| Description |
Cocurrent downflow is of major interest in gas-solids transport reactors, e.g. in the pyrolysis of coal particles, due to lower pressure drop than upflow and no minimum-gas-flow requirement. Pressure drop and rate of convective heat transfer in fully developed, downward flow of 100-micron and 300-micron (average screen size) suspensions of coal particles and air were measured in a 0.498-inch I.D. vertical tube. The gas Reynolds number varied from 10000 to 30000, and the solids mass loading ratio at the lower gas flow rate varied from 0 to approximately 20 with the larger particles and 0 to 12 with the smaller particles. The results of this investigation, which was the second phase of a two-part study on gas-solids suspensions in vertical downflow, support the conclusions obtained during the first phase with 329-micron spherical glass beads. Both studies seem to indicate that the mechanisms of heat and momentum transport are weaker in downflow than in upflow, and that particles of a size greater than 100 microns have an almost negligible effect on the wall-to-gas coefficient of heat transfer in the suspension. Particle velocity in the 300-micron suspension was measured by coating a few closely-sized particles (325- and 650-microns average screen size) with a phosphorescent substance and injecting them into the flowing suspension near the beginning of the flow test section. Two detectors located in the region of fully developed flow for the suspension provided a means of determining the velocity. The measured values agree closely with theoretical values predicted by solving the single-particle equation of motion. The particles were fully accelerated to the gas velocity at velocities less than approximately 80 fps and lagged the gas only slightly at higher velocities (up to 140 fps). Consequently, the slip phenomenon was neglected in calculating the frictional pressure drop from the measured pressure drop. Frictional pressure drop for fully developed flow of the suspension was correlated both in terms of a solids friction factor (for the increase in frictional pressure drop due to the solids) and a two-phase friction factor (for the total frictional pressure drop of the suspension). Friction factors were slightly lower than those obtained previously with glass beads in downflow, and the difference is attributed to the probability that the glass beads were not fully accelerated in the test section. Frictional pressure drop in both coal and glass-bead suspensions, however, was lower than that reported in the literature for upflow and less dependent on solids loading. The rate of gas-to-particle heat transfer was determined by measuring the gas-temperature profile at the outlet of a 4-foot-long heat-transfer section located in the region of fully developed flow. Particle impact heating of the thermocouple used to measure gas temperature in the 300-micron suspension was avoided by bending the tip downstream. Average particle Nusselt number in the suspension increased with increasing turbulence of the gas, increasing particle size, and decreasing solids loading. As a result of the dependence v of gas-to-particle rate of heat transfer on solids loading, particle temperature at the heat-transfer-section outlet depended strongly on particle size at values of solids-loading ratio less than 5 and was almost independent of size at higher solids loadings. This result should be of major interest in the design of a coal pyrolysis reactor where the rate of heat transfer to the coal particles is of primary importance. |
