Interpretation of mass transfer measurements in bubble columns considering dispersion of both phases

Mass transfer measurements in two bubble columns with an inner diameter of 100 resp. 140 mm with the systems air/water/carbon dioxide and nitrogen/n-propanol/carbon dioxide have been evaluated with the axial dispersion model. The dispersion coefficients of both phases have been determined in separate investigations. As the results revealed a strong influence of the liquid viscosity, additional dispersion coefficient measurements have been carried out with the system air/glycol. It could be shown that the liquid phase dispersion coefficient decreases with increasing viscosity while the gas phase dispersion coefficient increases with increasing liquid viscosity. Both coefficients are strongly dependent on the gas throughput and the column diameter. Using these coefficients, the mass transfer coefficients have been calculated by fitting the calculated concentration profile to the measured values and by splitting the volumetric mass transfer coefficient with the experimental value of the interfacial are a. The results agree best with a correlation of Calderbank and Moo-Young.     

Mass Transfer in a Flat Gas/Liquid Interface using non‐Newtonian Media

Gas/liquid mass transfer has been investigated using a stirred vessel gas/liquid contactor using non‐Newtonian media and carbon dioxide as absorbent and gas phase, respectively. The volumetric mass transfer coefficients at different operational variables have been determined. Non‐Newtonian media (liquid phase) were prepared as aqueous solutions of sodium carboxymethyl cellulose salt. The influence of the rheological properties, polymer concentration, stirring rate, and gas flow rate on mass transfer was studied for these liquid phases. Kinematic viscosity and density experimental data were used to calculate the average molecular weight corresponding to the polymer employed. The Ostwald model has been used to fit the rheological behavior of aqueous solutions of the polymer employed as absorbent phase. Reasonably good agreement was found between the predictions of the proposed models and the experimental data of mass transfer coefficients.     

Analysis of carbon dioxide gas/liquid mass transfer in aerated stirred vessels using non–Newtonian media

Gas/liquid mass transfer has been investigated in an aerated stirred tank using non‐Newtonian media and carbon dioxide as absorbent and gas phase respectively. The volumetric mass transfer coefficients at different operational variables have been measured. The non‐Newtonian media (liquid phases) used were aqueous solutions of two polymers, carboxymethyl cellulose and alginate sodium salts. The influence upon the mass transfer of the rheological properties, polymer concentration, stirring rate and gas flow rate was studied. Kinematic viscosity and density experimental data were used to calculate the average molecular weight corresponding to the polymers employed. Reasonable agreement was found between the predictions of proposed models and the experimental data. The results shown in the present paper allow us to understand carbon dioxide transfer to a non‐Newtonian liquid phase and to evaluate the effect of the liquid phase characteristics. The equations used in this paper allow accurate simulation of the transfer of a pure gas to a rheologically complex solution. Copyright © 2004 Society of Chemical Industry     

MEASUREMENT OF INTERFACIAL AREA AND MASS TRANSFER COEFFICIENTS BY CHEMICAL METHOD OF C02 ABSORPTION INTO AQUEOUS SOLUTIONS OF NaOH IN A MODEL COLUMN WITH EXPANDED METAL SHEET PACKING

The theory of gas absorption accompanied by fast pseudo-fast order reaction which considered dependences of diffusivity, kinetic constant and Henry's law constant on absolute temperature and ionic strength was used to obtain values of effective interfacial areas and mass transfer coefficients in gas and liquid phase.

Experimental measurement of carbon dioxide absorption from mixture with air was performed in a pilot-plant column with expanded metal sheet packing irrigated with sodium hydroxide solution.

Resulting liquid and gas-side mass transfer coefficients are compared with values obtained from physical Absorption measurement of carbon dioxide into water and with measurement of gas-side mass transfer coefficient for sulphur dioxide in the same column.

The differences between determined values are discussed.     

Diffusion coefficients and phase equilibria of carbon dioxide–ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) system

This article presents the mutual diffusion coefficients of a carbon dioxide–ionic liquid, [bmim][PF₆], system at temperatures of 313.15 and 323.15 K and pressures of 5 and 8 MPa. In order to estimate the diffusion coefficients, we have carried out experiments to find time-dependent carbon dioxide solubilities in the ionic liquid and then fit a transport model to the data. In a system containing high pressure carbon dioxide and ionic liquid, carbon dioxide dissolves in the liquid until its equilibrium mole fraction is reached. During this process, the position of the liquid–vapour interface and the density of the liquid phase change. To account for the variation in liquid density, an equation fit to the experimental density data is included in the transport model. To track the moving interface, the volume-of-fluid method is used. The diffusivities at dilute concentration and at thermodynamic phase equilibrium are determined and compared with the literature values and those obtained from correlations.     

Carbon dioxide stripping in ceramic hollow fibre membrane contactors

Ceramic hollow fibre membrane contactors have been applied to carbon dioxide stripping from monoethanolamine (MEA) solution at high temperature where most polymeric membranes would fail to operate. The experimental results show that the membrane contactors are immune from hydrodynamic problems, such as flooding and loading, since the gas and liquid phases can totally be separated by the hollow fibre membranes. The height of transfer unit (HTU) of the contactor was determined to be as low as 15 cm and is dependent on the fluid velocities. The mass transfer coefficients were theoretically predicted and found to be within a reasonable deviation. The mass transfer resistance in the liquid phase was found to be the majority of the total resistance.     

Two-phase flow—IV. Gas and liquid side mass transfer coefficients

Mass transfer coefficients in gas and liquid have been obtained for the case of cocurrent gas—liquid flow through a vertical tube 6 mm i.d. by absorbing sulphur dioxide into sodium hydroxide solution and carbon dioxide into sodium carbonate—sodium bicarbonate solution respectively. The liquid side mass transfer coefficient was found to increase with the gas velocity but showed a maximum when plotted against the liquid velocity. A model based on the analogy between momentum and mass transfer has been proposed for the rate of mass transfer in the liquid phase. The mass transfer coefficient in the gas phase increases with the gas velocity but the liquid velocity has an opposite effect. A correlation in terms of dimensionless groups is presented for the gas side mass transfer coefficient.     

Gas mass transfer to AOT-based microemulsions

The present paper analyses the gas/liquid mass transfer process employing carbon dioxide as gas phase and ternary water in oil microemulsions as absorbent liquid phases. The liquid phases were obtained by a direct mixing of water, 2,2,4-trimethylpentane and sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol OT). The characteristics of the microemulsions employed as liquid phase have been analysed to interpret the experimental results observed in the absorption process. More specifically, they have been analysed in relation to the percolation phenomenon and the effects produced by this phenomenon upon the different physical properties. Characteristic results have been observed for the gas/liquid mass transfer using microemulsions, because ternary microemulsions with high viscosity values in relation to pure water show a faster absorption process than the carbon dioxide/water system. This characteristic behaviour has been explained on the basis of the microemulsions internal dynamics.     

Gas/liquid mass transfer processes in a carbon dioxide/alkane system

A gas/liquid mass transfer process has been studied using carbon dioxide/alkane systems in a stirred vessel. Four linear alkanes (n‐heptane, n‐octane, n‐decane and n‐dodecane) have been used as the liquid phase in the present paper, and they have been employed to study the influence of the carbon length upon the mass transfer velocity. The mass transfer along the liquid phase has been studied using the mass transfer coefficient of the liquid phase, k_L. Pure carbon dioxide has been employed as the phase in all cases for this reason. The effects of the power supplied to the liquid phase and the gas flow rate upon the absorption process have also been analysed. Finally, the equations that allow calculation of the mass transfer coefficients have been applied for these systems, with acceptable results. Copyright © 2005 Society of Chemical Industry     

填料塔中氨水吸收二氧化碳的传质性能

在填料塔中对氨水吸收二氧化碳的传质性能进行了实验研究,测定了填料塔单位体积的有效传质面积aV,气相传质系数kG和体积总传质系数KGaV等,实验结果表明,随着气体流量增加,aV值略有上升,kGaV和kG值明显增大;体积总传质系数KGaV值也随着气体流量和氨水浓度的升高而增大。在总传质阻力中,气膜阻力所占比例相对较小,氨水吸收二氧化碳的过程主要受液膜阻力的控制。     

Recovery of carbon monoxide from flue gases by reactive absorption in ionic liquid imidazolium chlorocuprate(I):Mass transfer coefficients

Recovery of carbon monoxide from flue gases by selective absorption of carbon monoxide in an imidazolium chlorocuprate(I) ionic liquid is considered in this work as an alternative to the use of molecular volatile solvents such as aromatic hydrocarbons. The present work evaluates the CO mass transfer rates from the gas phase to the ionic liquid solutions in the absence of chemical reaction. To that end, carbon dioxide was employed as an inert model gas and absorption experiments were performed to assess the influence of different process variables in a batch reactor with flat gas–liquid interface. The experimental mass transfer coefficients showed significant var-iation with temperature, (3.4–10.9) × 10-7 m·s-1 between 293 and 313 K; stirring speed, (10.2–33.1) × 10-7 m·s-1 between 100 and 300 r·min-1;and concentration of copper(I), (6.6–10.2) × 10-7 m·s-1 between 0.25 and 2 mol·L-1. In addition, the mass transfer coefficients were eventually found to follow a poten-tial proportionality of the type kL∝μ-0.5 and the dimensionless correlation that makes the estimation of the mass transfer coefficients possible in the studied range of process variables was obtained:Sh=10-2.64·Re1.07·Sc0.75. These results constitute the first step in the kinetic analysis of the reaction between CO and imidazolium chlorocuprate(I) ionic liquid that determines the design of the separation units.     

Vapour- and liquid-side volumetric mass transfer coefficients measured in distillation column. Comparison with data calculated from absorption correlations

Volumetric mass transfer coefficients in liquid and vapour phases in distillation column were measured by the method consisting of a fitting of the concentration profile of liquid phase along the column obtained by the integration of a differential model to the experimental one. The mathematical model of distillation process includes mass and energy balances and the heat and mass transfer equations. The film model flux expressions with the convective transport contributions have been considered in the transfer equations. Vapour and liquid phases are supposed to be at their saturated temperatures along the column. Effect of changes of phase flows and physical properties of phases on the mass transfer coefficients along the column and non-ideal thermodynamic behaviour of the liquid phase have been taken into account. The concentration profiles of liquid phase are measured in the binary distillation of the ethanol-water and methanol-ethanol systems at total reflux on metal Pall Rings and Intalox saddles 25 mm in the column with diameter of 150 mm. The distillation mass transfer coefficients obtained by the fitting procedure are compared with those calculated from absorption data using Onda's, Billet's and Linek's correlations. The distillation heat transfer coefficients calculated from the model assuming saturated temperatures in both phases are compared with those calculated from the Chilton-Colburn and penetration model analogy between mass and heat transfer. The results have confirmed an agreement neither between distillation and from absorption correlations calculated mass transfer coefficients nor between analogy and from enthalpy balance calculated heat transfer coefficients. Also the concentration profiles obtained by the integration of the differential model of the distillation column using the coefficients from absorption correlation have differed from the experimental profiles considerably.     

Studies on cocurrent gas-liquid flow in helically coiled tubes. II. Theory and experiments on turbulent mass transfer with and without chemical reaction

Pure carbon dioxide was absorbed into distilled water and sodium hydroxide solution, in cocurrent two phase annular flow in helically coiled tubes in order to measure physical and chemical mass transfer coefficients and interfacial areas. (k*_La) was correlated by the pressure drop in the test sections and interfacial areas were found to vary with the liquid phase energy dissipation. According to a new theory, (k*_L) has been shown to be a function of the root mean square vorticity near the interface. The root mean square vorticity has been related to the pressure drop, gas density, liquid flow rate and liquid velocity. The physical mass transfer coefficients theoretically predicted are in good agreement with experimental results.     

A Simple Approach to Measuring the Gas Phase Heat and Mass Transfer Coefficients in a Bubble Column

A simple experimental approach was developed to measure the gas phase volumetric heat and mass transfer coefficients in a bubble column and a slurry bubble column employing a single gas nozzle. The experimental technique was based on a transfer model that simulates humidification and direct contact evaporation models in the case of a gas bubble rising in a liquid of uniform temperature. The temperature and relative humidity of the inlet and outlet gas in the column are the only measurements required in this technique. Experiments were carried out in a 0.15 m inner diameter column using water as the liquid phase, air as the gas phase, and cation resins of 0.1 mm diameter and a specific gravity of 1.2, as the solid phase. The results showed that, when using solid concentrations in the range of 7–10 wt %, both the volumetric gas‐phase heat and mass transfer coefficients increased with an increase in the gas superficial velocity and were further enhanced by increasing the solid load after a certain minimum superficial velocity had been reached in the column (0.044 m/s in the system used). Increasing the solid load beyond 10 wt %, did not contribute to a further increase in these coefficients. Furthermore, the gas holdup in the column increased with the superficial gas velocity and was further enhanced when the solid‐phase load was in the range of 7–10 wt %. These observations agree well with previously reported findings by other investigators.     

Simultaneous heat and mass transfer in binary distillation—I: Theory

Binary distillation in continuous contact equipment is modeled as a simultaneous heat and mass transfer process. In order to account for the interactions between heat and mass transfer between the two phases, the equations developed from enthalpy and material balances are analyzed simultaneously and it is revealed that the individual phase mass transfer coefficients can be evaluated rigorously by measuring liquid phase compositions and temperatures in distillation experiments. Using the theoretical relations, it is proved that the liquid phase in a distillation column will be saturated if and only if there is negligible resistance to mass transfer in the liquid film. For the case of comparable resistances in both phases, the possible amount of superheat in the liquid phase would be considerable and thus convenient to determine experimentally. The liquid phase temperatures are shown to be important in experimental analysis (i.e. in determining the individual phase transfer coefficients) but not in design applications. For the latter case, the model equations reduce to the conventional mass transfer relations which are not as sensitive to temperatures.     

Gas absorption into a turbulent liquid

Mass transfer coefficients were measured for the liquid phase for gas absorption into a turbulent liquid flowing down a long wetted-wall column. Helium, hydrogen, oxygen and carbon dioxide were absorbed into distilled water over a Reynolds number range of 1300–8300. The results indicate that the liquid phase mass transfer coefficient is proportional to the molecular diffusion coefficient raised to the 0·54 power. The results are interpreted in terms of an eddy diffusivity model and indicate that the eddy diffusivity increases as the square of the distance from the interface.     

The influence of molecular diffusion on mass transfer between turbulent liquids

The influence of molecular diffusion on liquid—liquid mass transfer in a stirred transfer cell has been found by measuring the rates transfer of helium and iso-butane from water to toluene and dekalin. These solutes have very different diffusion coefficients, their presence does not alter the physical properties of the liquids and, because their equilibrium distributions strongly favour the organic phases, the water phase mass transfer coefficient could be determined and was found to depend on the square root of the diffusion coefficient.The results are compared with the predictions of a model for liquid—liquid mass transfer under turbulent conditions, based on the approach of an eddy to the interface being restrained by interfacial tension and gravitational forces and taking into account eddy pressure fluctuations in both phases. This model provides a correlation for these results, as well as water phase mass transfer coefficients for the transfer of iso-butane from water to n-octanol, and previous stirred transfer cell results.     

Dye distribution in supercritical dyeing with carbon dioxide

The measurement of equilibrium dye uptake in PET fiber is carried out using a flow-type cylindrical vessel. Using the expanded liquid model in which both phases are supposed to be liquid mixtures, the equilibrium uptake of C.I. disperse red 60 may be calculated from each binary interaction parameter of carbon dioxide–dye, carbon dioxide–PET and dye–PET binary systems. The calculated uptake is compared with the observed one with the same order of magnitude, even though the former is overestimated. The binary interaction parameter of dye–PET system that is obtained from the equilibrium dye uptake in PET is smaller by about 1.5 than that obtained by the binary system. Nevertheless, it is found that the predicted uptake of dye with using the interaction parameter regressed from the ternary data is in excellent agreement with the experimental one. The distribution of C. I. disperse red 60 into supercritical fluid and PET phase at equilibrium can be predicted and compared with the experimental one. The estimated distribution coefficient increases with the pressure increase, because the sorption of dye in PET fiber increases slowly with the pressure than the dye solubility in carbon dioxide does. This tendency is weakened with increase of temperature.     

Gas-Liquid mass transfer in fixed beds with two-phase cocurrent downflow: Gas/newtonian and non-newtonian liquid systems

New data of gas-liquid mass transfer for cocurrent downflow through packed beds of porous and non-porous particles are presented. Mass transfer parameters for air/carbon dioxide/water, air/carbon dioxide/carboxymethylcellulose solution and air/carbon dioxide/sodium hydroxide systems were evaluated by least square fit of the calculated CO₂ concentration profiles in gas phase to the experimental values. The volumetric liquid-side mass transfer coefficient increases with the increase of the flow consistency index of the liquid. A comparison of the volumetric liquid-side mass transfer coefficient values evaluated with and without taking into account the axial dispersion shows that the influence of the liquid axial dispersion is significant at low liquid velocity and high CMC concentrations, and the influence of the gas axial dispersion is insignificant.     

对撞流反应器甲醇合成实验研究

液相法甲醇合成由于有惰性液体介质的存在,气液相间传质对反应起到了一定的阻碍作用,对撞流反应器使用喷嘴将催化剂浆料雾化从而强化了气液相间传质。文中在对撞流反应器内对甲醇合成温度、合成气比例、气流量、浆料循环量以及喷嘴个数进行了考察,结果表明,温度控制在230℃左右操作比较适宜,二氧化碳参与反应对甲醇合成较为有利,合成气流量在22.4 L/min以后时空产率几乎不再增加,增加浆料循环量和采用对置式二喷嘴或四喷嘴比单喷嘴时空产率和出口甲醇体积分数都有所增加。由结果可知,利用喷嘴雾化和液体对撞可以显著地增强气液传质从而达到增加液相甲醇合成时空产率的目的。