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.     

Axial Mixing and Mass Transfer Performance of an Annular Pulsed Disc-and-Doughnut Column

The annular pulsed disc-and-doughnut column (APDDC) is an important type of extraction equipment in the PUREX process, which has been used in several commercial reprocessing plants. As there are few mass transfer experimental results reported in the literature, the axial mixing and mass transfer performance of an APDDC was studied for both extraction and stripping processes in the present work. Two parameters in the axial dispersion model (ADM), namely, the axial dispersion coefficient of the continuous phase and the number of mass transfer units, were regressed by correlating ADM with experimental concentration profiles. The influence of flow rate and pulsation intensity on these parameters was also investigated. Models developed for the PDDC were tested for correlation with APDDC experimental data and suitable models and conditions were determined. The height of a mass transfer unit was also calculated, which highlights the impact of axial mixing on mass transfer performance. Moreover, the influence of internal wettability on mass transfer performance was discussed.     

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.     

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.     

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.     

聚并-分散脉冲筛板萃取塔的传质与轴向混合特性

A new configuration of coalescence-dispersed pulsed-sieve-plate extraction column (CDPSEC) was developed, and the mass transfer and axial mixing characteristics were evaluated with the two-point dynamic method.The influence of operation conditions was discussed with experimental results, showing that the mass transfer performance of CDPSEC mainly depends on the energy input and the holdup of dispersed phase. Higher energy input results in higher holdhp of the dispersed phase, the axial mixing of the continuous phase is suppressed, and the true height of mass transfer unit decreases markedly. On the other hand, higher energy input leads to more serious forward mixing of the dispersed phase, so the energy input should be limited. Accordingly the operation conditions were divided into two regions, and empirical correlations for predicting the mass transfer and axial mixing characteristics in different regions with a satisfactory accuracy were suggested.     

Mass Transfer and Axial Mixing Characteristics in a Coalescence-Dispersion Pulsed-Sieve-Plate Extraction Column

A new configuration of coalescence-dispersed pulsed-sieve-plate extraction column (CDPSEC) was developed, and the mass transfer and axial mixing characteristics were evaluated with the two-point dynamic method. The influence of operation conditions was discussed with experimental results, showing that the mass transfer performance of CDPSEC mainly depends on the energy input and the holdup of dispersed phase. Higher energy input results in higher holdup of the dispersed phase, the axial mixing of the continuous phase is suppressed, and the true height of mass transfer unit decreases markedly. On the other hand, higher energy input leads to more serious forward mixing of the dispersed phase, so the energy input should be limited. Accordingly the operation conditions were divided into two regions, and empirical correlations for predicting the mass transfer and axial mixing characteristics in different regions with a satisfactory accuracy were suggested.     

Mixing and solid‐liquid mass transfer characteristics in a three phase pulsed plate column with packed bed of solids in interplate spaces—a novel aerobic immobilized cell bioreactor

BACKGROUND: The pulsed plate column (PPC) with packed bed of solids in the interplate spaces finds use as a three phase aerobic bioreactor and is a potential heterogeneous catalytic reactor. Good knowledge of the extent of mixing in the liquid phase and solid‐liquid mass transfer coefficient are essential for modeling, design and optimization of these columns. The present work aims at the study of liquid phase mixing and solid–liquid mass transfer characteristics in a three phase PPC. RESULTS: Residence time distribution studies were performed. Dispersion number was found to increase with increase in liquid superficial velocities, frequency of pulsation, amplitude of pulsation and the vibrational velocities. Increase in frequency and amplitude of pulsation, and hence increase in vibrational velocity, resulted in increase of the solid–liquid mass transfer coefficient. CONCLUSIONS: The mixing behaviour in this contactor approximated a mixed flow behaviour. The three phase PPC was found to outperform many other kinds of three phase contactors in terms of solid liquid mass transfer characteristics. Empirical correlations developed can be used for the determination of solid–liquid mass transfer coefficients for three phase PPC and hence can facilitate the design, scale‐up and modeling of these columns, when used as chemical or biochemical reactors. Copyright © 2011 Society of Chemical Industry     

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.     

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.     

A dynamic forward mixing model for evaluating the mass transfer performances of an extraction column

It is well known that the droplet behavior of the dispersed phase in extraction equipments has a strong influence on the mass transfer performances. It is, and will continuously be a key project for design and scaling up of extraction columns. In this work, a dynamic mass transfer model, considering the effect of forward mixing led by the drop size distribution and the axial mixing of the continuous phase, has been developed, by which the axial mixing characteristic can be easily evaluated when a stimulus-response dynamic curve is obtained. In order to test the mass transfer model and to study in the effect of droplet coalescence on mass transfer performance, a typical experimental system of 30% tributyl phosphate (in kerosene)-nitric acid-water with interface intension of 0.00995 N/m was chosen to investigate the mass transfer in a coalescence-dispersion pulsed-sieve-plate extraction column (CDPSEC) with 150 mm in diameter. The two-point dynamic method was applied to get the stimulus-response curves. With these results the axial mixing of the CDPSEC were evaluated. The calculated results showed that the response curves could be predicted with the new mass transfer model very well. The model has marked advantages over the traditional diffusion model. It is closer to the practice, easier to solve for the mathematical equations and boundary conditions, and has only one parameter to be optimized. The calculated results also showed that the influence of local coalescence of droplets on mass transfer performances is obvious.     

Extraction of Uranium Nitrate with 30% (v/v) Tributyl Phosphate in Kerosene in a Pilot Annular Pulsed Disc-and-Doughnut Column—Part II: Axial Mixing and Mass Transfer Performance

Mass transfer experiments were carried out in an annular pulsed disc-and-doughnut column (APDDC) using 30% (v/v) TBP-kerosene + uranium nitrate + nitric acid + water system (uranium nitrate system) for both extraction and stripping processes. Parameters in the axial dispersion model (ADM) and plug-flow model (PFM), namely, the axial dispersion coefficient of the continuous phase and the number of mass transfer units, were regressed by correlating the respective model with the experimental concentration profile. The mass transfer coef?cient is calculated, and new correlations are developed to predict the axial mixing coefficient of the continuous phase and the volumetric mass transfer coefficient. The height of a transfer unit is also calculated. The influence of axial mixing on mass transfer performance for the uranium nitrate system is discussed.     

Liquid—liquid extraction column (rotating disc contactor)—model parameters from drop size distribution and solute concentration measurements

The practical application of an extraction column model which takes into account the influence of drop-size distribution (i.e. the ‘forward mixing’ model) is brought forward by the generation, from experimental data, of values of the mass transfer and axial dispersion coefficients required by the model. Values of these coefficients were generated from drop-size distribution and solute concentration profile measurements in a 22 cm diam. rotating disc contactor. The use of the Handlos-Baron drop mass transfer model is justified. The resulting continuous phase transfer coefficients were found to be dependent only on disc speed. Continuous phase axial dispersion coefficients were much higher than tracer-correlation predicted values at higher flows, and larger drop sizes. An explanation for this is presented.     

A New Pulsation Policy in a Disk and Doughnut Pulsed Column Applied to Solid‐Liquid Extraction of Andrographolide from Plants

Classically, sinusoidal oscillations are imposed to enhance mixing and mass transfer between two phases contacted. But, in any case of solid‐liquid contact, it was noticed that this pulsation mode was not efficient enough to allow a controlled behavior of the solid phase. The problem is particularly met during the treatment of raw plants or polydispersed populations with complex physical properties. The objective of this study is to demonstrate the viability of using a nonsinusoidal pulsation in a continuous contactor to replace a traditional batch sinusoidal mode. A review of the different pulsation techniques is firstly presented. The example of solid‐liquid extraction of andrographolide from plants has then been chosen to bring out the advantages of the new pulsation mode. The development of this application as a continuous process in a column has indeed encountered difficulties due to the important heterogeneity of the matter: one of these classes tends to float and the other to sink, which always leads to a definitive flooding in classical operations. Typically, the proposed signal is composed of two different periods: on the one hand, a classical sinusoidal pulsation step used to mix the liquid and solid phases in the active part of the column and allowing an optimal mass transfer and, on the other hand, an impulsion phase, generally used for the transport of solid. The extraction is carried out in a disk and doughnut column of 54 mm in diameter and 3.5 m in height. Liquid and solid are flowing concurrently and downwardly. Experiments have been performed to know the global characteristics of the process in steady state and to suggest some elements for industrial design. The results showed that an optimal tuning of the geometric characteristics of the column, the level of interface and the parameters of the pulsation could increase the operated domain where flooding is avoided.     

Hydrodynamics and mass transfer in a pulsed packed column

The hydrodynamics and mass transfer characteristics of a pulsed packed column (PPC) filled with a stainless steel super mini ring (SMR), ceramic and stainless steel Raschig rings have been studied using a 30% tributyl phosphate‐kerosene (dispersed phase)/acetic acid/water (continuous phase) system. Experiments were performed in a 100 mm internal diameter column with 1.0 m height of packing. The mass transfer and axial mixing parameters were estimated simultaneously from the measured concentration profiles of two‐phase based on the backflow model. It was found that pulsation has great influence on hydrodynamics and mass transfer characteristics of PPC with the SMR. H_oxp and H_ox decrease significantly with pulsation, whereas flooding velocity decreases only slightly. Comparison among the three types of packing showed that the SMR has superior characteristics both in terms of capacity and mass transfer efficiency. The influence of mass transfer on characteristics of PPC was also studied. New empirical equations of characteristic velocity, H_oxand H_oxd were proposed and good agreement between calculated and experimental data was obtained.     

Prediction of mass transfer coefficients in a pulsed disc and doughnut extraction column

A study of the mass transfer performance for a pulsed disc and doughnut extraction column has been presented for a range of operating conditions. The mass transfer performance has been investigated for both directions of mass transfer. This study has examined the mass transfer coefficients which has incorporated the effects of back‐mixing in the continuous phase. The effect of operating variables including pulsation intensity and dispersed and continuous phase velocities on volumetric overall mass transfer coefficient has been investigated. The experiments showed that mixer‐settler, transition and emulsion regimes exist in the column depending on the pulse characteristics. In the present work, effective diffusivity is substituted for molecular diffusivity in the Gröber equation for estimation of overall mass transfer coefficients. The enhancement factor is determined experimentally and there from a single empirical correlation is derived for prediction of enhancement factor in terms of Reynolds number, holdup and Eötvös number for all operating regimes and each mass transfer direction. The experimental results are in very good agreement with the values calculated by the proposed equation. © 2011 Canadian Society for Chemical Engineering     

Rates of extraction of mercaptan sulfur from pentane by caustic solution

The interphase rate process for the extraction of mercaptan sulfur from n-pentane with a 10% caustic solution is examined. The system represents a case of mass transfer with an equilibrium chemical reaction occurring in one phase. Experimental studies were made with both a continuous flow stirred tank reactor (CFSTR) and an annular flow jet reactor (AFJR). The interphase volumetric mass transfer coefficient (K_La) was derived based on Olander's theory. The K_La data for each contactor was found to be a function of the power input per unit volume. The AFJR exhibited higher K_La values and equilibration of mercaptan sulfur was achieved in about three milliseconds.     

Pulsed Disc‐and‐Doughnut Column Performance

Abstract: A study of the hydrodynamic variables, drop size, continuous phase axial dispersion, and mass‐transfer coefficients of a pulsed annular disc‐and‐doughnut liquid extraction column are presented for three different systems. The results indicate that the characteristic velocity plot of Gayler et al. (1953) can be used to describe the variation of holdup with flow rate for a range of pulsation velocities. The existence of several different operating regimes, namely streamline, mixer‐settler, and emulsion regimes, was observed when the input energy was altered. Mass‐transfer data from 72.5 mm i.d. and 2.5 m i.d. columns were interpreted in terms of the differential axial‐dispersion model; the number of transfer units in a unit length of column is proposed as the basis for scale‐up of the mass‐transfer performance. By considering the free areas in the column, a method is proposed for the geometric scale‐up of pulsed disc‐and‐doughnut columns.     

Mass transfer coefficients in a pulsed packed extraction column

The volumetric overall mass transfer coefficients have been measured in a pulsed packed extraction column using diffusion model for the toluene/acetone/water system. The experiments were carried out for both mass transfer directions. The effects of operational variables such as pulsation intensity and dispersed and continuous phases flow rates on volumetric overall mass transfer coefficients have been investigated. The experimental findings indicate that pulsation intensity and mass transfer direction have great influence on volumetric overall mass transfer coefficient. Significant, but weaker, are the effects of continuous and dispersed phase flow rates. The experimental results obtained in the present work are compared with some other types of extraction columns. Finally, two empirical correlations for prediction of the continuous phase overall mass transfer coefficient is derived in terms of Sherwood and Reynolds numbers. Good agreement between prediction and experiments was found for all operating conditions that were investigated.     

Development of Two‐point Dynamic Method for Evaluating Extraction Columns

The steady‐state method by measuring the concentration profile along the column height is an effective way, but it is a time and material consumption method for large extraction columns. In order to investigate the axial‐mixing and mass transfer performances in a large pulsed‐sieve‐plate extraction column with the diameter of 150mm, a two‐point dynamic method with mass transfer based on the diffusion model has been developed. The results proved that the two‐point dynamic method has the advantages of good accuracy, simple boundary equations and flexible sampling position over the traditional single‐point dynamic method. It is a reliable tool for studying the axial‐mixing and the mass transfer performances.