Rainer Buchholz

PROFILE OF LIQUID FLOW IN BUBBLE COLUMNS

Equations for the liquid velocity profile and the average gas hold-up in bubble columns including cocurrent flow are proposed. It is shown that the inversion point of liquid flow can be used as the characteristic parameter for calculating the liquid flow profiles and gas hold-up. A tracer method was developed to measure the inversion point of liquid flow in bubble column reactors. For water as the liquid phase this inversion point was found at a distance from the column axis of 0·70 0·73 times the column radius. Besides, bubble velocities and bubble diameters in water and methanol-water solutions were determined, using a 5-point conductivity microprobe. It was found that in dilute solutions of methanol the bubble velocity is lower than in pure water. With increasing superficial gas velocity, the bubble velocity steadily increases in pure water, whereas in methanol-water solution it first decreases and, after reaching a minimum, increases too.

COMPREHENSIVE STUDY OF THE GAS HOLD UP AND BUBBLE SIZE DISTRIBUTION IN HIGHLY VISCOUS LIQUIDS II. CMC SOLUTIONS

Following properties of short bubble columns employing CMC solutions (1.0, 1.4 and 2.0%) and perforated plates (0.5, 1.0 and 3.0 mm hole diameters) were determined: relative mean gas hold up, EG , bubble size distribution, Sauter bubble diameter, ds , and the specific geometrical bubble surface areas, a′ and a″ due to the “intermediate to large bubbles.” The a″ values were compared with the corresponding volumetric mass transfer coefficients, kLas and the mass transfer coefficients, kL , were estimated. The properties of these systems were investigated as function of the superficial gas velocity, WSG , CMC concentration and aerator type.

Determination of oxygen transfer from single air bubbles to liquids by oxygen microelectrodes

Abstract For the investigation of mass transfer from gas bubbles into liquids the concentration gradient of oxygen migrating from air bubbles was measured by means of oxygen microelectrodes. For this purpose a single air bubble was fixed by a platinum wire spiral with the liquid flowing downward. Thus the ascent of the bubble in an aerated liquid was simulated. Liquid-side mass transfer coefficients determined from concentration gradients were higher than values calculated from theory. Sherwood numbers obtained from experimental results for bubbles of larger diameters were distinctly higher than those for smaller bubbles (diameter ≅1 mm); the difference corresponds approximately to that predicted theoretically between bubbles with mobile and those with rigid interfaces.

Liquid-phase mixing model for hydrodynamics of bubble columns

Abstract In order to investigate the mixing of the liquid phase in bubble columns, the axial and radial distribution of a tracer substance in the water/air system was measured. To this end, the tracer was added at the top of the column as a Dirac pulsed signal evenly distributed over the column cross-section. It was established that the concentration distribution of the tracer was a function of the radial profiles of the mean axial velocity of the liquid and of the axial turbulence intensity. On this basis, a model was developed which takes into account the radial profiles of the mean axial velocity of the liquid as the deterministic parameter of mixing. The axial dispersion coefficient which is then obtained and which is adjusted with respect to the deterministic component can be described as the product of the radius of the bubble column and the stochastic component of the axial velocity component, that is, of the turbulence intensity.

Oxygen transfer from gas bubble into liquid.

Oxygen transfer from a gas bubble into the surrounding liquid is examined by measuring oxygen pressure with Po2-needle electrodes at different distances from the bubble. From there, the mass transfer coefficient can be calculated. First measurements yielded results in the range to be expected.