Analysis of gas holdup in bubble columns with non-Newtonian fluid using electrical resistance tomography and dynamic gas disengagement technique

This paper presents an experimental analysis of the influence of the liquid rheology on the gas flow pattern in a bubble column reactor. Aqueous solutions of xanthan are selected as an example of non-Newtonian shear thinning fluid. Averaged gas holdup is determined by two experimental techniques: parietal pressure probes and electrical resistance tomography (ERT). ERT is also used to provide 2D images of the gas phase distribution in a column cross-section. Bubble size distributions are evaluated by a gas disengagement technique using the parietal pressure probes. All these techniques clearly show the gas flow pattern is different in Newtonian and non-Newtonian fluids. Gas holdup values decrease when increasing the liquid viscosity and reach a minimum or a plateau. Homogeneous flow regime, observed in water at low gas velocities, tends to disappear when viscosity increases. This evolution is visualized by a much less isotropic distribution of the gas phase within cross-section of the column and by the appearance of much larger bubbles due to an increased coalescence phenomenon.     

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.     

Effect of antifoam agents on bubble characteristics in bubble columns based on acoustic sound measurements

In this paper, the effect of antifoam agents on bubble characteristics in bubble columns is studied. Specifically, the bubble characteristics of air in tap water are compared to those of air in 5% and 10% antifoam solutions. Bubble characteristics such as gas holdup, bubble diameter, bubble-size distribution, and damping ratio were investigated at various superficial gas velocities. These properties were deduced from the acoustic sound measurement. The study revealed that the addition of antifoam chemicals reduces the overall gas holdup and increases the average bubble diameter. The bubble-size distribution in tap water is found to be homogeneous while in antifoam solutions to be heterogeneous. It is also found that at low gas velocities the damping ratio for antifoam solutions is higher than that for tap water, while at high gas velocities the damping ratio is not affected. The results affirm that acoustic probes are excellent measuring tools over classical tools at moderate gas velocities.     

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.     

X-ray computed tomography in large bubble columns

X-ray computed tomography (CT) is used to explore the differences in a semi-batch bubble column operated at superficial gas velocities of U_g=3, 10, and 18 cm/s. Air-water or air-water-cellulose fiber systems comprise the multiphase flow, and the bubble column has a 32.1 cm internal diameter. A CT image of a phantom object composed of several air-filled tubes immersed in water is used to identify several characteristic features of the X-ray CT system. CT images are then compared between air-water and air-water-cellulose fiber systems. When the fiber mass fraction is 0.1%, gas holdup is slightly higher than that of the air-water system in the column center and near the column wall. In 1.0% cellulose fiber slurries, gas holdup is lower than that of air-water results at all radial positions.     

Efficient dispersion in a modified two-phase non-Newtonian downflow bubble column

A comprehensive study of dispersion of gas-non-Newtonian fluid has been made in a modified downflow bubble column. Distribution of gas holdup in axial location has been anticipated from hydrostatic force balance. The experimental gas holdup data in such modified bubble column have been analyzed by slip velocity and drift-flux models. A correlation in terms of various physical, geometric and operating parameters of the present system has also been developed to analyze the gas holdup.     

A gas holdup model for cocurrent air-water-fiber bubble columns

A gas holdup model is developed for cocurrent air-water-fiber bubble column flows using the drift-flux model. The model coefficients are estimated using a nonlinear least square method and systematically acquired experimental data. The model correlates gas holdup with superficial gas and liquid velocity, and fiber type and mass fraction. The model reproduces most experimental data within ±10% error and all but 3 of the 3839 experimental data points within ±15% error. It also accurately predicts air-water bubble column gas holdup data; these data were not used in estimating the model coefficients. The physical implications of the model coefficients are also discussed.     

Gas-liquid mass transfer in a slurry bubble column operated at gas hydrate forming conditions

Gas-liquid interphase mass transfer was investigated in a slurry bubble column under CO₂ hydrate forming operating conditions. Modeling gas hydrate formation requires knowledge of mass transfer and the hydrodynamics of the system. The pressure was varied from 0.1 to 4 MPa and the temperature from ambient to 277 K while the superficial gas velocity reached 0.20 m/s. Wettable ion-exchange resin particles were used to simulate the CO₂ hydrate physical properties affecting the system hydrodynamics. The slurry concentration was varied up to 10%vol. The volumetric mass transfer coefficient (k_la_l) followed the trend in gas holdup which rises with increasing superficial gas velocity and pressure. However, k_la_l and gas holdup both decreased with decreasing temperature, with the former being more sensitive. The effect of solid concentration on k_la_l and gas holdup was insignificant in the experimental range studied. Both hydrodynamic and transport data were compared to best available correlations.     

热态下鼓泡浆液反应器的气含率研究

本文在热态条件下，研究了鼓泡浆液反应器的气含率、反应器直径为0.098m，物系组成为氮气－液体石腊－石英砂。考察了气速、压力、温度、静床高及固体引入等因素对气含率的影响。对于53μm粒子的三相浆态体系，气含率与表观气速关系式为εG=0.053uG^1.2。     

Frictional loss in multiplate bubble columns

Gas holdup and pressure loss were measured for various gas-liquid systems in a batch, multistage bubble column. Experimental results show that the use of screen plates (α = 0.64) considerably increases the gas holdup but introduces a significant pressure loss for the two-phase mixture. The pressure loss was found to be independent of physical properties of the mixture, and predictable either from the modified separated flow model (Chen et al., 1986) or from the kinetic energy loss based on the liquid circulation velocity.     

Effect of surfactant on homogeneous regime stability in bubble column

Experiments were carried out to investigate the effect a surface active agent on homogeneous-heterogeneous flow regime transition in a laboratory scale bubble column. Air and water with various amount of CaCl₂ were the phases. The (voidage e) - (gas flow rate q) dependence was measured. The critical point where the homogeneous regime loses stability and the transition begins was evaluated by several methods. These methods are based on the slip speed concept and the drift flux model. The critical values of voidage and gas flow rate were taken as the quantitative measures of the homogeneous regime stability. They were plotted against the surfactant concentration. It was found that the surfactant has a dual effect on both the voidage and the regime transition: low concentration stabilizes and larger concentration destabilizes the homogeneous bubble bed. At present, we do not have an explanation to these observations. Possible physical mechanisms of the surfactant effect are expected to be revealed by further experiments, which are currently under way.     

AN EXPERIMENTAL STUDY OF GAS HOLDUP IN TWO-PHASE BUBBLE COLUMNS WITH FOAMING LIQUIDS

Gas-liquid upward flow experiments have been performed in two bubble columns of different diameters (0.10 and 0.29 m,) using air as gas phase and several liquids: water, aqueous solutions of ethanol and glycerine, kerosene, and a solution of a surfactant in kerosene. The main goal of the study is the analysis of foaming systems, including the comparison of their behavior with respect to non-foaming systems. The gas holdup was determined experimentally as a function of the gas and liquid superficial velocities in bubbling, churn-turbulent and foaming regimes. It was found that, for foaming systems, semi-batch operation enhances foam formation, yielding higher holdups than those obtained in continuous operation at very low liquid velocities. Opposite to what is observed in non-foaming systems, the liquid superficial velocity affects the gas holdup appreciably in foaming systems. An increase in column diameter results in a decrease in gas holdup for all the systems studied. In aqueous foaming systems, this trend is more drastic since foam is inhibited as the column diameter increases.     

The effect of operating and design parameter on bubble column performance: The LOPROX case study

It is known that the performances of multi-phase reactors depend on the operating parameters (the temperature and the pressure of the system), the phase properties, and the design parameters (the aspect ratio (AR), the bubble column diameter, and the gas sparger design). Hence, the precise design and the correct operation of multi-phase reactors depends on the understanding and prediction of the fluid dynamics parameters. This paper contributes to the existing discussion on the effect of operating and design parameter on multi-phase reactors and, in particular, it considers an industrial process (e.g., the LOPROX (low pressure oxidation) case study, which is typical example of two-phase bubble columns). Based on a previously-validated set of correlations, the influence of operating and design parameter on system performances is studied and critically analyzed. First, we studied the effects of the design parameter on the liquid–gas interfacial area, by keeping constant the fluid physical–chemical properties as well as the operating conditions; subsequently, we discussed for a fixed system design, the influence of the liquid phase properties and the operating pressure. In conclusion, this paper is intended to provide guidelines for the design and scale-up of multi-phase reactors.     

Measurement of gas phase characteristics using a monofibre optical probe in a three-phase flow

The study of gas-liquid-solid systems structure requires reliable measurement tools. In this paper, preliminary results on the potential use of a monofibre optical probe to investigate such flow are presented. This probe, manufactured at LEGI, allows the simultaneous measurement of the gas phase residence time and gas phase velocity. This specificity makes this probe more interesting than classical single tip probes (which measure only the gas residence time) or double tip probes (which are more intrusive). Although extensively used in two-phase gas-liquid, this probe was never used in gas-liquid-solid systems. First, the probe signal response is studied for three-phase flow conditions in the presence of solids. Results show that for soft solids, the probe tips can be contaminated when the probe pierces the solid. The signal processing procedure was modified accordingly to take into account these events. Second, the probe results are validated by comparing global results (global void fraction, gas flowrate) deduced from profile measurements with measurements performed by independent means. Lastly, void fraction profiles and interfacial area are studied more in detail. Depending on the solid loading, these profiles exhibit different behaviours. These features are associated to characteristics of the flow such as the transition from an homogeneous regime to an heterogenous regime, and are consistent with global observation performed by independent means. This demonstrates the ability of the probe to connect local information to the global behaviour and structure of the flow.     

Effect of liquid properties on the performance of bubble column reactors with fine pore spargers

This work is a study of the effect of liquid properties on the performance of bubble column reactors with fine pore spargers. Various liquids covering a range of surface tension and viscosity values are employed, while the gas phase is atmospheric air. A fast video technique is used for visual observations and, combined with image processing, is used for gas holdup and bubble size measurements. New data on average gas holdup values, bubble size distributions and Sauter diameters are presented and are consistent with existing physical models on coalescence/breakage. A correlation based on dimensionless groups for the prediction of gas holdup in the homogeneous regime is proposed and found to be in good agreement with available data.     

New insights into transient behaviors of local liquid-holdup in periodically operated trickle-bed reactors using electrical capacitance tomography (ECT)

Transient behavior of local liquid-holdup of air-kerosene fluids in periodically operated trickle-bed reactor (TBR) is investigated in an acrylic column (140 mm ID and a height of 980 mm) packed with 3.6 mm glass spheres using a nonintrusive technique of electrical capacitance tomography (ECT). Local liquid-holdups determined from ECT images of normalized permittivity are experimentally calibrated under the steady-state operation with that from the classic drainage method. The instantaneous ECT images are captured at several axial positions along the column for the periodically operated TBR with slow-mode. The effect of periodic operation parameters (split, period, and time-average flow rate) on the instantaneous profiles of local liquid-holdup is firstly examined compared with the previous results. Transient variations of radial distributions of liquid and their maldistribution factor are calculated and further analyzed to provide more liquid distribution information under periodic operations.     

Flow regimes and mass transfer in counter-current bubble columns

Counter current bubble columns have the feature that specific gas-liquid interfacial area and gas holdup are larger than those for standard and cocurrent bubble columns. In this study, three different flow regimes, churn-turbulent flow, bubble flow and bubble down-flow, have been observed in a counter-current bubble column and correlations of gas holdup and volumetric liquid-phase mass transfer coefficient have been proposed as functions of operating variables such as the superficial velocities of gas and liquid, the gas-liquid slip velocity and the liquid properties.     

Effect of bubble deformation on stability and mixing in bubble columns

The paper deals with hydrodynamics in bubble columns. The objective of the paper is to study stability and mixing in a bubble column. The modeling of parameters such as stationary drag and added mass is addressed. In addition, the effect of bubble deformation in terms of eccentricity is highlighted. In a previous paper, the transition between homogeneous and heterogeneous regimes in bubble column without liquid flow has been shown to be driven by the deformation of the bubbles associated to drag and added mass. In the present paper, this work is generalized to bubble column with liquid flow and to the transition from bubble flow to slug flow in a vertical pipe. Numerical simulations of gas-liquid reactors are presented. The numerical simulations are validated in the case of gas plume after the Becker et al. data (Becker, S., Sokolichin, A., & Eigenberg, G. (1994) Gas-liquid flow in bubble columns and loop reactors: Part II. Comparison of detailed experiments and flow simulations. Chemical Engineering Science, 49 (24B), 5747-5762. The numerical simulations are finally applied to a bubble column. The simulations of residence time distribution coupled to transient hydrodynamics are shown to be very sensitive to the modeling of interfacial transfer of momentum from the bubbles to the liquid in terms of drag and added mass, including the effect of bubble deformation.     

Effect of resistance internal on hydrodynamic behaviours and bubble characteristics in a laboratory-scale bubble column

A novel resistance internal is proposed to optimize the flow field and improve the gas–liquid contact in a co-hydrogenation reactor of coal and vacuum residuum. Local gas holdup, local liquid velocity, and characteristics of the bubble were investigated in a scaled-down laboratory model. The quantitative results showed that the resistance internals could reduce the thickness of the liquid reflux layer by a percentage up to 32% and reduce the difference in the local gas holdup at cross-sections of up to 44%. The Sauter mean diameter of the bubble decreased from 20.30 to 16.00 mm, which aroused the increase in bubble surface area by a percentage of up to 71.9%. The resistance internal promoted the breakup of the bubble with multiple mechanisms and provided diversion to fluid. In this work, improvement at multiple scales was realized, and the technical support for industrial application was provided.