GAS HOLDUP IN BAFFLED BUBBLE COLUMNS OF DILUTE SLURRIES OF FINE POWDERS AND VISCOUS LIQUIDS

Experimental gas phase holdup data reported by the authors as taken on two baffled bubble columns and involving slurries of fine powders (average particle diameter ≤90 μm) and dilute suspensions (mass fraction ≤ 20 percent) in viscous fluids are re-examined after correction for a small calculation error in superficial gas velocity. The two bubble columns are: a Plexiglas bubble column, 0.108 m in diameter and 2.25 m tall, equipped with seven 19 mm tubes arranged in equilateral triangular configuration with a pitch of 36.5 mm and a Pyrex glass column, 0.305 m in diameter and 3.24 m tall, equipped with thirty-seven 19 mm tubes arranged in the same configuration. Air and nitrogen are used as gas phase, water and Therminol as liquid phase, and iron oxide powders, glass beads and sand as solid phase. The small column data are at ambient temperature while those belonging to the large column extend from ambient to 473K. These data are examined to assess the influence of column diameter (scale-up), temperature, slurry concentration and viscosity, and superficial gas velocity on gas holdup for baffled columns.

It is found that in the 0.108 m diameter column the holdup is about the same for axial probes of different diameters, 19 mm, 31.8 mm and 50.8 mm, for two-phase systems involving liquids of small (water) and large (Therminol) viscosities. However, when a seven-tube bundle is installed, the holdup increases. This is consistent with the bubble dynamics and observed bubble sizes. This qualitative trend is also upheld by three-phase systems involving dilute slurries of fine powders. In the larger column when fully packed with a thirty-seven tube bundle, the holdup is found to be the same as for the small column. A loosely packed bundle gives smaller holdup at temperatures greater than the ambient due to the larger size of bubbles. The gas holdup is greater for a less viscous system and this is again due to the larger size of bubbles in a more viscous system. The influence of temperature is pronounced and is very characteristic of the nature of liquid involved. For example for water and Therminol the variations are in opposite directions with change in temperature. Bubble splitting and foaming control the nature of these dependencies. With the addition of solids the holdup is almost insignificantly altered at all temperatures.