Pure Component Adsorption Isotherms Research Articles

Vacancy solution model using the aNRTL activity coefficient for prediction of binary adsorption equilibria

Based on the Vacancy Solution Model (VSM) and the Adsorption Non-Random Two-Liquid (aNRTL) model of the activity coefficient, a novel model called VSM-aNRTL for gas adsorption equilibria is introduced. In this study, an innovative isotherm equation was applied to experimental monocomponent adsorption equilibrium data from the NIST/ARPA-E Database. The parameters derived from this application were subsequently utilized to predict the equilibrium data for 12 binary adsorption systems across three distinct temperatures, employing the extended model. The proposed model proved effective in correlating the adsorption isotherms of pure components, achieving a reduction in the number of adjustable parameters compared to the original VSM that employs the Wilson activity coefficient. It also outperformed the Ideal Adsorbed Solution Theory (IAST) model by incorporating only one additional parameter. Furthermore, the new model was employed to predict the equilibrium data of binary mixture adsorption, demonstrating results with lower deviations compared to both the IAST and the traditional VSM-Wilson models.

The influence of zeolite pore topology on the separation of carbon dioxide from methane

The influence of framework topology on the separation of carbon dioxide from methane has been studied in a set of small pore pure silica zeolites with CHA, IHW, ITW, LTA, MTF and RWR structures. To isolate this effect, the composition of the materials has been kept constant by using pure silica materials, which have neither aluminium in their framework nor extraframework cations that can alter the adsorption behaviour. Pure component adsorption isotherms and breakthrough experiments with synthetic mixtures resembling biogas and natural gas have been conducted. Materials Si-RWR and Si-ITW, which present channel-like topology, show higher IAST selectivities towards CO2 due to a kinetic exclusion of methane, which in the case of Si-RWR is pore-size based and in the case of Si-ITW, topology based. High heat of adsorption of CO2 on Si-ITW is explained through a maximisation of the dispersive interactions between the tight channels and the adsorbate. The extremely small pores of Si-RWR lead to a reduced diffusivity not only of CH4 but also of CO2 precluding its use as adsorbent for this separation. Si-ITW arises as a promising material in the separation of CO2 from CH4, having high selectivity towards CO2 while keeping high diffusion rates and a moderate maximum capacity. A comparison of the performance of these materials with that of aluminosilicate zeolites LTA with Si/Al ratio of 5 and 13X shows that Si-ITW can be more convenient by lowering the energetic requirements of the regeneration step. Moreover, the hydrophobicity of this material can be a further advantage over traditional zeolites.

The potential of direct air capture using adsorbents in cold climates.

Global warming threatens the entire planet, and solutions such as direct air capture (DAC) can be used to meet net-zero goals and go beyond. This study investigates using DAC in a 5-step temperature vacuum swing adsorption (TVSA) cycle with adsorbents' Li-X and Na-X, readily available industrial zeolites, to capture and concentrate CO2 from air in cold climates. From this study, we report that Na-X in cold conditions has the highest known CO2 adsorption capacity in air of 2.54mmol/g. This combined with Na-X's low CO2 heat of adsorption, and fast uptake-rate in comparison to other benchmark materials, allowed for Na-X operating in cold conditions to have the lowest reported DAC operating energy of 1.1 MWh/tonCO2. These findings from this study show the promise of this process in cold climates of Canada, Alaska, Greenland, and Antarctica to be part of the solution to global warming.

Natural Gas Storage Filled with Peat-Derived Carbon Adsorbent: Influence of Nonisothermal Effects and Ethane Impurities on the Storage Cycle.

Adsorbed natural gas (ANG) is a promising solution for improving the safety and storage capacity of low-pressure gas storage systems. The structural-energetic and adsorption properties of active carbon ACPK, synthesized from cheap peat raw materials, are presented. Calculations of the methane-ethane mixture adsorption on ACPK were performed using the experimental adsorption isotherms of pure components. It is shown that the accumulation of ethane can significantly increase the energy capacity of the ANG storage. Numerical molecular modeling of the methane-ethane mixture adsorption in slit-like model micropores has been carried out. The molecular effects associated with the displacement of ethane by methane molecules and the formation of a molecule layered structure are shown. The integral molecular adsorption isotherm of the mixture according to the molecular modeling adequately corresponds to the ideal adsorbed solution theory (IAST). The cyclic processes of gas charging and discharging from the ANG storage based on the ACPK are simulated in three modes: adiabatic, isothermal, and thermocontrolled. The adiabatic mode leads to a loss of 27-33% of energy capacity at 3.5 MPa compared to the isothermal mode, which has a 9.4-19.5% lower energy capacity compared to the thermocontrolled mode, with more efficient desorption of both methane and ethane.

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

In silico screening and experimental study of anion-pillared metal-organic frameworks for hydrogen isotope separation

Deuterium is a key energy source for fusion reaction and widely used in various application fields. Nevertheless, the separation of deuterium from its isotopic mixture with nearly same physicochemical properties is one of the greatest challenging tasks in industry. Compared with cryogenic distillation, quantum sieving effect based on nanoporous materials has been proposed as a promising strategy for isotopes separation, in which metal-organic frameworks (MOFs) are considered as an ideal platform because of their structural diversity and tailorable functionality. In this work, high-throughput molecular simulations were performed to identify optimal structural features of high-performance materials from a recently-constructed database of anion-pillared MOFs. As a proof-of-concept, the identified material SIFSIX-18-Cd was prepared using a proposed direct mixing method, which can achieve a goal of quickly preparation of such material on a large scale. The experimental pure-component adsorption isotherms of D2 and H2 confirmed the preferential adsorption of SIFSIX-18-Cd for D2 over H2 due to the quantum sieving effect. The advanced cryogenic thermal desorption spectroscopy (ACTDS) measurements further showed that the enrichment factor (EF) of an equimolar D2/H2 mixture in SIFSIX-18-Cd can reach 5.1 at 30 K and 1.0 bar, which is higher than the current cryogenic distillation method (1.5 at 24 K). Along with the proposed efficient preparation method, the high hydrophobic property of SIFSIX-18-Cd also demonstrated a broad industrial prospect of this material for hydrogen isotope separation.

Synthesis of Cu(I) doped mesoporous carbon for selective capture of ethylene from reaction products of oxidative coupling of methane (OCM)

There is a growing industrial interest in the recovery of ethylene from the gaseous products obtained in oxidative coupling of methane (OCM). The key challenge of employing OCM is the separation of ethylene from the product mixture that usually consists of ethylene, ethane, methane, carbon dioxide, carbon monoxide and hydrogen. In this work, we have synthesized Cu(I) doped mesoporous carbons as ethylene selective adsorbents to separate ethylene from the product mixture of OCM. The pristine mesoporous carbon was synthesized from resorcinol as carbon precursor and F127 as surfactant template. Cu(I) doping on mesoporous carbon was performed by CuCl as the precursor of Cu(I) with total Cu content of 0.3–3.3at.% in the resultant carbon. The structural identity of carbons was confirmed by pore textural properties, scanning electron microscopy (SEM), Energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). It was revealed that the selectivity towards ethylene is a function of Cu content of the mesoporous carbon indicating a possible π-complexation between ethylene and Cu(I). Pure component adsorption isotherms confirmed that ethylene adsorption is the highest followed by carbon dioxide, ethane, methane, carbon monoxide and hydrogen. IAST-based selectivity values for C2H4/C2H6, C2H4/CH4, C2H4/CO2, C2H4/CO and C2H4/H2 were calculated as 8-4, 80-48, 22-4, 251-36, 7644-18120767, respectively. The isosteric heat of adsorption of ethylene was in the range of 54-26 kJ/mol and it was higher than that of the other gases. The adsorbent also demonstrated good cyclability of ethylene adsorption. Finally, the simulated dynamic breakthrough results for fixed bed adsorption column demonstrated a large interval between the breakthrough times of ethylene and other gases, confirming its separation. The overall results confirm that Cu(I)-doped mesoporous carbons have a high potential for ethylene separation from OCM gas mixtures.

Isosteric heat of adsorption from thermodynamic Langmuir isotherm

The recently developed thermodynamic Langmuir isotherm model is used to estimate the isosteric heat of adsorption for pure component adsorption. Specifically, pure component adsorption isotherms at different temperatures are first correlated with the thermodynamic Langmuir isotherm model. Then the pure component isosteric heat of adsorption is calculated with the Clausius–Clapeyron equation. We show the thermodynamic Langmuir isotherm model correctly estimates the isosteric heat of adsorption as a function of surface loading for pure component adsorption.

A Rational Design of Microporous Nitrogen-Rich Lanthanide Metal-Organic Frameworks for CO2/CH4 Separation.

Three new lanthanide metal-organic frameworks IRHs-(1-3) supported by cyamelurate linkers have been synthesized and structurally characterized. The incorporation of numerous heteroatoms (N and O) into the pore walls and the relatively small microchannels of these porous solids enhance bonding force of the host-guest interactions, thus promoting the adsorption of carbon dioxide (CO2) over methane (CH4). The nonpolar covalent bonds in methane also favor the less uptake due to the hydrophilic walls of these frameworks. Grand canonical Monte Carlo simulations were performed to determine the origin of the adsorption. The density isocontour surfaces show that CO2 is mainly adsorbed on the walls composed of organic linkers and around the metal sites, whereas no specific adsorption site is observed for CH4, which indicates weak interactions between the framework and the adsorbed gas. As expected, the simulations show that CH4 is not observed around the metal center due to the presence of H2O molecules. The excellent selectivity of CO2/CH4 binary mixture was predicted by the ideal adsorbed solution theory (IAST) via correlating pure component adsorption isotherms with the Toth model. At 25 °C and 1 bar, the CO2 and CH4 uptakes for IRH-3 were 2.7 and 0.07 mol/kg, respectively, and the IAST predicated selectivity for CO2/CH4 (1:1) reached 27, which is among the best value for MOF materials.

Prediction of mixed‐gas adsorption equilibria from pure component adsorption isotherms

AbstractUsing only a single binary interaction parameter per adsorbate‐adsorbate pair, the adsorption nonrandom two‐liquid theory successfully correlates binary gas adsorption experimental data and predicts multicomponent gas adsorption equilibria. This work estimates the binary adsorbate–adsorbate interaction parameters from pure component isotherms and shows very promising agreement with the binary interaction parameters identified from regression of binary gas adsorption data. The results support the feasibility to predict binary and multicomponent gas adsorption equilibria from pure component isotherms for various adsorbents including silica gel, activated carbon, zeolites, and metal–organic frameworks.

Sustainable Biomass Glucose-Derived Porous Carbon Spheres with High Nitrogen Doping: As a Promising Adsorbent for CO2/CH4/N2 Adsorptive Separation.

Separation of CO2/CH4/N2 is significantly important from the view of environmental protection and energy utilization. In this work, we reported nitrogen (N)-doped porous carbon spheres prepared from sustainable biomass glucose via hydrothermal carbonization, CO2 activation, and urea treatment. The optimal carbon sample exhibited a high CO2 and CH4 capacity, as well as a low N2 uptake, under ambient conditions. The excellent selectivities toward CO2/N2, CO2/CH4, and CH4/N2 binary mixtures were predicted by ideal adsorbed solution theory (IAST) via correlating pure component adsorption isotherms with the Langmuir−Freundlich model. At 25 °C and 1 bar, the adsorption capacities for CO2 and CH4 were 3.03 and 1.3 mmol g−1, respectively, and the IAST predicated selectivities for CO2/N2 (15/85), CO2/CH4 (10/90), and CH4/N2 (30/70) reached 16.48, 7.49, and 3.76, respectively. These results should be attributed to the synergistic effect between suitable microporous structure and desirable N content. This report introduces a simple pathway to obtain N-doped porous carbon spheres to meet the flue gas and energy gas adsorptive separation requirements.

An activity-based formulation for Langmuir adsorption isotherm

This work presents an activity-based formulation for Langmuir adsorption isotherm. Treating adsorption as a chemical reaction between the gas molecule and the adsorption vacant site, the classical Langmuir isotherm model expresses the reaction in terms of the species concentrations. Designed to capture the surface heterogeneity, the proposed thermodynamic Langmuir isotherm model substitutes the species concentrations with the species activities and calculates the species activity coefficients with the adsorption non-random two-liquid activity coefficient model. The resulting isotherm model accurately represents pure component adsorption isotherms for gases with wide varieties of adsorbents including silica gels, activated carbons, zeolites and metal organic frameworks at various temperatures. With three physically meaningful parameters, the model outperforms the classical Langmuir isotherm model for the 98 isotherms of 33 systems examined.

Adsorption of binary hydrocarbon gas mixtures on activated carbon: Theoretical prediction of the binary equilibrium data

Experimental binary equilibrium data for the adsorption of hydrocarbon mixtures on activated carbon at given temperature are presented and discussed. Pure-component adsorption isotherms were fitted by using Langmuir model. Then adsorption isotherms were determined for different binary mixtures. Adsorption isotherms of binary mixtures were calculated using the real adsorbed solution theories, which requires the calculation of the activity coefficients for the components in the adsorbed phase. Activity coefficients are evaluated using Margules activity model and UNIQUAC model. We described the adsorption equilibria using proposed model parameters with taken to be equal to those of VLE. The results show that, except for a few systems, the calculated results were in good agreement with experimental adsorption equilibrium data.

Adsorption of propane and propylene on M-MOF-74 (M = Cu, Co): Equilibrium and kinetic study

In this study, adsorption equilibria and kinetics of propane and propylene on Cu-MOF-74 and Co-MOF-74 were investigated at temperatures of 303, 323, 343 and 363K and pressures up to 100kPa. Pure component adsorption isotherms and uptake curves were determined via volumetric method. Dual Site Sips model could better fit the experimental data than the Langmuir and Sips models. Isosteric heat of adsorption and IAST selectivity were calculated using Dual Site Sips model. Both adsorbents showed high affinity toward propylene over propane due to the presence of the open metal sites in their structure. Obtained results showed that Cu-MOF-74 exhibits higher selectivity (12.7) to propylene as well as higher adsorption capacity (7.6mmol/g) compared to Co-MOF-74, making this material as one of the most favorable adsorbents for propane/propylene separation which has been studied so far. The diffusion time constants of the adsorbates were also calculated by measuring transient uptake curves. Results indicated that the micropore diffusion model could predict the data points accurately. The rapid uptake of the adsorbates along with cyclable property suggest the Cu-MOF-74 as a highly reliable adsorbent for propane/propylene separation via vacuum pressure swing adsorption.

SYNTHESIS, CHARACTERIZATION, ADSORPTION AND THERMODYNAMIC STUDIES OF PURE AND BINARY CO2-N2 MIXTURES ON OXYGEN ENRICHED NANOSTRUCTURED CARBON ADSORBENTS

Abstract Oxygen enriched porous carbons have been synthesized by a nanocasting technique using mesoporous zeolite as template and epoxy resin as precursor. Characterization results show the effect of the nanocasting technique on the development of heterogeneous surface, high basicity, and high surface area of 686.37 m2g-1, beneficial for CO2 adsorption. Pure component adsorption isotherms were correlated with Langmuir, Sips, and dual-site Langmuir (DSL) models and found that Sips and DSL isotherm models fitted well, indicating thre heterogeneous nature of the adsorbent surface. Dynamic breakthrough data for the binary system CO2-N2 were obtained using a fixed-bed column at different adsorption temperatures (30-100 °C) and CO2 feed concentrations (5-12.5% by volume). The developed adsorbent shows high adsorption capacity with complete regenerability over four adsorption/desorption cycles. Prediction of binary components (CO2-N2) was made by using extended Sips, extended DSL and IAST (ideal adsorbed solution theory) by utilizing pure component adsorption isotherm data. Experimental and predicted equilibrium data were compared with breakthrough curve data and it was found that the extended forms (Sips and DSL) indicated under-predicted CO2 adsorption equilibria because of differences in adsorptive strengths of CO2 and N2 molecules. Also, adsorption equilibria were closely predicted using IAST theory. Asymmetric x-y diagrams from Raoult’s law indicated positive deviation, implying that as the CO2 gas phase molar fraction increases, total adsorbed amounts increase. Negative values of molar Gibbs free energy change suggested feasibility of the adsorption process. Formation of a more ordered configuration of CO2 molecules on the adsorbent surface was seen as a higher heat of adsorption was exhibited for CO2 as compared to N2.

Adsorption characteristics of light gases on basalt rock-based zeolite 4A

The adsorption characteristics of light gases on basalt rock-based zeolite 4A (BR zeolite-4A) were systematically investigated to evaluate its potential application as an alternative adsorbent for adsorption-based separation processes. We used alkali fusion and hydrothermal procedure to prepare the nanostructured adsorbent, BR zeolite-4A, which was characterized with field emission scanning electron microscopy, X-ray diffraction, and carbon dioxide adsorption apparatus. The single component adsorption equilibrium for CO2, CH4, N2 and H2 on the BR zeolite-4A was volumetrically determined using a nanoPOROSITY adsorption analyzer at the temperature range from (288.15 to 308.15) K and pressure range from (0.1 to 110) kPa. The experimental results indicate that BR zeolite-4A showed higher adsorption capacities for CO2 compared to other light gases, indicating the suitable porous material for selective separation by adsorption. Three different isotherm equations, Langmuir, Toth, and Sips, were used to correlate the adsorption isotherm data and the most reasonable results obtained from the Sips model irrespective of the adsorption isotherm types. Isosteric heat of adsorption and adsorption energy distribution function values were calculated and used to further examine the surface energetic heterogeneity of BR zeolite-4A. The pure component adsorption isotherm results were also used to predict the adsorption selectivity for CO2/N2, CO2/CH4, CO2/H2, and CH4/H2 binary mixtures (50:50) at different pressure ranges using ideal adsorbed solution theory.

Enhanced N-doped Porous Carbon Derived from KOH-Activated Waste Wool: A Promising Material for Selective Adsorption of CO₂/CH₄ and CH₄/N₂.

Separation of impurities (CO2 and N2) from CH4 is an important issue for natural gas alternatives (such as coalbed gas, biogas, and landfill gas) upgrading. It is notably challenging to synthesize high N-doped porous carbon with an appropriate porous structure. In this work, high N content (14.48 wt %) porous carbon with micropore size of 0.52 and 1.2 nm and specific surface area of 862 m2 g−1 has been synthesized from potassium hydroxide (KOH) activated waste wool upon the urea modification. Pure component adsorption isotherms of CO2, CH4, and N2 are systematically measured on this enhanced N-doped porous carbon at 0 and 25 °C, up to 1 bar, to evaluate the gases adsorption capability, and correlated with the Langmuir model. These data are used to estimate the separation selectivities for binary mixtures of CO2/CH4 and CH4/N2 at different mixing ratios according to the ideal adsorbed solution theory (IAST) model. At an ambient condition of 25 °C and 1 bar, the predicted selectivities for equimolar CO2/CH4 and CH4/N2 are 3.19 and 7.62, respectively, and the adsorption capacities for CO2, CH4, and N2 are 2.91, 1.01, and 0.13 mmol g−1, respectively. This report introduces a simple pathway to obtain enhanced N-doped porous carbon with large adsorption capacities for gas separation of CO2/CH4 and CH4/N2.

Prediction of adsorption isotherms from breakthrough curves

A mathematical model is used to predict adsorption isotherms from experimentally measured breakthrough curves. Using this approach, by performing only breakthrough experiments for a mixture of two (or more) components, one can obtain pure component adsorption isotherms up to the pressure of the experiment. As a case study, the adsorption of an equimolar mixture of CO2 and CH4 in zeolite ITQ-29 is investigated. Pure component linear adsorption isotherms for CO2 and CH4 are predicted by fitting the theoretical breakthrough curves to the experimental ones. Henry coefficients obtained from our approach are in excellent agreement with those measured experimentally. A similar procedure is applied to predict the complete Langmuir adsorption isotherm from breakthrough curves at high pressures. The resulting adsorption isotherms are in very good agreement with those measured experimentally. In our model for transient adsorption, mass transfer from the gas phase to the adsorbed phase is considered using the Linear Driving Force model and dispersion of the gas phase in the packed bed is taken into account. IAST is used to compute the equilibrium loadings for a mixture of gases. The influence of the dispersion coefficient and the effective mass transfer coefficient on the shape of breakthrough curves is investigated and discussed. Rough estimations of these values are sufficient to predict adsorption isotherms from breakthrough curves.

Molecular simulation of the preferential adsorption of CH4 and CO2 in middle-rank coal

ABSTRACTThe adsorption of the CO2/CH4 mixture in coal affects the CO2-enhanced coalbed methane recovery project. To gain a better understanding of CH4 and CO2 interaction with middle-rank coal, we developed a molecular concept with support for the sorption of CH4 and CO2 on Ximing-8 coal (XM-8) (1.8% vitrinite reflectance). A XM-8 coal model was built by using molecular dynamic (MD) simulations. The molecular simulations were established by the Grand Canonical Monte Carlo and MD methods to study the effects of the temperature, pressure, and species bulk mole fraction on the pure component adsorption isotherms, isosteric heat and adsorption selectivity. It turns out that the CO2 selectivity decreases as the pressure and its own bulk mole fraction increases, but it increases as temperature increases, and the selectivity values are not always greater than 1. The interactions between the small molecules and XM-8 were determined by using density functional theory. It was found that the interactions between the CO2 and XM-8 surface is greater, particularly for the heteroatoms than CH4. The adsorption selectivity and interaction were simultaneously used to reveal that the advantageously substituted range is high temperature, low pressure and a high content of heteroatoms.

Preparation and Evaluation of a Coconut Shell-Based Activated Carbon for CO2/CH4 Separation

Biomass is a widely distributed and renewable source of carbon. The main objective of this work is to produce an activated carbon from coconut shells with suitable characteristics to separate CO2 from biogas. The textural characterization of the adsorbent has been determined. Pure component adsorption isotherms of CO2 and CH4 at 30, 50 and 70 °C have been measured. The results reveal that the activated carbon had high CO2 adsorption capacity. Equilibrium of adsorption of CO2 and CH4 adsorption on the produced activated carbon reached 8.36 mmol/g and 4.63 mmol/g, respectively, at 30 °C and 10 bars. Moreover, the performance of the produced activated carbon, as a potential adsorbent for CO2 capture from a CO2/CH4 gas mixture, has been evaluated under dynamic conditions in a dedicated fixed-bed setup. The CO2 and CH4 adsorption capacities of the produced activated carbon are estimated to be 1.86 and 0.52 mol/kg, respectively, at 30 °C and 1 bar.