Carbon Dioxide in a Natural Gas-Condensate System FRED H. POETTIMANN AND DONALD L. KATZ University of Michigan, A n n Arbor, Mich.
Equilibrium constants for carbon dioxide i n a natural gas-condensate system have been determined over the range 1 to 10 mole % carbon dioxide. Densities and molecular weights have been determined for saturated vapor and liquid phases for twenty-four hydrocarbon mixtures containing carbon dioxide a t temperatures from 100" to 250" F. and pressures from 500 t o 2900 pounds per square inch absolute. It is s h o w n that the lower the
molecular weight of the hjdrocarbon in the binary carbon dioxide systems, the greater will be the deiiation of the carbon dioxide from ideal behaiior. Equilibrium constants of carbon dioxide in the natural gas-condensate system deiiate most from the ideal equilibrium constants. 9ince the multicomponent mixtures consisted of oier 60 mole yo methane, the methanecarbon dioxide system may be expected to show wide deviations from ideal solutions.
N
0 REPORT has been found in the literature on vaporequilibrium cell. The pressure on the cell ivbis obtained from a calibrated Bourdon tube gage having a range of 0-5000 pounds. liquid equilibria for carbon dioxide in a multicomponent The gage could be read probably to * 5 pouiitls per square inch of hydrocarbon system. Knowledge of this behavior would be useful in the petroleum industry for design calculations, since carbori thc true value. dioxide in varying amounts is found in most natural gases. Equilibrium constants for hydrocarbons such as nat,ural gsisTRAIN FOR A'VALYSES crude oil (1, 4 ) and natural gas-distillate (3) systems have been The mole fraction of carbon dioxide in a sample rms tlcterstudied exclusive of their carbon dioxide content. This paper reports equilibrium constants of carbon dioxide in a natural gasmined by weighing the carbon dioxide after absorption on condensate system and the comparison of these constants with Ascarite and also by determining the t,otal moles of sample anat,hose of the binary syst,ems described in a previous article (2). lyzed. The equilibrium phases were analyzed by the train shown The equilibrium phases were not analyzed for the concentrat,ion in Figure 1. A is a U-tube packed v-ith Ascarite and Dehydrite, of individual hydrocarbon constituents. R is the pycnometer containing the sample to be analyzed, C is a The apparatus used to obtain the vapor-liquid equilibrium liquid trap, D and E are 5-tubes packed with Drierite, F and G data \vas essentially that construct,ed by Standing and Katz ( 4 ) . are U-tubes packed with Ascarite and Dehydrit,c, and H is a UIt consist,ed of a steel equilibrium cell of about 750-cc. capacit,y. tube packed with Drierite. I is a small mercury bubbler to inThe cell was supported in a steel frame and surrounded by an dicate flow rate, J an open-tube mercury manometer, K a 10liter bottle serving as a reservoir, and L a water bath. The air bath with walls of asbestos board. The frame, in turn, was supported by trunnions which permit,ted the rotation of the bath volume of the entire system was accurately determincd. and thus provided agitation of the fluid within the cell. The pycnometer containing the sample to be analyzed was The temperature of the bath was maintained by manually placed in the position shown in Figure 1. U-tubes F and G were controlled 500-matt and 250-watt heaters. A second 500-watt then weighed on a n analytical balance. The entire system from heater was automatically controlled by a bimetallic thermoC to K , inclusive, was evacuated t o as low a pressure as possible (such as 1 mm. or less), and the pressure and temperature were regulator. Two thermocouple wells drilled into the cell walls noted. The lower valve of the pycnometer was then cracked permitt,ed measurement of the temperature of the cell by Calibrated copper-conopen, and the sample stantan thermocouallowed to enter the ples. It was posanalysis train. The sible to maintain this PYC NOMET ER gas passed through temperature within A the U-tubes, and the COz ABSORBERS carbon dioxide was +0.3" F. absorbed. -4ny cnTwo calibrated steel pycnometers of t,rained liquid was about 25-co. capacity caught in trap C. were used t o take About 10-15 minutes were allowed for the samples of the liquid and vapor phases by sample t o escape d i s p l a c e m e n t of from the pycnommercury. A mercury eters. After the hand pump provided mercury level in the the pressure. A manomet,er indicated surge cell of ahout that the sample had 350-cc. capacity y i s RESERVOIR completely escapcd used with the hand from the pycnometer, pump t o transfer pressure and temliquid samples to the Figure 1 . Analysis Train for Carbon Dioxide pcrature readings I
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With the volume of expanded gas, its temperature, and its pressure known, the number of moles of gas were calculated. From the weight of carbon dioxide absorbed in the Ascarite, the moles of carbon dioxide in the sample were determined. From the weight of the liquid residue and its molecular weight, the moles of liquid residue were calculated, The sum of the expanded gas, carbon dioxide, and liquid residue then gave the total moles of sample which, divided into moles of carbon dioxide, gave the mole fraction of carbon dioxide in the sample. There were no liquid residues in any of the vapor samples analyzed. The carbon dioxide came from Pure Carbonic Inc., and was stated to have a purity of 99.5% or better. Spccessive samples were completely absorbed in potassium hydroxide solution. The natural gas and weathered condensate came from the Erath field in Louisiana (well 3-2) and were furnished through the courtesy of The Texas Company which operates the Erath unit. Table I gives the analysfs of the Erath natural gas and weathered condensate, using boiling ranges to express the composition of the heavy ends of the condensate. Figure 2 shows the distillation curve for the Erath weathered condensate as obtained in a high-temperature Podbielniak column. Figure 3 gives curves for density and molecular weight us. mid-boiling point of the condensate fractions. Figure 4 correlates density with molecular weight for the condensate and fractions having the mid-boiling points shown on Figure 3.
600
500
400 k!
I
w
4
300
5 b u
2k 200 100
0
.
20
VOLUML
40 Ea eo PERCENT DlSTlLLLD
100
Figure 2. High-Tern erature Column Distillation o r Erath Weathered Condensate
.
were taken. The upper valve of the pycnometer was then cracked open, and air was drawn into the system to displace the gas in B, C, D, and E so as to absorb all of the carbon dioxide. U-tubes F and G were then weighed to determine the weight of carbon dioxide absorbed. The liquid residue in the trap was weighed, and care was taken to correct for any mercury going over in the sample. The molecular weight of the liquid residue was determined by the freezing point lowering in benzene saturated with water, with extrapolation to zero concentration of the solute. The density of the liquid was also determined.
Figure 4.
Molecular 'Weight-Density Correlation
GAS^ TABLE I. ANALYSISOF ERATHNATURAL
AND
WEATHERED
CONDENSATE* Component
Mole Fraction Natural Congas densate
.... ....
COI CHI CiHs CaHa ~SO-CIHIP n-CaHio iso-CIHii n-CrHtt CsHu C7Hl6' 100
200
300 400 TEMPERATURE P.
500
$00
Figure 3. Mid-boiling Point us. Density and Molecular Weight for Erath Condensate
--
Mol. wt.
in. a m .
180-300° F. 300-400 400-500 500-600 >600
.*..
Mple
Fraction of Condenrats
0.2666 0.2260 0.2040 0.1290 0.0466
.... 18.68,gravity = 0.640,p T c 139.2, density a t 60° F.
Mol. wt. 77O F. 1.329 oentipoises. b
Component Distilling over at:
-
P
368' R., = 670.7Ib./sq. 0.7875gram/co., viscosity at
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