Vertical Pneumatic Transport Research Articles

A Mathematical Simulation of the Vertical Pneumatic Transport of a Polydisperse Material

A mathematical model of the vertical pneumatic transport of a polydisperse bulk material is proposed based on the theory of interpenetrating continua and supplemented by equations of interfractional interaction that account for particle collisions of different fractions. The fractional interaction forces determined by the collision of particles are comparable in value with the interphase interaction forces of particles with a gas. For particles of fine fractions, collisions with large particles exert a decelerating effect, while larger particles are “pushed” by smaller particles. This leads to a decrease in the spread between particle speeds.

Performance of vertical diffusers carrying Gas-solid flow: experimental and numerical studies

In the present work, numerical and experimental studies are carried out to investigate the performance of vertical straight-walled conical diffusers carrying air-solid two-phase flow. Eulerian-Lagrangian approach is used to numerically simulate the two phases using the Chen-Kim k-ε turbulence model. The continuous phase (gas) is simulated using Eulerian frame by solving Reynolds-Averaged Navier-Stokes equations (RANS), while the dispersed phase (solids) is simulated using particle tracking method. Coupling between the two phases is established by adding particle source terms and void fraction in the continuous phase equations. A 4-way coupling is adapted to include the effect of particle-particle collisions. Lift forces, particle dispersion and particle-wall collisions are also considered in the simulation of solid-phase. The experimental study is carried out on a pilot scale vertical pneumatic transport system. Four different diffuser geometries are tested at various inlet-Reynolds numbers and mass loading ratios. Sand particles of different sizes and mass flow rates are used to represent the solid phase. Comparisons between numerical predictions and experimental results indicated good agreement. The effects of solid parameters are significant for small angled-diffusers and decrease as the diffuser angle increases. It is also found that, there is a significant decrease in the separation zone within diffuser due to the presence of solid particles. Energy is transferred from the gas phase to the solid phase in the upstream pipe. This energy is transferred again to the gas phase through the diffuser and its downstream tangent pipe. The rate of energy transfer is enhanced by increasing the solid mass flow rate and decreasing the particle size. The results show that the mass loading and size of solid particles have significant effects on the diffuser loss coefficient.

Swirling gas–solid flow through pneumatic conveying dryer

Numerical and experimental investigations of swirling pneumatic conveying dryer are performed. In the numerical study, the Eulerian–Lagrangian model is used to simulate the two-phases. The gas phase is simulated based on Reynolds Averaged Navier–Stokes equations (RANS) employing four turbulence models, namely: standard k–e model, RNG-based k–e model, extended k–e model and low-Reynolds number k–e model. Meanwhile 3-dimensional particle tracking procedure is used for the solid phase. The model takes into account the lift and drag forces and the effect of particle rotation as well as the particles dispersion by turbulence effect. The effects of inter-particles collisions and turbulence modulation by the solid particles, i.e. four-way coupling, are also included in the model. The experimental study is carried out on a pilot scale vertical pneumatic transport system. The swirl is imparted to the flow by axially rotating pipe of the same diameter as the drying pipe. Crushed limestone of different sizes is used to represent the solid phase. Measurements of pressure and temperature distributions along the dryer are performed at different inlet conditions. Comparisons between present model predictions and experimental results show a good agreement. The results indicate that the RNG-based k–e model gives better results compared with other tested turbulence models. In addition, it is found that the pressure drop of swirling flow is higher than that of non-swirling one and the swirl enhances the drying process.

Energy-saving pneumatic lift

This article examines the state of a gas suspension during vertical pneumatic transport upward. A comparison is made between pneumatic lifts with screw-type pneumatic pumps, chamber-type pneumatic pumps, and pneumatic-chamber air-lift pumps. The material is lifted in high concentrations and at low velocities in the gas mixture. Thus, the consumption of compressed gas is also low.

Simplified modeling of circulating flow of solids between a fluidized bed and a vertical pneumatic transport tube reactor connected by orifices

Interconnected and dual fluidized bed (FB) systems with various regimes of gas-solids flow are commonly studied with the aim to find and ensure suitable circulation of solids and control of flow between two chambers/reactors with different reacting gases. Dual separated fluidized beds or fluidized and transporting bed systems are convenient for cleaning of gases with regeneration of a solid sorbent or for gasification of solid fuels and production of fuel gas practically free of nitrogen. The paper concentrates on the development, comparison and checking of three simplified models for solid particle velocity, pressure drop and mass flux in a pneumatic transport reactor (riser). The simplified models have been used in a more general model for flow of solids via orifices between a FB near the incipient fluidization and a tube with vertical pneumatic transport (PT) of solids and recirculation to the FB. The model predictions have been compared with experimental data and validated in a limited range of operating conditions: for lower solids fluxes of sand particles ( d p = 0.3 mm, mass fluxes below 40 kg/m 2/s) at gas velocities higher than the terminal velocity U T of solids and for relatively high solid fluxes (over 160 kg/m 2/s) and gas velocities higher and lower than U T. Predictions from the models were used to estimate effects of particle size, gas velocity in the pneumatic transport column and height of the FB on circulating mass flux of solids. Comparison of the model-computed and experimental mass fluxes of solids with a theoretical forecast for the saturation carrying capacity of gas in pneumatic transport and a forecast of “choking” as computed by use of the Yang correlation has indicated possible problems and instabilities of solids flow in regimes with higher solids mass fluxes and/or lower gas velocities in the riser (PT-column). To control the circulation rate of a given particulate material in the dual FB-PT systems three main, practically important parameters should be considered: the superficial gas velocity in the riser (PT-column), height of the FB and cross sectional area of the orifices.

Experimental Studies of Dilute Vertical Pneumatic Transport

Dilute vertical pneumatic transport has been studied by using the experimental techniques LDA and PIV. LDA and PIV are two different techniques, but they are both common methods used to gain a better understanding of gas/particle multiphase flows. A comparison between the two methods has been performed. The main focus of this study was an experimental LDA investigation of ZrO2 particles and glass beads. The particles have approximately the same particle size distribution but different densities. The superficial gas velocity and the particle loading have also been varied in the experiments. The effects of superficial gas velocity, particle loading, and particle density on the particle mean velocities, particle u-rms, particle v-rms, and particle cross-moment were investigated. From the experimental investigation it was found that the axial particle velocities showed a dependence on the particle volume fraction for the ZrO2 particles. It was also observed that the axial fluctuations decreased for both glass and ZrO2 particles when the particle volume fraction increased. The axial fluctuations measured for the glass beads were higher than the fluctuations measured for the ZrO2 particles. The mean axial particle velocity measured by LDA and PIV showed good agreement.

Effect of Large Particles on the Augmentation of Wall Friction in Vertical Pneumatic and Hydraulic Transport in a Turbulent Fluid

This paper is concerned with prediction of the augmentation of wall friction by large particles (Fp) over that assuming turbulent flow in a pipe without particles (F f ) for vertical pneumatic and hydraulic transport in a turbulent fluid. Our measure of this is the value of the ratio F p /F f . Equations 29 and 32 are derived initially for predicting F p /F f for large particle parallel flow pneumatic transport based on the supposition that the particle phase friction factor (f p ) is solely a function of the Froude number, (gD) 1/2 /v where g is the gravitational acceleration, D is the tube diameter, and v is the particle phase velocity. The Froude number, gD/v 2 , is used to derive eq 40, its counterpart for hydraulic transport. These equations are shown to be qualitatively reliable and inconsistent only with the existence of a particle-free zone near the wall observed in experiments for large particle transport. To satisfy this requirement, f p is additionally assumed to be a function of the turbulence response parameter, t R /t e where t R is the particle relaxation time, and t e , the lifetime of the most energetic turbulent eddy. Equations 34 and 41 are our final correlating equations for F p /F f for pneumatic and hydraulic transport, respectively. When t R /t e is large, the particle-wall friction coefficient (f p ) goes to zero to conform with theory and experimental observations.

“Choking” Revisited

The classical “choking” phenomenon was revisited in this paper in light of confusion in the open literature. A coherent development was elucidated, with support of experimental data, to clarify the ever-present confusion in vertical pneumatic transport of solids. The importance of the distinction between the “small particles” and “large particles” in their manifestation of flow phenomena in vertical solids transport, especially at close to “choking” where low voidage and high solids concentration prevailed, was emphasized. A new operating map, which could be doubled as a regime transition map, was proposed for operating risers of circulating fluidized-bed systems. The “choking” and “nonchoking” transition proposed by Leung in 1980 was also reexamined.

LASER-BASED FLOW MEASUREMENTS OF DILUTE VERTICAL PNEUMATIC TRANSPORT

Dilute vertical pneumatic transport in a vertical lifter was studied using the sophisticated measurement techniques of laser Doppler anemometry (LDA) and phase Doppler anemometry (PDA). The vertical lifter consisted of a lower fluidized silo, an upper receiving tank, and a connecting vertical transport pipe made of clear glass. The experimental study was performed in order to get detailed information of the complex gas-particle flow behavior in a dilute vertical conveying system. Particle diameter, axial particle, and tangential particle velocities, as well as root mean square velocities, were measured simultaneously for different flow conditions. In addition, overall solid mass fluxes were obtained using weighing cells. Smooth and spherical zirconium oxide (ZrO2) solids were applied with two different particle size distributions. Measurements were performed using different flow rates of air. The air inlet condition was varied in order to study its effect on the flow behavior. The particle diameter measurements show that no axial or radial segregation by size occurs for this transport condition. The results show that the particle velocity is independent of the particle size as well. The axial velocity profiles at different heights are almost identical and flat, which indicates fully developed turbulent pipe flow. The turbulent velocity measurements show that turbulence is mainly caused by the velocity gradients, and not by particle-particle collisions in dilute flow. The solid mass flux measurements show the importance of optimum inlet condition and how this influences the mass flux.

Detection and Modeling of the Core-Annulus Transition in Industrial and Pilot Plant Risers

The core-annulus structure is essential to the modeling and optimization of riser reactors such as used in Fluid Catalytic Cracking. This paper presents results of measurements taken with various probes in a pilot plant and in an industrial riser. The instantaneous probe signals were analyzed with sophisticated signal analysis methods based on the detection of cycles and the determination of the correlation dimension.In a pilot plant riser, a core-annulus structure was identified with optical and momentum probe measurements. Cycle analysis of the optical probe measurements showed that the annulus was unstable: its thickness fluctuated with an average cycle time of 0.3 s. There were waves at the core-annulus boundary. In an industrial riser, a similar core-annulus structure could be identified with temperature and momentum probe measurements. Local temperature measurements are much easier to perform in an industrial riser than momentum probe measurements but can provide, with cycle analysis, the location of the core-annulus transition. Analysis of the momentum probe and temperature signals showed that the thickness of the wavy transition layer between core and annulus was about the same in the pilot plant and the industrial riser, meaning that the relative range of fluctuations in annulus thickness was much smaller in the larger industrial riser.A model was developed to predict the time-averaged transition between core and annulus. This model, which had been successfully used to predict the annulus thickness in dilute-phase vertical pneumatic transport lines, assumes that the annulus thickness is such that the riser pressure drop is minimized. Measurements and model predictions were in good agreement.

Conductive-Convective Heat Exchange in Disperse Systems with Suspended Particles

A film model of heat exchange of a disperse medium with a macrosurface under the conditions of ascending motion of a gas is suggested. A model equation is obtained which yields the dependence of the heat-transfer coefficient on determining factors. It is used to correlate experimental data in different systems: fluidized bed, circulating fluidized bed, and vertical pneumatic transport.

Thermodynamics of gas‐solid flow

AbstractNew thermodynamic fundamentals for vertical pneumatic conueying are proposed with derivation based strictly on mass conservation and continuum concept. For a 1‐D, vertical, steady gas‐solid flow, these new mass and energy balances allowed the derivation of a new nondimensional energy factor containing the pressure drop. This energy factor correlated quite well with the difference between the inlet gas velocity and solids terminal velocity, when tested against high‐pressure, 15‐m‐high, vertical pneumatic transport data from Institute of Gas Technology. The energy factor was also checked against atmospheric‐pressure data of the 5‐m lift line of the Pennsylvania State Uniuersity. The new energy factor covers quite well both the atmospheric‐ and high‐pressure sets of data (923 tests) including two heights, lean‐ and dense‐phase transports, several pipe diameters, broad and narrow particle‐size distributions, and different materials. The dissipation terms in gas‐solid flow were also clearly identified.

Phase Diagram of a Disperse System with Ascending Gas Flow

The phase diagram of an infiltrated disperse medium which includes a circulating fluidized bed in addition to fixed and fluidized beds and vertical pneumatic transport has been constructed. The expression to calculate the most important characteristic of a flow system, i.e., the transport velocity, has been obtained. The definition of a circulating fluidized bed has been given.

Pneumatic transport of granular materials with a stagnated dense bed in pipelines of complex configuration

In pneumat ic transport by a s t agna ted dense bed (SDB), the specific air f low rate is decreased, the solut ion to the p rob lem of removing dust f rom used air is s ignif icant ly s implif ied, and the granulometr ic compos i t ion o f the solid phase changes very little due to breakdown of the granules [1]. These advantages caused by the use o f low gas and t ransported material f low rates are responsible for its promise. We prev ious ly [ 1] revealed the industr ial possibi l i t ies o f using vertical SDB pneumatic transport. However , in the condi t ions of operating plants, the layouts of pneumatic t ransport lines are frequently a complex combina t ion of vert ical , horizontal , and curved sec t ions . Judging by the charac ter i s t ics descr ibed in the literature, the phys ica l condit ions of vert ical and horizontal pneumat ic transport d i f f e r significantly. For ver t ical SDB pneumatic t ranspor t , the condit ions [ 1] for stable, s teady-s ta te transport are de te rmined and the dependences are obtained for def in ing the basic pa rame te r s of the process; such data do not exist for SDB pneumatic t ransport in pipelines of c o m p l e x configurat ion. We de termined the design and process parameters of SDB pneumatic t ransport of granular mater ia ls in compl ica ted layouts. The studies were conducted on the spec ia l ly designed exper imenta l setup whose d iagram is shown in Fig. 1. Compres sed air from two mode l O-38B compressors enters receiver 12 and goes through cyl inder 13 for drying to feeder 2. At low pressure, the compressed gas is f i l tered through the bed of granular material in the feeder and pass ing into its pores a long the transmission p ipe l ine , is removed from the installation without mo v in g the material . The gas f low rate in the p ipe l ine increases with an increase in the pressure of the ~,,as 9 in the feeder, and at the end of the pipeline, removable thro t t l ing orifices having di f ferent internal d iameters are used as the regula t ing instrument . In upper hopper 9, the two-phase s t ream is separa ted into the individual components : gas is removed into the atmosphere through ro tameters 11, passing through a cloth filter; the granular material is sent through dis t r ibutor 6 to intermediate hopper 5. In s tudying vertical SDB pneuma t i c transport, it was found that al tering the level of the material in the feeder within certain limits does not a f fec t the transport pa rame te r s , so that no addit ional measures were inst i tuted to ensure a constant level of granular mater ia l in the feeder. In opera t ion of this instal lat ion, the level of the mater ia l in the feeder gradual ly decreases and accord ing to [1], at some t ime becomes lower than the critical value. For this reason, the pressure loss of the entire sec t ion decreases sharply, the steady state o f the process is per turbed, mater ia l is abrupt ly discharged from the p ipe l ine , and the gas f low rate increases s t rongly. A series of measures was inst i tuted to prevent these condi t ions. The intermediate hopper was equipped with a leve l -gauge glass with control marks . A certain amoun t of granular mater ia l is loaded into it before the unit is turned on, entering the feeder when cock 4 is opened. Since the t ransport system is closed, the rise in the level of granular material in the in te rmedia te hopper is con t ro l led during operat ion of the unit. The level of the material in this hopper approaching the control mark is the signal to totally empty the

A pseudo-Stokes representation of the effective drag coefficient for large particles entrained in a turbulent airstream

Recently published data by Littman et al., Powder Technol., 77 (1993) 267 and Powder Technol., 84 (1995) 49, for the effective drag coefficient in the vertical pneumatic transport of 1 and 2 mm glass and 1.99 mm rapeseed particles are represented by the pseudo-Stokes equation, C d = 24/ Re pc . The effects of freestream turbulence are represented using Lee's ( Int. J. Multiphase Flow, 13 (1987) 247) correlation for the apparent turbulent kinematic viscosity of the fluid felt by the particles in a suspension flow. These results not only confirm the importance of freestream turbulence in lowering the drag coefficient for those particles significantly below that on the standard drag curve but suggest that the turbulence intensities are high enough to largely or completely eliminate the particle wake. These low drag coefficients increase corresponding slip velocities above terminal suggesting that solids residence times in the pipe are much higher than predicted by use of the standard drag curve.

Hydrodynamic modeling of vertical non-accelerating gas-solids flow

The one-dimensional steady-state model for vertical non-accelerating liquid-solids flow of coarse spherical particles recently formulated and verified (Grbavčić et al., Powder Technol., 53 (1992) 182) has been extended to vertical pneumatic transport. Experiments were performed by transporting spherical glass particles of 1.20, 1.94 and 2.98 mm diameter with air in a 30 mm i.d. glass pipe. Slip Reynolds number for the particles ranged from 592 to 3813 and the pipe Reynolds number from 16 200 to 49 000. The loading ratio was varied from to 5.4 to 30.8. For these ratios, line voidage ranged from 0.9153 to 0.9913 and superficial gas velocity was in the range (0.91–2.2) U t, where U t represents single particle terminal velocity. Model predictions of fluid-particle interphase drag coefficient and voidages in the transport line are in good agreement with experimental data for particle mass flux W p<250 kg/(m 2 s). A new correlation for particle-wall friction coefficient in an air-spherical glass particle-glass pipe system is presented.

High-pressure vertical pneumatic transport investigation

Vertical pneumatic transport was investigated in a 0.026 m i.d. Lucite tube at various pressures. Nitrogen was the conveying gas at pressures of 101, 790, 2170 and 4238 kPa. Pressure drop, particle velocity, pressure fluctuations and flow patterns were measured or visually observed and recorded. Choking velocity, velocity at minimum pressure drop and the particle friction factor were also investigated at these elevated pressures. Glass beads (97 and 545 μm) and coal (89 and 505 μm) were used as the conveyed solids. The use of Lucite tubing was made possible by encapsulating the entire transport system in a high-pressure containment vessel and pressurizing the outside and the inside of the transport tube simultaneously. A Zenz-type diagram of pressure drop per unit length versus superficial gas velocity was plotted at all pressures investigated. As pressure increased, the curve shifted toward a higher pressure drop at a given gas velocity. Gas velocity at minimum pressure drop thereby decreases as the pressure increases. The same trend is observed for the choking velocity. Average particle velocity of the gas—solid flow mixture approaches the superficial gas velocity at higher pressures more readily than it does at lower pressures. Investigation of the friction factors for the small particles (89 and 97 μm) at elevated pressures revealed that the friction factors due to the gas and solid were dependent on the loading of the system. Expressions were developed for predicting the frictional pressure drop for the gas and solid at low loadings for small particles. Correlations for particle velocity, choking velocity, particle friction factor and velocity at minimum pressure drop are recommended for designing dilute-phase (i.e. ϵ>0.9) high-pressure transport systems.

Modeling the thermal cracking of ethane and propane in a non-isothermal vertical pneumatic transport reactor

A model has been formulated for noncatalytic thermal cracking of ethane and propane in a non-isothermal, adiabatic, vertical, pneumatic transport reactor. This model takes into consideration hydrodynamic properties of the gas-solid mixture, gas-particle heat transfer, and a kinetic scheme involving 81 radical reactions among 11 molecules and 11 radicals. Simulations of ethane cracking at a fixed conversion show that the particle diameter and solid to hydrocarbon ratio have the greatest effect on reactor performance; an optimum particle diameter of 50 μm was found

New approach to energetic optimization of pneumatic transport

In the present investigation we have been concerned with the description of a mathematical model for the energetic optimization of vertical pneumatic transport. This derived model also takes into account the influence of the length of the input transport pipe for the pure carrier medium on the value of the optimum mean velocity of the continuous phase. The influence of other quantities on the value of the optimum mean velocity of the carrier medium has also been studied. Values of the optimum mean velocity calculated from the model were compared with values of the mean velocity of air determined experimentally.

Computation of flow patterns in circulating fluidized beds

AbstractFlow regimes of dense‐phase vertical pneumatic transport of solids, referred to in the literature as circulating fluidized beds, have been computed using a generalization of the Navier‐Stokes equations for two fluids. In the less dense regime corresponding to volume fractions of solids of about 1%, the flow consists of centrally upward moving solids and downward moving clusters. The computations agree with observations made by high‐speed motion pictures and with measurements of radial solids concentrations and velocities. In the dense regime, corresponding to volume fractions of about 10%, a core‐annulus type of regime is obtained, with solids descending down at the wall. The computed voidage distributions and velocity profiles agree with measurement done at the Institute of Gas Technology.