Prediction of high-pressure adsorption equilibrium of supercritical gases using density functional theory

In this paper, we present the results of the prediction of the high-pressure adsorption equilibrium of supercritical gases (Ar, N2, CH4, and CO2) on various activated carbons (BPL, PCB, and Norit R1 extra) at various temperatures using a density-functional-theory-based finite wall thickness (FWT) model. Pore size distribution results of the carbons are taken from our recent previous work,(1,2) using this approach for characterization. To validate the model, isotherms calculated from the density functional theory (DFT) approach are comprehensively verified against those determined by grand canonical Monte Carlo (GCMC) simulation, before the theoretical adsorption isotherms of these investigated carbons calculated by the model are compared with the experimental adsorption measurements of the carbons. We illustrate the accuracy and consistency of the FWT model for the prediction of adsorption isotherms of the all investigated gases. The pore network connectivity problem occurring in the examined carbons is also discussed, and on the basis of the success of the predictions assuming a similar pore size distribution for accessible and inaccessible regions, it is suggested that this is largely related to the disordered nature of the carbon.     

Analysis of low vapor pressure region of adsorption isotherm

Summary In order to explain the low relative vapor pressure region in experimental adsorption isotherms of polar small molecules by polar adsorbents — as is the case with the protein-water system — the statistical thermodynamics of adsorption by a multi-component surface was studied on the basis of the BET model. The derived adsorption isotherm gives the same expression as the simple summation of the BET equation over all kinds of polar sites. The mathematical examination reveals a condition necessary for the approximation of the derived adsorption isotherm to the usual straight BET line. Besides, according to the present theory, it is possible to derive the number and weight averages of adsorption energy factors (C of BET theory) by the analysis of low vapor pressure region.

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Zusammenfassung Die Adsorption kleiner polarer Moleküle an polaren Adsorbentien, z. B. Wasser an Proteinen, wurde auf der Basis des BET-Modells im Bereich kleiner relativer Dampfdrücke mit den Methoden der statistischen Thermodynamik behandelt.Nach der entwickelten Theorie ist es möglich, die Anzahl und das durchschnittliche Gewicht von Adsorptionsenergiefaktoren zu ermitteln.

     

Enzymatic reactions in supercritical gases

The enzyme polyphenol oxidase has been found to be catalytically active in supercritical carbon dioxide and fluoroform: it readily oxidizesp-cresol andp-chlorophenol to their correspondingo-benzoquinones.     

Physical adsorption of gases

Summary Theoretically derived isotherms for monolayer and multilayer adsorption are used to interpret some earlier published experimental data. A satisfactory agreement is found in nearly all cases. To check the validity of the theoretical constant b it is plotted versus l/√T and this results in a straight-line dependence as it is foreseen by the theory. The b values found for the adsorption of n-butane on glass spheres are used to calculate the average settling times τ _L at different temperatures and subsequently τ _0(L) which is estimated to be ∼ 10⁻¹³ sec. It seems that the constant b is quasi-independent of the adsorbent properties which permits to obtain a characteristic plot for nitrogen isotherms.     

Rapid determination of vapor adsorption properties of ultramicroporous solids: An application to the zeolite affinity of adsorption

Adsorption capacities of zeolites are rapidly determined by a vapor sorption apparatus which can be used as an ultramicroporosimeter. From the sorption data, it is possible to calculate the zeolite affinities toward various adsorbates in comparison with water. The n-hexane affinity index of the new ZSM zeolites has values always higher than those of X, Y or Zeolon zeolites. This fact is attributed to the unusual framework structure of the silica-rich ZSM zeolites.

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, . . - ZSM , X, Y . ZSM , .

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Presented in part at the I⁰ Convegno nazionale di Catalisi-II⁰ Simposio Italo/Cecoslovacco di Catalisi, held in Bologna (Italy) on June 1979.     

Determination of the surface potential for adsorption layers of polyelectrolytes by the streaming potential method

A physicochemical model is proposed to describe electrokinetic phenomena in capillaries and pores the surface of which is coated with a charged porous adsorption layer. The use of this model makes it possible to explain experimental data on the surface potentials of polyelectrolyte adsorption layers upon their deformation resulting from solution flow. The commonly used Smoluchowski equation is shown to lead to large errors in the determination of the potential and charge of the surface of an adsorption layer.     

Determination of the vapor pressure of ethyl esters by Differential Scanning Calorimetry

Differential Scanning Calorimetry (d.s.c.) was used to determine the vapor pressure of the following ethyl esters: ethyl laurate, ethyl myristate, ethyl palmitate, ethyl stearate, ethyl oleate and ethyl linoleate in the range from 1.33 to 9.33 kPa. These esters are the major constituents of the biodiesel obtained from the transesterification of some vegetable oils with ethanol. Samples of 2 to 5 mg were used in the analysis, with a heating rate of 25 °C · min^?1 and a pinhole with a diameter of 0.25 mm. The results showed that Differential Scanning Calorimetry was a suitable technique for measuring the vapor pressure of organic compounds like fatty esters. The constants of the Antoine equation were determined from the experimental data and the validity of this equation in representing the vapor pressure and vaporization enthalpy of the compounds under study was examined.     

Determination of absolute gas adsorption isotherms: simple method based on the potential theory for buoyancy effect correction of pure gas and gas mixtures adsorption

The absolute adsorption isotherms are necessary to correctly evaluate the selectivity of the adsorbent material or to design adsorption processes at high pressure (e.g., H₂ purification from syngas processes, removal of acid gas from natural gas,…). The aim of this work is thus to propose an easy method to correct the buoyancy effect of the bulk phase on the adsorbed phase volume during both pure gas and gas mixtures adsorption for pressures up to 10 MPa. The potential theory of adsorption and the Dubinin–Radushkevich relation are adapted by introducing mixing parameters based on simple Berthelot rules. The concept of internal pressure used to characterize the adsorbed phase is also adapted for mixtures. The method is then improved on a commercial activated carbon (AC), when adsorbing pure H₂S and CH₄, and their mixtures up to 5 MPa. The study points out the importance to carefully consider the buoyancy effect of the bulk phase on the adsorbed phase volume. Its impact on the adsorbent material selectivity at high pressures could affect the design and the performances of PSA or TSA processes. For example, only considering the excess adsorption data leads to an apparent selectivity 13 % greater than the absolute one for a concentration of 6 ppm of H₂S in a CH₄ matrix at 5 MPa (298 K) on the AC.     

Determination of the degree of electrolytic dissociation of perchloric acid by vapor pressure

Analysis of the published data on the vapor pressure and degree of electrolytic dissociation of perchloric acid revealed that the molar fraction of nondissociated HClO₄ is determined by the Raoult—Henry law in a wide range of acidimetric concentrations and temperatures. The degree of dissociation of perchloric acid was calculated from its thermodynamic activity. The results of calculations agree satisfactorily with the known spectroscopic data.     

Determination of the average molecular weight of petroleum cuts by vapor pressure depression

The authors have constructed a static apparatus for the determination of the average molecular weight of petroleum cuts, by the vapor pressure depression method, also called tonometry. The solvent vapor pressure decrease, which appears when a compound is added in small quantities, is approximately proportional to the solute mole fraction. The average molecular weight of real petroleum cuts is given with a relative error of less than 2%. This error mainly results from the estimation method of the activity coefficients in the solution corresponding to the pseudobinary system “petroleum cut + solvent”.     

Determination of the saturation vapor pressure of silicon by Knudsen cell mass spectrometry

Silicon evaporation has been investigated by high-temperature mass spectrometry, and the saturation vapor pressure of silicon over its melt has been determined over the temperature range from 1739 and 2326 K. The saturation vapor pressure data obtained via silicon evaporation from Knudsen cells made of molybdenum, tungsten, molybdenum disilicide-lined molybdenum, and graphite silicided by the gas-phase method are compared. Among these materials, silicided graphite is the most inert toward silicon vapor. The silicon partial pressures measured in the silicided graphite cell are close to the recommended values.     

Extraction with supercritical gases in coupling with thin-layer chromatography

Summary Numerous substances can be extracted from samples by supercritical gases, e.g. either carbon dioxide or nitrous oxide in the fluid condition at temperatures of 40° C and pressures between 70 and 400 bar. An apparatus for such solvent-free extraction in the analytical field is described. It is coupled directly with a receiving apparatus for subsequent thin-layer chromatography. Three examples with corresponding chromatograms show the possibilities and advantages of this new coupling procedure.

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Extraktion mit überkritischen Gasen in Kopplung mit der Dünnschicht-Chromatographie1. Mitteilung: Aufbau der Apparatur, Handhabung und Anwendung

Zusammenfassung Zahlreiche Substanzen lassen sich mit überkritischen Gasen aus Proben fraktioniert abtrennen. Als Gase dienen z.B. Kohlendioxid und Distickstoffoxid im fluiden Zustand, d.h. bei Temperaturen von 40° C und Drücken zwischen 70 und 400 bar. Für derartige lösungsmittelfreie Extraktionen im analytischen Bereich wird eine Apparatur beschrieben. Sie ist direkt mit einer Auffangapparatur zur anschließenden Dünnschicht-Chromatographie gekoppelt. An 3 Beispielen mit entsprechenden Chromatogrammen werden die Möglichkeiten und Vorteile dieser neuen Kopplungsmethode demonstriert.

     

An application of mean-field perturbation theory for the adsorption of polar molecules in nanoslit-pores

In the present research study, we present the development of a model for characterizing and predicting the adsorption of polar molecules between two parallel plates based on mean-field perturbation theory. The electrostatic forces between fluid–fluid molecules in the slit shaped pore are modeled by considering permanent dipole–dipole interactions and permanent dipole-induced dipole moment interactions. The intermolecular potential for the electrostatic interactions was obtained by considering statistical averages over all possible orientations of the molecules. The proposed model is then used to study the sorption of water molecules in the slit shaped pore and an explicit equation for the Helmholtz free energy of the pore phase fluid is derived. Adsorption isotherms for different pore sizes are simulated and the relative contributions of fluid–wall and fluid–fluid interactions to the Helmholtz free energy are calculated as an illustration and compared with the results of existing models in the literature.AMS subject classification: 82B03, 82B05, 82B26, 82B30, 82D15, 31B10, 41A25     

Determination of the heat effects involved during toluene vapor adsorption and desorption from microporous activated carbon

In the present work the adsorption of toluene on microporous activated carbon was chosen as an illustrative example in order to show that different values of the heat effect might be obtained following the procedure used. Values ranging from 85 to 40 KJ/moL were obtained for the isosteric heat of adsorption at different adsorbate loadings using adsorption isotherm data measured under static conditions. However, the application of temperature programmed desorption (TPD) experiments carried out under dynamic conditions yields apparent energy of desorption values that cannot be systematically correlated with the heat of adsorption which is a thermodynamic parameter relevant to the adsorption equilibrium. This issue is of interest because the use of accurate values of the heat of adsorption is important for the correct designing and operating of adsorption facilities.     

Determination of vapor pressure curves of warifteine and methylwarifteine by using thermogravimetry

Warifteine and methylwarifteine are bisbenzylisoquinoline alkaloids from Cissampelos sympodialis Eichl., produces a reversible, nonspecific, and noncompetitive antagonism of histamine, anti-allergic therapy, and used in the folk medicine for the treatment of airway respiratory diseases. Thermal techniques are used for several purposes, e.g., for thermal characterization, and measurement of the vapor pressure with objective of the quantitative determination. The objective of this study was to evaluate of the vapor pressure of the curves of warifteine and methylwarifteine by thermogravimetry analysis, how a fast and sensible method for quantitative determination. This method is based on the ratio of the Arrhenius, Langmuir, and Antoine equations for a reference material and the sample under investigation. The results showed these alkaloids follow zero-order kinetics with respect to volatilization. The vapor pressure curves of warifteine and methylwarifteine were plotted for the rates of 10, 15, and 20 °C min⁻¹. The proposed method also possesses the potential.     

Calculation of isosteric heats of molecular gas and vapor adsorption on graphite using density functional theory

Isosteric heats of hydrogen, nitrogen, and methane adsorption on graphite surface have been calculated and analyzed using the density functional theory. The heats of adsorption have been calculated within wide ranges of pressures and subcritical temperatures. The peaks in the temperature dependences of isosteric heat of adsorption have been found to correlate with the degrees of filling of monolayers during adsorption.     

Saturated vapor pressure in the thallium-cadmium system

Component vapor pressures in the thallium-cadmium system determined by the method of boiling points (isothermal variant) and the flow method were used to calculate the thermodynamic characteristics of the vapor and condensed phases in the region of the existence of liquid solutions. The temperature-concentration dependences of the thermodynamic values were determined. The phase diagram was augmented by liquid-vapor phase transitions at atmospheric pressure and in a vacuum (100 and 10 Pa) with the determination of the boundaries for the coexistence region of the liquid and vapor phases.