Adsorption of Nitrogen on Silica Gel Over a Large Range of Temperatures

To study the mechanism of physical adsorption of supercritical gases, the adsorption equilibria of N₂ on silica gel for 103–298 K using 20 K increments and pressures up to 10 MPa were measured. A transition of the adsorption mechanism was proven on crossing the critical temperature, but the transition way observed is different from that observed with activated carbon. This causes a difference in the locations of the linear section of the n- _g isotherm at the near-critical temperature. Although the isotherm type is different on silica gel and on activated carbon in the sub-critical region, all isotherms in the supercritical region can be well modeled by a single model. It leads to the argument that the adsorption mechanism of supercritical gases is identical no matter what kind of adsorbent is used.     

Micropore Filling of Supercritical Xe in Micropores of Activated Carbon Fibers

The adsorption isotherms of Xe vapor at 196 K and supercritical Xe at 300 K on activated carbon fibers of different pore widths were gravimetrically measured. The adsorption isotherms of Xe vapor were compared with the N(2) adsorption isotherms. A Dubinin-Radushkevich (DR) plot of the adsorption isotherms of Xe vapor showed a good linearity, indicating that Xe vapor is adsorbed by the representative micropore filling mechanism. The adsorption isotherms of supercritical Xe were approximated by the Langmuir equation. The saturated adsorption amounts of supercritical Xe, W(L), were in the range of 0.14 to 0.22 ml g(-1). The adsorption isotherms of supercritical Xe were described by the supercritical DR equation, which provides the quasisaturated vapor pressure P(0q). Both P(0q) and W(L) lead to the reduced isotherm, which can describe three isotherms. The obtained reduced isotherm derived from the isotherms of supercritical Xe could describe even those of Xe vapor. Hence, both Xe vapor and supercritical Xe should be adsorbed by the same mechanism. The isosteric heat of Xe adsorption was greater than the enthalpy of vaporization of Xe by more than 12 kJ mol(-1). These results suggest that Xe molecules are stabilized in the form of a cluster in micropores even at 300 K. Copyright 2000 Academic Press.     

Adsorption of nitrogen on granular activated carbon: experiment and modeling

A carbon adsorbent was produced and used to volumetrically measure nitrogen adsorption isotherms from 93 to 298 K and up to 7 MPa. The isosteric heat of adsorption was determined to range between -9.5 and -16 kJ/mol. The excess adsorption isotherms were modeled using an approach based on a modified Dubinin-Astakhov adsorption model, adapted for excess adsorption, which provided an accurate fit for all supercritical isotherms. An expression for the differential energy of adsorption as a function of pressure was developed using the Dubinin-Astakhov isotherm. The energy of adsorption for the isotherms measured was found to range from -8 to -15 kJ/mol as a function of pressure.     

Determination of the Adsorbed Phase Volume and Its Application in Isotherm Modeling for the Adsorption of Supercritical Nitrogen on Activated Carbon

Experimental excess adsorption isotherms of nitrogen on activated carbon were measured in the range of 103-298 K and pressures up to 10 MPa. A method for determining the volume of the adsorbed phase from the experimental data for the supercritical temperatures was proposed. Such a volume with a new isotherm equation was used in modeling experimental excess isotherms, and a satisfactory fit was achieved. Copyright 2001 Academic Press.     

A linear comprehensive adsorption model of hydrogen on super activated carbon under supercritical conditions

The adsorption isotherms of hydrogen on super activated carbon were measured systematically, covering a temperature range of 93-293 K at 20 K intervals and pressures up to 7 MPa. All the experimental data were linearized by adopting the coordinates ln ln(n) vs 1/(ln P). The results indicate that the adsorption limit (P(lim), n(lim)) exists virtually at high pressure and has a certain physical meaning. Based on the adsorption limit, further analyses were carried out by modeling the adsorption isotherms in the coordinates ln(n(lim)/n) vs ln(RT(ln(P(lim)/P) and a linear comprehensive adsorption model was proposed in the form n (lim)=n.exp(psi T+lambda/T-c/) (beta).[RT ln( P (lim)P )] (b) which can predict the adsorption isotherm of hydrogen on activated carbon in supercritical conditions.     

Studies on the Physical Adsorption Equilibria of Gases on Porous Solids over a Wide Temperature Range Spanning the Critical Region—Adsorption on Microporous Activated Carbon

Adsorption equilibria of nitrogen and methane on microporous ( < 2 nm) activated carbon were measured for a wide temperature range (103‐298 K) spanning the critical region. Information relating to Henry constants, the isosteric heat of adsorption, and the amount of limiting adsorption were evaluated. All isotherms show type‐I features for both sub‐ and supercritical temperatures. A new isotherm equation and a consideration for the importance of the effect of the adsorbed phase volume allow this kind of isotherms to be modeled satisfactorily. The model parameter of the saturated amount of absolute adsorption (n⁰_t) equals the limiting adsorption amount (n^itm), leaving the physical meaning of the latter clarified, and the exponent parameter (q) proves to be an appropriate index of surface heterogeneity.     

超临界氢在活性炭上的吸附等温线研究

通过７７－２９８Ｋ范围内氢在ＡＸ－２１活性炭上的吸附数据，探讨如何用普通Ｉ－型等温线模型处理超临界条件下的吸附等温线，以获取关于超临界吸附系统的正确信息，结果表明，Ｌａｎｇｍｕｉｒ方程虽然可用来表达实验数据，但不能提供关于该吸附系统的任何可靠信息，Ｖｉｒｉａｌ方程虽不是整组数据的最好模型，但却能够可靠地确定Ｈｅｎｒｙ定律常数，然后可从ｖａｎｔＨｏｆｆ标绘决定等量吸附热，通过将实验数据拟合到Ｄｕｂｉ     

氢在A、X及ZSM-5型沸石上的高压物理吸附

The hydrogen adsorption properties and uptake capacities of the A, X and ZSM-5 types of zeolites were investigated at temperatures of 77, 195 and 293 K and pressures up to 7MPa, using a conventional volumetric adsorption apparatus. All hydrogen adsorption isotherms were basically type I, but the maximum in isotherm,a unique feature of supercritical adsorption, was observed at high pressures of 2-5 MPa at 77 K. The isosteric heats of adsorption were determined from the isotherms and the factors that influence their variations were discussed. Different types of zeolites exhibited remarkably different hydrogen uptake, based on both the framework structure and the nature of the cations present. The highest gravimetric storage capacity of 2.55wt% was obtained for NaX-type zeolite at 4 MPa and 77 K. In CaA, NaX and ZSM-5 types of zeolites,hydrogen uptakes were proportional to the specific surface areas, which were associated with the available void volumes of the zeolites. A threshold in hydrogen adsorption observed in NaA and KA was attributed to a pore blocking effect by large cations in KA. A ratio of the kinetic diameter of adsorbate to the effective opening diameter of zeolite was used to judge the blocking effect for physisorption.     

2,4-/2,6-二氨基甲苯在D151树脂上的吸附分离

从10种树脂中筛选出D151树脂对2,4-二氨基甲苯和2,6-二氨基甲苯的吸附分离及其热力学性质进行了研究。 测定了吸附等温线,Freundlich模型对实验的拟合度大于Langmuir模型,其相关系数大于0.99。 热力学研究结果表明,在293～313 K条件下,初始质量浓度为60～80 g/L时,2,4-二氨基甲苯的吸附焓变为-4.3490～-5.7558 kJ/mol,自由能变为-0.2911～-1.0346 kJ/mol,吸附熵变为-12.965～-16.150 J/(mol·K);而2,6-二氨基甲苯的吸附焓变为-2.9645～-3.6054 kJ/mol,自由能变为-0.1610～-0.6384 kJ/mol,吸附熵变为-7.939～-11.005 J/(mol·K)。 进一步研究了D151树脂对二氨基甲苯的动态吸附分离,可以将2,6-二氨基甲苯含量从20%提高至99.93%,将2,4-二氨基甲苯含量从80%提高至99.42%。     

一种测量超临界条件下苯酚吸附等温线方法

超临界吸附相平衡是超临界吸附／色谱分离过程设计的基础,通常,研究超临界吸附相平衡的实验不仅需要在高压下操作,而且需使用耐高压的检测器,本文提出一种测量超临界条件下吸附相平衡关系的“扩容减压吸收法”方法,它不需要耐高压检测器,以“苯酚－活性炭－超临界二氧化碳流体”为体系,测定了苯酚在活性炭－超临界二氧化碳流体之间的吸附相平衡关系,测定了苯酚在两种活性炭上的超临界吸附等温线,比较了苯酚在超临界条件和常     

Pattern of adsorption isotherms in Ono-Kondo coordinates

The Ono-Kondo lattice density functional theory is used to analyze adsorbate-adsorbate interactions for supercritical systems. In prior work, this approach has been used to study intermolecular interactions in subcritical adsorbed phases, and this has included the study of adsorbate-adsorbate repulsions in the regime of adsorption compression. In this paper, we present the general pattern of adsorption isotherms in Ono-Kondo coordinates; this has not been done in the past. For this purpose, experimental isotherms for adsorption of supercritical fluids (including nitrogen, methane, and carbon dioxide) are plotted in Ono-Kondo coordinates. In addition, we performed Grand Canonical Monte Carlo simulations of adsorption for Lennard-Jones molecules and plotted isotherms in Ono-Kondo coordinates. Our results indicate a pattern of isotherms with regimes of adsorbate-adsorbate attractions at low surface coverage and adsorbate-adsorbate repulsions at high surface coverage. When the generalized Ono-Kondo model is used over a wide range of pressures - from low pressures of the Henry's law regime to supercritical pressures - the slope of the isotherm varies from positive at low pressures to negative at high pressures. The linear sections of these graphs show when the adsorbate-adsorbate interaction energies are approximately constant. When these linear sections have negative slopes, it indicates that the system is in a state of adsorption compression.     

Studies on the transition behavior of physical adsorption from the sub- to the supercritical region: experiments on silica gel

Adsorption equilibria of nitrogen and methane on a mesoporous silica gel were measured over a temperature range from the sub- to the supercritical region. Determination of the compressibility factor shows a significant effect on the experimentally measured isotherms under conditions rho(g)/rho(c)>0.3. The transition of adsorption mechanisms in passing over the critical temperature was proven by the transformation of isotherms from type II to type I. The characteristic feature of subcritical adsorption was still observed at a temperature 8 K higher than the critical and the critical depletion phenomenon was clearly shown on such an isotherm. The limit quantities defined in different ways for the supercritical adsorption are in reasonable agreement, indicating the existence of an upper limit state of the supercritical adsorbate.     

氢气在A和X型沸石上超临界吸附的格子密度函数模型

采用基于三维Ono-Kondo方程的格子密度函数理论(LDFT)模型模拟了氢气在A和X型微孔沸石上的超临界吸附等温线. 根据沸石孔的尺寸和形状, LDFT模型将氢分子在孔中的吸附位分布近似为简单立方、面心立方和体心立方的团簇结构. 模拟结果表明, LDFT模型可有效地用于描述氢气在A和X型沸石上的单层或多层超临界吸附行为. 模拟所得的吸附等温线与实验测定结果吻合. 特别是, LDFT模型中的氢-沸石作用势能参数的准确性得到了Lennard-Jones(12-6)势能方法的有效验证. 因此, LDFT模型被用于预测了更宽温度和压力范围内氢气在X沸石上的超临界吸附.     

Adsorption of hydrogen on multiwalled carbon natotubes at 77 K

Adsorption of H₂ on multiwalled carbon nanotubes was measured at 77 K by a volumetric method. Adsorption and desorption isotherms are largely reversible. The adsorption capacity increased remarkably after receiving heat treatment at 400 °C and being pressed into pellets. The isotherms show typical feature of supercritical adsorption and were satisfactorily modeled by the model that applied for usual supercritical adsorption. The maximal adsorption capacity of hydrogen under the condition tested is less than 0.5 wt%.     

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.     

Adsorption phenomena at high pressures and temperatures 4. Isotherms of excess and absolute adsorption of Kr on NaA zeolite

The problem of conversion of an isotherm of excess adsorption measured experimentally into values of the total amount of the adsorbate (absolute adsorption) has been formulated. Five isotherms of excess adsorption of krypton on NaA zeolite were measured (at 334–500 K) at equilibrium pressures of -160 MPa. The corresponding isotherms of the total amount were calculated and the temperature dependences of the parameters of an equation describing the isotherms of excess adsorption were identified. It was shown that at high temperatures and pressures, krypton atoms diffuse into the -cavities of the NaA zeolite.For Part 3, seeRuss. Chem. Bull., 1996, 45, 534 (Engl. Transl.).Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1138–1142, May, 1996.     

Global statistical predictor model for characteristic adsorption energy of organic vapors-solid interaction: use in dynamic process simulation

Adsorption of Volatile Organic Compounds (VOCs) is one of the best remediation techniques for controlling industrial air pollution. In this paper, a quantitative predictor model for the characteristic adsorption energy (E) of the Dubinin-Radushkevich (DR) isotherm model has been established with R(2) value of 0.94. A predictor model for characteristic adsorption energy (E) has been established by using Multiple Linear Regression (MLR) analysis in a statistical package MINITAB. The experimental value of characteristic adsorption energy was computed by modeling the isotherm equilibrium data (which contain 120 isotherms involving five VOCs and eight activated carbons at 293, 313, 333, and 353 K) with the Gauss-Newton method in a statistical package R-STAT. The MLR model has been validated with the experimental equilibrium isotherm data points, and it will be implemented in the dynamic adsorption simulation model PROSIM. By implementing this model, it predicts an enormous range of 1200 isotherm equilibrium coefficients of DR model at different temperatures such as 293, 313, 333, and 353K (each isotherm has 10 equilibrium points by changing the concentration) just by a simple MLR characteristic energy model without any experiments.     

New thermodynamically consistent competitive adsorption isotherm in RPLC

A new equation of competitive isotherms was derived in the framework of the ideal adsorbed solution (IAS) that predicts multisolute adsorption isotherms from single-solute isotherms. The IAS theory makes this new isotherm thermodynamically consistent, whatever the saturation capacities of these single-component isotherms. On a Kromasil-C(18) column, with methanol-water (80/20 v/v) as the mobile phase, the best single-solute adsorption isotherm of both toluene and ethylbenzene is the liquid-solid extended multilayer BET isotherm. Despite a significant difference between the monolayer capacities of toluene (370 g/l) and ethylbenzene (170 g/l), the experimental adsorption data fit very well to single-component isotherms exhibiting the same capacities (200 g/l). The new competitive model was used for the modeling of the elution band profiles of mixtures of the two compounds. Excellent agreement between experimental and calculated profiles was observed, suggesting that the behavior of the toluene-ethylbenzene adsorbed phase on the stationary phase is close to ideal. For example, the concentrations measured for the intermediate plateau obtained in frontal analysis differ by less than 2% from those predicted by the IAS model.     

Adsorption of Supercritical Fluids

Adsorption isotherms for supercritical fluids are calculated using lattice theory. Results are compared with experimental data for methane on graphon and for carbon dioxide on coal. It is shown that the model reflects the peculiar features in adsorption isotherms of supercritical fluids (in particular, a maximum in the adsorption with increasing pressure). The analysis shows that there is two layer adsorption over a wide range of densities.     

Fine pore mouth structure of molecular sieve carbon with GCMC-assisted supercritical gas adsorption analysis

N₂ adsorption isotherms of molecular sieve carbon were measured at 77 K and 303 K. The Ar adsorption isotherms of molecular sieve carbon samples were also measured at 303 K. The grand canonical Monte Carlo (GCMC) simulation technique was applied to calculate the N₂ and Ar adsorption isotherms at 303 K using the ultramicropore volume determined by H₂O adsorption. The comparative method of experimental and simulated isotherms of supercritical N₂ and Ar at 303 K gave the width of the micropore mouth of the molecular sieve carbon, which can be applied to the ultramicropore width determination for other noncrystalline porous solids.