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CHEN 4470 Process Design Practice

Dr. Mario Richard EdenDepartment of Chemical EngineeringAuburn University

Lecture No. 16 Process Risk Assessment & Inherently Safe Process Design

March 6, 2012

Material Developed by Dr. Jeffrey R. Seay, University of Kentucky - Paducah

Process Safety and Design

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Importance of Process Safety

The safety record of the chemical process industry is

the responsibility of all of us in the profession.

Process safety is important for employees, theenvironment, the general public, and its the law.

As process design engineers we are tasked withreducing the risk of operating a chemical manufacturingprocess to a level acceptable to employees, regulatoryauthorities, insurance underwriters and the community

at large.

Recent chemical plant disasters underscore theimportance of this point in terms of both human and

financial losses.

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Recent Incidents

T2 Laboratories Inc Jacksonville, FL

December 19, 2007

4 Killed and 13 Wounded in reactor explosion in

manufacture of gasoline additive.

BP America Refinery

Texas City, TXMarch 23, 2005

15 Killed and 180 Wounded in isomerization unit

explosion and fire.

West Pharmaceutical Services

Kinston, NCJanuary 29, 2003

6 Killed and Dozens Wounded in dust cloud explosion

and fire from release of fine plastic powder.

Source: U.S. Chemical Safety Board, www.chemsafety.gov

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Hazard vs. Risk

HAZARD is a measure of the severity of theconsequences of a catastrophic failure of a givenprocess or system, regardless of the likelihood andwithout considering safeguards.

RISK is the combination of both the severity of theworst case consequence and the likelihood of theinitiating cause occurring.

In short, for an EXISTING PROCESS, we have littleinfluence on the HAZARD, but through the applicationof safeguards, we can reduce the RISK of operatingthe process.

Process Safety Terminology

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Process Hazard Analysis

Process Hazard Analysis (PHA) is a technique for

determining the RISK of operating a process or unitoperation.

PHAs are required by law for process handlingthreshhold quantities for certain listed Highly HazardousChemicals (HHC) or flammables.

Approved techniques for conducting PHAs:

HAZOP (Hazard and Operability)

What If? FMEA (Failure Mode and Effects Analysis)

In general, a PHA is conducted as a series of facilitated,team brainstorming sessions to systematically analyzethe process.

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Risk Assessment Example

Consider a low design pressure API storage

tank filled with cyclohexane.

Assume that the storage tank is equipped with

a pad/de-pad vent system to control pressure.

What If? Initiating Cause Consequence Safeguards

1. There is High

Pressure in the

Cyclohexane

Storage Tank?

1.1 Failure of

the pressure

regulator on

nitrogen

supply line.

1.1 Potential for pressure in tank to rise due

to influx of nitrogen through failed

regulator. Potential to exceed design

pressure of storage tank. Potential tank

leak or rupture leading to spill of a

flammable liquid. Potential fire should an

ignition source be present. Potential

personnel injury should exposure occur.

1. Pressure relief vent (PRV)

sized to relieve

overpressure due to this

scenario.

2. Pressure transmitter with

high alarm set to indicate

high pressure in

Cyclohexane Storage Tank.

Cyclohexane

Storage

Tank

PC

N2 Supply Vent Gas

- What hazard scenarios might occur from

this system?

- What are the consequences of thesescenarios?

- What Safeguards might we choose to

mitigate the risk?

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Mitigating Process Risk

The operating risk is determined by the PHA using an

appropriate Risk Assessment Methodology.

This risk is mitigated through the application of

safeguards that reduce the risk to an acceptable level.

ProcessR

is

InherentRisk

OperatingSafeguards

Level of

Acceptable

Operating Risk

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Layer of Protection Analysis

LOPA is a quantitativetechnique for reducing theRISK of a process.

The theory of LOPA is based

on not putting all your eggsin one basket.

The layers mitigate theprocess RISK as determinedby the PHA.

Each layer reduces the RISKof operating the process.

Core Process

1st Layer of

Protection

2nd Layer of

Protection

3rd Layer of

Protection

Each layer must be: Independent;

Effective; Reliable; Auditable.

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LOPA Example

Failure of Transfer Pumpleading to overfill of ProcessVessel.

Potential release of material tothe environment requiringreporting or remediation.

Potential personnel injury dueto exposure to material.

Severity would be based onproperties of the materialreleased.

Process Vessel

Liquid In

Liquid Out

LTLAH

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Inherently Safe Process Design

Inherent safety is a concept based on eliminating the

causes and/or reducing the consequences of potentialprocess upsets.

Inherently Safe Process Design is a technique applied

during the conceptual phase of process design.

Inherently Safe Process Design targets the HAZARD,rather than reducing the RISKafter the fact.

This technique is based on making inherently saferdesign choices at a point in the process developmentwhere the engineer has the most influence on the finaldesign.

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Inherently Safe Process Design

Definitions

Inherently safe process design can be grouped into 5categories

Each of these inherently safer design choices is appliedin the conceptual phase of development.

1 Intensification Continuous reactor vs. batch reactor

2 Substitution Change of feedstock

3 Attenuation Alternate technology

4 Limitation of effects Minimization of storage volume

5 Simplification Gravity flow vs. pumping

Category Example

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Inherently Safe Process Design

Azeotropic Distillation vs. Pervaporation

Azeotrope

Column

Entrainer

Vessel

Solvent

Column

1

2

3

4

5 6

7

8

Streams:

1 Solvent Feed2 Hexane Feed

3 Entrained Azeotrope

4 Waste Water

5 Aqueous Phase

6 Organic Phase

7 Hexane Recycle

8 Recovered Solvent

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Inherently Safe Process Design

Traditional Process

Sample Risk Assessment using What If? Methodology

Consider what types of safeguards would be required tomitigate the Process Risk due to these scenarios.

What If? Initiating Cause Consequence1. There is higher

pressure in the

Entrainment

Vessel?

1.1 External fire in the

process area.

1.1 Potential increased temperature and pressure leading to

possible vessel leak or rupture. Potential release of

flammable material to the atmosphere. Potential personnel

injury due to exposure.

1.2 Pressure regulator for

inert gas pad fails open.

1.2 Potential for vessel pressure to increase up to the inert gas

supply pressure. Potential vessel leak or rupture leading to

release of flammable material to the atmosphere. Potential

personnel injury due to exposure.

2. There is higher

level in the

EntrainerVessel?

2.1 Vessel level transmitter

fails and indicates lower

than actual volume.

2.1 Potential to overfill vessel with cyclohexane. Potential to

flood vent line with liquid leading to flammable liquid

reaching the vent gas incinerator. Potential to overwhelmincinerator leading to possible explosion. Potential

personnel injury due to exposure.

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Inherently Safe Process Design

Azeotropic Distillation vs. Pervaporation

Azeotrope

Column

Pervaporation

Unit

Solvent

Column

1

2

3

4

Streams:

1 Solvent Feed2 Azeotrope

3 Waste Water

4 Solvent Rich Phase

5 Water Rich Phase

6 Recovered Solvent

5

6

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Inherently Safe Process Design

Inherently Safer Process (Contd)

Based on this risk comparison, it is clear that multipleindependent protection layers would be required tomitigate the operating risk of the traditional process.

This risk can be reduced by designing an inherentlysafer, ie, less hazardous process.

Although a complete economic analysis would berequired, this example has illustrated that the need forindependent protection layers is reduced in theinherently safer process design.

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Summary 1:2

Conclusions

Clearly, process safety is a critical component of processdesign. In industry, no process is put into servicewithout a comprehensive risk assessment.

It is important to realize that the management ofoperating risk is the key focus of process safety. Asdesign engineers, we have responsibility for and themost influence on the overall hazard of a process.

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Summary 2:2

References1. R. Sanders, Chemical Process Safety

Learning from Case Histories, 3rd Edition,

Elsevier, Inc, 2005.

2. D. Nelson, Managing Chemical Safety, Government Institutes, 2003.

3. Environmental Protection Agency, Process Hazard Analysis, 40 CFR 68.67, 2005.

4. Occupational Safety and Health Administration, Process Safety Management of HighlyHazardous Chemicals, 29 CFR 1910.119, 2005.

5. Center for Chemical Process Safety, Layer of Protection Analysis Simplified ProcessRisk Assessment, AIChE, 2001.

6. T. Kletz, Process Plants: A Handbook for Inherently Safety Design, Taylor and Francis,1998.

7. Center for Chemical Process Safety, Guidelines for Engineering Design for ProcessSafety, AIChE, 1993.

8. Seay, J. and M. Eden, Incorporating Risk Assessment and Inherently Safer DesignPractices into Chemical Engineering Education, Journal of Chemical EngineeringEducation, 42(3), pp. 141-146, 2008.

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Next Lecture March 20

Role of design engineer in technology development

Bob Kline, Eastman Chemical

Control strategy development

Jennifer Kline, Eastman Chemical

Progress Report 2

Friday March 9?

Remember to turn in the team evaluation forms

Next Lecture March 22

Integration of design and control part I

SSLW 322-340

Other Business

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Zo Elizabeth Kline

Born Thursday, March 1, 10:03 AM

8 lb, 13 oz; 21 inches

Other Business

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Individual Team Assignments

Should be assigned this week

Choose one from the project description or

Email me two sentences describing what you would liketo investigate and I will respond with the officialproblem statement.

Other Business