Vacuum distillation mechanical pumps

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. 

Vacuum distillation of solvents is frequently undertaken to reduce the temperature of the sofution, thereby reducing the risk of decomposition. Vacuum distiffation also speeds the removal of solvents. In a pilot plant, setting vacuum is usually generated through a great variety of sealed mechanical pumps, or, simply, using steam... 

Fie. 40. Vacuum distillation equipment for substances which solidify easily (napnthols, phenylenediamine, etc.). The apparatus for large quantities (1000-30(X) kg.) is equipped with a stirring mechanism to prevent charrine. 1, distillation vessel 2, receiver with steam or water jacket 3, steam heated times to prevent solidification 4, trap to collect water and sublimate 5, line to pump. 

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. 

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. 

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. 

A basic apparatus similar to the one shown in Figure 16.1 may be used for microscale vacuum distillations. As is the case for simple distillation, this apparatus uses the Hickman head as a means to reduce the length of the vapor path. The major difference to be found when comparing this assembly to one for simple distillation (Technique 14, Figure 14.8) is that the opening to the atmosphere has been replaced by a connection to a vacuum source (top right-hand side). The usual sources of vacuum are the aspirator (Technique 8, Section 8.5), a mechanical vacuum pump, or a "house" vacuum line (one piped directly to the laboratory bench). The aspirator is probably the simplest of these sources and the vacuum source most likely to be available. However, if pressures below 10-20 mm Hg are required, a vacuum pump must be used. 

Preparation of silver maleate. Dissolve 65 g. of pure maleic acid (Section 111,143) in the calculated quantity of carefully standardised 3-5N aqueous ammonia solution in a 1-htre beaker and add, whilst stirring mechanically, a solution of 204 g. of silver nitrate in 200 ml. of water. Filter oflf the precipitated silver maleate at the pump, wash it with distilled water, and press well with the back of a large flat glass stopper. Dry in an electric oven at 50-60° to constant weight. The yield of the dry silver salt is 150 g. Store in a vacuum desiccator in the dark. 

We may encounter problems in the purification of substances with a high normal boiling point. If purification only requires a small number of theoretical stages. Short Path Distillation (SPD), in which pressures can be as low as 0.001 bar, can prove useful. Many vitamins and pharmaceuticals can be processed without deterioration of quality. It is now common to use mechanical vacuum pumps with proper condensers preceding the pump. 

Figure 1. Apparatus for the synthesis of borazine. (A) nitrogen supply from tank (B) oil bubbler (C) valve (D) 2-L, three-necked, round-bottomed flask (E) thermometer (F) distilling column (G) vacuum tubing (H) mercury manometer (I) four traps for the collection of the borazine product (J) valve to mechanical vacuum pump.

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