run away reactions

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GROUP MEMBERS

ARUN C S

C SRINESH

ROHITH KRISHNAN

SAFETY IN CHEMICAL ENGINEERING



CONTENTS

´ Introduction

´ Run away reactions in Engineering

´

Causes of run away reactions´ Risk assessment

´ Evaluating reaction hazards

´ Major incidents- Seveso and Chernobyl

´ Effects

´ Prevention



INTRODUCTION

´ Thermal runaway refers to a situation where anincrease in temperature changes the conditions ina way that causes a further increase intemperature, often leading to a destructive result

´ In the chemical process industry, raw materialsare converted into commercial products.Exothermic chemical reactions can lead to athermal runaway if the heat generation rate

exceeds the heat removal rate.´ The runaway itself is characterised by an

exponential increase in the rate of heatgeneration, temperature and pressure.



´ Pressure build-up during the runaway is caused byan increasing vapour pressure of liquidcomponents and by the production of non-

condensable gases.´ Apart from the loss of reactor inventory due to an

uncontrolled conversion process, a runawayreaction may lead to severely damaged equipment

or even a physical explosion if pressure build-upinside the reactor exceeds the design pressure.



RUNAWAY REACTIONS IN

ENGINEERING

´ In chemistry (and chemical engineering), this risk isassociated with strongly exothermic reactions that areaccelerated by temperature rise.

´ In electrical engineering, thermal runaway is typically

associated with increased current flow and powerdissipation, although exothermic chemical reactionscan also occur under some conditions.

´ Thermal runaway can occur in civil engineering,notably when the heat released by large amounts of curing concrete is not controlled.

´ In the science of astrophysics, thermal runawayof thermonuclear fusion in the cores of massive starscan cause Type I supernova explosions.



CAUSES OF RUNAWAY REACTIONS

Most possible occurrence of a runaway

reaction:

´ During a cooling problem like choking of

condenser used for cooling

´ Failure of cooling water due to electrical or

mechanical failure of pumps,

´ Inadequate cooling water pressure or high

cooling water temperature



An analysis of thermal runaways in the UK has

indicated that incidents occur because of:

´ Inadequate understanding of the process

chemistry and thermochemistry;

´ Inadequate design for heat removal;

´ Inadequate control systems and safety systems

´ Inadequate operational procedures, including

training.



Runaway reactions can also prevail from lessevident matters

´ Occurrence of hot spots in a chemical reactor orreactor wall

´ Failure of a stirrer´ Accumulation of reactants

´ Loss of solvent in reflux systems, fire etc,

´ An intentional chemical conversion process, self-heating

´ Failure of valves which can be used for addition of reactants at the required flow rate or discharge of hot materials.



CHEMICAL PROCESS RISK ASSESSMENT

A typical assessment will involve:

´ Defining the process, operating conditions and plant

´ Identifying the hazards

´ Evaluating the risks arising from the hazards anddeciding whether existing precautions are adequate ormore should be done

´ Selecting and specifying appropriate safety measures

´ Implementing and maintaining the selected safetymeasures.



´ The assessment should be sufficient to identify the

potential hazards and to investigate their causes. Where

possible, hazards should be avoided.

´

As the process design develops, foreseeable deviationsfrom the normal process, such as equipment failure or

operator error, should be considered. You may need tofollow a structured method for identifying hazards, such

as a hazard and operability study (HAZOP), particularly

when the plant or processes are highly hazardous,complex or involve new technology.



EVALUATING REACTION HAZARDS

´ In order to determine the hazards of a reaction, you

need information on the chemistry and thermochemistry

of the reaction. This includes:

´ The possibility of thermal decomposition of rawmaterials, intermediates, products and by-products;

´ Whether exothermic runaway can occur

´ The rate and quantity of heat and gas produced by the

reaction.´ As it is not safe to test unknown reactions in a full-size

reactor, various techniques and tests have been

developed to provide predictive data.



The main methods are:

´ Literature data and calculation, to give preliminaryinformation;

´ Basic screening tests, such as differentialscanning calorimetry or carius tube;

´ Isothermal calorimetry (mainly to measurereaction kinetics and heats of reaction);

´

Adiabatic calorimetry (mainly to examinerunaways); and

´ Relief vent sizing tests.



MAJOR INCIDENTS

´ 1947 Texas City disaster fromoverheated ammonium nitrate in a ship's hold

´ Disastrous release of a large volume of methyl

isocyanate gas from a Union Carbide plantin Bhopal India in 1984

´ Seveso disaster-where thermal runaway heated areaction to temperatures such that in addition to

the intended 2,4,5-trichlorophenol,poisonous 2,3,7,8-tetrachlorodibenzo-p-dioxin wasalso produced, and was vented into theenvironment after the reactor's rupture disk burst



CHERNOBYL DISASTER

´ The Chernobyl disaster was a nuclear

accident that occurred on 26 April 1986 at the

Chernobyl Nuclear Power Plant in Ukraine. It is

considered the worst nuclear power plant

accident in history, and is one of only two

classified as a level 7 event on

the International Nuclear Event Scale



´ Even when not actively generating power, nuclearpower reactors require cooling, typically providedby coolant flow, to remove decay heat. Pressurizedwater reactors use water flow at high pressure toremove waste heat. After an emergency shutdown,the core still generates a significant amount of residual heat

´ There had been concerns that in the event of a

power grid failure, external power would not havebeen immediately available to run the plant'scooling water pumps.



´ Chernobyl's reactors had three backup diesel

generators but there will be power gap and this

was unacceptable, and it had been suggested

that the rotational energy of the steam

turbine and residual steam pressure could be

used to generate electricity



´ The test focused on the switching sequences of theelectrical supplies for the reactor. The test procedurewas to begin with an automatic emergency shutdown.

No detrimental effect on the safety of the reactor wasanticipated, so the test program was not formallycoordinated with either the chief designer of the reactoror the scientific manager. Instead, it was approved onlyby the director of the plant (and even this approval was

not consistent with established procedures). According to the test parameters, the thermal output of the reactorshould have been no lower than 700 MW at the start of the experiment. If test conditions had been as planned,the procedure would almost certainly have been carried

out safely; the eventual disaster resulted from attemptsto boost the reactor output once the experiment hadbeen started, which was inconsistent with approvedprocedure.



´ As the experiment began, the four pumps wereactive. The steam to the turbines was shut off, anda run down of the turbine generator began.

´ As the momentum of the turbine generatordecreased, the water flow rate decreased, leading to increased formation of steam voids (bubbles) inthe core which reduced the ability of coolant toabsorb heat.

´

As the temperature went up the power output of the reactor also increased

´ Control rods were used to control the rate of reaction.



´

A bigger problem was a flawed graphite-tipcontrol rod design, which initially displaced

coolant before inserting neutron-absorbing

material to slow the reaction. As a result, the

control rods actually increased the reaction

rate in the lower half of the core.

´ A few seconds after the start of the insertion of

control rods, the core overheated, and secondslater this overheating resulted in the initial

explosion



NEW SAFETY LAWS AFTER

CHERNOBYL

´ Permission of technical experts were made

mandatory before any test run.

´

All reactors should undergo power failure testand other possible test before being used in a

plant.

´ More responsibility were given to technical

experts during test run.

´ Safety symbols were made mandatory.



SEVESO DISASTER

´ The Seveso disaster was an industrial accident

that occurred around 12:37 pm July 10, 1976,

in a small chemical manufacturing plant

approximately 15 km (9.3 mi) north of Milan in

the Lombardy region in Italy



´ The chemical 2,4,5-trichlorophenol was being

produced there from 1,2,4,5-tetrachlorobenzene by

the nucleophilic aromatic substitution reaction

with sodium hydroxide. The 2,4,5-trichlorophenol was

intended as an intermediate for hexachlorophene.



´ This reaction must be carried at a temperatureabove that of the normal process utilities that wereavailable, so it was decided to utilize the exhaust

steam from the electricity turbine on site, and passthat around an external heating coil on the reactor.This exhaust steam was at 12 bar and 190°C,resulting in a reaction mixture at 158°C (with a

boiling point of 1

60°

C). Safety testing showedonset of an exothermic side reaction at 230°C



´ On this occasion the batch process was interrupted prior

to finishing the final step of removal of ethylene

glycol by distillation, due to an Italian law requiring shutdown of plant operations over the weekend.

´ Other parts of the site started to close down as batches

finished, and no more were started. This caused theload on the turbine to fall dramatically, resulting in the

exhaust steam temperature rising to around 300°C,

heating the reactor wall above the level of the liquid tothe same temperature.



´ This batch was then stopped by isolating thesteam, and turning off the stirrer. The residualheat in the jacket then heated the upper layer

of the mixture next to the wall to the criticaltemperature (which was actually only 180°C,50°C lower than believed), starting a slowrunaway decomposition, and after seven hours

a rapid runaway reaction ensued when thetemperature reached 230°C



´ The relief valve eventually opened and 6 tonnes

of material were distributed over an

18 km2 area, including 1 kg of 2,3,7,8-

tetrachlorodibenzodioxin ² which is normally

seen only in trace amounts of less than

1 ppm (parts per million). However, in the

higher-temperature conditions associated withthe runaway reaction, TCDD production

apparently reached 100 ppm or more



EFFECTS

´ Emergency slaughtering commenced to

prevent TCDD from entering the food chain

´ 1600 people of all ages were found to suffer

from skin diseases

´ An excess mortality from cardiovascular and

respiratory diseases was uncovered,



EFFECTS OF THERMAL RUNAWAY

´ A runaway exothermic reaction can have a range of results from theboiling over of the reaction mass, to large increases in temperatureand pressure that lead to an explosion.

´ If flammable materials are released, fire or a secondary explosionmay result.

´ Hot liquors and toxic materials may contaminate the workplace orgenerate a toxic cloud that may spread off-site.

´ There can be serious risk of injuries, even death, to plant operators,and the general public and the local environment may be harmed

´ At best, a runaway causes loss and disruption of production, at worstit has the potential for a major accident, as the incidents at Sevesoand Bhopal have shown.



PREVENTION, INTERVENTION AND

EFFECT REDUCTION

The prevention of runaway reactions with advice on

issues such as:

´ Safe operation conditions, process monitoring or

intrinsically safer design.

´ In those cases where runaway hazards cannot be

eliminated, applicability of intervention techniques

such as inhibitor or coolant injection can be

investigated or requirements for effect reduction

(usually vent requirements for pressure relief) can be

determined.



SAFETY MEASURES

´ You can ensure safe operation in a number of

ways, by using:

´ Inherently safer methods, which eliminate or

reduce the hazard;

´ process control, which prevents a runaway

reaction occurring; and



ELECTRICAL FAULTS

´ Pharmaceutical industry or chemical industry uses a variety of flammable solvents which can be ignited electrical fault

´ Short circuits resulting in sparks, arcing, overheating of electricalcables light fixtures etc are some of the electrical faults that resulted

in fires´ Inappropriate electrical fittings in hazardous areas where specialized

electrical fittings like flame proof, intrinsically safe are to be usedhave also caused huge fires

´ Static charges generated by the transfer of solvent through a nonconducting medium have the potential to cause fires



INHERENT SAFETY

´ Where possible, you should first eliminate or reduce

hazards by inherently safer design. For example:

replace hazardous materials with safer ones:-have less unreacted material in the reactor, eg using a

continuous process instead of a batch reactor:

´ use a semi-batch method (in which one of the raw

materials is added over time) instead of a batchprocess; and/or.-

´ use a heating medium which has a maximum

temperature that is too low for the reaction mixture todecompose.



PROCESS CONTROL

´ Process control includes the use of

´ sensors

´ Alarms

´ trips and other control systems that either takeautomatic action or allow for manual interventionto prevent the conditions for uncontrolled reactionoccurring. Specifying such measures requires a

thorough understanding of the chemical processinvolved, especially the limits of safe operation.



PROTECTIVE MEASURES

Protective measures do not prevent a runaway but reduce theconsequences should one occur. They are rarely used on their own assome preventive measures are normally required to reduce thedemand upon them. As they operate once a runaway has started, adetailed knowledge of the reaction under runaway conditions is

needed for their effective specification. You can:´ design the plant to contain the maximum pressure-fit emergency

relief vents and ensure vented material goes to a safe place

´ crash cool the reaction mixture if it moves outside set limits;

´ add a reaction inhibitor to kill the reaction and prevent runaway; or

´ dump the reaction into a quenching fluid.



SELECTING THE BASIS OF SAFETY

´ The basis of safety for a chemical reaction is the combination of measures which are relied upon to ensure safe operation. Themeasures you choose for a particular case will depend on a numberof factors, including:

´ how easy it is to prevent runaway;

´ how applicable the various methods are´ how compatible the measures are with plant operation.

´ In practice, you may not be able to eliminate all hazards by inherentlysafer methods and may choose to add control measures to furtherreduce risk and back these up with protection, such as a vent, to dealwith the residual risk. Such a combination of methods is common. Asa runaway incident may affect the environment, you should alsoconsider whether your measures are adequate to comply withenvironmental law.



REFERENCES

´ Barton JA and Nolan PF 1989 Incidents in the

chemical industry due to thermal runaway

chemical reactions Hazards X: Process Safety

in Fine and Speciality Chemical Plants- IChem

115: 3-18

run away reactions

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