Several recent studies investigated the use of the novel Zeolitic-Imidazolate Framework (ZIF-8) membranes for olefin-paraffin separation. In this manuscript, a techno-commercial model is developed to examine the use of these membranes for separating propylene from propane. Single-stage and two-stage membrane processes were assessed for their performance compared to distillation. The assessment was conducted considering 70 wt% propylene feed, typically produced from the upstream depropanizer. The single-stage process was found technically capable and commercially competent to produce the chemical grade propylene (93 wt%), but not the polymer grade (99.5 wt%). Alternatively, the two-stage process was capable of producing both propylene grades at promising recovery and cost figures. The published propylene/propane selectivity of 35 appears adequate in meeting the separation demands, subject to the adoption of proper unit design. Future research should grant more attention towards aspects such as ZIF-8 membranes’ manufacturability, cost, and performance in real environments.
Mixed Matrix Membranes (MMMs) made from a porous covalent triazine piperazine polymer (CTPP) as filler embedded in poly ether-block-amide (PEBAX® 1657) were studied for the separation of CO2/N2 and CO2/CH4 gas systems. At a loading rate of 0.025 wt%, significant improvement was achieved for both CO2 permeability (from 53 to 73 barrer) and selectivity (from 51 to 79 for CO2/N2 and from 17 to 25 for CO2/CH4) that were measured at 293 K and 3 bars. Results of FTIR, DSC, WAXS, and SEM revealed a strong interaction between CTPP and PEBAX due to the high density of hydrogen bonding in CTPP, which led to chain rigidification of PEBAX at very low loading rate compared to other literature reported systems. On the other hand, CTPP contains rich nitrogen in the framework, which favourites the adsorption of CO2 more than N2 and CH4. Hence, although the chain rigidification decreased the CO2 adsorption sites in PEBAX matrix, the intrinsic porosity and high surface area of CTPP compensated the diffusivity and solubility which in turn improved the overall permeability and selectivity at a very low loading rate. CTPP is highly stable in acid, base, and high temperature up to 400 °C. Hence, this novel type material is a very promising filler for preparation of mixed matrix membranes for the separation of CO2/N2 and CO2/CH4 systems.
Anqi, Ali E.; Usta, Mustafa; Krysko, Robert; Lee, Jung Gil; Ghaffour, NorEddine; Oztekin, Alparslan(Journal of Membrane Science, Elsevier BV, 2019-10-25)[Article]
The performance of vacuum membrane distillation (VMD) modules can be optimized through careful selection of design parameters. The present study examines how the addition of cylindrical filaments in the feed channel increases momentum mixing and the overall performance of VMD modules under different operating inlet conditions. Three-dimensional transient Computational Fluid Dynamics (CFD) simulations are conducted using Wall-Adapting Local Eddy-Viscosity (WALE) subgrid-scale Large Eddy Simulation (LES) turbulence model. Local concentration, temperature, and flux are coupled at the membrane surface to predict the rate of water vapor diffused through the membrane by Knudsen and viscous diffusion mechanisms. The predicted and measured vapor flux agrees reasonably well; validating the employed model. The small-scale eddies induced by the presence of spacer filaments promote mixing in the module, thus the temperature and concentration polarization is alleviated and the water vapor flux is immensely improved. The insertions of filaments in the feed channel increase the water permeate rate by more than 50% at higher feed flow rates and inlet temperatures. The pressure drop by the spacer reduces the allowable module length by one order of magnitude, but the module length increases two folds at feed temperature 80℃. Even though the power consumption of the module containing the filaments is increased, the addition of filaments is strongly recommended since the required power for the process could be supplied from readily available low-grade heat source.
An intrinsically microporous hydroxyl-functionalized polyimide (PIM-PI) made from 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 2,6(7)-dihydroxy-3,7(6)-diaminotriptycene (DAT1-OH), was thermally converted to polybenzoxazole (PBO). The thermal rearrangement of the PIM-PI to PBO significantly increased the free volume, which was reflected by a boost in its microporosity as indicated by enhanced Brunauer-Emmett-Teller (BET) surface area from 167 to 405 m2 g−1. The increase in free volume noticeably improved the gas permeability but also resulted in reduced gas-pair selectivity. The fresh PBO membrane made by thermal treatment at 460 °C for 30 min (TRIP-TR-460-30) with a PBO conversion of 98% displayed a 20-fold higher CO2 permeability of 840 barrer than the initial value of 43 barrer for the 6FDA-DAT1-OH polyimide at the expense of ~ 60% decrease in pure-gas CO2/CH4 selectivity from 52 to 21. The TRIP-TR-460-30 PBO showed good performance for propylene/propane separation with pure-gas C3H6 permeability of 21 barrer and C3H6/C3H8 selectivity of 16 for a 28-days aged sample. When tested under mixed-gas conditions C3H6 permeability dropped to 12.8 barrer and C3H6/C3H8 selectivity of 8. TRIP-TR-460-30 PBO displayed mechanical properties comparable some rigid polyimides with tensile strength, Young's modulus and elongation at break of 58 MPa, 1.83 GPa and 4.3%, respectively.
Alabi, Adetunji; Cseri, Levente; Al Hajaj, Ahmed; Szekely, Gyorgy; Budd, Peter; Zou, Linda(Journal of Membrane Science, Elsevier BV, 2019-09-07)[Article]
We report the preparation of an electrostatically-coupled graphene oxide nanocomposite cation exchange membrane (CEM) based on sulfonic group containing graphene oxide (SGO) (45 wt % loading) and polyvinylidene fluoride (PVDF), where the ion exchange groups were provided by the SGO additive. SGO was prepared via the mixing of graphene oxide (GO) with a mixture derived from 3,4-dihydroxy-L-phenylalanine (L-DOPA) and poly(sodium 4-styrenesulfonate) (PSS). A mold-casting technique was developed to fabricate the free-standing nanocomposite CEM. The presence of sulfonic groups in the nanocomposite was confirmed with FTIR spectroscopy. Energy dispersive spectroscopy analysis showed the SGO was distributed across the entire membrane matrix, with minimal aggregation. The resultant SGO/PVDF nanocomposite CEM membrane demonstrated high hydrophilicity and high water uptake, but low swelling ratio. Furthermore, evaluation of the electrochemical properties of the nanocomposite CEM showed favorable ion exchange capacity (0.63 ± 0.08 meq/g), permselectivity (0.95 ± 0.04), and area resistance (2.8 ± 0.2 Ω cm2). The nanocomposite CEM show good potential for use in electromembrane desalination applications.
Falca, Gheorghe; Musteata, Valentina-Elena; Behzad, Ali Reza; Chisca, Stefan; Nunes, Suzana Pereira(Journal of Membrane Science, Elsevier BV, 2019-05-08)[Article]
Cellulose is the most abundant biopolymer, but it is difficult to process due to its low solubility in most of the solvents. In this work, we demonstrate the preparation, of self-standing and defect-free cellulose hollow fiber membranes made by a sustainable process for filtration in organic solvent medium. The hollow fibers were made by the simple spinning technique using ionic liquids as a solvent. The spun solutions were prepared with three different ionic liquids, having imidazolium-based cations and acetate or phosphates as anions. We used X-ray diffraction to evaluate the influence of the different ionic liquids on the crystallinity of the cellulose and the membrane solvent stability. We used cryo-scanning electron microscopy to investigate the porous structure of the hydrated membranes, distinguishing it from that of the dry membranes. The hollow fiber membrane performance was studied using dyes in water and ethanol solutions. The rejection of Congo Red (696 g mol−1) was higher than 90% in ethanol and even closer to 100% in water. The best results were obtained by using 1-ethyl-3-methyimidazolium diethyl phosphate and 1,3-dimethylimidazolium dimethyl phosphate. Our results indicate that by using greener process is possible to obtain solvent resistant cellulose hollow fibers.
Guo, Jiaxin; Lee, Jung Gil; Tan, Tian; Yeo, Joonho; Wong, Pak Wai; Ghaffour, Noreddine; An, Alicia Kyoungjin(Journal of Membrane Science, Elsevier BV, 2019-07-17)[Article]
Removing nitrogen from wastewater by conventional treatment methods requires substantial energy, only to release it back to the atmosphere as gaseous nitrogen. Herein, we investigated the applicability of membrane distillation (MD) in resource recovery from sludge digestate by controlling the volatility and pressure of the vapor transport across the membrane to concentrate ammonia in the permeate stream. A mixture of Nafion ionomer and Multiwall Carbon Nanotubes (MWCNTs) were incorporated into a Poly (vinylidene fluoride-co-hexafluoropropene; PVDF-HFP) nanofiber matrix to fabricate a nanoporous honeycomb Nafion membrane featuring high recovery and increased mechanical strength. Theoretical modeling was conducted to predict the expected performance of the fabricated Nafion membrane under different operation conditions and to reveal the mechanism behind the enhanced recovery of Nafion membranes in the MD process. The resultant Nafion (8%)/MWCNT (2.5%)/PVDF-HFP nanofibrous membrane showed up to three times higher ammonia recovery compared to the commercial PVDF membrane from a feed with an ammonia concentration of 300 mg/L. The theoretical analysis quantitatively revealed that the Nafion containing membrane can not only suppress the negative effect of membrane's structural resistance on the ammonia recovery efficiency but also enhance the efficiency. In addition, we also uncovered that the effect of Nafion on ammonia recovery efficiency was maximized when the Nafion 8% membrane was employed. This study demonstrated an innovative and realistically applicable MD treatment process for recovering resource, which integrates low-grade heat and has scaling-up potential for wastewater treatment plants.
Qamar, Adnan; Bucs,Szilard; Picioreanu, Cristian; Vrouwenvelder, Johannes S.; Ghaffour, Noreddine(Journal of Membrane Science, Elsevier BV, 2019-07-19)[Article]
A vital component of spiral-wound membrane modules is the spacer mesh. It not only structurally supports the membranes but also aids in mass-transport enhancement through the membrane surface. Fundamental understanding of hydrodynamics associated with these spacer designs is critical to improve the permeate flux performance by decreasing concentration polarization and minimizing (bio)fouling, as well as minimizing the axial pressure drop. In the present study, time and space resolved Direct Numerical Simulations (DNS) were performed for a commercial spacer geometry. The spacer geometry was reconstructed by measurements using Scanning Electron Microscopy (SEM). Computations were performed for three spacer cells, allowing elimination of stream-wise periodicity that was a major bottleneck in earlier studies. The numerical solver was well checked in terms of boundary layer profiles obtained from Particle Image Velocimetry (PIV) data and with pressure measurements corresponding to various flow channel velocities. Non-dimensional computations were performed for Reynolds Numbers (Re) ranging from 73 to 375 (inlet channel velocity of 0.073–0.375 m/s) covering the flow transition dynamics regime. Results indicate that flow transition from steady to unsteady regime occurs for Re > 250. The flow transition could be primarily attributed to the interaction between vortices attached to the spacer filaments and the screw-vortex that originates along the diagonal of the spacer cells. No turbulent transition was observed even at the highest investigated velocity (Re = 375). The frequency spectra of time-varying velocity signal shows that at Re > 350 a sudden shift of frequency spectra occurs from discrete to continuous mode indicating the onset of advanced instability. Spacer design criteria in terms of maximum principal stress is also proposed, which can potentially aid in minimizing biofilm seeding.
Asymmetric block copolymer membranes can be facilely prepared via non-solvent induced phase separation combined with self-assembly. The membrane is characterized by a thin layer of highly ordered and uniform cylindrical nano-channels on top of a non-ordered macroporous sponge-like layer. Up to date, most studies concerning block copolymer membranes are focused on separation applications. In this work, highly adsorptive and adhesive isoporous block copolymer membranes have been fabricated. Because of the functionalization with cyclodextrin, the membrane shows excellent affinity to guest molecules. Moreover, the membrane is biocompatible and is adhesive to various substrates (e.g. glass, silicon, gold and stainless steel). Triclosan as a model drug was utilized to demonstrate the applicability of the membranes as a drug reservoir. A high loading capacity (305.5 μg cm−2) was achieved. The release behavior was investigated under various pH values in vitro. A long-time extended drug release was achieved without showing an initial burst effect. Furthermore, pH-responsive release behavior was observed. The triclosan-loaded membrane exhibited a significant antibacterial effect due to the triclosan release; the diffusion out of the membrane was evaluated using a disc diffusion assay. This study provides great potential for isoporous block copolymer membranes as a delivery platform for a wide variety of biomedical applications
Siebdrath, Nadine; Siddiqui, Amber; Ding, Wei; Kruithof, Joop; Vrouwenvelder, Johannes S.(Journal of Membrane Science, Elsevier BV, 2019-05-18)[Article]
Biofouling development is affected by a variety of factors that change over the length of reverse osmosis (RO) membrane modules in pressure vessels. Spatially resolved biofouling formation was studied under conditions representative to practice using four one-meter Long Channel Membrane Test Cells (LCMTCs) in series, simulating an industrial pressure vessel.
Biofouling was induced by dosing an easily assimilable substrate to the feed water. The impact of biofouling on the sequential decline of RO membrane performance indicators (feed channel pressure drop, permeability and salt rejection) was investigated. Also, the temporal organic carbon (DOC) consumption was assessed spatially over the four test cells.
Results showed that all membrane performance indicators were impacted by biofouling formation. The feed channel pressure (FCP) drop increase was impacted earliest and strongest followed by permeability and salt rejection decline, underlining that FCP drop is a sensitive and early biofouling monitoring indicator. Spatially resolved biofouling investigations revealed that most biofouling was formed in the lead sections of membrane installation with a decreasing gradient over length, linked to DOC availability in the system. In this study, FCP drop played a crucial role: the FCP drop increase at the lead test cell of the membrane installation caused performance losses for the downstream test cells.
Minimizing the effect of biofouling on membrane performance should be pursued by a combination of strategies involving (i) early detection and preventive cleaning, (ii) substrate limitation for delaying biofouling built-up and (iii) optimized (early) cleaning procedures for more effective biofilm removal.
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