(379g) A Visual Comparison of Pneumatic Conveying of Soft and Hard Materials
Dilute phase pneumatic has been widely studied due to its importance in the transport of bulk solids, and its numerous applications in the industry. In general, most of the studies focus on the measurement and prediction of the conveying design parameters such us gas velocity and pressure drop, while visual analysis of particles dynamics is used only in limited experiments in which specific phenomenon like particle collisions are studied in a controlled manner. In this work, the particles dynamics were analyzed during normal operation of a 2 inch pneumatic conveying system, considering two testing materials: hard pellets and soft pellets. The visual analysis was done on straight sections and on two of the more common elbows: the long radius bend (LRB) and the blind T. Videos of particle movement during conveying were recorded with a high speed video camera in conjunction with pressure drop measurements. A number of conveying trials were performed in order to include high and low superficial gas velocities and several loadings (ì) for each material.
The experiments performed with the hard pellets showed the normal differences in pressure drop between the elbow and the straight sections, with the elbows as a significant component of the higher pressure drop. The difference between the LRB and the blind T was also as expected, with higher pressure drop for the latter. Pressure drop oscillations were detected during these trials, which were found to be related to the formation of particles clusters as shown by the high speed videos. The clusters originated from the rotary valve during the discharge of its pockets into the conveying line. As the hard pellets move through the LRB, they follow a trajectory along the outer wall of the elbow, showing little, if any bouncing. In this type of elbow the particle velocity does not decrease substantially, which would explain the low pressure drop. Through the blind T, the hard pellets showed a significant change in particle velocity as the pellets impacted the stationary particles in the blind T pocket. The required reacceleration of the particles should explain the higher pressure drop produced by this type of elbow.
The experiments with the soft pellets showed unexpected results. In this case, the bouncing of the particles was so intense that the particles did not significantly change their average axial velocity through the elbow. It appears that the elbow affects the conveying velocity of the particles upstream, so by the time the pellets get into the elbow, they have already reduced their velocity. Thus, the pressure differential per unit of length (ÄP/L) for the elbow and the straight section upstream was not different. It has to be noted that the ÄP across the elbows was measured by placing the pressure taps at sufficient distance downstream the elbow to permit the reacceleration of the particles. Through the LRB, the soft pellets bounced intensely so there was not a higher particle concentration close to the outlet wall of the elbow curvature. Similarly, the soft pellets did not show a significant change in velocity when passing through the blind T. For both elbows, the particles bounced creating a homogeneous distribution of particles inside the line. When comparing the LRB against the blind T using the soft pellets, the LRB showed lower pressure drop than the shorter elbow. However, its ÄP/L was similar to the straight section upstream for the same trial. This type of pellet did not show the particle clusters or the pressure drop fluctuations. In this case the bouncing of the particles increased the pressure drop due to the reduction of the effective conveying velocity, which increased the drag force.
From the visual analysis, it can be concluded that: When conveying hard pellets, the rotary valve creates clusters of particles that produce pressure oscillations. This phenomenon was not detected when conveying soft pellets. The hard pellets concentrate at the outer wall of the elbow when passing through the LRB. In this case, their velocity does not change substantially compared to the straight sections, which would explain the low pressure drop of this type of elbow. When passing through a blind T, the hard pellets significantly change their conveying velocity after impacting with the stationary particles in the blind T pocket. This change in velocity and the consequent reacceleration helps to explain the higher pressure drop developed by this type of elbow. When conveying soft pellets, the ÄP is higher than that for hard pellets due to the intense bouncing, which reduces the effective conveying velocity (the particles must be continuously reaccelerated), increasing the drag force. The bouncing of the soft pellets reduces the ÄP difference between the elbows and the straight sections and homogenizes the particle distribution inside the line during conveying.
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