A Pressure Regulator for Vacuum Distillation R. L. EMERSON AND R. B. WOODWARD Massachusetts Institute of Technology, Cambridge, Mass.
A
FFER considerable experience with various forms of
apparatus similar t o those described by Bertrand and others (1, 2,s) for regulating pressure in vacuum distillation, a pressure regulator was developed which is more compact and better adapted for practical usage, as shown in Figure 1. The apparatus is mounted by a buret clamp on the stem, L , in such manner that it may be rotated clockwise. The system to be evacuated is attached at A and the pump at B. When the desired vacuum is approached, the stopcock, D, is opened slightly and mercury is allowed to rise almost to the inlet, H . When the vacuum is within 1 or 2 mm. of that desired, the mercury is allowed to rise further to close off inlet H so that a globule of mercury, 1 to 2 mm. high, circulates in the system KL. If there is no bleed or introduction of gas into the distilling system, the circulation of the mercury will soon cease and the pressure will remain constant. If there is an intentional bleed, the circulation of the mercury in K L is dependent on the volume of the bleed and the capacity of the pump. If the capacity of the ump is far in excess of the gas entering as a bleed, a gradual recfuction in pressure may develop when the distillation is continued over several hours. This can be prevented by the addition of a side tube on the inlet tube, A , above the junction, H , which is controlled by a pinchcock and a piece of pressure tubing. This is opened sufficiently to balance approximately the urnp capacity and bleed. The circulation of the mercury in I?L is thus maintained constant, and the mercury does not tend to rise in inlet tube A . At the close of the distillation and
FIGURE 2
before the pump is shut off, the apparatus is rotated clockwise, stopcock D is opened, and the mercury is allowed to flow back into the manometer.
The apparatus can be constructed without a stopcock and the regulation controlled by the rotation of the apparatus held in the buret clamp. In this case it is best t o evacuate with the regulator rotated t o a nearly horizontal position. When the desired vacuum is approached, the apparatus is rotated anticlockwise t o form the 1- t o 2-mm. mercury seal circulating in KL. A flexible manometer head, attached just below stopcock D,as shown, obviates the use of a rotating clamp and can be used when a rigid apparatus is desired. The flexible manometer head is simply lowered at the close of the operation, the stopcock is opened, and the mercury is allowed to return to the bulb before shutting off the pump. The stopcock can also be omitted in this form, if desired, but the circulation of the mercury in K L is not so uniform. Another modification is shown in Figure 2. The manometer is replaced by a bulb, C’ which is controllable by the oblique three-way stopcock, E’. The circulation of mercury in K’L’ is the same as above described, but the manipulation of the mercury in C‘ requires considerable care. The regulator is connected as described above, with both stopcocks open and the air in C’ a t atmospheric Before starting the pump, both stopcocks are closed. %?;h:e desired vacuum is approached, stopcock D’ is opened and mercury is allowed to approach H’ as before. To lower the pressure, the air in reservoir 17’must be evacuated by the careful manipulation of stopcock E’. Unless considerable caution is exercised, the momentary increase in pressure in the regulator arms, K’ and L’,will force mercury through A’ into the system. When the pressure in C‘ is the same as that throughout the regulator, stopcock D‘ is carefully opened. When
tubing
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the level in K’ and L’ has fallen sufficiently, D’ is again closed, and the air is withdrawn from the system until equalization takes place at a lower pressure. If it is desired to raise the pressure, air must first be allowed to enter the system so that the mercury will not back into th,e system through A . Stopcock D‘ is then opened and the mercury is allowed to rise higher in tubes K’ and L’. To end the operation, the procedure is much the same as that described above, b e in g careful that the pressure in c‘ is the same as that of the other portion of the regulator. Then the mercury i s allowed t o fall from tubes K’ and L’ altogether, and assume a position of rest in the lower part of the regulator. The stopcocks are now shut, to avoid splashing a r i s i n g from any fortuitous change in pressure when shutting off the pump.
7 mm
ab..
A
2m id.-
D
If it is desired to FIGURE3
regulate the pressure within narrow limits, especially in
VOL. 9, NO. 7
the range of 1 to 10 mm., a larger size (Figure 3) can be used which is filled with n-butyl phthalate instead of mercury. The phthalate may be colored with Sudan 111 if desired. I n the phthalate type the inlet tube in the trap above E has a vent close to the ring seal. This allows the regulator to function, even though phthalate may have been carried over accidentally from the upper trap on L. Butyl phthalate may also be used in place of mercury in the types described above having one stopcock. I n these the dimensions shown to the left in Figure 3 may be used and a suitable manometer, either fixed or flexible, attached just below stopcock D. The type with the flexible manometer is much easier to operate than that shown in Figure 3 with two stopcocks. The proper functioning is dependent on a more or less constant exhaust system. Nitrobenzene was distilled a t variously reduced pressures and the resulting curve agreed with that derived from data in the International Critical Tables. Of the three forms shown, the second and third can be fused to other glass parts of the complete apparatus, if allglass construction is desired, while the first must be connected by pressure tubing to allow for the rotation of the regulator. All forms function satisfactorily, but in the third style the control of the mercury by the pressure in the bulb C‘requires some care and experience. Of the two sizes, the mercury one is more compact, the total range is higher, and the manometer fluid is not susceptible to the casual entrance of vapors, while in the phthalate type the adjustment can be regulated withid narrower limits.
Literature Cited (1) Bertrand, Bull. sot. chim.,(3)29, 776-8 (1903). (2) Godefroy, Ann. chim. phys., (6) 1, 138-44 (1884). (3) Reiff, 2. angew. Chem., 22, 1360 (1909). RECBIVED May 6, 1937, Contribution 98 from the Department of Blology and Public Health, Massachusetts Institute of Technology.
IN THISORQANICRESEARCHDEVELOPMENT LABORATORY SMALL-SCALE LABORATORY PROCESSES ARE STUDIED BEFOREA PRODUCT Is RELEASED FOR FACTORY PRODUCTION. LILLY RESEARCHLABORATORIES, INDIANAPOLIS, IND.
