Removal of impurities from vinyl chloride

Impurities are removed from vinyl chloride by passing it over finely divided calcium oxide.

The present invention relates to a process for removing impurities from 
vinyl chloride by passing it over an adsorbent. 
Monomeric vinyl chloride for the preparation of polyvinyl chloride must 
fulfill high purity requirements. Thus, for example, hydrogen chloride 
contained in the vinyl chloride leads to undesirable discoloration of the 
polymer. Moreover, it is found that iron salts which lead to discoloration 
of the monomer are formed by corrosion if monomeric vinyl chloride is 
stored for a relatively long period, due to the presence of traces of 
hydrogen chloride and water. 
Monomeric vinyl chloride is therefore usually purified by distillation 
before use. Hydrogen chloride and most of the water can be removed from 
the monomeric vinyl chloride by distillation, but because both hydrogen 
chloride and water (up to not more than 100 ppm) are present, it is 
necessary to use corrosion-resistant distillation columns and condensers. 
This type of purification of the monomer is therefore technically 
complicated. 
In another process, hydrogen chloride is removed from monomeric vinyl 
chloride by washing the monomer in sodium hydroxide solution or potassium 
hydroxide solution. The vinyl chloride purified in this manner is then 
dried by being passed over finely divided sodium hydroxide or potassium 
hydroxide. The disadvantage of this process is that reaction of traces of 
hydrogen chloride which are still present with the sodium hydroxide or 
potassium hydroxide gives smeary by-products which are partly soluble in 
vinyl chloride. Such a process is therefore still not completely 
satisfactory. 
It is an object of the present invention to provide a process for removing 
impurities from vinyl chloride, by passing it over an adsorbent, which 
does not have the disadvantages of the known processes. 
We have found that this object is achieved by using finely divided calcium 
oxide as the adsorbent. 
One advantage of the process is that iron salts can be removed from the 
monomeric vinyl chloride together with the water and hydrogen chloride. 
The fine-particled calcium oxide retains its geometric structure even 
after the adsorption, and does not disintegrate. Thus, the strength of the 
adsorbent is retained, and the pressure loss in the adsorber therefore 
remains constant. Th stoichiometric capacity of the calcium oxide can be 
utilized to the extent of more that 50%. 
Calcium oxide in fine-particled form with an average particle diameter of 
from 0.5 to 5 mm is used as the adsorbent. The calcium oxide obtained by 
burning chalk is advantageously used. Liquid vinyl chloride is passed over 
the calcium oxide at from 20.degree. to 70.degree. C., in particular from 
30.degree. to 50.degree. C., and under pressures of from 3.5 bar to 12 
bar, in particular from 4 to 8 bar. The residence time in the adsorber in 
advantageously from 1.0 to 5.0 minutes, so that from 60 tonnes/hour to 12 
tonnes/hour of vinyl chloride can be passed over 1 tonne of calcium oxide. 
The adsorber is advantageously a cylindrical reaction vessel filled with 
the finely divided calcium oxide and the vinyl chloride is advantageously 
passed downwards through the adsorber. 
We have found, surprisingly, that the vinyl chloride purified in this 
manner can be used directly for polymerization, to give vinyl chloride 
polymers containing no discoloring impurities.

EXAMPLE 
A cylindrical adsorber 146 l in volume and 88 cm in height is filled 
completely with calcium oxide having a particle diameter of 0.3 to 1.5 cm 
and a bulk density of 1 kg/l. 
The amounts of vinyl chloride given in the accompanying table are passed 
through the adsorber. The contents of hydrogen chloride and water before 
and after the adsorptive treatment are also given in the table. 
TABLE 
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Amount 
of vinyl 
chloride HCl H.sub.2 O 
(tonnes/ HCl H.sub.2 O 
down- down- 
hour) upstream upstream 
stream stream 
through of the of the of the of the 
Experi- 
the adsorber adsorber 
adsorber 
adsorber 
ment adsorber ppm ppm ppm ppm 
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I 1 2,400 800 
II 2 2,400 930 
III 3.0 22 20-25 2 10 
IV 1.5 18 20-25 1 10 
V 1.5 14 20-25 1 10 
VI 1.5 25 20-25 1 10 
VII 5.0 20 20-25 2 10 
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