Holdup measurements in a Scheibel column by means of an ultrasonic technique

An ultrasonic technique developed by Bonnet and Tavlarides for dispersed phase holdup determination in liquid–liquid contactors has been applied in a 0·10 m diameter Scheibel extraction column. The column consists of three alternate mixing and packing sections of 0·03 and 0·055 m in height, respectively. The packings were built with polypropylene mesh with 96% voidage. The liquid–liquid systems were toluene (dispersed)/water (continuous) and MIBK (dispersed)/water (continuous); the Rushton-type impellers were operated at 400, 500 and 600 rpm, and with four dispersed and continuous phase flow rates. In one of the mixing chambers two different holdup measuring devices were installed: two ultrasonic transducers and one controlled dispersion sampler. The values of dispersed phase holdup obtained by both methods were compared using statistical methods. It was found that at low agitation and for high interfacial tension, conditions for which the dispersion is not completely uniform, the difference was significant, whereas at high agitation and low interfacial tension the values obtained through both methods were statistically equal. This work demonstrates the applicability of the ultrasonic technique for holdup measurements to Scheibel columns, in which the only technique used so far was sampling. This ultrasonic technique allows us to solve the axial monitoring and control problems of these columns.     

Modeling the axial distribution of mass transfer coefficient in an agitated-pulsed extraction column

The dispersed phase holdup and drop size in solvent extraction columns vary along the column height and this affects the mass transfer coefficient and interfacial area. In this article, mass transfer study was performed experimentally using a 25 mm diameter agitated pulsed column. The axial distribution of mass transfer coefficient was determined by coupling population balance equation and axial dispersion model by taking the longitudinal variation in hydrodynamic performance into consideration. Feasibility of different mass transfer models in predicting concentration profiles was evaluated and a novel correlation based on effective diffusivity was developed. The results showed that both overall and volumetric mass transfer coefficients have significant change along the column height and greatly depends on the agitation speed and pulsation intensity. Increasing dispersed phase velocity also augments the overall mass transfer coefficient. The maximum number of transfer unit was measured to be 10 m⁻¹ at agitation speed of 1000 rpm.     

Use of axial dispersion and plug flow models for determination of axial mixing and mass transfer coefficient in an l‐shaped pulsed packed extraction column

In this study, the volumetric overall mass transfer and phases axial mixing coefficients have been investigated in a pilot plant of an L‐shaped pulsed packed extraction column by using two liquid systems of toluene/acetone/water and n‐butyl/acetone/water. The mass transfer performance has been evaluated using two methods of axial dispersion and a plug flow model. The effect of the operational variables and physical properties, including the dispersed and continuous phases flow rates, pulsation intensity, and interfacial tension, on mass transfer and phases axial mixing coefficients have been considered. It has been found that the pulsation intensity and the continuous phase flow rate seriously affect the mass transfer coefficient, however, the dispersed phase flow rate has a weaker effect. Also, the axial mixing of a phase is strongly affected by the pulsation intensity and the flow rate of the phase itself and it is not affected by the second phase flow rate. Finally, new correlations are proposed to accurately predict the mass transfer and axial mixing coefficients.     

“FORWARD MIXING” MODEL: APPLICATION TO PULSED PLATE EXTRACTION COLUMN OPERATION IN THE EMULSION REGION

The “Forward Mixing” model has been applied to data obtained from a 22 cm diameter pulsed plate extraction column. Measurements of drop size distributions, dispersed phase hold-up and concentration profiles for two systems (toluene-acetone-water and n-butanol-succinic acid-water) of quite different properties were made with the column operating in the emulsion region. Generated drop size distribution function parameters, size-dependent slip velocities and mass transfer coefficients, and continuous phase axial dispersion coefficients were accurate in predicting dispersed phase hold-up and extraction efficiencies (or the related plug flow number of transfer units). These parameters were correlated with phase superficial velocities and pulse velocities. The influence of continuous phase axial dispersion was much greater than the influence of drop size variation, and was not accurately predicted by most previous tracer-based correlations. An inlet dispersed phase distributor was beneficial to the performance with the high interfacial tension system.     

Characteristics of a countercurrent reciprocating plate bubble column. II. Axial mixing and mass transfer

Oxygen absorption from air into water and axial dispersion in the aqueous phase have been measured in a 5 cm diameter reciprocating plate bubble column. The volumetric mass transfer coefficients in semi-batch conditions were found to increase with agitation and were correlated with the specific power input and air flow rate. Under countercurrent conditions, it was found that axial mixing had little effect and conditions approached plug flow. The volumetric mass transfer coefficients were correlated with specific power input, air and water flow rates. Mass transfer coefficients were estimated using holdup and bubble diameter results. Comparison of the coefficients with the literature values indicated that the bubble surfaces were partially mobile.     

往复振动板式萃取塔水力性能研究

以上－煤油为介质,在内径为０．０３１ｍ的往复振动板式萃取塔内研究了此塔的分散相持液量、轴向混合和液滴大小。研究表明分散相持液量与板振幅、频率、连续相流速和分散相流速有关;Ｓａｕｔｅｒ平均直径与板振幅、频率有关。应用脉冲响应法测试此塔的轴向混合,以无因次方程对轴向混合系数进行关联。     

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.     

Flooding and axial dispersion in reciprocating plate extraction columns

The flooding and axial mixing characteristics of a 15 cm diameter reciprocating plate extraction column with six different plate arrangements handling kerosene dispersed in water are reported. A general correlation for flooding in the emulsion flow regime is developed and applied to the present results and to published data on other columns and systems. Under certain conditions of flow and agitation, the axial mixing is very high because of circulation, but it can be reduced by including baffles in the plate stack.     

Determining axial dispersion coefficients of pilot-scale annular pulsed disc and doughnut columns

In this study, a computational fluid dynamics(CFD) method was adopted to calculate axial dispersion coefficients of annular pulsed disc and doughnut columns(APDDCs). Passive tracer was uniformly injected by pulse input at the continuous phase inlet, and its concentration governing equation was solved in liquid–liquidtwo-phase flow fields. The residence time distributions(RTDs) were obtained using the surface monitoring technique. The adopted RTD–CFD method was verified by comparing the axial dispersion coefficient between simulation and experimental results in the literature. However, in pilot-scale APDDCs, the axial dispersion coefficients predicted by the CFD–RTD method were approximately three times larger than experimental results determined by the steady-state concentration profile method. This experimental method was demonstrated to be insensitive to the variation of the axial dispersion coefficient. The CFD–RTD method was more recommended to determine the axial dispersion coefficient. It was found that the axial dispersion coefficient increased with an increase in pulsation intensity, column diameter, and plate spacing, but was little affected by the throughput.     

Modelling of liquid-liquid extraction columns: Use of published model correlations in design

Design of several liquid-liquid extraction columns — packed, pulsed-packed, pulsed-plate, Oldshue-Rushton columns and the rotating disc contactor — was attempted utilizing available correlations for drop size, holdup of dispersed phase, flooding velocities, mass transfer coefficients and axial mixing coefficients. Correlations in many cases were vaguely defined and often based on very limited data. Results indicated that for given flow rates and extraction efficiency, the height of a packed or Oldshue-Rushton column must be considerably greater than the predicted minimum heights of the other three columns, which were comparable considering the limited data utilized in the developed correlations. A critical evaluation of the correlations should be carried out to guide the further experimental effort required to confirm the utility of the axial dispersion model in liquid-liquid extraction column design. Extension of the theory to include drop size variation is highly desirable.     

Prediction of axial mixing coefficients in rotating disc and asymmetric rotating disc extraction columns

The correlation of axial mixing in the continuous and dispersed phases of rotating disc and asymmetric rotating disc columns is presented. Published experimental results on continuous-phase axial mixing for both single- and two-phase flows, obtained with tracer injection methods and by solute concentration profiles, are considered. The correlation developed is based on 1055 data points for 32 liquid systems obtained by 19 different investigators. The axial mixing in rotating disc columns is found to be up to 20% larger than in asymmetric rotating disc columns. Data for the dispersed phase are harder to correlate than those for the continuous phase. Since the available results are often contradictory, the correlation for the dispersed-phase coefficient is thus less accurate than that for the continuousphase coefficient.     

ANALYSIS OF AXIAL MIXING IN A GAS-LIQUID RECIPROCATING PLATE COLUMN

Axial mixing in the continuous phase in a Landau reciprocating-plate column (LRPC) has been investigated for both single-phase and two-phase gas-liquid flow conditions. A hydrodynamic model is proposed in which axial mixing is described as a process consisting of a backflow through the plate plus longitudinal mixing within the stage. The region in the proximity of the plates is almost perfectly mixed, beyond which there is a low-intensity mixing zone that varies in height and degree of mixing depending on phase velocities as well as the plates design and oscillation velocity. The presence of the dispersed phase affects axial mixing in both the well- and poorly mixed regions of each stage in two opposite ways: it decreases the backflow between the stages due to the hindrance effect caused by the presence of gas bubbles, and it increases the axial dispersion coefficient in the second stage by increasing the turbulence and phase entrainment caused by circulation and bubbles rising. The model adjustable parameters were determined from an experimentally measured dispersion coefficient over a wide range of operating conditions using the transient tracer injection method. The predictions of the model compare favorably with experimental data and can be applied for describing axial mixing in the continuous phase in an LRPC with±14% accuracy.     

Axial Dispersion and Phase Holdup Characteristics in Reciprocating Plate Extraction Columns

Abstract

Axial dispersion and phase holdup characteristics have been determined in a 0.102-m i.d. × 3.5 m high QVF glass column. The axial dispersion coefficient decreases with increasing reciprocating frequency (f) and amplitude (A) in the inhomogeneous dispersed phase flow regime, whereas it increases in the emulsion flow regime. The axial dispersion coefficient with a perforated plate increases with continuous and dispersed phase velocities. However, the effect of phase velocities on axial dispersion is less pronounced with the fan plate. The axial dispersion coefficient can be correlated with A ² f, fluid velocities, and the free fractional opening area of the plates. The dispersed phase holdup increases with an increase in agitation intensity. Af, and decreases with the free opening area of the plate.     

New Approach in the Prediction of RDC Liquid‐Liquid Extraction Column Parameters

The liquid‐liquid extraction process is well‐known for its complexity and often entails intensive modeling and computational efforts to simulate its dynamic behavior. This paper presents a new application of the Genetic Algorithm (GA) to predict the modeling parameters of a chemical pilot plant involving a rotating disc liquid‐liquid extraction contactor (RDC). In this process, the droplet behavior of the dispersed phase has a strong influence on the mass transfer performance of the column. The mass transfer mechanism inside the drops of the dispersed phase was modeled by the Handlos‐Baron circulating drop model with consideration of the effect of forward mixing. Using the Genetic Algorithm method and the Numerical Analysis Group (NAG) software, the mass transfer and axial dispersion coefficients in the continuous phase in these columns were optimized. In order to obtain the RDC column parameters, a least‐square function of differences between the simulated and experimental concentration profiles (SSD) and 95 % confidence limit in the plug flow number of the transfer unit prediction were considered. The minus 95 % confidence limit and sum of square deviations for the GA method justified it as a successful method for optimization of the mass transfer and axial dispersion coefficients of liquid‐liquid extraction columns.     

A study on flow characteristics in a rotating screen blade extraction column

Studies have been carried out on the axial dispersion in the continuous phase and the hold-up of dispersed phase in a rotating screen-blade extraction column, by employing an axial dispersion model. Experiments on both single and two phase operations have been conducted with the plate spacing, the mesh size, the impeller speed (RPM), and the superficial velocities as the system parameters. By regression analysis of experimental data, empirical equations correlating the dispersion coefficients and the fractional hold-up of dispersed phase with the system parameters were obtained.     

Axial dispersion,holdup and flooding characteristics in pulsed extraction columns

The effects of interactions between the perforated and baffle plates, of pulse amplitude and of pulse frequency on axial dispersion in the continuous phase, on the dispersed phase holdup and on the flooding condition have been determined for a 10.2 cm 1D pulsed extraction column. The axial dispersion coefficient increased with both the pulse amplitude and the frequency, but it decreased with the decrease in baffle spacing in the unit module. Dispersed phase holdup increased with the number of perforated plates in the unit module at the lower pulsation velocities and increased with the pulse velocity. The baffle plate reduced the total throughputs, but baffle spacing did not have any significant effect on the total throughput.     

Effects of Gas‐Agitation and Packing on Hydrodynamics and Mass Transfer of Extraction Column

The effects of gas‐agitation and packing on hydrodynamics and mass transfer were investigated through experiments with air‐kerosene (benzoic acid)‐water system and corrugated‐packing of calendering plate with hole. The holdup of gas, holdup of dispersed liquid phase and mass transfer coefficient increase and the flooding velocity decrease with the increase in superficial gas velocity. Over‐agitation of gas causes over‐dispersion and emulsification of dispersed liquid phase, reduction of mass transfer performance and even flooding. The mass transfer performance of a packed column is far better than that of an unpacked column.     

Measurement of concentration profiles in a liquid-liquid extraction column

A novel experimental technique for withdrawing uncontaminated samples of each phase from a highly agitated two liquid phase system (primary dispersion) is presented. The technique has been applied in the study of the continuous and dispersed phase axial mixing characteristic of a mechanically agitated liquid Scheibel extraction column operating under different conditions treating the chemical system acetone-toluene-water. The column mixing compartments were separated by a mixed stainless steel-polypropylene knitted mesh packed bed which was completely ‘wetted’ by the organic dispersed phase. Several concentration profiles are presented and the non-ideal flow parameters as well as the mass transfer coefficients for the column and system under study are reported.     

Effect of system properties on the performance of Liquid—Liquid extraction columns—II: Oldshue—Rushton column

Mean drop size, fractional hold-up of dispersed phase and axial mixing characteristics have been determined in a 72 mm diameter mechanically agitated extraction column of Oldshue—Rushton type, using the two liquid—liquid mass transfer systems, toluene—acetone—water and MIBK-acetic acid—water. As for normal conditions of packed column operation described in Part I, solute presence and the direction of mass transfer has a significant effect on mean drop size, fractional hold-up and to a lesser extent, axial mixing in the dispersed phase. Probably the most dramatic effect however is the manner in which solute transfer affects dispersed phase behaviour. Highly coalescing conditions with transfer from the dispersed to the continuous phase can make the column practically unoperable. As for the packed column, axial mixing in the continuous phase is unaffected except in so far as solute presence and direction of mass transfer affect the hold-up of dispersed phase.     

Estimation of axial dispersion in pulsed-plate extraction columns

In this study, a two-region physical model was used to quantitatively estimate axial dispersion in pulsed-plate extraction columns, considering the operating conditions, geometry parameters and the physical properties of the experimental system. The calculated results fit the experimental axial dispersion coefficients well in nine geometries of pulsed-plate columns. This model is of importance to the design and scale-up of pulsed-plate extraction columns.