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Measuring and Modelling Residence Time Distribution of Low Density Solids in a Fluidized Bed Reactor of Sand Particles
Abstract
An experimental study was carried out to estimate the residence time distribution of low density particles injected into a fluidized bed reactor containing sand particles. Tracer experiments were performed at room temperature, using a pilot plant pyrolysis reactor. A novel technique was used to detect and measure the flow of solid tracer particles having different physical characteristics entering and leaving the fluid bed. The experimental results demonstrated that the degree of particle entrainment was a function of the fluidizing gas velocity, the particle size and the particle density. Solid mixing, segregation and entrainment were also studied as functions of physical and operating parameters. Various models were tested to characterize the non-ideal solids flow patterns within the fluid bed. A circulation model appeared to give a good description of the physical mechanism involved and to provide the best agreement with the experimental results.
... The experimental methods that have provided the most direct measurement of particle RTDs have relied on measurements of the exit times of distinctive particles (e.g., particles with some distinctive physical characteristic such as color, size, or chemical composition that can be readily detected) after they have been injected as pulses into experimental bubbling or circulating fluidized beds. Typically, the particles that were measured have been classified either as Geldart A or B type, which characterizes the flow patterns they tend to exhibit [Geldart (1973), Kunii and Levenspiel (1991)]. 1 Some of the most relevant articles on these experiments include: Yagi and Kunii (1961a), Helmrich et al (1986), Berruti et al (1988), Ambler et al (1990), Smolders and Baeyens (2000), Harris et al (2003a&b), Bhusarapu et al (2004), andAndreux et al (2008). All of these studies have reported some common features in the observed RTDs: ...
... Many of the general modeling approaches developed for chemical reactors have been adapted for modeling particle RTDs in bubbling and circulating beds. Some relevant articles in the literature that discuss RTD modeling in this context include the following: Yagi and Kunii (1961a), Verloop et al (1968), Berruti et al (1988), Ambler et al (1990), Smolders and Baeyens (2000), Harris et al (2002), Bhusarapu et al (2004), and Andreux et al (2008). As with the modeling approaches used for the more general problem in chemical reactors, particle RTD models for bubbling and fluidized beds have adopted one of three basic approaches, listed below in increasing order of complexity: ...
... • The 1D continuum dispersion model [see for example Berruti et al ((1988), Levenspiel (1999), and Smolders and Baeyens (2000]; ...
... Compared with gas mixing, the particle residence time distribution and particle mixing are more important for the CFB because they a ect chemical reaction rate and product distribution. Although there are quite a number of studies on particle mixing in the CFB (Bellgardt & Werther, 1986;Berruti, Liden, & Scott, 1988;Cen, Fan, Luo, Yan, & Ni, 1988;Ambler, Milne, Berruti, & Scott, 1990;Talukdar & Mathur, 1996;Fujima, Deguchi, & Endo, 1999;Kojima, Ishihara, Guilin, & Furusawa, 1989;Lin, Winell, Hansen, & Dam-Johansen, 1999;Mirgain, Briens, Pozo, Loutaty, & Bergougnou, 1998;Neathery, Schaefer, & Stencel, 1995;Niven, Khalili, & Hibbert, 2000a, b;Peters, Fan, & Sweeney, 1982;Sivashanmugam & Sundaram, 2000;Stellema, Kolar, Goeij, Schouten, & Bleek, 1997;Yang, 1988), only a few models of particle mixing are published (Berruti et al., 1988;Cen et al., 1988;Ambler et al., 1990). Due to the coreannulus structure, it is not suitable to use a one-dimensional dispersion equation to describe mixing in the CFB. ...
... Compared with gas mixing, the particle residence time distribution and particle mixing are more important for the CFB because they a ect chemical reaction rate and product distribution. Although there are quite a number of studies on particle mixing in the CFB (Bellgardt & Werther, 1986;Berruti, Liden, & Scott, 1988;Cen, Fan, Luo, Yan, & Ni, 1988;Ambler, Milne, Berruti, & Scott, 1990;Talukdar & Mathur, 1996;Fujima, Deguchi, & Endo, 1999;Kojima, Ishihara, Guilin, & Furusawa, 1989;Lin, Winell, Hansen, & Dam-Johansen, 1999;Mirgain, Briens, Pozo, Loutaty, & Bergougnou, 1998;Neathery, Schaefer, & Stencel, 1995;Niven, Khalili, & Hibbert, 2000a, b;Peters, Fan, & Sweeney, 1982;Sivashanmugam & Sundaram, 2000;Stellema, Kolar, Goeij, Schouten, & Bleek, 1997;Yang, 1988), only a few models of particle mixing are published (Berruti et al., 1988;Cen et al., 1988;Ambler et al., 1990). Due to the coreannulus structure, it is not suitable to use a one-dimensional dispersion equation to describe mixing in the CFB. ...
... On the basis of the core-annulus structure, the cell-mixing model is developed in the present work. In contrast with other published models (Berruti et al., 1988;Cen, et al., 1988; Ambler et al., 1990), particle mixing in the present work is simulated directly without the utilization of any parameters (e.g., dispersion parameters) determined from regressing experimental data. The dynamics of a chemical reaction for sulfur capture models can generally be divided into two groups (Mattisson & Lyngfelt, 1998a;Lyngfelt & Leckner, 1999): lime models and reactor models. ...
A mixing model has been developed to simulate the particle residence time distribution (RTD) in a circulating fluidized bed absorber (CFBA). Also, a gas/solid reaction model for sulfur dioxide (SO2) removal by lime has been developed. For the reaction model that considers RTD distribution inside the core and annulus regions of a CFBA, a macrochemical reaction can be simulated based on microchemical reaction dynamics. The presented model can predict SO2 and lime concentration distributions inside the CFBA, and give the amount of lime needed to remove a given percentage of SO2. It is found that SO2 concentration decreases with the increase of CFBA distance from the bottom in the core region. However, lime concentration exhibits a very slight variation in the core region. This means that lime is efficiently utilized to remove SO2. The model also predicts that SO2 partial pressure at the exit of the CFBA decreases with the increase in the percentage of fresh lime injected in the CFBA.
... The experimental setup allowed for both the identification and quantification of the major products. Although this experimental set-up may not fully represent "real-world" conditions experienced by the catalyst during CFP of biomass at the industrial scale, the micro-pyrolysis unit has been used a convenient tool by many research groups to evaluate prospective catalysts with high accuracy [28,29]. ...
The catalytic fast pyrolysis (CFP) of biomass represents an efficient integrated process to produce deoxygenated stable liquid fuels and valuable chemical products from lignocellulosic biomass. The zeolite ZSM-5 is a widely studied catalyst for the CFP process. However, its microporous structure may limit the diffusion of high molecular weight pyrolysis intermediates to its active sites. Mesoporous aluminosilicates such as Al-SBA-15 are promising materials with larger pore sizes that can overcome these diffusional limitations. Previous comparisons between mesoporous aluminosilicates and ZSM-5 for the CFP process have neglected the disproportionately high acidity of ZSM-5. In this study, an Al-SBA-15 catalyst has been synthesised with high acidity, comparable to that of a ZSM-5 catalyst with a Si:Al ratio of 15:1. The synthesised Al-SBA-15 catalyst was characterised by N2 physisorption, XRD and propylamine-TPD, and was compared to a ZSM-5 catalyst and a typical industrial equilibrium fluid catalytic cracking catalyst (e-FCC). All three catalysts were used at three different catalyst to biomass (C/B) ratios, to investigate the effect of varying concentrations of acid sites on the product distribution from the catalytic fast pyrolysis of beech wood. Interestingly, despite their dissimilar structural architectures, all three solid acid catalysts displayed similar reaction pathways towards the cracking of high molecular weight products such as levoglucosan and formation of intermediates including phenolics and furans. However, the selectivity towards the final catalytic products was dictated mainly by the structure of the catalysts. Despite their very similar surface area and acidity, the ZSM-5 exhibited high selectivity for the formation of desirable aromatic hydrocarbon products due to its shape-selective micropore structure, while Al-SBA-15 instead shifted the selectivity towards the formation of undesirable coke. The results highlighted the importance of catalyst shape-selectivity in the catalytic fast pyrolysis of biomass for the conversion of pyrolysis vapours into desirable products and the suppression of undesirable solid byproduct formation.
... Measurement and analysis of residence time distribution (RTD) provide necessary information pertaining to flow in industrial process systems (IAEA, 2008). Various authors have reviewed measurement of RTD of solid particles in laboratory-scale fluidized bed reactors (FBRs) (Berruti et al., 1988;Harris et al., 2002;Christophe et al., 2002). Different researchers have used different tracers such as chemical, magnetic, subliming, coloring and fluorescent tracers. ...
Radiotracer investigations were carried out for measurement and analysis of residence time distribution (RTD) of coal particles in a pilot-scale gasifier fitted with a flat air/steam distributor. Measurements were made at different operating conditions using gold-198 ( ¹⁹⁸ Au) labeled coal particles as a radiotracer. The measured RTDs were treated and mean residence times (MRTs) were determined. Furthermore, the treated RTDs were simulated using a suitably conceptualized mathematical model and detailed information about hydrodynamics of coal particles within the gasifier was obtained. Results of model simulation indicated that the gasifier behaved as an ideal mixer of fine coal particles exiting from the top of the gasifier. A small fraction of the coarser particles was found to be bypassing at ambient temperature.
... The use of correlation analysis is possible when simulating the process of mixing in vibration and centrifugal mixers [4,6]. We use a similar approach for a continuous drum mixer. ...
... To confirm the impact of inertia on particle entrapment, two successive numerical simulations with two particles of same diameter (40 μm), but made from different materials, steel (ρsteel=7800 kg/m 3 ) and sand (ρsand=2470 kg/m 3 [129]), are performed. The simulations show that the heavier the particle is, the more likely it will be entrapped. ...
Contact lubrication is essential in a wide range of mechanical systems like rolling element bearings (REBs). A minimum quantity of clean lubricant all along the bearing life is necessary but difficult to ensure. In fact, lubricants contain inevitably wear debris or external particles, like dust. Carried by the lubricant in the vicinity of elastohydrodynamic (EHD) contacts, particles can be entrapped with disastrous consequences for contacting surfaces. Entrapment of micrometric particles in submicrometric contacting gaps means irreversible damages for the surfaces. Damages weaken the surfaces and reduce significantly the REBs lifetime. The goal of this work is to analyze the critical particle entrainments in the contact inlet. Entrapment of steel spherical particles was investigated from the numerical and experimental point of view. Firstly, the phenomenology of entrapment was explored with a new experimental method based on Particle Image Velocimetry (PIV) technique installed on a tribometer. It enabled the evaluation of velocity profiles in the contact inlet and the tracking of particles within EHD contacts. Secondly, a numerical modelling of the inlet flow for EHD contacts, including the particle tracking, was developed. Finally, tests on a twin-disc machine with a controlled level of well-defined contamination were conducted to validate previous conclusions. A first set of results showed that particle entrapment is highly dependent on the lubricant velocity profile. Depending on contact geometry, from point to wide elliptical contacts, different entrapment probability were revealed. Surprisingly, increasing contact width with wide elliptical contacts leads to a drop of entrapped particles. It was demonstrated that this phenomenon is due to backflows occurring upstream from these contacts. Introducing a hybrid pair of contacting materials (silicon nitride–steel), dents on the surfaces due to entrapped particles were explored. It has been confirmed that silicon nitride surface offers a real ability to resist to indentation. It was also noticed that the entrapment probability for silicon nitride–steel contacts is equivalent to a steel–steel one.
... Although it may be useful to determine individual RTD of solid and liquid phases, it is fair to note that the existing analytical techniques to measure concentrations within solids are prone to severe measurement error and may not be truly relied upon to construct RTD response curve for the solid phase. A literature review indeed shows that most of the studies on G-S or L-S systems pertain to injection and measurement of tracer in the gas or liquid phase only [8][9][10]. Therefore, the results presented in the following section on the RTD data for the liquid (water) phase may be considered reasonably accurate enough to qualitatively corroborate the mechanistic steps proposed in Section 2.3 with regard to the types of solid flow patterns existing on the stage in two columns (new and conventional). ...
The design and development of a counter current multi-stage fluidized bed column for the removal of recalcitrant solutes such as fluorides, nitrates, arsenics and phosphates ions from wastewater are envisaged in this study. In treating wastewater using solid resins, the higher removal efficiency in multi-stage fluidized bed than that obtained in a single stage fixed and fluidized beds are experimentally demonstrated. In the proposed work, the many aspects of issues involved in the counter current multi-stage fluidized bed are studied. These include hydrodynamics, mass-transfer and scale-up. The design of the column includes column diameter, height of each stage, downspout size and water distributor. The hydrodynamics study predominantly focuses on determining loading/flooding criterion and the operating range of liquid-solid flows for proper fluidization on a single or multi-stage. The experimental results for determining the minimum fluidization conditions (flow rate, pressure drop and bed porosity) will be presented. The mass-transfer study will be carried out to determine the separation efficiency over a wide range of operating conditions under steady state such as different types of anions and cations such as fluorides, nitrates, arsenics and phosphates ions in waste water using stage-wise operation in fluidized beds. The effects of resin particle size and density on the hydrodynamics and mass-transfer will also be studied. The RTD analysis of the fluidized bed will be done to determine the channeling and mixing state of the particles in the bed.
Key words: Waste water, Ion-exchange, Fluidized bed, Multi-stage column, Counter-current flow
... Among the limited studies on the multiphase flow aspects of biomass fluidization, determination of characteristic fluidization velocities (Abdullah et al., 2003;Rao and Reddy, 2010;Rao and Bheemarasetti, 2001;Zhang et al., 2011), and the distribution and mixing pattern of biomass-inert materials (Berruti et al., 1988;Dos Santos and Goldstein, 2008;Shen et al., 2007;Yu et al., 2003) have been of particular interest to researchers. The impact of irregularly-shaped particles on the characteristics of the dilute and dense phases has received comparatively little attention, and it has generally been assumed that, due to the typically low ratio of biomass to sand in biomass processing units, the gas holdup in the bed is comparable to that of a bed of sand alone. ...
... Furthermore, rice husk is difficult to burn in a fluidised bed combustor where the continuous upcoming air or fluidising media force the particles to elutriate out of the system. The findings of Berruti et al. (1988) demonstrated that a low density biomass particle entrains from the fluidised bed and this depends on the fluidising velocity, particle size and density. Similarly, the cold study of various biomass particles and coal by Abdullah et al. (2003) concluded that the Geldart type B particles such as sawdust, shells, coal and bottom ash exhibited good fluidisation behaviour compared to that of Geldart type D particles (rice husk) and Geldart type A particles (palm fibres) which gave poor fluidisation quality. ...
The purpose of this study was to investigate the effect of bed height on the quality of rice husk ash in a 210-mm diameter pilot scale fluidised bed combustor. The degree of rice husk burning in the fluidised bed could be deduced from the temperature of the combustor and the particle size of the resulting ash. The turbulence in the bed would break down the char skeleton of the rice husk into finer size. From this study, the bed height of 0.5 Dc was found to give the lowest residual carbon content in the ash (1.9%) and the highest bed temperature (670°C). Moreover, the problem of contamination of amorphous rice husk ash with sand increased as the bed height was increased. Nevertheless, the results from the current study need to be validated in larger-scale fluidised beds to determine whether the bed height of 0.5 Dc is still applicable.
... It is fair to say that the existing analytical techniques to measure concentrations within solids are prone to measurement error and may not be truly relied upon to construct RTD response curve. A literature review indeed shows that most of the studies pertain to injection and measurement of tracer in the gas or liquid phase in G-S or L-S systems [19][20][21]. Only in recent years, thanks to the development of optical or other non-intrusive tomography techniques, including X-ray or IR, the visualization of solid voids and the measurement of solidvolume fractions in two-phase flow through packed columns have been made possible [22][23][24]. ...
In our recent work we carried out mass transfer study on the multi-stage fluidized bed ion exchanger column with solids and liquid flowing in the counter current directions and demonstrated improved separation efficiency of dissolved anions from waste water in comparison to that achieved in a fixed bed operation. In this study, we report the results pertaining to hydrodynamic study with a view to ascertaining the type of fluidization prevailing on the column's stage and the operating range of liquid and solid flow rates for the steady and stable operation of the column without loading or flooding with excess solid or water flow rates. Residence time distribution (RTD) study was carried out to investigate the extent of mixing on the stages. In addition, the experimental measurements for pressure-drops were made over a wide range of operating conditions including number of stages, height of the downspout on every stage, and the liquid and solid flow rates. Based on the data, empirical correlations were developed using scale-up analysis for predicting pressure-drop, bed porosity and average bed height during cross-flow fluidization apparently prevalent on the stage. The results in this study assume significance from the perspective of design and stable operation of staged fluidized bed ion exchangers.
... Although it may be useful to determine individual RTD of solid and liquid phases, it is fair to note that the existing analytical techniques to measure concentrations within solids are prone to severe measurement error and may not be truly relied upon to construct RTD response curve for the solid phase. A literature review indeed shows that most of the studies on G-S or L-S systems pertain to injection and measurement of tracer in the gas or liquid phase only [8][9][10]. Therefore, the results presented in the following section on the RTD data for the liquid (water) phase may be considered reasonably accurate enough to qualitatively corroborate the mechanistic steps proposed in Section 2.3 with regard to the types of solid flow patterns existing on the stage in two columns (new and conventional). ...
A hydrodynamic study was carried out on our patented radially cross-flow fluidized bed staged column, with the salt laden water and solid resins flowing counter-currently, to determine the loading/flooding criterion for a stable operation of the column, and also to ascertain the mal-distribution in flow. Residense time distribution measurements were taken to address the latter part of the study. The data collected from the hydrodynamic measurements show a wider range of liquid and solid flowrates that can be used for the stable operation of the column, in comparison to that obtained in the conventional multi-staged column. The extent of mal-distribution or channeling is also relatively smaller in the former. The observations are consistent with the mass-transfer results obtained in our recent study, where the separation of dissolved solids using the radially cross-flow ion exchange column was found to be 40% larger than that in the conventional column.
The characteristics of residence time distribution (RTD) of solid particles (Geldart B) with the pulse particle tracer approach in two cold rectangular bubbling fluidized beds (BFB) with continuous particle flow were experimentally investigated. The parameters tested were superficial gas velocity (U), feeding rate of particles (G s), particle bed height (H) and particle size (d p), and the used tracer was coal particles. The results showed that G s, H and dp were the major influential factors, while U affected little. The particle flow pattern in BFB reactor lied between those of the ideal plug flow reactor and completely stirred reactor (CSTR), it was close to the plug flow when the particle bed height was lower and G s, larger, whereas the flow was much closer to the full mixing flow of the CSTR at the higher particle bed height and smaller G s. The averaged residence time of particles was calculated as the value of 9%~18% below of the plug flow for the design of BFB based on the present experimental data.
This study proposes an innovative approach for the study of particle entrapment in rolling element bearings (REBs). Two couples of contacting materials were considered, the classical steel–steel and silicon nitride–steel used in hybrid bearings. Numerical simulations, as well as experiments, combine theoretical trajectories for incoming contaminant particles and effective entrapment ratios observed within a twin-disc machine. Linking both approaches allows the highlighting of some key parameters leading to particle entrapment under pure rolling conditions in elastohydrodynamic point contacts.
An accurate prediction of the minimum fluidization velocity is a crucial hydrodynamic aspect of the design of fluidized bed reactors. Common approaches for the prediction of the minimum fluidization velocities of binary-solid fluidized beds are first discussed here. The data of our own careful experimental investigation involving a binary-solid pair fluidized with water is presented. The effect of the relative composition of the two solid species comprising the fluidized bed on the bed void fraction at the incipient fluidization condition is reported and its influence on the minimum fluidization velocity is discussed. In this connection, the capability of packing models to predict the bed void fraction is also examined.
The mixing and segregation characteristics of model wastes such as a wood cylinder (ρ s=649 kg/ m 3) and an acrylic sphere (ρ s=1,120 kg/m 3) in a fluidized bed with an inclined distributor are investigated. In this experiment, the plenum chamber was divided into 12 sections so that the an flow rate distribution can be arbitrarily adjusted, and that particle circulation can be induced. After throwing a batch of model wastes, fluidization was kept for a given operation time. Then, the distribution of model wastes in the bed was investigated. Under the present experimental conditions, the model wastes were dispersed into the bed most evenly when the ratio of the higher superficial gas velocity to the lower u 0s/ u 0w was 2 and u 0w = u mf.
The gas distribution between the dilute (bubble) and dense (emulsion) phases of a fluidized bed is studied locally and globally in the bubbling regime for mixtures composed of sand and different weight fractions of biomass (2-16%). The dilute phase has been characterized by analyzing the pressure and voidage signals. A suite of pressure transducers was used to measure pressure fluctuations at different locations along the bed. A reflective optical probe measured local voidage signals and was placed at different radii (0<. r/. R<0.87) at a height of h=175. mm above the distributor plate. The mean voidage of the bed is increased with higher biomass loading, primarily because of dilution of the emulsion phase. Changing the quantity of biomass in the bed does not significantly affect the voidage of the bubble and emulsion phases. The void (bubble) fraction increases at the center of the bed, whereas it decreases and then increases at the wall region with increasing weight fraction of biomass. Higher quantities of biomass reduce the mean bubble size and boost the bubble frequency at the center of the bed. The core-annulus structure of the bed is intensified for mixtures with relatively low quantities of biomass, while increasing the biomass load leads to a more uniform distribution of small bubbles across the bed improving the fluidization quality. ?? 2013 Elsevier Ltd.
A two fluid model is extended to an N-phase, multi-fluid model, in which each particulate phase represents a collection of particles with identical diameter and density. The current N-phase model is applied to a fluidized bed classifier with six different particle sizes to investigate the effects of different operating conditions—fluidizing liquid flow rate, feed voidage, and particle size distribution in the feed stream—on the particle size distribution inside the classifier and the discharge streams. The predicted volume fraction of different particle sizes is compared with the experimental data reported by Chen et al. (2002) for two columns, 191 mm and 292 mm in diameter, each having different geometries and containing glass beads of different sizes fluidized with water. A fairly good agreement is observed between the measured and predicted values for mono- and poly-dispersed systems.
Biomass and bed material particles have been found to exhibit significant density differences during the operation of a circulating fluidized bed boiler. Further studies are required to analyze the mixed flow characteristics of these particles in a furnace. We built a test apparatus for a cold-operation, biomass-circulating fluidized bed and designed a space-storage sampler for furnace particle sampling based on modeling theory. The current study found that adding biomass particles nearly did not affect the flow characteristic and distribution of bed material particles because of density difference. However, biomass-particle enrichment phenomenon unexpectedly occurred in the lower furnace region and at the bottom of the top furnace outlet where biomass particle concentration was extremely low. Analyses and observations revealed that downward flow near the wall can effectively carry biomass particles to the bottom of the furnace, thus leading to the enrichment of biomass particles in the lower furnace region. The inhibition mechanism of dense, inert, bed material particles also limited the flow of light biomass particles into the upper furnace region. Different kinds of biomass exhibited similar particle distribution characteristics. Light biomass particles were enriched at the bottom of the furnace, and could hardly reach the furnace outlet.
Experimental investigation on RTD of solids is carried out in a single-stage fluidized bed provided with an internal, using uniformly sized particles and a binary solid mixture, varying gas flow rate, solids rate, bed height, dilution and the bed geometry. The effect of these variables on first and second moments as well as on F-curves has been determined. Using a binary solid mixture or an internal inside the bed is found to reduce backmixing of solids. The data is fitted to FTEM and the values of N obtained were compared for different variables.
Une étude expérimentale sur la distribution du temps de séjour des solides a été menée dans un lit fluidisé à un étage muni d'un interne; on a utilisé pour cela des particules de tailles uniformes et un mélange de solides binaire et fait varier le débit de gaz, la vitesse des solides, la hauteur de lit, la dilution et la géométrie du lit. L'effet de ces variables sur les premier et second moments, ainsi que sur les courbes F, ont été déterminés. On a trouvé que l'utilisation d'un mélange de solides binaire ou d'un interne à l'intérieur du lit réduisait le rétromélange des solides. Les données ont été calées au FTEM et les valeurs de N obtenues ont été comparées pour différentes variables.
The minimum fluidization behavior of five different binary-solid systems with a wide range of composition is experimentally investigated by carrying out slow defluidization of an initially fluidized bed. The size ratios of these binaries vary from 4 to 10 while their buoyed-density ratios vary from 0.22 to 0.52 such that their larger components are lighter and smaller ones are denser. The difference in the physical properties of the two constituent solid phases of the fluidized bed is found to strongly influence the evolution of the packing structure, and consequently the minimum fluidization velocity during the slow defluidization process. For binaries with the same size ratio, segregation increases with the decrease in their buoyed-density ratios. On the other hand, even for binaries with large difference in their densities, increasing the size ratio enhances the mixing. Depending upon the composition of the bed, a completely mixed defluidized structure sometimes develops for high size-ratio binaries. Finally, a simple correlation is proposed that can better describe the present minimum fluidization velocity data than other existing correlations.
The present study reports new experimental data on the expansion as well as the layer-inversion behavior of liquid-fluidized beds containing two particle species with almost 10-fold difference in the size and a significant difference in the density. The packing models, which are generally used for predicting the porosity of the packing of particle mixtures, are used here to describe the expansion and the phenomenon of the layer-inversion. Predictions of different averaging models, including the property-averaging model, the serial model, and various packing models, are compared with the present data as well as the layer-inversion data reported in the literature. The overall predictive capability of packing models is found to be superior.
Dense (or large) particles fall through liquid fluidized beds of light (or small) particles. In this work, a new contactor is described in which continuous, countercurrent transport of dense particles in a stationary, liquid fluidized bed of light particles is exploited to obtain selective and continuous transport of the dense phase. The system is refered to as the ‘trickle flow fluidized-bed reactor’. This sytem is evaluated in its application as a countercurrent adsorptive reactor. By selecting a suitable adsorbent as the dense phase, and a catalyst as the light phase, simultaneous reaction and countercurrent product removal may be achieved within one contactor. For a systematic design and optimization of such an adsorptive trickle flow fluidized-bed reactor, a predictive hydrodynamic model is required. In this work, the relation between the volume fractions and the fluxes of the three phases is investigated. A laboratory scale trickle flow fluidized-bed reactor has been constructed for experimental studies on hold-up and flux. A multi-component transport model to predict the volume fractions, fluxes and operating conditions is developed. The model is validated with the experimental hold-up and flux data from the trickle flow fluidized-bed reactor. To apply this concept as a countercurrent adsorptive reactor, a high countercurrent adsorbent flow, a high hold-up of the dense adsorbent and light catalyst are required, together with a stable operation. Hence, the reactor should operate at a minimum liquid fraction of approximately 50 vol% and a free area fraction of the supporting plate between 0.3 and 0.5. It is demonstrated that the system is very sensitive to the bulk density of the multi-component suspension. At a high hold-up of the dense particles, the bulk density of the bidisperse suspension may increase to a level which causes a complete wash-out of the light particles. Therefore, the difference in density between the light catalyst and dense adsorbent particles should be as small as possible.
A mathematical model, based on the core-annulus flow structure, is proposed to characterize the two-phase flow and the residence time distribution of solids within the riser of circulating fluidized beds. The model is tested against experimental data obtained in a laboratory unit utilizing a tracer technique. While the accuracy of the experimental technique is shown not to be sufficient to allow conclusive results, the model is capable of a good prediction of the observed trends of the characteristic bimodal residence time distributions with an excellent match of the times of arrival of the two peaks. Simulation runs have been performed to study the effects of key operating variables on the residence time distribution.
Biomass is important in energy conversion processes due to their favourable status with respect to greenhouse gas emissions. However, biomass particles have unusual properties which make them difficult to fluidize and handle. This paper reviews recent research on the hydrodynamics and mixing of biomass particles in fluidized beds. Whereas there has been considerable effort to develop new biomass gasification, combustion, pyrolysis and bio-conversion processes, relatively few authors have characterized the relevant flow characteristics of biomass particles in fluidized beds or investigated measures that could assist in resolving flow issues. The limited work that has been reported on biomass fluidization primarily treats means of achieving fluidization, mixing and segregation. Most of the work has been in low-velocity fluidized beds, although circulating fluidized beds are also important. Further research is needed to provide general understanding of interactions among heterogeneous particles and guidance on conditions that can lead to viable and sustainable processes.
In this paper we study stochastic models for the transport of particles in a fluidized bed reactor, and compute the associated residence time distribution (RTD). Our main model is basically a diffusion process in [0; A] with reflecting/absorbing boundary conditions, modified by allowing jumps to the origin as a result of transport of particles in the wake of rising fluidization bubbles. We study discrete time birth-death Markov chains as approximations to our diffusion model. For these we can compute the particle distribution inside the reactor as well as the RTD by simple and fast matrix calculations. It turns out that discretization of the reactor into a moderate number of segments already gives excellent numerical approximations to the continuous model. From the forward equation for the particle distribution in the discrete model we obtain in the diffusion limit a partial differential equation for the particle density p(t; x) @ @t p(t; x) = 1 2 @ 2 @x 2 [D(x)p(t; x)] Gamma @ @...
Regular circulation of the particulate phase can be induced in a shallow fluidized bed by means of an uneven distribution of fludizing fluid supplied to the base. Circulation is strongest with the particles traveling up at the walls and down at the middle of the bed, with a bed depth to bed width ratio around 0. 5. This ″Gulf Stream″ circulation provides a simple means of improving lateral mixing in shallow fluidized beds. Its application is discussed in relation to large fluidized bed reactors to which solid reactants are fed continuously.
A kinetically based prediction model for the production of organic liquids from the flash pyrolysis of biomass is proposed. Wood or other biomass is assumed to be decomposed according to two parallel reactions yielding liquid tar and ( gas + char) The tar is then assumed to further react by secondary homogeneous reactions to form mainly gas as a productThe model provides a very good agreement with the experimental results obtained using a pilot plant fluidized bed pyrolysis reactorThe proposed model is shown to be able to predict the organic liquid yield as a function of the operating parameters of the process, within the optimal conditions for maximizing the tar yields, and the reaction rate constants compare reasonably well with those reported in the literature
Elutriation characteristics of widely different solids (density from 920 to 5900 kg/m3) were measured in fluidized beds (up to 0.9 m in size) having high freeboard (7.5 m), using gas velocities up to 4 m/s.The experimental findings were compared with previously reported results and all the variables were well correlated with a simple empirical expression
A critical survey is given of the different models proposed for solids transport in gas-fluidized beds. It is shown that many of the models can fit the residence time distribution curve—the Ft-t curve—fairly well despite the fact that the physical behaviour of the bed is not recognized in these models. The intensity curve is recommended as a good tool for comparison of the models with the experiments.
A new mathematical model has been developed for the stirred tank reactor. The model assumes a circulating flow, created by the pumping action of the stirrer, and an isotropic homogeneous turbulence in the circulated fluid. For a continuously operating stirred reactor expressions are derived for the conversion of a first-order reaction, the spread in residence times and the distribution function. It is shown that at high circulation/feed rate ratios and/or at high turbulent diffusivities the model approaches the ideal mixer. Correlations are proposed for the circulation rate parameter and the turbulent diffusivity parameter. The mixing process in a stirred batch reactor is evaluated and described as a function of time and location. The expressions are useful for the correlation of some experimental results.
A bench-scale continuous flash pyrolysis unit using a fluidized bed at atmospheric pressure has been employed to investigate conditions for maximum organic liquid yields from various biomass materials. Liquid yields for poplar-aspen were reported previously, and this work describes results for the flash pyrolysis of maple, poplar bark, bagasse, peat, wheat straw, corn stover, and a crude commercial cellulose. Organic liquid yields of 60-70% mf can be obtained from hardwoods and bagasse, and 40-50% from agricultural residues. Peat and bark with lower cellulose content give lower yields. The effects of the addition of lime and of a nickel catalyst to the fluid bed are reported also. A rough correlation exists between has content and maximum organic liquid yield, but the liquid yield correlates better with the alpha-cellulose content of the biomass. General relationships valid over all reaction conditions appear to exist among the ratios of final decomposition products also, and this correlation is demonstrated for the yields of methane and carbon monoxide.
A continuous fluidized bed bench scale flash pyrolysis unit operating at atmospheric pressure and feed rates of about 15 g/h has been successfully designed and operated. A unique solids feeder capable of delivering constant low rates of biomass has also been developed. Extensive pyrolysis tests with hybrid aspen-popular sawdust (105–250 μm) have been carried out to investigate the effects of temperature, particle size, pyrolysis atmosphere and wood pretreatment on yields of tar, organic liquids, gases and char. At optimum pyrolysis conditions high tar yields of up to 65% of the dry wood weight fed are possible at residence times of less than one second.
On a conçu et employé avec succès, à l'échelle du laboratoire, une unité de pyrolyse-éclair à lit fluidisé continu; le dispositif fonctionnait à la pression atmosphérique et à des débits d'alimentation d'environ 15 g/h. On a aussi mis au point un dispositif unique d'alimentation en matières solides, capable d'assurer de faibles débits constants de biomasse. On a fait des expériences poussées de pyrolyse sur des sciures d'hybrides de peuplier-faux tremble (105—250 μm), dans le but d'étudier les effets de la température, de la granulométrie des particules, de l'atmosphère de la pyrolyse et d'un traitement préalable du bois sur les rendements en goudron, liquides organiques, gaz et matières carbonisées. Il est possible, dans les conditions optimales de pyrolyse, d'obtenir des rendements élevés en goudron, qui peuvent atteindre 65% du bois sec d'alimentation en poids pour des temps de séjour de moins d'une seconde.
A 0.27 m diameter fluidized bed reactor has been designed to allow experimental measurement of the axial and radial mixing behaviour of the solids.
A unique method has been developed which permits the continuous determination of solid tracer concentration with time at different radial and axial positions within the fluidized bed.
Solids mixing has been described by a model in which vertical mixing is instantaneous and lateral mixing occurs by dispersion.
The lateral solids dispersion coefficients have been evaluated at various operating conditions from the experimental results of tracer concentration versus time. Based on the results, a modification of an existing correlation is proposed.
A continuous atmospheric pressure flash pyrolysis process for the production of organic liquids from cellulosic biomass has been demonstrated at a scale of 1–3 kg/hr of dry feed. Organic liquid yields as high as 65–70% of the dry feed can be obtained from hardwood waste material, and 45–50% from wheat straw. The fluidized sand bed pyrolysis reactor operates on a unique principle so that char does not accumulate in the bed and treatment of the sand is not necessary. The product gas, about 15% of the yield, has a medium heating value. The liquid product is an acidic fluid, which pours easily and appears to be stable. A preliminary economic analysis suggests that if the pyrolysis oil can be used directly as a fuel, its production cost from wood waste is probably competitive with conventional fuel oil at the present time.
On a fait la démonstration d'un procédé continu de pyrolyse éclair à l'échelle de l à 3 kg/h d'alimentation sèche, à la pression atmosphérique, pour la production de liquides organiques à partir d'une biomasse cellulosique. On peut obtenir des rendements en liquide atteignant 65 à 70% de l'alimentation sèche à partir de déchets de bois dur et de 45 à 50% à partir de paille de blé. Le réacteur de pyrolyse, à lit de sable fluidisé, fonctionne sur un principe unique, de sorte que le charbon ne s'accumule pas dans le lit et qu'il n'est pas nécessaire de traiter le sable. Le produit gazeux, qui correspond à environ 15% de rendement, a une valeur calorifique moyenne. Le produit liquide est un fluide acide qui se déverse facilement et semble assez stable. Une analyse économique préliminaire indique que, si l'on peut employer directement l'huile de pyrolyse comme combustible, son coǔt de production à partir de déchets de bois est probablement compétitif actuellement avec celui de l'huile combustible classique.
Thesis (Ph. D.)--University of Waterloo, 1986. Includes bibliography.
Transverse mixing of particles in a fluidized bed with small ratio of the height of the bed to the linear transverse dimension
- Gel'perin
Residence time and dispersion of fine particles in a 2-D fluidized bed of large particles
- Valenzuela
Transport and residence time of particles in a shallow fluidized bed
- Heertjes