Carbon Dioxide Mass Transfer into Solvents in a Zeolite Hybrid Device

Carbon dioxide (CO₂) mass transfer processes are analyzed in hybrid equipment which involves a zeolitic membrane and a physical or chemical solvent. This separation device was chosen because the membrane can be used to produce a stream of higher CO₂ concentration to be treated by gas‐liquid absorption. The analysis of the mass transfer behavior of this gas through the solid phase is an important step before more complicated gas streams are applied. The combined use of both techniques can improve the global separation process because they allow performing a previous separation with a positive effect on the cost of the later separation operations. The influence of the liquid phase nature used in one chamber of the membrane contactor upon CO₂ global mass transfer is analyzed. Also the effect caused by the absorption regime, liquid and gas flow rates, and the pressure corresponding to the gas chamber on CO₂ mass transfer is studied to evaluate the importance of each variable.     

燃煤烟气中细颗粒物与共存气态组分对膜吸收CO2的影响

由于燃煤烟气中细颗粒物和气态污染物难以完全脱除,同时经湿法脱硫后烟气中水汽接近饱和状态,因此,有必要揭示细颗粒物及共存气态组分对膜吸收CO₂的影响。采用燃煤热态试验装置,考察了燃煤烟气中细颗粒物、SO₂、水汽对膜吸收CO₂性能的影响,并进行了实际燃煤湿法脱硫净烟气环境下的膜吸收CO₂试验。结果表明:细颗粒物随烟气通过膜组件后,部分细颗粒物可被膜截留,沉积于膜表面,导致膜吸收CO₂效率下降,其影响程度随细颗粒物浓度的降低而减弱,与细颗粒物物性有一定关系,通过有效降低烟气中细颗粒物浓度,可显著延长膜的稳定运行时间;SO₂的存在会与CO₂产生竞争吸收现象,但因烟气中SO₂含量远低于CO₂,对CO₂吸收效率影响不明显;对于水汽,只需在运行一段时间后对膜组件作气体干燥反吹,可基本维持膜组件的稳定运行。     

Absorption of SO<Subscript>2</Subscript> using PVDF hollow fiber membranes with PEG as an additive

In this study, removal of SO₂ from gas stream was carried out by using microporous polyvinylidene fluoride (PVDF) asymmetric hollow fiber membrane modules as gas-liquid contactor. The asymmetric hollow fiber membranes used in this study were prepared polyvinylidene fluoride by a wet phase inversion method. Water was used as an internal coagulant and external coagulation bath for all spinning runs. An aqueous solution containing 0.02 M NaOH was used as the absorbent. This study attempts to assess the influence of PEG additive, absorbent flow rate, SO₂ concentration, gas flow rate and gas flow direction on the SO₂ removal efficiency and overall mass transfer coefficient. The effect of liquid flow rate on SO₂ removal efficiency shows that at very low liquid flow rate, the NaOH available at the membrane surface for reacting with SO₂ is limited due to the liquid phase resistance. As liquid flow rate is above the minimum flow rate which overcomes the liquid phase resistance, the SO₂ absorption rate is controlled by resistance in the gas phase and the membrane. The SO₂ absorption rate with inlet SO₂ concentration was sharply increased by using hollow fiber membranes compared to a conventional wetted wall column because the former has higher gas liquid contacting area than the latter. The mass transfer coefficient is independent of pressure. When the gas mixture was fed in the shell side, the removal efficiency of SO₂ declined because of channeling problems on the shell side. Also, the addition of PEG in polymer dopes increased SO₂ removal efficiency. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

SULPHUR DIOXIDE ABSORPTION IN HOLLOW FIBRE MEMBRANE MODULES

The performance of non-porous silicone rubber and microporous hydrophobic polypropylene hollow fibre membranes coupled with liquid absorbent were assessed for the removal of SO₂; from a gas stream. This approach combines the advantages of absorption technology (high selectivity) with membrane systems (compactness of equipment). The advantages of such gas absorption membranes were evident in the increased selectivities of both membranes. A mathematical model which incorporates the effective permeabilities of the gases has been developed to simulate the separation process. Numerical simulations agreed well with the experiment. Further investigations were carried out to study the combined removal of CO₂ and SO₂ and any possible interactive effects of these gases during absorption in these contactors.     

Sulfur dioxide non‐dispersive absorption in N,N‐dimethylaniline using a ceramic membrane contactor

BACKGROUND: Removal of sulfur dioxide from gas emissions by selective absorption is a common method to separate and concentrate sulfur dioxide and to reduce air pollution and environmental risks. N,N‐dimethylaniline is an organic solvent used in some industrial applications for its sulfur dioxide affinity, leading to a regenerative process. However, the use of scrubbers and equipment in which direct contact between gas and liquid takes place leads to solvent losses due to evaporation and drops dragging. RESULTS: In this work, an innovative procedure based on non‐dispersive absorption in a ceramic hollow fibre membrane contactor was studied in order to avoid drops dragging. The absorption efficiency ranged between 40 and 50%, showing the technical viability of the process. The sulfur dioxide flux through the membrane has a linear relationship with the concentration of SO₂ in the gas stream and an overall mass transfer coefficient K_overall = (1.10 ± 0.11) × 10^?5 m s^?1 has been obtained. CONCLUSIONS: The mass transfer behaviour of a ceramic hollow fibre membrane contactor for sulfur dioxide non‐dispersive absorption in N,N‐dimethylaniline has been studied. The main resistance is found to be the ceramic membrane and the effective diffusivity has been inferred. The mass transfer model and parameters allow the evaluation of equipment design for technical applications. Copyright © 2008 Society of Chemical Industry     

Preparation of graphene oxide/polyamide composite nanofiltration membranes for enhancing stability and separation efficiency

Polyamide (PA) NF membranes are synthesized on a hollow fiber support by the interfacial polymerization (IP) of piperazine (PIP) and trimesoyl chloride (TMC). Then, GO is coated on the PA layer to decorate the NF membrane surface (denoted GO/PA-NF). This strategy aims to improve the hydrophilicity, chlorine resistance and separation stability of the membrane. The optimization, chemical composition, morphology, and hydrophilicity of the synthesized GO/PA-NF membrane are characterized. Results indicate that the optimized GO/PA-NF in terms of rejection rate and flux are with 0.05 wt% GO. The rejection of GO/PA-NF for Na2SO₄ and MgSO₄ is 99.4% and 96.9%, respectively. Even if the GO/PA-NF is immersed in 1000 ppm NaClO solution for 48 h, the NF membrane still maintains stable salt rejection. The developed NF membranes exhibit excellent treatment performance on dying wastewater. The permeate flux and rejection of GO/PA-NF toward Congo red solution are determined to be 44.2 L/m²h and 100%, respectively. Compared with the PA membrane, GO/PA-NF presents a higher rejection for Na₂SO₄ (99.4%) and a lower rejection for NaCl (less than 20%), which shows that the NF membranes have a better divalent/monovalent salt separation performance. This study highlights the superior performance of GO/PA-NF and shows its high potential for application in wastewater treatment.     

聚四氟乙烯膜气体吸收数学模型和孔隙率的影响

膜吸收是将膜分离与传统的吸收技术相结合的一种新型分离技术。在这些过程中经常使用多孔膜,多孔膜对过程的传质性能有一定的影响。对不同孔隙率的微孔聚四氟乙烯(PTFE)疏水性平板膜的膜气体吸收过程中液相传质性能进行了实验研究。当采用去离子水-CO2吸收体系时,多孔膜的孔隙率对液相传质性能没有影响;当采用NaOH水溶液-CO2吸收体系时,多孔膜的孔隙率对液相传质性能有明显的影响。在相同流速下,孔隙率大的膜液相传质系数高于孔隙率小的膜。以双膜理论为指导,建立了多孔膜气体吸收过程中液相传质模型。用该模型描述多孔膜孔隙率对液相传质系数的影响,其结果与实验数据具有良好的一致性。     

Removal of Water from Spent Phosphoric Acid Using a Sandwich Silicon Carbide Ceramic Membrane

It is important to develop an energy- and cost-efficient method of concentrating phosphoric acid because it is widely used in several industries. Three different ceramic membranes, namely, a silicon carbide (SiC) membrane, a TiO₂-coated SiC membrane, and a sandwich membrane of TiO₂ between SiC, were successfully applied for the selective separation of water from spent phosphoric acid. SiC was selected as raw material, TiO₂ as supporting material. The membrane was characterized by various instruments to check all parameters. Using the solution diffusion, statistical modeling of these membranes was performed and the membrane parameters, such as membrane diffusivity and mass transfer coefficient, were calculated. By reducing the porosity of the membrane, the desired separation can be improved.     

Theoretical and Experimental Studies on Acetylene Absorption in a Polytetrafluoroethylene Hollow‐Fiber Membrane Contactor

The separation of acetylene from a gas mixture was investigated using a polytetrafluoroethylene hollow‐fiber membrane contactor and 1‐methyl‐2‐pyrrolidinone as absorbent. The effects of the gas velocity, the liquid velocity, the feed gas concentration, and the module length on the acetylene mass transfer were investigated. The results showed that the acetylene mass transfer flux increased with increasing liquid velocity, gas velocity, and feed gas concentration, but decreased with increasing membrane module length. A mathematical model was used to predict the wetting extent of the membrane and the mass transfer resistance in the acetylene mass transfer process. The wetting extent of the membrane was found to increase with increasing liquid velocity and to be effectively restrained with increasing gas velocity. The liquid phase resistance and the wetted‐membrane phase resistance controlled the acetylene mass transfer in the acetylene absorption process. The acetylene absorption efficiency was maintained at 90 % for 114 h of the C₂H₂ membrane absorption–thermal desorption cycle process.     

Experimental study of CO2 absorption/stripping via PVDF hollow fiber membrane contactor

Gas–liquid hollow fiber membrane contactor can be a promising alternative for the CO₂ absorption/stripping due to the advantages over traditional contacting devices. In this study, the structurally developed hydrophobic polyvinylidene fluoride (PVDF) hollow fiber membranes were prepared via a wet spinning method. The membranes were characterized in terms of morphology, permeability, wetting resistance, overall porosity and mass transfer resistance. From the morphology analysis, the membranes demonstrated a thin outer finger-like layer with ultra thin skin and a thick inner sponge-like layer without skin. The characterization results indicated that the membranes possess a mean pore size of 9.6 nm with high permeability and wetting resistance and low mass transfer resistance (1.2 × 10⁴ s/m). Physical CO₂ absorption/stripping were conducted through the fabricated gas–liquid membrane contactor modules, where distilled water was used as the liquid absorbent. The liquid phase resistance was dominant due to significant change in the absorption/stripping flux with the liquid velocity. The CO₂ absorption flux was approximately 10 times higher than the CO₂ stripping flux at the same operating condition due to high solubility of CO₂ in water as confirmed with the effect of liquid phase pressure and temperature on the absorption/stripping flux.     

Selective Separation of Toluene/n‐Heptane by Supported Ionic Liquid Membranes with [Bmim][BF4]

Room‐temperature ionic liquids serve as alternative solvents for volatile organic compounds in liquid‐liquid extraction and liquid membrane separation. 1‐Butyl‐3‐methylimidazolium tetrafluoroborate ([Bmim][BF₄]) was applied for extraction and supported ionic liquid membranes (SILMs) to separate toluene and n‐heptane. A high separation factor of toluene was achieved due to the strong interaction between ionic liquid cations and toluene. The mass transfer performance of the SILM process was enhanced by higher operating temperature. With the increase of initial toluene concentration in the feed phase, the mass transfer flux and removal efficiency of the SILM process were improved, while the separation factor decreased. The mass transfer flux was growing with the increase of flow rate at both sides. The SILM process was stable over a long time period due to the high viscosity and low volatility of [Bmim][BF₄].     

Experimental studies on the influence of porosity on membrane absorption process

Eight kinds of flat membranes with different micro-structures were chosen to carry out the membrane absorption experiments with CO₂ and de-ionized water or 0.1 mol·L⁻¹ NaOH solution as the experimental system. According to experimental results, the membrane pores shape (stretched pore and cylinder pore) and membrane thickness do not affect the membrane absorption process, and the membrane porosity has only little influence on membrane absorption process for slow mass transfer system. However, the influence of porosity on the membrane absorption process became visible for fast mass transfer system. Moreover, the mass transfer behavior near the membrane surface on liquid side was studied. The results show that the influence of membrane porosity on mass transfer relates to flow condition, absorption system and distance between micro-pores, etc. __________ Translated from Journal of Chemical Engineering of Chinese Universities, 2007, 21(1): 14–19 [译自: 高校化学工程学报]     

Preparation and characterization of PVDF-montmorillonite mixed matrix hollow fiber membrane for gas–liquid contacting process

Porous PVDF-hydrophobic montmorillonite (MMT) mixed matrix membranes (MMMs) were fabricated via wet spinning method and used in membrane gas absorption process. The effects of hydrophobic MMT nano-clay loadings (1, 3 and 5 wt% of polymer) on the structure and performance were investigated. The fabricated membranes showed both finger-like and sponge-like structure with an increase in the length of finger-like pores in their cross-section, which resulted in higher permeability and lower mass transfer resistance compared to plain PVDF membrane. Also, significant improvements for surface hydrophobicity, critical entry pressure of water and porosity with the addition of filler were observed. The CO₂ absorption test was conducted through the gas–liquid membrane contactor and demonstrated a significant improvement in the CO₂ flux with MMT loading and the membrane with 5 wt% MMT presented highest performance. For example, at the liquid water velocity of 0.5 m s⁻¹, CO₂ flux of the MMM with 5 wt% MMT of 9.73 × 10⁻⁴ mol m⁻² s⁻¹ was approximately 56% higher than the PVDF membrane without nano-filler. In conclusion, MMMs with improved absorption properties can be a promising candidate for CO₂ absorption and separation processes through membrane contactors.     

Fabrication and characterization of poly(vinylidene fluoride)–polytetrafluoroethylene composite membrane for CO2 absorption in gas–liquid contacting process

The wetting resistance of poly(vinylidene fluoride) (PVDF) membrane is a critical factor which determines the carbon dioxide (CO₂) absorption performance of the gas–liquid membrane contactors. In this study, the composite PVDF–polytetrafluoroethylene (PTFE) hollow fiber membranes were fabricated through dry-jet wet phase-inversion method by dispersing PTFE nanoparticles into PVDF solution and adopting phosphoric acid as nonsolvent additive. Compared with the PVDF membrane, the composite membranes presented higher CO₂ absorption flux due to their higher effective surface porosity and surface hydrophobicity. The composite membrane with addition of 5 wt % PTFE in the dope gained the optimum CO₂ absorption flux of 9.84 × 10⁻⁴ and 2.02 × 10⁻³ mol m⁻² s⁻¹ at an inlet gas (CO₂/N₂ = 19/81, v/v) flow rate of 100 mL min⁻¹ by using distilled water and aqueous diethanolamine solution, respectively. Moreover, the 5% PTFE membrane showed better long-term stability than the PVDF membrane regardless of different types of absorbent, indicating that polymer blending demonstrates great potential for gas separation. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47767.     

Separation performance of thin-film composite nanofiltration membrane through interfacial polymerization using different amine monomers

Four kinds of thin-film composite (TFC) membranes were prepared via interfacial polymerization using diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA) and piperazidine (PIP) as water-soluble monomer, and trimesoyl chloride (TMC) as organic-soluble monomer. The surface chemical features of the resultant membranes were confirmed by contact angle measurement and Fourier transform infrared spectroscopy (FTIR). The membrane morphology and surface charges were investigated through Scanning electronic microscopy (SEM) and Zeta potential, respectively. Salt rejection was used to evaluate the separation performance of the four kinds of TFC membranes. The results showed that all the four kinds of TFC membranes exhibited typical negatively charged nanofiltration membrane characteristics. The salt rejections followed the sequence: Na₂SO₄ > MgSO₄ > MgCl₂ and the rejection of Na₂SO₄ was all over 80%. It was also found that the solubility of water-soluble monomer in organic solvent played an important role in manipulating the membrane structure, charge properties and thus the separation performance.     

Membrane Gas-Solvent Contactor Pilot Plant Trials of CO2 Absorption from Flue Gas

Membrane gas-solvent contactors have received much attention for CO₂ absorption, as the approach incorporates advantages from both solvent absorption and membrane gas separation. This study reports on pilot plant trials of three membrane contactors for the separation of CO₂ from flue gas. The contactors were porous polypropylene (PP), porous polytetrafluoroethylene (PTFE), and non-porous polydimethylsiloxane (PDMS), with the solvent PuraTreatTM F^TM. To enable performance comparison, laboratory measurements based on a gas mixture of 10% CO₂ in N₂ were also undertaken on the same contactor–solvent systems. It was found that the PP contactor experienced significant pore wetting in both laboratory and pilot plant studies. In contrast, the PTFE contactor experienced only minor pore wetting in the laboratory. However, in the pilot plant trial of the PTFE contactor extensive pore wetting was observed, and the overall mass transfer coefficient measured was comparable with the PP contactor. The non-porous PDMS contactor had an overall mass transfer coefficient two orders of magnitude less than the PP contactor, due to the greater mass transfer resistance of the polymeric film. However, the non-porous membrane does not experience pore wetting, which resulted in the overall mass transfer coefficient being similar for both laboratory and pilot plant measurements.     

Dual-phase membrane reactor for hydrogen separation with high tolerance to CO2 and H2S impurities

Catalytic membrane reactors based on oxygen-permeable membranes are recently studied for hydrogen separation because their hydrogen separation rates and separation factors are comparable to those of Pd-based membranes. New membrane materials with high performance and good tolerance to CO₂ and H₂S impurities are highly desired. In this work, a new membrane material Ce_0.85Sm_0.15O_1.925–Sr₂Fe_1.5Mo_0.5O_6-δ (SDC–SFM) was prepared for hydrogen separation. It exhibits high conductivities at low oxygen partial pressures, which is benefit to electron transfer and ion diffusion. A high hydrogen separation rate of 6.6 mL cm⁻² min⁻¹ was obtained on a 0.5-mm-thick membrane coated with Ni/SDC catalyst at 900°C. The membrane reactor was operated steadily for 532 h under atmospheres containing CO₂ and H₂S impurities. Various characterizations reveal that SDC–SFM has good stability in the membrane reactor for hydrogen separation. All facts confirm that SDC–SFM is promising for hydrogen separation in practical applications. © 2018 American Institute of Chemical Engineers AIChE J, 65: 1088–1096, 2019     

Effect of Feed Composition on the Gas Separation Performance of Binary and Ternary Mixed Matrix Membranes

Binary and ternary component mixed matrix membranes comprised of zeolite 4A and p-nitroaniline (pNA) in the polycarbonate (PC) matrix were prepared and appraised in gas separation. For comparison, homogenous membranes of PC and PC/pNA membranes were also investigated. The membranes were utilized to separate binary mixtures of CO₂/CH₄, H₂/CH₄, and CO₂/N₂. The effect of feed composition on the separation performance of membranes was investigated. Separation factors and ideal selectivities were similar for the PC membrane. A similar trend was also observed with the PC/pNA membrane. The separation factors of the PC/pNA membrane for CO₂/CH₄ were almost twice as high as those of the PC membrane regardless of the feed composition. The ideal selectivities were, however, higher than separation factors for PC/zeolite 4A and PC/pNA/zeolite 4A membranes. The PC/ pNA/zeolite 4A membrane has separation factors of 18 for 77% CO₂/ 23% CH₄ mixture, and 40 for 20% CO₂/ 80% CH₄ mixture, respectively. The separation factors of the mixed matrix membranes depended on the feed composition strongly. The PC/ pNA/zeolite 4A membrane had higher separation factors and lower permeabilities than the PC/zeolite 4A membrane. pNA assisted to eradicate partly the detrimental effects of interfacial voids and improved the molecular sieving effect of zeolite 4A dispersed in the PC.     

Process parameter optimization for ionic liquids as gas separation membranes based on a GWO–BPNN model

Gas separation membranes offer a cost-effective solution for capturing greenhouse gases, mitigating the global greenhouse effect. Ionic liquids (ILs) have emerged as one of the promising materials for greenhouse gas separation due to their strong affinity for CO₂. In this study, we propose a laboratory-scale method for preparing IL–PVDF blend membranes with high CO₂/N₂ selectivity. The separation performance of the membranes was evaluated using a custom gas permeation measurement system. The effects of casting solution composition, solidification method, and film-forming processes on separation performance were experimental investigated, and the obtained experimental data were used to train a back propagation neural network (BPNN) optimized by the Gray Wolf Optimizer (GWO) algorithm. This hybrid GWO–BPNN model was utilized to predict separation membrane efficiency, optimize the film-forming process, and identify the optimal range of process parameters. Notably, the GWO–BPNN model demonstrated a 2.76% higher prediction accuracy compared to a standalone BPNN. The results indicated that the GWO–BPNN algorithm has a great potential to accurately predict membrane separation efficiency and apply in optimal membrane process design (OMPD), and this method can significantly reduce the number of experimental trials required to achieve OMPD.