Sorbents carbon molecular sieves

Pore Size and Distribution. The pore diameter must be sufficient to allow the substances of interest to migrate into the pores to the adsorbing surface. Sorbents with very small pores, such as the carbon molecular sieves, are used to collect small molecules like permanent gases (e.g., methyl formate on Carbosieve B). 

Therefore, the solid sorbent should ideally have a low affinity to water, a characteristic easily met by the porous organic polymers. In contrast, the carbon-based sorbents and molecular sieves as well as some inorganic sorbents show a comparatively high uptake of water. If such sorbents are used in high-humidity environments special measures have to be employed to remove water during or after sampling. Possible methods are ... 

There are only four types of sorbents that have dominated the commercial use of adsorption activated carbon, molecular-sieve zeolites, sihca gel, and activated alumina. Estimates of worldwide annual sales of these sorbents are as follows (Humphry and Keller, 1997) ... 

GC/MS was used in a 1992 study (Phillips 1992) to detect carbon disulfide in human breath and environmental air with a detection sensitivity capable of 7.6lx 10"2 g/m3 (2.44xl0"2ppb). This highly sensitive technique can be rapidly accomplished by capturing the air sample on solid sorbent like molecular sieves to be later thermally desorbed in the laboratory with subsequent chromatography (Phillips 1992). 

Tenax GC sorbent has a known upper temperature limit of350 °C. It is a commonly used adsobent because of its high upper temperature limit and low background on desorption. Tenax is a porous polymer of 2,6-diphenyl phenol and has a packed density of approximately 0.22 g/mL (60/80 mesh). Chromosorb 106 is a cross-linked polystyrene porous polymer. It has a published upper temperature limit of 250 °C and a packed density of approximately 0.39 g/mL (60/80 mesh). Porapak N sorbent is a styrene/divinyl benzene porous polymer in wich vinyl pyrollidone is added to increase its polarity. The published upper temperature limit of Porapak N is 190 °C. Porapak N has a packed density of approximately 0.42 g/mL (60/80 mesh). Carbosieve B sorbent is a synthetic carbon molecular sieve and is one of the most retentive solid adsorbents available. It has an upper temperature limit of at least 400 °C and a packed density of approximately 0.22 g/mL (60/80 mesh). 

The sorbent may be passive (inert) or active depending on whether it is used only for moisture removal and then separated from the dried material or it becomes an integral part of the dry product. Typical inert sorbents are molecular sieves, zeolite, chabazite, activated carbon, bentonite, or silica gel. 

Alternative sorbent materials that are commonly used include granulized carbon black (GCB or Carbo-pack ), carbon molecular sieves (Carbosieve ), and Carboxen . These carbon-based materials are hydro-phobic. They also have higher thermal stability than Tenax, and can be used at 245°C. GCB has about the same trapping capacities as Tenax. It is often used in series with Carbosieve or Carboxen, which are strong adsorbents serving as an alternative to silica gel and charcoal. 

As discussed, for the full assessment of a sorbent, PSA simulation is needed. Kinetic PSA simulations with various degrees of simplifications are available. For instance, Schork et al. (1993) reported a kinetic PSA model with the assumption that the solid-phase concentration profiles along the bed are linear, and the computation can be substantially reduced. Their simplified model was used to evaluate carbon molecular sieves for N2 production from air. 

Carbon Molecular Sieves. Excellent carbon molecular sieve materials have been developed in industry. On these sorbents, the O2, N2, and Ar isotherms are approximately equal because they all adsorb by van der Waals interactions, and their polarizabilities are approximately the same. But the diffusivily ratio for 02(Ar)/N2 is approximately 30 due to differences in molecular size (Chen et al., 1994). 

A zeolite and carbon molecular sieves (CMS) have been examined for N2/CH4 separation. A process using 4A zeolite for this separation was developed by Habgood (1958), but this process was limited to low temperatures (—79 to 0°C) and a high-methane feed content (>90%). Ackley and Yang (1990) have demonstrated the use of carbon molecular sieve (CMS) for separation of N2/CH4 mixtures in pressure swing adsorption (PSA) processes but have also shown that the potential for CMS to achieve the desired pipeline quality (90% methane) is doubtful. The only two promising sorbents are clinoptilolites and titanosilicates, as discussed below. 

Carbon molecular sieves (CMS) are highly microporous materials having a preponderance of pores of < 1 nm. Among the various types of carbon, CMS materials represent one member of the family of activated carbons. CMS differ from activated carbons in the actual surface composition and the pore size distribution. Unlike CMS, activated carbons display far better detectable surface functionalities. CMS are finding a number of possible uses for the separation of air or other gases and in catalysis. CMS for use as air separation sorbents are usually made from activated carbons by a post-treatment that narrows the pore-size distribution to produce a material with a biomodal pore distribution having a predominance of pores < 0.6 nm [38]. Key to the performance of these materials is their size specific selectivity. CMS are similar to zeolites in that their porous structures have dimensions sized close to the critical dimensions of small to medium sized molecules, that is, the range between 0.3 and 1 nm. As a result, separations can be made on the basis of differences in molecular sizes and... 

The sorbent materials used to make the traps are mainly Tenax, silica gel, activated charcoal, graphitized carbon black (GCB or Carbopack), carbon molecular sieves (carbo-sieve such as Carbosieve-SIII), and Vocarb (Table 23.4). 

The purpose of this paper is to present such a model which can be regarded as a synthesis of fundamental considerations resulting from the activities in porous sorbents characterization and classical thermodynamics developments. This model is able to represent in a correct way adsorption data of oxygen, argon, nitrogen and methane on a given carbonaceous adsorbent using a unique pore size distribution function whatever the adsorbate. The procedure was applied on four different activated carbons and on a carbon molecular sieve. The adsorption isotherms were measured at 283 K, 303 K and 323 K and for pressures up to 2200 kPa. 

Thermally stable materials offering a wide range of sorbent strengths have been developed over the last 30 or 40 years for use in TD apphcations. These include standard carbon blacks (Carbographs and Carbopacks/traps), carbonised molecular sieves such as the Carboxen series and modified carbon blacks (Carbograph 5 TD and Carbopack X) which offer much of the strength of carbonized molecular sieves but with minimal water retention. All are now in widespread use as sorbents in thermal desorption tubes (Betz, et al., 1989). 

Characteristics of attrition and adsorption were investigated to remove CO2 in fluidized hed using activated carhon, activated alumina, molecular sieve 5 A and molecular sieve 13X. For every dry sorbent, attrition mainly still occurs in the early stage of fluidization and attrition indexs(AI) of molecular sieve 5A and molecular sieve 13X were higher than those of activated carbon and activated alumina. Percentage loss of adsorption capacity of molecular sieve 5A and molecular 13X were 14.5% and 13.5%, but that of activated carbon and activated alumina were 8.3% and 8.1%, respectively. Overall attrition rate constant (Ka) of activated alumina and activated carbon were lower than other sorbents. 

Therefore, in this study, activated carbon, activated alumina, molecular sieve 5A, and molecular sieve 13X were used as dry sorbents to control carbon dioxide in a fluidized bed. In addition, the attrition and percentage loss of adsorption capacity of the dry sorbents were investigated. 

Fig. 1 shows that minimum fluidization velocities of activated carbon, activated alumina, molecular sieve 5A and molecular sieve 13X are 8.0 cm/s, 8.5 cm/s, 6.2 cm/s and 6.5 cm/s, respectively. Also, theoretical calculation values of minimum fluidization velocity and terminal velocity of each dry sorbent were summarized in Table 1. 

Three general types of solid sorbents are mainly used for trapping VOCs in air inorganic sorbents like silica gels or molecular sieves, carbon-based porous materials and porous organic polymers. 

The main types of inorganic sorbents are silica gels, molecular sieves/zeolites, aluminum oxides and magnesium silicates. Carbon-based sorbents include activated charcoals, carbon blacks, graphihzed carbon blacks and graphitized molecular sieves. Styrene-divinylbenzene copolymers, ethylvinylbenzene/divinylbenzene... 

Carbon based sorbents and charcoal Molecular sieves... 

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