I
T. M. REED, JOHN F. WALTER, RICHARD R. CECIL, and R. D. DRESDNER Department of Chemical Engineering, University of Florida, Gainesville, Fla.
Quantity Purification of Fluorocarbons by Gas-liquid Chromatography Pure components were obtained from fluorocarbon mixtures with a simple and inexpensive feeding device for accurate charging of liquids and condensable gases. The variables of charging were studied; a diffusion equation applied to the elution curve gives diffusion coefficients
IN
CHROMATOGRAPHY, separations become poorer as the amount of charge per unit of column cross-sectional area is increased. To increase the separation rate b>- gas-liquid chromatography. several factors must be considered. For both analytical purposes and quantity separations. a stationary phase is selected primarily on the basis of the resolution it can produce. Some stationary media may produce the desired resolution by nonideal solution effects. ll’hen a solute in the stationary medium has an activity coefficient greater than unity, the capacity of the column is less than when the solution is ideal or deviates negatively from ideal behavior. ,411 other things being equal, rhe desirable stationary phase gives a partition coefficient as high as possible. This allows the maximum charge without affecting the spread of individual peaks ( 8 ) . The separation rate is determined also by the time required to develop the chromatogram. The appearance time of the total chromatogram and the resolution vary inversely as the tempera-
ture and as the flow rate of the carrier gas. Optimum conditions must be chosen to give reasonable yields at reasonable separation rates.
Apparatus The partitioning columns of the batch operating arrangement (Figure 1) were enclosed in a double-walled air thermostat made of 4- and 6-inch stovepipe. The thermostat, 4 feet long, was heated electrically with 53 feet of Nichrome wire (No. 18 B. Br S. gage, 0.410 ohm per foot), wrapped on the 4-inch stovepipe with a pitch of 1 inch per turn. The products were detected by a thermal conductivity cell (Gow-Mac LIT-128) of the self-purging type (2). The reference cell was of the diffusion type. Cut points \\ere determined as directed by Kirkland (70). The products were collected by manipulating the valves on the cooled traps. All lines were 0.125-inch copper tubing . Solenoid valves (Valvair Corp., Model LC-42-01) controlled the flow from the
feeder into the column. Valve S-1 supplied gas pressure to the feeder; S-2 was used in the liquid line to the column. S-2 was modified by drilling a hole into the bottom of the valve below the valve seat, mounted directly above the column on the thermostat. The carrier gas flowed through 5’2 continuously into the column, purging the volume below the seat. Parallel electrical connection of both solenoids and a timer provided simultaneous action when energized. The feeder was made from 1-inch copper tubing 10 inches long. The dip tube, of copper tubing 0.125 inch in diameter, extended to the bottom of the well. The total volume of material charged was approximately proportional to the product of the pressure difference across the feeder and the duration of the charging interval. The partitioning media were prepared (77), and packed in 1-meter lengths of steel electrical conduit, 1 inch in inside diameter. During packing the columns were mounted on it rack attached to an electromagnetic vibrator.
MANOMETER GAS SUPPLl
Calibration of Feeder
4
At each pressure differential the volumes charged of two substances fall
Figure 1. Partit i o n i n g columns were enclosed in a double-walled air thermostat
-
GAS REGULATOR FEEDER
’
DENSITY
VISCOSITY
0 WATER
3 400
1.76
FC 0-71
? ROTAMETER
A P = 2 2 7 MM.
30LUMN
1.38
JY
a
THERMOSTAT
I
CARRIER
GAS
Figure 2. Vol100 umes charged fall 9 on same curve when plotted against product 0 (iP)(f)/p .
I
I
I
I
I
I
2
3
4
5
VOLUME
CHARGED,
VOL. 51, NO. 3
ML. MARCH 1 9 5 9
271
b PARAMETER IS CHARGING DURATION IN SECONDS
Figure 4. Within 0.1 to 1.6 seconds asymmetry does not depend on charging duration
I
% OF P E A K HEIGHT
0 0
20 %
7
1, '
I /
40%
A
4
Figure 3. Both charging interval and loading influence spread of n-pentane peak
1 -inch column, 2 meters, n - h e x a d e c a n e on C., hyCelite, 39' drogen carrier gas at 2.5 cubic feei per hour
A
A
(UA
0
on the same curve when plotted against the product, ( A P ) ( t ) i p (Figure 2). Fluid viscosity was an insignificant variable. The fluorocarbon required 1 second at 112 mm. of mercury and 0.5 second at 227 mm.; these times were approximately 0.6 and 0 3 second for \\ater. The feeder \vas a t the level of valve S-2. Quantity of Charge and Duration of Charging
The quantity of charge and charging interval may be varied independently. Both should influence the quality of separation ( 6 ) . Their effects on the -ead of an elution curve are illustrated
in Figure 3. It \vas established by planimeter measurement that the amount of charge was proportional to the peak area. The effluent samples were condensed in liquid air traps and weighed to give the amount of charge. No effect on spread can be artributed to charging duration of 0.1 to 1.6 seconds. The quantity of charge (loading) is the important variable. The increase in spread accompanying loading limits the rate of processing material in difficult separations. Excessively long durations (10 to 1 5 seconds) increase the spread by 10 to 2 0 7 ~ , The intercept of 2.3 cg. per. cm. of
z
0
I-
V W
J LL
W 0
a W
0
I
*
A
A
2
1
80 %
I
a
0 V W
a
I
A I
1.0 2.0 S, C H A R G E I N GRAMS
3.
peak height on the ordinate of Figure 3 would be the spread in all cases if all the charge could be accommodated in the first "plate" of the partition column. The maximum height on such an ideal elution curve Lvould be proportional to the charge. The elution curves of n-pentane on n-hexadecane packing give distinctly unsymmetrical peaks. These chromatograms were studied in terms of a solution to Fick's diffusion equations (5) :
The same form of equation is obtained by the "rate theory'' and "plate theory" for chromatographic separations \rhen the number of "theoretical plates" is large ( 9 ) . The equation is for symmerrical peaks and cannot be applied directl>- to n-pentane chromatograms. The asymrcetry of a peak may be d r scribed by the ratio of t!ie x- distances on each side of the vertical line througf: the peak maximum. \tithin the range of 0.1 to 1.6 seconds there is n o dependence of as)-mmetry on charging duration (Figure 4). , i s the quantir) o l loading approaches zero. sy approaches unity. The curves seem t ~ ] approach the horizontal at about 1.3gram loading. Charging durations oi' 10 and 15 seconds produce more symmetrical peaks than chose for s!iorrer times a t the same quantity of charge. Column of Infinite Cross Section
M E
Figure 5 .
Two types of columns in series give both compounds pure in one operation I. II.
Perfluorobutyl sulfur pentofluoride Perfluorothiaphane tetrafluoride Temp. 30' C.
272
INDUSTRIAL AND ENGINEERING CHEMISTRY
These chromatograms obey the form of Equation 1 at infinite column cross section for finite charge. The height: h ; of the elution curve at any point is proportional to c? concentration of n-pentane in the carrier gas emerging from the column:
FLUOROCARBON P U R I F I C A T I O N (2)
c = kh
Here k is in grams of pentane per cubic centimeter of gas per centimeter of height on the chart. The spread is defined as 1 = S/AH
(3)
Csing Equations 2 and 3, and defining f = h 'H. Equation 1 may be written as In j
=
- xij4DB
- In
(2k&%B]A,)
(4)
Lvhere x , is the distance from the vertical through the peak maximum to the cur\-e a t height h : and A, is the limiting spread (0.46 cg. per sq. cm. of column area per cni. of chart, in this case), all for the symmetrical peak at infinite cross section. Plotting In 3 LIS. x B at constant ,f and extrapolating to the limiting -lo gives : (Cm. of Chart Let~gth)~ 0.09
sf.
f 0.8 0.6 0.4 0.2
1
I log,"f 0.904 0.779 0.602 0.301
0.34
0.70 1.30
A plot of IoglnJ PS. x i gi\-es a straight line. Diffusion coefficient D has a real significance in a column of infinite cross section, where the charge is initially all in one plane. As the material proceeds through the column, it is spread by diffusion. From the slope of Iogiof Ls. x,? D may be computed. Using 0 = 1 3 minutes (observed appearance time of n-pentane), and converting xi to time units from the chart speed (0.85 cm. per minute), D is calculated as 5.5 sq. cm. per minute. The diffusion coefficient of n-pentane in hydrogen a t 19' C. and a t the average pressure in the column of 1.5 atm. was computed by Equation 8.2-44 (3>p. 539) to be 5.80 q.cm. per minute. The intercept on the loglof axis gives the k value in Equation 2 ; k = 1.7 X 10-4 gram of n-pentane per cc. of gas per cm. of chart height. The unit of height here is for approximately 0.1 of the full bridge unbalance. If \ve choose as the IoJver limit of significant height a value of 0.1 cm., the detector cell and bridge circuit are apparently capable of indicating 1.7 X gram of pentane per cc. of gas. 'The chromatograms of n-pentane on n-hexadecane, in the absence of the distorting effect of excessive loadings, fall in the class of linear nonideal chromatography (9)-the phase distribution isotherms are linear and the mass transfer coefficients are finite, so that the peak spreads by diffusion effects. The comparable magnitudes of the experimental diffusion coefficient and the theoretical value for n-pentane in hydrogen indicate that spreading is essentially in the gas phase in this case. At finite loadings the asymmetry of the chromatograms and
T
the excess spreading arise from the fact that the charge initially occupies a finite length in the column. By increasing the column cross section, these adverse effects of loading can be reduced for a given total charge. A stationary phase. other than n-hexidecane, \vhich should have a greater pentane solubility, \vould alleviate the loading effects by reducing the initial column xolume required for a given charge.
+I0
M l N i
z
-
0
+ V W J U W
Typical Separations
0
a:
Perfluorobutyl Sulfur Pentafluoride and Perfluorothiaphane Tetrafluoride ( J ) . Figure 5 illustrates the importance
w
of the proper combination of partitioning columns in a separation. Using dinonyl phthalate plus n-hexadecane in series shows a small impurity between the main peaks, not resolved by dinonyl phthalate alone. By using the tlvo types of columns in series. both perfluoro compounds \vere obtained pure in one operation. It has been reported ( 7 7 ) that perfluorocarbons require perfluorocarbon partitioning media for separation. It was found, hoivever, that perfluorocarbon-sulfur compounds are better separated on hydrocarbon media. Although these compounds boil at approximately 70' C., they are eluted in short times at 30" C. from hydrocarbon media. The thermodynamics of the perfluorocarbon sulfur compounds in solution ivith hydrocarbons and Lvith perfluorocarbons have not been studied. Calculations from the chromatograms of perfluorobutyl sulfur pentafluoride sho\v that essentiallr ideal solutions are formed in Cl(CF?C:FCI)BCF?C;OOC:?HS (activity coefficient 0.9) ; nonideal, dinonyl phthalate (activity coefficienr 23).
a:
0.25-INCH
0
a:
0 0 W
C FH
AP= 14 PSI
PPsl8 PSI 0
TIME
Figure 6. Under actual operating conditions perfluorobutyl sulfur pentafluoride(1) and perfluorothiaphane tetrafluoride(l1) were separated at 2 ml. per hour
+
2 meters of dinonyl phtholate 2 meters of nhexodecane in series, I-inch tubes ot 3 0 ' C.
Mol. W t . n 125' C.)
Products
I. 11.
Found
1,2703 Solid
Theor.
346-349 308
346 3C6
Figure 6 sho\vs the chromatogram obtained under actual operating conditions. The rate of separation was 2 ml. per hour. The time of elution per charge was shortened by using high carrier %as rates (1.4 cubic feet per hour) during parts of the separation not critical to the resolution. The proper sequence of
TUBES
PARTITIONING MEDIUM
Cl(CFz CFCI),CFz
ORIGINAL MIXTURE L
I
20
COOC,H,
RESIDUAL
I
I
IO
0 TIME
I
30
MIXTURE
I
I
20
IO
J
0
IN MINUTES
Figure 7 . Analytical chromatograms of products from electrolysis of pyridine in anhydrous hydrogen fluoride Peak 4 was removed completely and peok 7 almost completely CiFllN Pertluoropiperidine C S F I P Pertluoropentone
VOL. 51, NO. 3
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OPERATING HELIUM RATES AND CUT POINTS AT 2 ML. LOADING PRODUCTS
I Figure 8. CFHCarrier gas rate in cubic feet per hour at column exit
FROM
a t 2.2 than a t 0.7 cubic foot per hour per unit of mixture separated. Acknowledgment
The authors thank F. \\-, Hoffman for the material containing perfluorobutyl sulfur pentafluoride and perfluorothiaphane tetrafluoride, and F. S.Tlumac for the electrolysis product of pyridine in anhydrous hydrogen fluoride.
Nomenclature .4 = cross-sectional area of partitioning
D f ,
F flo\v rates should be studied. I t may be better to use low rates while peak I is being developed, rather than while it is coming off the column. This separation could not be made by distillation. because one product is a solid.
Perfluoropentane and Perfluoropiperidine. Figure 7 shows the electrolvsis product of pyridine in anhydrous hydrogen fluoride before and after separation on a 1-inch partitioning column, 2 meters long, packed with CI(CF&FCl)aCF&OOC~H~on Celite. Eleven compounds are indicated in the original material. Peaks 4 and 7 , the main constituents, correspond, respectivelv. to perfluoropentane and to perfluoropiperidine. The center chart sholzs that peak 4 was removed completely and practically all of peak 7 was removed. These two products contain no impurities (right). The refractive indices a t 20’ C. were 1.2391 and 1.2701, respectively--lower than the reported values (72) of 1.245 for n-perfluoropentane and 1281 for perfluoropiperidine. The density of the perfluoropentane was 1.6036 grams per ml. at 25’ C., the same as the literature value (7). Figure 8 shows the operating chromatogram and the cut points in the 1inch tubes for the material of Figure 7. Rate of Separation
The rate of processing a mixture by such a batch operation is determined by
Table I.
R = S/Q,
(5)
et decreases with
carrier gas flow rate and temperature. I t is desirable to use a low temperature, as resolution usually decreases with increase in temperature. The maximum value of a charge is selected by the resolution desired in any particular case-in the perfluoropentaneperfluoropiperidine mixture, the resolution was selected as that required to produce essentially complete separation of the perfluoropentane peak from the adjacent peaks. The experimental value of S, at 30’ C. is given in Table I for various carrier gas flow rates. Approximate relationships are 0‘ c F-0.7. S, a F-2.5, so that R u F-I.8. The greater the carrier gas flow rate, the lower the rate of separation for the chosen resolution. If the carrier flow is 0.7 cubic foot per hour of helium, the separation is about 10 times faster than a t 2.2 cubic feet. There obviously must be a lower limit in carrier gas rate: below which the rate of separation \vi11 decrease. At zero carrier rate the separation rate is zero, because the height of one theoretical plate becomes infinite (7). The mixture spreads throughout the column by diffusion and essentially no separation is obtained. The total amount of carrier gas required to separate the material is also important. Low helium rates are overwhelmingly more economical (’Table I), 20 to 30 times as much gas i s required
Maximum Charge and Rate of Separation
0.10
Liters gas/ml. charge 9 peaks 5 peaks
3.3
7.2
0.08
13 6.5
...
40
*..
For complete resolution of peak 4 , perfluoropcntaile, from adjacent peaks.
274
INDUSTRIAL A N D ENGINEERING CHEMISTRY
h
H k
AP A
S t x
A 0 p
SUBSCRIPTS B = back of peak F = front of peak o
= symmetrical peak (infinite column
n2 t
= maximum value = total chromatogram
cross section)
of given mix-
ture Literature Cited (1) Burger, L. L., Cady, G. H., J . Am. Chem. SOC.73, 4243 (1951). (2) Dimbat, M., Porter,P. E., Stross, F. H., Anal. Chem. 28, 290 (1956). (3) Hirschfelder, J. O., Curtiss, C. F., Bird, R. B., “Molecular Theory of Gases and Liquids,” Wiley, New York, 1954
(4j‘H&man, F. LV., Simmons, T. c., others, J . Am. Chem. SOC. 79, 3424 (1957). (5) Jost, W., “Diffusion,” p. 17, *%cadernic Press, New York, 1952. (6) Keulemans, A. I. M., “Gas Chromatography,” p. 114, Reinhold, New York, I(
n c 7
17.71.
The greatqr the carrier gas flow rate, the lower i s the rate of separation Helium rate, cu. ft./hr. 0.7 1.1 1.7 2.2 s,, m l . a 4.0 2.0 1.0 0.2 O t for 9 peaks, min. 85 50 45 35 Rg, ml./min. 0.046 0.040 0.02 0.006 Bt for 5 peaks, min. 40 26 ... 18
Rj,rnl./min.
column, s q . cm. concentration in gas phase, grams per cc. = diffusion coefficient. sq. cni. per minute = ratio h/H = carrier gas flow rate, cubic feet per hour (C.F.H.) = chart height of elution curve any value of c, cm. = maximum value of h. = constant, c l h , grams per cc. per cm. of chart height = pressure difference across ferder, mm. Hg = rate of separation of mixture = quantity of charge, g r a m = charging interval duration, seconds = distance from vertical through peak maximum to elution curve at given G (or h ) ! minutes = spread of peak, SIAH. grams per sq. cm. per cm. of chart height = peak maximum appearance time, minutes = liquid density, grams per ml. =
U’
0.01 170 104
(7) Ibid.,p. 126. (8) Ibtd.; pp. 182-3. (9) Zbid., Chap. 4. (10) Kirkland, J. J., International Gas Chromatography Symposium, p. 173, Instrument Society of America, Pitts-
burgh, Pa., 1957.
( I l l Reed, T. M., Anal. Chem. 30, 221 (1958). (12) Simons, J. H., ed., “Fluorine Chem-
istry,” vol. I, Academic Press, New York, 1950.
RECEIVED for review June 16, 1958 ACCEPTED October 6, 1958
