Umedecimento de partículas em leito fluidizado com adição de ar pulsado

Abstract

The phenomenon of fluidization involves the suspension of solid particles in an upward flow of fluid, providing high rates of heat and mass transfer. To ensure effective performance in humidifying with inert particles, it is essential to maintain fluid dynamic stability, preventing particle agglomeration and the formation of preferential channels. This work aimed to study, using the central frequency technique based on pressure fluctuation measurements, the influence of adding pulsed air on the fluid dynamics of a fluidized bed during the humidification of solid particles. Glass beads were used as the bed material, and distilled water was used for wetting by atomization in the experimental tests. The beads have an apparent density of 2540 kg/m³, sphericity of 0.94, and a mean diameter of 0.389 mm. The experiments took place in a height of 1 m and 0.11 m internal diameter fluidized bed column, equipped with a solenoid valve for air pulsation. A superficial air velocity to minimum fluidization velocity ratio of 2.2 was used, characterizing bubbling regime conditions. Water atomization flow rates of 6, 10, and 15 mL/min were used, applying air pulsation frequencies of 0 (continuous mode), 1, 3, and 5 Hz simultaneously, totaling 12 experimental tests. The tests showed that the transition from the bubbling regime to defluidization occurs when the central frequency of the pressure fluctuation signal decreases. It was observed that without the addition of pulsed air, an increase in the atomization flow rate leads to a reduction in the time required for the onset of defluidization, although this increase in added water flow does not interfere with the time to subsequently reach the fixed bed state. With the addition of fluidized air pulsation, in most cases, there was an even greater reduction in the time required for defluidization, suggesting that pulsation helps to distribute the added water more rapidly in the bed volume. However, in some cases, it increased the time to reach the fixed bed state. In general, for low flow rates of added water, pulsation tends to promote defluidization and fixed bed conditions more rapidly. For slightly higher flow rates, a frequency of 3 Hz was sufficient to achieve significant results, like an increase in the time to reach defluidization and a longer duration of the experiments. The test with 10 mL/min and 3 Hz pulsation showed the best results, with a 3.6 times increase in the time required for defluidization and a 4.5 times increase to achieve the fixed bed compared to the test with 10 mL/min without pulsation added. This study proves that introducing pulsation in fluidized beds can improve bed homogeneity, moisture distribution, and depending on the applied frequency and atomization flow rates, it can be beneficial for drying using a higher-temperature fluidization gas.