Chemical Engineering October 2014 - 35

Hydrogen Cyanide
Production
By Intratec Solutions
H
ydrogen cyanide (HCN) is a chemical
precursor used in the production of
several industrially relevant compounds,
such as adiponitrile for nylon production,
methyl methacrylate for polymer manufacturing,
sodium cyanide for gold recovery and for
the production of methionine, which is used as
a feed additive.
HCN is mainly produced though the Andrussow
process, named for developer Leonid
Andrussow. The method involves reacting
ammonia, natural gas and air over a platinum
catalyst to form HCN. Alternatively, HCN can
be produced by the BMA process, which uses
ammonia and natural gas only.
The process
HCN production via the Andrussow process
is depicted in Figure 1 and described below,
based on information available in the literature.
The process can be divided into three
main areas: reaction, ammonia recovery and
product purification.
Reaction. Natural gas, ammonia and air are
fed into the reactor, where HCN is formed
through a catalytic reaction. The product
stream, containing HCN and unreacted ammonia,
must be cooled down to avoid HCN
decomposition. This is accomplished in a
waste-heat boiler located below the reactor.
The waste-heat boiler generates steam that can
be used elsewhere in the process.
Ammonia recovery. The product stream is fed
to the bottom of the ammonia absorber, where
phosphate is used to absorb ammonia. The
overhead stream of this column, which contains
mainly HCN, is sent to the purification section.
The bottoms stream, containing ammonia and
a small amount of HCN, is heated and sent to
the first HCN stripper.
In this column, HCN is separated in the overheads
and recycled to the ammonia absorber.
The bottom stream is fed to the top of the ammonia
stripper, where phosphate is separated
in the bottoms. Part of this stream is mixed with
phosphoric acid and recycled to the top of the
ammonia absorber. Ammonia, now free of
phosphate, is separated as the overhead stream
of the ammonia stripper. The ammonia is then
directed to the dryer, where it is concentrated
1250
1200
1150
1100
1050
1000
950
900
10
15
20
25
30
35
Plant capacity (thousands ton/yr)
FIGURE 2. Operating expenses for HCN production variation with plant capacity
and recycled to the reaction section.
Product purification. The HCN-containing
stream is fed to the HCN absorber, where
HCN is absorbed in cold water. The overhead
vent gas is incinerated, since it can still contain
some HCN. The bottom stream from the HCN
absorber is directed to the second HCN stripper,
where the product is separated from water,
and then purified in the HCN rectification
column to obtain 99.5%-pure HCN product.
Economic performance
An economic evaluation of the process was
conducted, taking the following assumptions
into consideration:
* A 22,800 ton/yr unit erected on the U.S.
Gulf Coast (the process equipment is represented
in the simplified flowsheet below)
* Outside battery limits units considered are
the cooling tower and the refrigeration system
* No storage was considered
The capital investment (including total fixed
investment, working capital and other capital
expenses) for the construction of this plant is
estimated to be about $80 million, while the
operating expenses are estimated to be about
$1,070 per ton of product. The variation of the
Vent gas
to incineration
CW
HCN
Natural
Gas
Air
3
NH3
make-up
ST
1
ST
BFW
4
CW
2
ST
Phosphoric acid
ST
ST
To waste
treatment
To waste
treatment
FIGURE 1. The HCN production scheme shown here is based on the Andrussow process
5
CW
10
BFW Boiler feed water
CW Cooling water
ST Steam
RF Refrigeration fluid
RF
9
ST
6
7
RF
ST
To waste
treatment
CW
8
ST
To waste
treatment
1 ) Reactor waste-heat boiler
2 ) Ammonia absorber
3 ) First HCN stripper
4 ) Ammonia stripper
5 ) Ammonia dryer
6 ) HCN absorber
7 ) Second HCN stripper
8 ) HCN rectification column
9 ) Refrigeration unit
10) Cooling tower
operating expenses according to plant capacity
is depicted in Figure 2.
Global perspective
HCN is a toxic compound and is dangerous
to the environment as well as to human
health. Additionally, it is flammable and, if not
properly stored, may undergo a polymerization
reaction that can be explosive. For these
reasons, HCN storage and transportation must
be avoided, so HCN is essentially a nontradable
product. Companies that use HCN as
a feedstock in their processes must produce it
themselves or have it supplied to them by an
integrated HCN facility. Also, companies may
purchase HCN from nearby acrylonitrile plants
that produce it as a byproduct. n
Editor's Note: The content for this column is
supplied by Intratec Solutions LLC (Houston; www.
intratec.us) and edited by Chemical Engineering.
The analyses and models presented herein are
prepared on the basis of publicly available and
non-confidential information. The information and
analysis are the opinions of Intratec and do not
represent the point of view of any third parties.
More information about the methodology for
preparing this type of analysis can be found, along
with terms of use, at www.intratec.us/che.
40
45
50
Operating expenses ($/ton)
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Chemical Engineering October 2014

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