I
DONALD F. OTHMER, ROGER GILMONT, and JAMES J. CONTl Polytechnic Institute of Brooklyn, Brooklyn 1, N e w York, N. Y.
An Adiabatic Equilibrium Still For M o r e Accurate Vapor-liquid Equilibrium Data By using a single unit with interchangeable accessories, the three major types of equilibrium stills can be compared
Editor's Note:
Detailed data for this article including graphs, tables, and discussion appear in the concurrent issue of the Jownal of Chemical and Engineering Data 5 (1960). 1
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FOR equilibrium stills and their operation have been discussed (7. 3, 4, 8-72, 74). Worth-while data are obtained when there is true equilibrium between vapor and liquid; a true sample of each ; no superheat; no entrainment; no heat loss, and hence no condensation in the vapor zone of the still ; true measurement of temperature and pressure; adequate mixing of liquid in the still; and no contamination of the materials. In designing distillation equipment the apparatus used perhaps most frequently to obtain accurate experimental data on vapor-liquid equilibrium has continuous recycle of condensate ( 4 ) . This still was simplified more than 30 years ago (9), and subsequently numerous modifications were devised, such as Cottrell boiling, external flash boiling. and boiling of two liquid phases. In the work described here, these modifications were integrated into a single unitary still with auxiliary interchangeable parts; thus, each working method could be compared using a single still. Partially miscible systems were investigated, and heat losses from the vapor space have been reduced to practically zero.
Description of Still Prime novelty of the still is its integral jacketing arrangement to minimize heat losses which, in the vapor portion of the still pot, could cause condensation and rectification of contacting vapors. The main still pot, A (page 626, left), is
surrounded in the upper portion by a vapor jacket, B. as had been standard for 30 years (9). There is always the possibility of air dissolving in the condensate if this is cooled; and if these gases are then boiled out on recycling, they may collect in vapor jacket space where there is minimum of vapor movement. Later stills (8) thus had a hotcondensate return to minimize this possibility. Even so, it was necessary to apply external electric heat to prevent heat losses. The whole pot of the new still is surrounded by a vacuum jacket, C, evacuated to a pressure below 1 micron which is the usual pressure in commercial Dewar vacuum bottles. This additional insulation reduces to what is really a third differential any error caused by rectification in the vapor space. Furthermore, there can be no dead spots in the vapor space. Also, the design gives very little contact of most of the vapors
with solid walls and possible liquid films thereon until they have been discharged to a space where all condensate goes to its receiver. Later models of the new still have had the glass surfaces of the vacuum jacket silvered to reduce radiation losses, as in the standard Dewar flask. A narrow vertical unsilvered strip on front and back allowed inspection of boiling taking place inside. Liquid boiled in A by the internal electric heater F generates equilibrium vapor, which on rising in the very small bubbles characteristic of this type of heating gives excellent agitation and contact with all of the liquid. Vapor leaves the main still body via the vapor jacket B, which extends down to the liquid level of the still pot. Any heat loss from the vapor jacket \vi11 cause condensation which will drain by way of the trough D,slightly sloping to the receiver. L'apor passes through the side arm and enters the condenser S.
PRESSURE '
DRYING TUBE
I
VACUUM
I
SIDE
SIDE
3
MANOMETER
MANOSTAT
EQUIL IB RlUM
GLASS WOOL
> DRYING TUBE
CWPRESSED AIR
SURE BOTTLE
/
F
VACUUM
Pressure was controlled by balancing a slight excess of carefully filtered and dried compressed air against a slight vacuum in a large surge bottle VOL. 52, NO. 7
e
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respectively. The three-way cock, I-! is normally set to allow condensate to return to the flask. I t is then turned to withdrawn condensate sample which is taken first; then turned to close off line W’ and prevent movement of liquid therein to or from the still while sample is taken from G. The pressure was controlled at all times at 760 =k 1 mm. of mercury by a system whereby a slight excess of carefully filtered and dried compressed air was balanced against a slight vacuum in a large surge bottle, regulated by a Cartesian manostat. Absolute pressure was determined with a calibrated mercury barometer, and all corrections were made. The system pressure was then set by adjusting the pressure difference between the still and the atmosphere to correspond to the difference between the barometer reading and 760 mm. of mercury. T o assess the adiabatic characteristics of the still under normal conditions of operation, data on the test system, acetone-methyl isobutyl ketone, was compared to rhe accurate data previously obtained ( 7 ) , using a previous still (8) definitely made adiabatic by a n auxiliary heating jacket. T o this end the Dresent still was fitted with an auxiliar>- electric heating jacket, the
Vapor passages are very short and large in cross section to minimize pressure drop, especially when operating at low pressures. A thermometer in a well at E indicates the equilibrium temperature of the vapor. Condensate a t its boiling point enters and fills the receiver L7,and flows back to the pot through stopcock V , the capillary return line I+’, and the groove H which is ground in the joint. The fixed volume of the receiver should be large enough to contain the sample for analysis. Either interchangeable receiver units are used as before ( 8 ) , or glass beads dropped into a receiver larger than necessary reduce its effective volume when smaller amounts of condensate are sufficient for analysis. Thus, advantage may be taken of the shorter time necessary to obtain equilibrium between condensate and still liquid. Operation of the still is essentially that which has been previously described (8),except that liquid volume is always maintained high enough to reach the vapor jacket. Thus, the vapor space inside the still is completely jacketed with vapor. When steady state is established, samples of the equilibrium liquid and vapor condensate are obtained through stopcocks G and V ,
purpose of which was to equalize the surface temperature of the still with the observed boiling point in the still, thus preventing any possible heat losses. This was made and operated substantially as before ( 7 ) with a carefully wound jacket heating unit and six thermocouples on the surface of the still; thus equal temperatures could be maintained. Equilibrium data for the system, acetonemethyl isobutyl ketone, were determined both with and without heat input to the auxiliary jacket. The materials were carefully prepared, stored, and utilized as before ( 6 ) , and all other precautions noted were also taken carefully. The two sets of data compare well with each other and with the previous data (7), indicating that the new still is effectively insulated without an auxiliary source of compensation for heat loss. The heaters, transformers, and thermocouple circuitry were thus all eliminated, and much time was saved in balancing temperatures through the insulating jacket. On page 628, data are shown in a plot which tends to amplify any differences. Modifications of Boiling
I t was desired to compare, in the same still, equilibrium data obtained using three basic methods of vapor generation:
1. Normal boiling. Vapor is generated inside body of boiling liquid. 2.
Cottrell-type
boiling.
Some
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U
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I
.
.
INCHES
Prime novelty of the adiabatic equilibrium still i s an integral jacketing arrangement which minimizes heat losses from the still pot
626
INDUSTRIAL AND ENGINEERING CHEMISTRY
Cottrell boiler modification. A special interchangeable boiler slips into the standard tapered joint at the bottom of the still
ADIABATIC E Q U I L I B R I U M S T I L L liquid. withdrawn from main body in the still: is heated to produce vapor in bubbles which forces slugs of the withdrawm liquid in contact with it u p through a tube. In this system ( 7 , 77) the best possible is for the vapor to be in equilibrium with the withdrawn liquid, rather than with the bulk of the liquid which is later analyzed. 3. Flash boiling. The condensate is entirely vaporized in an external flash boiler, and its vapor is bubbled through the liquid in the still to obtain vapors which are presumed to be more nearly in equilibrium with the liquid than vapors formed by normal boiling. This is contrary to industrial experience, where a plate in a column with vapors coming through may be less than 0.5 of an equilibrium unit, while the reboiler itself may be regarded usually as very close indeed to an equilibrium unit.
A few investigators have held, without any data whatsoever to support the position, that the latter two mechanisms offer better opportunities for equilibrium attainment between vapor and liquid than the simple vaporization from an electrically heated coil in the body of the liquid. Cottrell Boiler. T o compare Cottrell boiling with conventional boiling, a special interchangeable boiler was designed, which slips into the standard tapered joint at the bottom of the still. The immersion heater fits into its bottom. The heater produces vapor in the boiling
chamber, E;. M hich passes up the Cottrell tube Ji. and forces slugs of liquid along \$ith ii : the mixture of liquid and vapoi splashrs against the thermometer bulb. and disengages under the umbrella which is well above the liquid surface. The vapor passes off and the liquid drops back. Liquid recycles to the boiling chamber kia the annular space P, picking up the recycling condensate on the way. Vapor-liquid equilibrium data for the system, acetone-methyl isobutvl ketone were again obtained. using the still modified in this manner Results indicate no consistent difference betwem these data and the previous data determined with the conventional boiler. This suggests that the Cottrell boiler offers no refinement in vaporliquid equilibrium attainment. Indeed, as previously noted (2, 8, 77) any deviations would be in error. Flash Boiler. Another interchangeable external boiler was designed for use \vith the new still to study the third type of vapor generation, which has been
recommended by others. again without any data being given to justify its use or advantage. Another unit interchangeable with the still and the single electric heater was designed and built. A syphon, X , connected to a vent condenser (not shown in the illustration) was necessary to prevent surge back to the condensate receiver. \Vith this flash vaporizer. after a steady state is reached, the condensate which recycles to the boiler unit is completely flashed in L before passing up the central tube AV. This vapor which may even be somewhat superheated is then forced to go down and then bubble u p through the liquid surface in the still before passing to the condenser, because the umbrella (bubble cap in this case) is longer than that with the Cottrell boiler. This bubble action has been claimed, without evidence, to give a vapor more nearly in equilibrium with the liquid than does the normal boiling action. Other investigators with an external vaporizer (5) have needed a 19
m
ip. \
\
a
.. External vaporizer modification. Because of the nearly adiabatic nature of the unit, a second heater is not required
Partially miscible system modification. mixing was always obtained VOL. 52,
NO. 7
Excellent interphase
JULY 1960
627
second heater inside the still to make u p for heat losses, and some ( I ) have commented on the difficulty of exactly balancing the two heat inputs. Because of the almost completely adiabatic nature of the present unit, a second heater was not required. However, this basic type of still has objections. Temperature measurements of the rising vapor fluctuated somewhat and often indicated a considerable degree of superheat when the thermometer was directly in their path as in the case of the Cottrell unit. A more nearly correct temperature of the vapor leaving would be measured when the thermometer is in the path of the vapors after they had bubbled through the liquid, as shown. However, confidence in the external flasher is reduced by the fact that a flash vaporizer probably cannot be operated to get complete flash vaporization without superheating the vapors or alternately without liquid entrainment which may not be stopped by bubbling through the liquid. At low acetone concentrations the vapor entering the condenser appeared somewhat foggy, indicating a degree of entrainment passing to the condenser. This is borne out by the data which exhibit a lower than expected vapor concentration in the low concentration region. Thus, the external vaporizer may be considered to give vapors less near to equilibrium with the liquid than simple boiling.
be reached. If there is slight momentary holdup in one phase of the condensate, any condition of equilibrium would be upset. T o attain liquid-liquid equilibrium in the still, excellent mixing is required. Thermosyphon-type agitation provided an effective ansirer to this problem, using the boiler previously designed for Cottrell boiling with the Cottrell tube cut off. Excellent interphase mixing was obtained with any ratio of amount of upper layer to amount of lower layer. Seven modifications of two basically different designs of condensate receiver were tested before an entirely uniform condensate emulsion was maintained. T h e final cylindrical receiver has four internal baffles, BB, welded into the wall. T h e stirrer with four rectangular, slight1)- pitched blades operates at speeds u p to 4000 r.p.m., just below the baffles. and gives excellent mixing and a very slight lift. The receiver is higher than the still pot, and the slight lift of the condensate gives overflow. T h e capillary portion of the return line forms a continuous stream of small slugs of alternate phases, which pass through the annular space, P, into the boiler K. Vapor condensate falling from the condenser is drawn into the receiver by the slight pumping action therein. T h e stirrer is held on its axis by a small Teflon foot bearing, CC; in the cross sign. A standard section this is a Teflon gland snugly fits at the top of the receiver. .L\ larger spherical glass seal is necessary to insert the stirrer and provide flexibility for alignment. T h e final design shown was tested on the heterogeneous azeotropic systems, toluene-water (aqueous lower layer) and chloroform-water (aqueous upper layer). Results checked the accepted literature values. Numerous results were obtained on ternary systems which also correlated very well.
+
Equilibrium Still for Partially Miscible Systems
Special problems are created by systems giving two liquid phases in either the still or in the condensate receiver. A vapor in equilibrium with a heterogeneous liquid must be in equilibrium with both liquid phases which also must be in equilibrium with each other. Furthermore, the accumulated condensate must always be uniformly mixed SO that the overflow back to the pot is of exactly the same average composition as the vapors, or a steady state can never
Acknowledgment
T h e National
Science Foundation
04-
NEW EQUlLIERlUY STILL
8 NEW STILL P U S JACKET HEAT
A NEW
STILL WITH COTTRELL BOILER
NEW STILL WITH
VAPORIZER
DATA OF KARR.ET AL (71
X
Plotting (Y-X) vs. X for the system, acetone-methyl isobutyl ketone, shows some divergence from equilibrium when using the external vaporizer
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Fellowship awards which made this work possible are acknowledged. T h e Emil Greiner Co., New York, supplied all glass equipment. Key to letters i n Figures
Still body, 4 inch major i.d. Vapor jacket Vacuum jacket D. Trough draining to right E. Thermometer port F. Immersion electric heater G. Liquid sample cock H. Groove in heater joint J . Heater joint K . Boiling chamber L . Vaporizing chamber M . Cottrell tube -V, Vapor uptake tube 0 . Uptake tube P. Annular recycle to boiler Q. Annular condensate recycle R. Vapor line joint S. Condenser T . Condensate receiver L’. Condensate cooler V . Condensate sample cock W’. Capillary return line X. Vented syphon recycle line Y. Vent to receiver Z . Teflon gland for stirrer A A . Stirrer BB. Baffles CC. Teflon bearing for stirrer -4.
B. C.
References (1) Carney, T. P., “Laboratory Fractional
Distillation,” Macmillan, New York, 1949. (2) Ellis, S. R. M., Trans. Inst. Chem. Engrs. (London) 31, 96 (1953). (3) Fowler, R. T., J . SOL. Chem. Ind. (London) 6 8 , 131 (1949). (4) Hala, E., Pick, F., Fried, V., Vilim, O., “Vapor-Liquid Equilibrium,” Pergamon Press, New York, 1958. (5) Jones, C. A , Schoenborn, E. M., Colburn, A. P., IND. h c . CHEM. 35, 666 (1943). (6) Karr, A. E., Bowes, LV. M., Scheibel, E. G., Anal. Chem. 23,459 (1951). (7) Karr, A. E., Scheibel, E. G., Bowes, W. M.,Othmer, D. F., IND.ENG.CHEM. 43, 961 (1951). (8) Othmer, D. F., Anal. Chem. 20, 763 (1948). ( 9 ) Othmer, D. F., IND.ENG.CHEM.20, 743 (1928). (10) Ibid., 35, 614(1943). (11) Othmer, D. F., Gilmont, R., Encyc. of Chem. Technol., vol. 14, p. 611, Interscience, New York. (12) Ridgway, K., Indus. Chemist 32, 59 (1956). (13) Sameshima, J., J . Am. Chem. Soc. 40, 1483 (1918). (14) Schafer, W., Stage, H., Chenr. Ing. Tech. 21, 418 (1949). (15) Yamoguchi, Y., J . Tokyo Clzem. Soc. 34, 691 (1913). RECEIVED for review July 28, 1959 ACCEPTED February 8, 1960 Previous papers in this series have appeared in IND. ENG. CHEM.20, 743 (1928); 35, 614 (1943); 36, 1061 (1944); 37, 299, 895 (1945); 38, 751 (1946); 39, 1175 (1947); 40, 168 (1948); 41, 572 (1949); 43, 707, 711, 961, 1607 (1951); 44, 1864, 1872 (1952); 45, 1815 (1953). Ind. Eng. Chem., Anal. Ed. 4, 232 (1932j. Anal. Chem. 20, 763 (1948).
