United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-87/034n Sept. 1989
&EPA Project Summary
Prevention Reference Manual:
Chemical Specific, Volume 14:
Control of Accidental
Releases of Phosgene
D. S. Davis, G. B. DeWolf, S. F. Penrod, and J. D. Quass
Interest in reducing the probability
and consequences of accidental tox-
ic chemical releases that might harm
workers within a process facility or
people in the surrounding community
has prompted preparation of this
manual and a series of companion
manuals on the control of accidental
releases of toxic chemicals. This
manual, on phosgene, is one of sev-
eral chemical-specific Prevention Re-
ference Manuals.
Phosgene is a highly reactive and
corrosive liquid that has an IDLH (Im-
mediately Dangerous to Life and
Health) concentration of 2 ppm, mak-
ing it an acutely toxic hazard.
To reduce the risk of an accidental
release of phosgene, the potential
causes of releases in facilities handl-
ing phosgene must be identified. The
phosgene manual provides examples
of such causes, as well as of mea-
sures that may be taken to reduce
the accidental release risk. Such
measures include recommendations
on plant design practices; prevention,
protection, and mitigation techno-
logies; and operation and mainten-
ance practices. Cost estimates of
prevention, protection, and mitigation
measures are provided.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory, Re-
search Triangle Park, NC, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
Increasing concern about the potential-
ly disastrous consequences of accidental
releases of toxic chemicals has prompted
preparation of a series of manuals for
regulators and industry personnel on the
prevention of accidental releases of toxic
chemicals. The manual on phosgene is
one of several chemical-specific manuals
that address issues associated with the
storage, handling, and process operations
involving toxic chemicals as they are
used in the U.S.
In the U.S. and abroad, few significant
releases of phosgene have occurred;
however, in Hamburg, Germany, in 1928
a phosgene storage tank failed, killing 10
people, and in LaPorte, Texas, in 1969 an
accidental phosgene release at a chemi-
cal plant injured 2 people.
Phosgene is manufactured commer-
cially by reacting carbon monoxide with
chlorine over a carbon catalyst. Most of
the phosgene produced is used captively
at the production facility, although it is
also shipped in 150-lb (68kg) and 1-ton
(907 kg) cylinders. In the U.S., 85% of
the phosgene produced is used in the
manufacture of isocyanates, which are
precursors of polyurethane foam and rub-
ber. Phosgene is also used in the pro-
duction of polycarbonates and specialty
chemicals such as herbicides, pesticides,
dyes, and Pharmaceuticals.
Potential Causes of Releases
Phosgene releases can originate from
many sources, including ruptures in proc-
ess equipment, separated flanges, actu-
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ated relief valves or rupture discs, and
failed pumps or compressors. Phosgene
is not explosive or flammable; however, it
is highly reactive and highly toxic when
inhaled. When inhaled, phosgene slowly
hydrolyzes to hydrochloric acid in the
respiratory system.
The reactivity of phosgene makes it
corrosive. Evaporators and metering and
control equipment are especially sensi-
tive if they are in intermittent use where
moisture might enter the system. Either
liquid or vapor phosgene releases can
occur. Failures leading to accidental re-
leases of phosgene can be due to proc-
ess, equipment, or operational causes.
Possible process causes of phosgene
releases include:
• Overpressure of any storage or proc-
ess vessel containing phosgene, caus-
ed by decomposition of phosgene to
hydrogen chloride and carbon dioxide
from contamination with water;
• Overpressure of storage or process
vessels containing phosgene caused
by contamination of other phosgene
reactive materials;
• Excess phosgene feed leading to
overfilling or overpressuring of equip-
ment;
• Loss of agitation in batch reactor sys-
tems;
• Loss of cooling or temperature control
of reactor systems resulting in thermal
decomposition; and
• Loss of pH control in caustic slurry
reactors resulting in the decomposition
of phosgene to carbon dioxide and
hydrochloric acid, which could result
in overpressure and an accidental
release.
Equipment-caused accidental releases
result from hardware failure (e.g., exces-
sive stress due to improper fabrication,
construction, or installation); weakening of
equipment from excessive stress, exter-
nal loading, or corrosion; mechanical
fatigue and shock; creep failure in equip-
ment subjected to extreme operational
upsets, especially excess temperature;
stress corrosion cracking; and all forms of
corrosion.
Operation-caused accidental releases
result from incorrect operating and main-
tenance procedures or human errors
such as: overfilled storage vessels; errors
in loading and unloading procedures; in-
adequate maintenance; lack of inspection
and non-destructive testing of vessels
and piping to detect corrosion weakening;
and incomplete knowledge of the prop-
erties of a specific chemical or of the
process or chemical system.
Hazard Prevention and Control
The prevention of accidental releases
depends on a combination of technologi-
cal, administrative, and operational prac-
tices that apply to the design, construc-
tion, and operation of facilities where
phosgene is stored and used. Important
areas to be considered are process de-
sign, physical plant design, operating and
maintenance practices, and protective
systems.
The primary focus of process design is
on how the process is controlled in terms
of the basic process chemistry and the
variables of flow, pressure, temperature,
composition, and level. The process de-
sign must be evaluated to see how devia-
tions from expected design conditions
might start a series of events that could
result in an accidental release. A review
of the variables of process design might
lead to suggested modifications that
would enhance the integrity of the proc-
ess, such as changes in quantities of ma-
terials used, process pressure and temp-
erature conditions, unit operations used,
the sequence of operations, process
control strategies, and the instrumen-
tation used.
Physical plant design involves plant
equipment, siting and layout, and trans-
fer/transport facilities. The most important
considerations in selecting equipment
construction materials for phosgene ser-
vice are the temperature and moisture
content of the phosgene. Temperature is
important because phosgene decompos-
es to carbon monoxide and chlorine at
high temperatures, and most metals will
ignite at a given temperature in the pres-
ence of chlorine. Moisture content is
important because moist or wet phos-
gene hydrolyzes to hydrochloric acid,
which is very corrosive. Dry phosgene is
not considered corrosive.
Because of the large inventories con-
tained in phosgene storage vessels, they
are one of the most hazardous parts of a
phosgene system. A variety of safety fea-
tures are usually incorporated into phos-
gene storage vessels. Overpressure pro-
tection devices are routinely routed to a
containment vessel or to a caustic scrub-
ber. With liquid phosgene, overfilling and
overheating are important because of the
liquid's high coefficient of thermal
expansion. Since there is no way to
contain a phosgene discharge from
vessels used to transport phosgene (150-
and 2000-lb cylinders), great care muj
be taken in transporting and storing thes
vessels; exposure to temperatures abov
125°F (52°C) must be avoided.
Overfilling can be prevented by usini
level sensing devices, pressure relief de
vices, and adequately trained personne
Selection of such devices must take int
account the corrosiveness of phosgene
especially in contact with moisture. Con
tainers should be valved to allow th
vessel to be isolated from the process t
which the phosgene is being fed.
The design of piping systems shoul
be simple, minimizing the number c
joints and connections. Pipes should b
sloped, and drainage should be provide
at low points. When applicable, an expan
sion chamber should be installed t
prevent a rupture caused by thermal e>
pansion.
The siting of facilities and individu;
equipment should minimize personn<
exposure during a release. Ready ingres
and egress should be available in a
emergency. Large inventories of phos
gene should be kept away from potenti;
sources of fire or explosion. Phosgen
piping should not be located adjacent I
other piping under high pressure c
temperature. Storage facilities should b
segregated from the main process an
from control rooms, offices, utilities, sto
age, and laboratory areas.
Protection technologies for phosgeni
or those measures taken to capture <
destroy the chemical if it has breache
primary containment, include enclosure
scrubbers, incinerators, and flares. Enck
sures can capture phosgene spilled <
vented from storage or process equi|
ment, thereby preventing immediate di;
charge of the chemical to the enviroi
ment. Although specially designed encli
sures for phosgene service have n
been widely used, such containme
structures equipped with monitorir
equipment and alarms might be appropi
ate. The enclosures should be gastig
and have a ventilation system designe
to draw in air when the building is vente
to a scrubber. The bottom section
such a building should be liquidtight
retain spilled liquid phosgene.
Scrubbers are a traditional way to al
sorb toxic gases from process stream
Types of scrubbers that might be appr
priate for phosgene include spray towei
packed bed scrubbers, and Venturis.
Incinerators can be used to contr
phosgene releases from vents and pre
sure relief discharges, from proce
equipment, and from secondary co
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tainment structures. Phosgene dis-
|harges could be mixed with fuel and air
a thermal oxidation unit to convert the
toxic vapor to carbon monoxide and
chlorine since phosgene decomposes to
these gases at 1,472°F (800°C). The
system must be constructed of corrosion-
resistant materials. Also, an acid-gas
scrubber would be needed to remove the
hydrogen chloride and chlorine from the
vent gases.
Flares must be so designed that a
large phosgene release would not over-
whelm them and lead to a flame blowout
or to a temperature insufficient to com-
pletely destroy the phosgene. An acid-
gas scrubber would also be required for
this system.
If a large release of phosgene occurs,
workers must be rescued from the imme-
diate vicinity of the accident and people
downwind of the release must be evacua-
ted. The effects of the released chemical
on the plant and community must be mit-
igated by such measures as physical
barriers, water sprays and fogs, and
foams, where applicable. Such mitigation
measures divert, limit, or disperse the
chemical that has been spilled or releas-
ed to the atmosphere.
The foregoing techniques for prevent-
ing, containing, or mitigating an acciden-
al release of phosgene must also be
supported by management safety policy,
training, and proper operating and main-
tenance procedures.
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D. S. Daws, G. B. DeWolf, S. F. Penrod, and J. D. 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 14: Control of Accidental Releases of Phosgene," (Order No. PB 89-155
0481 AS; Cost: $21.95, 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
Official Business
Penalty for Private Use $300
EPA/600/S8-87/034n
00006583
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CHICAGO
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