Diffusion Barriers in the Kinetics of Water Vapor Adsorption/Desorption on Activated Carbons

Abstract

The adsorption of water vapor on a highly microporous coconut-shell-derived carbon and a mesoporous wood-derived carbon was studied. These carbons were chosen as they had markedly different porous structures. The adsorption and desorption characteristics of water vapor on the activated carbons were investigated over the relative pressure range p/p° = 0−0.9 for temperatures in the range 285−313 K in a static water vapor system. The adsorption isotherms were analyzed using the Dubinin−Serpinski equation, and this provided an assessment of the polarity of the carbons. The kinetics of water vapor adsorption and desorption were studied with different amounts of preadsorbed water for set changes in pressure relative to the saturated vapor pressure (p/p°). The adsorption kinetics for each relative pressure step were compared and used to calculate the activation energies for the vapor pressure increments. The kinetic results are discussed in relation to their relative position on the equilibrium isotherm and the adsorption mechanism of water vapor on activated carbons.

Effects of Textural Properties and Surface Oxygen Content of Activated Carbons on the Desorption Activation Energy of Water

“…The Boehm titration method can be used to determine the number of oxygen-containing groups on a sample surface (Müller et al 1996;Jorge et al 2002). Thus, 1 g samples of a given activated carbon was weighed carefully and placed into vials containing 50 ml of a 0.05 mol/l solution of hydrochloric acid, sodium hydroxide, sodium carbonate and sodium bicarbonate, respectively.…”

“…Since the water molecule has strong permanent dipole moments, its adsorption would be significantly enhanced by introducing oxide groups onto the sorbent surface. Many researchers (Salame and Bandosz 1999;Salame et al 1999;Müller et al 1996;Jorge et al 2002) have reported that the amount of water vapour adsorbed may be improved via surface oxidation of the activated carbons. Some researchers (McCallum et al 1999;Groszek and Aharoni 1999) have discussed the adsorption mechanism of the water vapour onto active carbons and have stated that water molecules first adsorb onto oxygenated surface sites via hydrogen bonding.…”

“…Cossarutto et al (2001) investigated the desorption characteristics of water vapour from activated carbons over the pressure range 0 < P/P 0 < 0.95 using a static adsorption system, and found that carbon surfaces enriched with nitrogen functions had a considerable influence on the adsorption/desorption rate of water vapour. Baudu et al (1993) used differential scanning calorimetry to study the desorption energies of water and organic molecules from activated carbon while Drobot et al (2001) and Harding et al (1998) investigated the desorption characteristics of water vapour from activated carbons through measurements of the kinetics of water vapour desorption and the determination of water vapour desorption isotherms, respectively.…”

Water vapour adsorption on lignin‐based activated carbons

“…53 -55 Harding et al 56 analysed the existence of diffusion barriers in the kinetics of water adsorption/desorption on activated carbons. Others 30,57 have studied the kinetics of adsorption with relative water vapour pressure, in terms of a rate constant.…”

“…This behaviour agrees fairly well with the results obtained in the literature. 30,56,57 In the low relative pressure region, the rate constant value decreases as the adsorption over the active centres proceeds and extent of covering increases, due to diffusion limitation of water molecules through the narrow micropores partially filled after adsorption on primary centres. D22G98 shows the highest rate constants at low relative pressure (up to p/p o < 0.4), probably due to the absence of the effect of diffusion of water vapour throughout the micropores, given the lack of porous structure of this sample.…”