3.5 Commercial Chemistry
Phosgene is obtained commercially by passing carbon monoxide and chlorine over
activated carbon. The reaction is exothermic, producing heat that must be removed
from the reactor.
The formula for the reaction to produce phosgene is:
Activated
CO + Cl2 COCl2 + Heat
Carbon Monoxide Chlorine Carbon Phosgene
Hydrogen and methane impurities in the carbon monoxide feed gas react with chlorine
to produce hydrogen chloride and carbon tetrachloride respectively. The formulas for
these two impurity reactions are:
H2 + Cl2 2 HCl + Heat
CH4 + 4Cl2 CCl4 + 4 HCl + Heat
Hydrogen and methane react with chlorine without catalyst, therefore the reaction can
take place in the piping prior to the reactors. Normally, these impurities are at very low
concentrations and the impurities formed are not significant. If a high concentration of
either impurity exists, these reactions can generate enough heat to melt the pipe. Since
chlorine is an oxidizer, and methane, hydrogen and carbon monoxide are fuels, a fire
can occur in the pipeline without oxygen. At temperatures above 250o
F, chlorine will
start reacting with steel, weakening the piping and vessels. At 483o
F, chlorine will ignite
iron and produce a fire. Detection of these impurity generated reactions can be noticed
by a rapid rise in the temperature of the feed gas after the carbon monoxide and
chlorine mixing point. The use of high mixing temperature automatic shutdown is a
useful method to help eliminate this type of failure.
Carbon tetrachloride and carbon dioxide can also be formed at high temperature by the
reaction of two phosgene molecules. In the center of the reaction tubes temperatures
are sufficiently hot to cause a small amount of this impurity reaction. The formula for
this reaction is:
2 COCl2 + Heat CCI4 + CO2