Safety Analysis of a Nitration Reaction by DSC and Reaction Calorimetry - METTLER TOLEDO

Safety Analysis of a Nitration Reaction by DSC and Reaction Calorimetry

Safety is an important aspect in process development in the chemical industry. This article describes how reaction calorimetry and DSC can be used to quickly assess the thermal hazard potential of chemicals and chemical reactions.

 

Introduction

In recent decades, numerous serious accidents have occurred in chemical production plants. Many of these accidents have resulted in serious injuries or even to the death of personnel and have often had a dramatic impact on the local environment. In many cases, the accidents are caused by processes that get out of control due to technical problems. Such so-called thermal runaways can result in disastrous explosions.

This article illustrates how important information for assessing the thermal safety of chemicals and processes can be obtained using reaction calorimetry and DSC analysis.

In the chemical industry, many chemical reactions are performed isothermally. Figure 1 shows a typical temperature-time diagram for an exothermic reaction. First, the reactants in the reactor are heated to the process temperature, TP. This is the temperature at which the reaction is to be carried out under control. In exothermic processes, the heat of reaction generated must be removed to keep the process temperature constant (endothermic reactions, that is, reactions requiring heat are usually not critical). If the cooling system fails, the heat of reaction cannot be completely removed.

As a result of this, the reaction temperature and hence the rate at which the reaction proceeds increase. The reaction product may then begin to decompose exothermically because of the high temperature (secondary reaction). This causes a further rise in the reaction temperature. In the worst case, the reactor system is no longer under control. This leads to a thermal runaway and possible explosion.

An important temperature for safety assessment is the so-called TD24. This corresponds to the temperature at which the time to maximum rate of the runaway reaction is 24 hours. The time it takes to reach the maximum rate under adiabatic conditions is known as TMR, the time to maximum rate.

This usually corresponds to the time when the reaction is finished and the maximum temperature has been reached in the reactor [2].

Other parameters are also important to assess the hazard potential or criticality of a reaction besides the TD24. These include the MAT, the maximum attainable temperature under adiabatic conditions or the MTSR, the maximum temperature of the synthesis reaction, the MTT the maximum technically allowable temperature, and the process temperature, TP.

The maximum technically possible temperature corresponds to the temperature at which the reaction mixture boils or at which the reactor can no longer withstand the pressure.

As shown in Figure 2, five classes of criticality may be distinguished depending on how the parameters TP, MAT/MTSR, MTT, TD24 compare [1]). In Classes 1 and 2, the risk of a thermal runaway is low. In Classes 3 to 5, the MAT/MTSR is clearly above the MTT or even above the TD24. In these cases, the cooling performance should be checked and additional cooling systems made available. There is a large risk of a thermal runaway.

To assess the criticality of a chemical process, the heats of reaction and the kinetics of all the reactions occurring in the process must be known. The reactions can be desired or undesired, such as the decomposition of the reaction product [3]. This information can be obtained without risk from DSC measurements using sample volumes of typically 20 μL.

Furthermore, the process itself (for example the process temperature, dosing rates, mixing, etc.) should be investigated and optimized. This is usually done using reaction calorimeters. Together, the two techniques provide rapid and reliable data that can be used to assess the criticality of chemical reactions. The following example describes the nitration of benzaldehyde and illustrates the combination of the two techniques. Figure 3 shows the equation for the nitration reaction.

Conclusions

Reaction calorimetry and DSC are techniques that are easy to use and safe. They can be used to estimate the thermal hazard potential of chemical reactions and reaction products. DSC enables the energy released when chemicals decompose to be quickly and reliably determined.

Furthermore, DSC provides information about the rate at which decomposition reactions proceed. Important safety-relevant temperatures (MAT/MTSR, TMR, TD24) can be determined when the DSC results are combined with data obtained from experiments using the RC1e reaction calorimeter.

The thermal criticality of a process can then be easily and safely estimated and the particular process optimized.

Whether a chemical process can be carried out safely on a larger scale depends not only on the thermal risks but also on factors such as the mixing behavior during dosing, the heat transfer in the reactor, changes in the viscosity of the reaction mixture or the phase behavior of the reaction mixture.

 

Safety Analysis of a Nitration Reaction by DSC and Reaction Calorimetry | Thermal Analysis Application No. UC 442 | Application published in METTLER TOLEDO Thermal Analysis UserCom 44