The effect of anisotropic dispersion on the convective mixing in long‐term CO2 storage in saline aquifers

Carbon dioxide storage in deep saline aquifers is considered a possible option to bring greenhouse gas emissions under control. The understanding of the underlying mechanisms, such as convective mixing and associated mechanisms, affecting this mixing may have an impact on the long‐term sequestration process in deep saline aquifers. One of the significant aspects of the flow of miscible species in porous media is velocity dependent dispersion. The effect of dispersion on dissolution of carbon dioxide (CO₂) into brine is investigated by full nonlinear numerical simulations. This study reveals that dispersion may dramatically change the trend of CO₂ dissolution into brine. It was found that the dissolution of CO₂ increases as dispersion strength increases. The mixing pattern also shows three different mechanisms: diffusion, convection, and a highly nonlinear interaction mechanism. However, the medium dispersivity ratios were found to slightly affect the mixing, while having an impact on the fingering pattern. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Discovery of novel zeolites for natural gas purification through combined material screening and process optimization

An efficient computational screening approach is proposed to select the most cost‐effective materials and adsorption process conditions for CH₄/CO₂ separation. The method identifies eight novel zeolites for removing CO₂ from natural gas, coalbed methane, shale gas, enhanced oil recovery gas, biogas, and landfill gas sources. The separation cost is minimized through hierarchical material screening combined with rigorous process modeling and optimization. Minimum purity and recovery constraints of 97 and 95%, respectively, are introduced to meet natural gas pipeline specifications and minimize losses. The top zeolite, WEI, can recover methane as economically as $0.15/MMBTU from natural gas with 5% CO₂ to $1.44/MMBTU from natural gas with 50% CO₂, showing the potential for developing natural gas reservoirs with higher CO₂ content. The necessity of a combined material selection and process optimization approach is demonstrated by the lack of clear correlation between cost and material‐centric metrics such as adsorption selectivity. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1767–1785, 2014     

Investigation of CO2 removal by immobilized carbonic anhydrase enzyme in a hollow‐fiber membrane bioreactor

Gas–liquid membrane contactors are compelling candidate bioreactors for implementing CO₂ capture because of large mass transfer rates and liquid–solid interfaces, low pressure drop, low axial dispersion and mixing, modularity, simple scale‐up or scale‐down, and operational suppleness. Binding the carbonic anhydrase (CA) enzyme on the membrane surface adds extra advantages due to the impressive large hydration turnover number and offers an attractive way for CO₂ capture. This novel approach to CO₂ removal by immobilized CA in a hollow‐fiber membrane bioreactor (HFMB) was investigated via a multiscale steady‐state model, under gas‐filled and partially liquid‐filled membrane pores conditions. The impact of CA loading, buffer acid‐base constant and concentration, membrane wetting, uncatalyzed/catalyzed CO₂ hydration in the wetted membrane zone, operating conditions, and cocurrent/countercurrent flow orientation on the HFMB performance was analyzed. The results showed that this low‐cost, green, and environmentally friendly technology could be an appealing alternative to CO₂ capture from stationary emissions sources. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2996–3007, 2017     

CHARACTERIZATION OF FLUIDIZED BED REACTORS WITH GAS TRACER MEASUREMENTS

In a steady state bench scale fluidized bed the decomposition reaction of NaHCO₃ was carried out. The residence times distributions, DRT, of carbon dioxide (the gaseous product) and non adsorbing argon (the reference tracer) were mass spectroscopically measured as a function of the bed temperature. By means of single-, two- and three-phase dispersion models as well as by a cell model, the DRT's were evaluated on line by a computer.

The steady state transverse and longitudinal concentration profiles of these tracers upstream from the plane source were also measured and evaluated by a dispersion model as well as by a counter current back mixing model. Comparison of the steady state and nonsteady state dispersion coefficient measurements indicate that the longitudinal gas mixing is only partially due to backmixing. The experimentally determined wake fractions agree well with those published in the literature. Since the adsorption rate of CO₂ on the pore surface area of the particles in the dense phase is high no interphase transfer from the interstitial gas of the dense phase into the bubble phase takes place.

The desorption of CO₂ and its return into the interstitial gas and than into the gas phase occurs only slowly and with an initial time lag. The on-line DRT can be used as a diagnostical technique for investigation of the reactor during its operation, if operation disturbances or breakdowns occur.     

Photocatalytic decomposition of 2-propanol in air by mechanical mixtures of TiO2 crystalline particles and silicalite adsorbent: The complete conversion of organic molecules strongly adsorbed within zeolitic channels

Fundamental photocatalytic behaviors were investigated for mechanical mixtures of TiO₂ crystalline particles (P25) and MFI type zeolite (silicalite) in the decomposition reaction of 2-propanol vapor in air for the first time. Mechanical mixing enables reliable comparisons to be made between photocatalysts because the contents of TiO₂ and the adsorbent can be widely varied (51 times in this study) while keeping the particle size and crystallinity of TiO₂ unchanged. That is, the use of mechanical mixture highlights the behavior of molecules adsorbed in the microporous crystals, keeping the TiO₂ unchanged. In the case of the mixed photocatalysts, the initial 2-propanol concentration in the gas phase was significantly reduced because of adsorption into the zeolite. After photo-irradiation started, 2-propanol was decomposed to CO₂ with no (or trace amount of) acetone detected in the gas phase. The analysis of final amount of CO₂ formed by the decomposition demonstrated that just by the mechanical mixing of TiO₂ and zeolite, the TiO₂ photocatalyst decomposed completely the reactant and intermediate molecules strongly adsorbed into the zeolite. On the other hand, in reference experiments in which TiO₂ and zeolite were not mixed and were separately placed in a photoreactor, the organic compounds strongly adsorbed in the zeolite could not be decomposed to CO₂ by the photocatalyst. It is notable that the CO₂ formation rates for the mixed photocatalysts were mostly constant for those comprising 40 wt% or larger amounts of zeolite, while being slower than for pure TiO₂. The rate-determining step was discussed based on these data. The present study showed that the mixed photocatalyst could remove organic vapors by adsorption in the dark and decompose completely to CO₂ at moderate reaction rates under photo-irradiation with minimized evolution of intermediate molecules into the gas phase.     

Modeling of CO2‐Hydrate Formation at the Gas‐Water Interface in Sand Sediment

Sub‐seabed geological storage of CO₂ in the form of gas hydrate is attractive because clathrate hydrate stably exists at low temperature and high pressure, even if a fault occurs by diastrophism like a big earthquake. For the effective design of the storage system it is necessary to model the formation of CO₂‐hydrate. Here, it is assumed that the formation of gas hydrate on the interface between gas and water consists of two stages: gas diffusion through the CO₂‐hydrate film and consequent CO₂‐hydrate formation on the interface, between film and water. Also proposed is the presence of a fresh reaction interface, which is part of the interface between the gas and aqueous phases and not covered with CO₂‐hydrate. Parameters necessary to model the hydrate formation in sand sediment are derived by comparing the results of the present numerical simulations and the measurements in the literature.     

An experimental evaluation and molecular simulation of high temperature gas adsorption on nanoporous carbon

A combination of experiments and molecular simulations has been used to further understand the contribution of gas adsorption to the carbon dioxide (CO₂) selectivity of nanoporous carbon (NPC) membranes as a function of temperature and under mixed gas conditions. Whilst there have been various publications on the adsorption of gases onto carbon materials, this study aims to benchmark a simulation model with experimental results using pure gases. The simulation model is then used to predict mixed gas behaviour. These mixed gas results can be used in the assessment of NPC membranes as a suitable technology for both carbon dioxide separations from air-blown syngas and from natural gas. The gas adsorption experiments and molecular simulations have confirmed that CO₂ is more readily adsorbed on nanoporous carbon than methane (CH₄) and nitrogen (N₂). Increasing the temperature reduces the extent of adsorption and the CO₂ selectivity. However, the difference between the CO₂ and N₂ heats of adsorption is significant resulting in good CO₂/N₂ separation even at higher temperatures.     

Numerical study of the effect of CO2/H2O dilution on the laminar burning velocity of methane/air flames under elevated initial temperature and pressure

Diluents have an essential effect during combustion. Discovering the influence of CO₂ and H₂O as diluents on laminar burning velocity (LBV) is helpful for combustion control and optimization. In this study, CH₄/air/CO₂/H₂O mixtures were investigated and validated using the FFCM-Mech 1.0 over extensive boundary conditions. The chemical effects of the diluents CO₂ and H₂O were separated using a decoupling method. It was found that an increase in initial temperature promotes the chemical effects, while an increase in initial pressure does the opposite. In addition, the inhibiting effect of CO₂ on LBV is stronger than that of H₂O. Sensitivity, mole fraction, and rate of production (ROP) analyses were used to reveal that the sum of the chemical effects of adding CO₂ and H₂O separately was greater than the chemical effects of adding equal amounts of CO₂ and H₂O simultaneously. This paper not only investigates the effect of CO₂ and H₂O on the LBV under wide boundary conditions, but also offers a valuable guide for studying the operating conditions and intensity settings of exhaust gas recirculation (EGR) and theoretical guidance for further research on the combination of EGR and in-cylinder water injection technology.     

Study the effects of Cloisite15A nanoclay incorporation on the morphology and gas permeation properties of Pebax2533 polymer

In this study, mixed matrix membranes (MMMs) were prepared using commercially available poly(ether‐b‐amide) (Pebax2533) as polymer matrix and organically modified montmorillonite (OMMt) as filler with the aim of investigating their gas permeation properties. The prepared membranes were characterized by Fourier‐transform infrared (FTIR) spectroscopy, X‐ray diffraction (XRD), scanning electron microscope (SEM), thermal gravimetric analysis, and tensile strength analyses. Gas permeation properties of all the prepared membranes were evaluated at different pressures and clay loadings. Results of FTIR and SEM confirmed the appropriate adhesion between polymer and nanoclays so that no void formation was observed in the polymer/clay interface. XRD results showed that in low loading, clay dispersion occurred as exfoliated‐intercalated and at high loading as intercalated‐phase separated. Results of gas permeation test showed that by adding layered and impermeable clay particles to the polymer matrix, the permeation of soluble CO₂ gas reduced by 28% for the highest clay loading. By increasing of pressure from 2 to 6 bar, CO₂/CH₄ permselectivity increased at all nanoclay loadings. The highest CO₂/CH₄ selectivity was obtained for 6 wt % clay MMM at all pressures, while the highest CO₂/H₂ selectivity was achieved for neat polymer at 6 bar. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45302.     

A numerical study of capillary pressure–saturation relationship for supercritical carbon dioxide (CO2) injection in deep saline aquifer

Carbon capture and sequestration (CCS) is expected to play a major role in reducing greenhouse gas in the atmosphere. It is applied using different methods including geological, oceanic and mineral sequestration. Geological sequestration refers to storing of CO₂ in underground geological formations including deep saline aquifers (DSAs). This process induces multiphase fluid flow and solute transport behaviour besides some geochemical reactions between the fluids and minerals in the geological formation. In this work, a series of numerical simulations are carried out to investigate the injection and transport behaviour of supercritical CO₂ in DSAs as a two-phase flow in porous media in addition to studying the influence of different parameters such as time scale, temperature, pressure, permeability and geochemical condition on the supercritical CO₂ injection in underground domains. In contrast to most works which are focussed on determining mass fraction of CO₂, this paper focuses on determining CO₂ gas saturation (i.e., volume fraction) at various time scales, temperatures and pressure conditions taking into consideration the effects of porosity/permeability, heterogeneity and capillarity for CO₂–water system. A series of numerical simulations is carried out to illustrate how the saturation, capillary pressure and the amount of dissolved CO₂ change with the change of injection process, hydrostatic pressure and geothermal gradient. For example, the obtained results are used to correlate how increase in the mean permeability of the geological formation allows greater injectivity and mobility of CO₂ which should lead to increase in CO₂ dissolution into the resident brine in the subsurface.     

Flow and mixing efficiency characterisation in a CO2-assisted single-screw extrusion process by residence time distribution using Raman spectroscopy

Hot-melt extrusion of a bio-sourced polyamide has been implemented in a single-screw extruder with supercritical carbon dioxide injection. CO₂ acts as a plasticiser in the extruder barrel and as a physical blowing agent at the die. To insure a better mixing and dissolution of the CO₂ into the polymer melt, addition of a static mixer between the screw tip and the die was tested. The effect of both the static mixing element and the CO₂ injection on the melt flow behaviour has been elucidated. A recent technique of in-line Raman spectroscopy was implemented to make a residence time distribution study, using titanium dioxide as a tracer. The use of a static mixer exerts a major modification on the flow behaviour: it improves mixing by enhancing dispersion. In addition, the structure of the manufactured products was studied: the static mixer led to more homogeneous porous structure. The broad range of CO₂ incorporation (up to 25%, w/w) into the melt led to the manufacture of foams with adjustable porosity from 15 to 70%.     

One‐dimensional productivity assessment for on‐field methane hydrate production using CO2/N2 mixture gas

The direct recovery of methane from gas hydrate‐bearing sediments is demonstrated, where a gaseous mixture of CO₂ + N₂ is used to trigger a replacement reaction in complex phase surroundings. A one‐dimensional high‐pressure reactor (8 m) was designed to test the actual aspects of the replacement reaction occurring in natural gas hydrate (NGH) reservoir conditions. NGH can be converted into CO₂ hydrate by a “replacement mechanism,” which serves double duty as a means of both sustainable energy source extraction and greenhouse gas sequestration. The replacement efficiency controlling totally recovered CH₄ amount is inversely proportional to CO₂ + N₂ injection rate which directly affecting solid ‐ gas contact time. Qualitative/quantitative analysis on compositional profiles at each port reveals that the length more than 5.6 m is required to show noticeable recovery rate for NGH production. These outcomes are expected to establish the optimized key process variables for near future field production tests. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1004–1014, 2015     

Effect of ship tilting and motion on amine absorber with structured‐packing for CO2 removal from natural gas

A gas‐liquid Eulerian porous media computational fluid dynamics (CFD) model was developed for an absorber with structured packing to remove CO₂ from natural gas by mono‐ethanol‐amine (MEA). The three‐dimensional geometry of the amine absorber with Mellapak 500.X was constructed to investigate the effect of the tilting and motion experienced on ships and barges for offshore plants. The momentum equation included porous resistance, gas‐liquid momentum exchange, and liquid dispersion to replace structured‐packing by porous media. The mass equation involved mass transfer of CO₂ gas into MEA solution, and one chemical reaction. Parameters of the CFD model were adjusted to fit experimental data measured in the CO₂‐MEA system. As the tilting angle increased, the liquid holdup and effective interfacial area decreased and CO₂ removal efficiency was lowered. The uniformity of liquid holdup deteriorated by 10% for a 3° static tilting, and a rolling motion with 4.5° amplitude and 12 s period, respectively. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4412–4425, 2015     

Axial gas dispersion in a fluidized bed of polyethylene particles

Gas mixing behavior was investigated in a residence time distribution experiment in a bubbling fluidized bed of 0.07 m ID and 0.80 m high. Linear low density polyethylene (LLDPE) particles having a mean diameter of 772 Μm and a particle size range of 200-1,500 Μm were employed as the bed material. The stimulus-response technique with CO₂ as a tracer gas was performed for the RTD study. The effects of gas velocity, aspect ratio (H₀/D) and scale-up on the axial gas dispersion were determined from the unsteady-state dispersion model, and the residence time distributions of gas in the fluidized bed were compared with the ideal reactors. It was found that axial dispersion depends on the gas velocity and aspect ratio of the bed. The dimensionless dispersion coefficient was correlated with Reynolds number and aspect ratio.     

A study on natural clinoptilolite for CO2/N2 gas separation

Based on its low cost and low water adsorption capacity, compared to synthetic zeolites (A-type, X-type and Y-type), natural, untreated clinoptilolite was examined as a potential adsorbent for a separation process targeting on removal of CO₂ from flue gas. Taking into consideration typical flue gas composition and temperature, adsorptive properties of binary CO₂/N₂ mixtures were tested in the temperature range of 268 to 403 K and compared with literature data. The results showed that CO₂ concentration, total pressure, and temperature strongly affect selectivity and working capacity, restricting the conditions under which the material could be used as an efficient adsorbent.     

Thermodynamic and molecular insights into the absorption of H2S,CO2, and CH4 in choline chloride plus urea mixtures

Deep eutectic solvents (DESs) are a class of promising media for gas separation. In order to examine the potential application of DESs for natural gas upgrading, the solubilities of H₂S, CO₂, and CH₄ in choline chloride (ChCl) plus urea mixtures were measured in this work. The solubility data were correlated with Henry's law equation to calculate the thermodynamic properties of gas absorption processes, such as Henry's constants and enthalpy changes. Grand-canonical Monte Carlo simulations and quantum chemistry calculations were also performed to examine the mechanism of gas absorption processes. It is found that the absorption of H₂S in ChCl + urea mixtures is governed by the hydrogen-bond interaction between Cl of ChCl and H of H₂S, whereas the absorption of CO₂ and CH₄ in ChCl+urea mixtures is governed by the free volume of solvents. Based on the different behavior of gas absorption, high H₂S/CO₂, H₂S/CH₄, and CO₂/CH₄ selectivities can be achieved by adjusting the ratio of ChCl/urea in mixtures.     

Numerical modeling of supercritical carbonation process in cement-based materials

In this paper, a mathematical model is developed to simulate the physical–chemical coupling process of supercritical carbonation in cement-based materials. This model takes into account the rate of chemical reaction, mass conservation for gas–liquid two phase flow, diffusion and dispersion of CO₂ in water, energy conservation for porous medium and the solubility of CO₂ in water. Numerical results are obtained and compared with experimental results. The degree of carbonation, temperature, gaseous pressure, moisture content and saturation of water within the material are predicted and presented. The influence of material saturation, temperature and pressure of supercritical CO₂ on carbonation depth is investigated through parametric studies. The comparisons with test results suggest that the coupled model can be used to predict carbonation process of cement-based materials under supercritical conditions.     

Synthesis and properties of sulfonated biphenyl poly(ether sulfone) and its mixed‐matrix membranes containing carbon nanotubes for gas separation

We prepared mixed‐matrix membranes (MMMs) composed of carboxylated single‐walled carbon nanotubes (f‐SWCNTs) and a sulfonated biphenyl poly(ether sulfone) (S‐PPSU) polymer matrix. The thermal stability and properties of the pores of the S‐PPSU and f‐SWCNTs were characterized by thermogravimetric analysis and sorption isotherm curves, respectively; these showed that the surface and pore diameter decreased after the introduction of carboxyl groups to the single‐walled carbon nanotubes (SWCNTs), and the pore properties did not restore original values even when the f‐SWCNTs were preheated to 350 °C to remove carboxyl groups. The gas‐separation measurement showed that the MMMs comprised of the S‐PPSU and f‐SWCNTs possessed better gas‐separation properties than the ones composed of biphenyl poly(ether sulfone) and SWCNTs. The permeability for N₂, O₂, He, and CO₂ and the selectivity for O₂/N₂ and O₂/CO₂ were enhanced simultaneously because of the good dispersion of f‐SWCNTs and the improved interaction between the two phases. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44995.     

Synthesis and CO2 adsorption behavior of amine‐functionalized porous polystyrene adsorbent

A solid amine adsorbent was prepared by modifying a porous polystyrene resin (XAD‐4) with chloroacetyl chloride through a Friedel–Crafts acylation reaction, followed by aminating with tetraethylenepentamine (TEPA). The adsorption behavior of CO₂ from a simulated flue gas on the solid amine adsorbent was evaluated. Factors that could determine the CO₂ adsorption performance of the adsorbents such as amine species, adsorption temperature, and moisture were investigated. The experimental results showed that the solid amine adsorbent modified with TEPA (XAD‐4‐TEPA), which had a longer chain, showed an amine efficiency superior to the other two amine species with shorter chains. The CO₂ adsorption capacity decreased obviously as the temperature increased because the reaction between CO₂ and amine groups was an exothermic reaction, and its adsorption amount reached 1.7 mmol/g at 10 °C in dry conditions. The existence of water could significantly increase the CO₂ adsorption amount of the adsorbent by promoting the chemical adsorption of CO₂ on XAD‐4‐TEPA. The adsorbent kept almost the same adsorption amount after 10 cycles of adsorption–desorption. All of these results indicated that amine‐functionalized XAD‐4 resin was a promising CO₂ adsorbent. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45046.     

Oxidation of the sulphurised dolomite produced in the desulphurisation of the gasification gases

Dolomite reacts with H₂S to produce calcium sulphide and has been broadly investigated as a desulphurisation agent due to its low-cost and favourable properties.Because CaS reacts with water or water vapour in the environment to regenerate hydrogen sulphide and, therefore, disposal is problematic and the chemical cannot be uses as a landfill material. One of the methods used to make this material inert is oxidation to convert calcium sulphide into calcium sulphate or calcium oxide.In our study, tests were carried out using dolomite from Granada, Spain, that was previously calcined and sulphurised at high temperature with a gas similar to that produced in gasification facilities. To approximate real-scale results, a relatively large amount of substance was used for each sample (100–150 g) and the samples were used in a fixed-bed position.The influence of different conditions, such as grain size, composition of the oxidation gas, gas velocity, bed length and temperature, was them investigated. The final solid products were characterised by X-ray diffraction and chemical analysis and the CO₂, SO₂, H₂S and COS concentrations in the gases produced during oxidation were analysed by gas chromatography.The results showed that the most influential factor was grain size and that the best oxidant was O₂ mixed with nitrogen.The presence of water vapour increases the residual concentration of CaS in the end product, but increased the CaO contentThe higher the oxygen concentration and the higher the gas velocity, the lower the residual content of CaS. CO₂ used alone oxidises CaS to produce SO₂ and COS, but at very low rates. It also produces some CS₂. Water vapour used alone can also oxidise the CaS to produce H₂S and SO₂ but also at very low velocity.At higher oxidation temperature, between 700°C and 850 °C, lesser residual CaS is obtained in the oxidised product.