338
ANALYTICAL CHEMISTRY
cedure the olefin mixture reached a maximum temperature of 45" C. compared with 60" and 70" C. for the A.S.T.M. procedure and the procedure using no preliminary shaking, respectively. Using 507' toluene in iso-octane, maximum temperatures of 16', 23', and 23" C. were obtained by the three shaking procedures. The temperatures of the reaction mixture were measured using an iron-constantan thermocouple in direct contact with the reaction mixture. The test method has been applied to a wide range of samples with satisfactory results. ACKNOWLEDGMENT
The authors wish to acknowledge the assistance of many members of the laboratory, particularly the following: S. C. T. McDowell for much of the recent laboratory work, especially that relating to temperature rise; R. E. Ledley and S. J. Hetzel for work on the solubility correction and proper method of reading meniscus; Miss D. J. Bellar for much accurate work; and C. C. Martin and \Ir,T. Harvey for helpful discussion. LITERATURE CITED
(1) Am. SOC.Testing Materials, Committee D-2, A n n u a l Report
D-2, 1946, Figure 1 .
(2) Am. SOC.Testing iaterials, Nethod ES45a (March 19, 1945). (3) Am. SOC.Testing Materials, Tentative Method D875-46T,(1946). (4) Berg, C., and Parker, F. D., A N ~ LCHEM., . in press. ( 5 ) Bond, G. R., Jr., IXD. ENG.CHEM.,ANAL.ED., 18, 692 (1946). (6) Brochet, A , , Compt. rend., 117, 115 (1893). (7) Brooks, B. T., and Humphrey, I., J . Am. Chem. SOC.,40, 856 (1918). (8) Fisher, C. H., and Eisner, A., Bur. Mines, Rept. Invest. 3356 (December 1937). (9) Garner, F. H., J . Inst. Petroleum Tech., 14, 695 (1928). (10) Hones, D. A., Ibid., 16, 54 (1930). (11) Ipatieff, 1'. N., and Pines, H., J. Org. Chem., 1, 464 (1936). (12) Kattwinkel, R., Brennstof Chem., 8, 353 (1927). (13) Kraemer, G., and Spilker, -4., Ber., 23, 3169 (1890). (14) Kurtz, S. S., Jr., Mills, I. W., Martin, C. C., Harvey, W.T., and Lipkin, M. R., ANAL.CHEM.,19, 176 (1947). (15) Lipkin, h4. R., Hoffecker, W. A., Martin, C. C., and Ledley, R. E., Ibid., 20, 130 (1948). (16) &fair,B. J., and Foreiati, A. F., J . Research 'Vatl. B u r . Standards, 32, 165 (1944). (17) Manning, A. B., J . Chem. SOC.,1929, 1014. (18) Ormandy, W.R., and Craven, E. C., J. Inst. Petroleum Tech., 13, 311 (1927). (19) Pritzker, J., and Jungkunz, R., Chem.-Ztg., 47, 313 (1923). (20) Scafe, et ai., . ~ A L .CHEx, 19, 971 (1947). (21) Thomas, C. L., Bloch, H. S., and Hoekstra, J., IND. ENQ. CHEM.,ANAL. ED., 10, 153 (1938). RECEIVED July 25, 1947.
Electrical Heating and Bottom Receivers for Distillation Equipment W. M. LANGDON', University of Zllinojs, C'rbana, 111.
T
H E procedures described here for The construction and application charge to be distilled to dryness; this is of electrical heating and bottom heating still flasks have been infrenot a serious disadvantage. If the contents are to be completely distilled over, receiversfor distillation equipment quently employed because of misconcepsome high-boiling scavenger liquid must be tions as to their operation and safety arediscussed. added to push the heavy ends through the characteristics and also because some of column. For total reflux operation the the modifications in design which make their operation practical have not been generally known among heaters can be run with approximately 25 ml. or less, which will usually allow cuts to be taken of the heavy ends. laboratory men. Correct boiling is best obtained by an arrangement that induces rapid circulation of the boiling liquid. A soft, even heat over a IMMERSION HEATER OF BARE WIRE large surface may give rise to bumping and foaming unless some An immersion heater of bare wire is illustrated in Figure 1, B. type of ebullator is used. A sharp of heat which The heating element consists of a 60-cm. length of No. 30 Chrovery rapid circulation gives very good boiling characteristics in m e wire ~ m,oundin a tight spiral spring of G ~diameter. ~ . The spring is stretched around three sides of a rectangular form of 4most cases. mm. glass rod. The ends of the element are connezted by twistin Ease of control is mainly dependent upon the heat capacity of and braiding two lengthsof copperwire, which we in turn fastene8 the heating device and the temperature a t which it runs. by small couplings to tungsten leads sealed through a joint. Safety depends upon a heating element which runs a t a relatively low temperature and is free from the possibility of breakThis heater is closest to the ideal type when vaporizing nonage, %%en large quantities of inflammable liquids are being discorrosiveliquids. Its negligible heat capacity allowsinstantaneous tilled, the vaporizer should contain only a small amount of liquid control response. There is no fire hazard rrith inflammable and should be so arranged that, in case of breakage, the main body liquids, as the fine wire burns through when out of contact with of liquid will be automatically isolated. little w e r h e a t i % even When the liquid. There is The boilup rate should be relatively constant. Half-shell element runs red hot, because it is surrounded by a rapidly movheaters usually have a decreased boilup rate as liquid is distilled ing blanket of vapors. The heater has been used in vacuum disfrom the flask, because a part of the heat is radiated to the surtillations without an ebullator although, in this case, it is advisroundings. ~vhole-shell heaters Tvill give a relatively constant able to furnish the major part of the heat by means of an external boilup but will run very hot as the still liquid becomes low, This shell heater. There is little tendency for liquids to froth, probis undesirable from the standpoint of safety. ably because of slight superheating of vapors surrounding the wire. Most of the heating flasks described below do not allolv the The main disadvantage, besides unsuitability for corrosive liquids, is carbonization of organic solvents if the element is esposed. 1 Present address, Rensselaer Polytechnic Institute, Troy, N. Y.
339
V O L U M E 20, NO. 4, A P R I L 1 9 4 8 Table 1. Heat Loss and Operating Temperatures of External Heating Elements Watts
Total I n p u t , Watts
490 345 120
350 225 105 20
T o t a l power input H e a t o u t in steam H e a t loss from entire flask (including boiling tube) H e a t loss from external heating wire
25
Thermocouple,
c.
97 128 156 183 197 212 232 244
EXTERNAL BOILING TUBE
One of the most useful heaters, because of its ease of construction consists of a 35-to 45-mm. tube sealed a t an angle t o one side of the bottom center of a round-bottomed flask (Figure 1, C ) . The heating element is preferably wrapped directly on the boiling tube with little outside insulation. Insulation is ineffective, as there is negligible heat loss from the element (Table I) and it may cause overheating and breakage if the flask runs dry. The heater gives good boiling, though not so smooth as bare wire, and requires a n ebullator for high-vacuum operation. In four years of use the writer has had no case of breakage during operation when using this type of heater. The location of the tube off-center causes the liquid to slide over the side of the tube and froth up, thus protecting the tube from thermal shock if the still flask runs dry.
The tests (Table I) were conducted TTith 1 liter of water in a 1-liter still flask supplipd with both an external boiling tube (Figure 1, C) and an immersion heater of bare wire (Figure 1, D). The heat-out in the steam was determined by condensing and removing the water. The heat loss from the still flask was determined by the difference between the power input and the heat-out Jvhen the immersion heater was used. The difference in heat-out for a given power input when the immersion heater and external boiling tube were used was taken t o be the heat loss from the latter heating element. The external boiling tube was 12 cm. long and 40 mm. in diameter. The heating element was 90 cm. of Chromel ribbon, 1.6 X 0.10 mm., covered with two thin strips of Alundum cement, 12 mm. wide. The temperatures were determined by placing a thermocouple over several turns of the winding and coating it with a little Alundum cement. The readings, as measured and recorded, are higher than the bare wire. Using the figures for heat loss above and assuming 1 0 0 ~ insulation o by the Alundum cement over the thermocouple, when the thermocouple reads 200" C. the bare wire would read 197" C. HEATER WITH EXTERNAL CLOSED CIRCUIT (FIGURE 2, B ) ( 8 )
The best results are obtained with a 15-mm. loop which gives rapid circulation and is resistant to thermal breakage if the heating is accidentally supplied above the liquid level. A heater of Chromel ribbon covering 20 cm. of 15-mm. tubing can supply more than 600 rvatts and myltiple loops can be used t o obtain greater capacity. The boiling characteristics are intermediate to the two types described above and it may he used for vacuum distillations, though it has an appreciable pressure drop. There is little tendency to foam or bump. The heater may be used to evaporate thermally unstable liquids in place of a steam jacket ( 2 , 6 ) . Two-phase systems whose components differ appreciably in boiling point give rise to violent explosions in this heater. PROTECTED IMMERSION HEATER (7)
The simplest construction consists of the element wound on a glass tube with flanged ends to center it inside the protection tube. Copper leads connected to the ends of the element are brought out of the protection tube through a rubber stopper. This seals the end of the protection tube, so that there is no fire hazard in case of breakage. An alternative construction method with more control lag is use of a loose ninding packed in dry Alundum pon-der. The leads are taken out through ceramic sleaves. S o breakage has occurred with these heaters as long as the heat is supplied below the liquid level. They are best used inside of an external boiling tube or a loop heater. Their boiling characteristics are almost as good as the bare wire heater. However, they are more difficult to construct than either external heater.
L-
IT:
Construction of Heating Elements. The heating element is wrapped directly on the glass and the ends are fastened with bands of copper wire wrapped over thin strips of asbestos paper. The ends of the element are braided into the twisted ends of copper wire. The coils of the element are held in place by thin strips of refractory cement; the writer prefers a temporary cement consisting of a thin slurry of Alundum cement and water. This can be painted on with a brush, sets almost immediately, and can be washed off. If a permanent cement is desired the Alundum slurry is made with diluted water glass. The glass is coated with paraffin wax or grease before the element is wound and cement applied. The heater can be made as a detachable shell.
@ ='E
Figure 1. B o t t o m receiver for total reflux, vented t o outside pressure by barometric legs Bare wire immersion heater External boiling tube D . B o t t o m receiver for finite reflux, recycle overflow line E . B o t t o m receiver for finite reflux, heat p u m p A.
B. C.
Khere autotransformers are not available, the element can be made a variable resistance by providing several taps of twisted copper wire, into which the element is braided. If the heating is to be distributed over the entire heating tube, the coils can be n-ound in multiple-pitch spirals. In even-pitch spirals the power leads are together a t one end of the element. They are more troublesome to wind and care has to be exercised that adjacent coils do not come loose and cause a short circuit.
340
A N A L Y T I C A L CHEMISTRY
Leads. The power leads are connected to the element by battery clips covered with rubber tubing. The rubber tubing prevents the clips from shorting out when disconnected and makes the connection t o the element incapable of accidental disconnection. The length of the twisted copper ends prevents the rubber tubing from overheating. SHELL HEATERS FOR ROUND-BOTTOAMED FLASKS
These heaters (1,3, 6) are preferably made as detachable shells, formed by winding asbestos rope around a flask and coating the outside with Alundum cement and water glass. A buried element can be wound on the outside of this shell, which is then covered with another similar shell. An exposed element, if desired, can be wound on the inside of the first shell and tacked in place with Alundum cement and water glass. These shells are light and strong and can stand a high operating temperature. They fit closely enough to be used interchangeably with differrnt flasks of the same size.
4
distillation rate can be obtained, independent of fluctuations in the power line and heat losses around the column base. While the liquid flow to the vaporizer may be varied instantaneously, the distillation rate lags because of condensation in the vapor lines to the column. The vaporizer has excellent safety characteristics, since there is very little liquid in it and the flow to it from the receiver may be stopped instantly. The possibility that heating liquid in the bath around the vaporizer may decompose if the distillation stops while the column is unattended, may be avoided by using a stable, high-boiling liquid for the heating bath. This liquid is contained in a jacket sealed to the vaporizer and vented at the top through an external condenser. A simpler but less effective method when a paraffin oil bath is used is to put a copper cooling coil just above the surface of the oil. This will remove most of the heat when the oil expands and covers the coil. BOTTO\I RECEIVERS FOR TOTAL REFLUX OPERATIONS
The simplest receiver (Figure 2, A ) consists of a holdup device inserted between the fractionating section and the still flask. Because the receiver illustrated runs under an atmosphere of the column vapors, the connecting lines must usually be sealed to the column. The fractionating effect of the still flask (one plate) is lost but it does not contain any heavy residues which accumulate in the still flask proper. In case of a leak elsewhere in the systrm, only the liquid contained in the still flask is lost. The same arrangement, except that the receiver is allowed to run under atmospheric pressure (Figure 1, A ) , allows more flexibility. The difference between the pressure a t the column base and the atmosphere is compensated for by small barometric legs. A third method (Figure 2, C) consists of maintaining the major portion of the still liquid in an outside receiver whose contents are circulated through the still flask by means of a pump. The arrangement illustrated allows, in the case of leakage, only the contents of the still and the small amount of overflow liquid to be lost. The above methods emphasize safety by isolating the major portion of the charge and limiting liquid loss to a small fraction. They permit any amount of liquid to be distilled without special oversized flasks, The heat loss is less from these receivers than for the usual operation with all the charge kept at the boiling point in the still flask, and cooling of the liquid recycle decreases the decomposition of thermally unstable liquids.
Figure 2. A . Bottom receiver for total reflux, running under a n atmosphere of still vapors B . External closed circuit beater C. Bottom receiver for total reflux, mixing with still flask contentn effected by m e a n s of a pump
Shell heaters have a great tendency towards bumping and are best used to insulate the still flask. They are very useful for supplying the major portion of heat in the case of thermally unstable liquids or viscous liquids, whose circulation is poor. They have considerable control lag and, unless the flask is totally enclosed by the shell, the boiling rate decreases with the liquid level. TOTAL VAPORIZER
A liquid may be vaporized in backward-feed distillations (4)in a packed or indented tube surrounded by a heated bath of liquid. This will vaporize any liquid that does not contain an appreciable amount of nonvolatile material to plug the vaporizer. I t has better heating characteristics than any of the other methods discussed. Since it is a film type of vaporizer, there is less superheating in the liquid phase and no bumping or frothing. Rate of vaporization is controlled by the flow of liquid from the receiver and is independent of the power to the vaporizer. This control is more troublesome than that by means of heat input. However, by using a calibrated orifice in the flow line a more constant
BOTTOM RECEIVERS FOR FINITE REFLUX
Batch operations may be effected as illustrated in Figure 1, E , by circulating through the still flask the contents of an outside receiver not supplied with a limiting overflow. Figure 1 shows the circulation being effected by means of a heat pump which permits the liquid recycle to enter at its boiling point. The pump consists of a U-leg from the receiver, the outer side wrapped with a heating element. The static liquid height, h,, is usually made at least 150% of the height, hf, to which the liquid is lifted. However, in small-bore tubing the former height may be severalfold smaller, An uninsulated 8-mm. tube with hl and hl both equal to 60 cni. (2 feet) will pump approximately 8 liters per hour of benzene with a powrr input of 600 watts. LITERATURE CITED
(1) Allen, I. C.,and Jacobs, W. A., J.SOC.Chem.I n d . , 31,lS(1912). (2) Kemmerer, K. S., IND.ENG.CHEM., ANAL.ED., 17,466(1945). (3) Kranta, H.A , , and Hufferd, R. W., Ibid., 12,752(1940). (4) Langdon, W.M . , Ibid., 17, 590 (1945). (5) Mitchell, D. T., Shildneck, P., and Dustin, J., Ibid., 16, 754 (1944). (6) Morey, G. W.,Ibid., 10, 531 (1938). (7) Podbielniak, W.J., Ibid., 13,639 (1941). (8) Raeder, M.G., Ibid., 1, 88 (1929). R W E I Y E D April 9, 1947.
