Kinetics of Water Adsorption/Desorption on Bituminous Coals

Abstract

The kinetics of water adsorption and desorption on nine bituminous coals of various ranks have been studied, mainly at 26 °C. To quantify the sorption kinetics, isobaric water uptake vs time data have been fitted by an empirical stretched-exponential model. This model provides good fits to water uptake data throughout the entire equilibration process at relative water vapor pressures (relative to saturation pressure) in the range of 0-0.9 for all coals investigated. This represents a distinct advantage oversimplified Fickian models which are valid for only a portion of the data within the short and long time limits. A flow rate kinetic parameter has been introduced to quantify the rate of water transfer from outside the coal particle to the intraparticle pore structure. Most of the bituminous coals investigated in this study have similar flow rates at relative pressures in the range of 0.2-0.9. However, for two coals of rank approaching that of semianthracites, the flow rate is significantly less than for all other bituminous coals investigated. It is believed this difference is caused by differences in the coal structure. Variations between coals in the flow rate at low relative pressures (0-0.2) appear to be caused by variations in the primary adsorption site density which is largely determined by the coal oxygen content. The density of oxygen containing functional groups also influences the flow rate at low relative pressures during water desorption.

Water sorption on coals

“…(5), kinetic data were plotted as function of square of time for M t /M 1 < 0.5. For water uptake, kinetic data show a slight delay in the beginning of water adsorption at each relative pressure, similar to the works of McCutcheon et al [9]. Experimental data were therefore fitted in the water uptake range of 0.05 < M t / M 1 < 0.50.…”

“…As reported by McCutcheon et al [9], few studies have reported on the characteristics of water diffusion into coals. Sevenster [10] and Kawecka and Lason [11] applied Fickian diffusion models to the kinetic experimental data of water sorption.…”

“…Constant rates on water diffusion on activated carbons have been estimated at about 10 À3 s À1 , from the LDF model [19,20,42]. Moreover, McCutcheon et al [9], using an empirical kinetic model, have found constant rates of about 10 À2 -10 À3 s À1 for bituminous coals. This discrepancy may be attributed at the experimental method and the fitting model used in this study, which could affect the value of water diffusion.…”

“…Sevenster [10] found a diffusion coefficient of water at about 10 À15 -10 À17 m 2 s À1 , for a grain size of coal about 250 lm. Recently, McCutcheon et al [9] investigated adsorption/desorption kinetics on different bituminous coals with an empirical kinetic model which has provided good fits in the entire equilibration process and at all relative pressures. They have argued that in a low relative pressure range, primary adsorption site and thus coal oxygen content influence the diffusion.…”

Retardation of Protein Dynamics by Trehalose in Dehydrated Systems of Photosynthetic Reaction Centers. Insights from Electron Transfer and Thermal Denaturation Kinetics

“…Non-exponential kinetics have been observed in different polymeric and amorphous systems for sorption/desorption processes not simply controlled by the energetics of surface attachment/detachment, but rate limited by different transport mechanisms, including diffusion. [66][67][68][69] It has been pointed out that for diffusion limited processes the sorption/desorption kinetics are well accounted for by a stretched exponential, and that the exponent β may be related to the geometry of the system. 66,68 The analysis of dehydration/rehydration kinetics support a diffusion limited water exchange between the RCtrehalose matrix and the atmosphere.…”