Dynamic DSC

Dynamic DSC scans of resole resins show two distinguishable reaction peaks, which correspond to formaldehyde addition and die formation of edier and metiiy-lene bridges characterized by different activation energies. Kinetic parameters calculated using a regression analysis show good agreement widi experimental values.75... 

Dynamic DSC, 409. See also Differential scanning calorimetry (DSC) Dynamic mechanical analysis (DMA), 138, 163, 241-242, 407, 409... 

Keller, A., Stark, D., Fierz, H., Heinzle, E. and Hungerbiihler, K. (1997) Estimation of the Time to Maximum Rate Using Dynamic DSC Experiments. J. Loss Prev. Process Ind. 10, 31-41. 

The most widely recommended calibration method for dynamic DSC operation involves the determination of the extrapolated onset temperature for the fusion of several standard substances, using various heating rates [255,256],... 

Figure 5.6. Dynamic DSC scan of PTFE (from Davis and Zimmerman7 4).

The increase of pore size with increasing amount of solvent can also be monitored with dynamic DSC-measurements. An endothermic peak at T=7 °C, corresponding to the melting point of crystalline cyclohexane, is observed in the opaque samples after the phase separation resulting from the formation of dispersed cyclohexane droplets (Fig. 53). 

But a striking behavior was observed when analyzing dynamic DSC runs at a constant heating rate of 10 K min-1. In these runs, the first-order behavior was found over the whole conversion range, but the activation energy was E = 110 kJ mol-1 (35-40 kJ mol-1 higher than the isothermal value). Therefore, the simple first-order kinetics seems to be an apparent... 

Model parameters were obtained by fitting dynamic DSC scans at 10 K min-1 for a formulation with I0 / M0 = 0.043. The following set of values was obtained ... 

The first step of the assessment is screening for the energy potential of a sample of a reaction mass, where a reaction has to be assessed, or of a sample of a substance, where the thermal stability has to be assessed. This may be obtained from a dynamic DSC experiment on samples of the reaction mass taken before, during, and after the reaction. Obviously, when the thermal stability of a sample has to be assessed, this is reduced to a representative sample of the reacting mass. If there is no significant energy potential, such as if the adiabatic temperature rise is less than 50 K and there is no overpressure, the study can be stopped at this stage. 

For chemical scenarios, the kinetic behavior of the reaction, the temperature and pressure increase rate must be known under runaway conditions in the interval between set pressure and maximum pressure. This implies a good knowledge of the thermo-chemical properties of the reaction mass. The required data are traditionally obtained from adiabatic calorimetric experiments [22, 25, 26]. Nevertheless, other calorimetric methods, especially dynamic DSC or Calvet experiments evaluated using the isoconversional approach, can also provide these data with accuracy and an excellent reliability for the temperature increase rate [27], as well as for the pressure increase [28, 29]. 

There may be great temptation to derive safe process conditions directly from the temperature at which a peak is detected in a dynamic DSC experiment. As an example, a so-called 50 K rule can be found in industrial practice. In fact, such a rule is equivalent to considering that at 50 K below the onset in DSC, no reaction occurs. This is scientifically wrong and may lead to catastrophically erroneous conclusions for two reasons ... 

The temperature determined in a dynamic DSC experiment strongly depends on the experimental conditions, especially on the scan rate (Figure 11.3), on the sensitivity of the experimental set up, and on the sample mass used. 

Figure 11.3 Dynamic DSC experiments for the same decomposition reactions with different scan rates. The peak position is a function of the scan rate. The apparent change of the peak surface is due to the fact that the temperature is a function of the scan rate that was changed.

Figure 11.10 Dynamic DSC thermograms of the same reaction recorded with different scan rates (1, 2, 4, and 8Kmirr ). The heat release rate q and the conversion are plotted as a function of temperature.

As a conclusion, it appears clear that measures must be taken to avoid omitting the charging of compound (A) the omission of (B) is not as critical, and requires no special measure. Thus, based on two dynamic DSC thermograms, important conclusions for the safety of the studied process can be drawn. 

Figure 11.19 Dynamic DSC thermograms of the diazotization reaction mass. Initial concentration in the upper thermogram and increased concentration in the lower.

Figure 11.20 Dynamic DSC thermogram of pure (A). Recorded at a scan rate of 4Kmirf in a closed pressure resistant crucible.

A pure solid reactant (A) is to be used in solution in a solvent for a reaction. The intended process temperature is 80 °C and the reactor is a stirred tank with a nominal volume of 10 m3. The dynamic DSC thermogram of pure (A) is depicted in Figure 11.20. 

Pastre, J., Worsdorfer, U., Keller, A. and Hungerbtihler, K. (2000) Comparison of different methods for estimating TMRad from dynamic DSC measurements with ADT 24 values obtained from adiabatic Dewar experiments, Journal of Loss Prevention in the Process Industries, 13 (1), 7. 

An autocatalytic decomposition can be followed by isothermal aging and periodic sampling for a chemical analysis of the substance. The reactant concentration first remains constant and decreases after an induction period (Figure 12.7). This is characteristic for self-accelerating or autocatalytic behavior. The chemical analysis may also be replaced by a thermal analysis using dynamic DSC or other calorimetric methods, following the decrease of the thermal potential as a function of the aging time. 

Figure 12.8 Dynamic DSC thermogram showing the difference in signal shape between autocatalytic (sharp peak) and nth-order reaction (flat peak).

At too low a temperature, the induction time may be longer than the experiment time, suggesting that there is no decomposition. This false interpretation can be avoided by comparing with the dynamic DSC experiment the measured energy must be the same in both experiments. 

A reaction mass is to be concentrated by vacuum distillation in a 1600 hter stirred tank. Before distillation, the contents of the vessel are 1500 kg, containing 500 kg of product. The solvent should be totally removed from the solution at 120 °C, with a maximum wall temperature of 145 °C (5 bar steam). In order to evaluate the thermal stability of the concentrated product, a dynamic DSC experiment was performed (Figure 12.12). 

Figure 12.12 Dynamic DSC thermogram of 12.3 mg concentrate in a gold plated high pressure crucible. The scan rate is 4Kmin l. The energy is 500JgT Temperature in °C, heat release rate in WkgT...

---