Equilibrium CO2 Research Articles

Converting CO2 from a Harmful Gas to a Renewable Source of Matter and Energy: A Review

The emission of greenhouse gases, specifically CO2, has reached non-acceptable levels causing global warming with adverse effects. Owing to rigorous ongoing research, these harmful substances might be converted to beneficial sources. The main objective of this study was to review the most effective processes that might utilize carbon dioxide to produce various substances, chemicals, and energy. For this, different existing and projected short-term and long-term strategies have been presented and discussed. Specifically, processes like the artificial tree of Lackner to capture CO2, chemical looping combustion (CLC), the application of CO2 in the food and processing industry, wastewater treatment, supercritical and refrigerant CO2, hydrogenation of CO2, the cultivation of microalgae, thermolysis, electrolysis, and photoelectrocatalysis techniques for producing hydrogen and biofuels, based on thermochemical processes, are the most promising ways to reduce and reuse CO2. Such behavioral changes can lead to the exchange of CO2 between natural reservoirs and help to maintain CO2 equilibrium among the atmosphere, the upper mixed layer of the sea, and the deep sea. Consequently, the mean global temperature and the climate are directly affected.

Effects of temperature, CO content, and reduction time on the selective reduction of a limonitic laterite ore

The production of nickel and cobalt from laterite ores has gained increasing attention as their demands continue to rise. This study evaluated the effects of temperature (700–1000 °C), CO content (10–70 vol%), and reduction time (10–90 min) on the selective reduction of a limonitic laterite ore and examined the mechanism of the selective reduction. SEM/EDS observation shows that the ore consists of goethite, hematite and small particles of clinochlore cemented by amorphous iron-rich silicate phase. Nickel and cobalt occur with goethite mineral in this ore. It was observed that fine grains of Fe-Ni-Co alloy dispersed within magnetite phase in reduced samples. XRD and XPS analyses show increased Fe2SiO4 generation with increasing reduction temperature, and elements nickel and cobalt were capatured in Fe2SiO4 according to SEM/EDS observation. Reduction at temperature above 850 °C resulted in a sharp increase in Fe2SiO4 generation and decrease in the specific surface area of the ore. The morphological and compositional observations indicate that CO content over 20 vol% promoted Fe formation from the laterite ore but hardly enhanced the formation of Ni and Co. The difference in equilibrium CO partial pressures for the formation of Fe, Ni, Co accounts for selective reduction. The current study suggests that selective reduction can be achieved by controlling reduction temperature and gas composition. Under the optimum condition of 850 °C, 20 vol% CO-80 vol% CO2 and 60 min, the degrees of metallisation of iron, nickel, and cobalt reduction were 17.5%, 87.3%, and 91.5%, respectively.

Evolving CO2 Rather Than SST Leads to a Factor of Ten Decrease in GCM Convergence Time.

The high computational cost of Global Climate Models (GCMs) is a problem that limits their use in many areas. Recently an inverse climate modeling (InvCM) method, which fixes the global mean sea surface temperature (SST) and evolves the CO2 mixing ratio to equilibrate climate, has been implemented in a cloud‐resolving model. In this article, we apply InvCM to ExoCAM GCM aquaplanet simulations, allowing the SST pattern to evolve while maintaining a fixed global‐mean SST. We find that InvCM produces the same climate as normal slab‐ocean simulations but converges an order of magnitude faster. We then use InvCM to calculate the equilibrium CO2 for SSTs ranging from 290 to 340 K at 1 K intervals and reproduce the large increase in climate sensitivity at an SST of about 315 K at much higher temperature resolution. The speedup provided by InvCM could be used to equilibrate GCMs at higher spatial resolution or to perform broader parameter space exploration in order to gain new insight into the climate system. Additionally, InvCM could be used to find unstable and hidden climate states, and to find climate states close to bifurcations such as the runaway greenhouse transition.

Rate-Based Absorption Modeling for Postcombustion CO2 Capture with Additively Manufactured Structured Packing

Carbon capture using amine-based solvents in an absorption process is a leading candidate for reducing greenhouse gas emissions in industrial flue gas streams. To reduce operating costs and associated parasitic energy of these processes, process intensification utilizing additively manufactured structured packing has emerged as a new technology to manage exothermic reactions during absorption while improving CO2 capture. A rate-based model framework has been developed for these novel packings that incorporates the mass and heat transfer phenomena for amine-based absorption of CO2. The rate-based model framework is first benchmarked using available solubility data and pilot plant data for aqueous monoethanolamine. The model validation shows accurate prediction of both CO2 equilibrium partial pressures and ion speciation for solubility data as well as CO2 capture, temperature profile, and solvent composition for pilot plant data in the absorption column. The model framework is then applied toward predicting the CO2 capture performance of additively manufactured structured packing. Simulations agree with experimental data in predicting the CO2 capture and the capture performance increase due to cooling within the structured packing device. Advantages of this rate-based model framework are the utilization of correlations that may predict mass transfer and heat transfer coefficients of the packing based on the geometric properties of the device and the implementation of the model framework in open-source programming.

Effects of modified atmosphere packaging on physicochemical properties of fresh‐cut ‘Deveci’ pears

This study aimed at determining the respiration rate of fresh-cut ‘Deveci’ pears and studying the effects of modified atmosphere packaging (MAP) on its quality. Respiration rate decreased with decreasing O2 level and it was determined as 1.04 ml O2/kg/hr or 1.16 ml CO2/kg/hr, under 4% O2+5% CO2 gaseous environment at 4°C. Michaelis-Menten with no inhibition was the best model to fit the respiration data. MAP under 2.15% O2+8.70% CO2 equilibrium atmosphere resulted in a lower total color change, a*-value, chroma, and a higher hue angle and L*-value compared to the air-control during 8-day storage at 4°C. The firmness of pear slices was better maintained in the MAP. The MAP treatment was successful in controlling the total soluble solids, pH, and titratable acidity of the fresh-cut pears. As a result, MAP of fresh-cut ‘Deveci’ pears has an important potential to preserve the physicochemical properties of the product. Novelty impact statement Respiration rate of fresh-cut ‘Deveci’ pears was reduced by 3–4-fold to 1.04 ml O2/kg/hr or 1.16 ml CO2/kg/hr values by modified atmosphere of 4% O2 and 5% CO2 at 4°C. Michaelis-Menten model with no CO2 inhibition was the best model for respiration rate as a function of O2 and CO2 concentrations. Modified atmosphere packaging improved texture and color, and maintained acidity and total soluble solids of fresh-cut ‘Deveci’ pears during storage.

Evaluating equilibrium and kinetics of CO2 and N2 adsorption into amine-functionalized metal-substituted MIL-101 frameworks using molecular simulation

The primary objective of the present research is to evaluate the equilibrium and kinetics of CO2 and N2 adsorption into different varieties of MIL-101 including Cr-, Al-, and Fe-based amine-functionalized MIL-101 metal organic frameworks (MOF) using grand canonical Monte-Carlo (GCMC) and molecular dynamics (MD) simulation methods. In the present study, an attempt is also made to clarify the adsorption mechanisms. To achieve this goal, the adsorption isotherms and self-diffusion coefficients of CO2 and N2 in these MIL-101 MOF adsorbents are computed at different temperatures using the universal force field (UFF). Isosteric heat of adsorption and the selectivity values of the CO2/N2 mixture are also calculated. Among the three computed atomic point charge models, including the QM-based Mulliken population analysis, electrostatic potential (ESP) derived, and semi-empirical charge equilibration (QEq) methods, the latter produces more accurate results compared to the others. The results show that molecular simulation using GCMC and MD methods with UFF and QEq can be an appropriate alternative to experiment for predicting equilibrium and kinetic data of the CO2 and N2 adsorption into different varieties of MIL-101. The radial distribution functions (RDF) indicate that the metal centers are dominant active sites compared to the amine groups in the amine-functionalized Cr-based MIL-101. However, it is not the case for the amine-functionalized Al- and Fe- based MIL-101 frameworks, for which the amine groups are more capable than metal sites to capture CO2 molecules.

Experimental and thermodynamic modeling of CO2 absorption into aqueous DEA and DEA+Pz blended solutions

The important environmental concern of the carbon dioxide (CO2) capture by the absorption process with amine solutions is studied in the current study. The experiments are performed at operating conditions of a temperature range of 25 65 °C, a pressure range of 1.5 6.5 bar, diethanolamine (DEA) concentration of 0 1.5 M, PZ concentration of 0 0.5 M, and stirrer speed of 0 300 rpm. The effects of operating conditions on CO2 loading, absorption rate, and mass transfer flux are investigated into two solutions of DEA and DEA+piperazine (Pz) blend in a stirred reactor. In the DEA+CO2+H2O system, by increasing stirrer speed from 0 to 300 rpm, the maximum values of CO2 loading and mass transfer flux at the same DEA concentration are increased 65% and 137%, respectively. The CO2 loading and mass transfer flux have higher values at higher initial pressures. The predicted values for species concentrations into the DEA+CO2+H2O system are also evaluated based on the Pitzer model, mass, and charge balance equations. The results showed that by increasing stirrer speed, the concentration of DEA molecule is decreased 40% but the concentrations of other ions and molecules are increased. In the DEA+Pz+CO2+H2O system, the results indicated that adding Pz to the absorbent solution was more efficient in CO2 capture relative to only the DEA solution. The equilibrium CO2 loading is increased by 20% and 16% for blend solutions with 4:6 and 6:4 molar ratios of DEA and Pz, respectively, but using only Pz solution is increased it up to 29%.

Experimental study of three-dimensional CO2-water drainage and fracture-matrix interactions in fractured porous media

Estimation of CO2 storage capacity in fractured porous reservoirs requires a better understanding of the CO2-water flow fundamentals in fracture-matrix systems. There are few core-flood experiments on three-dimensional CO2-water drainage in a fracture-matrix system, yet none have examined the impacts of the capillary continuity of fractures and fracture-matrix interactions. In this study, twelve drainage experiments were conducted in four fracture-matrix columns, each comprising a vertical stack of a cylindrical rock core, a filter paper serving as an analogue to a horizontal fracture, and a ceramic plate. The core sample was surrounded by open space at the top and circumferential sides to model fractures, allowing for 3-D CO2-water drainage in the rock core and displaced water draining across the horizontal fracture. X-ray computed tomography was conducted to visualize the dynamic invasion/drainage processes in four rock core samples showing contrasts in anisotropy, permeability, and heterogeneity. Experimental results show (1) the equilibrium CO2 saturations in the rock matrix vary from 0.10 to 0.60 at controlled capillary pressures up to 200 kPa, (2) the CO2 saturations in the matrix increaze with water saturation in the horizontal fracture resulting in better capillary continuity for water to drain across the fracture, and (3) the fracture water saturation can be enhanced by the non-uniform fracture capillary pressure and countercurrent flow of CO2 and water across the fracture-matrix interface. In the case of high fracture water saturation, matrix CO2 saturation largely depends on matrix anisotropy and heterogeneity. The core-scale experimental results contribute to understand the fracture-matrix interactions and CO2 storage efficiency in fractured porous media.

Evaluation of the Flamelet/Progress-Variable Approach and Flamelet-Generated Manifolds Method in Laminar Counter-Flow Diffusion Flame

To evaluate characteristics of the Flamelet/Progress-Variable approach (FPV) and Flamelet-Generated Manifolds method which can consider a detailed chemical reaction mechanism, a combustion simulation was performed in a laminar counter-flow diffusion flame. While the numerical solutions of the FPV reproduced the measurements and almost completely agreed with those of the detailed chemical reaction mechanism, the numerical solutions of the FGM method overpredicted the measurements of CO mole fraction and underpredicted the ones of CO2 especially in fuel-rich region, and differed from those of the detailed chemical reaction mechanism. This is because the flamelet table of the FGM indicates the state close to chemical equilibrium and overpredicts dissociation of CO2 when the combustion reaction sufficiently progresses.

Carbon Isotope Fractionation during the Formation of CO2 Hydrate and Equilibrium Pressures of 12CO2 and 13CO2 Hydrates.

Knowledge of carbon isotope fractionation is needed in order to discuss the formation and dissociation of naturally occurring CO2 hydrates. We investigated carbon isotope fractionation during CO2 hydrate formation and measured the three-phase equilibria of 12CO2–H2O and 13CO2–H2O systems. From a crystal structure viewpoint, the difference in the Raman spectra of hydrate-bound 12CO2 and 13CO2 was revealed, although their unit cell size was similar. The δ13C of hydrate-bound CO2 was lower than that of the residual CO2 (1.0–1.5‰) in a formation temperature ranging between 226 K and 278 K. The results show that the small difference between equilibrium pressures of ~0.01 MPa in 12CO2 and 13CO2 hydrates causes carbon isotope fractionation of ~1‰. However, the difference between equilibrium pressures in the 12CO2–H2O and 13CO2–H2O systems was smaller than the standard uncertainties of measurement; more accurate pressure measurement is required for quantitative discussion.

CO2 Absorption Using Hollow Fiber Membrane Contactors: Introducing pH Swing Absorption (pHSA) to Overcome Purity Limitation.

Recently, membrane contactors have gained more popularity in the field of CO2 removal; however, achieving high purity and competitive recovery for poor soluble gas (H2, N2, or CH4) remains elusive. Hence, a novel process for CO2 removal from a mixture of gases using hollow fiber membrane contactors is investigated theoretically and experimentally. A theoretical model is constructed to show that the dissolved residual CO2 hinders the capacity of the absorbent when it is regenerated. This model, backed up by experimental investigation, proves that achieving a purity > 99% without consuming excessive chemicals or energy remains challenging in a closed-loop system. As a solution, a novel strategy is proposed: the pH Swing Absorption which consists of manipulating the acido–basic equilibrium of CO2 in the absorption and desorption stages by injecting moderate acid and base amount. It aims at decreasing CO2 residual content in the regenerated absorbent, by converting CO2 into its ionic counterparts ( or ) before absorption and improving CO2 degassing before desorption. Therefore, this strategy unlocks the theoretical limitation due to equilibrium with CO2 residual content in the absorbent and increases considerably the maximum achievable purity. Results also show the dependency of the performance on operating conditions such as total gas pressure and liquid flowrate. For N2/CO2 mixture, this process achieved a nitrogen purity of 99.97% with a N2 recovery rate of 94.13%. Similarly, for H2/CO2 mixture, a maximum H2 purity of 99.96% and recovery rate of 93.96% was obtained using this process. Moreover, the proposed patented process could potentially reduce energy or chemicals consumption.

Kinetic approach to modelling CO2 adsorption from humid air using amine-functionalized resin: Equilibrium isotherms and column dynamics

Humidity can up to double the adsorption capacity of CO2 in conditions relevant to direct air capture (DAC) on amine-functionalized adsorbents, but the treatment of this phenomenon in isotherm or kinetic models has been all but neglected in the literature. In this work, a kinetic model based on reaction mechanisms of CO2 adsorption on supported amines in dry and humid conditions is proposed. The kinetic model was used in modelling of humid CO2 adsorption isotherms and dynamics of fixed-bed CO2 adsorption from air. The improvement of equilibrium CO2 capacity by humidity was captured well by the kinetic model, leading to good fits of CO2 isotherms. At best, the dynamic model closely represented the whole shape of CO2 adsorption breakthrough curves in several-hour fixed-bed adsorption experiments. Therefore, the proposed kinetic model is expected to be useful in simulation of the DAC process based on supported amine-adsorbents.

Experimental investigations and developing multilayer neural network models for prediction of CO2 solubility in aqueous MDEA/PZ and MEA/MDEA/PZ blends

AbstractIn this research, a new set of experimental data for CO2 solubility in aqueous blended amine solvents were investigated experimentally over the CO2 partial pressure range from 8 to 100 kPa at 40 °C and were compared with the benchmark aqueous 30 wt.% MEA solution. This work developed two multilayer neural network models named models A and B, for predicting the CO2 solubility in various aqueous blended amine solvents including 36 wt.% MDEA + 17 wt.% PZ, 24 wt.% MDEA + 26 wt.% PZ, and 6 wt.% MEA + 25 wt.% MDEA + 17 wt.% PZ. Models A and B were developed by using Levenberg–Marquardt back propagation algorithm with 427 and 301 of reliable experimental data sets gathered from the published data, respectively. The results indicate that the high accuracy prediction of the CO2 solubility in Methyldiethanolamine/Piperazine (MDEA/PZ) blends could be obtained by the network developed by Tan‐sigmoid transfer function with two hidden layers consist of eight and four neurons, while the network developed by Tan‐sigmoid transfer function with three hidden layers consist of 20, 10, and five neurons provided the highest accuracy for predicting the CO2 solubility in MEA/MDEA/PZ blends comparing to other model structures. The comparison results show that the neural network modeling provided more closer predictions to the experimental results than the simulator and other thermodynamic models when predicting the CO2 equilibrium solubility in blended amine solvents. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd.

Electrolyte solution of MDEA–PZ–TMS for CO2 absorption; response surface methodology and equilibrium modeling

This research aims to study the performance of MDEA–PZ–TMS​ blends solution in CO 2 reactive absorption via a stirrer reactor. Two modeling approaches of the response surface methodology (RSM) and equilibrium modeling using the modified Pitzer model were employed to predict process behavior at the equilibrium state. Five independent variables of temperature, pressure, MDEA, PZ, and TMS concentrations were considered in the ranges of 20–75 °C, 2–8 bar, and 1–5wt%, respectively. Based on the RSM modeling, both CO 2 loading and absorption percentage were simultaneously maximized with the degree of importance of 1 and 5, respectively, at operating conditions of 3.5 bar, 32.5 °C, 2wt% TMS, 4wt% MDEA, and 4wt% PZ concentrations. In the equilibrium modeling, the equilibrium CO 2 loading was predicted with an average relative error of 4.85% for all empirical data. The present modeling can be applied to various CO 2 absorption processes using different electrolyte solutions. • The CO 2 reactive absorption process is examined by MDEA-PZ-TMS blends solution. • The process is modeled by response surface methodology and equilibrium modeling. • Pressure, temperature, MDEA, PZ, and TMS concentrations are optimized by RSM. • Equilibrium modeling error is less than 5% makes suitable for different solvents. • The concentrations of all existing species in the electrolyte system are predicted.

Evaluation of CO2 Absorption by Amino Acid Salt Aqueous Solution Using Hybrid Soft Computing Methods.

Amino acid salt (AAs) aqueous solutions have recently exhibited a great potential in CO2 absorption from various gas mixtures. In this work, four hybrid machine learning methods were developed to evaluate 626 CO2 and AAs equilibrium data for different aqueous solutions of AAs (potassium sarcosinate, potassium l-asparaginate, potassium l-glutaminate, sodium l-phenylalanine, sodium glycinate, and potassium lysinate) gathered from reliable references. The models are the hybrids of the least squares support vector machine and coupled simulated annealing optimization algorithm, radial basis function neural network (RBF-NN), particle swarm optimization–adaptive neuro-fuzzy inference system, and hybrid adaptive neuro-fuzzy inference system. The inputs of the models are the CO2 partial pressure, temperature, mass concentration in the aqueous solution, molecular weight of AAs, hydrogen bond donor count, hydrogen bond acceptor count, rotatable bond count, heavy atom count, and complexity, and the CO2 loading capacity of AAs aqueous solution is considered as the output of the models. The accuracies of the models’ results were verified through graphical and statistical analyses. RBF-NN performance is promising and surpassed that of other models in estimating the CO2 loading capacities of AAs aqueous solutions.

Experimental Studies on Gas Hydrate‐Based CO2 Storage: State‐of‐the‐Art and Future Research Directions

Hydrate‐based CO2 storage is considered a potentially effective way of reducing greenhouse gas emissions and slowing down global warming. Herein, the locations in the ocean and permafrost that meet the requirements for hydrate‐based CO2 storage are summarized. Furthermore, research progress and shortcomings of hydrate‐based CO2 storage are analyzed. The two main methods for hydrate‐based CO2 storage are direct CO2 hydrate storage and CO2–CH4 hydrate replacement. Direct CO2 hydrate storage on the seabed and in the subsea (or permafrost) is proposed, and CO2 formation experiments on different scales are conducted. Various porous media are used to simulate the natural sediment and investigate the hydrate formation equilibrium and kinetics. CO2–CH4 replacement is preferred because it combines the benefits of CO2 storage and CH4 production. The hydrate equilibrium of different CO2–CH4 mixture hydrates is measured using different methods. Nuclear magnetic resonance and Raman spectroscopy are commonly used to study the hydrate structure characteristics and mass transfer properties. Macrokinetics are usually conducted in a high‐pressure vessel to analyze the effects of the experimental conditions on the replacement results. Nevertheless, the replacement rate and hydrate properties should be investigated further.

Vapor-liquid equilibrium and criticality of CO2 and n-heptane in shale organic pores by the Monte Carlo simulation

Thanks to the application of CO2 enhanced oil recovery (EOR) technology in shale oil production, phase behavior of CO2 and alkanes in nanopores has received considerable attention. Understanding the confinement effect on the vapor–liquid equilibrium and critical state of CO2 and alkanes is imperative to the calculation of phase behavior and prediction of oil recovery. In this work, the vapor–liquid equilibrium of pure CO2 and n-heptane (nC7) and their mixtures in shale organic pores are simulated using the Monte Carlo simulation. Compared to the bulk phase, the density of equilibrium gas is larger due to adsorption while the liquid phase density is lower because of capillary condensation. The influence of the confinement effect on the equilibrium phase composition results in the lower bubble point pressure and higher dew point pressure of the binary system. Changes in the equilibrium phase lead to a decrease in the critical temperature and pressure of single-component fluids and the minimum miscible pressure of binary system in nanopores. We fitted the critical property shift and analyzed the influence of pore size on fluid properties such as equilibrium pressure, composition, critical parameters, density distribution, etc. This study provides better insight into vapor–liquid equilibrium and critical state of CO2 and alkane in shale organic pores.

Effects of surfactant type and structure on properties of amines for carbon dioxide capture

Nonionic and cationic surfactants were tested for potential use as additives in monoethanolamine (MEA) solution to improve the wetting of solid packing materials (steel, stainless steel, or aluminum) by the solution. This is expected to improve the mass transfer of carbon dioxide (CO2) to the amine solution. The surface activities of surfactants were evaluated based on their ability to lower MEA’s surface tension, which is related to spreading and wetting of MEA on packing materials, to maximize the interfacial area for absorbing CO2. Effects of the surfactants on kinematic viscosity, foam ability and foam stability, CO2 equilibrium loading, and the initial CO2 absorption rate – deemed as the key properties of the MEA solution – were also measured. The results revealed that the surfactant could reduce surface tension of the MEA by 14–56%. Such reduction helped lower the contact angle of the MEA on the test materials by 40–55%. The surfactants did not significantly affect kinematic viscosity of MEA solution, but only slightly increased the amine’s viscosity. Neither were CO2 equilibrium loading nor initial CO2 absorption rate by MEA affected by any of the surfactants. A couple of surfactants, specifically a nonionic surfactant with a small hydrophilic head, and a cationic surfactant with a short linear chain, even slightly increased the initial CO2 absorption rate by MEA. However, the concentration of a surfactant in the MEA solution should be kept below 1 CMC to benefit from improved wetting while keeping foaming to a minimum.

Enhancing effect of UV activation of graphene oxide on carbon capture performance of metal-organic framework / graphene oxide hybrid adsorbents

• Novel CO 2 adsorbents by in-situ growth of Cu-BTC MOF on UV-irradiated GO developed. • UV treatment altered porosity characteristic and distribution of active sites. • The UV-GO/HKUST-1 adsorbents exhibited enhanced performance compared to pure MOF. • UV treatment is facile and can be easily scalable to produce highly-efficient hybrid adsorbents at industrial scale. Ability to capture carbon dioxide from diverse sources and conditions is vital in mitigating the impact of continuously increasing CO 2 levels into the atmosphere. Here, we developed a novel adsorbent based on Cu-BTC metal–organic framework (HKUST-1) and UV-irradiated graphene oxide (UV-GO) by in-situ growth, and assessed its CO 2 capture performance. The effect of the pre-growth UV activation of the GO counterpart on the chemical characteristics, microstructure, morphology, thermal behavior, and textural properties of the resulting hybrid adsorbent was evaluated. Equilibrium CO 2 adsorption isotherms and capture capacity were determined, while selectivity, regenerability, kinetics, and enthalpy of adsorption were evaluated as well. Water adsorption studies and stability, as well as CO 2 adsorption under the effect of humidity were also examined as to assess the capability of the hybrid adsorbents to capture CO 2 from humid mixtures. Formation of additional micro- and mesopores compared to pure HKUST-1 crystals was realized, which was enhanced upon 10-h UV treatment of the GO counterpart (HKUST-1@10UV-GO). Indicatively, the HKUST-1@10UV-GO hybrid adsorbent exhibited a CO 2 adsorption capacity of 5.14 mmol g −1 at 25 °C and 1 bar corresponding to a 45% increase compared to pure HKUST-1, accompanied with significantly enhanced CO 2 adsorption kinetics at higher temperatures. Comparing the adsorption performance at different temperatures (0, 25, 40, and 60 °C), a high CO 2 uptake of 9.5 mmol/g was evidenced for the HKUST-1@10UV-GO at 0 °C and 1 bar. Furthermore, the UV-treated hybrid adsorbents exhibited higher CO 2 /N 2 selectivity at 100 mbar rendering them promising for capture from low CO 2 concentration sources. Interestingly, the CO 2 uptake of HKUST-1@10UV-GO was not compromised in the presence of low amount of water (10% RH), rather it was slightly increased (by up to 3.2%), while hydrophilicity was suppressed compared to pure MOF, which brings an additional advantage for capture from humid mixtures.

Sensitivity analysis of mass transfer and enhancement factor correlations for the absorption of CO2 in a Sulzer DX packed column using 4-diethylamino-2-butanol (DEAB) solution

In this study, a rate-based model of post-combustion CO2 capture was developed for an absorption column packed with Sulzer DX. A new amine solution, 4-diethylamino-2-butanol (DEAB), was applied to the absorber as an active CO2 capture solvent, and a sensitivity analysis on mass-transfer coefficients in liquid and gas phases (kL and kG), effective interfacial area (ae), and enhancement factor (E) correlations was conducted to enhance the performance of the absorber. In the modeling, the absorber was divided into two sections—low capacity (less than 0.5 mol CO2/mol DEAB) and high capacity (more than 0.5 mol CO2/mol DEAB)—to improve model prediction accuracy, especially at high CO2 loading (αCO2). At αCO2 of less than 0.5, the CO2 equilibrium molar concentration (CCO2,e) was neglected, while at higher αCO2, the Deshmukh–Mather activity coefficient model was used to calculate CCO2,e. The solution procedure was validated against the axial experimental data of the CO2 mole fraction in the gas phase (yCO2) and the liquid-phase temperature (TL). Then, a sensitivity analysis was performed for the profiles of αCO2, yCO2, H2O mole fraction in the gas phase (yH2O), TL, and gas-phase temperature (TG) through various mass-transfer correlations for the Sulzer DX packing. The rate-based model with kL, kG and ae found that the correlation coefficient (R2) and average absolute relative deviation (AARD%) of yCO2 data were 0.9889 and 3.12, and those of TL data were 0.9685 and 2.09. The sensitivity analysis of various E correlations also revealed no significant difference between the calculated E values using the different relationships. Finally, the effects of αCO2, TL, and liquid flow rate (L) as the most important parameters of the absorber on the E value were investigated. The results indicated that αCO2 has a significant impact on the E values and consequently on the reaction rate.