Side Mass Transfer Research Articles

Nitrogen stripping of isopropyl-alcohol and toluene in a falling film micro reactor: Gas side mass transfer experiments and modelling at isothermal conditions

A falling film micro reactor has been investigated as a contactor for a separation process in order to be implemented in fine chemical and pharmaceutical production. Nitrogen stripping of a pure solvent was performed allowing straightforward mass transfer measurements due to the absence of resistance in the liquid side.Two mass transfer models were developed in this study, a semi-analytical one based on the mass balance equations combined with an existing Sherwood correlation for laminar flow in micro channels and a numerical model solving the hydrodynamics and mass transport equations. Very good agreement was found between the two models, which were then compared with experimental data. The experimental values of stripping efficiency agree with the calculated ones with an error of ±15% showing good modelling prediction. The calculated gas side mass transfer coefficients of the investigated micro contactor are in the range of 0.007–0.01ms−1.

Mass transfer and liquid hold-up determination in structured packing by CFD

Mass transfer and liquid hold-up in structured packing geometry are investigated using the volume of fluid method. Numerical simulations of two-dimensional co-current gas–liquid flow on structured packing with interfacial mass transfer are performed. The volume of fluid method is used to capture the gas–liquid interface motion. The mass transfer is computed by solving the concentration equation with an adapted modeling of the solubility (Haroun et al., 2010b). The liquid hold-up and the mass transfer are studied as function of liquid flow rate and structured packing geometry. Results show how the liquid flow rate and the complex geometry affect the liquid film flow topology and the interfacial mass transfer. For a specified packing geometry, it is demonstrated that for low liquid flow rate, the liquid film remains uniform and follow closely the profile of the structured wall. For uniform liquid film flow along packing wall, it is found that the liquid hold-up is in good agreement with the model proposed by Billet and Schultes (1999) and Raynal and Royon-Lebeaud (2007). When increasing the liquid flow rate, the liquid film does not follow the shape of the structured wall anymore, a static hold-up (recirculation zone) form in the cavities and grows as the Reynolds number increases until covering most of the packing cavities. The present work gives the liquid hold-up evolution for each liquid film flow regime according to the Reynolds number and the dimensionless amplitude of the corrugation. Concerning the liquid side mass transfer, it is found that the liquid side mass transfer is well predicted by the Higbie (1935) theory provided that adequate velocity and length scales are considered for exposure time determination. The exposure time of fluid element at the interface corresponds to the ratio between the curvilinear distance between two periodic corrugation contact point and the interface velocity. An exposure time model is proposed taking into the account physical and geometric parameters.

Statistical tool combined with image analysis to characterize hydrodynamics and mass transfer in a bubble column

Image analysis technique has been proved to be very effective in the quantification of particles size and morphology distributions in different work areas. In the present work this technique was combined with the discriminant factorial analysis (DFA) in order to allow the automatic identification of single bubbles (isolated bubbles without influence of surrounded bubbles) in multiphase systems. With the previous methodology it has been possible to distinguish online and automatically among three different classes of bubbles (single bubbles and medium complexity and large complexity bubbles groups), allowing for the first time the computation of the local bubble population complexity in the system. The automatic and correct characterization of the single bubbles allowed the correct determination of bubble size and, consequently, the specific interfacial area a at different experimental conditions. Agreement between automated and manual classification, measured in terms of a performance index, is 98% for single bubbles identification. Further, the present work describes the application of such methodology to the study of temperature, type of gas sparger, and liquid phase properties (viscosity and surface tension) influence on the individual components of volumetric liquid side mass transfer coefficient, k L a. The results show that the different experimental parameters and liquid properties act by a particular way on k L and a.

Experimental and theoretical mass transfer transient analysis of copper extraction using hollow fiber membrane contactors

Extraction of copper (II) from aqueous solutions using trifluoro-acetylacetone (TFA) diluted in 1-decanol, MIBK and tetradecane at 298 and 313 K was investigated experimentally and theoretically. Experimental part of the study concerns with the partitioning of copper (II) in both phases at equilibrium. Distribution of copper (II) in aqueous and organic phases was studied to investigate the hypothesis of constant partition coefficient for solvent extraction mass transfer analysis in hollow fiber membrane contactors (HFMCs). Partition coefficient of copper complexant was measured from extraction equilibrium analysis. Hatta number was calculated to define the kinetic regime. In the second part of experimentation, copper (II) extraction with TFA diluted in 1-decanol using hollow fiber membrane contactor closed system was examined based on aforementioned batch extraction kinetics. Previously developed theoretical model [M. Younas, S. Druon-Bocquet, J. Sanchez, Kinetic and dynamic study of liquid–liquid extraction of copper in a HFMC: experimentation, modeling and simulation, AIChE J. 56 (2010) 1469–1480] was modified to a more robust single-fiber flow model for mass transfer in non-dispersive solvent extraction in HFMC-based integrated plant. The model was, then, validated with the experimental results for dynamic responses of the solute concentration in reservoir. Experimental results were analysed with various shell side mass transfer correlations. It has been shown, by model simulations that the extent and speed of the extraction depend upon the copper (II) and TFA initial concentration, temperature and physical characteristics of solute and solvents. Likewise flow configuration affects the speed of extraction depending upon the nature of solute and solvent.

Kinetics of VOC absorption using capillary membrane contactor

The aim of this work was to experimentally study the acetone absorption in water by means of capillary membrane contactor. A “resistance in series” model was used to describe the rate of mass transfer. The experiments were designed in batch system for laminar and transient flow region. Results were approximated by mathematical model based on mass balance equations. The kinetic model enables to calculate two parameters: the equilibrium constant and mass transfer coefficient and takes into account the gas concentration profile along the equipment. The Wilson method was used to calculate membrane mass transfer resistance. Eight series of experiments for two types of modules and various flow arrangements were made for accurate approximation of membrane resistance and gas/liquid mass transfer coefficients. The results follow previous observations on advantages of the absorption process in the membrane contactors, additionally the process stability and a high performance compared to conventional absorption equipment were confirmed. However, there is still lack of appropriate description of shell side mass transfer in case of randomly packed polymeric fibers. A new correlation is proposed on the bases of conducted experiments.

Effect of some solid properties on gas–liquid mass transfer in a bubble column

Abstract The knowledge about the effects of solids on gas–liquid systems and the respective physical mechanisms are not yet totally clarified. In this work, the effect of the solids on the mass transfer characteristics in a bubble column was studied experimentally for the systems air/water/expandable polystyrene (EPS) beads and air/water/glass beads. Volumetric liquid side mass transfer coefficient, k L a, was determined under different solid concentrations (up to 30 vol.%), superficial gas velocities (up to 2.7 mm/s) and mean diameters (1100, 770 and 591 μm for EPS and 9.6 μm for glass beads). The presence of EPS solids affects negatively k L a being this effect more pronounced for the smaller particles. Also, a decrease in k L a occurs when the solid loading increases. Experiments done with large polystyrene particles ( d p ≥ 591 μm) contaminated with very fine EPS particles ( d p ≅ 0.1 μm) indicate that very fine particles play an important role on gas–liquid mass transfer. Mass transfer experiments in a hollow glass spheres three-phase slurry showed a dual effect of solids loading on k L a, contrarily to what happens with the previous particles. These results can be associated with the different surface properties of the particles studied. An empirical correlation for k L a on the experimental variables was developed.

Kinetics and mass transfer of carbon dioxide absorption into aqueous ammonia

Aqueous ammonia can be used to capture CO2 from flue gas of coal -fi red power plant with acceptable CO2 removal efficiency, high loading capacity of CO2, low corrosion, low cost, and less degradation, and it could remove other acid gas pollutants at the same time. However, the CO2 absorption rate and mass transfer in aqueous ammonia still need to study more deeply. The mass transfer characteristics of CO2 in aqueous ammonia were investigated in this paper. A small wetted wall column (WWC) was used to study the kinetics of CO2 absorbed in aqueous ammonia. The correlation of gas -side mass transfer coefficient kg was fitted as Sh=3.79(Re×Sc×d/h)0.486 based on the experimental data of CO2 absorption in 30%wt MEA solution for this WWC reactor. The reaction rates of CO2 in 1 wt%, 2.5 wt%, 5 wt% to 7.5 wt% aqueous ammonia were measured at the temperatures of 10 °C, 20 °C, 30 °Cto 40 °C, and the second-order reaction rate constant k2 was calculated. The termolecular mechanism model was used and discussed according to the experimental results. The kinetics constants of kTNH3 and kTH2O in termolecular mechanism were also fitted in this paper.

Gas–liquid mixing in a multi-scale micromixer with arborescence structure

A novel multi-scale micromixer with arborescent structure for high throughput gas–liquid mixing was evaluated by absorbing pure CO2 into alkaline solutions, and the volumetric mass transfer coefficient, interface area, liquid side mass transfer coefficient and pressure drop were determined for different configurations and operations of the micromixer. When the two fluids are first partitioned into sub-streams and impinging in opposite directions, and the mixing chamber has also an arborescent structure, the mixer (T-M) has a superior performance when the gas feeding rate is sufficiently high. For a small gas feeding rate, the mixer (J-M-G) will have a higher mass transfer coefficient if the gas is introduced from the center plate, which was used as the mixing chamber in T-M, but at the cost of larger pressure drop when compared with T-M. T-M has the best performance under all operating conditions if both mass transfer coefficient and pressure drop are taken into account.

Mass transfer and hydrodynamic characteristics of new carbon carbon packing: Application to CO 2 post-combustion capture

A novel structured packing, the 4D packing, has been characterized in terms of hydrodynamics, effective area and gas side mass transfer coefficient. The increase of the 4D opening fraction allows to reduce pressure drop and to get a better capacity than Mellapak 500Y and 750Y, for which the geometric areas are similar. The 50% open 4D packing, 4D-50%, leads to effective areas which are higher than Mellapak 500Y ones, and doubled compared with MellapakPlus 252Y ones. Effective areas for the 4D do not decrease when the opening fraction increases from 30 to 50%, this indicates that a non-negligible amount of droplets is generated at 50%. Gas side mass transfer coefficient had been measured with an original experimental method: water evaporation. Corresponding results seem to be in agreement with the literature, and with the fact that a large amount of droplets is generated. Correlations are proposed for both effective area and gas side mass transfer coefficient for the 4D-50%. The 4D-50% packing could be very interesting for post-combustion CO 2 capture since it generates low pressure drop and a very high interfacial area. This will be further confirmed by an economic study for which the absorber plant will be designed with a rate based model.

Hydrodynamic study and optimization strategy for the surfactant recovery from aqueous solutions

Foam fractionation for recovery of cetyl pyridinium chloride (CPC) from aqueous medium was studied. The effect of process parameters such as air flow rate, foam height, feed concentration, pH, and time was investigated to optimize the process. The maximum recovery of CPC obtained was 97.71% to 94.18% with an enrichment ratio of 3 and 1.21 for 8 CMC to 20 CMC. The average bubble size of the foam in the fractionation column was estimated by the photographic technique from which interfacial area was determined. Effect of surfactants on the theoretical liquid side mass transfer coefficient ( k L) was also investigated which is based on the determination of the diffusivity coefficient of surfactant on the surface of the liquid and was found to be 7.52 × 10 − 3 m/s whereas the experimental liquid side mass transfer coefficient was determined from the local volumetric mass transfer coefficient ( k La) and was found to be 7.01 × 10 − 3 m/s for the feed concentration of 20 CMC.

Marangoni effect in a falling film microreactor

Mass transfer in some gas–liquid systems may be significantly enhanced by the Marangoni effect. In the case of a falling film microreactor (FFMR) for gas–liquid reactions, the question whether the interfacial tension driven cells would develop in the limited space of a microchannel remains unsolved. One way of approaching this problem is to measure the mass transfer rate in a microreactor using a system known to exhibit the Marangoni effect in the macroscale, and to compare this rate with the value determined for the analogous process without this effect. Gas–liquid mass transfer in a FFMR with 29 microchannels (0.6 mm width each) was investigated. CO 2 was absorbed from a CO 2/N 2 gaseous mixture into an aqueous solution of NaOH, and the liquid side mass transfer coefficient as well as the gas side mass transfer coefficient were measured. Then the possibility of appearance of the Marangoni effect was investigated using aqueous monoethanolamine (MEA) solution. The existence of the cellular convection was observed. The enhancement of the overall rate of the mass transfer depends on both gas and amine concentrations.

Heat transfer during reflux condensation of an R134a/R123 mixture in vertical and inclined narrow tubular and rectangular channels

Heat transfer during reflux condensation in a narrow tube (0.007 m inner diameter, 0.5 m length) and a rectangular channel (0.007 m hydraulic diameter, 0.5 m length) was investigated. The basic mechanism of this process, i.e. removing high boiling point components from a stream of a more volatile fluid or even from inert gases, is still not sufficiently understood. The study presents experimental results with the zeotropic mixture of R134a and R123 at a constant composition of the incoming vapour of 50 mol%. The results show that both, gas side heat and mass transfer resistances and the resistances in the liquid film influence the condensation process. Channel inclination is less important in mixture condensation than in pure fluid condensation but not negligible, and increasing vapour mass flow enhances mixture condensation. Contrary to pure fluid results heat transfer in the rectangular channel with the same hydraulic diameter is much smaller than in the tube at the same vapour mass flow rate.

Temperature and solid properties effects on gas–liquid mass transfer

The knowledge about the effect of solid phase properties, at different temperatures, on gas–liquid transfer and the respective physical mechanisms has been poorly studied. In the present work, the temperature and solid properties (size and density) effects on the gas–liquid mass transfer characteristics in a bubble column were experimentally evaluated. Gas–liquid–solid systems formed by air/water/polyvinyl chloride (PVC) beads and air/water/expandable polystyrene (EPS) beads were used. For each system, volumetric liquid side mass transfer coefficient, k L a, was determined under different temperatures (20–35 °C), superficial gas velocities (up to 7.2 mm/s), solids sizes (210, 549 and 591 μm) and concentration (up to 5 vol.%). The results show that the temperature plays an important role on mass transfer phenomena as k L a increases as temperature is increased. However, temperature does not have, significantly, influence on the solids effect on k L a. In what concerns the effect of the solids on k L a, a negative effect of their presence is observed being this effect more pronounced for the largest particles (for PVC). In addition, for the same solid size, a decrease in k L a occurs when the solid loading increases (observed in both cases, PVC and EPS). Complementary, bubble/particle interaction studies, using an image analysis technique, were also done aiming a better understanding of the effects of the two types of particles – PVC and EPS – on k L a values, being clearly confirmed that physical (e.g. density) and chemical properties of the solids are important parameters to be taken into account on mass transfer and hydrodynamic studies. Based on theoretical models for k L and a, an empirical correlation for k L a dependence on the experimental variables was developed. The experimental results were fitted with a mean deviation of 5%, that is similar to the experimental error (5%).

Comparison of shell side mass transfer correlations in randomly packed hollow fiber membrane modules

In this work, the process of membrane-based solvent extraction was examined with 10% (v/v) tributyl phosphate in kerosene/phenol/water as experimental system, involving solute mass transfer from the aqueous phase (flowing in the shell or tube side) to the organic phase. Shell side mass transfer performance was determined in two randomly packed membrane modules with different packing densities. Experimental data of shell side mass transfer coefficient were compared with model predictions from nine existing empirical correlations. Results showed that the correlations of Gawronski et al. and Viegas et al. were found to give the good predictions in these modules.

Mass transfer characteristics of H 2S absorption from gaseous mixture into methyldiethanolamine solution in a T-junction microchannel

In this work, the mass transfer characteristics of absorption of H 2S (1200 ppmv) into 40 wt% methyldiethanolamine (MDEA) solution have been investigated experimentally in a horizontal cross-flow T-junction microchannel with a diameter of 1.00 mm. The interfacial area was determined by the absorption of CO 2 into NaOH solution. It was found that the high removal efficiency of H 2S (up to 99.5%) was achieved in the microchannel. Mass transfer resistance existed mainly on the gas side in this absorption process. It was shown that the gas side volumetric mass transfer coefficient and interfacial area were as high as 31,746 mol/(m 3 s atm) and 11,100 m 2/m 3, respectively. In addition, a comparison of mass transfer performance among different gas–liquid contactors was performed. It was revealed that the T-junction microchannel in this study could provide at least one or two orders of magnitude higher gas side volumetric mass transfer coefficient than others. However, the intensification of mass transfer process was mainly ascribed to the enhancement of interfacial area rather than the gas side mass transfer coefficient. Empirical correlations based on the experimental data were developed to predict gas side mass transfer coefficient and interfacial area.

Formaldehyde removal from air during membrane air humidification evaporative cooling: Effects of contactor design and operating conditions

Counter-current parallel flow membrane contactors were studied to estimate likely formaldehyde removal rates from air occurring during membrane air humidification and evaporative cooling applications. A mathematical model and experimental measurements of formaldehyde removal are presented to assess the impact of possible operating conditions and contactor configurations on expected formaldehyde removal rates. Removal efficiencies were predominately controlled by shell side mass transfer coefficients. However, removals decrease when water velocity within the fiber lumens is low and water supplied to the modules approach module evaporative losses. While removal rates were found to be adequate to control formaldehyde emissions in most space air-conditioning applications, large volumes of water usage appear to be necessary.

Effects of high hydrostatic pressure on distribution dynamics of free amino acids in water soaked brown rice grain

High hydrostatic pressure (HP) with approximately below 400 MPa can induce a transformation of food materials to an alternative form, where membrane systems are damaged but certain enzymes are still active. HP treatment of water soaked brown rice grain could modify the mass transfer inside and apparent activities of enzymes, resulting in HP-dependent change of distribution of free amino acids. Thus, the distribution of free amino acids in brown rice grain during preservation after HP treatment was analyzed. Just after HP treatment at 200 MPa for 10 min, the distribution of free amino acids was not apparently different from that of untreated control. In contrast, after 1 to 4 days preservation at 25°C, amino acids, such as Ala, Glu, Gly, Asp and Val, showed higher concentrations than those in control. This result suggested that HP treatment induced proteolysis to produce free amino acids. However, Gln, Thr and Cys, showed no apparent difference, suggesting that conversion of certain amino acids produced by proteolysis occurred. Moreover, the concentration of γ-aminobutyric acid (GABA) in HP-treated sample was higher than that in untreated control. These results suggested that HP treatment induced alteration of distribution of free amino acids of rice grains via proteolysis and certain amino acids metabolism pathways.

A numerical study of the assumptions underlying the calculation of the stationary zone mass transfer coefficient in the general plate height model of chromatography in two-dimensional pillar arrays

The present study investigates the validity of one of the key assumptions underlying the general plate height model of chromatography, i.e., the presumed independency of the individual band broadening contributions. More precisely, it is investigated under which conditions the mass transfer inside the stationary zone (e.g., porous pillars) is independent from the axial transport of species outside this zone, and how strongly any such dependency would affect the validity of the general plate height model of chromatography. For this purpose, detailed calculations of the species concentration distribution inside and outside the porous pillars of a computer-mimic of a porous pillar array column have been made. These simulations revealed a clear interplay between the mass transfer inside and outside the pillars, manifesting itself as an asymmetry of the species concentration distribution inside the pillars. The latter is in disagreement with the basic assumption used to calculate the value of the C s-term of the general plate height model. The asymmetry-effect is largest at low reduced velocities, high retention factors and high intra-pillar diffusion coefficients. Fortunately, these are conditions where the C s-term is relatively small, which might explain why the general plate height model of chromatography (and based on the symmetry assumption) can represent the band broadening in a porous pillar array within an accuracy on the order of some 1–2%.

Volume of fluid method for interfacial reactive mass transfer: Application to stable liquid film

A volume of fluid method is developed in order to simulate reactive mass transfer in two-phase flows and is applied to study reactive laminar liquid film. The thermodynamic equilibrium of chemical species at the interface is considered using Henry's law. The chemical species concentration equation is solved using primitive variables and local fluxes are locally directly calculated at the interface. The present treatment of jump discontinuity of chemical concentration is consistent with a volume of fluid approach and the difficulty to calculate accurate local mass flux across interface is overcome. For plane interface, the precision of the numerical simulation is found to be very satisfactory while for curved interface a special procedure has been developed to reduce the development of spurious fluxes at the interface. The algorithm is validated for different cases by comparison with available solutions. The method is then applied to study non-reactive and reactive mass transfer in a falling liquid film. The results show that the liquid side mass transfer is well predicted by the Higbie (1935) theory when the transfer is controlled by the film advection provided that adequate parameters are considered, i.e. the actual velocity at interface and not the average liquid film velocity. For situations controlled by diffusion, the Sherwood number tends to a constant value characteristic of purely diffusive situations. For the reactive mass transfer, first and second order irreversible chemical reactions in the liquid phase are considered. The numerical results are compared respectively, with Danckwerts (1970) and Brian et al. (1961) solutions and good agreement is observed. The proposed Volume of Fluid method is shown to be well adapted to deal with interfacial reactive mass transfer problems.

Direct numerical simulation of reactive absorption in gas–liquid flow on structured packing using interface capturing method

Direct numerical simulations are performed in order to study physical and reactive absorption in gas–liquid flow on structured packing. The volume of fluid method is used to capture the gas-liquid interface motion. The mass transport is computed by additional chemical species concentration transport equation with adequate modelling of solubility and chemical reaction. The numerical difficulties arise in imposing jump discontinuity for chemical concentrations at the interface due to different solubility. These difficulties are solved by an original method using a continuum mechanical modelling of two phases flow and Henry's law with constant coefficient. The present study shows how the mass transfer is affected by the complex geometry considered here and by the flow conditions. The results show firstly that the liquid side mass transfer is well predicted by the Higbie theory and the exposure time of a typical element of volume near the interface corresponds to ratio between characteristic length and velocity of the interface provided that the real velocity of the interface is used. For the considered geometry, the transfer is found to be increased compared to the transfer of a plane liquid film. Finally, for the case where the mass transfer is accompanied by second order irreversible chemical reaction in the liquid phase, the numerical results are compared to approximate solution presented by Brian et al. [1961. Penetration theory for gas absorption accompanied by a second order chemical reaction. A.I.Ch.E. J. 7, 226–231] and good agreement is observed.