Chemical Engineering January 2022 - 38

with loss potential, it is impossible to
make detailed estimates, so orderof-magnitude
quality has to suffice at
this point. Assign each hazard to one
of four levels of probability, which are
the following:
* Very Likely - occurring every 0-10
years
* Probable - occurring every 10-30
years
* Possible- occurring every 30-100
years
* Unlikely - occurring once every
100 years or more
As previously
mentioned,
larger
losses occur less often; smaller
losses are much more frequent. The
synthesis engineer, when assigning
one of the four probability levels to
the hazards, should consult with experienced
process safety experts and
refer to company incident history.
Step 4: Identifying major risks. Risk
is the combination of loss potential
and probability. When assessing risk
during synthesis, use the Risk Rating
Matrix (Table 5). First, enter the loss
potential and probability estimates
into the figure. If a material or other
hazard falls into one of the red cells,
it is important to work to eliminate, or
greatly reduce, risk by applying ISD.
Hazards falling into the yellow cells
are borderline candidates for ISD.
Step 5: Consider inherently safer
designs. The activities of hazard and
risk identification lead directly to the
question of what to do about highrisk
hazards. In general, there are
two options: use ISD principles or
add safeguards to the process. Of
the two, ISD is generally preferred
because it focuses on the elimination
or lessening of the hazard. It
also delivers simpler, more straightforward
designs. Kletz and Amyotte
state, " Traditional plant designs try to
reduce the risk by adding protective
equipment and following safe methods
of working. Inherently safer and
friendlier plants remove or reduce the
hazards " [6]. Protective equipment
and safe practices will eventually result
in an incident as equipment fails
or people make mistakes.
Three pillars of ISD apply to process
synthesis: substitution, minimization
and attenuation. These pillars
involve elimination of hazardous materials,
use of much smaller quanti38
ties
of them, and use of less hazardous
operating conditions.
Substitution. This focuses on substituting
non-hazardous or lesshazardous
materials for hazardous
ones. Consider the following:
* Using a reaction chain that doesn't
use hazardous materials (raw
materials or catalysts) or that
doesn't produce hazardous intermediates
or byproducts
* Pretreating the feed to eliminate
impurities that react to form hazardous
materials
* Using materials with higher flashpoints,
boiling points and other
properties that are well away
from the operating conditions of
the process
* Using different solvents, heattransfer
fluids or refrigerants to
eliminate those that may be toxic
or flammable
Minimization (intensification). The
focus here is to minimize the amount
of hazardous material in the system.
Consider the following:
* Continuous or semi-batch systems,
which usually hold smaller
volumes of material compared
to batch systems
* Changing reactor conditions -
catalyst, temperature, pressure
- to increase the reaction rate
and make the reactor smaller
* Using a column sequence that
minimizes the amount of hazardous
material held in the process
equipment
* Using
having
less
holdup. A few examples are:
divided-wall distillation columns,
packed versus tray columns,
thermosiphon versus kettle reboilers,
plate versus shell-andtube
exchangers, and tubular
versus pot reactors
Attenuation (moderation). Attenuation
stresses the use of less hazardous
operating conditions when
handling hazardous materials and
situations. Consider the following:
* Processing flammable materials
well below their flash points and
boiling points
* Handling explosive dusts as slurries
* Reducing the risks of a runaway
reaction
* Using a smaller reactor (for example,
a continuous versus batch
reactor)
* Controlling the reaction rate via the
order of chemical addition or by
catalyst choices
* Adding inert materials to flammable
mixtures
* Reducing process operating temperatures
to eliminate the need
for furnaces
Example
The following discussion about Bhopal
and MIC illustrates substitution
and minimization.
With an NFPA health rating of 4,
MIC is very hazardous. As such, it
would be placed in one of the red
cells of the Risk Rating Matrix (Table
5). Thus, the reaction that produces
it would be a prime candidate for
ISD. As understood today, substitution
is possible, because there are
two accepted routes for making carbaryl
(Sevin) [7]:
* The MIC route (used in Bhopal):
1. Methylamine + Phosgene ➔
MIC + 2 HCl
2. MIC + α-Naphthol ➔ Carbaryl
* The non-MIC route. This route uses
the same feedstocks, but reacts
them in a different order.
1. α-Naphthol + Phosgene ➔
Naphthol chloroformate + HCl
2. Naphthol chloroformate + Methylamine
➔ Carbaryl + HCl
Had the non-MIC route been used
at the Bhopal, India facility, there
would have been no incident - no
deaths and no injuries. Even safer
routes might be possible if something
less hazardous could be substituted
for phosgene, which has an
NFPA Health rating of 4 [8].
There are also minimization options.
Had the amount of MIC stored
been small, far fewer people would
have died or been injured. Recall,
over 40 tons of MIC were released.
It was this large release that caused
the high number of deaths and injuries.
Crowl and Louvar refer to a redesigned
process that could reduce
the MIC inventory to less than 20
lb [9]. In addition, the National Research
Council in their report " The
Use and Storage of Methyl Isocyanate
(MIC) at Bayer CropScience, "
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