In the second part of this article, we will explore other factors that affect durability, which include all the aspects influencing feed conditioning and compression.
Also, we will deepen our understanding of the influence of raw material particle size, moisture absorption capacity, the relation diameter/compression of the matrix and the performance of the granulator.
Meal conditioning should be considered as the key process of granulation, since everything that favors conditioning directly affects the quality of the granule and the performance of the granulator.
Conditioning is the process through which the meal is mixed with the moisture from steam and other liquids added to the conditioner (Figure 5). Then, a homogeneous mass will be formed, which will form the granules after passing through the matrix channels.
The higher the meal uniformity and the more embedded the liquids are, the better feed quality and granulator performance will be.
There are two factors closely linked to feed conditioning: temperature and moisture.
1/ As a general rule, considering equal moisture of the mass fraction to be granulated, the quality of the granule improves as temperature increases (Graph 1).
This effect is due to the fact that with a higher temperature both moisture and added liquids better penetrate into the meal, and the conditioning is more uniform.
2/ However, that is not true for the moisture (Graph 2).
Graph 2. Effect of the moisture on granule quality.
In a first stage, when the meal is dry, consistency slowly increases when moisture is added.
From a certain moisture level (different for each type of meal), the water takes up the interstitial spaces between meal particles, resulting in capillary attraction (due to the surface tension of water), cohesion between particles, and a rapid increase of consistency up to a maximum.
In the surroundings of this maximum the meal conditioning is ideal for granulation.
Over the maximum, the thickness of the water film between particles is increased, weakening the capillary action. Then, the mass goes from a plastic consistency to a liquid consistency, which makes the granulation impossible and forms the typical feed layers between the matrix and the rollers. This is observed when the granulator is clogged.
The problem we face during granulation is that moisture and temperature are connected, since we increase temperature through the steam and it incorporates the moisture.
For this reason, one of the key aspects for meal conditioning is to use dry steam.
To get dry steam in a feed mill, a steam ramp is used (Figure 2).
The steam ramp is a device of the steam circuit that, through a control valve, reduces the original steam pressure that comes from the boiler.
When the steam pressure is low, the amount of suspended water in the steam is lower. This excess water is removed thanks to the separator and returns to the boiler.
Therefore, the higher the difference between the pressure of steam produced in the boiler and the pressure of steam added to the conditioner, the lower the moisture incorporated into each cubic meter of steam and, consequently, the higher the temperature increase that can be reached for each point of moisture added.
The “drying” effect of the steam ramp is similar to the Foehn effect that occurs on the mountains.
A mass of humid air coming from the sea ascends over the mountainside and, as it goes up it cools, and consequently it can hold less water. Then, it condenses to form clouds and precipitates as rain. Once the air reaches the summit, it starts to descend on the other side of the mountain and becomes warmer again, and the result is a dry air (Figure 2).
Particle size is another factor of particular importance for the granulation process. It affects the granules at two levels: meal conditioning and final stickiness of the particles.
The smaller the size of the particle, the larger the contact surface area will be and, therefore, the higher the moisture absorption capacity and the easier steam penetration will be (Figure 3). Also, the greater the particle size, the lower the stickiness between particles, and the higher the number of breaking points.
Therefore, the smaller the particle size, the better the quality of the granule (Table 1).
The practical problem is that the current trend is towards increasing feed particle size due to its beneficial effect on animal health (Vukmirovic et al., 2017; Mingbin et al., 2015).
If we want to have a good granule, with a larger particle size, it is necessary to further control the other factors.
Matrix compression and its relation to matrix channel diameter is another important aspect of feed granulation.
Compression concerns the effective depth of the matrix compression channel (Graph 3), which is where the feed is compressed and transformed into granules.
The higher the compression, the better the granule quality, and the lower performance of the machine will be, as more friction occurs, and this makes it difficult for the feed to pass through the matrix (Figure 4).
We must have a compression and a relation compression/diameter which make it possible to optimize production with good quality of the granule.
When optimizing granule quality, we should not forget that the performance of the granulator is inversely associated with the quality, since with a higher performance and the other factors being equal, the time in the matrix channel is lower, hence the durability is lower (Figure 13).
The main factors that affect durability include the aspects influencing feed conditioning and compression.
Steam quality is essential, since it enables meal conditioning by providing temperature and moisture.
Meal temperature always has a positive impact on granulation, however meal moisture, which is affected by moisture from other sources (environment, raw materials, moisture added to the mixer, and moisture provided through steam), has an optimal point from which quality and performance show exponential decline, even stopping the granulator.
Other important factors are:
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