P h a se e q u ilib r ia d a ta are re q u ir e d for th e e ffic ie n t d e s ig n o f a b s o r p tio n a n d f r a c t io n a t in g e q u ip m e n t , a n d s in c e h y d r o g e n c h lo rid e is a s s u m in g a c o n s t a n t ly g r e a te r ro le in p e tr o le u m refin ery o p e r a tio n s , i t ap p ears d e sir a b le t o h a v e s u c h d a ta o n m ix tu r e s o f h y d r o g e n c h lo r id e w it h v a r io u s h y d r o c a r b o n s.
T h e c o m p o s it io n s o f e q u ilib r iu m v ap or- liq u id m ix tu r e s in t h e h y d r o g e n c h lo r id e - n - b u t a n e s y s t e m h a v e b e e n d e t e r m in e d a t te m p e r a tu r e s o f 7 0 °, 120°, a n d 180° F . for p r essu re s b e lo w 550 p o u n d s p er sq u a r e in c h , u s in g a c o n s t a n t v o lu m e ty p e a p p a r a tu s.
T h e d a ta , w h ic h are p r e s e n te d in b o t h t a b u lar a n d g r a p h ic a l fo r m as p r e s s u r e -c o m p o s i- tio n a n d e q u ilib r iu m c o n s t a n t-p r e s s u r e d ia g r a m s, in d ic a t e g e n e r a l a g r e e m e n t w it h R a o u lt ’s la w .
H YDROGEN chloride is used as a promoter for the aluminum chloride isomerization of n-butane to iso
butane. In a continuous isomerization unit it is necessary to introduce the hydrogen chloride with the n-
butane 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 frac
tionating 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
(2 ), hexane (5), benzene ( 1 ), octane (1), dodecane (f), and cyclohexane (7), but nothing was found for the system hy
drogen chloride-n-butane. This paper presents the equi
librium liquid and vapor compositions for the latter system at 70°, 120°, and 180° F. and at pressures up to 550 pounds per square inch.
A p p a r a tu s a n d M a te r ia ls
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 fines made of V8-inch steel tubing. To avoid excessive liquid holdup, the transverse area of the liquid sampling fine was
re-1 P r e s e n t a d d r e s s , P e n n s y l v a n i a S t a t e C o l le g e , S t a t e C o l le g e , P e n n a .
J. H . O T T E N W E L L E R 1, C L A R K H O L L O W A Y , J R ., AND W H IT N E Y W E IN R IC H
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 fine of the bomb.
The constant-temperature bath was equipped with immer
sion heaters and a cooling coil, and the fluid of the bath was constantly recirculated by a centrifugal pump. The tem
perature was controlled with a bimetallic expansion thermo
regulator, 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 em
ployed for the 180° F. determinations. The equilibrium chamber was supported vertically in the constant-tempera
ture 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 sam
ple 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. deter
minations 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 al
lowed to continue until a hydrogen chloride pressure in excess of that desired in the equilibrium chamber had developed.
M e th o d o f O p e r a tio n
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
208 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 35, No. 2 n-butane was then added for the series of isothermal deter
minations, 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 con
tinued until hydrogen chloride solution into the liquid phase became negligible at about the desired pressure. The pres
sure 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.
In 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 superatmos- pheric pressure, and no pressure drop on the system was no
ticeable 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 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. 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 at a minimum. When taking a vapor phase sample, the total pressure on the system did not drop more than 1 or 2 pounds
The equilibrium pressures in the system hydrogen chloride- n-butane were determined for 70°, 120°, and 180° F. at con
centrations 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 analy
ses and points taken as the pressure was decreased. De
viations in the liquid phase data between series of determina
tions made with increasing and with decreasing pressure were consistent; that is, in a series made by successively increas
ing the pressure, the concentration of hydrogen chloride would tend to be low, whereas in a decreasing series the con
centration would tend to 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 hydro
gen chloride, errors due to holdup and lack of equilibrium
., , ® over-all accuracy of the data is considered to be within the following limits:
February, 1943 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 209
MOL PERCENT H C L IN LIQUID
Fi g u r e 2 . Co m p a r i s o n o p Ex p e r i m e n t a l Da t a w i t h Ra o t j l t’s La w
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 de to be accurate within this limit, since it was calibrated with a dead weight tester at 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.
tems hydrogen chloride-n-butane and hydrogen chloride- propane, the critical locus for mixtures of hydrogen chloride and R-butane was drawn on Figure 1 similar in shape to the critical locus of mixtures of hydrogen chloride and propane
(2 ). The data at 70° and 120° F. were extended to the re
spective vapor pressures of hydrogen chloride, and little op
portunity is seen for errors of greater than 5 mole per cent system consists of a single phase. It is perhaps not generally realized that this and other retrograde conditions can be
easily demonstrated by actual calculation of the relative amount of liquid and vapor present at various points within the border curve. Thus, at point E where the total system in question contains 5 7 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 equa
tion:
0 . 2 6 3 x + 0 . 6 0 2 ( 1 0 0 - x) = 0 . 5 7 ( 1 0 0 )
Similar calculations based on the pressure-temperature type of diagram are possible, provided sufficient constant composi
tion envelopes are available.
The nearly linear relation of the composition to the pres
sure for the bubble-point curves in Figure 1 indicates approxi
mate 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 tane the value of K will approach infinity. For temperatures below the critical temperature of hydrogen chloride, the curves terminate at the vapor pressure of hydrogen chloride and a K
value of unity. For temperatures above the critical tem
perature 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-composi- tion type diagram. On this type of diagram the maximum pressure point is coincident with the critical point.
L ite r a tu r e C ite d