Restrictions on the Rate of Absorption When Evaluating Sorption Isotherms Obtained Using a Micro-Calorimetric Technique

Abstract

One interestingway of measuring sorption isotherms and heats of sorption is the newly developed micro-calorimetric method. It turns out, however, that the adsorption kinetics must be proven to be rapid enough to fulfill the assumptions behind the theory for the determination of the isotherm to be valid. Here, the lower limit of the accepted rate of adsorption will be analyzed by making calculations of the diffusion behavior in the calorimetric vessel. The evaluation of the adsorption isotherms is based on assumptions of a quasi-static nature. If the adsorption kinetics are slow the behavior in the calorimeter will turn to a more or less transient process. When the heat of adsorption is of interest and the sorption isotherm and kinetics of sorption can be determined by other methods, compensation for the transient effects can be made.

References

Wadsö, I. and Wadsö, L. (1996). A New Method for Determination of Vapour Sorption Isotherms Using a Twin Double Micro-Calorimeter. Thermochimica Acta, Vol. 271, pp. 179–187.

Wadsö, I. and Wadsö, L. (1997). A Second Generation Twin Double Micro-Calorimeter; Measurements of Sorption Isotherms, Heats of Sorption and Sorption Kinetics. Journal of Thermal Analysis, Vol. 49, pp. 1045–1052.

Johannesson, B. (2000). Transport and Sorption Phenomena in Concrete and Other Porous Media, Division of Building Materials, Lund Institute of Technology, Report TVBM–1019.

Weast, R.C. , Lide, D.R. Astle , M.J. and Beyer, W.H. (1989). Handbook of Chemistry and Physics, pp. F-50, 70th edition, CRC Press, Inc. Boca Raton, Florida.