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Thermal Safety

Laboratory Testing TÜV SÜD Schweiz Process Safety

Adiabatic Calorimetry

In an adiabatic calorimeter the heat exchange between the sample and the environment (i.e. the instrument) is reduced as much as possible, either by using insulating sample containers and/or by adjusting the temperature of the environment exactly to the temperature of the sample, so that there is no temperature gradient and thus no heat flow to or from the sample.

Adiabatic tests yield the following safety characteristics, which may be used in process risk analysis:

  • Adiabatic temperature rise (ΔTad)
  • Heat of reaction (ΔH)
  • Activation energy (Ea)
  • Time to Maximum Rate (TMR)

Adiabatic calorimetry is a challenging method. The following points are important for a successful application of this technology:

1. Small PHI factor:

In an ideal adiabatic system, 100% of the reaction's heat should be used to heat up the sample. In reality, heating of the sample container consumes part of the energy, which leads to a deviation from a truly adiabatic behavior. The PHI factor describes a ratio between the total heat capacity of the heated system (i.e. including the capacity of the container) and the heat capacity of a sample. Thus a low PHI factor (i.e. near 1) can be obtained if the heat capacity of the container is small compared to the heat capacity of the sample. This can be achieved either with a large sample mass or by using a light-weighted (but still pressure resistant) sample container.

2.  Excellent calibration of the thermal sensors:

Because the temperature of the instrument has to be exactly equal to the temperature of the sample, it is important that the thermal sensors in the sample and in the instrument be exactly matched. A non precise calibration may result in temperature drifts, which ultimately leads to wrong safety data.

3. Pressure resistance:

Since in adiabatic calorimetry reactions are observed under run-away conditions up to rather high temperatures, a very high pressure may occur. The required sample containers that withstand high pressures (which is difficult in view of point 1 above) or the pressure outside the sample container must be adjusted at any time to compensate the internal pressure.

TÜV SÜD Schweiz Process Safety offers a wide range of adiabatic testing methods:

  • in a Dewar flask or a wire basket with adiabatic control of the oven temperature (1)
  • in a 300bar autoclave with adiabatic control of the oven temperature (2)
  • in the Accelerating Rate Calorimeter (ARC, 3)
  • in the Vent Size Package (VSP2 manufactured by Fauske & Associates, 4)


Why using adiabatic calorimetry?

Only under adiabatic conditions, it is possible to study a reaction under run-away conditions (provided the conditions listed above are fulfilled)

Typical applications are:

  • Assessment of the risks of run-away reactions
  • Verification of model calculations derived from other thermoanalytical tests, e.g. verification of TMRad
  • Design of protection systems such as pressure relief systems, emergency quenching etc.
  • Visualization of possible failure scenarios

Quantitative determination of thermal data using reaction calorimetry

application

The use of reaction calorimetry enables us to determine quantitatively not only the reaction energy but also the amount of unreacted feedstock at any point in time . This data is essential for the design of the cooling system and the measures to be taken in the event of a cooling stirring failure.

the benefits for you

You receive a quantitative assessment of your reaction, which can include a reaction process and the action plan from a single source. Decades of industrial experience guarantee practical solutions.

our services

  • Testing of reactions in an RC1 reaction calorimeter (250 ml to 2 l) with reflux, in a temperature range from 0 C to 160 C, and under pressures up to 60 bar
  • Quantitative determination of reaction energy, heat accumulation and heat capacities
  • Determination of reaction kinetics using an FTIR probe
  • Testing of samples using microcalorimetry

Reaction calorimeter CPA202

You only have a small amount of starting material but you want to know the thermokinetic data of your reaction?

We have the perfect solution for you. We can perform reactions in our reaction calorimeter CPA 202 from the company ChemiSens. By its low volume reactor it is possible to determine the reaction energy and the heat flow using only 40 mL of reaction mass. This instrument is very versatile, it is possible to run reactions between -30°C and 200°C and up to 20 bar.

If you have any questions regarding this topic or if you want a quotation for a specific project, feel free to contact us.


Quantitative determination of heat accumulation hazards

application

Reactive substances that are stored under conditions of insufficient heat removal can accumulate heat internally, which in turn can start a self-ignition, with fire. A well-known example of this phenomenon is the haystack fire.

the benefits for you

Storage and transport of reactive materials optimized for safety with the best choice of containers.

our services

  • Quantitative determination of the heat generated depending on the temperature in the range 30 °C to 300 °C and 0,01 W/kg to 500 W/kg.
  • Estimation and measurement of the heat removal capacities of containers depending on their size, geometry and insulation.
  • Calculation of heat balances and consulting on the choice of container size and type plus safe storage and transport temperatures.

Quantitative screening of reactive mixtures for thermal hazards (microcalorimetry)

application

In the course of a chemical reaction various mixtures of feedstock and products arise. These must be further processed or stored as intermediates. If the thermal hazard of these subsequent processes is known quantitatively, the manufacturing process can then be designed for safe operation.

the benefits for you

You receive a quantitative assessment of your reaction, which can include a reaction process and the action plan from a single source. Decades of industrial experience guarantee practical solutions.

our services

  • Quantitative screening of reactive mixtures using Differential Scanning Calorimetry in gold-plated high pressure crucibles (Please note that TÜV SÜD Schweiz Process Safety sells these high pressure crucibles in our testing equipment shop). A set temperature program is used to heat up the sample. The various thermal processes are characterized by the temperature range where they occur and by measuring the heat they generate.
  • Determination of heat of mixing in a Setaram C 80 microcalorimeter with a mixing cell.
  • Determination of the pressure build-up due to reactions or decompositions using a Setaram C 80 microcalorimeter with a pressure cell.

dsc under oxygen

For the determination of safe drying conditions in convection dryers (fluidized bed, spray dryer) self-ignition tests in wire baskets, or air-over-layer tests are usually carried out. A widely used method is the screening test according to Grewer (VDI 2263).

In many cases, and in particular when safety data is to be determined in early stages of process development, the amount of available substance does not allow wire basket tests in the scale of tens or hundreds of milliliters. Moreover tests in wire baskets are critical with respect to laboratory hygiene.

TÜV SÜD Schweiz Process Safety has now developed a test method based on DSC measurements, estimates onset temperatures in the self-ignition test. DSC dynamic scans in crucibles closed under 5bar of pure oxygen are well suited for such approximations.

Based on calibration with phase transitions of ammonium nitrate the sensitivity of the set up according to Grewer has been found to be 60 mW/(g•K). DSC tests under oxygen are about 10 times more sensitive. Assuming an activation energy of 120kJ/mole at 150°C the test would yield a theoretical difference in the onset temperatures of 30°C.

The DSC method has the following advantages:

  • Small sample quantity
  • Allows testing of highly toxic or highly active substances
  • Quantitative data evaluation
  • Suitable for both liquids and solids
  • Possibility of determining oxidation reactions above the melting point of solids

The method has been used in many studies and is available also for your samples.

  

Stability measurements by Differential Scanning Calorimetry in glass cells

Differential Scanning Calorimetry is a standard method to screen for the thermal stability of substances and reaction mixtures. Usually closed pressure-resistant stainless steel crucibles are used. To prevent interactions between the steel and the sample, the crucibles are coated with a thin layer of gold, as gold is assumed to be inert against most substances.

However this is not always the case: some samples, for example substances that contain chloro- and nitro-groups can corrode the gold layer. In other cases, for example peroxides, it is known that gold catalyses their decomposition reactions.

In order to exclude these interactions, glass cells are sometimes used, which are closed by melting. Until now, the use of glass cells has however been quite cumbersome:

  • It was difficult to avoid contamination of the walls when filling the glass cell. During the melting process the substances were then pyrolized which changed the composition of the sample. Also the resulting local gas formation produced bubbles in the glass and so impaired the closing process by melting.
  • The melting process was also difficult to control; the crucibles were after of unequal height and weight. A reliable calibration of the instrument and therefore a quantitative evaluation of the measuring curve were therefore not possible.
  • The resulting glass cells had an unknown pressure resistance, so they could only be used at comparatively low temperatures. Even then some cells exploded and damaged the measuring head of the instrument resulting in expensive repairs and long down times.

New techniques

TÜV SÜD Schweiz Process Safety has developed a new technique to close glass cells, by which all the above mentioned drawbacks can be avoided: we can deliver measurements with the same reproducibility as with our high pressure stainless steel crucibles. A reliable calibration is now possible, as our controlled melting process results in a cell with highly reproducible dimensions and mass. Also the pressure range of the glass crucibles is comparable to stainless steel cells. The diagram shows the measurement of Ammonium nitrate both in gold-coated stainless steel cells and in glass cells.

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