The design-of-experiments analysis showed that the three responses were significantly affected by water content and extrusion type. The usable yield fraction also was significantly affected by the extrusion speed. Pellet size and usable yield were more influenced by the extrusion system, whereas pellet dispersion was more influenced by the water quantity (see Figure 4). The pellet size was greatest in the axial system, moderate in the dome system, and smallest in the radial system. The dome system allowed the highest usable yield and the narrowest pellet dispersion, compared with the other systems. Increasing the water content increased the particle size and the usable yield, and decreased the pellet distribution. Other studies observed the favorable effect of water quantity on other extrusion systems, notably the pellet-size increase by agglomeration, the pellet-dispersion decrease, and the usable-yield increase (2, 5, 9, 24, 28, 29, 32). Moreover, extrusion speed increased the yield (14). These results can be explained by the previous morphological observations. At lower water quantities, the rods observed after axial extrusion did not pass through a 2000-μm sieve, and the fine pellets observed after radial extrusion passed through the 1400-μm sieve, which in both cases decreased the usable yield fraction. A significant interaction between the extrusion system and the water quantity showed that water quantity had less influence on pellet size in the axial system (but great influence in the radial system), and little influence on pellet distribution and usable yield in the dome system. Presenting a compromise between the three responses, the dome system offered the best characteristics in terms of pellet-size dispersion and usable yield fraction. At high water quantities, the three extrusion systems' results were more similar.

Pellet elongation was influenced significantly by water quantity and, to a lesser extent, the extrusion system. An increase in water quantity improved pellet morphology by decreasing elongation. In the same way, some studies showed that increased water quantity yielded better roundness (2, 9, 11). A significant interaction between the extrusion system and water quantity showed that dome extrusion gave the best characteristics in terms of the morphology of the granules at lower water quantities. Water quantity had less influence on pellet elongation for this system. For the axial system, the elongation (manifested by rods) decreased as the water quantity increased. For the radial system, the elongation at the smallest water quantity (manifested by nonspherical pellets) also decreased with an increase in water content. Finally, the extrusion dome produced the least elongation at the lowest water quantity and was not affected by an increase in the water quantity. At high water quantities, the three extrusion systems provided pellets with similar morphology.

Figure 4: The effects of factors and interactions on various responses (Y1–Y8).

Figure 5: Response values according to extrusion method and water quantity for various responses (Y1–Y8).