Study On Adsorptive Enrichment Of Coalbed Methane

Coalbed methane (CBM) is a kind of natural gas buried in seams that have not been mined and it amounted up to 1x1014m3. The methane rich gases captured on methane drainage in operational mines contain different amounts of non-combustible gases, most of which is nitrogen. The concentration of methane varies for a wide range, but mostly in 20-45 %, which do not meet the specification on pipeline gas for the minimum heating value, for which the methane content is typically larger than 90 %. To utilize CBM and prevent from its emission into troposphere, the separation between CH4 and N2 has to be carried out. PSA is supossed to be a good choice for its low investment and energe cost. In this paper, we have focus on two respects of CH₄/N₂ separation. The frist part of the work is to study on the adsorption mechanism between on the CH4 and N2 on activated carbons; the second part of the work is to modify the conventional PSA cycle to achieve separation of CH₄/N₂.(1) The present work considered the separation on the basis of adsorption mechanism. The adsorption potential was calculated relying on the 10-4-3 potential model and the slit pore model of activated carbons. At the same time, the adsorption isotherms of CH₄/N₂ at 298 K are obtained for different pore widths by using a Grand Canonical Monte Carlo simulation (GCMC). The results of the calculation and simulations showed that the largest selectivity between CH4 and N2 occurred in pore size of 0.75 nm, and there was no difference between them for pores larger than 1.3nm. It has been known that supercritical gases follow the adsorption mechanism of monolayer surface coverage; however, the surface of adsorbent has not been fully covered when the partial pressure of CH4 or N2 is low, and the adsorption potential dominates the separation.(2) To study the effect of adsorption mechanism on the separation, eight carbon samples corresponding to different extent of activation were prepared .The separation coefficient between CH4 and N2 on the eight carbons was evaluated based on breakthrough curves .It was experimentally shown that the separation coefficient between CH4 and N2 increases following the increase of micropore volume, and decreases following the increase of pressure. The corresponding selectivity of CH₄/N₂ on the real activated carbon is predicted with the PSD determined by the DFT method of the adsorbents and simulation results and the simulation results match the experiment perfectly.(3) For the mixture of CH₄/N₂, methane, which is the strongly adsorbed component and obtained as bottom product in the typical PSA or VSA process, is the targeted product. However, methane is expected to be upgraded and obtained as top product, which is totally opposite with the typical PSA process in the role of each step. A new step, CO2 replacement, was implemented into conventional PSA cycle. The new cycle of pressure swing operation composed of four steps: pressurization, adsorption at an elevated pressure, CO2 replacement, and bed regeneration. The pressure swing adsorption experiments were carried out using activated carbon made in our laboratory, as adsorbent. A series of CH₄/N₂ mixtures containing 17.62 %,22.30 %,32.06 %,48.28 % and 51.33 % methane was used to simulate the methane rich gases. Effect of operational variables on the concentration and recovery of methane in the product stream was experimentally studied. It was shown that methane content in the product stream is higher than 88.5 % with methane recovery higher than 90 % for all feeding gases tested. It was shown also that replacement at ambient pressure is more efficient than the replacement at adsorption pressure. Both methane concentration in product and its recovery remarkably increased following the increase of methane content in feeding streams. When the displacement pressure at 0.4MPa , the recovery and concentration of methane reached 96.5% and 91.8%, respectively, for the methane rich gas containing 51.33 % CH4 and when the displacement pressure at 0.1MPa, the recovery and concentration of methane reached 96.5% and 91.8%, respectively. When the displacement pressure at 0.4MPa , the recovery and concentration of methane reached 91.4% and 84.4%, respectively, for the methane rich gas containing 17.62 % CH4 and when the displacement pressure at 0.1MPa, the recovery and concentration of methane reached 90% and 98%, respectively.