Daniel H. Chen

Separating positive and negative magnetoresistance for polyaniline-silicon nanocomposites in variable range hopping regime

This letter reports on unique room temperature organic magnetoresistance (OMAR) in the disordered polyaniline/silicon polymer nanocomposites in the variable range hopping regime. A transition from positive to negative OMAR was observed around 5.5 T. The theoretical analysis revealed that both wave-function shrinkage model and forward interference model contributed to the positive OMAR and only forward interference model was responsible for the observed negative OMAR. The obtained positive OMAR is well explained by the introduced localization length a0, density of states at the Fermi level (N(EF)), and average hopping length Rhop; and the negative OMAR is interpreted by the quantum interference effect.

Photocatalytic oxidation of butyraldehyde over titania in air: By-product identification and reaction pathways

In this study, gas phase photocatalytic oxidation (PCO) of n-butyraldehyde was carried out in a packed-bed reactor (titania supported on an inert packing). An on-line system consisting of two collection columns (one glass bead and one activated carbon) and a GC/MS was used for by-product identification. Various temperature levels (50, 90, 115, 170, 270, 320°C) were used in the by-product desorption from the activated carbon column. The oxidation yielded carbon dioxide as the end product. The by-products revealed many reaction pathways of the photocatalytic process. The major by-products (propionaldehyde, 1-propanol, ethanol, and acetaldehyde) were believed to undergo a C-C bond cleavage followed by hydrolysis. Secondary by-products (propyl formate, di-n-propyl ether, and 3-heptene) were caused by esterification, dehydration, and reductive coupling. Aldol condensation of the vapor phase aldehydes on TiO 2 surfaces followed by cyclization may be partially responsible for the formation of some minor by-products such as ethylbenzene.

A reduced reaction mechanism for the simulation in ethylene flare combustion

Industrial ethylene flares are considered to be a probable major source of volatile organic compounds (VOCs) such as formaldehyde. VOCs are chemicals that are responsible for the formation of other atmospheric pollutants like ozone. Due to the difficulty and cost of field measurements, on-line monitoring is not practical and other methods must be employed. Current methodologies for calculating speciated and total VOC emissions from flaring activities generally apply a simple mass reduction to the VOC species sent to the flare that does not consider the production of incomplete combustion or other intermediates. There arises a need of a speciation study for the inspection of these flare for their emission. However, most of the detailed kinetic mechanisms for the speciation study of flaring events are too complex, consist of large number of reactions and species, and also are computationally expensive. A reduced mechanism will thus be desirable for improving computational efficiency. In this study, a reduced mechanism for simulating ethylene flare combustion is presented. By retaining the important features of the detailed mechanism in the form of elementary reactions, and satisfying the species constraint of commercial CFD packages, the reduced mechanism, thereby, is useful for speciation study of flaring event.

PHOTOCATALYTIC OXIDATION OF ALDEHYDES/PCE USING POROUS ANATASE TITANIA AND VISIBLE-LIGHT-RESPONSIVE BROOKITE TITANIA

We have synthesized an annealed porous aerogel titania (LUAG2), which demonstrates a very high photocatalytic activity for aldehydes and perchloroethylene (PCE) photocatalytic oxidation (PCO) in gas phase under blacklight and fluorescent light irradiation. LUAG2 has a BET surface area of 237 m2/g and a porosity of 0.31 (volume fraction). X-ray diffraction (XRD) analysis shows LUAG2 is nearly pure anatase. It has improved the destruction of PCE and aldehydes as a group by 10–34% with black light compared to Degussa P-25. The optimum water vapor to butyraldehyde molar ratio is around 1/3. LUAG2 also shows better mineralization to CO2 than Degussa P-25 TiO2 does. Under irradiation of a 4 W fluorescent lamp LUAG2 gives a consistently higher conversion than that of Degussa P-25. The highly active photocatalyst indicates potential applications in indoor and outdoor environmental pollution control. A visible-light-responsive TiO2, NTB 200, is also investigated for comparison purposes.

TiO2 Photocatalytic Oxidation of Toluene and PCE Vapor in the Air

Abstract A thin film of TiO2 coated on a glass cylinder was used as the catalyst for the photo-oxidation of toluene and PCE. Two germicide mercury lamps, 4 watts each, were served as the UV source. Effects of light intensity, humidity, reactant concentration, and retention time were examined. The humidity effect indicated that the photocatalytic oxidation of toluene could be through hydroxyl free radicals which needed water molecules to produce. While for PCE, the oxidation mechanism could be through chlorine free radicals which did not need water molecules. The kinetic data of both toluene and PCE photocatalytic oxidation could be described by the Langmuir- Hinshelwood model. The oxidation rate constant of PCE was about 23 times higher than that of toluene. The intermediates identified during the photocatalytic oxidation of PCE were phosgene, trichloro-acetylchloride, and chloroform. For toluene, no significant amount of intermediates other than carbon dioxide could be detected.

Validation of a reduced combustion mechanism for light hydrocarbons

Due to the tremendous costs and difficulties associated with flare measurements, computational fluid dynamics (CFD) simulation could be a viable approach to predict the combustion efficiency as well as VOC/NOx emissions from industrial flaring activities. However, consisting of a large number of reactions and species, most of the detailed kinetic mechanisms for the speciation study of flaring events are too complicated to use in the CFD simulation of industrial-scale flares. A reduced combustion mechanism will lead to improved computational efficiency; however, its fidelity must be validated. This study uses 2D CFD simulations and 1D Chemkin simulations to validate a reduced mechanism developed for the combustion of light hydrocarbons up to C1–C3. This mechanism, consisting of 50 species and 337 reactions, is applicable to C1–C3 hydrocarbons and can be used to predict the combustion efficiency and fate of pollutants released from industrial flares composed of C1–C3 waste gases. In this article, experimental data reported in the literatures have been used to validate the reduced mechanism. The key performance indicators used for comparison are laminar burner-stabilized flames, laminar flame speeds, adiabatic flame temperatures, ignition delay tests, and temperature and concentration profiles of the critical species. The software package CHEMKIN 4.1.1 was used to verify the computational results of laminar flame speeds, adiabatic flame temperatures, and ignition delays. The axial profiles of various critical species are simulated using the commercial CFD software package FLUENT. It is demonstrated that simulation results using this reduced mechanism are in good agreement with reported experimental results.

Parametric Study of Ethylene Flare Operations Using Numerical Simulation

Abstract In addition to CO2 and H2O, industrial flares may also release Volatile Organic Compounds (VOCs), NOx, and CO among others. Since experimental measurements of these emissions are expensive, rigorous computational fluid dynamics (CFD) simulations and the accrued correlations are viable tools to understand and analyze factors affecting flare operations. In this paper, parametric studies of air and steam assisted ethylene flares based on CFD modeling were employed to investigate important flare operating parameters such as vent gas velocity, crosswind velocity, stoichiometric air ratio, steam-to-fuel ratio and heat content of the vent gas. The CFD modeling utilized a 50-species reduced mechanism (LU 1.1) based on rigorous combustion chemistry. Validation results of LU 1.1 are also presented. The destruction/removal efficiency and the combustion efficiency (DRE & CE) were computed along with HRVOCs/VOCs/NOx emission rates to quantify the flare performance. Correlations between DRE/CE and major parameters (crosswind, jet velocity, and combustion zone heating value) were developed using the results obtained from the case studies. A modified combustion zone heating value definition was proposed to compute a comprehensive heating value in the combustion zone.

SIMULATING VARIOUS SCHEMES OF THE CLAUS PROCESS

An interactive program based on the Gibbs free energy minimization and the FLOWTRAN system analysis was written to simulate various schemes of the Claus process. Dew point calculation and feedback control loops were provided. Certain species are treated as inerts in units like condensers, reheaters, and the waste heat boiler, Simulation of the Claus process air demand and simulation of two Claus plants (including a commercial one) were presented and the results analyzed.

Reduced combustion mechanism for C1–C4 hydrocarbons and its application in computational fluid dynamics flare modeling

ABSTRACT Emissions from flares constitute unburned hydrocarbons, carbon monoxide (CO), soot, and other partially burned and altered hydrocarbons along with carbon dioxide (CO2) and water. Soot or visible smoke is of particular concern for flare operators/regulatory agencies. The goal of the study is to develop a computational fluid dynamics (CFD) model capable of predicting flare combustion efficiency (CE) and soot emission. Since detailed combustion mechanisms are too complicated for (CFD) application, a 50-species reduced mechanism, LU 3.0.1, was developed. LU 3.0.1 is capable of handling C4 hydrocarbons and soot precursor species (C2H2, C2H4, C6H6). The new reduced mechanism LU 3.0.1 was first validated against experimental performance indicators: laminar flame speed, adiabatic flame temperature, and ignition delay. Further, CFD simulations using LU 3.0.1 were run to predict soot emission and CE of air-assisted flare tests conducted in 2010 in Tulsa, Oklahoma, using ANSYS Fluent software. Results of non-premixed probability density function (PDF) model and eddy dissipation concept (EDC) model are discussed. It is also noteworthy that when used in conjunction with the EDC turbulence-chemistry model, LU 3.0.1 can reasonably predict volatile organic compound (VOC) emissions as well. Implications: A reduced combustion mechanism containing 50 C1–C4 species and soot precursors has been developed and validated against experimental data. The combustion mechanism is then employed in the computational fluid dynamics (CFD) of modeling of soot emission and combustion efficiency (CE) of controlled flares for which experimental soot and CE data are available. The validated CFD modeling tools are useful for oil, gas, and chemical industries to comply with U.S. Environmental Protection Agency’s (EPA) mandate to achieve smokeless flaring with a high CE.

Computational fluid dynamics modeling of laboratory flames and an industrial flare

A computational fluid dynamics (CFD) methodology for simulating the combustion process has been validated with experimental results. Three different types of experimental setups were used to validate the CFD model. These setups include an industrial-scale flare setups and two lab-scale flames. The CFD study also involved three different fuels: C3H6/CH4/Air/N2, C2H4/O2/Ar, and CH4/Air. In the first setup, flare efficiency data from the Texas Commission on Environmental Quality (TCEQ) 2010 field tests were used to validate the CFD model. In the second setup, a McKenna burner with flat flames was simulated. Temperature and mass fractions of important species were compared with the experimental data. Finally, results of an experimental study done at Sandia National Laboratories to generate a lifted jet flame were used for the purpose of validation. The reduced 50 species mechanism, LU 1.1, the realizable k-ϵ turbulence model, and the EDC turbulence–chemistry interaction model were used for this work. Flare efficiency, axial profiles of temperature, and mass fractions of various intermediate species obtained in the simulation were compared with experimental data and a good agreement between the profiles was clearly observed. In particular, the simulation match with the TCEQ 2010 flare tests has been significantly improved (within 5% of the data) compared to the results reported by Singh et al. in 2012. Validation of the speciated flat flame data supports the view that flares can be a primary source of formaldehyde emission. Implications Validated computational fluid dynamics (CFD) models can be a useful tool to predict destruction and removal efficiency (DRE) and combustion efficiency (CE) under steam/air assist conditions in the face of many other flare operating variables such as fuel composition, exit jet velocity, and crosswind. Augmented with rigorous combustion chemistry, CFD is also a powerful tool to predict flare emissions such as formaldehyde. In fact, this study implicates flares emissions as a primary source of formaldehyde emissions. The rigorous CFD simulations, together with available controlled flare test data, can be fitted into simple response surface models for quick engineering use.