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Adsorption: Journal of the International Adsorption Society (v.14, #2-3)

Adsorption hysteresis in ordered mesoporous silicas by Kunimitsu Morishige (pp. 157-163).

We review some recent progress in experimental studies of the adsorption hysteresis of simple molecules in ordered mesoporous silicas. We show that the nature of the adsorption hysteresis due to capillary condensation can be examined with less ambiguity by measuring the hysteresis loop for the ordered mesoporous silicas with three types of pore geometries (cylindrical, interconnected cylindrical, and interconnected spherical) over a wide temperature range. The adsorption hysteresis arises from the metastability of a confined phase and the temperature at which the hysteresis disappears is lower than the critical temperature of vapor-liquid equilibrium in pores. The hysteresis occurs mainly on the desorption rather than adsorption branch, irrespective of the pore geometries.

Keywords: Adsorption hysteresis; Ordered mesoporous materials; Capillary condensation

Molecular simulation of condensation process of Lennard-Jones fluids confined in nanospace with jungle-gym structure by Satoshi Watanabe; Hayato Sugiyama; Minoru Miyahara (pp. 165-170).

We report grand canonical Monte Carlo simulations for a Lennard-Jones (LJ) fluid modeled on methane confined in nanospace with jungle-gym-like (JG) cubic structure, which is typically found in porous coordination polymers. Pillars composing the cubic structure were modeled as structureless smooth solid rods made of LJ carbon. We examined the effects of pore size, pore geometry, rod thickness, and rod potential onto the condensation phenomena in the JG pore structure. The simulations clarified that the condensation pressure and adsorption amount in the JG structure were influenced by pore size and rod potential, while the transition type was determined by rod thickness. The characteristics of the JG structure lie in the sensitivity to the slight changes in pore size, rod thickness, and rod potential owing to the combination of the packing effect of molecules and the superposition effect of rod potentials.

Keywords: Metal organic frameworks; Condensation phenomena; Jungle-gym cubic nanospace; GCMC simulation

Comparative features of ar adsorption on smooth and amorphous surfaces examined by density functional theory by E. A. Ustinov (pp. 171-179).

We analyze argon adsorption isotherms and isosteric heat of adsorption on graphitized and nongraphitized carbon black and silica surfaces by means of nonlocal density functional theory (NLDFT). It is shown that in the case of graphitized carbon black the behavior of the adsorbed phase is nearly identical to that in the bulk phase at a distance larger than about 3-4 molecular diameters from the surface. At a smaller distance argon forms solid-like molecular layers at a temperature at least 3.5 K above the triple point, with the interlayer distance being markedly smaller than the argon collision diameter. In the case of defected or amorphous surfaces adsorbed argon is liquid-like below its triple point. Our extension of the Tarazona NLDFT to amorphous solids (NLDFT-AS) and the Kierlik and Rosinberg version of NLDFT excellently fit argon adsorption isotherms and properly predict the isosteric heat of adsorption. We showed that the surface roughness affects the calculated heat of adsorption, which allowed us to adjust the width of the diffuse zone of the nongraphitized carbon black and the silica surface.

Keywords: Density functional theory; Adsorption isotherm; Heat of adsorption; Graphitized carbon black; Nongraphitized carbon black; Silica; Amorphous surface

Isomerization kinetics of small hydrocarbons in confinement by Erik E. Santiso; Marco Buongiorno Nardelli; Keith E. Gubbins (pp. 181-188).

Chemical reactions are often carried out in nano-structured materials, which can enhance reactions due to their large specific surface area, their interactions with the reacting mixture and confinement effects. In this work, we present a systematic study of the effect that the geometrical restrictions imposed by the pore walls can have on reactions that involve a three dimensional rearrangement of the atoms in a molecule. In particular, we consider the isomerization of three 4-membered hydrocarbons—n-butane, 1-butene and 1,3-butadiene confined in carbon nanopores of slit geometry. Our results illustrate the fact that, in the molecular sieving limit, the reaction rates change as the double exponential of the pore size (Santiso et al., in J. Chem. Phys., 2007a, submitted), and therefore the transition rates in nanopores can be many orders of magnitude different from the corresponding bulk values. These results can be used as a guideline for the molecular-level design of improved catalytic materials.

Keywords: Chemical reactions; Confinement; Carbon; Density functional theory; Variational transition state theory

Modelling of patterns during adsorption of chiral molecules on metallic surfaces by R. O. Uñac; A. M. Vidales; M. V. Gargiulo; J. L. Sales; G. Zgrablich (pp. 189-199).

A gas lattice model and Monte Carlo simulations have been used to model the behavior of chiral molecules adsorbed on clean metallic surfaces. The aim of this work is to characterize how adsorbed molecules organize on the surface, what their footprints are and which are the main mechanisms of interaction responsible for the different patterns observed. These patterns have already been depicted and illustrated by other authors through different techniques and here some of them have been simulated using simple models for adsorption. In these models, both inhibition effects (such as blockage of neighboring sites to simulate steric effects) and promotion of neighboring sites (to simulate adsorbate-adsorbate and adsorbate-substrate interactions) are used. These adsorption rules try to mimic the enantiomeric character of the adsorbed species.

Keywords: Chiral; Enantiomer; Adsorption

Modeling of adsorption in pores with strongly heterogeneous walls: parametric lattice-site wall model by B. Kuchta; L. Firlej; M. Marzec; P. Boulet (pp. 201-205).

We present results of grand canonical Monte Carlo simulations of adsorption in cylindrical pores with rough surface modeled by a parametric lattice-site approach. The sites are randomly distributed over the pore walls. They could be attractive, neutral or repulsive with respect to the smooth pore model. Each site is characterized by two amplitudes (structural and energetic) which modify locally the structure and energetic properties of the surface. The results presented here show how different parameters of the model affect the mechanism of adsorption and, consequently, the form of the isotherm.

Keywords: Pores; Heterogeneous walls; Lattice-site model

Adsorption of carbon dioxide in SAPO STA-7 and AlPO-18: Grand Canonical Monte Carlo simulations and microcalorimetry measurements by Irena Deroche; Lucia Gaberova; Guillaume Maurin; Philip Llewellyn; Maria Castro; Paul Wright (pp. 207-213).

The adsorption properties of carbon dioxide in SAPO STA-7 has been investigated by combining Grand Canonical Monte Carlo simulation and microcalorimetry. The modeling approach, based on both newly derived interatomic potentials for describing the interaction between CO₂ and the Brønsted acid sites, and a realistic description of the silicon distribution within the framework, provides isotherms and evolutions of the differential enthalpy of adsorption as a function of coverage for the STA-7 material characterised by a silicon fraction of 0.19. The simulated results are revealed to be in good agreement with the experimental data that allowed us to provide a possible microscopic mechanism of CO₂ adsorption in this material. These whole results are thus compared to those obtained for the purely aluminophosphate form of the AlPO-18. It is clearly emphasized that distinct adsorption behaviours are observed in these two systems, depending on the energetic characteristics of their surfaces.

Keywords: SAPO; STA-7; AlPO-18; CO₂ ; Adsorption; Grand Canonical Monte Carlo; Microcalorimetry; Isotherms and enthalpies of adsorption

Molecular simulation of adsorption and intrusion in nanopores by Benoit Coasne; Anne Galarneau; Francesco Di Renzo; Roland J. M. Pellenq (pp. 215-221).

This paper reports Monte Carlo simulations of the adsorption or intrusion in cylindrical silica nanopores. All the pores are opened at both ends towards an external bulk reservoir, so that they mimic real materials for which the confined fluid is always in contact with the external phase. This realistic model allows us to discuss the nature of the filling and emptying mechanisms. The adsorption corresponds to the metastable nucleation of the liquid phase, starting from a partially filled pore (a molecular thick film adsorbed at the pore surface). On the other hand, the desorption occurs through the displacement at equilibrium of a gas/liquid hemispherical interface (concave meniscus) along the pore axis. The intrusion of the non-wetting fluid proceeds through the invasion in the pore of the liquid/gas interface (convex meniscus), while the extrusion consists of the nucleation of the gas phase within the pore. In the case of adsorption, our simulation data are used to discuss the validity of the modified Kelvin equation (which is corrected for both the film adsorbed at the pore surface and the curvature effect on the gas/liquid surface tension).

Keywords: Adsorption; Intrusion; Porosimetry; Monte Carlo simulation; Nanoporous material

Nonlinear FR-ZLC method for investigation of adsorption equilibrium and kinetics by Menka Petkovska (pp. 223-239).

A new method for investigation of adsorption equilibrium and kinetics, named Nonlinear Frequency Response-Zero Length Column (NFR-ZLC) method, is introduced. It combines the advantages of the Nonlinear FR method (the potential to identify a model corresponding to the most probable kinetic mechanism and to estimate the equilibrium and kinetic parameters of the identified model) and of the ZLC method (the potential to derive direct information about the processes on the particle level, by eliminating the influence of the adsorber). The frequency response functions of a ZLC system, up to the third order, and for three simple kinetic mechanisms (film resistance control, micropore diffusion control and pore-surface diffusion control) are derived and simulated. The procedure for estimation of the equilibrium and kinetic parameters is defined and illustrated based on numerical simulations.

Keywords: Nonlinear FR; ZLC; Equilibrium; Kinetics; Mechanism identification; Parameter estimation

Adsorption and diffusion of propane and propylene in Ag⁺-impregnated MCM-41 by F. Iucolano; P. Aprea; D. Caputo; C. Colella; M. Eić; Q. Huang (pp. 241-246).

Equilibrium data and diffusion characteristics of propane and propylene were determined on mesoporous adsorbents modified with an organic molecule (APTES) and/or impregnated with AgNO₃, in order to obtain a separation by adsorption via π-complexation.Adsorption capacities were determined by a gravimetric technique, while diffusion characteristics were evaluated by the ZLC technique. The equilibrium isotherms data showed that the modification with an organic molecule will weaken the π-interaction between Ag⁺ and double C=C bond. On the other hand slightly higher adsorption capacities for propylene (about 1.5 mol/kg) were obtained for the sample prepared by a direct impregnation with larger amounts of AgNO₃ (M4 sample).Diffusion runs confirmed that the propane desorption rate on M4 sample was much higher compared to propylene. This evidence leads to a potential application of that adsorbent material for a kinetic separation.

Keywords: Propane; Propylene; MCM-41; Adsorption; Diffusion; Separation

The properties of gold catalysts precursors adsorbed on the MCM-41 materials modified with Mn and Fe oxides by Wojciech Gac; Jacek Goworek; Grzegorz Wójcik; Leszek Kępinski (pp. 247-256).

Silica mesoporous materials modified with manganese and iron were obtained by the hydrothermal method. Gold was introduced to pure and modified silica materials by the direct hydrothermal and impregnation methods. Nitrogen adsorption/desorption studies evidenced formation of the materials with large total surface area and mesoporous structure. Unmodified silica materials showed regular pore arrangement. The uniform porous structure was distorted in the iron or manganese containing samples. XRD, UV-Vis/DRS spectroscopy and temperature programmed reduction studies revealed changes of the nature of transition metal oxide and gold species on the different preparation stages. The oxide species after drying were strongly dispersed and partially incorporated to the silica framework. High temperature treatment led to the formation of extraframework Mn and Fe oxide species. Complex processes of gold deposition were observed during hydrothermal synthesis and impregnation of modified silica materials. The increase of the size of gold species was observed during calcination. The presence of transition metal oxides decreased sintering of gold crystallites.

Keywords: MCM-41; Gold; Manganese; Iron; TEM; XRD; TPR; UV-Vis

Adsorption properties and photocatalytic activity of TiO₂ and La-doped TiO₂ by Minji Jin; Yu Nagaoka; Kazuomi Nishi; Kinuyo Ogawa; Shoji Nagahata; Toshihide Horikawa; Masahiro Katoh; Tahei Tomida; Jun’ichi Hayashi (pp. 257-263).

Photocatalysts of TiO₂ and La-doped TiO₂ were prepared by calcining the pure TiO₂ sols and the sols mixed with La(NO₃)₃⋅6H₂O at 873 K, respectively. These photocatalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and N₂ adsorption-desorption isotherms measurement. As results, the BET surface area, pore diameter, mesopore volume and micropore volume slightly increased, while the crystallite size and the phase structure were little affected by lanthanum doping. The equilibrium adsorption of methylene blue (MB) on the photocatalysts were measured in a dark room. The adsorption isotherms were confirmed to fit to the Langmuir theory. Photocatalytic activities of the photocatalysts were studied by employing the photocatalytic degradation of MB in water and degradation of acetaldehyde in air under UV-irradiation using a black light. Kinetic analysis revealed that the rate controlling steps could be the surface reaction of the adsorbed MB on the catalyst surface for MB degradation and the reaction of adsorbed acetaldehyde with the gaseous acetaldehyde for degradation of acetaldehyde, respectively.

Keywords: Adsorption; Photocatalytic activity-TiO₂ ; La doping; Methylene blue degradation

Hydrogen adsorption in transition metal carbon nano-structures by Yun Xia Yang; Ranjeet K. Singh; Paul A. Webley (pp. 265-274).

Templated microporous carbons were synthesized from metal impregnated zeolite Y templates. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were employed to characterize morphology and structure of the generated carbon materials. The surface area, micro- and meso-pore volumes, as well as the pore size distribution of all the carbon materials were determined by N₂ adsorption at 77 K and correlated to their hydrogen storage capacity. All the hydrogen adsorption isotherms were Type 1 and reversible, indicating physisorption at 77 K. Most templated carbons show good hydrogen storage with the best sample Rh-C having surface area 1817 m²/g and micropore volume 1.04 cm³/g, achieving the highest as 8.8 mmol/g hydrogen storage capacity at 77 K, 1 bar. Comparison between activated carbons and synthesized templated carbons revealed that the hydrogen adsorption in the latter carbon samples occurs mainly by pore filling and smaller pores of sizes around 6 Å to 8 Å are filled initially, followed by larger micropores. Overall, hydrogen adsorption was found to be dependent on the micropore volume as well as the pore-size, larger micropore volumes showing higher hydrogen adsorption capacity.

Keywords: Carbon nanostructures; Hydrogen storage; Adsorption

Hydrothermal stability of pelletized zeolite 13X for energy storage applications by G. Storch; G. Reichenauer; F. Scheffler; A. Hauer (pp. 275-281).

Three samples of pelletized zeolite Na-13X from different industrial suppliers were hydrothermally treated in an open system for up to 3500 adsorption/desorption cycles. Before and after this aging procedure, the samples have been characterized by water uptake measurements, X-ray powder diffraction (XRD), Hg-porosimetry, N₂- and CO₂-adsorption and small-angle X-ray scattering (SAXS). Large differences in the degree of degradation were found between the different materials: The adsorbent with the best performance maintains 79% of its original water uptake capacity after 3500 cycles, whereas this value is reduced to 65% after only 1600 cycles in case of the most unstable sample. For all materials, the residual water adsorption capacity was found to be higher than it was to expect from XRD data. In addition to structural changes of the zeolite cages, Hg-porosimetry and SAXS reveal a modification of the sample morphology in the meso- and macropore range. CO₂ adsorption experiments evidence that as a result of the aging process mass transfer kinetics are slowed down significantly.These findings indicate that the influence of hydrothermal treatment on the water adsorption performance not only depends on the crystal structure of the actual adsorbent, but is indeed a result of a complex interplay with the system of larger pores. The crucial role of the binder material is underlined by the fact that the most stable sample was produced by a so-called binder-free method.

Keywords: Gas phase adsorption; Adsorbents; Energy applications

Potassium hydroxide modified active carbon for adsorptive refrigerators by Bronislaw Buczek; Eliza Wolak (pp. 283-287).

Active carbon-methanol pair appears to be suitable for adsorptive cooling systems. The porous structure of commercial active carbon was modified by potassium hydroxide treatment at 750 °C. Such treatment develops the specific surface area of carbon, changes nature of its surface and significantly increases the heat of wetting. The applied way for active carbon modification gives possibility of modeling porous structure, character surface and heat effects of carbonaceous materials for adsorptive refrigerators.

Keywords: Refrigeration; Active carbon-methanol pair; Heat effects of adsorption

Fundamental studies of gas sorption within mesopores situated amidst an inter-connected, irregular network by Sean P. Rigby; Peter I. Chigada; Emily L. Perkins; Matthew J. Watt-Smith; John P. Lowe; Karen J. Edler (pp. 289-307).

There has been little, or no, direct testing of theories of gas sorption within particular pores situated amidst a highly inter-connected pore network. The concept of thermodynamically independent pores within networks has also been challenged. In this work, a novel integrated nitrogen sorption and mercury porosimetry technique has been used to deconvolve the condensation and evaporation processes within a specific subset of pores contained within a larger, irregular network. The sizes and geometry of these pores were obtained completely independently of gas sorption, using mercury porosimetry and NMR cryoporometry, respectively. Hence, various theories of capillary condensation, such as the Kelvin equation, the Broeckhoff-de Boer method, Saam-Cole theory, and NLDFT could be directly tested, and the potential influence of any collective network phenomena detected. It was found that, even for a shielded pore, the Cohan equation for a cylindrical meniscus gave rise to the best prediction for the relative pressure of capillary condensation, once the effects of surface chemical heterogeneity on multi-layer build-up had been taken into account. The results were also found to be incompatible with the presence of particular collective adsorption effects, such as advanced condensation.

Keywords: Nitrogen adsorption; Capillary condensation; Independent pore; Network effects; NMR cryoporometry

Propylene/propane mixture adsorption on faujasite sorbents by Arjen van Miltenburg; Jorge Gascon; Weidong Zhu; Freek Kapteijn; Jacob A. Moulijn (pp. 309-321).

The adsorption of propylene and propane on zeolite NaX with and without a saturated (36 wt%) amount of CuCl have been investigated. The single component adsorption isotherms could be well described with a Dual-Site Langmuir model. The dispersion of CuCl results in a decrease of the maximum adsorption capacity of the zeolite for both components. For propylene a strong adsorption via π-complexation with CuCl is present, increasing the adsorption selectivity of the zeolite. The binary mixture (50:50) adsorption was determined via breakthrough/desorption experiments at 318, 358 and 408 K with a partial pressure of the two components between 0.8–54 kPa. For NaX the mixture loading could be well described with the IAS-theory and the single component isotherms, both qualitatively and quantitatively. A transition from an enthalpy controlled adsorption at lower loadings to an entropy affected adsorption at higher loadings was observed. The IAS-theory could only qualitatively describe the trends in the observed mixture adsorption for the CuCl/NaX adsorbent. The dispersion of CuCl in NaX results in a modest improvement of the adsorption selectivity for propylene over propane (from 3–7 to 15–30) but at the expense of a reduced capacity.

Keywords: Binary adsorption; Breakthrough; Faujasite; Propylene; Propane; Selectivity

Prediction of single component adsorption isotherms on microporous adsorbents by Ugur Akgün; Alfons Mersmann (pp. 323-333).

The adsorption of gases on microporous solids is a fundamental physical interaction which occurs in many technical processes, e.g. the heterogeneous catalysis or the purification of gases. In this context the adsorption equilibrium can determine the velocity and/or the capacity of the process. Therefore, it has to be known for designing purposes. The aim of this work has been the a priori prediction of the adsorption equilibria of arbitrary gases on microporous solids like zeolites and active carbon based only on the molecular properties of the adsorptive and the adsorbent. The adsorption isotherm is described completely from the Henry region over the transition zone to the saturation region. The quality of the model permits a first approximation of the planned process without further experimental effort.

Keywords: Henry coefficient; Zeolite; Active carbon; Hamaker constant; Interaction energy; Refractive index; Adsorption isotherm; Microporous adsorbents

The characterisation and evaluation of activated carbon in a cigarette filter by Michele Mola; Matthew Hallum; Peter Branton (pp. 335-341).

Cigarette smoke is an ever changing and extremely complex mixture of over 5000 chemicals. When the cigarette burns, thousands of chemical substances are generated, and these are distributed between the gas phase and the particles which constitute the smoke aerosol.Activated carbon when used in a filter can selectively remove a number of the vapour phase compounds to varying degrees of efficiency.Carbons of different activities (50–60% CTC and 90–100% CTC) have been characterised using nitrogen adsorption and also the sorption of a number of different vapours with different properties, using a dynamic gravimetric adsorption technique.Surface areas, pore volumes, diffusivities and heats of adsorption were calculated using nitrogen, benzene, heptane, ethyl acetate and water as the probe molecules.Smoke chemistries were measured using a screening approach on the characterised carbons as filter additives. It was seen that the higher activity carbon results in increased retention in the majority of the measured vapour phase smoke constituents.

Keywords: Activated carbon; Adsorption; Isotherms; Smoke

Naphthalene adsorption on activated carbons using solvents of different polarity by C. O. Ania; B. Cabal; J. B. Parra; A. Arenillas; B. Arias; J. J. Pis (pp. 343-355).

The hydrophobic-hydrophilic character of a series of microporous activated carbons was explored as a key factor in competitive adsorption of a non-polar compound from liquid phase. The selectivity of the carbon surface towards naphthalene was explored by performing the adsorption isotherms in water, cyclohexane and heptane. Solvent polarity and adsorbent hydrophobic character were found to strongly influence the adsorption capacity of naphthalene. In aqueous media, despite the non-polar character of the adsorbate, surface acidity lowered adsorption capacity. This is attributed to the competition of water from the adsorption sites, via H-bonding with surface functionalities and the formation of hydration clusters that reduce the accessibility and affinity of naphthalene to the inner pore structure. In organic media the uptake decreased due to competition of the hydrophobic solvent for the active sites of the carbon and to solvation effects. This competitive effect of the solvent is minimized in oxidized carbons as opposed to the trend obtained in aqueous solutions. The results confirmed that although adsorption of naphthalene strongly depends on the narrow microporosity of the adsorbent, competitive adsorption of the solvent for the active sites becomes important.

Keywords: Hydrophobicity; Surface polarity; Organic solvents; Naphthalene; Adsorption

Sorption kinetics for the removal of aldehydes from aqueous streams with extractant impregnated resins by Katarina Babić; Louis G. J. van der Ham; Andre B. de Haan (pp. 357-366).

The sorption kinetics for the removal aldehydes from aqueous solutions with Amberlite XAD-16 and MPP particles impregnated with Primene JM-T was investigated. A model, accounting for the simultaneous mass transfer and chemical reaction, is developed to describe the process. It is based on the analogy to the diffusion and reaction in a stagnant liquid sphere, but corrected for the porosity and particle properties influencing the diffusion. The developed model describes the kinetic behavior of the process in the low concentration region rather well. However, in the high concentration region, larger discrepancies are observed. Initially, the influence of the flow rate was investigated to eliminate the effect of the external mass transfer. The influence of the particle morphology was investigated for both physical and reactive sorption. Physical sorption experiments were used to determine the factor τ that takes the particle properties influencing the diffusion into account. It was shown that the diffusion is faster in XAD-16 than in MPP impregnated systems. Reaction rate constant k x was determined by fitting the model to the experimental data. Sorption of benzaldehyde appears to be significantly slower (k x∼10⁻⁴ l/mol s) than the sorption of pentanal (k x∼10⁻³ l/mol s) due to the slower chemical reaction. The influence of the particle size was investigated for the sorption of pentanal with XAD-16. It was observed that the particle size does influence the diffusion term, but does not have an effect on the reaction rate. On the other hand, the extractant loading influences the reaction rate slightly in the low concentration region, whereas the initial concentration of the solute has more pronounced effect.

Keywords: Extractant impregnated resins; Kinetics; Zero length column; Aldehydes; Amines

Adsorption of toluene, methylcyclohexane and neopentane on silica MCM-41 by Patrícia A. Russo; M. Manuela L. Ribeiro Carrott; Peter J. M. Carrott (pp. 367-375).

Adsorption-desorption isotherms of toluene, methylcyclohexane and neopentane were determined on a silica MCM-41 material of pore diameter ∼3.4 nm over the temperature range 258 K to 308 K (278 K for neopentane). The isosteric enthalpies of adsorption were determined from the isotherms at the various temperatures. It was found that the isotherms of toluene and methylcyclohexane have a similar variation with the temperature, exhibiting hysteresis at 268 K and at lower temperature, while the adsorption of neopentane is reversible at all temperatures. The three organic adsorptives interact differently with the silica surface and the isosteric enthalpies of adsorption indicated that methylcyclohexane has the weakest interaction and toluene the strongest. A slight increase in the adsorption enthalpy at the beginning of the capillary condensation step is observed with methylcyclohexane and neopentane but not with toluene.

Keywords: Hydrocarbon vapour adsorption; Model adsorbents; Hysteresis; Criticality; Isosteric enthalpies of adsorption

Adsorption of small uremic toxin molecules on MFI type zeolites from aqueous solution by D. Bergé-Lefranc; H. Pizzala; J. L. Paillaud; O. Schäf; C. Vagner; P. Boulet; B. Kuchta; R. Denoyel (pp. 377-387).

Adsorption properties of zeolites were investigated for the removal of p-cresol from aqueous solutions at 37 °C within the context of studying alternative methods to dialysis for removing uremic toxin from blood. MFI-framework type zeolites with different degrees of hydrophobicity and charge compensating cations were prepared: one pure silica MFI and four alumino-silicate MFIs (Si/Al = 30), with H⁺, Na⁺, K⁺ and Mg²⁺ as charge compensating cations. Adsorption isotherms and microcalorimetric measurements show a high affinity of p-cresol for all MFI type zeolites. The best capacity is obtained for the pure silica MFI, whereas the alumino-silicate samples show a higher affinity in the low concentration range. In the case of pure silica sample, the microscopic adsorption mechanism including the role of confined water is elucidated with the help of NMR, X-ray analysis (including Rietveld refinement) and Monte Carlo simulations. For all samples the high affinity is preserved in physiological serum solution, even in the presence of other toxin molecules such as urea. It is also shown that the compensating cation state of the samples is imposed by the physiological medium.

Keywords: Bioadsorption; Uremic toxins; p-Cresol; Zeolites; Monte-Carlo simulations

Adsorption and biotransformation of 17β-estradiol in biological activated carbon adsorbers by Fusheng Li; Akira Yuasa; Hidenori Tanaka; Yoshihiro Katamine (pp. 389-398).

As the first systematic study dealing with the adsorption of estrogens by granular activated carbon (GAC), the removal behavior of 17β-estradiol (E2) and its biotransformation product of estrone (E1) in fixed GAC columns was examined using four biological activated carbon (BAC) columns (BAC-1∼BAC-4) generated by coating four GAC columns with detached microorganisms from the riverbed sediment of a representative drinking river water source containing lower content of natural organic matter (NOM). For comparison, parallel adsorption experiments were also performed using another four GAC columns (GAC-1∼ GAC-4) packed by strictly following the configurations of four BAC columns. Adsorption experimental results obtained by intermittently spiking E2 over a total running period about 350 days into the river water mixed with or without a peaty water containing higher content of NOM showed that E2 was readily removed by adsorption and the combined adsorption/biodegradation. The vertical profiles of E2 and E1, which have great significance for better understanding and optimization of the adsorption process for removal of human estrogens, were also obtained.

Keywords: Estrogens; Hormones; NOM; Adsorption; Biodegradation

Heavy reflux PSA cycles for CO₂ recovery from flue gas: Part I. Performance evaluation by Steven P. Reynolds; Amal Mehrotra; Armin D. Ebner; James A. Ritter (pp. 399-413).

This study evaluated nine stripping PSA cycle configurations, all with a heavy reflux (HR) step, some with a light reflux (LR) step, and some with a recovery (REC) or feed plus recycle (F+R) step, for concentrating CO₂ from stack and flue gas at high temperature (575 K) using a K-promoted HTlc. Under the process conditions studied, the addition of the LR step always resulted in a better process performance; and in all cases, the addition of a REC or F+R step surprisingly did not affect the process performance except at low feed throughputs, where either cycle step resulted in a similar diminished performance. The best cycle based on overall performance was a 5-bed 5-step stripping PSA cycle with LR and HR from countercurrent depressurization (CnD) (98.7% CO₂ purity, 98.7% CO₂ recovery and 5.8 L STP/hr/kg feed throughput). The next best cycle was a 5-bed 5-step stripping PSA cycle with LR and HR from LR purge (96.5% CO₂ purity, 71.1% CO₂ recovery and 57.6 L STP/hr/kg feed throughput). These improved performances were caused mainly by the use of a very small purge to feed ratio (γ=0.02) for the former cycle and a larger one (γ=0.50) for the latter cycle. The former cycle was good for producing CO₂ at high purities and recoveries but at lower feed throughputs, and the latter cycle was useful for obtaining CO₂ at high purities and feed throughputs but at lower recoveries. The best performance of a 4-bed 4-step stripping PSA cycle with HR from CnD was disappointing because of low CO₂ recoveries (99.2% CO₂ purity, 15.2% CO₂ recovery and 72.0 L STP/hr/kg feed throughput). This last result revealed that the recoveries of this cycle would always be much lower than the corresponding cycles with a LR step, no matter the process conditions, and that the LR step was very important to the performance of these HR cycles for this application and process conditions studied.

Keywords: Pressure swing adsorption; Dual reflux; Stripping PSA; Rinse step; Recovery step; Feed plus recycle step; Heavy reflux; Light reflux; Carbon dioxide capture; Global warming

Capture of CO₂ from high humidity flue gas by vacuum swing adsorption with zeolite 13X by Gang Li; Penny Xiao; Paul Webley; Jun Zhang; Ranjeet Singh; Marc Marshall (pp. 415-422).

Capture of CO₂ from flue gas streams using adsorption processes must deal with the prospect of high humidity streams containing bulk CO₂ as well as other impurities such as SO x, NO x, etc. Most studies to date have ignored this aspect of CO₂ capture. In this study, we have experimentally examined the capture of CO₂ from a 12% synthetic flue gas stream at a relative humidity of 95% at 30 °C. A 13X adsorbent was used and the migration of the water and its subsequent impact on capture performance was evaluated. Binary breakthrough of CO₂/water vapor was performed and indicated a significant effect of water on CO₂ adsorption capacity, as expected. Cyclic experiments indicate that the water zone migrates a quarter of the way into the column and stabilizes its position so that CO₂ capture is still possible although decreased. The formation of a water zone creates a “cold spot” which has implications for the system performance. The recovery of CO₂ dropped from 78.5% to 60% when moving from dry to wet flue gas while the productivity dropped by 22%. Although the concentration of water leaving the bed under vacuum was 27%(vol), the low vacuum pressure prevented condensation of water in this stream. However, the vacuum pump acted as a condenser and separator to remove bulk water. An important consequence of the presence of a water zone was to elevate the vacuum level thereby reducing CO₂ working capacity. Thus although there is a detrimental effect of water on CO₂ capture, long term recovery of CO₂ is still possible in a single VSA process. Pre-drying of the flue gas steam is not required. However, careful consideration of the impact of water and accommodation thereof must be made particularly when the feed stream temperature increases resulting in higher feed water concentration.

Keywords: CO₂ capture; Pressure swing adsorption; Humid flue gas

Optimizing control of simulated moving bed separations of mixtures subject to the generalized Langmuir isotherm by Cristian Grossmann; Mohammad Amanullah; Manfred Morari; Marco Mazzotti; Massimo Morbidelli (pp. 423-432).

Simulated moving bed (SMB) is a cost-efficient separation technique that offers high productivity and low solvent consumption. SMB has gained importance in the pharmaceutical and fine chemical industry to perform complex separation tasks. However, an open and challenging problem is the optimal, robust operation of the SMB process. We have developed a control scheme that integrates the optimization and control of the SMB unit. A significant feature of the controller is that only minimal information of the system has to be provided, i.e. the linear adsorption behavior of the mixture to be separated and the average void fraction of the columns. Therefore, a full characterization of the adsorption behavior of the mixture and the columns is no longer required. In this ‘cycle to cycle’ control scheme, the measurements, optimization and control actions are performed once in every cycle. This paper presents simulation results of the control scheme applied to the separation of binary mixtures characterized by generalized Langmuir isotherms. The results are presented and analyzed in the frame of the triangle theory that has been recently extended to encompass these types of isotherms. Besides, online optimum performance of the SMB unit is compared with off-line optimization carried out using genetic algorithm. The results show that the controller fulfills the product and process specifications while operating the SMB unit optimally, regardless of the different types of Langmuir isotherms that the systems exhibit.

Keywords: Multicolumn processes; Preparative chromatography; Liquid phase adsorption

Comparison of configurations of a four-column simulated moving bed process by multi-objective optimization by Yoshiaki Kawajiri; Lorenz T. Biegler (pp. 433-442).

Configurations of a four-column simulated moving bed chromatographic process are investigated by multi-objective optimization. Various existing column configurations are compared through a multi-objective optimization problem. Furthermore, an approach based on an SMB superstructure is applied to find novel configurations which have been found to outperform the standard SMB configuration. An efficient numerical optimization technique is applied to the mathematical model of the SMB process. It has been confirmed that although the optimal configuration highly depends on the purity requirement, the superstructure approach is able to find the most efficient configuration without exploring various existing configurations.

Keywords: Chromatography; Simulated moving bed; Optimization; Nonlinear programming; Superstructure

Effective intraparticle diffusion coefficients of CoCl₂ in mesoporous functionalized silica adsorbents by Tanja Djekic (pp. 443-443).

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Adsorption: Journal of the International Adsorption Society (v.14, #2-3)

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Adsorption: Journal of the International Adsorption Society (v.14, #2-3)