A Simple Method for Producing Efficient Concentric Tube Distillation Columns Sherrel Smith Texas A & M University, College Station, TX 77843 Comparison of simple and fractional distillation is a common experiment in organic chemistry laboratories. For several years we have been using columns packed with glass heads for demonstrating fractional distillation. While these columns performed well, their use had some disadvantages. They display a relatively large holdup and are quite different from the equipment used in the simple distillation. The large holdup made it difficult to scale down the pot charge below 5 to 6 mL. Also, as a more effective teaching tool, we desire to use the same equipment for both the simple and fractional distillation hut markedly change the efficiency of the column in some simple fashion for the fractional distillation. The Ace Glass Inc. Hickman-Hinkle spinning hand still serves this purpose fairly well hut has some drawbacks. The sninnine hand would seem to he excessively expensive when i s e d for only one experiment. Also we have experienced some difficulty in operating these units with the heating mantles that we-use in our liboratories. The use of the heating mantle interferes with the spinning of the hand. These disadvantages were overcome by converting the Hickman-Hinkle column to a concentric tube column. This was accomnlished hv machining a nlastic insert to serve as the inner tube of a concentric t i h e column. We have used Delrin, UHMW polyethylene, and Kel-F for this purpose. These materials are suitable for a wide range of temperatures and distillates. The use of Kel-F for the insert provides the hest chemical compatibility hut is much more expensive than the other two plastics. The construction of these inserts is shown in Figures 1,2, and 3. They are machined to provideapproximatelv 0.03 in. clearance between the wall of the column and t,he i k e r t . T-~o k e e ~ the inserts ~ lv in the .--. . r o.~ e r oositioned columns, rings were machined at the top and bottom of the inserts. These rines " were sized to fit the inside diameter of the column loosely. Another ring was machined at the top to hold the insert in the column. Slots were cut in the rings to allow passage of vapor and liquid. The distillation awemhly with insert is shown in Figure 4. The Hickman-Hinkledistillation heads that we have received from Ace have shown sliaht variation in length and inside diameter. The dimensi& shown for the inserts provided a reasonable fit for most of the heads. Air condensers and Liebig condensers have also been converted t o efficient fractionating columns by machining an appropriate insert for them. A 14/20 condenser, with the water iacket evacuated. sealed. and fitted with an insert, had an effective column length of 18 cm. This rolumn oroduced 16 theoretical olates when ooerated with a distillation head that allowed reflux ratio adjustment.
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Figure 1. Crorsaectional view of an insert. Dimensions are in inches
Figwe 2. End view of top of insert showing slots for v a p a and liquid passage.
M e d line indicates positioning ring. Dimensions are in inches.
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Experlrnental Comparisons of separation efficiencies were made using two test mixtures. Both mixtures were made up 50150 hy volume. The first mixture was hexaneheptane, which has a boiling point difference of 29 "C. The second mixture was 2-methylpentane/cyclohexanewith a 20 OC difference in boiling point. Distillations were performed using 2 mL of the test mixtures in conical vials equipped with the Hiekmsn-Hinkle head. Heating was provided by a sand bath that was maintained at the same heat setting throughout the study. This provided essentially the same boilup rate for all distillations. Approximately 0.1-0.2-mL fractions were collected with Pasteur pipets from the well of the Hickman-Hinkle still. Composition of the 816
Journal of Chemical Education
Figure 3. End view of bottom of insert. Most of the positioning ring is cut away to allow vapor and liquid passage.
w Figure 4. Hickman-Hinkle still with insert in place.
fractions were determined by gas chromatography using %-in. X 5-ft columns packed with 8%Superox 20 on 80-100 mesh Chromosorh PAW. The chromatography was performed with aPerkin Elmer 8410 gas chromatograph. The results reported are the raw peak area percents as determined by the GC integrator. While these values are not true weight percentages, they are adequate for comparisons between the different setups. The values reported are for the first fractions collected of 0.1-0>-m~ volume. Spinning band columns can provide about twice the efficiency that concentric tube columns can produce. However, since the insert utilizes the full length of the tube in the distillation head, it produees essentially the same resolution as the spinning hand provided by Ace Glass, which uses only about half of the total tube length. The table shows a comparison of distilletion results using the Hickman-Hinkle still hy itself, with a spinning band, or with the inserts. The results shown are far the first fraction collected. Since it is very difficult to duplicate collection volumes with the Hiekman still, the results cannot be rigorously compared. However, both spinning band and inserts consistently demonstrated high resolution for bath
of the mixtures. Analysis of later fractions showed little change in composition until a fairly significant portion of the pot charge had been distilled. Results of Dlrtlllatlons with the Hbkman-Winkle S f l l distillation mixtured no inssn
Oelrin Kei-F HMWPE
hexane bp 89 heplane bp 98
2-methylpentane bp 80.3 cyclohexane bp 80.7
82.3bC 97.4 97.2 95.0 98.8
87.5 98.0 94.8 93.4 94.2
spinning-band 'me test mixtures wsre made up 50150 by volume.
'The result8ohown are for the first fraction with avolume of -0.1 mL. (The valueo are the area percent8 of the lower boiling component as reported by the imesrator.
Volume 68
Number 7
July 1991
617
