Vacuum distillation diffusion pumps

The so-called hydro-vac pump, shown in Fig. 11, 22, 2 (the upper half of the mercury reservoir and the column above it are insulated by a layer of asbestos), is an inexpensive, all-glass, mercury diffusion pump, which can be used in series either with an oil pmnp or with a water Alter pmnp (compare Fig. 11,21, 1) capable of producing a vacuum of at least 2 mm. It is accordingly of particular value in the organic laboratory for vacuum distillations, fractionations, sublimations and pyrolyses as well as for molecular distillations (see Section 11,26). The hydro-vac... 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

The operating principle of steam-jet ejectors is explained in Volume 1, Chapter 8. Then-specification, sizing and operation are covered in a comprehensive series of papers by Power (1964). Diffusion pumps are used where very low pressures are required (hard vacuum) for processes such as molecular distillation. 

AuC13 (45.5 mg, 0.150mmol) and AgSbF6 (103 mg, 0.300 mmol) were weighed into a 100-mL Schlenk-flask under argon. Nitromethane (20 mL) was added. The arene (100 mmol) and the alkyne (10 mmol solid alkyne as a concentrated solution in nitromethane) were added, and the reaction mixture was stirred for 4h at 50 °C. After cooling, the solvent and excess arene were removed under vacuum. The residue was purified by column chromatography (silica gel, hexanes-ethyl acetate). After removal of all the volatiles, an oil remained which was further purified by distillation under diffusion pump vacuum (<10 3 mbar). 

Mechanical Pumps. Perhaps the most common form of vacuum pump is a mechanical pump that operates with some sort of rotary action, with moving parts immersed in oil to seal them against back-streaming of exhaust as well as to provide lubrication. These pumps are used as forepumps for diffusion pumps. Other common laboratory applications are the evacuation of desiccators and transfer lines and distillation under reduced pressure. These pumps have ultimate pressures ranging from 10 to 0.05 Torr, and pumping speeds from 0.16 to 150 L s or more, depending on type and intended application. 

Attachment of the distillation apparatus through a joint to a glass high-vacuum line with forepump and mercury diffusion pump is recommended. Distillation may be carried out in ordinary apparatus with a mechanical pump only, but even minute traces of oxygen leaking into the apparatus will react at once with the hot diphenylphosphine vapors, causing smoke to appear in the still head and condenser. 

Occasionally a distillation will require a higher vacuum than that produced by a rotary pump. Higher vacuums are normally obtained by employing a mercury or oil vapour diffusion pump, in conjunction with a rotary pump. Oil vapour pumps are most commonly used today and various models are now commercially available which will produce a vacuum of about lO mmHg. These pumps are only used occasionally but it is very useful to have one shared between a large group or section. 

Containment of Materials. All experimental work with hexafluorides was carried out in a metal manifold constructed of nickel tubing and fittings which could be evacuated by both mechanical and diffusion pumps. The manifold incorporated Monel diaphragm valves and a number of 1 in. diameter valves with brass bodies. Monel bellows, and Teflon seats. The hexafluorides were transferred between vessels in the manifold by vacuum distillation at room temperature. 

In the high-vacuum range, fractionating vapour jet pumps are employed as diffusion pumps after oil seal vacuum pumps. These vapour jet diffusion pumps are equipped with especially constmcted nozzles with a diffusion slot. The working range of this type only starts in medium high vacuum and leads to high vacuum or molecular distillation. 

The vacuum pumps discussed in Chapter 6 are capable of reducing the pressure to about 0.1 torr when they are new, are well trapped, and have clean oil in them. This is not a sufficiently low pressure to permit a high efficiency molecular distillation. A previous example calculation indicated that 10" torr or less is necessary. The most common method for reducing pressures from 10" to 10" torr (13.3 - 0.013 Pa) is the diffusion pump. Multiple stage diffusion pumps are capable of reducing pressures to 1 O torr. A diagram of a down jet diffusion pump is shown in Figure 7-11, p. 79. 

Low vacuum (1 bar-1 mbar) Positive displacement pump Booster diffusion vacuum pump (particularly steam jet pumps) Low vacuum distillation Vacuum circulation evaporator Continuous evaporator with falling or wiped film Rotary evaporator... 

Medium vacuum (1 mbar-0.1 Pa) Pump combination for example positive displacement pump+ booster diffusion pump (oil ejector) Medium vacuum distillation Continuous evaporator with agitated film (film evaporator with fixed or swinging wipers, film evaporator with brushes or rolls) Rotary evaporator... 

High vacuum (0.1 Pa-ca. 0.001 Pa) Pump combination for example two stage positive displacement pump+ diffusion pump or single stage positive displacement pump +oil ejector + diffusion pump Molecular distillation (open path, short path distillation) Cup molecular distillator Rotary molecular distillator Film molecular distillator Centrifugal molecular distillator... 

The turnings are then placed in a stainless-steel, perforated basket of sufficient size to take the whole batch, i.e. about 36 in. deep by 24 in. diameter. The basket is placed inside the vacuum distillation retort shown in Fig. 6.11. This is about 8 ft deep and is divided into a lower, mild-steel portion, heated by a furnace, joined by a cooled neoprene gasket to an upper, stainless steel portion, which acts as a condenser. The furnace is in the form of an evacuated bell with a rating of 50 kW, although only about 20 kW is used in a distillation. The retort is capable of evacuation to a pressure of 100 /Li mercury equivalent by means of an oil diffusion pump, and the furnace can be evacuated to 1 mm mercury equivalent by a high-capacity mechanical pump, which serves five furnaces. 

The areas of application for these vacuum pumps are all within the rough and medium vacuum range, the principal vacuum range for chemistry processes. Only the short path and molecular distillation processes rely also high-vacuum pumps like the diffusion pump or even the turbomolecular pump (kinetic gas pumps). High-vacuum pumps are not covered in this book. For these refer to [2, 3]. 

Vacuum science started already in the seventeenth century (Otto von Guericke). With the invention of the diffusion pump (Wolfgang Gaede, 1916) the production of deeper vacuum became technically feasible and economical and, among others, the vacuum distillation technology could develop. 

---