Gas Residence Time Distributions in a Spouted Bed

In a spouted bed of 80mm in ID and 1700mm in height, the gas residence time distributions at different radial positions in both spout and annular area were measured with five different kinds of particles as spouting material, air as spouting gas, and hydrogen as tracer. The effects of superficial gas velocity, operating pressure, particle size and its category on gas residence time distribution were discussed. It was found that the gas velocity profile in spout was more uniform than that in annulus. It could be concluded that the gas flow in the spout could be treated as a plug-flow, while that in the annulus inhibited a strong non-ideal flow behavior. Increasing the superficial gas velocity and decreasing the operating pressure, the particle density and its size gave rise to spouting disturbance, thus the measured tracer concentrations vs. time curves fluctuated. The variances of residence time distribution curves could be taken as a measure of the gas fluctuation degree.

Particle residence time distribution and axial dispersion coefficient in a pressurized circulating fluidized bed by using multiphase particle-in-cell simulation

The particle residence time distribution(RTD)and axial dispersion coefficient are key parameters for the design and operation of a pressurized circulating fluidized bed(PCFB).In this study,the effects of pressure(0.1-0.6 MPa),fluidizing gas velocity(2-7 m·s^(-1)),and solid circulation rate(10-90 kg·m^(-2)·s^(-1))on particle RTD and axial dispersion coefficient in a PCFB are numerically investigated based on the multiphase particle-in-cell(MP-PIC)method.The details of the gas-solid flow behaviors of PCFB are revealed.Based on the gas-solid flow pattern,the particles tend to move more orderly under elevated pressures.With an increase in either fluidizing gas velocity or solid circulation rate,the mean residence time of particles decreases while the axial dispersion coefficient increases.With an increase in pressure,the core-annulus flow is strengthened,which leads to a wider shape of the particle RTD curve and a larger mean particle residence time.The back-mixing of particles increases with increasing pressure,resulting in an increase in the axial dispersion coefficient.

Modeling on Residence Time Distribution in Subsurface Flow Constructed Wetlands by Multi Flow Dispersion Model

As an important design factor for constructed wetlands,hydraulic retention time and its distribution will affect the treatment performance.Instantaneously injected sodium chloride tracers were used to obtain residence time distributions of the lab scale subsurface flow constructed wetland.Considering the presence of trailing and multiple peaks of the tracer breakthrough curve,the multi flow dispersion model(MFDM)was used to fit the experimental tracer breakthrough curves.According to the residual sum of squares and comparison between the experimental values and simulated values of the tracer concentration,MFDM could fit the residence time distribution(RTD)curve satisfactorily,the results of which also reflected the layered structure of wetland cells,thus to give reference for application of MFDM to the same kind of subsurface flow constructed wetlands.

Numerical simulation of flow field and residence time of nanoparticles in a 1000-ton industrial multi-jet combustion reactor

In this work,by establishing a three-dimensional physical model of a 1000-ton industrial multi-jet combustion reactor,a hexahedral structured grid was used to discretize the model.Combined with realizable k–εmodel,eddy-dissipation-concept,discrete-ordinate radiation model,hydrogen 19-step detailed reaction mechanism,air age user-defined-function,velocity field,temperature field,concentration field and gas arrival time in the reactor were numerically simulated.The Euler–Lagrange method combined with the discrete-phase-model was used to reveal the flow characteristics of particles in the reactor,and based on this,the effects of the reactor aspect ratios,central jet gas velocity and particle size on the flow field characteristics and particle back-mixing degree in the reactor were investigated.The results show that with the decrease of aspect ratio in the combustion reactors,the velocity and temperature attenuation in the reactor are intensified,the vortex phenomenon is aggravated,and the residence time distribution of nanoparticles is more dispersed.With the increase in the central jet gas velocities in reactors,the vortex lengthens along the axis,the turbulence intensity increases,and the residence time of particles decreases.The back-mixing degree and residence time of particles in the reactor also decrease with the increase in particle size.The simulation results can provide reference for the structural regulation of nanoparticles and the structural design of combustion reactor in the process of gas combustion synthesis.

THE RESIDENCE TIME DISTRIBUTION FOR MULTIFLOW SYSTEM

In this paper,the superposition rule of the residence time distribution functions for the general systemhaving multiple inlet and outlet streams has been described and proved rigorously.For the cascade ves-sels system where the processed material in separate stages may be nonideally mixed in various degrees andthe volumes of separate stages may not be equal,the overall residence time distribution function E(t)and eachE(t)of the flow systems have been derived.The applications of these results to various flow systems havebeen discussed.

Residence Time Distribution at Laminar Pulsatile Flow in a Straight Pipe

This paper deals with the problem of theoretical identification of the residence time distribution （RTD） characteristics of a straight pipe at laminar pulsatile flow, if tracer diffusion can be neglected. This situation is typical for micro-apparatuses （e.g. fluidic element） and also for flow in large arteries. Residence time distribution based on velocity profiles at pulsatile flow of a Newtonian liquid in a rigid pipe will be derived theoretically and compared with the well known results for a constant flow rate E（τ） = τ-^2/2τ^3 at τ 〉 τ^-/2, where E （τ） is differential distribution, x is residence time and τ^- is the mean residence time. The following part of the paper deals stimulus response experimental techniques using tracers. The principal problem related to laminar and convection dominated pulsatile flows is discussed： Can the impulse response also be identified with the actual residence time distribution in the case of variable flow？ The general answer is no, and differences between RTD and impulse responses are evaluated as a function of the frequency and amplitude of pulsatile flows.

Numerical analysis on the transport properties and residence time distribution of ribbon biomass particles in a riser reactor based on CFD-DEM approach

A bended ribbon biomass particle model was developed to explore the dynamic transport properties inside a riser reactor.Residence time distribution(RTD)of the particles was analyzed by using the Eulerian-Lagrange method.The effects of sampling height,particle density,particle size and gas-to-solid mass ratio on RTD were investigated.The coupled Computational Fluid Dynamics and Discrete Element Method(CFD-DEM)model was verified firstly by experimental data on pressure drop and residence time distribution density function.The simulation results demonstrated that the ribbon biomass particles display a typical annular-core spatial distribution during transportation.The RTD of particles exhibit an approximate single-peaked normal distribution.The mean residence time(MRT)can reach up to 0.7 s when the particle density is 1200 kg/m^(3).Particle with higher density has longer mean residence time.The flow patterns are closer to plug flow if particle length over 12 mm.The particle flow pattern is not sensitive to changes in particle density and size,while the gas-to-material mass ratio has a significant impact on it.

Analysis model of internal residence time distribution for fluid flow in a multi-strand continuous casting tundish

The external residence time distribution(RTD)curve is extensively used to characterise fluid flow within the single-strand continuous casting tundish.Dead volume fraction determination typically relies on the external RTD curve to reveal macroscopic fluid flow behaviour.Based on the external RTD to effectively assess dead volume fractions and other fluid characteristics under conditions of internal non-uniform flow,an internal RTD was introduced.In a smooth pipe under laminar flow conditions,the dead region occupies 25%of the total volume,which is defined as the space between the pipe wall and a radius of 0.866 R0(where R0 is the radius of pipe).Under turbulent flow conditions,the dead region only occupies 0.38%of the reactor’s internal volume,spanning from the pipe wall to a radius of 0.00189 R0.The results obtained using the external RTD method are consistent with the theoretical analysis.Experimental trials involving water were conducted to examine the flow of molten steel within a five-strand tundish.Subsequently,an analysis approach employing internal RTD was employed to evaluate fluid mixing within a multi-flow continuous casting tundish.Using the internal RTD method,the analysis revealed that the whole dead zone volume fraction of the intermediate package decreased from 26.9%to 18.9%after the addition of the flow control device.The dead volume fraction can be accurately depicted by utilising the internal mean RTD function.The association between the internal RTD function and the external average RTD can be effectively employed to scrutinise the response curve of the tracer within a system exhibiting uneven flow distribution.

Residence time distribution of wood chips in a semi-industrial multiple hearth furnace using RFID tracers Author links open overlay panel

In continuous biomass torrefaction plants,the products'yields,composition and homogeneity highly depend on the residence time of particles.A characterization of particle residence time distribution(RTD)was therefore carried out in an industrial-scale multiple hearth furnace on poplar wood chips using radio frequency identification tracers.The effects of operating conditions,namely,mass flow rate of biomass,shaft speed of the rabbling system and interdental length on the RTD were studied.The increase of shaft speed and mass flow rate reduces particles’mean residence time.Lowering the length between two successive teeth also increases the bed speed.Uncontrollable biomass accumulation(also called“bulldozing”)was observed during several tests.This phenomenon is favored by a high mass flow rate of resources,a small interdental length between the teeth and a low shaft speed.RTD measurements were compared to the axial dispersion model.For all tests,the Peclet number is ranging between 20 and 62,indicating that the multiple hearth furnace cannot be modelled as an ideal plug flow reactor.

Measurement,Modelling and Analysis of Residence Time Distribution Characteristics in a Continuous Hydrothermal Reactor

Understanding the residence time distribution(RTD)of a continuous hydrothermal reactor is of great significance to improve product quality and reaction efficiency.In this work,an on-line measurement system is attached to a continuous reactor to investigate the characteristics of RTD.An approach that can accurately fit and describe the experimental measured RTD curve by finding characteristic values is proposed for analysis and comparison.The RTD curves of three experiment groups are measured and the characteristic values are calculated.Results show that increasing total flow rate and extending effective reactor length have inverse effect on average residence time,but they both cause the reactor to approach a plug flow reactor and improve the materials leading.The branch flow rate fraction has no significant effect on RTD characteristics in the scope of the present work except the weak negative correlation with the average residence time.Besides,the natural convection stirring effect can also increase the average residence time,especially when the forced flow is weak.The analysis reveals that it is necessary to consider the matching of natural convection,forced flow and reactor size to control RTD when designing the hydrothermal reactor and working conditions.

Residence time distribution and heat/mass transfer performance of a millimeter scale butterfly-shaped reactor

A millimeter scale butterfly-shaped reactor was proposed based on sizing-up strategy and fabricated via femtosecond laser engraving. An improvement of mixing performance and residence time distribution was realized by means of contraction and expansion of the reaction channel. The liquid holdup was greatly increased through connection of multiple mixing units. Structure optimization of the reactor was carried out by computational fluid dynamics simulation, from which the effect of reactor internals on mixing and the influence of parallel branching structure on heat transfer were discussed. The UV–vis absorption spectroscopy was used to determine the residence time distribution in the reactor, and characteristic parameters such as skewness and dimensionless variance were obtained. Further, a chained stagnant flow model was proposed to precisely describe the trailing phenomenon caused by fluid stagnation and laminar flow in small scale reactors, which enables a better fit for the experimental results of the asymmetric residence time distribution. In addition, the heat transfer performance of the reactor was investigated, and the overall heat transfer coefficient was 110–600 W m^(-2)K-1in the flow rate range of 10–40 m L/min.

Application of computational fluid dynamic to model the hydraulic performance of subsurface flow wetlands

A subsurface flow wetland(SSFW)was simulated using a commercial computational fluid dynamic(CFD)code.The constructed media was simulated using porous media and the liquid resident time distribution(RTD)in the SSFW was obtained using the particle trajectory model.The effect of wetland configuration and operating conditions on the hydraulic performance of the SSFW were investigated.The results indicated that the hydraulic performance of the SSFW was predominantly affected by the wetland configuration.The hydr...

Onset velocity of circulating fluidization and particle residence time distribution:A CFD-DEM study

Until now, the onset velocity of circulating fluidization in liquid-solid fluidized beds has been defined by the turning point of the time required to empty a bed of particles as a function of the superfcial liquid velocity, and is reported to be only dependent on the liquid and particle properties. This study presents a new approach to calculate the onset velocity using CFD-DEM simulation of the particle residence time distribution （RTD）. The onset velocity is identified from the intersection of the fitted lines of the particle mean residence time as a function of superficial liquid velocity. Our results are in reasonable agreement with experimental data. The simulation indicates that the onset velocity is infuenced by the density and size of particles and weakly affected by riser height and diameter, A power-law function is proposed to correlate the mean particle residence time with the superficial liquid velocity. The collisional parameters have a minor effect on the mean residence time of particles and the onset velocity, but influence the particle RTD, showing some humps and trailing. The particle RTD is found to be related to the particle trajectories, which may indicate the complex flow structure and underlying mechanisms of the particle RTD.

Hydrodynamic characteristics of a four-compartment periodic anaerobic baffled reactor

Periodic anaerobic baffled reactor （PABR） is a novel reactor based on the design concept of anaerobic baffled reactor （ABR）. Residence time distribution （RTD） studies on both clean and working reactors at the same hydraulic residence time （HRT） of 2 d were carded out to investigate the dead spaces and mixing patterns in PABRs at different organic loading rates （OLRs） in various switching manners and frequencies. The results showed that the fraction of dead space in PABR was similar to that in ABR, which was low in comparison with other reactor designs. Dead space may be divided into two categories, hydraulic and biological. In RTD studies without biomass, the hydraulic dead space in the PABR run in an ＂every second＂ switching manner with T = 2 d was the lowest whereas that in the PABR run in a T = ∞ （ABR） switching manner was the highest. The same trend was obtained with the total dead space in RTD studies with biomass no matter what the OLR was. Biological dead space was the major contributor to dead space but affected decreasingly at higher OLR whichever switching manner the PABR run in. The flow patterns within the PABRs were intermediate between plug-flow and perfectly mixed under all the conditions tested,

Scale-up of bubbling fluidized beds with continuous particle flow based on particle-residence-time distribution

Few studies have investigated scale-up of the residence-time distribution （RTD） of particles in bubbling fluidized beds （BFBs） with continuous particle flow. Two approaches were investigated in this study： first, using well-known scaling laws that require changes in particle properties and gas velocity; second, using a simple approach keeping the same particles and gas velocity for different beds. Our theoretical analysis indicates it is possible to obtain similar RTDs in different BFBs with scaling laws if the plug-flow residence time （tpiug） is changed as m^0.5, where m is the scaling ratio of the bed; however, neither approach can ensure similar RTDs if tplug is kept invariant. To investigate RTD variations using two approaches without changing tplug, we performed experiments in three BFBs. The derivatives dE（θ）/dθ （where E（θ） is the dimensionless RTD density function and θ is the dimensionless time） in the early stage of the RTDs always varied with m 1, which was attributed to the fact that the particle movement in the early stage were mainly subject to dispersion. Using the simple approach, we obtained similar RTDs by separately treating the RTDs in the early and post-stages. This approach guarantees RTD similarity and provides basic rules for designing BFBs.

The Relationship Between Hysteresis and Liquid Flow Distribution in Trickle Beds

Experiments were conducted on a trickle bed with 0.283m ID to elucidate the relationship between hysteretic phenomena and liquid distribution. The hysteresis of pressure drop and the variance of radial liquid distribution were observed simultaneously. Residence time distribution (RTD), holdup and mean residence time (RT) of liquid phase were also found to demonstrate hysteresis of the same nature. RTD, liquid holdup and mean RT calculated with a simple model from the distribution of liquid flow rate show characteristics consistant with the experimental data, suggesting that the hyteretic phenomena originate from the multiplicity and nonuniformity of liquid flow distribution.

Optimization of residence time distribution in RCDG and an assessment of its applicability in continuous manufacturing

Knowledge of residence time is a critical aspect in developing control and material diversion strategies for continuous manufacturing processes in pharmaceutical manufacturing.Dry granulation is a promising continuous granulation technique as it is fast and economical.In this study,a step-change method to determine residence time in roll compaction/dry granulation is introduced.The factors roll speed and rotational speed of the impeller in the powder inlet unit of the compactor were evaluated using a central composite circumscribed statistical design of experiments in order to optimize the residence time.The fill volume in the compactor was varied exemplarily.It was found that high roll speed,low rotational speed of the impeller and low fill volume in the compactor are beneficial to generate fast transition through the compactor.The impact of roll speed increase was estimated.It can be concluded that despite fast residence time in the process,high roll speed and its subsequent high material throughput can generate a large amount of material that has to be discarded if material diversion is required.

Numerical Investigation of Residence Time Distribution for the Characterization of Groundwater Flow System in Three Dimensions

How to identify the nested structure of a three-dimensional(3D)hierarchical groundwater flow system is always a difficult problem puzzling hydrogeologists due to the multiple scales and complexity of the 3D flow field.The main objective of this study was to develop a quantitative method to partition the nested groundwater flow system into different hierarchies in three dimensions.A 3D numerical model with topography derived from the real geomatic data in Jinan,China was implemented to simulate groundwater flow and residence time at the regional scale while the recharge rate,anisotropic permeability and hydrothermal effect being set as climatic and hydrogeological variables in the simulations.The simulated groundwater residence time distribution showed a favorable consistency with the spatial distribution of flow fields.The probability density function of residence time with discontinuous segments indicated the discrete nature of time domain between different flow hierarchies,and it was used to partition the hierarchical flow system into shallow/intermediate/deep flow compartments.The changes in the groundwater flow system can be quantitatively depicted by the climatic and hydrogeological variables.This study provides new insights and an efficient way to analyze groundwater circulation and evolution in three dimensions from the perspective of time domain.

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.

Adsorption of Protein in the Expanded Bed

The influence of operation parameters on the adsorption performance of protein (bovin serum albumin, BSA) in an expanded bed was studied using Streamline diethyl aminoethyl (DEAE). The result of residence time distributions (RTD) and breakthrough curves showed that adsorption performance of the expanded bed could not be improved by increasing the flow velocity at the range from 16ml·min-1 to 26ml·min-1. The increase of protein concentration from 0.5 mg·ml to 2 mg·ml-1 resulted in poor adsorption performance. With increasing temperature from 5℃ to 30℃ and the sedimented bed height from 11.5cm to 22.5cm, the adsorption characteristics in the expanded bed were improved.