Gas-liquid Reaction System Research Articles

Mass transfer mechanism and relationship of gas–liquid annular flow in a microfluidic cross-junction device

Mass transfer performance of gas–liquid two-phase flow at microscale is the basis of application of microreactor in gas–liquid reaction systems. At present, few researches on the mass transfer property of annular flow have been reported. Therefore, the mass transfer mechanism and relationship of gas–liquid annular flow in a microfluidic cross-junction device are studied in the present study. We find that the main factors, i.e., flow pattern, liquid film thickness, liquid hydraulic retention time, phase interface fluctuation, and gas flow vorticity, which influence the flow mass transfer property, are directly affected both by gas and liquid flow velocities. But the influences of gas and liquid velocities on different mass transfer influencing factors are different. Thereout, the fitting relationships between gas and liquid flow velocities and mass transfer influencing factors are established. By comparing the results from calculations using fitting equations and simulations, it shows that the fitting equations have relatively high degrees of accuracy. Finally, the Pareto front, namely the Pareto optimal solution set, of gas and liquid velocity conditions for the best flow mass transfer property is obtained using the method of multi-objective particle swarm optimization. It is proved that the mass transfer property of the gas–liquid two-phase flow can be obviously enhanced under the guidance of the obtained Pareto optimal solution set through experimental verification.

Honeycomb reactor: a promising device for streamlining aerobic oxidation under continuous-flow conditions.

We report on the high potential of a honeycomb reactor for the use in aerobic oxidation under continuous-flow conditions. The honeycomb reactor is made of porous material with narrow channels separated by porous walls allowing for high density accumulation in the reactor. This structure raised the mixing efficiency of a gas-liquid reaction system, and it effectively accelerated the aerobic oxidation of benzyl alcohols to benzaldehydes under continuous-flow conditions. This reactor is a promising device for streamlining aerobic oxidation with high process safety because it is a closed system.

Micro interfacial Effect: The stability of catalytic activity under gas–liquid reaction pressure changes

The conversion of CO2 usually requires high temperature, high pressure and long reaction time, due to its chemical stability and gas–liquid mass transfer resistance. Based on the micro interfacial effect, the nano-micro interface reaction units (NMRUs) were designed and prepared for the efficient conversion of CO2 under mild conditions. NMRUs are made up of the micron-sized droplet inside and the nano-porous shell, which can significantly increase the gas–liquid contact area and thus intensify mass transfer from gas to liquid. Compared with the gas–liquid reaction system (G-L) in the traditional autoclave, the prominent intensification on CO2 cycloaddition by NMRUs is achieved at 25℃ and 0.1 MPa, with the equilibrium time shortened by more than a half. Noticeably, the influence of pressure on the gas–liquid reaction in NMRUs was nearly negligible, which was explained by the interface mass transfer mathematical model. The strengthening mechanism of NMRUs was further explored with the kinetic simulation.

Numerical modeling of reactive bubbly flows based on Euler-Lagrange approach

In this paper, the Euler-Lagrange approach was employed to simulate the gas–liquid reaction system in a rectangular bubble column by combining hydrodynamics, interfacial mass transfer and reaction kinetics. According to the satisfactory agreement with the experiment results, the detailed characteristics of the hydrodynamics were accurately reflected. For the chemisorption process of CO2 in aqueous NaOH, the results were more similar to the experimental values than those reported in the literature. Moreover, further improvements were successfully implemented: A visible inflection point, owing to which the shape of the curve was highly consistent with the experimental data, could be easily observed when taking into account the auto-dissociation of water. A relatively exact slope for the distribution curve of the mean Sauter diameter could be attained in the flow region below z/H = 0.3. In addition, an in-depth assessment was performed by comparing a series of typical mass transfer and enhancement factor models.

Microfluidics-based determination of diffusion coefficient for gas-liquid reaction system with hydrogen peroxide

In this work, we present a fluorescence-based microfluidic approach to study the transport characteristics in gas–liquid reaction system with hydrogen peroxide (H2O2). A system of nitrogen dioxide (NO2) and H2O2 was served as model to determinate diffusion coefficient of NO2. The diffusion coefficient was quantified through fading of fluorescent signals by increase of pH during the diffusion process. The diffusion coefficients were served as parameters to fit for a diffusion transport model as a function of time and determined to be 8.55 × 10−9 m2/s on average. The thickness of liquid film according to two film theory was obtained as a reasonable value of 180 μm given by the optimal fitting performance. The experimental operation for measurement of diffusion coefficient took only about 3 min. This paper provides an in situ, time-efficient, sample-saving and non-invasive approach to investigate microscale transport characteristics of gas–liquid systems.

Dynamics of Mobile Concentration Fronts in Gas–Liquid Reaction Systems: Analysis and Numerical Experiment

A mathematical model has been developed for the process of chemosorption with the moving front of an instantaneous irreversible reaction at small Reynolds numbers in a gas–liquid system before and after the reaction front formation moment. An analytical solution of this model has been found for the time period to the formation of a mobile chemosorption front with consideration for the effect of a reaction product on the dynamics of chemosorption. In contrast to the earlier known results, the model takes into account the effect of the diffusion transfer of a reaction product on the kinetic characteristics of chemosorption and different stages of the formation and motion of the concentration front. Some results of the processing and analysis of numerical experiment data are presented. The characteristic depths and times of the penetration of a captured component are plotted as functions of all the process-control parameters. Sherwood numbers and absorption acceleration and mass transfer coefficients are calculated for the captured component in the liquid phase.

Continuous synthesis of tetraethyl thiuram disulfide with CO2 as acid agent in a gas-liquid microdispersion system

Herein, a continuous synthetic method of tetraethyl thiuram disulfide (TETD) with CO2 as a weak acid reagent in a gas-liquid microdispersion reaction system has been described. A microdispersion reaction system comprising two microreactors for CO2 absorption and diethyldithiocarbamate acid subsequent oxidation reaction was developed. In the first step, the main products of absorption and oxidation reactions were identified. Next, the yield of TETD and selectivity of sodium diethyldithiocarbamate under different flow rates of raw materials, amount of CO2, aging effect, and pressure in the microreaction system were determined. The yield reached 89.03% based on 60% H2O2 dosage and the selectivity was 92.86% in less than 3 min of residence time, which was more than 30 min in the batch process. The space time yield in the microreaction system was ~20 times of that achieved in the batch process. The study provides a new successful reference for replacing batch synthesis by a continuous process using a microreaction system.

Gas-liquid reaction and mass transfer in microstructured coiled flow inverter

Microstructured coiled flow inverter (MCFI) as a helically coiled tubular device with 90° alternating bends provides enhanced radial mixing due to secondary flow (Dean vortices) in different planes. Liquid-liquid mass transfer characterization in MCFI revealed higher mass transfer rates compared to other capillary setups. However, the influence of Dean vortices and 90° bends on gas-liquid mass transfer has not been investigated and described yet. Different reactor setups, i.e. MCFI, straight capillary, helical coil, and bend reactor were fabricated from FEP tubes (ID=1mm). Gas-liquid mass transfer performance was investigated for a gas-liquid reaction system, i.e. cobalt (II) catalyzed air oxidation of sodium sulfite. Two-phase slug flow hydrodynamics and pressure drop were experimentally characterized, while the influence of operating and geometrical reactor parameters on conversion was investigated for reactor setups. MCFI offers up to 14% higher conversion in comparison to other capillary setups. Mixing within the liquid phase is enhanced by the formation of Dean vortices and additional direction change via 90° bends, which contribute to the internal diffusion in the liquid phase.

Experimental investigation of gas-liquid distribution in periodic open cellular structures as potential catalyst supports

Radial and axial liquid distribution was measured in a packed column of 500 mm length and 100 mm inner diameter under various gas- and liquid superficial velocities to achieve trickle flow regime. Liquid maldistribution was determined using a liquid collector at the outlet consisting of 21 collection zones of equal area. In this work, as packings inside the column, periodic open cellular structures (POCS) were manufactured and applied. Such POCS can be used as structured catalyst support and/or liquid distributor.The hydrodynamic behavior of packings with Kelvin cell, Diamond cell and a hybrid combination of both unit cells was studied to identify unit cells with high potential for achieving homogeneous liquid distribution over the entire reactor cross-section. The liquid superficial velocity had a strong influence on the liquid distribution, while an increasing gas flow rate did not change the flow patterns. Kelvin cell packings tend to distribute the liquid towards the center whereas Diamond cell packings predominantly spread the fluid towards the rim. As a consequence of these observations, in this work a novel POCS structure is proposed: The combination of Kelvin and Diamond unit cells, called hybrid DiaKel unit cell. With POCS consisting of this unit cell type, a significantly improved liquid distribution can be achieved. Thus, this structure features high potential as liquid distributor and/or catalyst support in gas-liquid reaction systems.

Analytical Sciences: Abstracts AS01-AS54

Identification of kinetic models and estimation of reaction and mass-transfer parameters can be performed using the extent-based identification method, whereby each chemical/physical process is handled separately [1-3]. This method is used here to analyze gas-liquid systems under unsteady-state mass transfer. Such a situation is common in the case of diffusion-controlled reactions and is modeled by the film theory, that is, transferring species accumulate in a liquid film. In both the gas and liquid bulks, mass-balance relations describe the species dynamics as ordinary differential equations (ODE) and serve as boundary conditions for the film. On the other hand, the dynamic accumulation in the film is described by Fick’s second law. The resulting partial differential equation (PDE) system is solved by discretization and rearrangement in ODEs. The estimation of diffusion coefficients follows a two-steps procedure. First, the extents of mass transfer are computed from measurements in the two bulks. Diffusion coefficients are then estimated individually by fitting each extent of mass transfer to the extent obtained by solving the corresponding PDE. Comparison of the estimated diffusion coefficients with their literature values serves to validate the models identified in the two bulks. The estimation of both kinetic parameters and diffusion coefficients is investigated for gas-liquid reaction systems with unsteady-state diffusion. The approach is illustrated with simulated examples. [1] Bhatt et al, Ind. & Eng. Chem. Res. 50, 12960-12974, 2011 [2] Srinivasan et al, Chem. Eng. J. 208, 785-793, 2012 [3] Billeter et al, Anal. Chim. Acta 767, 21-34, 2013

Extent-based kinetic identification using spectroscopic measurements and multivariate calibration

Extent-based kinetic identification is a kinetic modeling technique that uses concentration measurements to compute extents and identify reaction kinetics by the integral method of parameter estimation. This article considers the case where spectroscopic data are used together with a calibration model to predict concentrations. The calibration set is assumed to be constructed from reacting data that include pairs of concentration and spectral data. Alternatively, one can use the concentration- and spectral contributions of the reactions and mass transfers, which are obtained by pretreatment in reaction- and mass-transfer-variant form. The extent-based kinetic identification using concentrations predicted from spectroscopic data is illustrated through the simulation of both a homogeneous and a gas–liquid reaction system.

Extent-based incremental identification of reaction systems using concentration and calorimetric measurements

Extent-based incremental identification uses the concept of extents and the integral method of parameter estimation to identify reaction kinetics from concentration measurements. The approach is rather general and can be applied to open both homogeneous and gas–liquid reaction systems. This study proposes to incorporate calorimetric measurements into the extent-based identification approach for two main purposes: (i) to be able to compute the extents in certain cases when only a subset of the concentrations is measured and (ii) to estimate the enthalpies when all concentrations are measured. The two approaches are illustrated via the simulation of a homogeneous and a gas–liquid reaction system, respectively.

Fundamental Researches on the High-speed and High-efficiency Steelmaking Reaction

AbstractTraditionally, steelmaking reactions have been analyzed by thermodynamics. Recently, software packages that can be used to calculate the equilibrium conditions have improved greatly. In some cases, information obtained in this software is useful for analyzing the steelmaking reaction. On the other hand, in industrial operation, steelmaking reactions, i.e., decarburization, dephosphorization, desulfurization or nitrogen removal, do not reach the equilibrium condition. Therefore, the kinetic model is very important for gaining a theoretical understanding of the steelmaking reaction. In this paper, the following recent research activities were shown; 1) mass transfer of impurities between solid oxide and liquid slag, 2) simulation model of hot metal dephosphorization by multiphase slag, 3) evaluation of reaction rate at bath surface in gas-liquid reaction system and 4) condition for forming of metal emulsion by bottom bubbling.

Determination of Kinetics in Gas-Liquid Reaction Systems. An Overview

Determination of Kinetics in Gas-Liquid Reaction Systems. An Overview The aim of this paper is to present a brief review of the determination methods of reaction kinetics in gas-liquid systems with a special emphasis on CO2 absorption in aqueous alkanolamine solutions. Both homogenous and heterogeneous experimental techniques are described with the corresponding theoretical background needed for the interpretation of the results. The case of CO2 reaction in aqueous solutions of methyldiethanolamine is discussed as an illustrative example. It was demonstrated that various measurement techniques and methods of analyzing the experimental data can result in different expressions for the kinetic rate constants.

Incremental Identification of Reaction and Mass–Transfer Kinetics Using the Concept of Extents

This paper proposes a variation of the incremental approach to identify reaction and mass-transfer kinetics from concentration measurements for both homogeneous and gas–liquid reaction systems. Thi...

CFD simulation of the two-phase flow for a falling film microreactor

Falling film microreactors, which provide very high specific interfacial area, have become a promising solution to the fast and strongly exothermic/endothermic gas–liquid reaction systems. A computational fluid dynamic simulation of the two-phase flow for a falling film microreactor is presented using the volume of fluid (VOF) model. The hydrodynamic characteristics, from both 2-D and 3-D simulations, including liquid film thickness, velocity, pressure and shear stress profiles, are analyzed. 2-D simulation is adopted for the study of the relationship of liquid flow rate and film thickness, as well as the effects of gas flow rate, surface tension, liquid viscosity and pressure difference on the liquid flow rate. 3-D simulation is necessary to provide the comprehensive flow profiles. Although the system is in the laminar flow regime, the liquid film features a wavy structure and the velocity profiles are complex.

Extents of Reaction, Mass Transfer and Flow for Gas−Liquid Reaction Systems

For gas-liquid reaction systems with inlet and outlet streams, this paper proposes a linear transformation to decompose the numbers of moles vector into five distinct parts, namely, the extents of reaction, the extents of mass transfer, the extents of inlet flow, the extents of outlet flow, and invariants. Furthermore, several implications of being able to compute the extents of reaction, mass transfer, and inlet flow are discussed. The concept is illustrated via the simulation of various reactor configurations for the chlorination of butanoic acid.

Agitation Effects in a Gas-Liquid-Liquid Reactor System: Methyl Ethyl Ketazine Production

Our understanding of gas-liquid–liquid reaction systems is not well developed today despite their widespread use in the process industries. Research work is demanded or inspired in those fields where basic information is either lacking or poorly organized. The field of mixing and agitation in gas-liquid-liquid reaction system is a typical example. Methyl ethyl ketazine formation is a typical case of homogeneous catalyzed gas-liquid-liquid reaction. The parametric study of ketazine formation has been carried out in a semi batch reactor. Agitation effect on methyl ethyl ketazine formation has been studied. Two types of agitators are used - flat paddle stirrer and vaned disk stirrer. The studies show that agitation plays an important role on the reaction. Yield of ketazine improved with degree of agitation, categorizing the reaction as moderately fast. Vaned disk stirrer is found to be more effective compared to flat paddle stirrer. Rate of reaction increases up to 600 rpm indicating diffusion control, beyond 600 rpm there is no appreciable change in the yield, thus favoring kinetics controlled mechanism. This 600 rpm can be thus regarded as "the desired level of agitation."

Experimental studies and simulation of the dynamic behaviour of bubble column reactors

To meet the growing demand for dynamic simulation in chemical engineering a one-dimensional mathematical model for the dynamic behaviour of bubble column reactors is developed and implemented as a computer programmed. To validate some basic model assumptions and to show the abilities of the programme, experimental observations are compared with simulation results. The absorption of carbon dioxide into aqueous sodium hydroxide solutions is choosen as an example of a fast gas-liquid reaction system. For this model system a sufficiently good quantitative agreement is obtained.

An axial dispersion model for gas–liquid reactors based on the penetration theory

An axial dispersion reactor model for gas–liquid reaction systems is proposed in this paper based on the penetration theory. The mass transfer mechanism accompanied by a chemical irreversible first-order reaction is mathematically treated in a new way in order to use its results to develop the model conveniently. Analytical solutions can be obtained for the equation system involving linear differential equations by using of the eigenvalues of the equation system. In addition, an iteration procedure is given to solve the nonlinear differential equation system numerically. The influences of the important model parameters on the concentration profile, the mass transfer and the reactant conversion are also studied.