Effect of flotation frothers on bubble size and foam stability

Effect of surfactants on liquid-side mass transfer coefficients

Mixing behavior of three geometrically equivalent 0.06 m3 bubble column and airlift photobioreactors was compared for use in microalgal culture. Mixing was characterized in terms of the axial dispersion coefficient, mixing time, circulation time and Bodenstein number. Measurements were reported in tap water and sea water, as used in growing microalgal biomass. Compared with the two airlift reactors (a split-cylinder and a draft-tube sparged device, both of aspect ratio 9.1), the bubble column (aspect ratio = 9.1) had a shorter mixing time at any aeration rate (superficial aeration velocity of up to 0.05 m s−1 in all reactors). Values of the various mixing parameters in the two fluids were quite comparable at identical aeration rates in any specific geometry of the reactors. Existing correlations of mixing times satisfactorily correlated the data in the bubble column, but not in the airlift reactors. The following correlations were established for the airlift devices: B o LG = β ( F r 1 / 3 ) λ B o = k t m t c where Bo is the liquid-phase Bodenstein number, Fr is the Froude number, BoLG is the Bodenstein number based on the superficial velocity of the gas, tm is the mixing time, tc is the circulation time, and k, β and γ are constants that depend on the geometry of the reactor and the fluid used.

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Mixing in Bubble Column and Airlift Reactors

The size of bubbles generated using a single-hole sparger, a multi-hole sparger and a flotation cell were measured. While the size of bubbles generated from the single-hole sparger does not depend on frother concentration, the bubble size strongly depends on frother concentration when the multi-hole sparges, or flotation cell, are utilized. The bubbles size ceases to depend on frother at the concentrations exceeding the value characteristic for each frother and referred to as the critical coalescence (CCC). All the bubble size vs. frother concentration curves converge on a single curve. Aqueous solutions of the frothers that are characterized by low CCC values from foams quite stable under dynamic conditions. Since bubble coalescence can be inhibited by frother concentrations exceeding the CCC, sparing systems can be studied and compared only at frother concentrations that exceed CCC.



On a mesure la taille des bulles produites par un aerateur a orifice unique, un aerateur a orifices multiples et une cellule de flottation. Alors que la taille des bulles produites avec l'aerateur a orifice unique n'est pas liee a la concentration d'agent moussant, la taille des bulles depend fortement de la concentration d'agent moussant dans le cas des aerateurs a orifices multiples ou de la cellule de flottation. La taille des bulles cesse d'ede la valeur caracteristique de chaque agent moussant (concentration de coalescence critique ou CCC). Toutes les courbes de taille des bulles par rapport a la concentration d'agent moussant se superposent. Les solutions aqueuses des agents moussants qui sont caracterises par de faibles valeurs de CCC forment des mousses plutot stables dans des conditions dynamiques. Considerant que la coalescence es bulles peut etre inhibee par des concentrations d'agent moussant excedant la CCC, les systemes de dispersion peuvent etre etudies et compares seulemetn a des concentrations d'agent moussant qui excedent la CCC.

Bubble coalescence and its effect on dynamic foam stability

Studies of mixing in a concentric tube airlift bioreactor with different spargers

The performance of a new type of bubble generating device, the rubber sheet sparger, is contrasted and compared with rigid spargers. Impulse response tests were analyzed using the weighted‐moments method to determine voidage and dispersion coefficients in counter‐current bubble columns of nominal diameters two, four, six and twelve inches. The flexible rubber sheet sparger produced more uniform emulsions, smaller bubbles and larger voidages than perforated plates, while dispersion coefficients were reduced for a range of superficial gas velocities. The dispersion data seem to fit the isotropic turbulence model, with slight modifications. It is demonstrated that the rubber sheet sparger is self‐regulating, with hole size increasing in direct proportion to pressure drop across the sparger.

Robyn M. Hedge

Papers

Dispersion and hold-up in bubble columns — comparison of rigid and flexible spargers

Dispersion and Hold-Up in Bubble Columns