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Transcript of 1 Introduction to Inherently Safer Design Prepared for Safety and Chemical Engineering Education...
1
Introduction toInherently 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
Introduction to Inherently Safer Design
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What is inherently safer design?
Inherent - “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”
Introduction to Inherently Safer Design
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Why Inherently Safer Design?Flixborough, UK, 1974
Pasadena, TX, 1989
Bhopal, India, 1984
Introduction to Inherently Safer Design
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A subset of Green Engineering
Green Chemistryand Engineering
InherentlySafer
Design
Introduction to Inherently Safer Design
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History of inherently safer design
Not 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–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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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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To eliminate hazards:
Eliminate the material Change the material Change the conditions of use
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Chemical Process Safety Strategies
Inherent Passive Active Procedural
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Inherent
Eliminate or reduce the hazard by changing the process or materials which are non-hazardous 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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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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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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Procedural
Standard operating procedures, safety rules and standard procedures, emergency response procedures, training
EXAMPLE–Confined space entry procedures
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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
0.001Source: Swain, A.D., Handbook of Human Reliability Analysis, August 1983,
NUREG/CR-1278-F, U.S. Nuclear Regulatory Commission
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Batch Chemical Reactor Example
Hazard 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
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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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Inherent
TI
PI
VENT
REACTANT FEEDS
COOLING
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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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Passive
TI
PI
VENT
PRV
REACTANT FEEDS
COOLING
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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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Active
PAH
VENT
REACTANT FEEDS
COOLING
RUPTURE DISK WITH DISCHARGETO SAFE PLACE
TAH
SAFETY SYSTEMLOGIC ELEMENT
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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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Procedural
PAH
VENT
REACTANT FEEDS
COOLING
RUPTURE DISK WITH DISCHARGETO SAFE PLACE
TAH
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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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Layers of Protection
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Potential Incidents
Layers o
f Pro
tection
Actual Risk
Multiple Layers of Protection
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Degraded Layers of ProtectionPotential Incidents
Layers o
f Pro
tection
Higher Actual Risk
Degraded
Degraded
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“Inherently Safe” Process
No additional layers of protection needed
Probably not possible if you consider ALL potential hazards
But, we can be “Inherently Safer”
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Inherently Safer Process RiskPotential Incidents
Actual Risk
No Layers of P
rotectionN
eeded
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Managing multiple hazards – Process Option No. 1
Hazard 1 - Inherent
Hazard 2 – Passive, Active,
Procedures
Hazard 3 – Passive, Active,
Procedures
… Hazard n – ????
Toxicity Explosion Fire …..
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Hazard 1 - Inherent
Hazard 2 – Passive, Active,
Procedures
Hazard 3 – Passive, Active,
Procedures
… Hazard n – ????
Toxicity Explosion Fire …..Managing multiple hazards – Process Option No. 2
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Inherently Safer Design Strategies
Introduction to Inherently Safer Design
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Inherently Safer Design Strategies
Minimize Moderate Substitute Simplify
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Minimize
Use small quantities of hazardous substances or energy–Storage
– Intermediate storage
–Piping
–Process equipment “Process Intensification”
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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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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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Generic Nitration Reaction
Organic substrate (X-H) + HNO3
Nitrated Product (X-NO2) + H2O
Reaction is highly exothermic Usually 2 liquid phases – an aqueous/acid
phase and an organic/solvent phase
H2SO4
Solvent
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Semi-batch nitration process
Batch Reactor~6000 gallons
Organic Substrate andsolvents pre-charge
Nitric acid gradualaddition
Catalyst (usuallysulfuric acid) feed
or pre-charge
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What controls the rate of this reaction?
Mixing – bringing reactants into contact with each other
Mass transfer – from acid/aqueous phase (nitric acid) to organic phase (organic substrate)
Heat removal
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CSTR Nitration Process
Product
RawMaterialFeeds
Organic substrateCatalystNitric Acid
Reactor ~ 100 gallons
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Can you do this reaction in a tubular reactor?
RawMaterialFeeds
Organic substrateCatalystNitric Acid
Cooled continuousmixer/reactor
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“Semi-Batch” solution polymerization
Large (severalthousand gallons)
batch reactor
SolventAdditivesInitial Monomer "Heel"
Monomer andInitiator graduallyadded to minimize
inventory ofunreacted material
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What controls this reaction
Contacting of monomer reactants and polymerization initiators
Heat removal–Temperature control important for
molecular weight control
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Tubular Reactor
Product Storage Tank
Initiator Static mixer pipe reactor (severalinches diameter, several feet long,
cooling water jacket)
Monomer, solvent, additives
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Substitute
Replace a hazardous material with a less hazardous alternative
Substitute a less hazardous reaction chemistry
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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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Substitute Reaction ChemistryAcrylic Esters
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
RCHCO=CH HCl
)Ni(CO ROH + CO + CHCH 22
4
Reppe Process
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Alternate chemistry
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.
2 3 2 2 2 2CH = CHCH + 3
2O
Catalyst
CH = CHCO H + H O
2 2
+
2 2 2CH = CHCO H + ROH H
CH = CHCO R + H O
Propylene Oxidation Process
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Moderate
Dilution Refrigeration Less severe processing conditions
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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
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Effect of dilution
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Impact of refrigeration
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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
Introduction to Inherently Safer Design
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Simplify
Eliminate unnecessary complexity to reduce risk of human error–QUESTION ALL COMPLEXITY! Is it really
necessary?
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Simplify - eliminate equipment
Reactive distillation methyl acetate process (Eastman Chemical)
Which is simpler?
Reactor
SplitterExtractiveDistillaton
SolventRecovery
MethanolRecovery
Extractor
AzeoColumn
Decanter
FlashColumn
ColorColumn
FlashColumn
Water
Water
Heavies
MethylAcetate
Water
Catalyst
Methanol
Acetic Acid
ReactorColumn
ImpurityRemovalColumns
Water
Heavies
Acetic Acid
Methanol
SulfuricAcid
MethylAcetate
Introduction to Inherently Safer Design
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Modified methyl acetate process
Fewer vessels Fewer pumps Fewer flanges Fewer instruments Fewer valves Less piping ......
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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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Single, complex batch reactor
Condenser
DistillateReceiver
RefrigeratedBrine
LargeRupture
Disk
A
B
C
D
E
Condensate
Water Supply
Steam
Water Return
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A sequence of simpler batch reactors for the same process
A
B
C
D
E
DistillateReceiver
Condenser
Water Supply
Water Return
RefrigeratedBrine
Steam
Condensate
Large RuptureDisk
59
Inherent Safety Considerations through the Process Life Cycle
(Use manufacture of acrylate esters as an example)
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Research
Basic technology –Reppe process
–Propylene oxidation followed by esterification
–Other alternatives propane based Others - ????
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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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Preliminary Plant Design
Plant location–Plant site options
–Plant layout on selected site Consider
–People
–Property
–Environmentally sensitive locations
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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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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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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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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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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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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.
Introduction to Inherently Safer Design
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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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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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Resources
INSIDE Project and INSET Toolkit, Commission of the European Community, 1997 - available for download from:http://www.aeat-safety-and-risk.com/html/inset.html
Extensive journal and conference proceedings literature