Assessment of the sorption capacity and regeneration of carbon dioxide sorbents using thermogravimetric methods

Adsorption methods using solid sorbents are an alternative to the absorption technology in the processes of purification gases from carbon dioxide. There is a need to rapidly assess the suitability of sorbents for use it in PSA, TSA, or VPSA installations. Important parameters which determine the quality of the sorbent are the sorption capacity of sorbent, selectivity to CO₂ and the possibility of regeneration. This paper presents the results of sorption/desorption of CO₂ study on the impregnated porous materials using thermogravimetric methods. Thermogravimetry allows for rapid assessment of sorption capacity and regeneration of the sorbents. Specially selected temperature program allowed to determine the sorption capacity of sorbents depending on the concentration of CO₂ in the gas mixture and temperature. Degree of sorbent purification was determined in desorption process.     

An improved Sampling Strategy for the Measurement of VOCs in Air,Based on Cooled Sampling and Analysis by Thermodesorption-GC-MS/FID

Abstract

Reliable and comprehensive sampling methods are required to obtain accurate data for VOC concentrations in air samples. The major drawback of the adsorption tube sampling method, widely employed in environmental studies, is the fact that C₂ compounds are usually not trapped quantitatively.

The focus of this work was thus to improve sampling based on adsorption tubes packed with Molsieve and Carbosieve. To improve the sampling efficiency for the C₂ compounds, a cooling device, based on Peltier cooling was constructed, which could be operated at a temperature down to ?30°C.

Experiments under laboratory and field conditions were carried out to study the influence of the sampling temperatuie on the recovery of ethane and ethene as the most volatile VOCs. The results clearly demonstrate the need for a cooled sampling device for the analysis of C₂ compounds in air. Under the investigated conditions, the recoveries with ambient temperature sampling were only in the range of 38–46% for ethane and 33–59% for ethene respectively, in comparison to the cooled sampling device. These findings are only valid for the described conditions and can change significantly with temperature and concentration. A generalisation of the recovery is thus very difficult to give.

The use of the sampling device for a field study is reported, where samples were collected simultaneously at three different altitudes in a diurnal profile on the slope of the Schulterberg mountain in Tyrol (Austria).     

Microwave assisted vacuum regeneration for CO2 capture from wet flue gas

Small scale processing of flue gas with the goal of enriching the stream in CO₂ for sequestration or use is an interesting application area for adsorption technology. For example, boiler flue gas which may contain up to 10 % (v/v) CO₂ in air can be readily enriched to a stream containing >70 % CO₂ which may be ideal for use within a process such as acidification, precipitation, stripping, etc. The challenge in these applications is producing high purity CO₂ without excessive energy use and handling high concentrations of water vapor without the added complication of a pre-drying stage. In this study we have examined the use of microwave assisted vacuum as a way of rapidly directing thermal energy to the adsorbent surface to liberate water and CO₂. Preliminary “proof-of-concept” pump down experiments were conducted on a small transparent adsorption column of 13X zeolite pre-saturated with a 12 % CO₂ in N₂ gas mixture. Both wet and dry gas tests were conducted. The addition of microwave radiation improved the rapid desorption of CO₂ and water and improved the integrated CO₂ purity in the blowdown stream from 60 to 80 %. In the case of dry CO₂ mixtures, the enhancement is due to microwave heating of the 13X zeolite facilitated by the high cation density in the faujasite structure. In the case of water and CO₂ desorption, the temperature rise of the adsorbent upon microwave heating was much lower than that predicted by simple heating suggesting that the microwave radiation is absorbed primarily by the adsorbed water. A simplified energy analysis suggests that brief exposure of an adsorbent to microwave radiation will raise the required vacuum level for regeneration of high humidity flue gas streams and may lead to an overall lower energy penalty. The selective ability of microwave radiation to target different species provides scope for optimized, compact, flue gas treatment systems.     

A parametric study of CO2/N2 gas separation membrane processes for post-combustion capture

Capture of CO₂ from flue gases produced by the combustion of fossil fuels and biomass in air is referred to as post-combustion capture. Chemisorbent processes are considered to be the most feasible method and are already at an advanced stage of development, but gas separation membranes are attracting more and more attention as a possible alternative. This paper describes a detailed parametric study of mass and energy balances for a simulated single membrane process. Typical operating conditions (CO₂ concentration in the flue gas, pressure and temperature, etc.) together with the influence of the membrane quality (permeability, selectivity) and membrane area on membrane performance (CO₂ separation degree and CO₂ purity) are simulated over a wide range of parameters.     

Identifying Different Types of Catalysts for CO2 Reduction by Ethane through Dry Reforming and Oxidative Dehydrogenation

The recent shale gas boom combined with the requirement to reduce atmospheric CO₂ have created an opportunity for using both raw materials (shale gas and CO₂) in a single process. Shale gas is primarily made up of methane, but ethane comprises about 10 % and reserves are underutilized. Two routes have been investigated by combining ethane decomposition with CO₂ reduction to produce products of higher value. The first reaction is ethane dry reforming which produces synthesis gas (CO+H₂). The second route is oxidative dehydrogenation which produces ethylene using CO₂ as a soft oxidant. The results of this study indicate that the Pt/CeO₂ catalyst shows promise for the production of synthesis gas, while Mo₂C‐based materials preserve the C? C bond of ethane to produce ethylene. These findings are supported by density functional theory (DFT) calculations and X‐ray absorption near‐edge spectroscopy (XANES) characterization of the catalysts under in situ reaction conditions.     

Partial Oxidation of Methane to Methanol with Oxygen or Air in a Nonequilibrium Discharge Plasma

The partial oxidation of methane to methanol with oxygen or air was investigated experimentally and theoretically in a dielectric-barrier discharge (DBD). The predominant parameters of specific electric energy, oxygen content, flow rate, temperature, and gas pressure were determined in CH ₄ /O ₂ and CH ₄ /air mixtures. Optimum selectivities toward methanol formation were found at an oxygen concentration of about 15% in both feed gas mixtures. Low specific energy favors the selectivity toward methanol and suppresses the formation of carbon oxides. The experiments indicate that high methanol selectivities can be obtained at high methane conversion. The highest methanol yield of 3% and the highest methanol selectivity of about 30% were achieved in CH ₄ /O ₂ mixtures. In CH ₄ /air mixtures, as high as 2% methanol yield was also obtained. In addition, other useful products, like ethylene, ethane, propane, and ethanol, were detected. Experiment and numerical simulations show that the formation of H ₂ O and CO has a strong negative influence on methanol formation.     

Catalytic Oxidation of Xylene in Air using TiO<Subscript>2 </Subscript>under Electron Beam Irradiation

The oxidation of xylene and its irradiation byproducts in air using TiO₂ was studied under electron beam (EB) irradiation for the purification of ventilation gases emitted from paint factories. EB irradiation experiments were mainly performed under two different conditions: a TiO₂ pellet layer was placed in an irradiation or a non-irradiation space. The results revealed that xylene was decomposed and CO was formed in the gas phase of the irradiation space irrespective of the presence of TiO₂ pellets, while CO₂ was produced in the gas phase of the irradiation space and on the surface of TiO₂ pellets. The total CO₂ concentration increased when the pellet layer was in the non-irradiation space. On the other hand, the concentration of CO₂ produced on the surface of the TiO₂ pellets in the irradiation space was higher than that in a non-irradiation space.     

Capture of CO2 from high humidity flue gas by vacuum swing adsorption with zeolite 13X

Capture of CO₂ from flue gas streams using adsorption processes must deal with the prospect of high humidity streams containing bulk CO₂ as well as other impurities such as SO _x , NO _x , etc. Most studies to date have ignored this aspect of CO₂ capture. In this study, we have experimentally examined the capture of CO₂ from a 12% synthetic flue gas stream at a relative humidity of 95% at 30 °C. A 13X adsorbent was used and the migration of the water and its subsequent impact on capture performance was evaluated. Binary breakthrough of CO₂/water vapor was performed and indicated a significant effect of water on CO₂ adsorption capacity, as expected. Cyclic experiments indicate that the water zone migrates a quarter of the way into the column and stabilizes its position so that CO₂ capture is still possible although decreased. The formation of a water zone creates a “cold spot” which has implications for the system performance. The recovery of CO₂ dropped from 78.5% to 60% when moving from dry to wet flue gas while the productivity dropped by 22%. Although the concentration of water leaving the bed under vacuum was 27%(vol), the low vacuum pressure prevented condensation of water in this stream. However, the vacuum pump acted as a condenser and separator to remove bulk water. An important consequence of the presence of a water zone was to elevate the vacuum level thereby reducing CO₂ working capacity. Thus although there is a detrimental effect of water on CO₂ capture, long term recovery of CO₂ is still possible in a single VSA process. Pre-drying of the flue gas steam is not required. However, careful consideration of the impact of water and accommodation thereof must be made particularly when the feed stream temperature increases resulting in higher feed water concentration.     

Study of NOx and CO2 production of cultivated soil in closed microcosm experimental system

The aim of present project was to develop a microcosm experimental method for estimation of NO_x and CO₂ emission of microbial origin from cultivated soil. The effect of different factors (such as temperature, water supply, mineral-N source and organic matter addition, role of soil organisms and heavy metal contamination) that controlling the accumulation of N₂O and CO₂ in soil atmosphere and release to air was studied in closed microcosm laboratory model experiments. The headspace gas composition of closed glass vessels of 800-1200 cm³ containing 100-200 g brown forest soil sample was analysed. The N₂O and CO₂ concentration of gas samples was analysed by gas chromatographic methods and NO-content by means of chemiluminescent detection. Concerning the results, it can be stated that the applied microcosm experimental model proved to be a suitable tool for detecting the effect of factors influencing the NO_x and CO₂ release from agricultural soil. The temporal changes of N₂O and CO₂ concentration demonstrated the impact of the coupled microbial processes resulting in these greenhouse gases. The gas production depended on the soil moisture level, temperature and C/N ratio significantly. The inhibitory effect of toxic heavy metals (e.g. Cd) could also be affected by the C/N ratio. The appearance of NO as an intermediate of microbial processes was observed as well.     

New (p, ρ, T) data for carbon dioxide – Nitrogen mixtures from (250 to 400) K at pressures up to 20 MPa

Comprehensive (p, ρ, T) measurements on two binary mixtures (0.10 CO₂ + 0.90 N₂ and 0.15 CO₂ + 0.85 N₂) were carried out in the gas phase at seven isotherms between (250 and 400) K and pressures up to 20 MPa using a single sinker densimeter with magnetic suspension coupling. A total of 69 (p, ρ, T) data for the first mixture and 69 (p, ρ, T) data for the second are presented in this article. The uncertainty in density was estimated to be (0.02 to 0.15)%, while the uncertainty in temperature was 3.9 mK and the uncertainty in pressure was less than 0.015% (coverage factor k = 2). Experimental results were compared with densities calculated from the GERG equation of state and with data reported by other authors for similar mixtures. Results yielded that, while deviations between experimental data and values calculated from the GERG equation were lower than 0.05% in density for low pressures, the relative error at high pressures and low temperatures increased to about (0.2 to 0.3)%. The main aim of this work was to contribute to an accurate density data base for CO₂/N₂ mixtures and to check or improve equations of state existing for these binary mixtures.     

H2S-sensing properties of SnO2 produced by ball milling and different chemical reactions

In this work, the mechanochemical synthesis of a moderately agglomerated tin oxide (SnO₂) powders and the subsequent preparation of semiconductor gas sensors as prototypes, were studied. Tin (II) chloride (SnCl₂) powder was milled with calcium hydroxide (Ca(OH)₂) and potassium carbonate, (K₂CO₃) powder, respectively, in a ball mill at room temperature and in an air atmosphere. Heat treatment of milled mixtures at 400 °C resulted in the formation of a tetragonal phase, confirmed by X-ray diffraction (XRD). During milling in the presence of water, a high number of hydroxide (OH) groups are formed at the surface. When SnCl₂ was milled with K₂CO₃, no water was produced and the Fourier-transform infrared spectrum (FT-IR) of the powder has no surface hydroxyl deformations. On exposure to hydrogen sulfide (H₂S) gas, the particles, prepared from anhydrous powder, have higher sensitivity than these, prepared from hydrated powder. The SnO₂ thick film, prepared from anhydrous powder may be successfully applied to a H₂S gas sensor.     

Hydrogen gas mixture separation by CVD silica membrane

In this work we investigate the performance of high flux chemical vapour deposition (CVD) silica membranes for the separation of gas mixtures containing H₂ and CO₂ at various temperatures. The membranes were prepared by a counter diffusion CVD method where tetraethyl orthosilicate (TEOS) and O₂ were used as reactants. Single gas permeation resulted in activated transport for the smaller kinetic diameter gases (H₂ and He) whilst the larger kinetic diameter gases (CO₂ and N₂) showed negative activation energy. The single gas permeation of H₂ increased from 5.1 × 10⁻⁷ to 7.0 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ in the temperature range 100–400 °C, and H₂/CO₂ and H₂/N₂ selectivities reached 36 and 57 at 400 °C, respectively. The H₂ purity in the permeate stream also increased with temperature for H₂:CO₂ binary gas mixture, thus being beneficial for H₂ diffusion. H₂ competitively permeated through the membrane at a several range of gas mixtures, and a saturation level was achieved at H₂:CO₂ 60:40 feed concentration, where the diffusion of CO₂ molecules became negligible delivering ∼99% H₂ purity in the permeate stream. These results substantiate that the counter diffusion CVD method produced thin silica film membranes with a very precise pore size control, in particular suggesting a narrow pore distribution with average pore radius of about 3.1 Å.     

Studies on Gas Purification by a Pulsed Microwave Discharge at 2.46 GHz in Mixtures of N2/NO/O2 at Atmospheric Pressure

Pulsed microwave discharges operated at atmospheric pressure in gas mixtures containing N₂, O₂, and NO are investigated experimentally and theoretically for various gas mixture constituents and operating conditions with respect to the ability of exhaust gas purification. The rotational gas temperature and the vibrational temperature of N₂ are derived from CARS measurements. The composition of the exhaust gas after treatment is monitored using FTIR spectroscopy. The processes of the chemical, electronic, and vibrational kinetics are described by a model that has been developed to calculate the species densities. The results obtained show that in N₂/NO gas mixtures an overall reduction of NO_x takes place. In the case of N₂/O₂/NO gas mixtures, no net reduction of NO_x is achieved for a pulsed microwave power below 3600 W, a pulse length of 50 s, and a typical repetition frequency of 2 kHz.     

Electrode Potentials of Cells with an Electrolyte Based on Zirconium Dioxide in Gas Mixtures N2+ O2+ CO2+ CO: A Platinum Electrode

Steady-state potentials of various platinum electrodes are measured in cells containing electrolyte ZrO₂+ Y₂O₃(10 mol %) in the temperature range 673–773 K in binary equilibrium gas mixtures N₂+ O₂and CO + CO₂, as well as in four-component nonequilibrium gas mixtures N₂+ O₂+ CO₂+ CO containing 0–3 vol % CO and 0–10 vol % O₂. Adding CO to a gas mixture makes the electrode potential deviate from equilibrium, which is explained by chemisorption of CO on the electrode. The oxygen, which is adsorbed on platinum, interacts with CO; as a result, CO₂undergoes desorption and the surface concentration of CO drops.     

Adsorption of Sulfur Dioxide from Pseudo Binary Mixtures on Hydrophobic Zeolites: Modeling of the Breakthrough Curves

The adsorption of SO₂ from pseudo binary mixtures with water and CO₂ on hydrophobic zeolites (MFI and MOR type) was investigated using the breakthrough curve method. The SO₂ and water breakthrough curves were compared with theoretical ones based on an axially dispersed plug flow through the column and the linear driving force rate equation. In addition, different semi-predictive multi-component equilibrium equations were used for the breakthrough modeling: Langmuir 1, Langmuir 2 and Langmuir-Freundlich extended models. The overall mass transfer coefficients were derived by matching theoretical with experimental breakthrough curves for single component systems, i.e., water vapor or SO₂ in a carrier gas. They were also predicted from a simplified bi-porous adsorbent model and compared with experimentally derived values. The presence of CO₂ species in ternary mixtures with water vapor and SO₂, even at relatively high concentrations of 9 vol%, had no significant effect on the breakthrough behavior of the other two species. For that reason the CO₂ species was ignored in the analysis of the resulting pseudo binary mixtures. The breakthrough model was solved by finite element orthogonal collocation method using the commercial software gPROMS. Both extended Langmuir 1 and Langmuir 2 based models gave reasonable predictions of the water and SO₂ breakthrough curves for pseudo binary mixtures involving a mordenite sample for all water concentration levels used in this study (up to 3.5 vol%). However, the same models were successfully used to predict SO₂ breakthrough curves for a MFI sample only at low water concentrations, i.e., 1.5 vol%. At the higher water levels both models failed to describe equilibrium behavior in the MFI sample due to the introduction of multi-layer adsorption in the interstices between small MFI-26 crystals.     

Adsorption of CO<Subscript>2</Subscript>-containing gas mixtures over amine-bearing pore-expanded MCM-41 silica: application for CO<Subscript>2</Subscript> separation

Adsorption of CO₂, N₂, CH₄ and H₂ on triamine-grafted pore-expanded MCM-41 mesoporous silica (TRI-PE-MCM-41) was investigated at room temperature in a wide range of pressure (up to 25 bar) using gravimetric measurements. The material was found to exhibit high affinity toward CO₂ in comparison to the other species over the whole range of pressure. Column-breakthrough dynamic measurements of CO₂-containing mixtures showed very high selectivity toward CO₂ over N₂, CH₄ and H₂ at CO₂ concentrations within the range of 5 to 50%. These conditions are suitable for effective removal of CO₂ at room temperature from syngas, flue gas and biogas using temperature swing (TS) or temperature-pressure swing (TPS) regeneration mode. Moreover, TRI-PE-MCM-41 was found to be highly stable over hundreds of adsorption-desorption cycles using TPS as regeneration mode.     

Non-catalytic pyrolysis of ethane to ethylene in the presence of CO2 with or without limited O2

Influence of the presence of CO₂, which is a mild oxidant, on the performance of the thermal cracking of ethane to ethylene in the absence or presence of limited O₂ at different temperatures (750–900‡C), space velocities (1500–9000 h^-1) and CO₂/C₂H₆ and O₂/C₂H₆ mole ratios (0–2.0 and 0–0.3 respectively) has been investigated. In both the presence and absence of limited O₂, ethane conversion increases markedly because of the presence of CO₂, indicating its beneficial effect on the ethane to ethylene cracking. The increased ethane conversion is, however, not due to the oxidation of ethane to ethylene by CO₂; the formation of carbon monoxide in the presence of CO₂ is found to be very small. It is most probably due to the activation of ethane in the presence of CO₂.     

Adsorptive Separation of Acetylene from Light Hydrocarbons by Mesoporous Iron Trimesate MIL‐100(Fe)

A reducible metal–organic framework (MOF), iron(III) trimesate, denoted as MIL‐100(Fe), was investigated for the separation and purification of methane/ethane/ethylene/acetylene and an acetylene/CO₂ mixtures by using sorption isotherms, breakthrough experiments, ideal adsorbed solution theory (IAST) calculations, and IR spectroscopic analysis. The MIL‐100(Fe) showed high adsorption selectivity not only for acetylene and ethylene over methane and ethane, but also for acetylene over CO₂. The separation and purification of acetylene over ethylene was also possible for MIL‐100(Fe) activated at 423 K. According to the data obtained from operando IR spectroscopy, the unsaturated Fe^III sites and surface OH groups are mainly responsible for the successful separation of the acetylene/ethylene mixture, whereas the unsaturated Fe^II sites have a detrimental effect on both separation and purification. The potential of MIL‐100(Fe) for the separation of a mixture of C₂H₂/CO₂ was also examined by using the IAST calculations and transient breakthrough simulations. Comparing the IAST selectivity calculations of C₂H₂/CO₂ for four MOFs selected from the literature, the selectivity with MIL‐100(Fe) was higher than those of CuBTC, ZJU‐60a, and PCP‐33, but lower than that of HOF‐3.     

Effect of variation in argon content of calibration gases on determination of atmospheric carbon dioxide

Carbon dioxide (CO₂) is a greenhouse gas that makes by far the largest contribution to the global warming of the Earth's atmosphere. For the measurements of atmospheric CO₂ a non-dispersive infrared analyzer (NDIR) and gas chromatography are conventionally being used. We explored whether and to what degree argon content can influence the determination of atmospheric CO₂ using the comparison of CO₂ concentrations between the sample gas mixtures with varying Ar amounts at 0 and 18.6 mmol mol⁻¹ and the calibration gas mixtures with Ar at 8.4, 9.1, and 9.3 mmol mol⁻¹. We newly discovered that variation of Ar content in calibration gas mixtures could undermine accuracy for precise and accurate determination of atmospheric CO₂ in background air. The differences in CO₂ concentration due to the variation of Ar content in the calibration gas mixtures were negligible (<±0.03 μmol mol⁻¹) for NDIR systems whereas they noticeably increased (<±1.09 μmol mol⁻¹) especially for the modified GC systems to enhance instrumental sensitivity. We found that the thermal mass flow controller is the main source of the differences although such differences appeared only in the presence of a flow restrictor in GC systems. For reliable monitoring of real atmospheric CO₂ samples, one should use calibration gas mixtures that contain Ar content close to the level (9.332 mmol mol⁻¹) in the ambient air as possible. Practical guidelines were highlighted relating to selection of appropriate analytical approaches for the accurate and precise measurements of atmospheric CO₂. In addition, theoretical implications from the findings were addressed.     

Highly accurate gas chromatographic analysis of reference gases for use in vehicle emission measurements

A highly accurate gas chromatographic analytical method has been developed for the determination of the composition of gas mixtures. It was tested using a reference gas as an example consisting of 3.5% of CO, 14% of CO₂, 0.2% of propane and residual N₂ intended for the use in vehicle emission measurements. The method is based on comparison measurements with samples of a calibration gas, whose composition is iteratively adapted to that of the sample investigated using a gravimetric gas mixing method. For the gas chromatographic measurement, a molecular sieve column and a polymer column are used in parallel and in isothermal operation. All gas components can be determined by a single gas chromatographic measurement, and the relative uncertainty of measurement achievable is ?0.4%.