Ind. Eng. Chem. Fundam. 1983, 22, 495-499
405
EXPERIMENTAL TECHNIQUES
Vapor-Liquid Equilibrium Measurements up to 558 K and 7 MPa: A New Apparatus Jean-Luc Gulllevlc, Domlnlque Rlchon, and Henrl Renon Ecole Nationale Sup6rieure des Mines de Parls, Centre Rbcteurs et Processus, Equlpe Assoch5e au C.N.R.S. No. 768, 60 boulevard Saint-Michei, 75006 Park, France
A new static apparatus is described for the measurements of vapor-liquid equilibria up to 558 K and 7 MPa. The total composition of the mixture may be determined by weighing, while liquid and vapor mole fractions are obtained through GLC analysls on microsamples withdrawn from the equilibrium cell by means of capillaries coupled to sampling valves. The sampling system is tested independently for reproducibility and reliabiirty. The whole apparatus is found to be reliable and accurate when comparing results on the test system: propane-n-octane at several temperatures.
Introduction The design of absorption heat pumps depends closely on thermodynamics properties of the working fluid, especially vapol-liquid equilibria of the mixtures. Previous apparatuses (Legret et al., 1980; Meskel-Lesavre et al., 1981) were designed for similar measurements, but their ranges of temperatures were limited to 373 K. Furthermore, with the first one it is necessary to use an equation of state for calculating liquid mole fraction and with the second it is not possible to obtain experimental vapor mole fractions. The new apparatus extends the range of temperatures up to 558 K, allowing determinations of overall, liquid, and vapor mole fractions. Experimental Section Principles. An equilibrium cell is filled with each component of the mixture to be studied. The mass of each component introduced is determined by weighing. The cell is then kept a t constant temperaure in an air thermostat, and the mixture is stirred with a magnet. At equilibrium, pressure is read and samples of both phases are withdrawn and analyzed. Overall Setup. A flow diagram of the apparatus is given in Figure 1. Figure l a shows four main parts: an equilibrium cell with its sampling systems, the pressure and temperature measurement systems, the air thermostat, and the analysis setup. Figure l b represents another part of the equipment used to introduce known quantities of a gaseous component into the cell. A detailed drawing of the equilibrium cell is given in Figure 2. The cell has been designed to meet three requirements: it must be weighed on an analytical balance (Mettler H315, maximum load lo3 g, sensitivity 10" 9); it must withstand pressures up to 7 MPa and temperatures up to 558 K. The body (1)of the cell is made of magnetic stainless steel 316 LSS (AFNOR: 22 - CND - 17-12, Aubert et Duval). The parta A and B of the cell body (Figure 2) were Q196-4313/83/1022-0495$01.50IQ
machined separately and then welded together. The internal volume of the cell (A B) is about 50 cm3 with a horizontal diameter of 5.8 cm to allow a very fast mass transfer between vapor and liquid phases. The membrane pressure transducer (2) (Kaman, KP 1911,O-7 MPa) is located on the lower surface of the cell and very close to the mixture to avoid any dead volumes. On the same side are fixed the feeding valve (3) and the two sampling valves (4). A detailed drawing of a non rotating stem sampling valve is given on Figure 3. The carrier gas from the chromatograph enters the sampling valve through tubing (13) and goes out of the valve through the center of the stem (11).The stirriig magnet (5) is rotating around a fixed axis in order to keep it away from the capillary (6) connected to the gas sampling valve. The magnet is driven by a rotating magnetic field induced by four solenoids S (Figure 1)located outside the cell. In the air thermostat (MaGriel Physico-Chimique, Type Regulab 80) the cell is placed with the valves down-side; therefore the longest capillary is used to sample the gas while the shortest is used to sample the liquid phase. The length and the internal section of the capillaries, as well as the shape of the stems, result from an experimental study to obtain representative samples. Withdrawn samples are small enough to allow a gas-liquid chromatographic analysis and to yield a negligible change of the equilibrium conditions inside the cell. Such a sampling and analytical method requires the liquid sample to be in a vapor phase when flowing outside the sampling valve. This requirement defies the lowest possible experimental temperature. Temperature of the cell body is kept constant within 0.1 K and measured through two thermocouples inserted in wells (7) machined in the cell body. The analysis assembly is composed of a chromatograph (Girdel, Model 3000), an integrator (Delsi, Model Icap 5), and a recorder (Sefram, Model Servotrace). Calibrations. The pressure transducer is calibrated at each working temperature against a dead-weight balance (Budenberg, Type 280 H). It is associated with a differ-
+
0 1963 American Chemical Society
498
Ind. Eng. Chem. Fundam., Vol. 22, No. 4, 1983
a
9”
b
co
w
Figure 1. Flow diagram of the apparatus: AT, air thermostat; CG, carrier gas; Co, coupling; EC, equilibrium cell; GC, gas cylinder; GH, gas holder; GLC, gas-liquid chromatographic analysis setup; M, manometer; Ma, magnet; PD, pressure electronic display; PR, pressure regulator; PT,pressure transducer; S,solenoids; TD, temperature electronic display; Thl, Thz, thermocouples; VA, feeding valve; VS, sampling valves; V1,Vz,V3,valves.
CROSS SECTION X X
BOTTOM
E
VIEW
CR055 SECTION Y Y
Figure 2. Equilibrium cek 1, cell body; 2, location of the pressure transducer; 3, feeding valve; 4, sampling valve; 5, magnec 6, capillary; 7, well for a thermocouple; 8, welding.
ential pressure null transducer and an indicator (Ruska, Model 2413-705 and 2416-708) of maximum sensitivity lod MPa. The dispersion of pressure values for a given calibration curve is estimated to be less than MPa. Temperatures are measured within 0.2 K through a digital electronic instrument (Fluke, Model 2100). The thermal conductivitydetector of the chromatograph is frequently calibrated by introducing pure components through an automatic syringe (Hamilton, microlab). In this manner response coefficients are determined within 1%.
Figure 3. Sampling valve: 6, capillary; 8, welding; 9, carrier gas tubing (outlet); 10, sampling valve body; 11, nonrotating stem; 12, high-temperature resistant polymer O-ring; 13, carrier gas tubing (inlet).
Experimental Procedure The procedure consists of three steps: filling the cell, setting it in the apparatus to reach equilibrium conditions, and then making the equilibrium measurements. The filling procedure is as follows for a mixture made of one gaseous and one liquid component at room temperature. The cleaned and degassed cell is weighed; then the liquid component is introduced, cooled, and degassed by stirring under vacuum. After degassing, the cell is weighed to determine the mass of the liquid component. Then the cell is connected to the filling equipment (Figure lb) to introduce the gaseous component. A given amount of gas is introduced. Knowing the volume of the gas holder, GH, and ita internal pressure and temperature, the pressure drop in the gas holder is calculated corresponding to the number of moles to be transferred in the equilibrium cell. The exact mass of gaseous component is fmally determined by weighing the cell accurately. The filled cell is connected to the apparatus and fixed on a rigid support to allow easy handling of the sampling valves without constraints on the tubings and welded parts of the cell. The air thermostat and magnetic stirring are then activated to reach equilibrium conditions. When the temperature and pressure are constant, the pressure is read through the pressure transducer, and samples are withdrawn to perform phase analysis. A sample is obtained by opening the sampling valve during a short time. The pressure inside the cell is higher than that of the chromatographic circuit, and therefore the sample flows through the stem of the sampling valve. After the sampling valve is closed, the sample is taken out by the carrier gas up to the thermal conductivity detector. The response coefficients related to the thermal conductivity detector are used to calculate the x , y data corresponding to a given temperature which is modified after sampling to perform new P,ry measurements with the same loading of the cell. Tests and Results The origin and purity of the chemicals used are given in Table I. The tests of the sampling and analyzing procedure showed that the results are reproducible and
Ind. Eng. Chem. Fundam., Vol. 22, No. 4, 1983 497
Table I. Origin and Purity of the Chemicals Used chemical methane ethane propane n-octane n-nonane benzene % in volume.
origin
punty
Airgaz 99.950 Airgaz 99-95" 99.950 Airgaz Fluka 99.5b 99.0b Fluka Merck 99.7 GLC certified minimum purity.
Table 11. Reproducibility of the Liquid Sample Analysis with the Ethane-Benzene at 365.1 K and 2.76 MPa total no. of moles withdrawn ( x
0.78 1.13 1.40 2.57 7.24 8.11 10.09 20.63
ethane mole fraction
0.2079 0.2076 0.2079 0.2076 0.2078 0.2077 0.2076 0.2075
ETA
U
OL
Figure 5. Flow diagram of the equipment to test validity of sampling and analyses on a mixture under pressures lower than ita bubble pressure: AT, CG, GLC, Ma, PD, PR, PT, S, TD, VS, see Figure 1; OL, W C , see Figure 4; HP, hydraulic pump.
I'..
447.5 K
4.81
I
Figure 4. Flow diagram of the equipment to test validity of sampling and analyses on a mixture under pressures higher than ita bubble pressure: AT, CG, GLC, Ma, PD, PR, PT, S, TD, VS, see Figure 1; OL, operating lever; PG, pressurizing gas; VVC,variable volume cell.
the samples are representative of the sampled mixtures. Reproducibility and Reliability of Liquid Phase Analysis Results. For these purposes an ethane (1)benzene (2)mixture is made in a variable volume cell (see Figure 4). Its composition is determined by weighing each component within 10" g on analytical balance (Mettler H315 modified, maximum load 2 X lo3 g, sensitivity g). For a given temperature, data from Kay and Nevens (1952)are used to determine the bubble pressure of the mixture. Then, the pressure value inside the cell is set higher than the bubble pressure through the pressurizing gas, the pressure of which is read by means of a pressure transducer situated in the pressurizing circuit. Results of the analysis performed on several samples of increasing mass are reported in Table 11. The ethane mole fraction is found to be independent of sample mass and the results are reproducible within 0.2%. Taking account of the error on chromatographic calibration constant, the mole fraction given by analysis is estimated between 0.2056 and 0.2098, while the mole fraction determined by weighing is estimated between 0.2047and 0.2059. Barycenters of the two kinds of determinations are different by only 1% ,so the analysis coupled to the sampling valve is very well adapted for a liquid phase above its bubble pressure. To assure that the sampling system is also valid below the bubble pressures, another experiment (see the corre-
37
I
I
I
1
2
3
,
p
b
q "",to
Figure 6. Pressure as a function of the volume of hydraulic liquid introduced in the variable volume cell through the hydraulic pump for three different loadings (propane (1)-n-octane (2) system). Table 111. Test of the Sampling System in the Liquid Phase around the Bubble Pressure of the Propane (1)-n-Octane (2)System temp, K
427.4 447.5 447.5
bubble press., MPa
press. when sampling, MPa
XI
3.88
3.93 3.87 4.35 4.29 4.64 4.48
0.5804 0.5810 0.5473 0.5468 0.5663 0.5651
4*29 4.48
sponding flow diagram in Figure 5) was made, baaed in part on the work of Meskel-Lesavre et al. (1981). The variable volume cell is filled up with a propane (1)-n-octane (2) mixture. When equilibrium temperature is reached in the cell, a calibrated hydraulic pump is used to modify the internal volume of the variable volume cell and pressure is read as a function of the internal volume
Ind. Eng. Chem. Fundam., Vol. 22, No. 4, 1983
498
" t -r-+
38
i./-zz2
37
Figure 7. Pressure as a function of the volume of hydraulic liquid introduced in the variable volume cell through the hydraulic pump for four different loadings of given composition (propane (1)-n-octane (2) system) at 427.4 K.
4
0.54
0 55
0.57
0.56
X,
-
058
Figure 9. Bubble pressures at 447.5 K as a function of propane mole fraction in the propane (1)-n-octane (2) system: ( X ) and parallel curves, see Figure 8.
Table IV. Reproducibility Tests of the Vapor Sampling System: Methane-Ethane Mixture at Different Temperatures
-~
~
temp,
K 288.1 376.0 423.1 453.1 471.8
methane press., no. of mole MPa samples fraction
3.00 3.30 3.35 3.45 3.50
8 12 11 10 11
0.4801 0.4806 0.4804 0.4800 0.4798
dispersion *5 X +5 X +4 X ~3.5x 10'" k5 X
3.7.
1
0 57
L
0.575
0.58
0.565
X,
OS9
Figure 8. Bubble pressures at 427.4K as a function of propane mole fraction in the propane (1)-n-octane (2)system: (+) this work; experimental data with their incertitude rectangles (mole fraction obtained by weighing); (X) this work; experimental data with their incertitude rectangle (mole fraction obtained by sampling and analysis); parallel curves, incertitude range corresponding to the fitted data of Kay et al. (1974).
of the cell. Arbitrary units related to the volume of the hydraulic liquid displaced by the hydraulic pump are reported in Figure 6 which shows the bubble pressures: break points of the pressure-volume diagrams. Samplings and analyses have been performed at pressures apart but close to bubble pressures and results for three different fillings appear in Table 111. At the bubble pressure, the samples remain representative of the liquid phase from
which they are withdrawn. Other measurements have been performed at the bubble pressure on mixtures of known composition (by weighing) to determine the composition dependence of the bubble pressures at 427.4 K (see Figwe 7) and compare the results (see Figure 8) to data from Kay et al. (1974). In this figure the rectangles of errors, relative to pressures and mole fractions, are plotted in the two cases: mole fraction determined by weighing and mole fraction determined by sampling. There is an important overlap between the error surfaces related to the data of Kay et al. (1974) and our's. A similar experiment was made at 447.5 K (see Figure 9). Reproducibility of Vapor-Phase Analysis Results. The mixture methane-ethane has been selected to test the sampling and analyzing systems when working with a gaseous mixture. The results of the analyses on samples withdrawn at different temperatures are reported in Table W,their reproducibility is quite good, the deviation being about 0.1 % . Measurement on Test System. The propane-n-octane system already used in the Testa and Results section
Ind. Eng. Chem. Fundam., Vol. 22, No. 4, 1983
4Q9
Table V. Vapor-Liquid Equilibrium Data for the System Propane (1)-n-Octane (2) temp,
K 427.4 427.4 427.4 427.4 427.4 427.4 427.4 473.1
473.1 473.1
473.1 473.1 473.1 473.1
473.1 523.1
523.1 523.1
press., MPa
2.22 2.22 2.22 2.23 2.23 2.23 2.24 2.24 2.24 2.70 2.70 2.70 2.73 2.73 2.73 3.81 3.81 3.81 3.86 3.86 3.86 2.54 2.54 2.54 2.54 3.21 3.21 3.21 3.24 3.24 3.24 3.24 3.43 3.43 3.43 3.73 3.73 3.73 3.82 3.82 3.82 3.90 3.90 3.90 3.90 5.14 5.14 5.14 3.73 3.73 3.73 3.73 3.80 3.80 4.53 4.53 4.53
XI
Y1
0.3600 0.3608 0.3597 0.8780 0.8780 0.8786
total no. of mol ( X 10-2) n, = 6.054 n, = 7.937
6.0.
t
/
71'
n, = 6.088 n, = 7.943 n, = 6.120 n, = 7.949
0.3619 0.3620 0.3627 0.8860 0.8860 0.8870
lx
n, = 7.893 n, = 7.454 n, = 7.919 n, = 7.478
0.4327 0.4327 0.4336
X
3.0.
1
0.8967 0.8961 0.8937 0.5890 0.5883 0.5880 0.2600 0.2602 0.2594
n, = 12.034 n, = 5.205
0.7167 0.7162 0.7170 0.7172 0.7671 0.7576 0.7579
n, = 6.054 n, = 7.937 n, = 6.054 n, = 7.937
0.3499 0.3499 0.3503 0.3509 0.3640 0.3646 0.3647
n, = 6.714 n, = 8.565
0.7683 0.7680 0.7686 0.7775 0.7771 0.7778 0.4272 0.4274 0.4279 0.4276 0.5764 0.5774 0.5770 0.2780 0.2786 0.2790 0.2784 0.2830 0.2821 0.3685 0.3672 0.3673
n, = 4.505 n, = 8.880
n, = 7.680 n, = 7.336 n, = 7.682 n, = 7.337 n, = 7.727 n, = 7.423
0.7794 0.7784 0.7784 0.4942 0.4938 0.4944 0.4945 0.4975 0.4980 0.4970
n, n, n, n,
=
4.505
= 8.879 =
6.714
= 8.554
is studied at three temperatures: 427.4,473.1,and 523.1 K. Our experimental data are reported in Table V and Figure 10 along with smoothed data from Kay et al. (1974). The total number of moles for each component is given in column 5, the maximal error is estimated to 1unit on the last digit. As data from Kay et al. (1974)are obtained through their fitting treatment, we have no possibility to visualize if their raw data are scattered or not. What we can say is that ours are well-distributed on a regular curve.
x 3 .Yt a4 a6 08 Figure 10. Pressure as a function of propane mole fraction in the system propane (1)-n-octane (2):(0)this work at 427.4K (see Table V); (A)this work at 473.1 K (see Table V); (A)this work at 523.1 K (see Table V); ( 0 )this work at 427.4 K (see Table 111);(0) this work at 427.4K (see Figure 7);(X) smoothed data of Kay et al. (1974)at 427.4 K;(+) smoothed data of Kay et al. (1974)at 473.1 K (0) smoothed data of Kay et al. (1974)at 523.1 K. 1.0
.O
a2
The maximum deviation between the curves of the two author groups is about 0.02 mole fraction. We estimate that our incertitude on mole fraction is at maximum 1 % (see section entitled Testa and Results). The errors are mainly due to the chromatographic analysis (dx/dP)AP and ( d x / d T ) A T being negligible). Conclusion The new apparatus for determination of vapor-liquid equilibria through sampling by means of special valves coupled to capillaries was tested for reproducibility and reliability in different ways. Dispersions of the analysis results are less than 0.2% and comparisons with very accurate data obtained through a variable volume cell technique indicate an accuracy better than 1%in measured liquid and vapor mole fractions. The apparatus was used to study the propane-n-octane system at three temperatures and results were compared to those of Kay et al. (1974).There is good agreement, the greatest deviation, found in vapor phase, being 2%. Acknowledgment The authors acknowledge the financial support of Institut Francais du PBtrole and are grateful to P. Alali for machining and help in fitting the apparatus. Literature Cited Kay, W. B.; Qenco, J.; Flchtner, D. A. J . Chem. Eng. Data 1974, 19, 275. Kay, W. B.; Nevens, T. D. Chem. Eng. Rog. Symp. Ser. 1952, 48(3), 108. Legret, D.; Richon, D.; Renon, H. Ind. Eng. Chem Fundem. 1980, 19, 122. MeskeKesavre, M.;Rlchon, D.: Renon, H. Ind. Eng. chem.Fundem. 1981, 20, 284.
Received for review June 16,1982 Accepted June 1, 1983
