Model Gas Mixture Research Articles

Gd\u2013Bi\u2013Ce\u2013O materials as catalysts in CO oxidation

Ceria-based solid solutions doped by Gd and/or Bi were synthesized by co-precipitation method from acid aqueous solution of cerium, gadolinium, and bismuth salts, followed by calcination at the temperature of 500 °C. Characterization of the synthesized samples by many methods was carried out. It is shown that all obtained powders of solid solutions crystallized into a cubic structure of the fluorite type, with an average particle size of 10–17 nm, and for the sample obtained through intermediate acetylacetonate complexes was 5–8 nm. The samples had a mesoporous structure with a pore diameter of 2–5 nm. These systems were tested as the catalysts in CO oxidation in the model gas mixture by the flow method. Gd0.05Bi0.15Ce0.8O2 system obtained through intermediate acetylacetonates has the highest activity, so this sample can be used as a catalyst.

Monitoring and efficiency assessment of biofilter air deodorization using electronic nose prototype

Biofiltration is one of the techniques used to reduce odorants in the air. It is based on the aerobic degradation of pollutants by microorganisms located in the filter bed. The research presents the possibility of using the electronic nose prototype for monitoring and efficiency assessment of air biofiltration. The study was conducted using model gas mixtures containing representatives of three groups of chemical compounds: n-hexane (alkanes), acetone (ketones) and toluene (aromatic hydrocarbons). Based on the proposed linear models it has been shown that the determination of concentrations of these substances in the ternary mixture is possible at relatively low concentrations. It was also shown that the used filter medium (pine bark) had the highest removal efficiency of acetone.

Optimization of the Sodium Hydroxide–Assisted Hydrogen Sulfide Selective Removal from Natural Gas

Scrubbing of hydrocarbon-based gases such as natural gas or biogas is essential prior to its use. These gas mixtures when contaminated with hydrogen sulfide cause safety, environmental, and corrosion problems in pipelines. There are several methods of reducing the content of hydrogen sulfide in sour gases, but few of these are selective as well. Selectivity is important to avoid unnecessary chemical usage. Chemisorption in sodium hydroxide solution by applying a special spray technique appears to be an efficient and selective method for hydrogen sulfide removal. This study aims to determine the characteristic of system and thereby facilitating the design of gas-cleaning parameters. The studied parameters include the contact time, concentration of sodium hydroxide, absorbent volumetric flow rate, and gas composition. Model gas mixtures of hydrogen sulfide, carbon dioxide, and nitrogen were used for experiments. To define the optimal experiment setup, the Box–Behnken design (BBD) method was applied. Based on the method, the number and the condition of necessary absorption experiments were constructed. To rank the operating parameters of the absorption process from the point of view of which effect is more significant on the scrubbing performance, statistical analysis of variance (ANOVA) was done. The analysis shows contact time and concentration of sodium hydroxide as the two most influential factors in the absorption process. Carbon dioxide content of sour gas as influencing factor was examined for efficiency and order of priority parameters of process. An extended model has been created to support the optimization of operating conditions in the investigated removal system.

Catalytic Conversion of CO2 to CH4 with a Ru Catalyst: Application at Vehicle Exhaust

For the first time, catalytic conversion of CO2 to CH4 (Sabatier reaction) has been studied on real vehicle exhaust. Preliminary tests were carried out on model gas mixtures using as reactants a N2/CO2 gas mixture (CO2 = 9.5 vol% concentration similar to that of road vehicle exhaust) and pure hydrogen. Ruthenium powder (5 wt%) on alumina was used as catalyst, in a fixed bed cylindrical reactor. The operating conditions investigated are P = 1 atm, temperature in the range 160–320 °C, contact time 0.6–1.29 s, and H2/CO2 ratio 2.75–4.11. Experimental conditions were varied to study the effect of temperature, contact time, and H2/CO2 ratio on conversion, yield, and selectivity of the process. Once evaluated the performance of the catalytic process and identified the best operating conditions, real exhausts of a two-wheel motorcycle and a four-wheel gasoline vehicle were treated with the same experimental apparatus. Analyses of physical and chemical characterization of catalyst samples before and after the use were also performed. Results show that, even though best performances are generally obtained with model mixtures, high CO2 conversion together with high yield and selectivity in CH4 can be obtained with real vehicle exhausts too. Moreover, phenomena causing the deactivation of catalyst have not been observed. Results seem to indicate good opportunities for the development of the process.

Performance of Mn-based H2S sorbents in dry, reducing atmosphere – Manganese oxide support effects

High temperature H2S sorbents comprising manganese oxide (15wt.%) supported on TiO2, ZrO2, CeO2 and Al2O3 were prepared by wet impregnation using Mn(NO3)2·4H2O as Mn precursor. Upon activation in H2, the sorbent performance testing was conducted at 650°C in a model gas mixture with a composition of 0.4vol.% of H2S in 40vol.% of H2, the balance being inert gas. Sorbent regeneration was performed at 650°C with 5.2vol.% of O2 in N2. The initial sorption performance is first reported for the parent (Mn-free) supports showing a considerable H2S sorption on ZrO2 and CeO2 and a minor sorption on Al2O3 and TiO2. The Mn sorbents were subject to 13 sorption/regeneration cycles. The highest initial sorption capacity is reported for cerium supported Mn oxide, which is also the sample that deactivates most. A prolonged exposure to H2 at 650°C results in a reversible loss of the sorption capacity that was observed for all the samples except for Mn on zirconia suggesting its superior stability in H2.The supports, fresh and regenerated Mn sorbents were characterized by means of N2 sorption measurements, X-ray Diffraction and Raman spectroscopy. Temperature-Programmed Reduction was performed on the parent supports and fresh sorbents. The changes in material properties caused by the testing as well as a different degree of a metal oxide-support interaction influencing the sorption performance are discussed.

TREATMENT OF NATURAL GAS BY ADSORPTION OF CO2

Apart from burning, one of the possible uses of natural gas is as a fuel for motor vehicles. There are two types of fuel from natural gas — CNG (Compressed Natural Gas) or LNG (Liquefied Natural Gas). Liquefaction of natural gas is carried out for transport by tankers, which are an alternative to long-distance gas pipelines, as well as for transport over short distance, using LNG as a fuel for motor vehicles. A gas adjustment is necessary to get LNG. As an important part of the necessary adjustment of natural gas to get LNG, a reduction of CO<sub>2</sub> is needed. There is a danger of the carbon dioxide freezing during the gas cooling. This work deals with the testing of adsorption removal of CO2 from natural gas. The aim of these measurements was to find a suitable adsorbent for CO<sub>2</sub> removal from natural gas. Two different types of adsorbents were tested: activated carbon and molecular sieve. The adsorption properties of the selected adsorbents were tested and compared. The breakthrough curves for CO<sub>2</sub> for both adsorbents were measured. The conditions of the testing were estimated according to conditions at a gas regulation station — 4.0MPa pressure and 8 °C temperature. Natural gas was simulated by model gas mixture during the tests. The breakthrough volume was set as the gas volume passing through the adsorber up to the CO<sub>2</sub> concentration of 300 ml/m3 in the exhaust gas. The thermal and pressure desorption of CO<sub>2</sub> from saturated adsorbents were also tested after the adsorption.

Study of the Effect of the Degree of Overcooling During the Formation of Hydrates of a Methane-Propane Gas Mixture on the Equilibrium Conditions of Their Decomposition

A study is made of the effect of the initial degree of supercooling on the equilibrium conditions of decomposition of hydrates of a model gas mixture comprised of 95.66 mol % CH4 + 4.34 mol. % C3H8. To ensure a high degree of conversion of water to hydrate, 0.1 wt. % sodium dodecyl sulfate (SDS) solution was used as the liquid phase in experiments performed in an RCS6 Sapphire Rocking Cell instrument. The hydrates were produced by cooling under isochoric conditions in a GHA350 gas hydrate autoclave and under isochoric-isothermic conditions on the RCS6 instrument. The hydrated samples that were obtained were decomposed by heating under isochoric conditions at the rate of 0.2 K/h. Analysis of the decomposition curves indicates that, depending upon the initial conditions, both mixed methane-propane hydrates of various compositions and methane hydrate are formed when the test gas mixture is hydrated. It is demonstrated that the proportion of the CH4 hydrate which is formed increases with an increase in the initial degree of overcooling during hydrate formation. The curve depicting the equilibrium hydrate decomposition conditions for the gas mixture has a more complex configuration than the simple exponential curve proposed earlier. The data that is obtained points to the existence of a multiplicity of equilibrium conditions for the hydrate decomposition of gas mixtures, with the exact conditions in a specific case depending on the degree of overcooling of the system during hydrate formation.

Мокрая очистка воздуха от неорганических газов и пыли с применением щеточного распылителя

Brush type sprayer was applied to increase the efficiency of catching dust and inorganic gases. Molasses solution in various concentrations was used as an absorbent. Evaluation of capturing the model gas mixture components was carried out. The general scheme of air-purification unit is proposed.

Cobalt–zinc spinel dispersed over cordierite monoliths for catalytic N2O abatement from nitric acid plants

A series of monolithic catalysts with the 0.3wt.% loading of the (Co,Zn)Co2O4 spinel active phase dispersed on bare and ceria and zincite washcoated cordierite substrates was prepared by impregnation method: (Co,Zn)Co2O4/cordierite, (Co,Zn)Co2O4/ZnO/cordierite and (Co,Zn)Co2O4/CeO2/cordierite. The catalysts were thoroughly characterized (XRD, RS, SEM/TEM/EDX, XRF), and their catalytic deN2O activity was investigated using model gas mixture (2000ppm N2O/N2), and tail gases (1400±50ppm N2O, 900±100ppm NOx, 0.8±0.2vol.% H2O, 2.0±0.2vol.% O2) of the nitric acid pilot plant. The reported data points correspond to the measurements performed at possibly the closest tail gas composition. Morphological SEM analysis of the monolith cross-sections indicated the segregation of both ZnO and CeO2 washcoats in the form of islands covered by the (Co,Zn)Co2O4 active phase. The catalytic tests revealed that the monolithic catalysts exhibit high catalytic deN2O activity, reaching X>96% at 400°C (model gas) and 450°C (tail gases) for the best (Co,Zn)Co2O4/CeO2/cordierite system. It was also found that the specific reaction rate per cobalt spinel weight loading for the monolithic catalysts is even two orders of magnitude higher than in the case of the optimized bulk spinel phase. Yet, the beneficial effect of ceria, dominant in the model gas mixture, is largely dumped in tail gases at low temperature. It is however restored above 400°C, when the poisoning H2O and NOx molecules are desorbed.

Recovery of CH4 from coal mine model gas mixture (CH4/N2) by hydrate crystallization in the presence of cyclopentane

This work presents how water containing cyclopentane (CP) can facilitate the recovery of methane from the CH4/N2 coal mine gas mixture through hydrate crystallization. Gas hydrate formed in the CH4/N2/CP/H2O system has more favorable equilibrium conditions than that formed in pure water with the same gas mixture. The incipient equilibrium conditions at 4.0 and 13.0wt% CP were measured and are reported. The dynamics of hydrate formation using a 30mol% CH4/N2 gas mixture were studied at 283.4K, two CP concentrations (4.0 and 13.0wt%) and at 2.6 and 3.8MPa. These hydrate formation pressures correspond to 2.3 and 3.5MPa overpressures (driving forces) respectively. It was found that CH4 is preferentially incorporated into the hydrate crystal compared to N2. However, N2 competes with CH4 to enter the hydrate crystal at the higher driving force. The CH4 recovery substantially increases as the CP concentration is increased from 4.0 to 13.0wt%. The experimental condition of 283.4K, 2.6MPa, and 13.0wt% CP is preferred for the separation of CH4 from the 30mol% CH4/N2 gas mixture. The CH4 recovery obtained at this condition is approximately 46.1% and the corresponding CH4 content in the hydrate is 47.2mol%. A methane-rich stream (72mol% CH4/N2) is obtained after two stages of gas hydrate formation in the presence of cyclopentane.

Kinetics of steam reforming of ethane on Ni/YSZ (yttria-stabilised zirconia) catalyst

Experiments using a model gas mixture which was chosen to represent hydrocarbon fuel from hydrothermal gasification of biomass were carried out to study the kinetics of steam reforming of ethane over the Ni/YSZ (37wt.% Ni) anode of a solid oxide fuel cell (SOFC). For these studies gas total flow above 350mlmin−1 (STP) and catalyst particle size of 75–125μm were chosen in order to eliminate the pore and film diffusion. A rate equation of the power form was found from the measurements at various steam to carbon (S/C) ratios ranging from 2.5 to 4.5) and various temperatures ranging from 650 to 730°C. No significant rate drop due to coking on Ni/YSZ catalyst was observed under these conditions. The reaction order for ethane on Ni/YSZ was 1.58. For H2O, the reaction order tended to approach zero because the S/C ratio was always overstoichiometric. An activation energy of 80.5kJmol−1 was found for the steam reforming of ethane.

Removal of Hydrogen Sulfide from Natural gas,a Motor Vehicle Fuel

Compressed natural gas (CNG) is one of the fossil fuels, and is nothing other than the compressed version of the natural gas extracted by mining. CNG is mostly used for heating and the production of electricity. If used as a fuel for motor vehicles, it entails less emission of CO2 than does the use of gasoline or diesel. Irrespective of usage, the contents of commercial natural gas are strictly regulated by laws. Besides energy density, hydrogen sulfide content for example is a critical quality parameter of natural gas, hence it is strictly regulated. Hydrogen sulfide is extremely corrosive for the pipes. When burning such gases, hydrogen sulfide is converted to sulfur dioxide (SO2) which is harmful for the environment. We have devised a procedure for the replacement of solid adsorbents and amine-containing chemicals in hydrogen sulfide removal that can also be used with gases containing carbon dioxide. The essence of the method is that the natural gas is brought into contact with a dilute solution of NaOH in contact time of less than 1 second. At the beginning of chemisorption, hydrogen sulfide is absorbed 1-2 orders of magnitude faster than carbon dioxide. For the experiments, a laboratory Jet reactor was planned. The efficiency of purification was examined with different hydrogen sulfide containing model gas mixtures with respect to the following parameters: gas volume flow, alkali volume flow and alkali concentration.

Oxidation of Metallic Materials in Simulated CO<sub>2</sub>/H<sub>2</sub>O-Rich Service Environments Relevant to an Oxyfuel Plant

In the present study the oxidation behaviour of a number of candidate alloys for heat exchanging components was investigated in model gas mixtures containing high amounts of CO2 and/or water vapour in the temperature range 550-700°C up to exposure times of 1000 h. During exposure in Ar/CO2 and Ar/CO2/H2O base gas mixtures at 550-650°C the oxidation rates and scale compositions of martensitic 9-12%Cr steels were similar to those previously observed in steam environments. Thin and protective Cr-rich oxide scales which are commonly found during air oxidation was observed locally on the specimens surfaces after oxidation in Ar-(1-3%)O2-CO2. The tendency for protective chromia base scale formation increased when 3% oxygen was added, especially for the 12%Cr steel. When iron base oxide scales were formed on the metal surface, the martensitic steels tended to exhibit carburisation whereby the extent was reduced by increasing the water vapour and oxygen contents. All three studied austenitic alloys exhibited very slow scale growth rates at 550°C, however, at and above 600°C the steels with lower Cr content started to form two-layered iron rich surface oxide scales whereby the outer oxide was prone to spallation upon thermal cycling. The high-Cr austenitic steel 310N and the nickel base alloy 617 formed very thin, Cr-rich oxide scales at all used test temperatures and atmospheres. For those two materials the oxidation behaviour in gases containing water vapour in combination with intentionally added oxygen was affected by formation of volatile chromium oxyhydroxide.

Preferential CO oxidation over a platinum ceria alumina catalyst in a microchannel reactor

We have demonstrated the application of a Pt/Al 2O 3/CeO 2 catalyst into a microreactor to reduce the CO content by preferential oxidation in a H 2 rich model gas mixture. At temperatures between 120 °C and 260 °C and CO:O 2 ratios smaller than 0.7 CO conversion of up to 98% for CO 2-selectivities of 20%–35% were reached. The maximum space time yield was about 0.35 mmol g −1 min −1. To increase the CO conversion and CO 2-selectivity the concept of adding air or an air + N 2 mixture through microholes to the CO + H 2 mixture has been investigated with the design of an SSMR (staged supply microreactor). It was found that the CO 2 selectivity is higher compared to the case of feeding of all gases in one passage, but only for lower CO conversion rates. The best results for the preferential oxidation reaction were obtained at low reactant partial pressures. We found that the highest CO conversion rates are reached at reactant partial pressures in the range between 1 and 5 mbar. These values can be locally reached for total flows with 1–2 vol.-% CO and O 2 by feeding N 2 into the coated microchannels and the reactant gases into the uncoated microchannels. In this case, CO and H 2 are equally lowered and the CO oxidation may be favoured over the H 2 oxidation steps. A strong argument supporting this assumption is our measurements with varying the H 2 inlet stream.

Investigations on gas-phase reactions of ethane under SOFC conditions: An experimental and modeling study

Experiments using a model gas mixture which is chosen to represent hydrocarbon fuel from hydrothermal gasification of biomass were carried out under gas-phase conditions similar to those expected in the anode channel of a solid-oxide fuel cell (SOFC). Ethane conversion and product formation were evaluated from the measurements at various temperatures ranging from 610 °C to 860 °C and at a total pressure of 1.2 bar. The employed reactor was simulated using a plug-flow model coupled to a detailed gas-phase reaction mechanism. The measured temperature profile along the reactor was imported in the calculation routine. A satisfying agreement in both ethane conversion and product composition was reached between the model predictions and the experimental data. In order to determine which reactions were responsible for much of the observed kinetics a sensitivity analysis based on the rate of production (ROP) principle was performed. The results from the sensitivity analysis show that there are only six elementary reactions whose rates significantly affect the consumption or formation of the four species of interest, i.e. H 2, CH 4, C 2H 4 and C 2H 6.

A microkinetic model of ammonia decomposition on a Pt overlayer on Au(1 1 1)

Ammonia decomposition is important for a series of technological applications. For developing efficient catalysts for this reaction, basic understanding of underlying mechanisms is fundamental. We have investigated ammonia decomposition on platinum (1 1 1) and (1 0 0) surfaces, and on a platinum overlayer on a (1 1 1) gold substrate within a microkinetic model. The kinetic parameters in the corresponding rate equations have been estimated on the basis of ab initio calculations of reaction and activation energies for the adsorption and dehydrogenation processes on the catalyst surface. Steady-state coverages of species participating in the decomposition have been determined as solution of the rate equations in the limiting case of small ammonia concentrations of an N 2–NH 3 model gas mixture under flow conditions. Calculated turnover frequencies of ammonia decomposition as a function of the temperature reproduce characteristic features reported for platinum wires. The highest turnover frequency and hydrogen coverage have been obtained for the platinum overlayer on gold. Potential application of this system as ammonia-sensing device is discussed.

Integrated membrane/PSA systems for hydrogen recovery from gas mixtures

This paper is focused on the recovery of hydrogen from multicomponent gas mixtures by integrated membrane/PSA system. The paper presents the following results: computational estimation of the five-component mixture (H2/CO/CO2/N2/H2S) separation by means of membrane modules; estimation of the integrated membrane/PSA system efficiency; experimental results of gas mixtures separation by integrated membrane/PSA system with commercially available membranes (GENERON hollow fiber membrane module and PVTMS flat sheet membrane module). The influence of a purge to feed ratio, pressure, half cycle time and feed composition on the PSA performance was studied. The H2S, CO and H2O permeability through the membranes was estimated by means of the correlation analysis. It was shown that in some cases H2 recovery degree can achieve 90–97% for bio-synthesis gas and petrochemical gas if H2 concentration was equal 70% after pre-concentration at the membrane stage. The experiments were carried out using model gas mixtures (O2/N2 and He/CO2/O2 which imitate H2/CO2/CO mixture). Obtained results could be considerably improved by application of new membrane materials and new adsorbents.

The oxidation behaviour of the 9 % Cr steel P92in CO2- and H2O-rich gases relevant to oxyfuel environments

Abstract In oxyfuel plants metallic heat exchanging components will be subjected to service environments containing high amounts of CO2 and water vapour. In the present paper, the oxidation behaviour of the ferritic/martensitic 9 % Cr steel P92 was studied in a model gas mixture containing 70 % CO2-30 % H2O in the temperature range 550 – 650 °C. The results were compared with the behaviour in air, Ar–CO2 and Ar–H2O. In the CO2- and/or H2O-rich gases, the steel formed iron-rich oxide scales which possess substantially higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich oxide scales are formed as a result of a decreased flux of chromium in the bulk alloy toward the surface. This is the result of enhanced internal oxidation of chromium in the H2O-containing gases and carburisation in the CO2 gases. The oxide scales allow molecular transport of CO2 towards the metallic surface, resulting in carburisation of the alloy. The presence of water vapour induced buckling in the outer haematite layer, apparently as a result of compressive oxide growth stresses. Buckling did not occur in the H2O-free gas. This has been discussed in terms of the potential for H2O to increase growth stresses and accelerate crack propagation. The oxidation rates in CO2–H2O do not seem to be higher than those observed in flue gases of conventional fossil fuel fired power plants.

Scale formation mechanisms of martensitic steels in high CO2/H2O-containing gases simulating oxyfuel environments

In oxyfuel power plants, metallic components will be exposed to service environments containing high amounts of CO2 and water vapour. Therefore, the oxidation behaviour of a number of martensitic 9–12%Cr steels in a model gas mixture containing 70% CO2–30% H2O was studied in the temperature range 550–700°C. The results were compared with the behaviour in air, Ar–CO2 and Ar–H2O. It was found that in the CO2- and/or H2O-rich gases, the mentioned steels tended to form iron-rich oxide scales with significantly higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich scales were formed as a result of a decreased flux of chromium in the bulk alloy toward the surface because of enhanced internal oxidation of chromium in the H2O-containing gases and carbide formation in the CO2-rich gases. Additionally, the presence of water vapour in the exposure atmosphere led to buckling of the outer haematite layer, apparently as a result of compressive oxide growth stresses. The Fe-base oxide scales formed in CO2(–H2O)-rich gases appeared to be permeable to CO2 molecules resulting in substantial carburization of the steel.

Scale formation mechanisms of martensitic steels in high CO2/H2O-containing gases simulating oxyfuel environments

In oxyfuel power plants, metallic components will be exposed to service environments containing high amounts of CO2 and water vapour. Therefore, the oxidation behaviour of a number of martensitic 9–12%Cr steels in a model gas mixture containing 70% CO2–30% H2O was studied in the temperature range 550–700°C. The results were compared with the behaviour in air, Ar–CO2 and Ar–H2O. It was found that in the CO2- and/or H2O-rich gases, the mentioned steels tended to form iron-rich oxide scales with significantly higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich scales were formed as a result of a decreased flux of chromium in the bulk alloy toward the surface because of enhanced internal oxidation of chromium in the H2O-containing gases and carbide formation in the CO2-rich gases. Additionally, the presence of water vapour in the exposure atmosphere led to buckling of the outer haematite layer, apparently as a result of compressive oxide growth stresses. The Fe-base oxide scales formed in CO2(–H2O)-rich gases appeared to be permeable to CO2 molecules resulting in substantial carburization of the steel.