Adsorbents molecular sieve carbons

Despite the difference ia the nature of the surface, the adsorptive behavior of the molecular sieve carbons resembles that of the small pore zeoHtes. As their name implies, molecular sieve separations are possible on these adsorbents based on the differences ia adsorption rate, which, ia the extreme limit, may iavolve complete exclusion of the larger molecules from the micropores. 

Typical pore size distributions for these adsorbents have been given (see Adsorption). Only molecular sieve carbons and crystalline molecular sieves have large pore volumes in pores smaller than 1 nm. Only the crystalline molecular sieves have monodisperse pore diameters because of the regularity of their crystalline stmctures (41). 

Exit gases from the shift conversion are treated to remove carbon dioxide. This may be done by absorbing carbon dioxide in a physical or chemical absorption solvent or by adsorbing it using a special type of molecular sieves. Carbon dioxide, recovered from the treatment agent as a byproduct, is mainly used with ammonia to produce urea. The product is a pure hydrogen gas containing small amounts of carbon monoxide and carbon dioxide, which are further removed by methanation. 

MOLPSA-nitrogen [Molecular sieve pressure swing adsorption] A version of the PSA process for separating nitrogen from air, developed by Kobe Steel. Most PSA processes for nitrogen use molecular sieve carbon as the adsorbent, but this one uses zeolite X. Water and carbon dioxide are first removed in a two-bed PSA system, and then the nitrogen is concentrated and purified in a three-bed system. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

In order to determine the PSD of the micropores, Horvath-Kawazoe (H-K) method has been generally used. In 1983, Horvath and Kawazoe" developed a model for calculating the effective PSD of slit-shaped pores in molecular-sieve carbon from the adsorption isotherms. It is assumed that the micropores are either full or empty according to whether the adsorption pressure of the gas is greater or less than the characteristic value for particular micropore size. In H-K model, it is also assumed that the adsorbed phase thermodynamically behaves as a two-dimensional ideal gas. 

The points discussed above can be demonstrated by the case of simultaneous production of O2 and N2 enriched gases from ambient air. Air can be fractionated by selectively (thermodynamic) adsorbing N2 over O2 and Ar on a zeolite [3], or selectively (kinetic) adsorbing O2 over N2 and Ar on a molecular sieve carbon [4,5]. 

The cyclic steps of one of the PSA processes using the molecular sieve carbon as the adsorbent consist of (a) flowing compressed air through a packed bed of the carbon so that O2 can diffuse and adsorb into the carbon pores faster than N2 and At and produce a N2 rich product gas at feed air pressure (Pa), (b) pressure equalizing the adsorber with a companion adsorber, (c) counter-currently depressuring the adsorber to near ambient pressure to produce the O2 enriched gas, (d) pressure equalizing with another adsorber, and finally (e) repressurizing the adsorber to Pa with feed air [4]. 

The nanoporous carbon membrane consists of a thin layer (<10pm) of a nanoporous (3-7 A) carbon film supported on a meso-macroporous solid such as alumina or a carbonized polymeric structure. They are produced by judicious pyrolysis of polymeric films. Two types of membranes can be produced. A molecular sieve carbon (MSC) membrane contains pores (3-5 A diameters), which permits the smaller molecules of a gas mixture to enter the pores at the high-pressure side. These molecules adsorb on the pore walls and then they diffuse to the low-pressure side of the membrane where they desorb to the gas phase. Thus, separation is primarily based on differences in the size of the feed gas molecules. Table 7 gives a few examples of separation performance of MSC membranes. ° Component 1 is the smaller component of the feed gas mixture. 

Highly porous carbons can be produced from a variety of natural and synthetic precursors [11, 12]. Relatively inexpensive activated carbons are useful adsorbents, but generally their surface and pore structures are exceedingly complex [11, 13]. However, it is now possible to prepare a number of more uniform carbonaceous adsorbents. For example, molecular sieve carbons (MSCs) are available with narrow distributions of ultramicropores, which exhibit well-defined molecular selectivity [11], and carbon nanotubes, aerogels, and membranes are also amongst the most interesting advanced materials for research and development [12, 14]. 

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... 

Molecular sieving carbons (MSCs) have a smaller pore size with a sharper distribution in the range of micropores in comparison with other activated carbons for gas and liquid-phase adsorbates. They have been used for adsorbing and eliminating pollutant samples with a very low concentration (ethylene gas adsorption to keep fruits and vegetables fresh, filtering of hazardous gases in power plants, etc.) An important application of these MSCs was developed in gas separation systems [1-2]. 

Poly(m-phenylene isophthalamide) and other aramids have been proposed as precursor for activated carbon materials. These materials exhibit a very homogeneous nucropore size. This property makes them usable as adsorbents, molecular sieves, catalysts or electrodes. 

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. 

The Nitta et al. s equation (5.7-4) works satisfactorily for activated carbon and molecular sieving carbon with adsorbates of similar physical and chemical nature such as lower order paraffin hydrocarbons. This is exemplified with the experimental data of Nakahara et al. (1974) using methane, ethane, propane and n-butane on molecular sieving carbon 5A. The following table shows some typical parameters obtained using the Nitta et al. equation (5.7-1) assuming the interaction energy to be zero. 

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