The transition between dense and dilute flow in vertical conveying of Geldart D particles were investigated for risers of different diameters using a spouted bed as a solid feeding system. The transition and choking velocities were identified by combining analyses of pressure gradient versus air velocity diagrams, pressure fluctuation signals and voidage values. Experimental data were used to evaluate the effect of particle and riser diameters on the pressure gradient, mean mixture voidage, the regime transition and choking velocities. The transition velocity from dilute to dense phase could be identified, as well as the onset of the choking condition, which appeared as the air velocity was further reduced. Data obtained in the same experimental apparatus facility using a screw conveyor and a gravitational system as solid feeding devices have been used as a reference to be compared to those obtained using the spouted bed feeder.
The development of a bed is reported, in which characteristics from spouted beds and pneumatic conveying are combined. The apparatus can be described as a pneumatic transport bed modified by the introduction of a recirculation process for inert materials, which leads to a final configuration akin to that of spouted beds. The dependency of the particle recirculation rates on the air flow rates and on the distance between the air inlet and the lower end of the central tube (z0) was investigated. Also studied was the variation of the total pressure drop with air flow rate, for different loads of inert materials (glass spheres, dp = 2.8 mm) and different z0 values.
In the present work, one-dimensional two-phase continuum models were applied to simulate the pneumatic drying of porous alumina and solid glass particles. Pressure profiles, gas and solid temperature, and gas and solids moisture profiles were obtained in a 53.4-mm conveying tube. For both particles, maximum values of gas-to-particle heat transfer coefficients were obtained at air velocities close to the minimum pressure gradient velocity. Experimental temperature and moisture profiles of gas and solids were compared to simulated predictions, showing that models based on the twophase flow approach fail to predict all the observed physical phenomena in simultaneous momentum, heat, and mass transfer for pneumatic drying of coarse particles. However, using adequate correlations and constitutive equations to predict interaction forces and transport parameters, it was possible to obtain good predictions of gas and solid temperature profiles and of moisture content.
We measured and compared the flow properties of two alumina-based powders. The alumina powder (AP) is irregularly shaped and has a smooth surface and moisture content of 0.16% (d.b.), and the ceramic powder (CP), obtained after atomization in a spray dryer, is spherical and has a rough surface and moisture content of 1.07%. We measured the Hausner ratio (HR), the static angle of repose (AoR), the flow index (FI), the angle of internal friction, and the wall's friction angle. The properties measured using aerated techniques (AoR and HR) demonstrated that AP presents true cohesiveness (and therefore a difficult flow), while CP presents some cohesiveness and its flow might be classified as half way between difficult and easy flow. Their FI values, which were obtained using a nonaerated technique, enable us to classify the alumina as cohesive and the ceramic powder as an easy-flow powder. The large mean diameter and morphological characteristics of CP reduce interparticle forces and improve flowability, in spite of the higher moisture content of their granules. The angles of internal friction and of wall friction were not significantly different when comparing the two powders.
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