Physical adsorption of gases on graphitized carbon black

The Percus-Yevick equation of state is applied to a two-dimensional adsorbed fluid. Calculation of the adsorption isotherm is divided into two parts: the Henry's constant for gas-solid interactions, and the configurational contribution of the two-dimensional film. The theory is compared with experimental data for adsorption of argon and krypton on graphitized carbon black and agreement is excellent. This is the first successful calculation of adsorption isotherms over a wide range of temperature and surface density for which no adjustable parameters are used for the two-dimensional equation of state.     

Characterization of Cabot non-graphitized carbon blacks with a defective surface model: Adsorption of argon and nitrogen

A grand canonical Monte Carlo simulation (GCMC) is used to study the adsorption of argon and nitrogen on non-graphitized carbon black. The surface is assumed to be finite in length and composed of three graphene layers, the top layer of which contains defects. The isotherm obtained for the non-graphitized carbon shows a smooth S-shaped type while that obtained for the perfect graphene layer shows a wavy type. The isosteric heat is also affected by the defect; its behaviour versus loading exhibits a decrease at the beginning and then slightly increases once the first layer has been formed. The decreasing behaviour of isosteric heat at low loadings is not observed in the case of graphitized carbon black. The simulated results are compared against the experimental data of argon and nitrogen at 77 and 87.3 K on the Cabot carbon black BP 280, 460 and 2000. It is found that the defected finite surface describes well the data of these blacks. For the case of BP 2000 we have found that besides the defects of the surface, this sample contains a small population of small micropores having a width of 8.2 Å and its specific pore volume of 0.08 cm³/g.     

甲烷在石墨化炭黑上吸附平衡的分子模拟

采用GCMC法对不同温度和压力下纯CH4在石墨化炭黑上的吸附平衡进行了分子模拟研究,选用Lennard-Jones 12—6势能函数和联合原子力场参数（TraPPE）对体系进行势能计算,并将吸附平衡预测结果与实验数据进行了比较分析。结果表明,在所研究的温度范围内,除了在低压的一定区间外,GCMC模拟结果在中压和高压条件下与实验数据基本吻合,表明采用GCMC方法模拟甲烷分子在石墨化炭黑上的吸附平衡数据是可以进行准确预测的。在此基础上,利用模拟的吸附平衡数据计算不同温度下CH4的Henry常数进而到得极限吸附热,计算结果与实验数据接近。     

High resolution,quasi-equilibrium sorption studies of molecular sieves

Argon, nitrogen, and neopentane adsorption isotherms from molecular sieves are recorded at 87 K, 77 K, and 273 K, respectively, by a quasi-equilibrium, high resolution gas sorption technique. The molecular sieves used in this study are alkali exchanged zeolite X, AlPO₄-11, AlPO₄-5, VPI-5, KL, CaA, ZSM-5, and ZSM-11. Little relation is observed between the transition pressure for microporous nitrogen adsorption and pore size. Small changes in the effective pore size resulting from variations in cation size are detected in the transition pressure for argon adsorption. Large shifts in the transition pressure for argon adsorption are found for the 10-, 12-, and 18-membered ring pores of AlPO₄-11, AlPO₄-5, and VPI-5, respectively. Argon adsorption combined with neopentane adsorption on microporous materials provides additional information regarding transitions in the isotherm that result from dual pore systems and effects that may be due to adsorbate packing. The step in the nitrogen isotherm atP/P ₀> 0.1 from ZSM-5 is not observed in the nitrogen isotherm from ZSM-11.     

The prediction of adsorption equiliria for oxygen and nitrogen gas mixtures on 5a molecular sieves by ideal adsorbed solution theory(IAST)

Equilibria data for the adsorption of oxygen nitrogen and their binary mixtures were determined on 5A molecular sieve beads The single species isotherm data were approximated bv the Freundlich type isotherm in a piecewise manner. The binary equilibria were presented at the pressure of 1.5 atm abs and the three temperatures: 273, 283 and 293 K. The binary data were examined for values predicted by the ideal adsorbed solution theory using the piecewise Freundlich isotherms obtained from the single species isotherm data. Predicted equilibria data showed a good agreement with experimental values.     

吸附热预测吸附等温线

实验测定了N2 在沸石分子筛、C2 H6 在活性炭、CO2 在硅胶上的吸附等温线 ,研究用Clausius Clapeyron方程求得等量吸附热、再利用所得的吸附热预测其它温度的吸附等温线数据的方法。将吸附热预测的等温线与实验值及插值法内插得到的吸附等温线数据进行了比较 ,结果表明吸附热预测值与实验值吻合较好。此外还对文献数据利用等量吸附热预测较高压力 ( 65 0kPa)下的等温线 ,均与文献中的实验值一致。为吸附工业操作需要不同温度下的等温线数据和吸附过程的模拟与设计提供了简便、准确的计算方法     

Application of the general rate model with the Maxwell-Stefan equations for the prediction of the band profiles of the 1-indanol enantiomers

The adsorption isotherm data of R- and S-1-indanol and of their racemic mixture on cellulose tribenzoate were measured by frontal analysis. These experimental data were fitted to the single-component and the modified competitive Bilangmuir isotherms. The overloaded elution profiles of bands of the pure enantiomers and of the racemic mixture were calculated for different sample sizes, using the best competitive isotherm model and the General Rate Model of chromatography coupled with the generalized Maxwell-Stefan equation that describes the surface diffusion flux. The calculated and the experimental profiles were found to be in excellent agreement in all cases. The parameters of the model of the mass transfer kinetics were derived from the band profiles obtained for the pure enantiomers. The same values of these parameters give an excellent prediction of the profiles of multicomponent bands. The new model described here allows a satisfactory interpretation of the competitive mass transfer kinetics.     

Sorption of binary gas mixtures in zeolites: I. Sorption of nitrogen and carbon dioxide mixtures in silicalite

The adsorption of pure CO₂ and N₂ gases, and their mixtures have been determined on a very high silica form of the zeolite ZSM-5 using a newly developed isosteric technique. Unlike more traditional methods, this method allows one to measure mixture isotherms at fixed sorbed phase composition. The shape of the mixture isotherms and the isosteric heats of sorption for the mixture have been identified as functions of the equilibrium gas phase composition, which itself is a temperature dependent function of the sorbed phase composition. None of the isotherms approached saturation at the temperatures and pressures used. The ratio of Henry's constants for the two pure gases gives a reasonable estimate of the separation factor measured experimentally. Selective sorption of CO₂ is marked even at 70°C. Binary mixture sorption isotherms predicted from pure gas data using the ideal adsorbed solution (IAS) theory, are shown to be in close agreement with the experimental data.     

Adsorption Equilibria for Ethane and Propane Gas Mixtures on Activated Carbon

Abstract

We measured the transmission curves of binary gas mixtures of esthane and propane (and also of each gas alone) flowing through an adsorber bed at 25°C packed with (Columbia 4LXC 12/28) activated carbon. The adsorption isotherms for pure ethane and propane are described well by a modified Langmuir isotherm known as the Chakravarti-Dhar isotherm. Ethane in the binary mixtures exhibits smaller adsorption capacities than that of pure ethane; however, the presence of ethane does not significantly reduce the adsorption capacity of propane in the mixtures. The ideal adsorbed solution theory (IAST) of Myers and Prausnitz and the semiempirical method of Cook and Basmadjian (C-B) are applied to predict adsorption equilibrium properties of the binary mixtures from the known pure gas isotherms. The pure ethane isotherm data at pressures higher than 10 mmHg are obtained from a correlation formula. Good agreement between the predicted results from the IAST and C-B methods and the experimental results indicates (1) that the correlation formula can be used for estimating isotherms of related hydrocarbons with reliance and (2) that the two methods are useful in predicting the adsorption equilibrium properties for the ethane-propane mixtures on carbon.     

Statistical condensation adsorption isotherms of gas molecules adsorbed on porous adsorbents, surface monolayer adsorption isotherms and hysteresis phenomena

Condensation adsorption isotherms of type IV (or V) according to BDDT classification on porous adsorbents composed of one or two groups of adsorption sites are derived statistically. When Β_c1 (or Β_c2) is less than unity, the isotherm becomes type IV, and when it is greater than unity, the isotherm becomes type V. It is understood that the negative sign(-) of the additional adsorption energy, q, in the nth layer in deriving those theoretical isotherms plays a decisive role on the horizontally flat approach to the axis of the isotherm near the saturation vapor pressure. The values of the surface area can be calculated easily. The pore radii of all the adsorbents which we have obtained by using the derived isotherms with respect to the appropriately selected experimental data agree with those obtained by using the Kelvin equation. Many surface mono-layer adsorption isotherms are obtained in the process of deriving the various adsorption isotherms. From them we can learn that the surface sites are not adsorbed completely even near the saturated vapor pressure, and we can find the range of error by comparing them with v_ms of the BET equation. We could mention through judging the results of a great deal of the experimental isotherm data of types IV and V that “the cause of hysteresis phenomena of the condensation adsorption-desorption of gases is originated from the deviation from the thermodynamically reversible adsorption-desorption process in the condensation adsorption-desorption of gases”.     

A Separation Factor Method for the Analysis of Ideal Binary Mixtures in Gas-Solid Adsorption

Abstract

A correlation formula based on the separation factor is proposed for the mixture concentration in the adsorbed phase of an ideal binary solution in gas-solid adsorption. This formula is shown to apply to two binary systems (viz., 1,3-butadiene and n-butane on cross-linked polystyrene at 25°C, and acetylene and ethane on activated carbon at 25°C) with similar molecular and thermodynamic properties in the two components for each system. Comparison between the calculated and experimental values of the separation factor showed that the assumption of an ideal mixture is justified for each of these two binary systems. Mixture isotherms for the two ideal binary systems are calculated by the proposed correlation formula from the corresponding single-component isotherms. Good agreement between the calculated and experimental mixture isotherm data confirms that a binary system of two components with similar molecular properties (viz., molecular weight, normal boiling point, vapor pressure, number of carbon atoms in molecules, etc.) tends to form an ideal mixture (i.e., one with a constant separation factor).     

Three-dimensional mobile adsorption: evaluation of the density distribution in adsorbed phases

Brunet's three-dimensional model for mobile adsorbed phases has been modified to describe adsorption at submonolayer coverages, where Brunet's original theory fails. Here, use of Rudzinski's equation of state, obtained by summation of the virial expansion to infinity, allows us to consider the problem in terms of a radial distribution function. This distribution function in our approach depends parametrically on the Boyle temperature.In this way density profiles and the corresponding adsorption isotherm are evaluated for the system argon—carbon P33(2700) at 77.5 K. The theoretical density profiles behave correctly over a wide pressure range and the corresponding adsorption isotherm yields a good fit to the experimental data. The main conclusion which can be drawn is that the density distribution normal to the surface plays a more important role than was previously assumed.     

Evaluation of Adsorption Properties of Low Surface Area Carbons—Comparison of Experiments with a Theoretical Study

Adsorption isotherms of nitrogen, argon and methane were experimentally measured at 77 K on low surface area graphitised carbons and natural graphite. For direct comparison, theoretical isotherms for the same gases were calculated on a model, flat surface by using NL DFT. Vulcan (a graphitised carbon black, often used as a surface area standard) was shown to represent a good approximation of a flat graphite surface. It is a suitable material for evaluation of potential parameters in modelling of adsorption in micropores. It was also confirmed that the non-local density functional theory provided a good and reliable fit to experimental adsorption isotherms. SEM microscopy and x-ray diffraction revealed surface properties consistent with those evaluated by adsorption.     

Adsorption of mixed vapors on solids—I Graphon

The adsorption isotherms for ethanol-cyclohexane, ethanol-benzene, and benzene-cyclohexane vapor mixtures on Graphon at 20,30 and 40°C were measured at constant total pressure. The adsorption isotherms for the pure components were also obtained. Isosteric heats of adsorption were calculated for the pure adsorbates on Graphon. The binary vapor adsorption isotherms were compared to the pure component isotherms. The adsorbate-vapor composition diagrams for benzene-cyclohexane mixtures on Graphon all contained adsorption azeotropes. Very little selective adsorption of either component occurred. In general, benzene and cyclohexane were selectively adsorbed from benzene-ethanol and cyclohexane-ethanol mixtures on Graphon. The temperature dependence of the selectivity for the systems studied followed no consistent trend. Comparison of the binary vapor adsorption isotherms with the solution adsorption isotherms indicated that selectivity is generally higher in adsorption from solution. The experimental binary vapor adsorption isotherms compared favorably with those calculated from the pure vapor adsorption isotherms using the ideal adsorbed solution model. It was found that the adsorbed solutions were slightly non-ideal for benzene-cyclohexane mixtures on Graphon. The non-ideal behaviour observed for benzene-ethanol and cyclohexane-ethanol mixtures on Graphon was attributed to the presence of benzene or cyclohexane disrupting the hydrogen-bonded structure of ethanol on the Graphon surface.     

Adsorption and desorption of m-xylene vapor on organically modified montmorillonite

m-Xylene vapor was adsorbed to organically modified montmorillonite (organoclay). Hexadecyltrimethylammonium (HDTMA) was used to modify the surface of the clay. In adsorption experiments, the organoclay, along with the non-modified (washed) clay, was used. m-Xylene was adsorbed from a gaseous phase by using a fixed adsorption bed. The adsorption breakthrough curves and the adsorption isotherms were determined at three different temperatures (24, 34, and 44 °C). The adsorption data were modeled with the BET isotherm equation. It was found that the adsorption isotherms of washed clay exhibited a favorable Type I behavior, which implies that the adsorption capacity is strongly dependent on vapor concentration at low concentration ranges. In regards to the organoclay, isotherms showed a marginally favorable Type II behavior with a reduced adsorption capacity at low concentrations, and exhibited a linear increase at elevated vapor concentrations. The adsorption capacity of both washed clay and organoclay obviously decreased with the temperature. The desorption of m-xylene from the clays was also performed by using pure nitrogen, and the desorption rate constant k was on the order of 10⁻⁵ min⁻¹ for all types of clay.     

Equilibrium isotherms and adsorption heats analysis for ternary mixture of methane,ethane, and propane on 4A zeolite

Equilibrium isotherms were obtained experimentally and theoretically for adsorption of methane, ethane, and propane mixtures on 4A zeolite at 301 K. The experimental data were measured using constant volume method and analyzed by extended Langmuir, modified extended Langmuir, and extended Freundlich isotherm models. The best correlation was achieved with the extended Freundlich isotherm equation and the negative values for heats of adsorption indicate exothermic adsorption nature for these gases on 4A zeolite. The selectivity of 4A zeolite to adsorb gases was also studied with the observation that ethane was more selectively adsorbed than methane on 4A zeolite, while propane was less selectively adsorbed on this zeolite at the studied temperature. Also, it was found that an increase in initial partial pressure of ethane decreases the adsorbed amount of methane, while the increase in partial pressure of methane had no effect on adsorbed amount of ethane.     

Prediction of the band profiles of the mixtures of the 1-indanol enantiomers from data acquired with the single racemic mixture

The adsorption isotherm data of R- and S-1-indanol and of their racemic mixture on cellulose tribenzoate were measured by frontal analysis. These data were then fitted to the Langmuir, the Bilangmuir, the Toth, and the Langmuir-Freundlich isotherm models. The single component data fitted well to both the Bilangmuir and the Toth models. Combined with the lumped pore diffusion model (POR) of chromatography, these isotherms were used to calculate overloaded elution profiles of the pure enantiomers. The calculated and the experimental profiles agree excellently in all cases if the former are derived from the Bilangmuir model. The competitive experimental data also gave excellent agreement with the Bilangmuir model. The simultaneous fit of all the data, for the single components and the racemic mixture, gave again superior agreement with the bilangmuir model. The overloaded elution profiles of samples of the racemic mixture calculated with the Bilangmuir isotherm model combined with the POR model of chromatography gave results in very good agreement with the experimental band profiles of large samples of the racemic mixture. This confirms that in numerous cases the whole set of competitive isotherms of two enantiomers can be derived from the experimental data obtained only with the racemic mixture.