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Phase equilibria at high pressures for the ethane-heptane and ethane-triethylaluminum systems
Fluid Phase Equilibria, 82 (1993) 93-100
93
Elsevier Science Publishers B.V., Amsterdam
PHASE EQUILIBRIA AT H I G H PRESSURES FOR THE ETHANE-HEPTANE AND ETHANE-TRI ETHYLALUMINUM SYSTEMS A. M. Beard 1, J. C. Muilins 1 and G. A. Daniels 2 1Clemson University, Department of Chemical Engineering Clemson, South Carolina 29634 2Ethyl Corporation, P. O. Box 341 Baton Rouge, Louisiana 70821 ABSTRACT A continuous-flow apparatus was used to measure vapor-liquid phase equilibrium data for the ethane-heptane and ethane-triethylaluminum (TEA) systems. The apparatus includes a view cell to observe phase behavior at equilibrium conditions. On-line remote control and datalogging capabilities were provided by computers. Experimental data for the ethane-heptane system were obtained at a temperature of 394 K and pressures from 55.1 to 87.9 bar. The phase compositions obtained for this system are in good agreement with those reported previously in the literature. The ethane-TEA system was studied along the four isotherms of 348, 373, 398, and 423 K, and at pressures from 55 to 147 bar. Equilibrium vapor- and liquid-phase compositions were obtained along each isotherm at selected pressures. INTRODUCTION The importance of aluminum alkyls was established first by Karl Ziegler and coworkers (1960) in the early 1950s. Ziegler discovered an inexpensive method to synthesize triethylaluminum (TEA) from metallic aluminum, hydrogen, and ethylene. Reactions of commercial interest are ethylene addition (chain growth) to increase the length of the alkyl group, olefin displacement, used to produce alpha olefins by replacing the alkyl group by ethylene, and controlled oxidation of the aluminum carbon to produce the alkoxide, which upon hydrolysis will produce the primary alcohol. Vapor-liquid equilibrium data are required, both for the design of the high pressure growth reactors, as well as for the subsequent processing steps. No published vapor-liquid equilibrium data for hydrocarbon-aluminum alkyl mixtures exist. The electron-deficient nature of the aluminum atom in the aluminum alkyl is partially relieved by the formation of a dimer species. The dimer is formed by alkyl bridges that are weakly bonded to both of the aluminum atoms. The existence of the dimer species as the only associated aluminum alkyls species has been confirmed by infrared (Pitzer and Sheline, 1948), proton magnetic resonance and Raman spectra (Pitzer and Gutowsky, 1946), and also by X-ray crystal analysis (Lewis and Rundle, 1953). TEA is predominantly dimeric in the liquid phase and mostly monomeric in the vapor phase. The chemical equilibrium is shifted toward the monomer species
0378-3812/93/$06.00
©1993Elsevier Science Publishers B.V. All rights reserved
94
with increasing temperature and decreasing TEA concentration in hydrocarbon solutions (Smith, 1967 and 1972). Kinetic studies performed on the chain-growth reaction have indicated that the monomer is the primary reactive species (Smith, 1963). The objective of this study was to obtain vapor-liquid equilibrium data for the ethane-TEA system. Ethane, rather than ethylene, was used in this study to eliminate problems that would arise from the reaction of TEA with ethylene. These data will be used to model this system with the Peng-Robinson or other suitable equations-of-state. The system will be considered to consist of ethane and an equilibrium mixture of the TEA monomer and dimer species. Preliminary calculations indicate that good results are feasible. EXPERIMENTAL Materials The ethane (99.0% minimum) was supplied by Matheson Gas Products, Inc., the n-heptane was 'Baker' reagent grade (98.0% minimum) supplied by J. T. Baker, Inc., and the TEA was supplied by Ethyl Corporation. Chemical analysis of the TEA showed that it contained the following % by mass: TEA 95.1, tri-n-butyl aluminum 3.7, triisobutyl aluminum 0.4, diethyl aluminum hydride 0.4, ethylene 0.2, and 1butene 0.2. Apparatus A continuous-flow apparatus was used to measure vapor-liquid phase equilibrium data for the ethane-heptane and ethane-TEA systems. The experimental apparatus can be divided into three main sections: feed, mixing/equilibrium, and sampling. A simplified schematic diagram of the overall apparatus is given in Figure 1. A detailed description of the apparatus is given by Beard (1991). TO VENTp..
,C':~
r
DO1 "--
Figure 1. SchematicDiagram of the Overall Experimental Apparatus
,
95
The feed section consists of all equipment from the feed cylinders, FC1 and FC2, to the pumps, P1 and P2, (Milton Roy Minipumps, Model 2396-89) that are used to deliver the components at the required system pressure. The mixing/equilibrium section consists of a coiled length of preheater tubing (EC), a Kenics static mixer (SM), the equilibrium view cell (VC), and the vapor- and liquid-phase high-pressure Research control valves (PCV and LCV). The view cell functions primarily as a vapor-liquid separator. The 100 crn 3 view cell is a high-pressure, stainless steel PVT cell (Model 2329-800-000) manufactured by Ruska Instrument Corporation. Two tempered glass windows, mounted on opposite sides of the cell, are sealed to the cell body with Vespel gaskets. A fluorescent light and a video camera allow the vaporliquid interface level to be observed remotely. A concentric port located at the midpoint of the cell allows introduction of the equilibrated feed mixture and placement of the resistance temperature detector (RTD) inside the cell near the usual position of the interface. The view cell could not be operated at temperatures above 423 K due to the limitations of the Vespel gaskets. The view cell, equilibrium coil, and static mixer are contained within a Bemco heated air oven (OV). The air temperature within the oven is measured by an ironconstantan thermocouple and controlled by an LFE temperature controller (Currier & Roser, Inc., Model 2010) to +0.5 K. The system pressure in the view cell is controlled by an LFE pressure controller (Currier & Roser, Inc., Model 2002) and measured by a pressure transducer (PT3), (PSI Tronix). The pressure transducer was calibrated against a dead-weight pressure gauge tester to K-0.20 bar. The effluent streams from the cell flow into the sampling section after expansion through the control valves. The sampling section consists of all equipment downstream of the control valves, such as three-way valves (V8 and V9), a knockout pot (KO), sample containers, and wet test gas meters. The liquid samples accumulate in the sample containers (SC1 and SC2) while the vapor streams flow overhead to the wet test gas meters (GM1 and GM2), which measure the accumulated vapor volumes. The phase compositions are determined subsequently by material balance from the quantities of these liquid samples collected and vapor volumes measured for a given phase. The three-way valves, knockout pot, and sample containers are located in dry boxes (DB1 and DB2), which are maintained with an inert nitrogen atmosphere for safe handling of the aluminum alkyls. TEA and most other aluminum alkyls are pyrophoric and also react violently with water. MICROMAC Computer A ~MAC-5000 computer, interfaced with an IBM PC-AT-286, provides control and datalogging capabilities for real-time operation. Various menu screens for execution of process functions and control of process variables are displayed by the PC. These functions and variables include alarm and control loop setpoints, actuated valve positions, and recording options. Values of the process variables are updated approximately every 30 seconds. Specific examples of variables displayed or datalogged are given as follows: the
96
measured masses of the feed cylinders and their contents by scale balances (W1 and W2), pressure readings from the pressure transducers (PT1, PT2, and PT3), and the system temperature by the RTD. The operator performs experiments with the apparatus by means of the PC and the ~MAC computer. A diagram of the logic is given in Figure 2. Procedure
After initial startup was completed, the system was monitored until steady state was reached with the flows adjusted to maintain the liquid level at approximately the midpoint of the cell. The various temperatures and pressures were recorded at 30 s intervals. The feed rates of ethane and TEA were obtained by observing the decrease in the cylinder scale (Wl and W2) mass readings on the PC display screen over a period of several minutes. When the system reached the equilibrium conditions (steady state was maintained for 15 min previously), sample collection was initiated. The initial volume, temperature, and gauge pressure readings from both wet test meters (GM1 and GM2) were recorded. Both three-way valves (V8 and V9) were switched to allow flow of the cell effluent streams to the sample containers (SC1 and SC2) and the wet test meters. The Liquid samples were collected in the sample containers while the accumulated vapor volumes were measured by the wet test meters. When the experiment was completed, valves V8 and V9 were returned to their normal positions and the time recorded. Each experiment lasted approximately two hours. At the conclusion of the experiment, the Figure 2. Relationship Between the mass of liquid in each sample Operator, the PC, the ~MAC, container was determined. and the Experimental Apparatus RESULTS Ethane-Heptane System The ethane-heptane system was measured to ensure the reliability of the phase compositions obtained with this apparatus. Vapor-liquid equilibrium data were obtained at 394 K in the pressure range of 55.1 to 87.9 bar. The experimental data are presented in Figure 3. The mixture critical pressure of 87.9 bar at an ethane mole fraction of 0.829 in this work agrees very favorably with 88.1 bar and 0.830 reported by Dastur (1965). The agreement of the experimental data obtained in this work with those of both Kay (1938) and Dastur is good. The largest deviations between the data points of this work and those of Kay or Dastur range from three to six percent in the minor component.
97 Ethane-Triethylaluminum System Vapor-liquid equilibrium d a t a were obtained for the isotherms of 348, 373, 398, a n d 423 K, with the system pressure ranging from 55 to 147 bar. N o published data exist for comparison with the d a t a obtained in this investigation. The results are given in Table 1. A n overall d i a g r a m for the pressure-vs-compositions isotherms is presented in Figure 4. The mass fraction is used instead of the mole fraction because TEA exists as a mixture of m o n o m e r and dimer in equilibrium. The a p p r o x i m a t e mixture critical pressures for the isotherms were estimated b y extrapolation to be 98 bar at 348 K, 122 bar at 373 K, 138 bar at 398 K, and 148 bar at 423 K. TABLE 1. Experimental Results for the Ethane-TEA System
Temperature K
348 348 348 348 348 348 373 373 373 373 373 373 398 398 398 398 398 398 423 423 423 423 423 423 423
Pressure bar
W e i g h t Fraction Liquid Vapor Ethane TEA
55.16 75.84 82.74 89.63 93.08 96.53 55.16 82.74 96.53 110.32 117.21 120.66 68.95 96.53 110.32 124.11 131.00 136.52 75.84 96.53 110.32 124.11 137.89 144.79 146.86
0.176 0.313 0.385 0.449 0.512 0.553 0.125 0.205 0.262 0.351 0.448 0.537 0.144 0.228 0.277 0.361 0.425 0.515 0.137 0.193 0.234 0.279 0.353 0.427 0.441
0.0021 0.0195 0.0301 0.0602 0.0974 0.1811 0.0063 0.0266 0.0493 0.1056 0.1748 0.2899 0.0275 0.0538 0.0845 0.1458 0.1938 0.3023 0.0610 0.0850 0.1010 0.1490 0.2300 0.3150 0.3770
Two t r a n s f o r m e d variables, w h i c h p r o v i d e near linear plots, are used to test the internal consistency of the data. The natural logarithm of E, the m o d i f i e d
98 90"
140 "
8O 120'
z~ e."
100
8O
6O 60"
•
Kay (1938) 40
50 0.5
,
.
0.6
.
,
,
0.7
0.8
0.9
X,Y, Mole
Figure 3.
Fraction
0.0
.
i o'.2
"
oi,
"
D
373 K
•
398 K
O
423K
o'.6
o'.8
1.0
1.0 L v w I , Wl,
Ethane
Pressure versus Composition for the
Ethane-Heptane System at 394 K
Figure 4.
MaSS
Fraction
Ethane
Pressure versus Composition for the Ethane-TEA System
enhancement factor for the solubility of TEA in ethane, is plotted versus the total pressure in Figure 5. E is defined as E = V~2P
P~
(1)
where W~ = mass fraction of TEA in the vapor phase, P = total pressure, s P2 = saturated vapor pressure of pure TEA The vapor pressure of TEA (Hay, et al., 1969) was calculated from lOgl0P(bar) = 7.975-3613/T(K)
(2)
A plot of the total pressure divided by the liquid-phase ethane mass fraction versus the liquid-phase ethane mass fraction is presented in Figure 6. The largest deviations of the data points from the smooth curves in Figures 5 and 6 are in the range of three to five percent. D I S C U S S I O N OF RESULTS
The accuracy of the temperature and pressure reported in this study are limited by the basic measuring accuracy of the instruments (0.1 K and 0.2 bar) and the ability to control the temperature and pressure at the desired value.
99
The temperature was controlled to +-0.5 K. Various measurements taken under static and flowing conditions revealed that temperature gradients between the cell and the surrounding heated air are a maximum of 1 K. The critical temperature of ethane was measured as 305-306 K under flowing conditions by observing the critical opalescence region. 600 " 9-
500"
J
7"
°
6"
>Z
o.
5'
=,
4
400"
300'
3
2
1
I
348 K
e,
3~K
•
368 K
o
423 K
200'
100 0.0
,o
60
80
60
12o
1,o
•
348
r~
373K
•
398 K
o
423 K , 0.1
.
. . 0.2 L
W P, b a r
Figure 5. Modified Enhancement Factor for the Solubility of TEA in Ethane versus Pressure
.
. . 0.3
.
. 0.4
. 0.5
0.6
16o , Mass
Fraction
Ethane
1
Figure 6. P/w~I as a Function of Liquid Composition for the EthaneTEA System
The phase compositions were determined from the masses of the liquid samples collected and the accumulated gas volumes measured by the wet test meters. The total masses of the liquid sample bottles and their contents were determined to +0.1 g. Liquid holdup in the tubing and the sample containers was minimized through proper slope and shape designs. The wet test meters were calibrated and are estimated to be accurate to +0.5 percent. The data obtained in this work for the ethane-heptane system are in good agreement with those of both Kay and Dastur. This agreement supports the achievement of phase equilibria. The experimental conditions, i.e., temperature, pressure, and total feed rates, for the ethane-TEA system were in the same range as those for the ethane-heptane system. This indicates that phase equilibrium was achieved and that the data for the ethane-TEA system are accurate. Furthermore, both the component and overall material balances usually were closed satisfactorily to within +-5.0 percent.
100
The liquid-phase compositions for the ethane-TEA system are accurate to +5 percent of the minor component. The accuracies of the vapor-phase compositions in terms of TEA for various ranges of % ethane by mass are given as follows: 7092%, +-5 percent; 92-99%, +10 percent; 99-100%, +-50 percent. The large uncertainties in the very dilute region for TEA in the vapor phase are due to the very small liquid samples collected during those experiments. However, the results shown in Figure 5 indicate that these estimates are very conservative. ACKNOWLEDGEMENT The authors wish to express their appreciation to Dr. William H. Beaver for construction of the initial apparatus, and to Ethyl Corporation for the opportunity to perform this investigation by their participation in the Industrial Residency Program for M.S. students in the Chemical Engineering Department at Clemson University. REFERENCES 1.
2.
3. 4. 5.
6. 7. 8. 9.
10. 11.
Beard, A. M., 1991. High-Pressure Phase Equilibria for the EthaneTriethylaluminum System. M. S. Thesis, Clemson Univ. Dastur, S. P., 1965. Experimental Vapor-Liquid Equilibrium Study for the Ethane-n-Pentane-n-Heptane System. Prediction of the K-Values for Ternary and Quaternary Systems. Ph. D. Dissertation, Northwestern Univ. Hay, J. N., Hooper, P. G. and Robb, J. C., 1969. Kinetics of the Reaction of Metal Alkyl Compounds with Alkenes. Part 4. - The Dissociation of Aluminum Triethyl Dimer. Trans. Faraday Soc., 65: 1365. Kay, W.B., 1938. Liquid-Vapor Phase Equilibrium Relations in the Ethane-n-Heptane System. Ind. Eng. Chem., 30: 459. Lewis, P. H. and Rundle, R. E., 1953. Electron Deficient Compounds: (VII) Structure of the Trimethyl-A1 Dimer. J. Chem. Phys., 21: 986. Pitzer, K. S. and Gutowsky, H. S., 1946. Electron Deficient Molecules. II. Aluminum Alkyls. J. Amer. Chem. Soc., 68: 2204. Pitzer, K. S. and Sheline, R. K., 1948. The Infrared Spectrum and Structure of Aluminum Trimethyl. J. Chem. Phys., 16: 552. Smith, C. S., 1963. Reactions of Ethylene with Aluminum Triethyl: Effect of Variables and Kinetics of the System. Ph.D. Dissertation, Purdue Univ. Smith, M. B., 1967. The Monomer-Dimer Equilibria of Liquid Aluminum Alkyls. I. Triethylaluminum. J. Phys. Chem., 71: 364. Smith, M. B., 1972. The Monomer-Dimer Equilibria of Liquid Aluminum Alkyls. IV. Triethylaluminum in Mesitylene. J. Organometal. Chem., 46: 31. Ziegler, K., 1960. Organo-Aluminum Compounds. In: H. Ziess (Ed.), Organometallic Chemistry. Reinhold Publishing Corporation, New York: pp. 194-269.
93
Elsevier Science Publishers B.V., Amsterdam
PHASE EQUILIBRIA AT H I G H PRESSURES FOR THE ETHANE-HEPTANE AND ETHANE-TRI ETHYLALUMINUM SYSTEMS A. M. Beard 1, J. C. Muilins 1 and G. A. Daniels 2 1Clemson University, Department of Chemical Engineering Clemson, South Carolina 29634 2Ethyl Corporation, P. O. Box 341 Baton Rouge, Louisiana 70821 ABSTRACT A continuous-flow apparatus was used to measure vapor-liquid phase equilibrium data for the ethane-heptane and ethane-triethylaluminum (TEA) systems. The apparatus includes a view cell to observe phase behavior at equilibrium conditions. On-line remote control and datalogging capabilities were provided by computers. Experimental data for the ethane-heptane system were obtained at a temperature of 394 K and pressures from 55.1 to 87.9 bar. The phase compositions obtained for this system are in good agreement with those reported previously in the literature. The ethane-TEA system was studied along the four isotherms of 348, 373, 398, and 423 K, and at pressures from 55 to 147 bar. Equilibrium vapor- and liquid-phase compositions were obtained along each isotherm at selected pressures. INTRODUCTION The importance of aluminum alkyls was established first by Karl Ziegler and coworkers (1960) in the early 1950s. Ziegler discovered an inexpensive method to synthesize triethylaluminum (TEA) from metallic aluminum, hydrogen, and ethylene. Reactions of commercial interest are ethylene addition (chain growth) to increase the length of the alkyl group, olefin displacement, used to produce alpha olefins by replacing the alkyl group by ethylene, and controlled oxidation of the aluminum carbon to produce the alkoxide, which upon hydrolysis will produce the primary alcohol. Vapor-liquid equilibrium data are required, both for the design of the high pressure growth reactors, as well as for the subsequent processing steps. No published vapor-liquid equilibrium data for hydrocarbon-aluminum alkyl mixtures exist. The electron-deficient nature of the aluminum atom in the aluminum alkyl is partially relieved by the formation of a dimer species. The dimer is formed by alkyl bridges that are weakly bonded to both of the aluminum atoms. The existence of the dimer species as the only associated aluminum alkyls species has been confirmed by infrared (Pitzer and Sheline, 1948), proton magnetic resonance and Raman spectra (Pitzer and Gutowsky, 1946), and also by X-ray crystal analysis (Lewis and Rundle, 1953). TEA is predominantly dimeric in the liquid phase and mostly monomeric in the vapor phase. The chemical equilibrium is shifted toward the monomer species
0378-3812/93/$06.00
©1993Elsevier Science Publishers B.V. All rights reserved
94
with increasing temperature and decreasing TEA concentration in hydrocarbon solutions (Smith, 1967 and 1972). Kinetic studies performed on the chain-growth reaction have indicated that the monomer is the primary reactive species (Smith, 1963). The objective of this study was to obtain vapor-liquid equilibrium data for the ethane-TEA system. Ethane, rather than ethylene, was used in this study to eliminate problems that would arise from the reaction of TEA with ethylene. These data will be used to model this system with the Peng-Robinson or other suitable equations-of-state. The system will be considered to consist of ethane and an equilibrium mixture of the TEA monomer and dimer species. Preliminary calculations indicate that good results are feasible. EXPERIMENTAL Materials The ethane (99.0% minimum) was supplied by Matheson Gas Products, Inc., the n-heptane was 'Baker' reagent grade (98.0% minimum) supplied by J. T. Baker, Inc., and the TEA was supplied by Ethyl Corporation. Chemical analysis of the TEA showed that it contained the following % by mass: TEA 95.1, tri-n-butyl aluminum 3.7, triisobutyl aluminum 0.4, diethyl aluminum hydride 0.4, ethylene 0.2, and 1butene 0.2. Apparatus A continuous-flow apparatus was used to measure vapor-liquid phase equilibrium data for the ethane-heptane and ethane-TEA systems. The experimental apparatus can be divided into three main sections: feed, mixing/equilibrium, and sampling. A simplified schematic diagram of the overall apparatus is given in Figure 1. A detailed description of the apparatus is given by Beard (1991). TO VENTp..
,C':~
r
DO1 "--
Figure 1. SchematicDiagram of the Overall Experimental Apparatus
,
95
The feed section consists of all equipment from the feed cylinders, FC1 and FC2, to the pumps, P1 and P2, (Milton Roy Minipumps, Model 2396-89) that are used to deliver the components at the required system pressure. The mixing/equilibrium section consists of a coiled length of preheater tubing (EC), a Kenics static mixer (SM), the equilibrium view cell (VC), and the vapor- and liquid-phase high-pressure Research control valves (PCV and LCV). The view cell functions primarily as a vapor-liquid separator. The 100 crn 3 view cell is a high-pressure, stainless steel PVT cell (Model 2329-800-000) manufactured by Ruska Instrument Corporation. Two tempered glass windows, mounted on opposite sides of the cell, are sealed to the cell body with Vespel gaskets. A fluorescent light and a video camera allow the vaporliquid interface level to be observed remotely. A concentric port located at the midpoint of the cell allows introduction of the equilibrated feed mixture and placement of the resistance temperature detector (RTD) inside the cell near the usual position of the interface. The view cell could not be operated at temperatures above 423 K due to the limitations of the Vespel gaskets. The view cell, equilibrium coil, and static mixer are contained within a Bemco heated air oven (OV). The air temperature within the oven is measured by an ironconstantan thermocouple and controlled by an LFE temperature controller (Currier & Roser, Inc., Model 2010) to +0.5 K. The system pressure in the view cell is controlled by an LFE pressure controller (Currier & Roser, Inc., Model 2002) and measured by a pressure transducer (PT3), (PSI Tronix). The pressure transducer was calibrated against a dead-weight pressure gauge tester to K-0.20 bar. The effluent streams from the cell flow into the sampling section after expansion through the control valves. The sampling section consists of all equipment downstream of the control valves, such as three-way valves (V8 and V9), a knockout pot (KO), sample containers, and wet test gas meters. The liquid samples accumulate in the sample containers (SC1 and SC2) while the vapor streams flow overhead to the wet test gas meters (GM1 and GM2), which measure the accumulated vapor volumes. The phase compositions are determined subsequently by material balance from the quantities of these liquid samples collected and vapor volumes measured for a given phase. The three-way valves, knockout pot, and sample containers are located in dry boxes (DB1 and DB2), which are maintained with an inert nitrogen atmosphere for safe handling of the aluminum alkyls. TEA and most other aluminum alkyls are pyrophoric and also react violently with water. MICROMAC Computer A ~MAC-5000 computer, interfaced with an IBM PC-AT-286, provides control and datalogging capabilities for real-time operation. Various menu screens for execution of process functions and control of process variables are displayed by the PC. These functions and variables include alarm and control loop setpoints, actuated valve positions, and recording options. Values of the process variables are updated approximately every 30 seconds. Specific examples of variables displayed or datalogged are given as follows: the
96
measured masses of the feed cylinders and their contents by scale balances (W1 and W2), pressure readings from the pressure transducers (PT1, PT2, and PT3), and the system temperature by the RTD. The operator performs experiments with the apparatus by means of the PC and the ~MAC computer. A diagram of the logic is given in Figure 2. Procedure
After initial startup was completed, the system was monitored until steady state was reached with the flows adjusted to maintain the liquid level at approximately the midpoint of the cell. The various temperatures and pressures were recorded at 30 s intervals. The feed rates of ethane and TEA were obtained by observing the decrease in the cylinder scale (Wl and W2) mass readings on the PC display screen over a period of several minutes. When the system reached the equilibrium conditions (steady state was maintained for 15 min previously), sample collection was initiated. The initial volume, temperature, and gauge pressure readings from both wet test meters (GM1 and GM2) were recorded. Both three-way valves (V8 and V9) were switched to allow flow of the cell effluent streams to the sample containers (SC1 and SC2) and the wet test meters. The Liquid samples were collected in the sample containers while the accumulated vapor volumes were measured by the wet test meters. When the experiment was completed, valves V8 and V9 were returned to their normal positions and the time recorded. Each experiment lasted approximately two hours. At the conclusion of the experiment, the Figure 2. Relationship Between the mass of liquid in each sample Operator, the PC, the ~MAC, container was determined. and the Experimental Apparatus RESULTS Ethane-Heptane System The ethane-heptane system was measured to ensure the reliability of the phase compositions obtained with this apparatus. Vapor-liquid equilibrium data were obtained at 394 K in the pressure range of 55.1 to 87.9 bar. The experimental data are presented in Figure 3. The mixture critical pressure of 87.9 bar at an ethane mole fraction of 0.829 in this work agrees very favorably with 88.1 bar and 0.830 reported by Dastur (1965). The agreement of the experimental data obtained in this work with those of both Kay (1938) and Dastur is good. The largest deviations between the data points of this work and those of Kay or Dastur range from three to six percent in the minor component.
97 Ethane-Triethylaluminum System Vapor-liquid equilibrium d a t a were obtained for the isotherms of 348, 373, 398, a n d 423 K, with the system pressure ranging from 55 to 147 bar. N o published data exist for comparison with the d a t a obtained in this investigation. The results are given in Table 1. A n overall d i a g r a m for the pressure-vs-compositions isotherms is presented in Figure 4. The mass fraction is used instead of the mole fraction because TEA exists as a mixture of m o n o m e r and dimer in equilibrium. The a p p r o x i m a t e mixture critical pressures for the isotherms were estimated b y extrapolation to be 98 bar at 348 K, 122 bar at 373 K, 138 bar at 398 K, and 148 bar at 423 K. TABLE 1. Experimental Results for the Ethane-TEA System
Temperature K
348 348 348 348 348 348 373 373 373 373 373 373 398 398 398 398 398 398 423 423 423 423 423 423 423
Pressure bar
W e i g h t Fraction Liquid Vapor Ethane TEA
55.16 75.84 82.74 89.63 93.08 96.53 55.16 82.74 96.53 110.32 117.21 120.66 68.95 96.53 110.32 124.11 131.00 136.52 75.84 96.53 110.32 124.11 137.89 144.79 146.86
0.176 0.313 0.385 0.449 0.512 0.553 0.125 0.205 0.262 0.351 0.448 0.537 0.144 0.228 0.277 0.361 0.425 0.515 0.137 0.193 0.234 0.279 0.353 0.427 0.441
0.0021 0.0195 0.0301 0.0602 0.0974 0.1811 0.0063 0.0266 0.0493 0.1056 0.1748 0.2899 0.0275 0.0538 0.0845 0.1458 0.1938 0.3023 0.0610 0.0850 0.1010 0.1490 0.2300 0.3150 0.3770
Two t r a n s f o r m e d variables, w h i c h p r o v i d e near linear plots, are used to test the internal consistency of the data. The natural logarithm of E, the m o d i f i e d
98 90"
140 "
8O 120'
z~ e."
100
8O
6O 60"
•
Kay (1938) 40
50 0.5
,
.
0.6
.
,
,
0.7
0.8
0.9
X,Y, Mole
Figure 3.
Fraction
0.0
.
i o'.2
"
oi,
"
D
373 K
•
398 K
O
423K
o'.6
o'.8
1.0
1.0 L v w I , Wl,
Ethane
Pressure versus Composition for the
Ethane-Heptane System at 394 K
Figure 4.
MaSS
Fraction
Ethane
Pressure versus Composition for the Ethane-TEA System
enhancement factor for the solubility of TEA in ethane, is plotted versus the total pressure in Figure 5. E is defined as E = V~2P
P~
(1)
where W~ = mass fraction of TEA in the vapor phase, P = total pressure, s P2 = saturated vapor pressure of pure TEA The vapor pressure of TEA (Hay, et al., 1969) was calculated from lOgl0P(bar) = 7.975-3613/T(K)
(2)
A plot of the total pressure divided by the liquid-phase ethane mass fraction versus the liquid-phase ethane mass fraction is presented in Figure 6. The largest deviations of the data points from the smooth curves in Figures 5 and 6 are in the range of three to five percent. D I S C U S S I O N OF RESULTS
The accuracy of the temperature and pressure reported in this study are limited by the basic measuring accuracy of the instruments (0.1 K and 0.2 bar) and the ability to control the temperature and pressure at the desired value.
99
The temperature was controlled to +-0.5 K. Various measurements taken under static and flowing conditions revealed that temperature gradients between the cell and the surrounding heated air are a maximum of 1 K. The critical temperature of ethane was measured as 305-306 K under flowing conditions by observing the critical opalescence region. 600 " 9-
500"
J
7"
°
6"
>Z
o.
5'
=,
4
400"
300'
3
2
1
I
348 K
e,
3~K
•
368 K
o
423 K
200'
100 0.0
,o
60
80
60
12o
1,o
•
348
r~
373K
•
398 K
o
423 K , 0.1
.
. . 0.2 L
W P, b a r
Figure 5. Modified Enhancement Factor for the Solubility of TEA in Ethane versus Pressure
.
. . 0.3
.
. 0.4
. 0.5
0.6
16o , Mass
Fraction
Ethane
1
Figure 6. P/w~I as a Function of Liquid Composition for the EthaneTEA System
The phase compositions were determined from the masses of the liquid samples collected and the accumulated gas volumes measured by the wet test meters. The total masses of the liquid sample bottles and their contents were determined to +0.1 g. Liquid holdup in the tubing and the sample containers was minimized through proper slope and shape designs. The wet test meters were calibrated and are estimated to be accurate to +0.5 percent. The data obtained in this work for the ethane-heptane system are in good agreement with those of both Kay and Dastur. This agreement supports the achievement of phase equilibria. The experimental conditions, i.e., temperature, pressure, and total feed rates, for the ethane-TEA system were in the same range as those for the ethane-heptane system. This indicates that phase equilibrium was achieved and that the data for the ethane-TEA system are accurate. Furthermore, both the component and overall material balances usually were closed satisfactorily to within +-5.0 percent.
100
The liquid-phase compositions for the ethane-TEA system are accurate to +5 percent of the minor component. The accuracies of the vapor-phase compositions in terms of TEA for various ranges of % ethane by mass are given as follows: 7092%, +-5 percent; 92-99%, +10 percent; 99-100%, +-50 percent. The large uncertainties in the very dilute region for TEA in the vapor phase are due to the very small liquid samples collected during those experiments. However, the results shown in Figure 5 indicate that these estimates are very conservative. ACKNOWLEDGEMENT The authors wish to express their appreciation to Dr. William H. Beaver for construction of the initial apparatus, and to Ethyl Corporation for the opportunity to perform this investigation by their participation in the Industrial Residency Program for M.S. students in the Chemical Engineering Department at Clemson University. REFERENCES 1.
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Beard, A. M., 1991. High-Pressure Phase Equilibria for the EthaneTriethylaluminum System. M. S. Thesis, Clemson Univ. Dastur, S. P., 1965. Experimental Vapor-Liquid Equilibrium Study for the Ethane-n-Pentane-n-Heptane System. Prediction of the K-Values for Ternary and Quaternary Systems. Ph. D. Dissertation, Northwestern Univ. Hay, J. N., Hooper, P. G. and Robb, J. C., 1969. Kinetics of the Reaction of Metal Alkyl Compounds with Alkenes. Part 4. - The Dissociation of Aluminum Triethyl Dimer. Trans. Faraday Soc., 65: 1365. Kay, W.B., 1938. Liquid-Vapor Phase Equilibrium Relations in the Ethane-n-Heptane System. Ind. Eng. Chem., 30: 459. Lewis, P. H. and Rundle, R. E., 1953. Electron Deficient Compounds: (VII) Structure of the Trimethyl-A1 Dimer. J. Chem. Phys., 21: 986. Pitzer, K. S. and Gutowsky, H. S., 1946. Electron Deficient Molecules. II. Aluminum Alkyls. J. Amer. Chem. Soc., 68: 2204. Pitzer, K. S. and Sheline, R. K., 1948. The Infrared Spectrum and Structure of Aluminum Trimethyl. J. Chem. Phys., 16: 552. Smith, C. S., 1963. Reactions of Ethylene with Aluminum Triethyl: Effect of Variables and Kinetics of the System. Ph.D. Dissertation, Purdue Univ. Smith, M. B., 1967. The Monomer-Dimer Equilibria of Liquid Aluminum Alkyls. I. Triethylaluminum. J. Phys. Chem., 71: 364. Smith, M. B., 1972. The Monomer-Dimer Equilibria of Liquid Aluminum Alkyls. IV. Triethylaluminum in Mesitylene. J. Organometal. Chem., 46: 31. Ziegler, K., 1960. Organo-Aluminum Compounds. In: H. Ziess (Ed.), Organometallic Chemistry. Reinhold Publishing Corporation, New York: pp. 194-269.