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vapor—liquid and liquid-liquid equilibria and critical states of water + n-decane mixtures
Fluid Phase Equilibria, 59 (1990) 207-215
207
Elsevier Science Publishers B.V., Amsterdam
VAPOR-LIQUID AND LIQUID-LIQUID EQUILIBRIA AND CRITICAL STATES OF WATER -kn-DECANE MIXTURES QI WANG * and KWANG-CHU
CHAO
School of Chemical Engineering, Purdue University, West Lafayette, IN 47907 (U.S.A.)
(Received December 4,1989; accepted in final form March 17,199O)
ABSTRACT Wang, Q. and Chao, K.-C., 1990. Vapor-liquid and liquid-liquid equilibria and critical states of water + n-decane mixtures. Fluid Phase Equilibria, 59: 207-215. Vapor-liquid equilibrium and liquid-liquid equilibrium are observed in disjointed regions at 573.2, 593.2, and 613.2 K for mixtures of water+n-decane at pressures up to 230 bar. Vapor-liquid critical states are determined at the temperatures studied.
INTRODUCTION
Mixtures of water + hydrocarbons exhibit a rich variety of fluid phase behavior that is of technological interest because of the common occurrence of these mixtures in chemical processes, in oil reservoirs, in the environment, and other instances. The weak interaction between water and hydrocarbon molecules contrasts sharply with the strong interaction between water and water molecules in the same mixture. The extreme dissimilarity of the molecular interactions manifested as highly non-ideal solution behavior invites unusual attention. Vapor-liquid equilibria and vapor-liquid-liquid equilibria in water + ndecane mixtures have been studied by Sultanov and Skripka (1972) at 423-563 K. Roof (1970) reported the end point of the vapor-liquid-liquid coexistence states to be at 569.26 K and 96.25 bar. In this work we extend the measurement to higher temperatures and observe the vapor-liquid critical states. EXPERIMENTAL
We employ a flow apparatus for the high temperature experiments of this work in order to reduce and eliminate thermal decomposition of the hydro* Qi Wang is at Zhejiang University, Hangzhou, Zhejiang, China. 0378-3812/90/$03.50
Q 1990 - Elsevier Science Publishers B.V.
208
To Vacuum
Fig. 1. Scheme of apparatus.
carbon. The scheme of the apparatus is shown in Fig. 1. The pure liquids are supplied from two feed tanks to a duplex reciprocating plunger mini pump (Milton Roy Co., flow rate 46-460 ml h-i, maximum pressure 6000 psi). To dampen the flow fluctuation a dead-ended coil is connected to the tubing at the outlet of each plunger. The two streams are combined, mixed, and preheated in a coil of i in diameter tubing 6 ft long. The coil is heated by an electric resistance heating tape which is wrapped with insulation on the outside. A Variac controls the heating to bring the stream temperature to within a few degrees of the cell temperature. The stream then enters a bare coil in the nitrogen thermal bath. This coil, also made of d in diameter tubing 6 ft long, brings the stream to the thermal bath temperature. The equilibrium cell is made of a stainless steel bar stock in which a cylindrical space of $ in diameter is opened by drilling. Two transparent sapphire windows of 1 in diameter and f in thickness enclose the open space at both ends to permit visual observation of the cell contents and facilitate liquid level control. The enclosed horizontal cylindrical cell space has a volume of 10 cm3. Figure 2 shows a cell end closure. The seal against leakage of cell fluid is made by a gold O-ring backed up by a copper shim on the outside surface of the sapphire. Fluid pressure in the cell acts to increase the bearing pressure of the seal. The higher the cell pressure, the tighter the seal becomes. In conventional design the seal would be on the inside surface of the sapphire window. The action of the cell fhtid pressure would tend to weaken the seal.
209
CapA
\
Soft Gasket
Fig. 2. Cell end closure.
The temperature in the cell is measured with a thermocouple inserted into a therm0 well in the cell body. The temperature in the cell has been found to vary by no more than 0.3 IL The pressure is measured with a Heise Bourdon-tube gauge connected to the liquid sample line, and is accurate to within 0.5 bar from 0 to 500 bars. The overhead and bottom effluents. from the cell are sampled after cooling in the tubing. The collected samples are mixed with three times their volume of 1-propanol to obtain a homogeneous solution. The homogenized sample is analyzed with a Hewlett-Packard 5710 A gas chromatograph equipped with a thermal conductivity detector. The gas chromatograph (GC) column 2 m in length is packed with Chromosorb 102 from Analabs, Inc. The GC signal is digitized and integrated with a Hewlett-Packard 21MX mini computer. The GC output is calibrated with mixtures of known composition. The accuracy of composition determination is within 0.001 mole fraction units. Plug flow residence time of the stream in the bath coil and the cell is about 5-8 min in the reported experiments. Attainment of equilibrium in the apparatus is tested by varying the flow rate and hence the residence time. The effluent compositions obtained at the indicated residence times are found not to vary with any increase in residence time.
210 EXPERIMENTAL
RESULTS
In this work the phase equilibrium states of water + n-decane mixtures were determined at 573.2, 593.2, and 613.2 K at pressures up to 230 bar. The measured equilibrium phase compositions, temperature, and pressure are presented in Tables 1-3. Each table shows the results at one temperature and is in two parts. The first part shows the mole fractions of equilibrium liquid and gas phases, and the second part shows the two liquid phases at equilibrium, the unprimed phase being the denser liquid and the primed phase the lighter liquid. Figure 3 shows the isothermal pressure-composition diagram. At each temperature a region of vapor-liquid equilibrium is observed, and a separate region of liquid-liquid equilibrium is obtained at higher pressures. The vapor-liquid equilibrium region shrinks as the temperature is raised, tending toward one point at the critical state of n-decane. The liquid-liquid equilibrium region is raised to higher pressures as the temperature is increased, to be further removed from the VLE region. As pressure is increased at a
TABLE 1 Phases at equilibrium at 573.2 K p/b= 13.0 19.7 34.0 52.0 61.0 73.5 81.0 86.0 88.5 91.5 92.7
92.0 100.0 104.5 109.0 121.0 130.5 144.0 161.0 178.5 198.5
XV!
YW
0.0067 0.0338 0.1019 0.2077 0.2729 0.3782 0.4539 0.5178 0.5535 0.6062 0.6684
0.1002 0.3464 0.5775 0.6836 0.7118 0.7331 0.7403 0.7391 0.7315 0.7168 0.6725
XW
I %V
1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 0.9995 0.9970 0.9950
0.9563 0.8614 0.8161 0.7773 0.6826 0.6195 0.5751 0.5223 0.4917 0.4854
211 TABLE 2 Phases at equilibrium at 593.2 K p/b= 19.5 27.3 34.5 44.6 54.5 63.7 69.5 70.5
121.7 130.5 136.0 151.1 171.5 186.2 201.0
%I
YW
0.0208 0.0617 0.1048 0.1671 0.2389 0.3297 0.4147 0.4467
0.1653 0.3378 0.4318 0.5009 0.5444 0.5525 0.5331 0.5148
XXV
I XXV
1.0000 1.0000 1.0000 1.0000 0.9975 0.9968 0.9963
0.9507 0.8912 0.8526 0.7650 0.6794 0.6448 0.6184
fixed temperature the two equilibrium liquid phases become more and more different. While one liquid remains almost pure water, the other liquid becomes progressively richer in n-decane. This trend is continued to the highest pressure used in this work. A vapor-liquid critical state is obtained at each temperature by observing critical opalescence. With pressure gradually decreasing through the critical TABLE 3 Phases at equilibrium at 613.2 K
23.0 25.5 28.0
150.2 160.0 163.5 178.5 190.0 202.0 231.0
0.0321 0.0519 0.0731
0.0813 0.1002 0.1020 I
XW
I %V
1.0000 1.0000 0.9977 0.9961 0.9962 0.9963 0.9961
0.9920 0.9608 0.9503 0.8963 0.8587 0.8269 0.7801
0 573.2 K 0593.2 K A613.2 K
220
60 40 20 01
0
0.2
0.4
0.6 0.8
1.0
xw
Fig. 3. Isothermal pressure-composition
phase boundary curves.
region, the single-phase fluid at first turns brownish-red. Suddenly part of it becomes dark red and two phases appear. With gradually increasing pressure the two phases turn brownish-red, the phase boundary becomes unclear, the vapor phase disappears, and only one phase remains. Critical opalescence is observed in a narrow range of composition. The critical state is obtained by interpolation as the point of maximum pressure on the observed phase boundary. The directly observed points which are close together are not reported. The critical states are presented in Table 4 and are shown as filled points in Fig. 3. Figure 4 shows the critical locus on the p-T plane, The mixture vapor-liquid critical locus connects the critical point of n-decane to the end
TABLE 4 Critical states T/K
p/bar
Mole fraction of water
573.2 593.2 613.2
92.8 71.0 29.2
0.670 0.483 0.094
213
0 -
450
VLLE end point IRwf 19701 Crltical state and vapor presswe VL Critical locus.this work
500
550
600
650
:
0
T/K
Fig. 4. The critical states and VLLE coexistence states on the p-T
plane.
point of the vapor-liquid-liquid by Roof (1970).
three-phase coexistence curve determined
COMPARISON
BACK EQUATION
WITH AUGMENTED
OF STATE
Sultanov and Skripka (1972) measured vapor-liquid equilibria in water + n-decane at 548 K and vapor-liquid-liquid equilibria at 423-563 K. Lee and Chao (1988) found that the data agreed well with the augmented BACK equation of state when two interaction constants kujj and k,,, were determined for the representation of the data. We calculate K-values with the augmented BACK equation for the states observed in this work employing the two interaction constants previously reported. Figure 5 shows the calculated K-values in comparison with the results of this work. The vapor-liquid K-values ( = y/x) are on the left side of the figure and liquid-liquid K-values (= x’/x) are on the right. The two branches of K-values go through a reversal as the mixture flmd emerges from one type of phase equilibrium to the other. In vapor-liquid equilibrium water is the light component with K > 1 and decane is the heavy component with K < 1. In liquid-liquid equilibrium it is just the other way around. The switch of K-values coincides with a switch of the phases, In VLE the water-rich phase is lighter, while in LLE the decane-rich phase is lighter. The calculated and experimental K-values show an average absolute deviation of 5.3% for water in both regions, and a deviation of 2.7% for n-decane in vapor-liquid equilibrium. The good agreement indicates the
J
710
20
40
60
100 200 300
p/bar
Fig. 5. Experimental
and calculated K-values.
consistency of the new data with Sultanov and Skripka’s previous experiments. No experimental liquid-liquid K-values are shown in Fig. 5 for n-decane because the extremely small concentration of n-decane in the water-rich liquid was not accurately measured. The small concentration indicates that the experimental K-values for decane are very large. The equation of state calculated K-values are also very large, in the order of lo3 and higher, which is completely outside the range shown in Fig. 5. The fact that both the experimental and the calculated liquid-liquid K-values are very large, greatly exceeding the range in the figure, indicates an agreement in a qualitative sense. DISCUSSION
Van Konynenburg and Scott (1980), classified fluid phase diagrams of binary mixtures into six types. Streett (1983) discussed the behaviors of the various types in p-T space. Type II and Type III are commonly observed for water + hydrocarbon mixtures. In the p-T space of Type II the two pure-component critical points are connected by a vapor-liquid critical locus. There is a liquid-liquid coexistence region, and a liquid-liquid critical curve extends from the VLLE three-phase critical end point to higher pressures. In Type III mixtures, a vapor-liquid critical locus connects one pure-component critical point to the VLLE three-phase critical end point. A separate fluid-fluid critical line originating from the other pure-component critical point rises to very high pressures, sometimes passing through max-
215
ima and minima in pressure or temperature. In many cases, the critical line rises to temperatures above the critical point of water, leading to a highpressure region of gas-gas equilibrium. The experimental data in this work establish that the phase behavior of water + n-decane mixtures belongs to Type III described by van Konynenburg and Scott. The critical point of pure n-decane is connected with the VLLE three-phase coexistence curve by a vapor-liquid critical locus. Although unobserved at the temperatures so far explored, a separate branch of the ctitical curve of dilute n-decane in water-rich mixtures is expected to emanate from the critical point of pure water to proceed to higher pressures. Although almost completely immiscible at ambient conditions, water and n-decane become highly miscible in some region of high temperature and high pressure, and show vapor-liquid equilibrium and liquid-liquid equilibrium. The LLE states extend in the direction of higher pressure and appear to merge into gas-gas equilibrium states at higher temperatures. ACKNOWLEDGMENT
This work has been supported by the Office of Basic Energy Sciences, U.S. Department of Energy, through Grant DE-FG0284ER13288. Ho-Mu Lin contributed to the design of the equilibrium cell. REFERENCES Lee, M.J. and Chao, K.C., 1988. Augmented BACK equation of state for polar fluids. AIChE J., 34: 825. Roof, J.G., 1970. Three-phase critical point in hydrocarbon-water systems. J. Chem. Eng. Data, 15: 301. Streett, W.B., 1983. Phase Equilibria in Fluid and Solid Mixtures at High Pressure. In: M.E. PauIaitis, J.M.L. Penninger, R.D. Gray, Jr. and P. Davidson (Eds.), Chemical Engineering at Supercritical Fluid Conditions. Ann Arbor Science, Ann Arbor, MI, pp. 3-30. Sultanov, R.G. and Skripka, V.G., 1972. Solubihty of water in n-alkanes at elevated temperatures and pressures. Russ. J. Phys. Chem., 46: 1245. van Konynenburg, P.H. and Scott, R.L., 1980. Critical lines and phase equilibria in binary van der WaaIs mixtures. Philos. Trans. R. Sot., Ser. A, 298: 495.
207
Elsevier Science Publishers B.V., Amsterdam
VAPOR-LIQUID AND LIQUID-LIQUID EQUILIBRIA AND CRITICAL STATES OF WATER -kn-DECANE MIXTURES QI WANG * and KWANG-CHU
CHAO
School of Chemical Engineering, Purdue University, West Lafayette, IN 47907 (U.S.A.)
(Received December 4,1989; accepted in final form March 17,199O)
ABSTRACT Wang, Q. and Chao, K.-C., 1990. Vapor-liquid and liquid-liquid equilibria and critical states of water + n-decane mixtures. Fluid Phase Equilibria, 59: 207-215. Vapor-liquid equilibrium and liquid-liquid equilibrium are observed in disjointed regions at 573.2, 593.2, and 613.2 K for mixtures of water+n-decane at pressures up to 230 bar. Vapor-liquid critical states are determined at the temperatures studied.
INTRODUCTION
Mixtures of water + hydrocarbons exhibit a rich variety of fluid phase behavior that is of technological interest because of the common occurrence of these mixtures in chemical processes, in oil reservoirs, in the environment, and other instances. The weak interaction between water and hydrocarbon molecules contrasts sharply with the strong interaction between water and water molecules in the same mixture. The extreme dissimilarity of the molecular interactions manifested as highly non-ideal solution behavior invites unusual attention. Vapor-liquid equilibria and vapor-liquid-liquid equilibria in water + ndecane mixtures have been studied by Sultanov and Skripka (1972) at 423-563 K. Roof (1970) reported the end point of the vapor-liquid-liquid coexistence states to be at 569.26 K and 96.25 bar. In this work we extend the measurement to higher temperatures and observe the vapor-liquid critical states. EXPERIMENTAL
We employ a flow apparatus for the high temperature experiments of this work in order to reduce and eliminate thermal decomposition of the hydro* Qi Wang is at Zhejiang University, Hangzhou, Zhejiang, China. 0378-3812/90/$03.50
Q 1990 - Elsevier Science Publishers B.V.
208
To Vacuum
Fig. 1. Scheme of apparatus.
carbon. The scheme of the apparatus is shown in Fig. 1. The pure liquids are supplied from two feed tanks to a duplex reciprocating plunger mini pump (Milton Roy Co., flow rate 46-460 ml h-i, maximum pressure 6000 psi). To dampen the flow fluctuation a dead-ended coil is connected to the tubing at the outlet of each plunger. The two streams are combined, mixed, and preheated in a coil of i in diameter tubing 6 ft long. The coil is heated by an electric resistance heating tape which is wrapped with insulation on the outside. A Variac controls the heating to bring the stream temperature to within a few degrees of the cell temperature. The stream then enters a bare coil in the nitrogen thermal bath. This coil, also made of d in diameter tubing 6 ft long, brings the stream to the thermal bath temperature. The equilibrium cell is made of a stainless steel bar stock in which a cylindrical space of $ in diameter is opened by drilling. Two transparent sapphire windows of 1 in diameter and f in thickness enclose the open space at both ends to permit visual observation of the cell contents and facilitate liquid level control. The enclosed horizontal cylindrical cell space has a volume of 10 cm3. Figure 2 shows a cell end closure. The seal against leakage of cell fluid is made by a gold O-ring backed up by a copper shim on the outside surface of the sapphire. Fluid pressure in the cell acts to increase the bearing pressure of the seal. The higher the cell pressure, the tighter the seal becomes. In conventional design the seal would be on the inside surface of the sapphire window. The action of the cell fhtid pressure would tend to weaken the seal.
209
CapA
\
Soft Gasket
Fig. 2. Cell end closure.
The temperature in the cell is measured with a thermocouple inserted into a therm0 well in the cell body. The temperature in the cell has been found to vary by no more than 0.3 IL The pressure is measured with a Heise Bourdon-tube gauge connected to the liquid sample line, and is accurate to within 0.5 bar from 0 to 500 bars. The overhead and bottom effluents. from the cell are sampled after cooling in the tubing. The collected samples are mixed with three times their volume of 1-propanol to obtain a homogeneous solution. The homogenized sample is analyzed with a Hewlett-Packard 5710 A gas chromatograph equipped with a thermal conductivity detector. The gas chromatograph (GC) column 2 m in length is packed with Chromosorb 102 from Analabs, Inc. The GC signal is digitized and integrated with a Hewlett-Packard 21MX mini computer. The GC output is calibrated with mixtures of known composition. The accuracy of composition determination is within 0.001 mole fraction units. Plug flow residence time of the stream in the bath coil and the cell is about 5-8 min in the reported experiments. Attainment of equilibrium in the apparatus is tested by varying the flow rate and hence the residence time. The effluent compositions obtained at the indicated residence times are found not to vary with any increase in residence time.
210 EXPERIMENTAL
RESULTS
In this work the phase equilibrium states of water + n-decane mixtures were determined at 573.2, 593.2, and 613.2 K at pressures up to 230 bar. The measured equilibrium phase compositions, temperature, and pressure are presented in Tables 1-3. Each table shows the results at one temperature and is in two parts. The first part shows the mole fractions of equilibrium liquid and gas phases, and the second part shows the two liquid phases at equilibrium, the unprimed phase being the denser liquid and the primed phase the lighter liquid. Figure 3 shows the isothermal pressure-composition diagram. At each temperature a region of vapor-liquid equilibrium is observed, and a separate region of liquid-liquid equilibrium is obtained at higher pressures. The vapor-liquid equilibrium region shrinks as the temperature is raised, tending toward one point at the critical state of n-decane. The liquid-liquid equilibrium region is raised to higher pressures as the temperature is increased, to be further removed from the VLE region. As pressure is increased at a
TABLE 1 Phases at equilibrium at 573.2 K p/b= 13.0 19.7 34.0 52.0 61.0 73.5 81.0 86.0 88.5 91.5 92.7
92.0 100.0 104.5 109.0 121.0 130.5 144.0 161.0 178.5 198.5
XV!
YW
0.0067 0.0338 0.1019 0.2077 0.2729 0.3782 0.4539 0.5178 0.5535 0.6062 0.6684
0.1002 0.3464 0.5775 0.6836 0.7118 0.7331 0.7403 0.7391 0.7315 0.7168 0.6725
XW
I %V
1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 0.9995 0.9970 0.9950
0.9563 0.8614 0.8161 0.7773 0.6826 0.6195 0.5751 0.5223 0.4917 0.4854
211 TABLE 2 Phases at equilibrium at 593.2 K p/b= 19.5 27.3 34.5 44.6 54.5 63.7 69.5 70.5
121.7 130.5 136.0 151.1 171.5 186.2 201.0
%I
YW
0.0208 0.0617 0.1048 0.1671 0.2389 0.3297 0.4147 0.4467
0.1653 0.3378 0.4318 0.5009 0.5444 0.5525 0.5331 0.5148
XXV
I XXV
1.0000 1.0000 1.0000 1.0000 0.9975 0.9968 0.9963
0.9507 0.8912 0.8526 0.7650 0.6794 0.6448 0.6184
fixed temperature the two equilibrium liquid phases become more and more different. While one liquid remains almost pure water, the other liquid becomes progressively richer in n-decane. This trend is continued to the highest pressure used in this work. A vapor-liquid critical state is obtained at each temperature by observing critical opalescence. With pressure gradually decreasing through the critical TABLE 3 Phases at equilibrium at 613.2 K
23.0 25.5 28.0
150.2 160.0 163.5 178.5 190.0 202.0 231.0
0.0321 0.0519 0.0731
0.0813 0.1002 0.1020 I
XW
I %V
1.0000 1.0000 0.9977 0.9961 0.9962 0.9963 0.9961
0.9920 0.9608 0.9503 0.8963 0.8587 0.8269 0.7801
0 573.2 K 0593.2 K A613.2 K
220
60 40 20 01
0
0.2
0.4
0.6 0.8
1.0
xw
Fig. 3. Isothermal pressure-composition
phase boundary curves.
region, the single-phase fluid at first turns brownish-red. Suddenly part of it becomes dark red and two phases appear. With gradually increasing pressure the two phases turn brownish-red, the phase boundary becomes unclear, the vapor phase disappears, and only one phase remains. Critical opalescence is observed in a narrow range of composition. The critical state is obtained by interpolation as the point of maximum pressure on the observed phase boundary. The directly observed points which are close together are not reported. The critical states are presented in Table 4 and are shown as filled points in Fig. 3. Figure 4 shows the critical locus on the p-T plane, The mixture vapor-liquid critical locus connects the critical point of n-decane to the end
TABLE 4 Critical states T/K
p/bar
Mole fraction of water
573.2 593.2 613.2
92.8 71.0 29.2
0.670 0.483 0.094
213
0 -
450
VLLE end point IRwf 19701 Crltical state and vapor presswe VL Critical locus.this work
500
550
600
650
:
0
T/K
Fig. 4. The critical states and VLLE coexistence states on the p-T
plane.
point of the vapor-liquid-liquid by Roof (1970).
three-phase coexistence curve determined
COMPARISON
BACK EQUATION
WITH AUGMENTED
OF STATE
Sultanov and Skripka (1972) measured vapor-liquid equilibria in water + n-decane at 548 K and vapor-liquid-liquid equilibria at 423-563 K. Lee and Chao (1988) found that the data agreed well with the augmented BACK equation of state when two interaction constants kujj and k,,, were determined for the representation of the data. We calculate K-values with the augmented BACK equation for the states observed in this work employing the two interaction constants previously reported. Figure 5 shows the calculated K-values in comparison with the results of this work. The vapor-liquid K-values ( = y/x) are on the left side of the figure and liquid-liquid K-values (= x’/x) are on the right. The two branches of K-values go through a reversal as the mixture flmd emerges from one type of phase equilibrium to the other. In vapor-liquid equilibrium water is the light component with K > 1 and decane is the heavy component with K < 1. In liquid-liquid equilibrium it is just the other way around. The switch of K-values coincides with a switch of the phases, In VLE the water-rich phase is lighter, while in LLE the decane-rich phase is lighter. The calculated and experimental K-values show an average absolute deviation of 5.3% for water in both regions, and a deviation of 2.7% for n-decane in vapor-liquid equilibrium. The good agreement indicates the
J
710
20
40
60
100 200 300
p/bar
Fig. 5. Experimental
and calculated K-values.
consistency of the new data with Sultanov and Skripka’s previous experiments. No experimental liquid-liquid K-values are shown in Fig. 5 for n-decane because the extremely small concentration of n-decane in the water-rich liquid was not accurately measured. The small concentration indicates that the experimental K-values for decane are very large. The equation of state calculated K-values are also very large, in the order of lo3 and higher, which is completely outside the range shown in Fig. 5. The fact that both the experimental and the calculated liquid-liquid K-values are very large, greatly exceeding the range in the figure, indicates an agreement in a qualitative sense. DISCUSSION
Van Konynenburg and Scott (1980), classified fluid phase diagrams of binary mixtures into six types. Streett (1983) discussed the behaviors of the various types in p-T space. Type II and Type III are commonly observed for water + hydrocarbon mixtures. In the p-T space of Type II the two pure-component critical points are connected by a vapor-liquid critical locus. There is a liquid-liquid coexistence region, and a liquid-liquid critical curve extends from the VLLE three-phase critical end point to higher pressures. In Type III mixtures, a vapor-liquid critical locus connects one pure-component critical point to the VLLE three-phase critical end point. A separate fluid-fluid critical line originating from the other pure-component critical point rises to very high pressures, sometimes passing through max-
215
ima and minima in pressure or temperature. In many cases, the critical line rises to temperatures above the critical point of water, leading to a highpressure region of gas-gas equilibrium. The experimental data in this work establish that the phase behavior of water + n-decane mixtures belongs to Type III described by van Konynenburg and Scott. The critical point of pure n-decane is connected with the VLLE three-phase coexistence curve by a vapor-liquid critical locus. Although unobserved at the temperatures so far explored, a separate branch of the ctitical curve of dilute n-decane in water-rich mixtures is expected to emanate from the critical point of pure water to proceed to higher pressures. Although almost completely immiscible at ambient conditions, water and n-decane become highly miscible in some region of high temperature and high pressure, and show vapor-liquid equilibrium and liquid-liquid equilibrium. The LLE states extend in the direction of higher pressure and appear to merge into gas-gas equilibrium states at higher temperatures. ACKNOWLEDGMENT
This work has been supported by the Office of Basic Energy Sciences, U.S. Department of Energy, through Grant DE-FG0284ER13288. Ho-Mu Lin contributed to the design of the equilibrium cell. REFERENCES Lee, M.J. and Chao, K.C., 1988. Augmented BACK equation of state for polar fluids. AIChE J., 34: 825. Roof, J.G., 1970. Three-phase critical point in hydrocarbon-water systems. J. Chem. Eng. Data, 15: 301. Streett, W.B., 1983. Phase Equilibria in Fluid and Solid Mixtures at High Pressure. In: M.E. PauIaitis, J.M.L. Penninger, R.D. Gray, Jr. and P. Davidson (Eds.), Chemical Engineering at Supercritical Fluid Conditions. Ann Arbor Science, Ann Arbor, MI, pp. 3-30. Sultanov, R.G. and Skripka, V.G., 1972. Solubihty of water in n-alkanes at elevated temperatures and pressures. Russ. J. Phys. Chem., 46: 1245. van Konynenburg, P.H. and Scott, R.L., 1980. Critical lines and phase equilibria in binary van der WaaIs mixtures. Philos. Trans. R. Sot., Ser. A, 298: 495.