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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

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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”

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Why Inherently Safer Design?Flixborough, UK, 1974

Pasadena, TX, 1989

Bhopal, India, 1984

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A subset of Green Engineering

Green Chemistryand Engineering

InherentlySafer

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

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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

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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

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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

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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.

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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