J. Vanderschuren

Gas-to-particle and particle-to-particle heat transfer in fluidized beds of large particles

Abstract Gas-to-particle and interparticle transfer rates were measured, at room temperature, for three different materials, in the range 1 ≤ U / U mf ≤ 3.5 and 0.900 ≤ d p ≤ 2.250 mm by means of batch runs in which wet capillary-porous particles were dried in fluidized beds of similar dry ones. It was observed that gas-to-particle convective transfer coefficients vary only slightly with the air velocity. A “slow-bubble model” convenient for large particle fluidization, was developed to explain this finding and provided interesting data concerning the gas flow distribution in beds of large particles. The particle-to-particle heat transfer coefficients, nearly independent on the particle thermal properties, were correlated by the following equation: h pp = 206 d p −1.22 ( U / U mf ) −0.56 . A theoretical analysis of the interparticle heat transfer confirmed the observed trends and its application to our results allowed to estimate the dense phase expansion.

The absorption-oxidation of NOx with hydrogen peroxide for the treatment of tail gases

Abstract Absorption of NOx up to partial pressures of 500 Pa was experimented at 20°C and atmospheric pressure with aqueous and nitric acid solutions containing hydrogen peroxide. Various NOx oxidation ratios in the gas phase, and different concentrations of nitric acid and hydrogen peroxide in the scrubbing liquid were investigated. It was shown that the absorption rate was enhanced by H 2 O 2 but remains constant if increasing excess quantities of H 2 O 2 are used. Absorption of NOx into H 2 O 2 is considerably catalyzed by nitric acid. A mathematical model was developed in which all these experimental observations were included, and overall kinetic parameters for different NOx species were determined. Favorable agreement was shown between the model predictions and the experimental results obtained both in a laboratory-scale and a pilot-scale absorption column.

Nitrogen Oxides Scrubbing with Alkaline Solutions

The absorption of NOx into sodium hydroxide solutions was studied in a small packed column. A simple mathematical model developed for this absorption was used for the determination of rate parameters relative to NOx species in such solutions. While hydrolysis is the main controlling step for NO2, N2O4 and N2O3 species, nitrous acid HNO2 plays an essential role for the NOx absorption in NaOH solutions. Our mechanistic and kinetic findings were validated as the model has worked with fair success in predicting both NOx removal efficiencies and liquid phase compositions.

Particle-to-particle heat transfer in fluidized bed drying

Abstract Particle-to-emulsion and interparticle heat transfer rates were estimated in the range 1.5 ⩽ u / u mf ⩽3.5, 0.69 ⩽ d p ⩽ 2.15 mm by drying wet refractory particles in fluidized beds of similar dry particles of the same sizes. Overall particle-to-emulsion heat transfer coefficients decrease roughly as the inverse of the particle diameter. Particle-to-particle heat transfer coefficients vary with the power-2 of the particle diameter and decrease as the fluidization velocity increases.

Removal of tetravalent NOx from flue gases using solutions containing hydrogen peroxide

The absorption of NO x (IV) into nitric acid solutions containing a low concentration of hydrogen peroxide was studied in a small packed column. A simple mathematical model developed for this absorption was used for the determination of kinetic parameters relative to NO 2 and N 2 O 4 in such solutions. Results obtained at 10, 20, 30°C lead to the same interpretation: hydrolysis is the main controlling step for tetravalent nitrogen oxides absorption and there is no sensible effect of the acidity on the absorption efficicency. Hydrogen peroxide, however, plays an essential role in solution by preventing the HNO 2 decomposition. The mechanistic and kinetic findings were validated as the model has worked with fair success in predicting NO x removal efficiencies in a pilot-scale packed column.

Modeling of dilute sulfur dioxide absorption into calcium sulfite slurries

Abstract The absorption rate of SO 2 into calcium sulfite slurries is predicted by means of a model, based on the film theory, which considers absorption in a liquid film located at the gas-liquid interface and simultaneous dissolution of calcium sulfite in a liquid film surrounding the particles. The involved reactions are supposed to be reversible and instantaneous. Diffusive transport of ionic and molecular species in both films accounts for the effect of equilibrium and, due to unequal diffusivities of ions, for the effect of the electric potential gradient. The Nernst theory of stagnant film is therefore applied. Predicted overall gas mass transfer coefficients and mean absorption rates were found to be in good agreement with experimental values obtained at different conditions in a cable contactor.

The plate efficiency of multistage fluidized-bed adsorbers

Abstract The operation of a continuous five-stage fluidized-bed pilot adsorber, in the drying of air on activated alumina, gives Murphree plate efficiencies which increase with the ratio of solid to air flowrates and reach maximum values at twice the minimum fluidization velocity. The results are interpreted according to the Cholette and Cloutier model which considers dead zones and bypassing of particles on the plates. This solids flow pattern is confirmed by means of tracer studies.