Technically Speaking - Dr.
John Nordin, PhD
CHLORINE
SPILLS
People have asked us, are the models predicting release of a toxic gas release and
resulting dispersion any good? Do
the models accurately predict the real world situation? We
will take a look at chlorine as an example. Chlorine
is a highly toxic gas which has many uses. About
40 million tons of chlorine is produced worldwide annually, which includes about 13
million tons produced in the
United
States
and 12 million tons in
Europe
. Uses include water disinfection,
production of plastic products (about 30% of chlorine produced worldwide goes into
the production of PVC-based plastic materials), the pulp in paper industry, manufacture
of pesticides, liquid bleaches, textiles, manufacture of paints, hydrochloric acid,
petroleum products, and various other chemicals.
Chlorine Tank Containing Liquid
Chlorine
Chlorine is typically shipped and
stored as a liquid in a container under pressure. The
maximum-sized container (at least in the
United States
) shipped by rail is capable of holding 90 tons of liquid chlorine. The maximum size
chlorine tank shipped on a barge may have a capacity of up to 1100 tons (more likely,
4 smaller tanks will be shipped on one barge). Tank
cars shipped by motor vehicle may have a capacity up to 22 tons.
Under ambient temperature and not
under pressure, chlorine is a gas. One
pound of this gas at 70oF will occupy 5.45 cubic feet. This
is an uneconomical way of shipping and storing chlorine. But
if the gas is pressurized, it will liquefy. At
a pressure of 86.5 psig (101.1 pounds per square inch absolute) and 70oF,
chlorine will liquefy. One pound
of liquefied chlorine stored in a tank at 70oF and 86.5 psig will 0.0114
cubic feet rather than the 5.45 cubic feet for gaseous chlorine not under pressure. Clearly,
more chlorine can be stored as a liquid under pressure.
Typically, a chlorine tank when
full will contain maybe 85% or 90% by volume as a liquid and the rest as chlorine
vapor plus a pad of dry air or other non-reactive gas. Chlorine
is said to be stored under its own vapor pressure. The
system is self-regulating. If the
tank temperature increases, the pressure inside the tank will also increase. If
the tank temperature decreases, the pressure inside the tank will also decrease. At
0oF, the tank pressure will be about 13.9 psig (or 28.5 psi absolute). But
at 100oF, the tank pressure will rise to 140.2 psig (or 154.8 psi absolute). At
120oF, the tank pressure will rise to 201.65 psi absolute. In
this example, we are assuming that we are located at an elevation where the ambient
air pressure is 14.6 psi.
What happens if the temperature
gets too high? Tanks used to store or
transport chlorine have pressure release devices designed to vent the tank at the
top if the temperature or pressure gets too high. It
is better to vent some chlorine to the atmosphere than to have a catastrophic explosion
in case of a tank failure.
One-ton chlorine tanks in the
United
States
are equipped with fusible metal pressure release devices (six total, three at each
end of the tank) designed to yield or melt at a temperature between 158oF
and 165oF. Railroad tank cars
have a spring-loaded safety release device set to discharge at a gauge pressure of
225 psig (on cars marked 105A300W) or 375 psig (on cars marked 105A500W). Barge
tanks will also have several release devices for each tank, the ones designated 4
QJ are designed to release at 300 psig. Additional
details on safety devices are published in “The Chlorine Manual”, published by The
Chlorine Institute,
Washington
,
D.C.
Another safety feature on large
chlorine tanks is an excess flow valve, which is designed to close automatically if
the angle valve which regulates the discharge of chlorine is broken or sheared off. This
is not an emergency shutoff valve. The
excess flow valve is not activated by pressure or temperature, but is activated if
the discharge of liquid chlorine at the exit port exceeds some predetermined value.
What Happens If Liquid Chlorine
is Spilled?
The
owners of AristaTek while employed by the University of Wyoming Research Corporation
(UWRC), d/b/a/ Western Research Institute, made arrangements with the HazMat Spill
Center (HSC) located at the Nevada Test Site to spill liquid chlorine in a one square
meter pan. Two pans were used. This
work was part of a larger contract funded by the U.S. Department of Energy in 1995. The
pans were located inside a wind tunnel at the site; the wind tunnel allowed a controlled
environment by which measurements and video could be taken without the extraneous
complications of sunlight, wind shifts, and precipitation. A
large fan at the end of the wind tunnel allowed a controlled wind flow across the
pan. There were also turbulence promoters
upwind of the pans set in a predetermined pattern as recommended by several consultants
familiar with wind flow and atmospheric dispersion studies.
We (the people who later founded AristaTek) had available the HSC for a very short
time because of tight scheduling with other clients who also wanted to use the facility;
consequently there was time to do only three spill tests (two using ammonia and one
using chlorine) on April 4-6, 1995. The
major objective of the tests was to see if the pool evaporation rate for spilled cryogenic
hazardous chemicals agreed with the pool model evaporation predictions used by ALOHA. The
ALOHA model is used in EPA’s CAMEO software.
The chlorine was delivered to the
evaporation pan from a one-ton chlorine tank; a vent chamber in line ahead of the
pans allowed removal of chlorine gas so only liquid chlorine was delivered to the
pan. The system was heavily instrumented
(weight sensors, wind speed, temperature, video, etc.), and could be viewed and controlled
from the safety of a control room about 1 mile from the test. As
the pan was filled, solid chlorine hydrate quickly formed despite the low (10%) humidity. Before
the solid chlorine hydrate interfered seriously with the test, we were able to obtain
a chlorine pool evaporation rate of 0.7 kg/min/m2, which compares with
an ALOHA prediction of 0.85 kg/min/m2. The
wind speed for the test condition was 6 m/s. The
measured chlorine temperature was -60oC (-76oF).
As the chlorine evaporated, it extracted
heat from the chlorine liquid left in the pan. Therefore
the pool temperature dropped way below the ambient temperature of 25oC
(77oF) even though the temperature inside the one-ton supply tank was also
at ambient conditions.
However, the liquid chlorine quickly
absorbed water humidity from the air forming the solid chlorine hydrate. The
chlorine hydrate fairly quickly coated the temperature sensors in the pan and the
sides of the pan. As the pan filled with
the solid, the evaporation rate significantly decreased, and the pool temperature
increased. The chlorine hydrate could
not be pumped.
Liquid ammonia evaporation tests
were performed on April 5 and 6, 1995. We
were not troubled by the complication of formation of a solid hydrate during evaporation
of liquid ammonia. In one test, the evaporation
rates were measured at different pool temperatures ranging from -58oC to
-67oC at a wind speed of 5.57 m/s. The
measured evaporation rates agreed with the ALOHA model predictions. The
test was repeated the next day using approximately the same range of pool temperatures
except that the wind speed was 3 m/s. Again
the measured evaporation rate agreed closely with ALOHA model predictions between
0.11 and 0.22 kg/min/m2 between -70o and -60oC.
There were some differences in the
temperature measured within the evaporating pool and the surface. Temperatures
at the evaporating skin (as measured using an optical pyrometer) were several degrees
lower than the temperature of the bulk liquid. The
lowest skin temperature measured was -76oC. It
appeared that convection cells may be setting up within the evaporating pool, where
the ammonia as it evaporated chilled the surface, and the more dense chilled liquid
sank to the bottom of the pan. The sinking
of the surface liquid helped mix the liquid ammonia so the bulk temperatures were
fairly uniform (within about 2oC) in the pool, but the surface temperature
was lower.
The National Oceanic and Atmospheric
Administration (NOAA) out of
Seattle
,
WA
, sent a representative, Roy Overstreet, to observe the tests. NOAA
was responsible for writing the developmental document upon which ALOHA was based[1]. Roy
Overstreet concluded that the evaporation test results matched ALOHA predictions. He
was able to gather the measurements in the control room while the experiments were
underway. We also concluded the same
thing after processing the data gathered from the tests.
Because of the test agreement with
ALOHA predictions, the founders of AristaTek made a decision when the PEAC tool was
developed to use the same evaporation algorithms for pool evaporation that ALOHA used. The
pool evaporation algorithms are available in the public domain in peer-reviewed literature.
But there are complications. In
the case of chlorine, the chilled liquid extracts moisture from the air forming solid
chlorine hydrate, which retards evaporation. The
solid chlorine hydrate was observed in other outdoor chlorine field tests at the HSC. The
solid hydrate also has been seen in real-world chlorine spill accidents.
Another issue when using an evaporation
model is the matter of heat balance. If
there were there significant heat input, as what would happen if the chlorine or other
liquid were spilled on a hot surface, the evaporation rate would be higher than predictions,
at least initially. As the ground becomes
chilled, the evaporation rate will become closer to predictions unless there is an
extraneous circumstance as in a fire.
Modeling a Chlorine Rail Car
Accident.
In this hypothetical example, a
90-ton capacity rail car is involved in an accident, and a strong odor of chlorine
is present. As far as first responders
can tell, there are no large volumes of liquid chlorine coming from the tank, but
the accident scene is obscured by yellowish-greenish gas. Responders
can’t get near to see what exactly is happening. There
is no fire. There are no other rail cars
containing flammables or other hazardous substances which might react with chlorine
causing an explosion. There are
causalities. How should this situation
be modeled to estimate a public protective action zone? And
how do the models compare in their predictions?
In a real incident, responders rarely
have all the necessary information required to do gas dispersion modeling. Reasonable
guesses must be made.
On 6 January 2005, at about 2:30AM,
three 90-ton chlorine rail cars were involved in an accident at a crossing siding
in
Graniteville
,
SC.
The resulting chlorine gas leak killed
nine people and sent over 350 people to a nearby hospital for chlorine inhalation. About
5400 people were evacuated within a one-mile radius. Initially,
responders did not know how many rail car tanks were leaking chlorine. It
was established the next day that chlorine was leaking from only one rail car tank,
and that perhaps possibly 40% of the chlorine still remained in the tank. The
chlorine gas continued to escape from a fist-sized hole in the tank. On
January 9, when a temporary patch was used to plug the hole in the tank, it was estimated
that 30 tons of chlorine remained in the tank and 60 tons had escaped. More
details on the accident are available in an
accident investigation report generated by the National Transportation
Safety Board that can be viewed or downloaded by clicking
here.
There were reports that the ALOHA
gas dispersion model was used to estimate a protection action distance for the Graniteville
incident, and that this model grossly over predicted concentrations as a function
of distance downwind. If the modelers
assumed that all of the chlorine were released at once or during a relatively short
time period, e.g. 60 minutes, the default time in ALOHA, very large concentrations
would be predicted downwind. The
chlorine release rate was much slower in the Graniteville incident but still deadly.
The chlorine inside a 90-ton rail
car would be shipped as a liquid under its own vapor pressure. Probably
about 85% of the volume inside the tank would be liquid and the remaining amount vapor
and some nitrogen. Assuming an ambient
temperature of 50oF, the pressure (gage) inside the tank would be about
60 psi before the breach. If
the hole is at the top of the tank, chlorine gas will be released. The
drop in pressure inside the tank would cause the chlorine liquid to boil resulting
in more chlorine escaping. As the chlorine
boils, the tank will become chilled reducing the evaporation rate. Any
air moisture would result in chlorine hydrate formation, which could further reduce
the evaporation rate. It will take many
days to empty the tank.
On the other hand, if there is a
large hole at the bottom of the tank, the pressure will force chlorine liquid out
the hole. The tank will empty much
sooner. The chlorine liquid on
the ground will also evaporate quickly at least initially, but solid hydrate formation
will reduce the evaporation rate. Maximum
chlorine concentrations in the air will be much greater.
The 2004 Emergency Response Guidebook
lists Initial Isolation Zone and Protective Action Distances for hazardous chemicals
involved in transportation accidents. This
guidebook is published jointly by Transport
Canada
, U.S. Dept. of Transportation, and Secretariat of Transport (
Mexico
), and is available at http://hazmat.dot.gov/pubs/erg/erg2004.pdf.
The user need only consider four
categories for each chemical when looking up the Initial Isolation Zone and Protective
Action Distances in the Emergency Response Guidebook. The
categories are (1) small spills, daytime conditions, (2) large spills, daytime conditions, (3)
small spills, nighttime conditions, and (4) large spills, nighttime conditions. A
breach in a 90-ton railcar is a large spill. The
Emergency Response Guidebook lists the Initial Isolation distance for large spills
as 0.24 km, and the Protective Action Distance as 2.4 km (daytime conditions), or
7.4 km (nighttime conditions).
What is a large spill and what is a small spill? For
most hazardous chemicals, the Emergency Response Guidebook considers anything greater
than 55 or 60 gallon as a large spill. A
90-ton rail car is a large spill, but a breach in a one-ton chlorine tank would also
be considered a large spill. The information
displayed in the Emergency Response Guidebook for chlorine looks like the chart below. The
information can be displayed in English or metric units.
Gas dispersion models ask the user basic information such as (1) the chemical itself,
or at least information about the chemical such as molecular weight and boiling point;
(2) amount of chemical released, (3) information about the terrain, which is usually
expressed in terms of a “surface roughness”, and (4) meteorology. The
user then specifies a location downwind, and the model predicts a concentration at
that location. Alternatively, the
user may specify a concentration representing a “level of concern”, and the model
predicts a distance downwind corresponding to that concentration. Information
requested on meteorology could include (1) wind speed and direction, and (2) atmospheric
stability (time of day, geographical location, date, percent cloud cover, etc.).
What model should the responder
use? Some popular models in the
public domain are ALOHA and SLAB, Chlorine is a dense gas which tends to “hug” the
ground as it travels downwind until either solar heating creates atmospheric instability
or turbulence from wind eventually disperses the gas. Therefore
any model selected should have a “dense gas” component, which both ALOHA and SLAB
have. The model in the PEAC tool
also has a dense gas component.
Let us do a model comparison for
a chlorine release rate of 1 lb/second under daytime and nighttime conditions. We
don’t know the release rate but need to proceed with a reasonable guess.
The ALOHA model is available at no cost from the U.S. Environmental Protection Agency. Version
5.3.1 of the ALOHA model can be downloaded at http://www.epa.gov/ceppo/cameo/aloha.htm. The
PEAC tool is available from AristaTek, at http://www.aristatek.com/. SLAB
is a dense gas model developed by Lawrence Livermore National Laboratories under U.S.
Dept. of Energy contract. SLAB
is available from a number of sources such as http://www.weblakes.com/lakeepa4.html.
The models shown above were compared
at a surface roughness of 0.1 meters (cropland, light residential terrain), and the
wind speed was measured at a 2-meter height.
Usually, the Emergency Response
Planning Guideline Level 2 (ERPG-2) is used as the Level of Concern for public evacuation
distance. The ERPG-2 is also used in
the 2004 Emergency Response Guidebook for their Protective Action Distance. ERPG-2
is established by the American Industrial Hygiene Association and is defined as the
maximum airborne concentration below which it is believed that nearly all individuals
could be exposed for up to one hour without experiencing or developing irreversible
or other serious health effects or symptoms which could impair an individual’s ability
to take protective action. For
chlorine, the ERPG-2 is 3 ppm. For
the daytime condition at 10 mph wind, the corresponding distance ranges from 0.7 to
1 miles (1.1 to 1.6 km) depending upon
what model is used. For nighttime
conditions at 2 mph wind, the distances corresponding to 3 ppm varied from 2 to 7
miles (3.2 to 11.3 km) depending upon the model.
The lethal concentration of chlorine
(LC50) for a one-hour exposure based on rat studies is just under 300 ppm.
Note that the model comparisons
are plotted logarithmically. The level
of concern represents the ground level, plume cloud centerline concentration for different
distances (in miles) downwind. The
models agree fairly close to each other under windy or overcast conditions as represented
by the top graph, but depart from each other under the low-wind, clear nighttime condition. The
first graph represents a neutral, or “D” atmospheric stability; “neutral” meaning
that there is little ground cooling or solar heating resulting in a minimal temperature
gradient in the air. The
second graph represents a “stable” nighttime atmospheric condition where the cooler
air tends to sink to the ground, sometimes referred to as the “F” atmospheric stability. The
toxic gas cloud is not readily dispersed under stable, nighttime conditions, and as
a consequence, the distances matching up with Levels of Concern are much greater.
Why are the models predicting essentially
the same results under “neutral” conditions but depart under the stable, nighttime,
“F” stability condition? Models
in their development require data sets for calibration. The
data sets allow development of algorithms which predict how the chemical cloud will
spread and disperse as it travels downwind. For
example, the ALOHA model uses Briggs’ dispersion algorithms (called “sigmas”) which
were developed from sulfur dioxide releases tests in a Kansas field in the 1960’s. SLAB
used some data sets for chemical releases done at the Nevada Test Site near Mercury,
Nevada
. There are many data sets
developed under “D” or neutral stability conditions; the data sets can easily be done
in a wind tunnel under controlled conditions. But
data sets are few and far between under the stable, nighttime “F” stability condition. Consequently
the models differ depending upon what data sets are used.
The Aristatek founders completed
a series of chemical releases at the HSC located at the Nevada Test Site under a variety
of meteorological conditions ranging from the neutral “D” to the stable “F” stability. The
chemical was released and the air concentrations in the chemical cloud were measured
by a complex array of sensors placed downwind. Various
structures were in the cloud path mimicking conditions which might occur at an industrial
complex such as a refinery. Data
sets taken under neutral or “D” stability conditions were easy to characterize. The
“F” nighttime stability was much more difficult, the data collected was very much
dependent upon micrometeorology. There
can be a “near F” condition where there was enough local turbulence to disperse the
chemical cloud, and a “far F” condition where the chemical cloud remained. Also,
meteorological conditions were not uniform as the cloud moved from the source.
Therefore, it should not come as
a surprise if available models differ under the “F” stability condition. The
discrepancy becomes worse as the wind speed decreases. Available
models cannot handle the “zero” wind condition.
A small chlorine release incident
occurred in
Springfield
,
MA
during June 1988 under calm and overcast nighttime conditions. Based
on odor reports, the chlorine cloud appeared to move in all directions from the site,
bypassing some locations, with no discernable pattern. People
within about ¼ mile from the site were evacuated. A
couple of days later, on June 19, the release was much greater with an accompanying
fire. Under a daytime 8 mph wind
condition, the chemical cloud was described as several miles long and only a few city
blocks wide.
The concentrations graphed are maximum,
centerline concentrations. If a person
moves crosswind from the chemical cloud centerline, the concentrations will usually
rapidly drop off. But if conditions
are calm or a very low wind speed, especially after sunset, conditions are often unpredictable. If
the chemical is a dense gas such as chlorine and there is no fire, the person may
be advised to seek higher ground.
What can we conclude from all this?
-
In real accidents responders rarely
have all the necessary information required to run a gas dispersion model. Reasonable
guesses must be made. One of the biggest
unknowns is the release rate to the atmosphere. Usually
first responders can’t even get close to the site to determine exactly what is happening.
-
Reasonable guesses must be made
as to the release rate and meteorology. If
the responders guess too conservatively (e.g. all of the chemical released at once
or within a short period of time), critics may say that the model is too conservative.
-
The available popular models generally
give similar results under “neutral” atmospheric conditions or under light or moderate
wind conditions. There may be major
differences under low wind, clear, nighttime conditions.
-
The pool evaporation model used
by ALOHA and which is also in the PEAC tool accurately predict the evaporation rate
of liquid chlorine, but there are complications such as chlorine hydrate formation
(which reduces pool evaporation) or excessive heat input to the pool (which increases
the evaporation rate).
[1]
Reynolds, R.M., 1992, ALOHA™ Theoretical Description, Report NOS ORCA-65,
National Oceanic and Atmospheric Administration,
Seattle
,
WA
.
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