Paraffins, selective sorption

Iso-paraffins too can be separated from aromatics on H-ZSM-5, a process that cannot be accomplished with either the small-pore or large-pore zeolites, such as A and Y. While the selectivity is not nearly as large as it is for n-paraffins, selective sorption of 2-methylheptane relative to p-xylene (selectivity factor = 6.7) was nevertheless observed. 

Highly selective sorption of aromatic compounds from paraffin-containing solutions has been reported for the faujasites NaX, NaY, and HY under equilibrium conditions (2). Thus, benzene is preferentially adsorbed relative to n-hexane or n-decane, p-xylene relative to n-octane, and naphthalene relative to n-decane. The measured separation factors in these systems are so large (K>700) that essentially only one species exclusively occupies the internal volume of the zeolites. 

In distinct contrast to the faujasites, the intermediate-pore zeolites ZSM-5 and ZSM-11 exhibited a marked preference for n-paraffins relative to aromatics. As can be seen from Table II, both H-ZSM-5 and H-ZSM-11 preferentially sorbed n-nonane from mixtures of nonane and p-xylene dissolved in an inert, non-sorbable solvent, 1,3,5-tri-isopropylbenzene. Selectivity factors greater than 40 were observed, despite the fact that both compounds were readily sorbed when higher zeolite/sorbate ratios were used. Highly selective sorption of n-heptane relative to naphthalene, and n-tetradecane relative to 1-phenyloctane, was also observed with H-ZSM-5. 

Linear paraffins are sorbed at substantially higher rates than branched hydrocarbons. For example, n-hexane sorbs at a much higher rate than even 3-methylpentane, which in turn sorbs more rapidly than 2,3-dimethylbutane. Molecules larger than trimethylbenzene are not sorbed. Clearly ZSM-5 zeolite has properties which make it unique for shape-selective sorption and catalysis. ... 

It has been shown that olefin/paraffin selectivity of glassy polymer membrane materials was mainly of diffusive nature." Indeed, with the pressure and temperature conditions currently used in the literature, sorption isotherms of C2/C3 olefins and paraffins in glassy polymer are very similar." " " 114,125... 

Despite the fact that both normal and monomethyl-substituted paraffins readily enter the pores of ZSM-5 and ZSM-11, preferential sorption of the normal isomer is observed under thermodynamic equilibrium, non-kinetically controlled conditions. Whereas small-pore zeolites, such as 5A and erionite, totally exclude branched hydrocarbons, and large-pore zeolites exhibit little preference, the intermediate pore-size zeolites ZSM-5 and ZSM-11 show a marked preference for sorption of the linear paraffin, even under equilibrium conditions. Competitive liquid phase sorption studies at room temperature indicated selectivity factors greater than ten in favor of n-hexane relative to... 

From the previous discussion, it follows that the intracrystalline volume in zeolites is accessible only to those molecules whose size and shape permits sorption through the entry pores thus, a highly selective form of catalysis, based on sieving effects, is possible. Weisz and coworkers 7) have conclusively established that the locus of catalytic activity is within the intracrystalline pores when Linde 5A sieve ( 5 A pore diameter) was used, selective cracking of linear paraffins, but not branched paraffins, was observed. Furthermore, isoparaffin products were essentially absent. With the same catalyst, -butanol, but not isobutanol, was smoothly dehydrated at 230-260°. At very high temperatures, slight conversion of the excluded branched alcohol was observed, suggesting catalysis by a small number of active sites located at the exterior surface. Similar selectivity between adsorption of n-paraffins and branched-chain or aromatic hydrocarbons is shown by chabazite and erionite (18). 

Since basic equilibria, kinetic and fixed-bed data of sorption of nCs and nCe in pellets of 5A zeolite were obtained, we are able to simulate a cyclic PSA process for the separation of n/iso-paraffins. The case selected is the patent data shown by Minkkinen et al. [3]. In such process, isomerisation of Cs/Ce normal paraffins with recycling of normal paraffins is described. The recycling is performed in a selective adsorption containing 38Kg of 5A zeolite pellets. In the selective adsorption (lenght=4m i.d=12.7cm) unit a PSA cycle takes place at 300°C. Adsorption phase occurs at a total pressure of 15 bars with a duration of 6 minutes. Desorption phase is performed in 6 minutes at 2 bars countercurrent to adsorption with a fraction of the iCs rich product. To obtain continuous operation two columns are used. The effluent of the isomerisation reactor contains approximately 13.9 mole % nCs and 4.6 mole % nCo. The performance of the unit... 

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