Effect of product distribution geometry on drying of extruded maize pellets
Peters, Jacobus Du Toit
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In the drying of foodstuffs it is preferable that the product should be spread out flat and uniformly on the bed in order for the air to pass evenly through the product, ensuring uniform drying. However, many types of dryers distribute the product non-uniformly on the bed. This can cause some of the product to be over-dried or burnt while the rest is still too moist to be packaged. The problem therefore is to investigate the effect that the geometry of the product on the bed has on the drying of the product and secondly to make recommendations on the preferred floorplate length of a multi-level vertical dryer to dry the product under investigation satisfactorily. In order to investigate the effect that the product distribution geometry has on drying, two types of tests were done in which extruded maize product was dried under different conditions. For the first type, a non-uniform product bed geometry was dried at various air temperatures and air velocities. For the second type, the geometry was varied while keeping the air temperature and air velocity constant. With each test, samples for moisture analysis were taken before as well as after drying at the bed’s ridge, centre and valley. The experiments showed that drying at a higher temperature produces greater moisture removal from the product compared to drying at lower temperatures. In general the higher the air temperature is, the drier the product will be, provided that case hardening does not occur. The tests conducted showed that the difference in moisture content between the product at the ridge and the product at the valley increases as the air temperature increases. It was found that the higher the air velocity, the lower the product’s moisture content will be after drying. The difference in moisture content between the product at the ridge and the product at the valley is less if the product has been dried at a high air velocity compared to when the product has been dried at a lower air velocity. It was found that the further the distance is between the ridge and the valley, that is, the greater the amplitude is, the larger the difference will be between the moisture content at the ridge and the moisture content at the valley. This means that the lower the amplitude, the more uniform the bed geometry is. It was also found, however, that uniform drying can occur even though the geometry of the product on the bed is not uniform, provided that the amplitude is equal to or smaller than the maximum amplitude for uniform drying. If it is known how the air temperature and air velocity will affect the drying of the product, the dryer can be set up to dry the product successfully within the acceptable tolerance of the desired final moisture content, despite the fact that the product bed geometry is non-uniform. Furthermore, the difference in moisture content is dependent on the floorplate length of the multi-level dryer. This means the floorplate length needed to dry the product to a certain tolerance can be calculated beforehand and the dryer designed accordingly.
- Engineering