Modified BET Model Research Articles

Sorption isotherms and heat of sorption of pineapple

Sorption isotherms of pineapple were determined at 20°C, 30°C, 40°C and 50°C temperatures by using dynamic method. Six two-parameter and one three-parameter isotherm models were selected to fit the observed data, and the modified BET model was found to be the best-fitted model for pineapple. The heat of sorption of pineapple decreased with an increase in moisture content and the heat of sorption was found to be a power function of moisture content.

Determination of fractal dimensions of solid carbons from gas and liquid phase adsorption isotherms

The total surface area, micropore volume, and fractal dimensions of five different carbons (Sorbonorite 4, GAC 1240, and three amorphous carbons) were evaluated from analysis of gas (N2) and liquid (phenanthrene) adsorption isotherm data. The modified BET and fractal Frenkel–Halsey–Hill (FHH) models were used to estimate surface fractal dimensions. Micropore volumes were estimated from Dubinin–Radushkevich (DR) plots and were compared to those calculated from standard N2 adsorption isotherm data using de Boer’s t-method. The estimated surface fractal dimensions using the modified BET and FHH models (DS=3+3h, and P/P0 from 0.0 to 0.4) were (2.7, 2.6, 2.1, 2.4, and 2.1) and (2.5, 2.6, 1.9, 2.4, and 1.9), respectively. The FHH fractal analysis suggested that van der Waals forces are the dominant interaction forces between nitrogen and carbon surfaces. Depending on the method of analysis, the fractal dimensions of the carbons with suggested micropore structure, Sorbonorite 4 and GAC 1240, were 2.5–2.9 and 2.6–2.9, respectively. Analysis of the adsorption–desorption data suggested that amorphous carbons with fractal dimensions of 2.1 (from the modified BET model) have smooth surfaces, with respect to their micropore structure. Further analysis of the adsorption data showed that the slopes of the linear segment of the plots of adsorption potential versus relative amount adsorbed are dependent on the pore size range and surface structure (fractal dimension) of the carbons.

The onset of formation of the water liquid phase in pores of carbon adsorbents

A two-stage mechanism of adsorption, including nucleation and condensation, was proposed to describe the formation of the water liquid phase in carbon adsorbents. Adsorption is assumed to occur in cylindrical pores. Nucleation is described by a modified BET model, and condensation is treated by the model of a stretched liquid film on a bent surface. The onset of formation of the liquid phase (OFLP) is determined from the intersection of the adsorption isotherms for these stages. The theoretical value of the relative pressure of OFLP varies over a wide range, decreasing with a decrease in the pore radius and reactiing the relative vapor pressure of 0.178 for the spinodal state of water. The comparison method using isotherms of graphitized carbon black as the reference isotherms was applied for the determination of OFLP of water in real active carbons. This resulted in good agreement between theory and experiment.

Modified BET models for modeling water vapor sorption in hydrophilic glassy polymers and systems deviating strongly from ideality

The use of two modified BET models for modeling water vapor sorption in various nonideal systems involving strong interactions (textiles, polyamides, polyimide, vinyl polymers, proteins, and related compounds) has been investigated. Contrary to the original BET model derived for multimolecular adsorption and restricted to low water activities only, the three-parameter BET model, which considers sorption on a limited number of sorption layers, allowed the modeling for activities up to 0.9. This model has been shown to be an important extension of the original model but remains limited, due to the divergence of the model for a → 1. Releasing an other important assumption of the BET model, the GAB model was then investigated and its striking efficiency, in terms of high correlation coefficients and low residual sums of squares, was demonstrated for activities up to 0.95, thus covering all the usual experimental range. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1415–1430, 1998

Calculation of salt activities in molten salt hydrates applying the modified BET equation, I: Binary systems

On the basis of the modified BET model according to Stokes and Robinson, an equation for the calculation of salt activities in molten salt hydrates has been derived. The equation is used to describe successfully the liquidus curves of the hydrates of MgCl2, Mg(NO3)2 and CaCl2 inT-x diagrams. A promissing feature of the model is the small number of adjustable parameters and its extrapolative power.