Département de chimie

Gas absorption experiments in a pilot plant column with the sulzer structured packing mellapack

Brito, Marcelo Henriques de ; Stockar, Urs von (Dir.)

Thèse Ecole polytechnique fédérale de Lausanne EPFL : 1991 ; no 984.

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    Data for the volumetric mass transfer coefficient, kLa, and the effective area, of structured packings have rarely been reported in the literature. This is despite the ever-increasing importance of structured packings in industrial separation columns for absorption and distillation processes. This work verified that the CO2-NaOH reaction is suitable for the determination of effective mass transfer area. The kinetic parameter of this reaction which governs the absorption flux, was evaluated with a wetted wall column, which provides a known mass transfer area. Normally, this kinetic parameter is calculated with data obtained from other literature sources. Since there is a large disagreement, the value obtained with the wetted wall column was used in absorption columns packed with MELLAPAK for the determination of effective areas. The desorption of O2 from demineralized water into air is suitable for the determination of kLa values. The measured desorption rates are actually related to the overall volumetric liquid side mass transfer coefficient, which in turn is very close to kLa since O2 is sparingly soluble in water, even by comparison with CO2. Therefore the gas side resistance is negligible. The effective area for mass transfer and the kLa of the SULZER packings MELLAPAK 125.Y, 250.Y and 500.Y were measured under industrial-scale operating conditions, with a broad range of gas and liquid flows, in a pilot plant with a column having an internal diameter of 295 mm and a packing height of 420 mm. The experimental procedure and apparatus employed in this work were checked by making additional measurements of effective area for 25 mm ceramic ring packing; these values were compared with literature data. The influence of the drip point density of the liquid distributor and of a higher packing height of MELLAPAK 125.Y were also studied. This contribution shows that the effective mass transfer area for MELLAPAK 125.Y, 250.Y and 500.Y can be higher than the defined geometric area of the packing. The ratio of the effective area to the geometric area is a function of a Reynolds number defined with the liquid characteristic velocity and packing hydraulic diameter. For MELLAPAK with relatively low specific geometric area in particular, the space between the sheets is sufficient to permit droplet formation and turbulence enhancement, both of which magnify effective area. Empirical correlations for the kLa of MELLAPAK 250.Y and 50O.Y are proposed. The kLa values for MELLAPAK 250.Y and 500.Y are higher than for irregular packings with the same geometric area. The kLa for MELLAPAK 125.Y varies with the liquid and gas flowrates in a different way than that for MELLAPAK 250.Y, 500.Y and irregular packings. A model for predicting the liquid phase mass transfer coefficient, kL, for MELLAPAK 250.Y and 500.Y is proposed. In the development of this model, the experimental data from this work were considered together with the penetration theory. The value of kL increases with the liquid and gas flow and decreases with the geometric area of the packing. Higher kL values are associated with irregular packings with equal geometric area. Recommendations for improvements in the surface structure of MELLAPAK, enhancing liquid film renewal rate, as well as suggestions for future research projects end this thesis.