Vapor Take-Off Still Head Improvements in Laboratory Fractionation of Small Quantities at Reduced Pressures in Packed Columns R. A. HALL' Research Department, Nobel Division, Imperial Chemical Industries, Ltd., Stevenston, Ayrshire, Scotland
b A novel, variable-reflux still head utilizing vapor take-off is free of grease, metal, taps, or other moving parts. The holdup is low, and the still head temperature is accurately recorded even a t very low reflux rates. These features make it suitable for small scale work, particularly a t reduced pressures with reactive materials. Good fractionating efficiency is achieved a t low pressures with columns constructed in the form of a narrow annulus, packed with helices constrained to the vertical plane. An apparatus of related design but simpler construction is suitable for less exacting laboratory work a t reduced pressures. The purification and use of 1 -hexadecene in test mixtures are discussed.
6 plate equivalents, the total operating holdup for the nhole unit being not more than 0.5 1n1. and the pressure drop negligible. Tests of an othern ise similar but much longer column denionstrated that a packed annulus column of this type is as efficient, in ternis of plate equivalents, a t 1.0 mm. as a t atmospheric-pressure. SEMIMICRO UNIT Description and Operation of Still
Head.
Figure 1 illustrates the still head, incorporated in the semimicro distillation unit. The leads t o t h e still head thermocouple are encased in a narrow, thin-walled, removable thermocouple well, Trhich enters vertically through the still head, P . Being thinwalled, the thermocouple well has a lorn longitudinal heat conductance. furthermore it is immersed in vapor foi some distance above the junction A . Hence, accurate temperature readings are recorded even at lorn boil-up rates. Junction A is situated a t the reflux
T
H E V.4RIABLE-REFLUX still head describcld is unique in the following resperts: It is all-glass and of simple construction, contains no metal, grease. or any nioi.ing parts. and is operated with equal ease a t all pressures below atmospheric-. Vapor take-off is employed and holdup is thus minimized. The take-off rate is controlled by adjusting n scrm clip, and very slow take-off rntcs arc casily obtainable if rcquircd. It is especially suitable for use uith high efficiency, lon holdup, laboratory colunins dmigned for the fractionation of small quantities, because in such cases thc combination of low boil-up n ith high reflux ratio may nccessitatc tah-ofi rates of 1 drop per minutc or longer. The only extraneous items neccssary for thc operation of this still head are an cfficicnt inanostat system and a fine inert gas leak to the still pot: the forimr is. of course, required in any case for reduced pressure fractionations. A semimicro unit capable of fractionating chargcs as small as 2 ml. is describcd, in nhich the still head is combined n ith a 10-cm., packed annulus column. At 1.0 mm. and a take-off rate of 3 ml. pcr hour, the unit g a w around
Present address, Imperial Chemical Industries, Ltd., Bothwell St., Glasgow, Scotland.
w
Figure 1.
Semimicro distillation unit
A . Thermocouple well and reflux drip tip B. Jacket thermocouples B10, B14,B19. Ground-glass taper joints C. Lead to manometer D. Lead t o dry nitrogen supply
F. G. H. J.
Receiver Still pot, 10 ml. Center tube, sealed and evacuated Fractionating annulus, about 1.5 mm. wide, packed with 4-mm. glass helices
K . Metal casing L,, L,. Heater windings 31. Vacuum jacket, silvered except at -2
N.
P. Q. R.
S. Ti,
Inner reflector tube, silvered e\cept at A Still head, center tube Still head, annulus, (unpacked) Still pot capillary and nitrogen leak Screw clip Tp. Stopcocks
VOL. 31, NO. 3, MARCH 1959
437
Figure 2. Semimicro unit in position, with metal casing not yet wound with heater wiring
drip tip, and should, thercfore, record the required vapor-liquid equilibrium temperaturr, or as near as is practicably possible. The still head is dcsigned so that all condensate in i t returns to the column via the drip tip. When thc screw clip, S, is closed and the stopcocks, TIand T2,are open, the column is on total reflux, because the fine nitrogrn leak from the still pot capillary, R, is sufficient to carry all the vapor up central tuhe P. When the jacket temperatures have been adjusted (as described later), take-off is commenced by gradually opening screw clip S. The vapor efflux now divides, the part passing up P being totally condensed as before while the part passing up Q is partially condensed. Excellent and stable control of the take-off is afforded by adjusting S. A degree of extra fractionation probably takes place in the still head annulus. Construction Details. This unit has a IO-mi. pot capacity (Figure 1, C ) . Separate heating jackets ( L , and L,, respectively) surround the fractionating column proper and the still head. The vacuum jacket, M , is silvered over both sections, with a gap at A t o allow the reflux tip t o be seen. A separate, similarly silvered reflector cylinder, N , is incorporated inside t h e vacuum jacket in order to reduce heat
Table I.
Evaluation of Semimicro Unit with I-Hexadecene-n-Hexadecane Mixture
Still
Pressure, Mm. Hg 1.0 1.0 0.25
Pot Compn.,
Mole
Yo
1-Hexa-
deeenea 47.1 41.0 48.3
Heed Reflux,
Dis-
tillate Takeoff DisCompn., Rate, tillate Drops per Mole % MI. per Vohme, Minute 1-Heradeeene llour All. 8 to 10 8 to 10 8 to 10
56.5 52.0 55.5
Initial pot volume, 10 ml. in each run. To nearest 0.5 plate; total plates, column
438
transfer between the column and the heating jackets t o the absolute minimum -anessential where low boil-np columns are concerned. The jacket heating elements arc would around a steel casing, IC, made from two halves of a thin steel tuhe (13/1 inches in diameter) cut vith appropriate orifiers and fixed around the column in situ (Figure 2). There is an air space of inch between it and the vacuum jackct, in rr-hich are situated two tbcrmoconples, B , to record thc jacket temperaturrs. Thr two heating sections are each srnlcd top and bottom with ashcstos string and a suitable cement to prevent vcrtical air currents. Asbestos pnpcr inipregnatrri with siliconc resin is used to insulate the heatrr i!-iring from the steel tube. The upper ivinding, I,,, has a resistance of 26 ohms and is supplied with a maximum of 25 volts from a tapped transformer. Another transformer supplies a maximum of 70 volts to the lower winding, Ls, which has a resistance of 61 ohms. Input to both transformers is controlled by 0- to 2fi0-volt variahle transformers. Both windings are covered externally with silicone resin-impregnated asbestos paper and then with a l/rinch fiber glass insulating jacket. The latter completely encloscs the column, except that two '/i6-inch glass tubes are placed, diametrically opposed, a t the level of the reflux drip tip, A . A small electric flashlight bulb placed at the end of one tube enables the reflux to be viewed directly through the other, without appreciable heat loss. The method of changing receivers is self-evident from Figure 1. The receivers, F , are thin-walled sample tubes, supported as shown so that the take-off condensate falls directly into the receiver without touching the widebore tap. The fractionating section, J , consists of an annulus 10 em. long, 1.4 em. in inside diameter, and 1.7 cm. in outside diameter. The central tube is evacuated before sealing and is held in place by interlocking glass protuberances. The packing of 4-mm. glass helices is held on a crimped platinum wire ring, itself supported by the above-mentioned glass protuberances. The glass helices are constrained t o the vertical plane, partially overlapping, thus forming a network which affords excellent reflux distribution over the annulus walls without causing an undue pressure drop.
ANALYTICAL CHEMISTRY
3.0 1.0 3.0
+ pot.
0.8 0.2 0.8
Eqniv-
Holdup/Theoretical
dent
Plates6
Plate, MI.
5.5
0.1
6.5 4.5
0.1
0.1
Figure 3. Complete with oil bath
semimicro unit
The still pot is immersrd in an oil bath (a 250-ml. beaker), rvhich is heated and magnctically stirred by a combined hot platemagnetic stirrrr, commercially availahle. The beaker is insulated with asbestos string, and the whole is enclosed in an insulating jacket. Figure 3 illustrates the arrangement. The oil bath temperature, once adjusted, remains sufficiently constant for practical purposes, as judged by the rate of reflux at A . During the hexadecanehexadecene evaluation, no trouble I T n s experienced from bumping. It is useful to have the uitrog.cn leak, R, bent, S O that, if bumping is absent, it can he turned out of the liquid to keep foam bubblrs from entering the packed section of the column. Bumping liquids generally do not foam, so in these cases the leak can advantageously he immersed in the liquid. PERFORMANCE OF SEMIMICRO UNIT
At the commencement, the jacket temperatures and the oil bath temperature are adjusted so that a reflux of about 15 drops per minute is observed at A , and the lower column jacket temperature is at, or slightly above, the still head temperature. The still head (upper) jacket should be just hot enough to prevent a condensate mist from forming on the wall of the column a t A (usually about 6" to 8" C. below the reflux temperature). When steady conditions are attained, t a k e 4 is controlled as previously described by operating screw clip S. As successive fractions are removed, the oil bath temperature is raised by stages, the jacket temperatures of course being raised to correspond. The apparatus can he operakd on these lines from ahout 700 mm. down to less than 0.1 mm. The performance of this unit was tested at 1.0 mm. and 0.25 mm., using I-hexadecene-n-hexadecane mixtures.
-PUMP
@
Figure 4.
Simpler apparatus for routine use
B. Fractionating annulus packed with helices constrained to vertical plane and slightly overlapping C. Center tube Length of column wrapped with aluminum foil, then a heating E. tape, and lagged F . Upper, uninsulated section of colnmn G . Take-off condenser S,, S?. Screw clips Ti,T:. Thermometers
Tlie relati] e volatility was assumed to 1 x 5 the. wine a t 0.25 mm. as the known \:iluc, a t 1.0 mm.-namely. 1.07 ($2). This 1s a reasonable assumption, as the vapor pressure-temperature curves appear to be parallel donn to 0.5 Inm. ( 2 ) . The results, given in Table I, show that 4- to 6-plate equivalents can be expected under actual operating conditions in this pres>ure range. T h r pressure drop and the total operating holdup of the nhole unit were estiniatcd to be about 0.03 nini. and not inorc than 0.5 nil., respectively. These mtimatm are based on the performance of an 80-cm. column of slightly larger a n n u l u ~diameter, but otherwise built :md opclrated on the same principles. At 1.0 mni. a i t h hexadecane this long rolunin had a measured total operating lioldnp (liquid plus vapor) of not more than 4.0 ml., and a pressure drop of only 0.25 mm. (boil-up, 16 ml. per hour). I t is clear that the holdup and pressure drop of the 10-em. semimicro unit, operating a t a loner boiling rate, should certainly not exceed one eighth of the measured values for the 80-cni. column. PERFORMANCE OF LONGER COLUMN
The longer column was exhaustively tested a t pressures ranging from atmospheric to 0.1 mni. with four different t m t mixtures. At 1.0 mm. with the nhesadecane-1-hexadecene test mixture, 20 to 25 plate equivalents were obtained.
pears a t tlie top of the annulus a t I'. If annulus B is packed, it may be necessary to open Szfully and partially this depends on the dimensions close SI; of C, B , and the nature of the packing. The temperature of the jacket is now set a t about 10" C. below the temperature recorded by thermometer T1,and control clip Szis then further adjusted to give any required rate of take-off. Once adjusted, the take-off keeps remarkably constant and the unit can be left to run nith safety. The end of a fraction is usually signalled by a fall in tlie still head temperature, and take-off nil1 slow down and stop until the settings of screlv clip and oil bath are altered. The vapor temperaturc rccorded by the still head thermometer has been found to vary T\ ith thc oil bath teniperature, other things bcirig equal. Therefore, it is best to raise the oil bath ternperature in stagcs; by recording this temperature in conjunction n ith the still head tempcrature, the onset of different fractioiiq can bc recognized TT ithout difficulty. Satisfactory m aluation of this column was not possible, as quantitative estiniations of annulus boil-up, reflux ratio, and the like could not be made. However, some empirical tests using n-hexadecane-1-hesadecene mixtures indicated that H.E.T.P. values of the order of 5 to 8 em. n ere being obtained a t 1.O nini.
At atmospheric pressure with n-heptane-methylcyclohexane, 18 to 25 plate equivalents were obtained, while a t 20 mm. with ethylbenzene-chlorobenzene, the plate equivalence ranged from 28 to 34. All these results were obtained a t finite reflux ratios-that is, in conditions corresponding t o practical operationand the high performance a t the reduced pressures is noteworthy. Below 1.0 mm. the efficiency began to fall away. SIMPLER APPARATUS FOR ROUTINE USE
Figure 4 illustrates an apparatus of much simpler construction which can be adapted t o any size, and n hich has performed satisfactorily in normal laboratory distillations a t reduced pressures. I n this case, vapor from the still pot is divided a t the foot of the column; some passes up the center tube, C, and is totally condensed a t the top by a cold finger. A smaller flow of vapor passes up the helice-packed annular space, B , where part is condensed in the uninsulated top section, F , and refluxes back down tlie annulus. The remainder is taken off as product through condenser G. The relative amounts of vapor entering B and C are controlled by adjusting screw clips S1and Sz.The stopcocks facilitate receiver changing. T o operate this unit, the oil bath is set a t a temperature n hich provides a moderate reflux up C when S2 is closed. Szis then opened until condensate ap-
USE OF 1-HEXADECENE IN PLATE EVALUATION TEST MIXTURES
It was necessary to purify hexadecene by fractional freezing before material of satisfactory refractive index ( I , 3) and melting point could be obtained by subsequent distillation to constant refractive index. Fractionation to constant refractive index alone was not sufficient, probably due to the presence of 2-hexadecene. Moreover, there was some evidence that 1-hexadecene, during prolonged refluxing in tests, may partly isomerize t o 2-hexadecene, which causes an apparent reduction in plate efficiency. The isomerization could possibly have been catalyzed by the small platinum wire ring nhich held up the helice packing in all the columns used. This potential disadvantage attaching to the use of 1-hexadecene has not apparently attracted attention hitherto. LITERATURE CITED
(1) American Petroleum Institute, Research Project 44, "Selected Values of
Physical and Thermodynamic Properties of Hvdrocarbons and Related Compounds." (2) Benner, F. C., Dinardo, A,, Tobin, D. J., Znd. Er,g. Chem. 43, 722 (1951). 13) ~, Wibaut. J. P.. Geldof. , H.., Rec. Iraz'. chim. 65,'125 (1946).
RECEIVED for review May 24, 1957. Accepted October 3, 1958. VOL. 31, NO. 3, MARCH 1959
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