Diffusion propane, propene

The separation of olefin and paraffins, particularly the C2 (ethane/ethene) and C3 (propane/propene) pairs is an extremely important and demanding separation in die petrochemical industry (56). It is currently performed via cryogenic distillation and is thus energy (and capital) intensive. Thus, the use of zeolites to perform this separation has been studied intensely. While many zeolites have been investigated to selectively adsorb the olefin, PSA-type approaches are not currently used for this separation. More recently, small-pore zeolites have been reported wherein a clear kinetic separation is observed in that the diffusion of propylene is dramatically faster than that of propane (57, 58). This potentially represents a significant breakthrough in the field. 

Van Den Broeke and Krishna [56] compared the calculated and the experimental breakthrough curves of single components and of mixtures containing methane, carbon dioxide, propane, and propene on microporous activated carbon and on carbon molecular sieves. They ignored the external mass transfer kinetics and assumed that there is local equilibrium for each component between the pore surface and the stagnant fluid phase in the macropores. They also assumed that the surface-diffusion contribution is much larger than that of pore diffusion and they neglected pore diffusion. They used in their calculations three different... 

Typical applications of zeolite membranes in reactors include i) conversion enhancement either by equilibrium displacement (product removal) or by removal of catalyst poisons/ inhibitors and ii) selectivity enhancement either by control of residence time or by control of reactant traffic. A large number of examples are reported and discussed in [49,50,52], Several of them are reported in fable 3. The use of a zeolite membrane as a distributor for a reactant has been attempted for the partial oxidation of alkanes such as propane to propene [137], or n-butane to maleic anhydride [138]. Limited performances were obtained because the back-diffusion of the alkane is hardly controllable with this type of microporous membrane [139]. 

Zn species have also a significant effet on the distribution of aromatics. Whether it be from propene or from propane, the production of the less bulky compounds (benzene, toluene, paraxylene) is more favored )n ZnHZSMS than on HZSMS. This can be related to diffusion limitations created by the Zn species and shown by the adsorption of nitrogen and of m-xylene (Table 1). 

The recently reported tracer ZLC data for propane and propene in NaX show a similar discrepancy and for these systems there is evidently a clear difference in the trend of diffusivity with sorbate loading, as well as in the order of magnitude of the diffusivity values (Fig. 15a). 

In 2011, Agel et al. [82] reported the use of the silver-containing IL AgfNTfjj for the separation of propene from propane. Based on solubility and diffusion studies, they predicted the best membrane selectivity for propene at partial pressures between 0.1 and 0.2 bar. 

A mixture of propene and propane was used to study the enhancement of absorption and diffusion by chemical reaction, namely dimerization of the reactive propene to hexenes according to Scheme 21.1 [86]. 

Olson and coworker showed for ZIF-8, that at 30°C propene adsorbs much faster than propene (Fig. 7). The pore size diameter of ZIF-8 adsorbents could be fine-tuned by Olson using chlorination, bromination, and methylation. Since propene is about 0.2-0.3 A smaller in its critical diameter than propane, the ratio of the diffusivifies of D(propene) D(propane)= 125 [13]. Small changes, big effects. Ref. [13] shows the surprising results that modifying the pore size of the MOF ZIF-8 by linker substitution, the sorption uptake rates of propane and propene can be controlled dramatically. Propene is only slightly (0.2. .. 0.3 A) smaller than propane, but its diffusivity can be 100 times higher. 

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