Validation of CFD simulations of a stirred tank using particle image velocimetry data

Methods for validating CFD simulations based on the Reynolds Average Navier-Stokes equation (RANS) against Particle Image Velocimetry (PIV) measurements are investigated and applied to one of the most common problems in the chemical process industry — the prediction of flow field in a stirred vessel. A total of 1024 sequential instantaneous 2D velocity fields along the central axial plane of a stirred vessel with a P-4 axial impeller are obtained through PIV measurement. From the PIV data, the mean velocity, turbulent kinetic energy, Reynolds stresses and dissipation rate fields are extracted. By introducing several tools to quantify the similarities and differences between two-dimensional fields, CFD predictions of the flow field are validated against PIV data. Furthermore, using PIV and LDV data, the effect of boundary conditions on CFD simulation results is examined. The effect of different Reynolds stress closures on the flow prediction is also studied.     

新型转盘萃取塔研究开发与工业应用

采用激光多普勒测速仪(LDV)和计算流体力学(CFD)软件,对转盘萃取塔(RDC)内的单相流流场进行了测量和模拟。发现塔内存在沟流和级间的旋涡流动,级间返混严重,为此发明了一种装有级间转动挡板的新型转盘萃取塔(NRDC)。NRDC与传统的RDC的区别在于安装了设计独特的转动挡板。这些转动挡板安装在2个转盘之间,固定在转动轴上,并与固定环处于同一水平面。LDV测量和CFD模拟结果发现,NRDC可有效抑制沟流和级间旋涡流动。传质实验和流体力学表明,NRDC的传质效率比RDC高20%—40%,而液泛速度大致相当。成功地将NRDC用于引进RDC的扩能改造和新塔的设计中。     

基于均龄理论高效计算萃取塔轴向混合分布

在验证了CFD单相流场模拟的基础上,采用均龄理论计算了中试转盘塔内的轴向混合分布,并将计算结果和理论平均停留时间以及组分输运模型计算值进行对比。结果表明:均龄理论能准确预测转盘塔内的轴向混合信息,且其计算时间只需数十秒,远小于传统组分输运模型所需的两周时间,具有低计算量的特点;同时均龄理论克服了传统组分输运模型无法模拟轴向混合空间分布的缺陷,为萃取塔内部结构优化提供了更多信息,是一种高效的模拟方法。后续均龄理论模拟结果的分析预示着转盘塔内的流动近似呈现出级内全混、级间平推的特点,符合萃取操作的需求;而相对于转盘间良好的混合作用,静环间存在明显的流动死区,造成一定的非理想性,其结构有待于进一步的优化。     

Dispersed-phase holdup and mass transfer in a rotating disc contactor with perforated skirts

Dispersed-phase holdup and extraction efficiency were measured in a pilot-plant scale rotating disc contactor (RDC) with perforated skirts (RDCS) with kerosene-o-cresol-water system. The data for the RDCS were examined using the data in this work and the available correlations proposed for the plain RDC. A comparison between the data measured in the RDCS and those in the RDC indicated that the RDCS has rather better extraction efficiency compared with the RDC.     

Hydrodynamic and mass transfer parameter correlations for the rotating disc contactor

The normal, fragmented manner in which various model hydrodynamic and mass transfer parameters are measured and correlated separately, in the vain hope that accurate predictions of extraction column performance will be eventually possible, is avoided here. The ‘forward mixing’ model parameters are all determined simultaneously, in experiments with two sizes of rotating disc contactor where all required measurements, including drop size distributions and continuous phase profile compositions, are made during solute transfer between phases. Accurate predictions from the derived correlations of dispersed phase hold-up fraction, drop size distributions and extraction efficiency are the result.     

Investigation on local and average static hold‐ups in liquid–liquid systems in a rotary disc contactor

A rotating disc contactor with inner diameter of 91 mm and 21 compartments was used to investigate local and average static hold‐ups in the column. Three chemical systems, including distilled water as stagnant continuous phase and toluene, kerosene, and butyl acetate as dispersed phase, in case of no mass transfer, were applied. Different parameters consisting of drop size and static hold‐up were measured experimentally under various operating conditions, and two dimensionless correlations for estimation of local static hold‐up as a function of average mother drop size, physical properties, rotor speed, and stage number were proposed in two cases: (1) immovable and (2) rotating rotor discs. Furthermore, two dimensionless correlations were presented to estimate average static hold‐up as a function of average mother drop size, physical properties, rotor speed, and number of stages in the column in the two aforementioned conditions.     

Drop size distribution in a graesser contactor

An investigation was conducted of drop size distribution in a Graesser contactor, employing five liquid – liquid systems, viz., kerosene/water, benzene/water, xylene/water, hexane/water and n–butyl acetate/water. A 100 mm (4 inch) diameter Graesser contactor was used for this purpose It was found that the drop size distribution in a Graesser contactor obeys the upper – limit distribution expressed as: where A correlation was developed relating the Sauter mean diameter (d₃₂) to other effective groups      

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.     

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.     

The interaction of solute transfer,contaminants and drop break-up in rotating disc contactors: Part I. Correlation of drop breakage probabilities

The breakage process of single drops in RDC liquid-liquid extraction columns has been investigated. The breakage probability and daughter drop size distribution were the measured characteristics. Binary systems, non-equilibrated ternary systems with mass transfer in both directions (c → d and d → c) and systems with surface active agent added were used in the experiments. A model of the breakage probability was developed based on a modified Weber number, taking into account the applied shearing stress and the resisting interfacial tension force. It is shown that breakage probability can be estimated if interfacial tension is known as a function of interfacial conditions.     

基于粒子图像测速技术的流浆箱阶梯扩散器性能试验

采用粒子图像测速(PIV)系统对两种出口段型式阶梯扩散器的内流场进行了测试,得到了多种流量工况下阶梯扩散器轴向水平截面的速度分布、阻力损失及阶扩处空化等情况。测试结果表明,入口浆速为8～24m.s-1时,出口段横向速度分布在长径比为3～5时达到均匀,随后圆管出口段横向速度分布再由均匀发展到不均匀,而方管出口段的横向速度在达到均布后,后续流动没有明显不均匀的横向速度分布。入口浆速越大,对纤维的解絮越有利,但随着入口浆速的增大,流动阻力急剧增加,阶扩处空化区域增大,故在纤维充分分散的前提下,入口浆速以不超过空化的临界浆速为宜。研究表明,方管出口阶梯扩散器更适用于高速纸机,适当高的浆速和合适的形状尺寸是获得定量均匀、匀度好纸张的保证。     

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.     

Use of axial dispersion model for determination of Sherwood number and mass transfer coefficients in a perforated rotating disc contactor

The mass transfer process in a perforated rotating disk contactor (PRDC) using a toluene-acetone-water system was investigated.The volumetric overall mass transfer coefficients are calculated in a PRDC column.Both mass transfer directions are considered in experiments.The influences of operating variables containing agitation rate,dispersed and continuous phase flow rates and mass transfer in the extraction column are studied.According to obtained results,mass transfer is significantly dependent on agitation rate,while the dispersed and continuous phase flow rates have a minor effect on mass transfer in the extraction column.Furthermore,a novel empirical correlation is developed for prediction of overall continuous phase Sherwood number based on dispersed phase holdup,Reynolds number and mass transfer direction.There has been great agreement between experimental data and predicted values using a proposed correlation for all operating conditions.     

Deacidification of corn oil by solvent extraction in a perforated rotating disc column

The deacidification of corn oil by continuous liquid-liquid extraction was investigated in a rotating disc column. The solvent was ethanol containing approximately 6% water. The influence of rotor speed, oil phase flow, and column geometry upon the dispersed phase holdup and the mass transfer efficiency was studied. The dispersed phase holdup increased with the increase of rotor speed and oil phase flow. Pratt's equation was used for calculating the characteristic velocity. An inverse relation was observed between the characteristic velocity and rotor speed, which is different from data previously reported in the literature. The estimated volumetric mass transfer coefficients increased as rotor speed and oil phase flow increased. The experimental results proved that it is feasible to obtain a refined oil with an oleic acid content less than 0.3 wt% by continuous solvent extraction. They also indicated that the corresponding loss of neutral oil was less than 5 wt%. Such value for the loss of neutral oil is significantly lower than the results reported in the literature for alkali or physical refining of corn oil.     

运用粒子图像测速仪研究双层桨搅拌槽内流体流动

The flow fields in a dual Rushton impeller stirred tank with diameter of 0.48 m （T） were measured by using Particle Image Velocimetry （PIV）. Three different size impellers were used in the experiments with diameters of D = 0.33T, 0.40T and 0.50T, respectively. The multi-block and 360° ensemble-averaged approaches were used to measure the radial and axial angle-resolved velocity distributions. Three typical flow patterns, named, merging flow, parallel flow and diverging flow, were obtained by changing the clearance of the bottom impeller above the tank base （C1） and the spacing between the two impellers （C2）. The results show that while C1 is equal to D, the parallel flow occurs as C2≥0.40T, C2≥0.38T and C2≥0.32T and the merging flow occurs as C2≤0.38T, C2≤0.36T and C2≤0.27T for the impellers with diameter of D=0.33T, 0.40T and 0.50T, respectively. When C2 is equal to D, the diverging flow occurs in the value of C1≤0.15T for all three impellers. The flow numbers of these impellers were calculated for the parallel flow. Trailing vortices generated by the lower impeller for the diverging flow were shown by the 10° angle-resolved velocity measurements. The peak value of turbulence kinetic energy （ k/V^2tip = 0.12-0.15 or above） appears along the center of the impeller discharging stream.     

运用粒子图像测速仪研究双层Rushton桨搅拌槽内湍流特性

Particle Image Velocimetry （PIV） has been used to investigate turbulence characteristics in a 0.48 m diameter stirred vessel filled to a liquid height （ H = 1.4T ） of 0.67 m. The agitator had dual Rushton impellers of 0.19 m diameter （ D = 0.4T ）. The developed flow patterns depend on the clearance of the lower impeller above the base of the vessel, the spacing between the two impellers, and the submergence of the upper impeller below the liq- uid surface. Their combinations can generate three basic flow patterns, named, parallel, merging and diverging flows. The results of velocity measurement show that the flow characteristics in the impeller jet flow region changes very little for different positions. Average velocity, trailing vortices and shear strain rate distributions for three flow patterns were measured by using PIV technique. The characteristics of trailing vortex and its trajectory were described in detail for those three flow patterns.
Since the space-resolution of PIV can only reach the sub-grid rather than the Kolmogorov scale, a large-eddy PIV analysis has been used to estimate the distribution of the turbulent kinetic energy dissipation. Comparison of the distributions of turbulent kinetic energy and dissipation rate in merging flow shows that the highest turbulent kinetic energy and dissipation are both located in the vortex regions, but the maxima are at somewhat different lo- cations behind the blade. About 37% of the total energy is dissipated in dual impeller jet flow regions. The obtained distribution of shear strain rate for merging flow is similar to that of turbulence dissipation, with the shear strain rate around the trailing vortices much higher than in other areas.     

CFD simulations of single-phase flow in pulsed disc and doughnut columns: Axial dispersion and pressure drop

CFD simulations of pulsed disc and doughnut columns are performed to understand the effects of operating and geometric parameters on axial dispersion and pressure drop in single-phase flow. CFD simulations have been carried out using a two-step approach. In the first step, the flow field is obtained by solving the continuity and the momentum equations along with the equations of the standard k–ε model of turbulence. In the second step, the species transport equation is additionally solved to obtain the residence time distribution and hence the Peclet number and axial dispersion coefficient. The computational approach is validated by comparing its predictions with the experimental data reported in the literature and then used for detailed parametric analysis.     

Large eddy simulations and stereoscopic particle image velocimetry measurements in a scraped heat exchanger crystallizer geometry

The transport phenomena in scraped heat exchanger (HE) crystallizers are critical for the process performance. Fluid flow and turbulence close to the HE surface as generated by stirring elements and scraper blades are crucial in this respect as they aim at avoiding an insulating scale layer on the HE surface. For this reason we performed large-eddy simulations of the turbulent flow (at a Reynolds number of 5×10⁴) in a typical cooling crystallizer geometry with a focus on the bottom region where the heat exchanging surface was located. The flow simulations were validated with stereoscopic PIV experiments performed higher up in the crystallizer. Water at a constant temperature was the working fluid in the experiments as well as in the simulations. For reasons of optical accessibility being hindered by the scrapers, the experiments could not be done near the heat exchanging surface. The flow structures as revealed by the large-eddy simulations could explain the local occurrence of scaling on an evenly cooled HE surface, and its irreproducibility caused by instantaneous cold spots.     

用CFD研究气升式内环流生物反应器下降管中的流体力学性质

在对气升式内环流生物反应器内部流动分析基础上,全面考虑反应器下降管中气泡的并聚破碎、气液两相间相互作用和滑移等, 建立了能描述反应器下降管中复杂流动的CFD数学模型. 运用CFX-4.4对模型方程进行求解, 通过求解得到了包括气液两相速度场、局部气含率分布等详尽信息,并就液相流动速度与相应条件下的PIV测试结果进行了比较,主体流动速度的偏差在20%以下,且两者总的变化趋势一致.该模型能较好地预测反应器下降管内的复杂流场.     

Dearomatization of pyrolysis gasoline with an ionic liquid mixture: Experimental study and process simulation

The pyrolysis gasoline is the main source of benzene, toluene, and xylenes. The dearomatization of this stream is currently performed by liquid – liquid extraction using sulfolane. However, the sulfolane process has high operating costs that could be minimized by employing ionic liquids as solvents because of their non‐volatile character. In this work, we proposed a novel process to perform the dearomatization of pyrolysis gasoline using a binary mixture of 1‐ethyl‐3‐methylimidazolium tricyanomethanide ([emim][TCM]) and 1‐ethyl‐4‐methylpyridinium bis(trifluoromethylsulfonyl)imide ([4empy][Tf₂N]) ILs. The composition in the IL mixture was optimized considering their extractive and thermophysical properties. The Kremser method was applied using the experimental data to determine the number of equilibrium stages in the liquid – liquid extractor which provides the same extraction yields of aromatics using the IL mixture that those of the sulfolane process. The recovery section was designed and simulated from the experimental vapor – liquid equilibrium between the hydrocarbons and the IL mixture. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4054–4065, 2017