Adsorption-based Processes Research Articles

Adsorption of Carbon Dioxide and Nitrogen in Co3(ndc)3(dabco) Metal-Organic Framework.

Metal-organic frameworks (MOFs) are promising materials for processes such as carbon dioxide (CO2) capture or its storage. In this work, the adsorption of CO2 and nitrogen (N2) in Co3(ndc)3(dabco) MOF (ndc: 2,6-naphthalenedicarboxylate; dabco: 1,4-diazabicyclo[2.2.2]octane) is reported for the first time over the temperature range of 273-323 K and up to 35 bar. The adsorption isotherms are successfully described using the Langmuir isotherm model. The heats of adsorption for CO2 and N2, determined through the Clausius-Clapeyron equation, are 20-27 kJ/mol and 10-11 kJ/mol, respectively. The impact of using pressure and/or temperature swings on the CO2 working capacity is evaluated. If a flue gas with 15% CO2 is fed at 6 bar and 303 K and regenerated at 1 bar and 373 K, 1.58 moles of CO2 can be captured per kg of MOF. The analysis of the multicomponent adsorption of typical flue gas streams (15% CO2 balanced with N2), using the ideal adsorbed solution theory (IAST), shows that at 1 bar and 303 K, the CO2/N2 selectivity is 11.5. In summary, this work reports essential data for the design of adsorption-based processes for CO2 capture using a Co3(ndc)3(dabco) MOF, such as pressure swing adsorption (PSA).

Adsorption Modeling Based on Classical Density Functional Theory and PC-SAFT: Temperature Extrapolation and Fluid Transfer.

Adsorption is at the heart of many processes from gas separation to cooling. The design of adsorption-based processes requires equilibrium adsorption properties. However, data for adsorption equilibria are limited, and therefore, a model is desirable that uses as little data as possible for its parametrization, while allowing for data interpolation or even extrapolation. This work presents a physics-based model for adsorption isotherms and other equilibrium adsorption properties. The model is based on one-dimensional classical density functional theory (1D-DFT) and the perturbed-chain statistical associating fluid theory (PC-SAFT). The physical processes inside the pores are considered in a thermodynamically consistent approach that is computationally efficient. Once parametrized with a single isotherm, the model is able to extrapolate to other temperatures and outperforms the extrapolation capabilities of state-of-the-art models, such as the empirical isotherm models from Langmuir or Toth. Furthermore, standard combining rules can be used to transfer parameters adjusted to an adsorbent/fluid pair to other fluids. These features are demonstrated for the adsorption of N2, CH4, and CO2 in metal-organic frameworks. Thereby, the presented model can calculate temperature-dependent isotherms for various fluids by using data limited to a single isotherm as input.

Operability-economics trade-offs in adsorption-based CO2 capture processes

Dispatchable low-carbon power underpins the transition to a sustainable energy system, providing balancing load for the integration of intermittent renewable power. In such load-following operation, the post-combustion carbon capture process must be capable of highly transient operation. Here we have developed a computational framework that integrates process design, operability and techno-economic assessment of a pressure-vacuum swing adsorption process for CO2 capture. We demonstrate that the cost-optimal design has limited process flexibility, challenging reactiveness to disturbances in the flue gas conditions. Flexibility can be introduced by relaxing the CO2 recovery constraint on the operation, albeit at the expense of the capture efficiency of the process. We discover that adsorption-based processes can be designed to enhance flexibility, while improving performance with respect to the operational constraints on CO2 recovery and purity. The results herein demonstrate a trade-off between process economics and process operability, which must be rationalised to integrate CO2 capture units in low-carbon energy systems.

Rapid and Accurate Screening of the COF Space for Natural Gas Purification: COFInformatics.

In this work, we introduced COFInformatics, a computational approach merging molecular simulations and machine learning (ML) algorithms, to evaluate all synthesized and hypothetical covalent organic frameworks (COFs) for the CO2/CH4 mixture separation under four different adsorption-based processes: pressure swing adsorption (PSA), vacuum swing adsorption (VSA), temperature swing adsorption (TSA), and pressure-temperature swing adsorption (PTSA). We first extracted structural, chemical, energy-based, and graph-based molecular fingerprint features of every single COF structure in the very large COF space, consisting of nearly 70,000 materials, and then performed grand canonical Monte Carlo simulations to calculate the CO2/CH4 mixture adsorption properties of 7540 COFs. These features and simulation results were used to develop ML models that accurately and rapidly predict CO2/CH4 mixture adsorption and separation properties of all 68,614 COFs. The most efficient separation process and the best adsorbent candidates among the entire COF spectrum were identified and analyzed in detail to reveal the most important molecular features that lead to high-performance adsorbents. Our results showed that (i) many hypoCOFs outperform synthesized COFs by achieving higher CO2/CH4 selectivities; (ii) the top COF adsorbents consist of narrow pores and linkers comprising aromatic, triazine, and halogen groups; and (iii) PTSA is the most efficient process to use COF adsorbents for natural gas purification. We believe that COFInformatics promises to expedite the evaluation of COF adsorbents for CO2/CH4 separation, thereby circumventing the extensive, time- and resource-intensive molecular simulations.

High-throughput computational screening of MOF adsorbents for efficient propane capture from air and natural gas mixtures.

In this study, we used a high-throughput computational screening approach to examine the potential of metal-organic frameworks (MOFs) for capturing propane (C3H8) from different gas mixtures. We focused on Quantum MOF (QMOF) database composed of both synthesized and hypothetical MOFs and performed Grand Canonical Monte Carlo (GCMC) simulations to compute C3H8/N2/O2/Ar and C3H8/C2H6/CH4 mixture adsorption properties of MOFs. The separation of C3H8 from air mixture and the simultaneous separation of C3H8 and C2H6 from CH4 were studied for six different adsorption-based processes at various temperatures and pressures, including vacuum-swing adsorption (VSA), pressure-swing adsorption (PSA), vacuum-temperature swing adsorption (VTSA), and pressure-temperature swing adsorption (PTSA). The results of molecular simulations were used to evaluate the MOF adsorbents and the type of separation processes based on selectivity, working capacity, adsorbent performance score, and regenerability. Our results showed that VTSA is the most effective process since many MOFs offer high regenerability (>90%) combined with high C3H8 selectivity (>7 × 103) and high C2H6 + C3H8 selectivity (>100) for C3H8 capture from air and natural gas mixtures, respectively. Analysis of the top MOFs revealed that materials with narrow pores (<10Å) and low porosities (<0.7), having aromatic ring linkers, alumina or zinc metal nodes, typically exhibit a superior C3H8 separation performance. The top MOFs were shown to outperform commercial zeolite, MFI for C3H8 capture from air, and several well-known MOFs for C3H8 capture from natural gas stream. These results will direct the experimental efforts to the most efficient C3H8 capture processes by providing key molecular insights into selecting the most useful adsorbents.

Study of methane upgrading using an activated carbon 3D-printed adsorbent

Although Maxsorb is a commercially available powder-form material, to be used in adsorption-based processes, its handling is easier if used with a structured geometry. Indeed, in the monolithic form, activated carbons have shown the potential to be used in adsorption-based applications. The use of additive manufacturing (AM) allows the development of structured adsorbents with adaptable geometries. In this work, Maxsorb activated carbon was 3D-printed into a monolithic structure using a polymeric binding agent, carboxymethylcellulose (CMC). The monolith was obtained through direct writing of filaments of an ink containing the adsorbent and the binding agent in a layered way, according to a pre-established pattern. The material was comprehensively characterized by N2 physisorption at 77 K and CO2 adsorption at 273 K to analyze the porous structure, SEM, and mercury porosimetry to evaluate the macrostructure porosity. To examine the affinity of CH4 and N2 with the adsorbent, pure adsorption equilibrium isotherms of these two gases on the 3D-printed monolith were measured at 303, 333, and 373 K by a gravimetric method. The adsorption capacities were compared to the ones of the original powder. The activated carbon materials showed a higher affinity to CH4. Furthermore, the adsorption equilibrium data of CO2 and Ar were also assessed. For all gases, the adsorption capacities decreased upon shaping. The monolith's CH4/N2 ideal selectivity was compared with its pristine powder (2.84 vs. 3.54). The CH4/N2 separation performance was also assessed through dynamic breakthrough tests. The experimental breakthrough curves were well predicted using a mathematical model.

Adsorption Technologies for the Removal of Cytostatics in Water: A Review

One of the most common treatments for cancer disease requires the administration of cytostatics, which are very effective drugs in the elimination of cancerous cells, but are toxic for healthy tissues. After being administered to patients, they are excreted and frequently reach natural water bodies, due to their poor degradation in wastewater treatment plants (WWTPs), posing a global threat to the environment and human health. The study of proper treatment approaches for the effective removal of these hazardous drugs in WWTPs is thus a topic of concern and of utmost importance to ensure environmental integrity, resilience and sustainability. The aim of this work is to perform a comprehensive review of the application of adsorption-based processes for the treatment of aqueous matrices contaminated with cytostatics, which has never been addressed before. A detailed discussion on the operating conditions, type and concentrations of sorbents used, toxicity of the effluents and other relevant parameters is presented. This paper aims to help identify the most promising sorbents and conditions, the current knowledge gaps, and future challenges/perspectives on adsorption technologies (isolated or coupled with other processes) to tackle the problem of cytostatic fingerprints in water courses. Additionally, information concerning the implementation of these technologies from an environmental and economic (life cycle assessment) perspective is given.

Using zeolite filters to reduce activated carbon use in micropollutant removal from wastewater

Micropollutants (MPs) are a wide group of human-made chemicals, such as pharmaceuticals and/or industrial compounds, with a negative impact on ecosystems. As they are discharged through water lines, they are a common presence in wastewater treatment plants (WWTPs). Among the known MPs removal treatments, adsorption-based processes are mostly used given their high MP removal capacity, although their production costs and environmental impact can be high. This is the case of powdered activated carbon (PAC), that remains nevertheless often the best option. Therefore, the optimization of PAC adsorption through the combination with natural zeolites is an interesting perspective. In this study, a combined approach made of zeolite-based filter and PAC to reduce water concentrations of eleven micropollutants is reported. The addition of a zeolite-based removal step increased the overall average removal by 14 % compared to 55 % obtained by the sole PAC adsorption. The combined zeolite-PAC process, by having higher removal efficiency allows a reduction in the use of PAC with interesting potential savings in costs and CO2 emissions. Also, a zeolite step before conventional PAC can ease the organic matter loading on the carbon, allowing for an improved recirculation in the biology tank, and can make the use of renewable sources-based PAC more appealing.

Heavy Metals in Foods and Beverages: Global Situation, Health Risks and Reduction Methods.

Heavy metals are chemical elements with a toxic effect on the human body. The expansion of industries has led to significant increasing levels of these constituents in the environment. Intensive agriculture can also lead to an increased concentration of heavy metals as a result of using different fertilizers and pesticides. Heavy metal accumulation in soil and plants represents a serious issue because of the potential risks to consumers. There are several methods available for the removal of these toxic components from different substrates (chemical precipitation, electrodialysis, coagulation and flocculation, photocatalytic removal, and adsorption-based processes), but most procedures are expensive and difficult to perform. Thus, more research is needed on the development of low-cost methods in foods. This work represents a review on the heavy metal presence in different food substrates (such as fruits and vegetables, milk and dairy products, meat and meat derivatives, oils, and alcoholic beverages) and provides an overview of the current situation worldwide, taking into account the fact that risks for human health are induced by the intensification of industry and the high degree of pollution. Considering that the toxicological quality of food affects its acceptability, this work provides valuable data regarding the actual situation on the proposed topic.

Organic molecular gate in mordenite for deep removal of acetylene and carbon dioxide from ethylene under humid condition

Adsorption-based processes for the ternary separation of CO2/C2H2/C2H4 show great potential and attractiveness, but commercialization remains a challenge due to the lack of efficient adsorbents. In this study, organic amine-modified mordenite (MOR) was developed, serving not only as a goalkeeper to accurately control ostiole and diffusion channels but also increasing the resistance to the influence of water molecules. This modification significantly improved the CO2/C2H2/C2H4 separation performance compared to pristine MOR, as evidenced by experimental and simulation results. ETA-MOR achieved excellent results in the deep purification of C2H4 from ternary CO2/C2H2/C2H4 mixtures, even under high humidity conditions. Cycling breakthrough tests demonstrated that the synthesized ETA-MOR material exhibited exceptional separation and structural stability. The obtained adsorbents exhibited excellent separation performance, thereby validating the effectiveness and potential of simple amine modification on commercially viable zeolites for industrial light hydrocarbon adsorption.

A comparative exergy-based assessment of direct air capture technologies

The 6th Assessment of the IPCC underlined the need for urgent measures for carbon dioxide removal from the atmosphere, so as to meet the 1.5 °C goal by the end of this century. One option to achieve this is direct air capture (DAC) technologies. This work assesses the thermodynamic performances of different categories of DAC technologies, i.e., adsorption-based, absorption-based, ion exchange, and electrochemical. An exergy analysis is performed on the DAC processes in each category to identify hotspots for efficiency loss within the system. The results show that the consumption of materials is responsible for 5–40% of the exergy consumption of the most-developed DAC processes. Despite their greater use of materials compared to absorption-based processes, adsorption-based processes, together with ion exchange technologies, have the highest exergy efficiencies of the DAC technologies investigated. Moreover, the results highlight the importance of limiting material consumption and electrifying large-scale DAC plants, which cannot run exclusively on waste heat from industrial processes.

Guest-induced structural deformation in Cu-based metal-organic framework upon hydrocarbon adsorption

In a world where capture and separation processes represent above 10% of global energy consumption, novel porous materials, such as Metal-Organic Frameworks (MOFs) used in adsorption-based processes are a promising alternative to dethrone the high-energy-demanding distillation. Shape and size tailor-made pores in combination with Lewis acidic sites can enhance the adsorbate-adsorbent interactions. Understanding the underlying mechanisms of adsorption is essential to designing and optimizing capture and separation processes. Herein, we analyze the adsorption behaviour of light hydrocarbons (methane, ethane, ethylene, propane, and propylene) in two synthesized copper-based MOFs, Cu-MOF-74 and URJC-1. The experimental and computational adsorption curves reveal a limited effect of the exposed metal centers on the olefins. The lower interaction Cu-olefin is also reflected in the calculated enthalpy of adsorption and binding geometries. Moreover, the diamond-shaped pores' deformation upon external stimuli is first reported in URJC-1. This phenomenon is highlighted as the key to understanding the adsorbent's responsive mechanisms and potential in future industrial applications.

Agroindustrial waste conversion into ultramicroporous activated carbons for greenhouse gases adsorption-based processes

Activated carbons (ACs) for greenhouse gases adsorption were developed via KOH activation of waste twigs arising from processing of Ilex paraguariensis branches. The effect of KOH/carbonized twigs mass ratio (R = 1–3) and temperature (873–1073 K) was examined. The ACs showed BET areas between 1000 and 2150 m2 g−1, total pore volumes of 0.3–1.0 cm3 g−1, and completely microporous structures with large presence of ultramicropores (≤1.1 nm). The ACs obtained with R = 2 at 873 K attained the highest CO2 adsorption capacity (5.4 mmol g−1). An unprecedented correlation to estimate CO2 adsorption capacity in terms of ACs properties was developed. For the best sample, dynamic assays for CO2 and CH4 adsorption from single and binary equimolar gaseous streams were conducted at 101.32 kPa and 303 K. Regeneration showed that the ACs almost kept the initial dynamic adsorption capacity of both gases (1.4 mmol CO2 g−1 and 0.4 mmol CH4 g−1) after five adsorption-desorption cycles.

Carbon materials porosity analysis using DFT models for potential application in the recovery of methane from its low-concentration mixtures

One of the key factors in the adsorption-based processes such as Pressure Swing Adsorption is the proper selection of the adsorbent material. The primary motivation of this work is the thorough analysis of the porous structure of selected carbon materials for this process. Considering that the pore size distribution (PSD) determines the specific part of the adsorbent structure that is accessible to probe molecules of N2 and CO2 used for the analysis, we discuss the optimal pore size for the adsorption of CH4 at 293 K having molecular diameter is larger than N2 and CO2. To perform a detailed textural characterization of our carbon samples using the N2 and CO2 adsorption isotherms, we apply the 2D-NLDFT and QSDFT/NLDFT models available in the SAIEUS and ASiQwin software packages, respectively. To discuss the relationship between the PSD and the CH4 adsorption of our samples, we chose to use PSDs derived by the dual gas N2 & CO2 analysis method (SAIEUS software). We show that it is possible to estimate the experimental CH4 adsorption isotherms at 293 K based on the porous texture characteristics derived from N2 at 77 K and CO2 at 273 K and the application of appropriate 2D-NLDFT models. Furthermore, we demonstrate that the described carbon PSD characterization and the following prediction methods constitute valuable links between the textural properties and CH4 adsorption isotherms of the carbon materials. We believe that the results of this work will be a practical guide for future research on the selection of adsorbents for the separation of mixtures containing low concentrations of methane.

Adsorption of Carbon Dioxide, Methane, and Nitrogen on Zn(dcpa) Metal-Organic Framework

Adsorption-based processes using metal-organic frameworks (MOFs) are a promising option for carbon dioxide (CO2) capture from flue gases and biogas upgrading to biomethane. Here, the adsorption of CO2, methane (CH4), and nitrogen (N2) on Zn(dcpa) MOF (dcpa (2,6-dichlorophenylacetate)) is reported. The characterization of the MOF by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), and N2 physisorption at 77 K shows that it is stable up to 650 K, and confirms previous observations suggesting framework flexibility upon exposure to guest molecules. The adsorption equilibrium isotherms of the pure components (CO2, CH4, and N2), measured at 273–323 K, and up to 35 bar, are Langmuirian, except for that of CO2 at 273 K, which exhibits a stepwise shape with hysteresis. The latter is accurately interpreted in terms of the osmotic thermodynamic theory, with further refinement by assuming that the free energy difference between the two metastable structures of Zn(dcpa) is a normally distributed variable due to the existence of different crystal sizes and defects in a real sample. The ideal selectivities of the equimolar mixtures of CO2/N2 and CO2/CH4 at 1 bar and 303 K are 12.8 and 2.9, respectively, which are large enough for Zn(dcpa) to be usable in pressure swing adsorption.

Biomass Valorization to Produce Porous Carbons: Applications in CO2 Capture and Biogas Upgrading to Biomethane—A Mini-Review

Porous carbon materials, derived from biomass wastes and/or as by-products, are considered versatile, economical and environmentally sustainable. Recently, their high adsorption capacity has led to an increased interest in several environmental applications related to separation/purification both in liquid- and gas-phases. Specifically, their use in carbon dioxide (CO2) capture/sequestration has been a hot topic in the framework of gas adsorption applications. Cost effective biomass porous carbons with enhanced textural properties and high CO2 uptakes present themselves as attractive alternative adsorbents with potential to be used in CO2 capture/separation, apart from zeolites, commercial activated carbons and metal-organic frameworks (MOFs). The renewable and sustainable character of the precursor of these bioadsorbents must be highlighted in the context of a circular-economy and emergent renewable energy market to reach the EU climate and energy goals. This mini-review summarizes the current understandings and discussions about the development of porous carbons derived from bio-wastes, focusing their application to capture CO2 and upgrade biogas to biomethane by adsorption-based processes. Biogas is composed by 55–65 v/v% of methane (CH4) mainly in 35–45 v/v% of CO2. The biogas upgraded to bio-CH4 (97%v/v) through an adsorption process yields after proper conditioning to high quality biomethane and replaces natural gas of fossil source. The circular-economy impact of bio-CH4 production is further enhanced by the use of biomass-derived porous carbons employed in the production process.

Exploring periodic mesoporous organosilicas for ethane–ethylene adsorption–separation

Ethylene is a core building block in the chemical industry and its separation from ethane is very challenging due to high energy requirements. Adsorption-based processes can be an alternative to current processes based on cryogenic distillation. In this work, we explore for the first time the application of periodic mesoporous organosilica (PMO) materials, whose surface properties can be tuned with different functional groups. With the aim of correlating the PMO structure with ethane and ethylene adsorption, eight different PMO materials have been prepared, viz. the phenylene-bridged PMO, PMO aminated at the organic bridges, and PMO functionalized by silylation of free silanol (inorganic) moieties. High pressure adsorption isotherms were measured, and the separation selectivity and phase diagrams of a binary mixture of ethane and ethylene were estimated. Results have shown that, overall, the PMO studied tend to be more selective towards ethylene than ethane probably due to the interactions between the quadrupole moment of ethylene and the free silanols in the samples. After silylation, the novel materials presented surfaces with higher affinity towards ethane than those of the pristine material. For the aminated samples, functionalization with primary amines originated materials displaying better selectivity towards ethylene than those functionalized with secondary or tertiary amines.

Adsorption of organic molecules on carbon surfaces: Experimental data and molecular dynamics simulation considering multiple protonation states

Owing to their high specific surface and low production cost, carbon materials are among the most important adsorption materials. Novel usages, for instance in pharmaceutical applications, challenge existing methods because charged and strongly polar substances need to be adsorbed. Here, we systematically investigate the highly complex adsorption equilibria of organic molecules having multiple protonation states as a function of pH. The adsorption behavior depends on intermolecular interactions within the solution (dissociation equilibria) and between adsorbed molecules on the carbon surface (electrostatic forces). For the model substances maleic acid and phenylalanine, we demonstrate that a custom-made genetic algorithm is able to extract up to nine parameters of a multispecies isotherm from experimental data covering a broad pH-range. The parameters, including adsorption affinities, interaction energies, and maximum loadings were also predicted by molecular dynamics simulations. Both approaches obtained a good qualitative and mostly also quantitative description of the adsorption behavior within a pH-range of 2–12. By combining the determined isotherms with mass balances, the final concentrations and pH-shifts of batch adsorption experiments can be predicted. The developed modeling tools can be easily adapted to other types of pH-dependent, multispecies adsorbates and therefore will help to optimize adsorption-based processes in different fields.

Influence of the main operating parameters on the DRPSA process design based on the equilibrium theory

Among the adsorption-based separation processes for gaseous mixtures, those exploiting pressure variations, so-called Pressure Swing Adsorption (PSA) processes, are the most popular. In this work, we focus on the specific PSA configuration known as Dual Reflux-Pressure Swing Adsorption (DR-PSA) given its ability to achieve sharp separations. In the case of binary mixtures, an analytical approach based on Equilibrium Theory has been proposed to identify the operating conditions for complete separation under the assumption of linear isotherms. This same approach is not available when the separation is not complete. Accordingly, in this work we study the features of non-complete separations by solving numerically a general DR-PSA model with parameter values suitable to approach equilibrium conditions (no mass transport resistances, no axial mixing, isothermal conditions and no pressure drop), thus reproducing the analytical solution when complete separations are examined. Even for non-complete separations, triangularly shaped regions at constant purity can be identified on a plane whose axes correspond to suitable design parameters. Moreover, we found a general indication on how to select the lateral feed injection position to limit the loss in product purities when complete separation is not established, whatever is the composition of the feeding mixture. Finally, a sensitivity analysis with respect to pressure ratio, light reflux ratio and heavy product flowrate is proposed in order to assess how to recover product purities according to the specific degrees of freedom of a DR-PSA apparatus.

Generalized, Adsorbent-Agnostic, Artificial Neural Network Framework for Rapid Simulation, Optimization, and Adsorbent Screening of Adsorption Processes

Adsorption-based processes, such as pressure swing adsorption, because of their cyclic nature and the presence of multiple design variables, are challenging to simulate and optimize. Here, we intro...