MORAIS, V. L.; http://lattes.cnpq.br/0785740374540217; MORAIS, Vera Lúcia Meira de.
Résumé:
The drying of pastes and suspensions in spouted bed with inerts
is influenced by operative and geometric variables such as temperature and
flow rate of the input gas, bed height as well by the properties of the materials
to be dried. The knowledge of the interactions of these variables and the
performance of drying including the quality of the product is very important in
order to optimize the operation. This work deals with the drying of West Indian
cherry {Malpighia emarginata, DC) pulp in a spouted bed of 0,297m diameter,
with a conic base of 60° and inlet orifice diameter of 0,05m. Polystyrene
particles (d = 0,326cm, p = 1,045g/cm3) formed the bed of inerts with high
between 18cm to 28cm, including the conic to cylindrical regions. The feeding
of West Indian cherry pulp was intermittent, corresponding to 2% of the mass of
inerts. All the measures used to analyze the performance were made in the
constant rate period of drying. The temperature was limited by the termal
sensibility of the West Indian cherry and the velocity of the gas by the stability
of the spout. In this work the influences of the temperature (from 45°C to 54°C)
and flow rate of input air with the relation Q/Qms (flow normalized with the flow
of minimum spouting) between 1,5 and 1,8 were studied. To evaluate the
performance, the yield of powder of West Indian cherry collected in the cyclone
and its moisture content as well the thermal efficiency and the rate of
evaporation of water were analyzed. Using a simplified model the fraction of
wetted particles was calculated from the heat transfer coefficient. It was
observed that, for the experimental ranges used in the present work, the
temperature and velocity of gas has no significant influence on the yield of
powder but the moisture content of the product is influenced, showing a
maximum with the increasing of temperature and a minimum with the air flow.
To explain this behavior, it must be considered that the mechanisms involved are complex: the drying of suspensions proceeds by the covering of particles,
followed by evaporation of water, crushing of the film and entrainment of the
powder. So, na increasing of air flow, increases the rates of heat and mass
transfer, but reduces the residence time of the product in the column. The
temperature has a similar effect, improving the heat and mass transfer, but the
residence time of the product may also be reduced because an earlier crushing
of the film. The increasing of bed height improves the yield in powder.
Nevertheless the rate of evaporation of water and the thermal efficiency
decreases, due to the greater heat losses at the wall in the cylindrical region.
Only a fraction of the inert particles is effective for the heat transfer, because
the available area is limited by the wetted particles. The results obtained show
that the performance is impaired by the high retention of product on inert and
the low fraction of wetted particles, because the need of stability of the spout.
Improvement on the efficiency can be obtained through of decrease of the
adherence effects on the particles and the optimization of the food system.
