The effect of conditioning on pellet quality

Residence time

Another factor influencing pellet quality is the residence time of the feed mash in the conditioner. Within a conditioner, residence time is normally altered by changing the level of fill. Normally this is done by changing the angle of the paddles on the mixer shaft. Feed mash that is kept in the conditioner for a prolonged time normally has an increase in pellet durability (Huang et al.,2014).

Moisture diffusion into the feed particles is a relatively slow process compared to the diffusion of heat (Guerin., 2019). Therefore, particles that have been conditioned longer are less brittle and more plastic compared to particles with a shorter conditioning period, where the interior of the particle is still dry and brittle. More plastic particles result in better compaction of the particles to pellets. If sufficient moisture is still at the outside of the feed mash particles during pelletizing, binding is improved. Over conditioning leads to soft, pliable particles but lacks formation of bonds between particles due to absence of liquid bridges (Thomas et al., 1996). Liquid bridges are easy to form, weak bonds between particles. During cooling of the pellets these liquid bridges are transformed into strong solid bridges due to the removal of moisture from the pellets during which solid material is deposited at the contacting points and builds a strong bond between feed mash particles. This leads to a stronger pellet.

High pressure conditioning

In modern feed mills, besides normal conditioning high pressure conditioning is performed. Most commonly an expander, double pelleting or a BOA compactor is used in this process. High pressure conditioning uses mechanical energy to exert shear and increase the density of the feed mash, thus altering the structure. These changes in structure increase particle binding and decrease pellet porosity after pelleting which also results in higher quality pellets (Thomas et al., 1996). Additional effects of high-pressure conditioning are a further decrease in the size of the feed mash particles and decreasing the energy consumed of the pellet press. However, the total energy consumption for feed production including the expander, boa or the additional pelletizer will increase, usually by about 15%.

Take home messages

Conditioning is an important factor in creating good quality pellets. Optimizing pellet quality by altering conditioning parameters can be done by: making sure the steam is of high quality, increasing the temperature of the feed mash, prolonging the residence time in the conditioner and/or adding high pressure conditioning to the pelleting line.

Frequently checking the quality of the steam added into the conditioner is important. These checks encompass the whole steam delivery system, boiler functioning, effectiveness of the steam traps, the pressure drop over the steam reduction system and finally a visual check of the steam added to the conditioner.

There are several situations in which increasing conditioning temperature is beneficial: the current conditioning temperature is low, the steam is of a good quality, and the nutrients in the feed mash are not affected by degradation due to the higher temperature. The increased temperature plasticizes polymer feed components such as starch and protein which will improve pellet quality, increase capacity and reduce specific energy consumption.

Increasing the residence time of the feed mash in the conditioner affects the position of water in the feed mash particles. This in turn may lead to better plastication which may improve pellet quality. Too short a residence time will lead to excess moisture at the outside of the particle; this will lead to blockage of the pelletizer. Optimizing residence time to match particle size is therefore relevant to obtain the best possible pellet quality.

High pressure conditioning will increase pellet quality at the expense of increasing the mechanical energy used in the production process.