Page 58 - Chemical and process design handbook

Speight_Part 1_S&V 11/7/01 3:01 PM Page 1.44

1.44 REACTION TYPES
The α-sulfonic acid can be hydrolyzed to naphthalene by passing
o
steam at 160 C into the sulfonation mass. The naphthalene so formed
passes out with the steam and can be recovered. The pure β-sulfonic acid
left behind can be hydrolyzed by caustic fusion to yield relatively pure β-
naphthol.
In general, separations are based on some of the following considera-
tion:

1. Variations in the rate of hydrolysis of two isomers
2. Variations in the solubility of various salts in water
3. Differences in solubility in solvents other than water
4. Differences in solubility accentuated by common-ion effect (salt
additions)
5. Differences in properties of derivatives
6. Differences based on molecular size, such as using molecular sieves
or absorption.

Sulfonation reactions may be carried out in batch reactors or in contin-
uous reactors. Continuous sulfonation reactions are feasible only when the
organic compounds possess certain chemical and physical properties, and
are practical in only a relatively few industrial processes. Most commercial
sulfonation reactions are batch operations.
Continuous operations are feasible and practical (1) where the organic
compound (benzene or naphthalene) can be volatilized, (2) when reaction
rates are high (as in the chlorosulfonation of paraffins and the sulfonation
of alcohols), and (3) where production is large (as in the manufacture of
detergents, such as alkylaryl sulfonates).
Water of reaction forms during most sulfonation reactions, and unless a
method is devised to prevent excessive dilution because of water formed
during the reaction, the rate of sulfonation will be reduced. In the interests
of economy in sulfuric acid consumption, it is advantageous to remove or
chemically combine this water of reaction. For example, the use of reduced
pressure for removing the water of reaction has some technical advantages
in the sulfonation of phenol and of benzene.
The use of the partial-pressure distillation is predicated upon the ability of
the diluent, or an excess of volatile reactant, to remove the water of reaction
as it is formed and, hence, to maintain a high concentration of sulfuric acid.
If this concentration is maintained, the necessity for using excess sulfuric
acid is eliminated, since its only function is to maintain the acid concentra-