Liquid‐Solid Mass Transfer Behavior of a Vertical Array of Closely Spaced Horizontal Tubes in Relation to Catalytic and Electrochemical Reactor Design

The rates of mass transfer at a vertical array of closely spaced horizontal tubes were measured by the limiting‐current technique under single‐phase flow, gas sparging and two‐phase flow. The single‐phase flow data were correlated by the equation: Sh = 0.75 Sc^0.33 Re^0.59. The gas sparging data with no net solution flow were correlated by the equation: J = 0.31(Re_g.Fr)^–0.22. For two‐phase flow, the gas flow was found to enhance the rate of array mass transfer by a factor ranging from 1.25 to 5.25, depending on Re_g and Re. The enhancement ratio increases with decreasing Re and increasing Re_g. For Re ≥ 2500, the rate of mass transfer approaches the value of single‐phase flow, regardless of the value of Re_g, which ranged from 7 to 41. The importance of the present geometry in building electrochemical and catalytic reactors, where exothermic liquid‐solid diffusion‐controlled reactions take place, is highlighted. The present geometry offers the advantage that the outer surface acts as a turbulence promoter while the inner surface acts as a heat exchanger.     

Enhancement of the Rates of Liquid-Solid Heat and Mass Transfer in Annular Ducts by Using Circular Fins

Rates of solid-liquid mass and heat transfer (by analogy) were measured at the outer surface of the inner tube of an annular duct under developing flow conditions using the electrochemical method that elaborates the measurement of the limiting current of the reduction of K₃Fe(CN)₆ at the tested model surface. Variables examined were physical features of the solution (density ρ, viscosity μ, and diffusivity D), solution superficial velocity, tube active length, and tube diameter. The experimental results were correlated by a dimensionless equation involving all affecting parameters. Due to the presence of the circular fins at the tube surface, the degree of mass transfer was enhanced by a factor ranging from two to six subject to the operative settings. Applications of the current findings in the design of catalytic reactors, membrane dialyzers, and double-pipe heat exchangers were discussed.     

Mass Transfer Studies in Stirred AirLift Reactor

In the present work, water and three phase compositions of Solka-Floc, a cellulose fiber for simulating the biomass in bacteria, yeast, and fungal fermentation were studied in a 1.4?m³ stirred airlift reactor. The fractional dispersed phase holdup and the overall volumetric mass transfer coefficients were measured. The dispersed phase riser gas holdup and overall volumetric mass transfer coefficients both increased with increasing riser superficial dispersed phase velocity (0.02–0.1?ms^?1) and agitator speed in the range of 0–5?rs^?1. An increase in the Solka-Floc concentration (1–3% w/v) was found to reduce ?_GR and K _L a _L . Empirical correlations have been developed for fractional dispersed phase gas holdup and overall volumetric mass transfer coefficients.     

Mass Transfer Studies in Stirred AirLift Reactor

In the present work, water and three phase compositions of Solka-Floc, a cellulose fiber for simulating the biomass in bacteria, yeast, and fungal fermentation were studied in a 1.4 m³ stirred airlift reactor. The fractional dispersed phase holdup and the overall volumetric mass transfer coefficients were measured. The dispersed phase riser gas holdup and overall volumetric mass transfer coefficients both increased with increasing riser superficial dispersed phase velocity (0.02-0.1 ms^-1) and agitator speed in the range of 0-5 rs^-1. An increase in the Solka-Floc concentration (1-3% w/v) was found to reduce ε_GR and K_La_L. Empirical correlations have been developed for fractional dispersed phase gas holdup and overall volumetric mass transfer coefficients.     

Electropolishing of Vertical Plates under Intensified Mass Transfer Conditions in a Gas Stirred,Modified Parallel Plate Reactor

The rates of electropolishing of vertical copper plates in H₃PO₄ placed downstream of a H₂ evolving cathode were studied by measuring the limiting current of the diffusion controlled process under different conditions of H₂ discharge rates and H₃PO₄ concentrations. The mass transfer coefficient was found to increase with H₂ discharge rate raised to the exponent of 0.4. Within the studied experimental range of conditions, the rate of polishing increased by a factor ranging from 1.95–5.8, depending on the H₂ discharge rate and H₃PO₄ concentration compared to the natural convection value. The results are explained in terms of surface renewal theory. The proposed electrochemical reactor offers the advantage of increasing the rate of electropolishing without consuming external stirring energy as opposed to traditional electrochemical reactors.     

SMI聚合反应釜传质及传热的研究

提出了SMI耐热改性剂在聚合反应前期的放热曲线,指出该反应的特点。在150 L的聚合反应釜内研究了影响SMI聚合反应的关键参数Pv、Np/Nqd及传热系数K,重现了小试试验的传热、传质结果。借助水运实验验证了经验公式计算传热效果的可靠性,理论计算值与实际值吻合较好,实现了聚合反应的可控性,为进一步放大研究提供了参考依据。     

A Simple Approach to Measuring the Gas Phase Heat and Mass Transfer Coefficients in a Bubble Column

A simple experimental approach was developed to measure the gas phase volumetric heat and mass transfer coefficients in a bubble column and a slurry bubble column employing a single gas nozzle. The experimental technique was based on a transfer model that simulates humidification and direct contact evaporation models in the case of a gas bubble rising in a liquid of uniform temperature. The temperature and relative humidity of the inlet and outlet gas in the column are the only measurements required in this technique. Experiments were carried out in a 0.15 m inner diameter column using water as the liquid phase, air as the gas phase, and cation resins of 0.1 mm diameter and a specific gravity of 1.2, as the solid phase. The results showed that, when using solid concentrations in the range of 7–10 wt %, both the volumetric gas‐phase heat and mass transfer coefficients increased with an increase in the gas superficial velocity and were further enhanced by increasing the solid load after a certain minimum superficial velocity had been reached in the column (0.044 m/s in the system used). Increasing the solid load beyond 10 wt %, did not contribute to a further increase in these coefficients. Furthermore, the gas holdup in the column increased with the superficial gas velocity and was further enhanced when the solid‐phase load was in the range of 7–10 wt %. These observations agree well with previously reported findings by other investigators.     

Mass and Heat Transfer in a Continuous Plate Dryer

Abstract

A mathematical model for the mass-heat transfer in a continuous plate dryer has been developed. Along with a new mass transfer model, the formulas for several important parameters, such as height, volume of each granular heap and retention time, are provided. According to the penetration model of particle heat transfer, the average drying rate ([mdot]) is predicted together with the mean bulk temperature (T _out) and moisture content (X _out) by a straightforward stepwise calculation procedure. The only empirical parameter N _mix can be predicted by the method, provided that experimental data with various initial moisture contents (X _in) are given. According to the model, the optimization of a plate dryer should aim at the maximizing of the effective covering ratio (μ) and the total area-averaged heat transfer coefficient (α). The model and equations were applied in an experimental plant. In the result, the theoretical predictions are shown to be in satisfactory agreement with experimental data.     

Heat and Mass Transfer in Fry Drying of Wood

This article describes the coupled heat and mass (water, oil) transport phenomena in parallelepiped samples of beech (Fagus sylvatica) fried in peanut oil between 120 and 180°C. The aim was to evaluate the suitability of simultaneous fry drying and oil impregnation as an alternative wood treatment process. Water loss and oil impregnation were continuously assessed during the process. Temperature and pressure were measured at the center of the sample. The water in the peripheral layers of the wood vaporizes at atmospheric pressure. The water at the center of the wood vaporizes at overpressures of the order of 2.8 × 10⁵ Pa. High fluxes of water were recorded of about 0.006 kg/(m²s). The impregnated oil can amount to 20% of the mass of the removed water.     

Heat and Mass Transfer in Fry Drying of Wood

This article describes the coupled heat and mass (water, oil) transport phenomena in parallelepiped samples of beech (Fagus sylvatica) fried in peanut oil between 120 and 180°C. The aim was to evaluate the suitability of simultaneous fry drying and oil impregnation as an alternative wood treatment process. Water loss and oil impregnation were continuously assessed during the process. Temperature and pressure were measured at the center of the sample. The water in the peripheral layers of the wood vaporizes at atmospheric pressure. The water at the center of the wood vaporizes at overpressures of the order of 2.8 × 10⁵ Pa. High fluxes of water were recorded of about 0.006 kg/(m²s). The impregnated oil can amount to 20% of the mass of the removed water.     

Coupled Mass and Heat Transfer between a Cone and a Power‐Law Fluid

Coupled mass and heat transfer between a cone and a non‐Newtonian fluid was studied when the concentration level of the solute in the solvent is finite (finite dilution of solute approximation). Convective heat and mass transfer between a laminar flow and a stationary cone and between a rotating cone and a quiescent fluid is investigated. Solutions of both problems are found in the form of the dependencies of Sherwood number vs. Reynolds and Schmidt numbers. Coupled thermal effects during dissolution and solute concentration level effect on the rate of mass transfer are investigated. It is found that the rate of mass transfer between a cone and a non‐Newtonian fluid increases with the increase of the solute concentration level. The suggested approach is valid for high Peclet and Schmidt numbers. Isothermal and nonisothermal cases of dissolution are considered whereby the latter is described by the coupled equations of mass and heat transfer. It is shown that for positive dimensionless heat of dissolution, K > 0, thermal effects cause the increase of the mass transfer rate in comparison with the isothermal case. On the contrary, for K < 0 thermal effects cause the decrease of the mass transfer rate in comparison with the isothermal case. The latter effect becomes more pronounced with the increase of the concentration level of the solute in a solvent.     

Intensification of Rate of Diffusion Controlled Reactions in a Parallel Plate Electrochemical Reactor Stirred by a Curtain of Electrochemically Generated Gas Bubbles

Rates of mass transfer were measured at a vertical plate stirred by a rising curtain of oxygen bubbles generated electrochemically at an upstream plate by measuring the limiting current of the cathodic reduction of K₃Fe(CN)₆ in alkaline solution. The rate of mass transfer was found to increase over the natural convection value by a factor ranging from 2.4 to 25 compared to the previously reported range of 2–5 in the case of copper deposition from acidified solutions. The high tendency of oxygen bubbles to coalesce in alkaline solutions is believed to be responsible for the high rates of mass transfer in alkaline solutions. The rate of mass transfer at a plate stirred by a curtain of oxygen bubbles was found to decrease with increasing plate height and electrolyte concentration. Curtains of H₂ bubbles were found to be less effective in enhancing the rate of mass transfer compared to that of oxygen. Practical application of the results in designing a modified parallel plate electrochemical reactor stirred by a counterelectrode gas curtain was highlighted. The suggested design has the potential of saving part or all of the mechanical stirring energy as well as floor space since it extends vertically.     

Mass Transfer Coefficients in Mechanically Agitated Gas‐Liquid Contactors

There are a large number of correlations given in literature for the prediction of volume‐related liquid‐side mass transfer coefficients in mechanically agitated gas‐liquid contactors. Significant disagreement can be observed concerning the proposed correlations, so that no single correlation exists representing all of the mass transfer data given in the literature. The observed differences can mainly be ascribed to the differences in the geometry of the system, the range of operational conditions and the measurement method used. On the basis of a comparative study of mass transfer phenomena in agitated Newtonian and non‐Newtonian aerated liquids, a critical discussion of the literature results is presented in this review article, so that final conclusions can be drawn for the k_Lα values in the different single‐ and multiple‐impeller agitated systems studied in the literature.     

Gas‐Liquid Mass Transfer in a Bench‐Scale Trickle Bed Reactor used for Benzene Hydrogenation

The gas‐liquid mass transfer coefficients (MTCs) of a trickle bed reactor used for the study of benzene hydrogenation were investigated. The Ni/Al₂O₃ catalyst bed was diluted with a coarse‐grained inert carborundum (SiC) particle catalyst. Gas‐liquid mass transfer coefficients were estimated by using a heterogeneous model for reactor simulation, incorporating reaction kinetics, vapor‐liquid equilibrium, and catalyst particle internal mass transfer apart from gas‐liquid interface mass transfer. The effects of liquid axial dispersion and the catalyst wetting efficiency are shown to be negligible. Partial external mass transfer coefficients are correlated with gas superficial velocity, and comparison between them and those obtained from experiments conducted on a bed diluted with fine particles is also presented. On both sides of the gas‐liquid interface the hydrogen mass transfer coefficient is higher than the corresponding benzene one and both increase significantly with gas velocity. The gas‐side mass transfer limitations appear to be higher in the case of dilution with fine particles. On the liquid side, the mass transfer resistances are higher in the case of dilution with coarse inerts for gas velocities up to 3 · 10^–2 cm/sec, while for higher gas velocities this was inversed and higher mass transfer limitations were obtained for the beds diluted with fine inerts.     

Modeling the Heat and Mass Transfer in Microwave Drying of White Oak

A 2D comprehensive heat and mass transfer model was developed to simulate the free liquid, vapor, and bound water movement in microwave drying of white oak specimens. The experimental and model results showed that, for white oak, moisture movement was easily impeded and high gradient of internal vapor pressure occurred. The internal vapor pressure was affected by sample dimension (length and thickness). At the same input power density, the internal pressure generated in the core increased with the sample length and thickness. However, as compared with sample length, sample thickness has less effect on the pressure gradient because of the high ratio of permeability between longitudinal and transverse directions.     

Modeling of Gas‐Solid Reactions: Kinetics,Mass and Heat Transfer,and Evolution of the Pore Structure

Mathematical models for simulating heterogeneous gas‐solid reactions must describe a complex set of physicochemical and thermal phenomena. These include the chemical reaction itself, at an interface whose area varies during the conversion, the transport of gaseous species by diffusion in the pores of the solid, whose size and number generally change in the course of reaction, diffusional transport in the layer of solid product, the evolution or consumption of heat by the reaction and its transport in the porous solid, etc. The present paper gives details of the equations employed to model each of these processes. Some computed results illustrate how increasingly sophisticated recent models describe the gradual obstruction of pores during reactions, such as the sulfation of lime, or the thermal effects related to the exothermic nature of the oxidation of zinc sulfide.     

The Role of Mass Transfer in the Kinetics of Chemical Oxidation of Anthracine Slurry to Anthraquinone in a Batch Stirred Tank Reactor

The kinetics of chemical oxidation of anthracine powder by acidified dichromate in a batch agitated vessel stirred by a 45° four pitched blade turbine impeller was studied under different conditions. Variables studied were impeller rotation speed, solution physical properties and temperature. The rate of anthracine oxidation was found to increase with temperature according to the Arrhenius equation with an activation energy of 3.98 cal/mole. The rate of anthracine oxidation was found to increase with 0.56 power of impeller rotation speed. The value of the activation energy and the sensitivity of the rate of oxidation to stirring lend support to the diffusion‐controlled nature of the reaction. The data were correlated by the equation: Sh = 0.5 · 10^–3 Sc^0.33 · Fr^0.33 Implication of the above equation for the operation of industrial reactors was noted.     

Simultaneous Heat and Mass Transfer in Shrinkable Apple Slab During Drying

The goal of the study was to determine the influence of drying shrinkage on the kinetics of convection apple slab drying. The arbitrary Lagrange-Eulerian (ALE) method was used to enter a problem with moving boundaries. It was found that drying shrinkage had a major influence on the both simulated temperature and water content in the material. The lower the moisture content in particles during drying, the more pronounced the effect of shrinkage on simulation of heat and mass transfer. It was found that application of the arbitrary Lagrange-Eulerian method for shrinkage modeling leads to a relatively simple mathematical model of the drying kinetics of shrinkable materials.     

Simultaneous Heat and Mass Transfer in Shrinkable Apple Slab During Drying

The goal of the study was to determine the influence of drying shrinkage on the kinetics of convection apple slab drying. The arbitrary Lagrange-Eulerian (ALE) method was used to enter a problem with moving boundaries. It was found that drying shrinkage had a major influence on the both simulated temperature and water content in the material. The lower the moisture content in particles during drying, the more pronounced the effect of shrinkage on simulation of heat and mass transfer. It was found that application of the arbitrary Lagrange-Eulerian method for shrinkage modeling leads to a relatively simple mathematical model of the drying kinetics of shrinkable materials.     

Convective Heat and Mass Transfer Modeling in Gas‐Fluidized Beds

This review examines selected mechanistic and empirical models reported in the literature to predict convective heat and mass transfer coefficients in gas‐fluidized beds. The role of hydrodynamics in heat and mass transfer is briefly outlined before embarking on the modeling approaches. Both bed to wall and interphase heat transfer, are considered. In bed to wall heat transfer, the main focus of the review is the modeling of particle convective components, based on surface renewal. The concepts of transient and local heat transfer models are also discussed briefly. In the case of mass transfer, only interphase transfer is considered. Emphasis is placed on models based on combustion where mass transfer is seen to occur from a few active particles contained in a fluidized bed of inert particles.