Reflux Regulator for Laboratory Stills JOHANNES H. BRUUN, Sun Oil Company Research Laboratory, Norwood, Pa.
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original requirements and has been found satisfactory for laboratory stills.
T IS A w e l l - k n o w n fact
that the reflux ratio-moles of vapor returned to t h e column as refluxed liquid per minute divided by moles of vapor collected as distillate per minute-is o n e of the most important factors governing fractional distillations of mixtures c o n s i s t i n g of compounds with similar boiling p o i n t s . A number of reflux regulators have been described in the literature (1-16) in recent years, and while each of these may possess a certain advantageous feature, it was felt t h a t there was considerable room for improvement upon the present methods used for m e a s u r i n g a n d c,ontrolling the reflux ratio of laboratory stills. The most important requirements for an ideal reflux regulator may be s e t f o r t h a s follows:
As shown in Figure 1,the vapors from the top of the rectifying column will enter the reflux regulator at A and pass over thermometer bulb B into bulb C, where the vapors will be condensed into liquid by means of the reflux condenser, D. From bulb C the hot liquid will pass through a vertical capillary (to hold back uncondensed vapor) into the small dividing chamber, E. By means of a cylindrical metal plunger, F , movable horizontally by means of a micrometric screw, G, the flow of liquid from the dividing chamber, E, may be carefully c o n t r o l l e d . Thus, a certain amount (90 per cent, for example) of the liquid may be returned to the still as reflux by allowing it to pass through the vertical tube, H,,.and through the horizontal capillary, I,, back into the top of column A . The remaining part of the liquid (10 per cent) may be taken over as distillate I,,,svLnnoN by passing it through the vertic a l t u b e , Ha, a n d t h r o u g h t h e h o r i z o n t a l capillary, I d , into a cooler (not shown in the figure).
Inasmuch as the capillary tubes, I d and IT,are identical in size and length, it is evident t h a t t h e a m o u n t of liquid flowing through them will be 1. The refluxed liquid should CcL”n* d i r e c t l y proportional to the be a t its boiling point when it is FIGURE 1. REFLUXREGULATOR hydrostatic heads in the vertibeing returned to the column. cal tubes. H , and He,respecIf this is not the case. additional condensation of an unknown amount of vapor will take place tively, the numerical values of which may be read by means in the column with the obvious result that the actual reflux ratio of the vertical nlillimeter scale, would be larger than that measured by the reflux regulator. The numerical value of the reflux ratio employed a t any The “hold-upJJ of the reflux regulator be small in 2. time during a distillation is obtained by dividing the scale relation to the “hold-up” of the column. 3. I n order to avoid contaminating the distillates with disreading on the right, H,, by that on the left, Hd solved stopcock lubricant all stopcocks should be eliminated, An increase or decrease in the reflux ratio may be obtained particularly from lines through which liquid or hot vapors pass. simply by turning the micrometric control screw, G, to the 4. The reflux ratio should be easily adjustable during the left or right, respectively. When G and F are a t the extreme course of a distillation. Normally an infinite reflux ratio is desired until equilibrium in the column has been established, left, no distillate whatsoever is obtained; hence t.his set.ting whereafter the reflux ratio is maintained at a finite value which indicated by the corresponds to an infinite reflux ratio. may vary within wide limits (2:l to 1 O O : l ) during the distillahorizontal scale above the micrometer screw, i t is possible tion, de ending upon the difficulty encountered in the particular to obtain approximately any desired reflux ratio by means stage o?the fractionation. 5. The exact value of the reflux ratio should be easily determinable at any time during a distillation. In other words, it is not sufficient to have a regulator set at 10 to 1 and then simply vary this value to less than 10 to 1, more than 10 to 1, etc. 6. When distilling mixtures of unknown composition it is desirable to be able to maintain a definite value of the reflux ratio without necessitating any calculations and extrapolations f , Q u l D LEVEL based upon the heats of evaporation, molecular weights, composition, densities a t the boiling point, etc. 7. The reflux regulator, when once calibrated with one liquid, should be ready for use at any time for any other liquid or mixture of liquids without further calibration. 8. The reflux ratio should be independent of the rate of distillation, or, in other words, the regulator when once set at a definite value should maintain this reflux ratio even if the rate of boiling in the still pot should change within wide limits. FIGURE 2. DETAILOF THE DIVIDINGCHAMBER 9. The regulator should return the reflux to the column a t a uniform rate rather than operate intermittently on the siphon principle. of a single micrometer setting. The exact adjustment of the desired reflux ratio is obtained by a subsequent setting of the micrometer screw based upon the scale readings of H, With the above requirements as an ultimate goal, a number and H d . of different types of reflux regulators were developed in this Pressure equality in the vapor space on both sides of the laboratory. By combining the best features of each of these capillary tubes is insured by the equalizing lines, J . For a reflux regulator was finally developed which fulfills all the ’
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INDUSTRIAL AND ENGINEERING CHEiMISTRY D
VOL. 7, NO. 5
H, z, or the reflux ratio, are functions of the tem-
perature of the liquid. For this reason the reflux regulator in Figure 1 has been modified as shown in Figure 3 ( t h i s r e g u l a t o r is made by the Otto R. Greiner Company, 55 Plane Street, . Kewark, N. J,). J In Figure 3 the two horizontal capillary tubes have been placed together, so that they may be installed inside a small heating box, K , provided with a thermometer and heating coil (not shown in figure). In order t o conserve vertical space in the laboratory the condenser has been placed a t an angle with the column. This reflux regulator is more compact and practical than the one shown in Figure 1, which should be regarded more as an explanatory diagram than as an actual drawing. I n contrast to the conventional method, the thermometer has been placed in a position so that the vapors will strike it first a t a point above the bulb. By striking the stem of the thermometer the superheated vapors will have a tendency to cool down to the true boiling point before reaching the thermometer bulb. It should also be noted that the amount of FIGURE 3. REFLCXREGULATOR WITH TEMPERATURE vapor passing the thermometer bulb when the reflux ratio is CONTROLLED CAPILLARIES 20 to 1, for example, is 21 times the actual rate of distillation. Because of this fact an unusual constancy is obtained in the mixtures boiling near room temperature it may be desirable thermometer readings, even during distillations a t a very t o provide these equalizing lines with small condensers. low rate. Figure 2 is a detail sketch of the dividing chamber with the The important dimensions of Figure 3 are, naturally, iiieLaY plunger. From this it is evident that by moving the those of the capillary tubes, I d and I,. If these tubes are metal plunger toward the left the flow of liquid will be dimade of 0.9 mm. capillary tubing 100 mm. long, a hydroverted toward the right, and vice versa. It should be noted static head of 60 mm. will correspond to a rate of about 12 ml. that since the reflux ratio is determined by means of the scale per minute for a liquid such as benzene. For greater rates readings of Hd and H , (Figure l), irregularities in the diof flow the capillary tubes may be made correspondingly mensions of the cylindrical metal plunger or of the dividing shorter than 100 mm., or the height of Hd and H , may be chamber do not affect the accuracy of the reflux regulator. increased above 60 mm. For flows of benzene up to 50 ml. During its passage through the reflux regulator it was found per minute, capillary tubes of 1.0-mm. bore 50 mm. long will that the liquid in the two capillary tubes, I d and I,, would produce a hydrostatic head of about 60 mm. It is important cool down several degrees below its boiling point. In order that the two capillaries be of identical size and that they be t o avoid this it is necessary to heat the liquid in I , (by sur- . sealed perfectly horizontally. Figure 4 is a calibration curve of a reflux regulator of the type shown in Figure 3, with capillaries of 0.9 X 100 mm. As shown by this figure, the flow of liquid (benzene a t its boiling temperature) is directly proportional to the hydrostatic head.
Literature Cited
2 4 6 8 /O I2 14 FLOW THROUGH CAPILLARY I N ML PER MINUTE
FIGURE 4. CALIBRATION CURVE rounding the capillary with a heating coil) prior to its reentry into the column. For accurate work it is also important that the temperatures of the liquid in the two capillaries, Id and I,, be identical, since the viscosity and, therefore, also the flow, as well as the hydrostatic heads in Hd and H,, and consequently
Bruun, J. H., IND.ENG.CHEM.,Anal. Ed., 2, 187 (1930). Bruun, J. H., and Schicktana, 9. T., Bur. Standards J . Research, 7, 862-3 (1931). Clarke, H. T., and Rahrs, E. J., IND. ENG.CHEM., 18, 1092 (1926). Eddy, C. W., Ibid., Anal. Ed., 4 , 198 (1932). Fenske, M. R., Quiggle, D., and Tongberg, C. O., ISD.E N G CHEW,24, 411 (1932). Hickman, K., and Weyerts, W., J. Am. Chem. SOC.,52, 4714 (1930). Hill, J. B., and Ferris, S. W., IND. ENG.CHEM.,19, 379 (1927). Kester, E. B., and Andrews, R., Ibid., Anal. Ed., 3, 373 (1931). Leslie, E . H., and Geniesse, J. C., IND.ENG. CHEM., 18, 591 (1926). Leslie, R . T., and Schicktana, S. T.,Bur. Standards J . Research, 6, 379 (1931). Marshall, M. J., IND.ENG.CHEM.,20, 1379 (1928). Rothmann, 9. C., Ibid., Anal. Ed., 5, 338 (1933). Schwarta, A. M., and Bush, M. T., Ibid., 3, 138 (1931). Tongberg, C. O., Quiggle, D., and Fenske, M. R., IND.ENG. CHEM.,26, 1213 (1934). Wagner, E. C., and Simons, J. K., Ibid., Anal. Ed., 5, 183 (1933). RECEIVED June 18, 1935.
