An Automatic Pressure-Regulating Unit for Vacuum Distillation E. H. HUNTRESS AND E. B. HERSHBERG Research Laboratory of Organic Chemistry, Massachusetts Institute of Technology, Cambridge, Mass.
U
NTIL recently organic chemists have paid slight attention to the accurate regulation of pressure during vacuum distillation. This condition has been largely due to the fact that no simple satisfactory device was available for general laboratory use. Under these circumstances there has been a tendency to avoid distillation under reduced pressure wherever possible or to adopt temporary hand-regulated makeshifts where its use became imperative.
(Figures 6 and 7) which is so connected with a manostat that it continues to turn the pump motor off or on as further fluctuations of the pressure may require. I n order to make the apparatus of general utility this thermionic control has been so devieed that it will without change operate upon either alternating or direct current, according to local facilities and the nature of the pump motor. That is, with an alternating current pump motor the relay system also operates on the same alternating current line; with a direct current motor the relay system operates on the same direct current line. Thus, for a given motor the entire apparatus is controlled by a single switch, and in actual use is simply brought up to and attached to the distillation system, plugged in to the power line, and allowed to maintain whatever pressure is desired according to the setting. Associated with the pump and its thermionic control are the usual necessities of vacuum distillation, such as an absorption train (Figures 2 and 5 ) to protect the pump from
FIGURE 1. PRESSURE-REGULATING UNIT Having occasion to prepare numerous compounds requiring vacuum distillation and finding no suitable control devices at hand, the authors have constructed a simple portable regulator capable of maintaining to *0.15 mm. any pressure within the range of 2 to 30 mm., which has been used in these laboratories with great convenience and satisfaction. Before designing the apparatus here described a survey of the literature since 1840 showed that more than thirty published descriptions could be classified as follows: A. The pump operates continuously. 1. A valve is interposed between the pump and the system. Mercury valve (6, 16, 31). Mechanical valve (S6). 2. A valve bleeds in air at atmospheric pressure. Mercury valve (4,8, 15, 19, 26, 33, 3'4, 37). Mechanical valve (1, S, 6, 9, 11, 12, 17, 20, 21, 23, 24, 26, 27, 28, 89, 32, 36, 40).
B. The pump operates intermittently (7, 10, IS, 38, 59).
FIGURE2. PRESSURE-REGULATINQ UNIT, SHOWING ABSORPTION TRAIN AND RESERVOIR vapors, a manometer for pressure measurement, and a manostat to effect operation of the thermionic relay. The entire ensemble is mounted on a light four-wheeled carriage, affording ready mobility from one place to another.
THEMANOSTAT I n the present device (Figures 1 and 2) the pump operates This is essentially a U-manometer capable of being fastened intermittently. The pressure is reduced by means of a Cenco Hyvac pump operated by the usual 0.125-horsepower motor. in an inclined position. A fixed tungsten electrode contacts Once the desired pressure has been attained, the pump is with the mercury, completing the relay circuit which conswitched off by the action of a thermionic relay system trols the pump motor. Thus by tilting the manostat the 144
March 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
center of gravity is displaced, and the mercury will require a greater or lesser force, or vacuum, to bring it to that level a t which it will make contact with the fixed electrode. The manostat is of Pyrex glass, 14 em. high with arms 2.5 cm. between centers and 7 mm. internal diameter. One t u n g s t e n contact is sealed through the concave side of the U-bend a n d t h e o t h e r through the top of the arm w h i c h c o n n e c t s with the system. The m a n o s t a t is held near its upper end in a rotating clamp provided with a graduated r a d i o dial for observation of the s e t t i n g and a thumbscrew for clamping in a fixed position. Since the manostat is indep e n d e n t of v a r i a t i o n s of barometric pressure, a small graph relating the dial setting and pressure may be attached permanently to the panel for FIGURE3. MANOSTAT convenient reference. Constriction of the manostat tubing is unnecessary, for in operation the apparatus panel is clamped against the laboratory desk and oscillation of mercury is negligible. Lubrication of the mercury in the open arm is infeasible, since the organic lubricant is slowly decomposed by the small thermionic current. The slight lag induced by the cohesion of the mercury and the tungsten is not serious, but may be avoided by the use of iron or-nickel.
THEMANOMETER A small U-manometer 13 cm. high, with arms 4 om. betlween centers, was constructed of Pyrex tubing of 7 mm.
benzyl phthalate or di-n-butyl phthalate saturated with diphenyl mercury or di-p-tolyl mercury ( I @ , is recommended for increased sensitivity. A reading scale for the manometer is mounted on a square brass tube sliding on a square brass rod with a phosphor bronze friction spring. Parallax errors are avoided by observing the meniscus across brass rings blackened with ammonium sulfide and warmed over a flame
FIGURE5. ABSORPTION TRAINAND RESERVOIR (SO). The right-hand ring is split and fastened to the sliding scale, the lower edge being set a t the zero index. The left-hand ring is slipped over the closed arm and moves freely, with a friction spring holding it against the glass. To facilitate using the instrument a t night, the manometer is illuminated from behind by a 25-watt lamp mounted on the back of the panel and shining through a double ground-glass window which protects from any heat. Both the manometter and manostat are protected from sudden access of air by means of a 10-mm. tube tightly packed with cotton and
FIGURE4. MANOMETER
internal diameter so as to be suitable for the measurement of pressures between 0 and 30 mm. of mercury. As shown in Figure 4, the open end terminates in a tee, one side of which connects with the manostat and the other through 8 short soda lime-activated carbon absorption tube to a three-way stopcock. Constriction of the manometer tubing is unnecessary, but the use of a lubricant, such as butyl
145
PIGURE6.
RELAYASSEMBLY
146
ANALYTICAL EDITION A. C.
M
A . Struthers Dunn Inc. relay-type CXB51, coil H. Coil resisdance,’2700 ohms. Coil ratina. 2 milliamDeres. Contact ratinn. -. 2 amperesat 110 volts a. c. B . Struthers Dunn, Inc., relay-type CSBOO. Contact rating, 30 amperes. Universal alternating or direct current. When assembling i t is well t o mount on sponge rubber t o minimize noise. C. 4-mfd. condenser. Rating, 300 volts direct current 125 volts alternating current. M . Manoitat contacts. P. Pump motor, 0.125 t o 1h. p The rated operating current is usually much less than the starting current whioh must be broken a t the oontacts makina necessary the auxiliary heavyduty relay B . Ri. 25-watt, 115-volt tungsten filament, G. E. Mazda lamp. Rg. 40-watt, 115-volt tungsten filament, G. E. Mazda lamp. Ra. Variable series resistor, 25,000-250,000 ohms range. This resistor serves as a fine adjustment t o compensate for unavoidable variations in lamp? Ri Fnd Rz and in grid leak R 4 . I n practice this 1s adjusted t o the h~ghestreslstance (and hence the smallest operating current) that will give dependable operation. R4. Grid leak, 8 megohms. S. Sinale Dole flush tumbler switch and plug receptacle-foipump motor. R, T. 71-A type tube, 5-volt filament. The filament should operate at a dull red heat with UNIVERSAL~~O-VOLT lamps-Ri and R2 of the recommended rating.
FIGURE7. THERMIONIC RELAY
attached through a 1-mm. capillary side-arm to the threeway stopcock mentioned above.
Vol. 5, No.
cent lamps, r e s i s t a n c e s , and switch, are standard c o m m e r c i a l units. The relay may be enclosed in a wooden box or in a steel electrical cabinet 8 X 8 X 4 inches.
PUMPAND CARRIAGE The manostat, manometer, absorption train, reservoir, stopcock, and thermionic relay are all mounted upon a vertical wood panel as shown in Figures 1 and 2. This is supported by angle irons which join it to the frame of a small standard centrifuge carriage made by the International Centrifuge Corporation. The pump itself is mounted upon a wooden base, which rests in turn upon vibration insulating cushions of sponge r u b b e r glued to the floating base of the carriage. The weight of the pump is sufficient to balance that of the panel and its equipment, and the entire stand is mechanically st8able when being rolled from one place to another. When actually in use a t a given spot, the vertical wood panel may be anchored to the desk by means of an angle iron and screwclamp.
ABSORPTION TRAINAND RESERVOIR ACKNOWLEDGMENT For the protection of the pump it is necessary t o provide Grateful acknowledgment is made for assistance and ideas for the removal of all condensable vapors from the gas stream. to the members of the Research Laboratory of Organic ChemAfter considerable experimentation with various types and lengths of absorbents, the best combination was found to be istry, especially to G. S,Crandall, E. A. Averill, and A. A. a relatively short soda lime tower (Figure 2), for acid gases, Morton. together with a low temperature vapor trap, for water and LITERATURE CITED organic liquids. The latter consists essentially of a suitable (1) Auger, Bull. SOC. ehim., (3) 19, 731-3 (1898). glass bulb trap (Figure 5 ) around which is raised a Thermos (2) Beaver and Beaver, IND.ENG.CHEM.,15, 359-61 (1923). (3) Beckmann, 2. physik. Chem., 79, 565-76 (1912). bottle (Figure 2) containing solid carbon dioxide and some (4) Beckmann and Liesche, Ibid., 88, 13-21 (1914). low-freezing heat-transfer liquid. For the latter the authors (5) Bertrand, Bull. soe. chim., (3) 29, 776-8 (1903). prefer the use of a mixture of equal weights of chloroform and (6) Brown, Proc. Phys. Soe. (London), 3, 68-76 (1879-80). carbon tetrachloride (dR), which avoids all fire hazard and (7) Bruun and Schioktanz, Bur. Standards J . Research, 7, 875-8 (1931). upon whose surface the solid carbon dioxide floats. TO ( 8 ) Cohen, Chem. Weekblad, 23, 340-1 (1926). facilitate cleaning out condensate from the glass bulb trap (9) Cole, Philippine J . Sci., 44, 411-13 (1931). between runs without dismantling the train, a glass tube (10) Cox, IKD.ENG. CHEM.,Anal. Ed., 1, 7-8 (1929). joined to the bottom of the trap is bent so as to extend out (11) Dean, Hill, Smith, and Jacobs, Bur. Mines, Bull. 207, 24-29 (1922). of the top of the bath as indicated. By blowing back through (12) Druoker, 2. physik. Chem., 74, 612-19 (1910). the usual connection to the distillation system the trap may (13) Ellis, IXD.ENG.CHEM.,Anal. Ed., 4, 318-19 (1932). readily be emptied. (14) Fergusen, van Lente, and Hitohens, Ibid., 4, 218-19 (1932). Since the volume of the distillation system with which this (15) Garzino, Ann. Chim. Farmacologia, 17, 20-1 (1893). regulator is ordinarily employed is relatively small as com- (16) Godefroy, Ann. chim. phys., (6) 1, 138-44 (1884). (17) Hausser, BUZZ. SOC. chim., (3) 21, 253-4 (1899). pared with the capacity of the pump it has been found de- (18) Hiokman, J . Optical SOC.Am., 19, 190-212 (1929). sirable to include a reservoir in the system. For this pur- (19) Holterman, Chem.-Ztg., 32, 8 (1908). pose an ordinary 3-liter ring-neck Pyrex flask has served (20) Jacobs and Collins, Chem. & Met. Eng., 27, 1129 (1922). (21) Johnson, J . Chem. Soc., 1931, 2523-4. conveniently.
THERELAY The relay assembly shown in Figure 6 with wiring diagram in Figure 7 has been developed for the express purpose of controlling by means of a minute thermionic current the heavy starting Ioad taken by the induction motor operating the pump. It is characterized by use of standard reproducible units, differing in this respect from that of Fergusen, van Lente, and Hitchens ( 1 4 , by its relatively low cost, adaptability to common sources of power, and freedom from lag or fouling of the manostat contacts. Application of a negative grid bias to the normally unbiased thermionic tube T (Figure 7 ) so diminishes the plate current that reIease of the sensitive relay A is effected ( 2 ) . This action in turn controls the heavy-duty relay B operating the pump motor. The relay assembly is interchangeable on 110 to 115 volts alternating or direFt current and all parts, including vacuum tube, condenser, relays, incandes-
(22) (23) (24) (25) (26) (27) (28) (29) (30)
(31) (32) (33) (34) (35) (36) (37) (38) (39) (40)
Kanolt, Bur. Standards, Sei. Paper S-520, 619-33 (1926). Kuentzel, J . Am. Chem. SOC.,51, 3347-49 (1929). Mabery, Proc. Am. Acad. Arts Sei., 31, 10 (1895-6). Meyer, Ann., 165, 303-13 (1873). Miller and hlcKinney, IND.ENG. CHEM.,20, 552 (1928). Moschner, Chem.-Ztg., 12, 1243-4 (1888). Moye, 2. angew. Chem., 1898, 3. Perkin, J. Chem. SOC.,53, 689-94 (1888). Rechenberg, von, “Einfache und fraktionierte Destillation,” pp. 117-18, Sohimmel and Co., Leipzig, 1923. Reiff, Z. angew. Chem., 22, 1360 (1909). Richards, Rev. Sci. Instruments, 2, 49-52 (1931). Rutten, Chem. WeekbZad, 1, 635-8 (1904). Scofield, IND. EKG.CHEM.,18, 717 (1926). Smith, Ibid., 16, 22 (1924). Smits, 2. physik. Chem., 33, 39-46 (1900). Stlidel and Hahn, Ann., 195, 218-21 (1879). Sunier and White, IND. ENG.CHEM.,Anal. Ed., 3, 259 (1931). Swayze, Science, 76, 196-7 (1932). Wade and Merriman, J . Chem. SOC.,99, 984-97 (1911).
RECEIVEDOctober 29, 1932. Contribution No. 86 from the Research Laboratory of Organic Chemistry, Massac husetts Institute of Technology.
