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Runaway reactions, their courses and the methods to establish safe process conditions

J. Gustin

To cite this version:

J. Gustin. Runaway reactions, their courses and the methods to establish safe process conditions.

Journal de Physique III, EDP Sciences, 1991, 1 (8), pp.1401-1419. �10.1051/jp3:1991198�. �jpa-

00248666�

J. Phys. III France 1

(1991)

1401-1419 Ao0T 1991, _PAGE 1401

Classification

Physics

Abstracts 80.00

Runaway reactions, their

_courses

and the methods to establish safe process conditions

J. L. Gustin

Rh6ne-Poulenc Industrialisation, Ddcines, France

Rksumk. La

plus

grande

partie

de la littdrature _sur [es emballements

therrniques

traite des

consbquences

de l'accident telles que [es effets

mdcaniques,

les dmissions

toxiques

et inflamma- bles. Les travaux

publids

_par le DIERS fournissent des mdthodes perrnettant ^le dimensionnement d'dvents, ndcessitant des ddterminations

expdrimentales.

II y a moins d'inforrnation _sur la

manidre dont [es emballements

thermiques

peuvent survenir alors que ceux-ci peuvent avoir diffdrentes _causes. Le propos de cet article est de ddcrire les diffdrentes ddrives de

procddd qui

peuvent entrainer _un emballement

thermique

et de ddterrniner l'inforrnation

expdrimentale

ndcessaire _pour

l'analyse

des

risques

du

procddd,

le choix de conditions

opdratoires

s0res _et la rdduction des

consdquences

de l'emballement

therrnique. Chaque

dative de procddd

dangereuse,

est illustrde par des

exemples

connus dans l'industrie

chimique

et par des donndes

expdrimentales

obtenues dans des essais de laboratoire.

Les conditions de

procddd dangereuses prises

_en compte sont [es suivantes I) L'emballement

thermique homogbne

d0 I _une

tempdrature

excessive 2) L'emballement

thermique homogdne

par introduction d'un

catalyseur

ou d'un rdactif contr61ant; 3) L'emballement

therrnique hdtbrogdne

d0 I _une

tempdrature

locale excessive

4)

L'emballement

therrnique hdtdrogdne

db I

une faible conduction

therrnique

vers l'extdrieur 5) L'emballement

therrnique

d0 I _un temps ^de sdjour excessif I la

tempdrature

du

procddd

(Rdactions

autocatalytiques)

_;

6)

L'emballement

thermique

_par accumulation de rdactifs. La vitesse d'introduction d'un rdactif contr61ant est

supdrieure

I la vitesse de consommation de _ce rdactif, _{parce que} la

tempdrature

est trop basse _ou le catalyseur ^absent 7) L'emballement

therrnique

d0 I la

pressurisation

d'une enceinte par des interrnddiaires gazeux oxydants

(situation caractdristique

des

oxydations nitriques)

8) L'emballe- ment

therrnique

d0 I la

sdparation

de

phases

contenant des

espdces

instables

(liquides, solides)

par perte ^de

l'agitation

_{ou par} refroidissement

9)

L'emballement

thermique

_par

mdlange

de

produits incompatibles,

_se trouvant

prdcddemment

dans des

phases sdpardes

10) L'emballement thermique d0 I _un

chauffage

exteme ou I _un feu.

Considdrant les diffdrentes situations conduisant I _un emballement

therrnique,

l'intdrdt de

l'approche systdmatique

suivante est examind

Information

thdorique

et

expdrimentale

ndcessaire _pour ddterrniner les

risques

du

procddd.

Choix de conditions opdratoires addquates.

Choix de mdthodes convenables _pour le contr61e du

procddd.

Information

expdrimentale

ndcessaire pour le calcul d'dvent.

Abstract. Much of the literature _on runaway reactions deals with the consequences such _as mechanical damage toxic and flammable release. The DIERS literature provides effective methods for vent

sizing

where

experimental

information is

requested.

Thermal

stability

measurements

provide

information _on the onset temperature ^{and kinetic data for chemical}

reactions. There is less information _on the way the runaway reactions _occur whereas the runaway

reactions may have different _causes. The purpose of this _paper is to describe the various process deviations which _{can cause a} runaway react16n to _occur and to discuss the experimental information _necessary for nsk assessment, the choice of _a safe _process and the

mitigation

of the

consequences of the runaway reaction. Each

possible

hazardous process deviation is illustrated

by

examples from the process

industry

and/or relevant

experimental

information obtained from

laboratory experiments.

The

typical

hazardous situations to be considered _are the

following:

I) The

homogeneous

thermal runaway due to too

high

_a temperature.

2)

The

homogeneous

_runaway reaction

by

unintended introduction of additional reactants _or

catalyst.

3) The

heterogeneous

_runaway

reaction due to too

high

_a local temperature.

4)

The

heterogeneous

_runaway reaction caused by slow heat conduction to the outside. 5) The runaway reaction caused by excess residence time at the process temperature

(autocatalytic reactions).

6) The runaway reaction caused by reactant

accumulation. The

controling

reactant feed rate is

higher

than the

consumption

rate

perhaps

because the temperature ^is too low, _or the

catalyst

is absent. 7) The runaway reaction due to the pressurization of the enclosure by _gaseous

oxidizing

intermediates (typical of nitric oxidations).

8) The runaway reaction due to

phase

separation of unstable

species (liquids,

solids)

by

loss of

mixing

_{or on}

cooling.

9) The runaway reaction _on

mixing

of fast

reacting

chemicals in separate phases. 10) The runaway reaction due to fire _or external

heating.

Considering

the various runaway situations, the effectiveness of the

following approaches'is

discussed

Theoretical and

experimental

information

required

for hazard assessment.

Choice of

adequate

_process conditions.

Choice of

adequate

methods for process control.

Experimental information required for vent

sizing.

What is _a

Runaway

reaction ?

A runaway reaction is the consequence of the loss

of

control

of

the temperature of _a chemical

compound

_{or a}

mixture,

in _an enclosure.

This

phenomenon

is also called Thermal

Runaway

_or thermal

explosion.

The consequences of the loss of control of the temperature are . An increase of the rate of the chemical reactions.

. The _occurrence of unwanted exothermic reactions which _are not obtained in the normal

process conditions. Most of the time these reactions _are

decomposition

reactions of the reaction mixture.

. A pressure rise due _to two

phenomena

:

An increase of the reaction mixture vapor pressure due _to the temperature ^rise.

The

production

of _non condensable

decomposition

_gasses.

The pressure increase often _causes the enclosure to burst. This

explains

the _name thermal

explosion

_».

WHAT ARE THE CONSEQUENCES OF A RUNAWAY REACTION?

This loss of containment results in the emission of _a two

phase

mixture of gas and

liquid

to the outside

giving

_an aerosol.

If the reaction mixture is released in the open

air,

the aerosol _can

ignite

due to the oxidation of the hot gasses and

droplets

with

air,

_or

possibly

to the _occurrence of electrostatic

sparks.

The

ignition

of the cloud

gives

_a fire ball with _a weak _pressure effect at the

ground

level.

Pt 8 RUNAWAY REACTION. HAZARD IN THE PROCESS INDUSTRY 1403

If the reaction mixture is released in _a

housing,

the mixture of the gasses,

droplets

and air

can

explode

in this enclosed _area. In this _{case a} strong ^{blast effect} is

produced causing

the destruction of the

building

and

secondary

fires.

Often the reaction vessel bursts with emission of

projectiles

to a distance of several hundred meters.

The

projectiles

_can

damage

other apparatuses ^and

neighbouring

storage

causing again secondary

fires.

The loss of containment due to pressures

ranging

_up to several tens of bars _{causes a} short range blast effect which

pushes

all the

equipments

and

pipes

within 5 _to 15 _m distance.

In addition to the mechanical and thermal

effect,

the emission hazard due to the release of toxic

compounds

to the outside is to be considered. The immediate _or

long

term toxic hazard is the most

dangerous.

The toxic release caused _{a severe}

pollution

of the environment in

Seveso and thousands of deaths in

Bhopal.

WHAT ARE THE CAUSES OF THE RUNAWAY REACTIONS?

In the literature the

causes

_of

runaway reaction _are not _so well

investigated

_as the

consequences of the accidents. The _reason is that the _{causes are} often

complex.

Unless the incident is the result of _a

big

_process

deviation,

the determination of the _causes

requires

_a level of

knowledge

of the

chemistry

and of the process which is

only

available for _a few

people.

Most of the

time,

the _occurrence of _a runaway reactioi is _an

unexpected

event for those who _are in

charge

of

running

the

plants.

For this _reason it is of interest _to describe the various process deviations which _{can cause a}

runaway reaction to

happen,

and to discuss the

experimental

information _necessary for risk assessment, the choice of safe process and the

mitigation

of the consequences of the runaway.

It is sometimes difficult to

distinguish

between _a process deviation and _an hazardous process situation which could _{cause a} runaway reaction. For this _reason it is easier to describe the hazardous situation of _a process and then to mention the process deviation which initiates the runaway reaction.

At the moment, ten

dangerous

situations _or process deviations _can be

distinguished.

In _some

accident,

_{two or-} three of them could be present ^but any one of them could have caused _a runaway reaction.

We _now consider these ten situations.

1. The

homogeneous

tbermal _runaway due to too

high

a temperature.

It is the most famous runaway behaviour. It is

represented by

the Semenov

theory

and _can be described _as follows

A condensed

phase happens

to react at _some temperature, ⁱⁿ an enclosure with _{no or} not

enough

vent area.

The reaction is exothermic.

According

to the Arrhenius

law,

the rate of the reaction

and/or

the rate of heat

production

is _an

exponential

function of the temperature :

~~=@~= ^V.Q.k° exp(-

^~

.C~~exp(-£).

t RT

Whereas the

cooling capacity

is _a linear function of the temperature

@~=h.S. (T-T~)~T

where

4G

_" heat

$roduced _by

the exothermic reaction

(w)

4R

_" heat removed

by cooling (w)

V = volume of the condensed

phase (m~)

Q

" heat of reaction

ilmole)

k°

= constant factor of the Arrhenius law

((mole/m~)~ ~~/s)

S

= heat

exchange

_area

(m2)

T~ = ambient _or

cooling

fluid temperature

(K)

C ₌ concentration

(mole/m3)

n = order of the reaction

(dimensionless)

E ₌ activation energy

j/mole)

h

= heat transfert coefficient

(W/m2/K).

There is _some limit

temperature

above which the heat

produced by

the reaction is greater than the heat removed.

Above this limit temperature, ^the system temperature increases

steadily causing

the rate of the chemical reaction to accelerate

exponentially.

^~

Figures

I and 2

give

two

possible

deviations

by

which the temperature ^{control is lost}

figure

I

by

the decrease of the heat

exchange

coefficient _or of the heat

exchange

_area

figure2 by

_a too

high

temperature ^{for the heat}

exchange

fluid

in these

figures T~

is the heat

exchange

fluid temperature or the ambient

temperature

and To is the stable temperature ^{of the} system.

This model of Semenov is a

good representation

for batch stirred reactors

containing

_an

homogeneous

reaction mixture where the reactions follow the Arrhenius law.

The process deviation is the loss of the

cooling capacity

_or too

high

_a

temperature

^{of the} heat

exchange

fluid.

~

i~

,

,

i

,

, i

,

,

, ~

1j G T~~j~j~ai Tiimit

Fig.

I.-Semenov

representation. Temperature

control lost

by

the decrease of the heat

exchange

coefficient _or of the heat

exchange

_area.

Pt 8 RUNAWAY REACTION HAZARD IN THE PROCESS INDUSTRY 1405

.

~li~

i~

, ,

,

, i

,

, i

,

,

~

~A

~critical~limit

Fig.

2.- Semenov representation. Temperature control lost

by

too

high

_a temperature for the heat

exchange

fluid.

Example

in the process

industry

The runaway reactions

occurring

this way are

fortunately

seldom because the Semenov

representation

is well known. The basic

design

of _any process would meet the minimum

requirements

for _{a proper} control of the bulk

temperature.

Nevertheless there is still _a

typical

_process situation where runaway reactions _occur in conditions where the Semenov

representation

would

apply.

This

happens

in the

polyvalent plants

with batch processes. The wanted

product

is obtained in _a solvent which has to be removed

by

_a batch distillation with the

requirement

that _a low

solvent concentration should be left in the final

product.

Of _{course, a} batch reactor is not _a

good design

for this

operation

and it will take _a

long

time to reach _a low concentration of solvent in the

product.

In many instances several steam

pressures are available for

heating

and _one could _use steam with too

high

_a temperature to do the

job

within _a reasonable time.

For unstable chemical

compounds,

this is _{a very}

dangerous situation,

_a bad process and _a time

consuming operation.

Should the runaway

decomposition

reaction be

initiated,

the reactor

inventory

is very

high

and the consequences of

explosion

will be disastrous.

In any case the

quality

of the

product

will be poor because of the Arrhenius

decomposition

of the

product

at such _a

high temperature

over a

long

time of exposure.

When _an unstable

compound

is

processed,

like _a

nitrocompound,

_an oxime... the solvent is best removed in _a continuous film evaporator ^with a low

inventory.

Should the

decomposition

of the

product

_occur, the

inventory

^{is low} and the consequences of the runaway

unsignificant.

Experimental information requested for

_a

safety study

The

experimental

effort is devoted _to the obtention of the

following

information Determination of the onset temperature ^for the reaction _or

decomposition.

Heat of reaction _or heat release.

Kinetic behaviour

(Arrhenius.like

or

autocatalytic reaction).

Rate of reaction

(slow

_or fast

reaction).

Pressure effect.

Characterization of the

reacting

_system for vent

sizing (DIERS Methodology).

To collect this

information,

the

following

methods _are used

The thermal

stability

of the reactants, the

products

or the reaction mixtures is measured in DTA _or DSC

experiments

in closed cells.

The pressure effect and system behaviour _can be obtained from Autoclave

experiments.

These basic determinations allow the detection of

strongly

exothermic _or fast reactions.

The Arrhenius behaviour of the reaction must be checked _on temperature

scanning

DSC

experiments

_or

by

isothermal heat flux determinations with _a sensitive isothermal calorimeter.

The runaway behaviour _can be measured

using pseudo

adiabatic

calorimetry

:

An ARC

experiment

will

provide

a sensitive detection of the onset

temperature

and the kinetics _near the _onset.

Low

phi

factor closed Dewar and VSP

experiments

will

provide

information _on the

runaway behaviour for vent

sizing.

Check that the _same Arrhenius behaviour is obtained with the various

experimental

techniques

_at least _near the _onset temperature

Hazard _assessment

If the reaction is not too fast and exhibits Arrhenius type

kinetics,

the

safety

level is

evaluated

by comparing

the bulk _process temperature ^and the onset temperature ^{of the}

runaway reaction.

The bulk process temperature must be lower than the known onset temperature ⁱⁿ _any case

and the heat

exchange capacity

must exceed the

expected

heat

production.

Vent

sizing using

the DIERS methods will

provide

_an effective

protection.-

If the reaction may become _{a «} fast reaction _{» see case} 3 below.

2. The

homogeneous

_runaway reaction initiated _«

by

introduction _».

It is _a runaway reaction initiated in _an

homogeneous

reaction mixture

by

the

unexpected

introduction of _{a reactant or a}

catalyst.

A fast and exothermic reaction _occurs

immediatly

even

though

the mixture may be _at the normal temperature.

In this _case, the _process deviation is not too

high

_a

tdmperature

but an

operation

mistake.

Example

in the process

industry

A

good example

of this situation is the well known Phenol ₊

Formaldehyde

_runaway reaction

(Ref. [I]).

In this semi-batch process,

phenol

is allowed to react with

formaldehyde

in the _presence of

a

catalytic

amount of caustic soda to obtain _a condensation

product.

In this

step phenol

is loaded with the

catalytic

_amount of caustic soda in _a batch reactor and

formaldehyde

is introduced

slowly

under temperature control

(cooling)

at 60 °C. In the next step ^{of the} process a stoichiometric amount of caustic soda is added to obtain _a

phenate.

The process deviation is to

forget

the introduction of the

catalytic

amount of caustic soda in the first

step,

not to detect the

failure,

and to start the addition of caustic soda in the second

step

whereas all the

charges

of

phenol

and

formaldehyde

_are

_present.

A runaway reaction

then _occurs which will

pressurize

and burst the reaction vessel and

solidify

the reaction

mixture.

Pt 8 RUNAWAY REACTION HAZARD IN THE PROCESS INDUSTRY 1407

Experimental

determinations

The proper control of _a process prone ^to runaway reactions

by

introduction should include

safety

devices to

prevent

^{the deviation and} an emergency relief

system.

^{There is little} or no

interest in the determination of the _onset temperature ^{of the} _runaway reaction which _may be well below the process temperature.

The purpose of the

experimental

determinations is then to

provide

the information

necessary for vent

sizing

_;

using

the DIERS recommendations

characterization of the system ^behaviour

(High

_{vapor system, gassy}

reaction, Hybrid system),

determination of data

required

. Adiabatic heat rate and vapor pressure versus

temperature relationship,

. non condensable gas

production

rate.

These determinations should be made in conditions close to the adiabatic in low

phi

factor devices

~VSP

and close Dewar

flask).

The Phi Factor should _not exceed 1.2.

The

experiments

_are

usually

made

by loading

cool the reaction mixture and

rising slowly

the temperature to initiate the reaction.

Care should be taken if the measured onset temperature ^{is well below} the _process

temperature. This has _a strong effect _on the reaction rate

together

with the

phi

factor.

c~

°

-loco/limp -loco/<

Fig.

3. VSP.

ARC I° 208 209 PHtNOL + FORMALDEHYDE RtACTION DT/ot tdeg C/mini

t-'trim 1.~6

~

PHI- 1,2~7

w

&

(°1)

Fig.

4. ARC.

Figs.

3 and 4. Phenol ₊

Formaldehyde

_runaway reaction with the

recipe

of reference [I], measured in VSP and ARC. Note the difference in onset temperature and heat rate.

The measured kinetic data should be corrected to take into _account the value of the Phi factor and the difference in onset temperature. ^{A convenient correction} method for

simple

Arrhenius kinetics has been

given by

H. G. Fisher

(2).

Note that in many cases, a reaction which is

complex

when measured with _a

high phi

factor and _a sensitive onset detection

(in ARC)

turns to a

simple

_one

_step

Arrhenius reaction when measured in _a low

phi

factor device with _a

high

onset

temperature

due to a

high

heat rate base line

~VSP) (Figs. 3-4).

The process deviation is best

represented by

the introduction of the added reactant _or

catalyst

_on the reaction mixture at the

required

_process

temperature.

^This can be done with _a pressure resistant

syringe

_pump.

3. The

beterogeneous

_runaway reaction initiated

by

too

high

a local temperature.

The condensed

phase

is _not

homogeneous

but

heterogeneous

in temperature

and/or

composition,

due to _a poor

stirring

_{or no}

stirring

at all _:

Liquid

enclosed in _a

storage,

a

pipe,

_a valve.

Other

examples

include

stirring

which fails to maintain _an

homogeneous temperature

^due

to the presence of fast

propagating

reactions _or

deflagration.

M 8 RUNAWAY REACTION HAZARD IN THE PROCESS INDUSTRY 1409

In this _case, safe conditions _are not ensured

by

_a bulk

temperature

well below the reaction

onset

temperature

^but

by considering

_any

possible

local

temperature

^{which could initiate} a

fast

propagating

reaction wall

temperature

dead headed _pump...

The establishment of safe conditions should include

The

recognition

of

deflagrating compounds

_or

compounds

which could sustain fast

propagating

reactions.

The _use of

heating

fluids with _a temperature well below the onset

temperature

^{of the} reaction exotherm.

To avoid any cause of mechanical

heating.

Example

in the process

industry

Several accidents have been

reported involving

various aromatic nitro

compounds

_or nitric esters.

These

compounds

are prone ^to

deflagrate

and the

decomposition reaction,

_once initiated

on the reactor wall

by

too

high

_a

temperature,

will

instantaneously

_{assume a}

deflagrating

behaviour with _a strong ^{blast effect.}

j '

( emper~ture

i~0 30P

' 4°0 ~°°

~~~er ; ₁ ~~~

(2) DE

2j7°C/IJN

Experimental

determinations

A

special

attention is

paid

to the determination of the onset

temperature

^{of the fast}

decomposition using

DSC _or DTA measurements.

A fast and

highly

exothermic reaction is

usually

obtained _on the DSC

thermogram.

The exotherm exceeds 250

cal/g.

The

deflagrating

behaviour of

liquids

and solids _can be revealed

using

methods described

by

Grewer

[3].

The fast reaction is initiated _{on a}

sample

enclosed in _an

autoclave,

either

by

a

pyrotechnic ignition

source or

by temperature

scan. The pressure and temperature are

recorded

using

fast computer ^data

acquisition.

The reaction behaviour is then

analyzed by plotting

I)

The pressure in

log.

scale _versus the

temperature

ⁱⁿ

reciprocal

scale. See

figure

5.

2)

The _rate of pressure rise in

log.

scale _versus the _pressure in

log.

scale. See

figure

6.

Figure

5 shows the

production

of _non condensable

decomposition

_gasses, when the reaction is initiated

by heating.

Figure

6 _can show that the rate of pressure rise is _a power function of the pressure. Tl1is is considered _as the

proof

of _a

deflagrating

behaviour.

This is known for the

deflagration

of solid

propellants

in combustion chambers where the

deflagration

rate follows _a law

by Belyaev [4]

r=k.P~

10°

~

Pr«««ion .~arl

Fig..6. -Deflagration

of Ammonium Nitrate, _curve dP/dt vs. P.

Pt 8 RUNAWAY REACTION HAZARD IN THE PROCESS INDUSTRY 1411

Rates of pressure rise _as

high

_as 10 000

bar/s

have been measured _{at pressures} below 500 bar.

Note that this behaviour is influenced _by the

filling

ratio of the autoclave and that _some

reacting systems

may assume the _same

behaviour,

which _are

probably

not true

deflagrating

systems.

There is little interest in

experimental

determinations for _vent

sizing

unless the

deflagration

occurs after _some initial

pressurization

due _{to a} slow reaction.

4. The

heterogeneous

_runaway reaction

by

lack of heat conduction.

If _a stagnant solid _or

liquid

with _a low heat

conductivity

exibits _an

exotherm,

_a non~uniform

temperature profile

will appear.

For

example,

in _a

storage

the temperature ^{will rise} at the center of the containers faster than at the wall boundaries.

This

phenomenon

_{can cause} a runaway reaction. Convenient models have been

developed by

Frank Kamenetskii and Thomas. This

representation

is realistic for solid

storage

^whereas for

liquids,

convection will

help

to

equalize

the

temperature quicker

than

by

heat conduction.

Examples

in the process

industry

The

self-heating

of

large

coal

heaps

_or bulk storages ^is known. This is due to an oxidation reaction

involving

the diffusion of air into the solid bulk. In this _case the theoretical models

provide

_a

good representation

of the

phenomenon.

In the chemical

industry

the

explosion

of drums _or rail trucks is known.

If drums

containing

_a cristallized solid exhibit _a self

heating

the solid may melt _at the Center of the drums.

The

impurities

of the

product

will concentrate in the

liquid

because the less pure

product

melts first. This may cause the

decomposition

rate to accelerate faster than

expected

and the

drum to burst.

Heavy

chemicals _{or tars are} often loaded in rail trucks at

temperatures

^above

ambient,

to be

transported

to incineration facilities. Tl1ese

products

cristallize _on the container

walls, lowering

the heat

exchange capacity

of the container.

Again

the

product

that cristallizes first is the purer and the

impurities

concentrate in the

remaining liquid.

A minor exotherm in the

liquid

_{can cause} the temperature ^{to rise} at the center of the truck and the container to

explode.

Rules

of good practice

Never store unknown _or unstable chemicals in

huge

containers.

Fuel oil is stored in several thousands _tons containers but it is _a well known

product

which have been stored for

nearly

_one century. ^{This is} not the _case for your last _new chemical.

For

storage

^{above I} ton of any new chemical _or for unusual

large

bulk

storage

of any

substance, special

_care should be taken.

In the United States the S.A.D.T. is obtained from

experiments

_on full scale drums. More

conveniently

the adiabatic self-heat rate could be measured

by laboratory

determinations.

If

significant

adiabatic self

heating

^is

measured,

the

monitoring

of the

temperature

in the storage ^is necessary. Provision for

cooling

_or

emptying

the storage ^{should be made.}

Experimental

determinations

The adiabatic self heat rate for very slow exothermic reactions at ambient

temperature

is obtained from heat flux measurements in isothermal calorimeters. The _reason for this unusual

practice

is that sensitive isothermal calorimeters

provide

_a better detection of the exotherm than

pseudo

adiabatic methods.

The Setaram C80.isothermal calorimeter is able to _measure heat fluxes down to 10

mW/kg

which is

equivalent

to _a self-heat rate of 0.35

°C/day.

This

apparatus

seems to be _a

good compromise

:

Ten times this self heat rate would be _a hazardous situation for unattended

storage.

^Self- heat rates below 0.3

°C/day

_are of _no interest for

safety

_purposes.

Reaction kinetics obtained from

DSC/DTA

measurements at

higher temperatures

^should

not be

extrapolated

downwards to ambient temperature. ^{This is} as unsafe _as the

extrapolation

of kinetics to

higher

temperatures.

At least 3 determinations _at 3 different temperatures are necessary ^to ensure that the heat release follows _an Arrhenius law. A

plot

of the heat flux _or self-heat rate in

log.

scale _versus the temperature ⁱⁿ

reciprocal scale,

will

give

_a

straight

line.

S. The runaway reaction due to extended

reaction/cycle

^time.

Autocatalytic

reactions.

In _{some cases,} runaway reactions appear after _too

long

_a reaction _or

cycle

time. This is

probably

due to an

autocatalytic

_or self

accelerating

reaction.

An

autocatalytic

_or self

accelerating

reaction is a reaction which

produces

its _own

catalyst

or one of its

controlling

reactants. For this _reason, the reaction _can accelerate at constant temperature ^for chemical _reasons.

It is not necessary ^to rise the temperature to initiate the reaction. The reaction will appear under _{a constant}

temperature

exposure after _some induction time.

The induction time in

log.

scale is

usually

_a linear function of the

reciprocal

temperature.

In Kinetic

theory,

induction times refer to time to maximum rate. For

Safety considerations,

the time to the onset of the reaction is best considered.

Example

in the process

industry

A

good example

of this behaviour is the radicals chain

polymerizations

like the

vinylic polymerizations

_:

acrylic acid,

various

acrylates, acrolein,

vinyl

acetate.

A

large

number of

reacting systems showing autocatalytic

behaviour _are found in all fields of

organic chemistry.

In the _case of

vinylic polymerizations

the reaction

catalysts

_are

organic peroxids.

For _a

given

concentration of

peroxids

in the

vinylic

_monomers, the induction time is

only

_a function of the temperature. ^This

system

is _a rather

precise

chemical clock.

Another _process deviation

leading

to a

long

_{exposure to} the process

temperature,

^is to tolerate the _presence of

deposits

in the reaction vessel. These

deposits

_may last for _a

large

number of batches. If their

decomposition

is

autocatalytic,

it will appear ^at the end.

Experimental

determinations

Autocatalytic

_runaway reactions _can be detected

by considering

the unusual

dissymmetric shape

of their

DSC/DTA thermograms

obtained

by

_a

temperature

scan.

The

autocatalytic

behaviour _can then be checked

by

constant temperature exposures

using DSC/DTA

_or other

apparatuses

^{like close}

Dewar,

VSP _or ARC. The isothermal induction time must be measured _at least 3 times

using

different

temperatures

to obtain the

plot

of the

induction time _versus temperature. ^See

figure

7.

M 8 RUNAWAY REACTION HAZARD IN THE PROCESS INDUSTRY 1413

.m

<m ii

QJ

- ~m

o_ ⁱ

f LJJ

~

a

oo i iii iii

_{i11 iii-i}

i iii I till iii

_iJ

jot

_{(i ii iJ}

iifii iiN

Fig.

7.- Induction time of the

polymerization

of Vinyl ^Acetate initiated by a peroxide. VSP

measurement.

The

laboratory

determinations _can also

provide

the

experimental

information necessary for vent

sizing.

See

figure

8.

The

vinylic polymerization

_are often

high

_{vapor pressure} systems _as

long

_as the _monomers

are present.

There _{are some} theoretical and

practical

difficulties in

determining

the heat rate at a

given

set pressure or temperature, because this rate _{can assume} different values

depending

_on the

thermal

history

of the reaction.

Autocatalytic

reactions _can accelerate at constant tempera- ture.

In addition the methods to correct the results to adiabatic conditions _are not available for

autocatalytic

reactions. Much work is needed in this field.

Rules

of good practice

If there is _a

possibility

for _an

autocatalytic

reaction to appear or if this behaviour has not been

excluded,

the

operation

time of _{a process} should not be allowed to exceed the usual duration _or should be

kept

well below the known induction time of the

autocatalytic

reaction.

Emergency

relief _{vents can} be

provided

to

protect

^the enclosures.

They

must be

carefully

sized and _are not

absolutely

reliable

especially

for

vinylic polymerizations.

t

c,i

I" 1-

#

j- n

n

in-1

~=>

lo 50

1"1) ijii to i lo So

Iioff-.

blip !ll'fi

_/

Fig. 8. Heat rate of the polymerization of

Vinyl

Acetate initiated

by

_a

peroxide.

VSP measurement.

6. The runaway reaction

by

accumulation due to too low _a temperature.

In semi-batch processes the reaction is controlled

by

the slow introduction of _a

controlling

reactant. In these processes, if the introduction of this reactant is

interrupted

the reaction stops

readily.

If the temperature ^{of the reaction mixture is} too

low,

the rate of

consumption

of the

controlling

reactant is slower than the rate of introduction.

Consequently

the _reactant accumulates in the _reactor.

Later _on, a small increase in

temperature

or the

higher

concentration in

controlling

reactant will initiate the sudden reaction of the whole _reactant

inventory.

The heat

production by

the

exothermic reaction is then far above the

cooling capacity

of the reactor. The temperature rises and the reactor

pressurizes.

Example

in the process

industry

Let _us consider

again

the condensation

step

of the

phenol

₊

formaldehyde

_process.

This

reaction is made under temperature control at 60 °C.

Above 60 °C the

polycondensation (polymerization)

reaction takes

place

with _a

significant

rate.

Below 60 °C the rate of the condenshtion reaction is slow. It is very slow at ambient

temperature,

^The rate of

formaldehyde consumption

will be lower than the rate of

Pt 8 RUNAWAY REACTION HAZARD IN THE PROCESS INDUSTRY 1415

introduction.

Formaldehyde

will accumulate and react later

exothermically, triggering

the

polycondensation

reaction.

Rules

of good

practice

In semi-batch processes,- the rate of introduction of the

controlling

reactant must be

adjusted

to its rate of

consumption.

The batch temperature must be

kept

under control and the

cooling

demand must be

surveyed

to make _sure that the reactant is consumed.

The reactor _can be

equipped

with _{an emergency}

safety

vent.

Experimental

determinations

The reaction kinetics _can be obtained from

DSC/DTA

measurements and from

pseudo

adiabatic

techniques (ARC,

VSP closed

Dewar).

There is _no

particular

interest in the

precise

determination of the onset

temperature

^{of the} reaction. Processes _are

normally operated

at

temperatures

^{where the reaction is fast. Data for}

vent

sizing

is obtained from V.S.P. and

clbsed

Dewar

experiments.

7. The runaway reaction caused

by

the

pressurization

of _an enclosure

by

_an

oxidizing

_gaseous

intermediate.

This runaway situation is found in the nitric oxidation of

organic compounds.

A

typical example

is the nitric oxidation of

organic

diacids

present

^{in the} manufacture of

adipic acid,

a

nylon

intermediate.

In this

reaction,

the

controlling

reactants _are the

nitrogen

oxides

produced by

the reaction itself.

S5fExo

~~~~~ en verre

so

45

40

as

30

25

20

15

to

5

19S car./9.

-5

TEMPERATURE (Cl

_~~ 40 SO eO too 120 140 160 iBO 200 220 240 260 2eO

Fig.

9. DSC measurement of the heat release of nitric solutions of

organic

diacids.

As the reaction takes

place

in _an enclosed _area,

nitrogen

oxides _are

produced

and

pressurize

the vessel

causing

the reaction to

speed

_up.

Depending

_on the concentrations of nitric

acid, organic

fuel and diluent

(water)

the reaction _{can assume a more or} less violent behaviour

deflagrating

_or

detonating.

The _course of the runaway is influenced

by

the

filling

ratio.

Experimental

determinations

The reaction characteristic _can be

investigated considering

two separate contributions the heat

release,

the

production

of _non condensable

decomposition

_gasses

including NO~

The exotherm _can be measured

by DSC/DTA experiments.

The heat release onset

temperature

is

usually

found about 160 °C. The exotherm is Arrhenius-like _near the onset.

See

figure

9.

The non-condensable _gas

production

_can be measured

using thermomanometry

_or

Autoclave

experiments.

The onset temperature ^for gas

production

is influenced

by

the

filling

ratio and

by

the initial concentration in

NO~

in the

liquid phase.

Depending

_on the

sensitivity

of the

experimental technique,

_gas

production

is detected _at temperatures as low _as 60-70 °C. See

figure10.

I 11

lai I : LO@

do d.cawa.itian m ac

o

~

~ i

u

eo

Fig.

10. -Dewar flask measurement of the

production

of _non condensable _gas

by

nitric solutions of

organic

diacids.

Pt 8 RUNAWAY REACTION HAZARD _{IN THE} PROCESS INDUSTRY 1417

If the reaction is allowed to

pressurize

the

enclosure,

the reaction rate _can

speed

_up at such low

temperatures.

The

deflagration

behaviour of the reaction _can be studied in autoclave

experiments

where the reaction is initiated

by heating.

Rates of pressure rise _as

high

_as

10000bar/s

_can be

measured. The _rate of _pressure rise is _{a power} function of the _pressure. See

figure11.

The detonation behaviour _can be

investigated

in shock tubes. Detonations _are found to

occur in

high

_energy content

compositions (high

PEM

compositions).

~ Preaalon ~arl

Fig.

ll.-Deflagration

behaviour of nitric solutions of

organic

diacids measured in _an autoclave experiment.

Rules

of good practice

These solutions should _never be enclosed in

vessels, pipes

_or valves. The

pressurization

should _never be allowed.

Rupture

disks _can

possibly provide

_a relief _vent in _case of mistake.

Under runaway conditions the reaction behaves like

hybrid

_{or gassy systems.}

8. The runaway reaction

by segregation.

In this _case, the _{reaction mixture}

splits

in separate

phases

_one of which is unstable.

Liquid phases

_{can separate}

by

loss of

mixing.

A solid

phase

_{can separate}

by cooling.

Examples

in the process

industry

Runaway

reactions

by segregation

due to a loss of

mixing

_are known in several processes.

In emulsion

polymerizations

the _monomer

phase

_can separate from the aqueous

phase

and react with violence.

In nitration _processes where two

liquid phases

_are present, a

high

_energy content

liquid

_can

separate

^{and then}

decompose.

Runaway

reactions

by segregation

due to

cooling

_are

quite

_common.

Several

explosions

have occurred due to the cristallization of sodium

nitrophenate

_as

deposits

_on reactor walls

during cooling.

Sodium

nitrophenate

is _a very unstable and sensitive

compound

with _a

high

_energy content

(PEM).

Other

examples

cited _are

The cristallization of

performic

acid needles from _a concentrated solution _on

cooling

and their

subsequent decomposition.

The cristallization of sodium

hypochlorite

at

temperatures

below 15-17 °C from bleach solutions _{at a} concentration of100-105 chlorometric

degrees.

The solid is very unstable.

Safety

considerations

The main

problem

is _to be _aware that such unstable

compounds

_can

separate

in _a process

especially by

cristallization. If the unstable nature of the

deposits

is

known,

their formation must be avoided. If not, the

composition

of _any

deposit

_or

unexpected

_separate

liquid phase

must be

investigated

and their energy ^content evaluated

(PEM),'especially

when strong oxidizers and fuel _are present ^{in the} bulk

phase.

The appearance of unwanted unstable

phase

must be avoided

by

_{propper process} control.

Experimental

determinations

The thermal

stability

of any

separated phase

_or

deposit

_can be

investigated using DSC/DTA

and Autoclave

experiments.

9. The runaway reaction

by mixing incompatible

reactants,

previously

in separate

phases.

The deviation

causing

the runaway is

quite

^the

opposite

of the

previous

_case, for

mixing

two

liquid phases.

From a theoretical

point

of view the situation is the _{same as} for the runaway reaction

by

introduction _{» ;} the reaction is

immediately

fast at the initial system temperature.

A _{common case} in the process

industry

is the fast reaction of

incompatible

reactants with

water left in the process side after _a

cleaning operation.

Another

example

could be the reaction of water with acetic

anhydride previously

in separate

jhases,

on

mixing.

For the

experimental study,

the _same methods _as for the runaway reaction

by

introduction

can be followed.

10. The runaway reaction initiated

by

a fire or external

heating.

The _occurrence of _a fire _can initiate a runaway reaction

by raising

the temperature. In this

case the reaction rate is

greatly

increased

by

the external

heating.

The reaction is faster than in adiabatic conditions.

This

phenomenon

was known

long

_ago ^{but it has}

recently

been

pointed

_out

by

M. G.

Grolmes

[5]

^with some theoretical

support.

M 8 RUNAWAY REACTION HAZARD IN THE PROCESS INDUSTRY 1419

The external

heating

_{can cause runaway} reactions which would not appear in adiabatic conditions. This

phenomenon

is relevant to the _same

explanation by

Grolmes.

A

good example

is _{a runaway} reaction which

appeared

in _a

pilot plant

when _every

morning

the operators ^{raised the} temperature ^of a batch _to the process

value,

to resume the normal

operation

after _a

night long interruption.

The process

involving

the reaction of caustic soda in the presence of 1-2 dichlorethan _as solvent would _never exhibit _a runaway ^at a constant

process

temperature

^{but under fast}

heating conditions,

the reactor is

pressurized by

_a reaction

producing vinyl

chloride.

Safety

considerations

When enclosures _are

protected

for the fire _case

by

_pressure

safety valves,

the

possible

occurrence of _a runaway reaction should be taken into account for vent

sizing.

The vent area

should _not be based _on the rate of the runaway reaction measured under adiabatic conditions but _on the rate of the reaction under external

heating.

If the adiabatic rate of reaction is

known,

corrections should be made

using

Grolmes method. Otherwise the reaction rate should be measured under extemal

heating

conditions

using

_a closed Dewar _or the VSP.

Conclusion.

Much effort is devoted to the control of

homogeneous

_runaway reactions whereas most

incidents in the process

industry

dont

happen

this way.

In _many cases the situation is

complex

and involves several of the basic _cases

presented

here

above.

The

existing

theoretical basis can nevertheless

provide

useful

guidelines

for a careful

experimental investigation

of any

complex

case.

In any case, methods _are available either to prevent ^the occurrence or to

mitigate

the

consequences of _runaway reactions.

Hopefully

this

presentation

,vill

help

to

give

_a better

understanding

of these hazardous situations.

References

[1] ^{GUSTIN J.} L., 6th Int. Syrup. Loss Prevention and

safety promotion

in the process industry Oslo Nonvay, June 19-22

(1989)

_{paper n} 75.

[2] ^{FISHER H.} G., sth DIERS Users

Group Meeting,

Seattle, E.U. May 1989.

[3] ^GREWER Th., KLAIS O., _pressure rise during homogeneous decomposition ^and

deflagration,

Chem.

Eng. Symposium

series _n 102

(1987).

[4] ^{BELYAEV A.} F., Acia

physicochim.,

URSS 8

(1938)

763.

[5] GROLMES M. A., KING M. J., emergency relief

requirements

for certain

organic peroxides

based _on

DIERS

Methodology.

JOURNAL DE PHYSIQUE III T I, N 8,AQOT (W( 54