What Is Vacuum Distillation?

Vacuum distillation, also known as "decompression distillation", is one of the distillation methods. Under reduced pressure. It is generally used to separate substances that are liable to decompose or polymerize when heated to boiling under normal pressure or combined with other distillation methods (such as steam distillation) to reduce the temperature of distillation. Commonly used in chemical, petroleum and other industries. ^[1]

Distillation is a separation method that makes use of the differences in certain physical properties of the various components of the distillate. ^[2]

In the following, the basic principle of vacuum distillation will be explained using industrially prepared sponge zirconium as an example.

The products obtained by reduction are zirconium,

And the remaining magnesium. Not emitting

When the reaction product contains 29% ~ 3l%

,

64% 66%

, 4% 6%

;

Discharge by overflow method

When the reaction product contains 47% to 49%

, 42% to 45%

, 7% -9%

;

Discharge on top

When the reaction product contains: 58% to 61%

, 29% 33%

, 9% 1l%

.

In the above three types of reaction products, the melt surface contains a small amount of black powder.

The basic principle of vacuum distillation is the use of zirconium and

And the difference in vapor pressure of magnesium to achieve zirconium and

Effective separation from magnesium. Vacuum distillation includes evaporation (or sublimation) and condensation processes. The separation effect is mainly determined by their vapor pressure. Vapor pressure means the volatile energy of a substance. Each substance has its specific vapor pressure at a certain temperature.

The boiling point of magnesium is 1070 ° C.

Its boiling point is 1418 ; at normal pressure and 9000 , the average vapor pressure of magnesium is 1.3 × 104Pa,

It is 75Pa.

Zirconium has a boiling point of up to 3577 ° C and is not volatile at 900 ° C. It is compatible with magnesium and

The volatility varies widely and is good for separation. ^[4]

In the following, the industrial conditions for preparing sponge zirconium are used as examples to illustrate the process conditions of vacuum distillation.

(1) Equipment structure

Undercooling vacuum distillation furnace is generally used (as shown on the right)

The vacuum distillation process can be divided into two types: (1) the reactor is opened after the reactants are completely cooled; (2) the reactor is opened without cooling the reactants.

After the reducer with the reactants is sufficiently cooled on the cooler, the reducer is opened on a dedicated table, the cover is removed and the drain device is removed, and the tail of the still is welded with a steel plate to seal it. Purge with nitrogen and carefully remove the low-priced titanium chloride on the surface of the reaction with a metal brush. Do the same for the upper part of the inner wall of the reducer. The reducer with the reactants was then placed on the mounting bracket of the vacuum distillation apparatus.

The prepared return distillation tank is installed on the flange of the reactor. A conical heat insulation plate is pre-installed on the reactants in the reactor, and the connection between the reactor and the condenser of the distillation tank is sealed with a vacuum rubber gasket.

The installed distillation equipment is pressure tested with nitrogen, and the leak tightness is checked with a small residual pressure without leaking. To this end, the distillation equipment is connected to a vacuum system, which is composed of a primary vacuum pump BH-6 and a booster pump BI-t-2000H. When distilling to a residual pressure of not more than 70 microns of mercury, close the valve to isolate the equipment from the vacuum system and check for leaks. After 5 minutes, when the leakage is not more than 4 micrometers of mercury column / minute, the preparation of the distillation equipment is finished.

It usually takes less than one hour to install the reducer on the vacuum distillation unit.

When there is a blind plate of magnesium on the central tube of the reducer cover, do not cool or open the cover, put it on a special insulated trolley and install the return distillation tank, that is, install a vacuum distillation device. Welded the venting device of the reactor to the vacuum valve on the trolley. Prepare a water-cooled return distillation tank in advance on a dedicated operator's station. Check the sealing performance of the return distillation tank with a vacuum pipe with a residual pressure of not more than 70 mm mercury column, and the leakage does not exceed 4 micrometer mercury column / minute (after 5 minutes).

A set of vertical heat insulation plates and connecting pipes are mounted on the reducer cover on the trolley. The connecting pipes are sealed with asbestos. The return distillation tank with the cover is installed on the flange of the reducing cover. The vacuum rubber pad is used to make the connection tight.

The installed distillation equipment is transferred from the trolley to the distillation section and integrated into the distillation furnace. Switch on the vacuum pump BH-6, open the valves on the vacuum system and the pipeline, and start vacuuming. The furnace was evacuated to a 300 mm mercury column to prevent the vacuum still from deforming at high temperatures. ^[5]

(1) When installing a vacuum distillation instrument, a rubber stopper of a suitable size must be selected, and a mill-mouth vacuum distillation device is preferred.

(2) The distillate contains a large amount of low-boiling substances. It is necessary to first distill under normal pressure to evaporate most of the objects, and then use a water pump to distill under reduced pressure to remove the low-boiling substances.

(3) Stop heating, recover low boiling materials, check the connection of each part of the instrument, and make it close.

(4) Start the oil pump, and then close the safety valve slowly, and observe whether the pressure on the pressure gauge meets the requirements. If it fails, the safety valve can be used for adjustment.

(5) When the pressure reaches a constant and satisfactory level, start heating the distillation flask again. When distilling the monomers, a little zeolite should be added to the distillation flask (usually an oil bath is used, and high-boiling substances that are difficult to volatilize can be high in the later stage 30 ~ 50 ).

(6) After the distillation is completed, remove the heat source, wait for a bit to cool, and then gradually open the safety piston at the same time. When the mercury column balance in the pressure gauge drops, stop pumping. After the internal and external pressure of the system is balanced, remove the instrument and clean it. .