Hydrodynamic transition from fixed to fully fluidized beds for three-phase inverse fluidization

Hydrodynamic transition experiments, involving both visual observations and pressure measurements, were performed using a 127-mm diameter Plexiglas column for threephase inverse fluidized beds of 5.8-mm polyethylene spheres. Observations of interest not hitherto reported include: (1) A marked hysteresis effect (even when starting from a loose-packed condition) between inverse fluidization and defluidization which disappears when a wetting agent is added to the downflowing water. (2) An initially abrupt decrease of the minimum fluidization voidage, є_emf, followed by a gradual rise of є_emf with increasing superficial gas velocity, Ug. (3) Lower values of emf for three-phase systems than for the corresponding two-phase (liquid-solid) fluidized beds because local agitation by the gas bubbles causes bed compaction near the minimum liquid fluidization velocity, Ulmf. (4) Ulmf vs. Ug curves which, though they always show Ulmf decreasing as Ug increases, sometimes display concave-downward, sometimes concaveupward and sometimes S-shaped behavior.     

Voidage characteristics and prediction of bed expansion in liquid-solid inverse fluidized bed

Studies on voidage fluctuations, axial voidage profile and bed expansion are carried out by measuring the local void fraction using particles of wide ranging characteristics in liquid-solid inverse fluidized bed. The quality of fluidization is elucidated by the local voidage fluctuations. The RMS voidage fluctuation depicts a maximum with respect to average bed void fraction and increases with increase in Archimedes number. The fluidization quality has been quantified using average normalized RMS voidage fluctuation in terms of Transition number. The axial void fraction is almost uniform throughout the bed except for particles with size distribution. All the literature and present experimental data on bed expansion are unified in terms of Richardson and Zaki equation using experimental terminal velocities. A new correlation is proposed for predicting the wall effect corrected experimental terminal velocities, as a substitute for standard drag equation. The bed expansion data are also predicted using the drift flux model.     

Hydrodynamics of a superheated steam vacuum fluidized bed

Hydrodynamics of a superheated steam vacuum fluidized bed was experimentally studied. In these experiments, eight different types of large particles (1970–7430 μm) were used. In all cases, a behavior similar to that found in an air fluidized bed was observed. The minimum fluidization velocity was found to be increasing with decreasing operating pressure. In the case of employing superheated steam, the minimum fluidization conditions are established at a lower velocity than using air as the fluidizing medium. These tendencies are attributed to the variation of the mean free path of molecules. On the other hand, the experiments showed that the bed voidage in the minimum fluidization conditions is almost insensitive to the variation of the operating pressure. Several equations were developed to predict the minimum fluidization velocity. The values provided by these equations were compared with the experimental data as well as with the predictions of the correlations presented in the technical literature.     

Hydrodynamic study of a toroidal fluidized bed reactor

Hydrodynamic behavior of a newly developed toroidal fluidized bed reactor is studied in this work. The reactor has a gas distributor consisting of angled blades in an annular ring at the reactor bottom. The driving force for particles to move over the distributing blades comes from the velocity head of gas jets accelerated upon entering the blade spacing. Relevant hydrodynamic behaviors are measured with various inert materials in a pilot scale 400-mm toroidal fluidized bed reactor. The observed hydrodynamic behavior is found to be essentially predictable at ambient temperature by conventional hydrodynamic models. Fine particle tracking on the reactor wall is clearly observed through oxidation of zinc dross at a bed temperature of around 1120°C, and is simulated on the basis of a simplified mathematical model. Hydrodynamic issues, such as particle flying trajectory and retention time in the reactor, are discussed based on the developed model.     

反渗透技术在废水处理中应用的研究进展

反渗透技术是一种高效、易操作的液体分离技术,同传统的废水处理方法相比具有处理效果好,可实现废水的循环利用和对有用物质回收等优点。文章简要介绍了反渗透技术的基本原理,重点介绍了反渗透技术在垃圾渗滤液、矿区污水、钢铁工业废水、电厂废水处理中的应用研究进展状况。并讨论了反渗透膜技术的预处理、反渗透膜污染及清洗和反渗透技术的发展前景。     

Practical scaling considerations for dense gas fluidized beds interacting with the air-supply system

The present study addresses the problem of the scale-up/down of dense gas fluidized beds that may interact with their air-supply system (restricted here to the plenum and the distributor). An abundant literature about scaling issues yielded various sets of dimensionless numbers to be matched in order to ensure the invariance of the bed dynamics at different scales. All these sets embed the first indispensable requirement that is the geometrical similarity between scaled beds. Despite its self-evident character, this letter highlights important and new considerations about the applicability of this requirement for the design of the distributor and the plenum. For the distributor, it is shown that the geometrical similarity introduced by some authors is not always relevant and should only be used under particular conditions. For the scaling of the plenum, to our knowledge, nothing is mentioned in the literature. The present work shows that the ‘by-default’ geometrical scaling of the plenum is completely irrelevant. A practical rule to determine the plenum size at different scales is given so that similarity of the dynamic interaction between the bed and its air-supply system is ensured.     

A typical application of magnetic field inwastewater treatment with fluidized bed biofilm reactor

The effects of a magnetic field on wastewater treatment with a fluidized bed biofilm reactor was investigated. With glucose being the sole carbon source, the activated sludge obtained from a real wastewater treatment plant was used as a seed. Magnetically loaded polystyrene beads at the size of 500-595 w m were used as support materials for biofilm formation in a fluidized bed biofilm reactor. Magnetic field application allowed the operation of the column at high liquid flow rates, thus external diffusion limitations on the biofilm surface were lowered and the efficiency of biodegradation was increased. Denser, thinner, and more active biofilm was obtained with magnetic field application, especially in pulsed form. As expected, the system performance changed with operational parameters, and the increase in substrate removal reached up to 26% with pulsed application of a 17.8 mT DC-magnetic field under optimum conditions.     

Experimental study on the effects of gas permeation through flat membranes on the hydrodynamics in membrane-assisted fluidized beds

Recently, many novel reactor concepts based on membrane fluidized bed reactors have been proposed. In this work, the effects of gas permeation through flat membranes on the hydrodynamics in a pseudo-2D membrane-assisted gas–solid fluidized bed have been investigated experimentally. A combination of the non-invasive techniques (Particle Image Velocimetry (PIV) and Digital Image Analysis (DIA)) was employed to simultaneously investigate solids phase and bubble phase properties in great detail. Counter-intuitively, addition of secondary gas via the membranes, that constituted the confining walls of a gas–solid suspension at conditions close to incipient fluidization, did not result in a larger, but in a smaller equivalent bubble diameter, while gas extraction on the other hand, resulted in a larger equivalent bubble diameter, although in this case the effect was less pronounced. This could be explained by changes in the larger scale particle circulation patterns due to gas extraction and addition via the membranes.     

Flow structure of the solids in gas-solid fluidized beds

Existence of clusters in dense fluidized beds was investigated by analyzing the time-position data of a tracer obtained in several radioactive particle tracking experiments. It was found that in the case of sand particles, more gas passes through the bed as bubbles with increasing the superficial gas velocity and in the case of FCC powder, flow of the gas through the bed as bubbles does not increase in the turbulent fluidization regime. Cluster diameters were estimated from their velocities and found that descending clusters are generally larger than ascending ones and the size of both increases with increasing the superficial gas velocity. Bubble velocities evaluated in this work are in good agreement with the correlations in the bubbling regime of the fluidization available in the literature.     

Interpretation of the hydrodynamic behaviour in a conical fluidized bed dryer

Wet batches of placebo pharmaceutical granule were dried at inlet superficial gas velocities of 0.64 and 1.3 m/s in a Glatt GPCG-1 fluidized bed. Using pressure fluctuation analysis, the hydrodynamic behaviour indicates a transition from a multiple bubbling regime to a coalescence dominated regime as drying proceeds. The transitional fluidization behaviour is linked to the physical mechanisms associated with the constant and falling rate periods of drying porous materials. Excess surface moisture present during the constant rate period increases interparticle forces through liquid bridging. These liquid bridges stabilize the bed structure which limits bubble formation in the bed. Once the falling rate period is reached, the liquid bridges cannot be maintained and bubble coalescence increases. The resulting bubbling bed hydrodynamics can be explained using the simple two-phase model proposed by Toomey and Johnstone [1952. Gas fluidization of solid particles. Chemical Engineering Progress 48, 220-226] using the full support velocity and bed voidage characteristics of the granule at varying moisture contents.     

Distributor effects near the bottom region of turbulent fluidized beds

The distributor plate effects on the hydrodynamic characteristics of turbulent fluidized beds are investigated by obtaining measurements of pressure and radial voidage profiles in a column diameter of 0.29 m with Group A particles using bubble bubble-cap or perforated plate distributors. Distributor pressure drop measurements between the two distributors are compared with the theoretical estimations while the influence of the mass inventory is studied. Comparison is established for the transition velocity from bubbling to turbulent regime, U_c, deduced from the pressure fluctuations in the bed using gauge pressure measurements. The effect of the distributor on the flow structure near the bottom region of the bed is studied using differential and gauge pressure transducers located at different axial positions along the bed. The radial voidage profile in the bed is also measured using optical fiber probes, which provide local measurements of the voidage at different heights above the distributor. The distributor plate has a significant effect on the bed hydrodynamics. Owing to the jetting caused by the perforated plate distributor, earlier onset of the transition to the turbulent fluidization flow regime was observed. Moreover, increased carry over for the perforated plate compared with the bubble caps has been confirmed. The results have highlighted the influence of the distributor plate on the fluidized bed hydrodynamics which has consequences in terms of comparing experimental and simulation results between different distributor plates.     

Multifluid Eulerian modeling of dense gas-solids fluidized bed hydrodynamics: Influence of the dissipation parameters

Computational fluid dynamic (CFD) models must be thoroughly validated before they can be used with confidence for designing fluidized bed reactors. In this study, validation data were collected from a fluidized bed of (Geldart's group B) alumina particles operated at different gas velocities involving two fluidization hydrodynamic regimes (bubbling and slugging). The bed expansion, height of bed fluctuations and frequency of fluctuations were measured from videos of the fluidized bed. The Eulerian-Eulerian two fluid model MFIX was used to simulate the experiments. Two different models for the particle stresses—Schaeffer [Syamlal, M., Rogers, W., O’Brien, T.J., 1993. MFIX documentation: theory guide. Technical Report DOE/METC-94/1004 (DE9400087), Morgantown Energy Technology Centre, Morgantown, West Virginia (can be downloaded from Multiphase Flow with Interphase eXchanges (MFIX) website 〈http://www.mfix.org〉); Schaeffer, D.G., 1987. Instability in the evolution equations describing incompressible granular flow. Journal of Differential Equations 66, 61-74.] and Princeton [Srivastava, A., Sundaresan, S., 2003. Analysis of a frictional-kinetic model for gas-particle flow. Powder Technology 129(1-3), 72-85.] models—and different values of the restitution coefficient and internal angle of friction were evaluated. 3-D simulations are required for getting quantitative and qualitative agreement with experimental data. The results from the Princeton model are in better agreement with data than that from the Schaeffer model. Both free slip and Johnson-Jackson boundary conditions give nearly identical results. An increase in coefficient of restitution (e) from 0.8 to 1 leads to larger bed expansions and lower heights of fluctuations in the bubbling regime, whereas it leads to unchanged bed expansion and to a massive reduction in the height of fluctuations in the slugging regime. The angle of internal friction (φ) in the range 10-40^° does not affect the bed expansion, but its reduction significantly reduces the height of fluctuations.     

The influence of distributed reactant injection along the height of a fluidized bed reactor

A two-phase model is used to simulate spreading the introduction of reactant feed along the height of a fluidized bed reactor for oxidative dehydrogenation of ethane to ethylene. The reactor model is used to predict the reactor performance for different ethane-to-oxygen molar feed ratios, with premixed and non-premixed feed. The proposed model is used to simulate the premixed feed (without secondary injection), and for distributed feed with secondary injection at one, three and five injection levels above the primary distributor. Predictions from the model are shown to compare favourably with experimental data from an industrial pilot reactor of diameter 97 mm. A case study is then employed to explore a wider range of conditions than is possible experimentally. Oxidant distribution is shown to be beneficial in expanding the range of reactant compositions beyond those normally allowed by safety constraints. Distributing the feed over a number of levels improves the reactor performance, especially in reducing the selectivities of undesired by-products. Feeding gas at several levels is generally more promising than introducing feed at a single secondary injection level.     

Solid circulation rate in a circulating fluidized bed in the presence of fine powders

Fine powders (Geldart's group C) are added to a circulating fluidized bed (CFB) of coarse particles (Geldart's group A) and the solid circulation rate (SCR) is investigated with addition of fine powders of different sizes and different fractions (different hold-ups) to the bed. Experiments were carried out in a CFB of 2 m in height and 0.052 m in diameter, using FCC catalyst particles of as the coarse particles and cohesive aluminum hydroxide powders of 0.5- as the fine powders. The effects of hold-up of fine powders in the bed, fine powders size, and superficial gas velocity on the SCR were investigated.The SCR strongly depended on the hold-up of fine powders of 0.5- in size and noticeably decreased with increasing the hold-up of fine powders under constant gas velocity. This dependency disappeared when the size of fine powders was larger than . Thus, depending on the size of fine powders added to the CFB, two distinct regions for the changes of SCR could be clearly identified.     

陶瓷膜在染料及漂染废水处理中的应用

考察了以陶瓷膜为过滤介质的旋叶压滤机对絮凝后的废水进行处理的过滤速率及其影响因素,进行了动态过滤与终端过滤的比较,以及陶瓷膜与有机膜在废水处理效果上的对比研究。     

Analysis of pressure fluctuations in fluidized beds. II. Reconstruction of the data by the Fokker–Planck and Langevin equations

In this part of the sequel we develop a continuum representation of the pressure fluctuation time series p(t) for a fluidized bed (FB), analyzed in part I, by using stochastic methods based on the Markov processes. It is shown that the data may be represented by Markov series with a Markov time scale t_M that is estimated based on the data. Using the relation between the Markov processes and the Kramers–Moyal (KM) expansion that is a continuum equation that involves, in principle, an infinite number of coefficients, we represent the pressure fluctuation time series by a KM expansion. However, since the third and higher-order coefficients of the expansion are very small, the KM expansion reduces to a Fokker–Planck (FP) equation that represents p(t) in terms of a drift and a diffusion coefficients that are computed based on the data. The FP equation is, in turn, equivalent to a Langevin equation, which is used to reconstruct the data, i.e. generate the time series that mimic, in a statistical sense, the original data. Thus, the Langevin equation may also be used to make probabilistic predictions for the future values of the pressure over time scales that are of the order of the Markov time scale t_M. The accuracy of the reconstructed series and, hence, their continuum representation, is demonstrated. We also compute the frequency of level-crossing at a given level α, i.e. the relative frequency (probability) of occurrence of the event defined, for two times t_i−1 and t_i, by, , where P(x) is the probability of the event. Thus, yields the frequency that a given pressure fluctuation level may be expected. The average time that one should wait in order to observe the pressure at a given level again is also computed. The two quantities may be particularly important for modeling of the FBs. A relation is presented between the drift and diffusion coefficients of the FP equation and the Hurst exponent that has previously been used to describe the pressure fluctuation time series in terms of self-affine stochastic distributions.     

催化臭氧化技术在废水处理中的应用

阐述了均相催化臭氧化和非均相催化臭氧化技术在废水处理中的应用研究进展,并提出了技术发展趋势与存在的问题。非均相催化臭氧化技术是现代催化技术与环境化学处理技术的组合,其中催化剂具有活化臭氧、提高臭氧氧化降解能力的作用。该技术具有降解能力强、效率高、不产生二次污染等优点,将在环保领域发挥越来越大的作用。     

Comparison of fluidized bed flow regimes for steam methane reforming in membrane reactors: A simulation study

An important decision in the design of fluidized bed reactors is which of several flow regimes to choose. Almost all fluidized bed reactor models are restricted to a single flow regime, making comparison difficult, especially near the regime boundaries. This paper examines the performance of fluidized bed methane reformers with three models—a simple equilibrium model and two kinetic distributed models, based on different assumptions of varying sophistication. Membranes are incorporated to improve reactor performance. Eighteen cases are simulated for different flow regimes and membrane configurations. Predictions for the fast fluidization and turbulent flow regimes show that the rate-controlling step is permeation through the membranes. Bubbling regime simulations predict somewhat less hydrogen production than for turbulent and fast fluidization, due to the effects of interphase crossflow and mass transfer. Overall reactor performance is predicted to be best under turbulent fluidization operation. Practical considerations also affect the advantages, shortcomings and ultimate choice of flow regime.     

Preparation of dialdehyde cellulose hydrazone derivatives andevaluating their efficiency for sewage wastewater treatment

The cellulose hydrazone derivative is a novel product prepared from dialdehyde cellulose with 2-hydrazino-3,5,6,7tetrahydrocyclopentanethieno[2,3-d]-pyrimidin-4(4H)-one under suitably selected conditions. It was found that the reaction of dialdehyde cellulose with the 2-hydrazino derivative decreased progressively in the series cellulose powder, viscose wood pulp and cotton linter. The principal objective of this research was to evaluate the effectiveness of dialdehyde cellulose hydrazone derivatives on sewage wastewater settling, percentage of total suspended solids (TSS), carbon oxygen demand (COD), and iron and chromium removal. About 30-50% of chlorine was removed after treatment using dialdehyde cellulose hydrazone derivatives, thus indicating their good efficiency for halogen removal. Cellulose hydrazone derivatives were recommended as good coagulants and removed Fe and Cr by 73.9% and 66.7%, respectively. Jar tests revealed that the wastewater was best treated with the addition of chemical coagulants such as FeCl₃ or Al₂(SO₄)₃ combined with cellulose hydrazone derivatives at optimum doses. The optimum conditions produced better cleaning-up and improved removal of COD and TSS up to 77% and 97%, respectively.