Micropore structure of coal

The adsorption of carbon dioxide at 298 K, 273 K and 253 K by the vitrains of twelve Japanese coals and five foreign coals has been studied. The Dubinin-Polanyi equation was used in order to interpret the isotherms. Equilibrium points at different temperatures fell satisfactorily on a single straight line of the Dubinin-Polanyi equation. It was shown that it is possible to estimate the differential heat of adsorption of these coals from an isotherm obtained at a single temperature if the isotherm follows the Dubinin-Polanyi equation. The heats of adsorption of carbon dioxide on coals amounted to 6–7.5 kcal/mol regardless of rank. The rate of adsorption of carbon dioxide on coals was measured at a constant pressure of 260 mmHg at 298 K. The micropore volume, the gradient of the Dubinin plot and the rate of adsorption are discussed in relation to characteristics of their infra-red adsorption spectra. It is concluded that the micropore structure of coal is closely related to the number of hydrogen atoms bound directly to carbon atoms. A possible explanation of this is suggested.     

Micropore structure of activated carbons and predictions of adsorption equilibrium

A method for calculating micropore size distributions based on a molecular model of adsorption and analytical solution for the adsorption isotherm is presented in this study. Micropore volume filling is considered to be an evolution of two-dimensional condensation, which occurs on micropore walls at the critical condensation pressure. The treatment of adsorption isotherms of propane, propylene, acetylene, ethylene, cyclopentane and benzene shows that the method offers a quantitative approach to determining a reliable carbon texture, which is independent of adsorbate employed and adsorption temperature. The invariant parameters of porous structure coupled with molecular constants of adsorbate provide a prediction of the adsorption equilibrium in a wide range of pressures. Good agreement between experimental and calculated data is demonstrated for adsorption of both gases and vapors.     

Prediction of mixed-gas adsorption equilibria from pure component adsorption isotherms

Using only a single binary interaction parameter per adsorbate-adsorbate pair, the adsorption nonrandom two-liquid theory successfully correlates binary gas adsorption experimental data and predicts multicomponent gas adsorption equilibria. This work estimates the binary adsorbate–adsorbate interaction parameters from pure component isotherms and shows very promising agreement with the binary interaction parameters identified from regression of binary gas adsorption data. The results support the feasibility to predict binary and multicomponent gas adsorption equilibria from pure component isotherms for various adsorbents including silica gel, activated carbon, zeolites, and metal–organic frameworks.     

Calculation of single adsorption isotherms from gravimetrically measured binary gas mixture adsorption isotherms on activated carbon at high pressures

A method is developed for the calculation of single-component adsorption isotherms from gravimetrically measured binary gas mixture adsorption isotherms at high pressures, at two temperatures and for different mole fractions of the gas phase. The adsorption of nitrogen/methane on active carbon Norit R1 is taken as an experimental example.     

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     

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”.     

On selection of standard states in adsorption isotherms

The question of choice of appropriate standard states for adsorbed phases at electrode interfaces is reviewed and examined in relation to solvent displacement and the Langmuir and Flory-Huggins types of adsorption isotherms. Comparison with the situation for gas/solid adsorption is made. Standard potentials for electrochemisorption are considered for electrochemical isotherms with and without interaction/heterogeneity terms.     

Comparison of adsorption isotherms on Cu-BTC metal organic frameworks synthesized from different routes

Due to the differences in synthesis procedures used in preparation of metal organic frameworks, surface area and pore volume of the samples usually vary from one laboratory to another. The adsorption characteristics of the frameworks are also affected. In such cases, usually a scaling factor is employed to match isotherms obtained in different laboratories (or those from experiments and simulations). In an attempt to validate the use of a scaling factor, in this work we compare adsorption properties of N₂, O₂, Ar, CO₂, C₃H₈ and SF₆ on Cu-BTC (HKUST-1) framework synthesized using two different procedures. It was observed that Cu-BTC sample with higher surface area and pore volume has greater adsorption capacity for all gases at the two temperatures considered (298.25 and 318.15 K). For all gases, the ratio of specific adsorption capacities of the two samples can be broadly classified into two regions. In the low loading region this ratio was about 2.03 with in a 10% uncertainty envelope. However, in the high loading region, it increases to about 2.58. Thus, the scaling factor needed to compare adsorption isotherms of Cu-BTC having different surface areas may not be unique. This value seems to be strongly affected by loading, whereas it is relatively insensitive to the type of gas used, temperature or pressure.     

Rate equations and isotherms for two adsorption models

The rate equations and isotherms for two adsorption models have been formulated and discussed. The rate equations obtained have been compared to those of pseudo-first-order model and pseudo-second-order model, respectively. Calculated results indicated that the rate equations of single site model and dual site model could be approximated by those of pseudo-first-order model and pseudo-second-order model, respectively, in a limited ranges of the values of , K*and t*. The adsorption isotherm of single site model has been found to be the same as that of Langmuir type.     

An adsorption equilibrium model for Type 5 isotherms

Vapor adsorption on porous materials for non-wetting adsorbent–adsorbate pairs, characterized by a Type 5 isotherm, results in low loading at low relative pressure and increased loading due to capillary condensation behavior at higher relative pressure. The relationship between capillary condensation and the Kelvin equation suggests that a simple expression can be developed in which the adsorption equilibrium is obtained from an integrated pore size distribution function. This new expression is shown to describe the Type 5 isotherms of water adsorption on activated carbon. In addition, this expression has useful properties including: (1) finite saturation limit, (2) readily calculated Henry’s constant, and (3) readily calculated heat of adsorption, (4) can be written explicitly in terms of partial pressure or adsorbed phase loading and (5) parameter values attributable to the distribution function.     

Equilibrium adsorption isotherms for basic dyes onto lignite

The adsorption of two basic dyes in aqueous solutions onto lignite is reported. The adsorption equilibrium isotherms are recorded for the dyes in single component solution. The isotherms are plotted to obtain the Freundlich constants, the Langmuir constants, and the Redlich-Peterson constants.     

Nitrogen rejection from low quality natural gas by pressure swing adsorption experiments and simulation using dynamic adsorption isotherms

In order to remove N₂ from low quality natural gas, a mathematical model has been established by Aspen adsorption, using the CH₄-selective sorbent silicalite-1 pellets. The dynamic adsorption isotherm was first simulated by breakthrough simulation of a CH₄/N₂ mixture at different adsorption pressures and feed flow rates based on breakthrough experiments. The resulting simulated CH₄ dynamic adsorption amounts were very close to the experimental data at three different adsorption pressures (100, 200, and 300 kPa). Moreover, a single-bed, three-step pressure swing adsorption (PSA) experiment was performed, and the results were in good agreement with the simulated data, further corroborating the accuracy of the gas dynamic adsorption isotherm obtained by the simulation method. Finally, based on the simulated dynamic adsorption isotherm of CH₄ and N₂, a four-bed, eight-step PSA process has been designed, which enriched 75% (vol) CH₄ and 80% (vol) CH₄ to 95% (vol) and 99% (vol), and provided 99% (vol) recovery.     

Analysis of some adsorption isotherms on calcium silicate hydrates

Tricalcium silicate was hydrated in suspension at a water/solid ratio (w/s) of 9 and as a paste at a w/s of 0.7. Water contents after outgassing at room temperature were 23–25% for the suspension hydrates and 19–20% for the pastes. Outgassing was standardised so that subsequent increases in BET nitrogen surface area were small even when the pumping time was doubled. Nitrogen adsorption isotherms were similar for both types of product except for the greater volume of mesopores in the suspension hydrates filled at relative pressures greater than ca 0.9_p0(r_p > 100 ǎ). Electron microscopy suggested that the additional uptake of nitrogen involved capillary condensation between bundles of fibrils. Adsorption at low relative pressures would have occurred principally between the fibrils which were of the order of 20 ǎ thick. Pore sizes could be seen to extend continuously from the mesopore to micropore range and the fact that all the isotherms contained a hysteresis loop closing with an abrupt desorption at ca0.44p0 was regarded as a property of liquid nitrogen rather than as an indication of the pore size distribution. A more quantitative indication of the pore size range present was obtained using the α_s method with standard data for Fransil (silica). Plots for pastes were similar to those for suspension hydrates indicating a similar texture accessible to nitrogen in the pastes at the high w/s used. In order to characterise further the surface of the hydrates argon and carbon dioxide adsorptions were determined and the data compared using the Frenkel–Halsey–Hill equation.     

Characterization of single-walled carbon nanohorns using neon adsorption isotherms

Detailed volumetric neon adsorption isotherms were measured between 22.670 and 31.444 K on a sample of closed, mostly bud-like, aggregates of single-walled, carbon nanohorns, produced by NanoCraft, Inc. The results are compared to those recently reported on closed dahlia-like nanohorn aggregates. While for the closed dahlia-like aggregates the data shows that there are two groups of sites with different binding energies present at relatively low loadings, only one group of adsorption energies is present for the closed bud-like aggregates at comparable loadings. We measured the isosteric heat of adsorption and its dependence on substrate loading. From the low-loading limit of this quantity, we determined that the binding energy of the sites present on the bud-like aggregates have a value intermediate between that obtained for adsorption on planar graphite and that measured on the grooves of close-ended bundles on nanotubes for the same adsorbate.