Carbon molecular sieve post-treatment

The extent of purification depends on the use requirements. Generally, either intense aqueous extractive distillation, or post-treatment by fixed-bed absorption (qv) using activated carbon, molecular sieves (qv), and certain metals on carriers, is employed to improve odor and to remove minor impurities. Essence grade is produced by final distillation in nonferrous, eg, copper, equipment (66). 

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... 

To explore the difficulties in practical implementation of the above concepts, mixed matrix membranes, with 20% molecular sieves (by volume), were prepared by solution deposition on top of a porous ceramic support. The ceramic supports used were Anodise membrane filters which had 200 A pores that open into 2000 A pores and offer negligible resistance to gas flow. Initially the molecular sieve media, zeolites (4A crystals) or carbon molecular sieves, was dispersed in the solvent, dichloromethane, to remove entrapped air. After two hours, Matrimid was added to the mixture, and the solution was stirred for four hours. The solutions used varied in polymer content from 1-5 wt %. The solution was then deposited on top of the ceramic support, and the solvent was evaporated in a controlled manner. The membranes were then dried overnight at 90°C under vacuum. This was followed by a reactive intercalation post treatment technique 15) to eliminate defects. This technique involves imbibing a reactive monomer (e.g. diamine) from an inert solvent (e.g. heptane) into any micro defects. Next, a second reactive monomer (e.g. acid chloride) was introduced to reactively close defects by forming a low permeability polymer. The membranes were dried again to remove the inert solvent. Individual membrane thickness was determined by weight gain and varied from 5 to 25 Jim. 

To obtain carbon membranes with molecular sieving properties, pore diameters in the range of a few angstroms are required. The associated pyrolysis procedures and post treatments are more involved. This subject will be treated later under section 3.2.10 Molecular Sieving Membranes. 

Activated carbons have a spread of pore sizes. Consequently the possibility that they can show a partial molecular sieve effect cannot be overlooked when the components of the binary solution are not of the similar molecular dimensions. This factor would add a degree of preferential adsorption of the components of smaller size molecules irrespective of the competitive adsorption due to other factors. The composite isotherms would, therefore, be of the type obtained on heterogeneous surfaces. This competitive adsorption effect wiU be more prominent and visible when carbons are produced from the same source raw materials by different procedure or post preparation treatments. For example, carbons that have been produced after varying degrees of activation or carbons that are heat treated at varying temperatures after activation will have different porosities and pore size distribution. The extremely fine micropores get partially blocked as the final heat treatment temperature exceeds 800°C to 900°C, due to the calcinations of the pores. This will produce molecular sieve effect depending upon the heat treatment temperature. 

In addition, the effect of pre-treatments and post-treatments dnring the membrane fabrication process shoitld also be examined. As an example, post-oxidation treatments can be very useful to tailor the properties of a molecular sieving carbon membrane in order to make an adsorption-selective carbon membrane, which is suitable for the separation of hydrocarbon mixtures based on adsorption differences. In fact. 

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