Chemical Engineering January 2022 - 36

ment of the system requires
knowing the reactor volume,
the heat capacity of the reaction
mix, the heat of reaction
and the reaction rate. If the
last three pieces of information
are unknown, the data
can be obtained via reaction
calorimetry experiments.
❍
High-pressure or high-temperature
operations. These
are innately more hazardous
than lower-pressure and lower-temperature
operations.
High pressures increase the
odds of having an equipment
or piping leak. When a process
stream is within or near
its flammability range, or near
or above its flash or auto-ignition
temperature, leaks may
result in a fire or explosion.
The same is true of leaks
of flammable heat-transfer
fluids. A special category of
high-temperature operations
is furnaces. By their nature,
furnaces are risky. If a furnace
is used to heat a process or a
heat transfer fluid that is flammable,
tube leaks could result
in a fire or explosion. Also,
high-temperature operation
weakens the strength of the
tube materials increasing the
probability of leaks.
❍
Vacuum operation. While
vacuum operations are usually
safer than high-pressure
operations, air leaks into
a process stream that will
oxidize or that is combustible
could cause
an incident.
❍
Hazardous
material
inventories.
The issue
here is the
possibility
of an accidental
release.
The
larger the
inventory of
hazardous
materials,
the higher
the chance
TABLE 4. HARDWARE RELATED TO INCIDENTS FOR THE REFINERY
AND PETROCHEMICAL INDUSTRIES, 1974-2019
Hardware
Piping, including valves
Towers
Reactors
Storage tanks
Unknown
Instruments
Heat exchangers
Pumps
Compressors
Other
of having a catastrophic
incident. Inventory was a
major contributor to the large
number of deaths in Bhopal.
Inventories include both
in-storage and in-process
material. When considering
in-process inventory, keep in
mind that batch reactors generally
have higher volumes
than continuous reactors,
standard distillation columns
arranged in sequence contain
more liquid than a dividedwall
column does, and plate
towers hold more liquid than
packed towers do.
Step 2: Estimate loss potential.
Once the hazards in the process
have been identified, engineers
should assess the potential for loss
for each hazard. Because there
are few design details at this point,
these assessments will be " orderof-magnitude "
quality. As such, they
TABLE 3. THE NFPA RATING SYSTEM
Adapted from Figure 3-8 in Crowl and Louvar, Chemical Process Safety, 4th Ed. [5]
4
3
Health rating
Can be lethal
Can cause
permanent or
serious injury
Flammability
rating
Instability
rating
Special hazards
OX: Oxidizer
36
Rapidly
vaporizes and
burns at room
temperature
May detonate
Ignites easily at
ambient
temperatures
Shock and heat
may detonate
W: Violent reaction with water
SA: Simple asphyxiant
2
Can cause
temporary incapacitation
or
residual injury
Ignites when
moderately heated
Violent
chemical change
at elevated
temperatures
1
Can cause
significant
irritation
Must be
preheated to
ignite
Unstable if
heated
Not
hazardous
Will not
burn
Stable
% of Incidents
33
14
12
7
4
3
3
3
2
19
Source data: Marsh, The 100 Largest Losses in the Hydrocarbon Industry 1974-2019,
www.marsh.com/us/insights/research/100-largest-losses-in-the-hydrocarbon-industry.html
are only useful during synthesis.
They do not take the place of the
exacting safety studies done later
during detailed design.
Referring back to Table 1, note
that toxic releases and explosions
have the greatest potential for fatalities
and economic loss. To estimate
each hazard's potential for loss,
place each into one of the following
broad loss categories:
* Catastrophic - Those resulting
in deaths or permanent disabilities,
major community damage, or major
plant damage
* Major - Those resulting in hospitalizations,
significant
community
damage, or significant plant damage
* Moderate - Those resulting in
injuries needing medical treatment,
minor community damage, or moderate
plant damage
* Minor - Those resulting in minor
injuries, no community damage, or
minor plant damage
When assessing the loss potential
of materials, one can use NFPA
or HMIS ratings to define the severity
of health, flammability or instability
hazards. If these are not available,
use the Materials Safety Data
Sheets (MSDS) for each substance
or PubChem's Compound Summary
database [4]. If a material has an
NFPA or HMIS rating of 4, assign its
potential for loss as Catastrophic. A
3-rating should be assigned the loss
category of Major, and so on. To illustrate,
MIC, the toxic intermediate
in the Bhopal Sevin process, has an
NFPA health rating of 4. Thus, its loss
potential would be Catastrophic. Its
release was clearly that.
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Chemical Engineering January 2022

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