Membrane technologies polyimides

The development of asymmetric membrane technology in the 1960 s was a critical point in the history of gas separations. These asymmetric structures consist of a thin (0.1 utol n) dense skin supported on a coarse open-cell foam stmcture. A mmetric membranes composed of the polyimides discussed above can provide extremely high fluxes throuj the thin dense skin, and still possess the inherently hij separation factors of the basic glassy polymers from which they are made. In the early 1960 s, Loeb and Sourirajan described techniques for producing asymmetric cellulose acetate membranes suitable for separation operations. The processes involved in membrane formation are complex. It is believed that the thin dense skin forms at 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. 

Membrane technology has often been mentioned as the next technological generation for the prtrification of natural gases. Indeed, membrane systems are operated successfully for gas sweetening for decades. The best known examples include CO2 selective membranes that are based on pure polymers, e.g., cellulose acetate (Cynara membranes by Natco or Separex membranes by UOP) and polyimide (Ube). Despite their popularity, their performance at high pressures deteriorates as a result of CO2 induced plasticization. 

Since the early 1980s, membrane technology has advanced rapidly and continues to advance. In addition to cellulose acetate and polysulfone, the polymers used in making gas separation membranes include polyimides, polyamides, polyaramid, polydimethylsiloxane, silicon polycarbonate, neoprene, silicone rubber, and others. Today membranes can be designed to withstand a 2,000 psi pressure differential. Membranes used in hydrogen or carbon dioxide applications operate at temperatures up to 200°F, while those used in solvent applications can operate at temperatures up to about 400°F (Baker, 1985). 

Before implantation several in vitro tests were performed. For evaluation of a possible toxic reaction, we investigated the material and the whole devices in vitro with cell culture methods. Direct contact and extraction tests with a mouse fibroblasts cell line (L 929) and a neuroblastoma cell line (neuro-2-a) were performed according to the international standard ISO 10993 ( Biological Evaluation of Medical Devices ). The materials and devices showed no toxicity, i.e. no significant differences in membrane integrity of the cell membranes, mitochondrial activity and DNA synthesis rate. The neuro-2-a cell line is so sensitive that even small changes in process technology are detectable. The flexible polyimide structures proved to be non toxic. 

All the plants mentioned are operating with membranes based on hollow fiber polyimides or spiral-wound CA, which is considered proven technology. The environmental aspect related to CO2 as a green house gas has triggered the development of better membranes for CO2 removal—this is more closely discussed in Section 5.2.3. 

Du Pom, a leader in reverse osmosis technology built aronnd a unique class of tailored aromeik polyamides, was also an early leeder in the gas separation field.27,1 14,16 Molecuiariy engineered arometic polyimides were found by Du Pont to provide extraordinarily good flux and selectivity properties For hydrogen separations.27 Posttreataiem processes for these membranes were not reported. 

J. T. Macheras. Fluid separation membranes prepared from blends of polyimide polymers. US Patent 5 635 067, assigned to Praxair Technology, Inc. (Danbury, CT), June 3,1997. 

Data was obtained for three organic solvents, ethyl acetate, toluene and methanol - these solvents are commonly used in the pharmaceutical and chemical industries. STARMEM 122, an asymmetric OSN membrane with an active layer of polyimide, in a dry form but with a lube oil soaked into the membrane as a preserving agent, with a nominal MWCO of 220 g-moh (manufacturers data) was supplied by Membrane Extraction Technology Ltd (UK). 

Already in 1989, Spillman reported a first comparison of three different separation technologies in H2 recovery from refinery off-gas, by considering polyimide membranes (Table 19.6). This comparison is here provided by the... 

S. Kazama, S. Morimoto, S. Tanaka, H. Mano, T. Yashhna, K. Yamada, K. Haraya, Cardo polyimide membranes for CO2 capture from flue gases, in E.S. Rubin, D.W. Keith, C.F. Gilboy (Eds.), Greenhouse Gas Control Technologies, Vol. I, Elsevier, Amsterdam 75 (2005). 

Table 14.5 summarizes the data for H2 recovery from refinery off-gas by means of three different separation technologies, as reported by Spillman [80], considering polyimide membranes. A comparison among them is provided by the calculation of the relative three indicators. A lower investment cost than pressure swing adsorption (PSA) or cryogenic separation was estimated for the H2 recovery from refinery off-gas by polymeric membranes. This comparison is from 1989 however, since then polymeric membrane capital prices have dropped. 

H. Sun, C. Ma, B. Yuan, T. Wang, Y. Xu, Q. Xue, P. Li, Y. Kong, Cardo polyimides/ Ti02 mixed matrix membranes synthesis, characterization, and gas separation property improvement, Separation and Purification Technology 122 (2014) 367-375. 

Low-temperature process Production of relatively pure CO2 stream Both chemical and physical absorption are mature technologies Commercial polymeric membranes are available (polyimide, polysulfone, polyether-polyamide copolymer, etc.) and can be used with an adsorption liquid (e.g. MEA)... 

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