Adsorption Of N-butane Research Articles

Simple group contribution theory for adsorption of alkanes in nanoporous carbons

This paper develops a simplified form of the group-contribution theory of Russell and LeVan (Chem. Eng. Sci. 51 (1996) 4025) for the prediction of adsorption equilibria of n-alkanes on activated carbon. Molecules are separated into elements or groups, and adsorbate group-adsorbent interactions are the determining factor for adsorption equilibrium. The adsorbent surface is considered to be heterogeneous with a single dimensionless energy distribution. Model parameters are determined for pure-component adsorption of methane, ethane, n-butane, and n-hexane on BPL activated carbon at multiple temperatures. The resulting parameters are then used to predict adsorption equilibrium of propane and n-octane at multiple temperatures and mixtures of methane and n-hexane and ethane and n-hexane. The model gives a more accurate prediction of mixture adsorption than the ideal adsorbed solution theory. The model is also used to describe methane, propane, and n-pentane adsorption on BAX activated carbon (Ind. Eng. Chem. Res. 40 (2001) 338) and methane, ethane, and propane on Columbia Grade L carbon (Ind. Eng. Chem. 42 (1950) 1315) at multiple temperatures. The model is shown to describe adsorption equilibria of alkanes on all three activated carbons with good accuracy and can also be used as a predictive tool for components on which no experimental data are available.

Kinetics of Adsorption of n-butane on an Aggregate of Silicalite by Transient Non-equilibrium Molecular Dynamics

Here we present the results obtained by using a transient non-equilibrium molecular dynamics procedure (TNEMD) which allows us to study the kinetics of gas adsorption at different gas pressures. Our simulations were focused on the adsorption of n-butane on aggregates of silicalite at 300 K. These simulations give relevant information on both the equilibrium state and the kinetics of adsorption. The isotherms obtained are in agreement with previous experimental and simulated results. At the equilibrium state our simulations allowed us to identify five energetic sites on the aggregate: three inside the micropores (straight and zig-zag channels and their intersections) and two additional ones at the external surface. Moreover, our simulations show, as determined exprimentally, that the n-butane is adsorbed on the external sites of the zeolite only at high filling (high loadings) of the zeolite—i.e. close to the saturation. From the simulated kinetics we calculated the Fick's diffusion coefficients at different loadings.

Adsorption and breakthrough performance of carbon-coated ceramic monoliths at low concentration of n-butane

Carbon-coated ceramic monoliths have been studied for the dynamic adsorption of low-concentration n-butane. They exhibit a very sharp breakthrough performance, especially in the low-concentration regime, that illustrates the better breakthrough performance of monoliths with respect to shallow carbon packed beds. The low pressure drop of the monolithic system and its excellent performance under discontinuous flow conditions make it an attractive option for gas mask canister applications. A simulation model is presented, including terms of adsorption, diffusion and mass transfer. A parametric analysis has been carried out to study the influence of different variables on the breakthrough profile. The breakthrough performance is best described by considering a gas velocity distribution over the monolith cross-section. Stacking of the monolithic pieces, increasing cell density, and gas redistribution between pieces reduced this distribution and improved the breakthrough performance.

Co-adsorption of n-butane/water vapour mixtures on activated carbon fibre-based monoliths

Water– n-butane co-adsorption experiments on Activated Carbon Fibre based Monoliths (ACFMs) under equilibrium and dynamic conditions were performed at 30 °C. ACFMs proved to be better adsorbents than active carbon particles for the dry adsorption of n-butane at low concentration levels. The presence of water vapour in the gas stream has a negligible influence on n-butane adsorption for values of relative humidity (RH) equal to or below 25%. Higher water pressures cause a significant loss of adsorption capacity in ACFMs. Equilibrium water adsorption on ACFMs can be described by the Do and Do model [Carbon 38 (2000) 767] modified in order to account for both the variable size of the water aggregates on acid centres and the variable size of the clusters detached from the aggregates and inserted into the micropores. The model results show close agreement between the amounts of active sites for adsorption calculated by the model and the number expected from subjecting the ACFMs to different pre-treatments. The equilibrium of co-adsorption of water and n-butane can be described by a pore filling concept according to which the volume occupied by both gases adsorbed together from the gas mixture is approximately equal to the volume occupied by the pure component adsorbed separately to a greater extent. Pre-moistening of the monolith has little effect on its effective n-butane adsorption capacity (breakthrough time) during dynamic adsorption experiments. For values of relative humidity equal to or below 25%, the effective n-butane adsorption capacity of the monoliths was better than that of active carbon powder. Moreover, the monoliths produced a much lower pressure drop in the system. At these conditions, adsorption efficiency for the ACFMs never showed values below 85%.

A Study of Permeation of n-Butane through ZSM-5 Membrane by Using Monte Carlo and Equilibrium/Non-Equilibrium Molecular Dynamics Simulations.

Molecular simulations have been performed on permeation and adsorption of n-butane in a ZSM-5 zeolite membrane. In all simulations, a flexible potential model is used for n-butane since the smallest size of the permeating molecule is almost the same as the pore size of the ZSM-5 crystal. Equilibrium densities of n-butane in the ZSM-5 membrane calculated from the MC method are in good agreement with experimental adsorption data, being represented by the Langmuir adsorption isotherm model. Permeability of n-butane calculated from the μVT-NEMD method has a maximum against temperature, which agrees qualitatively with the experimental results. It is noted that the whole pores in the membrane are filled with molecules at room temperature. As compared with the local values of the Fick and the Maxwell–Stefan diffusion coefficients (DF, DMS) and the self-diffusion coefficients in the permeate direction (Ds, x), we confirm that the DMS is almost independent of molecular loadings in the membrane, and that DF > DMS > Ds, x at high loadings. In addition, the three diffusion coefficients are almost identical at the zero loading. By providing a modification for the distribution coefficient (S) and diffusion coefficient (D) in the case of nonlinear isotherms, we have clearly demonstrated that the permeation at low temperatures is controlled by the diffusion process, where whole pores are filled with molecules, and then the controlling step changes to the adsorption process at higher temperature where the pores are characterized by medium loadings.

An improvement of thermal conduction of activated carbon by adding graphite

This study was performed to improve thermal conduction of granular activated carbon by adding graphite powder. Granular activated carbons were fabricated from anthracites and graphite powders using a binder pitch, and then activated in a nitrogen-carbon dioxide gas at 850°C. Additions of graphite from 20 to 30 wt% were considered to be suitable for thermal conductivity and n-butane adsorption power. The thermal conductivities of such granular activated carbons were 20 times those of activated carbons without graphite. A fixed bed filled with the granular or powder activated carbons containing graphite were never ignited even though the inside of the bed was directly heated.

An Extension of the Consistent Valence Force Field (CVFF) with the Aim to Simulate the Structures of Vanadium Phosphorus Oxides and the Adsorption of n-Butane and of 1-Butene on their Crystal Planes

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Surface diffusion and adsorption of hydrocarbons in activated carbon

AbstractAdsorption and diffusion in a porous media were studied theoretically and experimentally with a differential transient permeation method. The porous medium is allowed to equilibrate at some specified loading, and then the time trajectory of the permeation process is followed after a small difference between the pressures at the end faces of the porous medium is introduced at time t = 0 +. Such a trajectory vs. time would contain adsorption and diffusion characteristics of the system. By studying this for various surface loadings, pore and surface diffusions can be fully characterized. Mathematical modeling of transient permeation is detailed for pure gases or vapors diffusion and adsorption in porous media. Effects of nonlinearity of adsorption isotherm, pressure, temperature and heat effects were considered in the model. Experimental data of diffusion and adsorption of propane, n‐butane and n‐hexane in activated carbon at different temperatures and loadings show the potential of this method as a useful tool to study adsorption kinetics in porous media. Validity of the model is best tested against the transient data where the kinetics curves exhibit sigmoidal shape, which is a result of the diffusion and adsorption rate during the initial stage of permeation.

Diffusion of binary mixtures across zeolite membranes: entropy effects on permeation selectivity

Diffusion of binary mixtures across zeolite membranes is strongly influenced by the sorption behaviour of the individual components. When the components in the mixture have significantly different molecular sizes, size entropy effects tend to favour the sorption of the smaller species at high molecular loadings. This is the case, for example, for adsorption of methane and n-butane in silicalite. For mixtures of linear and branched alkanes having the same number of carbon atoms, configurational entropy effects tend to favour the sorption of the linear alkane becuase these molecules “pack” more efficiently inside the ordered zeolite structure. In estimating the permeation of mixtures across zeolite membranes, both size and configurational entropy effects need to be properly accounted for. A model to estimate the permeation fluxes across zeolite membranes is developed by combining the Real Adsorbed Solution Theory to describe the mixture sorption, and the Maxwell-Stefan diffusion equations. The utility of the model is demonstrated by means of two case studies: (1) permeation of methane and n-butane, and (2) permeation of n-hexane and 2,2 dimethylbutane across a silicalite membrane.

Molecular adsorption and growth of n-butane adlayers on Pt(1 1 1)

The molecular adsorption of n-butane and the growth of n-butane adlayers on Pt(1 1 1) was investigated using molecular beam techniques, temperature-programmed desorption (TPD) and low-energy electron diffraction (LEED). It is found that as the surface coverage of n-butane increases, structural changes occur in the adlayer at surface temperatures near 98 K that are accompanied by changes in the binding energy and mobility of the adsorbed species. The film growth process can be divided into four distinct coverage regimes. At low coverages ( θ<0.14 ML, where 1 ML is defined as one butane molecule per Pt atom) a disordered monolayer forms in which the butane molecules prefer to lie parallel to the surface in order to minimize their binding energy. At coverages from 0.14 to 0.20 ML, ordered regions develop within the monolayer in which the butane molecules also lie parallel to the surface. The binding energy in the ordered phase is lower than that in the disordered phase due to repulsive intermolecular interactions. A more densely-packed ordered phase begins to form at 98 K after the low-coverage ordered phase saturates at 0.20 ML. The experimental results suggest that the n-butane molecules tilt away from the surface in the high-coverage ordered phase. Finally, a disordered second layer phase forms after the high coverage ordered phase saturates at 0.35 ML. The molecules in the second layer are very mobile at 98 K and rapidly diffuse to the edges of the beam spot. Interchange of molecules between the second layer and ordered monolayer is found to govern the net rate of second layer diffusion at surface temperatures less than 133 K. The adsorption probability of n-butane on Pt( 1 1 1 ) continuously increases with increasing coverage, with no significant dependencies on the structure of the n-butane adlayer. This finding indicates that the long-range arrangements and molecular orientations of a mobile alkane adlayer have a negligible influence on the intrinsic adsorption dynamics, suggesting that the energy transfer processes that facilitate adsorption are highly localized.

N-Butane isomerization on sulfated zirconia: active site heterogeneity and deactivation

The fast deactivation of sulfated zirconia (SZ) has limited its use in commercial processes such as n-butane isomerization. In order to investigate this deactivation, steady-state isotopic transient kinetic analysis (SSITKA) was utilized to study in situ changes in surface kinetic parameters for n-butane isomerization on a widely studied SZ at 150°C. Approximately 20% of the sulfate species was found to be n-butane adsorption sites, but only 1–2% of the sulfate species appeared to adsorb active surface reaction intermediates. The decrease in catalytic activity during deactivation could be attributed to the loss of active sites. The change in TOF ITK ∗ (TOF based on an average residence time of active surface intermediates) and the regeneration characteristics of the SZ catalyst suggest a possible active site heterogeneity. It appears that the high initial activity and the fast deactivation for TOS≤100 min were mainly due to the presence and deactivation of the more active sites, respectively. Following the loss of the more active sites, the less active sites provided the majority of the catalytic activity observed for TOS≥100 min . The less active sites appeared to be more easily regenerated than the more active sites as the catalytic activity at TOS≥100 min was recovered following regeneration at 315°C. Loss of active sites due to sulfur loss or migration seems unlikely. Site blockage by coke/oligomer formation appeared to be a significant contributor for catalyst deactivation for n-butane isomerization on SZ. The impact of sulfur reduction on catalyst deactivation cannot be ruled out at this point.

Separation of butane and xylene isomers with MFI-type zeolitic membrane synthesized by a vapor-phase transport method

MFI-type zeolitic membranes were prepared by a vapor-phase transport (VPT) method on porous α-alumina flat disks. Single- and mixed-gas permeation measurements of butane isomers were performed in the temperature range of 300–375 K. The separation factor was always greater than the ideal selectivity. This result is explained by the preferential adsorption of n-butane on MFI in the binary system. The pervaporation tests for xylene isomers were performed at 303 K. p-Xylene was the most permeable component in the unary system. In the permeation measurements of a binary mixture of p-xylene/ m-xylene and a ternary mixture of p-xylene/ m-xylene/ o-xylene, p-xylene predominantly permeated in the early stage, and then, the flux of p-xylene decreased gradually and finally became lower than that of the other xylene isomers. The adsorption of m-xylene in the pores of MFI seemed to block the permeation of p-xylene.

Comparative study of vanadium aluminophosphate molecular sieves VAPO-5, -11, -17 and -31

Vanadium aluminophosphate molecular sieves VAPO-5, -11, -17 and -31 were synthesized hydrothermally and fully characterized using XRD, A.A, SEM, N 2 and n-butane adsorption, diffuse reflectance UV-Vis, XPS, 27 Al , 31 P , 51 V NMR and ESR techniques. XRD data of VAPOs were consistent with their corresponding AlPOs structural patterns and indicated that all VAPOs were very well crystallized. The 51 V NMR spectra of calcined air-exposed (hydrated) VAPOs samples revealed that vanadium cations are present as V 5+ both in distorted octahedral and tetrahedral coordination and quite probably in a framework position. DRS UV-Visible spectroscopy shows that in low vanadium content calcined samples, V 5+ ions are mostly in tetrahedral coordination without any presence of polymeric extraframework oxy-vanadium phase. ESR and XPS spectroscopies indicated the presence of residual V 4+ in calcined samples. VAPOs were found to be active in vapor phase oxidation of toluene with molecular oxygen. VAPO-11 and -31 showed high oxidation catalytic activity and selectivity for benzaldehyde. The order of activity were VAPO-11 > VAPO-31 > VAPO-5 ≥ VAPO-17. VAPO-11 better selectivity is related to its unique structural properties compared to those of the other VAPOs synthesized in this study.

A Study of the Kinetics and Mechanism of the Adsorption and Anaerobic Partial Oxidation of n-Butane over a Vanadyl Pyrophosphate Catalyst

The interaction of n-butane with a ((VO)2P2O7) catalyst has been investigated by temperature-programmed desorption and anaerobic temperature-programmed reaction. n-Butane has been shown to adsorb on the (VO)2P2O7 to as a butyl–hydroxyl pair. When adsorption is carried out at 223 K, upon temperature programming some of the butyl–hydroxyl species recombine resulting in butane desorption at 260 K. However, when adsorption is carried out at 423 K, the hydroxyl species of the butyl–hydroxyl pair migrate away from the butyl species during the adsorption, forming water which is detected in the gas phase. Butane therefore is not observed to desorb at 260 K after we lowered the temperature to 223 K under the butane/helium from the adsorption temperature of 423 K prior to temperature programming from that temperature to 1100 K under a helium stream. Anaerobic temperature-programmed oxidation of n-butane produces butene and butadiene at a peak maximum temperature of 1000 K; this is exactly the temperature at which, upon temperature programming, oxygen evolves from the lattice and desorbs as O2. This, and the fact that the amount of oxygen desorbing from the (VO)2P2O7 at ∼1000 K is the same as that required for the oxidation of the n-butane to butene and butadiene, strongly suggests (i) that lattice oxygen as it emerges at the surface is the selective oxidant and (ii) that its appearance at the surface is the rate-determining step in the selective oxidation of n-butane. The surface of the (VO)2P2O7 catalyst on which this selective oxidation takes place has had approximately two monolayers of oxygen removed from it by unselective oxidation of the n-butane to CO, CO2, and H2O between 550 and 950 K and has had approximately one monolayer of carbon deposited on it at ∼1000 K. It is apparent, therefore, that the original crystallography of the (VO)2P2O7 catalyst will not exist during this selective oxidation and that theories that relate selectivity in partial oxidation to the (100) face of the (VO)2P2O7 catalyst cannot apply in this case.

Applications for activated carbons from used tires: butane working capacity

Activated carbons obtained from the pyrolysis (600–800°C) of used tires have been synthesized and studied for n-butane adsorption. To obtain optimal butane working capacity (BWC), higher burn-off was found necessary. It was also found that higher pyrolysis temperatures (∼800°C) allow mesoporosity to develop more readily and furthermore this porosity leads to higher BWC.

Performance evaluation of carbon adsorbents for automobile canisters

To assess the performance of carbon adsorbents for automobile canisters, we have studied the adsorption and desorption of n-butane and gasoline vapor experimentally in a column charged with pelletized carbons, Car-115 and DP-1. Norit Car-115, which is a well-known carbon adsorbent for canisters, was chosen as a standard adsorbent to compare with a domestic pelletized carbon (DP-1). The use of n-butane as a reference compound for the assessment of adsorbents was proven to be very reliable when compared with the adsorption of real gasoline vapors. According to the results obtained, the carbon adsorbent for automobile canisters requires at least two basic characteristics such as the development of well-defined mesopores and high specific area, which are essential for rapid uptake rate and high working capacity.

Chain Length Effects of Linear Alkanes in Zeolite Ferrierite. 2. Molecular Simulations

Configurational-bias Monte Carlo technique simulations are performed in the grand canonical ensemble to calculate the adsorption isotherms and the isosteric heats of adsorption of propane, n-butane, n-pentane, and n-hexane in ferrierite. The results are generally in good agreement with experiment results. The locations of the molecules in the ferrierite pore structure obtained from the simulations agree well with the locations obtained by earlier 13C NMR measurements. It is found that only short n-alkane molecules up to n-pentane can access the entire two-dimensional ferrierite pore structure; longer molecules adsorb only in the 10-ring channels and not in the ferrierite 8-ring cage. Adsorption of n-pentane in this cage is possible but only at relatively high pressures due to the high number of repulsive interactions with framework oxygen ions. The results presented are furthermore used to set limits to the size parameter describing the Lennard-Jones interaction between the n-alkane united atoms and the framework oxygen atoms.

TEOM: A Unique Technique for Measuring Adsorption Properties. Light Alkanes in Silicalite-1

A new and fast technique, the tapered element oscillating microbalance, TEOM, has been applied to accurately measure adsorption in microporous materials at conditions relevant for practical applications. With this technique, the experimental data are not influenced by factors such as buoyancy and flow patterns, which are encountered with conventional gravimetric methods. The equilibrium adsorption of light alkanes, methane, ethane, propane, n-butane, and i-butane in silicalite-1 has been investigated using the TEOM technique. Single-component adsorption isotherms are reported at temperatures in the range from 303 to 473 K and at pressures of up to 500 kPa. Either a conventional or a double Langmuir isotherm appropriately describes the equilibrium data. Thermodynamic properties calculated from the isotherms are in excellent agreement with literature data from other techniques. Isobaric and isothermal experiments demonstrate that i-butane exhibits a two-step adsorption behavior. Based on geometry and entrop...

ESR study of paramagnetic sites in sulfated zirconia

ESR spectroscopy was used to investigate paramagnetic sites in sulfated zirconia. Catalysts derived from zirconium oxide and zirconium hydroxide were studied. It was demonstrated that paramagnetic sites assigned to near-surface F-centers were formed during activation at temperatures above 573 K. The catalyst derived from zirconium hydroxide shows after activation at 873 K two types of paramagnetic sites: F-centers and Zr3+ sites. Both F-centers and Zr3+ sites in this catalyst form complexes with reagents upon n-butane or hydrogen adsorption at range of 423–523 K in contrast to paramagnetic sites of the oxide-derived catalyst.

Structure and initial interaction of butane and butane isomers in a [formula omitted] catalyst

The relationship between structure and catalytic performance of Pt-HMOR model catalysts towards the n-butane hydroisomerization reaction is studied employing a combination of simulation strategies. The n-butane binding energy increases with Pt loading. This behavior is explained based on the different locations obtained of low-energy n-butane adsorption sites at limiting Pt loadings. At high metal loading, the effective pore blocking by large Pt particles leads to the adsorption of n-butane in small side pockets. The high coordination with oxygen ions at these sites offers the best stabilization through optimization of adsorbate-zeolite dispersion forces. This structural explanation for the calculated binding energy change with the increase in platinum concentration is consistent with that proposed by a previous Monte Carlo study of the relatively high heat of adsorption of methane in mordenite. The lower the metal dispersion the more strongly bound the n-butane is, compared to iso-butane and butane isomers. Pt sites associated to clusters of up to six atoms in size, located in the main channel might be energetically accessible for a single n-butane molecule, as suggested by the calculated migration energy profiles.