Improved gas holdup in novel bubble column

This investigation reports the experimental and theoretical results carried out to evaluate the gas holdup for air–water system in a novel hybrid rotating and reciprocating perforated plate bubble column under countercurrent condition. The response of this hybrid column is found to be similar to that of reciprocating plate column (RPC) showing mixer‐settler, transition, and emulsion regions. The effect of agitation level, superficial gas velocity, superficial liquid velocity, perforation diameter, and plate spacing on gas holdup is studied and found to be significant. The gas holdup is found to be least in the range of agitation level of 1.3–1.5 cm/s. For all the superficial gas and liquid velocities considered in this present investigation, the critical agitation level at minimum gas holdup remains nearly same. The gas holdup in this hybrid column is 1.2–1.7 times higher in mixer‐settler region and 2.1–2.7 times higher in emulsion region than that of RPC. Correlations have been developed and found to concur with the experimental values. It can be used with 95% accuracy. © 2011 Canadian Society for Chemical Engineering     

Bubble size fractal dimension,gas holdup,and mass transfer in a bubble column with dual internals

As the scale of residual oil treatment increases and cleaner production improves in China, slurry bubble column reactors face many challenges and opportunities for residual oil hydrogenation technology. The internals development is critical to adapt the long-term stable operation. In this paper, the volumetric mass transfer coefficient, gas holdup and bubble size in a gas-liquid up-flow column are studied with two kinds of internals. The gas holdup and volumetric mass transfer coefficient increase by 120% and 42% when the fractal dimension of bubbles increases from 0.56 to 2.56, respectively. The enhanced mass transfer processing may improve the coke suppression ability in the slurry reactor for residual oil treatment. The results can be useful for the exploration of reacting conditions, scale-up strategies, and oil adaptability. This work is valuable for the design of reactor systems and technological processes.     

Bubble size fractal dimension,gas holdup,and mass transfer in a bubble column with dual internals

As the scale of residual oil treatment increases and cleaner production improves in China, slurry bubble column reactors face many challenges and opportunities for residual oil hydrogenation technology. The internals development is critical to adapt the long-term stable operation. In this paper, the volumetric mass transfer coefficient, gas holdup and bubble size in a gas–liquid up-flow column are studied with two kinds of internals. The gas holdup and volumetric mass transfer coefficient increase by 120% and 42% when the fractal dimension of bubbles increases from 0.56 to 2.56, respectively. The enhanced mass transfer processing may improve the coke suppression ability in the slurry reactor for residual oil treatment. The results can be useful for the exploration of reacting conditions, scale-up strategies, and oil adaptability. This work is valuable for the design of reactor systems and technological processes.     

浆态床鼓泡反应器中气含率的分布

为了解浆态床鼓泡反应器中气含率的分布规律,在浆态床鼓泡反应器冷模试验装置中,以空气-液体石蜡-氧化铝微球为试验介质对装置内部的气含率进行研究。利用压差法研究了表观气速、浆液固含量等操作条件对反应器床层总体气含率的影响,利用光纤探针法研究了浆态床反应器不同操作条件对局部气含率的影响,总结了反应器内部气含率的分布规律,并由此对工业浆态床鼓泡反应器的设计进行了研究。结果表明:浆态床反应器的总体气含率随表观气速的增大而增大,固体细颗粒的加入能适当降低总体气含率;在反应器底部,分布器对气体的均布作用明显,但表观气速的增大能够弱化分布器的作用;在反应器的中上部气含率不受分布器的影响,沿反应器径向呈现"中间高,边缘低"的分布趋势;在工业费托浆态床中,表观气速不宜低于0.12 m/s,内过滤系统适宜设置于反应器中上部靠近器壁的位置。     

鼓泡塔中气泡尺寸分布和局部气含率研究

在内径为0.38 m的鼓泡塔中采用双电导探针法对不同通气速率下的气泡尺寸分布和局部气含率进行了实验研究,分析了气泡尺寸的概率密度分布。结果表明:气泡尺寸随轴向高度的增加而增大,随径向距离增加而减小;鼓泡塔中气液流动可分为过渡流域和充分发展流域,在过渡流域气含率随轴向高度增加而增大,在充分发展流域气含率趋于均值,径向局部气含率分布呈抛物线型下降。高气速下气泡尺寸概率密度分布比低气速下宽,且随轴向高度的增加分布变宽。     

Time-dependent gas holdup variation in an air-water bubble column

Time-dependent gas holdup variation in a two-phase bubble column is reported with air and tap water as the working fluids. The results indicate that time-dependent gas holdup is closely related to the water, whose quality is unsteady and changes, not only during the two-phase flow, but also during idle periods. The significance and characteristics of the time-dependent gas holdup variation are influenced by the bubble column operation mode (cocurrent or semi-batch), the sparger orientation, the superficial gas velocity, and the superficial liquid velocity. It is proposed that a volatile substance (VS), which exists in the water in very small concentrations and inhibits bubble coalescence, evaporates during column operation and results in a time-dependent gas holdup. The influence of bubble column operation mode, sparger orientation, superficial gas velocity, and superficial liquid velocity on the time-dependent gas holdup variation are explained based on their effects on bubble size, bubble contacting frequency and mixing intensity. This work reveals that regular tap water may cause significant reproducibility problems in experimental studies of air-water two-phase flows.     

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.     

Influence of structured packing on gas holdup in a three-phase bubble column

In this work, the influence of structured packing on gas holdup in gas-liquid-solid dispersions has been studied. The experiments were carried out in an empty column and in column containing structured packing operating under identical conditions. Glass beads and silicon carbide particles were used as the solid material and the volumetric fraction of solids was varied from 0% to around 10%. The liquid viscosity was strongly modified using water, CMC solution and glycerol. The experimental results obtained with both columns were compared with previous results obtained in two-phase bubble columns. The influence of structured packing on the total gas holdup for different superficial gas velocities was found to be similar with and without suspended solids. Therefore, the results obtained in this work were analysed on the basis of correlations derived earlier for gas-liquid dispersions. Excepting the results obtained with glycerol, these correlations can predict the gas holdup of three-phase bubble columns with reasonable accuracy.     

Heat transfer and gas holdup studies in a bubble column: Air-water-sand system

Air-holdup and heat transfer coefficient data are reported for the air-water and air-water-sand system as a function of air velocity in the temperature range 297-343 K as measured in a 0.305 m diameter bubble column operating in semi-batch mode and equipped with either a five- or seven-tube bundle. A 65 μm average size sand powder is used at concentrations of 5 and 10 mass percent in the slurry. Available correlations of gas holdup and heat transfer coefficients are examined on the basis of these data. These are found inappropriate and inadequate for representing these experimental data. Gas holdup data are well represented by an approach based on Nicklin's (1962) work, and heat transfer data are adequately represented by a simple semi-empirical expression. Accurate experimental data on additional systems are needed to develop a reliable heat transfer theory particularly for process representation at temperatures higher than ambient.     

Effect of fiber length distribution on gas holdup in a cocurrent air-water-fiber bubble column

An experimental investigation is reported on the effect of fiber length distribution on gas holdup in a cocurrent air-water-fiber bubble column. Different combinations of 1 and 3 mm Rayon fibers are used to simulate different fiber length distributions. At a constant total fiber mass fraction, gas holdup generally decreases with increasing mass fraction of the 3 mm Rayon fiber while other conditions remain constant. Crowding factors estimated using four different methods (N_c=N_c,A, , N_c,L, and N_c,M) and the parameters and are tested on their performance to quantify the overall effects of fiber mass fraction and fiber length and its distribution on gas holdup. and provide the best characterization of the fiber effects on gas holdup in the cocurrent air-water-fiber bubble column. The crowding factor estimated using the model-based average fiber length (N_c,M) also provides a good characterization and is better than the other crowding factor definitions.     

The effect of bubble column diameter on gas holdup in fiber suspensions

Three bubble column diameters (D=10.2, 15.2, and 32.1 cm) are employed to study the scale-up effect on gas holdup in air-water and air-water-cellulose fiber (hardwood, softwood, and BCTMP) systems. The effect of column diameter depends on flow regime and fiber mass fraction. When , gas holdup decreases with increasing column diameter for the transitional and heterogeneous flow regime, and column diameter effects are negligible in the homogeneous flow regime. When , gas holdup is only affected by column diameter in the transitional flow regime for an air-water system and low fiber mass fraction suspensions (C?0.25%); column diameter effects disappear at medium fiber mass fractions (e.g., C=0.8%) but are significant at high fiber mass fractions (e.g., C=1.4%).     

Mixing characteristics and gas hold-up of a bubble column

The hydrodynamic behavior of a bubble column has been studied for various Newtonian and non-Newtonian liquids (water, glycerol, carboxymethylcellulose and polyacrylamide solutions). The mixing time, the power consumption, the circulation time and the gas hold-up have been measured in a cylindrical column (diameter: 0.254 m, height: 0.9 m) for three air sparger plates with different numbers and distributions of 1 mm diameter orifices. It is shown that the mixing efficiency decreases as the viscosity or the shear-thinning and elastic properties of the liquid increase. The viscosity of the liquid has little influence on the gas hold-up which is, however, strongly affected by the sparger plate characteristics and increases as the liquid phase becomes more elastic. A model for predicting gas hold-up is proposed.     

The investigation of gas holdup distribution in a two-phase bubble column using ultrasonic computed tomography

In this study, time-averaged gas holdup distributions were investigated in a 16 cm diameter bubble column for two-phase dispersed system of air–water and air–glycerol solution of 10 wt% by using ultrasonic computed tomography (UCT). A quantitative result of UCT – as a coupling of the ultrasonic transmission method and the iterative filtered backprojection (IFBP) image reconstruction technique – is presented. The UCT results are in a good agreement with those by the bed expansion method. A higher gas holdup in the air–glycerol 10 wt% solution than in the air–water system was observed. The distribution of gas holdup in the column with an attached baffle is also investigated by UCT.     

Gas holdup in a bubble column with immikible liquid mixtures

Experimental measurement of gas holdup was carried out in a medium-size gas-liquid-liquid bubble column with a multiple nozzle sparger plate using air, water and organic liquids. It was found that the fractional holdup depends on gas velocity, liquid properties, phase inversion in the liquid mixture as well as spreading coefficient of the organic liquid. In the presence of a liquid with a negative spreading coefficient the holdup is a minimum at the phase inversion point. but the reverse is true for a liquid with a positive coefficient of spreading. Observed bubble characteristics have been discussed. Correlations for gas holdup have been developed for different ranges of liquid composition.     

Gas holdup in a bubble column containing organic liquid mixtures

Some anomolous observations of the gas holdup behaviour in binary organic liquid mixtures in a bubble column are reported in this note, to supplement the recently published data⁽¹⁾. The gas holdup was higher for mixtures of organic liquids than for either pure component. It is shown that none of the existing theories fully explains the data.     

Liquid dispersion,gas holdup and frictional pressure drop in a packed bubble column at elevated pressures

The gas holdup, frictional pressure drop and liquid dispersion have been investigated in a packed bubble column at elevated pressures for the air–water system. The bubble column, which had an internal diameter of 0.15 m and which was packed with 15 mm plastic Pall rings was operated in the semibatch mode. The operating pressures ranged from 0.1 to 0.66 MPa. It was found that increasing the pressure increases both the gas holdup and the dispersion coefficient. In contradiction to the results obtained from packed bubble columns fed with a continuous net flow of liquid, a maximum point of the frictional pressure drop was observed at the transition point between bubble and pulse flow region.     

The role of gas phase momentum in determining gas holdup and hydrodynamic flow regimes in bubble column operations

Experiments conducted in 0.15 m diameter bubble columns using water and non-aqueous liquids have shown that the gas velocity at which transition from the bubbly flow to the churn-turbulent flow regime occurs is a function of gas density. The transition velocity increased with increasing gas density. The direct effect of gas density on gas holdup in the bubbly flow regime is small with only a slight increase in holdup being observed at higher densities (?_G α ρ_g ^0.04). In the churnturbulent region a much greater effect of gas density on gas holdup was observed. These differences were found to be a direct function of the differences in holdup values at the transition points. Gas holdup was found to be a function of the gas phase momentum. In the bubbly flow regime holdup was directly proportional to momentum while in the churn-turbulent regime holdup was proportional to momentum to the one third power. Reasons for this behaviour are discussed, as well as the implied effects on liquid mixing in bubble column slurry reactors. The effects of gas density may offer an explanation for some apparently anomalous published results.     

Bubble properties and mixing characteristics in an airlift bubble column with redistributor

The effects of gas velocity liquid velocity redistributor and their free surface area on bubble properties (size rising velocity size distribution) and mixing characteristics (mixing time circulation time) were studied in an internal airlift bubble column Bubble properties were measured by resistivity probe method, and mixing characteristics were obtained by salt-base impulse method in air-water system. The empirical correlations between bubble properties, mixing characteristics and various operating parameters were proposed The disk type redistnbutor was more effective to redisperse coalesced bubbles than the perforated plate type Especially, the disk type with 19.5% free surface area had given the best result.