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A Comparison between Several Commercial Polymer Hollow Fiber Membranes for Gas Separation
Abstract
Polyethersulfone (PES), polyetherimide (Ultem ® 1000), and polyimide (Matrimid ® 5218) are common commercial polymers used to produce hollow fiber membranes for different gas separation applications. In this work, asymmetric hollow fiber membranes were prepared using these polymers by a phase inversion technique. The effects of spinning parameters (composition of the dope and bore solution, bore flow rate, air gap distance, temperature of the spinneret and coagulation bath, as well as take-up speed) on the membrane structure and gas permeation properties were investigated. The membrane separation performances were characterized by measuring their gas permeation properties (permeance and selectivity) for different gases (H 2 , CO 2 , O 2 , N 2 , and CH 4 ) and by their cross-sectional morphology using scanning electron microscopy (SEM). The relationships between the gas separation performance of the hollow fibers and the intrinsic gas properties of the dense flat membranes made of the same materials were also studied. A comparison between the average apparent skin layer thickness calculated from O 2 permeability/permeance, and the results based on SEM images was made and good agreement was obtained between both results.
... The shape and diameter of these pores can vary over a wide range and depend on the method and conditions of formation [6,7]. It can be seen ( Figure 1) that in the transition from the surface layer to the porous one, the size of the pores changes relatively smoothly [8,9]. ...
... Chen et al. [8] presented several physicomechanical characteristics of commercial polymers used for the production of hollow fiber membranes (Table 1). Table 1. ...
... Table 1. Physical and mechanical characteristics of polymers [8]. ...
The porous layer of composite and asymmetric hollow fiber membranes acts as a support and is exposed to strong mechanical stresses. The effect of external pressure on the polymer structure and, as a consequence, the separation characteristics of the membrane remains unsolved. Based on the solution of the Lamé approach to the calculation of the stress state of a hollow cylinder, a method of calculation was proposed for hollow fiber membranes. Calculations were based on the approximation of the isotropic nature of the physical and mechanical characteristics of the selective layer and substrate. Permissible deformation of the membrane’s selective layer was determined from the linear sector of strain-on-stress dependence, where Hooke’s law was performed. For these calculations, commercial polyethersulfone membranes were chosen with an inner and/or outer selective layer and with the following values of Young’s modulus of 2650 and 72 MPa for the selective and porous layers, respectively. The results obtained indicate that the dependence of the maximum allowable operating pressure on the substrate thickness asymptotically trends to a certain maximum value for a given membrane. Presented data showed that membranes with outer selective layer can be operated at higher working pressure. Optimal parameters for hollow fiber gas separation membrane systems should be realized, solving the optimization problem and taking into account the influence of operating, physicochemical and physicomechanical parameters on each other.
... Then, the fibers were immersed in a 3% wt. silicone rubber solution (PDMS) for 5 min to seal the defects on the outer surface and dried at 60 o C for 4 h under vacuum [6]. 070003-2 ...
... The gas transport properties were obtained using a variable pressure (constant volume) method [5][6]. The permeability (1 Barrer = 10 -10 cm 3 (STP) cm / cm 2 s 1 cm Hg) for flat membranes was determined by Equation (2), while the permeance (1 GPU = 10 -6 cm 3 (STP) / cm 2 s 1 cm Hg) for hollow fiber membranes was calculated by Equation (3). ...
... They also show the absence of residual NMP (solvent with a boiling point of 204.5 o C). But in all cases, the Td5% of the hollow fibers are lower than the neat polymers [6]. This is likely associated to the crosslinked silicone rubber (PDMS) coating on the membranes. ...
Polymer hollow fibre membranes are becoming more and more used for gas separation. In this work, asymmetric hollow fibre membranes were prepared by a phase inversion technique with three commonly used commercial polymers: polyethersulfone (PES), polyetherimide (Ultem® 1000), and polyimides (Matrimid® 5218). The effect of spinning parameters (composition of the dope and bore solution, bore flow rate, air gap distance, temperature of the spinneret and coagulation bath, as well as take-up speed) on the membrane morphology and gas permeation properties was investigated. The membrane separation performances were characterized in terms of gas transport properties (perméance/selectivity) for different gases (H2, CO2, O2, N2, CH4) to relate this information with their morphology studied by scanning electron microscopy (SEM). Furthermore, dense flat membranes from the same materials were prepared by solvent casting to investigate the relationships between the gas separation performance and membrane configuration (hollow fibers vs. compact flat membrane). Finally, a comparison between the apparent skin layer thickness from O2 permeability/permeance and SEM image gave good agreement.
... One of the promising and desirable polymers of the polyolefin family is poly(4-methyl-1-pentene) (PMP), which has recently been applied in HF-GLMCs in the field of degasification of aqueous solutions (e.g., O2, CO2) due to their hydrophobic nature, low energy of intermolecular interactions, low bulk density, good chemical resistance, inertness, biocompatibility, and similar mechanical and structural properties [35][36][37]. As a thermoplastic material, PMP can be processed via melt technologies using green solvents [38], and a hollow-fibre membrane contactor (HF-MC) based on PMP can be produced using the non-reagent extrusion method [39]. Despite its use in gas separation, due to its semi-crystalline structure, adjustable permeability and reliable chemical and mechanical stability, the thick selective layer of HFs made from PMP polymers have not found extensive applications [39][40][41]. ...
... NH3 transport within the PMP-LLMC was studied as a function of the nature of the stripping acid (H3PO4 or HNO3), and overall NH3 mass transfer coefficient Km(NH3) can be calculated experimentally as described in Equation 1 [23,38]: The NH3 recovery efficiency (%) in the feed tank was estimated using Equation 2: ...
Two novel hollow-fibre liquid–liquid membrane contactor (HF-LLMC) modules named EF-010-A60 and EF-010-Q-A60 containing S-type (skin layer with low porosity) and Q-type (skin layer with higher porosity) fibres, respectively, with asymmetric, porous, and hydrophobic membranes made from poly(4-methyl-1-pentene) (PMP) were used for the efficient technology of ammonia from treated domestic wastewater to produce liquid fertilisers. The ammonia-rich stream was fed into the shell side of the HF-LLMC, while nitric acid or phosphoric acid were fed separately into the lumen to produce N-type (NO3–-NH4⁺) and N-P-type (NH4⁺-P2O5) fertilisers, respectively. The maximum NH3 recovery (>95%) was achieved in a closed-loop configuration with S-type fibres, while Q-type fibres showed a better performance in terms of the production of more concentrated N-type liquid fertiliser. With Q-type fibres, the highest values of N-P-type liquid fertilisers were achieved (8.0% N (NH4⁺) and 20.3% P2O5 (w/w)) using phosphoric acid, while the highest value of water flux across the PMP fibres was <0.01 kg m–2h–1. The highest overall mass transfer coefficient (Km) measured for solutions containing 5.0 gN-NH3 L–1 with a feed/stripping volume ratio of 60:1 was (2.9 ± 0.2) × 10⁻⁷ m s–1. Ultraviolet-visible spectroscopy and two-dimensional excitation-emission matrix fluorescence spectroscopy were employed to monitor the absence of pore-wetting events and the stability of the HF-LLMC under strongly acidic and basic conditions.
... [42] Polymers commonly employed in the fabrication of gas separation membranes include cellulose acetates, [43,44] polyethylene, polypropylene, polysulfones, [30,[45][46][47][48][49][50] fluoropolymers, [51,52] polyamides, and polyimides. [21,[53][54][55][56][57][58][59] Known for their microporous characteristics, polymers of intrinsic microporosity (PIMs) are better suited and more desirable for CO 2 capture applications due to their high permeability. [60][61][62] In order to fabricate PIMs, non-network macromolecular structures must be formed within the matrix material, while ensuring rigidity and non-linearity. ...
... Material extrusion is the simplest and most common 3D printing technique capable of processing a wide range of materials. [30,57] One type of 3D printing via material extrusion is the fused deposition modeling (FDM), where the nozzle is heated past the glass transition temperature to melt the polymer prior to layer-by-layer deposition. [39] As illustrated in Fig. 6, the extruded hot material hardens and subsequently adheres to the preceding layer. ...
Additive manufacturing (or 3D printing) is an evolving technology that shows great potential as a sustainable method for fabricating gas separation membranes for carbon capture applications. Compared to other gas separation techniques or membranes fabricated by conventional formative methods, the use of 3D-printed membranes is more advantageous because of their simplicity, higher energy efficiency, practicality, flexible and tailorable designs, and high separation efficiency. Although polymeric, cementitious, and gel-based materials have been exploited for the development and fabrication of robust and highly efficient CO2-capturing membranes, these materials require further innovation to become fit and suitable as feedstock for 3D printers. In this work, we review several membrane materials used for capturing CO2 and discuss their corresponding separation mechanisms and fabrication via 3D printing. We also summarize the challenges and limitations in using 3D-printed membranes and provide perspectives towards high-performance membrane fabrication and future industrial applications.
... One of the promising and desirable polymers of the polyolefin family is poly(4methyl-1-pentene) (PMP), which has recently been applied in HF-GLMCs in the field of degasification of aqueous solutions (e.g., O 2 , CO 2 ) due to their hydrophobic nature, low energy of intermolecular interactions, low bulk density, good chemical resistance, inertness, biocompatibility, and similar mechanical and structural properties [35][36][37]. As a thermoplastic material, PMP can be processed via melt technologies using green solvents [38], and a hollow-fibre membrane contactor (HF-LLMC) based on PMP can be produced using the nonreagent extrusion method [39]. Despite its use in gas separation, due to its semi-crystalline structure, adjustable permeability and reliable chemical and mechanical stability, the thick selective layer of HFs made from PMP polymers have not found extensive applications [39][40][41]. ...
... Details of the transport mechanism are detailed in section S1 of the supplementary information. NH 3 transport within the PMP-HF-LLMC was studied as a function of the nature of the stripping acid (H 3 PO 4 or HNO 3 ), and overall NH 3 mass transfer coefficient Km(NH 3 ) can be calculated experimentally as described in Equation (1) [23,38]: ...
The liquid-liquid membrane contactor (LLMC) is an innovative and eco-friendly technology for ammonium salts production; Electrodialysis (ED) can concentrate efficiently ammonium salts; Concentrated ammonium salts can be obtained by means of integration processes (LLMC and ED); Ammonium salts produced can be used as liquid fertilizers.
... This approach allows determining the optimum module number while minimizing the membrane separation plant footprint. In the case of the optimization of biogas separation, three polymer membranes previously produced in our laboratory (Chen et al., 2017) were chosen to show the effects of changes in the CO 2 permeance and CO 2 /CH 4 selectivity on the biogas separation cost, process layout, and modules number. ...
... The case study involves the design of a biogas upgrading process by which CO 2 and CH 4 are separated using hollow fiber polymer membranes (i.e. Ultem 1000) with CO 2 /CH 4 selectivities of 33.2 and 66.4 (Chen et al., 2017). In this case, the raw biogas as the product of an anaerobic digestion process is at first transferred to a separation plant and thereafter pre-treated to remove undesirable components. ...
Biogas as a sustainable energy source produced via different fermentation technologies needs upgrading prior use as fuel or for heat and electricity productions. Today, membrane technology is becoming more and more accepted to compete with other conventional biogas separation methods. We develop a membrane optimization model to find optimal values of operating parameters and the most effective layout while minimizing annual separation cost. To design a hollow fiber module, this model is also used to specify the optimal values of module packing fraction and dimensions while minimizing the required module number for a separation process. We also propose a new modeling approach to select membrane characteristics by which effects of CO2 permeance and CO2/CH4 selectivity on optimal process layouts are investigated. This approach provides a practical guideline for experimentalists to quickly verify the effect of modification techniques on membranes prior to using in a realistic process. The results show that the separation cost is less sensitive to the CH4 recovery (< 95%). For the same CO2/CH4 selectivity, not only the separation cost reduces by increasing the CO2 permeance by a factor of 2 but this also result in a 40% reduction in the total membrane area. The techno-economic analysis finally reveals that the membrane technology has a high potential either to displace the conventional methods or to be used in a hybrid process.
... Pertl et al. (2010) focused their analysis on the quantification of the global warming potential (GWP), assuming the substitution of natural gas with the produced biomethane, without any indication about its specific utilisation. They took into account four upgrading technologies, and data for membrane separation units derive from the available literature, and appear only partially in agreement with recent data (Chen et al., 2017;Munoz et al., 2015). The study carried out by Adelt et al. (2011) made a quantification of GWP and cumulative energy demand, with reference to biomethane production starting from energetic crops and upgrading the biogas by amine absorption. ...
... The biogas upgrading unit includes preliminary stages of biogas drying and compression, which is responsible of most of electrical consumptions, and hydrogen sul- phide removal, which requires 3.4 t/y of activated carbon (i.e., about 1 g for each normal cubic meter of treated raw biogas). The upgrading unit (Fig. 4) is equipped with a high-efficiency three- stage membrane separation system made of polyimide hollow fi- bres ( Chen et al., 2017), which provides a CO 2 removal of 98.0% and a methane slip limited to 0.69% ( Barbato, 2017). The base case configuration has three sources of emissions into atmosphere (Table 4): CHP system, biofilter, and upgrading unit. ...
The study aims to demonstrate the overall environmental sustainability of biomethane production by anaerobic digestion of the separately collected organic fraction of municipal solid waste. There is a great interest in the utilisation of biofuels produced from biowaste in the transport sector, due to the benefits of reduced pollutant emissions and diversified transport fuel supplies. An attributional, process-based life cycle assessment study quantifies and compares the potential environmental impacts of an anaerobic digestion plant, where the produced biogas is upgraded to biomethane for the transport sector instead that directly burned in a combined heat and power unit. The avoided impacts related to the utilisation of biomethane instead of diesel, petrol or natural gas have been evaluated with reference to a vehicle fleet made of passenger cars and small rigid trucks. They appear large enough to make the biomethane production the cleanest option for the management of biowaste. The global warming and non-renewable energy potentials of the Biowaste-to-Biomethane scenario improve of 79% and 36%, respectively, with reference to the Biowaste-to-Energy scenario. A sensitivity analysis evaluates the effect of several key parameters. Some of them are peculiar for the analysed application, such as the composition of the vehicle fleet, specific biomethane consumptions of these vehicles, and methane slip in the biogas upgrading unit. Some other parameters are more general, such as the final destination of solid digestate, gas engine efficiency, national electric energy mix. The results of the analysis provide data and information to policy-makers, planners and operators that would like or have to approach the management of the separately collected organic fraction of municipal solid waste. They also inform on the environmental advantages connected with the utilisation for road transportation of biomethane produced from this waste fraction.
... Polymeric and inorganic membranes may be configured in various ways that are not particular to a membrane material, although a configuration may be more suited to one membrane material over another. Common configurations include flat-sheet membranes [8,23,24], hollow-fiber membranes [25][26][27][28] and tubular membranes. Membranes may also have different morphologies depending on their structure. ...
This review presents a concise conceptual overview of membranes derived from organic chelating ligands as studied in several works. The authors’ approach is from the viewpoint of the classification of membranes by matrix composition. The first part presents composite matrix membranes as a key class of membranes and makes a case for the importance of organic chelating ligands in the formation of inorganic–organic composites. Organic chelating ligands, categorized into network-modifying and network-forming types, are explored in detail in the second part. Four key structural elements, of which organic chelating ligands (as organic modifiers) are one and which also include siloxane networks, transition-metal oxide networks and the polymerization/crosslinking of organic modifiers, form the building blocks of organic chelating ligand-derived inorganic–organic composites. Three and four parts explore microstructural engineering in membranes derived from network-modifying and network-forming ligands, respectively. The final part reviews robust carbon–ceramic composite membranes as important derivatives of inorganic–organic hybrid polymers for selective gas separation under hydrothermal conditions when the proper organic chelating ligand and crosslinking conditions are chosen. This review can serve as inspiration for taking advantage of the wide range of possibilities presented by organic chelating ligands.
... Permeance of pristine PES data obtained from Chen et al. [20]. The dense layer of PES substrate at 29 wt% PES is 5.87 × 10 −6 cm. ...
The current work predicted the permeance of CO2 across a ZIF-L@PDMS/PES composite membrane using two different models. The membrane was fabricated by dipping a PES hollow fiber membrane in a coating solution made using PDMS that contained ZIF-L. First, flat sheet ZIF-L@PDMS membranes were fabricated to verify the role of ZIF-L on the gas separation performance of the membrane. Based on the data, the presence of ZIF-L in the PDMS matrix allowed enhancement of both permeability and selectivity of CO2, where the maximum value was obtained at 1 wt% of ZIF-L. The performance of ZIF-L@PDMS layer, as a function of ZIF-L loading, was well-predicted by the Cussler model. Such information was then used to model the CO2 permeance across ZIF-L@PDMS/PES composite membrane via the correction factor, which was introduced in the resistance in series model. This work discovered that the model must consider the penetration depth and the inorganic loading (in the case of ZIF-L@PDMS/PES). The error between the predicted CO2 permeance and the experimental results was found to be minimal.
... On the other hand, substituting a flat sheet membrane configuration with the industrially preferred hollow fiber configuration provides various benefits, For example, an increase in surface area to volume ratio due to it being cylindrical, and thus an increase in the gas penetration through the membrane [47]. HFMMMs have the potential to increase the membrane performance further, provided that the materials selected are compatible to prevent particle agglomeration, which could potentially deteriorate the performance of the membrane [44]. ...
The increase in the global population has caused an increment in energy demand, and therefore, energy production has to be maximized through various means including the burning of natural gas. However, the purification of natural gas has caused CO2 levels to increase. Hollow fiber membranes offer advantages over other carbon capture technologies mainly due to their large surface-to-volume ratio, smaller footprint, and higher energy efficiency. In this work, hollow fiber mixed matrix membranes (HFMMMs) were fabricated by utilizing cellulose triacetate (CTA) as the polymer and amine-functionalized metal-organic framework (NH2-MIL-125(Ti)) as the filler for CO2 and CH4 gas permeation. CTA and NH2-MIL-125(Ti) are known for exhibiting a high affinity towards CO2. In addition, the utilization of these components as membrane materials for CO2 and CH4 gas permeation is hardly found in the literature. In this work, NH2-MIL-125(Ti)/CTA HFMMMs were spun by varying the air gap ranging from 1 cm to 7 cm. The filler dispersion, crystallinity, and functional groups of the fabricated HFMMMs were examined using EDX mapping, SEM, XRD, and FTIR. From the gas permeation testing, it was found that the NH2-MIL-125(Ti)/CTA HFMMM spun at an air gap of 1 cm demonstrated a CO2/CH4 ideal gas selectivity of 6.87 and a CO2 permeability of 26.46 GPU.
... More than in most other membrane processes, for gas separation the membrane material rather than the membrane morphology is the critical factor that determines its separation performance. The selection criteria for successful gas separation membranes [6,8] and the systematic comparison of different materials [157,158] have therefore been the topic of various studies. The first choice to make is that between glassy or rubbery polymers, and between temperature resistant, pressure resistant and/or plasticization resistant membranes, depending on the separation needs and the expected process conditions [159]. ...
This chapter discusses the application of hollow fiber (HF) membranes in the field of gas separation. HF allow to minimize energy cost, equipment size, waste production, and, in some cases, to convert by-products into new products with innovative value in the light of an increasingly necessary circular economy. Their main applications in gas separation are hydrogen recovery, oxygen-enriched air production, biogas upgrading, natural gas treatment, and post-combustion carbon capture from flue gas. HF membranes can be used to separate air for yielding either oxygen-or nitrogen-enriched streams or the pure gases as the final products. Biogas upgrading is a strongly emerging application field of polymeric gas separation membranes that has already reached a large-scale industrial application for the separation of biogas from organic waste, while thousands of smaller units are available worldwide. Helium recovery from natural gas by means of HF membrane separation is a valid alternative to the traditional cryogenic distillation.
... Hollow fiber configuration is more suitable for industrial gas separation due to their high surface to volume ratio and selectivity thus giving exceptional mass transfer properties for gas separations [86][87][88]. Hollow fiber mixed matrix membranes with a wide surface area and thin selective layers are recommended in most cases as the thickness of gas separation membranes is important [37,89]. To further enhance the performance of MMMs, incorporation of fillers into HFM configuration will form a hollow fiber mixed matrix membrane (HFMMM). ...
CO2 separation from raw natural gas can be achieved through the use of the promising membrane-based technology. Polymeric membranes are a known method for separating CO2 but suffer from trade-offs between its permeability and selectivity. Therefore, through the use of mixed matrix membranes (MMMs) which utilizes inorganic or hybrid fillers such as metal-organic frameworks (MOFs) in polymeric matrix, the permeability and selectivity trade-off can be overcome and possibly surpass the Robeson Upper Bounds. In this study, various types of MOFs are explored in terms of its structure and properties such as thermal and chemical stability. Next, the use of amine and non-amine functionalized MOFs in MMMs development are compared in order to investigate the effects of amine functionalization on the membrane gas separation performance for flat sheet and hollow fiber configurations as reported in the literature. Moreover, the gas transport properties and various challenges faced by hollow fiber mixed matrix membranes (HFMMMs) are discussed. In addition, the utilization of amine functionalization MOF for mitigating the challenges faced is included. Finally, the future directions of amine-functionalized MOF HFMMMs are discussed for the fields of CO2 separation.
... The surplus heat is assumed to be used for domestic hot water supply (Bacenetti et al., 2016a;Naroznova et al., 2016). The biogas upgrading unit was assumed to include a high-efficiency three-stage membrane separation unit made of polyimide hollow fibres for CO 2 capture (98% capture yield) (Ardolino et al., 2018;Chen et al., 2017). Electricity, chemical reactants, and emissions of biogas treatment units were accounted for each component of the system and discussed in section 2.4. ...
Anaerobic digestion (AD) of organic waste, although widely practiced, may require suitable accompanying treatments to enhance the degradability of complex materials. Since these may require significant efforts in terms of energy and chemical demand, careful assessment of their overall environmental sustainability is mandatory to evaluate their full-scale feasibility. The study aims to represent the environmental profile of ultrasonication (US) applied as a post- treatment of anaerobic digestion of agro-industrial organic residues. There is an interest in the US treatment for the processing of complex organic materials prior to AD in order to enhance the hydrolysis of complex organic substrates and increase the biogas yield of the biological process. An attributional, processbased life cycle assessment (LCA) study was applied to quantify and compare the potential environmental impacts of an AD plant, the biogas utilization options as well as the different digestate processing alternatives grouped into a set of 16 scenarios. Based on the results, upgrading of biogas and bio-methane use as vehicle fuel instead of energy generation from CHP or fuel cell was recommended due to the lower impact on GWP. Similarly, composting was a suitable option to reduce environmental impacts compared to belt drying. From the uncertainty analysis, AD without US as post-treatment proves to be more sustainable in terms of GWP compared to when US is used, showing net savings in GHG emissions especially when upgrading of biogas is applied. The analysis provides useful indications to policy makers to define sustainable management alternatives for organic residues as well as identify the environmental advantages associated with biogas utilization and digestate treatment and disposal alternatives.
... Reported membrane separation performance and rough price estimation for a membrane area unit allow the socio-economic analysis of the membrane-based CO 2 capture employing post-synthetically modified PE TFC membrane. [115]. e CA-PE price per unit weight is estimated based on the synthesis procedure of P[CA][Tf 2 N] with reagents supplied by commercial laboratory suppliers, such as Sigma Aldrich, Acros, TCI, etc. [43]. ...
Polymer-based CO2 selective membranes offer an energy efficient method to separate CO2 from flue gas. `Top-down’ polyelectrolytes represent a particularly interesting class of polymer materials based on their vast synthetic flexibility, tuneable interaction with gas molecules, ease of processability into thin films, and commercial availability of precursors. Recent developments in their synthesis and processing are reviewed herein. The four main groups of post-synthetically modified polyelectrolytes discern ionised neutral polymers, cation and anion functionalised polymers, and methacrylate-derived polyelectrolytes. These polyelectrolytes differentiate according to the origin and chemical structure of the precursor polymer. Polyelectrolytes are mostly processed into thin-film composite (TFC) membranes using physical and chemical layer deposition techniques such as solvent-casting, Langmuir-Blodgett, Layer-by-Layer, and chemical grafting. While solvent-casting allows manufacturing commercially competitive TFC membranes, other methods should still mature to become cost-efficient for large-scale application. Many post-synthetically modified polyelectrolytes exhibit outstanding selectivity for CO2 and some overcome the Robeson plot for CO2/N2 separation. However, their CO2 permeance remain low with only grafted and solvent-casted films being able to approach the industrially relevant performance parameters. The development of polyelectrolyte-based membranes for CO2 separation should direct further efforts at promoting the CO2 transport rates while maintaining high selectivities with additional emphasis on environmentally sourced precursor polymers.
... Currently, membranes are favored over other separation techniques due to their simplicity, low energy consumption and scalability. However they still suffer from low productivity, as well as low thermal and mechanical stability [1]. The basic materials used for most of the successful gas separation membranes are polymers. ...
In this study, mixed matrix membranes (MMM) based on poly(ether-b-amide) or Pebax® were prepared using a synthetized ZIF-67 and a commercial ZIF-8 (Basolite® Z1200) to determine the effect of particle content (0, 2, 3, 4 and 5 wt%) on CO2, CH4 and N2 single gas permeability as well as CO2/CH4 and CO2/N2 ideal selectivity. The MMM morphology was evaluated first by scanning electron microscopy (SEM) where excellent dispersion without aggregation was observed. The thermal properties were determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) showing the disruptive role of fillers on polymer chain mobility. Fourier transform infrared spectroscopy (FTIR) showed that no significant chemical interaction between the polymer and ZIF particles occurred. Finally, gas permeation results at 35 °C and 11 bar revealed higher CO2 permeability for all MMM, but especially for Pebax/ZIF-67 with a 130% increase (162 Barrer) compared to the pristine Pebax membrane (70 Barrer), while the Pebax/ZIF-8 produced a lower (85%) increase (130 Barrer). Due to the smaller pore aperture of ZIF-67, CO2 selectivity over CH4 and N2 was higher compared to ZIF-8. Overall, the Pebax/ZIF-67 system was able to overcome the Robeson upper bound for the CO2/N2 separation.
... Nevertheless, Ultrason (polyethersulfone) would be best for O 2 enrichment. But these conclusions might be dependent on the operating conditions like pressure and temperature, as well as the inlet gas composition [2]. ...
... The average outer skin layer thickness is estimated to be 0.64 µm. It is noticed that other reported hollow fibre membranes containing clay or PEI as one of the constituent, showed higher selectivity but lower permeability [38,[41][42][43]. However, in this study, by applying the performance enhancement strategies, all the developed hollow fibre membranes either crossed or were close to commercially attractive region. ...
Hollow fibre mixed matrix (HFMM) membranes with nano-filler embedded in polymer matrix offer an attractive route for the fabrication of high performance gas separation membranes. However, the quest to achieve high performance mixed matrix membranes remains a challenge without acquiring even filler distribution in polymer matrix. In this work, HFMM membranes comprising polyetherimide (PEI) with various inorganic and organic montmorillonite (I-MMT and O-MMT) loadings ranging from 1 to 4 wt%, were developed via phase inversion method and coated with PDMS for CO2/CH4 separation. Morphological, filler distribution, dispersion, surface topology and gas separation studies were carried out for developed hollow fibre (HF) membranes. Pure gases (CO2 and CH4) were used at varying pressure of 2–10 bars at ambient conditions. In addition, mixed gas test at CO2/CH4 composition of 50/50 v/v % was carried out for selected membranes. Upon incorporation of I-MMT, the developed mixed matrix membranes (MMMs) showed decrease in CO2/CH4 gas separation performance compared to neat PEI membrane. In contrast, the performance of asymmetric membrane was enhanced by incorporating O-MMT in PEI matrix to form MMMs. Uniform dispersion, void-free morphology and reduced surface roughness were observed for the aforementioned membranes. Furthermore, an increasing trend in ideal selectivity was observed up to 2 wt% O-MMT loading against all feed pressures. Thereafter, opposite trend was observed with increasing filler loading due to filler agglomeration. The maximum ideal selectivity achieved was 18.35 with 2 wt% loading at 4 bar pressure which is 52.2% higher than neat PEI hollow fibre membrane.
... Polyethersulfone (PESU) possesses good mechanical properties, good chemical resistance has been recognized and used in water purification [22][23][24][25][26]. The O 2 /N 2 selectivity of PESU (e.g. 6) [27] is comparable with Matrimid (e.g. ...
Carbon dioxide (CO2) is a greenhouse gas which is mainly found in natural gas (NG), biogas, and flue gas. Anthropogenic CO2 emissions are the direct result of burning fossil fuels. Meanwhile, pre‐ and postcombustion CO2 separation is a current state of CO2 removal method in an extensive manner. From environmental, economic, and transportation perspectives, removal of CO2 has driven the development of its separation process technology. Among the reported technologies, membrane‐based gas separation technologies have grown substantially, breakthroughs and advances in past decades. This review paper aims to provide an overview on competitive gas separation processes, different types of membranes available, gas transport mechanisms, and fabrication process of hollow fiber membranes, particularly dual‐layer hollow fiber membrane. The performance of the membranes in CO2 separation and effect of spinning parameters on the formation of hollow fiber membranes are highlighted. In addition, approaches to improve the dual‐layer adhesion, strategies to enhance the filler compatibility in the development of dual‐layer hollow fiber mixed‐matrix membranes, and effect of post‐treatments on the gas separation performance of membrane are also discussed. Finally, challenges and future perspectives of dual‐layer hollow fiber mixed‐matrix membranes toward CO2 capture, particularly on CO2/CH4 and CO2/N2 separation, are also included, due to its substantial and direct relevance to the gas separation industry.
In this research, simulation of an O2/N2 membrane separation process for N2 enrichment using an industrial polyimide hollow fiber (PIHF) membrane module was performed based on finite element method. A two-dimensional axial symmetric model was used to simulate the mass transfer, convection and diffusion phenomenon in the membrane module. In order to validate the model, the simulation results were compared with the industrial process data, and good agreement was observed. The effects of feed molar flow rate, feed pressure, and molar flow rate of sweep gas stream on the N2 enrichment percentage were investigated. As the feed molar flow rate increased from 1.2 to 1.8 kgmole/h, the N2 enrichment percentage in the membrane module diminished from 8.6 to 5.8 %. With feed pressure enhancement from 7 to 12 barg, the percentage of N2 enrichment increased from 6.4 to 10%. With increasing molar flow rate of sweep gas stream from 0.5705 to 1.0595 kgmole/h, the percentage of N2 enrichment enhanced from 7.4 to 8.9%. Besides the operational parameters, the effect of fiber length on the N2 enrichment percentage in PIHF membrane module was investigated for co-current and counter current flow patterns, respectively. As the fiber length increased, the N2 enrichment percentage augments for both patterns due to membrane surface area increment in the PIHF membrane module, which the percentage of N2 enrichment in the countercurrent pattern was higher than co-current. Moreover, Concentration Polarization Index (CPI) was investigated to show the degree of polarization expansion along the PIHF membrane. The effect of feed molar flow rate on the concentration polarization index was investigated, which showed the concentration polarization phenomenon is reduced when feed molar flow rate enhances.
Biogas serves a reliable renewable resource and energy carrier with growing potentials based on the number and size of plants in operation and planned for future. The technical viability of membranes for biogas valorization has attracted attention towards further advancements from the materials and process perspectives. The present review aims to meticulously analyze the extensive works carried out at laboratory, pilot, semi-industrial and industrial scales pertinent to biogas with the aid of membrane separation processes. Discussions are devoted to the performance characteristics and specifications of various membrane materials, processes and configurations employed spanning the entire value chain of biogas production, upgrading and conversion. These include recovery of dissolved methane and liquids (water, ammonia) at the production stage, as well as exploitation of semipermeable and gas-liquid membranes such as membrane contactors, membrane reactors and membrane bioreactors for upgrading of raw biogas to achieve quality biomethane. Besides, valuable experiences in integration of membranes in hybrid configurations for biogas upgrading are evaluated. Also, the emerging trends in biogas conversion are delineated by focusing on methanation and hydrogenation with the aid of membranes. Finally, guidelines for the design, integration and optimization of high-performance membrane systems are set by taking into account the economic considerations.
The CO2/CO mixed-gas separation performance of a polyimide (Matrimid®), polyetherimide (PEI) and polylactic acid (PLA) membrane, were characterized in the presence of CO2- rich ternary (CO2/ CO/ O2) and quaternary (CO2/ CO/ O2/ N2) feed gas mixtures mimicking the products of CO2 reforming conversion reactions. The membrane-based separation of this mixture is poorly characterized and original data were obtained in a novel mass spectrometric apparatus that permits to monitor the instantaneous permeate composition, thus allowing to evaluate both mixed gas diffusion and permeability coefficients of all gases. CO2, CO, O2andN2 permeability and diffusivity in single gas tests were measured between 298 and 353 K up to 1 atm feed pressure and relevant activation energies were evaluated. At 298 K Matrimid® exhibits CO permeability of 0.50 ± 0.03 Barrer and an ideal CO2/ CO selectivity of 16 ± 1. PEI and PLA exhibit similar ideal selectivity values but lower CO transport rates. In all examined polymer films the CO2/ CO selectivity has absorption-selective character that favours the permeation of CO2. The ideal CO2/ CO selectivity of all membrane samples decreases with temperature, reaching values of 10 ± 1 at 335 K in Matrimid®. The CO2/ CO selective performances of all examined membrane do not show markable variations exposing the membrane samples to CO2- rich gas mixtures as feed gas. The upper bound correlation among selectivity and permeability for the CO2/ CO gas couple is here for the first time proposed.
With the increasing global demand for energy, renewable and sustainable biogas has attracted considerable attention. However, the presence of various gases such as methane, carbon dioxide (CO2), nitrogen, and hydrogen sulfide in biogas, and the potential emission of acid gases, which may adversely influence the environment, limits the efficient application of biogas in many fields. Consequently, researchers have focused on the upgrade and purification of biogas to eliminate impurities and obtain high-quality and high-purity biomethane with an increased combustion efficiency. In this context, the removal of CO2 gas, which is the most abundant contaminant in biogas, is of significance. Compared to conventional biogas purification processes such as water scrubbing, chemical absorption, pressure swing adsorption, and cryogenic separation, advanced membrane separation technologies are simpler to implement, easier to scale, and incur lower costs. Notably, hollow fiber membranes enhance the gas separation efficiency and decrease costs because their large specific surface area provides a greater range of gas transport. Several reviews have described biogas upgrading technologies and gas separation membranes composed of different materials. In this review, five commonly used commercial biogas upgrading technologies, as well as biological microalgae-based techniques are compared, the advantages and limitations of polymeric and mixed matrix hollow fiber membranes are highlighted, and methods to fabricate and modify hollow fiber membranes are described. This will provide more ideas and methods for future low-cost, large-scale industrial biogas upgrading using membrane technology.
The increasing challenges in meeting the clean air demand have fuelled growing environmental awareness to secure effective air purification and separation techniques. Membrane separation has drawn interdisciplinary attention and has emerged as an indispensable gas purification technology. Nevertheless, the permeance/selectivity trade-off relationship, plasticization and physical aging are the main challenges in developing a promising membrane. There has been a substantial advancement in hollow fiber membranes in the last 10 years, owing to the attractive characteristics that favor industrial applications. In this review, we provide a critical summary of the latest discoveries of state-of-the-art polymeric hollow fiber membranes. The theoretical principles of spinning parameters and fabrication strategies on the membranes' final morphology and separation performance will be presented and discussed according to respective gas separation applications, namely CO2 capture, air purification, and hydrogen and propane/propylene separation. Besides, the latest progress in sulfur dioxide, hydrogen sulfide, water vapor and particulate matter removal will be discussed. The emerging R&D areas, particularly CO2-induced plasticization, physical aging and green solvents, will be highlighted. Lastly, this review will be concluded with challenges, perspectives and future directions. It is anticipated that this comprehensive review may stimulate a new research platform for developing next-generation gas separation hollow fiber membranes for sustainable energy and environmental applications. This journal is
The accurate assessment of the membrane gas separation properties requires improved metrologies when the feed gases are mixtures rich of components such as biogases, flue gases and mixtures produced by CO2 reforming. We present an original mass spectroscopy- based approach that permits to monitor the permeation kinetics of gas mixtures formed by components with overlapping mass signals. In the instrumental setup, sample-holder and quadrupole mass spectrometer are hosted in an Ultra High Vacuum chamber which is kept under dynamic pumping conditions during the experimental run. This setup permits to measure, as a function of time, the permeation flux of each mixture component monitoring kinetics with transient lasting down to the ∼ 1 s order. The flux detection limit is 10⁻⁶ cm³(STP) m⁻² s⁻¹ for species with unambiguously attributable mass signal and no worse than 10⁻⁵ cm³(STP) m⁻² s⁻¹ for species with overlapping mass signal. Tests were carried out at room temperature with ∼ 50 μm thick polymer films exposed to CO2- rich gas mixtures at total pressure below 10⁵ Pa. The analysis of permeation flux kinetics allows the evaluation of the diffusion constant of migrating species with ∼ 4 % indetermination. In steady-state conditions the permeation flux through a poly(lactic acid) film is measured with accuracy better than ∼ 3 % for components with unambiguously attributable mass signal (CO2, O2) and no worse than ∼ 6 % for those with overlapping mass signal (N2, CO).
In this article, the melt spinning behavior of poly(4‐methyl‐1‐pentene) (PMP) hollow fibers (HF) is examined. The melt spinning trials are carried out on a pilot scale melt spinning plant with different settings while a 10‐hole 2c‐shaped spinneret is used. It is found that the winding speed mainly affects the outer fiber diameter. The influence of different melt spinning parameters is investigated, in particular temperatures, take‐up velocities, and the use of quench air. For this purpose, the shape and crystalline structure of the fibers are analyzed using a light microscope, a scanning electron microscope, and wide‐angle X‐ray scattering. The shape of the fibers is mainly influenced by the temperature settings in the melt spinning process. As a reasonable lower limit, a melt spinning temperature of 280°C is identified. Concerning the crystallinity, a saturation going along with a slight reduction of the polymer chain orientation is observed at elevated take‐up velocities.
Polyetherimide (PEI) is an extraordinary type of polyimide with excellent thermal and mechanical properties. The polymer is also gas permeable and is considered one of the best membranes for gas separation. Despite the high selectivity, PEI suffers from low permeability due to the trade‐off between phenomena in polymers. To overcome this limitation, fillers are added during the membrane preparation to create voids for better gas transport. In this paper, permeability and selectivity data of PEI membranes for the separation of oxygen, carbon dioxide, and helium are discussed. The paper also summarizes the reported studies for adding fillers to improve the membrane performance. Developments of polyetherimide membranes for gas separation with the introduction of fillers for better performance
In the current work, effect of spinning conditions including, take-up speed and air-gap distance and post-treatment methods on the fabrication of cellulose acetate hollow fiber membranes (CA-HFMs) for CO2/N2 and CO2/CH4 separations have been reported. The gas permeation results obtained in this work revealed that permeances of gases were decreased with increase in take-up speed from free fall to 12.2 m/min. Meanwhile, gas pair selectivities increased with increasing take-up speed. Subsequently, increment in air-gap distance produced the “V” pattern for gases permeances and “A” pattern of gas pair selectivities for all CA-HFMs spun at different take-up speeds. Therefore, optimum take-up speed and air gap distance of CA-HFMs of 12.2 m/min and 5.0 cm were obtained, respectively. CA-HFM spun at optimum spinning conditions showed the highest CO2/CH4 and CO2/N2 ideal selectivities of 7.9 and 6.0, respectively. On the other hand, permeation results also demonstrated that the CO2/CH4 and CO2/N2 ideal selectivities of PDMS coated CA-HFMs were higher about 70.9% and 84.1%, respectively, compared to those values obtained from thermally treated CA-HFMs. Therefore, PDMS coating is considered as an effective approach to seal the macro-voids of HFMs compared to the thermal treatment in order to achieve higher permeation performance for CO2 separations. In addition, permeation results also manifested that the CA-HFM fabricated at optimum conditions has incredible worth from the prospective of industrial separations of CO2 from flue and natural gas.
Composite membranes with a thin selective layer based on poly[1-trimethylsilyl-1-propyne] (PTMSP) and crosslinked PTMSP containing 10 wt % of nanoparticles of porous aromatic frameworks (PAF-11) have been synthesized and studied. Monitoring of changes in the gas transport characteristics of the membranes under ambient conditions for 7500 h has revealed that for all the samples, the transport characteristics abruptly decrease within the first 1000–2000 h; after that, the mass transfer constants gradually change over time. In the case of a composite membrane with the selective layer based on crosslinked PTMSP and PAF-11 nanoparticles, stable permeability values after 7000 h are 2.1, 3.5, and 12.9 m³/(m² h atm) for N2, O2, and CO2,respectively (at an ideal selectivity of α(O2/N2) = 1.6 and α(CO2/N2) = 6.1); to date, this is the best published result for thin-film composite membranes based on highly permeable glassy polymers.
A mathematical model is developed to simulate a gas separation process using a hollow fiber membrane module. In particular, a new numerical technique is introduced based on flash calculation. Such analysis allows identifying the required membrane properties needed to reach module performance of interest. This model is validated for six different gas separation cases taken from literature. Then, the validated model is used to investigate the effect of O2 and N2 permeances on O2 recovery and O2 mole fraction in the permeate stream. A realistic two-stage air enrichment process is also proposed for O2 production using an industrial module with different fibers numbers. Moreover, this model is used to simulate a natural gas purification process using a single unit to determine the required membrane separation area and CH4 loss. Finally, a two-stage process is proposed to equally enhance CH4 retentate mole fraction and decrease CH4 loss. This article is protected by copyright. All rights reserved.
The manufacturing of low cost ceramic tubular support membrane via dry compaction method using cheap material, namely, sawdust along with kaolin and feldspar was demonstrated in this study. Thermogravimetric analysis, particle size distribution, image analysis, volumetric porosity and gas permeation experiments, acid–alkali test and three-point bend test were studied to characterize the fabricated membrane, systematically. The support membrane was casted on a cylindrical mold and sintered at three different temperatures of 550, 700 and 850 °C. The average pore diameter of the membrane was decreased with increasing temperature. Adversely, the membrane porosity was increased with increasing temperature. The chemical and mechanical stability of the fabricated membrane were appreciable. Based on raw materials, energy consumption and mold preparation, the cost of the support membrane was estimated as around $250/m2. Hereafter, these low cost support membranes with good physical and chemical properties may be applied for the processing of value-aided products.
An empirical model using response surface methodology (RSM) is proposed to investigate the role of parameters, such as, preparation pressure and binder contents on the porosity and flexural strength of a tubular low-cost ceramic membrane based on central compact design (CCD). Preparation pressure and binder contents are selected as input parameters to obtain controlled porosity with considerable strength of membrane. The optimum preparation pressure is found to be at 9.81 MPa with the sodium metasilicate and boric acid contents of 7.50 % each providing a microfiltration range membrane. The optimization study reveals that the errors between the experimental and predicted values are below 2 %. FESEM images clearly unveil a consolidated microstructure of the ceramic membrane. Acid-alkali test conveys that there is no major change in elemental composition due to the presence of binders in ceramic processing. The cost of the optimized membrane is estimated at around $332/m2.
A series of highly active Pt/TiO2catalysts were prepared by impregnation and deposition precipitation methods with different reduction processes. Their catalytic activities were evaluated by catalyticdecomposition of formaldehyde (HCHO) at room temperature. The effects of reduction treatment on structural properties and catalytic activity were studied. Reduced Pt/TiO2catalysts showed large differences in structural properties (such as particle size, oxidation state, surface content and electronic property of Ptnanoparticles, and surface oxygen) and catalytic activity for HCHOoxidation compared with the unreduced ones. Nearly 100% HCHO conversion was achieved on the former. Especially, sodium borohydride reduced Pt/TiO2catalysts even with 0.1% Pt loading showed nearly complete oxidation of HCHO. Well-dispersed and negatively charged metallic Ptnanoparticles, and rich chemisorbed oxygen are probably responsible for their high catalytic activities.
A novel type of reactor is developed for the catalytic reduction of NOx with NH3. In this reactor a porous membrane is used to keep the reactants separated from each other and to carry out the reaction in a controlled way inside the membrane. As the rate of reaction is fast compared to the diffusion rate of the reactants, the molar fluxes of both reactants are in stoichiometric ratio and slip of reactants to the opposite side of the membrane is prevented. The advantage of this reactor is the possibility of obtaining high conversions of NOx despite fluctuating concentrations of NOx without severe slip of NH3. This membrane reactor has been tested experimentally, and it is demonstrated that it is able to cope with a varying ratio of concentrations of NOx and NH3 without detectable slip of NH3 or NOx at a temperature of 569 K.
The modified Claus process is the most common method for the conversion to sulphur of the hydrogen sulphide contained in sour oil and natural gas. An important but relatively unstudied part of the Claus process are the reaction kinetics of the front-end reaction furnace in which sulphur production takes place, hydrocarbon contaminants are destroyed and reactions occur that prepare the sour gas for downstream catalytic processing. One of the key reactions that occurs in the front-end furnace is between H2S and SO2. This second part of the Claus reactions has been studied under catalytic but not thermal conditions. The purpose of this work was to study this reaction at actual Claus plant reaction furnace temperatures and residence times. The new kinetic data would then be used to develop a new reaction rate expression. Experiments were performed in a laboratory scale, isothermal, plug-flow reactor at temperatures between 850 and 1150°C and at residence times between 0.05 and 1.2 s. Overall conversion of H2S and SO2 were measured and results are presented in this paper. The newly developed kinetic rate expression is as follows:r=AfexpEaf/RTPH2SPSO20.5−ArexpEar/RTPH2OPS20.75,where Af=15,762(±1200)molcm−3s−1atm−1.5,Eaf=49.9(±0.3)kcalmol−1,Ar=506(±50)molcm−3s−1atm−1.75 and Ear=44.9(±0.5)kcalmol−1. The new rate expression correlates experimental H2S and SO2 conversion data within 12 and 18%, respectively. In addition, the predicted conversion for the new rate expression extrapolates correctly to equilibrium conversion values and the Arrhenius parameters predict the heat of reaction to within 0.05%.
There is an increasing challenge for chemical industry and research institutions to find cost-efficient and environmentally sound methods of converting natural resources into fuels chemicals and energy. Catalysts are essential to these processes and the Catalysis Specialist Periodical Report series serves to highlight major developments in this area. This series provides systematic and detailed reviews of topics of interest to scientists and engineers in the catalysis field. The coverage includes all major areas of heterogeneous and homogeneous catalysis and also specific applications of catalysis such as NOx control kinetics and experimental techniques such as microcalorimetry. Each chapter is compiled by recognised experts within their specialist fields and provides a summary of the current literature. This series will be of interest to all those in academia and industry who need an up-to-date critical analysis and summary of catalysis research and applications. Catalysis will be of interest to anyone working in academia and industry that needs an up-to-date critical analysis and summary of catalysis research and applications. Specialist Periodical Reports provide systematic and detailed review coverage in major areas of chemical research. Compiled by teams of leading experts in their specialist fields, this series is designed to help the chemistry community keep current with the latest developments in their field. Each volume in the series is published either annually or biennially and is a superb reference point for researchers.
A series of activated γ-alumina supported Mo/Co catalysts with different Mo loadings ranging from 8 to 20 wt.% have been prepared through the impregnation by soaking metal precursors over the alumina support followed by drying (120 °C) and calcination (350, 400 and 600 °C) for the fabrication of catalytic membrane. A comparative study on activated and γ-Al2O3with low surface area has been performed to understand the metal-support interaction and to select the suitable support in terms of metallic dispersion (MD) and metallic surface area (MSA) for better catalytic activity. Several characterization techniques, such as, BET surface area, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field-emission scanning electron microscopy (FESEM), electron spin resonance (ESR), temperature-programmed reduction (TPR), laser Raman spectroscopy (LRS), transmission electron micrograph (TEM), Energy dispersive X-ray (EDX) spectroscopy and CO chemisorption have been used to verify the interaction between Mo and activated γ-alumina. Based on characterization, 16% Mo–Co/activated γ-Al2O3 catalyst calcined at 400 °C is optimized and selected for activity test. Claus reaction has been chosen to study the catalytic activity. Overall as well as individual conversion of both H2S and SO2, selectivity and yield of product are measured in this study. The highest turnover frequency (TF) is observed as 3.80 min−1 at 300 min for the optimized catalyst.
Ceramic Membranes for Reaction and Separation is the first single-authored guide to the developing area of ceramic membranes. Starting by documenting established procedures of ceramic membrane preparation and characterization, this title then focuses on gas separation. The final chapter covers ceramic membrane reactors;- as distributors and separators, and general engineering considerations. Chapters include key examples to illustrate membrane synthesis, characterisation and applications in industry. Theoretical principles, advantages and disadvantages of using ceramic membranes under the various conditions are discussed where applicable.
Ion transport membranes (ITMs) offer promising technology for high purity oxygen production (up to 99%) without adversely affecting the efficiency of the oxy-fired plants. The permeation rate of such ITMs can be increased by replacing the conventional inert sweep gas with a reactant/diluent mixture (e.g. CO2, CH4) as this reduces the permeate partial pressure on the permeate side of the membrane, which, along with the temperature, governs the permeation flux. The significant limitation of this approach is that an uncontrolled, exothermic consumption of the permeated specie, can lead to membrane damage, and thus limits the potential of ITMs using reactive sweep gases (i.e. ITM reactors). By using a multichannel ITM reactor (isothermal reactor), it is proposed to operate the ITM reactor at, or near to isothermal conditions (i.e a spatially uniform temperature). This may be achieved by introducing a reactant into the permeate stream uniformly across the entire ITM reactor length from an adjacent channel with porous walls. The present work is aimed at modeling a nearly isothermal reactor using an ITM and a porous membrane to achieve uniform stoichiometric ratio of fuel/O-2 in order to have a uniform combustion all along the length of the membrane. A two-dimensional, computational fluid dynamics (CFD) model is solved to study the characteristics. The simulations are based on the numerical solution of the conservation of mass, momentum, energy and species transport equations of two dimensional flows. For the CFD calculations, the commercial solver FLUENT has been used. The models used have been validated against the experimental results found in the literature and are found to be in good agreement. The influence on the performance of oxygen separation through the ITM has been studied by varying the flow conditions at the permeate side. Results show that an approximate uniform combustion can be achieved along with effective thermal management of the ITM using the present isothermal reactor. It is also observed that for a constant mass flow rate of fuel mixture, the permeation rate of oxygen through ITM increases with the increase in CH4/CO2 ratio. The results indicate that the oxygen permeation flux increases by approximately 3 times for reactive case compared to separation only case. Moreover it is shown that using the present reactor model the reaction zone can be controlled. (C) 2013 Elsevier B.V. All rights reserved
Flow-through membrane reactors represent a strategy for process intensification, which benefits from the convective flow that is established due to a transmembrane pressure gradient. The interesting consequence from using these materials is the improved utilization of the catalyst dispersed in the membrane. We propose a theoretical analysis which quantifies the effectiveness factor ( ) and the degree of conversion. More importantly, the regime of operation which maximizes the enhancement from convective effects is identified. It corresponds to conditions of not only high internal Peclet number ( ), but also of comparable Thiele modulus ( ). We find that these two parameters are related by a simple analytically derived expression: . When this relationship holds, an upper limit to the enhancement in the effectiveness factor that can be observed is proportional to . This result also provides an answer to the effectiveness-conversion trade-off in ‘dead-end’ operation, when both objectives are important. The analytical solutions enable the complete description of the system in Peclet-Thiele diagrams, where the different reaction-transport regimes are identified. Moreover, issues that become particularly relevant in membrane reactors are discussed: curvature, flow direction and the ratio between the concentration distributions at both surfaces. The simplified design rules obtained bridge the gap between materials synthesis (with permeability and thickness as tunable properties) and process design (enhancement of the internal transport and productivity).
The extensive uses of pore-formers are considered to be a major factor for determining morphology in ceramic membrane fabrication, but the behavior of pore-formers during membrane fabrication remains unverified. Five different size ranges of sawdust screened through 30, 44, 60, 72 and 100 B.S.S. meshes (500, 355, 250, 212, and 150 μm) are utilized as a pore-former and are believed to influence on membrane porosity, pore size and surface texture. Two series of experiments have been conducted; the first set of experiment is planned to determine thermal behavior and particle size of sawdust as well as the change in physical properties of sawdust when burnt in the presence of air and the sustainability of raw and burnt-sawdust in acid and alkali media. The second set of experiment is the selection of appropriate sized sawdust particle for the fabrication of ceramic membrane based on first set of experiments for understanding the behavior of sawdust during membrane fabrication. Sawdust samples are characterized by thermogravimetric analysis (TGA), Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), field-emission scanning electron microscopy (FESEM) and energy dispersive x-ray (EDX). Chemical sustainability of sawdust is verified using acid-alkali test. Based on the experimental data, it is concluded that there is an obvious effect of sawdust on the morphology of fabricated membrane and the sawdust screened through 44 B.S.S. mesh is selected for the fabrication of membrane for our purpose. The mechanical strength of the membrane is also noteworthy.
A model was developed to simulate tubular enzymatic membrane reactors under three different configurations: dead-end, tangential flow with a porous enzymatic membrane and a non-permeable enzymatic wall. The simulations were applied to analyze the influence of reactor configuration, kinetics and mass transport conditions over the reactor performance in order to identify the main aspects to be taken into consideration for attaining optimal designs. The non-permeable enzymatic wall configuration under the evaluated conditions seems to be more valuable than the dead-end case in terms of substrate conversion and the tangential configuration looked more favorable to promote the best conversion in the permeate but not in the retentate. It was demonstrated that for a similar value, the Damköhler number can result in very dissimilar performances. The simulated results demonstrated that the most significant variable of the global performance of the enzymatic membrane reactors is the reaction kinetics: fast reactions attained very considerable conversion values under very different conditions.
Numerical simulations are presented to compare mass transfer at the bulk fluid-membrane interface of two types of membrane reactors, for arbitrary equilibrium reactions: the catalytic membrane reactor (CMR) in which the location of the reaction and separation coincide, and the inert membrane reactor (IMR) in which locations of reaction and separation distinct. The Maxwell-Stefan theory is adopted to describe this multi-component mass transport and to take friction between the species in the reaction mixture into account. Simulation results are presented that aid selection of the most appropriate reactor configuration for different reaction equilibrium characteristics. Effects of process conditions, membrane properties, and possibilities to optimize reactor design are discussed. Three regimes can be distinguished, based on the value of reaction equilibrium constant (K-eq). At very low K-eq, the CMR outperforms the IMR, and in particular a high membrane area/reactor volume ratio (A/V) a high product permeance, and a large residence time are required. At moderate K-eq, the CMR potentially outperforms the IMR, and conversion benefits in particular from a high A/V ratio and sufficiently high mass transfer. For high K-eq the performance of the IMR is superior as compared to the CMR. The simulation results indicate that, in particular for the CMR, a mass transport description that can properly address multi component mass transport characteristics is vital. The results predicted based the Maxwell-Stefan theory will not be captured adequately by a model based on, for instance, the law of Fick.
Ion transport membrane (ITM) based reactors have been suggested as a novel technology for several applications including fuel reforming and oxy-fuel combustion, which integrates air separation and fuel conversion while reducing complexity and the associated energy penalty. To utilize this technology more effectively, it is necessary to develop a better understanding of the fundamental processes of oxygen transport and fuel conversion in the immediate vicinity of the membrane. In this paper, a numerical model that spatially resolves the gas flow, transport and reactions is presented. The model incorporates detailed gas phase chemistry and transport. The model is used to express the oxygen permeation flux in terms of the oxygen concentrations at the membrane surface given data on the bulk concentration, which is necessary for cases when mass transfer limitations on the permeate side are important and for reactive flow modeling. The simulation results show the dependence of oxygen transport and fuel conversion on the geometry and flow parameters including the membrane temperature, feed and sweep gas flow, oxygen concentration in the feed and fuel concentration in the sweep gas.
The oxidative dehydrogenation of propane to propylene catalysed by vanadium supported by oxide, was carried out on a catalytic membrane reactor (CMR) in a reaction temperature range of 380–500 °C. Two different species of the internal layer (γ-Al2O3, ZSM-5) that supported the catalyst, were taken in consideration and two possible ways (premixed or separate) of feeding the reactants in the CMR were studied. The chief goal was to validate on an experimental basis, a simple phenomenological model to interpret the CMR performance. It has been found that the use of a thicker catalytic layer can enhance the selectivity to propylene when propane and oxygen are differentiated at the tube and the shell with respect to the premixed feed and when there is a sufficient thickness of the catalytic layer.
Methane steam reforming is one of the most important pathways for producing high purity hydrogen. In this context, the use of fixed-bed catalytic reactors equipped with hydrogen perm-selective membranes is an interesting alternative for producing high purity hydrogen in one single step. In this work, this reactor is studied by means of numerical simulations using a 2D model, consisting of mass, energy and momentum balances. The fixed-bed is considered to be formed by Ru/SiO2 catalyst particles, especially tailored for steam reforming at low temperature and steam-to-carbon ratio, whereas a composite palladium membrane was considered for hydrogen permeation. The model was validated with experimental data, and the adequacy of a simplified 1D model to simulate the membrane reactor was evaluated and discussed in comparison to the 2D model. Then, the model was used to study the influence of the main operating variables (inlet temperature, pressure, space velocity, steam excess and sweep gas rate in the permeate side) on the reactor performance. Finally, the optimum operating conditions, corresponding to a maximum hydrogen permeation rate, were determined, and the behaviour of the optimized reactor is analysed in detail.
A simple model is developed to examine the performance of a supported catalytic membrane within which occurs the consecutive-parallel reaction system given by A + B → R, with RATE = k1pA1ApBB, and A + R → P, with RATE = k2pA2ApRR. Closed-form solutions reveal that segregation of reactants A and B to opposite sides of the membrane is an effective strategy for increasing the desired product (R) point yield. However, increases in the component R yield come at the expense of the point catalyst utilization, due, in part, to depletion of reacting components B and R. The membrane performance is sensitive to the relative reaction orders with respect to component A for the special case in which the rates are zeroth-order with respect to B and R (B = R = 0). The segregation strategy is shown to be most beneficial if three requirements are met: (i) A1 < A2, (ii) k1, k2 sufficiently large and (iii) active layer sufficiently thin compared to support. Under favorable conditions [requirements (i)-(iii) met], component R is selectively produced near the active layer surface, and diffuses out of the membrane before further reaction to undesired product (P). The simulations indicate that the fractional increases in the R yield attained, as the degree of segregation is increased, exceed the fractional decreases in catalyst utilization. A secondary benefit of the membrane design is the confinement of reaction products in the bulk stream on the active layer side, thus reducing the downstream separation needs.
A kinetic study of the oxidation on H2S on γ-alumina by SO2 following a modified Claus process at low temperature (373–473 K) was carried out. To the analytical data, at varying SO2 and H2S concentrations, several Langmuirian type expressions for the reaction rate were applied. The models were submitted to statistical and physical criteria to establish the best fitting of the experimental results. The resulting kinetic expression applied to sulphurized surfaces shows the chemical reaction between adsorbed species—one of them H2S dissociatively adsorbed—as the controlling step of the process. The reaction was also effected under integral regime, the obtained results confirming the validity of the proposed kinetic expression.
Catalytic combustion of methane over Pd and Pt/SiO2/α-Al2O3 membranes was studied in the temperature range 300–650°C. Fuel and oxygen were fed at opposite membrane sides. In order to improve reactor controllability the α-Al2O3 membranes were impregnated with SiO2 sol resulting to smaller pore size. Methane conversions up to 100% for the palladium membrane and up to 42% for the platinum membrane were achieved. The results indicated a transition from kinetic to mass transfer control within the temperature range investigated. This was accompanied by reduction of methane slip from tube to shell side with increasing temperature. CO and H2 were detected in the product gases of the palladium membrane. Their concentration could be reduced by applying a trans-membrane pressure difference. Low concentrations of CO were observed for the Pt/SiO2/α-Al2O3 membrane, while no CO or H2 were detected for a Pd/α-Al2O3 membrane operating in dead-end configuration.
The catalytic properties of iron oxide supported platinum catalysts (Pt/Fe2O3), prepared by a colloid deposition route, were investigated for the complete oxidation of formaldehyde. It is found that all the Pt/Fe2O3 catalysts calcined at different temperatures (200–500°C) were active for the oxidation of formaldehyde. Among them, the catalysts calcined at lower temperatures (i.e., 200 and 300°C) exhibited relatively high catalytic activity and stability, which could completely oxidize HCHO even at room temperature. Based on a variety of physical–chemical characterization results, it is proposed that the presence of suitable interaction between Pt particles and iron oxide supports, which is mainly in the form of Pt–O–Fe bonds, should play a positive role in determining the catalytic activity and stability of the supported Pt/Fe2O3 catalysts.
An easily applied method for predicting binary gas-phase diffusivities is based on the use of special diffusion volumes coupled with extensive experiment and nonlinear least squares analysis of the data. Comparison with eight other correlations demonstrates the relative reliability and simplicity of the new method.
A mathematical model, based on the dusty-gas model extended with surface diffusion, is presented that describes mass transport owing to molecular diffusion and viscous flow, as well as an instantaneous reversible reaction inside a membrane reactor. The reactants are fed to opposite sides of the membrane, considering masstransfer resistances in the gas phase outside the membrane. The Claus reaction is chosen as a model reaction to study this membrane reactor.
The model is used to validate a previously presented simplified model. The simplified model predicts correct molar fluxes when it is very dilute and can therefore be considered a pseudo-binary system. Occurrence of a maximum or a minimum in the pressure profile inside the membrane, in the absence of an overall pressure difference over the membrane, depends not only on the stoichiometry of the reaction but on mobilities of the different species.
The Claus reaction is used to verify experimentally the transport model for a nonpermselective membrane reactor with a mean pore diameter of 350 nm. At 493 K and 542 K, molar fluxes experimentally determined are 10 to 20% lower than those predicted by the transport model. Conversions measured at pressures of 220 kPa and 500 kPa demonstrate that surface diffusion occurs as a transport mechanism despite the large pore diameter of the membrane. In the presence of a pressure difference over the membrane, there is a reasonable agreement between experimentally determined molar fluxes and those calculated by the transport model.
Catalytic membrane reactors are reviewed as applied to opportunities and applications within petroleum refineries. Since so many inorganic membranes take advantage of H2 permselectivity and H2 demands are increasing in a refinery, there are a number of interesting process applications being considered. H2 production can be enhanced by using Pd based membranes for dehydrogenation, oxydehydrogenation, and decomposition reactions. Permselective H2 membranes could be used for carrying out selective hydrogenations of organic substrates and coupled reactions. These membranes have been also considered for enhancing steam reforming reactions for the production of bulk H2, the water gas shift reaction, and the conversion of natural gas to syngas and liquid fuels. Dense oxide membranes are also being developed for the selective oxidation of CH4 to syngas. For many of these processes, the formation of carbon during steam reforming or dehydrogenation reactions will always be a huge hurdle towards any successful commercial application of Pd membranes to such processes. In any of these applications one has to understand production problems associated with the metal membranes, the refinery demands for high purity H2, and the reactor fabrication hurdles; these will be evaluated with recent examples. For all these applications, the critical issues that need to be resolved for the commercial use of catalytic membrane reactors will be discussed.
This paper reviews and classifies publications dealing with catalytic membrane reactors in flow-through mode. In contrast to other membrane reactor concepts, the membrane is operated in dead-end mode and no separation task is performed. As no permselectivity is required, catalytically active porous, mostly ceramic, membranes are applied. The task of the membrane is to provide for intensive contact between reactants and catalyst, combined with a short contact time and a narrow residence time distribution. The applications are categorized in three groups: Complete Conversion Integral Reactors, Selective Integral Reactors and Selective Differential Reactors.
The membrane reactor (MR) concept, combining in the same unit a conversion effect (catalyst) and a separation effect (membrane), already showed various potential benefits (increased reaction rate, selectivity and yield) for a range of reactions involving the membrane as extractor, distributor or contactor. Due to the generally severe conditions of heterogeneous catalysis, most MR applications use inorganic membranes, which can be dense or porous, inert or catalytically active. After a rapid overview of the working concepts of MRs, the main types of porous ceramic membranes, which have been developed for MR applications, are reported and discussed (characteristics and limitations). Starting from these general basis, our objective is to put recent developments into focus, with a special emphasis on porous composite infiltrated membranes and related synthesis methods. Some new ideas currently explored in our group, such as the ‘chemical valve membrane’ concept and the interest of nanophase materials for oxygen transport, will be also developed. An attempt in addressing the future developments of porous membranes for MRs will be finally proposed.
A membrane reactor with separated feed of reactants is demonstrated as a promising contractor type when dealing with heterogenously catalysed, very fast and exothermic gas phase reactions. Due to the separation of reactants a good control of the system is obtained, because process variables can be varied independently from each other. Transport of reactants is the rate governing process and because this is only slightly temperature dependent a thermal runaway will not occur. When dealing with e.g. combustion process no explosive mixtures will build up and safety is increased. Based on the dusty-gas model, the concentration profiles of components inside the membrane can be calculated together with the fluxes. However this is a calculation time consuming process and not necessary in all cases. In absence of a pressure drop and no slip of reactants to the opposite side a linearisation is possible leading to a simplified expression for the interfacial flux of a reactant and a criterion to evaluate the possibility of slip of reactants. Using the oxidation of carbon monoxide catalysed by platinum as a model reaction this approximation was experimentally verified by comparison of measured fluxes with the calculated results. Apart from flux measurements exploratory overall conversion measurements were carried out with the membrane reactor module in order to demonstrate its operation performance. From these studies it was concluded that conversion levels up to 90% carbon monoxide could easily be achieved.
A novel type of membrane reactor with separated feeding of the reactants is presented for chemical processes normally requiring strict stoichiometric feed rates of premixed reactants. The reactants are fed in the reactor to the different sides of a porous membrane which is impregnated with a catalyst for a heterogeneously catalyzed reaction. If the reaction rate is fast compared to the diffusion rates of the reactants, a small reaction zone inside the membrane occurs and slip of one of the reactants to the opposing side of the membrane is prevented. The location of this reaction zone will be such that the molar fluxes of the reactants are always in stoichiometric ratio. The features of this reactor are shown by means of mathematical modelling of molecular diffusion and viscous flow combined with an instantaneous, reversible reaction inside the membrane. As a model reaction the Claus reaction was selected and by conversion measurements the principle of a shifting reaction plane inside a porous membrane is demonstrated.
A pilot plant study on propane catalytic combustion in a membrane reactor with separate reactant feeds is presented. The membrane consisted of a porous alumina tube activated by insertion into its pores of a Ptγ-Al2O3 catalyst. The role of reactants concentration and of the feed flow rates were studied in the transport-controlled operating regime, where a number of interesting properties of this reactor setup can be exploited (absence of reactant slip through the membrane, lower risks of thermal runaways, possibility of increasing conversion by application of a pressure difference over the membrane, etc.). Attention is here focused on operation in the absence of trans-membrane pressure gradients. The reactor fluid-dynamics are investigated, too. The experimental results are in good agreement with the predictions of an isothermal model, analytically solved and based on the simplifying assumption that the reaction takes place in a limited zone inside the membrane (i.e. a surface for infinitely fast reactions).
This paper provides an experimental and modelling analysis of the performance of a membrane reactor with separate feed of reactants for the combustion of methane. In this reactor concept methane and air streams are fed at opposite sides of a Pt/γ-Al2O3-activated porous membrane which hosts their reaction. The effect of a number of operating parameters (temperature, methane feed concentration, pressure difference applied over the membrane, type and amount of catalyst deposited, time of operation) over the attainable conversion was assessed, while measuring any possible slip of unconverted methane to the air-feed side. The maximum specific heat power load which could be attained with the most active membrane in the absence of methane slip was approximately 15 kW m−2 with virtually no NOx emissions. Such potential might perhaps be exceeded if a properly designed membrane is tailored on purpose. For this sake a model, based on differential heat and mass balances throughout the membrane thickness, proved to be a promising design tool, since it allowed proper accordance with the experimental data with a single fitting parameter (pre-exponential kinetic constant).
Catalytic membrane reactors which carry out separation and reaction in a single unit are expected to be a promising approach to achieve green and sustainable chemistry with less energy consumption and lower pollution. This article presents a review of the recent progress of dense ceramic catalytic membranes and membrane reactors, and their potential applications in energy and environmental areas. A basic knowledge of catalytic membranes and membrane reactors is first introduced briefly, followed by a short discussion on the membrane materials including their structures, composition and strategies for material development. The configuration of catalytic membranes, the design of membrane reaction processes and the high temperature sealing are also discussed. The performance of catalytic membrane reactors for energy and environmental applications are summarized and typical catalytic membrane reaction processes are presented and discussed. Finally, current challenges and difficulties related to the industrialization of dense ceramic membrane reactors are addressed and possible future research is also outlined.
Catalytic oxidation of hydrogen sulphide
- T K Khanmamedov
- R H Weiland
Mass Transfer in Multicomponent Mixtures
- J A Wesselingh
- R Krishna