Fractionation of Alfalfa into Leaves and Stems using a Three Pass Rotary Drum Dryer

Fractionation/separation of chopped alfalfa into leaf and stem components was investigated using a small industrial three-pass rotary drum dryer. Both drying and fractionation of chopped alfalfa were successfully achieved in a single operational process. Generally, fractionation is carried out only as a separate process and, therefore, more costly overall. The success of fractionation was determined by the total amount of the desired component (leaf or stem) recovered, relative to the amount entering the process (i.e. separation efficiency) and by the purity (leaf or stem proportion) of the component in a product stream collected at the cyclone or drum exit. A series of experiments under different operating conditions were performed to establish the optimum process conditions leading to maximum separation efficiency and leaf purity. The operating variables used during experimentation were the outer drum end gate opening area, material feed rate, gas flow rate and gas temperatures at the drum inlet. The experimental results indicated that leaf purity decreased and separation efficiency increased with a decrease in dryer drum end gate opening area. A decrease in separation efficiency was observed with an increase in material feed rate. On the other hand, leaf purity increased and separation efficiency decreased with a decrease in gas flow rate. The decrease in separation efficiency was most significant at low gas flow rates. Separation efficiency decreased with a decrease in gas inlet temperature. Stem moisture content was observed to be consistently higher than the leaves. Lowering stem moisture levels to an acceptable level of 8% moisture content results in over drying of the leaves, leading to a loss in product quality. It was determined that the relative difference in moisture levels of stems and leaves at the drum and cyclone exit, respectively, can be averted by re-circulating in the order of 20% of the wet material (stems) from the drum exit back through the dryer inlet.