Measurements and Modeling of Excess Adsorption of Pure and Mixed Gases on Wet Coals

• Published in: Energy & fuels Vol. 26; no. 5; pp. 2899 - 2910
• Main Authors: Mohammad, Sayeed A, Sudibandriyo, Mahmud, Fitzgerald, James E, Liang, Xudong, Robinson, Robert L, Gasem, Khaled A. M
• Format: Journal Article
• Language: English
• Published: American Chemical Society 17.05.2012
• Subjects: Adsorption; Basins; Binary mixtures; Carbon dioxide; Coal; Isotherms; Mathematical models; Methane; Surface chemistry
• ISSN: 0887-0624; 1520-5029
• DOI: 10.1021/ef300197a

Pure-gas excess adsorption isotherms were measured for methane, nitrogen, and CO2 on two water-moistened coals at 319.3 K and pressures up to 12.4 MPa. The coals were lower basin Fruitland (LBF) coal, from the San Juan basin, and Illinois #6 coal, from the Illinois basin. Furthermore, gas-mixture adsorption measurements were made on the Illinois #6 coal for the three binary mixtures formed by methane, nitrogen, and CO2. The rationale for this study was (a) to provide data useful for development and testing of models for the competitive adsorption of mixed gases on coals and (b) to assess the ability of the simplified local-density model to describe the data. The pure gas measurements revealed that CO2 excess adsorption on both coals exhibited distinct maxima near the CO2 critical pressure. At a pressure of 7 MPa, the ratio of excess adsorptions for N2:CH4:CO2 was 1:2.8:4.4 for LBF coal and 1:2.7:6.3 for Illinois #6 coal. The LBF coal showed low levels of adsorption, which can be attributed to the high ash content of this coal sample. The mixed-gas adsorption isotherms were measured for the binary mixtures methane/nitrogen, methane/CO2, and nitrogen/CO2. For each system, measurements were made at a series of nominal feed compositions, specifically, at molar ratios of approximately 20%/80%, 40%/60%, 60%/40%, and 80%/20%. The results showed that the more strongly adsorbing component (based on pure-fluid measurements) exhibits higher component adsorption in the binary mixtures, even for feeds that are richer in the lower-adsorbing component. The only exception to this was the methane/nitrogen mixture at a 20%/80% feed composition. The simplified local-density (SLD) model was applied to the pure-gas adsorption data. Two alternative forms of the SLD framework describe the pure-gas adsorption data within the experimental uncertainties. The two forms were designed to investigate the relative accessibility and affinity of methane, nitrogen, and CO2 on each coal. Results indicated that CO2 exhibits both higher affinity as well as accessibility for the coal surface. The SLD model was also used to obtain a priori predictions of binary mixture adsorption based solely on pure-gas adsorption isotherms. Overall, the a priori predictions for mixtures were within two times the experimental uncertainties, based on model parameters obtained from pure-component adsorption data alone.