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.     

Dynamic Performance of Ozonation Treatment for Nonionic Surfactants (Polyoxyethylene Alkyl Ether) in a Bubble Column Reactor

The ozonation of a nonionic surfactant, Sannonic SS-90 (polyoxyethylene alkyl ether), which is one of polyoxyethylene nonionic surfactants, in water has been investigated using a bubble column. The effects of initial nonionic surfactant concentration, ozone gas flow rate, inlet ozone concentration in the gas-phase, liquid-phase temperature and hydrogen peroxide dose on decomposition of Sannonic SS-90 were systematically examined. The decomposition rate of Sannonic SS-90 decreased with the increase in the initial surfactant concentration and increased with increasing ozone flow rate and temperature. It was found that the rate of Sannonic SS-90 mineralization was weakly dependent on the gas-phase inlet ozone concentration in the range of the gas-phase inlet ozone concentration in this study. The oxidation rate increased with increasing concentration of H₂O₂, reached a maximum value and then decreased with further increasing of H₂O₂ concentration. The dynamic performance of the ozonation in a semi-batch bubble column was simulated using a mathematical model based on a tanks-in-series model. Reasonable agreement between the present experimental data and the simulated results was found.     

Model development,parameter identification,and simulation of SCP production processes in air lift tower reactors with external loop—II: Extended culture modelling and quasi steady state parameter identification

Yeast was cultivated in extended culture in a bench-scale 275 cm high air lift tower reactor 15 cm dia. with an external loop. Longitudinal dissolved oxygen concentration profiles, substrate and cell mass concentrations in the medium, O₂ and CO₂ concentrations in the gas phase, as well as gas flow rates and liquid recirculation rates were measured. A distributed parameter model was used to describe the cultivation process variation along the column, cell mass, substrate and oxygen balances in the medium, O₂ and CO₂ balances in the gas phase, variation of the volumetric mass transfer coefficient along the column due to bubble coalescence, as well as double substrate Monod kinetics. Based on simulation runs it was assumed that under non limited and oxygen transfer limited growth conditions, the cell mass and substrate concentrations are uniform in the reactor. The simulation was carried out by a hybrid computer. The unknown model parameters (volumetric mass transfer coefficient at the gas entrance, k_La^E, and coalescence factor K_ST) and two kinetic parameter R_Omax and K_O were identified by means of experimental results with quasi steady state simulation methods.     

Gas–liquid mass transfer in oil–water emulsions with an airlift bio-reactor

Biodesulfurization reaction must be performed in oil–water emulsions with an aerobic biocatalyst which demands oxygen. Different reactor configurations can be used for this purpose, but the bubble column bio-reactors with internal recirculation loop are usually not used. In the present work, the absorption of oxygen in water–dodecane emulsions was studied in a bubble column bio-reactor with internal recirculation loop, in operative conditions normally used for biodesulfurization. The K_La for oxygen was determined for several organic fractions from 0 to 100%, as well as at different gas flow rates. Estimation of K_La was done according to a fluid dynamic model based on an energy balance which takes into account the energy dissipated at the interfaces and on a mass transfer model based on the fluid dynamic model, the Higbie's penetration theory and Kolmogoroff's theory of isotropic turbulence. Experimental data of mass transfer coefficient were simulated with satisfactory accuracy, and differences were less than 20% for most cases. Mayor deviations were obtained for emulsions with 30 and 70% dodecane fraction. To obtain good agreement, assumptions of higher bubble diameter and slip velocity were done, evincing the effect of surface tension and liquid viscosity on the mass transfer coefficient.     

Bubble column research in Japan

Many experimental studies on the bubble column have been reported by Japanese researchers since around 1960. They include studies of bubble behaviour, bubble size distribution, transition from the homogeneous bubbly flow regime to the heterogeneous liquid circulation regime, liquid velocity distribution, longitudinal liquid mixing, hydrodynamic modelling, the gas holdup, and the volumetric coefficient of gas-liquid mass tranfer k_La. Studies covered various modified bubble columns, such as the airlift reactor with an external or internal loop, the packed bubble column, and others. Performance of three-phase bubble columns, which deal with suspensions or emulsions, and their use as bioreactors or chemical reactors were also studied.     

Dynamic modelling of a bubble column for particle formation via a gas–liquid reaction

This paper presents a dynamic model of a bubble column reactor with particle formation, accomplished by adopting a hybrid CFD-reaction engineering approach. CFD is employed for estimating the hydrodynamics and is based on the two-phase Eulerian–Eulerian viewpoint. The reaction engineering model links the penetration theory to a population balance that includes particle formation and growth with the aim of predicting the average particle size. The model is then applied to the precipitation of CaCO₃ via CO₂ absorption into Ca(OH)₂aq in a draft tube bubble column and draws insight into the phenomena underlying the crystal size evolution.     

Multiphase fluidization in large-scale slurry jet loop bubble columns for methanol and or dimethyl ether production

A solution methodology is proposed for the process development and process engineering of a continuously operated jet loop bubble column including integrated external or internal steam generation for, e.g., a high-efficiency large-scale medium pressure methanol and or dimethyl ether production, or other gas to liquid Fischer-Tropsch applications.A jet loop bubble column is defined in the present process development to study the combined integration of a jet-eductor draft tube system with an upper bubble column. The major advantages resulting from the integrated jet-eductor draft tube system in large-scale bubble columns are the guidance and good mixing efficiency of the multiphase flow up to the upper part of the bubble column. Reducing the bubble column operating liquid level at about 2.5-3.0 times of the column diameter above the upper end of the draft tube results in a classical jet-eductor draft tube reactor suitable for small and or medium-scale industrial applications.Methanol synthesis is usually executed catalytically in multistage packed beds at higher pressure, e.g. 26 MPa, and about 350-, resulting in a higher plant installation and operating cost. The successful scale-up of a slurry jet loop bubble column can achieve a higher catalytic selectivity at a lower pressure and temperature , and therefore reduce the overall plant investment and production cost [Toseland, 1999. Three phase flows under extreme conditions of pressure and temperature, Part II: industrial applications, Air products and Chemicals, Inc. Presented at the A.I.Ch.E. Annual Meeting, Dallas, TX; Fan, 1999. Three phase flows under extreme conditions of pressure and temperature, Part I: fundmental characteristics, Department of Chemical Engineering, The Ohio State University. Presented at the A.I.Ch.E. Annuxal Meeting, Dallas, TX]. In addition, the separate slurry production of dimethyl ether, or even coproduction with methanol, can be a more cost-effective process than the classical methanol dehydration process.The new Modified Slurry Process^© for large-scale methanol and or dimethyl ether production is presented including internal or external heat exchanger location for steam production.A process concept is developed of a Large Scale Slurry Jet Loop Bubble Column^© with external separator, auxiliary internal heat exchanger equipment and high-efficiency gas-liquid slurry jet-eductor mixing system including draft tubes and an upper bubble column. In addition, as comparison a simplified concept is discussed for a small-to-medium-scale slurry jet loop reactor including external steam production and bottom nozzle jet-eductor installation without the presence of an upper bubble column.The basic geometrical parameters of the proposed slurry jet loop bubble column and jet loop reactor are discussed. The influence of the selected geometrical parameters on the gas holdup, interfacial area and mixing is analyzed. Information about catalyst type and particle size distribution is also presented.The definition of optimal operating conditions related to the influence of the fluid dynamics and mixing on mass transfer efficiency and also information for the minimum required power input per unit volume for startup or stable reactor operation are discussed.A simplified estimation method is presented for the expected axial temperature difference across the overall length of the jet bubble column, and also the required heat transfer area of a new construction-type internal compact heat exchanger for efficient reactor cooling and operation.Scale-up is possible for large diameter jet loop bubble columns, typically up to 5 m diameter and 60 m height, including continuous three-phase slurry operation at higher power input and interfacial area, for more efficient synthesis gas absorption and reaction than in classical slurry bubble columns. Integration of suitable designed sieve trays can further guarantee an efficient operation of the lower jet loop draft tube system at higher column diameters and also achieve an efficient reactor operation in the upper bubble column section.     

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.     

Diamine versus amines blend for CO2 chemical absorption

Present work analyses the behavior of aqueous solutions of N,N-dimethylethylenediamine as chemical solvent for carbon dioxide separation by gas–liquid absorption. The interest of this molecule is centered on the presence of different types of amino centers that confer it the capability to act as a solvent based on amines blend. For this reason, a comparison between diamine and amines blend solvent has been carried out in order to understand the differences between these solvents using absorption and nuclear magnetic resonance studies. This experimental work analyses the influence of amine type, concentration, and ratio between different amines. Also, the effect of gas flow rate used in the bubble column reactor upon the absorption kinetics has been analyzed.     

CFD Modeling of Bubble Column Reactor Including the Influence of Gas Contraction

In this paper a CFD model for a bubble column reactor undergoing a first order reaction A → B is developed. The reactor operates in the homogeneous bubbly regime and has a diameter D_T = 1 m and height H_T = 5 m. The incoming gas stream contains inerts, varying in proportion from 10 % to 90 %. Three‐dimensional transient Eulerian simulations were carried out for an inlet superficial gas velocity U_G = 0.04 m/s. Due to the consumption of A, the gas phase suffers contraction along the height of the reactor and as a consequence there is a significant change in the gas velocity along the column height; this variation in gas velocity is stronger when the incoming gas contains a smaller proportion of inerts. The CFD simulations show that there is a considerable influence of gas contraction on both the bubble column hydrodynamics and on the reactor conversion. None of the conventionally used reactor models is capable of describing the reactor performance in the case of high gas phase contraction.     

Experimental Study on Ozone Synthesis via Dielectric Barrier Discharges

The characteristics of ozone generation using a dielectric barrier discharge reactor were investigated experimentally. Results indicate that ozone concentration increases with increasing applied voltage and gas residence time. In addition to applied voltage, ozone generation rate varies with reactor configuration as well. Optimum ozone generation rates can be reached at the specific gas residence time for a given applied voltage and gas composition. At the same applied voltage, the reactor with a single dielectric barrier results in a higher ozone generation rate in comparison with the reactor having double dielectric barriers. Given a constant N₂/O₂ ratio in the feed gas, NO_x concentration increases as applied voltage and gas residence time increase. Results indicate that maximum NO_x concentration is reached when the N₂/O₂ ratio of feed gas is 4.     

Performance of a bubble column reactor for oxidation of ethylene (wacker process)

An experimental investigation of the performance of a bubble column reactor for the liquid-phase catalytic oxidation of ethylene to acetaldehyde (Wacker Process) has been carried out by varying the different operating parameters, such as the inlet gas velocity and catalyst concentration. It has also been shown that, for a given set of conditions, a certain critical ratio of partial pressure of O₂ to that of ethylene in the reactor inlet is required for the overall catalytic cycle to work with maximum efficiency under steady state. The agreement between the experimental results and the predictions of a theoretical reactor model developed herein was found to be excellent.     

Performance of a slurry bubble column reactor for Fischer-Tropsch synthesis: Determination of optimum condition

The CO conversion and selectivity to C₁+ and C₁₁+ wax products over Co/Al₂O₃ as well as Ru/Co/Al₂O₃ Fischer-Tropsch (F-T)catalysts were investigated by varying reaction temperature (210-250 °C), system pressure (1.0-3.0 MPa), GHSV (1000-6000 L/kg/h), superficial gas velocity (1.7-13.6 cm/s) and slurry concentration (9.09-26.67 wt.%) in a slurry bubble column reactor (0.05 m diameter × 1.5 m height) to determine the optimum operating conditions. Squalane or paraffin wax was used as initial liquid media. The overall CO conversion increased with increasing reaction temperature, system pressure and catalyst concentration. However, the local maximum CO conversion was exhibited at GHSV of 1500-2000 L/kg/h and superficial gas velocity of 3.4-5.0 cm/s. The CO conversion in the case of Ru/Co/Al₂O₃ was much higher and stable than that in the case of Co/Al₂O₃. The selectivity to C₁₁+ wax products increased slightly with increasing GHSV; on the other hand, it decreased with increasing reaction temperature, system pressure, and solid concentration in a slurry bubble column reactor. It could be concluded that the optimum operating conditions based on the yield of hydrocarbons and wax products were; U_G = 6.8-10 cm/s, Cs = 15 wt.%, T = 220-230 °C, P = 2.0 MPa in a slurry bubble column reactor for F-T synthesis.     

Numerical Simulation of Absorbing CO2 with Ionic Liquids

Although separating CO₂ from flue gas with ionic liquids has been regarded as a new and effective method, the mass transfer properties of CO₂ absorption in these solvents have not been researched. In this paper, a coupled computational fluid dynamic (CFD) model and population balance model (PBM) was applied to study the mass transfer properties for capturing CO₂ with ionic liquids solvents. The numerical simulation was performed using the Fluent code. Considering the unique properties of ionic liquids, the Eulerian‐Eulerian two‐flow model with a new drag coefficient correlation was employed for the gas‐liquid fluid dynamic simulation. The gas holdup, interfacial area, and bubble size distribution in the bubble column reactor were predicted. The mass transfer coefficients were estimated with Higbie's penetration model. Furthermore, the velocity field and pressure field in the reactor were also predicted in this paper.     

A novel slurry bubble column membrane reactor concept for Fischer–Tropsch synthesis in GTL technology

Fischer–Tropsch synthesis (FTS) plays an important role in the production of ultra-clean transportation fuels, chemicals, and other hydrocarbon products. In this work, a novel combination of fixed-bed and slurry bubble column membrane reactor for Fischer–Tropsch synthesis has been proposed. In the first catalyst bed, the synthesis gas is partially converted to hydrocarbons in a water-cooled reactor which is fixed bed. In the second bed which is a membrane assisted slurry bubble column reactor, the heat of reaction is used to preheat the feed synthesis gas to the first reactor. Due to the decrease of H₂/CO to values far from optimum reactants ratio, the membrane concept is suggested to control hydrogen addition. A one-dimensional packed-bed model has been used for modeling of fixed-bed reactor. Also a one-dimensional model with plug flow pattern for gas phase and an axial dispersion pattern for liquid-solid suspension have been developed for modeling of slurry bubble column reactor. Proficiency of a membrane FTS reactor (MR) and a conventional FTS reactor (CR) at identical process conditions has been used as a basis for comparison in terms of temperature, gasoline yield, H₂ and CO conversion as well as selectivity. Results show a favorable temperature profile along the proposed concept, an enhancement in the gasoline yield and, thus a main decrease in undesirable product formation. The results suggest that utilizing this type of reactor could be feasible and beneficial. Experimental proof of concept is needed to establish the validity and safe operation of the proposed reactor.     

Enhancement of nitric oxide removal by ammonia on a low-rank coal based carbon by sulphuric acid treatment

This work presents a study of the effect of wet sulphuric acid treatment and gas-phase treatment with SO₂ + O₂ + H₂O on the catalytic activity of a low-rank coal-based carbon for the nitric oxide reduction with ammonia. Carbons were characterized by N₂ adsorption, TPD, and FTIR in order to assess how the surface chemistry and the texture of carbons change after the treatments. A great amount of oxygenated functional groups both CO₂ and CO evolving ones are produced by liquid-phase sulphuric acid treatment. However, the amount of those groups after gas-phase treatment with SO₂ + O₂ + H₂O is lower, in particular the CO₂ evolving groups. The catalytic activity of carbons was examined in a fixed bed reactor at 150 °C in a gas flow containing NO, O₂, N₂ and NH₃, the effluent concentration being monitored continuously during the reaction. The obtained results indicate that an appropriate balance between the type of oxygen functional groups and surface area available to the reactant gas are required to reach high levels of NO conversion.     

Physical absorption of single gas bubbles in degassed and preloaded waterPhysikalische Absorption von einzelnen Gasblasen in entgastem und vorbeladenem wasser

The absorption of single gas bubbles in degassed and preloaded water has been observed.Gases with a low solubility (e.g. O₂, He) absorb with a constant absorption rate. Gases with a high solubility (e.g. CO₂, N₂O) show an absorption rate which is three times higher at the beginning. Almost 70%–80% of the bubble volume absorbs with this higher rate. The rest can be described with the same liquid-side mass transfer coefficients as found for the gases with low solubility. They are of the order of magnitude of 1 × 10^?4 ?2 × 10^?4 m s^?1.Experiments with 2-propanol—water mixtures also show the enhanced absorption at the beginning.The result, using a dimensionless presentation, is that the point at which the rate of absorption changes depends on the gas density, the liquid density, the solubility coefficient, the degree of saturation, the liquid-side mass transfer coefficient and the initial radius.The reason for the enhanced absorption at the beginning of the absorption process is not yet known.     

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.     

Comparison of Hydrodynamics and Mass Transfer in Airlift and Bubble Column Reactors Using CFD

Computational Fluid Dynamics (CFD) is used to compare the hydrodynamics and mass transfer of an internal airlift reactor with that of a bubble column reactor, operating with an air/water system in the homogeneous bubble flow regime. The liquid circulation velocities are significantly higher in the airlift configuration than in bubble columns, leading to significantly lower gas holdups. Within the riser of the airlift, the gas and liquid phases are virtually in plug flow, whereas in bubble columns the gas and liquid phases follow parabolic velocity distributions. When compared at the same superficial gas velocity, the volumetric mass transfer coefficient, k_La, for an airlift is significantly lower than that for a bubble column. However, when the results are compared at the same values of gas holdup, the values of k_La are practically identical.     

Experimental Study and Mathematical Modeling of NO Removal Using the UV/H2O2 Advanced Oxidation Process

An experimental study on NO removal via UV/H₂O₂ process was conducted in a semi‐continuous bubble‐column reactor and the effect of some operation parameters including NO initial concentration and gas flow rates on removal efficiency was investigated. Applying UV light increased the efficiency significantly. The steady‐state removal efficiency was found to be higher at the lower gas flow rates. The bubble size as an important factor in mass transfer calculations and modeling procedure was determined at different gas flow rates using bubble photographs and image processing technique. In the ranges of flow rates studied here, the gas flow rate had no significant effect on the bubble diameter. A mathematical model was developed to describe the NO removal process. The model predictions were compared with existing experimental data, confirming a good agreement of the data.