Fast formation of a black inner α-Al2O3 layer doped with CuO on Al–Cu–Li alloy by soft sparking PEO process

Forming high-temperature α-Al₂O₃ phase under soft sparking is an intriguing phenomenon in plasma electrolytic oxidation (PEO) of Al alloys, which contradicts the low energy input of the process. In this study, α-Al₂O₃ doped with black CuO is formed beneath an amorphous white outer layer on Al–Cu–Li alloy by PEO in a dilute silicate electrolyte under soft sparking. In comparison, reddish coatings with dominating γ-Al₂O₃ are formed under the conventional plasma discharges, although blackish inner layer with α-Al₂O₃ can also be exposed by heavily polishing the samples. In order to know the underlying mechanism, temperatures at the coating surface and the underlying substrate have been monitored by a thermocouple under the conventional and soft sparking PEO regimes, respectively. Interestingly, high temperatures are detected in the case of soft sparking rather than PEO with strong discharges. The formation of CuO, quartz, and cristobalite within the soft sparking coating also supports the existence of high temperature. Hence, the formation of α-Al₂O₃ under soft sparking can be resolved to the conventional thermal activation mechanism, without the need of seeking other plausible explanations. Thermal condition evaluation for soft sparking PEO suggests that values of the effective thermal conductivity during PEO process for the outer layer and the barrier layer at the coating/substrate interface might be lower than ∼0.05 and ∼0.0017 W m⁻¹ K⁻¹, respectively. It is believed that the amorphous structure of the outer and barrier layers effectively blocks the heat dissipation, facilitating the formation of a highly wear-resistant inner layer with α-Al₂O₃, CuO, and the other high-temperature species under soft sparking.     

Modeling investigation of geometric size effect on pervaporation dehydration through scaled-up hollow fiber NaA zeolite membranes

A mass transfer model in consideration of multi-layer resistances through NaA zeolite membrane and lumen pressure drop in the permeate sidewas developed to describe pervaporation dehydration through scaled-up hollowfiber supported NaA zeolitemembrane. Itwas found that the transfer resistance in the lumen of the permeate side is strongly related with geometric size of hollow fiber zeolite membrane,which could not be neglected. The effect of geometric size on pervaporation dehydration could bemore significant under higher vacuumpressure in the permeate side. The transfer resistance in the lumen increaseswith the hollowfiber length but decreaseswith lumen diameter. The geometric structure could be optimized in terms of the ratio of lumen diameter to membrane length. A critical value of dI/L (Rc) to achieve high permeation flux was empirically correlated with extraction pressure in the permeate side. Typically, for a hollow fiber supported NaA zeolite membrane with length of 0.40 m, the lumen diameter should be larger than 2.0 mm under the extraction pressure of 1500 Pa.     

Pervaporation dehydration of ethylene glycol by NaA zeolite membranes

Home-made NaA zeolite membranes were used for pervaporation dehydration of ethylene glycol (EG)/water mixtures. Hydrothermal stability of the membranes in pervaporation was investigated for industrial application purpose. The membranes exhibited good stability for water content of less than 20 wt.% at 100 °C. The reduction of operating temperature was effective to improve membrane stability for operating at high feed water content (e.g. 30 wt.%). The influence of feed water content and operating temperature on dehydration of EG was extensively investigated. A permeation flux of 4.03 kg m⁻² h⁻¹ with separation factor of >5000 was achieved at 120 °C for the separation of the solution with 20 wt.% water content. A pilot-scale pervaporation facility with membrane area of 3 m² was built up for dehydration of EG with the water content of 20 wt.%, which showed technical feasibility for industrial application.     

Mass transfer simulation on pervaporation dehydration of ethanol through hollow fiber NaA zeolite membranes

A multi‐layer series‐resistance mass transfer model was developed to simulate mass transfer behaviors of water/ethanol mixture through hollow fiber NaA zeolite membranes. The mass transfer through zeolite layer was described by Maxwell‐Stefan mechanism based on adsorption and diffusion parameters obtained from molecular simulation. The mass transfer through asymmetric hollow fiber support was described by dusty gas model involving Knudsen diffusion and viscous flow. It was found that the sponge‐like layer of support besides of zeolite layer made an important contribution to overall membrane transfer resistance while the finger‐like layer had less effect. When permeate pressure shifted from 0.2 to 7.5 kPa, the mass transfer resistance contribution of sponge‐like layer varied from 27.1 to 17.8%. Effects of microstructure parameters of support on mass transfer through membrane were investigated extensively. Large pore size and thin thickness for sponge‐like layer of support were beneficial to improve water permeation flux. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2468–2478, 2016     

Preparation of zeolite NaA membranes on the inner side of tubular supports by means of a controlled seeding technique

Zeolite NaA membranes have been reproducibly prepared by seeded hydrothermal synthesis on the internal surface of porous -alumina tubular supports. A cross-flow filtration technique has been developed to allow a controlled seeding of zeolite NaA crystals from a suspension according to transmembrane pressure, pH, seed suspension flow rate, crystal size and concentration. The optimal seeding weight gain was found to be around 0.40 mg cm⁻² of membrane area. With this procedure, zeolite membranes with selectivities up to 600 at fluxes of 0.50 kg m⁻² h⁻¹ were obtained in the pervaporation of 92:8 wt.% ethanol/water mixtures at 323 K.     

二次生长法制备NaA型沸石膜及其在渗透汽化中的应用

首先采用负压法在氧化铝载体上引入晶种,再通过二次生长法合成了NaA型沸石膜,考察了各种因素对制膜的影响。结果表明,在90℃时反应晶化3h,能够形成最佳的NaA膜层。对于质量分数为90%的乙醇水溶液,所合成的膜的渗透汽化通量为0 19kg/(m2·h),分离因子达300以上。在此基础上研究了渗透汽化温度、渗透液浓度对于分离性能的影响,获得了渗透汽化过程的初步规律。     

Preparation of high‐performance zeolite NaA membranes in clear solution by adding SiO2 into Al2O3 hollow‐fiber precursor

Zeolite NaA membranes were prepared in a clear synthesis solution without the aid of nanoseeds. To improve the properties of the membranes formed in a clear solution, alumina hollow fibers were fabricated by adding silica powder to the conventional spinning slurry, resulting in hollow fibers with a mullite phase. Prior to the membrane synthesis, the hollow fibers were pretreated by dipping in an aged synthesis solution diluted with isopropanol. Dense zeolite NaA membranes on mullite‐containing alumina hollow fibers were successfully obtained at 100°C for 2 h without the aid of nanoseeds. The membranes have a good pervaporation performance with a high flux of 10.8 kg m^?2 h^?1 and a separation factor of over 10,000. The abundant mullite‐phase hydroxyl groups on the support surface promote the nucleation and growth of zeolite crystals on the support, resulting in dense membranes. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2679–2688, 2018     

含甲基纤维素的NaA沸石膜的制备和表征

NaA沸石膜具有规则的孔道结构,利于分子传输,在有机物脱水领域有一定的应用。为使沸石膜生长更连续均匀,提高渗透汽化性能,本文以甲基纤维素作为空间限制剂加入合成液,探究碱度、晶化温度以及晶化时间对膜的影响,按最优条件制备合成液,并依据质量比m(MC)∶m(H₂O)=1∶100添加甲基纤维素,制备NaA沸石膜。表征方法采用XRD、SEM和渗透汽化3种方式,结果表明添加甲基纤维素的沸石膜表面结构完整,生长致密且性能优良,在75℃下对0.6mol/L的NaCl水溶液做渗透汽化测试时,通量达8.33kg/(m²·h),盐离子截留率为99.95%。在0.6mol/L的NaCl的水溶液中测试72h,结果表明添加甲基纤维素的NaA沸石膜时间依存性更好,通量保持在8.30kg/(m²·h)左右,离子截留率稳定在99.90%。渗透汽化分离ω(C₂H₆O)=90%乙醇的水溶液,随着温度从60℃升高到75℃,沸石膜的通量由1.55kg/(m²·h)升高到2.56kg/(m²·h),渗透侧水含量保持在99.90%左右。     

Dehydration of water/1-1-dimethylhydrazine mixtures by zeolite membranes

In this research, dehydration of water/1-1-dimethylhydrazine (UDMH) mixtures by zeolite NaA and hydroxy sodalite membranes has been investigated. Support of these membranes has been tubular mullites that have been made by extruding a mixture of about 67–75% kaolin clay and 33–25% distilled water using an extruder. Zeolite NaA and hydroxy sodalite membranes have been coated on the external surface of the porous supports by the hydrothermal synthesis.

UDMH/water mixtures have been separated at ambient temperature and pressure by pervaporation (PV) using these zeolite membranes. These membranes showed very high selectivity of water for all UDMH mixtures. For the UDMH/water mixtures, separation factor as high as 10 000 has been obtained for UDMH feed concentration of 2%. Total mass fluxes of 1.05–0.2 kg/(m² h) have been also obtained.     

Silane‐modified NaA zeolite/PAAS hybrid pervaporation membranes for the dehydration of ethanol

To improve the pervaporation selectivity of poly(acrylic acid) sodium (PAAS) membranes incorporated with NaA zeolite, the interface compatibility between zeolite nanocrystals and the polymer matrix was improved by modifying NaA zeolite using 3‐aminopropyltriethoxysilane (APTES). Both X‐ray photoelectron spectra and FTIR confirmed the chemical modification, while the results of zeolite particle size analysis and scanning electron microscopy revealed the improved dispersion of the modified zeolite. Transmission electron microscopy images of these hybrid membranes indicated that the interface between the polymer and modified zeolite phases had improved. The effects of loaded NaA zeolite on the pervaporation performance of hybrid membranes were investigated. The selectivity of hybrid membranes made from APTES‐modified zeolite was higher than that using the original zeolite under the same conditions, because fewer voids resulted from the incompatibility between the zeolite and PAAS and the structure was more homogenous. Based on the Arrhenius plots, the activation energies of water and the ethanol ratio were lower for modified zeolite hybrid membranes, because water molecules experienced less restrictive passage through the membranes compared with the original zeolite‐based hybrid membrane. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

在粗孔a-Al_2O_3载体上合成NaA沸石膜

采用稀释的水玻璃作为分散介质配成0.5%(w)的NaA沸石悬浮液,对粗孔a-Al2O3(孔径3~5 mm)载体管修饰并预涂晶种,进一步采取原位水热晶化法在a-Al2O3载体管外表面制备NaA沸石膜. 重复合成5次后,在载体表面形成一层致密、连续的沸石晶体层. 由XRD确定该晶体为A型沸石,由SEM可观察到膜厚约15~20 mm,膜表面上的沸石晶体大小约为3~5 mm,晶体之间紧密孪生在一起,看不出晶间空隙. 制备的NaA沸石膜的H2渗透率为3.0510-6 mol/(m2sPa), 对H2/N2和H2/C3H8的理想分离因数分别为6.9和15.6,超过对应的努森扩散值3.74和4.69,说明所制备的NaA沸石膜具有分子筛分性能.