Feed Gas Concentration Research Articles

Carbon-free SiOx films deposited from octamethylcyclotetrasiloxane (OMCTS) by an atmospheric pressure plasma jet (APPJ)

The deposition of carbon-free, silicon oxide (SiOx) films with a non-thermal, RF capillary jet at 27.12 MHz at normal pressure is demonstrated. The gas mixture for film deposition is constituted of argon, oxygen and small admixtures of octamethylcyclotetrasiloxane (Si4O4C8H24, 0.4 ppm). Surface analysis of the deposited films reveals their exceptionally low carbon content. The XPS atom percentage stays at 2% and less, which is near detection limit. The parametric study reported here focuses on the optimization of the deposition process with regard to the chemical and morphological surface properties of the coating by varying oxygen feed gas concentration (0–0.2%) and substrate temperature (10–50 °C).

Emission characteristics of PAHs, benzene and phenol group hydrocarbons in O 2/RFG waste incineration processes

This study applies the oxygen/recycled flue gas (O 2/RFG) combustion technology for waste incineration in a laboratory-scale fluidized bed incinerator to investigate the effects of different RFG percentages and O 2 concentrations on the emission characteristics of organic pollutants (PAHs, phenol and benzene hydrocarbons). Experimental results show that most PAHs with high-ring structures were present in solid-phase and most low-ring PAHs were present in gas-phase. The major compounds of benzene and phenol hydrocarbons were benzene, toluene, trichlorobenzene and 2,4-dinitrophenol, phenol, dichlorophenol, respectively. As the O 2 concentration in feed gas was increased from 21% to 40%, the emissions of solid- and gas-phase PAHs and phenol compounds were decreased but not for benzene compounds. Increasing RFG percentages would decrease the emissions of gas-phase PAHs, benzene and phenol compounds, but increased those of solid-phase pollutants. The best operating conditions of such O 2/RFG combustion system to reduce the emissions of PAHs and phenol compounds were 40% O 2, 35% RFG, and that for benzene compounds was 21% O 2, 75% RFG. Comparing with conventional air combustion system, the best diminution efficiencies of PAHs, benzene and phenol compounds at such O 2/RFG conditions were 59.54%, 70.97% and 52.60%, respectively. With proper feed gas compositions and RFG percentages, the combustion efficiency and destruction efficiency of organic pollutants can be improved by this O 2/RFG combustion technology.

CO2 Adsorption on HZSM-5 Zeolite : Mass Transport Study in A Packed Bed Adsorber

Experimental and modeling have been done to study and to determine the diffusion parameters of CO2 adsorption on HZSM-5 zeolite in a packed­bed adsorber. Experiment works consisted of tracer and adsorption experiments. The feed gas concentrations were 40 and 80% CO2 in helium within various temperatures of 373, 423 and 473 K. The experiments were conducted by using transient step change adsorption. Tracer experiments using 20% Ar/He were conducted to measure dispersion and time lag effect of the packed bed system. A model of CO2 adsorption on HZSM­5 had been set up for transient packed­bed adsorber by assuming plug flow, isothermal and isobaric, single site Langmuir physisorption, no gas film mass transport resistance and Maxwell­Stefan mass transport in micropore applied. All the data were then optimized to get the best value of modified fitted parameter. The results indicated that at higher temperature, the quantities of gas adsorbed were decrease. This was due to lower adsorption capacity which occurs at higher temperature. The model was in a good agreement with the experiment data. Diffusivity tended to increase by increasing temperatures.

Emergency membrane contactor based absorption system for ammonia leaks in water treatment plants

Abstract Because of the suspected health risks of trihalomethanes (THMs), more and more water treatment plants have replaced traditional chlorine disinfection process with chloramines but often without the proper absorption system installed in the case of ammonia leaks in the storage room. A pilot plant membrane absorption system was developed and installed in a water treatment plant for this purpose. Experimentally determined contact angle, surface tension, and corrosion tests indicated that the sulfuric acid was the proper choice as the absorbent for leaking ammonia using polypropylene hollow fiber membrane contactor. Effects of several operating conditions on the mass transfer coefficient, ammonia absorption, and removal efficiency were examined, including the liquid concentration, liquid velocity, and feed gas concentration. Under the operation conditions investigated, the gas absorption efficiency over 99.9% was achieved. This indicated that the designed pilot plant membrane absorption system was effective to absorb the leaking ammonia in the model storage room. The removal rate of the ammonia in the model storage room was also experimentally and theoretically found to be primarily determined by the ammonia suction flow rate from the ammonia storage room to the membrane contactor. The ammonia removal rate of 99.9% was expected to be achieved within 1.3 h at the ammonia gas flow rate of 500 m3/h. The success of the pilot plant membrane absorption system developed in this study illustrated the potential of this technology for ammonia leaks in water treatment plant, also paved the way towards a larger scale application.

Application of seawater to enhance SO 2 removal from simulated flue gas through hollow fiber membrane contactor

In this paper, seawater was applied as absorbent for the removal of sulfur dioxide (SO 2) from flue gas in a hollow fiber membrane contactor. In order to verify the application feasibility of seawater on SO 2 removal, tap water, aqueous NaOH solution with a pH value of 8.35 and seawater were used as absorbents, respectively. The impacts of various factors such as three different absorbents, pressure gradient between gas and liquid, flow rate of absorbents, flow rate and concentration of feed gas on the overall mass transfer coefficient were investigated. The experimental results indicated that owing to the existence of complex CO 2–H 2O–HCO 3 −–CO 3 2− equilibrium system, the mass transfer coefficient of seawater was about twice as large as that of aqueous NaOH solution with pH value of 8.35. In addition, seawater could withstand the sudden increase of SO 2 loading rate. When the pressure gradient between gas and liquid kept under the penetration pressure, the membrane contactor could obtain high overall mass transfer coefficient, especially when seawater was used as absorbent. The height of transfer unit (HTU) of the membrane contactor was relatively small, and HTU increased hardly with increasing flow rate of flue gas when seawater was used as absorbent. Membrane contactor coupled with seawater absorption is a prospective technology for sulfur dioxide (SO 2) removal in coastal area.

Effect of process parameters on power requirements of vacuum swing adsorption technology for CO 2 capture from flue gas

This study focuses on the effects of process and operating parameters – feed gas temperature, evacuation pressure and feed concentration – on the performance of carbon dioxide vacuum swing adsorption (CO 2VSA) processes for CO 2 capture from gas, especially as it affects power consumption. To obtain reliable data on the VSA process, experimental work was conducted on a purposely built three bed CO 2VSA pilot plant using commercial 13X zeolite. Both 6 step and 9 step cycles were used to determine the influences of temperature, evacuation pressure and feed concentration on process performance (recovery, purity, power and corresponding capture cost). A simple economic model for CO 2 capture was developed and employed herein. Through experiments and analysis, it is found that the feed gas temperature, evacuation pressure and feed concentration have significant effects on power consumption and CO 2 capture cost. Our data demonstrate that the CO 2VSA process has good recovery (>70%), purity (>90%) and low power cost (4–10 kW/TPDc) when operating with 40 °C feed gas provided relatively deep vacuum is used. Enhanced performance is obtained when higher feed gas concentration is fed to the plant, as expected. Our data indicates large potential for application of CO 2VSA to CO 2 capture from flue gas.

Theoretical and experimental study on the emission characteristics of waste plastics incineration by modified O 2/RFG combustion technology

For mitigating the emission of greenhouse gas CO 2 from general air combustion systems, a clean combustion technology O 2/RFG is in development. The O 2/RFG combustion technology can significantly enhance the CO 2 concentration in the flue gas; however, using almost pure oxygen or pure CO 2 as feed gas is uneconomic and impractical. As a result, this study proposes a modified O 2/RFG combustion technology in which the minimum pure oxygen is mixed with the recycled flue gas and air to serve as the feed gas. The effects of different feed gas compositions and ratios of recycled flue gas on the emission characteristics of CO 2, CO and NO x during the plastics incineration are investigated by theoretical and experimental approaches. Theoretical calculations were carried out by a thermodynamic equilibrium program and the results indicated that the emissions of CO 2 were increased with the O 2 concentrations in the feed gas and the ratios of recycled flue gas increased. Experimental results did not have the same trends with theoretical calculations. The best feed gas composition of the modified O 2/RFG combustion was 40% O 2 + 60% N 2 and the best ratio of recycled flue gas was 15%. As the O 2 concentration in feed gas and the ratio of recycled flue gas increased, the total flow rates and pressures of feed gas reduced. The mixing of solid waste and feed gas was incomplete and the formation of CO 2 decreased. Moreover, the emission of CO was decreased as the O 2 concentration in feed gas and the ratio of recycled flue gas increased. The emission of NO x gradually increased with rising the ratio of recycled flue gas at lower O 2 concentration (<40%) but decreased at higher O 2 concentration (>60%).

Combustion Efficiency and CO2 Emission from O2/N2, O2/CO2, and O2/RFG Coal Combustion Processes

This study investigated the combustion efficiencies and emission characteristics of oxygen/recycled flue gas (O2/RFG) coal combustion processes. The effects of different operating conditions such as the feed gas compositions and concentrations, and the ratios of RFG were also discussed. Experimental results showed that the average concentration of CO2 produced in O2/CO2 combustion was higher than 92%, while the maximum concentration of CO2 was near 34% in O2/N2 combustion. This high concentration of CO2 in the flue gas of O2/CO2 combustion is beneficial for the control, separation, and recovery of CO2 during the combustion of fossil fuel. Upon increasing the O2 concentration in the feed gas of O2/RFG coal combustion, the formation of CO was decreased and the combustion efficiency was increased. Introducing the RFG into the second combustion chamber had higher combustion efficiency than that into the first combustion chamber. The mass flow rates of CO2, CO, and O2 were all increased with the ratios of RFG except for a few cases. With suitable gas compositions O2/CO2 and sufficient O2 concentration in the feed gas, high combustion efficiency and recovery efficiency of air pollutants could be achieved.

Predictions of the adsorption equilibrium of methane/carbon dioxide binary gas on coals using Langmuir and ideal adsorbed solution theory under feed gas conditions

Abstract Presently many research projects focus on the adsorption and predictions of methane, nitrogen, carbon dioxide and their mixtures on coals under equilibrium gas conditions. It is highly important to develop a method for prediction of CO 2 /CH 4 adsorption on coal under feed (source) gas conditions in the fields of enhanced coal bed methane recovery and CO 2 sequestration in coal bed with CO 2 injection. The objective is to develop reliable models to predict the adsorption behavior of mixtures on coals under feed gas conditions. In this context, the adsorption and predictions of methane, carbon dioxide and three binary mixtures of these gases on two China coals under feed gas conditions have been performed on the experimental data and derived equation. Firstly, the experimental measurements of CH 4 , CO 2 and their binary gas adsorption isotherms to investigate the adsorption behavior and use for prediction of adsorption equilibrium were conducted at 301 K at pressures up to 10 MPa. Secondly, a calculation formula of adsorbed amount of total gas was derived from relation between adsorbed amount and feed gas composition at standard state on condition of unknown equilibrium gas concentration during adsorption. Thirdly, the extended Langmuir, ideal adsorbed solution (IAS) theory in conjunction with Langmuir were used to predict the experimental data of binary gas adsorption based on an iterative algorithm. Finally, the prediction accuracy for experimental data of total and adsorbed amount of component gas and free gas compositions were analyzed. Experimental data indicate adsorbed amounts of the total gas and single-component CO 2 increase, but adsorbed amounts of CH 4 increase at low equilibrium pressure and decrease at high pressure with increase of CO 2 concentration in feed gas. Model predictions based on single-component isotherms and the calculation formula show that the quantitative formula obtained can be used to predict adsorption equilibrium of CH 4 /CO 2 binary gas under feed gas conditions. The binary gas prediction data are quite variable and depend upon the feed gas composition. The IAS–Langmuir provides, on average, a better fit to total gas adsorption but is variable for the single-component adsorption with the feed gas composition. The predictions of total and component gas adsorption and equilibrium gas compositions illustrate similar trends with their experimental data.

A discussion on transport phenomena and three-way kinetics of monolithic converters

A one-dimensional monolithic catalyst model is used to develop a global heterogeneous reaction mechanism for three-way applications. Separate conservation equations for the gas and the solid phase are coupled by the introduction of transport coefficients. Due to the relevance of transport phenomena on the overall conversion efficiency, the adequacy of the considered correlations for heat and mass transfer coefficients is elucidated, prior to the investigation of chemical kinetics. Resolved Nusselt ( Nu) and Sherwood ( Sh) number distributions in a monolith channel at steady-state and transient conditions, including catalytic reactions, are compared to local a priori correlations for constant and non-constant boundary conditions at the wall taken from literature. The reaction rate formulations are modified to achieve acceptable agreement with experimental data of palladium–rhodium catalysts for typical operating conditions of automobile applications. The conversion behavior of the lumped parameter model is validated against a wide range of air/fuel ratios and temperatures. Varying feed gas concentrations ( λ -sweep) and light-off experiments for stoichiometric and lean inlet conditions as well as a typical operating condition for secondary air injection are considered. The reaction mechanism is then used to predict the conversion performance of an aged catalyst. Instead of calibrating the complete kinetic parameter set, the adaption to the aged system is achieved by a physically meaningful reduction of the available reactive surface area only. Finally, the simulation results of an FTP75 drive cycle are compared to experimental data without further tuning.

Preparation and application of granular ZnO/Al 2O 3 catalyst for the removal of hazardous trichloroethylene

Trichloroethylene (TCE) is a volatile and nerve-toxic liquid, which is widely used in many industries as an organic solvent. Without proper treatment, it will be volatilized into the atmosphere easily and hazardous to the human health and the environment. This study tries to prepare granular ZnO/Al 2O 3 catalyst by a modified oil-drop sol–gel process incorporated the incipient wetness impregnation method and estimates its performance on the catalytic decomposition of TCE. The effects of different preparation and operation conditions are also investigated. Experimental results show that the granular ZnO/Al 2O 3 catalyst has good catalytic performance on TCE decomposition and the conversion of TCE is 98%. ZnO/Al 2O 3(N) catalyst has better performance than ZnO/Al 2O 3(O) at high temperature. Five percent of active metal concentration and 550 °C calcination temperature are the better and economic preparation conditions, and the optimum operation temperature and space velocity are 450 °C and 18,000 h −1, respectively. The conversions of TCE are similar and all higher than 90% as the oxygen concentration in feed gas is higher than 5%. By Fourier transform infrared spectrography (FT-IR) analyses, the major reaction products in the catalytic decomposition of TCE are HCl and CO 2. The Brunauer–Emmett–Teller (BET) surface areas of catalysts are significantly decreased as the calcination temperature is higher than 550 °C due to the sintering of catalyst materials, as well as the reaction temperature is higher than 150 °C due to the accumulations of reaction residues on the surfaces of catalysts. These results are also demonstrated by the results of scanning electron micrography (SEM) and energy disperse spectrography (EDS).

Comparison of MWPCVD diamond growth at low and high process gas pressures

Diamond growth by microwave plasma chemical vapour deposition (MWPCVD) at high temperature (∼1200 °C) and high process gas pressure (200 mbar) was studied and compared with growth at standard conditions (800 °C, 30 mbar). The growth rate was measured via laser reflection interferometry (LRI) and the gas phase was analysed by mass spectrometry (MS). The growth law was deduced from the variation of the growth rate with feed gas composition and additionally with the mass spectrometer signal of the relevant hydrocarbon species for all process conditions. It was found that etching of the surface by atomic hydrogen yields a significant modification of diamond growth at high gas pressure. One can treat it as process parallel to and independent of the growth which is controlled by the concentration of atomic hydrogen. At a substrate temperature of 800 °C the growth law shows a square root dependence on the CH 4,feed gas concentration and a saturation for high methane concentrations for both gas pressures. This behaviour is in accordance with a methyl based growth mechanism. At 1200 °C a low process gas pressure (30 mbar) resulted in deposition of graphitic phases. Increase to 200 mbar facilitated again single crystal diamond growth. The growth rate showed a linear relationship with the methane gas concentration and the C 2H x MS signal over more than one order of magnitude without any indication of saturation behaviour up to 20% CH 4. The change of the growth law indicates an acetylene based growth mechanism at high temperature. However, a growth mechanism based on a bimolecular reaction of CH x species cannot be excluded completely.

Effect of nitrogen addition on the properties and thermal stability of fluorinated amorphous carbon films

Continuous fluorinated amorphous carbon (a-C : F) films doped with nitrogen (a-C : F : N) were deposited by plasma enhanced chemical vapor deposition using CF 4 and C 2H 2 gases as precursors with the addition of N 2 gas. The surface morphologies, chemical compositions, deposition rates, thermal stability and mechanical properties of these films varied with the deposition parameters, including CF 4 and N 2 feed gas concentrations, processing pressure, plasma power and substrate temperature. With increasing N 2 feed gas concentration, the nitrogen content of the a-C : F : N films increased to about 6 at.% and contributed to higher mechanical properties. After thermal annealing, the a-C : F films with higher fluorine contents exhibited more obvious fluorine release and extensive film thickness shrinkage, whereas the a-C : F : N films with higher contents of nitrogen doping yielded less composition variations, smaller thickness shrinkages, higher mechanical properties, and conclusively better thermal stability.

Processing and Characterization of Fluorinated Amorphous Carbon Low-Dielectric-Constant Films

Continuous fluorinated amorphous carbon (a-C:F) films with low dielectric constants (low-k), relatively low surface roughness, and low refractive indices were deposited by plasma-enhanced chemical vapor deposition using and gases as precursors. A uniform composition was obtained except surface fluorination with an F-rich layer. The deposition rates, fluorine contents, and refractive indices of these films varied with the deposition parameters, including feed gas concentration, processing pressure, plasma power, and substrate temperature. The a-C:F films with higher fluorine contents had lower refractive indices but possessed smaller hardnesses and elastic moduli. During thermal annealing, fluorine was released from these films, but the interface adhesion to the substrates was improved. Plasma post-treatment altered the surface compositions of the films, while the interior compositions, refractive indices, and mechanical characteristics remained unchanged. © 2004 The Electrochemical Society. All rights reserved.

Kinetics of benzene to phenol oxidation over Fe-ZSM-5 catalyst

Benzene to phenol oxidation by nitrous oxide over the Fe-ZSM-5 catalyst was studied at 648–698 K and near atmospheric pressure using a bench scale plug flow reactor with a feed gas concentration of 30–80 mol% benzene, 1.5–5.0 mol% N 2O, and 0–4.7 mol% phenol. At these conditions, the selectivity of benzene and N 2O to phenol exceeded 98 and 95%, respectively. Benzoquinone, hydroquinone, catechol, and carbon oxides were the main by-products. The rate of N 2O consumption was observed to be first order with respect to N 2O concentration, and was observed to decrease with increasing benzene and phenol concentration. The apparent activation energy was 126 kJ/mol. A sorption kinetic model was developed. It was assumed that benzene and phenol are in sorption equilibrium between the gas-phase and the zeolite micropore volume. The limiting step of the overall reaction sequence is the reaction between sorbed N 2O and catalyst active sites bound with phenol. The interaction of sorbed benzene and phenol with oxidized α-sites is relatively fast. The equilibrium constant for benzene sorption on ZSM-5 was estimated from the kinetic data and was found to be in a good agreement with the sorption measurements reported in the literature. It was observed that the ratio of the specific rate of phenol formation to the specific rate of dihydroxybenzene formation is independent of temperature.

Gas-phase thermodynamic models of nitrogen-induced nanocrystallinity in chemical vapor-deposited diamond

Gas-phase thermodynamic equilibrium calculations involving H2/CH4/N2 mixtures were performed to investigate the chemical interactions leading to nitrogen-induced nanocrystallinity in microwave plasma chemical vapor deposition of diamond films. The strong influence of the CN radical in causing nanocrystallinity is confirmed by the correlation of its modeled composition in the gas phase with the degree of nanocrystallinity as determined experimentally for diamond films grown with different N2 additions. For a given CH4 feedgas concentration, there exists a critical N2 feedgas concentration, above which the change in the CH3/CN ratio is minimal and further induced nanocrystallinity is diminished. This is verified experimentally where it is observed that the same critical N2 feedgas concentration exists, above which a further decrease in diamond crystallinity and surface roughness of the grown diamond films is minimal.

Removal of H2S to ultra-low concentrations using an asymmetric hollow fibre membrane module

Removal of H2S to ultra-low concentrations using an asymmetric hollow fibre membrane module was studied from gas streams containing 17.9–1159 ppm H2S. The absorption medium used was 2 M sodium carbonate (NaCO3) aqueous solution. A porous polyvinylidene fluoride (PVDF) asymmetric hollow fibre membrane was used as the barrier between the liquid and gas phases. The effect of various operating conditions on H2S outlet concentration and mass transfer coefficient was examined. The operating conditions examined included the feed gas concentration, the gas velocity (resident time), the liquid velocity, and the gas pressure. The PVDF hollow fibre membrane module was found to be very efficient in purification of gas streams containing soluble toxic gases. With the feed concentrations of 17.9–1159 ppm H2S, complete removal of H2S can be reached at very short resistance time (<0.1 s) and low liquid/gas flow ratio. The mass transfer coefficient strongly depends on the gas phase pressure and gas flow rate.

Low temperature reforming of methane to synthesis gas with direct current pulse discharge method

Synthesis gas was produced by pulsed irradiation of electrons on a mixture of CH4 and CO2 (or H2O) at low temperature and atmospheric pressure without catalysts; especially in the CO2 reforming reaction, the H2∶CO ratio could be controlled and depended on the concentration of CO2 in the feed gas.

Use of periodic variations of reactant concentrations in time resolved DRIFT studies of heterogeneously catalysed reactions

For investigating the mechanism of solid-state catalysed reactions, sine wave modulation of feed gas concentrations was used to induce dynamic variations in the concentrations of products, intermediates and reactants, as monitored in situ in a DRIFT cell. The phase shift Δ ϕ between the external perturbation of the feed gas and the signals of products, intermediates and reactants was examined in dependence on the modulation frequency ω. A micro-kinetic model consisting of several simple steps was used to describe the process. A relationship between the phase shifts and the constants characterising the single reaction step can be derived from the applied micro-kinetic model. Therefore, the evolution of each phase shift as function of the modulation frequencies is characteristic for the individual reaction step. Repeating the experiment at different temperatures made it possible to assess the temperature dependent constants characterising the model, like the Arrhenius energy E A. This new method was tested for the CO oxidation over a Pd 25Zr 75 based catalyst. Possible reaction orders for the reactants CO and O 2 together with reaction constants were estimated.

γ-Alumina composite membranes modified with microporous silica for CO2 separation

γ-Al2O3 composite membranes have been modified with microporous silica layers to improve the separation factor of CO2 to N2. From the analysis of micropore volume fraction and CO2 adsorption behaviour of SiO2 unsupported membranes, it was found that the SiO2 membrane layer feasible to separate CO2 could be obtained from a sol prepared by hydrolysis of tetraethyl orthosilicate in aqueous nitric acid solution (acid concentration of 0.001 M and sol pH of 2.0). The unsupported membrane prepared from this optimum sol had a micropore volume fraction of 0.85 and CO2 adsorption amount of 27 cm3(STP)g−1 at 0.1 MPa and 25°C. Defect-free silica modified γ-Al2O3 membranes could besynthesized by dipcoating or pressurized coating from outside the support. The CO2/N2 separation factor of these membranes varied severely with the separation process parameters, such as transmembrane pressure, stage cut and CO2 concentration in feed gas. γ-Al2O3 membranes modified by dipcoating and pressurized coating had a CO2/N2 separation factor of 2.4 and 1.45, respectively, at ΔP = 0.3 MPa, stage cut = 0.1, and 25°C for a CO2 feed gas mole fraction of 0.5. The CO2/N2 separation factor at 25°C decreased with increasing heat-treatment temperature. The main mechanisms of CO2 permeation through silica modified membranes were surface diffusion and Knudsen diffusion.