Minimum Gas Velocity Research Articles

The effect of static mixer on mass transfer in draft tube bubble column and in external loop column

Measurement were carried out for pressure drops, liquid circulation velocities, minimum gas velocities for complete suspension of particles, rates of mass transfer from gas to liquid and from liquid to solid in bubble slurry columns with and without static mixers (motionless mixers). Two types of column, a draft tube bubble column (internal type) and an external loop column (external type), were compared. The static mixers were found to be effective to increase rates of mass transfer when a single nozzle was used as the gas distributor. When an optional pump was used in the external loop column, the mass transfer rate could be enhanced significantly.

High gradient magnetic separation of YBCO particles in a gas-fluidized bed

High gradient magnetic separation of copper-oxide superconducting particles in a gas-fluidized bed has been studied experimentally. Particles over the size range 40-105 mu m exhibit classical behavior when fluidized with He gas at 77 K, whereas the more cohesive, smaller particles were difficult to fluidize. Separation experiments on a uniform mixture of tetragonal and orthorhombic phases have shown that 30 min of exposure to the field gradient is sufficient to equilibrate the superconductor concentration profile. In addition, the equilibrium separation data reveal that best separation is achieved at three times the minimum fluidization gas velocity (U/sub mf/). This is consistent with the behavior of fine powders where the interparticle frictional forces are minimized at 3 U/sub mf/. Observed results are discussed in terms of a collective effect of Meissner and interparticle frictional forces. >

Mass transfer in bubble slurry column with static mixer in draft tube.

Systematic investigations are reported for pressure drops, minimum gas velocities for complete suspension of particles, and rates of mass transfer from gas to liquid and from liquid to solid in a draft-tube bubble column with and without a static mixer

Percolation theory of creation and mobilization of foams in porous media

AbstractA percolation model of foam mobilization in porous media is developed. This model indicates that there is a minimum pressure gradient or, equivalently, a minimum gas velocity required to initiate mobilization of foam. As a result, for most foam enhanced oil recovery processes, where the surface tension is not low, deep foam penetration depends on propagation of foam formed at a high pressure gradient near the well. Low surface tension makes mobilization of CO2 foams feasible, however, at pressure gradients found throughout much of the formation in a typical field application. The theory further predicts, and data confirm, that the minimum velocity for foam mobilization during steady flow of liquid and gas decreases as injected liquid volume fraction increases. The theory suggests a better strategy for foam generation: alternate injection of small slugs of liquid and gas.

MASS TRANSFER AND REACTION IN BUBBLE COLUMN SLURRY REACTOR WITH DRAFT TUBE

Gas-liquid and liquid-solid mass transfer coefficients were obtained in a draft tube bubble column slurry reactor (abbreviated as DTBCSR) from the measurements of gas absorption and ion-exchange, respectively. Oxidation of sulfur dioxide on activated carbon was carried out in the same reactor. The effects of gas flowrates, diameters of draft tube and solid concentrations were investigated. These results were compared with those in a bubble column slurry reactor without draft tube (abbreviated as BCSR). Minimum gas velocities for complete suspension of solid particles in DTBCSR were lower than those in BCSR. Gas-liquid and liquid-solid mass transfer coefficients in DTBCSR were higher than those in BCSR. Both coefficients were almost independent of solid loadings. Reaction rates in DTBCSR were higher than those in BCSR and a stirred slurry reactor (abbreviated as SSR) except for lower gas flow rates and smaller particles.

Simultaneous measurement of liquid film thickness, wall shear stress and gas flow turbulence of horizontal wavy two-phase flow

Wall shear stress, liquid film thickness and streamwise gas velocity are measured simultaneously, respectively, with a hot-film, conductance probe and hot-wire at a fully developed location of air—water wavy two-phase flow in a horizontal tube. A conditional sampling technique is applied to eliminate the effect of wire-wetting on the output signal of the hot-wire anemometer. A gradual decrease and successive rapid increase in the gas velocity are clearly observed just before large wave peaks are detected. Minimum gas velocities caused by sevral waves are lower than the propagation velocity of each wave. This may suggest the existence of a separation bubble attached to the front of large waves. An increase in the wall shear stress caused by the passing of large waves is also observed.

Necessary conditions for the existence of stable slugs

A consideration of the velocity with which the tail of a slug propagates downstream yields a minimum film height required to sustain stable slugs. Measurements of liquid heights and photographs for horizontal air/water flow in a 0.0953 m horizontal pipe provide support for this necessary condition for the existence of slugs, if the tail is considered to be an inviscid bubble. The representation of the front of a slug as a hydraulic jump yields a minimum gas velocity below which slugs cannot exist.

A study of particle motion in vertical pipe flows.

An experimental study has been made of particle motion and the minimum gas velocity to transport particles in a vertical pipe. Particles with different properties (material, diameter and shape) were used in the experiment. A measuring system based on a video camera and image processing computer were used to record particle trajectories. Experimental results show that the particle-wall collision is the most influential factor for the particle motion even in the vertical pipe flow. it was also found that the particle concentration is closely related to particle shape and the coefficient of restitution.

Characteristics of solid suspensions in a bubble column without and with draft tube

AbstractIn order to investigate the suspension behaviour of solid particles in bubble columns without and with draft tube, minimum gas velocities for generating and maintaining complete solid suspension were determined. Experiments were carried out in a bubble column with a diameter of 200 mm and a height of 2 500 mm. The bubble column could be equipped with a draft tube with a diameter of 120 mm and a length of 1 500 mm. The state of suspension was determined by measuring the hydrostatic pressure as a result of suspended solid particles with an inductive pressure gauge. Solid content εs, solid density ϱs, particle size dp, particle shape, particle size distribution, and solid wettability were varied. Furthermore measurements of integral gas holdup were carried out.

New model to calculate the choking velocity of monosize and multisize solids in vertical pneumatic transport lines

AbstractThe choking velocity is the minimum gas velocity required for the vertical dilute phase transport of a given solids flux. Experimental choking velocities obtained from the literature for both monosize and multisize particles were compared to choking velocities predicted with various published correlations. For monosize particles, Yang's correlation (1983) gave the most accurate predictions. For multisize particles, all the literature correlations gave rather poor predictions. Much more accurate predictions were obtained with a new simple physical model which assumes that an annulus forms along the pipe wall at low gas velocities. Choking then occurs when this annulus occupies approximately 25% of the total pipe cross‐sectional area.

Mechanism of incipient fluidization in fluidized bed at elevated temperature.

A new model describing the effect of the adhesive force of particles on the bed structure in a fluidized bed is proposed. Based on this model, equations for estimating the minimum fluidization bed voidage and gas velocity were analytically derived as functions of the particle size and the adhesive force of particles. From a comparison of theoretical values with experimental values obtained in silica sand beds with various particle sizes at temperatures up to 873 K, the analytical equations were found to give excellent predictions. The predicted values were also found to agree well with the experimental values of bed voidage at ambient conditions by Leva.

Simulation of a Major Oilfield Gas-Gathering Pipeline System

An engineering design study was performed on a gas-gathering system in a major offshore oilfield in Saudi Arabia. The dew point gas from separators will undergo retrograde condensation in the pipelines with reduction of pressure and temperature. Calculations presented for sizing the pipelines include phase behavior, pressure drop, temperature profiles, liquid volumes, flow pattern predictions, liquid flow rates and the minimum gas velocity required to continuously remove liquid from the risers.

Lateral solid mixing in the fluidized bed with multi-tubes internals.

The lateral mixing of fluidized particles in a fluidized bed with vertical or horizontal multi-tube internals was investigated by the unsteady state behaviour of tracer particles. The behaviour of the tracer particles is analyzed by a one-dimensional diffusion model. The tracer particles were activated alumina on which zinc acetate was adsorbed. The lateral diffusion coefficient of the particles was strongly affected by the hydraulic diameter of the internals, the gas velocity and the minimum fluidized gas velocity of the particles. Empirical equations for the lateral diffusion coefficient of the particles in a fluidized bed with vertical or horizontal multi-tube internals were obtained. The lateral diffusion coefficient of solid particles in an ordinary fluidized bed was correlated by an empirical equation. Thermal diffusivity obtained from lateral thermal conductivity in the fluidized bed with multi-tube internals agreed with the lateral diffusion coefficient at the same fluidized conditions.

Lateral solid mixing in a packed fluidized bed.

The lateral mixing of fluidized particles in a packed fluidized bed with cylindrical screen packings was investigated from the unsteady state behavior of tracer particles. The tracer particles were activated alumina particles on which zinc acetate was adsorbed. The lateral mixing characteristics were analyzed by a diffusion model. The lateral dispersion coefficient of the particles was strongly affected by packing size, fluidized gas velocity, and the minimum fluidized gas velocity of the particles. An empirical equation for the lateral dispersion coefficient of the particles in a packed fluidized bed with cylindrical screen packings was obtained.

THE EFFECT OF TEMPERATURE ON FLUIDIZED BED BEHAVIOUR

Most published correlations for the minimum fluidizing gas velocity have been derived from tests under ambient conditions and increasing discrepancy is found in their application over wider ranges of operating conditions. Up to 1000°C the Ergun equation is reliable but it requires a knowledge of the particle shape factor and bed voidage for its application. Bed voidage is found to vary with temperature for laminar gas flow conditions. Paralleling changes in gas flow conditions with operating temperature are changes in bed-to-surface heat transfer coefficients. There is a distinct transition from the interphase gas convective to the particle convective component of heat transfer being the dominant mechanism as the operating temperature increases and Remf reduces through 12,5 at Ar ∼ 26000. This is thought to be a consequence of change in bed bubbling behavior.

Lateral effective thermal conductivity in a fluidized bed with horizontal multi-tube internals.

The lateral effective thermal conductivity was measured in a fluidized bed with horizontal multitube internals. The tube diameter and tube pitch used in this work were in the range from 1.0 to 5.0 cm and 2.5 to 10 cm, respectively. The minimum fluidized gas velocity of fluidized particles and the superficial velocity were in the range from 4.0 to 27 cm/sec and 10 to 70 cm/sec, respectively. The lateral effective thermal conductivity in the bed is influenced by the properties of fluidized particles, the hydraulic diameter of the tube internals, the minimum fluidized gas velocity and the intensity of fluidization. An empirical equation for the lateral effective thermal conductivity in a fluidized bed with horizontal multi-tube internals is obtained.

KINETICS OF THE REACTION OF SULFUR TETRAFLUORIDE WITH URANIUM TRIOXIDE AND URANYL FLUORIDE1

The reactions between sulfur tetrafluoride and uranium trioxide and sulfur tetrafluoride and uranyl fluoride were studied between 255 and 370 deg by following the change in weight of the solid phase using a thermobalance. The rate of production of uranium hexafluoride was in agreement with the kinetics expected for reaction between a gas and a solid at a continuously diminishing spherical interface. The rate of reaction was temperature dependent. The activation energies for reaction of sulfur tetrafluoride with uranium trioxide and uranyl fluoride are, respectively, 6.0 and 32.0 kcal/mole. The rate of reaction with uranyl fluoride was dependent upon the partial pressure of sulfur tetrafluoride. A 1.55 exponent was found. Finally, a minimum gas velocity was found above which the reaction rate was independent of the velocity of gaseous reactant past the solid.

Flame Zone Studies. II. Applicability of One-Dimensional Models to Three-Dimensional Laminar Bunsen Flame Fronts

The applicability of one-dimensional models in flame theories has been investigated experimentally in the case of a lean quarter-atmosphere propane air flame. Studies of flow patterns through the flame fronts were made using particle track techniques. From this time-position information, normal gas velocity, stream tube area, and gas density were calculated throughout the flame front. If the density is a single valued measure of the state of the gas, then one-dimensional models can be tested for Bunsen flames. Unidimensionality was verified for this and several other propane-air flames under a number of conditions. In the reaction region preceding the luminous zone it was also found that the velocity and density profiles were invariant with respect to changes in flame geometry, gas flow, and burner diameter. Therefore, it is a good approximation to speak of single reaction path for this flame, which is a function only of the initial state of the gas. The measurements indicate that one-dimensional flame theories should be modified to include the effects of area expansion during the course of the reaction. Continuity, diffusion, and heat conduction equations should also be generalized to include the effect of expansion. It was noted that the hydrocarbon flame reaction consists of two regions: a fast propagating reaction associated with the luminous regions and the major portion of the heat release and a slow secondary reaction associated with the post luminous mantel of the flame. Flow in the primary region is a function only of the initial state of gas and independent of external influences such as gas velocity and flame geometry. Flow in the secondary region could be varied over wide limits with no measureable effect on the propagation of the flame. Burning velocity was found to be independent of gas velocity, flame geometry, and radius of curvature of flame front, and a function only of the initial state of the gas, providing it was defined as the minimum normal gas velocity ahead of the flame front. A discussion is given of the errors introduced by measuring burning velocity by flow measurements in the throat of the burner.