1. Fluidised bed granulation
2. Fluidised drum granulation
3. Re-cycle ratio and seeding
4. Spraying
5. Direction of rotation
6. Retention volume
7. Fluidisation table

Fluidised bed granulation

Fluidised bed granulation is a well-known process to produce fertiliser granules. Its use for ammonium nitrate (AN) and calcium ammonium nitrate (CAN) is rather limited and the process is used mainly for urea. The liquid urea solution is sprayed onto a fluidised bed of seed granules using special nozzles. The granules are fluidised by air. The finely sprayed droplets hit the seed, adhere to its surface and the water phase is removed by drying. Drying is achieved by the heat supplied by the hot fluidising air. Recycling particles are enlarged mainly by layers of melt crystallising on each particle. Hence, the granules grow, with undersized particles being recycled as seed.

The continuous process ensures that the layered granules build up to the target size. A fluidised bed process has the advantage that the granules are in constant motion with the aid of so-called fluidising air and behave in a fluid-like manner. On the one hand, this allows a very good exchange of heat and material at substantially homogeneous temperatures. On the other hand, it transports the granulate particles within the fluidised bed so that the already moistened particles from the spray area near the nozzle reach a drying area. Conversely, dried particles return to the proximity of the nozzle to be sprayed and continue to grow.

In a continuously operated granulator, seeds are introduced into the granulator at one end and, after a defined average residence time, the granules are discharged downstream of the granulator. The discharged crude product is graded by sieving. The meshes of the sieve inserts correspond to the desired minimum and maximum grain sizes of the product granules. The so-called undersize, which has not reached the minimum target size, is passed back into the granulator as seed. The so-called coarse or oversize grains, which have exceeded the maximum target size, are sent to a crusher and, after comminution, are also passed back into the granulator as seeds. This generation of new seeds from undersize and ground coarse granules is essential for the continuous and stable operation of spray fluidised bed granulators. Apart from heat and material balance, the particle balance inside the granulator also has to be controlled.

Fluidised drum granulation

Fluidised drum granulation combines drum granulation with fluidised bed technology, Figure 1. This produces a high quality consistent product with even granules that have a high crushing strength. It does so efficiently and with zero liquid emissions. The process is flexible enough to be able to produce a range of different products. Additives such as filler or micronutrients can be added with the sprayed granules. This process involves the spraying of the solution into a curtain falling down from the fluidised bed installed inside the granulator. Recycling particles and solution are fed to the granulator and the product is enlarged by layering. Atmospheric air is fed to the fluidised bed in the granulator. The particles are fluidised by air to cool them down and to finish crystallisation.

The granulator consists of a horizontal drum, fitted with internal lifters to elevate the product onto the fluidised table, fed with atmospheric air. The drum rotates around its axis in a conventional manner.

A double operation occurs inside the granulator: size increase and cooling. The granulator receives recycled material, which is crushed granules and undersize material. The granules entering the drum are lifted onto the table. As they fall from the table they are sprayed with the melt. The granules are again lifted onto the table where water removal and cooling occurs. The energy for the evaporation is supplied by the heat of crystallisation. This cycle is repeated as many times as necessary to reach the desired grain size, the granules being gradually formed with layers of material.

Re-cycle ratio and seeding.

The granulation is possible only in the presence of the liquid phase, which is either introduced with the feeding material or is fed additionally as melt or solution.

For the formation and the size of the granules it is very important to have the right proportion between the solid and liquid phases. If the liquid phase is missing the particles will be destroyed and there will be a build up of solid material. As soon as the liquid phase is sufficient, bridges form between the particles and bigger granules are formed. In this case the particle agglomeration is due to the attractive forces on the surface of the particles.

When the solid particles are completely wet (the humidity increases the attractive force between particles) then particles of the same size start to agglomerate.

Another key parameter is the seeding material used to maintain the granulation process. Processes for which the important recycle ratio is > 2, and where more than 10% of the oversize product is being crushed, can use these crushed particles as a seeding material for the granulation. However for processes that are more fine-tuned, with a low recycle ratio of 1 or < 2, and that have fewer oversize particles, there is a need to create these seeds. This is done in a specially designed crusher where the feed material could be marketable product. This approach allows precise control of the recycle loop.

During the granulation process there is an auto seeding phenomenon due to material which is broken in the granulator or in the recycle loop. However this cannot be controlled and an additional crusher to produce seeds is still needed.

One of the main advantages of the fluidised drum granulation process is its low recycle ratio. In this process the ratio is generally 1 and that means the quantities of sprayed liquid and recycled solid to granulator are equal. This assumption could be described by the simple expression:

The equation represents the ratio between the weight of the recycle (the solid product going back to the granulator) 𝜙1and the granulated product (product leaving the granulator and resulting the summed solid recycle and liquid melt product) 𝜙2.

This means that when the recycle ratio is 1 the grow up factor of each particle is 1.26. Table 1 shows how the grow up factor varies with the re-cycle ratio.

In order to maintain the granulation loop it is very important to permanently feed it with fresh seeding material. This is a variable in the process as during the granulation itself, the spraying of the melt, and the transport of the particles in the loop some granules break and form seeds that are used for further grow up. However this ‘natural’ process cannot be controlled. This is why a special crusher to produce seeds is generally installed to create seeding material and maintain stable levels in the granulation loop.

Spraying.

Spraying quality is one of the more important parameters to be considered during the granulation process.

Points to be considered are:

• The distance between the sprayers and the curtain – this distance varies depending on the behaviour of each product to be granulated.
• The minimum density of the falling curtain – this parameter defines the absorption percentage of the liquid by the solid. Experience has shown that bigger the diameter of the granules, the lower is the absorption rate. Other parameters also influence the absorption rate, such as the surface tension and temperature.
• The size and speed of the droplets going to the curtain – this is related to the type of nozzle to be used, along with its design and operating conditions.
• Generally the crossing of spraying jets has to be avoided as this generates dust, and encourages the formation of lumps.

Direction of rotation.

The direction of rotation should be such that when the lifters load the fluidised table they do so from the frontal side, see Figure 2.

Retention volume.

The definition of the retention volume is the amount of solid material inside the granulator that is necessary for the effective operation of the equipment.

This retention volume affects the design of the lifters, fluidised table, cylinder diameter, rotation speed and density of product.

It has been observed that the retention volume:

• increases with the feed material;
• increases with the number of lifters;
• decreases when the rotation speed increases;
• decreases when cylinder’s slope increases.

Fluidisation table.

The fluidised table is used to cool down the particles and finalise the crystallisation process from one side and from other side to form a curtain during the product fall, into which the melt is sprayed.

The table is a flat, slightly inclined plate with perforations through which fluidising air is blown.

The air fluidisation speed has to be carefully chosen in order to avoid stagnation of the product on the table when the speed is too low, or its entrainment with the air to the scrubber when the speed is too high.

Reference

Valkov, S. (2015). Update on Fluidised Drum Granulation Technology and its Applicability for Different Fertilisers. Proceedings International Fertiliser Society, 767.

Links to Related IFS Proceedings

235, (1985), Fluid Bed Granulation of Ammonium Nitrate and Calcium Ammonium Nitrate, J P Bruynseels

415, (1998), Fluid Drum Granulation for Ammonium Nitrate, L Dall’Aglio

451, (2000), Design of Rotary Driers and their Application in the Fertiliser Industry, I C Kemp, R J Milborne

782, (2016), Granulation Technology with Flexibility to Produce a Range of Specialist Products, N Kargaeva

783, (2016), Granulation of Complex Fertilisers, H Kiiski and A Kells

820, (2018), Fluidised bed granulation of ammonium sulphate – a new process, Christian Renk and Peter Banik