Article

Quantification of solids flow in a gas-solid riser: Single radioactive particle tracking

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Abstract

Solids in risers of circulating fluidized beds (CFB) exhibit local backflow and recirculation. Measurement of the concentration-time response to an impulse injection of tracer, even at two elevations cannot determine the residence time distribution (RTD) of solids uniquely. Hence, evaluation of RTD in risers from conventional tracer responses is difficult and often not possible. In addition, estimating the solids circulation rate in these closed loop systems, is a non-trivial problem. In this work, a single radioactive particle in the CFB loop is tracked during its multiple visits to the riser and, by invoking ergodicity, solids circulation rate, accurate solids RTD and additional information on the solids flow pattern in the riser are estimated. A calibration curve was established for the overall solids mass flux as a function of superficial gas velocity. A second peak in the probability density function (PDF) of the solids RTD curve in the riser was observed for operating conditions in the fast-fluidization regime.

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... Ambler et al. [2], Bhusarapu et al. [3] and Mostoufi et al. [48] carried out a series of solid RTD experimental studies using a radioactive pulse tracer technique. Bhusarapu et al. [3] used 46 Sc as tracer for measuring overall solid mass flux and solid RTD for the entire riser. ...
... Ambler et al. [2], Bhusarapu et al. [3] and Mostoufi et al. [48] carried out a series of solid RTD experimental studies using a radioactive pulse tracer technique. Bhusarapu et al. [3] used 46 Sc as tracer for measuring overall solid mass flux and solid RTD for the entire riser. The Scandium particles were coated with a layer of polymer (Parylene N) to have the same density as the solids used (Glass beads). ...
... Radioactive tracer Bhusarapu et al. [3] Riser of CFB A single radioactive particle ( 46 Sc) was used as a tracer. It was tailored by coating a layer of polymer (Parylene N) on the Scandium particle to achieve the same density as the solids used (glass beads) and the same diameter (150µm) as the mean particle size of glass beads. ...
Thesis
Full-text available
The aim of the present thesis is to develop a novel experimental technique for determining the residence time distribution (RTD) of solid particles in solid unit operations as well as model development. Initially, a novel optical method was developed to measure the particle RTD. Experiments are carried out with Silicon Carbide (SiC) and the pigment phosphorescent (Lumilux® Green SN-F50 WS) as tracer particle. A preliminary experimental study was conducted in a simple bubbling fluidized bed in order to validate the proposed RTD measurement methodology. In the second step, the developed technique of the concentration measurement was applied to measure the RTD of a deep fluidized bed. The particle RTD curves are determined experimentally in different operating conditions. Finally, a model consisting of the combination of the ideal reactors is proposed to predict the particle residence time distribution in the studied fluidized bed. The predicted output values are then compared with the experimental data to establish a good model fitting data
... 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: ...
... Based on the background summarized above, we hypothesize that it should be possible to combine a simplified reactor modeling approach such as that proposed by Liden et al (1988) with appropriate global reaction kinetics (modified for particle size and moisture) and low-order particle RTD functions to simulate the expected trends for fast pyrolysis in both bubbling bed and circulating bed reactors. To increase our confidence in the reliability of the RTD functions, we evaluated the ability of each function above to fit the experimental RTD results for Group B particles in both bubbling and circulating fluidized beds reported by the following investigators: 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), and Andreaux et al (2008). We observed that all of the above RTD functions can give reasonably close agreement to the reported RTDs using appropriate parameter values, especially considering the significant measurement errors that were explicitly reported or implied. ...
... For solids RTD in CFB risers, some significant discrepancies arose among numerous publications, and even seemingly contradictory conclusions were drawn by various researchers. First of all for the shape of RTD, some researchers observed the occurrence of a double-peak (Ambler et al., 1990;Bhusarapu et al., 2004a;Kojima et al., 1989;Lin et al., 1999;Patience et al., 1991;Wei et al., 1998;Weinell et al., 1997) or even multi-peak (Bhusarapu, 2005;Wang et al., 1996), whereas more researchers only measured a typical single-peak curve (Andreux et al., 2008;Bader et al., 1988;Harris et al., 2003c;Rhodes et al., 1991;Smolders and Baeyens, 2000;Viitanen, 1993;Zheng et al., 1992). Secondly, as the two widely used parameters to quantitatively measure solids RTD, solids dispersion coefficient, D, and Peclet number, Pe, have a huge discrepancy in magnitude for the tests measured in risers. ...
... Most of them may be not adequate for a fast response system or a circulating particulate system like a CFB riser. The most striking contradictions among RTD experimental studies include: (1) tracer particles do not have the identical material properties to the rest particles in the bed (Ambler et al., 1990;Andreux et al., 2008;Chesonis et al., 1990b;Du and Wei, 2002;Godfroy et al., 1999;Guío-Pérez et al., 2014;Patience and Chaouki, 1995;Patience et al., 1991;Rhodes et al., 1991;Smolders and Baeyens, 2000;Wang et al., 1996;Weinell et al., 1997); (2) boundary conditions at the inlet and outlet of tracer cannot be guaranteed to be closed-closed type (Bhusarapu et al., 2004a;Harris et al., 2003c); and (3) the duration of tracer injection in the pulse-response RTD experiment, which should be like a delta function, is too long (Chesonis et al., 1990a;Guío-Pérez et al., 2014;Patience et al., 1991;Smolders and Baeyens, 2000). In addition, a diverse set of models are applied to interpreting the measurements, leading to a variety of parameters. ...
... Thirdly, various segments of riser have been regarded as the test area by researchers with a some- CARPT computer automated radioactive particle tracking CFB circulating fluidized bed CFD computational fluid dynamics CPFD computational particle fluid dynamics DEM discrete element method LDV laser Doppler velocimeter PEPT positron emission particle tracking PRT particle radioactive tracking RTD residence time distribution what casual decision. Most of the earlier studies on solids RTD investigated solids mixing of the CFB riser as a whole (Andreux et al., 2008;Bader et al., 1988;Bhusarapu et al., 2004a;Chesonis et al., 1990a;Rhodes et al., 1991;Viitanen, 1993;Weinell et al., 1997;Yan et al., 2009;Zoonen, 1962). In this case, tracer injection was typically conducted at solids inlet or not far from gas distributor, and tracer detection was near the riser exit. ...
Article
Solids Residence Time Distribution (RTD) in Circulating Fluidized Bed (CFB) risers has received increasing attention due to its vital role in determining the operating condition, particle property, and reactor geometry in industrial CFB applications. In recent years, various solids RTD experimental techniques and theoretical models have been utilized and proposed to study CFB risers. Some controversial issues, however, also arose in the open publications. By means of exploring the advantages and disadvantages of each available RTD experimental technique and model when they are applied to particles in CFB risers, this study discussed the primary causes leading to the huge discrepancy in magnitude of solids dispersion coefficient and Peclet number, which can achieve 4 orders or span from 1 to 100. On the basis of the massive experiment data collected from the literature, the variations of average residence time, Peclet number and dispersion coefficient of solids with superficial gas velocity, solids mass flux and solids concentration were presented. By applying the transition of flow regime in CFB mode, we provided a helpful way to explain some existing contradictions in the reported effects of operating conditions on solids RTD. The possible reasons were also summarized to clarify why some researchers measured a double- or multi-peak solids RTD curve and the others could not in a similar situation.
... In this paper, two different CFB risers are simulated. The first one is operated with FCC particles 54 and the second one is with glass beads 55 . The detailed experimental setups are shown in Figure 1. ...
... The axial pressure profiles is measured with pressure probes at an acquisition frequency of 20 Hz. For the glass beads riser, the RTD and radial solids velocity profiles are measured by tracking a single radioactive particle 55 . The particle-based Reynolds number is in the range of 1 to 20 for both Geldart type A and B particles used in this study. ...
Article
Full-text available
For a long time, salt tracers have been used to measure the residence time distribution (RTD) of FCC particles. However, and due to limitations in experimental measurements and simulation methods, the ability of salt tracers to faithfully represent RTD’s has never been directly investigated. Our current simulation results using coarse grained CFD-DEM with filtered drag models show that the residence time of salt tracers with the same terminal velocity as FCC particles is slightly larger than that of FCC particles. This research also demonstrates the ability of filtered drag models to predict the correct RTD curve for FCC particles while the homogeneous drag model may only be used in the dilute riser flow of Geldart type B particles. Thus, the RTD of large scale reactors can be efficiently investigated with our proposed numerical method as well as by using the old-fashioned salt tracer technology.
... As such, a characterisation method that accounts for size should be implemented. Reactor characterisation studies present in the literature propose a number of possible solutions such as assuming that solids of different sizes travel at the same speed, using detectable radioactive or fluorescent tracers or even using laser diffraction (Pareek et al., 2001;Harris, Davidson and Thorpe, 2003;Bhusarapu, Al-Dahhan and Dudukovic, 2004;Sievers et al., 2016). Whilst no reports were found in the literature showing the implementation of laser diffraction to determine particle travel time based on size, this technique could be applied to characterize the ACR. ...
... Whilst soluble tracers with assumed rheological equivalence are used often, radioactive or phosphorescent particles have also been employed. A disadvantage of the latter is the time and effort required to ensure that the selected particles have the same properties as those being studied (Harris, Davidson and Thorpe, 2003;Bhusarapu, Al-Dahhan and Dudukovic, 2004;Gao, Muzzio and Ierapetritou, 2012). One study by Pareek et al. (2001) actually used laser diffraction to determine titania Time (min) powder contents at the outlet of a three phase annular bubble column reactor, but no investigation in the RTDs of different particle sizes was carried out (Pareek et al., 2001). ...
Conference Paper
Valorisation of agricultural wastes, such as Sugar Beet Pulp (SBP), for production of biofuels and value-added chemicals, has garnered increasing interest in recent years. Through physicochemical means, lignocellulosic material can be pretreated to release monosacharides which can then be upgraded by fermentative and biocatalytic routes. Previous UCL-led research has examined many aspects of utilisation of waste streams from sugar refineries. Vinasse, a glycerol-rich waste product of bioethanol production, was used as a nutrient source for enzyme production. Sugars from SBP, such as D-glucose, L-arabinose and D-galacturonic acid and which make up approximately 25% w/w, 21% w/w, and 20% w/w of the total pulp weight, respectively, were solubilised through operations such as steam explosion pretreatment and depolymerisation of the released polysaccharides. These SBP monosaccharides were then employed in bioconversion reactions using thermostable enzymes. This Thesis aims to study SBP as a feedstock for the enzymatic production of value added chemicals. It also aims to translate key reactions in the valorisation process from batch mode into a continuous flow process in a scalable, 100 mL, Agitated Cell Reactor (ACR). Initial Residence Time Distribution characterisation of the ACR showed that it provided excellent plug flow properties, equivalent to 13 stirred reactors in series. The ACR was able to handle SBP slurries over a range of solids loadings (1% w/v – 5% w/v) and residence times (3.8 min – 19.0 min). The SBP suspension was shown to be shear thinning with measured viscosities in the range of 0.0011 Pa.s at 1% w/v and 0.0339 Pa.s at 10% w/v. A set of correlations was developed that enable prediction of the feed viscosity as a function of SBP concentration and shear rate. The SBP particle size distribution ranged from 15.0 μm (D10) to 446 μm (D90) with a median size of 128 μm. Studies on the particle flow through the ACR demonstrated that steady state could be achieved, but that larger particles had longer residence times than smaller particles through the ACR. Dilute acid pretreatment (DAP) of SBP was investigated as an alternative to previous work on steam explosion as it would be more compatible with continuous operation. DAP using sulfuric acid at concentrations up to 75 mM and 80 °C was performed. These conditions showed good release of polymeric L-arabinose, which increased with higher temperatures and acid concentrations (70% w/w at 75 mM and 80 °C). Cellulose, which is more heat- and acid-resistant than SBP pectin, was only slightly hydrolysed into D-glucose, creating the potential for selective sugar fractionation. When compared to steam explosion pretreatment, flow DAP in the ACR obtained similar throughputs (3.5 and 3.1 g(L-arabinose).hr⁻¹, respectively), but productivity (throughput in terms of reactor volume) was an order of magnitude higher (3.5 and 25.6 g(L-arabinose).L⁻¹.hr⁻¹). Monomerisation of the polymeric L-arabinose could be achieved in a continuous flow enzyme-membrane as in previously described work. Finally, valorisation of the L-arabinose monomers by a continuous-flow, two-step enzymatic process in the ACR was demonstrated. L-gluco-heptulose is a rare ketoheptose which has potential cancer and diabetes treatment applications. The one-pot two-step production of L-gluco-heptulose using a thermostable transaminase (TAm) and transketolase (TK) both isolated from Deinococcus geothermalis DSM11300 was also carried out in the ACR. The initial goal was to use immobilized TK and TAm enzymes in order to intensify the bioconversion process. While TK could be successfully immobilized on both Nickel-chelated beads and Epoxymethacrylate resin, the TAm immobilization proved challenging with only low levels of retained activity. Consequently, the flow studies were performed with soluble TK and TAm enzymes. ACR bioconversions compared favourably with well-mixed batch reactions yields using the same reaction time (2 hours). Initial studies demonstrated the conversion of model substrates L-arabinose, L-serine and α-ketoglutaric acid into L-gluco-heptulose. Subsequently it was shown that L-gluco-heptulose could be synthesised equally well using SBP-derived L-arabinose. Concentrations of the intermediate product hydroxypyruvic acid (HPA) and L-gluco-heptulose obtained in continuous mode were 2.62 mM and 0.60 mM, respectively using SBP derived L-arabinose and 1.21 mM and 0.31 mM, respectively, using model solutes. This was equivalent throughputs of 170.5 µM.hr⁻¹ and 39.0 µM.hr⁻¹ for the SBP derived L-arabinose and 81.0 µM.hr⁻¹ and 20.0 µM.hr⁻¹ for the model solutes. Higher final L-gluco-heptulose concentrations could be obtained by increasing starting L-arabinose concentrations. The continuous process demonstrated here has clear potential for use within a SBP biorefinery. Future work needs to focus on alternative methods of TAm immobilization to enable process intensification and scale-up to pilot scale in order to demonstrate robust commercial operation.
... A schematic of the RPT set up is shown in Fig. 1. The general principle of RPT comprises two steps of measurements (Bhusarapu et al., 2004;Moslemian, 1987;Moslemian et al., 1992;Upadhyay et al., 2012;Jain et al., 2014). The first step is called the calibration step, in which the distance-count map is generated for each detector by placing the gamma-emitting radioactive particles at several known positions inside the process column. ...
... However, due to its long half-life and hygroscopic property, it may not be a preferred radioisotope for the RPT technique. A 46 Sc radioisotope having high neutron cross-section (σ: 27 b) for its production, suitable gamma energy, moderately long half-life and flexibility of production in various chemical forms is widely used for RPT studies (Devanathan, 1990;Degaleesan, 1997;Larachi et al., 1995;Rammohan, 2001;Roy et al., 2002;Bhusarapu, 2004;Upadhyay, 2010;Jain et al., 2017;Kamalanathan et al., 2017;Kalo et al., 2019b). ...
Article
Radioactive particle tracking (RPT) technique is a relatively newer technique for the characterization of flow of process materials (liquids, solids) in laboratory- and pilot-scale industrial systems. The technique uses a single particle labelled with a suitable radioisotope having similar physical properties to that of the bulk of the process material. The preparation of a representative radioactive microparticle is a challenging task in the implementation of the technique. There are no standard methods available for the preparation of radioactive microparticles. This paper discusses some of the methods of preparation of radioactive microparticles for RPT studies. A few examples of RPT applications using the prepared microparticles are also discussed.
... Different researchers have used different polymeric material for coating. 11,15 In the present study, the microspheres were coated with a composite of polypropylene (PP) and submicron sized iron particles. A known quantity of iron particles was mixed with the PP matrix with xylene as the solvent. ...
Article
This paper describes a novel approach for synthesis of scandium oxide (Sc2O3) microspheres based on sol-gel technique of internal gelation followed by neutron activation in a nuclear reactor to produce 46Sc for use as radiotracers in radioactive particle tracking (RPT) experiments. The synthesis of scandium oxide microspheres of regular and spherical shapes essentially consists of formation of concentrated colloidal sol of the Sc(OH)3 and transformation of this sol into a semi rigid gel followed by heating. The synthesized microspheres were characterized by X-ray diffraction, scanning electron microscopy, thermogravimetry analysis/differential thermal analysis, and surface area analyses. The neutron-activated microspheres were successfully used in RPT experiments to investigate hydrodynamics of solids in a pilot-scale liquid-solid fluidized bed.
... The solid phase concentration measurement of gas-solid two-phase flow is one of the most difficult parameters to be measured, which has always been the research focus and difficulty in industrial detection over the last decade [1]. Recently, many sensors, such as capacitance sensor used in process tomography [2,3], electrostatic sensor [4,5], optical sensor [6,7], microwave sensor [8,9], and thermal sensor [10,11], have been adopted in the research on gas-solid two-phase flow. A variety of data processing methods have been used in the measurement system to improve the accuracy, including neural network technique [12,13], soft-sensing technique [14], and data fusion method [15]. ...
Article
Full-text available
This paper mainly investigates the mass flow measurement of the gas-solid two-phase flow in pneumatic conveyor. A new data fusion method based on the thermal sensors is proposed, which can improve the overall accuracy of the flow rate of the gas-solid two-phase flow and the time resolution, that is, the overall response rate of the system. Based on this method, a model fusion used in time domain is obtained. Several examples are given to illustrate the advantages of the proposed method.
... In the reconstruction process, a reconstruction algorithm is applied on the actual tracking data to reconstruct the instantaneous positions of the tracer with the aid of the calibration data. Different reconstruction algorithms have been reported in the literature for such a purpose as follows: Data Reduction Scheme [10,14,15], Monte Carlo-Based Search Method [16,17], Neural Network-Based Method [18], and Cross-Correlation-Based Search Method involving Semi-Empirical Model [19]. Finally, the output of the reconstruction process is the Lagrangian time-series data of the tracer (i.e., time differentiation of instantaneous position). ...
Article
Full-text available
In the past two decades, the radioactive particle tracking (RPT) measurement technique has been proven to visualize flow fields of most multiphase flow systems of industrial interest. The accuracy of RPT, and hence the data obtained, depend largely on the calibration process, which stands here as a basis for two subsequent processes: tracking and reconstruction. However, limitations in the RPT calibration process can be found in different experimental constrains and in assumptions made in the classical Monte Carlo approach used to simulate number of counts received by the detectors. Therefore, in this work, we applied a GEANT4-based Monte Carlo code to simulate the RPT calibration process for an investigated multiphase flow system (i.e., gas–liquid bubble column). The GEANT4 code was performed to simulate the number of counts received by 28 scintillation detectors for 931 known tracer positions while capturing all the types of photon interaction and overcoming solids’ angle limitations in classical approaches. The results of the simulation were validated against experimental data obtained using an automated RPT calibration device. The results showed a good agreement between the simulated and experimental counts, where the maximum absolute average relative deviation detected was about 5%. The GEANT4 model typically predicted the number of counts received by all the detectors; however, it over-estimated the counts when the number of primary events applied in the model was not the optimal.
... The tracer trajectory was also used to characterize the particle-wall contacting parameters and the friction factor [178][179][180]. Based on the particle backflow phenomenon in a riser, Bhusarapu et al [105,146,181] measured the solids circulation rate, overall solids flux and turbulent parameter profiles. More dynamic properties were characterized by processing the RPT signals through statistical analysis, nonlinear dynamics analysis, symbolic dynamics analysis and data mining [182][183][184]. ...
Article
Gas–solid fluidization is a well-established technique to suspend or transport particles and has been applied in a variety of industrial processes. Nevertheless, our knowledge of fluidization hydrodynamics is still limited for the design, scale-up and operation optimization of fluidized bed reactors. It is, therefore, essential to characterize the two-phase flow behaviours in gas–solid fluidized beds and monitor the fluidization processes for control and optimization. A range of non-intrusive techniques have been developed or proposed for measuring the fluidization dynamic parameters and monitoring the flow status without disturbing or distorting the flow fields. This paper presents a comprehensive review of the non-intrusive measurement techniques and the current state of knowledge and experience in the characterization and monitoring of gas–solid fluidized beds. These techniques are classified into six main categories as per sensing principles, electrostatic, acoustic emission and vibration, visualization, particle tracking, laser Doppler anemometry and phase Doppler anemometry as well as pressure-fluctuation methods. Trends and future developments in this field are also discussed.
... 50% value of the cumulative solids residence time distribution), whereas a majority of investigators (Harris et al., 2003;Patience et al., 1991) considered t as the mean residence time (i.e. the first moment of the RTD curve). In addition some investigators even used the superficial gas velocity in place of u s in Eq. (28) (Bhusarapu et al., 2004;Rhodes et al., 1991;Smolders and Baeyens, 2000), under the assumption that Peclet number can be transformed if keeping L and D s unchanged. In fact for one solids RTD curve, only one convective velocity u s and one diffusion coefficient D s can be determined. ...
Article
Solids residence time distribution (RTD) in circulating fluidized bed risers is a critical parameter for evaluating reactor performances, however, it is still very difficult to be predicted via computational fluid dynamics (CFD) simulation due to the complexity of particle clustering phenomenon. This paper tries to establish an effective CFD model to reasonably predict solids RTD of gas-solids riser flows by means of properly addressing the paramount role of particle clusters in determining solids RTD. The gas-solids hydrodynamic characteristics were solved by Eulerian-Eulerian model, where an energy minimization multi-scale (EMMS) drag model was applied to modify the gas-solids drag force to account for the influence of particle clusters. The motion of tracer particles was calculated using species transport equation, where the diffusion coefficient of particles, a vital parameter indicating particle diffusion capacity, was investigated thoroughly. The established CFD model was validated against the available experimental data in the literature. It was shown that axial profiles of solids volume fraction and radial profiles of solids mass flux can be well predicted with EMMS drag model, but not with homogeneous drag model. The proper prediction of bed hydrodynamics is also very crucial to the success of solids RTD simulation. On the other hand, the effect of the diffusion coefficient of particles, the magnitude of which can span a range from 10-5m2/s to 10m2/s, is minor when compared with the convective transport mechanism, at least for the specific cases we studied. In addition, the importance of the sampling time resolution and tracer injection time for a RTD curve was addressed. The simulation results showed that a low time resolution often results in the loss of some micro-scale information, i.e. drastically smoothing the fluctuations of the RTD curve, and an inappropriate assessment of the tracer injection time can lead to a significant change of the RTD curve.
... 3 Though the riser offers better gas solids contacting as compared to that of fluidized bed reactor, it is well documented that it suffers from solids backmixing at walls under high flux operation conditions. 10,11 Also, counter-gravity flow sets up instabilities resulting in locally high slip velocities which leads to the nonuniform distribution of the catalyst particles, and in turn the desired products. Roy et al. 12 proposed a phenomenological model for the partial oxidation of n-butane in a gas-solids riser reactor using steady state kinetics, 13 as well as unsteady state kinetics. ...
Article
Selective partial oxidations represent an important class of reactions in the process industry. Of particular interest is the partial oxidation of n-butane to maleic anhydride (MAN), which is arguably the largest commercialized alkane partial oxidation process. Partial oxidation of n-butane, which uses vanadium phosphorous oxide (VPO) as a heterogeneous catalyst, is believed to operate through a unique mechanism in which lattice oxygen oxidizes n-butane selectively to MAN. Past work has shown that performing partial oxidation reactions in gas–solids riser configuration is realizable and commercially viable, which has lead to commercialization of this technology in the last decade. Though the riser configuration allows optimal and independent control of the oxidation and reduction steps, the riser unit suffers from solid backmixing at walls, which in turn result into lower conversion, nonoptimal selectivity and diminished overall yield of desired product. In recent years, there has been growing interest in downers involving cocurrent downflow of both solids and gas phases, hence offering relatively uniform flow characteristics. In this contribution, we explore through modeling the implications of effecting partial oxidation reactions in a downer (gas–solids cocurrent downflow) compared to that in a conventional riser reactor (gas–solids cocurrent up flow) operated under equivalent operating conditions. Further, we explore the operational space of downers for these reactions, suggesting ways for improving the productivity of downer for partial oxidation applications. © 2009 American Institute of Chemical Engineers AIChE J, 2010
... To eliminate the contribution to the tracer dispersion due to sections outside the packed bed, experiments were also performed with a short bed (L = 0.07 m). This deconvolution is permitted because the convection-dispersion equation is linear in concentration, and because the convective fluxes dominate the disperse fluxes at the boundary of deconvolution, i.e., 7 cm into the packed bed [14]. This last requirement was checked by using a colored dye injection: no pulsation or back-andforth movement of the tracer band was observed. ...
Article
Liquid holdup and dispersion are reported for a column of 2 mm internal diameter, filled with 0.1 mm spherical particles, for multiphase flows with hydrocarbon liquid flow rates of 10-100 mu L/min and nitrogen gas flow rates of 50-1000 mu L/min using different tracers with varying diffusion coefficients and vapor pressures. It was found that the liquid holdup (liquid volume/external void volume) was between 0.65 and 0.85, with variations between different experiments and limited impact of flow rate on the holdup. The dispersion characteristics were very similar to single-phase dispersion. The particle Peclet number for dispersion was close to 0.2. This value was of the same order of magnitude - just a factor of two to three lower - as the value that was obtained without gas flow. Tracer volatility did cause the tracer to elude earlier, but did not cause significant additional dispersion. The results suggest that the fluid mechanical interaction between the gas and the liquid was very limited.
... For experimental measurements, a certain amount of tracers were introduced into the apparatus, and then the solid residence time was determined via detecting tracer signals at the system exits [26,27]. Using this incentive-response technology of the tracer, researchers carried out many studies about solid residence time in the CFB riser [28][29][30][31][32][33][34][35]. The results showed that the solid residence time distribution could be descript by a plug flow with dispersion [34], and the solid residence time distribution curves had a sharp peak with a long tail [31]. ...
Article
The effect of superficial gas velocity (Uf) on solid behaviors in a full-loop circulating fluidized bed is numerically studied using computational fluid dynamics-discrete element method (CFD-DEM). Specifically, the solid mixing and dispersion, solid residence time, solid force and velocity, and particle granular temperature are comprehensively explored. The results show that increasing Uf slows the solid mixing. The solid axial dispersion in the riser is dominated among that in the three regions. Besides, increasing Uf decreases the solid cycle time and leads to a more uniform solid residence time (SRT) distribution in the riser and dipleg. The fluid force is an order of magnitude smaller than the collision force. Increasing Uf shows distinctive influences on the rotational speed and translational velocity in the three regions. Moreover, increasing Uf suppresses the particle granular temperature. The average particle granular temperatures in the riser at four superficial gas velocities (i.e., 5.5 m/s, 6.0 m/s, 6.5 m/s, and 7.0 m/s) are 0.09326 m²/s², 0.08040 m²/s², 0.06531 m²/s², and 0.05047 m²/s², respectively.
... Solid residence time (SRT) is a quantitative criterion to assess the solid back-mixing and the degree of chemical reaction in the CFBs Chen et al., 2017;Hua et al., 2014;Shi et al., 2015). In experiments, the incentive-response technology of tracer is usually used to obtain the time-related information (Bhusarapu et al., 2004;Guío-Pérez et al., 2014;Harris et al., 2003;Hua and Wang, 2018). However, the experimental methods suffer from high cost and hard operation, thus the numerical simulation is expected to be an important supplement. ...
Article
Gas-solid distribution is non-uniform in the circulating fluidized bed (CFB), which deteriorates gas and solid mixing, causing a lower system performance. In this work, the effect of ring baffles on system performance of a full-loop CFB is comprehensively studied using the computational fluid dynamics-discrete element method (CFD-DEM). Detailed information (i.e., solid velocity, solid flux, solid circulation rate, solid inventory height, solid residence time, and solid dispersion) are obtained and analyzed. Meaningful results include: (i) ring baffles improve the inter-particle and inter-phase interactions, and solid particles become more uniformly dispersed in the riser; (ii) introduction of ring baffles increases the solid holdup in local regions of the riser while has a slight influence on pressure drop; (iii) increasing the number of ring baffles and the spacing between ring baffles increases the solid circulation rate and solid inventory height, decreases solid cycle time in the system and SRT in the riser, increases SRT in the dipleg, and has a slight impact on SRT in the cyclone; (iv) the average solid dispersion coefficients in the riser along three directions are 0.675 × 10⁻³, 0.522 × 10⁻³, and 8.415 × 10⁻³ m²/s, and the insertion of ring baffles decreases the overall dispersion intensity in the axial direction while has a slight influence in the radial direction.
... A CFB riser CFD-Particle-in-Cell (PIC) Particles are grouped as a parcel-of-particle, which have the same properties, such as size, density, velocity, residence time, etc.; To simulate the lab-scale RTD experiments in (Bhusarapu et al., 2004) Hua et al. (2014 A CFB riser Two-phase Eulerian-Eulerian model and species transport equation ...
Article
The residence time distribution (RTD) of particles in a gas-solid dense fluidized bed with a continuous solid flow operation and baffles needs much more focus for the purpose of the wide industrial applications. The available models of solid RTD are not adequate in this case due to the additional convective diffusion induced by the cross-flow of the solid feeding and the complex geometry caused by the baffles. To address this problem, this work applied a two-phase Eulerian-Eulerian model combined with the species transport equation to predict solid RTD in a dense fluidized bed. The effects of the continuous solid feeding and baffles were considered. The solid dispersion coefficient D s in the species transport equation was calculated by an analytical solution from kinetic theory of granular flow. D s only needs the value at a molecular level due to the fact that the computational fluid dynamics (CFD) model is able to reproduce the RTD procedure exactly same as the practical experiments. To validate the established CFD model, a series of 3D lab-scale cold flow experiments were conducted in the free and baffled beds for the bed material with various severe non-spherical shapes. The measurement included the solid hydrodynamic characteristics at the outlet of the outflow pipe and solid RTD of the system. The reasonable agreement between the CFD prediction and experimental data demonstrated the good performance of the CFD model. 1D plug flow with dispersion model and the tanks-in-series model were further used to fit the calculated RTD curves. The estimated lateral dispersion coefficient of solids locates in a rational range compared with the data collected extensively from the literature. The results showed that the lateral dispersion coefficient of solids decreases when the baffles are installed.
... Research into radioactive particle tracing applications in multiphase flow system has a long history. There are a large number of published studies that describe the uses of radioactive particle tracking technique for gas-solid-liquid phase [1,12,13,14,15,16,17,18,19,20,21,22,23,24], solid phase in cylindrical tumbler reactor [25,26], gas-solid and gas-liquid phase in risers [27,28,29,30,31], liquid phase in circulating draft tube photobioreactor [32], solid-solid phase in V-blender reactor [33], and pebble bed reactors [34,35]. However, there is very little published research on the gas-liquid phase in quadrilateral bubble column reactors which need to be comprehensively investigated and examined using radioactive particle tracking technique. ...
Article
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Radioactive particle tracking (RPT) is one of the non-invasive techniques for monitoring and investigating multiphase flow system. This technique have been widely utilized in the field of chemical process engineering for better understanding and optimizing process hydrodynamics especially in the multiphase reactor such as bubble column reactor. Due to opaque nature of industrial process systems, especially in the case of multiphase flows, noninvasive methods based on ionizing radiation have been considered for evaluating the hydrodynamic parameters. The feasibility study of radioactive particle tracking techniques in quadrilateral bubble column reactor has been successfully achieved. The radioactive particle tracking facility and data acquisition system has been developed and experimental calibration using single particle radioactive particle ⁴⁶ Sc to investigate dynamics behaviour of quadrilateral bubble column reactor is completed. The results indicated that there is back mixing behaviour in the bubble column process. The results also reported that the radioactive particle ⁴⁶ Sc is still in good condition and there is no radiation contamination problem arises while performing radioactive particle tracking techniques. The RPT technique was performed to reveal the instantaneous velocity and time-averaged liquid velocity in the current bubble column reactor.
Article
Gas/solid and catalytic gas phase reactions in CFBs use different operating conditions, with a strict control of the solids residence time and limited back-mixing only essential in the latter applications. Since conversion proceeds with residence time, this residence time is an essential parameter in reactor modelling. To determine the residence time and its distribution (RTD), previous studies used either stimulus response or single tracer particle studies.The experiments of the present research were conducted at ambient conditions and combine both stimulus response and particle tracking measurements. Positron emission particle tracking (PEPT) continuously tracks individual radioactive tracer particles, thus yielding data on particle movement in “real time”, defining particle velocities and population density plots.Pulse tracer injection measurements of the RTD were performed in a 0.1 m I.D. riser. PEPT experiments were performed in a small ( I.D.) riser, using 18F-labelled sand and radish seed. The operating conditions varied from 1 to 10 m/s as superficial velocity, and 25– as solids circulation rate.Experimental results were compared with fittings from several models. Although the model evaluation shows that the residence time distribution (RTD) of the experiments shifts from near plug flow to perfect mixing (when the solids circulation rate decreases), none of the models fits the experimental results over the broad (U,G)-range.The particle slip velocity was found to be considerably below the theoretical value in core/annulus flow (due to cluster formation), but to be equal at high values of the solids circulation rate and superficial gas velocity.The transition from mixed to plug flow was further examined. At velocities near Utr the CFB-regime is either not fully developed and/or mixing occurs even at high solids circulation rates. This indicates the necessity of working at U> approx. ( to have a stable solids circulation, irrespective of the need to operate in either mixed or plug flow mode. At velocities above this limit, plug flow is achieved when the solids circulation rate . Solids back-mixing occurs at lower G and the operating mode can be described by the core/annulus approach. The relative sizes of core and annulus, as well as the downward particle velocity in the annulus (∼Ut) are defined from PEPT measurements.Own and literature data were finally combined in a core/annulus vs. plug flow diagram. These limits of working conditions were developed from experiments at ambient conditions. Since commercial CFB reactors normally operate at a higher temperature and/or pressure, gas properties such as density and viscosity will be different and possibly influence the gas–solid flow and mixing. Further tests at higher temperatures and pressures are needed or scaling laws must be considered. At ambient conditions, reactors requiring pure plug flow must operate at and . If back-mixing is required, as in gas/solid reactors, operation at and is recommended.
Article
Industrial CFB risers usually handle polydisperse mixtures with broad size distribution, which significantly influenced the performance of the reactors. However, traditional Computational Fluid Dynamics (CFD) models usually assumed that the particle followed the mono-disperse distribution. In the present work, the method of computational particle fluid dynamic (CPFD) was applied for simulating the complex hydrodynamics in the CFB riser with various particle size distributions (PSDs). Two kinds of PSDs, namely Gaussian and Lognormal distribution with various PSD widths, were implemented into the CPFD scheme. With the CPFD method, the present work extensively studied the effects of PSD on the hydrodynamics and on the solids back-mixing. The CPFD results showed that the PSD significantly affected particle's flow behaviors at the lower zone of the riser, while the PSD effects were negligible in the upper part of the riser. This is meaningful for the industrial riser reactors since most of the reaction and transport process occur in this lower zone of the riser. Besides, the simulation results showed that wider PSD dramatically weaken the particle's back-mixing behaviors in the riser. The significant effects of PSD predicted by the CPFD method imply that large errors will be introduced if the mono-disperse assumption is adopted to simulate the experimental CFB riser handling particles with broad size distribution.
Article
The effect of type of inlet conditions on the predictions of Eulerian–Eulerian simulations of a circulating fluidized bed riser has been investigated in both 2D and 3D domains. The 2D simulations were conducted using 3 different inlet configurations: (A) solids entering in radial direction from a two-sided inlet and gas entering axially from the bottom inlet, (B) solid entering axially from a two-sided bottom inlet near the wall and gas entering axially from a bottom inlet at the center, and (C) gas phase entering axially from a two-sided bottom inlet near the wall and solids entering axially from a bottom inlet at the center. In 2D simulations, it was found that both time-averaged axial velocity and solids volume fraction radial profiles were functions of the inlet kinetic energy as well as gas–solid mixing patterns at the inlet. Whereas 2D simulations using boundary conditions A and C showed significant deviations from experimental profiles, the boundary condition B as well as full-scale 3D simulations gave reasonable agreements with experimental observations.
Article
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A three-dimensional model for gas-solid flow in a circulating fluidized bed (CFB) riser was developed based on computational particle fluid dynamics (CPFD). The model was used to simulate the gas-solid flow behavior inside a circulating fluidized bed riser operating at various superficial gas velocities and solids mass fluxes in two fluidization regimes, a dilute phase transport (DPT) regime and a fast fluidization (FF) regime. The simulation results were evaluated based on comparison with experimental data of solids velocity and holdup, obtained from non-invasive automated radioactive particle tracking and gamma-ray tomography techniques, respectively. The agreement of the predicted solids velocity and holdup with experimental data validated the CPFD model for the CFB riser. The model predicted the main features of the gas-solid flows in the two regimes; the uniform dilute phase in the DPT regime, and the coexistence of the dilute phase in the upper region and the dense phase in the lower region in the FF regime. The clustering and solids back mixing in the FF regime were stronger than those in the DPT regime.
Article
To monitor the gas and solid streams in a downcomer of a J-valve in a CFB, the simultaneous measurement technique using the oxygen gas tracer, the hot particle tracer and the pressure drop was developed. By using this novel measurement technique, the dependence of the solid flow on the gas flow in the moving bed in the downcomer of J-valve was investigated. The pressure profile and the pressure balance in the J-valve as well as the CFB were measured. It was clarified that the circulating mass flux in a CFB or solid flow in a J-valve was governed by the pressure drop in the downcomer of the J-valve. The pressure drop in the downcomer of the J-valve was well estimated using the modified Ergun’s equation. To quantitatively control the circulating rate of the solid particles in the CFB, the monitor and tuning of the superficial gas velocity in the downcomer of the J-valve was important.
Article
Single radioactive particle tracking was used to measure the overall solids residence time (and its distribution) in the riser of a CFB, operating at superficial air velocities (U) of 1 to 9ms−1 and solids circulation fluxes (G) between 20 to 600kg m−2s−1.The results demonstrate that the particle motion and mixing differ according to the operating mode of the riser, with a fairly constant velocity throughout the riser achieved in the dilute or dense riser flow, but with a significant amount of back-mixing for intermediate values of U and/or G. This back-mixing is due to the core-annulus mode of particle flow. Whereas experimental results and theoretical predictions are in fair agreement for the dilute and dense riser flow, the core-annulus regime needs to account for a U and G dependent slip factor (φ), in excess of the commonly proposed value φ=2, especially at U–UTR
Article
The numerical simulation of CFBs is an important tool in the prediction of its flow behavior. Predicting the axial pressure profile is one of the major difficulties in modeling a CFB. A model using a Particle Based Approach (PBA) is developed to accurately predict the axial pressure profile in CFBs. The simulation model accounts for the axial and radial distribution of voidage and velocity of the gas and solid phases, and for the solids volume fraction and particle size distribution of the solid phase. The model results are compared with and validated against atmospheric cold CFB experimental literature data. Ranges of experimental data used in comparisons are as follows: bed diameter from 0.05 to 0.305m, bed height between 5 and 15.45m, mean particle diameter from 76 to 812μm, particle density from 189 to 2600kg/m3, solid circulation fluxes from 10.03 to 489kg/m2s and gas superficial velocities from 2.71 to 10.68m/s. The computational results agreed reasonably well with the experimental data. Moreover, both experimental data and model predictions show that the pressure drop profile is affected by the solid circulation flux and superficial velocity values in the riser. The pressure drop increases along the acceleration region as solid circulation flux increases and superficial velocity decreases.
Article
The solids mixing in a riser with a height of 10 m and 0.186 m inner diameter was investigated by using pneumatic phosphor tracer technique. Considering the shielding effect of the bed material on the light emitted from the phosphor tracer particle, a modified method for the phosphor tracer measurement is proposed. And then the curves of particle residence time distribution were obtained. The experimental results show that the particle diffusion mechanism can be explained by the dispersions of dispersed particles and particle clusters in the axial direction, and as well the core–annulus nonuniform distribution of the solids fraction in the radial direction of the riser. Moreover, based on the experimental results, a two-dimensional dispersion model was established to predict the solids axial and radial diffusion. Furthermore, the effects of superficial gas velocity and solids circulating flux on the axial and radial Peclet number of the particles were discussed; two empirical correlation formulas about the axial and the radial Peclet numbers were given; the calculated values agree well with the experimental results.
Article
This manuscript summarizes the plenary lecture delivered at the ISCRE 20 meeting in Kyoto on Tuesday, September 9, 2008. The scope, history and status of our chemical reaction engineering (CRE) discipline are briefly outlined complementing the broad review presented by Professor J. Schouten in a plenary lecture a day earlier (Schouten, 2008). It is argued here that the key challenge for CRE is the development of new more efficient and profitable technologies. This is to be accomplished via an improved science-based scale-up methodology for transfer of molecular discoveries to sustainable non-polluting processes that can meet the future energy, environmental, food and materials needs of the world. Available foundations for such scale-up are introduced and the role of science in developing the improved methodology for multiphase systems is described. It is also argued that progress in implementing the desired environmentally benign processes depends, in addition to overcoming technical challenges, on achieving changes in the socio-economic and political arena. Global regulations and peer pressures are needed to provide the economic incentives for cleaner and sustainable processes.
Article
The importance of reaction engineering in generating a myriad of products on which developed societies depend is outlined. The challenges of a political, economic, and technical nature that need to be addressed in rendering conversion of raw materials into desired products that are more environmentally friendly and sustainable are briefly discussed. It is shown that multiphase reactors are prevalent in all applications, and improvements in the reactor material and energy efficiencies lead to more environmentally benign processes. This requires, in addition to the selection of green process chemistry, systematic implementation of the multi-scale reaction engineering methodology to accomplish proper reactor type selection and scaleup for commercial applications. It is also illustrated that recent innovations in multiphase reaction engineering basically utilize two key concepts: process intensification (e.g., enhancement in mass and heat transfer rates) and simultaneous reaction and separation. Examples of these are discussed, such as micro-reactors, reactive distillation, etc. It is also shown that commercialization of bench-scale discoveries requires either scaleup in parallel or vertical scaleup. New tools for visualization of opaque multiphase flows and development of appropriate rational phenomenological multiphase reactor models for scaleup and design are also briefly discussed.
Article
The purpose of this report is to summarize the lecture given at the joint CAMURE-6 and ISMR-5 international symposium in Pune, India, in January 2007. The emphasis is on the pivotal role that reaction engineering has to play in addressing modern technological challenges. First the global challenges of reducing the environmental impact of our technologies are considered. Then the role of multiphase reaction engineering in enabling efficient transfer of molecular-scale discoveries to more benign and sustainable processes is outlined. Typical scale-up methodologies are introduced, and research needed for their further improvement is discussed. Examples of the importance of proper scale-up are provided.
Article
Solids back-mixing significantly influences the performance of circulating fluidized bed (CFB) risers. A better understanding of the solids back-mixing behavior is greatly essential for the design and operation of CFB risers. Computational particle fluid dynamics (CPFD) modeling based on the multiphase particle-in-cell (MP-PIC) method was applied to investigate the solids back-mixing behavior and its influencing factors in CFB risers operating in dilute phase transport (DPT) and fast fluidization (FF) regimes. The present work observed extremely long residence time and wide residence time distribution (RTD) curves for particles in the FF regime, indicating an extensive back-mixing of particles in the FF regime. The overall and local back-mixing behaviors of solids were further analyzed. The particles in the DPT regime have a little back-mixing in the lower part of the riser, while the particles in the FF regime experience a large-scale back-mixing throughout the riser. In addition, the factors influencing the back-mixing behavior of solids in different flow regimes were investigated. The results demonstrate that the solids back-mixing in the DPT regime is caused by the downward flow of particles near the wall region, while the severe back-mixing in the FF regime is due to the downflow particles and mesoscale structures (mainly particle clusters). The dynamic formation and dissolution of mesoscale structures could be the dominating factor, leading to the intense back-mixing for particles in the FF regime.
Article
The current work describes development and optimization of a process for preparation of cobalt-60 glass microspheres. These microspheres have potential for applications in radioactive particle tracking (RPT) studies in multiphase flow systems. In the first step of preparation, soda lime glass containing 5–10 wt% cobalt oxide was produced through melt-quench method. Subsequently, cobalt glass microspheres (CMSs) were prepared by microwave heating of tiny glass grains in presence of graphite. In the final step, radioactive cobalt-60 microspheres (RMSs) were produced by neutron irradiation of the CMSs in a nuclear reactor. The CMSs were characterized for surface morphology, elemental composition, homogeneity, crystalinity using SEM, EDX and XRD, respectively. The thermal behaviour of the microspheres was characterized by TG and DSC analysis. The size distribution of CMSs analyzed by SEM was found to be in the range 500–2000 μm. The preparation step was optimized to produce adequate activity in a single microsphere, so that they can be utilized for RPT applications.
Article
A Computational Particle Fluid Dynamics (CPFD) approach was applied to investigate the solids residence time distribution (RTD) and back-mixing behavior in a circulating fluidized bed (CFB) riser. The comparison between the simulation results and the experimental data indicates that the CPFD method was capable to predict the hydrodynamics and solids mean residence time. It was found that the solids residence time exhibited a non-uniform distribution both in the axial and in the radial directions of the riser. Even in the dilute phase transport (DPT) regime, the predicted solids RTD curves had the feature of an early peak and an extended tail, indicating that the solids flow deviates from plug flow and exhibits back-mixing. The solids back-mixing mainly occurred in the lower part of the riser, which provided significant implications for the industrial applications of the CFB reactors since most of the chemical reactions and heat/mass transport processes occur at the lower part of the riser. It is important to minimize the solids back-mixing at the lower part of the riser for the industrial CFB applications like the FCC.
Article
Ein neuartiges Verfahren, das magnetische Monitoring, wurde zur Bestimmung der Partikelbewegung eines magnetisch markierten Teilchens im Partikelkollektiv in diskontinuierlich sowie kontinuierlich arbeitenden Wirbelschichten eingesetzt. Die Position in x-, y- und z-Richtung und die räumliche Ausrichtung des Markers wird gemessen. Daraus lassen sich die Geschwindigkeit, die Beschleunigung, die auf die Partikel wirkenden Kräfte und Energien sowohl für die Translation als auch für die Rotation der Partikel berechnen. In der vorliegenden Arbeit wurden die Bewegungskennwerte der Partikel ermittelt. Des Weiteren wurde der Einfluss von verschiedenen Apparate- und Betriebsparametern auf die Bewegungskennwerte untersucht.A novel method, the magnetic monitoring was used for particle tracking in batch and continuous fluidized beds. By means of the magnetic monitoring system the spatial coordinates x, y and z and the orientation of the magnetic marked particle are measured. This allows calculation of the particle velocity, the particle acceleration and also the forces and energies acting on the particles both for translation and for the rotation. In the present work, the movement characteristics of the particle were determined. Moreover, the influence of different apparatus and operating parameters on the movement characteristics were investigated.
Article
Hydrodynamics plays a crucial role in defining the performance of circulating fluidized beds (CFB), and pressure fluctuation signal has been used to analyze the dynamic behavior in it. From this point of view, in the present study, the power spectral characteristic of pressure fluctuation signal in CFB is analyzed based on fractal theory. Analysis also takes into account the power spectrum and correlation analysis of pressure fluctuation signal. Results show that the pressure fluctuation signal has the characteristic of 1/f noise when the solid circulating flow is smaller. Elsewhere when the solid circulating flow is increased, pressure fluctuation signal has the characteristic of Fractal Brownian Motion (FBM). The value of power spectral exponent ? is between 2∼7, this value is fluctuant in the upper and bottom zone of riser, and it is somewhat steady in the middle zone of riser, which could be used to judge the transition of flow regimes from thin pneumatic flow to dense pneumatic conveying. Correlation analysis exteriorly indicated the chaos characteristic of pressure fluctuation signal in CFB.
Article
Successful scale-up of new multiphase reactions from the laboratory into practical processes is important to all sectors of the process industry. Business demands that process technologies involving molecular transformations maintain high profitability and operate safely within existing environmentally regulations. Current societal expectations and regulations require that all process technology should be environmentally responsible [1]. One key question to be answered is whether or not these expectations can be met in the foreseeable future with the current approaches to scale-up and technological workforce. In addition, advances in chemistry, physics, materials, and biology will continue to generate new potential reaction pathways for more efficient utilization of non-renewable and renewable resources. Another key question is whether the current methods for process scale-up incorporate the relevant scientific advances to ensure 'green technologies', or are they just extensions of previous largely empirical approaches having limited utility and reliability? Evidence suggests that only a science-based scale-up methodology can substantially reduce the risk of new process commercialization and provide reliable estimates of both profitability and environmental impact. We review briefly here the historical approach to scale-up and opine on the challenges of implementing improved approaches.
Article
The Topsøe Catalysis Forum was created as a framework for an open exchange of views on catalysis in fields of interest to Haldor Topsøe. The forum scope included a discussion of new catalytic reactions and new principles of catalysis in an attempt to jointly look beyond the horizon (. Topsoe catalysis forum, 2013). The 2013 meeting was dedicated to Modeling and Simulation of Heterogeneous Catalytic Processes and provided an opportunity to review and discuss the current state of the art in the engineering practice of heterogeneous catalytic systems (. Topsoe catalysis forum, 2013). The primary objective of this chapter is to capture key elements of our conference presentation (. Dudukovic, 2013) that were focused on multiscale reaction engineering concepts and to what extent these have been applied in the commercial implementation of multiphase heterogeneous catalytic reacting systems. Of particular interest is to identify common approaches and tools used in practice, and to examine their effectiveness in the scale-up and development of more efficient, environmentally friendly catalytic processes. Current practice is limited by the availability of experimental tools to increase the reliability of scale-up, and by the lack of more robust models for analysis and optimization of reactor systems for existing processes or the design of new reactor systems for implementation of new catalytic chemistries. From an economic perspective, the pursuit of short-term financial objectives favors the use of existing reactors with minimal modifications with performance analysis based upon simplified approaches. A longer-term perspective on the development and implementation of more advanced experimental techniques and modeling approaches for reactor analysis that are applicable to commercial reactor conditions would accelerate the development of new process technologies and result in reduced risk with associated lower costs.
Article
Hydrocyclone is a device for solid concentration or selection of solid particles from a liquid-solid mixture. It is widely used in the mineral industry for selection of solid particles from a few to a few hundred micrometers. This paper presents a radiotracer experiment and computational simulation of selection of solid particles in a hydrocyclone of Φ-500 mm, which is used in the industrial copper ore concentration process. The simulation, based on computational fluid dynamics (CFD) techniques, allowed obtaining the velocity and concentration distribution for a real mixture flowing in the hydrocyclone. The mixture was composed of water and nine solid phases of different grain sizes. Finally, the selection curve of solid grains was obtained and compared with the experimental radiotracer results.
Article
Often information on mean convective flow field and overall mixing, characterized by residence time distribution (RTD), is not sufficient for proper prediction of catalyst performance in Circulating Fluidized Beds (CFBs). In this paper, we report the overall solids motion as well as the characteristics of local solids mixing within a section of the riser of two pilot scale CFBs. The Lagrangian description of solids flow was obtained by the Computed Automated Radioactive Particle Tracking (CARPT) technique in a range of solids mass fluxes between 27 kg.m-2.s-1 to 145 kg.m -2.s-1 in two different flow regimes (Fast Fluidization (FF) and Dilute Phase Transport (DPT)). CARPT data was analyzed as Lagrangian tracks of single particle to characterize the local backmixing, which includes the effects of convection, local circulation and dispersion. Solids mixing in the transient flow field is characterized using residence time and trajectory length distributions, circulation and return time distributions, eddy diffusivities and parameters characterizing these distributions. The potential use of the above mentioned backmixing parameters in riser reactor models is discussed.
Article
Liquid phase residence time distributions (RTD) are reported by employing a novel experimental method of tracking a radioactive tracer particle during single phase and two-phase (gas-liquid) flows through a horizontal helical coil. Liquid and gas phase Reynolds numbers (NRe, L & NRe, G) were varied in the ranges from 1061 to 23150, and 130 to 100000, respectively. As part of this work, we tracked the entry and exit of the tracer particle in the coiled flow structure using an array of strategically placed scintillation detectors. From these experiments, it became possible to extract the residence time distribution (RTD) by enumeration of the trajectories of the tracer particle through the flow system. The investigations have resulted in explaining mixing performance for gas-liquid (two-phase) flow in coiled geometry, based on the trends in liquid phase Peclet number (NPe), over variable liquid and gas phase velocities.
Article
A set of dimensionless groups has been proposed in the literature by He et al. [He Y. L., Lim C. J., Grace J. R., Scale-up studies of spouted beds, Chemical Engineering Science, 52 (2), 329–339, 1997] to scale-up gas-solid spouted beds while maintaining their hydrodynamics similarity. The literature reported studies do not provide conclusive assessments about this methodology. Therefore, in this work, we have applied an advanced non-invasive radioactive particle tracking (RPT) technique for the first time to evaluate such scale-up methodology by measuring the local solids velocity, normal and shear stresses and the turbulent kinetic energy. The axial and azimuthal averaged radial profiles of solids velocity, normal stresses, shear stresses, and turbulent kinetic energy illustrate that the similarity of the hydrodynamics has not been attained when the proposed set of dimensionless groups has been matched using two sizes of spouted beds of 0.076 m and 0.152 m and sets of operating conditions. The conclusion is consistent with the recent reported findings by measuring cross sectional distribution and radial profiles of solids and gas holdups along the bed height using gamma-ray computed tomography and by the limited point measurements of solids velocity and holdup using optical fiber probe. It is clear that local measurements of hydrodynamic parameters are essential for detailed assessment of scale-up methodologies. The presented results of our work in terms of the components profiles of the particles radial velocities and turbulent parameters are also valuable for benchmarking computational fluid dynamics codes and models.
Article
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Radioisotope techniques are constantly and extensively used all over the world as a method to identify process systems malfunctions in various industries without requiring the shut down of the processing plant thus leading to high economical benefits to the plant owner. Different aspects of industrial radiotracer technology for troubleshooting, process control and optimization are evaluated through an exhaustive literature survey. The review covers the advantages of radiotracers, most commonly used radiotracers in industry for specific studies, applications of radiotracer techniques in various chemical industries, the design of radiotracer technology experiments, radiation detection and data acquisition in radiotracer technology as well as radiological safety aspects. Two industrial radiotracer techniques of residence time distribution (RTD) measurements and radioactive particle tracking (RPT) are discussed. The design of radiotracer technology experiments are also divided into two categories - radioactive particle tracking applications and residence time distribution applications.
Article
We implemented for the first time our radioactive particle tracking as an advanced noninvasive technique to further evaluate and validate our newly developed mechanistic scale-up methodology based on matching the radial profile of the gas holdup. Two spouted beds with diameters of 0.076 and 0.152 m were used. Three sets of conditions were implemented; i.e., conditions of the reference case, conditions that provided a gas-holdup radial profile similar to that of the reference case, and conditions that provided a gas-holdup radial profile dissimilar to that of the reference case. The results confirm the validation of the scale-up methodology in terms of obtaining closer dimensionless values and radial profiles of components of the particle velocity, normal stress, shear stress, and turbulent kinetic energy. The results further advance the understanding of gas–solids spouted beds, provide deeper insight into the solids dynamics of the beds and present important benchmarking data for validating computational fluid dynamics codes and models.
Chapter
It has been realized in recent decades that a proper investigation of gasification reactor requires the detailed information over the entire flow field, as well as time, at multiple scales. Such detailed information needs the use of sophisticated measuring techniques with capability to provide the required information over the entire flow field, as well as time, at multiple scales. Aside from the mean velocities and volume fractions, information about the flow fluctuations or dynamics (quantified in terms of cross-correlations and auto-correlations) is also desirable. In addition, it is preferable if such techniques are amenable to automation to reduce extensive human involvement in the data collection process. While such data are “stand-alone” sets of information, which can be used for design and scale-up strategies, it also provides information that is crucial to establish the validity of conventional models like phenomenological flow models describing residence time distribution (RTD), as well as more recent and sophisticated models like those based on computational fluid dynamics (CFD). In fact, it almost seems imprudent to validate CFD predictions on overall holdup and flow rates, because these spatial integrals of point properties are simply averages of a complete flow field that a CFD code is designed to and claims to compute. Thus, fair validation must involve validation at multiple scales, for which one needs experimental information also at multiple scales (and not just spatial and temporal averages). Several experimental techniques have been reported in past to quantify the flow field in gas–solid gasification reactors, with each technique having its own advantages and disadvantages. In this chapter, details of pressure, solid velocity, solid fraction, and RTD measurement techniques will be presented. Techniques will be divided majorly in two types, invasive and non-invasive. The postprocessing methods for each technique, advantages, and limitations will be discussed. Finally, some of the recent findings on gas–solids circulating fluidized bed using radioactive particle tracking (RPT) technique will be discussed in detail to explain the use of the experimental techniques for design and scale-up of these reactors.
Article
This work presents a novel technique with fast response for Residence Time Distribution (RTD) measurements in gas-solid unit operations (e.g., fluidized bed reactors). This technique is based on an optical method which eliminates the requirement of knowing the velocity and concentration profiles at the exit section of the system. Experiments were carried out with SiC particles and a phosphorescent pigment used as a tracer. A concentration measurement system was developed to measure the tracer concentration in SiC/pigment mixtures. The corresponding pigment concentrations were evaluated at the bottom of this system using a photomultiplier. The pigment concentration was derived from the integral of the signal intensity received by the photomultiplier. Then, a calibration curve was established which provided the empirical relationship between the integral and pigment concentration. In order to validate this RTD measurement technique, a series of experiments was performed in a bubbling fluidized bed and the effect of the bed height was studied. It was shown that the experimental RTD curves were in good agreement with the theoretical RTD of bubbling fluidized beds. This solids RTD measurement technique can be used to provide a better understanding of the hydrodynamics of complex solids unit operations.
Article
Owing to challenges in velocimetry techniques, flow hydrodynamics of binary fluidized beds has remained largely unexplored. Thus, the present study seeks to address unraveling the velocity distributions in the binary fluidized bed of glass beads (0.5 and 2mm) in fluidized with 10, 30, and 40% coarser fractions. In order to decipher the velocity distribution of the individual components, RPT was implemented, which yielded velocity distributions of both the 0.5 and 2 mm fractions. Distinct features of solids flow were observed with varying bed composition. The mean velocity of finer flotsam (0.5 mm particles) fractions were found to achieve higher velocity, which is reduced upon increasing the jetsam fraction (2 mm) due to variation in bed voidage. On the contrary, an enhanced coarser fraction of 2 mm particles in the bed diminished solids’ fluctuation, which led to a corresponding reduction in the RMS velocities. Similar trends were also noted when solids’ diffusivity was estimated for various bed compositions. Finally, the efficacy of this technique was independently verified qualitatively through mixing patterns obtained from ‘bed freezing’.
Article
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This paper is based on the lecture delivered at the First South East European Congress of Chemical Engineering (SEECCHE1) held on September 25-28, 2005 in Beograd, Serbia. Time constraints did not permit me to publish it in the proceedings of the conference, and since the topic considered is very much pertinent to the current developments in our profession, it seems appropriate to present it in this special issue of the same journal that carried the papers from that conference. In addition, the paper represents a small tribute to my brother Aieksandar for his dedicated work in emphasizing the importance of fundamentals in chemical engineering education. In this paper an attempt is made to illustrate to the younger generations of chemical and process engineers the importance of reaction engineering and its relevance to societal needs globally. The slides and comments of the talk given at SEECCRE1 are made available on a web site indicated at the end of the paper.
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Tests were conducted in a 30.5-cm-diameter riser with 76 μm FCC catalyst to determine the degree of gas/solid contacting in the “fast-fluid bed” regime. Data were obtained at superficial gas velocities of 3.7 to 6.1 m/s and solid fluxes of 98 to 195 kg/s-m². The conveying gas was air at ambient temperature and 103 kPa. Data were obtained on axial pressure profiles, particle velocity and mass flux as a function of radial and axial position, radial and axial gas dispersion, and solids residence time distributions. Density and voidage profiles were calculated from these data.
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Axial and lateral mixing of fine particles in a binary-solid riser have been investigated using a phosphor tracer method. The measured bimodal residence time distribution (RTD) demonstrated two types of axial dispersions of the fines: the dispersion of discrete particles and that of clusters. A proposed one-dimensional, bimodal dispersion model describes the bimodal RTDs very well. The axial Peclet number of the fines is not sensitive to the fraction of coarse particles, gas velocity and solids circulation rate. Lateral solids dispersion was determined by measuring the solids RTD at different radii using a point source tracer injection. A two-dimensional dispersion model describes the measured RTDs satisfactorily. Lateral solids mixing decreased as coarse particles were added into the riser. Correlations of the axial and lateral Peclet numbers obtained fit the experimental data well. On a étudié le mélange axial et latéeral de particules fines dans une colonne montante binaires-solides à l'aide d'une méthode de traçage au phospore. La distribution du temps de séjour (DTS) bimodale mesurée illustre deux types de dispersions axiales des particules fines: la dispersion de particules discrètes et la dispersion de grappes. On propose un modèle de dispersion unidimensionnel bimodal qui décrit très bien la DTS bimodale. Le nombre de Péclet axial des fins n'est pas sensible à la fraction des particules grossières, à la vitesse du gaz et a la vitesse de circulation des solides. On a déterminé la dispersion latérale des solides en mesurant la DTS des solides à différents rayons à l'aide d'une injection de traceur ponctuelle. Un modèle de dispersion bidimensionnel décrit les DTS de manière satisfaisante. Le mélange latéral des solides diminue avec l'ajout de particules grossières dans la colonne. Des correlations obtenues pour les nombres de Péclet axial et latéral montrent un bon accord avec les données expérimentales.
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A novel, non-intrusive method is described for measuring the particle residence time distribution (RTD) in a system with a short mean residence time. The method uses phosphorescent tracer particles activated by a high intensity pulse of light at the inlet. Tracer is detected using a light sensitive photomultiplier unit. Appropriate boundary conditions are maintained by using an annular feeder fluidised bed at the entrance boundary and an inline jet mixer installed at the exit boundary. This well defined arrangement of experimental boundary conditions represents a significant advance in the measurement of unbiased particle RTDs in these systems. The method was developed for measuring the particle RTD in a circulating fluidised bed (CFB) riser, but is applicable to other particle–fluid systems where a fast response measurement is required.