Page 17 - Chemical and process design handbook

Speight_Part 1_A 11/7/01 3:04 PM Page 1.3

ALKYLATION

Alkylation is usually used to increase performance of a product and
involves the conversion of, for example, an amine to its alkylated homologs
as in the reaction of aniline with methyl alcohol in the presence of sulfuric
acid catalyst:

C H NH + 2CH OH → C H N(CH ) + 2H O
6 5 2 3 6 5 3 2 2
Thus, aniline, with a considerable excess of methyl alcohol and a catalytic
o
amount of sulfuric acid, is heated in an autoclave at about 200 C for 5 or
6 hours at a high reaction pressure of 540 psi (3.7 MPa). Vacuum distilla-
tion is used for purification.
In the alkylation of aniline to diethylaniline by heating aniline and ethyl
alcohol, sulfuric acid cannot be used because it will form ether; conse-
quently, hydrochloric acid is employed, but these conditions are so corrosive
that the steel used to resist the pressure must be fitted with replaceable enam-
eled liners.
Alkylation reactions employing alkyl halides are carried out in an acidic
medium. For example, hydrobromic acid is formed when methyl bromide
is used in the alkylation leading, and for such reactions an autoclave with
a replaceable enameled liner and a lead-coated cover is suitable.
In the petroleum refining industry, alkylation is the union of an olefin
with an aromatic or paraffinic hydrocarbon:

CH =CH + (CH ) CH → (CH ) CCH CH
2 2 3 3 3 3 2 3
Alkylation processes are exothermic and are fundamentally similar to
refining industry polymerization processes but they differ in that only part
of the charging stock need be unsaturated. As a result, the alkylate product
contains no olefins and has a higher octane rating. These methods are
based on the reactivity of the tertiary carbon of the iso-butane with olefins,
such as propylene, butylenes, and amylenes. The product alkylate is a mix-
ture of saturated, stable isoparaffins distilling in the gasoline range, which
becomes a most desirable component of many high-octane gasolines.

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