Optimizing pellet cooling for quality, energy savings

Optimizing pellet cooling for quality, energy savings

French feed center launches Opsera project to optimize dryer-cooler performances in feed production

Several unitary operations are required in feed plants — not only grinding, dosage and pelleting, but also drying and cooling. The later occurs just behind grinding as the blended raw materials in powder form are mixed with steam prior to pelleting, in order to facilitate its shape through the die. But mixing powder and steam can lead to hot and wet pellets, which must be cooled and thoroughly dried for optimal storage (i.e. to prevent bacteria and mold development) and conform to regulations (moisture content must be under 14 percent, according to EU regulations).

The main objective of drying and cooling is to bring those wet and hot pellets to temperature and humidity equilibrium with ambient air as quickly and inexpensively as possible.

The difficulty presented by ambient air

The specificity of this unitary operation is precisely that it must conduct two functions at the same time: drying and cooling. Another difficulty is due to the fact that this operation uses the same vector, ambient air, as a drying and a cooling agent. Thus, air speed at the entrance of the machine will provide adverse actions on cooling and drying quality. Specifically, when air speed decreases, drying improves at the expense of cooling quality.

“Feed plants use two kinds of machinery to assure drying-cooling,” explains Fabrice Putier, director of Tecaliman, the French technical center for feed manufacturers. “They might be either horizontal or vertical, about two-third being vertical in France. Those dryers-coolers are made of compartments where wet and hot pellets are arranged in a bed. The depth may vary, but is generally about 50 cm.”

Ambient air is captured around the machine or outside the plant. It is moved by a fan, placed after the dryer-cooler. This air movement assures the drying and cooling of the pellets. The pelleting press continuously produces pellets, which enter the dryer-cooler. A level sensor checks the continuous flow: When pellets are detected, it automatically activates the extraction system placed at the bottom of the compartment. Thus, it extracts the pellets which are thought to be at the correct temperature and moisture levels.

pellet-cooling-system

The proper management of finished pellet temperature and moisture content improves its storage and safety qualities. | Tecaliman

The role of dryer-coolers

The shape of a horizontal dryer-cooler is different. At the unloading end of the pelleting press, pellets are laid down on a metal conveyor belt with holes. This conveyor runs regularly and then distributes the pellets on another metal conveyor below. Generally, those horizontal machines have two conveyors, but some providers choose to build dryers with four conveyors. The ambient air, coming from below, traverses the pellet layers to dry and cool them.

“Today, drying and cooling operations are not fully optimized,” says Putier. “Most dryers-coolers don’t provide any real time production control between product characteristics (products temperature and moisture on entry and exit) and machine operating parameters."

Adjustment of the layer height might be done a posteriori with moisture measurements on pellets when they exist in the machine. More rarely, this data is used to adjust fan speed.

Improving energy savings in the mill

Rising energy costs and sustainability awareness regarding energy consumption have driven feed manufacturers to request further support in order to improve their drying and cooling functions.

For example, in France, specific energy consumption of dryers-coolers is roughly 3.9 kWh/metric tons, that means 69 GWh/year for all the French feed plants, a global cost of €4.8 million (USD5.2 million) and the emission of 4,000 tons equivalent carbon (as measured in metric tons).

Managing the temperature and moisture content of finished product is also of great interest to maintain quality during storage either in presentation (brittleness) or sanitary quality. Furthermore, regulation demands that moisture content is below 14 percent. Nevertheless, a feed which is too much below this limit may result in a loss of revenue, which may even reach the level of plant energy consumption. The dryer-cooler is the key phase for mastering the moisture content in feed production.

Research aims to optimize process

To further explore this very important step in feed production, Tecaliman launched the Opsera project for optimization of dryer-cooler performance.

“The meaning of Opsera is, in French, the equivalent of optimization of dryer-cooler in feed production,” explains Putier. “It is a three-and-a-half-year project, funded by the national agency for energy saving, Ademe. The total project cost around €200,000 (USD215,266). We did conduct around 30 experiments, including a lot of measures on feed plants to better understand physical mechanisms. For example, moisture has to migrate from the heart of pellets to their periphery to be removed. Feed plants accepted to modify their process parameters to comply with our research aims and due to this strong collaboration between their teams and ours, we’ve been able to identify key parameters. We concentrated our work on vertical dryer-coolers as it is the most common shape here. We then simulated the mechanisms into the pilot plants of several engineering schools to obtain a drying-cooler modelling.”

Thus, a wide variation of air flow has only a small influence on moisture content (0.3 and 0.4 percent). The layer depth also has influence, but less on vertical machines than on horizontal machines. One key parameter is the drop height of the pellets between the level of the pelleting mill exit and the top of the layer bed in the dryer-cooler. The project helps researchers to better understand where each of the two functions (drying and cooling) occurs inside the machine.

Opsera’s research aimed to improve knowledge about the unitary operation. A decision support instrument is currently in the process of finalization to predict automatically the temperature and moisture content of pellets at the exit of the machine from their characteristics at its entrance. Such a tool will help to automatically control the process through the fan speed and the depth of a layer of pellets.

Tecaliman will release the first decision support instrument in the beginning of 2016.

“With the pellets' moisture and temperature at the entrance of the machine, the ambient air moisture and temperature, our computer tool gives our recommendation for layer height and fan speed,” explains Putier. “It will be the first step as fan speed impact is not yet fully understood. It’ll help feed plants to manage their energy consumption by better management of the process. It’ll also help them to resize fans. They are often too big to function at their real optimal operating point thus require too much energy and wear down faster.”