Chemical Engineering October 2014 - 51

Feature Report
A vaporizing (or tempered) system
often contains components
whose vapor-liquid equilibrium
controls the reaction temperature
and reaction rate for a fixed containment
pressure. This occurs
when components boil prior to the
gaseous decomposition, so heat
from the exothermic reactions is
removed by the latent heat of vaporization,
and the system pressure
is equal to the component
vapor pressure. The principal parameter
determining the vent size
is the rate of temperature at the
relief set pressure.
A gassy system contains a reaction
or decomposition that produces
non-condensable gases, with
continually increasing containment
pressure and a loss of tempering,
where the reaction temperature
may escalate rapidly. The
total pressure is controlled by the
gas generation, and the vent size
is determined by the maximum
pressure-rise rate. It should be emphasized
that a vapor system may
evolve into a gassy system when
the tempering is lost, which may
lead to a hybrid system.
A hybrid system has both tempering
and gassy characteristics, where
gaseous decomposition occurs prior
to boiling, but the rate of reaction
(the gas produced by decomposition)
is still tempered by vaporization.
The total pressure in the vessel is
the sum of the gas partial pressure
and the vapor pressure, and both
the rates of temperature and pressure
rise are needed to determine
the proper vent size.
Adiabatic calorimetry. Adiabatic
calorimeters provide useful information
on thermodynamic parameters.
Adiabatic calorimetry tests can record
temperature profiles over the
whole reaction process. The analysis
of the results includes the onset
temperature, adiabatic temperature
rise, self-heating rate, pressure-rise
rate, time to reaction, velocity constant,
activation energy and heat of
reaction. Information obtained from
the instruments can be used to calculate
runaway reaction kinetics
and conduct vent-sizing analysis. A
simple test using an APTAC's venting
capability can help to generate
pressure-relief sizing data, by providing
information about maximum
temperature and pressure, rates of
temperature and pressure rise and
gas and vapor production within
the system.
The APTAC can function in heatwait-search
mode, as well as heatsoak-search
mode, to determine
onset temperature and self-heating
rate for reactive materials. For example,
both modes have been used
to study the runaway behavior
of 25 wt. % hydroxylamine nitrate
(HAN). A comparison of the two
modes shows that HAN decomposition
exhibited a strong autocatalytic
behavior and that the
explosion state could start at a
much lower temperature following
a certain aging period. Based
on these experiments, it is recommended
that special work practices
be applied for handling HAN
at low temperatures, and that
longterm storage of HAN should
be avoided [22]. The APTAC has
also been used to study the effect
of metal contaminants on hydroxylamine
(HA), determining
that the presence of metal contaminants
or significantly higher
temperatures can greatly increase
the decomposition rate, creating
hazardous conditions [23].
Isoperibolic calorimetry. Another
form of calorimetry is isoperibolic
calorimetry, which can be considered
as a simple form of a heat-flow
calorimetry. With this technique,
the heat transfer medium is maintained
at a constant temperature,
and the temperature difference between
the sample and the medium
is recorded, providing data about
heat flow. In this type of calorimetry,
large amounts of sample can
be used. Experiments using isoperibolic
calorimetry on a 20 g solution
of HA in water demonstrate that
an increase in HA concentration
or temperature results in a faster
reaction rate. In this case, reaction
conditions dramatically affect the
global heat of reaction. Also, the
presence of metals or gases in the
environment can exacerbate the explosivity
of HA [19, 24].
Isothermal calorimetry. The
isothermal calorimeter is a kind
of heat-flow calorimeter where a
heat-transfer medium is applied
to remove heat as it is generated,
maintaining isothermal conditions
within a sample in the reactor. This
type of calorimeter (specifically the
HEL Simular model) has been used
to identify the intermediates that
form during the thermal decomposition
of HA, as spontaneous formation
of unstable intermediates can
trigger a runaway reaction [25].
Micro-calorimetry. For very small
sample sizes, a micro-calorimeter or
microreactor-based calorimeter can
be used to measure enthalpy and
heat capacity changes [26]. A microcalorimeter
can measure the heat
flux of a small thermal mass and
reagent quantities at the nano-scale.
In solutions, heat flux into or out of
the sample usually occurs during
reactions. This process involves an
interaction between two molecules,
the conformation of macromolecules
or the shifts in structure of multimolecular
colloidal systems [16]. The
advantage of a micro-calorimeter is
that it can provide uniform heating
and cooling while achieving a high
level of temperature homogeneity,
thus enhancing thermal sensitivity
and improving analysis capability. It
is also inexpensive and portable.
Process design example
There are many ways in which
calorimeters can be used to design
inherently safer processes. For example,
an important reaction in
the pharmaceutical industry, the noxidation
of alkyl pyridines, uses an
excess amount of hydrogen peroxide
in the presence of phosphotungstic
acid as a catalyst. Hydrogen peroxide
shows condition-dependent
decomposition and has the potential
to lead to a runaway reaction,
resulting in the generation of oxygen,
which presents further explosion
hazards. To achieve conditions
where the undesired decomposition
of hydrogen peroxide can be minimized,
the APTAC has been used
to determine the runaway behavior
of the reactants and the products
to ensure that altering process
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