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Adsorption: Journal of the International Adsorption Society (v.4, #3-4)
The Pore Structure Determination of Carbon Aerogels by Y. Hanzawa; K. Kaneko; N. Yoshizawa; R.W. Pekala; M.S. Dresselhaus (pp. 187-195).
The detailed adsorption isotherms of nitrogen on carbon aerogels at 77 K were measured. The N2 adsorption isotherm had a marked hysteresis. The adsorption isotherms were analyzed by high resolution αs-plots to evaluate their porosity. The αs-plots showed an explicit upward deviation from the linearity below αs = 0.5, suggesting the presence of micropores. The mesoporosity and microporosity were separately determined from the αs-plot. The predominant pores in carbon aerogels were mesopores and the percentage of micropores was in the range of 5 to 10% of the total pore volume. The N2 adsorption hysteresis was analyzed with the Saam-Cole theory under the assumption of the cylindrical pore shape. The parameters determined from the Saam-Cole method were associated with the carbon aerogel structure.
Keywords: mesopore structure; Saam-Cole theory; adsorption hysteresis; carbon aerogel; nitrogen adsorption
Analysis of the Micropore Structure of Various Microporous Materials from Nitrogen Adsorption at 77 K by A. Gil (pp. 197-206).
The micropore structure of four microporous materials (two zeolites, ZSM-5 and Y-82; an activated carbon and an alumina pillared clay) and their binary physical mixtures (50-50 wt%) have been examined by nitrogen adsorption at 77 K. Various micropore sizes have been considered from the stages on the micropore filling mechanism in the microporous materials. The application of the Dubinin-Astakhov (DA) equation to characterize and obtain the adsorption potential distributions of the microporous materials is presented.
Keywords: adsorption isotherms; adsorption potential distributions; micropore structure; structural heterogeneity
Capillary Condensation and Melting/Freezing Transitions for Methane in Slit Coal Pores by Aleksey Vishnyakov; Elena M. Piotrovskaya; Elena N. Brodskaya (pp. 207-224).
Grand canonical Monte Carlo method has been applied to study the adsorption behavior of methane in slit coal micropores for pore widths from 1.5 to 4.5 nm at temperatures 111–300 K. The adsorbent surface is considered to be inhomogeneous with C, N, S, O and H atoms, as well as vacancies, taken into account. The effect of surface microstructure on mechanism of the pore filling and saturation pressure is found to be more pronounced at relatively low and moderate strength of the adsorption field. It is also shown that irregular impurities and defects at the adsorbent surface contribute to the lowering of the freezing temperature and can change qualitively its dependence on the pore width.
Keywords: carbon adsorbents; pore condensation; computer simulation
Evaluation of the Secondary Pore Structure of Hydrothermally- and Acid-treated Faujasite Type Zeolites by Takae Kawai; Kazuo Tsutsumi (pp. 225-231).
With regard to H-Y type zeolites dealuminated by hydrothermal and acid treatments, their physical properties were characterized by measurements of 29Si- and 27Al-MAS-NMR, IR, and X-ray diffraction. The secondary pores were quantitatively analyzed by the t-plot method for nitrogen adsorption isotherm at liquid nitrogen temperature and were then compared with the results of samples dealuminated by SiCl4 treatment.The plateau region of the nitrogen adsorption isotherm diminished as hydrothermal and acid treatments proceeded, with the result that the shape of the t-plot changed to that of three straight sections. This t-plot shape suggested that the secondary pores with relatively consistent sizes could develop with the progress of the treatment. On the other hand, SiCl4 treatment was found to produce less secondary pores than hydrothermal and acid treatments. The surface area of micropores calculated from the t-plots gave a considerably higher value than values obtained from the BET equation and from that calculated geometrically. This is attributable to the micropore filling effect.
Keywords: faujasite; dealumination; hydrothermal treatment; acid treatment; secondary pore
Electric Fields on FSM-16 Surface by Ken-Ichi Honma; Tatsuya Yamazaki; Hiromichi Yoshida; Sentaro Ozawa (pp. 233-237).
The surface of mesoporous silicate, FSM-16, was investigated by infrared spectroscopy (IR) using methane and carbon monoxide as molecular probe. The appearance of ν1-peak of adsorbed CH4 indicated the presence of electric field (1.4 × 105 esu) which is attributed to a weakly polarized free silanol site (site-1). The site was located at the void of oxygen framework in FSM-16 pore wall. In addition, the presence of site-2 without any electric field was found by IR spectra of adsorbed CO. The latter site was assigned to a interacted silanol groups and/or a Si–O–Si site.
Determination of the Porous Structure of Activated Carbons Using the IAE Concept. Influence of the Local Adsorption Model by Marc Frère; Murielle Zinque; Karl Berlier; Roger Jadot (pp. 239-250).
In this paper we study a method for the determination of the micropore volume distribution function of activated carbons. This method is based on the Integral Adsorption Equation concept (IAE). The micropore volume distribution function is assumed to be a Gaussian of which the parameters are unknown. These parameters are determined using adsorption isotherms of carbon dioxide on a given activated carbon (F30/470 CHEMVIRON CARBON) at 278, 288, 298, 303, 308, 318 and 328 K and for pressures up to 100 kPa. Several local adsorption models are used (Langmuir, Volmer, Fowler-Guggenheim, Hill-de Boer). The influence of the choice of the local model on the pore volume distribution function is discussed. The physical validity of this function and the performances of the different models are presented. It appears that the effect of the temperature on the adsorption isotherms is difficult to model over a wide range of relative pressure. The Hill-de Boer and the Langmuir local models are the most efficient (average errors respectively equal to 3.53% and 2.80% in the studied range of temperature and pressure). They provide the most meaningful parameters for the pore volume distribution function.
Keywords: activated carbons; characterization; isotherms; integral adsorption equation
Studies on the Selectivity of Porous Polymers Containing Different Functional Groups by Barbara Gawdzik; Tadeusz Matynia (pp. 251-255).
Three types of porous polymers containing different functional groups were synthesized as stationary phases for gas chromatography. The influence of functional groups in the copolymer skeletons on their selectivities was studied. To determine the selectivities of these copolymers, two procedures were applied: the selectivity triangle and the general selectivity. As a reference phase Porapak Q which does not contain any functional groups was used.
Keywords: gas chromatography; porous polymer stationary phase; selectivity studies
Calculation of Adsorption Isotherms on Hard Solid Surfaces Using Measurements of Surface Tensions and Contact Angles by U. Messow; P. Braeuer; A. Schmidt; C. Bilke-Krause; K. Quitzsch; U. Zilles (pp. 257-267).
The adsorption excess isotherms of binary mixtures adsorbed on hard solids were calculated by means of surface tension and contact angle measurements using the Gibbs adsorption isotherm equation. The calculation procedure is described in detail using the authors' own measurements of mixtures containing ethylene glycol(1)/water(2) on Teflon and poly(vinyl chloride), and water(1)/n-propanol(2) on Teflon. On the basis of these results and also from surface tensions and contact angles on hard solids published by other authors, all types of isotherms were found as given for porous adsorbents in the classification of Schay and Nagy. In addition to those, new isotherm types are proposed.
Keywords: adsorption isotherms; surface excess; surface tension; contact angle; hard solid
Compensation Theory of Adsorption: Correlation and Prediction of Henry Constants for Linear Paraffins on Zeolite Adsorbents by Douglas M. Ruthven; Bal K. Kaul (pp. 269-273).
Experimental Henry's Law equilibrium data for linear paraffins show a well defined linear variation of adsorption energy with carbon number and a linear relationship between the entropy and energy of adsorption (compensation effect). Such correlations, are presented for sorption of linear paraffins on NaX, NaY, USY, silicalite and silica gel as well as for two commercial (faujasite based) catalysts and provide a concise way to predict the adsorptive behavior of the higher paraffins.
Keywords: zeolite; equilibrium; compensation effect; Henry constant; adsorption energies
Heat Effects in ZLC Experiments by Stefano Brandani; Celio Cavalcante; Anemir Guimarães; Douglas Ruthven (pp. 275-285).
The problem of nonisothermal desorption in a zero length column (ZLC) experiment is considered theoretically. Simple analytical expressions for the ZLC desorption curve are derived for certain limiting situations in which the governing equations reduce to a linear form. More general numerical solutions are calculated for a wide range of experimental conditions assuming both negligible mass transfer resistance and finite mass transfer resistance controlled by intraparticle diffusion. A simple criterion for negligible thermal effects is developed. It is shown that when the ZLC technique is applied to the measurement of diffusion in unaggregated zeolite crystals, as originally intended, heat effects are generally insignificant. However, when applied to the measurement of macropore diffusion in relatively large adsorbent particles heat effects can become important and may cause major modification of both the desorption rate and the shape of the desorption curve. A recent experimental ZLC study carried out with commercial adsorbent particles, under conditions of macropore diffusion control, showed an anomalous dependence of the desorption rate on both temperature and particle size. These effects can be qualitatively explained by the nonisothermal model. A more precise quantitative representation of these experiments will require a more refined model incorporating a nonlinear equilibrium isotherm as well as intraparticle diffusional resistance.
Keywords: diffusion zeolite; ZLC; heat effects
Calorimetric Effects of Simple Ion Adsorption at the Oxide-Electrolyte Interfaces: Seeking for a Simple Interpretation of the Data Obtained by Titration Calorimetry by W. Rudziński; R. Charmas; W. Piasecki; N. Kallay; J.M. Cases; M. François; F. Villieras; L.J. Michot (pp. 287-297).
The theoretical description of the enthalpic effects accompanying ion adsorption at the oxide-electrolyte interface, developed recently by Rudzinski and co-workers, is used here to analyze the Kallay's experimental procedure of determining nonconfigurational heats of proton adsorption in a simple experiment involving use of titration calorimetry. That theoretical analysis, based on considering a certain real adsorption system, leads to important recommendation at which conditions the Kallay's titration experiment yields the most reliable figures describing the heats of proton adsorption.
Keywords: calorimetry; electrolyte-oxide interface; ion adsorption
Full Analysis of Internal Adsorbate Redistribution in Regenerative Adsorption Cycles by M. Pons (pp. 299-311).
Adsorption cycles with heat regeneration are a promising CFC-free alternative to compression cycles, for refrigeration or heat pumping purposes. However, the process is complex and therefore requires efficient analysis tools to understand correctly. These tools are now developed and used here for studying the phenomenon of internal adsorbate redistribution. This phenomenon takes place during the periods when the adsorber is pressurised, or depressurised, under globally isosteric conditions. It can be easily thought that internal adsorbate redistribution reduces the cycle performance. In order to avoid this reduction, the adsorbate redistribution can be reduced by implementing separate vapour compartments inside the adsorbers, which also requires the installation of additive check valves between the adsorber, condenser and evaporator.The present study shows that, in fact, this phenomenon does not affect performance. In addition, thanks to the different analysis tools, a physical explanation of this result, based on first and second laws of thermodynamics is developed. This result is then valid for any heat regenerative cycle using an adsorption pair.
Keywords: entropy; heat pump; refrigeration; heat regeneration
Sorption of Water Vapour on Exinite Concentrates by Grażyna Ceglarska-Stefańska; Kinga Brzóska; Janusz Winnicki (pp. 313-319).
Maceral concentrates of the exinite group, isolated from the hard coals from the Janina and Brzeszcze Mines were studied. Sorption studies in the coal-water vapour system and the concentrates maceral-water vapour system were carried out using the volumetric method (under isothermal-isobaric conditions) at 298 K, at the following relative sorbate pressures: 0.19; 0.34; 0.52; 0.70 and 0.88.The presented sorption isotherms show that the exinites isolated from coals have higher water vapour sorption capacities than the coals from which they were isolated. Their sorption capacity is closely related to the accessibility of their porous structure during the interfacial-volumetric process of water vapour interaction in the coal substance.
Keywords: sorption of water vapour; coal; petrographic constitution
Effect of Enzyme Modification of Corn Grits on their Properties as an Adsorbent in a Skarstrom Pressure Swing Cycle Dryer by Kyle E. Beery; Manish Gulati; Eric P. Kvam; Michael R. Ladisch (pp. 321-335).
Corn grits have been tested as a desiccant in a pressure swing adsorption (PSA) system to produce dry air. Two sizes of unmodified corn grits were tested in the PSA system: 2.16 and 0.978 mm in diameter, which dried moist air to dew points of −42°C and −69°C, respectively. A modification technology has been developed for the corn grits that gives an increase in the operational adsorptive capacity in a pressure swing adsorption system, so that they remove as much moisture from air as molecular sieves at the same conditions. After modification, 2.16 mm corn grits dry moist air to a −56°C dew point and the 0.978 mm corn grits dry air to a −80°C dew point. The modification process creates surface modifications on the corn grits apparently making more adsorption sites easily available. The modification procedure increases the specific surface area of the grits and possibly decreases the crystallinity, which would make more hydroxyl groups available for adsorption of water. Possible applications of using corn grits in the pressure swing adsorption system include industrial gas dryers, sorptive cooling air conditioners, and recycling equipment for industrial solvents.
Keywords: materials; synthesis techniques; biochemical; pressure swing adsorption; characterization of properties
Comparison of Finite Difference Techniques for Simulating Pressure Swing Adsorption by Yujun Liu; Javier Delgado; James A. Ritter (pp. 337-344).
Three different finite-difference routines were compared for solving the nonlinear, coupled, partial differential and algebraic equations that describe pressure swing adsorption processes. A successive substitution method (SS), a block LU decomposition procedure (BLUD), and the method of lines approach with adaptive time stepping (DASSL) were used to simulate and compare the computation times required to reach the periodic state for two different PSA systems: PSA-air drying and PSA-solvent vapor recovery. For both systems, the results showed that DASSL was nearly twice as fast as BLUD, whereas SS was nearly an order of magnitude slower than BLUD. DASSL and BLUD were also very robust and accurate, as nearly identical bed profiles were obtained from both methods under both transient and periodic state conditions.
Keywords: pressure swing adsorption; mathematical models; numerical simulation; finite difference; adaptive time stepping
Pressure Transients in Gas Phase Adsorptive Reactors by Bhaskar K. Arumugam; Phillip C. Wankat (pp. 345-354).
The role of pressure and flow transients caused by strong adsorption on the behavior of gas phase adsorptive reactors was studied using a staged model. The general gas phase reaction A + B ⇌ C is considered for two cases: (1) the product C is adsorbed and (2) both reactants A and B are adsorbed. Strong adsorption of one or more components causes a decrease in the pressure(s) in the stage(s). The pressure decrease causes variations in the inlet and outlet flow rates and in the case of multiple stages, it causes variations in the flows between tanks. In accordance with Le Chatelier's principle, the pressure decrease aids or inhibits product formation depending on whether there is an increase or decrease in total moles by reaction. Reactant flow into the section where adsorption occurs increases because of increased pressure drop behind the adsorption front. However, the residence time of the reactants behind the adsorption front is lower because of the higher velocity. The flow variations can aid or hinder product formation depending on the specific conditions. Thus, the adsorption-caused pressure variations directly modify reaction rates and product concentrations and, also indirectly, by causing flow variations which affect reaction rates and product concentrations. This study highlights the need to include pressure variations when modeling gas phase adsorptive reactors if adsorption is strong irrespective of the net change in the total moles by reaction. It also demonstrates a method to incorporate axial pressure drop in staged models.
Keywords: adsorptive reactors; pressure transients
Separation of Bulk Carbon Dioxide-Hydrogen Mixtures by Selective Surface Flow Membrane by M. Paranjape; P.F. Clarke; B.B. Pruden; D.J. Parrillo; C. Thaeron; S. Sircar (pp. 355-360).
The separation performance of carbon dioxide-hydrogen mixtures by a nanoporous carbon membrane called selective surface flow membrane is described. The membrane selectively permeates CO2 from H2 and a H2 enriched gas is produced at the feed gas pressure. Extensive experimental data for the separation using feed gas pressures from 0.24 to 1.13 MPa and CO2 compositions from 5 to 75 (mol%) in H2 are reported. The data can be empirically correlated using a simple equation with a single adjustable-parameter. The adjustable parameter is found to be a linear function of the feed gas CO2 partial pressure.The membrane separates CO2-H2 mixture very efficiently even at a low total feed gas pressure (∼0.4 MPa). The membrane area required for a given separation decreases drastically with increasing feed gas pressure in the range of 0.24–0.92 MPa and then it becomes insensitive to the feed gas pressure.
Keywords: selective surface flow; nanoporous carbon membrane; gas separation; carbon dioxide; hydrogen
Application of Surface Diffusion Model to the Adsorption of Dyes on Bagasse Pith by Gordon McKay (pp. 361-372).
A homogeneous solid phase diffusion model (HSDM) has been developed using a computer to predict the performance of a batch adsorber. The computer program utilises a semi-analytical solution for a two resistance model based on external mass transfer and homogeneous solid phase diffusion. The model has been successfully applied to four adsorption systems, namely, the adsorption of AB25, AR114, BB69 and BR22 onto pith. The method produces excellent correlations between experimental and theoretical concentration decay curves in batch adsorbers. The model developed presents a solution using a single solid diffusion coefficient and a single external mass transfer coefficient which are sufficient to characterise the system within a range of initial dye concentration, 25–300 mg · dm3 and solid/liquid ratios (w/v) 0.25–2.
Keywords: dyestuffs; modeling; HSDM; equilibeium; film diffusion
Displacement Chromatography of Proteins using Low Molecular Weight Anionic Displacers by Amitava Kundu; Suresh Vunnum; Steven M. Cramer (pp. 373-381).
A major impediment to the implementation of displacement chromatography has been the lack of suitable displacer compounds. Recently, it has been shown that low molecular weight dendritic polymers, protected amino acids and antibiotics can be successfully employed for displacement purification in cation-exchange systems. In this paper, a variety of low molecular weight anionic displacers are identified for the resolution of a bovine β-lactoglobulin mixture into two closely related forms (A and B). A Dynamic Affinity plot is employed to evaluate the affinity of these low molecular weight compounds under various displacement conditions. In contrast to large polyelectrolyte displacers, the efficacy of these low molecular weight displacers are shown to be dependent on displacer concentration. In fact, the Dynamic Affinity Plot qualitatively predicts the transition from a displacement to a desorption regime with these low molecular weight displacers. In addition to the fundamental interest generated by low molecular weight displacers, it is likely that these displacers will have significant operational advantages as compared to large polyelectrolyte displacers. Furthermore, the ability to carry out selective displacement chromatography with these low molecular weight displacers offers significant potential for developing robust large scale displacement processes.