Real Adsorbed Solution Theory Research Articles

Elucidating the failure of the Ideal Adsorbed Solution Theory for CO2/H2O mixture adsorption in CALF-20

The use of CALF-20 for CO2 capture from humid flue gases has attracted a vast amount of attention from both academia and industry. The focus of this article is on CO2/H2O mixture adsorption for which published experimental data demonstrate the severe limitations of the Ideal Adsorbed Solution Theory (IAST) in providing a quantitative estimation of the component loadings. We use Configurational-Bias Monte Carlo (CBMC) simulations for elucidating the origins of thermodynamic non-idealities. The CBMC simulations reveal that two distinct regimes prevail during mixture adsorption. For low relative humidities less than say 20 %, the CO2 loadings in the adsorbed phase is significantly higher than anticipated by the IAST. CBMC simulations of intermolecular distances for guest pairs reveal that failure of the IAST can be traced to the phenomenon of segregated adsorption. The H2O-H2O pairs are close together at typical distances of about 3 Å. The CO2-H2O pairs are typically 8 Å apart, occupying adjacent adsorption sites corresponding to the O atoms of the oxalate group. This implies that the CO2 molecules face a less severe competitive adsorption with H2O than is anticipated by the IAST, whose applicability mandates a uniform and homogeneous distribution of adsorbates in pore space. The situation changes dramatically at high values of relative humidities, typically larger than about 50 %; in this scenario, the adsorbed phase is significantly richer in H2O. The CO2 molecules are compelled to share same adsorption site with pairs of H2O molecules that are hydrogen-bonded with each other. Consequently, the competition faced by CO2 is significantly higher than anticipated by the IAST, resulting in significantly lower CO2 uptakes.The important message that emerges from this investigation is the need to incorporate the Real Adsorbed Solution Theory (RAST) for quantitative modelling of fixed-bed adsorbers in CO2 capture with CALF-20.

Addressing spreading pressure dependence of real adsorbed solution theory with generalized Langmuir isotherm

Addressing spreading pressure dependence of real adsorbed solution theory with generalized Langmuir isotherm

The Rigid Adsorbent Lattice Fluid Model: Thermodynamic Consistency and Relationship to the Real Adsorbed Solution Theory.

The Rigid Adsorbent Lattice Fluid model has been shown to comply with all the requirements for thermodynamic consistency in the case of an adsorbent that does not undergo structural changes. This is achieved by introducing a correction to the reduced density function that multiplies the combinatorial term. A procedure to calculate the predicted adsorbed mixture activity coefficients has been presented that allows the production of excess Gibbs energy plots at a constant reduced grand potential. The predicted nonideality is structurally consistent with the Non-Ideal Adsorbed Solution Theory of Myers in terms of both its dependence on concentration and reduced grand potential. The ability to generate excess Gibbs energy values allows linking the new Rigid Adsorbent Lattice Fluid model to the traditional Real Adsorbed Solution Theory providing an alternative approach to predicting multicomponent adsorption based solely on pure component data.

Separation of Azeotropic Hydrofluorocarbon Refrigerant Mixtures: Thermodynamic and Kinetic Modeling for Binary Adsorption of HFC-32 and HFC-125 on Zeolite 5A.

Hydrofluorocarbons (HFCs) have been used extensively as refrigerants over the past four decades; however, HFCs are currently being phased out due to large global warming potentials. As the next generation of hydrofluoroolefin refrigerants are phased in, action must be taken to responsibly and sustainably deal with the HFCs currently in circulation. Ideally, unused HFCs can be reclaimed and recycled; however, many HFCs in circulation are azeotropic or near-azeotropic mixtures and must be separated before recycling. Previously, pure gas isotherm data were presented for both HFC-125 (pentafluoroethane) and HFC-32 (difluoromethane) with zeolite 5A, and it was concluded that this zeolite could separate refrigerant R-410A (50/50 wt % HFC-125/HFC-32). To further investigate the separation capabilities of zeolite 5A, binary adsorption was measured for the same system using the Integral Mass Balance method. Zeolite 5A showed a selectivity of 9.6-10.9 for HFC-32 over the composition range of 25-75 mol % HFC-125. Adsorbed phase activity coefficients were calculated from binary adsorption data. The Spreading Pressure Dependent, modified nonrandom two-liquid, and modified Wilson activity coefficient models were fit to experimental data, and the resulting activity coefficient models were used in Real Adsorbed Solution Theory (RAST). RAST binary adsorption model predictions were compared with Ideal Adsorbed Solution Theory (IAST) predictions made using the Dual-Site Langmuir, Tóth, and Jensen and Seaton pure gas isotherm models. Both IAST and RAST yielded qualitatively accurate predictions; however, quantitative accuracy was greatly improved using RAST models. Diffusion behavior of HFC-125 and HFC-32 was also investigated by fitting the isothermal Fickian diffusion model to kinetic data. Molecular-level phenomena were investigated to understand both thermodynamic and kinetic behaviors.

Activity coefficient models for accurate prediction of adsorption azeotropes

In this study seven adsorption azeotropes involving binary systems and zeolite-based adsorbents were systematically investigated. Pure component isotherms and mixed-gas adsorption data were taken from published literature except for the benzene–propene system on silicalite, which is newly presented in this work using molecular simulations. Experimental adsorbed phase composition and total amount adsorbed of the azeotropic systems were compared with the predictions of several models including: the ideal adsorbed solution theory (IAST), the heterogeneous ideal adsorbed solution theory (HIAST) and the real adsorbed solution theory (RAST) coupled with the 1-parameter Margules (1-Margules) and the van Laar equations. In the latter two models an additional loading parameter was incorporated in the expression of the excess Gibbs energy to account for the reduced grand potential dependency of the activity coefficients in the adsorbed phase. It was found that the HIAST and RAST–1-Margules models were able to predict the azeotropic behaviour of some systems with good accuracy. However, only the RAST–van Laar model consistently showed an average relative deviation below 3% compared to experimental data for both the adsorbed phase composition and the total amount adsorbed across the systems. This modified van Laar equation is therefore preferable in those engineering applications when the location of adsorption azeotropes is required with great accuracy and when there is lack of detailed characterization of the adsorbent that is needed to carry out molecular simulations.

Adsorption for efficient low carbon hydrogen production: part 2\u2014Cyclic experiments and model predictions

Hydrogen as clean energy carrier is expected to play a key role in future low-carbon energy systems. In this paper, we demonstrate a new technology for coupling fossil-fuel based hydrogen production with carbon capture and storage (CCS): the integration of CO2 capture and H2 purification in a single vacuum pressure swing adsorption (VPSA) cycle. An eight step VPSA cycle is tested in a two-column lab-pilot for a ternary CO2–H2–CH4 stream representative of shifted steam methane reformer (SMR) syngas, while using commercial zeolite 13X as adsorbent. The cycle can co-purify CO2 and H2, thus reaching H2 purities up to 99.96%, CO2 purities up to 98.9%, CO2 recoveries up to 94.3% and H2 recoveries up to 81%. The key decision variables for adjusting the separation performance to reach the required targets are the heavy purge (HP) duration, the feed duration, the evacuation pressure and the flow rate of the light purge (LP). In contrast to that, the separation performance is rather insensitive towards small changes in feed composition and in HP inlet composition. Comparing the experimental results with simulation results shows that the model for describing multi-component adsorption is critical in determining the predictive capabilities of the column model. Here, the real adsorbed solution theory (RAST) is necessary to describe all experiments well, whereas neither extended isotherms nor the ideal adsorbed solution theory (IAST) can reproduce all effects observed experimentally.

Application of pyridine-modified chitosan derivative for simultaneous adsorption of Cu(II) and oxyanions of Cr(VI) from aqueous solution

The bioadsorbent C1, which is a chitosan derivative prepared in a one-step synthesis, was successfully used to adsorb Cr(VI) and Cu(II) simultaneously. Here, for the first time the simultaneous adsorption of a cation and an anion was modeled using the Corsel model for kinetics and the Real Adsorbed Solution Theory model for equilibrium data. Batch studies of the adsorption of Cu(II) and Cr(VI) in single and binary aqueous solutions were performed as a function of initial solute concentration, contact time, and solution pH. The maximum adsorption capacities of C1 in single and binary aqueous solutions were 1.84 and 1.13 mmol g−1 for Cu(II) and 3.86 and 0.98 mmol g−1 for Cr(VI), respectively. The reuse of C1 was investigated, with Cu(II) ions being almost completely desorbed and fully re-adsorbed. For Cr(VI), the desorption was incomplete resulting in a lower re-adsorption. Energy-dispersive X-ray spectroscopy was used for mapping the distributions of Cr(VI) and Cu(II) adsorbed on the C1 surface in single and binary adsorption systems. Isothermal titration calorimetry experiments were performed for Cr(VI) and Cu(II) adsorption in single solutions. The thermodynamic parameters of adsorption showed that the adsorption of both metal ions was enthalpically driven, but entropically unfavorable.

A Thermodynamic Model for Pure and Binary Adsorption Equilibria of N2 and O2 on Lithium-Exchanged Low Silicon-to-Aluminum Ratio X Zeolite

To accurately describe the adsorption equilibria of N2 and O2 on LiLSX zeolite in the design of air separation by pressure swing adsorption, an applicable model (aNRTL–RAST) was developed. It was based on the real adsorbed solution theory (RAST) in which the activity coefficient was calculated by the nonrandom two-liquid theory (aNRTL). The binary adsorbate–adsorbate interaction parameters of the aNRTL theory were estimated from the pure component isotherms. The aNRTL–RAST model was used to predict the mixed gas equilibrium data under different adsorption conditions and compared with the extended Langmuir (E-L), two-dimensional equation of state (2D-EOS), and ideal adsorbed solution (IAST) models. aNRTL–RAST showed better accuracy than the other models, and the average errors for different adsorption conditions were <5%. Therefore, this model can be used to predict the adsorption equilibrium of mixed gases, avoiding complicated and tedious experiments. This study provides a theoretical basis for the selection of oxygen adsorbents for the separation process design of industrial air separation.

Adsorption in the context of clean hydrogen production: process intensification by integrating H2 purification and CO2 capture - A modeling and experimental study of multi-component adsorption

Coupling fossil-based H2 production with carbon capture and storage can produce H2 with low associated CO2 emissions, which is urgently needed to decarbonize industry, the energy sector, and the transportation sector. Vacuum pressure swing adsorption (VPSA) allows for the integration of two key separation tasks in this process, H2 purification and CO2 capture. In this work, we compare experimental and simulation results of a cyclic VPSA process. Our focus is the evaluation of different approaches for modeling multi-component adsorption. In particular, we use a three process Langmuir model for CO2 adsorption and Langmuir model for H2 and CH4 adsorption. We assess the different options for energetic site matching. The results indicate that energetic site matching of H2 and CO2 is of little importance. In contrast to that, only two models with a different energetic site matching for CO2 and CH4 result in a reasonable prediction of the experimental results, i.e. uncorrelated (UC) and uncorrelated unselective B (UCUS B). Both extended Sips isotherms and the use of the real adsorbed solution theory (RAST), however, result in a better prediction of the experimental results.

Selective adsorption of CO2 by Hex-star phosphorene from natural gas: Combining molecular simulation and real adsorbed solution theory

The performances of multilayer Hex-star phosphorene (HP) for the CO2 removal from natural gas were investigated by molecular simulation combined with real adsorbed solution theory (RAST). The influences of pressure (0.01–3.0 MPa), molar fraction of CO2 in bulk gas (0.05, 0.1, 0.2, 0.3, 0.4) and pre-adsorbed water content (0.0–9.9 mmol g−1) in adsorbents were thoroughly discussed at 300 K. The adsorption isotherms of both CO2 and CH4 predicted by RAST agree well with the simulation data. The adsorption selectivity calculations indicate that the multilayer HP exhibits an excellent separation performance under high pressure and high CO2 concentration. Meanwhile, under the low CO2 concentration, resulting from the sieving effect and stronger interactions between CO2 and water molecules, the CO2 selectivity can be significantly improved without impact of CO2 loading by pre-adsorbing some water molecules in the multilayer HP. This work suggests that the HP can act as a promising material for natural gas purification.

Coupling a Feedforward Network (FN) Model to Real Adsorbed Solution Theory (RAST) to Improve Prediction of Bisolute Adsorption on Resins

When predicting bisolute adsorption, the adsorbed solution theory (AST) and real adsorbed solution theory (RAST) either frequently show high prediction deviations or require bisolute adsorption data. Emerging feedforward network (FN) models can provide high prediction accuracy but lack broad applicability. To avoid those limitations, adsorption experiments were performed for a total of 12 single solutes and 55 bisolutes onto two widely used resins (MN200 and XAD-4). Different FN-based models were then built and compared with AST and RAST, based on which a new modeling strategy coupling FN to RAST and requiring only single-solute data was proposed. The root-mean-square error (RMSE) of predictions by the FN-RAST is 0.082 log units for 50 bisolute adsorption on MN200, much lower than that by AST (0.164) and slightly higher than that by RAST (0.069) or the best FN model (0.068). The FN-RAST model further provided satisfactory predictions for 5 bisolute adsorption on XAD-4 (RMSE = 0.10), which is comparable to that by RAST (0.10) and much lower than those by AST (0.26) and FN model (0.38). Therefore, the FN-RAST enjoys both satisfactory prediction accuracy and some broad applicability. The values of Abraham descriptors E and S were also founded to help assess/compare the nonideal behavior in different bisolute mixtures.

Water/Alcohol Mixture Adsorption in Hydrophobic Materials: Enhanced Water Ingress Caused by Hydrogen Bonding.

Microporous crystalline porous materials such as zeolites, metal–organic frameworks, and zeolitic imidazolate frameworks (ZIFs) have potential use for separating water/alcohol mixtures in fixed bed adsorbers and membrane permeation devices. For recovery of alcohols present in dilute aqueous solutions, the adsorbent materials need to be hydrophobic in order to prevent the ingress of water. The primary objective of this article is to investigate the accuracy of ideal adsorbed solution theory (IAST) for prediction of water/alcohol mixture adsorption in hydrophobic adsorbents. For this purpose, configurational bias Monte Carlo (CBMC) simulations are used to determine the component loadings for adsorption equilibrium of water/methanol and water/ethanol mixtures in all-silica zeolites (CHA, DDR, and FAU) and ZIF-8. Due to the occurrence of strong hydrogen bonding between water and alcohol molecules and attendant clustering, IAST fails to provide quantitative estimates of the component loadings and the adsorption selectivity. For a range of operating conditions, the water loading in the adsorbed phase may exceed that of pure water by one to two orders of magnitude. Furthermore, the occurrence of water–alcohol clusters moderates size entropy effects that prevail under pore saturation conditions. For quantitative modeling of the CBMC, simulated data requires the application of real adsorbed solution theory by incorporation of activity coefficients, suitably parameterized by the Margules model for the excess Gibbs free energy of adsorption.

Highlighting non-idealities in C2H4/CO2 mixture adsorption in 5A zeolite

Gasification of biomass at relatively low temperatures (770–880 °C) yields producer gas mixtures containing H2, CO and CO2, along with CH4, C2H6, and C2H4. It is of economic importance to recover C2H4 by selective adsorption using cation-exchanged zeolites such as LTA-5A. However, calculations of mixture adsorption equilibrium for LTA-5A using the Ideal Adsorbed Solution Theory (IAST) predict that the mixture adsorption equilibrium is selective to CO2, and recovery of C2H4 is not feasible. In sharp contrast to IAST predictions, transient breakthrough experiments with H2/CH4/C2H6/CO/CO2/C2H4 feed mixtures of different compositions show that selective recovery of C2H4 from the producer gas is feasible provided the C2H4/CO2 molar ratio in the feed mixture is below unity; for C2H4/CO2 molar ratios exceeding unity, the selectivity is in favor of CO2. The selectivity reversal phenomena signifies strong non-idealities in mixture adsorption. Such non-idealities can be quantified using the Real Adsorbed Solution Theory (RAST). This article underscores the need for performing transient breakthrough experiments with realistic producer gas mixtures for process modelling and development purposes.

Trimellitated sugarcane bagasse: A versatile adsorbent for removal of cationic dyes from aqueous solution. Part II: Batch and continuous adsorption in a bicomponent system

In the second part of this series of studies, the bicomponent adsorption of safranin-T (ST) and auramine-O (AO) on trimellitated sugarcane bagasse (STA) was evaluated using equimolar dye aqueous solutions at two pH values. Bicomponent batch adsorption was investigated as a function of contact time, solution pH and initial concentration of dyes. Bicomponent kinetic data were fitted by the pseudo-first-order and pseudo-second-order models and the competitive model of Corsel. Bicomponent equilibrium data were fitted by the real adsorbed solution theory model. The antagonistic interactions between ST and AO in the adsorption systems studied contributed to obtain values of maximum adsorption capacity in mono- (Qmax,mono) and bicomponent (Qmax,multi) lower than unity (Qmax,multi/Qmax,mono at pH 4.5 for ST of 0.75 and AO of 0.37 and at pH 7 for ST of 0.94 and AO of 0.43). Mono- and bicomponent adsorption of dyes in a fixed-bed column was evaluated at pH 4.5. The breakthrough curves were fitted by the Thomas and Bohart-Adams original models. Desorption of ST in a fixed-bed column was studied. The results obtained from the bicomponent batch and continuous adsorption showed that the presence of ST most affected the AO adsorption than the presence of AO affected the ST adsorption.

Equilibrium adsorption of bioethanol from aqueous solution by synthesized silicalite adsorbents: experimental and modeling

In this study, three silicalite adsorbents (silicalite a, silicalite b and silicalite c) for ethanol adsorption from aqueous phase were synthesized using tetraethylorthosilicate, Ludax solution and a new organic silica source, Tabasheer, respectively. Tetrapropylammonium hydroxide was used as primary structure directing agent and starch as secondary structure directing agent. XRD, BET, FT-IR and FE-SEM were used to characterize synthesized adsorbents. Single component adsorption for ethanol and water was investigated by non-adsorbing solvent concept. Binary adsorption isotherms of both components were also experimentally measured. A model based on real adsorbed solution theory was developed to predict experimental adsorption data. The XRD characterization confirmed that all silicalite adsorbents have the same MFI-type structure. FE-SEM results revealed that the different silica sources have impact on surface area and morphology. The BET surface area for adsorbents is in order of silicalite c > silicalite a > silicalite b. However, uptake rate toward ethanol is in order of silicalite a > silicalite c > silicalite b. The model based on real adsorbed solution theory predicted experimental adsorption isotherm data very well. In addition, results revealed that the binary adsorption uptake is lower than single component adsorption.

Description of Adsorption in Liquid Chromatography under Nonideal Conditions.

A thermodynamically consistent description of binary adsorption in reversed-phase chromatography is presented, accounting for thermodynamic nonidealities in the liquid and adsorbed phases. The investigated system involves the adsorbent Zorbax 300SB-C18, as well as phenetole and 4- tert-butylphenol as solutes and methanol and water as inert components forming the eluent. The description is based on adsorption isotherms, which are a function of the liquid-phase activities, to account for nonidealities in the liquid phase. Liquid-phase activities are calculated with a UNIQUAC model established in this work, based on experimental phase equilibrium data. The binary interaction in the adsorbed phase is described by the adsorbed solution theory, assuming an ideal (ideal adsorbed solution theory) or real (real adsorbed solution theory) adsorbed phase. Implementation of the established adsorption model in a chromatographic code achieves a quantitative description of experimental elution profiles, with feed compositions exploiting the entire miscible region, and involving a broad range of different eluent compositions (methanol/water). The quantitative agreement of the model and experimental data serves as a confirmation of the underlying physical (thermodynamic) concepts and of their applicability to a broad range of operating conditions.

Modeling Bisolute Adsorption of Aromatic Compounds Based on Adsorbed Solution Theories.

A large number of organic contaminants are commonly found in industrial and municipal wastewaters. For proper unit design to remove contaminant mixtures by adsorption, multicomponent adsorption equilibrium models are necessary. The present work examined the applicability of Ideal Adsorbed Solution Theory (IAST), a prevailing thermodynamic model, and its derivatives, i.e., Segregated IAST (SIAST) and Real Adsorbed Solution Theory (RAST), to bisolute adsorption of organic compounds onto a hyper-cross-linked polystyrene resin, MN200. Both IAST and SIAST were found to be less accurate in fitting the experimental bisolute adsorption isotherms than RAST. RAST incorporated with an empirical four-parameter equation developed in this work can fit the adsorbed phase activity coefficients, γi, better than RAST combined with the Wilson equation or the Nonrandom two-liquid (NRTL) model. Moreover, two polyparameter linear free energy relationships were developed for the adsorption of a number of solutes at low concentrations in the presence of a major contaminant (4-methylphenol or nitrobenzene). Results show that these relationships have a great potential in predicting γi of solutes when the adsorbed amounts are dominated by a major contaminant. To the best of our knowledge, this is the first study predicting γi for bisolute adsorption based on molecular descriptors. Overall, our findings have proved a major step forward to accurately modeling multisolute adsorption equilibrium.

Synthesis and application of a new carboxylated cellulose derivative. Part II: Removal of Co2+, Cu2+ and Ni2+ from bicomponent spiked aqueous solution

In the second part of this series of studies, the competitive adsorption of three binary systems Cu2+–Co2+, Cu2+–Ni2+ and Co2+–Ni2+ on a carboxylated cellulose derivative (CTA) was evaluated in binary equimolar (1:1) metal–ion aqueous solutions. Bicomponent adsorption studies were developed as a function of contact time and initial metal ion concentration. Bicomponent adsorption kinetic data was modeled by monocomponent kinetic models of pseudo-first- (PFO) and pseudo-second-order (PSO) and a competitive kinetic model of Corsel. Bicomponent adsorption isotherm data was modeled by the ideal adsorbed solution theory (IAST) and real adsorbed solution theory (RAST) models. The monocomponent isotherm models implemented into the IAST were the Langmuir and Sips models, whereas for the RAST model only the Langmuir model was implemented because this model provided the best prediction of the bicomponent isotherm data. The surface of the CTA adsorbent after bicomponent adsorption of metal ions was also examined by SEM-EDX. The effect of one metal ion on the adsorption capacity of another metal ion was discussed in detail with basis on the kinetic and thermodynamics parameters. The selectivity and performance of the CTA adsorbent for the removal of Cu2+, Co2+ and Ni2+ was also evaluated and discussed.

Adsorption equilibrium of methane and carbon dioxide on zeolite 13X: Experimental and thermodynamic modeling

A static, volumetric method has been used to determine the adsorption equilibrium of CH4 and CO2 and their binary mixtures on 13X molecular sieves at various temperatures between 273 and 343K. Pressures for the pure component data extend up to 10bar, while all binary data were obtained at 4bar and 6bar. The measured pure isotherms were regressed using different isotherm equations and the regressed parameters were applied to different thermodynamic models such as ideal adsorbed solution theory (IAST), vacancy solution theory (VST) and real adsorbed solution theory (RAST). CH4/CO2 system that deviate from ideality is not well represented by IAST and VST, whereas RAST which include activity coefficient in the adsorbed phase as characterizing parameter for non-ideality, show a much better representation of the binary equilibria. Experimental selectivities CO2/CH4 range from 26 to 2 at different pressure and temperatures. It is concluded that zeolite 13X suitable for natural gas industrial separation with yCO2<0.2 at 303K and 4bar. For landfill gas upgrading, where yCO2<0.45, zeolite 13X can be successfully applied at 303K, 4bar and 303K, 6bar and 323K, 4bar.

Adsorption equilibrium of binary mixtures of carbon dioxide and nitrogen on zeolites ZSM-5 and 13X

An investigation of the adsorption equilibrium of carbon dioxide (CO2) and nitrogen (N2) and their mixtures on zeolites ZSM-5 and 13X is presented. Pure component isotherms are measured at five different temperatures in the range of 25 °C and pressures up to 10 bar, and the resulting data are described with an appropriate isotherm equation. Adsorption equilibrium of CO2/N2 mixtures was measured at two temperatures (25 °C and 45 °C) and three pressure levels (1.2 bar, 3 bar and 10 bar) using three feed gas compositions on both materials tested. The isotherms obtained from the pure component measurements are used to predict the binary data using two approaches: An empirical extension of the Sips isotherm and the application of ideal adsorbed solution theory (IAST). In the context of the binary CO2/N2 adsorption, it was found that for ZSM-5 the system behaves close to ideal over the whole range of pressure and temperature investigated here and thus is well described by the IAST prediction. On the other hand, our results demonstrate non-ideal behavior in the adsorbed phase with increasing pressure for zeolite 13X, thus motivating the use of real adsorbed solution theory (RAST) to accurately describe the binary adsorption data.