Mechanical dewatering of chopped alfalfa
Date
1995
Authors
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Publisher
ORCID
Type
Degree Level
Masters
Abstract
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.
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Degree
Master of Science (M.Sc.)
Department
Agricultural and Bioresource Engineering
Program
Agricultural and Bioresource Engineering