Comparative analysis of isothermal adsorption models for coals with water content under supercritical conditions

The adsorption of methane in the water-bearing coal bed is mostly under supercritical conditions.To compare and analyze the applicability and accuracy of the classical adsorption models for coal with water and under supercritical conditions,the adsorption experiments of coalbed methane on coal samples with different water contents at different temperatures are carried out,and the Langmuir model,Dubinin-Radushkevich (D-R) model,Dubinin-Astakhov (D-A) model,also the modified Langmuir+k model,Dubinin-Radushkevich+k (D-R+k) model and Dubinin-Astakhov+k (D-A+k) model which take into account the dissolved gas and coals swell are used to fit the adsorption data of methane on coals with different water content,the fitting results are compared and analyzed,also the synthetic effect of water content and temperature on supercritical adsorption of methane on coal are discussed.The results show that the Langmuir model,the D-R model,and the D-A model deviate greatly from the measured results under medium and high pressure,and the root-mean-square error (RMSE) is between 0.01-0.06.However,the modified adsorption models which take into account factors such as dissolved gas and coals swell have higher fitting accuracy,and the D-A+k model is the most suitable model with minimal fitting errors and the highest accuracy for describing supercritical adsorption of methane on coals with water,and the RMSE is between 0.002-0.009.The saturated adsorption capacity of the coal decreases in varying degrees with the increase of water content at different temperatures,it tends to be a constant when the water content increases to a certain value,and it is decreased by 7.92% and 2.49% respectively when the water content is increased from 0 to 6% at 30 ℃ and 60 ℃.The saturated adsorption capacity of the coal with different water contents decreases in varying degrees with temperature increases,and it is decreased by 9.05%,4.61%,and 3.68% for coal with the water content of 0,2.5%,and 6% when the temperature is increased from 30 ℃ to 60 ℃.Therefore,the adsorption of methane on coal is influenced by both the temperature and water content.The saturated adsorption capacity of methane on coal decreases with increasing temperature and water content.The higher the temperature,the less the effect of water on the saturated adsorption capacity of methane on coal.The higher the water content,the less the effect of temperature on the saturated adsorption capacity of methane on coal.