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For difficult applications

Spray granulation of problematic products
For difficult applications

Fluidised bed spray granulation has evolved into an important drying process in the last few years, enabling a liquid containing solids to be turned into a dry product in a single process step. With many products, however, post-drying is indispensable. The A-WT combi technology with integral post-drying provides a compact solution here.

Dr. Hans Groenewold

A characteristic feature of the spray granulation process is that liquid is sprayed onto native particles, which are thoroughly mixed and dried in the fluidised bed. The particles grow as a result of being sprayed. As soon as they exceed a defined, adjustable size they are discharged. A continuous process is created, during which only liquid is added and only dry good product is delivered.
Spray granulation is a convective drying process, i.e. energy is supplied via heated air, which also ensures the evaporation of the water. The combination of liquid spraying and drying in the fluidised bed is the hallmark of this process. The spraying action causes the liquid to be dispersed into fine droplets, thus generating a large surface. One litre of water, for example, produces 15 billion droplets with a surface of 120 m2. These droplets are sprayed onto a fluidised bed, which likewise provides a very large drying surface for drying – or to be more precise for heat and mass transfer. This is demonstrated by another interesting equation: 1 kg of sugar (dp = 400 µm) consists of 20 million particles with a surface of 10 m2.
In the course of this dual process – spraying and fluidised bed – the material to be dried undergoes two stages of drying. The first stage is drying inside the spray jet. The partly dried – though still liquid – droplets meet the particles in the fluidised bed. They are distributed there over the particle surface and partially penetrate the solid, where the second drying step is effected.
While this process is adequate to provide a dry and storable product of the desired grain size for many materials, others require a further step, namely post-drying. Post-drying is necessitated by a combination of two material properties. On the one hand, the product is difficult to dry, often owing to chemically or sorptively bound water. On the other hand, sufficient growth of the particles is only ensured if the material contains enough unbound liquid. Without wishing to enter into the technicalities of particle growth, it can be asserted that particle humidity influences both growth and abrasion, so that the maximum particle size depends, amongst other things, on the content of unbound water.
Spray granulation with post-drying
A spray granulation plant containing a post-dryer is basically designed as illustrated in Figure 1 left. The complexity of the plant consisting of three main components – granulator, post-dryer and filter – is evident. Both the air and the solid must be handled between these components, with the result that there are numerous potential sources of disruption. Another drawback is less obvious. Only a small portion of the energy in the exhaust air from the post-dryer is used, as the product is difficult to dry. On the physical side, a state of thermodynamic equilibrium is reached at low humidity, so that the exhaust air absorbs comparatively little moisture.
The Austrian plant manufacturer Ammag has developed a further spray granulation stage for this application by combining the three above-mentioned plant components in a single unit (Figure 1 right). In this configuration the post-dryer is still a separate fluidised bed, but it is arranged below the granulator. The product granulated in the upper section of the plant, which is as yet too humid, is continuously conveyed into the post-dryer, where it is dried and then discharged. The principle of the plant thus resembles that shown on the left. The essential difference lies in the fact that the exhaust air from the post-dryer is utilised directly as supply air for granulation.
The exhaust air is available because the temperature in the post-dryer (where drying is difficult) is higher than in the granulator (where there should be sufficient humidity to ensure particle growth). Moreover, additional air can be supplied upstream of the granulation process, so that the two bed temperatures can be adjusted independently of one another.
Two advantages in one
There are two major benefits to be achieved by this arrangement. First, the low energy efficiency of post-drying has no negative effect on the overall balance. This positive influence on energy consumption is considerably enhanced by the fact that less air is required overall. Air consumption is reduced by 40 % on average.
The second benefit results from integrating the filters into the granulator, thus preventing recirculation of the filter dust into the granulator from the filter housing.
The A-WT combi technology could also be described as three-phase counter-current drying/granulation. The first phase consists of pre-drying in the spray jet. The second phase takes place simultaneously with particle growth in the granulation zone. The third drying phase is post-drying. The product moves through the plant from top to bottom. The air flows in the opposite direction, in other words from bottom to top. Once again, the combination of counter-current processes for heat and material transmission evidently offers numerous advantages.
Ammag has meanwhile demonstrated that this kind of plant also works in practice and installed several machines that operate according to this principle. Their overall size ranges from pilot scale to a distributor diameter of 3 m with capacities from 20 to 2000 kg/h.
Hall 4.1, Booth L10
cpp 428

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Achema 2006
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