Chemical Engineering September 2021 - 25

Technology Profile
Production of Adipic Acid
By Intratec Solutions
A
dipic acid (hexanedioic acid;
Figure 1) is among the most
important aliphatic dicarboxylic
acids, with about
2.5 billion kg produced annually. It is
isolated as odorless, colorless crystals
with an acidic taste. Adipic acid
undergoes the reactions of carboxylic
acids, reacting at one or both carboxylic
acid groups to form salts, esters,
amides, nitriles and so on. Because
of the bifunctional nature of this acid
(Figure 1), it is used in a number of
industrial polymerization processes.
Most adipic acid is used captively
by the producer in the manufacture
of nylon-6,6 polyamide, prepared by
reaction of adipic acid with 1,6-hexanediamine.
Adipic acid is also used
for a wide range of applications,
including: conversion to esters; in
placticizers, lubricants and polyurethane
resins; as an acidulant for
gelatins and jams; as a food additive
for buffering or neutralizing; to make
insect repellent; as a starch crosslinking
agent; and as an intermediate
for paints.
Commercial production of adipic
acid has been predominantly based
on cyclohexane and, to a lesser extent,
phenol. In light of shifts in hydrocarbon
pricing, alternative adipic
acid production routes have been researched,
mainly based on butadiene
and cyclohexene feedstocks. Adipic
acid may also be produced from renewable
sources (bio-adipic acid).
Adipic acid, when dispersed as a
dust, is subject to dust explosion, so
the acid is conveyed pneumatically
(or mechanically) from the drying
equipment to the shipping or storage
container. The material should be
stored in corrosion-resistant containers,
away from alkaline or strong oxidizing
materials. The containers can
be made of aluminum or stainless
steel, and the quantities range from
50-lb. bags to 200,000-lb hopper
cars. In storage, adipic acid tends
to cake, depending on particle size
and moisture content, so it is recommended
to use hermetically closed
containers in storage.
Process
The process for production of adipic
acid from cyclohexane comprises
two major sections: A first oxidation
step; and a second oxidation step
(Figure 2).
First oxidation step. Initially, cyclohexane
is fed to the first of several
staged air oxidation vessels, along
with metaboric acid. In the presence
of cobalt naphthenate catalysts,
the cyclohexane is oxidized in
the liquid-phase to cyclohexylhydroperoxide
(CHHP). In the presence of
boric acid, the CHHP is trapped as
cyclohexylperborate ester, which in
turn reacts with cyclohexane, yielding
borate ester and cyclohexanol.
The borate ester formed is relatively
stable, so the generation of
undesired byproducts is minimized.
Next, the ester is hydrolyzed, yielding
cyclohexanol and boric acid,
which is dehydrated and recycled
to the oxidation step. Subsequently,
the CHHP still present in the oxidation
product is decomposed by another
oxidation reaction over a fixed
bed of cobalt catalysts, yielding a
cyclohexanol-cyclohexanone mixFIGURE
1. Adipic acid is an industrially important
6-carbon dicarboxylic acid
ture known as KA oil.
Second oxidation step. The KA oil
is then reacted with nitric acid in the
presence of copper/vanadium catalysts
to form adipic acid. While this
second oxidation step is very exothermic,
a high ratio of nitric acid to
KA oil is established, so that the nitric
acid also works as a heat sink,
avoiding uncontrollable reactions.
This oxidation reaction is extremely
quick. It is carried out in two converters,
operating at different temperatures.
Most of the KA oil is oxidized
in the first, low-temperature
converter, with the remainder being
converted in the high-temperature
converter downstream.
Nitrogen
oxides and nitrogen are released
from the reaction. While N2O and
nitrogen can be viewed as nitric
acid losses, NO2 and NO are further
oxidized and recycled as nitric acid.
After separation of water and monobasic
acids from the oxidation product,
then crystallization under vacuum
and centrifugation, crude adipic
acid is obtained. Finally, the crude
adipic acid is re-dissolved in water,
recrystallized, centrifuged and dried
to yield fiber-grade adipic acid. ■
Editor's note: The content for this column is developed by Intratec
Solutions LLC (Houston; www.intratec.us) and edited by
Chemical Engineering. The analyses and models presented are
based on publicly available and non-confidential information. The
content represents the opinions of Intratec only.
1. Oxidizers
2. Decanter
3. Boric acid recovery
4. CHHP
decomposition
5. KA oil purification
6. Converters
7. Bleacher and
concentration still
8. Crystallization and
centrifugation steps
9. Rotary dryer
10. Purge treatment
11. Cooling tower
12. Boiler
CW Cooling water
ST Steam
FIGURE 2. The process diagram shows the production of adipic acid from cyclohexane
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