Biphasic Cellulose Acetate/Rtil Membranes and Functionalized Graphene Adsorbents for Natural Gas Processing: Experimental and Molecular Simulation Studies

摘要

In this dissertation, gas separation using membranes is investigated for natural gas upgrading. The main objectives of this study are separation of high value hydrocarbons such as propane (C3H8) from natural gas and carbon dioxide (CO2) separation from light gases such as nitrogen (N 2) and methane (CH4). To achieve these goals, supported ionic liquid membranes (SILMs), biphasic membranes, and nanoporous graphene (NPG) and graphene oxide (NPGO) membranes are studied.;Biphasic membranes are proposed to overcome SILMs issues for gas separation. The major issues with SILMs are low room temperature ionic liquid (RTIL) content and instability at high cross-membrane pressure. For this purpose, single and biphasic cellulose acetate (CA)/[emim][SCN] membranes were fabricated using the solution casting and solution casting/phase inversion methods, respectively. Infrared spectra and atomic force micrographs were generated to characterize the fabricated membranes. Moreover, the transport properties of CO2, N2, CH4, and C3H8 gases through the CA/[emim][SCN] dope membrane (single phase), cast biphasic CA/[emim][SCN] membrane, and supported [emim][SCN] membrane were determined using a batch gas permeance system and a continuous flow instrument. The results indicate that the SILM has the highest and the dope membrane has the lowest permeability for CO2 and C3H8. The cast biphasic membrane and SILM give almost similar permeabilities for these gases. The stability of the dope, biphasic, and SILM membranes are further determined, indicating there is a breakthrough point for all membranes. This point for the biphasic and SILM membranes corresponds to a similar pressure. This shows that biphasic membranes have potential to compete with SILMs for gas separation applications by improving casting procedure. The dope membrane is less stable at high pressures than the biphasic and SILM membranes, since it is in liquid state.;Molecular dynamics simulations were performed to gain fundamental molecular insights on the concentration-dependent adsorption and gas transport properties of the components in a CH4/CO2 gaseous mixture in single- and double-layered nanoporous graphene (NPG) and graphene oxide (NPGO) separation platforms. While these platforms are promising for a variety of separation applications, much about the relevant gas separation mechanisms in these systems is still unexplored. Based on the gas adsorption results in this work, at least two layers of CO2 are formed on the gas side of both NPG and NPGO, while no adsorption is observed for pure CH4 on the single-layered NPG. In contrast, increasing the CH4 concentration in the CH 4/CO2 mixture leads to an enhancement of the CH4 adsorption on both separation platforms. The through-the-pore diffusion coefficients of both CO2 and CH4 increase with an increase in the CH4 concentration for all NPG and NPGO systems. The permeance of CO2 is smaller than that of CH4, suggesting the NPG and NPGO platforms are more suitable as CO2 adsorbents or membranes for the CH4/CO2 (rather than the CO2/CH 4) separation. The highest observed selectivities for the CH4/CO 2 separation in the NPG and NPGO platforms are about 5 and 6, respectively.