PP(T, g&b
a7
P>)-x&yT
g,h
a7
PI) =o
(3)
where P designates measured pressure of the TBP distillation curve, Pi0 is the pure component vapour pressure of i-th model compound, xi is the mole fraction, T is temperature and gi is the group assignment to the i-th model compound. Equation (1) requires that the mid-volume boiling point of the fraction and the bubble point of the mixture of model compounds are equal. Equations (2) and (3) are the artificial constraints that ensure the pure component vapour pressures of model compounds do not differ substantially. The activity coefficients in eqn. (1) were omitted. This is a reasonable assumption as gasoline is a mixture of hydrocarbons only. n, (Yand /3 are three adjustable parameters; n is proportional to the number of aliphatic hydrocarbon groups in the paraffinic model molecule and in the alkyl chain attached to the naphthenic’ or aromatic ring; (Yand p are approximately proportional to the length of the alkyl substituent attached to the naphthenic and aromatic ring, respectively. The meaning of these parameters can be seen from Fig. 1. The computational procedure already described is referred to as minimisation procedure A. Pure component vapour pressures were calculated by the AMP group contribution method (Macknick and Prausnitz, 1979). Parameters were taken from Macknick and Prausnitz (1979) and from RiiZiEka (1983b). The AMP group contribution method is described in Appendix A. The structure of the model molecules issues from a rough estimate of the number of various kinds of groups usually present in a hydrocarbon molecule. The estimates reached by Smith et al. (1976) are: (1) alkyl carbons were assumed to be 10% methyl, 80% methylene and 10% tertiary carbons; (2) naphthenic carbons were assumed to be 80% methylene-type and 20% tertiary-type carbons; (3) aromatic carbons were assumed to be 70% single-ring carbons with attached hydrogen, 20% single-ring carbons attached to an alkyl carbon and 10% of fused-ring carbons. As gasoline is a low-boiling fraction, the fused-ring aromatic carbons were not taken into consideration. From several test calculations it was concluded that both the UNIFAC and the AMP group contribution models gave meaningful predictions of the pure component vapour pressure of artificial
35 paraffin
naphthene
0 aromatic
Fig. 1. Structure of model molecules.
model molecules and only if the number of terminal methyl groups is equal to two in the paraffinic molecules. In naphthenic and aromatic molecules the number of terminal methyl groups must equal the number of ring groups to which an alkyl chain is attached. On the basis of these observations the structure of model molecules depicted in Fig. 1 was proposed. Another modification of the computational procedure that results in the determination of the adjustable parameters n, cu and /3 was examined. Products of pure component vapour pressure and mole fraction in eqns. (2) and (3) were replaced by pure component vapour pressure, i.e., the equality of pure component vapour pressures of model compounds was required. This computational procedure is referred to as minimisation procedure B. A comparison of both procedures, which give slightly different sets of model compounds with regard to the prediction accuracy of LLE in the methanol + gasoline blends, is presented below.
0.3189 0.4076 0.5565 0.6520 0.7684
107.5
39.5 67.6 95.2 119.8 147.8
1
1 2 3 4 5
a Minimisation
0.6354
T,,
(“C)
Fraction no.
procedure
A-eqns.
0 0 -0.5998 - 0.3980 -0.2200 minimisation
5.2 6.1 7.6 8.5 9.7
- 0.5292
(l)-(3),
0 0 - 0.0305 0.1162 0.2456
8.4
B
procedure
0 0 7.0 8.0 9.1
7.1
Naph.
No. of C atoms
0.3172 0.4075 0.4983 0.5880 0.6955
0 0 5.1 5.8 6.7
0.0196
5.2 6.1 7.0 7.9 9.0 vapour
7.4
-0.2875 0 0 - 0.4040 - 0.1862 0.0037
Paraf. P
0 0 6.4 7.3 8.4
6.8
Arom.
pressures.
0 0 6.9 7.8 8.9
7.3
Naph.
No. of C atoms
shown in Fig. 1
of pure component
0 0 - 0.0677 0.0963 0.2392
a
(1) and equality
0.5446
5.0
B-eqn.
n
B a
of model molecules
Minim. procedure
Structure
Arom.
of groups in model molecules.
Paraf.
Aa
numbers
- 0.0230
Minim. procedure n a
n, a and p that define
5
Parameters
TABLE
37
Two types of gasoline representation that differ in the number of model compounds were examined. Firstly, gasoline was assumed to be characterised by a single boiling point. This means that gasoline was considered to be composed of three model compounds which represent three main families of chemical species comprising all light petroleum fractions; a paraffin, a naphthene and an aromatic. Secondly, gasoline was resolved into five fractions. The break&down of the TBP distillation curve into fractions was carried out using an algorithm that ensures an approximately constant increase of the boiling point of successive fractions. The boiling point of the first and of the second fraction was lower than that of the lightest physically meaningful naphthenic and aromatic model compound. Physically meaningful in this context means that the model molecule should not be built of a negative number of groups. Thus, the first two fractions were represented by a single paraffinic molecule. Each of the last three fractions was composed of a mixture of three model compounds; a paraffin, a naphthene and an aromatic. The paraffin-naphthene-aromatic composition of the last three fractions was adjusted so as to match the PNA composition of the parent mixture. In this type of representation gasoline was composed of 11 model compounds. Mid-boiling points of gasoline and of its fractions and the calculated parameters n, (Yand p are given in Table 5. The upper part of Table 5 presents calculated parameters when gasoline is assumed to be composed of three model compounds. The lower part pertains to the resolution of gasoline into five fractions. Prediction of conjugate phase composition
Composition of conjugate phases in the methanol + gasoline blends was predicted using the UNIFAC model (Fredenslund et al., 1977) for the calculation of liquid phase activity coefficients. The interaction parameters were taken from the LLE parameter table developed by Magnussen et al. (1981). The UNIFAC model is described in Appendix B. To calculate LLE the algorithm suggested by Novak et al. (1982) was used. It is based on solving the isoactivity criterion by the Newton-Raphson method. Application of the Newton-Raphson method requires that a good initial estimate of conjugate phase composition is provided. The initial estimates were obtained from the mutual solubility of a paraffinic model compound and methanol (Novak et al., 1986). A detailed comparison of experimental compositions of conjugate phases and of compositions predicted by applying different procedures of gasoline representation is summarised in Table 6 for a temperature of 20 oC. Calculated compositions of model compounds belonging to the three main fami-
6
A3 All B3 Bll
A3 All B3 Bll
A3 All B3 Bll
43.0
54.9
68.0
51.4
49.9
53.3
50.5
53.0 51.7 51.8 51.1
52.2 51.4 51.3 50.9
51.7 51.2 51.0 50.8
51.5 51.1 50.8 50.7
23.3
22.3
22.2
21.6
Exp.
Exp.
Pred. d
Naphth.
phase
Paraff.
Methanol-lean
and predicted
21.7 22.6 21.9 22.5
21.8 22.3 21.8 22.1
21.8 22.0 21.7 21.9
21.8 21.9 21.7 21.8
Pred.
4.8
5.4
4.0
6.9
Exp.
4.1 4.5 4.7 4.9
4.9 5.2 5.3 5.5
5.5 5.7 5.7 5.8
5.7 5.9 5.9 5.9
20.5
22.4
20.5
20.9
Exp.
21.2 21.2 21.7 21.5
21.1 21.2 21.6 21.5
21.0 21.1 21.6 21.5
21.0 21.1 21.6 21.5
5.4
5.5
5.5
4.8
Exp.
Paraff.
4.0 5.6 5.4 6.2
4.0 5.7 5.4 6.3
4.0 5.8 5.4 6.4
4.0 5.8 5.4 6.4
Pred.
Methanol-rich
in methanol+gasoline
Pred.
Methanol
compositions
Pred.
phase
Aromat.
conjugate phase
1.9
1.8
1.9
1.6
2.2 2.2 2.4 2.4
2.3 2.2 2.4 2.4
2.3 2.2 2.4 2.4
2.3 2.2 2.4 2.4
procedure,
Exp.
1.7 1.5 1.4 1.4
2.0 1.8 1.6 1.5
2.2 2.0 1.7 1.6
2.4 2.2 1.8 1.7
Pred.
91.8
91.7
91.5
92.7
Exp.
3 or 11 designates
0.9
1.0
1.1
1.0
Exp.
number
92.1 90.7 90.8 90.0
91.7 90.3 90.6 89.8
91.5 90.1 90.5 89.6
91.3 90.0 90.5 89.5
Pred.
of
in mol%) a
Methanol
(composition
Aromat.
at 20°C
Pred.
blends
Naphth.
a Owing to the round-off error, the percentage may not add to exactly 100.0. b Composition of bulk solution. ’ Minimisation procedure used to determine the set of model compounds (letter A or B designates model compounds that represent gasoline). d Composition predicted by the UNIFAC model.
A3 All B3 Bll
Procedure ’
of experimental
35.2
Bulk soln. mol% MeOH b
Comparison
TABLE
39
lies of hydrocarbons were added up separately in each family to give the total amount of paraffins, naphthenes and aromatics. A similar summation was also performed for principal classes of hydrocarbons determined by mass spectrometry. Table 6 also reports the bulk composition of the heterogeneous mixture. Different procedures of gasoline representation include the set of model molecules determined either by the minimisation procedure A (eqns. (l)-(3)) or by the minimisation procedure B. In both minimisation procedures gasoline was considered to be composed of three or 11 model compounds. It is apparent from Table 6 that the minimisation procedure, which results in the determination of model compounds, does not significantly influence the predicted conjugate phase composition. The results also indicate that the representation of gasoline by 11 model compounds is not superior to the very simplified representation by three model compounds. The same conclusions were reached for other temperatures. Thus, the experimental conjugate phase compositions determined over the whole temperature range studied are compared with predictions stemming from the minimisation procedure A and from the most simplified representation of gasoline by three model compounds. The comparison is summarized in Table 4. Based upon these data, the following general comments can be made: (1) the solubility of methanol and gasoline decreases with decreasing temperature; (2) methanol content in the methanol-lean phase gives the highest possible amount of methanol that can be added to gasoline. Exceeding this limit results in a split of the blend into two liquid phases. At 20°C up to 21 mol% of methanol can be blended with the gasoline investigated, whereas at - 20 oC only up to about 7 mol% of methanol can be added. (3) Prediction of conjugate phase composition by the UNIFAC model is at and above O°C in reasonable agreement with the experimental data. The prediction accuracy deteriorates with decreasing temperature and reaches the maximum difference of about 6 mol% in the methanol-lean phase at -20°C. This is, however, not surprising as the UNIFAC LLE parameter table was developed for the prediction of LLE at temperatures between 10 and 40 ‘C. To describe LLE properly in the methanol + gasoline system, we propose the estimation of the so-called ‘specific’ UNIFAC parameters that pertain to a limited set of carefully chosen substances comprising the systems of interest. It was demonstrated by Mukbopadhyay and Dongaonkar (1983) that by using specific UNIFAC parameters, prediction of LLE in multicomponent aromatics extraction systems can be improved. Gasoline and any other petroleum fraction is in principle composed of the three main families of hydrocarbons, viz. paraffins, naphthenes and
40
aromatics. Aromatic hydrocarbons containing from six to eight carbon atoms are completely miscible with methanol at temperatures above approximately -30°C. On the contrary, C, to C, paraffins and naphthenes exhibit a limited miscibility with methanol, the upper critical solution temperature varying roughly in the range 25 to 70 “C. Thus, the miscibility of methanol + gasoline blends is primarily determined by the content of aromatics in the base gasoline. The increasing concentration of aromatics in gasoline improves the miscibility characteristics. Gasoline used in this study contained approximately 8 mol% of aromatics. The region of complete miscibility will be larger for blends of methanol and reformates or other highly aromatic gasolines. The conventional approach to modelling LLE in systems containing a fossil fuel fraction is based upon representation of a multicomponent mixture by a few well-defined key compounds. This procedure was also examined in this work. The GE model used for the calculation of liquid phase activity coefficients was selected with regard to the availability of the interaction parameters. Sorensen and Arlt (1979) estimated ‘common’ UNIQUAC parameters for a large group of compounds. These UNIQUAC parameters are applicable in the temperature range 20 to 30 oC. From the group of hydrocarbons for which the ‘common’ UNIQUAC parameters are available, the following compounds were chosen to represent gasoline: n-heptane, cyclohexane and toluene. This choice is in approximate accordance with the number of carbon atoms in the three model molecules that were estimated by the previously described minimisation procedure A (see Table 5). Concentration of each key compound in the ternary mixture n-heptane + cyclohexane + toluene corresponds to the paraffinnaphthene-aromatic content of the base gasoline. Composition of conjugate phases in the mixture methanol + n-heptane + cyclohexane + toluene calculated by the UNIQUAC model is compared with experimental data and with predictions based upon representation of gasoline by three model compounds in Table 7 at a temperature of 20 oC. The results reported here indicate that in modelling LLE in methanol + gasoline blends the representation of gasoline by model compounds combined with the UNIFAC model is apparently superior to the application of the UNIQUAC model. This conclusion is further confirmed when inspecting the ternary diagram of the mixture n-heptane + toluene + methanol at 15 oC depicted in Fig. 2. The experimental tie lines measured by Kubic’ek (1984) are, in the region of interest of this study, more accurately described by the UNIFAC model than by the UNIQUAC model with ‘common’ interaction parameters. Composition of gasoline used in this work is expressed in terms of aromatics and non-aromatics and depicted by the full circle on the binary line n-heptane + toluene. From Fig. 2 it is evident that as soon as the
53.3
49.9
51.4
43.0
54.9
68.0
54.7 53.0
54.8 52.2
55.0 51.7
55.0 51.5
23.3
22.3
22.2
21.6
23.5 21.7
23.5 21.8
23.5 21.8
23.5 21.8
UNIQ UNIF
4.8
5.4
4.0
6.9
Exp.
Aromat.
7.0 4.1
6.8 4.9
6.7 5.5
6.7 5.7
UNIQ UNIF
20.5
22.4
20.5
20.9
Exp.
14.9 21.2
14.8 21.1
14.8 21.0
14.8 21.0
UNIQ UNIF
Methanol
5.4
5.5
5.5
4.8
Exp.
Paraff.
7.3 4.0
7.3 4.0
7.3 4.0
7.3 4.0
UNIQ UNIF
Methanol-rich
1.9
1.8
1.9
1.6
Exp.
Naphth.
phase
3.1 2.2
3.1 2.3
3.1 2.3
3.1 2.3
UNIQ UNIF
0.9
1.0
1.1
1.0
Exp.
Aromat.
0.7 1.7
0.7 2.0
0.7 2.2
0.7 2.4
UNIQ UNIF
91.8
91.7
91.5
92.7
Exp.
with
89.0 92.1
89.0 91.7
89.0 91.5
89.0 91.3
UNIQ UNIF
Methanol
+ toluene + methanol
a Owing to the round-off error, the percentage may not add to exactly 100.0. b Composition of bulk solution. ’ Predicted composition in n-heptane + cyclohexane + toluene + methanol mixture using ‘common’ UNIQUAC parameters. d Composition predicted by the UNIFAC model, gasoline represented by three model compounds determined by the minimization procedure A.
50.5
Exp.
Exp.
UNIQ ’ UNIF d
Naphth.
phase
Paraff.
Methanol-lean
of predicted conjugate phase compositions in the quatemary mixture n-heptane + cyclohexane and predicted data in methanol + gasoline blends at 20 “C (composition in mol%) a
35.2
B”Ik soln. mol% MeOH b
Comparison experimental
TABLE 7
c
42
,
3 Fig. 2. Experimental tie lines (KubiEek, 1984) and predicted binodal curves in the mixture 0 experimental tie lines; n-heptane (l)+ toluene (2)+methanol (3) at 15°C. O- - - binodal curve predicted by UNIQUAC using ‘common’ parameters; .-. -. binodal curve predicted by UNIFAC using LLE parameters.
content of aromatics in the gasoline is increased, both the UNIFAC and the UNIQUAC binodal curves will tend to deviate from the experimental tie lines. This clearly demonstrates the necessity to estimate and use ‘specific’ interaction parameters to correctly describe the limited miscibility of methanol + gasoline blends.
CONCLUSIONS
The experimental study of the limited miscibility of low aromatic gasoline and methanol blends have shown that a large miscibility gap exists already at temperatures above 0°C. The miscibility gap is further enlarged at subzero temperatures. At - 20 oC only up to about 7 mol% of methanol can be mixed with gasoline containing around 8 mol% of aromatics without being split into two liquid phases. The influence of the content of water on the miscibility characteristics of
43
methanol + gasoline blends has not been investigated. Both methanol and gasoline were carefully dried; they contained less than 0.01 wt.% of water. Two different procedures for the representation of gasoline and two different GE models have been examined with particular reference to the prediction of the conjugate phase composition in methanol + gasoline blends. At temperatures above 0 oC the UNIFAC group contribution model applied to a mixture of a few model compounds estimated on the basis of a distillation analysis and a paraffin-naphthene-aromatic content has been found to be superior to the UNIQUAC model applied to a mixture of well-defined key compounds. The disagreement between experimental data and compositions predicted by the UNIFAC method at subzero temperatures is due to the extrapolation problems of the GE model. A better description of the liquid-liquid equilibrium in methanol + gasoline blends over the whole temperature range may be expected when ‘specific’ interaction parameters become available. ACKNOWLEDGEMENTS
The authors thank Ing. J. Koudelka for his assistance in carrying out the TBP distillation analysis, N. SaSkovS for her aid in the measurement of densities and viscosities of gasoline and Dr. S. Kern&y for his helpful comments. APPENDIX
A
The pure component vapour pressures Pp are calculated by the AMP group contribution method (Ma&nick and Prausnitz, 1979) In Py=A+B/T+ClnT+DT+ET*
(Al)
Constants A, B, C, D and E are functions of three adjustable parameters VW,s and E, A = ln(R/I$) B=
+ (s - 0.5) ln( E,/R)
-E,/R
- ln[(s - l)!] - 2.337
(A2) (fw
C=lS-s
(A4
D = (s - I)/(WR)
(A5)
E = (s - 3)(s - I)/[2(W)*]
646)
44
where E,/R and T are in Kelvin, Pi0 is in atmospheres and the gas constant R = 82.06 cm3 atm (kmol))‘. The function (3 - l)! found in eqn. (A2) is the I? function. Parameters VW, s and E,/R can be found by summing group contributions according to
k s =
c
Vf’Sk
W)
= c z$‘)cOk/R
649)
k
E,/R
k
from the group k, vr) the where vWk, Sk and ‘cOk/R are contributions number of groups of type k in molecule i. The summation in eqns. (A7) to (A9) is over all groups.
APPENDIX
B
For activity coefficients yi the UNIFAC group contribution model is used (Fredenslund et al., 1977). The activity coefficients are given by a combinatorial and a residual term In y, = In Y,~+ In yIR The combinatorial In y,C= (In &/xi & = xirJcxjrj i summation r, = c vf)Rk k
(AlO)
part is given by + 1 - $Jxi)
- zqJ2(ln
e, = xiqi,/xjq, _i
+/0,
+ 1 - f&/0,)
(All) (AW
over all components
4, = c Vf’Qk
(A131
k
summation over all groups; R, is the volume parameter for group k; Qk is the surface area parameter for group k; vf) is the number of groups of type k in molecule i; xi is the liquid phase mole fraction of component i and z is the coordination number (z = 10). For the residual part the following equations are used In y: = C v:‘) (ln I, - In Ii’)) k
(Ala)
45
where summation is over all groups
k,
= exd -a,,/0
tn= Q2L'~
QnXn
(AW W7)
where a,,,,, is the group interaction parameter for interaction between groups m and n (umn # a,,,,,). Equations (A15) to (A18) also hold for In Pi’), except that the group composition variable, ek, is now the group fraction of group k in pure fluid i. LIST OF SYMBOLS
group interaction parameter A:%, C, 0, E constants of the AMP vapour pressure equation enthalpy of vaporisation of the hypothetical liquid at T = 0 E0 K (cm3 atm mol-‘) (1 cm3 atm mol-’ = 0.101325 J mol-‘) group assignment to i-th model compound g, adjustable parameter (eqns. (l)-(3)) n P pressure (kPa) Pf pure component vapour pressure of i-th component (atm) (1 atm = 101.325 kPa) molecular van der Waals surface area of i-th component 4i group van der Waals surface area of k-th group Qk R gas constant (cm3 atm (kmol-‘)) (R = 82.06 cm3 atm (kmol-‘) = 8.314 J (kmol-‘)) molecular van der Waals volume of i-th component r; group van der Waals volume of k-th group R, s number of equivalent oscillators per molecule contribution of group k to s ‘k T temperature (K) V wk contribution of group k to V, molecular hard-core van der Waals volume (cm3 mol- ‘) Yv mole fraction of i-th component xi a
46
Tn Z
m-th group fraction coordination number
Greek letters
Yi rk
%k +i
%I (I)
'k
adjustable parameter (eqns. (l)-(3)) adjustable parameter (eqns. (l)-(3)) activity coefficient of i-th component residual activity coefficient of group k contribution of group k to E, molecular volume fraction of i-th component parameter given by eqn. (A16) molecular surface fraction of i-th component (eqn. (A12)) m-th group surface fraction (eqn. (A17)) number of groups of type k in molecule i
Superscripts C R
combinatorial part residual part
REFERENCES Amadi, S.I. and Graham, E.E., 1983. Explosibility of high methanol fuel blends. Ind. Eng. Chem. Prod. Res. Dev., 22: 500-505. Anon, 1975. D2789-71. Standard method of test for hydrocarbon types in low olefinic gasoline by mass spectrometry. Annual book of ASTM standards, Part 24. Am. Sot. for Testing and Materials, Philadelphia. Bardon, M.F., 1979. Thermodynamic calculations for Otto cycle engines using methanol as a fuel. In: T.N. Veziroglu (Editor), Alternative Energy Sources II. Hydrocarbon Conversion. Hemisphere Publishing Corporation, Washington, Vol. 7, pp. 2887-2908. Carvalho, Jr., A.V., Rechtschaffen, E. and Goldstein Jr., L., 1979. Future of alcohol fuels program in Brasil. In: T.N. Veziro&t (Editor), Alternative Energy Sources II. Hydrocarbon Conversion. Hemisphere Publishing Corporation, Washington, Vol. 7, pp. 2867-2877. Fabries, J.F., Gustin, J.L. and Renon, H., 1977. Experimental measurements of phase equilibrium properties for systems containing n-heptane, benzene, N-methylpyrrolidone, and monoethanolamine. Representation by the NRTL equation. J. Chem. Eng. Data, 22: 303-308. Fredenslund, Aa., Gmehling, J. and Rasmussen, P., 1977. Vapor-liquid equilibria using UNIFAC. Elsevier, Amsterdam, Jawetz, P., 1979. The common sense approach in developing fuel alcohols. In: T.N. Veziroglu (Editor), Alternative Energy Sources II. Hydrocarbon Conversion. Hemisphere Publishing Corporation, Washington, Vol. 7, pp. 2861-2866. Jensen, T., Fredenslund, Aa. and Rasmussen, P., 1981. Pure-component vapor pressures using UNIFAC group contribution. Ind. Eng. Chem. Fundam., 20: 239-246.
KubiEek, V., 1984. Rovnovaha kapalina-kapalina v systemu methanol + n-heptan + toluen. MSc. Thesis, Institute of Chemical Technology, Praha. Leeper, S.A. and War&at, P.C., 1982. Gasohol production by extraction of ethanol using gasoline as solvent. Ind. Eng. Chem. Process Des. Dev., 21: 331-334. Macknick, A.B. and Prausnitz, J.M., 1979. Vapor pressures of heavy liquid hydrocarbons by a group-contribution method. Ind. Eng. Chem. Fundam., 18: 348-351. Magnussen, T., Rasmussen, P. and Fredenslund, Aa., 1981. UNIFAC parameter table for prediction of liquid-liquid equilibria. Ind. Eng. Chem. Process Des. Dev., 20: 331-339. McCallum, P.W., Timbario, T.J., Bechtold, R.L. and Ecklund, E.E., 1982. Methanol/ethanol: alcohol fuels for highway vehicles. Chem. Eng. Prog., 78: 52-59. Moreira, J.R. and Antal Jr., M.J., 1979. Gasification of biomass as a source of synfuels for developing countries. In: T.N. Veziroglu (Editor), Alternative Energy Sources II. Hydrocarbon Conversion. Hemisphere Publishing Corporation, Washington, Vol. 7, pp. 2823-2833. Mukhopadhyay, M. and Dongaonkar, K.R., 1983. Prediction of liquid-liquid equilibria in multicomponent aromatics extraction systems by use of the UNIFAC group contribution model. Ind. Eng. Chem. Process Des. Dev., 22: 521-532. Novak, J.P., Voiika, P., MatouS, J. and Rticka Jr., V., 1982. Liquid-liquid equilibrium. Calculation of equilibrium composition of coexisting phases. Collection Czech. Chem. Commun., 47: 1667-1685. Novak, J.P., MatouS, J. and Pick, J., 1986. Liquid-liquid equilibrium. Elsevier, Amsterdam, to be published. Rahman, M., Mikitenko, P. and Asselineau, L., 1984. Solvent extraction of aromatics from middle distillates. Equilibria prediction method by group contribution. Chem. Eng. Sci., 39: 1543-1558. Rajan, S., 1984. Water-ethanol-gasoline blends-physical properties, power, and pollution characteristics. J. Eng. Gas Turbines Power, 106: 841-848. Rhiicka Jr., V., Fredenslund, Aa. and Rasmussen, P., 1983a. Representation of petroleum fractions by group contribution. Ind. Eng. Chem. Process Des. Dev., 22: 49-53. RtXicka Jr., V., 1983b. Estimation of vapor pressures by a group contribution method. Ind. Eng. Chem. Fundam., 22: 266-267. Smith, G., Winnick, J., Abrams, D.S. and Prausnitz, J.M., 1976. Vapor pressures of high-boiling complex hydrocarbons. Can. J. Chem. Eng., 54: 337-343. Sorensen, J.M. and Arlt, W., 1979. Liquid-liquid equilibrium data collection. Vol. V, Parts 1, 2. Dechema Chemistry Series, Frankfurt.
LIQUID-LIQUID EQUILIBRIUM GASOLINE BLENDS VLASTIMIL
RU?ICKA,
Jr., RENATA
27 -
Printed
in The Netherlands
IN METHANOL
FR?DOVA
+
and JAROMIR
NOVAK
Departments of Physical Chemistry, and Petroleum Technology and Petrochemistry, Institute of Chemical Technology, 166 28 Prague 6 (Czechoslovakia) (Received
March 24, 1986; accepted
in final form June 16, 1986)
ABSTRACT RhiiEka Jr., V., Fjdovi, R. and Novak, J., 1986. Liquid-liquid gasoline blends. Fluid Phase Equilibria, 32: 27-47.
equilibrium
in methanol+
An investigation has been carried out to study the limited miscibility of methanol and gasoline blends over the temperature range - 20 to 20 o C. Two liquid phases in equilibrium were analysed by mass spectrometric methods and their composition reported, in addition to the methanol content, in terms of six principal classes of hydrocarbons. Liquid-liquid equilibrium was predicted using the UNIFAC group contribution model. In liquid-liquid equilibrium calculations, gasoline was represented by a set of model compounds. The number of different groups that comprise each model molecule was determined using the results of a distillation analysis and the paraffin-naphthene-aromatic composition. Estimation of conjugate phase composition using the UNIFAC model is reasonable at temperatures above 0°C. To describe correctly the limited miscibility of methanol+ gasoline blends over the whole temperature range studied, we found that ‘specific’ UNIFAC interaction parameters were necessary.
INTRODUCTION Pure alcohols, especially methanol and ethanol, and their blends with petroleum based fuels are receiving serious consideration as alternative fuels for both spark ignition and compression ignition engines (Jawetz, 1979; McCallum et al., 1982; Rajan, 1984). The simplest way of utilising alcohols in spark ignition engines is to blend up to 20 ~01% of alcohol with gasoline. The use of gasohol (a mixture of about 10 ~01% of methanol or ethanol and 90 ~01% of gasoline) has achieved widespread acceptance in some countries, e.g., Brazil (Carvalho et al., 1979). Methanol is probably the most attractive alternative vehicle fuel as it can be produced economically from coal or biomass using thermochemical conversion processes (Moreira and Antal, 1979). The use of pure methanol and its blends with gasoline is subject to investigation (Bardon, 1979; Amadi and Graham, 1983).
0378-3812/86/$03.50
0 1986 Elsevier
Science Publishers
B.V.
28
The main disadvantage in the utilization of alcohol + gasoline blends as vehicle fuel is the separation of the blend into two liquid phases at low temperatures. The miscibility characteristics of alcohol + gasoline blends is influenced strongly by the presence of water. The relative paraffinic, naphthenic and aromatic content of the gasoline and the structure and the concentration of the alcohol have a marked effect on the miscibility and on the water tolerance of blends. The miscibility of gasoline with alcohols increases with an increasing number of carbon atoms in the alcohol. This work has been concerned with obtaining experimental data on liquid-liquid equilibrium (LLE) in the methanol + gasoline blends and with the evaluation of different procedures for the prediction of LLE for these blends. Experimental data on the mutual solubility of methanol and saturated hydrocarbons are given by Sorensen and Arlt (1979). They include mixtures of methanol and paraffins (pentane, hexane, heptane, octane, nonane, decane, tetradecane, 2,2_dimethylbutane, 2,3-dimethylbutane, 3-methylpentane, 2,2,4_trimethylpentane, 2,2,5_trimethylhexane, 3-methylheptane) and of methanol and naphthenes (cyclopentane, methylcyclopentane, cyclohexane, methylcyclohexane). Sorensen and Arlt present only one ternary and one quaternary data set on systems containing methanol and a mixture of paraffinic, naphthenic and aromatic hydrocarbons similar to those contained in gasoline. The methanol + n-heptane + benzene system was studied at the temperatures 6.8, 13.8 and 32.8”C. LLE in the methanol + n-hexane + nheptane + benzene system was measured at 32.8”C. KubiCek (1984) measured LLE in the ternary system methanol + n-heptane + toluene at the temperatures -19.2, -0.1, 15.0 and 31.O”C. LLE in the above mentioned mixtures of well-defined compounds was most frequently correlated by using a suitable GE model (e.g., UNIQUAC, NRTL). The main difficulty in modelling LLE in systems that contain a fossil fluid, e.g., a petroleum fraction, is the proper representation of the complex multicomponent mixture. The conventional approach is based on representation of the complex fossil fluid by a few well-defined key compounds. Leeper and Wankat (1982) reported the use of heptane, toluene and a mixture of heptane and toluene to represent gasoline in an investigation of the extraction of ethanol from water using gasoline as solvent. In the study of the extraction of aromatics from a multicomponent hydrocarbon mixture, Mukhopadhyay and Dongaonkar (1983) approximated a multicomponent mixture by a paraffinic or a naphthenic and aromatic compound. Rahman et al. (1984), in an investigation of the solvent extraction of aromatics from medium petroleum fractions, considered the multicomponent hydrocarbon mixture to be composed of 72 (detailed model) and of 12 (simplified ‘model) components. The representative compounds were selected on the basis of mass spectrometric analysis which identified the principal classes of hydro-
29
carbons making up the parent mixture and their apparent carbon number. Very frequently, the choice of representative compounds is somewhat arbitrary. However, representative compounds should be selected preferably on the basis of some inspection property, e.g., distillation curve and paraffin-naphthene-aromatic (PNA) content (Ruiic’ka et al., 1983a) or mass spectrometric analysis (Rahman et al., 1984). To keep the computer time and memory requirements reasonably low, the number of representative compounds should be kept as low as possible. REiCka et al. (1983a) proposed a method for describing distillable fossil fuels in terms of model compounds determined by the application of the group-contribution concept. This method assumes that the fossil fuel fraction is composed of three representative compounds, viz. a paraffin, a naphthene and an aromatic. The number of different groups that each model molecule is made of is determined using the true boiling point distillation curve and PNA analysis. EXPERIMENTAL
Materials
Methanol (Lachema Brno, Czechoslovakia) was purified by the Grignard reaction and distilled in a packed column. It contained 6 x 10e3 wt.% of water as determined by the Fischer titration method. Gasoline was an atmospheric distillation cut which was dried by annealed calcium chloride. The amount of water in gasoline was 7 X lop3 wt%. Gasoline was thoroughly characterised using several experimental techniques. Firstly, two distillation assays, viz. the ASTM D86 and the ASTM D2892-73 true boiling point (TBP) distillation procedure were performed. During the ASTM D2892-73 procedure, we distilled the sample of gasoline at a constant rate of 5 cm3 s-l and divided it into 19 fractions of an approximately equal volume. Further, the density, viscosity and molar mass of gasoline and of each of its TBP fractions were measured. Molar mass was determined by the cryoscopic method. The ASTM D86 distillation curve and the TBP data are presented in Tables 1 and 2, respectively. Gasoline
TABLE 1 ASTM D86 distillation curve of gasoline Volume % distilled off Temperature (“C) Volume % distilled off Temperature ( ’ C)
0 30 55 99
5 46 60 100
10 56 65 110
15 59 70 115
20 70 75 119
25 73 80 125
30 81 85 125
35 85 90 137
40 86 94 148
45 93 100 151
50 97
30 TABLE 2 TBP distillation curve and properties of gasoline and of its fractions Fraction number
Volume % distilled off
IBP 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Gasoline
5 10 15 20 25 30 35 40 45 50 55 60 67.8 75 77.5 89.3 97 97.8 99.5
wt. % distilled off
TBP temperature (O C) 24.6 35.8 50.8 68.2 74.4 81.0 87.0 93.6 96.2 102.8 108.2 111.8 116.6 124.2 132.5 134.6 145.1 158.0 161.2
4.5 8.9 13.4 18.0 22.6 27.4 32.2 37.1 42.0 47.0 52.0 57.1 62.5 70.0 72.6 86.3 96.0 96.9 98.8
Density at 25°C (g cmm3)
Viscosity at 25°C (mPa s)
Molar mass (g mol- ‘)
0.6474 0.6437 0.6514 0.6730 0.6845 0.6872 0.6966 0.7028 0.7094 0.7153 0.7202 0.7246 0.7303 0.7356 0.7396 0.7416 0.7535 0.7600 0.7785 0.7018
0.250 0.253 0.266 0.311 0.337 0.347 0.374 0.395 0.417 0.435 0.456 0.474 0.498 0.528 0.549 0.565 0.660 0.692 1.040 0.397
15.3 76.8 19.3 85.2 92.7 89.2 93.9 92.7 97.1 91.2 98.9 105.0 104.9 110.3 111.9 115.4 119.6 129.0 130.4 96.4
was also analysed by mass spectrometry to determine the concentration of principal types of hydrocarbons. Using the ASTM D2789-71 (Anon, 1975) procedure, we calculated the concentration of the following compound types and their apparent carbon number: paraffins, monocycloparaffins, dicycloparaffins, alkylbenzenes, indans and tetralins, naphthalenes. Results of the mass spectrometric analysis are given in Table 3
TABLE 3 Concentration of principal types of hydrocarbons in gasoline
Cont. (mol%) Apparent carbon number
Paraffins
Monocycleparaffins
Dicycloparaffins
Alkylbenzenes
Indans tetralins
Naphthalenes
64.6
26.2
1.4
6.8
0.5
0.5
7.8
7.8
7.8
8.0
8.0
8.0
57.5
46.5 56.4 81.1
64.2 70.4 75.3
52.5 65.6 77.5
10
0 0 0
- 10 - 10 -10
-20 -20 -20
57.5 60.4 58.9
56.8 57.8 56.9
51.6 52.0 54.4
49.1
57.7 58.5 59.4
57.1 57.7 58.2
54.8 55.3 57.6
53.9
25.6 24.7 25.0
26.0 26.9 26.2
25.3 27.6 26.7
25.0
21.6 22.2 22.3 23.3
24.2 24.1 23.9
23.5 23.4 23.3
23.1 23.0 22.4
22.4
21.8 21.8 21.8 21.7
Pred.
7.8 7.3 8.6
6.4 4.7 6.7
8.7 7.2 5.5
7.6
6.9 4.0 5.4 4.8
5.4 4.6 3.8
4.6 4.1 3.7
5.5 5.0 3.1
4.9
5.7 5.5 4.9 4.1
Pred.
9.1 7.6 7.4
10.8 10.5 10.2
14.5 13.2 13.2
18.5
20.9 20.5 22.4 20.5
Exp.
12.8 12.8 12.9
14.7 14.8 14.8
16.6 16.7 16.9
18.8
21.0 21.0 21.1 21.2
Pred.
1.1 0.7 0.7
2.8 2.3 1.9
2.8 2.5 2.4
3.6
4.8 5.5 5.5 5.4
Exp.
2.9 3.0 3.0
3.2 3.2 3.2
3.4 3.4 3.5
3.7
4.0 4.0 4.0 4.0
Pred.
Exp.
51.5 51.7 52.2 53.0
Pred. ’
Exp.
50.5 53.3 49.9 51.4
Exp.
0.4 0.3 0.2
0.7 0.6 0.3
0.8 0.7 0.7
1.1
1.6 1.9 1.8 1.9
Exp.
1.8 1.8 1.7
1.9 1.9 1.8
2.0 2.0 1.9
2.1
2.3 2.3 2.3 2.2
0.4 0.3 0.1
0.6 0.3 0.2
0.8 0.4 0.4
0.6
1.0 1.1 1.0 0.9
Exp.
2.0 1.7 1.2
1.7 1.6 1.4
2.1 1.9 1.2
1.9
2.4 2.2 2.0 1.7
Pred.
Aromat.
98.2 98.7 99.0
95.9 96.9 97.6
95.5 96.3 96.4
94.6
92.7 91.5 91.7 91.8
Exp.
93.3 93.6 94.1
93.2 93.4 93.6
92.4 92.6 93.5
92.2
91.3 91.5 91.7 92.1
Pred.
Methanol
determined by the minimization
Pred.
Naphth.
Paraff.
Aromat.
Naphth.
Paraff. Methanol
Methanol-rich phase
Methanol-lean phase
a Owing to the round-off error, the percentage may not add to exactly 100.0. b Composition of bulk solution. ’ Composition predicted by the UNIFAC model, gasoline represented by three model compounds procedure A.
35.2 43.0 54.9 68.0
20 20 20 20
Bulk soln. mol% MeOH b
Comparison of experimental and predicted conjugate phase compositions in methanol + gasoline blends (composition in mol%) ’
TABLE 4
32
Procedure
Liquid-liquid equilibrium in methanol + gasoline blends was determined by the analytical method. The apparatus used was a modification of that described by Fabries et al. (1977). Briefly, all experiments were carried out in a glass cell provided with a magnetic stirrer and surrounded by a thermostatted jacket. To prevent condensation of air moisture on the outer surface of the vessel at subzero temperatures, we surrounded the thermostatted jacket by another evacuated jacket. LLE experiments were conducted in the temperature range - 20 to 20°C. Temperature was controlled to within 0.03” C at 20 oC and 0.05 oC at - 20 oC. Equilibrium phases were analysed by mass spectrometric methods as it enabled both the concentration of methanol and the composition of the hydrocarbon mixture, expressed in terms of six principal classes of hydrocarbons, to be determined. The gas chromatograph mass spectrometer LKB 9000, equipped with a heated inlet system kept at a temperature of 200°C was used. The amount of sample injected was 1 ~1. Ions were produced by electron impact with energies of 70 eV in the ion source which maintained a temperature of 250 oC and a pressure of lop4 Pa. The concentration of methanol in the equilibrium phases was determined by an internal standard method. A weighed amount of dioxane was added to a precisely known amount of the sample and the relative amount of methanol and dioxane in the mixture with gasoline was found from the calibration curve. The calibration curve gives the ratio of the intensity of ions m/t 32 (methanol) and m/z 88 (dioxane) versus the weight fraction of methanol in the binary mixture of methanol and dioxane. Methanol concentration measurement was reproducible to within +2-3 mol%. Reproducibility of measurements of the hydrocarbon type concentration is somewhat lower as noted in Anon (1975). Experimental conjugate phase compositions in methanol + gasoline blends are presented as a part of Table 4 at temperatures of 20, 10, 0, - 10 and - 20 oC. Composition of the bulk solution is also given here,
RESULTS
Representation of gasoline by model compounds
To incorporate gasoline in any standard computational procedure for calculation of LLE, we found it necessary to represent this multicomponent petroleum fraction, which contained around 100 different chemical sub-
33
stances, by a few properly averaged model molecules. Group contribution methods present a versatile and effective tool for predicting phase equilibrium in systems containing components for which no experimental data are available. The application of group contribution methods can even be extended to artificial compounds that do not exist in nature but have a defined molecular structure. If the structures of the artifical model compounds are determined on the basis of some inspection data (e.g., TBP distillation curve, molar mass distribution, paraffin-naphthene-aromatic content) these compounds can represent a complex fossil fluid. To estimate the structure of model compounds, we resolve the fossil fluid into a number of fractions. Each fraction may be represented by one or more model compounds depending on the number of different families of chemical species present in the mixture. The structure of model compounds is defined in terms of groups. The number of groups in each model molecule is then determined by the adjustment procedure that entails the minimisation of the selected objective function. The equality of the mid-volume boiling point of the fraction and of the bubble point of the mixture of model compounds was the minimisation criterion selected by RBZi&a et al. (1983a). Therefore, the group contribution models for predicting pure component vapour pressures and liquid phase activity coefficients were required. UNIFAC models (Fredenslund et al., 1977; Jensen et al., 1981) were used. As the characterisation method was developed for application to petroleum fluids that contain three main families of hydrocarbons, each fraction was represented by three model compounds, viz. a paraffinic, a naphthenic and an aromatic one. The composition of the model mixture was determined on the basis of the PNA analysis. The adjustment procedure, which results in the determination of the number of groups in each model molecule, was repeated for all fractions in which the original complex fluid was resolved. Thus, the complex fluid was represented by a mixture of model compounds; each model compound was characterised by a definite number of groups. In contrast to real molecules, the model molecules were often described by noninteger values of group numbers. The crucial point of the adjustment procedure was the definition of the structure of model molecules. Some group assignments gave unrealistic results, especially in the ratio of pure component vapour pressures of model compounds. To improve the representation of the parent multicomponent fluid by model compounds, in particular to ensure a better distribution of model compounds along the TBP distillation curve, we slightly modified the characterisation procedure previously developed (RtiZiEka et al., 1983). The number of different groups that comprise model molecules was determined
34
so as to fulfil the following three criteria P-
6
XiP,O(T,
g;(n,
a,
p))
=o
0) (4
x,PP(T, g&b
a7
P>)-x&yT
g,h
a7
PI) =o
(3)
where P designates measured pressure of the TBP distillation curve, Pi0 is the pure component vapour pressure of i-th model compound, xi is the mole fraction, T is temperature and gi is the group assignment to the i-th model compound. Equation (1) requires that the mid-volume boiling point of the fraction and the bubble point of the mixture of model compounds are equal. Equations (2) and (3) are the artificial constraints that ensure the pure component vapour pressures of model compounds do not differ substantially. The activity coefficients in eqn. (1) were omitted. This is a reasonable assumption as gasoline is a mixture of hydrocarbons only. n, (Yand /3 are three adjustable parameters; n is proportional to the number of aliphatic hydrocarbon groups in the paraffinic model molecule and in the alkyl chain attached to the naphthenic’ or aromatic ring; (Yand p are approximately proportional to the length of the alkyl substituent attached to the naphthenic and aromatic ring, respectively. The meaning of these parameters can be seen from Fig. 1. The computational procedure already described is referred to as minimisation procedure A. Pure component vapour pressures were calculated by the AMP group contribution method (Macknick and Prausnitz, 1979). Parameters were taken from Macknick and Prausnitz (1979) and from RiiZiEka (1983b). The AMP group contribution method is described in Appendix A. The structure of the model molecules issues from a rough estimate of the number of various kinds of groups usually present in a hydrocarbon molecule. The estimates reached by Smith et al. (1976) are: (1) alkyl carbons were assumed to be 10% methyl, 80% methylene and 10% tertiary carbons; (2) naphthenic carbons were assumed to be 80% methylene-type and 20% tertiary-type carbons; (3) aromatic carbons were assumed to be 70% single-ring carbons with attached hydrogen, 20% single-ring carbons attached to an alkyl carbon and 10% of fused-ring carbons. As gasoline is a low-boiling fraction, the fused-ring aromatic carbons were not taken into consideration. From several test calculations it was concluded that both the UNIFAC and the AMP group contribution models gave meaningful predictions of the pure component vapour pressure of artificial
35 paraffin
naphthene
0 aromatic
Fig. 1. Structure of model molecules.
model molecules and only if the number of terminal methyl groups is equal to two in the paraffinic molecules. In naphthenic and aromatic molecules the number of terminal methyl groups must equal the number of ring groups to which an alkyl chain is attached. On the basis of these observations the structure of model molecules depicted in Fig. 1 was proposed. Another modification of the computational procedure that results in the determination of the adjustable parameters n, cu and /3 was examined. Products of pure component vapour pressure and mole fraction in eqns. (2) and (3) were replaced by pure component vapour pressure, i.e., the equality of pure component vapour pressures of model compounds was required. This computational procedure is referred to as minimisation procedure B. A comparison of both procedures, which give slightly different sets of model compounds with regard to the prediction accuracy of LLE in the methanol + gasoline blends, is presented below.
0.3189 0.4076 0.5565 0.6520 0.7684
107.5
39.5 67.6 95.2 119.8 147.8
1
1 2 3 4 5
a Minimisation
0.6354
T,,
(“C)
Fraction no.
procedure
A-eqns.
0 0 -0.5998 - 0.3980 -0.2200 minimisation
5.2 6.1 7.6 8.5 9.7
- 0.5292
(l)-(3),
0 0 - 0.0305 0.1162 0.2456
8.4
B
procedure
0 0 7.0 8.0 9.1
7.1
Naph.
No. of C atoms
0.3172 0.4075 0.4983 0.5880 0.6955
0 0 5.1 5.8 6.7
0.0196
5.2 6.1 7.0 7.9 9.0 vapour
7.4
-0.2875 0 0 - 0.4040 - 0.1862 0.0037
Paraf. P
0 0 6.4 7.3 8.4
6.8
Arom.
pressures.
0 0 6.9 7.8 8.9
7.3
Naph.
No. of C atoms
shown in Fig. 1
of pure component
0 0 - 0.0677 0.0963 0.2392
a
(1) and equality
0.5446
5.0
B-eqn.
n
B a
of model molecules
Minim. procedure
Structure
Arom.
of groups in model molecules.
Paraf.
Aa
numbers
- 0.0230
Minim. procedure n a
n, a and p that define
5
Parameters
TABLE
37
Two types of gasoline representation that differ in the number of model compounds were examined. Firstly, gasoline was assumed to be characterised by a single boiling point. This means that gasoline was considered to be composed of three model compounds which represent three main families of chemical species comprising all light petroleum fractions; a paraffin, a naphthene and an aromatic. Secondly, gasoline was resolved into five fractions. The break&down of the TBP distillation curve into fractions was carried out using an algorithm that ensures an approximately constant increase of the boiling point of successive fractions. The boiling point of the first and of the second fraction was lower than that of the lightest physically meaningful naphthenic and aromatic model compound. Physically meaningful in this context means that the model molecule should not be built of a negative number of groups. Thus, the first two fractions were represented by a single paraffinic molecule. Each of the last three fractions was composed of a mixture of three model compounds; a paraffin, a naphthene and an aromatic. The paraffin-naphthene-aromatic composition of the last three fractions was adjusted so as to match the PNA composition of the parent mixture. In this type of representation gasoline was composed of 11 model compounds. Mid-boiling points of gasoline and of its fractions and the calculated parameters n, (Yand p are given in Table 5. The upper part of Table 5 presents calculated parameters when gasoline is assumed to be composed of three model compounds. The lower part pertains to the resolution of gasoline into five fractions. Prediction of conjugate phase composition
Composition of conjugate phases in the methanol + gasoline blends was predicted using the UNIFAC model (Fredenslund et al., 1977) for the calculation of liquid phase activity coefficients. The interaction parameters were taken from the LLE parameter table developed by Magnussen et al. (1981). The UNIFAC model is described in Appendix B. To calculate LLE the algorithm suggested by Novak et al. (1982) was used. It is based on solving the isoactivity criterion by the Newton-Raphson method. Application of the Newton-Raphson method requires that a good initial estimate of conjugate phase composition is provided. The initial estimates were obtained from the mutual solubility of a paraffinic model compound and methanol (Novak et al., 1986). A detailed comparison of experimental compositions of conjugate phases and of compositions predicted by applying different procedures of gasoline representation is summarised in Table 6 for a temperature of 20 oC. Calculated compositions of model compounds belonging to the three main fami-
6
A3 All B3 Bll
A3 All B3 Bll
A3 All B3 Bll
43.0
54.9
68.0
51.4
49.9
53.3
50.5
53.0 51.7 51.8 51.1
52.2 51.4 51.3 50.9
51.7 51.2 51.0 50.8
51.5 51.1 50.8 50.7
23.3
22.3
22.2
21.6
Exp.
Exp.
Pred. d
Naphth.
phase
Paraff.
Methanol-lean
and predicted
21.7 22.6 21.9 22.5
21.8 22.3 21.8 22.1
21.8 22.0 21.7 21.9
21.8 21.9 21.7 21.8
Pred.
4.8
5.4
4.0
6.9
Exp.
4.1 4.5 4.7 4.9
4.9 5.2 5.3 5.5
5.5 5.7 5.7 5.8
5.7 5.9 5.9 5.9
20.5
22.4
20.5
20.9
Exp.
21.2 21.2 21.7 21.5
21.1 21.2 21.6 21.5
21.0 21.1 21.6 21.5
21.0 21.1 21.6 21.5
5.4
5.5
5.5
4.8
Exp.
Paraff.
4.0 5.6 5.4 6.2
4.0 5.7 5.4 6.3
4.0 5.8 5.4 6.4
4.0 5.8 5.4 6.4
Pred.
Methanol-rich
in methanol+gasoline
Pred.
Methanol
compositions
Pred.
phase
Aromat.
conjugate phase
1.9
1.8
1.9
1.6
2.2 2.2 2.4 2.4
2.3 2.2 2.4 2.4
2.3 2.2 2.4 2.4
2.3 2.2 2.4 2.4
procedure,
Exp.
1.7 1.5 1.4 1.4
2.0 1.8 1.6 1.5
2.2 2.0 1.7 1.6
2.4 2.2 1.8 1.7
Pred.
91.8
91.7
91.5
92.7
Exp.
3 or 11 designates
0.9
1.0
1.1
1.0
Exp.
number
92.1 90.7 90.8 90.0
91.7 90.3 90.6 89.8
91.5 90.1 90.5 89.6
91.3 90.0 90.5 89.5
Pred.
of
in mol%) a
Methanol
(composition
Aromat.
at 20°C
Pred.
blends
Naphth.
a Owing to the round-off error, the percentage may not add to exactly 100.0. b Composition of bulk solution. ’ Minimisation procedure used to determine the set of model compounds (letter A or B designates model compounds that represent gasoline). d Composition predicted by the UNIFAC model.
A3 All B3 Bll
Procedure ’
of experimental
35.2
Bulk soln. mol% MeOH b
Comparison
TABLE
39
lies of hydrocarbons were added up separately in each family to give the total amount of paraffins, naphthenes and aromatics. A similar summation was also performed for principal classes of hydrocarbons determined by mass spectrometry. Table 6 also reports the bulk composition of the heterogeneous mixture. Different procedures of gasoline representation include the set of model molecules determined either by the minimisation procedure A (eqns. (l)-(3)) or by the minimisation procedure B. In both minimisation procedures gasoline was considered to be composed of three or 11 model compounds. It is apparent from Table 6 that the minimisation procedure, which results in the determination of model compounds, does not significantly influence the predicted conjugate phase composition. The results also indicate that the representation of gasoline by 11 model compounds is not superior to the very simplified representation by three model compounds. The same conclusions were reached for other temperatures. Thus, the experimental conjugate phase compositions determined over the whole temperature range studied are compared with predictions stemming from the minimisation procedure A and from the most simplified representation of gasoline by three model compounds. The comparison is summarized in Table 4. Based upon these data, the following general comments can be made: (1) the solubility of methanol and gasoline decreases with decreasing temperature; (2) methanol content in the methanol-lean phase gives the highest possible amount of methanol that can be added to gasoline. Exceeding this limit results in a split of the blend into two liquid phases. At 20°C up to 21 mol% of methanol can be blended with the gasoline investigated, whereas at - 20 oC only up to about 7 mol% of methanol can be added. (3) Prediction of conjugate phase composition by the UNIFAC model is at and above O°C in reasonable agreement with the experimental data. The prediction accuracy deteriorates with decreasing temperature and reaches the maximum difference of about 6 mol% in the methanol-lean phase at -20°C. This is, however, not surprising as the UNIFAC LLE parameter table was developed for the prediction of LLE at temperatures between 10 and 40 ‘C. To describe LLE properly in the methanol + gasoline system, we propose the estimation of the so-called ‘specific’ UNIFAC parameters that pertain to a limited set of carefully chosen substances comprising the systems of interest. It was demonstrated by Mukbopadhyay and Dongaonkar (1983) that by using specific UNIFAC parameters, prediction of LLE in multicomponent aromatics extraction systems can be improved. Gasoline and any other petroleum fraction is in principle composed of the three main families of hydrocarbons, viz. paraffins, naphthenes and
40
aromatics. Aromatic hydrocarbons containing from six to eight carbon atoms are completely miscible with methanol at temperatures above approximately -30°C. On the contrary, C, to C, paraffins and naphthenes exhibit a limited miscibility with methanol, the upper critical solution temperature varying roughly in the range 25 to 70 “C. Thus, the miscibility of methanol + gasoline blends is primarily determined by the content of aromatics in the base gasoline. The increasing concentration of aromatics in gasoline improves the miscibility characteristics. Gasoline used in this study contained approximately 8 mol% of aromatics. The region of complete miscibility will be larger for blends of methanol and reformates or other highly aromatic gasolines. The conventional approach to modelling LLE in systems containing a fossil fuel fraction is based upon representation of a multicomponent mixture by a few well-defined key compounds. This procedure was also examined in this work. The GE model used for the calculation of liquid phase activity coefficients was selected with regard to the availability of the interaction parameters. Sorensen and Arlt (1979) estimated ‘common’ UNIQUAC parameters for a large group of compounds. These UNIQUAC parameters are applicable in the temperature range 20 to 30 oC. From the group of hydrocarbons for which the ‘common’ UNIQUAC parameters are available, the following compounds were chosen to represent gasoline: n-heptane, cyclohexane and toluene. This choice is in approximate accordance with the number of carbon atoms in the three model molecules that were estimated by the previously described minimisation procedure A (see Table 5). Concentration of each key compound in the ternary mixture n-heptane + cyclohexane + toluene corresponds to the paraffinnaphthene-aromatic content of the base gasoline. Composition of conjugate phases in the mixture methanol + n-heptane + cyclohexane + toluene calculated by the UNIQUAC model is compared with experimental data and with predictions based upon representation of gasoline by three model compounds in Table 7 at a temperature of 20 oC. The results reported here indicate that in modelling LLE in methanol + gasoline blends the representation of gasoline by model compounds combined with the UNIFAC model is apparently superior to the application of the UNIQUAC model. This conclusion is further confirmed when inspecting the ternary diagram of the mixture n-heptane + toluene + methanol at 15 oC depicted in Fig. 2. The experimental tie lines measured by Kubic’ek (1984) are, in the region of interest of this study, more accurately described by the UNIFAC model than by the UNIQUAC model with ‘common’ interaction parameters. Composition of gasoline used in this work is expressed in terms of aromatics and non-aromatics and depicted by the full circle on the binary line n-heptane + toluene. From Fig. 2 it is evident that as soon as the
53.3
49.9
51.4
43.0
54.9
68.0
54.7 53.0
54.8 52.2
55.0 51.7
55.0 51.5
23.3
22.3
22.2
21.6
23.5 21.7
23.5 21.8
23.5 21.8
23.5 21.8
UNIQ UNIF
4.8
5.4
4.0
6.9
Exp.
Aromat.
7.0 4.1
6.8 4.9
6.7 5.5
6.7 5.7
UNIQ UNIF
20.5
22.4
20.5
20.9
Exp.
14.9 21.2
14.8 21.1
14.8 21.0
14.8 21.0
UNIQ UNIF
Methanol
5.4
5.5
5.5
4.8
Exp.
Paraff.
7.3 4.0
7.3 4.0
7.3 4.0
7.3 4.0
UNIQ UNIF
Methanol-rich
1.9
1.8
1.9
1.6
Exp.
Naphth.
phase
3.1 2.2
3.1 2.3
3.1 2.3
3.1 2.3
UNIQ UNIF
0.9
1.0
1.1
1.0
Exp.
Aromat.
0.7 1.7
0.7 2.0
0.7 2.2
0.7 2.4
UNIQ UNIF
91.8
91.7
91.5
92.7
Exp.
with
89.0 92.1
89.0 91.7
89.0 91.5
89.0 91.3
UNIQ UNIF
Methanol
+ toluene + methanol
a Owing to the round-off error, the percentage may not add to exactly 100.0. b Composition of bulk solution. ’ Predicted composition in n-heptane + cyclohexane + toluene + methanol mixture using ‘common’ UNIQUAC parameters. d Composition predicted by the UNIFAC model, gasoline represented by three model compounds determined by the minimization procedure A.
50.5
Exp.
Exp.
UNIQ ’ UNIF d
Naphth.
phase
Paraff.
Methanol-lean
of predicted conjugate phase compositions in the quatemary mixture n-heptane + cyclohexane and predicted data in methanol + gasoline blends at 20 “C (composition in mol%) a
35.2
B”Ik soln. mol% MeOH b
Comparison experimental
TABLE 7
c
42
,
3 Fig. 2. Experimental tie lines (KubiEek, 1984) and predicted binodal curves in the mixture 0 experimental tie lines; n-heptane (l)+ toluene (2)+methanol (3) at 15°C. O- - - binodal curve predicted by UNIQUAC using ‘common’ parameters; .-. -. binodal curve predicted by UNIFAC using LLE parameters.
content of aromatics in the gasoline is increased, both the UNIFAC and the UNIQUAC binodal curves will tend to deviate from the experimental tie lines. This clearly demonstrates the necessity to estimate and use ‘specific’ interaction parameters to correctly describe the limited miscibility of methanol + gasoline blends.
CONCLUSIONS
The experimental study of the limited miscibility of low aromatic gasoline and methanol blends have shown that a large miscibility gap exists already at temperatures above 0°C. The miscibility gap is further enlarged at subzero temperatures. At - 20 oC only up to about 7 mol% of methanol can be mixed with gasoline containing around 8 mol% of aromatics without being split into two liquid phases. The influence of the content of water on the miscibility characteristics of
43
methanol + gasoline blends has not been investigated. Both methanol and gasoline were carefully dried; they contained less than 0.01 wt.% of water. Two different procedures for the representation of gasoline and two different GE models have been examined with particular reference to the prediction of the conjugate phase composition in methanol + gasoline blends. At temperatures above 0 oC the UNIFAC group contribution model applied to a mixture of a few model compounds estimated on the basis of a distillation analysis and a paraffin-naphthene-aromatic content has been found to be superior to the UNIQUAC model applied to a mixture of well-defined key compounds. The disagreement between experimental data and compositions predicted by the UNIFAC method at subzero temperatures is due to the extrapolation problems of the GE model. A better description of the liquid-liquid equilibrium in methanol + gasoline blends over the whole temperature range may be expected when ‘specific’ interaction parameters become available. ACKNOWLEDGEMENTS
The authors thank Ing. J. Koudelka for his assistance in carrying out the TBP distillation analysis, N. SaSkovS for her aid in the measurement of densities and viscosities of gasoline and Dr. S. Kern&y for his helpful comments. APPENDIX
A
The pure component vapour pressures Pp are calculated by the AMP group contribution method (Ma&nick and Prausnitz, 1979) In Py=A+B/T+ClnT+DT+ET*
(Al)
Constants A, B, C, D and E are functions of three adjustable parameters VW,s and E, A = ln(R/I$) B=
+ (s - 0.5) ln( E,/R)
-E,/R
- ln[(s - l)!] - 2.337
(A2) (fw
C=lS-s
(A4
D = (s - I)/(WR)
(A5)
E = (s - 3)(s - I)/[2(W)*]
646)
44
where E,/R and T are in Kelvin, Pi0 is in atmospheres and the gas constant R = 82.06 cm3 atm (kmol))‘. The function (3 - l)! found in eqn. (A2) is the I? function. Parameters VW, s and E,/R can be found by summing group contributions according to
k s =
c
Vf’Sk
W)
= c z$‘)cOk/R
649)
k
E,/R
k
from the group k, vr) the where vWk, Sk and ‘cOk/R are contributions number of groups of type k in molecule i. The summation in eqns. (A7) to (A9) is over all groups.
APPENDIX
B
For activity coefficients yi the UNIFAC group contribution model is used (Fredenslund et al., 1977). The activity coefficients are given by a combinatorial and a residual term In y, = In Y,~+ In yIR The combinatorial In y,C= (In &/xi & = xirJcxjrj i summation r, = c vf)Rk k
(AlO)
part is given by + 1 - $Jxi)
- zqJ2(ln
e, = xiqi,/xjq, _i
+/0,
+ 1 - f&/0,)
(All) (AW
over all components
4, = c Vf’Qk
(A131
k
summation over all groups; R, is the volume parameter for group k; Qk is the surface area parameter for group k; vf) is the number of groups of type k in molecule i; xi is the liquid phase mole fraction of component i and z is the coordination number (z = 10). For the residual part the following equations are used In y: = C v:‘) (ln I, - In Ii’)) k
(Ala)
45
where summation is over all groups
k,
= exd -a,,/0
tn= Q2L'~
QnXn
(AW W7)
where a,,,,, is the group interaction parameter for interaction between groups m and n (umn # a,,,,,). Equations (A15) to (A18) also hold for In Pi’), except that the group composition variable, ek, is now the group fraction of group k in pure fluid i. LIST OF SYMBOLS
group interaction parameter A:%, C, 0, E constants of the AMP vapour pressure equation enthalpy of vaporisation of the hypothetical liquid at T = 0 E0 K (cm3 atm mol-‘) (1 cm3 atm mol-’ = 0.101325 J mol-‘) group assignment to i-th model compound g, adjustable parameter (eqns. (l)-(3)) n P pressure (kPa) Pf pure component vapour pressure of i-th component (atm) (1 atm = 101.325 kPa) molecular van der Waals surface area of i-th component 4i group van der Waals surface area of k-th group Qk R gas constant (cm3 atm (kmol-‘)) (R = 82.06 cm3 atm (kmol-‘) = 8.314 J (kmol-‘)) molecular van der Waals volume of i-th component r; group van der Waals volume of k-th group R, s number of equivalent oscillators per molecule contribution of group k to s ‘k T temperature (K) V wk contribution of group k to V, molecular hard-core van der Waals volume (cm3 mol- ‘) Yv mole fraction of i-th component xi a
46
Tn Z
m-th group fraction coordination number
Greek letters
Yi rk
%k +i
%I (I)
'k
adjustable parameter (eqns. (l)-(3)) adjustable parameter (eqns. (l)-(3)) activity coefficient of i-th component residual activity coefficient of group k contribution of group k to E, molecular volume fraction of i-th component parameter given by eqn. (A16) molecular surface fraction of i-th component (eqn. (A12)) m-th group surface fraction (eqn. (A17)) number of groups of type k in molecule i
Superscripts C R
combinatorial part residual part
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