A Pressure Regulator for Vacuum Distillation Toby J. Sommer Brandels University, Waltham. MA 02254
Each of the common manostats routinely used by organic chemists have some drawback. The device described by Emerson and Woodward is complicated to build and adjustment "requires considerable care!" Likewise, there is also some difficulty initializing the pressure in Cartesian diver manostats where the orifice can eet cloeeed. the movine parts can hind or stick, and the s e a f i u s u a l ~ m a d of e rubbed can deteriorate in the presence of organic v a ~ o r sThe . ~ Nester-Faust and Lewis-type manostats have variable gas ballasts that must he set in a hit-or-miss fashion while oumpine down the ~ y s t e mDepending .~ on the pressure being r e g d a c ed, system characteristics, orifice size, pump capacity, etc., the pressure may fluctuate noticeably with the diver or the liquid level in the manostat bulbs. Furthermore, all of the commercially available manostats are very expensive. The iiewman manostat is one of the simplest devices for regulating pressure in organic synthesis application^.^ It resembles a gas washing bottle in appearance: the gas inlet tube comes from the system and the gas outlet tube goes to the pump. I t operates on the principle that the gas in the system must overcome a hydrostatic head pressure before i t can escape through the exhaust tube to the pump. One simolv adds fluid to a heieht above the bottom of the inlet tube kc&,a~ent, a f t e r c o r r e e ~ i nfor ~ the fluid density, to the pressure desired in the system. One drawback of this design is that, once filled, the fluid level must remain fixed during operation. Because of leaks and other variables, the calrulated hydrostatic head pressure rarely equals the actual operating pressure and cannot he further adjusted. So. like others' before us and "after considerahle exoerience withvarious forms of apparatus . . . for regulating pressure in vacuum distillation, a pressure regulator was developed which is"' simple and inexpensive to construct and operate, compact and rugged in design, useful over a wide pressure range and, finally, well "adapted for practical usage."' We have modified the basic Newman manostat by incor-
' Emerson, R. L.; Woodward, R. B. Ind. Eng. Chem. Anal. Ed. 1937, 9,347. (a) Catalog G857. Gilmont Instruments, Inc., 401 Great Neck Road. Great Neck, NY 11021. (b) Catalog, Manostat Corporation,519 Eighth Ave.. New York. NY 10018. Wiberg, K. B. Laboratory Technique in Organic Chemistr): McGraw-HIIi: New York. 1960. Furnlss, B. S.: Hannaford. A. J.: Rogers, V.: Smith. P. W. G.; Tatchell, A. R. Vogel's Textbook of Practical Organic Chemistry, 4th ed.; Longman: London. 1978. Ace-Thred adapters are rated to hold a vacuum of lo-' torr. We have used them down to 0.002 torr without any difficulties. Ace Glass Co. 5028-30. 5261-16 or similar adapters are suitable. Dibutyl phthalate, d = 1.043:vacuum pump and silicone oils, d = 0.8to 1.1. Aoueous solutions can be used in some instances. Note that ~,~~~~~ same solutions ~ -sucrose. ~~~~. -Znl,. ~ -, etc.). , , Solutes ~ ~ . .have hlah ~~e densities -- - - - 1e.o.. can be used to trap vapors in some cases (Brown. L. R. Aldrichim. Acta 1981, 14, 42). but the pressure should be closely monitored and a cold trap should always be used to protect the pump in the usual way. Gilmont, R. Ind. Eng. Chem. Anal. Ed. 1948, 18, 633. ~~
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D TO PUMP
1. Theapparatus.A. 250-mL bulb; B.24140 S/Tjoint: C, IluMresaryoir. 22 cm X 18 mm; 0. Acbmred adapter 5261-16:E, tubing. 7 mm od.
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FROM SYSTEM
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ooratine an Ace-Thred or eauivalently adiustahle vacuumiight tcbing adapter as shown in ~ i ~ ; ri."uhrication e of the O-ring seals in the adapter D allows the system pressure to be easily adjusted unde; actual operating conditions. Bulb A contains any foaming or splatter of the working fluid until equilibrium is reached. The working fluid half fills elongation C and may he either mercury for control in the 2- to >loo-tom range or another fluid of low viscosity and vapor pressure for control in the 0.02- to 5-torr range.6 Other fluids may be used if the circumstances warrant.? The Ace-Thred adapter D consists of a plastic bushing with O-rine seals that make a vacuum-tinht connection to tubing or tiermometers and aground glassjoint that a d a p to standard-taoer ioints. The version illustrated has a huiltin hose conne&in for connection to the vacuum pump. Alternatively, a simpler adapter may be used in place of D hose connection may be incorporated directly and the into the bulb A. Tube E is common Pyrex tube that can he easily cut and replaced to suit a particular range of adjustability while maintaining sturdiness. If desired, the lower end of tube E can be constricted in a Bunsen burner so that minor pressure fluctuations can be d i m i n i ~ h e d . ~ , ~ Volume 66 Number 2
February 1969
181
vacuum by carefully sliding tube E in the lubricated O-rings until the desired pressure is obtained. T o shut the system down, the heat source is removed, and the system is allowed tocool. The tip of tube E is raised just above the fluid in tube C. The system is now vented to the desired atmosphere and the pump turned off without fluid backstreaming up tube l? 9 -.
Bv selection of anorooriate adanter and tube sizes. mano~.. stat sensitivity and efficiency can he matched ~o the characteristics of various systems. This regulator can he used with aspirator or mechanical vacuum pumps. The latter application renders it especially convenient for the distillation of moisture-sensitive compounds at intermediate pressures, ex., phosphoryl chloride (bp 38 'Cat 55 mm). It is simple to clean and maintain so that it is easv to chanee the fluid for different pressure ranges. ~owever,"thismanostat is so inexnensive that it mav be more convenient to keeo several in the iaboratory for different applications.'o ~
Figure 2. A typical anangemem incorporating a bypass valve. Figure 2 illustrates a typical arrangement incorporating a bypass valve even though such a valve is not always necessary with this device. (Tube E serves as a bypass when withdrawn from the fluid.) In ~ractice,for a typical distillation under reduced pressure, ihe pump and s & n are connected to the manostat as shown in the figures. The initial depth setting of tube E is approximated by a simple density-times-height calculation. The pump is turned on and, after the initial bumping of the fluid subsides. an eouilihrium oressure is reached. When the distillation heat so&e is turned on, the system will begin to ontgas and the pressure (gauge 2) will increase. As the distillation begins, a new equilibrium will be reached. The actual pressure (gauge 2) is always greater than the initially calculated value, but minor adjustments can now be made under
182
Journal of Chemical Education
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In Its slmplest form, as described here, the madified Newman manostat requires a minimum amount of common sense and caution to be used safely. However, the following additions might be considered: (1) A small Teflon washer with acenterinq hole that floatson the surfaceof the fluid cuts down on splattering a i d further secures the movable tube E within tube C. i2)Glass wool inside adaoter D helos to prevent splanering fluid frominier ngthe tube to the pump. (3)small trap tubes for tne fluid in the vacuum tram on eacn side of the manostat protect against contamnation due to loss of vacuum from either end of the system. 'O The manostat can be fabricated for - 3 4 0 , far less than the cost pf any commercial device currently available.
