Estudo do desempenho de um leito de jorro convencional para secagem de polpa de acerola.

Abstract

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.