Cranfield Water Science Institute, Cranfield University, Bedfordshire, UK.
Water Res. 2013 Jul 1;47(11):3688-95. doi: 10.1016/j.watres.2013.04.032. Epub 2013 Apr 29.
Secondary gas transport during the separation of a binary gas with a micro-porous hollow fibre membrane contactor (HMFC) has been studied for biogas upgrading. In this application, the loss or 'slip' of the secondary gas (methane) during separation is a known concern, specifically since methane possesses the intrinsic calorific value. Deionised (DI) water was initially used as the physical solvent. Under these conditions, carbon dioxide (CO2) and methane (CH4) absorption were dependent upon liquid velocity (V(L)). Whilst the highest CO2 flux was recorded at high V(L), selectivity towards CO2 declined due to low residence times and a diminished gas-side partial pressure, and resulted in slip of approximately 5.2% of the inlet methane. Sodium hydroxide was subsequently used as a comparative chemical absorption solvent. Under these conditions, CO2 mass transfer increased by increasing gas velocity (VG) which is attributed to the excess of reactive hydroxide ions present in the solvent, and the fast conversion of dissolved CO2 to carbonate species reinitiating the concentration gradient at the gas-liquid interface. At high gas velocities, CH4 slip was reduced to 0.1% under chemical conditions. Methane slip is therefore dependent upon whether the process is gas phase or liquid phase controlled, since methane mass transport can be adequately described by Henry's law within both physical and chemical solvents. The addition of an electrolyte was found to further retard CH4 absorption via the salting out effect. However, their applicability to physical solvents is limited since electrolytic concentration similarly impinges upon the solvents' capacity for CO2. This study illustrates the significance of secondary gas mass transport, and furthermore demonstrates that gas-phase controlled systems are recommended where greater selectivity is required.
在使用微孔中空纤维膜接触器（HMFC）分离二元气体时，已经研究了二次气体（甲烷）在分离过程中的传输。在这种应用中，二次气体（甲烷）的损失或“滑脱”是一个已知的问题，特别是因为甲烷具有内在的热值。最初使用去离子（DI）水作为物理溶剂。在这些条件下，二氧化碳（CO2）和甲烷（CH4）的吸收取决于液体速度（V(L)）。虽然在高 V(L) 下记录到最高的 CO2 通量，但由于停留时间短和气相分压降低，CO2 的选择性下降，导致入口甲烷的滑脱约为 5.2%。随后，氢氧化钠被用作比较的化学吸收溶剂。在这些条件下，通过增加气体速度（VG）增加了 CO2 的传质，这归因于溶剂中存在过量的反应性氢氧根离子，以及溶解的 CO2 快速转化为碳酸盐物种，在气液界面重新启动浓度梯度。在高气体速度下，在化学条件下甲烷滑脱降至 0.1%。因此，甲烷滑脱取决于过程是气相控制还是液相控制，因为在物理和化学溶剂中，甲烷质量传递都可以通过亨利定律充分描述。发现添加电解质通过盐析效应进一步减缓 CH4 的吸收。然而，它们在物理溶剂中的适用性有限，因为电解质浓度同样会影响溶剂对 CO2 的吸收能力。本研究说明了二次气体质量传输的重要性，并且进一步表明，在需要更高选择性的情况下，推荐使用气相控制的系统。
