Commercial applications carbon molecular sieve

New Adsorbent Materials. Silicalite and other hydrophobic molecular sieves, the new family of A1P04 molecular sieves, and steadily increasing families of other new molecular sieves (including structures with much larger pores than those now commercially available), as well as new carbon molecular sieves and pillared interlayer clays (PILCS), will become more available tor commercial applications, including adsorption. Adsorbents with enhanced performance, both highly selective physical adsorbents and easily regenerated, weak chemisorbents will be developed, as will new rate-selective adsorbents. 

Porous carbonaceous materials are important in many application areas because of their remarkable properties, such as high surface areas, chemical inertness, and good mechanical stability. Carbon molecular sieves that are amorphous and microporous are commercially important for the separation of nitrogen from air, and activated carbons with a wide pore size distribution are also useful adsorbents for various applications. 

By far the most fully developed application for carbon molecular sieves is in the separation of small gas molecules. A large number of patents describe claims for materials and processes that include carbon-based sieves. Separations that have been accomplished include oxygen and nitrogen, hydrogen and coke gases, methane and carbon dioxide, methane and xenon, ethylene and ethane, propylene and propane. A number of companies in Europe and the United States have recently offered commercial systems for the separation of nitrogen from air. A review of the use of carbon molecular sieves in separation technology is well beyond the scope of this article and the interested reader is referred to recent reviews and references sited therein (12,18.). ... 

For commercial applications, an adsorbent must be chosen carefully to give the required selectivity, capacity, stability, strength, and regenerability. The most commonly used adsorbents are activated carbon, molecular-sieve carbon, molecular-sieve zeolites, silica gel, and activated alumina. Of particular importance in the selection process is the adsorption isotherm for competing solutes when using a particular adsorbent. Most adsorption operations are conducted in a semicontinuous cyclic mode that includes a regeneration step. Batch slurry systems are favored for small-scale separations, whereas fixed-bed operations are preferred for large-scale separations. Quite elaborate cycles have been developed for the latter. 

One of the earliest publications on the preparation of carbon molecular sieves was done by Walker et al. [6]. They started from a Saran copolymer, but currently many different raw materials are being used either for research or commercial purposes. The main sources of carbon molecular sieves for commercial applications are coal and coconut shell. An extensive list of the work done in the preparation of carbon molecular sieves can be found elsewhere [7,8]. The typical steps in the preparation of carbon molecular sieves from coal are shown in Fig. 1 [8]. It can be seen that the main steps are oxidation, carbonization, followed either by steam activation or pyrolysis. During steam activation, pores are opened and the resulting carbon molecular sieve can be used in hydrogen purification, while during pyrolysis carbon deposition takes place, and a narrower pore size distribution is achieved. 

The commercial separation of air into N2 and O2, an industrially very important process, is achieved by either cryogenic distillation or pressure swing adsorption (PSA). The use of pillared clays forms an interesting alternative for the carbon molecular sieves and zeolites currently applied as adsorbents in PSA techniques. Both the capacity and the selectivity towards air components are very important features in gas adsorption applications. 

Until very recently, the use of adsorption systems (18) was generally limited to the removal of components present only in low concentrations. Recent progress in materials and engineering techniques has greatly extended the applications, as attested by Table 1.2, which lists only those applications that have been commercialized. Adsorbents used in effecting these separations are activated carbon, aluminum oxide, silica gel, and synthetic sodium or calcium aluminosilicate zeolite adsorbents (molecular sieves). The sieves differ from the... 

On carbon films prepared fi om commercially available poly(imide) films with a thickness of 0.1 mm, selective permeation of hydrogen gas was recently found [105]. This permselectivity may help to develop applications of these carbon films in fuel cells for vehicles, since CO in H2 gas supplied from an on-board reformer has to be removed through a molecular sieving membrane. 

Molecular sieves have also found application for desulfurization of natural-gas feed to ammonia plants. Removal of all types of sulfur compounds ahead of these plants is desirable because sulfur acts as a temporary poi.son to steam-hydrocarbon reforming catalysts and a permanent poison to expensive low-temperature shift conversion catalysts. An installation employing a standard dual bed adsorption system has been described by Lee and Collins (1968). The authors also describe comparative tests of a molecular sieve and a commercial grade of impregnated activated carbon in a dual-bed mobile pilot unit. The test results indicated that the molecular sieve could treat 2 to 4 times as much gas per unit volume of adsorbent as the carbon. The commercial plant consistently provided gas to the primary reformer containing less than 0.3 ppm (vol) peak total sulfur from a feed gas averaging about 0.6 ppm... 

These stiff polymeric materials, used mainly for commercial carbon dioxide removal applications (polyimides, cellulose acetate, polysulfones) have their selectivity mainly based on molecular sieving effects. Despite their affinity for polar compounds (those membranes are also recommended by membrane... 

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