Low Temperature Vapor-Liquid
Equilibria IN HYDROGEN-n-BUTANE SYSTEM HARRY J. AROYANl
AND DONALD L. KATZ University of Michigan, Ann Arbor, Mich.
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Equilibrium vapor and liquid phase compositions for the hydrogen-n-butane system were determined at pressures from 500 to 8000 pounds per square inch along isotherms of 75", 40°, loo, - Z O O , -50", -looo, -150°, and -200' F. Phase compositions and equilibrium constants are presented for this system. A graphical correlation for the prediction of the equilibrium constant for hydrogen in binary hydrogen-hydrocarbon systems is included. The phenomenon of a minimum in the bubble point curve is observed for this system.
D
ATA on the vapor-liquid equilibria relationships of hydro-
gen-hydrocarbon systems are relatively scarce. Listed in Table I are all the binary hydrogen-hydrocarbon phase equilibria data that could be found in the literature. While the tabulation may seem to be extensive, such is not the case, as,for the most part, the data consist of a few isolated values. Furthermore, a large portion of the investigations is in the form of solubility data in which only the composition of the liquid phase is reported.
Figure 2.
Flow Sheet o f Apparatus
A . Equilibriumdl B. Magnetic pump C. Pressure control cylinder D . Stirring motor E. Coolisgaoils F . Heater G. Refrigeration unit €2. Insulated jacket
TEMPERATURE
8
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No vapor-liquid equilibria data have been reported below room temperature for hydrogen in hydrocarbon systems above ethane in molecular weight. The reason for the absence of more complete investigations of the phase behavior of hydrogen-hydrocarbon systems has probably been the temperature limitations of the usual experimental equilibrium cells now in use. From a consideration of the low critical temperature of hydrogen, it is evident that a low temperature equilibrium cell is needed to study the phase behavior of hydrogen-hydrocarbon systems extensively. The apparatus used in this work was constructed so that investigations of phase behavior could be extended to low temperatures and high pressures. In the investigation of the hydrogen-petroleum naphtha sys1 Preaent
1000 lb./sq. inch gage
L. Hydrogen o y h d e r
M. n-Butame eylinder N. Pressure scrubbers P.
Thermowell
Q. n-Butane condenser
hem, Kay ( I O ) reports dew and bubble points of mixtures containing small amounts of hydrogen. He directs notice t o the anomaly of a minimum in the bubble point curve a t low hydrogen concentrations. Because of this unusual feature, he postulates the existence of two isobaric retrograde condensation PBgions, one of which is relatively far removed from the critical point for the mixture. Figure 1 shows Kay's visualization of the complete border curve for a mixture having a minimum in its bubble point curve. The temperature limitations of his apparatus prevented him from investigating this phenomenon below room temperature. In their work on the solubility of hydrogen in n-butane above room temperature, Nelson and Bonnell(I2) report data that indicate the presence of a minimum in the bubble point curve in the vicinity of room temperature. The hydrogenn-butane system was chosen for study a t low temperatures and elevated pressures, so that this unusual behavior could be studied further. The hydrogen used was reported by the manufacturer to have a purity of 99.9 mole %. Water vapor was removed by passing the hydrogen gas, a t cylinder pressure, through a drying tube containing anhydrous calcium sulfate. Pure grade +butane wm obtained through the courtesy of the Phillips Petroleum Company. This butane was reported to have a minimum purity of
Figure 1. Border Curve for Mixture Having Minimum in Bubble Point Curve (23')
1
J.
K. 10,000 Ib./sq. inch gage
address, California Research Corp., Richmond Calif.
185
186
INDUSTRIAL AND ENGINEERING CHEMISTRY
99.0 mole % n-butane, the major portion of the impurity being Lobutane (2-methylpropane). EXPERIRIENTA L APPARATUS
The experimental work on the hydrogen-n-butane system was carried out in a clnsed-flow type system patterned after the apparatus of Dodge and Duribar ( 3 )in their low temperature work on the nitrogen-oxygen system.
Vol. 43, No. 1
n-butane was added to the cell. Hydrogen was then injected, either directly from the cylinder or through the compression unit I n obtaining equilibrium, vapor from the equilibrium cell, A , was removed from the low temperature bath and circulated by the magnetic P m p , R. From B the vapor wvas passe through a cooling coil and injected into the bottom of A , where it bubbled through the liquid. Two baffles were located inside A to effect more complete separation of coexisting phases. Circulation was continued for approximately 2 hours prior to sampling The attainment of equilibrium was checked in two way8. First, a t each new isotherm, the composition of the liquid phase as a function of the time of circulation was determined. The leveling off of the composition with time served to indicate that equilibrium had been closely approached. As a second check, the compositions of coexisting phases were compared upon reaching equilibrium by two different paths. Equilibrium compositions obtained by increasing the pressure to a definite value were compared with equilibrium compositions obtained by decreasing the pressure to the same value. In all cases, agreements within the experimental error served to indicate that a state of equilibrium had been reached. Samples equivalent to 500 cc. of gas at room temperature and pressure were taken of each phase. Sample lines were stainless steel tubing 0.25 inch in outside diameter by 3 / 8 2 inch in inside diameter, into which a 14gage Chrome1 A wire had been inserted to reduce the volume of the sample line. During sample withdrawal, it was possible to maintain equilibrium pressure in the cell by injecting mercury from the compression unit into the pressure control cylinder, C .
The phase samples were analyzed by gas gravity methods. 'In the work of these investigators, vapor from the equilibrium cell was removed from the low temperature bath, passed through The sensitivity of the weighings was such that an accuracy of .a mercurypump a t room temperature, chilled to bath temperature, *0.0003 mole fraction was possible for the system under study. and bubbled through the liquid phase in the equilibrium cell. Pressures above 1000 pounds per square inch are believed to be The.mercury pum varied the enclosed volume of the system, accurate to *20 pounds per square inch, while pressures below causing pressure fuctuations that would become increasingly serious a t elevated operating pressures. Therefore, this arrangelo00 pounds per square inch are believed to be accurate to 1 3 ment was modified to eliminate pressure fluctuations during gas pounds per square inch. At temperatures down to -50" F.. circulation, by the use of a magnetically actuated high pressure temperature control of the bath was maintained within 10.2' F., circulating pump which maintained a constant volume during while for temperatures of -60" and -100" F., a control of movement of the gas hase. The type of magnetic pump used has been described by Exline and EnDean (4). 10.5" F. was possible. With the use of liquid nitrogen, bath Figure 2 shows a schematic flow diagram of the apparatus; the temperature control was maintained at *0.So F. Two liquid equilibrium cell was enclosed in a constant temperature air bath. samples and one vapor sample viere obtained for nearly all runs. By means of the cooling coil, E , enough Freon-12 was circulated to Experimental eyperience indicated that an accuracy of 10.0025 maintain a temperature lower than desired. Heater F was then adjusted to give the desired temperature. F was controlled by a bimetallic thermoregulator for temperatures down to TABLE I. HYDROGES-HYDROCARBOX EQUILIBRIUM DATA -50' F. For temperatures Pressure down to -200" F., a potenRange, Temperature Phase tiometric controller was used. Range, F. Reference Author Lb./Sq. I n r h .lbs. Hydrocarbon Compositions Liquid nitrogen replaced from ava 450-1130 - 140-- 175 Liquid-vapor Methane Freon-12 for the -150" and I T'erMethane 300-3000 - 20,;- - 3 10 Liquid-vapor -200' F. runs. Dry ice was hIethane used in the air bath to reach 600-3000 .230- - 20: Liquid-vapor -50" and -100" F. Teni450-1130 121--17> Liquid-vapor Ethane 500-3000 100-250 Liquid-vapor peratures were measured with Isobutane 300-1300 73-240 Liqiiid n-Butane c o p p e r - c o n s t a n t a n thermo440-1 170 130-420 Petroleum naohtha D e x aod bubble couples, which were calibrated points using the difference method 2,2.4-Trimethyl200-5000 100-300 Dean and Toolie Liquid-vapor pentane recommended bv the National joo-5000 200-300 Liquid-vapor Isomeric dodecane Dean and Tooke Bureauof Standirds (1). Stain15-2500 77 Liquid Frolich . Propane 15-1500 77 Liquid less steel, Type 304, was used Frolioh Butane 18-1300 77 Frolich Liquid Pentane for all units under stress a t low 15-1500 Liquid Frolich Hexane 27 temperature. 1..?-zxnn . _... ,i Liquid Frolich Octane 15-2800 Frolich ii Liquid Heavy naphtha To charge the cell, the sys15 2 8 0 0 77 Liquid Frolich Gas oil tem and lines were evacuated 730-2200 77-570 Liquid Ipatieff Gasoline to less than 1 mm. of mer71-870 730-2200 Liquid Ipatieff Kerosene Liquid Ipatieti Aroniatics cury absolute pressure and a measured amount of liquid
January 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
1 8
Figure 3 shows the solubility of hydrogen in n-butane a t vari-
ous temperatures; Figure 4 shows the composition of the equilibrium vapor phase as a function of pressure a t constant temperature. From Figure 3, it may be seen that the system exhibits reverse solubility over the range studied-Le., an increase in solubility results from m increase in temperature a t constant pressure. The vaporization characteristics of the components of a mixture may be conveniently expressed by means of the equilibrium constant, K ,
K = y/x
Figure 4. Saturated Vapor Compositions
mole fraction in the liquid phase and *0.001 mole fraction in the vapor phase was to be reasonably expect,ed. EXPERIMENTAL DATA
where y = mole fraction of component in vapor phase and x = mole fraction of same component in liquid phme. Figure 5 presents K for hydrogen in the hydrogen-n-butane system as a function of presaure at constant temperature, and Figure 6 presents a similar plot for n-butane. In considering these latter plots, it should be kept in mind that for low concentrations of hydrogen in the liquid and high concentrations of hydrogen in the vapor, the K values for hydrogen and n-butane, respectively, are greatly affected by small absolute changes in the composition. Below -50" F., the concentration of hydrogen in the vapor phase was greater than 99.8 mole % a t all pressures studied. Because of the inherent sensitivity mentioned above, it was deemed inadvisable t o give K values for n-butane below -50" F. Figure 7 shows the equilibrium constant for hydrogen in n-butane as a function of the temperature a t constant pressure. Bubble point curves for mixtures of constant composition as a function of temperature and pressure are shown in Figure 8. At temperatures down t o -200" F., the points of maximum pressure (cricondenbars) on the bubble point curves have not been reached for this system. The presence of the anomaly of a minimum
The experimental data presented in Table I1 consist of the compositions of coexisting equilibrium vapor and liquid phases a t the corresponding conditions of pressure and temperature.
PRESSURE
Figure 5. Equilibrium Constants for Hydrogen
(1)
, LBhQ.lN.ABS
Figure 6. Equilibrium Constants for n-Butane
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INDUSTRIAL AND ENGINEERING CHEMISTRY
point in the bubble point curve a t low hydrogen concentrations may also be noted in Figure 8. The occurrence of a solid phase at low temperatures prevented the formation of a complete bubble point curve of the unusual form predicted by Kay (Figure 1). The temperature corresponding to the cricondenbar has not been reached at -200" F. More than 70% of the temperature interval
Vol. 43, No. 1
between the critical temperatures of n-butane (306" F.) and of hydrogen ( - 400 'F.) has been covered by going down to -200 O F. without the appearance of cricondenbars for the range of compositions investigated CORRELATIOV OF EQUILIBRIU&l CONSTANTS
An attempt was made to correlate graphically the K values for hydrogen in binary systems of aliphatic hydrocarbons. The paucity of reliable vapor-liquid equilibrium data for systems of hydrogen and a hydrocarbon served as a great handicap in developing the correlation. It is to be expected that the equilibrium constant for hydrogen will be, for the most part, influenced by the variables of pressure, temperature, composition, and molecular weight of the heavier constituent Graphical representation of these four variables at one time is difficult and therefore an attempt was made to eliminate and/or combine variables so that graphical correlation could be simplified. The temperature ranges of the two phase data of the various systems that were available from the literature were so great that it was deemed necessary to represent temperature in a manner that would allow better comparison of the data. This was done by using as a parameter the reduced temperature with respect to the hydrocarbon present
TR = T/Tc
TEMPERATURE,
OF.
Figure 7. Equilibrium Constants for Hydrogen in Hydrogenn-Butane System
(2)
where T = equilibrium temperature, degrees Rankine, and TC = critical temperature of hydrocarbon, degrees Rankine. A plot of log K (for hydrogen) against log pressure at constant reduced temperature showed that the slopes of the lines corresponding to the various hydrogen-hydrocarbon systems were approximately equal (Figure 9). Consequently, the equilibrium constants for hydrogen in binary hydrogen-hydrocarbon systems may be expressed as a function of the equilibrium constant for hydrogen in a specified hydrogen-hydrocarbon system. The hydrogen+-butane system was chosen as the standard because it is the system upon which the most data are available. The equilibrium constant, K , for hydrogen in other hydrocarbon systems may be computed from the equilibrium constant, K B , for hydrogen in the hydrogen%-butane system and the factor, F , from Figure 10 with the equation
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