Side Mass Transfer Resistance Research Articles

CO2 absorption using benzylamine as absorbent and promoter in a hollow fiber membrane contactor: A numerical study

Benzylamine (BZA) is a novel amine that indicates a promising potential for CO2 absorption. In this work, a mathematical model has been presented to investigate the CO2 absorption using aqueous BZA compared to aqueous monoethanolamine (MEA), diethanolamine (DEA), and 4-diethylamino-2-butanol (DEAB) in a hollow fiber membrane contactor (HFMC). Several factors such as CO2 partial pressure, amine concentration, gas velocity, liquid velocity, temperature, wettability, and mass transfer resistance of phases have been studied. According to the results, increment in the amine concentration, CO2 partial pressure, liquid velocity, gas velocity, and temperature have enhanced the CO2 absorption flux. Also, adding BZA to aqueous MDEA and K2CO3 significantly increases the CO2 absorption flux in both solutions. Moreover, the mass transfer resistance of individual phases reveals that the liquid phase mass transfer resistance is the controlling phase in non-wetted mode, while with membrane wetting, contribution of membrane side mass transfer resistance increases and become the controlling phase. In general, BZA could be a prominent potential choice as absorbent and promoter for CO2 absorption in HFMCs.

Modeling of autocatalytic degradation of polymer microparticles with various morphologies based on analytical solutions of reaction-diffusion equations

Analytical solutions of transient concentration of degraded components inside cylindrical and slab-type PLGA particles immersed in infinite medium were derived by solving reaction-diffusion equations of autocatalytic reaction using eigenfunction expansion method. The resulting average concentrations were compared with the modeling results of spherical PLGA particles by Versypt and her colleagues to study the effect of particle morphology on the autocatalytic reaction. Mass transfer resistance inside and outside of the particles was also considered using Biot number, and its effects on the concentration inside particles with various morphologies were also studied by solving reaction-diffusion equation. To predict transient concentration in surrounding medium, coupled differential equations were solved for the three shapes of PLGA particles by assuming finite volume of the decomposition system. Mathematical solutions were obtained by Laplace transform, and the results were compared for the PLGA particles with different shapes depending on Thiele modulus and particle volume fraction.

Development of a novel quasi-3D model to investigate the performance of a falling film dehumidifier with CFD technology

The present study proposed a novel quasi-3D model for simultaneous simulation of heat and mass transfer process in a falling film dehumidifier with penetration mass transfer theory and CFD technology. Different from the existing 2D model, the new model took the shrinkage of falling film into consideration by introducing variable film wettability. Besides, an experimental system with a single channel falling film dehumidifier was designed and built up to validate the developed quasi-3D model. Results indicated that the shrinkage model could predict the flow pattern accurately, and the relative different between calculated and experimental wetting ratio was less than 4%. Combined with the shrinkage model, the CFD model was proved to be reliable to simulate the dehumidification process with the relative deviation of absolute moisture removal less than 10%. It was found that the non-wetting of falling film and mass transfer resistance in the air side hindered the dehumidification performance. Consequently, super-hydrophilic coating and curved plate were proposed to enlarge the wetting ratio and decrease the mass transfer resistance in the air side. By adopting the nano-TiO2 coating, the wetting ratio of dehumidifier increased from 81% to 97.8%, which led to a relative improvement of 15.2% for absolute moisture removal. In addition, the employment of curved dehumidifier could enhance the dehumidification performance by 70%.

Mathematical modeling and performance evaluation of a desiccant coated fin-tube heat exchanger

A solid-desiccant system that utilizes low grade heat is potentially a viable add-on to conventional HVAC systems since it can help reduce power consumption significantly, for achieving indoor thermal comfort conditions. In contrast to desiccant wheels which carry out adiabatic dehumidification, isothermal dehumidification process that may be realized by a cross-flow heat exchanger is much more efficient. In this paper, a novel mathematical model is developed to simulate heat and mass exchange phenomena of a desiccant coated fin tube heat exchanger (DCFTHX). This model takes solid side mass transfer resistance as well as fin efficiency into consideration. The model is validated using experimental results in the literature. It is also compared against simplified models to establish its utility. A parametric study is then conducted to investigate the effects of geometrical parameters as well as mass flow rate of water and air velocity on dehumidification and adsorption heat removal performance of the DCFTHX as well as the performance of the augmented air-conditioning system under warm and humid ambient conditions. Under the range of parameters and conditions simulated, if low grade waste heat (50 °C hot water) is available for regeneration, integration of DCFTHX sub-system with a conventional air conditioning system can yield as high as 31% energy savings (even when the additional power consumed by pumps and blower fans is accounted for).

CO2 Absorption with NaOH Solution through the Porous PVDF Hollow Fibre Membrane Contactor

In this study, porous hydrophobic polyvinylidene fluoride (PVDF) hollow fiber membranes were fabricated via a wet phase inversion process. In order to improve the phase inversion rate and provide porous membranes, 4 wt.% lithium chloride (LiCl) was used in the spinning dope. The prepared membrane morphology was studied using field emission scanning electron microscopy (FESEM). Chemical CO2absorption by NaOH solution (1M) was conducted through the PVDF hollow fiber membrane contactor. The effect of the main operating condition such as absorbent temperature, CO2 pressure and absorbent flow rate on the performance of CO2 absorption was investigated. From FESEM examination, the membrane possesses an almost sponge–like structure with ultra thin skin layer. Results of CO2absorption test showed that by increasing the absorbent flow rate the CO2 flux increased which confirmed the existence of liquid side mass transfer resistance. It was found that by increasing the absorbent temperature the CO2 flux considerably improved. Meanwhile, the effect of CO2 pressure on the absorption rate was insignificant. Therefore, it can be concluded that by applying a porous hydrophobic membrane with improved structure and optimizing the operating conditions, high CO2 removal efficiency can be achieved through gas–liquid membrane contactors

Novel dense skin hollow fiber membrane contactor based process for CO2 removal from raw biogas using water as absorbent

Abstract CO2 removal from raw biogas is required in order to meet standards for the gas grid or as a vehicle fuel. Among biogas upgrading available techniques, pressurized water absorption using packed column (PWA) is one of the most well-established technique. In this work, a novel absorption/desorption loop using dense based hollow fiber membrane contactor (HFMC) process and water as absorbent is proposed and investigated by simulation. Thanks to the ability of dense membranes to withstand a high transmembrane pressure, neither water depressurization before the desorber nor water recompression before the absorber is needed. 1D modeling based on a resistances in series approach is used for the modeling of both absorption and desorption units. HFMC process performances are compared to state of the art packed column (PWA) process reported in literature. Using commercially available dense based HFMC, the process is able to recover 96.6% of CO2 and reach biomethane purity of 98%. The corresponding energy requirement is of 0.17 kWh/Nm3 of raw biogas, which is 20 to 35% lower than that reported for packed column based process, under comparable gas inlet conditions and product specifications. HFMC process offers absorption intensification factor of about 1.68, corresponding to a volumetric reduction of about 68% of the absorption unit. Under the investigated operating conditions and due of the preponderant liquid side mass transfer resistance, process selectivity is mainly controlled by the absorbent selectivity. Without flash recovery, CH4 loss is of about 8%. No significant methane loss reduction is obtained from increasing membrane selectivity from 17 to 60 with membrane mass transfer coefficient of 5.10-4 and 5.10-5 m/s respectively. Perspectives for further process optimization are exposed.

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.

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.

Experimental and Theoretical Analysis of the Operating Parameters for Precipitating Acetaminophen and Tretinoin with Solution Enhanced Dispersion by Supercritical Fluids

The aim of this work is to analyze the experimental conditions for particle production with solution enhanced dispersion by supercritical fluids. Thermodynamics, atomization process, and mass transfer are separately studied in terms of phase equilibria, disintegration regimes, and mass transfer coefficients of liquid and vapor phases. These calculations are correlated with the precipitation data of acetaminophen and tretinoin. According to experimental results, the smallest and more spherical particles (with fewer aggregates) for both solids are obtained just above the mixture critical point of the antisolvent–solvent system with a high antisolvent–solvent mass flow rate ratio. Theoretical results indicate that a high mass transfer coefficient of the liquid phase and a high degree of supersaturation are produced at those conditions. That means a great antisolvent effect because of the low liquid side mass transfer resistance and a small particle size because of the high supersaturation. Furthermore, resul...

New liquid membrane technology for simultaneous extraction and stripping of copper(II) from wastewater

In this paper, a new liquid membrane technique, hollow fiber renewal liquid membrane (HFRLM), is presented, which is based on the surface renewal theory, and integrates the advantages of fiber membrane extraction, liquid film permeation and other liquid membrane processes. The results from the system of CuSO 4 + D 2 EHPA in kerosene + HCl show that the HFRLM process is very stable. The liquid membrane is renewed constantly during the process, the direct contact of organic droplets and aqueous phase provides large mass transfer area. These effects can significantly reduce the mass transfer resistance in the lumen side. Then the mixture of feed phase and organic phase flowing through the lumen side gives a higher mass transfer rate than that of stripping phase and organic phase, because the aqueous layer diffusion of feed phase is the rate-controlling step. The overall mass transfer coefficient increases with increasing flow rates and D2EHPA concentration in the organic phase, and with decreasing initial copper concentration in the feed phase. The overall mass transfer coefficient also increases with increasing pH in the feed phase, and reaches a maximum value at pH of 4.44, then decreases. Also, there is a favorable w / o volume ratio of 20:1 to 30:1 for this process. Compared with hollow fiber supported liquid membrane and hollow fiber membrane extraction processes, HFRLM process has a high mass transfer rate. Mathematical model for the HFRLM process based on the surface renewal theory is developed. The calculated results are in good agreement with experimental results under the conditions studied.

Forced convective boiling of ternary mixtures at high qualities

This paper deals with convective boiling of ternary mixtures in vertical tubes. Experiments were carried out in 8.58 m long, 25.4 mm ID, electrically heated test section using an n-pentane/ n-hexane/iso-octane mixture (0.31/0.22/0.47 overall mole fraction). Bulk and wall temperatures as well as local heat transfer coefficients were measured. The results were compared with those obtained using an extension to ternary (and multicomponent) mixtures of the methodology proposed by J.R. Barbosa and G.F. Hewitt [Int. J. Heat Mass Transfer 44 (2001) 1465, 1475] for prediction of forced convective boiling of binary mixtures in upward annular flow. Interphase transfer of heat and mass is dealt with using two formulations: (i) a film method in which the diffusive fluxes are calculated via a linearly generalised Fick's Law [H.L. Toor, AIChE J. 10 (4) (1964) 460; W.E. Stewart, R. Prober, Ind. Eng. Chem. Fundam. 3 (3) (1964) 224], and (ii) an effective diffusivity method. Droplet entrainment and deposition are modelled via an interchange model that takes into account the local concentration non-equilibrium between the liquid film and the entrained droplets. As for the binary case, it is claimed that the deterioration in the heat transfer coefficient is due to a combined effect of droplet interchange and mass transfer resistance in the vapour side.

Mass transfer performance of an endless belt counter-current ion exchanger for liquid phase adsorption

The performance of an endless belt counter-current liquid phase adsorption contactor has been studied experimentally for the removal of Cu 2+ from aqueous solution by a cation exchange resin. Although, such a device appears to offer the possibility of a highly efficient contactor, the performance is in fact severely constrained by the effect of solid side mass transfer resistance (diffusion of Cu 2+ within the resin) and by internal recirculation of the fluid, which becomes severe at high belt speeds. As a result of these constraints the system behaves effectively as a single theoretical stage and both the steady state and transient behaviour are adequately modeled on that basis.

Separation of CO2 by 2-amino-2-methyl-1-propanol aqueous solution in microporous hydrophobie hollow fiber contained liquid membrane

The absorption of CO2 from a mixture of CO2/N2 gas was carried out using a flat-stirred vessel and the polytetrafluoroethylene hollow fiber contained aqueous 2-amino-2-methyl-1-propanol (AMP) solution. The reaction of CO2 with AMP was confirmed to be a second order reversible reaction with fast-reaction region. The mass transfer resistance in the membrane side obtained from the comparison of the measured absorption rates of CO2 in a hollow fiber contained liquid membrane with a flat-stirred vessel corresponded to about 90% of overall-mass-transfer resistance. The mass transfer coefficient of hollow fiber phase could be evaluated, which was independent of CO2 loading.

Effect of Recycling of Feed Solution on the Efficiency of Supported Liquid Membrane Module

Abstract The effect of recycling of a feed solution from the outlet to the inlet of the feed channel of a plate-and-frame type supported liquid membrane module on the recovery of zinc was investigated under the condition that the feed side mass transfer resistance is predominant. Di(2-ethylhexyl)phosphoric acid was used as the carrier. It was found that at a given feed flow rate, recycling of the feed solution is effective for improving the recovery of zinc since recycling brings about an increase of the flow rate in the feed channel and also the mass transfer coefficient in the feed side. The experimental data could be satisfactorily simulated by a proposed theoretical model.

Modeling of multicomponent pervaporation for removal of volatile organic compounds from water

A resistance-in-series model was used to study the pervaporation of multiple volatile organic compounds (VOCs)-water mixtures. Permeation experiments were carried out for four membranes: poly(dimethylsiloxane) (PDMS), polyether-block-polyamides (PEBA), polyurethane (PUR) and silicone-polycarbonate copolymer (SPC) membranes. Three VOCs, i.e., toluene, 1,1,1-trichloroethane and methylene chloride were studied. Both organic and water permeabilities of the PEBA membrane for 1 VOC-water, 2 VOCs-water and 3 VOCs-water mixtures were found to be comparable with each other. Coupling effects for trace organic transfer through the membrane were not observed when the downstream pressure was close to zero. However, at high downstream pressure, if the downstream side mass transfer resistance dominated the overall mass transport, coupling effects might occur within the vapor phase. The downstream pressure effect for the PDMS membrane was determined. The experimental results were correlated very well by a simple mass transfer equation. The downstream pressure may have positive or negative effects on the separation factor, depending on the ratio of overall organic permeability over water permeability, β perm. The value of β perm is a function of the intrinsic organic and water permeabilities, liquid boundary layer mass transfer coefficient as well as membrane thickness. The vapor phase mass transfer resistance was found to be negligible at low downstream pressure (<15 mmHg). It was clearly shown in this work that the resistance-in-series model could be effectively to describe the pervaporation of dilute multiple VOCs-water mixtures through polymeric membranes.

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

Investigation of reactive extraction on single droplets

Extraction of dichlorphenol from freely suspended single drops into a continuous phase enhanced by an instantaneous chemical reaction was investigated using NaOH solutions of different concentrations. Also, the reverse mass transfer direction from the continuous phase into the dispersed phase was studied by extracting salicylic acid with the liquid ion exchanger LA-2. Concentrations were measured continuously without taking samples by means of a modified liquid scintillation technique. For appropriate concentrations of NaOH the continuous phase side mass transfer resistance vanishes with increasing extraction time and hence, for the first time, the individual mass transfer coefficients in the dispersed and continuous phase were determined from the overall value for exactly the same fluiddynamical conditions. Experimentally determined enhancement factors are larger than those predicted by the film theory.

Verification of multicomponent mass transfer models for condensation inside a vertical tube

This paper discusses and provides experimental evidence of the applicability of the following theories to multicomponent condensation. 1. (1) The Krishna—Standart method (1976) 2. (2) Toor's linearised theory (1964). 3. (3) An effective diffusivity model based on Colburn & Hougen (1934). It is found that all three models adequately quantify the vapour-side diffusional process for the experimental systems and conditions investigated but that multicomponent diffusional effects can be important. Criteria are discussed for the determination of the significance of diffusional interactions. The experimental data indicate that the analysis of Ackermann (1937) gives improved prediction of the sensible heat transfer from the bulk vapour phase and that the liquid side mass transfer resistance is not important under conditions studied. Computer simulation of the condenser predicts the condenser performence within an accuracy of ± 10 per cent.

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.