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Journal of Loss Prevention in the Process Industries 20 (2007) 7990

The integration of Dows fire and explosion index (F&EI) into process

design and optimization to achieve inherently safer design

Jaffee Suardina, M. Sam Mannana,, Mahmoud El-Halwagib

aMary Kay OConnor Process Safety Center, Artie McFerrin Department of Chemical Engineering, Texas A&M University,

College Station, TX 77843-3122, USAbArtie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122, USA

Received 14 March 2006; received in revised form 27 September 2006; accepted 16 October 2006

Abstract

For the processing industries, it is critically to have an economically optimum and inherently safer design and operation. The basic

concept is to achieve the best design based on technical and business performance criteria while performing within acceptable safety

levels. Commonly, safety is examined and incorporated typically as an after-thought to design. Therefore, systematic and structured

procedure for integrating safety into process design and optimization that is compatible with currently available optimization and safety

analysis methodology must be available.

The objective of this paper is to develop a systematic procedure for the incorporation of safety into the conceptual design and

optimization stage. We propose the inclusion of the Dow fire and explosion index (F&EI) as the safety metric in the design and

optimization framework by incorporating F&EI within the design and optimization framework. We first develop the F&EI computer

program to calculate the F&EI value and to generate the mathematical expression of F&EI as a function of material inventory and

operating pressure. The proposed procedure is applied to a case study involving reaction and separation. Then, the design and

optimization of the system are compared for the cases with and without safety as the optimization constraint. The final result is the

optimum economic and inherently safer design for the reactor and distillation column system.r 2006 Published by Elsevier Ltd.

Keyword: Fire and explosion index; Inherently safer design; Process safety; Process design and optimization

1. Introduction

Adapted from the Center for Chemical Process Safety

(CCPS), hazard is defined as physical or chemical

characteristic that has the potential for causing harm to

people, the environment, or property (Crowl, 1996). It is

very important to note that the hazards are intrinsic andare the basic properties of the material or its conditions of

use. For example, at a certain condition and concentration,

10,000 lb of propane holds the same amount of energy

which could be released by 28 tons of TNT. Those energies

are inherent to the propane, cannot be changed, and will be

released when equipment or other failure happens and

leads to an incident.

While an inherently safe plant infers a plant that has

no hazards on an absolute basis, such plant with zero

risk might be impossible to design and to operate.

Therefore, the need to manage hazards and risks strategi-

cally and systematically arises and one of the strategies is

the inherently safer design concept (as opposed to

inherently safe plant). In addition, the best strategy seeksto combine inherently safer design with process design and

optimization at the early stages of design where the degree

of freedom for modification is still high.

Mansfield and Cassidy (1994) presented an inherently

safer approach to plant design and general theory on how

it can be built into the design process. Palaniappan,

Srinivasan, and Tan (2004) applied inherently safety index

for identifying hazards and generating alternative designs.

Similar to the aforementioned efforts, others have been

concentrating on the safety analysis methodology without

ARTICLE IN PRESS

www.elsevier.com/locate/jlp

0950-4230/$ - see front matterr 2006 Published by Elsevier Ltd.

doi:10.1016/j.jlp.2006.10.006

Corresponding author. Tel.: +1 979862 3985; fax: +1 979845 6446.

E-mail address: [email protected] (M. Sam Mannan).
http://www.elsevier.com/locate/jlphttp://localhost/var/www/apps/conversion/tmp/scratch_15/dx.doi.org/10.1016/j.jlp.2006.10.006mailto:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_15/dx.doi.org/10.1016/j.jlp.2006.10.006http://www.elsevier.com/locate/jlp


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applying it into design and optimization in a single

framework. Therefore, there is a need to systematize the

incorporation of safety metrics in the design stage. This is

the focus of this paper. Section 2 describes the problem

statement and the proposed approach. Later, the proposed

method is applied to a case study and the results are

analyzed.

2. Inherently safer design

Inherently safer design infers the elimination of hazards

as much as possible out of a chemical or physical process

permanently as opposed to using layers of protection.

There are four primary principles of inherently safer design

concept proposed by Kletz (1991):

1. Intensificationto reduce the inventories of hazardous

materials as more inventory of hazardous chemicals

means more hazards.

2. Substitutionto use less hazardous materials in the

process.

3. Attenuationto operate a process at less dangerous

process conditions (pressure, temperature, flow rate,

etc.).

4. Limitation of effectsto design the process according to

the hazards offered by the process in order to reduce the

effects of the hazards.

In the US, inherently safer design started receiving more

attention following a highly-praised paper presented by

Kletz in 1985 at the 19th Loss Prevention Symposium of

the American Institute of Chemical Engineers (AIChE)(Hendershot, 1999).

3. Safety studies

The most common and traditional approach has focused

on layers of protection (LOP) where additional safety

devices and features are added to the process, as shown in

Fig. 1 (American Institute of Chemical Engineers (AIChE),

1994). The LOP method has been successful in analyzing

safety systems. However, this approach has several

disadvantages as listed below (Crowl, 1996):

LOP increase the complexity of the process, and hence thecapital and operating cost. In the oil and gas industries,

1530% of the capital cost goes to safety issues and

pollution prevention (Palaniappan et al., 2004).

The hazards within the process remain, even when LOPare installed and are built based on the anticipation of

incidents, as shown in Fig. 2(a). Since nature might find

creative ways to release hazards, there are always

dangers from unanticipated failure mechanisms that

the LOP are not ready for, as shown in Fig. 2(b).

Since no LOP can be perfect, failures or degradation inLOP may pose risks offered by the hazards that lead to

incidents, as shown in Fig. 2(c).

Other efforts by the industries and researchers toward

safety studies tend to focus on hazard identification and

control. There has been some work in developing more

advanced hazard and risk analysis methods such as Failure

Modes and Effects Analysis (FMEA), Fault Tree Analysis(FTA), Event Tree Analysis (ETA), CauseConsequence

Analysis (CCA), Preliminary Hazard Analysis, Human

Reliability Analysis (HRA), and Hazard and Operability

Study (HAZOP) in addition to traditional methods such as

check list, safety review, relative ranking, and Whatif

analysis (Wang, 2004).

Several inherent safety efforts taken by US corporations

and US affiliates of European companies are listed below:

Dow Chemical CompanyDeveloped the Dow Fireand Explosion Index (American Institute of Chemical

Engineers (AIChE) (1994)) and the Dow Chemical

ARTICLE IN PRESS

Basic

Control

Critical Alarms, Human/Manual Intervention

Process

Design

Feed,F,X

F

Condenser

Reboiler

Top

Product,X

D, D

BottomProduct,

B,XB

Safety

Instrumented

System

Physical Protection

(Relief devices, etc)

Emergency

Responses

Fig. 1. Typical layers of protection for CPI (adapted from Hendershot,

1997).

J. Suardin et al. / Journal of Loss Prevention in the Process Industries 20 (2007) 799080


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Exposure Index (AIChE, 1993) as hazard ranking

methodology based on inherent safety principles.

Exxon Chemical CompanyDescribed inherent safety,

health and environment review process based on a lifecycle approach (Hendershot, 1999).

Rohm and Haas Major Incident Prevention Programused checklist based on inherent safety principles for

hazard elimination and risk reduction (Hendershot,

1999).

In addition to actions taken by the CPI, actions have

also been taken by government in the form of federal

regulations such as the Process Safety Management (PSM)

regulation promulgated by the Occupational Safety and

Health Administration (OSHA) and the Risk Management

Program (RMP) regulation promulgated by the Environ-

mental Protection Agency (EPA).

Overall impression these efforts is that inherently safer

design principles have not been systematically applied. As

opposed to layers of protection concept, the concept of

inherently safer design is to reduce the inherent hazards

rather than to control them. There are two advantages

about having lower hazards: they need lesser LOP, less

complex LOP, and offer lower magnitude of hazards, as

shown in Figs. 3 and 4.

Another impression on the traditional approaches is that

the efforts focus on hazard identification and control. In

addition, currently optimization is performed as an attempt

to enhance the process design and the operation conditions

of equipment to achieve the largest production, the greatest

profit, minimum production cost, and the least energy

usage. Whereas, neither objective functions nor constraint

conditions contain safety parameters in the traditionalprocess optimization.

4. Hazard indices

There are several hazard indices available as tools for

chemical process loss prevention and risk management.

Although no index methodology can cover all safety

parameters, Dow fire and explosion index (F&EI), and

safety weighted hazard index (SWeHI) are found to be

robust (Khan & Amyotte, 2003). The F&EI is the most

widely known and used in the chemical industries. The

following are indices available in the industries and

research:

F&EI (American Institute of Chemical Engineers(AIChE), 1994) and Dows chemical exposure hazards

(Dow, 1993) as tools to determine relative ranking of

fire, explosion, and chemical exposure hazards. Etowa,

Amyotte, Pegg, and Khan (2002) have developed a

computer program to automate F&EI calculation and

perform sensitivity analysis using Microsofts Visual

Basic. However, their program was not intended to

determine business interruption and loss control credit

factors, to conduct process unit risk analyses, to

automate the sensitivity analysis in order to integrate

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Anticipated Potential Incidents Unanticipated Potential Incidents

unanticipated

Layers do not

work for

mechanisms

anticipated

LOP for

Degraded LOP

LOP

incidents

Actual Risk

Actual Risk

Potential Incidents

a b

c

Fig. 2. Layers of protection characteristics. (a) LOP reduces the anticipated potential incidents, (b) LOP does not reduce unanticipated potential incidents,

(c) degraded LOP does not reduce any potential incidents (adapted from Hendershot, 1997).

J. Suardin et al. / Journal of Loss Prevention in the Process Industries 20 (2007) 7990 81


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F&EI calculation into process design and optimization

framework.

SWeHI as a tool to define fire, explosion, and toxicrelease hazards (Khan, Sadiq, & Amyotte, 2003).

Environmental Risk Management Screening Tools(ERMSTs) from Four Elements, Inc. for ranking

environmental hazards including air, ground water,

and surface water pollution. (Hendershot, 1999).

Mond Index as a tool to define fire, explosion, and toxicrelease hazard (Hendershot, 1999).

Hazardous waste index (HWI) as a tool for flamm-ability, reactivity, toxicity, and corrosivity hazard of

waste materials (Khan et al., 2003; Khan & Amyotte,

2003).

Transportation Risk Screening Model (ADLTRSs

) as atool for determining risk to people and environment

posed by chemical transportation operations (Khan

et al., 2003).

Inherent safety index was developed by Heikkila (1999)of Helsinki University of Technology. This method

classifies safety factors into two categories: chemical and

process inherent safety. The chemical inherent safety

includes the choice of material used in the whole process

by looking at its heat of reaction, flammability,

explosiveness, toxicity, corrosivity, and incompatibility

of chemicals. The process inherent safety covers the

process equipment and its conditions such as inventory,

pressure, temperature, type of process equipment, and

structure of the process.

Overall inherent safety index was developed by Edwardand Lawrence (1993) to measure the inherent safety

potential for different routes of reaction to obtain the

same product.

Fuzzy logic-based inherent safety index (FLISI) wasdeveloped by Gentile (2004). One of the major problems

in applying inherent safety is that safety mostly based on

the qualitative principles that cannot be easily be

evaluated and analyzed. FLISI was an attempt to use

hierarchical fuzzy logic to measure inherent safety and

provide conceptual framework for inherent safetyanalysis. Fuzzy logic is very helpful for combining

qualitative information (expert judgment) and quanti-

tative data (numerical modeling) by using fuzzy

IFTHEN rules.

5. Problem statement and overall approach

The problem to be addressed in this paper may be stated

as follows: Given a processing system that requires

economic optimization, devise a procedure that achieves

optimum process design while insuring that the design

meets certain safety criteria. In order to address the

problem, several challenges have to be overcome. These

include the following:

What is the best design based on technical and businessperformance within acceptable safety level ?

How to quantify safety and incorporate the safetymetric during design?

How to perform the conceptual design in a computa-tionally efficient manner?

This paper attempts to perform this optimization and

analyze the result by modifying common process optimiza-

ARTICLE IN PRESS

LOP 1

LOP 2

ProcessDesign

Feed

,F,X

F

Condenser

Reboiler

TopProduct,

XD, D

BottomProduct,B, X

B

Fig. 3. Inherently safer process design requires no or less additional LOP

adapted from Hendershot (1997).

Potential Incidents

No or Less

LOP needed

by applying

ISDActual

Risk

Fig. 4. Potential incidents for inherently safer design (adapted from

Hendershot, 1997; Hendershot, 1999).



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tion which focuses only on the technical and business

performance. The modified procedure for the use of F&EI

as safety parameter in the optimization is given in Fig. 6.

The following four steps were conducted to illustrate the

proposal methodology:

1. Computerize Dows fire and explosion index calcula-tion.

2. Generate F&EI mathematical expressions as a function

of operating pressure and the amount of materials in the

process units.

3. Propose a general procedure for integrating safety

parameters into process design and optimization.

4. Optimize the reactor and distillation column as a case

study with economic, performance, and safety para-

meters as the constraints to verify the procedure.

Even though some of the data shown came from the

F&EI computer program that we developed, this paperfocuses only on the methodology, thus the development of

the F&EI computer program is not shown.

6. Dows fire and explosion index (F&EI) methodology

F&EI is the most widely used hazard index calculation

and has been used and revised for six times since 1967. The

last revision is the seventh edition which was published in

1994 and is applied for this research. Fig. 5 shows the

F&EI procedure.

The F&EI calculation is done as the following. First,

material factor (MF, the measure of the potential energy

released by material under evaluation) is obtained from

databases, material safety data sheet (MSDS), or manual

calculation (using flammability, NF, and reactivity value,

NR). Then, determine the sum of penalties that contributes

to loss probability and its magnitude (general process

hazard factor, F1) and the sum of factors that the factor

that can increase the probability and historically contri-

butes to major fire and explosion incidents (special process

hazards factor, F2).

General process hazards cover six items, namely,

exothermic chemical reactions, endothermic processes,

material handling and transfer, enclosed or indoor process

units, access and drainage and spill control, although itmay not be necessary to apply all of them. Special process

hazards cover twelve items: toxic material, sub-atmo-

spheric pressure, operation in or near flammable range,

dust explosion, relief pressure, low temperature, quantity

of flammable/unstable material, corrosion and erosion,

leakage-joints and packing, use of fired equipment, hot oil

heat exchange system, and rotating equipment. Each of the

items is represented in terms of penalties and credit

factors.

The fire and explosion index (F&EI) is calculated using

(American Institute of Chemical Engineers (AIChE), 1994)

F3 F1 F2, (1)

F&EI MF F3. (2)

The next step is business interruption calculation (BI)

that is done based on F&EI calculated. F&EI will

determine the radius and the area of exposure. Any

equipment within this area will be exposed to the hazard.

The damage factor is then calculated that represents the

overall effect of fire and blast damage produced by releaseof fuel or reactivity energy from unit equipment. By having

original equipment cost and value of production per month

(VPM) as an input, the actual minimum probable property

damage (Actual MPPD) can be determined and then BI is

calculated by Eq. (3) (American Institute of Chemical

Engineers (AIChE), 1994):

BI US$ MPDO

30 VPM 0:7. (3)

7. Case study: overview

Procedure in Fig. 6 is examined in order to support the

argument that integrating safety into process design and

optimization gives benefits without necessarily violating the

economic and technical parameter. Hence, the final design

is the optimum economics and the inherently safer design

for the reactor-distillation column system.

Basic chemical engineering processes include reaction,

separation, and mixing. It is very common in the chemical

process industries that reactor is followed by separator to

separate the un-reacted raw materials and the specified

products. Thus, reactordistillation column system is acommon system used in the chemical process industries and

studying its optimization is very important.

It is also a fact that in performing economic analysis of a

reactor, the separator should be included since there is

trade-off between reactorseparator systems as shown in

Fig. 7. Economic balance between a high reactor cost at

high conversion and a high separation cost at low

conversion will determine the optimum reactor conversion

based on the total cost. Therefore, it is necessary to have a

procedure to improve reactor performance and/or reac-

tordistillation column system to produce desired products

while in the range of acceptable economic profit and safety

level.

8. Case study: reactor and distillation column system

The reactordistillation system is shown in Fig. 8. In

addition, it is important to note that the data presented in

this problem statement are adapted from several sources

without specifically representing a certain process. The

reason behind it is that this research focuses on the concept

of integrating F&EI value, and not in the complexities of

the calculation of F&EI where expert judgment is really

needed for the optimization process which includes a lot of

variables.

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The reactor is to produce 645 million pounds of chemical

B per year from chemical A following the reaction of:

A ! B gas phase

The reaction properties allow only a portion of the

chemical A to be converted into chemical B. Then the output

of the reactor in the form of mixture of A and B will be fed to

the distillation column. Distillation column separates the

chemical A and B in order to have product A in a certain

number of purity. The data used in this case study are:

A-B (gas phase reaction)

Hazardous material: chemical A

Product of the reactor: 645 million pounds of chemicalB per year

Pressure range: 28 atm Isothermal and plug flow reactor Feasible optimum conversion:4070% Distillation column operating pressure:1016 atm

9. Objective function and optimization model

Optimization requires mathematical modeling. This

research employs F&EI as the safety parameter which its

mathematical expressions are available by using F&EI

program through its sensitivity analysis feature. The

ARTICLE IN PRESS

Select Pertinent Process

Unit

Calculate F1

General Process Hazards Factor

Detemine F & EI

F & EI = F3 x Material Factor

Calculate F2

Special Process Haxards Factor

Determine Area of Exposure

Determine Process Unit Hazards

Factor F3 = F1 x F2

Determine material factor

Calculate Loss Control

Credit Factor = C1 x C2 x C3

Determine Replacement Value in Exposure Area

Determine Base MPPD

Determine Actual MPPD

Determine MPDO

Determine BI

Determine Damage Factor

Fig. 5. F&EI procedure (AIChE, 1994).



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mathematical expressions are presented in the next section

along with other data required for the optimization.

9.1. Reactor optimization data

In this paper we use the plug flow reactor (PFR) as a case

study. The performance of the reactor can be determined

by using the following data:

1. The rate of reaction and the mass transfer characteristics

of the reacting fluid. This determines the volume of the

reactor needed to produce the specified product.

2. The constraints dictated by the reactor are set up such as

the type and geometry of the reactor. This determines the

cost of the reactor and thus the economic parameters.

Economic variables of reactor are type, diameter, height,

design pressure, materials of construction, and capacity

(Edgar, Himmeblau, & Lasdon, 2001).

PFR is a cylindrical vessel that can be determined in the

same way as that of the distillation column with several

modifications, only in different orientation. The reactor is a

horizontal pressure vessel in cylindrical form. The free on

board (f.o.b) purchase cost (CP) of horizontal pressure

vessel including the nozzles, the manholes, a skirt, and the

internals (not plates and/or packing) are described by

Seider, Seader, and Lewin (2004).

Conversion (X) is the measure on how far the reaction

has proceeded and is in the range of 01 (100%

conversion). In optimizing a reactor, the conversion might

not reach 100% conversion due to other constraints such

as economic factors. For reactions with more than one

reactant, the material which the conversion is based on

ARTICLE IN PRESS

Dows F & EI Method

By using F & EI Program

F & EI value < 128

DESIGN & OPTIMIZATION

ACCEPTABLE

LEVEL

FINAL DESIGN

OBJECTIVE FUNCTIONS:Economic Parameter

MODELING:Safety (Inherently Safer Design Principle )

Material Balance

Sizing and Costing of Equipment

Constraints:Safety (Inherently Safer Design Principle )

Technical Performance

Constraints Adjustment

Reactions and Materials Selection

Equipment Selection

Operating Condition Selection

etc

PROPOSED DESIGN

YES

NO

Red : improvement in optimization

Black : Current optimization

Fig. 6. The integration of safety parameter into process design and optimization.

Separator

Reactor

Total

1.0

X optimum

Cost($)

Reactor Conversion (X)

0

Fig. 7. Costs of reactor and distillation column as a function of reactor

conversion (Smith, 1995).



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must be specified. Conversion expression is

Conversion Conversion amountof material consumed

amount material provided Conversion amount in inlet streamamount in outlet streamamount in inlet stream .

4

The data for the reactor design and optimization are:

Objective function: Minimize Total reactor cost

Total cost Cv Cpl. (5)

Technical constraints (Fogler, 2002):

Volume fn X; FAo; CAo,

Volume p

4D2iL

FAo

kCAo2 ln

1

1 X X

. 6

Economic constraints (Seider et al., 2004)

Cv fn W,

CV expf8:717 0:2330 lnW 0:04333 lnW 2g,

(7)

W fnD; ts; L; Di,

W pDi tsL 0:8Di tsr, (8)

ts fnPd; Di,

tp PdDi

2SE 1:2Pd, (9)

Cpl fnDi,

CPL 1580Di0:20294, (10)

Pd fnPo,

Pd expf0:60608 0:91615lnPo 0:0015655lnP02g.

(11)

With Cv as cost of vessel, ts as vessel thickness, X as

conversion, Pd as design pressure, V as volume, CAo as initial

concentration ofA, Cplas cost of the platform, Po as operating

pressure, W as weight of the vessels, FAo as input material.

9.2. Distillation column optimization data:

Distillation column consists of tower vessel and plates/

packing. The capital cost of the distillation column is the

summation of the vessel cost and the installed plates/

packing cost. The data for the distillation column design

and optimization are:

Objective function Minimize Total reactor cost

Total distillation cost Cv Cpl Ct, (12)

Technical Constraint (Peters & Timmerhaus, 1991)

uf K1

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffirL rG

rG

r, (13)

Di 4Vt

uf rG p

0:5, (14)

L trayspacing N. (15)

ARTICLE IN PRESS

Steam

Cooling Water

Liquid Phase FlowVapor Phase Flow

F, XF

Condenser

Reboiler

Top Product,Chemical A

Reflux(Liquid)

Bottom Product,Chemical B

Vapor

Vapor

(Liquid)

(Liquid)

Tray

A

B (gas phase)Volume, Conversion

Chemical A

Chemical Aand B

Fig. 8. Reactor-distillation column system for the case study.



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Economic Constraints (Seider et al., 2004) Cv fn (W)

CV expf7:0374 0:18255lnW 0:02297lnW2g,

(16)

W fn (D, ts, L, Di)

W pDi tsL 0:8Di tsr, (17)

ts fn (Pd, Di)

tp PdDi

2SE 1:2Pd, (18)

Cpl fn(Di)

Cpl 237:1Di0:62216L0:80161, (19)

Pd expf0:60608 0:91615lnPo

0:0015655lnP02g, 20

With

Cpl 237:1Di0:62216L0:80161, (21)

CT NTFNTFTTFTMCBT, (22)

CBT 369 exp0:1739Di. (23)

With Cv as cost of vessel, ts as vessel thickness, Ct as cost of

the tray, Pd as design pressure, Cpl as cost of the platform,

Po as operating pressure, and W as weight of the vessels.

10. Result: F&EI mathematical expression

F&EI calculation is performed on the case study using

the F&EI program. The sensitivity analysis feature on the

F&EI program provides the mathematical expression of

F&EI as a function of pressure and material inventory. As

shown in Figs. 11 and 12, by varying the operating pressure

at constant material inventory and vice versa, the F&EI

program will automatically generate a chart. By using least

squares method, the mathematical expression of the chart

can be determined. This procedure is applied for both the

reactor and the distillation column.

For reactor, the expressions are:

F&EI 3 108Inventory2 0:0012Inventory 88:46,

(24)

F&EI 0:1176 pressure 109:8. (25)

For the distillation column, the expressions are:

F&EI 1 108Inventory2 0:0018

Inventory 101:16, 26

F&EI 5 105pressure2 0:1072

pressure 106:83. 27

Those expressions are the safety constraint in the

optimization and are applied according to the procedure

as shown in Fig. 6.

11. Result: optimization

Optimization is performed by LINGO optimization

software and uses principles of process integration (e.g.

El-Halwagi, 2006). For the reactor-distillation column

system, the total cost is the total of the reactor cost and

the distillation cost. As shown in Table 1, in the case study

of reactor and distillation column presented in this paper,

the feasible optimization solution without F&EI (safety

parameter) as the constraint is in the range of 4070% of

reaction conversion. American Institute of Chemical

Engineers (AIChE) (1994) recommends that Dows Fire

and Explosion Index as the safety constraint should not be

more than 128 as shown by the vertical line in the Fig. 10,

where the conversion is 49%. Therefore, applying F&EI

gives the new conversion range which is 4970%.

The vertical line also shows the conversion which gives

the F&EI value of 128. If the safety parameter is not

considered, the total cost will be available for the

conversion in the range of 4070%, as shown in Fig. 9.

However, safety parameter will not allow the process to

apply those conversions since at this point the process is

not inherently safer according to Dows F&EI methodol-

ogy. The feasible range of conversion after safety

parameter has been included is in the range of 4970%,

ARTICLE IN PRESS

5.40E+05

5.50E+05

5.60E+05

5.70E+05

5.80E+05

5.90E+05

6.00E+05

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Reactor Conversion

ReactorandDistillation

cost

1.08E+06

1.10E+06

1.12E+06

1.14E+06

1.16E+06

1.18E+06

1.20E+06

1.22E+06

Total CostRight Axis

Distillation Column Cost

Reactor Cost

Feasible Area

Fig. 9. Reactor-distillation column system cost without safety constraint.

Table 1Optimization result

Constraints Optimal conversion range

No safety constraint 4070%

F&EI o128 (recommended by

AIChE, 1994)

4970% (730% less)



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as shown in Fig. 10. This is 30% less than the original

conversion range which affects the economic performance

of the system. F&EI asks for higher conversion of reaction

due to the fact that higher conversion reaction produces

more products with less reactant compared to low

conversion reaction. In addition, the lower the conversion

the higher the reactant inventory needed thus the higher

material inventory required by the reactor and the higher

distillation column capacity required to perform specified

separation. This significant decrease in the conversion

range shows that the F&EI as safety constraint will affect

the overall system in this case study.

Fig. 10 also shows that the safety parameter is employedonly as one of the constraint for the optimization. It will

not change any of the design value such as the cost,

the reactor volume, the number of trays, etc. As a

constraint, safety will only limit the feasible area for the

optimization solution. Thus, if the optimization with

constraint is performed, the result will be unacceptable

and the designer has to adjust the constraint or the other

design variables.

In other word, F&EI is incorporated as a cutting point

between inherently safer and non-inherently safer based on

F&EI target value and not a variable. In this paper,

conversion range of less than 49% requires the amount of

reactant that produces F&EI value higher than 128 while

conversion range of bigger than 49% yields F&EI value of

less than 128. The F&EI values for conversion range of

4970% are absolutely less than 128 and considered as

inherently safer according to Dow F&EI method with

target value of 128. Hence the F&EI value for conversion

range after the cutting point does not affect design decision

significantly as it is already meet the objective function of

the optimization process, which are technical, business and

safety aspect of the system. However, it is important to

note that the F&EI value as safety constraint is not

restricted to 128 as it depends on the target value and will

change move the cutting point to a different one.

Figs. 11 and 12 show that F&EI value decreases with

lower material inventory. Hence, higher reaction conver-

sion produces lower F&EI value and it will move thecutting point of 128 showed in Fig. 10 more to the right,

and vice versa.

The advantages of integrating F&EI as safety parameter,

into process design and optimization are:

The final design is an inherently safer design. The trade off between safety and other constraints can

be adjusted according to the policy of the owner/

designer.

Since both safety studies and design and optimizationare performed at the same time, safety and other

constraints will affect each other significantly.

The safety level of the design is known even before theoptimal design is achieved. Thus, detail design is

worked after the safety level is acceptable.

F&EI value as safety constrain is flexible andcan be determine based on the objective of the

design and safety level needed. This changes the cutting

point.

12. Conclusion

Mathematical expression represented safety parameter is

required when safety is included in the optimization. A

ARTICLE IN PRESS

5.40E+05

5.50E+05

5.60E+05

5.70E+05

5.80E+05

5.90E+05

6.00E+05

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Reactor Conversion

ReactorandDistillationcost

1.08E+06

1.10E+06

1.12E+06

1.14E+06

1.16E+06

1.18E+06

1.20E+06

1.22E+06

Dow's F & EI= 128

Total Cost

Right Axis

Distillation Column Cost

Reactor Cost

Fig. 10. Reactor-distillation column with safety constraint.

Operating Pressure vs Fire and Explosion Index

REACTOR

REACTOR

Material Inventory vs Fire and Explosion Index

FireandE

xplosionIndex

FireandE

xplosionIndex

Pressure (Psig)

y = 3E-08x2 + 0.0012x + 88.46

180

160

15 25 35

140

120

100

10

108

110

112

114116

118

120

122

124

126

128

20 30

2040

60

80

5

20 40 60 80 100 120 140 160

00

0

y = 1E-16x2 + 0.1176x + 109.8

Material Inventory (1000 Ibs)

Fig. 11. Sensitivity analysis and F&EI mathematical expression for

reactor.



11/12

simple way to have it for Dows Fire and Explosion Index

can be done by using the F&EI computer program

developed for this paper.

The case study on reactor-distillation column system

proves that the proposed procedures of integrating

safety parameter (Dows F&EI in this research) into

process design and optimization framework quant-

itatively and systematically are very useful. The safety

parameter acts as a constraint rather than as the

process variable. It only limits the feasible area of the

conversion optimization solution. It does not change any

of the process variables. In the case study of reactor

and distillation column presented in this paper, the

feasible optimization solution without safety as the

constraint is in the range of 4070% of reaction conver-

sion. F&EI application as the safety parameter narrows the

conversion range into 4970%. The conversion range of

4049% is not inherently safer according to F&EI

methodology.

When F&EI value of 128 as constraint is applied, safety

constraint is not significantly affecting the decision making

any further within the conversion range of 49%70%. By

applying the different F&EI value as needed, one can find

different and the right cutting point for the design. Based

on the fact that lower reaction conversion demands higher

amount of reactant, lower F&EI value selected reduces the

conversion ranges. This changes the feasible conversion

range for the process under evaluation.

There are several contributions presented by this

methodology:

Getting safety parameter as a mathematical expressionhas been a problem in safety thus inhibits the integration

of safety parameter into process design systematically.

This paper presents a simple way of generating

expressions from available hazard analysis which can

be useful in modeling and predicting the hazard of the

specific process.

Proving that there is a possibility for Dows fire andexplosion index method to be integrated into process

design and optimization framework while satisfying the

specified technical and economic parameters.

Presenting general idea on how to integrate safety intoprocess design and optimization. Instead of using only

Dows fire and explosion index, reader might assign

other methodology that fits their specific process.

However, the idea is still the same which is having the

mathematical expression of the safety study undergo

and utilize it in the optimization.

ARTICLE IN PRESS

DISTILLATION COLUMN

Operating Pressure vs Fire and Explosion Index

DISTILLATION COLUMN

Material Inventory vs Fire and Explosion Index

FireandExplosionIndex

FireandExplos

ionIndex

Operating Pressure (Psig)

y = 5E-05x2 + 0.1072x + 106.83

160

150

150

150

250 350

140

130

120

110

100

100

100

200

200

300 400

90

8050

10 20 30 40 50 60 70 80

50

0

0

0

y = 1E-08x2 + 0.0018x + 101.16

Weight (1000 Ibs)

Fig. 12. Sensitivity analysis and F&EI mathematical expression for distillation column.



12/12

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