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