Run Away Reactions
-
Upload
abhishek-verma -
Category
Documents
-
view
223 -
download
0
Transcript of Run Away Reactions
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 1/37
GROUP MEMBERS
ARUN C S
C SRINESH
ROHITH KRISHNAN
SAFETY IN CHEMICAL ENGINEERING
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 2/37
CONTENTS
´ Introduction
´ Run away reactions in Engineering
´
Causes of run away reactions´ Risk assessment
´ Evaluating reaction hazards
´ Major incidents- Seveso and Chernobyl
´ Effects
´ Prevention
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 3/37
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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 4/37
´ 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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 5/37
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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 7/37
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
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 8/37
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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 9/37
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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 10/37
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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 11/37
´ 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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 12/37
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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 13/37
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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 14/37
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
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 15/37
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
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 16/37
´ 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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 17/37
´ 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
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 18/37
´ 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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 19/37
´ 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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 20/37
´
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
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 21/37
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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 22/37
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
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 23/37
´ 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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 24/37
´ 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
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 25/37
´ 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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 26/37
´ 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
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 27/37
´ 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
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 28/37
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,
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 29/37
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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 30/37
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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 31/37
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
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 32/37
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
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 33/37
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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 34/37
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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 35/37
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.
8/3/2019 Run Away Reactions
http://slidepdf.com/reader/full/run-away-reactions 36/37
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.