comparison of inherent safety indices in process concept evaluation

Comparison of inherent safety indices in process concept evaluation

Mostafizur Rahmana, Anna-Mari Heikkilab, Markku Hurmea,*

aLaboratory of Plant Design, Helsinki University of Technology, P.O. Box 6100, FIN-02015 Hut, FinlandbTechnical Research Centre of Finland, P.O. Box 1306, FIN-33101 Tampere, Finland

Abstract

In conceptual design, process routes can be compared and ranked by using inherent safety indices. In this paper, some inherent safety index

methods presented in literature are compared and their properties and limitations discussed. As a case study, an inherent safety evaluation of

methyl methacrylate process routes is presented. Three index based inherent safety evaluation methods are compared with expert evaluations

on methyl methacrylate process routes and their subprocesses. Also the index based inherent safety ranking of process routes is compared

with an expert ranking.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Inherent safety index; Index comparison; Methyl methacrylate process

1. Introduction

Inherent safety strives to enhance process safety by

introducing fundamentally safer characteristics into process

design. Implementation of inherent safety means selecting

and designing the process to eliminate hazards, rather than

accepting the hazards and implementing add-on systems to

control them. Therefore, inherently safer chemical plants

have less ‘built-in’ hazard potential than plants with a

conventional process concept.

Major decisions on process principles are done in the

process development and conceptual design phases. There-

fore, the preliminary design phases give the best opportu-

nities of implementing the inherent safety principles. In fact

the possibility of implementing inherent safety decreases as

the design proceeds. Thus, the inherent safety character-

istics should be evaluated systematically as early as

possible. The evaluation of inherent safety is however

difficult, since the lack of detailed information complicates

safety evaluation and decision-making in the early design

phases. At that time, much of the detailed information—on

which the decisions should be based—is still missing,

because the process is not designed yet. Once the process is

designed in detail, there would be all the information, but

0950-4230/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jlp.2005.06.015

* Corresponding author. Tel.: C358 9 4512632; fax: C358 9 4512694.

E-mail address: [email protected] (M. Hurme).

not the freedom to make conceptual changes. This paradox

makes it necessary to implement a dedicated methodology

for evaluating inherent safety in conceptual design to allow

early adoption of its principles.

2. Inherent safety indices

In early 1990s, there were already several existing

evaluation methods for process safety such as Dow and

Mond indices and Hazop studies. Unfortunately, they were

not directly suitable as analysis tools to be used in

preliminary process design. Most of the methods required

too detailed process information and were not directly

applicable, e.g. for conceptual design. Also not all methods

were suitable for computerised use with simulation and

optimization tools. Because of these reasons, the motivation

arose to develop new methods dedicated to inherent safety

evaluations in the early design phases. The main inherent

safety methods published in open literature for this purpose

are: Prototype Index of Inherent Safety (PIIS) developed by

Edwards and Lawrence (1993), Inherent Safety Index (ISI)

(Heikkila, 1999; Heikkila, Hurme, & Jarvelainen, 1996),

i-Safe index (Palaniappan, 2002; Palaniappan, Srinivasan,

& Tan, 2004), I2SI index (Khan & Amyotte, 2004), INSET

ISHE performance indices developed in the INSIDE Project

(2001) and EHS method (Koller, Fischer, & Hungerbuhler,

2000). Also Dow and Mond indices and Marshall’s

mortality index have been suggested as measurements of

Journal of Loss Prevention in the Process Industries 18 (2005) 327–334

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Table 1

Inherent safety index and its subindices (Heikkila, 1999)

Chemical inherent safety index Process inherent safety index

Subindices for reaction hazards Subindices for process condition

Heat of the main reaction IRM Inventory II

Heat of side reactions IRS Process temperature IT

Chemical interaction IINT Process pressure IP

M. Rahman et al. / Journal of Loss Prevention in the Process Industries 18 (2005) 327–334328

inherent safety though not originally developed for that

purpose (Kletz, 1991). Also other safety related indices exist

as discussed by Koller, Fischer, and Hungerbuhler (2001).

In this paper, ISI, PIIS and i-Safe inherent safety indices

will be compared with each other and with expert

evaluations. Also the general properties of the indices will

be discussed.
Subindices for hazardous
substances

Subindices for process system

Flammability IFL Equipment IEQ

Explosiveness IEX Process structure IST

Toxicity ITOX

Corrosiveness ICOR

2.1. Prototype inherent safety index

The first index published for evaluating the inherent

safety in process predesign was the Prototype Index for

Inherent Safety (PIIS) by Edwards and Lawrence (1993). It

is intended for analyzing the choice of a process route, i.e.

the raw materials used and the sequence of the reaction

steps. This method is reaction-step oriented, and it does not

consider much the other parts of the process. The PIIS is

calculated as a total score, which is a sum of a Chemical

Score and a Process Score. The Chemical Score consists of

inventory, flammability, explosiveness and toxicity, and the

Process Score includes temperature, pressure and yield.

2.2. Inherent safety index

The Inherent Safety Index (ISI) by Heikkila (1999) and

Heikkila et al. (1996) was developed to take into

consideration a larger scope of process steps—not only

the reaction route but also the separation sections, etc. ISI is

based on the evaluation of 12 parameters, which are selected

to represent major inherent safety factors and are already

available in the conceptual design phase (Table 1). Most of

the subindices of the method can be estimated quite easily

by using physical or chemical properties of compounds

present, or based on operating conditions and a concept of

the process. There is also one subindex that allows an

experience-based evaluation of the safety of the process

structure.

ISI consists of two main index groups. The Chemical

Inherent Safety Index describes the chemical aspects of

inherent safety, and the Process Inherent Safety Index

represents the process related aspects. For the index score

list, the reader should refer to Heikkila et al. (1996) or

Heikkila (1999).

Inherent safety index (IISI) is a sum of the chemical

inherent safety index (ICI) and the process inherent safety

index (IPI). These indices are calculated for each process

alternative separately and the results are compared with

each other. Table 1 describes the symbols of subindices

IISI Z ICI C IPI (1)

The chemical inherent safety index ICI (Eq. (2)) contains

chemical factors affecting the inherent safety of a process.

These factors consist of chemical reactivity, flammability,

explosiveness, toxicity and corrosiveness of the chemical

substances present in the process. Flammability,

explosiveness and toxicity are determined separately for

each substance in the process. Chemical reactivity consists

of the maximum values of indices for the heats of both main

and side reactions, and the maximum value of chemical

interaction, which describes the unintended reactions

between chemical substances present in the process area

studied

ICI Z IRM;max C IRS;max C IINT;max

C ðIFL C IEX C ITOXÞmax C ICOR;max (2)

The process inherent safety index IPI (Eq. (3)) expresses

the inherent safety of the process itself. It contains the

subindices of inventory, process temperature and pressure,

equipment safety and safe process structure

IPI Z II C IT;max C Ip;max C IEQ;max C IST;max (3)

The index for process structure gives an opportunity to

include earlier experience on similar or analog process

concepts in the evaluation. If this subindex is used, it is to be

estimated by an experienced designer or by using case-

based reasoning techniques on accident databases.

2.3. i-Safe index

The i-Safe Index developed by Palaniappan (2002) and

Palaniappan et al. (2004) compares process routes by using

sub-index values taken from ISI and PIIS. In addition, it

includes a NFPA reactivity rating values for the chemicals

present.

For the individual reaction steps (i.e. subprocesses) the

Overall Safety Index (OSI) includes Individual Chemical

Index ICI, Individual Reaction Index IRI and Total Reaction

Index TRI. The indices for the whole process are:

Hazardous Chemical Index (HCI), Hazardous Reaction

Index (HRI), Overall Chemical Index OCI, Overall

Reaction Index ORI, Overall Safety Index OSI, Worst

Chemical Index (WCI), Worst Reaction Index (WRI) and

Total Chemical Index (TCI).

Individual Chemical Index ICI is determined by the

properties of the chemicals involved in the reaction, and it is

calculated as a summation of indices assigned for

flammability (Nf), toxicity (Nt), explosiveness (Ne) and

Table 2

Criteria used in the inherent safety indices and their sources

Criteria PIIS ISI i-Safe

Heat of reaction C C

Heat of side reaction C

Chemical interaction L/M

Reactivity rating M

Flammability M M M

Explosiveness M M M

Toxicity M M M

Corrosiveness L

Inventory (C) C

Yield P P

Temperature P P P

Pressure P P P

Type of equipment P/E

Process structure E

C, calculated; E, engineering experience-based; L, literature; M, material

safety datasheets; P, patent & process literature.

M. Rahman et al. / Journal of Loss Prevention in the Process Industries 18 (2005) 327–334 329

NFPA reactivity rating (Nr). In ICI, all subindex values

come from ISI, except the reactivity rating, which comes

from NFPA reactivity rating values for chemicals.

Individual Reaction Index IRI is calculated as a

summation of subindices for temperature (Rt), pressure

(Rp), yield (Ry) and heat of reaction (Rh), which is quite

similar to the process score for PIIS except that the heat of

reaction is added. The index values, however, are taken

from ISI, except the yield, which comes from PIIS.

Total Reaction Index TRI for each reaction-step (i.e.

subprocess) is the sum of IRI and the max ICI for each step.

Overall Safety Index OSI is the sum of TRIs for each

reaction-step and describes the inherent safety of the whole

route.

2.4. Other index approaches

I2SI index by Khan and Amyotte (2004) comprises of

two main indices: a hazard index and an inherent safety

potential index. The first one measures the damage potential

of the process after taking into account the hazard control

measures. The latter accounts for the applicability of

inherent safety principles to the process. The authors have

also attempted to link inherent safety assessment to process

economics.

EHS method developed by Koller et al. (2000) is

particularly applicable to fine chemicals and batch

processing. The method integrates environmental, health

and safety assessments. It is flexible to adapt to the

information available and is still applicable even some

information is missing, since different calculation methods

are available.

INSET toolkit was developed in the INSIDE Project (2001)

to identify and evaluate inherently safer design options

throughout the process lifecycle. The ISHE performance

indices are methods to evaluate the safety, health and

environmental performance of processes. They involve

relatively simple calculations and allow rapid evaluations to

be made without attempt to combine the indices.

Gupta and Edwards (2003) have presented a graphical

approach for evaluating inherent safety by using the PIIS

index. The authors argued that the use of an overall index is

concealing the effects of different parameters and the

individual indices should be evaluated separately.

2.5. Criteria used in the index methods

Different index methods evaluate the processes by using

a different set of criteria. These criteria are measured by the

subindices used in the methods. Considering the differences

in criteria helps also to understand the differences between

the results of indices. Table 2 compares the criteria used in

the indices discussed in this paper.

Main difference is that PIIS uses a few very easily

available pieces of information. It does not consider reaction

or reactivity related data. i-Safe is quite similar to PIIS in

criteria but considers also reaction and reactivity related

information. ISI has the larges range of criteria considering

all the mentioned aspects plus the type of equipment,

corrosion and the process structure used. The inventory

criterion includes also the yield criterion. Chemical

interaction corresponds to some extent the reactivity rating,

but it is specific to the local subprocess conditions and is

therefore more elaborate. In Table 2, possible sources of

information for the subindices are presented.

3. Existing inherent safety index comparisons

Koller et al. (2001) compared methods for assessing

the hazard potential of processes during the design phase.

Many of the methods were index based such as PIIS, ISI,

and Dow Fire & Explosion Index. They divided the

comparison into two aspects: (1) fire, explosion, reaction

and decomposition hazard assessment; and (2) toxicity

hazard assessment. They used nine case processes as case

studies. Eight of the cases were batch processes and one

continuous process. The result was that some of the

methods (such as ISI and Dow F&E index) gave nearly

similar ranking for the fire, explosion and reaction

hazards. But for toxic hazards no similarities between

indices were found, since the toxicity assessment methods

vary much in different methods studied. The overall

hazard potential of the nine processes was compared with

pair-wise comparisons of different methods. The results

showed that only in 75% of cases two methods give the

same ranking of two processes when the aim was to

identify the more dangerous one. No comparison with

expert safety evaluations was made.

Another comparison is given by Khan, Sadiq, and

Amyotte (2003). They compared Dow and Mond indices,

Safety Weighted Hazard Index (SWeHI) (Khan & Abbasi,

1998) and PIIS (Edwards & Lawrence, 1993) for five


inherent safety guidewords for three units of an ethylene

oxide plant. None of the four indices were able to map

the inherent safety requirements completely and further

development of indices was proposed.

4. Methyl methacrylate process case study

The application of inherent safety indices for safety

evaluations is studied by using the manufacturing alterna-

tives for methyl methacrylate (MMA) as a case study.

Manufacturing MMA was selected as a case study to allow

better comparison of methods, since it was also used by

Edwards and Lawrence (1993), Lawrence (1996), and

Palaniappan (2002) as an example to demonstrate their

indices. The manufacturing routes are discussed in more

detail in Ullmann’s Encyclopedia (1990), by Rahman,

Heikkila, and Hurme (2005) and in the other above-

mentioned cases study articles.

Table 3 presents the process routes to methyl methacry-

late used in this case study and the subprocesses included in

each route.

Table 3

Methyl methacrylate process alternatives studied

Route/step Reactants Products Rea

pha

ACH Acetone cyanohydrin (ACH)

1 CH4, NH3, oxygen Hydrogen cyanide Ga

2 Acetone, HCN ACH Liq

3 ACH, sulphuric acid HMPA/HMPASE Liq

4 HMPA/HMPASE, CH3OH MMA Liq

5 H2SO4, NH4HSO4, O2, CH4 SO2, CO2, N2 Ga

6 Sulphur dioxide, oxygen Sulphur trioxide Ga

C2/PA Ethylene based via propionaldehyde

1 Ethylene, CO, hydrogen Propionaldehyde Ga

2 Propionaldehyde, CH2O Methacrolein Liq

3 Methacrolein, oxygen Methacrylic acid Ga

4 Methacrylic acid, CH3OH MMA Liq

C2/MP Ethylene based via methyl propionate

1 Ethylene, CO, methanol Methyl propionate Liq

2 Methanol, oxygen Methylal Ga

3 Methyl propionate, methylal MMA Ga

C3 Propylene based

1 Propylene, CO, HF Isobutyryl fluoride Liq

2 Isobutyryl fluoride, water Isobutyric acid Liq

3 Isobutyric acid, oxygen Methacrylic acid Ga

4 Methacrylic acid, methanol MMA Liq

i-C4 Isobutylene based

1 Isobutylene, oxygen Methacrolein Ga



TBA Tertiary butyl alcohol (TBA)

based

1 TBA, oxygen Methacrolein Ga



HMPA, 2-hydroxy-2methyl propionamide; HMPASE, 2-hydroxy-2-methyl pro

cyanide; CH3OH, methanol; CH2O, formaldehyde; CH4, methane; NH4HSO4, am

4.1. Index calculation for the MMA processes

ISI, PIIS and i-Safe indices calculated for the methyl

methacrylate subprocesses are presented in Table 3. For the

calculation details of PIIS and i-Safe indices, the

reader should refer to the above mentioned articles, and

Rahman et al. (2004) and Rahman, Heikkila, and Hurme

(2005) for the ISI calculation.

In this paper, all indices were calculated by using the

same consistent input data presented in Table 3. This was

necessary to allow the comparison on the same basis.

Different authors have previously used somewhat incon-

sistent data on the processes and chemicals: Lawrence

(1996) had some process operating condition and com-

ponent data missing, and Palaniappan (2002) used different

heats of reaction values than we did. The index values

calculated by us are shown in Table 4.

4.2. The approach for evaluation of the index based methods

The indices of subprocesses and process routes has been

compared with each other and with expert evaluations.

These expert evaluations were arranged by Lawrence

ction

se

Temperature

(8C)

Pressure

(bar)

Yield (%) DHr (kJ/kg)

s 1200 3.4 64 K3757

uid 29–38 1 91 K458

uid 130–150 7 98 v.exot

uid 110–130 7 100 Small

s 980–1200 1 100 K1520

s 405–440 1 99.7 K1229

s 100 15 90.7 K2162

uid 160–185 49 98 K1070

s 350 3.7 58 K2855

uid 70–100 6.8–7.5 75 653

uid 100 100 89 K2019

s 350–470 1–4.5 79 K1997

s 350 Low 87 483

uid 70 120 95 K835

uid 40–90 10 96 Exot

s 320–354 2.5–3 61 K883

uid 70–100 6.8–7.5 75 653

s 395 1–1.5 42 K1659

s 350 3.7 58 K1656

uid 70–100 6.8–7.5 75 490

s 350 4.8 83 K1165

s 350 3.7 58 K1656

uid 70–100 6.8–7.5 75 490

pionamide sulphate ester; MMA, methyl methacrylate; HCN, hydrogen

monium bisulphate; DHr, heat of reaction.

Table 4

Calculated subindex and index values for the MMA subprocesses

Process Hr Re Int FET Cor Invent Yield Temperat Pressure Eq Proc S

ISI iSa iSa ISI ISI PIIS iSa ISI ISI PIIS PIIS iSa ISI PIIS iSa ISI PIIS iSa ISI ISI ISI PIIS iSa

ACH1 4 4 2 0 10 12 10 0 2 1 4 4 4 10 4 0 1 0 3 2 25 28 24

ACH2 1 1 2 2 10 12 10 1 2 1 1 1 0 1 0 0 1 0 2 3 21 16 14

ACH3 3 3 2 2 6 5 6 2 2 1 1 1 1 2 1 1 2 1 2 2 21 11 14

ACH4 0 0 2 1 7 9 7 2 3 1 0 0 1 2 1 1 2 1 2 2 19 14 11

ACH5 3 3 0 0 5 7 5 2 0 1 0 0 4 10 4 0 1 0 4 1 19 19 12

ACH6 2 2 2 0 6 7 6 2 0 0 1 0 3 5 3 0 1 0 4 1 18 14 13

C2/PA1 3 3 0 1 10 14 10 0 2 1 1 1 1 2 1 1 3 1 3 2 23 21 16

C2/PA2 2 2 2 1 9 14 7 1 2 1 1 1 2 2 2 2 6 2 2 2 23 24 16

C2/PA3 3 3 2 1 6 7 6 1 3 1 5 5 3 4 3 0 1 0 2 2 21 18 19

C2/PA4 0 0 2 1 7 9 7 1 2 1 3 3 1 1 1 1 2 1 1 2 16 16 14

C2/MP1 3 3 0 0 10 14 10 0 2 1 2 2 1 2 1 3 7 3 3 2 24 26 19

C2/MP2 3 3 2 0 7 9 6 2 2 1 2 2 3 4 3 0 1 0 2 2 21 17 16

C2/MP3 0 0 2 0 7 9 7 0 2 1 2 2 3 4 3 0 1 0 3 2 17 17 14

C31 2 2 0 2 10 14 10 2 2 1 1 1 1 1 1 3 7 3 2 3 27 24 17

C32 3 3 2 2 6 6 6 2 2 1 1 1 1 1 1 1 3 1 3 2 22 12 14

C33 2 2 2 1 5 5 5 1 2 1 3 3 3 4 3 0 1 0 2 2 18 14 15

C34 0 0 2 1 7 9 7 1 2 1 3 3 1 1 1 1 2 1 1 2 16 16 14

C41 3 3 2 1 6 7 6 1 2 1 6 6 3 4 3 0 1 0 2 2 20 19 20

C42 3 3 2 1 6 7 6 1 3 1 5 5 3 4 3 0 1 0 2 2 21 18 19

C43 0 0 2 1 7 9 7 1 2 1 3 3 1 1 1 1 2 1 1 2 16 16 14

TBA1 2 2 2 1 7 7 6 1 2 1 2 2 3 4 3 0 1 0 2 2 20 15 15

TBA2 3 3 2 1 6 7 6 1 3 1 5 5 3 4 3 0 1 0 2 2 21 18 19

TBA3 0 0 2 1 7 9 7 1 2 1 3 3 1 1 1 1 2 1 1 2 16 16 14

Hr, heat of reaction; Re, reactivity; Int, interaction; FET, fire, explosiveness, toxicity; Cor, corrosiveness; Invent, inventory; Eq, equipment; Proc, process

structure.


(1996). The expert jury consisted of eight experts from

industry and academia including professors Kletz, Lees and

Duxbury. The experts evaluated the processes from three

point of views: (1) major accident; (2) medium scale

event (e.g. small explosion leading to loss of production);

(3) unplanned event that causes loss of production and

a disruption to local population but is not dangerous. The

expert scores presented in Table 5 are overall evaluations

and calculated as averages of the three above mentioned

point of views.

Since different index methods have different scales, and

their direct comparison is not possible directly. Thus, the

index scores of each method were normalized to allow

comparison. The normalization was done so that the index

values of each index type (PIIS, ISI and i-Safe) were

multiplied with a scaling parameter ai. This parameter ai

was optimized for each index type i so, that the sum of the

absolute values of differences of scaled index values aIi to

expert values Ie was minimized for the sum of all different

subprocesses j of the index i evaluated:

minX

j

jaiI;i;j K Ie;jj (4)

The scaling parameter values used for subprocess

evaluations were aPIISZ1.140, aISIZ1.074, and ai-SafeZ1.381. In subprocess evaluations only different subpro-

cesses were taken to comparison. If the same subprocess

existed in several process routes, it was included only

once in the subprocess comparisons and difference

calculations. For instance there were three similar

subprocesses to C2/PA4 and one similar subprocess to

C42 (see Table 5).

For the process route evaluations the parameters ai

were optimized once more. The values used for process

route evaluations were aPIISZ1.163, aISIZ1.0759, and

ai-SafeZ1.3673.

4.3. Comparison of index values

The comparisons of normalized index values with expert

evaluations of MMA subprocesses are shown in Table 5.

Table 5 presents the differences between index values and

expert values for both the different subprocesses present and

the process routes. On last line of Table 5, there are the

average relative differences of the index-based methods

compared to expert evaluations. It can be seen that the ISI

method has the smallest difference to expert values both in

subprocess and process route evaluations. The difference of

ISI to expert values is about 10%, and both PIIS and i-Safe

15% in the subprocess evaluations. In process route

evaluations the differences is only 3.5% for ISI and about

9% for PIIS and 10% for i-Safe. The result for ISI is

probably too optimistic for route evaluations, since it seems

that the negative and positive differences have cancelled

each other in many of the route evaluations. Therefore, the

route evaluations are generally better than the subprocess

evaluations—especially for ISI.

Table 5

Values of normalized index values, and differences to expert values for methyl methacrylate subprocesses and process routes

Sub-

process

Subprocess values Process route values

Index values Expert Differences to expert values Index values Expert Differences to expert values

ISI PIIS i-Saf ISI PIIS i-Saf ISI PIIS i-Saf ISI PIIS i-Saf

ACH1 26.85 31.93 33.14 29 2.148 2.93 4.143

ACH2 22.56 18.25 19.33 25 2.444 6.754 5.667

ACH3 22.56 12.54 19.33 19.33 3.222 6.789 0

ACH4 20.41 15.96 15.19 22.33 1.926 6.368 7.143

ACH5 20.41 21.67 16.57 17.33 3.074 4.333 0.762

ACH6 19.33 15.96 17.95 19.33 0 3.368 1.381 132.3 118.7 120.3 132.3 0 13.7 12.0

C2/PA1 24.7 23.95 22.1 22 2.704 1.947 0.095

C2/PA2 24.7 27.37 22.1 23.67 1.037 3.702 1.571

C2/PA3 22.56 20.53 26.24 21 1.556 0.474 5.238

C2/PA4 17.19 18.25 19.33 19.33 2.148 1.088 0 89.3 91.9 88.88 86 3.30 5.90 2.88

C2/MP1 25.78 29.65 26.24 28 2.222 1.649 1.762

C2/MP2 22.56 19.39 22.1 19.67 2.889 0.281 2.429

C2/MP3 18.26 19.39 19.33 19.33 1.074 0.053 0 66.7 69.8 67 67 0.29 2.80 0

C31 29 27.37 23.48 33.33 4.333 5.965 9.857

C32 23.63 13.68 19.33 27 3.37 13.32 7.667

C33 19.33 15.96 20.71 18 1.333 2.035 2.714

C34 17.19 18.25 19.33 19.33 S1 S1 S1 89.3 76.78 82.04 97.67 8.37 20.9 15.6

C41 21.48 21.67 27.62 21.67 0.185 0 5.952

C42 22.56 20.53 26.24 19.33 3.222 1.193 6.905

C43 17.19 18.25 19.33 19.33 S1 S1 S1 61.33 61.65 72.47 60.33 0.99 1.32 12.1

TBA1 21.48 17.11 20.71 18.33 3.148 1.228 2.381

TBA2 22.56 20.53 26.24 19.33 S1 S1 S1

TBA3 17.19 18.25 19.33 19.33 S2 S2 S2 61.33 57 65.63 57 4.32 0 8.63

Diff.% 9.9 15.0 15.5 3.5 8.9 10.2

S1, similar subprocess to C2/PA4; S2, similar subprocess to C42.

Table 6

Process rankings based on index and expert evaluations (the differences to

expert ranking are shown in italic)

Ranking ISI PIIS i-SAFE Expert

1 TBA & C4 TBA TBA TBA

2 TBA & C4 C4 C2/MP C4

3 C2/MP C2/MP C4 C2/MP

4 C2/PA & C3 C3 C3 C2/PA

5 C2/PA & C3 C2/PA C2/PA C3

6 ACH ACH ACH ACH


4.4. Discussion of the index methods

Some of the subprocess evaluations, which have large

differences to expert values, are discussed below and

conclusions are made on factors affecting these differences.

ACH3 and C32.: Large differences in PIIS values which

result from high heat of reaction and reactivity values,

which are not taken into account by PIIS. Therefore the PIIS

value is too small.

ACH1 and ACH5. PIIS gives too high values when the

reaction temperature is high. Temperature indexing in PIIS

is probably too steep.

ACH3. PIIS gives too low values, since by chance all the

FET (fire, explosiveness, toxicity) values are on the limit on

the small side. This is sometimes a common problem in

discrete indexing.

ACH4 and ACH2. i-Safe does not consider inventory at

all. PIIS considers it poorly, because of the bad index

scaling. Therefore, processes with large inventories get too

low index scores. Also equipment and process type hazards

are not considered.

ACH5. ISI gets a high equipment index values, because

the subprocess includes a furnace. Experts did not consider

this to be a dangerous operation itself. But a furnace can be a

source of ignition to leaks from other units. Maybe the

experts did not consider this possibility enough.

When using the expert values in evaluation of the

indices, it has to be remembered, that the in expert

evaluation limited and somewhat different information

was used than in index calculations. The level of

information available for the experts corresponded the

information needed for the PIIS index. For example the

process diagrams were not available. Experts also had

somewhat different opinions, and the spread of scores given

by them was in some cases quite large. However, average

values were used here.

4.5. Comparison of process rankings

MMA process alternatives can be ranked based on

index evaluations as shown by Table 6. The route index


values are presented in Table 5. The process routes with

a smaller index value are inherently safer. The expert

ranking of routes was originally given by Lawrence

(1996). It can be seen that the ISI evaluation is not

dissimilar to expert ranking, although in two cases ISI

gave the same index value for two processes. In PIIS

evaluation, there are two differences and in i-Safe four

differences to expert ranking. It should be noticed, that

the difference of the safest processes TBA and C4 is

quite small in expert evaluations (values 57.0 and 60.3,

respectively).

4.6. General observations

The indices have certain differences. PIIS is very

reaction-step oriented and does not consider, e.g. separation

sections at all. PIIS does not consider reaction hazards

directly but through yields, operating conditions and

physical properties. This causes errors when there are

significant reaction hazards. PIIS also lacks a right

inventory evaluation, because the inventory is for a constant

1 h residence time for a reaction-step and the number of

equipment is not considered at all. Temperature index may

have a too large scale, because it seems to cause too large

index values for high-temperature processes. PIIS is,

however, very straightforward and fast to use, since all the

input data can be got from material safety data sheets and

process literature (see Table 2). And still the results are

relatively good on average with the previously mentioned

exceptions.

i-Safe is also a reaction oriented index, which is quite

easy to use. It has a wider range of subindices than PIIS. It

has some reaction related subindices, so the reaction hazards

are covered. It is, however, also reaction-step oriented and

does not consider separation sections, since it lacks

inventory, equipment and process subindices. It is slightly

more elaborate to use than PIIS, since the heat of reaction

needs to be calculated. The results of the i-Safe index were

not better than PIIS values. The index seems to give wrong

answers, when the process indices (inventory, equipment

type, etc.) become important.

ISI has the largest set of subindices. The benefit is, that

more factors are covered, and therefore the method seems to

be most accurate. However, this has a backside; the

evaluation of the subindices is more laborious. A process

diagram is needed for the equipment index and inventory,

since the inventory index is based on the real number of

equipment. Heat of reaction needs also to be calculated.

Chemical interaction and corrosion indices are somewhat

laborious, since the information is not readily available.

Evaluation of the process concept index is very experience-

based. But if the information is available, it will probably

give a result in closest agreement with the expert

evaluations.

5. Conclusions

Inherent safety evaluations can be made in a reasonable

accuracy with the index methods discussed. For subprocess

evaluations the average difference to expert evaluations was

10.15%, and for process route evaluations 4.10%

depending on the method used. ISI, which is more elaborate

to use, gives the more accurate results. When the process

safety ranking is considered, only one method (ISI) gave

quite similar ranking to experts, although in two cases ISI

could make no difference between two processes. It has to

be noted that neither the experts were very unanimous on

the evaluations and rankings.

The inaccuracy of indices is related to the differences of

their subindex structure and properties. In PIIS the

evaluation is based on the reaction steps and it does not

consider separation sections directly at all. The reaction

hazards are not taken into account directly but through

pressure, temperature, physical properties and yields.

However, it has the merits of simplicity and therefore it is

most straightforward to use

i-Safe includes direct reaction hazard evaluation through

heat of reaction and reactivity rating. It does not have direct

inventory or process equipment related indices. Still the

accuracy was not better than with PIIS in this case study.

ISI has the widest range of indices and therefore it is most

elaborate to use. The results were closest to the expert

evaluations in this case study.

All index methods suffer to some extent from simplifica-

tions and lack of subindex interaction. For example, a large

inventory of dangerous or harmless chemical affects the

level of safety in reality. But both cases get the same

inventory index values, since inventory does not consider

the type of content. They also get the same chemical hazard

values, since the chemical hazard evaluations do not

consider the amount of the dangerous chemicals.

Despite of their lacks, inherent safety methods are quite

fast and reasonably accurate hazard evaluation methods in

early process development and conceptual design phases as

shown by the case study.

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