EFFECT OF DISTRIBUTOR ON BUBBLE SIZE AND BUBBLE RISE VELOCITY IN THE SLUGGING REGIME OF A SEMI-CYLINDRICAL GAS-SOLID FLUIDIZED BED†

A semi-cylindrical fluidized bed of 15 cm internal diameter, equipped with a transparent flat glass plate for the front wall, was employed to visually observe bubbles in the bubbling to slugging transition regime and in the slugging regime. Five kinds of perforated distributors were used to investigate the effect of distributor type on the bubble size and the bubble rise velocity. The average bubble size was not affected by distributor type in these flow regimes, and could be predicted by Darton et al's correlation (1974)of hole number 22. In other words, this comes from the inapplicability of the correlation to the slugging regime. The bubble rise velocity agreed well with Allahwala et al's correlation (1979) and was not affected by the type of distributor.     

BUBBLE FORMATION AT A PUNCTURE IN A SUBMERGED RUBBER MEMBRANE

Very large enhancements in volumetric mass transfer coefficients have recently been reported using a new type of sparger which is comprised of a punctured rubber membrane. The punctured sheet has been reported to produce very uniform emulsions of small bubbles, which leads to large apparent increases in gas voidage and mass transfer area. Flooding(slugging) is presumably repressed owing to the “elastic hole” phenomenom whereby the rubber sheet balloons and expands as applied pressure increases. Under conditions of expansion, a puncture in the sheet also expands thereby mitigating the occurence of jetting. In the present effort, we study a single puncture in the center of circular rubber sheets of 2, 3, and 4 inch diameters. By measuring bubble frequency and flow rate, we compute average bubble size. These results for flow rates from 0.01 to 2.0 cc/sec suggest that bubble size is practically constant over a nearly two decades of flow rate, until a flow of around 0.5 cc/sec, thence bubbles tend to follow the empirical correlation of L. Davidson and Amick (1956) and the inviscid theory of J.F. Davidson and Schuler (1960) both of which predict bubble volume increases as the 6/5 power of flow.

Using the “point source” model of J.F. Davidson for bubble growth, we include additional effects of surface tension to derive the required detachment-time. This leads to a theory which includes inertial, buoyancy and surface tension effects. The theory gives flow rate as a function of final bubble size and agrees quite well with the 180 experiments reported. The new theory approaches the inviscid models in the limit of large gas flow rates. Finally, we present results which clearly suggest that hole area increases linearly with increasing plenum chamber pressure.     

BUBBLE FORMATION AT A PUNCTURE IN A SUBMERGED RUBBER MEMBRANE

Very large enhancements in volumetric mass transfer coefficients have recently been reported using a new type of sparger which is comprised of a punctured rubber membrane. The punctured sheet has been reported to produce very uniform emulsions of small bubbles, which leads to large apparent increases in gas voidage and mass transfer area. Flooding(slugging) is presumably repressed owing to the “elastic hole” phenomenom whereby the rubber sheet balloons and expands as applied pressure increases. Under conditions of expansion, a puncture in the sheet also expands thereby mitigating the occurence of jetting. In the present effort, we study a single puncture in the center of circular rubber sheets of 2, 3, and 4 inch diameters. By measuring bubble frequency and flow rate, we compute average bubble size. These results for flow rates from 0.01 to 2.0 cc/sec suggest that bubble size is practically constant over a nearly two decades of flow rate, until a flow of around 0.5 cc/sec, thence bubbles tend to follow the empirical correlation of L. Davidson and Amick (1956) and the inviscid theory of J.F. Davidson and Schuler (1960) both of which predict bubble volume increases as the 6/5 power of flow.

Using the “point source” model of J.F. Davidson for bubble growth, we include additional effects of surface tension to derive the required detachment-time. This leads to a theory which includes inertial, buoyancy and surface tension effects. The theory gives flow rate as a function of final bubble size and agrees quite well with the 180 experiments reported. The new theory approaches the inviscid models in the limit of large gas flow rates. Finally, we present results which clearly suggest that hole area increases linearly with increasing plenum chamber pressure.     

EFFECT OF DISTRIBUTOR ON BUBBLE SIZE IN A CYLINDRICAL FLUIDIZED BED AT AN ELEVATED TEMPERATURE†

The effects of temperature and distributor on bubble diameter were investigated using a cylindrical fluidized bed of 147 mm in diameter. Three perforated distributors having different holes in diameter and the same ratio of holes to bed area were used. Eruption diameters of bubbles were measured using a high speed video-camera system under the following conditions: bed temperature = 300 and 600 K, bed particles = spherical glass beads of 272 μm in average size, excess gas velocity = 1-4 cm/s, and static bed height equals; 10-42 cm. The bubble diameter at 600 K was larger than that at 300 K. The difference became smaller with increasing the static bed height and with increasing the excess gas velocity. The distributor with larger holes gave larger bubbles. The effect of hole diameter of the distributor on the bubble diameter became insignificant with increasing the static bed height and with increasing the excess gas velocity.     

The motion mechanism and characteristic of bubble in a pseudo-2D tapered fluidized bed

The different carbon nanotube (CNT) particles (^@A and ^@V) were bed materials in the pseudo-2D tapered fluidized bed (TFB) with/without a distributor. A detailed investigation of the motion mechanism of bubbles was carried out. The high-speed photography and image analysis techniques were used to study bubble characteristic and mixing behavior in the tapered angle of TFB without a distributor. The fractal analysis method was used to analyze the degree of particles movement. Results showed that an S-shaped motion trajectory of bubbles was captured in the bed of ^@V particles. The population of observational bubbles in the bed of ^@V particles was more than that of ^@A particles, and the bubble size was smaller in the bed of ^@V particles than that of ^@A particles. The motion mechanism of bubbles had been shown to be related to bed materials and initial bed height in terms of analysis and comparison of bubble diameter, bubble aspect ratio and bubble shape factor. Importantly, compared to the TFB with a distributor, the TFB without a distributor had been proved to be beneficial to the CNT fluidization according to the study of bubble characteristic and the degree of the particle movement. Additionally, it was found that the mixing behavior of ^@V particles was better than ^@A particles in the tapered angle of TFB without a distributor.     

Bubble characteristics in shallow bubble column reactors

The bubble characteristics have been investigated in an air–water bubble column with shallow bed heights. The effect of bed height, location and the presence of solids on the bubble size, bubble rise velocity and overall and sectional gas holdup are studied over a range of superficial gas velocities. Optimal shallow bed operation relies on the combined entrance and exit effects at the distributor and the liquid bed surface. The gas holdup is found to decrease with an increase in H/D ratio but the effect is diminishing at high H/D ratios. A H/D ratio of 2–4 is found to be suitable for shallow bed operation. The presence of solids causes the formation of larger bubbles at the distributor and the effect is diminishing as the gas velocity is increased.     

Origin of pressure fluctuations in an internal-loop airlift reactor and its application in flow regime detection

The origin of pressure fluctuations in an internal-loop airlift reactor (ILALR) and its application in the flow transition detection are investigated. It is found that pressure fluctuations can be characterized as global pressure fluctuations and local pressure fluctuations by frequency domain analysis and wavelet analysis. The global pressure fluctuations generated by gas compression in the gas plenum and flow fluctuations in the gas-supply system have almost a linear attenuation in the downcomer and almost no attenuation in the riser, especially in heterogeneous flow regime. However, it is found that the pressure wave from bubble eruption at bed surface has little impact on the wall pressure fluctuations. The global pressure fluctuations may be explained by Sasic's model. The local bubble-induced pressure fluctuations generated by bubble passage, coalescence and breakage can be determined by bubble passage frequency bandwidth and lower coherence. After extracting the local bubble-induced pressure fluctuations from the origin wall pressure fluctuations, it is shown that the Hurst exponent of the local pressure fluctuations increases faster in the homogeneous flow regime than in the heterogeneous flow regime, which can be employed to indicate the flow regime transition.     

FORMATION AND FLOW OF GAS BUBBLES IN A PRESSURIZED BUBBLE COLUMN WITH A SINGLE ORIFICE OR NOZZLE GAS DISTRIBUTOR

The characteristics of gas bubbles in a 5 cm diameter bubble column equipped with a single orifice of 1,3 or 5 mm diameter were investigated under system pressure of 0.1-15 MPa. The formation of gas bubbles was strongly affected by the system pressure. Under high pressures a dispersed gas jet was formed at gas velocities where spherical gas bubbles would have been formed at atmospheric pressure. The critical gas velocity between the bubbling regime and the jetting regime was correlated with the liquid phase Weber number and the gas phase Reynolds number based on the gas velocity at the orifice. Bubble size and gas holdup in the main part of the bubble column were also affected by the bubble formation pattern at the distributor     

CHARACTERIZATION OF AXIAL NONUNIFORMITY IN FLUIDIZED BED BY THE AMPLITUDE OF LOCAL PRESSURE DROP FLUCTUATIONS

The amplitude of local pressure drop fluctuations was used to characterize the axial stability of a freely bubbling fluidized bed. The experiments were carried out in a bed of 0.318 m dia at normal and elevated temperatures. It was found that the transition between a freely bubbling zone, consisting of small bubbles distributed uniformly across the bed, and a zone of incipient slugging occurs if bubble to bed diameter ratio is about 0.15. The amplitude of local pressure drop fluctuations was found to be a linear function of gas velocity in a broad range of temperatures if measured in the freely bubbling zone     

Utilization of the pressure differential records from gas fluidized beds with internals for bubble parameters determination

Simulated pressure gradient records, based on the theoretical pressure field around a bubble in fluidized bed derived by Davidson, are analysed by statistical methods to determine significant bubble parameters. It is shown that from two records measured by two pairs of pressure differential probes located on common vertical axis the following can be computed: bubble velocity, bubble depth (diameter), vertical spacing of bubbles, bubbling frequency, distributions of bubble sizes and spacings, and local bubble phase fraction. Simulated and actual records of the passage of non-interacting single bubbles over four pairs of probes located in a fluidized bed with internals show good agreement.     

Non-intrusive monitoring of bubbles in a gas-solid fluidized bed using vibration signature analysis

A reliable method was developed to study bubble behavior by analysis of vibration signals in fluidized beds. The advantage of this method is that the vibration probe is in indirect contact with the process. Accelerometers were used to record vibration signals generated by particle flow through the fluidized bed at various superficial gas velocities and particle sizes. Measurement of vibration signals, sampled at 25 kHz for 30 s, enabled investigation of changes in flow structure related to flow regime transitions. To study bubble behavior under different conditions, different particle sizes were used in the experiments. The measurements were extensively analyzed using wavelet and fast Fourier transforms. Results indicate that the vibration frequency generated by bubbles is between 1000 and 3000 Hz. The vibration analysis was effectively used to detect minimum fluidization and transition from bubbling to slugging in gas-solid fluidized beds.     

Distributor effects near the bottom region of turbulent fluidized beds

The distributor plate effects on the hydrodynamic characteristics of turbulent fluidized beds are investigated by obtaining measurements of pressure and radial voidage profiles in a column diameter of 0.29 m with Group A particles using bubble bubble-cap or perforated plate distributors. Distributor pressure drop measurements between the two distributors are compared with the theoretical estimations while the influence of the mass inventory is studied. Comparison is established for the transition velocity from bubbling to turbulent regime, U_c, deduced from the pressure fluctuations in the bed using gauge pressure measurements. The effect of the distributor on the flow structure near the bottom region of the bed is studied using differential and gauge pressure transducers located at different axial positions along the bed. The radial voidage profile in the bed is also measured using optical fiber probes, which provide local measurements of the voidage at different heights above the distributor. The distributor plate has a significant effect on the bed hydrodynamics. Owing to the jetting caused by the perforated plate distributor, earlier onset of the transition to the turbulent fluidization flow regime was observed. Moreover, increased carry over for the perforated plate compared with the bubble caps has been confirmed. The results have highlighted the influence of the distributor plate on the fluidized bed hydrodynamics which has consequences in terms of comparing experimental and simulation results between different distributor plates.     

Characterization of spouted bed regimes using pressure fluctuation signals

This work compares time, frequency and phase space analyses of pressure measurements in different spouted beds. The experiments were carried out in different constructions of spouted bed apparatuses, operated under ambient conditions and under different spouting regimes. Spouted beds are used when the conventional fluidized beds fail to achieve a homogeneous and stable flow regime as, for example, in the case of non-spherical particles and in poly dispersed and finely dispersed systems. Different fluidization regimes in spouted beds have been characterized by the analysis of pressure fluctuation signals. Several flow regimes are found to exist as: fixed bed, channel formation, bubbling formation, stable spouting and slugging bed regimes. Analyses of standard deviation and chaotic time series on pressure fluctuation signals are conducted to determine the transition gas velocities. A treatment technique using the Fast Fourier Transformation of measured pressure fluctuations was developed to create plots describing the bed behaviour evolution from fixed to slugging bed. At the beginning of stable spouting the amplitude of pressure fluctuations is uniform and small.     

A critical review of the complex pressure fluctuation phenomenon in gas-solids fluidized beds

The complex pressure fluctuation phenomenon in gas-solid fluidized beds is systematically examined in this paper based on a comprehensive review of the literature data. The local pressure fluctuations are composed of multiple sources, including local bubble induced fluctuations, global bed oscillations and propagating pressure waves originating in other locations (e.g. bed surface, distributor and windbox). The interaction and coupling among bubble motion, under-damped oscillations of fluidized particles and bed surface, propagating compressible pressure waves and flow pulsation in gas-solid fluidized beds creates the complexity of local pressure fluctuations, and is likely responsible for the formation of complex but unique flow patterns. A few attempts have been reported in the literature on examining the interaction between bed oscillations, plenum chamber air pulsation and propagating pressure waves in fluidized beds, showing some promises on predicting the local pressure fluctuations. Future work should be focused on predicting local and global pressure fluctuations and the formation of unique surface flow patterns by coupling different contributing mechanisms.     

Population balance modelling of bubbling fluidized bed reactors—I. well-stirred dense phase

A detailed population balance model is presented for a fluidized bed reactor incorporating: the formation of bubbles at the grid plate, their rise with velocities governed by their sizes, random coalescence between bubbles, gas exchange between bubbles and the dense phase, and a first order chemical reaction in the dense phase under well-mixed conditions. Reaction conversion is calculated as a function of dimensionless parameters relating the rates of various competing processes such as coalescence, dense phase mixing, mass exchange between bubbles and dense phase and reaction rate. Comparison of conversions with those of Davidson et al. (1977) show significant variations indicating that the dynamics of bubble size distributions could have non-trivial effects on the extent of reaction. Fluctuations in bubble populations did not seem strong enough to translate to strong fluctuations in reaction conversion.     

THE EFFECTS OF DISTRIBUTOR DESIGN ON FLUIDIZED-BED HYDRODYNAMIC BEHAVIOR

should be addressed. The distributor was investigated for the purpose of design and scale up of large fluidized-bed combustors. Four orifice plates with different configurations were used to study the effect of distributor design on bubble formation and solid mixing. Experiments were carried out on a three-dimensional fluidized bed of 27.94 cm diameter and a two-dimensional bed with dimensions of 30.48cm ×1.27 cm. Motion pictures were used to study bubble formation and coalescence. Pressure profiles inside the three-dimensional bed were measured for several distributors to study bubble flow patterns, and tracer particles were used to study mixing patterns at various superficial velocities and particle sizes. The results show that the distributor plate with two-size orifices causes a non-uniform gas bubble flow inside the bed. This non-uniform gas bubble flow is associated with variations in local bed density and local voidage. Horizontal or radial solid circulation is also caused by this non-uniform gas bubble flow. The local bed density and voidage variations and the radial solid circulation cause the bubbles to move toward the area above the smaller orifices as the bubbles rise up and coalesce. This reduces the wall effect, and the bed is very uniformly fluidized when the two-size orifice plate with small holes in the center is employed.     

INVESTIGATIONS OF THE BUBBLE PHASE BEHAVIOR IN A HIGH TEMPERATURE FLUIDIZED BED REACTOR WITH COALCOMBUSTION

The properties of bubbles in a bench-scale fluidized bed reactor, 30 cm in diameter, during coal combustion were determined by means of a newly developed cooled bubble probe and a data processing system at temperatures up to 850°C and fluidization indexes up to 10 in axial and radial positions in the bed. The fluidization index above 4 and the temperature have only slight effect on the bubble properties. Their variation along the height above the gas distributor is dominant.

Oxygen and Co₂-concentration profiles were measured in the bed and in the freeboard, and the o₂-profiles were calculated by means of measured bubble data and bubble models. A comparison of measured and calculated o₂-profiles indicates that the mass transfer rates between the emulsion and bubble phases are larger than the ones calculated by the models.     

Improving Gas‐Liquid Mass Transfer in Bubble Columns by Applying Low‐Frequency Vibrations

We show that application of low‐frequency vibrations, in the 50–200 Hz range, to the liquid phase of an air‐water bubble column causes significantly smaller bubbles to be generated at the distributor plate. For bubble column operation in the homogeneous flow regime, measurements of the volumetric mass transfer coefficient using the oxygen absorption technique show that the increase in the k_La values ranges from 50–100 % depending on the flow rate. It is concluded that application of low‐frequency vibration has the potential of improving the performance of bubble columns.     

Bubble size distribution in the sparger region of bubble columns

It is well known that the gas distributor can play an important role on the evolution of the bubble size distribution (BSD) in gas-liquid reactors, strippers and absorbers. Therefore, the main subject of the present work was to study the influence of sparger design and process parameters on the BSD in the sparger region of the considered apparatus. For this purpose, both detailed measurements and prediction of the size of bubbles produced at the sparger were carried out in three different experimental apparatuses.The unique set of BSD curves were obtained by analyzing a large amount of bubbles with a measurement based on image analysis technique.Additionally, Colella's model of BSD evolution in bubble columns was further developed by implementing a detailed physical model for predicting the initial BSD at the sparger where the model input is only based on design/process parameters. A validation of the model was carried out using data from two different columns. The comparison between calculated and experimental BSD shows good agreement.     

多孔挡板流化床气泡行为的研究

本文在内径为(?)120mm 的多孔挡板流化床中,用光导纤维法和电容法测定了 Al(OH)_3粉、铜粉和 FCC 三种不同物料体系的气泡频率和气泡速度,对操作条件和挡板参数对气泡行为的影响作了研究和分析。结果表明,在一定的气速下,挡板的开孔率、孔径和板间距(级间高径比小于3)对气泡频率和气泡速度的影响较小;对属 B 类的 Al(OH)_3粉和铜粉物料,气速对气泡频率的影响可以忽略,而对属 A 类的 FCC 物料,气泡频率随气速的增大而增大。