CSA-LSSVM model for the estimation of solubility of n-alkane in supercritical CO2

Poor solubility of substantial hydrocarbons in CO₂ has constrained the use of CO₂-EOR (enhanced oil recovery) in the modern oil recovery industry to some extent. Subsequently, it is significant to research the solubility regularity of various hydrocarbons in supercritical carbon dioxide (scCO₂) in the first place. CO₂ injection as one of the popular methodologies in light of financially and environmentally friendly has wide applications in EOR. In this paper, our objective is to estimate the solubility of n-alkanes in scCO₂. This study highlights the application of a model based on least square support vector machine for estimation of solubility of n-alkanes in scCO₂. The tuning parameters of the developed model were determined by an optimization algorithm, namely coupled simulated annealing. A set of 184 data points of solubility was used to execute the new model. To assess the accuracy and effectiveness of the developed model for prediction of experimental data, statistical and graphical techniques were used. Moreover, the outcomes were compared with the results of literature correlations to predict the solubility of alkanes. Results demonstrate that the model is precise and viable for prediction of solubility data. The resulted values of R², root-mean-square error, SD, and % average absolute relative deviation for total data points are 0.99204, 0.12862, 0.6437, and 0.7753 for overall data, respectively.     

Phase equilibria prediction model for semiclathrate hydrates formed from CH4, CO2, and N2 in the presence of tetra-n-butyl ammonium bromide

In this work, a prediction model was studied and carried out to predict phase equilibrium conditions for semiclathrate hydrates formed from the ternary system of liquid water, tetra-n-butyl ammonium bromide, and CO₂, CH₄, or N₂. Based on the Soave–Redlich–Kwong equation of state along with the Chen–Guo model, the predictions of phase equilibria of semiclathrates of CO₂, CH₄, and N₂ + pure water/tetra-n-butyl ammonium bromide aqueous solution has been made. A new correlation for activity relating to the system temperature and tetra-n-butyl ammonium bromide concentration has been proposed. The prediction results were analyzed and showed good agreement with the experimental data.     

Prediction of CO2 adsorption on different activated carbons by hybrid group method of data-handling networks and LSSVM

CO₂ is a ubiquitous species that has received much attention recently. The adsorption of CO₂ by means of activated carbons is a well-tried technology that can be used on a large scale. Improvements of the prediction models with more accurate results and lower error are necessary for future development of the projects and the economic dispatch sector. The least square support vector machine, a relatively unexplored neural network known as group method of data handling (GMDH), were implemented to forecast the CO₂ adsorption on different activated carbons. This work aims to provide new methods to predict the adsorption equilibrium of pure CO₂ on a set of commercial activated carbons and to express it regarding textural properties such as Brunauer–Emmett–Teller (BET) surface area, total pore volume, and micropore volume. Results indicated that the utilized models are very accurate in predicting CO₂ adsorption on different activated carbons. Comparison of the outcomes of the two models shows that the GMDH model is more accurate with R² and mean squared error values of 0.8915 and 0.0001425, respectively.     

CFD simulation of CO2 removal from hydrogen rich stream in a microchannel

The main object of this research is to perform computational fluid dynamics simulation of CO₂ capturing from hydrogen-rich streams by aqueous DEA solution in a T-Junction microchannel contactor with 250 μm diameter and 5 mm length at dynamic conditions. To develop a comprehensive mathematical framework to simulate the flow hydrodynamics and mass transfer characteristics of system, the continuity and Navier-Stokes equations, two phase transport, and reaction rate model are coupled in COMSOL Multiphysics software. The developed model is solved and the effects of gas and liquid velocities as well as amine concentration on the CO₂ absorption rate, hydrogen purification fraction, and flow hydrodynamic are investigated. The absorption process consists of CO₂ diffusion from bubble bulk toward the bubble boundary, CO₂ solubility in the liquid boundary, diffusion from the boundary into the liquid bulk, and reaction with the amine molecules. The results show that when the gas and liquid streams are mixed in the junction point to form a bubble, the gas cross-section area becomes narrow, and the fluid velocity increases due to the applied force on the bubble by the liquid layers. It appears that increasing the DEA concentration in the inlet from 5% to 20% increases hydrogen purification fraction from 42.3 to 66.4%, and up to 96.7% hydrogen purity is achieved by 20% aqueous solution of DEA.     

The role of oxygen vacancies in the CO2 methanation employing Ni/ZrO2 doped with Ca

The Ni/ZrO₂ catalyst doped with Ca and Ni/ZrO₂ were employed in the CO₂ methanation, a reaction which will possibly be used for storing intermittent energy in the future. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS, reduction in situ), X-ray diffraction (XRD, reduction in situ and Rietveld refinement), electron paramagnetic resonance (EPR), temperature-programmed surface reaction, cyclohexane dehydrogenation model reaction, temperature-programmed desorption of CO₂ and chemical analysis. The catalytic behavior of these catalysts in the CO₂ methanation was analyzed employing a conventional catalytic test. Adding Ca to Ni/ZrO₂, the metallic surface area did not change whereas the CO₂ consumption rate almost tripled. The XRD, XPS and EPR analyses showed that Ca⁺² but also some Ni²⁺ are on the ZrO₂ surface lattice of the Ni/CaZrO₂ catalyst. These cations form pairs which are composed of oxygen vacancies and coordinatively unsaturated sites (cus). By increasing the number of these pairs, the CO₂ methanation rate increases. Moreover, the number of active sites of the CO₂ methanation rate limiting step (CO and/or formate species decomposition, rls) is enhanced as well, showing that the rls occurs on the vacancies-cus sites pairs.     

Effects of aqueous solubility and molecular diffusion on CO2-enhanced hydrocarbon recovery and storage from liquid-rich shale reservoirs

This study investigates the effects of aqueous solubility and aqueous molecular diffusion of gaseous components (CO₂ and CH₄) on the CO₂ huff and puff process in the liquid-rich shale reservoirs. In a gas-condensate reservoir, the effects negligibly enhance the hydrocarbon production, but increase the CO₂ storage with 4.4% by solubility trapping. In a shale oil reservoir, they increase the production and CO₂ storage up to 5% and 14% by more CO₂ imbibition and contacting between CO₂ and oil. This study clarifies the importance of aqueous solubility and aqueous-phase molecular diffusion of CO₂ huff and puff in shale reservoirs.     

Methanation of CO2 on bulk Co–Fe catalysts

The efficiency of CO₂ methanation was estimated through gas chromatography in the presence of Co–Fe catalysts. Scanning electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy were applied for ex-situ analysis of the catalysts after their test in the methanation reaction. Thermal programmed desorption mass spectroscopy experiments were performed to identify gaseous species adsorbed at the catalyst surface. Based on the experimental results, surface reaction model of CO₂ methanation on Co–Fe catalysts was proposed to specify active ensemble of metallic atoms at the catalyst surface, orientation of adsorbed CO₂ molecule on the ensemble and detailed reaction mechanism of CO₂→CH₄ conversion. The reaction step when OH group in the FeOOH complex recombined with the H atom adsorbed at the active ensemble to form H₂O molecule was considered as the rate-limiting step.     

Unveiling the mechanism of controllable CO2 hydrogenation by group VIB metal single atom anchored on N-doped graphite: A density functional theory study

CO₂ hydrogenation has raised considerable interest due to concerns about climate change. Realizing low-temperature reverse water gas shift (rWGS) reaction remains a significant challenge in the context of coupling it with the C–C growth reactions to convert CO₂ to C₂₊ fuels. We carried out systematic DFT simulations to unveil the underlying low-temperature mechanism for the selective hydrogenation of CO₂ to produce CO, over a variety of metal-based single atom catalysts (SACs) supported on the nitrogen-doped graphite. Group VIB metal-based SACs outperformed other 15 metal candidates in terms of versatile capacities in both selective activation of CO₂ molecule and facilitating escaping of CO and H₂O. Mo₁/N₃-Gt was especially outstanding by giving rise to spontaneous production of CO and O1 through an effective electron injection into the CO₂ molecule. Water formation has been identified as the potential rate-controlling step in such a catalytic reaction over Mo₁/N₃-Gt with an energy barrier of 1.10 eV. Herein, the H migration played a pivotal role and had tight affinity to the charge of H1 on the active site of catalyst. The dynamic coordination environment of Mo^δ+ was revealed to be the dominant factor affecting the surface H1 charge, leading to a variety of hydrogenation behaviors. The electron-deficient ligands of CO₂ and O1 on Mo₁/N₃-Gt, as well as additional adsorbed H₂, were effective in adjusting the 4d and 5s electronic structure of central Mo and consequently resulted in nearly electric neutral surface H1s, thus most benefiting the hydrogenation process. The optimal charge of the coordinated Mo for an outstanding selective hydrogenation performance in this scenario was found to be no less than +1.7e.     

Preliminary investigations of scaling and corrosion in high enthalpy geothermal wells in Hungary

A solubility equilibrium program GEOPROF was applied to the determination of the bubble-point depth, pressure and temperature, as well as the partial pressure profiles of the gases CO₂, CH₄ and N₂ between the bubble-point depth and the wellhead, in two high enthalpy geothermal wells, NSZ-2 and FAB-4 in southern Hungary. The pH, alkalinity, total carbonates, and equilibrium solubility for CaCO₃, CaSO₄, BaSO₄, and SrSO₄ along the well depth profile in the Na–K–Mg–Ca–H–Ba–Sr–Cl–Br–SO₄–OH–HCO₃–CO₃–CO₂–H₂O system were also determined and the concentrations of Ca²⁺, Ba²⁺, Sr²⁺, H⁺, OH⁻, HCO₃⁻, CO₃²⁻, and H₂CO₃* were computed at the actual temperature and CO₂ pressure using the Davies and Pitzer activity calculation methods. The calculated amounts of CaCO₃ scaling along the wells and at the surface were used in estimating service life. The results for well FAB-4 contain high uncertainties because of the estimated gas separation analysis data.     

Thermal and chemical contributions of added H2O and CO2 to major flame structures and NO emission characteristics in H2/N2 laminar diffusion flame

Numerical simulation with detailed chemistry has been carried out to clearly discriminate the thermal and chemical contributions of added diluents (H₂O and CO₂) to major flame structures and NO emission characteristics in H₂/N₂ counterflow diffusion flame. The pertinence of GRI, Miller–Bowman, and their recent modified mechanisms are estimated for the combined fuel of H₂, CO₂, and N₂. A virtual species X, which displaces the individual CO₂ and H₂O in the fuel sides, is introduced to separate chemical effects from thermal effects. In the case of H₂O addition the chain branching reaction, H + O₂ → O + OH is considerably augmented in comparison with that in the case of CO₂ addition. It is also seen that there exists a chemically super‐adiabatic effect in flame temperature due to the breakdown of H₂O. The reaction path of CH₂O→CH₂OH→CH₃ and the C1‐branch reactions become predominant due to the breakdown of CO₂. In NO emission behaviour super‐equilibrium effects caused by the surplus chain carrier radicals due to the breakdown of added H₂O are more superior to the enhanced effects of prompt NO with the breakdown of added CO₂. Especially, it is noted that thermal NO emission is directly influenced by the chemical super‐equilibrium effects of chain carrier radicals in the case of H₂O addition. As a result the overall NO emission in the case of the addition of H₂O is higher than that in the case of CO₂ addition. Copyright © 2002 John Wiley & Sons, Ltd.     

Gasification reactivity of biomass chars with CO2

In this study, carbon conversion was calculated from the data obtained with a real-time gas analyzer. In a lab-scale furnace, each biomass sample was pyrolyzed in a nitrogen environment and became biomass char. For preparation of the char, the furnace was electrically heated over 40 min up to the wall temperature of 850 °C, and maintained at the same temperature over 17 min. The furnace was again heated over 3 min to a temperature higher than 850 °C and then CO₂ was injected. The biomass char was then gasified with CO₂ under isothermal conditions. The reactivity of biomass char was investigated at various temperatures and CO₂ concentrations. The VRM (volume reaction model), SCM (shrinking core model), and RPM (random pore model) were used to interpret the experimental data. For each model, the activation energy (E) and pre-exponential factor (A) of the biomass char-CO₂ reaction were determined from gas-analysis data by using the Arrhenius equation. For the RPM, the apparent reaction order was determined. According to this study, it was found that the experimental data agreed better with the RPM than with the other two models. Through BET analyses, it was found that the structural parameter (ψ) of the surface area for the RPM was obtained as 4.22.     

Prediction of CO2 Solubility in Oil and the Effects on the Oil Physical Properties

Abstract

CO₂ solubility in oil is a key parameter in CO₂ flooding process. It results in oil swelling, increased oil density, and decreased oil viscosity. Laboratory studies needed to cover a wider range of data, and are time consuming, costly, and may be not available or possible in many situations. On the other hand, although various models and correlations are useful in certain situations, they may are not be applicable in many situations.

In this study, a new genetic algorithm- (GA)-based technique has been used to develop more reliable correlations to predict CO₂ solubility, oil swelling factor (SF), CO₂-oil density, and viscosity of CO₂-oil mixtures. Based on the Darwinian theory, the GA technique mimics some of the natural process mechanisms. Furthermore, GA-based model correlations recognize all the major parameters that affect each physical property and also well address the effects of CO₂ liquefaction pressure.

Genetic algorithm-based correlations have been successfully validated with published experimental data. In addition, a comparison of these correlations has been made against widely used correlations in the literature. It has been noted that the GA-based correlations yield more accurate predictions with lower errors than all other correlations tested. Furthermore, unlike other correlations that are applicable to limited data ranges and conditions, GA-based correlations have been validated over a wider range of data.     

Validation challenges in solid oxide electrolysis cell modeling fueled by low Steam/CO2 ratio

This study addresses some of the challenges in the modeling of solid oxide electrolysis cells (SOECs), particularly for feed gases containing low inlet steam-to-CO₂ concentration ratios. The common approach used for SOEC modeling is to neglect the CO₂ electrochemical reduction reaction. A comprehensive model validation versus experimental results presented by the Idaho National Laboratory (INL) is performed in this paper. Our validation results under various operating conditions show that the model deviation with experimental data increases above certain current densities, e.g., 1200 Am^?2. It can also be seen that the electrochemical reaction involving CO₂ can only be neglected when the steam flow supplied to the cell is high enough to support the water-gas shift reaction. Suppose the concentration of inlet water supply is not enough to support the reverse water gas shift reaction. In that case, the electrochemical reduction of CO₂ has to be considered to avoid model-predictive results that are far from available experimental observations.     

Photo-induced CO2 reduction by hydrogen for selective CO evolution in a dynamic monolith photoreactor loaded with Ag-modified TiO2 nanocatalyst

Ag-promoted TiO₂ nanoparticles immobilized over the cordierite monolithic support for dynamic and selective photo-reduction of CO₂ to CO by the use of hydrogen has been investigated. Ag-loaded TiO₂ NPs synthesized by a facile sol–gel method were coated over the monolith channels by dip-coating method. The samples were characterized by XRD, Raman, FTIR, SEM, TEM, XPS, N₂ adsorption–desorption, UV–Vis and PL spectroscopy. The photo-activity test of Ag-modified TiO₂ NPs was conducted for dynamic photocatalytic CO₂ reduction with H₂ as a reductant via a reverse water gas shift (RWGS) reaction in a cell type and monolith photo-reactors. Using 5 wt. % Ag/TO₂ NPs, CO₂ was energetically converted to CO with a yield rate 1335 μmole g-catal.^?1 h^?1, a 111 fold-higher than the amount of CO produced over the pure TiO₂ catalyst. More importantly, photo-activity of Ag/TiO₂ catalyst for CO evolution can be improved by 209 fold using monolith photo-reactor than the cell type reactor under the same operating conditions. This enactment was evidently due to the efficient light harvesting with larger illuminated surface area inside monolith micro-channels and efficient charges separation in the presence of Ag-metal. The reusability of Ag/TiO₂ NPs loaded over the monolithic support showed favorable recycling capability than the catalyst dispersed in a cell reactor. A possible reaction mechanism for this observation has been discussed in detail.     

A new modification on RK EOS for gaseous CO2 and gaseous mixtures of CO2 and H2O

To develop an equation of state with simple structure and reasonable accuracy for engineering application, Redlich–Kwong equation of state was modified for gaseous CO₂ and CO₂–H₂O mixtures. In the new modification, parameter ‘a’ of gaseous CO₂ was regressed as a function of temperature and pressure from recent reliable experimental data in the range: 220–750 K and 0.1–400 MPa. Moreover, a new mixing rule was proposed for gaseous CO₂–H₂O mixtures. To verify the accuracy of the new modification, densities were calculated and compared with experimental data. The average error is 1.68% for gaseous CO₂ and 0.93% for gaseous mixtures of CO₂ and H₂O. Other thermodynamic properties, such as enthalpy and heat capacities of CO₂ and excess enthalpy of gaseous CO₂–H₂O mixtures, were also calculated; results fit experimental data well, except for the critical region. Copyright © 2005 John Wiley & Sons, Ltd.     

Soft-templated NiO–CeO2 mixed oxides for biogas upgrading by direct CO2 methanation

The catalytic performance in the direct CO₂ methanation of a model biogas is investigated on NiO–CeO₂ nanostructured mixed oxides synthesized by the soft-template procedure with different Ni/Ce molar ratios. The samples are thoroughly characterized by means of ICP-AES, XRD, TEM and HR-TEM, N₂ physisorption at −196 °C, and H₂-TPR. They result to be constituted of CeO₂ rounded nanocrystals and of polycrystalline needle-like NiO particles. After a H₂-treatment at 400 °C for 1 h, the surface basic properties and the metal surface area are also assessed using CO₂ adsorption microcalorimetry and H₂-pulse chemisorption measurements, respectively. At increasing Ni content the Ni⁰ surface area increases, while the opposite occurs for the number of basic sites. Using a CO₂/CH₄/H₂ feed, at 11,000 cm³ h⁻¹ g_cat⁻¹, CO₂ conversions in the 83–89 mol% range and methane selectivities >99.5 mol% are reached at 275 °C and atmospheric pressure, highlighting the very good performances of the investigated catalysts.     

An adaptive neuro–fuzzy inference model for prediction of coal char conversion in CO2 gasification

Today, in industry, the gasification of coal as an important energy source is an interesting perspective. In this study, the application of adaptive neuro–fuzzy inference system (ANFIS) technique for estimation of char conversion in CO₂ gasification is investigated. The main variables affecting char conversion are particle size, reaction time, and reaction temperature, which are chosen as input variables of the proposed model. Experimental data which are gathered from the literature are applied for training, testing, and validation of developed ANFIS model. The results reveal the exact estimation of char conversions with the corresponding experimental values with the regression coefficients (R²) greater than 0.99.     

Lower flammability limit of H2/CO/air mixtures with N2 and CO2 dilution at elevated temperatures

The lower flammability limits of H₂/CO/air mixtures with N₂ and CO₂ dilution were systematic experimentally studied over a wide range of H₂ blending ratios (0–100 vol%) with N₂ (0–67 vol%) and CO₂ (0–67 vol%) dilution in the fuels under various elevated initial temperatures (298 K–473 K) and atmospheric pressure. The experimentation was conducted via an 8 L stainless steel cylindrical explosion vessel and using the metal wire fusing as the ignition source. The corresponding cases were also calculated using Kondo's correlation proposed based on a limiting flame temperature concept. To gain an insightful understanding of the effect of chemical kinetics at different H₂ fractions and CO₂ dilution ratios, sensitivity analysis and H mole fractions were carried out using Chemkin-Pro. The experimental results showed that the lower flammability limits decreased with the increase of H₂ fractions especially when the H₂ content was low (x_H2 ≤ 0.25). Attributable to the accelerated oxidation of CO by the greater generation of OH from H₂/O₂ reaction, Le Chatelier's Rule tended to relatively over-estimate the lower flammability limits of H₂/CO mixtures with a small amount of H₂. Because of the larger heat capacity, and the inhibition effect on the oxidation of CO and the generation of H radicals, CO₂ presented a stronger dilution effect on lower flammability limit than N₂. Moreover, the lower flammability limits for all measured syngas mixtures displayed great linear temperature dependence. A comparison between the experimental data and calculation results showed that, Kondo's correlation provided the satisfactorily accuracy predictions on the lower flammability limits of diluted syngas mixtures with lower H₂ fractions (x_H2 ≤ 0.5). However, when the H₂ fractions were high and the mixture was highly CO₂ diluted, Kondo's correlation over-estimated the lower flammability limits and the prediction error would reach to 30%. The considerably distinctions were not only attributed to the inadaptable assumption against to the growing and lower behaviour of H₂ flame temperature at lower flammability limit, but also caused by the preferential diffusion of H₂, as well as the variation of the chemical effects under high H₂ content and high CO₂ dilution conditions.     

High selective and efficient Fe2–N6 sites for CO2 electroreduction: A theoretical investigation

Developing high efficient and cheap electrocatalysts for carbon dioxide reduction reaction (CO₂RR) is the key to achieve CO₂ transformation into clean energy. Herein, a series of transition metal dimer and nitrogen codoped graphene (M₂N₆-Gra, M = Cr–Cu) acting as electrocatalysts for CO₂RR are investigated based on the density functional method. For M₂N₆-Gra (M = Cr, Mn), the selectivity is poor and CO poisoning is serious. Fe₂N₆-Gra is the best CO₂RR catalyst due to the good selectivity and catalytic activity. The calculated overpotential is very small, i.e., 0.03 V for COOH channel, 0.05 V for HCOO channel. Hydrogen evolution reaction is also refrained on the Fe₂N₆-Gra surface, which further supports its high catalytic performance. For M₂N₆-Gra (M = Co, Ni, Cu), the catalytic activity is poor due to large overpotentials. These results indicate that if designed carefully, the transition metal dimer and nitrogen codoped graphene would be good candidate for the high efficient and selective CO₂RR catalyst.     

Hydrate-based separation of the CO2 + H2 mixtures. Phase equilibria with isopropanol aqueous solutions and hydrogen solubility in CO2 hydrate

The gas hydrates' ability to preferentially bind one of the components of a gas mixture into a hydrate state makes it possible to consider hydrate-based technology as promising for the separation of gas mixtures. When a hydrate is obtained from a gas mixture, mixed hydrates with a complex composition inevitably occur. Issue of their composition determination stays apart. This a rather difficult task, which is complicated by the dissolution of small molecules such as hydrogen in the hydrate phase. This, in turn, impedes the analysis of the data obtained. In this work, the solubility of hydrogen in carbon dioxide hydrate in the range of 269.7–275.7 K and at partial H₂ pressure up to 4.5 MPa was experimentally determined. Hydrate composition was found to be CO₂·(0.01X)H₂·6.5H₂O at H₂ pressure of X MPa. The equilibrium conditions of hydrates formation in the systems of H₂O – CO₂ – H₂ and H₂O – 2-propanol – CO₂ – H₂ were also determined in a wide range of hydrogen concentrations. Hydrogen seems to be an indifferent diluent gas regarding CO₂ hydrate equilibrium pressure. The compositions of the equilibrium phases have been determined as well. It was shown that isopropanol does not form a double hydrate with СО₂, only sI СО₂ hydrate occurred in the studied systems. The obtained dependencies will be useful in analyzing the process of СО₂ + Н₂ gas mixtures separation by the hydrate-based method.