Mixed Gas System Research Articles

Iron and carbon codoped WO3 with hierarchical walnut-like microstructure for highly sensitive and selective acetone sensor

Current metal-oxide-based sensing materials are confronted with several challenges, especially in sensitivity, selectivity and stability, for their application in the breath acetone analysis. Herein, hierarchical walnut-like Fe-C-codoped WO3 microspheres were synthesized and characterized by X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The amount of Fe doping was optimized based on detecting the acetone responses dependent on the operating temperature. The sensor based on the optimal Fe-C-codoped WO3 (FW3) exhibited high response to acetone and very low responses to NH3, CO, toluene, methanol, ethanol and NO. The results indicate that the optimized material possesses high sensitivity and good selectivity toward acetone vapor. Besides, the FW3 sensor presented superior anti-interferential ability to various mixed-gas systems. More importantly, the responses of the sensor exhibited no obvious fluctuation over 12 weeks, implying good long-term stability of the synthesized material. We suggest that the phase, morphology and the increased number of oxygen vacancies induced by Fe doping are the underlying reason for the improved gas sensing performance of the Fe-C-codoped WO3 microspheres.

A unified phase equilibrium model for hydrogen solubility and solution density

For the transition to a clean and sustainable energy production from renewable sources like solar or wind power, large and secure storage of energy is required to compensate for the intermittent nature of these sources. Hydrogen could be a suitable energy carrier and hydrogen geological storage could provide the large capacities required. During storage hydrogen will be brought in contact with the formation fluids present, resulting in dissolution and possibly inducing geochemical reactions. Therefore in this work an accurate, consistent and reliable hydrogen solubility model is established, which allows to calculate the hydrogen solubility in the formation fluid and the corresponding variation of fluid density. The model accounts for system pressure, temperature and formation fluid salinity as well as the molar fraction, fugacity coefficient, Henry's constant, Poynting factor and activity coefficient of hydrogen. In the range of typical hydrogen geological storage conditions of 273–373 K, 1–50 MPa and 0–5 mol/kg NaCl this model can reproduce all available experimental data and predict hydrogen solubility in the formation fluid and the formation fluid density accurately. The model can predict hydrogen solubility within a maximum relative error of 5% for pure water and 15% for brines within the salinity range considered, which is in the range of uncertainty of measurement data. For realistic hydrogen gas geological storage, the model is extended to represent also H2-N2 or H2-CH4 mixed gas systems as well as mixed electrolyte solutions containing Na, K, Ca, Mg, Cl or SO4 and combinations of those. Model derivation, model calculations and implementation as well as an application example are presented to demonstrate the applicability of the developed methods and the model.

Iterative combining rules for the van der Waals potentials of mixed rare gas systems

An iterative procedure is introduced to make the results of some simple combining rules compatible with the Tang-Toennies potential model. The method is used to calculate the well locations Re and the well depths De of the van der Waals potentials of the mixed rare gas systems from the corresponding values of the homo-nuclear dimers. When the “sizes” of the two interacting atoms are very different, several rounds of iteration are required for the results to converge. The converged results can be substantially different from the starting values obtained from the combining rules. However, if the sizes of the interacting atoms are close, only one or even no iteration is necessary for the results to converge. In either case, the converged results are the accurate descriptions of the interaction potentials of the hetero-nuclear dimers.

Study on the Adsorption, Diffusion and Permeation Selectivity of Shale Gas in Organics

As kerogen is the main organic component in shale, the adsorption capacity, diffusion and permeability of the gas in kerogen plays an important role in shale gas production. Based on the molecular model of type II kerogen, an organic nanoporous structure was established. The Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) methods were used to study the adsorption and diffusion capacity of mixed gas systems with different mole ratios of CO2 and CH4 in the foregoing nanoporous structure, and gas adsorption, isosteric heats of adsorption and self-diffusion coefficient were obtained. The selective permeation of gas components in the organic pores was further studied. The results show that CO2 and CH4 present physical adsorption in the organic nanopores. The adsorption capacity of CO2 is larger than that of CH4 in organic pores, but the self-diffusion coefficient of CH4 in mixed gas is larger than that of CO2. Moreover, the self-diffusion coefficient in the horizontal direction is larger than that in the vertical direction. The mixed gas pressure and mole ratio have limited effects on the isosteric heat and the self-diffusion of CH4 and CO2 adsorption. Regarding the analysis of mixed gas selective permeation, it is concluded that the adsorption selectivity of CO2 is larger than that of CH4 in the organic nanopores. The larger the CO2/CH4 mole ratio, the greater the adsorption and permeation selectivity of mixed gas in shale. The permeation process is mainly controlled by adsorption rather than diffusion. These results are expected to reveal the adsorption and diffusion mechanism of gas in shale organics, which has a great significance for further research.

Prediction of phase equilibrium conditions for gas hydrates formed in the presence of cyclopentane or cyclohexane

In this work, we predicted the hydrate formation conditions for the pure and mixed gas systems of CH4, N2, O2 and CO2 in the presence of cyclopentane or cyclohexane. Chen-Guo model coupled with the predictive Soave-Redlich-Kwong (PSRK) group contribution method was used to calculate the three-phase equilibrium, and the UNIFAC model was employed to calculate the activity coefficient of the liquid phase. The pressure-corrected Henry’s law was employed to calculate the mole fraction of gas components existing in the aqueous phase except water. The prediction results and the percent of Absolute Average Deviation for the calculated hydrate formation pressures and temperatures were presented. It was found that the results predicted in this work agree well with those reported in the literature. Cyclopentane and cyclohexane in aqueous phase was assumed to evaporate into the gas phase and may change the gas components and the corresponding mole fractions in the gas phase. However, the results showed that the concentration variation of cyclopentane or cyclohexane has slight impact on the hydrate formation thermodynamics.

Amidine-Functionalized Poly(2-vinyl-4,4-dimethylazlactone) for Selective and Efficient CO2 Fixing

Development of novel polymeric materials capable of efficient CO2 capture and separation under ambient conditions is crucial for cost-effective and practical industrial applications. Here we report the facile synthesis of a new CO2-responsive polymer through postpolymerization modification of poly(2-vinyl-4,4-dimethylazlactone) (PVDMA). The reactive pendant azlactone groups of PVDMA are easily modified with 4-(N-methyltetrahydropyrimidine)benzyl alcohol (PBA) without any byproduct formation. FTIR and TGA experiments show the new PBA-functionalized polymer powder can reversibly capture CO2 at room temperature and under atmospheric pressure. CO2 capture was selective, showing a high fixing efficiency even with a mixed gas system (20% CO2, 80% N2) similar to flue gas. CO2 release occurred at room temperature, and release profiles were investigated as a function of temperature. Density functional theory (DFT) calculations coupled with modeling and simulation reveal the presence of two CO2 binding sites in the PBA-functionalized polymer resulting in a two-step CO2 release at room temperature. The ease of material preparation, high fixing efficiency, and robust release characteristics suggest that postpolymerization modification may be a useful route to designing new materials for CO2 capture.

Modeling of simultaneous competitive mixed gas permeation and CO2 induced plasticization in glassy polymers

Plasticization behavior of polymeric membranes, in presence of a second component in the feed is different from plasticization of pure CO2 due to competitive sorption. This study focused on a mathematical model derived for permeation of mixed gases in glassy polymers considering the plasticization case. It was assumed that diffusion coefficient for all components were exclusively a function of plasticizing component. In this study, the partial immobilization model was employed to determine fraction of mobile sorbed gases. This model accurately predicted the permeation behavior of CO2 as plasticizer and N2 in presence of plasticization. The model parameters were calculated by fitting experimental data from literature. Plasticization parameter (β) decreased for both CO2 and N2 by increasing fraction of N2 in the feed. It means that plasticization of glassy polymers was suppressed. This decrease was caused by competitive sorption between CO2 and N2. Indeed N2 in the feed acts as an anti-plasticizer. In addition, permeances of the feed gas components were declined in comparison to pure gases, which might be attributed to reduction of sorption and occupying Langmuir sites with the second component. The proposed model is capable of giving a useful tool to enhance our knowledge related to permeability and selectivity behavior of mixed gas systems in glassy polymeric membranes.

A method for simultaneous reconstruction of temperature and concentration distribution in gas mixtures based on acoustic tomography

The acquisition of the concentration and temperature distributions in a mixed gas system plays a crucial role in real industrial applications. Different from comment measurement method, in this paper an acoustic tomography (AT) based method is developed for the simultaneous reconstruction of the concentration and temperature distributions. The proposed method includes two stages. In the first stage, the correlations of the acoustic velocity and the intensity attenuation with the temperature and concentration are investigated. Then the approach that can simultaneously compute temperature and concentration is proposed. In the second stage, in order to achieve 2D tomography, the stochastic inversion method for solving the inverse problem is developed. Numerical simulations are implemented to validate the feasibility of the proposed reconstruction method.

Prediction of hydrate formation conditions using GE-EOS and UNIQUAC models for pure and mixed-gas systems

The aim of this study is to predict hydrate formation pressure and temperature through the three-phase (hydrate–liquid water–gas) equilibrium calculation using the GE-EOS and UNIQUAC activity coefficient models. Calculation of the hydrate formation conditions is carried out for pure gases such as CH4, C2H6, C3H8, iC4H10, CO2, H2S and N2, the gas binaries and ternaries; and some synthetic natural gas systems. Also calculations of the gas hydrate formation pressures are performed in the presence of methanol as inhibitor for the different gas hydrate systems. In this work, the Chen–Guo model is used for the hydrate phase and the UNIQUAC activity coefficient is applied for nonideality of the liquid phase. The SRK equation of state coupled with Huran-Vidal (HV) mixing rule is applied for the vapor phase. The UNIQUAC function is used not only for calculation of the activity coefficient of water in aqueous phase, but also is applied in the HV mixing rules. The UNIQUAC binary interaction parameters are adjusted through the hydrate experimental data directly so that there is no need to use the various vapor–liquid experimental data to obtain the parameters. To achieve more accurate results, the solubility of the gases in the aqueous phase is also taken into account using Henry's law. Finally, the present results are compared with the other available works and a very good agreement is observed with the experiment. The percent of Absolute Average Deviation (AAD%) for the calculated hydrate formation pressures and temperatures are considerably less than those results given by the other previous works.

Thin zeolite T/carbon composite membranes supported on the porous alumina tubes for CO2 separation

Thin zeolite T/carbon composite membranes (thickness about 3–4μm) with the favorable CO2 separation performances were successfully synthesized on the porous alumina tubes by dispersing zeolite T particles (particle size about 300nm) into polymeric precursor (polyfurfuryl alcohol (PFA)). The composite membrane is carbonized under the condition of 873K with a dwell time of 2h and ultra-high purity N2 flowing. The CO2 separation performance is influenced by the zeolite contents and dip-coating cycles of the composite membranes. The zeolite T/carbon composite membrane synthesized with zeolite contents of 2wt.% and two coating cycles displayed the best CO2 separation performance. The single and mixed gases permeation properties of the composite membrane were investigated in detail. Due to the competitive permeation effect between adsorbable gas (CO2) and non-adsorbable gas (N2 and CH4) in the membrane, the CO2/CH4 and CO2/N2 permselectivities (143 and 67.1) in the equimolar mixed gas system were higher than the results from single gas permeation test (97.3 and 51.6) at temperature of 298K. The effects of permeation temperature and pressure difference on CO2 separation performances of the composite membranes were also studied. The CO2/CH4 and CO2/N2 selectivities decreased with increasing the permeation temperature or increasing the pressure difference. The zeolite T/carbon composite membrane introduced in this paper can be a novel membrane candidate for industrial CO2 separation.

Gas sensing characteristics of MEMS gas sensor arrays in binary mixed-gas system

MOS gas sensor arrays based on MEMS gas sensor platforms were developed for the detection of carbon monoxide (CO), nitrogen oxides (NOx) and ammonia (NH3), and their gas sensing characteristics in binary mixed-gas system were investigated. Three gas sensing materials with nano-sized particles for these target gases, Pd–SnO2 for CO, In2O3 for NOx and Ru–WO3 for NH3 were synthesized using a sol–gel method. All the sensors showed good properties for their target gases at the optimum points for micro-heater operation. From the experimental data in MEMS gas sensor arrays in a binary mixed system, the gas sensing behavior and sensor response in mixed gas systems were scrutinized. The gas sensing behaviors to the mixed gas systems suggested that specific adsorption and selective activation of adsorption sites might occur in gas mixtures and offer the priority for the adsorption of specific gas. Thorough analysis of the sensing performance of the sensor arrays will make it possible to discriminate the components in gas mixtures as well as their concentrations.

Kinetics of the Formation and Dissociation of Gas Hydrates from CO2-CH4 Mixtures

Sequestration of carbon dioxide (CO2) in the form of its hydrates in natural methane (CH4) hydrate reservoirs, via CO2/CH4 exchange, is an attractive pathway that also yields valuable CH4 gas as product. In this paper, we describe a macroscale experiment to form CO2 and CH4-CO2 hydrates, under seafloor-mimic conditions, in a vessel fitted with glass windows that provides visualization of hydrates throughout formation and dissociation processes. Time resolved pressure and temperature data as well as images of hydrates are presented. Quantitative gas conversions with pure CO2, calculated from gas chromatographic measurements yielded values that range from 23 – 59% that correspond to the extent of formed hydrates. In CH4-rich CH4-CO2 mixed gas systems, CH4 hydrates were found to form preferentially.

Gas separation transport modeling for PDMS coatings on PEI-PEG IPN membranes

A caulking layer is frequently applied to gas separation membranes to alleviate the effects of structural defects. Caulking materials have high gas diffusivities, so as not to hinder the performance of the gas separation layer. Despite these considerations, for some bi-layer systems unaffected by plasticization, the mixed gas selectivity has been observed to be significantly lower than selectivity values calculated from pure gas permeabilities. The present object is to model and simulate two-layer caulked gas separation membranes at the fundamental level to discover the mechanistic basis for the observed behaviour. To this end, a transport based model considering intrinsic material properties was developed to simulate gas separation for a system of CO 2 and N 2 diffusing across a caulking layer of polydimethyl siloxane (PDMS) and then a polyetherimide-polyethylene glycol interpenetrating network (PEI-PEG IPN) membrane. The model was able to account for flux declines in mixed gas systems greater than those predicted by conventional series resistance models, such as have been observed experimentally.

The adsorption and desorption characteristics of a binary component system of toluene and methylethylketone on activated carbon modified with phosphoric acid

As adsorption characteristics, breakthrough curve and adsorption capacity were investigated using a flow-typed fixed-bed adsorption system for single or binary component, mixed gas systems of toluene and methylethylketone (MEK) on activated carbon (AC) modified with phosphoric acid (PA). In the results, the amount of toluene and MEK adsorbed on the PA modified AC (PA/AC) was larger than that on the purified AC. The breakthrough curve of the key component (toluene) was sigmoidal in the binary components. However, the breakthrough curve of the non-key component (MEK) exhibited the roll-up phenomenon, which was entirely different from the single component adsorption process because of the competitive adsorption of the key component. After passing a stream of toluene over the MEK adsorbed PA/AC, almost all of the MEK was easily substituted with toluene and desorbed at 25°C. Therefore, the roll-up of MEK was attributed to its lower adsorption intensity than toluene.

Carbon Dioxide Separation through Polymeric Membrane Systems for Flue Gas Applications

The capture and storage of carbon dioxide has been identified as one potential solution to greenhouse gas driven climate change. Efficient separation technologies are required for removal of carbon dioxide from flue gas streams to allow this solution to be widely implemented. A developing technology is membrane gas separation, which is more compact, energy efficient and possibly more economical than mature technologies, such as solvent absorption. This review examines the recent patented developments in polymeric based membranes designed for carbon dioxide separation from mixed-gas systems. Initially, the background to polymeric membrane separation is provided, with an overview of past polymeric designs. This is followed by a discussion on the current state of the art; in particular developments in mixed matrix polymeric membranes and facilitated transport polymeric membranes for improved carbon dioxide permeation and selectivity. Recent developments in other membrane types, carbon and inorganic, are reviewed for comparison purposes with polymeric developments. Finally, a brief comment on the future directions of polymeric membrane gas separation technologies is provided.

Gas-Phase Chemistry in Inductively Coupled Plasmas for NO Removal from Mixed Gas Systems

Inductively coupled rf plasmas were used to investigate the removal of NO from a variety of gas mixtures. Laser-induced fluorescence and optical emission spectroscopy were employed to measure the relative gas-phase density of NO as a function of the applied rf power, gas mixture, and catalytic substrate type. In general, the overall density of NO decreases as a function of applied rf power in both NO and N(2)/O(2) plasmas, but the addition of gases such as H(2)O vapor and CH(4), as well as the presence of Au-coated substrates, significantly affects the behavior of NO in these systems. Rotational and vibrational temperatures for NO were measured using laser-induced fluorescence excitation spectra and optical emission spectra. Results show NO vibrational temperatures are about a factor of 5 higher than rotational temperatures and indicate little dependence on applied rf power, feed gas composition, or overall system pressure. Possible mechanisms for the observed changes in [NO] as well as the rotational and vibrational temperature data are addressed.

Zeolite L/carbon nanocomposite membranes on the porous alumina tubes and their gas separation properties

With one step of vacuum coating and pyrolysis, the thin zeolite L/carbon nanocomposite membranes (about 3–4 μm) were obtained on the porous alumina tubes by incorporating nano-sized zeolite L (about 100 nm) into polymeric precursor (polyfurfuryl alcohol (PFA)). The permeation properties of pure H 2, CO 2, N 2 and CH 4, together with CO 2/CH 4 and CO 2/N 2 mixtures through the composite membrane were measured and analyzed. A notable increase was observed in the permeances of gas species in comparison with the pure carbon membrane owing to the enhancement of the pore structure properties, including pore volume, surface area, pore size and its distribution. Due to the restricted diffusion of non-adsorbable gas by adsorbable component in the micropores, the CO 2/CH 4 and CO 2/N 2 permselectivities in the mixed gas system were higher than the results from pure gas permeation. The effects of permeation temperature, pressure difference and CO 2 content in feed gases on separation performances of the composite membranes were also investigated. It was found that the CO 2/CH 4 and CO 2/N 2 separation factors decreased with increasing the permeation temperature and pressure difference, and increased with increasing CO 2 content in the mixtures.

활성탄 흡착탑을 이용한 휘발성 유기화합물의 흡착특성

VOCs (Volatile organic compounds) are known as one of the harmful chemicals, causing cancer and global warming. Therefore, the proper control, removal, and reduction of the emission of VOCs are important tasks for the environmental protection. Among the method of VOCs removal activated carbon bed is the most efficient and economical method. In this study, the adsorption performance of toluene gas was investigated using various activated carbons. To find out the adsorption efficiency, the H/D (Height/Diameter) of the activated carbon and GHSV (Gas Hourly Space Velocity) of the toluene gas were manipulated with various conditions. The effect of the temperature, humidity and toluene-MEK-IPA mixed gas on adsorption were also investigated. As a result, a high adsorption performance was found when GHSV is lower at room temperature and low humidity. It was also found that the adsorption efficiency of toluene-MEK-IPA mixed gas system was lower than that of toluene gas system.

Superior gas separation performance of dual-layer hollow fiber membranes with an ultrathin dense-selective layer

A concept demonstration has been made to simultaneously enhance both O 2 and CO 2 gas permeance and O 2/N 2 and CO 2/CH 4 selectivity via intelligently decoupling the effects of elongational and shear rates on dense-selective layer and optimizing spinning conditions in dual-layer hollow fiber fabrication. The dual-layer polyethersulfone hollow fiber membranes developed in this work exhibit an O 2/N 2 selectivity of 6.96 and an O 2 permeance of 4.79 GPU which corresponds to an ultrathin dense-selective layer of 918 Å at room temperature. These hollow fibers also show an impressive CO 2/CH 4 selectivity of 49.8 in the mixed gas system considering the intrinsic value of only 32 for polyethersulfone dense films. To our best knowledge, this is the first time to achieve such a high CO 2/CH 4 selectivity without incorporating any material modification. The above gas separation performance demonstrates that the optimization of dual-layer spinning conditions with balanced elongational and shear rates is an effective approach to produce superior hollow fiber membranes for oxygen enrichment and natural gas separation.

Carbon Dioxide Separation through Polymeric Membrane Systems for Flue Gas Applications

The capture and storage of carbon dioxide has been identified as one potential solution to greenhouse gas driven climate change. Efficient separation technologies are required for removal of carbon dioxide from flue gas streams to allow this solution to be widely implemented. A developing technology is membrane gas separation, which is more compact, energy efficient and possibly more economical than mature technologies, such as solvent absorption. This review examines the recent patented developments in polymeric based membranes designed for carbon dioxide separation from mixed-gas systems. Initially, the background to polymeric membrane separation is provided, with an overview of past polymeric designs. This is followed by a discussion on the current state of the art; in particular developments in mixed matrix polymeric membranes and facilitated transport polymeric membranes for improved carbon dioxide permeation and selectivity. Recent developments in other membrane types, carbon and inorganic, are reviewed for comparison purposes with polymeric developments. Finally, a brief comment on the future directions of polymeric membrane gas separation technologies is provided. Keywords: Polymeric membranes, gas separation, carbon dioxide, mixed matrix membranes, facilitated transport membranes