A.A.H. Drinkenburg

TRANSITION TO PULSING FLOW, HOLDUP AND PRESSURE-DROP IN PACKED-COLUMNS WITH CO-CURRENT GAS-LIQUID DOWNFLOW

Abstract Trickle beds of 1 meter in length and resp. 5, 10 and 20 cm in diameter were operated in the so-called pulsing flow regime. The packing was 2.5 resp. 4 mm Raschig rings. Air was taken as the gas phase. Several liquids were used. In this contribution we describe the transition from gas-continuous to pulsing flow, the liquid holdup and the pressure drop over the column. The transition can be described by an effective Froude-number. Correlations are set up for the transition point as well as for the hold-up. Pressure drop is shown to be linearly dependent upon the pulse frequency. In a separate series of experiments the transition to pulsing flow was measured in a 5 mm capillary tube, used as a wetted wall column. It became clear, that the onset of pulsing flow in such a capillary is not determined by the same parameter as the onset of pulsing flow in a packed column.

THE ENHANCEMENT OF THE PHYSICAL ABSORPTION OF GASES IN AQUEOUS ACTIVATED CARBON SLURRIES

The enhancement of the gas-liquid mass transfer rates in aqueous slurries containing small activated carbon particles was studied in a semi-batchwise operated stirred cell absorber with a plane interface. The maximum observed enhancement factors for absorption of propane, ethene and hydrogen in the aqueous slurries were 3.6, 3.3 and 2.0, respectively. It was shown that for our results and those reported in the literature the maximum enhancement factor, Emax, decreases with increasing liquid side gas-liquid mass transfer coefficient, kl. From all the experimental data the following relationship is found: (Emax−1) = 0.0019 (kt[m s−1])−0.71 although different types of activated carbon particles and differences in sizes were used by the various research groups. To describe these results a simple theory is presented.

EFFECTIVENESS OF MASS-TRANSFER IN A PACKED DISTILLATION COLUMN IN RELATION TO SURFACE-TENSION GRADIENTS

Abstract In packed columns large differences occur in the wetting of the particles and especially in the refreshing of the liquid on the wetted particles due to gradients in surface tension of the liquid/gas interface. Mass transfer rates may differ with a factor 2. In a column packed with Berl saddles distillation experiments were performed with a mixture n -heptane/cyclohexane. Ceramic Berl saddles of 4, 6 and 10 mm were used as well as aluminum Berl saddles of 4 mm. In some of the experiments the saddles were coated with PTFE (teflon). The driving force for mass transfer was varied over a wide range. Both negative and positive driving forces were realized. The influence of the surface tension driven refreshment of the interface is most pronounced for small particles; for larger liquid flow rates, that may be applied in beds with larger particles, the effect is obscured by the inertia of the downcoming liquid.

Absorption of gases into activated carbon—water slurries in a stirred cell

Abstract A surface-aerated stirred cell with a flat liquid surface was used to investigate the absorption of propane and ethene gas into slurries of activated carbon and water. Slurries with a solids concentration up to 4% by weight and particle diameters up to 565 μm were used. The experimental mass transfer data were interpreted using a three-resistance model (gas—liquid, liquid—solid and intraparticle resistance). The gas—liquid mass transfer coefficient was found to be enhanced by the presence of small carbon particles in the slurry, the maximum enhancement factor being 3.5. This enhancement was not influenced by the addition of 0.6% by weight of large particles ( d p = 565 μm) to the liquid phase. Intraparticle resistance to mass transfer is low, which can be explained by assuming surface diffusion of the adsorbed molecules on the porous carbon.

THE INFLUENCE OF SLIGHT DEPARTURES FROM VERTICAL ALIGNMENT ON LIQUID DISPERSION AND GAS HOLD-UP IN A BUBBLE COLUMN

Abstract The effects of vertical misalignment of a bubble column on the gas hold-up and the liquid-phase axial dispersion coefficient have been measured as a function of the superficial gas velocity in water-filled bubble columns of 22, 58 and 103 mm diameter with multi-holed gas spargers. From the measurements the following relationship between the axial dispersion coefficient E 1 , the column diameter d , and the angle of inclination α, of the bubble column is found: with α g −1 and C 1 = 1100 rad −1 m −1 . The influence of inclination on the gas holdup ϵ g in the bubble columns is given by: with α C 2 = 2.3 rad −1 .

Enlargement of wetted area and mass transfer due to surface tension gradients: the creeping film phenomenon

Abstract A two-dimensional model is proposed to describe the creeping of a liquid film against a vertical wall due to surface tension gradients. A steady solution of the governing equations is constructed through domain decomposition, scaling and matching. The creeping height is obtained as a function of the system parameters. The enhancement of mass transfer due to this creeping film is studied. From the results an existing discussion about a “measuring stick” for the mass transfer effectiveness of a creeping film can be settled.

Gas separation in a three-phase bubble column

Abstract Slurry sorption of mixtures of gaseous components has been studied as a gas separation method in a semibatchwise operated 0.3 m i.d. bubble column. Activated carbon (solid), water (liquid), propane and ethene (gas) were used as model components. The liquid phase surrounding the solid particles acts as a “dynamical filter” for gaseous components due to a difference in solubility of the gases in the liquid. On its (internal) surface the solid phase accumulates gas molecules that diffuse through the liquid. By proper selection of liquid and solid it is possible to obtain a slurry with both a high selectivity and a high capacity for one or more of the gaseous components. The influences of superficial gas velocity and solids holdup on the selectivity of the separation are reported. A model is proposed for the description of the mass transfer in the column. The equations were solved numerically with a finite-differences method. The results are in good agreement with the experimental data. Empirical correlations are developed to correlate the bubble diameter, gas holdup and volumetric mass transfer coefficient. The axial dispersion of the liquid phase appears to be equal to that of the dispersed solids.