United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-87/0340 Sept. 1989
f/EPA Project Summary
Prevention Reference Manual:
Chemical Specific, Volume 15:
Control of Accidental
Releases of Sulfur Trioxide
D. S. Davis, G. B. DeWolf, K. E. Hummel, and J. D. Quass
The accidental release of a toxic
chemical at Bhopal, India, in 1984 was
a milestone in creating an increased
public awareness of toxic release
problems. As a result of other,
perhaps less dramatic incidents in
the past, portions of the chemical in-
dustry were aware of this problem
long before this event These same
portions of the industry have made
advances in this area. Interest in re-
ducing the probability and conse-
quences of accidental toxic chemical
releases that might harm workers
within a process facility and people
in the surrounding community
prompted the preparation of a series
of technical manuals addressing ac-
cidental releases of toxic chemicals.
This project summary is for a chem-
ical specific manual for sulfur
trioxide (SO3). The manual summariz-
es information to aid regulators and
industry personnel in identifying and
controlling release hazards associ-
ated with SO3.
Reducing the risk associated with
an accidental release of SO3 involves
identifying some of the potential
causes of accidental releases that
apply to the process facilities that
handle and store the chemical. In this
manual, examples of potential causes
are identified as are measures that
may be taken to reduce the
accidental release risk. Such mea-
sures include recommendations on
plant design and maintenance prac-
tices. Conceptual cost estimates of
example 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
The accidental release of a toxic
chemical, methyl isocyanate, in Bhopal,
India, in 1984 was a milestone in creating
an increased public awareness of toxic
release problems. There have been other
less dramatic incidents of toxic chemical
release in the past, and the chemical
industry was aware of this problem long
before this event. Safety and loss
prevention has long been a standard part
of industry activity, and over the years
industry has made many advances in this
area. There is renewed interest, however,
in reviewing technology and procedures
for preventing, protecting against, and
mitigation of accidental releases.
As an aid to regulators and industry
personnel charged with reducing the
probability and consequences of acciden-
tal toxic chemical releases, a series of
technical manuals were prepared that
address prevention, protection, and miti-
gation measures for releases. This chem-
ical specific manual on sulfur trioxide
(S03) is part of that series.
S03 is a commodity chemical, pro-
duced by the catalytic oxidation of sulfur
dioxide (SO2). Data on the production of
S03 are not available but, based on the
relative production of surfactants, a use
-------
of 300 million Ib (136 million kg) per year
of SO3 is estimated for recent years. The
primary use of S03 is as a sulfonat-
ing/sulfating agent to produce anionic
surfactants, including linear alkylbenzene
sulfonates, alcohol sulfates, and alcohol
ether sulfates.
Storage systems for liquid SO3 include
55-gal. (0.21m3) drums and bulk storage
tanks.
In addition to anhydrous S03, oleum
(fuming sulfuric acid composed of sulfu-
ric acid and S03) is also used. This man-
ual focuses primarily on anhydrous S03,
but some considerations also apply to
oleum.
Potential Causes of Releases
Anhydrous S03 is a clear, colorless,
oily liquid with a strong, acrid odor. Liquid
S03 begins to freeze at around 90°F
(32°C). S03 is hygroscopic and fumes
upon exposure to moist air. Traces of
water or sulfuric acid can catalyze the
polymerization of liquid S03 to solid
forms that are difficult to remelt.
Because liquid anhydrous SO3 has a
large coefficient of thermal expansion, an
overpressurization hazard exists if stor-
age vessels have insufficient expansion
space or if pipelines full of liquid S03 are
sealed at both ends. In these situations,
thermal expansion of the liquid can result
in containment failure from the hydrostat-
ic pressure exerted by the liquid.
Failures leading to accidental releases
may be broadly classified as due to proc-
ess, equipment, or operational causes.
Causes discussed below are intended to
be illustrative, not exhaustive.
• Excess organic feed to a sulfona-
tion/sulfation reactor leading to exces-
sive exothermic reaction, combined
with failure of the cooling system;
• Backflow of process reactants to a
S03 feed tank;
• Inadequate water removal from
organic feeds to the sulfonation/sulfa-
tion process over a long period of
time, leading to progressive corrosion;
• Excess feeds in any part of the
system, leading to overfilling or over-
pressuring equipment;
• Loss of temperature control in cooling
units (reactor) or heating vents
(vaporizer); and
• Overpressure in S03 storage vessels
due to overheating or overfilling.
These situations may be caused by
exothermic reactions from contamina-
tion, fire exposure, or unrelieved over-
filling.
Equipment causes of accidental re-
leases result from hardware failures.
Some possible causes include:
• Excessive stress due to improper fab-
rication, construction, or installation;
• Failure of vessels at normal operating
conditions due to weakening of equip-
ment from excessive stress, external
loadings, or corrosion. Overheating is
also a possibility, especially for sulfon-
ation reactors or S03 vaporizers;
• Mechanical fatigue and shock in any
equipment. Mechanical fatigue could
result from age, vibration, or stress cy-
cling, caused by pressure cycling, for
example. Shock could occur from col-
lisions with moving equipment such as
cranes, or other equipment in process
or storage areas;
• Thermal fatigue and shock in sulfona-
tion reactors or heat exchangers;
• Brittle fracture in any equipment, but
especially in carbon steel equipment
subjected to extensive corrosion
where hydrogen embrittlement from
hydrogen release by sulfuric acid
attack may have occurred. Equipment
constructed of high alloys, especially
high strength alloys selected to reduce
the weight of major process
equipment, might be especially
sensitive where some corrosion has
occurred, or severe operating
conditions are encountered;
• Creep failure in equipment subjected
to extreme operational upsets, espe-
cially excess temperatures. This can
occur in equipment subjected to a fire
that may have caused damage before
being brought under control; and
• All forms of corrosion. External cor-
rosion from fugitive emissions of sul-
furic acid mist could lead to equip-
ment weakening.
Operational causes of accidental re-
leases are a result of incorrect operating
and maintenance procedures of human
errors, including:
• Overfilled storage vessels;
• Improper process system operation;
• Errors in loading and unloading proce-
dures;
• Inadequate maintenance in general,
but especially on water removal unit
operations, pressure relief systems,
and other preventive and protective
systems;
• Lack of inspection and nondestructive
testing of vessels and piping to detect
corrosion weakening; and
Incomplete knowledge of the proper
ties of a specific chemical, of the pro
cess, or of the chemical system.
Hazard Prevention and Control
Prevention of accidental releases relie
on the proper design, construction, am
operation of facilities where SO3 is storei
and used and on the protective system
that guard against an accidental release.
Process design involves the basi
chemistry of a process and how thi
chemistry is affected by the variables c
flow, pressure, temperature, compositior
and quantity. Any aspect of a proces
may be modified to enhance the integrit
of the system. Such changes could ir
volve the quantities of materials, proces
pressure and temperature conditions, th
sequence of operations, process contn
strategies, and the instrumentation used.
Physical plant design covers equip
ment, siting and layout, and tranj
fer/transport facilities. Mild steel is ger
erally satisfactory for storage and har
dling of S03. Where moisture may b
present, or for pumps, valves, and oth<
areas of high turbulence, Alloy 20, 3C
SS, or 316 SS is recommended. Gaske
or packing should be made of a fluon
carbon material.
The siting and layout of any facilii
handling S03 and of individual equipme
items should be designed to reduce pe
sonnet exposure during a release. Sitir
should allow ready ingress and egre:
and take advantage of barriers that n
duce release exposures. Considerab
distance between large inventories ar
sensitive receptors is desirable. Tt
ground under process equipment ar
storage vessels should be sloped so th
fire water and liquid spillage flow aw;
from equipment into drains. Storage fac
ities should be located in cool, dry, we
ventilated areas.
Because heat causes signified
thermal expansion of S03 and can lead
thermal decomposition, piping, storai
vessels, and other equipment should r
be located adjacent to piping contain!
flammable materials, hot process pipir
equipment, or other sources of direct
radiant heat. Special consideration shoi
be given to the location of furnaces a
other permanent sources of ignition in t
plant.
Protection technologies for containrru
and neutralization include enclosures a
scrubbers. Enclosures would capture z
S03 spilled or vented from storage
process equipment, containing the spill
liquid until it could be transferred to otl
-------
containment and discharged at a
ontrolled rate or to scrubbers for ab-
jrption.
Scrubbers can also be used for
controlling SO3 releases. S03, which is
soluble in sulfuric acid, can be absorbed
into sulfuric acid in scrubbing devices
such as spray towers, packed bed scrub-
bers, and Venturis.
If an accidental release occurs,
mitigation technologies can reduce the
consequences. Such measures include
physical barriers, water sprays and fogs,
and foams that will divert, limit, or dis-
perse the released chemical to the
atmosphere. Since S03 reacts exother-
mically with water, water sprays or water-
based foams should be used with caution
when controlling vapors from a spill.
Spills may also be absorbed onto
expanded clay or diatomaceous earth.
Since accidental releases of toxic
materials result not only from deficiencies
of design but also from deficiencies of
operation, safe operation of plants using
S03 requires competent, experienced
managers and staff trained in the proper
way to handle and store SO3.
-------
D. S. Daws, G. B. DeWolf, K. E. Hummel, 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 15: Control of Accidental Releases of Sulfur Trioxide," (Order No. PB 89-
155 055AS; 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/S8-87/0340
*GE«CY
------- |