Gas Holdup in Opaque Cellulose Fiber Slurries
Three different cellulose fiber types are used to study their effect on gas holdup and flow regime transition in a 10.2 cm semi-batch bubble column. The three natural fiber types include bleached softwood chemical pulp (softwood), bleached hardwood chemical pulp (hardwood), and bleached softwood chemithermomechanical pulp (BCTMP). Gas holdup is recorded over a range of fiber mass fractions (0 ≤ C ≤ 1.6%) and superficial gas velocities (Ug ≤ 23 cm/s). Experimental results show that gas holdup decreases with increasing fiber mass fraction. Homogeneous, transitional, and heterogeneous flow is observed for all three fiber types at low fiber mass fractions. All three fiber types produce similar results in the homogeneous flow regime while significant differences are recorded in the heterogeneous flow regime; those being low mass fraction hardwood (softwood) fiber slurries produce the highest (lowest) gas holdup. At higher fiber mass fractions, only pure heterogeneous flow is observed and softwood fiber slurries still produce the lowest gas holdup, although the differences in gas holdup between fiber types are small. The Zuber-Findlay drift flux model is used to describe the gas holdup results in cellulose fiber slurries when the flow conditions are heterogeneous. The Zuber-Findlay drift flux model is also used to identify the superficial gas velocity at which homogeneous flow is no longer observed with some success. Generally, the superficial gas velocity at which the flow deviates from homogeneous flow decreases with increasing fiber mass fraction.