Free butane/methane selectivity

It has been reported recently that flux and even selectivity of PMP and PTMSP can be enhanced by the addition of nanoparticles (285, 286]. Merkel et al. [285] added fumed sihca to PMP and observed a simultaneous increase of butane flux and butane/methane selectivity. This unusual behavior was explained by fumed-silica-induced disruption of polymer chain packing and an accompanying increase in the size of free volume elements through which molecular transport occurs. Gomes et al. [286] incorporated nanosized sihca particles by a sol-gel technique into PTMSP and found also for this polymer a simultaneous increase in flux and selectivity. It has to be studied, if physical aging of the polyacetylenes is reduced by the addition of nanoparticles. 

The mixed-gas transport behavior of PMP is qualitatively similar to that of PTMSP. The data in Table II show that PMP is significantly more permeable to -butane than to methane. For a feed gas mixture of 2 mol% n-butane in methane at a feed pressure of 150 psig and atmospheric permeate pressure at 25°C, the mixed-gas -butane/methane selectivity of PMP is 14 and the -butane permeability is 7,500 x 10 ° cm (STP) cm/cm s cmHg. This result indicates that the -butane/methane selectivity in PMP is dominated by a high solubility selectivity, similar to the behavior of high-free-volume, glassy PTMSP 5JO). The methane permeability of PMP in the mixture was reduced 5-fold by co-permeation of -butane. 

The n-butane/methane selectivity increased monotonically from 8 to 24 as the feed temperature was decreased from 50 to 2°C, as shown in Figure 5. It is important to note that both n-butane permeability and n-butane/methane selectivity increased in the PMP membrane as the feed temperature decreased. This temperature/permeability behavior is completely different from that in low-free-volume glassy polymers used... 

Interestingly, the performance enhancement was far greater when silica particles with hydrophilic surfaces were used. The authors attributed these results to unfavorable polymer/flller interaction, leading to an agglomeration of the long -alkyl groups at the surface of the polymer. An increase of butane permeability up to sixfold of unfilled polymer was obtained. On the other hand, when PTMSP (Lot 45-5641) of larger free volume was used, the addition of fumed silica increased the butane permeability but decreased the butane/methane selectivity. 

Nanocomposite membranes based on poly(4-methyl-2-pentyne) (PMP) and nano-sized fumed silica fillers have been put forward as a possible alternative to the less chemically resistant PTMSP for the removal ofC hydrocarbons (He et al, 2002). Specifically, an n-butane/methane separation factor of 26 and n-butane permeability of 19 000 Barrer were estimated when 30% hydrophilic silica was added to PMP, resulting in a simultaneous increase in n-butane/methane selectivity and n-butane permeability, in contrast to the more conventional trade-off relationships between selectivity and permeability in polymers. The addition of fumed silica in PMP enables this polymer to challenge PTMSP in terms of both efficiency and productivity, as it disrupts the molecular packing by fillers, which in turn causes a redistribution of the free volume (Merkel et al, 2002). 

Defect-free membranes comprising zeolites and amorphous glassy perfluoropolymers can be prepared by modifying the surface of the filler. The pure gas permeation experiments of a series of Teflon AF 1600 membranes with various amounts of 80 and 350nm silicalite-1 crystals cannot be interpreted on the basis of the Maxwell model, but are compatible with a model in which a barrier to transport exists on the zeolite surface and a lower density polymer layer surrounds the crystals. With a small zeolite size (80nm) the low density layers around the crystals may coalesce and form percolation paths of lesser resistance and less selectivity. Silicalite-1 crystals improve the CO2/CH4 selectivity of Hyflon AD60X, and drive the N2/CH4 selectivity beyond the Robeson s upper bound. It also turns out that the presence of silicaUte-l crystals, like fumed silica, promote the inversion of the methane/butane selectivity of Teflon AF2400 in mixed gas experiments. 

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