United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-87/034m Sept. 1989
&EPA Project Summary
Prevention Reference Manual:
Chemical Specific, Volume 13:
Control of Accidental
Releases of Methyl Isocyanate
D. S. Davis, G. B. DeWolf, R. A. Nash, J. D. Quass, and J. S. Stelling
Interest In reducing the probability
and consequences of accidental tox-
ic chemical releases that might harm
workers within a process facility and
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. The
manual on methyl isocyanate (MIC) is
one of several chemical-specific pre-
vention reference manuals. This man-
ual summarizes Information to help
regulators and Industry personnel
identify and control release hazards
associated with MIC.
To reduce the risk associated with
an accidental release of MIC, the
potential causes of accidental releas-
es in process facilities that handle
and store MIC must be identified. The
MIC manual provides examples of
such causes, as well as of measures
that may be taken to reduce the acci-
dental release risk. Such measures
include recommendations on plant
design; prevention, protection, and
mitigation technologies; and opera-
tion and maintenance practices.
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
potentially disastrous consequences of
accidental releases of toxic chemicals
resulted from the accident in Bhopal,
India, in 1984, that killed approximately
2,300 people and injured 30,000 to
40,000 more. A toxic cloud of methyl
isocyanate (MIC) was released from a
storage tank at a Union Carbide of India
pesticide plant. In another incident in
New York in 1984, a pump failure at FMC
Corporation led to the release of 45 gal.
(170 L) of MIC. Nearby areas, including a
school, had to be evacuated. Nine chil-
dren and two adults were treated for eye
irritation at a local hospital. Such acci-
dents have increased interest in reducing
the probability of accidental releases and
prompted preparation of a series of
manuals for regulators and industry
personnel on the prevention of accidental
releases of toxic chemicals. The manual
on 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.
MIC is a small volume chemical. Total
production in 1994 was estimated to be
30-35 x 106 Ib (14-16 x 1Q6 kg). Over
95% of MIC produced is used to man-
ufacture pesticides and herbicides. Since
the Bhopal release and tightening of
transportation regulations, MIC is no
longer shipped by rail car or in drums.
-------
MIC, which is primarily made from the
reaction of methylamine and phosgene is
an intermediate in the manufacture of
pesticides such as carbaryl (Sevin),
aldicarb (Tenik), carbofuran (Furadan),
and methamyl (Lannate).
Potential Causes of Releases
MIC is a highly volatile liquid at
ambient conditions. It is a colorless liquid
with a sharp odor at concentrations
greater than 2 ppm. Liquid MIC is slightly
less dense than water. Since the gaseous
form is twice as dense as air, it tends to
stay close to the ground if released in still
air. It is highly flammable and reactive,
particularly with compounds containing
active hydrogen atoms. Since it is an
unstable compound, extreme care should
be taken to avoid conditions that could
result in fires, explosions, and runaway
reactions. MIC is usually stored and
handled in dedicated stainless steel,
nickel, and glass-lined equipment. Other
materials could catalyze an undesirable
reaction that might lead to overpressure
releases. In some cases, the heat
generated by a water/MIC reaction is
sufficient to initiate self-polymerization.
Tanks and reactors should not be filled to
capacity to allow for possible expansion
of the contents. Because MIC might react
with water vapor in air, tank contents
should be protected by a blanket of dry
nitrogen. Also, MIC can react with itself in
the presence of a catalyst to form
trimethyl isocyanate. This reaction is also
exothermic and could result in a violent
runaway reaction.
The two basic causes of MIC releases
are: uncontrolled chemical reactions, and
spills and/or leaks that may occur during
normal operation such as routine
maintenance and cleaning of equipment.
Uncontrolled chemical reactions of MIC,
such as the one in Bhopal, are extremely
dangerous. While the quantities of MIC
likely to be spilled or leaked are not as
great as those released because of an
uncontrolled chemical reaction, the
probability of spills and leaks occurring is
much higher. Most of such MIC releases
in the U.S. were the result of equipment
failure. Failures leading to accidental MIC
releases can be due to process, equip-
ment, or operational problems.
Process causes are related to the fun-
damentals of process chemistry, control,
and general operation. Possible process
causes include:
• Contamination of MIC with rust from
the nitrogen supply line, resulting in a
runaway reaction;
• Contamination of MIC with a backflow
of caustic solution from a scrubber,
resulting in a runaway reaction;
• Contamination of MIC with brine used
as a cooling fluid, resulting in a run-
away reaction;
• Insufficient temperature and pressure
monitoring equipment to detect pro-
cess upsets or failure of instruments;
• Improper design of control devices
(flare, scrubber) to neutralize MIC
emissions caused by process upsets or
failure of control systems; and
• Insufficient cooling capacity to control
MIC storage temperature or failure of
cooling system.
Equipment causes of accidental releas-
es result from hardware failure such as:
• Improper materials of construction
operating as catalysts of highly exo-
thermic reactions;
• Improper materials of construction that
dissolve in MIC;
• Failure of equipment, and no backup
system;
• Excessive mechanical stress because
of improper fabrication, construction, or
installation; and
• Mechanical fatigue.
Operational causes of accidental
releases result from incorrect operating
and maintenance procedures or human
errors, including overfilling of storage
vessels, neglecting to purge process
lines with nitrogen before making repairs,
incomplete disaster plans, and inade-
quate maintenance in general.
Release Prevention and Control
To develop a thorough release preven-
tion plan, control must be maintained
over the following areas:
• Process design;
• Physical plant design;
• Protective systems; and
• Operating and maintenance practices.
Process design involves the basic
chemistry of a process and how this
chemistry is affected by the variables of
flow, pressure, temperature, and compo-
sition. The first concern in process
design is understanding how deviations
from expected conditions could result in
an accidental release. Any aspect of a
process may be modified to enhance the
integrity of the system. For example, the
quantities of materials used, the pressure
and temperature conditions, the type and
sequence of unit operations, contro
strategies, and the instrumentation used.
After the Bhopal release, for example, c
joint Federal/State Task Force investigat
ed the potential for a MIC release at the
Institute, West Virginia, MIC unit. The
major areas measured for this stud)
were:
• Whether the process design preventec
the likelihood of MIC concentration;
• Whether there was sufficient cooling o
MIC in process vessels;
• Whether MIC inventory was minimized
and
• Whether sufficient instrumentation ant
monitoring were provided for earh
detection of possible process upsets.
Physical plant design involve;
equipment, siting and layout, am
transfer/transport facilities. Because o
the extreme reactivity of MIC it shouk
always be handled and stored in i
protective environment. Many metals
such as carbon steel, iron, tin, and alumi
num, are commonly used by the chem
ical industry, but could act to catalyze
dangerously rapid trimerization of MIC
UCAPCO (formally Union Carbide) re
commends storing and handling MIC onl
in stainless steel, types 304 and 316
nickel; and glass-lined equipment. Glass
lined steel should be free of pinholes.
Of the four companies current!
producing MIC in the U.S., DuPont an
Sandoz in Texas consume MIC a
quickly as it is produced, making layoi
of the site less critical. The use c
underground storage tanks at UCAPO
was considered in the Task Force stud
and found to be adequate with existin
controls. The siting and layout of
particular MIC facility is complex, requi
ing careful consideration of factors ir
eluding: other processes in the vicinit
the proximity of population centers, pr<
vailing winds, local terrain, and the potei
tial for flooding and other natural event
Generally, large inventories of Ml
should be kept away from sources of fii
or explosion. Vehicular traffic near Ml
process or storage facilities should t
minimized. MIC piping should not t
located adjacent to other piping undi
high pressure or temperature or th
carries flammable materials. Storac
facilities should be segregated from tf
main process. During an emergenc
there should be multiple means of acce:
for emergency crews. Since all bulk shii
ments of MIC in the U.S. have be<
eliminated, loading and unloadir
facilities are no longer potential accide
areas.
-------
Protection technologies for the con-
ainment and neutralization of MIC
iclude enclosures, scrubbers, flares, and
other secondary control systems. En-
closures are containment structures that
capture any MIC spilled or vented from
storage or process equipment, thereby
preventing immediate discharge of the
chemical to the environment. Enclosures
contain the spilled liquid or gas until it
can be transferred to other containment,
discharged at a controlled rate, or trans-
ferred at a controlled rate to scrubbers for
neutralization. UCAPCO has an under-
ground enclosure capable of collecting
42,000 gal. (159,000 L).
Scrubbers can be used to control MIC
released from process vents and pres-
sure relief discharges from storage and
process equipment, Because of its
extreme reactivity with caustics, MIC is
destroyed rather than absorbed by a
scrubber. Scrubbers used to control MIC
emissions are designed to convert any
emissions associated with the processing
of MIC and to neutralize MIC emissions
in case of a catastrophic release.
Flares are routinely used in the chem-
ical processes to dispose of intermittent
or emergency emissions of flammable
waste gases. Flares used to control MIC
emissions serve as a secondary or back-
up control; they should be designed to
operate effectively during the course of a
worst-case event such as the failure of
the primary control device (scrubber).
Other control techniques currently in
service for reducing possible releases of
MIC include carbon adsorption and vapor
incineration.
If an accidental release occurs, mitiga-
tion technologies can reduce the conse-
quences. Such measures include phys-
ical barriers, water sprays and steam
curtains, activated carbon for liquid spills,
and evacuation. The purpose of a
mitigation technique is to divert, limit, or
disperse the spilled or released chemical.
Since accidental releases of toxic
materials result not only from inadequate
process design or equipment failure, but
also from deficiencies in operation and
maintenance, the safe operation of plants
processing MIC requires competent and
experienced managers end staff. Em-
ployees should be fully trained in the
important aspects of handling MIC, in
potential hazards, and in cleanup and
emergency procedures. Well-defined
procedures can decrease the possibility
of a hazardous release and reduce the
magnitude of any release that occurs.
-------
D. S. Daws, G. B. DeWolf, R. A. Nash, J. D. Quass, and J. S. Stalling 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 13: Control of Accidental Releases of Methyl Isocyanate," (Order No. PB
89-1161 483/AS; Cost: $15.95, 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/S8-87/034m
STREET
*l 6060«
------- |