588
JOURNAL OF
CHEMICAL EDUCATION
An Efficient Fractional Distillation Column An Experiment Suitable for Use i n Beginning Organic Laboratory Work H. R. SNYDER a n d R. L. SHRINER University of Illinois, Urbana, Illinois
M
ANY of the laboratory manuals used in beginning organic chemistry contain an experiment on"fractional distillation." The distillation of coal tar or petroleum is frequently carried out and such experiments are useful because they parallel industrial processes, but these. experiments lead to certain "cuts" or "fractions" which are mixtnres of compounds and do not illustrate the isolation of the pure components. In other manuals, the distillation experiments described do not demonstrate the separation of compeunds by true fractionation in a column. Instead, these experiments illustrate the process of multiple distillation. The fractionating devices used-Wurtz bulbs, Hempel tubes, and Vigreux columns-are very inefficient, the experiment is time-consuming and frequently requires more than one laboratory period for comnletion. The results are disappointing and nnsatisfactory to the student. He gets the idea that isolation of pure compounds distillation is either im-
possible or is such a tedious process that the result is not worth the effort. The technical developments in the chemical industries on fractional distillation have far outstripped the laboratory procedures customarily employed and are quite different from the experience which a student usually gets in his organic laboratory work. The chemical engineering students soon find out that there are efficientstills for the separation of compounds and naturally conclude that fractional distillations are possible on an industrial scale but not in the laboratory. The recent work of Fenske' and his collaborators has emphasized the value of using packed columns and demonstrated their efficiency. The studies emphasize the importance of the nature.of the packing, insulation of the column, and reflux coirfrol. There are excellent laboratory &actionatkg columns available from several c~mpanies,~ but the cost renders their use by students prohibitive. Hence, an efforthas been made to design a suitable column and provide an illustrative experiment for beginning students in organic chemistry. In designing a column for use by large numbers of students it is desirable to have one which is (a) rugged enough to avoid excessive breakage, (b) efficient enough to give good separations in one distillation, (c) fast enough in rate of distillation so that the experiment can be completed in two hours, and (d) as inexpensive as is compatible with the foregoing requirements. After several trials the column shown in Figure 1was adopted3. It is frankly a compromise which has been found suitable for use in beginning laboratory classes, organic preparations of various types and, with certain modifications, finds considerable application in research. For the experiment described below and for many separations where a 15' to 20' difference in boiling points of the components exists, i t has been found 1 FENS= AND CO-WOR~ERS,Ind. Eng. Chem., 24,408 (1932); 26, 1169, 1213 (1934); 28, 644 (1936); WHITMOREAND LUX, J. Am. Chem. Soc., 54,3448 (1932). 2 American Instrument Co., S~lverSpring, Maryland; Scientific Glass Apparatus Co.. Bloomfield, New Jersey. 8 This column without the side-arm test tube and carborundum may be obtained from Corning Glass Co., Corning, New York, at a cost of about 81.00.
that a packing of crystalline carborundum is satisfactory. The pieces should be about the size of grains of putfed rice. The following paragraphs represent the discussion and procedure given to the students in mimeographed form for their experiment. EXPERIMENT N0.-
Make sure that the ring holding the gauze is far enough above the table top to accommodate the burner. Provide the column with a thermometer and a water-cooled condenser, both fitted with clean, tight corks. Use a dry 100-cc. graduated cylinder as a receiver. Arrange the r&ux control in approximately the position shown in the diagram. Connect it to the condenser with rubber tubing and run a slow stream of water through both. Ask the instructor to inspect the apparatus.
FRACTIONALDISTILLAT~ON Discussion.-The basis of the separation of liquids by fractional distillation lies in the fact that when a liquid mixture (other than an azeotropic or canstant-boiling mixture) is partially vaporized, the resulting vapor is richer in the more volatile component than was theoriginal liquid. If this new vapor is then condensed and again partially vaporized, and the process repeated a sufficient number of times. the v a m r will eventuallv be tbat of the Dure The ordinary type of fractionating apparatus consists of a vertical tube mounted above the flask containing the mixture t o be separated. The column is arranged so that most of the vapor which enters it is condensed and returned to the flask. I t must also provide a large area of contact between the ascending vapor and the returning liquid. At the surface between these two phases part of the vapor condenses with the liberation of heat which causes partial vaporization of the liquid. The process is repeated continuously as the vapor traverses the tube. The material reaching the top of the column thus becomes highly concentrated with respect to the more volatile component. From the foregoing description it is obvious tbat i t is desirable to have a device for regulating the amount of liquid returning through the column. This may be accomplished by the use of a side-arm test tube as a "cold fineer" condenser. shown in Fieure 1. By ratitny or luwrring it 111ean.uunt of r r h x may Lc cuntrollcrl. T h c rqflux r l l i a i% the quorianr of tlw amuunt of liquid rrrwning lllrouyh rhc rolunm lo t h e arnuurlr collwtcd in lhc r w c i v r r $luring tlre :anw 1i0w interval. The factors of greatest importance in connection with the efficiency of a column are a q follows: (a) length, (6) reflux ratio, (c) packing material, and (d) temperature control. Increasing either the length of a column or its reflux ratio will result in a sharper separation. In practice both these factors are limited bv , Roodiw. ,.. which is discussed below. The packing mnterd nw,t provide a largc surface area snd yct Icarr 5uficient frcr sl.nrc to permit rhf counrcr-flow of liquid and vapor. Two packmy nmeri3ls of the sainc pnrtirle s i x m ~ d shape are not necessarily of identical efficiency. Apparently such factors as thermal conductivity and adsorption are also involved. Among the better packing materials for laboratory columns are crushed carborundum and glass helices. For maximum efficiency, a column should be so completely insulated that no condensation occurs along the sides of the tube below the reflux control. Such complete insulation would, of course, be very difficult t o attain. Fortunately, in the distillation of liquids boiling below about 110°, it is usually sufficient to wrap the column with several layers of asbestos paper. For higber-boiling liquids, or in the case of very long columns, it may be necessary t o provide the column with an electrical heating jacket. The presence in the column of an excessive amount of liquid results inflooding; the free space becomes filled with liquid, which may be forced up through the column by the pressure of the ascending vapor. Flooding may result from excessive length or poor insulation of the column. I t may also be due t o improper manipulation, particularly in connection with attempts to operate a t very high reflux ratios or t o distil too rapidly. Procedure.-Assemble the fractionating column as shown in Figure 1 and then set it up as shown in Figure 2. Mix 100 cc. of carbon tetrachloride and 100 cc. of toluene in a one-liter roundbottomed flask and add a few chips of clay plate. Connect the flask to the fractionating column with a good cork. Mount the apparatus on a ring stand with the flask resting on a wire gauze.
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Heat the liquid to gentle boiling. While the column is warming, prepare a sheet of graph paper for recording the boilingpoint and volume of distillate a t five-minute intervals throughout the distillation. After about five minutes the ,column will become hot and condensation on the cold finger will.begin. Adjust the burner until about 1.5 cc. per minute is being collected in the receiver. The amount of liquid condensed an the cold finger should be such that the returning stream just fails t o break into drops. Collect the carbon tetrachloride boiling between 75'-77O and record its volume. When about 90 cc. have been collected the temperature will rise above 77'. even though distillation is carried out very slowly and a t a high reflux ratio. At this point transfer the carbon tetrachloride to the bottle provided for this purpose and collect separately the material boiling a t 77'-108". Note the tendency of the column to flood as the temperature reaches the latter figure. After the temperature has remained a t 108" for about two minutes record the volume of the material collected between 77" and 108'. This portion is a mixture of the two components; if a quantitative separation were required it would be necessary to refractionate it. The refidue in the flask now consists of pure toluene. If time permits, distil it through the column after making any necessary adjustment of the reflux control, and record its volume and boiling range. Extinguish the burner and allow the apparatus to cool for about ten minutes before dismantling it. Measure and record the volume of the residue in the flask. This represents the "hold-up" of the column and flask. Combine the residue with the toluene fraction and transfer it t o the bottle provided on the side-shelf.
The plot of the boiling points versus volume of distillate made by the student results in a curve which is an
index of the performance of the column and the technic of the operator. A typical curve obtained with this carborundum-packed column is shown in Figure 3,
which also illustrates the results of a similar distillation with a Wurtz bulb. With the packed column, about 90 cc. of carbon tetrachloride (b. p. 75O-77O), and 71 cc. of toluene (b. p. 108°-1090) were obtained. The residue was 15 cc., which represents the "hold-up" of the apparatus. This residue is toluene, so the recovery of toluene was 86 cc. The intermediate mixture amounted to 23 cc. The only pure fraction obtained with the Wurtz bulb was about 31 cc. of toluene.
OTHER USES
The column shown in Figure 1 is rather large in diameter so that a fairly rapid rate of distillation may he used. The column can therefore be used in a considerable number of organic preparations where reasonably large amounts of material are t o . b e distilled. The hold-up in this column is about 15 cc. In certain research work it is desirable to use a column of 11-12 mm. diameter, in which the hold-up will be about one-fourth as great. For liquids which boil above 110' i t is advantageous to heat the column by wrapping No. 22 Nichrome wire around a layer of asbestos paper, spacing the turns about 0.5 cm. apart, and covering the heating element with a layer of asbestos paper. The heat input is controlled by means of a slide-wire resistance. It is also desirable to wrap the side tube with asbestos rope. The column may also he used for fractionation in vacuo. In this case all connections must be tight and a fraction cutter attached to the end of the condenser. For the separation of liquids which boil very close together the glass helices recommended by Fenske' should he used. In this case the column must be heated in order to prevent flooding. FENSKE, ref. 1. Also PRICEAND MCDERMOIT, Ind. Eng. C h m . . Anal. Ed.. 1 1 , 289 (1939). The helices may he purchased from the Scientific Glass Apparatus Co., Bloomfield. New Jersey, or American Instrument Co., Silver Spring, Maryland.
(From the Lyman C. Newell Colleclion)
OBVERSE AND Rsvgns~ OR
THE
FABRB C O ~ M O R A T IPLAQUETTE, VE STRUCK BY THE FRENCH MINT
