Drop formation mass transfer coefficients in extraction columns

Mass transfer coefficients (MTC) of single liquid drops during drop formation period, in the presence and absence of down flow of the continuous phase, were measured in an extraction column. The effects of formation time, needle size, and flow rates of the continuous and dispersed phase were evaluated experimentally. It was found that the drop size increases with increasing formation time and decreasing down flow of the continuous phase. The mass transfer coefficients are the largest in the initial stages of drop formation when convection is the most significant. Both flow rates have a significant effect on the rate of the mass transfer, and the convection caused by the dispersed phase flow is more important than the continuous phase. The mass transfer coefficient and the degree of extraction increase with increasing down flow rate of the continuous phase.     

The Interaction of solute transfer,contaminants and dro break-up in rotating disc contactors: Part II. The coupling of the mass transfer and breakage processes via interfacial tension

In order to investigate the influence of solute transfer and of surface active agents on the drop breakage process in liquih liquid extraction columns, their effect on the interfacial tension has to be studied in detail. The difficulty encountered is that the interfacial tension during solute transfer continuously changes and that no simple apparatus is commercially available which can measure these varying interfacial tension values. An attempt has been made here to theoretically predict them. The equations developed to predict the interfacial tension variation can be combined with a model for the breakage process and hence drop size distributions can be calculated from stage to stage. Applying a new combined film mass transfer coefficient model which takes into account the effect of contaminants, single drop extraction performance has been calculated for simplified conditions of constant bulk concentration in the continuous phase. Calculated efficiencies have been compared with experimental data and a good simulation of contaminant effects and dependency on drop size has been found. The calculations were restricted to low dispersed phase hold-up values, so that coalescence effects could be ignored. This work provides the required support for a procedure to be applied to counter-current flow extraction columns.     

Experimental investigation and numerical simulation of Marangoni effect induced by mass transfer during drop formation

Marangoni effect induced by interphase mass transfer plays an important role in liquid–liquid extraction and reaction processes. The interaction of Marangoni effect and interphase mass transfer during drop formation at different injection rates and different initial solute concentrations was investigated by experimental and numerical simulation. The extraction fraction was measured and the corresponding correlation was proposed. The level‐set method coupled with mass‐transfer equation is for the first time used to simulate the mass‐transfer induced Marangoni effect during drop formation. The simulated drop volume, shape, and extraction fraction are in good accordance with experimental data. Through the numerical simulation, it is found that the mass transfer in the first mass‐transfer period is the most efficient during drop formation when Marangoni convection occurs. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4424–4439, 2013     

A state-of-the-art review on single drop study in liquid-liquid extraction: Experiments and simulations

The experimental and numerical investigations of single drop in liquid/liquid extraction system have been reviewed with particular focus on experimental techniques and computational fluid dynamic simulation approaches. Comprehensive surveys of available experimental techniques and numerical approaches for single drop rising and falling were given. Subsequently, single drop mass transfer was also reviewed both experimentally and numerically. Additionally, single drop breakage and coalescence process and the influencing factors were summarized and compared, so as to establish sub-models for population balance model. Future directions on single drop mass transfer, drop breakage and coalescence were suggested. It is believed that the single drop is a powerful tool to assist extraction process design from lab-scale to pilot-scale.     

Computational techniques for liquid-liquid extraction column model parameter estimation,using the forward mixing model

Here is presented the first step toward the practical application of a model of liquid-liquid extraction column performance which includes the influence of drop size distribution, or of ‘forward mixing’. The theory, previously developed and described, has been used successfully to obtain model parameter values from experimental extraction data, including drop size distributions and solute concentration profiles. The presence of a significant settling zone height complicates the theory and poses difficulties. These were overcome by the reduction of the settling zone height to an insignificant level. Values of the continuous phase mass transfer, and axial dispersion, coefficients for an assumed (Handlos-Baron) drop-side model are reported. The overall mass transfer coefficients are confirmed to increase with drop size.     

液滴凝并端效应对单液滴传质测定的影响及消除

For the mass transfer to single drops during the stage of steady buoyancy-driven motion, experimental measurement is complicated with the terminal effect of additional mass transfer during drop formation and coalescence at the drop collector. Analysis reveals that consistent operating conditions and experimental procedure are of critical significance for minimizing the terminal effect of drop coalescence on the accuracy of mass transfer The novel design of a totally-closed extraction column is proposed for this purpose, which guarantees that the volumetric rate of drop phase injection is exactly equal to that of withdrawal of drops. Tests in two extraction systems demonstrate that the experimental repeatability is improved greatly and the terminal effect of mass transfer during drop coalescence is brought well under control.     

Experimental investigation and numerical simulation of mass transfer during drop formation

The solvent extraction features mass transfer between drops and the surrounding immiscible liquid. Previous study indicated that drop formation plays an important role in extraction because 10–50% of total mass transfer occurs in this stage. It is necessary to have thorough understanding about the mechanism of mass transfer between the drop phase and continuous phase during drop formation. In this work, the level set approach was adopted to capture the interface, and unsteady mass transfer during drop formation was formulated and numerically simulated in an axisymmetric cylindrical coordinate system by solving the fluid motion coupled with mass transfer equations. Drop formation time and mass transfer parameters from the numerical simulation were compared with experimental data in the MIBK–acetic acid–water solvent extraction system. The numerical predictions were found in good agreement with the experimental measurements.     

Drop-Size Distribution and Dispersed Phase Hold-up in a Large Rotating Disc Contactor

Abstract

The effect of drop size and size distribution and the dispersed-phase hold-up on the performance of an extraction column are the most important hydrodynamic characteristics, because under steady operating conditions, drop size and hold-up are proportional to the interfacial area. Correspondingly, the efficiency of mass transfer is a function of drop size as well as hold-up.

A number of experimental investigations using the rotating disc contactor (RDC) have reported the measurement of dispersed-phase hold-up, drop size, and size distribution. However, most of the published data are for very small RDCs of <7.5-cm diameter. All the correlations introduced to describe the column hydrodynamics give unreliable results when applied to large-scale RDC operation and with different systems to those studied. Therefore, RDC hydrodynamics in the absence of mass transfer have been studied on a 450-mm-diam column, 4.3-m high, and the results obtained have been compared with those reported previously from small extraction columns. Wide divergences have been found. The results of this study have been correlated to predict the drop size in each compartment. Agreement has been within 10% of the experimental data. When the results of this and previous studies are analyzed together by including the column dimensions, the agreement between predicted and experimental results is generally within 15%.     

Numerical simulation of unsteady mass transfer by the level set method

Unsteady mass transfer to/from a single drop in the continuous phase is formulated and numerically simulated in a moving reference coordinate system by solving the motion and mass transfer equations of an accelerating drop coupled with a level set equation for capturing the interface. Numerical simulation demonstrates the evolution of mass transfer rate and average drop concentration. Numerical simulation of the flow field and the concentration field simultaneously in each time step is compared with experimental data on single drop motion and mass transfer in two typical solvent extraction systems. The numerical predictions are found in good accord with the experimental measurements. The present numerical procedure in which the flow field is solved in a coupled way with the concentration field gives more accurate prediction than the previous decoupling algorithm by the authors.     

振动筛板槽中液滴分散动力学的研究(Ⅰ)实验研究及理论分析

本文通过实验观察发现,液滴的破碎只有在液滴与振动筛板孔口发生碰撞时才发生.基于这个实验现象,建立了振动筛板槽内液滴破碎的新模型.把筛板孔口附近的剪应力作为破碎力,破碎速率可表示为:G(d)=C_12Af/H(?)~n_4(d/d_h)~n_5[1-(d_(cr)/d)~(2n_1+1)]~0.5n(d)液滴的凝聚可以按气体分子碰撞过程来处理.凝聚速率可以用下式表示:ω(d_1,d_2)=C_Ⅱ(d_1+d_2)~(7/3)∈~（1/3)[β_d∈~(2/3)d_1+d_2/σ(d_1+d_2）~（1/3）]~n_6n(d_1)n(d_2)     

A new method for on-line controller tuning

A new method for tuning controllers on-line has been developed based on a single experimental test, a step change in controller set point. The set-point response data and analytical formulae are used to calculate model parameters for a first-order plus time delay transfer function. Controller settings can then be calculated using the model parameters and standard controller tuning relations. Simulation results demonstrate that the new method provides good initial values for PID controller settings despite gross modeling errors and unanticipated load disturbances that may occur during the experimental test.     

High flux mass transfer: Comparison of liquid-liquid extraction column and drop (Handlos-Baron) model predictions with rotating disc contactor performance

High flux mass transfer measurements have been made in a rotating disc contactor and the results compared with model predicted results. Extraction column and drop model equations for single solute transfer were modified to include the influence of the interphase convective, or drift, flux, previously neglected. The Handlos-Baron drop model was utilized and found to predict the correct trends with changes in drop size. Continuous phase axial dispersion measurements were carried out by pulse tracer injection and by concentration profile measurements at low flux, when simultaneous measurements were made of the continuous phase mass transfer coefficient. When these values were used in the high flux model, high flux extraction efficiencies were accurately predicted but concentration profiles were not.     

Hydrodynamics,axial mixing and mass transfer in open turbine rotating disc contactor (OTRDC)

An experimental study of hydrodynamics, axial mixing and mass transfer has been carried out in a newly developed liquid-liquid extraction contactor, namely the open turbine rotating disc contactor (OTRDC). It has been established that the OTRDC can be operated with larger holdups of the dispersed phase, larger interfaces and, hence, more efficient mass transfer than the conventional RDC. In correlating axial mixing data, a combined model has been applied in which both the forward mixing due to drop size distribution and the backmixing of droplets are taken into account. The RTD curves of dispersed phase predicted by the model agree well with the experimental data. Comparison of experimental mass transfer data with those predicted by the proposed axial mixing model and the theoretical single drop model shows that they are in good agreement.     

An improved dynamic combined model for evaluating the mass transfer performances in extraction columns

Dispersed phase droplet behavior research is very important for the design and scaling up of extraction columns. Recently, the droplet velocities at high holdup were found to be uniform, which means the conventional concept of forward mixing needs correction. The drop size distribution only influences the mass transfer coefficients and not the residence time distribution of droplets. In this work, an improved dynamic combined model considering the influence of drop size distribution has been developed, by which the axial mixing can be easily evaluated using a one-dimension search. A typical experimental system of 30% tributyl phosphate (TBP) (in kerosene)-nitric acid-water with interfacial tension of 0.00995 N/m was used to investigate the mass transfer performances in a coalescence-dispersion pulsed-sieve-plate extraction column (CDPSEC) with 150 mm in diameter. The two-point dynamic method was used to obtain the stimulus-response curves. With these results, the axial mixing in the CDPSEC was evaluated. The calculated results showed that the response curves could be predicted by the dynamic combined model with a deviation less than 0.001. This model has marked advantages over previous models in literature because of its accuracy, simple boundary conditions, and single parameter optimization.     

Filtration of solid and liquid aerosol mixtures: Pressure drop evolution and influence of solid/liquid ratio

Dust and droplet filtration is an essential process step in many industries. Although many studies have been dedicated to the mechanisms of dust filtration, little is known about the behaviour of an air filter when challenged with solid and liquid particles at the same time. This research article answers questions about the pressure drop during the filtration of a solid/liquid particle mixture, as well as on the influence of the respective solid and liquid particle concentrations.     

改进的液膜传递模型

本文提出了一种新的用来描述乳状液型的液膜,Ⅰ型促进传递分离体系的数学模型。引入了内相对流和分离过程中乳液平均滴径随时间变化的概念。提出了用两阶段模型来描述实际液膜分离体系的观点。即:在前一阶段应该考虑内相对流和乳液滴径变化,而后一阶段则可不考虑上述二因素的渐近前沿模型来描述。     

Drop formation in non-newtonian liquids under pulsed conditions

Drop formation from single nozzles under pulsed flow conditions in non-Newtonian fluids following the power law model has been studied. An existing model has been modified to explain the experimental data. The flow conditions employed correspond to the mixer—settler type of operation in pulsed sieve-plate extraction columns. The modified model predicts the drop sizes satisfactorily. It has been found that consideration of non-Newtonian behaviour is important at low pulse intensities and its significance decreases with increasing intensity of pulsation. Further, the proposed model for single orifices has been tested to predict the sizes of drops formed from a sieve-plate distributor having four holes, and has been found to predict the sizes fairly well in the absence of coalescence.     

Model Development of the Transient Solid Hold‐up and Pressure Drop in Fabric Filters

There are different non‐idealities related to cloth filters, which are cleaned by jet pulses. The well‐known phenomenon of cake compaction causes progressive curves of the pressure drop versus time. Some experiments show degressive shapes of the pressure drop, though. It will be shown, that this behavior can be explained by dividing the filter in segments. Each model segment can have a different cake thickness, whereas the pressure drop of every single segment is the same at any time during filtration. The capacity of this model is pointed out to determine the cake load on existing filters and to simulate unknown operating points.     

Effects of pressure drop in a PSA process

The effects of pressure drop on the dynamics of fixed-beds were theoretically studied. The system used was an H₂/CO mixture (70 vol% H₂, 30 vol% CO) in zeolite 5A. The pressure drop at the pressurization step affected the breakthrough time at an adsorption step in the PSA process. As a result, the combined effects of pressure drop during adsorption and pressurization steps led to earlier breakthrough compared to the case without a pressure drop. The effect of pressure drop at the adsorption step under the non-isothermal condition was slightly larger than that under the adiabatic condition. In the case of pressurization and blowdown steps with large pressure drop, the flow pattern near the open end during a short period of time had to be explained by the Ergun equation instead of Darcy’s law. However, there was only a slight difference in the results of a multi-bed PSA process depending on whether or not the pressure drops at the pressurization/depressurization steps as well as at the adsorption step were considered.     

Design of a two‐step pulsed pressure‐swing adsorption‐based oxygen concentrator

A two‐step pulsed pressure‐swing adsorption (PPSA) process has been modeled to assess the extent to which an oxygen concentrator might be miniaturized for medical applications. The process consists of a single bed of packed adsorbent particles that is alternately pressurized and depressurized at the feed end. An enriched oxygen product is withdrawn at ambient pressure from the product end when the bed is pressurized at the feed end. The product end remains closed during depressurization. The model development addresses the manner in which axial dispersion enters into the describing equations and the formulation of proper boundary conditions, both of which have not been handled rigorously in some prior modeling studies. The describing equations are solved using COMSOL® Multiphysics software. The effect on the performance of the adsorption time, desorption time, bed length, particle diameter, and imposed pressure drop across the bed have been investigated. An interesting novel result is that for a chosen particle size, bed length, and applied pressure drop, there is an optimum combination of adsorption and desorption times that maximizes the product purity. The results suggest that there are operating windows for both 5A and partially Ag‐exchanged Li‐substituted 13X zeolite adsorbents wherein the product oxygen purity is greater than 90%. At a given product flow rate within this operating window, the extent of miniaturization is limited by the (maximum) cycling frequency that is practically achievable. Sizing of an oxygen concentrator for personal medical applications is also discussed. A principal conclusion is that a compact oxygen concentrator capable of producing a highly oxygen‐enriched product is possible using commercially available adsorbents and implementable operating conditions. © 2009 American Institute of Chemical Engineers AIChE J, 2010