Preparation And Application Of Carbon Adsorbents For CH₄ Separation From Coal Bed Methane

Coalbed methane(CBM)is a type of unconventional natural gas resource with huge reserves in China.The exploitation and utilization of methane in coalbed methane can improve the shortage of natural gas in China and optimize the energy structure,which is of great significance.At the same time,methane in coalbed methane is one of the greenhouse gases that lead to global warming.Improving utilization technology of coalbed methane can reduce the direct emission of coalbed methane,which will help to slow down the trend of global warming.The low utilization rate of coalbed methane is mainly due to the fact that CBM is mixed with air during underground exploitation,resulting in low concentration of CBM,which cannot meet the requirements of industrial application.There is still a certain gap between CBM and pipeline natural gas.Therefore,it is necessary to purify and reuse CBM of medium and low concentration.Pressure swing adsorption technology is the industrial technology developed in recent years,due to the advantage of high degree of automation and low energy consumption,has caused wide public concern;but in the field of coalbed methane separation,how to develop adsorbent with good performance of adsorption and separation,understand the influence of adsorbent structure on CH₄/N₂ separation from coalbed methane,and grasp the competitive adsorption mechanism of gases need to be in-depth study since the physical and chemical properties of CH₄ is similar to that of N₂.In this paper,the secondary carbonization-steam activation process was used to prepare granular activated carbon from coke of coconut shell produced in wenchang,China.Thermogravimetric analyzer was used to investigate the pyrolysis reactivity of coke.Electronic balance was used to characterize the adsorption and separation performance of the samples.X-ray diffraction and scanning electron microscopy were used to characterize the microcrystalline structure and surface morphology of the adsorbent samples.The pore structure of adsorbent was characterized by N₂ adsorption and desorption curves at low temperature measured by fully automatic surface sorption instrument.The isotherm models were used to fit the equilibrium adsorption data.The fixed bed penetration curves illustrated the competitive adsorption and dynamic diffusion characteristics.The pressure swing adsorption evaluation device was used to investigate the enrichment effect and economy of each process condition.The main research work of this paper includes:1)the influence law of different carbon-activation preparation processes on the separation performance and pore structure;2)correlation between preparation technology index,pore structure,and separation performance;3)equilibrium adsorption and kinetic diffusion characteristics of gas components on adsorbent;4)the influence mechanism of different pressure swing adsorption process conditions on the enrichment effect and economic indexes was investigated,and establish an efficient four-tower vacuum pressure swing adsorption process.The main conclusions are as follows:1.Preparation of adsorbent and pore structure correlation law(1)The effects of activation temperature,water vapor flow rate,activation time and other control processes on the separation performance were discussed in detail,and the optimal preparation process was explored:carbonization(5?/min to 600?and then keep 600?for 30min),activation temperature 850?,water vapor flow rate2kg/h,activation for 40min.The equilibrium separation coefficient of the sample prepared in this preparation process can reach 3.95,the saturated adsorption capacity of CH₄ was up to 3.28mol/kg.(2)Pore structure parameters of activated carbon adsorbent changed regularly with the activation process parameters.When the activation temperature and activation time was increased,the specific surface area,total pore volume and micro pore volume increased gradually,and the average pore diameter decreased first and then increased.The specific surface area,total pore volume,micro pore volume and average pore size all increase with the increase of water vapor flow.(3)The simple technical indexes(iodine value and ignition loss rate)showed a regular increasing trend with the increase of activation parameters.The activation temperature and activation time had a more significant influence than water vapor flow.The iodine value and ignition loss rate were linearly correlated,with a correlation coefficient as high as 0.99.(4)The correlation between the single pore structure parameter index and the separation performance is poor,and the best correlation is the iodine value,with the correlation coefficient up to 0.90.The quantitative relationship between the pore size distribution of each microporous segment and the equilibrium separation coefficient is established,and the relative error is less than 10%.From the influence factors,it can be seen that pores with the pore size between 0.6 and 0.7nm contribute the most to the equilibrium separation coefficient and have the most significant influence,followed by those between 0.7 and 0.9nm,and then those between 0.4 and 0.6nm.After 0.9nm,the effect of pores is relatively weak.2.Equilibrium adsorption and diffusion kinetics of main gas components of coalbed methane on adsorbent(1)The order of adsorption amount of common gases in coalbed methane on activated carbon is CO₂>CH₄>O₂?N₂,and the most important role in determining the potential energy of these gases is polarization.Toth model,Sips model and MSL model are all superior to the classical Langmuir model to a certain extent,and the mean relative deviation ARE between the measured values of the adsorption experiment and the calculated results of the model is less than 5%.Toth model has the best fitting effect;the mean relative deviations of CH₄,N₂ and CO₂ is 0.8%,1.44%and 0.97%respectively.(2)The fitting results of three simplified adsorption kinetic models show that:the fitting effect is quasi-first-order dynamic model>second-order dynamic equation>internal diffusion model.It indicates that external mass transfer is the main control step to determine the adsorption process,while internal diffusion is not the only control step.Fick diffusion model can obtain the dynamic total diffusion coefficient and separation ratio,the results show that the dynamic separation coefficient of activated carbon is lower than 2,adsorbent is based on the equilibrium separation effect.The dynamic separation coefficient decreases with the increase of temperature and pressure.(3)N₂ penetration curve will show a"hump"shape when two components CH₄/N₂compete for adsorption.With the increase of concentration,the breakthrough time of the penetration curve was reduced,the peak position of the hump moved forward and the hump tended to be flat.There was a significant difference in CH₄/N₂ adsorption between the mixture with medium and high concentration above 50%and the mixture with low concentration above 20%,and the dynamic separation ratio decreased significantly when the mixture gas with high concentration was treated.The penetration time was significantly prolonged,the shape became slow,and the adsorption ratio of CH₄ and N₂ increased gradually with the increase of pressure.The total inlet gas flow increased,the diffusion speed is accelerated,and the penetration of both gases is shorter,which needs to be controlled within a certain limit.Higher flow rate is more favorable for N₂ adsorption,and the dynamic separation ratio has the best value at 1L/min.(4)According to the structure characteristics of the prepared adsorbent,the dual-dispersion and two-hole diffusion model was established,and the activation energy and limit diffusion coefficient of the micro pore diffusion were given.The model fitting result was good,which can realize the parameter calculation and the prediction of penetration curves.3.Study on pressure swing adsorption process(1)The pressure swing adsorption device of four towers was built.The influence of the process conditions on the enrichment effect index and economic index were studied.The optimal process conditions were found:the adsorption pressure was0.2MPa,the exhaust velocity was 3.2L/min,the adsorption time was 210s,the displacement time was 20s,and the pressure equalization time was 30s.The concentration of CH₄ in CH₄/N₂ mixture with 16%CH₄ content can be increased by 23percentage points,also CH₄ content can be increased by more than 30 percentage points in other feed gas with higher concentration.(2)Preliminary exploration was conducted on oxygen-containing CBM at different concentrations.The adsorbent can increase the concentration of CH₄ in simulated oxygen-containing CBM from 21%,35%and 66%to 48.70%,69.5%and95.4%,respectively.The content of CH₄ in tower top gas and produced gas far exceeds the explosion limit,indicating that the adsorbent not only has a good separation effect of CH₄/N₂,but also has a certain deoxidization effect,which can be applied to the treatment of oxygen-containing coalbed methane.