Compressibility of ButanePentane Mixtures below The compressibility, ( p o ) o / ( p ~ ) ~of~ ~ , n-butane-n-pentane mixtures at 30" C. has been determined. The compressibility of n-pentane at 30" C. has been obtained for pressures up to 500 mm. of mercury. The compressibility of propane has been determined at 0" and 50" C., and that of isobutane at 0" and 30" C.; the variation of the compressibility coefficient of propane and isobutane with temperature is presented. I t is shown that the additive rule for determining the compressibility of gas mixtures at low pressures holds for mixtures of butane and pentane and for gaseous hydrocarbon mixtures comparable to those ordinarily encountered in natural-gas pipe line operations and in the production of natural gasoline.
One Atrnosnhere F. W. JESSEN AND J. H. LIGHTFOOT Humble Oil & Refining Company, Houston, Texas
A 500-cc. flask was used to measure equal volumes of the gaseous components. It was filled to an absolute pressure of 200 mm. mercury by the component to be measured and then transferred by mercury displacement to an evacuated 3-liter flask which served as a mixing chamber and reservoir for the gases. After the proper gaseous mixture had been obtained in the reservoir, at least 24 hours were allowed t o elapse before the sample was introduced into the compressibility system. This assured thorough mixing of the gases. The mole per cent mixtures of butane-pentane corresponding to 25, 50, and 75 per cent pentane were obtained by displacing the gases from the 500-cc. flask to the 3-liter flask in ratios of 1volume pentane to 3 volumes butane, 2 volumes pentane to 2 volumes butane; and 3 volumes pentane t o 1volume butane, respectively. Allvolumes thus measured were introduced into the reservoir at a constant temperature. The temperature of the water bath was maintained-within *0.02' C. by means of a supersensitive thermoregulator and relay. Pressure measurements were observed on an absolute manometer, and the mercury column heights were read with a cathetometer to 0.03 mm. The gases used in this study were obtained from the Ohio Chemical and Manufacturing Company. Purity of the samples was as follows: butane 99.5 per cent, propane 99.5, isobutane 99.0; pentane, boiling point between 35" and 37" C. (corrected). The pentane was fractionated and the middle fraction boiling at 36" C. (corrected) was used in all experiments. Natural as samples from East Texas with the following composition ?determined by Podbielniak microfractionation) were employed for measuring the compressibility coefficient of rather "wet" gas:
T
HIS investigation was undertaken in conjunction with experiments carried out on natural-gas gathering systems and similar systems designed to supply the gases of readily liquefiable hydrocarbons as fuel for either industrial or domestic use. The compressibility of gaseous mixtures of butane-air below pressures of one atmosphere has been reported (7). This work is a continuation of the previous study on such gas mixtures; it has been extended to include compressibility measurements of butane-pentane mixtures and of a natural gas from East Texas. Data are also presented on the compressibility of propane, isobutane, and npentane a t pressures lower than one atmosphere. No data are available on the compressibility of gaseous mixtures of butane and pentane. In the course of the investigation it was found that the literature recorded no values for the compressibility of n-pentane or isobutane a t pressures below one atmosphere. Batuecas (1) calculated a value of (1 X) for propane from the formulas of Berthelot (2) and van Laar (8). Before a study of the compressibility of such gas mixtures could be inaugurated, it was necessqry to determine the compressibility of the individual gases to be used in the proposed gaseous mixtures. Burrell and Robertson (3) and Donaldson (6) studied the compressibility of natural gas from various sources but a t pressures above one atmosphere.
Sample Sample 749 3061 Per cent by volume Air Methane Ethane Propane Isobutane n-Butane Is0 entrtne n-Lane Hexane
+
-
-
1.1477
1.1337
100.00
Speoifia gravity
100.00
Results
F The results
of this investigation are presented in Tables I and 11. The (1 X) values tabulated represent the average of a t least three determinations except in the case of propane at 0 ' C., in which instance only one determination was made, in order to check the calculated value reported by Batuecas ( 1 ) . The maximum deviation of any two isolated (1 A)
Experimental Procedure The method of investigation was essentially that described in a previous paper (7'); however, a change pras made in the preparation of the gas mixtures, so that the exact composition of the mixtures under observation was;known.
+
+
312
MARCH, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
FlG.1.
C O M P R E S S I B I L I T Y Of BUTANEPENTANE M I X T U R E S A T 3 0 ~ c . a 5OOrnm.HG
TABLEI.
313
( p U ) ~ / ( ~ yVALUES ) ~ ~ ~ FOR
BUTANE-PENTANE
MIXTURESAT 30" C. (pv)o
Pentane Mole
PI
%
Pa
v2
. . ..
. . ....
.. . .
. .. . . .
238.03 237.94 237.97 237.90 237.95
657.7895 657.7928 657.7928 657.7829 657.7829
353.00 352.84 352.96 352.85 352.90
440.8408 440.8276 440.8243 440.8210 440.8210
0
25
Pa
VI
va
(PViSoo
.. .
. . ... .
689.45 689.00 689.19 689.13 689.06
222.1296 222.1164 222.1131 222.1098 222.1098
,
(1
+ A)
1.022310 1.03278 (7) 1.02758 1.04029 1.02697 1.02710 1.02739
1.03959 1.02811 1.03946 1.03931 1.03983
--
Average I .02743 1.03970
50
234.48 234.40 234.47 234.39 234.43
057.7862 657.7829 657.8061 657.8094 657.7895
347.49 347.44 347.41 347.33 347.40
440.8276 440.8243 440.8475 440.8542 440.8342
677.02 676.97 677.04 676.89 677.01
222.1164 222.1131 222.1329 222,1430 222.1213
1.03122 1.03022 1.03227 1.08161 1.03150
1.04569 1.04487 1.04615 1.04660 1.04560
--
Average 1.03136 1.04558
75
150.72 173.22 173.25 173.29 173.26
667.7829 657.7961 657.7862 657.7961 657.7961
223.68 256.92 256.93 256.94 257.00
440.8243 440.8309 440.8309 440.8308 440.8309
437.79 501.84 501.87 501.82 601.96
222.1098 222.1164 222.1164 222.1296 222.1197
100
160.29 150.34 148.62 148.58 148.57
657.7882 657.7894 657.7894 657.7928 657.7894
222.81 222.92 220.34 220.30 220.35
440.8296 440.8308 440.8308 440.8375 440.8375
435.39 435.51 430.50 430.49 430.47
222.1157 222.1157 222.1196 222.1249 222.1263
1.03578 1.03588 1.03646 1.03740 1.03551
A\ erage 1.03621 1.04178 and the maximum deviavalues is 23.3 X 1.04138 1.04214 values is 20.5 X tion in any two (pv)o/(pv)~~~ 1.04170 An error of one per cent in the prepara1.04177 tion of the gas mixtures would cause a difAverage 1.04175 v ) ~ ~ ~of ference of 0.00025 in the ( ~ ) v ) ~ / ( pvalues a Calculated from previous (1 + X) data for n-butane at 30' C. the butane-pentane mixtures. Figure 1 shows the c o m p r e s s i b i l i t y of butane-pentane m i x t u r e s . The ( ~ ~ ) O / ( ~ V ) ~ O O pressure term in calculating the (pv) isotherms for the butanevalues of the gas mixtures differ a maximum of 6.9 X pentane mixtures is within the experimental accuracy; howfrom the calculated additive values and are well within the ever, since such definite curvature is exhibited by n-pentane, limit of experimental error. Figure 2 shows the (pv) isothe (pv) isothermals of the various butane-pentane gas comthermals a t 30" C. These isotherms were calculated using the positions have all been plotted using the second-order presexpression sure term. Fighre 3 shows the variation of the compressibility coeffi(PV) = a bp cp2 cient of propane and isobutane with temperature. As in where a, b, c = constants determined from experimental data the case of n-butane (7) it is apparent that the calculated p = pressure, mm. of mercury v = volume, cc. values of the compressibility coefficient variation with temperature deduced from the relation given by Cawood and Patterson (4) lead to higher values than those experimentally There is evidence (7) that for butane some curvature of the determined. isothermals is indicated but that the results obtained when the p a term is not used in calculating (pv) is generally within the limit of experimental error. There is little curvature of the (pv) isothermals of isobutane and butane; however, for pentane the curvature is distinct and is beyond the accuracy of the tests. The error made in omitting the second-order ~~
+ +
PR O P 4 N E
1.m
,016
FIG.2
P V ISOTHERMALS AT 80'
C.
0..OZQ
0.S8
.o
2
8/
P
0 91
0.96
Y 0
100
200
100 P mm.up
400
so0
600
Too
":I //'
,-
I T
INDUSTRIAL AND ENGINEERING CHEMISTRY
314
TABLE11. (1 Temp., OC.
PI
+ A) VALUES
Pa
Dl
(1 (1
+ A)
pa
ua
582.46
221.9052
1.02073"
440.9674 440.9631 440.9610
690.14 690.27 690.31
222.2562 222.2519 222.2498
1.01201 1.01274 1.01245
u2
0
198.58
657.5750
Propane 295.45 440.6164
50
234.68 234.72 234.79
657.9281 657.9217 657.9196
349.46 349.44 349.60
then A.
657.5832 657.5920 657.5773 657.5843 657.5920
296.34 296.22 296.42 345.47 346.84
440.6240 440.6240 440.5852 440.6173 440.6328
579.86 579 89 580.35 674.24 676.50
221.9128 221.9122 221.8740 221.9163 221.9198
260.33 275.43 275.47 275.44 212.77
657.7829 657.7855 657.7829 657.7829 657.7974
386.53 408.96 408.93 408.95 316.17
440.8361 440.8204 440.8355 440.8375 440.8388
757.16 800.36 800.42 800.41 620.68
222.1230 222.1092 222.1243 222.1290 222.1276
1 ~
(1
- Ao)
+ AD (&)
where Ao = compressibility coefficient p = observed pressure P A += pressure at which A0 is known
1.04129 1.04176 1.04020 1.04062 1.04253
1
+ 0.02670
(g)
= 1.01843
V'
Hence the number of moles of the gas is
Average 1.04128 30
=
0.02670
1
Average 1.01240 199.67 199.62 199.70 232.87 233.86
=
+ A)
Thus the compressibility factor is:
Isobutane 0
VOL. 30, NO. 3
1.02949 1.02847 1.02932 1.02851 1.02900
Average 1.02896 0
One determination made t o check caloulated d a t a of Batuecas (f).
Figure 4 is a plot of the compressibility a t 0" and at 30" C. us. average molecular weight of gaseous hydrocarbons. The compressibility value for methane a t 0" C. is that given in International Critical Tables (6); the values for ethane and propane a t 0" C . are those reported by Batuecas (1). The natural gas compressibility a t 30" C. falls well within the experimental accuracy. It is noteworthy that the isobutane compressibility falls considerably below that of n-butane, and it is not improbable that the same relation exists between the compressibility values of isopentane and n-pentane. Figure 4 shows that the compressibility of any gaseous hydrocarbon mixture a t pressures near one atmosphere may be obtained from a fairly accurate measurement of the specific gravity of the gas. It is often necessary to know rather accurately the volume of gas being delivered a t pressures near one atmosphere or a t a somewhat lower pressure. For example, it may be desirous to ascertain the number of moles of a 75 per cent butane-25
X and A0 are generally quite small, and no great error will result if they are used interchangeably. Table I11 presents the variation of the (pv)o/ ( p ~values ) ~ from 100 to- 760 mm. mercury for the various gases and gaseous mixtures observed. ~
~~
~~
TABLE 111. VARIATION OF ( p ~ ) ~ / ( p v FROM ), 100
MERCURY
Temp.,
Gas Propane
O
Isobutane n-Butane 75% butane 25% pentane 50% 50% butane pentane 75% 25y0 butane pentane 100% pentane
+ + +
TO
760
MM.
(PV)O/ (PU)SOO
(pub
0 50
1 4- A 1.02073 1.01240
1 0159 1 00944
1.01059 1.00621
(pr)ioo 1.00387 1.00225
0 30
1,04128 1.02896
1.02900 l.01?76
1.01812 1.01217
1.00626 1.00421
C.
:
(PV)O/
(PO)O/
30
1.03277
1.02231
1.01372
1.00468
30
1.03969
1.02751
1.01700
1.00587
30
1.04558
1.03136
1.01941
1.00666
30 30
. . , .. , . ...
1.03621 1.04175
1.02350 1.02795
1.00815 1.01014
Acknowledgment The authors express their thanks to W. J. Frank, who aasisted with the calculations, and to the Humble Oil and Refining Company for permission to publish this paper.
FIG. 4
COMPRESSIBILITY VARIATION 0 F GASEOUS HYDROCARBONS 1.040
Literature Cited (1) Batuecas, J . chim. phys., 31, 165-83 (1934). (2) Berthelot, Trao. mem. bur. intern. poids mesures, 13, 38, 100 (1907). (3) Burrell and Robertson, Bur. Mines, Tech. Paper 158 (1917). (4)Cawood and Patterson, J . Chem. Soc., 1933,019-24. (6) Dooaldson, Western Gas, 6, No. 1, 46-9 (1930). (6) International Critical Tables, Vol. 111, p. 3,New York, McGrawHill Book Co., 1928. (7) Jessen and Lightfoot, IND.ENQ.C H ~ M28,870-7 ;., (1936). (8) Laar, van, "Die Zustandsgleichung von Gasen und Flussigkeiten," pp. 37-8, Leipzig, Leopold Voss, 1924.
1.oz.o
2 I.OP0
1.010
RECEIVIUD September 23. 1937. 1.000
0
Cl
E2
9
f4
c5
per cent pentane mixture occupying a volume of 1000 cc. at 345 mm. pressure and 30" C. From the compressibility data of the butane-pentane mixtures it is found that (pv)o/ (pv)~00 is 1.02743. Since Cawood and Patterson (4) showed that (1 A) is related to Ao, the compressibility coefficient, by the equation,
+
Purified Rubber for Electrical Insulation-Correction On page 649 of the June, 1937, issue of INDUSTRIAL AND EN-
CHEMISTRY an error occurs in the Literature Citations. Reference (7) to the patent issued t o C. R. Boggs should read: U. S. Patent 1,997,355. A. R. KEMP GINEERING
