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Transient gas diffusivity evaluation and modeling for methane and helium in coal
International Journal of Heat and Mass Transfer ( IF 5.2 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.ijheatmasstransfer.2020.120091
Ang Liu , Shimin Liu , Xiaowei Hou , Peng Liu

Abstract Gas diffusion in coal in known to be a transient process. A conceptualized cylindrical pore network was proposed in this study to quantify the transient mass transfer in coal at various pressures. Experimental measurements were carried out to measure the time-dependent mass transfer through the volumetric method. Based on the geometrical model, a new analytical model was proposed to describe and quantify the dynamic contributions of bulk diffusion, Knudsen diffusion and surface diffusion to the total gas mass transfer. In the model, the nanoscale pore deformation and dynamics were implicitly considered and modeled through the pore volume compressibility and the matrix shrinkage and swelling variations. This allows to model the gas transient process with pore structure modification. The analytical model is being solved through a numerical approach. The numerical model also combines the interactions between gas adsorption and desorption by recalling the converse source terms in the governing equations. The transient gas transport process can be divided into two stages, namely, the fast bulk and Knudsen diffusion controlled free pressure equilibrium and the slow sorption-controlled mass transfer due to the gradient of free gas pressure (pf) and adsorption equivalent pressure (pa). The numerical model was calibrated and validated against with the laboratory measurements. Comparing the cases that helium and methane injections at similar pressure (~315.98 psi for helium and ~319.90 psi for methane), the equilibrium time for helium is ~ 4.5% shorter than that of methane does attributed to the relative slow sorption kinetics. Also, the pore radius with methane injection will never recover to the initial pore radius, a 3.3% irreversible strain on pore radius induced by sorption effects was observed. A 3.6% difference between the relative probability of the molecular-wall collisions for helium (~97.7%) and the relative probability of molecular-wall collisions between methane molecules (~ 94.1%) was believed to be induced by the sorption effects. These results can ultimately provide the approach and data for analyzing the dynamic sorbing gas transport behaviors in sorptive coal.

中文翻译:

煤中甲烷和氦气的瞬态气体扩散率评估和建模

摘要 煤中的气体扩散是一个瞬态过程。本研究提出了一个概念化的圆柱形孔隙网络,以量化不同压力下煤中的瞬态传质。进行实验测量以通过体积法测量与时间相关的传质。基于几何模型,提出了一种新的分析模型来描述和量化体扩散、克努森扩散和表面扩散对总气体传质的动态贡献。在该模型中,纳米级孔隙变形和动力学被隐含地考虑并通过孔隙体积压缩率和基体收缩和膨胀变化建模。这允许模拟具有孔隙结构修改的气体瞬态过程。解析模型正在通过数值方法求解。数值模型还通过调用控制方程中的逆源项来结合气体吸附和解吸之间的相互作用。瞬态气体传输过程可分为两个阶段,即快速体积和克努森扩散控制的自由压力平衡以及由于自由气体压力(pf)和吸附当量压力(pa)的梯度而导致的缓慢吸附控制的传质过程. 数值模型通过实验室测量进行校准和验证。比较氦气和甲烷在相似压力下注入的情况(氦气为~315.98 psi,甲烷为~319.90 psi),氦气的平衡时间比甲烷的平衡时间短~4.5%,这归因于相对缓慢的吸附动力学。还,注入甲烷的孔隙半径永远不会恢复到初始孔隙半径,观察到由吸附效应引起的孔隙半径 3.3% 不可逆应变。氦的分子壁碰撞的相对概率 (~97.7%) 与甲烷分子之间的分子壁碰撞的相对概率 (~94.1%) 之间存在 3.6% 的差异被认为是由吸附效应引起的。这些结果最终可以为分析吸附煤中的动态吸附气体输运行为提供方法和数据。7%) 和甲烷分子之间的分子壁碰撞的相对概率 (~ 94.1%) 被认为是由吸附效应引起的。这些结果最终可以为分析吸附煤中的动态吸附气体输运行为提供方法和数据。7%) 和甲烷分子之间的分子壁碰撞的相对概率 (~ 94.1%) 被认为是由吸附效应引起的。这些结果最终可以为分析吸附煤中的动态吸附气体输运行为提供方法和数据。
更新日期:2020-10-01
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