Mass Transfer In Membrane Contactors Research Articles

Membrane assisted process intensification and optimization for removal and recovery of phenol from industrial effluents

The best method of preserving the environment is not to discharge any contaminant but to recover them as value from the effluent streams thereby adopting circular economy. This calls for developing new strategies including intensification of conventional processes to make the recovery of contaminants cost effective and environmentally friendly. In this context, experimental studies are reported with respect to conventional solvent extraction and membrane assisted solvent extraction for recovering phenol from effluent streams. Initial studies are carried out with aqueous solutions of phenol with concentrations up to 500 mg/L using hydrophobic polyvinylidene fluoride based hollow fiber membrane element in tandem with conventional solvent extraction using 1- hexanol as solvent. The operating parameters for the membrane assisted solvent extraction were optimized in parallel with conventional solvent extraction using the statistical tool, response surface methodology to provide a comparative assessment highlighting the advantages of membrane assisted solvent extraction in terms of energy consumption, operational simplicity and environmental friendliness. The studies were validated using real effluents obtained from a formaldehyde resin manufacturer. The comparative study highlights the specific advantages and demonstrates the superiority of membrane assisted solvent extraction using a real industrial effluent. It was observed that more than 98 % of phenol could be removed from effluent in membrane assisted solvent extraction and recover about 90 %, continuously allowing the recycle of the solvent. Being closed loop operation, no solvent is exposed to the environment making the process environmentally friendly operation. The mechanism of mass transfer in membrane contactors follows resistance in series model and the experimental values of mass transfer coefficient were estimated to be between 1.7 × 10 −7 and 2.2 × 10 −7 m/s, whereas the empirical values were around 1.3 × 10 −7 m/s.

Simulation of the enhancement of Dean flow on the liquid–liquid extraction in membrane contactors

Dean vortices generated through the curved paths have been demonstrated to be beneficial to enhance gas–liquid mass transfer in membrane contactors. However, the effect of Dean vortices in liquid–liquid membrane extraction was seldom studied. This work implemented a numerical model describing the liquid–liquid extraction process in curved membrane contactors with the software of COMSOL. The liquid–liquid extraction experiments were first performed verifying the applicability of the simulation. Firstly, the discussion about the role of membrane resistance revealed the fact that enhancement provided by Dean flows would be more pronounced for systems with lower membrane resistance. A parametric study was then conducted including geometric parameters and physical properties of the fluid, which indicated smaller pitch, smaller curvature diameter, and less viscous fluid led to a better enhancement. The intensification factor was in the range of 1 ∼ 1.8, corresponding to the Dean number range of 5 ∼ 30. Considering the energy consumption, the values of modified intensification factors were lower than 1 only when the Dean numbers were smaller than 8. Finally, the performance of four curved geometries: helically-wound, 90-degree bend, meander-shaped, knotted were compared. The presented model yielded new insights into the liquid–liquid exaction enhancement in the curved membrane contactors.

Absorption of CO2 from flue gas under oscillating gas flow conditions in gas-solvent hollow fibre membrane contactors

Membrane gas-solvent contactors are a viable approach for the absorption of carbon dioxide (CO2) into solvents, such as potassium glycinate, compared to traditional solvent absorption. The hollow fibre membrane design does have limitations, as the presence of the membrane adds extra resistance to mass transfer compared to traditional solvent absorption. To reduce the impact of this extra resistance it is necessary to increase mass transfer in the solvent and gas phase boundary layers. This investigation aimed to increase mass transfer in the gas phase boundary layer of a membrane contactor process by applying oscillating gas flow conditions. For an asymmetric polydimethylsiloxane membrane contactor undergoing oscillating gas flow conditions, an average increase of 19% in overall mass transfer coefficient (KG) compared to non-oscillating gas flow was observed, for 15 wt% potassium glycinate solvent. The oscillating frequency of the gas phase had only a minor impact on performance, above 2 Hz, implying that oscillation of the gas phase was important for mixing, not the rate of the oscillation. Alternatively, KG approximately doubled in value as a function of increased oscillating amplitude. This was due to the resulting pressure wave producing localized increases in the partial pressure driving force for mass transfer across the membrane. For the membrane contactors studied, mass transfer correlations, in terms of Sherwood number, were proposed for the oscillating and non-oscillating flow conditions and fitted to the experimental data. This outcome highlighted the importance of inducing mixing within the gas phase to maximise mass transfer in membrane contactors.

Influence of pH on gas phase controlled mass transfer in a membrane contactor for hydrogen sulphide absorption

The influence of pH on mass transfer in a hollow fibre membrane contactor applied to a dilute hydrogen sulphide (H2S) gas stream has been studied. When using sodium hydroxide to control pH, a threshold was established at pH 10 where the reaction between sodium hydroxide and H2S could be described as pseudo-first order with respect to H2S, indicating an excess of reactive hydroxide. For solvent pH above 10, the reactivity counteracted the low H2S solubility constant and negated liquid side resistance thus the process was predominantly gas phase controlled. Whilst the gas side hydrodynamics were firmly within the laminar regime, the experimentally determined overall mass transfer coefficient recorded in this study compared well to previous investigations at higher solvent concentrations. Based on the experimental data, a solvent pH of 11 was deemed most economically attractive. This sodium hydroxide concentration facilitates efficient H2S removal at solvent concentrations several orders of magnitude below those proposed in previous contactor studies. The data therefore illustrates that less concentrated chemical solvents can be used without demanding further membrane area to counterbalance the lower reactivity since [OH−] is already in excess at these solvent concentrations.

Implementation of the Finite Element Method for Simulation of Mass Transfer in Membrane Contactors

AbstractMathematical modeling and numerical simulation of gas separation by means of polymeric membrane contactors is presented. The finite element method is implemented for numerical simulation. COMSOL Multiphysics is used for simulation. Continuity equations are solved via computational fluid dynamics techniques based on the finite element method. A laminar velocity profile is applied for the solvent. Velocity distribution of the gas flow in the contactor is obtained by Happel's model. The predictions of percent CO2 removal obtained by the modeling were compared with the experimental values from literature for CO2 removal from CO2/N2 gas mixtures with amines. The modeling predictions were in good agreement with experimental data for different values of liquid flow rates.

Computer design of recycle membrane contactor systems for gas separation

The mathematical model of multicomponent mixture mass transfer in membrane contactor has been developed. The model is based on the calculation of mass balance in absorber and desorber taking into account the recirculation of the liquid carrier flow. Paper presents the results of simulation of recycle membrane contactor system (RMC) for biogas separation and air dehumidification application. The practical recommendations for optimal selection of contactor geometrical parameters and hydrodynamic regime are given. The optimal membrane area ratio in absorber and desorber in recycle system with physical absorbent as liquid carrier is determined.

Mass transfer studies in flat-sheet membrane contactor with ozonation

Ozone mass transport in the flat-sheet membrane contactor with chemical reaction in the liquid phase has been investigated. The ozone mass transfer coefficients without reaction in the liquid phase were indirectly determined by the oxygen transfer measurement with the steady state method. The oxidation of nitrite to nitrate was utilized to study mass transfer with ozonation. The ozone fluxes and the enhancement factors ( E) were predicted by the facilitation of the resistance-in-series model together with the film and surface renewal models. The performance of the designed membrane modules was assessed by the decolorization of NOM solutions and indigo reagent I. The ozone mass transfer coefficients from oxygen transport experiments were considerably higher than those calculated from the available correlations but provided good predictions for ozone fluxes for pure water. The ozonation accelerated mass transfer in membrane contactor by 6–100 times in this study. The experimental results validated the proposed methods for the predictions of the ozone fluxes and the enhancement factors. The average discrepancies between the experimental and calculated results were 6% for the ozone flux predictions and 3% for the enhancement factor predictions. The resistances to the ozone mass transfer with and without chemical reaction were controlled by the liquid film. The ozone consumption for the NOM decolouriration in the membrane contactor was considerably lower than that in a conventional bubble column.