低吸附量煤岩和页岩等温吸附实验的负吸附原因

Negative adsorption in the isotherm adsorption experiments of low-adsorption coal and shale

低吸附样品等温吸附实验经常出现反"V"形先升后降"负吸附"现象,成为困扰相关实验的测试难题。为此,基于前人的研究成果,引入大样量重量法等温吸附仪,分析了大样量重量法的测试原理和操作流程,对比了体积法、磁悬浮重量法和大样量重量法3种方法的优缺点,进而选取大样量重量法等温吸附仪对内蒙古二连盆地吉尔噶朗图凹陷吉煤2井低煤阶煤岩和川南地区某井下志留统龙马溪组页岩样品进行了等温吸附实验,以期在消除系统累计误差影响之后,探究出现"负吸附"现象的根本原因。实验结果表明:(1)大样量重量法测试中未出现"负吸附"现象,其对低煤阶煤岩和页岩样品的测试结果可靠,拟合度高(R^2>0.99);(2)电子传感器误差(天平、压力和温度等采集器件)、系统累计误差、吸附相密度取值差异等不会导致吸附曲线形态的反转性变化;(3)样品缸空体积误差是引起"负吸附"的根本原因。结论认为,在等温吸附解释算法中增加体积校正参数,可以提高测试的准确性。

Negative adsorption of an inverse V type which rises first and then drops is often observed in the adsorption test of low adsorption samples. In view of this, a gravimetric isotherm rig with a large sample quantity was introduced. Its testing principles and operation processes were analyzed based on the previous research results. Then, the volumetric method, magnetic-levitation gravimetric method and large-sample-quantity gravimetric method were compared in terms of their advantages and disadvantages. Accordingly, this gravimetric isotherm rig with a large sample quantity was adopted to perform isotherm adsorption experiments on the low-rank coal samples taken from Well Jimei 2 in the Jiergalangtu sag, Erlian Basin, Inner Mongolia, and shale samples from the Lower Silurian Longmaxi Fm in the southern Sichuan Basin, so as to explore the essential reason for the ＂negative adsorption＂ after the effect of a systematical cumulative error is eliminated. And the following research results were obtained. First, when the large-sample-quantity gravimetric method is adopted, the phenomenon of ＂negative adsorption＂ doesn＇t occur and the test results of low-rank coal and shale samples are reliable with a high fitting degree（R^2 0.99）. Second, an electronic sensor error（e.g. electronic balance and PT sensors）, a system accumulative error and an adsorbed phase density error are not the factors leading to the reversal of negative adsorption curves. Third, the void volume error of a sample cell is the essential reason for the negative adsorption. In conclusion, the test accuracy can be improved by introducing the volume correction factor into the isotherm adsorption interpretation algorithm.