November 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
2327
ACKNOWLEDGMENT
TABLE IV. STABILITYOF CAROTENE IN DEHYDRATED ALFALFA The authors are indebted to the following for supplying matMEAL WHENMXXEDWITH COTTONSEED'FLOUR AND COTTONSEEV terials used in this study: The W. J. Small Co., Inc., Kansas GLANDS City, Mo.; Hiram Walker and Sons, Inc., Peoria, 111.; Anheuser(Stored at 26O C.) Busch, Inc., St. Louis, Mo.;' Converted Rice, Inc., Houston, Peroentage Carotene Destroyed % of --Month8 in Storaie Tex.; American Rice Growers Cooperative Association, Houston, Material Added Mixture 1 3 4 6 6 8 Tex. ; Archer-Daniele-Midland Co., Minneapolis, Minn.; The Alfalfa only 11 46 62 70 74 81 Buckeye Cotton Oil Co., Cincinnati, Ohio; Wm. J. Stange Co., Cottonseed flour 10 11 37 66 66 67 77 Chicago, Ill.; Southbrn Regional Research Laboratory, New Orleans, La.
--.
..
Cottonseed glands
70
14
49
64
72
76
60
8
33
49
68
69
LITERATURE CITED
68
(1) DISCUSSION
The reason for the ability of some materials to stabilize carotene when mixed with alfalfa ie not clear, since the carotene is within the alfalfa particles and presumably does not come in contact with the active materials in the diluents. One possible explanation is that substances in the added materials are easily oxidized and thus cause a depletion of oxygen surrounding the particles of the mixture. This seems unlikely, since each sample jar was rolled before removing a portion for analysis, and each jar waa opened monthly. This should have supplied oxygen to replace that which might have been used up. Furthermore, there was a considerable head space in the jars, which should have s u p plied enough oxygen for considerably more oxidation than occurred. Another possible explanation is that some exchange of substances could have occurred from the diluenta to the alfalfa meal by way of the small quantity of oil that wm present. Since carotene is in the oil phase of alfalfa, it then would be in contact with the stabilizer. Further work will be needed to determine why such mixtures are more stable and what practical use can be realized from the phenomenon.
Bickoff, E., and Williams, K. T., IND. ENG.CHEM.,36, 320
(1944). (2) Brunius, E., and Hellstrom, V., Suensk. Kem. Tide, 57, 86 (1946). (3) Fraps, a. S., and Kemmerer, A. R., Tez. Am. Ezpt. Sta.Bull. 557 (1937). (4) Halverson, A. W., and Hart, E. B., 3. Dairy Sci., 30,245 (1947). ( 5 ) Mitchell, H. L., Schrenk, W.G.,and King, H. H., Arch. Biochem., 16,343 (1948).
(6) Mitchell. H. L., Schrenk, W. G., and King, H. H., IND.ENQ. CHEM.,41, 570 (1949). (7) Morgal, P. W., Byers, L. W., and Miller, E. J., Ihid., 35, 794 (1943). (8) O'Donnell, W. W., Food I d . , 19, 763 (1947). (9)
Silker, R. E., Schrenk, W. G., and King, H. H., IND. ENO.
C H l M . , ANAL. ED., 16, 513 (1944). (10) Vix, H. L. E., Spadsro, J. J., Westbrook, R. D., Crovetto, A. J., Pollard, E. F.,and Gastrook. E. A., 3. Am. Oil Chemists' SOC., 24,228 (1947).
RECEIVED April 10. 1960. Presented before the Division of Agricultural and SOCIETY,Chicago, Food Chemistry, 118th Meeting, AMERICANCREMICAL Ill. Contribution No. 408, Department of Chemistry, KEMW Agrioultural Industrial Experiment Station. This work waB supported by the K ~ M I U Development Commission.
Concentric-Tube Fractionating Columns CONARD K. DONNELL AND ROBERT M. KENNEDY Research Laboratory, Sun Oil Company, Norwood, Pa.
.
T h e concentric-tube fractionating column has been shown to be very useful for the precise fractionation of samples ranging from 10 to 50 ml. in volume. Details of constructing and testing such columns are given. The height of equivalent theoretical plate (H.E.T.P.) at total reflux of the columns ranged from 5.3 mm. at 30 ml. per hour boilup to 30 mm. at 300 ml. per hour. The two columns described had annular spaces 840 and 940 mm. long, giving separating powers in excess of 150 theoretical plates at low boiling rates. Pressure drop varied between 0.3 and 0.9 mm. of mercury and holdup between 0.01 and 0.05 ml. per theoretical plate. The width of the annulus in each column was 0.75 mm.
T
HE concentric-tube fractionating column was originated by Selker, Burk, and Lankelma (8) of the Standard Oil Company of Ohio for the precise fractionation of small samples of complex mixtures such as alkylates. These investigators constructed a still having three concentric annuli, each 1 mm. wide and 150 cm. long, which had a liquid holdup of 5 ml. and developed 85 theoretical plates at a boilup rate of 100 ml. per hour.
After conferences with M. d. Hartig and others of the Standard Oil Company of Ohio staff, the development work was continued in the authors' laboratory. Attention was concentrated on columns having only a single annulus, becauw the effort of properly distributing reflux to a number of annuli did not appear to be justified by the additional throughput that would have been obtained. The first successful columns were described in the Symposium on High Temperature Analytical Distillation conducted by the American Petroleum Institute (1) in November 1946. Since that time a number of major improvements have been made to facilitate the construction and operation of the columns. Good resulte have been achieved in the fractionation of alkylates with charges of 15 to 30 ml. and distillation times of 24 to 96 hours, depending on the siie and complexity of the sample. Fractions of 0.5-ml. volume are usually collected and then analyzed for their individual components by mesns of their infrared spectra. CONSTRUCTfON
The general design of the still is shown in Figure 1 and details of the upper and lower parts of the mlU& are shown in Figures 2 and 3.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2328
PROPERTIES OF COMPONENTS OF TESTMIXTURES TABLE I. PHYSICAL Methylcyclohexane n-Heptane Lit. ( 8 )
2,2,4-Trimethylpentane N.B.S. Lit. (8)
N.B.S. B.p., 760 mm., F.p., a C.
C.
d:o n v
98.42 -90.65 0.68371 1.38766
98.4 -90.5 0.68375 1.38774,
99.236 -107.48 0.69191 1,39147
lab.
99.3 -107.45 0.69194 1.39157
COLUMN.The fractionating section consists of the annular space between two concentric precision-bore tubes. The inner tube is 10.05 mm. in inside diameter with a 1.25-mm. wall and is sealed off a t both ends. The outer tube is 14.05 mm. in inside diameter, leaving an annular space of 0.75mm. The inner tube 1s eealed in place a t the top and is properly spaced by the technique of Naragon and Lewis ( 7 ) , which consists in winding the inner tube with No. 22 B. & S. gage copper wire with a pitch of about 0.5 inch, and sealing the tube in lace. After the remainder of the still is completed, it is annealeland the wire is removed by dissolvin it in acid. As a result, the annular space is uniform, even tfou h the tubes may not originally be perfectly straight. STILL&AD. Vapor from the column is condensed by a hairpin condenser and liquid reflux is returned to the inner tube through a calibrated dripper. Distillate is taken off through a capillary trap and is prevented from flowing continuously to the receiver by a slight pressure of nitrogen in the receiver, which is maintained by an adjustable bubbler. The nitrogen is vented intermittently by means of a solenoid valve operated by a cycle timer, during which interval distillate flows to the receiver. This system was suggested by Snyder and Steuber ( 9 ) of the Houdry Process Company. A thermocouple well is provided to determine the vapor temperature. STILL POT. The still ot is a pumping type designed by Robert E. Ledley, Jr., of tiis laboratory to ensure adequate mixing of the contents and to prevent bumping. Application of heat inside the inverted well causes liquid to circulate rapidly upward past the heater to the surface and down between the chimney and the outer wall. JACKET. The entire column is enclosed in a silvered vacuum jacket to reduce the heat loss. The drain stopcock is built into the wall of the jacket to reduce the possibility of breakage. A mercury plug in the stopcock is used to reduce losses. A narrow observation strip is provided a t the reflux dripper to permit observation of the reflux rate, and the lower part of the jacket is left bare to permit observation of the residue during the late stages of the distillation. This insulation is sufficient to permit distillin samples boiling to about 150" C. with no auxiliary jacket heat. #or higher boiling samples, a Glas-Col heating mantle (Figure 4) is provided. The upper section of the mantle around the fractionating section is generally maintained about 50 C. beO
in
Lit. (I)
.. .. 1,4230
low the vapor temperature, while the lower section around the still pot is maintained at the same temperature as the residue in the pot, With this jacket heater, vapor temperatures of 250" C. have been recorded.
..
,.
Vol. 42, No. 11
..
TESTING OF CONCENTRIC-TUBE COLUMNS
..
1.4230
The test data reported here were obtained in two columns which were alike except that one, designated as column A, had an annular space 940 mm. long while the second, column B, had an annular space 840 mm. long.
TESTMIXTURES AND ANALYSIS. The n-heptane was obtained from Westvaco Chlorine Products and was a grade used for octane ratings of motor fuels. The "iso-octane" was obtained from the Rohm & Haas Company and was also reference fuel grade. A sample was refluxed in a concentric-tube fractionating column and was found to contain no lower boiling component than 2,2,4-trimethylpentane. The hysical properties of these samples as determined by the aational Bureau of Standards and given by the supplier are recorded in Table 1. COLD TRAP WATER
OWING TUBE
/
NEEDLE VALVE NITROCEN
DISIILLATE TRAP
BuBELeR
TWERMOCWPLE WELL
INNER TUBE MSTILLATE RECEIVER
8Wk"
NEEOCE VALVE
WAS-COL MANTLE
-BUBBLER
m [ Y o N
SPIRAL
STOPCOCK
STILLPOT
BIUw-PRESblRL MANOMETER
TC. WELL
DROPIN CONCENTRIC-TUBE COLUMN A TABLE 11. PRESSURE (Charge. 90 vol. % methylcyclohexane, 10% n-heptane) Power Input Net" Net' Vapor Velocity, Cm./Sec. Pressure Drop T~ Watts watts At At ManMm. at at pot, head Av. ometer Hg. watts bottom head bottom 0 0 2.0 4.0 6.0 8.0 10.0 12.0 14.0
0 1.8 3.8 5.8 7.8 9.8 11.8 13.8 15.8
3.2 5.0 7.0 9.0 11.0 13.0 15.0 17.0 19.0
Inform of vaporizedliquid.
0 5.6 12. I 18.4 24.7 31.0 36.9 43.2 49.4
0 0 6.3 12.6 19.0 25.3 31.3 37.6 43.8
0 2.8 9.2 15.4 21.8 28.1 34.1 40.4 46.6
20 60 64 83 102 126 149 173 195
0.089 0.268 0.286 0.370 0.455 0.562 O r664 0.772 0.870
Annulus 0.75 mm. wide X 940 mm. long. Heat loss to bottom of annulus 3.2 watts; to still head with jaoket a t 30 C. = 5.0 watts. AHvap 76 9 tal./ ram for methylcyclohexane = 713' al.k&n . for . 1 c._.., .. n-hantane . = 68.3 cal./gram for iso:a&ne = 0.7692 0.0008671 for methylcyclohexane 6, = 0.68375 0.00084111 for n-heptane = 0.69194 0.00082991 for iso-octane 1 watt = 14.34 cral./min. For 50 mole % n-heptane-methylcyclohexane 1 watt 0.285 ml. of liquid/min. = 3.13 cm./sec. vapor velocity For 50 mole % iso-octane-methylcyclohexane 1 watt 0.298 ml. liquid/min. 3.25 cm./sec. vapor velocity
-
-
--
--
SOLEWOD WLVE
HEATER WELL STOPCOCK OW"
T
Figure 1. Concentric-Tube Fractionating Column
The methylcyclohexane was obtained from the Eastman Kodak Company. It was distilled through a 1Smm. Podbielniak column 36 inches long at a reflux ratio of 50 to 1 and the initial 10% and final 3.3% were discarded. Examination of the ultraviolet spectrum showed the Sam le to be aromatic-free. The refractive index of this sample is stown in Table I. The compositions of the test samples were determined from the refractive index as measured with a Valentine Precision refractometer accurate to *O.OOOl, using the refractive index-composition data of Willingham and Rossini (11). The Fenske equation (3)was used for the calculation of the number of theoretical plates. Yn = YB
A + 1) XX B
where A refers to lower boiling component, B refers to higher boiling component, Y = molal concentration at head, X = molal concentration a t pot, CY = volatility ratio, and n = number of theoretical plates.
November 1930
2329
INDUSTRIAL AND ENGINEERING CHEMISTRY
a t 30" C., the additional loss was 1.8 watts for both columns; at 60' C., 0.2 watt; and at 90"C., 0.0 watt. For each different set of conditions, the appropriate value for heat loss between pot and still head was deducted from the total heat input a t the pot to give the effective heat content of the vaporized liquid, a t the still head and at the bottom of the column, This permitted calculation of the linear vapor velocities at the bottom and top of the annulus from the known dimensions of the annulus and the data shown a t the bottom of Table 11. The perfect gas law was assumed in the calculations. The average of the velocities at the top and bottom was then taken as the average vapor velocity in the column. The heat input was measurer! with a Weston Type 310 wattmeter accurate to 0.25%. The power input required to maintain a given average boiling rate in each column is shown in Figure 0.
TRAP Omm. I 0.6 mm.
a
I,gl
SEo'rlON A-A
VAPOR TUBE
R m u x riel" 8 mm. 0. D.
OtNTCR TO 0.1 mn.
T.C. WELL O mm. O.D. I Imm. LD.
14.01 mm.
LO
PREOIWN-BORE TWIN5
--.-
I005 mm. I. D a
.
I O 3 8 mm. 0.R PR SON-BORE TU%
10/2 sPl4EnIGAL 4mm. MANOWVER GOWECTION
M mm. WCUUY JIOKET
3 SWINO LUW 120. APART
-3-TURN DOUBLE S P l R U 7mm. 0. D. TUB~N~'
Figure 2.
SILVER JACKET
A e m THIsLiNE OP mm.
Still-Head Details of Concentric-Tube Column
PRESSURE DROP. The pressure drop through a concentric17 mm. tube column under normal operating conditions is very low. To IO mm. measure this pressure accurately, a Roberta manometer is em- 3 mm. ployed. In operation a small increase in pressure in one of the . ORYN STWOOCK reeekvoira forces a relatively large volume of liquid through the capillary tubing between them and moves a smell air bubble in the capillary. The bubble travel is therefore proportional to the change in pressure, the proportionality factor being the ratio of the cross section of the reservoir to the cross section of the capilFilqure 3. Still-Pot Details of Concentric-Tube lary. For this application an amplification of about 16 is adeColumn quate. The use of water or a mobile oil in the manometer instead of mercury increases the over-all amplification to around HOLDUP. The operating holdup of column B was determined 220. A slow stream of nitrogen is bubbled into the manometer by refluxing a small charge of n-heptane containing about 25% of connections to purge the lines of condensable vapors. high-boiling oil. After a steady state had been reached, a small The pressure drop of column A at various boiling rates using a sample was withdrawn from the pot and the oil concentration was mixture of methylcyclohexane and n-heptane is shown in Figure 5 determined from the refractive index of the sample. The holdup and Table 11. At the boilup rates which are generally used, was calculated from the change in composition and the volume of around 60 ml. per hour, the pressure drop is about 0.3 mm. of the original charge, assuming that no oil had vaporiwd. The remercury. The boiling rates in all tests were determined by sults at various boilup rates are shown in Figure 7 and Table 111. accurately measuring the heat input to thestill pot and correcting The total holdup under normal operating conditions is from 2.0 to for the heat lost through the vacuum jacket. The heat lost 2.5 mh through the lower section of the jacket up to the bottom of the FRACTIONATING EFFICIENCY annular space waa determined by inserting a thermocouple into the manometer connection tube and gradually increasing the The height equivalent to a theoretical plate was determined by heat input till the vapors were detected by the thermocouple. refluxing a mixture of two components having nearly the same For columns A and B, the loss waa 3.2 and 5.0 watts, respectively. The additional heat TABLE 111. OPERATINQ HOLDUPFOR COLUMN B lost through the upper part of the jacket around the fracBoilCharge Residue tionating section was deterfined by the mixture at a rate of 2 to 3 drops per minute at the still head and then gradually reducing the heat input till the reflux just ceased. With the jacket
1ng
Rate
Vol
M I . / H & ~ mi.' 11 5 79 40 a
9.60 9.34 9.14 8.94
1'. sample
nV
oil
mi.
VOI.
n?9
1.4129 1,4126 1.4124 1.4121
27.3 27.1 26.9 26.6
2.62 2.53 2.46 2.38
0.26 0.20 0.20 0.20
1.4193 1.4200 1.4232 1.4221
From Figute7. Column 5
-
001.2 X ool. 4.
3l
Vol. sol.
Holdup, MI.
34.2 35.0 39.4 37.3
7.68 7.23 6.24 6.38
1.92 2.11 2.90 2.56
001.2
- 001.9.
Vola
Column 10
-
H.E.T.P.', Theot Mm. Plate; 5.8 6.7 8.5 7.4
145 147 99 113
Holdu
MlJT.9. 0.013 0.014 0.029 0.023
INDUSTRIAL AND ENGINEERING CHEMISTRY
2330
boiling points until equilibrium hab established. Small sitmpleb were then removed from the still head and the still pot and their compositions were determined from their refractive indexes. Two different test mixtures were used. The first mixture, n-heptane and methylcyclohexane, was used a t the higher boilup rates where the anticipated number of theoretical plates was less than about 80. Above this value the accuracy falls off rapidly because the concentration of methylcyclohexane in the head sample is BO near zero that an error of 0.0001 in the refractive index results in a large error in the Y A / Y Bratio and produces EL large error in the number of plates. At the lowest rates a mixture of methylcyclohexane and 2,2,4trimethylpentane was used. This mixture has a volatility ratio of 1.049 compared with 1.075 for n-heptane-methylcyclohexane. Consequently, the same number of theoretical plates produces a smaller concentration of low-boiling component in the head sample and the error in the Y A / Y Bratio is greatly reduced. At intermediate rates, the same number of plates waa found with both mixtures. The results of these tests are shown in Figure 7 and Table IV. Three series of runs were made, the first with the jacket at 30" C., the second at 60" C., and the third at 90" C. At 30' C. (solid curve), enough heat is lost through the jacket to condense about 30 ml. of liquid per hour, whereas at, 90' C. (dotted curve), practically no heat is lost and the column operates adiabatically. At high vapor velocities, around 50 cm. per second, the H.E.T.P. is approximately 30 111111. and is essentially independent of the jacket temperature, because only a small percentage of the vapoi is condensed in the column at any jacket temperature. At lower vapor velocities, the fraction of the total boilup which is condensed in the column increases and a significantly smaller H.E.T.P. is observed with the jacket at 30' C. The minimum H.E.T.P. is 5 mm. at an average vapor velocity of 5 cm. per second. These data with the jacket at different temperatures show that nonadiabatic operation is to be preferred to adiabatic operation.
IRON-CONSTANTAN
Vol. 42, No. 11
inew
I.D. TO FIT COLUMN YLASURINO 64mm.(LD,
COUPLES ATCENTER OF EACH HEATER.
T W O UNIFORM HEATING
COILS EACH CONSUYINO 170 WATTS AT 115 VOLTS 60 CYCLES A. C.
SfPARABLE 2IPPfR
1 2 m m D l A Y HOLE OPPOSITE ZIPPER
Figure 4, Glas-Col Mantle for Fractionating Column
The effect cannot be attributed merely to the reduced average boilup rate caused by condensation in the column, because all results have been based on average vapor velocities in the column. From a practical point of view these results show that for this type of column the jacket temperature should be about 50' C. below the vapor temperature.
2oi COLUMN 8 COLUMN A
AVERAGE B O l t l N O RATE, M L . P E R HOUR SO I20 180 e40 300
PO 30 $0 so VAPOR VELOCITY, OM. PER SEC.
m
o
04
so
0
io
Ib
3b
4b
sb
6b
AVERAQE VAPOR VELOCITY, CM. PER SEC.
Figure 5. Pressure Drop in Column A
Figure 6.
Heat Input us. Vapor Velocity
TABLE IV. FRACTIONATTNG EFFICIENCY TESTS ON CONCENTRIC-TUBE COLUMNS Col. A A A A
A A A A A B B B B
Test Mixt. 1 1
1 I
1
1
2 2 2 2 2 2 2
Heat Input Watt: 20 12 20 12 20 12 12 6 9 8.0 7.5 7.45 7.75
Jacket
pmp.,
C.
Total Reflux Houri 39 29 39 29 30 17 30 35 48 70 73 114 97
n?D"
Head
Pot
'
1.4218 1,4227 1.4203 1.4212 1.4221 1,4221 1,4217 1,4210 1.4204 1.4200 1.4225 1.4223 1.4228
T.P. 34 74 34 67 31 61 84.3 116.3 68.8 103.7 153 150 159
Net Heat Input to Column Bottom Top 16.8 8.8 16.8 8.8 18.6 8.8 8.8 2.8 5.8 2.5 2.0 1.95 2.25
15.0 7.0 16.6 8.6 16.8 8.8 7.0 1.0 5.8 1.4 0.9 0.85 1.15
Vapor Velocity, Cm./Seo. Bottom Top Av. 52.6 27.6 52.6 27.6 52.6 27.6 28.6 9.2
is.9
8.13 6.50 6.34 7.32
46.9 21.9 52.0 26.9 52.6 27.6 22.8 3.3 18.9 4.45 2.92 2.76 3.74
49.7 24.7 52.3 27.3 52.6 27.6 25.7 6.3 18.9 6.29 4.71 4.55 5.53
H,E,T,P,, Mm. 27.6 12.7 27.6 14.0 30.3 15.4 11.1 8.1 13.7 7.5 5.5 5.6 5.3
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1950
COMPARISON OF RESULTS WITH THEORY
In 1942 Westhaver (IO)and Kuhn (6, 6) derived equations which related H.E.T.P.at total reflux to the annulus width, vapor velocity, and vapor diffusion coe5cients of the components of the test mixture. Both equations, when used to calculate H.E.T.P. as a function of vapor velocity at a given annulus width, give the same results. Westhaver's equation, derived assuming adiabatic operation, laminar flow of vapor, and an ideal liquid fiim-ie., negligible surface transfer resistance and a uniform radial composition-is as follows: 17 (V,,WS) H.E.T.P., am. = - --
D
35
0
'1
f
e
D
+E
COLYMN A, JACPT 30'0. A 60. C. A S@C.
"
' "
8
(HOLDUP COLUMN E
30.0.
/
adiabatic and nonadiabatic cases. Two points are evident: the value of the dsusion coefficientused is not very critical; and the nonadiebatic curve conforms to the theoretical adiabatic curve more clo.sely than the experimental adiabatic curve. A change in the diffusion coefficient to a value between 0.10 and 0.15 sq. cm. per second is required to shift the theoretical curve enough to obtain agreement with the experimental adiabatic resultc1 at high values of linear velocity. However, this would not change the shape of the curve and consequently is of no utility in achieving better agreement at low values of vapor velority. Such a large uncertainty in the diffusion coefficient is unlikely und this cannot therefore be considered the fundamental cause of the observed disagreement, Nonuniform wetting of the antiulus walls cannot be responsible either, for observation of tho film shows that uniform wetting is obtained at rates as low m 20 ml. per hour, or linear velocitieg from 3 to 4 om. per second. It is probable that a significant deviation from ideality existfi in the liquid film, either a substantial surface transfer resistance, :L nonuniform radial composition, or both. The superiority of nonadiabatic operation over adiabatic operation is noteworthy and may be thought of as the result of an increase in the value of D produced by transport of material across the annular space due to condensation on the outer wall.
a a a
dX
2331
-
EXPERIMENTAL AOWATIC NONADIABATIC
"
0.01 AVVERAQE BOILIN6 RATE, YL. PER HOUR 60 I20 le0 e90 AMRAOE VAPOR VELOCITY,
C M PER SEG
Figure 7. Holdup and Separating Power us. Vapor Velodty where V . = linear vapor velocity, om. per second, 2W 5 widtb of annulus, cm., and D = diffusion coefficient, sq. om. per second. To compare the observed resultg with the predicted values of H.E.T.P. as a function of vapor velocity it was necessary to determine a value for the diffusion coefficient of the test mixtures. The diffusion coefficients were calculated using Gilliland's modification of the Maxwell equation (J), which is I-
whep D = diffusion coefficient, sq. cni. per second, T = absolute temperature, K., MA,M B = molecular weights of the two gases, P = total pressure, atmospheres, and VA,VB molecular volume. Using this equation, a value of 0.0375 sq cm. per second is obtained for the system n-heptane-methylcyclohexane a t its boiling point and 0.0348 sq. cm. per second for the system 2,2,4-trimethylpentane-methylcyclohexane. Westhaver suggested using the expression D = 1.34 n / p as an approximation for the diffusion coefficient where n is the vapor coefficient of viscosity and p is the vapor density. Using this equation, a value of 0.0326 sq. cm. per second was calculated for the system n-heptane-methylcycb hexane. This value compares reasonably well with that calculated by the Gilliland equation. In Figure 8, the H.E.T.P. has been plotted as a function of vapor velocity for the three diffusion coe5cienta which appear reasonable, all for an annulus width of 0.75 mm. Also plotted are the observed H.E.T.P.-vapor velocity curves for both the
.
Figure 8. Comparison of Experimental with Theoretical Separating Power
TYPICAL DISTILLATION RESULTS.Detailed results of alkylitk distillations in columns comparable to these have been reported (I), in which the separations obtained in routine operation were followed by quantitative analysis of the infrared spectra of the fractions. CONCLUSION
The concentric-tube type of fractionating column can att:rirr very small values of H.E.T.P. of the order of 5 to 6 mm. 1%:cause of ita low holdup it is ideally suited to the precise fractionrttion of small quantities of liquids boiling up to 250' C. The construction of such columns requires experience and care, but it is justified. In general, it is necessary that the annulus be very uniform over its entire length, that distribution of reflux to the inner tube be perfectly symmetrical, that the holdup in the head be negligible, and that return of reflux in the head be smooth and regular. High reflux ratios are required in actual distillations, but practical take-off rates can be achieved. Routine operation can be developed to a reliable procedure if care is exercised to minimize handling losses.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2332
LITERATURE CITED (l)
c. K., and
R*MS.
Vol. 42, No. 11
(7) Naragon, E. A., and Lewis, C.J., IND.ENC.CHEM.,ANAL.ED., 18, 448, (1946).
Am. Pet701eum2nst’,
(8)
2 6 , I I I , 2 3 (1946).
Egloff,Gustav, “Physical Constants of Hydrocarbons,” vel. 1, New York, Reinhold Publishing Corp., 1939. (3) Fenske, M. R., IND. ENC.CHEM.,24, 482 (1932). (4) Gilliland, E. R., Ibid., 26, 681 (1934). ( 5 ) Kuhn, W . , HeEw. Chim. Acta, 25, 252 (1942). ( G ) Kuhn, W., and Ryffel, A., Ibid., 26, 1693 (1943).
(2)
Selker, M. L., Burk, R.E., and Lankelma, H,p.,Ibid.,
12, 352
(1940). (9) Snyder, J. C., and Steuber W., Ibid., 16, 454 (1944).
(10)Westhaver, C. J., IND. ENO.CHEM,,34,126 (1942). (11) Willingham, C. B., and Rossini, F. D., J . Research Natl. Bur. Standards, 37, 15 (1946). RECEIVEDDecember 14, 1949. Presented a t the Meeting-in-Miniature, Philadelphia Section, AMERICAP;CHEMICAL SOCIETY, January 20, 1949.
ESTIMATION OF SURFACE TENSIONS Temarr Liauid Mixtures 1
J
ALAN S. MICHAELS, RICHARD S. ALEXANDER,
AND
COLMAN L. BECKER
Massachusetts Institute of Technology, Cambridge, Mass.
A
relation derived by Meissner and Michaels (7) for estimating surface tensions of liquid mixtures has been tested on three ternary mixtures-benzene-nitromethanen-propyl alcohol, toluene-isopropyl alcohol-furfural, and toluene-ethyl acetate-benzyl alcohol. Both the surface tensions and refractive indexes of these systems were determined experimentally; the refractive index data were then used, in conjunction with calculated values of the pa .ichor and molar refraction of the individual comporwnts, to calculate surface tensions by means of this relation. Comparison of the experimental and calculated \ alues of surface tension indicates that the parachor
molar refraction relation is as applicable to ternary mixtures as it is to binaries. The maximum deviation observed was about 19%; this occurred in the binary isopropyl alcohol-furfural, which appears to be abnormal because of strong adsorption of the alcohol a t the liquidvapor interface. The parachormolar refraction-refractive index relationship is, in general, more satisfactory for estimating surface tensions of ternary mixtures than linear interpolation of values for the pure components. As is true with binaries, however, the method fails in cases where strong surface adsorption of one or more components takes place.
I
(2)
as a function of composition for each mixture. The surface tensions of the pure liquid compounds selected for the mixtures differed widely, in order to provide an adequate test of the relations. Properties of the pure components employed are presented in Table I. Parachors for the components were calculated from the atomic and structural values of Sugden (8-8). Molar refractions were calculated similarly from the data of Eisenlohr (1, 6, 7). The experimentally determined refractive indexes of the mixtures were used directly in Equation 1.
(3)
EXPERIMENTAL
S A recent article ( 7 ) , Meissner and Michaels, using a modifi-
cation of a relation first proposed by Tripathi (9),were able to estimate accurately &face tensions of binary mixtures of organic liqriids from a knowledge of the refractive indexes of the liquids. The ,elations employed were as follows:
vh/,f
=
\PImtx * n L x
[Rim,, . 71.a,,,
-
1
+2
+ . . , etc. IRInLlA = [ R l ~ a+ [Rim + . . . , etc. +
[PIm2.= [PIISI [Plm
Khere y = surface tension of mixture in dynes per cm.; [PI = refractive index arachor; [ R ] = molar refraction; fsodium D-line); and z = mole fraction of component in liquid. Subscripts 1, 2, etc., refer to individual components.
LIQUIDCOMPOSITIONS. To cover adequately the field of each ternary mixture, the following procedure was employed: Binary mixtures of components A and B were prepared containing approximately 20, 40, 60, and 80 mole A , and the propertiesthat is, refractive index and surface tension-were determined. To each of these binaries, incremental amounts of C were added, and the desired properties measured for ternary mixtures con-
This relation was found to apply to a number of binary systems with considerable accuracy, although serious deviations were observed in systems where one c o m p o n e n t showed strong capillary activity with respect TABLEI. PROPERTIES AXD CONSTANTS OF PURELIQUIDCOMPOUNDS USED IN TERNARY to the other. MIXTURES The object of this inveetiga[RIt nD [PI, [PI’ Exptl. tion w&s to test the appliExptl. nd, Lit. Cited 36 78 Y5. Compound Caled. (8) dikd. (6) Exptl. (8) Caycd. Expa. Lit. Cited ( 4 ) cability of these relations to Nitromethane 131.1 132.1 12.44 12.49 1.3800*’.* 1.3818*0 35.5 35.5 36.1 A 0 . 5 ternary mixtures of organic Benzene 207.1 206.2(av.) 26.31 26.18 1.4970”,‘ 1.5014s 28.2 28.1 28.42 AO.06 n-Propanol 165.4 165.4 17.58 17.52 1.3842*6*2 1.3854m 23.5 23.5 23.4 *0.4 l i q u i d s . E i g h t common Isopro yl alooBol 165.4 165.8 17.58 17.54 1.3749” 1.3776” 21.5 21.3 21.6 t0.1 organic liquids (c.P. grade) Toluene 246.1 246.6(s~.) 30.93 31.06 1.4938’6.’ 1.4978’6.‘ 28.8 28.0 27.5 t 0 . 1 were used in the preparation Furfural 210.5 212.9 25.54 25.43 1.5239“ 1.5261” 40.4 43.0 43.2 t 0 . 4 Benzyl of three ternaries; surface alcohol 260.3 259.6 32.45 32.41 1.5388“.8 1.5396” 39.8 39.2 38.4 1.0 Ethylacetate 216.0 216.9(av.) 22.21 22.25 1.3700”.0 1.3722’V 23.4 23.4 22.9 k 0 . 4 tension and refractive index 1, were measured experimentally
