New predictive correlation for mass transfer coefficient in structured packed extraction columns

Developments in the area of packed columns, particularly structured packed columns, are ongoing, specifically in the area of liquid–liquid extractions in different industries. In the present study, mass transfer coefficients have been obtained experimentally in a structured packed extraction column to develop a new correlation for prediction of continuous phase Sherwood number. The experiments were carried out for toluene/acetic acid/water and n-butyl acetate/acetic acid/water systems with counter current flow in different heights of column. A new dimensionless parameter, d₃₂/h, is introduced in proposed equation. This number considers the effect of column height (h) and mean drop diameter (d₃₂) jointly. The main advantage of this approach is that the principal effect of column height is considered in correlation without which the experimental data could not be fitted with a acceptable accuracy.     

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.     

Modelling of liquid-liquid extraction columns: Use of published model correlations in design

Design of several liquid-liquid extraction columns — packed, pulsed-packed, pulsed-plate, Oldshue-Rushton columns and the rotating disc contactor — was attempted utilizing available correlations for drop size, holdup of dispersed phase, flooding velocities, mass transfer coefficients and axial mixing coefficients. Correlations in many cases were vaguely defined and often based on very limited data. Results indicated that for given flow rates and extraction efficiency, the height of a packed or Oldshue-Rushton column must be considerably greater than the predicted minimum heights of the other three columns, which were comparable considering the limited data utilized in the developed correlations. A critical evaluation of the correlations should be carried out to guide the further experimental effort required to confirm the utility of the axial dispersion model in liquid-liquid extraction column design. Extension of the theory to include drop size variation is highly desirable.     

规整填料塔多元蒸馏过程混合型模型的模拟

填料塔精馏过程的模拟设计大多采用平衡级模型,但由于单一板效率值的难以确定,很多场合模拟很不成功;而新提出的非平衡级模型认为塔内传质传热均处于非平衡状态,由于方程数和经验性参数多,模型求解非常困难。文章针对规整填料的特点,建立了规整填料塔蒸馏过程的一种混合型模型,模型的主要特征是认为气液二相传质处于不平衡状态,而传热处于平衡状态。模型建立在实际填料基础上,既舍去传统的平衡级模型不确定性,又省略了非平衡级模型中复杂的经验性传热系数和液相传质系数的计算。模型计算值和实验值符合较好,也证实混合型模型既反映实际,又使模型求解变得相对容易。     

The Influence of a Non-Uniform Distribution of the Liquid Phase on the Kinetics of Gase Side Mass Transfer in Absorption Packed Columns

In this paper is reported an experimental study, which aimed at quantifying the decrease in the packed tube columns efficiency when the liquid phase is distributed non-uniformly above the bed. The experiments were run in one-tube and multi-tube columns packed with stacked metallic 50 mm Pall rings, using the absorption system ammonia-air/0.5-1 N aqueous solution of H₂SO₄. For the calculation of the mass transfer coefficients in packed tube columns with a non-uniform distribution of the liquid phase a method was developed which enables a good approximation of the experimental results to be achieved.     

Modelling of mass transfer phenomena of associated systems in packed distillation columns

Mass transfer between liquid and vapour phases in packed distillation columns has been investigated by the two-film theory for binary associated systems. The effects of association reaction in systems with the association of one component in both phases, taking into account a concentration-dependent liquid-phase association constant, were examined. The mass transfer coefficients are modelled, taking into account the association constant. The experiments were carried out at atmospheric pressure with acetic acid-benzene and acetic acid-toluene systems in a column packed with Rasching rings.     

CONTRIBUTION TO PACKED COLUMNS SCALE-UP IN SEPARATION PROCESSES

Correlations will be presented to describe liquid hold up, loading and flooding capacity, volumetric mass transfer coefficients on liquid and gas or vapour phase and the pressure drop as a basis for computing packed columns, dumped and regular as well, including the influence of end effects on scaling. The relationships are found on a sound physical and mathematical basis and their validity were confirmed by experimental investigations of many packings; some of them were selected to be considered in this work.     

EFFICIENCY CONSIDERATIONS FOR CHOICE BETWEEN TRAYS AND PACKINGS

Mass transfer efficiency models for plate and packed columns in distillation applications were examined. The influence of operating conditions, hardware parameters and physical properties on the height equivalent of a theoretical plate (HETP) were investigated. Recommendations on the choice of packings or trays were developed for different cases to maximize column efficiency. Some inconsistencies in the efficiency models for plate and packed columns reported in the literature were also discussed.     

现代填料塔技术（四）填料塔的流体力学和传质（下）

从工程应用角度介绍填料塔传质发展概况２，重点归纳填料怪的有效面积和传质单元高度或传质系数研究的重要结论，并作扼要的分析和讨论；提出填料塔设计中确定安全系数所应的一些问题。     

Experimental investigation on the mean drop size and drop size distribution in an L-shaped pulsed sieve-plate column

Vertical pulsed extraction columns cannot be employed in applications with height limitations. On the other hand, the horizontal extraction columns have low throughput, which affects their applicability in industrial applications. Therefore, there is a need to design a new type of extractors for such circumstances. In this paper, an experimental study on drop sizes has been implemented in a novel L-shaped pulsed sieve-plate extraction column in the absence and presence of mass transfer. Moreover, new correlations are developed for prediction of the mean drop size and size distribution using the log-normal probability density function.     

Effective diffusivity in a structured packed column: Experimental and Sherwood number correlating study

Mass transfer coefficients along a structured packed column were experimentally determined to obtain a new correlation for dispersed phase Sherwood number based on molecular diffusivity. Then in a comparative investigation, the correlation was re-established based on effective diffusivity. The applied chemical systems were toluene/acetic acid/water (T/A/W) and butyl acetate/acetic acid/water (B/A/W). The effects of droplet size and packing height on experimental Sherwood number were also discussed. It was shown that local Sherwood number could be increased up to 188% with increasing the droplet size from 6 to 9 mm in fixed dispersed phase flow rate. It was also observed that when height of packing increased from 10 to 40 cm, local Sherwood number decreased by almost 48% for constant dispersed phase flow rate. The results have shown that the proposed correlation based on effective diffusivity can estimate the experimental drop Sherwood number with high accuracy (error of less than 5%). Moreover, current research shows that replacing molecular with effective diffusivity in some theoretical models can correct their estimation.     

Estimation of the efficiency of packed mass-transfer devices

The problems of assessing the effectiveness of packed mass-transfer devices in relation to distillation processes have been considered. The authors have shown that the existing dependences of determining the height equivalent to a theoretical plate cannot be used to calculate tray columns and columns with vortex packings, the contact surfaces of the phases and coefficients of mass transfer of which depend substantially on the water concentration (reflux ratio) and gas rate (vapor for distillation).     

Mass transfer in packed columns: The cylinder model

In order to predict mass transfer in packed columns, it is necessary to know the interfacial area. The well-known and often cited equation (Onda, Kolev, Zech, etc.) do not yield precise values for the mass transfer coefficient β_L a_e or the interfacial area a_e, especially for modern packing elements. A new model will be presented which takes into account the structure of packing (cylinder model) as well as the structure of the liquid hold-up. This model allows the separation of liquid flow through packings into its main constituents, i.e. rivulets and drops (freely falling). The determination of the structure of liquid flow allows the prediction of effective interfacial areas as well as of volumetric mass transfer coefficients for irrigated large packing elements manufactured from plastics, metals or ceramic. The applicability to novel future packings is one of the advantages of the new model.     

Prediction of Concentration Profiles in an L‐Shaped Pulsed Extraction Column

The L‐shaped extraction pulsed plate column is believed to be able to perform under operating conditions between those of the vertical and the horizontal pulsed plate columns. It has an extraction efficiency similar to the vertical pulsed plate column. Here, the mass transfer performance of this novel column type was investigated and the application of three different models, i.e., the plug flow, the axial dispersion, and the back flow models, was evaluated to predict the solute concentration profile along the column length. The water‐acetone‐n‐butyl acetate and the water‐acetone‐toluene systems were used. The influence of the operational parameters on the height of the mass transfer unit and the back flow coefficients was evaluated using the back flow model. New correlations were proposed to predict the height of the mass transfer unit along with the back flow coefficients in each phase, which were in satisfactory agreement with the experimental data.     

ABSORPTION OF NO AND SO2 IN FE(II)-EDTA SOLUTIONS II. PACKED COLUMN SIMULATION

Two methods were applied in simulating the height of a packed column required for the removal of NO and SO₂ by absorption into Fe(II)-EDTA solutions. The first method used a double-stirred cell to simulate a packed column by matching the gas- and liquid-phase mass transfer coefficients of the two device. The second method used the differential mass balance including reaction kinetics. Data taken in an 8 cm diameter, 40 cm packed height column and in a double-stirred cell were employed in illustrating the methodology. The two methods of prediction were quite satisfactory.     

DYNAMICS OF FRACTIONATORS WITH STRUCTURED PACKING

A generalized dynamic mode! of a distributed staged (packed) fractionator is developed in this work. In particular we consider the dynamics of fractionators which employ structured packings as a means of achieving mass and heat transfer in multi-component systems.

The mathematical model is described by a large set of partial and ordinary differential equations coupled with non-linear algebraic constraints (PODAEs). These equations arise from the mass and energy balances on a distributed column-section together with the fluid dynamic relations. A computational algorithm is developed which employs a polynomial approximation leading to a large differential-algebraic equation (DAE) system. This is solved using standard implicit DAE algorithms.

In this work, the liquid holdup and transfer coefficients (both mass and heat) are computed from established correlations and detailed thermodynamic relations. This differs from most of the previous work reported, which used very simple correlations based on a single variable such as liquid or vapour rate. The results show that the more rigorous computation of transfer coefficients is essential to the veracity of the model.

The model has been applied to the case of an industrial depropanizer which uses a Mellapak 250Y structured packing. The generalized model can be used to study the control and optimization of such fractionators, which are becoming more prolific in the petroleum and related industries.     

Separation performance investigation of packed distillation columns using simple NEQ approach based on packing multicomponent efficiencies and effective mass transfer coefficients

A simple non-equilibrium modeling approach is proposed to simulate multicomponent distillation process in packed columns. The real behavior of the column is simply considered by the evaluation of interphase mass transfer rate based on the overall mass transfer coefficient. Two distinct methods are used to calculate this overall coefficient including the effective mass transfer coefficient method and the packing efficiency method. The modelling procedure consists of an iterative segment-wise algorithm implemented in a MATLAB home-code. For verification, the obtained composition profiles from a structured and a random packed column are compared with reported experimental data. Comparisons show that the packing efficiency-based model could acceptably predict the experimental profiles with an average relative deviation of 18% and 25% for structured and random packed columns, respectively. This confirms that our simple non-equilibrium approach is a reliable and robust model for the performance evaluation of packed columns.     

板波填料萃取塔的实验研究(Ⅱ)传质性能

本文应用扩散模型并采用最优化方法拟合实测的稳态浓度剖面,在内径40mm、100mm的板波填料萃取塔中,用30%TBP(煤油)-醋酸-水和正丁醇-丁二酸-水两种体系研究了板波填料萃取塔的传质性能.     

CFD Simulation of Mass Transfer Efficiency and Pressure Drop in a Structured Packed Distillation Column

The pressure drop and mass transfer efficiency for two‐phase flow in a structured packed column were simulated using a commercial CFD package, CFX version 10. The distillation of the methanol/isopropanol system was carried out in a 0.073 m diameter column, with an element composed of a ceramic structured packing and 0.053 m in height. The Height Equivalent to Theoretical Plate (HETP) value varied from 0.106–0.146 m. Pressure drop experiments were measured with an air/water system. The pressure drops at the flooding and loading points were ca. 173 and 580 Pa/m of packing, respectively. HETPs and pressure drops calculated from the Computational Fluid Dynamics (CFD) model were compared to their experimental counterparts. The average relative error between CFD predictions and the experimental data for the prediction of dry pressure drop, irrigated pressure drop and mass transfer efficiency are 20.3 %, 23 % and 9.15 %, respectively. In all cases, the CFD predictions show a good agreement with the experimental data, indicating that CFD is a reliable, cost saving and suitable technique for the design and optimization of separation processes.     

新型均流填料塔的传质性能研究

提出了一种新型均流填料塔设备,在Φ600 mm的圆形冷模实验塔中,以空气-水-CO2为工质进行了CO2解吸试验,利用滴定法测定了多种气液负荷下新型均流填料塔的传质性能,并在相同工况下与鲍尔环填料塔进行了对比研究。试验结果表明:在试验喷淋密度下,新型均流填料塔与鲍尔环填料塔相比,液相传质单元高度降低了19%以上;随着喷淋密度的降低,液相传质单元高度降低更加显著。在正常操作范围内,新型均流填料塔的传质效率远高于鲍尔环填料塔,是一种传质性能优良的新型填料塔设备。