A review of recent advances in selective catalytic NOx reduction reactor technologies

Research on NOx treatment is extensive in recent years due to growing environmental awareness. Selec- tive catalytic reduction （SCR） of NOx, as a proven technology, offers higher NOx control efficiency than many other NOx treatment methods. The present work reviews the recent development of SCR reactor technologies. Firstly, catalysts and mechanism of different SCRs were briefly summarized. Different SCR reactors, e.g. structured reactor, fluidized bed reactor and moving bed reactor, were then discussed. As a more advanced technology, multifunctional reactors were also developed for SCR process and could be divided into two categories： decoupled adsorption-reaction process and combined SCR system. The mechanism and properties of these processes were discussed in detail. Some recommendations were given for the future work in SCR reactor design. SCR reactor technology for emerging energy processes was also addressed, such as oxyfuel combustion and biofuel conversion processes, which put forward new requirements for SCR technologies and also open new opportunities for advanced design of SCR reactors.     

A review of recent advances in selective catalytic NOx reduction reactor technologies

Research on NO_x treatment is extensive in recent years due to growing environmental awareness. Selective catalytic reduction (SCR) of NO_x, as a proven technology, offers higher NO_x control efficiency than many other NO_x treatment methods. The present work reviews the recent development of SCR reactor technologies. Firstly, catalysts and mechanism of different SCRs were briefly summarized. Different SCR reactors, e.g. structured reactor, fluidized bed reactor and moving bed reactor, were then discussed. As a more advanced technology, multifunctional reactors were also developed for SCR process and could be divided into two categories: decoupled adsorption-reaction process and combined SCR system. The mechanism and properties of these processes were discussed in detail. Some recommendations were given for the future work in SCR reactor design. SCR reactor technology for emerging energy processes was also addressed, such as oxyfuel combustion and biofuel conversion processes, which put forward new requirements for SCR technologies and also open new opportunities for advanced design of SCR reactors.     

Dynamic viscoelastic properties of free radical bulk polymerizing systems under near-isothermal and non-isothermal conditions

Dynamic viscoelastic moduli of an example polymerizing system namely the free radical bulk polymerization of methyl methacrylate (MMA), is studied using a Haake® rheometer–reactor assembly and an adapted Haake® HV-DIN cup-and-bob assembly. A series of experiments on the bulk polymerization of MMA under different temperature histories (isothermal, step-increase and step-decrease) and at two different initiator [2, 2′-azoisobutyronitrile (AIBN)] concentrations, have been carried out. The data on the storage modulus, G′, the loss modulus, G″, and the phase shift, δ, are measured during the course of polymerization, well into the gel effect region. A new correlation is developed for these properties. The kinetic model and the correlation for the zero-shear viscosity, presented in our earlier studies, are used in this generalized correlation. The correlation so developed is observed to represent experimental data quite well for a variety of temperature histories. The characteristic relaxation time, τ_i, representing the bulk polymerization of MMA is observed to be larger than the Rouse value, τ_R, by a factor of about seven. The findings are observed to be in agreement with several other studies for entangled non-polymerizing systems.     

Fluidized and vibrofluidized shallow beds of fresh leaves

The fluid dynamics behavior of shallow fluidized and vibrofluidized beds operating with fresh leaves was investigated with the aim of exploring drying applications in a modified conveyor belt (MCB) system,which may be operated in a fixedor fluidized-bed mode.Leaves of the specimens Duranta repens,Schinus molle,Coleus barbatus,Buxus sempervirens,and Bougainvillea spectabilis were tested with a range of sphericities from 0.063 to 0.213,bulk densities from 0.038 to 0.251 g/cm 3,apparent densities from 0.52 to ...     

Prediction of coating uniformity in batch fluidized-bed coating process

Coating of particulate materials in fluidized beds is a widely used technique to eliminate particle agglomeration, provide slow release of an active substance, or protect active ingredients. When thin polymer shells are applied on a particle surface, it is important to determine the process parameters that provide coating uniformity. In this study, the degree of coverage, defined as the fraction of the coated surface of the particles, is proposed as a quantitative criterion of coating uniformity. A new model for the batch fluidized-bed coating process is presented. The model allows prediction of the function of particle distribution according to the degree of coverage at a given process time and thereby enables assessment of coating uniformity. An algorithm for the numerical solution of model equations for a batch fluidized-bed coater is described. The influences of the main process parameters on the coating uniformity were shown.     

Theoretical models of vegetable drying by convection

The results of experiments are often used to model empirical phenomena. However, the term model is applied in various meanings. A model is usually treated as an abstract formal structure that can replace a material system considered as original, in respect to the aim of modeling. Certain formal structures may be treated as theoretical models of empirical phenomena. On the other hand, a material system can also be referred to as a model of an abstract system, e.g., a set of equations or a hypothesis. Such a material system, if it is a distinct empirical interpretation of the language of a given theory, is then called a real model. Both kinds of models are applied in drying technology, but the second one is more inventive. The mathematical structures are treated as empirical formulae or as theoretical models properly derived from true or legitimated promises of a given theory. The advantages of some mathematical theoretical models of drying processes versus empirical formulae are discussed. The creation of new mathematical theoretical models of convection drying kinetics of some shrinking solids is presented and analyzed. One of the above models was also hypothetically suggested for modeling the drying of cut vegetables in a fluidized-bed. Despite its initial acceptance due to peer empirical justification on cut carrots and celery, it still requires further theoretical analysis. Other models indicated here are theoretical models of vegetable drying in a tunnel drier. These models are created by deduction from laws of heat and mass transfer theory and its basic equations. XI Polish Drying Symposium, Poznań, Poland, 13–16 September 2005.     

Heat transfer mechanisms in poplar wood undergoing torrefaction

Torrefaction, a thermal treatment process of biomass, has been proved to improve biomass combustible properties. Torrefaction is defined as a thermochemical process in reduced oxygen condition and at temperature range from 200 to 300 °C for shorter residence time whereby energy yield is maximized, can be a bridging technology that can lead the conventional system (e.g. coal-fired plants) towards a sustainable energy system. In efforts to develop a commercial operable torrefaction reactor, the present study examines the minimum input condition at which biomass is torrefied and explores the heat transfer mechanisms during torrefaction in poplar wood samples. The heat transfer through the wood sample is numerically modeled and analyzed. Each poplar wood is torrefied at temperature of 250, 270, and 300 °C. The experimental study shows that the 270 °C-treatment can be deduced as the optimal input condition for torrefaction of poplar wood. A good understanding of heat transfer mechanisms can facilitate the upscaling and downscaling of torrefaction process equipment to fit the feedstock input criteria and can help to develop treatment input specifications that can maximize process efficiency.     

A novel coated-particle design and fluidized-bed chemical vapor deposition preparation method for fuel-element identification in a nuclear reactor

Particle coating is an important method that can be used to expand particle-technology applications. Coated-particle design and preparation for nuclear fuel-element trajectory tracing were focused on in this paper. Particles that contain elemental cobalt were selected because of the characteristic gamma ray spectra of ⁶⁰Co. A novel particle-structure design was proposed by coating particles that contain elemental cobalt with a high-density silicon-carbide (SiC) layer. During the coating process with the high-density SiC layer, cobalt metal was formed and diffused towards the coating, so an inner SiC–Co_xSi layer was designed and obtained by fluidized-bed chemical vapor deposition coupled with in-situ chemical reaction. The coating layers were studied by X-ray diffractometry, scanning electron microscopy, and energy dispersive X-ray spectroscopy techniques. The chemical composition was also determined by inductively coupled plasma optical emission spectrometry. The novel particle design can reduce the formation of metallic cobalt and prevent cobalt diffusion in the coating process, which can maintain safety in a nuclear reactor for an extended period. The experimental results also validated that coated particles maintain their structural integrity at extremely high temperatures (∼1950 °C), which meets the requirements of next-generation nuclear reactors.     

Heat Transfer and Pressure loss of a very shallow fluidized-bed heat exchanger Part 1. Experiment with a single row of tubes

A novel gas fluidized-bed heat exchanger with a very small static bed height has been developed for a heat-exchanging system using a low-pressure fan. This fluidized bed is composed of a multislit distributor, a single row of 8 mm diameter tubes, and glass beads 48–195 μm in diameter. The measured performance of heat transfer is excellent and that of fluidization is satisfactory, in spite of the static bed height being as small as 13 mm. In the best case, the test fluidized bed exhibited a heat transfer performance comparable to that of a conventional fluidized bed with a perforated plate distributor and a static bed height of 150 mm, and showed one-fourteenth the pressure loss.     

Clustering behavior of solid particles in two-dimensional liquid-solid fluidized-beds

In this paper, the clustering behavior of solid particles in a two-dimensional （2D） liquid-solid fluidized-bed was studied by using the charge coupled devices （CCD） imaging measuring and processing technique and was characterized by fractal analysis. CCD images show that the distribution of solid particles in the 2D liquid-solid fluidised-bed is not uniform and self-organization behavior of solid particles was observed under the present experimental conditions. The solid particles move up in the 2D fluidized-bed in groups or clusters whose configurations are often in the form of horizontal strands. The box fractal dimension of the cluster images in the 2D liquid-solid fluidized-bed increases with the rising of solid holdup and reduces with the increment of solid particle diameter and superficial liquid velocity. At given solid holdup and solid particle size, the lighter particles show smaller fractal dimensions.     

A structured packed-bed reactor designed for exothermic hydrogenation of acetone

Fixed-bed reactors randomly packed with catalysts have many disadvantages that may adversely affect the desired chemical reaction. The increasingly used monolithic reactor, in contrast, has many operational advantages; however, for a kinetically-controlled reaction, it does not contain sufficient catalyst to sustain the reaction. To address the problems associated with both randomly packed-bed reactor and the monolithic reactor, a structured packed-bed reactor was proposed and mathematical models were built for randomly packed-bed reactor and structured packed-bed reactor. Their respective performances were compared when applied to the exothermic reaction of the isopropanol–acetone–hydrogen chemical heat pump system. The results showed that the structured packed-bed reactor performed better in terms of pressure drop and heat transfer capacity, and had a lower radial temperature gradient, indicating that this reactor had a higher effective heat conductivity. Isopropanol on the catalyst particle surfaces was more concentrated near the tube wall because a wall effect existed in the boundary layer around the particle-wall contact points.     

A structured packed-bed reactor designed for exothermic hydrogenation of acetone

Fixed-bed reactors randomly packed with catalysts have many disadvantages that may adversely affect the desired chemical reaction.The increasingly used monolithic reactor,in contrast,has many operational advantages;however,for a kinetically-controlled reaction,it does not contain sufficient catalyst to sustain the reaction.To address the problems associated with both randomly packed-bed reactor and the monolithic reactor,a structured packed-bed reactor was proposed and mathematical models were built for randomly packed-bed reactor and structured packed-bed reactor.Their respective performances were compared when applied to the exothermic reaction of the isopropanol-acetone-hydrogen chemical heat pump system.The results showed that the structured packed-bed reactor performed better in terms of pressure drop and heat transfer capacity,and had a lower radial temperature gradient,indicating that this reactor had a higher effective heat conductivity.Isopropanol on the catalyst particle surfaces was more concentrated near the tube wall because a wall effect existed in the boundary layer around the particle-wall contact points.     

Relaxation of a fluidized bed to the state of a continuous medium

The kinetic equation proposed in [1,2] for describing the behavior of a system of particles in a gas flow differs from the usual Boltzmann equation with respect to the additional terms that take into account random variations of the particle velocity under the influence of the flow. As shown in [2], the collision operator and the Brownian-type operator in the starting kinetic equation describe essentially different simultaneous physical processes of change of state of the particle system: equalization of the mean kinetic energy of the particles and change of energy due to the action of the viscous forces associated with the suspending flow. Therefore the method of solving the kinetic equation used in [2], a direct generalization of the Chapman-Enskog method of solving the kinetic equation it is necessary to investigate method of solving the kinetic equation it is necessar y to investigate the relaxation processes in the system. Moreover, the relaxation of systems of the fluidized-bed type to the continuum state is also of independent interest in connection with the analysis of fast processes in the system, i.e., processes with a characteristic duration of the order of the mean free time.     

Thermomechanical instability in steady operating regime of continuous-flow chemical reactor with fixed catalyzer bed

A continuous-flow chemical reactor with fixed bed is a vessel filled with a porous catalyzer through which a liquid or gaseous reagent mixture is filtered. The motion of the mixture in this system is maintained by the pressure differential which compensates for the hydraulic resistance.The complete system of equations describing the mass-and heat-transfer processes in the continuous-flow chemical reactor includes the mass, momentum, and energy conservation equations [1], Usually, in the study of the existence and stability of steady reactor operating regimes, very simple reactor models are analyzed, in which only the mass-conservation equation in the form of the diffusion equation and the energy-conservation equation in the form of the heat-conduction equation are taken into account. The equation of motion drops out of consideration, since the velocity of the reagent mixture with the reaction products through the reactor is considered given and unvarying for disturbances of the steady operating regime.The purpose of the present study is to indicate the possibility of instability in the steady-process regime of a chemical reactor, which is associated with the fact that any temperature (and also concentration) variation within the reactor with disturbances of the steady regime will, generally speaking, lead to variation of the mixture viscosity and, consequently, to variation of the hydraulic resistance and the filtration velocity. This interconnection is an additional factor which may have a significant effect on the behavior of disturbances of the steady regime.In contrast with thermal, kinetic, and thermokinetic instability [2], it is natural to refer to the instability being investigated as thermomechanical instability. Let us consider an example.     

Flow reactor models for gas-liquid reactions based on the penetration theory

Flow reactor models for gas-liquid reaction systems are proposed in this paper based on the penetration theory in the isothermal case. The mass transfer mechanism accompanied by a chemical irreversible first-order reaction is mathematically treated in a new way in order to use its results to develop reactor design models conveniently. Analytical solutions can be obtained for the desgin equation system involving linear differential equations by using of either the eigenvalues or the Laplace-transformation and the superposition of the system. In addition, an iteration procedure is given to solve the nonlinear differential equation system numerically. Comparisons of the results from the analytical and numerical solutions are also made graphically.     

Auslegung eines Rohrreaktors für zweiphasige Flüssigkeits-Gas-Gemische

The fundamentals for a two-phase system in a tubular reactor had been studied. It can be demonstrated that the physical models derived for an exothermic reaction with gas development during the reaction are matching the practical results of the experiments performed in the laboratory. The complex hydrodynamics have been theoretically studied on the computer which also gave the geometry for the pilotplant tubular reactor.     

Memory access optimization for particle operations in computational fluid dynamics-discrete element method simulations

Computational Fluid Dynamics – Discrete Element Method is used to model gas-solid systems in several applications in energy, pharmaceutical and petrochemical industries. Computational performance bottlenecks often limit the problem sizes that can be simulated at industrial scale. The data structures used to store several millions of particles in such large-scale simulations have a large memory footprint that does not fit into the processor cache hierarchies on current high-performance-computing platforms, leading to reduced computational performance. This paper specifically addresses this aspect of memory access bottlenecks in industrial scale simulations. The use of space-filling curves to improve memory access patterns is described and their impact on computational performance is quantified in both shared and distributed memory parallelization paradigms. The Morton space filling curve applied to uniform grids and k-dimensional tree partitions are used to reorder the particle data-structure thus improving spatial and temporal locality in memory. The performance impact of these techniques when applied to two benchmark problems, namely the homogeneous-cooling-system and a fluidized-bed, are presented. These optimization techniques lead to approximately two-fold performance improvement in particle focused operations such as neighbor-list creation and data-exchange, with ∼ 1.5 times overall improvement in a fluidization simulation with 1.27 million particles.     

Dual characters of interaction between particles and fluid

The unique characteristics of gas-solids two-phase flow and fluidization in terms of the flow structures and the apparent behavior of particles and fluid-particle interactions are closely linked to physical properties of the particles, operating conditions and bed configurations. Fluidized beds behave quite differently when solid properties, gas velocities or vessel geometries are varied. An understanding of hydrodynamic changes and how they, in turn, influence the transfer and reaction characteristics of chemical and thermal operations by variations in gas-solid contact, residence time, solid circulation and mixing and gas distribution is very important for the proper design and scale-up of fluidized bed reactors. In this paper, rather than attempting a comprehensive survey, we concentrate on examining some important positive and negative impacts of particle sizes, bubbles, clusters and column walls on the physical and chemical aspects of chemical reactor performance from the engineering application point of view with the aim of forming an adequate concept for guiding the design of multiphase fluidized bed chemical reactors.One unique phenomenon associated with particle size is that fluidized bed behavior does not always vary monotonically with changing the average particle size. Different behaviors of particles with difference sizes can be well understood by analyzing the relationship between particle size and various forces. For both fine and coarse particles, too narrow a distribution is generally not favorable for smooth fluidization. A too wide size distribution, on the other hand, may lead to particle segregation and high particle elutriation. Good fluidization performance can be established with a proper size distribution in which inter-particle cohesive forces are reduced by the lubricating effect of fine particles on coarse particles for Type A, B and D particles or by the spacing effect of coarse particles or aggregates for Type C powders.Much emphasis has been paid to the negative impacts of bubbles, such as gas bypassing through bubbles, poor bubble-to-dense phase heat & mass transfer, bubble-induced large pressure fluctuations, process instabilities, catalyst attrition and equipment erosion, and high entrainment of particles induced by erupting bubbles at the bed surface. However, it should be noted that bubble motion and gas circulation through bubbles, together with the motion of particles in bubble wakes and clouds, contribute to good gas and solids mixing. The formation of clusters can be attributed to the movement of trailing particles into the low-pressure wake region of leading particles or clusters. On one hand, the existence of down-flowing clusters induces strong solid back-mixing and non-uniform radial distributions of particle velocities and holdups, which is undesirable for chemical reactions. On the other hand, the formation of clusters creates high solids holdups in the riser by inducing internal solids circulations, which are usually beneficial for increasing concentrations of solid catalysts or solid reactants.Wall effects have widely been blamed for complicating the scale-up and design of fluidized-bed reactors. The decrease in wall friction with increasing the column diameter can significantly change the flow patterns and other important characteristics even under identical operating conditions with the same gas and particles. However, internals, which can be considered as a special wall, have been used to improve the fluidized bed reactor performance.Generally, desirable and undesirable dual characteristics of interaction between particles and fluid are one of the important natures of multiphase flow. It is shown that there exists a critical balance between those positive and negative impacts. Good fluidization quality can always be achieved with a proper choice of right combinations of particle size and size distribution, bubble size and wall design to alleviate the negative impacts.