The aquaculture industry continues to grow-and with good reason. This trend pulls with it increased demand for high quality feed and feed manufacturing technology. It also calls for more feed plant design flexibility in terms of raw material choice.
Understanding customer and market requirements lays the foundation for a successful plant design. It is therefore vital to define accurately the scope of the project before proceeding to process flow design, specification of equipment and the general layout of a feed plant.
Few important key issues to be considered during the development and design of an aquatic feed plant are as follows:
Shaping the feed remains the heartbeat of the process
Whether a pellet press or extruder is used, the choice of technology is vital. A general mistake in the case of extrusion is failing to :
» Recognize the difference between various extrusion technologies. Extruders in the case of animal feeds are designed to be used in a very wide range of applications based on the formulations (and ingredients) to be extruded
» The expected nutritional outcome (digestibility, feed conversion ratio, specific growth rates, etc)
» The expected physical outcome (shape, size, buoyancy, water stability, structure, texture, etc)
Aquatic feeds in general call for single screw extruders of (single screw) medium shear design. In some applications (hatchery feeds for example) twin screw extruders might be the right choice.
The extruder choice and its capabilities to a large extend determines the up-stream and down-stream plant design. All design aspects up-stream and downstream from the extruder or pellet press should enjoy careful consideration. Some important issues as follows :
1. Raw material storage – developments such as square silos provide flexibility in terms of batching plant and loadout design. Apart from a smaller footprint and better hygiene compared to traditional round silos, it also encourages aesthetics of the superstructure or feed mill building itself (figure 3).
2. Size reduction – In general, specification for aquatic feed ingredient grist size ranges from an average diameter of 250 micron to 400 micron depending on the species. A sieve analysis should confirm that 90% to 95% of the raw ingredients falls within this range. Until recently this was achieved through a coarse grinding and fine grinding hammermill or pulveriser (double grinding). However, the latest developments in hammermill design allows for grinding ingredients to within this specification at grinding temperatures low enough to prevent any ingredient damage.
Down-Stream issues Special care is taken down-stream of the extruder as we now work with final product. Important issues down-stream worth mentioning are:
Liquid Application – The ability of an aquatic feed pellet to absorb fat or another liquid during coating is mainly a function of the structure of the pellet (the volume of voids inside the pellet) and the surface area of the pellet. Adding enough fat to a sinking or slow sinking pellet for species or culture conditions where high energy levels are required, could be challenging. A vacuum coater overcomes this challenge and helps to “layer” liquids from the core of the pellet outwards as needed. This way, liquid with highest palatability can be positioned towards the outer perimeter and surface of the pellet where it can act as an attractant.
Conveying and Transitional Equipment – choice of conveying equipment between down-stream operations should consider physical effect or damage to product and minimise chances of cross-contamination between runs of different formulations and products.
In addition the following are always key to good plant design and how it complies with design codes, environmental and other regulations :
» Online control of processing conditions such as moisture and water activity
» Cross-contamination and its effect on producing safe feed
» Dust Control
» Odor Control
» Noise Control
» The social or visual Impact of the feed mill in the zone where it is planned. Figure 3 proves that functional engineering can be artwork as well
» Control over the effect of output on the environment
Contributed by Ottevanger