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Adsorption: Journal of the International Adsorption Society (v.14, #4-5)

Foreword by Marco Mazzotti (pp. 445-445).

Water vapour adsorption and contrast-modified SAXS in microporous polymer-based carbons of different surface chemistry by Krisztina László; Cyrille Rochas; Erik Geissler (pp. 447-455).

The adsorption of water vapour on highly microporous activated carbons with different surface chemistry is investigated by small angle X-ray scattering (SAXS) as well as by adsorption isotherms. The water changes the intensity of the SAXS in a way that depends on how the pores are filled. With wetting liquids such as hexane, a pseudo binary model can be assumed in which pore-filling in reciprocal space q is described by a density function p(q). For water, clusters develop, even in the most oxidized carbon, creating a fully ternary system. In the Porod scattering region, however, the final slope is insensitive to the liquid-vapour interfaces. In this region, for the less oxidized samples, p(q) shows reasonable agreement with the adsorption isotherms. At low relative pressure P/P ₀, however, the SAXS results indicate a small degree of filling (about 10%) that is not reflected in the isotherms. The highly oxidized sample attains a degree of filling of about 70% that, unlike the corresponding isotherm, is constant for P/P ₀>0. These differences may be due to kinetic effects and/or ageing, involving either redistribution of the water molecules or modifications of the surface groups.

Keywords: Activated carbon; Surface treatment; Small angle X-ray scattering; Interfacial properties

Ellipsometry porosimetry (EP): thin film porosimetry by coupling an adsorption setting with an optical measurement, highlights on additional adsorption results by A. Bourgeois; Y. Turcant; Ch. Walsh; Ch. Defranoux (pp. 457-465).

Ellipsometry porosimetry (EP) is an emerging technique that is well adapted to porous thin films analysis; it is non contact and non destructive. EP tools developed at SOPRA, allows us to obtain adsorption isotherms with many different adsorptives at an ambient temperature. EP leads to the same results as classical adsorption experiments (e.g. porosity, pore size distribution …), but it also has some particular features leading to new information. For instance, our optical setup (Spectroscopic Ellipsometry) allows us to determine the variation of the thickness of the samples during the adsorption experiment. It is also very sensitive to interfaces; it is thus possible to detect a porosity gradient or to study a bi-layer sample and plot the two corresponding adsorption isotherms at the same time (Bourgeois et al. 2004). For porous thin films with a non porous barrier layer deposited on top, it is also possible to study the lateral diffusion phenomenon in the film (see figure below). In this paper, we will demonstrate a part of the different features of EP for adsorption on porous thin films.

Keywords: Gas phase adsorption; Diffusion

Energy-saving drying technology for porous media using liquefied DME gas by Hideki Kanda; Hisao Makino; Minoru Miyahara (pp. 467-473).

Process design and energy requirement for a practical plant are investigated for an energy-saving drying (dewatering) process invented by the authors in 2002 for high-moisture porous materials. The basic concept of the process involves the extraction of water from a high-moisture porous material by bringing it in physical contact with liquefied dimethyl ether (DME) at room temperature. Water content of DME asymptotically increases to the saturation value and the high-moisture porous material is dried almost perfectly. DME from the DME-water mixture is vaporized by decompression. DME and water are separated by flash distillation. DME vapor is compressed and cooled in a heat exchanger, and the latent heat of condensation is reused to vaporize the DME in the heat exchanger. Multistage compression and multistage flash distillation are employed. After compression, the temperature of DME is less than 50 °C. Because specific heat ratio of DME is only 1.11, the energy consumption of the compressor is reduced. Considering the adiabatic efficiency of the compressor and the net thermal efficiency, the total energy for dewatering is about 1100 kJ per 1-kg-water-content of the material being dewatered This process has significant potential and is compact than the existing dewatering processes.

Keywords: High moisture coal; Dewatering; Dimethyl ether; Process design; Energy requirement

Co-diffusion with a slow species in zeolites: cyclic experimentation and advanced modeling by K. Lettat; E. Jolimaitre; M. Tayakout; A. Methivier; D. Tondeur (pp. 475-484).

In this study, a new experimental method based on cyclic breakthrough curves is presented, in order to estimate the co-diffusion coefficients for mixtures at high adsorption loadings. For this purpose, cyclic liquid phase breakthrough curves of mixtures of 2-methylpentane 3-methylpentane (fast-diffusing species) and 2,2-dimethylbutane (slow-diffusing species) have been measured experimentally for different feed compositions at 185°C.Estimation of Langmuir coefficients and self-diffusivities was attempted from simple binary breakthrough curves with the above components using a modified Maxwell-Stefan-type model. However, for the slow-diffusing species, the parameters cannot be estimated accurately from such experiments, because the quantity of 22DMB entering the zeolite network in the experiment duration is not sufficient.On the other hand, a clear influence of the slow diffusing species (22DMB) on the fast diffusing species (3MP) breakthrough curves during cycles has been demonstrated. This phenomenon confirms that 22DMB slowly accumulates in the adsorbent during the cycles, and that is becomes therefore possible to estimate the 22DMB parameters from the cyclic data.

Keywords: Liquid-phase diffusion; Experimental study; Dusty gas model; Maxwell-Stefan equations; Silicalite; Paraffins

Liquid-liquid phase separation of binary Lennard-Jones fluid in slit nanopores by Hideki Kanda; Hisao Makino (pp. 485-491).

The capillary phase separation of a binary mixture of two truncated and shifted Lennard-Jones (LJ) Ar liquids in slit-shaped oxygen nanopores is examined. The LJ parameters—ε(Ar(A)–Ar(A))=ε(Ar(B)–Ar(B))=0.8ε(Ar(A)–Ar(B)) and 0.5ε(Ar(A)–O) = ε(Ar(B)–O)—were used to distinguish the two Ar liquids. The cut off distance for Ar was 3.5σ. We employed a molecular dynamics (MD) technique in which a pore space was connected with a bulk solution to easily determine the equilibrium bulk concentration. Liquid phase isotherms were obtained for pores with widths ranging from 5.5σ to 9.5σ, and the relation between the pore width and the phase separation concentration was determined. Each simulation was run until the bulk concentration attained equilibrium (1–2 μs). The MD results show that the Patrick model overestimates the bulk concentration for a given pore size. We proposed a modified Patrick model in which the pore wall potential is considered. In our model, the Gibbs-Tolman-Koenig-Buff effect is not considered for the interfacial tension since two surfaces of tension exist on both sides of the equimolar dividing surface of the two-Ar liquid phase. The two surfaces of tension neutralized Gibbs-Tolman-Koenig-Buff effect each other. The present simple model successfully describes the relation to prove its reliability.

Keywords: Liquid-liquid equilibria; Nanopore; Thermodynamic model; Lennard-Jones; Molecular simulation

Development of a new apparatus for gas mixture adsorption measurements coupling gravimetric and chromatographic techniques by Lomig Hamon; Marc Frère; Guy De Weireld (pp. 493-499).

An experimental apparatus which allows to measure adsorption isotherms for binary, ternary or quaternary mixtures for pressures between vacuum and 5.0 MPa and temperatures between 263 K and 373 K is presented. This system couples gravimetric and chromatographic techniques. The use of a mass spectrometer as gas analyzer allows to investigate a large variety of gases (such as carbon dioxide, hydrogen sulfide, mercaptans). In this paper, we measure the binary adsorption of hydrogen sulfide in a methane matrix on a commercial activated carbon at 1.0 MPa and at 298 K. Molar ratio in hydrogen sulfide is between 10 mol. ppm and 3 mol.%. Experimental results are then compared to simulated ones. The model which is tested is the classical Ideal Adsorbed Solution theory. This simulation step requires the pure gas equilibrium data obtained and fitted with the Langmuir’s law, which are also presented here.

Keywords: Fundamentals of adsorption; Gas phase adsorption; Bio, energy and environmental applications

Adsorption equilibria of binary ethylmercaptan/hydrocarbon mixtures on a NaX zeolite by J. P. Bellat; F. Benoit; G. Weber; C. Paulin; P. Mougin; M. Thomas (pp. 501-507).

We present here coadsorption data of ethylmercaptan with n-heptane or toluene on NaX at 298 K in the aim to use this adsorbent for removing mercaptans from natural gas. Results show that NaX has an adsorption affinity for sulphur compound strong enough to perform a deep desulphurization. NaX adsorbs preferentially ethylmercaptan over a large domain of pore filling. A displacement of n-heptane by ethylmercaptan is even observed. However, toluene becomes preferentially adsorbed at high filling.The dependence of filling and composition on selectivities and failed prediction of coadsorption equilibria by the IAS Theory indicate that the adsorbed mixture behaves as a non-ideal solution. Calorimetric measurements of coadsorption heats show that the coadsorption process is not governed by enthalpic effects but by entropic effects consecutive to steric hindrance of molecules in confined spaces.

Keywords: Coadsorption; Ethylmercaptan; n-heptane; Toluene; NaX; Calorimetry

Adsorption of volatile hydrocarbons on natural zeolite-clay material by Irina Breus; Alexandra Denisova; Sergey Nekljudov; Vladimir Breus (pp. 509-523).

This study investigated the vapor-phase sorption of hydrocarbons (HC) of various chemical nature (n-hexane, iso-octane, benzene, toluene and p-xylene) on zeolite-clay material (ZCM) of Tatarsky-Shatrashansky deposit (Tatarstan, Russia). For comparison, we also studied HC sorption on natural high-grade zeolite (Z) of St. Cloud Mine (New Mexico, USA), two local bentonite clays and synthetic zeolite-molecular sieve Ms5A. As a result, sorption capacity of ZCM towards aliphatic and aromatic HC was significantly higher than the sorption capacity of Z but lower than that of clays. In addition, the data showed that HC sorption on ZCM occurs as surface adsorption. The comparison with Ms5A and erionite ZAPS, a natural Mexican zeolite, revealed that both ZCM and Z do not interact as microporous selective sorbents with linear alkanes. The main differences in HC sorption on ZCM and Z were ZCM higher sorption capacity towards both aliphatic and aromatic HC, as well as its lower sorption sensitivity to the type of HC. We found that the reason for these differences is the presence of large amount of clay minerals in ZCM. To summarize, ZCM can be considered as a suitable adsorbent for protection and remediation of HC-contaminated soils.

Keywords: Hydrocarbons; Vapor-phase sorption; Zeolite-containing materials

Charge and discharge of methane on phenol-based carbon monolith by M. S. Balathanigaimani; Min-Joo Lee; Wang-Geun Shim; Jae-Wook Lee; Hee Moon (pp. 525-532).

Methane adsorptions on various synthesized and commercial activated carbons were assessed in a volumetric apparatus for the design of an efficient adsorbed natural gas storage system. Based on the methane adsorption equilibrium results from different carbon based materials, a monolith was also produced from RP-20. Dynamic studies were also performed for the prepared monolith and the pelletized commercial Norit-B4 activated carbon. The temperature variation in RP-20 monolith was analyzed and compared with those of Norit-B4 and a blank test, which consisted of a run without a sample. The temperature variation in RP-20 monolith was quite high compared to that observed with Norit-B4 and the blank test because of a higher isosteric heat of adsorption and a high packing density.

Keywords: Activated carbons; Methane adsorption; Monolith; Dynamic studies

Adsorption dynamics of hydrogen sulfide in impregnated activated carbon bed by Do-Young Choi; Jae-Wook Lee; Seong-Cheol Jang; Byoung-Sung Ahn; Dae-Ki Choi (pp. 533-538).

Potassium iodide (KI) impregnated activated carbons were prepared and applied for the removal of hydrogen sulfide. The adsorption dynamics of the prepared adsorbents were investigated in fixed-bed column as functions of the concentration of hydrogen sulfide and oxygen, and relative humidity. It was found that the adsorption capacity was highly dependent on the oxygen concentration because of the chemical adsorption of hydrogen sulfide on KI impregnated activated carbon. The adsorbents before and after adsorption of hydrogen sulfide were characterized by BET, SEM and EDS analysis.

Keywords: Adsorption; Hydrogen sulfide; Impregnation; Humidity; Activated carbon

Competitive adsorption equilibria of CO₂ and CH₄ on a dry coal by Stefan Ottiger; Ronny Pini; Giuseppe Storti; Marco Mazzotti (pp. 539-556).

Gases like CO₂ and CH₄ are able to adsorb on the coal surface, but also to dissolve into its structure causing the coal to swell. In this work, the binary adsorption of CO₂ and CH₄ on a dry coal (Sulcis Coal Province, Italy) and its swelling behavior are investigated. The competitive adsorption measurements are performed at 45 °C and up to 190 bar for pure CO₂, CH₄ and four mixtures of molar feed compositions of 20.0, 40.0, 60.0 and 80.0% CO₂ using a gravimetric-chromatographic technique. The results show that carbon dioxide adsorbs more favorably than methane leading to an enrichment of the fluid phase in CH₄. Coal swelling is determined using a high-pressure view cell, by exposing a coal disc to CO₂, CH₄ and He at 45 and 60 °C and up to 140 bar. For CO₂ and CH₄ a maximum swelling of about 4 and 2% is found, whereas He shows negligible swelling. The presented adsorption and swelling data are then discussed in terms of fundamental, thermodynamic aspects of adsorption and properties which are crucial for an ECBM operation, i.e. the CO₂ storage capacity and the dynamics of the replacement of CH₄ by CO₂.

Keywords: Supercritical adsorption; Multicomponent; Carbon dioxide; Methane; Coal; Swelling

H₂ storage in carbon materials by L. Zubizarreta; E. I. Gomez; A. Arenillas; C. O. Ania; J. B. Parra; J. J. Pis (pp. 557-566).

In this work a series of commercial carbons with different structural and textural properties were characterised and evaluated for their application in hydrogen storage. The results showed that temperature has a greater influence on the storage capacity of carbons than pressure. The highest H₂ storage capacity at 298 K and 90 bar was 0.5 wt%, while at 77 K and atmospheric pressure it was 2.9 wt%. It is also showed that, in order to predict the hydrogen storage capacity of carbon material both at cryogenic and ambient temperature, the only use of BET surface area or total micropore volume obtained from N₂ adsorption isotherm may be insufficient, the characterization of the narrow microporosity is needed due to its high contribution to hydrogen adsorption capacity. The process involved in hydrogen storage in pure carbon materials seems to be physisorption. Morphological or structural characteristics have no influence, at least on gravimetric storage capacity.

Keywords: Adsorbents; Carbon materials; Energy; Environmental systems

Optimisation-based simulation of a pressure swing adsorption process by M. A. Latifi; D. Salhi; D. Tondeur (pp. 567-573).

In this paper, an optimisation-based approach is developed for the determination of the cyclic steady-sate (CSS) of a pressure swing adsorption process (PSA). It consists in treating the simulation problem as a single dynamic optimisation problem where the performance index is the CSS condition, the decision variables are the state variables at the start of the cycle and the constraints are given by the process model equations with associated initial conditions. The resulting optimisation problem is solved using a gradient-based non linear programming (NLP) method, e.g. SQP method, where the gradients are computed by means of four different methods: finite differences, numerical and analytical sensitivities and adjoint system methods.

Keywords: PSA process; Cyclic steady-state; Simulation; Optimisation

Capture of CO₂ from flue gas streams with zeolite 13X by vacuum-pressure swing adsorption by Penny Xiao; Jun Zhang; Paul Webley; Gang Li; Ranjeet Singh; Richard Todd (pp. 575-582).

Vacuum swing adsorption (VSA) capture of CO₂ from flue gas streams is a promising technology for greenhouse gas mitigation. In this study we use a detailed, validated numerical model of the CO2VSA process to study the effect of a range of operating and design parameters on the system performance. The adsorbent used is 13X and a feed stream of 12% CO₂ and dry air is used to mimic flue gas. Feed pressures of 1.2 bar are used to minimize flue gas compression. A 9-step cycle with two equalisations and a 12-step cycle including product purge were both used to understand the impact of several cycle changes on performance. The ultimate vacuum level used is one of the most important parameters in dictating CO₂ purity, recovery and power consumption. For vacuum levels of 4 kPa and lower, CO₂ purities of >90% are achievable with a recovery of greater than 70%. Both purity and recovery drop quickly as the vacuum level is raised to 10 kPa. Total power consumption decreases as the vacuum pressure is raised, as expected, but the recovery decreases even quicker leading to a net increase in the specific power. The specific power appears to minimize at a vacuum pressure of approximately 4 kPa for the operating conditions used in our study. In addition to the ultimate vacuum level, vacuum time and feed time are found to impact the results for differing reasons. Longer evacuation times (to the same pressure level) imply lower flow rates and less pressure drop providing improved performance. Longer feed times led to partial breakthrough of the CO₂ front and reduced recovery but improved purity. The starting pressure of evacuation (which is not necessarily equal to the feed pressure) was also found to be important since the gas phase was enriched in CO₂ prior to removal by vacuum leading to improved CO₂ purity. A 12-step cycle including product purge was able to produce high purity CO₂ (>95%) with minimal impact on recovery. Finally, it was found that for 13X, the optimal feed temperature was around 67°C to maximize system purity. This is a consequence of the temperature dependence of the working selectivity and working capacity of 13X. In summary, our numerical model indicates that there is considerable scope for improvement and use of the VSA process for CO₂ capture from flue gas streams.

Keywords: Vacuum swing adsorption; CO₂ capture; Simulation

Hydrogen separation by multi-bed pressure swing adsorption of synthesis gas by Se-Il Yang; Do-Young Choi; Seong-Cheol Jang; Sung-Hyun Kim; Dae-Ki Choi (pp. 583-590).

The performance of multi-bed pressure swing adsorption (PSA) process for producing high purity hydrogen from synthesis gas was studied experimentally and theoretically using layered beds of activated carbon and zeolite 5A. Nonisothermal and nonadiabatic models, considering linear driving force model and Dual-site Langmuir adsorption isotherm model, were used. The effects of the following PSA variables on separation process were investigated: linear velocity of feed, adsorption time and purge gas quantity. As a result, we recovered a high purity H₂ product (99.999%) with a recovery of 66% from synthesis gas when the pressure was cycled between 1 and 8 atm at ambient temperature.

Keywords: Hydrogen separation; Pressure swing adsorption; Synthesis gas

Uptake of lead by carbonaceous adsorbents developed from tire rubber by M. Alexandre-Franco; C. Fernández-González; A. Macías-García; V. Gómez-Serrano (pp. 591-600).

The adsorption of lead in aqueous solution by discarded tire rubber (TR) and by carbonaceous adsorbents (CAs) prepared from it by thermal and chemical treatments is studied. TR was heated at 400 or 900 °C for 2 h in N₂ atmosphere or treated chemically with H₂SO₄, HNO₃, H₂SO₄/HNO₃, HCl, NaOH, HCl-NaOH or NaOH-HCl for 24 h. TR and CAs were characterized physico-chemically with regard to their texture, elemental composition and surface chemistry. The adsorption of lead was studied, mainly from the kinetic standpoint, using 4×10⁻³ mol L⁻¹ Pb²⁺ solutions at initial pH 2.0, 5.7 and 12.6. TR is practically a non-porous material. The heat treatment of TR mainly develops meso- and macroporosity. The effect on macroporosity is stronger when HNO₃ and H₂SO₄/HNO₃ are used. For all adsorbents, the adsorption of lead is not measurable when adsorptive solution at pH 2 is used. With an increase of pH from 5.7 to 12.6, the kinetics of adsorption becomes faster and the amount of lead adsorbed significantly increases. For the solution at pH 12.6 and an adsorption time of 8 h, the adsorption percentage is 36 wt% for TR. For the CAs prepared by heat treatment of TR, it is higher than 50 wt%. For longer adsorption times, the kinetics is much slower for the product of TR treatment with HNO_3. However, the corresponding adsorption percentage is as high as 93 wt% for an adsorption time of 264 h. TR and the CAs, in particular the product heated at 400 °C, are good adsorbents to be used in the rapid removal of a significant amount of lead from aqueous solution.

Keywords: Tire rubber; Carbonaceous adsorbents; Lead adsorption

Adsorption of mercury from single and multicomponent metal systems on activated carbon developed from cherry stones by M. Olivares-Marín; C. Fernández-González; A. Macías-García; V. Gómez-Serrano (pp. 601-610).

The adsorption of mercury from a single/multi-solute aqueous solution by activated carbon (AC) prepared from cherry stones (CS) by chemical activation with H₃PO₄, ZnCl₂ or KOH is studied. Three series of AC (i.e., P, H₃PO₄; Z, ZnCl₂; K, KOH) were prepared by controlling the impregnation ratio and carbonization temperature. The textural characterization of AC was carried out by gas adsorption, mercury porosimetry and density measurements. The surface chemistry was analyzed by the pH of the point of zero charge (pH_zpc), FT-IR spectroscopy and Boehm’s method. Experiments of mercury adsorption were conducted by the batch method, using aqueous solutions of mercury and of mercury, cadmium and zinc without pH adjustment. The ACs possess a wide range of pore volumes and sizes. Their microporosity is usually well developed. The meso- and macropore volumes are higher for the P carbons and K carbons, respectively. BET surface areas as a rule range between 1000 and 2000 m² g⁻¹. The pH_zpc is much lower for the P carbons. The content of acidic oxygen surface groups is lower for the K carbons, whereas the content of basic groups is higher for these carbons. The kinetics of the adsorption process of mercury is faster for ACs with high volumes of large size pores. However, the surface groups have a marked unfavorable influence on the kinetics. The pseudo-second order rate constant (k₂×10⁻³, g/mol h) is higher by the order Z-4-800 (67.69)>K-3-800 (43.45)>P-3.44-400 (36.98). The incorporation of zinc and cadmium to the mercury solution usually decelerates the adsorption process for the P carbons and Z carbons and accelerates it for the K carbons. The amount adsorbed of mercury is much larger for the K carbons than for the other ACs. For the Z carbons, competition effects of zinc and cadmium on the adsorption of mercury are negligible, which indicates that mercury adsorbs specifically on surface active sites of these adsorbents.

Keywords: Cherry stones; Activated carbon; Mercury adsorption

Separation characteristics of some phenoxy herbicides from aqueous solution by Tae-Young Kim; Seung-Sik Park; Seung-Jai Kim; Sung-Yong Cho (pp. 611-619).

The adsorption and desorption characteristics of some phenoxy herbicides (CPA 2,4-D, and MCPA) from an aqueous solution on the active carbon materials (GAC, F-400) were studied. Adsorption equilibrium capacities of the phenoxy herbicides increased with a decrease in pH of the solution. Adsorption equilibrium isotherms were represented by the Sips equation. Kinetic parameters were measured in a batch adsorber to analyze the adsorption rates of the phenoxy herbicides. The internal diffusion coefficients were determined by comparing the experimental concentration curves with those predicted from the surface diffusion model and the pore diffusion model. The adsorption model based on the linear driving force approximation (LDFA) was used to simulate the adsorption behavior of the phenoxy herbicides in a fixed bed adsorber. Over 95 percent desorption of the phenoxy herbicides was obtained using distilled water.

Keywords: Adsorption; Desorption; CPA; 2,4-D; MCPA; Activated carbon

Temperature-swing adsorption of aromatic compounds in water using polyampholyte gel by Shintaro Morisada; Hiroko Suzuki; Saki Emura; Yoshitsugu Hirokawa; Yoshio Nakano (pp. 621-628).

The adsorption property of the polyampholyte gel composed of sodium styrene sulfate (SSS) and vinylbenzyl trimethylammonium chloride (VBTA) has been investigated with several hydrophobic aromatic compounds as adsorbate. Using the N-isopropylacrylamide (NIPA) gel, the corresponding experiments were also performed for comparison. At room temperature, the NIPA gel hardly adsorbed the aromatic compounds, while it adsorbed them at higher temperatures. As for the SSS-VBTA gel, the adsorption amounts of the polyaromatic compounds decreased with increasing temperature, while the adsorption amounts of the monoaromatic compounds were almost independent of temperature and smaller than those of the polyaromatic compounds. These results indicate that the aromatic rings in the SSS-VBTA gel may play an important role in the adsorption of the aromatic compounds. Also, it has been demonstrated that the SSS-VBTA gel can repeatedly adsorb bisphenol-A at room temperature and desorb it at higher temperature by the temperature-swing operation: this behavior is diametrically opposite to that of the NIPA gel. This shows that the SSS-VBTA gel is much more suitable for the adsorption removal of the hydrophobic aromatic compounds from very dilute aqueous solutions, because a vast amount of energy is required for heating a large amount of water when using the NIPA gel.

Keywords: Polyampholyte gel; Temperature-swing adsorption; Aromatic compound; Hydrophobic interaction

Theoretical description of aggregation of cationic gemini surfactants in the bulk solution and on the silica surface by Mateusz Drach; Anna Andrzejewska; Jolanta Ciesla; Jolanta Narkiewicz-Michałek (pp. 629-638).

A theory of cationic dimeric (gemini) surfactant adsorption onto negatively charged surface is presented. In the proposed model it is assumed that the adsorbed phase is a mixture of singly dispersed molecules of surfactant and spherical, globular and cylindrical aggregates of different dimensions. Only the “excluded area” interactions between the adsorbed species are considered and the effects of surface heterogeneity on monomer adsorption are taken into account. The aggregation behavior of gemini surfactants is based on the additive free energy model proposed by Camesano and Nagarajan (2000). The calculated surfactant adsorption isotherms and the differential molar enthalpies of micellisation and adsorption are compared with the experimental results obtained for a series of gemini surfactants depending on the length of a spacer, temperature or the presence of electrolyte. On the basis of theoretical results the evolution of adsorbed phase of gemini surfactants with the increasing adsorption is discussed. It is shown that the evaluated cmc values and the dimensions of surfactant aggregates are in a good agreement with experiment. Unfortunately, the theoretical model does not describe properly the temperature dependence of micellisation process.

Keywords: Gemini surfactants; Surfactants adsorption; Surface aggregation

Measurement of adsorption of a single component from the liquid phase: modelling investigation and sensitivity analysis by M. J. Heslop; C. J. Schaschke; J. Sefcik; D. J. Richardson; P. A. Russell (pp. 639-651).

In this work, we consider an alternative approach for the measurement of adsorption from the liquid phase. Consider a mixture consisting of a non-adsorbed component (B) and an adsorbed component (A) present at some low concentration. Initially, a feed of component B only flows through a column packed with an adsorbent. Then, the feed is switched to the mixture of A and B. As soon as the mixture enters the column, there will be a reduction in the outlet flow rate as component A leaves the liquid phase and passes into the adsorbed phase. There are three stages to this work. The first is to develop overall and component balances to show how the amount adsorbed of component A can be determined from the variation in the column outlet flow rate. The second is to determine the actual variation in the column outlet flow rate for both plug flow and axial-dispersed plug flow. The final stage is to consider the suitability of a gravity-fed system to deliver the feed to the column. An analysis of the results shows that the experimental arrangement should be able to accurately monitor adsorption from the liquid phase where the mass fraction of the solute is of the order of 1%: the limiting experimental factor is how constant the volumetric flow rate of the liquid feed can be maintained.

Keywords: Adsorption; Ethanol; Liquid; Flow measurement; Theory

New experimental method for the determination of single-component isotherms: an application of the flow-rate retention time by M. J. Heslop; G. Mason; B. A. Buffham; A. McDonald; R. Low (pp. 653-663).

In this paper, we investigate adsorption of a single component (refrigerant) from the variation in the flow rate leaving the column. This requires evaluation of the flow-rate retention time—a measure of the net change in the amount adsorbed in the column, which can be positive or negative. The arrangement includes a system to deliver a fixed flow (30 mL/min) of helium through a column packed with 0.05 g of adsorbent. An experiment is initiated by adding a flow of refrigerant to the helium, and monitoring the outlet flow rate from the column. There are two main advantages of this approach: the experimental times are short (the order of 10 minutes) and the sensitivity is very high so that it can be used with very small as well as large amounts of adsorbent. Indeed, a sensitivity analysis suggests that the resolution is of the order of 10⁻⁵ g. The first results section considers the corrections required to the measured flow-rate retention time—these are small and determined by empty volume in the system. The second results section involves a determination of the adsorption isotherm of HFC-134A on an activated carbon at 35 °C up to a partial pressure of 0.25 bar.

Keywords: Adsorption; Activated carbon; HFC-134A; Isotherm; Flow rate

Standing wave optimization of SMB using a hybrid simulated annealing and genetic algorithm (SAGA) by Fattaneh G. Cauley; Stephen F. Cauley; Nien-Hwa Linda Wang (pp. 665-678).

In this paper we draw on two stochastic optimization techniques, Simulated Annealing and Genetic Algorithm (SAGA), to create a hybrid to determine the optimal design of nonlinear Simulated Moving Bed (SMB) systems. A mathematical programming model based on the Standing Wave Design (SWD) offers a significant advantage in optimizing SMB systems. SAGA builds upon the strength of SA and GA to optimize the 16 variables of the mixed-integer nonlinear programming model for single- and multi-objective optimizations. The SAGA procedure is shown to be robust with computational time in minutes for single-objective optimization and in a few hours for a multi-objective optimization, which is comprised of more than one hundred points.

Keywords: Simulated moving bed; Standing wave design; Simulated annealing; Genetic algorithm; Multi-objective

Statistical quantification of the influence of material properties on the oxidation and ignition of activated carbons by Thangavelu Jayabalan; Pascaline Pré; Valérie Héquet; Pierre Le Cloirec (pp. 679-686).

The influence of material properties on the reactivities of activated carbon materials have been studied on a laboratory scale. Carbon samples having diversified origin and properties were characterized using a thermogravimetry (TG) coupled with a differential scanning calorimetry (DSC). Reactivity parameters like the Point of Initial Oxidation (PIO) representing the beginning of the oxidation reactions and the Spontaneous Ignition Temperature (SIT) where the bed combustion takes place in a self sustaining manner were experimentally determined. The intrinsic properties of the activated carbons influencing oxidation and ignition were examined qualitatively followed by quantitative statistical correlations. Results from both qualitative and statistical correlations showed that increase in the oxygen content in the form of surface oxygenated groups increased the reactivity of activated carbons. It was by far the single most influential property discriminated from the analysis. The porosity characteristics like the specific surface area and pore volume did show some vague trends but could not be validated like that of the oxygen content. The effects of these individual properties on the oxidation and ignition reactivity are discussed.

Keywords: Adsorbents; Activated carbons; Thermal analysis

Optimization of synthesis procedures for structured PSA adsorbents by Alessandra Mosca; Jonas Hedlund; Firas N. Ridha; Paul Webley (pp. 687-693).

Structured adsorbents in the form of supported thin zeolite films may represent a competitive alternative to traditional zeolite adsorbents in form of beads or pellets used in PSA processes, due to the reduction of mass- and heat-transfer limitations typical of packed beds. Thin NaX films were grown by hydrothermal treatment using a clear solution on cordierite monoliths. Films grown by a multiple synthesis procedure were dense and uniform with a very small amount of sediments adjacent to the film, which may be an advantage in PSA applications. The CO₂ adsorption capacity and the pressure drop for the supported films were compared to those of a packed NaX bed. Although the adsorption capacity of the column filled with the structured adsorbents was 67 times lower than when the column was filled with zeolite beads, the pressure drop was 100 times lower for the structured adsorbent. The adsorption capacity can be increased by increasing the film thickness or the cell density of the monoliths without increasing the pressure drop significantly, indicating the potential advantage of structured adsorbents in PSA processes. Further investigations are needed in order to prove this hypothesis.

Keywords: Zeolite NaX; Traditional adsorbents; Structured adsorbents; PSA; Adsorption; Pressure drop

Structural and energetic properties of carbosils hydrothermally treated in the classical autoclave or the microwave reactor by J. Skubiszewska-Zięba (pp. 695-709).

Carbon/silica adsorbents (carbosils) prepared by pyrolysis of CH₂Cl₂ at 823 K and the reaction time from 0.5 to 6 h on the mesoporous silica gel surface (Si-60, Merck, granule size 0.2–0.5 mm) and then hydrothermally treated at 473 K with steam or liquid water by using the classical autoclave with traditional heating way or in the microwave reactor were studied by means of TG, adsorption and Q-TG methods. Changes in the structural characteristics of hybrid adsorbents before and after hydrothermal treatment were analyzed on the basis of nitrogen adsorption. Thermal properties of initial and modified samples as well as concentration of carbon deposit were studied using thermogravimetry (TG) in the range of 293–1273 K. The adsorbed water layers were investigated by means of thermodesorption of water under the quasi-isothermal conditions (Q-TG) in the range of 293–573 K. Concentration of weakly and strongly adsorbed water and the surface free energy on the interphase of adsorbent/water were calculated. It was stated that hydrothermal treatment in the microwave reactor, contrary to that in the classical autoclave, allows to obtain adsorbents with noticeably higher values of total pore volume in relation to the initial adsorbents and in majority with a higher specific surface area. Application of microwave energy allows to obtain adsorbents with lower values of surface free energy in relation to the initial adsorbents and those modified in the autoclave.

Keywords: Carbon-silica adsorbents; Hydrothermal treatment; Porous structure; Quasi-isothermal method; Adsorbed water layers; Gibbs free energy; Free surface energy

Textural and surface chemistry of activated bagasse and its role in the removal of chromium ions from solution by M. Valix; W. H. Cheung; K. Zhang (pp. 711-718).

The role of the surface chemical and physical properties of activated carbon in the removal of chromium was investigated. This was conducted by fitting the chromium removal by adsorption and reduction to Cr(III) to the physical properties including total surface and pore size of the carbon and its chemical property globally measured using carbon pH. The role of heteroatoms—sulfur, nitrogen, hydrogen and oxygen, to chromium removal was also investigated. This study showed that the structural and chemical properties displayed dual and conflicting properties in removing chromium. As such efficiencies gained in controlling the structure of the carbon are minimal. Optimal carbon properties which exhibited high chromium adsorption included high surface area, large pore size, high quantities of sulfur and nitrogen and minimal hydrogen and oxygen contents.

Keywords: Chromium; Activated carbon; Heteroatoms; Textural properties

Influence of quantum effects on the mechanism of adsorption and phase diagram of rare gases in carbon nanotubes by L. Firlej; B. Kuchta (pp. 719-726).

We present results of grand canonical Monte Carlo simulations of rare gases (He, Ne, Ar, Kr and Xe) adsorption in carbon nanotubes. The interaction model includes both quantum effects (via effective Feymann-Hibbs potential) and the atomic roughness of the tube. We show that the quantum contribution to interactions does not suppress the energetic corrugation of carbon nanotube but decreases only its average strength. In the case of Ne, the phase diagram and, in particular, the melting temperature for layers adsorbed on and within an individual tube does not depend on tube chirality. However, the structure of layers adsorbed on outer surface of the tube is strongly related to the atomic structure of the underlying tube.

Keywords: Molecular modeling; Adsorption mechanism; Quantum effects; Surface heterogeneity; Phase diagram

Direct calculations of the dispersion interaction between fullerenes and their equation for the potential energy by Tim S. Jakubov; David E. Mainwaring (pp. 727-732).

A discrete summation method has been developed to calculate the dispersion interaction potential between two C₆₀ fullerene molecules and applied to the fullerene–argon and fullerene–helium systems. The pair-wise atom-atom potentials for carbon–carbon, carbon–argon and carbon–helium interactions were described by the Lennard-Jones (6-12) expression and considered to be additive. Increased accuracy of these calculations was achieved by accounting for the radius and positions of each carbon atom in fullerene by taking into consideration the irregularity of hexagons in the fullerene structure.As we understand, a discrete summation method has not been employed for the calculation of the C₆₀–C₆₀ interaction. Additionally, the method provides a mechanism to calculate the interaction potential for different directions of approach and orientations of fullerene molecules. These differences reach up to 6% at the minimum energy position when comparing approaching hexagons compared to pentagons.The description of the calculated interaction potential energy of molecular C₆₀–C₆₀ fullerene that we propose is a physically grounded equation, which has provided excellent fit with the dispersion interaction data. We show that this equation is less complex in structure, while providing a higher accuracy (1% or less inaccuracy) than, for instance Girifalco’s (1992) equation, which gives inaccuracy in the range 4–5% when employed on our data. The proposed equation was further modified to provide a successful analytical description of the argon-fullerene interaction energy and an estimate of the resultant Henry constant and isosteric heat of adsorption.Finally these equations were used to calculate the variation of these Henry constants with temperature for the helium–fullerene system indicating that at room temperature the excess adsorption is negative and that the resulting error when used as a volumetric calibrant, although not unduly significant, is not negligible.

Keywords: Fundamentals of adsorption; Molecular modelling; Gas phase adsorption

Molecular simulations of water in hydrophobic microporous solids by Roland J.-M. Pellenq; Thomas Roussel; Joël Puibasset (pp. 733-742).

This work reports Grand Canonical Monte-Carlo molecular simulation (GCMC) results of water adsorption in a priori hydrophobic microporous solids such as silicalite, a purely siliceous zeolite (Ø_pore∼5 Å) and C-Y, a pure carbon replica of zeolite Y (Ø_pore∼1 nm). At a first step, in both cases, the water-water interactions are described with the SPC model (calibrated for bulk liquid water) while water-substrate interactions are calculated within the framework of the PN-TrAZ model. This adsorbate-zeolite potential decomposes into short range (repulsive, inductive and dispersive) interaction terms with transferable parameters plus, in the case of silicalite, an electrostatic interaction term based on SPC partial charges for water and ab initio charges for silicalite. With such a standard approach, we found that water fills the microporous volume in both materials at pressure value well below P ₀; hence does not show a strong hydrophobic behaviour at variance with reference experiments (V. Eroshenko et al. in C. R. Phys. 3:111, 2002). This indicates that common models used to describe confined polar molecules are far from being operative. We show on the basis of periodic ab initio calculations that confined water molecules in silicate have a dipole value ∼10% smaller than that in the 3D liquid phase indicating that the environment felt by a confined water molecule in silicalite pores is not equivalent to that in the bulk liquid. This implies that classical simulations of polar molecules in ultra confining environment should rely on polarizable potentials (K.S. Smirnov, D. Bougeard in Chem. Phys. 292:53, 2003) if one wishes to capture the underlying physics. Reducing the SPC water dipole moment by 5% in GCMC calculations does allow reproducing experimental data.

Keywords: Grand canonical Monte-Carlo; Ab initio periodic calculations; Water; Adsorption; Silicalite; Zeolite carbon replica

A molecular simulation study of the distribution of cation in zeolites by C. Abrioux; B. Coasne; G. Maurin; F. Henn; A. Boutin; A. Di Lella; C. Nieto-Draghi; A. H. Fuchs (pp. 743-754).

NVT Monte Carlo simulations are first used to describe the distribution of Na cations in Faujasite for several Si/Al ratios. These calculations were performed by combining two different sets of potential parameters combined with both T-atoms and explicit Si,Al models. Grand Canonical Monte Carlo simulations are then employed to investigate the influence of water adsorption on the distribution of cations in the case of a Faujasite sample with 56 cations (NaY56). These simulations data are compared to available experimental data and the influence of the choice of the forcefield for describing the cation/zeolite interaction on these results is discussed.

Keywords: Faujasite; Distribution of cations; Water adsorption; Grand canonical Monte Carlo simulation; Interatomic potential

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Adsorption: Journal of the International Adsorption Society (v.14, #4-5)

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Adsorption: Journal of the International Adsorption Society (v.14, #4-5)