Study Of Pressure Swing Adsorption Technologies And Processes For Methane Enrichment From Low Concentration Coalbed Methane

Limited by the geological conditions of the coal seams and the extraction technology,the air leakage of the coal body cracks often leads to the mixing of a large number of air（N₂,O₂,CO₂,H₂O,etc.）during the extraction process of coalbed methane（CBM）.This directly results in a low content of CH₄ in the extracted CBM,which seriously restricts the resource utilization of CBM.With the advantages of flexible operation,simple equipment and low energy consumption,adsorption separation technology has become one of the promising technologies for CBM separation and purification.However,existing adsorbents with low CH₄ adsorption capacity and limitted CH₄/N₂ selectivity lead to the unsatisfactory enrichment effect of the traditional pressure swing adsorption（PSA）process on low concentration CBM.Based on the theme of enrichment of low concentration CBM by PSA process,a series of systematic studies has been carried out with MFI type high-silica zeolite molecules sieve TUTJ-1 as the main research object.The separation performance of CH₄/N₂ and the influence of the presence of water molecules in CBM on its CH₄ enrichment performance were preliminarily confirmed in the laboratory scale study.In the pilot scale-up field,the performance of TUTJ-1in enriching low concentration CBM was evaluated and its excellent superhydrophobic nature was also tested on the spot.In view of the fact that it is difficult to directly enrich low concentration CBM to the pipeline natural gas specification in one step using conventional PSA processes or single adsorbent in practical application.The CO₂ displacement PSA process and the coupled nitrogen reflux displacement PSA process were designed and developed,which greatly improved the enrichment effect of low concentration CBM.The main content of this paper are as follows:（1）The adsorption isotherms of CH₄ and N₂ on shaped TUTJ-1 adsorbent were measured by static gravimetric method,and the IAST selectivity of CH₄/N₂ mixture was calculated.The separation performance of CH₄/N₂ mixture on TUTJ-1 adsorbent were tested by dynamic breakthrough experiments.Based on the above work,Aspen Adsorption software was used to simulate the performance of TUTJ-1 adsorbent applied to the separation and enrichment of low concentration CBM.The simulation results show that 20%and 30%CH₄ can be concentrated to 37–41%and 50–53%,respectively,after one-step enrichment of low concentration CBM,and meets the minimum methane content requirements of urban gas.（2）The potential application of TUTJ-1 used for separation and enrichment of low concentration CBM in the presence of water vapor was systematically studied.For the feed gas/moisture CH₄/N₂/H₂O（v）with different relative humidity,no change in CH₄ breakthrough time was found compared with the dry condition.While in the water vapor pre-saturated adsorption bed,only a very slight reduction was observed（9%）.All breakthrough experiments results show that when TUTJ-1 is used as adsorbent,the presence of water vapor has limited influence on CH₄/N₂ separation.PSA experiments were also conducted under dry and wet conditions（100%RH）.After reaching a cycle stable state,the fluctuation of purity or recovery was less than 2%,confirming negligible impact on CH₄ enrichment process in the presence of water.It can be concluded that the dehydration process before the adsorption unit can be omitted by using TUTJ-1 as the adsorbent,thus reducing the operation cost.（3）Considering the excellent CH₄/N₂ separation performance and hydrophobicity of TUTJ-1 adsorbent,the field test of low concentration CH₄ capture using mobile skid-mounted vacuum pressure swing adsorption（VPSA）system was successfully demonstrated in the coal mine gas drainage well.Field tests including breakthrough and VPSA experiments were conducted under a fluctuant CH₄ concentration range（16–30%）,and CH₄ content can increased to 43.3–70.7%with a maximum CH₄ productivity 43.6×10^-3 m³/（kg·h）.In particular,the fuel quality of the product gas meets the requirements of 4T grade urban gas and can be used for local households or power generation.Although the relative humidity in the feed gas is almost100%,it is not found that the presence of water has an impact on the separation performance of TUTJ-1.These results show that TUTJ-1 is more resistant to the influence of water than activated carbon,and has greater potential to capture CH₄ from low concentration CBM under humid conditions.（4）In view of limited methane concentration effect in low concentration CBM using conventional PSA technology,the process of low concentration CBM enrichment by CO₂displacement assisted VPSA process was studied on the TUTJ-1 adsorbent.The adsorption isotherms of CO₂,CH₄ and N₂ show that CO₂ is the strongest adsorption component,so it can be used as the displacer for CH₄ enrichment by CO₂ displacement process.The multi-component competitive adsorption behavior predicted by the mixture competitive adsorption model based on IAST and DSLF shows that the competitive adsorption capacity of CH₄ is almost the same as that of N₂ from the perspective of CO₂ during CO₂ displacement process.The experimental results of CO₂displacement elution show that the initial CO₂ displacement time and displacement flow rate have different internal influence mechanisms,and CO₂/CH₄mixture can also be used as displacer.Combined with CO₂ displacement elution experiment and process simulation analysis,the CH₄ displacement and migration mechanism in the CO₂displacement process was proposed.Through CO₂DIS-VPSA cycle experiment,high-purity methane product gas with CH₄ purity of 85–90%and recovery around 90%can be obtained by one-step enrichment of low concentration CBM.CO₂DIS-VPSA process with CO₂/CH₄ as the displacer performs better in enriching low concentration CBM compared with pure CO₂,and can be enriched to specifications of pipeline natural gas and can realize the joint utilization and enrichment of low quality methane resources from two sources.（5）In view of the fact that it is difficult to directly enrich low concentration CBM to pipeline natural gas specifications in the PSA process using single adsorbent,a two-stage PSA process coupled with CH₄ selective adsorbent TUTJ-1 and N₂ selective adsorbent MIL-100（Cr）was designed to enrich low concentration CBM.The adsorption isotherms of CH₄ and N₂ on the selected two adsorbents were measured at 298 K,and the IAST selectivity of the equimolar CH₄/N₂ mixture was calculated.The mixture breakthrough experiment indicates that the PSA process using a single adsorbent has the problem that the purity or recovery of CH₄ product gas is not high enough.The low concentration CBM enrichment performance of two two-stage PSA processes was compared.For the whole two-stage PSA process,the total CH₄ recovery of two-stage PSA process coupled with N₂ reflux displacement is 82.4%,while that of ordinary two-stage PSA process is only 43.2%.Therefore,the two-stage PSA process coupled with N₂ reflux displacement has great advantages in enriching CH₄ from low concentration CBM.