1. How urea cakes
2. The caking process between two urea particles
3. The factors that influence caking behaviour
4. Controlling the moisture content in urea granules

How urea cakes

Caking of urea is caused by the formation of crystal bonds between particles. Internal moisture or externally absorbed water is attracted, by capillary or thermal action forces, to the contact surface area of the particles. Saturated solutions then form at the contact surface of the particles. Due to a local temperature increase of urea as a result of thermal action, water in the liquid phase can evaporate, and by a crystallisation process, crystals are formed at the contact area of the particles, creating a more or less strong bond between the particles. The evaporation due to thermal action will increase the water vapour content of the warmed-up air, which can diffuse through the heap. When then in contact with a layer of urea of a lower temperature, the vapour is cooled down and moisture is re-condensed and absorbed again at the contact area of the particles. By this mechanism caking can take place through a large volume of a heap, Figure 1.

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Urea is hygroscopic, moisture is thus to be avoided. The concept of the Critical Relative Humidity (CRH) is of importance. The critical relative humidity of urea is around 73% at 30°C. In particular, in mixtures of urea and ammonium nitrate the critical relative humidity is reduced drastically (to around 18% at 30°C) and these two fertilisers ought not to be stored in bulk in the same silo. For the same reasons, contamination of urea is to be avoided and compatibility diagrams should be considered.

The caking process between two urea particles

Figure 2 shows two urea particles mounted in a laboratory test instrument before and after storage in an environmental (controlled humidity) chamber. The particles are prills and nearly round with a diameter of 1.6 mm. After storage at a set relative humidity, the small point contact of the left-hand picture has changed into a wide solid looking bridge. The particles were stored for 4 hours at 30°C and 67% relative humidity, which is below the critical humidity at this temperature. The particle surface appears to be rougher after storage than before. This is attributed to the adsorption and subsequent drying off of moisture during storage and after removal from the environmental chamber.

The differential picture from a microscope shown in Figure 3 illustrates this process. Material was dissolved mainly at the perimeter of the particles halfway between the contact point and the glue point that fixes the particles to the loading pins of the laboratory test instrument. These two points are also the receivers of the dissolved and re-crystallised material. This pattern suggests that the particles must have been covered with a moisture layer thick enough to actually allow flow along the particle surface. The size of the crystal bridge can be easily deduced from Figure 4.

Figure 4 shows the development of this bridge over time for urea prills stored at 70% relative humidity, 30°C, and a pressure loaded equivalent to 7.5 kPa. Even for short storage times, the bridge quickly reaches a size of about 0.9 mm and grows over time to 1.1 – 1.25 mm. Since the pins of the double particle systems (= laboratory test instrument) do not move — at least not measurably — during storage, all this material must be taken from the perimeter of the particles.

Therefore, the particles in the double particle system become less and less spherical and eventually the system might end up as “a cylinder”. This cylinder would be around 1.2 mm in diameter. Wahl et al. have shown that a similar behaviour can be found at other temperatures and relative humidities. Judging from the small increase of the bridge over time, the development of the bridge strength shown in Figure 5 is rather surprising.

After 4 hours at 30°C, 70% relative humidity and 7.5 kPa, the bridge supports a tensile force of about 20 mN. After 150 h, this has increased six-fold to almost 120 mN. The reason for this behaviour can be seen in Figure 6. It shows a cut through the crystal bridge between the two particles. A laser scanning microscope shows that after 4 hours (left-hand picture) a hollow bridge has formed which leaves a ring wall of re- crystallised material around the contact point. Inside the ring, the particle surface seems undisturbed. After 72 hours, the contact has filled out and two solid contact areas representing the bridge can be seen in the laser scanning microscope picture. This suggests that the size of the initial liquid bridge between particles is mainly defined by the humidity level in the gas phase. The amount of material dissolved in it, however, increases over time.

Figure 7 shows a diagram for the caking of urea as a function of the temperature and the relative humidity. The figure shows three distinct areas. For every temperature, the CRH gives the humidity level above which the particles start to uptake moisture and will eventually dissolve (forming in the end a saturated solution). Above this level, there will be no particles anymore in the laboratory test instrument after a couple of hours. Instead, a puddle of urea solution will be found. Below the CRH, a region can be seen where urea particles show caking. For this material, the width of this region seems to be an absolute 10% of relative humidity over the entire temperature range investigated. At even lower relative humidities, no caking could be detected.

Figure 7 clearly shows the strong dependency of the caking behaviour of urea on the prevalent temperature; the higher the temperature (meaning at lower relative humidities) the more likely caking will occur. The reader will appreciate that at higher temperatures the relative humidity may be lower even though the absolute moisture content in the air has increased. For urea with higher levels of formaldehyde, the CRH curve will be somewhat lower, resulting in a more hygroscopic behaviour of urea containing more formaldehyde. Further the graph shows that the higher the product temperature the lower the CRH. This implies it is important to produce – and maintain during shipping and further transport – urea granules with a temperature that is as low as possible and does not vary.

To determine the meaning of the crystal bridge size and strength measured in the laboratory test instrument, small bulk experiments were carried out in a laboratory uni-axial caking tester. Figure 8 shows a typical result for these measurements. The two diagrams show the dependence of the unconfined compressive yield strength on time, temperature and relative humidity.

At 25% relative humidity significant caking is observed only at 60°C after 24 h exposure. At 75% relative humidity, however, caking was much stronger and occurred even at the lowest temperature (25°C) at prolonged (24 h) exposure.

The factors that influence caking behaviour

Moisture and moisture differences between particles or batches of urea product are the most important factors causing caking. Also relevant is the relative humidity of the air in which the urea product is stored. The second important factor is temperature, and temperature differences between particles or batches of urea product, or between urea product and the surrounding air or outer walls. Other factors that influence caking behaviour are:

• free ammonia (and differences in the local concentration),
• formaldehyde content (and any local differences, note also that the formaldehyde content may vary in the granule itself),
• presence and type of coating,
• particle size distribution (size guide number),
• sphericity of the granules (uniformity index),
• dust and fines content,
• storage pressure (height of the urea pile),
• contamination with debris or other fertilisers,
• duration of storage.

It is emphasised that moisture is the key parameter.

The above restrictions, requirements and recommendations are not always easy to comply with, as ambient conditions can be unfavourable, while temperature and relative humidity levels may vary during day and night, and throughout the year. For example, storage of a warm product in a relatively cold environment is a situation that occurs often. Urea product is produced with a relatively high temperature and is transported at this temperature to storage. The average temperature in the storage is somewhat lower. The water vapour pressure of the warm urea product is higher than the water vapour pressure of the surrounding air, therefore some water will evaporate from the urea product into the air. At the same time the top layer of the urea heap will cool down to the lower temperature in the storage leading to a lower water vapour pressure in this top urea product. A moisture (and temperature) gradient is formed between the centre of the heap and the top layer and moisture will migrate from the centre to the top layer, which is supported by the air movement through the heap. As the temperature in the storage will vary somewhat due to day-night cycles, moisture ingress and moisture transport will occur in the top layer of the heap and liquid bridges are formed leading to caking of the top layer. Note that this temperature dependence is less critical when the moisture content of the urea product is lower. This emphasises the importance of keeping the moisture (and temperature) content of the urea as low as possible.

Controlling the moisture content in urea granules

Firstly, it is important to feed the fluid bed granulation with the right urea melt concentration. The right urea melt concentration again is determined to a significant degree by operating the vacuum section at the right design pressures and temperatures as shown by the P and T marks in Figure 9.

In the case of problems being encountered in consistently achieving the required design conditions, companies exist who can provide ‘troubleshooting’ services. In the granulation section several parameters can be adjusted to improve the moisture content as indicated in Figure 10. These are:

• product temperature in the fluid bed,
• product level in the fluid bed (proper bubbling and granule movement),
• control of the urea load nozzle,
• control and proper function of all air nozzles.

Arguably the most important parameter to influence the moisture content of the urea granules is the level in the fluid bed granulator. There needs to be a bubbling bed, which ensures that there is sufficient movement of the granules through the spraying zones. This is necessary to create the right circumstance and surface area to evaporate water from the urea melt and thus assure a sufficiently low moisture level in the granules.

Even when a urea plant produces a perfect product, the product may be mistreated during storage and in the logistics process, reducing its quality, as illustrated in Figure 11. It is  important to provide guidance on urea handling procedures to all parties involved in the logistic chain and customers.

References

M. Wahl, U. Bröckel, L. Brendel, H.J. Feise, B. Weigl, M. Röck, J. Schwedes. (2008). Understanding powder caking: Predicting caking strength from individual particle contacts. Powder Technology 188, 147–152.

Bibliography

IFDC. (2005). Fertilizer Dealer Handbook – Products, Storage, and Handling, Reference Manual IFDC-R-15, D. W. Rutland and J. Polo, IFDC, Muscle Shoals, Alabama, USA  .

Clayton, W. E. (1984). Humidity factors affecting storage and handling of fertilizers. International Fertilizer Development Center, Muscle Shoals, Alabama, USA.  

Links to related IFS Proceedings

131, (1972), Some Fundamental Aspects of Urea Technology, S M Lemkowitz, M G R T de Cooker, P J van den Berg

495, (2002), Urea Production Technology: Recent Developments, M J Brouwer

582, (2006), IPPC: The BAT Reference Document (BREF) for the Manufacture of Ammonia, Acids and Fertilisers, B Serr

725, (2013), Urea-based NPK Granulation – Examination of Constraints and Potential Solutions, S R Doshi