INDUSTRIAL AND ENGINEERING CHEMISTRY
2536
Vol. 46, No, 12
TABLE 11. S X O O TEXPERIMENTAL H DATAFOR C A R B ODIOXIDEX P R O P A KSYETEV E T
~
F.
0
32
-4
- 40
~
Pressure, . Fraction COz ~~ b, . , 1 ~ ~ Mole Inch Liquid Vapor
300 400 507
0,oCl0.000 0.260 0.480 0.712 1.00
0.00 0.330 0.683 0.818 0.882 1.00
34 100 200 250 285
0.00 0.190 0.518 0.820 1.00
0.00 0.656 0.851 0.928 1.00
15 50 100 145
0.00 0.165 0.528 1.00
0.00 0.710 0.882 1.00
67 100 200
Equilibrium _____ Ratio Con Propane
...
8.00 2.63
1.70
1.24 1 .oo
1.00 0.71 0.43 0.33 0.41
...
...
1.00 0.42 0.31 0.45
...
1.00 0.35 0.25
3.45 1.64 1.13 1 .00 4.30 1.67 1.00
...
...
propane beyond the experimental data because of the great possible error at the very l o a concentrations of propane; however, i t can be seen that there is a definite minimum in the equilibrium ratio for propane and the value tends to approach unity near the vapor pressure of the carbon dioxide. ACKNOWLEDGMENT
This work was made possible by a fellowship granted by the P a n American Refining Corp.
PRESSURE-PSIA
Figure 9. Equilibrium Ratio-Pressure Diagram for Carbon Dioxide-Propane System
“Chemical Process Prin( 5 ) Hougen, 0. A . , and Watson, I(. PI., ciples,” S e w York, John Wiley & Sons, 1947. (6) Kate, D. L., and Reasa, M. J., “Bibliography for Physical Behavior of Hydrocarbons under Pressure and Related Phenomenon,” h i 1 Arbor, Mich., J. W. Edwards, Inc., 1946. ( 7 ) Lange, A. Tu’., “Handbook of Chemistry,” Sandusky, Ohio, Handbook Publishera, Inc., 1941.
REFERENCES
(1) Brown, G. M., and Stuteman, L. F..C’hem. Eng. Progr., 45,
142-7 (1949). (2) Daynes, H. A., “Gas Analysis by Thermal Conductivity” Lon-
don, Cambridge University Press, 1933. (3) Dodge, B. F., “Chemical Engineering Thermodynamics,” New York, McGraw-Hill Book Co., 1944. (4) Guter, M., Newitt, D. M., and Ruhemann, XI., Proc. R o y . SOC. London, 176,140-52 (1940).
(8) Poettman, F. H., and Katz, D. L., IXD.ENG.CHEM.,37,847-53
(1945). Ruhemann, M., Proc. R o y . Soc. London, 171, 121-36 (1939). (IO) Sage. B. H., Lacey, W. N.,and Schaafsma, J. G., IND.ENQ. CHEM.,26,214-17 (1934). (9)
RECEIVSD for review September 14, 1953.
ACCEPTEDAugust 16, 1954. Presented in part a t the Seventh Annual Regional Meeting, AJIERICAN CHEXICAL S O C I E T Y , Austin, Tex.. December 1951.
Volumetric and Phase Behavior of Nitrogen-Hydrocarbon Systems NITROGEN-n-HEPTANE SYSTEM W, W. AKERS, D. M. KEHN1, AND C. H. #ILGORE2 T h e Rice Institute, Houston, Tex.
T
HE volumetric and phase behavior of the nitrogen-hydrocarbon systems, in addition t o being of academic interest, is of practical value in the production of natural gasoline and crude oil. With the utilization of natural gas from fields containing high fractions of nitrogen, these data become of increasing importance. The injection of nitrogen into a petroleum reservoir for pressure maintenance may have definite economic advantages in certain fields. n-Heptane was the hydrocarbon selected for this investigation because i t is representative of the light oil fractions encountered in many condensate systems. Numerous investigators have published the results of work on volumetric and phase behavior of systems consisting of two or three pure hydrocarbons and of carbon dioxide and hydrocarbons. Until the present, few data have been available on the nitrogen1
2
Present address, Humble Oil Co., Houston, Tex. Present address, C r o n n Central Petroleum Co.. Houston, Tex.
paraffin hydrocarbon systems. Beattie and Kay ( I ) have determined the comprewibility of n-heptane a t temperatures from 30” t o 250” C. and pressures up to 5000 pounds per square inch. The densities of mixtures of n-heptane and ethane have been investigated by Kay ( 4 ) over the entire range of composition. I n connection with a study of the natural gas-heptane system Boomer, Johnson, and Piercey (2) obtained solubility and phase density data on nitrogen in n-heptane a t 100.9 atmospheres and temperatures from 25” t o 115”C. Their data, althoughof a high order of accuracy, are of limited value bemuse results are reported a t only one preasure. MATERI4LS
The nitrogen uaed in this investigation was bone-dry prepurified nitrogen containing leEs than O.Olyoimpurities. The n-heptane (research grade) was furnished bv the Phillips Petroleum Co. The freezing point puiity was 99 58%.
December 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
2537
EQUIPMENT
A stainless steel cylinder having a capacity of 500 cc. was used for the equilibrium cell. It was connected directly to a mercury pump of 100-cc. capacity with a scale marked a t 1-cc. intervals and a vernier marked a t 0.01-cc. intervals. The pressure was read from a 16-inch Bourdon-type pressure gage connected directly to the pump. The gage was calibrated over the full pressure range, 0 to 10,000 pounds per square inch, a t frequent intervals during the work. The cell was held a t a constant temperature (rtO.5" F.) by an electrically heated oil bath and could be rocked back and forth a t a rate of 55 oscillations per minute. All tubing connections were made with 3/la-inch stainless steel tubing. A spiral of tubing permitted the pump to be connected directly to the pump while i t was being rocked. By means of a valve fitting a t the top, the equilibrium cell could be connected to a 600-cc. heptane storage cell, to the nitrogen cylinder, or to a sampling train. The temperature of the oil bath was measured by a mercury thermometer. This was taken as the temperature of the cell.
0'
2600
'
I
I
4000
I 6000
I
I 8000
I 10000
PRES SUR E -PSI A Figure 1. Phase Density at 90' F.
EXPERIMENTAL
Starting with the entire system filled with mercury a t 1000 pounds per square inch, the pump reading was recorded and a measured volume of mercury was withdrawn from the equilibrium cell. At the same time nitrogen was charged to the cell from the cylinder through the valve a t the top of the cell, a special needle valve with a long taper. The nitrogen was charged a t 1000 pounds per square inch and the second pump reading was recorded. Before the n-heptane was charged, more mercury was withdrawn from the cell. The equilibrium cell was then connected to the heptane storage cell by a short piece of tubing. The heptane was forced by mercury a t 1000 pounds per square inch absolute and room temperature into the equilibrium cell. Thus the volume of heptane charged was accurately measured a t 1000 pounds per square inch absolute. Room temperature variations during the charging period were of negligible effect. The pressure in the equilibrium cell was then adjusted by addition or removal of mercury by the pump and the entire contents were allowed to come to equilibrium by rocking for 30 minutes. The vapor sample was obtained by cracking the tapered needle valve a t the top of the equilibrium cell and allowing the gas to escape slowly a t atmospheric pressure. .4t the same time the pump was advanced a t a rate necessary to maintain constant pressure in the cell while the vapor phase was being removed. The end of the vapor phase in sampling was detected b a marked jump in pressure on the gage. At this point, whicK could be determined within 0.1 cc., pump and meter readings were recorded and the sampling was continued to the liquid phase. The volume of the gas phase a t atmospheric pressure was determined by metering it with a dry test meter. A gas density balance mas used to determine its density and hence its composition. This method of analysis proved unsatisfactory because of the small amounts of heptane contained in the vapor and it was abandoned after a few runs. Thereafter the heptane was adsorbed on activated carbon before the nitrogen was passed through the meter. Tests indicated that 15 grams of activated carbon in a drying tube would remove up to 1.8 grams of heptane from the gas. Most of the gas samples contained under 1 gram of the hydrocarbon. I n determining the composition of the liquid phase the saturated liquid was flashed to atmospheric pressure and the remaining liquid forced out of the cell and weighed. The flashed gas saturated with heptane was metered and the amount of heptane in it calculated from the vapor pressure of heptane at the temperature of the saturated gas. The soundness of this procedure was verified by a test run in which dry nitrogen was bubbled through heptane and the hydrocarbon collected on activated carbon. The amount of heptane carried over by the nitrogen was found to check within 1%of the amount calculated by assuming Raoult's law. The bubble point was determined for a mixture containing approximately 75 mole % nitrogen at 260" and 360' F. At lower temperatures the bubble point for this charge was above the capacity of the pressure gage. This mixture was charged to the cell and the pump advanced a few cubic centimeters a t a time.
Figure 2.
P RESSURE-PSIA Phase Density at 175' F.
After each advance the cell was oscillated for 30 minutes and the pressure and pump readings were recorded. A plot of pump reading against pressure gave a marked increase in slope a t the pressure when the last bubble of gas was forced into the liquid. The intersection of the two lines was taken as the bubble point and the corresponding pump reading was used to calculate the density of the saturated liquid. The density of a mixture containing 9.46% nitrogen was obtained up to 8000 pounds per square inch by advancing the mercury pump and decreasing the volume of the cell a few cubic centimeters a t a time. Each time the contents of the cell were allowed to come to thermal equilibrium and the resulting pump reading and pressure were recorded. In calculating the density, corrections were made for the thermal expansion of the mercury forced into the cell, for the expansion of the cell with pressure, and for the compressibility of mercury. RESULTS
The experimental data can be grouped into two parts: the phase density measurements and the phase equilibria measurements. The densities of the saturated liquid and vapor phases are tabulated in Table I and shown diagrammatically in Figures 1 to 4. In Table I1 are given liquid densities of a mixture containing 9.46 mole % nitrogen. A cross plot of the data a t 100.9 atmospheres is shown in Figure 5 , where the data of Boomer et at. ( 2 ) are also plotted. The agreement of the two works is satisfactory for the saturated vapor densities. For the liquid densities the data of Boomer are slightly higher below 120" F. and slightly lower a t higher temperatures. At 90" F. the density of the liquid phase increases with increasing pressure. A t the higher temperatures the greater solubility of the nitrogen in heptane causes the density to decrease with increasing pressure. The phase composition data for the four isotherms are presented in Figures 6 and 7, which show the marked increase in the
INDUSTRIAL AND ENGINEERING CHEMISTRY
2538 0.8
I
I
I
I
TABLE1.
PHASE
FOR
PRESSURE- P S l A Phase Density at 260" F.
-
L
J
I
! I
0 OS \
0
I
04
02
I
i .
-0
-
I
Liquid Density, G./Cc.
1,030 1,525 1,774 1,774 2,022 2,301 2,671 3,022 3,329 3,325 4,017 5,025 6,030 8,025 10,025
0.688 0 667 0.657 0.670 0.671 0 666 0 . fj6S 0 650 0.656 0.603 0.663 0.671 0.666 0.692
1,020 2.111 3,022 4,027 6,030 8,026 10,026
0.635 0.643 0.839 0.631 0.636 0.641 0.605
176" I'. 90.4 81.3 75.3 70.9 56.7 45.3 29.5
1 ,060 2,022 3,022 4,027 6,535 7,525 7,52d 8,005 9,005
0,597 0.591 0.583 0,586 0.542 0.526 0.548 0 . A49 0.522
1,131 2,220 3,025 3,525 4 027 4 027
0.546 0.507 0.495
0.692
260O
PRESSURE- PSlA
Figure 4.
DEXSITY -4XD EQUILIBRILX CoXCESTRATIoXS ~ITROGEK-n-I~EPT-4SE S Y S PEJI Mole R n-C in Liquid 90" F. 92.0 87.2 85.5 84.9 84.8 83.0 70.9 78.3 75 6 76.4 73.4 68.2 53.7 56.3 49.5
Pressure, Lb./Sq. Inch
Figure 3.
Phase DensiLy at 360' F.
Vol. 46, No. 12
4.495 6,323
...
0.073
0,724a 1.00a 0 . 3gn 0.25 1.20 0.50a 0.60 0.50" 1.00,J 0.70 0 79 1.07 1.39 2.18 3.00
0.'137 0.137 0.152 0.174 0.201 0.227 0.249 0 251 0.282 0.333 0.375 0.415 0.605
1.78
0.0716 0.142 0.190 0,271 0.354 0.431 0.449
1.13
1.31 1.54 2 43 4.08 5.88
0,0689 0.130 0.164 0.238
38.6
0,381
27.9 28.9 27 24.,
0.401 0 :4i1
360" F. 87.2 67.9 55.8 57,2
...
60
[Figure 5 .
I
I
I
I
I
1
I
I
100
140
180
220
260
300
340
380
I
0.144
0.201 0.191 0 231
...
44.6
...
18.2
1.0
420
TEMPERATURE-'E Phase Densit> at 100.9 atmospheres' Pressure
3.48 3.55 2.28 2.90 5.37 17.2 17.2 16.6
6.47 5.14 4.94 4.59 6.21
0.0666
0.'321 0 :4 i 7 24.8 Gas denrity balance used t o analyze vapor phase.
OL
lClole YGn-C; in Vapor
I?.
88.4 78 7 69.9 01 3
0:464
Vapor Density, G.iCc.
z 0.8
0
t-
solubilit,j-of the nitrogen at higher tempera,tures. This unusual phenomenon can be attributed to the nonidealit,y of the nitrogenheptane solutions. The vapor-liquid equilibrium ratios ( g 12) for nitrogen and heptane are plotted in Figures 8 and 9. Here again the effect of increasing solubilit,g ivith temperature is shonm. The eqiiilibrium ratio for the nitrogen decreascs rapidly as the temperature is increased. This effect is more pronounced a t the higher pressures. The K values for nitrogen are of a high degree of accuracy (1%-ithin 4%), and check very closely those reported by Johnson et a?.( 3 ) . The K values for n-heptane a t low pressures are niucli less accuratmebecause of the extremely small amounts (less than 1%)of heptane in the vapor phase. The experimental runs in which the gas dcnsity balance was used to determine the compoeit'ion of the vapor phase vere not used to compute the n-heptane equilibrium ratio values. Of special interest is the closed loop of the 3GO" F. composition curve on Figure 7 . The vapor and liquid p h a m become identical a t a pressure of 4800 pounds per square inch. The equilibrium ratios for both the nitrogen and the heptane approach unity a t this pressure and temperature (Figures 8 and 9). The phase density curve, Figure 4, also indicates an ext,rapolated critical pressure of approximately 4800 pounds per square inch at 360" F. Extrapolation of the data for the other isotherms indicates a critical pressure of 9000 pounds per square inch a t 260' F., 13,000 a t 175" F., and 23,200 a t 90' F. The approximate compositions
2 OB
[L
LL
-I or4
0
5 02
0 2000
4000
8000
8000
10000
PRESSU RE-PS IA Figure 6. Cornposition-~ressul.eDiagram I
Figure 7.
I
I
I
i
PRESSURE- PSlA Composition-Pressure Diagram
I1
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1954 10 9 8
2539 SOURCES O F ERROR
7 6
5 4
x $ 3 Y
2
I 1000
Figure 8.
2000
3000 4000
6000
10000
PRESSURE -PSIA Nitrogen Equilibrium Ratios
AT 9.46 MOLE% NITROGEN TABLE 11. LIQUIDDENSITY
90' F. Pressure, Density, lb./sq. inch g./cc. 1983 0.6776 3058 0.6851 4007 0.0915 5085 0.6975 6091 0.7058 7195 0.7084 8035 0.7128
175O F. ~Pressure, lb./sq. inch 3274 4107 5085 6101 7065 8045
Density, g./cc. 0.6551 0.6614 0.6684 0.0749 0.6811 0 6867
260' F. Pressure, Density,
Ib./sq. inch 2530 4027 6060 7035 8055
g./cc. o,6118
0.6269 0.6444 0.6514 ".6584
a t these critical points can be ascertained from Figures 6 and 7. rllthough these extrapolated values of the critical points are not of a high degree of accuracy, they give an insight into the phase boundaries of this nonideal mixture a t very high pressures.
The principal sources of errors in the experimental equipment are: measurement of pressure, measurement of temperature, measurement of volume of liquid and vapor samples, sampling of a pure phase, and analysis of the liquid and vapor phases. The pressure could be read t o within 1 pound per square inch. Throughout a run, the maximum variation of pressure was less than 6 pounds per square inch. Therefore, i t can be assumed that the error in pressure measurement is less than 3 pounds per square inch. The temperature PRESSURE was read directly from a mercury (1000PSIA) thermometer. The temperature Figure 9. n-Heptane variation throughout a run was Equilibrium Ratios less than 1 F., and the temperature a t the time of sampling was within 0.2" F. of the desired temperature. It is thus concluded that the temperature is within 0.5" F. of the equilibrium temperature. The volume measurements are within i O . O l % . The t,emperature variation has a maximum effect of approximately 35 pounds per square inch on the liquid density measurements and approximately 5 pounds per square inch on the vapor density measurements. The error in the phase composition is less than 0.1%. At high pressures i t was extremely difficult t o obtain checks on the phase density and composition. This reflects the difficulty of sampling a t near-critical conditions, where small changes in temperature or pressure can make relatively large composition changes. Moreover, a t these conditions, obtaining a homogeneous sample is extremely difficult.
(Volumetric and Phase Behavior of Nitrogen-Hydrocarbon Sys tems)
NITROGEN-BUTANE SYSTEM W. W. AKERS, L. L. ATTWELL', AND J. A. ROBINS0K2 The Rice Institute, Houston, Tex.
T
HE experimental equipment used with the nitrogen-nheptane system was employed in this study.
Xitrogen and butane were charged t o the cell a t 1000 pounds per square inch and room temperature in proportions to give a two-phase mixture a t the conditions under study. The heater was turned on and the pressure adjusted to the desired value. The cell was then rocked by the electric motor drive until the pressure and temperature remained constant for a t least 15 minutes. The cell was thcn etopped in a vertical poition and allowed to remain in this position for 30 minutes to allow separation of the two phases. The vapor was then sampled by cracking the needle valve on top of the cell, a t the same time maintaining cell pressure by the injection of mercury- into the cell with the pump. The gas density of the sample was determined directly by the Edwards density balance. Sampling was continued until three checks has been made on the gas density, then the remaining vapor was forced out of the cell. The liquid interface could be detected by a 1
2
Present address, Humble 011a n d Refining Go., Houston, Tex. Present address, Crown-Central Petroleum Co., Houston, Tex.
sudden jump in the cell pressure. After it was certain that only liquid remained in the cell, the cell pressure !?as increased approximately 1000 pounds per square inch and the liquid sampled in a manner similar to the vapor. RESULTS
The pressure-composition data for the two envelopes are given in Table I11 and plotted for the four isotherms in Figure 10. All four isotherms form closed loops. I n the 100" F. isotherm, the lowest point on the curve is 100% n-butane a t its vapor pressure. As the pressure is increased, the vapor rapidly becomes leaner in n-butane while the liquid very slowly decreases in butane. After the pressure has been increased to 1000 pounds per square inch absolute, the vapor composition remains almost constant up to 3200 pounds per square inch, then becomes richer in butane as the critical point is approached. This sudden change in the behavior of the vapor
