The pelletizing aim is to convert the feed milled into pellets. This physical transformation of feed has many benefits, such as feed densification (about 40%) which generates an increase in storage capacity and a reduction of transport costs, a reduction of dust emissions or a better feed preservation.
However, it has some disadvantages, such as expensive investments (silos, pellet mill, boiler, cooler…), additional energy costs (electricity and steam), or the change in water content with “free” water incorporation.
The pelletizing is characterized by a line composed of different successive steps :
– Storage in the “small storage silo” in various numbers, but generally 2,
– A screw conveyor for each small storage silo which is also the supply screws controlling the pelletizing flow rate,
– A cylinder mixer, also called conditioner, in which the feed is mixed with steam,
– The pellet mill.
After pelletizing, the feed passes by gravity through a cooler and possibly a crumbler or crusher and a sifter. The smaller particles are separated from the feed and come back to the pellet mill. In the line periphery, a boiler and a steam system are installed.
Ingredients usually are supplied to the conditioner by using a screw controlled by a speed variation device. The supply flow rate of the conditioner and the pellet mill is adjusted to a setpoint value for the current speed of the pellet mill motor.
The conditioning of the ingredients is performed by continuous incorporation of dry steam.
The steam injection, close to the inlet, is regulated at the output by a temperature sensor, allowing the feed treatment between 40 and 95°C. The residence time distribution in the conditioner can vary from few seconds to several hundred seconds.
The passing of meal through previously prepared dies affects the palletizing process. Dies can be either disc dies either ring dies (more typically on pellet mill with vertical dies) or rotating dies.
In the pelletizing process, meal is brought, after passing a magnetic device, inside the ring, a metal ring perforated of circular channels. After being compressed by rollers, the meal is oriented towards the die using scraper knives. Then the meal is extruded through the die channels. The product comes out in the form of a cylinder. At the exit of the die channels, two knives slice the cylinder slicing setting up the pellet length.
The steam amount injected into the conditioner affects the production flow rate and the energy consumption of the pellet mill. The pelletizing process is the first consumer of energy in a feed processing plant:
- 50 to 60% of electric energy consumption
- 80 to 90% of steam consumption