Type Of Contactor Research Articles

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

Abstract 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.

Investigating the process of liquid-liquid extraction by means of computational fluid dynamics

Liquid-liquid two-phase flow in extraction columns of the rotating disc contactor type is analysed using the multi-dimensional computational fluid dynamics (CFD) technique. Euler-Euler and Euler-Lagrange models are employed to give insight into the global flow structure and to analyse the turbulence related dispersion processes. Laser-Doppler velocity measurements are used to check the Euler-Euler results while a measured residence time distribution allows the assessment of the Euler-Lagrange approach. The results give rise to the expectation that CFD will become an accepted design tool in chemical engineering.

Modified Contactor for Experimental Studies of Mass Transfer and Chemical Reaction across a Liquid−Liquid Interface

A modified stirred-cell type contactor has been designed and tested for its use in experimental investigations of mass transfer and chemical reaction in liquid−liquid systems. Its design overcomes the shortcomings of previously proposed contactors. It is easy to operate and has the ability to investigate the effect of a wide range of operating conditions. The mixing performance of the contactor was tested using a dye injection and a tracer technique. Experiments to evaluate mass-transfer coefficients across a liquid−liquid interface have been conducted to determine its effectiveness. The results verified that the contactor could be successfully employed as a useful experimental research apparatus.

Evaluating a high‐efficiency ozone injection contactor

With sidestream venturi injection and a specially designed downflow tube, this recently developed ozone contactor is an attractive alternative to the fine‐bubble diffusion contactor for high ozone concentrations.A new type of ozone contactor, called the SVI‐DT (sidestream venturi injection with downflow tube) contactor, was evaluated at pilot scale and compared with the performance of a traditional countercurrent flow, fine‐bubble diffusion contactor. The specially designed downflow tube, equipped with one or more pressurized sidestream venturi injectors and installed inside an ozone reactor, facilitated the dissolution of ozone into water. The results showed that the SVI‐DT contactor achieved high mass transfer rates and outperformed the fine‐bubble diffusion contactor in removing color and yielding higher and longer‐lasting dissolved ozone residuals. The hydraulic behavior of the new contactor was also evaluated, and operating limits were established for maintaining stable two‐phase flow conditions in the downflow tube.

Design Procedure for Hooded Surface Oxygen Absorption Systems

AbstractA design procedure addressing effluent total dissolved gas pressure limits along with standard performance indicators such as oxygen absorption efficiency (kg absorbed/kg applied), and transfer efficiency (kg/kW.h), is presented for surface agitation equipment operating in an oxygen‐enriched atmosphere. Application of this contactor type in closed culture systems is attractive given its insensitivity to biological fouling and ability to operate without the need for a significant hydraulic gradient. Performance algorithms were developed through application of chemical reactor theory, Henry's Law and the Ideal Gas Law. In the analysis, gas and liquid phases were treated as being homogeneous. The design steps presented are unique in that required mass transfer coefficients (KLa), operating pressures (CP), and oxygen feed rates (G/L) are calculated for target changes in both dissolved oxygen and nitrogen without the use of iterative numerical procedures. A second calculation sequence establishes the sensitivity of system performance to changes in G/L when CP and KLa are known.

Modeling of Dissolved Ozone Concentration Profiles in Bubble Columns

Mathematical models previously employed to analyze concentration profiles of dissolved ozone in bubble‐column contactors often assumed either plug flow or complete mixing. In this paper a more realistic model, the axial dispersion model (ADM), was provided to integrate the nonideal mixing, mass‐transfer, and ozone‐decay process as a whole. The effects of various process parameters on the ozone profiles were analyzed numerically. It was found that a group of dimensionless quantities (PeL, Da, StG, and StL) is most convenient in describing the performance of ozone‐contacting systems. The different types of ozone contactors (countercurrent‐flow bubble columns, cocurrent‐flow bubble columns, and reactive contacting chambers) exhibited different patterns of concentration profile; as a result, different approaches are needed to relate their characteristic concentrations to readily observed measurements for predicting the disinfection efficiency. The applicability of the model is illustrated through the analysis...

Modelling Hydraulics of Ozone Contactors

The U.S. EPA published the surface water treatment rule which imposed stringent requirements for disinfection. The rule is based on the C × T concept. The detention time T10 must be determined for each type of contactor by performing tracer tests. Those tests have been performed for various ozone contactors. The results were evaluated using two types of models. For a classical diffuser-bubble contactor the ratio T10/τ is about 0.5. For an industrial deep U-tube this ratio is equal to 0.55, but it reaches 0.9 for a pilot system. These results are indicating that a deep U–tube can provide a greater T10/τ ratio than a diffuser–bubble contactor.

Design Considerations in Alternative Ozone Feed-Gas Systems

Abstract The design engineer and owner have multiple issues to address when planning to use an ozone process in a water or wastewater treatment plant. Considerations include feed-gas properties and equipment, ozone generator features and size, ozone contactor type and configuration, and methods of off-gas ozone destruction. Many successful ozone systems are in use today. Where performance has suffered, deficient feed-gas treatment often is the problem. The feed-gas to the ozone generator must be oil-free, particulate-free, and dry. In this paper, the factors involved in selecting and operating feed-gas equipment are presented.

Influence of operating and physical variables on interfacial area determination

AbstractThe influence of various operating and liquid‐phase physical variables on interfacial area ab in cylindrical and spherical gas‐liquid stirred contactors was studied using the sulfite method. Studied were stirring speed, sparger porosity, gas flow rate, liquid‐phase temperature, surface tension, and pH.The results show that the influence of operating variables (gas flow rate Qg, stirring speed ω, and the average pore radius of the sparger used rp) on ab, for both types of contactors, can be given by the equation: Dependency of ab on surface tension observed was believed to be caused by a rising rate of bubble coalescence with increased surfactant concentration. The higher the surfactant concentration, the lower ab, tending to a limit that depends on both the type of surfactant and the average size of sparger pore used. Due to the measurement method used, solution pH and temperature were also observed to influence ab.

On the evaluation of the interfacial turbulence (the Marangoni effect) in gas (vapour)—liquid mass transfer: Part I. A method for estimating the interfacial turbulence effect

Two widespread approaches for evaluation of the influence, upon gas (vapour)—liquid mass transfer in the case of ‘positive mixtures’ (dσ/d x < 0), of the interfacial turbulence caused by the surface tension gradients (Marangoni effect) have been surveyed and analysed critically. It is shown that the assumption of direct proportionality of mass transfer enhancement and Marangoni number has not been well justified. Evidence is given which supports the concept of defining the enhancement factor by the ratio R = Ma/Ma c) n . Some problems concerning the enhancement dynamics of mass transfer, the initiation threshold of the Marangoni instabilities, as well as the limits of saturation have been discussed thoroughly. By correlating a large amount of experimental data from the literature, equations have been derived concerning the critical Marangoni number Ma c and the exponent n. These equations are proposed for use when accounting for the Marangoni effect in gas (vapour)—liquid systems regardless of contactor type and solutions employed, the enhancement effect being obtained in relation to local mass transfer efficiency.

Gas liquid contacting in reciprocating plate columns: I. hydrodynamics

AbstractThe hydrodynamic characteristics of a 10‐cm reciprocating plate column used for counter‐current gas‐liquid contacting was investigated. The values of gas holdup, interfacial area and maximum throughputs achievable in this unit put it in a favourable position in comparison with other types of contactors.

Characterization of hydrodynamic parameters in a multistage column contactor

AbstractAccurate knowledge of hydrodynamic parameters is of major importance for the performance study of liquid‐liquid column extractors. The effects of operating parameters on dispersed phase holdup profiles, drop size distributions, and axial mixing in both phases were investigated in a 127 mm diameter multistage contactor of pilot plant scale for the toluene‐water physically equilibrated system. Correlations for the mean holdup, the mean drop size, and the continuous phase backmixing were obtained. A stronger dependence of holdup and drop size on the operating conditions and especially on the agitation speed was observed as compared to previous investigations for the same type of contactor. The axial mixing for the single phase flow was found to follow adequately an existing correlation, while the continuous‐phase axial mixing in two‐phase flows showed some deviations from other existing correlations. Also, flooding criteria, important for the control of the extraction process, were determined based on the shape of the holdup profiles.

Erosion behavior and 'erodibility' of Ag/CdO and Ag/SnO/sub 2/ contacts under AC 3 and AC 4 test conditions

Contact erosion depends on both duration and efficiency of interaction between arc and contacts. Arc motion in a magnetic field was recorded and the material loss caused by arc erosion was determined by weighing. The material loss related to 1 ms of arc interaction-named erodibility-was evaluated for the contact materials Ag/CdO and Ag/SnO/sub 2/. All tests were performed in one type of commercial contactor (rated current 110 A). The erodibility was determined for new and for partly eroded contacts prestressed during life tests under AC 3 and AC 4 conditions according to IEC Test Specification 158-1, AC 4 erosion was caused by break arcs mainly, the effect of make arcs being negligible, whereas AC 3 erosion was due to both break and make arcs; the latter was more effective in Ag/SnO/sub 2/ contacts. After AC 3 life tests the erodibility values determining the erosion of Ag/SnO/sub 2/ contacts at the site of arc formation were higher than those of Ag/CdO; after AC 4 life tests, however, there was hardly any difference between the erodibilities of the two materials.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Kinetics of Formation and Transport Resistances of Zirconium-Monobutyl Phosphate Films

The formation processes of monobutyl phosphate-zirconium films at a liquid-liquid interface have been investigated by observing changes in toluene absorption in a Lewis cell type contactor. Films were formed by adding monobutyl phosphate to the upper (organic) phase and allowing it to react with zirconium dissolved in the lower (aqueous) phase while monitoring the absorption of the organic solvent into the aqueous phase. Results indicate that the film formation occurs over a short period of time but may be preceded by a considerable lag period which depends on concentration, temperature and to a lesser extent, stirring. Mass transfer coefficients were decreased by as much as 40% at higher temperatures. Interfacial tension changes during film formation were also investigated and indicate that significant activity occurs at the interface.

HYDRODYNAMICS AND MASS TRANSFER IN A ROTATING BIOLOGICAL CONTACTOR

A model far predicting the hydrodynamics and mass transfer characteristics in a Rotating Biological type of Contactor is presented. Factors like the film thickness profile and saturation in predicting the rates of absorption of pure oxygen in water have been accounted for.

Influence of cations on crud formation in uranium circuits

Stable emulsions and crud are produced in many solvent extraction operations. The causes and prevention of the costly loss of solvent to the systems are of major concern, and seriously reflect on both the economics and the environment. Previous work by the CANMET laboratories consisted in characterization of the crud, determining the effect of other organic constituents, the effect of suspended solids and the influence of the shear and type of contactor. The present paper describes the effect of the following cations - Si, Al, Mg, Ca, Fe, U - on the generation of the crud.

Gas holdup and overall volumetric oxygen transfer coefficient in airlift contactors

The two major types of airlift contactors, concentric-tube and external-loop, were investigated for their gas holdup (riser and downcomer) and overall mass transfer characteristics. Results obtained in batch charges of tap water and 0.15 kmol/m(3) NaCl solution are reported for external-loop airlift contactors having downcomer-to-riser cross-sectional area ratios, A(d)/A(r), ranging from 0.11 <or= A(d)/A(r) <or= 0.69 and for concentric-tube types in the range 0.13 <or= A(d)/A(r) <or= 0.56.The results are compared and contrasted for the two types of airlift contactors and also with corresponding results obtained in a bubble column (A(d)/A(r) = 0). Both the riser gas holdup, (epsilon(G))(r), and the overall volumetric mass transfer coefficient, (K(L)a(D))(T), were found to increase with increasing gas superficial velocity and the corresponding specific gassing power input, (P(G)/V(D))(T). However, at a specified power input both (epsilon(G))(r) and (K(L)a(D))(T) decreased with increasing A(d)/A(r) due to a resultant increase in circulating liquid velocity. Unifying generalized correlations for both types of airlift contactors are given for both (epsilon(G))(r) and (K(L)a(D))(T). These correlations are expected to be applicable as well to continuous-liquid flow bubble columns at comparable liquid velocities.

Vacuum, SF6 and Air-Break Contactors for Medium Voltage Controllers

Vacuum, SF6 and air-break contactors are compared with emphasis on items of interest to users. Interruption of short- circuits over the full range of prospective currents is discussed and the advantage of high interrupting capability in a contactor is emphasized. The possibility of high peak voltages and high rates of rise of voltage with vacuum contactors is treated together with the means to reduce and control such transients. Comparisons are made among contactor types on size, weight, endurance, losses and maintenance. A summary is given of the advantages of each type of contactor.

Fluid dynamic study of a new type of solid-gas contactor: the fluidized/fixed or fluidized bed

Fluid dynamic study of a new type of solid-gas contactor: the fluidized/fixed or fluidized bed

Gas-liquid mass transfer in rotating perforated-disc contactors

This paper presents a numerical model for predicting the performance of liquid-gas mass transfer in a rotating perforated-disc type contactor. The device consists of a cylindrical section situated between two 45-degree conical sections. A liquid flows downward by gravity while a stream of air moves upward by buoyancy thus forming a counter-current flow situation in the contactor. A gas dissolved in the liquid transfers into air bubbles which are sheared to a tiny size as they rise through the perforations on the rotating disc. Both laminar and turbulent flows are treated. Utilizing the velocity distribution [10,11] and bubble trajectory [12] as the basis, the interphase mass transfer performance of carbon dioxide in the water-air system is numerically determined. It is disclosed that in both laminar and turbulent flow cases, the rate of interphase mass transfer increases significantly with a reduction in bubble size. Rotational speed does not affect mass transfer in laminar flow but causes an exponential mass transfer enhancement in higher turbulent flows. There exists an optimum through-flow rate of the liquid for the best mass transfer performance depending on the initial bubble size and disc speed. Test results [9] provide a qualitative confirmation of the theory.