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High temperature steam used in the pelleting process limits the ability to incorporate heat sensitive ingredient into the pellet feed. Adding the liquid feed ingredient after pelleting can reduce the damage of heat sensitive ingredient, however, in the traditional liquid coating system, most of the liquid ingredients are coated onto the surface only, and nearly two thirds of the added ingredient fall off the pellets during the following packaging, transportation, and storage processes due to friction. In the vacuum coating system, the liquid ingredient can penetrate into the core of the feed pellets after the release of vacuum.

A experimental vacuum coating system was developed. The liquid vitamin was sprayed into the pellets with different diameter under vacuum. A special mixing paddle was developed and incorporated into the coater chamber to improve the mixing uniformity. The effects of mixing time, vacuum pressure and spray pressure on the mixing uniformity were studied using the element tracing method. The special designed mixing paddle achieved a Coefficient of Variation (CV) for mixing uniformity less than 3% after 6 min of mixing. The effects of liquid dosing, vacuum pressure, vacuum release time and pellet size on the gradient of liquid concentration between the central and the outer region of pellet were studied. Decreasing of the vacuum pressure or the vacuum releasing time reduced the gradient of liquid concentration. Increase of liquid dosing caused a decrease of the gradient of liquid concentration. The smaller the pellet sizes the smaller the gradient of liquid concentration.