Ind. Eng. Chem. Res. 1996, 35, 4713-4717
4713
Multicomponent Solubilities of Reactants and Products of Cyclohexane Oxidation in Supercritical Carbon Dioxide M. Mukhopadhyay* and P. Srinivas Department of Chemical Engineering, Indian Institute of Technology, Powai, Bombay 400 076, India
Cyclohexane oxidation starting with a homogeneous mixture in supercritical carbon dioxide (SC CO2) medium encounters phase separation as the products are formed. Multicomponent solubility data in the reacting system are therefore needed to ascertain whether it is due to condensation of the reactants, oxygen and cyclohexane, or the products, cyclohexanone, cyclohexanol, and water. In the present work, solubility measurements have been made for cyclohexanone and cyclohexanol in supercritical CO2 at 410, 423, and 433 K and in N2 at 423 K (N2 is taken as a homomorph for oxygen to avoid reaction). Solubility data have been measured in SC CO2 at 423 K for each of the three binary mixtures, (i) cyclohexane-cyclohexanone, (ii) cyclohexane-cyclohexanol, and (iii) cyclohexanone-cyclohexanol, at 170 and 205 bar. These data have been utilized to regress for each binary pair two adjustable parameters in the PengRobinson (P-R) EOS for predicting the multicomponent solubility data in a SC mixture of CO2 and O2 in the proportion of 8:1. It was concluded that the solubility of water is suppressed in the presence of other organic products and would be the first to condense out of the reaction mixture. Introduction
Methodology
In recent years, supercritical fluids (SCF’s) have been employed in chemical reaction-separation schemes as a solvent medium, owing to their unique thermodynamic and transport properties. Their advantages may be better realized through a proper understanding of the relevant phase equilibria. A study on cyclohexane (C6H12) oxidation in a homogeneous supercritical carbon dioxide (SC CO2) medium starting with 10 mol % C6H12, 10 mol % O2, and 80 mol % CO2 to produce cyclohexanone (C6H10O) and cyclohexanol (C6H11OH) indicates the appearance of a cloud point in the reacting system (Srinivas and Mukhopadhyay, 1994). This phenomenon is attributed to the shift of the phase envelope due to the formation of the products. The phase behavior of reaction systems in CO2 is an extremely important problem, not just to explain the reaction kinetics but to determine the maximum concentration of reactants without any negative effects. It was not possible to analyze the condensate directly due to the limitations in the experiments. It is thus considered essential to know the solubility of the products C6H10O, C6H11OH, and water along with C6H12 in the SCF phase. While analyzing the phase behavior of such a reaction system, nitrogen was taken as a homomorph for oxygen for avoiding reaction during the phase equilibrium measurements. Thus, equilibrium data measured in nitrogen have been used to interpret the results that would be obtained in oxygen. Phase equilibrium data for the binary and ternary systems comprising C6H12, N2, and CO2 are reported in the literature (Shibata and Sandlar, 1989). Gallardo et al. (1987) reported the solubility of CO2 and N2 in C6H10O at moderate pressures. However, no data are available on the reaction system with products included at the desired conditions. The present paper elucidates the prediction of the multicomponent phase behavior for a cyclohexane oxidation reaction in SC CO2 by measuring binary and ternary high-pressure VLE and evaluating the binary interaction parameters to fit the P-R (Peng and Robinson, 1976) equation of state (EOS).
The solubility measurements of cyclohexanone and cyclohexanol in SC CO2 have been carried out in the range of pressures from 125 to 205 bar at 410, 423, and 433 K, the temperatures of interest to the reaction studies. Assuming the effect of temperature on the interaction constant with N2 to be insignificant within the temperature range, the solubilities in N2 have been measured only at one temperature, i.e., at 423 K at different pressures. The solubilities of three binaries in SC CO2, namely, (i) cyclohexane-cyclohexanone, (ii) cyclohexane-cyclohexanol, and (iii) cyclohexanonecyclohexanol, have been studied at 423 K at two pressures, 170 and 205 bar. The P-R EOS with the conventional quadratic mixing rules, as given below, has been used to represent the above systems:
S0888-5885(96)00115-7 CCC: $12.00
P)
a RT v - b v(v + b) + b(v - b)
(1)
a)
∑i ∑j zizjaij
(2)
b)
∑i ∑j zizjbij
(3)
where
and
aij ) (aiaj)1/2(1 - kij)
(4)
bi + bj (1 - nij) 2
(5)
bij )
The binary and ternary equilibrium data have been used to regress the binary interaction constants, kij and nij, in aij and bij, respectively, for each binary pair. The following indices have been employed to designate the components CO2 (1), C6H12 (2), O2/N2 (3), C6H10O (4), C6H11OH (5), and H2O (6). For example, k14 and n14 have been regressed from binary data of CO2-C6H10O, © 1996 American Chemical Society
4714 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 Table 1. Physical Properties of the Pure Components (Reid et al., 1987)
Table 2. Comparison of the Solubility of C6H12 in SC CO2 at 366 K
compd
MW
Tb, K
Tc, K
Pc, bar
ω
cyclohexane cyclohexanol cyclohexanone O2 CO2 N2
84.16 100.16 98.14 31.99 44.01 28.01
353.8 434.3 428.8 90.2 194.5 77.4
553.5 625.0 629.0 154.6 304.1 126.2
40.7 37.5 39.0 50.4 73.8 33.9
0.212 0.528 0.471a 0.025 0.239 0.039
a
yC6H12
a
pressure, bar
this study
lit.a
102.0 111.9 119.7
0.0579 0.0547 0.0580
0.0600 0.0550 0.0750
Shibata and Sandler, 1989.
Estimated.
k15 and n15 from binary data of CO2-C6H11OH at three temperatures, and k34 and k35 from binary data of N2C6H10O and N2-C6H11OH, respectively. However, k12, k32, and k16 have been regressed from the literature binary data. Further, ternary measurements on systems i, ii, and iii have subsequently been used to determine k24, k25, and k45 along with n24, n25, and n45, respectively, at one temperature. The interaction constants regressed have in turn been used to predict the multicomponent solubility data. The critical properties for all the compounds have been taken from Reid et al. (1987) as given in Table 1. The acentric factor for cyclohexanone is not available in the literature and, hence, has been estimated (Reid et al., 1987) from the critical temperature and pressure data. The probable product distribution in the SCF phase and the liquid phase and their selectivity of separation have been assessed to explain the nature of condensation. The distribution coefficient, Ki, and the selectivity, Sij, of separation of i with respect to j have been calculated as
Ki )
mole fraction of i in the fluid phase mole fraction of j in the liquid phase
(6)
Ki Kj
(7)
and
Sij ) Experimental Section
Apparatus. The experimental setup used in the present study has been described elsewhere (Srinivas and Mukhopadhyay, 1994) with the exception that it has provision for trapping both fluid and liquid samples each in a separate line in between a pair of consecutive valves. The variable-volume, equilibrium view cell is made of 316-L stainless steel with an o.d. of 63 mm and i.d. of 16 mm. The contents of the cell are viewed through a 25-mm-diameter and 25-mm-thick glass window. A Teflon-coated magnetic stirrer bar (10 mm × 4 mm) provides the necessary stirring. The required operating pressures are obtained by adjusting the piston position with the pressurizing medium (mercury) in a pressure generator (D. B. Robinson Associates, Edmonton, Alberta, Canada). The pressure of the system is measured in the mercury line with a pressure transducer (Sensotec, accuracy of (0.1 bar). The pressure difference across the piston is less than 2.0 bar. The temperature is maintained constant to within (1 K with a temperature controller (Arun Electronics, Bombay, India) and is measured by an iron-constantan thermocouple placed 5 mm deep into the body of the cell. Materials. Cyclohexane, cyclohexanone, and cyclohexanol with better than 99.5% purity were supplied by SISCO Research Labs, Bombay, carbon dioxide with
purity better than 99.0% was supplied by Speciality Gases Company Ltd, Bombay, and nitrogen with purity greater than 99.5% purity was supplied by M/s Indian Oxygen Ltd, Bombay. All the chemicals were directly used as received without further purification. Experimental Procedure. A typical run begins by evacuating the cell and transferring a known amount of liquid from a buret which is weighed before and after the transfer. Carbon dioxide is then gravimetrically taken into the cell from a small cylinder (50-cm3 capacity) which is also weighed before and after the transfer. The magnetic stirrer is switched on, and after the set temperature is attained, the pressure is adjusted to the desired value. The system is allowed to equilibrate for about 1 h, after which the stirrer is turned off and the system is allowed to settle for about 30 min. The vapor and liquid samples are collected in the lines between the respective valves, keeping the pressure constant by moving the piston forward. The samples are expanded slowly one after the other into cold traps containing 20 cm3 of ethyl acetate solvent. The expanding gas is quantified by measuring the volume of water displaced in a graduated cylindrical jar. The sample lines are washed thoroughly with a known amount of (30-40 cm3) fresh ethyl acetate used as an internal standard. A 0.16-µL portion out of this sample with the washed liquid was analyzed on a gas chromatograph (Schimadzu, GCRIA) containing a 10% SUPELCOWAXfused silica capillary column (30 m × 0.53 mm, Supelco) with a flame ionization detector. The concentrations obtained by the GC measurements are within (2% error. The pressure is then raised to a new value, and the same procedure is repeated. Results and Discussion Solubilities. The primary objective of this work is to predict the solubilities and selectivities of the reactants and products in the six-component system using the P-R EOS, requiring the knowledge of interaction constants kij and nij. These have been regressed from the experimental data of the corresponding binary and ternary systems. The solubility data of C6H12 in SC CO2 are given in Table 2 along with the literature data (Shibata and Sandlar, 1989) for comparison and are found to be in good agreement. The solubility data for C6H10O and C6H11OH are given in Table 3. The distribution coefficients Ki of C6H10O and C6H11OH are shown in Figures 1 and 2, respectively. The solubility is more for cyclohexanone than that for cyclohexanol, as the former is more nonpolar. In the temperature range studied, Ki’s sharply increase with pressure due to a higher association in the SC CO2 phase but decrease with temperature due to a reduced solvent-solute interaction at reduced density. The effect of temperature for cyclohexanone is slightly less significant than for cyclohexanol. The solubility data in N2 are shown in Table 4 for both C6H10O and C6H11OH at 423 K. The distribution
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4715 Table 3. Experimental Solubilities and Distribution Coefficient Data of Cyclohexanone (4) and Cyclohexanol (5) in CO2 (1) P, bar
y4
125.6 150.5 165.8 187.5 204.7
0.0197 0.0251 0.0298 0.0525 0.0906
139.1 169.3 190.5 204.9
0.0129 0.0302 0.0568 0.0820
142.8 157.5 170.6 190.9 204.8
0.0278 0.0281 0.0380 0.0546 0.0918
K4
P, bar
y5
K5
T ) 410 K 0.0277 145.9 0.0431 155.5 0.0529 172.3 0.0989 190.6 0.2048 205.4
0.0187 0.0175 0.0373 0.0541 0.0725
0.0222 0.0215 0.0474 0.0718 0.1125
T ) 423 K 0.0204 129.9 0.0504 153.8 0.1008 173.3 0.1603 190.8 204.3
0.0144 0.0222 0.0355 0.0539 0.0704
0.0178 0.0283 0.0456 0.0705 0.1061
T ) 433 K 0.0408 152.8 0.0433 164.6 0.0599 178.1 0.0904 191.8 0.1565 205.1
0.0154 0.0237 0.0335 0.0505 0.0724
0.0195 0.0298 0.0421 0.0657 0.1039
Table 4. Experimental Solubilities and Distribution Coefficient Data of Cyclohexanone (4) and Cyclohexanol (5) in N2 (3) at 423 K P, bar
103y4
103K4
P, bar
103y5
103K5
136.1 157.5 182.2 194.3 204.7
5.87 5.62 6.29 6.06 5.11
6.04 5.94 6.79 6.66 5.68
145.8 161.9 180.9 194.3 204.7
26.4 23.0 10.2 9.1 9.8
27.8 24.9 11.2 10.0 10.8
Figure 2. Experimental data on the distribution coefficient of cyclohexanol in CO2.
Figure 3. Experimental data on the distribution coefficient of cyclohexanol in N2 at 423 K.
Figure 1. Experimental data on distribution coefficient of cyclohexanone in CO2.
coefficient data are plotted against pressure in Figure 3. An interesting observation here is that the solubility of C6H11OH in N2 decreases with pressure up to about 190 bar after where it levels off, whereas the solubility of C6H10O remains more or less invariant with pressure. The solubility of C6H10O is lower than that of C6H11OH in N2 at any pressure, unlike that observed in CO2. The significantly higher solubility of cyclohexanol than cyclohexanone in N2 may perhaps be attributed to more self-association of more polar cyclohexanol in the vapor phase at moderate pressures. The decrease in solubility with increase in pressure is in line with the ideal behavior, as N2 behaves more ideally than CO2 under the same conditions. Interaction Constant. From the experimental data on the binary systems with CO2, a set of two adjustable
interaction constants, kij and nij, in the P-R EOS has been regressed at each temperature. The regressed interaction constants are given in Table 5. The values of kij are observed to be higher in magnitude as compared to the values reported by Occhiogrosso et al. (1986) for CO2-aromatics with alkyl groups. In the case of binary systems with N2, only one parameter in aij could represent the experimental data. As shown in Table 5, the interaction constants with N2 are higher than with CO2 due to the higher solute-solvent interactions caused by large asymmetry between the molecules. However, the deviations are relatively higher for N2 systems which are likely to be of less significance in the prediction of the multicomponent solubility data. The solubility data for the three binaries are shown in Table 6 and have been used to regress the binary interaction constants between them as given in Table 5. The phase envelopes generated from these regressed parameters are shown in Figure 4 for the system CO2C6H12-C6H10O, in Figure 5 for the system CO2-C6H12C6H11OH, and in Figure 6 for the system CO2-C6H10C6H11OH at 423 K. As can be seen from Figure 5, the phase envelope predicted with two interaction constants is closer to the experimental data as compared
4716 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 Table 5. Regressed Binary Interaction Constants in P-R EOS binary pair CO2-C6H10O CO2-C6H11OH N2-C6H10O N2-C6H11OH C6H12-c6H10O C6H12-C6H11OH C6H12O-C6H11OH CO2-H2O1 CO2-C6H12b N2-C6H12c
T, K
kij
nij
410 423 433 410 423 433 423 423 423 423 423 423 423 423
0.1752 0.1843 0.2000 0.1475 0.2018 0.1851 0.2999 0.3548 0.0153 0.0200 0.0535 0.0074 0.099 0.076
-0.0159 -0.0100 -0.0100 -0.1485 -0.0900 -0.1174
-0.3188 -0.0090 -0.0100
% AARD, Pa 4.09 6.07 5.82 1.59 6.71 4.94 25.31 12.55 0.16 4.71 8.64 6.81
Figure 4. Phase diagrams for the ternary system carbon dioxidecyclohexane-cyclohexanone at 423 K and two pressures.
a % AARD, P ) ∑(|P b exp - Pcal|/Pexp)/NDP × 100. Interpolated from the regressed data of Shibata and Sandler (1989) at 366.5 and 410.9 K and of Krichevski and Sorina (1960) at 473 K. c Same as that at 410.9 K from the data of Shibata and Sandler (1989).
Table 6. Experimental Ternary Equilibrium Data at 423 K mole fraction P, bar 170.0
170.0 204.9
170.0 204.9
liq phase
SC phase
CO2(1)-C6H12(2)-C6H10O(4) 1:0.5020 2:0.0791
1:0.9003 2:0.0418
CO2(1)-C6H12(2)-C6H11(OH(5) 1:0.3289 2:0.0465 1:0.3797 2:0.0304
1:0.9392 2:0.0157 1:0.8792 2:0.0217
CO2(1)-C6H10(4)-C6H11OH(5) 1:0.3008 4:0.2074 1:0.2694 4:0.1322
1:0.9562 4:0.0127 1:0.9287 4:0.0125
to the one predicted with a single interaction constant, namely, kij with nij ) 0, which in turn is narrower than the one with no predicted interaction constant (Figures 4-6). From the binary data of cyclohexane and water reported in the literature (Tsonopolous and Wilson, 1983), at 423 K, the solubility of water in cyclohexane is 2.15 mol % and that of cyclohexane in water is 1.135 × 10-2 mol %, which do not vary with pressure. The binary data of water and CO2 reported in the literature (Sukune and Kennedy, 1964) have been regressed for the binary interaction constants and are reported in Table 5. Distribution and Selectivities. The distribution coefficient data of cyclohexanone or cyclohexanol in the ternary mixtures with C6H12 and CO2, as seen from Table 7, are higher than those observed in the binary systems with CO2. This indicates that there is an increase in the solubility in CO2 in the presence of another organic component and it would probably further increase in a mixture of CO2 and N2 in place of only CO2. Consequently, separation of the products C6H10O and C6H11OH or the reactant C6H12 would not be possible from the SC CO2 medium unless their concentrations are significant and is less probable at higher pressures. Further, in SC CO2, the selectivities, S24 and S25, decrease with increasing liquid-phase concentration of cyclohexane as does the selectivity S45, which decreases with increasing concentration of cyclohexanone, as can be seen in Table 7. Further, S25 is always less than S24. Thus, it can be concluded from
Figure 5. Phase diagrams for the ternary system carbon dioxidecyclohexane-cyclohexanol at 423 K and two pressures.
Figure 6. Phase diagrams for the ternary system carbon dioxidecyclohexanone-cyclohexanol at 423 K and two pressures. Table 7. Calculated Distribution Coefficient and Selectivity Data in Ternary Systems at 423 K system: 1-2-4
system: 1-2-5
system: 1-4-5
K4a
K5b
K4c
S45
170 bar 0.051 4.18 0.063 4.02 0.079 3.41 0.099 2.92 0.127 2.49 0.226 1.83
0.084 0.076 0.067 0.063 0.063 0.056
1.76 1.34 0.91 0.75 0.65 0.56
205 bar 0.083 3.65 0.112 3.00 0.189 2.18 0.355 1.58 0.740 1.13
0.110 0.102 0.102 0.095 0.082
1.99 1.52 1.71 0.39 0.63
S24
0.115 0.161 0.187 0.367 0.788
8.46 4.69 3.77 1.82 1.21
0.214 0.299 0.407 0.579 0.806
4.73 2.92 2.06 1.49 1.16
S25
a With mole fraction of C H (2) in increasing order (downward). 6 12 With mole fraction of C6H12(2) in increasing order (downward). c With mole fraction of C H O(4) in increasing order (downward). 6 10 b
the above analysis that the degree of separability in the dilute SC reaction mixture, which consists of CO2 and O2 in the proportion of 8:1, would be of the order C6H12 < C6H10O < C6H11OH, provided the product mole fraction exceeds a significant value such as 0.073 for
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4717
C6H10O or 0.064 for C6H11OH or in between when both are present at 423 K and 205 bar, to cite an example. Taking the interaction constants between water and all the organics and that between CO2 and N2 to be zero as a first approximation, the multicomponent solubility of the six-component system has been predicted with the remaining regressed constants as given in Table 5. The selectivity S62 is estimated to be 1.037 × 10-4, and S64 and S65 are zero. Usage of nonzero interaction constants between water and all organics would result in a marginal decrease in the solubility of water in SC CO2 medium. This implies that in the presence of other products (organics), the solubility of water in SC CO2 is suppressed, resulting in condensation much before others as the reaction proceeds.
aij, bij ) interaction parameters in the P-R EOS kij, nij ) binary interaction constants in the P-R EOS Ki ) distribution coefficient of species i (yi/xi) NDP ) number of data points at each temperature P ) pressure, bar R ) gas constant, bar cm3 mol-1 K-1 Sij ) selectivity of species i with respect of species j (Ki/Kj) T ) temperature, K v ) molar volume, cm3 mol-1 xi ) mole fraction of species i in the liquid phase yi ) mole fraction of species i in the fluid phase zi ) mole fraction of species i ω ) acentric factor
Conclusion
Gallardo, M. A.; Melendo, J. M.; Urieta, J. S.; Losa, C. G. Can. J. Chem. 1987, 65, 2198. Krichervskii, I. R.; Sorinna, G. A. Liquid-Gas phase equilibria in the Cyclohexane-Carbon Dioxide and Cyclohexane Nitrous oxide systems. Russ. J. Phys. Chem. 1960, 34, 679. Occhiogrosso, R. N.; Igel, J. T.; McHugh, Mark, A. The phase behavior of Isopropyl Benzene-CO2 mixtures. Fluid Phase Equilib. 1986, 26, 165. Peng, D. Y.; Robinson, D. B. New two constant equation of state. Ind. Eng. Chem. Fundam. 1976, 15, 59. Reid, R. C.; Prausnitz, J. M.; Poling, P. E. Properties of gases and liquids; McGraw-Hill: New York, 1987. Shibata, S. K.; Sandler, S. I. High pressure vapor-liquid equilibria of mixture of Nitrogen, Carbon Dioxide, and Cyclohexane. J. Chem. Eng. Data 1989, 34, 419. Srinivas, P.; Mukhopadhyay, M. Oxidation of Cyclohexane in Supercritical Carbon Dioxide Medium. Ind. Eng. Chem. Res. 1994, 33, 3118-3124. Sukune, T.; Kennedy, G. C. The binary systems H2O-CO2 at high temperatures and pressures. Am. J. Sci. 1964, 262, 1055. Tsonopolous, T.; Wilson, G. M. High temperature mutual solubilities of hydrocarbons and water. AIChE J. 1983, 29, 990.
The present paper focuses attention on multicomponent phase behavior to determine whether reactants or products condense in the SCF medium. Multicomponent solubility data have been predicted from the experimentally measured solubility data of the reactants and products of cyclohexane oxidation in SC CO2 medium. It is observed that inclusion of two binary interaction parameters in the P-R EOS is needed to fit the solubility data of C6H10O and C6H11OH in CO2, whereas only one is needed to fit the data in N2. For ternary systems, inclusion of two interaction constants predicts shrinkage of the phase envelope as compared to one or zero constant, resulting in higher values of the solubilities in SC CO2. Cyclohexanone is more soluble in SC CO2 compared to cyclohexanol, which is reverse in N2. The solubilities of both products of oxidation, cyclohexanone and cyclohexanol, increase in the presence of cyclohexane. From the predicted multicomponent solubility data, it can be concluded that the solubility of water in SC CO2 is suppressed by the presence of all three organic compounds, and with the progress of the reaction, water condenses out of the solvent medium.
Literature Cited
Received for review February 27, 1996 Revised manuscript received August 14, 1996 Accepted August 20, 1996X IE9601155
Nomenclature a ) energy parameter in the P-R EOS b ) size parameter in the P-R EOS
X Abstract published in Advance ACS Abstracts, October 1, 1996.
