Hydrodynamics and ozone mass transfer in a tall bubble column

In the paper, major hydrodynamic parameters such as gas hold-up, phase velocities and axial dispersion as well as the ozone mass transfer coefficients in the liquid phase have been investigated in a tall bubble column for co-current, counter-current and semi-batch modes of operation. The major emphasis has been placed on evaluation of the dynamic characteristics of the combined system of experimental column and measuring sensors, which was applied in the subsequent determination of the axial dispersion and ozone mass transfer coefficients in the liquid phase. The ozone mass transfer coefficients have been estimated using two treatment methods of the recorded changes of ozone concentration in the liquid and gas phases with time during ozone absorption or stripping to an inert gas.     

Simulation of Marangoni convection effects on the hydrodynamics of liquid–liquid extraction drops

Concentration induced Marangoni convection (MC) effect has many applications in the chemical engineering processes. In this study, the MC effects have been investigated in the toluene/acetone/water (T/A/W) liquid–liquid extraction system, focusing on the mass transfer and hydrodynamics. For the first time, the VOF-CSS model has been used, and the mass transfer equation has been coupled with the hydrodynamic equations of the VOF model, using the surface tension force model of the CSS. A single mass transfer equation has been solved for the two phases, modifying the unsteady, convective, and diffusive terms based on the equilibrium distribution coefficient. The MC effect has been implemented in the CSS surface tension model with a concentration-dependent surface tension coefficient. Simulations have been carried out in 1, 1.5, and 2?mm drops in the wide range of concentration (0.9–30?g/L). Good agreements have been achieved for the concentration and reduced velocity values compared with the existing experimental data. Simulation results showed that the reduced velocity values remain almost constant for every drop diameter, in the initial concentration range from 3.7 to 30?g/L. The reacceleration time for the three drop sizes with the different initial concentrations has been obtained from the simulation results and compared. The MC effects on the axial velocity profiles and drag coefficients were also reported providing comprehensive information about the MC effects on the hydrodynamics.     

Mass Transfer in a Flat Gas/Liquid Interface using non‐Newtonian Media

Gas/liquid mass transfer has been investigated using a stirred vessel gas/liquid contactor using non‐Newtonian media and carbon dioxide as absorbent and gas phase, respectively. The volumetric mass transfer coefficients at different operational variables have been determined. Non‐Newtonian media (liquid phase) were prepared as aqueous solutions of sodium carboxymethyl cellulose salt. The influence of the rheological properties, polymer concentration, stirring rate, and gas flow rate on mass transfer was studied for these liquid phases. Kinematic viscosity and density experimental data were used to calculate the average molecular weight corresponding to the polymer employed. The Ostwald model has been used to fit the rheological behavior of aqueous solutions of the polymer employed as absorbent phase. Reasonably good agreement was found between the predictions of the proposed models and the experimental data of mass transfer coefficients.     

Modeling of annular reactors with surface reaction using computational fluid dynamics (CFD)

A CFD-based model for predicting the performance of annular reactors with surface reaction was developed. The capability of several hydrodynamic models to predict successfully the kinetic behavior of the reactor under diffusion limiting conditions was assessed against experimental data. The evaluation included five models: laminar, standard k–ε, realizable k–ε, Reynolds stress (RSM), and Abe–Kondoh–Nagano (AKN). The catalytic decomposition of hydrogen peroxide over a Mn/Al oxide catalyst coated on the reactor surface was used as a model reaction. The reactor was tested within a range of flow rates corresponding to 530<Re<11,000 and intrinsic reaction rate constants of 5×10^?5 to 1 m/s. The results demonstrated that the performance of the hydrodynamic models is associated with their capability to predict external mass transfer and ultimately, the level of mass transfer limitation present in the reacting system. For laminar flow conditions, the laminar model is capable of predicting the experimental behavior of the system. For transient and turbulent flow regimes, all the analyzed turbulence models provided good predictions of the system when the process was controlled by surface reaction. When the system presented some degree of mass transfer limitation, AKN and RSM exhibited better performance.     

Hydrodynamic analogy approach for modelling of reactive stripping with structured catalyst supports

Modelling of gas/vapour–liquid separation processes usually requires experimentally determined parameters, e.g., mass transfer coefficients. This results in expensive experimental work, especially for new types of column internals. A novel modelling approach based on hydrodynamic analogies (HA) has recently been developed for distillation units equipped with structured corrugated sheet packings. The HA model takes the packing geometry directly into account whereas the experimental determination of mass transfer coefficients is not required.In this work, the HA approach is extended to cover heterogeneously catalysed reactive stripping processes. Experimental investigations are performed with a test system, esterification of hexanoic acid and 1-octanol, using different types of catalytically coated supports as column internals (one corrugated sheet packing and three film-flow monoliths with different channel geometries). Simulation results obtained with the extended HA model are in a good agreement with experimental values.     

STUDY OF DOWNFLOW LIQUID JET LOOP REACTOR

1 INTRODUCTIONLiquid jet loop reactor(JLR)may be upflow(U-JLR)or downflow reactors(D-JLR)in design.The major differences between the two are the location of the nozzle andthe direction of the fluid flow.A large number of investigations on U-JLR havebeen published,but D-JLR with nozzles positioned on the top portion of the reac-tor was not much studied until recently.Up to now,only a few experimentalstudies on the hydrodynamics and mass transfer of D-JLR have been carried out[1-4].     

基于热不平衡考虑的溶液除湿/再生传热传质系数(Ⅰ)模型与Le-hD测量法

溶液除湿装置是溶液除湿空调系统中一个重要单元,其内部传热传质规律对系统性能起到决定性影响.在溶液绝热除湿模型基础上,提出考虑热不平衡的除湿模型用以计算溶液除湿过程的传热传质系数.模拟计算发现考虑热不平衡模型计算的溶液和空气出口参数与实验值能更好吻合,从而说明该模型能更准确计算除湿过程传热传质系数.当热容率比C*≥1.0...     

EDDY NUMBER DISTRIBUTION IN ISOTROPIC TURBULENCE AND ITS APPLICATION FOR ESTIMATING MASS TRANSFER COEFFICIENTS

The eddy number distribution for isotropic turbulent flows has been derived by manipulating well-established formulae for describing turbulence. The distribution has been used to derive an expression relating the fractional rate of surface renewal, and hence mass transfer coefficients across gas-liquid interfaces, to key process variables such as the local energy dissipation rate and the Kolmogorov scale. The expression has been compared with the previously published roll-cell and eddy-cell models, which were applied to correlating local mass transfer coefficients in a pipe and an open absorption channel. Excellent agreement has been found. The expression has also been applied to correlating the gas-side mass transfer coefficients for liquid evaporation in wetted wall pipe flows, and the mass transfer coefficient for solid dissolution adjacent to a solid-liquid interface in a stirred tank. The theoretical correlations have been validated by published experimental data.     

Investigations on hydrodynamics and mass transfer in gas–liquid stirred reactor using computational fluid dynamics

In this work, the hydrodynamics and mass transfer in a gas–liquid dual turbine stirred tank reactor are investigated using multiphase computational fluid dynamics coupled with population balance method (CFD–PBM). A steady state method of multiple frame of reference (MFR) approach is used to model the impeller and tank regions. The population balance for bubbles is considered using both homogeneous and inhomogeneous polydispersed flow (MUSIG) equations to account for bubble size distribution due to breakup and coalescence of bubbles. The gas–liquid mass transfer is implemented simultaneously along with the hydrodynamic simulation and the mass transfer coefficient is obtained theoretically using the equation based on the various approaches like penetration theory, slip velocity, eddy cell model and rigid based model. The CFD model predictions of local hydrodynamic parameters such as gas holdup, Sauter mean bubble diameter and interfacial area as well as averaged quantities of hydrodynamic and mass transfer parameters for different mass transfer theoretical models are compared with the reported experimental data of [Alves et al., 2002a] and [Alves et al., 2002b] . The predicted hydrodynamic and mass transfer parameters are in reasonable agreement with the experimental data.     

Design of stirred vessels with gas entrained from free liquid surface

In this review, the literature on surface aerators is critically analysed. In a number of studies, the critical speed for the onset of surface aeration has been correlated with the geometry of the system and the operating conditions. The studies on the rate of surface aeration, gas hold-up, mass transfer coefficients are inadequate for the design and scale-up. A comparison of the available designs, in terms of gas hold-up, and mass transfer coefficient is presented. A mathematical model has been proposed for surface aeration. Guidelines have been given for the selection of impeller configuration and overall design of surface aerators. Suggestions have been made for future studies in this area.     

NEURAL NETWORK–BASED HEAT AND MASS TRANSFER COEFFICIENTS FOR THE HYBRID MODELING OF FLUIDIZED REACTORS

The complex flow patterns induced in fluidized bed catalytic reactors and the competing parameters affecting the mass and heat transfer characteristics make the design of such reactors a challenging task to accomplish. The models of such processes rely heavily on predictive empirical correlations for the mass and heat transfer coefficients. Unfortunately, published empirical-based correlations have the common shortcoming of low prediction efficiency compared with experimental data. In this work, an artificial neural network approach is used to capture the reactor characteristics in terms of heat and mass transfer based on published experimental data. The developed ANN-based heat and mass transfer coefficients relations were used in a conventional FCR model and simulated under industrial operating conditions. The hybrid model predictions of the melt-flow index and the emulsion temperature were compared to industrial measurements as well as published models. The predictive quality of the hybrid model was superior to other models. This modeling approach can be used as an alternative to conventional modeling methods.     

MASS TRANSFER RATES IN LIQUID/LIQUID EXTRACTION WITH PARTIALLY MISCIBLE SOLVENTS

Abstract

Models for simulating the performance of extraction columns have been extended to include partially miscible solvents. Simultaneously, a comprehensive experimental study was carried out in which the concentration profiles of all components and all relevant hydrodynamic parameters in a pilot plant were measured, using liquid systems recommended by the European Federation of Chemical Engineers. True mass transfer coefficients were determined from the experimental data using a computer-aided curve-fitting technique to allow for the influence of backmixing. The coefficients obtained in this way were similar to those calculated for single drops.     

Experimental mass transfer coefficients in a pilot plant multistage column extractor

The volumetric overall mass transfer coefficients in a multistage column have been measured using axial dispersion model for toluene–acetone–water system. The effect of operating parameters on the volumetric overall mass transfer coefficients has been investigated for both mass transfer directions. The results show that the mass transfer performance is strongly dependent on rotor speed and mass transfer direction, although only slightly dependent on phase flow rates. In addition, empirical correlations to predict the overall mass transfer coefficients have been developed. The proposed correlations based on dimensionless numbers can be considered as a useful tool for the possible scale up of the multistage column extractor.     

Mass transfer in structured corrugated packing

The performance of structured corrugated packing has been simulated by establishing mechanistic models for liquid distribution, liquid flow on the packing surface and mass transfer. The models were used to investigate the effect of packing height, liquid load, initial maldistribution as well as differing initial distribution and solid-liquid contact angle on the packed column performance. Wetted surface area is the primary value of interest and the simulated results compare very well with those predicted by Onda (1968). The results clearly demonstrate that the wetted surface area is a strong function of the solid-liquid contact angle. Other predicted values such as mass transfer coefficients and overall height of a transfer unit show reasonable agreement with published data.     

Thermophysical Characteristics of Potatoes,Vegetables and Fruits

ABSTRACT

The main objective in this work is to study and deduce a governing equation for net mass transfer in moist air and turbulent flow. Development of simple and reliable steady state models for turbulent moist air-drying has been considered to be quite well covered in literature. However, the lack of necessary background information concerning classical drying models is now being rectified through research carried out with new approaches, which are initiated by advancement in laboratory equipment.

The known and trusted models are combined with coupled momentum, heat and mass transfer equations creating a reliable governing mass transfer equation for use in turbulent moist air drying processes, i.e. the advanced drying model (ADM). The ADM is a relatively user friendly and robust model, and it is well-suited for identifying transfer coefficients from boundary layer measurements, for example in modem high intensity paper drying machines.

The advanced drying model is analysed and verified with the specially designed experimental apparatus described in this article. The deduced mass transfer equation is then presented and experimentally verified to clarify why the use of Stefan's diffusion equation should be avoided when calculating high drying intensities in turbulent flow.

Finally, when applied to a wide drying range, the classical drying models require parameters which have been experimentally verified. Therefore, a comprehensive knowledge of governing mass transfer mechanisms will also reduce the large number of necessary drying experiments. The advanced drying model, which includes variable physical properties and transport coefficients, allows the simulation of many geometrical shapes and drying configurations and therefore provides a tool for optimising drying processes in a new manner.     

压缩空气溶液除湿中不同除湿剂除湿性能比较

在压缩空气溶液除湿实验平台上,分别以LiBr和LiCl水溶液作为除湿剂,实验研究了两种溶液在压缩空气溶液除湿系统中的除湿性能。以溶液表面水蒸气分压力作为比较基准,压缩空气出口含湿量和除湿量作为除湿性能的评价指标,对二者的除湿能力进行比较分析。同时基于压缩空气溶液除湿器传热传质模型,结合实验数据,研究了LiBr、LiCl溶液与压缩空气间的传质系数大小以及变化规律。结果表明：在相同的处理工况下,采用LiCl溶液对压缩空气进行除湿能得到更低的空气出口含湿量和更高的除湿量,LiCl溶液除湿过程的传质系数也高于LiBr溶液,即在压缩空气溶液除湿系统中LiCl溶液具有更优的除湿能力和传质性能。     

The role of diffusional resistance on crystal growth: Interpretation of dissolution and growth rate data

Experimental results on crystal growth and dissolutions kinetics were taken from literature to test two theoretical models describing the crystal growth from solutions. One of the models, designated by two-step model (TSM), is being used for many years to describe the relation between crystal growth rates with supersaturation, temperature and hydrodynamic conditions. The other model was presented in a previous work and will be called concurrent-step model (CSM). The chosen literature data allowed calculating the mass transfer coefficients during crystal growth, for different hydrodynamic conditions. The obtained results are interpreted taking into consideration well-established mass transfer theories. According to the TSM, the measured crystal growth kinetics can only be explained by means of an unrealistic variation of the mass transfer coefficient with the relative crystal-solution velocity. Conversely, mass transfer coefficients obtained by the CSM were confirmed by appropriate semi-theoretical correlations, both in their order of magnitude and in their behaviour. In addition, crystal growth and dissolution experiments of sucrose were carried out at in a batch crystallizer for different agitation speeds. The resulting kinetics are used to test the CSM in a system that is significantly different from the inorganic salts used in the analysed literature works. As predicted by this model, the existence of an adsorbed layer in the crystal surrounding is likely to have affected the solute molecular diffusivity in the medium. Based on this premise, the results obtained with sucrose are well described by the CSM.     

Interfacial area and mass transfer coefficients in liquid-gas ejectors

Measurements and correlations are reported for the interfacial area and mass transfer coefficients as a function of energy dissipation in a liquid-gas ejector. The correlations for interfacial area and mass transfer coefficients have been developed using Kolmogorov’s theory and Levich’s hydrodynamic derivations. The present developed correlations are validated using experimental results.     

Electrochemical study of liquid-solid mass transfer in packed bed electrodes with upward and downward co-current gas-liquid flow

Hydrodynamic and mass transfer parameters (pressure drop, gas and liquid hold-up, liquid-solid mass transfer coefficients) have been measured for porous electrodes with upward or downward co-current gas-liquid flow by means of several electrochemical techniques. The influence of the most important parameters (packing diameter, gas and liquid flow rates) and of the hydrodynamic flow regimes, has been studied. It is found that in the trickle flow regime the limiting current densities depend only on the liquid flow rates (with no measurable influence of the gas). In the upward flow configuration, the strong turbulence generated by the ascending gas bubbles leads to a sharp increase of current densities with the gas flow rate. A comparison between both configurations is presented.     

Liquid—liquid mass transfer in systems agitated by submerged jets

Mass transfer across a plane interface has been investigated for liquid—liquid systems, with the liquid phases agitated by vertically opposed, submerged jets, one in each phase. Mass transfer coefficients were determined for the systems ethyl acetate—water and butanol—water, and for the transfer of dissolved helium between water and toluene, and toluene between toluene and water. The latter two solutes provided a wide range of molecular diffusion coefficients. When turbulent eddies penetrated to the interface the mass transfer coefficients were found to be proportional to D^0.5, and could be represented by the Levich-Davies mass transfer model for mass transfer between turbulent liquids. The characteristic turbulence velocities in this model were related to the velocities of the liquids from the jet nozzles, and to the equipment dimensions (e.g. the distances of the jets from the interface, the radius of the vessel and the diameters of the jet nozzles) by an expression based on the hydrodynamic behaviour of jets.For the low interfacial tension system butanol—water, a flat disc had to be placed in the interface at the region of jet impingement, to prevent disruption of the interface, but, even so, only a limited range of jet flow rates could be used. Only at the highest flow rates were turbulent conditions obtained, and most of the experimental mass transfer coefficients for this system were between the values predicted by the Levich-Davies model and the Levich “three-zone” model for boundary layer flow.