Dependence of Holdup on Height in a Pulsed Plate Column Based on Binary Liquid‐Liquid Equilibrium Data

Holdup has been measured at various frequencies, amplitudes, continuous and dispersed phase flow rates for binary systems in a pulsed plate column capable of providing samples at various heights. The binary systems have been so selected as to cover a wide spectrum of interfacial tensions. Dispersed phase holdup was found to increase with height in a logarithmic fashion at conditions away from the flooding point and to become almost invariant with height near flooding conditions. The interfacial tension of the binary system has a large effect on the dispersed phase holdup. In systems having low interfacial tension, a small increase in any of the parameters can increase the holdup significantly and lead to flooding. In systems having high interfacial tension, on the other hand, variations in system parameters do not affect system performance significantly.     

Steady state hydrodynamics and mass transfer characteristics of a karr extraction column

The steady state hydrodynamics of the holdup in a 15 cm column agree quite well with mechanistic predictions at conditions approaching flooding. In the presence of mass transfer (acetic acid from the dispersed kerosene phase to the continuous water phase), the holdup data below flooding are not well predicted by the model due to enhanced droplet coalescence. The prediction of holdup at flooding is a function of the ratio of the flow rates but is independent of the physicochemical properties of the extraction system, and therefore can be used to estimate the flooding holdup regardless of the occurrence of mass transfer. In the second part of the paper, steady state mass transfer characteristics of the column were also simulated using a mechanistic model. Despite the difficulty of estimating parameters such as the mass transfer coefficient, the model curves could be arbitrarily adjusted to fit observed exit concentrations. These steady state results provide useful insights on the non-linearity of the system from a control viewpoint.     

Adaptive control of hydrodynamic holdup in a karr extraction column

An application of a Clarke-Gawthrop self tuning regulator to control the dispersed phase holdup in a Karr reciprocating plate extraction column is presented. The frequency of reciprocation was used to control holdup because of its speed of response and low cost. The relationship between the frequency of reciprocation and the dispersed phase holdup is highly nonlinear, and, therefore an adaptive controller has to be used to control this process. Previous work (Kusuma, 1981) on holdup control has been extended to cover a wider range of operating conditions and to prevent covariance windup by including an empirical discounting factor. The experimental results focus on the operating region near to flooding since this is the most difficult to control and is often closest to the optimal operating point. The advantages and limitations of the empirical discounting factor are discussed.     

Flooding and drop size in a pulsed disc and doughnut extraction column

The hydrodynamic behavior of a pulsed disc and doughnut column has been investigated using three different systems in the absence of mass transfer. Sauter-mean drop diameter (d₃₂), flooding velocity and holdup at flooding have been measured at different operating conditions. The following operating variables have been studied: pulsation intensity and flow rate of both liquid phases. As expected, smaller mean drop sizes are obtained with the increase of pulsation intensity. The results also show no significant effect of continuous phase flow rate on mean drop size, which increases with increase of dispersed phase flow rate for the operating conditions investigated. A single correlation for the prediction of d₃₂ in the mixer-settler, transition and emulsion regimes of operation is proposed with a mean deviation of 7.32%. The maximum throughput is influenced mainly by pulsation intensity and interfacial tension. Two precise correlations are proposed for predicting flooding velocities in this column. The first is based on operating variables, column geometry, and system physical properties. The second one considers the same variables, except column geometry. Good agreement between prediction and experiments is found for all operating conditions investigated.     

EFFECT OF MASS TRANSFER ON FLOODING AND HOLDUP IN A KARR COLUMN

This work considers flooding and dispersed phase holdup, with and without mass transfer, in a 7.6?cm diameter reciprocating plate extraction column using the liquid system toluene-acetone-water. The present flooding data in respect to the absence of mass transfer were well correlated by the hydrodynamic model based on the dispersion of drops by turbulent energy dissipation. During mass transfer the flooding characteristics were found to be dependent on the direction of mass transfer.

Dispersed phase holdup data were obtained by two different measuring techniques. Local holdup was found to vary with column height, while the overall holdup depended on agitation speed, phase throughputs and mass transfer direction. The holdup data thus obtained are compared with data published for the case without mass transfer, and new correlations are presented.     

气体搅动的筛板萃取塔性能实验研究

在筛板萃取塔中引入气体搅动,既能明显提高装置的传质效率,又能大幅提高装置处理能力。筛板塔的通量随气速的变化规律与填料塔有显著区别,其性能研究有重要意义。利用煤油(苯甲酸)-水-空气体系,考察了气体搅动和筛孔直径对萃取塔流体力学和传质性能的影响。结果表明,随着表观气速的增加,气含率、分散相含率、液泛速率和传质效率均明显增加。但过高的气速也会导致分散相的过于分散和乳化,传质性能下降,直至液泛。不同直径的筛孔相比,较小的筛孔使分散相停留时间延长,分散相含率和传质效率提高,但液泛速率和处理能力降低。     

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.     

Characterization of flooding in a wirz extraction column

The flooding behaviour of a 150-mm diameter Wirz-II extraction column has been investigated for six different liquid-liquid systems. Normally, flooding occurred by phase inversion but when the density difference was low could also occur by entrainment of dispersed-phase drops at the continuous-phase exit. Empirical expressions for flooding velocities and holdup at flooding are derived in terms of the operating variables, column geometry and physical properties of the liquid systems.     

Control of mass transfer and hydrodynamic holdup in a karr solvent extraction column

This paper reports an experimental application of multiloop control to a Karr solvent extraction column (a reciprocating plate column). The control objective was to keep the extract outlet concentration above some minimum level and to maintain a high value of dispersed phase holdup while keeping away from the flooding point. Transfer function models were identified from step tests about a nominal operating point and various interaction/operability analysis techniques were used to synthesize a control scheme. The control implemented in this study was a model inversion based scheme in which the extract outlet concentration was controlled by manipulating the continuous phase superficial velocity and the dispersed phase holdup was controlled by manipulating the frequency of reciprocation. Both servomechanism and regulatory responses are presented. A variable dead time Dahlin controller was used to control the extract outlet concentration and a Clarke-Gawthrop self tuning regulator was used to control the dispersed phase holdup. This multiloop control scheme is compared to single loop control of the extract outlet concentration.     

φ50折流板脉冲萃取柱水力学性能

以30％TRPO煤油溶液和1 mol·L^-1硝酸溶液为体系,在φ50折流板脉冲萃取柱中考察了折流板脉冲萃取柱的操作参数（脉冲振幅、脉冲频率、流速和流比）和结构参数（开孔率和板间距）对其水力学性能（液泛通量、液泛存留分数和正常操作时分散相存留分数）的影响.结果表明：在乳化操作区,液泛通量随着开孔率增加、脉冲强度和流比的减小而增大,与板间距无关;根据液泛存留分数的结果,给出了最优开孔率为23％;此时正常操作时分散相存留分数与连续相的流速、流比和脉冲振幅呈正比,而与脉冲频率无关,浸润性的影响造成了板间距对以硝酸溶液和30%TRPO煤油溶液为连续相时的分散相存留分数影响规律不同.     

Holdup and characteristic velocity in a Hanson mixer–settler extraction column

The hydrodynamic behavior of a pilot plant Hanson mixer–settler extraction column has been studied for three different liquid–liquid systems with and without mass transfer conditions. The dispersed phase axial holdup profile is investigated and an empirical correlation for prediction of dispersed phase holdup is recommended in terms of physical properties of liquid systems and operating conditions. A comparison between several correlations and experimental results shows that Pratt equation is more suitable for prediction of slip velocity because of its simplicity. On this basis, an empirical correlation is derived for prediction of characteristic velocity in terms of physical properties of liquid systems and operating conditions.     

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.     

Effect of Plate Wettability on Dispersed-Phase Holdup in a Pulsed Disc-and-Doughnut Solvent Extraction Column

The effect of plate wettability on the dispersed-phase holdup in a pulsed disc-and-doughnut solvent extraction column is presented. Teflon, nylon, and stainless steel plates have been used to simulate a change in the wetting characteristics of the plate material that can occur in an operating column due to ageing or deposits accumulating on the plate. Experimental holdup data have been measured over a range of operating conditions using a 1.0 m long glass column with an internal diameter of 72.5 mm containing alternating discs and doughnuts. The liquid-liquid system studied was tri-n-octylamine (TOA)-kerosene-water with sulphuric acid as the solute. Results show that there are noticeable changes in the characteristic velocity (determined from measured holdup) and operational regimes for the different plate materials, particularly at low pulsation intensities, when operating under dispersed aqueous conditions. Experimental holdup data from this study have also been compared to correlations from literature for predicting holdup. As none of these correlations for holdup incorporate plate wettability, a new correlation for predicting holdup has been proposed that incorporates the contact angle of the plate material to allow for changes in the wettability of the plate surface. This correlation is able to predict the holdup data from this study to within 10.5% for aqueous dispersed conditions.     

Dispersed phase holdup and slip velocity of phases in a pulsed packed column in conditions with and without mass transfer

Dispersed phase holdup and slip velocity of phases were measured in a pilot plant pulsed packed column with a diameter of 76.2 mm for two different chemical systems in conditions with and without mass transfer. The effects of pulsation intensity, dispersed and continuous phases flow rates, interfacial tension and solute concentration on dispersed phase holdup and slip velocity of phases were investigated. A new empirical correlation in terms of the above‐mentioned parameters developed from the measurements is given for the prediction of slip velocity. The dispersed phase holdup was calculated by means of this correlation and very good agreement between calculated and experimental values was obtained.     

Prediction of dispersed phase holdup in a modified multi‐stage bubble column scrubber

This paper reports on the experimental investigation carried out to evaluate fractional dispersed phase holdup for a gas‐liquid mixture in a modified multi‐stage bubble column (with contraction and expansion disks), which has been conceived, designed and fabricated as a wet scrubber for control of air pollution; in addition it has versatile use as a gas‐liquid contactor in chemical process industries. A correlation developed for predicting fractional dispersed phase holdup has been found to be encouraging and highly significant from statistical analysis.     

Experimental Investigation and CFD Modeling of Hydrodynamic Parameters in a Pulsed Packed Column

In this work, a combination of computational fluid dynamics (CFD) and droplet population-balance model (DPBM) in the framework of Fluent was applied to simulate the drop-size distributions and flow fields in a pilot-plant liquid–liquid extraction pulsed packed column. The three-dimensional unsteady-state liquid–liquid flow was modeled using the Eulerian two-fluid equations in conjunction with the realizable k – ε turbulence model. The classes method (CM) was chosen for solving population-balance equations. Two models for breakage and coalescence, the models of Luo and Garthe, were used in the CFD code. The model was validated by comparing the simulated drop-size distributions and holdup with experimental measurements. After the validation of the model, the effects of the operating conditions (feed rates and pulsation) on the dispersed phase holdup and drop-size distributions were studied. The results of linked CFD-DPBM model and experiments revealed that the dispersed phase holdup was increased when the organic and aqueous flow rates increased and when the intensity of pulse was increased, the holdup increased. Increasing the dispersed and continuous feed rates caused the Sauter mean diameter of the drops decreased and when the intensity of pulse was increased, because of high droplets break up rate, the Sauter mean diameter decreased. Results of linked CFD-DPBM model show that the CFD-DPBM tool is able to predict hydrodynamic parameters in a pulsed packed column.     

HOLDUP MEASUREMENT IN A SCHEIBEL EXTRACTION COLUMN

ABSTRACT The total holdup of a dispersed organic phase in a rotary-agitated Scheibel liquid-liquid extraction column of the first type is studied experimentally. The column is 2 inches in diameter and contains 11 stages with a 4-inch height packed section and a 1-inch height mixing section. The packed section is made of stainless steel wire mesh packing with a 93.74% free area and acts as a settler for the coalescence of droplets. Three systems are studied: benzene/water, methyl isobutyl ketone (MIBK)/water and 20% TBP-80% kerosene/1?M nitric acid. The effects of different phase flow rates and rotation speeds (300-670?rpm) on the dispersed phase holdup are investigated. The results show that the holdup increases with increasing dispersed phase flow rate but does not vary with continuous phase flow rate. For benzene, the holdup is large at low rotation speeds and decreases with increasing rotation speed until a minimum holdup is reached, after this transition point, the holdup increases with increasing rotation speed. This phenomena is analogous to that found in a pulse column. For MIBK and 20% TBP-kerosene, the holdup always increases with rotation speed in our experimental range and the transition point is not observed as M in benzene. This is attributed to the much larger interfacial tension of benzene-water as compared with the other 2 systems.     

Mass-transfer effects on droplet phenomena and extraction column hydrodynamics revisited

The effect of solute mass transfer between the two liquid phases on the drop size distribution and holdup profiles of the dispersed phase in a multistage extraction column is experimentally investigated in this article. Experimental results of the drop size distribution, dispersed-phase volume fraction (holdup), and concentration profiles are obtained for both directions of mass transfer. The drop size distribution is measured by a photomicrographic technique, the holdup profile is measured by an ultrasonic technique, and the concentration profiles are measured by refraction index measurements. A strong influence of the mass-transfer direction on drop size and holdup has been found. The results are in qualitative agreement with the observations of other investigators. Mass transfer of butyric acid from toluene (dispersed) to water (continuous) produced larger drop sizes and lower values of the dispersed-phase holdup than for the equilibrated toluene—water system. For mass transfer of butyric acid from water (continuous) to toluene (dispersed) smaller drop sizes and higher holdup values are observed than for the equilibrated toluene-water system. This behavior significantly affects the performance of the extractor and indicates the necessity for more studies to determine the physics of the phenomenon and to model these processes appropriately.     

Extraction of Uranium Nitrate with 30% (v/v) Tributyl Phosphate in Kerosene in an Pilot Annular Pulsed Disc-and-Doughnut Column—Part I: Hydraulic Performance

The annular pulsed disc-and-doughnut column (APDDC) is an important extraction contactor used in the PUREX process. In this work, hydraulic investigation was carried out in a stainless-steel APDDC, with 30% (v/v) tributyl phosphate (TBP)-kerosene + uranium nitrate + nitric acid + water as the test system. Two operation modes with aqueous phase and organic phase as the continuous fluid were performed separately. The equations, which were previously developed for 30% TBP-kerosene + nitric acid + water system in aqueous phase continuous operation mode, were presently used to predict the holdup, the Sauter drop diameter, the flooding holdup, the flooding throughput, and the operating regimes in both operation modes. Good agreement with the experimental data was observed because the wettability of the dispersed phase on the internals in both the operation modes was similar with that in the aqueous phase continuous operation mode in the previous work. An exception is the flooding properties of the organic continuous operation mode because of a different flooding mechanism. And so new equations were proposed to calculate the flooding properties for the organic continuous operation mode.     

Characterization of hydrodynamic parameters in a multistage column contactor

Accurate 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.