The use of preconditioners
Preconditioning
systems have generally been used in feed extrusion lines (e.g. dry pet food, fish-feed)
to overcome certain
limitations that single-screw extruders have in terms of mixing efficiency,
cooking and flow capacity. They are currently used in diverse food and feed
processing applications in both single-screw and co-rotating twin-screw extruders
(TSE).
For example
in TSE production of fish-feed pellets, preconditioners are recognized by
industry specialists as being efficient because they improve productivity and
quality and allow greater flexibility in controlling process parameters.
The function of preconditioners in extrusion
The
preconditioner, located directly upstream of the extruder, is an equipment
component that makes it possible to hydrate, pre-heat and homogeneously mix
powdery raw materials with injected fluids (steam, water and potentially oil or
slurries). This operation is carried out on a continuous basis and involves
facilitating heat and mass transfer as regularly as possible between the solid,
liquid and gaseous phases. The goal is to obtain a homogeneous pre-cooked product
that is best adapted to the next stages.
The quality
of this processing depends in large part on the intensity of the mechanical
mixing, but also on the specific thermal and water diffusivity for each product,
defined by the exposure time needed for the dry material to be in contact with
the steam so that it is properly hydrated and heated. Empirically, a residence
time of about two minutes was generally used upstream in the TSE for fish-feed
flours. New fish-feed recipes –richer in vegetable proteins and with reduced
starch– and new processing throughput and quality requirements for finished
products have led to seek preconditioning times around three minutes or more.
This
increase in preconditioning processing time results in energy and humidity being
disseminated to the core of the particles, which is more efficient than simple
surface treatment, even when apparently similar temperatures are reached.
Eventually, all of the work carried out to prepare cooking and start the starch
swelling is as much energy usefully gained by the product for the following
stages of the cooking-extrusion processing.
Innovations: description of operations
Clextral has
developed a new innovative preconditioner to respond to process requirements
and increase the capacity of existing systems. In the second part of this
article, we refer to this equipment as “preconditioner+” (see Picture n°1) to describe how it operates and to
introduce the main results of experimental tests.
With the
Preconditioner +, the main function - which is to continuously ensure an
homogeneous mix of the raw materials - is carried out by two horizontal
intermeshing counter-rotating shafts, fitted with adjustable paddles (see Picture
n°2), designed to
process the product flow in an optimal way. According to the specific throughput
and the physical and chemical properties of each material being processed, it
defines a distribution and filling ratio in the mixing unit, directly linked
with the residence time required in the end.
The key
innovation of the Preconditioner +, always aiming to improve heat and mass
transfer to the product, is to integrate an Advanced Filling Control device (AFC)
which interacts directly on the material inside the mixing chamber and which
enables the filling ratio to be adjusted. This AFC system uses a special
designed screw inside the tank, and modifies favorably the flow by enabling a
partial and controlled recycling of the material being processed, from the
outlet to the entry point, thus intensifying the specific preconditioning
functions.
In
addition, the AFC system does not just increase the filling ratio, but also
generates better product distribution, in particular in the first third of the
mixing chamber which is the preferential zone for steam injection. To achieve
this, the different adjustable water and steam injection ports have been
optimally positioned –axially and angularly– to enhance absorption and product
exposure time during the mixing stage.
Experimental measurements on the Preconditioner +
Extensive
experimental trials on processing effects have been carried out on the
preconditioner+ to assess the improvements generated by its AFC system on the
twin-screw extrusion process. In this case, a standard fish-feed recipe (basic salmon feed) was used,
without necessarily seeking optimal performance to begin with, but to set an
objective benchmark compared to traditional preconditioning systems.
The first
significant observation was a lowering of the SME (Specific Mechanical Energy)
required in the extruder
–an average of 3% with equal extrusion parameters– mainly due to the increase
of preconditioning time, and largely compensating the additional consumption of
the preconditioner. As mentioned previously, this efficient pre-cooking also
enables a reduction of the shearing and the torque used in the extruder, which
also is beneficial in reducing the wear of the screws and barrels.
By focusing
exclusively on the preconditioner, the experiment also demonstrated that the
new filling ratio (up to 75%) achieved by using the AFC, made it possible to
increase residence time by 60% for an equivalent fish-feed flow and therefore to go beyond the
average of three minutes sought. At the same time, the combination
filling/steam distribution made it possible to increase the product temperature
by 3-4°C at the outlet of the preconditioner, using comparable steam injection.
It is even possible to achieve product temperature of over 96°C with optimized
steam injection, while limiting energy loss thanks to the new design. Following
the improvements on the pre-cooking stage, we were able to measure a
significant increase of the gelatinization of starches before their insertion
into the extruder.
Finally, on
the finished product after extrusion, granulation and drying, these upstream
modifications resulted in increased hardness of the pellets of at least 10 to 15%,
an essential quality criterion required for their use in fish rearing farms.
Functional advantages of the Preconditioner + in industrial applications
Beyond the
process benefits on product quality, it is important to stress that this
equipment has also been designed to facilitate industrial applications, in
particular due to the increased flexibility provided by the AFC. This variable-speed
device makes it possible to adjust the residence time as required and thus
adapt the equipment to different types of recipes, thus responding to the need
to have multi-product extrusion lines. Moreover, this convenient design allows
the system to start up when fully charged, even after prolonged down time with
a full tank, thanks to the new kinematic feature.
In terms of
hygiene or food safety, the design of the preconditioner+ limits to a strict
minimum the retention areas and thus greatly facilitates cleaning. This feature
is further enhanced by an integrated “quick recipe change-over” function
through the reversible AFC system, used here as a conveying screw that enables
the entire tank to be emptied. These improvements in equipment cleaning
capabilities and hygienic design are non-negligible advantages, in particular
in food processing applications.
Eventually,
the intensification of the preconditioner provides improvement in the overall
process, energy savings and enhanced quality of finished products, as well as
greater flexibility and better control of operating parameters, whatever the
recipe.
Furthermore,
the preconditioner can be easily adapted to single-screw or twin-screw
extruders in various food processing applications that require a mix or the
pre-cooking of raw materials, such as those used for making pet foods, corn
flakes, pre-gelled flours or starches, pellets, etc.
Written By:
Samuel Raymond
R&D Engineer – CLEXTRAL
Technology Development Department
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