Gas-liquid Bubble Column Reactors Research Articles

Direct estimation of gas holdup in gas–liquid bubble column reactors using ultrasonic transmission tomography and artificial neural processing

Ultrasonic transmission tomography is an effective non-intrusive method for detecting gas–liquid two-phase flow patterns. A specific interest is the many processes whose reaction utilizes a bubble column, where the fast estimation of cross-sectional gas-holdup ratio is important for monitoring and control. In this study reference indirect image-based estimates were obtained from reconstructed tomographic data. Direct (non-image) estimation of the gas holdup ratio was also obtained using trained neural processing networks. Two forms were trialled: a generalized regression neural network (GRNN); and a long short-term memory (LSTM) network. Comparison trials were carried out for single-bubble, dual-bubble, circulation and laminar flows. Relative cross-sectional gas holdup error was selected for evaluation. For the image-based indirect trials the Tikhonov regularization algorithm had the lowest error range: 2.15%–15.64%. For direct methods the LSTM network had the lowest error range: 0.41%–9.63%, giving better performance than the image-based methods. The experimental data were used to verify the effectiveness of the network. The root-mean-square error of the test metrics for GRNN and LSTM network were 6.4260 and 5.4282, respectively, indicating that LSTM network has higher performance in processing the data in this paper.

독성가스 제거용 기포탑 반응기의 설계기법

Gas-liquid bubble column reactors are extensively used in industrial processes. A detailed knowledge of bubble size distribution is needed for determining the mass transfer in gas-liquid film. Experimental data on bubble size distribution and liquid-side mass transfer coefficient(kL) were used to calculate the estimated time to saturation in bubble column reactor. Also, the gas flux was evaluated to the liquid-side mass transfer coefficient(kL) and solubility data for hydrogen sulfide(H₂S) and chlorine(Cl₂) absorption into water. Simulation results show that H₂S absorption time to 50 % of saturation concentrations are 611 sec and 1,329 sec when bubble diameters are 0.5 mm and 4.5 mm, while absorbing 1 % H₂S gas. In case of Cl₂, absorption time range 657 to 1,400 sec when bubble size range 0.5 mm to 4.5 mm, while absorbing 1 % Cl2 gas. Calculated simulation results can be used in the design of emergency relief bubble reactors.

Ultrasonic Transmission Tomography Sensor Design for Bubble Identification in Gas-Liquid Bubble Column Reactors.

Scientists require methods to monitor the distribution of gas bubbles in gas-liquid bubble column reactors. One non-destructive method that can potential satisfy this requirement in industrial situations is ultrasonic transmission tomography (UTT). In this paper, an ultrasonic transmission tomography sensor is designed for measuring bubble distribution in a reactor. Factors that influence the transducer design include transmission energy loss, the resonance characteristics and vibration modes of the transducer, and diffusion angles of the transducers, which are discussed. For practical application, it was found that an excitation frequency of 300 kHz could identify the location and size of gas bubbles. The vibration mode and diffusion also directly affect the quality of the imaging. The geometric parameters of the transducer (a cylinder transducer with a 10 mm diameter and 6.7 mm thickness) are designed to achieve the performance requirements. A UTT system, based on these parameters, was built in order to verify the effectiveness of the designed ultrasonic transducer array. A Sector-diffusion-matrix based Linear Back Projection (SLBP) was used to reconstruct the gas/liquid two-phase flow from the obtained measurements. Two other image processing methods, based on SLBP algorithm named SLBP-HR (SLBP-Hybrid Reconstruction) and SLBP-ATF (SLBP-Adaptive Threshold Filtering), were introduced, and the imaging results are presented. The imaging results indicate that a gas bubble with a 3 mm radius can be identified from reconstructed images, and that three different flow patterns, namely, single gas bubble, double gas bubble with different diameters, and eccentric flow, can be identified from reconstructed images. This demonstrates that the designed UTT sensor can effectively measure bubble distribution in gas-liquid bubble column reactors.

Industrial radiotracer application in flow rate measurement and flowmeter calibration using 99mTc and 198Au nanoparticles radioisotope

The flow rate or fluid velocity measurement is important to maintain fluid flow quality performance in the systems. This study focuses on determination of volumetric flow rate measurement and to calibrate the conventional flowmeter using industrial radiotracer approach in quadrilateral gas-liquid bubble column reactor. In this work, two different radioisotopes which emit γ-ray have been chosen as radioactive tracer which is 99mTc produced from 99Mo/99mTc radioisotope generator and 198Au nanoparticle form neutron activation at research nuclear reactor TRIGA Mark II. Both radioisotopes representing liquid and solid tracer purposely designed for tracing liquid flow. The peak to peak radiotracer method known as pulse velocity method was applied to determine the volumetric flow rate. The radiation signals were monitored using 4 unit NaI scintillation detectors located at 4 different points nearby the inlet and outlet of the quadrilateral bubble column reactor process stream. The water volume inside the bubble column reactor was fixed at 0.04 m3 and liquid flow rates in this reactor were specified on installed flowmeter at different reference value which is 4 lpm, 8 lpm, and 12 lpm, respectively. The experimental result shows very good linearity and repeatability by following the theoretical equations with less uncertainty in volumetric flow rate measurement. The obtained results also validated the effectiveness of the proposed method for the installed flowmeter calibration efficiency.

Measurement method of bubble behavior by optical probe in gas-liquid bubble column reactor

Measurement method of bubble behavior by optical probe in gas-liquid bubble column reactor

The effects of liquid phase rheology on the hydrodynamics of a gas–liquid bubble column reactor

In this study, the effects of liquid phase rheology on the hydrodynamics of a pilot scale bubble column reactor is extensively investigated by applying various types of test liquids with different rheological characteristics as the operating fluids. Two fiber optic probes and several pressure transducers are used and different time-domain and frequency-domain analyses are applied to perform a comprehensive interpretation of the pressure signals and measure the hydrodynamic parameters of the gas phase. A new approach is proposed based on the dynamic moduli of viscoelastic solutions to better understand the simultaneous viscous and elastic effects. It was observed that the elasticity of the operating liquid reduced the average bubble chord length and increased the total gas holdup. The obtained results reveal that although the viscosity is more favorable for coalescence, the elasticity of the operating liquid can prevent bubble coalescence by showing a solid-like behavior at the interface of two bubbles.

Radial Profiles of Gas Bubble Behavior in a Gas‐Liquid Bubble Column Reactor under Elevated Pressures

AbstractThe effects of operating conditions on radial variation of gas holdups, bubble swarm rising velocity, and Sauter mean diameter were investigated in a bubble column reactor under elevated pressures using a conductivity probe method. Air served as gas phase and tap water as liquid phase with varying gas velocity and pressure. All three parameters increased constantly with higher superficial gas velocity. Maximum holdup, velocity, and Sauter mean diameter were found at the center of the cross section. Two different cases for Sauter mean diameter distribution were observed. The gas holdups increase continuously with higher system pressure, but decrease for bubble swarm rising velocity and Sauter mean diameter. According to experimental results, an empirical correlation of the gas holdup profiles is presented.

Experiments and CFD simulation of ferrous biooxidation in a bubble column bioreactor

In the present attempt a set of experiments and a 3D simulation using a commercially available computational fluid dynamics package (FLUENT) were adopted to investigate complex behavior involving hydrodynamics and ferrous biological oxidation in a gas–liquid bubble column reactor. By combining the hydrodynamics and chemical species transport equations, the velocity field, air volume fraction and ferrous biooxidation rate in the column were simulated. The kinetic model proposed by Nemati and Webb [Nemati, M., & Webb, C. (1997). A kinetic model for biological oxidation of ferrous iron by Thiobacillus ferrooxidans. Biotechnology and Bioengineering, 53, 478–486] was used to simulate the biooxidation rate in the column. Gas–liquid interactions were modeled using an Eulerian model in three dimensions. The effects of inlet air velocity and initial substrate (Fe 2+) concentration on the velocity field, air volume fraction and biooxidation rate of ferrous iron in the column were investigated. To validate the model, simulation was compared with the experimental data in the presence of Acidithiobacillus ferrooxidans in an aerated column where the superficial gas velocity was adjusted between 0 and 0.5 m/s. It was found that the initial ferrous concentration and the inlet air velocity had a pronounced effect on the ferrous biooxidation rate. The results indicated that the maximum biooxidation rate can be obtained at superficial air velocity of 0.1 m/s and initial ferrous concentration of 6.7 g/L.

Detailed modeling of hydrodynamics, mass transfer and chemical reactions in a bubble column using a discrete bubble model

A 3D discrete bubble model is adopted to investigate complex behavior involving hydrodynamics, mass transfer and chemical reactions in a gas–liquid bubble column reactor. In this model a continuum description is adopted for the liquid phase and additionally each individual bubble is tracked in a Lagrangian framework, while accounting for bubble–bubble and bubble–wall interactions via an encounter model. The mass transfer rate is calculated for each individual bubble using a surface renewal model accounting for the instantaneous and local properties of the liquid phase in its vicinity. The distributions in space of chemical species residing in the liquid phase are computed from the coupled species balances considering the mass transfer from bubbles and reactions between the species. The model has been applied to simulate chemisorption of CO 2 bubbles in NaOH solutions. Our results show that apart from hydrodynamics behavior, the model is able to predict the bubble size distribution as well as temporal and spatial variations of each chemical species involved.

Study of Bubble Formation Under Constant Flow Conditions

Bubble size is one of the key parameters in the design of two-phase gas-liquid bubble column reactors. Accurate knowledge of this parameter is essential for the prediction of gas holdup, heat and mass transfer coefficients. The previousfindings, particularly with respect to the infiuence of orifice size and physical properties of the liquid phase on bubble size, are often of contradictory nature. In this paper, extensive new experimental results are presented for regions where published data are insufficient. The suitability of artificial neural networks for identification of the process variables and modeling is evaluated. The Radial Basis Function (RBF) neural network architecture was used successfully to generate a nonlinear correlation for the prediction of bubble diameter. This correlation predicts the present data and the control data of other investigators with excellent accuracy.

Optimization of cyclical process for CO coupling regeneration reactions

In this paper the steady simulation and optimization were carried on for the synthesis of oxalate by carbon monoxide coupling-regeneration reaction in the gaseous phase with sequential modular approach modification software ASPEN, PRO/? and programs about two-dimension quasi-homogeneous reactor and gas—liquid bubble column reactor. At the same time, the influence of various operating parameters were investigated and the optimal operation range was determined. As well as the significant test about the simulation results and experimental results were carried out and the belief level is 95%.

Gas holdup in homogeneous and heterogeneous gas—liquid bubble column reactors

AbstractThe velocity‐holdup relationship is the most important design parameter for gas—liquid bubble column reactors, providing the basis for the prediction of heat and mass transfer coefficients and information on hydrodynamic conditions. A summary of the literature on gas holdup in bubble columns is supplemented by new experimental results which extend the data range. A criterion for the gas velocity leading to the transition between homogeneous and heterogeneous regimes for perforated plate gas distributors has been developed. Correlations for gas holdup in both regimes are developed and verified against both new and existing data.

ULTRASONIC CHARACTERIZATIONS OF GAS HOLDUP IN A BUBBLE COLUMN REACTOR

An indirect method of measuring gas holdup in gas-liquid bubble column reactors has been developed. This technique is based on the analysis of the ultrasonic wave transmitted through the two-phase flow. Gas holdup measurements have been made on water-nitrogen bubble system at ambient conditions. The data clearly show that the attenuation of the sound is a well-defined function of the gas holdup in the bubble column for homogeneous flow regime only (i.e., the superficial gas velocity is 4 cm/sec or less).

A unified approach to the scale-up of gas—solid fluidized bed and gas—liquid bubble column reactors

The aim of this work is to develop a unified approach to the scale-up of gas—solid (G—S) fluid bed and gas—liquid (G—L) bubble column reactors. The unified approach relies on analogies in the hydrodynamic behavior of G—S and G—L systems in both the homogeneous and heterogeneous flow regimes. The homogeneous G—S fluidization regime is to be identified with homogeneous G—L bubbly flow while the heterogeneous G—S fluidization regime is to be identified with the churnturbulent regime for G—L bubble columns. In the heterogeneous flow regime of operation, the classic two-phase theory developed for G—S fluidized beds can be applied with profit to describe also the hydrodynamics of G—L bubble columns provided the “dilute” phase is identified with the fast-rising large bubbles, and the “dense” phase is identified with the liquid phase containing entrained “small” bubbles. Quantitative analogies in the hydrodynamic behavior of G—S and G—L systems are demonstrated by the use of extensive experimental data obtained in five columns of three different diameters (2 × 0.1 m, 1 × 0.19 m and 2 × 0.38 m). The total expanded bed height in the experiments varied in the range 0.5–3.5 m. About 4,000 dynamic gas disengagement experiments were carried out with various systems to determine the total gas hold-up and the hold-ups of the “dilute” and “dense” phases. The gas phases used in the experiments were air, helium, argon and sulfur hexafluoride. Fluidized cracking catalyst (FCC) was used as the solid phase in fluid bed experiments. In bubble column operations the liquid phase used was water, paraffin oil or tetradecane. Sintered plates were used for gas distribution in all the columns. The gas hold-up in the “dense” phase, ɛ df was found to be practically independent of the scale of operation. The hold-up of the fast-rising “dilute” phase, ɛ b , on the other hand was found to be a significant function of the column diameter, D T , and of the total dispersion height, H. The “dilute” phase gas hold-up can be modeled for both G—S and G—L systems using a theory allowing for bubble growth in the region above distributor. The bubbles are assumed to grow in diameter up to a distance h *, at which the bubbles reach their equilibrium size. The equilibration height h * was found to increase with the superficial gas velocity through the dilute phase, U — U df ). For air—FCC, the value of h * varies in the range 0.4–1.2 m. For bubble columns the values of h * are significantly smaller and lie in the range 0–0.5 m. Increasing gas density increases the total gas voidage in both G—S and G—L systems but has no significant effect on the hold-up of the dilute phase. In G—L bubble columns, the liquid properties affect the total gas hold-up but have only a minor influence on the dilute phase hold-up. The unified model to describe the bubble hydrodynamics in G—S fluid beds and G—L bubble columns is a useful tool in scaling-up these two reactor types, because of the possibilities of cross-fertilization of design data.