A VISUAL DEMONSTRATION OF FRACTIONAL DISTILLATION JAMES 0.OSBURN State University of Iowa, Iowa City, Iowa
IN TEACAING chemical
techniques or chemical engineering unit operations, demonstrations are useful 111 creating interest and in aiding the student's memory. Demonstrations are also useful a s a means of attracting new students to the field, and of interpreting the profess~onto the public. Teachers who are concerned with student recruitment or public relations are always looking for simple demonstrations which dramatically ilIkstrate or explain the fundamental principles of chemical and engineering practices.
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watch, hut a simple packed column can serve almost as well. I n the past the liquids used for the demonstration have been colorless liquids, such as alcohol and water. Although these liquids demonstrate very well the physical nature of the operation they fail to show what the distillation process is designed to do, which is to separate materials into fractions of different compositions. The demonstrator must explain that the concentration of the more volatile component is higher a t the top of the column, since he has no simple way of showing this. The demonstration can be greatly improved if one of the components is colored, so that anyone can see that the concentration is different a t different points in the column. When such materials are used, the demonstration takes on added sienificance and interest. A search for such a system to he used in a demonstration for an annual Engineering College Open House showed that other requir&ents should he met. Most important, the makrials must be safe, both from the standwoint of inflammabilitv and toxicitv. Thev must be reasonably inexpensive: The boiling point must be n-ithin a certain range-if too high, the column will not operate without insnlation; if too low, there will be excessive loss of vapor. It is preferable t o have the colored material less volatile than the colorless one. since purity is assoriated with lack of color, and the demonstration is most effectiveif the "purest" material is a t the top of the column. The relative volatility of the two substances must also be neither too great, which would keep all of the rolored component in the still-pot, nor too small, in which case the concentration change through the column would be too small. he number of colored substances with the right, volatility is rather limited, but fortunately a pair of substances mere disrovered which meet all of the requirements listed. These are carbon tetrachloride and iodine. Although iodine is a solid, it has a high vapor pressure and dissolves in carbon tetrachloride to +e intensely colored solutions. The vapor in equilibrium with such solutions contains an a~wreciable amount of .. iodine, enough t o give it a violet color.
Demonstration Glass B u b b l e - C ~ DDistillation C d u m n
APPARATUS
One operation which has been a favorite for demonstrations is fractional distillation. This requires relatively simple equipment-a heat source, cooling water, and simple glassware. The audience can see the boiling t,aking place, the vapor bubbling up through the column, and t,he condensation taking place in the condenser. Glass huhble-cap columus are the most fascinating to
This system has been used for demonstrations for several years, with excellent. results. Two pieces of equipment have been used, both operating with total reflux. The first, shown in Figure 1, is a glass bubhlecap column constructed by the department glass blower a numher of years ago. Glass columns similar t o this are now available commercially. As can
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AUGUST, 1953
he seen in the photograph, the change in concentration from one plate to the next is easily apparent. The concentration in the still pot is 1.9 per cent iodine by weight, and the liquid on the top plate contains only a trace of iodine. These coiicentrations may be varied, hut this represents a satisfactory operating condition. A less expensive exhibit is shown in Figure 2. Here the fractionating is done in a glass tuhe filled with glass rings, and the color is seen to vary continuously along the tuhe. In this particular illustration the rings were cut from 4-mm. glass tubing with a glass-cutting saw; the exhibit could have been made just as well with glass beads, broken glass, or glass rings obtained from a supply house. A piece of glass wool a t the bottom of the column supports the parking. EQUILIBRIUM DIAGRAM
The demonstration call he made even more instructive if a vapor-liquid equilibrium diagram for the system is available. Equilibrium vapor and liquid concentrations were measured for solutions of several different concentrations. For this, an Othmer still was used. The iodine mas c.p. grade, and the carbon tetrachloride was a commerrial grade boiling a t 7o0C. The samples were analyzed by titratiou with thiosulfate solution, with starch indicator. Within the range of concentration studied, the equilibrium can he represented by Henry's law: Y = h X . With X and Y expressed in mol per cent of iodine in the liquid and in the vapor, the Henry's law constant, I$, has the value of 0.26. This is a t a substantially constant temperature of 7G°C., since the small amount of iodine present does not change the boiling point very much. Figure 3 shows the conventional equilibrium diagram for this system. Also shown on this chart are test tubes containing iodine solutions corresponding to typical compositions which would he obtained on adjacent plates in a bubble-'ap column with100 per cent efficient plates. The test tubes are lighted from behind, so that the concentration can be estimated from the amount of light transmission. Suppose one starts ~vitha 1.1 mol per cent solution in the flask. Its appearance is shown by the sample on the right, and it corresponds to a liquid composition as shown. The vapor composition is found from the equilibrium curve. This is done graphically by drawing t,he dotted line vertically until it intersects the equilibrium curve. If the column is operated a t total reflux, everything comes down that goes up, and the liquid coming down from the second plate is equal in amount and composition to the vapor going to the plate. This condition is obtained graphically by drawing a line horizontally until it intersects the line representing the condition X = Y, which is the 45-degree line. This gives the liquid composition for the next plate, and the construction is repeated. Liquid samples corresponding to four theoretical plates are shown, and the differences in concentration are readily apparent. At higher concentrations, the
Fisure 2.
Demonstration GI-5-Packed
Column
solutions are so dark that diflerences cannot he detected easily, and a t lower ones, t,hey are too light. With the aid of this equilibrium diagram, and a series of solutions of knomn composition, it mould he a simple matter to make a rough quantitative estimate of the individual plate efficiencies during, the course of a, demonstration. In a similar manner, hatch distillation
Figure 3.
Equilihrivrn Diagram for Carbon Tetrachloride-Iodine
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with less than total reflux can be demonstrated. The increased effectiveness of rectification compared with simple distillation can be demonstrated very vividly. The change in product composition as a batch distillation proceeds with constant reflux can also be shown, although this is limited by the solubility of iodine in carbon tetrachloride.
JOURNAL OF CHEMICAL EDUCATION
These materials may also be useful when it is desired to study the mixing of solutions. I n watching the operation of the equilibrium still, it was interesting to watch the mixing of the condensed vapor as it was returned to the still and mixed with the liquid in the still. This could easily be observed due to the differences in color intensity.
