Gas-liquid Stirred Tank Reactor Research Articles

Effect of novel mixed impeller on local bubble size and flow regime transition in pilot scale gas-liquid stirred tank reactor

Abstract The flow generated in a gas-liquid stirred tank reactor highly depends on the design of the impeller and sparger. To better understand the contact between the phases and the mass and heat transfer rates, especially when the mass transfer is the limiting step, it is crucial to investigate the hydrodynamics generated by the impellers and its impact on the bubble size and their distribution, and gas volume fraction. In this work, experimental and numerical studies are performed with a novel mixed impeller in a pilot scale (T = 0.486 m) gas-liquid stirred tank reactor. The Sauter mean diameter, mean bubble diameter and bubble size distribution is determined at the different radial and axial regions by using high-speed imaging technique. Further, Euler-Euler simulations are performed to find the detailed flow field of novel mixed impeller used in the current study. Finally, the gassed power to impeller swept volume ratio is determined from the CFD and correlated with the Sauter mean diameter measured in the experiment in the impeller discharge region. It is found that the novel mixed impeller used in current work shows the similar behavior as the Rushton impeller in the impeller discharge region and it also provide good axial mixing.

Circulating jet for enhancing the mass transfer in a gas–liquid stirred tank reactor

AbstractThere are some problems in gas–liquid stirred tank reactor (STR), such as low mass transfer efficiency, high exhaust gas oxygen concentration, and low product conversion rate, due to limitations of stirring speed and input power. This article summarizes the effect of adding circulating jet internals on bubble size distribution, overall gas holdup, gas–liquid volumetric mass transfer coefficient, and power per unit volume of the STR. When circulating jet is added, the average bubble Sauter mean diameter in the STR is reduced to 1.26 mm, and the overall gas holdup is increased to 8.23%, which is an increase of 3.62 times of the original STR. The gas–liquid volumetric mass transfer coefficient is increased to 0.05556 s−1, which is 4.84 times of the original STR. The power per unit volume is increased by only 1.4 times. These data provide references for the design and scale‐up of new jet STRs.

Hydrodynamics of gas phase under typical industrial gassing rates in a gas-liquid stirred tank using intrusive image-based method

The intrusive image-based method is used to probe local gas hydrodynamics, bubble size (BSD) and holdup distributions (BHD) in a gas-liquid stirred tank reactor (STR). A distinctive scheme combining automatic and manual processes is implemented to identify the object bubbles in the images. This method shows more accurate for judging the flow regime in gas-liquid STRs. Because of high intensity turbulence therein, the instantaneous local gas-liquid flow may present several very different patterns though the time-averaged bulk flow is homogeneous. More small bubbles are detected because of high resolution of the instrument, and consequently d32 obtained is smaller than those by other methods. The BSD and BHD at moderate gassing rates are reported and found closer to the physical reality. The measurements are of significance to understanding the fluid mechanics and provide benchmark data for the validation of the mathematical models in the gas-liquid STRs operating at industrial conditions.

Experimental study of power consumption, local characteristics distributions and homogenization energy in gas–liquid stirred tank reactors

In this paper, the power consumption, the vertical local void fraction and the local gas–liquid interfacial area are investigated in the aerated stirred tank reactors (STRs) equipped with a rigid-flexible impeller. Meanwhile, the regressive correlation based on power consumption and interfacial area is proposed. Then a novel homogenization energy (HE = RSDPtm) expression based on power consumption and local interfacial area is redefined and used to indicate the mixing efficiency. The optimal operating mode is selected based on the change of the HE value. This paper can provide research ideas for structural optimization of stirred reactors.

Electrical resistance tomography for gas holdup in a gas-liquid stirred tank reactor

Gas-Liquid flows in stirred tank reactors (STR) are used in many significant industrial operations such as hydrogenation, absorption, stripping, oxidation, hydrogenation, ozonation, chlorination, fermentation, etc. Gas-Liquid STRs are expected to perform several functions such as mixing, dispersing gas into liquid, mass and heat transfer and reactions. Gas hold up distribution and various flow regimes are the key parameters affecting performance of gas-liquid STRs. Various techniques such as visual analysis, photography, light attenuation, optical probe method are used to understand gas hold-up distribution within stirred tanks. Most of these techniques have some limitations with respect to measurement of gas hold up distribution. Electrical Resistance Tomography (ERT) is an upcoming technique for obtaining both qualitative and quantitative data on multiphase process units non-invasively and non-intrusively. In this work, an attempt was made to establish and validate the ERT technique for characterizing gas-liquid flows in a laboratory scale STR using the Rushton turbine impeller. ERT was used to study gas holdup and to identify flow regimes. The results were compared with the visual measurements as well as previously published correlations. The effect of gas flow rate, impeller speed on the mean gas holdup is discussed. The methodology and results presented in this work will be useful to effectively apply ERT for characterizing gas-liquid flows in stirred tanks.

Time-series analysis of optical probe measurements in gas–liquid stirred tanks

This work reports our findings on the time and frequency domain analysis of the optical probe measurements in a lab scale gas–liquid stirred tank reactor alternatively equipped with (i) a standard Rushton turbine and (ii) a half circular blades disk turbine. Optical measurements are collected from the tapered (conical) ends of optical fibers at five radial positions on the impeller discharge plane in the tank, r=0.4R, 0.5R, 0.6R, 0.7R, 0.8R, at a range of operating conditions. The collected data are processed and analyzed in the time and the frequency domains via MATLAB algorithms developed in house. The time domain analysis provided useful dispersion parameters, such as the local gas phase holdup and the bubble count, and the frequency domain analysis revealed information regarding the chaotic nature of bubble occupancy as a function of position and operating conditions. For both systems, the distinct pattern of the bubbles generated by the blades became chaotic beyond certain distances from the impeller at all operating conditions. In the vicinity of the impeller, however, different patterns are observed for the two turbines. These results suggest the optical probe׳s potential usage as a tool for impeller design and for in-situ identification/detection of the flow regime transitions to the fully recirculated regime.

Determination of flow regime and gas holdup in gas–liquid stirred tanks

This work provides an in-situ method for determining the flow regime in a lab scale gas–liquid stirred tank reactor based on optical probe measurements. Tapered (conical) end optical fibers, which can distinguish which phase their tips are surrounded by, were employed over the whole range of practical operating conditions achievable in our Chemical Reaction Engineering Laboratory (CREL). After checking for sources of error associated with the rise and fall times of the measured signals, gas holdup and bubble count profiles were obtained by processing the time-series data with appropriate in-house developed algorithms. The data were presented in terms of the two dimensionless numbers, the Flow Number (Fl) and the Froude Number (Fr). All experiments were executed with an air–water system but the technique can be employed with all liquids and gases. The results suggest that the optical probe, when strategically positioned, can successfully and readily determine which state of dispersion the reactor is in. This reveals the technique׳s potential usefulness as an important research and control tool.

Multidimensional population balance model for the simulation of turbulent gas–liquid systems in stirred tank reactors

The prediction of turbulent gas–liquid systems, and in general of multiphase flows, has been historically performed with the implicit assumption of separately considering fluid dynamics issues from the evolution of the dispersed phase (i.e., the gas bubbles). These two aspects can be simultaneously accounted for by means of a multidimensional population balance model, able to describe the interactions between the continuous liquid phase and the gas bubbles, as well as the interactions among different gas bubbles (e.g., coalescence and break-up), both in terms of momentum and mass coupling. The model has to quantify the effects of these interactions on the population of dispersed bubbles and has to estimate the distributions of bubble velocity, size and composition, as well as the state of the continuous phase. A novel approach based on the direct quadrature method of moments is here formulated, tested on several simplified cases and adopted to describe the evolution of the gas bubble in a realistic gas–liquid stirred tank reactor. Results are eventually validated through comparison with experimental data from the literature.

Evaluation of Drag Force Effect on Hold-Up in a Gas-Liquid Stirred Tank Reactor

A comprehensive computational method based on the Eulerian–Eulerian approach is presented for gas–liquid flows in a mechanically stirred vessel. Separate submodels were developed to investigate the influence of the drag coefficient on bubbles due to the interaction between bubbles and turbulence. A new model is used for the definition of drag coefficient and a detailed simulation method is used to estimate the interphase drag force. A standard k−e turbulent model is used and the impeller rotation is modelled by the multiple reference frame approach. Simulation results are compared with previous drag models and the experimental measurements. The results indicate that the new model is able to predict the influence of turbulent intensity on drag coefficient and the distribution of gas holdup. The computational model and the proposed correlation for drag in turbulent flow will be useful for simulation gas holdup distribution and flow regimes in stirred vessels.

06/01452 Oxidation mechanism of carbon black by NO2: Effect of water vapour: Jeguirim, M. et al. Fuel, 2005, 84, (14–15), 1949–1956

The aim of this paper is to model the simultaneous absorption mass transfer of NO2 and SO2 with chemical reactions. The absorption has been studied at low temperature (25 °C) in a gas–liquid stirred tank reactor, open for the gas flow and closed for the liquid phase. The kinetics of the reactions between sulphite and nitrite ions, obtained respectively from the absorption of SO2 and NO2, depend mainly on the pH of the solution. With a weakly acid solution (4 < pH < 5), at low temperature, after the simultaneous absorption and interaction between the ions generated, it is possible to transform NO2 and SO2 into two products: N2O and hydroxylamine disulfonic acid (HADS). In the first time, the mass transfer during the absorption of the two gases, without chemical reaction was characterized separately. Then the mass transfer during the simultaneous absorption with chemical reaction was quantified. The experimental conditions have been fixed at pH 4 and 5 for different gas flow rates. The mass transfer for NO2 was clearly improved by the interaction between nitrite and sulphite ions. The modelling of mass transfer in the liquid phase of the reactor allowed to specify the hydration kinetics of nitrogen dioxide.

Experimental study and modelling of mass transfer during simultaneous absorption of SO 2 and NO 2 with chemical reaction

The aim of this paper is to model the simultaneous absorption mass transfer of NO 2 and SO 2 with chemical reactions. The absorption has been studied at low temperature (25 °C) in a gas–liquid stirred tank reactor, open for the gas flow and closed for the liquid phase. The kinetics of the reactions between sulphite and nitrite ions, obtained respectively from the absorption of SO 2 and NO 2, depend mainly on the pH of the solution. With a weakly acid solution (4 < pH < 5), at low temperature, after the simultaneous absorption and interaction between the ions generated, it is possible to transform NO 2 and SO 2 into two products: N 2O and hydroxylamine disulfonic acid (HADS). In the first time, the mass transfer during the absorption of the two gases, without chemical reaction was characterized separately. Then the mass transfer during the simultaneous absorption with chemical reaction was quantified. The experimental conditions have been fixed at pH 4 and 5 for different gas flow rates. The mass transfer for NO 2 was clearly improved by the interaction between nitrite and sulphite ions. The modelling of mass transfer in the liquid phase of the reactor allowed to specify the hydration kinetics of nitrogen dioxide.

Prediction of Uncatalysed Calcium Bisulphite Oxidation Under Operating Conditions Relevant to Wet Flue Gas Desulphurization Plants

Wet limestone scrubbing is the most common flue gas desulphurization (FGD) process. In this process, oxidation of calcium bisulphite plays a major role both by determining the dewatering properties of the obtained sludge and the production of gypsum of high quality. In order to contribute to the simulation of the wet limestone FGD process, in the present paper a comprehensive model for isothermal gas–liquid stirred tank reactors has been used. Based on the assumption that the reactions are zeroth order with respect to gaseous reactants, the model requires appropriate physico-chemical parameters which are taken from the literature. The model was used to quantitatively interpret some literature experimental data concerning uncatalysed calcium bisulphite oxidation performed in a continuous stirred tank reactor. In particular, the model is capable of confidently predicting the transition from chemically to mass transfer controlled regime occurring when changing the operating conditions.

Time-series analysis approach for the identification of flooding/loading transition in gas–liquid stirred tank reactors

This work provides a simple method for the objective, quantitative and non-intrusive detection of flooding/loading transition in gas/liquid systems in agitated vessels starting from the time-series analysis of conductance fluctuations along a generic cross section of the vessel. The conductance probe is designed in order to introduce minimal perturbations of the flow field within the vessel. A thorough statistical characterization of the time series collected by the probe provides deeper insight into the fluid dynamics of gas–liquid systems in stirred vessels as well as a simple, objective way to detect flooding/loading transitions.

OPERATION POLICIES FOR A GAS—LIQUID STIRRED TANK REACTOR

The operation of a gas-liquid stirred tank reactor is simulated, including batch operation, steady and unsteady state, continuous operation and cyclic variable volume operation. A pseudo first order reaction is considered. Analytical expressions for the enhancement factor of gas absorption rate are derived for each case, and productivity obtainable under different operation policies is evaluated. Results obtained are shown graphically for specific cases for the purpose of demonstrating the influence of some of the parameters on the system The equations as presented can be easily extended to other kinetics, and serve as a useful guide for optimum operational policies in a gas-liquid stirred tank reactor.