Numerical Study of NOx Abatement Using Ozone Injection Integrated with Wet Absorption

Nitrogen oxides emitted from power plants and the chemical industry are poisonous to humans and animals, contribute to ozone depletion, and cause acid rain. More than 90% of nitrogen oxides (NO_x) consist of nitric oxide (NO), which is insoluble in water. Among the various available techniques of NO_x abatement, ozone injection is a promising method in which NO is oxidized to higher-order nitrogen oxides (NO₃, N₂O₃, N₂O₄, and N₂O₅), which can easily be absorbed in a wet scrubber. In this article, the ozone injection process integrated with an absorber column is numerically modeled and simulated at various operating conditions. The predicted results of NO_x oxidation with ozone injection and absorption in water agree with the published experimental results. The ozone injection process is modeled using a plug flow reactor, while the wet absorption is based on a rigorous rate-based RateFrac model. Detailed kinetic mechanisms of O₃-NO_x oxidation and absorption of nitrogen oxides in water are incorporated in the model to simultaneously predict the performance efficiency of the ozone reactor and absorber column. Thermodynamic properties of the components are estimated using an Electrolyte NRTL model. The influence of performance parameters (such as feed gas flow rate, inlet gas temperature, reactor configurations, ozone concentration, and NO/NO₂ molar ratio) on the oxidation efficiency of NO_x in the reactor and absorber column is investigated to predict the optimal operating conditions.     

Kinetic model of NOx ozonation and its experimental verification

The most of new technologies of reduction of NO_x emission, as literature survey (Skalska et al., 2010b) suggests is focused on NO_x emission control from power plants and mobile vehicles. Fewer investigations are conducted on the NO_x emission abatement from chemical industry. Recently, Chacuk et al. (2007) proposed the model for the nitrous acid oxidation with the use of ozone in gas–liquid contactor. It is well known that not all of NO_x can be totally absorbed in water or nitrous/nitric acid solution, as well as ozone is not totally consumed in the acidic liquid. The reaction between ozone and NO_x can take place also in the gas phase. The ozone injection into exhaust gas stream followed by absorption was proposed as the NO_x emission abatement. The objective of these studies was to propose kinetic model of the process and to determine the rate constants of NO_x ozonation in the laboratory scale batch reactor. The process was carried out in the 0.5 dm³ volume batch reactor for different concentrations of NO, and NO₂ and varying molar ratios of O₃/NO at temperature 25 °C. Gaseous reagents were analyzed using a Fourier Transform Infrared Spectrometer Jasco FTIR-4200. The kinetic model of NO_x ozonation process was proposed and rate constants were estimated based on experimental data.     

Nitrogen Oxides Scrubbing with Alkaline Solutions

The absorption of NOx into sodium hydroxide solutions was studied in a small packed column. A simple mathematical model developed for this absorption was used for the determination of rate parameters relative to NOx species in such solutions. While hydrolysis is the main controlling step for NO₂, N₂O₄ and N₂O₃ species, nitrous acid HNO₂ plays an essential role for the NOx absorption in NaOH solutions. Our mechanistic and kinetic findings were validated as the model has worked with fair success in predicting both NOx removal efficiencies and liquid phase compositions.     

Simultaneous oxidation of nitrogen oxides and sulfur dioxide with ozone and hydrogen peroxide

The use of ozone and hydrogen peroxide for the simultaneous oxidation of nitrogen and sulfur oxides was studied in experiments carried out in a stirred cell. It was found that in a gas mixture, containing both nitrogen and sulfur oxides, only the nitrogen oxides are oxidized by ozone. Contrary to earlier results, sulfur dioxide does not disturb the oxidation of nitrogen oxides under dry conditions. The consumption of ozone in the oxidation of nitric oxide was slightly below the stoichiometric level because the ozone was introduced into the reactor in the oxygen flow. When the molar ratio between ozone and nitric oxide was more than 0.4, some of the nitric oxide was oxidized to higher oxides of nitrogen, the final product being a solid mixture of N₂O₅ and (NO)₂S₂O₇. Some nitrosyl sulfuric acid was formed in the aqueous solution of hydrogen peroxide in addition to sulfuric acid under wet conditions. Some white solid was found on the walls of the reactor. This solid is said it the literature to consist of H₂SO₄, HNOSO₄ and (NO)₂S₂O₇.     

Photochemical Oxidation of Americium in Dilute Nitric Acid Solution with the Addition of Ozone

Abstract

A trial to oxidize americium (Am) from the trivalent to the hexavalent form in dilute nitric acid solution was undertaken by emitting light from a deuterium lamp as well as by blowing ozone into the solution. It was found out that trivalent Am in dilute nitric acid solution (～0.1 N) can be photooxidized to its hexavalent form by a deuterium lamp which emits lines below 170 nm. Photooxidation, however, cannot be effected unless the oxidation rate exceeds the rate of autoreduction of Am which is caused by radicals and ions formed by alpha radiolysis. Ozone was introduced into the solution to maintain Am in its hexavalent form because ozone, which does not oxidize Am³⁺ to Am⁶⁺ in acid media, readily oxidizes Am^{5 +} to Am⁶⁺ in HNO₃ solution. Photooxidation can be effectively carried out by a combination of photolysis and ozone. Its oxidation rate was about 5%/h in 0.1 N nitric acid solution at 65°C. The oxidation rate decreased with increasing nitric acid concentration.     

Regioselective nitration of o-xylene to 4-nitro-o-xylene using nitric acid over solid acid catalysts

The regioselective nitration of o-xylene to 4-nitro-o-xylene (4-NOX) has been studied in the liquid and vapor phase over zeolites H-beta, H-ZSM-5 and silica supported molybdenum oxide (MoO₃/SiO₂) catalysts. Zeolite H-beta showed the maximum conversion of 28% and 63% selectivity for 4-NOX in liquid phase nitration at 70 °C with 70% HNO₃. The conversion increased to 65% when the reaction was carried out in vapor phase at 150 °C using dilute 30% HNO₃. The formation of α-methylphenyl nitromethane by alkyl nitration in liquid phase was decreased in vapor phase reaction. The formation of oxidation products was also decreased in vapor phase reaction with minor amounts of dinitro and ipso-products. The influence of experimental parameters such as temperature, nitric acid concentration and WHSV on conversion and selectivity has been investigated. The use of dilute nitric acid and the selective formation of 4-NOX using dilute HNO₃ makes this process environmental friendly with a potential for commercialization.     

Experimental Evidence of Gas-Liquid Boundary Controlled Reactions in UV-Ozone Systems

Oxalic acid is chosen as a model compound to study the UV + O₃ reactions in an heterogeneous continuous gas sparging reactor. Oxalic acid has the advantage of not reacting significantly with ozone alone, and when oxidized with O₃ + UV radiation, the reaction is of the single-step type : HOOC-COOH + O₃ (UV) = 2 CO₂ + H₂O + O₂. The reactions are of zero order as long as no additional alkalinity is introduced into the system. Evidence is that the photolysis of ozone is produced in the gas phase and that the reaction occurs in the gas-liquid boundary layer.     

Removal of Tetravalent NOx from Flue Gases Using Solutions Containing Hydrogen Peroxide

The absorption of NO_x(IV) into nitric acid solutions containing a low concentration of hydrogen peroxide was studied in a small packed column. A simple mathematical model developed for this absorption was used for the determination of kinetic parameters relative to NO₂ and N₂O₄in such solutions. Results obtained at 10, 20 and 30 °C lead to the same interpretation: hydrolysis is the main controlling step for tetravalent nitrogen oxides absorption and there is no sensible effect of the acidity on the absorption efficiency. Hydrogen peroxide, however, plays an essential role in solution by preventing the HNO₂ decomposition. Our mechanistic and kinetic findings were validated as the model has worked with fair success in predicting NO_x removal efficiencies in a pilot-scale packed column.     

Oxidation mechanism of carbon black by NO2: Effect of water vapour

The oxidation mechanism of carbon by automotive exhaust gasses (O₂, H₂O and NO₂) was investigated in the range of temperature (300–400 °C) corresponding to the operation conditions of the continuous regeneration regime of the soot trap. No carbon gasification occurs when O₂ is injected in absence of NO₂ below 450 °C. However, when O₂ (10 vol.%) and NO₂ (100–600 ppmv) are both present in the gas mixture, two distinct carbon oxidation reactions take place. A direct reaction occurs between NO₂ and the carbon surface along with a co-operative one involving simultaneously O₂ and NO₂. In the latter case, NO₂ promotes the decomposition of the intermediate C(O) complex.Injection of water vapour increases the overall oxidation rate but does not modify the global mechanism. The promoting effect of water is attributed to the intermediate formation of traces of nitric and nitrous acids which enhance the direct reaction C–NO₂ rate. In contrast, water has no measurable effect on the co-operative C–NO₂–O₂ reaction. A relationship between the water promoting effect on the direct oxidation rate and the amount of theoretical formation of HNO₂ and HNO₃ was established.     

On the acidity of liquid and solid acid catalysts. Part 2. A thermodynamic and kinetic study for acid-catalysed nitrations

Solid acids prepared by adding sulfuric acid on silica gel have been used as catalysts in the nitration of nitrobenzenes and their properties have been tested by kinetic studies at 25°C. Nitration rates in concentrated aqueous solutions of sulfuric acid were also analysed and the catalytic efficiencies of sulfuric acid in liquid and solid phase were compared by using kinetic data of analogous compounds. The results show that the solid acid samples exhibit nitrating properties very similar to those observed in concentrated aqueous solutions of sulfuric acid (range of 90 wt%). The relationship between nitration rates and effective concentration of electrophilic species [NO ₂ ⁺ ], determined by studying the protonation–dehydration equilibrium of nitric acid in strong acids (HNO₃ + H⁺ ⇌ H₂O + NO ₂ ⁺ ), was tested to better understand the acidity properties of medium. This revised version was published online in November 2006 with corrections to the Cover Date.     

Energy Conversion and Temperature Dependence in Ozone Generator Using Pulsed Discharge in Oxygen

A detailed reaction kinetic model consisting of 10 species and 63 reactions is developed to investigate the energy conversion and temperature dependence in an ozone generator using oxygen pulsed discharge. The energy conversion ratios of total electric energy converted into reaction heat, heat carried by gas and heat loss to ambient, namely η_reaction, η_gas and η_loss, are obtained for the first time. The ratio of reaction heat η_reaction decreases substantially with increasing specific energy and inlet gas temperature, which represents how much energy is utilized effectively to synthesize ozone. Correspondingly, η_loss and η_gas increase gradually. η_reaction declines from 55.4% to 27.7% at inlet gas temperature of 298 K when specific energy changes from 0.06 J/cm³ to 0.78 J/cm³. The detailed reaction pathway including the degree of transformation among species for ozone formation is also obtained via kinetics simulation. Meanwhile, sensitivity analysis and rate-of-production analysis for the four most important species O₃, O, O(1D) and O₂(b¹∑) obtained from the reaction pathway are executed to understand quantitatively the temperature dependence of sensitivity coefficient and production rate for each individual reaction. The production rate of ozone via the most important ozone generation reaction O+O₂+O₂ = > O₃+O₂ increases linearly with the increase of gas temperature, as well as the destruction rates of ozone via the most important ozone decomposition reactions O₃+O₃ = > O₂+O₂+O₂ and O₃ + O = > O₂(b¹∑)+O₂.     

Advanced Oxidation of Tannic Acid in Aqueous Solution

Decomposition of tannic acid in aqueous solution in advanced oxidation processes has been studied. Different oxidizing agents: ozone, hydrogen peroxide and UV radiation have been used both as single and mutually combined components of the system. The course of reaction was examined by the changes of chemical oxygen demand (COD) and total organic carbon (TOC) in aqueous solutions. Particular attention has been paid to determine optimal concentration of hydrogen peroxide, when it is used alone, together with O₃ and in H₂O₂+O₃+UV combination. The most effective optimal concentration of H₂O₂ was found. The entire mineralization of tannic acid into final products CO₂ and H₂O can be accomplished in all combinations of advanced oxidation with ozone. Bacteriological test ToxAlert® with luminescence bacteria Vibro fisheri proved that toxicity of solutions decreased considerably during advanced oxidation of tannic acid solution.     

Mechanism and Kinetic Study on Elemental Mercury Oxidation in Flue Gas by Ozone Injection

The mechanism and kinetics of the elemental Hg oxidation in flue gas by ozone injection are investigated in detail by using quantum chemistry, kinetic simulation and experimental research. The reaction processes, activation energies and kinetic parameters are calculated and analyzed by quantum chemistry. From the comparison of activation energies, the Hg⁰ oxidation ability of oxidizing radicals is that: NO₃>O₃>NO₂. The calculated results are in good agreement with literature experimental results. The calculated kinetic parameters are employed for kinetic simulation. The results of kinetic simulation are in good agreement with the experimental results. Results show that, the Hg⁰ oxidization increases linearly when the mole ratio of O₃/NO becomes larger or the reaction temperature becomes higher. The reaction Hg+NO₃ = NO₂+HgO is the key elemental reaction and the concentration of NO₃ is the most important factor for affecting Hg⁰ oxidation.     

Comparison of Hydroxyl Radical Generation for Various Advanced Oxidation Combinations as Applied to Foundries

The authors monitored hydrogen peroxide (H₂O₂), ozone (O₃), and apparent hydroxyl radical (OH·) concentrations in the liquid phase, along with gas phase ozone when operating an advanced oxidation (AO) system that included H₂O₂, O₃, sonication, and underwater plasma (UWAP). The OH· radical converted non-fluorescent terephthalic acid to fluorescent hydroxyterephthalic acid (HTA). As determined from HTA formation, when a 500 ppm H₂O₂ dose in tap water was combined with O₃ and sonication, nearly twice as much OH· (0.72 ppm) accumulated than with H₂O₂ alone. When UWAP accompanied H₂O₂, O₃, and sonication, these together generated 15–35% more OH· than when UWAP was excluded. When ozone was introduced into this AO system, the AO system decomposed almost all the O₃. This research has been conducted as a part of a study that has appraised this advanced oxidation system (Sonoperoxone) in green sand foundries, where it has diminished volatile organic compound (VOC) and hazardous air pollutant (HAP) emissions by 20–75%; and clay and coal consumption by 20–35%.     

Ion‐Exchange Separation of Pu from Macro Concentration of Th in HNO3 Medium

Abstract

Sorption behavior of Th and Pu from anion‐ as well as cation‐exchange resin was investigated from nitric acid medium by both batch and column methods. The anion‐exchange studies involved anionic nitrate complexes of Pu⁴⁺ and Th⁴⁺ sorbed onto DOWEX 1x4 resin (50–100 mesh), and the cation‐exchange studies involved the sorption of Pu³⁺ and Th⁴⁺ onto BIORAD AG 50Wx8 (50–100 mesh) or DOWEX 50Wx4 (50–100 mesh) resin. The batch data gave a separation factor (K _d,Pu/K _d,Th) of 22 for the anion‐exchange method and 0.017 for the cation‐exchange method at 3 and 2 M HNO₃, respectively. A two‐stage ion‐exchange separation method was developed for the quantitative separation of Pu (8 g/L) from a macro amount of Th (200 g/L) in nitric acid medium. The first step involved the quantitative sorption of plutonium from the mixture while about 90% of Th could be washed in 6 column volumes. The plutonium, eluted (as Pu³⁺) using 0.5 M HNO₃ + 0.2 M hydrazinium nitrate (HN) + 0.2 M hydroxyl ammonium nitrate (HAN), and the residual (～10%) Th were subsequently loaded onto a cation‐exchange column in the second step. Greater than 99% Pu was recovered with 2 M HNO₃ (in ～8 column volumes) containing 0.2 M HN + 0.2 M HAN. The final elution of thorium from the cation‐exchange column was achieved in about 6 column volumes of 1 M α‐hydroxy isobutyric acid. A (Pu, Th)O₂ fuel scrap sample was dissolved in 16 M HNO₃ containing 0.005 M HF and was used subsequently as the feed for the anion‐exchange column. The eluted Pu was subsequently loaded onto a cation‐exchange column for final purification. The recovery of plutonium and thorium was found to be >99% and >98%, respectively, while the respective decontamination factors were estimated to be 215 and 292.     

Kinetic modeling and dynamic simulation for the catalytic wet air oxidation of aqueous ammonia to molecular nitrogen

A kinetic model for the catalytic wet air oxidation of aqueous ammonia over Ru/TiO₂ catalyst was developed considering the consecutive reaction steps as follows: (i) formation of active oxygen sites O* by the dissociative adsorption of aqueous O₂ on the catalyst, (ii) oxidation of aqueous NH₃ by the reaction with three O* sites to produce HNO₂, (iii) aqueous phase dissociation of HNO₂ into H⁺ and NO ₂ ^? , (iv) formation of NH ₄ ⁺ by the association of NH₃ with the HNO₂-dissociated H⁺, (v) formation of N₂ by the aqueous phase reaction between NO ₂ ^? and NH ₄ ⁺ , (vi) formation of NO₃ by the reaction of NO ₂ ^? with an O* site. For each reaction step, a rate equation was derived and its kinetic parameters were optimized by experimental data fitting. Activation energies for the reactions (ii), (v), and (vi) were 123.1, 76.7, and 54.5 kJ/mol, respectively, suggesting that the oxidation reaction of aqueous NH₃ to HNO₂ was a ratedetermining step. From the simulation using the kinetic parameters determined, the initial pH adjustment of the ammonia solution proved to be critical for determining the oxidation product selectivity between desirable N₂ and undesirable NO ₃ ^? as well as the degree of oxidation conversion of ammonia.     

Carbon coated monoliths as support material for a lactase from Aspergillus oryzae: Characterization and design of the carbon carriers

Tunable carbon-coated monoliths as carriers for enzyme adsorption are presented. Depending on enzyme properties and reaction conditions, the carrier can be adjusted to optimize enzyme loading. Carbon-ceramic composites were prepared by sucrose carbonization, polyfurfuryl alcohol (PFA) carbonization, and by growth of carbon nanofibers (CNFs) over deposited Ni. All carbons were treated in air and subsequently in 1 M HNO₃, and analyzed with respect to porosity, morphology and surface chemistry. The composites were applied as a carrier for a lactase from Aspergillus oryzae. The CNFs proved to be the best carrier, with respect to enzyme loading. Untreated fibers could adsorb 115 mg lactase/g carbon. After air/HNO₃ treatment this value increased to 360 mg/g. Porosity was not affected by air and air/HNO₃ treatment, implying that lactase adsorption mainly depends on surface chemistry. A clear trend was observed between oxygen content of different CNFs and lactase adsorption. Ni could be removed completely from the fiber tips of CNFs by different concentrated acids—nitric acid, hydrochloric acid, and oxalic acid. However, with HCl and HNO₃ the porosity and surface chemistry were affected. Treatment in oxalic acid removed Ni from the tips by complexation, without changing the porosity. For these samples, 30% of the Ni remained present in the sample as residual NiC₂O₄. This was confirmed by TGA-MS and XRD.     

Reduction of nitrogen oxides by ozonization-catalysis hybrid process

Treatment of nitrogen oxides (NO_x) by using a hybrid process consisting of ozonization and catalysis was investigated. The ozonization method may be an alternative for the oxidation of NO to NO₂. It was found that nitric oxide (NO) was easily oxidized to nitrogen dioxide (NO₂) in the ozonization chamber without using any hydrocarbon additive. In a temperature range of 443 to 503 K, the decomposition of ozone into molecular oxygen was not significant, and one mole of ozone approximately reacted with one mole of NO. A kinetic study revealed that the oxidation of NO to NO₂ by ozone was very fast, almost completed in a few tens of milliseconds. When the amount of ozone added was less than stoichiometric ratio with respect to the initial concentration of NO, negligible NO₃ and N₂O₅ were formed. The oxidation of a part of NO to NO₂ in the ozonization chamber enhanced the selective reduction of NO_x to N₂ by a catalyst (V₂O₅/TiO₂), indicating that the mixture of NO and NO₂ reacts faster than NO.     

Hydrodynamics of slurry bubble column during dimethyl ether (DME) synthesis: Gas-liquid recirculation model and radioactive tracer studies

Radioactive tracer measurements, using impulse injections of Ar⁴¹, powdered oxide of Mn⁵⁶ and real catalyst particles doped with an oxide of Mn⁵⁶, conducted at the Advance Fuels Development Unit (AFDU) slurry bubble column (BC) reactor during dimethyl ether (DME) synthesis (reactor pressure of 5.27 MPa, reactor temperature of , inlet superficial gas velocity of 17.1 cm/s, and a catalyst loading of 36 wt%) at LaPorte, Texas, are interpreted. The differences in the responses obtained by the catalyst and fine powdered Mn₂O₃ tracer injections are minimal indicating the validity of the pseudo-homogeneous assumption for the liquid plus solid (catalyst) phase mixtures. The gas-liquid recirculation model [Gupta et al., 2001a. Comparison of single- and two-bubble class gas-liquid recirculation models—application to pilot-plant radioactive tracer studies during methanol synthesis. Chemical Engineering Science 56(3), 1117-1125. 2001b. Hydrodynamics of churn turbulent bubble columns: gas-liquid recirculation and mechanistic modeling. Catalysis Today 64(3-4), 253-269], based on a constant bubble size, describing gas-liquid mass transfer superimposed on turbulent mixing of the gas and liquid phases, is used to simulate the gas, liquid and catalyst tracer responses acquired at the AFDU. The model is able to predict the characteristic features of the experimental responses observed for gas, slurry powder and catalyst tracers at different reactor elevations. The fact, that the same model was previously shown capable of predicting both gas and liquid radioactive tracer responses during methanol and Fischer-Tropsch (FT) synthesis, indicates that this model offers a relatively simple tool for assessing mixing and transport in bubble (BCs) for a variety of gas conversion processes and provides a phenomenologically based framework for BC reactor modeling.     

Interfacial area and liquid side mass transfer coefficient in trickle bed reactors operating with organic liquids

Interfacial area and liquid-side mass transfer coefficients measured in a 5cm diameter trickle-bed reactor operating with organic liquids are presented d_p≤ 2.4 mm and cylindrical catalyst of size 0.9 mm × 5 mm. A few data concern also 5.9 and 6.4 mm Raschig rings. Gas and liquid flowrates a Mass transfer parameters have been determined by the chemical technique using the carbamation of the reactants cyclohexylamine, monoethanolamine or die results obtained at low gas-liquid interaction with low liquid flowrate are reported for the ionic aqueous systems CO₂-NaOH and O₂-Na₂SO₃. The variation of the mass transfer data, the gas pressure drop and the liquid holdup with the gas and liquid flowrates show that there exists a strong connection between these parameters. This has led to correlate the with the liquid-solid friction factor within a $\text{+}$30% accuracy.