CO2/CH4在活性炭上吸附与分离的分子模拟

Monte Carlo 模拟(GCMC)研究了CO2和CH4在活性炭中的吸附情况.氢气和二氧化碳分子被模拟作Kennard-Jones球体,活性炭用一个裂缝气孔模型代替.研究了压力、温度、孔径活性炭对CO2(以CH4为基准)的选择性的影响.结果表明,低压、低温、孔径越小,选择性越大,即对分离CO2和CH4有利;233 K、孔径为2.29 nm以及低压下选择性达到10,此时分离效果较好.     

用于吸附分离CH4/CO2的碳分子筛的制备研究

以碳分子筛(CMS)作为吸附剂,探索其对CH4/CO2混合气的分离纯化性能。采用化学气相炭沉积法,以价格便宜的商品椰壳基活性炭为原料,以苯为沉积剂,制备碳分子筛。研究了沉积时间和沉积温度对CMS的吸附性质的影响。采用容量法对CMS吸附CH4和CO2性能进行测定。结果表明,在沉积温度为700℃,沉积时间为40min时制备的CMS平衡吸附选择性系数为58.03,远高于活性炭的5.124,对CH4/CO2有良好的分离效果。     

不同结构活性炭对CO_2、CH_4、N_2及O_2的吸附分离性能

制备了比表面积为1943 m2/g的纯微孔活性炭AC-1和比表面积为1567 m2/g,中孔比例为47.18%的活性炭AC-2.分别以AC-1及AC-2为吸附剂测定CO2、CH4、N2和O2的298 K吸附等温线,考察了两种活性炭对CO2/N2、CO2/CH4及CH4/N2气体混合物的吸附分离性能.实验结果表明,孔结构是影响吸附剂吸附分离性能的主要因素.富中孔活性炭AC-2较AC-1更适用于CO2/N2、CO2/CH4气体混和物的吸附分离,而微孔活性炭AC-1对CH4/N2混合体系的吸附分离性能优于AC-2.     

新型真空变压吸附法模拟分离N_2/CH_4/CO_2

依据活性炭对各组分吸附选择性差异,提出一种新型真空变压吸附(VPSA)工艺,可以在N2/CH4/CO2的三组分体系中富集CH4,该方法在传统的重组分提浓工序中加入了CH4和CO2产品气置换步骤,适用于对含低浓度CO2的煤层气甲烷富集。利用Aspen Adsorption软件对该过程进行模拟。原料气假设为体积分数9%CH4/6%CO2/85%N2,活性炭吸附剂对N2/CH4/CO2分离因子为1/4.15/10.62。在进料温度为298 K,吸附压力为0.6 MPa,解吸压力为-0.08 MPa条件下,运行稳态后能够将甲烷浓缩到体积分数77.172%,回收率高达97.923%。同时,CH4在轻重尾气中的体积分数分别仅为0.224%和0.673%。     

活性炭孔径分布对CH4/N2体系分离的影响

采用变压吸附分离的方法分离CH4/N2体系时,活性炭作为一种主要的吸附剂受到广泛研究.本文采用10-4-3模型计算得出,孔径为0.7～2.0nm的活性炭微孔对CH4/N2体系分离的效果最好；同时利用扩展Langmuir方程通过拟合4种活性炭对CH4/N2的吸附等温线,计算得出分离系数；最后讨论样品的孔径分布与分离系数的关系,进一步证明了10-4-3模型的计算结果.     

沼气变压吸附剂吸附性能比较

研究了工程上常用的3种吸附剂CO2吸附剂硅胶、13X分子筛和碳分子筛在高压(0.1~1 MPa)下对CO2和CH4的吸附容量、稳定性和选择性,以确定其在沼气变压吸附分离中应用的可行性. 结果表明,硅胶的吸附稳定性非常好,0.1 MPa时吸附选择性系数为8~10,能有效分离CH4?CO2;13X分子筛对CH4和CO2吸附容量最高、吸附选择性最好,重复使用5次CO2的吸附容量略有下降;以CH4的损失率作为评价标准时,13X分子筛吸附CH4的损失率最低. 降压不能使被碳分子筛吸附的CH4完全解吸,吸附位不能充分释放,不适用于CH4与CO2的分离.     

不同管径碳纳米管中CO_2/CH_4分离的分子模拟

生物甲烷路线在CO2减排和节能方面有很大的应用前景。而对生物沼气的分离是此路线的一个关键问题,特别是在60℃和0.1 MPa下。巨正则Monte Carlo(GCMC)和平衡分子动力学(EMD)的分子模拟方法研究CO2和CH4在不同管径的碳纳米管(CNT)中的吸附和扩散,可以从分子层面研究生物沼气的分离机理。分别计算了CO2/CH4二元混合物吸附量、吸附选择性、自扩散系数和渗透选择性等参数。模拟结果表明:由于碳管的受限空间和CO2与碳纳米管壁面分子之间强相互作用,导致二元等物质的量的混合物CO2/CH4的吸附量和扩散系数的差异。CO2的吸附量和自扩散系数都比CH4的大。渗透选择性在碳管管径达到最接近1 nm时达到最大值,此时混合物的分离过程是吸附控制,而非扩散控制。     

耐硫甲烷化催化剂的研究

煤制天然气采用耐硫甲烷化催化剂,减小了反应设备体积,对节省投资和降低能耗有积极意义。采用等体积浸渍法制备系列Mo-Ni/γ-Al2O3耐硫甲烷化催化剂,并对催化剂活性及耐硫性进行评价,考察浸渍液中不同Co和W元素添加量对催化剂活性的影响。结果表明,耐硫甲烷化催化剂活性中心MoS2和WS2的生成有利于提高CO转化率和CH4选择性,促进合成气生成CH4,Co的添加不利于提高催化剂的CO转化率和CH4选择性,而W元素的添加有利于提高催化剂的CO转化率和CH4选择性。在反应温度550℃、压力2 MPa和空速1 800 h-1条件下,n（H2）∶n（CO）=1∶1时,CO转化率为64.24%,CH4选择性为52.00%。n（H2）∶n（CO）=3∶1时,CO转化率为77.90%,CH4选择性为68.41%。     

稀土复合吸附剂的制备及变压吸附CO特性的研究

以活性炭负载Cu（I）为原料,添加适量稀土化合物制备成稀土复合吸附剂。实验测定了CO、CO2、CH4、N2在稀土复合吸附剂和5A分子筛上的吸附平衡,考察了CO、N2混合体系在稀土复合吸附剂和5A分子筛上的变压吸附性能。实验结果表明,该吸附剂对CO有很高的选择性,能满足变压吸附的要求,具有工业应用价值。     

无烟煤所制活性炭对CH4/N2的分离特性

以无烟煤为原料,采用预氧化-炭化-水蒸气活化法制备颗粒活性炭,用其变压吸附分离CH4/N2,以重量法测定了298 K下N2和CH4单组分及双组分气体在活性炭上的等温吸附曲线,并对活性炭的结构和表面性质进行了表征. 结果表明,以所制活性炭为吸附剂,采用单柱单循环变压吸附过程可将CH4/N2中CH4浓度最高提升30.7%(j). 所制活性炭孔结构以微孔为主且表面具有较多含氧官能团,对CH4/N2的分离效果较好,吸附选择性系数达3.4.     

Modeling the selectivity of activated carbons for efficient separation of hydrogen and carbon dioxide

Grand canonical Monte Carlo simulations (GCMC) are carried out to investigate the separation of hydrogen and carbon dioxide via adsorption in activated carbons. In the simulations, both hydrogen and carbon dioxide molecules are modeled as Lennard-Jones spheres, and the activated carbons are represented by a slit-pore model. At elevated temperatures (T = 505 and 923 K), the activated carbons exhibit essentially no preference over the two gases and the selectivity of carbon dioxide relative to hydrogen falls monotonically as the pore size increases. At room temperature, however, the selectivity of carbon dioxide relative to hydrogen reaches up to 90, indicating that hydrogen and carbon dioxide can be efficiently separated. Furthermore, the optimized pore sizes, of width H = 1.48 nm for the bulk mole fraction ratio of x_CO2/x_H2=1:2 and H = 1.18 nm for x_CO2/x_H2=1:8, are identified in which the activated carbons show the highest selectivity for the separation of hydrogen and carbon dioxide.     

巨正则系综蒙特卡罗法研究活性炭吸附

近年来,巨正则系综蒙特卡罗计算机模拟在研究炭素材料,特别是活性炭的吸附特性以及炭材料的结构表征和新材料设计方面得到应用。对巨正则系综蒙特卡罗法,目前在活性炭吸附特征研究中所普遍采用的活性炭微孔模型和分子与原子之间相互作用的模型进行较为详细的介绍。同时还给出了作者有关活性炭吸附氮和甲烷的基本特性的研究结果。     

Designed synthesis of porous carbons for the separation of light hydrocarbons

The separation of light hydrocarbon mixtures (C₁-C₃) generated from petrochemical industry is vital and challenging process for obtaining valuable pure chemical feedstocks. In comparison to the energy intensive conventional separation technologies (cryogenic distillation, absorption and hydrogenation), the adsorptive separation is considered as a low energy cost and high efficiency process. Porous carbons have been demonstrated as excellent adsorbents for the separation of light hydrocarbons, owing to their designable structure and tailorable properties. This review summarizes the recent advances of using porous carbons as adsorbents for the separation of light hydrocarbons, including methane/nitrogen, methane/alkane, methane/carbon dioxide, ethylene/ethane and propylene/propane. We discuss the separation mechanisms and highlight the material features including pore structure, surface chemistry and target molecular properties that determine the separation performance. Furthermore, the challenges and development direction associated with carbonaceous adsorbents for light hydrocarbon separation are discussed, meanwhile the guidelines for the design of porous carbons are proposed.     

Pore structure of activated carbons prepared by carbon dioxide and steam activation at different temperatures from extracted rockrose

The influence of the activation temperature on the pore structure of granular activated carbons prepared from rockrose (Cistus ladaniferus L.), extracted previously into petroleum ether, is comparatively studied. The preparation was carried out by pyrolysis of a char in nitrogen and its subsequent activation by carbon dioxide and steam (flow of water controlled to generate the same mol number per minute of water as well as carbon dioxide/nitrogen) at 700-950°C to 40% burn-off. The techniques applied to study the pore structure were: pycnometry (mercury, helium), adsorption (carbon dioxide, 298 K; nitrogen, 77 K), mercury porosimetry and scanning electron microscopy. The preparation by steam activation, especially at 700°C, yields activated carbons showing a total pore volume larger than those prepared by carbon dioxide activation. The pore structures present the greatest differences when the activations are carried out between 700 and 850°C and closer at higher temperatures. At high temperatures, the decrease of differences in pore development caused by carbon dioxide or steam is attributed to an external burn-off. The micropore structure of each activated carbon is mainly formed by wide micropores. At the lowest activation temperatures, especially at 700°C, steam develops the mesoporosity much more than carbon dioxide. At 950°C, a similar reduction of pore volume in the macropore range occurs.     

Molecular Simulations in Activated Carbons for Carbon Monoxide Removal From Wet Mixture of Hydrogen and Carbon Monoxide

Activated carbons are porous materials with a high surface area which are widely used in gases separation and catalysis. This work focuses on the understanding of the separation characteristics of activated carbons for purifying hydrogen from the wet mixture of hydrogen and carbon monoxide (in hundreds of ppm). This study would build a basic insight of the separation mechanism of the activated carbon and determine whether activated carbons are suitable for CO removal from the mixture. Systematic Grand Canonical Monte Carlo (GCMC) simulations have been carried out to assess the adsorption properties and selectivity of the activated carbon to the gaseous mixture with different gas phase compositions, temperatures, and pressures. The calculated adsorption isotherms, both for single and multi-component gas mixture, and isosteric heats of adsorption on the activated carbon showed reasonably good agreement with the experimental data available in the literature.     

Adsorption of methane and ethane into single-walled carbon nanotubes and slit-shaped carbonaceous pores

The adsorption equilibria of methane, ethane and their binary mixture in single-walled carbon nanotubes (SWNTs) and slit-shaped carbonaceous pores were studied by using a Grand Canonical Monte Carlo (GCMC) method. We used a slit-shaped pore for microporous structure of activated carbons and an armchair type of cylindrical pore for SWNTs. Methane was modeled as a spherical Lennard-Jones (LJ) model and ethane as two LJ sites with the unified methyl group. The isotherms of both components in micropore region displayed Type I adsorption by Brunauer et al., which corresponds to unimolecular adsorption. At low pressure the storage capacity of SWNTs for pure components of methane and ethane was higher than that for slit-shaped pore geometries of the same size, and the selectivities of equimolar bulk gas mixture were much higher. GCMC was shown to give good qualitative agreement with Ideal Adsorbed Solution Theory (IAST).     

Determination of pore size distribution and adsorption of methane and CCl4 on activated carbon by molecular simulation

A combined method of grand canonical Monte Carlo (GCMC) simulation and statistics integral equation (SIE) for the determination of pore size distribution (PSD) is developed based on the experimental adsorption data of methane on activated carbon at ambient temperature, T=299 K. In the GCMC simulation, methane is modeled as a Lennord-Jones spherical molecule, and the activated carbon pore is described as slit-shaped with the PSD. The well-known Steele’s 10-4-3 potential is used to represent the interaction between the fluid molecule and the solid wall. Covering the range of pore sizes of the activated carbon, a series of adsorption isotherms of methane in several uniform pores were obtained from GCMC. In order to improve the agreement between the experimental data and simulation results, the PSD is calculated by means of an adaptable procedure of deconvolution of the SIE method. Based on the simulated results, we use the activated carbon with the PSD as the prototype of adsorbent to investigate adsorption. The adsorption isotherms of methane and CCl₄ at 299 K in the activated carbon with the PSD are obtained. The adsorption amount of CCl₄ reaches 20 mmol/g at ambient temperature and pressure. The results indicate that the combined method of GCMC and SIE proposed here is a powerful technique for calculating the PSD of activated carbons and predicting adsorption on activated carbons.     

Advances in the study of methane storage in porous carbonaceous materials

This paper presents an overview of the results of our research group in methane storage, in which the behaviour of different carbon materials in methane storage has been studied. These materials include physically activated carbon fibres (ACFs), chemically activated carbons (ACs) and activated carbon monoliths (ACMs), all of them prepared in our laboratories. These results have been compared with those corresponding to commercial ACFs, commercial activated carbon cloths and felts, and a commercial activated carbon.An in depth analysis (different raw materials, activating agent and preparation variables) has been done in order to obtain the carbon material with the best methane adsorption capacity by unit volume of adsorbent. The important effect of the micropore volume, micropore size distribution (MPSD) and packing density of the carbon materials in the methane adsorption capacity and delivery has been analysed. After this study, activated carbons with volumetric methane uptake as high as 166 v/v and delivery of 145 v/v have been prepared. In addition, ACM with methane uptake of 140 v/v and a delivery of 126 v/v has also been obtained.Moreover, the results corresponding to preliminary in situ small angle neutron scattering (SANS) study of CD₄ adsorption under pressure in different porous carbons and a zeolite are also included. These experiments have established SANS as a viable technique to investigate high-pressure methane adsorption. CD₄ adsorption at supercritical conditions produces changes in the SANS curves. The changes observed are in agreement with theoretical speculations that the density of the adsorbed phase depends upon the pore size.     

Methane Steam Reforming in Hydrogen-permeable Membrane Reactor for Pure Hydrogen Production

Steam reforming of methane over a ruthenium catalyst has been carried out at 500 °C in a membrane reactor equipped with a palladium membrane supported on a porous stainless steel tube. Hydrogen and carbon dioxide are mainly produced in the reaction, while hydrogen is selectively permeated through the membrane. Since equilibrium-shift takes place by hydrogen separation from the reaction mixture, the methane conversion significantly exceeds the equilibrium value, which is low at 500 °C. The selectivity to carbon monoxide by-produced in the reaction is lower than that expected from the equilibrium. Although the equilibrium conversion decreases with an increase in the reaction pressure, the conversion with the membrane reactor can increase because the hydrogen separation is promoted by the pressure increase. The catalytic activity is an important factor to produce a sufficiently high methane conversion and it is enhanced at a high reaction pressure.     

Porosity studies on active carbons from anthracite

The pore structures of active carbons made from anthracite in a production plant have been investigated using low-temperature nitrogen sorption, mercury porosimetry and density techniques. Four carbons have been studied, two from the normal production range and two special carbons one of very low and one of very high activity. Total pore volumes in the range 510–630 mm³/g have been estimated for the production carbons, the greater part of this being in the micropore region. In general, the pore structures are similar to those found by other workers for anthracite activated by steam or carbon dioxide.