Comparison between the kinetics of cyclohexane absorption and desorption for heterogeneous bitumen nanofilms. (1st January 2021)

• Record Type: Journal Article
• Authors: Kislitsin, Vadim; Choi, Phillip
• Is Part Of: Fuel. Volume 283(2021)
• Publication Date: 2021-01-01
• Subjects: Bitumen nanofilms -- Desorption -- Absorption -- Gravimetric analysis; Fuel -- Periodicals; Coal -- Periodicals; Coal; Fuel; Periodicals; 662.6
• DOI: 10.1016/j.fuel.2020.118836
• Languages: English
• ISSNs: 0016-2361

Abstract

Graphical abstract: Abstract: We used a gravimetric technique to study the kinetics of cyclohexane absorption and desorption for bitumen films with thicknesses ranging from ~50 nm to ~1000 nm by monitoring their relative mass uptake and release rates at 25 ℃ and 1 atm, respectively. In the absorption experiments, bitumen films were saturated with cyclohexane by exposing them to a carrier gas containing cyclohexane vapor at two relative saturations (RS) – 20% and 90% separately. These films were then used in the following desorption experiments. The absorption rate was about two times faster than the desorption rate. Furthermore, the absorption rate exhibited a linear inverse film thickness dependence while desorption a non-linear one. The cyclohexane diffusivity in the films exhibited a linear positive film thickness dependence with it being higher in absorption than that in desorption. The observations implied that amounts of cyclohexane dissolved and evaporated controlled the overall absorption and desorption rates, respectively. The inverse film thickness dependence of such rates was due to the decreasing influence of the hydrophilic substrate on the concentration gradient of bitumen molecules with different degrees of polarity in the films. Mass uptake was observed in the early stage of the desorption experiments of the 20% RS samples, especially those with low bitumen coverages. This observation seemed to be a feature of nanoscale films. We speculated that cyclohexane evaporated into the carrier gas adjacent to the sample surface dissolved back into the bitumen film as a result of a cyclohexane concentration gradient reversal.