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Chemical Reaction Hazards
Reactivities of the elements and
structural groupings
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Reactivity of metals
• The elements of group 1 (lA) are the alkali
metals - lithium, sodium, potassium, rubidium
and caesium. Their atoms have a single
electron in their outer shells, which they part
with very readily to form positive ions.
Because of this they react violently with water
and most non-metals.
Reactivity of metals…
• The atoms of group 2 (2A) elements have two
electrons in their outer shells, which they also
lose readily. They are the alkali earth metals:
beryllium, magnesium, calcium, strontium and
barium, which also react vigorously but less
violently with water.
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Reactivity of metals…
• The atoms of group 3 elements have three
electrons in their outer shells and are also
active, but besides shedding electrons to form
positive ions, they may also achieve stability
by sharing their electrons with other atoms.
The first member, boron, is a reactive
nonmetal, followed by aluminium, a metal,
and the less common metals scandium,
yttrium and the rare earths.
Reactivity of metals…
• Most of the elements in periods IV, V and VI are
metals, and apart from the first three of each
period and the rare earths, they are more or less
stable in air and are used in engineering.
• They include the older metals -zinc, copper, tin,
iron, lead, bismuth, mercury, gold, silver and
platinum - and the newer ones - titanium,
vanadium, chromium, manganese, cobalt, nickel,
molybdenum, palladium, cadmium, tantalum,
tungsten, osmium and irridium.
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Reactivity of non-metals
• The elements of group 7 (7B) are the halogens,
fluorine, chlorine, bromine and iodine, whose
atoms have outer shells with one electron short
of a full set. They react strongly with atoms with
surplus electrons, to form negative ions.
• This activity decreases from fluorine to iodine.
Atoms which need two electrons to complete
their outer shells (oxygen, sulphur, selenium,
tellurium and the radioactive metal polonium)
are rather less active.
Reactivity of some structural
groupings
• Those which confer instability to the compounds in
which they are present, often rendering them explosive.
• Those which render the compound liable to polymerise,
with the evolution of heat.
• These are unsaturated linkages between
• carbon and carbon,
• carbon and oxygen, and
• carbon and nitrogen atoms,
• as well as three membered rings containing these
atoms.
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• Those which
render the
compound
liable to attack
by atmospheric
oxygen with the
formation of
peroxides,
hydroperoxides
and other
compounds
containing the -
0-0- grouping.
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The power of reactions
• The main clue to the possible violence of any
reaction lies in the heat liberated, the
temperature that may be reached and the
volume and nature of any gases and vapours
formed. Thermodynamics can tell us:
1. how much heat is given out or absorbed in the
reaction, and
2. whether it is equilibrium-limited or can proceed
virtually to completion.
how much heat is given out or
absorbed in the reaction?
• The first question requires a heat balance to be drawn
up between the starting materials and end products.
• This requires information on
• heats of reaction,
• specific heats,
• latent heats if there is a change of state,
• as well as work done on or by the reaction system,
e.g. through expansion, compression or
electrolysis.
• The heat of reaction is calculated from the heats of
formation of the reactants and products.
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whether it is equilibrium-limited or can
proceed virtually to completion?
• The second question is answered by the
equilibrium constant for the reaction at the
temperature in question.
• This is governed by the 'free energy change' of
the reaction which can be calculated from the
free energies of formation of the reactants
and products.
• Broadly speaking, any reaction (of which there
are many) that can lead to a rise in
temperature of 300°C and/or the production
of a significant amount of gas or vapour may
pose a significant hazard.
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Inorganic reactions
• Most of the more powerful inorganic reactions
come under the headings of:
– oxidation-reduction reactions
– acid-base reactions
– hydrations and hydrolyses
Oxidation-reduction reactions
• Reactions between the many oxidising and reducing
agents are usually powerful and sometimes explosive.
• Oxidising agents include
oxygen, chlorine, nitric, chloric and hypochloric acids
and their salts and hydrogen peroxide.
• Reducing agents include
hydrogen, carbon, most metals and several non-
metals, sulphur dioxide, sulphites, nitrites, ammonia
and hydrazine.
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Acid-base reactions
• These result whenever a 'base' such as a metal
oxide or hydroxide or ammonia reacts with an
acid to form a salt.
• Reactions between :
• 'strong' acids such as sulphuric and
hydrochloric and
• 'strong' bases such as sodium hydroxide are
very violent when the reactants are
concentrated.
Hydrations and hydrolyses
• Several acids and bases are formed industrially by
the combination of oxides with water e.g. the
production of sulphuric acid,
SO3 + H2O H2SO4
• and the slaking of lime,
CaO + H2O Ca(OH)2
• Many anhydrous salts add water to form
crystalline hydrates while titanium chloride is
hydrolysed by water to give titanium dioxide
pigment,
TiCl4 + 2H2O TiO2 + 4HCl
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Some hazardous organic reactions
and processes
• As with inorganic reactions, there are
powerful oxidation-reduction reactions (which
are mostly thought of as oxidations,
reductions and hydrogenations),
• acid-base reactions involving both organic and
inorganic acids, bases and anhydrides,
• and hydration reactions involving organic acid
anhydrides and acid chlorides.
Some hazardous organic reactions and processes
Two principal differences between organic and inorganic reactions are
described below.
• There is virtually no upper limit to the temperature at which inorganic
reactions are carried out, and many ore-reduction processes take place at
temperatures above 1000°C.
• Most organic reactions, however, have to be carried out at temperatures
ranging from ambient to 250°C, due to the thermal instability of the
materials.
• A few carefully controlled reactions such as ethylene production, with very
short contact times (one second or less), are, however, carried out at
temperatures of 800°C or more.
• There is nearly always a fire hazard with organic reactions, which are
mostly undertaken within totally enclosed plant and pipework, often
under pressure.
• Most inorganic reactions are free from serious fire hazards, and many of
them are carried out in plant which is open to the atmosphere.
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Reactivity as a process hazard
• As a potential process hazard, the chemical
reactivity of any substance should be
considered in the following contexts:
– its reactivity with elements and compounds with
which it is required to react in the process;
– its reactivity with atmospheric oxygen;
– its reactivity with water;
Reactivity as a process hazard…
– its reactivity with itself, i.e. its propensity to
polymerise, condense, decompose or explode;
– its reactivity with other materials with which it
may come in contact unintentionally in process,
storage or transport;
– its reactivity with materials of construction, i.e. its
corrosivity.
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Reactivity as a process hazard…
• The hazards of exothermic reactions occur in
several of these contexts, particularly in storage
of compounds which tend to polymerise or
decompose, and in process reactors themselves.
• The rates of most reactions increase rapidly with
temperature, leading to the danger of their
getting out of control, with large rises in
temperature and pressure and loss of
containment of the process materials.
Reactivity between reactants in
processes
• This must be carefully studied when the
reaction system is designed, both from the
thermodynamic and the kinetic aspects.
• The information is vital to the design of the
process in sizing heat exchangers and
determining heating and cooling
requirements, not to mention safety.
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• From the safety viewpoint, the main thing to
know is whether the reaction is strongly
exothermic, moderately exothermic, mildly
exothermic, thermally neutral or endothermic.
Reactivity with atmospheric oxygen
• In most continuous organic chemical reactors which
operate under pressure, air is automatically excluded,
except where it is deliberately introduced as oxidant for a
reaction.
• In some cases more stringent measures are taken, not
merely to prevent air entering plant while it is running but
also to remove it from the plant before starting up and to
remove oxygen from materials entering the process.
• This includes the use of oxygen scavengers such as sodium
nitrite, sodium sulphite, sulphur dioxide, hydrazine and
tertiary butyl catechol.
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Reactivity with water
• Many
chemicals
react violently
with water,
which is
seldom far
away, and is
widely used in
process plants
for cooling
and cleaning.
Self-reacting compounds (mainly
monomers)
• Monomers include styrene and substituted styrenes,
vinyl chloride, acrylonitrile, butadiene, isoprene and
cyclopentadiene, as well as methyl isocyanate whose
runaway reaction caused the Bhopal disaster of 1984.
• Most of these polymerise spontaneously at low rates at
ambient temperature even in the absence of a catalyst.
• All, however, polymerise much faster in the presence
of a catalyst, such as a peroxide formed by the action
of atmospheric oxygen on the material itself.
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Self-reacting compounds (mainly
monomers)…
• These reactions evolve considerable amounts of heat
and their rates increase exponentially as the
temperature rises.
• There is usually a critical temperature and/or a critical
mass for such materials in storage above which disaster
may lie imminent.
• In the case of monomers stored as liquids, the
temperature of the unpolymerised material will rise
until it reaches its boiling point, and then boil
vigorously as the rest of it polymerises.
Self-reacting compounds (mainly
monomers)…
• Most monomers must be stored out of
contact with air, with an inhibitor added which
will prevent polymerisation and/or destroy
any peroxide catalysts that may have formed.
• Inhibitors commonly used are hydroquinone
and its monomethyl ether, tertiary butyl
catechol and phenothiazine.
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Reactivity with adventitious materials
• While explosives are manufactured on a large
scale for military purposes, blasting, fireworks
and rockets, etc., many compounds which might
and sometimes do explode are made in the
chemical industry for entirely different purposes.
• They may possess other desirable properties in
themselves, or they may be intermediates in the
manufacture of other products.
Reactivity with adventitious
materials…
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Reactivity with adventitious
materials…
• A few compounds which readily release oxygen
and form explosive mixtures with organic matter
and other fuels, such as the nitrates and chlorates
of sodium, potassium and ammonium, hydrogen
peroxide and nitric acid.
•
• The fuels include porous or finely divided solids
such as charcoal, coal dust, sugar, sawdust,
sulphur and aluminium powder, and combustible
liquids such as fuel oil, ethanol and hydrazine.
Reactivity with adventitious
materials…
• there is always a need to screen industrial
chemicals and their mixtures for potential
explosivity, and to examine those with this
tendency for destructive power and sensitivity to
heat, friction, impact and other forms of
initiation.
• Only then can safe conditions for their
manufacture, storage, transport and use be
established.
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Structural groups which confer
instability
• 17 structural groupings, nine
of which contain nitrogen,
which confer instability to
the compounds in which
they are present, often
rendering them explosive.
• These groupings are known
as 'plosophors'.
Structural groups which confer
instability…
• Some more complex groupings which create
explosive tendencies in organic compounds
(including salts of organic bases) are:
• Primary nitramine -NH-N02
• Secondary nitramine > N-N02
• Nitroso -N=O
• Diazosulphide -N = N-S-N = N-
• Picrates [C6Hi(N02) 3.0]'
• Iodates [IO3]'
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Structural groups which confer
instability…
• Acetylene and acetylenic compounds are very
sensitive to heat, shock and abrasion, and
many are explosive.
• The Factory Mutual Engineering Corporation
classifies organic peroxides according to their
fire and explosion hazard into five classes, and
gives safeguards for their storage and use.
Structural groups which confer
instability…
• Those at the extremes of the range are:
– Class I. These present a high explosion hazard
through easily initiated, rapid explosive
decomposition. This group may include peroxides
that are relatively safe under highly controlled
temperatures or in a liquid solution, where loss of
temperature control or crystallisation from
solution can result in severe explosive
decomposition.
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Structural groups which confer
instability…
– Class V. These present a low or negligible fire
hazard, and with them combustible packing
materials may present a greater hazard than the
peroxide itself.
Preliminary screening of materials for
explosivity
• Two simple but effective preliminary tests can be
applied to materials whose composition is not
known, provided that effective precautions are
taken.
– The first, which should be done before all others, is to
take about one tenth of a gram of the material and
drop it onto a hot plate at above 300°C. If it goes pop,
bang, snap or crackle, one is warned of possible
trouble. If the sample decomposes or chars the test is
continued until no further decomposition is apparent.
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Preliminary screening of materials for
explosivity
– A test for instability in liquids is to place about 50 ml
in a small beaker located behind suitable barricades. A
small 50 watt coil of heating wire, attached by flexible
cable to a plug and socket switch in a safe location, is
then placed in the liquid. The observer having retired
to a safe observation point, the current is switched on
and left on until the sample has completely
evaporated, decomposed or burnt. The beaker should
not be approached again until it is cool and the
electricity has been disconnected. This simple test,
while not infallible, has detonated compounds whose
explosivity more sophisticated tests had failed to
reveal.
Thermochemical screening
• When any substance explodes, a considerable
amount of heat is usually released, partly in
the form of pressure waves and partly as a rise
in temperature of the explosion products. The
power of the explosion of the substance is
related to this quantity, known as its heat of
decomposition (ΔHd). Values of ΔHd and of
ΔHc, the heat of combustion, are quoted here
in MJ/kg 'net‘ at 25°C and atmospheric
pressure.
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Stability and sensitivity tests
• Tests for thermal and shock stability of
potentially explosive substances are needed to
establish safe processing conditions. Thermal
stability is important for operations such as
drying, reaction, evaporation, distillation and
other processes involving elevated
temperatures. Shock stability is important for
size reduction, pumping, blending and
transport.
Stability and sensitivity tests…
• Unfortunately, the stability of many of these
materials depends on their concentration,
purity and the nature of the impurities
present, as well as on their temperature and
physical environment. These tests are
therefore not infallible. Sometimes an
impurity may be present in an industrial
compound which considerably reduces its
stability.
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Thermal stability
• In addition to screening tests mentioned in
previous slides, a number of special tests are
used. These include the use of modified
melting point apparatus and thermal
analysers which can be employed for
differential thermal analysis (DTA) using
standard procedures.
Thermal stability…
• Since these work at atmospheric pressure and
may fail to detect exotherms on volatile
materials, confinement tests which use
thermal stability bombs have also been
developed. These tests also measure the
pressure rise in a confined space. Other more
refined methods include differential scanning
calorimetry (DSC) and accelerating rate
calorimetry (ARC).
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Shock sensitivity
• Thermally sensitive materials which decompose
exothermally are also sensitive to shock or
friction, although the stimulus needed may be
severe. Tests range from a simple hammer test,
through drop weight impact tests , to more
severe ones which use explosives for initiation,
such as card gap tests.
• A number of other tests, including the Cartridge
Case test for discriminating between detonating
and deflagrating explosives and a Bonfire test to
investigate the behaviour of the bulk material in a
fire.
Typical Hazardous Locations
Class I Locations
• Petroleum refining facilities
• Dip tanks containing flammable or combustible liquids
• Plants manufacturing organic coatings
• Spray finishing areas
• Solvent extraction plants
• Plants manufacturing or using pyroxylin (nitrocellulose) or other
plastics
• Utility gas plants, operations involving storage and handling of
liquified petroleum gas
• Petrochemical plants such as olefins, benzene, toluene, xylene,
vinyl, polypropylene
• Poly vinyl chloride/monoviny I chloride, met Hanoi, ammonia and
other related facilities
• Chemical plants making or using flammable organics
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Class Il Locations
• Manufacture and storage of magnesium and aluminum
powder
• Manufacture and storage of polyethylene fluff
• Manufacture and storage of starch
• Areas for packing and handling pulverized sugar and
cocoa
• Facilities for coal or coke preparation and handling
• Confectionery manufacturing plants
• Chemical plants making or using powders or bulk solids
Class III Locations
• Textile mills
• Cotton gins and cotton seed mills
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