With increasing use of extruded products in aquaculture, cooling of the pellets is not enough to bring the moisture content down to the desired level. Dryers are becoming more and more popular. Paul Douglas from Aeroglide emphasises that dryer design is of major importance in
establishing an energy efficient drying process.
The decision to purchase a drying system is one that many companies plan over months and sometimes years. This decision is usually associated with the consideration of current and future capacity requirements, increasing yield, reducing cost of operation and optimising product quality. The following questions are common: What are our product requirements today? What will they be in the future? What type of process controls should we install? How can a new drying system improve our position in the market? What is the optimum process technology for our specific product? How will we deal with emission and exhaust issues? How can we use the lowest possible amount of floor space and energy?
Regardless of the process, one major step in the production of any feed is moisture removal. You may ask: why does a feed manufacturer need to dry their product anyway, if it is only going to be put back in the water or openly exposed outside of the sealed packaging?
The primary objective is to improve stability or shelf life. There are other secondary benefits, such as reducing tackiness to improve handling, bringing about a chemical or physical change in the product, and reducing the weight of the product for shipping. The key is to dry the feed efficiently and economically.
Target moisture content
Reducing the feed to target moisture content provides the best conditions for coating where applicable, ideal shelf-stability during storage and transport, and a functional feed when delivered to the customer. All moist feeds can develop microbial growth if they are not processed and stored correctly, resulting in spoilage or the development of toxins.
The moisture content of a product can be defined as the percentage weight of water in relation to the total weight of the product (wet weight basis). Water is present in two types in a product, chemically bound moisture and free moisture. Water activity is a measure of the amount of available water in a product that will support microbial growth.
In practice, water activity values are more accurate than moisture contents as indicators of feed stability with respect to the potential for microbial growth. However, water activity is more difficult to measure than moisture content in normal production. For any product, an increase in water activity accompanies an increase in moisture content. Typically, when the relationship is established between water activity and moisture content, the feed moisture content is measured and monitored during production to give the optimum final moisture for a given feed at the sealing of the packaged product. This will allow for any moisture uptake that may occur with a post dry coating if applicable to the feed.
The main factors controlling drying are temperature, airflow, retention time and mixing/spreading, along with the physical and chemical characteristics of the product, and process air conditions. The driving force for evaporation is the difference in vapour pressure between the feed and the relatively dry process air. As the air is heated, its water holding capacity increases. The surface moisture of the product is absorbed by the dry process air, and carried away. Once the surface moisture has evaporated, the product must be heated to drive the core moisture to the surface where it evaporates. Free moisture is the first to evaporate, followed by internal bound moisture upon further heating. The entire process is a balance between the rate at which the feed particle gives up moisture, and the quality and condition of the air available to remove the moisture.
The nature of the product, controlled by the processor, plays a major part in the drying rate. Drying rate is directly proportional to the surface to volume ratio of each individual pellet. Flat products are the easiest to dry, while spherical products and micro feeds require more advanced drying techniques and processes.
The majority of feeds are produced by either pelleting or extrusion. Extrusion causes greater physical and chemical changes in the feed, resulting in a higher degree of bound moisture. Moisture can be removed from pelleted feed easier, faster and at lower temperatures. However, the fish feed and petfood industries today have a predominance of extruded feeds due to the benefits of better pasteurisation, nutrition and density control.
Product formulation also contributes to variation in drying rates. For example, products with a high fat content require more advanced drying techniques, while medicated or veterinary feeds may require lower drying temperatures to avoid thermal degradation. Incoming moisture content and desired final moisture determine the total workload required of the dryer.
The other major factors controlling drying relate to the equipment design. Retention time, temperature and airflow are the major variables around which dryer manufacturers design their process and equipment. Retention time should be long enough for uniform, efficient drying. For the same throughput capacity, longer retention times are directly related to deeper product bed depths. Process air temperature should be uniform and as high as the product permits.
Airflow is the most complex of the parameters. It includes airflow direction, velocity, temperature, humidity and distribution. Airflow is critical because it is the medium that carries the moisture from the feed. As air passes through and removes moisture from the extruded feed, it changes in quality and in its potential to remove additional moisture.
As it picks up moisture, the air’s humidity increases, and it cools from evaporative cooling, reducing its capacity to remove additional moisture. For this reason, it is desirable to reverse the airflow direction as the product dries, and to pass process air through the product bed only once before it is recirculated and reheated, with a portion of moisture laden air being exhausted.
The ultimate goal is that every individual extruded pellet sees equal quantities and qualities of process air, which results in all of the product being dried to a consistent level.
Further, product that is heated during drying must be properly cooled before bin storage or packing, or moisture will condense on the product and raise its water activity level during storage.
Dryer manufacturers balance process, energy and economic factors in determining the optimum dryer configuration for a feed manufacturer. The result is a variety of dryers to choose from, including tray dryers, single and multi-pass conveyor dryers, and vertical dryers. All dryer designs can be compared based on the fundamental drying factors, as well as the relationship to differences in the properties and quality of the finished feed.
Tray dryers are used for batch processes, normally for small quantities of feed. They are often employed as lab dryers to evaluate new formulations and drying parameters.
A vertical dryer can represent a semi-continuous batch or continuous process, depending upon the discharge mechanism. The simplest and oldest vertical dryer is actually a pellet cooler, which uses ambient air to remove free moisture from aquaculture feed pellets. More recent vertical dryers are multiple-stage, semi-continuous batch systems. Due to the height, access to the dryer for sanitation and maintenance may also be an issue.
Conveyor dryers and coolers are the only true continuous process dryers, with the greatest degree of control over precise drying parameters. Conveyor
dryers are the most commonly used dryers in the extruded feeds industry. They are either configured as single pass or multiple pass units. Conveyor dryers vary from simple, non-insulated dryers where air is not recirculated (very inefficient) to systems that have controls and instrumentation for all critical parameters in the drying process (to operate at the highest available levels of efficiency).
A properly designed dryer will operate efficiently and effectively, employing recirculated airflow, reversing airflow, and air passing through a product bed only once before being reheated with typically the optimum amount being exhausted for a combination of ideal drying and energy efficiency. Precise control of product depth, temperature, airflow, and residence time in the dryer ensures a uniformly dried, free-flowing feed with consistent moisture content.
In determining which dryer is best for their process, a feed manufacturer should look at factors that affect the output capacity of the dryer, the uniformity of the finished product, the durable, trouble-free operation of the dryer, and overall operating economics of the system. Quite often the dryer with the lowest price tag may end up costing the most in the long run.
The true price of a dryer includes the labour and time to install it and get it running at full production as well as the relative energy efficient operation, the level of moisture consistency in the final product and the ‘up-time’ for production over the life of the dryer. The real value of a good dryer is in many years of durable, dependable service producing a high quality feed. Ultimately the best dryer will be a system that will meet today’s objectives, yet be flexible enough to meet the challenges of the future.