Mathematical model of supercritical CO_2 adsorption on activated carbon Effect of operating conditions and adsorption scale-up

摘要

A general mathematical model for supercritical CO_2 adsorption on activated carbon has been developed.Adsorption curves obtained with a laboratory plant (adsorber of 10ml) have been fitted by with this model and the results have been extrapolated to adjust the breakthrough curves obtained with a pilot plant (adsorber of 11).Moreover a wide range of operating conditions has been covered,extraction pressures from 14.7 to 20.0MPa,temperatures between 30 and 50°C and CO2 flow-rates ranging between 0.12 and 0.36kg/h.The proposed model takes into account equilibrium (adsorption isotherm),diffusion in the solid (effective diffusion coefficient,D_e),mass-transfer from the bulk of the fluid phase to the surface of the solid (mass-transfer coefficient related to the supercritical solvent,k_f),axial dispersion (effective dispersion coefficient,D_(az)) and a first order reversible adsorption/desorption reaction at the solid surface site (adsorption and desorption kinetic constants,k_a and K_(d)).In order to get a suitable adjust,the experimental adsorption curves have been represented by this model by fitting the kinetic coefficients (k_a,D_e and D_(az)) to the experimental curves,obtained from the laboratory installation.The parameters k_f and K_d are calculated,k_f by well-known correlation and k_d by the determination of adsorption isotherm.A fair good fitting of all the available experimental laboratory results has been obtained using D_e values ranging between 2.4 and 4.8 x 10~9 m~2/s,D_(a2) values of 1.2-3.2 m2/s and k_a values of 1.4-1.5 m~3/kg s.With these optimised parameters it is possible to model the adsorption curves obtained with the pilot plant with reasonable accuracy (standard deviation,7-11%).