Mechanical dewatering of chopped alfalfa
Mechanical dewatering of cut alfalfa could replace the traditional method of field drying and would also be cheaper than total thermal dehydration in dryers. Juice from alfalfa also contains several compounds that would meet the growing demands of a low-calorie-fed population and also, provide compounds that could find a cure to some of the fatal communicable diseases. The major objective of this research is to find the pressure levels required to remove some of the initial moisture from shredded alfalfa and the quality of the pressed pulp and juice if alfalfa is mechanically dewatered in a pressure cell. The results from dewatering using the pressure cell are used to simulate the working of a continuous screw press for process optimization purposes. Experiments on dewatering of alfalfa were done on a screw press which is originally designed for extracting oil from oilseeds. The screw press was modified to simulate the field conditions of a continuous dewatering process for cut alfalfa. Measurements were done for dewatering power consumption and the pressed pulp was pelleted on a pilot scale pelleting mill. The quality of pressed pulp and expressed juice was analyzed at different screw speeds and choke openings. Using the available data from the above experiments, a quasimechanistic model was developed for the continuous dewatering process in a single-screw press. The model is based on a semi-empirical relationship given by Koegel et al. (1972) for dewatering of alfalfa in a batch-type hydraulic press. The model was tested against the experimental data and a sensitivity analysis was done by changing the variables. The agreement for predicted power and pressure is very poor. The results show that once the initial moisture is removed from fresh alfalfa at a low pressure of 2-4 MPa, further moisture removal from the dewatered pressed pulp becomes difficult even at considerably higher pressures. The efficiency of juice extraction can be enhanced by delaying the onset of the 'steady equilibrium moisture content state'. Comparison of energies using three different methods of pelleting shows that field drying can save energy by 13% over combination of mechanical dewatering and thermal drying. Combination of mechanical dewatering and thermal drying can save energy by 45% over total thermal dehydration. However considering the numerous value-added products obtained from the alfalfa juice and the fact that mechanical dewatering is a weather independent system, it seems to be a promising technology for the future.