Gas Side Mass Transfer Resistance Research Articles

Experimental and numerical investigation of gas-side mass transfer in Taylor flow

Taylor flows are of great interest for industrial application where efficient gas absorption is needed. Literature however scarcely relates to the gas-side mass transfer coefficient kG in Taylor flows. In this work, the authors selected and developed an experimental method, based on bubble expansion record, to estimate kG value in a millimetric channel of circular section, fed with nitrogen gas and pure ethyl acetate (EA). The experiments provided an order of magnitude of a few mm.s−1 at least for kG, for a two-phase velocity UTP ∼ 0.1 m.s−1.CFD simulations completed the experimental investigation and further estimated the order of magnitude, with kG∼10-2 m.s−1 when 0.09<UTP<0.18 m.s−1. The transport of EA inside the bubble suggests a significant contribution of diffusion in regard with the convective effects. kG is found to be sensitive to UTP, and to the temperature via the gas diffusivity and also via the bubble expansion leading to an increase in bubble velocity. The dependence of the gas-side Sherwood number to the Péclet number is similar to previously reported observations concerning a spherical bubble freely rising in quiescent liquid.This study thus provides the means to evaluate the gas-side resistance for any gas–liquid system in Taylor flows, a resistance that was usually simply dismissed in previous works. As an example, the authors demonstrated that, in Taylor flows, for a gas–liquid system of industrial interest (diluted CO2 in 30 wt.% N-methyldiethanolamine aqueous solution), gas-side mass transfer resistance is lower than the liquid-side one by at least one order of magnitude.

Mass transfer coefficients of carbon dioxide in aqueous blends of monoethanolamine and glycerol using wetted-wall column

There is an urgent need for CO2 capture development because of the global warming crisis. Recently CO2 absorption by the mixture of monoethanolamine (MEA) and glycerol, as an eco-friendly solvent, has been considered due to its promising performance and low technical and environmental impacts. However, more aspects of this process, especially mass transfer coefficients, need to be studied further. In this work, a bench-scale wetted-wall column was used to find the CO2 mass transfer coefficients in the aqueous blends of MEA (25 wt%) and glycerol (5–20 wt%). The experiments were performed nearly to the industrial conditions of flue gas at atmospheric pressure and three different temperatures (313, 323, and 333 K). The gas flow rate was maintained around 0.17 ± 0.01 stdL/s, and the CO2 partial pressure was in the range of 1–15 kPa. The findings revealed that increasing the glycerol to 10 wt% improves the overall mass transfer (KG), and adding more glycerol up to 20 wt% decreases the KG. The gas-side mass transfer resistance (1/kg) found to be negligible. Thus, the primary mass transfer resistance was in the liquid phase. It is also found that the solution with 10 wt% glycerol and 25 wt% MEA (10G25M) had the highest liquid-side mass transfer coefficient (kg′) among the other solutions. The 10G25M showed a comparable and even better absorption rate than solutions with a higher concentration of MEA studied in the literature. Compared with industrial-grade, the kg′ of the 10G25M was over two times higher than the 30 wt% MEA solution.

Mass transfer during bubble-free polymer devolatilization: a systematic study of surface renewal and mixing effects

This paper deals with a systematic study of bubble-free polymer devolatilization. Experimental degassing tests are combined with a surface renewal analysis. For this purpose, a simplified extruder model is used as representative rotating degassing machine. It is a partially filled agitator vessel with a blade stirrer. A model substance system consisting of highly viscous polydimethylsiloxane as polymer and 1,1,2-trichloro-1,2,2-trifluoroethane as volatile allows the experiments to be carried out at ambient temperature. The driving force for the mass transfer is achieved by reducing the partial pressure in the gas phase via an inert gas flow of nitrogen. This allows the specific investigation of the mechanism of film degassing. The temporal decrease in the concentration of volatiles in the polymer solution is measured thermogravimetrically at three different filling levels and three different rotational speeds in a total of 18 test series. At the same time, the free surfaces and renewal times are determined by video recordings and numerical simulations. For this, the free CFD software OpenFOAM is used with a VOF-based free surface solver. The numerical results are validated by the experiments. It is shown that the deviations of the measured mass transfer data from the theoretical predictions of the common surface renewal models depend on the degassing degree. Mixing is the decisive factor which can be taken into account in the form of a corrected model for the design of degassing machines. Furthermore, it is shown that the gas-side mass transfer resistance is not negligibly small in all cases.

Simulation of carbon dioxide absorption by monoethanolamine solution in wetted-wire column

This study is an attempt to investigate the chemical absorption of CO2 in aqueous mono-ethanolamine (MEA) solution in a multi-wire column. A rate-based process model based on the fast reaction for the CO2–MEA system was employed for modelling of mass transfer and chemical reaction inside the column. As the gas phase mass transfer resistance is not negligible comparing to the liquid phase mass transfer resistance, unlike previous models, gas side mass transfer resistance has also been considered in the model. In addition, the heat effects associated with the absorption and chemical reaction are included through energy balances in the gas and liquid phases. A computer program (MATLAB code) was developed to simulate CO2 absorption into aqueous solution of mono-ethanolamine in a multi-wire column. The model is capable of predicting essential information, such as overall mass transfer coefficient, gas absorption rate and efficiency of the column. The modelling results were compared with available experimental data conducted in columns that equipped with both single wire and multiple wires and good agreement was achieved. Thereby it was concluded that the developed model is capable to predict the chemical gas absorption performance of both multi-wire and one-wire gas–liquid contactors.

CO2 quality control through scrubbing in oxy-fuel combustion: An evaluation of operational pH impacts, and prediction of SO2 absorption rate at steady state

Oxy-fuel combustion is a promising CCS technology which is being demonstrated prior to commercialisation. While the flue gas in oxy-fuel combustion is concentrated in CO2, it contains impurities such as SO2. The elimination of SO2 can provide a clean CO2 stream ready for storage. This paper is to understand the absorption of SO2 in scrubbing relevant to those used in oxy-fuel technology.Steady state experiments were conducted in a continuous well stirred reactor to understand the absorption rate of SO2/CO2 into a total concentration of 0.28M of mixtures of NaHSO3 and NaHCO3 simulating liquids formed by scrubbers using NaOH as the reagent at solution pH values from 4 to 7 with the exiting gas concentrations of SO2 from 19ppm to 1500ppm and a constant CO2 concentration of 70%. Online measurement included gas phase SO2 and liquid pH, and offline measurement included CO2 (aq), HCO3−, S (IV), SO32− and S (VI) after each experiment. Three aspects investigated were the impacts of pH on the solution chemistry, the significance of solution pH and the concentration of gas phase SO2 on the absorption rate of SO2.The total sulphur concentration in liquid was found to be related to the effectiveness of Na+. The effective ratio of Na+ can be defined as the total sulphur to Na+ ratio and this effectiveness ratio of Na+ is pH dependent. At pH<5, Na+ is 99% effective. It reduces dramatically from 99% at a pH 5 to less than 15% at pH above 7. With regard to carbon based species also absorbed, super saturation of CO2 (aq) was observed at pH>5.5. The concentration of HCO3− increases dramatically above pH 6 and below this pH, the concentration of HCO3− is negligible.The absorption rate of SO2 was found to increase with pH with some increase with the concentration of SO2. The operational pH window for scrubbing may be defined by an upper limit pH where the absorption rate of SO2 starts to decreases from the maximum absorption rate of SO2 and the lower limit pH where the absorption rate of SO2 reduces to half of the maximum absorption rate of SO2. Both the upper limit and the lower limit decrease initially and stay stable with the concentration of SO2. This decrease is caused by the reversible reaction of the hydrolysis of SO2 and confirmed by equilibrium experiments of SO2 and sodium solutions. Operation within region 2 (pH 5–6) is recommended, depending on the scrubber design. The operation exit pH of the produced liquid can be varied within the region.The absorption rates of SO2 obtained in the steady state experiments were predicted by a model based on the instantaneous reaction assumption. This model generally overestimates the absorption rates of SO2 at pH values below 6 indicating a kinetic limitation of SO2 and water reaction at low pH values. The analysis on the controlling regions indicates that the gas side mass transfer resistance decreases with the concentration of SO2. Liquid side resistance becomes more important at a lower pH and a higher concentration of SO2.

Contribution of Liquid- and Gas-Side Mass Transfer Coefficients to Overall Mass Transfer Coefficient in Taylor Flow in a Microreactor

Abstract Gas-liquid microreactors find an increasing range of applications both in production, and for chemical analysis. The most often employed flow regime in these microreactors is Taylor flow. The rate of absorption of gases in liquids depends on gas-side and liquid-side resistances. There are several publications about liquid-side mass transfer coefficients in Taylor flow, but the data about gas-side mass transfer coefficients are practically non existent. We analysed the problem of gas-side mass transfer resistance in Taylor flow and determined conditions, in which it may influence the overall mass transfer rate. Investigations were performed using numerical simulations. The influence of the gas diffusivity, gas viscosity, channel diameter, bubble length and gas bubble velocity has been determined. It was found that in some case the mass transfer resistances in both phases are comparable and the gas-side resistance may be significant. In such cases, neglecting the gas-side coefficient may lead to errors in the experimental data interpretation.

Mass transfer studies on split-packing and single-block packing rotating packed beds

In this work, SO2 absorption in aqueous NaOH (gas side mass transfer resistance controlled system) and O2 desorption from the water (liquid side mass transfer resistance controlled system) are experimentally evaluated for enhancement in the controlling side volumetric mass transfer coefficient using the novel split-packing and conventional single-block rotating packed bed (RPB) designs. In the split-packing RPB design, mass transfer characteristics for co-rotation and counter-rotation of adjacent rings are studied. For the SO2 absorption system, results show a significant reduction in the controlling mass transfer resistance for split-packing over single-block packing for large RPBs. However, the mass transfer coefficients for co- and counter-rotation are comparable. For the O2 desorption system, at low rotation rates, the split-packing design gives higher mass transfer rates compared to the single-block packing. This difference disappears as the rotation rate is increased. Possible reasons for the experimental observations are speculated. The results suggest the split packing design to be more suitable for gas side mass transfer resistance controlled systems.

Scrubbing of CO2 Greenhouse Gases, Accompanied by Precipitation in a Continuous Bubble-Column Scrubber

A continuous bubble-column scrubber that absorbs carbon dioxide (CO2) gas using an alkaline solution under a pH-stat condition was used to explore the effects of the pH of the solution, gas concentration, gas-superficial velocity, and liquid-flow rate on the absorption rate and volumetric mass-transfer coefficient of CO2. When the scrubbing factor was evaluated, this scrubber outperformed other scrubbers. Correlations of local volumetric mass-transfer coefficients with process variables are presented here and the gas−liquid interfacial area and individual mass-transfer coefficient are discussed to provide insight into the mass-transfer mechanism. Investigation of the liquid-side mass transfer coefficient revealed that the kL/[OH−]0.5 value was a constant, thus demonstrating that the system obeyed second-order kinetics. Evidence also shows that liquid-side resistances are within the range of 27%−99%, indicating that gas-side mass-transfer resistance cannot be neglected in some cases. This implies that the absorption mechanism is flexible and can be shifted by adjusting the process parameters.

Mass diffusion in a spherical microporous particle with thermal effect and gas-side mass transfer resistance

An analytical solution for mass diffusion in a spherical microporous particle experienced with a small step change of gaseous phase adsorbate concentration is obtained. The mass diffusion in the solid is assumed to be micropore diffusion controlled. Thermal effect and gas-side mass transfer resistance are considered. The governing equations are solved by using the Laplace transformation method. Three factors, α, β and γ are defined to govern the heat and mass transfer. α and β are relevant to the thermal effect and γ dominates the gas-side mass transfer resistance. The applicable ranges of three simplified models are discussed. A limiting solution with mass diffusivity approaching infinity is obtained.

Behavior of Venturi Scrubbers as Chemical Reactors

Venturi scrubbers have been used for a long time as devices to remove particulates from gas streams. However, the large surface area of the drops also makes them effective gas-liquid reactors, particularly for scrubbing pollutant gases from an inert stream, and this aspect has been much less studied. Solutions to the equation of absorption with first-order chemical reaction in spherical polar coordinates are presented in graphical form as plots of dimensionless mass absorbed against dimensionless time, with dimensionless rate constant, gas volume, and gas-side mass-transfer coefficient as parameters. An asymptotic solution which combines the effects of finite gas volume and gas-side mass-transfer resistance is derived and its range of validity assessed; an extension to the case of infinite reaction rate is also proposed. The results are applied to the design of venturi scrubbers and compared with the experimental data of Ravindram and Pyla. Serious discrepancies are apparent, and possible reasons for these are discussed.

Modeling of batch catalytic chlorination of toluene in a bubble column

The chlorination of toluene was studied in a batch bubble column in the presence of ferric chloride. Mass transfer studies (in the absence of dissolved catalyst) showed that Cl 2 absorption into toluene was controlled up to 60% by the gas side mass transfer resistance. In addition, the experimentall obtained liquid side mass transfer coefficients were found to be very low, i.e. more than 10 times lower than the predictions from common correlations. A model was developed which describes the batch chlorination process with striking agreement on the basis of the mass transfer data measured in this study and the solubility and kinetic data reported recently. It was found that the first chlorination step took place in the transition from diffusiona to fast reaction regime. However, due to the dominating gas side resistance, the overall process was only slightly enhanced by the reaction in the liqu

The dry spinning process: Comparison of theory with experiment

AbstractTheoretical calculations based on a mathematical model of cellulose acetate dry spinning were compared with experimental data. Both experiment and theory indicate that the velocity in the spin line becomes essentially constant and equal to the take‐up speed at relatively short distances from the jet. Spin line diameter, however, continues to decrease with increasing axial position. Theory predicts that the filament diameter decreases rapidly at very short distances from the jet, but that the rate of this decrease tends to level off at greater distances. The experimental data exhibit similar trends. In addition, the theoretical and photographically observed spin line die swells are in good agreement. For a typical set of cellulose acetate extrusion conditions, computations show that within relatively short distances from the jet neither the diffusional resistance within the spin line nor the gas side mass transfer resistance in controlling. Both are significant. Calculations also indicate that both the filament and the gas side thermal resistances are important.