Liquid-Vapor Equilibrium Compositions in Hydrogen Chloride-n-Butane System Phase equilibria data are required for the efficient design of absorption and fractionating equipment, and since hydrogen chloride is assuming a constantly greater role in petroleum refinery operations, it appears desirable to have such data on mixtures of hydrogen chloride with various hydrocarbons. The compositions of equilibrium vaporliquid mixtures in the hydrogen chloriden-butane system have been determined at temperatures of 70°, 120', and 180' F. for pressures below 550 pounds per square inch, using a constant volunie type apparatus. The data, which are presented in both tabular and graphical form as pressure-coniposition and equilibrium constant-pressure diagrams, indicate general agreement with Raoult's law.
J. H. OTTENWELLERI, CLARKHOLLOWAY,JR., AND WHITNEY WEINRICH
Gulf Research and Development Company, Pittsburgh, Penna.
duced about 75 per cent by the insertion of a steel rod. The stainless-steel Bourdon-tube pressure gage was attached to the vapor-sampling line of the bomb. The constant-temperature bath was equipped with immersion heaters and a cooling coil, and the fluid of the bath was constantly recirculated by a centrifugal pump. The temperature was controlled with a bimetallic expansion thermoregulator, and the actual temperature reading was taken with a calibrated A. S. T. M. thermometer. Water was used in the bath for the 70" and 120" F. determinations, and oil was employed for the 180" F. determinations. The equilibrium chamber was supported vertically in the constant-temperature bath by side arms so that it could be rocked through an angle of about 90" and so that the sampling valves were below the level of the fluid in the bath. One-gallon bottles and 700-cc. burets, respectively, were used for sampling the liquid and vapor phases. A Cenco high-vacuum pump was available for evacuation of the sample containers, and mercury manometers were employed in sampling for pressure control and measurement. The c. P. n-butane used in this work was obtained from the Phillips Petroleum Company and was 99.6 per cent pure. The impurities were hydrocarbons boiling in the same range as n-butane, and since the solubility of hydrogen chloride in hydrocarbons of similar boiling points should not vary a great deal, no attempt was made to remove the 0.4 per cent impurity. The butane was passed through an activated alumina dryer to remove any traces of moisture. The hydrogen chloride for the experimental work at 70" F. was obtained in the dry anhydrous form from the Harshaw Chemical Company and was from 97 to 98 per cent pure. The hydrogen chloride used for the 120" and 180" F. determinations had a purity of over 99 per cent and was produced by the reaction of concentrated sulfuric acid with rock salt. A flanged steel bomb was used, and the generation was allowed to continue until a hydrogen chloride pressure in excess of that desired in the equilibrium chamber had developed.
H
YDROGEN chloride is used as a promoter for the aluminum chloride isomerizatidn of n-butane to isobutane. I n a continuous isomerization unit it is
necessary to introduce the hydrogen chloride with the nbutane charge in controlled amounts and also to recover it from the reactor effluent for recycling, since the hydrogen chloride consumption must be low if the process is to be economically feasible. Phase equilibria data for the hydrogen chloride-n-butane system are required if absorption and fractionating equipment handling these two components are to be designed with confidence. A literature search revealed some data on the solubility of hydrogen chloride in propane @), hexane (6),benzene ( I ) , octane ( I ) , dodecane ( I ) , and cyclohexane ( Y ) , but nothing was found for the system hydrogen chloride-n-butane, This paper presents the equilibrium liquid and vapor compositions for the latter system a t 70°, 120", and 180" F. and at pressures up to 550 pounds per square inch.
Apparatus and Materials The equilibrium chamber was a cylindrical one-gallon bomb constructed from a section of 6-inch, extra heavy, S. A. E. 1015 seamless steel tubing, and was provided with a thermowell and with both liquid and vapor sampling lines made of '/g-inch steel tubing. To avoid excessive liquid holdup, the transverse area of the liquid sampling line was re-
Method of Operation Before charging n-butane to the equilibrium chamber for each series of determinations, it was washed with acetone, dried, and flushed out several times with butane. Sufficient
Preaent address, Pennsylvania State College, State College, Penna
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
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n-butane was then added for the series of isothermal determinations, and air was swept from the line leading to the hydrogen chloride supply with a stream of butane. The equilibrium chamber was rocked steadily while hydrogen chloride was being admitted, and the admission was continued until hydrogen chloride solution into the liquid phase became negligible a t about the desired pressure. The pressure and temperature usually assumed constant values within an hour after the hydrogen chloride addition was stopped, but samples were never taken earlier than 8 hours after the last addition of hydrogen chloride. I n sampling, precautions were taken to remove stagnant materials from the internal lines and air or other foreign gases from the external lines. Gas samples from the liquid phase were taken into clean, dry, evacuated one-gallon bottles. They were flushed with three or four times their volume of sample in a slow continuous flow under slightly superatmospheric pressure, and no pressure drop on the system was noticeable during the sampling operation. For analysis, gas was allowed to flow from the bottle into 700-ml. evacuated (2 mm. mercury) gas burets which were constructed with a one-way glass stopcock a t one end and a three-way stopcock a t the other end. This three-way cock permitted evacuation and flushing of the connection between the buret and the onegallon bottle before the sample was withdrawn from the bottle. The buret samples were taken a t a pressure of 150200 mm. mercury, and the exact pressure together with the temperature and buret volume determined the weight of sample. The hydrogen chloride in the sample was neutralized with an excess of measured standard sodium hydroxide solution which was added through the three-way stopcock. This mixture was diluted with distilled water to provide adequate solution for absorbing the hydrogen chloride, and the buret was shaken vigorously. The excess sodium hydroxide was back-titrated with standard acid to the phenolphthalein end point.
L
I
I
I
20 40 60 M O L PERCENT HCL
I
00
I
FIGURE1. BUBBLEAKD DEW-POINT CURVES FOR
HYDROGEN CHLORIDE-%-BUTANE
SYSTEM
Gas samples of the vapor phase were taken and analyzed by the same procedure as for the liquid phase samples, except that the sample was passed directly into the buret from the equilibrium chamber in order to keep the amount withdrawn a t a minimum. When taking a vapor phase sample, the total pressure on the system did not drop more than 1 or 2 pounds per square inch.
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TABLE I. CONPOSITIONS OF BUBBLE-POINT LIQUID AND DEWPOINT VAPORFOR HYDROGEN CHLORIDE*-BUTANE SYSTEM Pressure, Lb./Sq. In.
---bIole Liquid
Abs.
yo HCI-
Vapor
A t 70' F. 66 114s 119a 200" 2118 283 323a 361 403
3.3 10.8
38.8 74.6
52.6 23.1 34.1 42.6 46.5 58.4
87.0 89.3 89.4 90.2 90.4 90.5
10 8
75 7
A t 120° F. 100 1065 137 l54a 186 213 215a 242 261a 319 321a
2.0 3.5 4.6 7.6 9.5 11.9 12.9 14.5 18.1 22.3 23.6 27.8 32.3 32.4
866Q
415" 417
a
199Q 205 254 257~ 306 317 366 385a 466a 484 53 1 Points in a decreasing series.
34:5 5417
..
6i:8
7i:5 76:3 79.0 78.5
A t 180' F 3.6 2.8 5 8 7.3 9.5 11.4 13.3 15.4 19.4 20.1 22.4
24.6 23.8 43.7 38.7 46.7 49.4 55.3 56.3 58.2 58.5 59.4
Experimental Data The equilibrium pressures in the system hydrogen chloriden-butane were determined for 70°, 120°, and 180" F. a t concentrations up to 56 mole per cent hydrogen chloride in the liquid phase, and they varied from 50 to 500 pounds per square inch absolute. The observed pressures, together with the corresponding liquid and vapor compositions for each of the three temperatures are presented in Table I and shown graphically in Figure 1. Distinction is made between points taken as the pressure was increased between successive analyses and points taken as the pressure was decreased. Deviations in the liquid phase data between series of determinations made with increasing and with decreasing pressure were consistent; that is, in a series made by successively increasing the pressure, the concentration of hydrogen chloride would tend to be low, whereas in a decreasing series the concentration would tend t o be high. This indicates either some holdup in the liquid sampling tube or a considerable lag in reaching complete equilibrium. Smooth curves for the liquid phase compositions were drawn through the mean of the two sets of data, and since the departure of individual points from the curve was never more than one mole per cent hydrogen chloride, errors due to holdup and lack of equilibrium should be nullified. Individual points in the vapor phase data failed t o lie on a smooth curve in some cases by as much as 2 mole per cent hydrogen chloride. This scattering of points from the curve exhibits no definite trend, although it is probably due to changes in the vapor samples caused by disruption of the equilibrium as the vapor was being withdrawn. The over-all accuracy of the data is considered to be within the following limits: Pressurc, lb.,/sq. in.
Temperature, F. HC1 in liquid, mole yo HC1 i n vnpoui role 70
1 2 10.4 10.4
t3
February, 1943
INDUSTRIAL AND ENGINEERING CHEMISTRY
The equilibrium chamber was designed with a capacity of one gallon so that the amount of material withdrawn for sampling purposes would be a small percentage of the total contents, and the amount actually withdrawn in sampling either phase amounted to less than one weight per cent of the phase. Sampling tubes were of small cross-sectional area, and care was taken to prevent sample contamination by adequate purging of the sampling lines. The analytical procedure as previously described is considered to involve less uncertainty than other details of the .work, I I I I I and as a further precaution, the hydrogen chloride content as d e t e r m i n e d by the acid ion equivalent was c h e c k e d by a c h l o r i d e ion a n a l y s i s . The unit of graduation o n the B o u r d o n tube gage was 5 pounds per I I I 1 0' IO 20 30 40 square inch, and the pressure MOL PERCENT HCL IN LIQUID could be read acOF EXPERIcurately t o 2 FIGURE2. COMPARISON MENTAL DATAWITH RAOULT'S LAW pounds per square inch. The gage is thought to be accurate within this limit, since it was calibrated with a dead weight tester a t frequent intervals as the work proceeded. All temperatures were taken with a calibrated mercury-in-glass thermometer which could be read accurately to 0.2" F. Bath temperature varied by not more than .t0.2' for the 70" and 120"F. determinations, and by not more than *0.4" for the 180" F. determination.
easily demonstrated by actual calculation of the relative amount of liquid and vapor present a t various points within the border curve. Thus, a t point E where the total system in question contains 57 mole per cent hydrogen chloride and where the equilibrium liquid and vapor as represented by points C and D contain 26.3 and 60.2 mole per cent hydrogen chloride, respectively, the mole per cent of the system in the liquid phase is represented by the unknown term in the equation : 0.2632 0.602 (100 - Z) 0.57 (100)
+
Similar calculations based on the pressure-temperature type of diagram are possible, provided sufficient constant composition envelopes are available. The nearly linear relation of the composition to the pressure for the bubble-point curves in Figure 1indicates approximate agreement with Raoult's law. This is illustrated to better advantage in Figure 2, where the experimental data are compared directly with the calculated predictions. The fugacities of hydrogen chloride were obtained by graphical integration of P-V-T data according to the method of Lewis and Randall (4), and the fugacities of n-butane were taken from the data of Sage, Webster, and Lacey (6).
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Phase Behavior I n a binary hydrocarbon system the critical locus for mixtures of the constituents is often in excess and may be far in excess of the critical pressure of either pure constituent. This phenomenon does not exist in the hydrogen chloridepropane system and, assuming an analogy between the systems hydrogen chloride-n-butane and hydrogen chloridepropane, the critical locus for mixtures of hydrogen chloride and n-butane was drawn on Figure 1 similar in shape to the critical locus of mixtures of hydrogen chloride and propane (2). The data a t 70" and 120" F. were extended t o the respective vapor pressures of hydrogen chloride, and little opportunity is seen for errors of greater than 5 mole per cent hydrogen chloride a t a given pressure in either the bubble point or the dew-point curves. I n order to present qualitatively the shape of a border curve for a temperature above the critical temperature of hydrogen chloride, the data a t 180" F. were extrapolated to the critical locus. Retrograde effects, as defined by Katz and Kurata (S), can occur a t this temperature. For example, if a gaseous mixture a t 180" F. and the pressure and composition defined by point A is compressed isothermally along line AEH, a t point B a drop of liquid will form. As compression is continued, the amount of liquid will increase until a maximum is reached a t some point, F . Under further compression, the amount of liquid will decrease (retrograde vaporization) until at point G the system consists of a single phase. It is perhaps not generally realized that this and other retrograde conditions can be
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FIGTJRE3. EQUILIBRIUM CONSTANT CURVES FOR HYDROGEN CHLORIDE I N ?+BUTANE Equilibrium constants, or the mole ratio of the hydrogen chloride concentration in the vapor phase to the concentration in the liquid phase, were calculated from the smooth curves in Figure 1 and are presented in Figure 3 for the three temperatures investigated. At the vapor pressure of n-butane the value of K will approach infinity. For temperatures below the critical temperature of hydrogen chloride, the curves terminate a t the vapor pressure of hydrogen chloride and a K value of unity. For temperatures above the critical temperature of hydrogen chloride, the curves terminate at a K value of unity and the maximum pressure attained by the dew point-bubble point border curve on a pressure-composition type diagram. On this type of diagram the maximum pressure point is coincident with the critical point.
Literature Cited Bell, J. Chem. SOC.,1931, 1371. Glockler, Fuller, and Roe, J. Chem. Phys., 1 , 714 (1933). Katz and Kurata, IND.ENG. CHEM.,32,817 (1940). Lewis and Randall, "Thermodynamics", p. 192, New York, McGraw-Hill Book Co., 1923. (6) O'Brien and Kenny, J. Am. Chem. SOC.,62, 1189 (1940). (6) Sage, Webster, and Lacey, IND.ENG.CHIDM., 29,1188 (1937). (7) Wiegner, 2.Elektrochem., 47, 163-4 (1941). (1) (2) (3) (4)
P R ~ B ~ Nbefore T~D the Division of Petroleum Chemistry at the 104th Meeting of the A M ~ R I C A CHEMICAL N SOCIETY, Buffalo, N. Y .
