Total Mass Transfer Resistance Research Articles

Reaction kinetics of CO2 capture into AMP/PZ/DME solid-liquid biphasic solvent

The non -aqueous solid-liquid biphasic solvent of 2-amino-2-methyl-1-propanol (AMP)/piperazine (PZ)/dipropylene glycol dimethyl ether (DME) features a high CO2 absorption loading, favorable phase separation behavior and high regeneration efficiency. Different with the liquid-liquid phase change solvent, the reaction kinetics of CO2 capture into solid-liquid biphasic solvent was rarely studied. In the present work, the reaction kinetics of CO2 absorption into AMP/PZ/DME solid-liquid biphasic solvent was investigated into the double stirred kettle reactor. The absorption reaction followed a pseudo-first-order kinetic model according to the zwitterion mechanism. The overall reaction rate constant (kov) and the enhancement factor (E) of CO2 absorption both increased with increasing temperature. The total mass transfer resistance of the absorbent decreased with increasing temperature and increased with increasing absorption loading, so the higher reaction temperature was conducive to the absorption, and the liquid phase mass transfer resistance was the main factor affecting the absorption rate.

Textural effect of Pt catalyst layers with different carbon supports on internal oxygen diffusion during oxygen reduction reaction.

One way to address the cost issue of polymer electrolyte membrane fuel cells (PEMFCs) is to reduce the amount of platinum used in the cathode catalyst layers (CLs). The oxygen mass transfer resistance of the cathode CLs is the main bottleneck limiting the polarization performance of low Pt-loading membrane electrodes at high current densities. Pt nanoparticles, ionomers, carbon supports, and water in cathode CLs can all affect their oxygen mass transfer resistance. From the perspective of carbon supports, this paper changed the texture of CLs by adding carbon nanotubes (CNTs) or graphene oxide (GO) into carbon black (XC72) and studied its impact on the oxygen mass transfer resistance. A mathematical model was adopted to correlate total mass transfer resistance and internal diffusion efficiency factor with CL structure parameters in order to determine the dominant textural effect of a CL. The results show that adding 30%CNT or 20GO to carbon black of XC72 improved the electrocatalytic performance and mass transfer capability of the composite carbon-supported Pt catalyst layers during oxygen reduction reaction. The study further reveals that the smaller particle-sized carbon material with tiny Pt nanoparticles deposition can minimize the internal oxygen diffusion resistance. A less dense CL structure can reduce the oxygen transfer resistance through the secondary pores. The conclusion obtained can provide guidance for the rational design of optimal cathode CLs of PEMFCs.

Effects of membrane characteristics on the evaporative cooling performance for hollow fiber membrane modules

The hollow fiber membrane-integrated evaporative cooler (MEC) is considered as a promising strategy for space air conditioning applications. For a counter-flow MEC, the appropriate fiber specification and flow configuration need to be identified to improve its comprehensive performance. A numerical model was developed to capture the coupled heat and mass transfer process, and its feasibility was verified by measured data. The influence of fiber specifications and flow configuration on its cooling, space-saving, energy and water conservation performance were investigated. The results revealed that a reasonable fiber specification required a packing fraction of about 0.1 and a length slightly longer than the air channel's entrance length, while the inner diameter and membrane thickness should be determined based on the priority index. The flow configuration of air flowing via shell-side and water going through tube-side was more suitable for counter-flow MEC, which was more energy efficient while maintaining cooling ability. In addition, the contribution of each component's resistance to the total heat and mass transfer resistance would fluctuate as the influencing factors changed. In general, the total heat transfer resistance was dominated by the air-side convective resistance, while the main contributor to the total mass transfer resistance was the membrane resistance.

Volatile methyl siloxane separation from biogas using hollow fiber membrane contactor with polyethylene glycol dimethyl ether: A numerical and experimental study

In this study, a mass transfer model for hollow fiber membrane contactors (HFMC) was developed to theoretically investigate the mass transfer performance of volatile methyl siloxane (VMS) separation by HFMC. In addition, the effects of operating parameters such as filling rate, liquid and gas velocities on the effectiveness of HFMC separation of VMS were experimentally investigated. The experimental results showed that the increase of filler fraction improved the removal of siloxane. When the absorbent velocity was 250 mL/min and the gas velocity was 500 mL/min, the removal rates of L2, D4, and D5 were 74%, 92%, and 89%, respectively. Additionally, the fluxes of L2, D4 and D5 also increased with the increase of gas and liquid velocities, which were in good agreement with the calculated results of the mass transfer model. The faster the gas and liquid velocities, the higher the total mass transfer coefficient, which indicated that the mass transfer resistance was reduced in the contactor system. The liquid phase mass transfer resistance contributed the most to the total mass transfer resistance in the non-wetting model. However, when the membrane was 10% wetted, the mass transfer resistance contribution of the membrane increased sharply, reaching a maximum of 38.2%.

Modeling CO2, H2S, COS, and CH3SH Simultaneous Removal Using Aqueous Sulfolane–MDEA Solution

In this study, a rate-based absorption model coupled with an improved thermodynamic model was developed to characterize the removal of acid components (CO2 and H2S) and organic sulfur (COS and CH3SH) from natural gas with an aqueous sulfolane–MDEA solution. First, the accuracy of the thermodynamic model was validated by comparing the calculated partial pressure of CO2, H2S, and CH3SH with those of the experimental data reported in the literature. Then, the industrial test data were employed to validate the absorption model and the simulation results agreed well with the experimental data. The average relative errors of the removal rates of CO2, COS, and CH3SH are 3.3%, 3.0%, 4.1%, respectively. Based on the validated coupled model, the total mass transfer coefficient and mass transfer resistance of each solute component at different column positions were analyzed. The effects of the gas–liquid ratio, overflow weir height, and absorption pressure on the absorption performance of each component were studied, and the influence of the acid component concentration in the feed gas on the removal efficiency of methyl mercaptan (CH3SH) was also discussed. It is found that the improved absorption model can better characterize the absorption performance and be conducive to the optimal design of the absorber column.

Mass and Heat Transfer Coefficients in a Thermophilic Membrane Distillation Bioreactor

AbstractThe mass and heat transfer mechanisms of the membrane distillation bioreactor (MDBR) system used for hospital wastewater treatment were determined. The initial flux and balance flux of the system were determined. The total mass transfer resistance calculated for the system increased by 72 % when the system reached equilibrium. Considering the temperature polarization coefficient, it is clear that the system was controlled by mass resistance.

Widening the applicability of AnMBR for urban wastewater treatment through PDMS membranes for dissolved methane capture: Effect of temperature and hydrodynamics

AnMBR technology is a promising alternative to achieve future energy-efficiency and environmental-friendly urban wastewater (UWW) treatment. However, the large amount of dissolved methane lost in the effluent represents a potential high environmental impact that hinder the feasibility of this technology for full-scale applications. The use of degassing membranes (DM) to capture the dissolved methane from AnMBR effluents can be considered as an interesting alternative to solve this problem although further research is required to assess the suitability of this emerging technology. The aim of this study was to assess the effect of operating temperature and hydrodynamics on the capture of dissolved methane from AnMBR effluents by DMs. To this aim, a commercial polydimethylsiloxane (PDMS) DM was coupled to an industrial prototype AnMBR (demonstration scale) treating UWW at ambient temperature. Different operating temperatures have been evaluated: 11, 18, 24 and 30 °C. Moreover, the DM was operated at different ratios of liquid flow rate to membrane area (QL:A) ranging from 22 to 190 Lh−1m−2 in order to study the resistance of the system to methane permeation. Methane recovery was maximized when temperature raised and QL:A was reduced, giving methane recovery efficiencies (MRE) of about 85% at a temperature of 30 °C and a QL:A of 25 Lh−1m−2. The study showed that high QL:A ratios hinder methane recovery by the perturbation of the DM fibers, being this effect intensified at lower temperatures probably due the higher liquid viscosities. Also, the performed fouling evaluation showed that not significant membrane fouling may be expected in the DM unit at the short-term when treating AnMBR effluents. A resistance-in-series model was proposed to predict the overall mass transfer of the system according to operating temperature and QL:A, showing that methane capture was controlled by the liquid phase, which represented up to 80–90% of total mass transfer resistance. The energy and environmental evaluation performed in this study revealed that PDMS DMs would enhance energy recovery and environmental feasibility of AnMBR technology for UWW treatment, especially when operating at low temperatures. When MRE was maximized, the combination of AnMBR with DM achieved net energy productions and net greenhouse gas reductions of up to 0.87 kWh and 0.216 kg CO2–eq per m3 of treated water.

Investigation of heat and mass transfer behavior of mannitol during vial freeze-drying

The vial design feature, such as the base curvature, acts as the primary heat transfer barrier in the lyophilization process. In this study, heat and mass transport characteristics of mannitol (5%) during vial freeze-drying are numerically investigated by including the vial base curvature effects. The influence of the product fill height, chamber pressure, and the vial base curvature on the product temperature, drying time, and mass transfer resistance is analyzed in detail. The diameter of the vial is taken as 15 mm, while the fill height of the product is varied from 3 to 5 mm. The chamber pressure is maintained between 5 and 50 Pa. The results revealed that the product with a 3 mm fill height experienced a 2.5 °C lower temperature than the product with a 5 mm fill height. As opposed to the flat vial predictions, if the vial base curvature effects are considered, the drying time is increased by 1.96 and 3.8 h for the 3 mm and 5 mm fill heights, respectively. The increased drying time is attributed to the reduced direct contact area between the vial and shelf and the vapor entrainment in the gap. The results showed that the dry layer mass transfer resistance contributes to 95% of the total mass transfer resistance; however, the contribution of semi-stoppered vial resistance is 1–2% only.

CO<sub>2</sub> Absorption/Desorption on Gas-Liquid Membrane Contactors Using Monoethanolamine Solvent: Comparison of Porous and Composite Hollow Fibers

The operating efficiency of asymmetric porous and composite membranes with a thin non-porous selective layer was compared in the processes of CO2 absorption and desorption in gas-liquid membrane contactors using aqueous solutions of monoethanolamine (MEA) with low concentration (<14 %). Composite membranes were prepared by direct deposition of poly (1-trimethylsilyl-1-propyne) (PTMSP) in a hollow fiber membrane module. The effects of gas flow rate and MEA solvent linear velocity on the CO2 mass transfer were evaluated. Porous membranes were shown to be more effective in the process of CO2 absorption, because they allow to remove more than 90 % of CO2 from the gas mixture during one pass of the solvent through the contactor. Composite membranes were more promising for CO2 desorption, since they provide half as much of the solvent vapor losses with comparable desorbed CO2 fluxes (0.12-(STP)/(m2·h)). The contributions of membrane and liquid phase to the overall mass transfer resistance during the CO2 absorption process were estimated. It was demonstrated that deposition of a thin selective layer from a highly permeable PTMSP with a thickness of only 3 μm increases the membrane contribution to the total mass transfer resistance from 10-20 % to 60-80 %.

Performance, mechanism, and kinetics of NO removal by combined ascorbic acid and FeIIEDTA reaction systems

FeIIEDTA complexation technology is one of the most promising processes to realize NO removal. but FeIIEDTA is easily oxidized to FeIIIEDTA losing activity, which limits its further application. To solve this problem, ascorbic acid (VC) combined with FeIIEDTA was used for NO removal. The influences of various operating parameters like FeIIEDTA concentration, VC concentration, original pH value and temperature on NO removal in bubbling device were examined preliminarily. The results revealed that the VC can greatly improve NO removal by FeIIEDTA. NO removal efficiency increased with the increasing of VC concentration as well as FeIIEDTA concentration. Low temperature or low O2 concentration was conducive to the absorption of NO. NO removal efficiency increased and then decreased with pH value increasing. For initial FeIIEDTA concentration of 0.016 M, pH and temperature had little effect on NO removal efficiency. In addition, the kinetic studies showed that compared with normal FeIIEDTA approach, NO absorption by mixed FeIIEDTA and VC was more effective in reducing total mass transfer resistance and enlarging the NO flux. The absorption process was controlled by the liquid film. Finally, N-containing product was determined as NO2− and NO3−, suggesting that NO was mainly translated into nitrite and nitrate.

The role of polyvinyl butyral additive in forming desirable pore structure for thin film composite forward osmosis membrane

A thin-film composite forward osmosis (TFC-FO) membrane was developed by combining a new substrate consisting of cellulose acetate butyrate (CAB) and polyvinyl butyral (PVB) with the conventional polyamide active layer. The incorporation of the PVB additive (0–4 wt%) was to improve the hydrophilicity and alter the pore formation mechanism of the substrate to reduce the internal concentration polarization (ICP) effect in FO. The results showed that promising structural parameters of the CAB/PVB substrate such as A value (1.08 L m−2 h−1 bar−1) and S value (363.5 μm) could be obtained for the M2 substrate with 2 wt% PVB, due to the combined effect of finger-like pores and high porosity with high pore connectivity. The evaluation of the TFC-FO membranes showed that the water flux of the TFC-M2 was up to 27.5 L m−2 h−1 with 1 M NaCl draw solution in active-layer facing draw solution (AL-DS) mode, which was 40% and 130% flux enhancement compared to the reported commercial CTA and pure CAB control TFC membrane, respectively, with lower specific salt flux Js/Jw (0.35 g/L). The fouling experiments using model feed containing BSA foulant showed that the TFC-M2 was almost unaffected (98%) by the dilutive ICP effect in the active-layer facing feed solution (AL-FS) mode; while it also exhibited the highest flux recovery of 88% in the AL-DS mode. Overall, the total mass transfer resistance Rt of the TFC-M2 was the lowest compared to all as-prepared TFC-FO membranes, indicating the key role of the PVB additive in tuning the CAB substrate structure for designing high performance FO membranes.

Characterization of kinetic parameters and mass transfer resistance in an aerobic fixed-bed reactor by in-situ respirometry

An airlift reactor filled with Raschig rings in the downcomer section only was designed to allow for the characterization of biofilm biomass at several superficial liquid velocities. The reactor was inoculated with activated sludge and operated with synthetic wastewater containing acetate. After steady state was reached, the kinetic parameters of the process were determined at each superficial liquid velocity by an in-situ pulse respirometry method. Subsequently, the biofilm contained in the reactor was disaggregated, resuspended in a mineral medium, and reintroduced in the reactor, without Raschig rings, i.e., a standard airlift reactor. The kinetic parameters of the bioprocess were determined again, by the same respirometry method. That strategy allowed for the estimation of apparent and intrinsic kinetic parameters; i.e., with and without mass transfer limitation, respectively. Moreover, the method also allowed for the determination of the internal and external effectiveness factors and mass transfer resistances. The results shown that the hydrodynamic conditions had a strong effect on the kinetic parameters but none on the stoichiometric parameters. The external mass transfer accounted for up to 70% of the total mass transfer resistance. It is concluded that respirometry is a useful tool for the characterization of biofilms, in actual random packing reactors.

Mild stir-assisted membrane dispersion for enhancing propionic acid extraction

Abstract Mild stir-assisted membrane dispersion extraction (MDE) method was employed to enhance propionic acid (HA) extraction and compared to the mechanical stirred extraction (MSE) method. Triocylamine (TOA) and tributyl phosphate (TBP) were chosen as model extractant to extract HA. Firstly, droplet size and the size distribution of organic phase were analyzed, and then the effects of phase ratio, extractant and HA concentration on extraction performance were investigated. Comparing the two extraction methods, the results show mild stir-assisted MDE method reduced the mass transfer equilibrium time compared to MSE method. The mass transfer mechanism was explored by analyzing mass transfer resistance. Mild stir-assisted MDE had less total mass transfer resistance than MSE. When the extractant concentration was 40%, the extraction process was controlled by organic phase mass transfer process with HA volume fraction was 1% and controlled by both of reaction process and organic phase mass transfer process when HA concentration increased to 5%. This work may provide a new type of extraction method for the recovery of organic carboxylic acid.

Biphasic solvent for CO2 capture: Amine property-performance and heat duty relationship

The main issue related to the deployment of the absorption based technology for CO2 capture is the intensive energy penalty. Biphasic solvent is regarded as a promising candidate for CO2 capture due to its potential in significant reduction of the energy consumption. However, currently, the development of biphasic solvents relies on the intensity of solvent screening work. To address this issue, this work investigated the relationships between the solvent properties and CO2 capture performance such as absorption capacity, phase separation behavior, kinetics, and thermodynamics. Ten primary/secondary amines and eight tertiary amines with different alkalinity (pKa) and hydrophobicity (Log P) were used as the main components of the biphasic solvent. Experimental results revealed that the tertiary amine’s alkalinity determines the absorption capacity of the biphasic solvent. The maximum CO2 absorption capacity was achieved by the tested biphasic solvent with a tertiary amine of a pKa around 9.8. Compared with the tertiary amine, the primary amine’s hydrophobicity was more sensible to the phase separation behavior of the solvent. The low Log P value of the primary amine and the high Log P value of the tertiary amine befitted the phase separation. Kinetic analysis indicate that the total mass transfer resistance of the rich solution decreased with the increase in the Log P value of the tertiary amine. Interestingly, the highest enhancement factor was achieved by the tertiary amine (2-(diethylamino)ethanol) with the pKa of 9.79. Further increase of the tertiary amine’s pKa decreased the enhancement factor of the biphasic solvent. This was attributed to the limitation of mass transfer of tertiary amine from the upper phase to lower phase. Furthermore, thermodynamic analysis showed that the sensible heat and the latent heat of the solvents decreased and increased, respectively, with the increase of log P value of the tertiary amines. The lowest reaction enthalpy and heat duty of the tested biphasic solvents were 45.8% and 52% lower than those of the benchmark monoethanolamine solution, respectively. Overall, compared with the primary amine, the selection of the tertiary amine is more important to achieve a great performance for CO2 capture. Therefore, the tertiary amine with a moderate pKa and Log P is beneficial to achieve a high absorption capacity, fast absorption rate, and low heat duty of the biphasic solvent.

Mass transfer studies on the dehydration of supercritical carbon dioxide using dense polymeric membranes

Continuous drying processes using supercritical CO2 (scCO2) as a water extraction agent require 24/7 operational dehydration units for scCO2 regeneration. Dehydration units using dense polymeric membranes are considered a cost effective, sustainable alternative to the current zeolite-based units. The focus of previous studies on the membrane-based dehydration of scCO2 was always on the membrane itself whereas boundary layer effects, e.g., concentration polarization, were not taken into account. To quantify the boundary layer effects, simulations were performed using three different membrane materials: SPEEK, Nafion® 117, and PEBAX® 1074. Process conditions during the simulations ranged from 8.0 to 18.0 MPa and 40 to 100 °C. Even though the three types of membranes examined differ in their H2O permeability and H2O over CO2 selectivity, in all cases 80% of the total mass-transfer resistance can be assigned to concentration polarization effects, making it the dominant parameter for water transport. Despite high but differing intrinsic water permeabilities of all three membranes materials, the H2O transport, thus H2O flux through the membrane is significantly reduced by concentration polarization down to similar levels. This makes it necessary to use larger membrane areas, that result in higher CO2 fluxes. As a consequence, material selection is predominantly based on the ability to reject CO2. Optimization of process conditions other than membrane material is briefly discussed.

Support mass transfer resistance of Pd/ceramic composite membranes in the presence of sweep gas

The present paper aims at reducing the general lack of fundamental understanding about the effects of sweep gas on membranes supported on porous substrates. This is necessary to design better performing membranes for hydrogen purification and CO2 capture applications. Sweep gas is a widely considered option for enhancing the separation performance of hydrogen-selective Pd membranes in e.g. pre-combustion carbon capture and H2 production via low-temperature steam reforming. However, additional mass transfer resistance arises when using sweep gas with composite membranes due to its diffusion into the porous support. In order to assess this issue quantitatively, the influence of N2 sweep gas was systematically studied over a wide permeate pressure range using a 5µm thick Pd membrane supported on an asymmetric ceramic support tube. Experiments were carried out between 300 and 500°C at a trans-membrane pressure difference of 100kPa. These measurements were complemented with mathematical modelling to elucidate the mass transfer resistance associated with the sweep gas. In general, the mass transfer resistance of the support grew substantially with permeate pressure and increasing sweep rate, contributing up to 40% to the total mass transfer resistance at the highest investigated permeate pressure (1MPa). The negative impact of higher permeate pressures on the permeating fluxes could be effectively compensated by using relatively small sweep flow rates up to 100mLmin−1. Further increase of the sweep gas amount improved the permeation flux only by moderate margins albeit the sweep gas effect was a little stronger at higher permeate pressures. For example, the H2 permeation rate improved by ca. 29% (38%) when raising the sweep flow rate from 100mLmin−1 to 500mLmin−1 at 400°C and 100kPa (900kPa) permeate pressure. The relative permeation rate improvement with sweep rate became weaker with increasing temperature. The reduction of the support mass transfer resistance appears to be a promising strategy for enhancing hydrogen permeation through Pd composite membranes in situations that require the application of sweep gas.

Drying characteristics of osmo-convectively dried sapota slices

Sapota is a highly productive, nutritious and tropical fruits of India contributing 1424 MT, on 160 ha area with the productivity of 8.9 MT/ha. In India, sapota is generally consumed fresh. Ripened sapota cannot be stored more than a day or two. However, it is highly perishable in nature, ripens faster and becomes unfit for consumption very soon. The post-harvest losses in sapota fruits is in the ranged from 25-30%. These losses mainly occur due to high ethylene evaluation and metabolic activity. To avoid the huge wastage, the surplus could be processed and preserved properly. Wastage of large quantities of sapota fruits before it reaches the consumer is due to poor quality transportation as well as storage facilities. One effective method of reducing this huge loss would be by converting it into various commercial sapota products like slices, powder, juice, concentrate, etc. Therefore, it is necessary to produce the final product with wholesome, soft and of acceptable quality. The products may be dehydrated, canned and refrigerated. Dehydrated fruit products have inherent advantages, such as prolong shelf-life, higher degree of resistance to bacterial attack and lower transportation, handling and storage costs. The use of faulty dehydration process causes quality defects of dehydrated fruits as tough woody texture, slow or incomplete rehydration, loss of juiciness and shrinkage. Osmotic dehydration is less energy intensive technique than air or vacuum drying process, since it can be conducted at low or ambient temperature. It is the process of water removal from a product by immersion in concentrated aqueous solution. The osmotic process prior to convective drying can be used as a pre-treatment for the dehydration of food prior to further processing such as freezing, vacuum drying, etc. Therefore, a study was proposed to investigate osmotic behaviour and drying characteristic of sapota slices under osmo-convective drying process. The optimum conditions for osmotic treatment were determined with the values of 27.72% WR and 8.25% SG for process parameters, syrup temperature of 47.36 °C, and sugar concentration of 53.53 °B for 167.85 min duration. After optimization, osmotically treated samples were dried in convective dryer at temperature 40, 50 and 60 °C. The convective drying behavior and drying characteristics was investigated for osmo-dehydrated sapota samples, at air temperature of 40, 50 and 60 °C. It was observed that the moisture content of the product was reduced exponentially with drying time and no constant rate period was observed. To determine the most suitable drying equation the the moisture ratio data of osmosed sapota dried at various air temperatures were fitted into the six thin-layer drying models. Among those two term model as best fitted for drying data. This indicated that the total mass transfer resistance was primarily due to the internal moisture diffusion within the sample.

Mathematical modeling and validation of CO2 mass transfer in a membrane contactor using ionic liquids for pre-combustion CO2 capture

Mass transfer and mathematical modelling of CO2 absorption in a tubular membrane contactor using 1-Butyl-3-methylimidazolium Tricyanomethanide ([Bmim][TCM]) for pre-combustion CO2 capture has been studied in this work. A 1D-model was developed based on resistance in series model and CO2 material balance. The developed model was validated with the experimental data, and good agreement was observed between the simulated and experimental results. A new mass transfer resistance term is added to reflect the non-flat concentration profile in the liquid phase. Simulation results indicate that the liquid phase resistance contributes 67% and 44% to the total mass transfer resistance for non-wetted and wetted modes of membrane respectively. The resistance that occurred due to considering transport in liquid phase contributes 31 and 20% for non-wetted and wetted modes of membrane contactor respectively. CO2 flux along the axial length of membrane contactor was modeled, giving the maxima at the gas outlet. The influences of operational constraints including liquid/gas flow rates, operation pressure/temperature, length of membrane contactor, and CO2 concentration in feed gas were also inspected.

Improving gas absorption efficiency using a novel dual membrane contactor

A dual membrane contactor containing two types of membranes (polypropylene membrane and silicone rubber membrane) was designed and constructed for gas absorption process. The module performance was evaluated based on permeation flux experiments. The experimental results were compared with the predictions from a numerical model developed in our previous studies. Furthermore, the mass transfer resistance in the fabricated module was investigated using resistance-in-series model. An empirical correlation was developed to describe mass transfer coefficient under different operational conditions.The performance tests completed with the proposed novel dual membrane design shows that by simultaneously stripping gaseous component from the solvent through nonporous membrane along with absorption process, permeation flux was improved by 12–78%, compared to conventional modules containing only one type of porous membrane. The enhancement can be further improved by optimizing the membrane design and operational conditions. The effect of various operational conditions on absorption performance of dual membrane module was also examined. The experimental results were found in good agreement with those generated by simulation model. It was found that mass transfer resistance in the dual membrane module existed mainly in the liquid phase while membrane's resistance contribution to the total mass transfer resistance was about 7%. Higher permeation flux was achieved when the module was operated with lower gas flow rates, higher liquid flow rates and a low vacuum pressure in nonporous membrane. When compared to dilute feed gas (solute concentration ≤5%), removal efficiency with concentrated gas is much higher when other operational conditions were kept the same. The proposed novel membrane design holds great potential for gas processing at remote locations such as offshore platforms.

On individual resistances of selective skin, porous support and diffusion boundary layer in water vapor permeation

The efficiency of gas permeation processes is typically determined by two main parameters: selectivity and permeance. While selectivity depends to a large extent on chemical nature of the selective polymer layer, permeance is often dominated by fluid dynamics next to the membrane interface. A well-known phenomenon in terms of diffusive transport limitation is concentration polarization. It occurs in both the stagnant boundary layer and the porous support. The latter is called internal concentration polarization. The more selective and permeable a material is, the more severe the impact of concentration polarization can be. As a result process optimization is an issue far beyond the mere optimization of the selective membrane layer. The accurate identification and quantification of the limiting resistances other than the selective layer is a major challenge. Hence we introduce a systematic characterization approach to deconvolute the total mass transfer resistance. By combining single and mixed gas measurements, we quantify contributions of the selective skin, the porous support and the stagnant boundary layer. All tests were performed with a system comprising water vapor and air, with water vapor as the preferentially permeating component. Depending on process parameters, boundary layer resistance was found to be larger or in the same order of magnitude as the one of the selective layer. The influence of the porous substrate varied with the materials used. The methodology developed is important for humidification as well as dehydration processes, in particular for enthalpy exchangers in building ventilation systems.