含水煤岩超临界等温吸附模型的对比分析

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

  • 摘要: 甲烷在煤层中的吸附多为含水条件下的超临界吸附。为了对比分析含水和超临界条件下经典吸附模型的适用性和准确性,开展了不同温度和不同含水率下煤层气的吸附实验,并分别采用Langmuir模型、Dubinin-Radushkevich(D-R)模型、Dubinin-Astakhov(D-A)模型,以及考虑了煤层气溶解及煤岩膨胀的改进Langmuir+k模型、Dubinin-Radushkevich+k(D-R+k)模型、Dubinin-Astakhov+k(D-A+k)模型对含水煤岩的吸附数据进行了拟合及对比分析,并讨论了水分和温度对煤岩超临界吸附的综合影响。结果表明:Langmuir模型、D-R模型和D-A模型在中压和高压下与实测结果偏差较大,均方根误差介于0.01~0.06。而考虑溶解气及煤岩膨胀等因素的改进吸附模型具有更高的拟合精度,其中D-A+k模型最适用于描述含水煤岩的超临界吸附,拟合误差最小,拟合精度最高,其均方根误差介于0.002~0.009。不同温度下含水煤岩的饱和吸附量均随含水率的增加出现不同程度的降低,当含水率增加至某一值后甲烷吸附量趋于稳定。在30和60 ℃下当煤岩含水率由0增至6%时,甲烷饱和吸附量分别降低7.92%和2.49%。不同含水率的煤岩其饱和吸附量随温度的增加出现不同程度的降低,当温度由30 ℃升高至60 ℃时,干煤样的饱和吸附量降低9.05%,而含水率为2.5%的湿煤样其饱和吸附量降低4.61%,含水率为6.0%的湿煤样其饱和吸附量降低3.68%。可见甲烷在煤岩中的吸附受水分和温度的综合影响,含水煤岩的饱和吸附量随温度和含水率的升高而降低,温度越高时煤岩的饱和吸附量受水分的影响越小,含水率越高的煤岩其饱和吸附量受温度影响越小。

     

    Abstract: 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.

     

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