Influence of high density particles in reduction of axial dispersion in liquid fluidized bed with residence time distribution curve studies

iquid phase RTD curves were investigated in classical fixed and fluidized bed regimes with high density particles. The effect of liquid velocity was studied on bed hydrodynamics. Using an impulse tracer injection technique in a column of 5 cm inner diameter and 1.2 m height, liquid RTD, mean residence time (MRT), axial dispersion coefficient (ADC) and vessel dispersion number (N _D ) were determined. ADC increases with liquid superficial velocity. It varied from 4.63 to 20.7 cm²/s for the particle Reynolds number of 43 to 279, respectively. The experimental results show that the hight density particles cause less ADC than the low density particles at an identical Reynolds number.     

Residence time distribution and dispersion of gas phase in a wet gas scrubbing system

Residence time distribution (RTD) of exhaust gas in a wet scrubbing system was investigated for application to the removal of SO _x , NO _x or dust included in exhaust gas. The mixing of gas phase in the wet scrubbing system was also examined by considering the axial dispersion coefficient of gas phase. Effects of gas amount (velocity), liquid amount (velocity) and solid floating materials on the residence time distribution (RTD) and axial dispersion coefficient of exhaust gas were discussed. The addition of solid floating materials could change the RTD and thus dispersion of exhaust gas in the scrubbing system. The mean residence time and axial dispersion coefficient of exhaust gas were well correlated in terms of operating variables.     

Radioisotope tracer study in trickle bed reactors

The residence time distribution (RTD) of liquid phase in trickle bed reactors has been measured for air‐water system using radioisotope tracer technique. Experiments were carried out in a glass column of internal diameter of 0.152 m packed with glass beads and actual catalyst particles of two different shapes. From the measured RTD curves, mean residence time of liquid was calculated and used to estimate liquid holdup. The axial dispersion model was used to simulate the experimental data and estimate mixing index, ie. Peclet number. The effect of liquid and gas flow rates on total liquid holdup and Peclet number has been investigated. Results of the study indicated that shape of the packing has significant effect on holdup and axial dispersion. Bodenstein number has been correlated to Reynolds number, Galileo number, shape and size of the packing.     

Characteristics and applications of flash metakaolins

This paper presents physical, chemical and mechanical characteristics of metakaolins obtained from an industrial flash calciner, in order to compare their properties with standard industrial metakaolin produced in a rotary kiln calciner. Three kaolins, with three levels of purity, were calcined by these two different methods to give six different metakaolins for the study. The results showed that the method of calcination did not affect the chemical composition of the metakaolins formed but did influence their physical properties and performance as a supplementary cementitious material when blended with Portland cement, and in geopolymer synthesis. Flash metakaolins have a lower water demand than rotary metakaolins, which can be explained by the morphological properties of the flash metakaolin, induced by the calcination process. Traditional rotary-calcined metakaolins tend to be angular layered particles, whereas flash metakaolins contain spherical particles. Mechanical test results showed that the two methods of calcination can lead to metakaolins with equivalent performance in the synthesis of construction materials.     

Axial and lateral dispersion of fine particles in a binary-solid riser

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.     

Measuring the residence time distribution in a twin screw extruder with the use of NIR‐spectroscopy

The mean residence time (MRT) and the residence time distribution (RTD) of polypropylene in a twin‐screw extruder was determined directly in‐line with the help of near infrared (NIR) spectroscopy and the use of an UV‐absorber as tracer. Different experiment alignments such as screw speed, mass throughput, back pressure as well as tracer content and their influence on MRT and RTD have been investigated. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39919.     

Material transport in a continuous rotary drum. Effect of discharge plate geometry

Experimental results on the influence of the discharge plate geometry on the dimensionless residence time distribution (RTD) for material transport in a continuous rotary drum are described. The RTD obtained by a stimulus-response technique for the different discharge plates can be described well by the axial dispersed flow model. Based on the characteristic Peclet number of the flow regime, material flow tended more towards the plug flow condition at an intermediate size discharge opening. Calculation of the axial dispersion coefficient in each case revealed that the open-ended drum behaved more like an ideal mixer. The implication of these results on the design of continuous rotary devices is discussed.     

Estimation of liquid phase mixing due to solids in a turbulent bed contactor

The mixing in two-phase gas-liquid and three-phase gas-liquid-solid system (turbulent bed contactor) is evaluated through residence time distribution (RTD) studies in terms of Peclet number. RTD experiments are conducted for various gas and liquid velocities, and number of stages for two- and three-phase systems. Since the mean residence time is very short in both the systems, a mixed flow tank with exponential decay RTD is used in series. After deconvolution, the RTD of the system is obtained. The experimental RTD curves are satisfactorily compared with the axial dispersion model and Peclet numbers are evaluated for all the experiments. The axial dispersion coefficients are calculated from Peclet numbers. With this study, it is thought that liquid phase mixing may be controlled by changing the quantity of solid particles in the bed.     

Green process technology for peroxycarboxylic acids: Estimation of kinetic and dispersion parameters aided by RTD measurements: Green synthesis of peroxycarboxylic acids

Synthesis of peroxypropionic and peroxyacetic acids from hydrogen peroxide and their carboxylic acids was carried out in a continuous fixed-bed reactor, demonstrating the concept of green process technology: biodegradable chemicals are made with the aid of heterogeneous catalysts. This reactor was constructed with several sampling locations along its length, allowing a better kinetic investigation. A profound study of the mean residence time distribution (RTD) was performed to analyze the behavior of the flow pattern at different liquid flow rates and catalyst loadings by using step and impulse methods with different tracer molecules. It was found that impulse experiments with HCl as a tracer is the most reliable method to study the RTD, because of this tracer does not interact with the catalyst. Based on the RTD study, it was demonstrated that the temperature (in the range 30–60 °C) and the catalyst distribution inside the reactor do not affect the flow pattern, but the ratio of the catalyst mass-to-liquid ratio affects the mean residence time and the liquid dispersion. Furthermore, it was shown that the presence of an axial dispersion is negligible when the average particle diameter is <1.38 mm, e.g., Péclet number exceeds 100. The kinetic study was carried out at different mean residence times (from 10 to 55 min), at different wet catalyst loadings (from 25 to 49 g) and reactor temperatures (from 30 to 60 °C). Mathematical models comprising kinetic and dispersion parameters were developed to compare the estimated parameters obtained from the continuous fixed-bed reactor with the ones obtained from a batch reactor.     

Liquid phase mixing in turbulent bed contactor

Axial mixing of the liquid phase in turbulent bed contactor (TBC) is studied through residence time distribution (RTD) experiments over a large range of variables such as flow rate of gas and liquid phases, static bed heights, diameter and density of particles and number of stages in presence of downcomer using air water system. Since all the liquid exits only through the downcomer, it enables the correct estimation of exit concentration of the tracer. The experimental RTD curves are satisfactorily compared with the axial dispersion model. The Peclet numbers evaluated by axial dispersion model and the Peclet numbers reported in the literature are used to propose a unified correlation in terms of operating and geometric parameters. Correlation is also developed for predicting the axial dispersion coefficient. It was observed in the present study that almost plug flow conditions can be achieved in multistage TBC.     

Mathematical modeling of an in-line low-NOx calciner

The reduction of the NO_x content in in-line-calciner-type kiln systems can be made by optimization of the primary firing in the rotary kiln and of the secondary firing in the calciner. Because the optimization of calciner offers greater opportunities the mathematical modeling of this reactor is very important. A heterogeneous, dynamic mathematical model for an in-line low-NO_x calciner based on non-isothermal diffusion-reaction models for char combustion and limestone calcination has been developed. The importance of the rate at which preheated combustion air was mixed into the main flow was particularly studied. The results of the simulations indicate that the external heat and mass transfer to the char particles is not limiting. Internal diffusion of O₂, CO, NO and CO₂ is important especially in the reducing zone and the first part of the oxidizing zone of the calciner and the internal heat transport limitation is significant for the endothermic limestone calcination. The rate at which preheated combustion air is mixed into the main flow directly influences the coal combustion rate, and thereby through the rate of heat release from combustion, it also influences the calcination rate and the temperature profile. The mixing rate has some influence on the CO concentration profile and an important influence on the overall degree of fuel-N to NO conversion.     

Residence time distribution and modeling of the liquid phase in an impinging stream reactor

Based on some experimental investigations of liquid phase residence time distribution (RTD) in an impinging stream reactor, a two-dimensional plug-flow dispersion model for predicting the liquid phase RTD in the reactor was proposed. The calculation results of the model can be in good agreement with the experimental RTD under different operating conditions. The axial liquid dispersion coefficient increases monotonously with the increasing liquid flux, but is almost independent of gas flux. As the liquid flux and the gas flux increase, the liquid dispersion coefficient of center-to-wall decreases. The axial liquid dispersion coefficient is much larger than that of center-to-wall, which indicates that the liquid RTD is dominated mainly by axial liquid dispersion in the impinging stream reactor.     

自热转化炉停留时间分布

以氢气作为示踪剂,运用脉冲法测定自热转化炉内停留时间的分布。实验结果表明:随着催化剂床层的增高,停留时间分布密度函数变窄,平均停留时间和量纲一方差均减小;当进口气量增大时,平均停留时间减小,量纲一方差增大。应用N个全混流反应器(CSTR)、轴向混合模型和平推流模型串联建立自热转化炉停留时间分布模型,由Laplace变换法和阻尼最小二乘法对模型参数进行估算,模型估计停留时间曲线与实验测量曲线吻合良好。     

Residence time distribution and modeling of the liquid phase in an impinging stream reactor

Based on some experimental investigations of liquid phase residence time distribution (RTD) in an impinging stream reactor, a two-dimensional plug-flow dispersion model for predicting the liquid phase RTD in the reactor was proposed. The calculation results of the model can be in good agreement with the experimental RTD under different operating conditions. The axial liquid dispersion coefficient increases monotonously with the increasing liquid flux, but is almost independent of gas flux. As the liquid flux and the gas flux increase, the liquid dispersion coefficient of center-to-wall decreases. The axial liquid dispersion coefficient is much larger than that of center-to-wall, which indicates that the liquid RTD is dominated mainly by axial liquid dispersion in the impinging stream reactor.     

双颗粒提升管中细颗粒的混合行为研究(Ⅰ)──轴向混合行为

利用磷光颗粒示踪技术对双颗粒提升管中细颗粒的轴向混合行为进行了研究测得反映细颗粒轴向扩散的停留时间分布曲线呈双峰分布，这表明细颗粒存在两种类型的扩散形式：弥散颗粒扩散和颗粒团扩散本文提出的一维两组分扩散模型可较好地描述停留时间分布曲线的双峰分布.实验结果表明细颗粒的两种轴向混合行为有较大的差别，而总包Peclet数几乎不随操作条件及床内粗细颗粒比例变化     

Axial gas phase dispersion in a molten salt oxidation reactor

Gas phase axial dispersion characteristics were determined in a molten salt oxidation reactor (air-molten sodium carbonate salt two phase system). The effects of the gas velocity (0.05–0.22 m/s) and molten salt bed temperature (870–970 °C) on the gas phase axial dispersion coefficient were studied. The amount of axial gas-phase dispersion was experimentally evaluated by means of residence time distribution (RTD) experiments using an inert gas tracer (CO). The experimentally determined RTD curves were interpreted by using the axial dispersions model, which proved to be a suitable means of describing the axial mixing in the gas phase. The results indicated that the axial dispersion coefficients exhibited an asymptotic value with increasing gas velocity due to the plug-flow like behavior in the higher gas velocity. Temperature had positive effects on the gas phase dispersion. The effect of the temperature on the dispersion intensity was interpreted in terms of the liquid circulation velocity using the drift-flux model.     

双颗粒提升管中细颗粒的混合行为研究(Ⅰ)──轴向混合行为

利用磷光颗粒示踪技术对双颗粒提升管中细颗粒的轴向混合行为进行了研究测得反映细颗粒轴向扩散的停留时间分布曲线呈双峰分布,这表明细颗粒存在两种类型的扩散形式：弥散颗粒扩散和颗粒团扩散本文提出的一维两组分扩散模型可较好地描述停留时间分布曲线的双峰分布.实验结果表明细颗粒的两种轴向混合行为有较大的差别,而总包Peclet数几乎不随操作条件及床内粗细颗粒比例变化     

Axial gas dispersion in a fluidized bed of polyethylene particles

Gas mixing behavior was investigated in a residence time distribution experiment in a bubbling fluidized bed of 0.07 m ID and 0.80 m high. Linear low density polyethylene (LLDPE) particles having a mean diameter of 772 Μm and a particle size range of 200-1,500 Μm were employed as the bed material. The stimulus-response technique with CO₂ as a tracer gas was performed for the RTD study. The effects of gas velocity, aspect ratio (H₀/D) and scale-up on the axial gas dispersion were determined from the unsteady-state dispersion model, and the residence time distributions of gas in the fluidized bed were compared with the ideal reactors. It was found that axial dispersion depends on the gas velocity and aspect ratio of the bed. The dimensionless dispersion coefficient was correlated with Reynolds number and aspect ratio.     

A physical description of solids transport in flighted rotary dryers

In this paper, a solids transport model for flighted rotary dryers is presented. Emphasis is placed on combining statistical and mechanistic modelling efforts to create a physically motivated compartment model involving pairs of perfectly mixed tank reactors linked in a series arrangement. Here, each tank pair, or cell, aims to physically describe a finite slice of a case study industrial rotary sugar dryer, and is hence governed primarily by flight geometry and dryer operational variables such as rotational speed and dryer inclination. Solids flow paths are structured to properly represent the different modes of transport in the rotary dryer, and values of transport coefficients are based on calculated rates of rotational and axial flows. A solids dispersion variable is used to correlate the model residence time distribution (RTD) prediction with available data from a tracer study conducted during industrial operation of a full-scale raw sugar dryer. RTD results from the model show intuitive responses to variations in solids feed rate, rotational speed and drum inclination.     

Mixing characteristics in the torus reactor

The hydrodynamic characteristics of propeller-induced toroidal flow in a loop reactor were investigated by performing an RTD analysis. The experimental determination of circulation time allows the calculation of the mean axial velocity with respect to the rotational speed of the impeller. RTD measurements are interpreted with the aid of the dispersion plug flow model, and it is shown that axial dispersion is relatively weak in the torus reactor. The mixing time was also determined experimentally and related to the circulation time. A direct relationship between mixing time and axial dispersion coefficient has been established, leading to a correlation for the mixing time in a torus reactor.