A model for a countercurrent gas—solid—solid trickle flow reactor for equilibrium reactions. The methanol synthesis

The theoretical background for a novel, countercurrent gas—solid—solid trickle flow reactor for equilibrium gas reactions is presented. A one-dimensional, steady-state reactor model is developed. The influence of the various process parameters on the reactor performance is discussed. The physical and chemical data used apply to the case of low-pressure methanol synthesis from CO and H₂ with an amorphous silica—alumina as the product adsorbent. Complete reactant conversion is attainable in a single-pass operation, so that a recycle loop for the non-converted reactants is superfluous.In the following article the installation and experiments for which this theory was developed will be described.     

The circulating fluid bed reactor — its main features and applications

After a brief review of the development of the circulating fluid bed (CFB) reactor principle, its main features as an efficient tool in performing reactions between gases and finely grained solids are discussed. Today, the circulating fluid bed reactor is used in numerous industrial processes or has passed the pilot or demonstration stage in many others. Large-scale CFB calciners have replaced the rotary kiln in the alumina industry. Combustion of carbon-containing fossil fuels and process residues for energy supply with accompanying low levels of noxious emissions is one of the most promising fields of future applications, especially in the power industry and municipal combined heat and power generation. Dry-scrubbing of process and power plant waste gases is also promising. Many more industrial applications of CFB reactors can be expected.     

N2O emissions from fluidised bed combustion. The effect of fuel characteristics and operating conditions☆

Fluidised bed combustion (FBC) is a versatile and relative clean technology except with respect to nitrous oxide (N₂O) emissions. The emissions of N₂O from FBCs are very dependent on a number of operating conditions (temperature, sorbent addition, excess oxygen, etc.), fuel characteristics and many homogeneous and heterogeneous reactions that take place.This paper describes the results obtained during the study of the effect of coal type on N₂O emissions from FBC. The combustion tests were performed in a circulating fluidised bed pilot plant, using two coals: a Spanish subbituminous (Puertollano) and a bituminous coal from Colombia (Carbocol). Using supporting laboratory-scale fluidised bed pyrolysis experimental data with these fuels the partitioning of fuel-N and the formation of the most important N₂O precursors, NH₃, HCN and char was followed. The pyrolysis tests results showed that the major part of the nitrogen remained in the char. Both coals a produced similar amount of HCN, but the amount of char-N was lower with Carbocol coal that with Puertollano coal. The combustion results showed that the conversion of fuel-N to N₂O was higher on the tests with Puertollano coal than with Carbocol coal. For this it was concluded that the formation of N₂O via char-N oxidation was the most important pathway. The temperature profile of the combustor and the sorbent addition strongly influence N₂O emissions.     

Hydrodynamics and mass transfer of gas–liquid flow in micropacked bed reactors with metal foam packing

Hydrodynamics and mass transfer of gas–liquid flow such as pressure drop, liquid holdup, and gas–liquid mass-transfer coefficient in micropacked bed reactors (μPBRs) with metal foam packing are investigated with an automated platform. Parametric studies are conducted varying gas and liquid superficial velocities, pore diameters of foam packing, and liquid physical properties. Experimental results show that μPBRs with foam packing have comparative mass transfer rate and 10 times lower pressure drop compared to the microparticles. The values of mass-transfer coefficient for three types of foam packing in μPBRs are 1–2 orders of magnitude larger than those in large-scale trickle bed reactors with foam packing. Furthermore, empirical correlations of pressure drop, liquid holdup, and gas–liquid mass-transfer coefficient in μPBRs with foam packing are proposed and the predicted values are found to be in good agreement with the experimental values.     

Hydrodynamics and mass transfer enhancement of gas–liquid flow in micropacked bed reactors: Effect of contact angle

Pressure drop, residence time distribution, dispersive behavior, liquid holdup, and mass transfer performance of gas–liquid flow in micropacked bed reactors (μPBRs) with different contact angles (CA) of particles are studied. The value of pressure drop for three types of beads can be obtained: copper beads (CA = 88.1°) > stainless steel beads (CA = 70.2°) > glass beads (CA = 47.1°). The liquid axial dispersion coefficient is 1.58 × 10⁻⁶ to 1.07 × 10⁻⁵ m²/s for glass beads and copper beads, which is smaller than those of trickle bed reactors. The liquid holdup of 400 μm copper beads is larger than that of 400 μm glass beads. The ratio of effective interfacial area enhancement is evaluated up to 55% for big contact angle beads compared with the hydrophilic glass beads. In addition, correlations of pressure drop, liquid holdup, and effective interfacial area in μPBRs with different wettability beads are developed and predicted values are in agreement with the experimental data.     

CFD simulation of fluidized bed reactors for polyolefin production – A review

This literature survey focuses on the application of computational fluid dynamics (CFD) in various aspects of the fluidized bed reactor. Although fluidized bed reactors are used in various industrial applications, this first-of-its-kind review highlights the use of CFD on polyolefin production. It is shown that CFD has been utilized for the following mechanisms of polymerization: governing of bubble formation, electrostatic charge effect, gas–solid flow behavior, particle distribution, solid–gas circulation pattern, bed expansion consequence, mixing and segregation, agglomeration and shear forces. Heat and mass transfer in the reactor modeling using CFD principles has also been taken under consideration. A number of softwares are available to interpret the data of the CFD simulation but only few softwares possess the analytical capability to interpret the complex flow behavior of fluidization. In this review, the popular softwares with their framework and application have been discussed. The advantages and feasibility of applying CFD to olefin polymerization in fluidized beds were deliberated and the prospect of future CFD applications was also discussed.     

A fundamental CFD study of the gas–solid flow field in fluidized bed polymerization reactors

A Eulerian–Eulerian model incorporating the kinetic theory of granular flow was applied to describe the gas–solid two-phase flow in fluidized bed polymerization reactors. The model parameters were examined, and the model was validated by comparing the simulation result with the classical calculated data. The effects of distributor shape, solid particle size, operational gas velocity and feed manner on the flow behavior in the reactor were also investigated numerically. The results show that with the increase of solid particle diameter, the bubble numbers decrease and the bubble size increases, resulting in a smaller bed expansion ratio. Bed expansion ratio increases with increasing the gas inlet velocity. Moreover, the final fluidized qualities are almost the same for the plane distributor case and the triangle distributor case. There exists a tempestuous wiggle from side to side in the bed at the continuous feed manner, which could not be obtained at a batch feed manner.     

Analysis of the flow pattern and periodicity of gas–liquid–liquid three-phase flow in a countercurrent mixer-settler

In contrast to the concurrent mixer-settler, the interaction between the mixing and settling chambers have to be taken into account in the simulation of the countercurrent mixer-settler, and no work has been reported for this equipment. In this work, a three-phase flow model based on the Eulerian multiphase model, coupled with a sliding mesh model is proposed for a countercurrent mixer-settler. Based on this, the dispersed phase distribution, flow pattern, and pressure distribution are investigated, which can help to fill the gap in the operation mechanism. In addition, the velocity vector distribution at the phase port shows an intriguing phenomenon that two types of vectors with opposite directions are distributed on the left and right sides of the same plane, which indicates that the material exchange in the mixing and settling chambers is simultaneous. Analysis of this variation at this location by a fast Fourier transform (FFT) method reveals that it is mainly influenced by the mixing chamber and is consistent with the main period of the outlet flow fluctuations. Therefore, by monitoring the fluctuation of the outlet flow and then analyzing it by the FFT method, the state of the whole tank can be determined, which makes it promising for the design of control systems for countercurrent mixer-settlers.     

Reduced graphene oxide for Li–air batteries: The effect of oxidation time and reduction conditions for graphene oxide

Reduced graphene oxide (rGO) has shown great promise as an air-cathode for Li–air batteries with high capacity. In this article we demonstrate how the oxidation time of graphene oxide (GO) affects the ratio of different functional groups and how trends of these in GO are extended to chemically and thermally reduced GO. We investigate how differences in functional groups and synthesis may affect the performance of Li–O₂ batteries. The oxidation timescale of the GO was varied between 30 min and 3 days before reduction. Powder X-ray diffraction, micro-Raman, FE-SEM, BET analysis, and XPS were used to characterize the GO’s and rGO’s. Selected samples of GO and rGO were analyzed by solid state ¹³C MAS NMR. These methods highlighted the difference between the two types of rGO’s, and XPS indicated how the chemical trends in GO are extended to rGO. A comparison between XPS and ¹³C MAS NMR showed that both techniques can enhance the structural understanding of rGO. Different rGO cathodes were tested in Li–O₂ batteries which revealed a difference in overpotentials and discharge capacities for the different rGO’s. We report the highest Li–O₂ battery discharge capacity recorded of approximately 60,000 mAh/g_carbon achieved with a thermally reduced GO cathode.     

Population balance modelling of bubbling fluidized bed reactors—I. well-stirred dense phase

A detailed population balance model is presented for a fluidized bed reactor incorporating: the formation of bubbles at the grid plate, their rise with velocities governed by their sizes, random coalescence between bubbles, gas exchange between bubbles and the dense phase, and a first order chemical reaction in the dense phase under well-mixed conditions. Reaction conversion is calculated as a function of dimensionless parameters relating the rates of various competing processes such as coalescence, dense phase mixing, mass exchange between bubbles and dense phase and reaction rate. Comparison of conversions with those of Davidson et al. (1977) show significant variations indicating that the dynamics of bubble size distributions could have non-trivial effects on the extent of reaction. Fluctuations in bubble populations did not seem strong enough to translate to strong fluctuations in reaction conversion.     

Multiplicity of the steady state in fluidized bed reactors—VIII. Partial oxidation of o-xylene

A model is derived for the case of a fluidized bed reactor in which partial oxidation of o-xylene occurs. The use of fluidized beds instead of fixed bed reactors allows for higher feed concentrations and lower feed temperatures. However, multiple steady states arise and it is shown that the maximum yield is obtained when the reactor is operated at the unstable middle steady state. However, for maximum productivity, the reactor must be operated at a lower temperature than that corresponding to maximum yield. This is due to the pathological dependence of the middle steady state on feed temperature and concentration.     

Simulation and experiment of gas–solid flow field in short-contact cyclone reactors

A short-contact cyclone reactor has been designed for the particular case of fluid catalytic cracking. The new type reactor mainly includes two parts: a reaction chamber and a separation chamber. So the cracking reactions and the separations between the products and catalysts could occur respectively and simultaneously. A three dimensional model was used to representing key parts of a laboratory cyclone reactor. The Eulerian–Eulerian computational fluid dynamics model with the kinetic theory of granular flow was adopted to simulate the gas–solid two-phase flow. The particle concentration distribution and pressure drop were measured by a PV-6A particles velocity measure instrument and a U-manometer, respectively. Simulated results show that in the reaction chamber solids can be transformed into a homogeneous dispersed flow, particles’ concentration becomes uniform gradually while catalysts flowing down, the concentration is a little higher near the wall because of boundary effect. After the gas–solid flowing into the separation chamber, the gas phase is separated with solids completely. The new reactor has a good contact and separation effect. Simulated results make a reasonable agreement with the experimental findings.     

Computational fluid dynamics study of kaolin–water flow in a T-junction using a novel shear-thinning fluid model

A recently proposed shear-thinning fluid model that mimics the response of seemingly viscoplastic materials is evaluated in computational fluid dynamics simulations by studying the steady flow of a kaolin–water suspension in a 2D T-junction. The velocity profiles for the kaolin–water suspension are reported at the mid-length of the main channel and the root of the bifurcation (where recirculation is expected to appear). The velocity profiles of the proposed model are compared with those from conventional viscoplastic models (Bingham plastic model and the Herschel–Bulkley model) at low (=100) and high Reynolds number (=2000). The new model predicts a recirculation zone (at the inner edge of the bifurcation arm) that conventional models do not. The effect of the variation in the model parameters (α₁ and α₂) on velocity profiles at low (=100) and high Reynolds numbers (=2000) is also documented. These indicate the disappearance of the recirculation zone at low Reynolds number as α₁ (equivalently, viscosity) increases, whereas the recirculation zone persists even for higher values of α₁ at high Reynolds number. Further, at low Reynolds number, the skewing of maximum velocity towards the outer edge of the bifurcation arm disappears as α₂ increases, whereas the skewing persists even at the highest value of α₂ used at the high Reynolds number.     

The behavior of gas flow ejected from two vertical nozzles in a fluidized bed

1 INTRODUCTIONIt has been generally accepted through industrial practices and laboratory experi-mentations that chemical reaction in a gas fluidized reactor takes place primarilyat the location within a few centimeters from its bottom.This is particularly no-ticeable for fast reactions.It has been known that under normal operating condi-tions in gas-solid fluidized reactors,the characteristics of bubble size and bubblemovements play important roles in affecting the mass transfer and contacts between     

The effect of processing conditions,amount of additives and composition on the microstructures and mechanical properties of α-SiAlON ceramics

A series of samples with yttrium α-SiAlON compositions and different amounts of additive has been fabricated from α-Si₃N₄, AlN, Al₂O₃ and Y₂O₃ starting powders, using gas pressure sintering and three different sintering procedures. One series of samples was heated up to 1825°C and then held for 3 h, another group of samples was held at a lower temperature (1500 or 1600°C) for 1 h and then heated up to 1825°C and held for 3 h. The results of investigations using scanning electron microscopy showed the effect of composition and sintering procedure on the morphology of α-SiAlON grains. It was found that the amount of elongated grains increased with increasing amount of liquid phase. The mechanical tests showed that all of the samples exhibited HV10 values in the range of 1800–1976 kg/mm and K_IC values in the range of 3.9–6.3 MPam^1/2.     

Development and Application of Hot Self—flow Repairing Mix for Trough

Crack of troughs for blast furnace was analyzed in this paper.Influences of crtical grain size,bonding agent and suspending agent on performance of the hot self-flow repairing mi was studied.Al2O3-SiC-C hot self-flow repairing mix was developed.     

Two-area model for bubble distribution in a turbulent fluidized bed of fine particles

The flow behavior of gas and solid was investigated in FCC simulator of φ710×4000/φ870×11000mm.The axial and radial distributions were detected with matrix fiber-opticprobes.It was found that the distribution of bubble diameter in the turbulent region of the fluidizedbed of fine particles was different from the results reported for lab-scale experiments.Radially therewere three areas,i.e.,the central(r/R=0-0.4),the intermittent or stable(r/R=0.4-0.8)and thenear wall(r/R=0.8-1.0)areas respectively.It was noticed that bubbles were almost non-existing atthe near wall area.Hence,according to the coalescence and splitting theory of bubbles,a two-areamodel of bubble diameter distribution was proposed and a dimensionless parameter(γ_M)regarded asan index for'quality'of fluidization was deduced.     

Nature and characteristics of gas–liquid flow regimes in a micro-packed bed reactor

Micro-packed bed reactors (μPBRs) have the advantages of high heat and mass transfer efficiency and excellent safety, and they have been successfully applied to hydrogenation and oxidation reactions. However, the study of gas–liquid flow regimes in the μPBR, which is essential for the mass transfer modeling and reactor scale-up, is still insufficient due to the limitation of micro-scale and complexity of capillary force. In this work, the flow regimes in the two-dimensional μPBR were systematically studied by visual method utilizing a high-performance camera. Four typical flow regimes and characteristics were captured, and flow regime transition was revealed. Effects of gas and liquid superficial velocities, liquid physical properties, and particle sizes on liquid spreading areal fraction and pressure drop were investigated. Flow regime transition correlation of churn flow and pseudo-static flow in the μPBR was provided for the first time based on the summary of the current and previous published results.     

A multi-period optimization model for optimal planning of China's power sector with consideration of carbon mitigation—The optimal pathway under uncertain parametric conditions

Optimal planning of power industry considering carbon mitigation for a long-term future is complex, involving many technical alternatives and infinite possible plants installations, retrofitting, and decommissions. Previously the authors built a multi-period superstructure optimization planning model of China's power sector, gaining the optimal pathway of China's power sector with fixed parametric input during 2010–2050. With that model, this paper attempted to optimize pathway of China's power sector under uncertainty, in which the most influential parameters were uncertain. A levelized optimal pathway of China's power sector was gained, reliability of which was verified by comparing it with optimal results for the stochastic samples. The levelized optimal pathway showed that in the presence of carbon tax, carbon emissions of the power sector were reduced significantly by developing low-carbon technologies including nuclear power, renewables, as well as carbon capture and sequestration (CCS), and CCS would be key to reduce carbon emissions from coal power.     

Methanation of carbon dioxide and fluidization quality in a fluid bed reactor—the influence of a decrease in gas volume

A large decrease of fluidization quality was observed when methanation of carbon dioxide was carried out in a fluid-bed reactor even if the catalyst particles had the optimal properties for good fluidization. The cause of this phenomenon was explored by measuring pressure fluctuations, bubble frequency and extent of CO₂ conversions. The results indicated that the decrease of the fluidity was caused by a reduction in volume of reactant gases due to the reaction. The voidage in the emulsion phase is considered to be an important factor affecting the fluidity. The fluidization quality and contacting efficiency could be improved by such devices as baffle internals or two-stage spargers.