Abstract
The aim of this paper was to investigate the repeatability of sorption processes occurring in the coal-methane system during multiple measurement series. For research purposes, three granular samples of coal sorbent were used. The samples were subjected to a series of three measurements, each time performed under the same conditions. During each series of measurements, the following stages took place: outgassing the sample in a vacuum, and then saturating it with methane under the pressure of 1, 3, and 10 bar. Sorption capacities and the values of effective diffusion coefficients were compared. The studies into the repeatability of measurements of sorption capacities, conducted by means of the IGA-001 (Intelligent Gravimetric Analyzer) gravimetric instrument for three measurement cycles, showed that the obtained measurement discrepancies in relation to the mean value did not exceed 1.23%. The discrepancies in question approximated the measurement uncertainty of the IGA-001 device. The evaluation of the recurrence of determining the values of effective diffusion coefficients showed that the obtained discrepancies were no larger than 10%, which might have resulted from problems with fulfilling most of the assumptions of the applied unipore diffusion model. The authors did not observe any changes of the sorption parameters of hard coal during consecutive cycles of methane saturation.
Highlights
The sorption capacities of porous media are of vital importance for numerous aspects of both scientific and engineering-related research
For the grain fraction of the analyzed samples (0.20–0.25 mm) and the values of the effective diffusion coefficients within the range of De = 10−10 − 10−8 cms, typical of the coals from the Upper Silesian Coal Basin, the 24-h period was considered long enough to reach the state approximating the conditions of the sorption equilibrium
In the final stage of the sorption process, the deviation from the state of the sorption equilibrium diminishes according to the formula resulting from (3): a∞ − a(t)
Summary
The sorption capacities of porous media are of vital importance for numerous aspects of both scientific and engineering-related research. One sorbent-sorbate system frequently occurring in nature is the hard coal-methane system. According to the suggestions of the authors, these changes are caused by the effect of the sorption-induced swelling of coal, which results in some structural changes in the coal substance, leading to the enlargement of the sorption area Such a phenomenon may be interpreted as a systematic increase in the availability of consecutive pores within the complex coal structure for penetration with gas molecules. This would result in changes to the parameters describing the kinetics of the methane sorption in coal
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