Dense Phase Flow Research Articles

Effect of regenerated standpipe flow pattern on catalyst transport in fluid catalytic cracking unit

Standpipe is a pipeline that constitutes one of the components in the downstream portion of the catalyst circulation loop in a fluid catalytic cracking (FCC) unit, facilitates catalyst transportation between reactor and regenerator. This study measured pressures of the regenerated standpipe on a 1.0 Mt/a FCC unit to elucidate the underlying cause of the observed catalyst transportation. The results indicated that multiple flow patterns coexist within the standpipe, rather than a single dense-phase flow. The characteristic parameters of the static and dynamic pressure were intimately connected to the aforementioned flow patterns. The dynamic pressure data indicated that pressure dominant frequency and amplitude could employed in the identification of flow patterns. The packed bed flow had a pressure frequency ranging from 0 to 3.3 Hz, and a dominant frequency amplitude of 0.8, 1.8 and 2.2 Hz, allowed diagnosis of poor catalyst transport. The study demonstrated that the standpipe structure and catalyst mass flow rate were the key factors affecting gas-solid flow degassing by allowing for multiple flow patterns in the standpipe. The influence of aeration on the flow patterns was also discussed here, with an improper aeration rate resulting in reaction temperature fluctuations ranging from 0.5 to 23 °C.

Modeling Dense Particle Flow in Multistage and Obstructed Flow Receivers Using High Fidelity Simulations

Particles are a leading contender for next-generation, concentrating solar power technologies, and the design of the particle receiver is critical to minimize the levelized cost of electricity. Falling particle receivers (FPRs) are a viable receiver concept, but many new designs feature complex particle obstructions that include dense discrete phase flows. This creates additional challenges for modeling as particle-to-particle interactions (i.e., collisions) and particle drag become more complex. To improve upon existing modeling strategies, a CFD-DEM simulation capability was created by coupling two independent codes: Sierra/Fuego and LAMMPS. A suitable receiver model was then defined using a traditional continuum-based model for the air and a granular model for the particle curtain. A sensitivity study was executed using this model to determine the relevance of different granular model inputs on important quantities of interest in obstructed flow FPRs: the particle velocity and curtain opacity. The study showed that the granular model inputs had little effect on the particle velocity magnitude and curtain opacity after an obstruction.

Experimental study on flow characteristics of gas–solid two-phase flow in inclined pipes

In a fluid catalytic cracking unit, inclined pipes are mainly used for particle transportation and to maintain the pressure balance between the reactor and regenerator. In this experimental inclined pipe device, the characteristics of gas–solid two-phase flow were studied by measuring and analyzing the dynamic pressure and particle concentration parameters in different flow states. Corresponding relationships between pressure fluctuation and particle concentration on the one hand and flow patterns on the other were discovered. These were used for the quantitative identification of flow patterns. Measurement data from an analysis of bubble and particle velocities in an inclined pipe revealed a critical mass flow rate, Gs, in the mutual transformation of different particle flow patterns. The critical mass flow rate Gsc1 of lean-phase and dense-phase flows was 149 kg/(m2·s). The critical mass flow rate Gsc2 of packed-bed and dense-phase flows was 240 kg/(m2·s). This study enriches the basic theoretical knowledge on flow patterns in inclined pipes and provides a theoretical basis for the adjustment of inclined pipe operation in industrial fluid catalytic cracking units.

A framework for modelling single slug horizontal pneumatic conveying

The complexity of dense phase flows generally results in a reliance on empirical or qualitative analysis, which lacks reliability and limits the adoption of these desirable low-velocity flows in practice. Within horizontal slug flow, an additional challenge has been relating the highly dynamic properties of individual slugs to steady feed rates of solids and gas — with the result being the widespread application of steady-state assumptions on slug properties. This paper attempts to address these gaps through the development of a numerical framework for modelling single slugs founded on the authors’ recent research on slug dynamics and stability. Based on a finite difference approximation, the developed framework provides a much-needed fundamental and quantitative tool for analysing slug dynamics and a foundation for research beyond single slug conveying.

Electrical capacitance tomography-based estimation of slug flow parameters in horizontally aligned pneumatic conveyors

Pneumatic conveying is a widely applied technique for the transportation of powders. Dense phase conveying such as slug flow is the preferable operating condition of such transport systems. Advantages are the reduced energy and conveying gas consumption and reduced material wear. The design and the operation of dense phase conveyors is a challenging task due to complex flow dynamics and the lack of reliable flow meters, which are crucial for process control. In this work the capabilities of electrical capacitance tomography (ECT) for slug flow parameter estimation are analysed. The estimation of flow parameters is addressed and demonstrated by means of laboratory experiments. Amongst other parameters it is shown that mass concentration distributions as well as particle velocity fields can be obtained from tomographic image reconstruction results, which are substantial parameters for potential mass flow metering. Additionally, the proposed estimation techniques are applied to signals provided by an ECT system, which is installed within a large-scale pneumatic conveyor of an industrial process plant. It is observed that also Geldart group A powders with Archimedes numbers in the scale of Ar≈1e0 are conveyable in slug-like structures which can be separated by stationary or non-stationary layers. Furthermore, various slug flow properties of powders with Ar>1e2 reported in literature are confirmed but also deviating properties are observed, leading to novel insights into horizontal dense phase flow processes of powders with Ar≈1e0. The results demonstrated in this work are pointing out the capabilities of ECT for flow analyses, which are enabling further insights into flow mechanisms and future developments of flow meters for dense phase flow processes.

Experimental study on heat transfer of dense phase flow through a horizontal tube under constant wall flux conditions

A novel application of particles is utilization as dense phase flow for heat capture, transportation and storage. The use of horizontal dense phase flow as heat transfer fluid in solar systems is discussed with respect to current technologies. The investigation aims to analyze the heat transfer behavior of a horizontal dense phase flow under the condition of uniform surface heat flux. The proposed technologies transport particles as semi-continuous plug flow with a bottom layer, particles at the bottom layer will regularly renew once plugs pass by. Tests were conducted in a self-pilot scale-up platform. The measured wall-to-mixture heat transfer coefficient varies from ∼303 to 813 W/m2K in the particle mass flux range from 125 to 777 kg/m2s. Three heat transfer equations will be provided with the experimental data. A heat transfer equation describes the wall-to-mixture heat transfer coefficient linearly increases with particle mass flux. Then, a general empirical correlation in a dimensionless form predicted results well with experimental data of the present work and other data reported in the literature. Furthermore, a mechanistic heat transfer model is proposed according to a partial penetration mechanism and multi-layer flow model. The predicted results agree with experimental results within 12% of the average error. Considering the findings in this paper, researchers may have ideas to improve the heat transfer of horizontal dense phase flow to utilize this technology to the parabolic concentrator for the solar thermal industry.

The characteristics of flow and wall-to-bed heat transfer for vertical dense-phase transport flow

The current work demonstrates the heat transfer capacity of dense-phase transport flow as a heat transfer fluid (HTF) in a vertical tube. Dense-phase transport flow is an ideal HTF that can be utilized in the industrial field, especially for concentrating solar power. Experiments were conducted with quartz particles (317 μm, 184 μm) and desert particles (164 μm) in a self-established 49 mm I.D. × 4 m industrial pipe under the uniform heat flux. The thermal behaviors are investigated for different particle mass fluxes and solid loading ratios, whilst covering the different hydrodynamic flow regimes of plug flow, slug flow, turbulent flow, and dense-riser upflow. The wall-to-bed heat transfer coefficients are calculated from experimental data, ranging from 300 W/(m2 K) to 1200 W/(m2 K). It is sensitive to the particle mass flux and hence flow regimes. A semi-empirical correlation is developed relating the Nusselt number to Froude number, Reynolds number, and particle-tube diameter ratio. It provides a fair estimate of the heat transfer coefficient compared with the empirical predictions of two other published dimensionless correlations. The application of the new semi-empirical correlation to other published experimental data is also investigated for an extrapolation analysis. With knowledge of heat transfer characteristics, the dense-phase flow is expected to open new opportunities for high-efficiency thermodynamic cycles in concentrating solar power.

On the Use of a Rotatable ECT Sensor to Investigate Dense Phase Flow: A Feasibility Study.

This paper presents the feasibility study of dynamic flow measurements using the concept of a rotatable electrical capacitance tomography (ECT) sensor. The experiment considered horizontal flow in a pneumatic conveying flow loop in the case of dense phase flow. Slugs and settled layers were imaged and a comparison was made between no rotation or rotation of the sensor for two image reconstruction schemas: linear back projection (LBP) and non-linear iterative back projection. Data were evaluated both qualitatively and quantitatively by estimating the solids concentration level for different hue levels.

Transport boundaries and prediction of the slug velocity and layer fraction in horizontal slug flow pneumatic conveying

The poorly understood mechanisms of slug flow remain an obstacle for widespread application of this dense phase flow. The parameters that characterise the flow have repeatedly been observed to have a bounded range of operating conditions; however, the ability to reliably predict these boundaries has not achieved the same level of repeatability. This paper presents a model to predict the absolute maximum transport boundaries for slug velocity and the absolute minimum transport boundaries for the layer fraction as a function of the gas mass flow rate. The predicted transport boundaries are supported by measurements, following which, the model is further developed as a prediction tool, where very good agreement with measurements is achieved. As the model provides relationships between all of the key parameters of slug flow, analysis is provided demonstrating the ability of the model to be applied as a convenient design tool for slug flow pneumatic conveying systems.

Estimate Mass Flow Rate of Dense Phased Powder Solids

The complex multiphase gas-solid flow has always been a point of attraction for researchers over the past decade to explore the sensing techniques to sense and measure the mass flow. Weather dilute or dense phase flow, the gas-solid flow measurement generally requires velocity profile and volumetric concentration measurement to find the mass flow rate. The nature of the solids, the environmental factors-specially moisture adversely affects the sensor readings-specifically when it is non-invasive capacitive sensors. Gas-solid flow finds its applications in power plants, food, chemical, automobile, and pharmaceutical industries. This paper aims to explore the evolution of a novel approach of using load cell in conjunction with capacitive electrodes for calculating the mass flow rate of the solids.

A study of energy loss due to particle to particle and wall collisions during fluidized dense-phase pneumatic transport

This paper results from ongoing research into the modelling of solids friction for fluidized dense-phase pneumatic transport of fine powders. Energy loss due to particle to particle and particle to wall collisions have been modelled. A new model for particle velocity has been introduced using numerical analysis and variable fluidized bulk density, which has shown that slip velocity decreases with an increase in the mass flow rate of air and dimensionless length. A new modified two-layer model for solids friction factor has been developed by introducing the collision effect and particle velocity model in the model of solids friction. The results of scale-up validation in larger and longer pipelines for four different products have shown that for the majority of the cases, the new model for solids friction factor has resulted in better predictions in dense-phase flow conditions compared to the predictions obtained by an existing two-layer model.

Multiscale analysis of flow patterns in the dense‐phase pneumatic conveying of pulverized coal

AbstractFour typical flow patterns in the vertical pipe of the dense‐phase pneumatic conveying of pulverized coal were obtained, namely packed bed flow, plug flow, churn flow and less dense phase flow. The electrical capacitance tomography (ECT) was used for space–time multiscale analysis. Firstly, Daubechies second order wavelet transform and rescaled range analysis(R/S) were applied for the scientific multiscale decompositions. The multiscale frequency range is that the microscale signal is d1–d5 (2.1875, 70.0000) Hz, the mesoscale signal is d6 and d7 (0.5469, 2.1875) Hz and the macroscale signal is d8–d11 (0.0342, 0.5469) Hz. The multiscale ECT signal characteristics of pure gas flow, particle–wall friction, and particle–particle collision were determined by experiments. The multiscale ECT signal energy matrix is proposed, based on the multiscale energy structure characteristics. Control mechanisms and multiscale statistical rules of four typical flow patterns are obtained. This study provides theoretical support for the study of flow pattern transformation of dense‐phase pneumatic conveying.

Modelling solids friction for fluidized dense-phase pneumatic conveying

Fluidized dense-phase pneumatic conveying of powders has a number of benefits in various industries such as coal-fired thermal power plants, chemical, cement, pharmaceutical, petrochemical plants, etc. However, reliable designing of fluidized dense phase pneumatic conveying system is significantly difficult due to the complex and turbulent nature of the flow. In the present work, power station fly ash was conveyed through different pipeline configurations. Governing equations were developed for the fluidized dense-phase flow in the pneumatic conveying system and were solved using Runge-Kutta-Fehlberg method (RKF45). The results revealed that the particle velocity and actual gas velocity and the ratio of the two velocities increase along the direction of flow, while an opposite trend was observed for the solids volumetric concentration. A particle velocity model has been developed using solid loading ratio and dimensionless velocity terms. This particle velocity model, when employed in the existing two-layer model format for solids friction factor, has provided improved scale-up predictions (under diameter and length scale-up conditions) compared to the original two-layer model.

Heat transfer from an immersed fixed silver sphere to a gas fluidised bed of very small particles

Results of unique heat transfer measurement in beds of fine, cracking catalyst particles, fluidized by air or helium gas, are compared with predictions from a theoretical model presented in the literature, and also with an earlier established empirical correlation. Moreover, the results have been related to dense phase flow conditions around a silver heat transfer probe by a simple turbulence model. A maximum heat transfer coefficient of h = 2300 W/m2K has been measured in a bed of 14?m (average diameter) particles, fluidized by helium gas. The data collected, and the model developed, can be used for the design of heat transfer tubes in fluidized beds of fine particles as for instance in fluid catalytic cracking (FCC) of crude oil heavy residues. The FCC is one of the most important conversion processes in the petroleum refineries.

Transient characteristics of fine powder flows within fluidized dense phase pneumatic conveying systems

The objective of this paper is to report the experimental findings related to the pressure fluctuations in the fluidized dense phase pneumatic conveying of fine powders in the pipeline. Investigation has also been carried out to understand the relation between pressure pulse characteristics and specific power consumption and pneumatic conveying parameters. In addition, the transition in mode of flow along the downstream of flow of fine powders has been assessed and discussed. The wavelet analysis (Daubechies db4 wavelet) of signals of air pulses revealed that the moving bed flow possesses the transient feature and pulsatile phenomenon characterized by multiple amplitudes and random frequencies. Variations of pulse structures along the length of pipeline have been observed due to occurring of frequent aeration and de-aeration of dunes during the conveying. Substantial variations in pulse structures have also been found with different types of bulk materials. Pulse velocities of air between two data points are found higher at high solid loading ratios leading to low voidage in case of dense phase flow.

On developing improved modelling for particle velocity and solids friction for fluidized dense-phase pneumatic transport systems

Pneumatic transport of fine powders in fluidized dense-phase pneumatic conveying of powders has become popular in several industries because it offers various advantages, such as reduced air flow and gas velocity, reduced pipeline sizing and wear rate, reduced size requirement of gas–solid separator unit etc. For the reliable design of a pneumatic conveying system precise estimation of the solids friction factor through horizontal straight pipes is essential, but it is a challenging task till date because of the highly concentrated, turbulent, and complex nature of the gas–solids mixture. In the present work, power station fly ash (median particle diameter: 22 μm; particle density: 2370 kg/m3; loose-poured bulk density: 660 kg/m3) and cement (median particle diameter: 19 μm; particle density: 2910 kg/m3; loose-poured bulk density: 1080 kg/m3) were conveyed through different pipeline configurations (i.e., 65-mm inner diameter × 254-m-long and 80/105-mm inner diameter × 407-m-long step-up pipeline). For the fluidized dense-phase flow in pneumatic conveying system, governing equations were developed and the same were solved using fourth-fifth-order Runge-Kutta-Fehlberg (RKF45) method. The results revealed that the particle velocity and actual gas velocity and the ratio of the two velocities increases along the direction of flow, while an opposite trend was found for the solids volumetric concentration. The particle velocity and actual gas velocity terms were then included in an existing pure dilute-phase model (for solid friction factor) to modify it, by incorporating sub-models for particle and actual gas velocities and impact and solids friction factor and make it suitable for dense-phase mode. The solids friction-factor model was then validated by comparing the experimental and predicted pneumatic conveying characteristics for different solids flow rates and by using it to predict the total pipeline pressure drops for larger and longer pipelines. The new model has shown reliable predictions in the dense-phase region.

Numerical simulation of fluidized dense-phase pneumatic conveying of powders to develop improved model for solids friction factor

Accurate prediction of the solids friction factor through horizontal straight pipes is important for the reliable design of a pneumatic conveying system, but it is a challenging assignment to date because of the highly concentrated, turbulent, and complex nature of the gas–solids mixture. Power-station fly ash was transported through different pipeline configurations. Numerical simulation of the dense-phase pneumatic conveying systems for three different solids and two different air flow rates have shown that particle and actual gas velocities and the ratio of the two velocities increases in the flow direction, whereas the reverse trend was found to occur for the solids volumetric concentration. To develop a solids friction-factor model suitable for dense-phase flow, we modified an existing pure dilute-phase model by incorporating sub-models for particle and actual gas velocities and impact and solids friction factor. The solids friction-factor model was validated by using it for scale-up predictions for total pipeline pressure drops in longer and larger pipes and by comparing experimental and predicted pneumatic conveying characteristics for different solids flow rates. The accuracy of the prediction was compared with a recently developed two-layer-based model. We discussed the effect of incorporating the particle and actual gas velocity terms in the solids friction-factor model instead of superficial air velocity.

Dune formation in dilute phase pneumatic transport system: PIV & PTV based analysis

Flow of gas-solid mixture through horizontal conveying section show a large variety of phenomena and is broadly classified into dilute and dense unstable regimes. Different types of instabilities are observed in the dense phase flow and are widely studied in literature; however, clustering instabilities are observed in the very dilute regime of flow with volume fraction ~0.001. A recent study has shown that regular, stable dune shaped clusters are formed in a small regime of the dilute phase of conveying. The dunes become unstable as the superficial gas velocity is decreased before it finally leads to the dense mode of conveying. The motivation of the current work is to investigate the velocity distribution on the surface of the stable and unstable dunes and thereby understand the cause behind the formation of the dunes in the conveying section. To that end, particle image and tracking velocimetry techniques are employed with the specific objective to determine the volume fraction and the velocity profile of the solid phase on the stable dune surface. A drastic change in the solid fraction within a few particle diameters from the dune surface suggests that PTV is more appropriate in the bulk whereas PIV is suitable for near surface investigation.

On developing improved modelling and scale-up procedures for pneumatic conveying of fine powders

Pneumatic transport of fine powders in fluidized dense-phase mode is becoming increasingly popular in various industries, such as power, chemical, cement, refinery, alumina, pharmaceutical, limestone, to list a few, because of the reasons of reduced gas flow rate and power consumption, decreased conveying velocities, improved product quality control, reduced pipeline sizing and wear rate, increased workplace safety etc. For the reliable design of a pneumatic conveying system, it is important to accurately predict the total pipeline pressure drop. However, accurate prediction of pressure drop from an improved understanding of the fundamental transport mechanism of fluidized dense-phase flow condition has only made limited progress till now because of the highly concentrated and turbulent nature of the gas-solids mixture. Power plant fly ash (median particle diameter: 30μm; particle density: 2300kg/m3; loose-poured bulk density: 700kg/m3) was conveyed through different pipelines (69mm I.D.×168m long; 105mm I.D.×168m long; 69mm I.D.×554m long). 8 different fly ash samples were tested in a fluidizing column for their deaeration characteristics and fluidized bulk densities were determined. Governing equations of flow for the dense-phase pneumatic conveying system of fine powders were solved using Runge-Kutta-Fehlberg (RKF45) method for different fluidized bulk densities of fly ash and air flow rates. The results have shown that the particle and actual gas velocities and the ratio of the two velocities increase in the direction of flow, while a reverse trend was apparent for the solids volumetric concentration. The results were compared against the predictions obtained using existing empirical relations for particle velocity. To develop an improved model for solids friction factor, an existing reliable pure dilute-phase model has been modified for dense-phase flow condition by incorporating sub-models for particle to actual gas velocity and impact and solids friction factor. The developed solids friction factor model was validated by using it to predict the total pipeline pressure drops for larger and longer pipelines and by comparing the experimental and predicted pneumatic conveying characteristics. The results have shown improved reliable predictions and that the model is capable of addressing the gradual transition of flow mechanism from dense- to dilute-phase. The accuracy of prediction is similar (in fact better in certain scale-up cases) when compared to a recently developed two-layer based model (developed by some of the authors). The results demonstrate the importance of incorporating particle and actual gas velocity terms in the model of solids friction factor instead of the prevailing techniques that overly depend on using superficial gas velocities.

Determination of a particle size distribution criterion for predicting dense phase pneumatic conveying behaviour of granular and powder materials

The purpose of this study was to evaluate the effect of particle size distribution on the modes of flow that particulate materials will support in a pneumatic conveying pipeline. It has long been known that some materials can be conveyed in dense phase flow, i.e. a condition wherein the superficial gas velocity is below the saltation value, whereas some materials will block the pipeline under such conditions. It has also been known for a long time that there are two distinct forms of dense phase flow, generally linked to whether the material is fine (such as cement powder), or coarse (such as pellets), but until now there has been no successful method to assess the conveyability of a material based on size distribution alone. Six materials with different size grades were conveyed in a 25mm bore pipeline, at a range of pressures up to 3bar and gas velocities from zero to 12m/s. The results showed that materials below a certain size would support a fluid-like dense phase mode of flow, whereas to support a low-velocity slug flow the key was to have a very narrow size distribution. Materials that satisfied neither of these criteria would not support flow at gas velocities below the saltation value. Clear trends for how throughput changes in the transition from lean to dense phase, are also demonstrated, which have major implications for pipeline sizing. A quantitative criterion for deciding on the likely conveyability of a material, based on size distribution alone, is proposed.