Hydrogen separation cellulose acetate

The SEPAREX system will recover over 90% of the hydrogen at a purity of 96+% for recycle, while increasing the heating value of the fuel gas from -550 BTU/SCF to -950 BTU/SCF. The projected flow rates and gas purities for the membrane separation are shown in Table II. Under the bone-dry feed conditions the cellulose acetate membrane is not affected by HCl. Special materials of construction and adhesives have been used in the fabrication of the spiral-wound elements to ensure their resistance to HCl in the gas streams. 

Concurrently with the work on carbon dioxide and hydrogen sulfide at General Electric, Steigelmann and Hughes [27] and others at Standard Oil were developing facilitated transport membranes for olefin separations. The principal target was the separation of ethylene/ethane and propylene/propane mixtures. Both separations are performed on a massive scale by distillation, but the relative volatilities of the olefins and paraffins are so small that large columns with up to 200 trays are required. In the facilitated transport process, concentrated aqueous silver salt solutions, held in microporous cellulose acetate flat sheets or hollow fibers, were used as the carrier. 

Eastman Chemical Company, together with Halcon, developed a commercial acetic anhydride process to an industrial scale [41b, 47]. This process starts with coal to make a hydrogen-rich synthesis gas, which is purified (Figure 4). A portion of the syn gas is separated to produce methanol from 2 1 H2/CO. Part of the methanol is used to scrub H2S from the coal-gasification step. The remainder of the methanol is combined with acetic acid to make methyl acetate. The methyl acetate is carbonylated to give acetic anhydride. The acetic anhydride is used to produce cellulose acetate in another process, and the resulting acetic acid is recycled to the esterification section. The acetic anhydride step of the pro-... 

Cellulose acetate membranes have been suggested for hydrogen recovery, C02/CH4 separations, gas dehydration and for nitrogen enrichment.41"43... 

The polyimide membrane is reported to be capable of operating at temperatures up to 150°C compared to upper limits of 100°C for PR ISM separators and 60°C for cellulose acetate.46 Permeabilities increase with temperature while selectivities normally drop. The second way to overcome the lower permeabilities is to operate the Ube system at higher temperatures than possible with polysulfone or cellulose acetate. Even at the high temperatures, the polyimide selectivity will remain high enough for the abovementioned hydrogen separations. Thus, one expects the Ube system to be competitive with PRISM separators for many hydrogen applications. 

S.P. DiMartino, J.L. Glazer, C.D. Houston and M.E. Shott, Hydrogen/ Carbon Monoxide Separation with Cellulose Acetate Membranes, Gas Separation and Purification (September 1988). 

In systems 5 and 6, this phenomenon is a result of hydrogen-bond formation between the polymer and solvent, which enhances the solubility. As hydrogen bonds are thermally labile, a rise in T reduces the number of bonds and causes eventual phase separation. In solutions, which are stabilized in this way by secondary bonding, the LCST usually appears below the boiling temperature of the solvent, but it has been found experimentally that an LCST can be detected in nonpolar systems when these are examined at temperatures approaching the critical temperature of the solvent. Polyisobutylene in a series of n-alkanes, polystyrene in methyl acetate and cyclohexane, and cellulose acetate in acetone all exhibit LCSTs. 

Commercial membranes for CO2 removal are polymer based, and the materials of choice are cellulose acetate, polyimides, polyamides, polysulfone, polycarbonates, and polyeth-erimide [12]. The most tested and used material is cellulose acetate, although polyimide has also some potential in certain CO2 removal applications. The properties of polyimides and other polymers can be modified to enhance the performance of the membrane. For instance, polyimide membranes were initially used for hydrogen recovery, but they were then modified for CO2 removal [13]. Cellulose acetate membranes were initially developed for reverse osmosis [14], and now they are the most popular CO2 removal membrane. To overcome state-of-the-art membranes for CO2 separation, new polymers, copolymers, block copolymers, blends and nanocomposites (mixed matrix membranes) have been developed [15-22]. However, many of them have failed during application because of different reasons (expensive materials, weak mechanical and chemical stability, etc.). 

Membranes for natural gas treatment have been employed since the 1980s, with the earlier critical discoveries that cellulose acetate membranes for reverse osmosis (RO) could be transformed into gas separation membranes [3,4]. Since membranes for treatment of water have a high affinity for water transport, a molecule with unique polar properties, there is also an affinity for other polar molecules such as carbon dioxide and hydrogen sulfide. Water, CO2 and HjS are impurities in natural gas (methane) termed acid gases that can promote corrosion of steel. Since pipelines are used in the transport... 

The last section Applied Aspects of Membrane Gas Separation contains three chapters. Brunetti et al. start their contribution with a brief review of membrane materials and membranes used in gas separation and survey the main directions of industrial applications of gas separation (hydrogen recovery, air separation, etc.). In the second part of their chapter they present a new concept for comparison of membrane and other, more traditional, methods for gas separation. Their approach includes a consideration of engineering, economical, environmental and social indicators. Something similar had been written 15 years ago [2] but this analysis is now rather outdated. White (Chapter 15) focuses on a specific but very important problem in industrial gas separation membrane separation of natural gas. The main emphasis is on cellulose acetate based membranes that have the longest history of practical applications. This chapter also contains the results of field tests of these membranes and considers approaches how to reduce the size and cost of industrial membrane systems. The final chapter is an example of detailed engineering... 

Gas permeation systems typically use hollow-fiber or spiral-wound membranes, although hollow-fiber systems are more common tBaker. 2004k Cellulose acetate membranes are used for carbon dioxide recovery, polysulfone coated with silicone rubber is used for hydrogen purification, and conposite membranes are used for air separation. The feed gas is forced into the membrane module under pressure. Retentate, which does not go through the membrane, will become concentrated in the less permeable gas. Retentate exits at a pressure that will be close to the input pressure. The more permeable species will be concentrated in permeate. Permeate, which has passed through the membrane, exits at low pressure. The operating cost for a gas permeator is the cost of conpression of the feed gas and the irreversible pressure difference that occurs for the gas that permeates the membrane. A typical hollow-fiber unit will contain 5000 m membrane area per m at a cost of approximately 200/m. ... 

GASEP Systems. This system was developed by Envirogenics and also uses cellulose acetate spiral-wound modules. These systems have been used for the sweetening of natural gas by removing CO2 and H2S, and the separation of C02/methane mixtures in landfills, and the separation of hydrogen in coat... 

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). 

Currently, the two most-commonly used polymers for gas separation membranes are polysulfone and cellulose acetate. Polysulfone, for instance, has a separation coefficient a (where a is the ratio of the permeability coefficient of the two gases) of 35 for the hydrogen/methane system and has a hydrogen permeability of 4.3 molrn s Pa Much research is currently being undertaken to develop other polymers which will exhibit higher selectivities and fluxes. 

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