CHEN 4470 – Process Design Practice

Dr. Mario Richard EdenDepartment of Chemical Engineering

Auburn University

Lecture No. 18 – Process Risk Assessment & Inherently Safe Process Design

March 19, 2013

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

Process Safety and Design

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, the environment, the general public, and it’s the law.

– As process design engineers we are tasked with reducing the risk of operating a chemical manufacturing process to a level acceptable to employees, regulatory authorities, insurance underwriters and the community at large.

– Recent chemical plant disasters underscore the importance of this point in terms of both human and financial losses.

Recent Incidents

T2 Laboratories Inc – Jacksonville, FLDecember 19, 20074 Killed and 13 Wounded in reactor explosion in manufacture of gasoline additive.

BP America Refinery – Texas City, TX March 23, 200515 Killed and 180 Wounded in isomerization unit explosion and fire.

West Pharmaceutical Services – Kinston, NCJanuary 29, 20036 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

• Hazard vs. Risk– HAZARD is a measure of the severity of the

consequences of a catastrophic failure of a given process or system, regardless of the likelihood and without considering safeguards.

– RISK is the combination of both the severity of the worst case consequence and the likelihood of the initiating cause occurring.

– In short, for an EXISTING PROCESS, we have little influence on the HAZARD, but through the application of safeguards, we can reduce the RISK of operating the process.

Process Safety Terminology

Process Hazard Analysis

– Process Hazard Analysis (PHA) is a technique for determining the RISK of operating a process or unit operation.

– PHAs are required by law for process handling threshhold quantities for certain listed Highly Hazardous Chemicals (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 analyze the process.

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 these scenarios?

- What Safeguards might we choose to mitigate the risk?

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.

Pro

cess

Ris

k

InherentRisk

OperatingSafeguards

Level of Acceptable Operating Risk

Layer of Protection Analysis

• LOPA is a quantitative technique for reducing the RISK of a process.

• The theory of LOPA is based on not “putting all your eggs in one basket”.

• The layers mitigate the process RISK as determined by the PHA.

• Each layer reduces the RISK of 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.

LOPA Example

• Failure of Transfer Pump leading to overfill of Process Vessel.

• Potential release of material to the environment requiring reporting or remediation.

• Potential personnel injury due to exposure to material.

• Severity would be based on properties of the material released.

Process Vessel

Liquid In

Liquid Out

LTLAH

Inherently Safe Process Design

– Inherent safety is a concept based on eliminating the causes and/or reducing the consequences of potential process 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 RISK after the fact.

– This technique is based on making inherently safer design choices at a point in the process development where the engineer has the most influence on the final design.

Inherently Safe Process Design

• Definitions– Inherently safe process design can be grouped

into 5 categories

– Each of these inherently safer design choices is applied in 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

Inherently Safe Process Design

• Azeotropic Distillation vs. Pervaporation

AzeotropeColumn

EntrainerVessel

SolventColumn

1

2

3

4

5 6

7

8

Streams:1 Solvent Feed2 Hexane Feed3 Entrained Azeotrope4 Waste Water5 Aqueous Phase6 Organic Phase7 Hexane Recycle8 Recovered Solvent

Inherently Safe Process Design

• Traditional Process– Sample Risk Assessment using What If?

Methodology

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

What If…? Initiating Cause Consequence 1. 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 Entrainer Vessel?

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 overwhelm incinerator leading to possible explosion. Potential personnel injury due to exposure.

Inherently Safe Process Design

• Azeotropic Distillation vs. Pervaporation

AzeotropeColumn

Pervaporation Unit

SolventColumn

1

2

3

4

Streams:1 Solvent Feed2 Azeotrope3 Waste Water4 Solvent Rich Phase5 Water Rich Phase6 Recovered Solvent

5

6

Inherently Safe Process Design

• Inherently Safer Process– When considering the potential upset scenarios

for the process, the benefits of the inherently safer process become clear.

Upset Scenario Traditional Process Inherently Safer Process External Fire Large volume of flammable liquid

circulating in process. Flammable volume limited to recovered solvent only.

Overfill Cyclohexane entrainer more volatile than 1-propanol.

Minimal liquid hold up in Pervaporation Unit.

Overpressure Larger liquid hold-up leads to higher severity in the event of a release.

Volume limited to solvent distillation hold-up.

Inherently Safe Process Design

• Inherently Safer Process (Cont’d)– Based on this risk comparison, it is clear that

multiple independent protection layers would be required to mitigate the operating risk of the traditional process.

– This risk can be reduced by designing an inherently safer, ie, less hazardous process.

– Although a complete economic analysis would be required, this example has illustrated that the need for independent protection layers is reduced in the inherently safer process design.

Summary 1:2

• Conclusions– Clearly, process safety is a critical component of

process design. In industry, no process is put into service without a comprehensive risk assessment.

– It is important to realize that the management of operating risk is the key focus of process safety. As design engineers, we have responsibility for and the most influence on the overall hazard of a process.

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 Highly Hazardous Chemicals, 29 CFR 1910.119, 2005.5. Center for Chemical Process Safety, Layer of Protection Analysis – Simplified

Process Risk 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

Process Safety, AIChE, 1993.8. Seay, J. and M. Eden, “Incorporating Risk Assessment and Inherently Safer

Design Practices into Chemical Engineering Education”, Journal of Chemical Engineering Education, 42(3), pp. 141-146, 2008.

• Next Lecture – March 21– Role of design engineer in technology

development– Bob Kline, Eastman Chemical

– Control strategy development– Jennifer Kline, Eastman Chemical

• Next Lecture – March 26– Property prediction and CAMD

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