Experimental study on determination of residual space volume of adsorption tank by vacuum filled He method

In order to study the effect of measuring the residual volume of adsorption tank with vacuum filling He method on the gas adsorption capacity, 10 coal samples were taken as research objects, and experiments such as vacuum filling He measurement, gas adsorption constant measurement and true density measurement were carried out with capacity method under low pressure and high pressure. The research shows that: ① coal hardly adsorbed He under high pressure; under negative pressure, He molecular dynamics diameter is smaller than CH₄ molecular dynamics diameter, and it is easy to be continuously sucked into smaller nanoscale coal pores by negative pressure difference, resulting in the remaining volume increasing with the extension of He filling time, forming an illusion of coal adsorbed He. ② Compared with the true densitometer algorithm, the residual volume of the adsorption tank of the vacuum filling He method is basically larger at 5, 30, 60, 90 and 120 min reading, and the absolute error at 5 min reading is 9.58% at the maximum and 1.52% at the minimum; The reading time of vacuum filling He method can be set to 5 min, which is the same as the reading time of the empty tank volume of the adsorption tank. ③ The value a of the gas adsorption constant calculated by the vacuum He method is generally small, and the value b is generally large. In the 5 min reading, the absolute error of a value is the maximum 17.69% and the minimum 4.24%, and the absolute error of b value is the maximum 33.59% and the minimum 7.51%, which is much larger than the error of low pressure gas adsorption capacity 0.45%～5.64%. Vacuum He filling method has little influence on gas adsorption capacity under 0.1 MPa pressure, but has a greater influence on the adsorption capacity at high pressure. The residual volume error is the root cause of “negative adsorption” of coal sample, and the residual volume of adsorption tank is the main factor affecting the gas adsorption capacity of coal. ④ The experiment shows that it is not scientific to use vacuum filling He method to determine the residual volume of different gases adsorbed by coal. In theory, the inert gas with the same molecular kinetic diameter as the measured gas should be used for calibration, but in practice it is difficult to select the inert gas with the same molecular diameter as the measured gas. ⑤ Establish a correction model of residual volume deviation and gas adsorption constant deviation, through which the a and b values determined by vacuum filling He method can be corrected, improve the accuracy and reliability of A and b values, and provide guidance and reference for the accurate prevention and control of mine gas disasters and the scientific research of coal seam CO₂ injection for CH₄ displacement.