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3.4.1 Procedures

3.4.1.1 - ‘Pure’ Experiments

The terminology of the ‘pure’ system is one without addition of biuret as the impurity. A copy of the solubility curve for urea produced by Speyers (74) is shown in Figure 3.5, along with the experimentally confirmed data points from this work. To confirm the solubility, set amounts of urea were accurately weighed and a set volume of distilled water added. This slurry was then heated to a set temperature and left mixing for around 1 hour. The slurry was then filtered and the weight of solid recovered measured and used to calculate the concentration at each temperature point.

For each experiment, a certain amount of technical grade urea was weighed using an electronic balance, according to the previously confirmed solubility curve (74). The solid urea particles were then poured into each vessel using a funnel to minimise loss and to this the required volume of distilled water was added. This gave a final solution with a supersaturation of 1.05 at a saturation temperature of 30 °C for both systems. The exact amounts utilized in making up the solutions in each system are shown in Table 3.2. The solution was then heated to 40 °C until all the solid had dissolved and was held at this temperature for one hour under the specific mixing conditions. The solution was then cooled at a given cooling rate to a final temperature of 24 °C, as shown in Table 3.2. Once the final temperature had been reached, the solution temperature was held for 30 minutes to allow the crystals to grow.

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Figure 3.5 – Solubility curve of urea in distilled water

Table 3.2 – Components added to the systems and system temperatures

System

3.4.1.2 – 1 wt. % Added Impurity Experiments

When 1 wt. % impurity was added into each system the basic procedure was the same as that of the ‘pure’ trials; this is termed as ‘1 wt. % added impurity’. The specific amounts of urea and distilled water were measured out in the same quantities shown in Table 3.2; to this 1 wt. % biuret was added before all solid particles were poured into each vessel using the funnel. As previously mentioned biuret has a much lower solubility in water than the urea within the experimental temperature range (Figure 3.6), therefore care was taken to ensure that the amount of biuret added was well below the biuret saturation point in both vessels. The required volume of distilled water was then added into each vessel, recovering any solid stuck to the vessel walls. Table 3.3 gives the exact quantities of components and crystallization temperatures. The solution was again heated to 40 °C to ensure the solid was fully dissolved and held at this temperature under the specific applied mixing conditions for one hour. The solution was then cooled at the given cooling rate to a final temperature of 24 °C. Once the final

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temperature had been reached, the solution temperature was again held for 30 minutes to allow the crystals to grow, before the crystals were removed for downstream filtration and washing.

Figure 3.6 – Solubility curve for biuret in water (81)

Table 3.3 – Components added to the 1 wt. % added impurity system and system temperatures

System

The procedures for preparing trials are identical to that of the 1% impurity system, except the amount of biuret was increased, see Table 3.4.

Table 3.4 – Components added to the 5 wt. % added impurity system and system temperatures

System

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It should be noted that the final temperature was lowered by 4 °C in these experiments, this was due to the effect of the added impurity on the nucleation temperature (79). When the amount of added impurity was increased the nucleation temperature dropped (23, 49), while this effect was observed in the 1 wt. % impurity trials, the decrease in temperature was small.

The final temperature of 24 °C was selected for all ‘pure’ experiments; however crystallization had not completely finished at this temperature. To investigate whether the final temperature could have an effect on purity and yield, tests were carried out on a ‘pure’ trial system with further cooling to 20 and 14 °C at a fixed mixing of 750 W/m³ and a fixed cooling rate of 0.25 °C/min in the OBC. A total of three repeats were conducted at each of the final temperatures and the data are given in Table 3.5, with all values being an average of the three trials.

Table 3.5 – Yield and purity results for various final temperatures for the ‘pure’ trials

Run temperature was lowered, there was very little change in crystal purity. In addition, excessive solids were noted in each crystallizer with lower final temperatures; this led to a detrimental effect on the capability of mixing of both systems. Large deposits of solid collected at the bottom of the vessel which could form agglomerates with time. As a consequence of this, 24 °C was chosen as the best compromise in terms of purity study and better overall mixing within both vessels for the ‘pure’ system.

A similar test was carried out with 1 wt. % added impurity and the same outcome was established, while 20 °C was chosen as the final temperature for the 5% impurity tests, this is due to the lower nucleation temperature noted in these tests, as impurity led to the retardation of crystal growth (14), the mixing provided in the two systems is still high enough to avoid excessive solid concentration within either vessel.

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