6 Discussion
6.4 Solvent crystallization of ADN
6.4 Solvent crystallization of ADN
Discussion 77 obtained from 1-propanol does not change under the chosen process conditions. It is noticeable that supersaturation is higher in the large-scale setup than in the small scale setup for both cooling rates.
The use of 1-octanol as solvent is leading to different supersaturation-depending crystal shapes and is discussed in the following.
Chapter 5.2.3.4, Figure 5-59 shows the EC measurements for all crystallization experiments. As there is no significant difference by comparing the trends of EC in O-5-L-#1 and O-5-L-#2 and the trends of EC in O-10-O-5-L-#1, O-10-L-#2 and O-10-L-#3, the supersaturation runs of the corresponding experiments are reproducible. For this reason only one data set is evaluated in terms of supersaturation for processes that are accomplished under the same conditions. The data for O-5-L is obtained from O-5-L-#2, the data for O-10-L from O-10-L-#2.
By cooling down with 5 K/h (experiments O-5-s and O-5-L), plate-shaped crystals are obtained from both experiments but with differently shaped basis areas (Chapter 5.2.3.4 Figure 5-45 to Figure 5-47). By taking a look at the supersaturations during the processes, it can be seen that the maximum supersaturation in O-5-s is S = 1.11 in contrast to the maximum supersaturation in O-5-L (S = 1.18). The variation of the experimental setup leads to a slight increase in supersaturation and to crystals with a different morphology.
The crystals obtained from cooling with 10 K/h are shown in Chapter 5.2.3.4, Figure 5-48 to 5-51. It is attracting attention that the crystals formed by the processes carried out with the higher cooling rate in the large-scale setup exhibit two different crystal morphologies. One fraction of the crystals is rod-shaped, the other plate-shaped similar to those obtained by slow cooling (O-5-L-#1 and O-5-L-#2) while the plates from O-10-L have a higher thickness. The variation of the morphologies resulting from one experiment can be explained by the range of supersaturation that is run through during the process. Two maxima of supersaturation are degraded both times by nucleation.
This is shown in Chapter 6.3.2, Figure 6-8. It is not clear which crystal fraction is formed first. As the rod-shaped crystals are more voluminous than the plate-shaped crystals, it is supposed that they are occurring during the degradation of the first maximum of supersaturation. An indication for this statement is that the amount of dissolved ADN available is higher than the amount that is available when the second maximum of supersaturation is decomposed: The concentration is reduced from c = 0.0180 g/g to c
= 0.0114 g/g (∆c1st maximum = 0.0066 g/g) at the first maximum. At the second maximum, the concentration is reduced from c = 0.0114 g/g to c = 0.0086 g/g (∆c2nd maximum = 0.0028 g/g). The concentration diagram is shown in Chapter 11, Figure 11-12. On the other hand, the number of plate-shaped crystals is larger compared to
the number of rod-shaped crystals. A large amount of small crystals is normally caused by nucleation at high supersaturations. This detail has to be clarified in future experiments by taking samples during the crystallization process.
It can be concluded that ADN from 1-propanol is of a plate-shaped morphology. It was not possible to influence the crystal shape by varying the experimental setup and the cooling rates that were used in this work. ADN from 1-octanol is sensitive to both the experimental setup and the cooling rates that induce different supersaturation during the processes. Different crystal shapes (rods, plate-shaped crystals with differently formed basis areas) are obtained from the processes.
6.4.3 Thermal analysis of recrystallized ADN
The thermal behaviour of ADN crystals resulting from crystallization experiments was analyzed by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) (see Chapter 11, Table 11-4).
For the DSC analysis, the focus was put on the temperature range from 20 °C to 120 °C where the solid-liquid phase transition occurs. Due to the fact that the melting peak of the original ADN used in this work is a sharp endothermic peak with an onset temperature TONSET = 92 – 93 °C, changes in product quality are observable by a reduced onset temperature of the melting peak, a broadening of the melting peak and the appearance of additional peaks.
The TGA was evaluated in that way that ML100 °C, the mass loss below 100 °C, is specified as well as the mass loss from 100 °C until the complete decomposition of the material. A clearly arranged overview on the DSC and TGA results of ADN obtained in the crystallization experiments is given in Table 6-7.
Table 6-7: Overview on the results of the thermal analysis of the recrystallized ADN
sample HMELT
[J/g]
TONSET [°C]
H2nd PEAK [J/g]
TONSET, 2nd PEAK
[°C]
ML100°C [%]
1-propanol,screening 145.8 93.47 - - 0.442
2-propanol, screening 112.4 91.90 2.802 58.08 1.444
1-pentanol, screening 26.76 71.64 - - 5.224
1-octanol, screening 145.2 93.43 - - 1.868
P-5-s 136.1 91.98 6.147 59.44 0.750
P-5-L 38.49 71.70 - - 7.170
P-10-s 153.6 93.72 - - 5.241
P-10-L 153.1 93.53 - - 0.344
O-5-s 119.4 91.58 6.336 59.02 0.956
O-5-L-#1 114.4 90.31 - - 0.430
O-5-L-#2 90.97 86.89 - - 1.559
Discussion 79
sample HMELT
[J/g]
TONSET [°C]
H2nd PEAK [J/g]
TONSET, 2nd PEAK
[°C]
ML100°C [%]
O-10-s 155.5 94.00 - - 0.923
O-10-L-#1 127.7 89.56 - - 0.529
O-10-L-#2 23.57 74.33 - - 6.888
O-10-L-#3 82.59 83.98 6.236 57.84 1.579
For some ADN samples, a second endothermic peak is observed at TONSET = 57 – 60 °C.
As pure ADN does not show solid-solid phase transitions, this is an indication for the presence of at least one additional substance. The appearance of a second peak was also observed by Löbbecke [LÖB99] who investigated the thermal behaviour of ADN by DSC and TGA in detail. According to him, the second peak at T = 55 – 65 °C is caused by an endothermic phase transition of an eutectic mixture of ADN and ammonium nitrate (AN). With an increasing AN concentration, the phase transition enthalpy of the eutectic mixture is increasing and at the same time, the melt enthalpy of ADN is decreasing. The exothermic decomposition of ADN to AN starting at TONSET = 127 °C was published by Löbbecke et al. [LÖB97] while the actual composition of the gaseous side products is more complex than formulated in the simplified reaction that is shown in Equation 6-5. The formation of AN from ADN in the liquid phase is proofed to happen.
In contrast to Löbbecke et al. [LÖB97], the ADN crystals that are investigated in this work are produced by solvent crystallization. This means that the temperature did not exceed TMAX = 45 °C for 1-propanol and TMAX = 55 °C for 1-octanol and is therefore much lower than TONSET = 127 °C. An energetic activation barrier EACTIVATION = 1633.4 J/g for the formation of a transition state, an energy gain EGAIN = -2210.5 J/g by the formation of the nitrate ion from the transition state and a resulting overall energy EOVERALL = -577.1 J/g was published by Politzer et al. [POL98] for the decomposition of ADN to AN. If it is possible to overcome the activation barrier in an ADN solution, the dissociation of ADN to AN will be possible.
NH4N(NO2)2 NH4NO3 + N2O Equation 6-5 The mass loss ML100°C is listed in Table 6-7 because ADN is a hygroscopic substance and the weight loss below 100 °C can be referred to water present in the ADN samples. The water content of the raw ADN and the solvents was measured by Karl-Fischer-Titration when the experiments were carried out (see Chapter 11, Table 11-1 and 11-2). All experiments are carried out in an air-conditioned work room with a relative humidity of approximately 30 %. The laboratories where the DSC and TG measurements are carried out are not equipped with an appropriate air-condition and are therefore not convenient for hygroscopic substances. According to Wingborg [WIN06], the critical
relative humidity at T = 25 °C is ϕCRITICAL,25°C = 55.1 %. This means that storing, handling and processing of ADN must be done at a relative humidity below ϕCRITICAL,25°C to maintain the original properties of an ADN sample. The ADN samples that show a significant ML100°C do all have a reduced melt enthalpy and a broadened melt peak except for P-10-s. The influence of water on the crystal quality was not examined more closely in this work as the water content is most likely caused by the handling in not air-conditioned laboratories.