Adam Vidoni

Date of Award

12-2011

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Advisor

Douglas Ruthven

Second Committee Member

William DeSisto

Third Committee Member

Michael Mason

Abstract

This thesis reports the results of an experimental study aimed at characterizing the transport properties of DDR crystals (a pure silica zeolite analog) by the “zero length column” technique. This material is potentially useful as a size selective molecular sieve adsorbent for separation of CH4 – CO2 in the upgrading of low grade natural gas (or biogas) as well as for the separation of C3H6 - C3H8 for production of polypropylene. In both these applications pure silica zeolites (such as DDR) have important practical advantages over the traditional cationic zeolites since they are hydrophobic and have low catalytic activity. Intracrystalline diffusivities of CH4 in DDR were measured for the single component system and in the presence of an excess of CO2. In contrast to the predictions from recent molecular simulations the experimental data show that the diffusivity of methane is increased (rather than decreased) by the presence of CO2. This is as expected from transition state theory if CH4 and CO2 are competitively adsorbed. In contrast the data for C2H6 (and C2H4) show no significant difference in diffusivity in the presence of CO2, suggesting non-competitive adsorption. This result can be explained if it is assumed that C2 hydrocarbon molecules occupy preferentially the window sites. The equilibrium isotherms provide tentative support for this hypothesis. Some of the samples showed evidence of significant surface resistance to mass transfer (in addition to intracrystalline diffusional resistance). This led to a further development of the mathematical model used to analyze the ZLC response curves and hence to an extension of the ZLC technique to allow the simultaneous measurement of both the surface rate coefficient and the intracrystalline diffusivity. A detailed study of CO2 equilibrium on several different samples of both DDR and silicalite (another pure silica zeolite) was also undertaken in order to determine the effect of surface hydroxyl content.

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