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Carousel Drying Technology
A good dryer design for production of high quality aquafeed
Maximum saving with good moisture uniformity and low energy consumption
By Dr. Jammy Ho. IDAH Co. Ltd.

 
Nowadays, in the fierce competitive market environment, every miller uses the similar process as well as ingredients, therefore, the product quality and manufacture cost becomes the thread of the competition. Overview the process, dryer performance play a critical role in this game. For selecting a suitable dryer, two main issues need to be considered, first is average moisture control from dried product and second is energy efficiency, both are related with the water!
Moisture uniformity is one of the critical factor when you judge the dryer performance. In the feed production process, the moisture uniformity is not only related with the product quality and energy efficiency but also it will impact of your profitable directly.  
Because products are sold by weight, a narrow moisture tolerance will improve product quality and avoiding raw material loss. As shown in Figure 1, black curve indicates the traditional dryer and red curve for the advantage dryer such as carousel dryer. A narrow range of moisture tolerance allows the moisture set point to be higher without the risk of exceeding the maximum moisture limit, the profit is shown as green zone. Suppose your final product maximum moisture content is 10%, in carousel dryer, the moisture control set point will be 9.5% rather than 8% in the traditional dryer. The result is you get 1.5% more product and saving drying energy at the same time. The value of this profits due to the moisture uniformity in combination with accurate moisture control can be surprising.

For instance, imagine a feed mill that produces 100,000 tons of extruded aquafeeds per year with a maximum moisture content of 10 %. The product cost $200 per ton. If we compare a dryer with ±0.5% and another with ± 2.0%, there is 1.5% more product to be sold with the same amount of raw ingredients. It is easy to calculate if you use a carousel dryer saving estimate $300,000 /year (100,000*1.5%*$200). In addition of the selling profits, a narrow moisture tolerance also avoid the energy waste from the over-dry. For the 10 Ton/hour capacity extrusion line for example, 1.5% less moisture evaporation will save 200 kg/hour steam consumption.    

    

Figure 1. Moisture uniformity comparison of carousel dryer with tranditional dryer Courtesy : IDAH Taiwan
In the short conclusion, with the carousel dryer, the product is handled with more stable moisture control in-time, less over-drying (avoid product loss) and with a higher average moisture content.
What are the design requirements to achieve a dryer with ± 0.5% moisture tolerance?    

  • The product layer height need to be lower (normally less than 20cm). A high layer will cause un-even drying in which the bottom product gets more drying as air enters than the product on the top where air leaves.

  • A round shape design in which optimize air flow distribution. Since it has no dead spots inside the dryer, also improves the air distribution and hygiene as well.

  • A counter-flow air stream, giving a optimize heat transfer goes through the perforated circular trays and product. Product is moving in circles, in a cross-counter air flow.

  • The product in the dryer is rotating and discharging into a lower deck and tumble the product for mixing. This will be done in each deck before the product is leaving the dryer with very uniform moisture, it also implies all the product gets the same treatment by passing different air volume and temperatures.

  • The product flow is First-in, First-out (FIFO). Every pellet is getting the same retention time that similar as continues production, the operator can easily identify the batch of product quality and correct it immediately if need be.

The another indicator for the dryer performance is energy efficiency, drying is a very energy-intensive operation. When extrudate discharge from extruder, the moisture content can be 20-30%, the dryer energy is significant and easily more than 50% of the plant’s energy consumption. Thus, it is a very challenging task to optimize the drying process efficiency without compromising the quality. The ultimate goal is the energy saving and product quality improving. 
Several studies in the literature show how the drying inlet/outlet parameters impact the energy efficiency. In conclusion, the inlet, outlet process air temperature and air volume are three major factors to determine the amount of water evaporation.Basically, the value of air volume equal to drying surfaces multiply air velocity. The drying surface is fixed by specific design, the velocity is limited by the size and density of pellets, therefore only the delta T is the flexible parameter for energy efficiency. The rule of thumb for delta T as following,

  • The air flow temperuture that leaves the dryer need to be lower: the lower the outlet air flow temperuture means the more air energy is used for water evaporation.

  • The air flow temperuture that enters the heater need to be higher : the higher inlet air temperuture meaning the air flow needs less heater energy to approach the set opint temperuture.

Therefore, a good air system like a carusel dryer has the following characteristics and as shown in Figure 2.

  • Using the recirculation air system: Using the cooler air or dryer exhaust air as the make up air.

  • Independent heating zone: The desire air volume and delta T could set in each specific zone.

  • Counter-flow design: the definition of counter-flow in the dryer design is the material drop from up to down and the air flow motion is bottom up, as some literatures showed, the counter-flow design will be more than 10% better efficiency than a cross-flow dryer like a belt or horizontal dryer.

  • Easy control: Since the delta temperature, air volume and retention time all of the parameters will impact the dryer performance, this is necessary to design a simple rule allow the operator easy to follow as shown in Figure 3. In the carousel dryer design, the operator only needs to verify the product outlet moisture, then adjust the inlet air flow temperature until approaching the target moisture. In practice, one percent moisture drop required 3temperatureraise.

                                                   

                                                       Figure 2. The characteristic of a carousel dryer design 

                                                                   Courtesy: IDAH Taiwan

                                                

                                                  Figure 3.  A sample of Individual heating section with independent control.
                                              The benefit is low exhaust air temperature and less steam consumption

                                              Courtesy: IDAH Taiwan
                                             
                                            

 
 
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