DIFFUSION COEFFICIENTS OF CARBON DIOXIDE WITHIN TYPE 13X ZEOLITE PARTICLES

The self-diffusion processes of CO₂ in a single particle of commercial type 13X zeolite have been studied by a new sorption rate method using a constant volume, variable pressure system. Experiments were carried out at 303.2 and 343.2 K. An inverse analysis of experimental uptake curves based on a macropore and micropore series diffusion model was performed to determine effective diffusion coefficients for both macropore and micropore diffusion simultaneously. Knudsen diffusion occurs within the macropore and the micropore diffusion coefficients having values of the order 10^?15–10^?14 m²/s, which are slightly greater than the previously reported micropore diffusion coefficients of type 5A zeolite crystals.     

Force fields for classical atomistic simulations of small gas molecules in silicalite-1 for energy-related gas separations at high temperatures

High temperature (>573 K) molecular dynamics studies of gas diffusion in microporous zeolites require consideration of the zeolite framework flexibility. Pore windows can expand and contract at high temperatures, affecting phase space and material properties. No studies to date have addressed the application of the condensed-phase optimized molecular potentials for atomistic simulation studies or the consistent valence force field to simulate gas diffusion and adsorption in siliceous MFI (silicalite-1). The current study seeks to validate these intramolecular and intermolecular potentials along with another zeolite-specific force field reported by Nicholas et al. (JACS 113:4792–4800, 1991) for silicalite-1, one of the most extensively investigated zeolites, with respect to diffusion of several gas molecules. The experimental diffusion coefficients of H₂, CO₂, CH₄, O₂ and N₂ in silicalite-1 obtained using pulse-field gradient-nuclear magnetic resonance and quasi-elastic neutron scattering methods were compared to theoretically derived diffusion coefficients employing these force fields in molecular dynamics simulations. The diffusion coefficients obtained using the three force fields for H₂, CO₂, CH₄, O₂ and N₂ agreed well with these experimental data. The zeolite-specific force field of Nicholas et al. was employed in grand canonical Monte Carlo simulations to obtain adsorption isotherms of these gases. The adsorption isotherms and isosteric heats of adsorption predicted were also in agreement with the expected range of available experimental and theoretical adsorption data reported in the literature.     

Kinetics of adsorption on activated carbon: application of heterogeneous vacancy solution theory

The kinetics of single component adsorption on activated carbon is investigated here using a heterogeneous vacancy solution theory (VST) of adsorption. The adsorption isotherm is developed to account for the adsorbate non-ideality due to the size difference between the adsorbate molecule and the vacant site, while incorporating adsorbent heterogeneity through a pore-width-related potential energy. The transport process in the bidisperse carbon considers coupled mass transfer in both macropore and micropore phases simultaneously. Adsorbate diffusion in the micropore network is modeled through effective medium theory, thus considering pore network connectivity in the adsorbent, with the activation energy for adsorbate diffusion related to the adsorption energy, represented by the Steele 10-4-3 potential for carbons. Experimental data of five hydrocarbons, CO₂ and SO₂ on Ajax carbon at multiple temperatures, as well as three hydrocarbons on Norit carbon at three temperatures are first fitted by the heterogeneous VST model to obtain the isotherm parameters, followed by application of the kinetic model to uptake data on carbon particles of different sizes and geometry at various temperatures. For the hydrocarbons studied, the model can successfully correlate the experimental data for both adsorption equilibrium and kinetics. However, there is some deviation in the fit of the desorption kinetics for polar compounds such as CO₂ and SO₂, due to the inadequacy of the L-J potential model in this case. The significance of viscous transport in the micropores is also considered here and found to be negligible, consistent with recent molecular simulation studies.     

Adsorptive separation of ethylene/ethane mixtures with CuCl@HY adsorbent: equilibrium and reversibility

In this work, we investigate the potential of CuCl-functionalized HY zeolite (CuCl@HY) as an effective adsorbent for the ethylene/ethane separation. The CuCl@HY adsorbents were prepared with CuCl₂ as precursor by a solid-state dispersion method, followed by the activation with CO. The CuCl@HY adsorbents with different CuCl loadings were investigated for ethylene and ethane adsorptions, and evaluated the reversibility for multiple ethylene adsorption/desorption cycles. The experimental results reveal that the optimal adsorbent with copper loadings of 5 mmol/g HY zeolite displays high ethylene adsorption capacity, high C₂H₄/C₂H₆ adsorption selectivity and good reversibility. In addition, the adsorption equilibrium isotherms of ethylene and ethane on CuCl@HY at temperatures up to 333 K can be well correlated by the Sips models, and the corresponding isosteric heats of adsorption are calculated using the Clausius–Clapeyron equation. The value of isosteric heat of adsorption suggests that the interaction of CuCl@HY with ethylene molecules is between physisorption and chemisorption.     

Surface Diffusion of Deuterium and Light Hydrocarbons in Microporous Vycor Glass

Abstract

The significance of the surface diffusion of helium through a microporous Vycor glass is emphasized, and the experimental evidence is presented. New permeability data of D₂, CH₄, C₂H₆, and C₃H₈ are reported over a wide temperature range, and the results are analyzed numerically. Interesting comparisons are made for D₂-H₂ and C₃H₈-CO₂ systems. Finally, the separation factors for CO₂-C₃H₈ and CH₄-C₃H₈ systems are discussed.

All the experimental work has been carried out with porous Vycor glass as the microporous medium. However, there is good reason to believe that the basic findings will hold for any other microporous barrier. Differences will only be a matter of magnitude and will be determined by the adsorption interaction of the respective gas-solid system.     

Analysis of the kinetics of regeneration of bidispersed activated granular carbon, by supercritical carbon dioxide

A desorption of m-xylene by supercritical CO₂ under different temperatures (40, 50 and 60 °C) and pressures (80, 100 and 128 bar) has been modelled using the analytical solution expressing the desorption yield for bidisperse granular activated carbon. This solution is in the form of an infinite double series. The coefficients of which were calculated by solving the transcendental equation using the method of Newton-Raphson. Solutions of first and second order of this equation determine the coefficients of the analytical solution. The results of this modelling, including macropore and micropore diffusion, show very good concordancy between experimental and simulated data for all operating conditions, which confirms the appropriateness of this model for this type of adsorbent considered. Only the equilibrium adsorption constant “K_C” was used as adjustable parameter. The values of K_C varied between 13.32 and 121.33 and the maximum average deviation between estimated and fitted values not exceeding 6.54%. On the other hand, it has been particularly highlighted for the experimental conditions studied, that the contribution of resistance due to external transfer and axial dispersion were negligible and that the resistance due to macropore diffusion was consistent and it was possible to reduce the time of desorption by reducing the size of the grain.     

Contact time optimization of two-stage batch adsorber systems using the modified film-pore diffusion model

In this paper, a contact time optimization methodology of a two-stage batch adsorber system taking minimum contact time as the objective function has been developed. The initial concentration of the second stage unit and adsorbent weight have been designated as variables and these have been studied under two conditions of the equilibrium solid-phase concentration, q_e, namely, when q_e is a variable and when q_e is a constant. Contact time optimization of a two-stage batch adsorber system has been demonstrated at three different conditions/cases for the adsorption of phenol on activated carbon and the adsorption of Astrazone Blue dye (Basic Blue 69) onto silica. A new concept of “pinch point” for the optimum design of batch adsorber system has been proposed. The optimization solutions show that there is a significant difference for minimum contact time at different process conditions.The diffusion mass transport model used to predict the concentration-time decay curve is a film-pore diffusion model. An analytical solution has been used for simplicity which assumes a constant capacity pseudo-irreversible isotherm.     

Equilibrium and Fixed Bed Adsorption of 1‐Butene,Propylene and Propane Over 13X Zeolite Pellets

Abstract

Propylene‐propane separation is one of the most difficult and demanding energetic operation currently practiced using cryogenic distillation. Extensive studies on various alternatives showed that cyclic adsorption processes, and particularly pressure swing adsorption (PSA), might be an option to replace the traditional distillation. In spite of the promising results of the PSA process, much attention is currently being paid to the simulated moving bed technology (SMB) for gas‐phase separations. The ingenious principle of this process is based on the choice of an adequate adsorbent‐desorbent couple. Thus, in the present work 1‐butene has been studied as an interesting desorbent to displace adsorbed propylene‐propane mixture on 13X zeolite. The measurements of pure 1‐butene adsorption isotherms over 13X zeolite were performed with a gravimetric experimental device for pressure ranging from 0 to 110 kPa and at temperature of 333, 353, 373, and 393 K. The experimental adsorption data were correlated using Toth model. The heat of adsorption at zero coverage and the maximum loading capacity of 1‐butene were found to be 54.4 kJ/mol and 2.10 mol/kg, respectively. The adsorption and desorption of 1‐butene on 13X zeolite packed on a fixed bed initially saturated either by a propane‐propylene mixture or a pure C₃ hydrocarbon has been studied. The performance of 1‐butene has been compared with isobutane that was previously proposed to be a highly effective desorbent for C₃H₆/C₃H₈ separation. A model based on a double LDF approximation for the mass transfer combined to a heterogeneous energy balance taking into account a variable velocity of the gaseous bulk phase, has been used to describe the breakthrough curves obtained experimentally at 373 K and 110 kPa.     

Effect of temperature on sorption of metals by silica-supported 2-(aminomethyl)pyridine. Part II: Sorption dynamics

The effect of temperature has been studied on metal-binding rates and dynamic column separation behavior of an adsorbent containing 2-(aminomethyl)pyridine groups supported on silica–polyamine composite. Binding kinetics of H₂SO₄ as well as copper and nickel sulfates has been measured by batch experiments at temperatures between 25 and 90 °C using synthetic sulfate solutions. The data were correlated with the non-ideal adsorption (NICA) model and a diffusion model of porous particles. The estimated model parameters were utilized in a dynamic column model to calculate the break-through curves obtained with the synthetic metal sulfate solutions and a concentrated ZnSO₄ process solution.The adsorption rate of sulfuric acid and metal sulfates is controlled by pore diffusion and increasing temperature markedly increases the sorption rate. The experimental batch uptake curves can be well correlated with the proposed diffusion model and with constant diffusion coefficients. The estimated activation energies of pore diffusion are about 9 and 20 kJ/mol for the acid and the metals, respectively. In column separation, increasing temperature substantially improves copper removal from the ZnSO₄ process solution. The improvement is predominantly due to enhanced intra-particle mass transport. The effect is further amplified by marked decrease in viscosity of the feed solution.     

Palladium,silver, and zinc oxide nanoparticles loaded on activated carbon as adsorbent for removal of bromophenol red from aqueous solution

The adsorption of bromophenol red (BPR) onto three adsorbents including palladium, silver and zinc oxide nanoparticles loaded on activated carbon (Pd-NP-AC, Ag-NP-AC and ZnO-NP-AC) in a batch system has been studied and the influence of various parameters has been optimized. The influence of time on removal of BPR on all adsorbent was investigated and experimental data were analyzed by four kinetic models including pseudo first and second-order, Elovich and the intraparticle diffusion equations. Following fitting the experimental data to these models, the respective parameters of each model such as rate constants, equilibrium adsorption capacities and correlation coefficients for each model were investigated and based on well known criterion their applicability was judged. It was seen that the adsorption of BPR onto all adsorbents sufficiently described by the pseudo second-order equation in addition to interparticle diffusion model. The adsorption of BPR on all adsorbent was investigated at various concentration of dye and the experimental equilibrium data were analyzed and fitted to the Langmuir, Freundlich, Tempkin, Dubinin, and Radushkevich equations. A single stage in batch process was efficient and suitable for all adsorbents using the Langmuir isotherm with maximum adsorption of 143 mg g^?1 for Pd-NP-AC, 250 mg g^?1 for Ag-NP-AC and 200 mg g^?1 for ZnO-NR-AC. Thermodynamic parameters such as ΔG°, ΔH°, and ΔS° for Pd-NP-AC adsorbent were calculated.     

Influence of intracrystalline diffusion on the selective catalytic reduction of NO by hydrocarbon over Cu-MFI zeolite

The influence of intracrystalline diffusion on the selective catalytic reduction of NO by hydrocarbons (propane, ethene) was investigated by using Cu-MFI catalysts of different zeolite crystal sizes. The influence of hydrocarbon as a reductant on the diffusion process was also investigated, employing kinetic and temperature-programmed desorption studies. In the NO–C₃H₈–O₂ reaction, the apparent reaction rate of NO conversion into N₂ did not depend on the zeolite crystal size, which indicates that the reaction is not controlled by intracrystalline diffusion. In the NO–C₂H₄–O₂ reaction, on the other hand, the apparent reaction rate evidently depended on the zeolite crystal size; the reaction rate over large crystal Cu-MFI (1.29 μm) was significantly less than that over a small one (0.09 μm), which indicates that the reaction is controlled by intracrystalline diffusion. The larger diffusion resistance in the NO–C₂H₄–O₂ reaction was attributed to the slower diffusion rate of ethene in zeolite channels than propane, which was due to much stronger interaction of ethene with Cu-MFI catalyst. Thus, the adsorption property of hydrocarbon on the Cu-MFI catalyst is revealed to play an important role in determining intracrystalline diffusivity and the diffusion influence on the selective reduction of NO over zeolite catalysts.     

Zur Adsorption von Dämpfen chlorierter C2-Kohlenwasserstoffe an Adsorberpolymeren und Aktivkohlen

Adsorption of Vapors of Chlorinated C₂-Hydrocarbons on Polymeric Adsorbents and Activated Carbons The adsorption (isotherms, kinetics, dynamics) of vapors of chlorinated ethanes and ethylenes from inert gas onto the novel crosslinked polymeric adsorbent Wofatit Y 77 was studied in comparison to traditional polymeric adsorbent Wofatit EP 61 and activated carbons B 23 and R 23 at 20°C. The crosslinked microporous polymeric adsorbent Wofatit Y 77 adsorbs the greatest amount. But the adsorption velocity is slightly lower than at activated carbons and the macroporous polymeric adsorbent Wofatit EP 61, respectively. The obtained steep adsorption breakthrough curves were evaluated according to the model of adsorption zone. Furthermore, the breakthrough curves were predicted from data of equilibrium and kinetics due to the equilibrium model and the model of film and pore diffusion by Rosen.     

DIFFUSION COEFFICIENTS OF CARBON DIOXIDE WITHIN TYPE 13X ZEOLITE PARTICLES

The self-diffusion processes of CO₂ in a single particle of commercial type 13X zeolite have been studied by a new sorption rate method using a constant volume, variable pressure system. Experiments were carried out at 303.2 and 343.2 K. An inverse analysis of experimental uptake curves based on a macropore and micropore series diffusion model was performed to determine effective diffusion coefficients for both macropore and micropore diffusion simultaneously. Knudsen diffusion occurs within the macropore and the micropore diffusion coefficients having values of the order 10^-15-10^-14 m²/s, which are slightly greater than the previously reported micropore diffusion coefficients of type 5A zeolite crystals.     

C2H2-SCR of NO over HZSM-5 affected by intracrystalline diffusion of NO x

The influence of particle size of HZSM-5 zeolite on selective catalytic reduction of NO by acetylene (C₂H₂-SCR) was investigated. The zeolite with nano-particle behaved considerable higher activity than the micro-particle one for the reaction. It was revealed that the large difference in the activity for the C₂H₂-SCR of NO arising from the particle size of zeolite was not caused by limited intracrystalline diffusion of the reductant, but that of NO₂, which was strongly supported by the adsorption results obtained over the zeolites.     

Micro-channel development and hydrogen adsorption properties in templated microporous carbons containing platinum nanoparticles

Ordered microporous carbons containing dispersed platinum nanoparticles were fabricated and chosen as suitable models to investigate micro-structure development and hydrogen transport properties of zeolite-templated carbons. X-ray photoelectron spectroscopy analysis revealed that the enhanced heat of adsorption is related to the narrow micro-channels templated from the zeolite and the presence of certain CO groups on the carbon. The lack of a well-defined and intense rotational transition line and the persistent broad H₂ recoil spectrum in neutron scattering results suggests a distribution of binding sites. Most interestingly, hydrogen diffusion occurs on two time scales, consisting of a fast liquid-like jump diffusion on the timescale of picoseconds along with an even faster bulk-like diffusion. The liquid-like motion is characterized by a diffusion constant of (2.1 ± 0.3) × 10⁻⁸ m²/s with an activation energy of ca. 77 K; both values indicate somewhat lower mobility than similar dynamics of H₂ on nanotubes, activated carbon XC-72, or Grafoil, yet greater mobility than that of bulk liquid. These unusual characteristics for hydrogen in carbons are believed to arise from the network of narrow pores in this zeolite-templated image of the zeolite. In fact, the diffusion constants of the templated carbons are extremely similar to those measured for zeolite 13X.     

The Adsorption of Pollutants by Peat,Lignite and Activated Chars

The ability of peat, lignite and activated chars made from peat and lignite to adsorb dyes and metals from wastewater and NO₂ from air was investigated. Equilibrium isotherms were determined to assess the maximum adsorption capacity of the adsorbents for the pollutants. Kinetic studies for the adsorption of dyes and metal ions onto the adsorbents were undertaken in agitated batch adsorbers. Mass transport models were tested to predict the concentration decay curves in batch adsorbers. The models tested were single resistance models based on the assumption of a single external mass transfer coefficient and two resistance models which included an internal diffusion coefficient and an external mass transfer coefficient. The surface phenomena which influence the extent and the rate of uptake have been studied. The equilibrium capacity data conform to Langmuir plots. A previously proposed model was used to evaluate the external single resistance mass transfer model and was successfully applied to predict the adsorption of metal ions in single component systems under batch conditions. It has been shown that the assumption of negligible intraparticle diffusion is valid and that external film diffusion is the rate limiting step in describing the adsorption processes at high sorbent loadings. The same type of result is not observed for the adsorption of coloured organic matter onto peat where the sorption processes cannot be successfully modelled by use of a single resistance model and a two resistance model incorporating internal diffusion is required. The surface phenomena which influence the extent and the rate of uptake of NO₂ have been studied. The type of chars produced and the activation processes affect the adsorption. As activation increases, micropore volume and surface area increase and the maximum capacity of the adsorbent increases. Surface area alone is not the only parameter which affects equilibrium uptake. © 1997 SCI.     

Removal of Ammonium from Wastewater by Pure Form Low-Silica Zeolite Y Synthesized from Halloysite Mineral

Pure form, single phase, and highly crystalline low-silica zeolite Y was synthesized from natural nanotubular halloysite mineral by the hydrothermal method. In the synthesis process, the halloysite consisted of SiO₂ and Al₂O₃ was used as starting material with adding supplementary silica and alumina sources. Ammonium adsorption properties of the as-synthesized zeolite Y were studied using batch experiments and the results revealed that its adsorption properties were strongly dependent on contact time, adsorbent dosage, pH, ionic strength, temperature, and initial concentration. The equilibrium data fit well with the Langmuir isotherm compared with the Freundlich isotherm. Kinetic studies showed that the adsorption followed the pseudo-second-order model. Thermodynamic parameters such as change in free energy (ΔG ⁰), enthalpy (ΔH ⁰), and entropy (ΔS ⁰) were also determined, which indicated that the adsorption of ammonium on zeolite Y was a spontaneous and exothermic process at ambient conditions. Due to its low cost, high adsorption capacity and fast adsorption rate, the zeolite Y synthesized from halloysite has the potential to be utilized for the cost-effective removal of ammonium from wastewater.     

Comparison of the diffusion coefficients of linear and cyclic alkanes

Monte Carlo simulations of linear and cyclic alkanes were performed on a coarse-grained high coordination lattice. The simulations were performed at 473 K for C_NH_2N+2 and C_NH_2N where N had the values of 60, 100, and 316. The results indicated: (i) at low molecular weights, cyclic alkanes have lower diffusion coefficients than linear alkanes, and (ii) at high molecular weights, they have higher diffusion coefficients than linear alkanes. The lower diffusion coefficient of the small cyclic alkanes was attributed to the high local density within the volume defined by the smaller mean square radius of gyration of the cyclic alkanes. The high local density of cyclic alkane segments resulted in a decrease in the mobility of the beads. The crossover in diffusion coefficients was observed around the entanglement molecular weight of linear alkanes, which suggests that the linear alkanes are more susceptible to the effects of entanglements than are the cyclic alkanes.     

The adsorption kinetics of sodium dodecylbenzenesulfonate on activated carbon. Branched-pore diffusional model revisited and comparison with other diffusional models

Abstract

The adsorption rate of sodium dodecylbenzenesulfonate (SDBS) on three commercial activated carbons (ACs) and an AC synthesized from almond shells was investigated in this study. The mechanisms controlling the overall adsorption rate of SDBS on ACs were found out by using the pore volume and surface diffusion model (PVSDM). The PVSDM showed that the intraparticle diffusion of SDBS in all ACs was mainly attributed to pore volume diffusion and surface diffusion. The surface diffusion coefficient, D_s, in all samples of ACs are influenced by the amount of surfactant adsorbed at equilibrium, q_e, as well as the mean micropore width, L₀. The contribution of surface diffusion to the overall intraparticle diffusion ranged from 45 to 70%, depending on the properties of AC. Moreover, the branched-pore diffusional model was revisited (BPDMR) assuming that the Fick diffusion is the only diffusion mechanism in the macropores and the diffusion in the micropores was represented by the micropore rate coefficient, K_C. Besides, it was proposed that the parameter f representing the mass fraction of SDBS adsorbed on macropores, can be estimated from the textural properties of ACs. Three new strategies were proposed to analyze the experimental data using BPDMR model, and it was demonstrated that the macropore diffusivity in BPDMR is close to the molecular diffusivity of SDBS in water solution. The micropore rate constant, K_C, ranged from 3.90?×?10^?6 to 10.6?×?10^?6 s^?1 and was affected by textural characteristics of ACs. Both models predicted the global adsorption rate of SDBS on ACs satisfactorily.     

Adsorption equilibrium and kinetics of H2O on zeolite 13x

The adsorption characteristics of H₂O on zeolite 13X were measured by a gravimetric method. The adsorption isotherm showed type II isotherm and was fitted by using both the excess surface work (ESW) model and Langmuir-Freundlich model. The results predicted by the Langmuir-Freundlich model were much smaller than the experimental results at higher-pressure region. However, the ESW model agreed well with the experimental data over the whole pressure region. In this case, a plot of the change in chemical potential versus the amount adsorbed gave two linear regions due to secondary effects such as capillary condensation. The experimental uptake curves were well fitted by several LDF models and solid diffusion model with the error range of 1.5-3.5%. Unlike the expectation that the more rigorous solid diffusion model would fit better, Nakao-Suzuki model showed the best agreement with experimental uptake data.