United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-87/034J Feb. 1988
Project Summary
Prevention Reference Manual:
Chemical Specific, Volume 10:
Control of Accidental Releases
of Hydrogen Cyanide
D. S. Davis, G. B. DeWolf, and J. D. Quass
Interest in reducing the probability
and consequences of accidental toxic
chemical releases that might harm
workers within a process facility and
people in the surrounding community
prompted preparation of a series of
reference manuals on loss prevention in
process industries. The manual on
hydrogen cyanide (HCN) is one of a
series of chemical-specific manuals that
addresses accidental release issues.
HCN has an IDLH (Immediately Dan-
gerous to Life and Health) concentration
of 50 ppm, making it an acute toxic
hazard.
To reduce the risk associated with an
accidental release of HCN. the potential
causes of such releases in facilities using
HCN must be identified. Examples of
such causes are discussed and measures
that can reduce the accidental release
risk are identified. Such measures in-
clude possible changes to plant design;
release prevention, protection, and
mitigation technologies; and more re-
sponsible operation and maintenance
practices. Conceptual cost estimates of
example prevention, protection, and
mitigation technologies are provided.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that Is fully docu-
mented In a separate report of the same
title (see Project Report ordering In-
formation at back).
Introduction
Increasing concern about the potentially
disastrous consequences of accidental
releases of toxic chemicals has prompted
the preparation of a series of reference
manuals on chemical loss prevention and
control. This manual compiles technical
information on HCN, specifically on pre-
venting, protecting against, and mitigating
accidental releases of HCN.
Two processes for manufacturing HCN
account for most of the chemical produced
in the U.S. The most widely used process
produces HCN by reacting natural gas
(methane), ammonia, and air. A second
process, called the BMA process, pro-
duces HCN by reacting methane with
ammonia. HCN is also produced as a by-
product of acrylonitrile manufacture. As
of 1983, the major use of HCN in the U.S.
was in the production of nylon-6.6. HCN
is also used in the production of poly-
methyl methacrylate, or Plexiglas ®, and
in the manufacture of various pesticides,
chelating agents, and sodium cyanide. In
the U.S., HCN is stored in small cylinders
(approximately 150 Ib or 68 kg), railroad
tank cars, and bulk storage tanks.
Process Hazards
Anhydrous HCN is a colorless or pale
yellow liquid with a mild odor of bitter
almonds. The liquid boils at 78.3°F
(25.7°C) at 1 atm (1 kPa) and forms a
colorless, flammable, toxic gas. Three
chemical properties of HCN contribute to
its accidental release potential: 1) it is
flammable in air at concentrations of 6 to
41 %; 2) the addition of alkaline chemicals,
water, and/or heat may promote exother-
mic self-polymerization and decomposi-
tion of the HCN; and 3) the addition of
large quantities of acid can cause the
rapid exothermic decomposition of the
HCN.
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Potential HCN releases may be in the
form of either liquid or vapor. Liquid spills
can occur when HCN is released at or
below its boiling point of 78.3°F, or when
a sudden release of the chemical above
its boiling point results in vapor flashing.
Failures leading to accidental releases
may be due to process, equipment, or
operational problems. Process causes are
related to the fundamentals of process
chemistry. Examples of possible process
causes of a HCN release include:
• Overheating of cyanide manufactur-
ing reactor, resulting in rapid thermal
decomposition;
• Loss of flow or composition control
when acid stabilizer is added to a
HCN stream, resulting in excess acid
(high acid levels can lead to rapid
decomposition and overpressure);
• Loss of flow or composition control,
as above, but resulting in low acid
levels that can lead to polymeriza-
tion-decomposition;
• Loss of pH control where acetone
cyanohydrin is present, resulting in
the decomposition to acetone and
HCN, which could lead to over-
pressure;
• Excess HCN feed leading to over-
filling or overpressuring equipment;
and
• Catalyst decay in HCN production
reaction, resulting in overheating of
the reactor.
Equipment causes of accidental re-
leases that result from hardware failure
include: excessive stress caused by im-
proper fabrication, construction, or in-
stallation; mechanical fatigue and shock
from age, vibration, stress cycling, or
collisions; creep failure in equipment
subjected to extreme operational upsets,
especially excess temperature; and
corrosion.
Operational causes of accidental re-
leases result from incorrect procedures
or human errors, including: overfilled
storage vessels; errors in loading and
unloading; inadequate maintenance,
especially of pressure relief systems and
other protection and prevention systems:
and lack of inspection and nondestructive
testing of vessels and piping to detect
weakening from corrosion.
Hazard Prevention and Control
To prevent, protect against, or mitigate
an accidental toxic chemical release, the
following areas must be considered:
process design, physical plant design,
operating and maintenance practices, and
protective systems.
2
Most large manufacturers of HCN assist
their customers in understanding proper
storage and handling procedures. Equip-
ment and procedures must accord with
applicable codes, standards, and regula-
tions. One company requires all cus-
tomers to comply with their safety
practices and routinely inspects cus-
tomers' facilities for compliance.
Process design and chemistry must be
evaluated to see how deviations from
expected conditions could initiate a series
of events that would lead to an accidental
release. The primary focus is on how the
process is controlled: by the basic process
chemistry; by the variables of flow,
temperature, pressure, composition, and
quantity; and by control instrumentation
and fire protection. Process design can
be modified to enhance the overall in-
tegrity of the system or that of specific
units within the system.
Physical plant design concerns equip-
ment (vessels, piping, valves, instru-
mentation), siting and layout, and
transfer/transport facilities. Anhydrous
HCN is not generally corrosive; therefore,
carbon steel is an acceptable construction
material appropriate for ambient storage
of the chemical. Elevated temperatures,
acid stabilizers, and water will affect the
corrosiveness of HCN solution, possibly
resulting in the stress corrosion cracking
of stainless steels and nickel-chromium
and nickel-copper alloys. Water solutions
of HCN containing sulfuric acid as a
stabilizer severely corrode carbon steel
(about 100°F or 38°C) and stainless steels
(above 175°F or 79°C).
Most HCN is stored in refrigerated
atmospheric storage vessels. Safety
features that address the ability of HCN
to undergo polymerization-decomposition
should be incorporated into the design of
any storage tank. All vessels that handle
HCN must be equipped with adequate
overpressure protection. The contents of
HCN vessels should be kept under an
inert atmosphere because of its flam-
ma bi I ity. Because HCN is so toxic, even
small leaks in a piping system could be
dangerous to operating personnel. Piping
should be constructed with welded con-
nections that are fully radiographed.
Threaded fittings should never be used.
Piping, valves, and fittings should all be
constructed of 316 stainless steel.
Facilities and equipment should be laid
out to reduce personnel exposure in the
event of a release. HCN piping should not
be adjacent to other piping, and inven-
tories of the chemical should be kept
away from sources of possible fire or
explosion. Storage should be situated
away from control rooms, offices, utilities, ^
other storage areas, and laboratory areas.
Multiple means of emergency access to
the facility should be provided.
Transfer and transport facilities should
be equipped with grounding connections
for rail cars and rails, drainage control
systems, rail car temperature monitor
and alarms, warning signs and lights,
and a deluge system or sufficient fire
hoses or monitors.
Protection technologies, used to con-
tain, treat, and neutralize a released
chemical, include: enclosures, flares, and
scrubbers.
If, in spite of all precautions, a large
amount of HCN is released, workers in
the immediate vicinity must be rescued,
and persons downwind must be evacu-
ated. The source of the release should be
determined and the leak should be
stopped, if possible. To reduce the effects
of the released chemical, mitigation mea-
sures (e.g., physical barriers, water sprays
and fogs, and foams) may be employed.
Physical barriers include dikes, high im-
pounding walls, and excavated and natural
basins. A spill of HCN can be diluted with
water, which will also reduce the vapor
generation rate. Water sprays and fogs
are also useful for dispersing and/or <
removing HCN vapor from the air. Large '
fans or blowers can direct the vapor
away from populated or other sensitive
areas, if the weather is calm.
Although quality hardware, contained
mechanical equipment, and protective
devices all increase plant safety, they
must be supported by the safety policies
of management and by appropriate train-
ing, operation, and maintenance proce-
dures that relate to the prevention of
accidental releases of HCN. Management
is responsible for such things as: ensuring
worker competency; developing and en-
forcing standard operating procedures;
adequately documenting policy and
procedures; communicating and promot-
ing feedback on safety issues; identifying,
assessing, and controlling hazards; and
conducting regular plant audits.
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D. Davis. G. DeWolf, andJ. Quass are with Radian Corp., Austin, TX 78766.
T. Kelly Janes is the EPA Project Officer (see below).
The complete report, entitled "Prevention Reference Manual: Chemical Specific,
Volume 10: Control of Accidental Releases of Hydrogen Cyanide," (Order
No. PB 88-107 032/AS; Cost: $18.95, subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
U.S.OFF]OALMAiL
~~U.S.PO$iTG'r
.22-:-
•• "> M' TEHl
£2561051
Official Business
Penalty for Private Use $300
EPA/600/S8-87/034J
0000329 PS
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