batch reactor hazards

Batch reactor hazards and their control

Phillip Carson and Clive Mumford

Introduction

Batch processes are used in many industries, such as

dyestuffs, pharmaceuticals, synthetic resins, in some

fermentation processes, and in speciality chemicals

production. Because of their unique problems, this

themed issue is dedicated to the hazards of, and safety

controls for, batch processes.

As with continuous processes, accidents with batch

operations may result in re, explosion, employee ill-

health, property and environmental damage, nancial

and business loss, or harm to consumers.

Example 1A 5% solution of dextrose infusion uid was

manufactured by a batch process and autoclaved. One

third of the bottles failed to reach sterilizing temperature

due to retention of air in the autoclave. Evidence of the

shortcomingwas clear to the operators since the autoclave

thermometer failed to indicate a temperature rise. This

was ignored in contravention of the operating procedure,

not for the rst time, on the grounds that the recording

thermometers had a history of unreliable operation.

Quality control checks on the batch failed to show

bacterial contamination and the product was released for

sale. During the interval between manufacture and use,

the bacteria in the product multiplied to dangerous levels.

Five hospital patients treated with this infusion uid

during surgical operations died1 . Subsequent examination

indicated that the product was contaminated and a major

product recall exercise was instigated. Issues identied as

requiring attention included:

thermometer calibration and logs; autoclave maintenance and logs; inadequate procedures generally; permitting unofcial procedures to develop; inadequacy of procedures to cope with all eventualities; inadequate equipment=poor use of existing equipment; inadequate procedures for cleaning the autoclave; poor monitoring of autoclave conditions; lack of adequate training; poor communication;

inadequate corrective action by management followingprevious government inspections;

inadequate follow-up inspections by governmentagency;

lack of internal quality control=assurance system.

Seriousmishaps can occur throughout any stage of the

batch processing life-cycle, from laboratory through to

production-scale, and during any operations, such as

material storage, mixing, product isolation, drying etc.

However, this paper concentrates on batch chemical

reactions.

Example 2An explosion in a conical drier blender killed one man

and caused 1.75m damage. At the time of the explosion

about 1300kg of poultry food additive had remained in

the closed drier for a period of 27 hr after the drying

process was completed at a temperature of 120130C.The explosion was caused by thermal decomposition,

although tests using differential thermal analysis had

shown it was safe at the drying temperature. Other tests

included shock sensitivity, ammability and

thermodynamic computations, all of which failed to

identify a potential hazard. Afterwards, accelerating

calorimetry showed that a typical batch could self-heat to

destruction if held under adiabatic conditions at 120

125C for 24 hr2 . This incident is discussed in more detailon page 25.

Table 1 lists the main causes of accidents with batch

reactions3 . An analysis of the basic reasons for such

incidents in the UK identied inadequacies in

understanding the process chemistry and

thermochemistry, in design for heat removal, in systems

and safety systems and in operational procedures

including training4 . This, together with the following

examples, plus those in the other papers all highlight the

need to fully understand:

the properties of all materials involved, including thepotential risks of thermal decomposition of all materials;

the kinetics and thermodynamics of the processwhether an exothermic reaction is a risk, the rate and

0260-9576=03=$17.63C 0.00www.ingentaselect.com=titles=02609576.htm # Institution of Chemical Engineers 2003

LOSS PREVENTION BULLETIN 171

13

quantity of energy release, and the rate and quantity of

any gas release are important considerations;

the design of the reaction process and associatedequipment.

Materials involved

The physico-chemical and toxicological properties of the

materials used must be fully understood and accounted

for in design and operation. Typical requirements are

listed in Table 2. Clearly, for hazardous or nuisance

materials charging is preferably enclosed, or partially-

enclosed with local exhaust ventilation; risk may also be

reduced by handling them in a different physical form.

Example 3After a period of work at a plastics company mixing

batches of granules for subsequent processing, a man

developed nose and eye discomfort. This was associated

with chest tightness and wheezing, often delayed for

several hours. His level of physical tness reduced. The

cause proved to be exposure to dust from sacks of

anhydride cross-linking agent. The managers were aware

of the hazards of this class of substance but did not have a

material safety data sheet. Workers were not provided

with any information, instruction or training. The man

was diagnosed as suffering from occupational asthma and

advised to avoid further exposure. He remained

unemployed and physically unt after four years away

from exposure5 .

Consideration must be given to both in-process

materials such as feeds, catalysts, intermediates, products,

by-products, contaminants (including water and

corrosion products), solvents, additives, and to processing

aids such as heat-transfer uids, recycles, ow-aids,

refrigerants, re-ghting chemicals.

Example 4An explosion in a 3.1 tonne reactor resulted in it being

projected 300m. A polyether-alcohol intermediate at

approximately 100C had been pumped at 30 hr and 14hrprior to the incident where it remained unagitated and

exposed to air via an open vent. Subsequent thermal

stability tests indicated this could cause an oxidizing

reaction sufcient to raise the temperature to 300C. Atabove 300C a rapid exothermic reaction takes placegenerating large volumes of gasthis was believed to be

self-sustaining until the kettle ruptured6 .

This reinforces the need to simulate plant conditions

when designing plant and operating exothermic

reactions.

Example 5A diazonium compound in sulphuric acid was left for

45 hr at ambient temperature in a closed, jacketed enamel-

lined vessel. The agitator was started. After 25 minutes

the pressure was 2.8 kgcm2 and rising fast and the

temperature was 160C. The operator immediatelyopened the vent valve and turned on cooling water to the

jacket but, within a minute, the chargehole cover was

blown off and the contents of the vessel blown out7 .

Diazo solutions of this type are stable at ambient

temperature and the mixture must be heated to at least

115C before decomposition becomes spontaneous. Athigher temperatures decomposition becomes violent.

However, if contaminated by traces of water,

decomposition becomes spontaneous at 100C. Theaccident was attributed to rain water having entered the

vessel via a vent pipe during a violent thunderstorm and

forming an unmixed layer of water on the surface of the

vessel contents. On starting the agitator the heat of mixing

was sufcient to raise the temperature of the

contaminated solution to above 100C.

Example 6A reactor exploded due to an unexpected exothermic

reaction during the manufacture of

2,4-diuoronitrobenzene from 2,4-dichloronitrobenzene

and potassium uoride in the presence of

dimethylacetamide solvent8 . The plant was partially

destroyed, with missiles and blast damage extending to

500m and secondary res. Six operators were injured and

one subsequently died.

The runaway was caused by contamination of the

recycled solvent with acetic acid, formed by the reaction

of the solvent with water that passed into the tank in

which the reaction product was stored. The azeotrope of

dimethylacetamide and acetic acid has the same boiling

point as dimethylacetamide itself, and hence acetic acid

was recycled with recovered solvent.

On previous occasions water contamination had been

removed at the start of batch distillation, but this time the

amount was greater and it formed a separate layer,

favouring acetic acid production. Moreover, although the


14

TABLE 1: CAUSES OF INCIDENTS INVOLVING BATCH

REACTIONS

Cause Percentage of accidents

Mischarging 21

Reaction chemistry or

thermochemistry

20

Temperature

control=cooling

19

Maintenance 15

Agitation 10

Material specication 9

Human factors 6

TABLE 2: INFORMATION REQUIRED FOR HAZARDOUS CHEMICALS

Name of chemical other names

Supplier

Uses

General description of hazards

Range of incompatible chemicals. Any incompatibility with air, water

General description of precautions

Fire ghting methods

Regulations

Sources of advice on precautions

Characteristics: evaluate as appropriate under all process conditions

Formula (chemical structure)

Purity (identity of any contaminants), physical state, appearance, other relevant information

Concentrations, odour, detectable concentration, taste (analytical methods)

Physical properties

Molecular weight Particle size, size distribution

Vapour density Foaming=emulsication characteristics

Specic gravity Critical temperature=pressure

Melting point Expansion coefcient

Boiling point Surface temperature

Solubility=miscibility (with water; in general) Joule-Thompson effect

Viscosity Caking properties

Corrosivity

Contamination factors (incompatibility); oxidizing or reducing agent; dangerous reactions

Flammability data

Flash point Vapour pressure

Fire point Dielectric constant

Flammability limits (LEL=UEL) Electrical resistivity

Ignition temperature Electrical group

Spontaneous heating Explosion properties of dust

Toxic thermal degradation products in a re

Reactivity (instability) information

Acceleration rate calorimetry Drop weight test

Differential thermal analysis (DTA) Thermal decomposition test

Impact test Self-acceleration test

Thermal stability Card gap test (under connement)

Lead block test JANAF

Explosion propagation with detonation Critical diameter

Pyrophoricity

Toxicity information

Toxic hazard rating

Hygiene standard (OEL, TLV, MAC)

LD50Biological properties

Exposure effects

Inhalation (general) Respiratory irritation

Ingestion Respiratory sensitization

Skin=eye irritation Skin sensitization

Carcinogenicity Mutagenicity

Teratogenicity

Radiation information

Radiation survey

Alpha, beta, gamma, neutron emission


15

equilibrium in the acetic acid formation equation is well

to the left, any unconverted raw material acts as a

scavenger and by removal of the dimethylamine formed

with the acetic acid moves the equilibrium to the right.

CH3CONMe2 CH2O D CH3COOHCHNMe2In the runaway reaction itself potassium acetate is

formed initially by reaction of potassium uoride and

acetic acid. This reacts with the 2,4-dichloronitrobenzene

to form acetoxychloronitrobenzenewhich is unstable

under the reaction conditions and reacts further to

produce a ketene, carbon dioxide, polyaryl ethers and tars.

Incidents have occurred in batch operations either

because the incorrect material was added by mistake,

because of changes in material specication, or because

materials had been added in the wrong sequence.

Example 7The wrong material was charged to a reactor because of

confusion over chemical names. Three operators who

were involved in moving and charging the material failed

to notice that it was not triethanolamine but

triethylamine9 .

Example 8An explosion in a batch process in an agrochemical plant

destroyed a ve storey building in June 1999. Potassium

hydroxide, instead of potassium carbonate, was

mistakenly heated with 2-chloro-5-nitrotoluene and

dimethylsulphoxide.

The blast from the explosion caused a nearby house

and several roofs to collapse and resulted in a power cut

which disrupted rail transport for several hours. People

required treatment for eye and respiratory problems

following exposure to the cloud of black smoke. Damage

costs were estimated to exceed 38m10 .

Example 9In a reaction of an aromatic amine with a chloronitro

compound, synthetic soda ash was used as an acid

acceptor to prevent the formation of ferric chloride, which

was known to catalyse exothermic side reactions. After 20

years of successful operation the synthetic soda ash was

replaced by natural soda ash. In the non-aqueous reaction

medium the difference in crystallinity of natural soda ash

rendered it less efcient as an acid acceptor. This allowed

the acid to build-up and ferric chloride formed from

reaction with the mild steel vessel. This catalysed

exothermic side-reactions and resulted in over-

pressurization of the reactor and a serious explosion. A

weakness in the manhole closure caused the cover to

blow-off despite operation of a relief valve on the reactor,

which was designed for 1000psi. (Connement tests

subsequently indicated that the side reactions could

develop pressures of 400600psi). Thrusts of gases from

the manhole propelled the vessel downwards releasing

gases into the building, where combustion caused the

explosion11 .

Example 10A batch operation involved charging an aldehyde to a

solution of caustic, an aromatic solvent and a phenolic

compound. The operator anticipated a problem with

fumes so he altered the sequence of addition, adding the

aldehyde before the caustic. Whilst the solvent was being

vacuum-charged via a dip pipe the reactor contents

exothermed and the pressure reached 25psig. The

ammable solvent splashed into the process area, and

onto the operator, fortunately without igniting9 .

A proper safety review would have revealed the

potential fume problem, cautioned against altering the

order of addition, and hence avoided the incident.

Clearly, problems may also arise if the wrong

quantities of materials are used.

Example 11An inorganic salt was used as a buffer to control the pH of

a batch reaction. Lack of control would result in a violent,

exothermic side reaction. Due to a mistake in the

calculation of the weights of raw materials required,

insufcient buffer was added to one batch. The reactor

exploded.

Additional protective measures were incorporated on

the modied plant and the formulation changed to allow

for addition of twice the theoretical buffer requirements12 .

Kinetics and thermodynamics ofthe process

The characteristics of chemical reactions resulting in

intermediates or nal products vary widely. They may

involve:

reaction in gas, liquid, (neat or in solution, suspensionor emulsion) or solid phase;

catalytic or non-catalytic; exothermic, endothermic, or negligible heat loss=gain; reversible or irreversible; rst-order (with the rate directly proportional to theconcentration of reactants), second-order (with the rate

depending on two concentration terms) or complex

kinetics.

A thorough knowledge of the reaction kinetics and

thermodynamics of the processes is a prerequisite for safe

operation.


16

Chemical reaction rate is generally a function of

reactants concentration and temperature. For example,

with the reaction:

AC B D product (1)

the reaction rateD kaabbwhere a, bD concentration of reactants

kD rate constanta,bD constants depending upon reaction

Usually

k D Aexp (E=RTr) (2)

Where ED activation energy, specic to the reactionTrD absolute temperature of reactantsAD integration constant

Thus in the case of an exothermic batch reaction unless

the heat of reaction is removed an increase in temperature

may lead to `run-away conditions. (For most

homogeneous reactions, the rate doubles for every 10Crise in temperature). If the sole means of heat removal is

an external jacket or internal coil through which coolant

ows at temperature Tc, the rate of heat removal is

proportional to (Tr-Tc). Clearly the rate of reaction

increases exponentially with temperature, whereas the

rate of heat removal is only linear. Thus a critical value of

Tr will exist at which control is lost and temperature can

then rise rapidly. This may result in a boil-over of the

reaction mass, over-pressurization of the reactor due to

rapid gas generation, or violent boiling leading to an

explosion. Elevated temperatures may initiate secondary

runaway reactions or thermal decomposition.

Hence, temperature is an important process variable

and typical reaction rate versus temperature relationships

are shown in Figure 1. Many chemical reactions are

exothermic, and particular concern for safety arises with

those reactions characterized by, for example, (a) and (c).

Examples of exothermic processes are given in Table 3.

Careful selection of operating temperature and the

provision of reliable means for heat removal, plus

emergency back-up, are therefore needed. The removal of

heat from a batch reactor may be by a combination of:

an external jacket, an internal coil or array of tubes witheither a ow of cooling media or vaporization of a

refrigerant;

an external heat exchanger, with re-circulation of thereactants;

reactants vaporization-cooling with a reux condenser.Potential instabilities during operation need to be

identied. For example contaminants such as impurities

in the reactants or corrosion products may act as catalysts,

which may promote unexpected reactions or accelerate

the rate of the desired reaction. Alternatively, process

conditions may change for some reason during operation,

or reactants may accumulate in the system. Possible

implications of the lack of understanding of the chemical

kinetics are illustrated by the following case histories.

Example 12A compound was produced from N-substituted aniline

and epichlorohydrin. The reaction was carried out so that

an agitated mixture of these two reagents was heated to

60C by means of an internal steam coil. When theexothermic reaction started a switch was made to cooling

water to maintain the temperature at 60C. On oneoccasion13 an operating error allowed the temperature to

exceed 70C and, even with full water ow, thereaction continued to accelerate. The temperature

increased slowly over about 10 minutes so that a full

evacuation was possible before the ensuing explosion at

120C.While a pressure relief system may have avoided an

explosion, because of the relatively slow rate of pressure

rise, this example clearly illustrates how the controllable

range of an exothermic reaction is a function of Tr.

Example 13A nitric acid charge was added during the nitration of an

intermediate. The reactant mass was agitated for 4 hr,

with cooling applied, to allow for complete reactant

consumption. The nal shift before the weekend had

nished so cooling was shut off with the agitation left on.

The temperature was approximately ambient. The

temperature rose linearly over the next 35 hr reaching

80C by Sunday morning. That evening the batcherupted. Subsequently, a series of adiabatic heat ow

FIGURE 1: TYPES OF REACTION=TEMPERATURE CURVE.(A) RAPID INCREASE WITH TEMPERATURENORMALCHARACTERISTICS WHERE IS THE HEAT REMOVALRATE FOR A SPECIFIC COOLANT TEMPERATURE ANDTHROUGHPUT; R IS THE POINT OF RUNAWAY. (B) SLOWINCREASE IN RATE WITH TEMPERATURECHARACTERISITIC OF SOME HETEROGENEOUS REACTIONS.(C) VERY RAPID INCREASE AT ONE POINTTHE IGNITIONPOINT IN AN EXPLOSION. (D) DECREASE IN RATE ATHIGHER TEMPERATURECHARACTERISTIC OF CATALYTICREACTIONS. (E) DECREASE IN RATE AT INTERMEDIATETEMPERATURE FOLLOWED BY AN INCREASE. (F) SLOWDECREASE IN RATE WITH TEMPERATURE


17

calorimeter experiments in which energy input from

agitation was measured conrmed that this had increased

the batch temperature into its decomposition range and

resulted in the incident14 .

If the consequences of failure include thermal

runaway, it is important to ensure by an appropriate

system of work reinforced by instrumentation=alarms

that the agitator is on when required during charging:

if layering of reactants due to differences in liquiddensity or miscibility or temporary crust formation

occurs, as these may subsequently result in a hazardous

reaction; or

if agitation is necessary for effective heat removal viaa jacket or coil.

Example 14600kg of `dry xylene was charged to a reactor. The

moisture content was checked using the Karl Fisher

method, but due to inadequate agitation there was a layer

of water beneath the xylene. When almost 300kg of

phosphorus oxychloride was added, a violent reaction

generated an internal gas pressure of 2 bar causing the

relief valve to lift. A number of joints leaked and

hydrogen chloride vented into the building15 .

Example 15An operator was preparing a mixture of phenol and

liquid caustic soda. The phenol was added without

agitation resulting in layering in the mixing vessel. When

the agitator began operation, a violent reaction occurred

and sufcient heat was generated to cause explosive

liquid boiling. About half of the reactor contents were

projected through the hinged lid onto the operator who

suffered extensive phenol burns16 .

Example 16Chalk slurry was used to neutralize acidic efuent in an

agitated tank. When an excess acidity was detected in the

discharge efuent an operator found that the agitator had

stopped. When it was restarted the resulting violent

reaction blew-off a manwaycover and lifted the bolted lid17 .

Example 17In a routine batch operation chlorosulphonic acid was to

be charged to a kettle followed by 98% sulphuric acid. An

operator failed to switch on the agitator prior to charging

the sulphuric acid; when this was noticed the agitator was

started. The chlorosulphonic acid had formed a layer over

the sulphuric acid. Hydrogen chloride was produced

spontaneously on mixing and this ejected acid out of the

man-way18 .

In the case of heat removal via a jacket=coil, the

process side offers the major resistance to heat transfer

and, as is commonly the case, the overall heat transfer

coefcient depends upon the speed of rotation of the

agitator (for example, overall heat transfer rate is

approximately proportional to (speed)2=3 , neglecting dirt

TABLE 3: POTENTIALLY HAZARDOUS EXOTHERMIC PROCESSES

Materials Subject to explosive reaction or detonation

Which react energetically with water or common contaminants

Subject to spontaneous polymerization=decomposition=combustion

Processes Exothermic

Contain ammables and operated at high pressure or temperature, or both

In which intrinsically unstable compounds are present

Operating in, or near, the explosive range

Involving highly toxic ingredients

Subject to a dust, gas, mist or vapour explosion

With a large inventory of stored pressure energy

Examples Hydrogenation (addition of hydrogen atoms to both sides of a double- or triple-bond)

Hydrolysis (reaction of a chemical with water)

Isomerization (rearrangement of atoms within a molecule)

Sulphonation (introduction of an SO3H radial into an organic moiety e.g. by reaction with H2SO4)

Neutralization (reaction between acid and base)

Alkylation (addition of alkyl group to a compound)

Esterication (reaction between alcohols and acids)

Oxidation (combination of oxygen with substances)

Polymerizations (linking small molecules to produce large ones)

Condensations (joining together two or more molecules with elimination of water)

Halogenations (substitution of, for example, H atoms in organic molecules by halogen)


18

factors) and agitator failure may result in local hot-

spots=thermal runaways.

Selected causes of runaway in batch reactors are given

in Figure 2.

Equipment design

Batch arrangements are exible since each reactor can be

used to produce different products and quality assurance

is simplied by the ability to identify a specic batch.

Design details are however critical. For example, the scale

is criticalif reliance is placed upon a jacket for heat

removal the reactor volume increaseswith (diameter)3 but

the heat transfer area increases approximately with

(diameter)2 .

A comparison of some safety features with those for

continuous reactors is given in Table 4.

Example 18A new product was prepared in a laboratory glass

reactor19 . The process involved a rapid strongly

exothermic catalysed isomerization. A modied version

of the reactor was used on a pilot plant scale and feed was

to be introduced in small increments after the initial

charge had reacted. The arrangement had approximately

5-litre volume beneath the isomantle, from which a

pumping stirrer drew liquid into the bulk. However the

10 litres required to provide a reasonable level was

considered too large. The base of the reactor was therefore

lled with glass beads to immobilize the stirrer.

A 5-litre charge of reactant to a level just above the

glass beads was warmed-upwith catalyst by switching on

the lowest zone of the isomantle. When the vapour-space

temperature reached 60C, an internal vapour explosionoccurred which pushed apart the reactor base ange.

The reactant had a ash point of 28C and anautoignition temperature of 250C. The explosion wascaused by autoignition of vapour by the hot glass surface

above the liquid level. Subsequent modications which

proved successful involved use of a heal to ensure that the

liquid level completely covered the isomantle zones in use,

use of a stirrer, andmonitoring of the liquid temperature21 .

Such isomantles may reach surface temperatures of

300C and it is important that glass surfaces alwaysremain submerged.

TABLE 4: SAFETY FEATURES OF BATCH AND CONTINUOUS REACTORS

Continuous

smaller quantities of materials are held up in the system; the scale of any potential re=explosion or toxic release hazard is reduced; any hazardous intermediate products may be consumed as fast as they are produced, hence minimizing their hazard; less requirement for cleaning=entering vessels or equipment; any potentially hazardous intermediates can be processed without storage; less start-up and shutdown operations; therefore less unsteady state operation; steady-state operation aids automatic control; the reactor is less subject to cyclical uctuations in pressure and temperature; the probability of operator error is reduced*.Batch

reactors may be isolated from one another, so spread of re can be minimized by the use of small, isolated parallelunits;

analytical control can be applied to each batch of raw materials, materials in process, and products before use ortransfer;

identication of material sources and process conditions assists in Quality Assurance.

*Computer control also reduces the potential for operator error with batch reactors as do detailed instructions=

operating procedures, training and supervisions


19

FIGURE 2: SELECTED CAUSES OF RUNAWAY IN BATCHREACTORS

Many processes involve a high degree of technology

and require equipment designed to rigid specications

coupled with sophisticated automatic control and safety

devices. With some reactions that are difcult to control it

is particularly important to provide protection against

failure of cooling media, agitation, control or safety

instrumentation etc. Obviously, the reactor itself and

associated pipework=agitator must be adequately

designed for the operating conditions of pressure,

temperature, corrosive environment etc.

Example 19In the production of benzyl formate, equal volumes of

benzyl alcohol and phosgene were reacted in an excess of

toluene at 1216C. On one occasion the glass-lined kettleruptured due to an internal explosion during vacuum

distillation of the toluene. Corrosion of a ferrous alloy

valve by the phosgene had provided ferric ions, which

catalysed decomposition of the ester20 .

Example 20A 0.45m3 vessel was used to chlorinate an aromatic

monomer dissolved in carbon tetrachloride at 50C. Whenonly 10% of gaseous chlorine had been added to one batch,

the top of the vessel was blown off and the polymer

solution was ejected over the working area21 . Iron

chlorides had entered the reactor via stainless steel feed

lines. These then catalysed a very rapid side-reaction

betweenmonomer and the chlorine, or hydrogen chloride,

evolving gas and producing a polymeric residue.

Example 21An exothermic, liquid-phase reaction was performed

batchwise in a 379-litre agitated pilot-plant reactor. The

vessel was provided with a steam jacket equipped with

isolation valves to allow for multi-purpose use. At the end

of the day the reaction was left to progress with isolation

valves closed. Thermal expansion of the water retained in

the jacket occurred during the night due to the predictable

temperature rise associated with the exotherm. This

caused the vessel wall to buckle inwards and fouled the

agitator22 .

These points are further illustrated by the Seveso

incident, described briey below (and in more detail

elsewhere23).

Example 22In July 1976 a bursting disc ruptured on a reactor at a

chemical works near Milan. The reactor was in use to

manufacture trichlorophenol at a temperature of 170

185C. It was heated by steam at 190C. It had beenassumed that the reactants could not attain the runaway

temperature of 230C, at which temperature the hyper-

poison TCDD (tetrachlorodibenzodioxin) would be

produced as a by-product. The reactor was listed as

working at atmospheric pressure and the bursting disc,

rated at 3.5 bar, was to protect it from over-pressure

during transfer of the contents using air pressure; it

vented directly to atmosphere just above the roof of the

single-storey, reactor building.

At about 6 am on the day of the accident, a Saturday,

the reactor was shut down before the acidication stage,

which released the trichlorophenol product. All external

power was shut off including the temperature recorder,

which indicated 160C. The exact cause of the ensuingexothermic reaction was unproven but the reactor

contents were estimated to have reached 300C before thebursting disc ruptured. Approximately half of the reactor

contents escaped in 20 minutes; an estimated 0.253 kg of

TCDD was released to the atmosphere.

Although no fatalities were directly attributable to the

incident the drifting poison cloud caused 750 people to be

evacuated from their homes, development of chloracne in

large parts of the exposed population, widespread

damage to crops, pollution of the Rivers Seveso and Po,

and the death of many domestic animals. More than

12 km2 were contaminated with TCDD at or above a

concentration of 5 mg=m2 .In this case the safeguards against over-heating

included provision for cooling using the steam coils, for

dumping 3000 litres of cold water into the reactor and for

using the reux condenser. These all required manual

operation but even if they had been automatic they would

have remained inoperative with the power shut off.

Suppliers of the bursting disc advised the provision of a

second receiver tank to cover the discharge of materials of

high value or toxicity. No such tank was provided.

Considerations for reaction process selection and

design include those listed in Table 5.

Example 23In a batch reactor system glycerol was charged and

circulated through a heat exchanger, which served as

either a heater or a cooler24 . It was rst used as a heater

until the temperature reached 115C at which point thefeed of ethylene oxide commenced; since the reaction was

exothermic, the heat exchanger was switched to cooling.

To start the ethylene oxide feed pump required:

Circulating pump operational

Temperature> 115C (otherwise no reaction)Temperature< 125C (otherwise reaction rate

excessive)

On one occasion when the ethylene oxide feed

commenced, the reactor pressure increased indicative

of the ethylene oxide not reacting. The operator deduced

that the temperature point was probably reading low and

therefore adjusted the trip setting to provide more heat to

initiate the reaction. He allowed the indicated


20

temperature to reach 200C but still the pressure did notfall. He suspected his theory was wrong and, on checking,

found that the valve at the bottom of the vessel was still

closed. He opened the valve and three tonnes of ethylene

oxide together with the glycerol passed through the

heater and catalyser. There was a violent runaway

reaction which ruptured the reactor; the escaping gases

exploded and two men were injured. (The indicated rise

in temperature was unreal. A pump was running with a

closed suction valve and the heat generated affected the

nearby temperature point. The ow indicator and low-

ow alarm were both out of order).

In general, depending upon the nature of the

reactants, the types of reactor, the reactor conguration

and the reaction conditions, typical events and operator

errors requiring consideration to ensure safety are listed

in Table 6. The consequences of wrong material

transfer may simply be a ruined batch, or be so serious as

to indicate a potential requirement for valve

interlocking. In some processes (such as those in Table 3)

it may be so serious as to necessitate emergency shut-off

of in-ows, perhaps as indicated in the N-substituted

aniline case history described earlier. If the feed system

itself incorporates any heat exchange facilities, such as

pre-coolers or pre-heaters, then the effect of their

failure possibly leading to unusually high or low

temperatures of the reactor must be considered.

Example 24An unusual `runaway occurred in a resin manufacturing

plant when, because of sub-zero weather, steam was

applied to a catalyst weigh-tank associated with a batch

reactor. Excess temperature initiated the reaction in

the weigh-tank and, since it had no cooling provisions,

the exotherm caused a boil-over of the tanks contents.

The vapour cloud ignited and the explosion disabled the

sprinkler protection. The reactor area and an adjoining

warehouse were destroyed by re25 .

Numerous measures may be applied to minimize any

hazard with exothermic reactors including:

If possible, avoiding operation in which all reactants areinitially mixed together and any catalyst and heat is

applied to the start of the reaction. (Experience shows

that dangerous runaway reactions are more likely with

such an arrangement.)

Limiting the size of reactor (this limits inventory andcan provide a more favourable ratio of heat transfer area

to volume). As noted earlier, this ratio becomes

increasingly unfavourable as the size of the reactor is

increased, and there may be a critical diameter of

practical use for any specic reaction. This means that a

reaction which is easily controlled in a laboratory or

small pilot-plant reactor may be hazardous on a large

scale unless additional control measures are introduced.

Operating with reactants diluted either as solutions,suspensions or emulsions reduces the reaction rate as

per equation (1) and effectively increases the ratio of

heat transfer area to reactants volume.

Controlling the rate of addition of one componentaccording to the temperature of reactants i.e. using

semi-batch operation.

Providing efcient agitation to distribute reactantshomogeneously throughout the reactor volume to avoid

hot spots and to improve the reactor-side heat transfer

lm coefcient. (Without good agitation a radial

temperature gradient may exist. If, for example, the

temperature at some point in the reaction mixture were

10C higher than at the wall the reaction rate at thispoint could be twice that at the wall. This may result in

the production of a different chemical).


21

TABLE 5: CONSIDERATIONS IN REACTION PROCESS SELECTION AND DESIGN

consider whether less-hazardous materials can be used; investigate potentially unstable reactions and side reactions such as spontaneous combustion or polymerization; consider the risk that poor mixing or inefcient distribution of reactants and heat sources may, by malfunction or dueto design error, give rise to undesirable side reactions, hot spots, reactor runaway, fouling etc.;

consider whether the reaction can be made less hazardous by changing the relative concentration of reactants or otherreactor operating conditions, such as having less unreactive material in the reactor;

assess whether side reactions produce poisonous or explosive material, or cause dangerous fouling; investigate whether the materials absorb moisture from the air and then swell, adhere to surfaces, form toxic orcorrosive liquid or gas, etc.;

determine the effect of all impurities on chemical reactions and characteristics of process mixtures; ensure that the materials of construction are compatible with each other and the chemical process materials; allow for all aspects of catalyst behaviour, such as ageing, poisoning, disintegration, activation, and regeneration; investigate whether hazardous materials can build up in the process (such as traces of combustible and non-condensable materials, toxic or explosive intermediates, or by-products);

consider whether any heating medium used should have a maximum temperature signicantly lower than that atwhich decomposition of the reaction mixture may be initiated.

Providing a reux condenser to remove latent heat ofvaporization.

Ensuring all instruments for temperature, pressure, andow control are actuated by properly located,

uncontaminated, correctly adjusted sensors that are

highly reliable. Critical instruments may be duplicated.

Limiting the temperature difference between coolantsand reactants to, for example, a maximum of 10C.

Providing concise, up-to-date operating instructionsand training.

Example 25A nitration reaction involving substituted benzoic acid

had been run many times in the laboratory and once on a

50-gallon pilot plant scale. Fuming nitric acid was used as

the nitronium ion source and fuming sulphuric acid as

solvent at 8090C. On a 200-gallon scale a runawayoccurred. The nitric acidsulphuric acid mixture was

being slowly added to the substituted benzoic acid when,

a few minutes in to the run, the temperature suddenly

increased and the rupture disc blew. The dome gasket also

ruptured as personnel evacuated the area. Subsequent

procedures for running reactions in the pilot plant were

reviewed by a Batch Record Review Board; they required

more information and instructions to be included in the

written batch record26 .

Example 26The agitator on a batch-operated nitration reactor stopped,

but the instrumentprovided to alarmand shut off acid feed

ow when the agitator ceased to rotate failed to operate.

An explosion occurred when the agitator restarted27 .

Example 27After the laboratory had closed for the weekend, re

broke out in a 23-litre insulated pot containing ammable

liquid. The liquid was undergoing a test at elevated

temperature, maintained by a hot plate equipped with a

temperature control. Failure of the temperature control

was responsible for over-heating and ignition of the

liquid. Fortunately a watchman discovered the re in time

and it was extinguished28 .

One reason that batch reactors may be preferred is

because the interval between batches is available for

cleaning procedures to ensure no deleterious

intermediates accumulate. However, this itself may pose

hazards due to inadequate preparation for cleaning,

inadequate cleaning procedures, or incomplete removal of

the liquids used for cleaning purposes.


22

TABLE 6: BATCH REACTORPOSSIBLE EVENTS AND OPERATOR ERRORS

Event

agitator failure (mechanical or electrical); instrument failure (pressure, ow, temperature, level or a reaction parameter such as concentration); failure of instrument air or electricity; loss of inert gas blanket; failure of relief devices (for example, pressure relief valves or rupture discs); restricted or blocked vent; leakage of materials out (for example, due to gasket failure) or air inwards; attainment of abnormal reaction conditions (over-pressure, over-temperature, segregation of reactants, excessivereaction rate, initiation of side reactions, layering);

failure of coolant, refrigerant, or other utilities; restriction of material ows in=out; failure of high- or low-pressure alarms=cut-outs; power failure affecting agitator, pumps, instruments; emergency elsewhere on plant=site.Selected operator errors

failure to clean, purge vessel or lines; addition of wrong material or wrong quantities (more, less or zero); failure to add catalyst or other material35 ; addition of materials in incorrect sequence; failure to add material (for example, short-stop or inhibitor) at correct stage; error in valve, switch or associated equipment operation; spillage of material; improper venting to atmosphere; failure to actuate agitation at the proper time or use of incorrect speed.

Example 28The procedure for washing out a reactor after discharging

a batch of aluminium chloride melt into water involved

lling it with water. On one occasion, the wash water was

added before the vessel was completely empty, no

dipping or visual inspection having been performed.

There was a violent reaction and the hydrogen chloride

evolved ruptured the glass vent29 .

Example 29A commercial cleaning solution was used to remove

polymer build-up from a 50-gallon reactor. Some uncured

material beneath the hard crust reacted with the solution

causing a pressure rise. The relief valve being blocked

with polymer solids impeded pressure relief and pressure

blew out a 5 cm glass elbow. Such an incident could have

been anticipated by prior testing of the cleaning agent and

reaction residue9 .

In addition to measures for the control of possible

events and errors such as those listed in Table 6,

consideration should be given to limiting their

consequences. In some circumstances this may involve

designing the plant to contain the maximum pressure that

could be developed as a consequence of a runaway.

Depending on the reaction, reactor type and

conditions, emergencymeasures include a combination of:

vent to blow-down facilities or other safe place. Detailedadvice is available on relief system design32 ;

emergency heat removal by supplementary cooling,such as by pumping reactants through an external heat

exchanger loop or by the use of lower temperature

cooling medium;

dilution with compatible gas or liquid; dumping contents (for example, into a ash drum,reservoir, or quench tank containing an appropriate

quenching uid);

shut-off feed; increase off-take; destroy catalyst (i.e. add inhibitor); inert gas purging; deluge reactants with compatible liquid (such as solventor water);

increased agitation provision, so increasing the rate ofheat removal. With a semi-batch operation an obvious

measure is to shut off the ow of reactants.

Detailed examples of these measures are provided in

reference 33. The selection of the correct combination of

emergency measures and their speed of response are

clearly important factors.

Example 30A batch chlorination reactionwas controlled by automatic

regulation of the ow of chlorine. One day the

thermocouple failed. The coolant (brine) was shut off and

the stirrer stopped while the instrument was repaired. A

delay in shutting off the chlorine ow allowed high

temperatures to develop and the resulting decomposition

reaction caused an explosion in the reactor with eight

fatalities and extensive damage34 .

In some cases it is likely to be very difcult to regain

control.

Example 31To start a reaction in an agitated vessel the contents had to

be heated by steam in a jacket. At 60C the steam shouldhave been isolated, the jacket purged, and cooling water

applied to remove the heat of reaction. This change-over

was the responsibility of the operator.

The operator was distracted at the crucial time. On his

return shortly afterwards the temperature was 70C. Heswitched to full cooling but the temperature continued to

rise. The building was evacuated and the reactor

ultimately exploded at 120C.This incident should not be dismissed as `operator

error. It could have been identied by some formalized

hazard evaluation technique, and a system of work or

automatic control introduced to minimize it.

Conclusion

Accidents involving batch reactors can be costly in terms

of injuries to personnel, damage to property or the

environment, lost business=reputation and in settling

claims for compensation. Loss prevention requires a

strategy to:

Plan reactions (understand the chemistry; plan as semi-batch; consider side reactions; collect data; conduct desk

screening; carry out laboratory studies and pilot-plant

trials; use HAZOPs prior to scale-up to production

mode, as discussed by Bickerton on page 10).

Control reactions (charging of the correct materials ofthe right specication, in the correct order, in the correct

amount at the correct time and temperature;

provision of material and line labelling; provision of

reactor cooling, agitation, temperature and pressure

control; operator instruction, training and

supervision).

Plan for loss of control (provide detection and alarms,reactor venting, additional cooling, crash cooling,

dumping, quenching; stop feed supply, stop catalyst

feed; add reaction inhibitor; train operators to recognise

major deviations and the action to take, and rehearse

emergency actions and area evacuations).

Further guidance and case histories are provided by

references 33 and 3642 and in an excellent video

entitled `Control of Exothermic Chemical Reactions,

produced by the UK Health and Safety Executive


23

in conjunction with the Institution of Chemical

Engineers.

References1. (a) C. M. Clothier, Report to the Committee appointed

to inquire into the circumstances, including theproduction, which led to the use of contaminatedinfusion uids in the Davenport Section of thePlymouth General Hospital, 1972, HMSO.(b) Anon, The Daily Telegraph, March 7th 1972, 1;J. Owen, ibid, March 8th 1972.(c) Anon, The Pharmaceutical Journal, 1972, 209, 1, 5.

2. Health and Safety Executive, The Explosion at the DowChemicals Factory, Kings Lynn, 27th June 1976, 1977,HMSO.

3. P. F. Nolan, Report to HSE on case histories ofRunaway Reactions.

4. J. A. Barton and P. F. Nolan, Hazards X: Process Safetyin Fine and Speciality Chemical Plants, Institution ofChemical Engineers Symposium Series, No 115, 3

5. Anon, Health and Safety Commission Newsletter, 1991,(77) June, 9.

6. C. H. Vervalin, (ed), Fire Protection Manual forHydroprocessing Plants, 2nd edn, Gulf, Houston, Texas,1973, p. 82

7. A. J. D. Jennings, Courses on Process Safety. Theory andPractice, Institution of Chemical Engineers, Universityof Durham, 1116 July 1982.

8. T. A. Kletz and J. Redman, The Chemical Engineer, 28thFeb 1991, 15; T. A. Kletz, Loss Prevention Bulletin,Institution of Chemical Engineers, 1991, (100), 21.

9. M. A. Capraro and J. H. Strickland, Plant OperationsProgress, 1989, 8(4), 189.

10. Anon, The Chem. Eng. 24th June 1999, 5.

11. W. W. Russell, Loss Prevention, 1976, 10; Anon,Manufacturing Chemists Assoc Case History, 1911.

12. T. A. Kletz, An Engineers View of Human Error,Institution of Chemical Engineers, 1985, 21.

13. Anon, Loss Prevention Bulletin, Institution of ChemicalEngineers, 1975, (001), 3.

14. E. D. West, G. W. Gravenstone and T. F. Hoppe, PlantOperations Progress, 1986, 5(3), 142.


16. Annual Report of HM Inspector of Factories, HMSO,1967.



19. Anon, Loss Prevention Bulletin, Institution of ChemicalEngineers, 1981, ( ), 21.


21. Anon, Loss Prevention Bulletin, Institution of ChemicalEngineers, 1977, (012), 4.2.

22. P. A. Carson and C. J. Mumford, Loss PreventionBulletin, Institution of Chemical Engineers, 1989, (089),1; ibid. 1989 (090) 3; ibid, 1990; (093), 5; J. Jones, et al,Chemical Engineering, 1993, (April), 136.

23. Anon, The Economist, 17 June 1998, 101, V. C. Marshall,The Chemical Engineer, 1980, 499.

24. R. Rushford, NE Coast Inst Eng Trans, 1977, 93, 117.

25. F. A. Manuele, One Hundred Largest Losses, LossPrevention Bulletin, Institution of Chemical Engineers,1984, (058), 1.

26. R. N. Brummel, Plant Operations Progress, 1989, 8(4),228.

27. E. J. Fritz, Loss Prevention, 1969, 3, 41.

28. National Fire Protection Association, Occupancy FireRecord: Laboratories, NFPA, Boston, 1958, 6.

29. Anon, Loss Prevention Bulletin, Institution of ChemicalEngineers, 1980, (035), 2.2.2.

30. J. C. Etchells and J. Wilday, Workbook for ChemicalReactor Relief System Sizing, HSE Contract ResearchNo136, HSE Books, 1998.

31. J. A. Barton and R. L. Rogers, Chemical ReactionHazards, 2nd edn, Institution of Chemical Engineers,1997.

32. Anon, Manufacturing Chemists Assoc. Case History1962, 1, case history 371.


34. T. Yoshida, Safety of Reactive Chemicals, Elsevier, 1987.

35. Association of British Pharmaceutical Industry,Guidance Notes on Chemical Reaction Hazard Analysis,1989.

36. L. Bretherick (ed), Brethericks Handbook ofReactive Chemical Hazards, Peter Urben. 6th edn,1999.

37. National Fire Protection Association,Manual ofHazardous Chemical Reactions, 1986, NFPA, Boston.

38. P. A. Carson and C. J. Mumford, The Safe Handlingof Chemicals in Industry Volumes 1 and 2, Longman,1988.

39. P. A. Carson and C. J. Mumford, The Safe Handling ofChemicals in Industry Volume 3, Longman, 1996.

40. P. A. Carson and C. J. Mumford, HazardousChemicals Handbook, 2nd edn, Butterworth Heinemann,2002.


24

batch reactor hazards

Documents