Analysis Of Mass-transfer Resistances Research Articles

Modeling CO2 adsorption dynamics within solid amine sorbent based on the fundamental diffusion-reaction processes

A rigorous dynamic adsorption model, considering the realistic diffusion-reaction processes of CO2 adsorption onto the amine functionalized mesoporous silica, was proposed in this work. Three fundamental adsorption steps at different length scales were took into account in model development and a numerical algorithm was developed to solve the coupled partial differential equations of the multi-scale model. TGA (Thermal Gravity Analysis) experiments at different temperatures and CO2 partial pressures were conducted to acquire the dynamic adsorption curves for model fitting and validation. The simulation results based on the proposed model agreed well with the experimental data under all conditions. Through the developed model, the species distribution, mass transfer phenomena and reaction mechanisms at different scales within the sorbent were quantitatively predicted. The results indicated that the bulk of the sorbent captured CO2 simultaneously, and no adsorption front was observed. After the very early stage, the whole sorbent reacted with CO2 uniformly. Further mass transfer resistance analysis showed that chemical reaction was the rate-limiting step, and CO2 diffusion in the amine layer would play an important role in the later stage of adsorption, especially at high temperatures or high CO2 partial pressures. These findings provided a quantitative insight into dynamic evolution of CO2 adsorption within the sorbent particle, which would guide the design of sorbents with optimal pore features and amine loadings.

Performance of a polypropylene membrane contactor for the recovery of dissolved methane from anaerobic effluents: Mass transfer evaluation, long-term operation and cleaning strategies

A polypropylene membrane contactor was used for the recovery of dissolved methane from an anaerobic reactor effluent. Effect of operational parameters, operation mode and fouling on long-term operation was studied using vacuum pressure or N2 as sweep gas. Results were analyzed based on the mass transfer estimations. Lower performance was observed in the shell-side mode due to the lower liquid velocity and the probable channeling. Membrane pore wetting was observed with the increase in QL in the vacuum-pressure mode. This was confirmed with mass transfer resistance analysis, resulting in an estimated wetted pore fraction of between 0.25 and 0.53. The highest removal efficiencies were obtained with the liquid flowing in the lumen side and sweep-gas operation (between 98% and 67% for QL between 4.1 and 27.2 L h–1), with negligible effect of the N2 flow rate. In the long-term operation, the impact of membrane fouling was less intense in the lumen side, with longer operation time and more reversible fouling. A complete characterization of the fouling based on water sample analysis concluded that both inorganic and organic foulants were present, probably with higher biofouling presence. A combination of water and chemical cleanings resulted in a recommended protocol based on daily water cleaning.

Analysis of Mass Transfer Resistance for Adsorption of Phosphate onto Industrial Waste Materials in Plug-flow column

The understanding resistance of mass transfer for adsorption of phosphate (PO4 3-) onto industrial waste such as clay brick and ceramic as an adsorbent is important to verify. This study presented the removal of PO4 3- from a synthetic solution by adsorbing onto the clay brick and ceramic wastes in a plug flow column (PFC). The experimental results showed that increasing the flow rate was decreases the breakthrough time. The mass transfer factor (MTF) models were used to study the behaviours of breakthrough curve and to determine the resistance of mass transfer. The MTF models verified that the resistance of mass transfer could be dependent on porous diffusion until the percentage of outflow reaches 72% for clay brick waste and 86% for ceramic waste, even though film mass transfer can play a minor role in controlling the movement of PO4 3- from the bulk water to film zone. The results of MTF models coefficient could be indicated to increase the capacity of clay brick and ceramic wastes to adsorb PO4 3- from synthetic solution, it needs to develop the porosity of these adsorbents by either physical or chemical modification.

Performance evaluation and mass transfer study of CO2 absorption in flat sheet membrane contactor using novel porous polysulfone membrane

The performance of gas-liquid membrane contactor for CO2 capture was investigated using a novel polysulfone (PSF) flat membrane prepared via non-solvent phase inversion method. Polyvinyl pyrrolidone (PVP) was used as an additive in the dope solution of PSF membranes. Morphological studies by scanning electron microscopy (SEM) analysis revealed that PSF membrane with PVP has a finger-like structure, but the PSF membrane without PVP has a sponge-like structure. Also, characterization results through atomic force microscopy (AFM) and contact angle measurement demonstrated that the porosity, surface roughness and hydrophobicity of the PSF membrane increased with addition of PVP to the dope solution. Mass transfer resistance analysis, based on CO2 absorption flux, displayed that addition of PVP to the dope solution of PSF membrane decreased membrane mass transfer resistance, and significantly improved CO2 absorption flux up to 2.7 and 1.8 times of absorption fluxes of PSF membrane without PVP and commercial PVDF, respectively.

Effect of long-term operation on the performance of polypropylene and polyvinylidene fluoride membrane contactors for CO2 absorption

The reduction of CO2 emission to the atmosphere is necessary with regard to the global warming mitigation. Gas absorption membrane (GAM) contactor for CO2 capture is an emerging technology and exhibits great advantages compared to conventional chemical CO2 absorption processes. In this study, to better understand the GAM process, especially its long-term performance for CO2 absorption, both polypropylene (PP) and polyvinylidene fluoride (PVDF) hollow fiber membrane contactors were investigated for a 30-day operation period. The CO2 flux and recovery declined with time for both PP and PVDF membrane systems and the situation was worse for PVDF membrane. Analysis of mass transfer resistance based on Wilson plot method suggested that after long-term operation the resistance for CO2 intramembrane transport increased by 160% and 290% for PP and PVDF membrane systems, respectively. The significant increase in the membrane mass transfer resistance means the membrane has been wetted.The contact angle measurements indicated that the membrane hydrophobicity declined significantly for PP and PVDF after 30days of operation. Scanning electron microscope and the overall membrane porosity analysis proved that the change of the membrane pore structure were responsible for the deterioration. After long-term contacting with MEA solution, physical or chemical dissolving of membrane surface occurred for both PP and PVDF. Some pinholes appeared on the outer surface of the PVDF membrane after use for 3days, and both their number and size significantly increased after 30days; however, in the case of PP membrane, the pores changed from originally a long and narrow shape to oval, and the pore size also became uneven. Therefore, the variation in physical and chemical membrane properties during the long-term run should be paid attention to in order to comprehensively evaluate the GAM contactor for CO2 absorption.

Simultaneous absorption of CO 2 and H 2S from biogas by capillary membrane contactor

This work presents the study on the simultaneous absorption of H 2S and CO 2 from biogas using a capillary membrane contactor. The synthetic biogas contained 250–1000 ppm H 2S, 20–40% CO 2 and CH 4. The absorbents used were water and monoethanolamine (MEA) solution. The effects of liquid and gas velocities, gas composition, on the absorption performance and selectivity of H 2S were investigated together with the detailed analysis of the mass transfer resistances of the membrane contactor system. The use of MEA solution gave much higher absorption fluxes of both CO 2 and H 2S compared to water. The absorption flux of H 2S significantly increased with increasing gas flow rate and slightly increased with liquid velocity and MEA concentration, while the absorption flux of CO 2 moderately increased with liquid velocity and was highly enhanced with increasing MEA concentration. The increase in CO 2 concentration obviously decreased the H 2S flux. The results of H 2S selectivity and the mass transfer resistance analysis of the non-wetted mode showed that the gas phase resistance played the important role on the mass transfer of H 2S. The opposite was found for the mass transfer of CO 2, i.e., the liquid phase resistance controlled the mass transfer. For the partially wetted mode, the wetted membrane resistance controlled the CO 2 absorption. On the contrary, for H 2S absorption, the wetted membrane resistance was insignificant and the gas phase resistance controlled the mass transfer. The observation of CO 2 and H 2S flux for a period of 6 h in order to investigate the effect of membrane wetting showed that the CO 2 flux dropped approximately 7.6%, while change of H 2S flux was negligible.

Mass Transfer Resistance Analysis and Semipermeable Membrane Optimization for Pressure Retarded Osmosis

Salinity power is a promising renewable energy with a large amount, which can be tapped by pressure retarded osmosis(PRO), however, efficient membranes are in lack. It was reported subtrates of asymmetrical reverse osmosis(RO) membranes had severe concentration polarizations in PRO process, degrading performance greatly and limiting PRO application. Based on solute transportation equations, the paper presented a common mass transfer resistance model of osmosis process, and disclosed essential differences between PRO and RO by the comparison of resistances. In PRO water permeates through membranes against solute diffusion, which facilitates solute to accumulate in porous subtrate, as a result, subtrate mass transfer resistance is big, water flux and power density is small, especially in the case of trivial subtrate mass transfer coefficient. Commercial RO membrane CA-3000 was studied for PRO application, whose subtrate resistance was found much bigger in PRO, taking the majority of total resistance. Subtrate was optimized for PRO process and membrane performance was projected. When subtrate mass transfer coefficient was improved to 4×10-6 m/s, power density of 4.75 W/m2 was obtained with average sea water, which approached the threshold of commercial salinity power exploitation.

Simultaneous Transport and Separation of Cu(II) and Zn(II) in Cu-Zn-Co Sulfate Solution by Double Strip Dispersion Hybrid Liquid Membrane (SDHLM)

A double strip dispersion hybrid liquid membrane (SDHLM) was successfully used in the simultaneous extraction and separation of Cu(II), Zn(II), and Co(II) from Cu-Zn-Co dilute feed phase. In the double SDHLM system, Acorga M5640-loaded membrane was placed between the 1st and the 2nd compartment, whereas the mono(2-ethylhexyl) 2-ethylhexyl phosphonate [HEH(EHP)]-loaded membrane was placed between the 1st and the 3rd compartment of the transport cell. The feed solution was filled in the central feed compartment(1st compartment) of the transport cell. The effect of the different experimental variables on separation was examined. The optimum separation conditions were summarized. An analysis of mass transfer resistances in the double SDHLM system shows that the mass transfer resistance for the diffusion of Zn(II) ions in the microporous membrane phase is dominant and the mass transfer resistances for the diffusion of copper (II) ions in the aqueous boundary layer and in the microporous membrane phase are dominant in comparison with the overall mass transfer resistance. The experiments verify that the double strip dispersion hybrid liquid membrane (SDHLM) possesses the nonequlibrium mass transfer characteristic.

Simultaneous removal and recovery of cadmium (II) and CN − from simulated electroplating rinse wastewater by a strip dispersion hybrid liquid membrane (SDHLM) containing double carrier

A novel liquid membrane system, denoted strip dispersion hybrid liquid membrane (SDHLM) with a double carrier (TRPO, trialkyl-phosphine oxide, and D2EHPA, di-(2-ethylhexyl) phosphoric acid) membrane phase, basic stripping aqueous phase and weak acidic waste water phase, was reported for the simultaneous removal and recovery of Cd(II) and CN − from simulated electroplating rinse wastewater. The basic stripping solution containing the concentrated Cd(CN) 4 2− could be again used in the electroplating tanks. The experiments verified that CN − was more quickly transported from the feed phase into the stripping solution than cadmium (II) when the organic phase only contains TRPO. In order to reach the same transport rate for CN − and Cd(II) D2EHPA was added in the organic phase only containing TRPO. Effect of various factors on the transport of CN − and Cd(II) were systematically investigated by orthogonal test of the extraction or membrane transport. The analysis of mass transfer resistances in SDHLM indicates that the mass transfer resistance due to the membrane-phase diffusion is dominant in the overall mass transfer process. The accumulation of coordination compound of CN −-carrier or Cd(II)-carrier in membrane for SDHLM shows that the transfer in SDHLM possesses the characteristic of nonequlibrium mass transfer in this study. According to the optimized condition, after the transport of 6 h for SDHLM the content of CN − or Cd(II) in the feed phase is 0.05 and 0.02 mg/L, respectively. The uptake of membrane solution into an adjacent aqueous phase for SLM is 2.1–2.6 times as high as that in SDHLM. In the experimental comparison of permeability coefficient, recovery percentage or concentration of solute in stripping solution, loss of membrane solution, the accumulation of coordination compound between CN −-carrier or Cd(II)-carrier in membrane, and transport efficiency of membrane for the two types of liquid membrane, SDHLM has superiority over SLM.

Heat and mass transfer resistance analysis of membrane distillation

Expressions for the mass transfer resistances of all the physical domains composing the air-gap membrane distillation (AGMD) and direct contact membrane distillation (DCMD) processes are developed and their absolute and relative effects are evaluated to improve the process understanding and identify promising ways for its improvement. The resistances are computed based on the authors’ two-dimensional conjugate model in which a simultaneous numerical solution of the momentum, energy and diffusion equations of the feed and cold solutions have been carried out, and the results of which were validated in comparison with available experimental results. Some of the main conclusions are that: (1) the use and examination of process domain mass transfer resistances is indeed an effective method for understanding the process and identifying ways to improve it, (2) the air/vapor gap dominates the mass transfer resistances of the AGMD domains, and while increasing the air/vapor gap width reduces the parasitic heat transfer by conduction, increasing the width beyond 2 mm has thus not improved the process thermal efficiency, (3) the hot solution inlet temperature and the air gap width have by far the strongest effect on the domain mass transfer resistance, mainly as a consequence of their effect on the air/vapor gap mass transfer resistance, (4) the inlet velocities of the hot and cold solutions have a small effect in AGMD, where the effect of the hot solution velocity is the higher one, (5) the concentration of the solution has a slight effect on the process, (6) the material used for the membrane should have a small thermal conductivity for a more efficient MD process and (7) efforts to minimize the mass transfer resistance of the cold solution will have a relatively small effect on the permeate flux.

Strontium Removal by New Alkyl Phenylphosphonic Acids in Supported Liquid Membranes with Strip Dispersion

We have synthesized a family of new extractants, alkyl phenylphosphonic acids, for strontium removal by supported liquid membranes (SLMs) with strip dispersion. The stability of the SLMs has been ensured by the improved SLM with strip dispersion technique. The SLMs removed radioactive Sr-90 to the target of 8 pCi/L or lower from feed solutions of 300−1000 pCi/L. Overall mass transfer coefficient data indicate that the mass transfer resistance for the membrane phase is negligible. An analysis of mass transfer resistances shows that the interfacial resistances due to extraction and stripping reactions are dominant. Stripping with HCl was nearly quantitative and more effective than that with H2SO4. The extractant was more effective than di(2-ethylhexyl) phosphoric acid.

ANALYSIS OF LEAKAGE EFFECT IN COUPLED TRANSPORT THROUGH LIQUID SURFACTANT MEMBRANE

Considerable effort has been directed toward describing, through analysis of mass-transfer resistances for solute diffusion and reaction in an emulsion drop, the permeation mechanism in liquid membrane systems based on the advancing reaction front model. However, very little attention has been paid to the loss of extraction efficiency often encountered in these systems due to rupture of the emulsion globules. In the present study, an unsteady-state mathematical model for batch extraction of monovalent HCrO4 − and divalent CrO4 2− from aqueous solution, through the use of Aliquat 336 as extractant and sodium hydroxide as stripping reagent, is reported. The rate of change of internal-phase volume due to membrane breakage was assumed to be independent of time, and an instantaneous and irreversible reaction between the solute and the internal reagent at the membrane–internal droplet interface was presumed to create a reaction front within the emulsion globule. The leakage coefficient was determined experimentally and was found to be dependent on internal-phase volume fraction. Batch experiments were performed for separation of Cr(VI) from aqueous potassium dichromate solution and the validity of the model was tested through the comparisons of model predictions with experimental data for different internal-phase volume fractions. The effect of external-phase pH on overall extraction in presence of leakage was also studied with semiempirical equations of the solute distribution coefficient that were obtained through experiments.

Mass Transfer Resistance Analysis of L-Tryptophan Extraction in an Emulsion Liquid Membrane System

The extraction of L-tryptophan with an emulsion liquid membrane containing di-2-ethylhexy phosphoric acid as a carrier, Span 80 as a surfactant, kerosene as a solvent, and hydrochloric acid solution as an internal phase stripping reagent, was studied. The effects of external phase pH, carrier concentration, internal stripping reagent concentration, and external initial solute concentration on the mass transfer flux were examined. The fractional resistances of external phase diffusion and emulsion globule diffusion to the overall process were defined and calculated by the proposed model. Thus, the rate-controlling steps for the overall process were quantitatively identified. For higher external phase pH and/or higher carrier concentration as well as lower external initial solute concentration, the overall process is mainly determined by the external phase diffusion. On the other hand, for lower external pH and/or lower carrier concentration, the emulsion diffusion is a rate-controlling step. However, in the majority of cases, the overall process is governed by the combined effects of both the external phase diffusion and the emulsion globule diffusion. Compared with the external phase pH and carrier concentration, the concentrations of internal stripping reagent and external initial solute are unimportant factors determining the overall process.

Mass transfer resistance in ASFF reactors for waste water treatment

Analysis of mass transfer resistances was performed for an aerated submerged fixed-film reactor (ASFF) for the treatment of waste water containing a mixture of sucrose and ammonia. Both external and internal mass transfer resistances were considered in the analysis, and characterized as a function of feed flow-rate and concentration. Results show that, over a certain operating regime, external mass transfer resistance in the system was greater for sucrose removal than ammonia. This is because the reaction rates for carbon removal were much larger than those of nitrogen. As a result, existence of any form of mass transfer resistance caused by inadequate mixing or diffusion limitations, strongly affects the overall removal rates of carbon more than nitrogen. Effects of the internal måss transfer resistance were virtually non-existent for ammonia removal. This behaviour was found over two orders of magnitude range for the effective diffusivity for ammonia, and one order of magnitude for the film specific surface area. However, over the same parameters' range, it is found that sucrose removal was strongly affected upon lowering its effective diffusivity and increasing the film specific surface area.