Keith C. Behnke
Kansas State University
Pelleting has been, and continues to be, a popular processing technique in feed manufacturing. In basic
terms, pelleting converts a finely ground blend of ingredients into dense, free flowing agglomerates
(pellets). There are many reasons used to justify the process, but it may be appropriate to list just a few:
- improved animal performance,
- decreased feed wastage,
- reduced selective feeding,
- improved bulk density,
- better material handling characteristics,
- destruction of deleterious organisms, and
- customer expectations.
Pelleting operations are not without cost. It is a fairly expensive process in terms of both capital and
variable costs, but the expense is usually justified in improved plant profit as well as animal performance.
The purpose of this paper is to discuss the pelleting process in terms of operations and to describe
how the success or failure of the operation can impact on profits as well as animal performance.
The formation of the pellet actually occurs at the "nip"
between the rolls and the die. All other activities associated
with the operation such as conditioning, cooling, etc. really
support and augment the action at that point in the system.
In order to understand the process and be in a position to
make intelligent decision to improve throughput, quality or
appearance, one must have a thorough understanding of
what happens at the nip point. Figure 1 is a representation
of the die-roll assembly and will help the reader understand
Depending upon the physical characteristics of the feed, a
lesser or greater proportion of the work done by the pellet
mill is used for compression. For example, if the formula
contains a high level of fibrous ingredients such as bagasse,
bran, or ground alfalfa, the mill will expend a large amount
of energy simply compressing the mash to the density of
the subsequent pellet. Conversely, for a relatively dense feed
such as high grain and soy meal, the mill will expend a lesser
amount of energy for compression and a greater amount
The "extrusion area" shown in the figure is the point at which
the mash has reached pellet density and begins to flow
through the die holes. There are many physical forces that
must be dealt with in the pelleting process. Figure 2 is included
to familiarize the reader with some of the major forces involved.
The primary purpose of the roll is to provide a force on the mash to densify the feed and cause it to
flow toward the die. The gap between the roll and the die, the roll surface characteristics and the
physical properties of the mash determine how great this potential force might be.
The die provides, not only the final diameter of the pellet, but the resistance force on the feed and
has a direct influence on throughput rate and pellet quality. These two forces (roll and die) are apposite
each other as shown in the diagram, but must work together to provide quality pellets at an acceptable
production rate. The force generated by the roll must be greater than the resistive force provided by
the die; if not, throughput is zero.
With a general understanding of the process inside the pellet chamber, it is appropriate to move to
a discussion of various factors that affect both throughput and pellet quality.