J. A. D A V I S NEWELL RODEWALD F. K U R A T A
A N APPARATUS FOR PHASE STUDIES BETWEEN 20" K. AND 300" K. Phase and volumetric behavior can be studied simultaneously
eneral interest in low temperature processing and handling of materials has increased greatly in recent years. The natural gas industry is becoming more cognizant of the possibilities offered by low temperature processing and handling of natural gas. Governmental action relating to helium conservation and the use of large quantities of cryogenic fluids by the space and missile industries have drawn attention to the low temperature field. There is a need in all of these areas for experimental information on phase behavior not only for direct application in engineering calculations, but, more importantly, to add to the general understanding of phase behavior a t low temperatures. The data will serve as bases for developing or extending theoretical procedures
G
36
INDUSTRIAL AND ENGINEERING CHEMISTRY
to predict phase behavior. In view of the great need for phase behavior data a t low temperatures, an apparatus was constructed to obtain such information. The system shown in Figure 1 is designed for the determination of phase and volumetric behavior in the region between 20' K. and 300' K. The test section is a stirred cell which permits visual observation of the sample. The cell is immersed in a t h m o s t a t e d bath which operates with either liquid or gas as the heat transfer medium, using liquid nitrogen or liquid helium as refrigerant. T o verify the precision of the apparatus, experimental data were taken on the 77' K. isotherm of the heliumnitrogen system. These values were compared to data previously published in the literature, and were in good
general agreement. However, lack of complete agree ment among previously published data makes it difficult to make a quantitative statement regarding the precision of the apparatus. Measurements of saturated volumetric behavior of nitrogen indicated that the apparatus could be used to Aetermine liquid densities within a few tenths of a per mt. Saturated vapor phase compressibility factors umld be determined with an accuracy of about 2%. Exprimenml Apparatus SHIELD Fltl TUBE 2-8ATH VENT 3-BATH FILL TUBE 1-COOLANT VENT 5-COOLANT INPUT 6-HEATER POWER PlUG 7-VACUUM JACKET VALVE 8-RESISTANCE THERMOMETER LEADS ~ - U ~ P E NITROGEN R SHIELD F11L TUBE IO-UPPER NITROGEN
Y
g 0
-I
500
-I
w
500
2 Lo
a (L
w
5
400
4oa
VI
w
n 30 0
300
2 00
200
I
1 0 80
70
80
Figure 9.
100
90 TEMPERATURE
110
0 081
120
0.90
0 95
*ti.
2 *
I10
I15
I20
125
PV/NRT
Figure 10. Saturated vapor compressibility factor, 77.2' K.
Bubble point behavior of a 0.7y0helium mixture
balances for both runs can be written and solved directly for the compositions and densities of the liquid and vapor phases. T h e resulting equations are:
105
IO0
compressibility factor of the mixture a t metering temperature and pressure,
2
=
p
= density, g. moles/ml.
PV/nR T Su bsrripts 1 = run number 1 2 = run number 2 a = component a L = liquid phase v = vapor phase
where :
P Q
= =
R
=
T -
=
x
= =
V
= VeV,,1l = W =
y
metering pressure, lb./sq. in. abs. moles of gas mixture metered to equilibrium cell gas constant, PV/nT metering temperature, OK. volume per cent volume of equilibrium cell, ml. mole fraction of component a in gas mixture composition liquid phase, mole fraction composition vapor phase, mole fraction
I n applying Equations 1 through 4, it is desirable to keep the differences in the numerators and denominators as large as possible. This can be accomplished by selecting the compositions of the gas mixtures so that one run measures volume per cent liquid near the bubble-point and the second measures volume per cent liquid near the dew-point. Experimental Verification of Equipment Operation
T h e apparatus was operated from below 14' K. to above 300' K. Operation was satisfactory with either liquid or gas bath medium. Lack of sufficient vertical agitation in the gas bath caused a temperature stratifi(Continued on next page) VOL. 5 5
NO. 1 1
NOVEMBER 1 9 6 3
41
Table IV.
I
Smoothed Liquid Phase Density
i
Saturated Liquid Density
I
Pressure, L b /Sq. In. Abs.
.
100 200 300 400 500 600 700 800 900 1000
0.14 0.31 0.46 0.62 0.77 0.91 1.05 1. I 8 1.28 1.38
0.02898 0.02910 0.02921 0.02930 0.02938 0.02945 0,02952 0.02959 0.02965 0.02971
0.8109 0.8131 0.8151 0.8165 0.8177 0.8186 0.8196 0.8206 0.8215 0.8225
0.8097 0.8114 0.8131 0,8148 0.8165 0.8182 0.8198 0,8214 0,8230 0,8246
From Van Itterbeek and Verbeke (15).
cation of about 0.5’ K. across the thermostated portion of the bath, as measured by moving a resistance thermometer vertically through the bath. Median temperatures are reported. To check the operating characteristics and reliability of the apparatus, experimental data were taken on the helium-nitrogen system. Data are presented here only in a region where previously published values are available for comparison. Using the technique based on Equations 1 through 4, the saturated phase and volumetric behavior of the helium-nitrogen system was determined along the 77’ K . isotherm to 1000 lb./sq. in. A combination of five gas mixtures was used. Three of the mixtures were of low helium content, yielding high volume per cent liquid, and two mixtures were of high helium content, yielding low volume per cent liquid. Pressure and volume were recorded only after the pressure remained constant for at least 20 minutes. This required about 30 minutes when the quantity of liquid in the cell was small, and more than an hour when the quantity of liquid in the cell was large. Compressibility factors required in Equations 1 through 4 were calculated from the virial coefficients for heliumnitrogen mixtures reported by Kramer and Miller ( 7 7 ) . Vapor and liquid phase compositions are listed in Table I. Figure 7 indicates the behavior of the vapor phase of the helium-nitrogen binary along the 77 ’ K . isotherm. The behavior a t lower pressures was determined from dew-point measurements on four helium mixtures. The values are listed in Table 11. These dew points were obtained by noting the break in a plot of the quantity of gas metered to the equilibrium cell us. cell pressure. The break in such a curve becomes less discernible as the helium content increases, making it impractical to use this approach above about 96 per cent helium, at least on the 77’ isotherm. The data of Buzyna, Macriss, and Ellington (2) and Kharakhorin (70) are plotted in Figure 7 for comparison purposes. Figure 8 indicates the behavior of the liquid phase along with the data of Buzyna (2) and Kharakhorin (70). It is interesting to note that the scatter in the data here 42
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
and that reported by Buzyna and Kharakhorin are of thc same order of magnitude. The data, however, were obtained by three different techniques, Buzyna having obtained his data by static sampling, and Kharakhorin by a recirculation technique. Comparison of the data of various investigators of the helium-nitrogen system is difficult, since few of the investigators report data for the same isotherms. Figure 9 is a cross-plot of data showing the bubble-point behavior of a 0.7 per cent helium mixture. The values at 69’ and 65’ K . labeled “this study,” and the solid-vapor locus are from a later part of the present project, and are reported by Rodewald (73). These are presented here to show the trend in the data and the lower termination point of the bubble-point locus. Data presented by DeVaney, Dalton, and Meeks (4),and Gonikberg and Fastowsky (6) are shown in addition to values for Buzyna and Kharakhorin. Fedoritenko and Ruhemann (5) have published plots of liquid composition for the heliumnitrogen system, but presented no tabular data. Table I11 lists the volumetric behavior for the binary as calculated from Equations 3 and 4. Since at 1000 lb./ sq. in. the saturated liquid phase contains less than 1.5% helium, the density of the liquid phase should correspond quite closely to that of pure nitrogen. Table IV is a comparison of the saturated liquid densities of the binary compared to the densities of nitrogen as reported by Van Itterbeek and Verbeke (75). The maximum difference between the liquid nitrogen densities of Van Itterbeek and Verbeke and the densities of the binary is about 0.24 per cent. The behavior of the saturated vapor compressibility factors is shown in Figure IO. At the higher pressures, the vapor phase is largely helium, and the compressibility factors for the vapor phase agree with those of helium, as shown in Figure 10. Data taken on pure nitrogen indicated that the apparatus was capable of determining liquid phase densities with an accuracy of the order of a few tenths of a per cent. Accuracy of vapor phase compressibility factors for pure nitrogen was about two per cent.
REFERENCES (1) Bloomer 0. T. Rao K. N., "Thermodynamic Properties of Kiticigen,~’ Inrt. Gar ?echnoi. R k . Buii. 18, 1952. (2) Buzyna, G., hlacriss, R. 4.,Ellington, R. T., Chem. Eng. Pmg. S’rn/>. Str 59,h‘o. 44, 101 (1963). ( 3 ) Davis, J. A,, Ph. D. thesis, Uniwrsity of Kansac, 1963. ( 4 ) DeVaney, JV. E., Dalton, B. J., hleeks, J. C., Jr., J. Ctrenr. lirtg. D o t n 8, No. 4, 473-477 (October, 1963). (5) Fedoritenko, A , , Ruhemann, Zh. Tekhn. Fir. 4, 36-43 (1937). (6) Gonikberg, M. G., Fantowsky, \V, G.: Acta Phyiicochim. U.R.S.S. 12) 67-72 (1940). (7) Hill, A . E., J . Am. Chem. Soc. 45, 1143 (1923). (8) Hoge, H. J., Rev. Sci. Insit. 21, 815-16 (1950). (q) Johnson, V. J., (general ed.) “A Compendium of the Properties of Mntcrials a t Low Temperatures,” Nat. Bur. Std. Cryogenic Engineering Laboratory Boulder. Colo . 1961. (10) Kharakhorin, F. F., Foreign Pelrol. lechnol. 9, 397-410 (1941), translation from Zk. Tekhn. F i z . 10, 1533-40 (1940). (11) Kramer, G. M,,Miller, J. G., J . P/zys.C/iem. 61,785-88 (1957). (12) Kurata,F., Kohn, J. P., Petrol.Procers. 11,No. 12, 57 (1956). (13) Rodewald, N. C., Ph. D. thesis, University ofKansas, 1963. (14) Strohridge, T . R., “Thermodynamic Properties of Kitrogen from 64’ L i i 300’ K. Between 0.1 and 200 Atmospheres,” N a t . Bu7, Std. Tech. N o t e 129,1962. (15) VanItterbeek,A., Verheke, O., C7yogenics2,79-80 (1961). ~j
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