Vemuri Balakotaiah

Bio: Vemuri Balakotaiah is an academic researcher from University of Houston. The author has contributed to research in topics: Adiabatic process & NOx. The author has an hindex of 45, co-authored 225 publications receiving 6633 citations. Previous affiliations of Vemuri Balakotaiah include Schlumberger.

Two‐scale continuum model for simulation of wormholes in carbonate acidization

Abstract: A two-scale continuum model is developed to describe transport and reaction mechanisms in reactive dissolution of a porous medium, and used to study wormhole formation during acid stimulation of carbonate cores. The model accounts for pore level physics by coupling local pore-scale phenomena to macroscopic variables (Darcy velocity, pressure and reactant cup-mixing concentration) through structure-property relationships (permeability-porosity, average pore size-porosity, and so on), and the dependence of mass transfer and dispersion coefficients on evolving pore scale variables (average pore size and local Reynolds and Schmidt numbers). The gradients in concentration at the pore level caused by flow, species diffusion and chemical reaction are described using two concentration variables and a local mass-transfer coefficient. Numerical simulations of the model on a two-dimensional (2-D) domain show that the model captures the different types of dissolution patterns observed in the experiments. A qualitative criterion for wormhole formation is developed and it is given by Λ ∼ O(1), where Λ = . Here, keff is the effective volumetric dissolution rate constant, DeT is the transverse dispersion coefficient, and uo is the injection velocity. The model is used to examine the influence of the level of dispersion, the heterogeneities present in the core, reaction kinetics and mass transfer on wormhole formation. The model predictions are favorably compared to laboratory data. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Effect of medium heterogeneities on reactive dissolution of carbonates

Abstract: The influence of medium heterogeneities on wormhole formation in carbonates is studied using a two-scale continuum model. The model describes the coupling between the transport and reaction processes occurring at the pore and Darcy scales. The medium heterogeneity is represented through initial porosity (or permeability) field by introducing a randomly generated normal distribution of local porosity values. Heterogeneity in the rock is characterized by the magnitude of maximum variation in local porosity value from the average porosity and by the length scale over which this variation occurs. It is found that heterogeneity in a rock affects not only the structure of the patterns formed during reactive dissolution but also the amount of acid required to achieve a given increase in permeability. The volume of acid required decreases as the heterogeneity magnitude or length scale are increased and this is particularly noticeable at high injection rates of acid. At intermediate injection rates, the required acid volume decreases gradually and an optimum value in heterogeneity magnitude may exist. This has been attributed to excessive branching in a pattern when the medium becomes extremely heterogeneous. In addition, the amount of acid required to breakthrough is found to depend on the initial rock porosity and dimensions of the rock being acidized. Finally, a novel way to characterize heterogeneity is defined, where heterogeneity at the core-scale is expressed using a heterogeneity parameter, ϖ as a product of the heterogeneity magnitude and length scale, and is validated for a given rock type at different injection conditions.

Experimental and kinetic study of NO oxidation on model Pt catalysts

Abstract: Modeling and experimental studies on model Pt/Al 2 O 3 and Pt/BaO/Al 2 O 3 catalysts are performed to elucidate the kinetics of NO oxidation, which is a key step during the lean phase of NO x trap operation. Experiments show that a steady-state is never truly achieved during NO oxidation; a continuous decrease in the reaction rate with time is observed on both the catalysts. This decrease is distinct from and beyond the prompt inhibition of the NO oxidation reaction observed with NO 2 in the feed or product. NO oxidation carried out after catalyst pretreatments with H 2 , O 2 , and NO 2 indicates that NO 2 is responsible for the deactivation while NO 2 storage plays a negligible role. Experiments with NO 2 as the feed elucidate its role in the production of NO, either by storage or by decomposition, for a wide range of temperatures. The highly oxidizing nature of NO 2 suggests that the Pt surface could be covered with oxygen, either as chemisorbed O or as Pt oxides, which results in slow poisoning of the catalyst. Microkinetic analysis of the NO oxidation reaction shows O 2 adsorption as the rate-determining step and predominant surface species to be adsorbed NO and O. Based on the microkinetic studies, a global kinetic model is proposed which includes the inhibiting effect of NO 2 on the NO oxidation reaction. The importance of including coverage of NO in the global model at low temperatures is shown, which is neglected in the current literature global models. The model predicts the experimental observations for a wide range of temperatures within acceptable error limits. However, prediction of the transient data requires modeling of NO 2 storage, decomposition and the complex NO 2 inhibition chemistry in addition to other surface reactions.

Modeling and analysis of wormhole formation in reactive dissolution of carbonate rocks

Abstract: A two-scale continuum model is used to simulate reactive dissolution of carbonate rocks in radial flow. The three main types of patterns observed in linear and radial flow experiments, namely, compact, wormhole and uniform patterns are numerically simulated. The fractal nature of wormholes observed in laboratory experiments is established and confirmed through simulations and the fractal dimension is quantitatively matched. The dependence of wormhole fractal dimension, optimum injection rate and minimum pore volumes required to breakthrough the medium on heterogeneity magnitude and aspect ratio is investigated. A new criterion to predict the optimum injection rate for wormhole formation in radial flow is derived and validated. It is observed that the wormhole penetration depth increases with injection time as t b , where the exponent b is found to be approximately 0.66, as observed in the experiments. A critical level of heterogeneity magnitude seems to exist below which the minimum pore volumes required to breakthrough are much higher.

Experimental and kinetic modeling study of NH3-SCR of NOx on Fe-ZSM-5, Cu-chabazite and combined Fe- and Cu-zeolite monolithic catalysts

Abstract: A comprehensive experimental and modeling study of selective catalytic reduction of NO x with NH 3 was carried out on Fe-ZSM-5 and Cu-chabazite (CHA) catalysts. The experiments reveal that Cu-CHA catalyst has a higher NH 3 storage capacity and activity for NH 3 oxidation and standard SCR compared to Fe-ZSM-5. The NO x reduction activity on the Fe-ZSM-5 catalyst was found to be strongly dependent on the NO 2 feed fraction in contrast to Cu-CHA catalyst for which NO x conversion was much less sensitive to NO 2 . In the presence of excess NO 2 , both N 2 O and ammonium nitrate were produced on both catalysts although Fe-ZSM-5 catalyst had a higher selectivity towards these byproducts compared to Cu-CHA. For different feed conditions (NO 2 /NO x =0–1), Cu-CHA was a more active NO x reduction catalyst at lower temperatures ( 400 °C). Global kinetic models were developed to predict the main features of several SCR system reactions investigated experimentally. The models account for NH 3 adsorption, NH 3 oxidation, NO oxidation, standard SCR, fast SCR, NO 2 SCR, ammonium nitrate formation and its decomposition to N 2 O, N 2 O decomposition and N 2 O reduction by NH 3 . The 1+1 dimensional reactor model accounts for potential washcoat diffusion limitations. The model accurately predicts the steady state NO x and NH 3 conversions and the selectivity of the different products formed during these reactions. The model was used to predict the performance of standard and fast SCR reactions on combined systems of Fe- and Cu-zeolite monolithic catalysts which were found to have higher NO x conversion activity over a wider temperature range than with individual Fe- and Cu-zeolite catalysts as reported in our earlier study ( Metkar et al., 2012b ). Among various configurations of the combined catalysts, either a single brick made up of a dual-layer catalyst with a thin Fe-zeolite layer on top of a thick Cu-zeolite layer or a sequential arrangement of short Fe-ZSM-5 brick followed by longer Cu-CHA brick resulted in high NO x removal efficiency over a wide temperature range of practical interest.

Nitric Oxide and Peroxynitrite in Health and Disease

Abstract: The discovery that mammalian cells have the ability to synthesize the free radical nitric oxide (NO) has stimulated an extraordinary impetus for scientific research in all the fields of biology and medicine. Since its early description as an endothelial-derived relaxing factor, NO has emerged as a fundamental signaling device regulating virtually every critical cellular function, as well as a potent mediator of cellular damage in a wide range of conditions. Recent evidence indicates that most of the cytotoxicity attributed to NO is rather due to peroxynitrite, produced from the diffusion-controlled reaction between NO and another free radical, the superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins via direct oxidative reactions or via indirect, radical-mediated mechanisms. These reactions trigger cellular responses ranging from subtle modulations of cell signaling to overwhelming oxidative injury, committing cells to necrosis or apoptosis. In vivo, peroxynitrite generation represents a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. Hence, novel pharmacological strategies aimed at removing peroxynitrite might represent powerful therapeutic tools in the future. Evidence supporting these novel roles of NO and peroxynitrite is presented in detail in this review.

Dynamics and stability of thin liquid films

Abstract: The dynamics and stability of thin liquid films have fascinated scientists over many decades: the observations of regular wave patterns in film flows down a windowpane or along guttering, the patterning of dewetting droplets, and the fingering of viscous flows down a slope are all examples that are familiar in daily life. Thin film flows occur over a wide range of length scales and are central to numerous areas of engineering, geophysics, and biophysics; these include nanofluidics and microfluidics, coating flows, intensive processing, lava flows, dynamics of continental ice sheets, tear-film rupture, and surfactant replacement therapy. These flows have attracted considerable attention in the literature, which have resulted in many significant developments in experimental, analytical, and numerical research in this area. These include advances in understanding dewetting, thermocapillary- and surfactant-driven films, falling films and films flowing over structured, compliant, and rapidly rotating substrates, and evaporating films as well as those manipulated via use of electric fields to produce nanoscale patterns. These developments are reviewed in this paper and open problems and exciting research avenues in this thriving area of fluid mechanics are also highlighted.

Selective Catalytic Reduction of NOx with NH3 by Using Novel Catalysts: State of the Art and Future Prospects.

Abstract: Selective catalytic reduction with NH3 (NH3-SCR) is the most efficient technology to reduce the emission of nitrogen oxides (NOx) from coal-fired industries, diesel engines, etc. Although V2O5-WO3(MoO3)/TiO2 and CHA structured zeolite catalysts have been utilized in commercial applications, the increasing requirements for broad working temperature window, strong SO2/alkali/heavy metal-resistance, and high hydrothermal stability have stimulated the development of new-type NH3-SCR catalysts. This review summarizes the latest SCR reaction mechanisms and emerging poison-resistant mechanisms in the beginning and subsequently gives a comprehensive overview of newly developed SCR catalysts, including metal oxide catalysts ranging from VOx, MnOx, CeO2, and Fe2O3 to CuO based catalysts; acidic compound catalysts containing vanadate, phosphate and sulfate catalysts; ion exchanged zeolite catalysts such as Fe, Cu, Mn, etc. exchanged zeolite catalysts; monolith catalysts including extruded, washcoated, and metal-mesh/foam-based monolith catalysts. The challenges and opportunities for each type of catalysts are proposed while the effective strategies are summarized for enhancing the acidity/redox circle and poison-resistance through modification, creating novel nanostructures, exposing specific crystalline planes, constructing protective/sacrificial sites, etc. Some suggestions are given about future research directions that efforts should be made in. Hopefully, this review can bridge the gap between newly developed catalysts and practical requirements to realize their commercial applications in the near future.

Recent advances in automotive catalysis for NOx emission control by small-pore microporous materials

Abstract: The ever increasing demand to develop highly fuel efficient engines coincides with the need to minimize air pollution originating from the exhaust gases of internal combustion engines. Dramatically improved fuel efficiency can be achieved at air-to-fuel ratios much higher than stoichiometric. In the presence of oxygen in large excess, however, traditional three-way catalysts are unable to reduce NOx. Among the number of lean-NOx reduction technologies, selective catalytic reduction (SCR) of NOx by NH3 over Cu- and Fe-ion exchanged zeolite catalysts has been extensively studied over the past 30+ years. Despite the significant advances in developing a viable practical zeolite-based catalyst for lean NOx reduction, the insufficient hydrothermal stabilities of the zeolite structures considered cast doubts about their real-world applicability. During the past decade renewed interest in zeolite-based lean NOx reduction was spurred by the discovery of the very high activity of Cu–SSZ-13 (and the isostructural Cu–SAPO-34) in the NH3-SCR of NOx. These new, small-pore zeolite-based catalysts not only exhibited very high NOx conversion and N2 selectivity, but also exhibited exceptionally high hydrothermal stability at high temperatures. In this review we summarize the key discoveries of the past ∼5 years that led to the introduction of these catalysts into practical applications. This review first briefly discusses the structure and preparation of the CHA structure-based zeolite catalysts, and then summarizes the key learnings of the rather extensive (but not complete) characterisation work. Then we summarize the key findings of reaction kinetic studies, and provide some mechanistic details emerging from these investigations. At the end of the review we highlight some of the issues that still need to be addressed in automotive exhaust control catalysis.

Modeling and Simulation of Lithium-Ion Batteries from a Systems Engineering Perspective

Abstract: The lithium-ion battery is an ideal candidate for a wide variety of applications due to its high energy/power density and operating voltage. Some limitations of existing lithium-ion battery technology include underutilization, stress-induced material damage, capacity fade, and the potential for thermal runaway. This paper reviews efforts in the modeling and simulation of lithium-ion batteries and their use in the design of better batteries. Likely future directions in battery modeling and design including promising research opportunities are outlined.