Local Gas Holdup Variation in a Fiber Slurry

Air-water-fiber flows are found in the pulp and paper industry in a variety of unit operations. Understanding the complex hydrodynamics found in such flows is beneficial to flotation deinking (i.e., a separation process found in paper recycling) and fiber bleaching. This paper experimentally investigates the effects of superficial gas velocity, fiber type, fiber length, and fiber weight percent on the gas flow regime and the overall and local gas holdup in a 32.1-cm-diameter semibatch bubble column. Experiments are performed using three different rayon fiber lengths (3, 6, and 12 mm) and three different cellulose (natural) fiber types over a range of superficial gas velocities (Ug ≤ 20 cm/s) and fiber weight percents (0 < C ≤ 1.8%). The local gas holdup is determined by pressure drop measurements at several axial locations spanning a height of 10 column diameters (H = 3.21 m), while the overall gas holdup is determined from the overall pressure drop. Results show local gas holdup maxima at two different axial locations, which suggest the existence of recirculation cells. The location of the upper local gas holdup maximum, and thus the size of the recirculation cells, depends on the fiber type.