(241f) Facilitated Transport of Thiophenes through Ag2O-Filled P D M S Membranes
AIChE Annual Meeting
Tuesday, November 14, 2006 - 2:35pm to 3:00pm
Organosulfur impurities present in gasoline produce SOx in automotive exhaust and consequently lead to severe environmental problems. Worldwide concerns over environment have inspired an increasing interest both in academia and industry for deep desulfurization of gasoline. As a membrane based separation technique, pervaporation possesses great predominance and potential for desulfurization of gasoline, in which the separation mechanism is based on the difference of sorption and diffusion properties of the feed compounds. In our previous work, polydimethylsiloxane (PDMS) was selected for pervaporative desulfurization of model gasoline based on the solubility parameter principle and PDMS/PAN composite membranes were prepared for the purpose. With n-octane/thiophene as the primary components making up model gasoline, the fundamental problems in relation to the mechanism, model and contributing factors in the desulfurization process of gasoline by pervaporation were investigated. Experimental results showed that PDMS membrane is selective for thiophenes and considerable fluxes were gained. However, the separation of thiophene/hydrocarbon mixtures has not been much effective because the physico-chemical properties of thiophenes and various hydrocarbons are not efficiently distinguishable. According to the analysis of Robeson, greater permeability and higher selectivity couldn't be achieved simultaneously within solution-diffusion mechanism when using conventional polymeric membranes. But in heterogeneous membranes consisted of a polymeric phase and certain carrier particles, the reversible reactions between carrier and penetrant provide another transport mechanism through the membrane in addition to the solution-diffusion. Thanks to the complexation reaction between silver ion (Ag+) and the double bonds of the solute, Ag+ is mostly used as the carrier for the facilitated transport of unsaturated hydrocarbons. Investigation on desulfurization of transport fuels by adsorption also confirmed the existence of strong ?Î-complexation bond between Ag+ and thiophene or substituted thiophenes. Therefore, Ag+-containing salts or oxides could be reasonably employed to promote the transport of thiophenes within the membrane. In current work, silver oxide (Ag2O) was taken as the filler for preparation of the fixed-site carrier PDMS membranes, in which the facilitated transport of thiophenes was expected. Membranes with the following degree of Ag2O loadings were used: 0, 2, 5, 8 and 10wt. %. With n-octane/thiophene/2-methyl-thiophene as the model gasoline, the pervaporative desulfurization performance of the above membranes was investigated. Contributing factors including Ag2O contents in the membranes, feed temperature were evaluated experimentally. With the increase of Ag2O loadings in PDMS membranes, total fluxes decreased and higher feed temperature would make this trend more evident. Owing to the ultra low concentrations of thiophene and 2-methyl-thiophene in the model gasoline (corresponding sulfur contents about 1000µg/g), the majority of feed and permeate was n-octane. Within the fixed-site carrier PDMS membranes, the primary transport mechanisms for permeating molecules include the diffusion through the polymeric phase, diffusion through pores of the carrier, sorption onto the carrier, surface diffusion in the carrier and desorption from the carrier. For n-octane, the filled Ag2O particles are inert and completely impermeable. The diffusivity of n-octane would decrease due to the tortuosity effect and the total flux decrease accordingly. On the other hand, complexation between Ag+ and double bonds in thiophenes would accelerate their sorption onto and desorption from the carrier particles and lead to facilitated transport fluxes of thiophenes. According to RC circuit model, the continuous reversible reaction between the carrier and solute would result in fluctuation of local concentration in the membrane, which leads to a higher free energy, a higher chemical potential and a higher flux for the desired component. Selectivity to thiophenes would increase because of the facilitation effect in Ag2O filled membranes. With the membranes having a normalized PDMS layer of 15µm, when Ag2O loadings increased from 0 to 5wt.%, the total fluxes decreased from 3.31 to 2.85 kg/m2h at 50°C, while the corresponding enrichment factors increased from 3.55 to 4.46 and from 2.24 to 2.61 for thiophene and 2-methyl-thiophene respectively. Effect of feed temperature on the pervaporative desulfurization efficiency using Ag2O-filled PDMS membranes was also evaluated. Without exception, all the experimental evidence confirmed an increase in permeability with increasing feed temperature for both filled and unfilled membranes. Two parallel impacts of temperature on the permeant and membrane should be responsible for this: one is the increase of the mobility of individual permeating molecules which promotes their movement both in the bulk feed solution and within the membrane, one is the enhanced mobility of the polymer segments which offers more free volume for permeating molecules to occupy. As a minor contributing factor, changes occurring on the carrier-polymer interface played a certain role in the results obtained. At the same time, the increase in the degree of swelling of the membrane with temperature resulted in a decrease in selectivity to thiophenes. Additionally, experimental results indicated that extent to which feed temperature affected the enrichment factors of thiophenes was different in filled and unfilled membranes. It is so mainly because, with the variation in feed temperature, the facilitated fluxes of thiophenes in Ag2O filled membranes were dominated by the reversible reaction between Ag+ and thiophenes, although solution-diffusion fluxes both in filled and unfilled membranes followed nearly the same relationship. Based on the experimental data gained in our earlier work, the upper bound curves for separation of n-octane/thiophene mixtures using unfilled PDMS membranes were constructed. There is a trade-off between the permeability and selectivity of thiophenes. Being consistent with the quantitative analysis, Ag2O filled PDMS membranes can surpass the upper bound for the facilitated transport mechanism within the process. Acknowledgement: The authors want to express their gratitude to Ministry of Science and Technology of China for the financial support (National 973 Project, No. 2003CB615701).
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