Introduction to
Inherently Safer Design
Prepared for Safety and Chemical
Engineering Education (SACHE) by:
Dennis C. Hendershot
Rohm and Haas Company, retired
©American
Institute of Chemical Engineers, 2006
1
Introduction to Inherently Safer Design
What is inherently safer
design?
- “existing in something as a
permanent and inseparable element...”
Eliminate or minimize hazards rather
than control hazards
 Safety based on physical and chemical
properties of the system, not “add-on”
safety devices and systems
 “Safer” – not “Safe”
 Inherent
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Introduction to Inherently Safer Design
Why Inherently Safer Design?
Flixborough, UK, 1974
Bhopal, India,
1984
3
Pasadena, TX, 1989
Introduction to Inherently Safer Design
A subset of Green Engineering
Inherently
Safer
Design
Green Chemistry
and Engineering
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Introduction to Inherently Safer Design
History of inherently safer
design
really a new concept – elimination of
hazards has a long history
 Second half of 20th Century chemical
industry – increased hazards from huge,
world scale petrochemical plants
 Not
–Concern about cost and reliability of
traditional “add on” safety systems
–Trevor Kletz – ICI (1977) – Is there a better
way?
 Eliminate
or dramatically reduce hazards
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Introduction to Inherently Safer Design
Hazard
An inherent physical or chemical
characteristic that has the potential for
causing harm to people, the environment, or
property (CCPS, 1992).
 Hazards are intrinsic to a material, or its
conditions of use.
 Examples

– Phosgene - toxic by inhalation
– Acetone - flammable
– High pressure steam - potential energy due to
pressure, high temperature
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Introduction to Inherently Safer Design
To eliminate hazards:
 Eliminate
the material
 Change the material
 Change the conditions of use
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Introduction to Inherently Safer Design
Chemical Process Safety
Strategies
 Inherent
 Passive
 Active
 Procedural
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Introduction to Inherently Safer Design
Inherent
Eliminate or reduce the hazard by changing
the process or materials which are nonhazardous or less hazardous
 Integral to the product, process, or plant cannot be easily defeated or changed without
fundamentally altering the process or plant
design
 EXAMPLE

– Substituting water for a flammable solvent (latex
paints compared to oil base paints)
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Introduction to Inherently Safer Design
Passive
 Minimize
hazard using process or
equipment design features which reduce
frequency or consequence without the
active functioning of any device
 EXAMPLE
–Containment dike around a hazardous
material storage tank
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Introduction to Inherently Safer Design
Active
Controls, safety interlocks, automatic shut
down systems
 Multiple active elements

– Sensor - detect hazardous condition
– Logic device - decide what to do
– Control element - implement action
Prevent incidents, or mitigate the
consequences of incidents
 EXAMPLES

– High level alarm in a tank shuts automatic feed
valve
– A sprinkler system which extinguishes a fire
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Introduction to Inherently Safer Design
Procedural
 Standard
operating procedures, safety
rules and standard procedures,
emergency response procedures,
training
 EXAMPLE
–Confined space entry procedures
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Introduction to Inherently Safer Design
Human Reliability

Available Response
Time (minutes)
1
10
20
30
60

Probability of
incorrect diagnosis –
single control room
event
~1.0
0.5
0.1
0.01
0.001
Source: Swain, A.D., Handbook of Human Reliability Analysis, August 1983,
NUREG/CR-1278-F, U.S. Nuclear Regulatory Commission
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Introduction to Inherently Safer Design
Batch Chemical Reactor
Example
of concern – runaway reaction
causing high temperature and pressure
and potential reactor rupture
 Example – Morton
International,
Paterson, NJ
runaway reaction in
1998, injured 9
people
 Hazard
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Introduction to Inherently Safer Design
Inherent
 Develop
chemistry which is not
exothermic, or mildly exothermic
–Maximum adiabatic reactor temperature
< boiling point of all ingredients and onset
temperature of any decomposition or other
reactions, and no gaseous products are
generated by the reaction
–The reaction does not generate any
pressure, either from confined gas products
or from boiling of the reactor contents
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Introduction to Inherently Safer Design
Inherent
VENT
REACTANT FEEDS
PI
COOLING
TI
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Introduction to Inherently Safer Design
Passive
 Maximum
adiabatic pressure for
reaction determined to be 150 psig
–From vapor pressure of reactor contents or
generation of gaseous products
 Run
reaction in a 250 psig design
reactor
 Hazard (pressure) still exists, but
passively contained by the pressure
vessel
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Introduction to Inherently Safer Design
Passive
VENT
REACTANT FEEDS
PRV
PI
TI
COOLING
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Introduction to Inherently Safer Design
Active
Maximum adiabatic pressure for 100%
reaction is 150 psig, reactor design pressure
is 50 psig
 Gradually add limiting reactant with
temperature control to limit potential energy
from reaction
 Use high temperature and pressure interlocks
to stop feed and apply emergency cooling
 Provide emergency relief system

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Introduction to Inherently Safer Design
Active
VENT
RUPTURE DISK WITH DISCHARGE
TO SAFE PLACE
REACTANT FEEDS
PA
H
SAFETY SYSTEM
LOGIC ELEMENT
TA
H
COOLING
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Introduction to Inherently Safer Design
Procedural
 Maximum
adiabatic pressure for 100%
reaction is 150 psig, reactor design
pressure is 50 psig
 Gradually add limiting reactant with
temperature control to limit potential
energy from reaction
 Train operator to observe temperature,
stop feeds and apply cooling if
temperature exceeds critical operating
limit
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Introduction to Inherently Safer Design
Procedural
VENT
RUPTURE DISK WITH DISCHARGE
TO SAFE PLACE
REACTANT FEEDS
PA
H
TA
H
COOLING
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Introduction to Inherently Safer Design
Which strategy should we
use?
 Generally,
in order of robustness and
reliability:
–Inherent
–Passive
–Active
–Procedural
 But
- there is a place and need for ALL
of these strategies in a complete safety
program
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Introduction to Inherently Safer Design
Layers of Protection
COMMUNITY EMERGENCY REPSONSE
PLANT EMERGENCY REPSONSE
PHYSICAL PROTECTION (DIKES)
PHYSICAL PROTECTION (RELIEF DEVICES)
AUTOMATIC ACTION SIS OR ESD
CRITICAL ALARMS, OPERATOR
SUPERVISION, AND M ANUAL INTERVENTION
BASIC CONTROLS, PROCESS ALARMS,
AND OPERATOR SUPERVISION
PROCESS
DESIGN
I
LAH
1
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Introduction to Inherently Safer Design
Multiple Layers of Protection
Potential Incidents
Layers of Protection
Actual Risk
25
Introduction to Inherently Safer Design
Degraded Layers of Protection
Potential Incidents
Degraded
Higher Actual Risk
Layers of Protection
Degraded
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Introduction to Inherently Safer Design
“Inherently Safe” Process
 No
additional layers of protection
needed
 Probably not possible if you consider
ALL potential hazards
 But, we can be “Inherently Safer”
PROCESS
DESIGN
I
LAH
1
27
Introduction to Inherently Safer Design
Inherently Safer Process Risk
Potential Incidents
No Layers of Protection
Needed
Actual Risk
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Introduction to Inherently Safer Design
Managing multiple hazards – Process Option No. 1
Toxicity
Explosion
Fire
…..
COMMUNITY EMERGENCY REPSONSE
PLANT EMERGENCY REPSONSE
PHYSICAL PROTECTION (DIKES)
PHYSICAL PROTECTION (RELIEF DEVICES)
AUTOMATIC ACTION SIS OR ESD
PROCESS
DESIGN
I
CRITICAL ALARMS, OPERATOR
SUPERVISION, AND MANUAL INTERVENTION
BASIC CONTROLS, PROCESS ALARMS,
AND OPERATOR SUPERVISION
LAH
1
PROCESS
DESIGN
AUTOMATIC ACTION SIS OR ESD
CRITICAL ALARMS, OPERATOR
SUPERVISION, AND MANUAL INTERVENTION
BASIC CONTROLS, PROCESS ALARMS,
AND OPERATOR SUPERVISION
PROCESS
DESIGN
I
I
LAH
1
Hazard 1 Inherent
Hazard 2 –
Passive,
Active,
Procedures
LAH
1
Hazard 3 – …
Passive,
Active,
Procedures
Hazard n –
????
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Introduction to Inherently Safer Design
Managing multiple hazards – Process Option No. 2
Toxicity
Explosion
Fire
…..
COMMUNITY EMERGENCY REPSONSE
PLANT EMERGENCY REPSONSE
PHYSICAL PROTECTION (DIKES)
AUTOMATIC ACTION SIS OR ESD
CRITICAL ALARMS, OPERATOR
SUPERVISION, AND MANUAL INTERVENTION
BASIC CONTROLS, PROCESS ALARMS,
AND OPERATOR SUPERVISION
PHYSICAL PROTECTION (RELIEF DEVICES)
AUTOMATIC ACTION SIS OR ESD
CRITICAL ALARMS, OPERATOR
SUPERVISION, AND MANUAL INTERVENTION
BASIC CONTROLS, PROCESS ALARMS,
AND OPERATOR SUPERVISION
PROCESS
DESIGN
I
I
LAH
1
Hazard 3 –
Passive,
Active,
Procedures
PROCESS
DESIGN
I
PROCESS
DESIGN
LAH
1
LAH
1
Hazard 2 –
Passive,
Active,
Procedures
…
Hazard 1 Inherent
Hazard n –
????
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Inherently Safer Design
Strategies
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Introduction to Inherently Safer Design
Inherently Safer Design Strategies
 Minimize
 Moderate
 Substitute
 Simplify
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Introduction to Inherently Safer Design
Minimize
 Use
small quantities of hazardous
substances or energy
–Storage
–Intermediate storage
–Piping
–Process equipment
 “Process
Intensification”
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Introduction to Inherently Safer Design
Benefits
 Reduced
consequence of incident
(explosion, fire, toxic material release)
 Improved effectiveness and feasibility of
other protective systems – for example:
–Secondary containment
–Reactor dump or quench systems
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Introduction to Inherently Safer Design
Opportunities for process
intensification in reactors
 Understand
what controls chemical
reaction to design equipment to
optimize the reaction
–Heat removal
–Mass transfer
 Mixing
 Between
phases/across surfaces
–Chemical equilibrium
–Molecular processes
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Introduction to Inherently Safer Design
Generic Nitration Reaction
Organic substrate (X-H) + HNO3
H2SO4
Solvent
Nitrated Product (X-NO2) + H2O
 Reaction
is highly exothermic
 Usually 2 liquid phases – an
aqueous/acid phase and an
organic/solvent phase
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Introduction to Inherently Safer Design
Semi-batch nitration process
Catalyst (usually
sulfuric acid) feed
or pre-charge
Nitric acid gradual
addition
Organic Substrate and
solvents pre-charge
Batch Reactor
~6000 gallons
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Introduction to Inherently Safer Design
What controls the rate of this
reaction?
– bringing reactants into contact
with each other
 Mass transfer – from acid/aqueous
phase (nitric acid) to organic phase
(organic substrate)
 Heat removal
 Mixing
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Introduction to Inherently Safer Design
CSTR Nitration Process
Raw
Material
Feeds
Organic substrate
Catalyst
Nitric Acid
Reactor ~ 100 gallons
Product
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Introduction to Inherently Safer Design
Can you do this reaction in a
tubular reactor?
Raw
Cooled continuous
Material
mixer/reactor
Feeds
Organic substrate
Catalyst
Nitric Acid
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Introduction to Inherently Safer Design
“Semi-Batch” solution
polymerization
Solvent
Additives
Initial Monomer "Heel"
Large (several
thousand gallons)
batch reactor
Monomer and
Initiator gradually
added to minimize
inventory of
unreacted material
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Introduction to Inherently Safer Design
What controls this reaction
 Contacting
of monomer reactants and
polymerization initiators
 Heat removal
–Temperature control important for
molecular weight control
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Introduction to Inherently Safer Design
Tubular Reactor
Initiator
Static mixer pipe reactor (several
inches diameter, several feet long,
cooling water jacket)
Monomer, solvent, additives
Product Storage Tank
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Introduction to Inherently Safer Design
Substitute
 Replace
a hazardous material with a less
hazardous alternative
 Substitute a less hazardous reaction
chemistry
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Introduction to Inherently Safer Design
Substitute materials
 Water
based coatings and paints in
place of solvent based alternatives
–Reduce fire hazard
–Less toxic
–Less odor
–More environmentally friendly
–Reduce hazards for end user and also for
the manufacturer
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Introduction to Inherently Safer Design
Substitute Reaction Chemistry
Acrylic Esters

Reppe Process
Ni(CO )4
CH  CH + CO + ROH
 CH 2 = CHCO2 R
HCl
Acetylene - flammable, reactive
 Carbon monoxide - toxic, flammable
 Nickel carbonyl - toxic, environmental hazard
(heavy metals), carcinogenic
 Anhydrous HCl - toxic, corrosive
 Product - a monomer with reactivity
(polymerization) hazards

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Introduction to Inherently Safer Design
Alternate chemistry
Propylene Oxidation Process
3
Catalyst
 CH 2 = CHCO2 H + H 2 O
CH 2 = CHCH 3 + O2
2
CH 2 = CHCO2 H + ROH
H+
 CH 2 = CHCO2 R + H 2 O
Inherently safe?
 No, but inherently safer. Hazards are primarily
flammability, corrosivity from sulfuric acid
catalyst for the esterification step, small
amounts of acrolein as a transient
intermediate in the oxidation step, reactivity
hazard for the monomer product.
47

Introduction to Inherently Safer Design
Moderate
 Dilution
 Refrigeration
 Less
severe processing conditions
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Introduction to Inherently Safer Design
Dilution
 Aqueous
ammonia instead of anhydrous
 Aqueous HCl in place of anhydrous HCl
 Sulfuric acid in place of oleum
 Wet benzoyl peroxide in place of dry
 Dynamite instead of nitroglycerine
49
Ce
Conc
28%
Aqueous
Ammonia
Introduction to Inherently Safer Design
0
0
Distance, Miles
5
Effect of dilution
Centerline Ammonia
Concentration, mole ppm
20,000
(B) - Release Scenario:
2 inch transfer pipe failure
10,000
Anhydrous
Ammonia
28%
Aqueous
Ammonia
0
0
Distance, Miles
1
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Introduction to Inherently Safer Design
Impact of refrigeration
Monomethylamine
Storage
Temperature
(°C)
10
3
-6
Distance to
ERPG-3 (500 ppm)
Concentration,
km
1.9
1.1
0.6
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Introduction to Inherently Safer Design
Less severe processing
conditions

Ammonia manufacture
– 1930s - pressures up to 600 bar
– 1950s - typically 300-350 bar
– 1980s - plants operating at pressures of 100-150
bar were being built
Result of understanding and improving the
process
 Lower pressure plants are cheaper, more
efficient, as well as safer

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Introduction to Inherently Safer Design
Simplify
 Eliminate
unnecessary complexity to
reduce risk of human error
–QUESTION ALL COMPLEXITY! Is it really
necessary?
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Introduction to Inherently Safer Design
Simplify - eliminate equipment
 Reactive
distillation methyl acetate
process (Eastman Chemical)
 Which is simpler?
Acetic Acid
Methyl
Acetate
Methanol
Catalyst
Methyl
Acetate
Acetic Acid
Methanol
Recovery
Reactor
Solvent
Recovery
Splitter
Sulfuric
Acid
Methanol
Extractive
Distillaton
Water
Reactor
Column
Impurity
Removal
Columns
Decanter
Extractor
Heavies
Color
Column
Azeo
Column
Flash
Column
Water
Heavies
54
Flash
Column
Water
Water
Introduction to Inherently Safer Design
Modified methyl acetate
process
 Fewer
vessels
 Fewer pumps
 Fewer flanges
 Fewer instruments
 Fewer valves
 Less piping
 ......
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Introduction to Inherently Safer Design
But, it isn’t simpler in every
way
 Reactive
distillation column itself is
more complex
 Multiple unit operations occur within
one vessel
 More complex to design
 More difficult to control and operate
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Introduction to Inherently Safer Design
Single, complex batch reactor
Large
Rupture
Disk
A
B
C
Condenser
D
E
Distillate
Receiver
Steam
Refrigerated
Brine
Water Return
Water Supply
57
Condensate
Introduction to Inherently Safer Design
A sequence of simpler batch
reactors for the same process
A
B
C
Large Rupture
Disk
Refrigerated
Brine
D
Water Return
Water Supply
Condenser
E
Distillate
Receiver
Steam
Condensate
58
Inherent Safety Considerations
through the Process Life Cycle
(Use manufacture of acrylate
esters as an example)
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Introduction to Inherently Safer Design
Research
 Basic
technology
–Reppe process
–Propylene oxidation followed by
esterification
–Other alternatives
 propane
based
 Others - ????
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Introduction to Inherently Safer Design
Process Development

Implementation of selected technology
– Oxidation catalyst options





Temperature
Pressure
Selectivity
Impurities
Catalyst hazards
– Esterification catalyst options


Sulfuric acid
Ion exchange resins or other immobilized acid
functionality catalysts
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Introduction to Inherently Safer Design
Preliminary Plant Design
 Plant
location
–Plant site options
–Plant layout on selected site
 Consider
–People
–Property
–Environmentally sensitive locations
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Introduction to Inherently Safer Design
Detailed Plant Design
 Equipment
size
 Inventory of raw materials
 Inventory of process intermediates
 One large train vs. multiple smaller
trains
 Specific equipment location
…
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Introduction to Inherently Safer Design
Detailed Equipment Design
 Inventory
of hazardous material in each
equipment item
 Heat transfer media (temperature,
pressure, fluid)
 Pipe size, length, construction (flanged,
welded, screwed pipe)
 ……
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Introduction to Inherently Safer Design
Operation
 “User
friendly” operating procedures
 Management of change
–Consider inherently safer options when
making modifications
–Identify opportunities for improving
inherent safety based on operating
experience, improvements in technology
and knowledge
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Introduction to Inherently Safer Design
When to consider Inherent
Safety?
 Start
early in process research and
development
 NEVER STOP looking for inherently
safer design and operating
improvements
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Introduction to Inherently Safer Design
Questions designers should ask
when they have identified a hazard
Ask, in this order:
1. Can I eliminate this hazard?
2. If not, can I reduce the magnitude of the hazard?
3. Do the alternatives identified in questions 1 and 2
increase the magnitude of any other hazards, or
create new hazards?
(If so, consider all hazards in selecting the best alternative.)
4.
At this point, what technical and management
systems are required to manage the hazards which
inevitably will remain?
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Introduction to Inherently Safer Design
Inherently Safer Design and
Regulations

Contra Costa County, CA Industrial Safety Ordinance
(1999)
– Requires evaluation of inherently safer technologies
– Reviewed by enforcement agencies
– Allows consideration of feasibility and economics

New Jersey Department of the Environment (2005)
– Facilities covered by the New Jersey Toxic Catastrophe
Prevention Act (TCPA) must review the practicality of
adopting inherently safer technology as an approach to
reducing the potential impact of a terrorist attack

United States Federal requirements
– Several “chemical security” bills which include requirements
for consideration of inherently safer design have been
introduced in Congress, but, as of June 2006 none of these
have been enacted.
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Introduction to Inherently Safer Design
Resources
 Kletz,
T. A., Process Plants - A
Handbook for Inherently Safer Design,
Taylor and Francis, London, 1998.
 Inherently Safer Chemical Processes - A
Life Cycle Approach, American Institute
of Chemical Engineers, New York, 1996.
– Note: A second edition is being written in 2006.
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Introduction to Inherently Safer Design
Resources
 Guidelines
for Engineering Design for
Process Safety, Chapter 2 “Inherently
Safer Plants.” American Institute of
Chemical Engineers, New York, 1993.
 Guidelines
for Design Solutions for Process
Equipment Failures, American Institute of
Chemical Engineers, New York, 1998.
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Introduction to Inherently Safer Design
Resources
 INSIDE
Project and INSET Toolkit,
Commission of the European
Community, 1997 - available for
download from:
http://www.aeat-safety-andrisk.com/html/inset.html
 Extensive
journal and conference
proceedings literature
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