278
ANALYTICAL CHEMISTRY SEAL A N D BEARING
/
Table 1. Evaluation of Freezing Point Depression Constant of Ethylene Carbonate Compound Azobenaene
Molecular Weight 182
Benzoic acid BenaoDhenone
122 182
Hydroquinone
110
Maleic anhydride
98
p-Nitroaniline
138
Phenanthrene Vanillin
178 132
Mole/ 1000 Grams of Solvent
h-
0.017 0.0181 0 020 0.041 0.062 0.0229 0.0427 0.0268 0.0566 0 0163 0.0359 0.013 0,0234 0.0393
7.16 7.13 6.95 6.93 7.10 6.95 7.01 7.13 6.89 7.08 7.08 6.89 7.09 6.92
T HERMOMETE
UPPER COLLAR
TEFLON TETRAFLUOROETHYLEN E RESIN REFLUX VALVE -TAKE-OFF VA L V E
Xelv York, 1946). Although ethylene carbonate reacts with a variety of functional groups, it is stable under the conditions used. EXPERIMENTAL
Ethylene carbonate (Jefferson Chemical Co., Inc.) was purified by fractional distillation at pressures of approximately 10 mm. of mercury through a column packed with single-turn helices. Fractions of freezing points within 0.02" C. m-ere combined and allolved to solidify in small crystals. The solid material was well mixed and stored in fairly small, sealed bottles. The freezing points were determined in a standard Beckmann freezing point depression apparatus. No special precautions to ensure high accuracy mere taken. After supercooling, a definite plateau in the cooling curve was reached. The temperature remained constant for approximately 4 minutes, thus giving a constant freezing point and making an extrapolation to the liquid cooling line unnecessary. The results are summarized in Table I. The cryoscopic constant was equal to 7.03 Z!Z 0.14.
Spinning Band Still for Vacuum Operation Ralph G. Nester, E. I. du Pont de Nemours & Co., Wilmington, Del. URING
Figure 1.
Spinning band still Patented
cating plastic such as Teflon tetrafluoroethylene resin, nylon (polyhexamethyleneadipamide), polymonochlorotrifluoroethylene, or polyethylene may be used. The bearing is a glass or metal spherical or inverted conical bead mounted on the shaft of the spinning band and forms a very effective rotary seal. The assembly adjusts itself for slight wear or misalignment.
recent years there has been much interest in the
D development of laboratory stills having rotating members
as packing in view of desirable performance characteristics such as low pressure drop, low operating holdup, and high efficiency. Provision of a trouble-free seal for the rotating shaft to permit operation under high vacuum has given considerable trouble in the past, although in 1947 Birch, Gripp, and Nathan ( 1 ) described a spinning band column operable a t 0.01 mm. with a total pressure drop of only 0.04 nim. More recently Foster and Green ( a ) have reported a magnetic drive for spinning band columns that makes possible continuous distillations of 120 hours at pressures as low as 0.1 mm. Another variation of spinning band type columns is reported by Murray ( 3 ) . The spinning band still described (Figure 1) offers certain advantages over previous designs, especially from the standpoint of a simple, highly effective combination seal and suspension bearing for the rotating band, an improved reflux and take-off needle valve that requires no grease and is free from leaks, and a spinning spiral wire gauze band that is particularly efficient. Stills of this design are easy to fabricate, are sturdy, and have proved to be very practical pieces of laboratory research equipment that in almost all instances are far superior to other types of laboratory stills in ability to fractionate heat-sensitive or high-boiling, high-viscosity liquids. Seal and Bearing. The unique combination seal and suspension bearing seat is made of a plug of a self-lubricating plastic that fits tightly the top of the still column. The plug has a passage through it for the shaft of the spinning band. This passage is wide a t the top, narrows donm to form an inverted conical or hemispherical bearing seat, and continues as a narrovi passage to the bottom of the plug (Figure 2). Any self-lubri-
L F - LUBRICATING LASS
OR
METAL
Figure 2. Detail of seal and suspension bearing seat
Spinning Band. The spinning band is formed by attaching 80- to 100-mesh wire gauze spirally to the shaft; usually there are about 3 to 16 spirals per foot of shaft length. There are three sections of spirally twisted wire gauze on the shaft, one in the condenser section of the column, one in the fractionating section, and the third in the still pot. The two outer longitudinal strands of x-ire are removed from the gauze, so that the transverse wires ext,end beyond the outer longitudinal wire and come in
279
V O L U M E 2 8 , NO. 2, F E B R U A R Y 1 9 5 6 contact n i t h the inner column wall. This construction provides for violent agitation of the reflux liquid and for intimate contact of descending liquid and ascending vapors. This contact is enhanced by the spiral construction of the band which throws both liquid and va or to the column wall. The pitch and direction of rotation o f the spiral create a downward pumping action 1% hich accelerates the return of the descending liquid to the still pot. The section of band in the still pot ensures smooth boiling and uniform boil-up rates. All of these factors contribute to give a still of high flexibility and efficiency. A guide bearing a t the bottom of the still pot prevents vibration of the shaft. The construction whereby the transverse wires of the spiral band brush the column wall also helps keep the spinning band centered and free of vibration. Take-Off Valve. Reflux ratio of the still is controlled very accurately by means of an adjustable needle valve (Figure 3). The valve is made by threading a stainless steel rod through a plug of Teflon tetrafluoroethylene resin fitted tightly into the top of a glass tube. The lower end of the rod has a Teflon tip which fits into the valve seat located between arms which connect with the still column and the take-off tube. Adjustment of the valve is made by means of a brass turn nut a t the top of the rod.
To give a more precise idea. of thc operatioiial characteristics of a typical still, data are given in Table I. The still used had an interior column diameter of 10 mm., column hand length of 29.75 inches with 12 spirals per foot, and condenser band length of 5.25 inches. The band %*asoperated a t 3300 r.p.m. using a mixture of 25% n-heptane aiid 75% methylcyclohexane by volume. AChNOW LEUGXIENT
The author nishcs to express his acknowledgment axid :tppi vriation to J. TV. Robson for assistance in obtaining distillation data, and to T. *J. Uhrig for help in fabrication of the still. LITERATURE CITED
. S. F., Gripp, I-.,and Nathan, W. S.,J . SOC. C h c ! ~ Irid. 66,33-40 (1947). (2) Foster, N. G., and Green. L. E., J r . , AXLL. CHEM.24, 1869 (1952). ( 3 ) hlurray, Ii. E., J . A m . Oil Chemists' SOC.28, 235- 9 (1951 ). (1) Birch,
CONTRIBUTIOX 340, Chemical Department. Experimental Station, E. I. dri
TEFLON 1 E 1 R AF 1. UO R O E TH Y L E N E
P o n t de h'emours 8; Co.
Simple large Volume Fraction Collector
for Column Chromatography Victor Ginsburg, Department of Plant Biochemistry, University of California, Berkeley, Calif.
-
~ V P L inexpensive E
d Figure 3.
fraction collector has bren devised for the automatic fractionation of the eluate from columns. With the increasing use of column chromatography for chemical separations, many automatic fraction collectors have been described. The receivers in these collectors are usually changed by means of electric motors that are activated by impulses from timers or by the closing of circuits after certain volumes of eluate have been collected.
b
Detail of adjustable needle valve
Use of such a valve eliminates the possibility of air leaks or bubbling back such as may occur with a stopcock. OPERATIONAL CHARACTERISTICS
Stills of this design have been made with interior column diameters ranging from 6 to 50 mm. and with column heights of 1 to 6 feet Such stills have been operated readily a t pressures as low as 10-4 mm. Dependent on still size and operating conditions, a height of equivalent theoretical plate as Ion- as 0.74 inch and a pressure drop of 0.04 mm. have been obtained. k column measuring 23 mm. in diameter by 36 inches in height has been used for a large number of distillations a t a through-put late of 4 liters per hour using a 12-liter distillation flask. ii 1-liter distillation flask may be used with this column, provided an adapter of correct length is used for connecting the column and flask. Microstills with columns 6 mm. in diameter and height from 12 to 36 inches are characterized by small holdup, as low as 0.3 ml., and high boil-up rates. Such microcolumns can he operated with a few milliliters of starting material..
Table I.
Operational Characteristics of Spinning Band Still at Different Boil-Up Rates
Boil-up rate, ml./hour Operating hold-up, ml. Pressure drop, inm. H g H.E.T.P., inches
1622 4 3 1.09 3 50
333 0 8 0 23 1.07
20.5 0 7 0 23 0 74
For the separation of nucleotides by column chromatography, it is often necessary to collect up to 100 liters of eluate in 500-ml. fractions or less. In this laboratory, a simple versatile apparatuP
