Page 35 - Chemical and process design handbook

Speight_Part 1_H 11/7/01 3:03 PM Page 1.21

HALOGENATION

Halogenation is almost always chlorination, for the difference in cost
between chlorine and the other halogens, particularly on a molar basis, is
quite substantial. In some cases, the presence of bromine (Br), iodine (I),
or fluorine (F) confers additional properties to warrant manufacture.
Chlorination proceeds (1) by addition to an unsaturated bond, (2) by
substitution for hydrogen, or (3) by replacement of another group such as
hydroxyl (–OH) or sulfonic (–SO H). Light catalyzes some chlorination
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reactions, temperature has a profound effect, and polychlorination almost
always occurs to some degree. All halogenation reactions are strongly
exothermic.
In the chlorination process (Fig.1), chlorine and methane (fresh and recy-
cled) are charged in the ratio 0.6/1.0 to a reactor in which the temperature
o
is maintained at 340 to 370 C. The reaction product contains chlorinated
hydrocarbons with unreacted methane, hydrogen chloride, chlorine, and
heavier chlorinated products. Secondary chlorination reactions take place
at ambient temperature in a light-catalyzed reactor that converts methylene
chloride to chloroform, and in a reactor that converts chloroform to carbon
tetrachloride. By changing reagent ratios, temperatures, and recycling
ratio, it is possible to vary the product mix somewhat to satisfy market
demands. Ignition is avoided by using narrow channels and high velocities
in the reactor. The chlorine conversion is total, and the methane conversion
around 65 percent.
Equipment for the commercial chlorination reactions is more difficult to
select, since the combination of halogen, oxygen, halogen acid, water, and
heat is particularly corrosive. Alloys such as Hastelloy and Durichlor resist
well and are often used, and glass, glass-enameled steel, and tantalum are
totally resistant but not always available. Anhydrous conditions permit
operation with steel or nickel alloys. With nonaqueous media, apparatus
constructed of iron and lined with plastics and/or lead and glazed tile is the
most suitable, though chemical stoneware, fused quartz, glass, or glass-lined
equipment can be used for either the whole plant or specific apparatus.

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