Chemical Engineering July 2021 - 21

Technology Profile
Production of Caprolactam from Benzene
By Intratec Solutions
C
aprolactam (Figure 1) is a
cyclic amide widely used
as a chemical intermediate.
The main forms of caprolactam
are molten (liquid) and flakes.
At ambient temperatures, it is a white,
hygroscopic, crystalline solid.
Almost all caprolactam is used as
monomer in the production of polycaprolactam,
also known as nylon 6.
Fibers, sheets, filaments and bristles
made from nylon 6 can be used, in
turn, in a broad range of products,
including apparel and home furnishings;
carpets; and industrial uses
(tires, reinforced rubber products).
The uses and applications of caprolactam
may vary according to the
product grade. Commercial caprolactam
is produced with high purity,
where water is usually the main contaminant
with concentrations around
0.1 wt.%. Caprolactam can be used
in the manufacture of other products,
including 6-aminocaproic acid;
caprolactam disulfide; hexamethyleneimine;
polyamide 6 terpolymers;
poly(ether-amide) elastomers; n-vinyl
caprolactam and lysine.
The process
Caprolactam
production involves
four major
sections: (1)
benzene hydrogenation;
(2)
cyclohexane
oxidation; (3) oximation
& Beckmann rearrangement;
and (4) ammonium sulfate purification
(Figure 2).
Benzene hydrogenation. First,
dried benzene is reacted with hydrogen
in two steps, in the presence
of platinum-based and zinc-oxide
catalysts, to form cyclohexane. The
intermediate is purified in two columns
and hydrogen is recycled to
the reactor.
Cyclohexane oxidation. The cyclohexane
is converted to a mixture of
cyclohexanone and cyclohexanol by
liquid-phase air oxidation in the presence
of a soluble cobalt catalyst.
Subsequently, the cyclohexanol in
the mixture is converted to cyclohexanone
by vapor-phase dehydrogenation
in the presence of a coppermagnesium
catalyst.
FIGURE 1. Caprolactam
structure is shown here
Oximation and Beckmann rearrangement.
Ammonia is oxidized
by oxygen in the presence of steam,
yielding nitric oxide, which is absorbed
in a solution. This nitric oxide is hydrogenated
over a palladium catalyst, in
the presence of dilute sulfuric acid,
producing hydroxyl ammonium sulfate
solution. It reacts with cyclohexanone
in stirred reactors in series to form
the oxime. The heterogeneous outlet
is neutralized with ammonia. Finally,
the cyclohexanone oxime (Figure 3)
is converted to caprolactam by Beckmann
rearrangement, in the presence
of oleum (concentrated sulfuric acid).
The product of the rearrangement,
caprolactam, is purified by neutralization,
extraction in the
presence of toluene, and
distillation. The caprolactam
melt is solidified and
converted into flakes.
Ammonium
sulfate
purification. The ammonium
sulfate
soluFIGURE
3.
The structure
of the oxime
intermediate is
shown here
tion that is removed in
the neutralization steps
is concentrated by
evaporation. Then it is crystallized,
centrifuged from the mother liquor
and dried.
Production pathways
This organic compound can be produced
commercially from cyclohexanone,
cyclohexane, or toluene as
starting materials. Most caprolactam
production is based on the cyclohexanone
process. To a lesser extent,
caprolactam is produced commercially
by the photonitrosation of cyclohexane
or by nitrosation of cyclohexane
carboxylic acid (derived from
toluene) in the presence of sulfuric
acid. Ultimately, the main commercial
processes employed for caprolactam
production are based on benzene or
toluene from BTX (benzene, toluene,
xylenes), and generate ammonium
sulfate as a byproduct.
n
Edited by Scott Jenkins
Editor's note: Content for this column was originally developed by
Intratec Solutions LLC (Houston; www.intratec.us) and is edited by
Chemical Engineering. The analyses presented are based on publicly
available and non-confidential information. The content represents
the opinions of Intratec only. More information about the methodology
for preparing the analyses can be found, along with terms of use,
at www.intratec.us/che.
1. Benzene drying column
2. Hydrogenation reactors
3. Benzene purification
4. Cyclohexane recovery
5. Cyclohexane oxidation
6. Cyclohexanone purification
7.
Oximation
8. (NH4OH)2SO4 production
9. Beckmann arrangement
10. Caprolactam purification
11. NH4SO4 recovery
12. Refrigeration unit
13. Heat transfer fluid
14. Cooling tower
15. Boiler
16. Air separation unit
HF Heat transfer fluid
CW Cooling water
RF Refrigeration fluid
ST Steam
BFW Boiler feed water
N2 Nitrogen
FIGURE 2. This process diagram depicts the production of caprolactam from benzene
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