甲烷在活性炭上吸附平衡模型的研究

比较吸附模型分析甲烷在活性炭上吸附平衡的适用性,为吸附式天然气(ANG)的工程应用提供准确的预测模型。基于在温度268.15～338.15 K、压力0～12.5 MPa测试的甲烷在Ajax活性炭上的吸附平衡数据,选择Langmuir、Langmuir-Freundlich和Toth方程,应用非线性回归拟合方程参数后,确定绝对吸附量和甲烷吸附相态,并比较方程在不同压力区域内的预测精度。结果表明,甲烷吸附相密度随平衡温度和压力变化;由绝对吸附量确定的甲烷在Ajax活性炭上的平均等量吸附热为15.72 kJ/mol,小于由过剩吸附量的标绘结果;Langmuir、Langmuir-Freundlich和Toth方程预测结果在0～0.025 MPa的累积相对误差为6.449 8%、7.918 4%和0.910 0%,在1～10 MPa为0.491 1%、0.161 3%和0.369 4%。Toth方程在整个压力范围内的预测结果最为准确,但Langmuir-Freundlich方程在较高压力区域内具有较高的预测精度。     

超临界甲烷在活性炭上的吸附平衡分析

为分析由吸附平衡时的热力参数确定吸附量、吸附模型和等量吸附热精度的影响因素,选择在温度268.15~338.15 K和压力0~13.5 MPa测试的甲烷在Ajax活性炭上的吸附平衡数据,通过引入甲烷分子可进入活性炭吸附空间内的容积和可以不考虑甲烷在孔内吸附的临界孔宽的概念,依据甲烷在吸附平衡前后的总量守恒,确定甲烷在吸附池内的总量、绝对吸附量和过剩吸附量三者之间的关系式。结果表明,在引入吸附质分子可进入吸附空间内的容积和临界孔宽后,经由活性炭的孔径分布(PSD),可以准确计算甲烷在活性炭上的过剩吸附量;应用实验数据非线性回归Toth方程参数后,可由Gibbs关于吸附的定义确定甲烷在活性炭上的绝对吸附量。比较结果时发现,由于未考虑本体相中甲烷分子对吸附甲烷分子的影响,采用过剩吸附量的等量吸附线标绘确定的等量吸附热数值偏高,工程应用时应由绝对吸附量来确定等量吸附热。     

甲烷在石墨化热解碳黑和活性炭上的吸附

为研究影响碳基吸附剂吸附超临界温度气体的主要因素,选择石墨化热解碳黑BP280和Ajax活性炭,分析超临界温度高压甲烷在其上的吸附平衡。应用容积法,在压力0~20.5 MPa、温度253 K~313 K测定甲烷的吸附平衡数据,并由等量吸附线标绘和亨利定律常数确定等量吸附热。引入通用吸附等温方程,再由方程的Langmuir标绘确定最大吸附容量,进而通过方程的线性化计算吸附平衡态中甲烷分子的作用能。结果表明,甲烷在两种吸附剂上的最大吸附容量均随温度而变化,并都小于液态甲烷的密度;甲烷在碳黑和活性炭上的等量吸附热分别为11.9 kJ/mol~12.5 kJ/mol和17.5 kJ/mol~22.5 kJ/mol,体现了两种吸附剂不同的表面能量分布;甲烷分子间作用能随吸附量的变化特点反映了超临界温度甲烷以类似于压缩气体状态聚集的特点和吸附剂结构上的差异。碳基吸附剂的比表面积和微孔容积是影响其储存甲烷容量的重要因素。     

甲烷在层状石墨烯和活性炭上的吸附平衡

以吸附式天然气（ANG）吸附剂的工程应用为目的,以0-10 MPa、283.15-303.15 K甲烷在层状石墨烯（GS（3D）,比表面积2062 m²/g）和活性炭SAC-01（比表面积1507 m²/g）上的吸附平衡数据作分析。首先,在77.15 K下由氮气吸附表征样品的孔径大小及分布（PSD）和比表面积。其次,选择极低压力下的吸附平衡数据标定亨利定律常数,确定甲烷在两吸附剂上的极限吸附热,并由维里方程和10-4-3势能函数计算甲烷与两吸附剂壁面之间的相互作用势。最后,依据测试的甲烷在吸附剂上的高压吸附平衡数据,比较了Langmuir系列方程的关联数据后的拟合精度,并由绝对吸附量计算了甲烷的等量吸附热。结果表明,甲烷在GS（3D）和活性炭SAC-01上的平均极限吸附热为23.07、20.67 kJ/mol;283.15 K下甲烷分子与GS（3D）和活性炭SAC-01之间的交互作用势ε_sf/k为67.19、64.23 K,与洛伦混合法则的计算值64.60 K相近;Toth方程关联甲烷在活性炭SAC-01和GS（3D）上吸附平衡数据的拟合累计相对误差为0.25%和2.29%;甲烷在活性炭SAC-01和GS（3D）上的等量吸附热平均值为16.8和18.3 kJ/mol。相对于活性炭SAC-01,比表面积和微孔容积均较高的GS（3D）对甲烷的吸附更具有优势。     

Dynamics of joint adsorption of mutually soluble substances by activated carbons

A model for the dynamics of isothermal absorption of a binary mixture of an organic substance, soluble in water, and water vapor in a fixed bed of activated carbon was proposed. It includes the equations of material balance and the Myers—Prausnitz model for equilibrium adsorption. The possibility of formation of the condensed phase during the adsorption of an organic substance on moist activated carbon was shown. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1491–1495, August, 1998.     

Measurement and theoretical analysis of the adsorption of supercritical methane on superactivated carbon

Adsorption/desorption isotherms of supercritical methane on superactivated carbon have been measured in the range of 0-10 MPa and 233-333 K (20 K interval). The reversibility of the physical adsorption process is acknowledged. The heat of adsorption of 16.5 kJ/mol is determined from the isotherms, and a new modeling strategy for isotherms with maximum is presented. The model yields fits to the experimental isotherms with precision of ?%, maintaining the constancy of the characteristic energy of adsorption. The exponent of the model equation expresses the pore size distribution feature of the adsorbent. The density of the supercritical adsor-bate is evaluated as a parameter of the model. It is shown that the conventional isotherm theory works too at supercritical condition if the limit state of supercritical adsorption is introduced into isotherm modeling.     

甲烷在活性炭上裂解制氢研究

在连续流动石英固定床反应器上研究了甲烷在活性炭上裂解制氢的反应,并对反应前后活性炭的比表面积以及孔径分布等的变化进行了测定。结果表明,甲烷在五种活性炭上的裂解行为基本相同,反应初期转化率最高,随着反应进行转化率逐渐降低直至一个平稳的状态;降低甲烷分压和增加甲烷与活性炭的接触时间可提高甲烷转化率;温度的升高有利于初始转化率的提高,但不利于活性炭的稳定性;反应后活性炭比表面积、孔容及微孔孔容都明显降低,平均孔径增大,孔径分布向中孔方向迁移,说明甲烷的裂解导致了活性炭孔特别是微孔内的炭沉积以及进一步的孔堵塞。     

Preparation of activated carbon with high surface area for high-capacity methane storage

Activated carbon （AC） was fabricated from corncob, which is cheap and abundant. Experimental parameters such as particle size of corncob, KOHlchar weight ratio, and activation temperature and time were optimized to generate AC, which shows high methane sorption capacity. AC has high specific surface area （3227 m^2/g）, with pore volume and pore size distribution equal to 1.829 cm^3/g and ca. 1.7-2.2 nm, respectively. Under the condition of 2℃ and less than 7.8 MPa, methane sorption in the presence of water （Rw = 1.4） was as high as 43.7 wt% methane per unit mass of dry AC. The result is significantly higher than those of coconut-derived AC （32 wt%） and ordered mesoporous carbon （41.2 wt%, Rw = 4.07） under the same condition. The physical properties and amorphous chaotic structure of AC were characterized by N2 adsorption isotherms, XRD, SEM and HRTEM. Hence, the corncob-derived AC can be considered as a competitive methane-storage material for vehicles, which are run by natural gas.

Key words     

Thermodynamics of methane adsorption on the microporous carbon adsorbent ACC

The behavior of the thermodynamic functions (differential molar isosteric heat of adsorption, entropy, enthalpy, and heat capacity) of the adsorption system methane—microporous carbon adsorbent ACC was analyzed at different adsorption equilibrium parameters in the temperature interval from 177.65 to 393 K and in the pressure range from 1 Pa to 6 MPa. The influence of the nonideal character of the gas phase and noninertness of the adsorbent were taken into account, which resulted in the appearance of the temperature dependence of the isosteric heat of adsorption, especially in the region of high pressures of the adsorptive. For the system studied, nonideality of the gas phase exerts the main effect on the thermodynamic functions of the adsorption system. In this interval of the parameters of the adsorption system, the correction to the noninertness of the adsorbent is not higher than 2.5%. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1765–1771, September, 2008.     

On the mechanisms of phenol adsorption by carbons

The removal of phenol and related compounds from dilute aqueous solutions by activated carbons corresponds to the coating of the micropore walls and of the external surface by a monolayer. This process is described by an analog of the Dubinin—Radushkevich—Kaganer equation. On the other hand, as suggested by immersion calorimetry at 293 K, in the case of concentrated solutions, the mechanism corresponds to the volume filling of the micropores, as observed for the adsorption of phenol from the vapor phase. The equilibrium is described by the Dubinin—Astakhov equation. It follows that the removal of phenol from mixtures with water depends on the relative concentrations, and the limiting factor for adsorption is either the effective surface area of the carbon, or the micropore volume.     

Nitrogen adsorption on fluorinated activated carbon fiber

Nitrogen adsorption isotherms for fluorinated activated carbon fiber (F-ACF) and fluorinated carbon black (F-CB) were measured at 77 K. Surface structures of F-ACF and F-CB were examined by _s -plot analysis using the adsorption data on the nonporous carbon black (CB) and F-CB. The surface energy of F-ACF was lower than that of ACF. The micropore structure of ACF was preserved even after fluorination, although the limiting adsorption amount and the micropore width decreased with fluorination.     

Prediction of the micropore structure parameters and adsorption properties of activated carbons

A regularity govering variations of volume and linear size of micropores in carbon adsorbents during their vapor-gas activation was found. A parameter was proposed that characterizes the degree of development of the micropore system in activating carbons and an initial carbonized material. The parameter is defined as the number (or surface area) of micropores in the volume unit of the micropore zones. This parameter allows one to rationalize the choice of carbonized materials for the preparation of activated carbons with specified adsorption properties and to establish the range of activation beyond which the structure of the micropores loses stability. Furthermore, the parameter serves to predict how activation affects micropore structure parameters and adsorption properties of carbons. This in turn indicates the optimal degrees of microporosity of carbons needed to attain required adsorption properties.     

活性炭液相吸附去除噻吩硫化物的研究

迄今为止,国内外降低汽油硫含量的方法主要有原料加氢脱硫、汽油加氢脱硫、溶剂抽提脱硫、催化裂化脱硫、氧化脱硫、生物脱硫、吸附脱硫及组合技术,同时一些非常规技术如膜过程脱硫、等离子体和光脱硫也在积极探索之中。     

Determination of the micropore volume distribution function of activated carbons by gas adsorption

A new method for the determination of the micropore volume distribution function of activated carbons is presented. It is based on the treatment of pure gas adsorption isotherms by a theoretical model derived from the Hill-de Boer theory. Adsorption data (isotherms and heat curves) for carbon dioxide, ethane and ethylene on activated carbon (F30/470 CHEMVIRON CARBON) have been provided by a thermobalance coupled to a calorimeter (TG-DSC 111 SETARAM) at different temperatures (233, 273, 303 and 323 K) for pressures up to 100 kPa. Adsorption isotherms of carbon dioxide and ethane at 303 and 323 K have been used for the determination of the micropore volume distribution function of the activated carbon of interest. The knowledge of its structure has then allowed the simulation of adsorption isotherms and heats for the same adsorbates at the same temperatures as those experimentally studied. Similar calculations have been conducted for ethylene. Whatever the adsorbate (carbon dioxide and ethane used for the determination of the micropore volume distribution function or ethylene), the mean deviation between experimental and calculated isotherms does not exceed 4% at quasicritical and supercritical temperatures (303 and 323 K). In the same temperature conditions, discrepancies between calculation and experiment reach about 10% for adsorption heats. For both isotherms and heats, large discrepancies appear at low temperature (233 and 273 K). This method allows the determination of the micropore volume distribution function of activated carbons. The validity of the results is insured using several isotherms of several adsorbates and taking into account the calorimetric effect of the phenomenon. That is the reason why this method can also be seen as a new possible model for pure gas adsorption data prediction. This paper also presents a brief summary of the state of the art in this field.     

Empirical Relationships Describing Energetics of Adsorption at Low Coverages on Microporous Carbons

The fundamental empirical relationships that correlate the adsorption energy with physicochemical parameters of adsorbates are discussed. Based on the experimental data of the adsorption enthalpy of different organic adsorbates on microporous activated carbons some new correlations between adsorption enthalpy and entropy at zero surface coverage and physicochemical parameters of adsorbed molecules are proposed. It is shown that they can be applied for the calculation of carbon porosity. The influence of carbon surface oxidation on its energetic heterogeneity is also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Carbon dioxide adsorption on the microporous ACC carbon adsorbent

Adsorption isotherms of carbon dioxide on the microporous ACC carbon adsorbent and the adsorption deformation of the adsorbent were measured. The heats of adsorption at temperatures raising from 243 to 393 K and pressures from 1 to 5⋅10⁶ Pa were measured. In the low-temperature region (243 K), an increase in the amount adsorbed is accompanied by adsorbent contraction, and at high micropore fillings (a > 10 mmol g⁻¹) the ACC carbon adsorbent expands. At high temperatures, adsorbent expansion is observed in the whole region of micropore filling. At 243 K in the low filling region (a < 1 mmol g⁻¹), the heat of adsorption decreases smoothly from 27 to 24 kJ mol⁻¹. The heat of adsorption remains virtually unchanged in the interval 2 mmol g⁻¹ < a < 11 mmol g⁻¹ and then decreases to 8 kJ mol⁻¹ at a = 12 mmol g⁻¹. Taking into account the nonideal character of the gas phase and adsorbent deformation the heats of adsorption are strongly temperature-dependent in a region of high pressures. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1331–1335, June, 2005.     

Sorption removal of cephalexin by HNO3 and H2O2 oxidized activated carbons

Cephalexin’s traces within pharmaceutical effluents have toxic impact toward ecological and human health.Low-cost activated carbon derived from Trapa natans husk was oxidized with hydrogen peroxide and nitric acid,and tested for their ability to remove cephalexin from aqueous solutions.Oxidization with H2O2 showed negative effect on the cephalexin sorption,whereas HNO3 oxidization improved the adsorption.The cephalexin adsorption isotherms on the native and HNO3 oxidized carbons correlated well with the Freundlich equation while their kinetics followed the pseudo-second order model.The removal of cephalexin by the native and HNO3 oxidized carbons was found to be most favored at low ionic strength and strong acidic conditions.Based on the thermal and FTIR analyses,the interaction mechanisms of the interaction between cephalexin and the carbons were proposed.Electrostatic attraction,hydrophobic interaction and chemical bonding with surface functional groups were demonstrated as primary mechanisms for cephalexin removal.The nitrogen functionalities on the carbon surface were considered to be an important factor affecting the adsorption process.     

Nanoporous carbons as promising novel methane adsorbents for natural gas technology

Nanoporous carbons were synthesized using furfuryl alcohol and sucrose as precursors and MCM-41 and mordenite as nanoporous templates.The produced nanoporous carbons were used as adsorbent for methane storage.The average pore diameter of the samples varied from 3.9 nm to 5.9 nm and the BET surface area varied from 320m2/g to 824m2/g.The volumetric adsorption experiments revealed that MCM-41 and sucrose had better performance compared with mordenite and furfuryl alcohol,correspondingly.Also,the effect of precursor to template ratio on the structure of nanoporous carbons and their adsorption capacities was investigated.The nanoporous carbon produced from MCM-41 mesoporous molecular sieve partially filled by sucrose shows the best methane adsorption capacity among the tested samples.     

Influence of analysis conditions on low pressure adsorption measurements and its consequences in characterization of energetic and structural heterogeneity of microporous carbons

Nitrogen adsorption isotherms on nonporous and microporous carbons were thoroughly studied at low relative pressures. For nonporous carbons low pressure measurements seem to be unaffected by analysis conditions. However, these measurements on microporous solids may be affected by analysis conditions at relative pressures below 10^–4. It was shown that selection of proper equilibration time is crucial for correct measurements of equilibrium pressures during adsorption on microporous carbons. The isotherm shift induced by insufficient equilibration of the system may affect the surface heterogeneity and microporosity analysis. A comparison of the adsorption energy and pore volume distribution functions calculated from low pressure nitrogen adsorption isotherms measured at different equilibration times on a microporous carbon shows that this effect is smaller than it was expected.     

Improving methane storage on wet activated carbons at various amounts of water

Different mesoporous activated carbons were prepared by both chemical and physical activation processes and were examined for methane uptake in the presence of water.Methane isotherms were obtained at ...