Mass transfer characteristics of liquid films flowing down a vertical wire in a counter current gas flow

The wetted-wire packing, mainly consisting of a bundle of vertical parallel wires, is a promising concept for the use in separation columns. To investigate the multiphase flow inside the packing in detail and to estimate the performance of the packing, experiments on liquid films on a single vertical wire in a counter current gas flow were carried out. To get information about the interfacial area, an optical measurement of the film thickness was carried out with a digital high speed camera and image recognition tools. By measuring the evaporation of water and aqueous polyvinylpyrrolidone solutions into air, the gas-side mass transfer was determined. The liquid-side mass transfer was examined by measuring the desorption of CO₂ from water into air. The results show that the mass transfer coefficients are comparable to those appearing in common structured packings. When assuming a sufficiently high wire packing density, a specific interfacial area similar to corrugated sheet structured packings can be reached. Previous studies predicted a low pressure drop per packing height and extended capacity limits compared to common packings. In consideration of these results, the wetted wire packing therefore is shown to be suitable especially for absorption processes where a low pressure drop is favourable.     

Hydrodynamic analogy approach for modelling of reactive stripping with structured catalyst supports

Modelling of gas/vapour–liquid separation processes usually requires experimentally determined parameters, e.g., mass transfer coefficients. This results in expensive experimental work, especially for new types of column internals. A novel modelling approach based on hydrodynamic analogies (HA) has recently been developed for distillation units equipped with structured corrugated sheet packings. The HA model takes the packing geometry directly into account whereas the experimental determination of mass transfer coefficients is not required.In this work, the HA approach is extended to cover heterogeneously catalysed reactive stripping processes. Experimental investigations are performed with a test system, esterification of hexanoic acid and 1-octanol, using different types of catalytically coated supports as column internals (one corrugated sheet packing and three film-flow monoliths with different channel geometries). Simulation results obtained with the extended HA model are in a good agreement with experimental values.     

新型垂直板规整填料流体力学及传质性能

采用氧解吸实验,在直径190 mm的有机玻璃塔内,液相喷淋密度10～38 m³·m^-2·h^-1,F因子0.2～3.2 m·s^-1·（kg·m^-3）^0.5的实验条件下测定了一种新型垂直板规整填料的流体力学及传质性能。实验结果表明：垂直板填料的操作压降及传质性能均显著优于商业波纹填料。通过与几种经改进的250型波纹填料相比发现,两者泛点F因子整体上相当;在较高液体喷淋密度下,垂直板填料传质性能及压降均高于改进250型波纹填料;在低喷淋密度下,垂直板填料可实现压降低于改进250型波纹填料,而两者传质性能相当。此外,对填料结构改进对其性能的影响进行了单因素考察。     

Mass transfer performance of structured packings in a CO2 absorption tower

This paper studies the mass transfer performance of structured packings in the absorption of CO2 from air with aqueous NaOH solution. The Eight structured packings tested are sheet metal ones with corrugations of different geometry parameters. Effective mass transfer area and overall gas phase mass transfer coefficient have been measured in an absorption column of 200 mm diameter under the conditions of gas F-factor in 0.38–1.52 Pa0.5 and aqueous NaOH solution concentration of 0.10–0.15 kmol·m?3. The effects of gas/liquid phase flow rates and packing geometry parameters are also investigated. The results show that the effective mass transfer area changes not only with packing geometry parameters and liquid load, but also with gas F-factor. A new effective mass transfer area correlation on the gas F-factor and the liquid load was proposed, which is found to fit experiment data very well.     

CFD Study on the Local Mass Transfer Efficiency in the Gas Phase of Structured Packing

Visualization of local mass transfer coefficients over the dry surface of corrugated‐sheet structured packing is essential for optimizing the existing geometry of structured packing and for improving mass transfer efficiency to develop new structured packing. The local flow patterns between packing sheets and the gas‐phase mass transfer coefficient at each point over the surface are illustrated by employing a wall‐surface reaction model. Different turbulence models are utilized, i.e., a standard κ‐? model and three different low‐Re‐κ‐? models. The numerical calculation results with the Lam‐Bremhorst low‐Re‐κ‐? turbulence model is found to agree well with experimental data. There are three similar regions with enhanced mass transfer efficiency in each mass transfer unit cell of structured packing.     

Mass Transfer In Trickle‐Bed Reactors With Structured Packing

An experimental investigation was carried out to examine the fluid dynamic and mass transfer behavior of structured packing, with the liquid and gas phase flowing co‐currently downwards in the column. Liquid to packing mass transfer coefficients for three positions within the pack were measured by an electrochemical method, varying both the liquid and gas loads as well as the physical properties of the liquid phase. Due to the high void fraction of structured packing, much higher liquid flow rates can be used than in traditional particle trickle‐beds. It was found that in the range studied, the gas superficial velocity has no effect on the mass transfer rate and thus, a general mass transfer correlation in terms of liquid Reynolds number only, was developed. Wetted areas and the radial distribution of liquid through the packing element were determined by a colorimetric method. Within the range of liquid flow rates investigated, complete wetting is not achieved, even with the low viscosity solutions. The measured ratios of hydraulic to geometric area, agree reasonably well with values predicted by existing relationships.     

Transient gas–liquid mass transfer model for thin liquid films on structured solid packings

This paper describes a model for gas–liquid mass transfer through thin liquid films present on structured packings for gas–liquid operations under dispersed gas flow regime. The model has been derived for two cases: the absorption (or desorption) of a gaseous component into the liquid film and the transfer of the gaseous component through the liquid film to the packing surface where an infinitely fast reaction takes place. These cases have been solved for three bubble geometries: rectangular, cylindrical, and spherical. For Fourier numbers below 0.3, the model corresponds to Higbie’s penetration theory for both cases. The Sherwood numbers for cylindrical and spherical bubbles are 20% and 35% higher, respectively, than for rectangular bubbles. In case of absorption and Fourier numbers exceeding 3, the effect of bubble geometry becomes more pronounced. The Sherwood numbers for cylindrical and spherical bubbles now are 55% and 100% higher, respectively, than for rectangular bubbles. In case of an infinitely fast reaction at the packing surface, the Sherwood number corresponds to Whitman’s film theory (Sh=1$Sh=1$) for all bubble geometries. In this paper also practical approximations to the derived Sherwood numbers are presented. The approximations for both cases and all bubble geometries describe all the model data within an error of 4%. The application of the model has been demonstrated for three examples: (1) gas–liquid mass transfer for a structured packing; (2) gas–liquid mass transfer in a microchannel operated with annular flow; (3) gas–liquid mass transfer in a microchannel with Taylor flow.     

Gas‐Liquid Mass Transfer in a Bench‐Scale Trickle Bed Reactor used for Benzene Hydrogenation

The gas‐liquid mass transfer coefficients (MTCs) of a trickle bed reactor used for the study of benzene hydrogenation were investigated. The Ni/Al₂O₃ catalyst bed was diluted with a coarse‐grained inert carborundum (SiC) particle catalyst. Gas‐liquid mass transfer coefficients were estimated by using a heterogeneous model for reactor simulation, incorporating reaction kinetics, vapor‐liquid equilibrium, and catalyst particle internal mass transfer apart from gas‐liquid interface mass transfer. The effects of liquid axial dispersion and the catalyst wetting efficiency are shown to be negligible. Partial external mass transfer coefficients are correlated with gas superficial velocity, and comparison between them and those obtained from experiments conducted on a bed diluted with fine particles is also presented. On both sides of the gas‐liquid interface the hydrogen mass transfer coefficient is higher than the corresponding benzene one and both increase significantly with gas velocity. The gas‐side mass transfer limitations appear to be higher in the case of dilution with fine particles. On the liquid side, the mass transfer resistances are higher in the case of dilution with coarse inerts for gas velocities up to 3 · 10^–2 cm/sec, while for higher gas velocities this was inversed and higher mass transfer limitations were obtained for the beds diluted with fine inerts.     

液膜性质的小尺度研究

Structured packing is a good candidate for CO₂ capture process because of its higher mass transfer efficiency and lower pressure drop.Now,the challenging problem of CO₂ capture and storage demands more and more efficiency equipment.The aim of the present study is to investigate the liquid film characteristics under counter current gas phase and throw some insight into the enhancing mechanism of mass transfer performance in structured packing.A high speed digital camera,non-intrusive measurement technique,was used.Water and air were working fluids.Experiments were carried out for different gas/liquid flow rates and different inclination angles.The time-average and instantaneous film widths for each set of flow parameters were calculated.It is shown that the effects of gas phase could be neglected for lower flow rate,and then,become more pronounced at higher flow rate.According to instantaneous film width,three different stages can be distinguished.One is the constant width of liquid film.The second is the slight decrease of film width and the smooth surface.This kind of character will lead to less interfacial area and deteriorate the packing mass transfer performance.For the third stage,the variation of film width shows clearly chaotic behavior.The prediction model was also developed in present work.The predicted and experimental results are in good agreement.     

液相流动方式对波纹规整填料性能的影响

根据液相在波纹规整填料片上呈现渗流、膜状流等不同的流动方式,选择5种不同的波纹规整填料对其流体力学和传质性能进行研究,以探究液相在波纹片上的流动方式对波纹规整填料性能的影响.研究结果表明,液相呈渗流流动的泡沫碳化硅波纹规整填料(SCFP型)有利于液体横向扩散和液膜均匀分布,当液相喷淋密度和气相F因子均较小时,其压降最低,传质效率最高;液相主要呈渗流流动、兼有膜状流动的双层错孔丝网填料(DMⅢ型)有利于波纹片两侧液体交换,强化液体在流动过程中的扰动,其压降及传质性能略逊于SCFP型填料;液相主要呈膜状流动的BX型、DMⅠ型及DMⅡ型填料波纹片表面液膜较厚,横向扩散能力差,其传质效率低于SCFP型和DMⅢ型填料.研究揭示了依靠渗流作用的波纹规整填料具有较好的应用性能,为波纹规整填料的进一步发展开拓了新思路.     

Gas holdup and mass transfer in solid suspended bubble columns in presence of structured packings

Electrochemical gas absorption or biotechnical purification processes using structured packing as electrode or as biological support, respectively, may operate in bubble columns in presence of suspended solids. In both systems the knowledge of mass transfer rates from the liquid to the packing is important for the design of equipment. In the present investigation, the fluid dynamic behavior of a simple bubble column and a bubble column containing small size particles, both in presence of structured packing, was studied. Furthermore, mass transfer coefficients between the liquid and the structured packing were obtained by the electrochemical method. The influence of physical properties of the liquid phase, gas flow rate, kind and concentration of the suspended particles on both gas holdup and mass transfer was investigated. Correlations of the experimental data of mass transfer using dimensionless groups were derived and compared to previous correlations. Similarity with a heat transfer expression already used in two-phase systems was found.     

Liquid Flow on Structured Packing: CFD Simulation and Experimental Study

A CFD model of the two‐phase countercurrent flow in the geometry of the plate‐type structured packing Mellapak 250.Y was built, tested and verified. The model was applied to determine the effect of liquid and gas flow rates and physicochemical properties of the flowing liquids on the interfacial area formed on structured packing. The CFD model allowed us to determine the minimum liquid flow rate at which an unbroken liquid film was observed on the packing surface. The simulations confirmed that with an increase of the wetting rate the surface of the packing covered with a liquid film increased until the surface was totally covered up, while further slight changes of an interfacial area were the result of wave formation. The effect of gas load (F factor) on the film surface was in the range of a calculation error. Results of the CFD simulation allow us to predict the stages of film formation during liquid flow, to follow local velocity oscillations, film thickness and velocity profiles of phases.     

空分上塔规整填料结构优化——波纹规整填料倾角的确定

空气深冷分离是一个多精馏塔偶合过程 ,其中上塔处于核心位置 ,具有许多进出料口 ,各段填料层内气液两相负荷变化相当大。通过改变波纹规整填料的倾角可以避免塔体变径。该文从双膜理论出发 ,推导出规整填料的重要结构参数之一———波纹倾角对效率影响的关联 ,同时给出了规整填料的通用泛点关联式。     

Gas side controlled mass transfer in a new packing with stamped horizontal lamellae operating at extremely low liquid loads

The gas side controlled mass transfer coefficient of an easy to make packing, with stamped horizontal lamellae, designed for operating at extremely low liquid superficial velocity, between 0.04 and 1 m³/m² h is investigated. The absorption of SO₂ in sodium hydroxide water solution is used as a model process. Based on the obtained experimental data, an equation for calculating the gas side controlled mass transfer coefficient, with deviation of ±8% is presented.The comparison between the new packing and different highly effective packings shows that the new one has lower pressure drop at the same specific area and mass transfer coefficient, respectively higher mass transfer coefficient at the same pressure drop and specific surface.Of all compared highly effective packings, only the crossed strips packing, quite difficult to produce, has close, yet higher, pressure drop at similar specific surface area and volumetric mass transfer coefficient.     

错流降膜液体干燥剂除湿/再生传热传质数学模型及分析

分析了错流降膜液体干燥剂除湿及再生传热传质过程 ,建立了基于实际除湿系统的描述再生和除湿过程的数学模型 ,考虑到除湿过程中产生的热效应 ,以氯化钙溶液为除湿剂时 ,对气侧和液侧的传热传质系数进行了理论和数值求解 .计算结果表明 ,传热传质系数与气流流动状态、除湿剂的热物理性质等因素有关     

环境化工中的气/液膜接触分离过程及其特性

阐述了新型膜接触器由于避免了传统分离设备的一系列缺点和不足,具有操作范围宽,分离效率高,气液两相的流速可独立控制,可直接线性放大和结构紧凑等众多优点,日益成为分离科学研究的热点。并着眼于3种气/液界面膜接触分离过程(膜吸收、膜蒸馏、膜结构填料替代)在废气、废水等环境治理与化工分离方面的研究,从膜接触器结构、膜材料、传质、分离效率等方面详细分析和探讨了三种膜过程的分离特点,并对其作用原理、操作参数及分离性能进行了综合比较。     

In‐situ‐Stoffübergangsmessungen mit Computertomographie: Auswahl des Testsystems

For the in‐situ measurement of the mass transfer of a gas phase into a liquid film on a packing using X‐ray computed tomography, a substance combination is needed. The thickness of the liquid film was measured using computed tomography, to ensure that enough pixels in the film are available for the mass transfer experiments. In order to decide if the substance combination is distinguishable in the CT images, the X‐ray activity of gases and of dissolved gas in the liquid phase was investigated. For the determination of the maximum solubility the quantum chemical model COSMO‐RS was used to simulate the vapor‐liquid equilibria.     

Hydrodynamik und Stofftransport in einem Perlschnurreaktor für Gas/Flüssig/Fest‐Reaktionen

In this work, a pellet string reactor was characterized with respect to hydrodynamics and mass transfer. The catalyst packing consists of a cylindrical channel with a diameter of 1.41 mm, which was filled with spherical catalyst particles, having an outer diameter of 0.8 mm. Under reaction conditions (liquid phase hydrogenation of α‐methylstyrene) overall (gas‐liquid‐solid) volumetric mass transfer coefficients for hydrogen between 0.8 and 5.5 s^–1 were computed. Due to high mass transfer rates and simple reactor geometry, pellet string reactors can be applied in industry as highly efficient reaction units.     

Comparison of mass transfer efficiency in horizontal rotating packed beds and rotating biological contactors

In this study, the mass transfer efficiencies of a novel horizontal rotating packed (h‐RPB) bed and the conventional disc‐type rotating biological contactor (RBC) were studied at four speeds and seven submergences. Pall rings of two different sizes (25, 38 mm), superintalox saddles and a wiremesh spiral bundle were used as packings in the h‐RPB. Volumetric gas–liquid mass transfer coefficients were determined by unsteady state absorption of atmospheric oxygen in de‐aerated water. Power consumption per unit liquid volume has been found for all geometries tested. The oxygen transfer efficiency values for the h‐RPB were found to be 2–5 kg kWh^?1 and for the disc RBC were found to be 1–2 kg kWh^?1. The performance of the h‐RPB was also compared with other gas–liquid contactors such as surface aerators. The study proves that the h‐RPB is a energy efficient alternative to conventional contactors. Copyright © 2005 Society of Chemical Industry     

Mass transfer in cocurrent gas-liquid flow: Gas side mass transfer coefficients in upflow,interfacial areas and mass transfer coefficient in gas and liquid in downflow

A comparison of the values of interfacial area in cocurrent downward flow of gas and liquid with those obtained in upward flow revealed very little difference.Volumetric as well as true liquid side mass transfer coefficients in downflow were found to be several times lower than in upflow. Only in the smallest 10 mm tube a dependence of the mass transfer coefficient on the gas flow rate could be detected, no effect of the gas velocity was observed in the 15 and 20 mm tubes. A correlation for the liquid side mass transfer coefficient was obtained in terms of the Reynolds number of the film.The volumetric gas side mass transfer coefficient in upflow was generally independent on the liquid flow rate, except at high gas velocities in the two smaller tubes. Correlations were obtained for the volumetric mass transfer coefficient in terms of the specific rate of energy dissipation, and of the true mass transfer coefficient in terms of dimensionless groups. Much lower values were obtained for the gas side mass transfer coefficient in downflow. Only the data for the 10 mm tube could be correlated with some success by the formulas proposed for upflow.