Gas centrifugation with wall condensation

Current large throughput gas cleaning processes are not able to deal with highly contaminated gas fields. Amine treatment is typically used for removing these contaminants — CO2 and H2S — from gas, but is not economic or practicable for highly contaminated gas, that is, where the concentrations of CO2 or H2S are above ca. 10 and 5%, respectively.1,2 Another major disadvantage is the low pressure at which waste gas is produced — compression is then required for waste disposal processes, such as reinjection. An alternative candidate process which has recently been evaluated is centrifugal gas separation.3 A gas centrifuge is basically a cylinder, filled with a gas mixture, and rotating at high speed. Due to the large centrifugal forces, the gas is pushed to the wall, resulting in a pressure gradient. Gases with different molecular weights have different partial pressure profiles and their mole fraction profiles vary with radius. In a methane/carbon dioxide mixture, the concentration of the CO2 increases near the wall by diffusion along the concentration gradient. We have shown experimentally that a purely gaseous process is too slow for practical application. Increasing the pressure is not that helpful for component separation in the gas phase, because to a first approximation the product of diffusion constant and pressure pD is constant that is, at high pressures the diffusion constant decreases.4 However, at the higher pressures generated in a centrifuge, there is a second much more dominant mechanism which will also cause separation — namely condensation due to the radial compression.5 This effect has some similar physical properties to what happens in the so-called evaporative centrifuge which has been previously analyzed for isotope separation, and which we have recently shown to be quite different from natural gas separation behavior in a centrifuge.6 In this study we examine whether condensation speeds up the separation process for a contaminated natural gas scenario. We identify two mechanisms for this condensation. A model is constructed which simulates the effect of condensation by centrifugal enrichment. The results of this model are compared to results of simulations of a gas/gas centrifuge. It is investigated in how far the concept of wall condensation leads to a significant increase of the separator performance in comparison with that of pure gas/gas separation by centrifugation.