Analysis of volume expansion mechanism of CO2-acetate systems at 40 °C

We measured the density and liquid phase CO₂ mole fraction (xCO₂) of CO₂-expanded acetates (methyl acetate, ethyl acetate, propyl acetate, butyl acetate, i-propyl acetate, and t-butyl acetate) at 40 °C and carried out molecular dynamics (MD) simulations. The pressure dependence of xCO₂ was almost the same for all measured acetates. The expansion coefficient and the partial molar volume estimated using the Peng-Robinson equation of state was found to have regions: a nearly constant region and a rapidly changing region that seem to be caused by the interspaces. When the length of the alkyl chain increased, the interspaces became larger. CO₂ molecules existed in the interspaces while the volume remains nearly constant in the lower xCO₂ region. However, there were no interspaces in the higher xCO₂ region where volume expanded rapidly and these trends were supported by the MD simulations. The fraction from the center of mass of CO₂ to the carbonyl oxygen atom was highest in regions of lower xCO₂, while the distance from the center of mass of CO₂ to the carbonyl oxygen atom was shortest regardless of region or mixture. The results show that CO₂ molecules tend to aggregate around the carbonyl oxygen in the acetate.     

Modeling the selectivity of activated carbons for efficient separation of hydrogen and carbon dioxide

Grand canonical Monte Carlo simulations (GCMC) are carried out to investigate the separation of hydrogen and carbon dioxide via adsorption in activated carbons. In the simulations, both hydrogen and carbon dioxide molecules are modeled as Lennard-Jones spheres, and the activated carbons are represented by a slit-pore model. At elevated temperatures (T = 505 and 923 K), the activated carbons exhibit essentially no preference over the two gases and the selectivity of carbon dioxide relative to hydrogen falls monotonically as the pore size increases. At room temperature, however, the selectivity of carbon dioxide relative to hydrogen reaches up to 90, indicating that hydrogen and carbon dioxide can be efficiently separated. Furthermore, the optimized pore sizes, of width H = 1.48 nm for the bulk mole fraction ratio of x_CO2/x_H2=1:2 and H = 1.18 nm for x_CO2/x_H2=1:8, are identified in which the activated carbons show the highest selectivity for the separation of hydrogen and carbon dioxide.     

High pressure density and solubility for the CO2 + 1-ethyl-3-methylimidazolium ethylsulfate system

The solubility and density of the CO₂ + 1-ethyl-3-methylimidazolium ethylsulfate system were investigated. The carbon dioxide solubility in the IL was measured in the temperature range 273–413 K, for pressure up to 5 MPa and CO₂ mole fractions ranging from 0.02 to 0.5 using the isochoric method, while the system density was carried out at temperatures ranging from 278.15 K to 398.15 K, pressures from 10 MPa to 120 MPa and 0.2, 0.4, 0.7 and 0.8 CO₂ mole fractions. Similar to what was previously observed for phosphonate-based ILs, the ionic liquid high polarity leads to positive deviations from ideality resulting from unfavorable interactions with the CO₂.The results from the density and solubility derived properties show that the system presents important negative excess molar volumes, over the whole range of compositions and temperatures, and a negative entropy of solvation that suggests an increase in ordering of the solvent molecules surrounding the solute. The observed negative excess molar volumes result from the large difference between the molecular volumes of the species involved, with the small carbon dioxide molecules occupying the empty spaces between the larger IL ions, supporting the notion that the carbon dioxide, upon dissolution, occupies essentially the bulk free volume since the IL does not significantly expand upon gas absorption. These results portray ionic liquids as a porous media, like a soft sponge, with a huge free volume in which large amounts of carbon dioxide are able to accommodate during the dissolution process.     

Phase Equilibrium Measurements of Sacha Inchi Oil (<Emphasis Type="Italic">Plukenetia volubilis</Emphasis>) and CO<Subscript>2</Subscript> at High Pressures

New data on phase equilibria for Sacha inchi seed oil in carbon dioxide have been measured using a variable volume cell phase equilibria system at temperatures of 303, 313 and 323 K and at pressures ranging from 4.3 to 27.7 MPa. The CO₂ mole fraction varied from 0.7488 to 0.9997. At the studied concentrations, phase transitions of vapor-liquid, liquid-liquid-vapor and liquid-liquid were observed. Sacha inchi oil contains 47% of omega-3 fatty acids, with a ratio of 0.76:1 for omega-6:omega-3, which is good for human health. The Peng-Robinson equation of state was used to describe the experimental data. A qualitative agreement was obtained between experimental and calculated data for the binary system CO₂ and Sacha inchi seed oil.     

Parameters controlling supersaturation by DELOS using carbon dioxide

The present work pertains to estimation of the maximum degree of supersaturation that can be attained in an organic solution by the DELOS process using carbon dioxide (CO₂) as a cosolvent. The paper analyzes the effects of initial mole fraction of carbon dioxide, temperature and pressure on the degree of supersaturation of cholesterol in a CO₂—acetone—cholesterol solution. It has been observed that owing to liberation of large amounts of CO₂ very large temperature drops may be attained by depressurization, resulting in attainment of very high supersaturation. Within the ranges of the parameters studied in this work, the degree of supersaturation is higher with higher values of initial temperature and initial CO₂ mole fraction of the solution due to inverse interdependence of the final temperature and the residual CO₂ mole fraction in the depressurized solution. Copyright © 2006 Society of Chemical Industry     

Bubble point pressures and densities of hexamethyldisiloxane–carbon dioxide binary mixture using a constant volume view cell

The phase behavior of hexamethyldisiloxane (HMDS)–carbon dioxide (CO₂) binary mixture was investigated using a constant volume view cell. The accuracy of the measurement technique was inspected against the bubble point pressure data in the literature for ethanol (C₂H₅OH)–carbon dioxide (CO₂) binary mixture. The bubble point pressures for C₂H₅OH–CO₂ agreed well with the literature values. The bubble point pressures of HMDS–CO₂ binary mixture were determined at five different temperatures (T = 298.2 K, 308.2 K, 313.2 K, 323.2 K, 333.2 K) and at various compositions. The bubble point pressures increased with increasing temperature and CO₂ mole fraction in the binary mixture. The phase behavior of the binary mixture was modeled using the Peng–Robinson Stryjek–Vera equation of state (PRSVEoS). The binary interaction parameters were regressed from experimental bubble point pressures at each temperature and were found to exhibit a linear dependency on temperature. The HMDS–CO₂ binary mixture was also found to exhibit Type II phase behavior. Additionally, P–T–ρ measurements for the same binary system were conducted and excess molar volumes were calculated.     

Diffusion coefficients of l-menthone and l-carvone in mixtures of carbon dioxide and ethanol

The molecular diffusion coefficients of l-menthone and l-carvone in supercritical carbon dioxide (SCCO₂) and carbon dioxide containing 5 and 10 mol% ethanol as a modifier were measured by the Taylor-Aris chromatographic peak broadening (CPB) method over the ranges of temperature from 308.15 to 338.15 K and pressure from 15 to 30 MPa. It was found that the correlation relationships between diffusion coefficients and the temperature, pressure, viscosity, and density, such as the linear correlation between the D₁₂ and ρ, and between the D₁₂ and T/η, which were valid in binary systems, were also suitable for ternary systems of carbon dioxide containing modifier. The diffusion coefficients in modified SCCO₂ decreased with increasing the ethanol mole fraction due to the chemical association between the two solutes and ethanol. Of several models used to predict experimental data in pure carbon dioxide, the two models of Funazukuri-Ishiwata-Wakao and He-Yu-1998 were the best with the AAD less than 3.2%. Furthermore, the models of modified Wilke-Chang, Scheibel, Reddy-Doraiswamy, Lusis-Ratcliff, Hayduck-Minhas, Tyn-Calus, and Lai-Tan overestimated the diffusion coefficient in ethanol modified SCCO₂ with the AAD values increaseing with the percentage of ethanol, which were probably due to the increase of the volume of solvaton sphere as a true diffusion unit with the percentage of ethanol. Moreover, the free volume model of Dymond is good for predicting the experimental data in pure carbon dioxide and ethanol modified SCCO₂ with the AAD values range from 3.21 to 1.90%.     

The role of the methyl ester moiety in biodiesel combustion: A kinetic modeling comparison of methyl butanoate and n-butane

Growth of the biodiesel industry has motivated increased study of the combustion characteristics of its constituent molecules and building combustion modeling capability. Understanding how these characteristics differ between bio-derived and conventional diesel fuels can help in evaluating biodiesel performance. A kinetic modeling comparison of methyl butanoate and n-butane, its corresponding alkane, contrasted the combustion of methyl esters and normal alkanes, towards understanding the effect of the methyl ester moiety. Utilizing a combined n-heptane and methyl butanoate kinetic mechanism in shock tube simulations, the results predicted no region of negative temperature coefficient (NTC) behavior for methyl butanoate, compared to a well defined NTC region for n-butane. We observed that oxidation pathways associated with the methyl ester moiety inhibited NTC behavior, through increased production of hydroperoxy radicals (HO₂) instead of hydroxyl radicals (OH). In addition, we compared the evolution of carbon monoxide, carbon dioxide, ethylene and acetylene. The early formation of CO and CO₂, directly from methyl butanoate, revealed unique reaction pathways that also influenced a reduction in soot precursor formation. Overall, these results will help to understand how combustion processes change with the inclusion of oxygenated fuels, which will inform the study and design of combustion technologies.     

Continuous production of fatty acid methyl esters from corn oil in a supercritical carbon dioxide bioreactor

Continuous production of fatty acid methyl esters (FAMEs) from corn oil was studied in a supercritical carbon dioxide (SC-CO₂) bioreactor using immobilized lipase (Novozym 435) as catalyst. Response surface methodology (RSM) based on central composite rotatable design (CCRD) was employed to investigate and optimize the reaction conditions: pressure (11-35 MPa), temperature (35-63 °C), substrate mole ratio (methanol:corn oil 1-9) and CO₂ flow rate (0.4-3.6 L/min, measured at ambient conditions). Increasing the substrate mole ratio increased the FAME content, whereas increasing pressure decreased the FAME content. Higher conversions were obtained at higher and lower temperatures and CO₂ flow rates compared to moderate temperatures and CO₂ flow rates. The optimal reaction conditions generated from the predictive model for the maximum FAME content were 19.4 MPa, 62.9 °C, 7.03 substrate mole ratio and 0.72 L/min CO₂ flow rate. The optimum predicted FAME content was 98.9% compared to an actual value of 93.3 ± 1.1% (w/w). The SC-CO₂ bioreactor packed with immobilized lipase shows great potential for biodiesel production.     

Butyl rubber–Ba0.7Sr0.3TiO3 composites for flexible microwave electronic applications

Butyl rubber–Ba_0.7Sr_0.3TiO₃ composites (BR–BST) were prepared by sigma mixing followed by hot pressing. The stress–strain studies show the good flexibility of the composite. The dielectric properties of the composites were investigated at both radio and microwave frequencies. The relative permittivity (ε_r) and loss tangent (tan δ) improved with filler loading at both the frequencies. The relative permittivity and loss tangent of the BR–BST composites at a maximum filler loading of 0.39 volume fraction (v_f) are 13.1 and 0.009 respectively at 5 GHz measured by Split Post Dielectric Resonator (SPDR). The effective relative permittivity of the BR–BST composites is compared with theoretical models. The variation of ε_r with temperature was also investigated in the range 22–80 °C at 1 MHz. The microwave dielectric properties of the composites are also studied after repeated bending. The coefficient of thermal expansion (CTE) of the butyl rubber–BST composites decreased with the addition of the BST ceramic.     

Investigation of CO2 sorption in molten polymers at high pressures using Raman line imaging

CO₂ sorption in molten poly(ε-caprolactone) (PCL) has been investigated by temporally resolved Raman line imaging, which is introduced here as a new technology for the direct measurement of gas mass fraction profiles inside polymers during transient sorption experiments. Molten PCL was exposed to pressurized carbon dioxide in an optically accessible pressure cell at 80 °C and pressures up to 7.1 MPa. During sorption, Raman spectra were acquired temporally and spatially resolved across the PCL drop, allowing the evaluation of the PCL/CO₂ mutual diffusivity, of the CO₂ solubility in PCL, and the determination of temporal evolution of mass fraction profiles of carbon dioxide inside PCL.     

The effect of oxygen and carbon dioxide concentration on soot formation in non-premixed flames

The influence of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation was studied by Time Resolved Laser Induced Incandescence (TIRE-LII) and TEM photography in non-premixed co-flowing flames. TIRE-LII method was used to measure the distribution of two-dimensional soot volume fraction and primary particle size. The soot was directly sampled by the thermophoretic method, and its diameter was examined by TEM photography. Two suitable delay times of the TIRE-LII method affecting measurable range and sensitivity were determined by comparing TEM photographs with the TIRE-LII signal. The effects of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation were investigated with these calibrated techniques. An O₂+(CO₂, N₂, and [Ar+CO₂]) mixtures in co-flow were used to isolate carbon dioxide effects systematically. The primary particle number concentration and soot volume fraction were abruptly decreased by the addition of carbon dioxide to co-flow. This suppression was resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of carbon dioxide. The increase of oxygen concentration in the co-flow caused an enhancement of soot nucleation and thus the residence time increase, but the specific growth rate showed almost the same value regardless of the co-flow mixture in the growth region. This result suggests that the specific growth rate has a weak dependence on the relative change of co-flow conditions in non-premixed co-flowing flames.     

Effect of various porous nanotextures on the reversible electrochemical sorption of hydrogen in activated carbons

The reversible hydrogen storage capacity of three series of activated carbons (ACs) prepared from different precursors by KOH, CO₂ and steam activation is determined by electrodecomposition of an alkaline water solution and is correlated with the nanotextural parameters of ACs. Galvanostatic charge/discharge appears as a precise quantitative method for estimating the hydrogen sorption capacity, whereas, cyclic voltammetry supplies a very useful information on the electrosorption mechanism. For the ACs studied, the hydrogen sorption capacity is not linearly related with any of the porosity parameters commonly used in other publications, such as the Dubinin-Radushkevich micropore volumes determined by nitrogen or carbon dioxide adsorption, VDR N₂ and VDR CO₂. In particular, an important discrepancy is observed for the KOH activated materials, suggesting that this treatment may provoke changes of pore shape. A better correlation is found considering the nanopore size distribution obtained from CO₂ adsorption by the DFT method. The amount of hydrogen reversibly adsorbed demonstrates a proportional trend with the volume of micropores smaller than 0.6-0.7 nm. However, in all cases, a part of the micropore volume estimated by CO₂ adsorption is ineffective, suggesting that some ultramicropores are involved in irreversible trapping of hydrogen.     

Determination of kinetics of CO2 absorption in solutions of 2‐amino‐2‐methyl‐1‐propanol using a microfluidic technique

The kinetics for the reactions of carbon dioxide with 2‐amine‐2‐methyl‐1‐propanol (AMP) and carbon dioxide (CO₂) in both aqueous and nonaqueous solutions were measured using a microfluidic method at a temperature range of 298–318 K. The mixtures of AMP‐water and AMP‐ethylene glycol were applied for the working systems. Gas‐liquid bubbly microflows were formed through a microsieve device and used to determine the reaction characteristics by online observation of the volume change of microbubbles at the initial flow stage. In this condition, a mathematical model according to zwitterion mechanism has been developed to predict the reaction kinetics. The predicted kinetics of CO₂ absorption in the AMP aqueous solution verified the reliability of the method by comparing with literatures’ results. Furthermore, the reaction rate parameters for the reaction of CO₂ with AMP in both solutions were determined. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4358–4366, 2015     

Preparation and characterization of chars and activated carbons from Saskatchewan lignite

Saskatchewan lignite was used as a precursor to prepare carbonaceous adsorbents for use as SO₂ adsorbent from flue gases. The lignite was carbonized producing char in a fixed bed microreactor system at different temperatures from 350 to 550 ^°C in nitrogen atmosphere. The chars obtained at 475 ^°C for 120 min exhibited the highest micropore surface area (136 m²/g) and volume (0.062 cm³/g) and the smallest median pore diameter (∼ 0.7 nm). Carbon dioxide and steam were used as activating agents. Activation of char at optimum conditions of 650–675 ^°C for 15 min with carbon dioxide and steam resulted in a further increase in micropore surface area (220 and 186 m²/g for CO₂ and steam, respectively) and volume (0.090 and 0.085 cm³/g for CO₂ and steam, respectively). The yield of char was 64 wt.%, while the yields of activated carbon were 60 and 57 wt.% for CO₂ and steam activation, respectively; all based on the mass of original lignite.     

Catalytic chain transfer polymerisation of CO2-expanded methyl methacrylate

The catalytic chain transfer polymerisation of methyl methacrylate expanded with dense CO₂ is reported. Experimental values of the chain transfer constant (C_s) are presented at 50 °C and in the range of pressure from 0.1 to 6 MPa, using a cobaloxime complex as the chain transfer catalyst. The effect of small quantities of poly(methyl methacrylate) on the volumetric expansion of the monomer with CO₂ is considered. Experimental data are presented on the pressure limit for homogeneous expansion as a function of the molecular weight of the polymer. It is shown that the values of C_s in CO₂-expanded methyl methacrylate are significantly higher than that in bulk monomer. This effect is mainly attributed to the enhancement of the chain transfer rate coefficient, arising from the reduction in the viscosity of the medium, and is consistent with a diffusion-controlled rate-determining step in the chain transfer process.     

Challenge to fortyfold expansion of biodegradable polyester foams by using carbon dioxide as a blowing agent

This paper presents a new foaming technology using supercritical carbon dioxide as a blowing agent to obtain large volume expansions of biodegradable polyester foams of over fortyfold. The basic approach for the promotion of a large volume expansion ratio with carbon dioxide was to prevent cell coalescence by using a branched material, to dissolve carbon dioxide completely in the melt by promoting convective diffusion under a high processing pressure, to reduce the diffusivity of gas by lowering the melt temperature, and to optimize the processing conditions in the die to maximize volume expansion. The desirable composition of the materials includes dehydrated branched biodegradable polyester (polybutylene succinate), CO₂ (blowing agent), and tale (nucleating agent). A single‐screw extrusion system was used for foam processing. A large volume expansion ratio of up to forty‐fivefold was achieved from the biodegradable polyester foams. The morphologies and volume expansion ratios of biodegradable polyester foams at various processing temperatures and pressures were studied.     

Comparative study of the influence of CO<Subscript>2</Subscript> and H<Subscript>2</Subscript>O on the chemical structure of lean and rich methane-air flames at atmospheric pressure

A comparative study of the influence of CO₂ and H₂O on both lean and rich CH₄-air laminar flames is performed. Six premixed flames are stabilized on a flat flame burner at atmospheric pressure: lean (with the equivalence ratio maintained constant at ? = 0.7) and rich (with the equivalence ratio maintained constant at ? = 1.4) CH₄-air, CH₄-CO₂-air, and CH₄-H₂O-air flames. These flames are studied experimentally and numerically. The [CO₂]/[CH₄] and [H₂O]/[CH₄] ratios are kept equal to 0.4 for both flames series. Species mole fraction profiles are measured by gas chromatography and Fourier transform infrared spectroscopy analyses of gas samples withdrawn along the vertical axis by a quartz microprobe. Flames structures are computed by using the ChemkinII/Premix code. Four detailed combustion mechanisms are used to calculate the laminar flame velocities and species mole fraction profiles: GRI-Mech 3.0, Dagaut, UCSD, and GDFkin®3.0.     

Interfacial tension measurements and modelling of (carbon dioxide + n-alkane) and (carbon dioxide + water) binary mixtures at elevated pressures and temperatures

Supercritical carbon dioxide (CO₂) is often used as a process fluid for enhanced oil recovery. The storage of carbon dioxide in underground formations is a potential way of mitigating climate change during a transition period to more sustainable energy sources. Combining injection with subsequent trapping of the non-wetting supercritical carbon dioxide phase in the pores of a depleted reservoir is a promising scheme for allowing the continued use of fossil fuels with minimal environmental consequences. The design of such processes is ultimately linked to the confined behaviour of the fluids in question at reservoir conditions, which is largely controlled by interfacial forces. Measurements of the relevant interfacial tensions for systems containing alkanes, carbon dioxide and water are currently limited and inconsistent while models usually fail to capture the pressure dependence of the interfacial tension. In this work, a density functional theory based on the SAFT-VR equation of state was used to predict the interfacial tension of (H₂O + CO₂ + n-alkane) binary systems over wide ranges of temperature and pressure. The comparison with a new set of reported experimental data of three (n-alkane + CO₂) systems at pressures up to the critical points, as well as with the (H₂O + CO₂) system at pressures up to 60 MPa, for a temperature range of (298-443) K, is discussed.     

Measurement and thermodynamic modeling of partition coefficients in N,N-dimethylacetamide-water-carbon dioxide system

The partition coefficients of N,N-dimethylacetamide (N,N-DMA) between the water and the supercritical and near-critical carbon dioxide (CO₂) phases were measured in the temperature range of 298.15-328.15 K and the pressure range of 8.3-24.1 MPa. The measurements were carried out in a 56 ml vessel by contacting the carbon dioxide and the aqueous phases. The partition coefficients of N,N-DMA increased from 0.05 to 0.150 with increasing pressure at a constant temperature and increased with temperature at a constant density. The bubble point pressures of N,N-DMA-CO₂ mixtures were measured at 298.15 K, 308.15 K and 318.15 K and were found to increase with increasing mole fraction of CO₂. The partition coefficients were modeled using the Peng-Robinson Equation of State (PREOS) combined with modified van der Waals mixing rule. The binary interaction parameters for the CO₂-H₂O pair were taken from the literature and were regressed for CO₂-N,N-DMA and H₂O-N,N-DMA pairs by fitting partition coefficients data. The binary interaction parameter for CO₂-N,N-DMA pair was found to depend linearly on temperature. The bubble point pressures of N,N-DMA and CO₂ were also measured and could be predicted fairly well using the regressed binary interaction parameters.