Maximum stable bubble size and gas holdup in high-pressure slurry bubble columns

Experiments of pressure effects on gas holdup and bubble size in slurry bub ble columns at 5.6 MPa and at gas velocities up to 45 cm/s indicate that th e gas holdup increases with ail increase in pressure especially at high slu rry concentration. At ambient pressure, a higher solids concentration signi ficantly lowers gas holdup over rite entire gas-velocity range, while at 5. 6 MPa, the effect of solids concentration on gas holdup is relatively small at gas velocities above 25 cm/s. Ail empirical correlation was developed b ased on these data and those in the literature to predict gas holdup in bub ble and slurry bubble columns over a wide range of operating conditions. An analysis Of bubble flow characteristics during dynamic gas disengagement i ndicates that large bubbles play a key role in determining gas holdup due t o the large bubble and wake volumes that induce the acceleration Of small b ubbles. Direct measurements of bubble size shows that elevated pressures le ad to smaller bubble size and narrower bubble-size distributions. Bubble si ze increases significantly with increasing solids concentration at ambient pressure, while at high pressures this effect is less pronounced. A theoret ical analysis of circulation of gas inside die bubble yields an analytical expression for maximum stable bubble size in high-pressure slurry bubble co lumns. Based on this internal circulation model, the maximum stable bubble size at high pressures is significantly smaller due to the high gas inertia and low, gas-liquid surface tension. The smaller bubble size and its reduc ed bubble rise velocity account for the observed pressure effect on gas hol dup.