Effect of Initial Solid Purity

Two different sources of urea were initially utilized within this research; an initial batch from Prolabo UK with an average purity of 89 % (Urea1) and a second batch of industrial grade urea sourced from VWR with an average initial purity of 95 % (Urea2). All initial purities were confirmed in house using the offline FTIR solution method described in Section 4.1.3.

The urea starting purity was thought to be an important factor with a possible effect on nucleation and growth; this must be verified before valid comparisons could be commenced.

Consequently several experiments were undertaken using the ‘pure’ system. Following that the tests were repeated with 1 % added impurity.

3.7.1 Results and Discussion

The first results to be compared are the nucleation temperatures obtained in the ‘pure’ trials, Table 3.8 presents the average nucleation temperature from both urea sources in the OBC, with the standard deviation and Kolmogorov-Smirnov Statistical Probability factor (P). Further discussion of the statistical analysis is provided in Section 4.2. A 'P' value of less than 0.1 is considered to be statistically significant.

Table 3.8 – Average nucleation temperatures (NT) for the OBC system with varying initial urea purity, were n = 3

52

As can be seen in Table 3.8, the nucleation temperatures for the crystallization of Urea1 is statistically lower than that for Urea2 at every condition investigated. This suggests that the lower the initial urea purity, the lower the nucleation temperature and the larger the MSZW.

ensure that the cooling rates are similar, examples of the temperature cooling profiles are shown in Figure 3.8 at a specific cooling rate of 0.50 °C/min and a mixing intensity of 170 W/m³. These curves show a similar cooling slope with the Urea1 having a peak, indicating nucleation at a lower temperature than Urea2. The curves are consistent for all conditions investigated above.

Figure 3.8 – Temperature profiles obtained with a cooling rate of 0.50 °C min^-1 and mixing intensity of 170 W m^-3 for the crystallization of Urea1 and Urea2 in the OBC

To ensure that the nucleation temperature differences noted are due to the initial purity and are not vessel dependent, the same analysis was repeated in the STC. The average nucleation temperatures achieved are shown in Table 3.9, along with the Kolmogorov-Smirnov test result and the standard deviations.

20 25 30 35 40 45

0 1000 2000 3000 4000 5000

Temperature (°C)

Time (s)

Urea1 Urea2

53

Table 3.9 – Average nucleation temperatures (NT) for STC with varying initial urea purity where n = 3

NT + standard

The results again indicate that the urea with the higher initial purity (Urea2) nucleates at a statistically higher temperature than the lower starting purity (Urea1). The consistent trend obtained here suggests that the initial purity of the crystallizing solid has an effect on the nucleation temperature.

Following completion of these comparisons for the ‘pure’ systems, it is important to verify that this trend is still present in an ‘impure’ starting environment. Therefore, crystallization was carried out for both starting urea purities (Urea1 at 89 % and Urea 2 at 95 %) with the addition of 1 % biuret. Table 3.10 shows the nucleation temperatures at five conditions for the OBC, while Table 3.11 shows the results for the STC under the same conditions. Again these values are the average of at least three trials and are shown with the Kolmogorov-Smirnov probability factor and the standard deviations.

Table 3.10 – Average nucleation temperatures (NT) for1 % AI from the OBC system with varying initial urea purity where n = 3

54

Table 3.11 – Average nucleation temperatures (NT) for 1 % AI from the STC system with varying initial urea purity where n = 3

Both Tables 3.10 and 3.11 have the same trend as the ‘pure’ systems with Urea2 showing a much higher nucleation temperature than the lower initial purity system (Urea1). Therefore the starting purity of the compound affects the nucleation temperature obtained in this system, independent of the experimental condition and added impurity level.

The previously noted trends of decreasing nucleation temperature with increased cooling rate (70) are still apparent in Tables 3.10 & 3.11, as are the trends with increasing mixing intensity (31). Table 3.12 shows the nucleation temperature differences calculated from the average values between ‘pure’ and ‘impure’ trials, in both the OBC and STC using both Urea1 and Urea2.

Table 3.12 – Differences in average nucleation temperatures [ANT] between 0 and 1 % AI

Differences in ANT [°C] OBC – 0% to 1% AI STC – 0% to 1% AI

‘impure’ systems are generally larger for the lower initial purity urea (Urea1). When we compare the solid FTIR traces, Figure 3.9, of both compounds we can see that while the two profiles

55

appear similar, there are subtle differences as the material possibly contains different impurities.

This could have led to the larger difference from Urea1, possibly due to extra nucleation retardation thorough the addition of impurity. Explanation of how these spectra were produced can be found on section 4.1.3.

Figure 3.9 – FTIR spectra of Solid Urea1 (grey line) and Urea2 [+0.2 Absorbance offset] (black line)

3.7.2 Conclusion

In conclusion, the initial purity of the compound has an effect on the nucleation temperature obtained, with a lower nucleation temperature being noted with a lower starting purity. This trend is verified as independent of the applied cooling rate and mixing intensity effects.

The addition of a specific impurity lowers the nucleation temperature regardless of the initial purity. It has also been noted that a lower initial purity leads to a larger temperature drop when impurity is added, this is most likely due to the variation in impurity profile between the urea sources. The trends are confirmed for all cooling rates and mixing in both vessels. These results highlight the importance of knowing the initial purity of the substance. Note that all experiments presented in the remaining thesis were completed using Urea2.

0 0.2 0.4 0.6 0.8 1 1.2

600 800

1000 1200

1400 1600

1800 2000

Wavenumber (cm^-1)

Urea1 Urea2

56

STC OBC

PT 100 and Turbidity Probe

PT 100 and Turbidity Probe