Membrane Mass Transfer Resistance Research Articles

Investigating the influence of CaCO3 nanoparticles on the performance of CO2 absorption in the PVC membrane contactors

The membrane contactor application to purify acid gases is a significant advancement in environmental protection and engineering processes. Notwithstanding the numerous privileges of membrane contactors, the wetting of the polymeric membrane is considered the main worry in developing the mentioned technology. The polymeric membrane wetting by liquid absorbents increases the membrane mass transfer resistance and decreases the gas absorption. In this study, hydrophobic PVC membranes including various amounts of CaCO3 nanoparticles were manufactured to reduce the wetting problem. The prepared membranes were evaluated using AFM, TEM, XRD, contact angle, SEM, and tensile strength measurements. The contact angle results illustrated that adding the nanoparticles enhanced the membrane contact angle; so that for nanocomposite membranes comprising 1.5 wt.% nanoparticles, the contact angle increased from about 77°–94°. Investigating the tensile strength results showed that the nanoparticle presence in the membrane structure raised the tensile strength by 10 MPa. Finally, the nanocomposite and pure membrane performance in CO2 absorption was evaluated in the system of membrane contactors. The findings exhibited that after 25 days of the gas absorption process, the permeate flux of pure membranes significantly decreased. However, the gas absorption flux in nanocomposite membranes containing 1.5 wt.% nanoparticles remained constant during this period.

A novel membrane-based absorber for sorption heat transformers: A comparative experimental study of a thin-film absorbent and salt-impregnated adsorbents

The present study proposes a novel membrane-based sorber bed with stationary solution film for oscillatory sorption heat transformers. A custom-built gravimetric large pressure jump setup is used to experimentally compare the performance of the novel membrane-based sorber bed with a Lithium Bromide/Silica gel solid composite sorbent synthesized using the salt-impregnation method. It is shown that the current membrane-based sorber bed provides up to about four times the specific cooling power and more than twice the cooling power density of solid sorbents, demonstrating the capability of the present sorber bed as a promising alternative to solid sorbents. The specific cooling power, cooling power density, and energy storage density of about 2.8 kW/kg, 470 kW/m3, and 269 MJ/m3 are experimentally obtained, respectively. According to experimental results, it is observed that the film thickness plays a significant role in sorption dynamics, while the effect of membrane mass transfer resistance on sorption dynamics is less than 10%. In addition, the present membrane-based sorber bed is analytically modeled, and the results are validated with present experimental data. The present analytical model is also used to investigate the effects of design parameters on the performance metrics.

A computational fluid dynamics study on TPMS-based spacers in direct contact membrane distillation modules

Feed spacers are incorporated in membrane modules to enhance fluid mixing, thereby enhancing membrane flux. Previous experimental work showed that triply periodic minimal surfaces (TPMS)-based spacers induce significant improvements in membrane distillation (MD) performance, which was attributed to the suppression of membrane boundary layer, reducing heat and mass transfer resistance. To corroborate those findings, in this study, we assess the performance of TPMS spacers in direct contact MD (DCMD) modules using computational fluid dynamics (CFD). Results showed that the Fischer-Koch S TPMS spacer topology achieved twice the flux of the empty channel case and exhibited a 13 % increment in flux compared to the net-type spacer, but this improvement was accompanied by a higher pressure drop in the feed channel. Therefore, a normalized power ratio (NPR) was used to evaluate the overall performance of spacers. For Reynold number (Re) ranging between 200 and 1200, Fischer-Koch S spacer had a 12 % improvement in NPR compared to the net-type spacer and resulted in higher shear stress near the membrane surface, indicating a promising mean for mitigating fouling in membrane distillation. This research highlights the importance of utilizing novel spacers in membrane distillation and showcases the power of CFD tools in optimizing the process.

A comparative method for estimating the membrane mass transfer resistance of a ceramic hollow fiber membrane contactor using a Wetted‐Wall Column

In this study, an experimental method for estimating the relative membrane mass transfer resistance (RmHFMC) by comparing the results obtained using a ceramic hollow fiber membrane contactor (HFMC) with those obtained using a wetted-wall column (WWC) was proposed. The method was successfully applied for the determination of the optimal pore structure to increase the CO2 absorption. To find a relationship between relative RmHFMC in the ceramic HFMCs and its pore characteristics, the method was tested on two ceramic HFMCs prepared by non-solvent induced phase separation method. The pore structures were analyzed by scanning electron microscopy, capillary flow porometry, and gas permeability measurements. The results obtained show that the relative RmHFMC contribution ranged from ∼90 to ∼27% of the overall mass transfer resistance (RoHFMC) and was related to the variation of the pore structure properties. To the best of our knowledge, this is the first study to estimate the relative RmHFMC element of the ceramic HFMC through an experimental comparison with a WWC performed in the same experimental conditions. The proposed method is expected to provide a practical solution for estimating the relative RmHFMC of the ceramic HFMCs.

Superhydrophobic Surface-Constructed Membrane Contactor with Hierarchical Lotus-Leaf-Like Interfaces for Efficient SO2 Capture.

An organic-inorganic polyvinylidene fluoride/polyvinylidene fluoride-silica (PVDF/PVDF-SiO2) mixed matrix membrane contactor is fabricated via a facile and efficient hydrophobic modification method. The solubility parameters of the PVDF particle are precisely regulated, the PVDF particles are blended with SiO2 nanoparticles to form PVDF-SiO2 suspension, and then the suspension is introduced onto the surface of the PVDF substrate by an in situ spin coating strategy. The PVDF particles are partly etched and incorporated to construct the adhesive PVDF-SiO2 core-shell layer on the PVDF substrate, which results in a more stable PVDF-SiO2 coating layer on the substrate. The surface structure is precisely regulated by changing the etching morphology of PVDF particles and amount of doped PVDF and SiO2 particles, forming an integrated porous PVDF-SiO2 layer and constructing hierarchical lotus-leaf-like interfaces. The resultant PVDF/PVDF-SiO2 membrane contactors display the relatively regular distribution of pore size with ∼420 nm and excellent hydrophobic property with a water contact angle of ∼158°, which noticeably lightens wetting phenomena of membrane contactors. The SO2 absorption fluxes can reach as high as 1.26 × 10-3 mol·m-2·s-1 using 0.625 M of ethanolamine (EA) as liquid absorbent. The high stability of the SO2 absorption flux test indicates the excellent interface compatibility between the PVDF-SiO2 coating layer and the PVDF substrate. The versatile organic-inorganic layer exhibits super hydrophobic property, which prevents wetting of membrane pores. In addition, the membrane mass transfer resistance (H/Km) and membrane phase transfer coefficient (Km) are explored.

Electrospinning in membrane contactor: manufacturing Elec-PVDF/SiO2 superhydrophobic surface for efficient flue gas desulphurization applications

In membrane contactors, maintaining a high SO2 absorption flux and an excellent wetting resistance are crucial for hazardous gas removal. In this study, we adopted an electrospinning strategy to fabricate highly robust superhydrophobic dual-layer Elec-PVDF/SiO2 composite membrane contactors used for flue gas desulfurization. The composite membrane contactor consisted of a durable and ultrathin three-dimensional (3D) superhydrophobic surface and a porous supporting layer, where the formulation was optimized by regulating the PVDF concentration, solvent ratio and SiO2 particles content in electrospinning solution. The scanning electronic microscopy (SEM), EDS-mapping, water contact angle (WCA) and surface roughness of as-prepared Elec-PVDF/SiO2 composite membrane contactors were conducted to explore the physical and chemical structure. The SiO2 nanoparticles were uniformly loaded in Elec-PVDF/SiO2 composite membrane contactor, and constructed micro-nano dual-coarse lotus-leaf-like morphology, which noticeably elevated surface roughness (Ra). The SiO2 nanoparticles also functioned as hydrophobic modifiers, which boosted the WAC up to 155°. The SO2 absorption fluxes and SO2 removal efficiencies were investigated. In particular, the membrane contactor doped with 20 wt% SiO2 nanoparticles significantly elevated the stability of desulfurization performance. Besides, the membrane mass transfer coefficient (Km) and corresponding membrane mass transfer resistance (H/Km) were explored.

Increasing lithium extraction performance by adding sulfonated poly (ether ether ketone) into block‐copolymer ethylene vinyl alcohol membrane

AbstractBACKGROUNDHigh efficiency membrane extraction (ME) of lithium from brine has been constrained by membrane mass transfer resistance. We report here a new solvent‐resistant membrane produced from ethylene vinyl alcohol (EVAL) and sulfonated polyether ether ketone (SPEEK).RESULTSThe effect of the SPEEK on the thermodynamics and kinetics of demixing to the membrane morphology was investigated systematically. Addition SPEEK created extra resistance to the movement of the polymer chains in solution which was reflected in the reduced gelation diffusion coefficient. About 0.5 wt% of SPEEK was sufficient to reduce the mass transfer resistance of lithium ions (Li+). Membrane thickness showed linear correlation with Li+ flux, but was limited by its weak mechanical strength. Finally, a composite EVAL/SPEEK membrane supported by a thin loose nonwoven fabric showed a stress of 12.5 MPa, nearly 10‐fold greater than the unsupported membrane, and the highest lithium extraction flux of 4.80 × 10−8 mol cm−2 s−1.CONCLUSIONThe work provides a rationale for developing mechanically stable, high extraction flux membranes for ME. © 2020 Society of Chemical Industry

Theoretical modeling of the mass transfer performance of CO2 absorption into DEAB solution in hollow fiber membrane contactor

4-diethylamino-2-butanol (DEAB), as a novel tertiary amine, shows a promising potential for CO2 capture. In this study, the mass transfer performance of CO2 absorption into aqueous DEAB solution in a non-wetted and partially-wetted mode of hollow fiber membrane contactor (HFMC) was theoretically investigated in comparison with that of monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA) and 2-amino-2-methyl-1-propanol (AMP). A 2D mathematical model based on finite element method (FEM) was established to solve the steady-state continuity equations for the shell, tube and membrane sides simultaneously. The influences of the operating parameters on the CO2 absorption flux in HFMC including liquid and gas velocity and CO2 partial pressure were comprehensively investigated. The numerical results show that the CO2 absorption flux can increase with the increasing liquid velocity and CO2 partial pressure, and slightly increases with the increasing gas velocity. Moreover, the CO2 absorption performance of aqueous DEAB solution was further compared with different amine solutions, which reveals that the CO2 absorption flux of DEAB is higher than those of DEA, MDEA and AMP, and is also comparable to MEA. The analysis on the mass transfer resistance indicates that the proportion of the membrane mass transfer resistance increased rapidly from 13.7% to 75.3% as membrane wetting ratio increased from 0% to 20%. Instead of the liquid phase, the mass transfer in wetted membrane phase becomes the rate-controlling step ultimately. The increase in the membrane wetting leads to the significant decrease in CO2 absorption performance with 49.4% and 80.5% decrease in CO2 absorption flux and the overall mass transfer coefficient at membrane wetting ratio of 5% and 50%, respectively. Based on the analysis on enhancement factor, it demonstrates that the chemical reaction between CO2 and DEAB for the non-wetted mode generally occurs in the intermediate fast-instantaneous regime and gradually transfers to the instantaneous regime with membrane wetting.

Mixed matrix membrane contactor containing core-shell hierarchical Cu@4A filler for efficient SO2 capture.

Achieving high flux membrane contactor is significantly important for hazardous gas removal. In this study, we prepared poly(vinylidene fluoride) (PVDF)-based mixed matrix membrane contactor (MMMC) that contained a core-shell hirarchical Cu@4A composite filler (Cu@4A). On one hand, the Cu@4A regulated the physical structure of MMMC, which enhanced gas permeation and thus resulted in the increment of physical SO2 absorption flux. On the other hand, Cu@4A changed the chemical environment of MMMC by remarkably increased SO2 facilitated transport sites, which elevated SO2 concentration around Cu@4A by the enhancement of adsorption and oxidation of SO2, resulting in the increase of chemical SO2 absorption flux. Moreover, the copper nanosheets on 4A helped to construct facilitated transport pathways along the Cu@4A fillers at polymer-filler interface. The results showed that Cu@4A loaded MMMC exhibited increased SO2 removal efficiency and SO2 absorption flux compared with PVDF control membrane. Specifically, the M1040 MMMC loaded with 40 wt% Cu@4A and PVDF concentration 10 wt% exhibited the highest SO2 removal efficiency and SO2 absorption flux, which was up to 73.6% and 9.1 × 10-4 mol·m-2·s-1 at the liquid flow rate of 30 L/h. Besides, the overall SO2 mass transfer coefficient (Ko) and membrane mass transfer resistance (H/Km) were investigated.

A novel catalytically active membrane with highly porous catalytic layer for the conversion enhancement of esterification: Focusing on the reduction of mass transfer resistance of the catalytic layer

A composite catalytically active membrane was prepared focusing on the reduction of membrane mass transfer resistance to achieve a better esterification-pervaporation coupling performance. The effect of membrane preparation conditions on the membrane morphology was first evaluated. Under an optimized membrane preparation conditions, a “sandwich-like” composite membrane with a highly inter-connected sponge-like catalytic layer on a polyvinyl alcohol / polyethersulfone bilayer was obtained. The porosity of the membrane was found to be as high as 81.6%. A simple resistance-in-series model was developed to analyze the mass transfer resistance distribution in a traditional inert membrane reactor (IMR), a catalytically active membrane reactor (CAMR) with dense catalytic layer and a catalytically active membrane reactor with porous catalytic layer, respectively. Results showed that the preparation of a highly porous catalytic layer decreased the resistance of catalytic layer from 48.5% to 20.6% of overall resistances, leading to an enhanced water removal ability for the composite membrane. Finally, reaction-separation coupling experiments in IMR, CAMR with porous catalytic layer and CAMR with dense catalytic layer showed that, with a faster reaction kinetics and water removal rate, CAMR with porous catalytic layer exhibited a best coupling performance.

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.

Novel method for incorporating hydrophobic silica nanoparticles on polyetherimide hollow fiber membranes for CO2 absorption in a gas–liquid membrane contactor

In a gas–liquid membrane contactor, a larger pore size can result in a lower membrane mass transfer resistance. However, the membrane pore size is usually limited by the concern of pore wetting, e.g. a large pore size means a higher wetting tendency. As a breakthrough, this paper reported a porous polyetherimide (PEI) hollow fiber membrane with high surface porosity and large pore size to minimize the membrane mass transfer resistance by using a triple-orifice spinneret in the hollow fiber spinning process, and followed by a novel approach of fluorinated silica (fSiO2) nanoparticles (NPs) incorporation to make the membrane surface highly hydrophobic and chemical resistant to prevent the membrane from wetting caused by the large pore size on the membrane surface. The newly developed composite hollow fiber membranes showed the advancing contact angle value of 123.3°, receding contact angle value of 107.2°, and contact angle hysteresis of only 15.9°, indicating the high water resistant property. The composite membrane also exhibited a higher rigidity property compared with the original PEI substrate. The CO2 absorption flux of the composite membranes was investigated in both physical and chemical absorptions in a gas–liquid membrane contactor system. The membrane contactor showed a stable performance throughout the 60d long-term operation using a 2M sodium taurinate aqueous solution as the liquid absorbent.The highly hydrophobic composite hollow fiber membrane was able to outperform a conventional polymeric hydrophobic membrane in term of superior gas absorption flux and outstanding long-term stability, suggesting that the formation of organic–inorganic composite membranes is an effective way to enhance the feasibility of membrane contactor processes for practical applications. The results demonstrated the important role of membrane fabrication and modification techniques in facilitating the commercialization of membrane contactor technology.

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.

Pertraction performance of phenol through PDMS/PVDF composite membrane in the membrane aromatic recovery system (MARS)

Pertraction properties of phenol from aqueous solution through a plate composite membrane (poly (dimethylsiloxane) (PDMS)/polyvinylidene fluoride (PVDF)) in the membrane aromatic recovery system (MARS) were investigated systematically. The effects of liquid flow status, system temperature, feed concentration and pressure on the overall mass transfer coefficient (Kov), and the permeability for phenol through the membrane were also discussed. The feed-side liquid boundary layer mass transfer coefficient was proportional to Re0.46, membrane mass transfer coefficient was 15.0×10−7ms−1, which exhibited five times higher than that of tubular silicone rubber membrane (3.5×10−7ms−1), and possessed higher mass flux (2.38×10−2kgm−2h−1) and lower activation energy of permeation (9.7kJmol−1). Whereas, the permeability (5.9×10–12m2s−1) and diffusion coefficients (2.4×10–11m2s−1) of phenol were lower than those of nonporous silicone rubber. Kov increased with the rise of feed concentration and finally reaches a stable level (5.0–15.0gl−1). The diffusivity of phenol in the membrane was decreased with pressure. The membrane mass transfer resistance was found to be the rate controlling step in the transport process of phenol.

Kinetics of VOC absorption using capillary membrane contactor

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

Design of Multi-layer Anode for Direct Methanol Fuel Cell

To improve the catalyst utilization and reduce the catalyst amount, we designed the anode structure. The superior catalyst utilization of direct methanol fuel cell (DMFC) was obtained by localized catalyst loading on the reaction sites. Hence the multi-layer anode with localized catalyst loading was prepared by the sputtering method. The designed multi-layer electrode showed a high cell performance and mass activity because of the good contact at the interface between the catalyst and the electrolyte membrane and low mass transfer resistance. The ultra-low catalyst loading electrode preparing by the sputtering method was a very effective method for reducing the catalyst amount and enhancing the mass activity.

A mathematical model for gas absorption membrane contactors that studies the effect of partially wetted membranes

A mathematical model was developed to simulate the concentration profile in a gas absorption membrane (GAM) system. Carbon dioxide (CO 2) absorption into an aqueous solution of monoethanolamine (MEA) was investigated in the GAM system. Three GAM modules were potted with polytetrafluoroethylene (PTFE) membranes and connected in series to measure CO 2 concentration and CO 2 loading profiles along the length of the GAM system. The model predictions for CO 2 concentration and CO 2 loading profiles along the length of GAM column were in excellent agreement with the experimental results. The average absolute deviation between the model and experimental results was 1.49%. The Wilson plot method was used to determine the membrane resistance, which was compared with a theoretical membrane resistance. It was found that the membrane mass transfer resistance calculated using the Wilson plot method could predict the CO 2 concentration profile with a higher accuracy than the theoretical method. Partial membrane wetting was modeled to investigate the effect of membrane mass transfer resistance on the absorption performance and the overall mass transfer coefficient. The results showed that an acceptable membrane wetting for CO 2 absorption in MEA solutions in GAM systems was 40%. A higher lean solution temperature increased the membrane wetting in the GAM system. The membrane mass transfer resistance in completely liquid-filled membrane pores accounted for 92% of overall mass transfer resistance.

Mass transfer study and modeling of gas–liquid membrane contacting process by multistage cascade model for CO 2 absorption

The objective of this work was to characterize the main mass transfer resistance for CO 2 capture in the gas–liquid membrane contacting process by both physical and chemical absorption conditions. The characterization was performed based on the resistance-in-series model as well as the Wilson-plot method. In addition, a multistage cascade model, which is able to predict the time for the system to reach a steady-state condition, was developed to describe CO 2 absorption in the membrane contacting process. The cascade model was numerically solved by using the MATLAB program. It was found that the main mass transfer resistance of the physical absorption (using pure water as an absorbent) and the chemical absorption (using 2 M NaOH as an absorbent) was in the liquid phase and in the membrane, respectively. The membrane mass transfer resistance in the case of physical absorption presented approximately 36% of the total resistances at a liquid velocity of 2.13 m/s. For the chemical absorption condition applied, the membrane mass transfer resistance occupied around 99% of the total resistance. The results of simulation by the cascade model agreed well with the experimental results when the overall mass transfer coefficient obtained form the experiment was employed. The model can potentially be used with various operating conditions including the liquid velocity, gas concentration, and reactive absorbent used.

Fouling and its reversibility in relation to flow properties and module design in aerated hollow fibre modules for membrane bioreactors

Nowadays, most membrane bioreactors are using membranes submerged in the biomass and aeration in the concentrate compartment to limit or to control fouling. An important issue for the design of modules or membrane bundles in MBRs is to understand how the air/liquid flow is behaving and influencing fouling and its reversibility in relationship to the module properties. This paper focuses on an innovative and very specific process, in which HF membranes are put in a cartridge outside the activated sludge tank and a recycling loop is associated to the cartridge in order to decrease concentration of foulant species at the membrane surface and mass transfer resistance. Recycling operates with a very low liquid velocity in the module (a few cm.s(-1)) which constitutes a specificity of this process in terms of filtration operation. The aim of this study is to characterise two-phase flow and its effects on fouling and fouling reversibility at the scale of a semi-industrial bundle of outside/in hollow fibres, and as a function of bundle properties (packing density, fibre diameter), using specific methods to characterise the flow and fouling effects. Two modules were used showing a different packing density. Filtration was operated at constant permeate flux with clay suspension at 0.65 g.l(-1) in the same hydrodynamic conditions. Fouling kinetics and irreversibility were characterised by an adapted step method, and gas and liquid flows were characterised at global scale by residence time distribution analyses and gas hold-up. Fouling velocities are clearly influenced by gas velocity. The proportion of dead to total volume in the module is mainly affected by the liquid flow velocity and module design. The module with the higher fibre diameter and the lower packing density showed better performances in terms of fouling which was correlated with better flow properties.

Membrane thickness reduction effects on direct contact membrane distillation performance

This work refers to the simulation and analysis of membrane thickness reduction effects on direct contact membrane distillation (DCMD) performance. A resistances-in-series model has been used which allows the coupling of simultaneous heat and mass transfer in DCMD systems providing in this way an easier description of transport than more usual polarization models. In the study two types of membranes, namely supported and unsupported porous membranes, have been considered. The response to membrane thickness reduction has been analysed considering in turn both NaCl and sucrose aqueous solutions as feeds. The results show that, for both feeds, there is a critical membrane thickness such that below this critical value the negative effects of thickness reduction (associated with the reduction of the membrane conduction heat transfer resistance) are higher than the positive ones (associated with the reduction of the membrane mass transfer resistance). So there is a membrane thickness that maximizes the water flux and this thickness depends on operating conditions and other membrane characteristics. The model used has been particularly adequate for explanation of thickness dependence results because it includes expressions for the dependence of transport resistances on process parameters. It has been concluded that the manufacturing of supported membranes can be a promising way to fabricate DCMD membranes mechanically strong provided with a thin functional hydrophobic layer in the optimum range.