Grenzbelastung von unpulsierten Füllkörper-Kolonnen bei der Flüssig/Flüssig-Extraktion

Determination of the Limit Load of Non-pulsated Packed Columns for Liquid-Liquid Extraction. Mersmann developed a flood point diagram to determine the limit load of different types of extractors. This contains the rising or falling velocity of an individual droplet as the intermediate quantity. In the flood point diagram valid for packed columns, there is a shaded area of possible load limits instead of a loading curve. In this paper statements about the limit of load of non-pulsated packed columns are focussed by giving a limit line for the mass transfer direction C → D and another for that of D → C. Both the limit lines are within the shaded area given by Mersmann. In the modified flood point diagram, a new relationship for the rising or falling velocity of the individual droplets in a packed bed is used as the intermediate quantity. The validity of this has already been proved for gas-liquid systems.     

Begasen höherviskoser Flüssigkeiten

Aeration of relatively viscous liquids . The article considers the flooding point and stirrer performance, as well as mass transfer in agitator vessels and bubble columns, in relatively viscous (aqueous glucose and glycerol solutions as well as millet broth) and non-Newtonian liquids (aqueous CMC and PAA solutions). Numerous experimental results published in the available literature and several obtained by the author provide a basis for these considerations. All the experimental results are presented in the framework of similarity theory. Thus it also proves possible to present the k_La values in sorption characteristics. In order to achieve this aim, a representative viscosity is introduced for non-Newtonian liquids, both in agitator vessels and in bubble columns. The comprehensive and comparative account reflects the current state of the art. Design data and dimensioning criteria are given for certain systems and working ranges. Comparison between agitator vessels and bubble columns on the basis of the design data derived permits predictions to be made regarding the suitability of various articles of equipment. Some of the results have already been reported [1].     

板网填料液泛研究

将用于在管两相流动的Wallis'方程根据规整填料的结构特征加以改进,周末关联规整填料的泛点数据,计算结果与板网填料的泛点实验数据相差很小。将该方程重新整理可以反映规整填料泛点能力参数和流动参数的关系,计算结果与实验值相当。因此该方程大一种比常用的Bian－Hoagen关联式更好的关联规整填料泛点的方法。     

Vermischungseffekte in Flüssig/Flüssig-Extraktionskolonnen

Axial mixing in liquid/liquid countercurrent extractors . Methods are considered for the determination of longitudinal mixing in many countercurrent extractors. It is found that only the single phase mixing of the continuous phase can be accurately determined; with certain reservations this also applies to the disperse phase. In the case of RDC and ARD in particular, correlations are available which permit calculation of Bodenstein numbers for both phases. Such scale-up calculations are not recommended for many extractors. It is very important to establish adequate correlations for calculation of mixing of the disperse phase in order to obtain exact dispersion heights in connection with those of the continuous phase.     

Fortschritte auf dem Gebiet der Flüssig/Flüssig-Extraktion

Progress in liquid/liquid extraction . Liquid/liquid extraction continues to increase in importance. Classical areas of application, such as aromatic chemistry, hydrometallurgy, and reprocessing of spent nuclear fuels are being supplemented by new uses in biotechnology, in pharmaceutical production, and in water treatment. This upswing would be unthinkable without intensified R & D in chemistry and process engineering. Deeper knowledge of the physical chemistry of solvents permits purposeful choice of solvents for special separation tasks, advances in engineering elucidation of flow and mass transfer in proven and in newly designed extractors creates a basis for calculation or at least assessment criteria for the choice and operating behaviour of equipment. Yet much remains to be clarified. This article describes the current state of the art in the analysis of flow and mass transfer in countercurrent gravity feed columns with and without pulsation or agitator energy and in centrifugal extractors. A brief look at new designs demonstrates that scientific knowledge gave birth to the new ideas expressed therein.     

A physical model for the prediction of liquid hold-up in two-phase countercurrent columns

This contribution presents the determination of liquid hold-up in gas/liquid two-phase countercurrent columns filled with random or structured packings. The equations resulting from the established physical relationships are varified against the values for liquid hold-up determined experimentally on 56 different column packings and 16 gas/liquid systems. The experimental and calculated results agree well, with only slight deviations. This also applies to the range between the loading and flooding points for two-phase countercurrent flow.     

Cocurrent vacuum de-aeration of water for injection into oil reservoirs

Conventional vacuum stripping to reduce the oxygen content of injection water for secondary recovery of oil is carried out in packed columns with the released gases and water flowing countercurrently; hence large column diameters (normal liquid load circa 100 m³/(m²h)) and foaming that requires the addition of defoaming agents. Measurements with cocurrent and countercurrent flow at the same flow rate produced practically identical mass transfer coefficients. With flooding excluded in cocurrent flow, columns can be operated with higher liquid loads than in countercurrent flow. In addition, nitrogen released in the top part of the packing is better utilized as stripping gas, and foam is withdrawn as soon as it increases the pressure drop.     

Inertgaseinsatz in Füllkörperkolonnen für die Flüssig/Flüssig-Extraktion

Gas is fed into static countercurrent extractors with the goal of introducing more energy into the binary liquid/liquid system. Increasing the turbulence raises the particle dispersion and thus increases the mass transfer area. This in turn should lead to improved mass transfer. Decision aids for more or less promising use of inert gas in liquid/liquid extraction are presented on the bases of recent publications and the authors' own studies and correlations.     

Untersuchungen zur Betriebscharakteristik pulsierter Füllkörperkolonnen für die Flüssig/Flüssig-Extraktion

Studies on the operating characteristics of pulsed packed-columns for liquid/liquid extraction. The dependence of capacity limit and extraction efficiency on the operating conditions, the properties, and the packing material is illustrated with the aid of experiments on two test mixtures in pilot plant columns. The direction of mass transfer has a large influence on the capacity limit and extraction efficiency. From the practical point of view, the operating characteristics of pulsed packed-columns provide the chemical engineer with reference data for the estimation of column dimensions and rational planning of pilot plant experiments. Methods for the precalculation of capacity limits and extraction efficiencies of pulsed packed-columns are reported in the literature. A comparison of the results obtained by these methods with the experimental data indicates the feasibilities and limits of such a calculation.     

Flüssigphase-Rückvermischung in gepackten Blasensäulen

Liquid phase backmixing in packed bubble columns . Correlations for the axial liquid phase dispersion coefficient in bubble columns packed with metal Raschig rings and Pall rings are given as Pe_g = f (Ga, Re_g, H/D). The dependencies on physical and operational properties are discussed in detail with the aid of diagrams. Pall rings are not able to completely suppress greater turbulences and backmixing in columns of diameters D > 20 cm. A rule of thumb is also given for the apparent dispersion coefficient in this range. Raschig rings, however, are well suited for suppressing backmixing. The problems of adequate fulfilling of the model and undisturbed measurement of the backmixing behaviour are dealt with in detail.     

Flooding in inclined pipes — Effect of entrance section

Flooding in countercurrent flow of air and water in inclined tubes has been investigated. Data on flooding inception in the whole range of inclinations have been collected and predictive models for calculating the flooding conditions as a function of the flow rates and pipe inclination were proposed. Special attention is placed on the effect of the liquid injection mode. It has been shown that the porous injection system causes a local disturbance and enhances flooding at relatively shallow inclinations and high liquid flow rates.     

Flow phenomena and flooding limitations in a phase transition extraction column

The flooding limitations of liquid-liquid countercurrent two-phase flow in a phase transition extraction (PTE) column were investigated both experimentally and theoretically. The solvents used in the PTE process are partially miscible and posses a critical point of miscibility. In the experiments, the effect of the settlers’ inclination on the onset of flooding in non-isothermal PTE column was investigated. It was found that flooding conditions in the column can be postponed by using off-horizontal settlers. This led to the development of a novel structure of a PTE column with improved throughputs.A model is suggested for predicting flooding conditions in the PTE column. It combines a recently developed two-fluid model with improved theory-based closure relations for laminar stratified flow, with a non-isothermal model for evaluating the composition in the mixing section and the internal streams in the column. The results of the combined model favorably predict the experimental flooding flow rates.     

Numerical solution of multicolumn adsorption processes under periodic countercurrent operation

Numerical solutions of the dimensionless equations describing multicolumn adsorption processes are presented. The columns are operated in series and move periodically countercurrent to a continuous fluid flow. Results are given for intraparticle mass transfer resistance separately and in combination with film resistance assuming a linear adsorption isotherm. Comparisons are made with columns in periodic operation and in continuous countercurrent operation. The utilization of the particles in a multicolumn arrangement, or equivalently in a pulsed bed system, compared to that in a column in continuous countercurrent operation is commonly about 80% for two columns in series and more than 90% for four columns in series, when the total column lengths and the fluid flowrates are the same.     

Modellbildung für Mehrphasenströmungen in Reaktoren

Modelling multiphase flow in reactors. This paper is intended to enhance the knowledge of two phase reactors. Their performance depends not only on reaction kinetics but essentially on fluid-dynamically fixed quantities such as volume fractions, dispersion coefficients in the fluid and in the gas phase, as well as the volumetric mass transfer coefficient. These are basically dependent on the slip velocity of the phases and on the internal circulation speed of the fluid. The circulation either results from the fluid passing through the vessel (as in a bubble column) or it has to be effected separately in order to avoid separation. Suspension of solids or aeration of liquids in loop and stirred reactors can serve as examples. Dimensionless groups that characterize the main material and geometric properties can be derived using the slip velocity of the largest stable bubble and a certain circulation parameter. The power of this method will be shown by the characterization of bubble columns and gas-liquid mixers. Simple laws comparable with those applying to bubble columns emerge when approaching the flooding point of a disc turbine in a tank. Power numbers, gas volume fractions, and volumetric mass transfer coefficients can be bundled over the whole spectrum of the non-flooded states, when their characteristic quantities are divided by the quantities relating to the flooding point.     

Hydraulic Calculations for Cross‐channeled Packings in Distillation Unit Based on a Physical Model

A physically based calculation model has been developed in order to describe the liquid and the gas flow in column packings with any arbitrary cross‐channel structure. An equation system is presented which characterizes the film flow on the surface as influenced by the countercurrent flowing gas stream and the respective geometric parameters of the packing. The considered hydraulic operating parameters are the pressure drop, the film thickness, and the radial liquid distribution as a function of the column load up to the flooding point. Care was taken to introduce only constants that can be interpreted physically. Their number was reduced to a minimum of three in order to provide the possibility of easy extrapolation to other packing dimensions. Numerical simulations have been carried out for different liquids assuming a fully wetted packing surface. A distribution width is introduced as the parameter characterizing the radial liquid distribution. Its value together with the respective gas split factors are important variables for the inclusion of maldistribution in the calculation of a distillation column. The numerical simulations up to the flooding point correspond well to the experimental data obtained from a test column.     

Gas/Flüssig-Reaktoren

Gas-liquid reactors . The gas hold-up in bubble columns increases in proportion to the gas flux density in the homogeneous flow regime and rises less than proportionally in the heterogeneous flow regime. Both the gas and the liquid axial dispersion coefficient increase with gas flow. Gas phase dispersion becomes more intensive with increasing liquid viscosity, while liquid dispersion drops slightly. Experimental results for mass transfer in low viscosity liquids show that the two-range turbulence model best fits experimental data. When aerating highly viscous Newtonian and non-Newtonian liquids, mass transfer in the liquid phase is well described by known relations valid for very low bubble-Reynolds number and very high Schmidt number.     

Mathematical simulation of the performance of the solid liquid extractors—I: Diffusion batteries

The extraction in purely countercurrent solid liquid extractors is extensively discussed in literature. In practice, the local mode of phase-contact often deviates from the overall counter-currence as is the case in e.g. belt type extractors and diffusion batteries. In the diffusion battery the local phase contact in each of the columns is, in essence, an instationary column process. However, from an overall point of view the extraction approaches a purely contercurrent process, dependent on the choice of the number of columns in the system and the cycle time. The differential equations describing mass transfer in this extractor were solved by numerical methods. The exit concentrations in cyclic steady state are averaged over one time cycle to obtain the extraction efficiency. Results of the calculations are presented as a correlation between the number of “plug flow” and “true” transfer units. It is shown that deviations from pure countercurrent are small when 4 or more columns are used under normal operating conditions.     

Prediction of Flooding in Packed Liquid-Liquid and High-Pressure Extraction Columns Using a Gaussian Process

Reliable prediction of flooding conditions is needed for sizing and operating packed extraction columns. Due to the complex interplay of physicochemical properties, operational parameters and the packing-specific properties, it is challenging to develop accurate semi-empirical or rigorous models with a high validity range. State of the art models may therefore fail to predict flooding accurately. To overcome this problem, a data-driven model based on Gaussian processes is developed to predict flooding for packed liquid-liquid and high-pressure extraction columns. The optimized Gaussian process for the liquid-liquid extraction column results in an average absolute relative error (AARE) of 15.23 %, whereas the algorithm for the high-pressure extraction column results in an AARE of 13.68 %. Both algorithms can predict flooding curves for different packing geometries and chemical systems precisely.