Simplified Local-density Modeling of Pure and Multi-component Gas Adsorption on Dry and Wet Coals

Abstract

Generalized correlations for the model parameters in the modified simplified local-density/Peng-Robinson (SLD-PR) model were develop to provide reliable predictions for the equilibrium adsorption of methane, nitrogen, CO2 and their mixtures on dry and wet coals in the range of conditions encountered in coalbed methane (CBM) production and CO2 sequestration. The adsorption of pure methane, nitrogen and CO2 and their mixtures on Argonne premium coals and OSU coals were considered in this study. The coals used included five Argonne premium coals (Illinois #6, Beulah Zap, Wyodak, Upper Freeport, Pocahontas coal) and five OSU coals (Illinois #6, Fruitland OSU #1 and #2, Tiffany and Lower Basin Fruitland coal). The SLD-PR model parameters (coal surface areas and solid-solid interaction energy) were regressed to obtain precise representation of pure-gas adsorption on each coal. The results obtained indicate that the SLD-PR model is able to represent the puregas adsorption on these coals within expected experimental uncertainties. The regressed model parameters were correlated (generalized) in terms of the excess adsorption of adsorbates (methane or nitrogen or CO2) at 400 psia and the coal characteristics, including the fixed carbon and the equilibrium moisture. The generalized parameters facilitate the SLD-PR model prediction of the pure-gas adsorption on these coals within twice the experimental uncertainties. The generalized model parameters from the pure-gas adsorption were used to predict mixture adsorption of these gases on wet coals. Specifically, the mixed-gas adsorption on wet Illinois #6, Fruitland OSU #1 and wet Tiffany coal were modeled. With few exceptions, the model was able to predict the mixture adsorption within three times the experimental uncertainties. Furthermore, inclusion of binary interaction parameters (BIPs) in the SLD-PR model improves the generalized prediction for mixture adsorption. Using generalized model parameters from the pure gases, the BIPs were regressed to obtain a better correlation for the mixture adsorption. When generalized in terms of coal characterization or gas properties, the BIPs resulted in predictions of the mixed-gas adsorption, on average, within twice of the experimental uncertainties.