Stability, membrane performance

Zeolite A is a very successful membrane for separation of water from alcohols, but it suffers from stability issues under acid conditions [23]. Usually, a Hquid phase should be avoided and, for this reason, vapor permeation is preferred. Recent developments show that the hydrophilic MOR [23] and PHI [50] membranes are more stable under acidic conditions in combination with a good membrane performance. 

A wide variety of polymeric membranes with different barrier properties is already available, many of them in various formats and with various dedicated specifications. The ongoing development in the field is very dynamic and focused on further increasing barrier selectivities (if possible at maximum transmembrane fluxes) and/ or improving membrane stability in order to broaden the applicability. This tailoring of membrane performance is done via various routes controlled macro-molecular synthesis (with a focus on functional polymeric architectures), development of advanced polymer blends or mixed-matrix materials, preparation of novel composite membranes and selective surface modification are the most important trends. Advanced functional polymer membranes such as stimuli-responsive [54] or molecularly imprinted polymer (MIP) membranes [55] are examples of the development of another dimension in that field. On that basis, it is expected that polymeric membranes will play a major role in process intensification in many different fields. 

Nation ionomers are produced by copolymerization of a perfluorinated vinyl ether comonomer with tetrafluoroethylene resulting in the chemical structure shown in Figure 8.25 [162,166], This polymer and other related polymers consist of perfluorinated, hydrophobic, backbones that give chemical stability to the material. The material also contains sulfonated, hydrophilic, side groups that make hydration possible in the acidic regions, and also allow the transport of protons at low temperatures, since the higher limit of temperature is determined by the humidification of the membrane, since water is a sine qua non for conduction [166], The material exhibits a proton conductivity of 0.1 S/cm at 80°C [162], The membrane performance is then based on the hydrophilic character of the sulfonic acid groups, which allow proton transport when hydrated while the hydrophobic... 

For a zeolite T (OFF stmcture, 0.68 nm XRD pore diameter), Tanaka et al. [131], observed that the separation factor of a water/acetic acid (50/50 wt%) measured at 75°C decreased monotonically after the immersion of the membrane into the acetic acid mixmre. Initially, the separation factor and water flux were 182 and 1.46 kg/m h, respectively, and after 32 h these values changed to 86 and 1.77 kg/m h, showing a deterioration of the membrane. Cui et al. [130] also smdied the stability of crystals and membranes of zeolite T in acid medium. The powders were immersed in a 50/50 wt% water/acetic acid mixture for 7 days at 75°C and also in HCl solutions 0.5 and 1 M for 1 h at 50°C. The analysis of the samples after the treatment by ICP and XRD indicated that the sample treated in the acetic acid solution maintained its original Si/Al ratio equal to 4 however, the hydrochloridric acid treatment with the 1 M solution destroyed the zeolite stmcmre and the 0.5 M solution dealuminated the zeolite to a Si/Al equal to 8.9 and the XRD analysis corresponded to zeolite T. The membrane performance, after being used for 1 week at different water/acetic acid concentrations, remains almost unchanged and the separation factor of the membrane treated in HCl dramatically decreased as was expected. 

Asaeda M, Sakou Y, Yang J, and Shimasaki K. Stability and performance of porous siUca-zirconia composite membranes for pervaporation of aqueous organic solutions. J Membr Sci 2002 209 163-175. 

S.2.2.10 Membrane Performance Guidelines The key concept used in this chapter on membrane performance in PEFC is the perception of present-day PEMs as phase-separated systems. A hydrophobic phase of polymer backbones that provides mechanical stability and a hydrophilic phase of water-containing pathways for proton and water mobility are distinguished. 

The PEC-1000 membrane of Toray Industries, Inc., has been described by Kurihara et al (21). This membrane was characterized as a thin-film composite type made by an acid catalyzed polymerization on the surface. Membrane performance reported for seawater tests was 99.9 percent TDS rejection at fluxes of 5.0 to 7.4 gfd (8.3 to 12.3 L/sq m/hr) when tested with 3.5 percent synthetic seawater at 800 psi (5516 kPascals). The membrane was stable in 1500-hour tests in spiral-wrap elements and exhibited stability in a temperature range of 25 to 55°C and in a pH range from 1 to 13. High organic rejections were reported for the PEC-1000 membrane rejection of dimethylformamide from a 10 percent solution was 95 percent and similar tests with dimethylsulfoxide showed 96 percent rejection. The composition and conditions for preparation of PEC-1000 membrane is not disclosed in Reference 21. Apparently it is a dip-cast membrane related to compositions described by Kurihara, Watanaba and Inoue in Reference 18. 

Several publications on the processing of membranes based on these materials could be found in the literature [5-28]. The selection of membrane material for a given application could be divided in to two parts Screening of materials based on bulk properties and screening based on thin film properties. In the former case, intrinsic material properties such as stability and conductivity will decide the outcome of the research work. In the latter case, the defect free formability of thin film will be the deciding part. The method of film formation as well as the quality of the support substrates could become important in this respect. In supported membranes, material stability and membrane performance are very much related. The most important issue - the application of membranes in high temperature environments - is therefore the study of the stmcture of the membrane/material and its correlation with the stability/durability. 

Fig. 16.1 Temperature range of application of various hydrogen selective membranes/materials. Schematic prepared based on reported membrane performance or material stability data...

Yoshida K, Hirano Y, Fujii H, Tsuru T, Asaeda M. Hydrothermal stability and performance of siLica-zirconia membranes for hydrogen separation in hydrothermal conditions. J Chem Eng Jpn. 2001 34(4) 523-30. 

Commercial applicahons have advanced to the pilot plant scale so far. It can be speculated that in the overall scheme the aforementioned issues of catalyst stability and membrane stability and performance are critical issues. In the particular case of rhodium-catalyzed hydroformylation (for higher aUcenes or functional olefins) for the synthesis of fine chemicals it can be assumed that, as a nonscientific and nontechnical driver, the price of rhodium will contribute to the commercial success. 

Y. Su, X. Jian, S. Zhang, and C. Yan. Preparation of novel PPES-B UF membrane with good thermal stability The effect of additives on membrane performance and cross-section morphology. J. Membr. ScL, 271(l-2) 205-214, March 2006. 

Li et al. [30] synthesized membrane reactor with three Pd tubes by varying the thickness from 5.6 to 6.1 pm. They evaluated membrane reactor for WGS reaction for 30 days. The H2 permeance results at two pressures are presented in Figure 6.16. It can be seen that the pure H2 permeance remained stable during the reaction test of 27 days, showing a good chemical and mechanical stability of the Pd membranes used in this study. It is implied that there was no degradation of the membrane performance due to, e.g., carbon formation on the... 

New materials with improved CO2/CH4 separation selectivity and membrane stability under realistic NG conditions have been developed however, even after three decades of development, only three membrane material types have been commercialized cellulose acetate-based Separex (Honeywell s UOP), Cynara (Cameron) membranes, polyimide-based membranes from Medal (Air Liquide) and Ube, and per-fluoropolymer-based Z-top membranes from Membrane Technology and Research, Inc. (MTR). The key reasons for the selection of the desired polymer for commercialization are the cost of material, ease of fabrication into commercially viable form, effect of impurities on membrane performance, and gas selectivity under realistic feed conditions. 

Novel membrane materials with improved selectivities, permeabilities, or stability could alter the performance envelope for membrane systems, provided the right targets are met. Care must be taken to focus R D efforts on the critical limitations for particular options. For example, improvements in selectivity for applications where the membrane performance is flux limited will have limited impact. 

In order to achieve a high membrane performance, both selectivity and permeability should be high. However, the function of a membrane depends also on its structural integrity and stability, and hence limitations exist for reducing membrane thickness to reduce membrane resistance and increase permeability. This is not a critical problem in microfiltration where the pores enable a size-based selection for... 

The conventional membrane architecture (CA), the one where the catalyst is placed within the wall of the membrane, is the most studied on the laboratory scale and most frequently reported in the literature. However, the application of the conventional architecture to H2S decomposition is limited by the current ceramic membrane thermal stability [72] and a mismatch between the membrane performance, in terms of flow through and the heat flux, which could be applied to the catalytic tubes. Such imbalance would, from an engineering point of view, require a large and impractical heat transfer surface. 

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