Gas Residence Time Distribution Research Articles

Solid-fluid mixing behavior of conical spouted beds with internal devices

The effect of draft tubes and fountain confiner on the gas and solids mixing behavior is studied in conical spouted beds. Accordingly, the bed porosity has been determined in different hydrodynamic regimes of 1.1,1.25 and 1.5u/ums, with beds equipped with draft tubes of 0%,56% and 100% opening ratio. These devices significantly affect the gas residence time and particle cycle time distributions, which are further improved by using a fountain confiner, specially when high inlet gas flow rates are desired. The addition of this device was found to remove stagnant gas pockets over the annular zone, while reducing the particle cycle time by 15% with the OSDT configuration at 1.5u/ums. This reduction heavily depends on the distance between the bed surface and the fountain confiner. By including draft tubes, the expected particle cycle time can be more than doubled due to the reduced annular-spot solids circulation. Therefore, it was found that a combination of internal devices and operating flow rate present a promising strategy to control the gas flow pattern, while keeping the distribution of particles cycle times required for each application.

A convenient method to validate the gas flow of a CFD-CT simulation applied on a packed bed used in gas biofiltration through residence time distributions

In this work, the validation of a Computational Fluid Dynamics (CFD) model coupled with a 3D computational tomography (CT) bed description of the gas flow field inside a packed column used for gas biofiltration was conducted using a low-cost metal oxide sensor. The validation was carried out in terms of gas residence time distribution (RTD), which was constructed from the sensor measurements using a mathematical model to filter the transient signal behavior. The coupled CFD-CT model was used to obtain the steady-state velocity field inside the packed bed by numerically solving the incompressible Navier-Stokes equations. Later, the tracer injection was simulated over the obtained velocity field by solving a transport equation for a passive scalar. Finally, the experimental and simulated RTD were compared to validate the model. The comparison between the experiments and the CFD simulations showed good agreement between both shapes of the RTD distribution with a relative difference of 4.167% for the mean RTD, denoting the potential of the proposed methodology to validate the CFD model and predict the moments of the RTD. This methodology can become a very useful tool for the validation of CFD simulations with the final purpose of studying the processes at the microscale undergoing inside packed bed biofiltration reactors.

Hydrodynamics and mass transfer enhancement of gas–liquid flow in micropacked bed reactors: Effect of contact angle

AbstractPressure drop, residence time distribution, dispersive behavior, liquid holdup, and mass transfer performance of gas–liquid flow in micropacked bed reactors (μPBRs) with different contact angles (CA) of particles are studied. The value of pressure drop for three types of beads can be obtained: copper beads (CA = 88.1°) > stainless steel beads (CA = 70.2°) > glass beads (CA = 47.1°). The liquid axial dispersion coefficient is 1.58 × 10−6 to 1.07 × 10−5 m2/s for glass beads and copper beads, which is smaller than those of trickle bed reactors. The liquid holdup of 400 μm copper beads is larger than that of 400 μm glass beads. The ratio of effective interfacial area enhancement is evaluated up to 55% for big contact angle beads compared with the hydrophilic glass beads. In addition, correlations of pressure drop, liquid holdup, and effective interfacial area in μPBRs with different wettability beads are developed and predicted values are in agreement with the experimental data.

On-line reset control of a commercial-scale opposed multi-burner coal-water slurry gasification system using dynamic reduced-order model

The entrained flows from four opposed burners enhance the mixing in a coal-water slurry (CWS) gasifier, which increases the extent of carbon conversion. Moreover, the operation of the opposed multi-burner (OMB) gasifier is more flexible because one couple of the opposed burners is allowed to be shut off for a certain period as long as the other couple is still running. Thus, the operation lifetime of the gasification system is determined by the ability to return the shut-off burners to operation, referred to as online reset control (ORC). However, the ORC is manual, which increases the operation uncertainty. In this study, we demonstrate an automatic control strategy using a dynamic reduced-order model (ROM). A reactor network model (RNM) of the gasifier, which considers several sub-models, is developed based on a flow field analysis and particle and gas residence time distributions (RTDs). The dynamic ROM and simulated manual ORC are verified using industrial data (5.8 MPa, 1500 metric tons per day (TPD)). An automatic control scheme for the ORC is proposed. The dynamic ROM is validated to be reliable when operating with the commercial-scale OMB CWS gasification system, as well as with its manual ORC process. The automatic ORC is promising for industrial gasification because the fluctuation ranges of the operation temperature and pressure of the gasifier are acceptable, which indicates that the reliable dynamic ROM can simulate a closed-loop control for a complex reactor.

Kinetic investigation of N2 flowing DC discharges

Nitrogen flowing DC discharges were generated between two side-armed electrodes in a drift tube. The discharges operated at gas residence times (t) of ∼4 × 10−4 s, reduced electric fields (E/N) between 90 and 118 Td, and electron densities (ne) between 1010 and 1011 cm−3. A kinetic numerical model was elaborated to study the discharge kinetics. The model calculates the densities of 18 electronic states of nitrogen in the discharge, including the 45 vibrational levels of the N2(X1Σ+g) molecules, as functions of the gas residence time. The model is employed to describe the density profiles of neutral and excited atomic and molecular species, and nitrogen ions, along with the N2(X1Σ+g) vibrational distributions for our experimental conditions. The N2(X1Σ+g) vibrational and gas temperatures, E/N, ne, and the N2(B3Πg), N2(C3Πu), and N2+(B2Σ+u) relative densities were measured in the discharge by optical emission spectroscopy and double probes. The experimental determined gas temperature (Tg), electron density, and reduced electric field were used in the calculations of the electron energy distribution function and reaction rate constants. The vibrational temperature (Tv) and excited species densities measured were compared to the calculated values from the model. Although much attention has been devoted to the study of nitrogen DC discharges in the last few years, this work presents for the first time the N+ – N4+ and N2+(B2Σ+u) ion density distribution together with the densities of 13 atomic and molecular nitrogen states as functions of the discharge gas residence time and N2(X1Σ+g) vibrational distributions calculated for experimental conditions of low pressure DC discharges operating at short residence times.

Validation and application of a multiphase CFD model for hydrodynamics, temperature field and RTD simulation in a pilot-scale biomass pyrolysis vapor phase upgrading reactor

Accurate prediction of transport phenomena is critical for VPU reactor design, optimization, and scale-up. The current study focused on the validation and application of a multiphase CFD model within an open-source code MFiX for hydrodynamics, temperature field, and residence time distribution (RTD) simulation in a non-reacting circulating fluidized bed riser for biomass pyrolysis vapor phase upgrading (VPU). First, an Eulerian-Eulerian approach three-dimensional CFD model was employed to simulate the pilot-scale VPU riser on the supercomputer Joule. Excellent quantitative agreement between experimental and simulated results was achieved for pressure drops and temperature field in a range of operating conditions. Then the validated multiphase CFD model was applied to predict gas and solid residence time distributions (RTDs) since prediction and analysis of RTD is an important tool to study the complex multiphase flow behavior and mixing inside chemical reactors. The predictions show that solid mean residence time is 3.5 times the gas residence time; the solid RTD is more sensitive to the process gas flow rate than the solids circulation rate.

CFD Analysis of Gas Flow Characteristics and Residence Time Distribution in a Rotating Spherical Packing Bed

Rotating packed beds (RPBs) have attracted widespread attention due to their high mass transfer efficiency. In this paper, gas flow characteristics in a rotating spherical packing bed (RSPB) were i...

Model analysis of gas residence time in an ironmaking blast furnace

Gas residence time distribution (RTD) is an effective and convenient indicator for evaluating the performance of complex multiphase chemical reactors including ironmaking blast furnaces (BFs). However, in the open literature, there lacks the systematic RTD research for BFs. In this study, an integrated mathematical model is developed for describing the gas RTD of a BF. The model combines a steady multi-fluid model for describing the in-furnace state of flow and thermo-chemical behavior of gas-solid-liquid phases and a transient model for describing the dynamic behavior of tracer materials. The results show that the gas flow field inside the BF is quite complex, resulting from many factors such as furnace geometry and coupled thermo-chemical behaviors of other phases. The tail of gas RTD curve resulted from the lagging phenomenon of tracer flow inside BFs, is captured. The gas RTD is discussed by using mean residence time, dispersion of molecules distribution, cumulative distribution function and residence time intensity function. Under the given BF conditions, mean residence time and space time of gas fluids are predicted as 13.5 s and 16.3 s, respectively. The existence of stagnant flow in the BF can be both derived and directly identified. Moreover, it is indicated the gas flow patterns in the BF are composed of piston-type flow, stagnant flow and limited mixing flow. This study provides a cost-effective tool for better understanding BF gas flow dynamics and optimizing BF operations.

Conditioning micro fluidized bed for maximal approach of gas plug flow

We have for the first time investigated the gas-solid fluidization behavior in size-reduced beds called micro fluidized bed (Liu. X et al. Chem. Eng. J., 2008, 137: 302–307), but up to now there is not yet any clear definition for the micro fluidized beds (MFBs). This study intends to characterize MFBs in terms of gas back-mixing. The residence time distribution (RTD) and extent of back-mixing of gas were investigated in beds with inner diameters of up to 21 mm for particles of FCC, glass beads and silica sand. The RTD curves of gas, shown as E(t) versus time t, in such beds were determined on the basis of axial dispersion model to obtain the mean residence time t¯, variance σt2 and peak height of a given E(t) curve. In terms of these parameters the degree of gas back-mixing and its variation were evaluated with respect to particles, bed diameter (D), superficial gas velocity (Ug) and static particle bed height (Hs). It was found that the RTD of gas is subject to a unique correlation between the height of E(t) peak and the variance σt2, which makes it clear that the gas flow in an MFB has limited gas dispersion and is highly approaching to a plug flow if σt2 is below 0.25 or the height of E(t) peak is larger than 1.0. This refers to the feature that is most desired for an MFB and can thus be the criterion for defining MFB.

Simulations and pre-experiment uncertainty analyses for a hydrogen diffusion experiment using a “two side purged membrane” setup

An experiment termed “Q-PETE” representative for the situation in the Helium Cooled Pebble Bed (HCPB) breeding zone and suitable for the validation of relevant tritium transport codes has been devised. In a temperature controlled setup a hydrogen loaded gas is directed over a steel membrane (several millimeter thickness) into which it can permeate. On the other side of the membrane a sweep gas flow collects the permeated hydrogen and transports it to a quadrupole mass spectrometer for quantitative time resolved detection. This paper introduces methods to handle time resolved permeation experiments with sweep gas, considering the residence time distribution of the sweep gas. The methods are used to quantitatively simulate the planned experiments and expected signal distortion effects. Additionally, an uncertainty model is applied to find optimized process- and geometry parameters for the experiments.

Gas residence time distribution in a conical spouted bed

Conical spouted beds (CSB) are considered as promising contacting devices for several applications such as drying. One of the most critical parameters influencing the performance of a CSB is the gas partitioning between the spout and annulus regions. This study presents results from gas residence time distribution (RTD) experiments carried out using a radioactive tracer measurement in a fully instrumented laboratory CSB. RTD measurements were done in the conical empty vessel and in the CSB operating with and without a draft tube. Comparisons between the RTD curves obtained clearly show the influence of the operating parameters on the gas partitioning between the spout and annulus regions. It is highlighted that the fraction of total gas injected in the CSB that passes through the annulus reaches values up to 90% for large bed height. The fraction of total flow that passes through the spout and the annulus regions appears to be independent of the operating value of U/Ums. The gas penetration in the annulus occurs along the entire interface between these two regions.A predictive model for the partitioning of gas between the annulus and the spout is also developed based on the expression of mass and momentum balances for the gas in the different regions of the conical spouted bed. Experimental results are compared to the model's predictions in term of gas residence time distribution in the annulus and spout regions. The model is also successfully used to highlight gas-partitioning parameters.

Effectiveness of purging on preventing gas emission buildup in wood pellet storage

Storage of wood pellets has resulted in deadly accidents in connection with off‐gassing and self‐heating. A forced ventilation system should be in place to sweep the off‐gases and control the thermal conditions. In this study, multiple purging tests were conducted in a pilot scale silo to evaluate the effectiveness of a purging system and quantify the time and volume of the gas needed to sweep the off‐gases. To identify the degree of mixing, residence time distribution of the tracer gas was also studied experimentally. Large deviations from plug flow suggested strong gas mixing for all superficial velocities. As the velocity increased, the system dispersion number became smaller, which indicated less degree of mixing with increased volume of the purging gas. One‐dimensional modelling and numerical simulation of the off‐gas concentration profile gave the best agreement with the measured gas concentration at the bottom and middle of the silo.

Kinetic analysis of TiO2-catalyzed heterogeneous photocatalytic oxidation of ethylene using computational fluid dynamics

Photocatalytic oxidation of ethylene was carried out over a TiO2 catalyst at 295K using two types of non-ideal fixed-bed flow reactors: a rectangular reactor and a cylindrical reactor. Computational fluid dynamics (CFD) analysis using a low Reynolds number-type k-ε turbulence model was conducted to investigate the ethylene oxidation behavior in the reactors at various gas flow rates and ethylene concentrations in the inlet gas (ethylene concentration of 50–250ppm and gas flow rate of 50–250mL/min). In the rectangular reactor, steady-state activities were obtained for ethylene oxidation under all conditions. The rate of ethylene oxidation on the TiO2 surface was analyzed in terms of Langmuir–Hinshelwood (L–H) type kinetics. The kinetic parameters for the surface reactions obtained via CFD analysis fit well with the experimental data. The CFD analysis also revealed that the ethylene concentration distribution in the reactor depended on the gas residence time distribution. We also carried out CFD analysis for the cylindrical reactor and compared the ethylene oxidation behavior with that in the rectangular reactor.

CFD Simulation of Gas Residence Time Distribution in Agitated Tank

Gas residence time is an important parameter for gas-liquid agitated tank. Two approaches, i.e., Euler-Tracer method and CFD-DPM method are proposed for predicting gas residence time distribution (RTD) in an aerated agitated tank by using a Fluent 6.2 software package. The simulation results show that the characteristic of the gas-RTD is a curve with single peak and long tailing. Bubble size, stirring speed and gas inlet flow rate have great effect on gas-RTD in the stirred tank. Small bubbles have wider residence time distribution and stay in the vessel longer than the large bubbles and tend to complete mixing. With increasing of impeller speed or decreasing of gas inlet rate, gas-RTD become wider and have longer average gas residence time, which is in favor of gas effectively utilization.

Role of thermal intensity on operational characteristics of ultra-low emission colorless distributed combustion

This paper examines the development of ultra-low emission colorless distributed combustion (CDC) for gas turbines, operating at thermal intensity in the range of 5–453MW/m3atm. Higher thermal intensity combustors are desirable for increased performance with minimal increase in hardware costs in terms of both weight and volume of gas turbine combustors. Most land based gas turbine combustors operate at thermal intensity of about 15MW/m3atm but operation at higher intensity can provide increased performance. Design of high thermal intensity CDC combustor requires control of critical parameters, such as gas recirculation, fuel/oxidizer mixing, air preheats, and residence time distribution characteristics via proper selection of different air and fuel injection configurations to achieve desirable combustion characteristics. Initially, various flow field configurations were investigated at a low thermal intensity of 5MW/m3atm. This effort was followed by investigations at higher thermal intensity ranges of 20–40MW/m3atm by reducing the combustor volume for a forward flow configuration under more favorable novel non-premixed conditions. Further investigations were performed for a simpler combustor having single injection ports for air and fuel with detailed investigation of various flow field configurations performed at a thermal intensity of 28MW/m3atm. Further reduction in combustion volume resulted in thermal intensity of 57MW/m3atm. For the flow configurations investigated, reverse cross-flow configuration was found to give more favorable results with air injected from exit end and fuel injected in a cross-flow direction to the air flow. This reverse cross-flow geometry was investigated in detail by further reducing the combustor volume. Air injection diameter was increased to reduce the pressure drop across the combustor. The combustor was investigated at thermal intensity in the range of 53–85MW/m3atm. This geometry resulted in 4ppm NO emission and CO emissions of about 27ppm under novel non-premixed flow conditions at thermal intensity of 53MW/m3atm. The pressure drop at this operational point was less than 5% to meet the conventional combustor requirements. Further reduction in combustor volume resulted in very high thermal intensity in the range of 156–198MW/m3atm and at the most desirable operational point NO and CO emissions were 8ppm and 82ppm, respectively under novel non-premixed mode at a thermal intensity of 170MW/m3atm. Even further reduction in combustor volume resulted in combustor operation at ultra-high thermal intensity of 283–453MW/m3atm and at the desirable operating point the NO and CO emissions were 4ppm and 115ppm respectively at a thermal intensity of 340MW/m3atm. The sequential efforts reveal the possibility of significantly increasing the thermal intensity while still achieving desirable emission levels.

CFD-DPM Modeling of Gas-Liquid Flow in a Stirred Vessel

Gas-liquid flow in a stirred vessel was simulated numerically with computational fluid dynamics(CFD). Gas was treated as discrete phase and described by discrete phase model (DPM), while the liquid was considered as a continuum and solved under Euler reference frame. The liquid velocity, gas holdup and gas residence time distribution in the stirred vessel were predicted. The simulation results show that gas dispersion in the stirred vessel is very non-uniformity and high gas holdup is found in the centre of the stirred vessel and vortexes while relatively low in bottom region and region between two impellers. Liquid velocity has great influence on bubble residence time and size distributions.

Insights into large‐scale cell‐culture reactors: II. Gas‐phase mixing and CO2 stripping

Most discussions about stirred tank bioreactors for cell cultures focus on liquid-phase motions and neglect the importance of the gas phase for mixing, power input and especially CO(2) stripping. Particularly in large production reactors, CO(2) removal from the culture is known to be a major problem. Here, we show that stripping is mainly affected by the change of the gas composition during the movement of the gas phase through the bioreactor from the sparger system towards the headspace. A mathematical model for CO(2)-stripping and O(2)-mass transfer is presented taking gas-residence times into account. The gas phase is not moving through the reactor in form of a plug flow as often assumed. The model is validated by measurement data. Further measurement results are presented that show how the gas is partly recirculated by the impellers, thus increasing the gas-residence time. The gas-residence times can be measured easily with stimulus-response techniques. The results offer further insights on the gas-residence time distributions in stirred tank reactors.

Coupling between countercurrent gas and solid flows in a moving granular bed: The role of shear bands at the walls

We extended the standard approach to countercurrent gas–solid flow in vertical vessels by explicitly coupling the gas flow and the rheology of the moving bed of granular solids, modelled as a continuum, pseudo-fluid. The method aims at quantitatively accounting for the presence of shear in the granular material that induces changes in local porosity, affecting the gas flow pattern through the solids. Results are presented for the vertical channel configuration, discussing the gas maldistribution both through global and specific indexes, highlighting the effect of the relevant parameters such as solids and gas flowrate, channel width, and wall friction. Non-uniform gas flow distribution resulting from uneven bed porosity is also discussed in terms of gas residence time distribution (RTD). The theoretical RTD in a vessel of constant porosity and Literature data obtained in actual moving beds are qualitatively compared to our results, supporting the relevance under given circumstances of the coupling between gas and solids flow.

Simulation Study of Gas-Solid Macro-Mixing Characterization in Small Fixed-Fluidized Bed Reactor

In fixed-fluidized bed reactor, laboratory evaluation of different catalyst, raw materials and process parameters can be implemented, so it has wide applications in the refining process. In this study, we focused on small fixed-fluidized bed reactor, using Eulerian-Eulerian two-fluid model, simulated the gas-solid flow behavior in it. Gas residence time distribution was measured in order to characterize macro-mixing. At the same time, by changing the reactor structure and operating conditions, we studied their effects on gas-solid macro-mixing characterization. The results show that the effects of structural parameters are larger than operating conditions, and different parameters have different effects. Different parameters can be adjusted to change the gas-solid macro-mixing characterization in small fixed-fluidized bed reactor. Therefore, the small fixed-fluidized bed reactor can provide better results in more application areas.

Gas flow behavior in industrial FCC disengager vessels with different coupling configurations between two-stage separators

The gas flow behaviors and gas residence time distribution in industrial FCC disengager space are numerically studied with the combined consideration of the product gas, stripping gas and anti-coking steam. Although purge steam is used in the top dome in order to alleviate the coke formation in the refinery plants, its effectiveness is still obscure. In this study, the Reynolds stress model is applied to calculate the gas flow field and the scalar transport equations are used to obtain the gas residence time distribution. The species transport equations are used to get the local mass fractions of steam and oil vapors and further to verify the effectiveness of the anti-coking steam. In view of the wide use of “open” configuration, the optimizing position of the primary cyclone outlet is proposed for an industrial case in Qianguo refinery. Based on the results, suggestions are made for improved operation of the industrial units.