Extrusion systems for aqua feed and pet food: Single- or twin-screw?
by Diego Clivio, Clivio Solutions, Argentina
The extrusion process is a thermo-mechanical process that consists of forcing a product to pass through one or more holes of a specific size under pressure and temperature thanks to the Archimedes screw concept (also known as "endless screw").
The extrusion processes in the processing of extruded pet food and aquaculture aim to cook a mixture of raw materials and ingredients, giving it the form of croquette (pellet/kibble) that facilitates its subsequent handling.
The most important processes that take place inside an extruder is the gelatinisation of starches and texturisation of proteins. Both processes help aid in greater digestibility, as well as to form a structure and texture that hosts other ingredients, minerals and fats in a stable and uniform way.
For a good gelatinisation of starches and texturisation of proteins, moisture is needed (which is added in the form of liquid water and steam in pre-conditioning and/or extrusion). Suitable temperatures are also needed, which can be provided through the addition of steam, as well as being produced by the mechanical work and friction produced by the screws. Mechanical energy is also needed, granted by the shearing action of the screws.
Today, there are two technological forms of extrusion, single-screw (SSE) and twin-screw (TSE). In this article we will talk about the features, advantages and disadvantages of each of these technologies.
In single-screw extruders we observe as a characteristic that the screw moves the materials through the channel by developing a 'drag flow', the speed of which is directly proportional to the speed of the screw. Due to the constraint of the die in the discharge of the extruder, there is a "backpressure flow', which goes in the opposite direction to the drag flow. Therefore, the flowrate in single-screw extruders is equivalent to the drag flow rate minus the backpressure flow rate.
The flow dynamics in the screw channel shows us that the fluid particles travel at different speeds and that they do not interact in their entirety, resulting in a dispersion of dwell times and an uneven mixture.
Consequently, the heat transfer, as well as the entry of mechanical energy in the cooking section, are very limited. In addition, the length of the shear depends on the space, which generates heterogeneities of the melting properties (cooking duration, temperature and pressure).
In short, single-screw technology features:
- A single processing section
- A unique dependence on performance and screw speed
- A single operating point (performance combined with maximum screw speed and die opening area)
- Limitations in mixing, which limits heat transfer, mechanical energy input, and generates heterogeneities in particle fusion (cooking duration, composition, temperature and pressure)
- The higher the screw wear, the lower the extruder performance. A reduction of 10-20 percent can be observed during the life of the screw
- 'Lubrication factors' in the composition of the mixture generate runoff, which reduces the performance of the extruder and the addition of mechanical energy.
Twin-screw extruders have multiple processing sections, (melting/cooking, vapours/gas extraction, vapours/positive displacement pumping), thanks to the different restrictions generated by the work of the screws.
In twin-screw extruders, very thorough mixing is observed in the coupling area of the screws (via the use of macro-mixing and micro-mixing).
Consequently, the heat transfer coefficient in these sections is very high. Homogeneous fusions can be obtained with an optimised linking of lipids. Expansion develops consistently, leading to an excellent density, texture and shape of the final product, as well as a uniform density and colour.
In short, twin-screw technology involves:
- Multiple processing sections
- The performance and speed of the screw are unlinked. Therefore, for a formulated mixture, twin-screw extruders are characterised by their many operating points. In addition, the screw profile can vary extensively to modulate the addition of mechanical energy
- The mixing process is very thorough, providing great additional benefits in relation to the quality of the product
- The slippage of the mixture is compensated by the positive displacement pumping of the screws
- Less sensitivity to changes in raw materials
- Increased tolerance to changes in moisture, fibre aggregates and fats
- Better quality and consistency of final product
- Greater overall process flexibility.
In aqua feed production, the best possible uniformity and high level of degassing decreases the chances of floating pellets when sinking aqua feed is produced. Thanks to the action of the two screws, twin-screw extrusion allows us to produce micro-pellets of up to 500 microns (0.5 mm).
In the particular case of extrusion experts Clextral, significant energy savings in drying are achieved by their ability to work with two-to-three percent less humidity. This results in much improved end products.
Both extrusion technologies are widely used in the pet food and aqua feed production sector. Single-screw technology typically requires a lower investment cost and lower spare parts costs.
While twin-screw extruders require a higher investment, this investment brings the possibility of working with a wider range of raw materials with more fibres, more vegetal proteins and higher fat content.