(44l) Beyond DSC, Improved Thermal Hazards Screening Methodology

Thermal Hazards Screening is a primary part of
any chemical process safety program. 
Differential Scanning Calorimetry (DSC) is used extensively in studying
the thermal properties of materials and is often used to screen for thermal
hazards. This method is quick and inherently safe as it can be done with a
small amount of material (typically milligrams).  These two benefits cannot be understated as
there may be hundreds of screening test which a lab may have to do in a given
year and sometimes little or nothing is known about its thermal behavior, its
toxicity, or the toxicity of the decomposition products.  DSC also has some short comings as it is
unable to measure pressure, which can be a significant hazard, even when the
heat release is relatively low.  Also it
can be difficult to handle multiphase samples or samples which react together
at room temperature. 

The Multiple Mode Calorimeter (MMC) is an
excellent tool to be used in conjunction with the DSC to provide a more
comprehensive look at thermal hazards. 
The MMC uses samples sizes up to about 3 grams so it is possible to do
injection and stirring.  In the MMC
temperature and pressure are recorded as a function of time.  Sample energies measurement of both
endotherms and exotherms is possible. Heat capacity can be calculated as a
function of temperature. This method offers a quick and inexpensive way to
perform isothermal tests which can be used to help design larger scale tests
done in 2L+ reaction calorimeters or pilot plant operations.

The Multiple Mode Calorimeter consists of an
electronic base and a detachable calorimeter assembly.  There are multiple types of calorimeters
available which can be operated by the electronic base.  This allows the user to select the right
calorimeter for the type of experiment that is needed.  The calorimeters can also be tethered to the
electronic base with an electronic cable. 
This allow for the calorimeter to be placed remotely from the electronics.  The calorimeter can then be easily placed in
the hood and/or behind explosion barriers or even placed inside a freezer for
sub-ambient operations. This instrument has been described in detail elsewhere.

There are a number of calorimeters available;
the best two for Thermal Hazards Screening is shown in Figure 1 & 2. The
first calorimeter (Figure 3) is capable of operating as a low phi adiabatic
calorimeter but more importantly, for thermal hazards screening, can run in
constant power or constant temperature rate mode. The MMC external heater
module does not run in adiabatic mode but can be operated in scanning
mode.  Both MMC modules incorporate the
VariPhi technology and can run true isothermal calorimetry tests. These MMC
modules can measure endotherms and exotherms as well as calculating heat
capacity during those periods where no transitions or reactions are
occurring.  The MMC measures sample
pressure as a function of time/temperature which is critical in screening for
thermal hazards.

        
Figure 1                          Figure 2                                         Figure 3                                                    

   MMC VariPhi Calorimeter      MMC External Heater               Netzsch DSC 200 F3 Maia^®

DSCs, like shown in Figures 3, have been used
routinely as an initial step in the thermal hazards screening process.  They have some unique advantages which make
them suitable for this task.  Scanning
tests can be done very rapidly and with an auto-sampler they can be used to
screen many samples efficiently.  They
sample size is small which makes them intrinsically safe in case of explosion
or toxic gas production.  These systems
are small, inexpensive and have low operating costs.  There are numerous DSCs available on the
marketplace and they are used in many different applications.  Two features are important from the thermal
screening application.  The first is an
auto-sampler as discussed earlier.  The
second is the material of construction used in the DSC.  It is likely that a DSC system used for
thermal hazard screening will see explosions and the release of corrosive
materials.  DSC systems that can run
hundreds of polymer melt tests without fail may have a limited life in the
thermal hazards screening field.  This is
often unlooked.  Figure 5 shows the
cut-away of a robust DSC system.

Their continues to be a need to be able to
safely and effectively screen thermal hazards for organizations involved in the
manufacture, processing, storage or transportation of chemicals. Numerous
screening tools are available in the marketplace, perhaps the most widely used
is the Differential Scanning Calorimeter (DSC). 
This method has many advantages but the largest disadvantage is that the
sample pressure is not measured.  Thermal
hazards are normally associated with pressure increases and these pressure
increases are what ultimately provide the hazard to equipment and
personnel.  A storage drum does not melt
due to a thermal runaway it ruptures due to the pressure increase because of
the thermal runaway.  Screening tools
should be sensitive enough to see small exotherms that in near adiabatic
conditions can fuel a runaway, they should be easy to use, provide quick
results, and be as flexible as possible to handle different scenarios.

[1] Chippett, A New,
Low Φ Factor, Scanning, Adiabatic Calorimeter, 18^th Annual CCPS
International Conference, 2003

[2]Chippett, US Patent
Number 7,021,820 B2, April 4, 2006