This work involves the detection and monitoring of solvent interactions with heavy oil and bitumen. Two nondestructive methodslow-field nuclear magnetic resonance (NMR) and X-ray computer-assisted tomography (CAT)were used. It is shown that low-field NMR can be a very useful tool in understanding the relationship of viscosity, density, and asphaltene precipitation in bitumen−solvent mixtures. Such mixtures are present in solvent-related heavy oil and bitumen recovery processes, such as vapor extraction (VAPEX). As a solvent comes into contact with a heavy oil or bitumen sample, the mobility of hydrogen-bearing molecules of both solvent and oil changes. These changes are detectable through changes in the NMR relaxation characteristics of both the solvent and the oil and can be correlated to mass flux and concentration changes. Based on Fick's second law, diffusion coefficients were calculated for combinations of three oils and six solvents. X-ray CAT scanning was also used in parallel for analysis of solvent diffusion into the bitumen. As the solvent was diffusing into the bitumen, a concentration gradient was obtained. Concentration values at certain times were used to calculate diffusion coefficients, which were compared with results obtained from NMR data, using both an analytical method and a numerical method. The diffusion coefficients were considered either as constants or as functions of solvent concentration in two models that have been developed during this research. The overall diffusion coefficients calculated for several pairs of oils and solvents at different ratios, both by NMR data and X-ray tomography, were on the order of 10-6 cm2/s.

A frequency response method using concentration variation is developed theoretically and experimentally and applied to investigate mixture diffusion in nanoporous adsorbents. The method is based on periodically time-varying species feed concentrations with a constant total molar inlet flow rate. It can be used without the need of a carrier gas. A mathematical model is formulated considering nanopore diffusion, surface barrier resistance, external film resistance, and axial dispersion. The related analytical solutions for frequency response are derived. For nanopore diffusion, a theory for nonconstant mixture Fickian diffusivity with cross-terms is developed from irreversible thermodynamics and shows that mixture Fickian diffusivities can be expressed as the product of corrected diffusivities and a thermodynamic factor that accounts for concentration dependence. The number of unknown variables for Fickian diffusivities is the same as the number for Onsager coefficients or Maxwell-Stefan diffusivities. Adsorption of CO 2 , CH 4 , and their mixtures on carbon molecular sieve (CMS) is investigated systematically for equilibrium and mass transfer rates. The mass transfer mechanisms for pure CO 2 and CH 4 on CMS measured by a pressure-swing frequency response method are found to be differentsthe rate-controlling mechanism for CO 2 is only nanopore diffusion, whereas the diffusion rate for CH 4 is limited mainly by a surface barrier resistance at the pore mouth of the CMS. All of the mixture experimental data are measured by the new concentration-swing frequency response method and are described well by the nonconstant Fickian diffusivity model with the thermodynamic factor derived for a multicomponent multisite-Langmuir isotherm.

Two flow-through frequency response methods of concentration-swing frequency response (CSFR) and pressure-swing frequency response (PSFR) are applied to study the adsorption of chloroethane on BPL activated carbon over a wide range of concentrations. Chloroethane at low concentrations in helium, 500 ppm V and 10 mol %, are studied using CSFR. Two models which treat an adsorption bed with axial dispersion or a simplified well-mixed concentration have been developed for both isothermal and nonisothermal conditions. The results show that heat effects can be negligible for the CSFR method. Pure chloroethane at relatively high concentrations is studied using PSFR. The experimental data are described well by a nonisothermal nanopore diffusion model. The diffusivities obtained from these two distinct methods are identical for the same concentration. The modeling of the concentration behavior depends quite sensitively on the isotherm model and obeys Darken's equation for the D-R isotherm. The results show that the frequency response method can distinguish the importance of heat effects and the relative importance of the mass-transfer mechanisms.

A simple apparatus using the pressure-swing frequency response method has been extended to measure multicomponent diffusivities. Transfer phenomena are investigated for different compositions of a binary mixture of nitrogen and oxygen in a carbon molecular sieve. The paper has two objectives. First, we consider the applicability of some predictive multicomponent diffusion models, which predict the mixture diffusivities based on pure-component information.The results show that these models can provide qualitative but not quantitative descriptions. Second, we determine mixture diffusivities, including Fickian diffusivities and Maxwell-Stefan surface diffusivities, from the data using a new mathematical model. These diffusivities are concentration-dependent. A simple relationship that introduces a thermodynamic factor is used to describe the concentration dependence of the Fickian diffusivities. It reduces to Darken's equation for pure components. This approach, based on nonconstant Fickian diffusivities, provides an excellent description for all of the experimental data. An alternative approach is also developed for obtaining the Maxwell-Stefan surface diffusivities without using the empirical Vignes relationship.