Gas Holdup in a Cocurrent Air-Water-Fiber Bubble Column

Abstract

Effects of superficial liquid velocity (Ul ), superficial gas velocity (Ug ), and fiber mass fraction (C) on gas holdup (ε) and flow regime transition are studied experimentally in well-mixed water-cellulose fiber suspensions in a cocurrent bubble column. Experimental results show that the gas holdup decreases with increasing Ul when C and Ug are constant. The gas holdup is not significantly affected by C in the range of C < 0.4%, but decreases with increasing C in the range of 0.4% ≤ C ≤ 1.5%. When C > 1.5%, a significant amount of gas is trapped in the fiber network and recirculates with the water-fiber slurry in the system; as a result, the measured gas holdup is higher than that at C = 1.5%. The axial gas holdup distribution is shown to be a complex function of superficial gas and liquid velocities and fiber mass fraction. The drift-flux model is used to analyze the flow regime transitions at different conditions. Three distinct flow regimes are observed when C ≤ 0.4%, but only two are identified when 0.6% ≤ C ≤ 1.5%. The superficial gas velocities at which flow transition occurs from one regime to another are not significantly affected by Ul and slightly decrease with increasing C. ABSTRACTEffects of superficial liquid velocity (U/), superficial gas velocity (U g ), and fiber mass fraction (C) on gas holdup (c) and flowregime transition are studied experimentally in well-mixed water-cellulose fiber suspensions in a cocurrent bubble column. Experimental results show that the gas holdup decreases with increasingU/ when C and U g are constant. The gas holdup is notsignificantly affected by C in the range of C < 0.4%, but decreases with increasing C in the range of 0.4%~C ::; 1.5%.WhenC > 1.5%, a significant amount of gas is trapped in the fibernetwork and recirculates with the water-fiber slurry in the system; as a result, the measured gas holdup is higher than that atC::: 1.5%. The axial gas holdup distribution is shown to be a complexfunction of superficial gas and liquid velocities and fibermass fraction. The drift-flux model is used to analyze the flowregime transitions at different conditions. Three distinct flowregimes are observed when C~0.4%, but only two are identifiedwhen 0.6% s C s 1.5%. The superficial gas velocitiesat which flow transition occurs from one regime to another are not significantly affected by U/ and slightly decreasewith increasing C. KEYWORDS:Bubble column; Cellulose fiber; Gas holdup; Hydrodynamics;Multiphase flow NOMENCLATUREBo coefficient in Eq. (3), Eo = COUI +U 1m C fiber mass fraction

Similitude Analysis for Gas-Liquid-Fiber Flows in Cocurrent Bubble Columns

“…(16) is 4/5, which is significantly . different from the value (-4) found in the experimental study of Tang and Heindel [23], indicating that n; alone is not sufficient to characterize the fiber effects on gas holdup. This is ,because rr; only accounts for the ratio of bubble buoyancy force to fiber network strength.…”

“…The slight difference between the physical implications of Nr and Nc (i.e., Nc only accounts for the average fiber length while Nr together with Nc provides both average and standard deviation of fiber length distribution) was used to justify the application of the combination of these two parameters in characterizing the fiber effects. Although fiber length distribution is an important reason that Nc should be used together with Nc to quantify the fiber effects on gas holdup, it is not a sufficient one, i.e., there may be other reasons because Nc alone does not characterize gas holdup effects when the fibers have a uniform length (1, 3, and 6 mm Rayon), while N~-·N; with a~ 0.2 does [23].…”

“…Tang and Heindel [23] investigated the influences of fiber mass fraction and type on gas holdup in the same cocurrent airliquid-fiber bubble column used in the present study. They reported that for different fiber types, neither the crowding factor (Nc) nor the fiber number density (Nr) was sufficient in quantifying fiber effects on gas holdup.…”

“…(18) and (19) come from the conditions imposed by Tang and Heindel [23] Equation (20) shows that the dimensionless parameter II~ deviates from the fiber effect characterization parameter reported in Tang and Heindel [23] by a multiplier,…”

“…Bubble size distribution in gas-liquid-fiber flows and its variation with fiber mass fraction and fiber type have been investigated in semi-batch [8-1 0] and cocurrent bubble columns [11 ]. Gas holdup in gas-liquid-fiber systems has also been studied in both semi-batch [2,[12][13][14][15][16][17][18] and co current [2,[19][20][21][22][23] bubble columns. Effects of superficial gas and liquid velocity, fiber mass fraction, fiber type, and gas ·Corresponding author, Phone: 515-294-0057, Fax: 515-294-3261, Email: theindel@iastate.edu distribution method on gas holdup were studied in these investigations.…”