Heat transfer coefficients in coiled stirred-tank systems

Steady-state and transient heat transfer coefficients were determined in coiled, stirred-tank systems with single-tank and two-tanks-in-series arrangements, using flat-blade turbine impellers and water. Flowrates, hot water temperature and agitator speed were varied for Re between 19,000 and 77,000 for the agitated side, covering the Re = 20,000–40,000 range for which published data are not available.     

Mass transfer from discrete gas bubbles in a reciprocating plate column

Measurements have been made of the uptake of oxygen in water from a series of discrete bubbles released into a 5 cm diameter reciprocating plate column. The rate of increase of dissolved oxygen in the water has provided data on the average mass transfer coefficients under the following conditions; (a) free rise in the absence of plates, (b) bubbles rising through a static plate assembly and (c) bubbles rising through moderately agitated plates (frequency up to 1.28 Hz). It was found that the mass transfer coefficients were higher under condition (b) than (a), and were further increased under condition (c). The results were compared with earlier data on mass transfer to agitated bubble dispersions.     

Desorption of carbon dioxide from supersaturated water in an agitated vessel

The rates of desorption of carbon dioxide from supersaturated water solutions into pure carbon dioxide or nitrogen gas stream were measured at 15,25, and 35°C in a baffled agitated vessel with a flat gas-liquid interface operated in a continuous manner. The volumetric liquid-phase mass transfer coefficients for the bubbles generated in the agitated liquid and the enhancement factors of the volumetric liquid-phase mass transfer coefficient for the free liquid surface due to the bubbling were calculated from the measured desorption rates and correlated as functions of the relative supersaturation of the solution and the liquid-phase Reynolds number.     

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.     

Design of a laboratory experiment on heat transfer in an agitated vessel

A novel teaching laboratory experiment is described, which demonstrates heat transfer under agitation. The experiment involves a simple and inexpensive apparatus with necessary basic components. The laboratory deals with the experimental determination of the heat transfer rates and the overall heat transfer coefficient between steam and water in an agitated vessel. The apparatus can be operated under both static and flow conditions, which affords the student an improved understanding of heat transfer during transient and steady-state modes. Further, student is trained to study the effect of the impeller speed and water flow rate on the rate of heat transfer. By performing the experiment, the student is able to determine the overall heat transfer coefficients experimentally, and compare the results with those obtained from theoretical calculations using correlations available in the literature. In addition, while working in groups, students develop team work and technical writing skills in preparing a comprehensive laboratory report.     

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.     

INFLUENCE OF MIXING INTENSITY ON THE MASS TRANSFER COEFFICIENT ACROSS LIQUID-LIQUID INTERFACES

The influence of mixing intensity on the liquid-liquid mass transfer have been investigated using a binary system (water/n-butanol) in a modified stirred cell type contactor which overcomes the shortcomings associated with the previous contactors. It is observed that the mass transfer coefficient is affected by the degree of mixing intensity in both phases. The dependence of the aqueous and organic mass transfer coefficients on the mixing rate is about the same when both phases are agitated at the same speed and these coefficients become asymptotic values for the coefficients obtained at non identical agitation rate in both phases.     

Characterization of isotherms and thin-layer drying of red kidney beans,Part II: Three-dimensional finite element models to estimate transient mass and heat transfer coefficients and water diffusivity

Three-dimensional finite element models with consideration of shrinkage and irregular shape were developed to estimate the relationships among the transient heat and mass transfer coefficients, the transient water diffusivity, and the temperature and moisture content of the red kidney beans being dried under different drying conditions. An equation was developed to calculate the transient mass transfer coefficient using the measured time–moisture content data. This calculated transient mass transfer coefficient was further used to calculate the transient heat transfer coefficient. To verify the predicted temperature on the surface of the red kidney beans, surface temperature was measured using a handhold infrared thermometer. These measured temperature and time–moisture content data were used to determine the transient water diffusivity using the least square method when the red kidney bean kernel experienced a shrinkage during drying. Strong relationship among the transient heat and mass transfer coefficients, the water diffusivity, and the ratio of the transient heat and mass transfer coefficients was revealed. This relationship could be used to predict temperature and moisture content of the red kidney beans during the entire drying period. The Lewis number?=?27, and the ratio of the transient heat over mass transfer coefficients was 10765?J?m^?3?k^?1 at 30 and 40°C, and 10729?J?m^?3?k^?1 at 50°C. Shrinkage did not significantly influence the value of the estimated transient water diffusivity.     

Comparison Of Ozonation Kinetic Data From Film and Danckwerts Theories

Danckwerts and film theories have been compared through the aqueous ozonation kinetic study of three organics: crotonic, cinnamic and 0hydroxycinnamic acids. At the conditions investigated, these compounds are removed from water exclusively by direct reactions with ozone and the kinetic regime of ozone absorption corresponds to a fast second order irreversible reaction. In this regime, Danckwerts theory provides wider possibilities than film theory for the determination of the rate constant and individual liquid side mass transfer coefficients. The ozonation reactor was operated as an agitated cell to determine the rate constant and as an agitated tank to obtain the mass transfer coefficient. Rate constants were found to increase with increasing pH. At _pH 7 the ozone reactivity of these compounds was in the following increasing order: Cinnamic acid <? crotonic acid <? o‐hydroxycinnamic acid. At _pH 3 differences in reactivity were not significant.     

Power Input Prediction in Agitated Gas‐Liquid Contactors with Non‐coalescent Batch

Volumetric mass transfer coefficients, k_La, just as power input are considered as essential parameters for mechanically agitated gas‐liquid contactors in relation to their optimization and design. The knowledge of power input is crucial for the prediction of other mass transfer characteristics. A power input correlation is created for the industrial design of the process with a non‐coalescent batch that would be appropriate for a broad range of operational conditions. The recommended resulting correlation is able to predict the power input for impellers in industrial‐scale design for a significant scope of operational conditions.     

The dynamics of extraction processes: Part III. The determination of steady state and transient concentration profiles

Techniques for determining concentration profiles during the transient and final steady state operation of a countercurrent extraction column are described and evaluated. The column is mechanically agitated and the stages are separated using Teflon sieve trays. The paper discusses numerical procedures for analyzing the behaviour of such columns, and the errors introduced by the measurement techniques studied. Mass transfer coefficients for the column under study are reported, and there is a discussion of the effect of concentration (or position along the column) on their value.     

Heat transfer in an indirect-heat agitated dryerWärmeübertragung in indirekt beheizten Trommeltrocknern

The heat transfer coefficients between the heating plane and the granular material under agitation in a small indirect-heat agitated dryer with both moving and stationary heating planes were measured in order to obtain data for its design and to analyse the heat transfer mechanism. Based on the ‘particle heat transfer model’ suggested by Schlünder and partly revised by Mollekopf and Martin, heat transfer models for both types of heating planes were proposed to correlate the observed heat transfer coefficients. Comparison between the experimental and calculated heat transfer coefficients showed no serious deviation in either case. Using these models, it is possible to estimate the heat transfer coefficient between the heating plane and the granular material in this type of dryer.     

Heat and Mass Transfer in an Agitated Contact-Convective Heated Dryer

For the investigation of the drying process of a pharmaceutical fermentation waste and for determining specific heat and mass transfer coefficients an agitated contact-convective heated dryer was constructed. This dryer is also suitable for drying of other granular solids with high moisture content. Hence we investigated the drying of a by-product from bio-ethanol production, as well. The pilot-plant agitated dryer makes possible continuous measurement and data-acquisition. Data-acquisition of heated wall temperature, inlet and outlet air temperatures and humidity, mass reduction of the material makes possible the determination of transfer coefficients by the heat and mass balance of the dryer. The measured heat and mass transfer coefficients serve as proper input parameters for the simulation calculations.     

Absorption and desorption of carbon dioxide in several water types

This study investigated the effect of water type on the rate of CO₂ transfer from/to an aqueous phase with varying degree of water salinity. The absorption and desorption experiments were conducted on reverse osmosis product, brackish well, and brackish water reverse osmosis reject waters as well as seawater in a mechanically agitated tank. Results show that the direction of mass transfer has a major impact on the value of the volumetric mass transfer coefficient, k_La, with the absorption experiments always rendering higher values. Furthermore, k_La values always decreased with salinity in both absorption and desorption experiments until a certain critical salinity value was reached, beyond which mass transfer increased again. However, k_La values were found to decrease continuously with an increase in the water alkalinity in absorption experiments, while no clear conclusion could be drawn for the alkalinity effect in the case of desorption experiments. These observations suggest that the effect of alkalinity should be further investigated to elucidate its impact along with the salinity on the volumetric mass transfer rate.     

An effect of the impeller eccentricity on the process characteristics in an agitated vessel—experimental and numerical modeling

An effect of the impeller eccentricity on the process characteristics in an agitated vessel was analyzed on the basis of our own experimental and numerical results obtained within the turbulent range of the Newtonian liquid flow. Mixing time, power consumption, local and mean values of the heat transfer coefficients and distributions of the transport coefficients (shear rate and friction coefficient) at the vicinity of the vessel wall were studied experimentally within the wide range of the operating and geometrical parameters of the agitated vessel equipped with eccentrically located high-speed impeller. Numerical simulations of hydrodynamics in an agitated vessel with eccentrically located axial flow impeller (up-pumping propeller or downpumping HE-3 impeller) were carried out using CFD method.     

氢气和一氧化碳在混二甲苯中的传质系数

在１Ｌ表面进气式高压机械搅拌釜中,采用气体间歇吸收技术,在不同的温度,气体压力下研究了Ｈ２和ＣＯ在混二 的体积传质系数,结果表明,Ｈ２和Ｃ类混二甲苯中的体积传质系数均随温度的升高而升高,随压力增加而增加,而且在桢的温度和压力下,ＣＯ在混二甲苯中的体积传质系数大于Ｈ２的体积传质系数,但数量级是相同的,同时随着搅拌速率的升高,体积传质系数也在增加。     

Axial mixing and mass transfer in a vibrating perforated plate extraction column

The axial mixing and countercurrent mass transfer characteristics of a 5 cm diameter extraction column agitated by vibrating perforated Teflon plates have been investigated. The dispersed phase was an organic liquid (usually kerosene) and the continuous phase was water. Axial mixing was measured in both phases using pulse tracer techniques; in the continuous phase the axial mixing was estimated to have a significant effect on mass transfer, but axial mixing in the dispersed phase had a negligible effect. Mass transfer was measured for several different solutes; n-butyric acid, benzoic acid and phenol. The overall heights of a transfer unit (cont. phase) were in the order of 10-20 cm for the organic-acids but higher for transfer of phenol from very dilute solutions. The characteristics of the vibrating plate column have been compared with those of other types of extractor and suggestions are made for further development.     

Bubble swarm behaviour and gas absorption in non-newtonian fluids in sparged columns

Mean relative gas hold up, slip velocity, bubble size distribution, and volumetric mass transfer coefficient of oxygen were measured in sparged columns of highly viscous non-Newtonian fluids (CMC solutions) as a function of the gas flow rate, and CMC concentration (fluid consistency index k, and flow behaviour index n).By comparison of the measured bubble swarm velocities with those calculated by relations for single bubbles the bubble swarm behaviour was investigated. It could be shown that small bubbles in swarm have higher rising velocities than single bubbles, expecially in highly viscous media. Large single bubbles rise with high velocity due to the change of their shape caused by the swarm of the smaller bubbles. No large bubbles with spherical cap shape could be observed. The volumetric mass transfer coefficient decreases rapidly with increasing CMC-concentration.A comparison of the volumetric mass transfer coefficients with those measured in mechanically agitated vessels indicates, that the performance of sparged columns is comparable with the one of agitated vessels. Because of their lower energy requirement sparged columns are more economical than mechanically agitated vessels. It is possible to improve the performance of sparged columns by the redispersion of large bubbles in a multistage equipment.     

Solid-liquid mass transfer coefficients in gas-solid-liquid agitated vessels

The research on mass transfer coefficients in solid-liquid agitated systems has received substantial attention in the past, due both to the interest in fundamental aspects of mass transfer between particles and turbulent fluids and to the importance of practical applications. In contrast, little information is available on solid-liquid mass transfer when a third gaseous phase is also dispersed into the system, in spite of the importance of the applications of gas-solid-liquid agitated systems. In this work a suitable dissolution technique was used to measure the solid-liquid mass transfer coefficient in gas-solid-liquid vessels stirred by either radial or axial impellers. The mechanical power dissipated by the stirrers at various agitation speeds and gas flow rates was also measured by means of a new technique. The mass transfer data obtained were found to be well correlated to the 0.25 power of the specific power dissipation, indicating that the Kolmogorov's theory of mass transfer applies to these systems, while no clear influence of the gas hold-up was ascertained.