Thermal Analysis for Safety Evaluation of Chemical Processes
On Demand Webinar

Thermal Analysis for Safety Evaluation of Chemical Processes

On Demand Webinar

This webinar is an introduction to thermal analysis for safety assessment in the chemical industry

Thermal Analysis for Safety
Thermal Analysis for Safety

Over the last few decades severe accidents have occurred in chemical production plants leading to high casualties and significant impacts on the environment. In many cases, thermal runaway reactions were the primary cause of such accidents. Using thermal analysis and reaction calorimetry, thermal safety aspects of chemicals and of chemical processes can be assessed and thermal hazards can be identified at an early stage of process development.

In this Webinar, we will show how thermal analysis is applied to investigate hazardous compounds and perform safety studies. We will present some typical examples of samples measured by DSC and TGA.

42:43 min

The Webinar covers the following topics:

  • Basic concepts of safety analysis:
    - Time to maximum rate
    - Criticality of a reaction
  • Thermal analysis and reaction calorimetry applications
  • Instrumentation and applications
    - Differential Scanning Calorimetry (DSC)
    - Thermogravimetry (TGA)
  • Summary

This webinar describes the various uses of thermal analysis for safety assessment in the chemical industry.

Safety issues in the chemical industry

In recent years, a number of serious accidents have occurred in chemical production facilities. Many of these accidents have resulted in severe injuries, or even to the death of plant operators, and have often had a dramatic impact on the local environment.
In many cases, thermal runaway reactions were the primary cause of the accidents. An efficient thermal screening system for chemicals and chemical processes is therefore crucial for reducing accidents in the chemical industry.

Thermal analysis for safety assessment

In general, DSC is used if the focus is on thermal safety aspects of individual chemicals. Reaction calorimetry is used to provide safety information with respect to processing, for example, it allows us to determine the heat flows that occur during the dosing of reactants or during stirring.

The main applications for TGA have to do with evaporation, desorption and vaporization behavior, thermal stability, and the kinetics of decomposition.
Information about the products evolved during a decomposition reaction can be obtained if the TGA is combined with an evolved-gas-analysis technique such as FTIR, MS, or GC-MS. In addition, the DSC signal simultaneously recorded with some of METTLER TOLEDO's TGA instruments measures the melting point or melting range of a material.

Safety Screening of Chemicals and Chemical Processes

Slide 0: Safety Screening of Chemicals and Chemical Processes


Ladies and Gentlemen,

Welcome to the METTLER TOLEDO webinar on the “Safety Screening of Chemicals and Chemical Processes”.


In recent years, a number of serious accidents have occurred in chemical production facilities. Many of these accidents have resulted in severe injuries, or even to the death of plant operators, and have often had a dramatic impact on the local environment.

In many cases, thermal runaway reactions were the primary cause of the accidents. An efficient thermal screening system for chemicals and chemical processes is therefore crucial for reducing accidents in the chemical industry.


Slide 1: Contents

The slide lists the topics that I want to cover.

First, I would like to discuss some basic concepts related to thermal safety of chemicals and chemical processes.

I will then outline some of the fundamental principles of reaction calorimetry and thermal analysis.

After this, I want to present several examples related to the thermal safety of chemicals. This will be followed by a short case study that describes safety aspects of a chemical process using both reaction calorimetry and thermal analysis techniques.


Finally, I will summarize the use of the different techniques and list a number of useful references for further information and reading.


Slide 2: Introduction


During the course of this webinar, I would like to describe how safety aspects of chemicals and chemical reactions can be investigated using reaction calorimetry and thermal analysis techniques.

The focus is on different aspects of thermal safety.

These include the determination of important key values such as

-       reaction enthalpies or specific heat capacities and the kinetic analysis of reactions,

-       and the calculation of factors relevant to safety such as the time-to-maximum-rate, and the TD24, that is, the temperature at which the time to maximum rate is 24 hours.



Slide 3: Introduction

Most of the processes carried out in the chemical and pharmaceutical industry are performed in the batch or semi-batch mode. The hazard potential and risk of a chemical process is related to the reactivity and toxicity of the chemicals involved and the design of the particular process.

While the toxicity of the reactants cannot be influenced, the appropriate design of a process is essential for keeping the reaction under control at any time.


As a typical example, the slide shows a temperature-versus-time-profile of an exothermal batch or semi-batch reaction. Initially, the reactants are fed into the reactor vessel at a certain temperature and are then heated to the process temperature while stirring. From then onward, the reaction usually proceeds isothermally until the initial mixture has been fully converted to the desired product.


Since we are assuming that the reaction is exothermic, the reactor has to be cooled to maintain the reaction temperature. If a cooling failure occurs, the reaction temperature will increase and eventually reach the maximum achievable temperature of the synthesis reaction, MAT. At this temperature, undesired secondary reactions may occur such as the decomposition of the reaction products. The heat produced during these undesired secondary reactions results in a further increase of the temperature in the reactor. This may finally end in a serious hazardous event, a so-called thermal runaway. An important temperature is the temperature at which the time-to-maximum-rate of the runaway reaction is 24 hours (TD24). The time-to-maximum-rate is the time it takes for the reaction rate to reach its maximum value at a particular temperature.


Undesired thermal runaway reactions can be prevented by studying the reaction process and optimizing the process parameters. This is usually done using reaction-calorimetry equipment. The method allows process conditions such as stirring, the rate of addition of reactants, and reaction temperatures to be varied using sample volumes ranging from 40 milli-liters to 6 liters.


Undesired reactions, however, are potentially more dangerous than the desired reaction itself. Fortunately, thermal analysis offers sensitive techniques that allow sample volumes of just 20 microliters to be quickly analyzed. This type of analysis considerably limits the damage that could occur due to a thermal runaway reaction.

Thermal analysis techniques are powerful screening tools that enable thermal safety data to be obtained which can be used to make a sound qualification of the criticality of a product.

The next slide shows a simple risk-assessment scheme based on a DSC measurement.



Slide 4: Simple risk analysis

The heat produced by reactions is a prime factor for the safety-screening of chemicals. Fortunately, reaction- or decomposition-enthalpies can be easily measured by DSC.

The rate at which heat is generated is also very important and can be investigated using a reaction kinetics program such as model-free-kinetics. As a simple rule of thumb, you can say that a reaction is potentially hazardous if the adiabatic temperature increase due to a reaction or decomposition reaction is greater than 50 kelvin and if the rate of increase is high.

In such cases, more-sophisticated tests must be performed, and the entire process will probably have to be reviewed. The adiabatic temperature increase corresponds to a worst-case scenario, that is, the heat released is exclusively used to heat up the reactants. The screening scheme shown in the slide is particularly useful for assessing the thermal safety of products.


A key value frequently used to decide whether an isothermal reaction is fast or slow is the time-to-maximum-rate, that is, the time that the reaction takes to reach its maximum rate. This is illustrated in the next slide.


Slide 5: Time to Maximum Rate                                     Adiabatic conditions

An important characteristic of a thermal runaway reaction is the time it takes to complete. If adiabatic conditions are assumed, the time is called the time-to-maximum-rate-under-adiabatic-conditions.

The scenario behind this concept is to assume that a reaction starts at a certain temperature. If the heat produced by the reaction cannot be removed, the reactants will heat up. The higher temperature will accelerate the reaction so that still more heat is produced. This results in a final temperature being reached after a certain time. After this time, the reaction is completed and the reaction rate has reached its maximum value.

This process is illustrated in the slide. Initially, the reaction temperature was 50 degrees Celsius. If the heat produced at this temperature is not removed, for example because of a failure in the cooling system, the reactants will begin to heat up. After about 99 minutes, the reaction is complete and the temperature of the reaction products has reached almost 190 degrees.

Based on some simplified assumptions about the operating conditions of the reactor and the reaction rate (for example a zero order reaction), the time-of-no-return or TNR can be estimated as 0.632 times the time-to-maximum-rate. The time-of-no-return corresponds to the time after which the runaway reaction can no longer be stopped. This situation can only be prevented if an emergency cooling system comes into operation before the TNR is reached.



Slide 6: Estimating the Criticality of a Reaction

During chemical processing, the criticality of a reaction not only depends on the adiabatic temperature increase that might occur during the reaction or decomposition but also on several other temperatures. This is illustrated in the slide.


The relevant temperatures used to assess the thermal criticality of a process are:

-       the process temperature, Tp (colored green),

-       the maximum achievable temperature under adiabatic conditions, MAT (colored blue),

-       the temperature at which the time-to-maximum-rate is 24 hours, TD24 (colored red), and

-       the maximum-temperature-for-technical-reasons, MTT (colored yellow).


The process temperature, Tp, is the temperature at which the process is carried out.

The maximum-temperature-for-technical-reasons, MTT, is determined by the boiling point of the reactants in an open reactor, or the temperature at which the pressure in the reaction vessel exceeds a certain value.


Class 1 and 2 reactions are reactions in which no special measures have to be taken, that is, the process presents a low thermal risk. In this case, the MAT is well below MTT and also below TD24.

With Class 3 to 5 reactions, MAT is well above MTT or even above TD24 - if a cooling failure occurs, the temperature in the reactor may exceed the MTT. This will definitely lead to a serious hazard.


Further details on this subject can be found in the book by Francis Stoessel entitled “Thermal Safety of Chemical Processes”.


In the following sections, I want to describe some of the techniques used in thermal safety analysis. I will first explain the basic setup of a reaction calorimeter and then discuss some thermal analysis techniques relevant to the field of thermal safety.



Slide 7: Reaction Calorimetry

This slide shows the basic setup of a reaction calorimeter.

A reaction calorimeter consists of reaction vessel in a thermally controlled environment. This allows experiments to be performed both under isothermal and dynamic conditions. The reaction vessel can be equipped with a stirrer, feed units and different sensors, for example for temperature measurement, pH, pressure, and so on.

The upper two pictures in the slide show the RC1 reaction calorimeter. This uses an oil-based high performance thermostat system that enables the heat of reaction to be accurately measured. Its design guarantees fast heating and cooling, precise temperature control and a large heat removal capacity. It can be operated in the temperature range minus 90 to plus 300 degrees Celsius using reactors with volumes of 40 milliliters to 6 liters at pressures of up to 100 bar.

The pictures below show the OptiMax and the EasyMax reaction calorimeters. Here the heating and cooling capacity is provided by Peltier elements. This limits the temperature range from minus 40 to plus 180 degrees. If temperatures below minus 25 degrees are required, additional cooling capacity is needed. The reaction volumes of the Optimax and EasyMax range from 30 milliliters up to 1 liter.


Slide 8: iC Safety – Characterizing the Risk

iC Safety™ is a software package that complements the RC1 control software.

Based on experimental data from the RC1e™ Reaction Calorimeter or a DSC instrument, the iC Safety™ program allows the criticality of chemical reactions and processes to be assessed. The probability and the severity of a runaway reaction are directly linked to the stability and energy potential of the reactants.

Thermal accumulation, the adiabatic temperature rise in case of a cooling failure, the maximum temperature of the synthesis reaction (MTSR) or the maximum achievable temperature (MAT) are some of the quantities that can be calculated by iC Safety™.

This information is visualized in charts called “Safety Runaway Graphs” and “Criticality Graphs”. Typical graphs are shown on the right side of the screen shot of the iC Safety™ program screen. They provide a quick overview of the safety risk of a chemical reaction.



Slide 9: Thermal Analysis

What is Thermal Analysis

The ICTAC definition is:

“A group of techniques in which a physical property of a substance is measured as a function of temperature whilst the substance is subjected to a controlled temperature program”.

The schematic diagram on the right shows a simple linear temperature program with isothermal segments at the beginning and at the end.


The lower half of the slide illustrates typical events that occur when a sample is heated. For example, initial melting, in which the sample changes from the solid to the liquid state. If the sample is exposed to air or oxygen, it will start to oxidize and finally decompose.

Thermal analysis techniques can be used to measure properties such as the heat capacity, thermal expansion, mechanical modulus, softening, changes in sample mass and chemical stability to name just a few.


Slide 10: Thermal Analysis                             Techniques for safety screening

The slide shows the two most important thermal analysis techniques used for assessing thermal safety, namely:


Differential Scanning Calorimetry, or DSC. This is the most widely used thermal analysis technique. The upper picture shows a DSC sensor with a crucible containing a sample, colored red, and a reference crucible.




Thermogravimetric Analysis, or TGA. The picture below shows an inside view of the TGA balance.


I will explain these techniques in more detail in the following slides.



Slide 11: What is DSC?

Let’s begin with DSC. This technique allows us to determine the energy absorbed or released by a sample as it is heated or cooled.


DSC instruments are available in different versions depending on their temperature range, the type of sensor they use, and their heating and cooling rates.


The standard METTLER TOLEDO DSC instrument operates from minus one hundred and fifty degrees Celsius to plus seven hundred degrees at heating rates of up to three hundred Kelvin per minute. Samples are normally measured in small crucibles made of aluminum, alumina or other materials, typically using sample amounts of one to twenty milligrams.



Another useful DSC technique is high-pressure DSC or HPDSC for short. The METTLER TOLEDO HP DSC 1 instrument can analyze samples under inert or reactive gases at pressures of up to ten megapascals. This suppresses any undesired vaporization of samples or enables the stability of samples to be studied under increased oxygen pressures.


The schematic DSC heating curve shows typical effects that are observed when a semi-crystalline polymer is measured. Exothermic effects point in the upward direction and endothermic effects downward. The effects are numbered next to the curve and are explained in the table, namely:

One, the initial deflection or start-up transient of the DSC;

Two, the baseline where no thermal effects occur;

Three, a glass transition with enthalpy relaxation;

Four, cold crystallization;

Five, melting of the crystalline fraction; and finally

Six, oxidative exothermic decomposition.


Slide 12: Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry is used to investigate thermal behavior and study processes such as melting, boiling, crystallization or chemical reactions.

The table summarizes effects relevant to the thermal safety of chemicals. The main applications have to do with reaction kinetics and reaction enthalpies. DSC measurements can also provide information about the specific heat capacity and about other safety and process-relevant effects such as melting, boiling, crystallization or vitrification.


The picture on the right side of the slide shows a view of an open DSC furnace with sample and reference crucibles. The standard crucibles are made of aluminum. High-pressure crucibles are normally used for safety screening experiments in order to prevent vaporization. The crucibles can be used at pressures of up to 200 bar. The small picture in the bottom right corner of the slide shows such a high-pressure crucible.


Slide 13: Application 1: Nitro Compounds by DSC

I now want to discuss some examples in which the focus is on the thermal safety screening of chemicals.

The slide shows DSC curves of 2-nitrophenol, and pure and impure 4-nitrotoluene. The diagram on the left shows the DSC melting curves of the three substances. The diagram on the right displays their exothermic decomposition curves.

The reaction enthalpies can be used to assess the hazard potential of each substance. If we assume a typical sample-heat-capacity of 2 joules per gram kelvin, a reaction enthalpy of 2400 joules per gram would result in a temperature increase of about 1200 kelvin under adiabatic conditions. The sample would decompose completely with the production of large amounts of gases. In a closed reaction vessel, the pressure would increase and the vessel would most likely burst.

The shape of the 2-nitrophenol curve (colored green) indicates that several decomposition steps overlap. Comparison of the reaction peaks of the pure and impure 4-nitrotoluene samples shows that the decomposition enthalpy of the impure sample is about 20% less than that of the pure sample. More important, however, is the fact that the reaction temperature of the impure nitrotoluene sample is almost 50 kelvin lower than that of the pure sample.



Slide 14: Application 2: Nitrocellulose by DSC

Isothermal DSC experiments show how quickly a reaction proceeds and indicate potentially dangerous auto-acceleration behavior. The problem with such measurements is that they take a long time to perform. An alternative is to use the so-called iso-step method.

Here the temperature program consists of a series of short heating segments separated by somewhat longer isothermal segments. This method was developed to determine the critical starting temperature of a reaction more clearly.

The slide shows the measurement of nitrocellulose using this technique. The step-like green curve displays the temperature program. The large black curve is part of the complete DSC curve shown in the small diagram in the upper part of the slide. The dotted blue line corresponds to the isothermal heat flow without a reaction.

We see that an exothermic reaction begins below 120 degrees Celsius as indicated by Arrow 1 on the left. Initially, the heat flow increases continues to increase in each isothermal segment, for example see Arrow 2. This indicates auto-acceleration behavior. From about 155 degrees onward, the reaction begins to behave “normally”, that is, the reaction accelerates in the heating segments but then slows down during the isothermal segments, as show for example by Arrow 3.



Slide 15: Model Free Kinetics

The investigation of the kinetics of a reaction is very useful for estimating the reaction rate and the timescale of a reaction under practical isothermal conditions. This information can be obtained by evaluating several DSC measurements using the so-called "model free kinetics" program. The main points to mention concerning model free kinetics are:

1.     The method makes use of the isoconversional principle. This principle assumes that the reaction rate at a particular conversion depends solely on the temperature. The temperature dependence of the reaction rate is described by the Arrhenius function.

2.     The activation energy in the Arrhenius function is not constant but depends on the conversion. The term “apparent” activation energy is therefore used; “apparent” because it cannot be assigned to an individual process but describes all the processes that occur simultaneously at any time, for example reactions or transport processes.

3.     No additional mathematical model is needed to describe the kinetics.


Slide 16: Application3: Polymerization of Ethyl Acrylate

The slide illustrates the use of kinetic analysis to investigate thermal stability and predict thermal behavior. It summarizes measurements performed on stabilized ethyl acrylate as an example. The liquid is stabilized because it would otherwise immediately polymerize. It is a well-known fact that polymerization reactions also show a tendency to cause thermal runaways.

The slide shows the steps involved in the investigation of decomposition reactions using model free kinetics. The three DSC curves in the left-hand upper corner were measured at different heating rates and show that the production of heat rapidly increases after a certain temperature increase or time. This indicates complex reaction behavior. We assume that, at this point, the stabilizer has been used up and that polymerization suddenly begins with great intensity. The reaction enthalpy of about 720 joules per gram is sufficiently large to decompose the polymer if no cooling is applied. Since the polymerization process proceeds so rapidly, the system behaves practically adiabatically.


The kinetics software calculates the apparent activation energy as a function of conversion from the three DSC curves. The resulting activation energy curve is shown below left in the diagram.

The activation energy is then used to make predictions, for example for the isothermal behavior of ethyl acrylate at different temperatures. Predictions for temperatures of 110, 120 and 130 degrees Celsius are shown in the large diagram on the right. We can see that at 110 degrees it takes about 5 hours before the stabilizer is consumed and the reaction begins.

The accuracy of the prediction of course has to be checked. In this case, it was confirmed practically by performing an isothermal DSC measurement at 110 degrees as show by the dashed green curve. The result shows that there is good agreement between the measured curve and the prediction: for example, the measured curve indicates that it takes 337 minutes to reach a conversion of 60% compared with the predicted time of 304  minutes. A risk assessment will then have to be performed to decide whether, or not, this difference is acceptable.



Slide 17: What is TGA?

Now let’s turn our attention to TGA.

In this technique, the mass of a sample is continuously measured as it is heated or cooled in a defined atmosphere. We simply put a few milligrams of the sample in a crucible, heat it and record the weight change.

The schematic TGA heating curve shows the typical effects observed when a polymer is measured by TGA. In this technique, it is very common to first heat a sample under inert conditions and then to switch the atmosphere to oxidative conditions at a certain temperature. Evaluation of the different mass-loss steps allows us to obtain information about the composition of the sample.

The individual mass-loss-steps and are numbered next to the curve and explained in the table, namely:

One, heating begins and volatile components vaporize;

Two, pyrolysis of organic substances and polymers;

Three, at six hundred degrees Celsius the atmosphere is switched from nitrogen to oxygen to obtain oxidative conditions;

Four, carbon black or carbon fibers burn;

Five, inorganic fillers such as silicates are left behind as a residue.


Some METTLER TOLEDO TGA instruments also simultaneously record DSC or DTA curves. These curves represent effects due to heat exchange and are often very helpful for the interpretation of the TGA curve.


Slide 18: Thermogravimetric Analysis (TGA)

Thermogravimetric analysis is used to investigate processes like vaporization or decomposition. Evolved gases can be analyzed online using hyphenated techniques such as TGA-MS, TGA-FTIR or TGA-GC/MS


The table summarizes the main applications of TGA for the safety screening of chemicals.

The picture on the right is a view of the open furnace and shows a sample holder with positions for the sample and reference crucibles in a TGA/DSC instrument. The standard crucibles are made of alumina.



Slide 19: Application 1: NH4NO3 by TGA and DSC

Chemical substances that contain nitro groups are well-known for their high explosive potential. Some of these substances are used as propellants or explosives, for example nitrocellulose or ammonium nitrate. Ammonium nitrate is also used in large quantities as a fertilizer.

The slide displays DSC and TGA measurement curves of ammonium nitrate. The curves illustrate the influence of the measurement conditions on the course of the reaction.

The DSC curves in the upper diagram show the rate of heat production (heat flow) measured in open and in hermetically sealed high-pressure crucibles.

The TGA curve below shows the loss of volatile material from an open crucible.

If a substance is measured in an open crucible, or in a crucible sealed by a lid with a small hole, the decomposition and evaporation processes may overlap. The apparent reaction enthalpy is then much smaller than when the substance is measured in a hermetically sealed crucible.

In this particular example, we can see from the TGA curve that the sample evaporates almost completely before the decomposition temperature is reached. In a sealed container, a large amount of heat is liberated. At the same time, the pressure in the container increases due to the formation of gaseous decomposition products. In fact, many serious explosions have occurred due to the decomposition of ammonium nitrate.



Slide 20: Case study (TA-RC): Nitration of Benzaldehyde

In the following slides, I want to illustrate how a chemical process can be investigated using reaction calorimetry and how undesired reactions of the reaction product can be screened with respect to their hazard potential by DSC.

The reaction process we chose was the nitration of benzaldehyde. Nitration reactions are often performed in the chemical industry to obtain reactive intermediates. Nitrated products therefore have a high hazard potential because of their inherent thermal instability.

The nitration of benzaldehyde was performed in an RC1e reaction calorimeter equipped with a 500-milliliter reactor vessel, a stirrer, and feed lines for nitric acid and benzaldehyde. This allowed us to simulate a semi-batch process. The reaction was performed at 10 degrees Celsius. Secondary or decomposition reactions were characterized by analyzing samples of the final mixture obtained in the RC1 by DSC.



Slide 21: Case Study (TA-RC): Nitration of Benzaldehyde

The slide summarizes the results of the RC1 experiment.

The upper curve records the total mass of the reactants in the reaction vessel. Initially, the vessel was loaded with 38 grams of sulfuric acid. Nitric acid was added after 20 minutes and benzaldehyde 95 minutes later. The addition of benzaldehyde was stopped after 30 minutes.

The lower curve displays the heat flow measured during the experiment. The curve shows that the nitration reaction starts as soon as benzaldehyde is fed into the nitric-acid-sulfuric-acid solution. When the addition is completed, the reaction stops immediately.

The noisy heat flow curve observed during the reaction is due to the fact that the addition was discontinuous. The small peaks on the heat flow curve at about 70 and 110 minutes are caused by online calibration measurements needed to calculate the heat capacity of the reactants. The enthalpy of nitration was about 177.4 kilojoules per mole benzaldehyde.

In a worst-case scenario with a cooling failure and adiabatic conditions, we estimate that the temperature increase of the reaction mixture due to the nitration reaction would be about 57 kelvin. This temperature is referred to as the “maximum achievable temperature” or MAT. The specific heat capacity of the reaction mixture at the end of the experiment was about 1.66 joules per gram kelvin.


Slide 22: Case Study (TA-RC): Nitration of Benzaldehyde

DSC measurements were performed to study the decomposition reaction of the final reaction mixture. The reaction product is nitrobenzaldehyde and is in solution in a mixture of nitric and sulfuric acids. The measurements were performed in high-pressure crucibles to prevent the solvents from evaporating.

The DSC curve in the lower part of the diagram shows the result of a heating experiment measured at 5 kelvin per minute. The curve shows that an exothermic reaction occurs from 60 degrees Celsius onward. This reaction overlaps a second reaction that produces a large exothermic heat flow above 270 degrees.

The enthalpy of the reaction with a maximum around 132 degrees is about 222 joules per gram sample mass. Using this reaction enthalpy and assuming n’th order kinetics, we can calculate the time to maximum rate (TMR) for any temperature as well as the temperature at which the time to maximum rate, TD24, corresponds to 24 hours.

In this example, we obtain a TD24 of 19 degrees. This is consistent with the TD24 calculated using the iC Safety software package available with the RC1e reaction calorimeter. This allows us to assess the criticality of the secondary reaction.



Slide 23: Case study: Nitration of Benzaldehyde

The slide shows a so-called criticality graph for the process temperature of 10 degrees Celsius used in this experiment.

The relevant temperatures for assessing thermal criticality have been calculated using the activation energy obtained from n'th order kinetics. A value of 90 degrees was chosen for MTT; this corresponds to the boiling point of nitric acid.

TD24 is well below MAT and is very close to the processing temperature. This means that even short interruptions of the cooling system could trigger the secondary reaction and lead to a system runaway. The process therefore has a high criticality and requires additional safety measures such as a redundant cooling system and additional safety valves.


Slide 24: Summary

The table summarizes the most important thermal characteristics used for assessing the thermal safety of chemicals and chemical processes as well as the techniques recommended for obtaining the information.


In general, DSC is used if the focus is on thermal safety aspects of individual chemicals. Reaction calorimetry is used to provide safety information with respect to processing, for example, it allows us to determine the heat flows that occur during the dosing of reactants or during stirring.


The main applications for TGA have to do with evaporation, desorption and vaporization behavior, thermal stability, and the kinetics of decomposition.

Information about the products evolved during a decomposition reaction can be obtained if the TGA is combined with an evolved-gas-analysis technique such as FTIR, MS, or GC-MS. In addition, the DSC signal simultaneously recorded with some of METTLER TOLEDO's TGA instruments measures the melting point or melting range of a material.


Slide 25: For More Information on Safety

Finally, I would like to draw your attention to information about process safety applications that you can download from the Internet.

METTLER TOLEDO publishes articles on thermal analysis and applications from different fields twice a year in UserCom, the well-known METTLER TOLEDO biannual technical customer magazine.

Back issues can be downloaded as PDFs from as shown in the slide. A compilation of applications can be found in the “Thermal Analysis in Practice” handbook.


Slide 26: For More Information on Thermal Analysis

In addition, you can download details about webinars, application handbooks or information of a more general nature from the Internet address given on this slide.



Slide 27: Thank You

This concludes my presentation on the safety screening of chemicals and chemical processes. Thank you very much for your interest and attention.

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