No abstract

This work deals with a thermodynamic and kinetic experimental study of adsorption of gaseous single Cl/Br-VOCs (1,2-dichloroethane, 1-bromo-2-chloroethane, and 1,2-dibromoethane) onto siliceous ZSM-5 at 25 °C. The adsorbed amounts, adsorption heats, and diffusivities were simultaneously obtained. The isotherms were of the type I and verified Henry's law in a very large domain of filling. The adsorption heats and the diffusivities of the brominated molecules displayed specific behaviors probably owing to a transport resistance. The comparison of the relative position of isotherms and the adsorption heats allowed us to predict that the competitive adsorption of their mixture would be similar to a distillation. The breakthrough curves verified this prediction.

The adsorption/co-adsorption of 1,2-dichloroethane (DCA) and 1-bromo,2-chloroethane (BCA) from the vapour and the liquid phases by zeolite ZSM-5 at ambient temperature is reported, using Dubinin's theory and the recent Myers-Prausnitz-Dubinin (MPD) theory. For adsorption from the liquid phase, the activity coefficients in the adsorbed and the liquid states are the same and no selectivity is observed. This is confirmed by the absence of an excess enthalpy of immersion of ZSM-5 into the mixtures. Adsorption from the vapour phase proceeds in two stages, as indicated by double Dubinin-Astakhov (DA) plots. If one considers only the domain of high relative pressure, co-adsorption is described by the MPD theory. The agreement between calculated and experimental compositions of adsorbate can be improved by using, as a first and good approximation, the activity coefficients of the liquid-solid equilibrium at the same composition. A good agreement is also observed between the enthalpies of immersion calculated from the DA equation and the experimental integral heats of adsorption. However, for vapour mixtures ZSM-5 shows selectivity in favour of the more volatile compound, 1-bromo,2-chloroethane. This probably results from adsorption at low pressures and could be due to differences in kinetics.