PHYSICAL QUALITY OF PELLETED ANIMAL FEED .2. CONTRIBUTION OF PROCESSES AND ITS CONDITIONS

The effects of changes in process parameters and their effect on pelle t quality in terms of hardness and durability are discussed, The pelle ting process in this respect is the combination of conditioning, pelle ting and cooling. The parameters discussed with respect to the conditi oning process are process variables such as steam and water and system parameters such as residence time and pressure. Parameters during the pelleting process that can be adjusted or influence pelleting propert ies of a feed mash include layout (e.g. flat-bed vs. ring-die pellet p ress) and dimensions, roller and die assembly and die velocity of the pellet press, The effect of the changes in one or more parameters and its effect on pellet quality (durability and hardness) is however ofte n a matter of the judgement and experience of the operator. For instan ce, a certain increase in the amount of steam added to a feed mash gen erally improves pellet hardness and durability. Increasing the amount of dissipated power in the feed mash, generally has a similar favourab le effect on pellet quality. The extent to which pellet hardness and d urability rise is however dependent on feed formulation and other para meters as temperature of the feed mash and cooling air characteristics . The latter parameters can be measured but their relation to the pell eting process is often not clear. Pellet quality, as affected by cooli ng, is mainly determined by the bed-height of the pellets in the coole r, pellet-size, air-flow and air-characteristics during the cooling pe riod. The use of modern conditioning systems to operate at a wide rang e of processing and system variables means that pellet quality is dict ated more by equipment than by diet formulation. However, use of such conditioning systems must always be justifiable in terms of cost and p ellet quality, rather than the dictates of fashion. It is concluded th at the amount of steam is a more decisive factor than steam pressure. In addition, it seems that the individual steam supply of a factory ha s a larger influence on the measured pellet hardness and durability th an would be expected from theoretical relationships concerning steam i n an ideal situation. Although water has binding properties as well, i t is concluded that steam is far superior to water in producing good q uality pellets. The additional heat included in the meal permits chang es in physico-chemical properties which lead to more durable and hard pellets. Equipment which incorporates some form of hold-time enhances the possibility to incorporate more liquids, without detrimental effec ts on pellet quality. The use of pressure to alter physico-chemical pr operties of the feed in combination with water and heat, and the use o f pressure to pre-densify the feed mash prior to pelleting seems to be important in obtaining a good quality pellet. Pelleting is not the su m of conditioning, pelleting and cooling steps, but should be consider ed as an integral system which performance is dependent on interrelati ons between the three unit operations: conditioning, pelleting and coo ling. However, these interrelations with respect to the different form ulations used, are still poorly understood. The use of decision suppor t systems and process optimization procedures may nowadays greatly enh ance the opportunity to obtain the best possible quality of pelleted f eeds with minimal use of labour and energy for a given feed formulatio n.