High Pressure Adsoprtion of Pure Coalbed Gases on Dry Coals

Abstract

High pressure adsorption of pure methane, nitrogen, CO2 and ethane at 328.2 K (131F) and pressures to 13.8 MPa (2000 psia) were measured on dry Illinois #6, Wyodak, Pocahontas #3, Beulah Zap, and Upper Freeport coals. Uncertainties associated with the temperature, pressure, and volume measurements as well as the calculated fluid densities were propagated to estimate the uncertainty of each adsorption datum point.The Langmuir model, Loading-Ratio correlation (LRC) and the Ono-Kondo (OK) lattice model were used to represent these pure-gas, high pressure adsorption data of the near-critical and supercritical regions. The average expected uncertainties of the newly-acquired data for the methane, nitrogen, ethane, and CO2 adsorption were approximately 2.4% (0.02-0.03 mmol/g), 2.7% (0.03-0.04 mmol/g), 10.1% (0.09-0.13 mmol/g) and 5.4% (0.06-0.12 mmol/g), respectively. Nitrogen, methane, ethane, and CO2 exhibited an increasing order in the amount of absolute gas adsorbed. Moreover, methane and nitrogen showed far less variation in adsorption amounts than either CO2 or ethane. Little variation in isotherm reproducibility was shown for methane after the coal has been subjected to CO2 and ethane gas adsorption. The Langmuir model and the LRC can represent the absolute adsorption data within their experimental uncertainties. The OK monolayer model appears effective in modeling pure-gas adsorption on dry coal matrices at near-critical and supercritical regions. The model can describe the present adsorption data within their expected experimental uncertainties. Further, the one-parameter generalized OK model, which relates the model parameters to gas properties and accessible adsorbent characteristics, can represent the adsorption isotherms on dry coals with 7% average absolute deviation or twice the expected experimental uncertainties.